ZnO Core-shell Structures Research Articles

Oxygen Adsorption on Polar and Non-Polar Zn:ZnO Heterostructures from First Principles.

Zn:ZnO nanostructures have been studied extensively due to their potential use in many applications, such as oxygen scavengers for food packaging applications. Under atmospheric conditions, ZnO grows on the surface of Zn via an oxidation process. The mechanisms governing Zn oxidation are still not fully understood, with classical oxidation models, such as the Cabrera Mott, underestimating the oxide thickness of Zn:ZnO core-shell structures. In this work, Ab initio DFT calculations were performed to assess the adsorption properties of oxygen molecules on Zn:ZnO heterostructures to help elucidate the mechanisms involved in the growth of a ZnO film on a Zn substrate. Results suggest that the charge transfer mechanism from the Zn:ZnO heterostructures to the adsorbed oxygen layer can be promoted by two different processes: the electronic doping of ZnO due to the formation of the Zn:ZnO interface and the excess surface charge due to the presence of dangling bonds on the as cleaved ZnO.

The effect of different rates of ultra-thin gossamer-like rGO coatings on photocatalytic performance in ZnO core-shell structures for optoelectronic applications

In the study, core-shell-structured Al/(ZnO:rGO)/pSi/Al photo-diodes were successfully fabricated using a sol-gel spin-coating method by varying the concentration of reduced-graphene oxide (rGO) from 1 % to 9 % (wt). The ZnO:rGO composite solution was coated on a silicon (p-Si) wafer at 1000 rpm and 300 K. Both aluminum back-ohmic and front-rectifier contacts were performed on the p-Si wafer by physical-vapor-deposition (PVD). The morphological and chemical structure of the photo-diodes were determined by using field-emission scanning electron microscopy (FE-SEM), energy dispersive spectrometry (EDS), and X-ray diffraction (XRD). The current-voltage (I-V) analysis in dark and under ultraviolet (UV, 365 nm) wavelength was utilized in detail. Basic electrical parameters, including the ideality factor (n), barrier height (BH) and series-shunt resistances (R s , R sh ), were calculated using a variety of methods and compared to each other. The Card-Rhoderick method was used to extract energy-dependent profiles of interface traps (N ss ). The core-shell-structured (ZnO-7 % rGO) photo-diode exhibited the best photocatalytic performance both in dark and under various illumination intensities (50–250 mW/cm 2 ). The ZnO:rGO interlayer at the metal-semiconductor (M/S) interface leads to improvement of the photo-diode in respect of low-ideality factor/Nss/leakage-current and high-rectification and BH. • The ZnO:rGO core-shell structures were synthesized and coated successfully and homogeneously using sol-gel spin coating. • This study is pioneer due to the fact that there is no detailed study about the Al/ZnO:rGO/pSi/Al photo-diodes from ZnO:rGO interface as wrapped different rGO rates ZnO core-shell. • The rGO nanosheets were provided bridging and this may be a conductivity mechanism between ZnO spheres that increases the conductivity of the photo-diodes. • The 7 % rGO included ZnO core-shell structure has a strongly positive effect on the photo-diode performance. • The 7 % rGO included ZnO:rGO interlayer can be succesfully used instead of traditional insulator layer in the future.

Discharge intermittency considerably changes ZnO spatial distribution in porous Zn anodes

Porous Zn anodes are ubiquitous in primary batteries and are under development for low-cost rechargeable batteries. Spatial distribution of ZnO discharge product is a critical factor in these, because it can passivate the active material. In rechargeable cells this is related to a major failure mechanism called shape change, in which ZnO is relocated to inactive locations. In this work we demonstrate rest steps during discharge of primary Zn anodes dramatically alter the placement of ZnO in the anode. In alkaline electrolyte, ZnO discharge product is typically modeled as precipitating close to Zn particles, forming a porous ZnO shell around the Zn core. Anodes discharged continuously at low-rate are compared to anodes similarly discharged intermittently, using in situ computed tomography from a synchrotron source. Zn–ZnO core-shell structures are produced during continuous discharge but are not found in cells discharged intermittently. Continuously discharged cells showed that ZnO was formed most strongly near the separator, in agreement with Zn anode battery models. In pulse-discharged cells, ZnO was more radially distributed and was in large formations not physically connected to Zn particles. Thus, discharge intermittency changes spatial distribution of ZnO in ways that are unpredicted by Zn anode models.

Improved Photoresponse Characteristics of a ZnO-Based UV Photodetector by the Formation of an Amorphous SnO2 Shell Layer

Although ZnO nanostructure-based photodetectors feature a well-established system, they still present difficulties when being used in practical situations due to their slow response time. In this study, we report on how forming an amorphous SnO2 (a-SnO2) shell layer on ZnO nanorods (NRs) enhances the photoresponse speed of a ZnO-based UV photodetector (UV PD). Our suggested UV PD, consisting of a ZnO/a-SnO2 NRs core–shell structure, shows a rise time that is 26 times faster than a UV PD with bare ZnO NRs under 365 nm UV irradiation. In addition, the light responsivity of the ZnO/SnO2 NRs PD simultaneously increases by 3.1 times, which can be attributed to the passivation effects of the coated a-SnO2 shell layer. With a wide bandgap (~4.5 eV), the a-SnO2 shell layer can successfully suppress the oxygen-mediated process on the ZnO NRs surface, improving the photoresponse properties. Therefore, with a fast photoresponse speed and a low fabrication temperature, our as-synthesized, a-SnO2-coated ZnO core–shell structure qualifies as a candidate for ZnO-based PDs.

