ZnO Nanotetrapods Research Articles

Synthesis of semiconductor oxide nanosheets, nanotetrapods and nanoplane-suite like grown on metal foil using different method

ZnO production nanostructure using different method: first method, electrochemical deposition on Zn foil using 0.3 M zinc sulfate heptahydrate (ZnSO4·7H2O) and sulfuric acid aqueous solution at a current density of 30 and 35 mA/cm2 for deposition time 40 min at room temperature and second method, Zn foils were thermally oxidized in a conventional tube furnace at a temperature equal range of 700–900 °C in air for 5 h or less in static air to prepared semiconductor nanomaterials ZnO nanorods, nanotetrapods and nanoplane. The XRD diffraction of higher intensity peaks at (101) and (002) miller indices for two methods can be recognized to a hexagonal wurtzite structure unit cell. Surface morphology images with different magnifications which clearly shows that the whole Zn foil and rod substrate obtained ZnO nanosheets, nanotetrapods, nanorods and growth nanoplanes were also found. The length of these nanotetrapods and nanorods lies in the equal range of 1–1.5 µm with an average diameter of 80 nm. It was well known that ZnO nano crystal exhibited two emission peaks. One was located at about 365 nm wavelength (UV luminescence band), and the other peak position at 475 nm wavelength (green luminescence band).

Piezoelectric effect of 3-D ZnO nanotetrapods

ZnO nanotetrapods could be designed as multiterminal strain sensors for enhancing sensitivity and directivity.

An enzymatic biosensor based on three-dimensional ZnO nanotetrapods spatial net modified AlGaAs/GaAs high electron mobility transistors

We designed and constructed three dimensional (3D) zinc oxide Nanotetrapods (T-ZnOs) modified AlGaAs/GaAs high electron mobility transistors (HEMTs) for enzymatic uric acid (UA) detection. The chemical vapor deposition synthesized T-ZnOs was distributed on the gate areas of HEMTs in order to immobilize uricase and improve the sensitivity of the HEMTs. Combining with the high efficiency of enzyme immobilization by T-ZnOs and high sensitivity from HEMT, the as-constructed uricase/T-ZnOs/HEMTs biosensor showed fast response towards UA at ∼1 s, wide linear range from 0.2 nM to 0.2 mM and the low detect limit at 0.2 nM. The results point out an avenue to design electronic device as miniaturized lab-on-chip device for high sensitive and specific in biomedical and clinical diagnosis applications.

Enhanced Charge Collection for Splitting of Water Enabled by an Engineered Three-Dimensional Nanospike Array

Photoelectrochemical (PEC) water splitting is a promising method of converting solar energy to hydrogen fuel from water using photocatalysts. Despite much effort in preparing mesoporous thin films on planar substrates, relatively little attention has been paid to their deposition on three-dimensional (3D) substrates, which could improve electron collection and enhance light-trapping. Here, we report the first synthesis of hierarchically branched anatase TiO2 nanotetrapods, achieved by dissolution and nucleation processes on a ZnO nanotetrapods template. When used as a photoanode for efficient PEC water splitting, the unique branched anatase TiO2 nanotetrapods yielded a photocurrent density of 0.54 mA cm–2 at applied potential of 0.35 V vs RHE, much higher than that of commercial TiO2 nanoparticles under otherwise identical conditions. Moreover, when the nanotetrapods were deposited on an ordered, purposely engineered 3D F-doped tin oxide (FTO) nanospike array, the photocurrent density was upgraded to 0.72 mA cm–2. This large photocurrent enhancement can be attributed to the ultrahigh contact surface area with the electrolyte, which is bequeathed by the hierarchically branched TiO2 nanotetrapods with a skin layer of vertically aligned ultrathin nanospines, as well as the short charge transport distance and enhanced light-trapping due to the peculiar 3D FTO nanospike array we have engineered by design.

