PREPARAÇÃO E INDUÇÃO DE DEFEITOS EM NANOPARTÍCULAS DE TiO2: APLICAÇÃO EM CÉLULAS A COMBUSTÍVEL FOTOCATALÍTICAS E FOTOELETRÓLISE DA ÁGUA.

Abstract

The high levels of global CO2 emissions have been of concern to the global environmental authorities, since the global warming caused by the greenhouse effect can generate major environmental, economic and social impacts. To reduce the energy dependence on fossil fuels, the use of solar energy has proved to be a viable strategy. Two interesting options that deserve to be highlighted are the conversion of solar energy into chemical energy – in the form of H2 through water photoelectrolysis – in a photoelectrochemical cell (PEC), and the conversion of solar energy into electrical energy using a photo fuel cell (PFC). Both processes use semiconductors to absorb sunlight and, in this respect, much fundamental research is still needed to make these technologies feasible for practical use, optimizing efficiency and reducing the costs involved in the production of materials. In this work we present the study and optimization of TiO2 reduction for use in photoelectrochemical cells and photo fuel cells aiming the conversion of solar energy. TiO2 nanoparticles were prepared by the hydrolysis of titanium isopropoxide and defects were introduced into the material structure through a thermochemical reduction with different amounts of NaBH4. As a result, nearly spherical TiO2 nanoparticles of pure anatase crystalline phase were obtained. The reduction process did not change the semiconductor's morphology or its crystalline structure, but did induce drastic changes in its optical, electronic and photoelectrochemical properties. The white TiO2 bandgap was 3.2 eV while the reduced samples showed a slight decrease in the bandgap in addition to an increase in visible-light absorption. The reduced samples with less NaBH4 achieved better PEC and PFC results because, by reducing TiO2, defects such as oxygen vacancies and Ti3+ were induced in the material. These defects form energy states inside the forbidden semiconductor band, which explains the increased absorption of visible light, however, the excess of defects end up as sites of electron/hole trapping and, therefore, samples prepared with 5% m/m of NaBH4, among those studied, showed better photocatalytic performances.