We performed ab initio calculations on the basic set of MoS₂ single-layer materials, namely the 1H, 1T and 1T′ polytypes, to lay a theoretical framework on the emerging breakthrough-discoveries of high activity towards the hydrogen evolution reaction (HER) in exfoliated MoS₂ and related materials. Our calculations show that for exfoliated MoS₂, 1T′ is the most HER active polytype, with active sites both on the basal plane and at the edges of the layered grains. In comparison, the basal planes of the 1H and 1T polytypes are HER inactive and their edge-sites are not as active as those of the 1T′ polytype. We also found that 1T-MoS₂ is unstable and easily transforms into 1T′-MoS₂, and the 1T′ phase is metastable with a considerable barrier >0.7 eV to bar its transformation into the most stable 1H phase. Further, unlike the case of exfoliated WS₂, the HER activity of exfoliated MoS₂ is not so critically affected by the lattice strain. In addition, in contrast to the 1T′-WS₂, the 1T′-MoS₂ is not metallic but has a very small bandgap of 0.1–0.2 eV. Hence, 1T′-MoS₂ should have a high enough conductivity and other suitable properties for it to function as an electrochemical HER catalyst.

Surface engineering is known as an effective strategy to enhance the catalytic properties of Pt-based nanomaterials. Herein, we report on surface engineering Ni-Pt nanoalloys with a facile method by varying the Ni doping concentration and oleylamine/oleicacid surfactant-mix. The alloy-composition, exposed facet condition, and surface lattice strain are, thereby manipulated to optimize the catalytic efficiency of such nanoalloys for methanol oxidation reaction (MOR). Exemplary nanoalloys including Ni_0.69Pt_0.31 truncated octahedrons, Ni_0.45Pt_0.55 nanomultipods and Ni_0.20Pt_0.80 nanoflowers are thoroughly characterized, with a commercial Pt/C catalyst as a common benchmark. Their variations in MOR catalytic efficiency are significant: 2.2 A/mg_Pt for Ni_0.20Pt_0.80 nanoflowers, 1.2 A/mg_Pt for Ni_0.45Pt_0.55 nanomultipods, 0.7 A/mg_Pt for Ni_0.69Pt_0.31 truncated octahedrons, and 0.6 A/mg_Pt for the commercial Pt/C catalysts. Assisted by density functional theory calculations, we correlate these observed catalysis-variations particularly to the intriguing presence of surface interplanar-strains, such as {111} facets with an interplanar-tensile-strain of 2.6% and {200} facets with an interplanar-tensile-strain of 3.5%, on the Ni_0.20Pt_0.80 nanoflowers.

The growth of highly oriented (110) γ-CuI films on copper substrates with the best photoluminescence properties among all known CuI films are demonstrated, in the context of improving the current techniques in the preparation of CuI films for electronic and optoelectronic applications. The comparison of the photoluminescence spectra of CuI films with different growth orientations shows that the highly oriented (110) films prepared are less defective and give only a sharp exciton band.

The electronic structures and photocatalytic properties of bismuth oxyhalide bilayers (BiOX1/BiOX2, X1 and X2 are Cl, Br, I) are studied by density functional theory. Briefly, their compositionally tunable bandgaps range from 1.85 to 3.41 eV, suitable for sun-light absorption, and all bilayers have band-alignments good for photocatalytic water-splitting. Among them, heterogeneous BiOBr/BiOI bilayer is the best as it has the smallest bandgap. More importantly, photo-excitation of BiOBr/BiOI leads to electron supply to the conduction band minimum with localized states belonging mainly to bismuth of BiOBr where the H⁺/H₂ half-reaction of water-splitting can be sustained. Meanwhile, holes generated by such photo-excitation are mainly derived from the iodine states of BiOI in the valence band maximum; thus, the O₂/H₂O half-reaction of water splitting is facilitated on BiOI. Detailed band-structure analysis also indicates that this intriguing spatial separation of photo-generated electron-hole pairs and the two half-reactions of water splitting are good for a wide photo-excitation spectrum from 2–5 eV; as such, BiOBr/BiOI bilayer can be an efficient photocatalyst for water-splitting, particularly with further optimization of its optical absorptivity.