Thermochromic VO2 films from ammonium citrato-oxovanadate(iv) with excellent optical and phase transition properties

Abstract

Thermochromic VO₂ film is a potential material for energy-saving windows in future buildings. Considering the difficulty in controlling the valence state and polymorphism during the formation of VO₂ (M) film, we propose a facile and safe solution method of fabricating monoclinic (M) VO₂ film directly from the newly synthesized ammonium citrato-oxovanadate(IV) compound [(NH₄)₄[V₂O₂(C₆H₄O₇)₂]·2H₂O, denoted as CA-V(IV)], as a vanadium(IV) precursor to stabilize vanadium in the 4+ valence state without utilizing V₂O₅ as an intermediate or complex post-treatment in vacuum. This new ambient-stable compound CA-V(IV) contains centrosymmetric dinuclear complex anions [{VO(C₆H₄O₇)}₂]⁴⁻, in which the carboxylate groups are coordinated to the V⁴⁺ ions in a monodentate fashion and each vanadium atom exhibits distorted octahedral geometry. With a perfect monoclinic phase, the VO₂ films possessed excellent thermochromic and visible transmissive properties. Accompanying a change in film thickness from 119 nm to 41 nm, the integral visible transmittance T_vis at 25 °C of VO₂ films ranged from 38.4% to 70.0%. The maximum visible transmittance (T_max) reached 77.2% for VO₂ film with a thickness of 41 nm, which shows that the phase transition of VO₂ did not greatly affect the visible transmittance of the film. For the VO₂ film with a thickness of 41 nm, of which the visible transmittance modulation (ΔT_vis = 2.1%) was the largest among the three samples, the solar energy modulation (ΔT_sol) reached 11.3%. Moreover, the transition temperature of our VO₂ films was far below that of bulk VO₂ (68 °C), and the best transition temperature was as low as 50.8 °C.