Growth Of ZnO Nanowires Research Articles

NO2 sensor based on Al modified ZnO nanowires

Hydrothermally grown ZnO nanowires (NWs) have been successfully modified by an ultrathin layer of Al resulting in an improved and highly selective NO2 sensor. ZnO NWs containing 1.8 at.% of Al (thickness - 5 nm) could reversibly detect NO2 in the range from 0.5 to 5 ppm. The sensor film showed a linear dependence for all the concentrations between 0.5 and 5 ppm. A sensor response of 6.2 towards 4 ppm of NO2 was reversibly achieved at 230 °C with swift response kinetics exhibiting response and recovery times of 18 and 65 s, respectively. Modification with Al causes the resistance of the sensor film to decrease with an analogous improvement in the sensor response. The observed decrease in sensor resistance agrees well with the findings of work function studies. Sensor film modified with Al showed 60 meV decreases in work function. NO2 being an oxidizing gas causes an increase in work function by 0.25 and 0.19 eV for pure and Al modified ZnO NW films, respectively. A gas sensing mechanism has been proposed based on gas sensing, Kelvin probe and X-ray photoelectron spectroscopy measurements.

Rational Strategy for Space-Confined Seeded Growth of ZnO Nanowires in Meter-Long Microtubes

Seeded crystal growths of nanostructures within confined spaces offer an interesting approach to design chemical reaction spaces with tailored inner surface properties. However, such crystal growth within confined spaces tends to be inherently difficult as the length increases as a result of confinement effects. Here, we demonstrate a space-confined seeded growth of ZnO nanowires within meter-long microtubes of 100 μm inner diameter with the aspect ratio of up to 10 000, which had been unattainable to previous methods of seeded crystal growths. ZnO nanowires could be grown via seeded hydrothermal crystal growth for relatively short microtubes below the length of 40 mm, while any ZnO nanostructures were not observable at all for longer microtubes above 60 mm with the aspect ratio of 600. Microstructural and mass spectrometric analysis revealed that a conventional seed layer formation using zinc acetate is unfeasible within the confined space of long microtubes as a result of the formation of detrimental residual Zn complex compounds. To overcome this space-confined issue, a flow-assisted seed layer formation is proposed. This flow-assisted method enables growth of spatially uniform ZnO nanowires via removing residual compounds even for 1 m long microtubes with the aspect ratio of up to 10 000. Finally, the applicably of ZnO-nanowire-decorated long microtubes for liquid-phase separations was demonstrated.

Formation and optical characteristics of ZnO:Eu/ZnO nanowires grown by sputtering-assisted metalorganic chemical vapor deposition

We report on the growth and optical characteristics of Eu-doped ZnO (ZnO:Eu) films on ZnO nanowires (NWs) by sputtering-assisted metalorganic chemical vapor deposition. ZnO:Eu films are grown by sputtering Eu2O3 targets during the growth of ZnO NWs. The crystal quality of ZnO host materials is improved by using the NW configuration due to a strain relaxation effect, which is elucidated by optical characterization. An enhancement of Eu3+ luminescence at 613 nm is observed at room temperature for the ZnO:Eu/ZnO NWs as compared to a conventional film. Site-selective spectroscopy by direct excitation of Eu3+ ions by a tunable dye laser reveals that the most intense emission peak is located at a wavelength of ∼613.5 nm, which coincides with the main peak under indirect excitation. This result indicates that the observed enhanced Eu luminescence is due to a relative increase in the number of Eu centers which can effectively be excited via the host material.

Surface Modification of ZnO Nanowires with CuO: A Tool to Realize Highly-Sensitive H2S Sensor

Hydrothermally grown ZnO nanowires (NWs) have been successfully synthesized and surface modified by an ultrathin layer of CuO using dip coating technique to achieve a highly sensitive H2S sensor. XRD analysis confirmed the hexagonal structure of ZnO without any Cu sub-oxide peaks. After CuO modification, the peak shift was observed in the electronic states of O and assigned to the defects and increase in adsorbed oxygen species. Similarly, a red shift was also observed in the band edge absorption after CuO modification arising due to defects. The sensor film showed an overall n-type character as confirmed using I(V) characteristics. Interestingly, sensor response kinetics towards H2S were enhanced after CuO modification. The highest sensor response value of 298 was measured towards 10 ppm H2S at 150°C for CuO:ZnO NWs sample having 1.26 at % of Cu. This improved sensor response has been attributed mainly to the formation of randomly distributed p–n nano-hetero-junctions between p-type CuO and n-type ZnO over the sensor surface. In particular, the p–n nano-hetero-junctions collapsed due to conversion of semiconducting CuO into metallic CuS after the unique interaction with H2S.

