ENHANCEMENT OF TITANIUM DIOXIDE PHOTOCATALYSIS WITH POLYHYDROXY
FULLERENES
By
VIJAY B. KRISHNA
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
2007
1
© 2007 Vijay B Krishna
2
To my parents, B.S. Krishna and B.K. Saroja, for their unwavering encouragement, support and
motivation to pursue my dreams
3
ACKNOWLEDGMENTS
I would like to express my sincere gratitude to my advisor Dr. Brij Moudgil and my cochair Dr. Ben Koopman for their valuable guidance, intellectual stimulation and support
throughout this study. Their virtues of critical assessment and raising the bar for achievement
helped me expand my potential. I would like to thank Dr. Wolfgang Sigmund for his invaluable
suggestions and fruitful discussions. I would also like to express my sincere gratitude to Dr.
Seymour Block and Dr. Sam Farrah, who introduced me to the exciting field of microbiology. I
would like to thank my committee members Dr. Chris Batich, Dr. Hassan El-Shall and Dr.
Laurie Gower for their time, guidance and constructive comments. I would also like to thank
National Science Foundation (NSF Grant EEC-94-02989), Particle Engineering Research Center
(PERC) and its industrial partners for financial support.
Appreciation is also extended to PERC staff members Dr. Vic Jackson, Dr. Kevin Powers,
Gill Brubaker, Gary Scheiffele, Greg Norton, Jo-Anne Standridge and Donna Jackson for their
assistance throughout the course of this study. I am also thankful to Dr. Kerry Siebein, Lynda
Schneider, Chuck Garretson and members of Water Reclamation Facility for their help.
I gratefully acknowledge my colleagues Scott Brown, Madhavan Esayanur, Suresh
Yeruva, Kyoung-Ho Bu, Yunmi Kim, Amit Vohra, Jue Zhao, Steve Tedeschi, Parvesh Sharma,
Rhye Hamey, Georgios Pyrgiotakis, Maria Palazuelos, Anna Fuller, Ivan Vakarelski, Marco
Verwjis, Dauntel Specht, Smithi Pumprueg, Sajit Daosukho, Dushyant Shekawat, Marie
Kissinger, Witcha Imaram, Gautam Kini, Ashutosh Agrawal, and Amit Singh for their valuable
insights, intellectual discussions, friendship and moral support.
Finally, I would like acknowledge my parents, who encouraged me to pursue my dreams.
4
TABLE OF CONTENTS
page
ACKNOWLEDGMENTS ...............................................................................................................4
LIST OF TABLES...........................................................................................................................7
LIST OF FIGURES .........................................................................................................................8
ABSTRACT...................................................................................................................................10
CHAPTERS
1
INTRODUCTION AND BACKGROUND ...........................................................................12
General Overview of Inactivation Techniques .......................................................................13
Photocatalysis with TiO2 ........................................................................................................16
Modification of Electronic Properties of Titanium dioxide ...................................................18
Microbial Inactivation by Photocatalysis ...............................................................................20
Structure of Bacterial Endospores and Vegetative Bacterial Cells .................................20
Mechanism of Photocatalytic Inactivation ......................................................................21
Carbon Nanotubes and Fullerenes ..........................................................................................22
2
MATERIALS AND METHODS ...........................................................................................30
Synthesis of TiO2 Coated Multi-wall Carbon Nanotubes.......................................................30
Culturing and Purification of Bacillus Spores ........................................................................31
Culturing of Escherichia coli..................................................................................................31
Experimental Apparatus .........................................................................................................31
Photocatalytic Inactivation of Bacillus Spores .......................................................................31
Photocatalytic Inactivation of Escherichia coli......................................................................32
Optimization of PHF Concentration with Dye Degradation Experiments .............................33
Adsorption Experiments .........................................................................................................34
HR-TEM Imaging of PHF-TiO2 Nanocomposite...................................................................36
Electron Paramagnetic Resonance Spectroscopy for Hydroxyl Radical Determination........36
Synthesis of Polyhydroxy Fullerenes .....................................................................................37
Dye Degradation Experiments with Synthesized PHF...........................................................38
Characterization of PHF .........................................................................................................38
Mass Spectroscopy ..........................................................................................................38
Gaussian Modeling..........................................................................................................38
FTIR and XPS .................................................................................................................39
3
RESULTS AND DISCUSSION.............................................................................................43
Enhancement of TiO2 Photocatalysis with Carbon Nanotubes ..............................................43
Optimization of Photocatalyst Concentration .................................................................44
Photocatalytic Inactivation of Bacterial Endospores.......................................................44
5
B. cereus spores........................................................................................................45
B. subtilis spores.......................................................................................................46
Photocatalytic Inactivation of Vegetative Bacterial Cells...............................................46
Effect of Size and Aspect Ratio of Photocatalyst............................................................48
Approach-1: Mutants with and without surface appendages ...................................49
Approach-2: Size reduction of TiO2 coated MWNT ...............................................49
Enhancement of TiO2 Photocatalysis with Polyhydroxy Fullerenes......................................50
Preliminary Dye Degradation Experiments with Self-assembled PHF-TiO2
Nanocomposites...........................................................................................................51
Optimization of PHF-TiO2 Nanocomposites for Photocatalysis.....................................52
Enhancement of E. coli Inactivation with PHF-TiO2 Nanocomposite............................54
Adsorption of PHF on TiO2 Nanoparticles .....................................................................54
Detection of Hydroxyl Radicals with EPR......................................................................56
Influence of Composition of Polyhydroxy Fullerenes on Photocatalytic Enhancement ........59
Photocatalytic Dye Degradation with Fresh and Aged PHF ...........................................59
Mass Spectroscopy for Fresh and Aged PHF..................................................................60
FTIR Analysis of Fresh and Aged PHF ..........................................................................60
Gaussian simulation .................................................................................................61
Literature values .......................................................................................................62
XPS Analysis for Fresh and Aged PHF ..........................................................................62
TGA Analysis for Fresh and Aged PHF..........................................................................63
Effect of Impure Functional Groups................................................................................64
4
CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH ...............................97
Conclusions.............................................................................................................................97
Suggestions for Future Research ..........................................................................................101
APPENDIX
CALCULATION OF SURFACE COVERAGE ................................................105
LIST OF REFERENCES.............................................................................................................108
BIOGRAPHICAL SKETCH .......................................................................................................123
6
LIST OF TABLES
Table
page
3-1
Zeta potential values of photocatalysts and E. coli............................................................94
3-2
Peak assignments of FTIR peaks for fresh and aged PHF based on results from
Gaussian simulation of C60(OH)24 and literature. ..............................................................95
3-3
Elemental composition of fresh and aged PHF obtained with XPS analysis. ...................96
3-4
Peak position and elemental composition of fresh and aged PHF obtained with XPS
analysis...............................................................................................................................96
7
LIST OF FIGURES
Figures
page
1-1
Comparison of current disinfection techniques. ................................................................25
1-2
Steps in photocatalytic generation of reactive species.......................................................26
1-3
Complex shell structure of bacterial endospores. ..............................................................27
1-4
Cell-wall structure of non-motile E. coli. ..........................................................................28
1-5
Photocatalytic inactivation of a bacterium.........................................................................29
2-1
High-resolution TEM image of TiO2 coated on multi-wall nanotube. ..............................40
2-2
Experimental setup for photocatalysis experiments. .........................................................41
2-3
High resolution TEM images of PHF coated on TiO2 nanoparticles.................................42
3-1
D value estimation for photocatalytic inactivation of E. coli with Degussa P25. .............66
3-2
Optimum concentration of Degussa P25 for photocatalytic inactivation of E. coli.. ........67
3-3
Photocatalytic inactivation of B. cereus spores. ................................................................68
3-4
Photocatalytic inactivation of B. subtilis spores. ...............................................................69
3-5
Photocatalytic inactivation of E. coli. ................................................................................70
3-6
Interaction of TiO2 coated MWNT and Degussa P25 with surface appendages of E.
coli......................................................................................................................................71
3-7
Two different approaches undertaken to test the hypothesis that surface appendages
sterically hinder contact of high-aspect ratio photocatalyst with bacterial cell-wall.........72
3-8
Photocatalytic inactivation of S. aureus mutants with and without surface
appendages.........................................................................................................................73
3-9
First-order degradation kinetics of Procion Red MX-5B upon UVA irradiation with
PHF, TiO2 and a mixture of TiO2 and PHF.. .....................................................................74
3-10 Normalized pseudo-first-order rate coefficient for dye degradation as a function of
the ratio of added polyhydroxy fullerenes (PHF) to TiO2. ................................................75
3-11 Photocatalytic inactivation of E. coli plotted as a function of survival ratio vs. time. ......76
3-12 D values for E. coli inactivation with Degussa P25 alone and a mixture of Degussa
P25 and PHF. .....................................................................................................................77
8
3-13 Absorption spectrum of polyhydroxy fullerenes. ..............................................................78
3-14 Calibration curves for polyhydroxy fullerenes (PHF) at three different pH values. .........79
3-15 Adsorption density of polyhydroxy fullerenes (PHF) on titanium dioxide and shift in
zeta potential of titanium dioxide nanoparticles with adsorption of polyhydroxy
fullerenes at different pH.. .................................................................................................80
3-16 Zeta potential shifts as a function of adsorption density....................................................81
3-17 Electron paramagnetic resonance spectra obtained upon UVA irradiation of DMPO
and TiO2 alone and TiO2+PHF. .........................................................................................82
3-18 Effect of PHF on generation of hydroxyl radicals by UVA irradiation of titanium
dioxide................................................................................................................................83
3-19 Hypothetical photocatalytic reactions occurring upon UV irradiation.. ............................84
3-20 First-order degradation kinetics of Procion Red MX-5B upon UVA irradiation with
TiO2 alone, TiO2 + aged PHF and TiO2 + fresh PHF.. ......................................................85
3-21 APCI-MS of fresh and aged Polyhydroxy Fullerenes. ......................................................86
3-22 FTIR spectrum of fresh Polyhydroxy Fullerenes...............................................................87
3-23 FTIR spectrum of aged Polyhydroxy Fullerenes. ..............................................................88
3-24 Gaussian simulation of C60(OH)24. ....................................................................................89
3-25 Gaussian simulation of vibrational spectrum for C60(OH)24..............................................90
3-26 Experimental C1s XPS spectrum (top curve) of fresh Polyhydroxy Fullerenes with
fitted curves representing three different oxidation states of carbon.................................91
3-27 Experimental C1s XPS spectrum (top curve) of aged Polyhydroxy Fullerenes with
fitted curves representing three different oxidation states of carbon.................................92
3-28 TGA spectra for fresh and aged Polyhydroxy Fullerenes. Numbers refer to different
stages of weight loss. .........................................................................................................93
A-1
Estimated surface coverage and observed enhancement as a function of dosed ratios
of PHF to TiO2. ................................................................................................................107
9
Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
ENHANCEMENT OF TITANIUM DIOXIDE PHOTOCATALYSIS WITH POLYHYDROXY
FULLERENES
By
Vijay B. Krishna
May 2007
Chair: Brij Moudgil
Cochair: Ben Koopman
Major: Materials Science and Engineering
Semiconductor photocatalysts, particularly TiO2, are attracting extensive research for
destruction of environmentally hazardous chemicals (e.g., organic pollutants, greenhouse gases)
and hazardous bioparticulates (e.g., bacterial endospores, emerging pathogens) because they can
achieve complete mineralization without generation of toxic byproducts. Several attempts have
been made to improve the quantum efficiency of TiO2 by conjugating it with conductors such as
metals and organic molecules for scavenging the photo-generated electrons. Another class of
materials well known for their electron accepting properties is carbon nanotubes and fullerenes.
TiO2 (anatase polymorph) was coated on multi-wall carbon nanotubes by sol-gel coating
and the resulting nanocomposites were found to inactivate bacterial endospores two times faster
than Degussa P25 (gold standard), but were ineffective against Escherichia coli. This was
attributed to their high aspect ratio, which prevented contact with the fimbriae covered cell-wall
of E. coli.
Water-soluble and non-toxic polyhydroxy fullerenes (PHF) were employed as alternate to
the TiO2 coated MWNT. Adsorption of PHF molecules onto TiO2 by electrostatic interaction
was demonstrated. PHF-TiO2 nanocomposites enhanced the photocatalytic activity of TiO2 for
10
dye degradation and E. coli inactivation. Surface coverage of TiO2 nanoparticles by PHF
molecules determined the extent of enhancement, with an optimum at 2–7% surface coverage.
The rate of photocatalytic dye degradation by the TiO2-PHF nanocomposite was 2.6 times the
rate found with TiO2 alone.
The hypothesis that scavenging of photo-generated electrons and therefore higher
generation of hydroxyl radicals is the mechanism for the observed enhancement was validated.
The concentration of hydroxyl radicals generated by PHF-TiO2 nanocomposite was up to 60%
greater than the concentration obtained with TiO2 alone as determined with EPR.
Influence of functional groups of PHF on its electron scavenging ability and stability was
determined. Fresh and aged forms of PHF were characterized by MS, FTIR, XPS and TGA.
Higher concentrations of impure groups were detrimental to stability and electron scavenging
ability of PHF. A ratio of impure groups to hydroxyl groups of 0.27 was associated with
successful enhancement by PHF, whereas a ratio of 1.66 was associated with no enhancement.
Guidelines for effective formulation of PHF-TiO2 nanocomposites were developed.
11
CHAPTER 1
INTRODUCTION AND BACKGROUND
Semiconductor photocatalysts are attracting extensive research for destruction of
environmentally hazardous chemicals (e.g., greenhouse gases, organic pollutants) and hazardous
bioparticulates (e.g., bacterial endospores and emerging pathogens) because they can achieve
complete mineralization (complete oxidation of bacterial cell to carbon dioxide and water)
without generation of toxic byproducts (Hoffmann et al., 1995; Fujishima et al., 1999; Block,
2001; Atrih and Foster, 2002). Titanium dioxide (TiO2) has been commercially applied as a selfcleaning coating on buildings and glass materials, especially in Japan, South Korea and
Singapore. However, the potential for such applications is limited by the low quantum efficiency
(10%) of TiO2 photocatalysis (Hoffmann et al., 1995). Several attempts have been made to
improve the efficiency by conjugating titanium dioxide with metals or organic molecules. Metals
such as silver, gold and platinum are either deposited on titanium dioxide particles by reduction
of their salts and electron beam evaporation, or co-synthesized with titanium dioxide precursors
(Vamathevan et al., 2002; Arabatzis et al., 2003a; Subramanian et al., 2003a; Sun et al., 2003;
Sreethawong and Yoshikawa, 2005). Doping of metals has also been achieved with ion beam
implantation. Organic compounds, which can conduct electrons, are covalently conjugated to
titanium dioxide particles for scavenging of photo-generated electrons (Rajh et al., 1999;
Gratzel, 2001; Paunesku et al., 2003; Brune et al., 2004). All of the above mentioned approaches
for enhancing photocatalysis require complex conjugation chemistry and therefore additional
unit operations in the synthesis processes. Furthermore, contradictory results have been reported
in the literature for enhancement with metals. The observed enhancement is usually
demonstrated for organic dye degradation and seldom for bacterial inactivation. Certain metals
are also undesirable where health effects and ultimate disposal are concerned. Therefore there is
12
a need for environmentally benign materials that demonstrate consistent enhancement of
photocatalysis for inactivation of microorganisms as well as degradation of organic pollutants.
This chapter provides a general overview of available inactivation techniques and
identifies the need for a new inactivation methodology based on photocatalysis. The current
understanding of titanium dioxide photocatalysis and efforts for improving its quantum
efficiency is discussed. Next a general picture of mechanism of photocatalytic inactivation of
microorganisms is provided. Finally, the potential of carbon nanotubes and fullerenes as
enhancers for titanium dioxide photocatalysis is provided.
