Well-aligned ZnO Nanowire Arrays Research Articles

Solution-derived 40 µm vertically aligned ZnO nanowire arrays as photoelectrodes in dye-sensitized solarcells

Well-aligned ZnO nanowire arrays with a long length of more than 40 µm were prepared successfully by using the polyethylenimine (PEI)-assisted preheatinghydrothermal method (PAPHT). Several important synthetic parameters such as PEIcontent, growth time, preheating time and zinc salt concentration were found to determinethe growth of ultralong ZnO nanowire arrays, including length, diameter, density andalignment degree. The photoluminescence (PL) spectrum of as-grown ultralong ZnOnanowire arrays revealed a UV emission and a yellow emission, which was attributed to theabsorbed hydroxyl group based on the peak shift after annealing in various atmospheres.The performance of dye-sensitized solar cells (DSSCs) increased with increasing length ofZnO nanowire arrays, which was mainly ascribed to the aggrandized photocurrent andreduced recombination loss according to electrochemical impedance spectroscopy(EIS). A maximum efficiency of 1.3% for a cell with a short-circuit current density(Jsc) = 4.26 mA cm2, open-circuit voltage(Voc) = 0.69 V and (fill factor)FF = 0.42 was achieved witha length of 40 µm.

Enhanced field emission of well-aligned ZnO nanowire arrays illuminated by UV

Vertical zinc oxide nanowires, used as electron emitters, were synthesized on ZnO:Ga/glass substrate by a vapor–liquid–solid process without a catalyst. The nanowires, 5 μm long and 50–250 nm in diameter, were grown with a preferred (0 0 2) orientation and a small XRD full-width at half-maximum intensity. Illumination of ZnO nanowires by UV improved their field emission attributes, reducing their turn-on electric field to 2.1 V/μm at a current density of l μA/cm 2. This phenomenon is attributed to the generation of a large number of hole–electron pairs by UV illumination.

Well-Aligned ZnO Nanowire Arrays Prepared by Seed-Layer-Free Electrodeposition and Their Cassie−Wenzel Transition after Hydrophobization

We report a facile electrochemical route for the one-step fabrication of ZnO nanowire (NW) arrays. The method is seed-layer-free, and the NWs are directly attached to a fluorine-doped tin oxide (FTO) substrate. The effects of growth temperature, precursor concentration, substrate etching, and deposition time on the layer morphology and structure are analyzed. The ZnO NWs are vertically well-aligned and textured with the c-axis normal to the substrate. The growth of the more vertically oriented initial wires is favored by a self-alignment process, and the layer texturing with the c-axis oriented normal to the surface is increased upon deposition. NWs with aspect ratios higher than 30 have been synthesized. The as-grown layers were superhydrophilic, and they were converted to superhydrophobic by surface derivatization with stearic acid (SA). The surface could be commuted back from superhydrophobic to superhydrophilic by a simple acetone washing. The present work demonstrates the importance of oxide NW length to control the hydrophobic state of the surface. By increasing the NW length (and then the aspect ratio), a transition between a Wenzel state and a Cassie−Baxter state was found. For long and homogeneous ZnO NWs, the hysteresis is low (5.5°), and the advancing and receding contact angles are high (168.3°/162.8° max). The role of wire density is discussed. The superhydrophobic layers are of interest for self-cleaning surfaces, biological experiments, and nano/microfluidics.

Ultra-low turn-on field from ultra-long ZnO nanowire arrays emitters

Abstract Ultra-long and well-aligned ZnO nanowire arrays have been synthesized on p-Si (8–13 Ω cm) substrates by vapor phase transport (VPT) method. The length of the nanowire arrays can be modified by adjusting the distance between the source and substrates. Their field emission properties were investigated and the lowest turn-on voltage was observed at 1.08 V/μm. The exceptional field emission performances are attributed to the intrinsically high aspect ratios of ultra-long ZnO nanowire arrays. Our results show another feasible route to enhance field emission performance of ZnO nanowire arrays.

