Nanocomposites of TiO2 and Reduced Graphene Oxide as Efficient

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Nanocomposites of TiO2 and Reduced Graphene Oxide as Efficient Photocatalysts for Hydrogen Evolution Wenqing Fan, Qinghua Lai, Qinghong Zhang,* and Ye Wang* State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ABSTRACT: Nanocomposites of titanium dioxide (P25) and reduced graphene oxide (RGO), which were prepared by several techniques including UV-assisted photocatalytic reduction, hydrazine reduction, and hydrothermal method, were studied as photocatalysts for the evolution of hydrogen from alcohol solution under UV
vis irradiation. The incorporation of RGO into P25 significantly enhanced the photocatalytic activity for H2 evolution, and the P25
RGO composite prepared by the hydrothermal method exhibited the best performance. The optimum mass ratio of P25 to RGO in the composite was 1/0.2. The P25
RGO composite was stable and could be used recyclably, and it could also catalyze the evolution of H2 from pure water. Our characterizations suggested that P25 nanoparticles with diameters of 20
30 nm were dispersed on the RGO sheet in the composite, and the stronger interaction between P25 and RGO provided a better photocatalytic activity. The intimate contact between P25 and RGO was proposed to accelerate the transfer of photogenerated electrons on P25 to RGO, suppressing the recombination of charge carriers and thus increasing the photocatalytic performance.

1. INTRODUCTION The utilization of solar energy for the conversion of water to hydrogen and oxygen has been considered as an ultimate solution to energy and environmental problems. Many photocatalysts have been reported to catalyze the evolution of H2 from aqueous solutions with and without sacrificial reagents.1
7 Among these photocatalysts, TiO2 is one of the most promising catalysts because of its superior photocatalytic performance, easy availability, long-term stability, and nontoxicity.1,8,9 Typically, photoexcited electron
hole pairs can be generated under the irradiation with wavelength lower than that corresponding to the band gap energy of TiO2 (3.2 eV for anatase). The photogenerated electrons can then work for the reduction of Hþ in the aqueous solution to form H2. The recombination of the electrons and holes is one of the main reasons for the low efficiency of photocatalysis. Therefore, one of the most challenging issues in photocatalysis is to overcome the quick recombination of photogenerated electrons and holes. Several strategies have been proposed to increase the efficiency of TiO2 photocatalyst. The preparation of TiO2-based composites is an efficient means to enhance the photocatalytic performance via retarding the charge recombination.10 For examples, the loading of noble metal nanoparticles onto TiO2 is known to enhance the photocatalytic performances owing to the roles of metal nanoparticles in storing and shuttling photogenerated electrons from TiO2 to an acceptor.11
14 The combination of TiO2 with other semiconductors such as MoO3, WO3, and SnO2 may also enhance the photocatalytic performance.15
17 The composite of TiO2 and carbon materials, particularly carbon nanotubes (CNTs), has attracted much attention in recent years.18 r 2011 American Chemical Society

Hoffmann et al.19 proposed that the photogenerated electrons in the space-charge regions may be transferred into CNTs, and the holes remain on TiO2, thus retarding the recombination of electrons and holes. In addition, CNTs may also provide high surface areas or peculiar functional groups for the efficient adsorption of reactants and may act as photosensitizers. The possible formation of Ti
O
C bonds may also affect the photocatalytic behaviors of the TiO2
CNT composites. Furthermore, CNTs could function for controlling the morphology of TiO2 nanoparticles.20 Graphene is a two-dimensional sp2-hybridized carbon nanosheet, which possesses many unique properties such as a very high theoretical specific surface area (∼2600 m2 g
1), high mobility of charge carriers, and good mechanical strength.21
23 Very recently, several limited papers have been contributed to study the catalytic or photocatalytic properties of graphene or reduced graphene oxide (RGO).24
30 Kamat and co-workers31 have demonstrated that the photogenerated electrons from UVirradiated TiO2 can be transferred to graphene oxide (GO), reducing GO to RGO. The photogenerated electrons could also be transported across the RGO to reduce Ag ions to Ag nanoparticles at a location distinct from the TiO2 anchored site on RGO.32 Li and co-workers25 demonstrated that the TiO2 (P25)
RGO composite was a highly efficient photocatalyst for the degradation of methylene blue. However, a very recent study by Xu and co-workers30 pointed out that the TiO2
RGO composite Received: January 27, 2011 Revised: April 23, 2011 Published: May 09, 2011 10694

