A sustainable production of H2 by water splitting and photo-reforming of organic wastes on Au/TiO2 nanotube arrays

Photoelectrochemical (PEC) water splitting and photo-reforming of biomass derivatives are emerging as promising and sustainable methods for solar H2 generation. Notwithstanding the recent advances in designing photoanode materials, PEC systems have still not been implemented, due to the difficulty of synthesizing a photo-catalyst able to absorb efficiently the visible part of sunlight and convert it into H2 [1]. The more known example is titania (TiO2), a promising semi-conductor material but limited by its high band gap (3.2 eV) which sets its light absorption within the ultraviolet region of the solar spectrum. Several attempts have been in literature to extend the cut-off wavelength of this catalyst, including doping the crystalline lattice with hetero-atoms or decorating its surface with metal nano-particles, but only slight improvements have been resulted. Among the other metals, gold (Au) represents a special case. Though it is well known that Au nanoparticles (NPs) positively influence solar light harvesting of titania through surface plasmon resonance, the mechanism of interaction of the plasmons with the conduction band of titania has not unequivocally interpreted yet. In this contribution we have studied the enhancement of titania photo-response under visible irradiation, by depositing size-controlled Au NPs onto ordered arrays of TiO2 nanotubes. The Au NP size and dispersion on titania surface, together with a highly ordered nanoarchitecture of TiO2-based substrate, are of paramount importance to enhance the activity of these photo-catalysts. In order to obtain Au NPs with different dimensions, ranging from 2 to 40 nm, we started to synthesize colloidal solutions of Au NPs by reduction of an aqueous solution of HAuCl4 with NaBH4 or sodium citrate. By varying some parameters, such as the i) nature of reducing/capping agent, ii) ratio of gold ion concentration to stabilizer/reductant and iii) reaction temperature, a high grade of control on dimension and size distribution can be obtained. In a second step an appropriate volume of Au colloidal solution was contacted with a thin layer of TiO2 nanotubes prepared by anodic oxidation of Ti foils, in order to obtain a Au loading of 0.5 mg per cm2 of titania. The samples were characterized by Transmission Electron Microscopy, UV-Visible Diffusive Reflectance Spectroscopy, Cyclic Voltammetry and Chronoamperometry. Then, they were tested within our homemade PEC reactors [2] in water photo-splitting as well as in photo-reforming of ethanol for H2 production. A Xe-arc lamp (300 W), equipped with different kinds of filters, were used to evaluate their photo-responses in different absorption regions (ultraviolet or visible part). Results evidenced that smaller Au NPs (2-5 nm) strongly enhanced titania performances under visible light irradiation, opening the route of a feasible implementation of the PEC devices in the sustainable production of energy. References [1] C. Ampelli, R. Passalacqua, C. Genovese, S. Perathoner, G. Centi, T. Montini, V. Gombac, J.J. Delgado, P. Fornasiero, 2013, RSC Advances, 3 (44), 21776-21788. [2] C. Ampelli, C. Genovese, R. Passalacqua, S. Perathoner, G. Centi, 2012, Theor. Found. Chem. Eng., 46 (6), 651-657.