Growth mechanism and diameter control of well-aligned small-diameter ZnO nanowirearrays synthesized by a catalyst-free thermal evaporation method

Well-aligned small-diameter ZnO nanowire arrays have been synthesized on Si wafers pre-coated withc-oriented ZnO thin films by a catalyst-free thermal evaporation method. The morphology ofthe products has been found to be greatly affected by the oxygen flow rate. Theself-catalyzed VLS mechanism is proposed to interpret the growth of the ZnO nanowiresand the change in the product morphology. Classical nucleation theory is employed toanalyze the growth process of the ZnO nanowires, and Zn vapor supersaturation isproposed to be a key factor to affect the diameter and the areal density of the ZnOnanowires. The average diameter of the ZnO nanowires can be finely controlled in the rangeof 12–31 nm by controlling the oxygen flow rate and hence the Zn vapor supersaturation.This result is consistent with our theoretical prediction. X-ray photoelectron spectroscopymeasurements were performed to characterize the stoichiometry of nanowires withvarious average diameters. It also revealed that there are many OH species on thenanowire surface. Photoluminescence measurements showed that the deep-levelemission of the nanowires with average diameter of 12 nm peaks in the yellowregion, which may be attributed to the presence of OH species and surface effects.

Effect of seed layer on the growth of well-aligned ZnO nanowires

We demonstrate that vertical well-aligned crystalline ZnO nanowire arrays were grown on ZnO/glass substrates by a low-temperature solution method. Different thicknesses of ZnO seed layers on glass substrates were prepared by radio-frequency sputtering. In this work it was found that the morphology of ZnO nanowires strongly depends on the thickness of ZnO seed layers. The average diameter of nanowires is increased from 50 to 130 nm and the nanowire density is decreased from 110 to 60 μm −2 while the seed layer thickness is varied from 20 to 1000 nm. The improved control of the morphology of ZnO nanowire arrays may lead to an enhanced carrier collection of hybrid polymer photovoltaic devices based on ZnO.

Superhydrophobic ZnO Nanowire Surface: Chemical Modification and Effects of UV Irradiation

We report a facile wet-chemical route for the fabrication of a superhydrophobic ZnO nanowire surface modified with fatty acids. A systematic study was performed on the relationship between carbon chain length of fatty acid and wetting states of well-aligned ZnO nanowire arrays. The wettability of the ZnO nanowire array was monotonically converted from hydrophilicity to hydrophobicity by increasing the carbon chain length of the chemisorbed fatty acid. The superhydrophobic surface obtained with stearic acid (SA, C18) showed a maximum water contact angle (CA) of 167°, which decreased gradually under UV illumination, due to UV-enhanced decomposition of SA monolayer. XPS analysis confirmed the photodecomposition of SA molecules through UV irradiation. Also, a selective area UV photopatterning was demonstrated using a photomask on the modified ZnO nanowire array.

Diameter-Controlled Vapor−Solid Epitaxial Growth and Properties of Aligned ZnO Nanowire Arrays

A facile, template-free method was used to grow large areas of well-aligned ZnO nanowire arrays on amorphous SiO2 substrates. The arrays are composed of vertically aligned, single-crystalline, wurtzitic [001] ZnO nanowires whose diameters were easily controlled by growth temperature, adjusted by changing the distance between the substrate and the precursor material in the growth chamber. A vapor−solid epitaxial growth mechanism is proposed by which ZnO nanocrystals, nucleated on a NiO catalytic film, seed the growth of the ZnO nanowires. Photoluminescence spectra indicate broad visible wavelength emission, likely caused by near surface traps, whose intensity relative to band edge ultraviolet emission grows as nanowire radii decrease. UV photoconductivity measured for individual ZnO nanowire devices demonstrates their potential as a UV light nanosensor.

