(410g) Remarkable Enhancement of Photocatalytic Hydrogen Production in Sensitized and N2/Ar Plasma Treated, Nanoporous TiO2 Films

The optical and electronic properties of TiO₂ thin films provide tremendous opportunities in several applications including photocatalysis, photovoltaics and photoconductors for energy production. Despite many attractive features of TiO₂, critical challenges include the innate inability of TiO₂ to absorb visible light and the fast recombination of photoexcited charge carriers. In this study, we prepared ordered nanoporous TiO₂ films co-modified by graphene quantum dot sensitization and nitrogen doping (GQD-N-TiO₂) for hydrogen production from photoelectrochemical water splitting under visible light irradiation. First, cubic ordered nanoporous TiO₂ films were prepared by a surfactant templated sol-gel method. Then, TiO₂ films were treated with N₂/Ar plasma for the incorporation of substitutional N atoms into the lattice of TiO₂. GQDs were prepared by chemically oxidizing carbon nano-onions. The immobilization of GQDs was accomplished by reacting carboxyl groups of GQDs with amine groups of N-TiO₂ developed by the prior immobilization of (3-aminopropyl)triethoxysilane (APTES). Successful immobilization of GQDs onto N-TiO₂ was probed by UV-Vis, FT-IR, zeta potential and contact angle measurements. The nitrogen-doped TiO₂ (N-TiO₂) showed the significant reduction of band-gap (3.5 eV to 2.88 eV) and the unprecedented enhancement (240 times) of photocatalytic hydrogen production visible light illumination. The GQD-immobilized N-TiO₂ film showed about 2 times enhancement compared to N-TiO₂, showing the important role of GQDs. This outstanding enhancement is attributed to the high surface area of nanoporous films and synergistic effects of nitrogen doping and GQD sensitization resulting in enhanced visible light absorption, efficient charge separation and transport.