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.     

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.     

Structural and optical properties of MgyNi1–yHx gradient thin films in relation to the as-deposited metallic state

Thin Mg_yNi_1−yH_x films with a gradient in chemical composition are investigated by optical spectrophotometry, dc resistivity and X-ray diffraction measurements before and after exposure to hydrogen. In the metallic state crystalline Mg₂Ni is present for 0.6 ≤ y ≤ 0.8 and coexists with amorphous Mg and/or Ni. The hydride state is mainly characterized by the presence of Mg₂NiH₄ around the stoichiometric [Mg]/[Ni] = 2 composition and some MgH₂ on the Mg-rich side. The abrupt microstructural changes found in the as-deposited metallic state around the Mg–Mg₂Ni eutectic point correlate well with the compositional dependence of the optical properties in the hydride state. We conclude that the formation of the hydride depends directly on the detailed nature of the metallic parent phase. Furthermore, we demonstrate that high-throughput compositional screening via fiber-optic spectrophotometry is useful for hydride identification. When no structural long-range order is present, this provides a new tool for the search for hydrogen storage materials.     

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.     

Structure–property relationships in electrochromic WO3 films deposited by reactive sputtering

WO_x electrochromic (EC) films deposited by DC magnetron sputtering technique were investigated by XRD and STM measurements. The reversible microstructure changes of the WO_x film between the bleached and colored EC states were revealed. The study indicates that the amorphous as-deposited WO_x film (a-WO_x) is of amorphous microstructure both in bleached and colored states; however, the crystalline WO_x (c-WO_x) is stoichiometric triclinic lattice WO₃ in bleached state (the lattice parameters: a=7.2944 Å, b=7.4855 Å, c=3.7958 Å, α=89.38°, β=90.42°, γ=90.80°), and changes into nonstoichiometric tetragonal lattice WO_2.9 in colored state (a=b=5.336 Å, c=3.788 Å, α=β=γ=90°). The surface morphologies of the colored WO_x films are very different from those of the bleached WO_x films.     

Thermochemical two-step water splitting by ZrO2-supported NixFe3−xO4 for solar hydrogen production

A thermochemical two-step water-splitting cycle using a redox metal oxide was examined for Ni(II) ferrites or Ni_xFe_3−xO₄ (0  x  1) for the purpose of converting solar high-temperature heat to hydrogen. The Ni(II) ferrite was decomposed to Ni-doped wustite (Ni_yFe_1−yO) at 1400 °C under an inert atmosphere in the first thermal-reduction step of the cycle; it was then reoxidized with steam to generate hydrogen at 1000 °C in the second water-decomposition step. Although nondoped Fe₃O₄ powders formed a nonporous, dense mass of iron oxide by the fusion of FeO and its subsequent solidification after the thermal-reduction step, Ni(II)–ferrite powders were converted into a porous, soft mass after the step. This was probably because Ni doping in the FeO phase raised the melting point of wustite above 1400 °C. Supporting the Ni(II) ferrites on m-ZrO₂ (monoclinic zirconia) alleviated the high-temperature sintering of iron oxide; as a result, the supported ferrites exhibited greater reactivity and assisted the repeatability of the cyclic water splitting process as compared to the unsupported ferrites. The reactivity increased with the doping value x, and was maximum at x = 1.0 in the Ni_xFe_3−xO₄/m-ZrO₂ system.     

Electrochromic properties of NiOx prepared by low vacuum evaporation

Nickel oxide (NiO_x) thin films were prepared by a low vacuum evaporation (LVE) onto fluorine doped tin oxide (FTO) coated glass substrates. Electrochromic test device (ECTD) was constructed by using these films as working electrodes, together with the FTO/glass as an opposite (counter) electrode in alkaline environment (0.1 M NaOH aqueous solution). Those films exhibited anodic electrochromism, changing color from transparent white-yellowish to dark brown. The coloration and bleaching times, defined as time to reach of the final change in transmittance, were found to be 1.7 and 4.2 s, respectively.The electrochromic characteristics were recorded by using cyclic voltammetry. The optical transmission spectra of the bleached and colored states were recorded in the visible part of the spectrum. The Fourier-transform infrared (FTIR) reflection spectra of the sample in the bleached and colored states were measured at near normal incidence angle. The dependence of the transmission of the sample at wavelength λ=670 nm on the electric charge transported to the anode were also examined. The coloration efficiency was calculated to be 32.4 cm²/C from the slope of the graphical presentation of the optical density as a function of the charge density, during the charge extraction. The spontaneous ex situ change of coloration with time and stability of the sample in the bleached and colored states were also examined.     

