Photocatalytic Nanocomposites based on TiO2 and Carbon Nanotubes

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Photocatalytic Nanocomposites based on TiO2 and Carbon Nanotubes
LEE, SUNG-HWAN ( Author, Primary )
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Anatase ( jstor )
Carbon ( jstor )
Carbon nanotubes ( jstor )
Chemicals ( jstor )
Dyes ( jstor )
Electrons ( jstor )
Irradiation ( jstor )
Nanoparticles ( jstor )
Nanotubes ( jstor )
Surface areas ( jstor )

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University of Florida
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Copyright Sung-Hwan Lee. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
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Copyright 2004 by SUNG-HWAN LEE


This document is dedicated to my wife, and daughter with love.


iv ACKNOWLEDGMENTS I would like to sincerely thank Dr. Wolf gang Sigmund for serving as my adviser and giving me the opportunity to perform th is research. Additiona lly I would like to acknowledge and thank Drs. Brij Moudgil and Ben Koopman for giving me advice on many occasions and their fruitful conversa tions. Many of the achievements during my doctoral research would not have been po ssible without their ex cellent guidance and support. I also would like to acknowledge th e Particle Engineeri ng Research Center (PERC) for the financial support. I would like to thank Drs. Dinesh Sha h, Ellsworth Whitney, Susan Sinnott, Darryl Butt for serving as my advisory committee. I would like to thank PERC graduate stude nts, Georgios, Smithi, and Vijay who have collaborated on diverse experiments. Cagri is appreciated for the operation of atomic force microscope (AFM). I am grateful to Peter for the experimental support in electrospinning. Last, but not least, I wish to offer my sincere thanks to my parents, wife and daughter who encouraged me whenever I felt exhausted. I especially thank my wife, ImYoung, for being with me.


v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES............................................................................................................vii LIST OF FIGURES.........................................................................................................viii ABSTRACT....................................................................................................................... xi 1 INTRODUCTION............................................................................................................1 Titanium Dioxide (TiO2) Photocatalysis......................................................................3 Photocatalytic Disinfection of Biological Contaminants.............................................8 Design and Synthesis of Highly E nhanced Photocatalyst System.............................13 Design of TiO2-Carbon Nanotube System..........................................................13 TiO2 Nanocoating on Carbon Nanotubes............................................................19 Electrospinning of Photocatalytic Nanofibers.....................................................21 2 EXPERIMENTAL AND METHODOLOGY................................................................25 Experimental Parameters in Phot ocatalytic Efficiency Tests.....................................25 Preparation of Photocatalytic Nanocomposite Particulate.........................................28 TiO2 Nanocoated Carbon Nanotubes..................................................................28 Benchmark: Degussa Aeroxide® P25..................................................................32 Dye Degradation Test.................................................................................................33 Spore Inactivation Test...............................................................................................37 Electrospun Photocatalytic Nanofibers.......................................................................41 3 RESULTS AND DISCUSSION.....................................................................................45 Material Characterization...........................................................................................45 TiO2 Nanocoated Carbon Nanotubes..................................................................45 Characterization of Electrospun Photocatalytic Nanofibers................................53 Dye Degradation Test.................................................................................................60 Photocatalysis with UV-A Irradiation.................................................................63 Photocatalysis with Visible Light and Post UV-A Reaction...............................67 Spore Inactivation Test under UV-A irradiation........................................................70


vi YoungÂ’s Modulus of Electrospun TiO2-Carbon Nanotube composite fibers.............73 4 CONCLUSIONS.............................................................................................................80 LIST OF REFERENCES...................................................................................................83 BIOGRAPHICAL SKETCH.............................................................................................95


vii LIST OF TABLES Table page 1-1. Bandgap energy of various photocatalysts..................................................................4 1-2. Primary process and ch aracteristic time of TiO2 photocatalysis in H2O.....................8 1-3. Modes of microorganism removal or inac tivation action for various disinfection methods.....................................................................................................................12 1-4. Work functions of noble metals and carbon materials..............................................19 3-1. Diameters, suspended lengths, and YoungÂ’s moduli of nanofibers..........................77


viii LIST OF FIGURES Figure page 1-1. Schematic diagram; overall process of semiconductor photocatalysis in an aqueous system......................................................................................................................... 7 1-2. Schematic diagram; photogenera tion of charge carriers in TiO2 and electron trapping by fullerene (reduction of C60)...................................................................16 1-3. Schematic diagram; photogenera tion of charge carriers in a TiO2 shell and electron trapping by a carbon nanotube core and fo llowing reactions where CNT is a carbon nanotube, hVB is a hole in TiO2 valence band, eCB is a electron in TiO2 conduction band, and et is trapped electrons...............................................................................18 2-1. Influence of the different experimental parameters which govern the reaction rate r ; (a) amount of catalyst, (b) wavelength, (c) temperature, and (d) radiant flux [101]27 2-2. potential of as received a nd functionalized carbon nanotubes vs. pH....................30 2-3. Flow chart of TiO2 sol-gel nanocoating on carbon nanotubes..................................31 2-4. Molecular structure of azo dye (Procion Red MX-5B).............................................34 2-5. Experimental setup for photocatalytic dye degradation............................................35 2-6. Dye degradation by photo catalytic reaction; absorption intensity decrease in UV-Vis spectra because of photodegradation by Degussa Aeroxide® P25...........................36 2-7. SEM image of (a) endospores and (b) bact eria, and (c) structur e of endospore: core; cellular components, DNA, UV resistance, co rtex; heat resistance, peptidoglycan, ~200 nm, inner spore coat: acid resistan t proteins, 20-40 nm, outer spore coat; alkali resistant proteins, 40-90 nm...........................................................................38 2-8. Experimental setup fo r spore inactivation test..........................................................41 2-9. Flowchart of electrospinni ng of photocatalytic nanofibers.......................................43 2-10. Schematic diagram of electrospinning....................................................................44 2-11. Schematic diagram of AFM three point bending test on electrospun polycrystalline nanofiber...................................................................................................................44


ix 3-1. TEM images of surface functiona lized multi-walled carbon nanotube.....................46 3-2. SEM images (a) anatase nanocoated carbon nanotubes and (b) anatase coating fragments after carbon nanotube bur n-out, and (c) TGA/DTA analysis..................48 3-3. XRD pattern of anatase nanocoating fragments after carbon nanotube removal......49 3-4. XRD pattern of rutile nanocoating fr agments after carbon nanotube removal.........49 3-5. TEM images of (a), (b) individual, (c) agglomerated anatase nanocoated carbon nanotubes, and (d) SAD pattern...............................................................................50 3-6. TEM images of (a), (b) individual, (c) agglomerated rut ile nanocoated carbon nanotubes, and (d) SAD pattern...............................................................................51 3-7. FTIR spectra of anatase nanocoated carb on nanotubes (a) before heat treatment, (b) after heat treatment (500 C, 3 hours), and (c) anatas e coating layer (carbon nanotubes removed by thermal oxidation at 750 C)................................................52 3-8. XRD patterns (a) TiO2 nanofibers and (b) TiO2-Ag nanofibers................................55 3-9. Electron microscopy images of PVP-TiO2 continuous nanofibers (a-c) SEM and (d) TEM.........................................................................................................................56 3-10. Electron microscopy images of TiO2 continuous nanofibers (a-c) SEM and (d) TEM.........................................................................................................................57 3-11. Electron microscopy imag es and XRD pattern of TiO2-carbon nanotube continuous composite nanofibers (a) SEM, (b) TEM, and (c) XRD..........................................58 3-12. Electron microscopy images and EDS sp ectra of TiO2-Ag continuous composite nanofibers (a, b) SEM, (c) TEM image, and (d) EDS..............................................59 3-13. UV-Vis spectra of (a) anatase na nocoated carbon nanotubes and (b) Degussa Aeroxide® P25 dispersed in the dye solution without irradiation............................61 3-14. Dye degradation by anatase – carbon nanotube mixture as a function of carbon nanotube amount......................................................................................................62 3-15. Direct comparison of dye degradatio ns by anatase nanocoated carbon nanotubes, rutile nanocoated carbon nanotubes and Degussa Aeroxide® P25 with UV-A irradiation.................................................................................................................64 3-16. Curve fitting (the first order of e xponential decay) and extrapolation of dye degradation data by anatase nanocoate d carbon nanotubes and Degussa Aeroxide® P25 with UV-A irradiation.......................................................................................65


x 3-17. Schematic band diagrams and charge carrier separation mech anisms of (a) hole trapping in Degussa Aeroxide® P25 and (b) electron trapping in anatase nanocoated carbon nanotubes......................................................................................................66 3-18. Photocatalytic dye degradation by anatase nanocoated carbon nanotubes with visible light irradiation.............................................................................................68 3-19. Post UV-A dye degradation by anatase nanocoated carbon nanotubes...................69 3-20. Photocatalytic endospor e inactivation by anatase nano coated carbon nanotubes and Degussa Aeroxide® P25 with UV-A irradiation......................................................72 3-21. Curve fitting (the first order of e xponential decay) and extrapolation of spore inactivation data by anatase nanocoate d carbon nanotubes and Degussa Aeroxide® P25 with UV-A irradiation.......................................................................................72 3-22. Images of sample nanofibers (a) SEM image of nanofiber deposited on alumina membrane, (b) AFM image of nanofiber on alumina membrane, (c) TEM image of polycrystalline TiO2 electrospun fibers, and (d) TEM image of TiO2-carbon nanotube composite fibers........................................................................................74 3-23. Actual AFM scanning data on (a) fiber and (b) pore; force (F) is applied at the middle of the fiber lying on a pore with a diameter (L) for three point bending.....75 3-24. Actual AFM force curves of alumina substrate, Si wafer, and TiO2 nanofiber......76 3-25. Young’s Modulus vs. diameter of TiO2 and TiO2-carbon nanotube fibers.............79


xi Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy PHOTOCATALYTIC NANOCOMPOSITES BASED ON TiO2 AND CARBON NANOTUBES By Sung-Hwan Lee December, 2004 Chair: Wolfgang M. Sigmund Major Department: Materials Science and Engineering Nanocomposite particles and fibers based on TiO2 and multi-walled carbon nanotubes were developed for photocatalytic appl ications, such as pur ification of organic contaminants and disinfection of hazardous microorganisms. TiO2 (either anatase or rutile) nanocoated carbon nanotubes were synthesized via sol-gel. Their nanostructures were characterized with HRTEM, SAD, and XRD. The anatase nanocoated carbon nanotubes had 172 m2/g BET specific surface area (SSA) which was more than three times larger than the best photocatalytic nanoparticles in market (Degussa Aeroxide® P25, SSA 50 m2/g). The anatase nanocoated carbon nanotubes showed higher photodegradation of dye molecu les than Degussa Aeroxide® P25 under UV-A irradiation. In addition, the anatase nanocoated carbon nano tubes were reactive with visible light irradiation and, consequently, dye degrada tion was observed with visible light while Degussa Aeroxide® P25 was not effective. Any signifi cant sign of photocatalysis was not observed when rutile nanocoated carbon na notube was used. Effective inactivation of


xii bacterial endospores ( Bacillus Cereus ) in an aqueous medium was achieved with anatase nanocoated carbon nanotubes with UV-A irra diation. The inactivation with Degussa Aeroxide® P25 was slower in the same experi mental condition. The rutile nanocoated carbon nanotube was not considerably func tioning in dye degradation and spore inactivation. A few obstacles still remain for production and practical applications of the anatase nanocoated carbon nanotubes; (a) low yield from the sol-gel process and (b) limited use as a particulate system because of potential toxicity of particles. These problems were overcome by electrospinning. Anatase, anatas e-silver, and anatase-carbon nanotubes continuous nanofibers were produced succe ssfully. For photocatalytic filtration applications, it was important to unders tand the mechanical properties of the photocatalytic nanofibers because of the britt le nature of cerami c materials. Young’s moduli (E) of electrospun fibers were dete rmined by three point bending test using atomic force microscopy (AFM). It was obser ved that polycrystal line anatase nanofiber had lower E than bulk. Moreover, the mechan ical reinforcement by carbon nanotubes was observed in the anatase-carbon nanotube composite nanofibers.


1 CHAPTER 1 INTRODUCTION The purpose of this study was to develop novel photocatalytic systems to provide for the solution of more than 99% clean up of organic and biological contaminants that impact the health and safety of the civilia n, commercial, and defense sectors. Since the 1970s, scientists began to realize that the susceptibility of ti tanium dioxide (TiO2) to absorb photon energy from the UV end of the so lar spectrum and then react with water molecules to produce radicals could be used to create surfaces that were, for all practical purposes, self-cleaning. Photo catalysts can break down al most any organic compound, and a number of research groups have been s eeking to take advantag e of its reactivity by developing a wide range of environmentally be neficial products [1-3]. Not only organic contaminants can be destroyed by TiO2 particles but also microorganisms can be inactivated. When a microorganism is in contact with the TiO2 surface that is exposed to UV light of a wavelength below 385 nm, hydr oxyl radicals are formed and break down the cell wall and outer membrane, allowi ng cell contents to leak out and TiO2 particles to enter, thereby causing cell damage and death in the presence of water [4]. One of the largest fields of interest and growth in th e upcoming century is th at of photocatalysis. From October 15-17, 2003, the International Photocatalysis Technology Convention and Exhibition was held in Tokyo, Ja pan. There it was predicted that by 2005 the market of photocatalysis for the environment will r each 10 billion dollars . The concept of benefiting mankind in a helpful manner has great moral a nd ethical worth.


