Growth Mechanism for the Controlled Synthesis of MgH₂/Mg Crystals via a Vapor–Solid Process

In this paper, we continue the work on the controlled growth of MgH₂/Mg nano/microstructures, in which we highlight the growth mechanism from thermodynamics and kinetics, by a catalyst-free vapor–solid method of hydriding chemical vapor deposition (HCVD), which is a process of the evaporation–condensation of Mg under high-pressure H₂. We easily achieved control of both the morphology and composition, including nano/microsized powders of MgH₂ or a mixture of MgH₂ and Mg, nanosized straight and curved MgH₂ fibers, irregular bulk microstructures of Mg, hexagonal microplates of Mg, and microdendritic Mg, of the products by adjusting three experimental parameters: evaporation temperature, deposition temperature, and H₂ pressure. The products were compositionally separated into three categories, Mg, MgH₂, and a mixture of both, based on the growth temperature within the thermodynamic P–T diagram of the Mg–H system. A growth mechanism for the HCVD process was proposed from the perspectives of thermodynamics and kinetics, comprising a two-step reaction on the deposition substrate of the first Mg condensation, Mg_(g) → Mg_(s), with subsequent hydrogenation, Mg_(s) + H_2(g) ↔ MgH_2(s). The composition of the products was controlled by controlling the reaction rates of these two steps, in which these reaction rates were determined by the atmospheric conditions of deposition temperature, H₂ pressure, and evaporation temperature. These Mg/MgH₂ nano/microstructures may find applications in hydrogen storage and batteries. By choosing suitable experimental parameters, such as evaporation temperature, deposition temperature, and H₂ pressure, this simple vapor-solid method may be extended to the synthesis of nano/microstructures of other metal hydrides.