W. Wlodarski

XPS study of Nb-doped oxygen sensing TiO2 thin films prepared by sol-gel method

Abstract Titanium dioxide (TiO2) thin films have been prepared using the sol-gel method and subsequently doped with niobium oxide (Nb2O5) for use in oxygen sensing applications. The chemical composition of the resultant film sensor surface has been investigated using X-ray photoelectron spectroscopy (XPS). Films are essentially stoichiometric with carbon as the dominant impurity at the surface. The film electrical resistance has been examined for detection of oxygen at concentrations of 1 ppm to 1%. Doping resulted in a 40% increase in the oxygen gas sensitivity of the thin films at an operating temperature, as low as 190°C.

Investigation on the O3 sensitivity properties of WO3 thin films prepared by sol–gel, thermal evaporation and r.f. sputtering techniques

WO3 thin films have been deposited on alumina substrates provided with platinum interdigital electrodes by sol–gel (SG), r.f. sputtering (RFS), and vacuum thermal evaporation (VTE) techniques and annealed at temperatures between 500°C and 600°C for 1 to 30 h in static air. The morphology, crystalline phase and chemical composition of the films have been characterised using SEM, glancing XRD and XPS techniques. The electrical response has been measured exposing the films to O3 (10–180 ppb), NO2 (0.2–1 ppm), NOx (27 ppm NO and 1 ppm NO2) at different operating temperatures ranging between 200 and 400°C and humid air at 50% R.H. SG prepared films have shown bigger responses (S=IAir/Igas) with respect to VTE and RFS for all the investigated gases and operating temperatures. RFS prepared has resulted to be less sensitive, but faster in the response and more stable in terms of signal reproducibility. The response to O3 has been found to be at maximum at 400°C. At this temperature the response to 80 ppb of ozone has been: S=35 (SG), S=18 (VTE) and S=5 (RFS). The NO2 and NOx response reached the maximum at 200°C and becomes negligible at 400°C. Improvements on the O3 gas sensitivity and selectivity can be achieved by fixing the operating temperature of the films at 400°C.

Titanium dioxide thin films prepared for alcohol microsensor applications

Abstract Nanosized TiO2 thin films with different doping concentrations on alumina substrates were prepared using a sol–gel process for alcohol sensing. Experimental results indicated that the sensor is able to monitor alcohols selectively at ppm levels. The samples are insensitive (S

Nanorod based Schottky contact gas sensors in reversed bias condition

There has been significant interest in using electronically contacted nanorod or nanotube arrays as gas sensors, whereby an adsorbate modifies either the impedance or the Fermi level of the array, enabling detection. Typically, such arrays demonstrate the I-V curves of a Schottky diode that is formed using a metal-semiconductor junction with rectifying characteristics. We show in this work that nanostructured Schottky diodes have a functionally different response, characteristic of the large electric field induced by the size scale of the array. Specifically, they are characterized by a low reverse breakdown voltage. As a result, the reverse bias current becomes a strong function of the applied voltage. In this work, for the first time, we model this unique feature by describing the enhancement effect of high aspect ratio nanostructures on the I-V characteristics of a Schottky diode. A Pt/ZnO/SiC nanostructured Schottky diode is fabricated to verify the theoretical equations presented. The gas sensing properties of the Schottky diode in reversed bias is investigated and it is shown that the theoretical calculations are in excellent agreement with measurements.

Sensitivity enhancement towards ethanol and methanol of TiO2 films doped with Pt and Nb

Abstract This paper presents a possibility to improve the sensitivity towards ethanol and methanol of nanocrystalline TiO2 thin films by doping with Nb and Pt. The thin films were prepared using the sol–gel process by the spin coating technique on alumina substrates. The microstructure and the chemical composition of the thin films were characterised using XRD, SEM, RBS and XPS techniques. The experimental results of the alcohol sensing characteristics indicated that the doped TiO2 thin film has an enhanced sensitivity with respect to the undoped ones. The relative change of the conductance of the thin films due to the introduction of 500 ppm of ethanol is as high as 2370% at an operating temperature of 300°C, making them feasible for development of breath analysers (detection limit is 200 ppm).

Investigation on ozone-sensitive In2O3 thin films

Abstract Indium oxide (In2O3) thin films have been prepared by sol–gel and RF sputtering techniques. The sol–gel film appeared to be much more sensitive to ozone compared to the RF sputtered film. The morphology of both films was examined by SEM, while their chemical composition was analyzed using X-ray photoelectron spectroscopy (XPS) in order to understand the properties responsible for the high sensitivity of sol–gel films. The examination results showed that the sol–gel films are very porous and uniform in grain size, but the RF films are relatively dense and of coalescent grains. XPS analysis also highlighted that there may be oxygen vacancies on the surface of sol–gel films. The large surface areas of sol–gel films and oxygen deficiency in the film structure are responsible for its higher sensitivity.

