Narrow Results

ZnO and Zn_1-xMg_xO thin films were prepared on glass and silicon substrates by spin coating method using 2-methoxyethanol solution of zinc acetate dihydrate and magnesium acetate dihydrate stabilized by monoethanolamine. The effects of drying and annealing condition of structural and optical properties of the films were studied. It was found that the samples annealed at 650°C improves the crystallographic orientation of the ZnO films grown by the sol-gel process significantly. Two types of substrates were used to examine the substrate effects of the growth of Zn_1-xMg_xO thin films. In corning glass substrates, the lattice constant decreased by a little with increasing concentration of Mg, whereas in the case of silicon substrates, the lattice constant decreased rapidly with x when compared to the glass substrates. The optical band energy gaps of Zn_1-xMg_xO thin films were slightly increased with increasing concentration of Mg.

The growth mode and electronic structure of the vapor-deposited Ag overlayer on the surface is investigated by angle-resolved photoemission spectroscopy utilizing synchrotron radiation. The coverage dependent measurements of the two-dimensional band structure by the Ag 4d states reveal that the bands with a dispersing feature are developed from the low coverage region, suggesting that the Ag adatoms aggregate to form clusters with a laterally ordered structure. The Ag clusters grow in size with increasing the Ag coverage whereas the ordered atomic structure of the Ag cluster is preserved. From the analysis of the Ag 5sp band, the Ag clusters have a nonmetallic electronic structure at low coverages, and the metallic nature is developed as the size of the clusters becomes enlarged.

ZnO thin films were prepared on silicon (001) and corning glass substrates using Pulsed laser deposition (PLD) technique with different oxygen pressures. The microstructure, crystallinity, and resistivity of the films depend on the oxygen pressure used. The effects of the films grown at room temperature and at 500°C with different oxygen pressures have been investigated by analyzing the optical and electrical properties of the film. The XRD analysis showed that the high intensity of c-axis orientation of ZnO thin films was obtained under high oxygen pressure and this leads to greater electrical and optical properties. By applying high pressure oxygen, the resistivity value was decreased and optical transmittance became higher in the visible region. The surface morphology of the films showed that the smooth surface was observed without any cracks.

Highly (002)-oriented nanocrystalline ZnO thin films were deposited on glass substrates at a substrate temperature of 500°C in air atmosphere by low-cost spray pyrolysis method. Room temperature photoluminescence spectral studies show the presence of near band edge emission and defect level emission (DLE) from the samples. The DLE wavelength is blueshifted about 22 nm and further emission intensity enhancement is observed in (002)-oriented sample as compared to that of wurtzite ZnO phase. The modifications in local defect levels and change in crystalline symmetry are attributed to the blue shift in the DLE region of c-axis oriented sample. The increased emission intensity of (002)-oriented samples is due to improvement in crystalline quality, preferential oriented growth and spherical morphology of nanocrystalline grains.

In this paper, stearic acid/Zn²⁺ monolayer were formed at air–water interface and then multilayers were deposited onto a glass slide by Langmuir–Blodgett method. After annealing at 300°C for 0.25 h and 550°C for 2 h, ZnO thin films were fabricated. The optimized parameters for monolayer formation and multilayers deposition were determined by the surface pressure-surface area (Π-A) isotherm and transfer ratio (t.r.), respectively. The results of X-ray diffraction showed that multilayers changed into ZnO thin films with hexagonal structure and of high crystallization after annealing at high temperature. The observation of SEM images indicated that ZnO thin films had a uniform and flat surface with compact arrangement.

Zinc oxide (ZnO) and aluminum (Al) doped ZnO nanostructures with and without surfactant have been successfully prepared via sol-gel route. The effect of the surfactant glyoxalic acid and various concentration of Al on the structural property of ZnO was analyzed by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR). The morphology of the samples was recorded using field emission scanning electron microscopy. The uniform distribution of ZnO nanostructures with hexagonal facets is facilitated by the surfactant and the grain growth is further inhibited by the increase in concentration of Al. The ethanol (0–300ppm) sensing characteristics of the as-prepared samples were systematically investigated at room temperature. Surfactant-assisted ZnO/Al:ZnO nanostructures show higher sensitivity of 94% at room temperature than ZnO/Al:ZnO nanostructures without surfactant. Faster response at 68s and recovery at 50s is also achieved by the samples. The surfactant-assisted ZnO nanostructures exhibit sharp selective detection towards ethanol when compared to the samples without surfactant. The enhanced ethanol sensing property may be ascribed to the larger surface area which is due to uniform and smaller crystallite size of the surfactant-assisted sample.

