Publication date: 31st March 2013
Capturing as much of the solar spectrum as possible is a must for any processes, as for example those using heterogeneous photocatalysis, which try to harvest and store solar energy in form of fuel. The prototypical photocatalysts TiO2 and ZnO are unable to fulfill this aim, therefore other materials are sought. Enough chemical redox potential must be built in the photocatalyst to drive the desired reaction; in the case of water splitting (the "Holy Grail" in this field) this implies, considering the unavoidable losses, having a bandgap of ca. 1.8 eV or higher, implying that IR light cannot be used. This restriction can be circumvented, however, if one can use in photocatalysis the intermediate band (IB) principle, recently proposed for photovoltaics [1]. In this scheme, insertion of a delocalized and partially occupied band inside the gap of a semiconductor allows exciting electrons across the full bandgap using two sub-bandgap energy photons, mimicking the photosynthesis Z-scheme but allowing using as well higher energy photons for the direct bandgap excitation.
In recent years we have suggested, based on solid state chemistry concepts and quantum calculations, several materials which develop an IB through insertion of a well-chosen transition metal in a typical semiconductor: Ti:GaP, Cr:CuGaS2, Ti:CuGaS2, V:In2S3 and V:SnS2 among others [2]. We have synthesized the two latter materials as nanocrystalline solids, evidencing that the inclusion of V in them leads to sub-bandgap photon absorption. Furthermore we have proved, using a simple photocatalytic reaction (degradation of aqueous formic acid), that sub-bandgap photons can drive in them the photocatalytic process with a quantum efficiency similar to that of over-bandgap photons [2d, 3]. In particular, In2S3 (a thiospinel having an intrinsic bandgap of 2.0 eV) becomes able, upon 10% substitution of In by V, to carry out the photocatalytic reaction with photons having wavelengths up to 750 nm (see Figure). These results, which would be presented in this contribution, show the promise of the IB concept for enhancing efficiencies in solar fuel production with photocatalysts once coupled to appropriate co-catalysts.
Spectral dependence of reaction rate of aqueous HCOOH photocatalytic degradation, obtained for pure and V-substituted In2S3, compared with the corresponding absorption spectra
[1] Luque, A.; Martí, A. Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels. Phys. Rev. Lett. 1997, 78, 5014-5017. [2] a) Palacios P.; Fernández, J.J.; Sánchez, K.; Conesa, J.C.; Wahnón, P. First-principles investigation of isolated band formation in half-metal TixGa1-xP compound at different dilutions. Phys. Rev. B 73 (2006) 085206; b) Palacios, P.; Sánchez, K.; Conesa, J.C.; Fernández, J.J.; Wahnón, P. Theoretical modelling of intermediate band solar cell materials based on metal-doped chalcopyrite compounds. Thin Solid Films 2007, 515, 6280-6284; c) Palacios, P.; Aguilera, I.; Sánchez, K.; Conesa, J.C.; Wahnón, P. Transition Metal Substituted Indium Thiospinels as Novel Intermediate Band Materials: Prediction and Understanding of their Electronic Properties. Phys. Rev. Lett. 2008, 101, 046403; d) Wahnón, P.; Conesa, J.C.; Palacios, P.; Lucena, R.; Aguilera, I.; Seminovski, Y.; Fresno, F. V-doped SnS2: a new intermediate band material for a better use of the solar spectrum. Phys. Chem. Chem. Phys. 2011, 13, 20401-2407. [3] a) Lucena, R.; Aguilera, I.; Palacios, P.; Wahnón, P.; Conesa, J.C. Synthesis and Spectral Properties of Nanocrystalline V-substituted In2S3, a Novel Material for More Efficient Use of Solar Radiation. Chem. Mater. 2008, 20, 5125-5127; b) ibid., submitted.
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