A Comparative Investigation into the Electronic Structure and Optical Response of NiMoO ₄ Modulated by Transition Metal Doping

This study systematically investigates the effects of 3d, 4d, and 5d transition metal (TM) doping on the electronic structure and optical properties of NiMoO₄. Based on the bandgap modulation efficiency and cost-effectiveness, three representative dopants, Mn, Nb, and W, were selected for an in-depth analysis. Density functional theory calculations revealed that TM doping significantly narrows the band gap of pristine NiMoO₄ (1.13 eV), reducing it to 0.56, 0.40, and 0.34 eV for Mn-, Nb-, and W-doped systems, respectively. Band structure (BS) and density of states (DOS) analyses demonstrated that the band gap reduction was primarily attributed to the introduction of dopant-induced hybridized states within the original gap region. Furthermore, charge density difference plots and Bader charge analysis indicate substantial electron redistribution upon doping, with oxygen atoms serving as electron-rich centers and forming charge-sharing O-TM bonds. This redistribution facilitates charge separation and transfer, which is beneficial for the photocatalytic activity. Optical property evaluations revealed that W@NiMoO₄ exhibited the strongest visible-light absorption, reaching up to 4.06 × 10⁵ cm^-1. These findings suggest that TM doping not only offers tunable band gaps but also modulates carrier localization and light-matter interactions, rendering TM@NiMoO₄ promising for applications in photovoltaics and photocatalysis.