Ultraviolet electroluminescence from nanostructural SnO2-based heterojunction with high-pressure synthesized Li-doped ZnO as a hole source

We report an ultraviolet (UV) electroluminescence (EL) in n-SnO₂/p-ZnO heterojunction light-emitting diodes with the nanostructural SnO₂ as an n-type layer and the Li-doped ZnO (ZnO:Li) synthesized by high-temperature high-pressure (HTHP) method as a high hole concentration p-type layer. Two kinds of SnO₂ nanostructures including nanobelts (NBs) and nanowires (NWs) were used to fabricate n-type layers in the heterojunctions. The two heterojunctions with different SnO₂ nanostructures demonstrate different light-emission feature in EL measurements. The SnO₂ NBs/p-ZnO heterojunction shows a blue emission band centered at 416?nm under forward-bias voltage. A strong UV emission peak located at 391?nm was observed for the SnO₂ NWs/p-ZnO heterojunction. Photoluminescence (PL) spectra indicate that the difference in EL is attributed to morphology-dependent light-emission feature in nanostructural SnO₂ layer. Our results suggest that the nanostructural SnO₂/ZnO:Li heterojunction is a potential and promising system in the UV optoelectronic field.     

Synthesis and Characterization of ZnO Nanowire–CdO Composite Nanostructures

ZnO nanowire–CdO composite nanostructures were fabricated by a simple two-step process involving ammonia solution method and thermal evaporation. First, ZnO nanowires (NWs) were grown on Si substrate by aqueous ammonia solution method and then CdO was deposited on these ZnO NWs by thermal evaporation of cadmium chloride powder. The surface morphology and structure of the synthesized composite structures were analyzed by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The optical absorbance spectrum showed that ZnO NW–CdO composites can absorb light up to 550 nm. The photoluminescence spectrum of the composite structure does not show any CdO-related emission peak and also there was no band gap modification of ZnO due to CdO. The photocurrent measurements showed that ZnO NW–CdO composite structures have better photocurrent when compared with the bare ZnO NWs.     

Well-aligned ZnO nanowires 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.     

Field emission characteristics of zinc oxide nanowires synthesized by vapor-solid process

Vertically aligned ZnO nanowire (NW) arrays have been synthesized on silicon substrates by chemical vapor deposition. The growth of ZnO NWs may be dominated by vapor-solid nucleation mechanism. Morphological, structural, optical, and field emission characteristics can be modified by varying the growth time. For growth time that reaches 120 min, the length and diameter of ZnO NWs are 1.5 μm and 350 nm, respectively, and they also show preferential growth orientation along the c-axis. Room-temperature photoluminescence spectra exhibit a sharp UV emission and broad green emission, and the enhanced UV-to-green emission ratio with increasing growth time might originate from the reduced concentration of surface defects. Furthermore, strong alignment and uniform distribution of ZnO NWs can also effectively enhance the antireflection to reach the average reflectance of 5.7% in the visible region. The field emission measurement indicated that the growth time plays an important role in density- and morphology-controlled ZnO NWs, and thus, ZnO NWs are expected to be used in versatile optoelectronic devices.     

Investigations into the impact of the substrates with ultra thin seed layers deposited by atomic layer deposition on ZnO Nanowire Arrays

ABSTRACT: The impact of various substrates and ZnO ultra thin seed layers prepared by atomic layer deposition on the geometric morphology of subsequent ZnO nanowire arrays (NWs) fabricated by the hydrothermal method was investigated. The investigated substrates included B-doped ZnO films, Indium Tin Oxide films, single crystal silicon (111), and glass sheets. Scanning electron microscopy and X-ray diffraction measurements revealed that the geometry and aligment of the NWs were controlled by surface topography of the substrates and thickness of the ZnO seed layers, respectively. According to atomic force microscopy data, we suggest that the substrate, fluctuate amplitude and fluctuate frequency of roughness on ZnO seed layers have a great impact on the alignment of the resulting NWs, wherease the influence of the seed layers' texture was negligible. Transmission Electron Microscopy and Photoluminescence spectroscopy showed that crystal defects were influenced greatly by substrates instead of seed layers.     

