The in-gap band concept in photocatalysis: mimicking the photosynthesis Z scheme with a single material

Trabajo presentado en el 9th Joint Programme Steering Committee (AMPEA workshop: From Power to Gas), celebrado en Nancy (Francia) el 20 y 21 de junio de 2016.Recent work of ours has proposed, based on DFT results, several formulations of single-phase materials which, due to an electronic structure having a narrow, partially filled band inside the gap (in-gap band, IGB), may couple two low energy photons to achieve a higher energy electronic excitation (like in the Z-scheme used by nature in photosynthesis), allowing use of a wider spectral range of solar energy and therefore higher efficiency in photovoltaic or photocatalytic systems. We have realized the concept in some of these cases with transition metal-substituted main group sulfides. Thus adding V to In2S3 (with gap=2.0 eV) extends to 2.0 eV (as happens with pure In2S3), but also with those in the said <1.6 eV range, i.e. upconversion occurs [1]. The observed PL intensity grows linearly, not quadratically, with the intensity of the exciting photons in the EE>1.0 eV. Both things discard that it is due to frequency doubling, being explained instead by partial filling of a properly placed IGB, confirmed by advanced GW quantum calculations. Furthermore, both PL and photocatalytic activity with exciting photons having energy>2.0 eV are not degraded by adding V, i.e. the latter does not lead to higher recombination, explained by the delocalization of the IGB. Same principle should be utilizable in H2 photogeneration. Work to allow the latter process by coupling the sulphide photocatalyst to a hydrogenase enzyme is under way, and promising results are being obtained. [1] R. Lucena et al., J. Mater. Chem. A 2 (2014) 8236.N