Photocatalytic Reduction of Graphene Oxide Nanosheets on TiO2

Oct 29, 2009 - The slope of the fitted line yields a relative rate of reduction of the number of viable bacteria (here, called antibacterial activity)...
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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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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,

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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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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. References and Notes (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films. Science 2004, 306, 666–669. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Twodimensional gas of massless Dirac fermions in graphene. Nature 2005, 438, 197–200. (3) Zhang, Y. B.; Tan, Y.; Stormer, H. L.; Kim, P. Experimental observation of quantum Hall effect and Berry’s phase in graphene. Nature 2005, 438, 201–204. (4) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. The structure of suspended graphene sheets. Nature 2007, 446, 60–63. (5) Katsnelson, M. I.; Novoselov, K. S. Graphene: New bridge between condensed matter physics and quantum electrodynamics. Solid State Commun. 2007, 143, 3–13. (6) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection of individual gas molecules adsorbed on graphene. Nat. Mater. 2007, 6, 652–655. (7) Robinson, J. T.; Perkins, F. K.; Snow, E. S.; Wei, Z. Q.; Sheehan, P. E. Reduced graphene oxide molecular sensors. Nano Lett. 2008, 8, 3137– 3140. (8) Arsat, R.; Breedon, M.; Shafiei, M.; Spizziri, P. G.; Gilje, S.; Kaner, R. B.; Kalantar-zadeh, K.; Wlodarski, W. Graphene-like nano-sheets for surface acoustic wave gas sensor applications. Chem. Phys. Lett. 2009, 467, 344–347. (9) Wang, X.; Zhi, L. J.; Tsao, N.; Tomovic, Z.; Li, J. L.; Mullen, K. Transparent carbon films as electrodes in organic solar cells. Angew. Chem., Int. Ed. 2008, 47, 2990–2992. (10) Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Electronic transport properties of individual chemically reduced graphene oxide sheets. Nano Lett. 2007, 7, 3499–3503. (11) Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B. A chemical route to graphene for device applications. Nano Lett. 2007, 7, 3394–3398. (12) Liang, X.; Fu, Z.; Chou, S. Y. Graphene transistors fabricated via transfer-printing in device active-areas on large wafer. Nano Lett. 2007, 7, 3840–3844. (13) Stampfer, C.; Schurtenberger, E.; Molitor, F.; Gu¨ttinger, J.; Ihn, T.; Ensslin, K. Tunable graphene single electron transistor. Nano Lett. 2008, 8, 2378–2383. (14) Bao, W.; Zhang, H.; Bruck, J.; Lau, C. N.; Bockrath, M.; Standley, B. Graphene-based atomic-scale switches. Nano Lett. 2008, 8, 3345–3349. (15) Mohanty, N.; Berry, V. Graphene-Based Single-Bacterium Resolution Biodevice and DNA Transistor: Interfacing Graphene Derivatives with Nanoscale and Microscale Biocomponents. Nano Lett. 2008, 8, 4469–4476. (16) Dresselhaus, M. S.; Dresselhaus, G. Intercalation compounds of graphite. AdV. Phys. 2002, 51, 1–186. (17) Hirata, M.; Gotou, T.; Horiuchi, S.; Fujiwara, M.; Ohba, M. Thinfilm particles of graphite oxide 1: High-yield synthesis and flexibility of the particles. Carbon 2004, 42, 2929–2937. (18) Yu, M. F.; Lourie, O.; Moloni, K.; Kelly, T. F.; Ruoff, R. S. Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load. Science 2000, 287, 637–640. (19) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-dimensional atomic crystals. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451–10453. (20) Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater. 2007, 6, 183–191.

