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Photocatalysis of Cerium-Zirconium Dioxide Solid Solution Nanoparticle Free Radical Scavengers

Permanent Link: http://ufdc.ufl.edu/UFE0024463/00001

Material Information

Title: Photocatalysis of Cerium-Zirconium Dioxide Solid Solution Nanoparticle Free Radical Scavengers
Physical Description: 1 online resource (70 p.)
Language: english
Creator: Qing, Rui
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: bandgap, ceria, free, nanoparticle, photocatalysis, ros
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Free radical scavenging nanoparticles are based on semiconductors. In principal they should also be photo-catalytically active since they have a band gap. This work aimed to verify the level of photocatalytic activity for a series of CexZr1-xO2 nanoparticles which were amongst the best free radical scavenging particles to date, even surpassing the activity of commercialized enzymes. The nanoparticles were synthesized by the reverse micelle method and characterized with various techniques. The photocatalytic activity was determined by the degradation of Procion? Red MX-5B dye and compared to a commercially available photocatalyst, i.e. Aeroxide? TiO2 P25 under UV light illumination with different wavelengths. It was found that free radical scavengers have the ability to act as free radical generators under UV-light, but the photocatalytic reaction constants were found to be one to two orders of magnitude lower than Aeroxide? TiO2 P25 for dye degradation with 302 nm UV irradiation. No measureable reaction was observed for longer wavelengths. This suggests that the generation of free radicals by exposure of free radical scavenging particles under sunlight should be insignificant.
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 Rui Qing.
Thesis: Thesis (M.S.)--University of Florida, 2009.
Local: Adviser: Sigmund, Wolfgang M.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024463:00001

Permanent Link: http://ufdc.ufl.edu/UFE0024463/00001

Material Information

Title: Photocatalysis of Cerium-Zirconium Dioxide Solid Solution Nanoparticle Free Radical Scavengers
Physical Description: 1 online resource (70 p.)
Language: english
Creator: Qing, Rui
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: bandgap, ceria, free, nanoparticle, photocatalysis, ros
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Free radical scavenging nanoparticles are based on semiconductors. In principal they should also be photo-catalytically active since they have a band gap. This work aimed to verify the level of photocatalytic activity for a series of CexZr1-xO2 nanoparticles which were amongst the best free radical scavenging particles to date, even surpassing the activity of commercialized enzymes. The nanoparticles were synthesized by the reverse micelle method and characterized with various techniques. The photocatalytic activity was determined by the degradation of Procion? Red MX-5B dye and compared to a commercially available photocatalyst, i.e. Aeroxide? TiO2 P25 under UV light illumination with different wavelengths. It was found that free radical scavengers have the ability to act as free radical generators under UV-light, but the photocatalytic reaction constants were found to be one to two orders of magnitude lower than Aeroxide? TiO2 P25 for dye degradation with 302 nm UV irradiation. No measureable reaction was observed for longer wavelengths. This suggests that the generation of free radicals by exposure of free radical scavenging particles under sunlight should be insignificant.
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 Rui Qing.
Thesis: Thesis (M.S.)--University of Florida, 2009.
Local: Adviser: Sigmund, Wolfgang M.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024463:00001


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1 PHOTOCATALYSIS OF CERIUM ZIRCONIUM DIOXIDE SOLID SOLUTION NANOPARTICLE FREE RADICAL SCAVENGERS By RUI QING A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2009

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2 2009 Rui Qing

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3 ACKNOWLEDGMENTS There are many people to thank for. Without the ir crucial support and help, I would not be able to complete this work and finish my masters degree. First of all, I would like thank Dr. Wolfgang Sigmund for his compassion, inspiration, consideration and guidance towards this work. The enthusiasm, devotion, and philosophy he delivered to students are the key elements lead to the completion of this research. His mentoring and knowledge helped me in every aspect in accomplishing the goal of the project. I also like to thank my committee member, Dr. Dempere and Dr. Baney for their help all through my masters career. They are great examples for my development in the future. I would also like to thank current and previous students in Dr. Sigmunds Group. They have helped me not only in sparking my interests and insights into the academic research, but also accompanying my life in the University of Florida. I thank Dr. Yi Yang Tsai and Dr. Georgios Pyrgiotakis who helped me a lot in building the system and discussing the results. I would give a warm thanks to ShuHau Hsu, Yi Chung Wang, Karron Woan, Hyo ungjun Park, Raymond Scheffler Dr. Joshua Taylor and Michael Laudenslager who have shared the precious memories with me here. I would like to recognize the help of the staff i.e. Ms Kerry Siebein, Mr. Eric Lambers in MAIC (Major Analytical Instrumentation Center) and Gill Brubaker PERC (Particle Engineering Research Center ) for their help in training me using the equipments and characterizing my samples. Finally I would like to acknowledge my mother for her financial as well as mental support all through my life in the US. I also owe sincere thanks for all my friends who have been supportive in my life.

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4 TABLE OF CONTENTS page ACKNOWLEDGMENTS ...............................................................................................................3 LIST OF TABLES ...........................................................................................................................6 LIST OF FIGURES .........................................................................................................................7 ABSTRACT .....................................................................................................................................9 CHAPTER 1 INTRODUCTI ON ..................................................................................................................10 1.1 Introduction .......................................................................................................................10 1.2 Motivation .........................................................................................................................11 1.3 Objective ...........................................................................................................................11 2 BACKGROUND ....................................................................................................................13 2.1 P s hotocatalytic Materials ..................................................................................................13 2.1.1 Basics of Semiconductor Photocatalysts ................................................................13 2.1.2 Nanosize Impact on Semiconductor Photocatalysts ...............................................15 2.2 Ceria and Cerium Zirconium Dioxide Solid Solutions ....................................................17 2.2.1 CeO2 Nanoparticles in Biomedical Field ...............................................................17 2.2.2 Cerium Zirconium Solid Solutions ........................................................................18 2.3 Cerium Zirconium Solid Solution Application in Free Radical Scavenging ...................19 3 SYNTHESIS OF CERIUM ZIRCONIUM DIOXIDE NANOPARTICLES .........................23 3.1 Reverse Micelle Method Synthesis ..................................................................................23 3.2 Synthesis of CexZr1 xO2 Nanoparticles .............................................................................24 3.3 Part icle Dispersion ............................................................................................................25 4 CHARACTERIZATION OF CERIUM ZIRCONIUM DIOXIDE NANOPARTICLES ......29 4.1 Chemical Ratio Determination .........................................................................................29 4.1.1 Inductively Couple Plasma Spectrometry (ICP) ....................................................29 4.1.2 Compositional Results ............................................................................................30 4.2 Particle Size Analysis .......................................................................................................31 4.2.1 Dynamic Light Scattering Method .........................................................................31 4.2.2 Size and Size Distribution of CexZr1 xO2 nanoparticles .........................................32 4.3 Structural Characterization by TEM .................................................................................33 4.3.1 HighResolution Transmission Electron Microscopy ............................................33 4.3.2 TEM Results and Discussion ..................................................................................34

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5 4.4 Structural Characterization by XPS ..................................................................................35 4.4.1 X Ray Photoelectron Spectroscopy ........................................................................35 4.4.2 Experimental ...........................................................................................................36 4.4.3 Results and Discussion ...........................................................................................36 4.5 Electronic Properties Characterization by UV/Visible Spectroscopy ..............................37 4.5.1 Absorption S pectrum by UV/Visible Spectroscopy ...............................................37 4.5.2 Tauc Relation and Bandgap Calculation ................................................................38 5 PHOTOCATALYSIS BY CERIUM ZIRCONIUM DIOXIDE NANOPARTICLES ..........48 5.1 Experimental Setup and Procedures .................................................................................48 5.1.1 Reactor Chamber Setup ..........................................................................................48 5.1.2 Dye Selection ..........................................................................................................49 5.1.3 Procedures ..............................................................................................................49 5.2 Theory for P hotocatalytic Degradation Reaction .............................................................50 5.3 Results and Discussion .....................................................................................................52 5.3.1 Photocatalysis of CexZr1 xO2 nanoparticles under 302 nm UV irradiation ............52 5.3.2 Photocatalysis of CexZr1 xO2 nanoparticles under 365 nm UV irradiation ............53 5.3.3 Rate Constant Calculation and Implications ..........................................................54 5.4 Summary ...........................................................................................................................55 6 IMPLICATIONS AND CONCLUSIONS ..............................................................................61 6.1 Implications and Future Work ..........................................................................................61 6.2 Conclusions .......................................................................................................................62 LIST OF REFERENCES ...............................................................................................................64 BIOGRAPHICAL SKETCH .........................................................................................................70

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6 LIST OF TABLES Table page 41 Cerium/Zirconium ratio for synthesized CexZr1 xO2 nanoparticles by inductively coupled plasma ...................................................................................................................40 42 Size and specific surface area of CexZr1 xO2 nanoparticles by dynamic light scattering method ...............................................................................................................40 43 XPS data for CeO2 and Ce0.4Zr0.6O2 nanoparticles. ............................................................40 44 The calculated bandgap for direct band transition of CexZr1 xO2 nanoparticles. ................41 51 Photocatalytic reaction intermediates summarized for different stages .............................57 52 Normalized rate constants of photocatalytic degradation of Procion Red MX 5B for the synthesized CexZr1 xO2 nanoparticles under 302 nm and 365 nm UV i llumination ....57

