Organometallic sol/gel chemistry of metal sulfides

This paper reviews organometallic chemistry relevant to the sol/gel processing of metal sulfides. It focuses on the synthesis of ZnS wherein different classes of precursors were evaluated for their solution/gelation behavior. These involved the reaction of Zn-thiolate/mercaptide compound precursors with hydrogen sulfide or organo-polysulfides. Only two reactions were identified where polymeric residues or gels were created. One of these used a pentamer species - [EtZn(SBu^t)]₅ - which yielded an amorphous, fibrous ZnS residue after reaction with controlled quantities of H₂S. A subsequent heat-treatment of this residue under flowing H₂S created a mixture of β-ZnS powder and α-ZnS single crystal whiskers. The other reaction exhibiting sol/gel behavior used a polysulfide - (BzS)₂S - to deliver sulfur to the Et₂Zn precursor. In this case, a clear high viscosity solution was created. This solution gelled and dried to form an amorphous solid which exhibited a glass transition. At higher temperature, a fine-grained ZnS was generated. It remains to be determined whether this chemistry can be used for sol/gel processing of ceramics; i.e., net-shaping of the residue, and then consolidation/transformation in H₂S to create a ZnS ceramic, or in the case of the pentamer reaction, a ZnS whisker-reinforced ZnS composite.