TiO2 thin films - Influence of annealing temperature on structural, optical and photocatalytic properties

Nanostructured TiO₂ thin films were deposited on glass substrates by sol-gel dip coating technique. The structural, morphological and optical characterizations of the as deposited and annealed films were carried out using X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), and UV-vis transmittance spectroscopy. As-deposited films were amorphous, and the XRD studies showed that the formation of anatase phase was initiated at annealing temperature close to 400 °C. The grain size of the film annealed at 600 °C was about 20 nm. The lattice parameters for the films annealed at 600 °C were a = 3.7862 ? and c = 9.5172 ?, which is close to the reported values of anatase phase. Band gap of the as deposited film was estimated as 3.42 eV and was found to decrease with the annealing temperature. At 550 nm the refractive index of the films annealed at 600 °C was 2.11, which is low compared to a pore free anatase TiO₂. The room temperature electrical resistivity in the dark was of the order of 4.45 × 10⁶ ohm-cm. Photocatalytic activity of the TiO₂ films were studied by monitoring the degradation of aqueous methylene blue under UV light irradiation and was observed that films annealed above 400 °C had good photocatalytic activity which is explained as due to the structural and morphological properties of the films.     

Study of the Li insertion into V2O5 films deposited by CVD onto various substrates

Vanadium oxide films were synthesised by chemical vapour deposition (CVD) from pure of triisopropoxyvanadium oxide (VO(OC₃H₇)₃) and oxygen as precursors. The influence of the substrate on the crystallinity of the vanadium oxide films was studied before and after annealing at 500 °C. On mica substrates, as-deposited film was composed of crystalline V₂O₅ as revealed by XRD. On Pt, Ti, stainless steel, glass and F-doped SnO₂ substrates, an annealing procedure was required to get V₂O₅. SEM investigations have clearly evidence V₂O₅ plates but the kinetics growth seems to be strongly dependent on the nature of the substrate. The insertion/extraction of Li⁺ into the host structure was investigated in 1 M LiClO₄-PC with annealed V₂O₅ films deposited on Ti, Pt and stainless steel substrates. The best electrochemical performances were obtained in the potential range 3.8–2.8 V versus Li/Li⁺ with V₂O₅ films deposited onto stainless steel substrate: the reversible capacity reaches after subsequent cycles was about 115 mAh g⁻¹ (rate C/23). In a wider potential range (between 3.8 and 2.2 V versus Li/Li⁺), V₂O₅ deposited onto Ti substrate exhibited the higher electrochemical performances (220 mAh g⁻¹ for a rate of C/23).     

Highly-efficient electrochromic performance of nanostructured TiO2 films made by doctor blade technique

Electrochromic TiO₂ anatase thin films on F-doped tin oxide (FTO) substrates were prepared by doctor blade method using a colloidal solution of titanium oxide with particles of 15 nm in size. The films were transparent in the visible range and well colored in a solution of 1 M LiClO₄ in propylene carbonate. The transmittances of the colored films were found to be strongly dependent on the Li⁺ inserted charges. The response time of the electrochromic device coloration was found to be as small as 2 s for a 1 cm² sample and the coloration efficiency at a wavelength of 550 nm reached a value as high as 33.7 cm² C⁻¹ for a 600 nm thick nanocrystalline-TiO₂ on a FTO-coated glass substrate. Combining the experimental data obtained from in situ transmittance spectra and in situ X-ray diffraction analysis with the data from chronoamperometric measurements, it was clearly demonstrated that Li⁺ insertion (extraction) into (out of) the TiO₂ anatase films resulted in the formation (disappearance) of the Li_0.5TiO₂ compound. Potential application of nanocrystalline porous TiO₂ films in large-area electrochromic windows may be considered.     

