Enhancing the Photocatalytic Activity of TiO₂ Through the Use of Selective Contacts Based on Photovoltaic Solar Cells

Photocatalysis is defined as the acceleration of chemical reactions under illumination and in the presence of a catalyst, which absorbs light and is involved in the chemical transformation of the reaction partners. Photocatalytic activity is highly dependent on the ability of the catalyst to efficiently create and separate photogenerated charge carriers. The electrochemical energies of photogenerated electrons and holes are used to oxidize or reduce compounds to make useful materials, including hydrogen and hydrocarbons, and to remove contaminants on wall surfaces and water. Since the illumination source is from the sun, photocatalysis is considered a green technology for converting solar energy into chemical energy.<br/>The working principles behind the operation of photocatalysis and the photovoltaic solar cell are similar. In both cases, the objective is to generate energetically separated electrons and holes, preventing them from recombining before accessing the external circuit. In the solar cell, the photogenerated electrons and holes recombine through the external circuit through metallic cables connected to an electrical load, while in photocatalysis electrons and holes participate in reduction and oxidation reactions. In both cases, avoiding recombination of the carriers before reaching the external circuit (solar cell) or the active surface (photocatalysis) is of paramount importance.<br/>In this work we optimize the generation of hydrogen from TiO₂ using an approach widely used in solar cells, that is, using selective contacts of holes and electrons. In particular, the generation of hydrogen has been studied using three different systems, all of them based on TiO₂ sol-gel. These have been: TiO₂ alone, TiO₂/Au and a-Si:H(n)/c-Si/MoO_3-x/TiO₂/Au. In the latter case, the a-Si:H(n)/c-Si/MoO_3-x structure corresponds to a crystalline silicon solar cell, without electrical contacts.<br/>The overall mean hydrogen production was 1.6 <i>mmol/h g_titania</i> for the TiO₂ absorber alone, whereas in sample with Au the overall mean production increases to 4.3 <i>mmol/h g_titania</i> due to the catalytic effect of the Au nanoparticles. However, for the sample deposited on top of the electrodeless silicon solar cell the generation of hydrogen increased an extra 33 % with respect to the TiO₂/Au sample, with a mean overall hydrogen production of 5.7 <i>mmol/h g_titania</i>.<br/>This experimental result can be explained by taking into account the energy diagram of the different layers. The final conclusion is that the TiO2/Au interface acts as a selective contact of holes, and the a-Si (n)/c-Si/MoO3-x structure acts as a selective contact of electrons.<br/>Additional characterization of the samples was performed by HRTEM, SEM and EDX.