Effects of Water and Cell Culture Media on the Physicochemical Properties of ZnMgO Nanoparticles and Their Toxicity toward Mammalian Cells

ZnMgO nanoparticles have shown potential for medical applications as an efficient antibacterial agent. In this work, we investigate the effect of water and two commonly used cell culture media on the physicochemical properties of ZnMgO nanoparticles in correlation with their cytotoxicity. In vacuum, ZnMgO nanopowder consists of MgO (nanocubes) and ZnO (nanotetrapods and nanorods) particles. Upon exposure to water or the Luria-Bertani solution, ZnO characteristic shapes were not observable while MgO nanocubes transformed into octahedral form. In addition, water caused morphological alternations in form of disordered and fragmented structures. This effect was directly reflected in UV/vis absorption properties of ZnMgO, implying that formation of new states within the band gap of ZnO and redistribution of specific sites on MgO surfaces occurs in the presence of water. In mammalian culture cell medium, ZnMgO nanoparticles were shapeless, agglomerated, and coated with surrounding proteins. Serum albumin was found to adsorb as a major but not the only protein. Adsorbed albumin mainly preserved its α-helix secondary structure. Finally, the cytotoxicity of ZnMgO was shown to strongly depend on the environment: in the presence of serum proteins ZnMgO nanopowder was found to be safe for mammalian cells while highly toxic in a serum-free medium or a medium containing only albumin. Our results demonstrate that nanostructured ZnMgO reaches living cells with modified morphology and surface structure when compared to as-synthesized particles kept in vacuum. In addition, its biocompatibility can be modulated by proteins from biological environment.

Piezoelectric Effect of Quaic-Nanotetrapods ZnO Nanostructure

Quaic-nanotetrapods ZnO nanostructure were fabricated by a simple chemical vapour deposition method. The morphologies, crystalline qualities and optical characteristics of the quaic-nanotetrapods ZnO nanotetrapods were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL) analytic technology, respectively. A piezoelectric nanogenerator based on the ZnO nanotetrapods was fabricated by a simple technique. And the energy harvesting ability is studied by experimental characterization methods and theoretical calculations.

Controlled morphology of ZnO nanostructures by adjusting the zinc foil heating temperature in an air-filled box furnace

By heating zinc foil in an air-filled box furnace, one-dimensional ZnO nanorods, two-dimensional ZnO nanoplates and three-dimensional ZnO nanotetrapods were prepared by adjusting the temperature in the furnace in the ranges of 500–600, 650–750 and 800–900 °C, respectively. The morphologies, structures and emissions of the synthesized ZnO nanostructures were investigated by scanning electron microscopy, X-ray diffractometry, transmission electron microscopy, selected area electron diffraction and photoluminescence spectroscopy. Mechanisms on the control of the morphology and photoluminescence were discussed in terms of the crystal growth habits combined with the temperature-dependent diffusions of zinc and oxygen atoms in the ZnO lattices.

Solution-Processed, Barrier-Confined, and 1D Nanostructure Supported Quasi-quantum Well with Large Photoluminescence Enhancement

Planar substrate supported semiconductor quantum well (QW) structures are not amenable to manipulation in miniature devices, while free-standing QW nanostructures, e.g., ultrathin nanosheets and nanoribbons, suffer from mechanical and environmental instability. Therefore, it is tempting to fashion high-quality QW structures on anisotropic and mechanically robust supporting nanostructures such as nanowires and nanoplates. Herein, we report a solution quasi-heteroepitaxial route for growing a barrier-confined quasi-QW structure (ZnSe/CdSe/ZnSe) on the supporting arms of ZnO nanotetrapods, which have a 1D nanowire structure, through the combination of ion exchange and successive deposition assembly. This resulted in highly crystalline and highly oriented quasi-QWs along the whole axial direction of the arms of the nanotetrapod because a transition buffer layer (Zn(x)Cd(1-x)Se) was formed and in turn reduced the lattice mismatch and surface defects. Significantly, such a barrier-confined QW emits excitonic light ∼17 times stronger than the heterojunction (HJ)-type structure (ZnSe/CdSe, HJ) at the single-particle level. Time-resolved photoluminescence from ensemble QWs exhibits a lifetime of 10 ns, contrasting sharply with ∼300 ps for the control HJ sample. Single-particle PL and Raman spectra suggest that the barrier layer of QW has completely removed the surface trap states on the HJ and restored or upgraded the photoelectric properties of the semiconductor layer. Therefore, this deliberate heteroepitaxial growth protocol on the supporting nanotetrapod has realized a several micrometer long QW structure with high mechanical robustness and high photoelectric quality. We envision that such QWs integrated on 1D nanostructures will largely improve the performance of solar cells and bioprobes, among others.

