Phase evolution and crystal growth of VO2 nanostructures under hydrothermal reactions

Abstract

The phase evolution and crystal growth of VO₂ nanostructures against reaction time in a high-pressure V₂O₅–oxalic acid hydrothermal system were systematically investigated. It was found that the rather thin VO₂ (B) nanobelts were first obtained, then stacked to form large belt-like structures and subsequently phase transformed into VO₂ (A), based on an oriented attachment-recrystallization mechanism. The large VO₂ (A) belt-like structures could further assemble into novel “snowflake” VO₂ (M) microcrystals with even bigger sizes and nearly well-defined six-fold symmetry. Due to the Ostwald ripening effect regarding crystal size discrepancy, the VO₂ (M) phase could further grow at the cost of the gradual dissolution of VO₂ (A) and full elimination of VO₂ (B). The phase evolution from VO₂ (B) first to VO₂ (A) and then to VO₂ (M), is actually a step-by-step thermodynamically downhill process, owing to the gradual relaxation of structural tension within the VO₂ crystal lattice. Thus, our investigation, for the first time, demonstrated the feasibility of the well-known Ostwald's step rules towards the phase evolution process of VO₂ and could provide unprecedented new insight to promote understanding of the synthesis and properties of vanadium oxide compounds.