Elucidation of the electrochromic mechanism of nanostructured iron oxides films

Nanostructured hematite thin films were electrochemically cycled in an aqueous solution of LiOH. Through optical, structural, morphological, and magnetic measurements, the coloration mechanism of electrochromic iron oxide thin films was elucidated. The conditions for double or single electrochromic behavior are given in this work. During the electrochemical cycling, it was found that topotactic transformations of hexagonal crystal structures are favored; i.e. α-Fe₂O₃ to Fe(OH)₂ and subsequently to δ-FeOOH. These topotactic redox reactions are responsible for color changes of iron oxide films.     

Transformation of amorphous iron oxide thin films predeposited by spray pyrolysis into a single FeSe2-phase by selenisation

The present paper investigates a simple and non-toxic method to transform amorphous iron oxide pre-deposited by spray pyrolysis of FeCl₃·6H₂O (0.03 M)-based aqueous solution onto glass substrates heated at 350 °C into FeSe₂ thin films. The amorphous iron oxide films were heat treated under a selenium atmosphere (10⁻⁴ Pa) at different temperatures for 6 h. X-ray diffraction (XRD) was used to investigate the structure of the obtained films. Single FeSe₂-phase films having good crystallinity were obtained at a selenisation temperature of 550 °C. Optical analyses of the FeSe₂ films obtained at 550 °C enabled us to deduce a large absorption coefficient (, ). Surface scanning electron microscopy (SEM) observations show inhomogeneous films. Electrical conductivity of the as-prepared films was measured at high and low temperatures.     

Electrosynthesis and characterization of iron selenide thin films

Iron selenide thin films have been deposited onto stainless steel and fluorine-doped tin oxide (FTO) glass substrates by the electrodeposition process, in potentiostatic mode using ferric chloride (FeCl₃) and selenium dioxide (SeO₂) salts. The deposition mechanism and growth of the films were investigated by cyclic voltammetry and chronoamperometry. The structural, morphological, compositional and optical properties of the deposited films have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), EDAX and optical absorption techniques, respectively. XRD studies reveal that the films are polycrystalline with monoclinic crystal structure. The surface morphology study shows that grains are uniformly distributed over the entire surface of the substrate. EDAX study reveals that the iron selenide films are rich in iron. Optical absorption study shows the presence of direct transition with bandgap energy of 1.23 eV.     

Electrochromic behaviour of Nb2O5 thin films with different morphologies obtained by spray pyrolysis

Two different procedures to stabilize the precursor NbCl₅ have been applied to obtain Nb₂O₅ thin films by spray pyrolysis. Depending on the procedure used, determined by the way in which the precursor solution was injected into the air stream of the spray nozzle, niobium oxide thin films with different surface morphologies can be obtained. The structural properties of the Nb₂O₅ thin films depend on the post-annealing temperature because as-deposited films are amorphous, independently of the synthesis procedure used. The electrochromic behaviour has been estimated for all films, where monochromatic colouration efficiency (at 660 nm) of 25.5 cm²/C and a cathodic charge density close to 24 mC/cm² were found to give the best results to date for niobium oxide thin films obtained by spray pyrolysis.     

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.     

Formation and electronic structure of interface

In the present work the formation of the interface between polycrystalline silver and thin films of titanium oxide was studied with photoelectron spectroscopy (XPS, UPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Titanium oxide was deposited stepwise on 100 nm thick silver films by reactive magnetron sputtering allowing to study the evolution of the interface formation process. The process involves two steps: formation of thin layer of silver oxide and subsequent growth of the TiO₂ film. For better understanding of the silver oxidation process, pure silver films were exposed to a low temperature Ar/O plasma for different time intervals providing a possibility to investigate early stages of the oxide film growth.     

Photoactive iron pyrite films for photoelectrochemical (PEC) cells

Photoactive iron pyrite (FeS₂) films have been grown by sulfurization of sprayed iron oxide films. The structural properties were studied using X-ray diffraction. The films showed good crystallinity with grain size ≈500 Å. These films have been used as photoanode in photoelectrochemical cells.     

