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Abstract Most nanomaterials, such as transition metal carbides, phosphides, nitrides, chalcogenides, etc., have been extensively studied for their various properties in recent years. The similarly attractive transition metal borides, on the contrary, have seen little interest from the materials science community, mainly because nanomaterials are notoriously difficult to synthesize. Herein, a simple, general synthetic method toward crystalline transition metal boride nanomaterials is proposed. This new method takes advantage of the redox chemistry of Sn/SnCl₂, the volatility and recrystallization of SnCl₂at the synthesis conditions, as well as the immiscibility of tin with boron, to produce crystalline phases of 3d, 4d, and 5d transition metal nanoborides with different morphologies (nanorods, nanosheets, nanoprisms, nanoplates, nanoparticles, etc.). Importantly, this method allows flexibility in the choice of the transition metal, as well as the ability to target several compositions within the same binary phase diagram (e.g., Mo₂B, α‐MoB, MoB₂, Mo₂B₄). The simplicity and wide applicability of the method should enable the fulfillment of the great potential of this understudied class of materials, which show a variety of excellent chemical, electrochemical, and physical properties at the microscale. 

Abstract Abundant transition metal borides are emerging as substitute electrochemical hydrogen evolution reaction (HER) catalysts for noble metals. Herein, an unusual canonic‐like behavior of theclattice parameter in the AlB₂‐type solid solution Cr_1–xMo_xB₂(x= 0, 0.25, 0.4, 0.5, 0.6, 0.75, 1) and its direct correlation to the HER activity in 0.5 M H₂SO₄solution are reported. The activity increases with increasingx, reaching its maximum atx= 0.6 before decreasing again. At high current densities, Cr_0.4Mo_0.6B₂outperforms Pt/C, as it needs 180 mV less overpotential to drive an 800 mA cm⁻²current density. Cr_0.4Mo_0.6B₂has excellent long‐term stability and durability showing no significant activity loss after 5000 cycles and 25 h of operation in acid. First‐principles calculations have correctly reproduced the nonlinear dependence of theclattice parameter and have shown that the mixed metal/B layers, such as (110), promote hydrogen evolution more efficiently forx= 0.6, supporting the experimental results.