Enhanced UV photosensitivity from rapid thermal annealed vertically aligned ZnO nanowires

We report on the major improvement in UV photosensitivity and faster photoresponse from vertically aligned ZnO nanowires (NWs) by means of rapid thermal annealing (RTA). The ZnO NWs were grown by vapor-liquid-solid method and subsequently RTA treated at 700°C and 800°C for 120 s. The UV photosensitivity (photo-to-dark current ratio) is 4.5 × 10³ for the as-grown NWs and after RTA treatment it is enhanced by a factor of five. The photocurrent (PC) spectra of the as-grown and RTA-treated NWs show a strong peak in the UV region and two other relatively weak peaks in the visible region. The photoresponse measurement shows a bi-exponential growth and bi-exponential decay of the PC from as-grown as well as RTA-treated ZnO NWs. The growth and decay time constants are reduced after the RTA treatment indicating a faster photoresponse. The dark current-voltage characteristics clearly show the presence of surface defects-related trap centers on the as-grown ZnO NWs and after RTA treatment it is significantly reduced. The RTA processing diminishes the surface defect-related trap centers and modifies the surface of the ZnO NWs, resulting in enhanced PC and faster photoresponse. These results demonstrated the effectiveness of RTA processing for achieving improved photosensitivity of ZnO NWs.     

Modification of the optical and structural properties of ZnO nanowires by low-energy Ar+ ion sputtering

The effects of low-energy (≤2 kV) Ar⁺ irradiation on the optical and structural properties of zinc oxide (ZnO) nanowires (NWs) grown by a simple and cost-effective low-temperature technique were investigated. Both photoluminescence spectra from ZnO NW-coated films and cathodoluminescence analysis of individual ZnO NWs demonstrated obvious evidences of ultraviolet/visible luminescent enhancement with respect to irradiation fluence. Annihilation of the thinner ZnO NWs after the ion bombardment was appreciated by means of high-resolution scanning electron microscopy and transmission electron microscopy (TEM), which results in an increasing NW mean diameter for increasing irradiation fluences. Corresponding structural analysis by TEM pointed out not only significant changes in the morphology but also in the microstructure of these NWs, revealing certain radiation-sensitive behavior. The possible mechanisms accounting for the decrease of the deep-level emissions in the NWs with the increasing irradiation fluences are discussed according to their structural modifications.     

Efficient solar light photocatalytic degradation of commercial pharmaceutical drug and dye using rGO-PANI assisted c-ZnO heterojunction nanocomposites

The major requirement of a solar light-activated photocatalyst is the effective utilization of the light-induced electron-hole pair and exciton lifetime. Herein, the reduced graphene oxide and polyaniline assisted carbon doped porous ZnO (c-ZnO) heterojunction nanocomposites (RPZ nanocomposites) were designed for enhanced photocatalytic degradation of the commercially available pharmaceutical antibiotic drug amoxicillin and clavulanate potassium (ACP) and methylene blue (MB) dye using natural sunlight. The surface morphology, phase purity, and bonding environment of the prepared RPZ heterojunction nanocomposite were analyzed using scanning electron microscopy, X-ray diffraction, and soft X-ray absorption spectroscopy, respectively. In comparison with pure ZnO, a doping and composite formation reduced the bandgap energy from 3.34 to 2.80 eV, calculated using the Tauc plot. From photocatalytic degradation studies, the as-prepared RPZ heterojunction nanocomposite efficiently degraded 95% and 47% of MB dye and ACP in 100 min under natural sunlight with the reaction rates of 0.0296 and 0.0055 min^?1, respectively. The removal efficiency of the photocatalyst was obtained to be 95% and 46.14% for MB dye and ACP, respectively.     

Structural and photoluminescence studies on catalytic growth of silicon/zinc oxide heterostructure nanowires

