Surface Of ZnO Crystals Research Articles

Effect of CdS on microstructure and performance of ZnO composite photocatalyst

AbstractIn this study, Nd‒Dy‒ZnO and CdS‒Nd‒Dy‒ZnO photocatalysts were prepared by microwave hydrothermal method to improve the photocatalytic performance of ZnO. The microstructure and performance of samples were characterized by X‐ray diffraction, scanning electron microscopy, ultraviolet (UV)‒visible, and X‐ray photoelectron spectroscopy, and evaluated with the methylene blue. The results showed that CdS attached to the ZnO crystal surface in the form of flocculent particles, and the combination with CdS promoted the growth of ZnO crystals, while an excess of CdS was found to be detrimental to the crystallization of ZnO. When nCdS/nZnO = 0.30 mol%, composite materials achieved the optimal photocatalytic performance, which made the degradation rate of methylene blue reach 93.42% under UV irradiation for 4 h. CdS‒Nd‒Dy‒ZnO composite photocatalytic material can generate active oxygen groups such as hydroxyl and superoxide radicals simultaneously, which greatly enhances the photocatalytic efficiency of the composite material.

High-index crystal plane of ZnO nanopyramidal structures: Stabilization, growth, and improved photocatalytic performance

While the low-index planes of Wurtzite ZnO, such as {0001} and {101¯0} are well-understood, the high-index crystal surfaces have not yet been thoroughly researched despite possessing structural characteristics that make them suitable for many important surface chemistry processes. The high surface energy of high-index ZnO crystal surfaces makes synthesis challenging to achieve due to their instability during crystal growth. In this work, we present a combined experimental and theoretical analysis of growth and photocatalytic activity of ZnO high-index crystal facets. Density functional theory calculations are performed to determine the thermodynamic conditions necessary to stabilize the high-energy semi-polar {112¯2} facets of pyramidal ZnO nanostructures grown via chemical vapor deposition (CVD). The photocatalytic properties of as-synthesized nanopyramidal structures for water splitting applications showed a 73% improved photocatalytic performance compared to CVD-grown hexagonal ZnO nanorods with dominating low-index {101¯0} facets.

Alcoholic Solvent Influence on ZnO Synthesis: A Joint Experimental and Theoretical Study

Zinc oxide nanoparticles were prepared by a solvothermal synthesis using various alcoholic reaction solvents including ethanol, 1-propanol, 1-butanol, 1-pentanol, and 1-octanol, at 170 °C. The nucleation and growth processes of the ZnO nanoparticles were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) characterization, and they were corroborated by means of quantum chemical calculations at the density functional theory level (DFT). On the basis of the results of joint microstructural and theoretical study, the nucleation and preferential growth mechanism of ZnO nanoparticles is proposed. The effect of various alcoholic solvent on the overall shape and growth rate along the preferential c-axis of ZnO nanoparticles were corroborated by means of theoretical simulations of interface-solvent interaction by using nanoclusters (ZnO)n (n = 12 and 36). The (ZnO)36–CH3(CH2)nOH (n ≤ 7) interaction has been found to be spontaneous exergonic process only in the presence of 1-butanol as reaction medium with ΔG*INT = −4.95 kcal mol–1, whereas endergonic in the presence of all remaining alcohols used (ΔG*INT > 0). The results of X-ray diffraction size-strain analysis reveal that solely the use of 1-butanol as solvent leads to rather isotropic crystallite shape (Dv⊥c-ax ∼12 nm, Dv∥c-ax ∼12 nm), whereas in the presence of all remaining alcohols used ZnO nanoparticles grew prevailing in the c-direction to form nanorods. The alcohols of different size and polarity acts as a solvent and reactant as well as controlling agents for crystal growth, providing different binding interactions involved in both the nucleation processes and preferential growth of ZnO nanoparticles. The calculated values of EO···H and EZn–O of (ZnO)36–CH3(CH2)7OH (1-octanol) interaction indicate a weaker interaction of the nonpolar 1-octanol and polar a little positively charged Zn surface of ZnO crystal as well as a higher preferential growth rate along the c-axis in comparison to polar alcoholic media.

