ZnO Bulks Research Articles

High-precision strategy for piezoelectric characterization of nano/microwire

Investigating piezoelectricity of nano/microwire is important for their applications in nanogenerator, actuator, sensor and many more. Up to now, accurately probing the piezoelectric constant of nano/microwire along its axial direction has remained a challenge. Here, we propose a strategy that can high-precisely characterize the piezoelectric constant of nano/microwire along its axial direction through constructing a lateral structure with short connection of two ends of nano/microwire as one electrode and probe tip as another electrode, making the nano/microwire not buckle and its length unchanged during measurements. Theoretical analysis and simulations show that this strategy has ultra-high precision, in which the error can be as low as 2% (aspect ratio ∼100), 0.3% (∼500) and less than 0.2% (∼1000). Using this strategy, the measured piezoelectric constant d33 of ZnO microwires is 12.31 ± 0.57 pm/V, highly consistent with widely accepted values in ZnO bulks and films, further verifying the correctness and accuracy of this strategy. This simple, universal and high-precision strategy contributes to the piezoelectric characterization of nano/microwires and their applications in energy harvesting, human-machine interfacing, active electronics/optoelectronics and many more.

Impacts of Ga doping on structural and thermoelectric properties of ZnO bulks sintered by solid-state reaction

Introduction: ZnO-based bulks have been known as high-temperature thermoelectric materials due to their high thermal stability and large Seebeck coefficient. The biggest handicaps of pure ZnO are normally low electrical conductivity and weak thermoelectric power factor. This work aims to improve the electrical and thermoelectric properties of pure ZnO bulks by Ga doping. Methods: The pure ZnO and Zn0.98Ga0.02O bulks were fabricated by solid-state reaction in air at high temperature. The crystalline and microstructural properties of the samples were analyzed by using X-ray diffraction and field-emission scanning electron microscopy, respectively. The dependences on temperature of electrical conductivity, Seebeck coefficient, and power factor were recorded by using a commercial LSR-3 system. Results: The Ga doping increases carrier concentration and density-of-state effective mass, leading to simultaneous improvements in electrical conductivity and Seebeck coefficient of ZnO. At 500oC, the thermoelectric power factor of the Zn0.98Ga0.02O bulk is remarkably enhanced by 36% to 169.8 µV/mK2 as compared to the ZnO. Conclusion: The Ga doping shows not only the significant improvement in power factor, but also the potential for reducing thermal conductivity due to spinel phase segregation. Thus, this work provides a promising solution for controlling the thermoelectric performance of ZnO by doping Ga.

Co-catalyst-free large ZnO single crystal for high-efficiency piezocatalytic hydrogen evolution from pure water

Piezocatalytic materials have been widely used for catalytic hydrogen evolution and purification of organic contaminants. However, most studies focus on nano-size and/or polycrystalline catalysts, suffering from aggregation and neutralization of internal piezoelectric field caused by polydomains. Here we report a single crystal ZnO of large size and few bulk defects crafted by a hydrothermal method for piezocatalytic hydrogen generation from pure water. It is noteworthy that single-side surface areas of both original as-prepared ZnO and Ga-doped ZnO bulk crystals are larger than 30 cm2. The high quality of ZnO and Ga-doped ZnO bulks are further uncovered by high-resolution transmission electron microscope (HRTEM), photoluminescence (PL) and X-ray diffraction (XRD). Remarkably, an outstanding hydrogen production rate of co-catalyst-free Ga-doped ZnO bulk crystal (i.e., a maximum rate of 5915 μmol h−1 m−2) is observed in pure water triggered by ultrasound in dark, which is over 100 times higher than that of its powder counterpart (i.e., 52.54 μmol h−1 m−2). The piezocatalytic performance of ZnO bulk crystal is systematically studied in terms of varied exposed crystal facet, thickness and conductivity. Different piezocatalytic performances are attributed to magnitude and distribution of piezoelectric potential, revealed by the finite element method (FEM) simulation. The density functional theory (DFT) calculations are employed to investigate the piezocatalytic hydrogen evolution process, indicating a strong H2O adsorption and a low energy barrier for both H2O dissociation and H2 generation on the stressed Zn-terminated (0001) ZnO surface.

