Tetrapod-shaped ZnO Research Articles

Transparent Conductive Oxide Layer-Less Three Dimensional Dye Sensitized Solar Cells: Fabrication of Ionic Path in Three Dimensional Ti Electrode

A transparent conductive oxide-less dye-sensitized solar cell (TCO-less DSC) consisting of a thick and porous Ti electrode is reported. The Ti electrode was contacted with the back of a stained porous titania layer. A thick Ti electrode prepared by conventional sputtering prevented ionic diffusions through the Ti electrode and decreased photovoltaic performance. Tetrapod-shaped ZnO crystals were used to prepare straight and continuous nanopores in the Ti electrode to maintain the ionic diffusion. 8% efficiency of the TCO-less DSC is also reported.

Self-catalytic Synthesis, Structures, and Properties of High-Quality Tetrapod-Shaped ZnO Nanostructures

High-quality tetrapod-shaped ZnO (T-ZnO) nanostructures were synthesized through the direct reaction of Zn and zinc acetate (ZAc) via a thermal evaporation method in Ar at 650 °C without any catalyst. Controlling the experimental parameters showed that ZAc precusor plays an important self-catalytic role in the vapor transport process. The individual legs of as-synthesized ZnO nanotetrapods were characterized using field emission scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray diffraction. The photoluminescence (PL) measurements showed the different PL features of T-ZnO nanostructures. Two typical emission peaks at ∼387 nm and ∼495 nm were observed. Especially, the emission peak at 455−495 nm includes four subordinate peaks. Field emission investigation revealed that the T-ZnO nanostructures with different morphologies possess good field emission property with the largest turn-on field of 1.85 V/μm, and the field emission density reaches 1.92 × 10−4 A/cm2 when the field is 5.5 V/μm. The growth mechanism was discussed in detail. A self-catalysis vapor−liquid−solid growth mechanism was proposed for the formation of the ZnO nanotetrapods.

Synthesis of tetrapod-shaped ZnO nanowires by directive oxidation of Al-Zn mixture

Tetrapod-shaped ZnO nanowires were synthesized by the directive oxidation process of Al-Zn mixtures containing Mg. Mg played a crucial role in the formation of the ZnO nanowires. X-ray diffraction data and SEM images of the nanowires revealed that the ZnO nanowires were single crystalline with wurtzite structure of hexagonal phase. With increasing Mg concentration in Al-Zn mixture from 2 mass% to 8 mass%, the tip morphology of the nanowires changed from spherical shape to needle-like shape, and went back to spherical shape. The quantity of the nanowires first increased and then decreased with Mg concentration. From Energy dispersive X-ray spectrum and SEM image taken for the tip of the ZnO nanowire, it is supposed that the growth of the nanowires proceeded in vapor-solid mechanism. The green emission centered at 510 nm was observed at room temperature, which resulted from high density of oxygen vacancies in the nanowires.

Formaldehyde sensor based on Ni-doped tetrapod-shaped ZnO nanopowder induced by external magnetic field

The sensors based on Ni-doped ZnO nanopowder with tetrapod-shape (T-ZnO) were fabricated by screen-printing technique with external magnetic field in different direction. The morphologies and crystal structures of the thick film were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM), respectively. Gas-sensing property of sensors responded to 100 ppm formaldehyde was also detected. The results show that the direction of magnetic field has crucial effect on the sensor sensitivity. The sensors based on 5 wt% Ni-doped T-ZnO induced by magnetic field in parallel direction to the thick film surface, has the optimization sensitivity, the shortest response and recovery time, which are 10.6, 16 and 15 s, respectively. The magnetic-field induction model and the gas-sensing mechanism of the Ni-doped T-ZnO are proposed.

