Marco Faccio

NO2 sensitivity of WO3 thin film obtained by high vacuum thermal evaporation

Abstract The gas sensitivity, selectivity and stability properties of WO 3 thin films for the detection of NO 2 gas in the concentration range 0.2–5 ppm, have been evaluated and discussed in the light of the preparation conditions and working temperature. Thin films were obtained by evaporating high purity WO 3 powder by an electrically heated crucible at about 5 × 10 −4 Pa on sapphire substrates provided with Pt interdigital type sputtered electrodes and annealed for 1 h at 400, 500 and 600°C. The film morphology, crystalline phase and chemical composition were characterised through AFM, low angle XRD and XPS. The electrical response was measured by means of DC current mode. The annealed films showed crystallographic orientation belonging to the triclinic structure of WO 3 , while the as-deposited films were found to be amorphous. The binding energies of O 1s and W 4f confirmed the existence of the WO 3 phase, with a stoichiometric ratio close to the theoretical one. All the films showed the highest sensitivity to NO 2 at a working temperature of 200°C. The 500°C annealed film was found to be the most sensitive to NO 2 gas, compared to those annealed at 400 and 600°C. No cross sensitivity effects were found by exposing the sensors to CO, CH 4 . WO 3 films showed strong sensitivity to C 2 H 5 OH and H 2 O. Long term stability test at a working temperature of 350°C, performed by cycling the films in dry air and 5 ppm NO 2 revealed no substantial change in the electrical properties in terms of drift and sensitivity.

An electronic nose for food analysis

Since the first developments of electronic noses, food analysis has been considered as one of its most useful applications. In this paper an electronic nose based on quartz microbalances coated with metallo-porphyrins and related compounds is presented and illustrated. Extensive tests on various substances playing key roles in food analysis show that sensing properties of the sensing materials (in terms of sensitivity and selectivity) can be exploited for electronic nose applications devoted to the analysis of various kinds of foods. The versatility of this system has been successfully tested on different kinds of foods, such as fish, meat, vegetable and wine for which results are shown.

The application of metalloporphyrins as coating material for quartz microbalance-based chemical sensors

Abstract The results of both optimization and tests to prove the suitability of an array of quartz microbalance sensors (QMBs) modified with various metalloporphyrins for the determination of food freshness are presented and discussed. As far as optimization is concerned, it was found that a minimum amount of 50 μg of metalloporphyrin must be used for the modification of the quartz microbalance sensors in order to obtain the maximum sensitivity. The sensory behavior of five different porphyrins was subsequently studied. QMBs were modified using four different meso -tetraphenylporphyrins: phenyl, p -nitrophenyl, p -bromophenyl, p -methoxyphenyl and an octa-alkylporphyrin ( etio -porphyrin I), all loaded with a Co 2+ metal ion. A clear decrease in the sensitivity for the etio -porphyrin I was observed whereas for the meso -tetraphenyl-porphyrins the best response was obtained for the p -nitrophenyl derivative. These results can be attributed to the different electron densities which are present at the metal center of the macrocycle. The determination of the response behavior with respect to methanol, thiophene, diethylamine and triethylamine of a sensor array consisting of rhodium, ruthenium, cobalt, and manganese meso -tetraphenylporphyrin revealed that there is a clear difference in terms of the sensitivity and hence, the selectivity for the various QMBs. The rhodium and the cobalt-based QMBs were alike and demonstrated a preference for the gases with soft donating sites, i.e. thiophene and the amines. The QMBs based on ruthenium and manganese demonstrated distinctly different behavior. The ruthenium-based QMB demonstrated no clear preference for gases with either hard or soft donating sites, whereas the manganese-based QMB preferred gases with hard donating sites, i.e. methanol. These results led to the overall conclusion that this sensor array could be used for the analysis of complex gas mixtures, where the most prevalent gases fall under the categories of the amines, the alcohols and the sulphides.