Electrochromic behavior of Ni---W oxide electrodes

A new electrochromic material, Ni---W oxide, was fabricated by a reactive sputtering method and the effects of tungsten concentration in Ni---W oxide thin films on the electrochromic behavior were investigated. It has been found that the charge transfer resistance which has been known to be very high in NiO_x becomes significantly low with the addition of tungsten in NiO_x. It turned out that the charge transfer density during the cyclic voltammetry increased considerably with the addition of tungsten in NiO_x such that at an optimum concentration of tungsten (atomic ratio of W/Ni = 0.33), a fast optical switching with the insertion and removal of lithium in Ni---W oxide thin films could be expected. The electrochromic display device composed K_0.3WO_3.15 and Ni---W oxide was fabricated and its optical switching characterized.     

Photoelectrochemical characterization of sprayed tungsten oxide thin films

A simple and inexpensive method was used for the preparation of tungsten oxide thin films. Ammonium tungstate was sprayed onto the F.T.O. coated glass substrates and then heated to convert it into tungsten oxide thin films. Photoelectrochemical studies were carried out by fabricating a PEC cell of configuration WO₃/0.1 M Na₂SO₄/C. Different parameters such as flat band potential, junction ideality factor, decay time constant, stability of the cell etc. of the system are estimated.     

Potential dependent growth of Cu(OH)2 nanostructures on Cu and their thermal conversion to mixed-valent copper oxides p-type photoelectrode

Herein, we report the anodic growth of nanostructured Cu(OH)₂ thin films on the copper surface by potentiostatic and potentiodynamic methods. The phase formation, crystalline feature, morphology and the progressive growth of Cu(OH)₂ thin films were controlled by the applied potential and concentrations of alkali utilized in anodization process. Electrochemical investigations suggest the rapid growth of Cu(OH)₂ and passivation at higher anodization potentials. In contrast, the lower anodization potentials favours the progressive growth of Cu(OH)₂ nanorod like features. The thermal treatment of Cu/Cu(OH)₂ reveals the formation of crystalline mixed copper oxide film with predominantly Cu₂O phase at 673 K and 773 K. Photoelectrochemical investigations of these copper oxide thin films exhibit the p-type behavior with repeatable photovoltage (55 mV) and stable photocurrent responses (20–60 μA cm⁻²). The hydrogen evolution studies show better activity with nanostructured Cu(OH)₂ and copper oxides than with aggregated thin films and bare substrate.     

Zirconium nitride based transparent heat mirror coatings —preparation and characterisation

Transparent heat mirror coatings based on thin zirconium nitride films have been prepared using reactive magnetron sputtering. The zirconium nitride films have been sandwiched between layers of zirconium oxide. It is shown that the multilayer configuration ZrO₂/ZrN/ZrO₂ can be used as solar control coatings on window glazings. A visible transmittance of around 60% and a thermal emittance lower than 0.2 can be obtained, and the ratio between visible transmittance and total solar transmittance can be as high as 1.7. The influence of substrate temperature on the optical quality of the films is evaluated and it is shown that the crystal structure of the first oxide layer is of importance for the optical quality of the nitride. The influence of preparation conditions and accelerated ageing has been modelled using the optical constants of thin films prepared under identical conditions as the films in the multilayer coatings.     

Electrochromic films of tungsten oxyfluoride and electron bombarded tungsten oxide

Reactive DC magnetron sputtering of W was performed in a plasma of Ar + O₂ with and without CF₄ addition and a positive substrate bias. The durability under extended Li⁺ intercalation/ deintercalation was excellent for electron bombarded oxide films and poor for oxyfluoride films, while the electrochromic colour/bleach dynamics was slow for the electron bombarded oxide but fast for the oxyfluoride. The range of optical modulation was large. Tandem films, with a thin protective layer of electron bombarded oxide covering a thicker oxyfluoride layer, were able to combine rapid dynamics with good durability.     