Silicon/zinc oxide (Si/ZnO) core-shell nanowires (NWs) were prepared on a p-type Si(111) substrate using a two-step growth process. First, indium seed-coated Si NWs (In/Si NWs) were synthesized using a plasma-assisted hot-wire chemical vapor deposition technique. This was then followed by the growth of a ZnO nanostructure shell layer using a vapor transport and condensation method. By varying the ZnO growth time from 0.5 to 2 h, different morphologies of ZnO nanostructures, such as ZnO nanoparticles, ZnO shell layer, and ZnO nanorods were grown on the In/Si NWs. The In seeds were believed to act as centers to attract the ZnO molecule vapors, further inducing the lateral growth of ZnO nanorods from the Si/ZnO core-shell NWs via a vapor-liquid-solid mechanism. The ZnO nanorods had a tendency to grow in the direction of [0001] as indicated by X-ray diffraction and high resolution transmission electron microscopy analyses. We showed that the Si/ZnO core-shell NWs exhibit a broad visible emission ranging from 400 to 750 nm due to the combination of emissions from oxygen vacancies in ZnO and In₂O₃ structures and nanocrystallite Si on the Si NWs. The hierarchical growth of straight ZnO nanorods on the core-shell NWs eventually reduced the defect (green) emission and enhanced the near band edge (ultraviolet) emission of the ZnO.     

Growth of ZnO nanowires on carbon fibers for photocatalytic degradation of methylene blue aqueous solutions: An investigation on the optimization of processing parameters through response surface methodology/central composite design

In this work, effects of hydrothermal (HT) synthesis method parameters, temperature, concentration and growth time, on the formation of zinc oxide nanowire structures on carbon fibers (ZnO NWs/CFs) were evaluated. Morphological, structural, photocatalytic properties were determined through scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV–Visible spectrophotometer. In addition, response surface methodology (RSM) and central composite design (CCD) were applied to optimize the hydrothermal synthesis parameters. The results pointed out that, the change in hydrothermal solution concentration (from 3.2 to 37 mM ZnNO_3·6H₂O) and process time (from 2.6 to 9.2 h) lead to the increase in thickness and decrease in aspect ratio of zinc oxide nanowires. Whereas, the temperature increases from 80 to 130 °C had a minute effect on the structure. ZnO nanowires with zincite structure were obtained for all processing conditions. Finally, photocatalytic activity of ZnO NWs/CFs on the degradation of aqueous methylene blue solution (MB) were recorded comparatively. ZnO NWs/CFs structure exhibited photocatalytic activity in the degradation of methylene blue (MB). The most effective structure was obtained at 120 °C, 30 mM Zn(NO₃)_2·6H₂O and 4 h HT synthesis parameters.     

Chemical Precipitation Synthesis and Optical Properties of ZnO/SiO2 Nanocomposites

SiO₂ nanowires, made from natural chrysotile, were used to synthesize zinc oxide (ZnO)/SiO₂ composites by chemical precipitation. The as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy, and Fourier transform infrared. Their optical properties were studied by a ultraviolet-vis spectrophotometer and a fluorescence spectrophotometer. Structural analysis revealed that the crystal size of ZnO crystallites is <20 nm, and the leached SiO₂ nanowires were amorphous. TEM analysis showed that the size of ZnO particles in SZ8 was mainly in the range from 15 to 20 nm and dispersed uniformly on the surface of SiO₂ nanowires. The photoluminescence spectra showed that ZnO/SiO₂ composites have stronger emitting intensity at the blue–green band than pure ZnO synthesized under the same reaction conditions. Therefore, the composites will be of great interest in the application of luminescence material. The as-prepared ZnO/SiO₂ composites can be used as photocatalysts for waste water treatment because they separate much more easily away from solution.     

吸附相反应技术制备纳米Zno/SiO2

采用吸附相反应技术制备ZnO/SiO2纳米复合材料,考察了反应途径、体系中水含量和NaOH浓度对ZnO/SiO2纳米复合材料的影响,并采用透射电子显微镜(TEM)、X-射线衍射(XRD)、酸碱滴定和络合滴定等手段对ZnO/SiO2纳米复合材料进行了表征.结果表明,利用吸附相反应技术能有效调控ZnO粒子粒径,吸附质种类与浓度以及吸附层厚度与性质均会影响粒子的粒径.随着吸附层厚度的增大,粒径逐渐增大;随着物理吸附层的形成以及吸附质浓度增高,粒径逐渐变小.与Ag纳米粒子制备过程对比,发现制备Ag和ZnO粒子的反应机理不同导致两种粒子的形成和生长过程有较大差异.     

