The design of a highly efficient electrocatalyst for oxygen evolution reaction (OER) is of great significance to the clean energy conversion system. Herein, novel Mo-doped NiFe phosphide (Mo-NiFe-P) nanoflowers are developed as robust high-activity catalysts for OER via the phosphidation of MoO₄²⁻ intercalated NiFe-layered double hydroxide (NiFe-LDH). The introduction of high valence Mo can significantly promote the catalytic activity of OER because of the strong electronic interactions with Ni and Fe. By tailoring the amount of molybdate intercalated into NiFe-LDH, the optimal phosphide shows outstanding overpotentials of 261 and 272 mV to drive current densities of 50 and 100 mA cm⁻² in 1 mol L⁻¹ KOH. This work demonstrates that the amount of molybdate influences the structure of phosphide prepared by the intercalated LDHs and also affects the electrocatalytic behavior. In addition, density functional theory (DFT) calculations show that introducing Mo could alter the intrinsic electronic structure of NiFe-P, which, in turn, could accelerate the reaction kinetics. This approach could be extended to the preparation of other cost-efficient phosphides for OER.

Beyond graphene, two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention owing to their potential in next-generation nanoelectronics and optoelectronics. Nevertheless, grain boundaries are ubiquitous in large-area as-grown TMD materials and would significantly affect their band structure, electrical transport, and optical properties. Therefore, the characterization of grain boundaries is essential for engineering the properties and optimizing the growth in TMD materials. Although the existence of boundaries can be measured using scanning tunneling microscopy, transmission electron microscopy, or nonlinear optical microscopy, a universal, convenient, and accurate method to detect boundaries with a twist angle over a large scale is still lacking. Herein, we report a high-throughput method using mild hot H₂O etching to visualize grain boundaries of TMDs under an optical microscope, while ensuring that the method is nearly noninvasive to grain domains. This technique utilizes the reactivity difference between stable grain domains and defective grain boundaries and the mild etching capacity of hot water vapor. As grain boundaries of two domains with twist angles have defective lines, this method enables to visualize all types of grain boundaries unambiguously. Moreover, the characterization is based on an optical microscope and therefore naturally of a large scale. We further demonstrate the successful application of this method to other TMD materials such as MoS₂ and WSe₂. Our technique facilitates the large-area characterization of grain boundaries and will accelerate the controllable growth of large single-crystal TMDs.