Ellipsometric spectroscopy study of cobalt oxide thin films deposited by sol–gel

Due to their unique optical properties, solar selective coatings enhance the thermal efficiency of solar photothermal converters. Hence it seems to be interesting to study the optical properties of promising materials as solar selective coatings. In an earlier work, it was demonstrated that sol–gel deposited cobalt oxide thin films possess suitable optical properties as selective coatings. In this work, cobalt oxide thin films were prepared by same technique and their optical properties were analyzed as a function of the dipping time of the substrate in the sol, using the spectroscopy ellipsometry, atomic force microscopy and X-ray photoelectron spectroscopy techniques. The optical constants (n and k) for these films, in the 200–800 nm range, are reported as a function of the dipping time. The fitting of ellipsometric data, I_s and I_c, for the glass substrate and the cobalt oxide thin film, as modeled with the Lorentz and Tauc–Lorentz dispersion relations, indicated that the film microstructure resembles a multilayer stack with voids. From these results, the Co₃O₄ and void percentages in the film were estimated. Both, thin film thickness and void/Co₃O₄ percentage ratio, were determined to be strongly dependent on the immersion time. Furthermore, the total thickness of a multilayered film was found to be the sum of thickness of each individual layer.     

Structural characterization and electrochemical properties of RF-sputtered nanocrystalline Co3O4 thin-film anode

Nanocrystalline Co₃O₄ thin-film anodes were deposited on Pt-coated silicon and 304 stainless steel by radio frequency (RF) magnetron sputtering. The as-deposited and annealed cobalt oxide thin films showed smooth and crack-free morphologies. Both the as-deposited and annealed films exhibited spinel Co₃O₄ phase with nanocrystalline structure. High-temperature annealing enhanced the crystallinity of RF-sputtered cobalt oxide films due to rearrangement of cobalt and oxygen atoms. Electrochemical characterization of RF-sputtered films was carried out by cyclic voltammetry and charge/discharge tests in the voltage range of 0.3–3.0 V. Cyclic voltammetry plots showed that the RF-sputtered Co₃O₄ thin films were electrochemically active. X-ray photoelectron spectrometer (XPS) showed that the fresh cobalt oxide films had two peaks of Co₃O₄. In addition to the binding energy of cobalt oxide, the XPS spectrum of discharged film presented two additional binding energies correspond to Co metal. The first discharge capacities of as-deposited, 300, 500, and 700 °C-annealed films were 722.8, 772.5, 868.4, and 1059.9 μAh cm⁻² μm⁻¹, respectively. High-temperature annealing could enhance the capacity and cycle retention obviously. After 25 cycles discharging, the annealed films showed better cycle retention than as-deposited film. The 700 °C-annealed film exhibited excellent discharge capacity approximated to the theoretical capacity.     

Substrate temperature effect on transparent heat reflecting nanocrystalline ITO films prepared by electron beam evaporation

In this study, indium tin oxide (ITO) thin films were preparedon glass substrate by electron beam evaporation technique and then were annealed in air atmosphere at 350 °C for 30 min. Increasing substrate temperature (T_s) from 25 to 380°°C reduced sheet resistance of ITO thin films from 150(Ω/□) to 14(Ω/□). The UV-visible-near IR transmittance and reflectance spectra were also confirmed that the substrate temperature has significant effect on the properties of heat reflecting thin films. High transparency (83%) over the visible wavelength region of spectrum and (over 90%) reflectance in near-IR region were obtained at T_s = 300° C. Plasma wavelength, carrier concentrations (n_e) and refractive index of the layer were also calculated. The allowed direct band gap at the temperature range 100–300° C was estimated to be in the range 3.71–3.89 eV. Band gap widening due to increase in substrate temperature was observed and is explained on the basis of Burstein-Moss shift. XRD patterns showed that the films were polycrystalline. High quality crystalline thin films with grain size of about 40 nm were obtained.     