Synthesis of tetrapod-shaped ZnO whiskers and microrods in one crucible by thermal evaporation of Zn/C mixtures

Tetrapod-shaped ZnO whiskers and microrods were synthesized in one crucible by thermal evaporation of Zn/C mixtures at 930 °C in air without any catalyst. The digital camera, optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction techniques were used to study the morphologies and crystal structures of these tetrapod-shaped ZnO microcrystals. The results show that these two types of ZnO tetrapods are grown at different heights within the same crucible. The legs of these tetrapod-shaped ZnO crystals are hexagonally faceted. Some tetrapod-shaped ZnO whiskers show hierarchical structures. A short button-like hexagonal ZnO microcrystal is observed at the triple junctions of some tetrapod-shaped ZnO whiskers. The tetrapod-shaped ZnO microrods are capped by two sets of hexagonal pyramids with two different groups of crystal planes for the surfaces. These two types of tetrapod-shaped ZnO microcrystals have different side faces and aspect ratio, which are believed to be the result of their different growth behaviors. The octa-twin model was used to discuss the different growth behaviors of these two types of ZnO tetrapods. The crystal planes of the legs and the pyramids were determined.

Preparation of tetrapod-shaped ZnO nanomaterial field emission cathodes by deposition method

Tetrapod-shaped ZnO nanomaterial field emission cathode was preparated by deposition method. To the high-vacuum flat capsulation with phosphor screen and cathode was applied a voltage to test the cathode emission current and the phosphor screen brightness. For the cathode with total area 13×15 cm2, the test results showed that when the volume percentage of K2SiO3 ranged from 50 × 10-3 to 83× 10-3, concentration of Ba(NO3)2 arranged from 50 × 10-4 to 77×10-4 M, and concentration of tetrapod-shaped ZnO arranged from 82×10-4 to 12×10-3M, the cathode emission current was the largest, and phosphor screen at peak brightness could reach 60 cd/cm2 The average brightness was 48 cd/cm2

Effect of humidity on the gas sensing property of the tetrapod-shaped ZnO nanopowder sensor

The testing chamber humidity and the storage circumstance humidity effects on the tetrapod-shaped ZnO nanopowder (named as T-ZnO) thick film sensors were investigated by measuring the resistance and sensitivity. The resistance increases gradually with increasing relative humidity (RH) in a range of 32%–75% RH in testing chamber, while in a range of 75%–96% RH, decreases gradually with the increase of RH. The sensitivity to ethanol 100 ppm in testing chambers with different humidity is in the order of 50% RH > of 75% RH > of 32% RH > of 96% RH. The sensitivity change in the storage circumstance with different RH is similar to the change in testing chamber with different RH. The stability of T-ZnO sensor is influenced evidently by the storage circumstance humidity. The water vapour in the tetrapod-shaped ZnO nanopowders was investigated by thermogravimetric analysis (TGA). The testing gas and the reactant adsorbed in the sensitive film were characterized by infrared spectrum (IR). The explanation of the observed effects was given and the mechanism of interaction of the ZnO sensing layer with H 2O was proposed.

Tetrapod-shaped ZnO microtubes synthesized from Zn/C mixtures

We report for the first time the growth of tetrapod-shaped ZnO microtubes synthesized via a simple method. The tetrapod-shaped ZnO microtubes were synthesized from mixtures of high-purity Zn powder and activated carbon in a crucible at 930–940 °C in air without the presence of any catalyst. The ZnO microtubes were found to be grown on the arrays of ZnO microcrystals, which were horizontally grown from the inner walls of crucibles. Optical and scanning electron microscopy were utilized to observe the morphologies of the tetrapod-shaped ZnO microtubes, and X-ray diffraction was employed to study the crystal structure. The microtubes were found to be hexagonally shaped with well-defined side faces. A growth model was proposed to explain the formation of the tubular ZnO microcrystals.

Preparation and Characterization of Tin Doped Tetra-Pod Shaped Zinc Oxide

Tin doped tetra-pod shaped ZnO (T-ZnO) were fabricated by equilibrant airflow reaction method. The microstructure of tin doped T-ZnO was characterized by X-ray Diffraction analysis and Scanning Electron Microscope. The results demonstrated that most of tin acted as a catalyst during the ZnO crystal growth process and the others reacted with ZnO to generate Zn2SnO4. Tin had catalytic influences on the morphology of T-ZnO, which promoted predominant growth on the c axis of ZnO crystal at the addition of 15% in weight and advanced the growth on the a axis at higher weight proportion.