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The Journal of Physical Chemistry C was in essence the same as the TiO2
CNT composite on enhancement of photocatalytic performance in the gas-phase degradation of benzene. Dai and co-workers33 reported that a TiO2
RGO composite prepared by growing TiO2 nanocrystals on GO through hydrolysis followed by hydrothermal treatment efficiently catalyzed the photocatalytic degradation of rhodamine B, and there existed strong interactions between TiO2 and the graphene sheets. Cui et al.26 demonstrated that the TiO2
RGO composite prepared by a sol
gel method could catalyze the H2 evolution from aqueous solution containing Na2S and Na2SO3 as sacrificial agents. The maximum H2 formation rate of the TiO2
RGO composite (8.6 μmol h
1) was about twice that of P25 alone (4.5 μmol h
1).26 Although graphene possesses many unique properties, its applications in catalysis field are still scarce. Herein, we report the preparation, characterization, and photocatalytic performance of P25
RGO composites for the evolution of H2. Typically, composites containing RGO sheets can be obtained by reducing the suspension containing colloidal GO solution. Several techniques such as chemical reduction using hydrazine,28 UV-assisted photoreduction,31,32,34 sol
gel method,26 and hydrothermal method25,30,33 have been employed for the preparation of semiconductor (including TiO2)
RGO composites. This paper contributes to study the photocatalytic behaviors of the P25
RGO composites prepared by several different methods.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Graphene Oxide (GO). GO was prepared from graphite powder according to the modified method reported by Hummers and Offeman.35 In brief, 1.0 g of graphite powder and 0.5 g of NaNO3 were added into 23 mL of cooled (273 K) concentrated H2SO4. Then, 3 g of KMnO4 was added gradually with continuous stirring and cooling, and the temperature of the mixture was maintained below 293 K. After the ice bath was removed, the mixture was stirred at 308 K for 30 min. 46 mL of distilled water was added slowly to cause an increase in temperature to 371 K, and the mixture was maintained at that temperature for 15 min. The reaction was terminated by addition of 140 mL of distilled water followed by 10 mL of 30% H2O2 aqueous solution. The solid product was separated by centrifugation and washed repeatedly with 5% HCl solution until sulfate anion could not be detected with BaCl2. The resultant solid was dried in vacuum at 323 K to obtain GO. 2.2. Preparation of P25
RGO Composites. P25, which contained 20% of rutile and 80% of anatase, was purchased from Degussa. Three methods, i.e., UV-assisted photoreduction,31,32,34 hydrazine reduction,28 and hydrothermal method,25,30 were employed for the preparation of P25
RGO composites. UV-Assisted Photoreduction Method. GO was first dissolved in EtOH by ultrasonic treatment for 1 h to yield a yellow-brown solution. Then, P25 was added into the GO colloidal solution, and the mixture was stirred for another 1 h to obtain a homogeneous suspension. The suspension was kept agitated during UV irradiation to reduce GO and to deposit P25 on the RGO sheets. The resultant composite was collected by centrifugation, washed repeatedly with water, and dried in vacuum at 333 K. The sample prepared by this procedure is denoted as P25
RGO-photo. Hydrazine Reduction Method. GO was first dissolved in H2O by ultrasonic treatment for 1 h to yield a yellow-brown solution, and P25 was added into the GO colloidal solution. The mixture was stirred for another 1 h to obtain a homogeneous suspension.

