(260af) Ordered Nanoporous Titania Thin Films for Energy Conversion and Storage

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 work, nanostructured TiO₂ thin films are modified by doping using hydrogen, nitrogen and titanium, and sensitization using graphene quantum dot sensitization. For all of these modifiers, well-ordered nanoporous titania films were synthesized by surfactant templated sol-gel process. Two methods: hydrazine and plasma treatments have been developed for nitrogen and hydrogen doping in the nanoporous titania films for band gap reduction, visible light absorption and enhancement of photocatalytic activity. The plasma treated nitrogen doped nanoporous titania showed about 240 times higher photoactivity compared to undoped film in hydrogen production from photoelectrochemical water splitting under visible light illumination. In addition to the intrinsic modification of titania by doping, graphene quantum dot sensitization in nanoporous titania film was also investigated for visible light photocatalysis. Graphene quantum dot sensitization and nitrogen doping of ordered nanoporous titania films showed synergistic effect in water splitting due to high surface area, band gap reduction, enhanced visible light absorption, and efficient charge separation and transport. As a stationary energy storage system, supercapacitors were prepared using hydrogen doped nanoporous titania films. The capacitance of plasma-treated hydrogen doped titania films was 300 and 176 times higher than the pristine film in aqueous and organic electrolytes, respectively, due to increased conductivity, oxygen vacancy formation and increased hydroxyl group formation in titania. This study suggests that plasma based doping and graphene quantum dot sensitization are promising strategies to reduce band gap and enhance visible light absorption of high surface area surfactant templated nanoporous titania films, leading to superior visible-light driven photoelectrochemical hydrogen production and electrochemical capacitance.