Microwave-assisted low temperature fabrication of nanostructured α-Fe2O3 electrodes for solar-driven hydrogen generation

It is demonstrated for the first time that significant enhancement of photoelectrochemical performance could be achieved by using microwave-assisted annealing for the fabrication of α-Fe₂O₃ thin films. The process can also lead to significant energy savings (>60% when compared with conventional methods). Different types of Fe thin films were oxidized using both microwave and conventional heating techniques. The photoelectrochemical performance of electrodeposited, undoped and Si-doped iron oxide samples showed that microwave-annealing resulted in superior structural and performance enhancements. The photocurrent densities obtained from microwave annealed samples are among the highest values reported for α-Fe₂O₃ photoelectrodes fabricated at low temperatures and short times; the highest photocurrent density at 0.55 V vs. V_Ag/AgCl, before the dark current onset, was 450 μA cm⁻² for the Si-doped films annealed at 270 °C for 15 min using microwave irradiation (and 180 μA cm⁻² at 0.23 V vs. V_Ag/AgCl) while conventional annealing at the same temperature resulted in samples with negligible (3 μA cm⁻²) photoactivity. In contrast, a 450 °C/15 min conventional heat treatment only resulted in a film with 25% lower photocurrent density than that of the microwave annealed sample. The improved performance is attributed to the lower processing temperatures and rapidity of the microwave method that help to retain the nanostructure of the thin films whilst restricting the grain growth to a minimum. The lower processing temperature requirements of the microwave process can also open up the possibility of fabricating hematite thin films on conducting, flexible, plastic electronic substrates.     

H2 sensors based on WO3 thin films activated by platinum nanoparticles synthesized by electroless process

Platinum (Pt) nanoparticles were successfully synthesized on tungsten oxide (WO₃) thin films by electroless process without any further post-treatment. The prepared Pt nanoparticles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and field-emission scanning electron microscopy. Gas sensors based on the Pt–WO₃ films were found to provide repeatable and significant responses to ppm-level H₂. The size of Pt nanoparticles increases with the deposition time and has improved the sensing characteristics of the sensors. The work in this paper paves a facile way to the fabrication of Pt nanoparticles on metal oxide surface at a low temperature (68 °C).     

Iron pyrite FeS2 for flexible solar cells

We used the vacuum thermal evaporation technique to grow iron thin films on heated glass substrates. Thick fresh iron layers (1000–3000 Å) sulfurized under nitrogen flux were converted into iron pyrite FeS₂ phase identified by X-ray study.Reproducible 1–2 kω (10⁻² ω-cm) FeS₂ layers showed P type coductivity. Nevertheless, iron films previously oxidized showed N type behaviour when submitted to the same sulfurization process. Optical transmission measurements had been carried out on several thicknesses before and after layer conversion. The results confirmed the highly absorbing character of the pyrite FeS₂ and also the presence, in the explored spectral range (UV-Visible) of two competitive transitions: the first at 1.31 eV (indirect gap) abd the second at 1.45 eV (direct gap). Some interpretations are given concerning the origin of the opacity of the FeS₂ obtained from Fe films. Comparison is made with iron pyrite results obtained by spray pyrolysis as reported by some workers.     

Raman and XPS characterization of vanadium oxide thin films deposited by reactive RF sputtering

In this paper we report on Raman and XPS characterization of vanadium oxide thin films deposited by RF-sputtering. The samples were deposited by using a vanadium target in different oxygen fluxes, so that the stoichiometry (O/V ratio) of the oxide was varied. Several physical parameters of the films indicate a strong structural difference between the sample deposited at lower oxygen flux (1 scc m) and those obtained with higher flux (from 1.25 to 9 scc m). The increase of O/V ratio corresponds to a lower crystallinity of the thin films as indicated by the initial lowering and the final disappearance of the characteristic Raman mode of V₂O₅ (crystal) at about 140 cm⁻¹. For the highest flux samples new broad bands develop, typical of amorphous materials, both in polarized as well as in depolarized Raman spectra.     

Sputtering of thin pyrite films

Argon and reactive ion beam sputtering of thin pyrite (FeS₂) and other iron sulfide films is reported. Films were characterized by X-ray diffraction, Rutherford backscattering, measurements of the temperature-dependent conductivity and optical spectroscopy.