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

Monoclinic vanadium dioxide VO 2 (M) shows a reversible first-order metal−insulator transition (MIT) to metallic rutile phase VO 2 (R) at a temperature of T c = ∼68 °C. Synthesis of micro- and nanocrystals of VO 2 (M) by a direct one-step hydrothermal method is reported herein. The reaction employs a combination of a hydrolyzed precipitate of VO 2+ 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 VO 2 (M) microcrystals or hexagon shaped nanocrystals were obtained depending on the reaction conditions, admixed with VO 2 (B) nanowires. Higher temperature, pH or NaOH concentration during the precursor formation favored VO 2 (M) and VO 2 (B) nanocrystals. The VO 2 (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 VO 2 nanocrystals to infiltrate opaline lattices of polystyrene beads to generate inverse opals.