Y.X. Li

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.

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 of sol-gel prepared CeO2-TiO2 thin films for oxygen gas sensing

Abstract The oxygen gas sensing performance of semiconducting CeO 2 –TiO 2 thin films have been investigated. These thin films have been prepared by the sol–gel process utilizing a non-alkoxide as the main precursors. For gas sensing measurements, the films were deposited by the spin coating technique onto alumina substrates with interdigital transducers located on the top and a micro-heater on the bottom. For the microstructural characterization, the thin films were deposited onto single crystal silicon substrates. X-ray photoelectron spectroscopy (XPS), Auger electron spectrometry (AES) and scanning electron microscopy (SEM) were employed to analyze the films. These films were exposed to various concentrations of O 2 gas and their electrical responses were measured.

Semiconductor MoO3–TiO2 thin film gas sensors

Abstract The O 2 , CO and NO 2 gas sensing properties of MoO 3 –TiO 2 thin films have been studied. The sol–gel process was employed to fabricate MoO 3 –TiO 2 thin films onto sapphire and alumina transducers for gas sensing measurements. It was found the MoO 3 dominated sensors have a lower optimal operating temperature of 370°C than the TiO 2 dominated sensors. The response of the sensors was stable and reproducible at operating temperatures below 400°C. However, once the films were exposed to temperatures higher than 400°C, repeatable gas sensing results could not be achieved. The low evaporation temperature of MoO 3 component of the mixed system is believed to be the cause of the sensors instability and irreversibility at high operating temperatures.

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.

A Pt/Ga/sub 2/O/sub 3/-ZnO/SiC Schottky diode-based hydrocarbon gas sensor

In this paper, a novel metal-reactive insulator-silicon carbide device with a catalytic layer for hydrocarbon gas-sensing is presented. This structure, employed as a Schottky diode, utilizes sol-gel prepared Ga/sub 2/O/sub 3/-ZnO layer as the reactive insulator. The sensor has been exposed to propene gas, which lowers the barrier height of the diode. The responses were stable and repeatable at operating temperatures between 300 and 600/spl deg/C. The response to propene in different ambients was examined. The effect of diode bias has been investigated by analyzing the sensors response to various propene concentrations when held at constant currents of 2 and 8 mA.

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...

O/sub 2/ sensing properties of Zn- and Au-doped Fe/sub 2/O/sub 3/ thin films

Zn- and Au-doped iron oxide thin films have been prepared by liquid phase deposition. These films have been characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Their performance as oxygen gas sensors has been measured. It has been shown that both the Zn and Au dopants increase the oxygen response of the pure iron oxide films. The XRD and SEM results show that Zn changes both the microstructure and the particles size of the sensing layer through the formation of a solid solution with iron oxide. However, the strong increase in sensitivity to oxygen of the Au-doped Fe/sub 2/O/sub 3/ film has been related to the more favorable chemisorption of oxygen on the small gold particles at the interface with the semiconductor oxide. The results show that Au-doped iron oxide sensors are most promising for oxygen gas sensing.

A layered structure surface acoustic wave for oxygen sensing

A novel layered structure surface acoustic wave (layered SAW) transducer has been employed for an oxygen sensing application. It is a SiO/sub 2/(0.36 /spl mu/m)/ST-cut quartz crystal transducer. The dominant mode propagating in the transducer is a combination of Rayleigh and Love modes. Such a structure has the advantage of confining the acoustic wave energy to the top selective layer, which increases the sensitivity of the device. A TiO/sub 2/ thin film deposited by the sol-gel process has been used as the oxygen sensitive layer.

A Pt/Ga/sub 2/O/sub 3//SiC Schottky diode based hydrocarbon gas sensor

This paper presents the propene gas sensing performance of Pt/Ga/sub 2/O/sub 3//SiC based Schottky diodes. The metal oxide semiconducting Ga/sub 2/O/sub 3/ thin films were doped with Zn and prepared by the sol-gel process. The thin films were deposited onto the SiC by the spin coating technique and a Pt layer was deposited on the top of the metal oxide forming the Schottky diode. The sensor responses were stable and repeatable towards propene at operating temperatures between 300 and 600/spl deg/C. The diodes were biased at a constant current of 2 mA. When exposed to 1,900 ppm of propene at an operating temperature of 525/spl deg/C, a shift of 85 mV was observed.