All solid state electrochromic devices on glass and polymeric foils

Tungsten oxide (WO₃), vanadium and nickel-hydroxide (VO_xH_y and NiO_xH_y)-films were evaporated on glass and polymeric substrates covered with indium-tin oxide (ITO). Films of nickel-oxide (NiO_x) were reactively sputtered from a nickel target. In order to obtain electrochromic devices the WO₃ film was used as one electrode and with a polymeric solid state electrolyte (PSSE) glued to each of the other films which served as different counter electrodes. The films for themselves and the complete devices were investigated by optoelectrochemical and other methods. The most stable device was the WO₃–VO_xH_y system which even improved the electrochromic properties after 3×10⁴ cycles.     

Isothermal transient ionic current study of laminated electrochromic devices for smart window applications

The isothermal transient ionic current (ITIC) in three different kinds of laminated electrochromic devices has been determined. The devices consisted of one 300 nm thick layer of nickel oxide and one 300 nm thick layer of WO₃ deposited onto separate In₂O₃:Sn (ITO) covered glass substrates by DC magnetron sputtering at room temperature. They were then laminated with a polymeric ion conductor, acting as electrolyte, in symmetric and asymmetric configurations, i.e. WO₃/WO₃, NiO_xV_yH_z/NiO_xV_yH_z and WO₃/NiO_xH_y. The electrolyte was prepared by mixing polyethylene glycol of average molecular weight 400 (PEG 400) and lithium triflouromethanesulfonate (LiSO₃CF₃) for 12 h at 70°C and with a ratio oxygen/lithium (O/Li)=10. The samples were first polarized, i.e. the ions are transported to one of the electrodes, which in the asymmetric devices is the nickel oxide electrode. At the first applied potential step, the ions move through the electrolyte towards the opposite electrode. The potential is then switched and the ions move back to the first electrode. The ITIC curves are found to depend on the electrode material and in the asymmetric case also the direction of the ion current.     

Electrochromic properties of NiOxHy thin films

NiO_xH_y films were prepared by DC magnetron sputtering in H₂/O₂ atmosphere. NiO_xH_y coatings with transparency and high electrochromic efficiency were obtained by changing H₂ content. A 60 nm thick NiO_xH_y film with transmittance of 0.57 (as-deposited state), 0.78 (bleached state) and 0.24 (coloured state) at wavelength of 550 nm was deposited in an atmosphere of H₂(60%)+O₂(40%). Analysis of infrared spectra (60002400 cm⁻¹) showed that the absorption peaks for bleached and colored states are associated with free ‘OH’ and OH stretching vibrations, respectively. XPS Ni2p core level spectra of colored NiO_xH_y film exhibited a peak at 856.2±0.2 eV which is attributed to Ni³⁺. Ni2p core level spectra of the bleached and as-deposited films exhibited two peaks at 856.4±0.2 and 854.6±0.2 eV which are attributed to Ni³⁺ and Ni²⁺.     

Improved electrochromic films of NiOx and WOxPy obtained by spray pyrolysis

Electrochromic films of NiO_x and WO_xP_y were produced by the spray pyrolysis technique. The nickel-oxide-based coatings were obtained from both an alcoholic solution of nickel nitrate and aqueous solution of the mixture nickel nitrate/cobalt nitrate. Coatings obtained from alcoholic solutions showed a noticeable contrast of optical transmittance from fully bleached to colored state. X-ray diffraction analysis showed a slight crystallization in NiO_x after electrochemical treatment: one diffraction peak for as-deposited films turned to three diffraction peaks for electrochemical treated samples. Coatings obtained from aqueous solution of mixture nickel nitrate/cobalt nitrate showed an optimized electrochromic behavior at a Ni:Co proportion of 90:10. At this condition an optical contrast of 50% is found. X-ray diffraction showed that these samples comprised a phase mixture of Co₃O₄ and NiO.WO_xP_y samples were obtained from polytungsten gel in which H₃PO₄ was added. We found that for 8.3 at% of P:W, the electrochromism was optimized. Pyrolytic coatings of WO_xP_y show superior behavior than those of WO_x obtained by spray pyrolysis, both in optical contrast and durability.     

