Effect of cobalt doping and annealing on properties of titania thin films prepared by sol-gel process

Undoped and cobalt doped titania (TiO₂) thin films have been prepared on Si(1 0 0) monocrystal and quartz substrate using the sol-gel deposition method and annealed in air at 450, 550, 650, 750, 850, 950 and 1050 °C. Several experimental techniques (AFM, XRD, Raman spectroscopy, XRR, EDX, XPS, XAS, UV-VIS spectroscopy) have been used to characterize these films. Further more the degree of light induced hydrophilicity was estimated by measuring the contact angle of a water droplet on the film. Increase of the annealing temperature and in smaller degree also cobalt doping predispose titania crystallite growth. The rutile phase was detected at lower temperatures in the cobalt doped films than in the undoped titania films. Cobalt in the cobalt doped TiO₂ was seen to be in Co²⁺ oxidation state, mainly in CoTiO₃ phase when films were annealed at temperatures higher than 650 °C. Cobalt compounds segregated into the sub-surface region and to the surface of the titania, where they formed islands. Cobalt doping red-shifted the fundamental absorption edge further into the visible range, however it did not enhance the light induced hydrophilicity of the thin film surface as compared to the undoped titania thin films.     

Effect of annealing temperature on photocatalytic activity of ZnO thin films prepared by sol-gel method

Zinc oxide thin films are deposited on Si and quartz substrates using the sol-gel method. The thin films, annealed at 400, 600 and 800 °C respectively, are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), ultraviolet-visible spectrophotometer (UV-Vis), fluorescence spectrometer (FL) and the photocatalytic activity is tested by the decomposition of methyl orange dye under UV illumination. The results show that the mean grain size, surface-to-volume ratio, rms roughness and degradation efficiency of the thin films increases with increasing annealing temperature. In particular, ZnO thin film annealed at 800 °C exhibits the highest photocatalytic activity, degrading methyl orange by almost 88% in 180 min. Photocatalytic reaction mechanism of the ZnO thin films is discussed in detail, and the oxygen defects are proposed to be the active sites of the ZnO photocatalyst.     

Photoluminescence study of pure and Li-doped ZnO thin films grown by sol-gel technique

The effect of annealing atmosphere, temperature and aging on the photoluminescence of pure and Li-doped ZnO thin films has been investigated. Annealing the pure ZnO in N₂ and He above 800 °C results in green emission centered at ca. 500 nm; however annealing in air red-shifts the green emission to 527 nm. The visible emission of the Li-doped ZnO is found to be largely dependent on the annealing atmosphere. Warm-white photoluminescence with a broad emission band covering nearly the whole visible spectrum is obtained for the Li-doped ZnO films annealed in helium. The substitutional and interstitial extrinsic point defects created by lithium doping may mediate the relative concentration of the intrinsic defects and thereby tune the intrinsic-defect-related visible emission. The enhanced intensity ratio of near-band-edge ultraviolet emission to deep-level visible emission with aging time may be ascribed to both in-diffusion of oxygen from air and self-diffusion of oxygen interstitials to heal the oxygen vacancies during the aging process.     

Influence of glucose on the structural and optical properties of ZnO thin films prepared by sol-gel method

In this work, ZnO thin films were prepared by sol-gel method on glass substrates followed by calcinations at 500 °C for an hour. The effect of glucose on the structure and optical properties of the films was studied. The structural characteristics of the samples were analyzed by X-ray diffractometer (XRD) and atomic force microscope (AFM). The optical properties were studied by a UV-visible spectrophotometer. The results show that some of the prepared ZnO thin films have a high preferential oriented c-axis orientation with compact hexagonal wurtzite structure due to a proper amount of glucose introducing. After introducing the glucose additive in ZnO colloids, the intensity of (002) peak, the transmittance, and the optical band gap of the ZnO thin films increases because of the enhanced ZnO crystallization. On the contrary, the absorbance, the film thickness, and the surface root-mean-square (RMS) roughness of the ZnO thin films decreases. The glucose additive could not only improve the surface RMS roughness and microstructure of ZnO thin films, but also enhance the transmittance and the energy band gap more easily.     

