The electrochromic properties of nickel oxide thin films prepared by chemical vapor deposition

Electrochromic nickel oxide thin films were prepared in air by an atmospheric-pressure chemical vapor deposition method. The raw material was nickel acetylacetonate. Amorphous nickel oxide thin films were obtained at a substrate temperature 250°C. Reduction and oxidation of the films in 1 M KOH resulted in bleaching and coloration, respectively. Coulometry indicated that the coloration efficiency was 44 cm²/C.     

The electrochromic properties of sputtered nickel oxide films

Electrochromic nickel oxide films were prepared by reactive RF sputtering from a nickel target in an oxygen atmosphere. The films were deposited as a compact 40 nm layer of trivalent nickel oxide, Ni₂O₃. Reduction and oxidation of the films in 1 M KOH resulted in bleaching and coloration, respectively. Voltammetry indicated that the eventual electrochromic reaction involved the β-Ni(OH)₂/β-NiOOH couple. In situ visible spectra showed electrochromic modulation of the transmittance throughout the visible range with a peak change in transmittance of about 60% at a wavelength of 500 nm. In situ spectra in the near-infrared region indicated improved electrochromic switching in this region; the sputtered nickel oxide film exhibited about a 30% change in transmittance in comparison to less than 10% for a similar electroprecipitated nickel hydroxide film. The sputtered nickel oxide films exhibited durable electrochromic switching for over 2500 coloration/bleaching cycles, a significant improvement over the less than 500 switching cycles exhibited by electroprecipitated nickel hydroxide films.     

Optical, structural and electrochromic properties of nickel oxide films produced by sol-gel technique

Nickel oxide films have been deposited from nickel acetate precursor using a sol-gel dip coating method, onto glass and conducting fluorine doped tin oxide (FTO) glass substrate. The direct energy gap (E_gd) values for the 2-10 layered films are in the range of 3.62 eV-3.72 eV. X-ray diffraction (XRD) analysis reveals that films consisting of 2-6 layers are amorphous, while films consisting of 8-10 layers are poly-crystalline with cubic grains of around 12 nm-20 nm and preferential growth along the (1 1 1) and (2 0 0) planes. Fourier Transform Infrared (FTIR) spectrum confirms the formation of Ni-O. Electrochromic properties of the nickel oxide coatings were studied using cyclic voltammetric (CV) technique. The 8 layered NiO films exhibit the anodic/cathodic diffusion coefficient of 16.7/5.73 × 10⁻¹³ cm²/s and the change in optical transmission is ΔT_630nm = 53% with a photopic contrast ratio of 2.87.     

Studies on electrochromic properties of nickel oxide thin films prepared by spray pyrolysis technique

Electrochromic nickel oxide thin films were prepared by using a simple and inexpensive spray pyrolysis technique (SPT) onto fluorine-doped tin oxide (FTO) coated glass substrates from nickel chloride solution. Transparent NiO-thin films were obtained at a substrate temperature 350°C. The films were cubic NiO with preferred orientation in the (1 1 1) direction. Infrared spectroscopy results show presence of free hydroxyl ion and water in nickel oxide thin films. The electrochromic properties of the thin films were studied in an aqueous alkaline electrolyte (0.1 M KOH) using cyclic voltammetry (CV), chronoamperometry (CA) and spectrophotometry. The films exhibit anodic electrochromism, changing colour from transparent to black. The colouration efficiency at 630 nm was calculated to be 37 cm²/C.     

Ion transport in electrochromic nickel oxide thin films

Dynamic process of ion injection into nickel oxide film in both KOH and LiClO₄–PC electrolyte were studied using electrochemical measurement. Cottrell and Butler–Volmer current–time relations were applied to obtain ion diffusion coefficient D and exchange current I_ex by fitting the experimental data. Results showed that the film displays complex transport behavior which depends on the deposition rate of the film, the environment and sequence of heat treatment, the adopted electrolyte. The change of the number of the oxygen and nickel vacancies are suggested to be responsible for the phenomena.     

The electrochromic properties of chemically deposited tungsten oxide films

Thin films of tungsten oxide were prepared by deposition from a solution of tungsten chloride, and, for comparison, by the acidification of a solution of sodium tungstate. The electrochromic behavior of these films was investigated by studying their current-voltage behavior, absorbance spectra, response time, and the diffusion coefficient of lithium ions in the film.     