Characterization and electrochromic properties of CuxNi1−xO films prepared by sol–gel dip-coating

Cu_xNi_1−xO electrochromic thin films were prepared by sol–gel dip coating and characterized by XRD, UV–vis absorption and electrochromic test. XRD results show that the structure of the Cu_x Ni_1−xO thin films is still in cubic NiO structure. UV–vis absorption spectra show that the absorption edges of the Cu_xNi_1−xO films can be tuned from 335 nm (x = 0) to 550 nm (x = 0.3), and the transmittance of the colored films decrease as the content of Cu increases. Cu_xNi_1−xO films show good electrochromic behavior, both the coloring and bleaching time for a Cu_0.2Ni_0.8O film were less than 1 s, with a variation of transmittance up to 75% at the wavelength of 632.8 nm.     

Composition, surface topography, structure, Raman, and electrochemical/photoelectrochemical characterisation of CdxHg1−xTe films

Cd-rich Cd_xHg_{1 − x}Te films have been electrodeposited under potentiostatic conditions on conducting glass and Ti substrates from an acidic solution containing the respective ions as Cd²⁺:Hg²⁺:HTeO₂⁺ = 100:1:2. Six films one after another have been prepared from a single electrochemical cell. EDAX analysis of the air annealed films show decreasing Hg content in the deposit as the number of film preparation increases. SEM analysis indicate undulatory surface with Hg-rich clusters at the top surface. XRD analysis indicate the presence of Cd_xHg_{1 − x}Te along with . The Cd_xHg_{1 − x}Te alloy formation have been confirmed from Raman shift measurements which change with composition, x. The as-deposited films are n-type but converts to p-type after air annealing. Spectral response measurements gave band gap values that change with Hg content in the deposit. Band gap values ranging from 1.1 eV to 1.45 eV have been estimated. Photoelectrochemical solar cells using polysulphide electrolyte have been fabricated which gave an open-circuit photovoltage and short-circuit photocurrent, respectively, as 325 mV and 5.5 mA/cm² under 60 mW/cm² intensity of illumination.     

Synthesis and assessment of La0.8Sr0.2ScyMn1−yO3−δ as cathodes for solid-oxide fuel cells on scandium-stabilized zirconia electrolyte

Perovskite-type La_0.8Sr_0.2Sc_yMn_1−yO_3−δ oxides (LSSMy, y = 0.0–0.2) were synthesized and investigated as cathodes for solid-oxide fuel cells (SOFCs) containing a stabilized zirconia electrolyte. The introduction of Sc³⁺ into the B-site of La_0.8Sr_0.2MnO_3−δ (LSM) led to a decrease in the oxides’ thermal expansion coefficients and electrical conductivities. Among the various LSSMy oxides tested, LSSM0.05 possessed the smallest area-specific cathodic polarization resistance, as a result of the suppressive effect of Sc³⁺ on surface SrO segregation and the optimization of the concentration of surface oxygen vacancies. At 850 °C, it was only 0.094 Ω cm² after a current passage of 400 mA cm⁻² for 30 min, significantly lower than that of LSM (0.25 Ω cm²). An anode-supported cell with a LSSM0.05 cathode demonstrated a peak power density of 1300 mW cm⁻² at 850 °C. The corresponding value for the cell with LSM cathode was 450 mW cm⁻² under the same conditions. The LSSM0.05 oxide may potentially be a good cathode material for IT-SOFCs containing doped zirconia electrolytes.     

Oxidation states in nickel oxide electrochromism

Nickel oxide, displaying anodic colouration, shows promise as a complementary counter electrode for tungsten oxide in electrochromic devices. Consideration of the thermodynamic possibilities for nickel, as expressed in the Pourbaix diagram, suggests that the commonly-accepted reaction schemes for the colouration of nickel oxide (NiOx): Ni(OH)₂ + OH⁻ ↔ NiOOH + e⁻ H₂O and Ni(OH)₂ ↔ NiOOH + H⁺ + e⁻ may not represent the whole picture. Electrochemical Impedance Spectroscopy (EIS) plots for both sputtered and electrodeposited NiOx films have been obtained. These suggest that two distinct reactions appear to occur at different overvoltages. These seem consistent with conversion of Ni₃O₄ to Ni₂O₃; and of the latter to NiO₂. Differences, as yet to be elucidated, are apparent between the sputtered and the electrodeposited films. The EIS data agree well with the results obtained by others [15,16]. Potentiostatic polarization of sputtered NiOx in KOH solution lends support to the idea, derived from EIS, of two transitions. By progressively raising the electrode potential it has proved possible to isolate these steps. Both visual and spectrophotometric observations indicate that the colouration appears to occur in two steps. The transmittance data agree well with those of others [18]. The species involved appear to be Ni₃O₄ as the bleached state with Ni₂O₃ and NiO₂ as dark states. The band structure implications remain to be worked out. It is suggested, however, that the present concept may assist in the consideration of NiOx electrochromism.     

