One-pot synthesis of amorphous high entropy Mo–C–N–O–S solids with ultradispersed Mo sites

Abstract

Reaction of urea with ammonium heptamolybdate (HMA) at 550° in inert atmosphere leads to nanocrystalline molybdenum carbide Mo₂C, whereas reaction of HMA with elemental sulfur gives MoS₂ sulfide nanoparticles. Heating of sole urea in the same conditions is a well-known method to prepare C₃N₄. However, none of these phases is formed upon heating of a mixture of sulfur, HMA and urea, but the product has a complex high entropy Mo–C–N–O–S composition in which four light elements are randomly distributed around the Mo atoms. These solids remain amorphous and keep high surface area even at 550 °C, but disproportionate at 750 °C. The Mo–C–N–O–S solids prepared via this extremely simple one-pot synthesis route were studied by several techniques including X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM). The materials are mesoporous and show promising HER activity in acidic conditions. Reaction mechanism leading to the mixed phase was studied by thermal analysis coupled with mass-spectrometry. XAS study demonstrated that Mo is surrounded by light atoms and has a small number of Mo neighbors. Remarkably, Mo species remain ultradispersed in the solids containing even 40 wt% of the metal. This study opens a new pathway toward high-entropy amorphous materials.

Graphical abstract