2 Highly efficient TiO2 photocatalytic particulate a nd continuous fibrous systems were developed by modifying the TiO2 electronic structure and increasing reactive surface area of the catalysts with multi-walled carbon nanotubes in this study. Nanotechnology (photocatalyst desi gn, synthesis and characteri zation) and environmental engineering sciences (microbi al evaluation) were assembled to improve conventional water and air purification systems. The novelty of the approach was in the integration of the most reactive photocatalyst, TiO2, with carbon nanotubes that can provide high surface area and excessive quantum yield because of their high aspect ratio and charge carrier scavenging. Both the nanoparticulate system (TiO2 nanocoated carbon nanotubes) and the continuous nanofiber system (elect rospun photocatalytic nanofibers) were successfully developed and divers e properties were determined. Specific objectives were as follows: 1. The design of photocatalytic nanocom posite systems which have higher photocatalytic efficiency (both surface prope rties and quantum yield) than the best commercial photocatalyst (Degussa Aeroxide® P25) with UV-A (wavelength of 350 nm), or visible light irradiation. 2. The synthesis and the characterizatio n of nanostructured photocatalytic nanocomposites. 3. The determination of photocatalytic effi ciencies in organic dye degradation and spore inactivation using Degussa Aeroxide® P25 as a standard. It has been well established how to determine the dye degr adation efficiency of photocatalysts of Degussa Aeroxide® P25. However, it was challenging to inactivate spores because of their multi-protective layers and there has not been a published work of spore inactivation of Degussa Aeroxide® P25. The water-borne spore inactivation tests were attempted and the efficiency of each photocatalyst is measured. 4. The preparation of continuous photocataly tic (composite) nanofiber to overcome the potential toxicity of the nanoparticulat e system and the determination of the mechanical property (Young’s Modulus) by atomic force microscopy (AFM) three point bending.


3 Titanium Dioxide (TiO2) Photocatalysis TiO2 is a ubiquitous opaque white pigment. This stable filler and colorant has been widely used to make products as diverse as paper, plastics, lipstick, toothpaste, and pharmaceutical tablets [5]. Moreover, TiO2 nanoparticles in the 10-50 nm range take on unusual properties and can be used in various applications, such as self-cleaning window glass, air and water purification systems, and antibacterial coating by tapping the photocatalytic properties of th ese particles. Scientists have adapted them to remove nitrogen oxides from power plant exhausts, and they are looking at wa ys to harness these environmental catalysts to treat diesel vehicle emissions. TiO2 is a wide bandgap semiconductor and researchers ar e looking at it as a substitute for silicon to make solar power cells, as well as battery storage media [6]. According to IUPAC (International Uni on of Pure and Applied Chemistry) compendium of chemical terminology, photocatal ysis is defined as a catalytic reaction involving light absorption by a catalyst or by a substrate [7]. In 1972, Fujishima and Honda discovered the photocatal ytic splitting of water on TiO2 electrodes [8]. Since then, research efforts in understanding the funda mental processes and in enhancing the photocatalytic efficiency of TiO2 have come from extensive research performed by scientists. In recent years, environmental clea nup applications have been one of the most active areas in heterogeneous photocatalysis. This is inspir ed by the potential application of TiO2-based photocatalysts for the total destru ction of organic compounds in polluted air and wastewaters [9,10]. There are semiconductors which can be used for photocatalysis, such as WO3, Fe2O3, CeO2, ZnO2 and ZnS [11]. Metal sulfides are not stable enough for catalysis in aqueous medi a due to photoanodic corrosion and they are also toxic. Fe oxides undergo photocathodic corrosion. ZnO is unstable in water and


4 forms Zn(OH)2 on the particle surface [12]. In this study, the mo st important purpose is the modification and utilization of TiO2 for an advanced photocatalysis. Thus, complete understanding of the fundamental nature of TiO2 for photoelectric and photochemical properties is necessary before modification. The properties are related to the atomic structure. TiO2 crystallizes in thr ee major different structures: rutile (tetragonal /mnm P4 D2 14 4h , a = b = 4.584 Å, c = 2.953 Å), anatase (tetragonal, /amd P4 D1 19 4h , a = b = 3.782 Å, c = 9.502 Å) and brookite (rhombohedrical, Pbca D15 2h , a = 5.436 Å, b = 9.166 Å, c = 5.135 Å) [13]. Only rutile and anatase play any role in th e applications and are of interest here. Photocatal ysis is attributed to the el ectrical characteristics of TiO2. The band gap energy of rutile is 3.0 eV a nd that of anatase is 3.2 eV. The electron effective masses ( m*) in rutile and anatase are approximately 20 m0 and 1 m0, respectively ( m0 is electron rest mass). The mobility of the el ectrons in rutile is about 89 times lower than that in anatase according to µ ( m*)-3/2 T1/2 for polar semiconductors. The diffusivity of the electrons in rutile is al so ~89 times smaller than in anatase [14]. Table 1-1. Bandgap energy of various photocatalysts Photocatalyst Bandgap energy (eV) Photocatalyst Bandgap Energy (eV) Si 1.1 ZnO 3.2 TiO2 (rutile) 3.0 TiO2 (anatase) 3.2 WO3 2.7 CdS 2.4 ZnS 3.7 SrTiO3 3.4 SnO2 3.5 WSe2 1.2 Fe2O3 2.2 -Fe2O3 3.1 The wide band-gap semiconductors can act as sensitizers for light-induced redox processes due to their electronic structure, which is characterized by a filled valence band and an empty conduction band. Wh en a photon with energy of h matches or exceeds the


5 bandgap energy of the semiconductor, an electr on is excited from the valence band into the conduction band leaving a hole behind. Exc ited state conduction-band electrons and valence-band holes can recombine and dissi pate as heat. The valence-band holes are powerful oxidants, while the conduction-band el ectrons are good reductants. For organic compound degradation the redox potential of H2O/OH must be within the bandgap of the semiconductor. When the aqueous solution of the semiconductor photocatalyst is excited with ultraviolet light, electron-hole pairs de velop. These electron-hole pairs have an oxidizing potential of 2.9 V vs. normal hydrogen electrode (NHE), which is enough to oxidize most pollutants. The general TiO2 photocatalytic reactions can be described by the equations 1-1 and 1-2. The most impor tant reaction involves hydroxyl ions (OH-) on the surface of TiO2 reacting with the holes, fo rming hydroxyl radicals (OH), which is the main cause of the photodegradation of orga nic contaminants and the inactivation of hazardous microorganisms [12,15]. TiO2 + h ecb + hvb + (1-1) hvb + + Ti OH Ti OH (1-2) In the absence of suitable electron and/or hole scavengers, the stored energy is dissipated within a few nanoseconds by recombination. Recombination of the photoexcited electron-hole pair needs to be retarded for an efficient charge transfer process to occur on the photocat alyst surface. The efficiency of the photocatalytic process can be measured as a quantum yield, which is defined as the number of defined events occurring per photon absorbed by the system or as the amount (mol) of reactant


6 consumed or product formed per amount of phot ons (Einstein) absorbed [7]. The ability to measure the actual absorbed light is practi cally not feasible in heterogeneous systems due to scattering of light by the semiconduc tor surface and the recombination. It is usually assumed that all the light is absorbed and the capacity is quot ed as an apparent quantum yield. If several products are formed from the photocatalyt ic reaction, then the efficiency is sometimes measured as the yiel d of a particular product. The quantum yield for an ideal system () can be given by the simple relationship [15]: R CT CTk k k (1-3) where kCT is the charge transfer process rate and kR is the electron-hole recombination rate Assuming that diffusion of the products occurs quickly without the reverse reaction, is proportional to the ra te of the charge transfer processes and inversely proportional to the sum of the charge transfer rate and the electron-hole recombination rate. Without recombination the quantum yield would take on the ideal value of 1 for the photocatalytic process. Char ge carrier trapping would suppress recombination and increase the lifetime of the separated electr on and hole. Surface and bulk defects naturally occur during the synthesis and they may he lp suppress the recombination by trapping charge carriers. The hole produced by irra diation reacts with water or surface-bound hydroxyl ion producing hydroxyl radical. El ectron released by irradiation of photocatalyst combines with dissolved molecular oxygen, producing the superoxide radical, O2 . Hydrogen peroxide possibl y added acts as an oxidant, but also as an e-


7 scavenger instead of diss olved molecular oxygen. H2O2 dissociates to hydroxyl radical and hydroxide ion even easier than H2O2, due to an extra electron [12-16]. Primary process and characteristic time of TiO2 photocatalysis in an aqueous system is listed in Table 1-2 [16]. The photocatalyst can be used for the photodegradation of organic molecules denotes the conversion of organic compounds to CO2, H2O, NO3 -, or other oxides, halide ion, phosphate, etc. for environmental remediation. Ofte n degradation begins with partial oxidation, and mechanistic studies relevant to oxidative photoca talytic degradation frequently focus on early stages involv ing photooxygenation, photooxidative cleavages, or other oxidative conversions . Environmental decontaminat ion by photocatalysis can be more appealing than conventional chemical oxidation methods because semiconductors are inexpensive, nontoxic, a nd capable of extended use without substantial loss of photocatalytic activity. Figure 1-1. Schematic diagram; overall pro cess of semiconductor photocatalysis in an aqueous system Conduction Band Valence Band h h+eO2 O2 TiOH TiOH


8 Table 1-2. Primary process and characteristic time of TiO2 photocatalysis in H2O Primary Process Characteristic time (second) Charge-carrier generation CB VB 2e h TiOhv 10-15 (very fast) OH + formation at the TiO2 surface } OH Ti { OH Ti hIV IV VB 10-9 (fast) Electron trapping OH} Ti { OH Ti eIII IV CB III IV CBTi Ti e 10-10 (shallow trap; dynamic equilibrium) 10-8 (deep trap; irreversible) Charge-carrier recombination OH Ti } OH Ti { eIV IV CB OH Ti OH} Ti { hIV III VB 10-7 (slow) 10-9 (fast) Interfacial charge transfer Red OH Ti Red } OH Ti {IV IV x IV x tO OH Ti O e 10-7 (slow) 10-3 (very slow) Photocatalytic Disinfection of Biological Contaminants Bioweaponry has its roots from the ancient past and became a science in the early 20th century following the break through discoveries in mi crobiology and immunology of the late 1800s. Direction of bioweapons agains t the military is biowarfare and direction against civilians is bioterrorism [17]. The biological warfare agent was first used during the French and Indian Wars by British forces in North America ( 1754-1767) [18]. Since the anthrax epidemic caused by the escape of an aerosol of Bacillus anthracis (anthrax) at the military facility in Un ion of Soviet Socialist Re publics in 1979 [19] and by the anthrax mail via US Postal Service in the United States in 2001 [20]. The non-natural epidemic by biological warfare agents has been great concern over the entire society [21]. It costs more than $ 100 million to clear anthrax out an contaminated building (Brentwood post office, Washington D.C. a nd American Media building, Boca Raton,


9 Florida) and there are immediate needs to develop a highly efficient method for clean-up of biologically contaminated area [22]. The concern over chemical and biological warf are agents, such as anthrax, has been greatly increased since the terrorist attack in the U.S. on September 11th 2001. Bacillus anthracis, the bacterium that causes anthrax, emerged as one of the most threatening biological agents that may be used as wea pons. Anthrax spores were mailed to several locations via the US Postal Service resulting in tw enty-two confirmed or suspected cases of anthrax infection [23]. Th e photocatalytic destruction of warfare agents, nerve agent stimulant (organophosphorus compounds) and mustard gas stimulant (organosulfur compounds), has been studied in the past decade [24-28]. The chemical compounds can be completely mineralized via multiple steps involving several intermediate products. In most cases, CO2, H2O, and inorganic salts are the final products and no hazardous final byproducts are formed. However, photocatalytic oxidation over TiO2 can be kinetically retarded due to the accumulation of partially oxidized intermediate species on the catalyst surface, a poisoning process which can occur ra pidly in some cases if the photocatalytic removal rate is not high enough [29]. Si nce the 1980s, antimicrobial treatment has become a prevalent and widely accepted re mediation strategy to control harmful organisms [30-36]. While contributing to the health and safety of the public and the preservation of materials by decreasing th e number of microorganisms, antimicrobial pesticides involve risks of s ilent failure, causing potential exposure hazards. Biological warfare agents include bacteria, viruses, f ungi, and other living mi croorganisms that can kill or incapacitate. Bacterial diseases are considered the most likely avenue of attack, because bacteria can be easily produced in fermenters, the infectious organisms can be


10 easily spread through air or water, and the di seases have a short incubation time and high lethality. In this study, photocatalytic na nocomposite systems, which are composed of TiO2 shell and carbon nanotube core, are deve loped to destroy microorganisms and toxins in the environment as well as chemi cal agents. Photocataly tic technology of TiO2 is an attractive approach for controlli ng environmental pollutants because of the following characteristics: TiO2 is an environmentally benign material The same basic technology can be applied to both water and air media The use of a catalyst eliminates the need for chemical oxidants Microbes are completely mineralized High destruction rates enable the system to be compact Either black lights or solar energy can serve as the excitation source The scientific basis of the technology has already been established The greatest bacterial threats may be anthrax (caused by Bacillus anthracis), plague (caused by Yersinia pestis), and Tularemia (caused by Francisella tularensis). Spores of Bacillus anthracis are particularly dangerous because th ey are stable for years or decades in the environment [37]. Anthrax is tran smitted through inhalation, contact with skin, and ingestion. Importantly, an tibiotics can treat bacterial diseases. The U.S. government is taking steps to ensure that adequate s upplies of antibiotics are available and can quickly be transported to combat disease outbr eaks. Viral diseases might be considered a less likely bioterrorism agent because pathoge nic viruses can only reproduce inside host cells and thus require sophisticated tiss ue culture techniques for mass production. However, antibiotics are not effective agains t viruses and vaccines are not available for most severe viral diseases. Among these are the viral hemorrhagic fevers (VHFs) that include Ebola, Marburg, Lassa, and South Amer ican VHF viruses. VHFs can result from


