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Concept
Vanadium(IV) oxide
Summary
Vanadium(IV) oxide or vanadium dioxide is an inorganic compound with the formula VO2. It is a dark blue solid. Vanadium(IV) dioxide is amphoteric, dissolving in non-oxidising acids to give the blue vanadyl ion, [VO]2+ and in alkali to give the brown [V4O9]2− ion, or at high pH [VO4]4−. VO2 has a phase transition very close to room temperature (~). Electrical resistivity, opacity, etc, can change up several orders. Owing to these properties, it has been used in surface coating, sensors, and imaging. Potential applications include use in memory devices, phase-change switches, passive radiative cooling applications, such as smart windows and roofs, that cool or warm depending on temperature, aerospace communication systems and neuromorphic computing.
At temperatures below Tc = , VO2 has a monoclinic (space group P21/c) crystal structure. Above Tc, the structure is tetragonal, like rutile TiO2. In the monoclinic phase, the V4+ ions form pairs along the c axis, leading to alternate short and long V-V distances of 2.65 Å and 3.12 Å. In comparison, in the rutile phase the V4+ ions are separated by a fixed distance of 2.96 Å. As a result, the number of V4+ ions in the crystallographic unit cell doubles from the rutile to the monoclinic phase.
The equilibrium morphology of rutile VO2 particles is acicular, laterally confined by (110) surfaces, which are the most stable termination planes. The surface tends to be oxidized with respect to the stoichiometric composition, with the oxygen adsorbed on the (110) surface forming vanadyl species. The presence of V5+ ions at the surface of VO2 films has been confirmed by X-ray photoelectron spectroscopy.
In 2022, a to date unique and unknown feature of the material was reported – it can "remember" previous external stimuli (via structural rather than electronic states), with potential for e.g. data storage and processing, potentially including in neuromorphic computing.
At the rutile to monoclinic transition temperature (), VO2 also exhibits a metal to semiconductor transition in its electronic structure: the rutile phase is metallic while the monoclinic phase is semiconducting.
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