General Overview of Inactivation Techniques
Microorganisms are ubiquitous in nature and are essential components of our ecosystem.
They are the only organisms known to exist in extreme environments, such as at temperature as
high as 150°C (NSF), at alkaline and acidic pH or in saline environments (Perry et al., 2002).
Microorganisms are present in depths of ocean floor and can survive extreme conditions in outer
space, which led to the theory that earth biota was originally colonized from space (Mileikowsky
et al., 2000; Horneck et al., 2001; Mastrapa et al., 2001).
Pathogenic microorganisms are major cause of concern for the well being of society. Total
global spending on healthcare exceeds $ 4.1 trillion (WHO, 2007). The most common pathogen,
influenza virus, accounts for $12 billion every year in the U.S. for medical expenses and
productivity losses (Solvay, 2007). An average loss of 6.5 working days is associated with the
common cold (Keech et al., 1998). The recent outbreak of bird flu virus (H5N1), an example of
emerging pathogen, has cost $10 billion globally (USAID).
Another area of concern is emergence of antibiotic resistant strains of microorganisms such
as methicillin resistant Staphylococcus aureus, which are major contributors to healthcare-
13
associated infection. An estimated 2 million nosocomial infections are reported annually, which
results in 90,000 deaths and $4.5 billion in healthcare costs (Haley et al., 1985; CDC, 2006a).
Other non-pathogenic microorganisms present especially in indoor environment are major
contributors to allergy and respiratory problems, which are the leading cause of school and work
absence. The Center for Disease Control and Prevention (CDC) Summary Health Statistics for
U.S. children state that nine million U.S. children under the age of 18 have been diagnosed with
asthma. An estimated $16 billion is spent (medical expenses + productivity loss) in U.S. alone
annually due to asthma (LungUSA, 2005).
Apart from the natural threats from pathogens present in the environment, a new cause for
concern is biological warfare. The anthrax attack in the U.S. resulted in 5 deaths and 22
infections (CDC, 2006b). The U.S. budget for homeland security related programs for fiscal year
2007 is $5.2 billion, indicating the seriousness of this threat.
As suggested above, there is a need for development of better microbial inactivating
agents. The current global market for disinfectants is $3.5 billion and is expected to rise to $6
billion by 2010 (Kaiser, 2006). The prevailing technology for inactivation of microorganisms can
be classified into three categories—heat, radiation and chemical based inactivating agents (Fig.11). These inactivation techniques along with their shortcomings are presented below.
Heat based inactivation: Thermal inactivation of microorganisms is the most commonly
employed disinfection technique. In fact, heat is the oldest known method for disinfecting
drinking water and is still widely used. Heat can kill most of the common disease causing
waterborne bacteria and thus is a common method for treating water. However, there are certain
microorganisms, such as bacterial endospores, which are not easily affected by heat.
14
Furthermore, thermal techniques cannot be applied for inactivation of all microorganisms present
on surfaces (Block, 2001).
Radiation based inactivation: Ultraviolet (UV) and Gamma ray radiation are the two
major types of radiation utilized for inactivation. UV radiation is commonly used for water
treatment and has also been applied for inactivation of microorganisms in air and on surfaces.
Gamma radiation is employed for sterilization of food. The major disadvantage associated with
this technique is cost. Radiation at high intensity is required to ensure complete inactivation of
microorganisms, which needs higher energy consumption (Block, 2001). Inactivation with
Gamma radiation requires separate building units as they can be hazardous for human exposure.
Although no special enclosure is necessary for UV radiation, they are harmful to humans.
Chemical inactivating agents: The chemical disinfectants are in widespread use and they
can be further classified in to mild and strong chemical agents. Mild chemical inactivating agents
such as those based on alcohols, aldehydes and surfactants are commonly employed for cleaning
surfaces in hygiene products for washing hands. Strong chemical agents such as those based on
chlorine and peroxygen compounds find applications in water treatment. Chlorine bleach is
commonly employed for treating water in swimming pools. Strong chemical agents are not
recommended for treating surfaces as they can easily oxidize them. Chemical agents are also not
applicable for inactivation of microorganisms present in air and food. Gaseous chemical agents
such as ozone, ethylene oxide and chlorine dioxide are employed as sterilants for treating
medical devices. However, their cost and safety issues with handling restrict their application. A
major disadvantage of chemical agents is that they generate toxic byproducts, including
mutagens and carcinogens (Block, 2001).
15
Photocatalysis is a relatively new technology which overcomes the disadvantages of the
above mentioned inactivating agents. Photocatalyst based technology can be applied for
inactivation of microorganisms in air and water and on surfaces. Semiconductors such as
titanium dioxide, zinc oxide, cadmium sulfide, vanadium oxide and cerium oxide are employed
as photocatalysts (Liu and Yang, 2003; Hernandez-Alonso et al., 2004; Karunakaran and
Senthilvelan, 2005). Titanium dioxide, in particular anatase crystal structure, has an advantage
over other semiconductor photocatalysts as it has higher quantum efficiency and in addition is
low cost and is generally recognized as being non-toxic (Zeng et al., 2005; Wang et al., 2006).
Photocatalysis with TiO2
In 1972, Fujishima and Honda first demonstrated electrochemical photolysis of water with
titanium dioxide (Fujishima and Honda, 1972). Since then extensive research has been conducted
on titanium dioxide photocatalysis with elucidation of its mechanism.
Titanium dioxide is a semiconductor, which naturally exists in three different crystalline
forms—anatase, brookite and rutile. Rutile is the most stable of three polymorphs. The calculated
structure energy for rutile is more stable than anatase by 3.8 kJ mole-1 and brookite by 24.8 kJ
mole-1 (Post and Burnham, 1986). Anatase and brookite are known to exist at room temperature
as metastable states. In fact due to very small difference between structure energy of anatase and
rutile, the former can be considered as stable.
Anatase structure has attracted substantial research for photocatalysis even though it has a
higher band gap of 3.2 eV compared to 3.11 eV and 3.0 eV for brookite and rutile, respectively
(Li et al., 2007). The higher band gap also reduces the recombination probability and therefore
anatase exhibits better photocatalytic activity than rutile (Yan et al., 2005; Li et al., 2007).
16
Titanium dioxide (TiO2) has a band gap of 3.2 eV. When ultraviolet (UV) light of
wavelength less than 380 nm irradiates TiO2 particles, the photon energy is sufficient for
excitation of electrons from valence band to conduction band leaving a hole behind. Various
reactions occurring during photocatalysis are summarized below, where e-CB represents electrons
in conduction band; h+VB represents holes in valence band; D represents electron donors and A
represents electron acceptors.
Electron-hole pair generation
TiO2 + hν → e-CB + h+VB
(10-15 s)
(1-1)
e-CB + Ti4+ → Ti3+ (trapped electron)
(10-10 s)
(1-2)
h+VB + O2- → O- (trapped hole)
(10-10 s)
(1-3)
(10-9 s)
(1-4)
(10-7 s)
(1-5)
Charge localization
Electron-hole recombination
e-CB + h+VB → phonons
Surface reactions
TiO2 (h+) + D → TiO2 + D•
Specific examples of above reaction
TiO2 (h+) + -OH → TiO2 + •OH
(1-6)
TiO2 (h+) + H2O → TiO2 + •OH + H+
(1-7)
TiO2 (e-) + A → TiO2 + A-•
(10-3 s)
(1-8)
Specific examples of above reaction
TiO2 (e-) + O2 → TiO2 + O2-•
(1-9)
TiO2 (e-) + H2O2 → TiO2 + OH- + •OH
(1-10)
17
The generation of electron-hole pairs (Eq. 1-1) is fast, occurring in 10-15 seconds
(Hoffmann et al., 1995). The photo-generated electrons and holes are either localized in shallow
traps (Eq. 1-2 and 1-3) or recombine (Eq. 1-4). Charge localization is much slower (10-10
seconds) than generation of electron-hole pairs (Berger et al., 2005). The localized electrons and
holes can recombine or take part in redox reactions, as shown in Figure 1-2. More than 90% of
the electron-hole pairs recombine at a timescale of 10-9 seconds (Hoffmann et al., 1995; Berger
et al., 2005). The recombination energy is dissipated as phonons. The remaining electron-hole
pairs migrate to the surface, where they react with adsorbed electron acceptors and donors (Eq.
1-5 to 1-10). The migration and interfacial charge transfer processes are the slowest of all the
steps, with an overall timescale in the range of 10-8–10-3 seconds (Hoffmann et al., 1995).
Recombination, which is faster than charge migration and interfacial charge transfer, is the
biggest single factor limiting the quantum efficiency of photocatalysis.
Modification of Electronic Properties of Titanium dioxide
Electronic property modification is usually done by creating an additional band (metastable state) near the conduction band, where the electron can remain for a longer time
(Hoffmann et al., 1995). The photon-generated electron-hole pairs recombine at a faster rate (109
seconds) than the generation of reactive species (10-8 to 10-3 seconds) (Hoffmann et al., 1995).
Decreasing the recombination rate, by trapping the photon-generated electrons or holes, will
increase the production of reactive species and hence the overall efficiency of photocatalysis.
Several attempts have been made to separate the photo-generated electrons and holes to reduce
recombination. Titanium dioxide photocatalysts have been conjugated with electron scavenging
agents such as metals or organic molecules(Hoffmann et al., 1995; Keleher et al., 2002; Wang et
al., 2002; Arabatzis et al., 2003a; Arabatzis et al., 2003b; Hu et al., 2003c). Metals such as
silver, gold and platinum are generally preferred due to their high conductivity and corrosion
18
resistance. They are either deposited on titanium dioxide particles by reduction of their salts and
electron beam evaporation, or co-synthesized with titanium dioxide precursors (Vamathevan et
al., 2002; Arabatzis et al., 2003a; Subramanian et al., 2003a; Sun et al., 2003; Sreethawong and
Yoshikawa, 2005). Doping of metals has also been achieved with ion beam implantation (Wang
et al., 2002). However, contradictory results have been reported with metal as enhancers.
Experimental investigations with gold–titanium dioxide nanocomposite particles showed that the
response of photocatalyst is extended to the visible region with a significant decrease in their
photocatalytic performance under ultraviolet radiation (Arabatzis et al., 2003a). Silver doped
titanium dioxide nanocomposite exhibited increase in degradation rate for sucrose, however no
enhancement was observed for degradation of salicyclic acid (Vamathevan et al., 2002). Organic
compounds, which can conduct electrons, are covalently conjugated to titanium dioxide particles
for scavenging of photo-generated electrons with applications including solar cells and visible
light photocatalysis(Rajh et al., 1999; Gratzel, 2001; Paunesku et al., 2003; Brune et al., 2004).
Another class of conductors used recently is the carbon nanotube. Titanium dioxide
coated multi-wall carbon nanotubes (MWNT) have been employed to increase the photocatalytic
degradation of organic pollutants and inactivation of microorganisms (Lee, 2004; Pumprueg,
2004; Krishna et al., 2005; Lee et al., 2005; Yu et al., 2005a; Pyrgiotakis, 2006). It was
hypothesized that the photo-generated electrons are scavenged by the MWNT. Use of coadsorbents such as silica, alumina, zeolites and activated carbon have also been explored for
increasing the efficacy of photocatalysis (Matos et al., 2001).
19
Microbial Inactivation by Photocatalysis
The extent of inactivation was observed to be inversely proportional to the thickness and
complexity of the cell wall. Therefore it is important to understand the structure of
microorganisms before examining the mechanism of inactivation.
Structure of Bacterial Endospores and Vegetative Bacterial Cells
Endospores of bacteria are the most resistant microorganisms against all disinfection and
sterilization techniques (Nicholson et al., 2000; Block, 2001; Atrih and Foster, 2002). Their high
degree of resistance is governed by a unique spore structure, as shown in Figure 1-3. Each level
of spore structure provides resistance to different disinfectants. The small acid soluble proteins
(SASP) in the core protect the DNA from UV radiation, whereas cortex provides heat resistance
and the spore coat layers protects the spore from chemical attack (Driks, 1999; Henriques and
Moran, 2000; Nicholson et al., 2000; Riesenman and Nicholson, 2000; Block, 2001; Atrih and
Foster, 2002). Endospores of Bacillus cereus and Bacillus anthracis have an extra outermost
layer called exosporium, which is not present in Bacillus subtilis. Filaments are also present in
case of B. cereus spores (Mizuki et al., 1998).
In contrast to bacterial endospores, the vegetative bacterial cells such as Escherichia coli
have simple cell-wall structure (Fig. 1-4) (Perry et al., 2002). The cytoplasmic membrane is
surrounded by a thin layer of peptidoglycan (2 nm) with periplasmic space in between. The
peptidoglycan layer is surrounded by an outer protein membrane (8 nm) again with periplasmic
space in between. E. coli, which is a Gram negative bacterium, also possesses surface
appendages. Fimbriae are long filamentous polymeric protein structures anchored inside plasma
membrane (Klemm, 1994; Mol and Oudega, 1996). Fimbriae can be as long as 2 microns and
can vary in diameter from 2 to 10 nm. They can be either thick and rigid or thin and flexible,
20
however their exact structure in natural surrounding is not known. The total cell-wall thickness is
approximately 20 nm.
Mechanism of Photocatalytic Inactivation
A better understanding of microbial inactivation has evolved since the first proposed
mechanism by Matsunaga and co-workers (Matsunaga et al., 1985). They proposed that the cell
death was caused by decrease in respiratory activity due to photocatalytic oxidation of
intracellular coenzyme A. Saito et al. proposed that the cell death occurs due to photocatalytic
disruption of cell membrane, evident from leakage of intracellular K+ ions (Saito et al., 1992c).
Leakage of Ca+ ions has also been observed with cancer cells. Sunada et al. found that
endotoxin, an integral component of the outer membrane was degraded by photocatalytic action
of TiO2, which leads to membrane damage (Sunada et al., 1998). Maness et al. (Maness et al.,
1999) showed that TiO2 photocatalytic reaction causes the lipid peroxidation reaction, which
results in disruption of normal activities associated with intact cell membrane, such as respiration
(Jacoby et al., 1998). The loss of membrane structure was proposed to be the cause of cell death.
Lu et al. also showed that cell death was caused by the decomposition of the cell-wall and cellmembrane resulting in leakage of intracellular components, as shown in Figure 1-5 (Lu et al.,
2003).
Since the cell death is caused by photocatalytic degradation of cell-wall, the inactivation
time is proportional to the complexity and density of cell wall structure (Kuhn et al., 2003).
Endospores, with their complex and dense shell structure, have the longest inactivation time (in
hours compared to minutes for simple bacteria). The photocatalytic inactivation time can be
decreased by increasing the generation of reactive species, which can be achieved by delaying
the recombination process.
21
Carbon Nanotubes and Fullerenes
Fullerenes and carbon nanotubes are allotropes of carbon with unique properties. The
existence of fullerenes was first proposed by Osawa in 1970 (Osawa, 1970). Although Rohlfing
and coworkers were first to spectroscopically determine the existence of fullerenes (Rohlfing et
al., 1984), Kroto et al., were the first to synthesize and purify fullerenes, which they named
Buckminsterfullerenes (Kroto et al., 1985). The presence of carbon nanotubes was first observed
by Iijima in 1991 (Iijima, 1991). Since then carbon nanotubes and fullerenes have attracted
substantial research on their physical, electronic and chemical properties.
