R. Giorgi

Enhancement of sensitivity in gas chemiresistors based on carbon nanotube surface functionalized with noble metal (Au, Pt) nanoclusters

Multiwalled carbon nanotube (MWCNT) films have been fabricated by using plasma-enhanced chemical vapor deposition system onto Cr–Au patterned alumina substrates, provided with 3nm thick Fe growth catalyst, for NO2 and NH3 gas sensing applications, at sensor temperature in the range of 100–250°C. Nanoclusters of noble metal surface catalysts (Au and Pt) have been sputtered on the surface of MWCNTs to enhance the gas sensitivity with respect to unfunctionalized carbon nanotube films. It was found that the gas sensitivity of Pt- and Au-functionalized MWCNT gas sensors significantly improved by a factor up to an order of magnitude through a spillover effect for NH3 and NO2 gas detections, respectively. The metal-functionalized MWCNT sensors exhibit very high gas sensitivity, fast response, reversibility, good repeatability, and sub-ppm range detection limit with the sensing properties of the MWCNT films tailored by surface catalyst used to functionalize the MWCNT sensors.

H2 and H2/CO oxidation mechanism on Pt/C, Ru/C and Pt–Ru/C electrocatalysts

The oxidation kinetics of H2 and H2 + 100 ppm CO were investigated on Pt, Ru and Pt–Ru electrocatalysts supported on a high-surface area carbon powder. The atomic ratios of Pt to Ru were 3, 1 and 0.33. XRD, TEM, EDS and XPS were used to characterize the electrocatalysts. When alloyed with ruthenium, a decrease in mean particle size and a modification of the platinum electronic structure were identified. Impedance measurements in H2SO4, at open circuit potential, indicated different mechanisms for hydrogen oxidation on Pt/C (Tafel–Volmer path) and Pt–Ru/C (Heyrowsky–Volmer path). These mechanisms also occur in the presence of CO. Best performances, both in H2 and H2 + CO, were achieved by the catalyst with the ratio Pt/Ru = 1. This is due to a compromise between the number of free sites and the presence of adsorbed water on the catalyst. For CO tolerance, an intrinsic mechanism not involving CO electroxidation was proposed. This mechanism derives from changes in the electronic structure of platinum when alloyed with ruthenium.

Effect of growth catalysts on gas sensitivity in carbon nanotube film based chemiresistive sensors

Multiwalled carbon nanotube (MWCNT) films have been deposited by using plasma enhanced chemical vapor deposition (PECVD) system onto Cr–Au patterned alumina substrates for NO2 and NH3 gas sensing applications, at operating temperature of 200°C. Nanoclusters of different MWCNT-growth catalysts (Fe and Co) have been predeposited onto substrates to promote the growth of the carbon nanotube films with different structural properties. It is demonstrated that the gas sensitivity of the MWCNT films depends on catalyst used for their growth with highest gas sensitivity achieved for Co-grown MWCNT films. The chemiresistor gas response is attributed to the p-type conductivity in semiconducting MWCNTs and the electrical charge transfer is found to be the major sensing mechanism. The results obtained demonstrate that the MWCNT-based sensors exhibit high gas sensitivity, fast response and reversibility, good repeatability and reproducibility, and sub-ppm range detection limit with the gas sensing properties of the MWC...

Formation process of nanocrystalline materials from x-ray diffraction profile analysis: Application to platinum catalysts

It’s well known that x-ray line profile analysis is a powerful and convenient method to probe the microstructural characteristics of nanocrystalline samples. In the literature well-documented techniques are normally used to obtain crystalline size distributions from x-ray line-broadening analysis. However, it is less considered that the shape of such size distributions may be a means to determine by which mechanism the particles have grown. A simple method is presented here to distinguish between two different growth mechanisms: the coalescence and Ostwald ripening process. An application of the method to platinum nanoparticle electrocatalysts with different size distributions, dispersed on high-surface-area carbon blacks, is discussed.

Wettability of HF-CVD diamond films

Wettability of HF-CVD diamond films grown in different conditions has been investigated. Wettability depends on surface tension: solids with high surface tension, as diamond, should be hydrophilic, while solids with low surface tension should be hydrophobic. In spite of these arguments, natural diamond exhibits a moderate hydrophobicity [J. Coll. Inter. Sci. 130 (1999) 35], depending on surface termination (hydrogen or oxygen terminated). In this work we find that CVD diamond films show wettability behaviours ranging from a small, up to clear hydrophobicity, probably according to surface carbon termination functionalities. Wettability does not seem to be influenced by characteristics as film structural morphology or growth orientation direction, which were analysed by SEM, while it seems dependent on surface reconstruction, as detected by Raman and XPS analysis. Moreover, in contrast with natural diamond [J. Coll. Inter. Sci. 130 (1999) 35] we found an enhancement to water wettability when CVD diamond films were treated in a hot filament activated hydrogen atmosphere. We argue that this effect may be due to the hydrogen etching of reconstructed surface layers with lower surface tension.

