Crystalline ZnO Research Articles

Electronic and thermal properties of Ag-doped single crystal zinc oxide via laser-induced technique

The doping of ZnO has attracted lots of attention because it is an important way to tune the properties of ZnO. Post-doping after growth is one of the efficient strategies. Here, we report a unique approach to successfully dope the single crystalline ZnO with Ag by the laser-induced method, which can effectively further post-treat grown samples. Magnetron sputtering was used to coat the Ag film with a thickness of about 50 nm on the single crystalline ZnO. Neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was chosen to irradiate the Ag-capped ZnO samples, followed by annealing at 700 °C for two hours to form ZnO:Ag. The three-dimensional (3D) information of the elemental distribution of Ag in ZnO was obtained through time-of-flight secondary ion mass spectrometry (TOF-SIMS). TOF-SIMS and core-level x-ray photoelectron spectroscopy (XPS) demonstrated that the Ag impurities could be effectively doped into single crystalline ZnO samples as deep as several hundred nanometers. Obvious broadening of core level XPS profiles of Ag from the surface to depths of hundred nms was observed, indicating the variance of chemical state changes in laser-induced Ag-doped ZnO. Interesting features of electronic mixing states were detected in the valence band XPS of ZnO:Ag, suggesting the strong coupling or interaction of Ag and ZnO in the sample rather than their simple mixture. The Ag-doped ZnO also showed a narrower bandgap and a decrease in thermal diffusion coefficient compared to the pure ZnO, which would be beneficial to thermoelectric performance.

Green Synthesis of ZnO Nanostructures Using Pyrus pyrifolia: Antimicrobial, Photocatalytic and Dielectric Properties

In this study, zinc oxide nanostructures (ZnO NS) were synthesized using Pyrus pyrifolia fruit extract. Biophysical characterization results confirmed that the synthesized materials are crystalline wurtzite ZnO structures. Field emission scanning electron microscopy (FESEM) revealed that the ZnO NS are cubical, and the sizes range 20–80 nm. Transmission electron microscopy (TEM) and XRD results revealed a crystal lattice spacing of 0.23 nm and (101) the crystalline plane on ZnO NS. UV-Visible spectrophotometer results showed an absorbance peak at 373 nm. The ZnO NS demonstrated significant antibacterial activity analyzed by metabolic activity analysis and disc diffusion assay against Escherichia coli and Staphylococcus aureus. FESEM analysis confirmed the bacterial membrane disruption and the release of cytoplasmic contents was studied by electron microscopy analysis. Further, ZnO NS achieved good photocatalytic activity of decolorizing 88% of methylene blue (MB) in 60 min. The dielectric constant and loss of ZnO were found to be 3.19 and 2.80 at 1 kHz, respectively. The research findings from this study could offer new insights for developing potential antibacterial and photocatalytic materials.

Effect of Thermal Treatment on the Structure and Morphology of Vanadium Doped ZnO Nanostructures Obtained by Microwave Assisted Sol-Gel Method.

In this paper, we conducted a fundamental study concerning the effect of thermal treatment on the structure and morphology of 2 mol% vanadium doped ZnO nanopowders obtained by microwave assisted sol-gel method (MW). The samples were analyzed by DTA, FTIR, XRD, SEM, and UV-Vis spectroscopy. The DTA results showed that above 500 °C, there was no mass loss in the TG curves, and ZnO crystallization occurred. The XRD patterns of the thermally treated powders at 500 °C and 650 °C showed the crystallization of ZnO (zincite) belonging to the wurtzite-type structure. It was found that in the 650 °C thermally treated powder, aside from ZnO, traces of Zn3(VO4)2 existed. FTIR spectra of the annealed samples confirmed the formation of the ZnO crystalline phase and V-O bands. The micrographs revealed that the temperature influenced the morphology. The increase in the annealing temperature led to the grain growth. The SEM images of the MW powder thermally treated at 650 °C showed two types of grains: hexagonal grains and cylindrical nanorods. UV-Vis spectra showed that the absorption band also increased with the increasing temperature of thermal treatment. The MW sample annealed at 650 °C had the highest absorption in ultraviolet domain.