Quasi-solid-state dye-sensitized solar cells employing nanocrystalline TiO2 films made at low temperature

Quasi-solid-state dye-sensitized solar cells with enhanced performance were made by using nanocrystalline TiO₂ films without any template deposited on plastic or glass substrates at low temperature. A simple and benign procedure was developed to synthesize the low-temperature TiO₂ nanostructured films. According to this method, a small quantity of titanium isopropoxide (TTIP) was added in an ethanolic dispersion of TiO₂ powder consisting of nanoparticles at room temperature, which after alkoxide's hydrolysis helps to the connection between TiO₂ particles and to the formation of mechanically stable thick films on plastic or glass substrates. Pure TiO₂ films without any organic residuals consisting of nanoparticles were formed with surface area of 56 m²/g and pore volume of 0.383 cm³/g similar to that obtained for Degussa-P25 powder. The structural properties of the films were characterized by microscopy techniques, X-ray diffractometry, and porosimetry. Overall solar to electric energy conversion efficiencies of 5.3% and 3.2% (under 1sun) were achieved for quasi-solid-state dye-sensitized solar cells employing such TiO₂ films on F:SnO₂ glass and ITO plastic substrates, respectively. Thus, the quasi-solid-state device based on low-temperature TiO₂ attains a conversion efficiency which is very close to that obtained for cells consisting of TiO₂ nanoparticles sintered at high temperature.     

Correlation of electrical and physical properties of photoanode with hydrogen evolution in enzymatic photo-electrochemical cell

In this study, the electrical and physical properties, including the current density, open-circuit voltage, morphology and crystalline structure, of an anodized TiO₂ electrode on a titanium foil are correlated with the hydrogen production rate in an enzymatic photo-electrochemical system. The effect of light intensity at ca. 74 and ca. 146 mW cm⁻² on the properties is also examined. Anodizing (20 V; bath temperature 5 °C; anodizing time 45 min) and subsequent annealing (350–850 °C for 5 h) of the Ti foils in an O₂ atmosphere led to the formation of a tube-shaped, or a compact layered, TiO₂ film on the Ti substrate depending on the annealing temperature. The annealing temperature has a similar effect on the properties of the sample and the hydrogen evolution rate. The generated electrical value, the chronoamperometry (CA), is +13 to −229 and +13 to −247 μA for light intensities of ca. 74 and ca. 146 mW cm⁻², while the corresponding open-circuit voltage (OCV) is in the range of −41 to −687 and −144 to 738 mV, respectively. In the absence of light (dark), the CA is 13–29 μA and the OCV is +258 to −126 mW cm⁻². The trend in the electrical properties for the different samples is well matched with the rate of hydrogen evolution. The samples with higher activities (450, 550, and 650 °C) have similar X-ray diffraction (XRD) patterns, which clearly indicates that the samples showing the highest evolution rate are composed of both anatase and rutile, while those showing a lower evolution rate are made of either anatase or rutile. Increasing the intensity of the irradiated light causes a remarkable enhancement in the rate of hydrogen production from 71 to 153 μmol h⁻¹ cm⁻².     

Flame spray deposition of La0.6Sr0.4CoO3−δ thin films: Microstructural characterization, electrochemical performance and degradation

Thin films of La_0.6Sr_0.4CoO_3−δ (LSC) were deposited by flame spray deposition at a deposition temperature of 200 °C. The as-deposited LSC films were dense, particle-free and amorphous. Upon annealing above 600 °C, the films crystallized into the rhombohedral perovskite LSC phase. For isothermal annealing at higher temperatures parabolic grain growth kinetics occur in parallel to densification of the thin films. Electrochemical measurements on symmetrical cells with LSC films on CGO pellets showed lowest area specific resistance (ASR) of 0.96 Ω cm² at 600 °C for films of 38 nm in grain size annealed at 700 °C. Degradation of the ASR of the LSC films of 3.9% was found after 5 days operation at 550 °C in ambient air.     