Carbon nanotubes, especially single-wall nanotubes, are known to possess metallic as well
as semiconducting properties depending on the angle of folding graphene sheets (Saito et al.,
1992a; Saito et al., 1992b). Multi-wall carbon nanotubes (MWNT) are generally
semiconducting, with their outermost layer sometimes dictating the electronic properties. In fact
MWNT are speculated to be the first above room temperature superconductor (Zhao and Wang,
2001). The unique electronic properties of carbon nanotubes have been researched for electronic
applications, as reviewed by Anantram and Leonard (Anantram and Leonard, 2006). Carbon
nanotubes have been employed as electron acceptor for bio and nanobio-sensor applications
(Guiseppi-Elie et al., 2002; Chen et al., 2003; Goswami et al., 2004). Kymakis and Amaratunga
exploited the electron scavenging ability of single-wall carbon nanotubes in photovoltaic cells
(Kymakis and Amaratunga, 2003). MWNT have also been employed for enhancing titanium
dioxide photocatalysis (Lee, 2004; Pumprueg, 2004; Krishna et al., 2005; Lee et al., 2005; Yu et
al., 2005b; Pyrgiotakis, 2006).
Fullerenes, which are spherical version of nanotubes, have also been exploited for their
unique electronic, optical and magnetic properties (Guldi, 2000b; Gust et al., 2000; Makarova,
2001). Fullerenes, specifically C60, which have truncated icosahedra structure, possess three22
dimensional symmetry. Fullerenes have also been reported as three dimensional electron
acceptors (Guldi, 2000a). Fullerenes were experimentally shown to accept up to six electrons per
molecule (Xie et al., 1992). Fullerenes can accumulate and discharge electrons, thus acting as
electron relays (Guldi and Prato, 2000). Electrochemical sensors based on fullerenes and carbon
nanotubes have been reviewed by Sherigara and coworkers (Sherigara et al., 2003). Kamat and
coworkers have demonstrated the transfer of photo-generated electrons from titanium dioxide to
fullerenes with ethanol/benzene mixture as solvent (Kamat et al., 1994).
Fullerenes are not soluble in water and are also reported to be toxic (Oberdorster, 2004),
limiting their potential for many applications. The water-solubility of fullerenes is improved by
coupling the fullerene cage with hydrophilic molecules (Taylor, 1999). Functionalized fullerenes
such as 6,6-phenyl-C60-butyric acid (PCBM) has been successfully employed as an electron
acceptor in solar cells (McNeill et al., 2004). Self-assembled nanocomposites of functionalized
water-soluble fullerenes with CdTe nanoparticles have been prepared, in which fullerenes
scavenge electrons from CdTe nanoparticles (Guldi et al., 2004). These functionalized fullerenes
usually possess a few hydrophilic chains, and thus are somewhat water soluble.
Fullerenes have also been functionalized with 12–42 hydroxyl groups per molecule. The
advantage of hydroxylated fullerenes is that they are reported to be non-toxic and have been
researched for antioxidant applications. Polyhydroxy fullerenes were reported to reduce
oxidative stress on cells by scavenging reactive oxygen species (Chen et al., 2004; Djordjevic et
al., 2004b). Furthermore, they are patented as therapeutics (US5994410) and are used in
cosmetics (www.vc60.com).
As mentioned earlier, carbon nanotubes and fullerenes are known for their unique
electronic properties including electron accepting ability (Guldi, 2000a; Chen et al., 2003;
23
Sherigara et al., 2003; Guldi et al., 2005). Their potential, however, is restricted by limited
solubility in water. Chemical functionalization of nanotubes and fullerenes can promote their
dispersal in aqueous system and thus increase their range of application. However, loss in
electron scavenging ability of fullerenes upon hydroxylation has been reported (Mohan et al.,
1997), questioning their applicability for enhancing photocatalysis. Although carbon nanotubes
have been employed for enhancement of titanium dioxide photocatalysis (Lee, 2004; Pumprueg,
2004; Krishna et al., 2005; Lee et al., 2005; Pyrgiotakis, 2006), no previous attempts have been
made to harness the electron-scavenging ability of fullerenes as a means of enhancing
photocatalysis. Accordingly, the goal of the present research was to develop photocatalytic
nanocomposites based on electron accepting properties of fullerenes. The specific objectives
were fourfold: (1) verify if carbon nanotubes could enhance the activity of TiO2 against
microorganisms, (2) determine if hydroxylated fullerenes could enhance TiO2 photocatalysis and
avoid steric hindrance encountered with TiO2-carbon nanotube nanocomposites, (3) confirm
mechanism of photocatalytic enhancement with PHF, and (4) evaluate effect of PHF
composition on its performance.
The results obtained from accomplishing the four objectives will be further analyzed to
develop a set of design guidelines for enhancing TiO2 photocatalysis for applications of societal
significance, such as coatings, pollution control, healthcare and control of confined
environments.
24
Systems
Heat
Radiation
Chemical agents
Photocatalysis
(Mild)
(Strong)
Water
Air
Surfaces
25
Food
Disadv. Some
microbes
are
resistant
High intensity Toxic by
reqd.,
products
expensive
Figure 1-1. Comparison of current disinfection techniques.
Toxic by
products
Slow
A O2
hν
e-
10-3 s
.A .-O
2
TiO2
10-7 s
10-15 s
Conduction Band
26
Valence Band
h+
.D .OH
10-7 s
D -OH
Figure 1-2. Steps in photocatalytic generation of reactive species.
A – e- acceptors
D – e- donors
Outer Spore Coat
Core
Exosporium
(40–90 nm)
Cortex
(200 nm)
SASP
27
DNA
Germ Cell-wall
Inner Spore Coat
(20–40 nm)
Figure 1-3. Complex shell structure of bacterial endospores.
Fimbriae
(0.2–2.0 µm length;
2–10 nm thickness)
Lipopolysaccharides
Outer
membrane
Porin
(8 nm)
28
Periplasmic
space
Peptidoglycan
layer (2 nm)
Plasma
membrane
(7 nm)
Membrane
proteins
Figure 1-4. Cell-wall structure of non-motile E. coli.
Cytoplasm
Phospholipids
UV
Light
TiO2
nanoparticle
29
Bacterium
Figure 1-5. Photocatalytic inactivation of a bacterium.
Cell
membrane
disruption
Bacterium
Leakage of
intracellular
constituents
Lysed Cell
CHAPTER 2
MATERIALS AND METHODS
Synthesis of TiO2 Coated Multi-wall Carbon Nanotubes
Commercially available arc-discharged multi-wall carbon nanotubes (Alfa Aesar, 3–24 nm
outer diameter, 0.5–5 μm) were used as base material. Multi-wall carbon nanotubes (MWNT)
have unique electronic properties that can trap photon-generated electrons. Although electron
transfer from titanium dioxide nanoparticle to uncoated carbon nanotubes is feasible, covalent
linkage increases the efficiency of electron transfer and hence photocatalysis. Covalent linkage
of multi-wall carbon nanotubes with titanium dioxide is also desirable for strong and stable
coating. Since carbon nanotubes have smooth walls without any defects, it is necessary to add
functional groups on the surface, which can act as reaction sites for sol-gel coating of titanium
dioxide. The nanotubes were functionalized by chemical oxidation method (Tsang et al., 1994).
Functionalization was carried out by dispersing 100 mg of MWNT in HNO3, using 30 minute
sonication, followed by refluxing at 140°C for 10 hours. The functionalization process involves
oxidation of surface to yield –COOH, >C=O, –OH groups on the surface of MWNT, as also
determined by FTIR (Hiura et al., 1995; Ebbesen et al., 1996). After functionalization step,
MWNTs were washed several times with deionised water until no residual acids in the wash
water were detected. In order to coat functionalized surface with titanium dioxide, 20 μL of
Titanium(III) sulfate (99.9+%) solution was added to MWNTs, dispersed in deionized water,
with constant stirring for 1 hour. The TiO2 coated MWNTs were then washed and dried at 60°C
for 2 days. The crystallization of titanium dioxide coating was done by heat treatment at 500°C.
Figure 2-1 shows high resolution transmission electron micrograph of anatase coated multi-wall
nanotubes (Lee, 2004; Krishna et al., 2005; Lee et al., 2005).
30
Culturing and Purification of Bacillus Spores
Endospores of Bacillus cereus ATTC 2 was used as a surrogate of Bacillus anthracis. The
spores were cultured according to ASTM E2111 protocol. Presence of vegetative cells can
induce artifacts in the experiments, due to their higher susceptibility towards photocatalytic
inactivation. Spores were harvested and purified using the lysozyme treatment (Harwood and
Cutting, 1990). The lysozyme treatment was followed by heat shock treatment (80°C, 10 min)
for complete killing of vegetative cells. Finally spores were suspended in sterile deinoized water
and refrigerated at 4°C until further use.
Culturing of Escherichia coli
E. coli C3000 was cultured for 24 hours at 37°C and harvested to give a stock solution
according to the protocol of Rincon and Pulgarin (Rincon and Pulgarin, 2003). E. coli was stored
in deionized water at 4°C for a maximum of 24 hours.
Experimental Apparatus
Dye degradation experiments were performed inside an ultraviolet (UV) chamber with 16
solar UV lamps (Southern New England Ultra Violet Company, Branfield, CT). Cold air was
passed over the reaction mixtures to limit heating. The surfaces of the reaction mixtures were
positioned 100 mm below the lamps. A multiplate magnetic stirrer was used to provide
agitation.
Photocatalytic Inactivation of Bacillus Spores
The photocatalytic inactivation efficiency comparison of anatase coated multi-wall carbon
nanotubes (MWNT) and commercial titanium dioxide nanopowders (Degussa P25) was done on
surface area basis. The surface area of anatase coated MWNT and Degussa P25 were found to be
172 m2 g-1 and 50 m2 g-1 respectively, by BET analysis (Pumprueg, 2004; Krishna et al., 2005;
31
Lee et al., 2005). 0.01% of commercial titanium dioxide (Degussa P25) was used for disinfection
experiments, as this is shown to be the optimum concentration by Block et al. (1997). The
different reaction slurries prepared are listed below.
•
•
Control – 20ml of water + 10ml of spore suspension
Commercial titanium dioxide (Degussa P25) – 3mg of commercial TiO2 P25 in 20ml of
water + 10ml of spore suspension
•
TiO2 coated multi-wall carbon nanotubes – 0.8mg of TiO2 coated nanotubes in 20ml of
water + 10ml of spore suspension.
•
Functionalized multi-wall carbon nanotubes – 0.8mg of functionalized nanotubes in 20ml
of water + 10ml of spore suspension.
The suspension was transferred to a sterile Petri dish containing a sterile magnetic stirring
bar. The Petri dish containing the suspension was then exposed to ultraviolet (UV) light with
sixteen 350 nm UV lamps. A cooling fan was used to avoid increase in temperature of the
suspension. The experimental set up is shown in Figure 2-2. The UV lamps were stabilized for
30 min prior to the start of the experiment to obtain constant intensity. Samples were collected at
regular intervals of time, including a sample at the start of the experiment. The samples were
diluted appropriately and plated to determine the survival concentration. The results were
analyzed by plotting the survival ratio against UV irradiation time. D values are determined as
the time required for one log reduction in exponential portion of the curve and LD90 is the time
required for 90% reduction of population (Block, 2001).
Photocatalytic Inactivation of Escherichia coli
The photocatalytic inactivation experiments were conducted with Degussa P25. The
photocatalyst suspension was prepared at a concentration of 30 mg L-1 and sonicated (Misonix
Sonicator 3000, Farmingdale, NY) for one hour at 165 W. Subsequently 0.03 mg/L of PHF was
added to 20 mL of the photocatalyst suspension followed by addition of 10 mL of stock E. coli
32
suspension. The initial concentration of E. coli in the reaction mixture was 3.6×107 cfu mL-1.
The reaction mixture was stirred using a magnetic stirrer, with irradiation by UVA at intensity of
86 W m-2. Three 0.333 mL samples were collected every 15 minutes for one hour. The collected
samples were then serially diluted with phosphate buffered saline. Appropriate dilutions were
plated on tryptic soy agar and incubated at 37°C for 20 hours prior to counting. Semilog graphs
of survival ratio (concentration of E. coli at any given time normalized with initial concentration)
vs. time were plotted to determine the first-order inactivation rate coefficients and D-value (time
required for one log reduction in linear region of the graph).
Optimization of PHF Concentration with Dye Degradation Experiments
Titanium dioxide was dispersed in deionized water as described previously to give a
concentration of 30 or 100 mg L-1 in the reaction mixture. Immediately after sonication, PHF
was added, followed closely by addition of dye. Different dosed weight ratios of PHF to
titanium dioxide (0, 0.0002, 0.001, 0.003, 0.005, 0.05 and 0.093) were tested. The initial dye
concentration in the reaction mixture was 3 mg L-1. The pH of the reaction mixture was
approximately 6. The reaction mixture was poured into a 35×100 mm Petri dish and mixed for
10 minutes in the dark using a magnetic stirrer. Two 0.5 mL samples were collected and pipetted
into a cuvette. The UV lamps were then turned on for the duration of the experiment. Samples
were collected every 15 minutes for one hour. Because of the need to process multiple samples,
the titanium dioxide was separated by sedimentation in the dark over a 10-day period. Reaction
mixture containing a representative PHF concentration (150 µg L-1) and no titanium dioxide was
carried through the test procedure as a control. The supernatants were analyzed for dye
concentration by UV-Vis spectroscopy at peak heights at 512 and 538 nm. The pseudo-firstorder rate coefficients for photocatalytic degradation of Procion red dye were determined from
33
semi-log plots of normalized absorbance (absorbance of dye solution at any given time divided
by the dye absorbance at time zero) versus time.
The statistical significance of adsorption and dye degradation results were analyzed by the
Tukey HSD and Tukey-Kramer tests at α = 0.01(Sokal and Rohlf, 1997; Montgomery, 2005).
Adsorption Experiments
Polyhydroxy fullerenes (C60(OH)n, n=18–24) were purchased from BuckyUSA (Houston,
TX). Titanium dioxide (anatase polymorph, 5 nm particle size) was obtained from Alfa Aesar
(Ward Hill, MA). All other chemicals were acquired from Fisher Scientific (Hampton, NH). The
triazine monoazo organic dye, Procion Red MX-5B, was used in dye degradation studies. Stock
solution of the organic dye was formulated at a concentration of 100 mg L-1 with sterile
deionized water. A stock solution of polyhydroxy fullerenes was prepared by dissolving 1.4 mg
of C60(OH)n in 10 mL of sterile deionized water.
The concentration of PHF was measured by UV absorbance (Perkin-Elmer Lambda 800,
Wellesley, MA). In order to construct calibration curves, PHF solutions were prepared in
supernatants of titanium dioxide suspensions to be consistent with the methodology of adsorption
experiments and to verify the efficacy of centrifugation step. Each titanium dioxide suspension
(50, 100, or 200 mg L-1) was prepared in deionized water. The pH of the suspension was adjusted
with HCl or NaOH solution so that a separate calibration curve could be prepared for each target
pH value (3.3, 6.1, 9.9) used in the adsorption experiments. The suspension was sonicated at 165
mW in a water bath sonicator (Misonix Sonicator 3000, Farmingdale, NY) for one hour and the
pH was measured again. Titanium dioxide was separated from the suspension by centrifugation
at 15,000×g for 15 minutes. Supernatant was collected and centrifuged again for 25 minutes.
The final supernatant was found to be free of titanium dioxide particles based on results from
inductively coupled plasma (Perkin-Elmer Plasma 3200, Wellesley, MA). PHF was added to the
34
supernatants to give desired final concentrations. Calibration curves (PHF absorbance at 254 nm
vs. concentration) were constructed at the selected pH values. Results of experiments carried out
to determine if the concentration of titanium dioxide had an effect on the calibration curves
indicated that initial concentration of titanium dioxide in the range of 50–200 mg L-1 had no
effect (data not shown).