Synthesis of ZnO nanowire arrays and their photoluminescence property

Well-aligned ZnO nanowire arrays were directly fabricated on the photoresist coated Si substrates instead of general metal catalyst coated substrate or some special buffer layers. An important feature of this catalyst-free growth process is that the immiscibility of the zinc oxide–carbon system can provide a thermodynamic driving force for the radial growth of one-dimensional (1D) nanowire arrays. In addition, the influence of thickness of the photoresist on the morphology and photoluminescence (PL) properties of the ZnO nanowire arrays was investigated in detail. It is found that thicker photoresist is contributed to the growth of well-aligned ZnO nanowire arrays with good ultraviolet performance.

Facile route to well-aligned ZnO nanowire arrays

Well-aligned ZnO nanowire arrays were fabricated on the photoresist SPR6112-B coated (111) Si substrates by a facile vapor transport and condensation method. The structure and growth mechanism of the ZnO nanowire arrays were investigated in detail. It is found that the immiscibility of the zinc oxide–carbon system is responsible for the self-catalysis vapor–liquid–solid (VLS) growth of nanowire arrays. The photoluminescence measurement only presents a strong near-band-edge ultraviolet (UV) emission band centered at 379.6 nm (3.266 eV), exhibiting that the nanowire arrays are of stoichiometric composition and have good optical performance.

P-Type ZnO Nanowire Arrays

Well-aligned ZnO nanowire (NW) arrays with durable and reproducible p-type conductivity were synthesized on alpha-sapphire substrates by using N2O as a dopant source via vapor-liquid-solid growth. The nitrogen-doped ZnO NWs are single-crystalline and grown predominantly along the [110] direction, in contrast to the [001] direction of undoped ZnO NWs. Electrical transport measurements reveal that the nondoped ZnO NWs exhibit n-type conductivity, whereas the nitrogen-doped ZnO NWs show compensated highly resistive n-type and finally p-type conductivity upon increasing N2O ratio in the reaction atmosphere. The electrical properties of p-type ZnO NWs are stable and reproducible with a hole concentration of (1-2) x 10(18) cm(-3) and a field-effect mobility of 10-17 cm2 V(-2) s(-1). Surface adsorptions have a significant effect on the transport properties of NWs. Temperature-dependent PL spectra of N-doped ZnO NWs show acceptor-bound-exciton emission, which corroborates the p-type conductivity. The realization of p-type ZnO NWs with durable and controlled transport properties is important for fabrication of nanoscale electronic and optoelectronic devices.

The influence of growth temperature on ZnO nanowires

ZnO nanowires have been successfully synthesized on Si(100) substrate by a simple physical vapor deposition method. Thin film ZnO layer used as the nucleation site can avoid the contamination from the metal catalysts, and it can also control the growth direction of ZnO nanowires. Well-aligned ZnO nanowire arrays along the normal direction of the substrate can be obtained by controlling different growth temperature, which was demonstrated by XRD and FESEM analysis. A strong ultraviolet emission at room temperature was observed in all ZnO nanostructures. In addition, the growth mechanisms of the ZnO nanowires with different growth temperature is discussed in details.

Seed-layer controlled synthesis of well-aligned ZnO nanowire arrays via a low temperature aqueous solution method

A simple and controllable method has been developed to synthesize well-aligned ZnO nanowire arrays on silicon and glass substrates in aqueous solution. A thin ZnO seed-layer coated with colloidal ZnO nanocrystals was introduced to control the density and orientation of ZnO nanowires. X-ray diffraction (XRD) and scanning electron microscopy (SEM) result show the high dense and well-aligned character of ZnO nanowires. The photoluminescence (PL) reveals the high crystal quality of the nanowires. It was found that the ZnO seed-layer can effect the size distribution, density and crystal structure of the nanowires. The growth mechanism of ZnO nanowires was also discussed.

