Photodegradation of Graphene Oxide Sheets by TiO2 Nanoparticles

J. Phys. Chem. C 2010, 114, 12955–12959

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Photodegradation of Graphene Oxide Sheets by TiO2 Nanoparticles after a Photocatalytic Reduction O. Akhavan,*,†,‡ M. Abdolahad,§ A. Esfandiar,‡ and M. Mohatashamifar| Department of Physics, P. O. Box 11155-9161, Sharif UniVersity of Technology, Tehran, Iran, Institute for Nanoscience and Nanotechnology, P. O. Box 14588-89694, Sharif UniVersity of Technology, Tehran, Iran, Nano-Electronics and Thin Film Laboratory, Department of Electrical and Computer Engineering, North Kargar AVenue, P. O. Box 14395/515, UniVersity of Tehran, Tehran, Iran, and Electronic Research Center, Tehran, Iran ReceiVed: April 18, 2010; ReVised Manuscript ReceiVed: June 6, 2010

TiO2 nanoparticles were physically attached to chemically synthesized single-layer graphene oxide nanosheets deposited between Au electrodes in order to investigate the electrical, chemical, and structural properties of the TiO2/graphene oxide composition exposed to UV irradiation. X-ray photoelectron spectroscopy showed that after effective photocatalytic reduction of the graphene oxide sheets by the TiO2 nanoparticles in ethanol, the carbon content of the reduced graphene oxides gradually decreased by increasing the irradiation time, while no considerable variation was detected in the reduction level of the reduced sheets. Raman spectroscopy indicated that, at first, the photocatalytic reduction resulted in a significant increase in the graphitized sp2 structure over the disorders in the graphene oxides. After that, as the carbon content decreased by UV irradiation, further disorders appeared in the reduced graphene oxide sheets, confirming degradation of the reduced sheets after the photocatalytic reduction. Based on the current-voltage characteristic, the optimum time for the photocatalytic reduction resulted in a sharp decrease in the electrical resistivity of the reduced graphene oxide. However, longer photocatalytic processes caused a high increase in the resistivity, due to dominating the photodegradation process over the nearly completed photocatalytic reduction. 1. Introduction In the universe of carbon nanostructures, graphene as a oneatom-thick sheet made up of sp2-bonded carbons in a hexagonal lattice with unique physical and chemical characteristics has attracted special attention. In fact, after empirical discovery of graphene in 2004 by Novoselov et al.,1 it has rapidly come into view as an extraordinary nanomaterial with promising capability in some research and technological fields in material science and condensed-matter physics.2-14 Among the various fabrication procedures of graphene, the chemical exfoliation method has been extensively used as an effective, reliable, and low-cost method.15,16 In this method, the chemically oxidized graphite is cleaved by ultrasonic dispersion or rapid thermal expansion, and the synthesized graphene oxide nanosheets should be reduced to obtain graphene nanosheets. After reduction of graphene oxides, high electrical conductance and transparency can be achieved, so that reduced graphene oxides are known as promising candidates for largearea transparent conductors.17 However, due to the toxicity of chemical reductants such as hydrazine and high temperatures (>500 °C) required in the thermal reductions, the usual chemical and thermal reductions are not completely compatible with the current electronic and chemistry technologies, and so, their extensive applications have been limited. Furthermore, it was shown that in the Au-doped graphene sheets,18 the Au nanostructures act as antireduction resist during a chemical reduc* Corresponding author. Tel.: +98-21-66164566. Fax: +98-21-66022711. E-mail address: [email protected]. † Department of Physics, Sharif University of Technology. ‡ Institute for Nanoscience and Nanotechnology, Sharif University of Technology. § University of Tehran. | Electronic Research Center.