Cross sensitivity and stability of NO2 sensors from WO3 thin film

The H2O, C2H5OH, CO, CH4, NO and SO2 cross sensitivity to NO2 gas, as well as the long term stability of the electrical response of WO3 thin films have been evaluated and discussed in the light of different preparation conditions and working temperatures. Thin films have been obtained by evaporating high purity WO3 powder at 5 × 10−4 Pa on sapphire substrates provided with Pt interdigital sputtered electrodes and annealed at 500°C for 6, 12 and 24 h. The film morphology, crystalline phase and chemical composition have been characterized through AFM, glancing angle XRD and XPS. The as-deposited film is amorphous with WO3 stoichiometry on the surface, after annealing at 500°C the films are well crystallised but with preferential orientation of WO3 along the (200) plane. The increasing of the annealing time shows a positive effect on the crystallite and grain size of the film, while the mean roughness and surface area difference slightly decrease. The binding energies of the annealed films are close to that of WO3 and small downshifts from the characteristic binding energy of W 4f72 reflects the formation of oxygen vacancies on the longer time annealed films. All the films show the highest sensitivity to NO2 gas (0.7–5 ppm concentration range), at 250°C working temperature. At this temperature and 1.7 ppm NO2 the calculated sensitivities yield S = 12, S = 43 and S = 45 for 6, 12 and 24 h annealed films, respectively. No cross sensitivity has been found by exposing the WO3 films to CO and CH4. Negligible H2O cross to NO2 has resulted for the 24 h annealed film in the 40–80% relative humidity range, as well as to 300 ppm SO2 and 10 ppm NO. Only 1000 ppm C2H5OH has resulted in a significant cross to the NO2 measure. The increase in the annealing time had positive effects on the sensitivity, cross sensitivity and long term stability properties. The 45-fold increase in the resistance of the 24 h annealed on exposure to 1.7 ppm of NO2, as well as the good long term stability properties of its electrical response, suggest the possibility of utilising the sensor for air-quality monitoring.

NO2 gas sensitivity of sol-gel-derived α-Fe2O3 thin films

Abstract α-Fe 2 O 3 thin films have been coated on silicon and sapphire substrates with Ag, Au and Pt electrodes by sol-gel processing. Differential thermal analysis and X-ray diffraction have highlighted that the film crystallization occurs at temperatures higher than 400 °C. After heat treatments at 400, 500 and 600 °C for 30 min, the 200 nm-thick films have α-Fe 2 O 3 cystallites about 50 nm in mean size, estimated from the half-width of the (104) diffraction peak. The electrical response of the films to various gases including NO 2 and CO has been tested in the temperature range 100–300 °C. The sensors with Pt and Au electrodes show the largest resistance changes for NO 2 at 210 °C and for CO at 300 °C, respectively. For instance, the sensitivity, defined as the ratio between the resistances in gas and in air, is 36 for a Pt-electrode sensor in 100 ppm NO 2 , and 1.6 for an Au-electrode sensor in 600 ppm CO. No CH 4 and NO sensitivities have been found in the investigated samples. The electrode effect on the gas sensitivity has been reasoned in terms of the Pt and Au catalyst reaction to NO 2 and CO on α-Fe 2 O 3 films.

New ADC with piecewise linear characteristic: case study-implementation of a smart humidity sensor

In this paper, the architecture of a new analog-to-digital converter (ADC) with a piecewise linear characteristic (PLADC) is proposed. In this device, some discrete points of the characteristic can be modified to fit a requested profile. This converter facility can be utilized to gain remarkable advantages in a wide variety of applications, such as the implementation of a sensor linearization technique. In the paper the internal architecture of a two-stage 11-b flash PLADC prototype is briefly described. A representative demonstrative application, namely, the implementation of a linear digital humidity sensor, is discussed showing the effectiveness and usefulness of this device. The problem of the characterization of this converter is also discussed, reporting some remarks about the implemented solutions.