11 the inhalation of aerosolized virus. Case fa tality rates for Ebola and Marburg are 25-88% according to Centers for Disease Co ntrol and Prevention (CDC). Gerba and coworkers reported that the ra pid emergence of wa terborne pathogens, such as Cryptosporidium parvum and Escherichia coli O157:H7, have created a threat to the drinking water industry and there is a gr owing need to develop a strategy [38]. He also claimed that toilet seats could be cleaner than kitchen and office areas. For example, work stations contain nearly four hundred times as many microbes than lavatories. Thus, the surfaces must be regularly disinfected to prevent the spre ad of viruses and bacteria responsible for disease. If photocatalysts, wh ich are reactive under sunl ight or room light are available and surfaces are co ated with those, infected su rfaces can be self-cleaned. Because of the widespread use of antibiot ics and the emergence of more resistant and virulent strains of microor ganisms, there is an immediat e need to develop alternative sterilization technologies [33] . Although a wealth of inform ation has demonstrated the efficacy of the biocidal actions of the TiO2 photocatalyst, the fundamental mechanism underlying the photocatalytic inactivation of microorganism has not yet been well established. In 1985, Matsunaga and coworker s reported that microbial cells in water could be killed by contact with a TiO2-Pt catalyst upon illuminati on with near-UV light for 60 to 120 min [30]. Later, the same gr oup of workers successfully constructed a practical photochemical device in which TiO2 powder was immobilized on an acetyl cellulose membrane. An Escherichia coli O157:H7 suspension flowing through the device was completely sterilized [39]. They observed that the extent of killing was inversely proportional to the thickness and structure of the cell wall. Their findings occupied the attention for ster ilization and resulted in atte mpts to use this technology for


12 disinfecting drinking water a nd removing bioaerosols from indoor air environments [3136]. When irradiated TiO2 particles are in dir ect contact with or close to microbes, the microbial surface is the primary target of the initial oxidative attack of OH generated by the TiO2 photocatalyst. Polyunsatur ated phospholipids are an in tegral component of the bacterial cell membrane, and many functions, such as semipermeability, respiration, and oxidative phosphorylation reacti ons, rely on an intact membra ne structure. Therefore, lipid peroxidation is detrimental to all forms of life. The TiO2 photocatalytic reaction indeed causes the lipid peroxidation reac tion to take place and normal functions associated with an intact membrane are lost. The loss of membrane structure and membrane functions is the root cause of cell death when photocatalytic TiO2 particles are outside the cell [33]. Secondary wa stewater effluent containing 103 to 106 CFU/ml of bacteria and viruses wa s treated through TiO2 photocatalysis under s unlight or simulated sunlight and two-log inactivation, which is similar to the disinfection rates obtained by Matsunaga et al. was observed. Fungi, tumor cells a nd even cancer cells have been successfully inactivated by TiO2 photocatalysis as well [31]. The disinfection actions of various methods are summarized in Table 1-3 [4]. Table 1-3. Modes of microorganism re moval or inactivation action for various disinfection methods Method OH O2 -, H2O2 Cl hv Adsorption Capture Photocatalysis † ‡ UV (254nm) Chlorine HEPA filter † UV-A irradiation may have some inactivation effect on sensitive organisms ‡ In some catalyst configurations, photocatalyst may act as a filter


13 Design and Synthesis of Highly Enhanced Photocatalyst System Design of TiO2-Carbon Nanotube System One of the very basic results of the physics and chemistry of solids is the insight that most properties of solids depend on the microstructure, i.e. the chemical composition, the arrangement of the atoms (ato mic structure) and the size of a solid in one, two or three dimensions. If one of these parameters is changed, the properties of a solid change accordingly. The synthesis of novel materials with new properties by means of the controlled manipulation of their micros tructure on the atomic level has become an emerging interdisciplinary field based on so lid state physics, chemistry, biology and materials science. Novel materials may i nvolve (isolated, substrate-supported or embedded) nanometer-sized particles, thin wi res or thin films with reduced dimensions and/or dimensionality. Size effects result if th e characteristic size of the building blocks of the microstructure is redu ced to the point where critical length scales of physical phenomena (e.g. the mean free paths of electrons or phonons, a coherency length, a screening length, etc.) become comparable with the characteristic size of the building blocks of the microstructure [40,41]. The size effect on photocat alytic applications is still controversial. The relationship between the TiO2 particle size and phot ocatalytic activity has been addressed and the si gnificant disagreements requi re a careful approach. Anpo et al. reported an increase in the TiO2 photocatalytic activity for the hydrogenation of CH3CCH with decreasing particle size [42] . They associated the pronounced activity enhancement for particles smaller than 10 nm with the combined effects of larger surface area and size quantization. A similar observa tion was also made fo r the photocatalytic degradation of methylene blue in aq ueous suspension for a series of TiO2 particles larger than 30 nm. However, other reports showed that the photocatalytic efficiency does not


14 monotonically increase with decr easing particle size [43]. An optimal particle size of about 10 nm was observed for nanocrystalline TiO2 photocatalysts in the decomposition of chloroform [44,45]. Rivera et al. had reported a linear increase in photocatalytic oxidation of trichloroethylene with increasing anatase crystal size [46]. However, the agglomeration of the TiO2 primary nanoparticles is diffi cult to avoid. The morphology and size of these aggregates or secondary particles can affect the light-scattering properties of the catalyst, as well as the degree of photon penetration. The transport properties of the reactants and products w ithin the aggregate can also alter the effectiveness of the catalyst. A photocatalytic nano material with a high aspect ratio is desirable because the needle-like structure may retain a high photo catalytic surface area and a high degree of photon absorpti on even after the agglomeration.In 1981, the photocatalytic na nocomposite system of Pt-RuO2-TiO2 was developed in Switzerland [47]. When TiO2 nanoparticles are loaded with Pt and RuO2 nanoparticles, the high photocatalytic activity led to water decomposition in visibl e light experiments. Duonghong et al. concluded that excited electrons in the CB were channeled to Pt sites where hydrogen evolution occurs. It was also asserted that the role of RuO2 is to accelerate the hole transfer from the valence band of TiO2 to the aqueous solution. The system behaves as a short-circuited micro photoeletcrochemical cell where Pt is the cathode and RuO2 is the anode. Band-gap excitation in the TiO2 substrate injects negatively charged electrons into the Pt part icles and positively charged holes into the RuO2 particles. TiO2 structure can be tailored in order to accomplish higher efficiencies with a large specific surface area in many photocatalytic applications. TiO2 nanostructured


15 materials with a high photocatalyt ic surface area can be synthesized via diverse methods, such as sol-gel, or electrospinning [48-56]. Si nce the photocatalytic reaction occurs at the catalyst surface, porous photocatalysts are often used to adsorb target contaminants more efficiently in environmental remediati on. The electronic structure of TiO2 can be refined and modified during the synthesis (e.g., refinement of atomic st ructure by heat treatment). However, it can be more effectively modi fied by coupling of conductive materials, typically novel metals, with photocatalysts [51-61]. In this study, multi-walled carbon nanotubes were selected and used as a templa te not only to increase the (photocatalytic) specific surface area but also to enhance apparent quantum efficiency owing to their high aspect ratio and unique electrical properties. Kamat et al. reported the one-electron reduction of fullerenes (C60) using TiO2 nanoparticles by both steady-state and la ser flash photolysis when the TiO2 nanoparticles were UV irradiated first before the addition of C60 [62]. Multi-walled carbon nanotubes, relatives of fullerenes, can be ideal for photogenerated electron scavenging due to the multiple graphene layers where electrons can flow through when in contact with a photocatalyst. Carbon nanotube core – TiO2 shell structure can be readily obtained by functionalization of carbon nanotubes and so l-gel nanocoating on them [63]. It can provide a truly nanosized phot ocatalytic composite system with a high aspect ratio. Consequently, a high specific surface area for photocatalytic reaction, and a high specific interfacial area for the efficien t electron trapping can be prov ided. During the synthesis of multi-walled carbon nanotubes, a scroll of a given helicity, which converts into the thermodynamically more stable multi-walled st ructure, composed of nested cylinders. This conversion process is assisted by numer ous defects, such as atomic vacancies and


16 slip planes. Therefore, multi-walled carbon nano tubes have defects, atomic vacancies and slip planes. Although an ideal graphene laye r is chemically inert, the surface can be functionalized from the defect sites at the surfaces and chemically reactive for the TiO2 sol-gel nanocoating. Kamat et al. reported the one-electron reduction of fullerenes (C60) using TiO2 nanoparticles by both steady-state and la ser flash photolysis when the TiO2 nanoparticles were UV irradiated first before the addition of C60 (Figure 1-2) [62,64]. 60 2 60 2C TiO C TiOhv (1-4) Figure 1-2. Schematic diagram; photoge neration of charge carriers in TiO2 and electron trapping by fullerene (reduction of C60) The deposition of a noble metal on semiconductor nanoparticles is an essential factor for maximizing the efficiency of photocatalytic reactions. It is comm only assumed that the noble metal acts as a sink for photoinduced ch arge carriers and promotes interfacial charge transfer processes. However, unlik e bulk metals, the nanoparticles do not often exhibit ohmic contact with the semiconducto r surface which retains the charge before transferring them to the re dox species. During extended photo catalysis, electron trapped by metal islands of Ag, Au and Cu becomes inhibited as their Fermi-level shifts close to EG C60 eVB CB TiO2 h h+


17 the conduction band of the semiconductor. Pt on the other hand act s as an electron scavenger and fails to achieve Fermi-level equilibration. The carbon nanotube is a good candidate for scavenging of photogenera ted electrons because of its unique dimensionality, high aspect ratio, and electri cal properties. Unlike metal islands, carbon nanotubes can be chemically bonded to TiO2 and this may enhance the photogenerated electron flow to the carbon na notube. Therefore, it is possi ble to improve the catalytic properties of traditional photocatalysts by designing photocatalytic composite nanoparticles using carbon na notubes as a core and TiO2 as a shell. Contact of metallic multi-walled carbon nanotubes with the se miconductor indirectly influences the interfacial charge transfer proc esses in a favorable way. This core-shell structure can be readily obtained by the surface functionaliza tion of multi-walled carbon nanotubes and TiO2 sol-gel nanocoating. This design can pr ovide a truly nanosized photocatalytic composite system with a high aspect ratio, and, thus, high specific surface area for photocatalytic reaction, and high specific interfacial area for th e efficient electron trapping. In this study, multi-walled carbon nanotube was used as a catalytic supporter in order to increase not only the specific su rface area providing more hydroxyl radicals, but also the quantum efficiency by retarding ch arge carrier recombination and scavenging photogenerated electrons thr ough the interface between TiO2 and carbon nanotube and graphene structures. Work f unctions of noble metals, that are resistant to corrosion and oxidation, ranges from 4.63 to 5.55 eV and t hose of carboneous materials vary from 4.8 to 7.87 eV (Table 1-4) [65]. The work f unction is the energy needed to remove an electron from the Fermi level in a metal to a point at infinite distance away outside the


18 surface and the values of nobel metals and car boneous materials are greater than that of TiO2, providing a Schottky barrier which facilita tes the transfer of electrons from TiO2 [66]. The work function of multi-walled carbon nanotubes is approximately 5 eV and, therefore, the photo exci ted electrons of TiO2 conduction band can be readily scavenges through multi-walled carbon nanotubes when they are coupled (Figure 1-3). Moreover, carbon nanotubes may modify the electronic structure of TiO2 by narrowing the bandgap, rendering TiO2 more sensitive to the visible light. CNT / ) e h ( TiO CNT / TiOCB VB 2 2 hv (1-4) ) e ( CNT / ) h ( TiO CNT / ) e h ( TiOt VB 2 CB VB 2 (1-5) surface TiO t 2 surface TiO t VB 22 2OH ) e ( CNT / TiO OH ) e ( CNT / ) h ( TiO (1-6) Figure 1-3. Schematic diagram; photoge neration of charge carriers in a TiO2 shell and electron trapping by a carbon nanotube core and following reactions where CNT is a carbon nanotube, hVB is a hole in TiO2 valence band, eCB is a electron in TiO2 conduction band, and et is trapped electrons h eh+ TiO2 (Shell) Carbon Nanotube (Core) TiO2 (Shell)


19 Table 1-4. Work functions of noble metals and carbon materials Material Work function (eV) Pt 5.55 Au 5.38 Ag 4.63 Graphite 4.91 C60 (fullerene) 7.87 Carbon nanotube ~ 5.0 TiO2 Nanocoating on Carbon Nanotubes Dimensionality becomes a crucial factor in determining the photocatalytic properties of TiO2 including surface area and, therefor e, efforts have been made to increase the specific surface area of TiO2 by decreasing particle size or constructing (meso)porous TiO2. TiO2 sol-gel nanocoating on carbon na notubes is a promising way not only to increase the specific surface area b ecause of the needlelike structure and but also to increase the quantum efficiency by re tarding charge carrier recombination because of the electron scavenging through the gra phene layers. Typically multi-walled carbon nanotubes have been used as templates for metal oxide nanocoatings [67-70]. Carbon nanotubes can be thought of as cy lindrical hollow micro-crystals of graphite having their own unique properties [71,72]. There are two main types of carbon nanotubes that can have high st ructural perfection. Single-wa lled nanotubes consist of a single graphite sheet seamlessly wrapped into a cylindrical tube. Multi-walled nanotubes comprise an array of such nanotubes that are concentrically nested like rings of a tree trunk [73,74]. Carbon nanotubes are usually made by carbon-arc discharge, laser ablation of carbon, or chemical vapor depositi on (CVD) and they can be aligned via diverse CVD methods [75-81]. 20 cm long carbon nanotube strands were recently produced by the catalytic pyrolysis of n-hexane with an enhanced vert ical floating technique [82].


