Surfactant-controlled composition and crystal structure of manganese(II) sulfide nanocrystals prepared by solvothermal synthesis

Abstract

We investigated how the outcome of the solvothermal synthesis of manganese(II) sulfide (MnS) nanocrystals (NCs) is affected bythe type and amount of long chain surfactant present in the reaction mixture. Prompted by a previous observation that a larger thanstoichiometric amount of sulfur is required [Puglisi, A.; Mondini, S.; Cenedese, S.; Ferretti, A. M.; Santo, N.; Ponti A. Chem.Mater. 2010, 22, 2804-2813], we carried out a wide set of reactions using Mn(II) carboxylates and Mn2(CO)10 as precursors withvarying amounts of sulfur and carboxylic acid. MnS NCs were obtained provided that the S/Mn ratio was larger than the L/Mnratio, otherwise MnO NCs were produced. Since MnS can crystallize in three distinct phases (rock salt a-MnS, zincblende ß-MnS,and wurtzite γ-MnS), we also investigated whether the surfactant affected the NC polymorphism. We found that MnS polymorphismcan be controlled by appropriate selection of the surfactant. γ-MnS nanocrystals formed when a 1:2 mixture of long chaincarboxylic acid and amine was used, irrespective of the presence of carboxylic acid as a free surfactant or ligand in the metalprecursor. When we used a single surfactant (carboxylic acid, alcohol, thiol, amine), α-MnS nanocrystals were obtained. The peculiarrole of the amine seems to be related to its basicity. The nanocrystals were characterized by TEM and electron diffraction; ATR-FTIR spectroscopy provided information about the surfactants adsorbed on the NCs.