Photoelectrochemical properties of spray deposited n-CdSe thin films

Polycrystalline cadmium selenide (CdSe) thin films have been prepared by spraying a mixture of an equimolar aqueous solutions of cadmium chloride and selenourea on preheated fluorine doped tin oxide (FTO) coated glass substrates at different substrate temperatures. The cell configuration n-CdSe/1 M (NaOH + Na₂S + S)/C is used for studying the capacitance-voltage (C-V) characteristics in dark, current-voltage (I-V) characteristics in dark and under illumination, photovoltaic power output and spectral response characteristics of the as deposited films. Photoelectrochemical study shows that as deposited CdSe thin films exhibits n-type of conductivity. The spectral response characteristics of the films at room temperature show a prominent sharp peak at 725 nm. The measured values of efficiency (η) and fill factor (FF) are found to be 0.50% and 0.44 respectively for film deposited at 300 °C. Electrochemical impedance spectroscopy studies show that the CdSe film deposited at 300 °C shows better performance in PEC cell.     

Investigations on electron beam evaporated Cu(In0.85Ga0.15)Se2 thin film solar cells

CIGS bulk with composition of CuIn_0.85Ga_0.15Se₂ was synthesized by direct reaction of elemental copper, indium, gallium and selenium. CIGS thin films were then deposited onto well-cleaned glass substrates using the prepared bulk alloy by electron beam deposition method. The structural properties of the deposited films were studied using X-ray diffraction technique. The as-deposited CIGS films were found to be amorphous. On annealing, the films crystallized with a tetragonal chalcopyrite structure. An intermediate Cu-rich phase precipitated at 200 °C and dissociated at higher annealing temperatures. Average grain size calculated from the XRD spectra indicated that the films had a nano-crystalline structure and was further corroborated by AFM analysis of the sample surface. The chemical constituents present in the deposited CIGS films were identified using energy dispersive X-ray analysis. CIGS based solar cells were then fabricated on molybdenum and ITO coated glass substrates and the efficiencies have been evaluated.     

High efficiency flexible dye-sensitized solar cells by multiple electrophoretic depositions

A multiple electrophoretic deposition (EPD) of binder-free TiO₂ photoanode has been developed to successfully fill the crack occurring after air-drying on the first EPD-TiO₂ film surface. With the slow 2nd EPD, high quality TiO₂ thin films are acquired on flexible ITO/PEN substrates at room temperature and the device efficiency of the dye-sensitized solar cell achieved 5.54% with a high fill factor of 0.721. Electrochemical impedance spectroscopy measurements analyze the great enhancement of the photovoltaic performance through multiple EPD. The electron diffusion coefficient improved by about 1 order of magnitude in crack-less multiple-EPD TiO₂ films. With the scattering layer, the device reveals a high conversion efficiency of up to 6.63% under AM 1.5 G one sun irradiation, having a short circuit current density, open circuit voltage, and filling factor of 12.06 mA cm⁻², 0.763 V and 0.72, respectively.     

Action of oxygen and sodium carbonate in the urea-SNCR process

Experimental researches are focused on the effects of O₂ concentration and sodium carbonate on Selective Non-Catalytic Reduction (SNCR) performance in a tube reactor, and plug flow reactor model and perfectly stirred reactor model in CHEMKIN are adopted to simulate the reactions processes. It is found that there is a conversion temperature point (CTP), on the two sides of which oxygen performs different effect. Below CTP, which is 1273 K in our experiments, higher NO reduction efficiency can be gained with higher oxygen concentration because more O₂ results in more radicals to drive the reduction chain reactions by speeding up the reactions O₂ + H = O + OH and H₂O + O = 2OH. At 1473 K without oxygen, 60% of NO reduction efficiency can be achieved and a 15 ppm Na₂CO₃ addition improves it to 90%. In this case the reaction H₂O + H = OH + H₂ becomes fast enough to provide the radical OH without the aid of O₂ to produce NH₂ which reduces NO. And H₂ is the byproduct of this reaction. Na₂CO₃ addition shifts the optimal temperature of SNCR 50 K towards lower temperature and more NO is removed in the temperature window. The reactions NaO + H₂O = NaOH + OH and NaOH + O₂ = NaO₂ + OH and NaOH + M = Na + M + OH offer new pathways to produce OH radical, which results in more OH and more NH₂ to reduce NO. The promotion effect of Na₂CO₃ is significant when temperature is lower or O₂ concentration is lower, which means the radicals are insufficient.     

