Hydrothermal synthesis of vanadium dioxide polymorphs from vanadium alpha-hydroxycarboxylato complexes

Abstract

Vanadium dioxide (VO2) is a versatile material with notable applications in fields such as energy storage and energy-efficient smart windows. The versatility of VO2 is related to its extensive crystallography with many multiple polymorphic forms. VO2 can undergo a metal–insulator phase transition (MIT) at a critical temperature of 68 °C, which is accompanied by a reversible structure transformation from high-temperature metal phase VO2(R) to low-temperature insulator phase VO2(M). This phase transition results in a distinct change of the infrared transmittance, rendering VO2 suitable for thermochromic windows. Additionally, VO2(B) is frequently proposed as a promising cathode material in electrochemical batteries. Its crystals are constructed from distorted octahedral, sharing both corners and edges to form V-O channels that are perpendicular to the c-axis, allowing the insertion/extraction of ions. For the synthesis of VO2 a one-step hydrothermal method would be straightforward and cost effective. However, due to the flexible redox chemistry of vanadium and given the variety of polymorphs in VO2 the directed synthesis of a specific VO2 crystal structure is an arduous undertaking. Most reported one-step hydrothermal methods start from a vanadium(V)-compound (V2O5 or NH4VO3), which is reduced to a VO2 polymorph in hydrothermal conditions by oxalic acid or hydrazine. [Li et al. Small 2017, 13, 1701147] Unfortunately, little or no attention is given to the role of the intermediate species on the VO2 phase formation. In the present work we start with the synthesis of precursor solutions containing vanadium alpha-hydroxycarboxylato complexes. In this context, citric acid and tartaric acid are compared to oxalic acid. These respective acids initiate the formation of diverse complexes that produce a different VO2 polymorph after hydrothermal treatment. The results, presented here, demonstrate that a clear knowledge of the precursor chemistry is an essential first step towards the selective hydrothermal synthesis of VO2 polymorphs.