Quantum chemical simulations of doped ZnO nanowires for photocatalytic hydrogen generation

Zinc oxide (ZnO) is considered in general as a promising material for solar water splitting. Its wurtzite-structured bulk samples, however, can be considered as active for photocatalytic applications only under UV irradiation, where they possess ∼1% efficiency of sunlight energy conversion due to their wide band gap (3.4 eV). Although pristine ZnO nanowires (NWs) possess noticeably narrower band gaps than the bulk, the tendency of band gap reduction with increasing NW diameter is insufficient, and further modification is required. We have contributed to filling this gap by performing a series of ab initio calculations on ZnO NWs of different diameters (d_NW), which are mono-doped by metal (Ag) and non-metal atoms (C, N) or contain oxygen vacancies with varied concentration (∼3 vs. ∼6%). To reproduce qualitatively the energies of one-electron states of nanowires in our calculations, the hybrid DFT + HF Hamiltonian has been used, based on the PBE0 exchange-correlation functional. We have analyzed changes in the electronic structure induced in a few defect composition scenarios, showing that, for specific concentrations and locations of the dopants, the optical absorption peak of doped ZnO can be shifted to the visible light range with promising efficiency. In agreement with experimental observation, the most significant results have been achieved for carbon-doped ZnO nanowires. They possess the highest photocatalytic suitability, since the band gap is reduced in this case down to 2.1–2.2 eV (for nanowire diameters of 2.9–3.5 nm), which corresponds to optimal 15–17% efficiency of solar energy conversion.