Free-standing ZnO-CuO composite nanowire array films and their gas sensing properties

In vitro influence of PEG functionalized ZnO-CuO nanocomposites on bacterial growth

Polyethyleneglycol-coated biocompatible CuO-ZnO nanocomposites were fabricated hydrothermally varying Zn:Cu ratios as 1:1, 2:1, and 1:2, and their antibacterial activity was determined through the well diffusion method against the Gram-negative Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and the Gram-positive Staphylococcus aureus. The minimum inhibitory concentration and the minimum bactericidal concentration values of the synthesized samples were determined. Subsequently, the time synergy kill assay was performed to elucidate the nature of the overall inhibitory effect against the aforementioned bacterial species. The mean zone of inhibition values for all four samples are presented. The inhibitory effect increased with increasing concentration of the nanocomposite (20, 40 and 60 mg/ml) on all the bacterial species except for S. aureus. According to the MBC/MIC ratio, ZnO was found to be bacteriostatic for E. coli and P. aeruginosa, and bactericidal for S. aureus and K. pneumoniae. Zn:Cu 2:1 was bactericidal on all bacterial species. A bacteriostatic effect was observed on E. coli and P. aeruginosa in the presence of Zn:Cu 1:1 whereas, it showed a bactericidal effect on S. aureus and K. pneumoniae. Zn:Cu 1:2 exhibited a bacteriostatic effect on E. coli while a bactericidal effect was observed for E. coli, P. aeruginosa, and K. pneumoniae. The metal oxide nanocomposites were found to be more sensitive towards the Gram-positive strain than the Gram-negative strains. Further, all the nanocomposites possess anti-oxidant activity as shown by the DPPH assay.

Morphology and Luminescence of Flexible Free-Standing ZnO/Zn Composite Films Grown by Vapor Transport Synthesis

A method for fabricating flexible free-standing ZnO/Zn composite films from the vapor phase using a regular array of silicon microwhiskers as a substrate is presented. The structural and morphological peculiarities, as well as luminescent properties of the films, were studied. The films have a hybrid structure consisting of two main microlayers. The first layer is formed directly on the tops of Si whiskers and has a thickness up to 10 µm. This layer features a polycrystalline structure and well-developed surface morphology. The second layer, which makes up the front side of the films, is up to 100 µm thick and consists of large microcrystals. The films show good bending strength-in particular, resistance to repeated bending and twisting-which is provided by a zinc metallic part constituting the flexible carrier of the films. ZnO photoluminescence was observed from both surfaces of the films but with conspicuous spectral differences. In particular, a significant weakening of ZnO green luminescence (more than 10 times) at an almost constant intensity of UV near-band edge emission was found for the polycrystalline side of the films as compared to the microcrystalline side. A high degree of homogeneity of the luminescent properties of the films over their area was demonstrated. The results obtained emphasize the relevance of further studies of such ZnO structures-in particular, for application in flexible devices, sensors, photocatalysis and light generation.

Nanowire-based sensor electronics for chemical and biological applications

Detection and recognition of chemical and biological species via sensor electronics are important not only for various sensing applications but also for fundamental scientific understanding. In the past two decades, sensor devices using one-dimensional (1D) nanowires have emerged as promising and powerful platforms for electrical detection of chemical species and biologically relevant molecules due to their superior sensing performance, long-term stability, and ultra-low power consumption. This paper presents a comprehensive overview of the recent progress and achievements in 1D nanowire synthesis, working principles of nanowire-based sensors, and the applications of nanowire-based sensor electronics in chemical and biological analytes detection and recognition. In addition, some critical issues that hinder the practical applications of 1D nanowire-based sensor electronics, including device reproducibility and selectivity, stability, and power consumption, will be highlighted. Finally, challenges, perspectives, and opportunities for developing advanced and innovative nanowire-based sensor electronics in chemical and biological applications are featured.

Enhanced room temperature gas sensing properties of low temperature solution processed ZnO/CuO heterojunction

The development of room temperature gas sensors having response towards a specific gas is attracting researchers nowadays in the field. In the present work, room temperature (29 °C) ethanol sensor based on vertically aligned ZnO nanorods decorated with CuO nanoparticles was successfully fabricated by simple cost effective solution processing. The heterojunction sensor exhibits better sensor parameters compared to pristine ZnO. The response of the heterojunction sensor to 50 ppm ethanol is, at least, 2-fold higher than the response of the ZnO bare sensor. Also the response and recovery time of ZnO/CuO sensor to 50 ppm ethanol are of 9 and 420 s whereas the values are 16 and 510 s respectively for ZnO sensor. The vertical alignment of ZnO nanorods as well as its surface modification by CuO nanoparticles increased the effective surface area of the device and the formation of p-CuO/n-ZnO junction at the interface are the reasons for the improved performance at room temperature. In addition to ethanol, the fabricated device has the capability to detect the presence of reducing gases like hydrogen sulfide and ammonia at room temperature.