ZnO nanowires decoration on carbon fiber via hydrothermal synthesis for paper-based friction materials with improved friction and wear properties

Two-step growth of ZnO nanowires (NWs) on carbon fiber (CF) surface via hydrothermal synthesis was studied and their application in the preparation of paper-based friction materials by wet-forming process was also investigated. SEM and EDS results showed a dense and uniform ZnO NWs layer with vertical alignment was well established on surface of CFs. UV–vis spectra and XRD characterization further confirmed the formation of ZnO NWs on CFs surface. In comparison with control sample (paper-based friction material containing pristine CFs), the modified sample (paper-based friction material containing modified CFs) exhibited higher and more stable dynamic friction coefficient and greater wear resistance. It was concluded that the CFs@ZnO NWs had excellent tribological properties and was highly promising for wet paper-based friction material.     

Preparation and characterization of PS‐PMMA/ZnO nanocomposite films with novel properties of high transparency and UV‐shielding capacity

High transparent and UV‐shielding poly (styrene)‐co‐poly(methyl methacrylate) (PS‐PMMA)/zinc oxide (ZnO) optical nanocomposite films were prepared by solution mixing using methyl ethyl ketone (MEK) as a cosolvent. The films were characterized by X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–vis) spectra, high‐resolution transmission electron microscopy (HR‐TEM), and atomic force microscope (AFM). Cross‐section HR‐TEM and AFM images showed that the ZnO nanoparticles were uniformly dispersed in the polymer matrix at the nanoscale level. The XRD and FTIR studies indicate that there is no chemical bond or interaction between PS‐PMMA and ZnO nanoparticles in the nanocomposite films. The UV–vis spectra in the wavelength range of 200–800 nm showed that nanocomposite films with ZnO particle contents from 1 to 20 wt % had strong absorption in UV spectrum region and the same transparency as pure PMMA‐PS film in the visible region. The optical properties of polymer are greatly improved by the incorporation of ZnO nanoparticles. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Electrodeposition of Cu-doped ZnO nanowire arrays and heterojunction formation with p-GaN for color tunable light emitting diode applications

Copper-doped zinc oxide (ZnO:Cu) nanowires (NWs) were electrochemically deposited at low temperature on fluor-doped tin oxide (FTO) substrates. The electrochemical behavior of the Cu–Zn system for Cu-doped ZnO electrodeposition was studied and the electrochemical reaction mechanism is discussed. The synthesized ZnO arrayed layers were investigated by using SEM, XRD, EDX, photoluminescence and Raman techniques. X-ray diffraction analysis demonstrates a decrease in the lattice parameters of Cu-doped ZnO NWs. Structural analyses show that the nanomaterial is of hexagonal structure with the Cu incorporated in ZnO NWs probably by substituting zinc in the host lattice. Photoluminescence studies on pure and Cu-doped ZnO NWs shows that the near band edge emission is red-shifted by about 5 or 12 nm depending on Cu(II) concentration in the electrolytic bath solution (3 or 6 μmol l⁻¹). Cu-doped ZnO NWs have been also epitaxially grown on Mg doped p-GaN single-crystalline layers and the (ZnO:Cu NWs)/(p-GaN:Mg) heterojunction has been used to fabricate a light-emitting diode (LED) structure. The emission was red-shifted to the visible violet spectral region compared to pure ZnO. The present work demonstrates the ability of electrodeposition to produce high quality ZnO nanowires with tailored optical properties by doping. The obtained results are of great importance for further studies on bandgap engineering of ZnO, for color-tunable LED applications and for quantum well preparation.     

Preparation of nano-ZnO/PMMA composite particles via grafting of the copolymer onto the surface of zinc oxide nanoparticles

The grafting of polymers onto the surface of zinc oxide nanoparticles and radical copolymerization of methyl methacrylate (MMA) and methacrylic acid (MAA) were investigated. The copolymer chains encapsulating nanoparticles were anchored onto the surface of nano-ZnO through reactions of carboxyl groups with ZnO. Grafting percentage and grafting efficiency of composite particles were investigated by employing thermogravimetric analysis (TGA). FT-IR and ¹³C NMR showed that there existed a strong interaction at the interface of nano-ZnO and copolymer, which implied that the copolymer chains were grafted onto the surface of ZnO nanoparticles. Nano-ZnO being encapsulated by copolymer was confirmed by using transmission electron microscopy (TEM). Additionally, TGA plots showed that the presence of ZnO nanoparticles improved the thermal stability of copolymer to a certain extent. Another important finding is the copolymerization and grafting reaction did not alter the crystalline structure of the ZnO nanoparticles according to the X-ray diffraction patterns. It can also be seen from scanning electron microscope (SEM) that grafted polymer chains on nanoparticles interfere with the aggregation of ZnO nanoparticles in polymer matrix and improve their compatibility with the polymeric matrix.     