Organic frameworks induce synthesis and growth mechanism of well-ordered dumbbell-shaped ZnO particles

Coordination bonds of ligands and metal ions are generally considered for improving agglomeration during the synthesis of metal oxides. However, the strong interactions always limit the performance due to the requirement of additional energy and calcination. A new strategy of synthesizing ZnO particles is proposed through a hydrothermal inducing redox method by using 2-methyl imidazole and Zn(NO3)2. In situ direct redox between this organic ligand and NO3− in a hydrothermal condition is confirmed radical change instead of the simple coordination between organic ligands with Zn2+ in traditional method. The directly acquired ZnO particles possessing uniform dumbbell-like morphology have no obvious agglomeration. The yield reaches >95%. In particular, the synthesis mechanism and morphology evolution during the ZnO aggregates growth were carefully explored and proposed. Density functional theory (DFT) and surface energy calculations demonstrated the effects of activity variation of reactive groups and preferred growth surface of ZnO crystals. This work gives new insights into new synthesis strategy of ZnO particles via hydrothermal condition.

Enhanced ferromagnetism in graphite-like carbon layer-coated ZnO crystals

We report on the preparation of graphite-like carbon-coated ZnO nanoparticles by ball milling a powder mixture of ZnO and C. Magnetic measurements indicated that carbon coating can strongly enhance the room-temperature ferromagnetism of ZnO nanoparticles, depending on preparation conditions. This enhancement may stem from robust ferromagnetism at the ZnO/C interface. The origin of interfacial ferromagnetism is attributed to crystallographic defects at the surface of ZnO crystals which are stabilized by the carbon shell layer.

Three-dimensional epitaxy of single crystalline semiconductors by polarity-selective multistage growth

Despite recent advances in three-dimensional (3D) fabrication, the epitaxial growth of finely controlled, single crystalline 3D structures is still constrained. In this study, we demonstrate the step-by-step growth of hierarchical 3D architectures composed of single crystalline semiconductors. To achieve this goal, we first established control of the preferential growth direction of ZnO crystals by polarity-selective crystallization during the low-temperature solution-phase synthesis. The time-dependent analysis showed that the binding of a citrate additive to the positively charged, top (0001) surface of ZnO crystals led to an inversion of the growth anisotropy from predominantly the vertical direction to the lateral direction with more than a 10 times increase in the width-to-height ratio. With further fine optimization of the charge balance, we minimized the interruption of citrate ions for epitaxial and iterative stacking of Zn and O ions, and achieved single-crystalline hexaplates. This feature was further combined with multistage epitaxial growth of the nanocrystal constituents, producing a 3D, single crystalline semiconductor with excellent luminescent characteristics.

Low-Index ZnO Crystal Plane-Specific Binding Behavior of Whole Immunoglobulin G Proteins.

Crystallographic surface-resolved examination of protein-ZnO interactions can greatly enhance the fundamental understanding of protein adsorption on these technologically important solid surfaces which, in turn, will be tremendously valuable for the emerging applications of ZnO-based biomaterials and biosensors. We examine experimentally and via computer simulations the intriguing differences in the adsorption preferences and binding behavior of whole immunoglobulin G (IgG) proteins to various, low-index ZnO crystal surfaces at the individual biomolecule level. By performing direct atomic force microscopy imaging, we determine that IgG predominantly binds to the ZnO plane of (101̅0) relative to the other three low-index planes of (0001), (0001̅), and (112̅0). This phenomenon is highly unusual, particularly considering the fact that the average binding energy of amino acids (AAs) on the ZnO (0001) facet is higher than that on the (101̅0) plane. In conjunction with combined Monte Carlo-molecular dynamics simulations, we further explain the possible origins of our unusual experimental findings with critical factors such as the specific spatial locations of strongly binding AAs in the protein and their spatial distributions on the exterior surface of the protein.

