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Enhancing the Photoactive Surface Area of Dye Sensitized Solar Cells using Supercritical Fluids

Permanent Link: http://ufdc.ufl.edu/UFE0042416/00001

Material Information

Title: Enhancing the Photoactive Surface Area of Dye Sensitized Solar Cells using Supercritical Fluids
Physical Description: 1 online resource (124 p.)
Language: english
Creator: Rajab, Fahd
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: fto, hipco, ito, n719, n749, nanotubes, supercritical, swan, titania
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The contribution of this dissertation is primarily related to surface interfaces and methods for enhancing materials properties. I discuss optimum extraction conditions for purifying single wall carbon nanotubes using supercritical carbon dioxide without altering their unique properties. Hipco and Swan single wall carbon nanotubes (SWCNTs) were purified using sc-CO2 at temperatures ranging from 35 to 75degreeC and pressures ranging from 250 to 400 atm. Single and multiple extractions were performed with up to 80 % removal efficiency. Also, I discuss preparation of thick defect free titania films on fluorine-doped tin oxide (FTO) and tin-doped indium oxide (ITO) substrates for dye sensitized solar cells. The sintering and annealing steps were controlled by reducing the pressure and the rate of temperature change to reduce the stresses generated during film preparation, allowing the generation of thick titania films on both FTO and ITO with minimal defects. Films as thick as 25 microm were prepared on FTO substrates with a defect density of only 6.0 %. DSSCs fabricated with titania films on FTO substrates at different vacuum levels showed enhanced device performance compared with standard cells. I show a nondestructive dye analysis technique to measure dye absorbance on solid films. Using dye absorbance in the solid film and profilometry to measure film thicknesses, extinction coefficients and mole concentrations for N-719 and N-749 dyes adsorbed onto titania were determined. The nondestructive approach was able to generate concentration and current spatial profiles, which illustrate the effect of multilayer dye formation on device performance. Moreover, I show improvement in device efficiency based on stable thick titania films impregnated with N-719 and N-749 ruthenium dyes for DSSCs using conventional dip and supercritical fluid methods. DSSCs fabricated with these films show up to 9.44 % and 6.57 % efficiency for N-719 and N-749, respectively. Although these efficiency levels remain below recorded efficiency values, the results of this research not only provide better understanding of the important factors but also suggest that higher efficiencies can be obtained if thicker films are impregnated with a suitable dye using the supercritical fluid approach.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Fahd Rajab.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Ziegler, Kirk.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042416:00001

Permanent Link: http://ufdc.ufl.edu/UFE0042416/00001

Material Information

Title: Enhancing the Photoactive Surface Area of Dye Sensitized Solar Cells using Supercritical Fluids
Physical Description: 1 online resource (124 p.)
Language: english
Creator: Rajab, Fahd
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: fto, hipco, ito, n719, n749, nanotubes, supercritical, swan, titania
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The contribution of this dissertation is primarily related to surface interfaces and methods for enhancing materials properties. I discuss optimum extraction conditions for purifying single wall carbon nanotubes using supercritical carbon dioxide without altering their unique properties. Hipco and Swan single wall carbon nanotubes (SWCNTs) were purified using sc-CO2 at temperatures ranging from 35 to 75degreeC and pressures ranging from 250 to 400 atm. Single and multiple extractions were performed with up to 80 % removal efficiency. Also, I discuss preparation of thick defect free titania films on fluorine-doped tin oxide (FTO) and tin-doped indium oxide (ITO) substrates for dye sensitized solar cells. The sintering and annealing steps were controlled by reducing the pressure and the rate of temperature change to reduce the stresses generated during film preparation, allowing the generation of thick titania films on both FTO and ITO with minimal defects. Films as thick as 25 microm were prepared on FTO substrates with a defect density of only 6.0 %. DSSCs fabricated with titania films on FTO substrates at different vacuum levels showed enhanced device performance compared with standard cells. I show a nondestructive dye analysis technique to measure dye absorbance on solid films. Using dye absorbance in the solid film and profilometry to measure film thicknesses, extinction coefficients and mole concentrations for N-719 and N-749 dyes adsorbed onto titania were determined. The nondestructive approach was able to generate concentration and current spatial profiles, which illustrate the effect of multilayer dye formation on device performance. Moreover, I show improvement in device efficiency based on stable thick titania films impregnated with N-719 and N-749 ruthenium dyes for DSSCs using conventional dip and supercritical fluid methods. DSSCs fabricated with these films show up to 9.44 % and 6.57 % efficiency for N-719 and N-749, respectively. Although these efficiency levels remain below recorded efficiency values, the results of this research not only provide better understanding of the important factors but also suggest that higher efficiencies can be obtained if thicker films are impregnated with a suitable dye using the supercritical fluid approach.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Fahd Rajab.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Ziegler, Kirk.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042416:00001


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1 ENHANCING THE PHOTOACTIVE SURFACE AREA OF DYE SENSITIZED SOLAR CELLS USING SUPERCRITICAL FLUIDS By FAHD MOHSIN RAJAB A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010

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2 2010 Fahd Mohsin Rajab

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3 To my parents and wife: Thank you for your unconditional support with my studies

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4 ACKNOWLEDGMENTS I would like to thank my advisor Kirk J. Ziegler and the faculty in my committee for their support and guidance. I would also like to thank the undergraduate students who assisted with this work especially David Loaring and Jake Hyvonen. I would like to acknowledge the faculty and staff at Major Analytical Instrumentation Center, Interdisciplinary Center for Biotechnology Research, Particle and Engineering Research Center and Chemical Engineering Department at University of Florida especially Kerry Siebein, Valentin Craciun, Karen Kelley and Jim Hinnant. I would also like to acknowledge the University of Florida Opportunity Fund and the Donors of the American Chemical Society Petroleum Research Fund for support of this research.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................. 4 LIST OF TABLES ............................................................................................................ 7 LIST OF FIGURES .......................................................................................................... 8 ABSTRACT ................................................................................................................... 11 CHAPTER 1 INTRODUCTION .................................................................................................... 14 Problem Description ............................................................................................... 14 Objectives and R esults ........................................................................................... 16 Dye Sensitized Solar Cells ...................................................................................... 17 Supercritical Fluids ................................................................................................. 18 Electrodeposition .................................................................................................... 20 2 SUPERCRITICAL FLUID PURIFICATION OF HIPCO AND SWAN SINGLE WALL CARBON NANOTUBES (SWCNTS) ............................................................ 24 Initial Remar ks ........................................................................................................ 24 Materials and Experimental .................................................................................... 25 Results and Discussion ........................................................................................... 28 Closing Findings ..................................................................................................... 31 3 PREPARING THICK, DEFECT FREE FILMS OF ANATASE TITANIA FOR DYE SENSITIZED SOLAR CELLS ................................................................................. 42 Preliminary Remarks ............................................................................................... 42 Experimental Section .............................................................................................. 44 Results .................................................................................................................... 47 Discussion .............................................................................................................. 50 Conclusionary Findings ........................................................................................... 53 4 NONDESTRUCTIVE MEASUREMENT OF DYE ADSORPTION ONTO TITANIA FILMS ...................................................................................................... 64 Preliminary Remarks ............................................................................................... 64 Materials and Methods ............................................................................................ 65 Results and Discussion ........................................................................................... 67 Concluding Remarks .............................................................................................. 70

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6 5 ENHANCING THE PHOTO ACTIVE SURFACE AREA OF DYE SENSITIZED SOLAR CELLS ....................................................................................................... 78 In itial Remarks ........................................................................................................ 78 Materials and Methods ............................................................................................ 80 Results and Discussion ........................................................................................... 83 Concluding Remarks .............................................................................................. 86 6 CONCLUSION ........................................................................................................ 96 APPENDIX A SUPERCRITICAL FLUID PURIFICATION OF (PREPURIFIED) SINGLE WALL CARBON NANOTUBES (SWCNTS) ...................................................................... 98 B ELECTRO DEPOSITION AND TEMPLATE REMOVAL USING SUPERCRITICAL CARBON DIOXIDE ................................................................. 105 Nanowire Electrodeposition Using Supercritical Carbon Dioxide .......................... 105 Template Removal Using Supercritical Carbon Dioxide ....................................... 109 Conclusions and Future Avenues of Research ..................................................... 109 C ANODIZING ALUMINUM AND NANOWIRE ELECTRO DEPOSITION ................ 113 LIST OF REFERENCES ............................................................................................. 116 BIOGRAPHICAL SKETCH .......................................................................................... 124

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7 LIST OF TABLES Table page 1-1 Diffusivities and viscosities of liquids, supercritical fluids and gases. ................. 22 2-1 HiPco carbon nanotubes metal oxide content (wt %) with residual tri butyl phosphate (TBP). ............................................................................................... 32 2-2 HiPco carbon nanotubes metal oxide content (wt %) after subtraction of r esidual tri butyl phosphate TBP ......................................................................... 32 2-3 Metal ions removed from HiPco and Swan SWCNTs.. ....................................... 33 3-1 Experimental conditions for preparing thick, defect free titania films on ITO and FTO substrates. ........................................................................................... 54 3-2 Efficiency of DSSCs made with 10 20 m titania films on FTO subst rates sint ered at varied pressures at 450C.. .............................................................. 55 4-1 Measured molar extinction coefficients for N 719 and N 749 dyes. .................... 72 5-1 The efficiency of the best DSSCs made with FTO substrates with titania films of various thicknesses. ....................................................................................... 87

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8 LIST OF FIGURES Figure page 1-1 Capillary condensation/ evaporation for liquid solvents in nanopores. ................ 23 2-1 Setup of carbon nanotubes metal extraction using supercritical carbon dioxide.. .............................................................................................................. 34 2-2 TGA of raw and treated HiPco carbon nanotubes.. ............................................ 34 2-3 Tri butyl phosphate (TBP) residuals in HiPco carbon nanotubes.. ..................... 35 2-4 Final metal oxide content of treated HiPco carbon nanotubes.. .......................... 35 2-5 TGA of raw and treated Swan carbon nanotubes .............................................. 36 2-6 TGA of raw and treated HiPco carbon nanotubes by multiple extractions.. ........ 36 2-7 Final metal oxide content of treated Swan carbon nanotubes.. .......................... 37 2-8 TEM images of HiPco and Swan carbon nanotubes treated by sc CO2 extraction. ........................................................................................................... 38 2-9 TEM image of HiPco carbon nanotubes treated by the one pot method.. .......... 38 210 Raman spectroscopy of raw and treated HiPco carbon nanotubes.. .................. 39 211 Raman spectra of raw and treated Swan carbon nanotubes.. ............................ 39 212 Fluorescence of raw and treated HiPco carbon nanotubes.. .............................. 40 213 Fluorescence of (10 %) Swan and treated Swan carbon nanotubes.. ................ 41 3-1 The preparation steps for making thick titania films on transparent conductive oxide (TCO) substrates.. .................................................................................... 56 3-2 S EM image of titania film (~25 m thick). ........................................................... 57 3-3 Xray diffraction of 20 m thick titania films. ....................................................... 57 3-4 Raman spectra of m titania films on FTO substrates. ....................................... 58 3-5 Microscopic images of the final titania films on FTO. .......................................... 59 3-6 The effect of sintering pressure on the defect density of titania films. ................ 60 3-7 Microscopic images of the final titani a films cooled at a rate of 1 C/min.. ........ 61

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9 3-8 The effect of the cooling rate during annealing on the defect density of titania films. ................................................................................................................... 61 3-9 Performance characteristics of DSSCs fabricated from photoanodes sintered at different pressures.. ........................................................................................ 62 310 SEM images of 20 m titania films sintered at varied pressures.. ...................... 63 4-1 N719 and N 749 dye extinction coefficient in NaOH.. ....................................... 73 4-2 Spectroscopic peaks of N 719 and N 749 dyes in NaOH solution and in solid titania ................................................................................................................. 74 4-3 Absorbance of N 719 and N 749 dyes adsorbed onto the solid titania film or dissolved in NaOH. ............................................................................................. 75 4-4 Compariso n of the concentration of N 719 and N 749 dye determined by solid state or NaOH solution ............................................................................... 76 4-5 Spatial profile s of N719 dye absorption and short circuit current in 10 m titania film.. ......................................................................................................... 77 5-1 Electron transport and recombination paths between the semiconductor and the electrolyte.. ................................................................................................... 88 5-2 Absorbance of 7, 10 and 20 m titania films in N719 dye ................................. 88 5-3 Solubility of N719 dye in sc CO2 using in situ UV VIS measurements. ............. 89 5-4 Solubility of N719 in scCO2 using cloud point measurements. ......................... 90 5-5 Absorbance of impregnated titania films by UV VIS spectroscopy. .................... 91 5-6 Per formance characteristics of DSSCs fabricated from photoanodes in N 749.. ................................................................................................................... 92 5-7 Efficiency and electron life time of DSSCs fabricated from photoanodes in N 749.. ................................................................................................................... 93 5-8 Performance characteristics of DSSCs fabricated from photoanodes in N 719.. ................................................................................................................... 94 5-9 The effect of t butanol and concentration of N 719 on the efficiency of DSSCs.. .............................................................................................................. 95 A-1 Raman spectra of raw and treated prepurified carbon nanotubes.. ................. 103 A-2 Fluorescence of raw and treated prepurified carbon nanotubes.. .................... 103

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10 A-3 Final metal oxide content of treated prepurified carbon nanotubes.. ............... 104 B-1 High pressure system for electrodeposition.. .................................................... 112 B-2 SEM image of 200 nm whatman anodisc 13. ................................................... 112 B-3 TEM image of nickel deposits from a watt bath solution. .................................. 112 C-1 Substrates after deposition of Cr and Al layers, wrapped in Al foil for silica deposition. ........................................................................................................ 115

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11 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ENHANCING THE PHOTOACTIVE SURFACE AREA OF DYE SENSITIZED SOLAR CELLS USING SUPERCRITICAL FLUIDS By Fahd Mohsin Rajab December 2010 Chair: Kirk Jeremy Ziegler Major: Chemical Engineering The contribution of this dissertation is primarily related to surface interfaces and methods for enhancing materials properties. I discuss optimum extraction conditions for purif ying single wall carbon nanotubes using supercritical carbon dioxide without altering their unique properties. Also, I discuss preparation of thick defect free titania films on different substrates for dye sensitized solar cells by reducing film stresses. I show a nondestructive dye analysis technique to measure dye absorbance on solid films and generate concentration and current spatial profiles, which illustrate the effect of multilayer dye formation on device performance. Moreover, I show improv ement in device efficiency based on stable thick titania films impregnated with a ruthenium dye using supercritical carbon dioxide. One of the most important challenges the world faces today is clean and affordable energy. Photovoltaic devices such as dye sensitized solar cells (DSSCs) present a promising solution for conversion of light to energy efficiently. The recorded ef ficiency of these devices is 10 to 11 %, which currently makes them uneconomical for production at large scale. Fundamental to such devices is the harvesting of sunlight molecules by a thin film of wideband semi conductor nanoparticles coated with a dye.

