Doped 1D Nanostructures of Transition-metal Oxides: First-principles Evaluation of Photocatalytic Suitability

The splitting of water molecules under the influence of solar light on semiconducting electrodes is a clean and renewable source for the production of hydrogen fuel. Its efficiency depends on the relative position of the band-gap edges or the induced defect levels with a proper band alignment relative to the redox H⁺/H₂ and O₂/H₂O potentials. For example, TiO₂ and ZnO bulk, as well as thick slabs (whose band gaps are ∼3.2–3.4 eV), can be active only for photocatalytic applications under UV irradiation (possessing ∼1 % solar energy conversion efficiency). Nevertheless, by adjusting the band gap through formation of nanostructures and further doping, the efficiency can be increased up to ∼15 % (for 2.0–2.2 eV band gap). We analyse results of DFT (density functional theory) calculations on TiO₂ nanotubes and ZnO nanowires, both pristine and doped (e.g., by Ag_Zn, C_O, Fe_Ti, N_O and S_O substitutes). To reproduce the energies of one-electron states better, we have incorporated the Hartree-Fock (HF) exchange into the hybrid DFT+HF Hamiltonian. Both the atomic and electronic structure of nanomaterials, simulated by us, are analysed to evaluate their photocatalytic suitability, including positions of the redox potential levels inside the modified band gap, the width of which corresponds to visible-light energies. Analysis of the densities of states (DOS) for considered nanostructures clearly shows that photocatalytic properties can be significantly altered by dopants. The chosen hybrid methods of first-principles calculations significantly simplify selection of suitable nanomaterials possessing the required photocatalytic properties under solar light irradiation.