Michele Sacerdoti

MoO3-based sputtered thin films for fast NO2 detection

Abstract The authors report about preparation and characterization of thin films of MoO3 as a material for gas-sensing applications. Structural investigation of the films is carried out by electron microscopy and X-ray diffraction techniques. The sensing behavior of the MoO3 films was tested to NO2, showing capability to detect a few ppm of NO2 with considerably short response time.

Sol-gel processed TiO2-based nano-sized powders for use in thick-film gas sensors for atmospheric pollutant monitoring

Sol-gel routes were used to prepare pure and 5 at% and 10 at% Ta- or Nb-dope TiO2 nano-sized powders. The thermal decomposition behaviour of the precursors was studied using simultaneous thermogravimetric and differential thermal analysis (TG/DTA). X-ray diffraction (XRD) analysis showed that the powders heated to 400°C were crystalline in the anatase TiO2 structure. The pure TiO2 powder heated to 850°C showed the rutile structure. The addition of Ta and Nb inhibited the anatase-to-rutile phase transformation up to 950–1050°C. Ta was soluble in the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films were fabricated with these powders by screen printing technology and then fired for 1 h at different temperatures in the 650–1050°C range. Scanning electron microscopy (SEM) observations showed that the anatase-to-rutile phase transformation induces a grain growth of about one order of magnitude for pure TiO2. The addition of Ta and Nb is effective to keep the TiO2 grain size at a nanometric level even at 950°C, though grain growth was observed with increasing temperature. The gas-sensitive electrical response of the thick films were tested in laboratory, in environments with CO in dry and wet air. Conductance measurements showed a good gas response only for the nanostructured titania-based films. For field tests, the prototype sensors were placed beside a conventional station for atmospheric pollutant monitoring. The electrical response of the thick films was compared with the results of the analytical instruments. The same trend was observed for both systems, demonstrating the use of gas sensors for this aim.

Characterization of a molybdenum oxide sputtered thin film as a gas sensor

We report about preparation and characterization of sputtered MoO3 thin film as sensing layer for gas detection. We show its capability to detect CO concentrations lower than 10 ppm in wet air, a feature that allows direct usage of this material for environmental monitoring. The research also highlights some criteria for selecting a suitable material and shows which features are important for a thin film to be a good candidate as a chemical sensor.

Doping of a nanostructured titania thick film: structural and electrical investigations

Abstract Thick films of nanostructured pure TiO 2 , Nb/TiO 2 , Ga/TiO 2 , and Ta/TiO 2 have been fabricated by screen-printing technology. Structural, morphological, and optical studies have been carried out in order to correlate the measurements to electrical performance. We showed that addition of dopants inhibits grain growth due to firing and hinders conversion of anatase to rutile. The films were characterised electrically in terms of conduction properties and grain-to-grain energy barrier. The films proved sensitive to CO and NO 2 . It was also shown that firing temperature and dopant content strongly influence the gas response of the films.

Gas sensing through thick film technology

Abstract We report in our research on semiconductor-based sensing layers deposited via thick-film technique. Particular focus was devoted to achieve nanosized films through proper processing and to study their morphological and structural features. Nanosized powders were prepared by sol–gel method or laser-assisted spray pyrolysis. We also considered some techniques to maintain the stability of a nanostructure for long-term usage of the sensing layers. We detailed the preparation of screen printing pastes suitable for gas sensing application. Implementation of the sensing film on a low-power-consumption micromachined hotplate has also been addressed. The performance of such devices is presented and compared to that of conventional units.

Sol-Gel Nanosized Semiconducting Titania-Based Powders for Thick-Film Gas Sensors

Pure, 5 at%, and 10 at% Ta- or Nb-doped TiO2 nanosized powders were prepared by the sol-gel method. The powders heated to 400°C have the crystalline anatase structure. While the pure TiO2 powder heated to 850°C has the rutile structure, the addition of Ta and Nb inhibited the anatase-to-rutile phase transformation at this temperature. Ta was soluble in the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films were fabricated with these powders by screen printing technology and then fired at 650°C and 850°C for 1 h. SEM observations showed that the anatase-to-rutile phase transformation induces a grain growth of about one order of magnitude for pure TiO2. The addition of Ta and Nb is effective to keep the TiO2 grain size at the nanometric level even at 850°C. Conductance measurements showed that a good gas response is observed only for the nanostructured titania-based films. The CO response of these materials is only slightly affected by humidity.