Visualizing Structural and Chemical Transformations of an Industrial Cu/ZnO/Al2O3 Pre‐catalyst during Activation and CO2 Reduction

AbstractThe typical industrial catalyst used for methanol synthesis is a multi‐component catalyst based on Cu/ZnO/Al2O3. The synergies between various phases of this catalyst play a vital role in defining the overall catalytic function and performance. To gain insights into the role and interaction between the relevant components and phases, ex situ and in situ transmission electron microscopy (TEM) was deployed to investigate the structures and phases of an industrial Cu/ZnO/Al2O3 in its precursor, activated and reaction states. High structural inhomogeneity in this material is revealed, i. e. the presence of various phases with different morphologies and compositions. It is shown how structural and compositional changes occur during hydrogen treatment and how compositional inhomogeneity in the starting material translates into differences in the local composition of the activated and working catalyst. The formation of defective metallic copper particles (stacking faults and twins) that are in an intimate contact with zinc oxide (poorly crystalline, partially reduced ZnOx and crystalline ZnO), alumina, Zn−Al oxide and carbon‐zinc‐oxygen containing phases (such as zinc formate) is observed. It is also uncovered that alumina plays a potentially important role in stabilizing cationic zinc species. This work provides atomic‐level insight into the relevant state of an industrial methanol synthesis catalyst and the associated synergistic interplay between the involved phases in reactive atmosphere.

Bottom-Gated ZnO TFT Pressure Sensor with 1D Nanorods.

In this study, a bottom-gated ZnO thin film transistor (TFT) pressure sensor with nanorods (NRs) is suggested. The NRs are formed on a planar channel of the TFT by hydrothermal synthesis for the mediators of pressure amplification. The fabricated devices show enhanced sensitivity by 16~20 times better than that of the thin film structure because NRs have a small pressure transmission area and causes more strain in the underlayered piezoelectric channel material. When making a sensor with a three-terminal structure, the leakage current in stand-by mode and optimal conductance state for pressure sensor is expected to be controlled by the gate voltage. A scanning electron microscope (SEM) was used to identify the nanorods grown by hydrothermal synthesis. X-ray diffraction (XRD) was used to compare ZnO crystallinity according to device structure and process conditions. To investigate the effect of NRs, channel mobility is also extracted experimentally and the lateral flow of current density is analyzed with simulation (COMSOL) showing that when the piezopotential due to polarization is formed vertically in the channel, the effective mobility is degraded.

Exciton-Assisted UV Stimulated Emission with Incoherent Feedback in Polydisperse Crystalline ZnO Powder

A comparative analysis of the features of UV-stimulated emission (SE) of various disordered active materials based on ZnO crystallites for a random laser (RL) was carried out. The superlinear increase in the intensity of the UV photoluminescence (PL) band of polydisperse nano-micro-crystalline (PNMC) ZnO powder at a wavelength of λ = 387 nm and some narrowing of its halfwidth in the range of 20 ÷ 15 nm with increasing pump intensity indicates random lasing with incoherent feedback (FB). The properties of similar UV PL bands under the same conditions of a thin film containing hexagonal ZnO microdisks, as well as samples of monodisperse ZnO nanopowder with nanoparticle sizes of 100 nm, indicate stimulated radiation with coherent feedback. It is shown that, among the studied materials, PNMC ZnO powder with widely dispersed crystallites ranges in size from 50 nm to several microns, which in turn, consists of nanograins with dimensions of ~25 nm, is the most suitable for creating a random laser with incoherent feedback at room temperature. The dominant factor of UV SE in PNMC ZnO powder is radiation transitions under exciton–exciton scattering conditions. The possible mechanisms of this random emission with the continuous spectrum are discussed. The average optical gain coefficient αg at λ = 387 nm in this RL system is estimated as αg~150 cm−1.

Facile green synthesis, analysis, in vitro antidiabetic and antimicrobial activity of ZnO macropores.

