Synthesis and characterization of aluminum-doped CdO thin films by sol–gel process

Aluminum-doped cadmium oxide (CdO:Al) thin films are deposited on glass substrates by the sol–gel dip-coating method, taking cadmium acetate dihydrate as the precursor material. Aluminum nitrate has been taken as a source of Al-dopant. XRD pattern reveals the good crystallinity of CdO thin films. SEM micrograph showed the presence of faceted crystallites. Optical study shows 40–85% transparency with a bandgap value lying in the range 2.76–2.52 eV, depending upon the Al content in the films. Optimum percentage of Al was 5.22 for a maximum room temperature conductivity of 2.81×10³ (Ω cm)⁻¹. Hall measurement confirmed that the material is of n-type, with mobility and carrier concentrations lying in the range 413–14.7 cm²/V s, and 3.4×10¹⁹–8.11×10²⁰ cm⁻³, when percentage of Al varies in the range 1.32–7.24.     

Electrochromic thin films prepared by sol–gel process

New methods are shown for lower temperature preparation of amorphous tungsten oxide thin film and preparation of crystalline iridium oxide thin film by sol–gel process using metal chloride as the starting materials and ethanol as a solvent. These electrochromic materials were combined with gel solid electrolyte, and preparation of fully solid-state electrochromic display (ECD) was made. The transmittance of the ECD could be made to change by 35% by applying a voltage of 3 V for 0.2 sec.     

Investigation of sol–gel derived thin films of titanium dioxide doped with vanadium oxide

Thin films of titanium dioxide and titanium–vanadium oxide were obtained by a sol–gel method. The coatings are uniform, smooth with very good optical properties. The solutions of both kinds are stable for more than a year. Structure and vibrational properties were studied with the help of X-ray diffraction (XRD) analysis and infrared spectroscopy (IR). The refractive indices and film thicknesses were measured by an ellipsometer at a wavelength of 638.2 nm, as a function of annealing temperature. The optical properties were investigated by ultraviolet–visible (UV–VIS) spectroscopy.     

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.     

Electrical characterization and Poole–Frenkel effect in sol–gel derived ZnO:Al thin films

Electrical properties of ZnO:Al thin films, prepared by sol–gel dip-coating technique were studied in the range of 0.32% to 1.62% Al concentrations in the films. Room temperature electrical conductivity was found in the range of 0.08 to 1.39 S/cm for different aluminium concentrations in the films. I–E characteristics of the films at a constant temperature showed non-linearity, while non-linearity becomes more and more pronounced with increase in temperature and this could be explained by Poole–Frenkel model of thermionic emission. Presence of adsorbed oxygen and excess Al atoms at grain boundaries is assumed to be the cause of this effect. These atoms produce defect levels, which trapped electrons and created a potential barrier across the grain boundaries. In the presence of an external field, the barrier height was attenuated, resulting in the thermionic emission of electrons from the trapped level to the conduction band. The trapped potentials (φ_t) were calculated for different doping concentrations in the films. The thermoelectric power (TEP) measurement confirmed the n-type nature of the films and the room temperature Seebeck coefficient was found to be −91.65 μV/K.     

Titanium–tin oxide protective films on a black cobalt photothermal absorber

We report the effect of covering an electrodeposited black cobalt absorber film with a Ti : Sn oxide film at various atomic ratios prepared by the sol–gel dip process. The resulting composite was characterized in its optical, structural and morphological properties. After thermal treatment at 400°C, the uncoated black cobalt film is oxidized and Co₃O₄ is formed. Samples covered with Ti : Sn films and thermally treated at the same temperature suffered lower oxidation maintaining in great extent the original metallic cobalt structure. The optical properties of the resulting material were affected by the presence of the Ti : Sn coating, and the best protective film obtained was a transparent Ti : Sn (8 : 2) atomic ratio oxide film, with a 210 nm thickness. This composite system exhibits an absorptance value of 0.91 and an emittance value of 0.34 after a 100 h, 400°C thermal treatment. A photothermal material composed of a layer of black cobalt and a protective oxide film coating seems then a promising solar energy absorber capable of withstanding high operating temperatures (400°C).     