Rapid synthesis of ZnO nanowires and nanoplates with highly enhanced photocatalytic performance

ZnO nanowires and nanoplates with extremely enhanced photocatalytic performance were prepared via one-step facile thermal decomposition of zinc acetate dihydrate and well characterized by FESEM, TEM, STEM-HAADF, EDX, XRD, PL, XPS, UV–Vis and Raman spectroscopy. FESEM and TEM studies revealed ZnO nanowires, nanoplates and few nanorods in the sample prepared at 300 °C, while increase in the thermal decomposition temperature to 500 °C led to the formation of ZnO nanoparticles and nanorods. XRD results revealed hexagonal wurtzite ZnO nanostructures with crystallite size variation from 47 to 34 nm with increase in thermal decomposition temperature, while PL studies showed decrease in band gap from 3.31 to 3.26 eV along with significant quenching of defect emissions as the thermal decomposition temperature was increased from 300 to 500 °C. ZnO nanowires and nanoplates prepared at 300 °C exhibited extremely enhanced photocatalytic performance and led to almost complete decomposition of methylene blue (MB) and methyl orange (MO) in only 6 and 32 min, respectively. The extremely strong photocatalytic performance of ZnO nanowires and nanoplates makes them exciting candidates for solar-driven water purification. The possible mechanisms of growth of ZnO nanowires and nanoplates and their enhanced photocatalytic performance were proposed.

Size-controllable zinc oxide nanowires fabricated via the combination of die-casting and oxidation process

Herein, we demonstrate a facile approach in the fabrication of one-dimensional ZnO nanowires (NWs) from the vacuum die-casting method in assistance with anodic aluminum oxide (AAO) template. The casting method enables molten Zn to embed uniformly over the AAO nanomould under high pressure and temperature of 500 °C. Besides, the significant influence of the oxidation time and temperature stimulates the ZnO NWs growth inside the AAO nanochannels. The oxidation in the nanochannels was profoundly controlled by the inward diffusion of oxygen atoms and the ZnO grows perpendicular towards the surface which obeys the parabolic law. The TEM and XRD results revealed the ZnO wurtzite crystalline structure with preferential orientation along the c-axis. The relationship between ZnO geometry and function of oxidation parameters is demonstrated by microscopic and spectroscopic analysis. The obtained NWs from the chemical etching process resulted in high crystalline, homogeneous, and high-aspect ratio ZnO NWs in relationship with the AAO pore dimension. Further, the as-prepared ZnO NWs electrical properties were measured in a four-electrode system through the single nanowire device fabricated by FIB. The obtained current-voltage characteristic curves exhibited hysterias loop in consequence with the space charge limited current effect of ZnO NW, which confirms its potential to be used in practical electronic applications.

Enhancing the interyarn friction properties of kevlar and glass fabrics through ZnO nanowire coating

The interyarn friction properties of fabrics can be enhanced by appropriate surface treatment of fibers. This study focuses on evaluating the interyarn friction properties of Kevlar and Glass fabrics that were coated with ZnO nanowires through different growth cycles. ZnO nanowires were coated onto woven Kevlar and Glass fabrics through a low temperature hydrothermal solution method. Longer growth time coupled with periodic refreshing of growth solution and washing of fabrics was found to be a favorable condition for uniform and precipitate free growth of ZnO nanowires. The effects of ZnO nanowire coating on the tensile and interyarn friction properties of fabrics were measured. In general, after ZnO nanowire coatings, Kevlar fabrics remained equally flexible as that of bare fabric while Glass fabrics became relatively stiff and brittle. The interyarn friction properties of Kevlar fabrics were found to be high under 100 N transverse tension while the transverse tension was found to have negligible or insignificant effect on the interyarn friction properties of Glass fabrics that were used in this research. Compared to bare fabric, Kevlar fabric coated with ZnO nanowires for extended duration showed 266% and 293% increase in yarn pull out load and energy, respectively under 100 N tension. Compared to bare fabric, Glass fabric coated with ZnO nanowires for extended duration showed 517% and 376% increase in yarn pull out load and energy, respectively under 5 N tension. These significant improvements in interyarn friction properties were attributed to mechanical interlocking and accumulation of ZnO nanowires at the intersection of yarns.

