TiO2 thin films for dyes photodegradation

The aim of the present study is to investigate the influence of the TiO₂ specific surface (powder, film) on the photocatalytic degradation of methyl orange. Porous TiO₂ films were deposited on transparent conducting oxide substrates by spray pyrolysis deposition. The films were characterized by X-ray diffraction (XRD), Scanning Electronic Microscopy, and the UV-Vis spectroscopy. The XRD spectra of nanoporous TiO₂ films revealed an anatase, crystalline structure that is known as the most suitable structure in photocatalysis. The average thickness of the films was 260 nm and the measured band gap is 3.44 eV. The influence of the operational parameters (dye concentration, contact time) on the degradation rate of the dye on TiO₂ was examined. There were calculated the kinetic parameters and the process efficiency. Using thin films of TiO₂ is technologically recommended but raises problems due to lowering the amount of catalyst available for the dye degradation.     

Effect of SnO2 on the photocatalytical properties of TiO2 films

TiO₂/SnO₂ thin films with different tin atomic percentages were successfully prepared on glass substrates by the spray pyrolysis method from an alcoholic solution of TiO[C₅H₇O₂]₂ with different concentrations of SnCl₄. The TiO₂/SnO₂ thin films prepared at 450 °C presented the anatase phase in polycrystalline configuration from %Sn = 0 in the starting solution up to %Sn = 20, at higher tin content the films present an amorphous configuration. The resulting thin films have a homogeneous surface structure with some porosity. The photocatalytical properties of the films were evaluated with the degradation of methylene blue. The products of the degradation reaction were identified by ¹H nuclear magnetic resonance and the film properties were studied by atomic force microscopy, scanning electron microscopy, UV–Vis spectroscopy, and X-ray diffraction.     

Undoped and Cr-doped TiO2 thin films obtained by spray pyrolysis

Undoped and chromium doped titanium oxide thin films were fabricated by spray pyrolysis by using a solution of titanium tetrachloride and ethyl alcohol. The films have been deposited on heated glass substrates at 373 K. After annealing for 90 min at 723 K, the initially amorphous films became polycrystalline with a predominant anatase structure and average crystallite sizes depending on dopant (Cr) concentration. The repartition of chromium impurities in the matrix of titanium oxide films, analyzed by electron paramagnetic resonance and X-ray photoelectron spectroscopy showed that the entrance of chromium into the anatase structure is mainly achieved by substitution. A decrease in unit cell parameters ratio (c/a) with the increase of chromium content sustains this assertion. The wetting properties of the titanium oxide films were evaluated from contact angle measurements between de-ionized water and films surface during- and post-irradiation with UV light. The correlation between the concentration of the dopant, film structure, surface morphology and wettability characteristics is discussed.     

Structural, optical, and electrical properties of tin sulfide thin films grown by spray pyrolysis

Tin sulfide (SnS) thin films have been prepared by spray pyrolysis (SP) technique using tin chloride and N, N-dimethylthiourea as precursor compounds. Thin films prepared at different temperatures have been characterized using several techniques. X-ray diffraction studies have shown that substrate temperature (T_s) affects the crystalline structure of the deposited material as well as the optoelectronic properties. The calculated optical band gap (E_g) value for films deposited at T_s = 320-396 °C was 1.70 eV (SnS). Additional phases of SnS₂ at 455 °C and SnO₂ at 488 °C were formed. The measured electrical resistivity value for SnS films was ∼ 1 × 10⁴ Ω-cm.     

Study of structural, electrical, optical, thermoelectric and photoconductive properties of S and Al co-doped SnO2 semiconductor thin films prepared by spray pyrolysis