Prior to an adsorption experiment; a suspension of titanium dioxide (100 mg L-1) was
prepared as described previously at a selected pH. Immediately after sonication, PHF was added
to the suspension to give a final concentration of 10 mg L-1 titanium dioxide and the suspension
was mixed on an orbital shaker at 200 rev/min and 35°C for 12 hr. Zeta potential and pH of the
suspensions were measured before addition of PHF and after completion of the agitation period.
Zeta potential measurements were performed with Brookhaven ZetaPlus (Holtsville, NY). To
separate unadsorbed PHF from the suspension, the mixture was centrifuged as described
previously. The concentration of PHF in the supernatant was determined by UV spectroscopy (λ
= 254 nm). The quantity of adsorbed PHF was then calculated as the difference between the
dosed quantity and the amount remaining in the supernatant. The adsorption density was
calculated as the number of molecules of PHF adsorbed per unit surface area of titanium dioxide
nanoparticles. The surface area of titanium dioxide nanoparticles was determined by BET
(Quantachrome Autosorb 1C-MS, Boynton Beach, FL). Surface coverage of titanium dioxide by
PHF was calculated using a PHF diameter of 1.3 nm(Ozawa, 1995), which gives a projected
surface area of 1.33 nm2 per molecule. The extent of adsorption was estimated from adsorption
results at neutral pH. Control experiments were conducted without titanium dioxide.
35
HR-TEM Imaging of PHF-TiO2 Nanocomposite
Samples for high resolution transmission electron microscopy (HR-TEM) were prepared
by sonicating 30 mg L-1 of TiO2 suspension for one hour, followed by ten fold dilution of the
suspension. The diluted suspension was again sonicated for 30 minutes and PHF was added to
the suspension to give 3 mg L-1 of TiO2 and 0.3 mg L-1 of PHF in the final suspension. A 10 µL
drop of suspension was pipetted on top of TEM grid (Lacey carbon grids, EMS, Hatfield, PA)
and allowed to dry overnight. The samples were imaged with field emission HR-TEM (JEOL
2010F) at 15 kV accelerating voltage. The PHF-TiO2 ratio was chosen to be 100 times higher
than that employed for photocatalysis experiments to increase the probability of detection of
PHF on TiO2 surface. HR-TEM images of PHF coated on TiO2 are presented in Figure 2-3. TiO2
is in the form of agglomerates of 20–30 nm size. A single crystal size of 5 nm is apparent from
other images (not shown). PHF is present as clusters of 2–3 nm with apparent lattice diffraction.
Electron Paramagnetic Resonance Spectroscopy for Hydroxyl Radical Determination
EPR experiments were carried out to investigate the radical generation capability of PHFTiO2 nanocomposite. EPR spectra were recorded at room temperature using a commercial
Bruker Elexsys E580 spectrometer employing Bruker’s high-Q cavity, ER 4123SHQE, and using
quartz capillaries of approximately 1x2 mm (IDxOD). Spectral parameters were typically 100
kHz modulation frequency, 1 G modulation amplitude, 2 mW microwave power, 9.87 GHz
microwave frequency, 20.48 ms time constant and 81.92 conversion time/point.
A photocatalyst-spin trap mixture was made up by combining 190 µL of photocatalyst
suspension containing 30 mg L-1 TiO2 and 0.03 mg/L of PHF with 10 µL of 5,5-dimethyl-1pyrroline N-oxide (DMPO) (Alexis) and 6 quartz capillaries were filled with the mixture. EPR
spectra were obtained before and after a predetermined time of exposure of each tube to UVA
lamps (13 W m-2). To quantify the concentration of hydroxyl radicals generated, the integrated
36
peak area of the DMPO-OH˙ spectra was compared with the integrated peak area of a spectrum
using 10 µM 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy radical (HTEMPO) (Fluka) at each
UVA irradiation time (Uchino et al., 2002). Analysis and double integration of the spectra was
performed with Grace, version 5.1.20, a public domain 2D-plotting and data analysis package
released under the GNU public license [http://plasma-gate.weizmann.ac.il/Grace/].
EPR experiments were also carried out to investigate the radical generation capability of
PHF alone. Procedure was same as described above except that TiO2 was omitted from the
reaction mixture. Control experiments with neither PHF nor TiO2 present were performed in the
dark and UVA irradiation.
Synthesis of Polyhydroxy Fullerenes
Polyhydroxy fullerenes (PHF) were synthesized via alkali route similar to Li et al. A
solution of fullerenes was prepared by adding 80 mg of C60 (95%, BuckyUSA, Houston TX) to
60 mL of benzene (HPLC grade, Fisher). A mixture of 2 mL of NaOH solution (1 g/mL) and 0.3
mL of tetra butyl ammonium hydroxide (40% solution) was prepared in a separate Erlenmeyer
flask. The fullerene solution was added to the alkali-surfactant solution under vigorous stirring.
After 30 minutes, the stirring was stopped and the mixture was allowed to phase separate. The
top clear phase was decanted and remaining slurry was stirred with additional 12 mL of
deionized water for 24 hours. The mixture was then filtered through Whatman 40 filter paper and
the filtrate was concentrated to 5 mL in a vacuum oven at 60ºC. The resultant slurry was washed
four times with 50 mL of methanol by alternate centrifugation (5000 g, 10 min) and
resuspension. After the final wash, PHF were suspended in 20 mL of methanol and dried under
vacuum at 60ºC. The mass of PHF obtained was 120 mg.
37
Dye Degradation Experiments with Synthesized PHF
Dye degradation experiments were conducted with anatase (5 nm, Alfa-Aesar) titanium
dioxide as the photocatalyst. A photocatalyst suspension was prepared by sonicating 30 mg L-1
of anatase for 1 hour. PHF was added to the suspension to give a final concentration of 0.03 mg
L-1. Procion Red MX5B was then added to the photocatalyst suspension to give a final
concentration of 3 mg L-1. The reaction mixture was transferred to a Petri dish with a magnetic
stirrer and placed 100 mm below a bank of 16 UVA lamps (Southern New England Ultra Violet
Company, Branfield, CT). The mixture was stirred in the dark for 10 minutes and then exposed
to UVA at 86 W m-2 intensity. Immediately prior to turning on the lights, two 1.5 mL samples
were pipetted into a plastic vial. Subsequent samples were collected at 15 min intervals for one
hour. Each collected sample was centrifuged twice at 10,000 g for 15 min. and the final
supernatant was transferred to a plastic (PMME) cuvette. The UV-Vis spectrum was then
obtained, with absorbance at 512 nm and 538 nm used for data analysis. The log of normalized
sample absorbance was plotted vs. irradiation time and the slopes measured to obtain pseudofirst order degradation coefficients.
Characterization of PHF
Mass Spectroscopy
PHF samples were analyzed with atmospheric pressure chemical ionization (APCI) mass
spectroscopy (ThermoFinnigan, San Jose, CA).
Gaussian Modeling
Structure optimization and vibration frequency prediction for PHF was performed with
Gaussian software. To simplify the computation, PHF was assumed to have 24 hydroxyl groups
based on information from the supplier. Becke-style 3-Parameter Density Functional Theory
38
(using the Lee-Yang-Parr correlation function) 6-31 G* basis set was employed. B3LYP is a
hybrid of density functional and Hartree-Fock theory (Foresman, 1996).
FTIR and XPS
Both fresh and aged form of PHF was characterized by Diffuse Reflectance Infrared
Fourier Transform (DRIFT) and X-ray Photoelectron Spectroscopy (XPS). DRIFT experiments
were carried out with Thermo Electron Magna 760 with potassium bromide as background. XPS
experiments were performed with Kratos Analytical Surface Analyzer XSAM 800 in survey and
multiplex mode. The C1s spectrum was subjected to peak fitting analysis with Grams 7.01
software (Thermo Fisher Scientific, Waltham, MA) to determine the various oxidation states of
carbon.
39
Anatase coated MWNT
MWNT
40
TiO2
Figure 2-1. High-resolution TEM image of TiO2 coated on multi-wall nanotube.
Fan
Fan
UV Lamps
41
Spore/E. coli
suspension in
Petri dish
Figure 2-2. Experimental setup for photocatalysis experiments.
Magnetic
stirrer
TiO2
agglomerate
5 nm
5 nm
42
PHF Lattice
Diffraction
PHFclusters
Figure 2-3. High resolution TEM images of PHF coated on TiO2 nanoparticles.
CHAPTER 3
RESULTS AND DISCUSSION
The present work explores utilization of electron accepting character of carbon nanotubes
and fullerenes for scavenging photo-generated electrons from titanium dioxide for accelerating
photocatalysis. The photocatalytic activity of carbon nanotubes coated with titanium dioxide was
studied initially. This was followed by assessment of self-assembled nanocomposites of
polyhydroxy fullerenes and titanium dioxide. Photocatalytic activity and mechanisms of
interaction and enhancement were characterized. Finally, the influence of the composition of
polyhydroxy fullerenes on electron scavenging ability was evaluated.
Enhancement of TiO2 Photocatalysis with Carbon Nanotubes
Carbon nanotubes are known for their unique electronic properties. They have been employed
as electrodes for sensing applications (Popov, 2004; Guldi et al., 2005; Anantram and Leonard,
2006). Guldi et al. has also developed photoelectrochemical devices based on electron accepting
ability of carbon nanotubes (Guldi et al., 2005). This ability was exploited in the present study to
scavenge photo-generated electrons from titanium dioxide. Photocatalytic nanocomposites were
prepared by coating titanium dioxide on functionalized multi-wall carbon nanotubes (Lee, 2004;
Lee et al., 2005; Pyrgiotakis, 2006). The photocatalytic activity of nanocomposite was tested
against the best commercially available photocatalyst—Degussa P25, on equal surface area
basis, since contact area between the photocatalyst particle and microbial surface governs the
inactivation process. The concentration of Degussa P25 was first optimized for inactivation of
vegetative bacterial cells. The photocatalysts were then tested for their efficacy to inactivate
bacterial endospores, which are the most resistant life forms on the planet. This was compared to
performance against vegetative bacterial cells. Finally, size and aspect ratio of photocatalyst was
proposed as an important factor in the photocatalytic inactivation process.
43
Optimization of Photocatalyst Concentration
The optimum concentration of Degussa P25 was determined by inactivating E. coli with
different concentrations of photocatalyst. The performance of the photocatalyst was
characterized in terms of D value, which is the time required for one log reduction in viable cell
concentration. A typical example for D value estimation is presented in Figure 3-1.
Photocatalytic inactivation of E. coli was carried out with Degussa P25 at 10, 50, 100,
1000 and 5000 mg L-1 concentrations. The D values for different concentrations of Degussa P25
are plotted in Figure 3-2. Initially D value decreases with increase in concentration of
photocatalyst as expected, with the minimum of 4 min observed at the concentration of 100 mg
L-1. Further increase in concentration of Degussa P25 appears to exhibit a trend towards higher D
values. Increase in D value at higher concentrations of photocatalyst can be attributed to multilayer photocatalyst coating of bacteria, which blocks the UVA irradiation activating the
photocatalyst layer in contact with the bacterial surface. The shielding phenomenon has been
previously reported by Block et al. (1997). The optimum concentration of Degussa P25 at 100
mg L-1, which is similar to reported values (Onoda et al., 1988; Block et al., 1997), was chosen
for subsequent experiments involving photocatalytic inactivation of bacterial endospores as well
as vegetative bacterial cells.
Photocatalytic Inactivation of Bacterial Endospores
Bacterial endospores are the most resistant microorganisms against all disinfection
techniques. Bacillus cereus and Bacillus subtilis were employed as surrogates for Bacillus
anthracis, which causes anthrax. Bacillus cereus is very close to Bacillus anthracis in genetic
makeup (Ivanova et al., 2003; Read et al., 2003) and has thus become popular as a surrogate.
Bacillus subtilis spores have previously been employed as a surrogate for B. anthracis spores
44
(Nicholson and Galeano, 2003). TiO2 coated MWNT nanocomposites were compared with
Degussa P25 on the basis of equal surface areas. The concentration of Degussa P25 was 100 mg
L-1, as determined by optimization experiments. The BET surface area of TiO2 coated MWNT
was three times that of the Degussa P25 and therefore the mass concentration of 33 mg L-1 of
TiO2 coated MWNT was employed to give equal surface area.
B. cereus spores
The D values for inactivation of B. cereus spores are presented in Figure 3-3. The B.
cereus spores are susceptible to UVA irradiation with a D value of 170 min (Control).
Photocatalytic inactivation with Degussa P25, at an optimum concentration of 100 mg L-1, has a
D value similar to UVA inactivation. The time required for spore inactivation is significantly
longer as compared to that for a bacteria, such as E. coli, which has a D value of ~ 5 minutes (Lu
et al., 2003). The longer time for photocatalytic inactivation of spores can be explained on the
basis of its complex and thick shell structure (~ 200 nm), which needs to be degraded for
inactivation. It has been reported that increase in complexity and density of cell wall structure
increases the photocatalytic inactivation time (Zheng et al., 2000; Kuhn et al., 2003).
Photocatalytic inactivation experiments using TiO2 coated multi-wall carbon nanotubes yielded
D value of 72 minutes for inactivation, which is half that of best commercial titanium dioxide—
Degussa P25 (Lee, 2004; Pumprueg, 2004; Krishna et al., 2005; Lee et al., 2005). Experiments
conducted with functionalized and uncoated MWNT yielded a D value of 240 min, which is
higher than UVA inactivation. The functionalized MWNT can shield spores from UVA
irradiation thereby increasing the time required for photolytic inactivation and thus D value. The
results indicate that anatase coated multi-wall carbon nanotubes are able to increase the
photocatalysis efficiency, which is hypothesized to be due to delay in recombination process
thereby increasing generation of reactive species.
45
B. subtilis spores
A single set of inactivation experiments were conducted with B. subtilis spores. The D
values obtained for photocatalytic inactivation experiments are presented in Figure 3-4. The D
value for UVA inactivation was 240 min. This is higher than that obtained for B. cereus spores.
Photocatalytic inactivation with Degussa P25 gave a D value of 90 min, which suggests that B.
subtilis spores are susceptible to photocatalysis with P25, whereas B. cereus spores are not. The
higher resistance of B. cereus spores compared to B. subtilis spores towards photocatalytic
inactivation could be hypothesized to reflect the presence of exosporium. TiO2 coated MWNT
exhibited a D value of 56 min, indicating that these photocatalytic nanocomposites inactivate B.
subtilis spores faster than Degussa P25.
Photocatalytic Inactivation of Vegetative Bacterial Cells
Based on the encouraging results obtained from photocatalytic inactivation of bacterial
endospores with TiO2 coated MWNT, experiments were carried out on bacterial cells, which are
generally much more susceptible to disinfectants than endospores. Escherichia coli, which are
common enteric bacteria, were selected as a model microorganism for inactivation studies. E.
coli are rod shaped Gram negative bacteria that have a cell membrane comprising an inner
membrane, a middle peptidoglycan layer and an outer cell wall. The cell-wall of E. coli is about
10 times thinner than that of bacterial endospores. Thus the photocatalytic inactivation time is
expected to be shorter.
The D value for photocatalytic inactivation of E. coli with commercial titanium dioxide
nanoparticles (Degussa P25) was found to be 4 min (Fig. 3-5), which is in agreement with the
literature (Lu et al., 2003; Rincon and Pulgarin, 2003). This illustrates the greater susceptibility
of E. coli compared to endospores, considering that Degussa P25 had no effect on B. cereus
46
endospores and acted 20 times more slowly against B. subtilis spores. In contrast, no statistical
difference was observed between inactivation of E. coli with TiO2 coated MWNT and the control
containing no photocatalyst. These results suggests that TiO2 coated MWNT were not active
against E. coli.
Failure of TiO2 coated MWNT to inactivate E. coli can be rationalized on the basis of
mechanism of photocatalytic inactivation. The first crucial step involved in the process of
photocatalytic inactivation of a bacterium, as discussed in Chapter 2, is the contact of the
photocatalyst with cell-wall of the bacterium. It is hypothesized that limited or no contact of
TiO2 coated MWNT with cell-wall of E. coli is the cause for lack of photocatalytic inactivation.
