Optoelectronic Structure and Photocatalytic Applications of Na(Bi,La)S 2 Solid Solutions with Tunable Band Gaps

Abstract

NaLa 1-x Bi x S 2 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 Na 2 CO 3 , La 2 O 3 , and Bi 2 O 3 as precursors and via sulfurization with flowing CS 2 at 800 °C for 2 h. The Vegard's law behavior of cell parameters showed a perfect Bi/La solid solution in the cubic NaLa 1-x Bi x S 2 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 NaLa 1-x Bi x S 2 - 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 NaBiS 2 . Systematic DFT calculations on NaLa 1-x Bi x S 2 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 NaLa 1-x Bi x S 2 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.