In this paper, the effect of S and Al concentrations on the structural, electrical, optical, thermoelectric and photoconductive properties of the films was studied. The [Al]/[Sn] and [S]/[Sn] atomic ratios in the spray solutions were varied from 10 at.% to 40 at.% and 0 to 50 at.%, respectively. X-ray diffraction analysis showed the formation of SnO₂ cassiterite phase as a main phase and the numerous sulfur phases including S, SnS, SnS₂ and Sn₂S₃ in SnO₂:Al films. Scanning electron microscopy studies showed that in the absence of S, increasing the Al content results in a smaller grain size and with the addition of S, the films appear to contain small cracks and nodules. The minimum resistance of 0.175 (kΩ/□) was obtained for S-doped SnO₂:Al (40 at.%) film with 20 at.% S-doping. From the Hall effect measurements, the majority carrier concentration was obtained in order of 10¹⁷-10¹⁸ cm^− 3. The thermoelectric measurements showed that majority carriers change from electrons to holes for S-doping in SnO₂:Al (40 at.%) thin films. The maximum Seebeck coefficient of + 774 μV/K (at T = 370 K) was obtained for S-doped SnO₂:Al (10 at.%) film with 50 at.% S-doping. The band gap values were obtained in the range of 3.8-4.2 eV. The S-doped SnO₂:Al (40 at.%) films have shown considerably photoconductivity more than S-doped SnO₂:Al (10 at.%) with increasing S-doping. The best photoconductive property was obtained for co-doped SnO₂ thin film with 40 at.% Al and 5 at.% S concentration in solution.     

CuInS2 thin films obtained by spray pyrolysis for photovoltaic applications

Copper indium disulphide, CuInS₂, is a promising absorber material for thin film photovoltaic which has recently attracted considerable attention due to its suitability to reach high efficiency solar cells by using low-cost techniques. In this work CuInS₂ thin films have been deposited by chemical spray pyrolysis onto glass substrates at ambient atmosphere, using different composition solutions at various substrate temperatures. Structural, chemical composition and optical properties of CIS films were analysed by X-ray diffraction, energy dispersive X-ray spectroscopy and optical spectroscopy. Sprayed CIS films are polycrystalline with a chalcopyrite structure with a preferential orientation along the <112> direction and no remains of oxides were found after spraying in suitable conditions. X-ray microanalysis shows that a chemical composition near to stochiometry can be obtained. An optical gap of about 1.51 eV was found for sprayed CIS thin films.     

Effect of thermal annealing time on optical and structural properties of TeO2 thin films

TeO₂ thin films were deposited on quartz substrates by rf reactive sputtering technique from a Te metal target. The obtained samples were annealed in an argon atmosphere at 450 °C for different annealing times up to 90 min. X-ray diffraction studies revealed that the as-grown samples were amorphous and there was no appreciable change in structure for a short annealing time. Thin films became polycrystalline with the tetragonal (α-phase) structure of tellurium dioxide crystal with the increase of the thermal annealing time. The refractive index and optical energy gap of the films were calculated by modelling transmittance spectra. The optical energy gap decreased continuously from 3.83 eV to 3.71 eV with increasing thermal annealing time.     

Thermal properties of TiO2 films grown by atomic layer deposition

We study the thermal properties of amorphous TiO₂ thin films of various thicknesses t, grown by atomic layer deposition. The thermo-optic coefficient dn/dT and the temperature coefficient dρ/dT of film density ρ are determined from ellipsometric data in wavelength range 380 < λ < 1800 nm with the Cauchy model and the Lorentz-Lorenz relation. It is found that dn/dT exhibits negative values for films with t < 150 nm and positive values for thicker films, while no significant changes in the two coefficients take place if t < 200 nm. A qualitative physical explanation based on porosity of the thin films is suggested. Films with t = 60 nm are illustrated in detail at λ = 640 nm: the room-temperature values of the coefficients are found to be dn/dT = − 3.1 × 10^− 5 °C^− 1 and dρ/dT = − 4.8 × 10^− 5g cm^− 3° C^− 1.     

Structural investigation of thin TiO2 films prepared by evaporation and post‐heating

Thin films of TiO₂ have been prepared by reactive evaporation of Ti₂O₃ at substrate temperatures from 150 °C to 350 °C and by post‐heating at 150 °C to 850 °C. The mass density of the films increases with increasing substrate and annealing temperature. The crystalline structure of the film prepared at 350 °C is anatase and becomes rutile upon annealing at 850 °C. All other films are amorphous as‐prepared and become anatase upon annealing above 250 °C. The crystallinity is higher for films prepared at lower temperature and does not increase with annealing temperature. Coatings with reproducible optical properties are obtained when deposited and post‐annealed at 250 °C.     

