Dopant distributions and band-edge positions in Sr-doped NaTaO3: a first-principles study.

Sr doping is known to enhance the water-splitting activity of NaTaO3 photocatalysts and is often accompanied by surface segregation of Sr, yet, the electronic-structural origin remains unclear. Here, we employ first-principles calculations with explicit enumeration of all Sr configurations in bulk NaTaO3 (NTO) and TaO2-terminated (001) surfaces. In the bulk, the enumeration shows that shorter Sr-Sr separations are energetically preferred. At surfaces, Sr segregates and accumulates strongly. Layer-resolved local density of states indicates that in-gap states are confined to the outermost TaO2 layer, while interior layers retain a near-bulk gap. In Sr-rich surface layers, both band edges shift upward relative to the interior. Population analyses link the valence-band rise to O-O contraction and the conduction-band rise to Ta-Ta elongation. Sr-doping and segregation yields near-surface band bending and a built-in electric field that can promote electron-hole separation and suppress recombination, rationalising the observed activity gains.