VO2 nanostructures based chemiresistors for low power energy consumption hydrogen sensing

Mott-type VO₂ oxide nanobelts are demonstrated to be effective hydrogen gas sensors at room temperature. These nanobelts, synthesized by hydrothermal process and exhibiting the VO₂ (A) crystallographic phase, display room temperature H₂ sensitivity as low as 0.17 ppm. The nanobelts (ultralong belt-like) nanostructures could be an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and for building functional devices based on individual nanobelts.     

Gas sensing study of hydrothermal reflux synthesized NiO/graphene foam electrode for CO sensing

Nickel oxide nanosheets have been successfully synthesized on the graphene foam (GF) using hydrothermal reflux process for their application as carbon monoxide (CO) gas sensor. X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, energy dispersive spectroscopy, and gas sorption analysis were used to characterize the structure and morphology of the samples. The morphology (SEM), crystal structure (XRD and Raman), and elemental composition (EDS) analysis of NiO/GF composite confirmed the cubic crystal structure of NiO and elemental composition (i.e., Ni, O, and C) of NiO/GF composite. The results reveal that the incorporation of graphene into NiO nanosheets not only improved the surface area of NiO/GF composite, but also enhanced the performance of the composite on CO sensing by improving its conductivity. These results indicate that NiO/GF has potential as electrode material for CO gas sensor.     

Solar absorption and thermal emission properties of multiwall carbon nanotube/nickel oxide nanocomposite thin films synthesized by sol–gel process

Multiwall carbon nanotubes (MWCNTs)/nickel oxide (NiO) nanocomposites were successfully prepared by a sol–gel process and coated on an aluminium substrate. The MWCNTs were chemically functionalized and then added into NiO alcogels, and magnetic stirred for homogeneous dispersion into the NiO matrix solution. The morphology of the resulting nanocomposite thin films showed that the MWCNTs were embedded in the NiO nano-particle matrix, while HRTEM confirmed that the MWCNTs were surrounded by the NiO nano-particles. Raman spectra for functionalized MWCNTs displayed a red shift from the pristine MWCNTs suggesting successful purification/functionalization. The spectrum for the MWCNTs/NiO nanocomposite indicated the presence of both the TO and LO phonons of NiO, and the D and G bands of the MWCNTs. Red and blue shifts of the NiO phonons and the MWCNT phonons suggested that the vibrational properties of both materials were changed to form new nanocomposite vibrational properties. Despite unoptimized layer thickness and composition, the solar absorptance of the functionalized MWCNTs/NiO nanocomposite films was 0.84 (for a single layer). The thermal emittance at 100 °C was approximately 0.2. These results suggest that MWCNTs/NiO nanocomposite materials are suitable for solar thermal applications.     

FTIR and Raman Spectroscopy of Carbon Nanoparticles in SiO2, ZnO and NiO Matrices

Coatings of carbon nanoparticles dispersed in SiO₂, ZnO and NiO matrices on aluminium substrates have been fabricated by a sol–gel technique. Spectrophotometry was used to determine the solar absorptance and the thermal emittance of the composite coatings with a view to apply these as selective solar absorber surfaces in solar thermal collectors. Cross-sectional high resolution transmission electron microscopy (X-HRTEM) was used to study the fine structure of the samples. Raman spectroscopy was used to estimate the grain size and crystallite size of the carbon clusters of the composite coatings. X-HRTEM studies revealed a nanometric grain size for all types of samples. The C–SiO₂, C–ZnO and C–NiO coatings contained amorphous carbon nanoparticles embedded in nanocrystalline SiO₂, ZnO and NiO matrices, respectively. Selected area electron diffraction (SAED) showed that a small amount of Ni grains of 30 nm diameter also existed in the NiO matrix. The thermal emittances of the samples were 10% for C–SiO₂, 6% for the C–ZnO and 4% for the C–NiO samples. The solar absorptances were 95%, 71% and 84% for the C–SiO₂, C–ZnO and C–NiO samples, respectively. Based on these results, C–NiO samples proved to have the best solar selectivity behaviour followed by the C–ZnO, and last were the C–SiO₂ samples. Raman spectroscopy studies revealed that both the C–ZnO and C–NiO samples have grain sizes for the carbon clusters in the range 55–62 nm and a crystallite size of 6 nm.     

Self Assembly and Properties of C:WO3 Nano-Platelets and C:VO2/V2O5 Triangular Capsules Produced by Laser Solution Photolysis

Laser photolysis of WCl₆ in ethanol and a specific mixture of V₂O₅ and VCl₃ in ethanol lead to carbon modified vanadium and tungsten oxides with interesting properties. The presence of graphene’s aromatic rings (from the vibrational frequency of 1,600 cm⁻¹) together with C–C bonding of carbon (from the Raman shift of 1,124 cm⁻¹) present unique optical, vibrational, electronic and structural properties of the intended tungsten trioxide and vanadium dioxide materials. The morphology of these samples shows nano-platelets in WO _x samples and, in VO _x samples, encapsulated spherical quantum dots in conjunction with fullerenes of VO _x . Conductivity studies revealed that the VO₂/V₂O₅ nanostructures are more sensitive to Cl than to the presence of ethanol, whereas the C:WO₃ nano-platelets are more sensitive to ethanol than atomic C.     

