Room temperature ferromagnetism in vacuum-annealed TiO2 thin films

Thin films of pure TiO₂ have been prepared using both spin-coating and sputter-deposition techniques on sapphire and quartz substrates. The structural characteristics of the films have been investigated in detail using Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM). When annealed in vacuum, all films demonstrate room temperature ferromagnetism, while the air-annealed samples show much smaller, often negligible, magnetic moments. The magnetization of the vacuum-annealed sputtered samples depends on film thickness, with the volume magnetization decreasing monotonically with increasing thickness. Furthermore, the magnetization per unit area also decreases slightly with increasing film thickness. These results suggest that ferromagnetism in the vacuum-annealed TiO₂ films is mediated by surface defects or interfacial effects, but does not arise from stoichiometric crystalline TiO₂.     

Crystallisation of TiO2 thin films induced by excimer laser irradiation

Titanium dioxide thin films have been deposited by reactive magnetron sputtering on glass substrate and subsequently irradiated by UV radiation using a KrF excimer laser. In this work, we have study the influence of the laser fluence (F) ranging between 0.05 and 0.40 mJ/cm² on the constitution and microstructure of the deposited films. Irradiated thin films are characterized by profilometry, scanning electron microscopy and X-ray diffraction. As deposited films are amorphous, while irradiated films present an anatase structure. The crystallinity of the films strongly varies as a function of F with maximum for F = 0.125 J/cm². In addition to the modification of their constitution, the irradiated areas present a strongly modified microstructure with appearance of nanoscale features. The physico-chemical mechanisms of these structural modifications are discussed based on the theory of nucleation.     

Growth of ultra-thin TiO2 films by spray pyrolysis on different substrates

In the present study TiO₂ films were deposited by spray pyrolysis method onto ITO covered glass and Si (1 0 0) substrates. The spray solution containing titanium(IV) isopropoxide, acetylacetone and ethanol was sprayed at a substrate temperature of 450 °C employing 1-125 spray pulses (1 s spray and 30 s pause). According to AFM, continuous coverage of ITO and Si substrates with TiO₂ layer is formed by 5-10 and below 5 spray pulses, respectively. XPS studies revealed that TiO₂ film growth on Si substrate using up to 4 spray pulses follows 2D or layer-by-layer-growth. Above 4 spray pulses, 3D or island growth becomes dominant irrespective of the substrate. Only 50 spray pulses result in TiO₂ layer with the thickness more than XPS measurement escape depth as any signal from the substrate could not be detected. TiO₂ grain size remains 30 nm on ITO and increases from 10-20 nm to 50-100 nm on Si substrate with the number of spray pulses from 1 to 125.     

Characterization of TiO2/Au/TiO2 films deposited by magnetron sputtering on polycarbonate substrates

Transparent and conducting TiO₂/Au/TiO₂ (TAuT) films were deposited by reactive magnetron sputtering on polycarbonate substrates to investigate the effect of the Au interlayer on the optical, electrical, and structural properties of the films. In TAuT films, the Au interlayer thickness was kept at 5 nm. Although total thickness was maintained at 100 nm, the stack structure was varied as 50/5/45, 70/5/25, and 90/5/5 nm.In XRD pattern, the intermediate Au films were crystallized, while all TAuT films did not show any diffraction peaks for TiO₂ films with regardless of stack structure. The optical and electrical properties were dependent on the stack structure of the films. The lowest sheet resistance of 23 Ω/□ and highest optical transmittance of 76% at 550 nm were obtained from TiO₂ 90 nm/Au 5 nm/TiO₂ 5 nm films. The work function was dependent on the film stack. The highest work function (4.8 eV) was observed with the TiO₂ 90 nm/Au 5 nm/TiO₂ 5 nm film stack. The TAuT film stack of TiO₂ 90 nm/Au 5 nm/TiO₂ 5 nm films is an optimized stack that may be an alternative candidate for transparent electrodes in flat panel displays.     

X-ray multiple diffraction in the characterization of TiNO and TiO2 thin films grown on Si(0 0 1)

TiO₂ and TiN_xO_y thin films grown by low pressure metal-organic chemical vapor deposition (LP-MOCVD) on top of Si(0 0 1) substrate were characterized by X-ray multiple diffraction. X-ray reflectivity analysis of TiO₂[1 1 0] and TiNO[1 0 0] polycrystalline layers allowed to determine the growth rate (−80 Å/min) of TiO₂ and (−40 Å/min) of TiNO films. X-ray multiple diffraction through the Renninger scans, i.e., ?-scans for (0 0 2)Si substrate primary reflection is used as a non-conventional method to obtain the substrate lattice parameter distortion due to the thin film conventional deposition, from where the information on film strain type is obtained.     

