Synthesis and optical properties of pencil-like and shuttle-like ZnO microrods

The pencil-like and shuttle-like ZnO microrods have been fabricated on Si (100) substrates by chemical vapor deposition. Structure characterization results show that the microrods are perfect single crystals with the wurtzite structure along the [0001] growth direction and have diameters ranging from 100 nm to 2 μm and lengths up to 10 μm. Room-temperature photoluminescence measurements of ZnO microstructures exhibit an intensive ultraviolet peak at 390 nm and a broad peak centered at about 526 nm, which can be attributed to the free exciton emission and the deep level emission, respectively. Cathodoluminescence measurements show the same ultraviolet and green emissions as seen in the photoluminescence results. A possible growth mechanism of ZnO microrods is finally proposed.     

Hierarchical ZnO microrods: synthesis,structure, optical and photocatalytic properties

Hierarchical ZnO microrods have been prepared by cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal process at low temperature. Sheetlike or fluffy ZnO nanostructures have been developed in a large density on the surface of the microrods. The ZnO cores grow preferentially along [001]; the nanosheets are single crystalline and grow in two dimensions along the [001] and [120] directions. The formation of hierarchical structures has been attributed to the presence of CTAB in the solution, which facilitates the secondary nucleation and growth. The nanosheet-covered hierarchical microrods have been analyzed by Raman and photoluminescence spectroscopy, and they exhibit a blue emission at 465 nm and a strong orange emission at 640 nm. Photocatalytic activities of the products have been examined, and the nanosheet-covered hierarchical microrods display the best activity in photodegradation of phenol due to their unique surface features and high surface area.     

Room-temperature synthesis, photoluminescence and photocatalytic properties of SnO nanosheet-based flowerlike architectures

Flowerlike architectures constructed with SnO nanosheets were synthesized by a reaction of SnCl₂⋅2H₂O with NaOH in water at room temperature. The diameters of the flowerlike architectures are in the range of 7 to 9 μm. The constituent nanosheets with diameters of 3 to 5 μm and the thicknesses of 200 to 500 nm are single crystalline with tetragonal architecture. A possible mechanism was also proposed to account for the formation of the flowerlike superstructure. The as-synthesized SnO nanosheet-based flowerlike architectures display a strong emission at 390 nm and a very weak emission at 482 nm and a high photocatalytic activity in the photodegradation of malachite green.     

制备一维可控形貌的ZnO微米棒

用化学溶液法以醋酸锌和六亚甲基四胺为原料在玻璃衬底上生长出不同形貌的亚微米和微米ZnO棒。探讨了反应液的酸碱度和反应液浓度对生成的ZnO棒形貌的影响，并分析了其生长机制。通过控制一定的酸碱度和溶液浓度，可以得到规则的六角ZnO棒状阵列。这种规则的六角棒沿着[002]方向生长。测量了样品的XRD，扫描电镜像(SEM)，并对其发光性能进行了测量分析。其中规则六角ZnO棒的光致发光光谱中有一很强的峰值650nm红色宽谱带和一峰值约387nm的激子发光峰。激子发光峰加宽，实际上是自由激子的发光峰(380nm)和Zni的发光峰(430nm)的叠加。而红色发光峰可能是Vo2+中的电子和价带中的空穴辐射复合所致。     

Excitation wavelength dependent photoluminescence intensity of six faceted prismatic ZnO microrods

This article presents the investigation on the large-scale synthesis of ZnO microrods with a simple low temperature hydrothermal method without using surfactants, organic solvents, or catalytic reagents. The synthesized ZnO powder is characterized with different techniques. The X-ray diffraction study reveals the excellent crystal quality of the ZnO product possessing the hexagonal (wurtzite-type) crystal structure. The scanning electron microscope observation confirms the formation of six faceted prismatic hexagonal ZnO microrods with the aspect ratio of 10. It also reveals that the ZnO microrods grow along the (0 0 0 1) direction and finally emerge with a sharp tip because of the existence of polar faces. The UV–vis spectrum shows a sharp absorption peak centered at 370 nm, which is in a good agreement with the equivalent bulk band gap value. The strong UV absorption peak implies the excellent crystal quality of the synthesized ZnO microrods. Room temperature photoluminescence spectroscopic study of the ZnO microrods with different excitation wavelengths reveals a strong band edge emission peak centered at 398 nm and a defect related visible blue emission peak at 460 nm. The decrease in photoluminescence intensity with negligible red shift in peak position is observed with increasing excitation wavelength.     