Sol–gel deposited nickel oxide films for electrochromic applications

The electrochromic (EC) behavior, the microstructure, and the morphology of sol–gel deposited nickel oxide (NiO_x) coatings were investigated. The films were produced by spin and dip-coating techniques on indium tin oxide (ITO)/glass and Corning glass (2947) substrates.The coating solutions were prepared by reacting nickel(II) 2-ethylhexanoate as the precursor, and isopropanol as the solvent. NiO_x was heat treated at 350 °C for 1 h. The surface morphology, crystal structure, and EC characteristics of the coatings were investigated by scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), atomic force spectroscopy (AFM), X-ray diffractometry (XRD), and cyclic voltammetry (CV).SEM and AFM images revealed that the surface morphology and surface characteristics of the spin- and dip-coated films on both types of substrate were different. XRD spectra revealed that both films were amorphous, either on ITO or Corning glass substrates. CV showed a reversible electrochemical insertion or extraction of the K⁺ ions, cycled in 1 M KOH electrolyte, in both type of film. The crystal structure of the cycled films was found to be XRD amorphous. Spectroelectrochemistry demonstrated that dip-coated films were more stable up to 1000 coloration–bleaching cycles, whereas spin-coated films gradually degraded after 500 cycles.     

Electrochemically induced electrochromic properties in nickel thin films deposited by DC magnetron sputtering

Nickel oxide thin films and other electrochromic materials receive particular attention due to the great variety of practical applications in energy conservation and in semitransparent optical devices. In this work, nickel thin films were produced by DC magnetron sputtering on ITO substrates. The nickel–ITO thin films were studied by electrochemical techniques, and electrochromic properties were induced in the films after several different cyclic voltammetry runs. The cyclic potential range was set from −400 to 600 mV and the scan rates were varied from 6.6 to 10 mV/s. The electrochromic phenomena was observed just after 80 cycles as derived from voltammograms and color changes in the nickel oxide films were observed close to 100 cycles. The optical properties of as-deposited films and of the ones tested in the electrochemical cell were determined by optical spectrophotometry in the visible range. The structural properties of the films were studied by X-ray diffractometry, scanning and transmission electron microscopy in conventional and high-resolution modes. The electrochemical properties were studied principally by the cyclic voltammetry technique. Noticeable differences in induced electrochromic behavior were observed between the nickel films deposited on two sets of ITO substrates, prepared by DC magnetron sputtering and spray pyrolysis.     

Thickness-dependent electrochromic properties of solution thermolyzed tungsten oxide thin films

Tungsten oxide (WO₃) thin films are prepared by using a simple and inexpensive solution thermolysis technique. Thin film samples of different thickness are obtained by varying quantity of ammonium tungstate solution sprayed onto the preheated conducting glass substrate. A simple three-electrode cell has been formed to study the electrochemical and electrochromic properties. The electrochemical parameters of the cell such as anodic peak current, anodic peak potential, threshold voltage, amount of H⁺ ions intercalated into and deintercalated out of the WO₃ samples are calculated. The effect of film thickness on these parameters are studied. The extent of electrochromism and reversibility of the colouration/bleaching processes of various WO₃ samples are described. The colouration efficiencies at 633 nm are calculated. The maximum colouration efficiency obtained for thicker film, is 56 cm²/C. The samples were found to be stable in 0.05N H₂SO₄ electrolyte up to 1×10³ colour/bleach cycles.     

The electrochromic and photocatalytic properties of electron beam evaporated vanadium-doped tungsten oxide thin films

The electrochromic and photocatalytic properties of vanadium-doped tungsten trioxide thin films prepared at room temperature (300 K) by the electron beam evaporation technique are reported in this paper. The vanadium to tungsten ratio (V/W) in these films are 0.003, 0.019, 0.029 and 0.047. The optical band gap of the vanadium-doped tungsten oxide (WO₃) thin film initially increases from 3.16 to 3.28 eV for V/W ratio 0.003 then decreases to 3.15 eV for V/W ratio 0.047. These vanadium-doped films switch between neutral gray and transparent states. The coloration efficiency (CE) decreases from 82 cm² C⁻¹ (pure WO₃) to 27 cm² C⁻¹ for the film containing V/W ratio 0.047. The photocatalytic activity has enhanced with vanadium doping and maximum activity of 15% (percentage change in optical density of methylene blue due to photo degradation) has been observed for the film containing V/W ratio of 0.019. The Kelvin probe measurements show that the work function of pure WO₃ films is 4.07 eV and vanadium doping initially increases the work function to 4.19 eV for V/W ratio 0.019 and then decreases it to 3.97 eV for film with V/W ratio 0.047.     