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7 LIST OF FIGURES Figure page 21 Schematic representation of the reactions taking place in a semiconductor photocatalyst ......................................................................................................................21 22 Schematic diagram showing the potentials for various redox processes occurring on the TiO2 surface at pH 7 .....................................................................................................21 23 Schematic graph of CeO2 fluorite structure ........................................................................21 24 Phase diagram of CexZr1 xO2 system ..................................................................................21 25 Reduction percentages of the CexZr1 xO2 solid solutions as a function of reduction temperature ........................................................................................................................22 26 Rate constant for superoxide radical scavenging by CexZr1 xO2 nanoparticles ..................22 31 Schematic view of reverse micelle method ........................................................................26 32 Overall procedure of CexZr1 xO2 nanoparticles synthesis ...................................................27 33 Optical image: CexZr1 xO2 suspensions in different solvents .............................................28 41 Constructive and destructive effect of light scattering .......................................................42 42 Size and distribution of CexZr1 xO2 nanoparticles by dynamic light scattering method ....43 43 TEM image of CeO2 nanoparticles .....................................................................................44 44 TEM image of Ce0.6Zr0.4O2 nanoparticles ..........................................................................45 45 TEM image of ZrO2 particles .............................................................................................46 46 UV/VIS absorption spectra for synthesized CexZr1 xO2 nanoparticles ..............................47 47 Fitting curve for direct band transition of ceria nanoparticles. ..........................................47 51 S ketch of the reactor chamber ............................................................................................58 52 The chemical structure of Procion Red MX 5B ..............................................................58 53 The absorption spectrum for 5 ppm solution of the Procion Red MX 5B dye ...............59 54 Time dependence of normalized residual dye concentration for irradiation with 302 nm UV light .......................................................................................................................59

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8 55 Time dependence of normalized residual dye concentration for irradiation with 365 nm UV light .......................................................................................................................60 61 Schematic structure for CeO2 and CexZr1 xO2 crystals. ......................................................63 62 The reaction pathway for CexZr1 xO2 nanoparticles free radical scavengers ......................63

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9 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science PHOTOCATALYSIS OF CERIUM ZIRCONIUM DIOXIDE SOLID SOLUTION NANOPARTICLE FREE RADICAL SCAVENGERS By Rui Qing May 2009 Chair: Wolfgang M. Sigmund Major: Materials Science and Engineering Free radical scavenging nanoparticles are based on semiconductors. In principal they should also be photocatalytically active since they have a band gap. This work aim ed to verify the level of photocatalytic activity for a series of CexZr1 xO2 nanoparticles which were amongst the best free radical scavenging particles to date, even surpassing the activity of commercialized enzymes. The nanoparticles were synthesized by the reverse micelle method and characterized with various techniques The p hotocatalytic activity was determined by the degradation of Procion Red MX 5B dye and compared to a commercially available photocatalyst, i.e. Aeroxide TiO2 P25 under UV light illumination with different wavelengths. It was found that free radical scavengers have the ability to act as free radical generators under UV light, but t he photocatalytic reaction constants were found to be one to two orders of magnitude lower than Aeroxide TiO2 P25 for dye degradation with 302 nm UV irradiation. No measureable r eaction was observed for longer wavelengths. This suggests that the generation of free radicals by exposure of free radical scavenging particles under sunlight should be insignificant.

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10 CHAPTER 1 INTRODUCTION 1.1 Introduction Traditionally used for their mechanical property as ceramic polishing materials and glass sensitizers, ceria [cerium (IV) dioxide, CeO2] are now being exploited extensively for its electronic properties in various applications. The unique capability of cerium atom to switch between Ce3+ and Ce4+ ionic state in oxidized and reduced condition enables it to be used widely in various catalytic and functional systems. Examples in clude histochemistry [1] three way catalytic converters [2] electrolytes in solid oxide fuel cells [3, 4] gas sensors [5] oxygen storage materials [6] and water decomposition catalysts [7] From the last decade, researchers started to relate many human disorders and malfunction s to the detrimental effect caused by reactive oxygen species (ROS ) [8, 9] The accret ing impact of ROS could lead to the damage of living cells, and finally cause various kinds of human diseases, such as cancers, obesity, cardiovascular dysfunct ion, Parkinsons disease, etc.. Based on the nonbiological applications of ceria based materials, scientists tested them also in vivo systems utilizing their oxygen exchange properties. Potentially a panacea for harmful reactive oxygen species limiting the lifespan of living cells, ceri a was investigated by many groups regarding these applications [8, 9] Y.Y Tsai et al had successfully established a reverse micelle method to synthesize nano size ceria particles. They also tested the particles capability in scavenging detrimental intracellular free radicals in vivo [8, 9] Similar works also include Das et al.s research in cerias protective effect to spinal cord neurons [8, 9] However, the performance of pure ceria in such applications was lim ited by its oxygen change capability involved in the process. The ir overall activity was constrained by the equilibrium ratio between Ce3+ and Ce4+ in ceria materials. In 1950, Duwez and Odell

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11 established the first phase diagram of cerium zirconium dioxide quasi binary system [10] Proved to be effective in catalytic applications [11, 12] cerium zirconium dioxide solid solution nanoparticles we re now studied and reported to be an effective reactive oxygen species scavenger [13] It was demonstrated by Y.Y. Tsai et al that cerium zirconium dioxide solid solution nanoparticles are effective in scavenging peroxide and superoxide free radicals. Even compared to the current commercialized enzyme scavenger SOD (Superoxide Dismutase), CexZr1 xO2 nanoparticles had displayed a higher activity under similar conditions. 1.2 Motivation This research focuses on a new aspect of these solid solution nanoparticles. Though widely acknowledged as a potential candidate for bio protections, ceria and its solid solution with zirconia are still semiconductors. Such compounds are expected to behave as semiconductor photocatalysts when irradiated with light with photonic energy higher than their bandgap. Therefore, CexZr1 xO2 should generate free radical species when exposed to UV light. To date there is no publication showing the magnitude of the ir photocatalytic activity. Furthermore, the quantum confinement in small particles additionally alters the band structure. This means that experimental data are of importance to allow calculation o f bandgap and photocatalytic reaction constants. The ability for CexZr1 xO2 nanoparticles to perform these reactions potentially hinders its application in free radical scavenging systems. Actually, it has been demonstrated that cerium dioxide is a potent ial photocatalyst that can be used to decompose water to produce oxygen in aqueous system with electron acceptors, i.e. Ce4+ or Fe3+ [7] 1.3 Objective As has been mentioned above, CeO2 is an excellent free radical scavenger when used in extending the lifespan of living cells in vivo. Thus there is huge potential that they could be widely used in the medical field. However, the semiconductor nature of these materials might

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12 hinder their application as biocures. In order to test the feasibility of CexZr1 xO2 nanoparticles scavengers in different wo rking conditions, and better understand the science behind this semiconductor scavenger, the objectives in this thesis are as follows: Synthesize CexZr1 xO2 nanoparticles applicable for free radical scavenging application Investigate the electronic propert y of CexZr1 xO2 nanoparticles. Test the photocatalytic activity of CexZr1 xO2 nanoparticles Explain the behavior of the materials To achieve the objectives, CexZr1 xO2 nanoparticles need to be synthesized via reverse micelle method using the same setup as Y.Y. Tsais method [8, 9] A direct comparison between the photocatalytic property and free radical scavenging activity of nanoparticles synthesized through the same route would gi ve us a visual description of their characteristics. To assist the understanding of CexZr1 xO2 nanoparticles behaviors, the electronic property of these particle s need to be determined. As photocatalytic reactions often involve the generation of electronhole pairs, the bandgap structure of the nanoparticles need to be measured. Also, to determine the performance of CexZr1 xO2 nanoparticles under different working conditions, different UV light sources need to be applied. The catalytic activity is done via dye degradation method, which yields an accurate and fast result. The final results need to be correspondingly analyzed with other applications of CexZr1 xO2 nan oparticles, which will help to build a mutual understanding of the overall system.

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13 CHAPTER 2 BACKGROUND 2.1 Photocatalytic Materials 2.1.1 Basics of Semiconductor Photocatalysts Photocatalysis is defined to be the chemical reaction induced by photoirradiation with the presence of a catalyst [14] The materials that could facilitate these reactions without being transformed are named photocatalysts. Photosynthesis by plants is a well known example for photocatalysis, wher e chlorophyll serves the photocatalyst. There are different kinds of photocatalysts, such as organometallic complexes and semiconductors. They are being widely used in various fields environmentally and catalytically [15 18] Semiconductor photocatalyst are exploited extensively nowadays for their optimal performance and nonpollutive nature. A number of photocatalytic reactions ha ve already been applied for many practical us e s e.g. the removal of pollutants in water, self cleaning glasses, etc. [15 18] Theoretically all semiconductors can display photocatalytic properties due to the presence of a bandgap, but generally metal oxides and compounds exhibit better catalytic behaviors [19, 20] Although e xtensively investigated for better performance in photocatalytic activity, the basic working principle of semiconductive photocatalysts is rather simple. First a photon with energy higher than the bandgap of the material strikes on the sample. An electron in valence band (VB) is then excited to the conduction band (CB), leaving a hole in the VB with positive charge The electronhole pair is named an exciton. After the irradiation, the exciton either rec ombine s and releases the absorbed photonic energy or it may react with chemicals present on the surface of photocatalyst to generate free radical species. During

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14 reaction, photon energy and the excitons are consumed, while the structure of semiconductors r emains unchanged. The whole reaction pathway is described in Figure 2 1. Based on the above discussed mechanism, several parameters in the reaction would contribute to the overall photocatalytic efficiency of a semiconductive material: light absorbing property of the semiconductor recombination rate of agitated electrons and holes rate for oxidative and reductive reactions that transform electrons and holes to free radicals The light absorbing property is largely determined by the type and lattice structure of materials. Though sometimes surface modification could enhance the activity of photocatalysis, such an impact is negligible especially for powder materials. The second and third factors decisive for photocatalysis should be considered accordingly Whi le the oxidative and reductive reactions on materials surface contribut e to photocatalysis, the recombination of excitons is destructive, producing nothing but heat to the whole system. However, though acting divergently to the photocatalysis efficiencies both recombination progress and oxidation/reduction on the surface depend largely on the potential of those redox processes, which could be attributed to the band structure of a semiconductor. Here titania [titanium dioxide, TiO2] the most used semiconductor photocatalyst, is cited as an example to illustrate the effect of the two parameters. Figure 2 2 is a schematic diagram showing the potentials for various redox processes on TiO2 surface from A. Fujishima et al [21] th e discoverer of titania based photocatalysts. The main reactions and time needed for these processes are summarized as follows [22 24] : Exciton generation 2 + ++ 1015s (2 1)