20 The unrefined multi-walled carbon nanotube s contain not only nanotubes but also nanoparticles with a weight ratio of about 2: 1 in the best cases of carbon arc discharge syntheses. Thus, prior to a sol-gel TiO2 nanocoating process, the surface of carbon nanotubes must be modified to remove the unwanted nanoparticles. The scientists of NEC Corporation, where carbon nanotubes were first discovered, developed a chemical oxidation method, refluxing multi-walled carbon nanotubes in sulfuric acid. Potassium permanganate was added in situ and the yield of the purified nanotubes was approximately 40 % and revealed that 15 % of the surface of the carbon nanotubes was covered with carboxylic (-COOH), carbonyl (CO) and hydroxylic (-COH) species in a ratio of 9:4:2 [83,84]. Satishkumar et al. observed that the chemi cal oxidation thinned the multi-walled carbon nanotubes [85]. They calcu lated concentration of the surface acid groups by acid-base titrations. When a con centrated strong acid is boiled, many free oxygen atoms are produced. When an oxygen atom encounters a carbon nanotube, an oxidation reaction occurs. The oxidized mu lti-walled carbon nanotube surface groups can be determined and confirmed with Fourier transform infrared Spectroscopy (FTIR) [8587]. The acidic sites of commercially availa ble full-length purified single-walled carbon nanotubes were also calculated with the same method and the total percentage of acidic sites were only 1-3 %, approximately [88]. Thus, the multi-walled carbon nanotubes are more appropriate as a template for metal oxi de coatings not only because of the lower cost, but also because of the higher con centration of the functional groups. Once the functionalization process is completed, it should therefore be possible to use the functional groups on the carbon nanotube surf aces as initiation points for chemical reactions.


21 After the surface of carbon nanotubes is modified and stabilized in aqueous solution, they can be used as (remova ble) templates for sol-gel processing. Nanocomposite structures based on carbon na notubes can be built by coating nanotubes uniformly with TiO2 (either anatase or rutile) stru ctures. This unique composite is expected to have interesting mechanical as well as photochemical properties due to a combination of dimensional ef fects and interf ace properties. Previous works regarding the carbon nanotube templates by Rao and co-workers led to SiO2, Al2O3, and V2O5 nanotubes [68-70]. The SiO2 coating was obtained by stirring the functionalized carbon nanotubes in tetra-ethyl-ortho silicate (TEOS) and heating initially in a vacuum at 100 C followed by 500 C in air. For the Al2O3 and V2O5 coating the nanotubes were st irred in a gel of aluminum isopropoxide and water or a vanadium pentoxide gel, respectively. Af ter washing and sintering processes, Al2O2 and V2O5 coated carbon nanotubes were obtained. Tr ansition-metal ions were added to the SiO2 coated carbon nanotubes by mixing the carbon nanotubes in a TEOS/ethanol solution containing transition-metal compounds. After the drying and heating procedures, Ni, Cr. Cu, or Co doped SiO2 coating layers were obtained. In this study, titanium sulfate and titanium (IV) chloride were used as an anatase and a rutile precursor, respectively. Electrospinning of Phot ocatalytic Nanofibers There are a few obstructions for photocatalytic particulate system, such as potential toxicity if nanoparticles ar e inhaled and deposited in alveoli of human lung. If photocatalytic nanocomposite is prepared with a continuous fibrous form, it will benefit the global community by higher filtration/inac tivation efficiency of health hazardous microorganisms and higher mineralization/de gradation of chemical contaminants. To


22 date, electrospinning method has been widely studied because of its economical efficiency and simplicity for mass producti on of nanostructured materials [89-91]. In the early 1900’s several experiments were performed to study electrically create droplet formation and the firs t patent was awarded for elect rostatically created polymer fibers in 1934 to Anton Formhals [92-94]. Taylor discovered the equilibrium condition that occurs when a conducting fl uid droplet was exposed to an electric field and the shape was specifically a cone with a semi-vertical angle of 49.3 or apex angle of 98.6 [95]. This conical shape has been coined the Taylor Cone. Recently metal oxide nanofibers have been produced as well from solutions containing polymers and metal precursors via sol-gel kinetics [96-101]. Fi bers with a diameter of 100nm will have 1000 times the specific surface area as fibers with a diamet er of 100µm. This means that with 1,000 times less material one can have equivalent surface areas for photocatalysis reactions to occur and filtration in general. Electrospinni ng of sol-gel precursors has proved to be an excellent alternative method and is the only me thod to fabricate con tinuous nanofibers of infinite aspect ratio with a simple setup. The components include a power source capable of forming a large electric field (>0.5 kV/cm) , a counter electrode, a viscous solution, and a means of pumping the solution. While a solu tion is forced out of the capillary tube, either by gravity or an external force, th e immediate forces acting on the liquid are gravity, surface tension, and electrical stre sses. These forces compete and balance each out to form a Taylor cone, and depending on th e status of equilibrium the cone will eject droplets and/or a jet of liquid. Once a jet starts to eject from the apex of a Taylor cone, it will remain stable for a certain distance (insta bility region) which is specific to each solution and electrical configuration.


23 No theory exists that can take into account all variab les and describe the process with quantitative accuracy. However Rutledge et al. created the most in-depth theory of the jet and instability regions to date. Three main instabilities exist; (a) Rayleigh instability, (b) axisymmetric conducting instab ility, (c) whipping inst ability, and each can be thought of as acting independently and competing for stability [102-106]. Whipping instability caused the decreased diameter of the fibers and the jet is further decreased in diameter (up to 3 orders of magnitude) by solv ent evaporation and, in the case of ceramic fibers, polymer burnout and cr ystallization. The electros pun fibers were typically collected on a flat collector but a rotating dr um can be used to collect aligned fibers. Aligned fibers were also collected on a pa tterned substrate where two electrodes were placed parallel to each other with a small ga p in between, held at the same potential. The original process used w ith high molecular weight po lymers has been combined with sol-gel chemistry to pr oduce continuous ceramic nanofibers. The generality of the sol-gel process opened broad opportunities to produce and use novel inorganic and hybrid nanofibers from a variety of materials. It has been generally recognized that cer amics frequently offer advantages over polymers and metals in their use, such as chem ical resistivity, hardness, wear resistance, high melting temperature, low density and low pr ice. Ceramic nanofibers can be used as a thermally and chemically stable filtering medium. However, their applications have been often limited by their brittleness at low temperature. Gleiter et al. demonstrated nanocrystalline ceramics can be ductile [107] . They observed that conventionally brittle ceramics, including TiO2, became ductile permitting large (~100%) plastic deformation at a low temperature if a polycrystalline ceramic was generated with a crystal size of a few


24 nanometers. Carbon nanotubes have been predicte d to have not only fascinating electrical but also remarkable mechanical properties [73]. Thus, carbon nanotubes can be used for not only photogenerated electr on trapping but also mechanical reinforcement in the photocatalytic nanocomposite system. Salvetat et al. reported that arc-discharged multiwalled carbon nanotubes had YoungÂ’s modulus close to 1 TPa [108]. In this study, electrospun polycrystalline TiO2 and TiO2-carbon nanotube composite nanofibers were prepared. The mechanical property was studi ed with atomic force microscopy (AFM) three point bending to dete rmine the YoungÂ’s modulus.


25 CHAPTER 2 EXPERIMENTAL AND METHODOLOGY Experimental Parameters in Ph otocatalytic Efficiency Tests Photocatalytic efficiency strongly depe nds on physical experimental parameters, such as specific surface area (photocatalytic surface area) and quantum efficiency. The chemical characteristics, such as hydrophilicity and chemical stability, are also important. However, the surface chemical properties of TiO2 nanocoated carbon nanotubes and Degussa Aeroxide® P25 were assumed to be the same except the quantum efficiency. The dye degradation and the spore inactivation experiments were performed based on the same surface area of each photocatalysts to determine the relative efficiency. There are five major physical parameters governing the kinetics of photocatalytic reaction in an aqueous system; (a) mass of cat alyst, (b) wavelength, (d) temperature, (e) radiant flux, and (f) quantum yield (Figure 21) [109]. The initial rates of reaction are directly proportional to the mass m of catalyst. This indicat es a true heterogeneous catalytic regime. However, above a certain value of m, the reaction rate levels off and becomes independent of m. This limit corresponds to the maximum amount of TiO2 in which all the particles (the entire surface exposed) are totally illuminated. For higher quantities of catalyst, a screen ing effect of excess particles occurs, which masks part of the photoreactive surface. The optimum mass must be determined in order to avoid excess of catalyst and to ensure a total absorption of efficient photons. The variations of the reaction rate as a functi on of the wavelength follows th e absorption spectrum of the


26 catalyst, with a threshold corresponding to its band gap energy. In order for TiO2 to be photocatalytically reactive, at least UV-A is required. Th e photonic activation process makes systems not required heating and they are operating at a room temperature range (20 C T 80 C). At very low temperatures below 0 C or at very high temperature above 80 C, the photocatalytic activity decreases. Thus, a photocatalytic experimental setup requires coolers for the optimum temp erature. The photocatalytic reaction rate, r, is proportional to the radiant flux,, and this confirms the photo-induced nature of the activation of the catalytic process, with th e participation of phot o-induced electrical charges to the reaction mechanism. However, above a certain point, the reaction rate becomes proportional to1/2. According to the kinetic defi nition of quantum yield, it is equal to the ratio of the r eaction rate in molecules per second to the efficient photonic flux in photons per second. Theoretically, th e maximum value is equal to 1 and it may vary with a wide range depending on the experimental conditions. In this study, the same photocatalyst surf ace area of each photocatalyst is used and the optimum amount is selected in order fo r commercial photocatalyst nanoparticles to inactivate bacter ial endospores (Bacillus Cereus). Degussa Aeroxide® P25 was used as a standard and the efficiency of TiO2 nanocoated carbon nanotube was directly compared with the values of Degussa Aeroxide® P25 obtained in the same experimental conditions. The radiant flux was also optimized for commercial nanopartic les and the UV-A, wavelength of 350 nm, was selected for photocat alytic reactions with UV irradiation in order to minimize the biocidal effect of UV. Moreover, visible light was used for the photocatalytic activation of TiO2 nanocoated carbon nanotube to determine the effect of the modified (reduced) TiO2 bandgap by conductive carbon nanotubes. The temperature


27 was maintained at room temperature at 25 C by air cooling system for both dye degradation and spore inactiva tion experiments. Therefore, it was possible to compare the quantum yield for each photocatalyst by eval uating the photocatalytic reaction rates (r) of each test assuming the oxygen pressure in the system is constant. Figure 2-1. Influence of the different expe rimental parameters which govern the reaction rate r; (a) amount of catalyst, (b) wavelengt h, (c) temperature, and (d) radiant flux [101] r r m r r T 20-80C r r UV m r (a) (b) (c) (d)


28 Organic dye photodegradation and bacteria l endospore inactivation of anatase nanocoated carbon nanotubes, rutile nanoc oated carbon nanotubes, Degussa Aeroxide® P25, and Ishihara TTO-51 nanoparticles were tested and their rela tive efficiencies are compared. Preparation of Photocatalytic Nanocomposite Particulate TiO2 Nanocoated Carbon Nanotubes Usually catalytic supports are classified by their chemical nature to organic and inorganic supports. No matter what the s upport is, it plays an important role in immobilizing active catalyst. Principally, the support has three main functions: (1) to increase the surface area of cat alytic material, (2) to decr ease sintering and to improve hydrophobicity and thermal, hydrolytic, and chemi cal stability of the catalytic material, and (3) to govern the useful lifetime of the catalyst [110]. Support may also improve the activity of the catalyst by acting as a co-cat alyst. Reducing particle size increases surface area. Other possibilities to in crease the active surface area are to increase porosity or to apply appropriate support. By increasing th e porosity the surface area of many common supports may be increased to a great exte nt. However, controlling reaction conditions inside the particles is difficu lt and porosity may decrease selectivity in some cases. With fibrous support the active surface area of cat alyst may be relatively high without any significant pressure loss. Zeolite (hydrous aluminum silicate minerals, whose molecules enclose cations of sodium, pot assium, calcium, strontium, or barium, or a corresponding synthetic compound), glassand carbon fibe rs, ceramic materials, polymers, and activated carbon have been widely used as a catalytic support. However, single-walled and multi-walled carbon nanotubes have generated an intense effervescence due to not


29 only their unique electrical, mechanical, a nd thermal properties, but also their high specific surface areas [111]. The chemically inert carbon nanotube surfaces must be functionalized for sol-gel nanocoating. In this study, the carbon nanot ube surfaces were chemically oxidized for the creation of chemically reactiv e at the surface. The chemical oxidation treatment of multiwalled carbon nanotubes was originally performed to open the tips and to fill the cavities of carbon nanotubes [112, 113]. In order to c ovalently decorate the outside surface of multi-walled carbon nanotubes with either inorganic or organic moieties, functionalization of outer graphene layer is required to overcom e the inertness of graphene layers. The oxidation process can cover the carbon nanotube surface with carboxylic (-COOH), carbonyl (-C=O), and hydroxy lic (-COH) species in an approximate ratio of 4:2:1 which changes the chemical make -up of the reactive edge of the tips and the outer layer of the nanotube [83,84]. It was also reported that the chemical oxidative treatment is effective in stabilization of the carbon nanotube dispersion at higher concentrations [86]. In this study, ar c-discharged multi-walled carbon nanotubes (Aldrich) were used. They were re fluxed in boiling nitric acid (HNO3) at 140 °C for 10 hours as a chemical oxidation process. Afte r washing with deionized water until the supernatant attained pH 7, the samples were diluted and dried in an oven at 60 °C. The obtained multi-walled carbon nanotubes were cove red with acidic sites, such as carboxyl and carbonyl groups. A small amount of the samp le was added to aqueous solutions of various pH values to determine isoelectri c point (IEP) change due to the functional groups of carbon nanotube surfaces. Prior to the potential measurement (ZetaPlus, Brookhaven Instrument Corp.), the susp ensions were sonicated for 15 min. potentials of