Structural, optical and luminescent characteristics of sprayed fluorine-doped In2O3 thin films for solar cells

Fluorine-doped indium oxide thin films, F-In₂O₃, prepared by the spray pyrolysis technique on glass substrates have been studied using cathodoluminescence spectroscopy, X-ray diffraction and spectrophotometry. These films, deposited at the optimal substrate temperature (T_s=450 °C), crystallize in a cubic structure with a preferential orientation along the (4 0 0) direction. For this temperature, the electrical resistivity is in the order of 6×10⁻³ Ω cm and the average optical transmission in the visible region is larger than 95%. At room temperature, the cathodoluminescence spectra of F-In₂O₃ present two emission peaks: blue indirect band gap peak at 410 nm and a red emission at 650 nm.     

Effect of different annealing conditions on the properties of chemically deposited ZnS thin films on ITO coated glass substrates

The effects of different annealing conditions such as atmospheres, temperatures and times on the structural, morphological and optical properties of ZnS thin films prepared on ITO coated glass substrates by chemical bath deposition were studied. Aqueous solutions of zinc acetate and thiourea were used as precursors along with stable complexing agents, such as Na₂EDTA and Na₃-citrate, in an alkaline medium. X-ray diffraction patterns showed that the as-deposited and as-annealed ZnS films had an amorphous structure or poor crystallinity below the optimized annealing conditions of 500 °C and 60 min with the exception of the films annealed in N₂+H₂S annealing atmosphere. The ZnS thin films annealed in N₂+H₂S atmosphere for 1 h at 500 °C showed three sharp peaks for the (1 1 1), (2 2 0) and (1 1 3) planes of polycrystalline cubic ZnS without any unwanted secondary ZnO phases. X-ray photoelectron spectroscopy revealed Zn-OH and Zn-S bonding in the as-deposited ZnS thin film. However, the ZnS thin films annealed at 500 °C showed Zn-S bonding regardless of the annealing atmosphere. The sharp absorption edge and band gap energy of the as-deposited and as-annealed ZnS thin films varied from 295 to 310 nm and 3.5 to 3.89 eV, respectively.     

Synthesis and characterization of nanosized LiNiVO4 electrode material

Inverse spinel LiNiVO₄ thin films were prepared by rf-sputtering, fallowed by films annealed at 300, 450 and 600 °C for 2 h to induce the crystallization of the films. The films were characterized by X-ray diffraction, Rutherford backscattering spectroscopy, nuclear reaction analysis, Auger electron spectroscopy, atomic force microscopy and scanning electron microscope techniques. The Anodic electrochemical performance films have been cycled in the range of 0.02–3.0 V, at room temperature, at a current density 75 μA cm⁻². Galvanostatic cycling and cyclic voltammetry results shows characteristic cycling curves with respect to annealing temperature. The films annealed at 450 °C showed best electrochemical performance and excellent capacity retention during cycling was observed due to its nanosized morphology.     

p-Type CuSbS2 thin films by thermal diffusion of copper into Sb2S3

We report the preparation of copper antimony sulfide (CuSbS₂) thin films by heating Sb₂S₃/Cu multilayer in vacuum. Sb₂S₃ thin film was prepared from a chemical bath containing SbCl₃ and Na₂S₂O₃ salts at room temperature (27 °C) on well cleaned glass substrates. A copper thin film was deposited on Sb₂S₃ film by thermal evaporation and Sb₂S₃/Cu layers were subjected to annealing at different conditions. Structure, morphology, optical and electrical properties of the thin films formed by varying Cu layer thickness and heating conditions were analyzed using different characterization techniques. XRD analysis showed that the thin films formed at 300 and 380 °C consist of CuSbS₂ with chalcostibite structure. These thin films showed p-type conductivity and the conductivity value increased with increase in copper content. The optical band gap of CuSbS₂ was evaluated as nearly 1.5 eV.     