Properties of sol-gel dip-coated zinc oxide thin films

Zinc oxide (ZnO) films were deposited on glass substrates by the sol-gel dip coating method using acrylamide route. The films were characterized by X-ray diffraction studies which indicated wurtzite structure. Optical absorption measurements indicated band gap in the range 3.17-3.32 eV. XPS studies indicated the formation of ZnO. The resistivity of the films were in the range 1000-10,000 ohm cm.     

Correlation between microstructure and optical properties of nano-crystalline TiO2 thin films prepared by sol-gel dip coating

Titanium dioxide thin films have been prepared from tetrabutyl-orthotitanate solution and methanol as a solvent by sol-gel dip coating technique. TiO₂ thin films prepared using a sol-gel process have been analyzed for different annealing temperatures. Structural properties in terms of crystal structure were investigated by Raman spectroscopy. The surface morphology and composition of the films were investigated by atomic force microscopy (AFM). The optical transmittance and reflectance spectra of TiO₂ thin films deposited on silicon substrate were also determined. Spectroscopic ellipsometry study was used to determine the annealing temperature effect on the optical properties and the optical gap of the TiO₂ thin films. The results show that the TiO₂ thin films crystallize in anatase phase between 400 and 800 °C, and into the anatase-rutile phase at 1000 °C, and further into the rutile phase at 1200 °C. We have found that the films consist of titanium dioxide nano-crystals. The AFM surface morphology results indicate that the particle size increases from 5 to 41 nm by increasing the annealing temperature. The TiO₂ thin films have high transparency in the visible range. For annealing temperatures between 1000 and 1400 °C, the transmittance of the films was reduced significantly in the wavelength range of 300-800 nm due to the change of crystallite phase and composition in the films. We have demonstrated as well the decrease of the optical band gap with the increase of the annealing temperature.     

Excimer laser processing of ZnO thin films prepared by the sol-gel process

ZnO thin films were prepared on soda-lime glass from a single spin-coating deposition of a sol-gel prepared with anhydrous zinc acetate [Zn(C₂H₃O₂)₂], monoethanolamine [H₂NC₂H₄OH] and isopropanol. The deposited films were dried at 50 and 300 °C. X-ray analysis showed that the films were amorphous. Laser annealing was performed using an excimer laser. The laser pulse repetition rate was 25 Hz with a pulse energy of 5.9 mJ, giving a fluence of 225 mJ cm⁻² on the ZnO film. Typically, five laser pulses per unit area of the film were used. After laser processing, the hexagonal wurtzite phase of zinc oxide was observed from X-ray diffraction pattern analysis. The thin films had a transparency of greater than 70% in the visible region. The optical band-gap energy was 3.454 eV. Scanning electron microscopy and profilometry analysis highlighted the change in morphology that occurred as a result of laser processing. This comparative study shows that our sol-gel processing route differs significantly from ZnO sol-gel films prepared by conventional furnace annealing which requires temperatures above 450 °C for the formation of crystalline ZnO.     

Microstructures, optical and electrical properties of In-doped ZnO thin films prepared by sol-gel method

ZnO and indium-doped ZnO (I_xZO) thin films were prepared on silica-glass substrates by the sol-gel method. The thin films were crystallized at 600 °C and 700 °C for 1 h in 6.9 × 10⁻¹ Torr under pure O₂ atmosphere. The analyzed results were compared to investigate the structural characteristics and optical properties. The surface morphology of the I_xZO films was different from that of the ZnO films, and showed a thin overlay structure. In addition, the crystallization of I_xZO film was depleted at higher crystallized temperatures. From XRD analysis, the ZnO and I_xZO thin films possessed hexagonal structures. Notably, micro-In₂O₃ phases were observed in the I_xZO thin films using EDS. Both of In₂O₃ phases and the crystallization mechanism not only improved the peeling of structure, but also improved the electrical conductivity of I_xZO thin films. For the PL spectrum, the optical property of the I_xZO film was raised at a higher crystallization temperature. Although the In₂O₃ phases reduced the structural defects of I_xZO thin film, the optical effect of the residual In³⁺ was not enhanced completely at higher crystallized temperatures.     