tion.19 Hence, in addition to improving the current reducing processes, other effective reducing methods, including microwave and (photo)catalytic reductions, are considered as some of the important research subjects for production of high quality reduced graphene oxide nanosheets at lower and lower temperatures. For example, an improved thermal reduction method by which an effective reduction of graphene oxide sheets occurred at temperatures lower than 500 °C was studied previously.20 Microwave reduction of graphene oxide sheets was also investigated by Chen et al.21 Concerning the (photo)catalytic reduction processes, Xu et al.22 showed that Au, Pt, and Pd metallic nanoparticles adsorbed on graphene oxide sheets can reduce the graphene oxide sheets with ethylene glycol in a catalytic process. In other works, photocatalytic reduction of graphene oxide sheets by TiO2 nanoparticles and thin films in ethanol solution was investigated.23,24 However, it is well-known that metal oxide semiconductors, such as TiO2, can easily decompose carbonaceous bonds in a photocatalytic process.25,26 Therefore, photocatalytic reduction of graphene oxides by using metal oxide semiconductor photocatalysts and its effects on the quality of the reduced graphene sheets should be examined in more detail. In this research, based on the procedure reported by Williams et al.23 (only applied in the photocatalytic reduction of graphene oxide), gradual photodegradation of chemically synthesized graphene oxide sheets physically combined with TiO2 nanoparticles was systematically investigated, particularly after completion of the photocatalytic reduction of the oxide sheets. In this regard, the effect of UV irradiation and its duration on the oxygen-containing carbonaceous bonds, carbon contents including disorder and the graphitized carbons, and the current-voltage characteristic of the TiO2/graphene oxide sheets were examined for different periods of irradiation time.

10.1021/jp103472c  2010 American Chemical Society Published on Web 07/14/2010

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2. Experimental Section The improved Hummers method15,27 was applied to oxidize natural graphite powder (particle diameter of 45 µm, SigmaAldrich) in order to synthesize the graphite oxide. In a typical procedure, 50 mL of H2SO4 was added into a 500 mL flask including 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 and allowed to warm to room temperature. The suspension was stirred continuously for 2 h at 35 °C. Then, it was cooled in an ice bath and subsequently diluted by 350 mL of deionized (DI) water. Then H2O2 (30%) was added to reduce the residual permanganate to soluble manganese ions, that is, until stopping the gas evolution. Finally, the resulting suspension was filtered, washed with 1 M HCl and twice with DI water, and dried at 60 °C for 24 h to obtain brownish graphite oxide powders. The obtained graphite oxide powder was dispersed in water (1 mg/ mL) to obtain a suspension. Then the suspension was sonicated for 30 min to obtain a graphene oxide suspension. The graphene oxide samples were prepared by drop-casting the graphene oxide suspension onto two chemically patterned Au electrodes (with ∼1.5 µm space between the electrodes) deposited on SiO2/Si substrates by using e-beam evaporation. After drying in air, the samples were annealed in air at 200 °C for 30 min. Then, the annealed samples were immersed in a prepared TiO2 suspension (40 mg/mL commercial TiO2 nanoparticles with particle diameter of 15 nm and surface area of 240 ( 50 m2 g-1 (Hurricane Co., Iran) in DI water), and the suspension was sonicated for 30 min to physically hybridize the TiO2 nanoparticles on the surface of the deposited graphene oxide sheets. Then, the TiO2/graphene oxide samples were annealed at 200 °C for 30 min. To study the photocatalytic effect of the TiO2 nanoparticles on the graphene oxide sheets, at first, the TiO2/graphene oxide samples were dispersed in ethanol (C2H5OH, Merck, >99.9%)). Then, the samples 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. Surface morphology of the TiO2/graphene oxide sheets deposited on the Au electrodes (especially those that were deposited on the hollow space between the two electrodes) was studied by using a Philips XL30 scanning electron microscopy (SEM). Atomic force microscopy (AFM) images were obtained by using a Park Scientific model CP-Research (VEECO). The substrates used for AFM imaging were freshly cleaved mica substrates. X-ray photoelectron spectroscopy (XPS) was employed to study the relative concentration of carbon and the chemical states of the TiO2/graphene oxide sheets irradiated at different irradiation times. The data were obtained using a hemispherical analyzer with an Al KR X-ray source (hν ) 1486.6 eV) operating at a vacuum better than 10-7 Pa. In the XPS data analysis, peak deconvolution was performed using Gaussian components after a Shirley background subtraction. Raman spectroscopy was performed at room temperature using a Raman Microprobe (HR-800 Jobin-Yvon) with 532 nm Nd: YAG excitation source. Current-voltage curves of the TiO2/ graphene oxide sheets exposed to UV irradiation for the different times were obtained by using a Keithley 485 Autoranging Picoammeter. 3. Results and Discussion Figure 1 shows surface morphology of the TiO2/graphene oxide sheets deposited on the Au electrodes, especially those located on the hollow space between the two electrodes, as

