akhavan2009

20214
J. Phys. Chem. C 2009, 113, 20214–20220
Photocatalytic Reduction of Graphene Oxide Nanosheets on TiO2 Thin Film for
Photoinactivation of Bacteria in Solar Light Irradiation
O. Akhavan* and E. Ghaderi
Department of Physics, Sharif UniVersity of Technology, P.O. Box 11155-9161, Tehran, Iran
ReceiVed: July 04, 2009; ReVised Manuscript ReceiVed: September 12, 2009
Graphene oxide platelets synthesized by using a chemical exfoliation method were deposited on anatase TiO2
thin films. Postannealing of the graphene oxide/TiO2 thin films at 400 °C in air resulted in partial formation
of a Ti-C bond between the platelets and their beneath thin film. By using atomic force microscopy and
X-ray photoelectron spectroscopy analyses, UV-visible light-induced photocatalytic reduction of the graphene
oxide platelets of the annealed graphene oxide/TiO2 thin films immersed in ethanol was studied for the different
irradiation times. After 4 h of photocatalytic reduction, the vertical space between the platelets decreased
from about 1.1 to less than 0.8 nm and the concentration of the CdO bond was reduced 85%, indicating
effective reduction of the graphene oxide platelets to the graphene ones. The graphene oxide/TiO2 thin films
reduced at different irradiation times were utilized as nanocomposite photocatalysts for degradation of E. coli
bacteria in an aqueous solution under solar light irradiation. The photocatalytic reduction of the graphene
oxide platelets for 4 h caused an improvement of the antibacterial activity of the TiO2 thin film by a factor
of about 7.5. The reduced graphene oxide platelets were chemically stable after photoinactivation of the
bacteria.
1. Introduction
After experimental discovery of graphene in 2004,1 as a oneatom-thick sheet of sp2-bonded carbon atoms in a hexagonal
two-dimensional lattice, it has quickly appeared as a highly
promising nanomaterial with unique properties to open up a
new research area for material science and condensed-matter
physics,2-6 and to aim for a wide rang of technological
applications.6-15 Concerning the unique properties of graphene,
it has been predicted that the thermal conductivity and mechanical stiffness of graphene sheets may remarkably compete with
the in-plane values for graphite (∼3000 W m-1 K-1 and ∼1060
GPa, respectively) and their fracture strength should be comparable to that of carbon nanotubes (CNTs) for similar types of
defects.16-18 In addition, they exhibit an extremely high specific
surface area (∼2600 m2/g)19-21 and their electrons can move
ballistically in a high quality graphene sheet without scattering
with mobilities exceeding ∼15 000 m2 V-1 s-1 at room
temperature.1,2,22-24
Since one possible way to utilize these properties in applications would be to incorporate graphene sheets in a composite
material, recently grapheme-containing composite materials have
been attracting much attention (see, for example, refs 21 and
25-27). Fabrication of such composites requires not only the
high-quality production of graphene sheets but also their
effective incorporation in various and desirable matrices. For
production of graphene, the π-stacked graphene sheets are
usually exfoliated (with the exfoliation energy of 61 meV/C
atom) by micromechanical cleavage of highly oriented pyrolytic
graphite2,22,23,28 and/or chemical exfoliation from bulk graphite.29-41
In manufacturing of compositions, carbon nanostructures (e.g.,
CNTs) have drawn much attention due to their unique electrical
and structural properties and ability to improve catalytic
* To whom correspondence should be addressed. E-mail: oakhavan@
sharif.edu. Phone: +98-21-66164566. Fax: +98 -21-66022711.
properties.42-45 For example, since CNTs present good electron
conductions, high surface areas, and high adsorption capacities,
they were applied as excellent dopants and supports for TiO2based materials to be used as photocatalysts.46-52 Hence,
graphene as an unrolled CNT and as a counterpart of graphite
with well-separated two-dimensional aromatic sheets may be
applied as an excellent sensitizer of semiconductor photocatalyst
such as TiO2. On the other hand, recently, it was shown that
graphene oxide nanosheets of TiO2-graphene suspensions in
ethanol can be reduced in a UV-assisted photocatalytic process.53
Therefore, manufacturing of grapheme-TiO2 nanocomposition
can result in at least two advantages: (1) controlling the reduction
of the graphene oxide nanosheets incorporated in the composition by using UV irradiation and (2) more sensitizing of the
photocatalytic activity of TiO2 thin films for more effective
applications in solar light irradiation.
