Hydrothermal Synthesis of Monoclinic VO₂ Micro- and Nanocrystals in One Step and Their Use in Fabricating Inverse Opals

Monoclinic vanadium dioxide VO2(M) shows a reversible first-order metal−insulator transition (MIT) to metallic rutile phase VO2(R) at a temperature of Tc = ∼68 °C. Synthesis of micro- and nanocrystals of VO2(M) by a direct one-step hydrothermal method is reported herein. The reaction employs a combination of a hydrolyzed precipitate of VO2+ with hydrazine and NaOH. The morphology and composition of the product depends on the conditions used for preparing the precursor, including pH, temperature, and NaOH concentration. Asterisk or rod shaped VO2(M) microcrystals or hexagon shaped nanocrystals were obtained depending on the reaction conditions, admixed with VO2(B) nanowires. Higher temperature, pH or NaOH concentration during the precursor formation favored VO2(M) and VO2(B) nanocrystals. The VO2(M) microcrystals showed the expected metal−insulator transition at around 67 °C which was monitored by optical microscopy. As a demonstration of their potential use, we used the VO2 nanocrystals to infiltrate opaline lattices of polystyrene beads to generate inverse opals.