Figure 1. SEM image of the TiO2/graphene oxide sheets on Au electrodes deposited on SiO2/Si substrate.

connecting bridges of the electrodes. The width of the electrodes was 250 µm, and the average distance between them was about 1.5 µm. After deposition of the TiO2/graphene oxide sheets, we observed 41 sheets connecting the two electrodes. Here, the SEM image shows only two graphene oxide sheets connecting the electrodes. The dimension of the majority of the graphene oxide sheets was found to be a few micrometers, as can be seen for some of them in Figure 1. The light spots on the graphene oxide sheets can be assigned to the TiO2 nanoparticles attached on the surface. To better observe and characterize the topography of the TiO2/ graphene oxide sheets, AFM was used as an appropriate technique, as shown in Figure 2. The AFM image (Figure 2a) shows two partially overlapped platelets nearly covered by nanoparticles with average size of 18 nm. The diameter histogram of the surface particles was presented in Figure 2b. Although, based on the average size, the surface particles can be attributed to the TiO2 nanoparticles, a fraction of them can be also assigned to the residual carbons and/or solvents attached to defect sites of the graphene oxide sheets. The height profile diagram of the AFM image (Figure 2c) containing the sharp peaks (with the height of ∼10-20 nm) confirmed attachment of the TiO2 nanoparticles (with the average size of 15 nm) on the surface of the graphene sheets. The height profile also showed that the thickness of the graphene oxide sheets was ∼0.9 nm, which is in good consistency with the typical thickness of the single-layer graphene oxides (∼0.8 nm).16 In fact, the typical thickness of graphene oxide shows a ∼0.44 nm increase in graphene thickness (∼0.36 nm) because of the presence of epoxy and hydroxyl groups on both sides of the oxide surface.16,28 To study the effect of UV irradiation and its duration on the chemical state of the TiO2/graphene oxide sheets, XPS was utilized. The deconvoluted C(1s) XPS spectra of the TiO2/ graphene (oxide) samples have been shown in Figure 3. The deconvoluted peak centered at the binding energy ranging from 284.8 to 285.0 eV was assigned to the CsC, CdC, and CsH bonds. The deconvoluted peaks centered at the binding energy ranges of 286.0-286.5, 287.4-287.7, and 289.0-289.5 eV were attributed to the CsOH, CdO, and OdCsOH oxygencontaining carbonaceous bands, respectively.29-31 Before the light irradiation (Figure 3a), high amounts of the oxygencontaining carbonaceous bands were detected in the carbon peak, consistent with the presence of the graphene oxides on the

Photodegradation of Graphene Oxide Sheets

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Figure 2. (a) AFM image of the TiO2/graphene oxide sheets on a mica substrate, (b) diameter histogram of the surface particles, and (c) height profile diagram of the line shown in the AFM image. The sharp peaks in the depth profile correspond to the TiO2 nanoparticles, and the vertical distance of each couple of the markers is given above them.

TABLE 1: Peak Area (A) Ratios of the Oxygen-Containing Bonds to the CC Bonds and the Total Carbon Bands to the Au Band (Obtained by XPS) and the Peak Intensity Ratios of ID/IG (Obtained by Raman Analysis) of the TiO2/ Graphene (Oxide) Samples at the Different Irradiation Times XPS ACOH/ ACC

ACO/ ACC

AOCOH/ ACC

normalized (AC/AAu)a

ID/IG

0 1 2 4 10 24

0.68 0.23 0.16 0.15 0.13 0.14

1.26 0.37 0.18 0.18 0.21 0.20

0.16 0.06 0.03 0.03 0.03 0.03

1 0.82 0.73 0.63 0.32 0.15

1.26 0.92 0.81 0.95 1.10 1.55

a

Figure 3. Peak deconvolution of C(1s) XPS core level of the TiO2/ graphene (oxide) sheets after (a) 0, (b) 1, (c) 2, (d) 4, (e) 10, and (f) 24 h UV irradiation time.