Impact of substrate material and annealing conditions on the microstructure and chemistry of yttria-stabilized-zirconia thin films

Si-diffusion from Si-based substrates into yttria-stabilized-zirconia (YSZ) thin films and its impact on their microstructure and chemistry is investigated. YSZ thin films used in electrochemical applications based on micro-electrochemical systems (MEMS) are deposited via spray pyrolysis onto silicon-based and silicon-free substrates, i.e. Si_xN_y-coated Si wafer, SiO₂ single crystals and Al₂O₃, sapphire. The samples are annealed at 600 °C and 1000 °C for 20 h in air. Transmission electron microscopy (TEM) showed that the Si_xN_y-coated Si wafer is oxidized to SiO_z at the interface to the YSZ thin film at temperatures as low as 600 °C. On all YSZ thin films, silica is detected by X-ray photoelectron spectroscopy (XPS). A particular large Si concentration of up to 11 at% is detected at the surface of the YSZ thin films when deposited on silicon-based substrates after annealing at 1000 °C. Their grain boundary mobility is reduced 2.5 times due to the incorporation of SiO₂. YSZ films on Si-based substrates annealed at 600 °C show a grain size gradient from the interface to the surface of 3 nm to 10 nm. For these films, the silicon content is about 1.5 at% at the thin film's surface.     

Structure, composition and optical properties of Cu2ZnSnS4 thin films deposited by Pulsed Laser Deposition method

Polycrystalline Cu₂ZnSnS₄ (CZTS) thin films have been directly deposited on heating Mo-coated glass substrates by Pulsed Laser Deposition (PLD) method. The results of energy dispersive X-ray spectroscopy (EDX) indicate that these CZTS thin films are Cu-rich and S-poor. The combination of X-ray diffraction (XRD) results and Raman spectroscopy reveals that these thin films exhibit strong preferential orientation of grains along [1 1 2] direction and small Cu_2−xS phase easily exists in CZTS thin films. The lattice parameters and grain sizes have been examined based on XRD patterns and Atom Force Microscopy (AFM). The band gap (E_g) of CZTS thin films, which are determined by reflection spectroscopy varies from 1.53 to 1.98 eV, depending on substrate temperature (T_sub). The optical absorption coefficient of CZTS thin film (T_sub=450 °C) measured by spectroscopic ellipsometry (SE) is above 10⁴ cm⁻¹.     

Performance of (La,Sr)(Co,Fe)O3−x double-layer cathode films for intermediate temperature solid oxide fuel cell

In this study the performance evaluation of (La,Sr)(Co,Fe)O_3−x (LSCF) double-layer films characterized by impedance spectroscopy between 403 and 603 °C to be used for intermediate temperature solid oxide fuel cells (IT-SOFCs) is presented. Two LSCF layers with different microstructures were sequentially deposited onto Ce_0.9Gd_0.1O_1.95 (CGO) substrates in a symmetrical fashion. A first layer of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−x with a thickness of 7 μm and a nano-scaled particle size was deposited by electrostatic spray deposition (ESD) technique. Different deposition conditions were used in preparing the ESD films to evaluate the influence of film morphology on the electrochemical performance. After annealing, a current collector layer of La_0.58Sr_0.4Co_0.2Fe_0.8O_3−x with ∼45 μm in thickness and a larger particle size was deposited by screen printing. Area specific resistances (ASRs) were determined from impedance spectroscopy measurements performed in air between 403 and 603 °C, at 25 °C steps. A dependence of electrochemical performance on the morphology of the LSCF layer deposited by ESD was observed. The lowest ASR, measured during 130 h of isothermal dwelling at 603 °C, averaged 0.13 Ω cm² with negligible variation and is the lowest reported value for this composition, to the best of our knowledge. Reported results assure an excellent suitability of this type of assembly for IT-SOFCs.     