Microstructure and electrical properties of Mn-doped barium strontium titanate thin films prepared on copper foils

Ba_0.7−xSr_0.3Mn_xTiO₃ (x = 0, 0.025, 0.05) thin films have been prepared on copper foils using sol-gel method. The films were processed in an atmosphere with low oxygen pressure so that the substrate oxidation is avoided and the formation of the perovskite phase is allowed. XRD and SEM results showed that Mn doping enhanced the crystallization of the perovskite phase in the films. The Mn substitution prevents the reduction of Ti⁴⁺ to Ti³⁺, which is supported by XPS analysis. The Ba_0.7−xSr_0.3Mn_xTiO₃ film with x = 0.025 (BSMT25) exhibits preferred dielectric behavior and a lower leakage current density among the three thin films. The dielectric constant and loss of the BSMT25 film are 1213.5 and 0.065 at 1 MHz and around zero field, which are mostly desired for embedded capacitor applications. The mechanism of Mn doping on improving the electrical properties of barium strontium titanate (BST) thin films was investigated.     

Red luminescence in ZnO films prepared by a glycol-based Pechini method

ZnO thin films on fused quartz substrates were prepared by a glycol-based Pechini method. The structural and optical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical transmittance spectrum, and photoluminescence (PL) spectrum. A red emission around 700 nm was found in PL spectrum, and its peak intensity gained a strong enhancement (140%) while annealing temperature increased from 700 °C to 800 °C. The red emission was ascribed to the possible high defect density in boundary layers of nanocrystalline grains.     

FT-IR, XPS and PEC characterization of spray deposited hematite thin films

Hematite thin films were prepared by spraying ethanolic solution of ferric trichloride and have been characterized by using Fourier transform infra-red (FT-IR) and X-ray photoelectron spectroscopic (XPS) techniques. The film prepared by spray consists of a single phase of α-Fe₂O₃. The XPS studies confirm that chemical states of Fe³⁺ and O²⁻ in the film; thereby confirming the formation of the hematite thin films. The photoelectrochemical (PEC) studies have been carried out by forming a three-electrode system using 1 M NaOH electrolyte. The junction is illuminated with white light to obtain I-V characteristics in chopped light. The studies indicate the films exhibit n-type conductivity.     

Effects of annealing temperature on the structural and photoluminescence properties of nanocrystalline ZrO2 thin films prepared by sol-gel route

Highly transparent nanocrystalline zirconia thin films were prepared by the sol-gel dip coating technique. XRD pattern of ZrO₂ thin film annealed at 400 °C shows the formation of tetragonal phase with a particle size of 13.6 nm. FT-IR spectra reveal the formation of Zr-O-Zr and the reduction of OH and other functional groups as the temperature increases. The transmittance spectra give an average transmittance greater than 80% in the film of thickness 262 nm. Photoluminescence (PL) spectra give intense band at 391 nm and a broad band centered at 300 nm. The increase of PL intensity with elevation of annealing temperature is related to reduction of OH groups, increase in the crystallinity and reduction in the non-radiative related defects. The luminescence dependence on defects in the film makes it suitable for luminescent oxygen-sensor development. The “Red shift” of excitation peak is related to an increase in the oxygen content of films with annealing temperature. The “Blue shift” of PL spectra originates from the change of stress of the film due to lattice distortion. The defect states in the nanocrystalline zirconia thin films play an important role in the energy transfer process.     