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15 Electron hole trapping ++ ( ) 108s (2 2) + ( ) 109s (2 3) ++ 109s (2 4) Electron hole recombination: ++ ( ) 107s (2 5) + ( ) ( ) 108s (2 6) Oxidation and reduction: ( )+ 0 ( ) + + 107s (2 7) + ( ) + + 103s (2 8) F rom the equations it is easy to conclude that the rate of recombination is much higher than the rate of r edox reactions and the photocatalytic ac tivity of titania should be insignificant. However, the recombination rate could be retarded by various means. Researchers have successfully enhanced the catalytic activity of titania by doping with transition metals (e.g. Cr, Fe), coupling with metals (e. g. Pt) and coupling with a semiconductor (e.g. Aeroxide TiO2 P25: anatase phase titania with rutile phase titania). On the other hand, if the recombination of excitons is enhanced, or the redox reactions on the surface are retarded, the photocatalytic act ivity of the material will be controlled. 2.1.2 Nanosize Impact on Semiconductor Photocatalysts Due to the unique property of nanosize particles, the photocatalytic activity for these nanoparticles will deviate from, though still correlate to, their bulk counterparts. There have been many research report s focusing on the use of nanosize materials as photocatalysts [25 27] These nanosize materials inhabit the region between single molecules and large r colloids, considered

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16 parts of the bulk phase. T hey often exhibit unexpected behavior during application and it would be necessary to investigate them separately. On e size impact worth y to be noticed during photocatalysis is the quantum confinement effect. First discussed by Frohlich in 1937 [28 30] this size effect had been concluded in several reviews [31, 32] It had a wide range impact on electrical, optical and photocatalytic properties of materials. Generally speaking, quantum confinement effect refers to the bandgap change or absorbance onset of a semiconductor due to the decrease of particle size. Specifically, when the particle size is comparable to the de Broglie wavelength of electronic carriers or excitonic Bohr radius, spatial confinement on carrier movements occurs. Correspondingly, quantization of energy levels and presence of discrete states could be expected. Consequently, the optical bandgap of the material will be increased. Quantum confinement effect often accompanies a blue shift in materials absorption spectrum. As quantum confinement is capable of trapping the electrons in discrete states, the redox potentials for photocatalysis reactions are subjected to a change. One factor need to be taken into account concerning the effect is the diff erential confining potential for electrons and holes [33] Electrons are often more mobile than holes in most of the materials due to the different effective mass of these carriers. It is believed the critical size for quantum confinement effect is greatly influenced by the ba lance of different charge carrier mobility [33] Whether such an impact would be beneficial or detrimental to photocatalytic reaction s will be dependent on actual existing species. On the other hand, nanosize particles display a much higher specific surface area than their bulk count erparts. As photocatalysis is a surface sensitive process, more surface sites will be

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17 available for redox reactions and absorbing target reactants. In this aspect, nanosize materials would enhance the photocatalytic efficiency of catalyst on a same concentration basis. 2.2 Ceria and Cerium Zirconium Dioxide Solid Solutions As has been discussed above, all semiconductor materials are theoretically potential photocatalysts. CeO2 is a fluorite type semiconductor crystal with bulk bandgap estimated t o be 3.16 eV [34] That means a photoi rradiation with wavelength lower than 388 nm would be capable of intriguing the photocatalytic activity in bulk ceria compounds. Actually, though not yet been intensively studied and used as photocatalysts, CeO2 and CeO2based materials have already been exploited widely in various catalytic systems. The unique capability of exchanging lattice oxygen and environmental oxygen species due to their nonstoichiometric switch between Ce3+ and Ce4+ ionic states render s them effective catalysts as catalytic conve rters in automobiles, electrolytes in solid oxide fuel cells, gas sensors, etc. [14 20] 2.2.1 CeO2 Nanoparticles in Biomedical Field The biomedical use of CeO2 nanoparticles was initially discovered by Rzigalinski et al. [35, 36] and confirmed by many other groups [8, 37, 38] Different cultures and tissues were tested in vitro and in vivo. It wa s reported that the introduc tion of CeO2 nanoparticles would extend lifespan of neuron cells up to six fold [35] Other experiments dealing with different kind of cells were consistent with previous findings. It is now believed by scientists that the protective effects CeO2 nanoparticles have for living cells is due to their free radical scavenging capability. Though this property is not fully studied in living bodies, it has long been investigated in a variety of nonbiological systems [5, 39] As has been discussed above, CeO2 have strong oxygen exchanging activity between lattic e and environment. During these processes, oxygen radicals are generated as intermediates. The reaction pathway for oxygen as published by Tsai et al. is as follows [9] :

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18 2 ( ) 2 ( ) 2 ( ) 2 ( ) 2 2 ( ) 2 2 ( ) 2 in which the process for oxygen molecules to be incorporated into ceria lattice is described. The oxygen molecules are subjected to a transformation through the intermediate state including peroxide and superoxide species before finally entering the lattice site oxygen ions while the number of vacant site attributed to the existence of Ce3+ ions in ceria structure is also a function of oxygen pressure in the environment. 2.2.2 Cerium Zirconium Solid Solutions T he i on exchange reactivity and oxygen storage capacity is highly limited for pure ceria system due to the equilibrium ratio between Ce3+ and Ce4+ ions in room temperature. To improve the ir free radical scavenging activity and protective power on living systems scientists have selected zirconium to substitute cerium to achieve the enhancement [40]. Though found to be a relatively new concept in the biomedical field, CexZr1 xO2 solid solution were studied as oxygen storage materials [6] and catalysts for soot decomposition [12] Thus it has already been investigated by many research g roups and its transitions between different phases have been confirmed. Generally speaking, CexZr1 xO2 solid solution would undergo three major phase transformations at room temperatures, according to Figure 2 4 [40] A monoclinic phase is expected when the CeO2 content is less than 10%, while a rigid cubic structure exists for these crystals with CeO2 content more than 80%. Other than that, the solid solution displays tetragonal symmetry which could be further divided into three stable and metastable phases. The presence of these different phases is suggested by XRD an d Raman c haracterization [41, 42] However, the boundar ies for the transition between these phases rather approximate both due to the metastable nature of these phases and the sizesensitivity o f structure distortions.

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19 The structural change due to the introduction of zirconium substitutions also induces changes in stoichiometric properties within the crystals. Several studies have shown that the relative concentration of Ce3+ ions over Ce4+ ions in CexZr1 xO2 solid solution is promoted as the zirconium content increases, especially at high temperature [43, 44] These nonstoichoimetric changes are also confirmed by T. Sasaki et al using electron diffraction pattern through TEM tests [45] Thus the concentration of charge carriers is also subjected to an increase and so are their catalytic activities, theoretically. 2.3 Cerium Zirconium Solid Solution Application in Free Radical Scavenging Stated in Section 1.3, the objective of this thesis and research is to determine the photocatalytic activity of CexZr1 xO2 nanoparticles with free radical scavenging properties Thus in this section, the previous study of free radical scavenging activity on these nanoparticles are briefly reviewed [9, 13] Y.Y. Tsai et al successfully synthesized a series of CexZr1 xO2 nanoparticles with uniform size distribution and well defined crystalline state [13] The synthesized CeO2 nanoparticles we re experimentally determined to be effective in improving the cell cultures viability by scavenging dangerous oxidative stress [9] Zirconium substitution of cerium in the solid solution wa s proved to be benefi cial in increasing the free radical scavenging capability for the synthesized nanoparticles. CexZr1 xO2 nanoparticles were determined to be efficient in scavenging peroxide and superoxide reactive oxygen species (ROS) in vitro, and the results were reprodu ced and summarized in Figure 2 6 [13] It is suggested by Figure 26 that even compared to the currently commercialized ROS scavenger, superoxide dismutase (SOD) enzyme CexZr1 xO2 nanoparticles with zirconium content more than 30% exhibited a better efficiency. Y.Y. Tsai et al attribute d the superior efficiency of CexZr1 xO2 nanoparticles to a higher mobile electronic carrier

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20 concentration in the solid solutions and reveale d an analogous trend between the free radical scavenging properties and oxygen storage capacities for these compounds.

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21 Conduction Band Valence Band UV light Oxidation: e.g.: H2O or OHOH Reduction: e.g.: O2 O e cb h vb Bandgap Excitation Recombination Figure 2 1. Schematic representation of the reactions taking place in a semiconductor photocatalyst Figure 2 3 Schematic graph of CeO 2 fluorite structure Figure 2 2 Schematic diagram showing the potentials for various redox processes occurring on the TiO2 surface at pH 7 Figure 2 4 Phase diagram of Ce x Zr 1 x O 2 system [26]

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22 Figure 2 5 Reduction percentages of the Ce x Zr 1 x O 2 solid solutions as a function of reduction temperature (1 h at each temperature) [ 33] Figure 2 6. Rate constant for superoxide radical scavenging by Ce x Zr 1 x O 2 nanoparticles [13] 1.0 0.8 0.6 0.4 0.2 0 10000 20000 30000 40000 50000 60000 Rate constant for superoxide scavenging of SODReaction Constant (M-1s-1)Cerium Content