Insufficient contact of TiO2 coated MWNT with cell-wall can be due to either electrostatic
repulsion or steric repulsion.
Electrostatic interactions were investigated by measuring zeta potential for TiO2 coated
MWNT, Degussa P25 and E. coli. The zeta potential values are presented in Table 3-1. TiO2
coated MWNT has higher negative zeta potential than Degussa P25 and thus can be
electrostatically repelled from E. coli surface. One approach to identify the role of electrostatic
interactions is to conduct photocatalysis experiments with TiO2 coated MWNT at a lower pH,
where the zeta potential of the photocatalyst has a smaller negative value. However,
susceptibility of E. coli to inactivating agents at lower pH can introduce an artifact in this
approach. Alternatively, photocatalysis experiments can be conducted with Degussa P25 at
higher pH where the zeta potential of the photocatalyst has a higher negative value.
Accordingly, photocatalysis experiments were carried out with Degussa P25 at pH 7.5. The zeta
potential values of Degussa P25 and E. coli at pH 7.5 are -25 mV and -40 mV, respectively. The
large negative values of zeta potential suggests that favorable electrostatic repulsion exists and
47
therefore Degussa P25 should not contribute towards photocatalytic inactivation of E. coli.
However, the D value increased from 4 min at pH 6 to 6 min at pH 7.5. A marginal increase in D
value suggests that electrostatic interactions may not be the dominant factor in limiting the
contact of TiO2 coated MWNT with cell-wall of E. coli.
Effect of Size and Aspect Ratio of Photocatalyst
Given that electrostatic interactions were shown not to be responsible for lack of contact,
steric repulsion would seem to be the most likely cause of insufficient contact between TiO2
coated MWNT and bacterial cell-wall. The cause of steric repulsion could be the surface
appendages of E. coli, such as fimbriae and flagella. Fimbriae are a characteristic feature of
Gram negative bacteria, such as E. coli and are usually 1–4 microns in length and 2–10 nm in
diameter (Mol and Oudega, 1996). Based on the TEM image (Klemm, 1994) of E. coli, the
average distance between two fimbriae can be estimated to be 100 nm. Therefore the size of the
photocatalyst (or radius of gyration in case of high-aspect ratio particles) must be less than 100
nm for particles to penetrate fimbriae and contact with cell-wall of E. coli. The dynamic nature
of fimbriae, due to thermal vibrations, can further reduce maximum size limit of particles for
penetration. TiO2 coated MWNT are high-aspect ratio photocatalysts with 25 nm diameter and
2–10 µm length. Since fimbriae are known to hinder interaction of macrophages (10 µm longest
dimension) with E. coli cells, it is plausible that fimbriae can also sterically hinder contact
between the TiO2 coated MWNT and cell-wall, since MWNT have a length of 2–10 µm. Steric
hindrance of TiO2 coated MWNT by fimbriae is depicted in Figure 3-6. In contrast, Degussa P25
has a primary particle size of 30 nm and therefore can easily penetrate between fimbriae and
adsorb on surface of E. coli, as shown in Figure 3-6.
Two different approaches were undertaken, as illustrated in Figure 3-7, to test the
hypothesis that surface appendages sterically hinder contact of high-aspect ratio photocatalyst
48
with bacterial cell-wall. In the first approach, high-aspect ratio photocatalyst were tested for
inactivation of mutants with and without surface appendages. According to the hypothesis, the D
value for inactivation of mutant with surface appendages should be higher than that for the
mutant without surface appendages. In second approach, TiO2 coated MWNT were ground to
decrease the aspect ratio, which should result in more effective penetration between fimbriae,
thereby increasing contact between photocatalyst and the bacterial cell-wall and, concomitantly,
bacterial inactivation (lowering of D value).
Approach-1: Mutants with and without surface appendages
Staphylococcus aureus was used as a model for bacteria with and without surface
appendages. E. coli could not be used as model as no mutants are known that are completely
devoid of surface appendages. Furthermore genetic engineering approaches to producing mutants
completely without fimbriae have been unsuccessful. The strains of S. aureus with surface
appendages used in this study were CP5 and Smith Diffuse. The mutant strains without surface
appendages were CP- and Smith Compact. The results with CP5 and CP- (Fig. 3-8) indicate that
these two strains are highly susceptible to UVA radiation. In fact an increase in D value was
observed in the presence of photocatalyst, which may be due to the UVA shielding by
photocatalyst particles. In a different experiment, the Smith diffuse and Smith Compact strains
were also found to be highly susceptible (D value = 4 min) to UVA radiation. No further
experiments were conducted for this approach due to high sensitivity of these strains to UVA
irradiation.
Approach-2: Size reduction of TiO2 coated MWNT
As mentioned earlier, TiO2 coated MWNT photocatalyst are 2–10 µm long and 20–30 nm
in diameter. Stirred media mill was employed for size reduction of this photocatalyst with
zirconia beads as grinding media. Grinding of TiO2 for 2–4 hours leads to phase transformation
49
from anatase (photocatalytically active) to rutile or brookite (photocatalytically inactive) (Criado
and Real, 1983). Therefore, the grinding time in the present study was limited to four hours. The
particle size of ground TiO2 coated MWNT was experimentally determined to be 500 nm, which
is larger than the estimated size of 100 nm required for sufficient adsorption of photocatalyst on
to cell-wall of E. coli. The Photocatalysis experiments with size-reduced TiO2 coated MWNT
particles did not inactivate E. coli. Failure of photocatalytic inactivation can be attributed to
either insufficient reduction of size or loss in photocatalytic activity. Grinding of TiO2 coated
MWNT can also result in delamination of TiO2 coating from the surface of carbon nanotubes,
which reduces the photocatalytic activity of the nanocomposite. No further experiments were
performed with size-reduction approach due to inherent flaws in the methodology, as mentioned
above.
The electron accepting ability of carbon nanotubes was successfully exploited for
enhancing TiO2 photocatalysis. However, the photocatalytic nanocomposites were unable to
inactivate E. coli due to their large size. Fullerenes, which are a spherical version of carbon
nanotubes, have electron accepting properties similar to carbon nanotubes (Sherigara et al.,
2003). Another advantage of fullerenes is their small molecular size of 1 nm, which overcomes
the size constraint for inactivation of microorganisms with surface appendages.
Enhancement of TiO2 Photocatalysis with Polyhydroxy Fullerenes
Fullerenes (C60) are known for their unique electronic properties (Guldi, 2000b; Gust et
al., 2000; Makarova, 2001). Kamat and coworkers have demonstrated the transfer of photogenerated electrons from titanium dioxide to fullerenes with ethanol/benzene mixture as solvent
(Kamat et al., 1994). Fullerenes are not soluble in water and toxic, limiting their use in aqueous
media for enhancing photocatalysis. The water-solubility of fullerenes is improved by coupling
hydroxyl groups to the molecules, creating the possibility of utilizing their electronic properties
50
in aqueous systems. Hydroxylated fullerenes are reported to be non-toxic and have been
proposed for therapeutic applications. However, addition of hydroxyl groups to the fullerene
structure modifies the electronic properties of the fullerenes. In one study, polyhydroxy
fullerenes synthesized via acid route could not scavenge electrons from electron rich amines such
as diazabicyclooctane (DABCO) and hexamine (Mohan et al., 1997). Furthermore, Brownian
motion of water-soluble fullerenes could impede their effective contact with titanium dioxide
surface for electron transfer process.
A nanocomposite of PHF and TiO2 can be synthesized either by coating a cluster of
fullerene molecules with TiO2 or by coating TiO2 with fullerenes. In the latter case, the
negatively charged PHF could potentially adsorb to the TiO2 near its isoelectric point (pH 5–6),
in effect representing self-assembly of photocatalytic nanocomposite. This approach is attractive
because it does not require chemical bonding of PHF to TiO2, and may also be more practical.
Accordingly, experiments with self-assembled nanocomposites of PHF and TiO2 were conducted
to test the ability of PHF to enhance the photocatalytic activity of TiO2.
Preliminary Dye Degradation Experiments with Self-assembled PHF-TiO2 Nanocomposites
Preliminary experiments were conducted to verify if PHF can enhance the photocatalytic
activity of TiO2 for degradation of an organic pollutant. Procion Red MX 5B, which is an
aromatic triazine monoazo compound, served as a model organic pollutant as it is relatively
resistant to oxidation. Anatase (99%) was chosen as the photocatalyst for dye degradation
experiments.
Measured trends in dye absorbance versus time, representing PHF alone, titanium dioxide
alone, and a mixture of PHF and titanium dioxide under solar UVA irradiation are presented in
Figure 3-9. The photocatalytic dye degradation reaction follows first-order reaction kinetics and
has been characterized with pseudo-first-order rate coefficients (So et al., 2002; Hu et al., 2003b;
51
Hu et al., 2003a; Yu et al., 2005a). Pseudo-first-order relationships were fitted to the data by
least squares linear regression of log absorbance vs. time. The fitted relationships, in which the
exponents give the photocatalysis rate coefficients, are shown adjacent to the respective curves.
The rate of degradation with self-assembled PHF-TiO2 nanocomposite, as indicated by the
slope of the fit, was statistically more significant than the rate with TiO2 alone (alpha=0.01). The
pseudo-first order rate coefficient of nanocomposite (0.0119±0.0001 min-1) was 1.6 times higher
than the rate coefficient with TiO2 alone (0.0073±0.001 min-1). It is noteworthy that the rate of
dye degradation with PHF alone (6 x 10-5 min-1) was two orders of magnitude less than obtained
with TiO2 alone. Previous studies have reported that PHF can generate superoxide radicals with
UVA and visible light (Kamat et al., 2000; Vileno et al., 2004; Pickering and Wiesner, 2005).
However, the rate of generation of superoxide radicals is dependent on the pH; the rate at pH 7
(9 x 10-5 min-1) is 100 times slower than at pH 5 (Pickering and Wiesner, 2005). In the present
study, at pH 6, the generation of superoxide radicals may not be significant and therefore no dye
degradation was observed.
Optimization of PHF-TiO2 Nanocomposites for Photocatalysis
Polyhydroxy fullerenes were successfully employed to enhance the photocatalytic activity
of TiO2. Preliminary dye degradation experiments with a self-assembled nanocomposite showed
that amount of PHF required to enhance the photocatalytic activity was three orders of
magnitude less than TiO2 concentration. Assuming that electron scavenging is the mechanism of
observed enhancement, the magnitude of enhancement is dependent on the surface coverage of
titanium dioxide nanoparticles with PHF. Therefore, an optimum concentration of PHF is
expected which will result in maximum enhancement for degradation of dye.
Further dye degradation experiments were carried out to optimize PHF concentration. The
photocatalysis rate coefficients for experiments performed with different dosed weight ratios of
52
PHF to titanium dioxide are shown in Figure 3-10. Ratios in the range of 0.001 to 0.003 gave the
highest rate coefficients, which were 1.64–1.68 times higher than the rate coefficient for titanium
dioxide alone. Further increase in the ratio caused a decrease of dye degradation rate, with
finally no dye degradation observed at a PHF to titanium dioxide ratio of 0.09.
The presence of optimum dosed weight ratio of PHF to titanium dioxide can be explained
on the basis of surface coverage of titanium dioxide nanoparticles by PHF. At the ratio of
0.0002, the surface coverage of titanium dioxide nanoparticles is very low (< 1%) and therefore
few photo-generated electrons are scavenged by PHF. Further increase in PHF concentration
leads to greater surface coverage and therefore the number of scavenged photo-generated
electron also increases, resulting in higher photocatalysis efficiency (lower recombination). This
phenomenon is observed at PHF to titanium dioxide ratios of 0.001–0.003 (2–7 %). Further
increase in the ratio can lead to complete monolayer coverage of titanium dioxide surface
exposed to light, resulting in no apparent enhancement. The reduction in enhancement can be
due to blocking of active sites for radical generation and shielding of TiO2 from UVA irradiation.
The calculated surface coverage (232%) at which photocatalysis is almost completely suppressed
represents multi layer coating of PHF on TiO2. Fullerenes are well known for their optical
limiting ability—higher absorption of light in excited (triplet) state—so that a few layers of PHF
should be sufficient to block the UVA radiation entirely [ref]. These calculations assumed that
PHF molecules were in non-aggregated state, since aggregation has been predicted to occur only
at much higher concentrations (Jeng et al., 1999).
After successful application of PHF to enhance photocatalytic activity of anatase for dye
degradation, experiments were conducted to test the ability of PHF to enhance the activity of the
best commercially available photocatalyst, Degussa P25. The optimum PHF/TiO2 ratio obtained
53
with dye degradation studies was employed for accelerating inactivation of E. coli with Degussa
P25.
Enhancement of E. coli Inactivation with PHF-TiO2 Nanocomposite
Kinetics of the photocatalytic inactivation of E. coli are presented in Figure 3-11.
Inactivation rate in the absence of P25 (i.e., control and PHF alone) was essentially zero. The
inactivation rate coefficient for P25 + PHF (0.177±0.022 min-1) is 1.9 times faster than the
inactivation rate for P25 alone (0.094±0.027 min-1). Figure 3-12 shows that the D value (time
required for 1 log reduction of E. coli) with P25 + PHF is 7.1 min, which is significantly less (α
= 0.05) than the time of 11.7 min required with P25 alone. Complete destruction (7 log10
reduction) of E. coli is achieved in 40 minutes with P25 + PHF, compared to 60 minutes with
P25 alone. These results are noteworthy, since P25 is the best commercially available
photocatalyst. These results indicate that the PHF is capable of enhancing the action of both
anatase and Degussa P25 (mixture of anatase and rutile) and may also enhance photocatalytic
activity of materials such as zinc oxide, vanadium oxide and cerium oxide.
In order to gain a better understanding of the interactions between PHF and TiO2, the
mechanisms of interaction both in terms of self-assembly and photocatalysis need to be
examined. Thus the adsorption of PHF was first studied, followed by analysis of free hydroxyl
formation.
Adsorption of PHF on TiO2 Nanoparticles
Adsorption experiments were carried out to investigate self-assembly of PHF on TiO2.
Predetermined concentrations of PHF were added to TiO2 suspensions and the amount of
adsorbed PHF was determined by depletion method. The concentration of PHF remaining in the
solution was ascertained with UV-Vis spectroscopy. The UV-Vis absorption spectrum of PHF in
54
the range of 200–800 nm wavelengths is shown in Figure 3-13. The absorbance at 254 nm was
selected for calibration, as organic carbon analysis is usually performed at this wavelength
(Deflandre and Gagne, 2001; Potter, 2005). The wavelength of 254 nm minimizes the influence
of interferences while maximizing sensitivity. Examples of calibration curves at three different
pH values are shown in Figure 3-14.
The experimental data indicate that PHF molecules adsorb to titanium dioxide particles.
Adsorption density of PHF on titanium dioxide nanoparticles increased with lowering of pH
(Fig. 3-15). Correspondingly, the shift in zeta potential (difference between zeta potential of
nanocomposites and titanium dioxide alone) became more negative with lowering of pH. The
zeta potential shifts were highly correlated (α = 0.01) with adsorption density (Fig. 3-16).
PHF molecules have affinity towards titanium dioxide, as indicated by adsorption density.