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Then, 85% hydrazine aqueous solution was added into the suspension. The reduction process was carried out at 323 K for 12 h. The resultant composite was collected by centrifugation, washed repeatedly with water, and dried in vacuum at 333 K. The sample prepared by this method is denoted as P25
RGO-hydrazine. Hydrothermal Method. GO was first dissolved in H2O by ultrasonic treatment for 1 h to yield a yellow-brown solution, and P25 was added into the GO colloidal solution. After stirred for another 1 h, the obtained homogeneous suspension was transferred to a Teflon-lined autoclave and was subjected to hydrothermal treatment at 453 K for 6 h. GO could be reduced to RGO during the hydrothermal process.36,37 The resultant composite was collected by centrifugation, washed repeatedly with water, and dried in vacuum at 333 K. The sample prepared by this procedure is denoted as P25
RGO-hydrothermal. 2.3. Preparation of P25
CNT Composite. Multiwalled CNTs with outer diameters of 20
60 nm and inner diameters of 3
5 nm were synthesized by an established method.38 For comparison, a similar procedure to that used for the preparation of the P25
RGO-hydrothermal was employed for the preparation of the P25
CNT. In brief, both P25 and CNTs were added into H2O, followed by ultrasonic treatment for 1 h to yield a homogeneous suspension. Then, the suspension was transferred to a Teflon-lined autoclave and was subjected to hydrothermal treatment at 453 K for 6 h. The same post-treatment procedure was adopted as that used in the preparation of the P25
RGOhydrothermal. 2.4. Characterization. Powder X-ray diffraction (XRD) patterns were collected on a Panalytical X0 pert Pro diffractometer using Cu KR radiation (40 kV, 30 mA). X-ray photoelectron spectroscopic (XPS) measurements were performed on a Quantum 2000 Scanning ESCA Microprobe (Physical Electronics) using Al KR radiation (1846.6 eV) as X-ray source. Transmission electron microscopy (TEM) measurements were performed on a JEM-2100 electron microscope operated at an acceleration voltage of 200 kV. Transmission FT-IR spectra were recorded on a Nicolet Avatar 330 instrument equipped with an MCT detector with a resolution of 4 cm
1. The sample was pressed into a self-supporting wafer and was placed in an IR cell. Raman spectroscopic measurements were carried out with a Renishaw Inva Raman System 1000. A laser output of 40 mW was used, and the maximum incident power at the sample was ∼4 mW in each measurement. Diffusion reflectance UV
visible (UV
vis) spectra were recorded on a Varian-Cary 5000 spectrometer equipped with a diffuse reflectance accessory. The spectra were collected with BaSO4 as a reference. 2.5. Photocatalytic Measurement. The photocatalytic reactions were performed on a Pyrex reaction cell connected to a closed gas circulation and evacuation system. The light source was a 200 W Xe arc lamp (PLS-SXE300, Beijing Trusttech Co., Ltd.) at UV
vis. Typically, the photocatalyst (100 mg) was dispersed in an aqueous solution containing H2O (80 mL) and an alcohol (20 mL). The products were H2 and CO2. The amount of H2 evolved was determined with an online gas chromatograph (Techcomp; GC-7890II) equipped with a molecular sieve 5 A column and a TCD detector, and N2 was used as the carrier gas.

3. RESULTS AND DISCUSSION 3.1. Photocatalytic Behaviors of P25
RGO Composites. Figure 1 shows the rates of H2 evolution in methanol aqueous solution over the P25
RGO composites with a mass ratio of 10695

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Figure 1. Comparison of photocatalytic performances of P25, P25
CNT composites with different mass ratios of P25/CNT, and P25
RGO composites (mass ratio of P25/RGO = 1/0.2) prepared by different methods for the evolution of H2 from methanol aqueous solution. Catalyst, 100 mg; volume of solution, 100 mL; CH3OH concentration, 20 vol %.