(21) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-based composite materials. Nature 2006, 442, 282–286. (22) Zhang, Y.; Tan, Y.; Stormer, H. L.; Kim, P. Experimental observation of quantum Hall effect and Berry’s phase in graphene. Nature 2005, 438, 201–204. (23) Zhang, Y.; Small, J. P.; Amori, M. E. S.; Kim, P. Electric Field Modulation of Galvanomagnetic Properties of Mesoscopic Graphite. Phys. ReV. Lett. 2005, 94, 176803. 1-4). (24) Berger, C.; Song, Z.; Li, T.; Li, X.; Ogbazghi, A. Y.; Feng, R.; Dai, Z.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Ultrathin epitaxial graphite: two-dimensional electron gas properties and a route toward grapheme-based nanoelectronics. J. Phys. Chem. B 2004, 108, 19912–19916. (25) Kotov, N. A. Materials science: Carbon sheet solutions. Nature 2006, 442, 254–255. (26) Watcharotone, S.; Dikin, D. A.; Stankovich, S.; Piner, R.; Jung, I.; Dommett, G. H. B.; Evmenenko, G.; Wu, S. E.; Chen, S. F.; Liu, C. P.; Nguyen, S. T.; Ruoff, R. S. Graphene-silica composite thin films as transparent conductors. Nano Lett. 2007, 7, 1888–1892. (27) Wang, D.; Choi, D.; Li, J.; Yang, Z.; Nie, Z.; Kou, R.; Hu, D.; Wang, C.; Saraf, L. V.; Zhang, J.; Aksay, I. A.; Liu, J. Self-Assembled TiO2-Graphene Hybrid Nanostructures for Enhanced Li-Ion Insertion. ACS Nano 2009, 3, 907–914. (28) Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Making graphene visible. Appl. Phys. Lett. 2007, 91, 063124(1-3). (29) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y; Nguyen, S. T.; Ruoff, R. S. Synthesis of graphemebased nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007, 45, 1558–1565. (30) Niyogi, S.; Bekyarova, E.; Itkis, M. E.; McWilliams, J. L.; Hamon, M. A.; Haddon, R. C. Solution properties of graphite and graphene. J. Am. Chem. Soc. 2006, 128, 7720–7721. (31) Schniepp, H. C.; Li, J. L.; McAllister, M. J.; Sai, H.; HerreraAlonso, M.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. Functionalized single graphene sheets derived from splitting graphite oxide. J. Phys. Chem. B 2006, 110, 8535–8539. (32) McAllister, M. J.; LiO, J. L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; CarO, R.; Prud’homme, R. K.; Aksay, I. A. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chem. Mater. 2007, 19, 4396– 4404. (33) Yu, A. P.; Ramesh, P.; Itkis, M. E.; Bekyarova, E.; Haddon, R. C. Graphite nanoplatelet-epoxy composite thermal interface materials. J. Phys. Chem. C 2007, 111, 7565–7569. (34) Li, X. L.; Wang, X. R.; Zhang, L.; Lee, S. W.; Dai, H. J. Chemically derived, ultra-smooth graphene nanoribbon semiconductors. Science 2008, 319, 1229–1232. (35) Kudin, K. N.; Ozbas, B.; Schniepp, H. C.; Prudhomme, R. K.; Aksay, I. A.; Car, R. Raman spectra of graphite oxide and functionalized graphene sheets. Nano Lett. 2008, 8, 36–41. (36) Worsley, K. A.; Ramesh, P.; Mandal, S. K.; Niyogi, S.; Itkis, M. E.; Haddon, R. C. Soluble graphene derived from graphite fluoride. Chem. Phys. Lett. 2007, 445, 51–56. (37) Stankovich, S.; Piner, R. D.; Chen, X. Q.; Wu, N. Q.; Nguyen, S. T.; Ruoff, R. S. Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate). J. Mater. Chem. 2006, 16, 155–158. (38) Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets. Carbon 2006, 44, 3342–3347. (39) Wu, Z. S.; Ren, W.; Gao, L.; Liu, B.; Jiang, C.; Cheng, H. M. Synthesis of high-quality graphene with a pre-determined number of layers. Carbon 2009, 47, 493–499. (40) Wang, G.; Wang, B.; Park, J.; Yang, J.; Shen, X.; Yao, J. Synthesis of enhanced hydrophilic and hydrophobic grapheme oxide nanosheets by a solvothermal method. Carbon 2009, 47, 68–72. (41) Li, D.; Mu¨ller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotechnol. 2008, 3, 101–115. (42) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Carbon Nanotubes-The Route Towards Applications. Science 2002, 297, 787– 792. (43) Kamat, P. V. Harvesting photons with carbon nanotubes. Nanotoday 2006, 1, 20–27. (44) Kongkanand, A.; Kamat, P. V. Interactions of single wall carbon nanotubes with methyl viologen radicals. Quantitative estimation of stored electrons. J. Phys. Chem. C 2007, 111, 9012–9015. (45) Kongkanand, A.; Kamat, P. V. Electron storage in single wall carbon nanotubes. Fermi level equilibration in semiconductor-SWCNT suspensions. ACS Nano 2007, 1, 13–20.

