ZnO Microstructures Research Articles

Visualizing Intrinsic 3D‐Strain Distribution in Gold Coated ZnO Microstructures by Bragg Coherent X‐Ray Diffraction Imaging and Transmission Electron Microscopy with Respect to Piezotronic Applications

AbstractNovel devices ranging from bio magnetic field sensors to energy harvesting nano machines utilize the piezotronic effect. For optimal function, understanding the interaction of electrical and strain phenomena within the semiconductor crystal is necessary. Here, studies of a model piezotronic system are presented, consisting of a ZnO microrod coated by a thin layer of gold, which forms a Schottky contact with the piezoelectric ZnO material. Coherent X‐ray diffraction imaging (CXDI) and transmission electron microscopy (TEM) are used to visualize the structure and strain distribution, showing that the ZnO microrod exhibits strains of multiple origins in the bulk and at the interface. Strain values of −6 × 10‐4 have been measured by CXDI at the ZnO/Au interface. The origin is shown to be a combination of an interface strain, possibly caused by the Schottky contact formation, and distinct, localized electrical fields inside the crystal which are assigned to electron depletion and screening in a bent ZnO/Au piezotronic rod. These findings will contribute to sensor development and to a better understanding of piezotronic applications.

Vapor-solid growth ZnO:ZrO2 micro and nanocomposites

Microstructures of ZnO and ZrO2 and the corresponding composites have been grown by the Vapor-Solid method. A detailed characterization of their structural, compositional and optical properties has been performed. X-ray diffraction (XRD) and Raman spectroscopy measurements, let us identify two main phases, wurtzite ZnO and monoclinic ZrO2, reinforcing the hypothesis of ZnO/ZrO2 composite formation. Scanning Electron Microscopy observations reveal the presence of different morphologies depending on the synthesis conditions. Chemical composition was analyzed by energy dispersive X-ray (EDX) spectroscopy. Luminescence properties were studied by photoluminescence and cathodoluminescence. Our results indicate that doping is occurring in both phases: ZnO is doped with Zr and ZrO2 is doped with Zn. However important differences between Zr rich and Zn rich structures are encountered. While the Zn rich structures are single phase with the characteristic wurtzite crystal structure of ZnO, the Zr rich structures are multiphase with both monoclinic and wurtzite crystal structures present, then leading to formation of the composites with interesting luminescence properties, with intense emissions in the near UV and the visble range, even at room temperature.

Local Strain Distribution in ZnO Microstructures Visualized with Scanning Nano X‐Ray Diffraction and Impact on Electrical Properties

The fast and contact‐free detection of biomagnetic vital signs can benefit clinical diagnostics in medical care, emergency services, and scientific studies, hugely. A highly sensitive magnetoelectric sensor for the detection of biomagnetic signals combined with the piezotronic effect is a promising path to increase the signal detection limit. Herein, the results of three ZnO microrods examined by nano X‐ray diffraction and current–voltage curves to investigate the crystalline structure influence on the Schottky contact properties are presented. The measurements reveal different strain distributions for the three rods and that these are linked with the electrical properties, showing that the crystalline quality has a direct influence on the Schottky contact properties. An analytical model is created to determine the influence of the stress. Although rotation of the strain orientation changes the strain appearance in the measurement, it does not affect the Schottky contact properties.

The effect of annealing temperature on hydrothermally grown sisal-like ZnO microstructures

This study has extensively described the fabrication and characterization of sisal-like ZnO microstructures. HMT aqueous solution containing zinc nitrate tetrahydrate as crystalline zinc source has been utilized to synthesize sisal-like ZnO samples via a controlled hydrothermal route. A variety of characterization techniques including FESEM, XRD, FT-IR, and Raman spectroscopy have been employed to investigate the effects related to varying annealing temperatures on morphological and structural properties. The FESEM observations have revealed a transition from hexagonal microrods to modified microrods and then to pointed microrods for sisal-like ZnO architectures as the annealing temperature increases. XRD results have indicated a considerably high purity for as-synthesized ZnO microstructures. The crystalline quantities of ZnO samples have been estimated using various X-ray line-broadening analysis methods including Debye-Scherrer and Williamson-Hall techniques. UV-Vis DRS measurements have been carried out to study optical properties. Results have shown improved light harvesting related to both enhanced absorption intensities and decreased band-gap energies with the increase of annealing temperatures.

