Mixed oxides for photo-electrochemical applications

The photoexcitation of an n-type photo-anode and a p-type photo-cathode provides electrons and holes in photo-electrochemical processes. Depending on the use of photosensitive material as anode, cathode or both, different photo-electrochemical (PEC) system configurations and experimental conditions have to be taken into account to consider "green" the photo-electrochemical process when solar energy and water are use to drive the process. Several studies have been done by our group in order to obtain a photo-anode driven PEC cell in which a lowering of the applied cell voltage was obtained from the photo-potential generated at TiO₂ nanotubular structures. Among other p-type semiconductors, Ni oxides based photo-cathodes, coupled to different photo-anodes are proposed in the literature as inexpensive semiconductors in photo-catalysis. However, the absorption spectrum of these metal oxides is limited to ultraviolet region due to their large band gap and therefore efforts to activate Ni oxide and TiO₂ for a visible light range have to be done. In particular, the literature proposed the combination of different electronic structures, based on mixed metal oxide photocatalysts to enhance the light-response range to the visible region, thereby promoting the photo-catalytic efficiency. The present work proposes the synthesis of hierarchical structures of Ti/Al and Ni/Al mixed oxides. TiO₂ nanotubular structures were obtained by oxidation of a Ti foil in deionized water and glycerol organic solution with NH4F at room temperature. Ni oxide films were obtained supported onto Indium Tin Oxide (ITO) coated glass. Doctor-blade method and subsequent annealing treatment were adopted to obtain a stable structure. Commercially available Nickel oxide nanopowders and nanowires were used, as well as alcoholic solution of nickel-nitrate salt, all dissolved into N,N-dimethylformamide (DMF) were used as precursor solutions to obtain home-made Ni oxide. Aluminum oxide, embedded in the Ni oxide and Ti oxide structures by repeated dipping/thermal cycles, is proposed to enhance the photo-catalytic performance of the bare Ti and Ni oxides. The materials were morphologically and electrochemically characterized by scanning electron microscopy, cyclic voltammetry and chrono-amperometry analysis. The stability and photo-catalytic activity of the synthesized samples were studied.