Realization of high thermoelectric power factor in Ta-doped ZnO by grain boundary engineering

Electrical conductivity in nanostructured ZnO bulks is limited by the inherently low carrier mobility caused by the high density of grain boundaries and interfaces. In this study, Zn1−xTaxO (x = 0, 0.01, 0.02, 0.03) with micro/nanoplatelet structures composed of nearly coherent dense grain boundaries with a low misorientation angle of ∼4° between the grains was successfully fabricated. Despite the presence of a significant amount of grain boundaries and interfaces in the sintered bulk material, a high carrier mobility (52.2 cm2 V−1 s−1) was obtained in the composition Zn0.99Ta0.01O, which is comparable to the value shown by ZnO single crystals and far higher than their ordinary nanostructured counterparts (<15 cm2 V−1 s−1). In addition, the distortion of the density of states increased effective mass induced by Ta 5d hybridization in ZnO caused a Seebeck coefficient of −290 μV K−1 at 1002 K resulting in a high power factor of 15.2 × 10−4 W m−1 K−2 at 1002 K.

Morphological and Optical Properties of Cobalt Ion-Modified ZnO Nanowires

In this study, we prepared cobalt (Co) ion-modified ZnO nanowires using hydrothermal synthesis with zinc acetate dehydrate and Co (II) acetate hydrate precursors in ethanol solutions. Their morphological and optical properties were investigated with varying Co precursor concentration. The morphological changes of the ZnO nanowires depended positively on the concentration of the Co precursor. The ZnO nanowires showed modified crystal orientations and nanostructure shapes depending on the Co concentration in the solutions. Variations in the optical properties of the Co ion-modified ZnO nanowires could be explained by the interaction of the Co ions with the band electrons, oxygen vacancies, and zinc interstitials. The overall growth and characteristics of ZnO nanowires synthesized in solutions containing low levels of Co ions were related to Co doping into the ZnO bulks. In solutions containing high levels of Co ions, these were additionally related to the Co oxide cluster.

Effects of Au nanoparticles and ZnO morphology on the photocatalytic performance of Au doped ZnO/TiO2 films

Au doped ZnO nanocomposite films on TiO2 seeding layer (AuZ/T) were fabricated by hydrothermal processing and their photocatalytic performance was investigated. It could be found that the AuZ/T with micrometer(μm)-sized, lying ZnO bulks revealed optimal photocatalytic performance toward methyl orange under simulated sunlight, whose apparent degradation rate constant Kapp of 1.31 was about 20% higher compared to that of ZnO/TiO2 and 3 times higher compared to that of ZnO. The Au nanoparticles, TiO2 seeding layer and hydrothermal processing time imposed vital influence on the morphology of ZnO nanostructures, which played key roles in the formation of ZnO/TiO2 heterojunction and charge transfer (CT) inside it, as demonstrated by kinetics of transient photoluminescence (PL) decaying. The incorporation of Au nanoparticles not only induced the variations of ZnO crystallinity and reduction of ZnO band gap (Eg), but also generated the Schottky heterojunction of metal–semiconductor, which would be beneficial to the CT inside nanocomposite films and separation of photo-generated electron–hole pairs, as verified by the remarkable PL suppression. The mechanism responsible for photocatalysis enhancement, which was resulted from the hybrid effects of Au nanoparticles and the ZnO morphology was discussed in details.

The investigation of the defect structures for Co2+ in ZnO microwires, thin films and bulks

The defect structures for Co2+ in ZnO microwires, thin films and bulks are theoretically investigated by analyzing the electron paramagnetic resonance (EPR) parameters (zero-field splitting D, g factors g// and g⊥ and hyperfine structure constants A// and A⊥) for trigonally distorted tetrahedral 3d7 clusters in a consistent way. The impurities Co2+ are found to suffer the displacements (≈0.046, 0.044 and 0.045Å) away from the ligand triangles along the C3 axis in ZnO microwires, thin films and bulks, respectively, which considerably reduce the trigonal distortions of the Zn2+ sites in the hosts. Apart from the contributions from the conventional crystal-field (CF) mechanism, those from the charge-transfer (CT) mechanism are important and should be included in the EPR analysis. The defect structures and spectral properties of the three ZnO:Co2+ systems are discussed.

Enhanced Al/Ni co-doping and power factor in textured ZnO thermoelectric ceramics prepared by hydrothermal synthesis and spark plasma sintering

In this work, we report a significant enhancement of thermoelectric power factor in textured Al/Ni-doped ZnO bulks, which were prepared by spark plasma sintering (SPS) Zn0.98-xAl0.02NixO (x = 0, 0.01, 0.02, 0.03, 0.04) microrod-shaped powders synthesized by hydrothermal synthesis (HS). The co-doped Ni2+ takes up usually the interstitial ZnO lattice cell and expands its lattice during the HS process, which favors the interstitial solution for Al3+ in the powders. The improved solubility of Al3+ in the powders was retained in the bulks after SPS due to its short sintering time, leading to a high carrier concentration of ca. 1 × 1020 cm−3 at 0.01 ≤ x ≤ 0.04. The co-doping mechanism of Al3+ and/or Ni2+ during the HS and SPS process was also discussed in detail concerning their solution types. The textured microstructure along with a high solubility of Al3+ contributes to an enhanced power factor up to 6.16 × 10−4 Wm−1K−2 at 673 K in the Zn0.97Al0.02Ni0.01O bulk and an optimized ZT value of 0.057 at 773 K in the Zn0.96Al0.02Ni0.02O bulk.

