Janus MoSSe/WSSe Heterobilayers as Selective Photocatalysts for Water Splitting

Abstract

Identifying materials that simultaneously straddle the water redox potentials and possess an intrinsic electric field is crucial for achieving high solar-to-hydrogen (STH) efficiency. Using state-of-the-art first-principles calculations, including a range-separated hybrid functional and spin-orbit coupling, we investigate MoXY/WXY (X, Y = S, Se) Janus bilayers for overall water splitting. We find that the Se-Se interfaced heterobilayer is intrinsically capable of driving water splitting, while its S-S counterpart can meet the redox requirements through pH modulation. For both configurations, a remarkable STH efficiency of 17.1\% is predicted. Compared with homo-bilayers, hetero-bilayers benefit from the chemical potential difference between Mo and W, which generates a built-in electric field and promotes spatial separation of photogenerated carriers, suppressing recombination and overall enhancing hydrogen production. These results demonstrate the promise of Janus heterobilayers for efficient solar-driven water splitting.

Figures (5)

• Figure 1: Side views of the optimized AB-stacked MoSSe/WSSe heterobilayers in the 2H phase: (a) SMoSE|SWSe, (b) SeMoS|SWSe, (c) SMoSe|SeWS, and (d) SeMoS|SeWS. The blue arrow in panel (a) indicates the metal–metal distance $d$. The gray dashed bars guide the eyes to recognize the stacking configuration.
• Figure 2: (a) Layer-projected electronic band structures of the considered Janus hetero-bilayers calculated using the HSE06 hybrid functional with spin-orbit coupling. (b) Total (gray area) and atom-projected density of states. In all graphs, the Fermi level is set to 0 eV.
• Figure 3: Spin-projected electronic band structures ($s_z$ component) of the Janus heterobilayer configurations, calculated at the HSE06+SOC level of theory. The bands are color-coded based on their $s_z$ spin expectation values to visualize spin-splitting and potential Rashba-like features. In all panels, the Fermi level ($E_F$) is shifted to 0 eV, indicated by horizontal dashed line.
• Figure 4: One-dimensional profile of the planar-averaged Hartree potential for various heterobilayers. Red arrows denote the potential difference ($\Delta \Phi$) between the two surfaces of the bilayer, while blue arrows indicate the intrinsic internal electric field within each Janus monolayer.
• Figure 5: Band alignment of homo-metal (gray background) and hetero-metal Janus bilayers relative to the water redox potentials referenced to the vacuum level set to 0 eV. The scheme on the right illustrates the band offsets and the direction of the built-in electric field in the SeMoS$\mid$SeWS heterobilayer.