Growth mechanism and photocatalytic evaluation of flower-like ZnO micro-structures prepared with SDBS assistance

Flower-like ZnO microstructures were successfully produced using a hydrothermal method employing ZnSO4/(NH4)2SO4 as a raw material. The effect of the operating parameters of the hydrothermal temperature, OH−/Zn2+ molar ratio, time, and amount of dispersant on the phase structure and micromorphology of the ZnO particles were investigated. The synthesis conditions of the flower-like ZnO microstructures were: hydrothermal temperature of 160°C, OH−/Zn2+ molar ratio of 5:1, reaction time of 4 h, and 4 mL of dispersant. The flower-like ZnO microstructures were comprised of hexagon-shaped ZnO rods arranged in a radiatively. Degradation experiments of Rhodamine B with the flower-like ZnO microstructures demonstrated a degradation efficiency of 97.6% after 4 h of exposure to sunshine, indicating excellent photocatalytic capacity. The growth mechanism of the flower-like ZnO microstructures was presented.

Effect of Doping Rare-Earth Element on the Structural, Morphological, Optical and Photocatalytic Properties of ZnO Nanoparticles in the Degradation of Methylene Blue Dye

ZnO has been widely explored as a remarkable photocatalyst for the degradation of wide ranges of organic pollutants. However, its photocatalytic activity could be enhanced through metal doping. In this study, a rare-earth element, Cerium was doped into the microstructure of ZnO nanoparticles (NPs) using deposition precipitation method to enhance its photocatalytic activity towards methylene blue degradation via UV light irradiation. The resulting degradation efficiencies (DEs) recorded in this study are ∼ 64 %, 85 % and 55 % respectively. Similarly, the recorded rates are also found to be 9 x 10 −3, 21 x10 −3 and 6 x 10 −3 respectively, which are well correlated with the DE values. This superior photocatalytic performance achieved by 1 % Ce doping could be attributed to the reduction in band gap of the pure ZnO NPs from 2.69 eV – 2.60 eV. Therefore, 1 % Ce was the optimum doping of Ce4+ ion into ZnO microstructure, which demonstrates that Ce4+ ion could be used as an effective tool for stabilizing the generation of the reactive oxygen species, slowing down the recombination of the photo-generated charge carriers as well as enhancing their separation. Therefore, the optimum Ce doping into the ZnO NPs could play a remarkable role in facilitating the structural, microstructural, optical and electronic properties of the ZnO semiconductor, and eventually enhanced its photocatalytic activity.

Needle- and cross-linked ZnO microstructures and their photocatalytic activity using experimental and DFT approach

Thermal oxidation is a simple and cost-effective method that allows the production for large-scale of ZnO microstructures. By modifying the etching conditions (with HNO3) of the precursor, the morphology of the as-oxidized samples can be controlled, giving way to needle- and cross-linked ZnO microstructures. Electron microscopy results revealed that the (100) are dominating planes in the cross-linked structures, while the (101) are dominating planes in the needle structures. Density functional theory studies demonstrated that the ZnO interaction with HNO3 is a selective process which depends on the ZnO surfaces involved, whereas the reactivity of such surfaces can be attributed to the p orbitals associated with oxygen. Finally, photocatalytic results proved that these ZnO microstructures can be applied to degrade organic pollutants; i.e., methylene blue.