surface of the as-prepared samples. No peaks relating to formation of Ti-C and or Ti-O-C bonds were found in the XPS spectra, indicating physical (not chemical) attachment of the TiO2 nanoparticles to the graphene oxide sheets. To quantitatively investigate and compare the change in concentration of the oxygen-containing carbonaceous bands, the peak area ratios of the CsOH, CdO, and OdCsOH bonds to the CsC, CdC, and CsH bonds were calculated and presented in Table 1. By exposing the TiO2/graphene oxides immersed in ethanol to the UV light irradiation for 1 h (Figure 3b), the concentration of the oxygen-containing bonds substantially decreased, indicating photocatalytic reduction of the graphene oxide sheets in the TiO2/graphene oxide composition. After 2 h irradiation (Figure 3c), the relative concentration of the CsOH, CdO, and OdCs OH bonds showed about 76, 85, and 81% reduction relative to the corresponding concentrations of the sample before irradiation, respectively. The remarkable decrease in the concentration of the oxygen-containing bonds of the graphene oxides after 2 h UV irradiation indicated their effective photocatalytic reduction in the TiO2/graphene (oxide) composition. By increasing the time of the UV exposure to 4, 10, and 24 h, no considerable change in the concentration of the oxygen-

Raman

irradiation time (h)

AC ) ACC + ACOH + ACO + AOCOH.

containing bonds was observed. But, based on the decrease in the peak area ratio of the C(1s) to Au(4f) core levels (AC/AAu), it was found that the amount of carbon on surface of the samples also decreased during the photocatalytic reduction, particularly after 10 and 24 h irradiation. In fact, after 24 h irradiation, 85% of the surface carbons of the TiO2/graphene (oxide) composition disappeared, while no considerable change in their chemical states was found. This means that the TiO2 nanoparticles in the TiO2/graphene (oxide) composition gradually degraded the graphene oxide sheets after reducing them in a photocatalytic process. To further examine the degradation of the reduced graphene oxide sheets of the TiO2/graphene (oxide) composition in the photocatalytic process, Raman spectroscopy was utilized, as shown in Figure 4. In fact, Raman spectroscopy is known as a suitable technique to study the ordered/disordered crystal structures of carbonaceous materials. The usual characteristics of carbon materials in Raman spectra are the G band (∼1580 cm-1), which is usually attributed to the E2g phonon of C sp2 atoms, and the D band (∼1350 cm-1) as a breathing mode of κ-point phonons of A1g symmetry,32,33 which is attributed to local defects and disorders, particularly located at the edges of graphene and graphite platelets.34 Concerning this, a smaller ID/IG peak intensity ratio in a Raman spectrum can be assigned to lower defects and disorders of the graphitized structures, smaller fraction of sp3/sp2-bonded carbon, and/or larger size of the in-plane graphitic crystallite sp2 domains. The Raman spectra shown in Figure 4 display the G line at about 1585 cm-1 and the D line at 1350 cm-1. The obtained values of the ID/IG ratio were also presented in Table 1. It was found that the ID/IG ratio decreased from 1.26 to 0.81 after 2 h UV irradiation. This can

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Figure 4. Raman spectra of the TiO2/graphene (oxide) sheets after (a) 0, (b) 1, (c) 2, (d) 4, (e) 10, and (f) 24 h UV irradiation time.

