One-pot synthesis of TiO2/graphene nanocomposites for excellent visible light photocatalysis based on chemical exfoliation method
Graphical abstract
Introduction
The advantages of TiO2 as a photocatalyst are identified by its superior properties including chemical stability [1], strong oxidizing power [2], and nontoxicity, as well as a wide spectrum of applications in solar cell [3], gas sensor [4], optoelectronic devices [5], and so on. However, the broad band gap (3.2 eV) and a short lifetime of photoexcited electron-hole pairs (2–3 μs) [6] are the major pitfalls of TiO2 in visible-light photocatalysis. Since the early 2000's, considerable efforts have been devoted to alleviate the said shortfalls of TiO2. For example, implanting with foreign metal ions or non-metals [[7], [8], [9], [10], [11], [12]] can narrow the band gap of TiO2. Tailored heterojunction with other semiconductors [13,14], noble metal doping [[15], [16], [17]], or grafting of organic chelating ligands [[18], [19], [20]] are effective ways to improve the charge carrier separation. Among these modifications, graphene is the newest material to be paired with TiO2. In addition to the extremely high specific surface area as a support [[21], [22], [23]], graphene plays a substantial role in collecting photoexcited electrons from the conduction band of TiO2 [24]. The appropriate work function of graphene (4.42 eV) [25], as well as the 2D π-conjugate structure, ensure the effective charge carrier separation.
Due to the expected advantages of the TiO2/graphene nanocomposites in visible-light photocatalysis, a variety of synthetic routes have been developed, in which a majority of the fabrication processes have employed graphene oxide (GO) as a starting material. GO was coupled with TiO2 and then or simultaneously reduced to graphene by chemical [[26], [27], [28], [29]], thermal [30,31], solvothermal [25,32], microwave-assisted [33], or photochemical [34,35] reduction. Nevertheless, the use of a reduction process to prepare graphene can generate a non-negligible amount of defects on the graphene framework including vacancies [36] and residual oxygen-containing functional groups [37,38]. A large amount of defects on the graphene framework inevitably affect the electronic properties of the nanocomposites by introducing scattering centers and decreasing the ballistic transport path length. Thus, the defect-rich graphene can impede the charge carrier separation probability in the TiO2/graphene nanocomposites [39]. In addition, the sensitivity of titanium alkoxide to water (GO usually contains) significantly obstructs the uniform and controlled growth of TiO2 on graphene [40]. To the best of our knowledge, these problems are still challenges in the fabrication of TiO2/graphene nanocomposites. It is expected that minimization of defects on graphene sheets lets the photoexcited electrons be more successfully stored and shuttled away from the TiO2/graphene interfaces [41,42]. In addition, the interfacial contact between graphene sheets and nano-sized TiO2 particles or films is also considered as an essential factor for the effective transfer of photoexcited electrons [43,44].
Herein, we propose a novel GO-free route to fabricate TiO2/graphene nanocomposites, starting from graphene dispersion in titanium alkoxide. The dispersion was obtained by the chemical exfoliation of graphite in titanium tetra-n-butoxide with the aid of ultrasonication. To the obtained dispersion, aqueous solution of a catalyst was added for executing the sol-gel reaction in the presence of dispersed graphene sheets, eventually affording TiO2/graphene nanocomposites with defect-less graphene sheets uniformly and thinly covered by a TiO2 nano layer. The nanocomposites presented a dramatic extension of the absorption edge into the visible light region as well as the significantly enhanced electron-hole separation efficiency. The visible-light photocatalytic ability of the nanocomposites was tested in the degradation of methylene blue, in which the developed TiO2/graphene nanocomposites exhibited the activity 15 times greater than TiO2-P25 and 5 times greater than a conventional GO-based nanocomposite.
Section snippets
Materials
Graphite powder (≥98%, 45 μm) and benzylamine (>98%) were obtained from Wako Pure Chemical Industries, aqueous ammonia solution (NH3) (28–30%) from Kanto Chemical, and titanium tetra-n-butoxide (Ti(OnBu)4, ≥97%) from Sigma Aldrich. Graphene nanoplatelets (GNP) (Strem Chemicals) with the surface area of 750 m2 g−1 and anatase TiO2-TKP 101 (Tayca) with the primary crystallite size of 6 nm were used to estimate the amount of graphene content in TiO2/graphene nanocomposites. A commercial
Synthesis of the TiO2/graphene nanocomposites
The quality of graphene, that was obtained by chemical exfoliation, was investigated by Raman spectroscopy (Fig. 1). It was found that the quality of graphene produced in Ti(OnBu)4 was superior to that produced in benzylamine, in which a sharp 2D band with a relatively low FWHM value (28 cm−1) suggested the successful preparation of a few-layer graphene [59] in Ti(OnBu)4. In the case of graphene prepared in benylamine, the broadening of the 2D band together with a relatively high FWHM value
Conclusion
In conclusion, a new synthetic route for the preparation of TiO2/graphene nanocomposites was disclosed herein, which was based on the chemical exfoliation of graphite in titanium alkoxide with the aid of ultrasonication, and subsequent sol-gel reaction in the presence of a specific catalyst, i.e. benzylamine. It was found that the titanium alkoxide afforded the graphene dispersion of a high quality in terms of a trace amount of defects and a few layers of dispersed graphene. Moreover, the
Acknowledgements
N.N.T.T and D.X.T are grateful for the scholarship of the Ministry of Education, Culture, Sport, Science and Technology, Japan.
Authors would like to offer special thanks to Mr. Tung Thanh Nguyen for helping with the AFM measurements.
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