Tungsten Oxide Films of High Electrochromic Efficiencies Obtained by Deposition

Abstract

Amorphous tungsten oxide films of high electrochromic efficiency were produced by a chemical vapor deposition technique in which the precursors of film formation are generated at the surface of a heated tungsten filament by reactions of tungsten with oxygen. The film deposition rate was determined as a function of the filament temperature and oxygen pressure. Molecular structure investigations using X-ray photoelectron spectroscopy revealed that the tungsten oxide was in the WO 3 stoichiometry. The electrochromic properties of the films were studied for Li ϩ intercalation. Optical efficiencies as high as 78 and 125 cm 2 /C at 632.8 and 950 nm, respectively, and absorbance changes from 0.07 to 1.1 were measured between the bleached and the colored states at 632.8 nm.Tungsten oxide films find application in electrochromic devices, 1 gas sensors, 2 and electrocatalysis. 3 The electrochromic effect, particularly, has been extensively investigated in these films as they present one of the best optical performances among the inorganic electrochromic compounds. 4,5 Several deposition techniques such as sputtering, 6 thermal evaporation, 7 and plasma-enhanced chemical vapor deposition 8 have been used to obtain electrochromic tungsten oxide films. Recently, we developed a new deposition method, called hot filament metal oxide deposition ͑HFMOD͒, using a tungsten filament heated in a rarefied oxygen atmosphere. The film is formed on a substrate positioned near the filament and the deposition rate is controlled by the filament temperature and the oxygen pressure. Both the thermochemistry of the process and the kinetics of film formation are currently under investigation. It is clear, however, that the film is formed from volatile W x O y precursors generated on the heated tungsten surface in reactions between oxygen and tungsten.This work describes the electrochromic properties of tungsten oxide films obtained by HFMOD. Because the deposition rate is of practical interest in thin-film deposition and the crystalline and molecular structure usually play important roles in electrochromic properties, this paper also includes studies of these parameters. The deposition rate was investigated as a function of the filament temperature and oxygen pressure while the crystalline and molecular structure were characterized by X-ray diffractometry ͑XRD͒ and X-ray photoelectron spectroscopy ͑XPS͒, respectively. The electrochromic properties of the coatings were studied for Li ϩ intercalation, using an electrochemical cell coupled with an optical setup. The films exhibited wide changes in optical absorbance, excellent optical and electrochemical reversibility, and very large electrochromic efficiencies. Figure 1 is a schematic representation of the deposition apparatus ͑tungsten filament, substrate, and other accessories͒ mounted inside a vacuum chamber. The filament was heated resistively by a current supply to high temperatures while oxygen was admitted into the chamber via an electronic mass flowmeter. Pressure measurements were made u...

Li diffusion and electrochromism in amorphous and crystalline vanadium oxide thin film electrodes

“…10 Recently we developed a thin film deposition process, the hot filament metal oxide deposition (HFMOD), 11 with which tungsten and molybdenum oxide film electrodes were produced, and their electrochromic properties for Li intercalation were studied. 12,13 In this paper, we investigate the optoelectrochemical properties of vanadium oxide films synthesized onto ITO-coated glass substrates by the HFMOD technique. The investigations were carried out in the amorphous as-deposited films and in those thermally-treated at 200, 300, 400 and 500 o C. The electrochemical characterization was made by cyclic voltammetry and chronopotentiometry.…”

“…Details of the deposition apparatus can be found elsewhere. 11,12 The deposition chamber was kept at a pressure of 0.24 Pa of oxygen, introduced via a high precision mass flowmeter. During deposition, the chamber was continuously pumped.…”

Berreman Effect in Amorphous and Crystalline WO3 Thin Films

“…Tungsten oxide films have been the focus of extensive scientific investigations due to their prospective technological applications in electrochromic devices, [1][2][3] high energy density microbatteries, 4 gasochromic sensors, 5,6 and electrocatalysis. 7,8 Although very simple in terms of stoichiometric formula, WO 3 is known to be very complex as far as structure and phase transitions are concerned.…”

“…Tungsten oxide films have been the focus of extensive scientific investigations due to their prospective technological applications in electrochromic devices, − high energy density microbatteries, gasochromic sensors, , and electrocatalysis. , …”

“…Thus, in this paper we describe an investigation on the structural and phase changes of WO 3 films as a function of the annealing temperature using IRRAS, Raman Spectroscopy, and XRD. The films were obtained by a new deposition method developed in our laboratory, called hot filament metal oxide deposition (HFMOD) described in previous articles. ,, Briefly, a tungsten filament is heated in a rarefied oxygen atmosphere. The film is formed on a substrate positioned near the filament from volatile oxide species generated on the heated tungsten surface from reactions between oxygen and tungsten.…”