The Deep Eutectic Solvent Precipitation Synthesis of Metastable Zn₄V₂O₉

A precipitation method involving a deep eutectic solvent (DES)─a mixture of hydrogen bond donor and acceptor─is used to synthesize a ternary metal oxide. Without toxic reagents, precipitates consisting of Zn₃(OH)₂V₂O₇·nH₂O and Zn₅(OH)₆(CO₃)₂ are obtained by simply introducing deionized H₂O to the DES solution containing dissolved ZnO and V₂O₅. Manipulation of the synthetic conditions demonstrates high tunability in the size/morphology of the two-dimensional nanosheets precipitated during the dynamic equilibrium process. According to differential scanning calorimetry and high-temperature powder X-ray diffraction, Zn₃V₂O₈ and ZnO obtained by the annealing of the precipitate are intermediates in the reaction pathway toward metastable Zn₄V₂O₉. Intimate mixing of the metal precursors achieved by the precipitation method allows access to the metastable zinc-rich vanadate with unusually rapid heat treatment. The UV-vis and surface photovoltage spectra reveal the presence of sub-band gap states, stemming from the reduced vanadium (V⁴⁺) center. Photoelectrochemical measurements confirm weak photoanodic currents for water and methanol oxidation. For the first time, this work shows the synthesis of a metastable oxide with the DES-precipitation route and provides insight into the structure-property relationship of the zinc-rich vanadate.