V-substituted In2S3: an intermediate band material with photocatalytic activity in the whole visible light range

Abstract

We proposed in our previous work V-substituted In₂S₃ as an intermediate band (IB) material able to enhance the efficiency of photovoltaic cells by combining two photons to achieve a higher energy electron excitation, much like natural photosynthesis. Here this hyper-doped material is tested in a photocatalytic reaction using wavelength-controlled light. The results evidence its ability to use photons with wavelengths of up to 750 nm, i.e. with energy significantly lower than the bandgap (=2.0 eV) of non-substituted In₂S₃, driving with them the photocatalytic reaction at rates comparable to those of non-substituted In₂S₃ in its photoactivity range (λ ≤ 650 nm). Photoluminescence spectra evidence that the same bandgap excitation as in V-free In₂S₃ occurs in V-substituted In₂S₃ upon illumination with photons in the same sub-bandgap energy range which is effective in photocatalysis, and its linear dependence on light intensity proves that this is not due to a nonlinear optical property. This evidences for the first time that a two-photon process can be active in photocatalysis in a single-phase material. Quantum calculations using GW-type many-body perturbation theory suggest that the new band introduced in the In₂S₃ gap by V insertion is located closer to the conduction band than to the valence band, so that hot carriers produced by the two-photon process would be of electron type; they also show that the absorption coefficients of both transitions involving the IB are of significant and similar magnitude. The results imply that V-substituted In₂S₃, besides being photocatalytically active in the whole visible light range (a property which could be used for the production of solar fuels), could make possible photovoltaic cells of improved efficiency.