Hydrothermal synthesis of pure α-phase manganese(II) sulfide without the use of organic reagents

Recent studies exploring the role of metal sulfides as (photo)catalysts in prebiotic synthesis reactions provide the impetus for finding carbon-free synthesis methods for metal sulfides. The decomposition of organosulfur and organometallic precursor compounds is often the protocol for synthesizing bulk metal chalcogenides, such as manganese sulfide (MnS). Here we report a hydrothermal synthesis method for the formation of MnS in which a MnCl ₂ solution is injected into a preheated sulfide solution. By varying the temperature of injection and subsequent aging time, we can control the specific crystal phase of the product. Three MnS polymorphs are known, and two of these, α-MnS and γ-MnS, form as pure phases in aqueous systems. The initial precipitate formed upon mixing of aqueous solutions of Mn ²⁺ and S ^2- at ambient temperature is nanocrystalline and is composed of a mixture of γ-MnS (wurtzite structure) and β-MnS (zinc blende structure). β-MnS has not previously been identified as forming under aqueous conditions. The initial binary-phase precipitate can be transformed to pure, highly crystalline γ-MnS by aging at temperatures as low as 150°C within 3 days. Aging to yield pure α-MnS requires temperatures in excess of ∼200°C for 3 days. Characterization of the products was performed using powder X-ray diffraction, total scattering and pair distribution function analysis, dynamic light scattering, and transmission and scanning electron microscopy. Chemical analyses were performed using colorimetric techniques.