US EPA

4837730 

Journal Article 

The evolution of the Mo5(+) oxidation state in the thermochromic effect of MoO3 thin films deposited by rf magnetron sputtering 

Morales-Luna, M; Tomas, SA; Arvizu, MA; Perez-Gonzalez, M; Campos-Gonzalez, E 

2017 

Yes 

Journal of Alloys and Compounds
ISSN: 0925-8388 

722 

938-945 

10.1016/j.jallcom.2017.06.149 

WOS:000405520400121 

Molybdenum trioxide (MoO3) thin films were deposited on glass substrates by rf reactive magnetron sputtering using a metallic molybdenum target. The thermochromic effect was induced by exposing the samples to either argon or air at specific temperatures within the range 25-300 degrees C. Optical absorption spectra indicated that the samples annealed in argon show a higher thermochromic response than the samples annealed in air. Moreover, the average luminous transmittance T*(lum) was investigated as a function of the annealing temperature, T-a. The highest change of T*(lum) was found in samples treated in argon, where T*(lum) shifted from 71.69% at room temperature (RT) to 44.58% at 300 degrees C. For samples treated in air, T*(lum) decreased from 71.69% at RT down to 67.47% at 200 degrees C, but increased up to 79.08% at 300 degrees C. In addition, the annealing in argon at 300 degrees C revealed the formation of two coloration bands around 550 nm and 900 nm, which are associated with an oxygen vacancy defect state (Mo5+O5) and an intervalence charge transfer transition of a hexa-coordinated defect state (Mo5+O6), respectively. Raman spectra, HR-TEM images, and X-ray diffraction measurements showed that the samples treated in air displayed mainly the orthorhombic alpha-MoO3 phase with small indications of the monoclinic beta-MoO3 phase, whereas the treatment in argon led to a mixture of beta-MoO3 and the Magneli phase Mo9O26. The dependence of the Mo5+ oxidation state on Ta, was analyzed by X-ray photoemission spectroscopy (XPS) using deconvolution methods. For the annealing in air, it was found that the Mo5+ signal of the Mo 3d(3/2) peak increased slightly from 25 to 200 degrees C but exhibited a steep decrease at 300 degrees C; on the contrary, this signal showed a slight increase in the whole range for the anoxic atmosphere. These results show that the Mo5+ oxidation state and the optical density display a similar dependence on Ta for both types of annealing atmosphere. (C) 2017 Elsevier B.V. All rights reserved. 

Molybdenum oxide; Thin films; Thermochromism; Sputtering deposition