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.     

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.     

Preparation and characterisation of aluminium zirconium oxide for metal-oxide-semiconductor capacitor

A functional type metal-oxide-semiconductor (MOS) based capacitor was fabricated and studied by using aluminium zirconium oxide (Al_xZr_yO_z) as a potential high dielectric constant (k) gate oxide, which was transformed from as-sputtered Al-Zr alloy after undergoing a wet oxidation at 400°C, 600°C, 800°C, and 1000°C in the presence of nitrogen as a carrier gas. A mixture of tetragonal ZrO₂-monoclinic Al_xZr_yO_z phases were present at 600°C while stablized tetragonal Al_xZr_yO_z phases were detected at higher temperatures with a minute micro strain change. The largest k value (21) was obtained by the film oxidised at 600°C, followed by 800°C while the lowest one at 1000°C. The discrepancy was due to the absence of tetragonal ZrO₂ in the latter films. The attainment of a k value closer to the reported value for ZrO₂ at 600°C suggested that the tetragonal ZrO₂ phase was one of the factors yielding a high k value at 600°C. However, further investigation was required for this sample because the slow trap density and total interface trap density was high despite a high k value, mainly attributed to the presence of negatively charged traps as the scattering centre in the film. The film obtained at 1000°C was not encourageable to be deployed as a passivation layer for Si MOS device due to its low k controlled by the thick interfacial layer.     

Optical properties of electrochromic all-solid-state devices

We have investigated the optical properties of an all-thin-film electrochromic device, with a thin film of ZrO₂ acting as an ion conductor. The device also employed electrochromic layers of amorphous or crystalline WO₃ and NiV_xO_yH_z. Transmission (T) and reflection (R) spectra were recorded in the wavelength range 300–2500 nm at different intercalation levels, both for single films and complete devices. The results show that T decreases significantly upon intercalation in the WO₃ thin films as well as in the devices. The reflectance only shows minor changes.     

Electrochromism of sol–gel derived niobium oxide films

Niobium oxide films are promising cathodic electrochromics that in many aspects can compete with the more frequently studied WO₃ films. The films reported herein were prepared using the sol–gel route from a NbCl₅ precursor. The electrochromic properties were pronounced for crystalline films heat-treated at 500°C and exhibited transmittance changes between coloured and bleached states of 60% in the ultraviolet (UV) and 80% in the visible (VIS) and near-infrared (NIR) regions. Improved bleaching and more reversible electrochromism of thick niobium oxide films (thickness (d)>250 nm) were obtained by lithiation.Electrochromic (EC) devices were also prepared by assembling niobium oxide and lithiated niobium oxide films of different thicknesses with a hybrid inorganic/organic Li⁺ ionic conductor (organically modified electrolyte-ormolyte) and a molybdenum and antimony doped tin oxide (SnO₂ : Sb(7%) : Mo(10%) counter electrode films. The EC devices exhibited adequate colouring/bleaching kinetics (<2 min) and colouring/bleaching changes up to 40–50%.     

Pre-existence of HxWO3 in e-beam deposited WO3 films

Structural and optical properties of e-beam deposited tungsten trioxide (WO₃) films in as-deposited and electrochemically coloured states were investigated by spectrophotometric and XRD techniques. These investigations have shown the as-deposited WO₃ films to be porous and with small amount of H_xWO₃ pre-existing in them. The films further facilitate insertion of H⁺ ions on colouration resulting in tetragonal H_xWO₃ with a = 4.74Å and c = 3.19Å.     

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.     

