In situ growth, structure characterization, and enhanced photocatalysis of high-quality, single-crystalline ZnTe/ZnO branched nanoheterostructures

Single-crystalline, high-quality branched ZnTe-core/ZnO-branch nanoheterostructures were synthesized by an in situ strategy in an environmental scanning electron microscope. Composition and structure characterization confirmed that ZnO nanowires were perfectly epitaxially grown on ZnTe nanowires as branches. Noticeably, growth temperature plays a crucial role in determining the density and diameter of the ZnO nanobranches on ZnTe nanowires: a higher growth temperature leads to ZnO nanowires with higher density and smaller diameter. It was demonstrated that ZnO nanobranches exhibited a selective nucleation behavior on distinct side facets of ZnTe nanowires. Highly ordered ZnO nanobranches were found epitaxially grown on {211} facet of ZnTe nanowires, while there was no ZnO nanowire growth on {110} facet of ZnTe nanowires. Using first-principles calculation, we found that surface energy of distinct side facets has a strong impact on ZnO nucleation, and confirm that {211} facet of ZnTe nanowires is energetically more favorable for ZnO nanowire growth than {110} facet, which is in good agreement with our experimental findings. Remarkably, such unique ZnTe/ZnO 3D branched nanowire heterostructures exhibited improved photocatalytic abilities, superior to ZnO nanowires and ZnTe nanowires, due to the much enhanced effective surface area of their unique architecture and effective electron-hole separation at the ZnTe/ZnO interfaces.     

Growth of catalyst-free high-quality ZnO nanowires by thermal evaporation under air ambient

ZnO nanowires have been successfully fabricated on Si substrate by simple thermal evaporation of Zn powder under air ambient without any catalyst. Morphology and structure analyses indicated that ZnO nanowires had high purity and perfect crystallinity. The diameter of ZnO nanowires was 40 to 100 nm, and the length was about several tens of micrometers. The prepared ZnO nanowires exhibited a hexagonal wurtzite crystal structure. The growth of the ZnO nanostructure was explained by the vapor-solid mechanism. The simplicity, low cost and fewer necessary apparatuses of the process would suit the high-throughput fabrication of ZnO nanowires. The ZnO nanowires fabricated on Si substrate are compatible with state-of-the-art semiconductor industry. They are expected to have potential applications in functional nanodevices.     

Plasma effects in semiconducting nanowire growth

Three case studies are presented to show low-temperature plasma-specific effects in the solution of (i) effective control of nucleation and growth; (ii) environmental friendliness; and (iii) energy efficiency critical issues in semiconducting nanowire growth. The first case (related to (i) and (iii)) shows that in catalytic growth of Si nanowires, plasma-specific effects lead to a substantial increase in growth rates, decrease of the minimum nanowire thickness, and much faster nanowire nucleation at the same growth temperatures. For nucleation and growth of nanowires of the same thickness, much lower temperatures are required. In the second example (related to (ii)), we produce Si nanowire networks with controllable nanowire thickness, length, and area density without any catalyst or external supply of Si building material. This case is an environmentally-friendly alternative to the commonly used Si microfabrication based on a highly-toxic silane precursor gas. The third example is related to (iii) and demonstrates that ZnO nanowires can be synthesized in plasma-enhanced CVD at significantly lower process temperatures than in similar neutral gas-based processes and without compromising structural quality and performance of the nanowires. Our results are relevant to the development of next-generation nanoelectronic, optoelectronic, energy conversion and sensing devices based on semiconducting nanowires.     

Preparation and electrochemical performances of ZnO nanowires as anode materials for Ni/Zn secondary battery

ZnO nanowires were synthesized by a hydrothermal route without any substrate or template. Structure analyses through XRD, SEM, TEM and HRTEM indicated that ZnO nanowires had high purity and perfect crystallinity, and grew along [0 0 0 1]. The diameter was 50-80 nm, the length was about several micrometers and length-diameter ratio was more than 100. As electrode materials of Ni/Zn batteries, ZnO nanowires showed the obviously improved cycle stability, average discharge capacity of 609 mAh g⁻¹, higher discharge voltage/lower charge voltage. Slow rate cyclic voltammetry showed that electrochemical activity of ZnO nanowires was superior to that of the conventional ZnO. The improvements of electrochemical performance were ascribed to the unique nanowire structure. During the charging/discharging cycles, nanowires were broke, grew in diameter, and changed into nanorods. Nanowires lying parallel to the anodes could suppress the growth of dendrite clusters perpendicular to the anodes.     