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12 The amount of photons harvested depends on dye thickness. Cell efficiency is a measure of electrons transfer through a wideband semiconductor s uch as titanium dioxide (TiO2), to generate current between the two electrodes. T he typical conductive electrodes can be replaced with an alternative conductive film made w ith carbon nanotubes. My effort was focused on purifying the carbon nanotubes from their inherent metal impurities using supercritical carbon dioxide (scCO2) Hipco and Swan single wall carbon nanotubes (SWCNTs) have been purified using scCO2 at temperatures ranging from 35 to 75C and pressures ranging from 250 to 400 atm. Single and multiple extractions were performed with up to 80 % removal efficiency. E nhancing the photoactive surface area of DSSCs using a supercritical fluid was examined. First, the solubility of two ruthenium dyes in scCO2 was investigated. This part of the study required purchasing of all the equipments and parts needed to carry out the experiments as well as understanding the limitations of the setup. The solubility of ruthenium dyes in supercritical carbon dioxide, performed with the inclusion of cosolvent in a concentration of 5 to 30 % at 35 to 55C and 100 to 400 atm, was measured by UV VIS spectroscopy and cloud point measurements. Second, a natase titania nanoparticle films were prepared on fluorine-do ped tin oxide (FTO) and tin doped indium oxide (ITO) substrates T he sintering and annealing steps were controlled by reducing the pressure and the rate of temperature change to reduce the stresses generated during film preparation, allowing the generation of thick titania films on both FTO and ITO with minimal defects. Films as thick as 25 m were prepared on FTO substrates with a defect density of only 6.0 % DSSCs fabricated with

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13 titania films on FTO substrates at different vacuum levels showed enhanced device performance compared with standard cells. The cells prepared at intermediate pressures showed higher short circuit current, opencircuit voltage and efficiency. Third, a nondestructive method was developed to quantify the adsorption of dyes in titania films for DSSCs UVVIS spectroscopy showed that th e absorbance of dyes in the solid film scales with the absorbance from the typical destructive method used to quantify dye adsorption. Using dye absorbance in the solid film and profilometry to measure the thicknesses of titania films on FTO substrates, extinction coefficients for N 719 and N 749 dyes adsorbed onto titania were determined. The nondestructive approach was able to calculate mole concentrations of both dyes for film thicknesses of 7 to 20 m. This method also provided spatial profiles of dye concentration and short circuit current of an impregnated film which showed optimum coverage was required for best device performance. Finally, N719 and N749 ruthenium dyes were used to impregnate up to 20 m of a thick anatase titania films on FTO for DSSCs using conventional dip and supercrit ical fluid methods. The dyes were dissolved in different solvent mixtures includin g scCO2. DSSCs fabricated with these films show up to 9.44 % and 6.57 % effic iency for N 719 and N 749, respectively. Although these efficiency levels remain below recorded efficiency values the results of this thesis not only provide better understanding of the important factors but also suggest that higher efficiencies can be obtained if thicker films are impregnated with a suitable dye using the supercritical fluid approach.

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14 CHAPTER 1 INTRODUCTION Problem Description Nanomaterials most important characteristic is high surface area. Transport in high surface area and porous structures, is essential for utilization of nanomaterials unique properties. However, due to capillary effects and surface forces, transport in nanopores can be limited at the nanosize level. This hinders use of nanomaterials in many applications where purity and integrity are needed for performance and better efficiencies. An important application that presents these problems is dye sensitized solar cells (DSSCs). Fundamental to this device is harvesting of sunlight molecules by a thin film of wide band semi conductors nanoparticles coated with a dye. The amount of photons harvested depends on dye thickness. Cell efficiency is a measure of electrons transfer through the wideband semiconductor to generate current between the two electrodes [1] DSSC electrodes and carbon nanotubes. DSSC electrodes are transparent conducting oxide (TCO) films on glass substrates. Film preparation techniques such as physical vapor deposition are costly and time consuming. The conductivity of the currently used films, fluorine-do ped tin oxide (FTO) or tin doped indium oxide (ITO) is reduced as films are made more transparent. Single w all carbon nanotubes (SWCNTs) can be used to make cost effective, highly transparent and conductive films for charge transport [2] H owever, they have inherent metal impurities that limit use of their electronic properties. Most purification techniques use acids which affect the integrity of the SWCNT s structure or give poor extraction efficiencies due to capillary effects.

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15 NanoparticleDSSC. A key factor for efficiency in DSSCs is the high photo active surface area of the semiconductor. However, most DSSCs utilize only a 10 m film with a monolayer of dye coverage because of electron diffusion length and reduced electron collection efficiency. A thick semiconductor nanoparticle film can provide a high photoactive surface area if dye coverage is enhanced. Thick films in general show defects due to compressive or tensile forces that lead to film failure. NanowireDSSC. E lectron transport was found to be low in a nanoparticle film due to electrons percolation through a random network of the polycrystalline material. Nanowires enable faster electron transport but lack the high surface area. Therefore, synthesis of high density nanowire array s is essential for achieving both high surface area and faster electron transfer. Template synthesis of nanowires by electrodeposition in electrolyte solution can lead to incomplete pore inclusion of templates channels due to hydrogen hydrolysis and surface tension of electrolyte solution. It can also produce polycrystalline materials with larger grain sizes and pin holes due to hydrogen hydrolysis which can create electron trap sites. When nanowire arrays are synthesized, the template is dissolved in an acid solution. However, template removal by acids allows surface forces to cause aggregation of the deposited nanowires, which prevents their effective use where a uniform high density nanowire array is required. Dye coverage efficiency When films are impregnated with dye, they are typically dissolved in a solution to measure their concentration from their absorbance. This limits ability to investigate DSSC efficiency based on film dye coverage. The time range for optimum impregnation varies depending on film properties and solvent mixtures. Optimum dye coverage of the semiconductor can increase the photo active surface

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16 area and promote light harvesting efficiency. However, capillary effects and s urface forces of liquid solvents can limit transport of dye and lead to poor dye coverage. Objectives and Results In c hapter 2, purification of carbon nanotubes using supercritical carbon dioxide is investigated with nanotubes of different properties made with d ifferent synthesis processes The results of an initial study on purification of carbon nanotubes using supercritical carbon dioxide is discussed in appendix A. In c hapter 3, anatase thick titania films on transparent conductive glass are prepared with minimum defect density by controlling sintering and cooling conditions. DSSCs are prepared with these films and show higher device performance. In chapter 4, a nondestructive dye adsorption technique is developed to allow concentration analysis of dye on solid titania films used in fabricated DSSCs Optimal dye adsorption time ranges are determined for titania films with different thickness. In c hapter 5, w et and gas dye coverage methods are examined for ruthenium dyes dissolved in various (co)solvent mi xtures for impregnating thick titania films. The efficiencies of fabricated DSSCs are determined and parameters show importance of optimum transport and film thickness for device efficiency The rest of this chapter discusses background on dye sensitized s olar cells, supercritical fluids (SCF) and electrodeposition. Miscibility of hydrogen in SCF miniemulsions should prevent pin holes formations and pores blockage in nanowire synthesis by hydrogen hydrolysis. Electrodeposition with SCF miniemulsions is s imilar to pulse electrodeposition, which should enhance smaller grain sizes and better crystallinity. Template removal using a SCF should prevent capillary effects that cause aggregation. In appendix B the theory and preparation work for synthesis of nanowires using scCO2 is discussed. This part of my

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17 research was halted due to equipment shortage and interest in pursuing the typical method for synthesis of nanowire s, which is discussed in appendix C Dye Sensitized Solar Cells Dye sensitized solar cells (DSSCs) are potential future devices for converting light to electricity at la band gap semiconductor film such as titania (TiO2) or zinc oxide (ZnO) nanoparticles, which provides a large internal surface area for sufficient chromophore to yield high light absorption in the 400 800 nm region, where much of the solar flux is incident, a monolayer of organic dye molecules absorbed onto the semiconductor, and a liquid electrolyte containing the redox couple I /I3which interpenetrates the dyecoated nanoparticles [1] When a photon excites the electron in the dye, it is injected into the conduction band of a wide band gap semiconductor and is diffused across it to the current collector (anode) and carried to the cathode where it reduces I3in the electrolyte. Then, the dye is regenerated by the I thereby closing the cyclic conversion of ligh t to electricity [3,4] Under sunlight, an injected electron may experience many trapping events before either penetrating to the electrode or recombining with an oxidizing species, primarily I3in the electrolyte. The main parameters of the electron transport in DSSCs were determined to be the light intensity of the irradiation light, semiconductor film thickness, concentration of iodine, and nanoscale titania materials that make the semiconductor electrode [5] A high electron diffusion coefficient and a low recombination rate constant are required for highly efficient dyesensitized solar cells. Attempts to improve cell efficiency above its record of 1011 % by thickening the nanocrystalline film to improve its light absorption at red wavelengths have failed because increasing film thickness is

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18 limit ed by electron diffusion length. Also, higher dye loadings for photon harvesting show low diffusion and high recombination [1,6] A typical dye sensitized solar cell is fabricated on transparent conducting oxide (TCO) glass. The conductive coating is either fluorinedoped tin oxide or tindoped indium oxide. Screen printing transparent titanium dioxide is deposited by spreading with a blade or by spin coating. Sintering is done at 450C for inter particle connectivit y. Film thickness is between 10 to 20 65 %, average pore size is 15 nm and particle diameter is 15 to 20 nm. Ruthenium dyes such as ruthenium 620 are used to sensitize wide bandgap oxide semiconductors like titanium dioxide efficiently up to a wavelength of 920 nm. The counter electrode is fabricated from FTO coated glass, with a platinum layer as catalyst for the reduction of the redox electrolyte at the counter electrode. The redox electrolyte iodide, tri iodide (I /I3) is used for electrical contact between the electrodes and for dye regeneration [7] Cell efficiency n is given by equation (1 1): in sc ocP jV FF n (1 1) where Pin is light density, Voc is voltage at open circuit, and Jsc is current at short circuit Fill factor ( FF) is given by equation (12) : sc oc mpJV P FF (1 2) where Pmp is power at maximum point (Vmp*Jmp) [1,8] Supercritical Fluids Supercr itical fluid (SCF) is a substance above its critical pressure, temperature and density but below the pressure required to condense it to a solid. It has properties

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19 of both liquids and gases; it can dissolve solid compounds like liquids because of its solve nt strength and it has high mass transfer rate like gases due to its high diffusivity and low viscosity. Solvent strength of a SCF depends on density and can be adjusted by tuning the density of fluid phase by varying temperature and pressure. Therefore, conducting reactions in SCFs enables control of viscosity, diffusivity and surface tension. Nanomaterials of better properties have been synthesized using SCFs [9,10] Diffusion is inversely proportional to viscosity as given by Einstein Stokes relation in equation (1 3) : r TK DB6 (1 -3) where KB diffusing particle [11] S upercritical fluids viscosities are one to two orders of magnitude less than liquids, hence diffusivities are much higher see Table 1-1 [12] SCFs are environmentally friendly and have been investigated as replacements for hazardous organic solvents. The most common supercritical fluids are water and carbon dioxide. Supercritical carbon dioxide (sc CO2) critical pressure and temperature are 72.8 bars 31.1 [12] Sc CO2 is nontoxic, nonflammable, and cheap (second least expensive after water). SCFs most important property is their zero surface tension. Transport in nanopores is critical for reactions and synthesis in nanomaterials. Surface tension, vapor pressure and potential energy from pore wall can affect phase properties within nanopores, causing capillary effects such as capillary condensation or evaporation. The Kelvin equation (1 4) describes the effect of surface tension and vapor pressure on condensation or evaporation in cylindrical pores with zero contract angles:

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20 tR V P P KTp sat v 2 ln (1 -4) where K is Boltzmanns constant, T is temperature, P is pressure, and the subscript v indicates vapor pressure and sat indicates saturation, is the surface tension of the liquid, Vp is the volume per molecule of liquid, R is the radius of the pore and t is the thickness of the phase inside the pore [1316] Capillary evaporation and condensation of various liquid solvents in cylindrical nanopores was modeled using the Kelv in equation. As seen in Figure 1-1 capillary evaporation drops in nanopores of less than 60 nanometers (nm). However, the drop is at maximum in pores less than 10 nm. SCFs can be utilized in such nanopore sizes especially for their zero surface tensions. Electrodeposition The main components of an electro deposition cell are electrolyte containing metal ions e lectrodes ( cathode and anode) and a power source. The metal ions in the solution are reduced to the metal form by gain of electrons at the cathode surface. The metal solution interface is the locus of electro deposition process. It is where the interaction between the electrically conductive electrode and the ionic conductive solution takes place. The properties of water and ionic solutions are of great relevance to the properties of the two metal solution interfaces [17,18] In an ionic water solution, there are ionion interactions and ionwater interactions. In ionion interactions, each positive ion is surrounded by atmosphere of negative ions and vice versa. The solution is l argely neutral but cations and anions are not distributed uniformly in an ionic solution. T he main forces in ion water interaction are iondipole interactions, which cause the orientation of water molecules in the immediate vicinity of an ion. This in effect causes the formation of primary shell of with completely oriented

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21 water molecules and secondary region of partially oriented water molecules. The description of the metal ionic solution interface and the two regions and the prediction of potentials are best described in Grahame Triplelayer model, which has two planes of closest approach; ( IHP ) inner plane of partially or fully dehydrated ions and ( OHP) outer plane of closes approach of fully dehydrated ions [1 8] Transfer of ionic species can happen in an electrochemical cell due to migration, convection and diffusion effects. Migration of ions of electrolyte occurs due to change in electric field and is always present. Supply of excess of nonelectro active i ons (electrolyte solution), minimizes the contribution of migration and decreases solution resistance, which improves working electrode potential control. Convection happens due to stirring and should be minimized. The main driving force of an electrodepos ition process is diffusion of analyte by a concentration gradient. Nernst equation provides a link between electrode potential and the concentrations of participants in the electrode process. For electrodeposition when only an oxidized species is present, the relationship is given by the following expressions (1 5) : ),0(ln0tC NF RT EE (1 -5) where E is overall potential needed, E is standard potential, R is the thermal rate constant, T is temperature, N number of moles, F is Faraday constant and Co ( 0,t) is the concentration of oxidized species at the electrode surface. Electrodeposition rate can be controlled by current density through over potential [17] The thickness can be controlled and it is the product of molecular weight and the reciprocal of number of electrons, Faradays constant, area and density. The maximum length of a nanowires array is limited by the thickness of the template channels.

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22 Table 11. Diffusivities and viscosities of liquids, supercritical fluids and gases. Property D(cm2/sec) (g/cm/s) Liquids 10-5 10-2 SCF 10-3 10-3 Gases 10-1 10-4

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23 0.4 0.5 0.6 0.7 0.8 0.9 1 0 10 20 304050 60 7080 water (20oC) water (50oC) water (100oC) acetic acid (20oC) acetone (20oC) ethanol (20oC) n-Hexane (20oC)Capillary EvaporationPore diameter (nm) Figure 1-1 Capillary evaporation for liquid solvents in nanopores using the Kelvin equation.

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24 CHAPTER 2 SUPERCRITICAL FLUID PURIFICATION OF HIPCO AND SWAN SINGLE WALL CARBON NANOTUBES (SW CNTS) Initial Remarks S ingle walled carbon nanotubes (SW C NTs) have been the focus of recent research due to their unique electronic and mechanical properties. Their properties can be utilized for materials s cience applications and electronic devices. For example, they can be used as conductive films in dye sensitized solar cells (DSSCs) instead of fluorine-do ped tin oxide (FTO) or tin doped indium oxide (ITO) SWCNTs films can be made transparent while remaining highly conductive. SWCNT films show high transparency in a wide spectral range from the UV visual to the near IR range. Lagemaat et al. [19] purified SWCNTs using an acid reflux method and produced films of 70 % transmittance at 650 nm. Films can be prepared by spraying or spin coating on glass substrates [20] However, there are several technical difficulties that must be overcome before they can be used. Purification of SWCNTs from inherent metal impurities [21] is important for making conductive films. Most purification processes use strong acids or oxidative environments to remove the impurities, which can oxidize the carbon nanotube sidewalls. The intensity of the carbon nanotubes are reduced by acid treatment. Blackburn et al. showed the effect of protonation on the florescence intensity of carbon nanotubes [22] Wang el al. [23] showed the highly selective purification one pot technique. The carbon nanotubes are dissolved in an aqueous mixture of hydrogen peroxide (H2O2) and hydrogen chloride (HCl) at 40 70C for 4 8 h However, this method causes defects to the carbon nanotubes. Magnetic purification of carbon nanotubes has been studied by Wiltshire et al. [24] Although this method can be promising, but scalability to the

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25 industry level can be challenging. Chiang el al. [25] used a wet gas method by oxidizing at 225C, sonicating in HCl and annealing at 800C in Ar. Raman spectroscopy of the trea ted sample in their study show defects to the structure of the nanotubes. Wang el al. [26] reported high metal extraction efficiency from carbon nanotubes using electrochemical pretreatment step with ethylenediaminetetraacetic acid (EDTA) electrolyte, in a three electrode setup with AgCl reference electrode. However, this method can be limited by onion shells formation around the metals preventing oxidation and effective metal extraction. Inefficiencies of wet methods are attributed to high surface tension of liquids solvents In this chapter supercritical fluid carbon dioxide (scCO2) is used to purify different types of carbon nanotubes from their inherent metal impurities at different temperatures and pressures. M ul tiple extractions are performed for optimum extraction condition, to enhance metal removal efficiency The carbon nanotubes samples are analyzed by TGA, Raman spectroscopy and fluorescence to assess metal removal and properties of the carbon nanotubes. Transmission electron microscopy is used to evaluate metal size and distribution before and after purification. Materials and Experimental Materials Tri -nbutylphosphate (TBP 99% purity from SigmaAldrich), and hexafluoroacetylacetone (HFA 95% purity f rom Sigma Aldrich) were used as received. Hipco and S wan single wall carbon nanotubes (SWCNT s) were used as received. Carbon dioxide was received from Airgas and used as received. Micro Reactor MS 16 was purchased from High Pressure Company. Experimental HiPco and Swan c arbon nanotubes were processed to remove metal impurities used in their synthesis by supercritical fluid extraction at temperatures