A Copper Oxide/Zinc Oxide Composite Nano-Surface for Use in a Biosensor

In this study, biosensors based on zinc oxide–copper oxide composite nano-surfaces were prepared using a simple and inexpensive distributed colloidal technique. Combinations of mixed dispersions with volume ratios of 1:1, 1:2 and 2:1 ZnO:CuO were compared. The uniform nano-crystalline sensor surfaces on polyethylene terephthalate (PET) were analysed using scanning electron microscopy (SEM), Atomic Force Microscopy (AFM) and Raman Spectroscopy. The ZnO–CuO composite biosensor nano-surfaces showed a significantly increased impedimetric signal compared with pure ZnO nanocrystals, and the maximum output was achieved with a volume ratio of 1:2 ZnO/CuO. The antibody capture of C-reactive protein (CRP) on the nano-surfaces was used to demonstrate the enhanced signal generated with increasing amounts of CuO in the nano-surface.

Study on room temperature gas-sensing performance of CuO film-decorated ordered porous ZnO composite by In2O3 sensitization

For the first time, ordered mesoporous ZnO nanoparticles have been synthesized by a template method. The electroplating after chemical plating method was creatively used to form copper film on the surface of the prepared ZnO, and then a CuO film-decorated ordered porous ZnO composite (CuO/ZnO) was obtained by a high-temperature oxidation method. In₂O₃ was loaded into the prepared CuO film-ZnO by an ultrasonic-assisted method to sensitize the room temperature gas-sensing performance of the prepared CuO/ZnO materials. The doped In₂O₃ could effectively improve the gas-sensing properties of the prepared materials to nitrogen oxides (NO _x ) at room temperature. The 1% In₂O₃ doped CuO/ZnO sample (1 wt% In₂O₃-CuO/ZnO) showed the best gas-sensing properties whose response to 100 ppm NO _x reached 82%, and the detectable minimum concentration reached 1 ppm at room temperature. The prepared materials had a good selectivity, better response, very low detection limit, and high sensitivity to NO _x gas at room temperature, which would have a great development space in the gas sensor field and a great research value.

Hybridization of Zinc Oxide Tetrapods for Selective Gas Sensing Applications

In this work, the exceptionally improved sensing capability of highly porous three-dimensional (3-D) hybrid ceramic networks toward reducing gases is demonstrated for the first time. The 3-D hybrid ceramic networks are based on doped metal oxides (Me_xO_y and Zn_xMe_1-xO_y, Me = Fe, Cu, Al) and alloyed zinc oxide tetrapods (ZnO-T) forming numerous junctions and heterojunctions. A change in morphology of the samples and formation of different complex microstructures is achieved by mixing the metallic (Fe, Cu, Al) microparticles with ZnO-T grown by the flame transport synthesis (FTS) in different weight ratios (ZnO-T:Me, e.g., 20:1) followed by subsequent thermal annealing in air. The gas sensing studies reveal the possibility to control and change/tune the selectivity of the materials, depending on the elemental content ratio and the type of added metal oxide in the 3-D ZnO-T hybrid networks. While pristine ZnO-T networks showed a good response to H₂ gas, a change/tune in selectivity to ethanol vapor with a decrease in optimal operating temperature was observed in the networks hybridized with Fe-oxide and Cu-oxide. In the case of hybridization with ZnAl₂O₄, an improvement of H₂ gas response (to ∼7.5) was reached at lower doping concentrations (20:1), whereas the increase in concentration of ZnAl₂O₄ (ZnO-T:Al, 10:1), the selectivity changes to methane CH₄ gas (response is about 28). Selectivity tuning to different gases is attributed to the catalytic properties of the metal oxides after hybridization, while the gas sensitivity improvement is mainly associated with additional modulation of the electrical resistance by the built-in potential barriers between n-n and n-p heterojunctions, during adsorption and desorption of gaseous species. Density functional theory based calculations provided the mechanistic insights into the interactions between different hybrid networks and gas molecules to support the experimentally observed results. The studied networked materials and sensor structures performances would provide particular advantages in the field of fundamental research, applied physics studies, and industrial and ecological applications.

