Due to the earth abundance and tunable electronic properties, etc., transition metal oxides (TMOs) show attractive attention in oxygen evolution reaction. O‐vacancies (Vo) play important roles in tailoring the local surface and electronic environment to lower the activation barriers. Herein, an effective strategy is shown to enhance the oxygen evolution reduction (OER) performance on Co3O4 ultrathin nanosheets via combined cation substitution and anion vacancies. The oxygen‐deficient Fe‐Co‐O nanosheets (3–4 nm thickness) display an overpotential of 260 mV@10 mA cm−2 and a Tafel slope of 53 mV dec−1, outperforming those of the benchmark RuO2 in 1.0 m KOH. Further calculations demonstrate that the combined introduction of Fe cation and Vo with appropriate location and content finely tune the intermediate absorption, consequently lowering the rate‐limiting activation energy from 0.82 to as low as 0.15 eV. The feasibility is also proved by oxygen‐deficient Ni‐Co‐O nanosheets. This work not only establishes a clear atomic‐level correlation between cation substitution, anion vacancies, and OER performance, but also provides valuable insights for the rational design of highly efficient catalysts for OER.

Shape engineering plays a crucial role in the application of two-dimensional (2D) layered metal dichalcogenide (LMD) crystalline materials in terms of physical and chemical property modulation. However, controllable growth of 1T phase tin disulfide (SnS2) with multifarious morphologies has rarely been reported and remains challenging. Herein, we report a direct synthesis of large-size, uniform, and atomically thin 1T-SnS2 with multiple morphologies by adding potassium halides via a facile chemical vapor deposition process. A variety of morphologies, i.e., from hexagon, triangle, windmill, and dendritic to coralloid, corresponding to fractal dimensions from 1.01 to 1.81 are accurately controlled by growth conditions. Moreover, the Sn concentration controls the morphology change of SnS2. The edge length of the SnS2 dendritic flake can grow larger than 500 μm in 5 min. Potassium halides can significantly reduce the surface migration barrier of the SnS2 cluster and enhance the SnS2 adhesion force with substrate to facilitate efficient high in-plane growth of monolayer SnS2 compared to sodium halides by density functional theory calculations. More branched SnS2 with higher fractal dimension provides more active sites for enhancing hydrogen evolution reactions. Importantly, we prove that potassium halides are preferable for 1T-phase LMDs structures, while sodium halides are more suitable for 2H-phase materials. The growth mechanism proposed here provides a general approach for controllable-phase synthesis of 2D LMD crystals and related heterostructures. Shape engineering of 2D materials also provides a strategy to tune LMD properties for demanding applications.

Crystal phase control still remains a challenge for the precise synthesis of 2D layered metal dichalcogenide (LMD) materials. The T′ phase structure has profound influences on enhancing electrical conductivity, increasing active sites, and improving intrinsic catalytic activity, which are urgently needed for enhancing hydrogen evolution reaction (HER) activity. Theoretical calculations suggest that metastable T′ phase 2D Sn 1−x W x S 2 alloys can be formed by combining W with 1T tin disulfide (SnS 2 ) as a template to achieve a semiconductor-to-metallic transition. Herein, 2D Sn 1−x W x S 2 alloys with varying x are prepared by adjusting the molar ratio of reactants via hydrothermal synthesis, among which Sn 0.3 W 0.7 S 2 displays a maximum of concentration of 81% in the metallic phase and features a distorted octahedral-coordinated metastable 1T′ phase structure. The obtained 1T′-Sn 0.3 W 0.7 S 2 has high intrinsic electrical conductivity, lattice distortion, and defects, showing a prominently improved HER catalytic performance. Metallic Sn 0.3 W 0.7 S 2 coupled with carbon black exhibits at least a 215-fold improvement compared to pristine SnS 2 . It has excellent long-term durability and HER activity. This work reveals a general phase transition strategy by using T phase materials as templates and merging heteroatoms to achieve synthetic metastable phase 2D LMDs that have a significantly improved HER catalytic performance.

Different two-dimensional (2D) materials, when combined together to form heterostructures, can exhibit exciting properties that do not exist in individual components. Therefore, intensive research efforts have been devoted to their fabrication and characterization. Previously, vertical and in-plane 2D heterostructures have been formed by mechanical stacking and chemical vapor deposition. Here, we report a new material system that can form in-plane p-n junctions by thermal conversion of n-type SnSe to p-type SnSe. Through scanning tunneling microscopy and density functional theory studies, we find that these two distinctively different lattices can form atomically sharp interfaces and have a type II to nearly type III band alignment. We also demonstrate that this method can be used to create micron-sized in-plane p-n junctions at predefined locations. These findings pave the way for further exploration of the intriguing properties of the SnSe-SnSe heterostructure.

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We present a systematical study of atomic structures and electronic properties of various dimension tellurium (Te) with broken intrinsical screw symmetry by applying reasonable strain. It is demonstrated that (i) bulk trigonal Te has degenerate Weyl nodes around the H point near the Fermi energy, and this degeneracy will be broken by introducing the selenium (Se) atom through creating the inner unsymmetrical strain, instead of external shear strain. (ii) 2D structures of tetragonal Te (t-Te) and 1T-MoS-like Te (1T-Te) show direct and indirect band gap, respectively. Under the uniform biaxial compressive (BC) strain, monolayer of t-Te shows the direct-to-indirect band gap transition, while 1T-Te monolayer has a band gap transition firstly from indirect to direct and then from direct to indirect. Their effective masses of hole and electron can be effectively tuned by BC strain. (iii) One-dimensional (1D) structures of single helix, triangular Te and hexagonal Te nanowires display the obvious quantum confinement effect on the band structure and different sensitivity to the effect of uniaxial compressive strain.

The development of inexpensive metal-free catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desirable for fuel cells and rechargeable metal–air batteries. Black phosphorus (BP), as a new kind of two-dimensional (2D) layer material, has recently shown excellent OER electrocatalytic activity. However, atomistic understanding of the catalytic mechanism is lacking. Here, on the basis of ab initio calculations, we find that pristine phosphorene shows poor ORR/OER performances. However, oxidation can effectively tune the adsorption strength of reactive intermediates and thus change its OER/ORR electrocatalytic performance. For OER, the higher the local oxidation degree (D local O) of phosphorene, the better the OER activity. Therefore, the oxidized phosphorene site with highest D local O shows the best OER catalytic property. In contrast, there exists an optimum D local O for ORR. These findings provide new insights for better understanding and improving the catalytic performances of BP-based electrocatalysts and could stimulate more theoretical and experimental studies in this area.

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