In this study, surface-enhanced Raman spectroscopic (SERS) substrates with reusability were carefully fabricated and investigated. Based on a simple and cost-effective hydrothermal process, zinc sheets were used as a base for growing zinc oxide nanorods (ZnO NRs) with hexagonal structures as templates for the SERS substrates. In the experimentation, the authors explored a variation of the physical NR structures based on precursors of zinc nitrate (Zn(NO${}_3{)}_2)$: hexamethylenetetramine (HMTA) at 1:1 ratio, in aqueous solution with DI water at a concentration of 2.5–20mM. The prepared zinc oxide templates were finally decorated with gold nanoparticles (Au NPs) with the sputtering deposition for 90s in order to promote the SERS-active surface. From physical observations, the scanning electron microscopic (SEM) results showed that the ZnO NRs exhibited an increase in size from 56.4nm to 244.06nm as the solution concentration was increased. Further investigations also demonstrated that the Au-decorated SERS-active samples had the gold nanoparticles covering the top of the ZnO NRs. The prepared SERS substrates were finally measured for the Raman enhancement with methylene blue (MB) as the test molecules. The results showed that the SERS substrates could detect the Raman peaks of the MB at the limit of detection of $1\times 10^{-6}$M. In addition, the SERS substrates were tested for reusability with the UV exposure, up to at least nine cycles. This work therefore reported the progress of the fabrications of the SERS-active materials with the reusable potentials in several SERS applications.

Dye sensitized solar cells (DSSCs) provide promisingly, organic–inorganic, clean hybrid, cost effective and efficient molecular solar cell devices. Due to their distinct and multifunctional qualities, zinc oxide (ZnO) nanostructures are promising materials used to create photoanodes for DSSCs due to the availability of larger surface area than bulk sheet substance, effectual light-dispersing centers, and when mixed with titanium dioxide they produce a core–shell formation that diminishes the coalition rate and provide direct charge. Moreover, ZnO thin sheets have been broadly observed due of its potential application in various fields i.e. piezoelectric, photovoltaic, pyroelectric and optoelectronic utilization. This review studies the recent advances in the fabrication of zinc oxide-based photovoltaics; synthesis of ZnO nanostructures with variable morphologies including thin sheets, nanotubes, nanorods, nanoflowers, nanofibers and factors that control the growth and morphologies of these nanospecies and part of crystallographic planes for the fabrication of various zinc oxide nanoshapes. In the next part of this paper, numerous fabrication routes — doped and undoped ZnO thin films — are discussed and different parameters of photovoltaics are investigated, e.g. efficiency pre and post annealing temperatures, fill factors spinning speed and coating time, additives, nature of precursor which impacts on morphological and optical parameters of these sheets. In short, this review is dedicated to the ZnO photoanode, its properties, issues related to ZnO photoanode, various improvement approaches, fabrication methods successfully trialled so far followed by market potential of the DSSC technology, conclusion and recommendations

Undoped and europium doped ZnO films were electrodeposited from precursor solution on indium tin oxide (ITO) surfaces. Undoped ZnO nanorods were successfully obtained and grown under diffusion control perpendicularly ($c$ axis) on the ITO surface. The X-ray diffraction (XRD) analysis showed a wurtzite structure with sizes of the hexagonal unit cell: $a=3.2045$Å $=b$ and $c=5.2033$Å. After insertion of Eu${}^{3+}$ into the ZnO matrix, XRD analysis shows a shift of preferential peak to lower angles (2$\theta )$ indicating that a dislocation occurs into the unit cell with modification of its parameters. The grain sizes of doped ZnO decrease with increasing Eu${}^{3+}$ rate. Scanning electron microscopy (SEM) images show a clear change of morphological structure from nanorods to a nanosheet structure. Photoluminescence tests clearly showed an improvement in emission bands and the corresponding intensities are important in Eu${}^{3+}$ doped ZnO.

Zinc oxide and germanium multilayer films have been deposited on glass substrate using electron beam evaporation and resistive heating system, respectively, for alternate layers. The structural optical and electrical parameters have been investigated for the deposited films. The layer formation was confirmed by employing Rutherford back-scattering technique. Optical properties exhibit quantum confinement effect by showing the separate band gaps for ZnO and Ge. Electrical conductivity increases due to combined effect of all six layers (six alternate layers of Ge and ZnO).

