Intermediates in the formation of microporous layered tin(IV) sulfide materials

Abstract

The formation pathway of microporous layered tin(iv) sulfides A₂Sn₃S₇ and A₂Sn₄S₉ (A=cation), respectively denoted SnS-1 and SnS-3, synthesized from elemental Sn and S sources in the presence of templates and mineralizers has been studied. Three types of reaction intermediates, dimeric [Sn₂S₆ ]^4– , polysulfides [Sn(S₄)₃ ]^2– /[Sn(S₄)₂(S₆)]^2– and thiosulfate [S₂O₃ ]^2– , have been isolated and characterized by single crystal X-ray diffraction structure analysis. UV–VIS and ¹¹⁹Sn NMR studies of mother-liquors of representative SnS-n systems indicate that the dimeric [Sn₂S₆ ]^4– anion is the predominant solution tin-containing species and a likely basic building unit for the SnS-n frameworks. The role of template cations has also been examined. The pH dependent condensation–polymerization of the dimeric [Sn₂S₆ ]^4– precursor in the presence of template cations is believed to be responsible for the formation of the SnS-n structures. The dissolution and redox reactions of the elemental Sn metal and S₈ powders are found to be extremely sensitive to pH, mineralizers and temperature. Reagents, such as sulfide, fluoride, hydroxide and amines that can react with tin, sulfur and various tin sulfide and polysulfide intermediates are good mineralizers. To produce large single crystals of the SnS-n materials, the concentration and strength of mineralizers need to be kept low, however, to prepare phase pure materials, the presence of excess mineralizer is required. In general, amine templates are strong mineralizers for tin metal particles, presumably through coordination to various tin polysulfide and sulfide intermediates. As a result, amine templates/mineralizers provide cleaner and smaller product crystals under reaction conditions that are similar to those used with tetraalkylammonium hydroxide templates. Surprisingly, it was found that (DABCOH)₂Sn₃S₇ , denoted DABCOH-SnS-1, can be formed using ‘very soft chemistry’ at room temperature and atmospheric pressure conditions, from an aqueous solution of [Sn₂S₆ ]^4– simply by the precise control of pH. The hydrothermal reaction conditions that are generally employed in the synthesis of SnS-n materials only serve to digest the starting material.