Stages in the catalyst-free InP nanowire growth on silicon (100) by metal organic chemical vapor deposition

Catalyst-free InP nanowires were grown on Si (100) substrates by low-pressure metal organic chemical vapor deposition. The different stages of nanowire growth were investigated. The scanning electron microscopy images showed that the density of the nanowires increased as the growth continued. Catalyzing indium droplets could still be fabricated in the nanowire growing process. X-ray diffraction showed that the nanowires grown at different stages were single crystalline with <111 > growth direction. The photoluminescence studies carried out at room temperature on InP nanowires reveal that the blueshift of photoluminescence decreased as the growing time accumulates, which is related to the increase in the diameter, rather than the length. Raman spectra for nanowires at different growing stages show that the quality of the nanowire changes. The growth of InP nanowires at different growing stages is demonstrated as a dynamic process.     

Low temperature growth of ZnO nanowire arrays with enhanced high performance photocatalytic activity and reusability

Aligned ZnO nanowire arrays have been grown on the silicon substrates with ZnO seed layer by an aqueous chemical growth process at low reaction temperature. The volumes of ammonium hydroxide solution are not only controlled by the dimension and density of ZnO nanowires, but are also influenced by their optical emission property. The ZnO nanowire arrays exhibit very strong and broad green emission from defect in the cathodoluminescence spectrum. In addition, the ZnO nanowires grown on the silicon substrates can serve as effective and convenient recyclable photocatalysts. Only a very slight decrease in the photodecomposition rate was observed after ten cycles of the photocatalysis experiment.     

Synthesis and characterisation of self-assembled SiC nanowires and nanoribbons by using sol–gel carbothermal reduction

ABSTRACT

Hexagonal-shaped 3C-SiC nanowires were grafted onto SiC nanoribbons by a sol–gel technique using ferrocene as catalyst. The nanowire diameter (～200?nm) and the nanoribbon width–thickness ratio (20:1) are uniform along their entire length. Their length is about several tens to several hundreds of micrometres. Meanwhile, single SiC nanostructure (nanowire or nanoribbon) was obtained by adjusting temperature field. A novel cooperative growth mechanism of vapour–liquid–solid and vapour–solid was proposed for the self-assembled SiC nanostructure. The self-assembled SiC nanowires and nanoribbons exhibit two strong broad photoluminescence peaks at wavelengths of about 373 and 471?nm, which are significantly shifted to the blue compared with the reported luminescence of SiC nanowires. This study will pave a way for the controllable synthesis of SiC nanowires and nanoribbons, and provide a simple method to connect them together firmly as potential applications for nanodevices in future.     

Synthesis of ZnO nanowires on steel alloy substrate by thermal evaporation: Growth mechanism and structural and optical properties

ZnO nanowires having a diameter in the range of 15–40 nm and several tens of micrometers in length were grown on steel alloy substrates by the thermal evaporation technique without the use of any catalyst or additives. A detailed structural analysis revealed that the as-grown ZnO nanowires are single crystalline with wurtzite hexagonal structures and preferentially oriented in the c-axis direction. Origination of a strong and sharp Raman-active E₂ mode at 436.6 cm^-1 indicated that the grown ZnO nanowires have good crystal quality with the hexagonal wurtzite phase. Photoluminescence spectra also exhibited a sharp and strong peak in UV and a suppressed and weak band in the visible region, confirming the good optical properties and less structural defects for the deposited products. Additionally, a systematic growth mechanism is also proposed in detail to acquire a better understanding for the growth of nanowires on steel alloy substrate.     

First-Principles Study of Magnetic Properties of 3d Transition Metals Doped in ZnO Nanowires

The defect formation energies of transition metals (Cr, Fe, and Ni) doped in the pseudo-H passivated ZnO nanowires and bulk are systematically investigated using first-principles methods. The general chemical trends of the nanowires are similar to those of the bulk. We also show that the formation energy increases as the diameter of the nanowire decreases, indicating that the doping of magnetic ions in the ZnO nanowire becomes more difficult with decreasing diameter. We also systematically calculate the ferromagnetic properties of transition metals doped in the ZnO nanowire and bulk, and find that Cr ions of the nanowire favor ferromagnetic state, which is consistent with the experimental results. We also find that the ferromagnetic coupling state of Cr is more stable in the nanowire than in the bulk, which may lead to a higher T _c useful for the nano-materials design of spintronics.     

Nanoparticulate PdZn as a Novel Catalyst for ZnO Nanowire Growth

ZnO nanowires have been grown by chemical vapour deposition (CVD) using PdZn bimetallic nanoparticles to catalyse the process. Nanocatalyst particles with mean particle diameters of 2.6 ± 0.3 nm were shown to catalyse the growth process, displaying activities that compare well with those reported for sputtered systems. Since nanowire diameters are linked to catalyst morphology, the size-control we are able to exhibit during particle preparation represents an advantage over existing approaches in terms of controlling nanowire dimensions, which is necessary in order to utilize the nanowires for catalytic or electrical applications.     