20220

J. Phys. Chem. C, Vol. 113, No. 47, 2009

(46) Kuo, C.-S.; Tseng, Y.-H.; Lin, H.-Y.; Huang, C.-H.; Shen, C.-Y.; Li, Y.-Y.; Shahd, S. I.; Huang, C. P. Synthesis of a CNT-grafted TiO2 nanocatalyst and its activity triggered by a DC voltage. Nanotechnology 2007, 18 (1-5), 465607. (47) Wang, W.; Serp, P.; Kalck, P.; Faria, J. L. Photocatalytic degradation of phenol on MWNT and titania composite catalysts prepared by a modified sol-gel method. Appl. Catal., B 2005, 56, 305–312. (48) Wang, W.; Serp, P.; Kalck, P.; Faria, J. L. Visible light photodegradation of phenol on MWNT-TiO2 composite catalysts prepared by a modified sol-gel method. J. Mol. Catal. A: Chem. 2005, 235, 194–199. (49) Yu, H.; Quan, X.; Chen, S.; Zhao, H.; Zhang, Y. TiO2-carbon nanotube heterojunction arrays with a controllable thickness of TiO2 layer and their first application in photocatalysis. J. Photochem. Photobiol., A 2008, 200, 301–306. (50) An, G.; Ma, W.; Sun, Z.; Liu, Z.; Han, B.; Miao, S.; Miao, Z.; Ding, K. Preparation of titania/carbon nanotube composites using supercritical ethanol and their photocatalytic activity for phenol degradation under visible light irradiation. Carbon 2007, 45, 1795–1801. (51) Chen, L.-C.; Ho, Y.-C.; Guo, W.-S.; Huang, C.-M.; Pan, T.-C. Enhanced visible light-induced photoelectrocatalytic degradation of phenol by carbon nanotube-doped TiO2 electrodes. Electrochim. Acta 2009, 54, 3884–3891. (52) Akhavan, O.; Abdolahad, M.; Abdi, Y.; Mohajerzadeh, S. Synthesis of titania/carbon nanotubes heterojunction arrays for photoinactivation of E. coli in visible light irradiation. Carbon 2009, 47, 3280–3287. (53) Williams, G.; Seger, B.; Kamat, P. V. TiO2-Graphene Nanocomposites. UV-Assisted Photocatalytic Reduction of Graphene Oxide. ACS Nano 2008, 2, 1487–1491. (54) Akhavan, O. Lasting antibacterial activities of Ag-TiO2/Ag/a-TiO2 nanocomposite thin film photocatalysts under solar light irradiation. J. Colloid Interface Sci. 2009, 336, 117–124. (55) Hummers, W. S.; Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80, 1339–1339. (56) Osva´th, Z.; Darabont, Al.; Nemes-Incze, P.; Horva´th, E.; Horva´th, Z. E.; Biro´, L. P. Graphene layers from thermal oxidation of exfoliated graphite plates. Carbon 2007, 45, 3022–3026. (57) Wang, G.; Yang, J.; Park, J.; Gou, X.; Wang, B.; Liu, H.; Yao, J. Facile Synthesis and Characterization of Graphene Nanosheets. J. Phys. Chem. C 2008, 112, 8192–8195. (58) Yang, D.; Velamakanni, A.; Bozoklu, G.; Park, S.; Stoller, M.; Piner, R. D.; Stankovich, S.; Jung, I.; Field, D. A., Jr.; Ruoff, R. S. Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy. Carbon 2009, 47, 145–152. (59) Chiang, T. C.; Seitz, F. Photoemission spectroscopy in solids. Ann. Phys. 2001, 10, 61–74.

