Preferential adsorption of NH3 gas molecules on MWCNT defect sites probed using in situ Raman spectroscopy

The preferential adsorption of NH3 gas molecules on multiwalled carbon nanotubes (MWCNTs) was studied using in situ Raman spectroscopy. It was observed that the full widths at half maximum of the G band and the intensity ratio I2D/IG of the MWCNTs decreased significantly during NH3 gas adsorption at elevated temperatures. These observations were explained in terms of suppressed second-order-defect associated Raman vibrations resulting in a lower disorder Raman band due to ammonia adsorption on the defect sites. Another corresponding effect was a temporary increase in electron doping levels due to ammonia adsorption. This behaviour was accompanied by a drop of ca. 2% in the resistance of the MWCNTs corresponding to the occupancy of most of the defect sites. We suggest preferential adsorption of ammonia gas molecules on the thermally activated defect sites of MWCNTs as an appropriate gas sensing mechanism. This knowledge can be used to design and tune the selectivity of ammonia gas sensor

Semiconducting Metal Oxide Based Sensors for Selective Gas Pollutant Detection

A review of some papers published in the last fifty years that focus on the semiconducting metal oxide (SMO) based sensors for the selective and sensitive detection of various environmental pollutants is presented

Crystallite size effect on the structural transitions of WO₃ studied by Raman spectroscopy

International audienceNanocrystallites of tungsten oxide samples of 2, 4, 16, 35 and 60 nm of diameter were prepared by cryosol and pyrosol techniques. The pressure- and temperature-induced phase transitions of these samples were monitored by Raman spectrometry from 0.1 MPa to 34 GPa and from 77 to 1200 K. The tetragonal (α)–orthorhombic (β)–monoclinic (γ) transitions in these nanometric samples are strongly downshifted in temperature by comparison with the bulk WO3. For instance, the tetragonal phase which exists above 1171 K for the bulk tungsten oxide can be stabilized at 700 K for the 35 nm sample. In the same way, the monoclinic P21/n-monoclinic P21/c high-pressure-induced transition is slightly shifted from 0.1 GPa to a higher pressure (1.5 GPa). The discussion of these transition-line shifts is based on thermodynamic considerations in which the surface energy of crystallites plays an important role. </p

REACTIVITE CHIMIQUE ET PROPRIETES PHYSIQUES DES OXYDES SNO 2 ET WO 3 CRISTALLINS ET NANOCRISTALLINS

GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

X-ray diffraction study of WO₃ at high pressure

International audienceThe high-pressure behaviour of microcrystalline tungsten oxide (WO3) has been investigated with angle-dispersive synchrotron x-ray powder diffraction in a diamond anvil cell up to 40 GPa at room temperature. Up to 21 GPa, the pressure dependence of the volume of the monoclinic high-pressure (P21/c) phase is described by a third-order Birch–Murnaghan equation of state with parameters V0 = 210.9(7)Å3, KT = 27(2)GP a and K' = 9.4(5). At 24 GPa, a first-order phase transition occurs with an approximate ΔV of 7.4% to a monoclinic P21/a unit cell with a = 6.1669(8)Å, b = 4.5758(6)Å, c = 5.3159(6)Å, β = 101.440(9)ˆ. A second transition is observed at pressures higher than 31 GPa with an approximate ΔV of 12% to a phase described by a third monoclinic unit cell, with a = 10.3633(22)Å, b = 3.9065(8)Å, c = 9.3459(18)Å and β = 98.539(14)

High-pressure Raman study of microcrystalline WO₃ tungsten oxide

International audienceA high-pressure Raman study of microcrystalline tungsten oxide was performed in the 0.1 MPa-30 GPa pressure range under hydrostatic and non-hydrostatic conditions. Two phase transitions are evidenced; they take place below 0.1 GPa and at about 22 GPa and are of first order. Two spectral anomalies are observed at about 3 and 10 GPa; they may be related to diffuse weak structural transitions. The number of observed Raman bands remains practically unchanged in the 0.1-30 GPa range and thus the symmetry changes are likely to be small. Surprisingly, the non-hydrostatic conditions do not induce inhomogeneous band broadening and do not modify the transition sequence observed in hydrostatic conditions. The compressibilities of the different observed phases are estimated from spectral data and discussed within Hazen's polyhedral approach

High-pressure study of microcrystalline tungsten trioxide phase transitions by Raman spectroscopy

A high-pressure Raman study of microcrystalline tungsten oxide was performed in the 0.1 MPa-31 GPa pressure range under isostatic and non-isostatic conditions. The phase transitions, which take place below 0.1 GPa and at 22 GPa are first order, while two further structural changes, observed at about 3 and 10 GPa, are diffuse. Surprisingly, the non-isostatic conditions do not induce inhomogeneous band broadenings and do not modify the transitions sequence observed in isostatic conditions. Compressibility of the different phases is estimated from spectral data

Nanopowders and nanostructured oxides : phase transitions and surface reactivity

International audienceWe report on experimental characterizations of nanocrystalline oxides by Raman spectroscopy, which has proven to be a reference technique when the scale of the coherent domains becomes too small for X-ray diffraction. Crystallite-size modification of ZrO2 and WO3 phase diagrams along with surface modes, surface reactions and adsorbed species characterization on WO3 and SnO2 will be reviewed