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26 ranging from 35 to 65C and pressures between 250 and 400 atm TBP was used as an oxidizing agent and HFA was used as a chelating agents. Moreover, reagent and solubility limitation wa s probed by using TBP with an acid complex and 5 % ethanol as a cosolvent in supercritical carbon dioxide extraction. 0.5 ml HFA and 1.04 ml of TBP complex (1.5 ml TBP and 1.5 ml of 70 % HNO3) were used for HiPco carbon nanotubes scCO2 extraction at 60C and at 200 atm. Static e xtraction was performed for 1 h and 30 min followed by dynamic extraction for 15 min. Also, the extent of metal oxidation wa s tested by an electrochemical pretreatment step. Pretreatment was performed in ethylenediaminetetraacetic acid (EDTA) electrolyte, ethanol and water solutions and 215 mg of CNTs. The three electrode setup included a platinum foil as the working electrode, platinum wire as the counter electrode and AgCl reference electrode. Chronoamperometry was run for 4000 seconds at 0.41 V. Solution was mixed for 5 min and then centrifuged for 12 h at 10,000 rpm. The effect of acid treatment on nanotube properties wa s shown by using the one pot method with a sample of HiPco carbon nanotubes. The tubes are dissolved in an aqueous mixture of hydrogen peroxide (H2O2) and hydrogen chloride (HCl) at 40 70C for 4 8 h. ScCO2 extraction. Isco syringe pump was used to pressurize CO2 to the micro reactor. The reactor (9/16 I.D. 9 in length, 24 mL) of stainless steel was immersed in a water bath heated to the desired temperature by a temperature regulator in internal circulation mode, see Figure 21. Approximately 22 mg of Hipco and 40 mg and Swan carbon nanotubes and a small stir bar were wrapped in P8 filter paper and inserted into the reactor. A magnetic stirrer was used to stir the magnetic stir bar. The oxidizing and

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27 chelating agents were loaded according to the raw carbon nanotube metal content (approximately at 2:1 ratio) into the reactor using a syringe with a leur lock connection. The reactor was loaded with CO2 to the desired pressure and allowed to reach equilibrium. CO2 was again loaded until the desired pressure was reached. The reactor was h eld at the extraction conditions for an hour. The CO2 was then slowly vented and bubbled through water, yielding purple droplets in the water solution [2628] The carbon nanotubes were recovered from the reactor and bath sonicated in ethanol for 30 min. The suspension was then filtered through a 0.1 m PTFE membrane and washed with hexane. To simplify the filtration process, alternatively, the carbon nanotubes were flushed multiple times with sc CO2 at constant flow mode to flush out the used reagents. This method was tested with some hipco carbon nanotubes samples. Characterization. Thermogravimetric (TGA) analysis was conducted in air up to 800C at a ramp rate of 10C/min. Inductively Coupled Plasma Spectroscopy (ICP) analysis was performed on homogenized collected effluent. Raman spectra were recor ded on the solid carbon nanotubes using a Reinshaw Invia Bio Raman with excitation from a 785 nm diode laser. For fluorescence measurements, carbon nanotubes were dispersed in 1 wt % S DBS surfactant. The suspension was homogenized with 10,000 rpm for 1 hour followed by tip sonication for 10 min T oluene wa s added to the suspension and then mixed with vortex at 1,500 rpm for 30 s econds Finally, the suspension was allowed to settle for more than 2 h. The suspension was characterized using an Applied NanoFluorescence Nanospectrolyzer (Houston, TX) with excitation from 662 and 784 nm diode lasers.

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28 Results and Discussion Effect of pressure and temperature in scCO2 extraction. Pretreatment of HiPco carbon nanotubes with an electrochemical technique showed small oxidation peaks from iron catalyst. This indicates inaccessibility of metals for oxidation due to poor penetration of the solvents in the carbon nanotubes. For the raw Hipco carbon nanotubes, initial metal content ranged from 25 to 30 %. TGA of purified H i pco carbon nanotubes showed final ash content ranging from 21.6 to 43.5 %, see Figure 22 and Table 2-1 This increase in ash content was attributed to residual amounts of reagents in samples, see Figure 2-3 Final ash content of H ipco carbon nanotubes treated with TBP wa s higher than that of TBP or HFA alone. Analysis using TGA results was performed to determine the actual amount of metal oxide left in each treated sample, see Table 22. TBP weight is the difference of weight between 110C and 307C; the f irst sample temperatures after the boiling points of water and TBP, respectively, during the TGA ramp program. The percentage of TBP left in a carbon nanotube sample was 11.87 %, see Figure 2 -3 Based on these calculations, the final metal oxide content without TBP is found for each sample. The best removal efficiency was found at 325 atm, see Figure 2-4 ICP showed optimum iron removal of around 40 ppm at this pressure. The other metals present such as Mg, Mo and Ni are at low concentrations below 3.5 ppm see Table 2-3 On the other hand, Swan carbon nanotubes, initial metal content ranged from 84 to 90 %. The treated samples at the range of temperatures and pressures showed best removal at 55C and 325 atm, see Figure 2-5 ICP showed optimum magnesium removal of around 500 ppm at this condition. The other metals present such as Fe, Mo and Ni are at low concentrations, see Table 2-3.

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29 Effect of multiple extr actions and higher temperatures. Multiple extractions with fresh chelating agent and oxidant were perf ormed at optimum conditions for both HiPco and Swan carbon nanotubes. Hipco carbon nanotubes did not show better metal removal efficiency at any of the conditions tested, see Figure 2-6 This confi rms that efficiency limitation wa s not due to lack of reagents. Hence, a single extraction step was sufficient under these conditions. However, swan carbon nanotubes showed enhanced metal removal efficiency at 45C and 325 atm. The extraction was repeated twice at this condition and the final ash content was reduc ed from 30 % to 20 %, see Figure 2-7 The extraction at 55C and 325 atm showed a similar final ash content of 20 % with a single extraction. Extraction was repeated 4 times at 55C and 325 atm but did not show further reduction in metal content Further, the efficiency was performed at 65C and 325 with a single extraction and showed final ash content of 16.7 %. Extraction was repeated at that condition but did not show efficiency improvement. Moreover, the extraction was performed at 75C and 325 atm. However, this showed reduction in efficiency with final ash content of 25 %. These results show the best extraction conditions are at higher temperatures around 65C and 325 atm. This is primarily due to optimum oxidation and chelating conditions for removal of magnesium HiPco carbon nanotubes have higher density of metal impurities in the 3-5 nm range. On the other hand, Swan carbon nanotubes have impurities of different particle sizes; see Figure 2-8 a and c The high metal content in Swan carbon nanotubes allows high initial oxidization and removal of metal s, which makes more inaccessible metal sites available for oxidization. Unlike S wan carbon nanotubes, HiPco low metal content

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30 allows low removal which prevents solvents penetration to oxidize more metals sites The difficulty in removing all metal content wa s due to the onion shell formation around the metals; see Figure 28b and d. Supercritical carbon dioxide extraction was very efficient in removing the majority of the metal impurities. Figure 29 shows impurity remaining in similar onion shell formation even with the one pot method using acid for treatment The solubility of the reagents in supercritical carbon dioxide was found to have no effect on further removal. Adding 5 % of ethanol as a cosolvent did not enhance extraction efficiency Moreover, the processing condition of the carbon nanotubes had no effect on removal; two Hipco carbon nanotubes sample were processed with and without crushing and both samples showed the same final ash content. Finally, combining supercritical extraction with acid treatment did not enhance oxidation and metal removal This shows that the supercritical fluid method wa s sufficient in a single extraction step when performed at optimum conditions. Carbon nanotube integrity assessment Raman spectra were performed on the solid carbon nanotubes. The Raman spectra for the HiPco nanotubes shows the characteristic peaks at 1594 and 1295 cm-1. The low D/G ratio is indicative of high quality carbon nanotubes with few defects, see Figure 210. After purification using TBP and HFA in sc CO2, there are no changes to the Raman spectra. The Raman spectra of raw and treated Swan nanotub es are comparable, see Figure 211. These results show that there was no sidewall damage caused to the nanotubes during the supercritical carbon dioxide extraction process. T he one pot method showed final ash content of 12 % However, R aman spectra shows lower D/G ratio, ind icative of nanotube defects.

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31 Fluorescence is a sensitive analysis technique to probe any oxidative environment on the carbon nanotubes. The peaks of fluorescence of HiPco carbon nanotubes correspond to the different types of the carbon nanotubes. The spectra showed no decrease in intensity after purification, see Figure 212 Fluorescence of one pot treated carbon nanotubes show the least intensity compared with other treated samples. The fluorescence of treated Swan carbon nanotubes at 65C and 325 atm wa s compared with the 10 % purified sample provided by the manufacturer. This wa s due to difficulty of taking fluorescence of the raw high metal content Swan carbon nanotubes. F luorescence peaks correspond to different types of carbon nanotubes and show no decrease in intensity after purification, see Figure 213. Closing Findings Supercr itical carbon dioxide extraction showed that metal impurities were most effectively removed at an intermediate pressure of 325 atm and at high temperatures for both HiPco and Swan carbon nanotubes. This wa s due to enhanced oxidation of the main metal catal yst in each carbon nanoube material. The effective extraction was due to sc CO2 low surface tension and high diffusivity. The carbon nanotubes were purified from their impurities without sidewall damage or decreased fluorescence intensity. The final metal oxide content of 16 % for both HiPco and Swan carbon nanotubes was attributed to the onion shell formation around the metals

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32 Table 2-1 HiPco carbon nanotubes metal oxide content (wt %) with residual t ri butyl phosphate ( TBP) after supercritical carbon dioxide treatment with TBP and HFA. Temp, Pressure 35C 45C 55C 250 atm 31.01 32.22 43.49 325 atm 30.85 28.38 25.29 400 atm 21.64 30.97 35.46 Table 2-2 HiPco carbon nanotubes metal oxide content (wt %) after subtraction of residual t ri butyl phosphate TBP. TGA analysis is used to estimate the amount of residual TBP TBP initial wt is the difference of wt between 110C and 307C (wt of CNT) the first sample temperatures after the boiling points of water and TBP. TBP wt % is 11.87 % (ratio of TBP i n CNT from TGA) times the ratio of TBP/CNT of a sample. TBP final wt for each sample is found by multiplying TBP wt % by TBP initial wt. This value is subtracted from final wt reading of TGA to calculated adjusted final metal oxide content. T (C), P (at m) a sh wt (w/TBP) wt at 110 C wt at 307 C TBP initial wt TBP/CNT TBP wt % TBP final wt final a sh wt 35, 250 31.01 10.17 5.28 4.89 0.93 0.11 0.54 20.80 35, 325 30.85 7.47 3.72 3.75 1.01 0.12 0.45 18.69 35, 400 21.64 4.76 3.17 1.59 0.50 0.06 0.10 18.6 3 45, 250 32.22 11.00 5.45 5.55 1.02 0.12 0.67 19.81 45, 325 28.38 11.81 5.87 5.94 1.01 0.12 0.71 16.07 45, 400 30.79 11.26 5.67 5.59 0.99 0.12 0.65 19.15 55, 250 43.49 20.00 8.45 11.55 1.37 0.16 1.87 21.14 55, 325 25.29 8.28 5.20 3.08 0.59 0.07 0.22 21.09 55, 400 35.46 18.34 8.76 9.58 1.09 0.13 1.24 21.27

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33 Table 2-3 Metal ions removed from HiPco and Swan SWCNTs ICP analysis was performed on effluent from the different supercritical carbon dioxide extraction conditions. The detection level is 0.004, 0.003, 0.012 and 0.026 ppm (mg of metal/ Kg of solvent) for Mo, Ni, Fe and Mg, respectively. BDL stands for below detection level. SWCNTs T (C), P (atm) Mo (ppm) Ni (ppm) Fe (ppm) Mg (ppm) HiPco 35, 325 0.22 0.26 36.13 3.53 45, 325 0.07 0.31 37.49 1.77 55, 325 0.24 0.06 39.35 1.21 45, 250 0.18 0.27 38.51 0.54 45, 400 0.11 0.19 40.43 BDL Swan 45, 325 BDL 61.64 8.57 124.31 55, 325 0.05 0.18 5.14 292.34 65, 325 BDL 0.31 9.24 500.92 75, 325 BDL 0.17 11.37 258.65 45, 400 BDL 0.42 4.02 72.3 3

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34 Figure 21. Setup of carbon nanotubes metal extraction using s upercr itical carbon dioxide. Sc CO2 is pressurized from isco pump to the cell with oxidizing and chelating agents Static e xtraction is performed under pressure and temperature control before purging through a collection vial. 0 20 40 60 80 100 120 0 100200300400500600700800 Raw HiPco One Pot 35oC, 250 atm 35oC, 325 atm 35oC, 400 atm 45oC, 250 atm 45oC, 325 atm 45oC, 400 atm 55oC, 250 atm 55oC, 325 atm 55oC, 400 atmFinal Ash (wt %) Temperature (oC) Figure 2-2 TGA of raw and treated HiPco carbon nanotubes S upercritical carbon dioxide extraction was performed at temperature from 35 to 55C and pressures from 250 to 300 atm. Raw HiPc o shows metal oxide (ash) content of 25 wt %. Treated sa mples by sc CO2 show metal oxide of 20 to 40 wt % due to remaining tri butyl phosphate (TBP) in carbon nanotubes. One pot purification shows final metal oxide content of 12 wt %.

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35 0 20 40 60 80 100 120 0 2004006008001000 HiPco SWCNT w/ TBP (wt %) HiPco SWCNT (wt %) TBP (wt %) Final Ash (wt %)Temperature (oC) Figure 23. T ri butyl phosphate (TBP) residuals in HiPco carbon nanotubes. TGA of carbon nanotubes with TBP shows final metal content of ~40 % compared with 25 % and 8.5 % of carbon nanotubes and TBP, respectively. This result indicates 11.87 % residuals of TBP in carbon nanotubes. 15 16 17 18 19 20 21 22 200250300350400450 35oC 45oC 55oCFinal Ash (wt %)Pressure (atm) Figure 2-4 Final metal oxide content of treated HiPco carbon nanotubes S upercritical carbon dioxide extraction was performed at temperature from 35 to 55C and pressures from 250 to 400 atm. A single extraction at 45C and 325 atm show s 16 % of final ash content

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36 0 20 40 60 80 100 120 0 200400600800 Raw Swan (45oC & 325 atm) 1 ext (45oC & 325 atm) 2 ext 55oC & 325 atm 65oC & 325 atmFinal Ash (wt %)Temperature (oC) Figure 2-5 TGA of raw and treated Swan carbon nanotubes S upercritical carbon dioxide was p erformed at temperature from 35 to 55C and pressure from 250 to 400 atm Raw Swan shows metal oxide of 84 wt %. Purified samples by sc CO2 show metal oxide of 20 to 30 wt %. A single extraction at 65C and 325 atm shows 16.7 wt % of final ash content compared with 20 wt % for m ultiple extractions at 45C and 325 atm 0 20 40 60 80 100 120 300400500600700800 Raw HiPco 35oC, 400 atm 1 ext 35oC, 400 atm 2 ext 55oC, 400 atm 1 ext 55oC, 400 atm 2 extFinal Ash (wt %)Temperature (oC) Figure 2-6 TGA of raw and treated HiPco carbon nanotubes by multiple extractions. Multiple extractions at 35 and 55C and at 400 atm using supercritical carbon dioxide do not show significant metal removal. Raw HiPco shows metal oxid e content of ~ 25 wt %.