30 the carbon nanotubes (before and after the chemical oxidation) as a function of pH are given in Figure 2-2. The samples ar e added to an aqueous solution of 10-3 M KCl and their suspensions are treated by ultrasonication. The supernan tants are titrated using 0.1 M HCl and 0.1 M NaOH. The isoelectric point (IEP) shift from pH 7 to pH 3.5 was observed. Thus, the functionalized carbon nanot ubes were stabilized by the electrostatic repulsion of the negatively charged surfaces in deionized water (pH7) for a sol-gel process [114]. The carbon nanotube dispersi on was also aided by magnetic stirring. Those functional groups were confirmed by Fourier transform infrared (FTIR) spectroscopy. The tota l number of functional groups can be determined by titration with NaOH [85,88] and was 51019/g approximately. Figure 2-2. potential of as received and functionalized carbon nanotubes vs. pH 123456789101112 0 + potentialpH As received MWNTs Functionalized MWNTs-


31 Figure 2-3. Flow chart of TiO2 sol-gel nanocoating on carbon nanotubes Rutile nanoparticles can be prepared by oxidation of titanium (IV) chloride (TiCl4) [115], and the phase transformation of TiO2 from anatase to rutile at temperatures higher than 450°C [118]. For rutile nanocoating on the functionalized multi-walled carbon nanotubes, titanium (IV) chloride (TiCl4) was added dropwise to the prepared carbon nanotube dispersion in an iced bath. The so l was stirred for one hour and washed with H2O. The prepared particles were dried at 60° C for one day and heat treated at 500°C for the crystallization. The rutile in bulk is the stable phase under ambient pressure while the anatase metastable phase transf orms to the rutile as long as the transition is kinetically possible. However, as size of the phases decreases, nanocrystalline anatase phase Functionalization of carbon nanotubes in HNO3 at 140C Washing with H2O Carbon nanotube dispersion in H2O Sol-gel Coating Anatase: Ti2(SO4)2, 25C Rutile: TiCl4 at ~0C Washing with H2O Drying at 60C for 1 day Heat Treatment at 500C for >2 hours TiO2 nanocoated carbon nanotubes


32 becomes the stable phase [117, 118]. In situ TEM studies of anatase nanoparticles with average domain sizes of 6 nm showed that there was no phase transformation to the rutile phase up to 1000 °C [119]. It was reported that an anatase crystalline can be precipitated by the hydrolysis of titania precursor when an extremely high water:precursor molar ratio (20) is present during the r eaction [120]. For the synthesis of an anatase nanocoated multi-walled carbon nanotube, 10-20 L of titanium (III) sulfate (Ti2(SO4)3, 99.9+%) solution was added and stirred with 1 00 mg of carbon nanotubes dispersed H2O (200 mL) for 1 hour as a sol-gel nanocoating process. After the sol-gel process, excessive TiO2 precursor is removed by centrifugi ng and washing repeatedly. TiO2 nanocoated carbon nanotubes are dried at 60C for two days and heat treated at 500C for the crystallization of TiO2 coating layer. Benchmark: Degussa Aeroxide® P25 Degussa Aeroxide® P25, which is known as the be st photocatalyst in the market [121], was used as a benchmark. Degussa Aeroxide® P25 is synthesized by a high temperature flame pyrolysis method and consists of anatase and rutile phases with a ratio of 3:1. Intensive TEM studies showed that it was not characterized by the coexistence of more than one crystalline phase on the indi vidual particles. Indi vidual single crystal particles of either anatase or rutile were observed. The average diameters of the anatase and rutile particles are approximately 25 and 85 nm, respectively, and they exist separately by forming their agglomerates that range between 50 and 200 nm [122]. However, the good interparticle contacts are formed between those two phases in water [123]. The specific surface area is ~50 m2/g. The high photocatalytic efficiency of Degussa Aeroxide® P25 can be contributed to the we ll-developed crystallinity which may


33 results in less recombination cen ters [124]. It is also believed that the rutil e phase plays a role of charge separation pr oviding oxidation sites. [61] The photodegradation of ali phatic, inorganic, aromatic compounds, surfactants and dyes have been demonstrated successfully using Degussa Aeroxide® P25. Mercury (Hg), xenon (Xe), mercury-xenon lamps, and black fl uorescent light have been typically used to activate Degussa Aeroxide® P25 particles. They can be immobilized on various catalytic supports or used as a form of coating. The modification using noble metals (i.e. platinum and silver) have widely applied to enhance the photocatalytic efficiency [125]. The disinfection of Degussa Aeroxide® P25 has been also intensively studied since Matsunaga’s work [126-132]. Even killing of tumor and cancer cells was reported. HeLa cells (cervical carcinoma) were killed by exposure to light from an unfiltered 500 W mercury arc lamp in the presence of Degu ssa Aeroxide® P25. Cells were cultured in Minimum Essential Medium (MEM) with De gussa Aeroxide® P25 for 24 hours, then exposed to light from a high pressure mercury lamp. HeLa cells were killed even after the external TiO2 was removed by washing and TEM analysis showed that TiO2 particles had adsorbed onto the cell surface or that they were ingested by the cells. Tumors caused by transplanting HeLa cells into mice were s uppressed by photocatalysis. T-24 cells (bladder transitional cell carcinoma) died after the UVA irradiation in the presence of Degussa Aeroxide® P25 because the cell functions were damaged [4]. Dye Degradation Test The monoazo dye Procion Red MX-5B (C19H10Cl2N6O7S2), also called Reactive Red 2 (C.I. number 18200) was used as a surrog ate for an organic contaminant. It is an environmental friendly dye that belongs to the family of “cold” reactive dyes (Figure 24). 100 ml of this dye solution with a concentration of 5mg/L was prepared. Degussa


34 Aeroxide® P25 (anatase), Ishihara TTO-51A (rutile) and TiO2 (anatase or rutile) nanocoated carbon nanotubes were added to th e dye solution based on the same surface area (0.15 m2). The as prepared dispersion was placed in a black box under UV radiation (wavelength 350 nm, photon density 30 W/m2) unde r constant stirring (Figure 2-5). 3 ml of the solution was collected and the dye concentration of each sample was characterized by UV-Vis spectroscopy as a function of time. The Post UV-A behavi or of each sample was studied as well monitoring the dye c oncentration with UV-Vis spectroscopy. Moreover, the same experiment was perfor med under visible light irradiation using halogen lamps (photon density 50 W/m2) to determine the visible-light-sensitivity of each photocatalysts. Figure 2-4. Molecular structure of azo dye (Procion Red MX-5B) Since the photocatalytic oxidation process with Procion Red MX-5B, like other azo dyes and other organic compounds apparently followed first-order kinetics with the pseudo-first-order rate consta nt [133]. Degradation and mine ralization can be monitored by determining sulfate ion and total organi c carbon concentrations in the reaction solution. In this study, the reduction in th e concentration of the dye molecule was determined with the absorbance value at th e maximum of the absorption spectrum at 538 NN Cl N N N Cl Na SO3S NaO3 N H OH


35 nm for every dye by monitoring UV-Vis spectrum in 400-800 nm using a spectrophotometer (Figure 2-6). Figure 2-5. Experimental setup fo r photocatalytic dye degradation To compare photocatalytic efficiency of each photocatalytic material, it was assumed that the absorption intensities of UV-Vis data were linearly proportional to the dye concentration. 100 C C C (%) n Degradatio0 0 (2-1) where C0 is the initial dye concentration and C is the dye concentration after the treatments as concentration unit, mgL-1. The destruction rates of photocatal ytic oxidation over illuminated TiO2 are often fitted to the Langmuir-Hinshelwood (L -H) kinetics model [16] which: Magnetic stirrer UV Lamps Dye solution with photocatalyst Cooling Air


36 KC kKC t C r 1 d d (2-2) where r is the oxidation rate of the reactant (mg/L min), C the concentration of the reactant (mg/L), t the irradiation time, k is the reaction rate constant (mg/L min), and K the adsorption coefficient of the reactant (L /mg). L-H model represents a mechanism for Figure 2-6. Dye degradation by photocatalytic reaction; abso rption intensity decrease in UV-Vis spectra because of photode gradation by Degussa Aeroxide® P25 surface catalysis in which the reaction occurs between species that are adsorbed on the surface. When the chemical concentration C0 is small the equation can be simplified to an apparent first-order equation. In general firs t-order kinetics are appr opriate for the entire 450500550600650700750800 AbsorptionWavelength (nm) 10min 20min 30min 40min 50min 60min


37 concentration range up to few ppm. The LH model was established to describe the dependence of the observed reaction rate on the initial solute concentrations. t k kKt C Capp. 0ln (2-3) It has been agreed that the expression fo r the rate follows the L-H law for the five possible situations; (a) photocatalytic reactions must be initiated by photoabsorption, (b) excited electron and hole are formed, (c) elec trons and holes react with surface adsorbed species, (d) otherwise recombin e each other to give no chemi cal reaction but heat, and (e) if any chemical or photoinduced reaction(s) fo llows, its (their) kinetics must be included. However, it is not possible to say whether the process takes place on the surface in the solution or at the interface [134]. Thus, Degussa Aeroxide® P25 was used as a standard photocatalyst and the each dye degradation e fficiency was empirically determined and compared based on the same photocatalytic surface area. Spore Inactivation Test Bacterial endospore suspension prepared by a lysozyme-treatment method was used in this study. Spore suspension were also prepared by ASTM E2111-00 standard and a heat-treatment method. However, a significant amount of vegetative cells was observed in the sample suspensions obtai ned by other methods. The presence of the vegetative cells in an endospore suspension can lead to misleading results when the spores are the focus of an investigation, b ecause the vegetative cells may protect spores from photocatalytic inactivati on. Consequently, the protectio n by the vegetative cells may increase the survival of spores. Moreover, the vegetative cells are more sensitive to photocatalytic reaction than spores. This would tend to decrease the concentration colony forming units (CFUs) in an irradiated suspension below what would be expected


38 if only spores were present. The presen ce of the vegetative cells would negate the desired one to one correspondence between CF Us and viable spores. For example, a vegetative filament of several cells might contain several unreleased spores. The filament could yield a single CFU if one or more or the spores were viable. This situation would also increase survival be yond what would be expected if only spores were present. Figure 2-7. SEM image of (a) endospores and (b) bacteria, and (c) structure of endospore: core; cellular components, DNA, UV resistance, cortex; heat resistance, peptidoglycan, ~200 nm, inne r spore coat: acid resistant proteins, 20-40 nm, outer spore coat; alkali resistant proteins, 40-90 nm (a) (b) Nucleoid Inner spore coat Core Cortex Outer spore coat (c)


39 The lysozyme-treatment method included se veral washes of bacterial suspension with alternative salts that caused vegetative cells to lyse. Heat-treatment (80°C, 10 minutes) was included at the end of the pr ocess to eliminate the remaining vegetative cells and/or injured spores. The lysozyme-tr eatment method resulted in pure spores in the final suspension. No trace of vegetati ve cells was detected by scanning electron microscopy and particles of 3 µm or greater in size, which are expected to be vegetative cells, are less than 1 % of the total particle number in all samples. Bacillus cereus ATTC 2 was used as a surrogate of Bacillus anthracis in this study. The bacteria are inocul ated into 500 mL Erlenmeyer flasks containing 99 mL of Columbia broth supplemented with 1 mL of MnSO4·H2O (10 mM). Foam plugs were used to allow air access and prevent contamin ation. Liquid cultures were incubated for 3 days at 35 ± 2 °C in orbital incubator-shake r (Model C24, New Brunswick Scientific) at 250 rev/min. Spores were harvested and purif ied using the lysozyme treatment [135]. The heat shock treatment (80°C, 10 min) wa s applied following the purification process to ensure killing of vegetative cells. Spore suspensions were kept in sterile deionized water and refrigerated at 4°C until use. The photocatalyst suspension was prepared by adding 3 mg of Degussa Aeroxide® P25 or 0.8 mg of TiO2 nanocoated carbon nanotubes into a sterile flask containing 20 mL sterile deionized water followed by sonicati on for 30 minutes in an ice water bath sonicator (Cole-Parmer 8890). The suspensi on mixed with a spore suspension (10mL) was then transferred into a 100 ×15 mm sterile Petri dish and agitated by magnetic stirrer. Three types of samples were prepared; (i ) the control sample by suspending 10 mL of spore suspension in 20 mL of sterile deioni zed water, (ii) the experimental sample with


40 3 mg of Degussa Aeroxide® P25 TiO2 added into 20 mL of sterile deionized water, sonicated (30 min) in an ice water bath , and adding a volume of 10 mL of spore suspension giving the total amount of 30 mL of spore plus ph otocatalyst suspension, and (iii) the experimental sample with TiO2 (either anatase or ru tile) nanocoated carbon nanotubes by adding 0.8 mg of the nanopa rticles (BET specific surface area of 172 m2/g) into 20 mL of sterile deionized water, sonicating, and adding of spore suspension as (ii). Each sample was transferred to a sterile 100 × 15 mm sterile Petri dish with a sterile magnetic stirring bar. A UV chamber consisting of a bank of sixteen 350 nm UV lamps (RPR-3500A, Southern New England), a lamp cooling fan, and an adjustable sample holder was used for the spore inactivation tests (Figure 2-8). A magnetic stirrer was placed on the sample holder at the center of irradiation area to provide mixing of experimental suspension. The sample holder was adjusted to give a di stance of 10 cm measured from the lamp surface to the initial suspen sion surface. The UV intensity was measured using a radiometer (Model 30526, Eppley Laborator ies Inc.) and a correction coefficient specifically to solar UV was a pplied [136]. The UV lamps were stabilized for 30 min to obtain constant intensity (92 W/m2) before the Petri dish containing the prepared suspension was placed on top of the magnetic stirrer. Samples were collected immediately afte r the suspension was exposed to UV and every subsequent 30 min. For each sampling, a volume of 0.25 mL of the suspension was collected four times resulting in the tota l volume of 1 mL into a sterile culture tube, which was wrapped with aluminum foil. The tube was capped and refrigerated immediately after sampling until use.