Growth of well-aligned ZnO nanowire arrays on Si substrate

Well-aligned ZnO nanowire arrays have been successfully synthesized on Si(100) substrateby a vapour transport process. A ZnO thin film was used as the nucleation sites, which cancontrol the growth orientation of the nanowires. By observation of the initial process ofnanowire growth, a vapour transport solid condensation mechanism was proposed for ZnOnanowire growth, in which the role of ZnO thin film was to provide nucleation sites fornanowire growth. It was also found that the nanowire density could be adjusted by varyingthe thickness of the ZnO thin film. The synthesized ZnO nanowires, which had asingle-crystalline wurtzite structure, had diameters of 50–120 nm and lengths of around5 µm. The strong ultraviolet emission and weak deep level emission reflect the high opticalquality of the nanowires.

Simple approach to highly oriented ZnO nanowire arrays: large-scale growth,photoluminescence and photocatalytic properties

A simple and low-cost soft route has been developed to the directlarge-scale production of highly oriented ZnO nanowire arrays at60 °C. It has been found that the growth of well-aligned ZnO nanowire arrays is dependent onseveral parameters, such as the reaction time and the concentration of ammonium ions.The environmentally benign ZnO nanowires obtained, with significant photoluminescenceproperty and interesting photocatalytic activity, are single crystals and have a low defectconcentration, which could be expected to find promising potential for optoelectronic andenvironmental application.

A New Method for Bulk-Quantity Synthesis of Patterned Well-Aligned ZnO Nanowire Arrays

Large area well-aligned ZnO nanowire arrays have successfully been synthesized on an unconventional substrate: stainless-steel mesh at a low growth temperature of 400oC. The as-grown ZnO nanowires have uniform diameters about 20 nm and a strong UV peak was observed in photoluminescence spectra. The growth method provides a novel way to produce high quality ZnO nanowire arrays, and can be also used to direct the controllable growth of other nanomaterials.

Low‐Temperature Wafer‐Scale Production of ZnO Nanowire Arrays.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Low-temperature wafer-scale production of ZnO nanowire arrays.

Since the first report of ultraviolet lasing from ZnO nanowires, substantial effort has been devoted to the development of synthetic methodologies for one-dimensional ZnO nanostructures. Among the various techniques described in the literature, evaporation and condensation processes are favored for their simplicity and high-quality products, but these gas-phase approaches generally require economically prohibitive temperatures of 800–900 8C. Despite recent MOCVD schemes that reduced the deposition temperature to 450 8C by using organometallic zinc precursors, the commercial potential of gas-phase-grown ZnO nanowires remains constrained by the expensive and/or insulating (for example, Al2O3) substrates required for oriented growth, as well as the size and cost of the vapor deposition systems. A low-temperature, large-scale, and versatile synthetic process is needed before ZnO nanowire arrays find realistic applications in solar energy conversion, light emission, and other promising areas. Solution approaches to ZnO nanowires are appealing because of their low growth temperatures and good potential for scale-up. In this regard, Vayssieres et al. developed a hydrothermal process for producing arrays of ZnO microrods and nanorods on conducting glass substrates at 95 8C. Recently, a seeded growth process was used to make helical ZnO rods and columns at a similar temperature. Here we expand on these synthetic methods to produce homogeneous and dense arrays of ZnO nanowires that can be grown on arbitrary substrates under mild aqueous conditions. We present data for arrays on four-inch (ca. 10 cm) silicon wafers and two-inch plastic substrates, which demonstrate the ease of commercial scale-up. The simple two-step procedure yields oriented nanowire films with the largest surface area yet reported for nanowire arrays. The growth process ensures that a majority of the nanowires in the array are in direct contact with the substrate and provide a continuous pathway for carrier transport, an important feature for future electronic devices based on these materials. Well-aligned ZnO nanowire arrays were grown using a simple two-step process. In the first step, ZnO nanocrystals (5–10 nm in diameter) were spin-cast several times onto a four-inch Si(100) wafer to form a 50–200-nm thick film of crystal seeds. Between coatings, the wafer was annealed at 150 8C to ensure particle adhesion to the wafer surface. The ZnO nanocrystals were prepared according to the method of Pacholski. A NaOH solution in methanol (0.03m) was added slowly to a solution of zinc acetate dihydrate (0.01m) in methanol at 60 8C and stirred for two hours. The resulting nanoparticles are spherical and stable for at least two weeks in solution. After uniformly coating the silicon wafer with ZnO nanocrystals, hydrothermal ZnO growth was carried out by suspending the wafer upside-down in an open crystallizing dish filled with an aqueous solution of zinc nitrate hydrate (0.025m) and methenamine or diethylenetriamine (0.025m) at 90 8C. Reaction times spanned from 0.5 to 6 h. The wafer was then removed from solution, rinsed with deionized water, and dried. A field-emission scanning electron microscope (FESEM) was used to examine the morphology of the nanowire array across the entire wafer, while single nanowires were characterized by transmission electron microscopy (TEM). Nanowire crystallinity and growth direction were analyzed by X-ray diffraction and electron diffraction techniques. SEM images taken of several four-inch samples showed that the entire wafer was coated with a highly uniform and densely packed array of ZnO nanowires (Figure 1). X-ray diffraction (not shown) gave a wurtzite ZnO pattern with an enhanced (002) peak resulting from the vertical orientation of the nanowires. A typical synthesis (1.5 h) yielded wires with diameters ranging between 40–80 nm and lengths of 1.5–2 mm.