Electrochemical deposition of branched hierarchical ZnO nanowire arrays and its photoelectrochemical properties

Branched hierarchical ZnO nanowire arrays are synthesized on fluorine-doped tin oxide (FTO) substrate via a two-step electrochemical deposition process, which involves the electrodeposition of ZnO nanowire arrays on conductive glass substrate, followed by the electrochemical growth of ZnO nanorod branches on the backbones of the primary ZnO nanowires. The formation mechanism of the branched hierarchical nanostructure is discussed. It is demonstrated that coating the primary nanowire arrays with ZnO nanoparticles seed layer plays a key role in synthesising the branched hierarchical ZnO nanostructure. By adjusting the concentration of Zn(CH₃COO)₂ colloid in coating process and the reaction time of the second-step deposition, the density and the length of the secondary nanorod branches in the hierarchical nanostructures can be both varied. Moreover, the photoelectrochemical properties of the dye-sensitized solar cell (DSSC) based on branched hierarchical ZnO nanowire arrays are investigated. Due to the enlargement of the internal surface area within the branched nanostructure photoelectrode, the DSSC consisting of branched hierarchical ZnO nanowire arrays yields a power conversion efficiency of 0.88%, which is almost twice higher than that of the DSSC fabricated using bare ZnO nanowire arrays.     

Direct synthesis of vertically aligned ZnO nanowires on FTO substrates using a CVD method and the improvement of photovoltaic performance

In this work, we report a direct synthesis of vertically aligned ZnO nanowires on fluorine-doped tin oxide-coated substrates using the chemical vapor deposition (CVD) method. ZnO nanowires with a length of more than 30 μm were synthesized, and dye-sensitized solar cells (DSSCs) based on the as-grown nanowires were fabricated, which showed improvement of the device performance compared to those fabricated using transferred ZnO nanowires. Dependence of the cell performance on nanowire length and annealing temperature was also examined. This synthesis method provided a straightforward, one-step CVD process to grow relatively long ZnO nanowires and avoided subsequent nanowire transfer process, which simplified DSSC fabrication and improved cell performance.     

Angle-dependent photodegradation over ZnO nanowire arrays on flexible paper substrates

In this study, we grew zinc oxide (ZnO) nanowire arrays on paper substrates using a two-step growth strategy. In the first step, we formed single-crystalline ZnO nanoparticles of uniform size distribution (ca. 4 nm) as seeds for the hydrothermal growth of the ZnO nanowire arrays. After spin-coating of these seeds onto paper, we grew ZnO nanowire arrays conformally on these substrates. The crystal structure of a ZnO nanowire revealed that the nanowires were single-crystalline and had grown along the c axis. Further visualization through annular bright field scanning transmission electron microscopy revealed that the hydrothermally grown ZnO nanowires possessed Zn polarity. From photocatalytic activity measurements of the ZnO nanowire (NW) arrays on paper substrate, we extracted rate constants of 0.415, 0.244, 0.195, and 0.08 s^-1 for the degradation of methylene blue at incident angles of 0°, 30°, 60°, and 75°, respectively; that is, the photocatalytic activity of these ZnO nanowire arrays was related to the cosine of the incident angle of the UV light. Accordingly, these materials have promising applications in the design of sterilization systems and light-harvesting devices.     

Synthesis of ZnO nanowires on Si substrate by thermal evaporation method without catalyst: Structural and optical properties

Synthesis of ZnO nanowires was achieved on Si(100) substrate by the thermal evaporation of high purity metallic zinc powder without the use of any metal catalyst or additives. The diameter and length of the as-grown nanowires were in the range of 20–35 nm and few micrometers, respectively. The shapes and sizes of ZnO nanowires were dependent on the growth time. The high resolution transmission electron microscopy and selected area electron diffraction patterns indicated that the as-grown products are single crystalline with wurtzite hexagonal phase. Room temperature photoluminescence studies exhibited a strong UV emission and a suppressed green emission, confirming the good optical properties for the deposited nanowires.     

Effects of Seed Layer Characteristics on the Synthesis of ZnO Nanowires

Single crystalline zinc oxide (ZnO) nanowires were synthesized on sputter-deposited ZnO seed layers via hydrothermal reactions in an equimolar (20 mM) aqueous solutions of Zn(NO₃)₂·6H₂O and C₆H₁₂N₄ at 90°C. The sputter-deposited ZnO seed layers were prepared to exhibit different crystalline structures in order to examine their effects on the growth of ZnO nanowires. It was found that the nanowire diameter depends on the size of the (002) grains of the seed layer. This is attributed to the epitaxially growth of the nanowires from the columnar grains of the seed layer which is shown by the TEM analysis.     

