Crystallization trigger of Mn-doped zinc silicate in supercritical water via Zn, Mn, Si sources and complexing agent ethylenediamine tetraacetic acid

Crystallization of Mn-doped zinc silicate, α-Zn₂SiO₄:Mn²⁺ was studied using several Zn, Mn, and Si sources in supercritical water with and without ethylenediamine tetraacetic acid (EDTA). Oxalate, hydroxide, oxide, and sulfate were used as the Zn and Mn sources and amorphous, crystalline, and colloidal SiO₂ and tetraethoxyorthosilicate (TEOS) were used as the Si sources. Zn and Mn sources acted as a trigger for Zn₂SiO₄ crystallization through dissolution, while the solubility of the Si source affected reaction velocity. The oxalate source gave single phase α-Zn₂SiO₄:Mn²⁺, while the hydroxide source provided mainly β-Zn₂SiO₄:Mn²⁺ with a small amount of α-Zn₂SiO₄:Mn²⁺. The oxide sources did not readily react and only gave small amounts of α- and β-Zn₂SiO₄:Mn²⁺. Although the sulfate source did not give any Zn₂SiO₄ phases, addition of EDTA as a complexing agent for Zn²⁺ ions caused α-Zn₂SiO₄ crystallization. Remarkably, addition of EDTA into the sulfate source and TEOS gave botryoidal druses of rod-like shaped α-Zn₂SiO₄ crystals, which are similar to natural α-Zn₂SiO₄ deposits (willemite) that only occur in La Calamine, Belgium (Moresnet). In conclusion, Zn and Mn oxalates and amorphous SiO₂ were the best combinations for crystallizing single phase α-Zn₂SiO₄:Mn²⁺ from supercritical water.