Intense orange emission from hydrothermally synthesized ZnO flower-like structure: effect of charge carrier—LO phonon interaction on emission characteristics

Easy control of electronic and optical properties of ZnO is pivotal for its various optoelectronic applications. The present manuscript illustrates preparation of flower- and needle-like microstructures of ZnO by simple hydrothermal technique using Zn(NO3)2 and NaOH as precursors. The method is found to be suitable to generate different ZnO microstructures just by changing NaOH concentration. Other parameters such as reaction temperature, time have no significant effect other than change of size of the microstructures. After careful analyses of the microstructures by Raman spectroscopy, it appears that charge carrier—LO phonon interaction which plays important role in optoelectronic properties decreases with increasing NaOH concentration. Photoluminescence spectroscopic data depict that the synthesized microstructures exhibit intense and tunable orange emission at 620 nm, attributed to interstitial oxygen defects. The present protocol is found to be very effective to control defects of ZnO microstructures; particularly, it seems to be very sensitive to control oxygen interstitial. Full width at half maxima (FWHM), an important parameter of any optical emission, is observed to be decreasing with increasing NaOH. A theoretical model, developed on the basis of Huang—Rhys ‘S’ factor, has been developed to correlate the trend of FWHM considering charge carrier—LO phonon interaction and orbital radii of the charge carriers.

Investigating the effects of hydrothermal temperature on morphology-controlled synthesis of flower-shaped ZnO microstructures

The hydrothermal synthesis and morphology-dependent properties of flower-shaped ZnO microstructures have been extensively described in this study. The growth of flower-shaped ZnO microstructures has been conducted in HMT aqueous solutions containing zinc nitrate tetrahydrate precursor in three different hydrothermal temperatures (100 °C, 150 °C, and 200 °C). Morphological observations and structural analysis have been performed using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transfer infrared spectroscopy (FT-IR), and Raman spectroscopy. Morphological studies have indicated a transition from sheet-like petals to rod-like structures for flower-shaped architectures with the increase of reaction temperatures. XRD patterns have revealed single-crystalline behavior for as-synthesized ZnO microstructures. Crystalline quantities have been investigated using a variety of X-ray line broadening analysis techniques including Debye-Scherrer and Williamson-Hall methods. UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) has been employed to carry out the optical studies and estimate optical band gap energies. The optical results have suggested morphology-dependent responses for flower-shaped ZnO microstructures.

The growth mechanism of zinc oxide micro-rods via vapor-phase method

Two different ZnO micro-structures were deposited by a Chemical Vapour Deposited method in a small quartz tube. The micro-rods and micro-wires were found on the edge of the tube respectivly. The morphology and structural properties were texted by scanning electronic microscopy technique and X-ray diffraction. The results shows that the micro-rods have typical hexagonal wurtzite structure without any defect and the micro-wires have two types, the single ZnO crystal and the double-like one. The X-ray diffraction gives some information through the intense 002 peak which means the samples have a great wurtzite structure and grown up along the c-axis. The mechanism of the two samples also discussed, it is in relation to the substrate temperature and the concentration of the mixed gas.

Facile growth of high transmittance ZnO micro-rod clusters on indium tin oxide by a wet-chemical synthesis method

A simple wet-chemical synthesis method was developed to fabricate Zinc oxide micro-rod clusters. The synthesis process involved the rapid dilution of a zinc-bearing alkaline solution at 150°C in a convection oven on indium tin oxide (ITO) substrates. The synthesis was carried out by immersing an unseeded ITO substrate in a mixture of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine ((CH2)6N4) aqueous solution. The obtained sample was annealed at 400°C for 2 h. The structural, morphological and optical properties of the synthesized ZnO microstructures were investigated by X-ray diffraction, scanning electron microscopy and ultraviolet-visible spectroscopy, respectively. The ZnO rod clusters are hexagonal phase of the wurtzite structure. The crystal grain sizes of the films were found to be 72.6, 84.3 and 66.3 nm for the (100), (002) and (101) crystal planes, respectively. The optical bandgap of the ZnO was determined to be 3.147 eV.