P25/RGO of 1/0.2 prepared by different methods. For comparison, the results obtained for P25 alone and the P25
CNT composites with different mass ratios are also listed in Figure 1. The controlled blank reaction in the absence of any catalyst or in the presence of RGO or CNT alone showed no H2 formation. The use of P25 alone could form H2. However, the P25
RGO composites prepared by all the three methods exhibited significantly higher rates of H2 evolution than P25 alone (Figure 1), clearly demonstrating that the presence of RGO could enhance the photocatalytic performance of P25 for H2 evolution. Among the P25
RGO composites, the P25
RGO-hydrothermal afforded the highest rate of H2 evolution. The photocatalytic activity decreased in the order of P25
RGO-hydrothermal > P25
RGOphoto > P25
RGO-hydrazine. Thus, the hydrothermal method is the most effective for preparing RGO-promoted photocatalysts, whereas the hydrazine reduction is the least effective. Figure 1 also reveals that the rate of H2 evolution over the P25
RGO-hydrothermal was significantly higher than that over the P25
CNT catalyst with an optimized mass ratio of P25/ CNT (1/0.3). Li and co-workers25 observed a 15% increase in the degradation of methylene blue when the P25
RGO was compared with the P25
CNT. On the other hand, Xu and co-workers30 only found a slight difference between these two composites in the photodegradation reaction. The distinct difference between the two composites in our case suggests that the composite catalyst based on RGO is more suitable for H2 evolution reactions. We further investigated the effect of the ratio of P25/RGO on photocatalytic performances of P25
RGO-hydrothermal series of samples. Figure 2 shows that the amount of H2 evolved over each catalyst increased proportionally with reaction time, indicating that the photocatalytic reaction proceed steadily in each case. The presence of RGO with an appropriate content in the composite significantly accelerates the evolution of H2 from methanol aqueous solution. The amount of H2 evolved was the largest over the composite with a mass ratio of P25/RGO of 1/0.2. The rate of H2 evolution calculated for this composite was 74 μmol h
1, which was 1 order of magnitude higher than that for P25 alone (6.8 μmol h
1). This increase in H2 formation rate by combining RGO with P25 is much more significant than that reported by Cui et al.,26 who used a sol
gel method for preparing the TiO2
RGO composite and Na2S and Na2SO3 as sacrificial agents. A further

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Figure 2. Photocatalytic performances of P25
RGO-hydrothermal series of composites with different mass ratios of P25/RGO for the evolution of H2 from methanol aqueous solution. Catalyst, 100 mg; volume of solution, 100 mL; CH3OH concentration, 20 vol %.

increase in RGO content to a mass ratio of P25/RGO of 1/0.3 rather decreased the amount of H2 evolved. In the cases of photocatalytic degradation of methylene blue, the optimum mass ratio of RGO to P25 was 5 wt %.30 In our case, a higher content of RGO was required for obtaining the optimum activity. This may indicate that RGO plays more important roles in the evolution of H2. We examined the stability of our composite catalyst. After each run, the P25
RGO-hydrothermal catalyst was evacuated for 30 min and was reused in the next run. The result in Figure 3 clearly shows that no deactivation occurs in the repeated uses, and our P25
RGO-hydrothermal catalyst can be used recyclably. Figure 4 shows the effect of the concentration of CH3OH in aqueous solution on the rate of H2 evolution catalyzed by the P25
RGO-hydrothermal (P25/RGO = 1/0.2) composite. In the absence of a sacrificial agent (i.e., in pure H2O), the H2 evolution rate was quite low. The presence of methanol significantly accelerated the evolution of H2. The rate of H2 evolution was saturated at CH3OH concentration of ∼20 vol %. We further compared the rates of H2 evolution from the aqueous solutions of different kinds of alcohols. The result in Figure 5 reveals that CH3OH is a better sacrificial agent for H2 evolution than C2H5OH and i-C3H7OH. Nada et al.39 also observed that CH3OH aqueous solution was more efficient than C2H5OH aqueous solution for H2 evolution in the presence of a TiO2-based photocatalyst. It would be highly desirable to perform H2 evolution in the absence of any sacrificial agent. Figure 6 compares the amounts of H2 evolved from pure water catalyzed by P25 and the P25
RGO-hydrothermal (P25/RGO = 1/0.2) composite. As compared to P25 alone, a significantly larger amount of H2 was obtained over the P25
RGO composite. The rate of H2 evolution from pure H2O increased for ∼5 times by incorporation of RGO into P25. This further demonstrates the significant promoting effect of RGO in H2 evolution photocatalysis. 3.2. Characterizations of P25
RGO Composites. We performed structural characterizations for the P25
RGO composites as well as GO and RGO. The XRD pattern of GO showed a strong and sharp diffraction peak at 2θ of 10.6
(Figure 7), corresponding to a d-spacing of ∼8.4 Å. This is in agreement with the lamellar structure of GO. For the RGO obtained from the hydrothermal reduction, this diffraction peak disappeared and a very broad peak at 2θ of ∼25
(corresponding to a d-spacing of 3.6 Å) was observed, suggesting the reduction of GO to graphene sheets.36 The P25
RGO-hydrothermal composite exhibited diffraction peaks only ascribed to P25, which contained both 10696

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Figure 3. Changes of photocatalytic performances of P25
RGO-hydrothermal (P25/RGO = 1/0.2) during the repeated uses. Catalyst, 100 mg; volume of solution, 100 mL; CH3OH concentration, 20 vol %.

