A. Bonavita

CO and NO2 sensing properties of doped-Fe2O3 thin films prepared by LPD

Abstract The CO and NO2 sensing properties of iron oxide thin films doped with Au and Zn and prepared by a liquid-phase deposition method (LPD) have been investigated. The undoped Fe2O3 sensor was found to be sensitive to NO2 but not to CO. The addition of Zn increases the sensitivity at low temperature towards NO2 and decreases strongly the intrinsic resistance of the iron oxide film. The addition of gold was necessary to obtain a detectable response to CO, the Au-doped ZnO sensor was found the most sensitive to CO. By operating at an appropriate temperature, these sensors could be able to detect selectively CO and NO2, with negligible humidity cross-sensitivity.

Vanadium Oxide Sensing Layer Grown on Carbon Nanotubes by a New Atomic Layer Deposition Process

A new atomic layer deposition (ALD) process was applied for the homogeneous coating of carbon nanotubes with vanadium oxide. It permits the coating of the inner and outer surface with a highly conformal film of controllable thickness and, hence, the production of high surface area hybrid materials at a so far unprecedented quality. The ALD-coated tubes are used as active component in gas-sensing devices. They show electric responses that are directly related to the peculiar structure, i.e., the p-n heterojunction formed between the support and the film.

Humidity sensing properties of Li–iron oxide based thin films

Abstract Li-doped iron oxide thin films deposited on a porous ceramic substrate by a liquid-phase method (LPD) were investigated as humidity sensors. Large variations in the resistance, up to about 4–5 order of magnitude, were observed by changing the relative humidity (RH) between 10 and 90%. The investigated sensors show a quick and reversible response to cyclic variations in the RH. The effects of pretreatment and Li content were investigated. The role of Li on the response to water vapour of iron oxide thin films is discussed.

Synthesis, Characterization and Gas Sensing Properties of Ag@α-Fe2O3 Core-Shell Nanocomposites

Ag@α-Fe2O3 nanocomposite having a core–shell structure was synthesized by a two-step reduction-sol gel approach, including Ag nanoparticles synthesis by sodium borohydride as the reducing agent in a first step and the subsequent mixing with a Fe+3 sol for α-Fe2O3 coating. The synthesized Ag@α-Fe2O3 nanocomposite has been characterized by various techniques, such as SEM, TEM and UV-Vis spectroscopy. The electrical and gas sensing properties of the synthesized composite towards low concentrations of ethanol have been evaluated. The Ag@α-Fe2O3 nanocomposite showed better sensing characteristics than the pure α-Fe2O3. The peculiar hierarchical nano-architecture and the chemical and electronic sensitization effect of Ag nanoparticles in Ag@α-Fe2O3 sensors were postulated to play a key role in modulating gas-sensing properties in comparison to pristine α-Fe2O3 sensors.

Gold promoted Li-Fe2O3 thin films for humidity sensors

Abstract A study of iron oxide thin films doped with Li and Au for humidity sensing applications is reported. The characteristics of sensitivity, linearity and stability of humidity sensors based on these films were investigated and correlated to the structural and textural properties of the sensing layer. Such properties are influenced by the nature of the dopant and the thermal treatment. Li creates preferential adsorption sites for the H2O molecules thus increasing the sensitivity at low RH. Gold instead reduces the drift of the resistance baseline stabilizing the presence of amorphous iron oxide phases. By optimizing the thermal treatment of Au-doped Li–Fe2O3 thin films, humidity sensors with high sensitivity, linearity and good stability have been developed.

A comparison of the ethanol sensing properties of α-iron oxide nanostructures prepared via the sol–gel and electrospinning techniques

Haematite (α-Fe2O3) nanostructures were synthesized via a Pechini sol-gel method (PSG) and an electrospinning (ES) technique. Their texture and morphology were investigated by scanning and transmission electron microscopy. α-Fe2O3 nanoparticles were obtained by the PSG method, whereas fibrous structures consisting of interconnected particles were synthesized through the ES technique. The crystallinity of the α-Fe2O3 nanostructures was also studied by means of x-ray diffraction and Raman spectroscopy. Gas-sensing devices were fabricated by printing the synthesized samples on ceramic substrates provided with interdigitated Pt electrodes. The sensors were tested towards low concentrations of ethanol in air in the temperature range (200-400 °C). The results show that the α-Fe2O3 nanostructures exhibit somewhat different gas-sensing properties and, interestingly, their sensing behaviour is strongly temperature-dependent. The availability of active sites for oxygen chemisorption and the diffusion of the analyte gas within the sensing layer structure are hypothesized to be the key factors responsible for the different sensing behaviour observed.

HREELS study of Au/Fe2O3 thick film gas sensors

Abstract CO gas sensors based on Au-doped/Fe 2 O 3 thick films were studied in ultra-high-vacuum (UHV) by high resolution electron energy loss spectroscopy (HREELS). Several adsorbed species were identified on HREEL spectra collected at room temperature on thick films annealed at different temperatures. Among these, carboxylate and formate species were postulated as intermediates of the complex reactions occurring on the surface between carbon monoxide and oxygen. The catalytic and sensing properties of the Au/iron oxide samples towards CO have been also investigated showing the better performances of the co-precipitated samples in comparison to impregnated ones and the influence of annealing temperature. Data reported were correlated to the presence of oxygenate intermediates detected by HREELS.

ZnO:Ca nanopowders with enhanced CO2 sensing properties

Calcium doped ZnO (CZO) nanopowders with [Ca]/[Zn] atomic ratios of 0, 0.01, 0.03 and 0.05 were prepared via a sol-gel route and characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction and Fourier transform infrared spectroscopy (FT-IR). Characterization data showed that undoped and Ca-doped ZnO samples have a hexagonal wurtzite structure with a slight distortion of the ZnO lattice and no extra secondary phases, suggesting the substitution of Ca ions in the ZnO structure.Chemo-resistive devices based on a thick layer of the synthesized CZO nanoparticles were fabricated and their electrical and sensing properties towards CO2 were investigated. Sensing tests have demonstrated that Ca loading is the key factor in modulating the electrical properties and strongly improving the response of ZnO matrix towards CO2. An increased CO2 adsorption with Ca loading has been also evidenced by FT-IR, providing the basis for the formulation of a plausible mechanism for CO2 sensing operating on these sensors.

Design and Development of a Breath Acetone MOS Sensor for Ketogenic Diets Control

The design of a handy and noninvasive measurement of acetone in the human breath, which is useful for ketosis monitoring and control, by means of MOS sensors is reported. For this aim, highly sensitive resistive sensors in thick film configuration, fabricated depositing sensing layers of In2O 3 and Pt-In2O 3 nanopowders by screen-printing, have been developed. The devices were calibrated using standard gases in the laboratory, showing high sensitivity and a linear response in the range of acetone concentration investigated (1-100 ppm). The results obtained indicate that MOS sensors based on Pt-In2O 3 nanopowders are promising as fast and quantifiable means of determining acetone in the breath, posing the advantages of real time measurements and low costs devices for the control of ketogenic diets.

Preparation, characterization and CO sensing of Au/iron oxide thin films

The preparation and characterization of Au-doped iron oxide thin films is reported. Doped-thin films have been prepared by a liquid-phase deposition (LPD) method from aqueous solutions in the presence of a reactive atmosphere containing NH3. The morphology and microstructure of the prepared films are described. The effect of some process parameters (precursor solution concentration, calcination temperature) on the film formation, thickness and microstructure are investigated. Sensor devices based on Au/Fe2O3 thin films deposited on alumina by the LPD method, are sensitive to CO at operating temperatures between 200–400 °C.