In this work, graphene oxide nanosheets prepared by using a
chemical exfoliation method were deposited on the surface of
an anatase TiO2 thin film. UV-assisted photocatalytic reduction
of the deposited graphene oxide nanosheets was investigated
by using an X-ray photoelectron spectroscopy (XPS) analysis
for the different irradiation times. The effect of the presence of
the reduced graphene oxide nanosheets as the sensitizers of the
TiO2 photocatalyst was studied by testing the antibacterial
activity of the graphene (oxide)/TiO2 composition thin film
against E. coli bacteria under solar light irradiation.
2. Experimental Section
Synthesize of Anatase TiO2 Thin Films. The sol-gel
method was applied to synthesize anatase TiO2 thin films on
glass substrates with a dimension of 10 × 10 mm2. TiCl4 was
added dropwise to ethanol (50 mL) with a volume ratio of 1/10
while the solution was being stirred. The TiO2 films were
obtained by the dip-coating method. At first, the substrates were
cleaned by abluent, deionized (DI) water (18 MΩ), and acetone
10.1021/jp906325q CCC: $40.75  2009 American Chemical Society
Published on Web 10/29/2009
J. Phys. Chem. C, Vol. 113, No. 47, 2009 20215
in turns. Then, they were immersed in the sol for about 1 min
and pulled up vertically at a speed of 1 mm/s. After the samples
were dried at room temperature for 24 h, they were subsequently
heated at 100 °C for 1 h. The crystallization of the films occurred
by heat treatment at 450 °C for 60 min. Elsewhere, we showed
that this procedure is suitable for production of anatase TiO2
thin films.54 The thickness of the films was measured at about
200 nm by a profilometer.
Preparation of Graphene Oxide Nanosheets. The modified
Hummers method29,55 was utilized to oxidize natural graphite
powders (45 µm, Sigma-Aldrich) for the synthesis of graphite
oxide. In a typical procedure, 50 mL of H2SO4 was added to a
500 mL flask containing 2 g of graphite at room temperature.
The flask was cooled to 0 °C in an ice bath. Then 6 g of
potassium permanganate (KMnO4) was added slowly to the
above mixture, which was allowed to warm to room temperature. The suspension was stirred continuously for 2 h at 35 °C.
After that, it was cooled in an ice bath and subsequently diluted
by 350 mL of DI water. Then H2O2 (30%) was added in order
to reduce residual permanganate to soluble manganese ions, i.e.,
until the gas evolution ceased. Finally, the resulting suspension
was filtered, washed with water, and dried at 60 °C for 24 h to
obtain graphite oxide.
By rapidly heating the as-prepared graphite oxide in a tube
furnace, it was thermally exfoliated. After the tube furnace was
heated to 1050 °C, an alumina boat loaded with the graphite
oxide was quickly moved into the heating zone of the furnace,
kept there for 30 s, then rapidly removed.
Deposition of Graphene Oxide on TiO2. Thin films containing graphene oxide platelets were prepared by spreading an
aqueous suspension of the prepared graphene oxide nanosheets
onto the TiO2 thin film. Then the deposited samples were dried
at 60 °C in air for 24 h (as-deposited graphene oxide/TiO2 thin
films). For better adhesion of the graphene oxide platelets to
the TiO2 layer, the dried thin films were postannealed at
400 °C in air for 30 min (annealed graphene oxide/TiO2 thin
films). Before the graphene oxide deposition, the TiO2 thin films
were carefully cleaned by DI water and methanol and 2 h of
UV-visible irradiation of a mercury lamp.
