Controllable morphology, structure and optical properties of W–Eu Co-doped VO2 with a tunable phase transition temperature and band gap

Uncontrollable morphology, high phase transition temperature and vague phase transition mechanism of vanadium dioxide (VO2) hinder its large-scale application in smart windows and optoelectronic switches. In this paper, controllable VO2(B) powders co-doped with W–Eu were synthesized via a hydrothermal method using CH3COOH·2H2O and H2O2 as the reductant and assistant reductant, where W and Eu were derived from H2WO4 and Eu(NO3)3·6H2O, respectively. Most VO2(B) crystals transformed into VO2(M) after annealing. Pristine VO2 had a regular rectangular shape, and it would change to be fabric-like, spongy and coralloid after doping with W and Eu. Furthermore, the surface of W–Eu co-doped samples presented an urchin-like structure and nanorods appeared after annealing. Importantly, doping and annealing also have significant effects on the performances of VO2 samples. Upon W–Eu co-doping, the phase transition temperature of VO2 (M) samples decreased by an average of approximately 15 °C, and the energy band gap decreased from 0.70 eV to 0.33 eV corresponding to the change of phase transition temperature. Not only the thermochromic performances of VO2 were optimized, but also the mechanisms were explained accordingly in this paper.