Rapid increase in population and development in industry causes many problems such as microbial contaminations and chronic diseases such as diabetes. Materials synthesized at nanoscale are novel antidiabetic and antimicrobial agents. ZnO nanoparticles with macropores characteristics are synthesized by green methods. Turmeric, clove buds and green tea extracts are used as additives. X-ray diffraction results confirmed the hexagonal wurtzite structure of ZnO nanoparticles and crystallinity was quit high in case of green tea extract. Sample synthesized with clove shows relatively higher crystallite size (10.64) which is pertaining to variation in Zn2+ and OH- ions. The nanoparticles are more or less spherical in nature, macropores and clustered together revealed by SEM images. Macroporosity of the sample was further confirmed by nitrogen adsorption-desorption isotherm. The deep absorption band at 605cm-1 in FTIR spectra attributed the wurtzite-type ZnO. The major dominating sharp peak was detected at 437cm-1 in Raman spectra which is a feature of the wurtzite hexagonal phase ZnO. UV-Vis spectra showed red shift from wavelength 362 to 375nm with different plant extracts. Impedance analysis showed a high dielectric constant and low tangent loss in case of green tea extract. ZnO synthesized using green tea exhibited ~ 95% α-glucosidase inhibition activity and 91% α-amylase inhibition activity. Antibacterial results revealed that synthesized ZnO nanoparticles showed activity against Bacillus subtilis and E. coli with inhibition zone 35mm and 29mm, respectively.

Development of surface-modified galvanised steel as an immobilised photocatalyst for textile wastewater treatment

The quest for cost-effective photocatalytic material and a simple process for wastewater degradation has inspired the development of immobilised ZnO photocatalyst by surface-modifying the commercial galvanised steel via plasma electrolytic oxidation process (PEO). In this work, galvanised steel underwent oxidation via PEO using a suitable base electrolyte system (borax and sodium aluminate). The optimum time for effectively converting zinc in the galvanised steel to ZnO is investigated. The 90 s PEO process effectively converted ZnO with the bandgap value of 3.30 eV. Additionally, vanadium was doped into the ZnO by adding sodium metavanadate (0.3 to 1.2 g/L) to the base electrolyte to reduce the bandgap. The effect of dopant concentration on the surface morphology, phase analysis, optical properties, recombination rate and separation efficiency of charge carriers were investigated. Also, the photocatalytic efficiency was measured by evaluating methylene blue dye's degradation under UV light irradiation. It was found that 0.9 g/L of sodium meta vanadate addition to the base electrolyte system was shown crystalline ZnO with near-visible light absorption (3.15 eV) along with reduced electron-hole recombination. Finally, the vanadium-doped ZnO photocatalyst showed 95 % MB dye degradation with a rate constant K = 0.014 min−1 under UV light.

Synthesis of ZnO/Au Nanocomposite for Antibacterial Applications.

Annually, antimicrobial-resistant infections-related mortality worldwide accelerates due to the increased use of antibiotics during the coronavirus pandemic and the antimicrobial resistance, which grows exponentially, and disproportionately to the current rate of development of new antibiotics. Nanoparticles can be an alternative to the current therapeutic approach against multi-drug resistance microorganisms caused infections. The motivation behind this work was to find a superior antibacterial nanomaterial, which can be efficient, biocompatible, and stable in time. This study evaluated the antibacterial activity of ZnO-based nanomaterials with different morphologies, synthesized through the solvothermal method and further modified with Au nanoparticles through wet chemical reduction. The structure, crystallinity, and morphology of ZnO and ZnO/Au nanomaterials have been investigated with XRD, SEM, TEM, DLS, and FTIR spectroscopy. The antibacterial effect of unmodified ZnO and ZnO/Au nanomaterials against Escherichia coli and Staphylococcus aureus was investigated through disc diffusion and tetrazolium/formazan (TTC) assays. The results showed that the proposed nanomaterials exhibited significant antibacterial effects on the Gram-positive and Gram-negative bacteria. Furthermore, ZnO nanorods with diameters smaller than 50 nm showed better antibacterial activity than ZnO nanorods with larger dimensions. The antibacterial efficiency against Escherichia coli and Staphylococcus aureus improved considerably by adding 0.2% (w/w) Au to ZnO nanorods. The results indicated the new materials' potential for antibacterial applications.