Photocatalytic Reduction of Graphene Oxide on TiO2. For
the photocatalytic reduction of the graphene oxide nanosheets
deposited on the TiO2, at first, the thin films were immersed in
ethanol solution. Then they were irradiated by a 110 mW/cm2
mercury lamp (peak wavelengths at 275, 350, and 660 nm) for
different periods of time at room temperature.
Material Characterization. Surface topography of the thin
films was studied by atomic force microscopy (AFM) obtained
by using a Park Scientific model CP-Research (VEECO) with
a contact force setting of 1 nN. XPS was employed to study
the chemical states of the prepared samples. The data were
obtained by using a hemispherical analyzer with an Al KR X-ray
source (hν ) 1486.6 eV) operating at a vacuum better than 10-7
Pa. SDP Ver. 4.0 software was utilized to analyze and
deconvolute the XPS peaks. Peak deconvolutions were performed with Gaussian components after a Shirley background
subtraction.
Antibacterial Test. The antibacterial activity of the TiO2 and
the graphene (oxide)/TiO2 thin films against the Escherichia
coli (E. coli, ATCC 25922) bacteria was studied with use of
the so-called antibacterial drop-test. Before the microbiological
experiment, all glass ware and samples were sterilized by
autoclaving at 120 °C for 15 min. The microorganisms were
cultured on a nutrient agar plate at 37 °C for 24 h. The cultured
bacteria were added in 10 mL of saline solution to reach the
concentration of bacteria of ∼108 colony forming units per
milliliter (CFU/mL) corresponding to the MacFarland scale. A
portion of the saline solution containing the bacteria was diluted
to ∼106 CFU/mL by DI water. For the antibacterial drop-test,
each thin film was placed into a sterilized Petri dish. Then 100
µL of the diluted saline solution containing E. coli was spread
on the surface of the thin film. After exposing the thin films to
solar light irradiation (during the months of May-September
in Tehran (IRAN) at around noon), the bacteria were washed
from the surface of the thin film with 5 mL of phosphate buffer
solution in the sterilized Petri dish. Then 10 µL of each bacteria
suspension was spread on a nutrient agar plate and incubated
at 37 °C for 24 h before counting the surviving bacterial
colonies.
3. Results and Discussion
To observe and characterize the graphene (oxide) nanosheets
deposited on the TiO2 thin film, AFM was utilized as an
effective technique. The AFM images of the graphene (oxide)/
TiO2 thin films have been shown in Figure 1. It is seen that the
films consist of overlapping platelets. The graphene (oxide)
platelets deposited on the surface showed a relatively smooth
planar structure. The surface of the annealed TiO2 thin film was
also smooth so that its root-mean-square surface roughness was
measured at 0.54 nm. The as-deposited graphene oxide thin film
exhibited in Figure 1a shows two overlapped graphene oxide
platelets and a number of particle-like features on the film
surface. These surface features can be attributed to the residual
carbon or solvent attached to defect sites of the platelets and/or
the film surface, as also observed by others (see for example
ref 56). In fact, we also observed that these particle-like features
could be removed from the surface by heat treatment of the
graphene oxide thin films at temperatures higher than 500 °C,
which can be assigned to combustion of the particles to carbon
dioxide, as similarly reported by Wang et al.57 The height profile
diagram of the AFM image showed that the height of the
graphene oxide layer was about 1.7 nm, which is larger than
∼0.8 nm as the typical thickness of the observed single-layer
graphene oxides.31 It is known that the typical thickness of
graphene oxide shows a ∼0.44 nm increase in graphene
thickness (∼0.36 nm) due to the presence of epoxy and hydroxyl
groups on both sides of the oxide surface.31,32 The larger
thickness observed in this work (∼1.6 nm) may be due to the
presence of the particle-like features between the platelets56 in
addition to the presence of the functional groups adsorbed on
both sides of a single-layer graphene oxide sheet.34 By annealing
the as-deposited graphene oxide/TiO2 thin film at
400 °C, the surface concentration of the particle-like features
on the surface decreased, as can be seen in Figure 1b. Moreover,
the height profile measurement showed that the thickness of
the graphene oxide layers decreased to about 1.1 nm. This
indicated that the residual materials initiated evaporation not
only from the surfaces, but also from the space between the
platelets. After exposing the graphene oxide/TiO2 thin films to
UV irradiation for 4 h (Figure 1c), the spacing between the
platelets was measured in the range of about 0.75-0.80 nm, as
shown in the height profile diagram of Figure 1c. The decrease
of the thickness of the platelets down to values smaller than
the theoretical value for the thickness of graphene oxide
nanosheets may refer to reduction (but, likely not a complete
reduction) of the graphene oxide platelets to graphene ones. This
matter will be discussed in more details by using XPS analysis.