Structural and electrical properties of magnetron sputtered Ti(ON) thin films: The case of TiN doped in situ with oxygen

Incorporation of oxygen into TiN lattice results in formation of titanium oxynitrides, TiO_xN_y that have become particularly interesting for photocatalytic applications. Elaboration as well as characterization of TiN and in situ oxygen-doped thin films is the subject of this paper. Thin films, 250-320 nm in thickness, have been deposited by dc-pulsed magnetron reactive sputtering from Ti target under controllable gas flows of Ar, N₂ and O₂. Optical monitoring of Ti plasma emission line at λ = 500 nm has been implemented in order to stabilize the sputtering rate. Scanning electron microscopy (SEM), X-ray diffraction in grazing incidence (GIXRD), micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), optical spectrophotometry and four-point probe electrical resistivity measurements have been performed in order to follow evolution of film physical parameters as a function of the oxygen flow rate ηO₂ at which the films were deposited. The relationship between ηO₂ expressed in standard cubic centimetres per minute, sccm and the nitrogen/oxygen content in thin films has been established by means of the analysis of the XPS spectra. GIXRD studies indicate that incorporation of oxygen results in a progressive loss of preferential orientation in 〈1 1 1〉 direction, a change in the grain size from 16 nm for TiN to about 3 nm for films deposited at ηO₂=1.32 sccm and a decrease in the lattice constant. A systematic shift of all X-ray diffraction (XRD) lines towards higher diffraction angles is consistent with substitution of oxygen for nitrogen. Micro-Raman investigations indicate amorphisation of thin films upon oxidation. Binding energies determined from fitting of the XPS results concerning the N1s and Ti2p lines give evidence of the presence of TiO_xN_y compound. Red-shift of the plasma reflectance edge upon TiN oxidation is correlated with a decreased carrier concentration. Metal-semiconductor transition can be expected on the basis of the electrical conductivity decrease and development of the fundamental absorption across the forbidden band of TiO₂ upon increase in the oxygen flow rate. Additional absorption feature in the visible range, being a consequence of coexistence of free-electron and interband absorption within almost the same spectral range (λ = 400-600 nm) seems to be very promising for photocatalytic applications of titanium oxynitride thin films.     

Fabrication of bilayered YSZ/SDC electrolyte film by electrophoretic deposition for reduced-temperature operating anode-supported SOFC

Bilayered Y₂O₃-stabilized ZrO₂ (YSZ)/Sm₂O₃-doped CeO₂ (SDC) electrolyte films were successfully fabricated on porous NiO–YSZ composite substrates by electrophoretic deposition (EPD) based on electrophoretic filtration followed by co-firing with the substrates. In EPD, positively charged YSZ and SDC powders were deposited directly on the substrates, layer by layer from ethanol-based suspensions. Delamination between YSZ and SDC films was avoided by reducing the SDC films’ thickness to ca. 1 μm. A single cell was constructed on the bilayered electrolyte films composed of ca. 4 μm-thick YSZ and ca. 1 μm-thick SDC films. As a cathode in the cell, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−x (LSCF) was used. Maximum output power densities greater than 0.6 W cm⁻² were obtained at 700 °C for the bilayered YSZ/SDC electrolyte cells thus constructed.     

Effect of the sintering temperature on the photocatalytic activity of ZnO+Zn2TiO4 thin films

ZnO+Zn₂TiO₄ thin films were obtained by the sol–gel method, the precursor solutions were prepared using two Ti/Zn ratios: 0.49 and 0.69. The films were deposited on glass slide substrates and sintered at temperatures in the 200–600 °C range in increments of 50 °C, with the goal of studying the dependence of the photocatalytic activity (PA) on the annealing temperature. The films were characterized by X-ray diffraction and UV–Vis spectroscopy. The PA was evaluated by measuring the UV–Vis absorption spectra of the methylene blue in aqueous solution before and after photobleaching, using the Lambert–Beer's principle. The higher photocatalytic activities were obtained from the films with sintering temperature around 450 °C, for both Ti/Zn ratios studied.