针状ZnO的纳米纤维素诱导制备及光催化性能

以纳米纤维素（NCC）为形貌诱导剂,乙酸锌为反应前驱物,采用水热法合成针状ZnO。通过X射线衍射仪（XRD）、扫描电子显微镜（SEM）和能量色散X射线光谱仪（EDS）对ZnO的形貌、微观结构、结晶特性进行了表征,并探讨了其形成机理及光催化降解性能。结果表明水热条件下,NCC能诱导ZnO在其表面沉积和生长,可形成直径260nm、长度10μm的针状结构,诱导制备的针状ZnO室温下2h内对孔雀绿的紫外光降解效率为88.5%。针状ZnO在光催化剂等领域具有潜在应用价值。     

Direct conversion of β-Ga2O3 thin films to β-Ga2O3 nanowires by annealing in a hydrogen atmosphere

We report the annealing process of Au/β-Ga₂O₃ thin films in a hydrogen atmosphere leading to a direct conversion of β-Ga₂O₃ thin films to β-Ga₂O₃ nanowires (NWs). Annealing in a hydrogen atmosphere results in the evaporation of β-Ga₂O₃ thin films, which are subsequently converted to β-Ga₂O₃ NWs through the vapor-liquid-solid (VLS) process assisted by Au nanocrystals. The VLS growth starts at 600?°C and progresses with increase in the annealing temperature to 800?°C. β-Ga₂O₃ NWs are formed on the surface of the host β-Ga₂O₃ thin films, resulting in the formation of a homogeneous β-Ga₂O₃ NW/β-Ga₂O₃ thin film structure. Based on structural analyses using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, a possible mechanism for the growth of β-Ga₂O₃ NWs is presented.     

The nitridation of ZnO nanowires

ZnO nanowires (NWs) with diameters of 50 to 250 nm and lengths of several micrometres have been grown by reactive vapour transport via the reaction of Zn with oxygen on 1 nm Au/Si(001) at 550°C under an inert flow of Ar. These exhibited clear peaks in the X-ray diffraction corresponding to the hexagonal wurtzite crystal structure of ZnO and a photoluminescence spectrum with a peak at 3.3 eV corresponding to band edge emission close to 3.2 eV determined from the abrupt onset in the absorption-transmission through ZnO NWs grown on 0.5 nm Au/quartz. We find that the post growth nitridation of ZnO NWs under a steady flow of NH₃ at temperatures ≤600°C promotes the formation of a ZnO/Zn₃N₂ core-shell structure as suggested by the suppression of the peaks related to ZnO and the emergence of new ones corresponding to the cubic crystal structure of Zn₃N₂ while maintaining their integrity. Higher temperatures lead to the complete elimination of the ZnO NWs. We discuss the effect of nitridation time, flow of NH₃, ramp rate and hydrogen on the conversion and propose a mechanism for the nitridation.     

Assorted analytical and spectroscopic techniques for the optimization of the defect-related properties in size-controlled ZnO nanowires

In this article, the important role of the intrinsic defects in size-controlled ZnO nanowires (NWs) which play a critical role in the properties of the NWs, was studied with a combined innovative experimental analysis. The NWs prepared by both the aqueous solution method and chemical vapour deposition process were of increasing length and decreasing size-to-volume (S/V) ratio. The combined approach involved different analytical and spectroscopic techniques and from the correlation between the different measurements, the concentration of the oxygen vacancies jointly with the zinc interstitials defects and the zinc vacancy defects was observed to be positively or negatively correlated, respectively, with the magnitude of the photoluminescence intensity and radiative lifetimes. Furthermore, the experimental results also suggest that the oxygen vacancy defects are not only spatially located on the surface of the NW but an increasing fraction of the total oxygen vacancy defects connected with the green emission is also located in an annulus region beneath the surface as the ZnO NWs elongate. On the other hand, as the donor concentration plays a critical function in the properties of the ZnO NWs, an analytical model was derived for the calculation of the donor concentration of the NWs directly from its reverse-biased current-voltage characteristics obtained from the conductive atomic force microscopy measurements.