Influence of water concentration on the solvothermal synthesis of VO2(B) nanocrystals

Abstract

Nanocrystals of VO₂(B) have attracted significant attention for their promising performance in electrochemical energy storage applications. Phase-purity and nanocrystal morphology are both crucial factors for the performance of these materials, but given the large number of crystal polymorphs available to VO₂, achieving simultaneous control over crystal phase and nanocrystal size is a significant synthetic challenge. This paper describes the impact of water concentration on the synthesis of VO₂(B) nanocrystals via solvothermal reaction of a molecular V(IV) precursor in toluene. Using controlled, stoichiometric amounts of water (20 equivalents per vanadium centre) enables access to short nanorods of VO₂(B) whose lengths follow a Gaussian distribution with an average and standard deviation of 110 ± 30 nm. Decreasing the amount of water present to two or four equivalents results in formation of VO₂(A) nanocrystals and eliminating it entirely results in no reaction. Increasing the amount of water to more than 20 equivalents increases the average length of the VO₂(B) nanorods and causes the distribution in rod lengths to evolve from Gaussian to lognormal. This evolution in the size distribution is consistent with changes in a model Gaussian distribution observed upon simulated end-to-end oriented attachment events. These results demonstrate that control over the concentration of water is a useful strategy for tuning the morphology and crystal phase of VO₂ nanocrystals.