Maria Luisa Di Vona

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.

Durability of sulfonated aromatic polymers for proton-exchange-membrane fuel cells.

As a key component of proton-exchange-membrane fuel cells (PEMFCs), proton-exchange membranes (PEMs) must continuously withstand very harsh environments during long-term fuel cell operations. With the coming commercialization of PEMFCs, investigations into the durability and degradation of PEMs are becoming more and more urgent and interesting. Herein, various recent attempts and achievements to improve the durability of sulfonated aromatic polymers (SAPs) are reviewed and some further developments are predicted. Extensive investigations into inexpensive SAPs as alternative electrolyte membranes include modification of available polymer materials; design, synthesis, and optimization of new macromolecules; durability testing; and exploring the degradation mechanisms.

Sol-gel preparation and characterization of Ag-TiO2 nanocomposite thin films

Ag-TiO2 thin films were prepared with a sol-gel route, using titanium isopropoxide and silver nitrate as precursors, at 0.03 and 0.06 Ag/Ti nominal atomic ratios. After drying at 80°C, the films were fired at 300°C and 500°C for 30 min. The films were analysed by X-ray diffraction (XRD) with glancing angle, and X-ray photoelectron spectroscopy (XPS), with depth profiling of the concentration. XPS analysis showed the presence of C and N as impurities in the nanocomposite films. Their concentration decreased with increasing the firing temperature. Chemical state analysis showed that Ag was present in metallic state, except for the very outer layer where it was present as Ag+. For the films prepared with a Ag/Ti concentration of 0.06, depth profiling measurements of the film fired at 300°C showed a strong Ag enrichment at the outer surface, while composition remained almost constant within the rest of the film, at 0.019. For the films heated to 500°C, two layers were found, where the Ag/Ti ratios were 0.015 near the surface and 0.026 near the substrate.

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.

Photoelectrochemical properties of sol-gel processed Ag-TiO2 nanocomposite thin films

Ag-TiO2 thin films were prepared with a sol-gel route, using titanium isopropoxide and silver nitrate as precursors, at 0, 0.03 and 0.06 Ag/Ti nominal atomic ratios. After drying at 80°C, the films were fired at 300°C, 500°C, and 600°C for 30 min and 5 h. Glancing angle X-ray diffraction (XRD) analysis and X-ray photoelectron spectroscopy (XPS), with depth profiling of the concentration, were used to study the films. XPS analysis showed the presence of C and N as impurities in the nanocomposite films. Their concentration decreased with increasing the firing temperature. Chemical state analysis showed that Ag was present in metallic state, except for the outer layer where it was present as Ag+. For the films prepared with a Agt/Ti concentration of 0.06, depth profiling measurements of the film fired at 300°C showed a strong Ag enrichment at the outer surface, while composition remained almost constant within the rest of the film, at Ag/Ti atomic ratio of 0.02. Two layers were found for the films heated to 500°C, where the Ag/Ti ratios were 0.015 near the surface and 0.03 near the substrate. The photoelectrochemical properties of Ag-TiO2 were studied for thin films deposited on ITO substrates. Photocurrents of Ag-TiO2 nanocomposite electrodes fired at 300°C were observed even at visible light, for wavelengths longer than 400 nm.

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.

Sulfonated aromatic polymers as proton-conducting solid electrolytes for fuel cells: A short review

Abstract This review describes main strategies for the development of sulfonated aromatic polymers (SAP) with optimal properties for medium temperature (90–120 ºC) polymer electrolyte membrane fuel cell applications. SAP are promising economical polymer electrolytes, but there still exist some challenges about these materials due mainly to excessive swelling in water, poor mechanical strength and low dimensional stability. Here, the state-of-the-art of SAP is reviewed and the main focus will be directed to properties of SAP, including proton conductivity, water uptake, mechanical strength, permeability. Especially three approaches to improve the performances are addressed: the formation of copolymers, the formation of hybrid materials and the polymer reticulation.

Fluoride-ion-conducting Polymers: Ionic Conductivity and Fluoride Ion Diffusion Coefficient in Quaternized Polysulfones

We describe the three-step synthesis of a new polymeric fluoride ion conductor based on the fully aromatic polymer polysulfone (PSU). In the first step, PSU is chloromethylated (CM-PSU) using reagents (i.e., stannic chloride, paraformaldehyde, and trimethylchlorosilane) that are less toxic than those used in the standard procedure. In the second step, CM-PSU reacts with a tertiary amine (trimethylamine or 1,4-diazabicyclo[2.2.2]octane) to form quaternary ammonium groups fixed on the PSU backbone and mobile chloride counter-anions. The chloride ions can, in a third step, be exchanged with fluoride ions by immersion of the ionomer in NaF solution. The fluoride ion conductivity reaches 3-5 mS cm(-1) at 25 °C and 5-10 mS cm(-1) at 40 °C. We determined the F(-) diffusion coefficient in these ionomers by pulsed gradient spin-echo (PGSE) high-resolution magic angle spinning (HRMAS) nuclear magnetic resonance (NMR) spectroscopy and by impedance spectroscopy using the Nernst-Einstein relation. The diffusion coefficients determined by the two methods are in good agreement, ranging from 2 to 4×10(-10)  m(2)  s(-1) . The porosity and tortuosity of the ionomer membranes can be estimated.

Synthesis and Characterization of Novel Ionoconductor Gels for Biomedical Applications in Space

Under the framework of the project Anomalous Long Term Effects in Astronauts, an experiment to study the risk for functional brain anomalies due to cosmic radiation during long manned missions, a new gel-type polymer electrolyte membrane for recording bioelectric signals in space has been developed. Ionic conducting gels have been formed by immobilizing liquid solutions, containing LiX salts (X 5 ClO4 ,C F 3SO3) dissolved in 1,2-diethoxyethane ~gly!, using poly~methyl methacrylate !~ PMMA! as the polymeric matrix. The electrical properties of these membranes have been studied using impedance spectroscopy. The membranes exhibit good mechanical and chemical stability and high conductivity at room temperature (10 23 to 10

Stabilized sulfonated aromatic polymers by in situ solvothermal cross-linking

The cross-link reaction via sulfone bridges of sulfonated polyetheretherketone (SPEEK) by thermal treatment at 180 °C in presence of dimethylsulfoxide (DMSO) is discussed. The modifications of properties subsequent to the cross-linking are presented. The mechanical strength as well as the hydrolytic stability increased with the thermal treatment time, i.e., with the degree of cross-linking. The proton conductivity was determined as function of temperature, IEC, degree of cross-linking and hydration number. The memory effect, which is the membrane ability to “remember” the water uptake reached at high temperature also at lower temperature, is exploited in order to achieve high values of conductivity. Membranes swelled at 110 °C can reach a conductivity of 0.14 S/cm at 80°C with a hydration number () of 73.