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.