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23 CHAPTER 3 SYNTHES I S OF CERIUM ZIRCONIUM DIOXIDE NA NOPARTICLES In previous chapters, the main properties and applications of ceria and cerium zirconium dioxide solid solution were reviewed. This chapter briefly introduces the preparation method used to synthesize the CexZr1 xO2 nanoparticles in our group for photocatalysis tests. Overall, 2 steps were involved in the preparation: particles synthesis via reverse micelle method and the dispersion of nanoparticles in an aqueous system. The synthesis of CexZr1 xO2 nanoparticles would be a crucial step for precise evaluation of their photocatalytic property. To ensure a reliable analysis on the catalytic activity, the uniformity and homogeneity of CexZr1 xO2 nanoparticles with various compositions needed to be maintained. Reverse micelle method could provide a system with relatively narrow range distribution of nanoparticles and better uniformity [46] Saline/Tri sodium cit rate buffer was used to disperse the CexZr1 xO2 nanoparticles. According to Dr. Y.Y. Tsai [9] the IEP (isoelectric point) of CeO2 nanoparticles is subjected to a shift from pH= 8 to approximately pH= 2 when stabi lized. The modification on the particles surface was believed to provide them sufficient electr ic double layer repulsion against agglomeration. The dispersed suspension was able to maintain its transparency for several months. 3.1 Reverse Micelle Method S ynthesis Reverse micelle method is considered to be one of the most suitable candidates for preparing highly size and surface controlled nanosize particles [46] A reverse micelle emulsion is a stable and isotropic mixture of oil, water and sur f actant. Within the system, at least two immiscible phases co exist with each other, while the surfactant molecules form a monolayer at the interface. The reverse micelle is so named because of the reverse alignment of surfact on

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24 mi celle surfaces W here each micelle serves as a single isolated reactor, the formation of product particles is highly modulated. The achievable uniformity of synthesized nanoparticles and flexible control of their surface structure render reverse micelle method a n ideal way in synt hesizing various kind of particulate system s Actually, it has already been applied widely to the preparation of quantum dots system s such as CdSe [47], CdS [48 50] and ZnS Cd S alloys. Presently there are several w ater/oil systems applicable for reverse micelle method. Besides the commonly used AOT ( Aerosol OT ) / t oluene system, re searchers have also applied NP 5 (Igepal CO 520)/cyclohexane solution [51] or even quaternary CTAB ( Cetyl trimethylammonium bromide ) /n hexanol/heptanes [46] to form reverse micelles. By carefully controlling the [water]/[surfactant] ratio, [water]/[non polar solvent] ratio and reaction time, nanoparticles with different f eatures can be achieved [52] 3.2 Synthesis of CexZr1 xO2 Nanoparticles In our system, reagent grade zirconyl(IV) nitrate hydrate (99.5%, M.W.=231.23g/mol) and cerium(III) nitrate hexahydrate (99.5%, M.W.=434.22 g/mol) from Acros Organics were diluted to 0.1 M in aqueous system as precursor solution during the synthesis. 1.5M ammonium hydroxide was used as additives. Laboratory grade 100% solid sodium bis(2ethylhexyl) sulphosuccinate Aerosol OT from Fisher scientific was added into laboratory grade toluene to assist the formation of nano size reverse micelles. 1.5 g AOT solid was dis solved in 100ml toluene. The p recursor solution was prepared by mixing 0.1 mM cerium nitrate and 0.1 mM zirco nyl nitrate aqueous solution according to their atomic ratio. 5 ml precursor was then titrated into the AOT/toluene solvant. The system was then stirred in a constant rate for 1 hr to a llow the uniform generation of aqueous reverse micelles. After that, 10 ml 1.5 M ammonium hydroxide solution was titrated into the system to initiate crystal nucleation and growth. The system wa s

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25 then again stirred for 1 hour with constant rate for full reaction The reaction and precipitation of the nanoparticles in the reverse micelle and wa s considered complete after this time During the reaction, the system went through a brownish color and finally becom ing yellowish, indicating the presence of cerium dioxide compounds. The r everse micelle reaction is described in Figure 31. The synthesized particles in the reverse micelle system were then collected by centrifugation with an Eppendorf 5810 Centrifuge at 9400 rpm (rounds per minute) for 50 min In order t o remove AOT surfactan t, the particles were then rinsed successively by methanol, ethanol and d.i water (twice). The pullutants introduced in previous steps were washed away by d.i. water also. Finally, the purified particles were suspended and stabilized in a saline/trisodium citrate buffer solution for storage. The overall synthesis procedure is illustrated by Figure 3 2. 3.3 Particle Dispersion 0.05 M saline/tri sodium citrate buffer solution made from solid state chemicals from MP Biomedical was prepared according to manufacturers instruction. With a pH at around 7.4, the solution was used to disperse the synthesized CexZr1 xO2 nanoparticles. Ultrasonication was used to break potential flocculation and assist dispersion. The stabilized nanoparticles in diff erent solvents were shown in Figure 33. The dispersed nanoparticle system wa s able maintain their transparency for more than 6 month.

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26 Figure 3 1. Schematic v iew of r everse m icelle m ethod precursor + water reactant B added crystal + water reaction + nucleation

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27 Step 1: Reverse m icelle s ynthesis Step 2: Particle wash and purification Step 3: Particle dispersion A B (c) (d) (e) (f) (g) Figure 3 2 Overall procedure of Ce x Zr 1 x O 2 nanoparticles synthesis: A: reaction beaker on a magnetic stirrer; B: syringe used to titrate precursor and additive; C: precursor solution; D: eppendorf 5810 centrifuge; E: misonix S3000 ultrasonicator; F: washing solvents; G: dispersed nanoparticles. C D E

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28 Figure 3 3 Optical image: Ce x Zr 1 x O 2 suspensions in different solvents ( Left: S.C buffer Rig ht: d. i water )

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29 CHAPTER 4 CHARACTERIZATION OF CERIUM ZIRCONIUM DIOXIDE NANOPARTICLES This chapter discusses the characterization of synthesized nanoparticles via a variety of techniques. Both the structural properties and electronic properties of the particulate system are to be presented. In this chapter, each technique used to determine the characteristics of particles will be briefly introduced. The results from these characterization techniques will also be displayed. 4.1 Chemical Ratio Determination 4.1.1 Inductively Couple Plasma Spectrometry (ICP) Inductively Coupled Plasma Spectrometry is a sophisticated analytical technique aiming to detect the trace materials in target samples. Inductively coupled plasma refers to a plasma source where the energy is supplied by electromagnetically induced electrical currents. An ICP typically consist of several components: a sample introduction system, a torch, high frequency generator, transfer optics and spectrometer and a computer interface. A torch in ICP is comprised by three concentric tubes, usually made from silica. The torch is situated w ith a watercooled coil of radio frequency magnetic field generator. During operation, a flow of argon gas is introduced into the torch, where the radio frequency field activates the electrons and their collision with argon gas makes them electrically cond uctive [53, 54] Thus the plasma within the torch contains mostly argon atom s with a small proportion of free electrons and argon ions. The temperature of the plasma is rather high. In order to prevent possible detrimental damage to other parts of the instrument, the plasma is insulated by several flow s of inert gas, typically a rgon and nitrogen [55, 56] In ICP experiments, it require s the elements to be in a solution state before entering the ins trument. Normally an aqueous system is preferred. During operation, the sample solution is

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30 introduced into the system by a carrier gas (argon, sometimes helium). Then the target elements are injected into the argon plasma and ionized. Atomic emission of the samples is converted into electrical signal by means of diffraction grating. The light intensity with different wavelength is monitored by photomultiplier. As each element has specific value of emission lines in the argon plasma, the electrical signal of these lines are compared with standard sample with known concentrations. By comparing the intensity of standards and target samples the concentration of specific elements are to be determined. 4.1.2 Compositional Results Perkin Elmer Plasma 3200 Inductively Coupled Plasma Spectroscopy (ICP) in Particle Engineering Research Center (PERC) was used to confirm the actual chemical composition of our samples. To prepare solution feasible for ICP analysis, 200 ml particles suspension w as allowed to be fully diss olved by sulfuric acid (95%, Sigma Aldrich) overnight, and then diluted with same amount of deionized water. The ionic concentration of cerium over zirconium in the solution was determined afterwards in plasma state. The standard deviation of the measurements was established by 1 ppm, 10 ppm and 100 ppm cerium and zirconium ICP standards provided by Ricca Chemical. The detection limit of the equipment wa s less than 1ppm. The chemical ratio of final products is shown in Table 41. Formula column indicated the expect ed formulation of the product based on the reagent ratio. From the results shown in the table, it was suggested that though a deviation no more than 0.06 was achieved away from the calculated formula, the actual component of the nanoparticles ge nerally correspond well with theoretical formulation. The deviation could either be induced during the process of synthesis or from the experimental error in ICP equipment. Generally speaking, the overall quality of the particles was acceptable for further analysis in characterizing the photocatalytic activities of particles with distinguishable components.

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31 For comparison purpose, the atomic ratios for commercial particles used in parallel experiments, directly reported from their companies, were also inc luded in the table. 4.2 Particle Size Analysis 4.2.1 Dynamic Light Scattering Method In this section dynamic light scattering method is described as it was applied to evaluate the size and size distribution of CexZr1 xO2 nanoparticles. The hydrodynamic diameter of equivalent sphere to the nanoparticles are measured and reported. Dynamic light scattering is a commonly used technique for determining size and size distribution of small particles in suspensions [57] The fundamental for this technique is the random movement of particles due to their collision with solvent molecules, also known as Brow nian motion. The velocity of Brownian motion could be defined by the translational diffusion coefficient of particles, and it is indirectly proportional to the size of particles. When an impinging laser hits suspended particles sufficiently small, they are to be scattered in different directions. Depending on the movement of the target particles, the coherent scattering light could either be constructive or destructive to the impinging light [58] as shown in Figure 4 1. The fluctuation in intensity of the light is collected by a correlator Given s ufficient time an autocorrelation function could be plotted from the random motion of small particles By fitting the intens ity curves the size and size distribution of particles are able to be obtained. Normally, l arge r particles often display a smooth curve and smaller ones have more noisy curves. However, by applying the technique it is worthy to be aware that dynamic light scattering is actually measuring the hydrodynamic diameter of the particle. The hydrodynamic diameter consists of both the particles diameter and thickness of electrical double layer. The impact of electrical double layer on the results depends on the impact it has on the motion of the particles. Also, the hydrodynamic diameter is determined based on the equivalent sphere of target particles.