Akhavan and Ghaderi (60) Yumitori, S. Correlation of C1s chemical state intensities with the O1s intensity in the XPS analysis of anodically oxidized glass-like carbon samples. J. Mater. Sci. 2000, 35, 139–146. (61) Huang, Y.; Ho, W.; Lee, S.; Zhang, L.; Li, G.; Jimmy, J. C. Effect of carbon doping on the mesoporous structure of nanocrystalline titanium dioxide and its solar-light-driven photocatalytic degradation of NOx. Langmuir 2008, 24, 3510–3516. (62) Sun, H.; Bai, Y.; Cheng, Y.; Jin, W.; Xu, N. Preparation and characterization of visible-light-driven carbon-sulfur-codoped TiO2 photocatalysts. Ind. Eng. Chem. Res. 2006, 45, 4971–4976. (63) Hoffman, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. ReV. 1995, 95, 69–96. (64) Ollis, F. D.; Al-Ekabi, H., Eds. Photocatalytic Purification and Treatment of Water and Air; Elsevier Science Publishers: Amsterdam, The Netherlands, 1993. (65) Kikuchi, Y.; Sunada, K.; Iyoda, T.; Hashimoto, K.; Fujishima, A. Photocatalytic bactericidal effect of TiO2 thin film: dynamic view of the active oxygen species responsible for the effect. J. Photochem. Photobiol., A 1997, 106, 51–56. (66) Kamat, P. V.; Bedja, I.; Hotchandani, S. Photoinduced Charge Transfer Between Carbon and Semiconductor Clusters. One-Electron Reduction of C60 in Colloidal TiO2 Semiconductor Suspensions. J. Phys. Chem. 1994, 98, 9137–9142. (67) Chen, H.; Mu¨ller, M. B.; Gilmore, K. J.; Wallace, G. G.; Li, D. Mechanically Strong, Electrically Conductive, and Biocompatible Graphene Paper. AdV. Mater. 2008, 20, 3557–3561. (68) Sato, T.; Taya, M. Copper-aided photosterilization of microbial cells on TiO2 film under irradiation from a white light fluorescent lamp. Biochem. Eng. J. 2006, 30, 199–204. (69) Kuo, Y.-L.; Chen, H.-W.; Ku, Y. Analysis of silver particles incorporated on TiO2 coatings for the photodecomposition of o-cresol. Thin Solid Films 2007, 515, 3461–3468. (70) Vamathevan, V.; Amal, R.; Beydoun, D.; Low, G.; McEvoy, S. Photocatalytic oxidation of organics in water using pure and silver-modified titanium dioxide particles. J. Photochem. Photobiol., A 2002, 148, 233– 245. (71) Murakata, T.; Higashi, Y.; Yasui, N.; Higuchi, T.; Sato, S. Photocatalytic preparation of noble metal nanoparticles with use of ultrafine TiO2 particles. J. Chem. Eng. Jpn. 2002, 35, 1270–1276. (72) Akhavan, O.; Ghaderi, E. Bactericidal effects of Ag nanoparticles immobilized on surface of SiO2 thin film with high concentration. Curr. Appl. Phys. 2009, 9, 1381–1385. (73) Akhavan, O.; Ghaderi, E. Enhancement of antibacterial properties of Ag nanorods by electric field. Sci. Technol. AdV. Mater. 2009, 10, 015003(1-5).

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