To study the chemical state variations of the graphene (oxide)/
TiO2 thin films, XPS analysis was utilized. Figure 2 shows XPS
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J. Phys. Chem. C, Vol. 113, No. 47, 2009
Akhavan and Ghaderi
Figure 1. AFM images of the graphene (oxide)/TiO2 thin films (a) as-deposited, (b) annealed at 400 °C, and (c) exposed to UV-visible light
irradiation for 4 h in the photocatalytic reduction process.
spectra at the Ti(2p) binding energy region. There are two bands
in this region. The bands located at binding energies of 464.5
and 458.9 eV were assigned to the Ti(2p1/2) and Ti(2p3/2)
spin-orbital splitting photoelectrons in the Ti4+ chemical state,
respectively. The slitting between these bands was also found
at 5.6 eV. All of these findings refer to the presence of the
normal state of Ti4+ in the as-deposited graphene oxide/TiO2
film, as seen in Figure 2a. The peak deconvolution of the Ti(2p)
spectrum of the films annealed at 400 °C indicated two other
weak peaks centered at 465.8 and 460.2 eV (relating to the
Ti(2p1/2) and Ti(2p3/2) peaks) which were attributed to formation
of a TisC bond on the film surface at this annealing temperature. Formation of the TisC bond also can be examined and
confirmed by analysis of the C(1s) core level of the XPS spectra,
as studied in the following.
The deconvoluted C(1s) XPS spectra of the graphene (oxide)/
TiO2 thin films has been shown in Figure 3. The binding energy
of 285.0 eV is attributed to the CsC, CdC, and CsH bonds
on the film surface. The deconvoluted peaks centered at the
binding energies of 286.0, 287.7, and 289.2 eV were assigned
to the CsOH, CdO, and OdCsOH functional groups,
respectively (see, for example, refs 58-60). Moreover, the band
located at 283.7 eV (observed after annealing the films at
400 °C) was assigned to the presence of the TisC bond (see,
for example, refs 51, 61, and 62). For the as-deposited graphene
oxide/TiO2 thin film (Figure 3a), no contribution relating to the
TisC bond was observed. Instead, it shows considerable
contributions of the functional groups in the carbon peak,
indicating deposition of graphene oxide platelets on the film
surface. By post annealing the as-deposited films at 400 °C in
air (Figure 3b), the peak corresponding to the TisC bond
appeared and the concentration of the CdO bond increased.
But, a slight reduction in concentration of the OH-containing
functional groups was observed. These results showed that post
annealing of the graphene oxide/TiO2 film at 400 °C led to
formation of the TisC bond between carbon of the graphene
oxide platelets and/or the residual material containing the
carbonaceous bond and the titanium of the TiO2 film. In addition,
J. Phys. Chem. C, Vol. 113, No. 47, 2009 20217
Figure 2. Peak deconvolution of the Ti(2p) XPS core level of graphene
oxide/TiO2 thin films (a) as-deposited and (b) annealed at 400 °C in
air.
