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

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 multiple... [ view full 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 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 octahedrals, 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, glycolic 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.