Perovskite Nanoparticle-Sensitized Ga2O3 Nanorod Arrays for CO Detection at High Temperature

Noble metal nanoparticles are extensively used for sensitizing metal oxide chemical sensors through the catalytic spillover mechanism. However, due to earth-scarcity and high cost of noble metals, finding replacements presents a great economic benefit. Besides, high temperature and harsh environment sensor applications demand material stability under conditions approaching thermal and chemical stability limits of noble metals. In this study, we employed thermally stable perovskite-type La(0.8)Sr(0.2)FeO3 (LSFO) nanoparticle surface decoration on Ga2O3 nanorod array gas sensors and discovered an order of magnitude enhanced sensitivity to carbon monoxide at 500 °C. The LSFO nanoparticle catalysts was of comparable performance to that achieved by Pt nanoparticles, with a much lower weight loading than Pt. Detailed electron microscopy and X-ray photoelectron spectroscopy studies suggested the LSFO nanoparticle sensitization effect is attributed to a spillover-like effect associated with the gas-LSFO-Ga2O3 triple-interfaces that spread the negatively charged surface oxygen ions from LSFO nanoparticles surfaces over to β-Ga2O3 nanorod surfaces with faster surface CO oxidation reactions.

High-response H2S sensor based on ZnO/SnO2 heterogeneous nanospheres

Compared to SnO₂ and ZnO gas sensors, the ZnO/SnO₂ heterogeneous sensors showed exceptional electrical responses to H₂S gas at 300 °C.

Low temperature solution processed ZnO/CuO heterojunction photocatalyst for visible light induced photo-degradation of organic pollutants

Morphology controlled hierarchical ZnO/CuO architecture was obtained on both flexible and rigid substrates, which exhibited excellent photocatalytic performance by virtue of favourable heterojunction formation at nanostructure interfaces.

Fabrication and photoresponse of ZnO nanowires/CuO coaxial heterojunction

The fabrication and properties of n-ZnO nanowires/p-CuO coaxial heterojunction (CH) with a photoresist (PR) blocking layer are reported. In our study, c-plane wurtzite ZnO nanowires were grown by aqueous chemical method, and monoclinic CuO (111) was then coated on the ZnO nanowires by electrochemical deposition to form CH. To improve the device performance, a PR layer was inserted between the ZnO buffer layer and the CuO film to serve as a blocking layer to block the leakage current. Structural investigations of the CH indicate that the sample has good crystalline quality. It was found that our refined structure possesses a better rectifying ratio and smaller reverse leakage current. As there is a large on/off ratio between light on and off and the major light response is centered at around 424 nm, the experimental results suggest that the PR-inserted ZnO/CuO CH can be used as a good narrow-band blue light detector.

The Influences of CuO/ZnO Ratios on the Crystallization Characteristics Electrical and Magnetic Properties of CuxZn1−xO Powders

This study synthesizes CuxZn1−xO powders using an aqueous solution method. The CuxZn1−xO powders with different content ratios of CuO and ZnO (CuO : ZnO = 1 : 2, 1 : 1, and 2 : 1) were formed. The crystalline characteristics and electrical and magnetic properties depended primarily on the mixing effect and oxygenation. The electrical resistance of C0.5Z0.5O (1.5×105 Ω/□) powder was lower than that of CuO (5.82×105 Ω/□) powder after ZnO mixing in CuO. This reduction was attributed to the substitution of Cu+ ions at Zn2+ sites or the formation of electron trapping defect centers. The concentration ratio of Cu2O phase in CuxZn1−xO powder mainly dominated the electrical resistance. The CuxZn1−xO has a diluted ferromagnetism (DFM) and paramagnetism (PM). The electrical resistance of CuxZn1−xO decreased; the magnetic behavior increased instead. This study also analyzes the chemical binding of Cu0.5Zn0.5O powders to confirm the contribution of Cu+ ions to the electrical and magnetic properties.

Substrate-free fabrication of self-supporting ZnO nanowire arrays

Thin films composed of self-supporting ZnO nanowire arrays are fabricated via a hydrothermal approach without the presence of any substrates. The films can be transferred and bonded to an arbitrary substrate for device applications. As a demonstration, a piezoelectric converter is made which is able to generate electric charge under compressive forces.

(Zn,H)-codoped copper oxide nanoparticles via pulsed laser ablation on Cu-Zn alloy in water

Nanosized (5 to 10 nm) amorphous and crystalline nanocondensates, i.e., metallic α-phase of Zn-Cu alloy in face-centered cubic structure and (Zn,H)-codoped cuprite (Cu2O) with high-pressure-favored close-packed sublattice, were formed by pulsed laser ablation on bulk Cu65Zn35 in water and characterized by X-ray/electron diffractions and optical spectroscopy. The as-fabricated hybrid nanocondensates are darkish and showed photoluminescence in the whole visible region. Further dwelling of such nanocondensates in water caused progressive formation of a rice-like assembly of (Zn,H)-codoped tenorite (CuO) nanoparticles with (001), (100), and {111} preferred orientations, (111) tilt boundary, yellowish color, and minimum bandgap narrowing down to ca. 2.7 eV for potential photocatalytic applications.