Elucidation of the role of ZnO in sulfur cure in novel EPDM-CTS blends

A new concept is introduced in this study, suggesting the role played by the ZnO crystal in sulfur crosslinking polymers, particularly, rubber blends of EPDM-CTS. The study is conducted in a polymer blend of ethylene propylene diene terpolymer (EPDM) and cyclic tetrasulfide (CTS). CTS is a reactive low molecular weight polymer, which can act as a sulfur donor. It is proposed that the sulfur crosslinking occurs at the surface of the ZnO crystals. A reaction site is created on the surface of the ZnO crystal by the reaction with stearic acid, which creates a “template” or active site on the surface as a catalytic site for the CTS reaction to take place. Various experiments are performed to substantiate the newly proposed role the crystalline ZnO structure plays in influencing the initiation of the sulfur crosslinking. The investigation is conducted in rubber compounds in the absence of fillers and other additives and additionally in chemical model studies. The analyses conducted in rubber compounds are performed on fully or partially cured rubber to study the evolution of the chemical process involved in the CTS reaction and the curing of the rubber blends. Step-cured compound allows analyses of the transformation of the ZnO crystal size during the sulfur crosslinking process, and the ability to gauge the step transformation that the ZnO crystal is undergoing during the crosslinking process. Additional chemical model studies are used in the study of the influence of the ZnO crystal on sulfur crosslinking, to confirm the reaction path and by-products of reaction. Lastly, quantum mechanical (QM) and molecular mechanics (MM) calculations are applied in support of the suggested mechanism of reaction.

Metal-Organic Framework@Metal Oxide Heterostructures Induced by Electron-Beam Radiation.

Metal organic frameworks (MOFs) are a distinct family of crystalline porous materials finding extensive applications. Their synthesis often requires elevated temperature and relatively long reaction time. We report here the first case of MOF synthesis activated by high-energy (1.5 MeV) electron beam radiation from a commercially available electron-accelerator. Using ZIF-8 as a representative for demonstration, this type of synthesis can be accomplished under ambient conditions within minutes, leading to energy consumption about two orders of magnitude lower than that of the solvothermal condition. Interestingly, by controlling the absorbed dose in the synthesis, the electron beam not only activates the formation reaction of ZIF-8, but also partially etches the material during the synthesis affording a hierarchical pore architecture and highly crystalline ZnO nanoparticles on the surface of ZIF-8. This gives rise to a new strategy to obtain MOF@metal oxide heterostructures, finding utilities in photocatalytic degradation of organic dyes.

Metal‐Organic Framework@Metal Oxide Heterostructures Induced by Electron‐Beam Radiation

AbstractMetal organic frameworks (MOFs) are a distinct family of crystalline porous materials finding extensive applications. Their synthesis often requires elevated temperature and relatively long reaction time. We report here the first case of MOF synthesis activated by high‐energy (1.5 MeV) electron beam radiation from a commercially available electron‐accelerator. Using ZIF‐8 as a representative for demonstration, this type of synthesis can be accomplished under ambient conditions within minutes, leading to energy consumption about two orders of magnitude lower than that of the solvothermal condition. Interestingly, by controlling the absorbed dose in the synthesis, the electron beam not only activates the formation reaction of ZIF‐8, but also partially etches the material during the synthesis affording a hierarchical pore architecture and highly crystalline ZnO nanoparticles on the surface of ZIF‐8. This gives rise to a new strategy to obtain MOF@metal oxide heterostructures, finding utilities in photocatalytic degradation of organic dyes.

29.3: Perovskite‐Oxide Heteronanocrystals for Light Emitting Application

Colloidal lead halide Perovskite nanocrystals (PNCs) have excellent optical properties that endow the PNCs with great application advantages in the display field. However, the ionic surface structure makes PNCs easy to degrade in light, heat, and air, which restricts their application. Here we report ligand-induced growth of crystalline ZnO on CsPbX3 (X = Br, I) PNCs for LED application based on the design of PNCs with carboxylate ligand capping. ZnO was grown on the surface of PNCs through in situ organometallic reactions, and a series of Type-II PNC/ZnO heteronanocrystals (PZNCs) were prepared. Crystalline ZnO effectively passivates the surface of colloidal PNCs and significantly improves their stability under various environmental conditions. White light-emitting diodes were fabricated with green- and red-emission PZNCs that achieved no obvious decay for long-time operation in air.

