Electrochromic Niobium Oxide Nanorods

Niobium oxide (Nb₂O₅) is an interesting active material for technologies ranging from catalysis and sensors to energy storage and electrochromic devices owing to its unique optical, electronic, and electrochemical properties. These properties vary between different phases and morphologies in the Nb₂O₅ system, but systematic studies that correlate properties to phase and morphology are limited by current synthetic methods, which require postsynthetic high temperature treatments and suffer from a lack of direct and precise control over morphology, crystal structure, and stoichiometry. Here, we report a heat-up colloidal synthesis method that produces orthorhombic Nb₂O₅ nanorods 1 nm in width by 31 nm in length that preferentially grow along the [001] direction. The synthesis is based on aminolysis of niobium oleate in octadecene, and nanorods are formed through three distinct steps: aminolysis-driven formation of niobium oxo clusters, condensation into amorphous Nb₂O₅ seeds below the reaction temperature (240 °C, under atmospheric pressure), and crystallization and growth of Nb₂O₅ nanorods. We investigated the electrochromic behavior of nanorod thin films upon Li⁺ intercalation and observed predominantly near-infrared coloration, fast switching kinetics, and durability for at least 500 charge–discharge cycles.