Optical and hydrophilic properties of Cr doped TiO2-SiO2 nanostructure thin film

Cr doped TiO₂-SiO₂ nanostructure thin film on glass substrates was prepared by a sol-gel dip coating process. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the structural and chemical properties of the films. A UV-vis spectrophotometer was used to measure the transmittance spectra of the thin film. The hydrophilicity of the thin film during irradiation and storage in a dark place was measured by a contact angle analyzer. The results indicated that Cr doping has a significant effect on the transmittance and super-hydrophilicity of TiO₂-SiO₂ thin film.     

Methylene blue photoelectrodegradation under UV irradiation on Au/Pd-modified TiO2 films

In this work TiO₂ thin films were modified with gold/palladium (Au/Pd) bimetallic paticles by sputtering method. TiO₂ films were deposited on ITO (SnO₂:In) by Doctor Blade method and post-anneling. The properties of the films were studied through measurements of XRD (X-ray diffraction) and AFM (atomic force microscopy). The degradation of methylene blue was studied by UV-irradiated pure TiO₂ and Au/Pd-modified TiO₂ in aqueous solution. Langmuir-Hinshelwood model was used to obtain kinetic information. Photocatalytic study indicated that Au/Pd-modified TiO₂ photocatalytic activity was better than TiO₂ pure; the best half-life time for Au/Pd-modified TiO₂ in photodegradation was 2.8 times smaller than TiO₂ pure; finally the efficiency in methylene blue photodegradation was improved from 23% to 43% when Au/Pd-modified TiO₂ films were used.     

Uniform thin films of TiO2 nanoparticles deposited by matrix-assisted pulsed laser evaporation

We report morphological and optical properties of a colloidal TiO₂ nanoparticle film, deposited on a quartz substrate by using the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique. Atomic Force Microscopy demonstrated that a good uniformity of the deposition can be obtained. The presence of agglomerates with dimensions of about 1 μm in size was noticed. Form UV-vis transmission spectra, recorded in the 200-800 nm range, the optical constants and the energy gap were determined besides the film thickness. The optical constants resulted in agreement with the values reported in literature for TiO₂ nanoparticle thin films.     

Vapor phase modification of sol-gel derived titania (TiO2) surfaces

Chemical vapor deposition (CVD) method was used in titania surface modification. Titania layers were obtained in sol-gel process and prepared as thin films on silicon wafers in dip-coating method. In order to define the influence of modification on titania surface properties (e.g., friction), various types of fluoroalkylsilanes were used. The effectiveness of the modification was monitored by FT-IR spectroscopy. The topography and frictional measurements were investigated with the use of atomic force microscopy (AFM).     

Structural, magnetic and electronic structure studies of Mn doped TiO2 thin films

We report structural, magnetic and electronic structure study of Mn doped TiO₂ thin films grown using pulsed laser deposition method. The films were characterized using X-ray diffraction (XRD), dc magnetization, X-ray magnetic circular dichroism (XMCD) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. XRD results indicate that films exhibit single phase nature with rutile structure and exclude the secondary phase related to Mn metal cluster or any oxide phase of Mn. Magnetization studies reveal that both the films (3% and 5% Mn doped TiO₂) exhibit room temperature ferromagnetism and saturation magnetization increases with increase in concentration of Mn doping. The spectral features of XMCD at Mn L_3,2 edge show that Mn²⁺ ions contribute to the ferromagnetism. NEXAFS spectra measured at O K edge show a strong hybridization between Mn, Ti 3d and O 2p orbitals. NEXAFS spectra measured at Mn and Ti L_3,2 edge show that Mn exist in +2 valence state, whereas, Ti is in +4 state in Mn doped TiO₂ films.     

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.     

