Understanding the Photocatalytic Water Reduction Capability of Na<i>M</i>(WO₄)₂ (M = Ga, Sc) by the Electronic Structure and Bond Characteristic Analysis

<i>m</i>-WO₃ has a narrow band gap, making it a promising photocatalyst. However, its antibonding W-O orbitals limit water reduction capability due to insufficient reduction potential. Incorporating a weakly bonded <i>M</i>-O in Na<i>M</i>(WO₄)₂ (M = Ga, Sc) enhances W-O covalency compared to that in <i>m</i>-WO₃, resulting in a more negative conduction band minimum (CBM) potential, sufficient for photocatalytic water reduction. Here, Na<i>M</i>(WO₄)₂ (M = Ga, Sc) were synthesized via high-temperature solid-state reactions, and their precise structures were determined through Rietveld refinement of high-resolution XRD data. Density functional theory (DFT) calculations were performed to analyze the electronic structures, and bond characters were further examined using the crystal orbital Hamilton population (COHP), crystal orbital bond index (COBI), and electron localization function (ELF). Experimentally, NaGa(WO₄)₂ loaded with 0.63 wt % Pd and NaSc(WO₄)₂ loaded with 0.94 wt % Pd exhibited photocatalytic H₂ generation rates of 2.45 and 6.30 μmol/h, respectively, under UV irradiation in a methanol aqueous solution. Apparent quantum yields at 295 nm are estimated to be 0.66 and 2.21%, respectively. Notably, NaSc(WO₄)₂ displayed a superior photocatalytic activity, which can be attributed to its more negative CBM potential, as discussed through a comparison of their electronic structures.