Synthesis and Characterization of Lanthanide Hydroxide Single‐Crystal Nanowires.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis and characterization of lanthanide hydroxide single-crystal nanowires.

Nanowires represent a class of quasi-one-dimensional materials, in which carrier motion is restricted in two directions so that they usually exhibit significant photochemical, photophysical, and electron-transport properties which differ from that of bulk or nanoparticle materials.[1±11] In the last decade, many important materials have been prepared in the form of nanorods or nanowires to generate some unexpected properties[2±5] based on which, many new potential applications have been explored. For example, wellcontrolled and monodisperse CdSe nanorods can be used in solar cells,[2] and can form liquid-crystal phases with orientational order and positional disorder in organic solvents;[3] single-crystalline, well-defined ZnO nanowires can serve as natural resonance cavities, and well-aligned ZnO nanowire arrays may serve as room-temperature ultraviolet nanolasers.[4] Along with chemical composition and crystal structure, shape and dimensionality are now regarded as particularly important factors that influence the chemical and/or physical properties of materials. As a consequence of their unique electronic structures and the numerous transition modes involving the 4f shell of their ions, lanthanide compounds usually have outstanding optical, electrical, and magnetic properties,[12±22] and have been widely used as high-quality phosphors,[12] up-conversion materials,[13] catalysts,[14] and time-resolved fluorescence (TRF) labels for biological detection.[15] There have been extensive studies regarding lanthanide chemistry at bulk or atomic levels,[16±19] and the synthesis of lanthanide monothiooxides,[20] lanthanide-doped oxides or sulfides, organolanthanide compounds, the complexation behavior of lanthanide ions or atoms,[16,17] and the emission phenomena from lanthanides when bound to biomolecules (including calcium-binding proteins and nucleic acids).[18,19] Very recently, the synthesis of lanthanide oxide[21] and fluoride[22] nanoparticles with enhanced luminescence and photomagnetic properties has also been reported. However, to the best of our knowledge, few studies have focused on the synthesis of nanowires or nanorods of lanthaniderelated compounds. Such materials would be of great significance because of the possible novel properties induced by their reduced dimensionality. Herein, we report the synthesis of lanthanide hydroxide nanowires (La(OH)3, Pr(OH)3, Nd(OH)3, Sm(OH)3, Eu(OH)3, Gd(OH)3, Dy(OH)3, Tb(OH)3, Ho(OH)3, Tm(OH)3, and YbOOH) through a facile solution-based hydrothermal synthetic pathway. The synthesis of Ln(OH)3 nanowires was based on the preparation of colloidal Ln(OH)3 at room temperature, with subsequent hydrothermal treatment at 180 8C for about 12 h. Figure 1A shows typical XRD patterns for La(OH)3 nanowires. All of the reflections could be readily indexed to the hexagonal phase (space group P63/m (no. 176)) of La(OH)3