Investigation of defect states in light-irradiated single-crystal ZnO by low-temperature positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy of single crystalline ZnO during light illumination has been performed in conjunction with electron spin resonance (ESR) to investigate defects related to persistent photoconductivity. The intensity of the ESR signal changes depending on the light wavelengths and the apparent positron lifetime during red-light illumination is approximately 6 ps lower on average than that during blue-light illumination. The results suggest that the excitation of V O during blue-light illumination increases positively ionized V O, while the relaxation of V O during red-light illumination decreases positively ionized V O, leading to a decrease in the apparent positron lifetime.

The Influence of Al Doping on the Optical Characteristics of ZnO Nanopowders Obtained by the Low-Cost Sol-Gel Method

In this work, the influence of Al (0, 2, 4, and 6 wt.%) on the optical properties of ZnO has been briefly investigated and described. The undoped and doped samples were characterized using a UV-visible spectrophotometer and Photoluminescence (PL). The X-ray diffraction (XRD) data in our preceding study is also indexed, and materials are said to have a wurtzite-structured hexagonal phase and exhibit no impurity phases. The average crystallite sizes of the pure ZnO were found to increase, i.e., from 14.19 nm to 34.17 nm with an increase in temperature, and in the case of Al-doped ZnO, at a constant temperature, from 35.05 nm to 18.89 nm, respectively. The average crystallite size of AZO (Al-doped ZnO) decreases with increasing Al content. With the increasing temperature, increases in crystallinity and size of the pure ZnO have been observed. In the case of Al doping, with an increase in the concentration of doping the crystallite size is seen to be decreased, without any change in the temperature. Al doping improves the formation of the well-ordered crystalline structure of ZnO up to a certain limit of doping. The absorbance spectra were used to determine the optical band gap of the samples. The optical photoluminescence (PL) spectra of both ZnO and AZO nanopowder are obviously influenced by the increasing temperature and Al doping content, respectively. Different optical properties of ZnO and AZO were observed for different contents of Al in ZnO. The band gap of AZO nanopowder with different concentrations has been seen to be lower than that of undoped ZnO (3.10 eV), suggesting broad application potential. Also, the optical properties of ZnO were tailored by Al doping in the near visible region, suggesting various potential uses.

The study of polychromatic luminescence mechanism of ZnO and ZnO/NiS@NiO heterojunction under different alkali source concentrations

ZnO and ZnO/NiS@NiO (ZNN) composites with different precursor concentration ratios (Zn(CH3COO)2•2H2O: NaOH = 1:1, 1:5 and 1:10) were prepared using a hydrothermal method. The study found that as the concentration of NaOH decreased, the crystallinity of ZnO gradually decreased, the internal defects increase significantly, and the visible light emission intensity in the photoluminescence (PL) spectra also increased. Meanwhile, pure ZnO prepared under the condition of the ratio of 1:1 exhibited completely different defect types of PL characteristics from the products prepared at the ratio of 1:5 and 1:10. The PL emission intensity of ZNN composites prepared by combining various pure ZnO and NiS@NiO has been significantly enhanced, and all ZNN composites exhibited the same type of luminescence characteristics. It is concluded that with the addition of NiS@NiO, the green emission of ZnO is significantly enhanced, and the emission of some defects is shielded. Based on the discussion and analysis, a possible mechanism for the PL process of ZnO and its ZNN composites prepared with different alkali source concentrations was presented.

Low-Cost, High-Yield ZnO Nanostars Synthesis for Pseudocapacitor Applications.

Energy storage devices based on earth-abundant materials are key steps towards portable and sustainable technologies used in daily life. Pseudocapacitive devices, combining high power and high energy density features, are widely required, and transition metal oxides represent promising building materials owing to their excellent stability, abundance, and ease of synthesis. Here, we report an original ZnO-based nanostructure, named nanostars (NSs), obtained at high yields by chemical bath deposition (CBD) and applied as pseudocapacitors. The ZnO NSs appeared as bundles of crystalline ZnO nanostrips (30 nm thin and up to 12 µm long) with a six-point star shape, self-assembled onto a plane. X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence spectroscopy (PL) were used to confirm the crystal structure, shape, and defect-mediated radiation. The ZnO NSs, dispersed onto graphene paper, were tested for energy storage by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) analyses, showing a clear pseudocapacitor behavior. The energy storage mechanism was analyzed and related to oxygen vacancy defects at the surface. A proper evaluation of the charge stored on the ZnO NSs and the substrate allowed us to investigate the storage efficiency, measuring a maximum specific capacitance of 94 due to ZnO nanostars alone, with a marked diffusion-limited behavior. The obtained results demonstrate the promising efficacy of ZnO-based NSs as sustainable materials for pseudocapacitors.

