(658d) Ordered Mesoporous TiO2 Thin Films Sensitized with Graphene Quantum Dots for Visible-Light-Driven Photocatalysis

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, graphene quantum dots (GQDs) were used as sensitizers to extend the photo-response of cubic ordered mesoporous TiO₂ thin films. TiO₂ films with cubic ordered mesopores were prepared by a surfactant templated sol-gel method using TiCl₄ as precursor and triblock copolymer Pluronic F-127 as the template. To provide a site for coupling to GQDs, some of the calcined TiO₂ films were functionalized with amines using (3-aminopropyl)triethoxysilane (APTES). The amine functionalized titania films were then dipped into a GQD solution containing ethyl(dimethylaminoproplyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS) for covalent bonding between titania and GQDs. UV-vis absorbance spectra of GQD sensitized mesoporous TiO₂ (GQD-TiO₂) films indicate enhancement of the absorption of visible light consistent with sensitization. The GQD-TiO₂ films were characterized using SEM and TEM analysis and found to exhibit open, accessible ordered mesopores. Finally, the visible-light photocatalytic activity of GQD-TiO₂ films was determined from the photocatalytic degradation of methylene blue under illumination with a visible-light LED (455 nm wavelength). The GQD-TiO₂ films showed enhanced photocatalytic activity, with a first-order methylene blue rate coefficient 4 times greater than that of undoped TiO₂ films. The GQD-TiO₂ films also showed about 4 times greater photocurrent in visible-light photoelectrochemical activity than undoped TiO₂ for hydrogen production by water splitting. The use of electrochemical impedance spectroscopy to understand charge carrier kinetics contributing to enhanced photocatalysis in the GQD-TiO₂ films will be discussed.