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37 10 20 30 40 50 60 70 80 200250300350400450 35oC 45oC 1 ext 45oC 2 ext 55oC 65oCFinal Ash (wt %)Pressure (atm) Figure 2-7 Final metal oxide content of treated Swan carbon nanotubes S upercritical carbon dioxide extraction was performed at temperature from 35 to 65C and pressures from 250 t o 400 atm. A single extr action at 65C and 325 atm shows 16.7 wt % of final ash content compared with 20 wt % for multiple extractions at 45C and 325 atm.

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38 Figure 2-8 TEM images of raw HiPco (a) treated HiPco (b) raw Swan (c ) and treated Swan carbon nanotubes (d) HiPco carbon nanotubes have higher density of metal impurities in the 35 nm range (a), where as Swan carbon nanotubes have impurities of different particle sizes (c). Onion shell formation around the metals show prevents their removal (b) and (d). Figure 2-9 T EM image of HiPco carbon nanotubes treated by the one pot method. The sample shows remaining metal content.

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39 0 0.2 0.4 0.6 0.8 1 1.2 0 500100015002000250030003500 Raw HiPco One Pot 35oC, 250 atm 35oC, 325 atm 35oC, 400 atm 45oC, 250 atm 45oC, 325 atm 45oC, 400 atm 55oC, 250 atm 55oC, 325 atm 55oC, 400 atmNormalized IntensityRaman shift (cm-1) Figure 210 Raman spectroscopy of raw and treated HiPco carbon nanotubes S upercritical carbon dioxide treated smaples show comparable spectra to the raw sample suggesting no side wall damage to the nanotubes One pot method shows lower D/G ratio (D peak at 1295 cm-1, G peak at 1594 cm-1), indicative of side wall damage. 0 0.2 0.4 0.6 0.8 1 50010001500200025003000 65oC & 325 atm Raw SwanNormalized IntensityRaman shift (cm-1) Figure 211 Raman spectra of raw and treated Swan carbon nanotubes. S upercritical carbon dioxide treated sample at 65C and 325 atm show s comparable spectra to the raw sample indicating no side wall damage to the carbon nanotubes.

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40 0 2x10-124x10-126x10-128x10-121x10-111.2x10-1190010001100120013001400 Raw One pot 45oC, 400 atm 55oC, 400 atmIntensity of Excitation at 660 nmwavelength (nm) Figure 212 Fluoresc ence of raw and treated HiPco carbon nanotubes. S upercritical carbon dioxide extraction s at 45 to 55C and 400 atm show comparable intensity to the raw sample. One pot method shows least intensity.

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41 0 5x10-141x10-131.5x10-132x10-132.5x10-139001000110012001300140015001600 Swan (10%) 65oC, 325 atmIntensity of Excitation at 660 nmwavelength (nm) Figure 213. Fluorescence of (10 %) Swan and treated Swan carbon nanotubes. S upercritical carbon dioxide extraction at 65C and 325 atm shows comparable intensity.

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42 CHAPTER 3 PREPARING THICK, DEF ECT FREE FILMS OF ANATASE TITANIA FOR DYE SENSITIZED SOLAR CEL LS Preliminary Remarks The anatase phase of titania receives the most interest out of the three polymorphs of titanium (IV) oxide because of its photo catalytic activity. The unusual properties associated with nanostructured films of titania provide numerous applications in the fields of solar cells, optics, environmental remediation [29] antibacterial coatings [30] photocatalytic reactor s [31] electrochromic displays [32] and gas sensing. [33] Interestingly, titania also exhibits photoinduced superhydrophilicity when exposed to UV irradiation [34] ; the titania reverts back to its hydrophobic nature once left in a dark state for several hours. Carp et al. [35] provide a good review of the interesting properties and applications of titania. Titania films have been prepared by numerous techniques, including chemical vapor deposition [36] sputtering [37] atomic layer deposition [38] and deposition of sols [39] The structure and properties of titania films are strongly dependent on the substrate and preparation method In many applications, the deposition of sols is preferred because it is cost effective and prepares porous films of titania. The conventional method used to prepare these porous TiO2 films is to deposit the nanoparticles onto a substrate and heat the film to temperatures of 450 550 C. The heating step removes the solvent used to aid dispersion of the nanoparticles onto the substrate. As the solvent evaporates, the nanoparticles adhere to the substrate and form an electrically connected network. Multiple coat and bake [40] steps are often used to prepare thick films (~ 10 m) of titania.

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43 The difficulty in preparing thick films from multiple cycles comes from the stresses that are generated during the rapid heating and cooling cycles, resulting in widespread cracks in the film during the evaporation and annealing process [33] Strain develops in the film because it is bonded to the substrate during the evaporation process [41] Compressive or tensile stresses can then develop in the films because of different thermal expansion between the substrate and the film [41,42] Once these stresses become too large, the film starts to crack and it becomes fragile. Typically, film delaminatio n is attributed to compressive stresses while surface cracks are associated with tensile stress [43,44] Several research groups have investigated different methods to avoid the formation of defects in the preparation of thick titania films. Carotta et al. showed that slowing the evaporation rate by polymerizing the solvent helped reduce the formation of cracks in titania films up to 10 m thick [33] Ito et al reported a repetitive coating technique to make ~ 20 m titania films by sintering multiple layers while the substrate was hot [34] However, delamination and film cracking still readily occurred during solvent evaporation. In a similar approach, Halme et al prepared 322 m thick films by depositing, drying, and mild pressing the TiO2 film at 100 kg/cm2; multiple cycles were used to build the film with a final press of 100 0 kg/cm2 [7] However, some researchers have found that it is difficult to control the thickness and uniformity of the titania film prepared by this approach. For this reason, Grinis et al filled the void space between particles with a nonpolar volatile liquid to maintain a uniform pressure on the particles during compression [45] Although ~ 25 m thick titania films were prepared, numerous heating and pressing cycles were required.

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44 A simple method for preparing thick and robust (> 15 m) TiO2 films on conducting substrates with minimal defects remains elusive. In th is chapter I use the simple doctor blade technique [39] to spread a sol onto substrates for the preparation of ~ 25 m thick films of titania. By controlling the evaporation of the solvent and the temperature cycling (heating and cooling rates), the stresses that caus e film defects are reduced. This technique is applied to titania films of various thicknesses on both FTO and ITO coated glass substrates. The final titania films are stable and have significantly lower defect densities than films prepared by the conventional approach, i.e., without controlled evaporation or temperature cycling. These anatase titania films are utilized as the photoanode in dyesensitized solar cells and show improvements in short circuit current, opencircuit voltage and device efficiency. Experimental Section Materials A paste containing ~18 wt % nanocrystalline titanium dioxide (Ti Nanoxide T/SP ) was obtained from Solaronix [46] According to the manufacturer, the size of the titania particles is 13 nm. ITO and FTO on glass substrates were obtained precut from Thin Film Devices, Inc. and had conductive film thicknesses of 450 15 nm and 300 2, respectively. The surface roughness of the films was reported to be between 0.220.33 % The optical transmission was 75 % and 78 % (measured at 550 nm) for FTO and ITO, respectively. Deposition of Ti O2 film Impurities were removed from the surface of the ITO and FTO substrates prior to TiO2 deposition by sonicating in soa py deionized water, acetone and ethanol for 20 min each Titania films of various thicknesses (7, 10, 20, and 25 m) on conductive glass were prepared by using one or more layers of adhesive 3M

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45 Scotch tape. The thickness of one tape layer is about 50 m and typically results in a ~10 m film after annealing. A variety of processing conditions were used to prepare titania films, as s hown in Table 31 and Figure 31. The first step was to spread the titania paste onto the substrate using a doctor blade, as shown in Figure 31a. Three titania films with diameters of 7 mm (surface area of 0.38 cm2) were deposited on a single ITO or FTO substrate. The adhesive tape was gently removed before sintering, leaving little residue on the glass. Sintering of film The film w as sintered with either an ultra high vacuum tube furnace (Barnstead type 79300 Thermolyne) or a high vacuum oven ( Barnstead 3608) which have minimum pressure ratings of 8.010-3 and 1.3 kPa, respectively The vacuum level was controlled by bleeding nitrogen (tube furnace) or air (high vacuum oven) into the chamber. As shown in 31b, sintering of the film was done in two stages at constant pressure ranging from 5.310-2 to 101.3 kPa. The heating rate (dTi/dt) to each temperature set point (Ti) was controlled as well as the sintering time (ti) at each stage of the heating cycle. Films were first placed in the vacuum chamber and then heated once the pressure was achieved. The heating rate to the first set point (dT1/dt) is low (2 5 C/min) to prevent film failure due to rapid solvent evaporation. All films were cooled by convection at the same pressure. Annealing of the film T itania films sintered at lower pressures were discolored, likely due to a deficiency of oxygen in the titania [47] However, the films became transparent by annealing at atmospheric pressure on a hot plate The annealing rate was 28.3C/min to a temperature of 450C. Annealing time at this temperature ranged from 40 to 105 min depending on initial film color; the time required to achieve

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46 transparent films was longer for darker films. After the films were transparent, the annealed films were cooled at a rate (dT3/ dt) that ranged from 1 to 100C/min, as shown in Figure 31d and Table 31. Characterization techniques. Xray diffraction ( XRD Philips APD, Cu source ) was used to verify the formation of anatase titania after annealing. S canning electron microscopy images ( JEOL JSM 6400) and profilometry were used to determine film thickness. Raman spectroscopy ( Renishaw Invia Bio Raman with excitation from a 785 nm diode laser) was used to probe film strain and confirm the formation of anatase titania Optical microscopy (Nikon O ptiphot) equipped with a digital camera (Amscope MD800) was used to assess the defects within the films. Six images were taken for each of the three titania films on a substrate at each stage of film preparation. RGB/8 i mages were converted to black and white using Adobe Photoshop to estimate the defect density using the pixel count. First, the high filter pass feature was used to improve the contrast of film defects. The threshold value (a tool to shift image coloration between black and white) was then adjusted to reflect the actual image defect density. The mode of the image was next changed to Black and White. Therefore, the color density was forced to black and white and a histogram plot obtained the relative amount of pixels associated with each. The defect density was defined as the ratio of pixels from film defects to the total pixel count In addition, the surface area of the film was varied (0.38, 1, 2, and 3 cm2) for titania films on FTO (sintered at 5.310-2) and found to have minimal changes to the defect density (7.7, 3.1, 5.1, and 2.9 %, respectively), yielding an average of 4.7 2.23 % for all film surface areas.

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47 DSSC device fabrication. The titania films are impregnated in 3.0104 M solution of the N 749 dye in acetonitrile and t butanol (1:1 vol %) for 24168 h. Counter electrodes are painted with Platisol (Solaronix) using a brush and fired in a tube furnace at 400C ) was cut and placed between the electrodes and the electrolyte was injected into the cell by capillary action. The current voltage characteristics of the prepared DSSC were measured under simulated AM 1.5 solar light using a source meter (VersaSTAT 3 type from Princeton Applied Research) and a solar simulator (Solar Light Co.) Open circuit voltage decay measurements are used to assess the recombination processes in DSSCs [48] After full sun illumination on the DSSC at open circuit (for 5 sec), illumination is shut off and voltage decay is measured with time. The voltage decay is fit to a decay function and the derivative is evaluated at (t = 30 sec) to calculate electron lifetime from equation (3 1): 1 t V e TkB n (3-1) where KB is the Boltzmann constant, T is temperature and e is the electron charge. The parameter KBT/e is thermal voltage, which is 0.026 V at room temperature. Results Characterization of titania films Scanning electron microscopy was used to meas ure the thickness of titania on FTO and ITO substrates. As shown in Figure 32, the films are uniform with a maximum thickness of ~ 25 m. Xray diffraction (XRD) was used to determine the final phase of titania formed at different processing conditions. F igure 33 shows the XRD peaks for titania films sintered at atmospheric pressure

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48 ( 101.3 kPa) and vacuum (8.010-3 kPa). XRD peaks between 20 and 70 degrees correspond to anatase peaks, confirm ing that both films a re anatase titania. The XRD also shows that the particle size of the titania does not change when sintered in vacuum. The Raman spectra of the titania films in Figure 34 show peaks at 400, 514 and 640 cm-1. These peaks are known to correspond to the anatase phase of titania. This data also shows that the phase does not change with film thickness. Furthermore, there are no peak shifts in the spectra that are associated with strain in the film [41,49] Table 31 presents a summary of the r esults for each processing condition. Control samples of ITO and FTO substrates (i.e., no titania film) show no defects, confirming that defects are due to titania film formation. In addition, 20 m titania films on ITO and FTO were prepared at atmospheric pressure and annealed directly to 450C for comparison. These control samples had defect densities of 38.3 and 14.2 %, respectively. The defect density of each film was measured at each stage of the process. The most critical steps for minimizing the defect density were found to be the sintering pressure and annealing cooling rate. In general, FTO films have lower defect densities than ITO substrates. For example, under optimized conditions, the defect densities for 20 m thick titania are 2.5 and 7.8 % for FTO and ITO substrates respectively. Effect of sintering conditions on defect density The optical images in Figure 35 show the effect of the sintering pressure on the defect density of the titania films. The films show a significant amount of defects at atmospheric pressure. However, the amount of defects steadily decreases as the sintering pressure is lowered. These changes in defect density are summarized in Figure 36 for the titania films on both FTO

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49 and ITO substrates. The defect densities after sintering of titania films on FTO and ITO substrates decayed exponentially as the sintering pressure decreased; defect densities ranged from 0.4 15.0 and 0.1 12.6 % for FTO and ITO substrates, respectively. A signif icant increase in defect density was seen for titania on both FTO and ITO based substrates after the annealing step. However, the effect of the sintering pressure is still observed in the defect density of titania on FTO substrates. The films prepared at the lowest pressure had a final defect density of 2.5 %. On the other hand, the final defect density of titania on ITO was nearly constant, despite the significant differences in defect density after sintering. Although the defect density was weakly dependent on pressure, controlling the sintering step for titania on ITO does significantly improve the defect density from the control sample (16.3 vs. 38.3 %, see Table 31). These differences indicate that the slower heating rate during controlled sintering of titania films on ITO helps reduce the defect density. Setting the heating rates and the temperature set points slightly higher during sintering at the lowest pressure of 5.310-2 kPa had minor changes to the defect density of titania films on ITO and FTO respectively (see Table 31). Effect of annealing conditions on defect density Figures 37 and 38 show the effect of the cooling rate (dT3/dt) after the films sintered at l ow pressure are annealed at 450C. While the defect density associated with the titania films on FTO was constant within experimental error, the titania films on ITO had defect densities more sensitive to this processing step. The defect density decreased from 21.9 to 7.8 % as the cooling rate (dT3/dt) was reduced from 100 to 1 C/ min.