ZnO nanowires based schottky contacts of Rh/ZnO interfaces for the enhanced performance of electronic devices

The surface/interface studies of semiconductor materials are of particular interest, having a significant impact on nanoscience and nanotechnology in the development of advanced nanoelectronic devices. In this research work, design and fabrication of ZnO nanowires (NWs) based Schottky contacts of metal/semiconductor (Rh/ZnO) interfaces for superior diode characteristics of electronic devices has been investigated. The morphological and structural investigation suggest that smart thermal CVD is a promising method for the synthesis of high-quality catalyst free ZnO NWs on n-type Si<100> substrate with 700–1300 nm in length and 35–45 nm in diameter. Successful fabrication of controlled growth of ZnO NWs with high crystallinity, smooth surface and growth along [002] direction was achieved. X-ray photoelectron spectroscopy (XPS) confirm the purity and chemical states of ZnO NWs. The electrical nature of the nanoscale contact between Rh and ZnO NW was measured using conductive atomic force microscopy (C-AFM) at ultra-high vacuum. The fabricated Schottky contacts of Rh/ZnO interfaces show superior diode characteristics with good rectifying behavior, relatively small turn-on voltage (~ 0.99 eV) and good ideality factor (~ 1.95). This study develops a unique strategy for design and fabrication of ZnO NWs based Schottky contacts of Rh/ZnO interfaces for various future optoelectronic applications.

Low-temperature large-area fabrication of ZnO nanowires on flexible plastic substrates by solution-processible metal-seeded hydrothermal growth

We have developed the low-temperature conformal ZnO nanowire fabrication on flexible plastic substrates by utilizing the solution-processible metal seed-assisted hydrothermal ZnO crystallization. Structural evolution of ZnO nanowires controlled by major parameters involving growth temperature, growth time, and seed coating condition, has been systematically investigated towards uniform and large-area growth of conformal ZnO nanowires. Direct ZnO nanowire growth on flexible plastics without undergoing the high-temperature seed sintering has been realized by developing the low-temperature Ag-seeded hydrothermal ZnO nanowire growth. The nanoporous Ag layer favorable for ZnO crystal nucleation and continued nanowire growth can be reduced from the Ag ion solution coating at the temperature as low as 130 °C. This tactfully enables the selective hydrothermal growth of ZnO nanowires on the Ag patterns on flexible plastics. Such an all-solution-processible low-temperature fabrication protocol may provide an essential and practical solution to develop many diverse applications including wearable and transparent electronics, sensors, and photocatalytic devices. As one example, we demonstrate that a transparent UV sensor can be devised based on the ZNW growth on the Ag micromesh electrode.

Growth and NO2-Sensing Properties of Biaxial p-SnO/n-ZnO Heterostructured Nanowires.

Biaxial p-SnO/n-ZnO heterostructured nanowires (average length of 10 μm) were grown onto a glass substrate by thermal evaporation in vacuum. These nanowires had spherical ball tips, and the size of the SnO part increased gradually from the top to the bottom of the nanowire, but the corresponding size of ZnO varied slightly. The Sn-Zn alloy formed in the tips resulted in determined as the catalyst of the growth of the ZnO nanowires. The growth process of the p-SnO/n-ZnO biaxial nanowires is discussed based on vapor-liquid-solid (VLS) based on the subsequent growth process: the VLS catalytic growth of the ZnO nanowire and subsequent epitaxial SnO growth on the sidewall of the pregrown ZnO nanowire. An epitaxial relationship, (001)SnO//(110)ZnO and [110]SnO//[002]ZnO, was observed in the biaxial p-SnO/n-ZnO heterostructured nanowires. The gas-sensing properties of the as-synthesized p-SnO/n-ZnO nanowires were investigated. The results show that the device exhibit a good performance to the ppb-level NO2 at room temperature (25 °C) without light illumination. The detection limit of the p-SnO/n-ZnO sensor to NO2 is 50 ppb. Moreover, the NO2-sensing properties of the p-SnO/n-ZnO device were investigated under various relative humidity. Finally, the NO2-sensing mechanism of the p-SnO/n-ZnO nanowires was proposed and discussed.

Significance of Ni doping on structure-morphology-photoluminescence, optical and photocatalytic activity of CBD grown ZnO nanowires for opto-photocatalyst applications

In this work, we study bare and Nickel (Ni) doped ZnO nanostructured thin films synthesized by the chemical bath approach. The as-prepared thin films were characterized extensively by XRD, SEM, PL, and UV–visible spectroscopy before and after Ni incorporation. Finally, the photocatalytic action of all catalysts was assessed using methylene blue (MB) dye degradation under the visible light irradiation. X-ray diffraction analysis approves wurtzite hexagonal structure with predominant orientation along (002) plane, and crystallite size was noticed to reduce from 34 to 32 nm by Ni-doping. The SEM images proved the formation of well-defined nanowire structures. Photoluminescence analysis confirmed the band-edge and defect-related emission peaks. The intensity of PL spectra ensured a decreased value after Ni-doping, suggesting delayed electron-hole pair recombination, which influences the decolorization activity. The 3 wt% Ni-doped ZnO thin film possessed the least optical bandgap value of 3.21 eV. It was observed that the Ni (3%) doped ZnO nanowires showed a significantly improved photocatalytic performance (96%) compared to the pure ZnO nanostructure (83%).