Effect of S/In concentration ratio on the physical properties of AgInS2-sprayed thin films

 AgInS₂ thin films have been prepared on glass substrates by the spray pyrolysis process using an aqueous solution, containing silver acetate (AgCH₃CO₂), thiourea (SC(NH₂)₂) and indium chloride (InCl₃) as precursors. The depositions were carried out at the substrate temperature of 420 °C. The value of the concentration ratio in the spray solution of indium and silver elements x=[Ag⁺]/[In³⁺] was equal to 1.3, whereas y=[S²⁻]/[In³⁺] varied between 4 and 7. The structural study (XRD, EPMA and AFM ) shows that all films obtained using y=4 with a nearly stoichiometric composition consist essentially of AgInS₂ chalcopyrite compound and they exhibit in the as-deposited state, the best crystallinity with a (1 1 2) preferential orientation. On the other hand, films obtained using y higher than y=4 exhibit p-type character. Moreover, the optical analysis via the transmittance, reflectance reveals that the band-gap energy E_g increases slightly as a function of y composition (E_g varies from 1.87 to 2.07 eV).     

Changes in electronic structure of the electrochemically Li-ion deintercalated LiNiO2 system investigated by soft X-ray absorption spectroscopy

The electronic structures of Li_1−xNiO₂ system after electrochemically deintercalated to various “x” values have been investigated using soft X-ray absorption spectroscopy (XAS) for oxygen K-edge and Ni L_II,III-edge. By comparing the O K-edge XAS spectra of Li_1−xNiO₂ electrode collected simultaneously by the partial electron yield (PEY) mode and florescence mode (FY), it was found that the surface electronic structure of Li_1−xNiO₂ electrode is different from the bulk. Therefore, if the PEY mode, which is a surface-sensitive technique, is used alone, the reliability of the results is limited to the surface structures only. Ni L_II,III-edge XAS results of Li_1−xNiO₂ electrode at different charge states, in both the FY and PEY modes, show that Ni²⁺ ions at the surface are mostly oxidized to Ni³⁺ ions during Li deintercalation, whereas the process in the bulk is the oxidation of Ni ions from Ni³⁺ to Ni⁴⁺.     

Properties of CuIn1−xGaxSe2

Polycrystalline bulk samples of CuIn_1−xGa_xSe₂ weregrown with nominal x = 0.15, 0.25 and 0.5. Mobility, conductivity and band gap were measured at room and low temperatures. Mobilities for x = 0.21 were several hundred cm² V⁻¹s⁻¹ at room temperature and for x = 0.15 were 10³ cm² V⁻¹ s⁻¹, all n type. The band gaps were estimated from the spectra of photoelectrochemical cells at room temperature (with 8.5 K photoluminescence estimates shown in brackets), as 1.10 eV (1.14) for x = 0.21, and 1.07 eV (1.093) for x = 0.15. Crystal mechanical properties as regards cracks were not as good as for CuInSe₂, using similar growth techniques.     

High-temperature optical properties and stability of AlxOy–AlNx–Al solar selective absorbing surface prepared by DC magnetron reactive sputtering

Al_xO_y–AlN_x–Al selective absorbing surface was prepared by DC magnetron reactive sputtering with aluminum alloy (LY13)¹ in air and argon. The studies were carried out to access the high-temperature (400°C–600°C) optical properties and stability of the coatings. The coatings were found to withstand heating at 600°C for 30 min in 4.5×10⁻³ Pa vacuum with absorptance 0.94 and emittance 0.07 after annealing. After heating at 450°C for 10 h, the specimen still had good performance whose absorptance and emittance was 0.93 and 0.07, respectively. Auger electron spectroscopy was used to analyse the structure of the solar selective surface before and after annealing.     

Investigations on ZnxCd1−xO thin films obtained by spray pyrolysis

Zn_xCd_1−xO thin films were prepared on glass substrates by spray pyrolysis technique. The precursor solutions were obtained by varying the concentration of Zn(NO₃)₂·6H₂O and Cd(NO₃)₂·4H₂O in bi-distilled water. The structural properties have been studied using X-ray diffraction spectra. All the structures include the basic compounds, i.e. ZnO and CdO. The orientation and the crystalline phases of the deposited films were specified. With the addition of Zn to the precursor solution, we can observe the preferential orientation of the CdO in the [2 0 0] direction. The electrical measurements were performed using method of four contacts. Thin films transmittances, in the 1.5–4.3 eV range, for different compositions have been measured and the optical gaps have been determined. The variations are explained considering the gaps of the two pure films. The influence of increased Cd concentration in the films on the structural, electrical and optical properties is investigated in this study.     