Chemical bath deposition and electrochromic properties of NiOx films

Nickel oxide (NiO_x) thin films were prepared by the chemical deposition method (solution growth) on two kinds of substrates: (1) glass and (2) glass/SnO₂ : F. Films were thermally treated at 200°C for 10 min in atmosphere. The texture, microstructure and composition were examined by optical microscopy, X-ray diffraction patterns (XRD) and X-ray photoelectron spectroscopy (XPS) analysis of the surface layer. The films exhibited anode electrochromism. The optical properties of the bleached and colored state were examined with transmittance spectroscopy in the visible region and reflectance FTIR spectroscopy. An electrochromic test device (ECTD), consisting of SnO₂/NiO_x/NaOH–H₂O/SnO₂, was assembled and tested by cyclic voltammetry combined with a simultaneous recording of the change of transparency at λ=670 nm. The coloration efficiency was evaluated to be 24.3 cm²/C. The spontaneous ex-situ change of coloration with time of the colored and bleached NiO_x/SnO₂/glass was also examined.     

Optical properties of electrochromic iridium oxide and iridium-tantalum oxide thin films in different colouration states

Electrochromic iridium oxide (IrOx) and iridium-tantalum oxide (IrTaOx) thin films were prepared by sputtering. Complex refractive indices were determined for samples deposited on indium-tin oxide covered glass in different colouration states, and for as-deposited samples on sapphire and Corning glass. The refractive index was found to be practically constant for both IrOx (∼1.3) and IrTaOx (∼2). The extinction coefficient was found to vary between the coloured and bleached states with ∼35% for IrOx and ∼55% for IrTaOx at 660 nm. This is believed to be a result of the removal of intraband transitions within the Ir t_2g band during bleaching.     

Structure and optical properties of electrochromic copper oxide films prepared by reactive and conventional evaporation techniques

Copper oxide films (Cu_xO) are deposited by thermal evaporation techniques using copper oxide (CuO) or copper (Cu) as starting material. By varying the deposition parameters, two main types of Cu_xO film exhibiting different optical properties form. These are reddish gray and colorless films. The samples are characterised optically and morphologically. X-ray diffraction spectra reveal that evaporated Cu_xO films are amorphous. Fourier-transform infrared spectra of the samples were studied to evaluate chemical identification. The refractive index, the extinction coefficient and the thickness of the films are evluated from transmittance characteristics in the ultraviolet, visible and near-infrared regions. The refractive indices of the samples are between 2.9 and 3.1. The values determined for the optical constants are in aggreement with the results found in the literature. We report for the first time that Cu_xO films show reversible optical switching from the colored to bleached state. Optical transmittance measurements of the copper oxide film relative to indium tin oxide coated glass varied during coloring from spectral transmittance T_s = 85−40%     

Comparison of electrochromic amorphous and crystalline tungsten oxide films

A detailed systematic study of the tungsten oxide thin films has been carried out using WO₃ films after they were annealed at progressively increasing temperatures ranging from 350°C to 450°C in oxygen environments. The structural properties of the films were characterized using X-ray diffraction and Raman spectroscopy. The amorphous WO₃ films remain as an amorphous phase up to 385°C and begin to crystallize at 390°C and then are completely crystallized at 450°C. Absorption peaks of the films are found to shift to a higher energy side with increasing annealing temperature up to 385°C and then shift abruptly to a lower energy as the films begin to crystallize at 390°C. Deconvolution of the absorption spectra shows that there are two different polaron transitions in the amorphous WO₃ films.     

Optical properties of tungsten and titanium oxide thin films prepared by plasma sputter deposition

Tungsten oxide and titanium oxide thin films were prepared by RF reactive magnetron sputter deposition. The stationary and rotating substrate holders were applied to analyze the rotating effect. The optical properties and thicknesses of oxide films were determined by a proposed optical model and the measured transmittance spectra. The dispersed refractive indices of thin films have a wide range distribution in different sputtering conditions. In the situation of rotating substrate holder, the refractive index was lower than that of the stationary substrate holder. Also, amorphous TiO₂ structure can be prepared by using rotating substrate holder. The transmittance spectrum of crystalline TiO₂ reveals that the textured structure on the film surface affects the transmittance characteristic.     

