Coloration Mechanism in Proton-Intercalated Electrochromic Hydrated NiO[sub y] and Ni[sub 1−x]V[sub x]O[sub y] Thin Films

Abstract

Electrochromic ͑EC͒ films of nickel oxide, with and without vanadium, were prepared by reactive dc magnetron sputtering. They were characterized by electrochemical and optical measurements and studied by X-ray photoelectron spectroscopy ͑PES͒ using synchrotron radiation. The films were analyzed under as-deposited conditions and after bleaching/coloration by insertion/ extraction of protons from a basic solution and ensuing charge stabilization. Optical measurements were consistent with a coloration process due to charge-transfer transitions from Ni 2+ to Ni 3+ states. The PES measurements showed a higher concentration of Ni 3+ in the colored films. Moreover, two peaks were present in the O 1s spectra of the bleached film and pointed to contributions of Ni͑OH͒ 2 and NiO. The changes in the O 1s spectra upon coloration treatment indicate the presence of Ni 2 O 3 in the colored film and necessitated an extension of the conventional model for the mechanism of EC coloration. The model involves not only proton extraction from nickel hydroxide to form nickel oxyhydroxide but also participation of NiO in the coloration process to form Electrochromic ͑EC͒ materials are able to change their optical properties reversibly and persistently upon charge insertion/ extraction under the application of an external voltage.1 Materials that color upon charge insertion/extraction are called cathodic/ anodic. These materials can be implemented in energy-saving and comfort-enhancing architectural "smart windows," and other application areas include mirrors with variable specular reflectance, nonemissive information displays, surfaces with variable thermal emittance, eyewear, etc.1-4 EC technology has been used for some niche applications for several years. However, the situation is now changing and electrochromism will reach wider applications. 5Thin films of Ni oxide possess anodic electrochromism, as discovered in the mid-1980s 6-9 and subsequently studied in a large number of investigations. [1][2][3][4] The visual appearance goes between transparent and brown upon extraction and insertion of charge. In EC devices incorporating cathodically coloring W oxide films, the brown color of Ni oxide is complementary to the blue color of W oxide, both together yielding a neutral ͑gray͒ color in transmittance for dark films. Ni oxide films produced for EC applications exhibit a cubic polycrystalline structure 1,9-14 and a density between 3 and 6.5 g/cm 3 depending on deposition conditions. 1,14,15 The low density is indicative of porosity. 10,14,16 The grain size depends strongly on film deposition conditions and can range from 3 to 300 nm. 9,10,12,14,15,17 The porosity and small grain size, leading to a large surface-to-bulk ratio, increase the EC activity of the films, 12,18,19 and the reaction that produces the color takes place in the outermost parts of the grains. 12Several varieties of reactively sputter-deposited EC hydrated nickel-based oxide thin films have been reported. [1][2][3][4]20 Optimized Ni 1−x V x O y films made by sputter...

Electrochromism in nickel oxide and tungsten oxide thin films: Ion intercalation from different electrolytes

“…By examining the carbonate contribution to C1 at 289 eV by XPS it was concluded that about 10% of the oxygen was bound to carbon. The films are probably not pure NiO z , but also contain hydroxide phases, as shown by Avendano et al [16]. NiO z was compatible with all three electrolytes.…”

Spectroscopic ellipsometry characterization of electrochromic tungsten oxide and nickel oxide thin films made by sputter deposition

“…Figure 3 displays corresponding cross-sectional views on fractured samples. The latter SEM micrographs give clear evidence for columnar structures and porosity on the nanometer scale; these features are conducive to ionic transport and are hence advantageous with regard to EC performance [1,14]. Film thicknesses for samples on Si were found to be in the approximate thickness range 350 < d < 370 nm.…”

“…Corresponding bulk densities are 7.16 g/cm 3 for monoclinic WO 3 and 6.67 g/cm 3 for NiO single crystals, thus showing that the films have a large amount of porosity. The Ni oxide film, in particular, is expected to be strongly hydrous [14].…”

“…Electrochromic Ni oxide also has been known for years [11,12]; here the optical absorption-at least in sputter deposited films-is dominated by transitions between Ni 2+ and Ni 3+ in hydrous surface layers on NiO-type nanoparticles forming nanoporous films [13,14].…”

Structure and composition of sputter-deposited nickel-tungsten oxide films

“…For nickel oxides the Ni 2p spectra contain multiple peaks at binding energies between about 852 and 870 eV, including the main peak and satellites [20][21][22] NiOOH, since the sample has a darkish brown colour that indicates the presence of some Ni 3+ , which possibly is responsible for the asymmetry of the XPS peaks.…”