Optoelectronic Structure and Photocatalytic Applications of Na(Bi,La)S₂ Solid Solutions with Tunable Band Gaps

NaLa1–xBixS2 solid solutions with tunable band gaps were synthesized, and their optoelectronic structures and photocatalytic performance were investigated via experimental and theoretical approaches. The solid solution powders with various La/Bi ratios were synthesized with Na2CO3, La2O3, and Bi2O3 as precursors and via sulfurization with flowing CS2 at 800 °C for 2 h. The Vegard’s law behavior of cell parameters showed a perfect Bi/La solid solution in the cubic NaLa1–xBixS2 with the associated linear variation of the lattice constants. On the contrary, the combination of diffuse reflectance ultraviolet–visible spectroscopy with density functional theory (DFT) calculations employing the HSE06 functional reveals a monotonic but nonlinear variation of the band gap of the solid solution. While consistent valence band maximum was obtained in NaLa1–xBixS2consisting mainly of S 3p orbitalsthe conduction band minimum was contributed by discrete Bi orbitals present at more positive potential than La. As a result, the slight inclusion of Bi caused a drastic shift in the band gap, and 24% Bi substitution provided an absorption edge closer to that of pure NaBiS2. Systematic DFT calculations on NaLa1–xBixS2 determined the optoelectronic properties for improved photovoltaic and photocatalytic performance with a Bi-rich sample rather than a La-rich counterpart; that is, there were larger absorption coefficients, smaller effective masses, and larger dielectric constants for Bi-rich samples versus La-rich samples. The NaLa1–xBixS2 particles decorated with Pt nanoparticles show maximum hydrogen evolution performance with x = 0.02–0.06 of Bi samples consistent with the compensating effects between photon absorption capacity and loss of electromotive force with decreasing band gap