M. Radecka

SNO2-TIO2 SOLID SOLUTIONS FOR GAS SENSORS

Abstract Mixed oxide gas sensors of SnO 2 –TiO 2 have been prepared in the form of polycrystalline ceramics and rf-sputtered thin films. The influence of the chemical and phase composition on the crystallographic structure, microstructure and sensor performance is discussed. The measurements of the electrical resistance as a function of hydrogen partial pressure have been performed with air or argon as a reference gas. The results are analyzed in terms of the surface and bulk interaction models. It is proposed that hydrogen detection in air involves the preadsorbed O − species. Hydrogen interaction with the SnO 2 –TiO 2 system in Ar atmosphere is governed by bulk diffusion of oxygen vacancies.

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

Abstract The effect of Nb and Cr dopants as well as Sn 4+ additions on the electronic structure of rf-sputtered TiO 2 thin films and its subsequent influence on gas sensor performance is reported. The changes in the electrical conductivities of TiO 2 thin films doped with up to 10 at.% Nb, 4 at.% Cr and TiO 2 -SnO 2 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 TiO 2 is shown to be affected by doping.

SEMICONDUCTING PROPERTIES OF UNDOPED TiO2

Abstract Both electrical conductivity and thermopower were studied for undoped [001] oriented TiO 2 single crystals within the temperature ranges 985–1387 K and oxygen partial pressure 10 −15 − 10 5 Pa. Using a Jonker-type analysis the energy gap in TiO 2 was determined ( E g = 3.09 − 1.33 × 10 −3 T/eV). It was shown that the transport of both electrons and electron holes occurs according to the small polaron model. The electrical properties determined in this work indicate that below and above 1300 K, respectively, the predominant defects in TiO 2 are doubly ionised oxygen vacancies and interstitial Ti 4+ ions.

Study of the TiO2–Cr2O3 system for photoelectrolytic decomposition of water

Abstract Titanium dioxide thin films doped with up to 16 at.% of chromium were prepared by rf reactive sputtering. The effect of doping concentration on the structural, optical and electrochemical properties was studied. Modification of the electronic structure of titanium dioxide by chromium doping was established. The allowed states due to Cr were found to be situated in the forbidden band gap at about 2.5 eV below the bottom of the conduction band. Electrochemical measurements were performed in a three-electrode cell with the TiO 2 –Cr acting as anode. The impedance spectra and capacitance of the electrode/electrolyte interface versus voltage (Mott–Schottky plots) were recorded over a wide frequency range of 1–10 6 Hz. The flat band potential, effective donor density and depletion layer thickness were obtained from capacitance data as a function of the applied potential at 10 kHz.

Electrical properties of Cr- and Nb-doped TiO2 thin films

The series of thin films of solid solutions of TiO2+xCr2O3 (0 < x < 0.013) and TiO2+yNb2O5 (0 < y < 0.05) were deposited on fused silica by means of reactive RF co-sputtering. The obtained films remain in the amorphous state up to 673 K. Annealing at 1173 K produces a well-developed rutile structure while formation of the anatase structure has not been observed. The high temperature electrical properties such as electrical conductivity, thermopower and work function were investigated within the wide range of oxygen activity (10-10–105 Pa). The effect of both Cr and Nb concentrations and film thickness on the measured parameters was studied. A mechanism for the incorporation of dopants (Cr and Nb) and defect model has been proposed. The influence of both intrinsic and extrinsic defects on the electrical properties has been discussed. The distribution of main elements (Ti, Si, O) as well as impurities were determined by the SIMS method.