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32 Thus it might not be applicable for t hose particles with high aspect ratio, where the ease of motion for different directions is highly different 4.2.2 Size and Size Distribution of CexZr1 xO2 nanoparticles Figure 42 showed the size and size distribution results of synthesized CexZr1 xO2 na noparticles from Nanotrac Particle Size Analyzer in PERC. Both frequency and cumulative frequency for the particles were included in the diagrams as a function of diameter. Except for the zirconia particles, all the particles synthesized with our setup displayed a generally narrow range distribution at around 4 nm but no more than 10 nm. From the cumulative frequency curves d50 and d95 for those compounds were also determined and reported in Table 42. The equivalent sphere diameters based on mean area and mean volume was calculated in the table. Specific surface area (SSA) was critical in this research as photo assisted reactions were often surfacedependent, involving both chemical a dsorption on the particles and reaction proc ess. The theoretical SSA was r elated to dNS and dNV with the equation: = ( 2 ) (4 1) In which A was the total area of the sample, V was the corresponding volume and was the density which could be calculated by 1st order Vegards law with reported density of ceria and zirconia. F ro m the calculation, SSA s of CexZr1 xO2 nanoparticles w ere determined to be 10920 m2/g, approximately two times of the reported SSA of 50 15 m2/g for Aeroxide TiO2 P25 from Evonik Corporation and NanoActive Cerium Oxide. However, in this table, the theoretical SSA for zirconia was not included. It was mainly because of a relatively agglomerate d state of zirconia particles, which had been di scovered in TEM image s Furthermore, t hey did not exhibit a single crystalline state according to the electron diffraction

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33 pattern, which will be covered in a latter part of this chapter. Thus the SSA for these particles could not be simply calculated as ha d been done to nanoparticles with other compositions. For comparison purpose s the corresponding data for commercial particles used in parallel experiments, directly reported from their companies, were also included in Table 42. 4.3 Structural Characteri zation by TEM 4.3.1 High Resolution Transmission Electron Microscopy In this se ction HighResolution Transmission Electron Microscopy (HR TEM) data is reported, which i s applied to determine the particle shape, crystal structure and crystallinity of CexZr1 xO2 nanoparticles. TEM is a scientific instrument that utilizes transmitted electrons instead of light to scrutinize objects at very fine resolutions [59] When a highenergy electron passes through the specimen, it interacts with the sample and yield s signals representing the information it ha s collected There are two main modes applicable in TEM technique: image mode and diffraction m ode. By adjusting the intermediate lenses, different portion s of electron s that pass through the back focal plan will be collected Either the second image of the sample or a diffraction pattern will be obtained on the image screen like photographic film or CCD camera. For image mode, contrast is generated directly by surface absorption of electrons on the sample. The thicker the sample, or the higher the atomic number the sample has, the darker the image will be. If there is the sample in the beam path th e image will appear to be bright. Thus it is also named bright field imaging [60] For diffraction mode, DeBroglies model is used to explain the behaviors of electrons. With the initial acceleration through electric field, electrons could be considered as a wave with equivalent wavelength described by Equation 42:

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34 = = 2 (4 2) where h is Plan c ks constant; p is momentum; m is the mass for an electron; e is the electron charge and V is the acceleration voltage. By substituting real values in Equation 42: ( ) = 1 505 ( ) (4 3) Similar as other types of diffraction techniques, such as XRD (X Ray Diffraction), LEED (Low Energy Electron Diff raction) and RHEED (Reflective High Energy Electron Diffraction), the diffraction in TEM follows Laues rule and only dots on Ewa l ds sphere in reciprocal lattice are able to be displayed. Thus it is easy to conclude diffraction pattern with distinguishabl e rings have better crystallinity than ones with diffusive lunar pattern For a large single crystal, discrete diffraction spots will be displayed in the pattern. 4.3.2 TEM Results and Discussion Figure 43 to Figure 45 represented the selected TEM grap hs of CexZr1 xO2 nanoparticles (x=1, 0.6, 0). Both their bright field image and electron diffraction pattern we re included. As the nanoparticles were synthesized using the same method as in Dr. Y.Y. Tsais dissertation, only three distinguishable compositions were chosen for the TEM test. The bright field images of CeO2 and Ce0.6Zr0.4O2 nanoparticles displayed generally uniform shape with size around 23 nm. Some selected crystal lites were highlighted with red circles. The relatively uniform sphere like shape of the particles proved the feasibility of dynamic light scattering method. It was suggested from the latti ce fringes that each nanoparticle exhibited a single crystalline state. The distinguishable rings in electron diffraction pattern also proved the well defined crystallinity within the samples. However, for ZrO2 samples, the particles seem ed to be highly agglomerated with porous structure. It wa s believed that the difference of interaction between oxide particles and the surfactant (sodium citr ate buffer) caused

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35 the divergent behaviors between ZrO2 and other compounds. The diffusive electron diffraction patt ern also indicated a poor crystalline state within ZrO2 particles. Due to the agglomeration and the deviation of diameter measured by dynamic light scattering method resulting from the nonuniformity of ZrO2 particles, the SSA was not calculated in Table 4 2. 4.4 Structural Characterization by XPS 4.4.1 X Ray Photoelectron Spectroscopy To confirm the actual bonding state and chemical shift due to the variation of composition and UV radiation, X Ray Photoelectron Spectroscopy (XPS) was used for the synthesized nanoparticles. Also known as Electron Spectroscopy for Chemical Analysis (ESCA) [61], XPS is a semi quantitative, surface sensitive [62] technique based on the highenergy version of photoelectric effect, allowing both chemical and elemental identification of compounds. XPS could detect all elements except for hydrogen and helium with concentration higher than 0.1 atomic%. During operation, X ray with energy of 1~2 keV is generated and impinge on the sample, penetrating several microns below the surface; incident X ray photon interacts with an core level electron of an atom, while energy of the X ray photon is transferred to the electron; the electron with sufficient energy is then ejected from the atom and collected by a n electron energy analyzer. The energy of excited electron could be described by Equation 43: (4 4) where hv is the photonic energy of primary xray; Ik is the binding energy of a core electron; Ek is the kinetic energy of ejected electron and is the w ork function of the equipment (3~4 eV). To prevent contamination and provide an unobstructed path for the generated photoelectrons, XPS system is operated under an ultra high vacuum ( 9 Torr). Thus sample must be stable in the chamber, either in a soli d state or undergoing extra pre treatment. E I hvk k

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36 4.4.2 Experimental The well suspended CexZr1 xO2 (x=1, 0.4) nanoparticles were re dispersed in ethanol and collected by centrifuge. To ensure there was no more sodium citrate group bounded to the particle surface, the particles were washed by ethanol twice Finally, particles suspension in ethanol was dipped on to a 1cm1cm square cut silicon substrate. After the evaporation of ethanol, the nanoparticles we re deposited on the substrate. Multi layer atoms were applied to substrate to ensure there was sufficient sample to generate XPS signals. In order to determine the change in structure attributed to UV radiation, another CeO2 sample pretreated with UV light was prepared. 20ml sample and 20 5000 ppm Procion MX 5B dye used for the photocatalysis experiment, which will be introduced in Chapter 5, were added into a transparent glass vial, and placed in reaction chamber for 6 hours. 302 nm UV light was used as irradiation sou rce. After that, the samples were prepared with same routine as has been described above to make deposition on silicon substrates. 4.4.3 Results and Discussion By integrating the area under corresponding XPS peaks, the relative intensity of each element is able to be achieved. Table.43 summarizes the intensity of carbon 1s, cerium 3d and oxygen 1s peaks for unirradiated CeO2, unirradiated Ce0.4Zr0.6O2 and irradiated CeO2, respectively. From the cerium/oxygen equivalent ratios calculated from the integration it is easy to suggest though not quantitatively, that the substitution of cerium by zirconium would favor the generation of Ce2O3 bonding states, increasing the concentration of oxygen vacancies in the CeO2 lattice, while in the other case, UV irradiation would contribute to the incorporation of environmental oxygen species into CeO2 lattice sites and eliminate its intrinsic nonstoichiometric defects.

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37 4.5 Electronic Properties Characterization by UV/V isible Spectroscopy 4.5.1 Absorption S pectrum by UV/Visible Spectroscopy UV/Visible spectroscopy is a spectroscopic technique commonly used to examine the solid or solution of transition metal ions or organic compounds. It utilizes the electronic transition from ground state t o an excited state for target compounds. Within the transition, electrons are promoted from valence band to the conduction band after absorbing the energy in an impinging photon. Only photons with energy that match the energy gap between molecular orbitals are to be absorbed, and an absorption peak is generated in the spectrum. Though this information is also displayed more or less by the color of the samples, a UV/VIS measurement will allow a more precise quantitative determination of samples properties. The concentration of sample s and the readings of UV/VIS are correlated by Beer Lamberts Law, shown in Equation 4 4: (4 5) where A is the measured absorbance; I and I0 are the transmitted and original intensity of light, respectively; is the extinction coefficient; c is the concentration of the sample; l is the length of light path. Beer Lamberts Law is applicable for most homogeneous samples. With UV/VIS, the type of functional groups the concentration of certain species and many other information could be obtained. In this section, the absorption spectra of CexZr1 xO2 nanoparticles were measured by Perkin Elmer Lambda 800 UV/Vis spectrometer. Before the experiment, the CexZr1 xO2 nanoparticles were diluted to a concentration of 0.75 mM and titrated into quartz cuvettes for scanning. The scanning range was 220 nm to 800 nm. For comparison purpose, commercial NanoActive Cerium Oxide particles from Nano Scale corporation were also included. l c I I A 0 10log

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38 Figure 46 show ed the absorption spectra for CexZr1 xO2 nanoparticles with different compositions (x=1, 0.8, 0.7, 0.6, 0.4, 0.2, 0). In the diagram, a strong blue shift of the absorption edge was presented with increasing zirconium component. While for metals, absorption peaks w as correlated to the interband transition of electrons, for semiconductors, the position of absorption edge is determined by separation between valence band and conduction band. Also, with increas ing zirconium component, the ability of absorbing UV l ight wa s also decreasing. This corresponded well to the UV absorbing ability of ceria nanoparticles. However, surprisingly, despite that the absorption edge was moved to a lower energy position, a lower intensity of absorption peak for commercial ceria was displayed in the diagram. This might be cause d by the relatively lower surface area available on commercial ceria in the same concentration of synthesized CexZr1 xO2 nanoparticles. 4.5.2 Tauc Relation and Bandgap Calculation To better understand the elec tronic property and their performance under UV light, the bandgap of the nanoparticles were also calculated from their absorption curves. According to the Tauc relation, the absorption coefficient (extinction coefficient) is related to the bandgap of a semiconductor material with Equation 45 [34, 63] : ( ) = ( ) 2 (4 6) where A is a constant, is the absorption coefficient; hv is the photonic energy; Eg is the bandgap of the sample; m=1 for direct band transitions and m=4 for indirect band transitions [64] In following calculation m=1 and it is presumed for a direct band transition in ceria. Figure.47 displayed the plot of ( )2 versus transformed from UV/VIS curves. The energy intercept of fitted line with x axis yielded the Eg value for the sample.

