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

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)]center dot 2H(2)O, 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 degrees 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 (Lambda T-vis = 2.1%) was the largest among the three samples, the solar energy modulation (Delta T-sol) reached 11.3%. Moreover, the transition temperature of our VO2 films was far below that of bulk VO2 (68 degrees C), and the best transition temperature was as low as 50.8 degrees C.