Polymorph-Induced Reducibility and Electron Trapping Energetics of Nb and W Dopants in TiO₂

Controlling the formation of electron polarons in TiO₂ doped with transition metals is important for the design of transparent conducting oxides for high-efficiency photovoltaics and photocatalysts with tunable reaction selectivities. In this work, EPR spectroscopy is combined with Hubbard-corrected density functional theory (DFT+U), with refined atomic-like Hubbard projectors, to show the sensitivity of charge compensation in substitutionally doped Nb-TiO₂ and W-TiO₂ with respect to the TiO₂ polymorph (i.e., anatase or rutile). Both EPR magnetic tensors and DFT+U-predicted Nb 4d and W 5d orbital occupancies show the formation of differing dopant charge states depending on the TiO₂ polymorph, with nonmagnetic Nb⁵⁺ and W⁶⁺ in doped anatase and paramagnetic Nb⁴⁺ and W⁵⁺ in doped rutile. The results provide an example of how a coherent experimental and theory-validated framework can be used to understand and predict the reducibility of dopants and electron trapping energetics in TiO₂ polymorphs. The outcome enables greater control over the electronic and magnetic properties of metal oxide semiconductors, which are crucial for the rational design of next-generation materials for energy conversion and catalytic applications.