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39 The Eg values for each synthesized nanoparticles are summarized in Table 4 3. Although bulk ceria had only an Eg=3.19 eV [34] nanosize ceria particles displayed a much larger bandgap Eg=3.58 eV. These might be attributed to the effect of quantum confinement. Due to the lack of sufficient electrons i n each single energy state, the electrons were restrained to the potential wells in lateral dimensions so that quantized energy levels took place of continuous bands structure in these nanoparticles. The relationship between quantum confinement and particl e size could be inferred by the contrast between commercialized NanoActive ceria and synthesized ceria in our lab. The commercial ceria had a larger crystallite size of 7nm, and correspondingly exhibited a smaller bandgap than ceria obtained from reverse m icelle method. Thus a smaller crystal would induce a more severe quantum confinement effect in the experiments. Also, it can be seen from Table 4 3 that the bandgap energy exhibited an increasing trend as the ratio of zirconium in the oxide increased. This corresponded to the fact that zirconia (Eg=4 5 eV depending on the synthesis method) has a much larger bandgap than ceria. The nonlinear increase of bandgap could be explained by a higher order Vegards law as a result of bowing effect.

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40 Formula Cerium atomic ratio Deviation % (Cerium) Zirconium atomic ratio Deviation % (Zirconium) Commercial Ceria >0.997* Aeroxide TiO 2 P25 CeO 2 0.97 0.036 0.03 0.075 Ce 0.8 Zr 0.2 O 2 0.78 0.046 0.22 0.044 Ce 0.7 Zr 0.3 O 2 0.70 0.036 0.30 0.037 Ce 0.6 Zr 0.4 O 2 0.58 0.057 0.42 0.038 Ce 0.4 Zr 0.6 O 2 0.34 0.20 0.66 0.037 Ce 0.2 Zr 0.8 O 2 0.16 0.14 0.84 0.034 ZrO 2 0.03 0.99 0.97 0.036 Comparison Sample d50 Average crystallite size(nm) Specific surface area: SSA(m 2 /g) Commercial Ceria 9.5 7 >=50 Aeroxide TiO 2 P25 21 5015 Sample d50(nm) d95(nm) Area mean diameter: d NS (nm) Volume mean diameter: d NV (nm) Specific surface area: SSA(m 2 /g) CeO 2 3.0 4.0 3.6 3.6 116 Ce 0.8 Zr 0.2 O 2 2.6 4.4 3.4 3.6 107 Ce 0.7 Zr 0.3 O 2 3.5 4.9 4.3 4.5 89 Ce 0.6 Zr 0.4 O 2 3.2 4.5 3.8 3.9 109 Ce 0.4 Zr 0.6 O 2 3.0 4.1 3.5 3.6 123 Ce 0.2 Zr 0.8 O 2 2.9 4.3 3.5 3.6 128 ZrO 2 75.9 148.9 106.9 118.2 Sample C 1s Ce 3d O 1s Ce / O equivalent area ratio Height Area Height Area Height Area unirradiated CeO 2 2305 13185 11665 86806 13001 51703 1.679 unirradiated Ce 0.4 Zr 0.6 O 2 4885 22402 10890 72381 20888 69782 2.593 irradiated CeO 2 2155 11028 9629 61705 15922 57939 1.065 Table 4 1. Cerium/Zirconium ratio for synthesized Ce x Zr 1 x O 2 nanoparticles by i nductively c ouple d plasma Table 4 2. Size and specific surface area of Ce x Zr 1 x O 2 nanoparticles by dynamic light scattering method These data were directly reported from their company Table 4 3. XPS data for CeO 2 and Ce 0.4 Zr 0.6 O 2 nanoparticles

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41 Sample Bandgap (eV) Commercial CeO 2 3.400.06 CeO2 3.580.03 Ce0.8Zr0.2O2 3.690.02 Ce0.7Zr0.3O2 3.700.02 Ce0.6Zr0.4O2 3.700.04 Ce0.4Zr0.6O2 3.730.05 Ce0.2Zr0.8O2 3.830.01 ZrO 2 4.550.09 Table 4 4. The calculated bandgap for direct band transition of Ce x Zr 1 x O 2 nanoparticles.

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42 Figure 4 1. Constructive and destructive effect of light scattering

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43 Figure 4 2 Size and distribution of Ce x Zr 1 x O 2 nanoparticles by dynamic light scattering method. A : CeO2; B : Ce0.8Zr0.2O2; C : Ce0.7Zr0.3O2; D : Ce0.6Zr0.4O2; E : Ce0.4Zr0.6O2; F : Ce0.2Zr0.8O2; G : ZrO2. A B C D E F G

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44 Figure 4 3 TEM image of CeO 2 nanoparticles: A: CeO 2 nanoparticles at magnification 400000X, scale bar 10nm; B: CeO2 nanoparticles at magnification 600000X, scale bar 5nm; C: CeO2 nanoparticles at magnification 600000X, scale bar 5nm; D: e lectron diffraction pattern of CeO2 nanoparticles. A B C D

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45 A B C D Figure 4 4 TEM image of Ce 0.6 Zr 0.4 O 2 nanoparticles: A: Ce 0.6 Zr 0.4 O 2 nanoparticles at magnification 400000X, scale bar 10nm; B: Ce0.6Zr0.4O2 nanoparticles at magnification 600000X, scale bar 5nm; C: Ce0.6Zr0.4O2 nanoparticles at magnification 600000X, scale bar 5nm; D: e lectron diffraction pattern of Ce0.6Zr0.4O2 nanoparticles.

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46 Figure 4 5 TEM image of ZrO 2 particles: A: ZrO 2 particles at magnification 15000X, scale bar 10nm; B: ZrO2particles at magnification 40000X, scale bar 5nm; C: e lectron diffraction pattern of ZrO2 particles. A B C

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47 240 320 400 480 560 640 720 800 A B C D E F G Habsorbance (arbitrary unit)Wavelength (nm) Figure 4 6 UV/VIS absorption spectra for synthesized Ce x Zr 1 x O 2 nanoparticles. A : CeO2; B : Ce0.8Zr0.2O2; C : Ce0.7Zr0.3O2; D : Ce0.6Zr0.4O2; E : Ce0.4Zr0.6O2; F : Ce0.2Zr0.8O2; G : ZrO2; H: NanoActive Cerium Oxide 2.0 2.5 3.0 3.5 4.0 4.5 5.0 (ahv)2Photonic Energy (eV) Figure 4 7 Fitting curve for direct band transition of ceria nanoparticles.

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48 CHAPTER 5 PHOTOCATALYSIS BY CERIUM ZIRCONIUM DIOXIDE NANOPARTICLES In this chapter, experiments performed to determine the photocatalytic activity of CexZr1 xO2 nanoparticles are described. Dye degradation method was used to achieve the objectives [65, 66] In the experiments an organic dye was photocatalytically degraded and its concentration was monitored as a function of time. This technique was chosen over other tests mainly because of its fastness, accuracy and particle property dependency, while other methods might introduce many variables not able to be fully controlled that would possibly alter the final results [67] In the following section, the experimental setup is described, followed by the theory of 1st order reaction (Langmuir Hinshelwood theory). Finally, the results of the experiments are presented and corresponding derivation through the theory is also illustrated. 5.1 Experimental Setup and Procedures 5.1.1 Reactor Chamber Setup To determine the photocatalytic activity for CexZr1 xO2 nanoparticles, a reactor was designed an d built by our lab, as shown in Figure 51. The inner wall of the chamber was covered by UV absorptive sheets. Those protective materials aimed to reduce the scattered radiation during the experiment. As temperature wa s also a factor which could contribute to the results of photocatalytic reaction, a 5 W electric fan was placed on top of the chamber for convective heat transport in order to keep a constant temperature in the chamber. With this set up the temperature in the reaction vials was monitored to be 26.00.5 after 2 increase in the first 20 minutes. To initiate the reaction, three 8 W UV lamps were used as the irradiative light sources. Depending on the experiment, lamps with peak wavelength at 302nm or 365nm were used for the test. Samples were kept in transparent glass vials 20 cm below the lamps. The light intensity received by samples in this configuration was determined to be approximately 15 W/m2.