Transport properties of (Sn,Ti)O2 polycrystalline ceramics and thin films

Polycrystalline ceramics and thin films of (Sn,Ti)O 2 were prepared with the aim of investigating the mass and electronic charge transport as well as the electronic structure. The re-equilibration kinetics were monitored by measuring the dc electrical conductivity as a function of temperature, 850 K < T < 1050 K, and oxygen partial pressure, 10 - 15 Pa <p(O 2 )< 10 5 Pa. The chemical diffusion coefficient was found to be independent of p(O 2 ). The energy of the forbidden band gap, evaluated on the basis of optical measurements, is presented as a function of tin dioxide mole fraction. The influence of the chemical and phase composition on the microstructure, electronic and optical properties of polycrystalline ceramics and thin films of (Sn,Ti)O 2 is discussed.

Noble metal/titanium dioxide nanocermets for photoelectrochemical applications

Abstract Noble metal/TiO2 nanocermets were deposited by rf sputtering from Ag:Ti and Au:Ti mosaic targets. Results of studies of the microstructure and the optical properties of Ag:TiO2 and Au:TiO2 thin films are discussed. Nanometric inclusions of Au and Ag in dielectric matrices create conditions for additional absorption band due to the surface plasmon resonance (SPR). The SPR occuring in the visible region of the light spectrum, at λR=430–450 nm for Ag:TiO2 and at λR=580–630 nm for Au:TiO2, improves the spectral characteristics of TiO2 photoanodes. The presence of SPR is related to the concentration, size and distribution of the noble metal particles.

Microstructure and gas-sensing properties of (Sn,Ti)O2 thin films deposited by RGTO technique

Abstract The application of the RGTO (rheotaxial growth and thermal oxidation) technique to the deposition of (Sn,Ti)O 2 thin films is described. Phase composition, microstructure, surface roughness and gas-sensing properties of thin films were studied. The comparison between the films grown by RGTO and those obtained in the reactive rf sputtering is given. The structure modelling has been performed to account for the presence of two crystallographic phases: tetragonal; and orthorhombic in the RGTO-thin films. It is shown that RGTO yields oxides of extremely rough surfaces. Illumination of such a layer with an electromagnetic wave of the wavelength comparable with dimensions of surface irregularities results in an enhanced light scattering. The electrical responses to 200–10000 ppm H 2 and 150–1000 ppm CH 4 are reproducible and significant at the sensor operating temperature of 400°C.

Structural evolution of SnO2TiO2 nanocrystalline films for gas sensors

Thin films (50‐200 nm) of SnO2TiO2 were deposited on SiO2:(001)Si substrates by RF-sputtering and by molecular beam before they were annealed in vacuum at 200‐900°C. In-situ TEM, XRD, SEM, Raman and IR-spectroscopy were used to analyze the structure transformations in the SnO2TiO2 films. In the as-deposited state, the films are amorphous. They crystallize at higher temperatures (starting at about 500°C) forming nanosized grains. The problem of the spinodal decomposition in the SnO2TiO2 system observed earlier at high temperatures is discussed also for low-temperature processing. The stoichiometry of the films of both groups (reactive ion sputtered and high-vacuum e-gun sputtered) is being compared.

Structural and optical properties of Sr1−xBaxTiO3 thin films prepared by rf sputtering

Abstract Sr1−xBaxTiO3 thin films (0≤x≤1) were deposited at T=723 K by the rf sputtering method from targets comprised of ceramic, SrTiO3–BaTiO3 mosaic. The influence of barium content and the post-deposition annealing at T=1173 K on the structural and optical properties were studied. It has been found that the crystallinity of the as-sputtered (at T=723 K) thin films decrease with the increase of Ba content (x). The perovskite structure with preferred orientation (changing from (1 1 0) for x=0 to (h 0 0) for x=1) was detected for the post-deposition annealed (at T=1173 K) Sr1−xBaxTiO3 thin films. The annealing of Sr1−xBaxTiO3 thin films affects the optical transmission spectra and absorption edge position. The observed changes are the strongest in case of BaTiO3 films and those of related compositions. The refractive index derived for the well-developed perovskite structure was found to be independent on the chemical composition (x) and was n=2.1, for λ=1.55 μm.