Optical properties of ITO/TiO2 single and double layer thin films deposited by RPLAD

Indium tin oxide (ITO) and titanium dioxide (TiO₂) single layer and double layer ITO/TiO₂ films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser for applications in dye-sensitized solar cells (DSSCs). The films were deposited on SiO₂ substrates either at room temperatures (RT) or heated to 200-400 °C. Under optimized conditions, transmission of ITO films in the visible (vis) range was above 89% for films produced at RT and 93% for the ones deposited at higher temperatures. Increasing the substrate temperature from RT to 400 °C enhances the transmission of TiO₂ films in the vis-NIR from about 70% to 92%. High transmission (≈90%) was observed for the double layer ITO/TiO₂ with a transmission cut-off above 900 nm. From the transmission data, the energies gaps (E_g), as well as the refractive indexes (n) for the films were estimated. n ≈ 2.03 and 2.04, respectively for ITO films and TiO₂ film deposited at 400 °C in the visible region. Post-annealing of the TiO₂ films for 3 h at 300 and 500 °C was performed to enhance n. The refractive index of the TiO₂ films increases with the post-annealing temperature. The direct band gap is 3.6, 3.74 and 3.82 eV for ITO films deposited at RT, 200, and 400 °C, respectively. The TiO₂ films present a direct band gap of 3.51 and 3.37 eV for as deposited TiO₂ films and when annealed at 400 °C, respectively. There is a shift of about 0.1 eV between ITO and ITO/TiO₂ films deposited at 200 °C. The shift decreases by half when the TiO₂ film was deposited at 400 °C. Post-annealing was also performed on double layer ITO/TiO₂.     

Structural, optical and electrochemical properties of TiO2 thin films grown by APCVD method

Atmospheric pressure chemical vapor deposition (APCVD) of TiO₂ thin films has been achieved onto glass and onto ITO-coated glass substrates, from the reaction of TiCl₄ with ethyl acetate (EtOAc). The effect of the synthesis temperature on the optical, structural and electrochemical properties was studied through spectral transmittance, X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS) measurements. It was established that the TiO₂ films deposited onto glass substrate, at temperatures greater than 400 °C grown with rutile type tetragonal structure, whereas the TiO₂ films deposited onto ITO-coated glass substrate grown with anatase type structure. EIS was applied as suitable method to determine the charge transfer resistance in the electrolyte/TiO₂ interface, typically found in dye-sensitized solar cells.     

Substrate effects on the oxygen gas sensing properties of SnO2/TiO2 thin films

In this study, SnO₂/TiO₂ thin films are fabricated on SiO₂/Si and Corning glass 1737 substrates using a R.F. magnetron sputtering process. The gas sensing properties of these films under an oxygen atmosphere with and without UV irradiation are carefully examined. The surface structure, morphology, optical transmission characteristics, and chemical compositions of the films are analyzed by atomic force microscopy, scanning electron microscopy and PL spectrometry. It is found that the oxygen sensitivity of the films deposited on Corning glass 1737 substrates is significantly lower than that of the films grown on SiO₂/Si substrates. Therefore, the results suggest that SiO₂/Si is an appropriate substrate material for oxygen gas sensors fabricated using thin SnO₂/TiO₂ films.     

Density, thickness and composition measurements of TiO2SiO2 thin films by coupling X-ray reflectometry, ellipsometry and electron probe microanalysis-X

Mixed TiO₂SiO₂ thin films were deposited by aerosol atmospheric CVD method by using di-acetoxi di-butoxi silane (DADBS) and Ti tetra-butoxide as precursors. By varying the deposition temperatures between 470 and 600 °C and the ratios between the Si and Ti precursors (Si/Ti) from 2 up to 16, films with different compositions and thicknesses were deposited. The coupled analysis of the results of different characterisation methods was used in order to determine the variation of the composition, the thickness and the density of the films. First EPMA measurements were performed at different acceleration voltages with a Cameca SX50 system. By analysing, with specific software, the evolution of the intensity ratio I_x/I_std versus the voltage, the composition and the mass thickness (product of density by the thickness) were determined. In order to measure independently the density, X-ray reflectometry experiments were performed. By analysing the value of the critical angle and the Kiessig fringes, the density and the thickness of the layers were determined. The refractive index and the thickness of the films were also measured by ellipsometry. By assuming a linear interpolation between the index value of the pure SiO₂ and TiO₂ films, the film composition was deduced from the refractive index value. XPS measurements were also performed in order to obtain an independent value of the composition. A good agreement between the ways to measure the density is obtained.     

