As-prepared ZnO Nanowires Research Articles

Robust ZnO nanowire photoanodes with oxygen vacancies for efficient photoelectrochemical cathodic protection

Oxygen vacancy engineering of metal oxides is an effective strategy to modulate electronic structures and regulate active sites for improving their photoelectrochemical performances. Herein, a robust ZnO nanowire photoanode with an appropriate amount of oxygen vacancies was synthesized through a seed-assistant hydrothermal approach. Under intermittent sunlight illumination, the as-prepared ZnO nanowire photoanode shifts the open circuit potential of the coupled 304 stainless steel to − 770 mV with a potential drop of 503 mV relative to steel’s corrosion potential (−267 mV), suggesting a superior photocathodic protection performance. In a durability test, the photocathodic protection potential further drops and stabilizes at − 930 mV for hours under continuous illumination, which is attributed to the formation of ZnO/ZnS core/shell heterostructures as well as the in-situ creation of anionic vacancies. This work demonstrates an advantageous effect of oxygen vacancies on the metal oxide photoanodes during photocathodic protection process.

Tailoring the Hydrothermal Synthesis of Stainless Steel Wire Sieve-Supported Ag-Doped ZnO Nanowires to Optimize Their Photo-catalytic Activity

Batches of un-doped and Ag-doped ZnO nanowires (ZnONWs) were prepared hydrothermally on stainless steel wire sieves at varied Zn2+ concentrations of the growth solution and at different Ag+ concentrations of the silver nitrate solution. Methylene blue solution was degraded with these as-prepared ZnONWs in the presences of ultraviolet irradiation. It is found that both the processing parameters greatly affect the surface textures, wettability, and photo-activity of the ZnONWs. The latter synthesizing parameter is optimized only after the former one has been finely regulated. The un-doped and Ag-doped ZnONWs at Zn2+ concentration of 75 mM of the growth solution and at Ag+ concentration of3 mM of the silver nitrate solution both produce Gaussian rough surfaces and in each batch are most hydrophilic. Therefore, in the related batch the contacting surface area of the catalyst is the largest, the hydroxyl radicals attached on the top ends of corresponding ZnONWs the most, and the catalytic activity of these catalysts the optimal. Besides these, the latter synthesizing parameter affects the photo-activity of Ag-doped ZnONWs more significantly than the former one does that of un-doped ZnONWs.

Synthesis and thermal stability of ZnO nanowires

ZnO nanowires (NWs) were synthesized on Au-coated Si (100) substrates by vapor transport method. The effect of high temperature annealing on the structural and chemical composition as well as thermal stability was studied. The as-prepared ZnO NWs was nearly stoichiometric and identified as hexagonal ZnO phase. After annealing at 1,473 K, the atomic ratio of O/Zn, the intensity of the diffraction peaks, and the diameter of nanowires were increased. The ZnO NWs were fragmented into nanocrystals and the fragments coalesced with each other after annealing at 1,673 K. The thermal stability of ZnO NWs was studied by thermo-gravimetric (TG) analysis. A sharp increase in the TG curves was observed and can be attributed to the oxidation of some possibly presented Zn atoms. The activation energy of oxidation of Zn interstitial atoms was found to be 484.81 kJ mol−1. A mass gain peak was observed after annealing at 1,473 K, but it was completely eliminated after annealing at 1,673 K.

Strain-enhanced cable-type 3D UV photodetecting of ZnO nanowires on a Ni wire by coupling of piezotronics effect and pn junction

A 3D-sensitified and strain-enhanced UV wire-photodetector has been fabricated with ZnO NWs grown on an oxidized Ni wire by chemical vapor deposition method. The photoluminescence (PL) spectra of the device shows a sharp UV peak at around 380 nm and a negligible peak at around 520 nm, which proves that the as-prepared ZnO nanowires were well-crystallized. The current-voltage (I-V) and current-time (I-T) characteristics under different rotation angles of the heterojunctions show good rectifying diode behaviors and stability under different angles which make 3D detection possible. The sensitivity of the device is enhanced by strains due to piezotronic effect of ZnO nanowires. These performances of the device demonstrates a promising approach to 3D photodetecting and strain sensing and also provide a prospective application to the development of weaving single wire into fabrics technology.