Mn doped ZnO nanostructured thin films prepared by ultrasonic spray pyrolysis method

Mn-doped ZnO thin films with different percentage of Mn content (0, 1, 3 and 5 at.%) and substrate temperature of 350 °C, were deposited by a simple ultrasonic spray pyrolysis method under atmospheric pressure. We have studied the structural and optical properties by using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and ultra-violet visible near infrared (UV–Vis-NIR) spectroscopy. The lattice parameters calculated for the Mn-doped ZnO from XRD pattern were found to be slightly larger than those of the undoped ZnO, which indicate substitution of Mn in ZnO lattice. Compared with the Raman spectra for ZnO pure films, the Mn-doping effect on the spectra is revealed by the presence of additional peak around 524 cm⁻¹ due to Mn incorporation. With increasing Mn doping the optical band gap increases indicating the Burstein–Moss effect.     

Luminescence properties of Tb and Eu-doped alumina films prepared by sol-gel method under various conditions and sensitized luminescence

Rare earth ion (Tb³⁺ and Eu³⁺)-doped alumina films were prepared by the aqueous sol-gel method under various conditions. The influences of the OH groups (phonon relaxation) and rare earth ion concentration (cross-relaxation) on luminescence were examined. In regard to the former relaxation, at treatment temperature above 600°C, reciprocal lifetime decreased with OH concentration, and below 500°C, decreased markedly and nonlinearly. On the other hand, in regard to the latter relaxation, there was negligible effect on luminescence for these doped films. The quantitative treatment was tried to lifetime considering these influences. Tb³⁺ and Eu³⁺ co-doped alumina films showed enhanced Eu³⁺ luminescence by the energy transfer from Tb³⁺ to Eu³⁺. Eu³⁺ luminescence intensity increased with a greater Tb³⁺ concentration.     

Influence of the heating temperature on the properties of nickel doped TiO2 films prepared by sol-gel method

Formation and properties of nickel doped TiO₂ films prepared by sol-gel method were studied using X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy, and energy dispersive X-ray analysis. The results demonstrate that sizes of TiO₂ crystallites increase with increasing heating temperature. Also, at temperatures above 800 ° C diffusion of nickel onto the surfaces results in increased concentrations of nickel compounds on the surfaces. Similar to pure TiO₂ films the light-induced modification of hydrophilicity is observed also in the case of nickel doped TiO₂ films.     

Effects of the substrate and oxygen partial pressure on the microstructures and optical properties of Ti-doped ZnO thin films

Ti-doped ZnO (ZnO:Ti) thin films were deposited on the glass and Si substrates using radio frequency reactive magnetron sputtering. The effects of substrate on the microstructures and optical properties of ZnO:Ti thin films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and a fluorescence spectrophotometer. The structural analyses of the films indicated that they were polycrystalline and had a hexagonal wurtzite structure on different substrates. When ZnO:Ti thin film was deposited on Si substrate, the film had a c-axis preferred orientation, while preferred orientation of ZnO:Ti thin film deposited on glass substrate changed towards (1 0 0). Finally, we discussed the influence of the oxygen partial pressures on the structural and optical properties of glass-substrate ZnO:Ti thin films. At a high ratio of O₂:Ar of 18:10 sccm, the intensity of (0 0 2) diffraction peak was stronger than that of (1 0 0) diffraction peak, which indicated that preferred orientation changed with the increase of O₂:Ar ratios. The average optical transmittance with over 93% in the visible range was obtained independent of the O₂:Ar ratio. The photoluminescence (PL) spectra measured at room temperature revealed four main emission peaks located at 428, 444, 476 and 527 nm. Intense blue-green luminescence was obtained from the sample deposited at a ratio of O₂:Ar of 14:10 sccm. The results showed that the oxygen partial pressures had an important influence for PL spectra and the origin of these emissions was discussed.     