Zinc Vacancy–Hydrogen Complexes as Major Defects in ZnO Nanowires Grown by Chemical Bath Deposition

Crystal defects in unintentionally doped ZnO nanowires grown by chemical bath deposition (CBD) play a capital role on their optical and electrical properties, governing the performances of many nan...

Why is it difficult to grow spontaneous ZnO nanowires using molecular beam epitaxy?

Surface diffusion is known to be of prime importance in the growth of semiconductor nanowires. In this work, we used ZnMgO layers as markers to analyze the growth mechanisms and kinetics during the deposition of ZnMgO/ZnO multilayered shells by molecular beam epitaxy on previously grown ZnO nanowire cores (so called core–shell heterostructures). Specifically, the influence of the O2 flow sent into the plasma cell on the adatom surface mobility was investigated. By carefully measuring the growth rate on the lateral facets as well as on the top of the nanowires, it is concluded that the surface diffusion length of adatoms, within the used MBE growth conditions, is very low. Such poor surface mobility explains why so few works can be found related to the spontaneous growth (without catalyst) of ZnO nanowires by MBE, contrary to other deposition techniques.

Morphological and Optical Properties of Cobalt Ion-Modified ZnO Nanowires

In this study, we prepared cobalt (Co) ion-modified ZnO nanowires using hydrothermal synthesis with zinc acetate dehydrate and Co (II) acetate hydrate precursors in ethanol solutions. Their morphological and optical properties were investigated with varying Co precursor concentration. The morphological changes of the ZnO nanowires depended positively on the concentration of the Co precursor. The ZnO nanowires showed modified crystal orientations and nanostructure shapes depending on the Co concentration in the solutions. Variations in the optical properties of the Co ion-modified ZnO nanowires could be explained by the interaction of the Co ions with the band electrons, oxygen vacancies, and zinc interstitials. The overall growth and characteristics of ZnO nanowires synthesized in solutions containing low levels of Co ions were related to Co doping into the ZnO bulks. In solutions containing high levels of Co ions, these were additionally related to the Co oxide cluster.

Evidence of O‐Polar (0001¯) ZnO Surfaces Induced by In Situ Ga Doping

Controlling the facet orientation and polarity of semiconductor nanostructures is a major issue for achieving devices requiring specific surface properties. Herein, new facets are created during the growth of ZnO nanowires (NWs) by metal‐organic chemical vapor deposition, as a consequence of the trimethylgallium flow introduced in the gas phase. This in situ Ga‐doping induces original shapes such as “Christmas tree”‐like and “Taipei tower”‐like nanostructures, developed along the C‐axis direction. In particular, lateral facets exhibit polar surfaces normal to this growth direction, which can be seen either as overhangs (for Christmas trees) or as terraces (for Taipei towers). In both cases, convergent beam electron diffraction reveals that those surfaces are O‐polar (000) planes, so that the two kinds of nano‐objects grow with opposite polarities, i.e., Zn‐polar (+C) for Christmas trees and O‐polar (−C) for Taipei towers. By confirming previously published theoretical calculations, this work provides an experimental evidence that Ga doping favors the growth of O‐polar ZnO facets instead of the nonpolar M‐surfaces usually developed for undoped ZnO NWs. Nano‐cathodoluminescence studies emphasize the intense near band edge emission of the nanostructures in the ultraviolet range, demonstrating their high optical quality.

Effect of intense pulsed light on hydrothermally grown ZnO nanowires

Hydrothermal growth of ZnO nanowires has been of great interest as it allows the use of organic substrates for flexible electronics applications. However, there are drawbacks on the growth technique as it is known to introduce defects in the nanowires. In this work, the use of intense pulsed light on hydrothermally grown ZnO nanowires on polyimide substrate was performed to enhance the physical and electronic properties of the nanomaterials. Such photonic annealing technique offers a rapid and effective improvement in material properties and importantly it is suitable for use on a wide variety of organic substrates, which are vital for flexible electronics.

