Nguyen Minh Vuong

Optimization of a zinc oxide urchin-like structure for high-performance gas sensing

Zinc oxide (ZnO) hollow hemisphere (HS) and urchin-like (UL) structures were fabricated and examined for application to a gas sensor. Films of hollow ZnO-HS arrays floating over substrates were synthesized via Zn sputtering onto the template of a polystyrene sphere array followed by oxidation. Growing ZnO nanorods upon HS surfaces via a hydrothermal method formed hollow ZnO–UL structures. The thicknesses of the HS films and the lengths of nanorods in the UL structures were varied to obtain the maximum response to NO gas. Both sensor structures showed a sensing of tens of parts per billion of levels of NO concentrations with good response and gas selectivity. The highest response was realized through the thinness and the open porosity of the structures. The surface depletion determined the sensor response signal for the sensor geometry with the highest response.

Porous Au-embedded WO3 Nanowire Structure for Efficient Detection of CH4 and H2S

We developed a facile method to fabricate highly porous Au-embedded WO3 nanowire structures for efficient sensing of CH4 and H2S gases. Highly porous single-wall carbon nanotubes were used as template to fabricate WO3 nanowire structures with high porosity. Gold nanoparticles were decorated on the tungsten nanowires by dipping in HAuCl4 solution, followed by oxidation. The surface morphology, structure, and electrical properties of the fabricated WO3 and Au-embedded WO3 nanowire structures were examined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and current–voltage measurements. Formation of a nanowire structure resulted in significant enhancement in sensing response to H2S and CH4 gases. Furthermore, Au embedment into the WO3 nanowire structures remarkably improved the performance of the sensors. The increase in response performance of sensors and adsorption–desorption kinetic processes on the sensing layers were discussed in relation with the role of Au embedment.

Realization of an open space ensemble for nanowires: a strategy for the maximum response in resistive sensors

Here, we used NO as a test gas to propose a strategy for a nanowire gas sensor with the maximum response—the lowest detection limits. The apparatus uses an open space ensemble structure of nanowires with diameters at near total-depletion. For this purpose, a series of open space nanowire structures of WO3 was fabricated with diameters varying from 35 to 82 nm, and a corresponding conduction nanowire sensor model was proposed. The nanowire structures revealed the highest response and a lowest detection limit of 30 ppb. Furthermore, the sensor response was maximum with nanowires of ∼40 nm, which is the diameter corresponding to total depletion conditions; the response was decreased at smaller diameters. The sensor model successfully explained the ultimate lower limits of the size effect in the nanowire sensors. To realize optimum sensor performance with the practical ensemble type nano-structures, an open space morphology is critical to remove the effect of gas diffusion throughout the structure.

Electrochromic properties of porous WO3–TiO2 core–shell nanowires

A highly porous TiO2–WO3 core–shell nanowire structure for an electrochromic (EC) coating was fabricated by sputter deposition of titanium and tungsten on a porous single-walled carbon nanotube template. This process was followed by thermal oxidation. The morphology and crystalline quality of composite materials were investigated by scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The electrochemical and EC properties were also examined and compared with thin TiO2–WO3 composite films as well as individual WO3 and TiO2 nanowire structures. The highly porous composite nanowire structure showed a highly enhanced proton intercalation capacity with good reversible electrochemical cycling of intercalation–deintercalation. The nanostructure also showed significantly improved EC contrast and coloration efficiency. This enhancement was observed with high chemical stability of the material. We proposed an atomic model of proton insertion into oxygen vacancies to explain the EC property and gas sensing simultaneously.

CuO-Decorated ZnO Hierarchical Nanostructures as Efficient and Established Sensing Materials for H 2 S Gas Sensors

Highly sensitive hydrogen sulfide (H2S) gas sensors were developed from CuO-decorated ZnO semiconducting hierarchical nanostructures. The ZnO hierarchical nanostructure was fabricated by an electrospinning method following hydrothermal and heat treatment. CuO decoration of ZnO hierarchical structures was carried out by a wet method. The H2S gas-sensing properties were examined at different working temperatures using various quantities of CuO as the variable. CuO decoration of the ZnO hierarchical structure was observed to promote sensitivity for H2S gas higher than 30 times at low working temperature (200 °C) compared with that in the nondecorated hierarchical structure. The sensing mechanism of the hybrid sensor structure is also discussed. The morphology and characteristics of the samples were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis absorption, photoluminescence (PL), and electrical measurements.

An edge-contacted pn-heterojunction of a p-SWCNT/n-WO3 thin film

We report a heterojunction formed between single-wall carbon nanotubes (SWCNTs) and a WO3 thin film. WO3 thin films were fabricated by sputter deposition of W followed by oxidation and SWCNTs were fabricated by the arc-discharge method. The morphology of the structures was examined by scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The current–voltage characteristics of WO3 thin films, SWCNTs films, and SWCNT/WO3 heterojunctions were measured in darkness and under ultraviolet light. A rectifying heterojunction formation of p-SWCNT/n-WO3 was confirmed. The observed unconventional optoelectronic properties were analyzed, and the results were used to explain the photoconduction phenomena occurring at the heterojunction. The heterojunctions and resistors were also examined for their photodetection performance.

Ferromagnetism in Zn 1−x Mn x O nanoparticles prepared by mechanical milling

In this paper, we have prepared a Zn0.98Mn0.02O paramagnet by solid-state reaction, and then created defects by mechanical milling. Changing the milling time (tm) from 0.5 to 20 h (used the grinding zirconia medium), we obtained nanoparticles (NPs) with different crystallite sizes (d) of 30-157 nm (calculated from XRD patterns and the Williamson-Hall method). A decrease of d increases the lattice strain (ε), and density of lattice defects. This broadens and blurs the spectral lines of Raman scattering and electron spin resonance (ESR), not shown. Interestingly, magnetization studies versus magnetic field at 300 K reveal the samples d <; 150 nm exhibiting FM order. The FM order becomes largest for d = 72 nm, corresponding to saturation magnetization Ms ≈ 6×10-3 emu/g. Apart from this d value, Ms will be gradually decreased. To find out the origin/nature of the observed phenomenon, we recorded X-ray absorption fine structure (XAFS) spectra. It appears that there is the coexistence of Mn2+ and Mn3+ ions in NPs. Their concentration ratio is slightly changed with decreasing d, due to a slight shift of absorption edge. Based on the features of Fourier-transformed XAFS and ESR spectra, we suggest that ferromagnetism in NPs is mainly related to oxygen vacancies. Lattice distortions can lead to the presence of zinc interstitials for the samples d <; 72 nm, which decrease Ms.

Enhancement of Dye-Sensitized Solar Cell Efficiency by Spherical Voids in Nanocrystalline ZnO Electrodes

Light scattering enhancement is widely used to enhance the optical absorption efficiency of dye-sensitized solar cells. In this work, we systematically analyzed the effects of spherical voids distributed as light-scattering centers in photoanode films made of an assembly of zinc oxide nanoparticles. Spherical voids in electrode films were formed using a sacrificial template of polystyrene (PS) spheres. The diameter and volume concentration of these spheres was varied to optimize the efficiency of dye-sensitized solar cells. The effects of film thickness on this efficiency was also examined. Electrochemical impedance spectroscopy was performed to study electron transport in the electrodes. The highest power conversion efficiency of 4.07 % was observed with film thickness. This relatively low optimum thickness of the electrode film is due to the enhanced light absorption caused by the light scattering centers of voids distributed in the film.