Shuttle-like ZnO nano/microrods: Facile synthesis, optical characterization and high formaldehyde sensing properties

Shuttle-like ZnO nano/microrods were successfully synthesized via a low temperature (80 °C), “green” (without any organic solvent or surfactant) and simple hydrothermal process in the solution of zinc chloride and ammonia water. X-ray diffraction and Raman spectroscopy indicated that the ZnO nano/microrods are a well-crystallized hexagonal wurtzite structure. Yet photoluminescence analysis showed that abundant intrinsic defects (52.97% electron donor defects and 45.49% electron acceptor defects) exist on the surface of ZnO crystals. Gas sensors based on the shuttle-like ZnO nano/microrods exhibited high sensitivity, rapid response-recovery and good selectivity to formaldehyde in the range of 10-1000 ppm at an optimum operating temperature of 400 °C. Through applying linear fitting to the plot of sensitivity versus formaldehyde concentration in logarithmic forms, the chemisorbed oxygen species on the ZnO surface were found to be O²⁻ (highly active among O₂, O₂⁻ and O⁻ species). Notably, formaldehyde can be easily distinguished from acetaldehyde with a selectivity of about 3. The high formaldehyde sensitivity is mainly attributed to the synergistic effect of abundant electron donor defects (52.97%) and highly active oxidants (surface adsorbed O²⁻ species) co-existed on the surfaces of ZnO.     

Pressure dependence of the near-band-edge photoluminescence from ZnO microrods at low temperature

The temperature and pressure dependences of band-edge photoluminescence from ZnO microrods have been investigated. The energy separation between the free exciton (FX) and its first order phonon replica (FX-1LO) decreases at a rate of k_BT with increasing temperature. The intensity ratio of the FX-1LO to the bound exciton (BX) emission is found to decrease slightly with increasing pressure. All of the exciton emission peaks show a blue shift with increasing pressure. The pressure coefficient of the FX transition, longitudinal optical (LO) phonon energy, and binding energy of BX are estimated to be 21.4, 0.5, and 0.9 meV/GPa, respectively.     

Solvothermal synthesis, characterization and optical properties of ZnO, ZnO-MgO and ZnO-NiO, mixed oxide nanoparticles

ZnO-MgO and ZnO-NiO mixed oxides nanoparticles were produced from a solution containing Zinc acetate, Mg and Ni nitrate by Solvothermal method. The calcination process of the ZnO-MgO and ZnO-NiO composites nanoparticles brought forth polycrystalline two-phase ZnO-MgO and ZnO-NiO nanoparticles of 40-80 nm in diameters. ZnO, MgO and NiO were crystallized into würtzite and rock salt structures, respectively. The optical properties of ZnO-MgO and ZnO-NiO nanoparticles were obtained by solid state UV and solid state florescent. The XRD, SEM and Raman spectroscopies of these nanoparticles were analyzed.     

Photoluminescence and field emission properties of Sn-doped ZnO microrods

Sn-doped ZnO (SZO) microrods have been fabricated by a thermal evaporation method. Effect of Sn dopant on the microstructure, morphological and composition of as-prepared SZO microrods have been investigated by X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. The influence of the doping concentration on the morphological of the microrods has been investigated. Photoluminescence (PL) of these SZO microrods exhibits a weak ultraviolet (UV) emission peak at around 382 nm and the strong green emission peak at around 525 nm at room temperature. Field emission measurements demonstrate that the SZO possess good performance with a turn-on field of ∼1.94 V/μm and a threshold field of ∼3.23 V/μm, which have promising application as a competitive cathode material in FE microelectronic devices.     