A simple preparation of Co0.75Fe0.25 hydrous oxide nanoparticles as active electrocatalysts for water oxidation reaction

A serious of CoFe hydrous oxide nanoparticles (Co_1−zFe_zO_xH_y NP [z = 0-1]) were synthesized by stirring CoCl₂ and FeCl₂ in aqueous KOH solutions for 0.5 hours, and the resulting nanoparticles are active electrocatalysts for water oxidation reaction (WOR) in alkaline solutions. The Co_0.75Fe_0.25O_xH_y NP, which are the most active electrocatalysts, have a layered double hydroxide (LDH) structure, with the average particle sizes about 16 nm. When using Ni foam as the substrate, the electrode with Co_0.75Fe_0.25O_xH_y NP as catalysts exhibited an overpotential of 284 mV for WOR in 1 M KOH at 10 mA cm⁻². The Tafel slope was 43 mV dec⁻¹, and the TOF was 1.30 s⁻¹ at 350 mV overpotential (loading Co + Fe = 0.133 mg cm⁻²). The Co_0.75Fe_0.25O_xH_y NP are robust toward long-term galvanostatic test and showed less than 2% dissolution in 10 hours. Preliminary kinetic study suggests that the presence of Fe in Co_1−zFe_zO_xH_y NP alters the WOR mechanistic pathways. The increased number of electrochemically active sites endowed by the LDH structure and the electronic interactions between Co and Fe as suggested by XPS, both contribute to the enhanced WOR activity.     

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.     

Hydrogen evolution reaction on single crystal WO3/C nanoparticles supported on carbon in acid and alkaline solution

Single crystal tungsten oxide (WO₃) nanoparticles were prepared via a microwave-assisted method. Electrochemical activity for hydrogen evolution reaction (HER) on WO₃ supported on carbon black (WO₃/C) electrocatalyst was first studied in acid solution (0.5 M H₂SO₄) and alkaline solution (1.0 M KOH) at room temperature. The overall experimental results revealed that the electrocatalytic activity for HER on WO₃/C is one order magnitude higher than those obtained with carbon black in 0.5 M H₂SO₄ and is six times than in the case of carbon black in 1.0 M KOH. These results demonstrated that WO₃ could enhance the electrocatalytic activity for hydrogen evolution reaction in acid solution (0.5 M H₂SO₄) and alkaline solution (1.0 M KOH). On the other hand, the kinetic reaction mechanisms were discussed on WO₃/C electrocatalysts and carbon black in acid solution and alkaline solution for HER. Consequently, the rate-determining step changed from Tafel to Volmer due to the incorporation of WO₃ in acid solution. However, the rate-determining step carries through Tafel reaction on both electrocatalysts in alkaline solution though WO₃ was introduced.     

Electrochromism in NiOx and WOx obtained by spray pyrolysis

Electrochromic films of NiO_x and WO_x were produced by the spray pyrolysis technique. The nickel-oxide-based coatings were obtained from an aqueous solution of nickel nitrate. Those obtained below 300° C did not show any diffraction peak when subjected to X-ray diffraction analysis, and those obtained above 400° C showed a diffraction pattern corresponding to cubic NiO. Films obtained below 300° C showed an electrochromic effect with an electrochromic efficiency of 30 cm²/C.Tungsten-oxide-based coatings were obtained from a solution of H₂WO₄ in aqueous ammonia. The films were grown at 150° C, and they showed a diffraction pattern corresponding to monoclinic WO₃ when subjected to a post-heat treatment at 400° C during ten minutes. The WO_x films showed a noticeable electrochromism under cation insertion, and presented an electrochromic efficiency of 42 cm²/C. Both as-deposited and heat-treated samples showed good electrochromism.     

Annealing effect on electrochromic properties of tungsten oxide nanowires

The effect of thermal annealing on the electrochromic properties of the tungsten oxide (WO_3−x) nanowires deposited on a transparent conducting substrate by vapor evaporation was investigated. The X-ray diffraction (XRD) indicated that the structures of the nanowries annealed below 500 °C had no significant change. The X-ray photoelectron spectroscopy (XPS) analysis suggested that the O/W ratio and the amount of W⁶⁺ ions in the annealed nanowire films could be increased as increasing annealing temperature. Increased annealing temperature could promote the coloration efficiency and contrast of the nanowire films; however, it could also affect the switching speed of the nanowire films.     