Electrical and photoluminescence properties of evaporated CuIn1−xGaxTe2 thin films

Polycrystalline thin films of CuIn_1−xGa_xTe₂ have been deposited by flash evaporation on Corning glass 7059 substrates at T_s=200°C. Hall and resistivity measurements have been carried out down to 77 K. These films are p-type and the variation of the resistivity may be linked to defects, disorder of the material or grain boundaries. The PL spectra of these films after annealing in argon atmosphere at T_a=450°C have showed a broad band emission between 0.98 and 1.12 eV in which the main peak appears at 1.05 eV (at 4.2 K).     

Characterization of Pr1−xSrxCo0.8Fe0.2O3−δ (0.2 ≤ x ≤ 0.6) cathode materials for intermediate-temperature solid oxide fuel cells

Cathode materials consisting of Pr_1−xSr_xCo_0.8Fe_0.2O_3−δ (x = 0.2–0.6) were prepared by the sol–gel process for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The samples had an orthorhombic perovskite structure. The electrical conductivities were all higher than 279 S cm⁻¹. The highest conductivity, 1040 S cm⁻¹, was found at 300 °C for the composition x = 0.4. Symmetrical cathodes made of Pr_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (PSCF)–Ce_0.85Gd_0.15O_1.925 (50:50 by weight) composite powders were screen-printed on GDC electrolyte pellets. The area specific resistance value for the PSCF–GDC cathode was as low as 0.046 Ω cm² at 800 °C. The maximum power densities of a cell using the PSCF–GDC cathode were 520 mW cm⁻², 435 mW cm⁻² and 303 mW cm⁻² at 800 °C, 750 °C and 700 °C, respectively.     

X-ray absorption fine structure and neutron diffraction analyses of de-intercalation behavior in the LiCoO2 and LiNiO2 systems

Variations of electronic and local structures of Ni and Co in Li_{1 − x}NiO₂ and Li_{1 − x}CoO₂ as a function of x were clarified for the first time by in situ X-ray absorption fine structure analysis (XAFS). Chemical shifts of the X-ray absorption near edge sructure spectra of the Ni K-edge in Li_{1 − x}NO₂ as a function of x were continuous while an abrupt change was observed for the Co K-edge spectra of Li_{1 − x}CoO. Radial structure functions obtained by Fourier-transform of the Ni K-edge EXAFS of LiNiO₂ exhibited abnormally low height of the Ni-O peak. This phenomenon is explained by the Jahn—Teller distortion of the NiO₆ octahedron due to the low spin Ni³⁺ (d⁷) ion. De-intercalation of the Li ion caused oxidation of Ni³⁺ (d⁷) to Ni⁴⁺ (d⁶) and reduced the distortion, hence the Ni-O peak increased with increasing x value. The crystal structures of LiNiO₂ treated with D₂O, and electrochemically de-intercalated Li_0.34NiO₂ were solved by Rietveld analysis of neutron diffraction data.     

PtxNi1−x nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane

We report on Pt_xNi_1−x (x = 0, 0.35, 0.44, 0.65, 0.75, and 0.93) nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane (NH₃BH₃). The Pt_xNi_1−x catalysts were prepared through a redox replacement reaction with a reverse microemulsion technique. The structure, morphology, and chemical composition of the obtained samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) equipped with energy dispersive X-ray (EDX), and inductively coupled plasma emission spectroscopy (ICP). The results show that the diameters of the Pt_xNi_1−x nanoparticles are about 2–4 nm, and the Pt atomic contents in the catalysts were 35%, 44%, 65%, 75%, and 93%, respectively. It is found that the catalytic activity toward the hydrolysis of NH₃BH₃ is correlated with the composition of the Pt_xNi_1−x catalysts. The annealing of Pt_0.65Ni_0.35 at 300 °C for 1 h increases the crystallinity of the nanoparticles, but shows almost the same activity as that without annealing. Among the as-prepared Pt_xNi_1−x nanoparticles, Pt_0.65Ni_0.35 displays the highest catalytic performance, delivering a high hydrogen-release rate of 4784.7 mL min⁻¹ g⁻¹ and a low activation energy of 39.0 kJ mol⁻¹.