Nanonetworks: Rapid Fabrication Technique for Interpenetrated ZnO Nanotetrapod Networks for Fast UV Sensors (Adv. Mater. 10/2014)

Flame transport synthesis (FTS) offers a unique fabrication technique for different nano-microtetrapods and their interconnected networks. Using the FTS method, interconnected ZnO nanotetrapod networks are fabricated directly bridging two patterned electrodes in a chip. It offers rapid and cost-effective synthesis with a direct integration of nano-microtetrapod networks in a chip, ready for device applications. Fast UV-detection response from these interconnected ZnO nanotetrapod networks is demonstrated by R. Adelung, Y. K. Mishra and co-workers on page 1541.

The atomic origin of high catalytic activity of ZnO nanotetrapods for decomposition of ammonium perchlorate

Distinct from the common well faceted ZnO nanorods (R-ZnO), ZnO nanotetrapods (T-ZnO) exhibited a remarkable catalytic activity for the thermal decomposition of ammonium perchlorate (AP): the activation energy at high temperature decomposition (HTD) was significantly decreased to 111.9 kJ mol−1, much lower than 162.5 kJ mol−1 for pure AP and 156.9 kJ mol−1 for AP with R-ZnO. This was attributed to more abundant atomic steps on the surface of T-ZnO than that of R-ZnO, as evidenced by HRTEM and density function theory (DFT) calculations. It was shown that the initiation step of perchloric acid (PA) decomposition happened much faster on stepped T-ZnO edges, resulting in the formation of active oxygen atoms from HClO4. The formed oxygen atoms would subsequently react with NH3 to produce HNO, N2O and NO species, thus leading to an obvious decrease in the activation energy of AP decomposition. The proposed catalytic mechanism was further corroborated by the TG-IR spectroscopy results. Our work can provide atomic insights into the catalytic decomposition of AP on ZnO nanostructures.

Low-temperature CVD synthesis of patterned core-shell VO2@ZnO nanotetrapods and enhanced temperature-dependent field-emission properties.

VO2 nanostructures are attractive materials because of their reversible metal-insulator transition (MIT) and wide applications in devices. When they are used as field emitters, a new type of temperature-controlled field emission device can be fabricated. Vapor transport methods used to synthesize traditional VO2 nanostructures are energy-intensive, low yield, and produce simple morphology (quasi-1D) that exhibits substrate clamping; thus they are not suitable for field emission applications. To overcome these limitations, ZnO nanotetrapods were used as templates, and patterned core-shell VO2@ZnO nanotetrapods were successfully grown on an ITO/glass substrate via a low-temperature CVD synthesis. SEM, TEM, EDX, XPS analyses and X-ray diffraction revealed that the cores and shells of these nanotetrapods were single crystal wurtzite-type ZnO and polycrystalline VO2, respectively. The VO2@ZnO nanotetrapods show strongly MIT-related FE properties, the emission current density at low temperature is significantly enhanced in comparison with pure VO2 nanostructures, and the emission current density increased by about 20 times as the ambient temperature increased from 25 to 105 °C at a fixed field of 5 V μm(-1). Although the VO2@ZnO nanotetrapods show a worse FE performance at low temperatures compared with pure ZnO nanotetrapods, the FE performance was substantially improved at high temperatures, which was attributed to the MIT-related band bending near the interface and the abrupt resistance change across the MIT.

Ultrafast and mass production of ZnO nanotetrapods by induction-heating under air ambient

ZnO nanotetrapods have been large-scale prepared through a facile and ultrafast induction-heating method. The morphologies of ZnO nanostructures can be adjusted by modifying the induction-heating supply power. The ZnO products are characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope and selected area electron diffraction. The results reveal the presence of octahedral structured nucleus in the center of tetrapods with wurtzite phase branches. The possible formation mechanism for these ZnO nanostructures is proposed.