Thick-film gas sensors based on vanadium–titanium oxide powders prepared by sol-gel synthesis

Two titania powders modified by 10 at.% of vanadium were prepared by two different sol-gel routes. The powders fired at 650 °C had the rutile structure. These powders were used to produce prototype thick-film sensors. Four series of thick-film samples were fabricated by screen-printing, fired for 1 h at 650 and 850 °C. The morphology and gas-sensing properties were examined and compared with those of pure and Ta-added titania films, previously studied by the authors. Ta addition inhibited the anatase-to-rutile phase transformation during heating and was also effective in keeping the TiO2 grain size in the nanometre range. On the contrary, V addition facilitated the anatase-to-rutile phase transformation. Thick films obtained from the two powders had similar conductance behaviour vs. temperature. The gas response of the films was affected by both the grain size and firing temperature.

In situ time resolved synchrotron powder diffraction study of mordenite

The step by step thermal dehydration process of the zeolites mordenite from Pashan (Poona, India) (Na 3.51 K 0.14 Ca 1.89 Mg 0.09 Sr 0.01 )[Fe 3+ 0.03 Al 7.40 Si 40.53 O 96 ] · 27.26H 2 O, has been studied in situ by synchrotron powder diffraction. The time-resolved experiment was performed using a translating imaging plate system. The structure refinements by full profile Rietveld analysis were performed in the Cmcm space group in the temperature range from 25 to 830°C. The results of structure refinements indicate that the slight cell-volume contraction (∼ 1.9%) is related to the release of water molecules from the channels: above 375°C, the water loss lead to an enlargement of the 8-membered ring parallel to [010], which occurred by a flattening of T3-O9-T3 and T4-O4-T3 bridges. The dehydration process is reflected not only in the content of water molecules in the zeolite channel, but also in the temperature behaviour of the unit cell parameters. The cell parameters b and c decreased regularly as the temperature rose, whereas a decreased up to 400°C then increased up to 630°C, and finally decreased until the end of experiment. The removal of water molecules was accompanied by a spreading of the initial Ca sites into many positions bonded to the framework oxygens. The increased interaction with the framework oxygens of Ca sites was intimately related to the distortion of the 12-ring which is in turn related to the lengthening of the a cell parameter.

Single crystal neutron diffraction study of the natural zeolite barrerite in its ND 4 -exchanged form

Barrerite single crystals from Kuiu Island (Alaska) were treated in a ND 4 Cl aqueous solution to obtain the ND 4 -exchanged form. The crystal structure of the exchanged barrerite (Na 0.85 K 0.24 Mg 0.38 Ca 1.30 Ba 0.02 (ND 4 ) 10.52 Al 15.01 Si 56.99 O 144 ·41D 2 O, a = 13.601 A, b = 18.232 A, c = 17.810 A, space group Fmmm ) was studied by single crystal neutron diffraction on data collected at 20 K at the D19 line at ILL (Grenoble). A comparison with a previously studied NH 4 -exchanged barrerite showed that the framework was identical in the two refinements, and that a substantial agreement occurred also in the extraframework content in the residual cations, in D 2 O molecules and in ND 4 + ions. Surprisingly, no evidence was found of D atoms either around nitrogen or around the oxygens of water molecules. This result could be interpreted as a consequence of the partial occupancy of the extraframework sites and the large distances of these sites from framework oxygens, with consequent weak hydrogen bonds, which make a disorder in the orientation of D 2 O and ND 4 groups highly probable.

Enhanced Gas Sensing Properties of Different ZnO 3D Hierarchical Structures

Six different ZnO nanomorphologies were synthesized trough wet chemical routes starting from a water solution of zinc nitrate hexahydrate, obtaining two types of morphologies: bidimensional nanocrystals and nanoparticles aggregates. Powders and films characterizations have been carried out by means of TG–DTA, SEM, and X-ray diffraction analysis. Finally, electrical measurements were performed with the aim to compare conductive properties of the thick films, surface barrier heights and gas sensing features, mainly versus acetone and other VOCs related to the breath gas analysis. Among the different morphologies tested, it turned out that the samples constituted by nanoparticle aggregates exhibited the best performances versus all gases, but especially toward acetone at sub-ppm level.