Synthesis, Characterization, and Photocatalytic Performance of ZnO–Graphene Nanocomposites: A Review

ZnO is an exciting material for photocatalysis applications due to its high activity, easy accessibility of raw materials, low production costs, and nontoxic. Several ZnO nano and microstructures can be obtained, such as nanoparticles, nanorods, micro flowers, microspheres, among others, depending on the preparation method and conditions. ZnO is a wide bandgap semiconductor presenting massive recombination of the generated charge carriers, limiting its photocatalytic efficiency and stability. It is common to mix it with metal, metal oxide, sulfides, polymers, and nanocarbon-based materials to improve its photocatalytic behavior. Therefore, ZnO–nanocarbon composites formation has been a viable alternative that leads to new, more active, and stable photocatalytic systems. Mainly, graphene is a well-known two-dimensional material, which could be an excellent candidate to hybridize with ZnO due to its excellent physical and chemical properties (e.g., high specific surface area, optical transmittance, and thermal conductivity, among others). This review analyses ZnO–graphene nanocomposites’ recent advances, addressing the synthesis methods and the resulting structural, morphological, optical, and electronic properties. Moreover, we examine the ZnO–graphene composites’ role in the photocatalytic degradation of organic/inorganic pollutants.

Al-Doped Dumbbell-Like ZnO for Enhanced Ethanol Sensing Performance

A dumbbell-like Al doped ZnO(AZO) microstructure was successfully synthesized via a facile, controllable and one-pot hydrothermal method. The samples were characterized by XRD, SEM, EDS and TEM. It showed that all the samples present dumbbell-like structure composed of highly crystalline hexagonal wurtzite ZnO. The dumbbell-like structure is formed through the self-assembly of Al-dopled ZnO nanodisks. Gas sensing tests reveal that the Al-doped dumbbell-like ZnO microstructure (3 at %) shows excellent gas sensing response (5.37 for ethanol with 1 ppm) for ethanol with shorter response-recovery time (53 s/21 s). The notable sensing performance could be mainly attributed to the increased content of oxygen vacancy in ZnO due to the Al doping. Thus, the Al-doped dumbbell-like ZnO microstructures could provide an inspiration for the design and preparation of the highly sensitive ethanol gas sensor with the controllable electronic structure.

Novel ZnO microwoods with ultrahigh selectivity for detection of trimethylamine vapor

On basis of specific surface acid/base interaction, a sensor fabricated using acidic oxide is inclined to detect alkaline vapor. Herein, a novel wood-shaped ZnO microstructure consisted of two microrods has been successfully demonstrated using a facile hydrothermal approach, and is further employed as a channel for triethylamine (TEA) sensing. Owing to the certain surface acidity and ultralow specific surface area (<1 m2/g) of this ZnO product, the as-fabricated sensor showed a remarkable response towards alkaline TEA compared with that of the other interfering gases. In addition, a rapid response rate (6 s for detecting 100 ppm TEA) and actual detection limit (1 ppm) were also observed.

Observation of an electrical breakdown at ZnO Schottky contacts in varistors

We report an electrical breakdown (EBD) at metal/ZnO junctions in varistors with Ag-Pd and Pd electrodes. It was found that these junctions show Schottky behaviour with a mean Schottky barrier height (SBH) of 0.70 ± 0.07 eV for Bi-based varistors with Ag-Pd (80−20 %) electrodes and a mean SBH of 0.47 ± 0.03 eV for Pr-based varistors with Pd electrodes. All investigated electrode-to-grain junctions (EGJ) exhibited an EBD of the Schottky barrier in the reverse current direction, whereas the mean EBD voltage for the Bi-based varistors is 3.5 V and 2.5 V for the Pr-based varistors. Observation of electroluminescent light at the EGJ interface clearly indicates that the EBD is related to electron-hole generation, similar to the EBD at the grain boundaries in the ZnO microstructure. This shows that a similar good but asymmetric varistor effect, as it is observed at grain boundaries, can be observed at EGJ.

Columnar structure growth of Mn-doped ZnO (MZO) thin films by radio frequency co-sputtering and studies on films properties

ABSTRACT The present investigation deals with the columnar structured growth of Mn doped ZnO (MZO) thin films by radio frequency sputtering technique. The X-ray diffraction and the processed electron diffraction pattern confirmed the hexagonal wurtzite structure and the film growth direction along (101) plane. The structure of grown film was compared with the pristine ZnO film and the JCPDS card No. 00-36-1451. The microstructures of the pristine ZnO and MZO films were studied by scanning electron microscopy and atomic force microscopy. The columnar structure growth of MZO was confirmed by transmission electron microscopy using TEM cross-section sample. The presence of constituent elements and their concentrations in the films were recorded by energy dispersive X-ray diffraction spectroscopy. The optical transmittance, absorption edge and band gap of the material were studied by UV-Visible transmission spectroscopy. The field dependent magnetization at room-temperature was analysed by using vibrating sample magnetometer and magneto-optic Kerr effect.