Photoluminescence Studies of ZnO, ZnO:Eu and ZnO:Eu Nanoparticles Covered with Y<SUB>2</SUB>O<SUB>3</SUB> Matrix

Development of advanced display and lighting technology such as field emission displays and plasma display panels requires phosphor which has a high efficiency and low degradation. Particle sizes and the locations of dopants in the hosts take an important role in the luminescence emissions of phosphors. ZnO nanoparticles are widely employed in plasma field emission display devices and well investigated; however, lanthanide (Ln3+) doped ZnO needs more investigations. In ZnO:Eu the lanthanide ions (Eu3+) may occupy either Zn2+ lattice site or on surface of ZnO crystal. The emissions of Eu3+ ion on the surface are the characteristic of Eu2 O3 , which leads to weak luminescence emission. To observe such phenomena, nanoparticles of ZnO, 2 at.% Eu3+ doped ZnO (ZnO:Eu) and ZnO:Eu covered with yattria matrix were prepared by wet chemical method at low temperature. The prepared nanoparticles were characterized by XRD and TEM. XRD data reveal the significant phase segregation of the annealed nanoparticles compared with lower heated samples. This phase segregation of Eu3+ ion establishes responsible for the decrease luminescence intensity of annealed ZnO:Eu nanoparticles compared with the as-prepared ZnO:Eu nanoparticles. Improvement on luminescence emissions could be achieved only for the as-prepared ZnO:Eu nanoparticles while covered with yattria matrix.

Direct writing of 150 nm gratings and squares on ZnO crystal in water by using 800 nm femtosecond laser.

We present a controllable fabrication of nanogratings and nanosquares on the surface of ZnO crystal in water based on femtosecond laser-induced periodic surface structures (LIPSS). The formation of nanogrooves depends on both laser fluence and writing speed. A single groove with width less than 40 nm and double grooves with distance of 150 nm have been produced by manipulating 800 nm femtosecond laser fluence. Nanogratings with period of 150 nm, 300 nm and 1000 nm, and nanosquares with dimensions of 150 × 150 nm2 were fabricated by using this direct femtosecond laser writing technique.

Infrared femtosecond laser-induced great enhancement of ultraviolet luminescence of ZnO two-dimensional nanostructures

Two-dimensional (2D) complex nanostructures on the surface of ZnO crystal are fabricated by the interference of three 800 nm fs laser beams. The 2D nanostructures exhibit a great enhancement of UV emission excited by infrared fs laser with central wavelengths ranging from 1,200 nm to 2,000 nm. We propose that the defect states in the band gap of 2D nanostructures induced by 800 nm fs laser ablation cause the great enhancement of UV emission. We make theoretical calculations and explain well with the experimental results.

Large‐Scale Synthesis of Vertically Aligned ZnO Hexagonal Nanotube‐Rod Hybrids Using a Two‐Step Growth Method

Zn‐polar (0001) surfaces are more chemically reactive than other surfaces of ZnO crystals and drive preferential anisotropic and asymmetric growth along the [0001] direction, which facilitates growth of c‐axis oriented, one‐dimensional ZnO nanostructures. Accordingly, capping the top (0001) surface of ZnO crystals can impede c‐axis growth and thus serve to modulate growth habits. In this study, we generated vertically aligned ZnO hexagonal nanotube‐rod (h‐NTR) hybrids by modulating growth habits during a second‐stage process. Electron microscopy studies revealed the formation of very thin (10–20 nm) single‐crystalline nanotube walls along the edges of underlying hexagonal rod tops capped with Si. In addition, spatially resolved investigation of ZnO h‐NTR indicated an abrupt increase in the measured bandgap across rod‐tube junctions, which was ascribed to a quantum confinement effect and Burstein–Moss effect of carriers within the very thin nanotube walls.

ZnO nanocrystals with a high percentage of exposed [formula omitted] reactive facets for enhanced gas sensing performance

Zinc oxide single crystals with a high percentage of exposed {4 2¯ 2¯ 3¯} reactive facet were prepared by a facile hydrothermal route. X-ray diffraction, field emission scanning electron microscopy, and high resolution transmission microscopy confirmed the faceted single crystal structure. Through the density functional theory (DFT) calculations, it is validated that the {4 2¯ 2¯ 3¯} surface of ZnO crystals has high surface energy. When used as a sensing material in gas sensors, ZnO crystals with dominating exposed {4 2¯ 2¯ 3¯} planes exhibited a superior sensitivity toward toxic and flammable gases.