Figure 4. Number of bacteria cultured from the viable E. coli on the
surface of the graphene oxide/TiO2 thin films reduced by UV-visible
light-assisted photocatalytic process for (a) 0, (b) 0.5, (c) 1, (d) 2, and
(e) 4 h irradiation time, as compared to (f) number of bacteria on bare
TiO2 thin film and (g) on graphene oxide/glass film, under solar light
irradiation. (h) Number of bacteria on the graphene oxides applied in
part g but in the dark, as a control sample.
the surface of the graphene oxide/TiO2 thin film. By increasing
the time of the UV-visible light exposure to 4 h (Figure 3f),
the relative concentration of the CsOH, CdO, and OdCsOH
bonds decreased to 73%, 85%, and 72% of the corresponding
concentrations of the thin film annealed at 400 °C, respectively.
These substantial decreases in the concentration of the oxygencontaining bonds indicated an effective chemical reduction of
the graphene oxide platelets to graphene ones on the TiO2 thin
film.
The photocatalytic activities of TiO2-based material are
well-known.54,63-65 When TiO2 is exposed to UV light, the
photoinduced electron-hole pairs are generated. In the presence
of ethanol the separated holes are scavenged to produce ethoxy
radicals, according to the following reaction:53
TiO2(h-+e-) + C2H5OH f TiO2(e-) + •C2H4OH + H+
Figure 3. Peak deconvolution of the C(1s) XPS core level of graphene
(oxide)/TiO2 thin films (a) as-deposited, (b) annealed at 400 °C in air,
reduced by the UV-visible light-assisted photocatalytic reduction for
(c) 0.5, (d) 1, (e) 2, and (f) 4 h of irradiation time in ethanol, as
compared to (g) the XPS spectrum of the thin film (f) immersed in the
aqueous solution containing the bacteria and under solar light irradiation
for 80 min.
it indicated that the annealed graphene oxide platelets were
chemically bonded to the film surface resulting in a better
adhesion. To study and compare the change of the concentration
of the functional groups, the peak area ratios of the CsOH,
CdO, and OdCsOH bonds to the CsC, CdC, and CsH
bonds were calculated and summarized in Table 1. By exposing
the graphene oxide/TiO2 thin films immersed in the ethanol to
the UV-visible light irradiation for 0.5 h (Figure 3c), the relative
concentration of the CsOH, CdO, and OdCsOH bonds
showed 25%, 41%, and 6% reduction from the corresponding
concentrations for the thin film annealed at 400 °C, respectively.
Hence, the light exposure resulted in a chemical reduction on
This means that the photoexcited electrons accumulate on the
surface of the TiO2 thin film. These electrons serve to interact
with the graphene oxide platelets to reduce the functional groups.
It was previously shown that the electrons stored in TiO2
nanoparticles are readily scavenged by carbon nanostructures
such as fullerenes, CNTs, and graphene platelets.45,53,66 Here,
based on the XPS analysis, we also showed that the graphene
oxide platelet with its oxygen-containing functional groups
readily interacts with the beneath TiO2 thin film and undergoes
reduction under the UV-visible light irradiation. The reduced
graphene oxide/TiO2 thin films exposed to UV-visible light
irradiation for 4 h (now, the graphene/TiO2 thin films) were
also utilized for an antibacterial test in an aqueous solution (see
the next paragraph). After the antibacterial test, the XPS analysis
of the C(1s) core level of the graphene/TiO2 thin film (Figure
3g) showed the continuation of the decrease in the concentration
of the CdO bond (28% reduction relative to concentration of
the bond before the antibacterial test (see Table 1)). But, a very
slight increase in the concentration of the CsOH bond was
observed that may be assigned to the abundance of the OH
active groups in the aqueous solution containing the bacteria.
These results showed that the reduced graphene platelets applied
for the antibacterial test were chemically stable on the TiO2
thin film.
