photocatalytic nanocomposites based on tio2 and carbon nanotubes

PHOTOCATALYTIC NANOCOMPOSITES BASED ON TiO2 AND CARBON NANOTUBES By SUNG-HWAN LEE A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2004 Copyright 2004 by SUNG-HWAN LEE This document is dedicated to my wife, and daughter with love ACKNOWLEDGMENTS I would like to sincerely thank Dr Wolfgang Sigmund for serving as my adviser and giving me the opportunity to perform this research Additionally I would like to acknowledge and thank Drs Brij Moudgil and Ben Koopman for giving me advice on many occasions and their fruitful conversations Many of the achievements during my doctoral research would not have been possible without their excellent guidance and support I also would like to acknowledge the Particle Engineering Research Center (PERC) for the financial support I would like to thank Drs Dinesh Shah, Ellsworth Whitney, Susan Sinnott, Darryl Butt for serving as my advisory committee I would like to thank PERC graduate students, 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 iv TABLE OF CONTENTS page ACKNOWLEDGMENTS iv LIST OF TABLES vii LIST OF FIGURES viii ABSTRACT xi INTRODUCTION Titanium Dioxide (TiO2) Photocatalysis Photocatalytic Disinfection of Biological Contaminants .8 Design and Synthesis of Highly Enhanced Photocatalyst System .13 Design of TiO2-Carbon Nanotube System 13 TiO2 Nanocoating on Carbon Nanotubes 19 Electrospinning of Photocatalytic Nanofibers 21 EXPERIMENTAL AND METHODOLOGY 25 Experimental Parameters in Photocatalytic 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 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 v Young’s Modulus of Electrospun TiO2-Carbon Nanotube composite fibers .73 CONCLUSIONS 80 LIST OF REFERENCES 83 BIOGRAPHICAL SKETCH .95 vi LIST OF TABLES page Table 1-1 Bandgap energy of various photocatalysts 1-2 Primary process and characteristic time of TiO2 photocatalysis in H2O 1-3 Modes of microorganism removal or inactivation action for various disinfection methods .12 1-4 Work functions of noble metals and carbon materials 19 3-1 Diameters, suspended lengths, and Young’s moduli of nanofibers 77 vii LIST OF FIGURES page Figure 1-1 Schematic diagram; overall process of semiconductor photocatalysis in an aqueous system .7 1-2 Schematic diagram; photogeneration of charge carriers in TiO2 and electron trapping by fullerene (reduction of C60) 16 1-3 Schematic diagram; photogeneration 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 .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 ζ potential of as received and 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 photocatalytic 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) bacteria, and (c) structure of endospore: core; cellular components, DNA, UV resistance, cortex; heat resistance, peptidoglycan, ~200 nm, inner spore coat: acid resistant proteins, 20-40 nm, outer spore coat; alkali resistant proteins, 40-90 nm 38 2-8 Experimental setup for spore inactivation test 41 2-9 Flowchart of electrospinning 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 viii 3-1 TEM images of surface functionalized multi-walled carbon nanotube 46 3-2 SEM images (a) anatase nanocoated carbon nanotubes and (b) anatase coating fragments after carbon nanotube burn-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 fragments 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 rutile nanocoated carbon nanotubes, and (d) SAD pattern .51 3-7 FTIR spectra of anatase nanocoated carbon nanotubes (a) before heat treatment, (b) after heat treatment (500°C, hours), and (c) anatase 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 images 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 spectra of TiO2-Ag continuous composite nanofibers (a, b) SEM, (c) TEM image, and (d) EDS 59 3-13 UV-Vis spectra of (a) anatase nanocoated 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 degradations 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 exponential decay) and extrapolation of dye degradation data by anatase nanocoated carbon nanotubes and Degussa Aeroxide® P25 with UV-A irradiation .65 ix 3-17 Schematic band diagrams and charge carrier separation mechanisms 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 endospore inactivation by anatase nanocoated carbon nanotubes and Degussa Aeroxide® P25 with UV-A irradiation 72 3-21 Curve fitting (the first order of exponential decay) and extrapolation of spore inactivation data by anatase nanocoated 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 x 81 nanocoated carbon nanotubes and other commercial rutile nanoparticles, 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 recombination was greatly retarded; (2) the anatase shell and the carbon nanotubes were chemically 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 and the anatase shell could modify the electronic structure of the 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 dispersion because of the black carbon nanotube core The photodegradation of azo dye molecules under visible light occurred in the presence of the anatase nanocoated carbon nanotubes, but the efficiency was lower than the photodegradation under UV-A Moreover, the post UV-A photodegradation was observed in the anatase nanocoated carbon nanotubes 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 probably 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 inactivated the spores with UV-A irradiation faster than Degussa Aeroxide® P25 The increased number of CFU is observed because of the UV screening effect when the same surface area of rutile nanocoated carbon nanotubes or Ishihara TTO 51-A was used The syntheses of anatase, anatase-silver, and anatase-carbon nanotube nanofibers were successfully demonstrated for the continuous (composite) nanofiber system via electrospinning Three point bending tests of the electrospun continuous fiber (anatase and anatase-carbon nanotube) were performed with atomic force microscopy The mean Young’s modulus (E) of anatase nanofibers (grain size 10-20 nm) was 75.6 ± 23.2 GPa and significantly different 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 values (>250 GPa) might represent carbon nanotube reinforced region and were well matched to the data in the reference while lower E values (