Synthesis, Characterization,and Photocatalytic Hydrogen Production By TiO₂Composite Semiconductors

With the increasing concerns of clean energy demand and environmental issues after combustion of fossil fuels, photocatalytic hydrogen production via water splitting has attracted much attention as a promising way to solve these problems. Photocatalytic water splitting for hydrogen production could be achieved over a variety of semiconductors, mainly including TiO2, Cu2O, ZnO, CdS, CdSe, CdTe, and g-C3N4. However, due to inefficient performance, these materials for solar driven hydrogen production are still far from pratical application. It is highly desirable for a semiconductor photocatalyst to have long-lived electron-hole pairs, low photogenerated charge recombination, efficient electron transfer process with or without cocatalysts, long-term chemical stability, and photostability, low-cost, and good absorption in the visible region. According to the standard Gibbs free energy change (AGo) for water splitting, the band gap energy (Eg) of a photocatalyst should be>1.23eV. Titanium dioxide (TiO2) fits above requirements and is widely studied because of its many advantages including strong oxidizing powder, non-toxicity, low-cost, high efficiency, good thermal, and chemical stability. Over the past few decades, tremendous efforts have been devoted to improve the catalytic activity of TiO2-based photocatalysts. However, TiO2can only absorb UV light (5%of the solar spectrum) due to its high band gap (3.0eV for rutile and3.2eV for anatase), resulting in low efficiency in the visible region.Many approaches have been reported in the literature to enhance the photocatalytic activity of wide band gap semiconductors like TiO2for hydrogen production. These approaches include sensitization the materials with low band gap materials, noble metal deposition, metal elements doping, nonmetal elements doping, functionalization with cocatalytsts, and constructing heterojunctions with other narrow band gap semiconductors, etc. In the present study, we report the TiO2/CdS and TiO2photocatalysts modified by CoOx and Ni(OH)2cocatalysts for visible light-driven hydrogen production from water.The semiconductor composites were prepared by simple solvothermal methods and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), and X-ray photoelectron spectroscopy (XPS) to identify their physical properties and absorption properties in the visible region. The results are as follows:(1) CoOx and Ni(OH)2can serve as efficient cocatalysts to enhance the photocatalytic activity.(2) Photocatalysts modified by CoOx exhibited significant improvement of visible light absorption.(3) Modified by Ni(OH)2and CoOx, the semiconductor materials showed long-term photocatalytic activity.(4) We also investigated the photocatalytic hydrogen production of CdS/rGO composites modified by Ni(OH)2cocatalyst.