Acidity of Carboxylic Acid Ligands Influences the Formation of VO2(A) and VO2(B) Nanocrystals under Solvothermal Conditions

Vanadium dioxide (VO₂) can adopt many different crystal structures at ambient temperature and pressure, each with different, and often desirable, electronic, optical, and chemical properties. Understanding how to control which crystal phase forms under various reaction conditions is therefore crucial to developing VO₂ for various applications. This paper describes the impact of ligand acidity on the formation of VO₂ nanocrystals from the solvothermal reaction of vanadyl acetylacetonate (VO(acac)₂) with stoichiometric amounts of water. Carboxylic acids examined herein favor the formation of the monoclinic VO₂(B) phase over the tetragonal VO₂(A) phase as the concentration of water in the reaction increases. However, the threshold concentration of water required to obtain phase-pure VO₂(B) nanocrystals increases as the pK_a of the carboxylic acid decreases. We also observe that increasing the concentration of VO(acac)₂ or the concentration of acid while keeping the concentration of water constant favors the formation of VO₂(A). Single-crystal electron diffraction measurements enable the identification of vanadyl carboxylate species formed in reactions that do not contain enough water to promote the formation of VO₂. Increasing the length of the carbon chain on aliphatic carboxylic acids did not impact the phase of VO₂ nanocrystals obtained but did result in a change from nanorod to nanoplatelet morphology. These results suggest that inhibiting the rate of hydrolysis of the VO(acac)₂ precursor either by decreasing the ratio of water to VO(acac)₂ or by increasing the fraction of water molecules that are protonated favors the formation of VO₂(A) over VO₂(B).