A low-temperature limit for growth of ZnO nanowires by using of laser ablation processes

The contribution deals with growth of ZnO nanowires on metal catalysts by using of pulsed laser deposition and with the influence of growth temperature. The process of nanowires preparation comprised two technological steps—both were based on pulsed laser ablation processes: (1) production of metal nanoparticles by laser ablation in liquids and (2) pulsed laser deposition of ZnO nanowires by ablation of ZnO target on substrate with metal nanoparticles. Nanoparticles from various metals (Au, Ag, Ni, Cu, Al, Mg, Zn, Sn and BiSn alloy) were prepared by pulsed laser ablation at 1064 nm in deionised water. Colloids contained metal nanoparticles were applied on Si (100) substrates, and after drying, nanoparticles served as catalysts of VLS crystallisation. Temperatures in interval 600—200 °C were experimentally compared for the nanowires growth with applied ablation laser working at 248 nm. The lowest achieved temperature value for growth of ZnO nanowires was 425–450 °C. However, among applied metals Cu and Al nanoparticles only successfully catalysed ZnO nanowires at this temperature. Properties of prepared samples were investigated by scanning electron microscopy and photoluminescence. Experimental results revealed that along with the growth temperature, selection of proper metal catalyst is also important factor for nanowires crystallisation.

Controllable lateral growth and electrical properties of nonpolar ZnO nanowires

The iodide interplay with polar Zn2+ and O2− induced nonpolar [101¯0] ZnO nanowires is fabricated via a simple vapor phase transport at a temperature of as low as 250 °C that is compatible with the nanodevice processing technique. As-fabricated nanowires exhibit single crystalline hexagonal wurtzite structures and grow along the [101¯0] direction instead of the conventional polar [0001] direction. The growth evolution can be explained by the synergy of the vapor–liquid–solid process and iodide direction-modulation. The electrical measurements demonstrate that the mobility of the PbI2-induced [101¯0] nanowires is significantly improved in comparison with that of the BiI3-modulated [112¯0] ones. These unique nonpolar nanowires are promising for improved high efficiency nanodevices.

Effect of Hydrothermal Solutions on Optical and Electrical Properties of ZnO Nanowires Grown on a ZnO:Al Seed Layer for Improved Mechanical Performance

ZnO nanowire arrays were fabricated by hydrothermal growth on Al-doped ZnO (ZnO:Al) seed layers coated on soda-lime silicate glass by sol–gel coating. The properties of the ZnO:Al seed layer were evaluated to obtain ZnO nanowires with the optimal size and length and to realize suitable adhesion of the ZnO:Al grains to the substrate. The optimal mechanical performance (adhesion and abrasion resistance) of the ZnO:Al seed layer was obtained at Al 1 at.%. The seed layers annealed between 400 and 500 C exhibited enhanced ZnO nanowire growth. Increasing the annealing temperature within this range improved the electrical and optical properties of the nanowires. Additionally, two chemical compounds, zinc acetate (ZA) and zinc nitrate (ZN), were used to compare the effects of the solution type on the hydrothermal growth. The nanowires grown in the ZA solution were thicker and had higher electrical conductivity compared to the ZN solution. The gamma transmission technique was used to determine the agglomeration of ZnO:Al nanospheres and to examine the crystallite size and density of the ZnO:Al seed layers.

Seed layer effect on morphological, structural, and optical properties of electrochemically grown ZnO nanowires over different SnO2:F/glass substrates

Transparent conductive oxide electrodes and specifically SnO2:F/glass are widely employed substrates in the preparation of optoelectronic devices. This study deals with the surface effect of commercially SnO2:F/glass electrodes on the properties of ZnO nanowires grown onto them by using an electrochemical method. Four types of commercially SnO2:F/glass with sheet resistances in the range of 7–15 Ω cm−2 and roughness from 9 to 39 nm were used. The deposition process of ZnO nanowires was performed with zero, one, and two ZnO seed layers, respectively. The seed layer was grown from an ethanolic solution of zinc acetate as a Zn2+ precursor, and further, a thermal treatment was performed. The formation of the nanomaterial was carried out by using the potentiostatic method at − 1 V and until reaching a total charge of − 1.5 C at a temperature of 75 °C. Different densities of nanowires were obtained for each case, with diameters ranging from 30 to 400 nm. The SnO2:F/glass roughness affects the diameter and alignment of the ZnO nanowires. Measurements of diffusive spectra in the optical range, as well as the determination of polarization properties, show that the seed layer deposition is indispensable to obtain optimal optical quality. The findings here discussed constitute another critical factor to be considered in the synthesis of well-aligned nanowires.