Diffusion-Induced Redox Gradients for the Concurrent Synthesis of MoS₂ and MoO₃ in a Single Reactor: A Green Pathway for Hydrogen Evolution

The present work demonstrates the single reactor, diffusion-controlled, stratified, and simultaneous synthesis of molybdenum oxide (Mo^(VI)O₃) and molybdenum sulfide (Mo^(IV)S₂) using the reaction-diffusion (RD) framework. Ammonium molybdate heptahydrate was used as an inner electrolyte immobilized in agar gel, while sodium sulfide with dissolved oxygen served as the outer electrolyte. We have optimized the pH levels and the <i>oxic</i>, <i>hypoxic</i>, and <i>anoxic</i> environments inside the reactor, which lead to local redox gradients and the competitive diffusion of oxygen and sulfide, facilitating species-specific and spatially stratified synthesis of these materials. The redox gradients exert thermodynamic control, as the diffusion coefficient of oxygen is smaller than that of sulfide. <i>Oxic</i> conditions resulted in the preferential formation of MoO₃, even in the presence of sulfide. Conversely, <i>hypoxic</i> and <i>anoxic</i> conditions resulted in the formation of a mixture of MoO₃ and MoS₂ and pure MoS₂, respectively. The materials were characterized for their morphological, structural, and chemical composition and speciation. The analysis revealed that MoO₃ and MoS₂ are both layered two-dimensional materials. We then assessed the electrocatalytic properties of the materials for the hydrogen evolution reaction (HER). It was observed that MoS₂ demonstrated superior HER kinetics than MoO₃.