TiO2–graphene nanocomposites for photocatalytic hydrogen production from splitting water

https://doi.org/10.1016/j.ijhydene.2011.11.004Get rights and content

Abstract

TiO2 (P25)–graphene (P25–GR) hybrids were prepared via solvothermal reaction of graphene oxide and P25 using ethanol as solvent. The as-prepared P25–GR nanocomposites were characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, photoluminescence emission spectroscopy and ultraviolet-visible (UV–vis) diffuse reflectance spectroscopy. The results indicated that P25–GR nanocomposites possessed enhanced light absorption ability and charge separation efficiency. As photocatalysts, P25–GR hybrids were much better than the bare P25, when they were used in the hydrogen evolution from aqueous methanol solution under Xe-lamp illumination. A significant enhancement in the rate of hydrogen production was achieved through using P25–GR as photocatalysts, comparing to bare P25. The optimum mass ratio of GR to P25 in the hybrids was 0.5 wt%. The higher mass ratio of GR in P25–GR would decrease the photocatalytic activity of P25.

Highlights

► P25–graphene hybrids are prepared via solvothermal reaction of GO and P25. ► The hybrids possess high light-absorption ability and charge-separation efficiency. ► The hybrids show the higher rate of hydrogen production than bare P25.

Introduction

The increasingly serious energy crisis and the environmental contamination caused by the burning of fossil fuels have led to an aggressive search for renewable and environmental-friendly energy recources. Hydrogen energy has been recognized as a potentially significant form of storable and clean energy for the future. Since the discovery of the first water-splitting system based on TiO2 and Pt in 1972 by Fujishima and Honda [1], many kinds of materials and derivatives have been discovered as photocatalysts for this reaction [2]. Currently, TiO2 is still one of the most widely used photocatalysts due to its exceptional optical and electronic properties, strong oxidizing power, non-toxicity, chemical stability, and low cost [3]. Typically, photoexcited electron-hole pairs can be generated under the light irradiation with wavelength lower than that corresponding to the band gap energy of TiO2. The photogenerated electrons then drive the water-splitting reaction to produce hydrogen [4]. However, the photogenerated electrons and holes in TiO2 may experience a rapid recombination, which is one of key factor limiting further improvement of its photocatalytic efficiency [5]. Therefore, one of the most challenging issues on photocatalysis is to overcome the quick recombination of photogenerated electrons and holes.

Several strategies have been employed to improve the photocatalytic performance of TiO2, for example, textural design [6], [7], coupling TiO2 with metal or other semiconductors [8], [9], etc. In particular, great interest has been devoted to combining carbon nanomaterials [10], [11], particularly carbon nanotubes (CNTs) [12], with TiO2 to enhance its photocatalytic performance.

Graphene (GR) as a new carbon nanomaterial has many exceptional properties, such as high electron mobility, high transparency, flexible structure, and large theoretical specific surface area [13], [14], [15]. Thus, the combination of TiO2 and graphene is promising to improve the photocatalytic performance of TiO2. Most recently, Li et al. [16] demonstrated that TiO2–graphene shows an enhancement of photocatalytic activity for the degradation of methylene blue. Xu et al. [17] pointed out that the TiO2–graphene composite was a highly efficient photocatalyst for the degradation of gas-phase benzene. Dai et al. [18] reported that a TiO2–graphene composite prepared by growing TiO2 nanocrystals on graphene oxide (GO) through hydrolysis of Ti(BuO)4 has improved the photocatalytic activity of TiO2 for the degradation of rhodamine B. However, few reports have been focused on the application of graphene in the photocatalytic hydrogen production field. Recently, Cui and co-workers [19] prepared the TiO2–graphene composite by a sol-gel method and demonstrated that TiO2–graphene showed higher photocatalytic activity for H2 evolution from aqueous solution containing Na2S and Na2SO3 as sacrificial agents than P25. It has been reported that GO can be reduced to GR by solvothermal reaction of GO in the ethanol solvent [20]. Herein, TiO2 (P25)–GR composites have been prepared via a facile solvothermal reaction. The microstructures of the resultant hybrids have been characterized. The photocatalytic performance for H2 evolution from water splitting has been analyzed as well.

Section snippets

Synthesis of P25–graphene hybrids

Graphene oxide (GO) was prepared using natural graphite powder through a modified Hummers method [21], [22]. In a typical experiment, 2 g graphite (500 mesh), 1 g sodium nitrate and 50 mL concentrated H2SO4 were put into a 250 mL flask in an ice bath. After the mixture was stirred continuously for 2 h, 7.3 g KMnO4, 7 mL H2O2 (30 wt%) and 150 mL deionized water were added into the above reaction mixture. Then the mixture was filtered through 0.45 μm cellulose membrane film. The resultant yellow-brown

Results and discussion

Fig. 1(a) indicated the typical AFM image of the graphene sheets obtained by solvothermal method. The thickness, measured from the height profile of the AFM image, as shown in Fig. 1(b), was about 1 nm, which was consistent with the data reported in the literature, indicating that the formation of the single-layered GR [23]. Fig. 1(c) showed the UV–vis absorption spectra of GO and GR aqueous dispersions obtained by solvothermal reduction. The absorption peaks centered at 230 and 300 nm were

Conclusion

In summary, we have prepared P25-GR hybrids with different mass ratios of GR by a solvothermal treatment of graphene oxide and P25 nanoparticles using ethanol as solvent. The experimental results suggested that graphene oxide can be reduced to graphene sheets by solvothermal reduction. Due to its two-dimensional π-conjugation structure, graphene served as an acceptor of the photogenerated electrons of P25 and transporter to separate the photogenerated electron–hole pairs effectively. P25-GR

Acknowledgements

The authors acknowledge financial support from National Nature Science Foundation of China (50802051), National Key Basic Research and Development Program of China (no. 2009CB220004), and Chinese Postdoctoral Science Foundation (20060400055).

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