The influece of water vapour on carbon monoxide sensitivity of α-Fe2O3 microporous ceramic sensors

Abstract The influence of humidity on the carbon monoxide (CO) sensitivity of α-Fe2O3 porous ceramic sensors was investigated by precision volt-amperometric and impedance spectroscpy techniques in the 0-300 ppm CO range. The humidity varied from 0 to 95% r.h. The sensors were prepared by sintering a 130 m2/g specific area α-hematite powder at 800, 850 and 900 °C, respectively. The microstructural properties were characterized by X-ray diffraction, N2 adsorption and mercury intrusion porosimetric techniques. The variation of the resistance and conductivity activation energy were evaluated in high vacuum, dry and wet air conditions, as a function of the temperature. The resistance versus temperature relationship was explained, according to literature data, as thermally activated adsorption/desorption mechanisms of the surface oxygen-related species like O-, O2- and OH-. α-Fe2O3 reacts as an n-type semiconductor when exposed to CO and water vapour atmospheres. The maximum Co and water vapour sensitivity, expressed as ΔR/R variations, were found at 300 °C for both gases. Cross-sensitivity tests carried out at 300 and 400 °C activation temperatures showed that water vapour interferes with Co increasing the ΔR/R variation. A catalytic action of the water vapour at 300 ° C is proposed.

Microstructure and electrical properties of an α-hematite ceramic humidity sensor

Abstract α-Hematite porous ceramic material has been investigated as a humidity sensitive material, responding in the 0–95% relative humidity (r.h.) range. α-Hematite powders, with controlled microporosity, were obtained through the topotactic decomposition reaction of α-goethite and sintered at 50 °C steps in the 850–1100 °C range. SEM, TEM and Hg intrusion porosimetry techniques were employed in the microstructural characterization of the sintered compacts. The electrical response was investigated by means of the volt-amperometric technique, at 300 frequency Hz and 1Vpp signal generator, in the 0–95% r.h. range. The use of different types of electrode configurations showed that the electrical response was largely affected by their geometry. The humidity sensitivity and the response time were also found to be influenced by the microstructure, i.e. by the sintering temperature. All the results are highly reproducible.

Microstructure and electrical properties of Si-doped α-Fe2O3 humidity sensor

Abstract The humidity sensitivity of Si-doped α-hematite sintered compacts was investigated by precision volt-amperometric and impedance spectroscopy techniques in the 0–95% relative humidity (r.h.) range. 130 m2/g specific area α-hematite powders were dispersed in 3-amino-propyl-trihydroxy-silane solution and dried at 120 °C to yield 2 wt.% Si. Sintering was performed at 50 °C steps in the temperature range 850–1100 °C. Compacts were characterized by SEM, mercury intrusion and nitrogen adsorption techniques. The electrical response of Si-doped α-Fe2O3 was affected by the microstructure and sintering temperature. The 850–950 °C sintered compacts showed a linear response of impedance, in the logarithmic scale, in the whole investigated r.h. range. The variation of impedance was measured from 108 (0% r.h.) to 104 ohm (95% r.h.). The response times of the sensors were evaluated by 0–60 r.h. variations. The results were compared with undoped sintered compacts.

Advances in food analysis by electronic nose

Electronic noses have been designed and utilized for a variety of different applications. Undoubtedly, among these, food analysis has gained the major attention. In fact in food analysis there is a double opportunity for electronic nose developers. The first is that the chemical patterns considered are sometimes rather complex, so the introduction of an instrument able to consider at the same time, in an easy experimental procedure, all the chemical patterns, is certainly appealing. The second aspect of food analysis concerns the wide utilization of natural olfaction and taste. Panels of well trained tasters and smellers are daily utilized to certify the goodness of foods and their fitting with the human taste. Therefore food analysis also represents a practical field where performances of natural and artificial olfaction and taste can be compared and where an electronic nose can be utilized as an essential support of the human capabilities. In this paper some key issues concerning the application of electronic noses to food analysis are examined and examples of applications, related to the electronic nose developed at the University of Rome Tor Vergata are illustrated and discussed.