Controlled growth of superhydrophobic films by sol-gel method on aluminum substrate

Superhydrophobic surface was prepared by sol-gel method on aluminum substrate via immersing the clean pure aluminum substrate into the solution of zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetraamine (C₆H₁₂N₄) at different molar ratios and unchanged 0.04 mol/L total concentration, then heated at 95 °C in water bath for 1.5 h, subsequently modified with 18 alkanethiols or stearic acid. When the molar ratios of Zn(NO₃)₂·6H₂O and C₆H₁₂N₄ were changed from 10:1 to 1:1 the contact angle was higher than 150°. The best prepared surface had a high water contact angle of about 154.8°, as well as low angle hysteresis of about 3°. The surface of prepared films using Zn(NO₃)₂·6H₂O and C₆H₁₂N₄ composed of ZnO and Zn-Al LDH, and Al. SEM images of the film showed that the resulting surface exhibits different flower-shaped wurtzite zinc oxide microstructure and porous Zn-Al LDH. The special flowerlike and porous architecture, along with the low surface energy leads to the surface superhydrophobicity.     

Nanostructure and optical properties of M doped ZnO (M=Ni, Mn) thin films prepared by sol-gel process

The transparent thin films of undoped, Mn-doped, and Ni-doped zinc oxide (ZnO) have been deposited on glass substrates via sol-gel technique using zinc acetate dehydrate, nickel chloride, and manganese chloride as precursors. The structural properties and morphologies of the deposited undoped and doped ZnO thin films have been investigated. X-ray diffraction (XRD) spectra, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and microstructure of the thin films. Optical properties of the thin films were determined by photoluminescence (PL) and UV/vis spectroscopy. The analyzed results indicate that the obtained films are of good crystal quality and have smooth surfaces, which have a pure hexagonal wurtzite ZnO structure without any Mn or Ni related phases. The band gap energy was estimated by Tauc's method and found to be 3.28, 3.26, and 3.34 eV for ZnO, Ni-doped ZnO, and Mn-doped ZnO thin films at room temperature, respectively. Room temperature photoluminescence is observed for the ZnO, Ni-doped ZnO, and Mn-doped ZnO thin films.     

Reversible wettability of nanostructured ZnO thin films by sol-gel method

Nanostructured ZnO thin films were deposited on Si(1 1 1) and quartz substrate by sol-gel method. The thin films were annealed at 673 K, 873 K, and 1073 K for 60 min. Microstructure, surface topography, and water contact angle of the thin films have been measured by X-ray diffractometer, atomic force microscopy, and water contact angle apparatus. XRD results showed that the ZnO thin films are polycrystalline with hexagonal wurtzite structure. AFM studies revealed that rms roughness changes from 2.3 nm to 7.4 nm and the grain size grow up continuously with increasing annealing temperature. Wettability results indicated that hydrophobicity of the un-irradiated ZnO thin films enhances with annealing temperature increase. The hydrophobic ZnO surfaces could be reversibly switched to hydrophilic by alternation of UV illumination and dark storage (thermal treatment). By studying the magnitude and the contact angle reduction rate of the light-induced process, the contribution of surface roughness is discussed.     

Excimer laser processing as a tool for photocatalytic design of sol-gel TiO2 thin films

Nanostructured sol-gel TiO₂ thin films spin coated on silicate glass plates are subjected to excimer (KrF^*) pulsed laser irradiation in order to tailor their structure and photocatalytic properties. The surface morphology of virgin and laser-processed films are followed applying electron optical imaging and atomic force microscopy. The evolution of the surface roughness and pore formation are shown to be accompanied by optical absorption edge shift to infrared wavelength range. Conventional X-ray diffraction analysis and high-resolution transmission electron imaging are applied in order to obtain information on the phase composition. Co-existence of amorphous and anatase TiO₂ phases in nonirradiated sol-gel films is found. It is established that after laser processing the most intense XRD anatase peak is shifted to lower 2θ range. The analysis of high-resolution transmission electron images of film profiles evidences for the laser induced phase transitions. Formation of rutile and brookite TiO₂ accompanied by evolution of oxygen deficient Ti_nO_2n−1 phases are identified in the subsurface region. The contribution of laser processing for increasing the photocatalytic efficiency of laser-modified films toward the oxidation of methylene blue water solution is demonstrated. The results obtained reveal a novel-processing route for designing sol-gel titania films with improved photocatalytical activity.