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.     

Solar photocatalytic degradation of gaseous formaldehyde by sol–gel TiO2 thin film for enhancement of indoor air quality

Solar photocatalytic degradation of formaldehyde in the gaseous phase has been investigated. The tested photoreactor is made of a borosilicate glass tube with the inner surface coated with a sol–gel TiO₂ thin film. In a pseudo-first-order Langmuir–Hinshelwood (L–H) model, the maximum reaction rate constant obtained is 0.148 min⁻¹ under an exposure to sunlight with solar UVA irradiance of 1.56 mW/cm². The solar photolysis effect is found to be negligible. It is also found that the sol–gel TiO₂ thin film has a lower apparent photonic efficiency of solar photocatalysis than a Degussa P25 TiO₂ coating. However, for the photonic efficiency taking into account the absorbed and scattered photons only and, in other words, excluding the transmitted photons, the thin film has a higher value. Based on a total of 28 measured data, an empirical-correlation equation has been developed to express the reactant residue with respect to the solar UVA irradiance and exposure time. A reasonable agreement between the correlation and experimental data is obtained. The findings of this investigation can be applied to design optimization of a honeycomb photoreactor made up of TiO₂-coated glass tubes or polygonal cells.     

Towards electrochromic stability in sol–gel-derived tungsten oxide films: cyclic voltammetric and spectrophotometric investigations

Hitherto unexplored irreversible changes during initial coloration/bleaching cycles for sol–gel-derived tungsten oxide (WO₃) films have been investigated using cyclic voltammetric and spectrophotometric techniques. Non-ideal features appearing in the initial five anodic (deintercalation) cycles in the voltammogram with simultaneous decreased optical transmission of the bleached films have been explained in terms of possible stoichiometric variations affecting the coloration efficiency (CE) of the films and the associated mechanisms. Electrochromic stability attained thereafter manifests in retraceable voltammograms and almost invariant value of the CE.     

Optical properties of mixed V/Ce oxide films prepared via inorganic sol–gel route

New mixed V/Ce films at 55, 38, and 32 at% of V were prepared via inorganic sol–gel route by dip-coating technique. The absorption edge of prepared films shifts towards higher wavelengths at higher concentration of added vanadium pentoxide. The indirect-allowed band gap (E_G) also changes in dependence of added vanadium oxide from 2.8 up to 2.3 eV.     

Sol–gel derived CuCoMnOx spinel coatings for solar absorbers: Structural and optical properties

A novel black coloured coating with the composition CuCoMnO_x was prepared using sol–gel synthesis. The coatings were deposited using the dip-coating technique from alcoholic sols based on Mn-acetate and Co- and Cu-chloride precursors. Thermogravimetric analysis showed that xerogels become crystalline at 316°C while X-ray diffraction analysis revealed that the coatings and powders correspond predominantly to CuCoMnO_x spinels. Rutherford back scattering (RBS) and transmission electron microscopic (TEM) studies combined with energy dispersive X-ray spectroscopy (EDXS) measurements confirmed that Cu, Mn and Co are present in the films in stoichiometric ratios close to that in the initial sols. IR spectroscopy has been employed to study the formation of sols by following the changes in the vibrational bands of the acetate groups during both thermal hydrolysis and the ageing of sols to xerogels. It was found that ageing of xerogels was accompanied by the formation of −COO⁻ bridging units, which at 250°C are no longer visible in the IR spectra but substituted by the vibrational modes characteristic for CuCoMnO_x. The solar absorptance (a_s) and thermal emittance (e_T) of the coatings when deposited on an Al-substrate are a_s=0.9 and e_T=0.05, which rank deposited black sol–gel CuCoMnO_x spinels among the promising candidates for spectrally selective absorber coatings for solar collectors and solar facades.     