Photoelectrochemical Bioanalyte Sensor Based on Engineered One-Dimensional Nanostructured Oxide

Photoelectrochemical (PEC) non-enzymatic sensing systems are beneficial for the detection of low concentrations of biological molecules. Among different potential nanostructured materials, one dimensional TiO2 and ZnO nanorods are promising semi-conductor which can be successfully applied in many photoelectrochemical applications, mainly due to their photoactive properties, and long-term stability. In spite of their advantages, the wide band gap of 1D TiO2 and ZnO along with their high recombination rate confine their applications dramatically. These properties, prevent 1D TiO2 and ZnO from absorbing visible light; therefore, the semiconductor is only photoactive in the UV range. Here, we describe novel non-enzymatic sunlight-driven PEC sensors based on a modified 1D TiO2 nanorod array with cocatalyst and 1D ZnO core-shell structure for the aim of improved PEC non-enzymatic detection of glucose and H2O2, respectively. The 1D TiO2 and ZnO nanorods array were prepared through facile hydrothermal at and electrodeposition methods, respectively. The modified electrodes were characterized through FESEM, HRTEM, XRD, and UV-vis spectroscopy. The novel 1D TiO2 and 1D ZnO@TiO2 core shell electrodes exhibited enhanced PEC sensing properties with a low limit of detection and a high sensitivity, over a linear range of 0.1 – 1000 nM of glucose and 1 pM-100 mM of H2O2 in PBS. Additionally, the sensor displayed high selectivity, high stability, and high reproducibility. Overall, our engineered PEC sensor demonstrated ultrasensitive detection of glucose and H2O2 and a capable platform for continued development into the field of PEC non-enzymatic sensors.

Synthesis of ZnO[sbnd]Al2O3 core-shell nanocomposite materials by fast and facile microwave irradiation method and investigation of their optical properties

Alumina (Al2O3) coated ZnO core-shell structures were synthesized by a novel, fast, and facile route utilizing microwave (MW) irradiation to control photocatalytic property of ZnO. The phase analysis and the core–shell structure development were corroborated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray fluorescence (XRF), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray (EDX) analysis and Fourier transform infrared spectroscopy (FT-IR). The XPS results affirmed that elements on the coated surface were Al and O. Zeta potential analysis predicted the presence of Al2O3 layer on ZnO due to almost similar zeta potential curve for pure Al2O3 and Al2O3 coated ZnO nanoparticles. There was no significant change in band gap energy of ZnO after amorphous Al2O3 coating as obtained from derived data of the reflectance spectra but gradual decreasing of reflectance in the visible range, measured by UV–vis spectroscopy, of the prepared core-shell nanoparticle may be due to the coating of amorphous Al2O3 on ZnO. The photocatalytic efficiency of ZnO was reduced after amorphous Al2O3 layer as confirmed by the photodegradation of methylene blue under UV irradiation.

Efficient and durable ZnO core-shell structures for photocatalytic applications in aqueous media

Core-shell nanostructures were synthesized by aerosol assisted chemical vapor deposition method. A ZnO nanorod (ZNR) was the core material, while the shell coating was individually film of Ti, Cu-Zn or Fe oxide. The crystalline phases of the synthesized materials were identified as oxides in the form of wurtzite (ZnO), anatase (TiO2), tenorite (CuO) and hematite (Fe2O3). Only for samples prepared using single Ti and Fe sources, amorphous shells were detected. The photocatalytic performance of all samples was tested by the discoloration of methylene blue. Findings confirmed that bare ZnO nanorods and commercial photocatalyst (Degussa P25) are comparable photocatalysts since 99% of dye discoloration was achieved in 60min. Additionally, discoloration tests performed with the core-shell nanostructures revealed that the ZNR/TiOx and ZNR/CuO-ZnO samples exhibited more than 93% of discoloration in 60min. Nevertheless, with samples involving ZNR/Fe2O3 and ZNR/FeOx/TiO2/Fe2O3, a maximum of 33% of dye discoloration was measured. Microstructural changes of the samples before and after the photocatalysis essays were examined. Results showed that the bare ZnO and CuO-ZnO coated nanorods dissolved in the aqueous medium. Conversely, the integrity of the ZnO core was evidenced through the morphological and structural analysis, when single TiOx and multiple FeOx and Fe2O3 layers, were employed. Furthermore, quantitative measurement of Zn solubility was performed in selected samples to confirm dissolution inhibition. Noteworthy, outcomes suggest that the functionality of ZnO when applied in aqueous systems could be assured solely with the developed ZNR/TiOx material.

