(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.