D.E. Motaung

Highly selective NH3 gas sensor based on Au loaded ZnO nanostructures prepared using microwave-assisted method

ZnO nanorods synthesized using microwave-assisted approach were functionalized with gold (Au) nanoparticles. The Au coverage on the surface of the functionalized ZnO was controlled by adjusting the concentration of the Au precursor. According to X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) results, it was confirmed that Au form nanoparticles loaded on the surface of ZnO. The small Au loading level of 0.5wt% showed the highest response of 1600-100ppm of NH3 gas at room temperature (RT) whereas further increase of Au loading level resulted in poor detection of NH3. All Au loaded ZnO (Au/ZnO) based sensors exhibited very short recovery and response times compared to unloaded ZnO sensing materials. The responses of ZnO and Au/ZnO based sensors (0.5-2.5wt%) to other flammable gases, including H2, CO and CH4, were considerably less, demonstrating that Au/ZnO based sensors were highly selective to NH3 gas at room temperature. Spill over mechanism which is the main reason for the observed enhanced NH3 response with 0.5 Au loading level is explained in detail.

Correlating the magnetism and gas sensing properties of Mn-doped ZnO films enhanced by UV irradiation

In this study, we report on the correlation between the magnetism and gas sensing properties of Mn-doped ZnO films grown via aerosol spray pyrolysis. The evolution of the structure, morphology, optical properties, and chemical state of ZnO with the Mn concentration was also investigated. ZnO doped with Mn (0.1 at%) demonstrated room-temperature ferromagnetism (RTFM) due to the uncompensated surface spins primarily originating from structural defects and oxygen vacancies (VO) on the surface, which act as active sites for the adsorption of oxygen species. The undoped ZnO structure revealed both FM and paramagnetism (PM) at the near surface of the film. Increased Mn doping destroyed the RTFM ordering due to improved PM features induced by Mn clusters on the ZnO surface and reduced amount of VO on the surface. However, ZnO films doped with Mn (0.1 at%) exhibited an improved sensing response to oxidizing gases compared to their counterparts, showing that films with RTFM with no PM contribution exhibit improved sensing properties. These analyses revealed that the nature of the film surface plays a substantial role in both their magnetic and sensing behaviors.

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.

In situ optical emission study on the role of C2 in the synthesis of single-walled carbon nanotubes

In situ optical emission spectroscopy was used to study the temporal and spatial behavior of laser induced plasmas in the laser-furnace synthesis of single-walled carbon nanotubes (SWCNTs). A graphite composite target located within a sealed quartz tube with a chemical stoichiometric composition of 95:4:1 at. wt % of carbon, yttrium, and nickel, respectively, was ablated by a Q-switched Nd:YAG laser delivering colinear, focused laser pulses of 1064 and 532 nm temporarily separated by 20 ns. The ablation process was done at a furnace temperature of 1273 K in a flow of argon gas at either 150 or 200 SCCM (SCCM denotes cubic centimeter per minute at STP). The pressure was varied (100, 400, and 600 Torr) for each gas flow setting. The temporal and spatial behavior of the emission intensity associated with C2 Swan bands (d Π3g−a Π3u) was investigated and found to be influenced by the pressure and flow rate of the argon gas. At conditions optimal to SWCNT production, a sharp drop in C2 intensity followed by a r...

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.

Life cycle assessment of facile microwave-assisted zinc oxide (ZnO) nanostructures

The life cycle assessment of several zinc oxide (ZnO) nanostructures, fabricated by a facile microwave technique, is presented. Key synthesis parameters such as annealing temperature, varied from 90°C to 220°C, and microwave power, varied from 110W to 710W, are assessed. The effect of these parameters on both the structural characteristics and the environmental sustainability of the nanostructures is examined. The nanostructures were characterized by means of X-ray diffraction (XRD), focused ion beam scanning electron microscopy (FIB-SEM), ultraviolet-visible spectroscopy (UV-Vis), Photoluminescence (PL) and Brunauer-Emmett-Teller (BET) analysis. Crystalline size was found to be 22.40nm at 110W microwave power, 24.83nm at 310W, and 24.01nm at 710W. Microwave power and synthesis temperature were both directly proportional to the surface area. At 110W the surface area was 10.44m2/g, at 310W 12.88m2/g, and at 710W 14.60m2/g; while it was found to be 11.64m2/g at 150°C and 18.09m2/g at 220°C. Based on these, a life cycle analysis (LCA) of the produced ZnO nanoparticles was carried out, using the ZnO surface area (1m2/g) as the functional unit. It was found that the main environmental weaknesses identified during the production process were; (a) the use of ethanol for purifying the produced nanomaterials and (b) the electricity consumption for the ZnO calcination, provided by South Africas fossil-fuel dependent electricity source. When the effect of the key synthesis parameters on environmental sustainability was examined it was found that an increase of either microwave power (from 110 to 710W) or synthesis temperatures (from 90 to 220°C), results in higher sustainability, with the environmental footprint reduced by 27% and 41%, respectively. Through a sensitivity analysis, it was observed that an electricity mix based on renewable energy could improve the environmental sustainability of the nanoparticles by 25%.

The Effect of Reducing Agents on the Electronic, Magnetic and Electrocatalytic Properties of Thiol-Capped Pt/Co and Pt/Ni Nanoparticles

The electronic, magnetic and electrocatalytic properties of bimetallic thiol-capped Pt/Co and Pt/Ni nanoparticles were synthesised using two reducing agents, NaBH4 and N2H4. X-ray diffraction analysis of the nanoparticles showed Pt lattice contraction upon the addition of Co or Ni to Pt indicating the formation of an alloy structure, more apparent when N2H4 was used. XPS data analysis revealed Pt metal and Pt(II) (assigned to PtO) and a higher concentration of surface metallic Ni and Co for the NaBH4-reduced samples. Both the NaBH4- and N2H4-reduced catalysts were active for the methanol oxidation reaction (MOR), with the Pt-Co-N2H4 catalyst revealing the highest activity. The N2H4 significantly affected the magnetic properties of Pt/Co and Pt/Ni particles by controlling the morphology and crystalline structure of the nanoparticles. In general, the type of reducing agent affected the final properties of the nanoparticles.

Ferromagnetic resonance characterization of nano-FePt by electron spin resonance

Electron spin resonance (ESR) measurements at room temperature and X-band microwave frequency were performed on highly crystalline FePt system thin films. Fairly high DC static magnetic field absorption of about 300 mT was observed in these films. We attribute the high field absorption to ferromagnetic resonance (FMR). Upon increasing iron content in FePt system, no detectable spin waves modes were identified already at room temperature. This signifies a homogeneous distribution of the magnetization across the films. We qualitatively attributed such homogeneity distribution in the films to self-assembly of these Fe–Pt system nanoparticles. The results revealed that the FePt system contains hyperfine coupling with sextet exhibiting a phase reversal behaviour compared to FMR line. Both iron content and crystallite size increased the FMR intensity making the films good candidates for large data storage mediums and spintronics.

Effect of additional electron acceptor in hybrid P3HT:PCBM:ZnO spin-coated films for photovoltaic application: Effect of additional electron acceptor in hybrid P3HT:PCBM:ZnO spin-coated films

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