GaN nanostructures-poly(vinyl alcohol) composite based hydrostatic pressure sensor device

The wide band-gap semiconductor material gallium nitride was synthesized using a one step microwave-assisted solution phase technique. The synthesized GaN nanocrystals showed an intense ultraviolet-blue emission typical of GaN materials. Hydrostatic pressure sensors were fabricated using a GaN/polyvinyl alcohol (PVA) composite film deposited onto an interdigitated electrode and studied by measuring the change in alternating current conductance of the devices at varied applied pressures. Three different GaN concentrations of 29, 50 and 67% were used. A very high sensitivity in the range 100–200 kPa was observed for these devices. The composite devices demonstrated both response and recovery times of less than 16 s.     

Optimization, Yield Studies and Morphology of WO3 Nano-Wires Synthesized by Laser Pyrolysis in C2H2 and O2 Ambients—Validation of a New Growth Mechanism

Laser pyrolysis has been used to synthesize WO₃ nanostructures. Spherical nano-particles were obtained when acetylene was used to carry the precursor droplet, whereas thin films were obtained at high flow-rates of oxygen carrier gas. In both environments WO₃ nano-wires appear only after thermal annealing of the as-deposited powders and films. Samples produced under oxygen carrier gas in the laser pyrolysis system gave a higher yield of WO₃ nano-wires after annealing than the samples which were run under acetylene carrier gas. Alongside the targeted nano-wires, the acetylene-ran samples showed trace amounts of multi-walled carbon nano-tubes; such carbon nano-tubes are not seen in the oxygen-processed WO₃ nano-wires. The solid–vapour–solid (SVS) mechanism [B. Mwakikunga et al., J. Nanosci. Nanotechnol., 2008] was found to be the possible mechanism that explains the manner of growth of the nano-wires. This model, based on the theory from basic statistical mechanics has herein been validated by length-diameter data for the produced WO₃ nano-wires.     

Formation of tungsten oxide nanostructures by laser pyrolysis: stars,fibres and spheres

In this letter, the production of multi-phase WO₃ and WO_{3- x} (where x could vary between 0.1 and 0.3) nanostructures synthesized by CO₂-laser pyrolysis technique at varying laser wavelengths (9.22-10.82 mm) and power densities (17-110 W/cm²) is reported. The average spherical particle sizes for the wavelength variation samples ranged between 113 and 560 nm, and the average spherical particle sizes for power density variation samples ranged between 108 and 205 nm. Synthesis of W₁₈O₄₉ (= WO_2.72) stars by this method is reported for the first time at a power density and wavelength of 2.2 kW/cm² and 10.6 μm, respectively. It was found that more concentrated starting precursors result in the growth of hierarchical structures such as stars, whereas dilute starting precursors result in the growth of simpler structures such as wires.     

Comparative study: the effect of annealing conditions on the properties of P3HT:PCBM blends

This paper presents a detailed study on the role of various annealing treatments on organic poly(3-hexylthiophene) and [6]-phenyl-C₆₁-butyric acid methyl ester blends under different experimental conditions. A combination of analytical tools is used to study the alteration of the phase separation, structure and photovoltaic properties of the P3HT:PCBM blend during the annealing process. Results showed that the thermal annealing yields PCBM “needle-like” crystals and that prolonged heat treatment leads to extensive phase separation, as demonstrated by the growth in the size and quantity of PCBM crystals. The substrate annealing method demonstrated an optimal morphology by eradicating and suppressing the formation of fullerene clusters across the film, resulting in longer P3HT fibrils with smaller diameter. Improved optical constants, PL quenching and a decrease in the P3HT optical bad-gap were demonstrated for the substrate annealed films due to the limited diffusion of the PCBM molecules. An effective strategy for determining an optimized morphology through substrate annealing treatment is therefore revealed for improved device efficiency.     

Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD

We report on the thermally induced changes of the nano-structural and optical properties of hydrogenated nanocrystalline silicon in the temperature range 200–700 °C. The as-deposited sample has a high crystalline volume fraction of 53% with an average crystallite size of ~3.9 nm, where 66% of the total hydrogen is bonded as ≡Si–H monohydrides on the nano-crystallite surface. A growth in the native crystallite size and crystalline volume fraction occurs at annealing temperatures ≥400 °C, where hydrogen is initially removed from the crystallite grain boundaries followed by its removal from the amorphous network. The nucleation of smaller nano-crystallites at higher temperatures accounts for the enhanced porous structure and the increase in the optical band gap and average gap.