Sonochemical synthesis, photocatalytic activity and optical properties of silica coated ZnO nanoparticles

In this paper, we report the synthesis of silica coated ZnO nanoparticles by ultrasound irradiation of a mixture of dispersion of ZnO, tetraethoxysilane (TEOS), and ammonia in an ethanol-water solution medium. The silica coating layer formed at the initial TEOS/ZnO loading of 0.8 for 60 min ultrasonic irradiation was uniform and extended up to 3 nm from the ZnO surface as revealed from HR-TEM images. Silica coated ZnO nanoparticles demonstrated a significant inhibition of photocatalytic activity against photodegradation of methylene blue dye in aqueous solution. The effects of silica coating on the UV blocking property of ZnO nanoparticles were also studied.     

Template-free hydrothermal synthesis of ZnO microrods for gas sensor application

Zinc oxide microrods with controlled diameter were prepared without the addition of template and additive by a simple hydrothermal route only using Zn(CH₃COO)₂·2H₂O as a precursor. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electron diffraction (ED). The crystal structure of prepared ZnO microrods is hexagonal phase polycrystalline with zincite structure. With the increase of the precursor concentration from 0.05 M to 0.6 M, the diameter of the ZnO microrods increased from 1 μm to 5 μm. A localized oriented attachment mechanism was prepared to account for the formation of ZnO microrods. The gas-sensing performance experiments indicated that the prepared ZnO microrods exhibited highly sensitive, selective gas-sensing properties, and good stability to acetone vapor. The response and recovery time of ZnO-based gas sensor to 100 ppm acetone vapor are 12 s and 18 s, respectively. The mechanism of the ZnO-based sensor was investigated.     

ZnO micro- and nano-structures: microwave-assisted solvothermal synthesis,morphology control and photocatalytic properties

ZnO with various morphologies have been prepared by a simple microwave-assisted solvothermal method in a short period of time. In this synthetic method, Zn(CH₃COO)₂⋅2H₂O is used as the reactant, a mixture of water and ethylene glycol is used as solvents, and no other additives are used. The morphology of the prepared samples can be controlled by changing the mixing ratio of ethylene glycol to water. The photocatalytic activities of the prepared samples are also investigated.     

Structural, morphological and photoluminescence properties of W-doped ZnO nanostructures

W-doped ZnO nanostructures were synthesized at substrate temperature of 600 °C by pulsed laser deposition (PLD), from different wt% of WO₃ and ZnO mixed together. The resulting nanostructures have been characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy and photoluminescence for structural, surface morphology and optical properties as function of W-doping. XRD results show that the films have preferred orientation along a c-axis (0 0 L) plane. We have observed nanorods on all samples, except that W-doped samples show perfectly aligned nanorods. The nanorods exhibit near-band-edge (NBE) ultraviolet (UV) and violet emissions with strong deep-level blue emissions and green emissions at room temperature.     

Luminescence anisotropy of ZnO microrods

The local features of light emission from ZnO microrods were studied: it is revealed that ZnO luminescence spectra are significantly influenced by the crystal morphology. It is shown that the near and edge ultraviolet emission occurs primarily from the top (0001) planes of ZnO microrods; while the defect related visible emission was found to occur dominantly from the side facets. The room temperature cathodoluminescence analysis revealed that visible emission consists of a few overlapping peaks, arising due to recombination on common points and surface defects (Zn_i, V_o, V_o⁰/V_o^?*_, V_o^** and surface defects.). While at low temperature, only the luminescence due to neutral donor bound exciton (D⁰X) emission is observed. The data obtained suggest that the light emission spectra of ZnO material of diverse morphology cannot be directly compared, although some common spectral features are present.     

Solvothermal synthesis and characterization of sandwich-like graphene/ZnO nanocomposites

Graphene-based nanocomposites are emerging as a new class of materials that hold promise for many applications. In this paper, we present a general approach for the preparation of sandwich-like graphene/ZnO nanocomposites in ethylene glycol (EG) medium using graphene oxide as a precursor of graphene and zinc acetylacetonate as a single-source precursor of zinc oxide. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy and thermogravimetry analysis. It was shown that the as-formed ZnO nanoparticles with a diameter of about 5 nm were densely and uniformly deposited on both surfaces of the graphene sheets to form a sandwich-like composite structure and as a result, the restacking of the as-reduced graphene sheets was effectively prevented. The ZnO-coated graphene nanocomposites can be expected to effectively improve the photocatalysis and sensing properties of ZnO and would be promising for practical applications in future nanotechnology.     

