Designing novel multifunctional materials at the nanoscale is vitally important for flexible electronics. Here, we have uncovered a twodimensional metal dichalcogenide PbS 2 with intriguing negative Poisson ratio behavior and favorable optical and photocatalytic water splitting properties. The calculations indicate that the Poisson ratio of the PbS 2 monolayer is −0.061 along both x and y lattice directions, which is attributed to its unique tetrahedral motif and the ligand field of the local PbS 4 units in the PbS 2 monolayer. The electronic band structures show that the narrow band gap (1.59 eV) of the PbS 2 monolayer could be effectively modulated by strain engineering. Most importantly, the strain-induced tunability of optical absorbance and suitable band edge alignment make the PbS 2 monolayer a promising catalyst for photocatalytic water splitting, which is further confirmed by the reaction free energies. These findings offer an effective avenue for the design and synthesis of a novel optoelectronic functional material.

With the increasing demands for high-performance energy storage materials, rechargeable Li-ion batteries (LIBs) have attracted widespread attention. However, the lack of large-capacity anode materials hinders the advancement of LIBs. Based on the first-principles calculations, hexagonal boron ring structures with metal coverage M4B6 (M = Mg, Zr, Nb, Mo, Hf, Ta, and W) are studied. Especially, the stable Mg4B6 monolayer is found with planar B rings and excellent electrical conductivity. More excitingly, the Mg4B6 monolayer can maintain steady average open-circuit voltage and an ultrahigh theoretical capacity up to 4299 mA h g–1 for LIBs, which are distinctly better than those of the other two-dimensional materials. All of these properties imply that the Mg4B6 monolayer is a promising candidate for anode material with good rate capacity.