Black TiO_{2- x}Nanoparticles Decorated with Ni Nanoparticles and Trace Amounts of Pt Nanoparticles for Photocatalytic Hydrogen Generation

A cocatalyst plays an essential role in photoassisted hydrogen generation, and it is an almost inevitable component of a photocatalyst. Costly noble metal (e.g., Pt) cocatalysts exhibit almost irreplaceable efficiencies, and finding a suitable replacement is a challenging proposition. Controlled synthesis of a nanoparticle cocatalyst on semiconductors at the nanoscale level is one of the most promising approaches to accomplish the Pt equivalent activity. Herein, a photodeposited metallic Ni-based cocatalyst containing a small amount of Pt (<2 atom % with respect to Ni) on reduced/black TiO2-x is introduced. The developed cocatalyst (2.21 wt % Ni and 0.094 wt % Pt with respect to TiO2-x) exhibits better charge separation efficiency and photoassisted hydrogen generation rate than an only-Pt (0.91 wt %) cocatalyst from methanol-water. The rates are 69 and 3.1 mmol g-1 h-1 for a Ni-based cocatalyst, while 65 and 2.5 mmol g-1 h-1 for a Pt cocatalyst, respectively, under ultraviolet-visible and visible light. A small amount of Pt ensures the photodeposition of Ni nanoparticles adjacent to Pt nanoparticles, enhancing the charge migration from the reduced TiO2-x surface for hydrogen evolution. It is found that in the absence of Pt, the photodeposited Ni(OH)2 is obtained instead of metallic Ni nanoparticles, which exhibits a comparatively low hydrogen generation rate. The present study opens an alternative way to cocatalyst design and fabrication by the controlled synthesis of nanoparticles for a wide range of photocatalytic conversions facilitated by enhanced charge separation.