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49 5.1.2 Dye Selection The photocatalytic efficiency of CexZr1 xO2 nanoparticles was tested by a series of experiment s using a dye degradation method. Brilliant Procion Red MX 5B (C19H13Cl2N6Na2O7S2) with its chemical structure shown in Figure 5 2 was chosen for our experiment s [68] This dye ha s a three stage degradation mechanism in response to the presence of hydroxyl groups. In the 1st stage, the most active bonds, including C N bond and C S bond are hydroxylated. After that, groups linked to the triazine ring are replaced by a hydroxyl group to produce cyanuric acid. Finally, further oxidation of the yields from previous steps would form CO2, water and minerals to complete the reaction. The intermediate compounds in the reaction are summarized in Table 5 1, according to reference [69] More detailed degradation mechanism of the dye has been developed by several groups like H. Lachheb et al. [66] and C.M. So et al. [70] and will not be covered in this thesis. The use of Procion Red MX 5B could mainly be attributed to its moderate rate of degradation under common conditions [71] It i s a critical issue for photocatalysis since too fast a reaction often causes difficulty i n system stabilization and the parameters for reaction are hard to control, while in slow reaction water evaporation would yield uncertainties in aqueous systems. The absorption peak for Procion Red MX 5B lie s in between 510 nm to 540 nm, and is experimentally determined to be 533 nm, as shown in Figure 53. 5.1.3 Procedures In the experiment, Procion Red MX 5B was diluted to 5 ppm and mixed with 0.75 mM CexZr1 xO2 nanoparticle suspensions. 0.05 s uspension to maintain the pH value during the whole reaction. The UV lamps were turned on 30 minutes ahead to allow the stabilization of light intensity. A series of samples were prepared and fully mixed before being placed in the chamber for reaction. The chamber was isolated from

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50 outer light sources and the samples are irradiated by UV light. Portions of the samples w ere obtained every 40 minutes from the reaction vials and titrated in 1.5 ml plastic cuvettes. The residual dye concentration in the cuvette s was monitored by UV/VIS spectrometer as a function of peak absorbance at 533 nm. The total reaction time was 360 minutes and reaction constant is calculated based on Langmuir Hinshelwood theory. Since nanosize particles were well dispersed and also capab le of scattering light, control samples with only CexZr1 xO2 nanoparticles in a concentration equal to the ongoing experimental sets were prepared as reference to determine the absorbance solely contributed by the remaining dye. For comparison of photocat alytic activity between currently commercialized photocatalyst and the particles synthesized in our lab, Aeroxide TiO2 P25 from Evonik Industries was used. It is one of the most powerful commercial photocatalysis system s [72] and is considered an ideal material to compare the photocatalytic efficiencies. As an important parameter for photocatalytic reactio ns, the specific surface area for Aeroxide TiO2 P25 is reported to be 5015 m2/g from Evonik Industries and was directly used for calculation. Additionally, a parallel experiment employing NanoActive Cerium Oxide from NanoScale Corporation was also conducted as comparison between our particles and commercial nanosize ceria particles. The s pecific surface area was reported to be >50 m2/g and was also directly us ed. 5.2 Theory for Photocatalytic Degradation Reaction Langmuir Hinshelwood theory is the most commonly used kinetic model for describing photocatalytic behaviors [67, 68, 73, 74] In this model, it is assumed that the overall reaction rate depend s on the a dsorption of dye molecules on catalytic particles as well as the reaction rates. The phot ocatalytic oxidation in Langmuir Hinshelwood theory is given by equation 51: t t tKC kKC dt dC r 1 (5 1)

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51 or tkKC k r 1 1 1 (5 2) where r is the reaction rate, k is the overall rate constant, Ct is the dye concentration, and K is the adsorption coefficient [7 4] The solution for Equation 51 is given out by Equation 5 3: kKt C C K C Ct t ) ( ln0 0 (5 3) where C0 is the initial concentration of the dye. Equation 53 could be solved for t by a series of experi mental data with discrete value of Ct from C0 to 0. It is a nonlinear solution but when C0 approaches 0, ) ( ln0 0C C K C Ct t (5 4) and a pseudo first order solution could be assumed, as shown in Equation 55: t t tkKC C k dt dC r (5 5) where k is the overall reaction constant in the unit of time1, and the e quation could be integrated as: t k kKt C Ct' ln0 (5 6) or t k te C C' 0 (5 7) in which a clearly exponential decay could be expected However, the photocatalytic reactions are also affected by many other parameters. For reactions conducted under different experimental conditions, variations in other factors, such as

PAGE 52

52 catalyst s load, pH, and light intensity will also be taken into consider ation. Additional terms must be added into the solution for the general Langmuir Hinshelwood model to represent these variations. Additionally, in the Langmuir Hinshelwood model the reaction is assumed to be a single step reaction, which is not true for the case in reality. However, for the simplification of analysis, it is assumed that there is a ratedetermining step within all the reactions and k represents the rate constant of this slowest one. 5.3 Results and Discussion 5.3.1 Photocatalysis of CexZr1 xO2 nanoparticles under 302 nm UV irradiation This set of experiments aimed to investigate the photocatalytic efficiency of CexZr1 xO2 nanoparticles under the agitation of UV B (302 nm) lights, which far exceeded the bandgap 3.40 eV~4.55 eV of target nanoparticles. The experiments were conducted with a same concentration ba sed on cerium/zirconium ions Samples were prepared according to the protocols described in section 5.1.3. However, for the commercial ceria particle from NanoScale Corp oration, the concentration wa s 2 mM instead of 0.75 mM as no notic e abl e photocatalytic acti vity had been observed for the system with that concentration. Suspensions with higher concentrations were also tested but due to severe agglomeration and sedimentat ion, their activity degraded again. Figure 54 show ed the photocatalytic degradation results for Procion Red MX 5B dye under 302 nm UV light radiation as a function of time. Y axis denote d the resi dual dye compared to its original concentration. The res idual concentration of the dye was evaluated by UV/VIS spectroscopy with Beer Lambert s Law (Equation 44) The upper most curve showed the dye performance under 302nm UV radiation when there was no photocatalyst s present. It wa s worthy to note that the wa ter itself could not generated sufficient free radical groups to cause the degradation of the dye even under such harsh

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53 exposure s for as long as 6 hours. It was crucial since external contributions to photocatalysis could cause difficulty in evaluating the activity among synthesized nanoparticles. Besides the no particle control sample, photocatalytic activity had been observed with the presence of different particles. Proved to be a nonefficient photocatalyst, ZrO2 particles displayed some catalytic act ivity under our experimental condition but were insignificant S urprising ly, NanoActive ceria from NanoScale Corporation, performed secondworst in all sets of experiments with the presence of particles. Its photocatalytic activity barely exceeded that fro m the synthesized zirconia particles. Such a result might either be explained by the higher concentration in the experimental suspensions or the inadequate surface area due to agglomeration. The drastic drop observed for both the zirconia and NanoActive ce ria samples were most probably caused by experimental errors. Regarding the catalytic results for CexZr1 xO2 nanoparticles, better catalytic activity had be en displayed than zirconia and NanoActive ceria samples. Interestingly, a noticeable trend could also been found that compounds with higher zirconium composition showed a better photocatalytic activity. The reasons for the tendency wa s largely unknown but the speculation wa s that there might be more [Ce3+] in these compounds. This issue will be discussed in a latter part of the chapter. In the bottom of the diagram, the Aeroxide TiO2 P25, a currently commercialized photocatalyst, show ed the best catalytic a ctivity within all the experimental sets. Also, in this series of experiment, the Aeroxide TiO2 P25 was the only photocatalyst be able to degrade the entire dye in the suspension. 5.3.2 Photocatalysis of CexZr1 xO2 nanoparticles under 3 65 nm UV irradiation This series of experiment was designed and run to evaluate the performance of CexZr1 xO2 nanoparticles under the irradiation of 365 nm UV light. 365 nm lie s within UVA region and

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54 construct ed portion of the sunlight in routine life. It was an ideal wavelength for testing the nanoparticles if they were feasible for outdoor uses. The samples were also prepared according to the protocols in 5.1.3 except for NanoActive ceria, which was prepared in a concentration of 2 mM as in the 302 nm UV irradiation test. The degradation results for the dye were presented in Figure 55. As had been expected, water itself could not cause dye degradation under 365 nm UV radiations Also, except for the Aeroxide TiO2 P25 commercialized photocatalyst, all the other experimental sets did not exhibit any photocatalytic activity under such condition. The lack of photocatalysis in synthesized CexZr1 xO2 nanoparticles could be explained by the bandgap value achieved in section 4.5.2. Even the ceria with lowest bandgap 3.58 eV among all the CexZr1 xO2 as low as 348 nm to generate excitons. Thus it was reasonable to conclude the synthesized CexZr1 xO2 nanoparticles have no photocatalytic activity under such wavelength. On the other hand, the catalytic activity results also proved the correctness of the calculation in the previous chapter. For the two comparison sets, Aeroxide TiO2 P25 and NanoActive ceria, Aeroxide TiO2 P25maintained to be most photoactive but its activity was not as good as that under the irradiation under 302 nm UV light. Interestingly, though the photonic energy of impinging light (3.40 eV) was theoretically sufficient to overcome the bandgap of NanoA ctive ceria samples, they still did not exhibit any photocatalytic activity. The drastic drop during the first 40 minutes might rather be caused by experimental error than photocatalytic response. 5.3.3 Rate Constant Calculation and Implications As has be en discussed above, photocatalytic reactions generally follow a first order reaction model and could be described by Langmuir Hinshelwood model.