The increase in demand for petroleum fuels attracted the attention of the research community to identify a control measure for petroleum source depletion. On the other hand, the heavy usage and burning of petroleum products cause environmental pollutants like oxides of nitrogen, carbon monoxide, hydrocarbons, etc. Therefore, it is important to control all the problems caused by the increasing use of petroleum products, such as diesel, petrol, Kerosene, fuel oil, etc. This investigation aims to reduce the usage of fossil-based petroleum diesel by producing an effective substitute bio-diesel from Juliflora seeds, which exhibit similar properties to diesel. Furthermore, the operational and emission behaviors of the biodiesel in the Reactive Controlled Compression Ignition (RCCI) engine are analyzed by including nanoadditives such as ZnO, ${\mathrm{\text{Al}}}_2{\mathrm{\text{O}}}_3$ and ${\mathrm{\text{CeO}}}_2$. Furthermore, the obtained performance and emission results are optimized using a hybrid deep neural network (DNN) using the Aquila optimization algorithm (AOA). The algorithm chooses the diesel-biodiesel blend with 75 ppm of alumina nanoparticle as the optimum blend. This considered blend provided better performance and emission results at 72.86% loading condition. The obtained results are 28.19% for brake thermal efficiency (BTE), 446.14 g/kWh for brake-specific fuel consumption, 0.105% for carbon monoxide emission, 19.82 ppm of unburnt hydrocarbon, 457 ppm of NO${}_x$ emission and 21.98% of smoke emission.

Morphology evolution of inorganic/organic crystals during crystallization is a universal growth phenomenon. In this work, we have developed a capping agent-assisted strategy to clearly identify the whole process of morphology evolution in solution growth system. One kind of morphology evolution trend with three types of morphologies of cuprous oxide (Cu₂O) was kinetically observed at varying the molar ratio of EDTA/Cu(II) under three different pH values. Two kinds of morphology evolution trends of zinc oxide (ZnO) were also kinetically observed in the presence of H₂O₂ and CH₃COOH (HAc), respectively. Simulation results show that the morphology evolution of nano- to micro-scale crystals is strongly dependent on the bonding characteristics of a growth system. The present strategy positively explores the interesting principles of morphology evolution of functional materials, and can be widely extended to nano- to micro-scale devices research.

A combination of the remarkably simple technique of electrospinning, developed to fabricate polymer nanofibers, and sol–gel processing has been utilized to produce fine zinc oxide nanofibers with an average diameter of 70 nm. A non-toxic precursor solution of polyvinyl alcohol and zinc acetate was electrospun and the resulting fibers were then calcined at a relatively low temperature to produce ZnO nanofibers. Simultaneous thermal analyses were used to study the formation of ZnO nanofibers from the precursor material. X-ray diffraction was employed to analyze the phases and different microscopy techniques, such as scanning electron microscopy, transmission electron microscopy and atomic force microscopy were used to study the morphology and size of the fibers. Fourier transform infrared spectroscopy was employed to investigate the composition of the precursor and ZnO fibers. The specific surface area of the electrospun nanofibers was determined using the Brunauer–Emmett–Teller method and optical properties were measured by UV-Vis and PL spectroscopy. The very high specific surface area of the ZnO fibers makes them potential candidates for nanodevice applications in gas sensing, dye-sensitized solar cells, and UV/blue emission devices.

Zinc oxide film has been fabricated by converting flower-like structure of zinc carbonate hydroxide made by chemical bath deposition technique. Flower-like structure is employed as charge transport network of dye-sensitized photoanode. Analysis of current density–voltage characteristic shows deposition temperature and deposition time of chemical bath deposition influence photovoltaic performance. Analysis of electrochemical impedance spectroscopy reveals short electron lifetime and high effective electron diffusion coefficient of zinc oxide-based dye-sensitized solar cell.

The degraded performance of annealed ZnO-based photodetector can be improved by embedding an Ag interlayer. The Ag interlay converted to Ag₂O during ZnO deposition. After Ag₂O nanoparticles formed, the interface between ZnO and Ag₂O produced band-bending to redistribute charges and increase the Schottky barrier between the electrical contact and ZnO. The excess Ag interlayer created lattice defects, causing the dark current to increase slightly. The increasing visible PL intensity of annealed ZnO, with the increasing Ag interlayer, clarified the transfer of the photo-generated electron.