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J. Phys. Chem. C, Vol. 113, No. 47, 2009
Akhavan and Ghaderi
TABLE 1: The Peak Area (A) Ratios of the Oxygen-Containing Bonds to the CC Bonds, for the Graphene (Oxide)/TiO2 Thin
Films at the Different Experimental Conditions, and the Antibacterial Activity of the Various Thin Films
graphene (oxide)/TiO2 thin film
as-deposited
annealed at 400 °C
0.5 h
ACOH/ACC
ACO/ACC
AOCOH/ACC
antibacterial activity (×10-3 min-1)
0.61
1.32
0.13
0.52
1.46
0.11
11
0.39
0.85
0.08
17
The bactericidal activity of the graphene (oxide)/TiO2 thin
films against E. coli bacteria was investigated under solar light
exposure, as shown in Figure 4a-e. To have a benchmark,
antibacterial activity of the bare TiO2 thin films and the asdeposited graphene oxide/glass thin films was also studied under
solar light irradiation (see Figure 4f,g). Moreover, the graphene
oxide as-deposited on a glass substrate was applied as a control
sample in the dark (Figure 4h). It was previously shown that
graphene (oxide) itself is a biocompatible material67 applicable
as a biosensor with a single-bacterium sensitivity.15 Here, we
also observed that the bacteria slowly grew on the graphene
oxide thin films in the dark (see Figure 4h). Although the
sunlight exposure prevented the growth of the bacteria on the
graphene oxides deposited on a glass substrate, it only resulted
in a very slight decrease in the number of the viable bacteria
on the surface of the reduced graphene oxides. However, the
amount of viable bacteria on the surface of the thin films
containing the TiO2 photocatalyst exponentially was reduced
under solar light irradiation. The slope of the fitted line yields
a relative rate of reduction of the number of viable bacteria (here,
called antibacterial activity). The bare TiO2 thin film showed a
weak antibacterial activity with the relative rate of reduction of
about 8.6 × 10-3 min-1 under solar light irradiation. It is seen
that the graphene oxide/TiO2 thin film annealed at 400 °C
improved the antibacterial activity up to about 25% of the
activity of the bare TiO2 thin film under sunlight irradiation.
However, after the photocatalytic reduction of the annealed
graphene oxide/TiO2 thin film under UV-visible light irradiation
for 0.5 h, the antibacterial activity was substantially improved
up to about 60%. By increasing the time of the photocatalytic
reduction to 4 h, the solar light-induced antibacterial activity
of the reduced graphene oxide/TiO2 thin films excellently
increased by a factor of about 6 (7.5) relative to the activity of
the annealed graphene oxide/TiO2 (the bare TiO2) thin film.
It was observed that the bacteria could slowly grow on the
surface of the as-deposited graphene oxide in the dark indicating
the biocompatibility of the graphene oxide platelets, as previously reported.67 Meanwhile, the TiO2 thin film itself did not
show strong antibacterial activity under solar light irradiation,
because only ∼5% of the solar energy is in the UV region
suitable for photoexcitation of electron-hole pairs in TiO2. It
was established that the photocatalytic and bactericidal activities
of TiO2-based materials can be enhanced by incorporating noble
metal (nano)particles54,68-71 due to decreasing the optical band
gap energy of TiO2 and/or decreasing the recombination rate
of the photoexcited pairs. In fact, oxidant reduction by electrons
(milliseconds) is much slower than the oxidation of reductants
by holes (100 ns) in the TiO2 photocatalytic process.63 Hence,
an increase in the rate of electron transfer to the oxidant can
result in an increase of the quantum yield of the photocatalytic
process. In the metal/semiconductor oxide composites the
photoexcited electrons accumulate on the incorporated metal
and holes remain on the photocatalyst surface leading to a
reduction in recombination rate of the pairs, because of a better
charge separation between them. In this work, it was observed
after UV exposure for a period of time
1h
2h
4h
4 h + antibacterial test
0.25
0.42
0.05
30
0.19
0.30
0.04
45
0.14
0.21
0.03
65
0.17
0.15
0.03
that the reduced graphene oxides deposited on the surface of
the TiO2 thin film could improve the photocatalytic performance
of the TiO2. In fact, the reduced graphene oxide platelets (as
the conductive and transparent sheets incorporated on the surface
of the TiO2 thin film) could play a role similar to the role of
the incorporated metallic particles in TiO2-based materials. But,
using UV-visible spectrophotometry, we found that the optical
band gap energy of the TiO2 thin films was not significantly
changed by incorporating the graphene (oxide) platelets. Therefore, during the photocatalytic activity of the TiO2 thin film
under sunlight irradiation, the reduced graphene oxide platelets
could act as electron acceptors to effectively decrease the rate
of recombination of the photoexcited pairs and so increase the
quantum efficiency of the photocatalytic process, as previously
indicated by Williams et al.53 In this regard, here we have shown
that better reduction of the graphene oxide (better conductivity
of the platelets and so more accumulation of the photoexcited
electrons on them) resulted in better photocatalytic performance
of the TiO2 thin film in solar light irradiation.