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50 Discussion The results indicate that the mechanisms of defect formation in titania films are different on ITO and FTO substrates. The critical step for maintaining defect free films of titania on FTO is the sintering step, which is highly dependent on the sintering pressure. While lower pressure improved defect density, a majority of the benefits can be achieved at a pressure of 20 kPa, as seen in Figure 36a. The reduced pressure and controlled heating rate during sintering offers some benefits to the titania films prepared on ITO; however, the cooling rate from the annealing step was the most crucial step in determining the final defect density. The formation of defects or cracks in the film during heating and cooling are associated with compressive or tensile stresses that build in the film because of differences in thermal expansion between the film and substrate. The thermal expansion coefficients of the film and substrate should be comparable so they can expand and contract together to avoid mechanical failure from the stresses. The consistent trend of defect density with sintering pressure shown in Figure 36 for titania films before and after annealing indicates that defects formed during the sintering step propagate across the film once it is su bjected to stronger stresses during cooling. Therefore, the forces that generate defect s in the titania films occur at the initial stages of the sintering process. Titania films sintered at low pressures on both ITO and FTO substrates show significant decreases in defect density after the sintering step (circles in Figure 36). These changes indicate that the stresses in titania films on both substrates has been reduced at low pressure. Jagtap et al. have shown that the thermal expansion coefficient of anatase titania at low pressures can be approximately twice the values at

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51 atmospheric pressure [50] Therefore, the sintering in vacuum could allow the titania film to match the thermal expansion of the ITO or FTO substrate better, reducing the stresses generated in the film during sintering. This conclusion is supported by the fact that titania films still show a dependency on pressure after annealing (squares in Figure 36), indicating that this step has helped to reduce the stresses within the film. Although improvements to the defect density of titania on ITO substrates are observed from thermal expansion differences at low pressure, these films still have a significant amount of residual stress that is generated in the film during sintering. This stress is released during the cooling step of the annealing process, causing widespread defects across the film. For this reason, the heating and cooling steps become much more important for titania films on ITO. This effect is first observed by comparing (see Table 31) the titania films prepared at atmospheric pressure on ITO with either a slower (controlled) or faster heating rate (control sample). The slower heating rate allows the stresses generated within the film to relax during sintering. The lower stored stresses within the film after sintering results in less defects created by the subsequent release of the stress during cooling. Similarly, the slower cooling rates after annealing provide more time for the stresses generated within the film to be dissipated before they generate defects or cracks, as shown in Figure 38. These results provide a guide for preparing thicker titania films with minimal defects. It is clear that a low pressure and slower heating rate should be maintained during sintering to reduce the defect density generated within the film. The cooling rate was also controlled (chosen to be 1 C/min) for ITObased films because of the strong dependency of the cooling rate after annealing on defect density. These conditions

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52 yielded 25 m titania films on ITO and FTO with defect densities of 22.8 and 6.0 %, respectively (see Table 31). Interestingly, the defect density on FTO substrates was found to be almost linearly dependent on the titania thickness (see Table 31). Finally, DSSCs were fabricated using the titania films prepared under controlled sintering and annealing as the photoanode. The short circuit current and efficiency steadily improve as the thickness of the titania increases from 7 to 20 m, whi ch is attributed to the additional surface area of the device. As shown in Figure 39, the titania fabricated under vacuum yield better device characteristics than the standard method for making photoanodes (control). The titania photoanodes fabricated under vacuum have higher short circuit current and opencircuit voltage but lower fill factors. The vacuum pressure used to prepare the photoanodes also has an effect on the device characteristics, as seen in Figure 39bf. The best performance is seen at int ermediate vacuum levels. These results indicate that the lowest pressures used to make the photoanode are good for minimizing defect density but not optimizing device performance. It is likely that the lowest pressures reduce the porosity of the film, see Figure 310 Films with lower porosity may prevent adequate impregnation of the photoanode with dye, resulting in more back reactions between the titania and electrolyte. This conclusion is supported by the fact that the lowest pressure (5.310-2 kPa) has about the same opencircuit voltage as the highest pressure (71.3 kPa) but lower short circuit current and shorter electron lifetime. Similar behavior is observed with DSSCs m ade with 10 m films, see Table 3-2 Further improvements to device efficiency may be possible if better dye impregnation can be achieved, which is discussed in chapter 5.

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53 Conclusionary Findings TiO2 films were prepared on transparent conducting oxide (TCO) substrates using a simple deposition technique with controlled sintering and annealing conditions. XRD and Raman spectroscopy confirmed the formation of anatase titania films after the annealing step. Stresses generated within the film during sintering and annealing cause defects or cracks to form within the film. These defects are initiated during the sintering step and continue to propagate once the film is subjected to stronger stresses during cooling. Reducing the stresses within the film results in better quality films that can be used for various photo applications. Sintering the titania films at low pressures reduces the stresses on both ITO and FTO substrates. The effect is more prominent in titania films prepared on FTO, resulting in significantly lower defects in the films. The preparation of defect free titania films on ITO requires further control of the heating and cooling steps. Titania films up to 25 m thick with minimal defects could then be prepared on both ITO and FTO substrates by minimizing the stresses generated during film preparation. Defect free films of titania on FTO were used as the photoanode in DSSCs. The photoanodes prepared under vacuum had better overall device characteristics than those made by the standard method. Therefore, the larger surface area provided by the thicker films increased the efficiency of the cells. The pressure during sintering also affected the device performance, which was attributed to lower dye coverage because of porosity changes.

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54 Table 31. Experimental conditions for preparing thick, defect free titania films on ITO and FTO subs trates. TCO Sintering Annealing Thickness dT1/dt T1 t1 dT2/dt T2 t2 P Defect density dT3/dt Defect density ( ) (C/min) (C) (min) (C/min) (C) (min) (kPa) (%) (C/min) (%) ITO 20 10 38.3 FTO 20 10 14.2 ITO 20 2.4 120 1 2 220 30 101.3 12.5 10 26.1 ITO 20 2.4 120 1 2 220 30 71.3 12.6 10 18.9 ITO 20 2.4 120 1 2 220 30 34.7 2.2 10 16.3 ITO 20 2.4 120 1 2 220 30 21.3 0.8 10 16.7 ITO 20 2.4 120 1 2 220 30 5.310 2 0.1 10 17.7 FTO 20 2.4 120 1 2 220 30 101.3 15.0 10 15.2 FTO 20 2.4 120 1 2 220 30 71.3 8.5 10 9.6 FTO 20 2.4 120 1 2 220 30 34.7 2.7 10 5.7 FTO 20 2.4 120 1 2 220 30 21.3 0.7 10 4.2 FTO 20 2.4 120 1 2 220 30 5.310 2 0.4 10 2.5 ITO 20 5 180 15 10 450 30 5.310 2 0.2 10 14 .0 FTO 20 5 180 15 10 450 30 5.310 2 1 10 7.7 ITO 20 5 180 15 10 450 30 5.310 2 0.3 1 7.8 ITO 20 5 180 15 10 450 30 5.310 2 0.5 100 21.9 FTO 20 5 180 15 10 450 30 5.310 2 0.8 1 3.5 FTO 20 5 180 15 10 450 30 5.310 2 0.4 100 9.1 ITO 25 5 180 15 10 450 30 1.210 2 1 22.8 FTO 7 5 180 15 10 450 30 5.310 2 10 1.9 FTO 10 5 180 15 10 450 30 5.310 2 10 2.7 FTO 25 5 180 15 10 450 30 8.010 3 10 6.0

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55 Table 32. Efficiency of DSSCs made with 10 20 m titania films on FTO substrates sintered a t varied pressures at 450C. The films are impregnated in 3.0 10-4 M of N 719 in ethanol for 30 h. Pressure Thickness Isc Voc FF Efficiency (kPa) ( m) (mA/cm 2 ) (V) (%) (s) 101.3 20 4.83 0.62 0.6 1.8 0.851 71.3 20 7.78 0.68 0.43 2.27 1.786 34.7 20 10.1 0.76 0.34 2.62 0.121 5.3 x10 2 20 6.16 0.68 0.46 1.92 0.080 101.3 10 1.37 0.58 0.52 0.42 0.809 5.3 x10 2 10 3.52 0.62 0.65 1.43 0.019

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56 Figure 31. The preparation steps for making thick titania films on transparent conductive oxide (TCO) substrates. Titania is deposited using the doctor blade technique (a) followed by sintering at pressures ranging from 8.010-3 to 101.3 kPa (b) annealing at 450C (c) and controlled cooling rates ranging from 1 to 100C/min (d)

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57 Figure 32. SEM image of titania film (~25 m thick) sintered at 8.010-3 kPa and 450C and cooled at 10C/min. 20304050 60 70-2 degrees)Intensity (a.u.)+ + ++ + + + + + ++ + + +2 1 Figure 33. X ray diffraction of 20 m thick titania films annealed at atmospheric pressure at 450C on an ITO substrate (1) and sintered in vacuum at 220C at 8.010-3 kPa on a FTO substrate (2) The peaks correspond to anatase titania (+) and ITO ( ). In (2), the titania film was removed from the FTO substrate so that only titania peaks were observed.

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58 200300400500600700Intensity (a.u.)Raman shift (cm-1)3 2 1 Figure 34. Raman spectra of 7 (1) 10 (2) and 25 (3) m titania films on FTO substrates. The peaks at 400, 514 and 640 cm-1 confirm the formation of anatase titania and the spectra show no changes in position associated with strain.

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59 Figure 35. Microscopic images of the final titania films on FTO which were sintered at 220C and a pressure of 101.3 (a) 71.32 (b) 34.66 (c) 21.32 (d) and 5.310-2 (e) kPa. The dark regions in the images are defects.

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60 Figure 36. The effect of sintering pressure on the defect density of titania films on FTO (a) and ITO (b) The films were prepared using a sintering temperature of 220C and a cooling rate of 10C/min. The circles and squares indicate defect densities after sintering and annealing, respectively. The shaded and open symbols correspond to sintering in the tube furnace and vacuum oven, respectively. The lines show exponential fits to the data. 1 10 0.01 0.1 1 10 100 Pressure (kPa)Defect density (%)(a)FTOsintered annealed 0.1 1 10 0.01 0.1 1 10 100Defect density (%)Pressure (kPa)(b)ITOsintered annealed

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61 Figure 37. Microscopic images of the final titania films on FTO (a) and ITO (b) which were sintered at 450C and 5.310-2 kPa and then cooled at a rate of 1 C/min. The defect densities of these films are 3.5 and 7.8 %, respectively. 0 5 10 15 20 25 -1 -10 -100Defect density (%)Cooling rate (oC/min)2 1 Figure 38. The effect of the cooling rate during annealing on the defect density of titania films on I TO (1) and F TO (2) which were sint ered at 5.310-2 kPa and 450C. The dashed lines indicate the defect density after sintering.

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62 Figure 3-9 Performance characteristics of DSSCs fabricated from photoanodes sintered at different pressures compared to the standard preparation method (cont rol). (a) Currentvoltage measurements; (b) Short circuit current; (c) Opencircuit voltage; (d) Fill factor; (e) Efficiency; (f) Electron lifetime. All titania photoanodes are 20 m thick and the control experiment is designated by the in (b) (f). 0 2 4 6 8 10 0 0.10.20.30.40.50.60.70.8Current Density (mA/cm2)Voltage (V)20 mcontrol 5.3x10-2 kPa 71.3 kPa 34.7 kPa 4 8 12 50100isc (mA/cm2)Pressure (kPa) 0.6 0.7 0.8 50100Voc (V)Pressure (kPa) 0.3 0.5 0.7 50100FFPressure (kPa) 1 2 3 50100 (%)Pressure (kPa) 0 1 2 50100 (s)Pressure (kPa) *(a) (b) (c) (d) (e)(f)

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63 Figure 310 SEM images of 20 m titania films sintered at varied pressures. The films are sintered at 101.3 (a), 34.7 (b) and 5.3x10-2 (c) kPa at 450C for 30 min followed by annealing at atmospheric pressure for transparenc y. The films sintered at lower pressures show closer particles and lower porosity

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64 CHAPTER 4 NONDESTRUCTIVE MEASUREMENT OF DYE ADSORPTION ONTO TITANIA FILMS Preliminary Remarks Dye sensitized solar cells (DSSCs) are not limited by the high purities and, consequently, high capital costs associated with silicon based devices [51] These devices are centered around a chromophore sensitizer typically a ruthenium based dye, that harvest s photons. P hotons adsorbed by the dye are excited to the lowest unoccupied molecular orbital where the electron can then be injected into the conduction band of a large band gap semiconductor, which is typically titania A majority of the research of DSSCs has focused on the use of N719 [ bis(tetrabutylammonium) cis di(thiocyanato) N,NAbis(4 carboxylato4A carboxylic acid 2,2A bipyridine) ruthenium (II) ] or N749 [(2,2':6',2' 'terpyridine4,4',4''tricarboxylate) ruthenium (II) tris (tetrabutylammo nium) tris(isothiocyanate)] as the photosensitive dyes. The impregnation of the dye into the titania photoanode occurs by soaking the matrix in a solution of the dye for several hours. The dye is adsorbed to the surface of titania by carboxylate groups which bind in different anchoring modes [52] It is important to obtain uniform dye coverage on titania for enhanced electron transfer and low recombination. Excess dye loading can lead to multilayer formation, which cause quenching of the photoexcited electrons and reduction of charge injection. On the other hand, poor dye coverage leads to low photoactive surface area and electron trap sites. Although the dye is critical to the operation of DSSCs, often the dye concentration is not reported or correlated to device performance. The reason for this lack of valuable information is that the dye concentration can only be obtained by dissolving the dye in a base solution, destroying the cell. This destructive method is simple but the expensive

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65 dyes are wasted in the process. In many cases, undissolved dye remains on the films after dissolution of the titania, which is an inherent source of error in determining the concentration or dye coverage from the destructive method. This destructive method also provides no information about the uniformity of dye coverage. Finally, the fractional coverage and distribution of dye are also important parameters in models that describe the performance of DSSCs [5 3,54] Obtaining accurate measurements of dye coverage that correlate to specific device performance will be important to developing more accurate models. In this chapter, I discuss a nondestructive method to quantify the concentration of N719 and N 749 dyes adsorbed onto titania photoanodes. The method is able to characterize the concentration in photoanodes ranging between 7 and 20 m in thickness. Spatial profiles of dye adsorption can also be correlated to local performance. The results show that the dye coverage is often nonuniform and that excessive dye concentrations lead to lower short circuit currents. Materials and Methods Materials. A paste containing ~18 wt % nanocrystalline titanium dioxide (Ti Nanoxide T/SP) as well as the N 719 and N 749 Ruthenium based dyes [55] were purchased from Solaronix. The particle size of the titanium dioxide is reported to be 13 nm. FTO on glass substrates were obtained precut from Thin Film Devices, Inc. Acetonitile and ethanol were purchased from Fisher Scientific. Sodium hydroxide (97 %) was purchased from Sigma Aldrich. Titania film preparation and dye impregnation. Titania films of various thicknesses were pasted onto FTO substrates, as described in chapter 3. Briefl y, titania

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66 paste is used to make 7, 10 and 20 m thick films (surface area of 0.384 cm2) using a simple doctor blade method. After the titania is pasted onto FTO, the films are sintered in a tube furnace (Barnstead type 79300 Thermolyne tube furnace) at a pressure of 5.310-2 kPa at 450C for 30 minutes. The films are then annealed at the same temperature on a hot plate until they become transparent. Finally, the films are cooled to room tempera ture at a controlled rate of 10C/min. This method was shown to prepare defect free titania films for thickne sses ranging from 7 to 25 m. Solutions of the N749 and N 719 dyes (1.6810-4 M) were dissolved in acetonitrile and ethanol, respectively. Titania films were impregnated by soaking in the dye solution for times ranging between 6 hours and 1 week. Determining the molar extinction coefficient of the dyes in the NaOH solution. The liquid extinction coefficient l dye was obtained experimentally. N719 and N 749 dyes are dissolved in a mixture of sodium hydroxide and ethanol (1:1 vol %) The spectra is taken for each sample by dilution with 0.5 ml of NaOH:EtOH mixture to collect absorbance of different dye concentrations. The pH of the solutions did not change in these measurements. The absorbance at the peak was plotted as a function of concentration (see Figure 41a and b), which yielded straight lines as expected by Beers Law. The slope of the line was used as the molar extinction coefficient of each dye. Measurement of molar extinction coefficients in the solid state. SEM and profilometry were used to measure the f ilm thickness. UVvis spectroscopy of the titania films or dye solutions were collected from 350 to 650 nm on a PerkinElmer Lambda 9 UV/VIS/NIR spectrometer. The spectra for dyes in the titania films were measured first.