Figure 4. Rate of H2 evolution as a function of CH3OH concentration over P25
RGO-hydrothermal (P25/RGO = 1/0.2). Catalyst, 100 mg; volume of solution, 100 mL.

Figure 5. Rate of H2 evolution as a function of type of sacrificial agent over P25
RGO-hydrothermal (P25/RGO = 1/0.2). Catalyst, 100 mg; volume of solution, 100 mL; alcohol concentration, 20 vol %.

anatase and rutile. The same phenomenon was observed in previous papers,25,30 suggesting that GO was reduced to RGO during the hydrothermal process. The P25
RGO composites prepared by the other methods showed similar XRD patterns. Figure 8 shows the C 1s and Ti 2p XPS spectra for the P25
RGO composites together with GO and the RGO obtained from hydrothermal reduction. In the C 1s spectra, two main peaks were observed for GO: the peak at 284.6 eV was assignable to the sp2 carbon species, while the peak at higher binding energies (286
288 eV) was ascribed to oxygenated carbon species such

Figure 6. Comparison of photocatalytic performances between P25 and P25
RGO-hydrothermal (P25/RGO = 1/0.2) for H2 evolution from pure H2O. Catalyst, 100 mg; volume of H2O, 100 mL.

as hydroxyl, carboxyl, and epoxide species on GO surfaces.37 For the RGO and the P25
RGO composites prepared by different methods, the peak of oxygenated carbon species disappeared or decreased significantly. This also indicates the reduction of GO to graphene sheets in the P25
RGO composites.37 The C 1s XPS spectrum for the P25
CNT composite with a mass ratio of P25/CNT of 0.3 is also shown in Figure 8, and the peak of oxygenated carbon species at higher binding energies is quite weak. The binding energy of Ti 2p3/2 was observed at 458.5 eV in all these samples (Figure 8B), which is typical of that for TiO2. From FT-IR measurements, a broad absorption band below 1000 cm
1 was observed for the P25
RGO composites prepared by different methods (Figure 9). This band could be attributed to the vibrations of Ti
O
Ti bonds in P25, which showed a similar band in the same region. Furthermore, an absorption band at ∼1570 cm
1 was clearly observed for the P25
RGO composites prepared by different methods (Figure 9). This band can be attributed to the skeletal vibration of the graphene sheets.25,36 All these characterization results confirm the reduction of GO to graphene sheets in the P25
RGO composites. However, these characterizations did not provide information about the differences in the physicochemical properties of the P25
RGO composites prepared by different methods. Raman spectroscopy was used to characterize the P25
RGO composites as well as GO. The typical features in Raman spectra are the G band at ∼1595 cm
1 and the D band at ∼1350 cm
1 (Figure 10). These bands could be assigned to the E2g phonon of C sp2 atoms and one breathing mode of k-point phonons, 10697

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Figure 7. XRD patterns of GO, RGO, P25, and P25
RGO-hydrothermal (P25/RGO = 1/0.2).

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Figure 10. Raman spectra. (a) GO, (b) P25
RGO-photo (P25/RGO = 1/0.2), (c) P25
RGO-hydrazine ((P25/RGO = 1/0.2), (d) P25
RGO-hydrothermal ((P25/RGO = 1/0.2).

Figure 8. (A) C 1s and (B) Ti 2p XPS spectra. (a) GO, (b) RGO, (c) P25
RGO-photo (P25/RGO = 1/0.2), (d) P25
RGO-hydrazine ((P25/RGO = 1/0.2), (e) P25
RGO-hydrothermal ((P25/RGO = 1/ 0.2), (f) P25
CNT (P25/CNT = 1/0.3), (g) P25.

Figure 11. Pictures of suspensions of P25 and P25
RGO composites after reduction or hydrothermal process. From left to right: suspensions of P25, P25
RGO-photo, P25
RGO-hydrazine, and P25
RGO-hydrothermal. From top to bottom: instantly after reduction or hydrothermal process, after resting for 1 h and after resting for 2 h.