Effects of Annealing on the Structural and Photoluminescent Properties of Ag-Doped ZnO Nanowires Prepared by Ion Implantation

ZnO nanowires deposited on Si substrates were prepared by thermal evaporation of a mixture of ZnO and carbon powder. Ag ions with an energy of 63 keV and a dose of 5 × 1015 ions/cm2 were implanted into the as-prepared ZnO nanowires. After ion implantation, the Ag-implanted ZnO nanowires were annealed in air at different temperatures from 600°C to 1000°C. Effects of ion implantation and thermal annealing on the structural and photoluminescent (PL) properties of the ZnO nanowires were investigated by transmission electron microscopy (TEM), selected area energy dispersive X-ray spectroscopy (SAEDX), X-ray diffraction (XRD), and fluorescence spectrophotometry. TEM, HR-TEM, and SAEDX analyses demonstrated that efficient doping of Ag was achieved by ion implantation and the subsequent annealing process. XRD patterns revealed that the hexagonal wurtzite structure of ZnO nanowires was maintained after ion implantation. Photoluminescent emissions of ZnO nanowires were decreased significantly by Ag implantation but could be recovered by thermal annealing. The mechanism of the influence of ion implantation and annealing on the PL intensity was assessed.

Growth behavior of ZnO nanowires on Au-seeded SiO2–GaN co-substrate by vapor transport and deposition

Au-seeded amorphous SiO2 layer with crystalline GaN dot windows was fabricated on c-plane sapphire wafer. In order to compare the growth of ZnO nanostructures on amorphous SiO2 with that on crystalline GaN directly, the Au-seeded SiO2–GaN co-substrates were used for the growth of ZnO nanowires by the vapor transport and deposition (VTD) technique. As-prepared ZnO nanowires were characterized by a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The results show that there is an Au particle on the grown ZnO nanowire and the formation of ZnO nanowire on crystalline GaN is easier than that on amorphous SiO2. The experimental results were explained by the change of Zn contents in an Au–Zn droplet with the co-substrate positions during VTD process and ZnO epitaxial growth.

Functionalised zinc oxide nanowire gas sensors: Enhanced NO2 gas sensor response by chemical modification of nanowire surfaces

Surface coating with an organic self-assembled monolayer (SAM) can enhance surface reactions or the absorption of specific gases and hence improve the response of a metal oxide (MOx) sensor toward particular target gases in the environment. In this study the effect of an adsorbed organic layer on the dynamic response of zinc oxide nanowire gas sensors was investigated. The effect of ZnO surface functionalisation by two different organic molecules, tris(hydroxymethyl)aminomethane (THMA) and dodecanethiol (DT), was studied. The response towards ammonia, nitrous oxide and nitrogen dioxide was investigated for three sensor configurations, namely pure ZnO nanowires, organic-coated ZnO nanowires and ZnO nanowires covered with a sparse layer of organic-coated ZnO nanoparticles. Exposure of the nanowire sensors to the oxidising gas NO2 produced a significant and reproducible response. ZnO and THMA-coated ZnO nanowire sensors both readily detected NO2 down to a concentration in the very low ppm range. Notably, the THMA-coated nanowires consistently displayed a small, enhanced response to NO2 compared to uncoated ZnO nanowire sensors. At the lower concentration levels tested, ZnO nanowire sensors that were coated with THMA-capped ZnO nanoparticles were found to exhibit the greatest enhanced response. ΔR/R was two times greater than that for the as-prepared ZnO nanowire sensors. It is proposed that the ΔR/R enhancement in this case originates from the changes induced in the depletion-layer width of the ZnO nanoparticles that bridge ZnO nanowires resulting from THMA ligand binding to the surface of the particle coating. The heightened response and selectivity to the NO2 target are positive results arising from the coating of these ZnO nanowire sensors with organic-SAM-functionalised ZnO nanoparticles.

Well-aligned ZnOnanowires with excellent field emission and photocatalytic properties

Well-aligned ZnO nanowires (NWs) were successfully synthesized on Si(100) by the process of carbothermal reduction and vapor-liquid-solid method. Scanning electron microscopy and transmission electron microscopy results confirmed that ZnO NWs were single crystalline wurtzite structures and grew along the [0001] direction. The influences of substrate temperature and total pressure on the growth were discussed. The well-aligned ZnO NWs show good field emission properties, and the emitter constructed of pencil-like ZnO NWs exhibited a low turn-on field (3.82 V μm(-1)) and a high field enhancement factor (β = 2303). Finally, we demonstrated that the as-prepared ZnO NWs with small diameter on the substrate have good photocatalytic activity toward degradation of methylene blue. Using ZnO NWs with Au nanoparticles (NPs) would decrease the recombination rate of hole-electron pairs due to the great shift of the Fermi level to the conduction band. Hence, adding Au NPs was a promising method to enhance the photocatalytic performance of ZnO NWs. It is significant that photocatalyst fabricated by ZnO NWs can apply to the degradation of organic pollution, and solve the environmental issues.