41 Figure 2-8. Experimental set up for spore inactivation test The sample was analyzed for survival ratio of Bacillus cereus spores as a function of UV-A irradiation time. Colony forming units (CFU) were enumerated by spread plating the culture onto tryptic soy agar plat es. The culture was serially diluted using sterile phosphate buffered saline solution cont aining 2 mM of sodium dodecyl sulfate. The presence of surfactant in di luting media was crucial since Bacillus cereus spore is the most hydrophobic among Bacillus species and tends to agglomerate in water [137]. Experimental studies showed that the coefficient of variation (CV) of Bacillus cereus CFU was maintained below 10% when 2 mM of sodium dodecyl sulfate was added to the diluting media (phosphate buffered saline solution ). The plated dishes were incubated at 35°C for 12 hours. Electrospun Photocatalytic Nanofibers An electrospinning set-up was built and the components were a power source capable of creating an electric field, a counter electrode, a vi scous solution, and a syringe pump (Fig 2-9). In this study, Ti(IV)-isopropoxide (Ti(O i Pr)4, Aldrich) was used as an anatase precursor. One of the biggest advantag es of electrospinning pr ocess is that multiStirrer Air Flow UV Lamps Spore suspension mixed with photocatalyst


42 component composite system is readily ac complished simply by mixing different sols. Small amount of silver nitrate solution (AgNO3, Aldrich) or functionalized multi-walled carbon nanotubes were mixed with Ti(O i Pr)4-ethanol solution in order to electrospin silver or carbon nanotube embedded TiO2 nanocomposite fibers. 1.5 mL Ti(O i Pr)4, and 3 mL of acetic acid were added to 10 mL of ethanol containing poly(vinyl pyrrolidone) (PVP, Aldrich, Mw 1,300,000). The sol was stirred in a capped bottle for 30 minutes. Subsequently, 50-200 mg of silver nitrate so lution was added and th e sol was stirred for another 30 minutes. The prepared sol was ej ected from the conductive stainless steel capillary (22 gauge) at a flow rate of 0.5 cc/ hour with an electric fi eld of 2 kV/cm, where the voltage and distance were set to 10 kV, and 5 cm respectively. Commercial aluminum foil was grounded and used to collect a prec ursor composite nanofiber mat. Electrospun nanofibers were also collected on diverse s ubstrates, such as glass fiber filters and anodized mesoporous alumina. No significant e ffect of the substrates was observed. The collected samples were dried in air for one da y to reduce the impact of stress in the fiber by synereses. As a final step, the sa mple was heat treated in air at 500 C for 2 hours in order to remove PVP and to crystallize TiO2. Single component (TiO2) nanofibers were also prepared for comparison. Several methods have been performed to determine the mechanical properties of polymeric nanofibers and carbon nanotubes incl uding observing the thermally or electric field induced vibration with transmissi on electron microscopy (TEM) [138,139], and measuring the lateral bending or axial comp ression with atomic force microscopy (AFM) [108,140-142]. The former method was limited due to reliable detection of vibrations by TEM, thus restricting the sample size. The latter one, which enabled the direct


43 determination of the applied fo rce as a function of displacem ent, can be used by applying a nanoscale three-point bending te st with AFM. Elastic properties can be measured with three-point bend test by preparing a single nanofiber suspended over an empty place and applying a small deflection at the middle of th e nanofiber along its suspended length with an AFM cantilever in contact mode. The inde ntation of the AFM probe into a sample surface offers a direct approach to determin e elastic properties by measuring the sensor response to the z-piezo displacement. Dimension 3100 Scanning Probe microscope (SPM, Veeco Instruments) was used to appl y a load at the middle of the suspended nanofiber on the alumina membrane (Whatman Anodisc 13, pore size 0.2 µm) with a silicon nitride cantilever. Figure 2-9. Flowchart of electrosp inning of photocatalytic nanofibers Sol preparation Ti(O i Pr)4 as TiO2 precursor (with AgNO3 or carbon nanotubes) Heat Treatment at 500 C for >2 hours Drying at 25 C for 1day Electrospinning Anatase nanofibers (with Ag or carbon nanotubes)


44 Figure 2-10. Schematic diagram of electrospinning Figure 2-11. Schematic diagram of AFM three point bending test on electrospun polycrystalline nanofiber kV Jet region Instability (whipping) region Taylor cone Pore diameter (L) Nanofiber Si3N4 pyramidal tip (F)


45 CHAPTER 3 RESULTS AND DISCUSSION Material Characterization TiO2 Nanocoated Carbon Nanotubes Either anatase or rutile nanocoated mu lti-walled carbon nanotubes were prepared via a sol-gel process. The functionali zed carbon nanotubes were used and the morphology is shown in Figure 3-1. After each nanocoating process, the thickness of TiO2 coating layer was less than 5 nm for an atase phase and less than 10 nm for rutile phase. Because both the TiO2 surface to the bulk ratio a nd the interface to the bulk ratio are very high, it was expected for the photoge nerated electron to be easily and quickly trapped at the interface or diffuse to the core carbon nanotube graphene layers. The remaining holes at the valance band al so could rapidly diffuse to the TiO2 coating surface for the hydroxyl radical to be formed also because of the high surface to bulk ratio. A nitrogen adsorption isotherm method at 77 K was used to obtain the specific surface area of TiO2 nanocoated carbon nanotube and the BET (Brunauer – Emmett – Teller) specific surface area was ~172 m2/g. Thermogravimetry and differential thermal analyses (TGA/DTA, Netzsch STA 449C) under air flow showed that carbon nanotube started to decompose over 500 C (Figure 3-2). However, an exothermic peak near 700 C indicated that the decom position of carbon nanotube mainly occurred over 700 C in air. Carbon nanotubes were removed by ther mal oxidation under air to study the nanostructure by comparing Fourier transfor m infrared spectroscopy and x-ray diffraction data of TiO2 nanocoated carbon nanotube s with those of TiO2 coating layer. The field


46 emission gun scanning electron microscopy (FEGSEM) images of TiO2 nanocoated carbon nanotubes and TiO2 coating fragments are shown in Figure 3-2 (b). The morphologies of TiO2 fragments indicate that the carbon nanotube is required to retain the needle-like structure. Figure 3-1. TEM images of surface f unctionalized multi-walled carbon nanotube The multi-walled carbon nanotube (002) reflection overlapped anatase (101) or rutile (110) reflections and the intensity of carbon nanotube (002) was much higher than that of TiO2 peaks because of a rela tively large amount of carbon nanotube. In order to deconvolute TiO2 reflections from carbon nanotub e reflection, multi-walled carbon nanotubes were burnt out at 750 C and residual TiO2 coating was analyzed by XRD. The XRD pattern obtained from the sol-gel nanocoa ting process using titanium sulfate as a TiO2 precursor (Figure 3-3) clear ly indicates the coating la yer was anatase before the carbon nanotube removal at 750 C. Rutile phase is thermodynamically more stable form of TiO2, and, consequently, anatase can transform to rutile but rutile cannot transform to anatase by heat treatment in an ambient condi tion. When titanium tetrachloride was used


47 as a precursor, rutile coating layer was obtai ned and the XRD pattern is given in Figure 3-4. The nanostructure of TiO2 nanocoated carbon nanotube were analyzed with high resolution transmission electron micros copy (HRTEM, JEOL 2010F). There was no significant morphology difference in the HRTEM images between the anatase or rutile nanocoating layer and the atomic structure of TiO2 nanocoating layer was also confirmed by selected area diffraction patt erns (Figure 3-5 and 3-6). The chemical and structural properties of anatase nanocoated multi-walled carbon nanotubes were analyzed with Fourier Transf orm Infrared spectroscopy (FTIR). In FTIR spectra (transmission mode) the absorption bands of functional groups created on the carbon nanotube surfaces were observed (Figur e 3-7). However, the high absorbance of the nanotubes necessitated a very low sample concentration and theref ore there was likely to be significant water contamination. It has been generally al so accepted that the interpretation of surface groups can only be qualitative, as they cannot be expected to behave as isolated functional groups. Absorption bands at 1550 cm-1 and 1200 cm-1 can be assigned to the carbon nanotube skeleton. The signature of >C=O functional groups is evident at 1750 cm-1 and –OH functional groups appear at 3500 cm-1. Absorption bands of O–H bending of carboxyl groups ar e not significantly evident at 1400 cm-1 probably because of the convolution with the abso rption band of phenolic groups and strong absorption of the carbon nanotubes. The intens ities of these bands became very weak after the heat treatment because of the consumption of the functional groups. The weakened absorption bands were the sign of the completion of the covalent bonding between the carbon nanotube and the TiO2 nanocoating layer. The hump at 500 cm-1 signified the amorphous TiO2 phase in Figure 3-7 (a). When it was crystallized, the split


48 of the band into two sharp transmission bands was observed in Figure 2-7 (b) and (c). The shoulder at 1100 cm-1 became shaper when the carbon nanotubes were removed. This indicated that most likely Ti-O-C ab sorption band was present at the spectrum in Figure 3-7 (b). [86] Figure 3-2. SEM images (a) anatase na nocoated carbon nanotubes and (b) anatase coating fragments after carbon nanotube burn-out, and (c) TGA/DTA analysis 0100200300400500600700 80 85 90 95 100 Temperature (C)TG (%)TGA DTA0 1 2 3 4 5 DTA (mW/mg)exo (a) (b) (c)


49 Figure 3-3. XRD pattern of anatase na nocoating fragments after carbon nanotube removal Figure 3-4. XRD pattern of rutile nanocoa ting fragments after carbon nanotube removal 203040506070 Intensity (a.u.)2 (deg)(101) (004) (200) (105)(211) (204)203040506070 Intensity (a.u.)2 (deg)(110) (101) (111) (211) (220) (002)(310)


50 Figure 3-5. TEM images of (a), (b) indi vidual, (c) agglomerated anatase nanocoated carbon nanotubes, and (d) SAD pattern (a) (b) (c) (d)


51 Figure 3-6. TEM images of (a), (b) individua l, (c) agglomerated rutile nanocoated carbon nanotubes, and (d) SAD pattern (a) (b) (c) (d)


52 Figure 3-7. FTIR spectra of anatase na nocoated carbon nanotubes (a) before heat treatment, (b) after heat treatment (500 C, 3 hours), and (c ) anatase coating layer (carbon nanotubes remove d by thermal oxidation at 750 C) 200016001200800400 200016001200800400 (a) (b)TransmissionWavenumber (cm-1)(c)


53 Characterization of Electros pun Photocatalytic Nanofibers Anatase nanofibular mesostructures with various components were successfully obtained. The morphologies and the nanostructu res of anatase, anat ase-silver (Ag), and TiO2 (anatase)-(multi-walled) carbon nanotube were studied with FEGSEM and HRTEM. TiO2 (anatase phase) crystallization wa s completed by a heat treatment at 500 C for longer than 2 hours in air and polym eric matrix was simultaneously removed by thermal oxidation. After the heat treatment , the anatase grain size ranged from 10 to 20 nm. In anatase-Ag nanocomposite fibers, anat ase grain size was similar to that of the anatase nanofiber. The silver nanoparticle s were individual entities in the TiO2 nanofiber and they have a mean diameter of 10 nm. Anatase – multi-walled carbon nanotubes were obtained basically by dispersing functionali zed multi-walled carbon nanotubes in the viscous liquid containing TiO2 precursor. All the electros pun nanofibers were randomly oriented and entangled to form a mat char acterized by a nonwoven textile structure. The mats were collected either on aluminum foil for electron microscopy analyses or on mesoporous aluminum oxide membrane (Whatman Anodisc) for atomic force microscopy (AFM) three point bending test. Carbon nanotubes can be dispersed and so lubilized by wrapping with poly(vinyl pyrrolidone) (PVP) even without functi onalization of their hydrophobic graphene surface. PVP was selected for the electrospi nning process because of its good solubility in alcohols and water and compatibility with TiO2 precursors even when its molecular weight was up to M w 1,300,000. Smalley et al. proposed that the wrapping of the carbon nanotubes by (water-)soluble polymers was a general phenomenon, driven largely by a thermodynamic drive to eliminate the hydrophob ic interface between the tubes and their


54 medium [143]. Solubilization with near m onolayer coverage of associated PVP around individual carbon nanotubes might occur as helical wrapping. Multi-helical wrapping might also take place allowing high surface ar ea coverage with low backbone strain of PVP. It was also reported that high molecu lar weight polymer could disperse individual carbon nanotubes. Cryo-HRTEM images of r ope-to-single-tube transition due to adsorption of polymer confirmed that the a gglomeration of carbon nanotubes could be effectively avoided [144]. In this study, functionalized carbon na notubes were used to enhance the chemical reactivity in order for anatase phase to be chemically bonded with carbon nanotube surfaces for not only the enha ncement of the photocat alytic efficiency, but also the mechanical reinforcemen t of continuous ceramic nanofibers. X-ray diffraction patte rn confirmed TiO2 had anatase phase after heat treatment in TiO2, TiO2-Ag, and TiO2-carbon nanotube nanofibers (Fig ure 3-8 and 3-11 (c)). The PVP-anatase TiO2 nanofiber in electron microscopy im ages confirmed that the fiber was continuous and nonwoven. Anatase crystalline nanoparticles did not appear clearly in HRTEM analysis indicating that the anatas e nanoparticles were embedded in the PVP fibular matrix. The nonwoven mat was easily peel ed off from the aluminum foil after the drying and calcinations process. The grain size was typically less than 20 nm in the HRTEM image (Figure 3-10 (d)). Similar morphology was observed in anatase – carbon nanotube composite nanofiber samples (Figur e 3-11). The anatase nanofiber decorated with silver nanoparticles was analyzed with energy dispersive x-ray spectroscopy (EDS). The darker region in HRTEM image was confirme d as individual or agglomerated silver nanoparticles (Figure 3-12 (c) and (d)).