Figure 9. FT-IR spectra. (a) P25, (b) GO, (c) P25
RGO-photo (P25/RGO = 1/0.2), (d) P25
RGO-hydrazine ((P25/RGO = 1/0.2), (e) P25
RGO-hydrothermal ((P25/RGO = 1/0.2).

respectively, and the intensity ratio of D band to G band (ID/IG) is proposed to be an indication of disorder in GO or RGO, originating from defects associated with vacancies, grain boundaries, and amorphous carbons.37 Loh and co-workers37 observed a lower 10698

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Figure 12. TEM images. (a) RGO, (b) P25
RGO-photo (P25/RGO = 1/0.2), (c) P25
RGO-hydrazine (P25/RGO = 1/0.2), (d) P25
RGO-hydrothermal (P25/RGO = 1/0.2), (e) P25
CNT (P25/CNT = 1/0.3), (f) P25
RGO-hydrothermal (P25/RGO = 1/0.05), (g) P25
RGO-hydrothermal (P25/RGO = 1/0.1), (h) P25
RGO-hydrothermal (P25/RGO = 1/0.3).

Figure 13. (A) Diffuse reflectance UV
vis spectra of P25, P25
RGO composites (P25/RGO = 1/0.2) prepared by different methods, and P25
CNT composite (P25/CNT = 1/0.3). (B) Corresponding plot of transformed Kubelka
Munk function versus the energy of the light.

ID/IG for the RGO obtained from hydrothermal reduction of GO compared to that from hydrazine reduction and speculated that

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the former reduction method was advantageous to preparing RGO with less defects.37 In our case, the ID/IG was 2.06 for GO, and it decreased slightly to 2.02 for the P25
RGO-hydrothermal (Figure 10). On the other hand, the ID/IG rose slightly to 2.10 and 2.18 for the P25
RGO-photo and the P25
RGO-hydrazine, respectively. Although the ID/IG was slightly lower for the P25
RGO-hydrothermal, which might suggest the lower concentration of defects in this composite, the differences among the P25
RGO composites prepared by different methods were not very significant. Figure 11 displays the sample pictures of the P25
RGO suspensions obtained after reduction or hydrothermal process. For comparison, the suspension containing P25 alone was also shown. Instantly after the reduction or hydrothermal process, the suspension of P25
RGO-hydrothermal was the most homogeneous, whereas the suspension of P25
RGO-hydrazine was the least homogeneous. After keeping resting for 1 h, in the case of P25
RGO-hydrazine suspension, almost all of the RGO (black) rose up to the surface, leaving the colloidal P25 suspension. This separation implies that the interaction between P25 and RGO is the weakest in the P25
RGO-hydrazine. Such a separation also occurred for the suspension of P25
RGO-photo, but the degree is less serious than that in the case of P25
RGO-hydrazine. On the other hand, in the case of P25
RGO-hydrothermal suspension, RGO (black) began to sink to the bottom after resting for a while. After 2 h, the solution became clear, suggesting the simultaneous subsiding of RGO and P25. This suggests a strong interaction between the two components in the P25
RGO-hydrothermal. Thus, the interactions between RGO and P25 may decrease in the order of P25
RGO-hydrothermal > P25
RGO-photo > P25
RGO-hydrazine. This order is the same as that observed for the photocatalytic activity for H2 evolution (Figure 1). Therefore, we speculate that the interaction between P25 and graphene sheets in the composite is a key factor in determining the photocatalytic behavior. Figure 12 shows the TEM images of the RGO, the P25
RGO composites prepared by different methods, the P25
RGOhydrothermal with different mass ratios of P25/RGO, and the P25
CNT composite. The sheetlike structure of graphene was observed for the RGO. The TEM images of the P25
RGO composites show that the composites are composed of graphene sheets and P25 nanoparticles. The size of P25 nanoparticles were in the range 20
30 nm. However, the contact between P25 and graphene was different for the composites prepared by different methods. In the P25
RGO-hydrothermal, P25 nanoparticles were more highly dispersed on graphene sheets, whereas the segregation of P25 nanoparticles from graphene sheets or the aggregation of P25 nanoparticles was serious in the P25
RGOhydrazine or the P25
RGO-photo composite. These may lead to the differences in catalytic behaviors among the P25
RGO composites prepared by different methods. Moreover, the dispersion of P25 nanoparticles over CNTs was found not as good as that over the graphene sheets in the P25
RGO-hydrothermal composite. Our TEM measurements further reveal that the increase in RGO content in the P25
RGO-hydrothermal composites increases the dispersion of P25 nanoparticles on the graphene sheets (Figure 12, curves f
h). Diffuse reflectance UV
vis spectroscopy was applied to further study the interaction between P25 and RGO in the composites prepared by different methods. Li et al.25 and Xu et al.30 both observed a red shift of the absorption edge of P25 upon the addition of RGO, indicating the narrowing of the band gap of P25. This was proposed to correspond to the formation of 10699