Large-Scale and Rapid Synthesis of Ultralong ZnO Nanowire Films via Anodization

In this study, we report a facile and rapid synthesis strategy for preparing ultralong ZnO nanowire (NW) arrays by anodizing zinc foil at room temperature in a slightly basic solution, followed by annealing treatment. Detailed structure and composition characterizations are presented, including scanning electron microscopy, energy-dispersive X-ray spectrometry, transmission electron microscopy, and X-ray diffraction. We also explore the effects of temperature, applied voltage, and pretreatment on the anodization process. On the optimized conditions, dense NW film can be achieved, resulting in the enhanced optical absorption. More importantly, the as-prepared ZnO NWs have exhibited reasonable electronic properties. The zinc anodization technique, which does not involve complex chemicals and steps, is a low-cost and high-efficiency approach to large-scale synthesis of ultralong ZnO NW film, suggesting the great potential in dye-sensitized solar cells and gas sensors.

Reproducible Growth of Ultralong ZnO Nanowire Arrays in the Metastable Supersaturated Solution

Vertical aligned ZnO nanowire arrays with a long length and high aspect ratio were prepared successfully in a metastable supersaturated solution which was formed by the introduction of aluminum-iso-propylate into the growth solution. Investigations on the growth revealed that ZnO nanowires were preferentially grown along the c-axis in the metastable supersaturated solution and growth kinetics was subjected to reaction-limited growth. By the cyclic growth in the metastable supersaturated solution, the ZnO nanowire arrays with a length of 10 μm and aspect ratio of 70 could be prepared. The as-prepared ZnO nanowire arrays have potential applications in the next generation nanodevices.

Fabrication of ZnO nanowire arrays by cycle growth in surfactantless aqueous solution and their applications on dye-sensitized solar cells

Large-scale ZnO nanowire arrays vertically aligned on the substrates were achieved from cycle growth without surfactants. The 8 μm long ZnO nanowire arrays were prepared by 20 cycles. The aspect ratio of ZnO nanowire can be increased with increasing the growth cycle. As displayed by microstructures and photoluminescence (PL) analysis, the ZnO nanowire was good single crystal and the defects in the as-prepared ZnO nanowire arrays were controlled at a low concentration. By increasing the length and aspect ratio of ZnO nanowire, the performances of dye-sensitized solar cells based on the ZnO nanowire arrays were improved. As-prepared ZnO nanowire arrays have potential applications in fabricating next generation nanodevices.

Molten Salt Route toward the Growth of ZnO Nanowires in Unusual Growth Directions

ZnO nanowires with unusual growth directions, such as the approximate 102 and the 100 directions, were prepared by using the LiCl molten salt synthetic method. Intrinsic crystallographic structures and the growth directions of the as-prepared ZnO nanowires were investigated by using selected area electron diffraction and high-resolution transmission electron microscopy. In the present case, Li+ and Cl- ions of molten salts may bind with O2- and Zn2+ ions, respectively, of the {101} and {001} polar surfaces of the ZnO crystals, resulting in the decrease of their surface energies and tuning the growth directions by blocking the growth on the polar surfaces. A combination of the growth along the <102>, <100>, and <210> directions may lead to the formation of complex tree like ZnO dendrites. Strong green light emission was observed from room-temperature PL spectra of the as-prepared ZnO nanowires. This molten-salt synthetic process could be extended to synthesize other kinds of unusual 1D nanomaterials with specific crystal structures and properties.

Formation of ZnO nanostructures by a simple way of thermal evaporation

Mass production of ZnO nanowires, nanoribbons, and needle-like rods has been achieved by a simple method of thermal evaporation of ZnO powders mixed with graphite. Metallic catalysts, carrying gases, and vacuum conditions are not necessary. Temperature is the critical experimental parameter for the formation of different morphologies of ZnO nanostructures. Zn or Zn suboxide plays a crucial role for the nucleation of ZnO nanostructures. The as-prepared ZnO nanowires consist of single crystalline cores and thin amorphous shells. As determined by electron diffraction, the growth direction of ZnO nanowires is [001], which has no orientation relationship with the substrate. A strong room-temperature photoluminescence in ZnO nanostructures has been demonstrated.

ZnO nanowires fabricated by a convenient route

A convenient microemulsion-mediated hydrothermal process was employed to synthesize ZnO nanowires, which exhibited a strong ultraviolet emission and a relatively weak defect emission. X-Ray diffraction, scanning electron microscopy, transmission electron microscopy and fluorescence spectroscopy were used to characterize the as-prepared ZnO nanowires.