Chemical protection of ZnO nanorods at ultralow pH To form a hierarchical BiFeO3/ZnO core-shell structure.

ZnO is an interesting material for photoactive and optoelectronic devices because of the wide range of available nanostructures and advantageous semiconducting properties. However, a significant drawback of ZnO is the low stability in high or low pH solutions. This has limited the development of ZnO core-shell materials for use in Z-scheme systems or photovoltaics, where any secondary phase is produced using chemical solution processing at low or high pH. Here, we show a simple process to produce an organic capping layer of 3-aminopropyltriethoxysilane that can successfully stabilize nanostructured ZnO for processing below pH 1. We demonstrate that this process can be used to produce a ZnO-BiFeO3 (BFO) core-shell structure by a sol-gel process. Using a range of physical and analytical techniques, we show that BFO is highly crystalline and produces a conformal coating with a thickness of 2.5 nm. X-ray photoelectron spectroscopy and X-ray diffraction confirm the phase and expected chemical composition of BFO. Finally we are able to demonstrate that diodes produced using the ZnO-BFO core-shell structure have improved performance with a rectification ratio at ±3 V of 2800 because of the reduction in reverse current typically associated with surface recombination on ZnO. Our process opens a route to producing a range of hitherto prohibited ZnO core-shell structures that may have applications ranging from photovoltaic devices to core-shell photocatalysts.

Preparation and luminescent properties of CdS/ZnO core shell quantum dots

The ZnO,CdS,and CdS / ZnO quantum dots have been successfully prepared through a facial sol-gel method. After annealing treatment,the CdS / ZnO core-shell structure with a ZnO shell of 10 nm was acquired. The CdS / ZnO core-shell structure resulted in the obvious red shift in the absorption band edge. Compared to those of ZnO and CdS,obvious changes were found in the excitation and emission spectra of CdS/ZnO and the luminescent intensity of CdS / ZnO increased due to the core-shell structure. Also,it was verified that the core-shell structure led to the change of the energy level structures.

Synthesis and characterization of multi-functional ZnO nano ceramic phosphor.

This manuscript reports on the synthesis and characterization of a multi-functional ZnO core-shell structure with CoFe2O4 as magnetic materials. To form a combination between optical (ZnO) and magnetic (CoFe2O4) materials, the surfaces of both materials were modified by amine and silica. First, ZnO raw mertials were coated silica using sol-gel methods and -OH group of their surface were modified using amines. Hydrophobic CoFe2O4 nanoparticle was coated with silica and was found to have hydrophilic properties. CoFe2O4 attached silica-coated ZnO core-shell structures were prepared by self-assembly technique and have blue luminescence and magnetic properties.

Microwave hydrothermal synthesis and gas sensing application of porous ZnO core–shell microstructures

A ZnO core–shell structure with a movable core inside a hollow shell has been rapidly prepared by an efficient microwave hydrothermal method without any template. The synthesis was performed in an aqueous solution using zinc acetate dihydrate as the precursor. The observations of field emission electron microscopy and transmission electron microscopy showed that these ZnO core–shell structures were composed of numerous primary particles with size of tens of nanometers. A series of time-dependent experiments were carried out in order to have a closer inspection of the evolution processes of such structures. When evaluated as a sensing material for gas sensors, the as-synthesized ZnO core–shell structures displayed a high response towards ethanol and good response-recovery properties.

Facile one-step synthesis and photoluminescence properties of Ag–ZnO core–shell structure

Ag–ZnO core–shell structures were gained via one-step solvothermal process. The products were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, photoluminescence (PL) and UV–vis spectroscopy, respectively. It was shown that the properties were greatly changed compared to pure ZnO from the PL and Raman spectra, which indicated the strong interfacial interaction between ZnO and Ag. The work provides a feasible method to synthesize Ag–ZnO core–shell structure photocatalyst, which is promising in the further practical application of ZnO-based photocatalytic materials.

Eu2O3-functionalized ZnO/palygorskite

Eu2O3-functionalized ZnO/palygorskite was synthesized in a two-step method, and was thoroughly characterized. Energy was transferred from Eu2O3 to ZnO, but not from ZnO to Eu2O3, in contrast to literature reports. The interaction between Eu2O3 and ZnO directly influenced the energy transfer process. Eu2O3 was sporadically loaded on the ZnO surface, rather than doped in the ZnO crystal lattice as Eu3+ or assembled as a Eu2O3–ZnO core–shell structure as is usually reported. Nanoparticle surface interactions and the ZnO morphology were possible factors affecting energy transfer. Particle-to-particle contact between Eu2O3 and ZnO benefited the energy transfer from Eu2O3 to ZnO. Eu2O3 acted as an ‘energy package’, first absorbing then transferring energy to ZnO. This process increased the number of excited carriers within ZnO, thus enhancing the luminescence and photocatalysis properties.