Structural and electrochromic properties of tungsten oxide prepared by surfactant-assisted process

By virtue of gemini surfactant template, nanostructured tungsten oxides thin films were prepared from the modified tungsten hexachloride sol-gel techniques. Temperature was varied as it is an important factor for crystallization, surface morphology and microstructure of tungsten oxides, from the studies of X-ray diffractions, scanning electron microscopy and transmission electron microscopy. The mesoporous sample calcined at 300 °C has tri-dimensional vermicular mesopores with nanocrystallites embedded in the pore wall, while such uniform structure would be destroyed by higher calcination temperature of about 400 °C. X-ray photoelectron spectroscopy was used for analyzing the surface-binding states and the stoichiometry for the oxides. Electrochromic characterization was implemented by simultaneous voltametric and spectrophotometric measurements of tungsten oxides/indium tin oxide (ITO) electrodes. The investigation results showed that organized pore-wall nanostructure has strong effects on the electrochemical and chromogenic properties depending on the specific surface area and the impacts from the evolved crystallization.     

Electrochromic properties of nickel oxide films prepared by introduction of hydrogen into sputtering atmosphere

Colorless as-deposited films of nickel oxide were prepared by introduction of hydrogen into sputtering atmosphere, which is mixture of argon and oxygen. The electrochromic characteristics of the films thus obtained were investigated. Change in optical density depended linearly on the density of injected charges. The coloration efficiency decreased with increasing RF power of sputtering. On the other hand, the coloration efficiency increased a little with increasing hydrogen content in sputtering atmosphere up to 40%, but decreased steeply at 50%. The maximum coloration efficiency of 36 cm²/C was obtained at 8 Pa total pressure with hydrogen content of 40%.     

Preparation and properties of electrochromic tungsten oxide film

Tungsten oxide films have been formed by poly-complex. The dip-coating solution was prepared by adding hydrogen peroxide to a tungsten acid solution which was obtained from ion exchange. Amorphous and crystalline films can be obtained at different firing temperature. Optical and electrochemical measurements of these films have been carried out. The XRD, IR, TG-DTA analysis and SEM morphologies observation are also described. The results show that these coatings have good electrochromic properties.     

Effect of humidity on structure and electrochromic properties of sol–gel-derived tungsten oxide films

The influence of varying relative humidity (RH55 and 75%) during thin film deposition from an oxalato-acetylated peroxotungstic acid sol by dip coating, on the microstructure and electrochromic properties of pristine tungsten oxide (WO₃) films obtained upon annealing is presented. The films fabricated under a relative humidity of 55% are amorphous whereas the ones cast under a substantially humid atmosphere (RH75%) are characterized by interconnected nanocrystallites with a triclinic phase and a nanoporous surface morphology as well. Upon lithium insertion, larger integrated values of transmission modulation and coloration efficiency are observed over the photopic and solar regions, for the films prepared under a RH75% as compared to that observed for the films deposited under a RH of 55%. Functional improvements are due to the larger surface area of nanocrystallites and a porous microstructure, a consequence of a higher degree of hydration and hydroxylation in the former films in contrast to the non-porous and a rather featureless structure of the latter films. Faster switching kinetics between the clear and blue states, a greater current density for lithium intercalation, a higher diffusion coefficient for lithium and a superior cycling stability, again shown by the film fabricated under a 75% RH confirm that the WO₃ film microstructure is most conducive for a more facile ion insertion–extraction process, which hints at its potential for electrochromic window applications.     

The electro-optical properties of amorphous indium tin oxide films prepared at room temperature by pulsed laser deposition

The electrical and optical properties of pulsed laser deposited amorphous indium tin oxide films at room temperature are discussed. The films were grown from indium oxide (In₂O₃) targets of different tin (Sn) doping content (0, 5 and 10 wt%) at different oxygen pressures (P_O2) ranging from 1×10⁻³ to 5×10⁻² Torr. The electrical and optical properties of the films were examined by Hall measurements and optical spectrophotometry. It was found that high conductivity amorphous films could be prepared at room temperature irrespective of the Sn doping content. The properties of these films deposited from 0, 5, 10 wt% Sn-doped In₂O₃ targets show a similar response to changes in P_O2. The maximal conductivity of (4.0, 2.1 and 1.8)×10³ S/cm and optical transmittance (visible) higher than 90% were obtained at P_O2 region of (1–1.5)×10⁻² Torr. An undoped In₂O₃ film produced the highest conductivity of 4×10³ S/cm in these studies.