Fabrication of tin oxide film by sol–gel method for photovoltaic solar cell system

Transparent conducting tin(IV) oxide thin films have been developed with a sol–gel method, which is a low-cost process for the electrode materials of solar cell substrate. The precursor solution was made of tin isopropoxide dissolved in isopropyl alcohol. The hydrolysis rate was controlled by the addition of triethanolamine. Dipping and spin-coating technique were applied to coat tin oxide on borosilicate glass. The resistivity of the thin film was lower than 0.01 Ω-cm and the transmittance was higher than 90% in a visible range.     

Preparation of c-axis-oriented zinc-oxide thin films and the study of their microstructure and optical properties

c-axis-oriented zinc-oxide (ZnO) thin films were prepared on microscope glass substrates by sol–gel deposition. A homogeneous and stable solution was prepared by dissolving zinc-acetate 2-hydrate (ZnAc) in 2-propanol and diethanolamine. Crystalline ZnO thin films were obtained following an annealing process at temperatures between 450°C and 550°C for 1 h. Increasing annealing temperature increased the grain size and the c-axis orientation. The X-ray diffraction analysis revealed single-phase ZnO with hexagonal zincite structure. The absorption edge analysis revealed that the optical band-gap energy for the films were between 3.26 and 3.28 eV and electronic transition was direct transition type.     

Optical transmittance and photoconductivity studies on ZnO : Al thin films prepared by the sol–gel technique

Polycrystalline ZnO : Al thin films have been prepared by the (Sol–gel) chemical deposition method. The ZnO : Al thin films are very transparent (90%) in the near UV, VIS and IR regions. The films are oriented along the c-axis ([0 0 2] direction) in the hexagonal structure. It is known that pure ZnO thin films are not chemically stable in corrosive media, but aluminium stabilizes the ZnO system and increases its electrical conductivity. Finally, the ZnO : Al thin films are reasonably stable under storage in air and in reactive atmospheres like O₂, H₂O, H₂ or in weak acids. Dark- and photo-conductivity of the ZnO : Al films are very high (1–100 Ω⁻¹ cm⁻¹), so that they can be used as transparent conductors in solar cells or in electrochromic devices.     

Electrical and optical properties of highly conducting CdO:F thin film deposited by sol–gel dip coating technique

Highly conducting fluorine-doped cadmium oxide (CdO:F) thin films were deposited by sol–gel dip coating technique on glass and Si substrates. F concentration in the films was varied from 2.0% to 13.8% as determined from energy dispersive X-ray analysis. X-ray diffraction pattern showed that the films were polycrystalline in nature. The optimum F concentration for obtaining maximum conductivity was found to be 9.7%. The corresponding electrical conductivity was found to be 1.088×10⁴ S/cm and mobility 60.41 V/cm². Analysis of UV-VIS-NIR spectrum of the film with F concentration 9.7% showed a direct band gap energy of 2.3 eV.     

Optically passive counter electrodes for electrochromic devices: transition metal–cerium oxide thin films

Films of W–Ce oxide and Ti–Ce oxide were made by reactive DC magnetron sputtering. Li⁺ intercalation/deintercalation showed that the electrochromic coloration efficiency depended on the Ce/W and Ce/Ti ratio, and at proper values of these ratios the films remained transparent irrespective of their ion content. Ti–Ce oxide combines this property with good electrochemical cycling durability, and this material can serve as a counter electrode in transparent electrochromic devices.     

Optical and electrochemical characteristics of sol–gel deposited iron oxide films

Homogenous, crack free iron oxide films are prepared by the sol–gel spin coating technique from a solution of iron iso-propoxide and isopropanol. The films were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV-visible (UV–Vis) spectroscopy and cyclic voltammetry (CV). XRD of the films showed that they had an amorphous structure. The optical constants refractive index (n) and extinction coefficient (k) were measured by scanning spectrometer in the wavelength range of 390–990 nm. The n and k values were found n =2.3±0.01 and k =0.2±0.002 at 650 nm. The electrochemical behavior investigated in 0.5 M LiClO₄ propylene carbonate (PC) electrolyte-CV examinations showed good rechargeability of the Li⁺/e⁻ insertion extraction process beyond 300 cycles. Spectroelectrochemistry showed that these films exhibit weak cathodic coloration in the spectral range of 350–800 nm.     