Carbon monoxide response of molybdenum oxide thin films deposited by different techniques

Abstract MoO3 thin films have been deposited on alumina substrates by radio frequency (RF) sputtering from a metallic molybdenum target in a reactive atmosphere and by the sol–gel (SG) technique using molybdenum ethoxide solutions. The as-deposited RF films have been annealed at 500°C for 1 h, while the SG films have been annealed at temperature range between 400°C and 700°C for 1 h. The formation of a well-developed nanoparticle structure for the RF films with respect to the SG ones was suggested by scanning electron microscopy (SEM) characterisation. X-ray diffraction (XRD) has confirmed the formation of crystalline orthorhombic MoO3 structures (JCPDS 5-508) for both the RF and SG films after annealing. The gas sensing properties towards CO have been examined. MoO3-based gas sensors developed are capable of CO down to few ppm with a very fast response.

NO2 response of In2O3 thin film gas sensors prepared by sol-gel and vacuum thermal evaporation techniques

In2O3 thin films have been prepared by high vacuum thermal evaporation (HVTE) and by sol–gel (SG) techniques. The deposited HVTE and SG films have been annealed at 500°C for 24 and 1 h, respectively. After annealing at 500°C, the films are highly crystalline cubic In2O3. XPS characterization has revealed the formation of stoichiometric In2O3 (HVTE) and nearly stoichiometric In2O3−x (SG). SEM characterization has highlighted substantial morphological differences between the SG (highly porous microstructure) and HVTE (denser) films. All the films show the highest sensitivity to NO2 gas (0.7–7 ppm concentration range), at 250°C working temperature. Negligible H2O cross has resulted in the 40–80% relative humidity range. Only 1000 ppm C2H5OH has resulted in a significant cross to the NO2 response.

Investigation on the cross sensitivity of NO2 sensors based on In2O3 thin films prepared by sol-gel and vacuum thermal evaporation

In2O3 thin films have been prepared from commercially available pure In2O3 powders by high vacuum thermal evaporation (HVTE) and from indium iso-propoxide solutions by sol-gel techniques (SG). The films have been deposited on sapphire substrates provided with platinum interdigital sputtered electrodes. The as-deposited HVTE and SG films have been annealed at 500°C for 24 and 1 h, respectively. The film morphology, crystalline phase and chemical composition have been characterised by SEM, glancing angle XRD and XPS techniques. After annealing at 500°C the films’ microstructure turns from amorphous to crystalline with the development of highly crystalline cubic In2O3−x (JCPDS card 6-0416). XPS characterisation has revealed the formation of stoichiometric In2O3 (HVTE) and nearly stoichiometric In2O3−x (SG) after annealing. SEM characterisation has highlighted substantial morphological differences between the SG (highly porous microstructure) and HVTE (denser) films. All the films show the highest sensitivity to NO2 gas (0.7–7 ppm concentration range), at 250°C working temperature. At this temperature and 0.7 ppm NO2 the calculated sensitivities (S=Rg/Ra) yield S=10 and S=7 for SG and HVTE, respectively. No cross sensitivity have been found by exposing the In2O3 films to CO and CH4. Negligible H2O cross has resulted in the 40–80% relative humidity range, as well as to 1 ppm Cl2 and 10 ppm NO. Only 1000 ppm C2H5OH has resulted to have a significant cross to the NO2 response.

Characterization of sol-gel prepared WO3 thin films as a gas sensor

Tungsten trioxide (WO3) thin films have been prepared by the sol-gel process and annealed at different temperatures of 400, 500, 600, and 700 °C for 1 h. The morphology, microstructure, crystalline structure, and composition of the films have been analyzed using scanning electron microscopy (SEM), x-ray diffraction, Rutherford backscattering spectroscopy (RBS), and x-ray photoelectron spectroscopy (XPS) techniques. The SEM analysis showed that the films annealed at 400 °C are smooth and uniform. However, these evolved as granular at an annealing temperature of 500 °C. The films annealed at still higher temperatures have two distinct grains of different shapes and sizes. The films annealed below 400 °C are amorphous. Annealing at 500 °C resulted in the films having polycrystalline structure. RBS and XPS characterization have revealed that the films annealed at 400 °C are stoichiometric. Annealing above this temperature resulted in the films becoming off-stoichiometric. The electrical resistance of the film...