Structural, thermal and optical characterization of TiO2:ZrO2 thin films prepared by sol-gel method

We have studied the structural and optical properties of thin films of TiO₂, doped with 5% ZrO₂ and deposited on glass substrate (by the sol-gel method). The dip-coated thin films have been examined at different annealing temperatures (350 to 450 °C) and for various layer thicknesses (63-286 nm). Refractive index and porosity were calculated from the measured transmittance spectrum. The values of the index of refraction are in the range of 1.62-2.29 and the porosity is in the range of 0.21-0.70. The coefficient of transmission varies from 50 to 90%. In the case of the powder of TiO₂, doped with 5% ZrO₂, and aged for 3 months in ambient temperature, we have noticed the formation of the anatase phase (tetragonal structure with 14.8 nm grains). However, the undoped TiO₂ exhibits an amorphous phase. After heat treatments of thin films, titanium oxide starts to crystallize at the annealing temperature 350 °C. The obtained structures are anatase and brookite. The calculated grain size, depending on the annealing temperature and the layer thickness, is in the range (8.58-20.56 nm).     

Structural and electrochromic properties of TiO2 thin films prepared by metallorganic chemical vapor deposition

Titanium dioxide thin films were deposited by Metallorganic Chemical Vapor Deposition at substrate temperatures ranging from 250 °C to 450 °C over soda lime glass and indium tin oxide coated glass substrates. X-ray diffraction studies show that films have a crystalline anatase structure at all the deposition temperatures. Particle size decreases and texture changes with the increase in substrate temperature. X-ray photoelectron spectroscopy confirms the appearance of a new well resolved state in the core level of Ti 2p spectrum shifted by 1.16 eV to lower binding energy due to the reduction of Ti^+ 4 to Ti^+ 3 upon litheation. Chronoamperometery, cyclic voltammetery and in situ UV-Vis spectrophotometeric studies were carried out on the prepared samples. Particle size and crystallinity control the electrochromic performance. The 350 °C film shows the highest ion storage capacity and the highest optical modulation along with an appreciable band gap broadening.     

Influence of spray pyrolysis deposition parameters on the optoelectronic properties of WO3 thin films

The electrochromic (EC) properties of tungsten oxide (WO₃), such as coloration efficiency, cyclic durability and reversibility strongly depend on the structural and morphological properties, which are influenced by the deposition method and parameters.This paper presents the steps for optimizing the deposition parameters (substrate temperature, air flow pressure and precursor solution molarity) for improving the optical and electrical properties of WO₃ thin films for EC applications. WO₃ thin films were deposited by spray pyrolysis using tungsten hexachloride (WCl₆) dissolved in ethanol as precursor solution. The EC properties of optimized films were tested in two different electrolytes (H₂SO₄ 1 M and acetic acid/sodium acetate buffer with pH = 4) and changes in structure, composition and morphology of the films after coloration/bleaching cycles were discussed.The deposition temperature, carrier gas pressure and solution molarity were optimized at 250 °C, 120 kPa and 0.14 M respectively. Under these condition a dense, uniform film, with homogenous distribution of particles, good adhesion to the substrate, low roughness (9.02 nm), high transparency (> 70% in the 500-1100 nm range) and conductivity was obtained. Transmission modulation is higher for the sample cycled in H₂SO₄ 1 M (64% at 630 nm) compared to that cycled in the buffer (21% at 630 nm), whereas opposite results were obtained for coloration efficiencies 28 cm² C^− 1 (at 630 nm) and 35 cm² C^− 1 (at 630 nm), respectively. Changes in surface chemistry and morphology of the optimized sample were observed after cycling in H₂SO₄.     