Controllable synthesis and shape-dependent photocatalytic activity of ZnO nanorods with a cone and different aspect ratios and of short-and-fat ZnO microrods by varying the reaction temperature and time

ZnO nanorods with a cone and different aspect ratios and short-and-fat ZnO microrods were synthesized via a hydrothermal reaction of Zn with Zn(CH₃COO)₂ and H₂O. The control over these ZnO nanocrystals with a wurtzite structure and different shapes was achieved by adjusting only the reaction temperature and time. A possible kinetic mechanism was proposed to account for the growth of these ZnO nanocrystals with different shapes. Photocatalytic activities of ZnO nanocrystals with distinctive shapes in the degradation of methyl orange were investigated. The results indicate that the photocatalytic ability of the ZnO nanorods with a cone and different aspect ratios is stronger than that of the short-and-fat microrods.     

Large-scale synthesis of ZnO nanowires using a low-temperature chemical route and their photoluminescence properties

Single-crystalline zinc oxide (ZnO) nanowires were synthesized from zinc powder and H₂O through a simple chemical route at 730 °C in Ar atmosphere. The potential exists for bulk synthesis of ZnO nanowires at temperatures significantly less than the 200–300 °C of thermal evaporation methods reported formerly. Scanning electron microscopy and transmission electron microscopy observations reveal that the ZnO nanowires are structurally uniform, have lengths up to several hundreds of micrometers and diameters of about 40–60 nm and crystallize in a hexagonal structure. The growth of ZnO nanowires is controlled by the vapor–solid crystal-growth mechanism. Photoluminescence measurements show that the ZnO nanowires have a strong near-band ultraviolet emission at 380 nm and a green light emission at 520 nm caused by oxygen vacancies. PACS 81.05.Ys; 78.55.Et     

Synthesis and photoluminescence properties of ZnO nanorods and nanotubes

Different morphologies of zinc oxide (ZnO) nanorods and nanotubes, which were grown under the same conditions but different dissolving processes, are prepared in our experiment through hydrothermal method. After the growth process, cooling down the reactor naturally or dissolving at a constant temperature of 40 °C, preferential dissolution will occur at different places on the tip of ZnO nanorods. During the dissolution process, different dissolution rates on the entire surface of nanorod will lead to different nanostructures. ZnO nanorods and nanotubes on Cu substrates display the same PL property with strong green emission but weak UV emission, while ZnO nanorods on Si substrates exhibits a relatively strong UV emission.     

Structural, optical and magnetic properties of Cr doped ZnO microrods prepared by spray pyrolysis method

A series of Cr-doped ZnO micro-rod arrays were fabricated by a spray pyrolysis method. X-ray diffraction patterns of the samples showed that the undoped and Cr-doped ZnO microrods exhibit hexagonal crystal structure. Surface morphology analysis of the samples has revealed that pure ZnO sample has a hexagonal microrod morphology. From X-ray photoelectron spectroscopy studies, the Cr 2p3/2 binding energy is found to be 577.3 eV indicating that the electron binding energy of the Cr in ZnO is almost the same as the binding energy of Cr³⁺ states in Cr₂O₃. The optical band gap E_g decreases slightly from 3.26 to 3.15 eV with the increase of actual Cr molar fraction from x = 0.00 to 0.046 in ZnO. Photoluminescence studies at 10 K show that the incorporation of chromium leads to a relative increase of deep level band intensity. It was also observed that Cr doped samples clearly showed ferromagnetic behavior; however, 2.5 at.% Cr doped ZnO showed remnant magnetization higher than that of 1.1 at.% and 4.6 at.% Cr doped samples, while 4.6 at.% Cr doped ZnO samples had a coercive field higher than the other dopings.