To quantitatively compare the bactericidal activity of the
graphene (oxide)/TiO2 with some other corresponding antibacterial surfaces, we selected the previously investigated TiO2 [this
work], Ag-SiO2,54,72 Ag nanorod,73 and Ag-TiO2/Ag/a-TiO254
thin films as corresponding photocatalytic and antibacterial thin
films applicable in sunlight irradiation. It can be found that for
the graphene oxide/TiO2 thin film in which the graphene oxide
platelets were reduced by UV-visible light irradiation for 4 h,
the antibacterial activity was significantly improved as compared
to the TiO2, Ag-SiO2, Ag nanorod, and Ag-TiO2/Ag/a-TiO2 thin
films, by factors of 7.5, 3.7, 1.7, and 1.1, respectively. Therefore,
the reduced graphene platelets can be considered as one of the
excellent sensitizers of the TiO2-based materials to develop them
for more efficient solar light-induced photocatalytic processes.
4. Conclusions
The graphene oxide platelets synthesized by chemical oxidation and exfoliation of graphite were deposited on the sol-gel
anatase TiO2 thin films. The postannealing of the graphene
oxide/TiO2 thin films at 400 °C in air resulted in initiation of
evaporation of residual materials and partial formation of the
TisC bond between the platelets and their beneath thin film.
The photocatalytic reduction of the graphene oxide platelets of
the annealed graphene oxide/TiO2 thin films immersed in ethanol
caused a decrease of spacing of the platelets and a reduction of
the concentration of the oxygen-containing functional groups
on the film surface. By increasing the irradiation time of the
photocatalytic reduction to 4 h, the vertical space between the
platelets decreased from about 1.1 to less than 0.8 nm and the
concentration of the CsOH, CdO, and OdCsOH bonds
decreased 73%, 85%, and 72%, respectively, indicating effective
reduction of the graphene oxide platelets to graphene ones.
Moreover, as the irradiation time of the photocatalytic process
increased, the antibacterial activity of the graphene (oxide)/TiO2
thin film was enhanced under solar light irradiation. After
J. Phys. Chem. C, Vol. 113, No. 47, 2009 20219
photocatalytic reduction for 4 h, the antibacterial activity of the
graphene oxide/TiO2 thin film was improved by a factor of about
6 (7.5) relative to the activity of the annealed graphene oxide/
TiO2 (the bare TiO2) thin film. The reduced graphene oxide
platelets were chemically stable after the antibacterial tests.
These results indicated a direct interaction between the TiO2
thin film and the graphene (oxide) platelets, showing formation
of graphene (oxide)/TiO2 nanocomposition. In addition, they
provided development of the nanocomposite photocatalysts
which can recover themselves and work efficiently under solar
light irradiation.
Acknowledgement. .
O.A. would like to thank the Research Council of Sharif
University of Technology and also the Iran Nanotechnology
Initiative Council for financial support of the work.
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