Fabrication of different morphologies of ZnO superstructures in presence of synthesized ethylammonium nitrate (EAN) ionic liquid: synthesis, characterization and analysis

ZnO particles were synthesized by hydrothermal route at 95 °C, with different ethylammonium nitrate (EAN) : water volume ratio (0.01 to 1) in reaction media. Morphology of ZnO particles changed from initial cylindrical to intermediate spindle and finally to spherical with increasing concentration of EAN in reaction media whereas pH of aliquots remained within range of 7-7.5. Aggregates of EAN bind to Zn(2+) enriched both terminal planes as well as to Zn(2+) and O(2-) enriched side planes of basis units which finally resulted to formation of spherical ZnO superstructures. Favorable H-bond and electrostatic interaction helped to bind EAN aggregates with surfaces of ZnO crystals. It was found that the spherical ZnO superstructures showed novel photoluminescent property and enhanced photocatalytic activity compared to that of the commercially available ZnO.

Verification of surface polarity of O-face ZnO(0 0 0 [formula omitted]) by quantitative modeling analysis of Auger electron spectroscopy

Is crystalline ZnO(0001¯) O-face surface believed to be enriched by Zn atoms? This study may get the answer. We proposed a simplified model to simulate surface concentration ratio on (0001¯)-O or (0001)-Zn surface based on the hard-sphere model. The simulation ratio was performed by integrating electron signals from the assumed Auger emission, in which the electron mean free path and relative atomic layer arrangements inside the different polarity ZnO crystal surface were considered as relevant parameters. After counting more than 100 experimental observations of Zn/O ratios, the high frequency peak ratio was found at around 0.428, which was near the value predicted by the proposed model using the IMFP database. The ratio larger than the peak value corresponds to that observed in the annealed samples. A downward trend of the ratio evaluated on the post-sputtering sample indicates the possibility of a Zn-enriched phase appearing on the annealed O-face surface. This phenomenon can further elucidate the O-deficiency debate on most ZnO materials.

Highly sensitive and selective dimethylamine sensors based on hierarchical ZnO architectures composed of nanorods and nanosheet-assembled microspheres

Abstract Hierarchical ZnO architectures composed of nanorods and nanosheet-assembled microspheres were successfully synthesized through a one-pot, cost-effective and environment-friendly hydrothermal process at low-temperature. X-ray diffraction and Raman spectroscopy indicated that the ZnO architectures are well-crystallized in a hexagonal wurtzite structure. Electronmicroscopy observations and N 2 adsorption–desorption results confirmed that plenty of meso-pores imbed in the nanosheets and considerable interspaces exist among adjacent nanosheets. Photoluminescence analysis showed that abundant intrinsic defects (52.5% electron donor defects and 47.5% electron acceptor ones) are present on the ZnO crystal surfaces. X-ray photoelectron spectrometry revealed that a large mount of oxygen species (23.77%) chemisorbs on the surface of the as-prepared ZnO structures. The gas sensing property of the ZnO architectures displayed high response, fast response-recovery, good selectivity and long-term stability to 1–1000 ppm dimethylamine at 370 °C. Especially, even 1 ppm dimethylamine could be well detected with a high response value ( S = 16.8). More interestingly, dimethylamine could be easily distinguished from a trimethylamine, methylamine or ammonia atmosphere with high selectivity. The high dimethylamine response is mainly attributed to the high contents of electron donor defects ( Z n i and V O ) on the ZnO surfaces.

Surface plasmon on topological insulator/dielectric interface enhanced ZnO ultraviolet photoluminescence

It has recently been predicted that the surface plasmons are allowed to exist on the interface between a topological insulator and vacuum. Surface plasmons can be employed to enhance the optical emission from various illuminants. Here, we study the photoluminescence properties of the ZnO/Bi2Te3 hybrid structures. Thin flakes of Bi2Te3, a typical three-dimensional topological insulator, were prepared on ZnO crystal surface by mechanical exfoliation method. The ultraviolet emission from ZnO was found to be enhanced by the Bi2Te3 thin flakes, which was attributed to the surface plasmon – photon coupling at the Bi2Te3/ZnO interface.