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67 The titania films were mounted to an X Z translational stage (Newport) and multiple measurements (~15) of the initial and impregnated titania film were obtained. The spectra of the initial and impregnated titania films were averaged for analysis. The titania films were then dissolved in 4 mL of 0.1 M NaOH and ethanol (1:1 vol %) for 30 minutes for spectral analysis. It is worth noting that the spectra used to obtain the molar extinction coefficients were taken from a set of data that showed complete dissolution of the film and no residual dye remaining on the substrate. Spatial profiles. The spectra for dyes in the titania films were measured in a grid by moving the X Z translational stage across the titania film in 0.5 mm increments. Absorbance values at the peak position are collected and rescaled with values between 0 and 1 and assigned a color between blue and white, where blue corresponds to the highest concentrations. The assigned colors are used to create a square mesh to represent the concentration profile in the titania film. T he ti tania film was then covered with a iodide/triiodide electrolyte solution (purchased from Solaronix) in an open cell, which was completed with a platinum wire (diameter of 0.3 mm) as the working electrode. The platinum wire was used to measure the short cir cuit current at local points under simulated AM 1.5 solar light using a source meter (VersaSTAT 3 type from Princeton Applied Research) and a solar simulator (Solar Light Co.). Once again, the values were scaled and assigned a color to obtain the short cir cuit current profile. Results and Discussion The absorption spectra for N719 and N 749 dyes in the NaOH solution are shown in Figure 4-2 respectively. The N719 dye shows characteristic peaks at 375 and 512 nm (see Fig. 4-2 a) while the N749 dye has characteristic peaks at 397 and 586 nm

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68 (see Fig. 4-2 b). The molar extinction coefficients of each dye were determined using the 512 and 586 nm peak for N719 and N 749 (see supplementary data), respectively, as shown in Table 41. The spectra for each dye redshifts once they are adsorbed onto the tita nia film, as shown in Figure 4-2 For example, the peaks used to determine the molar extinction coefficients of each dye shift ed to 525 and 620 nm for N719 and N 749, respectively. Shifts in peak position can be expected because of the effect of the localized environment on the excited state [56] Although the spectra of each dye adsorbed onto the titania has shifted, Beers Law can still be used to measure the concentration provided there are no other optical effects that alter the transmission of light. A comparison of the absorbance for each dye measured in the NaOH solution and titania show a nearly constant ratio, a s shown in Figure 4-3 The slope of the curve yields the absorbance ratio, A dye,l / Adye,s, where the subscripts l and ss correspond to the liquid NaOH and solid titania film, respectively. A dye,l / Adye,s equals ~3 and ~9 for the N719 and N 749 dyes, respectively The linearity o f this ratio indicates that any optical effects are negligible. Therefore, Beers Law given by equation (41) : V N lAdye dye dye (4 1) can be applied to to the absorbance of the dye in both the NaOH solution and titania films. In writing this expres sion, A dye is the absorbance from the dye, l dye is the molar extinction coefficient of the dye, ll is the path length for light, N dye is the moles of dye that absorb light, and VV is t he volume of the medium hosting the dye. If the dye adsorbed onto the titania film is dissolved completely in the NaOH solution, then the

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69 moles of dye in NaOH must be equal to the number of moles in the solid titania film as in equation (42) : s dye s dye ss dye dye l dye dyeN t VA l VA N, , (4 -2) where is the thickness of the titania film. By rearranging equation ( 4-2 ) and substituting s satV where sa is the planar surface area of the film, an expression for the molar extinction coefficient of the dye adsorbed onto the solid titania films is obtained, l dye l s dye s dye s dyeV a l A A, (4-3) Equation ( 4-3 ) shows that the molar extinction coefficient for the dye adsorbed onto the titania films, s dye is a function of the constants l ,s a ,l V and l dye and the ratio of dye absorbance, l dye s dyeAA, ,/ Equation (43) was then used to determine the molar extinction coefficients of each dye adsorbed onto the titania films, as shown in Table 41. These molar extinction coefficients can be used to calculate the concentration of dye adsorbed onto the titania film. As shown in Figure 4 -4 the molar extinction coefficients are able to accurately determine the amount of dye adsorbed at a variety of concentrations and film thicknesses. Once the molar extinction coefficient is known for the dye, spatial profiles of dye adsorption can be obtained. The X Z translational stage was used to collect the absorbance in a grid using steps of 0.5 mm. Spatial profiles show significant deviation in

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70 the adsorption of dye across the film. As shown in Figure 4-5 (a) for a 7 m film impregnated with N719 dye, the difference in concentration across the film can be quite significant, ranging between 4.3 10-6 and 1.2 10-5 mol cm-3. The spatial distribution shows that there is a higher concentration of dye in the center of the film. Profilometry shows that the titania film thickness was higher at the center than the edges of the film (ranges between 6.2 and 6.8 m). This spatial technique provides valuable information about surface coverage and impregnation uniformity. This information cannot be provided in the typical destructive method and may help explain variations in DSSC device efficiency within the film. To demonstrate, the short circuit current was probed with a platinum wire for this film along a similar grid. Comparison of parts (a) and (b) of Figure 4-5 shows that higher short circuit current typically corresponds to intermediate dye concentrations in the film. These trends can be associated with the effect that the dye coverage has on device performance. If the dye coverage is too low, then excessive back reactions between the electrons in the titania and the electrolyte occur, reducing the current generated. On the other hand, higher concentrations of dye can influence electron transfer. Ono e t al showed that DSSCs exhibited high charge transfer resistance with multilayer adsorption of dye on the surface of the photoanode [57] P hotoexcited electrons are quenched as multiple layers of the dye build up on the surface, reducing the injection of electrons into the conduction band of the semiconductor Concluding Remarks The molar extinction coefficients of both N-7 19 and N 749 dyes adsorbed onto titania were determined. These molar extinction coefficients can be used to determine

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71 the concentration of dye adsorbed in films of varying thickness without destroying the film. This nondestructive technique provides an accurate measurement of the dye adsorbed onto titania and can be used to relate the local concentrations with device performance through spatial profiling.

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72 Table 41. Measured molar extinction coefficients for N 719 and N 749 dyes. N 719 N 749 (nm) ( L cm -1 mol -1 ) (nm) ( L cm -1 mol -1 ) NaOH/ethanol solution (pH =13.09) 512 12406 586 6320.5 titania film 525 6916 620 6454

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73 Figure 41. N 719 and N 749 dye extinction coefficient in NaOH. The liquid extinction coefficient was obtained experimentally using calibration data in NaOH and equation (4 1). The areas of the various thick films is 3.8510-5 m2, l is 1 cm and the volume of the NaOH solution is 410-3L. 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 7x10-58x10-59x10-50.00010.000110.000120.000130.00014 y = 12406x Absorbance at 512 nmConcentration (moles/L) N-719 (a) 0.25 0.3 0.35 0.4 0.45 0.5 0.55 4.5x10-55x10-55.5x10-56x10-56.5x10-57x10-57.5x10-58x10-58.5x10-5 y = 6320.5x Absorbance at 586 nmConcentration (moles/L) N-749 (b)

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74 Figure 4-2 Spectroscopic peaks of N 719 and N 749 dyes in NaOH solution (a) and in solid titania (b) 0 0.5 1 1.5 2 2.5 3 3.5 4 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 350400450500550N-719AbsorbanceWavelength1 2(a) 400450500550600650 0 0.5 1 1.5 2 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7AbsorbanceWavelength2 1(b)N-749

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75 0 0.1 0.2 0.3 0.4 0.5 0.6 0 0.1 0.2 0.3 0.4 0 0.05 0.1 0.15 0.2 0 0.010.020.030.040.050.06 Absorbance in solid TiO2Absorbance in NaOHAbsorbance in solid TiO2Absorbance in NaOH N-719 N-749 Figure 43. Absorbance of N 719 and N 749 dyes adsorbed onto the solid titania film or dissolved in NaOH.

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76 1x10-82x10-83x10-84x10-81x10-82x10-83x10-84x10-8Dye in solid TiO2 (mol)Dye in NaOH solution (mol) Figure 4-4 Comparison of the concentration of N 719 (red symbols) and N 749 (black symbols) dye determined by measuring the absorbance in the solid state (nondestructive) or NaOH solution (destructive) for titania films ranging in 20 m thick films. The dashed line represents equimolar values.

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77 Figure 45. Spatial profile of N 719 dye absorption in 10 m titania film by performing 0.5 mm step measurements using X Z stage over a square area (a) Scale bar shows concentration ranges from 4.310-6 (white) to 1.210-5 mol/cm3(blue). Short circuit current profile of N 719, 10 m film. Scale bar shows short circuit current ranges from 5.610-5 to 1.410-3 mA (b) (a)(b) 5.6x10-51.4x10-3 mA 4.3x10-61.2x10-5 mol/cm3

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78 CHAPTER 5 ENHANCING THE PHOTO ACTIVE SURFACE AREA OF DYE SENSITIZED SO LAR CELLS Ini tial Remarks The most important factor for enhancing DSSC efficiency is to increase the photoactive surface area of the semiconductor. Enhancing dye coverage for DSSCs can enhance the photon generation and electron collection efficiency. The incident photon to current conversion efficiency is given by equation (51) coll injn LHE IPCE )()( (5 1) inj is the quantum yield for electron injection from the excited sensitizer in the conduction band of coll is the electron collection efficiency [4] To improve nanowire structure or nanoparticle film, dye uptake becomes essential in harvesting of photons and higher IPCE efficienc y. It should be stated that it is enhanced monolayer and uniform dye coverage that is required to enhance performance. Multilayer formation and poor dye coverage hinders kinetics of electron transfer. Ono el al shows formation of agglome ration of dyes mol ecules on TiO2 surfaces causes high charge transfer resistance [57] D ye aggregates cause quenching of photoexcited electrons which can prevent injection in the semiconductor, and causes recombination of electrons w ith the electrolyte. The electron diffusion length is the distance in which injected electrons in the conduction band of the semiconductor will travel before they will likely recombine with holes in the semiconductor, with the dye in its ground state or backreact with the electrolyte, see

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79 Figure 5-1 The electron diffusion length is limited by dynamic competition betw een electron diffusion and lifetime as given by equation (52) nn nDL (5 2) where Ln is electron diffusion length, Dn is electron diffusion and n is electron lifetime. It is understood that the measured lifetime does not in general correspond to the free carrier lifetime but is rather an average of characteristic times for survival of free and trapped elec trons [48] Peter et al. showed optimum electron diffusion length for typical DSSCs to be 10 m [58] In order to achieve higher device efficiency, it is important to utilize thicker titania films while maintaining higher diffusion length and low recombination. Thicker semiconductor films can ideally enhance the photoactive surface area. However, poor penetration and dye aggregation in thicker films causes recombination and electron traps due to transport limitation with liquid solvents. Reduced electron diffusion coefficient and increased recombination will not allow electron diffusion length to increase beyond 10 m. Supercritical fluids (SCFs) have low surface tension and high diffusivities [59] and are expected to deliver dye to small pore sizes and inaccessible spaces and enhance uptake. Ogomi et al. [60] showed that dye injection in SCFs can yield 20 % increase in efficiency compared with conventional dip coating processes. In this chapter, solubility measurements of ruthenium dyes were performed using a spectroscopic technique to confirm dye dispersion in a SCF at a range of temperatures and pressures. Dye coating of the semiconductor by wet and gas methods is in vestigated for up to 20 m thick titania with different solvent mixtures including sc -

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80 CO2. DSSCs are tested to obtain short circuit current, open circuit voltage, efficiency and electr on lifetime. Materials and Methods Materials A paste containing ~18 wt % nanocrystalline titanium dioxide (Ti Nanoxide T/SP ), N 719 and N 749 Ruthenium based dyes, as well as Iodolyte AN 50 and Platisol were purchased from Solaronix. The particle size of the titanium dioxide is reported to be 13 nm. FTO on glass substrates were obtained precut from Thin Film Devices, Inc. Acetonitrile, ethanol and t butanol were purchased from Fisher Scientific. thick sheets by Ran Castle Co. Titania film preparation. Titania films of various thicknesses were pasted onto FTO substrates as described in chapter 3. Briefly, titania paste was used to make 7, 10 and 20 m thick films (surface area of 0.384 cm2) using a simple doctor blade method. After the titania was pasted onto FTO, the films were sintered in a tube furnace (Barnstead type 79300 Thermolyne tube furnace) at pressure in the range of 5.310-2 to 101.3 kPa at 450C for 30 min. The films were then annealed at the same temperature on a hot plate until they become transparent [61] Finally, the films were cooled to room temperature at a controlled rate of 10C/min. This method was shown to prepare defect free titania films for thicknesses ranging from 7 to 25 m. Dye solubility in scCO2. N749 and N 719 ruthenium dyes were dissolved in a cosolvent for solubility in sc CO2 [62] The amount of cosolvent was limited to less than 5 % (vol %) with sc CO2. 3 .010-4 M of dye in a cosolvent wa s injected in the volume variable view cell (purchased from Metallic Creations Co.) at a given

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81 temperature. Then, Isco syringe pump was used to pressurize sc CO2 to the view cell at the desired pressure while stirring using an egg shaped stir bar to reach equilibrium. The view cell wa s aligned vertically with the UV VIS spectrometer light source for UV VIS measurements. N 719 and N 749 dyes solubility in sc CO2 were determined at temperatures ranging from 35 to 55C and at pressures ranging from 100 to 400 atm. Cloud point measurements were collected at the same conditions by varying pressure at a given temperature. Dye impregnation by dipping. 3.010-4 M solutions of N 719 and N 749 were prepared in ethanol and acetonitrile-tbutanol (1:1 vol %), respectively. The effect of t butanol as a cosolvent on device performance was tested by preparing different N 719 and N 749 dip samples in ethanol -tbutanol (1:1 vol %) and acetonitrile respectively. These films were impregnated by dipping at room temperature from 6 h to 1 week. The optimum impregnation time for the various thick films was found to vary depending on film thickness, see Figure 5-2 The 7, 10 and 20 m films were impregnated at 24, 48, and 180 h, respectively for optimum dye coverage. Dye impregnation by scCO2. The ruthenium dye concentration in a cosolvent was increased to 30 % for film coverage. The optimum concentration of the dye in sc CO2 ranged from 0.5010-4 to 0.910-4 M at pressur es ranging from 10 to 15 MPA [63] 7 ml of the 3.010-4 M solutions of N 719 and N 749 in ethanol and a cetonitrile -tbutanol (1:1 vol %), respectively, was injected in the view cell. Isco syringe pump wa s used to delive r pressurized carbon dioxide to the view cell using a 6way valve injection loop at 40C and 100 atm a nd was allowed to reach equilibrium. Cell pressure was increased to 250 atm by injecting sc CO2 from the back of the view cell to maintain dye molality in the

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82 solvent mixture. Titania films placed in the view cell were impregnated for times ranging between 1 to 1.5 h. Characterization. Profilometry and SEM were used to measure the film thickness. Dye solubility wa s measured using the PerkinElmer Lambda 9 U V/VIS/NIR spectrometer in situ. Cloud point measurements were obtained at the operating conditions. Dye concentration was measured using a nondestructive technique instead of destructive dye adsorption in NaOH [64] as de scribed in chapter 4. Briefly, UVVIS spectroscopy of the titania films were collected from 350 to 650 nm on a PerkinElmer Lambda 9 UV/VIS/NIR spectrometer. The spectra for dyes in the titania films were measured first. The titania films were mounted to an X Z translational stage (Newport) and multiple measurements (~15) of the initial and impregnated titania film were obtained. The spectra of the initial and impregnated titania films were averaged for analysis. DSSC device fabrication and testing. FTO count er electrode was painted with Platisol (Solaronix) using a brush and fired in the tube furnace at 400C for 5 min. Surolyn spacer was cut and placed between electrodes and the electrolyte was injected by capillary action. The current voltage characteristics of the prepared DSSC were measured under simulated AM 1.5 solar light using a source meter (VersaSTAT 3 type from Princeton Applied Research) and a solar simulator (Solar Light Co.) Open circuit voltage decay measurements were used to assess the recombination processes in DSSCs [48] When illumination was shut off at open circuit, electrons transfer ed by recombination only and voltage decay wa s related to electron lifetime. The voltage

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83 changes were fit to a decay function and the derivative was used to calculate electron lifetime using equation (53): 1 t V e TkB n (5-3) where KB is Boltzmann constant, T is temperature and e is electron charge. KBT/e is thermal voltage and at room temperature is 0.026 V It has been shown [3,65,66] that the recombination reaction in DSSCs is nonlinear and is of second order with respect to concentration of electrons, e.g. U is pro portional with n2. In general, nkUr (5 4) w here U is the rate of recombination, Kr is the net recombination rate constant, n is electron concentration and is recombination order. The recombination order can be calculated using equation (55) [48] : ndt d 1 (5 5) It has been suggested that the value of increases when interfacial potential drop increas es in DSSCs. Deviation of the value toward unity suggests reduction of electron recombination. Results and Discussion Solubility using scCO2. Ruthenium dyes have low solubility in scCO2 with up to 5 % (vol %) of ethanol as a cosolvent. UV VIS spectroscopy measurements at different pressures and temperatures showed dye solubility is in the range of ~10-7 moles, see Figure 5-3 This wa s attributed to the weak solvent strength of the non polar carbon dioxide. Cloud point measurements confirmed solubility of dye increases at higher