A facile and green method was utilized to synthesize core-shelled Zn/ZnO microspheres by boiling Zn microparticles in water for improving the photocatalytic activity of ZnO. The synthesized Zn/ZnO core-shells were investigated by means of scanning electron microscope, X-ray diffractometer and photoluminescence spectrometer, respectively. The morphology analysis showed that the metallic Zn core was about 6 μm in diameter while the ZnO shell was about 600 nm in thickness. Compared to ZnO nanoparticles, the core-shelled Zn/ZnO microspheres exhibited improved photocatalytic activity in degrading methyl orange in water. Our results suggest that the metal–semiconductor junction formed at the Zn/ZnO interface is responsible for the enhanced photocatalytic activity of ZnO.

F, C-codoped ZnO nanoparticles were synthesized by the precipitation method. X-ray photoelectron spectroscopy spectra (XPS) measurements confirmed the existence of F–Zn, C–F, –CF₂– and O–C–O bonds in the lattices of ZnO nanoparticles. The band gap of ZnO was narrowed due to F and C dopants, which should be beneficial for the improvement of the photocatalytic activity. However, our experiments demonstrated that F, C-codoping restrained the photocatalytic activity of ZnO nanoparticles. To detect the possible microstructural defects, the analysis of electron paramagnetic resonance (EPR) was performed. It was suggested that the positive-charged defects were formed by the substitution of F ions for O lattice sites. defects are deep donors and act as recombination centers for photo-generated electrons and holes, which could result in the decrease of the photocatalytic activity. Although the photocatalytic activity of F, C-codoped ZnO is depressed, the antibacterial activity still keeps a comparable level in comparison with that of pure ZnO. Therefore, this material has a potential application in textiles.

Zinc oxide (ZnO) nanorods (ZNRs) and hierarchical ZnO nanosheet–spheres (ZNSs) were prepared through a simple aqueous chemical growth process and a low-temperature solid-phase method, respectively. The prepared ZNRs and ZNSs were mixed to obtain a composite structure by using a circumference oscillator. After structure and morphology characterizations via X-ray diffraction and scanning electron microscopy, the mixture of ZNRs and ZNSs was used as a photoanode in dye-sensitized solar cells (DSSCs). Photovoltaic performance and optimal mixture ratio were investigated. The results indicated that the photovoltaic properties of DSSCs depended on the microstructures, morphologies and mixture ratios of the electrodes. In addition, the mixture of ZNRs and ZNSs (molar ratio of 1:12) yielded an overall light conversion efficiency of 6.02%, with a fill factor of 65.0%, a short-circuit current of 13.49 mA/cm², and an open-circuit voltage of 0.69 V. These values are higher than those of pure ZNRs or pure ZNSs.

Sn–ZnO nanorod array coatings with a low concentration of tin dioxide (0, 0.5, 1, 5 mol.%) were obtained by hydrothermal method on glass or polycore substrates. The synthesized materials are crystallized in the wurtzite structure, which was confirmed by X-ray diffraction analysis. According to scanning electron microscopy arrays of nanorods have an average length 120–180 nm and a diameter of 57–77 nm. It was established that an increase in the temperature of hydrothermal synthesis (from 95${}^{\circ }$C to 200${}^{\circ }$C) leads to a decrease in particle sizes of pure ZnO. All obtained materials have photocatalytic activity in the visible light range. The best photocatalytic activity — 82% degradation of methylene blue in 3 h of visible light irradiation — was shown for material containing 1 mol.% of tin dioxide. This hydrothermal method opens a new pathway for developing new nanostructured photocatalysts.

In this paper, Carbon-doped Zinc Oxide (C-ZnO) samples were prepared using the solid-state reaction method. The influence of carbon-doping on the structural and dielectric properties of ZnO samples was studied. The shift in the highest peak position (101) in XRD patterns of carbon-doped samples was observed. The Raman peak at 581cm${}^{-1}$ in undoped ZnO was shifted and broadened in carbon-doped ZnO samples. The ZnO samples doped with carbon show higher values of dielectric constant ($\sim 2400$ at 1kHz) compared to pure ZnO($\sim 9$ at 1kHz) which was due to increase in native point defects in the samples. The ac conductivity (${\sigma }_{\mathrm{\text{ac}}}$) value of the carbon-doped sample was enhanced by 10³ times for ((ZnO)${}_{0.9}$${\text{C}}_{0.1}$) sample.