Shuttle-like ZnO nano/microrods: Facile synthesis, optical characterization and high formaldehyde sensing properties

Shuttle-like ZnO nano/microrods were successfully synthesized via a low temperature (80°C), “green” (without any organic solvent or surfactant) and simple hydrothermal process in the solution of zinc chloride and ammonia water. X-ray diffraction and Raman spectroscopy indicated that the ZnO nano/microrods are a well-crystallized hexagonal wurtzite structure. Yet photoluminescence analysis showed that abundant intrinsic defects (52.97% electron donor defects and 45.49% electron acceptor defects) exist on the surface of ZnO crystals. Gas sensors based on the shuttle-like ZnO nano/microrods exhibited high sensitivity, rapid response–recovery and good selectivity to formaldehyde in the range of 10–1000ppm at an optimum operating temperature of 400°C. Through applying linear fitting to the plot of sensitivity versus formaldehyde concentration in logarithmic forms, the chemisorbed oxygen species on the ZnO surface were found to be O2− (highly active among O2, O2− and O− species). Notably, formaldehyde can be easily distinguished from acetaldehyde with a selectivity of about 3. The high formaldehyde sensitivity is mainly attributed to the synergistic effect of abundant electron donor defects (52.97%) and highly active oxidants (surface adsorbed O2− species) co-existed on the surfaces of ZnO.

Ultraviolet luminescence enhancement of ZnO two-dimensional periodic nanostructures fabricated by the interference of three femtosecond laser beams

We have developed a simple and rapid method for fabricating ZnO periodic nanostructures. By changing the laser polarization combinations, we fabricated different types of two-dimensional (2D) nanostructures on ZnO crystal surfaces by the interference of three femtosecond laser beams. The 2D nanostructures became more regular and uniform when increasing the cross angles between any two laser beams. Compared with the case of the plane surfaces of ZnO crystals, the 2D nanostructures revealed an ultraviolet (UV) luminescence enhancement excited by an 800 nm femtosecond laser beam. We studied the photoluminescence properties of the 2D nanostructures and the mechanisms of the UV luminescence enhancement. Our results indicated that the enhancement was caused by an increase in optical absorption with respect to that of the unaltered ZnO plane surface and by the formation of surface defect states.

Growth of Highly Orientated and Well-Aligned ZnO Nanowhiskers Using Aqueous Solutions

Length-tailored, monodisperse, highly orientated, single-crystalline hexagonal and aligned ZnO nanowhiskers were grown onto F-doped SnO2 conductive glass (FTO) substrate at 88 °C using an aqueous solution. The aqueous solution for growth of ZnO nanowhiskers included zinc nitrate hexahydrate, hexamethylenetetramine and polyethylenimine. The addition of branched polyethylenimine, which may be adsorbed on the nonpolar surface of ZnO crystals, improved the growth of ZnO nanowhiskers along the c-axis.

Effects of polyethylenimine on morphology and property of ZnO films grown in aqueous solutions

Morphologies and properties of well-aligned ZnO films were controlled using zinc nitrate-hexamethylenetetramine aqueous solutions with the addition of polyethylenimine as a surfactant. Porous and dense ZnO films were fabricated with and without polyethylenimine, respectively. The addition of polyethylenimine proceeded to form porous ZnO whiskers film by preferential adsorption to nonpolar crystal faces and modifications of the surface free energy and growth rate. Dense ZnO film showed high transmittance of 80%, and low intensity of fluorescence and photo-induced current. Porous ZnO whiskers film showed low transmittance of 70%, while high intensity of fluorescence and high photo-induced current were detected because the porous ZnO nanowhisker film possessed a large interior surface area which can capture large amounts of DNA molecules labeled with dye molecules on the surface of ZnO crystals. High performance dye-sensitized sensors can be produced using ZnO whisker films prepared from an aqueous solution.