55 Figure 3-8. XRD patterns (a) TiO2 nanofibers and (b) TiO2-Ag nanofibers 203040506070 Intensity (a.u.)2deg)(101) (111) (112)Silver Anatase(200) (200)(105) (006) (204)(220)(a) (b)


56 Figure 3-9. Electron micr oscopy images of PVP-TiO2 continuous nanofibers (a-c) SEM and (d) TEM (a) (b) (c) (d)


57 Figure 3-10. Electron microscopy images of TiO2 continuous nanof ibers (a-c) SEM and (d) TEM (a) (b) (c) (d)


58 Figure 3-11. Electron microscopy images and XRD pattern of TiO2-carbon nanotube continuous composite nanofibers (a) SEM, (b) TEM, and (c) XRD (a) (b) 203040506070 Intensity (a.u.)2deg)(101) (111) (200) (105) (006) (204) AnataseCarbon Nanotube(c)


59 Figure 3-12. Electron micr oscopy images and EDS spectra of TiO2-Ag continuous composite nanofibers (a, b) SEM, (c) TEM image, and (d) EDS (a) (b) (c) (d)


60 Dye Degradation Test The anatase coating layer was very thin (characteristically less than 5nm) and, consequently, it was expected for carbon nanotube core to help TiO2 layer absorb visible light due to its black color. As a prelimin ary experiment before the sol-gel anatase nanocoating on functionalized carbon nanotube s, anatase nanoparticles with mean diameter of 5 nm were mixed with func tionalized carbon nanotubes. The mixture with various amount of carbon nanotubes were disper sed in dye solution (5 mg/L of Procion MX-5) under UV-A irradiation. 1 mg of carbon nanotube with 3 mg of anatase nanoparticles showed the best photocatalytic dye degradation result destroying up to 95% of organic dye molecules within 80 minutes . When more carbon nanotubes were added, slower dye degradation rate was observed b ecause of screening of UV-A irradiation by carbon nanotubes. Carbon nanotubes could be coupled with the anat ase nanoparticles and the photocatalytic efficiency was enhanced . It was expected for carbon nanotubes to effectively trap electrons photogenerated in the TiO2 nanocoated carbon nanotube system. However, the excessive amount of carbon nanotubes could block the UV-A irradiation and, consequently, the photocat alytic reaction, such as photogeneration of charge carriers, could be hindered. Therefore, the carbon nanotube core-TiO2 shell structure was desired to avoid the screeni ng effect by carbon nanotubes. In the core-shell system, the carbon nanotube core may play a role of reservoir for photogenerated electrons and the charge carrier recombina tion can be effectively avoided during the photocatalysis. It was also attempted to de grade dye molecules by photoinduced charging and dark discharging mechanism because hydrogen peroxide (H2O2) can be formed when the trapped electrons by the car bon nanotube core are discharged.


61 Figure 3-13. UV-Vis spectra of (a) anat ase nanocoated carbon nanotubes and (b) Degussa Aeroxide® P25 dispersed in the dye solution without irradiation 400450500550600650700750800 AbsorptionWavelength (nm)400450500550600650700750800 AbsorptionWavelength (nm)(a) (b)


62 Figure 3-14. Dye degradation by anatase – carbon nanotube mixtur e as a function of carbon nanotube amount 020406080 0.0 0.2 0.4 0.6 0.8 1.0 Anatase (5 nm) with 0 mg CNT 1 mg CNT 2 mg CNT 3 mg CNTC/C0Time (min) 020406080 0.01 0.1 1 C/C0Time (min) Anatase (5 nm) with 0 mg CNT 1 mg CNT 2 mg CNT 3 mg CNT(a) (b)


63 Photocatalysis with UV-A Irradiation It was attempted to degrade dye molecule s with photocatalytic reaction with UV-A irradiation. Neither commer cial rutile nanoparticles (Ishihara TTO-51A) nor rutile nanocoated carbon nanotubes showed any eff ectiveness in photodegradation of dye molecules because the photoreactivity of rutile phase is much lower than anatase phase because of the higher electron-hole recombinat ion rate on rutile due to lower capacity to adsorb O2. ~90% and >99% of dye degradati on was achieved with Degussa Aeroxide® P25 and anatase nanocoated carbon nanotubes, respectivel y, with UV-A irradiation within 70 minutes (Figure 3-15). It must be noted that the anatase nanocoated carbon nanotube had more than three times larger specific surface area approximately than Degussa Aeroxide® P25. Therefore, only 0.8 mg of anatase nanocoated carbon nanotubes and 3 mg of Degussa Aeroxide® P25 were used based on the assumption that the photodegradation occurs at the surface of photocatalysts. High er photocatalytic reaction in anatase nanocoated carbon nanotubes denoted effective electron trapping by the carbon nanotube core. The interfacial area (contac t area between photocatalyst and electron scavenger) is also important in trapping of photogenerated electron. Higher interfacial area between TiO2 and carbon nanotube due to the high aspect ratio could provide more electron scavenging through car bon nanotube and the charge carrier recombination can be effectively retarded. The dye degradation ra te was well fitted with the first order of exponential decay curve (Figure 3-16). The time required for 99 % of dye degradation by Degussa Aeroxide® P25 was obtained by extrapolati ng the data and was 131 minutes while the anatase nanocoated carbon nanotube s achieved 99 % dye degradation within 70 minutes.


64 Figure 3-15. Direct comparison of dye degradations by anatase nanocoated carbon nanotubes, rutile nanocoated car bon nanotubes and Degussa Aeroxide® P25 with UV-A irradiation 010203040506070 0.0 0.2 0.4 0.6 0.8 1.0 C/C0Time (min) Rutile nanocoated carbon nanotubes Degussa P25 Anatase nanocoated carbon nanotubes 010203040506070 0.01 0.1 1 C/C0Time (min) Rutile nanocoated carbon nanotubes Degussa P25 Anatase nanocoated carbon nanotubes(a) (b)


65 Figure 3-16. Curve fitting (the first order of exponential decay) and extrapolation of dye degradation data by anatase nanoc oated carbon nanotubes and Degussa Aeroxide® P25 with UV-A irradiation Degussa Aeroxide® P25 is basically a (wide bandgap semiconductor – wide bandgap semiconductor, rutile – anatase) co mposite material and there is a barrier between two different phases due to band bending (Figure 3-17 (a)) [123,124]. Electrons in valence bands of each phase are excited with UV irradiation and it is very difficult for photogenerated electrons flow through the barr ier. However, holes can migrate from anatase to rutile, and if those holes re act with the hydroxyl ions at the surface, photocatalytic oxidation may occur. Because of the charge separation, the recombination rate is slower than pure anatase or rutile particles, but there is still a chance for photogenerated electrons in the conduction ba nd of rutile recombine with holes before 020406080100120140 0.01 0.1 1 C/C0=e-t/16C/C0Time (min) Degussa Aeroxide P25 Anatase nanocoated carbon nanotubeC/C0=e-t/29


66 photocatalytic reactions at th e rutile surfaces. Oxygen is re duced by the electrons at the anatase surface. Figure 3-17. Schematic band diagrams and ch arge carrier separation mechanisms of (a) hole trapping in Degussa Aeroxide® P25 and (b) electron trapping in anatase nanocoated carbon nanotubes Rutile Anatase Valence Band Conduction Band Carbon Nanotube Anatase Conduction Band (a) (b) Valence Band


67 Higher efficiency of anatase nanocoated carbon nanotube can be also explained by a band bending when a multi-walled carbon nanotube coupled with the anatase phase (Figure 3-17 (b)). In this system, photoge nerated electrons in the conduction band of anatase flow through the conductiv e carbon nanotube and the holes in the valence band of anatase can generate hydroxyl radicals at th e surface. The carbon nanotube core – anatase shell structure can minimize the number of ch arge carrier recombination events and, consequently, maximize the number of hydr oxyl radicals at the anatase surface. Photocatalysis with Visible Li ght and Post UV-A Reaction A goal for many researchers in the field of photochemistry has been to modify TiO2 to reduce its bandgap energy so it can abso rb sunlight over a broader range of wavelengths. The utilization of TiO2 photocatalysts requires UV irradiation and, therefore, only makes use of 3-5% of the sunl ight that reach the earth. To establish clean and safe photocatalytic systems using the mo st chemically and environmentally ideal energy source, solar energy, the development of photocatalysts whic h can operate under visible light is crucial. Anpo et al. have developed a photocatalyst, a metal ion-implanted (V, Cr) TiO2 which enables the absorption of visible light up to a wavelength of 500-550 nm [145]. This was used for the decomposition of NOx into N2 and O2 using only outdoor sunlight. Asahi et al. reported that yellowish TiO2-xNx films prepared by sputtering and a heat treatment showed photocatalytic activit ies under visible light (wavelength < 500 nm) [146]. They observed the photodegradations of methylene blue and gaseous acetaldehyde and hydrophilicity of the film surface because of the reduction of the band-gap. Sulfur doped TiO2 also showed photocatalytic dye degrada tion under visible light irradiation. In 2003, carbon doped TiO2 powders, which were also visible light sensitive, were reported [147,148]. Carbon-doped TiO2 powders were synthesized e ither by the thermal oxidation


68 of TiC powders under O2 flow or by the hydrolysis of titanium tetrachloride with tetrabutylammonium hydroxide followed by cal cinations. It was claimed that the carbon doped TiO2 could break down dissolved pollutants even in diffuse interior daylight. Figure 3-18. Photocatalytic dye degradat ion by anatase nanocoated carbon nanotubes with visible light irradiation The dye degradation by the anatase nanocoa ted carbon nanotube with visible light irradiation was observed but there was no sign of photocat alysis by Degussa Aeroxide® P25 and rutile nanocoated carbon nanotube in the same experimental condition (Figure 318). The >50% of photodegradation with vi sible light irradiat ion by the anatase nanocoated carbon nanotube was achieved within 120 minutes. When wide bandgap semiconductor is coupled with conductive mate rials, the bandgap re duction occurs and, consequently, the photon energy of visible light is sufficient to exci te electrons in the 020406080100120 0.4 0.6 0.8 1.0 C/C0Time (min)


69 valence band of semiconductor. In Figure 3-13 (a) anatase nanocoated carbon nanotube absorb visible light because of the black carbon nanotube core. The anatase nanocoated carbon nanotube had higher re lative efficiency in the photodegradation test by UV-A irradiation. Moreover, highly energy efficien t photocatalysis will be possible because the anatase nanocoated carbon nanotubes ar e reactive with less photon energy. Figure 3-19. Post UV-A dye degradati on by anatase nanocoated carbon nanotubes Kamat et al. reported similar results in a silver core-anatase shell system. They observed charging of the silver core with electrons under UV excitation [149]. Discharging of electrons was also monito red from the shift in maximum plasmon absorption. They claimed the maximum number of electrons that could be stored in each silver core was dictated by the Fermi level shif t that was necessary to equilibrate with the anatase Fermi level. Excellent photodegradatio n efficiency of anat ase nanocoated carbon nanotubes confirm charging of carbon nanotube co re with UV-A irradiation as well. Post 024681012141618 0.4 0.6 0.8 1.0 C/C0Time (103 min)


70 UV-A dye degradation was expected because of electron discharging from the carbon nanotube. Only with 5 minutes of UV-A i rradiation, the phot odegradation of dye molecules proceeds relatively slowly a nd >50% of dye degradation within 17.28 103 minutes and this result confirm the photoindu ced charging and dark discharging effects in anatase nanocoated carbon nanotube system (F igure 3-19). However, the characteristic time for the reduction of oxygen and the consequent H2O2 formation is a multi-step process and approximately much slower than that for the hydroxyl radical formation. Spore Inactivation Test under UV-A irradiation TiO2 photocatalysis has received widesp read attention for the microbial sterilization and ther e are a number of advantages incl uding strongly oxidizing reactions at the photocatalytic surface and the possibi lity of minimal production of disinfection byproducts. Under severe e nvironmental conditions Bacillus can produce spores with multiple protective layers (endospores). The nature of the complex, along with its dehydrated protoplast, and the extent of mi neralization account for the sporeÂ’s extreme resistance to many adverse physical and ch emical agents. The physical and chemical durability allows them to survive for a long ti me and to germinate when nutrients become available. The thick wall of spores is impermeable to most damaging agents. The degradation of the outer coat by radicals is not lethal to the subsequent germination of the de-coated spores. Nevertheless, once the protective coat is removed, a lethal event may occur. Therefore, in order for photocatal ysts to inactivate sp ores, photocatalytic efficiency must be high enough to destroy a ll the multiple protective layers of spores. Choi et al. reported that the inactivat ion rate of Coliform bacter ia is linearly proportional


71 to the OH concentration in the UV-A/TiO2 process [132]. Thus, the quantum yield of photocatalyst can be the direct measure of biocidal efficiency and vice versa. Each survival ratio was calculated base d on the CFU and normalized. >99 % of spores in the suspension were inactivated by anatase nanocoated carbon nanotubes while only ~ 76 % of spore inactivation was observed by Degussa Aeroxide® P25 within 2 hours (Figure 3-20). When rutile nanoparticle s (Ishihara TTO-51A) or rutile nanocoated carbon nanotubes were used, the increased number of CFU was observed because screening of UV irradiation occurred. The spor e inactivation rate also followed the first order of exponential decay. In order to obtain the time re quire for 99 % inactivation of spores by Degussa Aeroxide® P25, the data were fitted to the first order of exponential decay curve and extrapolated. For Degussa Aeroxide® P25 to inactivate 99 % of spores it could take 361 minutes while the anatase na nocoated carbon nanotubes inactivated more than 99 % within 120 minutes. 020406080100120 0.0 0.2 0.4 0.6 0.8 1.0 Survival RatioTime (min) Anatase nanocoated carbon nanotubes Degussa Aeroxide P25(a)