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84 pressures and lower temperatures, see Figure 5-4 a. This shows that solubility is proportional with carbon dioxide density; see Figure 5-4 b. Dip vs. scCO2 impregnation. 7, 10 and 20 m thick titania films on FTO substrates, impregnated by dip and scCO2 methods in both N 719 and N 749, were probed for film concentration using the nondestructive technique, described in chapter 4. The scCO2 method showed comparable absorbance to the dip methods with much lower time scales. Effecti ve coverage with scCO2 can be achieved in 11.5 h compared with up to 180 h for the dip method. The films impregnated in the alternative cosolvent mix ture for N 719 in ethanol -tbutanol show ed higher absorbance than those in ethanol see Figure 5-5 a. The films impregnated in N 749 in a cetonitrile showed comparable absorbance to those in a cetonitrile-tbutanol see Figure 5-5 b. N749DSSC device performance. Current voltage characteristics of DSSCs with photoanodes of different thicknesses prepared at 5.3 10-2 kPa and impregnated in N 749 using different solvent mixtures are shown in Figure 5-6 a. The increase in short circuit current in the thicker films for both dip and SCF films was due to higher photoactive surface areas. The fill factors ranged from 0.5 4 to 0.58 f or all the cells, see Figure 5-6 b. The three (co)solvents mixtures used with N 749 show ed increase in efficiency from 2.23.5 % to 4.7 5.8 % going from 7 to 10 m in film thickness; see Figure 5-7 a. The higher efficiency of the DSSCs prepared wi th a cetonitrile -tbutanol wa s likely due to reduced recombination. However, when film thickness was increased to 20 m, the efficiency of the two liquid (co)solvent mixtures dropped to 2.53.5 %. This was due to poor penetration of solvents in the small pore sizes reducing coverage electron generation and collection efficiency T he 20 m SCF showed lo wer absorbance, see

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85 Figure 5-5 b. This wa s likely due to better coverage of the film using sc CO2 with a monolayer of dye on most of the film surface. The relatively lower electron life time indicate overall better coverage of the thicker films; see Figure 5-7 b. The thicker films gave highest efficiency of 6.57 % due to improved absorption of light between 700900 nm [ 67] which N749 is capable of sensitizing efficiently. N719DSSC device performance. Current voltage characteristics of DSSCs with photoanodes of different thicknesses prepared at 5.310-2 kPa and impregnated in N 719 using different solvent mixtures a re shown in Figure 5-8 a. N 719 in ethanol gave efficiency around 1.77 % for all the thicknesses, see Figure 5-8 b. N 719 in ethanol and scCO2 gives 4.83 % at 7 m. However, at higher thickness of 1020 m efficiency drops to around 1.5 %. In a similar trend but with much higher efficiency, N 719 in ethanol -tbutanol gave 9.43 % at 7 m and dropped to 6.42 % at 20 m. This increase in efficiency for the co solvent mixtures of ethanol -tbutanol wa s due to reduced recombination. The higher efficiency at the 7 m thick films for the co solvent mixtures wa s because N719 dye sensitized efficiently up to 750 nm and thinner films provided higher electron collection efficiency. The effect of tbutanol wa s probed by changing the percentage of ethanol to tbutanol fro m 25 % to 100 %. DSSCs prepared with 7 m titania flms impregnated in ethanol -tbutanol (25:75 %) show ed best device performance. This was likely due to minimized recombination. It i s understood that t butanol reacts with titania surface and reduces recombination between electrons and the electrolyte. The effect of multilayer dye formation on titania wa s probed by changing the concentration of N 719 in ethanol tbutanol (25:75 %) from 1.0 10-4 to 4.0 10-4 M. Efficiency wa s enhanced by increasing

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86 concentrat ion of dye and was found optimum at 3.0 10-4 M. However, it dropped at 4.0 10-4 M and this was likely due to multilayer formation of dye molecules on titania surface, see Figure 5-9 The lower efficiency of the 50:50 vol % compared with the previous result in Figure 5-8 b wa s related to FTO reduced transmission. The properties of the best dye sensitized solar cells fabricated in different solvent mixtures are summarized in Table 51. DSSC s made with t he 7 m film s impregnated with N 719 showed best efficiency of 9.44 % and 4.84 % DSSCs made with th ick films of 20 and 10 m impregnated with N 749 showed best efficiency of 6.57 % and 5.82 % Concluding Remarks DSSCs made with thick photoanodes using N749 showed high efficiency due to improvement of absorption of light between 700 to 900 nm Although using tert butanol helped enhance efficiency of the 10 m films by reducing recombination, it was only sc CO2 mixture that allowed enhancing the photoactive surface area and higher efficiency for the 20 m thick fi lms. DSSCs made with 7 m thick photoanodes using N 719 dye showed higher efficiency because N 719 dye sensitizes efficiently up to 750 nm and it is likely that the thinner films allow higher electron collection efficiency. This shows importance of dye sen sitization for enhancing performance of DSSCs.

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87 Table 51. The efficiency of the best DSSCs made with FTO substrates with titania films of various thicknesses with N 719 and N 749 dissolved in various solvent mixtures. Dye Solvents Thickness Isc Voc FF Efficiency ( m) (mA/cm 2 ) (V) (%) (s) N 719 Ethanol :t But 7 22.43 0.77 0.55 9.44 0.018 N 749 CO 2 :AcN:t But 20 16.92 0.73 0.53 6.57 0.029 N 749 AcN:t But 10 13.27 0.77 0.57 5.82 0.509 N719 CO2: Ethanol 7 10.16 0.74 0.64 4.84 0.076

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88 Figure 5-1. Electron transport and recombination paths between the semiconductor and the electrolyte. After light has excited dye molecules to states of different energy (black), electrons are injected into the conduction band of the semiconductor and complete the circuit to reduce oxidized electrolyte (green). Electrons can recombine with the oxidized species of the electrolyte or with unexcited dye molecules (red). 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0 50 100150200 7 m 10 m 20 mAbsorbanceTime (hrs)N-719 Figure 5-2 Absorbance of 7, 10 and 20 m titania films in N 719 dye. The adsorption peaks vary depending on film thickness.

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89 (a) 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 440460480500520540560 60oC 50oC 40oCAbsorbanceWavelength (nm) (b) Figure 5-3 Solubility of N719 dye in sc CO2 and 4-5 % of ethanol as a cosolvent using in situ UVVIS measurements Solubility a t 200 atm and temperatures ranging from 40 to 60C showing higher absorbance (lower solubility) at 60 C (a). Solubility at 50C and pressures ranging from 100 to 400 atm showing higher absorbance (lower solubility) at 100 atm (b)

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90 Figure 5-4 Solubility o f N 719 in carbon dioxide using ethanol as a cosolvent. Cloud point measurements of N 719 in sc CO2 and less than 5 % of ethanol as a cosolvent (a) Solubility of N719 is proportional to density of carbon dioxide (b) 50 100 150 200 250 300 350 400 3035404550556065 2.27x10-7 dye moles (co-solvent 4.1 %) 1.82x10-7 dye moles (co-solvent 4.6 %) 2.83x10-7 dye moles (co-solvent 4.0%)Pressure (atm)Temperature (oC)(a)2 1 N-719 0.2 0.4 0.6 0.8 1 1.2 3035404550556065 2.27x10-7 dye moles (co-solvent 4.1 %) 1.82x10-7 dye moles (co-solvent 4.6 %) 2.83x10-7 dye moles (co-solvent 4.0%)Carbon dioxide density (g/cm3)Temperature (oC)(b)N-719

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91 Figure 5-5 Absorbance of impregnated titania films by UVVIS spectroscopy Absorbance of 7,10 and 20 m anatase titania film in N 719 dye dissolved in solvent mixtures of ethanol tbutanol and scCO2 (a) Absorbance of similar titania films in N749 dye dissolved in solvent mixtures o f acetonitrile, tbutanol and sc CO2 (b) 0 0.2 0.4 0.6 0.8 1 1.2 0.1 1 10 100 1000 7 m 10 m 20 mAbsrobanceTime (hrs) N-719Black: ethanol:CO2Red: ethanol Green: ethanol:t-butanol(a) 0 0.2 0.4 0.6 0.8 1 0.1 1 10 100 1000 7 m m 20 mAbsorbanceTime (hrs)N-749Black: acetonitrile:t-butanol:CO2Red: acetonitrile:t-butanol Green: acetonitrile(b)

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92 Figure 5-6 Performance characteristics of DSSCs fabricated from photoanodes in N749. IV curve s of DSSCs prepared with 7, 10 and 20 m thick titania films impregnated with N 749 dye in a cetonitrile tbut anol and scCO2 (a). Fill factors of the DSSCs at various film thicknesses (b) 0 5 10 15 20 0 0.10.20.30.40.50.60.70.8 7 m DIP1 10 m DIP1 20 m DIP1 7 m SCF 10 m SCF 20 m SCF 7 m DIP2 10 m DIP2 20 m DIP2Current Density (mA/cm2)Voltage (V) N-749 (a) 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 68 10121416182022 SCF (AcN:t-But:CO2) DIP1 (AcN:t-But) DIP2 (AcN) N-749Fill FactorTiO2 film thickness ( m) (b)

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93 Figure 5-7 Efficiency and electron life time of DSSCs fabricated from photoanodes in N749. Efficiency (a) and electron life time (b) of DSSCs made with titania films of various film thicknesses impregnated with N 749 in acetonitrile, tbutanol and sc CO2. 0 1 2 3 4 5 6 7 8 7 1020 SCF (AcN:t-But:CO2) DIP1 (AcN:t-But) DIP2 (AcN)Efficiency (%)TiO2 film thickness ( m) (a)N-749 0 0.5 1 1.5 2 7 1020 SCF (AcN:t-But:CO2) DIP1 (AcN:t-But) DIP2 (AcN) TiO2 film thickness ( m)Electron life time (s)N-749(b)

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94 Figure 5-8 Performance characteristics of DSSCs fabricated from photoanodes in N719. IV curve s of DSSCs prepared with 7, 10 and 20 m thick titania films impregnated with N -7 19 dye in ethanol, tbutanol and sc CO2. Efficiency of the DSSCs at various film thicknesses (b) 0 5 10 15 20 25 30 0 0.10.20.30.40.50.60.70.8 7 m DIP 10 m DIP 20 m DIP 7 m SCF 10 m SCF 20 m SCF 7 m DIP2 10 m DIP2 20 m DIP2Current Density (mA/cm2)Voltage (V) N-719 (a) 0 2 4 6 8 10 12 7 1020 DIP1 (EtOH) DIP2 (EtOH:t-But) SCF (EtOH:CO2)Efficiency (%)TiO2 film thickness ( m) (b)N-719

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95 1.5 2 2.5 3 3.5 4 4.5 0 20 4060 80 100 Molarity Tert-butanol (%) 0 1x10-42x10-43x10-44x10-4Efficiency (%)Molarity (mol/L) Tert-butanol (%) N-719 Figure 5-9 The effect of tbutanol and concentration of N 719 on the efficiency of DSSCs The DSSCs are made with 7 m titania films si ntered at 2.3 kPa and at 450C The optimum concentration is 3.0 10-4 M in ethanol and t butanol at (25:75 vol %).

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96 CHAPTER 6 CONCLUSION The work in this dissertation explores a unique approach to transport in nanomaterials utilizing supercritical carbon dioxide as an alternative cosolvent. The improved transport in carbon nanotubes allowed their benign purification without altering their unique properties. The extraction efficiency of carbon nanotubes made with different processes was related to the nonotube properties and accessibility of metal catalyst for oxidation. Thick titania films on FTO and ITO substrates were prepared to increase the semiconductor surface area. The effect of sintering pressures and cooling rates were probed to avoid film defects. Sintering the titania films at low pressures reduced the stresses on both ITO and FTO substrates. The effect was more prominent in titania films prepared on FTO, resulting in significantly lower defects in the films. The preparation of defect free titania films on ITO required further control of the heating and cooling steps. DSSCs fabricated under vacuum yield better device characteristics than the standard method for making photoanodes. DSSCs fabricated with these FTO films showed best performance at intermediate vacuum levels. These results indicate that the lowest pressures used to make the photoanode are good for minimizing defect density but not optimizing device performance. The nondestructive method for analysis of dye adsorption was developed for device analysis with respect to dye coverage. The molar extinction coefficients of both N719 and N 749 dyes adsorbed onto titania were determined. These molar extinction coefficients can be used to determine the concentration of dye adsorbed in films of varying thickness es without destroying the film.

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97 The photoactive surface area of DSSCs thick semiconductor films wa s increased by performing wet and gas impregnation at optimal adsorption times for both N 719 and N749. U tilizing supercritical carbon dioxide for impregnation of thick films overcame transport limitation by wet methods Device characteristics such as efficiency and electron lifetime were analyzed with respect to film surface area, porosity, and dye coverage. While DSSCs made with thicke r photoanodes worked better with N749, those made with thinner photoanodes worked better with N719 due to sensitization properties of the different dyes

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98 APPENDIX A SUPERCRITICAL FLUID PURIFICATION OF ( PREPURIFIED) SINGLE WALL CARBON NANOTUBES (SW CNTS) Introduction Supercritical carbon dioxide is shown to be a good solvent environment for removing metal impurities from carbon nanotube samples. The best processing conditions tested were at the moderate conditions of 45C and 400 atm using hexafluoroacety lacetone as a chelating agent, yielding a final ash (metal oxide) content of 2.4 wt % or an estimated metal content of 1.5 wt % or less. Not only did these conditions efficiently remove the metal from the carbon nanotubes, the process had no effect on the inherent properties of the carbon nanotubes. Both Raman and fluorescence spectroscopy are sensitive to impurities and showed no increase in sidewall damage or reduction in emission intensity. Triton X 100 was demonstrated as an alternative chelating agent. Experimental Procedure Materials and Reagents. Tri -nbutylphosphate (TBP 99 % purity from Sigma Aldrich),Triton X 100, and hexafluoroacetylacetone (HFA 95 % purity from Sigma Aldrich) were used as received. Carbon nanotubes were used as received. Carbon dioxide was received from Praxair and used as received. Procedure. The stainless steel extraction cell (9/16 I.D. 9 in length, 24 mL) was immersed in a water bath and temperature controlled using a recirculating bath. Pressure was controlled to 2 bar using a 260 mL Isco syringe pump (Lincoln, NE). An external magnetic stirrer drove a PTFE coated magnetic stir bar. Approximately 20 mg of carbon nanotubes were wrapped in Whatman filter paper and inserted into the reaction cell. The chelating agent and oxidizing agent were loaded into a boat and

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99 inserted into the chamber. The chelating agent and oxidizing agent were added at a 1:1 ratio for Triton X 100 and 1:2 ratio for HFA. The cell was loaded with CO2 to approximately 100 atm and then heated to the desired temperature. Once equilibrium was achieved, CO2 was again loaded until the desired pressure was reached. The chamber was held at the extraction conditions for 1 h. The CO2 was then slowly vented and bubbled through water, yielding a turbid, purple solution. The nanotubes were recovered from the chamber and bath sonicated in ethanol for 30 min. The suspension was then filtered through a 0.1 m PTFE membrane and washed with hexane. Characterization. Thermogravimetric (TGA) analysis was conducted in air up to 800C at a ramp rate of 10 C/min. Raman spectra were recorded on the solid carbon nanotubes using a Reinshaw Invia Bio Raman with excitation from a 785 nm diode laser. For fluorescence measurements, carbon nanotubes were dispersed in 1 wt % SDBS surfactant using a probe tip ultrasonicator and characterized using an Applied NanoFluorescence Nanospectrolyzer (Houston, TX) with excitation from 662 and 784 nm diode lasers. Results Effect of Pressure and Temperature on Extraction. The ash (metal oxide) content of the asreceived carbon nanotubes before purification was 9.3 %. Carbon nanotubes collected from the extraction chamber were analyzed using TGA to measure the remaining metal content in the sample. All purification conditions showed reduced metal content in the carbon nanotubes. Figure A -1 show s the effect of temperature and pressure on the final ash content of carbon nanotubes purified using HFA as the chelating agent. As can be seen in the figure, higher pressures and temperatures yield