Effects of Co doping on structure and optical properties of TiO2 thin films prepared by sol-gel method

Co-doped TiO₂ thin films were synthesized on quartz substrates by sol-gel method. Atomic force microscopy results indicate that the surfaces of the Ti_1 − xCo_xO₂ (0 ≤ x ≤ 0.10) films become smooth and compact with increasing Co content. X-ray diffraction results show that all the films are rutile phase structure and Co doping leads to lattice contraction. X-ray photoelectron spectroscopy results reveal that the predominant oxidation state of Co is divalent. Peak positions of Raman-active modes (B_2g, A_1g and E_g) shift to lower frequency with increasing Co content. The refractive index n at 670 nm from transmittance spectra increases with increasing Co content. The OBG varies between 3.10 and 3.26 eV. Note that optical band gap (OBG) first increases and then decreases with increasing Co content, reaching its maximum value when x is 0.03. These results suggest that the increasing mechanism of OBG is related to the decrease of grain size, compressive stress, and reduction of rutile TiO₂, and the decreasing mechanism of OBG is involved with defect and impurity. The competition of the two mechanisms leads to the strange change of OBG.     

Effect of temperature on structural, optical and photoluminescence properties of antimony (Sb) doped polycrystalline CuInS2 thin films prepared by spray pyrolysis

Copper indium disulphide (CuInS₂) is an absorber material for solar cell and photovoltaic applications. By suitably doping CuInS₂ thin films with dopants such as Zn, Cd, Na, Bi, Sn, N, P and As its structural, optical, photoluminescence properties and electrical conductivities could be controlled and modified. In this work, Sb (0.01 mole (M)) doped CuInS₂ thin films are grown in the temperature range 300-400 °C on heated glass substrates. It is observed that the film growth temperature, the ion ratio (Cu/In = 1.25) and Sb-doping affects the structural, optical and photoluminescence properties of sprayed CuInS₂ films.The XRD patterns confirm that the Sb-doping suppresses the growth of CuInS₂ polycrystalline thin films along (1 1 2) preferred plane and in other characteristic planes. The EDAX results confirm the presence of Cu, In, S and Sb. About 60% of light transmission occurs in the wavelength range 350-1100 nm. The absorption coefficient (α) is found to be in the order of 10⁵ cm⁻¹. The band gap energy increases as the temperature increases from 300-400 °C (1.35-1.40 eV). SEM photographs depict that large sized crystals of Sb-doped CuInS₂ (1 μm) are formed on the surface of the films. Well defined sharp blue and green band emissions are exhibited by Sb-doped CuInS₂ thin films. Defects-related photoluminescence emissions are discussed. These Sb-doped CuInS₂ thin films are prepared by the cost effective method of spray pyrolysis from the aqueous solutions of CuCl₂, InCl₃, SC(NH₂)₂ and SbCl₃ on heated glass substrates.     

Influence of Nb dopant on the structural and optical properties of nanocrystalline TiO2 thin films

In this study, preparation of Nb-doped (0-20 mol% Nb) TiO₂ dip-coated thin films on glazed porcelain substrates via sol-gel process has been investigated. The effects of Nb on the structural, optical, and photo-catalytic properties of applied thin films have been studied by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Surface topography and surface chemical state of thin films was examined by atomic force microscope and X-ray photoelectron spectroscopy. XRD and Raman study showed that the Nb doping inhibited the grain growth. The photo-catalytic activity of the film was tested on degradation of methylene blue. Best photo-catalytic activity of Nb-doped TiO₂ thin films were measured in the TiO₂-1 mol% Nb sample. The average optical transmittance of about 47% in the visible range and the band gap of films became wider with increasing Nb doping concentration. The Nb⁵⁺ dopant presented substitutional Ti⁴⁺ into TiO₂ lattice.     

Thin TiO2 films for an electrochromic system

Thin electrochromic titanium dioxide films were obtained by spray pyrolysis on glass sheets coated with SnO₂: F using titanium (IV) oxide acetylacetonate and were characterized for the morphology and chemical nature. Electrochromic coloration–bleaching reactions with a Li-ion conducting hybrid organic–inorganic electrolyte and a WO₃ coated electrochromic electrode resulted in a reversible optical properties modulation in the visible and near infrared range of the spectrum. An XPS analysis showed the presence of lower oxidized states in the films.     