Laser‐driven synthesis of nanocrystalline alumina powders from gas‐phase precursors

Carbon dioxide laser synthesis from gaseous precursors has been successfully applied to produce nanosized Al2O3 particles. Trimethylaluminum [Al(CH3)3] and nitrous‐oxide (N2O) were used as gas phase reactants. Ethylene (C2H4) was added as sensitizer gas. The as‐synthesized powder particles showed considerable carbon contamination and an amorphouslike structure. After thermal treatment at 1200–1400 °C, the powder transformed to hexagonal α‐Al2O3 with very low carbon contamination, confirmed by x‐ray diffraction, x‐ray photoelectron spectroscopy, and chemical analysis. The thermally treated powder was composed of spherical single‐crystal nanoparticles with a mean size 〈D〉 of 15–20 nm, as determined by x‐ray diffraction, electron microscopy, and Brunauer–Emmett–Teller specific surface measurements. The laser synthesized Al2O3 particles are suitable dispersoids for intermetallic alloy technology.

Synthesis of TiC and SiC/TiC nanocrystalline powders by gas-phase laser-induced reaction

In the development of advanced ceramic nanocomposites, the production of titanium-based nanopowders has assumed importance, especially since recent results showed that mechanical properties of SiC matrices could be significantly improved by the incorporation of TiC second phase and vice versa. Nanosized TiC and TiC/SiC powders were synthesized by CO2 laser-induced reactions. A TiCl4 and hydrocarbons mixture was irradiated for titanium carbide production; the addition of SiH4 gave rise to the composite powders. The results of the powder characterization, obtained mainly by X-ray photoelectron spectroscopy, X-ray-induced Auger Spectroscopy and X-ray diffraction, are discussed in detail to correlate the reaction parameters to the composition and morphology of the products. Both TiC and SiC/TiC nanopowders showed a high reactivity to the air exposure, as evinced by the surface oxidation of the particles. Nevertheless, a quite good yield was obtained for nanocomposites of β-SiC matrix with the addition of up to 20% TiC phase.

Composite ceramic powders obtained by laser induced reactions of silane and amines

Ultrafine, ternary (Si/C/N) ceramic powders have been produced after CO 2 laser initiated reactions between silane and mono- or dimethylamine. Various spectroscopic diagnostics and chemical analyses, including ir spectrophotometry, x-ray photoelectron spectroscopy, and x-ray diffraction analysis have been employed to correlate the parameters involved in gas-phase reactions and the powder final composition.

Carbon Nanotube Growth on PAN‐ and Pitch‐Based Carbon Fibres by HFCVD

Abstract Carbon nanotubes (CNT) were grown on carbon fibres, both PAN‐ and pitch‐based, by hot filament chemical vapour deposition (HFCVD) using H2 and CH4 as precursors. Nickel clusters were electrodeposited on the fibre surfaces to catalyse the growth, and uniform CNT coatings were obtained on both PAN‐ and pitch‐based carbon fibres. Ni cluster features varied, depending on the deposition parameters, showing on average larger dimensions and denser distribution on pitch fibres. Multi‐walled CNTs with smooth walls and low impurity content were grown. The morphological features, both before and after the growth process, were characterised by SEM. This novel material based on carbon fibres coated with CNT, shows a potential for applications in polymeric matrix composites.

Nanosized metal catalysts in electrodes for solid polymeric electrolyte fuel cells: an XPS and XRD study

Solid polymeric fuel cells (SPFC) are devices that are able to convert chemical energy into electric energy continuously without any ambient pollution. The electrodes of this type of cells are layered structures, composed of a carbon support, a diffusive layer and a catalytic layer, usually platinum clusters dispersed on amorphous carbon microparticles, whose properties, at atomic and molecular level, determine the catalytic behaviour. In this work, microstructural and compositional properties of the catalytic layer are studied by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Firstly, the electronic properties of Pt clusters are compared with those of Pt bulk and the different response to the photoemission process is evidenced. Moreover, changes in the Pt photoemission spectral features from electrodes subjected to different treatments are correlated with particle size variation.