Novel fabrication of net-like and flake-like Fe doped TiO2 thin films

New morphologies of net-like and flake-like TiO₂ thin films with different concentrations of Fe dopant were successfully fabricated by micro-arc oxidation (MAO) process of Ti plates and a subsequent chemical treatment of the as-prepared MAO-TiO₂ thin films. It was found that Fe ions can be easily introduced into the MAO-TiO₂ samples with the increase concentration of K₄(FeCN)₆·3H₂O precursor, and the amount of Fe determined the morphologies of TiO₂ thin films after chemical treatment; net-like morphology was observed with low Fe dopant, while it transformed to a flake-like one when Fe exceeds 1.7 at.%. UV-vis spectroscopy test showed that the absorption edge of the Fe ions doped TiO₂ thin films with new morphologies has an obvious red shift.     

Oblique angle deposition of TiO2 thin films prepared by electron-beam evaporation

Optical, structural and photocatalytic properties of TiO₂ thin films obliquely deposited on quartz glass substrate using an electron-beam evaporation method were investigated. The photocatalytic activity of the films was evaluated by photodecomposition of methylene blue. An increase in incident deposition angle increased the porosity and surface roughness of the TiO₂ films. As a result, the photocatalytic activity was enhanced with incident deposition angle up to 60°. However, a further increase in incident deposition angle to 75° reduced the photocatalytic activity due to a lack of the crystalline phase.     

XPS characterization of sensitized n-TiO2 thin films for dye-sensitized solar cell applications

TiO₂ thin films, employed in dye-sensitized solar cells, were prepared by the sol-gel method or directly by Degussa P25 oxide and their surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The effect of adsorption of the cis-[Ru(dcbH₂)₂(NCS)₂] dye, N3, on the surface of films was investigated. From XPS spectra taken before and after argon-ion sputtering procedure, the surface composition of inner and outer layers of sensitized films was obtained and a preferential etching of Ru peak in relation to the Ti and N ones was identified. The photoelectrochemical parameters were also evaluated and rationalized in terms of the morphological characteristics of the films.     

Preparation and characterization of amine-functionalized SiO2/TiO2 films for formaldehyde degradation

This paper investigated the gaseous formaldehyde degradation by the amine-functionalized SiO₂/TiO₂ photocatalytic films for improving indoor air quality. The films were synthesized via the co-condensation reaction of methyltrimethoxysilane (MTMOS) and 3-aminopropyltrimethoxysilane (APTMS). The physicochemical properties of prepared photocatalysts were characterized with N₂ adsorption/desorption isotherms measurement, X-ray diffraction (XRD) and Fourier Transform Infrared spectroscopy (FT/IR). The effect of amine-functional groups and the ratio of MTMOS/APTMS precursors on the formaldehyde adsorption and photocatalytic degradation were investigated. The results showed that the formaldehyde adsorption and photocatalytic degradation of the APTMS-functionalized SiO₂/TiO₂ film was higher than that of SiO₂/TiO₂ film due to the surface adsorption on amine sites and the relatively high of the specific surface area of the APTMS-functionalized SiO₂/TiO₂ film (15 times higher than SiO₂/TiO₂). The enhancement of the formaldehyde degradation of the film can be attributed to the synergetic effect of adsorption and subsequent photocatalytic decomposition. The repeatability of photocatalytic film was also tested and the degradation efficiency was 91.0% of initial efficiency after seven cycles.     

Structural, electrical and optical properties of TiO2 doped WO3 thin films

TiO₂ doped WO₃ thin films were deposited onto glass substrates and fluorine doped tin oxide (FTO) coated conducting glass substrates, maintained at 500 °C by pyrolytic decomposition of adequate precursor solution. Equimolar ammonium tungstate ((NH₄)₂WO₄) and titanyl acetyl acetonate (TiAcAc) solutions were mixed together at pH 9 in volume proportions and used as a precursor solution for the deposition of TiO₂ doped WO₃ thin films. Doping concentrations were varied between 4 and 38%. The effect of TiO₂ doping concentration on structural, electrical and optical properties of TiO₂ doped WO₃ thin films were studied. Values of room temperature electrical resistivity, thermoelectric power and band gap energy (E_g) were estimated. The films with 38% TiO₂ doping in WO₃ exhibited lowest resistivity, n-type electrical conductivity and improved electrochromic performance among all the samples. The values of thermoelectric power (TEP) were in the range of 23-56 μV/K and the direct band gap energy varied between 2.72 and 2.86 eV.