Ultrathin molybdenum boride films for highly efficient catalysis of the hydrogen evolution reaction

All molybdenumborides with various phases such as Mo2B, alpha-MoB, beta-MoB, and MoB2 have been found to possess excellent electrocatalytic hydrogen evolution reaction (HER) activity. Ultrathin two-dimensional (2D) borides are expected to have both maximized surface active sites and fast electron transport, ensuring higher HER activity. Here we report the large-area preparation of ultrathin hexagonal Mo3B films of 6.48 nm thickness on Mo foils by chemical vapour deposition using amixture of boron and boron oxide powders as the boron source, and hydrogen gas as both the carrier and reducing gas. The ultrathin film exhibits fantastic stability in acidic solution and has a small Tafel slope of 52 mV dec(-1) which is the lowest value so far reported for molybdenum boride catalysts. Furthermore, our first-principles calculations show that the ultrathin Mo3B film is metallic, which facilitates fast electron transport along the active edges of the thin film for enhancing the HER activity.