72 Figure 3-20. Photocatalytic endospor e inactivation by anatase nanocoated carbon nanotubes and Degussa Aeroxide® P25 with UV-A irradiation Figure 3-21. Curve fitting (the first order of exponential decay) and extrapolation of spore inactivation data by anatase na nocoated carbon nanotubes and Degussa Aeroxide® P25 with UV-A irradiation 020406080100120 0.01 0.1 1 Survival RatioTime (min) Anatase nanocoated carbon nanotubes Degussa Aeroxide P25(b) 060120180240300360 0.01 0.1 1 C/C0=e-t/19Survival RatioTime (min) Degussa Aeroxide P25 Anatase nanocoated carbon nanotubesC/C0=e-t/78


73 YoungÂ’s Modulus of Electrospun TiO2-Carbon Nanotube composite fibers TiO2 and TiO2-carbon nanotube continuous nanofibe rs with mean diameters of 53 and 63 nm, respectively, were prepared via a sol-gel method in combination with an electrospinning process. Elastic properties of individual nanofibers were calculated as measured by atomic force microscopy (AFM) three point bending test using a mesoporous aluminum oxide (Al2O3) membrane (Whatman Anodisc) as a support. Figure 3-22 shows the morphology of prepared sa mple. Figure 3-23 shows the actual AFM scanned data of the TiO2 sample fiber and the Al2O3 membrane pore. The pore diameter (L) and fiber diameter (D) was obtaine d by AFM scanning data (Table 3-1). A complete force curve contains four regi ons (Figure 3-24); (a) the tip was far from the sample, and there was no interaction and no cantilever deflection and longand shortrange tip-sample interactions caused the can tilever to deflect (snap-in) as the stage extends and brings the sample closer to the tip (b) when the tip contacts the surface, the stage movement and cantilever deflection beca me coupled, which appeared in the curve as a straight line (contact line) (c) at this point, the direction of the sample motion was inverted and the tip was withdrawn from the sample. (d) At a certain distance the tip was detached from the sample (snap-out) and the cantilever came back to its equilibrium position. The upper force limit was set to 6 nN. All experiments were performed in ambient conditions. The force was measured by collec ting a force curve, which was a plot of cantilever deflection, as a function of sa mple position along the z-axis. A reference cantilever deflection-piezo displacement curve was obtained by measuring on an Al2O3 substrate and a Si wafer. As shown in Figure 3-24, the deflection of the fiber ( ) is the difference between the loading and the reference curve, where z0 and d0 are for the


74 Figure 3-22. Images of sample nanofiber s (a) SEM image of nanofiber deposited on alumina membrane, (b) AFM image of nanofiber on alumina membrane, (c) TEM image of polycrystalline TiO2 electrospun fibers, and (d) TEM image of TiO2-carbon nanotube composite fibers reference and z and d for the test sample. It was assumed that a simple relationship between the force (F), and the defl ection given by Equation 3-2, where k is the spring constant of the cantilever. ) d (d ) z (z 0 0(3-1) ) d (d F0-k (3-2) (a) (b) (c) (d)


75 The elastic (YoungÂ’s) modulus (E) of nanofibers was calculated from beam bending theory given by Equation 3-3, where is the deflection of the beam at midspan, F is the applied force, L is the suspended length, E is the YoungÂ’s modulus, I is the second moment area of the beam, and =192 for a clamped beam. I L F E3 (3-3) where, 64 D I4 for a filled cylinder Figure 3-23. Actual AFM scanning data on (a) fiber and (b) pore; for ce (F) is applied at the middle of the fiber lying on a pore with a diameter (L) for three point bending (a) (b) F L


76 Figure 3-24. Actual AFM force curves of alumina substrate, Si wafer, and TiO2 nanofiber The mean YoungÂ’s modulus (E) of TiO2 nanofibers was 75.6 GPa, respectively. However, as seen from Table 3-1, the E value of TiO2 was considerably low compared to bulk TiO2 which is approximately 282 GPa. Th is difference can be explained by following reasons: (i) the effect of the crystal orientation, (ii) diffu sional creep at room temperature, and (iii) neglected shear deform ations and possible errors during the tests. As seen from Figure 3-25, the scatter in E values obtained from Equation 3-3, might be due to the applied point being a littl e off-center with respect to the beam span and the uncertainties in fiber dimensions and spring constant of the cantilever which affected the measured load. Although shear de formations were neglected in this study, the increase in fiber diameter significantly decreased E value. This could be attributed to the fact that at relatively low length-to-diameter (L/D) ratios shear deformations become an important contribution. -80 -40 0 40 80 120 160 D (nm)Z (nm) Alumina Si waferSample(a) (b) (c) (d)


77 Table 3-1. Diameters (D), suspended lengt hs (L), and Young’s moduli (E) of nanofibers Sample D (nm) L (nm) E (GPa) 53 323 64.6 50 349 64.0 46 252 124.6 42 332 72 51 280 61.3 72 286 52.0 59 342 70.0 61 371 61.3 53 294 70 TiO2 Nanofiber 38 276 116.6 Mean 53 9 311 36 75.6 23.2 26 252 78.4 39 216 133.7 55 228 442.4 33 246 266.7 91 276 63.7 57 222 357.3 23 344 102.4 124 352 52.2 72 297 22.9 CNT/TiO2 Nanofiber 113 245 48.9 Mean 63 34 268 46 156.9 138.8 represents standard deviation and CNT represents carbon nanotube AFM can be also used for the tensile stre ngth test of indivi dual nanofibers. Yu et al. performed the tensile loading experiment on multi-walled carbon nanotubes within a scanning electron microscope [150]. They obs erved that the carbon nanotubes broke in the outermost layer (sword-in-sheath failure), and the tensile strength of this layer ranged from 11 to 63 GPa for the set of 19 carbon nanotubes. The Young’s modulus of the outermost layer varied from 270 to 950 GPa. Forró et al. noted that their modulus measurements of carbon nanotubes in 1999 us ing AFM did not strongly depend on the diameter [108]. They claimed that the modulus of multi-walled carbon nanotubes correlates to the amount of disorder in the nanotube walls.


78 The YoungÂ’s moduli of anatas e-carbon nanotube composite fibers were scattered and composed of two groups. Higher E values were well matched to those of carbon nanotubes obtained by Yu et al. Thus, higher E values represented carbon nanotube reinforced region. Lower E values clearly corresponded to the an atase region without carbon nanotubes. The alignment of carbon nanotubes could be the main factor determining the mechanical properties. Ko et al. observed agglomerated nanostructure in polylactic acid (PLA)-(single-walled) carbon na notube and relatively well aligned carbon nanotubes in polyacrylonitrile (PAN)-(singl e-walled) carbon nanotube co-electrospun continuous fiber samples [151]. Significantly hi gher elastic modulus was calculated from the measurement of PAN-carbon nanotube nano composite fiber with AFM measurement. Therefore, to reinforce the electrospun co mposite nanofibers, the alignment of carbon nanotubes in the TiO2 nanofiber matrix must be thoroughly studied. Moreover, inhomogeneous transient concentration profile s could substantially affect the local rheological and other properties of the sols during the electrospi nning process. Carbon nanotubes might not be unifor mly distributed in the TiO2 matrix and, consequently, seven out of ten measurements show ed non-reinforcement of TiO2 nanofibers because carbon nanotubes were not dispersed perfectly (agglomeration could occur) and the low concentration of carbon nanotubes in the sol.


79 Figure 3-25. YoungÂ’s Modulus vs. diameter of TiO2 and TiO2-carbon nanotube fibers 20406080100120 0 100 200 300 400 500 E (GPa)Diameter (nm) TiO2 TiO2-carbon Nanotube


80 CHAPTER 4 CONCLUSIONS Two types of photocatalytic nanocomposites, particulat e and continuous nanofiber systems, were successfully synthesized for the photodegradation of organic contaminants (azo dye, Procion red MX-5B) and the inactivation of bacterial endospores ( Bacillus Cereus , anthrax surrogate). For the particulate system, anatase or rutile nanocoating on functionalized multi-walled carbon nanotubes were achieved via a sol-gel process. Anatase nanocoated carbon nanotubes and rutile nanocoated carbon nanotubes were prepared as a highly efficient photocatalytic nanoparticles system. Titanium sulfate for anatase and titanium tetrachloride soluti on for rutile phase were used as TiO2 precursor and multi-walled carbon nanotubes were functionalized through a chemical oxidation process prior to the sol-gel process. Wh en conductive carbon nanotubes were coupled with wide bandgap semiconductors (photocatalys ts) higher photocatal ytic efficiency was observed than the best photocatalyst (Degussa Aeroxide® P25) in the market. These nanoparticles were tested to determine the re lative efficiencies of photodegradation using Degussa Aeroxide® P25 as a standard material under UV-A irradiation. TiO2 nanocoated carbon nanotubes had more than three times larger specific surface area than Degussa Aeroxide® P25. The experiments were performed based on the same surface area and the experimental conditions were optimized for the best performance of Degussa Aeroxide® P25. Procion MX-5 was used and the con centration change of dye molecules was determined with UV-Vis spectroscopy. The anatase nanocoated carbon nanotubes showed a higher photodegradation rate than Degussa Aeroxide® P25 while the rutile


81 nanocoated carbon nanotubes and other commerci al rutile nanopartic les, Ishihara TTO 51-A, were not reactive in the same experimental condition. The outstanding performance of anatase nanocoated carbon nanotubes was because; (1) electron trapping occured at the interface between the anatase shell layer and the carbon nanotube core and, consequently, the electron – hole recombina tion was greatly retarded; (2) the anatase shell and the carbon nanotubes were chemi cally bonded (good contact between a shell and a core); (3) the interface area was large; and (4) photon absorption was higher in the presence of carbon nanotube core. The photodegradation of azo dye molecules was also performed under visible light irradiation. There was no significant change in dye concentration when the rutile nanocoated carbon nanotubes and Degussa Aeroxide® P25 were used. The covalent bonding between the carbon nanotube core a nd the anatase shell could modify the electronic structure of th e photocatalytic shell (TiO2 nanocoating layer) so that visible light irradiation could excite the electrons of the valence band of anatase shell. Moreover, UV-Vis spectra confirmed that the spectrum in the visible light region was absorbed by the anatase nanocoated carbon nanotube disper sion because of the black carbon nanotube core. The photodegradation of azo dye mol ecules under visible light occurred in the presence of the anatase nanocoated carbon na notubes, but the efficiency was lower than the photodegradation under UV-A. Moreove r, the post UV-A photodegradation was observed in the anatase nanocoated carbon nano tubes after a five minute exposure to UVA irradiation. The photogenerated electrons could be accumulated at the carbon nanotube core during photocatalysis under UV-A. Because discharging of trapped electrons could generate H2O2 for dye degradation, H2O2 formation by discharged electrons might


82 contribute to the degradation of dye molecules. However, the post UV-A dye degradation in the dark occured very slowly. Approximately 50% dye degradation by the anatase nanocoated carbon nanotubes in the post UV-A experiment was achieved after 10 days. After 10 days no progress was observed probabl y due to the depletion of photogenerated electrons. Bacterial endospores ( Bacillus cereus ) prepared by lysozyme and heat treatments were used as a surrogate of anthrax spores for the spore inactivation test. The anatase nanocoated carbon nanotubes effectively inactiv ated the spores w ith UV-A irradiation faster than Degussa Aeroxide® P25. The increased number of CFU is observed because of the UV screening effect when the sa me surface area of rutile nanocoated carbon nanotubes or Ishihara TTO 51-A was used. The syntheses of anatase, anatase-silv er, and anatase-carbon nanotube nanofibers were successfully demonstrated for the continuous (composite) nanofiber system via electrospinning. Three point bending tests of the electrospun con tinuous fiber (anatase and anatase-carbon nanotube) were performed with atomic force microscopy. The mean Young’s modulus (E) of anatase nanofi bers (grain size 10-20 nm) was 75.6 23.2 GPa and significantly differe nt from E of bulk TiO2 (282 GPa) probably because of the diffusional creep in room temperature. E of anatase-carbon nanotube fibers was scattered and composed of two groups. Higher E va lues (>250 GPa) might represent carbon nanotube reinforced region and were well matc hed to the data in the reference while lower E values (<100 GPa) clearly corres ponded to the anatase region without carbon nanotubes. The AFM three point bending result s opened up the possibility of mechanical reinforcement of electrospun TiO2 nanofibers by carbon nanotubes.


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95 BIOGRAPHICAL SKETCH Sung-Hwan Lee was born in Seoul, Korea, on March 7, 1971. He was raised there with two sisters and graduated from Yoido High School. He enetered Inha University (Incheon, Korea) in the Department of Cera mic Engineering with a scholarship in 1990. After his sophomore year, he served in an artillery regiment in the Korean military service for more than two years. Then he re turned to the school, earning a Bachelor of Science degree in February, 1997. He then cont inued his education at the New York State College of Ceramics (NYSCC) at Alfred University (Alfred, New York) studying amorphous SiO2 materials with Dr. Alexis Clare. He was awarded for an outstanding academic performance and earned a Master of Science in materials science in April, 1999. He transferred to the University of Fl orida (Gainesville, Florida) in pursuit of a Doctor of Philosophy degree in the Department of Materials Science and Engineering. He earned the degree in December, 2004 specializin g in ceramics. He was very fortunate to have had great academic opportunities with Drs. Wolfgang Sigmund and Brij Moudgil at the Particle Engineering Research Center (PERC) at the Univer sity of Florida and is very grateful for all those who helped him become who he is today.