Growth and formation mechanism of sea urchin-like ZnO nanostructures on Si

Sea urchin-like nanostructures of ZnO consisting of ZnO nanowires with blunt faceted ends were grown on Si (100) substrates by oxidation of metallic Zn at 600 °C. ZnO nanowires having a diameter of 30–60 nm and length of 2–4 Μm were in similar shape with uniform diameter along its entire length with well faceted blunt ends. X-ray diffraction and transmission electron microscope analysis showed that the as-grown nanostructures were highly crystalline with wurtzite hexagonal structure having lattice constants of a=b=3.25 å and c=5.21 å. Room temperature photoluminescence (PL) measurements showed a weak near band-edge emission at 380 nm, but a strong green emission at 500–530 nm. A model for vapor-solid (VS) growth mechanism of ZnO nanowires was presented, in which nucleation of ZnO is crucial for the growth of the nanostructures.     

Fabrication and characterization of hexagonally patterned quasi-1D ZnO nanowire arrays

Quasi-one-dimensional (quasi-1D) ZnO nanowire arrays with hexagonal pattern have been successfully synthesized via the vapor transport process without any metal catalyst. By utilizing polystyrene microsphere self-assembled monolayer, sol–gel-derived ZnO thin films were used as the periodic nucleation sites for the growth of ZnO nanowires. High-quality quasi-1D ZnO nanowires were grown from nucleation sites, and the original hexagonal periodicity is well-preserved. According to the experimental results, the vapor transport solid condensation mechanism was proposed, in which the sol–gel-derived ZnO film acting as a seed layer for nucleation. This simple method provides a favorable way to form quasi-1D ZnO nanostructures applicable to diverse fields such as two-dimensional photonic crystal, nanolaser, sensor arrays, and other optoelectronic devices.     

Direct Synthesis of Alumina Nanowires on Ceramic Substrates Using Sn Catalysts

Direct integration of nanostructures into macroscopic substrates is very important for their practical applications. In this work, we report a simple method that can be introduced for the Sn‐catalyzed growth of alumina nanowires on ceramic substrates such as porous disk, monolith, and foam. Our study focuses on the role of the Sn catalysts in the formation mechanisms governing nanowire growth. Using the proposed approach, hair‐ or grass‐like tufts of 20 nm diameter nanowires grow on the surface of the ~3 μm diameter Sn particles, in a tip growth mechanism. The nanowires of α‐phased polycrystalline structure grow and are packed via a complex process involving batch‐by‐batch, branching, and amalgamation growth. The detailed observations reveal that the Sn catalyst is key to tailoring the growth patterns of the nanowires. In addition, cathodoluminescence studies highlight the potential optical applications of the alumina nanowires.     

In situ synthesis of SiC nanowire porous layer on carbon/carbon composites

SiC nanowire porous layer was synthesized in situ by a simple two‐step technique involving slurry painting and high temperature heat‐treatment without catalyst assist. The proposed method is effective and low‐cost, which can prepare plentiful and high‐purity SiC nanowires (SiCNWs). The microstructure of SiCNWs and the effects of heat‐treatment temperatures on the synthesis of SiCNWs were investigated in detail. Results show that the as‐synthesized NWs consisted of 3C–SiC, with the length of up to several hundred micrometers and the diameter of 50‐100 nm at 1500°C. Meanwhile, the diameter of SiCNW increases with the increasing of heat‐treatment temperature. The growth process of the SiCNWs can be controlled by the vapor‐solid growth mechanism. After introducing SiCNWs into the SiC coating prepared by pack cementation, the elastic modulus of the nanowire‐toughened SiC coating was increased by 35% compared to SiC coating without SiCNWs.     

ZnO纳米线阵列膜的自组装生长及其金属接触特性

以磁控溅射制备的ZnO纳米晶薄膜作为籽晶层,用水热法在80℃氧化铟锡(indium tin oxide,ITO)玻璃衬底上,实现了大面积ZnO纳米线阵列膜的取向生长,制备了3种金属-半导体-金属(metal-semiconductor-metal,MSM)结构的ZnO半导体纳米线阵列膜样品,测试了薄膜样品的光学特性和I-V特性。结果表明:在相同的生长液浓度下,籽晶层对所生长的纳米线尺度分布有显著影响。所制备的纳米线薄膜在室温下具有显著的紫外带边发射特性。ZnO纳米线/Ag和ZnO纳米线/Al的金属-半导体接触均具有明显的Schottky接触特性,而ZnO纳米线/Au的金属-半导体接触具有明显Ohmic接触特性。