Effect of Oxidation Temperature on the Morphology of ZnO Nanowires Fabricated by Directed Melt Oxidation of Al-Zn Alloy

ZnO nanowires were synthesized by directed melt oxidation of Al-Zn alloy at the temperature over 900°C in an air. Straight type nanowires were formed on the surface of the alloy oxidized at 950°C. However for the alloys oxidized over 1000°C, tetrapod-shaped ZnO nanowires with four legs, extended from center, were found in high yield and distributed uniformly on the whole surface. No other shaped crystals and powders were found. X-ray diffraction (XRD) pattern revealed that the nanowires were ZnO with wurtzite structure of hexagonal phase. The diameter of nanowires was controlled to 80 nm and the length to 20 micrometers. Higher temperature produced nanowires with larger diameter and longer length.

The gas-sensing properties of thick film based on tetrapod-shaped ZnO nanopowders

Tetrapod-shaped ZnO nanopowders were prepared by the method of vapor-phase oxidation from metallic zinc as raw materials. The gas-sensing properties of thick film based on tetrapod-shaped ZnO nanopowders to volatile organic compounds (VOCs), benzene, toluene, xylene, alcohol and acetone were measured, and compared with that of commercial ZnO powders with granular shape. The results showed that tetrapod-shaped ZnO had the better gas-sensing properties: the maximum sensitivity temperature was reduced, the gas sensitivity was improved and the time of response–recovery was shortened. The differences in gas-sensing properties between the thick films were discussed in according to the morphological characteristics, size and agglomeration of raw powder as well as microstructure of sintered thick films.

Synthesis and properties of multipod-shaped ZnO nanorods for gas-sensor applications

We reported the synthesis and optical properties of ZnO nanorods with multipod shapes, and their applications as gas sensors. The multipod-shaped ZnO (M-ZnO ) nanorods, having tens of needle-like nanorods united at a common junction, exhibited different structural and optical characters from those of the tetrapod-shaped ZnO (T-ZnO) nanorods. Employing the M-ZnO and the T-ZnO nanorods, respectively, gas sensors were prepared and tested for the detection of ethanol in air. It was found that the M-ZnO devices had an improved sensing performance in comparison with that of the T-ZnO sensors, of which a possible sensing mechanism related to a surface reaction process was discussed.

Two-step evaporation process for formation of aligned zinc oxide nanowires

Two-step gas flow controlled evaporation process was introduced as a simple technique to fabricate aligned ZnO nanowires via the VLS growth mode. Using this method, well-aligned single crystalline ZnO nanowires was successfully synthesized on Au coated Si substrates. The diameter of ZnO nanowires was controlled from 40 to 150 nm and the ratio of length to diameter was observed to be larger than 30. Tetrapod-shaped ZnO nanostructures were formed using a mixture of Ar and O 2 gases, and it indicated that the partial pressure of oxygen during the synthesis is crucial for the growth of ZnO nanowire structures. Green emission induced from singly ionized oxygen vacancies was generated from ZnO nanowires with the excitonic emission in UV range. Higher intensity of the green emission was observed in thinner nanowires, which is attributed to their higher surface-to-volume ratio. Patterned structure of ZnO nanowires with a high fidelity was achieved by pattering the Au catalyst films.

Studies on the antistatic mechanism of tetrapod-shaped zinc oxide whisker

Antistatic composites of polyurethane, polyvinyl and natural rubber with the electrical resistivity of 10 6– 10 10 Ω cm were prepared using tetrapod-shaped zinc oxide whisker as the conducting additives. The conductive behavior of this system has been analyzed and the results show that the tetrapod-shaped ZnO whisker is an effective antistatic additive for polymer materials. A formula of critical volume fraction of ZnO whisker for the formation of the conducting paths of the polymer/ZnOw composite has been put forward and compared with the experimental results. The analysis of the conductive mechanism indicates that the charge concentrating effect at the needles’ tips of the whisker will be distinct if the material is under an electric field. The tunnel effect may exist and cause leakage current in this kind of composites.