The isoelectric point (i.e., the pH at which the zeta potential is zero) of titanium dioxide is in the
pH range 5–6 (Kosmulski, 2002; Miyauchi et al., 2004; Liufu et al., 2005). Upon adsorption of
PHF molecules at pH 3.3, the zeta potential of titanium dioxide changed from +25 mV to -30
mV indicating that PHF molecules are negatively charged. The trends of adsorption density and
shifts in zeta potential values with varying pH are consistent with the PHF molecules having a
negative charge (Mohan et al., 1997) based on the following reasoning. At acidic pH values, the
surfaces of titanium dioxide nanoparticles have a net positive charge and larger amount of PHF
molecules are electrostatically attracted towards the titanium dioxide resulting in higher
adsorption density. Since some of the positive sites on titanium dioxide surface are covered by
PHF molecules, the net charge displayed by the surface decreases leading to lowering of zeta
potential and hence a pronounced shift in zeta potential towards negative values. Above IEP of
TiO2, the PHF molecules are electrostatically repelled from the negatively charged titanium
55
dioxide surfaces and hence the adsorption density is low. At very low adsorption, the surface
charge remains essentially unaltered and therefore the shift in zeta potential is negligible.
Electrostatic forces govern the adsorption of PHF molecules onto surface of titanium dioxide
nanoparticles.
Detection of Hydroxyl Radicals with EPR
One approach to validate the hypothesis that PHF enhances TiO2 photocatalysis by
electron scavenging is by measuring the concentration of hydroxyl radicals. The concentration of
hydroxyl radicals can be determined by either fluorescence techniques or by detecting spin
trapped radicals with electron paramagnetic resonance spectroscopy (EPR). The former approach
comprises of organic molecules that become fluorescent when attacked by hydroxyl radicals.
However, superoxide radicals, which are produced during photocatalysis, can also react with the
organic molecules thereby contributing to the fluorescent signal. In contrast, the latter approach
utilizes spin-trap agents, which are not susceptible to attack by superoxide radicals. Accordingly,
EPR experiments were conducted to test the electron scavenging hypothesis with 5,5-dimethyl-1pyrroline N-oxide (DMPO) as spin-trap for hydroxyl radicals.
The EPR spectra obtained after UVA irradiation of DMPO and TiO2 alone or TiO2+PHF
(Fig. 3-17) were identical in terms of peak locations and the characteristic 1:2:2:1 relative peak
magnitudes associated with trapped hydroxyl radicals in the form of DMPO-OH (Uchino et al.,
2002). Presence of PHF in the reaction mixtures had no effect on peak locations, but
substantially increased peak magnitudes. This shows that addition of PHF increases the
concentration of DMPO-OH˙, indicating higher rates of free hydroxyl radical generation.
56
Plots of the DMPO-OH˙ concentrations achieved after UVA irradiation of various reaction
mixtures is presented in Figure 3-18. No production of DMPO-OH˙ was observed in the dark
control. A concentration of 1.2 µM DMPO-OH was formed in the UVA control over 60 min,
corresponding to a yield of 0.33 M cm2 J-1. This is comparable to the irradiated control yield of
0.5 M cm2 J-1 obtained by Uchino et al. (Uchino et al., 2002). Production of DMPO-OH˙ by
irradiation of PHF alone was similar to the UVA control, indicating that PHF alone does not
generate more hydroxyl radicals than UVA alone. Substantially higher DMPO-OH˙
concentrations were obtained when TiO2 was present in the reaction mixture, reflecting the
contribution of photocatalysis. The rate of DMPO-OH˙ production in the presence of both TiO2
and PHF was in the range of 20–60% higher than the rate obtained with TiO2 alone.
The concentration of DMPO-OH˙ generated exhibits a maximum as a function of UVA
irradiation time followed by a gradual decrease. Similar observations have been reported by
others for ceria (Hernandez-Alonso et al., 2004) and TiO2 photocatalysts (Dvoranova et al.,
2002). The gradual decrease after reaching a maximum may be attributed to degradation of
trapped radicals, accompanied by a drop-off in the rate of DMPO-OH˙ production. DMPO-OH˙
is known to be unstable, with a half-life of 20 min (Grela et al., 1996). One possible mechanism
of degradation is multiple additions of hydroxyl radicals on DMPO (Dvoranova et al., 2002).
However, Hernandez-Alonso et al. (Hernandez-Alonso et al., 2004) showed experimental results
that conflict with the multiple addition mechanism, and argued that direct oxidation of DMPOOH˙ radicals by photo-generated holes could better explain their results. A slow-down in the
production rate of DMPO-OH˙ could occur due to consumption of oxygen (Brezova et al.,
1991), resulting in accumulation of electrons thus promoting electron-hole recombination. Direct
57
oxidation of DMPO-OH˙ radicals by photo-generated holes would lead to a slow-down in free
hydroxyl generation from holes and thus decreased DMPO-OH˙ production.
Contrary to Mohan and coworkers’ conclusion that electron accepting ability of fullerenes
is lost upon hydroxylation (Mohan et al., 1997), the observed enhancement in dye degradation,
E. coli inactivation and hydroxyl radical generation indicates that PHF molecules retain their
electron scavenging ability. The enhancement is speculated to proceed in the following way.
The photo-generated electrons and holes in titanium dioxide particles take part in redox reactions
at the surface or recombine, as depicted in Figure 3-19a. The recombination process has a faster
kinetics and controls the efficiency of photocatalysis (Hoffmann et al., 1995). In the presence of
adsorbed PHF molecules, photo-generated electrons are scavenged by PHF molecules (Fig. 319b), decreasing the number available for recombination. The scavenged electrons ultimately
enter reduction reactions, assuming that PHF act as an electron relay similar to fullerenes as
suggested by Guldi and Prato (Guldi, 2000b).
The discrepancy in electron scavenging ability of PHF between our study and the literature
can be attributed to the molecular composition. The PHF employed in the present study was
synthesized via alkali route, whereas Mohan et al. produced PHF via acid synthesis route. The
methodology employed for synthesis can dictate the composition of functional groups present on
the fullerene cage. The estimated number of hydroxyl groups for PHF synthesized via alkali
route is usually higher (24–42) than those synthesized via acid route (18–24). PHF typically
contains impure groups such as hemiketal, epoxide and carbonyl. As discussed below, increasing
the concentration of certain functional groups can either enhance or result in loss of electron
scavenging ability.
58
Influence of Composition of Polyhydroxy Fullerenes on Photocatalytic Enhancement
Recent literature has emphasized the need for extensive characterization of PHF as the
type and concentration of functional groups present on fullerene cage can significantly influence
its properties (Xing et al., 2004; Rodriguez-Zavala and Guirado-Lopez, 2006). PHF usually
contains impure groups such as hemiketal, epoxide and carbonyl. The functionality of PHF can
also influence its electron scavenging ability and, hence, its role in photocatalysis. Increasing the
concentration of certain functional groups can either enhance or result in loss of electron
scavenging ability. Accordingly, experiments were carried out with two different PHF molecules
for enhancement of titanium dioxide photocatalysis. A freshly synthesized batch of PHF
molecules, which was able to show enhancement, was termed “fresh PHF,” whereas a batch of
PHF molecules aged for 18 months and showing no enhancement were termed “aged PHF.”
Both types of PHF were extensively characterized to determine the functional groups present and
correlate composition with electron scavenging ability.
Photocatalytic Dye Degradation with Fresh and Aged PHF
Photocatalytic degradation of Procion Red dye with TiO2 alone, TiO2 + fresh PHF and
TiO2 + aged PHF are presented in Figure 3-20. The ratio of PHF to TiO2 was set at the optimum
value of 0.001 as previously determined. The activity of PHF was determined by its ability to
enhance photocatalytic degradation rate. The pseudo-first order rate coefficient with fresh PHF
(0.0128±0.0029 min-1) was 2.6 times higher than the rate coefficient without PHF
(0.0048±0.0005 min-1). The enhancement (2.6×) with the synthesized batch of fullerenes is
higher than previously reported by Krishna et al. (2006). Presence of aged PHF gave a
photocatalytic rate that was not significantly different than TiO2 alone.
59
Mass Spectroscopy for Fresh and Aged PHF
Mass spectroscopy was employed to investigate the stability of fullerene cage of fresh and
aged PHF. The portions of the MS spectra in the range of 700–850 m/z for fresh and aged PHF
are presented in Figure 3-21. The base fullerene (C60) peak at 720.6 m/z is present at 100
relative intensity in both fresh and aged PHF, indicating that fullerene cage is intact in both PHF
samples. The presence of peaks in the range expected for PHF ions (approximately 1600 m/z
based on XPS data) would assist in estimating the molecular weight of the PHF. However, no
peaks were observed in this range (data not shown). The absence of peaks representing PHF ions
has also been reported by Xing et al. (2004), Chen et al. (2001) and Chiang et al. (1993).
FTIR Analysis of Fresh and Aged PHF
FTIR spectroscopy has been employed in literature to identify various functional groups
present on fullerene cage (Chiang et al., 1993; Li et al., 1993; Husebo et al., 2004; Xing et al.,
2004). The FTIR-DRIFT spectra for fresh and aged forms of PHF are presented in Figures 3-22
and 3-23, respectively. (Note that the wavenumbers in this plot are increasing from right to left
as is commonly portrayed in the literature.) The fresh PHF exhibits five peaks at 3300, 1591,
1450, 1062, 881 and 471 wavenumbers along with shoulders at 1661, 1357 and 1165
wavenumbers. The aged form of PHF has peaks at 3300, 1595, 1408 and 1074 wavenumbers
with a shoulder at 1680 cm-1.
The vibrational modes of PHF were assigned to FTIR peaks of fresh and aged PHF based
on the information obtained from Gaussian simulation and the literature (Table 3-2).
60
Gaussian simulation
Semi-empirical computation (PM3) has been employed in the literature for structure
optimization of various hydroxylated fullerenes and possible stable isomers are reported (Slanina
et al., 1996; Guirado-Lopez and Rincon, 2006; Rodriguez-Zavala and Guirado-Lopez, 2006) .
However no reports are present on theoretical prediction of vibrational peaks for hydroxylated
fullerenes. The purpose of utilizing hybrid quantum chemical computation in the present study
was to identify the vibration modes responsible for experimental FTIR peaks. For ease of
simulation PHF was assumed to have 24 hydroxyl groups and no carbonyl, hemiketal and
epoxide functionalities. The structure optimization and vibration spectrum generation were
performed with hybrid quantum-chemical basis set (B3LYP 6-31G*) (Foresman, 1996). The
optimized structure (Fig. 3-24) is similar to the reported C60(OH)24 structure obtained by a semiempirical quantum-chemical optimization (PM3) (Slanina et al., 1996). The hydroxyl groups are
present as intramoleculary hydrogen bonded islands on fullerene cage similar to the results
obtained in literature (Guirado-Lopez and Rincon, 2006; Rodriguez-Zavala and Guirado-Lopez,
2006). The average C–O and O–H bond length are 1.43 and 0.98 Å, respectively, and the
average C–O–H bond angle is 107°. These results are similar to the values obtained with PM3
optimization for C60(OH)26 by Guirado-Lopez and Rincon (2006). Additionally, weak
intramolecular hydrogen bonding was observed with average bond length of 1.78 Å, which is
similar to the hydrogen bond length in water.
The vibration spectrum generated for C60(OH)24 with B3LYP 6-31 G* basis set is
presented in Figure 3-25, with wavenumbers increasing from left to right on x axis. The
simulated FTIR peak exhibits four major peaks. The broad peak at 3450 cm-1 originates from O–
H stretching. The peak at 1450 cm-1, 1070 cm-1 and 370 cm-1 represents C–O–H bending, C–O
stretching and O–H rocking vibrations, respectively.
61
In reality, PHF molecules synthesized via alkali route have hemiketal and epoxide groups
along with hydroxyl groups as revealed by FTIR and XPS analysis in the present study and
reports in the literature (Husebo et al., 2004; Xing et al., 2004). Rodriguez-Zavala and GuiradoLopez (2006) have performed theoretical studies on hydroxylated fullerenes with different
numbers of epoxide groups. They found that presence of epoxide groups on hydroxylated
fullerenes can have significant effects on their structure and electronic and optical properties.
Further theoretical and experimental research is required to elucidate the effect of different
functionalities on physical, electronic and chemical properties of PHF.
Literature values
The peak at 1591 was not observed in the simulation and was ascribed to C=C vibrations
as reported in the literature (Chiang et al., 1993; Li et al., 1993; Xing et al., 2004). The shoulders
at 1658 cm-1, 1357 cm-1 and 1165 cm-1 were attributed to hemiketal (Xing et al., 2004), epoxides
and esters (Schraff et al., 2005)respectively.
XPS Analysis for Fresh and Aged PHF
Polyhydroxy fullerene (PHF) molecules have hemiketal, epoxide and sometimes carbonyl
functional groups in addition to hydroxyl groups. Therefore extensive characterization of PHF as
well as determination of its empirical formula is necessary to compare the properties with those
reported in the literature. Unfortunately, there is a lack of consistency in techniques employed for
determining the empirical molecular formula of PHF. The three common methodologies
employed are elemental analysis (Li et al., 1993; Chen et al., 2001; Xing et al., 2004; Vileno et
al., 2006), thermo gravimetric analysis (Goswami et al., 2004; Alves et al., 2006) and XPS
(Husebo et al., 2004). However, as Husebo et al. reported, the presence of residue in TGA and
elemental analysis can influence the empirical formula. The present study therefore utilizes XPS
data, similar to Husebo et al., to determine the empirical formulas for fresh and aged PHF.
62
XPS analysis indicated presence of C, O and Na for both fresh and aged forms of PHF.
The relative atomic concentrations determined for each element are provided in Table 3-3. The
C1s region was further analyzed to reveal three oxidation states of carbon as shown in Figures 326 and 3-27, which is in agreement with the literature (Chiang et al., 1993; Xing et al., 2004).
Since no carbonyl peak was present in FTIR spectra, the highest oxidation state was assumed to
be due to hemiketal structure as revealed from FTIR. Curve fitting analysis was performed for
the C1s spectrum of fresh and aged PHF to calculate the relative concentration of each carbon
oxidation state (Table 3-4). The relative concentration of mono-oxygenated carbon is three times
higher in fresh PHF than aged form.
The relative atomic concentrations of C, O and Na and along with concentrations of monoand di-oxygenated states of carbon were employed to deduce the composition of PHF according
to Husebo et al. (2004). The molecular formula for fresh PHF was calculated as
C60O8(OH)28Na10 and for aged PHF as C60O16(OH)10Na24. As seen from the empirical formulas,
the number of functional groups added to fullerene cage in this study is within the range (24–42)
of reported values (Xing et al., 2004; Vileno et al., 2006) however, fresh PHF has a higher
number of functional groups per molecule than aged PHF.
TGA Analysis for Fresh and Aged PHF
Thermo gravimetric analyses (TGA) of fresh and aged PHF are presented in Figure 3-28.
The weight loss generally occurs in 4–5 stages, similar to that reported by Chiang et al. (1993).
The first stage in weight loss is attributed to desorption of physically adsorbed water and
accounted for 16% of the weight of both fresh and aged PHF. The second stage is attributed to
desorption of hydroxyl functional groups and was 20% and 21% of the weight of fresh and aged
PHF respectively. The weight loss in stage 3 is attributed to desorption of hemiketal functional
groups and accounted for 11% of weight of fresh PHF and 34% of the weight of aged PHF. A
63
small weight loss (6%) in stage 4 was observed for fresh PHF and is attributed to desorption of
carbonyl or epoxide groups. No weight loss in stage 4 was observed in agede PHF suggesting
absence of carbonyl or epoxide groups. Finally aged PHF exhibited thermal degradation of
structure resulting in further 19% weight loss (stage 5). No loss in stage 5 was observed for fresh
PHF. The total weight loss up to a temperature of 1000 C was 54% for fresh PHF, which is close
to the 52% weight loss reported by Husebo et al. (2004). TGA shows that the ratio of the weights
of hemiketal to hydroxyl groups is higher for aged PHF. The same trend in terms of number of
hemiketal to hydroxyl groups is also observed in XPS results.