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55 By applying Equation 56 and fitting the degradation curves, the calculated rate constant for CexZr1 xO2 nanoparticles were presented in Table 5 2. Contrast samples were also included in the table. As photocatalytic activity wa s a surfacesensitive property, the final calculation of rate constants were normalized to the surface area presented in Table 52. The t able clearly showed a photocatalytic activity from CexZr1 xO2 nanoparticles 1~2 orders weaker than that of commercial Aeroxide TiO2 P25 from Evonik Industr ies under 302 nm UV radiation. The low catalytic activity for the synthesized particles was a good i mplication for their bio medical applications especially under sunlight exposures, which has portions in the UV range. As has been mentioned above, another interesting trend displayed in the rate constant was the increase in photocatalytic activities with increase in zirconium content in CexZr1 xO2 nanoparticles. A sixfold raise was achieved by Ce0.2Zr0.8O2 particles over pure ceria. The mechanism for the zirconium content in increasing the photocatalytic activity was largely unknown. However, analogic trends had been discovered in the superoxide scavenging activity of these nanoparticles. Also, high tempera ture oxygen storage property of ceria zirconia solid solutions displayed a similar tendency. With both of the two above mentioned properties depending largely on the [Ce3+]/[Ce4+] ratio in solid solution lattice, suspicion was raised that the presence of C e3+ site might also play an important role in the photocatalysis process, e.g. retarding the recombination of excitons. Yet more research into it will still be needed for fully understanding of the whole mechanism. 5.4 Summary In this chapter a series of experiments w ere performed to evaluate the photocatalytic activity of synthesized CexZr1 xO2 nanoparticles. The goal was achieved by the degradation of Procion Red MX 5B dye. Several control samples were made to eliminate the external factors

PAGE 56

56 that might a ffect the final results. C atalytic rate constant solely contributed by the activity of synthesized particles were obtained. To compare the activity of particles from our lab and those commercialized ones, Aeroxide TiO2 P25 from Evonik Industr ies and NanoA ctive Cerium Oxide from NanoScale Corporation were used for parallel experiments. By carefully analyzing and comparing the results, following conclusions w ere confirmed: CexZr1 xO2 nanoparticles displayed photocatalytic activity under the radiation of 302 nm UV light. Their performance exceeded that of NanoActive Cerium Oxide but was 1~2 orders lower to that of Aeroxide TiO2 P25, a commercialized photocatalyst. The photocatalytic activity of CexZr1 xO2 nanoparticles increased as the zirconium component increased under 302 nm UV irradiation. CexZr1 xO2 nanoparticles did not display any photocatalytic activity under 365 nm UV light radiation. The behavior of photocatalysis of CexZr1 xO2 nanoparticles resembled their performance in free radical scavenging an d oxygen storage capacity. This phenomenon indicated that there might be some common mechanisms contributing to each of their applications.

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57 Stage Photocatalytic Reaction Intermediates 1 p Hydroxy phenyl 3 hydroxy propanedioic acid 3 Hydroxy benzeneacet ic acid 2Hydroxybenzoic acid, pHydroxycinnamic acid 1,2 Benzenedicarboxylic acid 2 Cyanuric acid 1 Propene 1,2,3 tricarboxylic acid Propanedioic acid Propanoic acid Malic acid Butenedioic acid Oxalic acid Acetic acid 3 CO 2 and H 2 O Minerals (sulfuric compounds) Sample Rate const. for 302 nm UV light radiated sample ( 5 m 2 s 1 ) Rate const. for 365 nm UV light radiated sample ( 5 m 2 s 1 ) Blank 0 0 Aeroxide TiO 2 P25 61080 37080 CeO 2 72 0 Ce 0.8 Zr 0.2 O 2 193 0 Ce 0.7 Zr 0.3 O 2 183 0 Ce 0.6 Zr 0.4 O 2 203 0 Ce 0.4 Zr 0.6 O 2 323 0 Ce 0.2 Zr 0.8 O 2 445 0 Table 5 1. Photocatalytic r eaction i ntermediates summarized for different stages [ 69 ] Table 5 2. Normalized rate constants of photocatalytic degradation of Procion R ed MX 5B for the synthesized CexZr1 xO2 nanoparticles under 302 nm and 365 nm UV illumination

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58 Figure 5 1. Sketch of the reactor chamber Figure 5 2 The chemical structure of Procion Red MX 5 B [ 68 ]

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59 400 450 500 550 600 650 0.00 0.02 0.04 0.06 0.08 0.10 0.12 533 (nm)Wavelength (nm)Absorbance (a.u) Figure 5 3 The absorption spectrum for 5 ppm solution of the Procion Red MX 5B dye 0 40 80 120 160 200 240 280 320 360 400 440 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 C(t)/C(0)Time / min No Particle Zirconia NanoActive Cerium Oxide Ceria Ce0.8-Zr0.2 Ce0.7-Zr0.3 Ce0.6-Zr0.4 Ce0.4-Zr0.6 Ce0.2-Zr0.8 Aeroxide TiO2 P25 Figure 5 4 Ti me dependence of normalized residual dye concentration for irradiation with 302 nm UV light

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60 0 40 80 120 160 200 240 280 320 360 400 440 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Time / minC(t)/C(0) No Particle Ceria Ce0.8-Zr0.2 Ce0.7-Zr0.3 Ce0.6-Zr0.4 Ce0.4-Zr0.6 Ce0.2-Zr0.8 Zirconia NanoActive Cerium Oxied Aeroxide TiO2 P25 Figure 5 5 Time dependence of normalized residual dye concentration for irradiation with 3 65 nm UV light

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61 CHAPTER 6 IMPLICATIO NS AND CONCLUSIONS 6.1 Implications and Future Work CexZr1 xO2 nanoparticles are found to possess the photocatalytic activity to degrade the Procion Red MX 5B azo dye. The catalytic activity of these nanoparticles, though not comparable to that of the current commercialized photocatalyst Aeroxide TiO2 P25 has displayed a n increasing trend when the content of zirconium increases. Proved to be effecti ve in increasing the efficiency of CeO2 as three way catalyst [40] and free radial scavengers [13] zirconium substitution also seems to be beneficial in enhancing CeO2s photocatalytic activity. Though the mechanism for the tendency is not fully investigated, analogue trends imp ly a common mechanism, or at least shared components for those different applications. Figure 61 shows the equilibrium single cell structure for fluorite CexZr1 xO2 compound [40] It is suggested that the introduction of zirconium into CeO2 lattice would assist the generation of oxygen vacancies due to the difference in ionic radius between Zr4+ and Ce4+ ions The increase in oxygen vacancies in the lattice will in turn give raise to the ratio of [ Ce3+] to [ Ce4+] XPS data from section 4.4.4 also confirm the increase of oxygen content in the same set of particles after the photocatalytic experiment. The envir onmental oxygen species are possibly incorporated into the lattice during the photocatalysis reactions. According to Y.Y. Tsais efforts on the biomedical application of CexZr1 xO2 nanoparticles [13] the reaction pathway for CexZr1 xO2 nanoparticles free radical scavenging properties is summarized in Figure 6 2. It should be noticed that there are many species also existing in the redox reactions for semiconductor photocatalysis. Thus the implication is that equilibrium oxygen vacancies also play an important role in the photocatalysis of CexZr1 xO2 nanoparticles,

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62 and the photocatalysis and free radical scavenging properties might be counter reactions for these nanosize particles. However, the speculation made above is largely not proved and more research would be necessary to investigate into the mechanism between their photocatalytic activity and free radical scavenging properties. Future works including the tests of C exZr1 xO2 nanoparticles in the presence of both UV light irradiation and pre generated reactive oxygen species might help to generate a better understanding of these nanoparticles. 6.2 Conclusions According to experiments and discussions in previous chap ters, the following conclusion for this thesis could be summarized: CexZr1 xO2 nanoparticles synthesized through reverse micelle method. S pecific surface area of 10920 m2/g has been calculated for these nanoparticles from their dynamic light scattering diameters The synthesized CexZr1 xO2 nanoparticles display a well defined crystalline state. Zirconium substitution induces the generation of oxygen vacancies and Ce3+ sites in the CeO2 lattice. UV irradiation promote s the incorporation of environmental oxygen species into the CeO2 lattice. These nanosize particles displayed an absorption band in the UV region. Their bandgaps were larger than bulk materials due to the quantum confinement effect, which affect s the ir photocatalytic activities. The catalytic activity of synthesized nanoparticles was 1~2 orders lower than commercialized Aeroxide TiO2 P25 photocatalyst, but better than NanoActive Cerium Oxide from NanoScale Corporation, under 302 nm UV light illumination. No measurable activity has been observed for CexZr1 xO2 nanoparticles under 365 nm UV light Thus their photocatalytic activity under sunlight should be insignificant. Zirconium substitution enhance s the photocatalytic activity of ceria nanoparticles. As there are analog ous trend s in the reaction rates of free radical scavenging and the oxygen storage capacity of cerium zirconium dioxide solid solutions, a common mechanism explaining both is hypothesized.

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63 Figure 6 1. Schematic s tructure for CeO 2 and Ce x Zr 1 x O 2 crystals. Intrinsic O x x Intrinsic X O x x Intrinsic O x x Intrinsic O x x IntrinsicO x x Intrinsic O x xe V O Zr Ce O O O O ZrCe O V O Zr Ce O e V O Zr Ce O e V O Zr Ce O e VO Zr Ce O 2 2 1 2 2 2 2 22 1 2 2 2 1 2 2 1 2 2 1 2 1 2 2 2 1 2 25 4 3 2 1 1 Figure 6 2 The reaction pathway for Ce x Zr 1 x O 2 nanoparticles free radical scavengers [13]

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70 BIOGRAPHICAL SKETCH Rui Qing was born in Hanshan, a small town in Anhui, China in 1985. He moved to Hefei, the capital of Anhui province after 2 years elementary education in his hometown. Having attended the e ntrance e xamination for higher education in China, he was admitted in 2003 by Tongji University, Shanghai, China, majoring in m aterials s cience and e ngineering. He received his bachelors degree in s cience in June 2007 after 4 years undergraduate study. He then sought for a further education overseas and joined the D epartment of M aterials S cience and E ngineering in University of Florida from August 2007. Up unti l now he has been working in Dr. Sigmunds group and expecting to receive his m asters degree in May 2009. In the undergraduate education in Tongji University, Rui specialized in nonmetallic, inorganic materials He had worked on a project aimed to investigate the glass forming region of tellur ide niobium phosphate ternary glass system and their properties. These materials could potentially be used as lenses material in digital camera and op tical microscopy. He was also awarded 3rd rank scholarship for excellent students in Tongji University for academic year 20032004 and 20042005. In his masters study under Dr. Sigmunds supervision, Rui advanced the understanding of photocatalytic activity of cerium zirconium dioxide solid solution nanoparticles. Widely used as three catalyst in automobiles and electrolyte materials in solid oxide fuel cells, cerium zirconium dioxide solid solution s were investigated for their free radical scavenging property in biomedical uses. However, as a semiconductor these materials might be able to generate ROS (Reactive Oxygen Species) though they are capable of scaveng ing them. In the project Rui and his advisor successfully proved the photocatalysis of cerium zirconium dioxide nanoparticles and compare them to the commercialized photocatalyst. They are also preparing a paper for publication from the result s achieve