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100 better metal removal. The lowest ash (metal oxide) content achieved to date is 2.4 wt %. Without knowing the exact atomic content of the catalyst, it is difficult to convert this to the true metal content. However, it is estimated that this corresponds to a metal content of 1.5 wt % or less. Multiple extraction steps. The carbon nanotubes were first purified at 45C and 400 atm. The extraction was then repeated with fresh chelating agent and oxidant at the same operating conditions. The final metal content after the second extraction did not improve (results not shown). The lack of any changes suggests that a singlestep extraction is feasible provided the ratios of all components are optimized. Assessment of carbon nanotube damage. The most critical parameter to characterize in the purification of carbon nanotubes is the damage to the sidewall and the amount of residual compounds remaining that affect their properties. Most purification processes use strong acids or oxidative environments to remove the impurities, which can oxidize the carbon nanotube sidewalls. Raman spectroscopy provides a measure of the sp3/sp2 carbon atoms (~1300 and ~1600 cm-1, respectively) in the carbon nanotube sidewall. This ratio of bands, also known as the D/G band ratio, is often used to describe the quality of the carbon nanotubes. The Raman spectra for the raw nanotubes in Figure A 2 show a low D/G ratio indicative of high quality carbon nanotubes with few sp3 defects. After purification using HFA in sc CO2, there are no changes to the Raman spectra. These results indicate that there has been no sidewall damage introduced to the nanotubes during the purification process. While Raman spectroscopy is an accurate means of indicating changes to the structure of the nanotubes, some carbon nanotube properties, such as fluorescence, are very sensitive

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101 to the presence of even small amounts of impurities. Most purification processes result in substantial reductions in the fluorescence emission of carbon nanotubes. These reductions are possibly due to the remaining acid, which tends to quench the fluorescence. In many purification processes, the reduction in emission intensity can be an order of magnitude or more. Figure A 3 shows the fluorescence spectra for the raw and the highest purity carbon nanotubes obtained using HFA as a chelating agent. As can be seen, both have good fluorescence spectra with peaks corresponding to different types of carbon nanotubes. The purified nanotubes show similar fluorescence intensity to the raw sample. Triton X 10 0 as a chelating agent. Triton X 100 was tested as a cheaper alternative to HFA for the chelating agent in the removal of metal impurities in sc CO2. TX 100 was capable of removing the metal from the as received carbon nanotubes. While this chelating agent did not perform as well as HFA, it did remove a considerable amount of the metal impurities. The differences in extraction efficiency are likely related to the reduced solubility of the surfactant in sc CO2. It is anticipated that better metal removal could be obtained with multiple extraction steps or optimized extraction conditions. Conclusions Carbon nanotubes were purified in supercritical CO2 using an oxidant and chelating agent. The carbon nanotubes were characterized to determine the amount of metal remaining and the effect of purification on the final properties of the nanotubes. The metal content was reduced at all extraction conditions using a singlestep extraction. However, it was found that higher pressures and temperatures yielded the

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102 lowest m etal content. The best extractions were obtained when using HFA as the chelating agent at 45C and 400 atm. The final ash content was 2.4 wt % or an estimated metal content of 1.5 wt % or less. The purified carbon nanotubes showed no changes to their inherent properties during processing. Raman spectra, especially the D band which measures the amount of sidewall defects, showed no changes. The fluorescence of the carbon nanotubes also showed no decrease in intensity after purification. Therefore, the purification of carbon nanotubes using chelating agents and oxidants in a supercritical environment provides a benign process capable of achieving nanotubes with low metal content.

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103 Figure A -1 Raman spectra of raw and treated carbon nanotubes. Supercritical carbon dioxide treated sample at 35 to 45C and 300 to 400 atm show comparable spectra to the raw sample indicating no side wall damage to the carbon nanotubes. Figure A 2. Fluorescence of raw and treated carbon nanotubes. Supercritical carbon dioxi de extraction shows comparable intensity.

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104 Figure A 3. Final metal oxide content of treated carbon nanotubes. Supercritical carbon dioxide extraction was performed at temperature from 35 to 45C and pressures from 300 to 400 atm. A single extraction at 45C and 400 atm shows 2. 4 wt % of final ash content.

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105 APPENDIX B ELECTRO DEPOSITION AND TEMPLATE REMOVAL USING SUPERCRITICAL CARBON DIOXIDE Nanowire E lectrodeposition Using Supercritical Carbon Dioxide The main factor for eff iciency in DSSCs is the high photo active surface area of the semiconductor. Semiconductor nanoparticle film s have high surface area but electron diffusion is low In nanowire dye sensitized solar cells, electron diffusion is increased by replacing the nanoparticle film with an array of oriented singlecrystalline nanowires. Electron transport in crystalline wires was found to be several orders of magnitude faster than percolation through a random polycrystalline network due to excellent crystallinity and radial electric field in each nanowire which helps in repelling electrons from the surrounding electrolyte [1,6] Nanowires enable faster electron transport but lack the high surface area. Therefore, synthesis of high density nanowires is essential for achieving both faster electron transfer and high surface area. Template synthesis by electrodeposition has been the best method for synthesis of highly ordered nanomaterials. However, this technique can lead to incomplete pore inclusion and pin holes formation due to hydrogen hydrolysis. Moreover, it can produce polycrystalline materials with larger grain sizes. CO2 in water miniemulstions. Miniemulsions are defined as aggregates of one phase dispersed in another phase with sizes between microemulsions and emulsions. Sc CO2in water miniemulsions can be used in electrodeposition for synthesizing arrays of high aspect ratio nanowires with better inclusion and conductivity. Psathas et al. showed that sc CO2in wa ter miniemulsions of 175 nm droplets can be formed with the phase inversion temperature (PIT) emulsification method with low shear followed by

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106 rapid cooling. The temperatureinduced inversion in the curvature of the system occur with a minimum in interfacial tension [68] Pore inclusion has been problematic in nanomaterials synthesis by techniques such as chemical vapor deposition (CVD) or electrodeposition. This can be due to release of hydrogen during hydrolysis that can block or plug pores entrances in electrodeposition. Miscibility of hydrogen in SCF can prevent hydrogen hydrolysis and allow complete inclusion. Yoshida et al. showed that pin holes formation in the order of microns can be eliminated in nickel electroplating using sc CO2 [69,70] Electrodeposition using scCO2. Electrodeposition in SCF miniemulsions is similar to pulse electrodeposition; nucleation and crystal growth cannot occur when SCF comes in contact with the cathode, therefore grain size is limited and better crystalline materials are formed. Tian et al. have prepared Ag, Au and Cu single crystal nanowires by template electrodeposition utilizing a slow multi step nucleation and growth mechanism by lower over potential and higher temperature and a gelatin additive to enhance the growth of single crystal metal nanowires [71,72] These conditions can enhance surface diffusion of atoms and favors the growth of existing crystal nuclei. Anodized alumina oxide templates. The best method for synthesis of highly ordered nanowire arrays has been utilizing the ordered templates of anodized alumina oxide (AAO), which are available commercially in 50 to 200 nm in diameter and up to 60 microns in length. Alumina oxide is anodized in phosphoric acid to enlarge pore size and then etched in phosphoric, oxalic or sulfuric acids [72,73] at different temperatures to form cylindrical channels of varying diameters. Electrodeposition is confined to the walls of the AAO template.

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107 Equipment Built To perform electrodepositioin using a SCF, a high temperature and pressure system was designed, high pressure chamber (HPC) insulated safety box was built and initial experiments were performed. Isco pump is used to deliver pressurized carbon dioxide from a CO2 tank to the HPC. A homogenizer pump should be used to homogenize sc CO2 and the electrolyte solution and circulate the fluids to the high pressure chamber through the volume variable view cell as seen in Figure B1a Volume variable view cell with side sapphire windo ws can be used with UV -vis spectroscopy for phase verification. For safety a doublewall aluminum box was constructed for the 1L high pressure chamber (Autoclave engineering). High temperature insulation blanket was installed between the double walls of the chamber box. The set up of our chamber includes temperature controller and solid state relays connected by high temperature wiring to six heating bands (Tempco) on the chamber, a thermocouple inserted in the cap of the chamber for temperature control and a potentiostat, which can be used to ensure a constant applied voltage for a three electrode cell configuration, see Figure B-1b Two 12 electrodes and two s ample h olders were coated with a micron of platinum against corrosion, see Figure B-1c. Electrodes length enables minimum separation distance between the electrodes during electrodeposition. A thin platinum wire in 1/16 tubing can be used as a reference electrode. Temperature controller PID values were manually tuned for our system. Inert beads are to be used as volume fillers inside the high pressure chamber to minimize working volume by 50 %. Experimental Sc CO2in water environment are utilized by formation of emulsions or miniemulsions [70,7478] using ionic or nonionic surfactants. The steric

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108 forces between the head groups of molecules of an ionic surfactant are stronger than those in a nonionic surfactant. Therefore, they should form stronger and more stable emulsions. AOT and decane as a cosolvent (99.0 % purity Fisher Scientific Co.) and Octaethylene Glycol Monododecyl Ether OGME (99.7 % purity Wako Pure Chemicals Co.) have been used to make emulsions as ionic and nonionic surfactants, respectively [7981] Sc CO2in water miniemulsions of 175 nm can be formed by using a triblock copolymer of poly (dimethylsiloxane) (PDMS) and poly (ethylene oxide) (PEO), PEO PDMSPEO [68] Emulsion formation by an ionic surfactant Emulsions of sc CO2in water were formed in a volume variable view cell (volume 23 ml). 1.0 g of AOT, an ionic surfactant, was dissolved in 15 ml of double di water by stirring at 70C Then, 0.2 ml of decane was added as a cosolvent to the volume variable view cell, which was set in the water bath at 45C. After system reached equilibrium, 8 ml of supercritical CO2 at 75 atm was injected. After agitation by stirring using cross shaped stir bar, emulsions started to form. Complete emulsions of sc CO2in water were formed in 10 min Electrodeposition of metal nanowires. Nickel was chosen for initial studies of electrodeposition [69,77,79,82,83] Nickel electrolyte watt bath was prepared by using nickel chloride, nickel sulphate and boric acid. Nickel electrodeposition was performed on 0.1 micron AAO Whatman template at atmospheric pressure and room temperature [69,82] see Figure B-2 Brass and nickel metal electrodes were used as a cathode and an anode, respectively. The current density used was 2.0 mA/dm2. Depositio n time was 40 min based on template thickness of 13 microns. After experiment was completed, nickel was plated on brass metal and templates were taken for analysis. AAO template

PAGE 109

109 was dissolved in 25 % phosphoric acid for 24 h TEM confirms dep osition of nic kel, see Figure B-3 However, deposits are less than 0.1 micron, less than pore diameter, which is attributed to etching of the deposits in the phosphoric acid. Template Removal Using Supercritical Carbon Dioxide Capillary evaporation and surface forces can cause aggregation of nanowires when a template is dissolved and dried in air Zheng et al. observed small cracks in ZnO nanowires arrays and suggested that the cause is the mechanical forces during the processes of dissolution and drying [84] Pressure drop across the interface is related to surface curvature and wetting angle according to Youngs equation: a P cos 2 (B-1) contact angle. As the wetting angle decreases during evaporation, the pressure drop rises and the surface forces cause the nanowires to be pulled closer and aggregate. This hinders their use in DSSCs applications where a uniform structure is required for optimum photon collection and electron transport. Conclusions and Future Avenues of Research Electrodeposition can be controlled better without ionic species transfer by stirring. However, aggregation formation requires agitation. Therefore, miniemulsions stability in the view cell using PEO PDMSPEO surfactant is required to ensure that experiments can be carried out in stable miniemulsions [68,78,85,86] prior to electrodeposition of nanomaterial s. Homogenizer pump should be run initially for homogenization of the fluids. Then it can be stopped during electrodepositi on. Miniemulsion stability study can be performed using the phase inversion method (PIT) [87] with 50:50 C/W (CO2in -

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110 water) at initial temperatures between 40 and 90 min and pressures between 300 to 150 atmospheres. Metal and semiconductor nanowires such as ZnO [88] and Ni of 200 nm in diameter and 30 to 60 microns in length can be synthesized using AAO templates to study the effect of hydrogen hydrolysis and miniemulsions on pore inclusion, pin hole formation and crystallinity. Pore inclusion and conductivity of individual nanowires can be tested using conductive AFM. The widely used wide band gap semiconductors for electron transport in DSSCs, titania (TiO2) and zinc oxide (ZnO), should ideally be single crystalline to enhance electron transfer and reduce entrapment events and percolation in the semiconductor. However, they can hardly be synthesized as single crystalline due to their high melting points. Single crystalline materials require high temperature or low time for atoms to preferentially probe most favorable sites, which cannot be achieved by electrodeposition in an aqueous environment. Alternatively, a single or polycrystalline metal coated with a small layer of semiconductor can be used for faster electron diffusion. Dobrev et al. synthesized single crystal nanowires such as Cu, Pb, Bi, Ag and Sb by pulsed electrodeposition [72] Polycrystalline metals such as Co, Ni, Rh and Pt can be synthesized. Deposition of titania on metal nanowires by electrophoretic deposition can be utilized for DSSCs fabrication [45,8991] Capillary evaporation and surface forces effects on aggregation can be studied using SCF drying with nanowire arrays of 50 to 100 nm in diameter and 10 to 30 microns in length. Nanowire arrays of c an be placed in a micro chamber immersed in a water bath set to a desired temperature by a thermoregulator. Carbon dioxide can be supplied to a micro reactor to the desired pressure by isco pump. Drying using sc CO2

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111 can be performed in a static process at temperatures between 35 and 55C and pressures between 100 and 200 atm. Then sc CO2 can be vented slowly out of the system. Analysis can be carried out by scanning electron microscopy.

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112 (a) (b) (c) Figure B-1 High pressure system for electrodepositi on. Setup for electrodeposition using sc CO2 in high pressure chamber (a) H igh pressure chamber and temperature control system (b) Electrodes coated for corrosion (c). Figure B-2 SEM image of 200 nm whatman anodisc 13. Fig ure B-3 TEM image of nickel deposits from a watt bath solution.

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113 APPENDIX C ANODIZING ALUMINUM AND NANOWIRE ELECTRO DEPOSITION Electrodeposition in electrolyte solution using anodized alumina templates wa s investigated as an alternative structure for dye sensitized solar cells. First, ITO substrates are degreased in soapy water, acetone and ethanol for 30 min each to remove contamination. Then, deposition of chromium or titanium interlayers, along with aluminum onto ITO substrates is performed using Ebeam evaporator at Microfab facility These aluminum substrates are then anodized to make AAO on ITO for the templated deposition of nanowires. The procedure includes deposition of 10 nm titanium or chromi um layer to enhance adhesion, 2 to 10 m thick alumi num layer and 10 m silica layer strip on the film near the top of the substrate to prevent excessive oxidation during anodization at the air solution interface, see Figure C-1 The materials used in this procedure are 99.99 % aluminum shots (9.5 mm), tita nium, chromium and silica (glass pieces), and sample graphite crucibles, all provided by Microfab. Reproducibility of these substrates was problematic due to variability in ITO substrates quality as well as Ebeam operations. However, the successful films were anodized in 11 wt % phosphoric acid. Some films failed either at the beginning or end of the anodization due to poor adhesion. The properly anodized films were pore widened in 5 wt % phosphoric solution for 5 min and 30 seconds. Electrodeposition of gold nanowires was performed. Most films showed uneven deposition possibly due to uneven anodization. The AAO template was then removed by etching in 25 wt % phosphoric acid. The nanowires remained in solution until they were taken for electrostatic repulsion to prevent nanowire aggregation. This method was developed in

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114 the lab. However, non aggregated nanowires could not be synthesized with this method. This could be due to all the variability in the process. The operation of the Ebeam evaporator was changed after major maintenance and Al deposition at the 200300 angstroms per second could not be achieved. The rate of deposition was related to good adhesion of Al on the ITO substrates.

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115 Figure C-1 Substr ates after deposition of Cr and Al layer s, wrapped in Al foil for silica deposition.

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124 BIOGRAPHICAL SKETCH Fahd Mohsin Rajab finished high school in 1995 with recognized athletic and academic accomplishments. He joined Aramco Oil Company and was among a group of successful candidates to study bachelors in chemical engineering with a full scholarship. Fahd graduated in 2001 from Vanderbilt University and started a working career at Aramco in refining, planning and management for more than 4 years before he decided to pursue graduate studies. Fahd received his masters degree in chemical engineering from the University of Florida in 2007. He then started his doctorate studies focusing on nanomaterials and its applications. He worked in various areas including purification of carbon nanotubes, transport in nanomaterials using supercritial fluids thin fil m preparation, and fabrication and testing of dye sensitized solar cells. Fahd received his Ph.D. in chemical engineering from the University of Florida in the fall of 2010. Fahd plans to be involved in the educational and industry sectors. Fahds background in refining and solar energy will help him continue active research for solutions to such important global problems