M.C. Carotta

Screen-printed perovskite-type thick films as gas sensors for environmental monitoring

Abstract Thick films of LaFeO 3 and SmFeO 3 were fabricated by screen-printing technology on alumina substrates with comb-type Au electrodes. The perovskite-type oxide powders used for the preparation of the thick films have been prepared by the thermal decomposition at 700°C of hexacyanocomplexes, Ln[Fe(CN) 6 ] ·  n H 2 O. These powders are ultrafine, homogeneous, and free of intragranular pores. The films have been fired at different temperatures in the 750–1000°C range, in N 2 and air atmospheres. The gas-sensitive electrical response of the thick films have been tested in laboratory, in environments with different gases (CO and NO 2 ) in dry and wet air. For field tests, the prototype sensors have been placed beside a conventional station for environmental monitoring. The electrical response of the thick films has been compared with the results of the analytical instruments for environmental monitoring. The same trend was observed for both systems, with very promising results.

NANOSTRUCTURED PURE AND NB-DOPED TIO2 AS THICK FILM GAS SENSORS FOR ENVIRONMENTAL MONITORING

Abstract Thick films of nanostructured TiO 2 and Niobium-doped TiO 2 have been fabricated by screen-printing technology starting from pure Titania and Niobium-doped Titania powders; the powders were prepared by laser pyrolysis method which provides nanosized particles. The laser powders are crystalline with anatase structure and an average specific surface area of approximately 100 m 2 g −1 ; their grain size ranges from 10 up to 25 nm, the particles are spherical monocristalline and without internal porosity. In this work we evidentiated that TiO 2 -based thick film sensors exhibit a suitable sensitivity to atmospheric environmental monitoring provided that the microstructural properties of the materials are suitably correlated to the required electrical features. Moreover nanostructured particles were obtained at high firing temperature by adding a proper metal ions to inhibit the grain sintering. Finally, prototype sensors based on pure titania sensing film have been prepared and tested in field for environmental monitoring application.

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.

Model for Schottky barrier and surface states in nanostructured n-type semiconductors

A semiclassical model for Schottky contacts to be applied to nanosized polycrystalline n-type semiconductors was developed. To this purpose we determined the density of surface states as a function of the mean grain radius, which establishes the Schottky barrier height. The intergranular potential shape was investigated in depletion approximation under spherical geometry and a critical revision of this method was proposed. The model was then extended to also include nanostructured materials, which could not be considered in the previous approach. Thus we were able to explain the flattening of the band bending and the decrease in the surface state density, which are experimentally observed when the granulometry is very fine.

Preparation and Characterization of Nanostructured Titania Thick Films

The extraordinary modification of bulk-material properties provided by nanosized materials, due to magnification of surface effects, has encouraged the scientific community to seek novel methodologies of synthesis. Among these, particular emphasis has been given to the techniques that envisage large-scale production and could improve the present-day technology. Nanostructured titania films possess an immense range of applications, e.g. in the field of optics, electrical insulation, photovoltaic solar cells, electrochromic displays, antibacterial coatings, photocatalytic reactors, high-performance anodes in ion batteries, and for gas sensing. Indeed, the production of nanostructured titania thin films has been recently carried out by several methods. It was also shown that some applications greatly benefited from a nanostructured phase for TiO2. [11]

Gas-sensitive electrical properties of perovskite-type SmFeO3 thick films

Abstract Ultrafine SmFeO 3 powders have been prepared by the thermal decomposition at 700°C of the corresponding hexacyanocomplex, Sm[Fe(CN) 6 ]·4H 2 O. These powders have been used for the preparation of thick films, screen-printed on alumina substrates with comb-type Au electrodes. The electrical response of the films fired at 750°C, in N 2 and air atmospheres, prepared from pastes with different composition of the organic vehicle (o.v.), showed that they were sensitive to CO and NO 2 in dry and wet air. The composition of the o.v. strongly influenced the electrical conductivity and its activation energy and the gas response of the SmFeO 3 films, which had similar microstructure. The presence in the o.v. of components with hydroxyl groups causes a reaction with adsorbed oxygen species, which results in an increase in the films resistivity, being SmFeO 3 a p -type semiconductor and in their NO 2 response. The thick film processing parameters are thus of primary concern for the NO 2 sensing properties of the SmFeO 3 thick films.

Near-infrared photoluminescence in titania: Evidence for phonon-replica effect

The photoluminescence of rutile and anatase TiO2 has been investigated in the range of 360–890 nm at several temperatures. An unexpected intense near-infrared (1.53 eV) photoluminescence band was recorded for both phases. At low temperatures, the resulting bands form a substructure of equally spaced peaks irrespective of the phase. The spectra were interpreted in the framework of the single-configuration-coordinate model as the phonon-replica effect, originating from ionization of oxygen vacancies.

Preparation and characterization of SnO2 and MoOx–SnO2 nanosized powders for thick film gas sensors

Abstract This work gives results about the characterization of SnO 2 materials, prepared via the sol–gel route, pure and Mo 6+ -added. The materials were characterized as powders or thick films using a variety of techniques. The morphology of the powders was analyzed by XRD, SEM, TEM and HRTEM, their texture by volumetric measurements. The morphology of the thick films was analyzed by SEM. The goal of obtaining powders and films made by regularly shaped and nanosized (30÷50 nm) particles, even after thermal treatments at 850°C is attained. FT-IR spectroscopic and electrical measurements were employed on powders and films, respectively, to obtain information on the electronic effect due to the molybdenum addition. FT-IR results show that Mo lowers the intensity of the light scattered by free electrons and the intensity of a broad absorption, previously assigned to the photoionization of V O + [V O + + hν →V O 2+ +e − (c.b.)]. Accordingly, electrical data show that molybdenum markedly lowers (of about 2 orders of magnitude) the conductance of the films in air. Electrical measurements show that Mo lowers the response of tin oxide towards CO, but leaves almost unaltered or enhances its ability to sense NO 2 , depending on the thermal pretreatments. Both pure and Mo-added materials treated at 650°C show the same response to NO 2 . However, for the pure material treated at 850°C the response to NO 2 is halved, while it is almost unaffected by the thermal treatment on the Mo-added materials. The sensing temperature of maximum response is in any case 150°C. FT-IR spectroscopy was also employed to obtain information on the Mo species present on the surface of the materials after treatments in oxygen and on how they are affected in the presence of the different testing gases. Furthermore surface species formed by NO 2 interaction were carefully investigated.

Preparation of nanosized titania thick and thin films as gas-sensors

Abstract Nanosized titania thick and thin films were produced and shown to possess sensing capability. Correlation between electrical performance and structural features is inferred, with particular emphasis to the role played by the grain size. The nanostructured nature of the layers allows the films to be operated within a remarkably lower temperature range (350–800°C) with respect to previously existing titania bulk sensors. The films are sensitive to CO and NO2.

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.