Structural characterization and novel optical properties of defect chalcopyrite ZnGa2Te4 thin films

Stoichiometric thin film samples of the ternary ZnGa₂Te₄ defect chalcopyrite compound were prepared and characterized by X-ray diffraction technique. The elemental chemical composition of the prepared bulk material as well as of the as-deposited film was determined by energy-dispersive X-ray spectrometry. ZnGa₂Te₄ thin films were deposited, by conventional thermal evaporation technique onto highly cleaned glass substrates. The X-ray and electron diffraction studies revealed that the as-deposited and the annealed ZnGa₂Te₄ films at annealing temperature t_a ≤ 548 K are amorphous, while those annealed at t_a ≥ 573 K (for 1 h), are polycrystalline. The optical properties of the as-deposited films have been investigated for the first time at normal incidence in the spectral range from 500 to 2500 nm. The refractive index dispersion in the transmission and low absorption region is adequately described by the Wemple–DiDomenico single oscillator model, whereby, the values of the oscillator parameters have been calculated. The analysis of the optical absorption coefficient revealed an in-direct optical transition with energy of 1.33 eV for the as-deposited sample. This work suggested that ZnGa₂Te₄ is a good candidate in solar cell devices as an absorbing layer.     

Spray deposition and property analysis of anatase phase titania (TiO2) nanostructures

Anatase phase titanium dioxide thin films have been deposited at various substrate temperatures by chemical spray pyrolysis of an aerosol of titanyl acetylacetonate. Deposited TiO₂ films were nanocrystalline and preferentially oriented along [101] direction, uniform and adherent to the glass substrate. Best films processed at 450 °C were characterized to analyze its phase composition, texture, roughness, optical and electrical properties. X-ray photoelectron spectroscopy revealed that the surface of the film has only the Ti⁴⁺ cations to form perfect TiO₂ stoichiometry with less amount of hydration. Atomic force microscopy image demonstrated the existence of homogeneous and rough surface, suitable for electrocatalytic applications. The film has an optical transmittance more than 90% and the refractive index of 2.07 was recorded at the wavelength 633 nm. Due to nano-sized grains, obtained optical band gap (3.65 eV) of the TiO₂ thin film was larger than that of the bulk TiO₂ (3.2 eV). Calculated porosity of the films 0.44, revealed the porous nature of the films. Hall measurements indicated that these materials are p-type and yield a carrier density of the order 8.8 × 10²⁰ cm⁻³ and a carrier mobility of 0.48 × 10⁻⁶ cm²/Vs. The dc electrical conductivity was therefore very low (8.91 × 10⁻⁶ S/cm) because of lower value of mean free path of the charge carriers (4.36 × 10⁻¹¹ cm). It gives an impression that the process of spray pyrolysis provides an easy way to tailor make thin films possessing superior properties.     

Effect of Nb, Cr, Sn additions on gas sensing properties of TiO2 thin films

The effect of Nb and Cr dopants as well as Sn⁴⁺ additions on the electronic structure of rf-sputtered TiO₂ thin films and its subsequent influence on gas sensor performance is reported. The changes in the electrical conductivities of TiO₂ thin films doped with up to 10 at.% Nb, 4 at.% Cr and TiO₂-SnO₂ in the full range of compositions upon exposure to hydrogen and oxygen are demonstrated. The spectral dependence of the absorption coefficient in the vicinity of the band gap transition of TiO₂ is shown to be affected by doping.     

Structural, electrical and optical properties of thermochromic VO2 thin films obtained by reactive electron beam evaporation

We present the structural and physical characterization of vanadium dioxide (VO₂) thin films prepared by reactive electron beam evaporation from a vanadium target under oxygen atmosphere. We correlate the experimental parameters (substrate temperature, oxygen flow) with the films structural properties under a radiofrequency incident power fixed to 50 W. Most of the obtained layers exhibit monocrystalline structures matching that of the monoclinic VO₂ phase. The temperature dependence of the electrical resistivity and optical transmission for the obtained films show that they present thermoelectric and thermochromic properties, with a phase transition temperature around 68 °C. The results show that for specific experimental conditions the VO₂ layers exhibit sharp changes in electrical and optical properties across the phase transition.