Pd–WO3/reduced graphene oxide hierarchical nanostructures as efficient hydrogen gas sensors

Pd–WO₃ nanostructures were incorporated on graphene oxide (GO) and partially reduced graphene oxide (PRGO) sheets using a controlled hydrothermal process to fabricate effective hydrogen gas sensors. Pd–WO₃ nanostructures showed ribbon-like morphologies and Pd–WO₃/GO presented an irregular nanostructured form, while Pd–WO₃/PRGO exhibited a hierarchical nanostructure with a high surface area. Gas sensing properties of thin films of these materials were studied for different hydrogen concentrations (from 20 to 10,000 ppm) at various temperatures (from room temperature to 250 °C). Although adding GO in the Pd–WO₃, after hydrothermal process could increase the film conductivity, gas sensitivity was reduced to half, due to lower surface area of the irregular morphology in comparison with the ribbon-like morphology. The Pd–WO₃/PRGO films showed an optimum sensitivity (∼10 folds better than the sensitivity of Pd–WO₃/GO), and a fast response and recovery time (<1 min) at low temperature of 100 °C. Moreover, the Pd–WO₃/PRGO-based gas sensor was sensitive to 20 ppm concentration of hydrogen gas at room temperature. The results confirmed the effect of residual oxygen-containing functional groups of PRGO on the growth and morphology of Pd–WO₃ as well as gas sensing properties of metal oxide/graphene based hybrid nanostructures.     

Promotion of electrochromism in spray-deposited molybdenum oxide-doped iridium oxide thin films

Electrochromic molybdenum oxide-doped iridium oxide thin films were prepared by using a pneumatic spray pyrolysis technique onto fluorine-doped tin oxide (FTO) coated conducting glass substrates. An aqueous solution of 0.01 M ammonium molybdate was mixed with 0.01 M iridium trichloride in different volume proportions and the resultant solution was used as a precursor for spraying. An aqueous electrolyte (0.5 N H₂SO₄) was used to study electrochromic properties of thin films using cyclic voltammetry (CV), chronoamperometry (CA) and spectrophotometry. During the potential scan the iridium oxide electrode switches between coloured and bleached state due to Ir⁺⁴–Ir⁺³ intervalency charge transfers. The optical density difference (ΔOD)_λ=630 nm and colouration efficiency was maximum for 2% molybdenum oxide-doped sample. Moreover, loss in charge density during extended cycling is less than undoped and other doped (>2%) samples.     

Properties of mixed-oxide MoO3/V2O5 electrochromic films coated from peroxo-polymolybdovanadate solutions

Peroxo-polymolybdovanadates with various mole ratios of Mo/V are formed by direct reaction of a mixture of metallic Mo and V with hydrogen peroxide solution. Homogeneous amorphous thin films about 0.4 μm are fabricated on an ITO glass substrate by a spin-coating technique using the peroxo-polymolybdovanadate solutions. After heat-treatment at a desired temperature (80–120°C) in air for 1 h, they show reversible electrochromism in an organic Li-electrolyte solution, and change color between greenish yellow and violet. The potential (E) versus composition (x, Li content per mole of Mo_yV_1−yO_z) diagrams for these mixed films could be approximated by two straight lines with different slopes, which suggests that in these films there are two kinds of sites with different site energies. The changes in electrochromism with heat-treatment are also discussed in relation to the micro-structure of the films.     

Effect of solution processed graphene oxide/nickel oxide bi-layer on cell performance of bulk-heterojunction organic photovoltaic

We report the solution processed graphene oxide (GO), NiO_x and GO/NiO_x bi-layer used as an anode interfacial layer in organic bulk-heterojunction solar cells. The bulk-heterojunction solar cells using GO, NiO_x and GO/NiO_x bi-layer exhibited the conversion efficiency of 2.33%, 3.10% and 3.48%, respectively. The cell efficiency is correlated with the matching of energy levels between ITO, hole transport layer and P3HT and thus a well-matched stack layer of ITO/GO/NiO_x/P3HT:PCBM/LiF/Al shows the best cell efficiency of 3.48% with the J_SC of 8.71 mA/cm², V_OC of 0.602 V and FF of 66.44%.