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using hydrazine, and hydrothermal method. These composites have shown better photocatalytic performances for H2 evolution from methanol aqueous solution than P25 alone. We have demonstrated that the composite prepared by the hydrothermal method is the most efficient photocatalyst for the evolution of H2. The ratio of P25/RGO in the composite also determines the photocatalytic performance, and the optimum mass ratio has been found to be 1/0.2. Under proper conditions, the rate of H2 evolution over the P25
RGO composite is 1 order of magnitude higher than that over P25 alone. Our comparison has revealed that the P25
RGO composite is also more effective than the P25
CNT composite for the evolution of H2. The P25
RGO composite is stable and can be used repeatedly. Moreover, the P25
RGO composite also demonstrates significantly higher rate of H2 evolution from pure water than P25 alone. Our characterizations suggest that GO can be reduced to graphene sheets by each of the preparation method. The hydrothermal method results in stronger interaction between P25 and graphene sheets in the composite. We propose that the intimate contact between P25 and RGO may accelerate the transfer of photogenerated electrons on P25 to RGO, suppressing the recombination of charge carriers. This mainly causes the higher photocatalytic performance of the P25
RGO composite prepared by the hydrothermal method.

’ AUTHOR INFORMATION Figure 14. (A) Diffuse reflectance UV
vis spectra of P25 and P25
RGO-hydrothermal with different mass ratios of P25/RGO. (B) Corresponding plot of transformed Kubelka
Munk function versus the energy of the light.

Corresponding Author

Ti
O
C bond between P25 and RGO,25,30 similar to that observed for the carbon-doped TiO2 composites.40 Figure 13 shows UV
vis spectra of P25
RGO composites prepared by different methods together with P25 and the P25
CNT composite. We confirmed the red shift of the absorption edge after the addition of RGO and CNT to P25 (Figure 13A). From the plot of the modified Kubleka
Munk function, i.e., [F(R¥)hν]1/2, versus the energy of exciting light (hν), the band gap narrowing was clearly observed for all the composites (Figure 13B). Among these samples, the degree of band gap narrowing was the highest for the P25
RGO-hydrothermal, further suggesting that the interaction between P25 and RGO in this composite was the strongest. We further performed diffuse reflectance UV
vis spectroscopic studies for the P25
RGO-hydrothermal with different mass ratios of P25/RGO. Figure 14 shows that the degree of band gap narrowing increases with the content of RGO in the composite. This indicates that the interaction between P25 and RGO increases when the mass ratio of P25/RGO decreases from 1/0.05 to 1/0.3. However, Figure 2 shows that the optimum mass ratio of P25/ RGO for the photocatalytic evolution of H2 is 1/0.2. We believe that this is because P25 is the main photocatalyst for the generation of electron
hole pairs, whereas RGO functions as a promoter for increasing the transport of photogenerated electrons and retarding the charge recombination. The too high content of RGO would decrease the photocatalytic activity by decreasing the photogenerated electron
hole pairs.

’ ACKNOWLEDGMENT This work was supported by the NSF of China (Nos. 20873110, 20923004, and 21033006), the National Basic Research Program of China (No. 2010CB732303), the Key Scientific Project of Fujian Province (No. 2009HZ0002-1), and the Research Fund for the Doctoral Program of Higher Education (No. 20090121110007).

4. CONCLUSIONS We have prepared P25
RGO composites by three different methods, i.e., UV-assisted photoreduction, chemical reduction

*E-mail: [email protected] (Q.Z.), [email protected] (Y.W.). Tel: þ86-592-2186156. Fax: þ86-592-2183047.

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