Effect of Impure Functional Groups
Stability: As mentioned earlier, functional groups other than hydroxyls can influence the
properties of PHF. Xing et al. (2004) has synthesized PHF with different concentrations of
functional groups by controlling the concentration of alkali. The various PHF were then
characterized extensively to determine the effect of presence of impure groups (hemiketal,
epoxide and carbonyl) on stability of fullerene cage. Laser induced dissociation and XPS
analyses indicated that the stability of the fullerene molecule decreases with higher
concentrations of impure groups. Xing et al. define a parameter R (ratio of impure groups to
hydroxyl groups as determined by XPS analysis) and suggest that PHF molecules with R less
than 0.2 and number of hydroxyl groups less than 36 are mostly stable. In the present study, the
values of R (as determined by XPS analyses) for fresh and aged PHF are 0.27 and 1.66
respectively, suggesting lower stability of aged PHF. The lower stability of aged PHF is
confirmed by thermal degradation observed at temperatures greater than 800 °C (Fig. 3-28).
Electron scavenging ability: Theoretical simulation by Rodriguez-Zavala and GuiradoLopez (2006) suggest that presence of impure groups (hemiketal and epoxide) can significantly
influence the electronic property of PHF molecules. In absence of impure groups, the PHF
64
molecules were able to take up to 6 electrons without rupture of carbon structure. It should be
noted that addition of six electrons resulted in cleavage of a single hydroxyl bond, suggesting
further addition of electrons can promote dehydroxylation of fullerene cage. In presence of
impure groups, addition of electrons did not result in desorption of hydroxyl moieties, instead
leading to surface reconstruction where C–C bonds containing an adsorbed oxygen were
replaced by C–O–C bonds. Based on these theoretical studies and experimental investigations by
Xing et al. (2004), the relative concentration of impure groups can be proposed to influence the
electron scavenging ability of PHF molecules. PHF molecules with lower ratios of impure
groups to hydroxyl groups (and with fewer than 36 hydroxyl groups) are needed for efficient
scavenging of electrons leading to greater enhancement in titanium dioxide photocatalysis. These
observations indicate that more efficient design of PHF is possible
.
65
0
66
Log (Survival Ratio)
-1
D value = ∆t
∆S
-2
∆S
-3
Rate Coefficient =
∆S
∆t
-4
∆t
-5
y = -0.1647x + 0.0004
-6
-7
0
10
20
30
40
Time (min)
Figure 3-1. D value estimation for photocatalytic inactivation of E. coli with Degussa P25.
50
60
80
D Value (min)
70
60
50
40
30
67
20
10
0
10
100
1000
10000
Titanium dioxide concentration (mg/L)
Figure 3-2. Optimum concentration of Degussa P25 for photocatalytic inactivation of E. coli. Error bars are ±1.0 SD; some of the
error bars are too small to be visible.
300
D Value (min)
250
200
150
100
68
50
0
Control
Funct. MWNT
Figure 3-3. Photocatalytic inactivation of B. cereus spores.
Degussa P25
TiO2 coated
TiO2
coated
MWNT
MWNT
250
D Value (min)
200
150
100
69
50
0
Control
Figure 3-4. Photocatalytic inactivation of B. subtilis spores.
Degussa P25
TiO2
TiO2 coated MWNT
MWNT
100
D Value (min)
80
60
40
70
20
0
Control
Figure 3-5. Photocatalytic inactivation of E. coli.
Degussa P25
TiO
TiO2
coated MWNT
MWNT
2 coated
2 – 10 microns
TiO2 coated nanotubes (Dia = 25 nm)
Degussa P25 (Dia = 30 nm)
71
Fimbriae
Figure 3-6. Interaction of TiO2 coated MWNT and Degussa P25 with surface appendages of E. coli.
Hypothesis
High aspect ratio photocatalysts cannot
inactivate microbes with surface appendages
Experiments with high aspect ratio
photocatalysts on microbes with &
without surface appendages
Experiments with low aspect
ratio photocatalysts on microbes
with surface appendages.
72
Experiments with mutant
strains of bacteria with and
without surface appendages
UV was sufficient to inactivate
S.aureus mutants
Grinding of TiO2
coated multi-wall
carbon nanotubes
Fullerenes as eacceptors
They did not
inactivate E. coli
Figure 3-7. Two different approaches undertaken to test the hypothesis that surface appendages sterically hinder contact of highaspect ratio photocatalyst with bacterial cell-wall.
10
CP5
CP-
D Value (min)
8
6
4
73
2
0
Control
Degussa P25
TiO
TiO2
coatedMWNT
MWNT
2 coated
Figure 3-8. Photocatalytic inactivation of S. aureus mutants with and without surface appendages. The strain CP5 has surface
appendages and CP- is mutant strain without appendages.
PHF
0
y = -6E-05x - 0.0003
-0.1
TiO2
log (C/Co)
-0.2
y = -0.0073x - 0.0398
-0.3
TiO2 + PHF
-0.4
-0.5
74
-0.6
-0.7
y = -0.0119x - 0.0174
-0.8
0
15
30
45
60
Time (min)
Figure 3-9. First-order degradation kinetics of Procion Red MX-5B upon UVA irradiation with PHF, TiO2 and a mixture of TiO2 and
PHF. Error bars are ±1.0 SD; some of the error bars are too small to be visible.
TiO2
75
Normalized Pseudo-FirstOrder Rate Coeffecient
TiO2
1.8
TiO2
b
1.6
1.4
1.2
a
1
0.8
0.6
0.4
c
0.2
0
-0.001
0
0.001
0.002
0.003
0.004
0.005
0.1
PHF/ TiO2 Ratio (Initial wt. basis)
Figure 3-10. Normalized pseudo-first-order rate coefficient for dye degradation as a function of the ratio of added polyhydroxy
fullerenes (PHF) to TiO2. Values above 1.0 indicate enhancement of photocatalysis relative to that achieved with TiO2
alone. Means of the normalized pseudo-first order rate coefficient at PHF ratios of 0, 0.001 and 0.093 were significantly
different from each other at α = 0.01, as indicated with the different letters above the respective data points. The error bars
at point b are too small to be seen.
PHF
0
Control
76
Log (Survival Ratio)
-1
-2
P25
-3
-4
-5
-6
P25+PHF
-7
-8
0
10
20
30
40
50
60
Time (min)
Figure 3-11. Photocatalytic inactivation of E. coli plotted as a function of survival ratio vs. time. Error bars are ±1.0 SD; error bars for
PHF and control are too small to be visible. The rate with P25+PHF is significantly greater than the rate with P25 alone at
α = 0.05.
15
77
D value (min)
12
9
6
3
0
P25
P25 + PHF
Figure 3-12. D values for E. coli inactivation with Degussa P25 alone and a mixture of Degussa P25 and PHF. Error bars are ±1.0 SD.
0.4
Absorbance
0.3
0.2
78
0.1
0
200
300
400
500
Wavelength (nm)
Figure 3-13. Absorption spectrum of polyhydroxy fullerenes.
600
700
800
0.4
2
R =1
79
Absorbance at λ=254 nm
0.35
2
R =1
0.3
pH 6.1
0.25
pH 9.9
2
R = 0.995
0.2
0.15
pH 3.3
0.1
0.05
0
0
2
4
6
Concentration of PHF (mg/l)
Figure 3-14. Calibration curves for polyhydroxy fullerenes (PHF) at three different pH values.
8
10
b
2.5E+17
-10
2E+17
-20
1.5E+17
-30
-40
1E+17
b
B
a
-50
B
b
5E+16
Δ Zeta Potential (mV)
80
(# of molecules of PHF/ m2)
A
a
Adsorption Density
0
b
-60
0
-70
0
2
4
6
8
10
12
pH
Figure 3-15. Adsorption density of polyhydroxy fullerenes (PHF) on titanium dioxide and shift in zeta potential of titanium dioxide
nanoparticles with adsorption of polyhydroxy fullerenes at different pH. Means labeled with same letter are not
significantly different at α=0.01.
81
Δ Zeta Potential (mV)
-70
-60
-50
-40
y = -3E-16x + 1.2002
R2 = 0.9621
-30
-20
-10
00
-5E+16
0
5E+16
1E+17
1.5E+17
2E+17
Adsorption Density (# of molecules of PHF/ m2)
Figure 3-16. Zeta potential shifts as a function of adsorption density.
2.5E+17
1200
82
EPR Intensity (arb. units)
TiO2
TiO2
TiO2 ++ PHF
TiO2
900
600
300
0
-300
-600
-900
-1200
3470
3500
3530
3560
Magnetic Field (Gauss)
Figure 3-17. Electron paramagnetic resonance spectra obtained upon UVA irradiation of DMPO and TiO2 alone and TiO2+PHF.
DMPO-OH (µM)
4
TiO2+PHF
3
83
2
TiO2
1
PHF
UVA Control
Dark Control
0
0
10
20
30
40
50
Time (min)
Figure 3-18. Effect of PHF on generation of hydroxyl radicals by UVA irradiation of titanium dioxide.
60
.A
A
A
A
PHF
hν
e-
10-3 sec
.A
hν
Conduction Band
10-7 sec
10-15 sec
Conduction Band
TiO2
TiO2
84
Valence Band
h+
.A
e-
Valence Band
h+
.D
10-7 sec
.D
b)
a)
D
D
Figure 3-19. Hypothetical photocatalytic reactions occurring upon UV irradiation. a) with TiO2 alone and b) in presence of adsorbed
PHF molecules. The time-scale for photocatalytic processes are from Hoffman et al. (1995).
Note: A and ˙A represents acceptor molecules before and after reduction (for e.g., O2 and ˙O2). D and ˙D represents donor
molecules before and after oxidation respectively (for e.g., OH¯ and ˙OH).
0
y = -0.0043x - 0.0063
-0.1
TiO2 + aged PHF
-0.2
y = -0.0053x - 0.0186
log (C/Co)
-0.3
TiO2 alone
-0.4
-0.5
y = -0.0128x + 0.0089
-0.6
85
-0.7
TiO2 + fresh PHF
-0.8
-0.9
0
15
30
45
60
Time (min)
Figure 3-20. First-order degradation kinetics of Procion Red MX-5B upon UVA irradiation with TiO2 alone, TiO2 + aged PHF and
TiO2 + fresh PHF. Error bars are ±1.0 SD; some of the error bars are too small to be visible.
720.6
100
90
80
70
721.5
60
776.5
50
777.5
40
722.7
30
775.1
20
10
723.1
0
700
710
743.7
735.4
718.9
700.6
736.6
720
730
740
752.3
761.8
750
760
770.3
770
Fresh PHF
778.7
779.1
792.1
802.5 808.5
780
790
720.6
100
800
/
810
820.5 827.9
820
830
843.2 846.3 84
840
850
86
90
Relative Abundance
80
70
721.5
60
50
776.4
40
777.5
Aged PHF
30
10 702.3
0
700
727.5
715.3
737.2
742.6
707.7
710
720
730
740
751.3 754.5 757.7
750
760
766.3
778.3
774.9
790.1
786.3
770
Figure 3-21. APCI-MS of fresh and aged Polyhydroxy Fullerenes.
780
841.6
792.1
795.1
20
790
838.4
801.8 808.3 812.6 816.4 820.5 829.3 836.5
800
810
820
830
840
849.3
850
C–O
0.14
0.12
Kubelka-Munk
C–O–H
0.1
C–C
0.08
C=C
0.06
87
0.04
Hemi-ketal
O–H
0.02
0
3600
2800
2000
Wavenumber (cm-1)
Figure 3-22. FTIR spectrum of fresh Polyhydroxy Fullerenes.
1200
400
0.12
C=C
0.1
Kubelka-Munk
C–O–H
0.08
0.06
Hemi-ketal
C–O
0.04
88
O–H
0.02
0
3600
2800
2000
Wavenumber (cm-1)
Figure 3-23. FTIR spectrum of aged Polyhydroxy Fullerenes.
1200
400
Carbon
Oxygen
Hydrogen
89
Figure 3-24. Gaussian simulation of C60(OH)24.
2 800
C–O
2 600
2 400
2 200
IR Intensity
2 000
C–O–H
1 800
1 600
1 400
1 200
O–H
1 000
90
800
O–H
600
400
200
0
0
500
1 000
1 500
2 000
Wavenumber (cm-1)
Figure 3-25. Gaussian simulation of vibrational spectrum for C60(OH)24.
2 500
3 000
3 500
180000
Experimental
Fitted
Non-oxygenated
Carbon
91
Counts per second
150000
Mono-oxygenated
Carbon
120000
90000
Hemi-ketal
60000
30000
292
290
288
286
284
282
Binding Energy (eV)
Figure 3-26. Experimental C1s XPS spectrum (top curve) of fresh Polyhydroxy Fullerenes with fitted curves representing three
different oxidation states of carbon.
180000
Non-oxygenated
Carbon
Counts per second
150000
120000
90000
Experimental
Fitted
Mono-oxygenated
Carbon
Hemi-ketal
92
60000
30000
292
290
288
286
284
282
Binding Energy (eV)
Figure 3-27. Experimental C1s XPS spectrum (top curve) of aged Polyhydroxy Fullerenes with fitted curves representing three
different oxidation states of carbon.
100
90 1b
1a
2a
80
Weight (%)
70
2b
60
3a
Fresh PHF
4a
50
3b
40
Aged PHF
30
93
20
5b
10
0
20
170
320
470
620
770
920
Temperature
Figure 3-28. TGA spectra for fresh and aged Polyhydroxy Fullerenes. Numbers refer to different stages of weight loss.
Table 3-1. Zeta potential values of photocatalysts and E. coli.
Systems
Zeta Potential (mV)
pH 6.0
pH 7.5
Escherichia coli
-35±5
-40±5
TiO2 coated MWNT
-20±5
-
Degussa P25
-15±5
-25±5
94
Table 3-2. Peak assignments of FTIR peaks for fresh and aged PHF based on results from
Gaussian simulation of C60(OH)24 and literature.
Peak Assignments based on
Vibration
modes
Peak Location based on
Gaussian
Simulation
O–H stretching
Hemiketal
3450
-
Gaussian simulation + literature
Literature
3420a, 3430b,
3410c, 3300d,
1658c,
1595a, 1600b,
Fresh PHF
Aged PHF
3300
3300
1661
1680
1591
1595
C=C stretching
-
C–O–H bending
1450
1392a, 1412c,
1450
1408
-
1376e
1357
-
1062
1074
Epoxides
C–O stretching
1070
c
d
1593 , 1585 ,
1084a, 1070b,
1065d,
Esters
-
1197e
1165
-
C–C
-
490f
471
-
a – Chiang et al., 1993; b – Li et al., 1993; c – Xing et al., 2004; d – Vileno et al., 2006; e –
Schraff et al., 2005; f – Alves et al., 2006.
95
Table 3-3. Elemental composition of fresh and aged PHF obtained with XPS analysis.
Relative Atomic Concentration
Elements
Fresh PHF
Aged PHF
Carbon
61.23 %
47.33 %
Oxygen
28.44 %
33.82 %
Sodium
10.33 %
18.85 %
Table 3-4. Peak position and elemental composition of fresh and aged PHF obtained with XPS
analysis.
Oxidation States of
Carbon
Peak Position
Relative Concentration
Fresh PHF
Aged PHF
Non-oxygenated
284.8 eV
284.8 eV
40.2 %
57.1 %
Mono-oxygenated
286.16 eV
286.14 eV
47.1 %
16.1 %
Di-oxygenated
288.55 eV
287.91 eV
12.7 %
26.8 %
96
Fresh PHF
Aged PHF
CHAPTER 4
CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH
Conclusions
Titanium dioxide photocatalysis has immense potential for pollution remediation and
inactivation of microorganisms as it has several advantages over current technology including
complete mineralization (complete oxidation of bacterial cell to carbon dioxide and water) of
microorganisms and no release of toxic by-products. The intrinsic disadvantage of low quantum
efficiency for titanium dioxide has motivated researchers to integrate titanium dioxide with
metallic and organic components as enhancers with contradictory results reported (Vamathevan
et al., 2002; Arabatzis et al., 2003a). Other materials which have been investigated for their
unique electronic properties are carbon nanotubes and fullerenes. They possess electron
accepting character—a requirement for enhancers.
