(Invited) Role of Heterojunctions in Activation of CO2 Molecule

Abstract

Photo-electro-reduction of CO2 is continuously gaining a considerable interest in view of possible production of carbon-based energy carriers. Given the periodic availability of solar light, of primary importance is to orient the CO2 conversion versus formation of easily storable fuels such as, e.g. methanol or ethanol. Thus, it is important to diversify the CO2 reduction products not only by the applied potential but by careful optimisation of the working system. A default approach, consists in using p-type semiconductors as a main component of the CO2-reduction cathodes. However, the intrinsic drawback of p-type semiconductor photo-cathodes lies in low electron density in the CB and, as generally observed, the lack of catalytic properties towards CO2 reduction. Thus, in an attempt to increase the conduction band electron population, we will present herein a build-up hybrid photo-electrodes including p-type - n-type semiconductors junctions. An additional advantage of creating such p-type – n-type semiconductor heterojunction is protection of the p-type component against photocorrosion induced by largely negative potentials that offers, at the same time, the possibility to enlarge the choice of employed p-type semiconductors to relatively unstable ones. The requirements for the n-type semiconductors are fulfilled by several metal oxides such as TiO2 or SrTiO3, or KTaO3. Apart the role of heterojunction in CO2 activation, we will also point out a particular aspect of using a model system involving Au NPs of different sizes deposited on a TiO2/Ti substrate by examining the electro-reduction of carbon monoxide. This reaction appears as being the crucial, kinetically difficult, step in the electro-reduction of CO2 to alcohols and hydrocarbons. Recognition and understanding of an individual process might already give a picture of a whole system.