be assigned to formation of further sp2 bonds after the UVassisted photocatalytic reduction of the graphene oxide sheets, in good consistency with the XPS results. But, by further increasing the time of UV irradiation, the ID/IG ratio increased so that after 24 h irradiation the ID/IG ratio increased to a high value of 1.55. Such considerable increase in the ID/IG ratio was assigned to the appearance of further carbonaceous defects in the reduced graphene oxide sheets due to the photocatalytic degradation of them by the TiO2 nanoparticles. In fact, since the ratio of IG/ID is proportional to the in-plane graphitic crystallite size,35,36 it can be concluded that the crystallite size of sp2 domains of the graphene sheets photodegraded by the TiO2 nanoparticles after 24 h decreased about 19 and 48% of the crystallite size of the as-prepared graphene oxide sheets and the graphene oxide sheets photocatalytically reduced in 2 h, respectively. Raman spectroscopy is also utilized to examine the single-, bi-, and multilayer characteristics of graphene and/or graphene oxide layers. For example, it was previously reported that the peak position of the G band of the single-layer graphenes (1585 cm-1) shifts about 6 cm-1 into lower frequencies after stacking further graphene layers (for 2-6 layers G band shifts to 1579 cm-1).28,34-39 In addition, shape and position of the 2D band are the key parameters indicating formation and the layer numbers of graphene sheets.28,37-40 The 2D peak position of the single-layer graphene sheets is usually observed at 2679 cm-1, while the 2D band of multilayer (2-4 layers) shifts to higher wavenumbers by 19 cm-1.34 Here, the observed 2D bands of the graphene (oxide) sheets with a nearly symmetrical shape were centered at around 2680 cm-1, indicating formation of single-layer graphene (oxide) sheets. Therefore, the Raman analysis confirmed presence of single-layer graphene (oxide) sheets, as also found by using the height profile analysis of the AFM image. The current-voltage (IV) characteristic of the TiO2/graphene (oxide) sheets irradiated for the different periods of time was also studied, as shown in Figure 5. The linear behavior of the current-voltage curve of the TiO2/graphene (oxide) sheets indicated the metallic nature of the sheets and formation of ohmic contact between the sheets and the electrodes. Using the obtained IV curves, sheet resistance (Rs) of the TiO2/graphene (oxide) sheets was calculated for different irradiation times, as shown in the inset of Figure 5. The sheet resistance of the asprepared TiO2/graphene oxides was evaluated to be as high as ∼1011 Ω/sq. The IV diagram of the as-prepared graphene oxides was very similar to that of the as-prepared TiO2/graphene oxides (shown in Figure 5). This similarity indicated that the TiO2 nanoparticles connected only physically (not chemically) to the surface of the graphene oxide sheets, consistent with the XPS

Akhavan et al.

Figure 5. Current-voltage diagram of the TiO2/graphene (oxide) samples at different irradiation times. The inset shows sheet resistance of the TiO2/graphene (oxide) nanosheets at the different irradiation times.

analysis. After the photocatalytic reduction of the graphene oxide sheets, the sheet resistance sharply decreased, so that, for example, the Rs value of the TiO2/graphene oxide sheets decreased to 4.6 × 106 Ω/sq, after 2 h photocatalytic reduction. The increase in the conduction of the graphene (oxide) sheets can be assigned to decrease in the oxygen contents of the sheets, as previously studied for the graphene sheets exposed to oxygen plasma.41 Moreover, comparing our optimum Rs value of the photocatalytic reduced graphene oxides with the ones reported for graphene oxides reduced by hydrazine (∼108 Ω/sq) and graphene oxides reduced by both hydrazine and thermal reduction at 400 °C (∼105 Ω/sq)42,43 shows that application of the photocatalytic process in a suitable time (here, 2 h) can be substantially effective in chemical reduction of the graphene oxide sheets. However, longer times of UV irradiation resulted in increase in the sheet resistance of the reduced graphene oxide sheets (for example, after 24 h irradiation, the Rs value increased to the large value of 5.1 × 109 Ω/sq). Since, the XPS analysis showed that the level of reduction of the reduced graphene oxide sheets was unchanged for the irradiation times longer than 2 h, the high increase in the Rs value of the graphene oxide sheets irradiated for the longer times was assigned to photodegradation of the reduced graphene oxide sheets by the TiO2 nanoparticles, consistent with the analysis of Raman spectra. 4. Conclusions The chemically synthesized graphene oxide sheets were physically combined by TiO2 nanoparticles after deposition between Au thin film electrodes. Both Raman and AFM analyses confirmed formation of single-layer graphene oxide sheets. Based on the significant reduction of the oxygen-containing carbonaceous bands and the ID/IG ratio of the XPS and Raman spectra, it was found that the photocatalytic reduction of the graphene oxides by the TiO2 nanoparticles was nearly completed after 2 h UV irradiation, respectively. In addition, using XPS no considerable change was observed in the reduction level of the reduced graphene oxides for the longer irradiation times. However, the longer irradiations resulted in decrease of the carbon content of the reduced graphene oxides (based on the XPS) and increase of the carbon defects (based on the Raman), indicating degradation of the reduced sheets by the TiO2 nanoparticles in the photocatalytic process. Consistently, the IV measurements showed that the Rs value of the graphene oxide sheets substantially decreased from ∼1011 to 4.6 × 106 Ω/sq after the photocatalytic reduction for 2 h. But, by increasing the irradiation time from 4 to 24 h, the Rs value highly increased

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