The present study utilizes the electronic properties of carbon nanotubes and fullerenes by
functionalization and integration with titanium dioxide to accelerate photocatalysis. TiO2 coated
MWNT inactivated bacterial endospores twice as fast as the best commercially available
photocatalyst (Degussa P25), suggesting that MWNT can enhance the photocatalytic activity of
TiO2. Photocatalytic inactivation of microorganisms with TiO2 coated MWNT revealed that size
and aspect ratio of photocatalyst are important parameters to be considered while designing the
photocatalyst. TiO2 coated MWNT, which are high aspect ratio photocatalyst with one
dimension 2–10 µm length, were not able to photocatalytically inactivate Escherichia coli. The
surface appendages of Gram negative bacteria (E. coli) can prevent contact of TiO2 coated
MWNT with cell-wall of the bacterium thereby hindering photocatalytic inactivation. Although
the role of surface appendages (such as fimbriae and capsules) in evasion from recognition and
phagocytosis by immune system is well known (Henderson et al., 2003), its role in sterically
97
hindering photocatalysis was unknown until now. The present studies suggest that an important
design criterion for developing photocatalysts is the size, which should be less than 100 nm to be
effective against microorganisms with surface appendages, such as E. coli.
Consequently fullerenes, which are 1 nm spherical version of carbon nanotubes and
therefore expected to share the same electron scavenging ability, were employed to enhance the
photocatalytic activity of TiO2. Pristine fullerenes are not soluble in water and are reported to be
toxic. However, hydroxylation of the fullerenes renders them water soluble and eliminates
toxicity. Nanocomposites of the hydroxylated (i.e. polyhydroxy) fullerenes and TiO2 were
formed by self-assembly. The PHF-TiO2 nanocomposites were successfully employed to
enhance the photocatalytic efficacy of titanium dioxide nanoparticles for degradation of Procion
red dye, which because of its aromatic structure is relatively resistant to oxidation.
The affinity of PHF molecules towards titanium dioxide surface enables the electron
scavenging properties of PHF to be exploited without the need for chemical bonding between
PHF molecule and titanium dioxide through sol-gel or other coating process. Earlier work on
improving the photocatalytic activity of titanium dioxide with various metals (Ag, Pt, Au)
employed composite synthesis steps such as chemical reduction or electron beam evaporation,
which adds cost and unit operations for synthesis of modified photocatalysts. Our approach is
simpler and the degree of enhancement observed is above average of reported values for
metal/TiO2 or metal oxide/TiO2 composites (Li and Li, 2002; Vamathevan et al., 2002; Arabatzis
et al., 2003a; Hong et al., 2003; Sun et al., 2003; Zhang et al., 2003; Sung-Suh et al., 2004;
Ozawa et al., 2005). Although the cost of fullerenes is presently high relative to other chemicals
used for enhancing titanium dioxide photocatalysis, their cost has decreased since the beginning
98
of commercial large-scale production and we expect further reduction in cost. Furthermore, the
quantity of PHF employed is 10 to 100 times lower than other enhancers.
The present study suggests that there is an optimum ratio of PHF to TiO2 at which fastest
rate for dye degradation is observed. The optimum ratio is much less than monolayer coverage of
PHF on TiO2 surface. Fullerenes are known for their optical limiting ability, which can
potentially block the UV light reaching TiO2 surface.
The quite substantial degree of enhancement in photocatalysis achieved with PHF in the
present study would seem to contradict previous literature. The role of PHF as electron
scavenger was confirmed with measurement of production of hydroxyl radicals with electron
paramagnetic resonance. The concentration of spin trapped hydroxyl radicals was higher with
PHF than TiO2 alone. PHF have also been reported to generate radicals upon irradiation with
UVA or visible light (Kamat et al., 2000; Vileno et al., 2004; Pickering and Wiesner, 2005). As
previously discussed, free radical generation by PHF alone is orders of magnitude lower than
production by TiO2 alone and is thus not responsible for the observed enhancement. This leaves
electron scavenging by PHF as the most plausible explanation.
PHF have been employed as antioxidants for therapeutic applications due to their ability to
neutralize both hydroxyl and superoxide radicals (Bogdanovic et al., 2004; Djordjevic et al.,
2004a; Mirkov et al., 2004). Scavenging of these radicals might be expected to decrease, not
increase the rate of photocatalysis. However, the lifetime of hydroxyl radicals is very short (20
ns, (Grela et al., 1996)), therefore only those radicals generated close to PHF could be
scavenged. Superoxide radicals, which are generated by photo-generated electrons, have a
longer lifetime and thus may diffuse to the PHF. However, PHF is adsorbed to the TiO2 surface
at the beginning of the experiment (Krishna et al., 2006), presenting an immediate and direct
99
route for electron transfer. Thus, it is likely that the preferred route of electron transfer to the
PHF is directly from the TiO2, rather than through the diffusion-mediated superoxide route.
PHF usually contains impure groups such as hemiketal, epoxide and carbonyl, in addition
to hydroxyl groups. The presence of impure groups has been shown to affect the electronic,
physical and chemical properties of PHF (Xing et al., 2004; Rodriguez-Zavala and GuiradoLopez, 2006). The present study with aged form of PHF and freshly synthesized form of PHF
revealed that the higher ratio of impure groups to hydroxyl groups is detrimental to both electron
accepting ability and stability of PHF molecules. The electron scavenging ability of PHF
molecules can be maximized by employing a synthesis methodology, which completely
eliminates impure groups on PHF molecules. Another important parameter to be considered is
the number of hydroxyl groups present on PHF molecules. Although higher number of hydroxyl
groups is preferred to enhance water solubility, numbers greater than 36 has been experimentally
and theoretically shown to reduce the stability of PHF molecules.
Based on the results obtained from present study, information about three important design
parameters—size of the photocatalytic nanocomposite, ratio of impure groups to hydroxyl
groups in PHF and surface coverage of TiO2 by PHF—was obtained. The design parameters are
relevant to the case where either degradation of an organic dye or inactivation of microorganisms
is the performance measure.
There is a critical size of photocatalyst that is related to the surface morphology of the
target particles, for example the spacing of fimbriae on a bacterial cell-wall. Photocatalyst that
exceeds the critical size cannot contact the surface of the target particle because of steric
hindrance and therefore would be ineffective in photocatalytic degradation of that particle. In the
case of E. coli as the target particle this size is 100 nm.
100
The degree of hydroxylation and the number of impure groups needs to be controlled and
maintained in order to ensure favorable performance of the PHF. In the present study, a ratio of
impure groups to hydroxyl groups of 0.27 was associated with successful enhancement by PHF,
whereas a ratio of 1.66 was associated with no enhancement.
There is a critical surface coverage of the photocatalyst by the enhancer molecules. Where
the enhancer molecules have high optical limiting property such as PHF, the surface coverage
should be less than monolayer. In the case of PHF and a TiO2 agglomerate size of 80 nm, a
surface coverage in the range of 2–7% gives the fastest rates of dye degradation.
Overall, PHF-TiO2 combination has the potential to significantly enhance the
photocatalytic activity of TiO2 on a commercial scale. Handling and processing of PHF does not
pose any health or environmental hazard considering that hydroxylated fullerenes are reported to
be non-toxic in nature. Application of this novel nanocomposite for inactivation of
microorganisms that are harmful to human health could improve the well-being of society.
Suggestions for Future Research
Based on the findings of the current research the first proposed task for future research is
optimization of PHF molecules based on theoretical and experimental approach to improve their
electron scavenging ability. The impure groups can be eliminated from PHF molecules by
employing suitable synthesis parameters (such as alkali concentration, duration and temperature
of reaction), which needs to be determined experimentally. Xing et al (2004) have
experimentally shown that the concentration of impure groups can be reduced by increasing the
concentration of alkali employed for hydroxylation. A theoretical approach, similar to
Rodriguez-Zavala and Guirado-Lopez (2006), should be employed to predict the stability and
electron accepting ability of PHF molecules and to identify the optimum number of hydroxyl
101
groups. Results of this work would have implications for shelf-life of materials that incorporate
PHF.
The optimized PHF molecules can then be utilized to prepare self-assembled
photocatalytic nanocomposites and tested for enhancement in inactivation of bacterial
endospores. These results will corroborate the shape effect hypothesis suggesting that nano-sized
photocatalytic nanocomposites can accelerate inactivation of toughest microorganisms as well as
microorganisms with surface appendages.
The present study revealed that PHF enhances the photocatalytic activity of two different
TiO2 photocatalysts. Since the mechanism of enhancement is electron scavenging, PHF should
be applicable for enhancement in activity of other photocatalysts such as zinc oxide, vanadium
oxide and cerium oxide (Liu and Yang, 2003; Hernandez-Alonso et al., 2004; Karunakaran and
Senthilvelan, 2005). Zinc oxide is commercially used as an antimicrobial agent in deodorants
and air sanitizers. Considerable research has been carried out on integrating zinc oxide with
electron scavengers such gold, platinum and silver to improve its photocatalytic activity (Gouvea
et al., 2000; Subramanian et al., 2003b). Zinc oxide has also been coated on multi-wall carbon
nanotubes to enhance its photocatalytic activity (Jiang and Gao, 2005). The potential of PHF to
enhance these photocatalyst needs to be determined experimentally. PHF can also be applied as
electron relay for improving the efficacy of solar cells.
Another potential area for application is in photocatalytic coatings. The spores of bacteria
and fungi in the indoor environment are a major cause for allergy and respiratory problems and
are also responsible for nosocomial infections (Utrup et al., 2003; Schwab and Straus, 2004;
Chauhan et al., 2006; Paterson, 2006; Rice, 2006). The 2004 Center for Disease Control (CDC)
summary health statistics for U.S. children states that nine million children under the age of 18
102
have been diagnosed with asthma (Dey et al., 2004). Surfaces of windows, walls, tables etc. in
buildings are repositories of bacterial and fungal spores, which can survive extreme
environments and germinate on return of favorable conditions. Our preliminary assessment with
swab analysis of walls, windows and table-tops in our meeting rooms and laboratories indicated
that spores of bacteria and fungi are present in a concentration range of 1–2 cfu/cm2. Common
alcohol or hypochlorite disinfectants are ineffective against spores (Sagripanti and Bonifacino,
1999).
Titanium dioxide has been commercially applied as a self-cleaning coating on buildings
and glass materials, especially in Japan, South Korea and Singapore (Fujishima et al., 1999). The
current state-of-the-art photocatalyst coatings are active against specific organic pollutants.
Preliminary studies with the best commercially available photocatalyst (Degussa P25) indicated
that the photocatalysts can be successfully applied as a spray-on coating for degradation of
organic pollutants. However they were not effective for inactivation of bacterial endospores. The
self-assembled nanocomposites with optimized PHF molecules can be applied as spray-on
coating, which should improve the rate of inactivation of bioparticulates present on various
illuminated surfaces. The photocatalyst coating on outdoor surfaces can potentially reduce
atmospheric pollutants including greenhouse gases thereby contributing to a safer and cleaner
environment. Furthermore, the self assembled photocatalytic nanocomposites can be applied as a
coating on filter surfaces (illuminated with UVA light) present in air circulating ducts. The
photocatalyst coating will degrade the bioparticulates accumulated on the filter surface leading to
reduction in pressure drop across the filter and therefore reducing the energy loss. Reduction in
bioparticulates responsible for asthma, allergy and sick building syndrome will improve the
indoor living conditions.
103
Another avenue for potential application of PHF-TiO2 self-assembled nanocomposite is
visible light photocatalysis. Preliminary dye degradation and bacterial inactivation experiments
with these nanocomposites suggested that they are photocatalytically active in visible light and
further optimization is needed to accelerate the rate of photocatalysis. Kamat and Gevaert have
demonstrated charge transfer between fullerenes and titanium dioxide in visible light. In contrast
to UVA photocatalysis, electrons are transferred from fullerenes to titanium dioxide under
visible light illumination. Therefore the functional groups contributing to generation of charged
state in PHF may be different and should be identified by theoretical and experimental approach.
PHF with optimized composition can then be utilized for preparing self-assembled
nanocomposites. Again optimization of the ratio of PHF to TiO2 is required since the mechanism
of electron transfer is different. Alternatively PHF can be integrated with different electron
scavengers. The visible light photocatalytic nanocomposites can be then tested for their efficacy
in degradation of organic pollutants and inactivation of microorganisms. These nanocomposites
can also be applied as photocatalytic coatings for indoor and outdoor surfaces.
104
APPENDIX
CALCULATION OF SURFACE COVERAGE
The surface coverage of PHF on TiO2 was calculated on the basis of following
assumptions:
•
•
PHF molecules are in non-aggregated state.
PHF molecules have affinity towards TiO2 surface and all PHF molecules adsorb on TiO2
particles.
•
The molecular weight of PHF is 1626 Daltons, based on the average molecular weight
determined from empirical formula of fresh and aged PHF.
•
The molecular size of PHF is 1.3 nm (Jeng et al., 1999).
The surface coverage was calculated as follows
Basis: 1 gm of TiO2
SurfaceCoverage =
n × CS PHF
× 100%
TSATiO 2
Where
n=
WPHF × 6.023 × 10 23
MWPHF
CS PHF
⎞
⎛d
= π ⎜ PHF ⎟
⎝ 2 ⎠
2
⎛ 1 × 10 21
TSATiO 2 = ⎜⎜
⎝ ρ TiO 2 × VTiO 2
3
VTiO 2
4
⎛d
⎞
= × π × ⎜ TiO 2 ⎟
3
⎝ 2 ⎠
2
ATiO 2
⎛d
⎞
= 4 × π × ⎜ TiO 2 ⎟
⎝ 2 ⎠
⎞
⎟⎟ × ATiO 2
⎠
WPHF = Dosed weight of PHF per gm of TiO2
105
MWPHF = Molecular weight of PHF = 1626 gm mole-1
dPHF = 1.3 nm
ρTiO2 = 3.8 gm cm-3
dTiO2 = 80 nm (size of agglomerate as determined by particle size measurement)
Estimated surface coverage for different ratios of dosed PHF to TiO2 are presented in
Figure A-1, along with enhancement observed for degradation of Procion Red MX 5B.
106
1.8
250
200
1.4
1.2
150
1
0.8
100
0.6
0.4
50
0.2
0
0.0001
0
0.001
0.01
0.1
PHF/TiO2 Ratio
Figure A-1. Estimated surface coverage and observed enhancement as a function of dosed ratios of PHF to TiO2.
Surface Coverage (%)
107
Enhancement (A.U.)
1.6
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BIOGRAPHICAL SKETCH
Vijay Krishna was born on August 28, 1979, in Bangalore, India. He completed his high
school education in 1996 from Kendriya Vidyalaya, Delhi. In September 2000, he graduated
from B.M.S. College of Engineering (BMSCE), Bangalore University, India with a bachelor’s
degree in chemical engineering. In October 2000, he joined Unilever Research India and worked
with the global laundry bar research team. He conducted research on development of novel
structuring systems and density reduction of laundry bars, which resulted in three patents. Vijay
decided to pursue higher studies and joined the Department of Materials Science and
Engineering at University of Florida in August 2002. He earned the degree of Master of Science
in 2004 with specialization in biomaterials. He has been working on the topic of this dissertation
and expects to graduate with a Doctor in Philosophy in the spring of 2007.
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