Selective H2S detection by CuO functionalized ZnO nanotetrapods at room temperature

ZnO nanotetrapods have been synthesized by carbothermal method. The structure, phase, morphology of the synthesized sample were investigated by X-ray diffraction and X-ray photoelectron spectroscopy, Scanning electron microscopy, Transmission electron microscopy and Selected area electron diffraction. The gas-sensing characteristics of thick films of pure and CuO-functionalized ZnO Nanotetrapods have been compared. Pure ZnO nanotetrapod films were found to be sensitive to both H2S and NO with similar sensitivities, at a temperature of 250–300 °C. It is demonstrated that functionalization of ZnO nanotetrapods with CuO, results in selectivity towards H2S at a lower temperature of 50 °C.

Rapid Fabrication Technique for Interpenetrated ZnO Nanotetrapod Networks for Fast UV Sensors

Two flame-based synthesis methods are presented for fabricating ZnO-nanostructure-based UV photodetectors: burner flame transport synthesis (B-FTS)and crucible flame transport synthesis (C-FTS). B-FTS allows rapid growth of ZnO nanotetrapods and in situ bridging of them into electrical contacts. The photo detector made from interconnected ZnO nanotetrapod networks exhibits fast response/recovery times and a high current ratio under UV illumination.

ZnO nanorods: morphology control, optical properties, and nanodevice applications

Research interest in ZnO nanostructures derives from their excellent luminescent properties and availability of low cost fabricating and processing, which hold promise for the development of electronic and optoelectronic nanodevices. In this review, we focus on the progress in synthesis, properties and nanodevices of ZnO nanorod (NR) arrays and nanotetrapods (NTPs). Recent work done by the authors are also presented. After a brief introduction to the controlled fabrication methods for the highly-ordered ZnO NR arrays and NTPs, we present some aspects of the fundamental properties, especially optical performance, of ZnO NRs/NTPs. Then, we provide an overview of the applications to functional nanodevices based on individual NR and NTP of ZnO. It is demonstrated that different morphologies of ZnO nanostructures have salient effects on their properties and applications. Although much progress has been achieved in the fundamental and applied investigations of ZnO NRs/NTPs over the past decade, many obstacles still remain, hampering further development in this field. Finally, some longstanding problems that warrant further investigation are addressed.

The synthesis and optical properties of different zinc oxide nanostructures

The tetrapod-like whiskers (T-ZnO), nanoaeroplanes, nanocombs, nanobelts, bead-like nanoforms and many other forms of hexagonal zinc oxide were synthesized by the chemical deposition process (CVD). From the experimental results it can be seen that the composition of source gases and the speed of oxidation are essential to the growth of each of these morphologies of zinc oxide. By controlling the growth conditions, one novel morphology of ZnO (bead-like nanoform) has been obtained. The ZnO tetrapod-like whiskers were grown without catalysts on the walls of quartz tube. The structure and morphology of the T-ZnO was characterized. All four legs of as-grown ZnO nanotetrapods are needle-like and hexagonal shaped and grow in the ±[0001] direction. The effect of synthesis conditions on the morphology and size T-ZnO was observed. Possible mechanisms of growth were investigated. The lengths of the legs of the tetrapods range from 1 to 15 μm and diameter varying from 0.03 to 1 μm during the synthesis. In the photoluminescence spectra T-ZnO clearly shows two maxima at UV and visible regions. The oxygen impurity, particularly, oxygen flow rates mainly influences on increasing (decreasing) at PL peaks. The maximum near of 590–610 nm (∼2 eV), confirming the influence of native defects (oxygen vacancies and interstitials) on the PL intensity and optical quenching exciton peak.

Self-Catalyzed Thermal Chemical Vapor Deposited ZnO Nanotetrapods

ZnO in various nanostructures forms have been widely studied for the application such as in solar cells, light emitting diodes, UV sensors and so on. In this paper, we have successfully deposited ZnO nanotetrapods using thermal chemical vapour deposition (TCVD) technique on layer-by-layer ZnO seeded catalyst, with Zn powder and O2 gas as source materials. We demonstrate that by using double furnace TCVD system, ZnO nanotetrapods can be deposited at lower temperature than the vapour temperature of the Zn powder. In this paper we report the effect of different deposition temperature (450 °C to 600 °C) on the surface morphologies, crystalline structure and optical properties of the ZnO nanotetrapods. FE-SEM micrographs show that the length of the nanotetrapods arms decreases with the increase of the deposition temperature. PL spectra show that the visible emission are very low compared to the UV emission which indicates that the ZnO tetrapod have very low intrinsic defect. The highest UV emission intensity is given by the sample deposited at 500 °C.

