K. Zakrzewska

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.

The influence of thermal annealing on the structural, electrical and optical properties of TiO2-x thin films

Abstract Amorphous thin films of TiO2-x with a departure from stoichiometry x in the range from 0.08 to -0.2 were deposited by plasma-emission-controlled DC magnetron and conventional RF reactive sputtering. The influence of the post-deposition annealing at 573-1373 K on the structural, electrical and optical properties was studied. It was established that substoichiometric films with x = 0.08 when subjected to the annealing changed their overall spectral characteristics while in the case of overoxidized samples (x = -0.2) the transmission spectra were strongly modified in the region of the absorption edge (3.0-3.7 eV). It was found that the refractive index of TiO2-x films increased with annealing temperature up to a saturation level of about 2.39. The analysis of the absorption data revealed that direct allowed transitions at the energy of 3.44-3.51 eV prevailed in rutile samples.

Plasma-emission-controlled D.C. magnetron sputtering of TiO2-x thin films

A simple, low cost plasma-emission-controlled system was designed and built in our laboratory. The system was applied to d.c. reactive magnetron sputtering of TiO2-x films. The film properties such as composition, electrical resistivity, refractive index and absorption coefficient were studied in relation to thereference voltage being a controllable system parameter. It was found that there existed a correlation between the reference voltage which was a measure of the intensity of the titanium emission line and the film composition. The changes in the electrical and optical parameters of TiO2-x films with the reference voltage could be accounted for by the variation in the departure x from stoichiometry.

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.

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.

Reactively sputtered TiO2−x thin films with plasma-emission-controlled departure from stoichiometry

Thin films of TiO 2-x with reproducible optical and structural properties were deposited by d.c. reactive magnetron sputtering. The automatic control of the sputtering conditions was attained by monitoring the emission line of metallic Ti at A = 500 nm and by using this signal as a feedback to operate the gas admission system. Grazing incidence diffraction (GID) revealed formation of the anatase-rutile mixtures in all deposited films. Spectrophotometric measurements showed a shift in the fundamental absorption edge from 3.4 to 3.2 eV that could be traced back to the diminishing amount of the anatase phase in films deposited at high sputtering rates. The change in the refractive index from 2.3 to 2.45 was also observed.

Photocatalytic Activity of W-Doped TiO2 Nanopowders

Abstract Polycrystalline tungsten-doped titanium dioxide nanopowders within a dopant concentration of 0-1 at.% were prepared by a one-step flame spray synthesis (FSS). Mixtures of titanium tetra-isopropoxide dissolved in ethanol and tungsten hexacarbonyl solubilized in tetrahydrofuran were used as precursors. The specific surface area (SSA) of the powders was between 40 and 130 m2/g depending on the process parameters, namely the flame combustion stoichiometry and their chemical composition. Irrespective of their composition the powders consisted of spherical particles, as visible by transmission electron microscopy (TEM). They are highly crystallized with anatase as the predominant phase, according to X-ray diffraction (XRD). The band gap energy was determined by spectrophotometric measurement and was between 3.29 eV and 3.34 eV. The photocatalytic performance of the powders was studied by the degradation of methylene blue (MB) and methyl orange (MO) in aqueous suspensions under UV-A (365 nm) as well as visible (400-500 nm) irradiation. The catalytic activity was determined as a function of the tungsten concentration, the specific surface area and the flame combustion stoichiometry. It was found that the TiO2-0.7 at %W particles with high SSA have the highest photoactivity for degradation of MB and MO. This can be explained by the incorporation of the tungsten into the TiO2 crystal lattice as well as modification surface properties.

Nanocrystalline TiO2/SnO2 heterostructures for gas sensing

The aim of this research is to study the role of nanocrystalline TiO2/SnO2 n–n heterojunctions for hydrogen sensing. Nanopowders of pure SnO2, 90 mol % SnO2/10 mol % TiO2, 10 mol % SnO2/90 mol % TiO2 and pure TiO2 have been obtained using flame spray synthesis (FSS). The samples have been characterized by BET, XRD, SEM, HR-TEM, Mössbauer effect and impedance spectroscopy. Gas-sensing experiments have been performed for H2 concentrations of 1–3000 ppm at 200–400 °C. The nanomaterials are well-crystallized, anatase TiO2, rutile TiO2 and cassiterite SnO2 polymorphic forms are present depending on the chemical composition of the powders. The crystallite sizes from XRD peak analysis are within the range of 3–27 nm. Tin exhibits only the oxidation state 4+. The H2 detection threshold for the studied TiO2/SnO2 heterostructures is lower than 1 ppm especially in the case of SnO2-rich samples. The recovery time of SnO2-based heterostructures, despite their large responses over the whole measuring range, is much longer than that of TiO2-rich samples at higher H2 flows. TiO2/SnO2 heterostructures can be intentionally modified for the improved H2 detection within both the small (1–50 ppm) and the large (50–3000 ppm) concentration range. The temperature T max at which the semiconducting behavior begins to prevail upon water desorption/oxygen adsorption depends on the TiO2/SnO2 composition. The electrical resistance of sensing materials exhibits a power-law dependence on the H2 partial pressure. This allows us to draw a conclusion about the first step in the gas sensing mechanism related to the adsorption of oxygen ions at the surface of nanomaterials.