Optimization of active antireflection ZnO films for p-GaAs-based heterojunction solar cells

We have optimized the photovoltaic properties of active ZnO/p-GaAs heterojunction solar cells by pulsed laser deposition method by varying the oxygen pressure from 0 to 50 mTorr during the fabrication process. We observed the crystallinity and grain size of ZnO enhanced with increasing oxygen pressure from 0 to 30 mTorr. In addition, with this increase of oxygen flux, the intensity of E1(LO) modes obtained by Raman measurements declines significantly and almost disappears under an oxygen pressure of 50 mTorr. The change of intensity is assumed to be the change of oxygen vacancy and zinc interstitial concentrations in ZnO films with oxygen pressures. Current-voltage measurements and extractions show that ZnO grown at 30 mTorr displays the best performance with the ISC of 24.3 mA/cm2, the efficiency of 8.746 %, and FF0 of 68.73 %. The high performance of the heterojunction solar cell grown at the oxygen partial pressure of 30 mTorr might be due to the reduction of oxygen vacancies by increasing oxygen during the deposition. The results reveal the importance of the oxygen processing gas in promoting devices performance.

Preparation and characterization of ZnO/SiO2@n-octadecane nanocapsule for ultraviolet absorbing and photothermal conversion energy storage

In this work, PCM nanocapsules composed of ZnO/SiO2 shell and n-octadecane core were successfully synthesized by emulsion polymerization and hydrothermal method. The crystallinity of ZnO was enhanced on the SiO2 shell by MPTS modification. The granule-like ZnO were distributed on the SiO2 shell. The encapsulation rate of n-octadecane in nanocapsules was about 45.2 % with enthalpy of 95.0 J/g melting at room temperature 28.3 °C. Furthermore, due to the presence of ZnO, the synthesized PCM nanocapsules had excellent UV absorption properties for whole wavelengths, and the absorbed UV light can be converted into heat energy for storage, and the photothermal conversion efficiency had reached 25.0 %. Hence, the prepared ZnO/SiO2@n-octadecane nanocapsules had broad application prospects due to its thermal energy storage, UV protection and photothermal conversion performance.

Development of antibacterial and superabsorbent wound composite sponges containing carboxymethyl cellulose/gelatin/Cu-doped ZnO nanoparticles

This study aimed to develop a novel antibacterial and superabsorbent dressing by introducing the Cu-doped ZnO nanoparticles into the carboxymethyl cellulose/gelatin glutaraldehyde-crosslinked composite sponge that is fabricated by lyophilization method. Undoped and Cu-doped ZnO (Zn1−xCuxO, x = 0.03 and 0.05) nanoparticles were synthesized through the stabilizing agent-used precipitation process and characterized by XRD, FESEM, FTIR, and ICP-OES techniques. The XRD evaluation determined that the concentration of copper in ZnO is limited to below 5%. Additionally, The ICP-OES analysis confirmed the effect of the doping process on the ZnO crystalline structure by releasing more zinc and copper ions from Cu-doped ZnO, which resulted to improve antibacterial activity against Staphylococcus aureus and Escherichia coli bacterial strains. The effect of ZnO nanoparticles on the physical and mechanical performance of the optimized composite sponge indicated that the incorporation of 3 wt% ZnO nanoparticles produces a well-interconnected porous structure (~156 µm) with high water absorption (~3089%) and proper elongation (~49%) in a wet medium. The incorporation of Cu-doped ZnO nanoparticles enhanced antibacterial potential of the composite sponge. Meanwhile, all sponge groups are safe for viability, proliferation and adhesion of human dermal fibroblast cells. Overall, the obtained data has proved the potential of carboxymethyl cellulose/gelatin/Cu-doped ZnO dressing as a promising candidate for managing infected wounds.