A soft chemistry approach to preparing (de)sodiated transition-metal hydroxy molybdates
A soft chemistry approach was applied to synthesize de(sodiated) transition-metal hydroxy molybdates under hydrothermal conditions. Different NaM2(H3O2)(MoO4)2 and M2MoO4(OH)2 compositions (M = Ni, Zn) were prepared at mild conditions of temperature (110-250°C), for times ranging from 10 min to 24 h. Sodiated compounds were obtained after the microwave-assisted hydrothermal process at 150°C (10-120 min), for zinc-based materials. Besides, conventional heating destabilized the sodiated NaZn2(H3O2)(MoO4)2 phase at 110°C, thus increasing the amount of the Zn2MoO4(OH)2 up to 250°C. NaNi2(H3O2)(MoO4)2 phases were noted only for 10 min under microwaves, while longer times promoted a chemical reaction towards a new desodiated phase. This novel Ni2MoO4(OH)2 phase was also obtained under conventional hydrothermal processing (110-250°C, 24 h). Slow precipitation rates yielded a hydrated nickel molybdate (triclinic, P-1) after coprecipitation followed or not by microwave irradiation. Raman spectroscopic analyses showed good agreement with experimental results and group-theory calculations for both sodiated as well as for desodiated materials. A temperature-induced polymorphic transformation was observed from NiMoO4.xH2O (triclinic, P-1) to alfa-NiMoO4 (monoclinic, C2/m). For all obtained polymorphs, the relevant Raman modes were depicted, allowing us to determine their spectroscopic fingerprints. The results constitute an important basis on the understanding of the synthesis of (de)sodiated transition-metal hydroxy molybdates under hydrothermal conditions.