A new method for preparation of zinc oxide whiskers

Tetrapod-shaped ZnO whiskers were prepared with high yield and good purity in a short reaction time by a new method, the oxidation of pellets or otherwise shaped metallic zinc at 500–800°C under atmospheric pressure. The reaction conditions were analyzed and compared with other methods. The products were identified by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The results show that this is a practical and economical method for manufacturing ZnO whiskers. A suitable ratio between the volume of the furnace chamber and the quantity of the material zinc is suggested.

Tetrapod-shaped ZnO whisker and its composites

Abstract Tetrapod-shaped zinc oxide whisker has been prepared with high purity and good yield in a short reaction time using a new method, the gas expanding method, which is the oxidation of pellets or other shaped metallic zinc at 500–800°C and at atmospheric pressure. The results show that this is a practical and economical method for the manufacturing of ZnO whiskers with a peculiar shape of tetrapod. A suitable ratio for the volume of the furnace chamber and the quantity of the metallic zinc has been suggested. The structures of the products were studied by SEM and X-ray diffraction analysis. The microwave absorption of the composites of ZnO whisker/chlorobutadiene rubber has been investigated, and the result indicates that the composites are of excellence with respect to microwave absorption. The results of vibration tests of polyvinyl chloride show that material containing tetrapod-like ZnO whisker appears to have superior damping characteristics.

Co-stimulation with ATP and PTH induces oscillations in intracellular free calcium concentration in single Fura-2 loaded primary human osteoblasts

Tetrapod-shaped ZnO crystals were synthesized by an oxidation process of Zn powder at temperatures over 930 °C in air. The ZnO crystals were formed in an alumina crucible in which metallic Zn powder was put. X-ray diffraction patterns revealed that the tetrapods were ZnO with wurtzite structure. Any impurities including Zn were not detected, indicating that the ZnO crystals were of high quality. As the oxidation temperature increased, the size of tetrapods was getting larger and longer. However at 1100 °C, tetrapod morphology was disappeared and changed to irregular shape. Strong UV emission at 380 nm was observed in the CL spectra taken for the ZnO tetrapods. The increase in the intensity of green emission at 510 nm was also observed with increased oxidation temperature.

Some evidence for the octa-twin model of tetrapod ZnO particles

Transmission electron microscopy at the central part of tetrapod-shaped ZnO particles, produced by oxidizing zinc vapor in air, has revealed that there exist trigonal-pyramid crystals other than the leg crystals at the center. Scanning electron microscopy of fractured tetrapod particles has shown characteristic facetted fracture surfaces. These results, together with the etch patterns on the surfaces of the particles which exhibit polarities of the constituent crystals, are consistent with the previously proposed growth model from an inversion-type octa-twin embryo.

Octahedral multiple-twin model of tetrapod ZnO crystals

Abstract The characteristic morphology of tetrapod-shaped ZnO particles grown by oxidizing zinc vapour in air has been thoroughly interpreted by a growth process from octahedral multiple-twin embryos. The origin of the tetrapod-shape arises from firstly the formation of octahedral multiple twins due to the small c/a ratio of wurtzite ZnO, secondly the presence of in version-type twin boundaries and thirdly a large growth anisotropy with respect to the crystal polarity.

Electrical Conductivity of Tetrapod-Shaped ZnO Whiskers

The electrical conductivity of tetrapod-shaped ZnO whiskers was measured under different atmospheric conditions. Apparent resistivity of 104∼108 Ω·cm was obtained in the form of the compressed whisker powder. The conductivity was found to depend strongly on the compressive pressure, the electrical field strength and the atmosphere. The rectification effect was observed in current-voltage measurements of the whiskers using a pair of parallel-plane electrodes for powder material. The rectification was interpreted to be mainly caused by the depletion layer formed near the crystal surface due to the oxygen chemisorption on the crystal surface.