Building High-Efficiency CdS/CdSe-Sensitized Solar Cells with a Hierarchically Branched Double-Layer Architecture

We report a double-layer architecture for a photoanode of quantum-dot-sensitized solar cells (QDSSCs), which consists of a ZnO nanorod array (NR) underlayer and a ZnO nanotetrapod (TP) top layer. Such double-layer and branching strategies have significantly increased the power conversion efficiency (PCE) to as high as 5.24%, nearly reaching the record PCE of QDSSCs based on TiO2. Our systematic studies have shown that the double-layer strategy could significantly reduce charge recombination at the interface between the charge collection anode (FTO) and ZnO nanostructure because of the strong and compact adhesion of the NRs and enhance charge transport due to the partially interpenetrating contact between the NR and TP layers, leading to improved open-circuit voltage (Voc) and short-circuit current density (Jsc). Also, when the double layer was subjected to further branching, a large increase in Jsc and, to a lesser extent, the fill factor (FF) has resulted from increases in quantum-dot loading, enhanced light scattering, and reduced series resistance.

Low Temperature Sensing Properties of a Nano Hybrid Material Based on ZnO Nanotetrapods and Titanyl Phthalocyanine

ZnO nanotetrapods have recently been exploited for the realization of high-sensitivity gas sensors, but they are affected by the typical drawbacks of metal-oxides, i.e., poor selectivity and a relatively high working temperature. On the other hand, it has been also demonstrated that the combined use of nanostructured metal oxides and organic molecules can improve the gas sensing performance sensitivity or selectivity, even at lower temperatures. A gas sensor device, based on films of interconnected ZnO nanotetrapods properly functionalized by titanyl phthalocyanine (TiOPc), has been realized in order to combine the high surface to volume ratio and structural stability of the crystalline ZnO nanostructures with the enhanced sensitivity of the semiconducting TiOPc molecule, especially at low temperature. The electronic properties of the resulting nanohybrid material are different from those of each single component. The response of the hybrid nanostructure towards different gases has been compared with that of ZnO nanotetrapod without functionalization in order to highlight the peculiar properties of the hybrid interaction(s). The dynamic response in time has been studied for different gases and temperatures; in particular, an increase in the response to NO2 has been observed, even at room temperature. The formation of localized p-n heterojunctions and the possibility of exchanging charge carriers at the hybrid interface is shown to be crucial for the sensing mechanism.

Processing–microstructure–property correlations of gas sensors based on ZnO nanotetrapods

After gas sensors based on ZnO nanotetrapods (T-ZnO) were processed by sintering at different temperatures from 350 to 850°C, a strong correlation was interestingly found among the sintering processing, material microstructure and gas-sensing properties. With increasing sintering temperature from 350 to 750°C, the feet and cross of T-ZnO became gradually shorter and bigger, respectively. And subsequently tetrahedron-shaped ZnO nanoparticles were produced instead of T-ZnO at 850°C. The morphological evolution was explained by a new physical model involving Thomson effect, leading to a decrease in the specific surface area. In addition, the contact between feet got better and then became poorer. Meanwhile, surface defects of T-ZnO were also altered: zinc interstitial (Zni••) was decreased in its amount while oxygen vacancy (VO×) showed an inverse trend as sintering temperature increased. Moreover, the best gas-sensing performance toward formaldehyde and methanol was obtained after sintering at 450°C. This was mainly attributed to the synergetic effect between the best grain contact (meaning that more T-ZnO can make contributions to the sensor response) and more zinc interstitial as well as larger specific surface area (supplying more chemisorbed oxygen). Our work could offer important guidance for the process selection and material design to develop nanostructure-based sensors.