Electronic structures of ZnX (X = O and S) nanosheets from first-principles energy loss near edge structure studies

This paper tries to study the core energy loss spectra of zinc based nanosheets (ZnO and ZnS) in density functional theory using the FPLAPW method. We have calculated the energy loss near edge structure (ELNES) spectra of zinc K- and L2,3-edges, and oxygen and sulfur K-edges in ZnO and ZnS nanosheet at magic angle conditions and compare to those of ZnO and ZnS wurtzite bulk structures. As a result of the smaller bond lengths in the nanosheet structures, all the ELNES spectra in nanosheets, including Zn K- and L2,3-edges, and O and S K-edges show a shift to the higher energies. The calculations reveal that in comparison to Zn edges in ZnO structures, all the ELNES spectra of ZnS structures including the bulk and sheet show a shift to lower energy region. This is a result of larger bond lengths in ZnS structures, and that it can be used to fingerprint each structure. The comparison of ELNES spectra and unoccupied symmetry-projected density of states (local DOS) confirms that Zn K-edges of both ZnO and ZnS nanosheets have main features including the electron transition to π* and σ* states which this is a sign of the strong sp2 hybridization and it is absent in Zn K-edge of ZnO and ZnS bulks. Moreover, in comparison to bulks, the energy differences between some main features in Zn based nanosheets decreases. The accordance of ELNES spectra and unoccupied local DOS shows that the main contributions in Zn L-edges ELNES of ZnO and ZnS nanosheets refer to transitions to mostly d-symmetry states and p-d hybridization. Orientation dependencies of the spectral features in nanosheet structures were also consideable. The overall dispersions of oxygen and sulfur K-edges in bulks and nanosheets are different. Since the inclusion of core-holes and super-cells is essential for accurate reproduction of features in ZnO and ZnS bulks and due to the rather wide band gaps of Zn based nanosheets, it may be essential as well for all the ELNES spectra of nanosheet structures.

Effects of Oxygen Vacancy on Optical and Electrical Properties of ZnO Bulks and Nanowires

Based on the generalized gradient approximation (GGA) in density functional theory (DFT) and using the first-principle plane wave ultrasoft pseudopotential method, we construct and optimize the structures of intrinsic and oxygen vacancy (VO) ZnO bulks and nanowires (NWs) in the Castep module. Moreover, the calculation of band structures and the optical properties are carried out. The calculated results exhibit that the oxygen vacancy exerts a more significant influence on the electronic structures of the ZnO bulks instead of the NWs. What is more, the influences of the VO on the optical properties are mainly embodied in the ultraviolet region, and the main optical parameters of ZnO bulks and NWs with VO are anisotropic.

Effects of N doping on photoelectric properties of along different directions of ZnO bulk and nanotube

The electronic structures and optical properties of N-doped ZnO bulks and nanotubes are investigated using the first-principles density functional method. The calculated results show that the main optical parameters of ZnO bulks are isotropic (especially in the high frequency region), while ZnO nanotubes exhibit anisotropic optical properties. N doping results show that ZnO bulks and nanotubes present more obvious anisotropies in the low-frequency region. Thereinto, the optical parameters of N-doped ZnO bulks along the [100] direction are greater than those along the [001] direction, while for N-doped nanotubes, the variable quantities of optical parameters along the [100] direction are less than those along the [001] direction. In addition, refractive indexes, electrical conductivities, dielectric constants, and absorption coefficients of ZnO bulks and nanotubes each contain an obvious spectral band in the deep ultraviolet (UV) (100 nm ∼ 300 nm). For each of N-doped ZnO bulks and nanotubes, a spectral peak appears in the UV and visible light region, showing that N doping can broaden the application scope of the optical properties of ZnO.

Young׳s modulus of individual ZnO nanowires

We used a contact-mode atomic force microscopy (AFM) to study the mechanical properties of an individual ZnO nanowire in the open air. It is noteworthy that the Young׳s modulus can be determined by an AFM tip compressing a single nanowire on a rigid substrate, which can bring more repeatability and accuracy for the measurements. In particular, the calculated radial Young׳s modulus of ZnO nanowires is consistent with the data of ZnO bulks and thin films. We also present the Young׳s modulus with different diameters, and all these are discussed deeply.

