Carboxyl-Functionalized Graphene for Highly Efficient H2-Evolution Activity of TiO2 Photocatalyst

: The use of semiconductor photocatalysts (CdS, g-C 3 N 4 , TiO 2 , etc .) to generate hydrogen (H 2 ) is a prospective strategy that can convert solar energy into hydrogen energy, thereby meeting future energy demands. Among the numerous photocatalysts, TiO 2 has attracted significant attention because of its suitable reduction potential and excellent chemical stability. However, the photoexcited electrons and holes of TiO 2 are easily quenched, leading to limited photocatalytic performance. Furthermore, graphene has been used as an effective electron cocatalyst in the accelerated transport of photoinduced electrons to enhance the H 2 -production performance of TiO 2 , owing to its excellent conductivity and high charge carrier mobility. For an efficient graphene-based photocatalyst, the rapid transfer of photogenerated electrons is extremely important along with an effectual interfacial H 2 -production reaction on the graphene surface. Therefore, it is necessary to further optimize the graphene microstructures (functionalized graphene) to improve the H 2 -production performance of graphene-based TiO 2 photocatalysts. The introduction of H 2 -evolution active sites onto the graphene surface is an effective strategy for the functionalization of graphene. Compared with the noncovalent functionalization of graphene (such as loading Pt, MoS x , and CoS x on the graphene surface), its covalent functionalization can provide a strong interaction between graphene and organic molecules in the form of H 2 -evolution active sites that are produced by chemical reactions. In this study, carboxyl-functionalized graphene (rGO-COOH) was successfully modified via ring-opening and esterification reactions on the TiO 2 surface by using an ultrasound-assisted self-assembly method to prepare a high-activity TiO 2 /rGO-COOH photocatalyst. The Fourier transform infrared (FTIR) spectra, X-ray photoelectron spectroscopy (XPS), and thermogravimetric (TG) curves revealed the successful covalent functionalization of GO to rGO-COOH by significantly enhanced ― COOH groups in FTIR and increased peak area of carboxyl groups in XPS. A series of characterizations, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), XPS, and UV-vis adsorption spectra, were performed to demonstrate the successful synthesis of TiO 2 /rGO-COOH photocatalysts. The experimental data for the hydrogen-evolution rate showed that the TiO 2 /rGO-COOH displayed an extremely high hydrogen-generation activity (254.2 μ mol · h − 1 · g − 1 ), which was 2.06- and 4.48-fold higher than those of TiO 2 /GO and TiO 2 , respectively. The enhanced photocatalytic activity of TiO 2 /rGO-COOH is ascribed to the carboxyl groups of carboxyl-functionalized graphene, which act as effective hydrogen-generation active sites and enrich hydrogen ions owing to their excellent nucleophilicity that facilitates the interfacial hydrogen production reaction of TiO 2 . This study provides novel insights into the development of high-activity graphene-supported photocatalysts in the hydrogen-generation field.