Wet Synthesis of Elongated Hexagonal ZnO Microstructures for Applications as Photo-Piezoelectric Catalysts.

It is well known that energetic demand and environmental pollution are strictly connected; the side products of vehicle and industrial exhausts are considered extremely dangerous for both human and environmental health. In the last years, the possibility to simultaneously photo-degrade water dissolved pollutants by means of ZnO nanostructures and to use their piezoelectric features to enhance the photo-degradation process has been investigated. In the present contribution, an easy and low-cost wet approach to synthetize hexagonal elongated ZnO microstructures in the wurtzite phase was developed. ZnO performances as photo-catalysts, under UV-light irradiation, were confirmed on water dissolved methylene blue dye. Piezoelectric responses of the synthetized ZnO microstructures were evaluated, as well, by depositing them into films onto flexible substrates, and a home-made layout was developed, in order to stimulate the ZnO microstructures deposited on solid supports by means of mechanical stress and UV photons, simultaneously. A relevant increment of the photo-degradation efficiency was observed when the piezopotential was applied, proposing the present approach as a completely eco-friendly tool, able to use renewable energy sources to degrade water solved pollutants.

Electronic Tuning of Zinc Oxide by Direct Fabrication of Chromium (Cr) incorporated photoanodes for Visible-light driven Water Splitting Applications

Herein, we report the synthesis of Cr incorporated ZnO sheets arrays microstructures and construction of photoelectrode through a direct aerosol assisted chemical vapour deposition (AACVD) method. The as-prepared Cr incorporated ZnO microstructures were characterized by transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, powdered X-ray spectroscopy, X-ray photoelectron spectroscopy and UV-Vis diffused reflectance spectroscopy. The Cr incorporation in ZnO red shifted the optical band gap of as-prepared photoanodes. The 15% Cr incorporation in ZnO has shown enhanced PEC performance. The AACVD method provides an efficient in situ incorporation approach for the manipulation of morphological aspects, phase purity, and band structure of photoelectrodes for an enhanced PEC performance.

A facile controllable self-assembly of 3D elliptical ZnO microspheres from 1D nanowires for effective detection of acetone

3D elliptical ZnO microspheres self-assembled from 1D nanowires were synthesized via a facile solvothermal route. Their morphology and structures were characterized by X-ray diffraction, scanning electron microscopy and N2 adsorption-desorption techniques, and the gas-sensing properties were investigated. The results indicate that as-synthesized elliptical wool ball-like ZnO can be controlled by adjusting alkali concentrations and it exhibits enhanced sensitivity and high selectivity to acetone. Notably, the response to 10 and 100 ppm reached 14 and 36 respectively, which are 4 and 1.5 times as much as that of the nanorods-composed ZnO, implying wool ball-like ZnO microstructures can be served as sensor material for acetone detection.

3D porous flower-like ZnO microstructures loaded by large-size Ag and their ultrahigh sensitivity to ethanol

3D porous flower-like ZnO microstructures loaded by large-size Ag were successfully synthesized via a simple two-stage method. XRD and XPS results proved that the synthesized Ag/ZnO was a pure phase. SEM and TEM results showed that the Ag nanoparticles with large size (about 400 nm) were successfully loaded on the porous flower-like ZnO. The sensitivity of Ag/ZnO was 268 to 200 ppm ethanol at 300 °C, which was 6 times higher than that of pure ZnO. In addition, the sensor of Ag/ZnO exhibited a faster response/recovery behavior and a lower detection limit. The enhanced gas sensing performance of Ag/ZnO has been explained from three aspects: the “spillover effect” of Ag, the formation of metal-semiconductor heterojunction and the role of “electronic sensitization”.