Comprehensive study of the surface morphology evolution induced by thermal annealing in single-crystalline ZnO Films and ZnO bulks

We report on the evolution of the surface morphology induced by thermal annealing in N2 ambient over a wide temperature range of 500–1200 °C in single-crystalline ZnO films and ZnO bulks. The surface morphology is seriously changed by the annealing temperature, and the evolution can be categorized into three regions: island growth, island agglomeration, and pit formation. Island growth at low temperatures below 700 °C, is ascribed to the atomic migration to reduce surface energy, which causes surface roughening. Island agglomeration at intermediate temperatures of 700–900 °C is ascribed to the migration and the evaporation of surface atoms, which causes surface flattening. Pit formation at high temperatures above 900 °C is ascribed to the atomic evaporation by high vapor pressure, which causes surface destruction. On the other hand, the bulk lattice is continuously improved with increasing annealing temperature in the temperature regions before the surface-destruction region, which is attributed to the reduction in the numbers of point and line defects caused by recrystallization. As a result, the best surface morphology and the best bulk lattice are obtained at an annealing temperature of 900 °C. The common surface-morphology evolution of ZnO films and ZnO bulks with increasing annealing temperature can be summarized using the three steps of surface roughening by island growth, surface flattening by island agglomeration, and surface destruction by pit formation.

High temperature annealing treatment on Co doped ZnO bulks

Zn0.95Co0.05O Bulk samples were synthesized by solid state reaction method and annealed by different processes. The Zn treated samples show ferromagnetism，whereas the vacuum annealed samples do not show ferromagnetism. The XRD patterns show that there is Co impurity phase in the sample after Zn treatment at 1100℃，which is the reason of the ferromagnetism observed. At low temperature，both the positive and negative magnetoresistance （MR） effect can be observed for all the samples. We conclude that the s-d exchange_interaction_induced spin-splitting gives rise to positive MR， while the formation of bound magnetic polarons and the magnetic field suppressed spin-disorder scattering are responsible for the negative MR behavior under high field.

Comparative study of photoluminescences for Zn-polar and O-polar faces of single-crystalline ZnO bulks

The authors have an extensive study of photoluminescences for Zn-polar and O-polar faces of single-crystalline ZnO bulks. In the photoluminescence (PL) spectra at 10 K, Zn-polar and O-polar faces show a common emission feature: neutral donor-bound excitons and their longitudinal-optical (LO) phonon replicas are strong and free excitons are very weak. However, in the PL spectra at room temperature (RT), Zn-polar and O-polar faces show extremely different emission characteristics: the emission intensity of Zn-polar face is 30 times larger than that of O-polar face and the band edge of Zn-polar face is 33 meV redshifted from that of O-polar face. The temperature dependence of photoluminescence indicates that the PL spectra at RT are closely associated with free excitons and their phonon-assisted annihilation processes. As a result, it is found that the RT PL spectra of Zn-polar face are dominated by the first-order LO phonon replica of A free excitons, while that of O-polar face is determined by A free excitons. This is ascribed to the difference in exciton-phonon coupling strengths that Zn-polar phase has larger coupling strength than O-polar face.

Room temperature ferromagnetism in two-step-prepared Co-doped ZnO bulks

We have prepared Co-doped ZnO bulks with a two-step method. The bulk samples are sintered at the temperature above 1000°C to get pure single phase and then treated by Zn vapor in a vacuum. Based on such two-step preparation, clear carrier induced ferromagnetism is found in Zn treated Co-doped ZnO bulks, which suggests that the magnetism mechanism can be of codoping effect. The onset of ferromagnetism is associated with the further increase of the hybridization of the impurity band with 3d states near the Fermi level when additional carrier induced to let the Stoner criterion for ferromagnetism to be satisfied.

Enhancement effect of photoluminescence in assemblies of nano-ZnO particles/silica aerogels

SiO 2 aerogels were prepared by a sol-gel method and supercritical drying. Introduction of nano-ZnO into nanopores of the SiO2 aerogels is performed by immersion, chemical reaction, and hydrolysis. Photoluminescence of these assemblies of nano-ZnO particles/SiO2 aerogels were systematically measured. For each assembly, a very strong photoluminescence (PL) band with the peak position of about 500 nm was observed. The luminescence intensities of the assemblies of nano-ZnO particles/SiO2 aerogels are 10–50 times higher than that of nanostructured ZnO bulks. The quantum efficiency of the PL band for the assemblies is in the range of 0.2%–1%. This luminescence enhancement effect is caused by the increase of the singly ionized oxygen vacancies in nano-ZnO particles, which are located in nanopores of the SiO2 aerogel.