Rational Selection of Transition Metal Co-dopant in Sulfur-doped Titanium Dioxide

This paper investigates the general trend in structural, electronic, and optical properties of anatase TiO₂ photocatalysts co-doped with transition metals and sulfur. We attempt to rationalize co-dopant selection by employing molecular dynamics and density functional theory calculations. The structural properties of the first-row transition metal co-dopants were determined. TM-TiO₂ and TM/S-TiO₂ were structurally stable, with minimal changes in their lattice parameters, cell volume, density, and XRD profiles relative to pristine TiO₂. However, only Fe and Mn, among the first-row transition metals, are thermodynamically-favorable, i.e., their substitutional energies are lower relative to pristine TiO₂. Intermediate energy levels (IELs) are formed during transition metals and sulfur co-doping on TiO₂. In particular, Fe and Co form two IELs between the VBM and CBM, resulting in improved optical properties, especially in the visible-light region, mainly attributed to the unsaturated nonbonding transition metal d orbitals and the half-filled Ti–O bonding orbitals. On the other hand, Cu and Ni form three IELs close to each other due to the M–O anti-bond orbitals, half-filled p orbitals of S, and the Ti–S anti-bonding orbitals. These IELs in co-doped systems can serve as “stepping stones” for the photogenerated electrons, facilitating easier charge mobility. Among the investigated co-doped systems, Fe/S-TiO₂ was shown to be the most promising for photocatalytic applications.