Studies of tin–transition metal–carbon and tin–cobalt–transition metal–carbon negative electrode materials prepared by mechanical attrition

Samples of Sn₃₀TM₃₀C₄₀ and of Sn₃₀Co₁₅TM₁₅C₄₀, with TM = 3d transition metals, were prepared by vertical-axis attritor milling. The structure and performance of these samples were studied by X-ray diffraction (XRD) and by electrochemical testing. The XRD patterns of Sn₃₀TM₃₀C₄₀ show an amorphous-like diffraction pattern only for the sample with TM = Co. The other prepared samples show broadened Bragg peaks of their main starting material, along with an amorphous-like background, even after 32 h of milling. Samples with TM = Co and TM = Ni show stable differential capacity versus potential plots and stable cycling for at least 100 cycles with reversible capacities of 425 and 250 mAh g⁻¹, respectively. All samples prepared with 15 at.% Co show good capacity retention for at least 100 cycles ranging from 270 mAh g⁻¹ for samples with TM = Ni to 500 mAh g⁻¹ for samples with TM = Ti. The differential capacity versus potential plots for all the prepared Sn₃₀Co₁₅TM₁₅C₄₀ samples show similar structure to that of Sn₃₀Co₃₀C₄₀ except when TM = Cu. This shows the possibility of preparing tin-based negative electrode materials using a combination of cobalt and TM, especially if one looks to reduce the cobalt content.     

Optical and structural properties of – thin films

In this study, the sol–gel spin-coating method has been used to make Ta₂O₅–CeO₂ thin films. These films have been prepared in various composition ratios to observe changes in their optical and structural properties. Reflectance and transmittance spectra were collected in the spectral range of 300–1000 nm and were accurately fit using the Tauc–Lorentz model. Film thicknesses, refractive indices, absorption coefficients, and optical band gaps were extracted from the theoretical fit. The highest refractive index value was found at 5% CeO₂ doping. The structure of the films was characterized by X-ray diffractometry and Fourier transform infrared spectrometry, while the surface morphology was examined through atomic force microscopy.     

Aging effect of diethanolamine stabilized sol on different properties of TiO2 films: Electrochromic applications

Diethanolamine derived clear precursor sol has been utilized for the deposition of TiO₂ films annealed at 470 °C for 5 min. Effect of the precursor sol's aging on different properties of the films has been examined in the present study. Films obtained from aged sol have exhibited superior electrochemical (diffusion coefficient—2.46×10⁻¹⁰ cm² s⁻¹) and electrochromic characteristics due to enhanced Li ion insertion upon application of electric field. The aged sol derived films have exhibited a higher optical modulation (40% at 550 nm) between the colored and bleached states. The ion storage capacities of the films derived from freshly prepared and aged sols are 4.1 and 8.1 mC cm⁻², respectively, upon applied voltage of ±1.5 V. X-ray diffraction studies have affirmed an increase in the TiO₂ crystallite size upon the use of aged sol for the deposition of films. FTIR investigations have confirmed the conversion of Ti–O–Ti to Ti–O network in the aged sol derived films. SEM studies have evidenced porosity changes in films obtained from the sol aged for different durations. The index of refraction as measured by the ellipsometry method corroborates the SEM results and shows reduced porosity (pore size—38 nm) in films derived from the sol just reaching the state of gelation. Thickness of the aged sol derived film is measured to be the highest i.e. 350 nm. Energy bandgaps of the films for both direct and indirect transitions tend to decrease as a function of sol's aging.