Temperature‐Controlled Catalytic Growth of ZnS Nanostructures by the Evaporation of ZnS Nanopowders

ZnS nanostructures with different morphologies, sizes, and microstructures were synthesized by the evaporation of ZnS nanopowders. Based on the appearance of the as‐synthesized products, we show that substrate temperature and catalyst are the critical factors for controlling the size and the structure of various kinds of ZnS nanostructures, such as nanorods, nanowires, nanobelts, and nanosheets. Within a certain temperature range, products with a specific morphology can be obtained. Therefore, it may be possible to obtain ZnS nanostructures with a specific morphology by controlling the reaction temperature and catalyst. This represents an important step toward the design and control of nanostructures. High‐resolution electron microscopy revealed that most of the nanorods and nanowires grew along the [100] direction, whereas most of the nanobelts and nanosheets grew along [001]. Photoluminescence properties and growth mechanisms of these as‐synthesized ZnS nanostructures are discussed.     

Controlled Growth of ZnO Nanowires and Their Optical Properties

This article surveys recent developments in the rational synthesis of single‐crystalline zinc oxide nanowires and their unique optical properties. The growth of ZnO nanowires was carried out in a simple chemical vapor transport and condensation (CVTC) system. Based on our fundamental understanding of the vapor–liquid–solid (VLS) nanowire growth mechanism, different levels of growth controls (including positional, orientational, diameter, and density control) have been achieved. Power‐dependent emission has been examined and lasing action was observed in these ZnO nanowires when the excitation intensity exceeds a threshold (∼40 kW cm^–2). These short‐wavelength nanolasers operate at room temperature and the areal density of these nanolasers on substrate readily reaches 1 × 10¹⁰ cm^–2. The observation of lasing action in these nanowire arrays without any fabricated mirrors indicates these single‐crystalline, well‐facetted nanowires can function as self‐contained optical resonance cavities. This argument is further supported by our recent near‐field scanning optical microscopy (NSOM) studies on single nanowires.     

Generating Blue and Red Luminescence from ZnO/Poly(ethylene glycol) Nanocomposites Prepared Using an In‐Situ Method

The peak of the luminescence spectrum of zinc oxide (ZnO) is usually observed above 500 nm (yellow region). By in‐situ growth of ZnO nanoparticles in a poly(ethylene glycol) (PEG) matrix, we have succeeded in producing ZnO/polymer composites with stable luminescence peaks down to 465 nm (blue region). The unbalanced precursor molarity approach, where the molarity of one precursor (LiOH) is several times larger than the molarity represented by a chemical reaction balance, was used. The blue luminescence, which was accompanied by an enhancement of luminescence intensity, was observed at very high LiOH concentrations. This was probably due to the simultaneous reduction in the crystalline size and improvement in the crystallinity. Doping ZnO nanoparticles with europium also generated a red luminescence at 616 nm, due to the ⁵D₀ →⁷F₂ transition of Eu ions.     

From Layered Basic Zinc Acetate Nanobelts to Hierarchical Zinc Oxide Nanostructures and Porous Zinc Oxide Nanobelts

Novel hierarchical ZnO nanostructures, porous ZnO nanobelts, and nanoparticle chains are prepared from a precursor of synthetic bilayered basic zinc acetate (BLBZA) nanobelts. BLBZA nanobelts are obtained by a simple synthetic route under mild conditions. X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, infrared spectroscopy, and thermal analysis are used to characterize the BLBZA nanobelts and ZnO nanostructures. The obtained BLBZA precursor consists of a lamellar structure with two interlayer distances of 1.33 and 2.03 nm, exhibits a beltlike morphology, and has widths of 200 to 600 nm, thicknesses of 10 to 50 nm, and lengths of up to 50 μm. Refluxing an aqueous dispersion of BLBZA nanobelts at 120 °C for 12 h leads to the formation of well‐defined hierarchical ZnO nanostructures. The time‐dependent shape‐evolution process suggests that spindlelike ZnO particles form first, and then the ringlike nanosheets grow heterogeneously on the backbone of these spindles. In addition, calcination in air can remove ligand molecules and intercalated water molecules from BLBZA nanobelts, resulting in the formation of porous ZnO nanobelts and nanoparticle chains. The BLBZA nanobelts serve as templates during the transformation to form ZnO beltlike nanoparticle chains without morphological deformation. Photoluminescence results show that both the as‐synthesized hierarchical ZnO nanostructures and porous ZnO nanobelts show a narrow and sharp UV emission at 390 nm and a broad blue–green emission at above 466 nm when excited by UV light.     

Halide‐Transport Chemical Vapor Deposition of Luminescent ZnS:Mn2+ One‐Dimensional Nanostructures

ZnS:M²⁺ (M = Mn, Co, or Cu) single‐crystal one‐dimensional nanostructures have been prepared via a simple halide‐transport chemical vapor deposition (HTCVD) process at a relatively low temperature. The obvious phase transition suggests that doping with Mn favors the formation of the hexagonal phase at a relative low temperature. The strong photoluminescence from blue to green and the yellow–orange emission, which was caused by the doping of various elements in ZnS nanowires and nanobelts, suggests possible applications of the one‐dimensional nanostructures in nanoscale optoelectronic devices.     

Co3O4 Nanostructures with Different Morphologies and their Field‐Emission Properties

We report an efficient method to synthesize vertically aligned Co₃O₄ nanostructures on the surface of cobalt foils. This synthesis is accomplished by simply heating the cobalt foils in the presence of oxygen gas. The resultant morphologies of the nanostructures can be tailored to be either one‐dimensional nanowires or two‐dimensional nanowalls by controlling the reactivity and the diffusion rate of the oxygen species during the growth process. A possible growth mechanism governing the formation of such nanostructures is discussed. The field‐emission properties of the as‐synthesized nanostructures are investigated in detail. The turn‐on field was determined to be 6.4 and 7.7 V μm^–1 for nanowires and nanowalls, respectively. The nanowire samples show superior field‐emission characteristics with a lower turn‐on field and higher current density because of their sharp tip geometry and high aspect ratio.     

Well‐Aligned ZnO Nanorods via Hydrogen Treatment of ZnO Films

The formation of well‐aligned ZnO nanorods has been achieved via H₂ treatment of as‐grown ZnO films. Structural analyses reveal that the ZnO nanorods on the ZnO films are preferentially oriented along the c‐axis direction and exhibit a single‐crystalline wurtzite structure. To investigate the mechanism of formation of ZnO nanorods on the film, further H₂ treatment of the as‐grown ZnO nanorods was performed. Thinner and longer ZnO nanorods were obtained after certain periods of H₂ treatment. It is proposed that both etching and re‐deposition processes are taking place during the process, resulting in the aspect‐ratio enhancement of the ZnO nanorods and the formation of ZnO nanorods on the ZnO films. It is suggested that an appropriate concentration of the etching products remaining from the initial rod‐forming H₂ treatment allows subsequent re‐deposition of the ZnO nanorods with enhanced differentiation of the growth rates on the 〈001〉 and 〈100〉 crystal facets.     

Hierarchical Shelled ZnO Structures Made of Bunched Nanowire Arrays

The size‐ and morphology‐controlled growth of ZnO nanowire (NW) arrays is potentially of interest for the design of advanced catalysts and nanodevices. By adjusting the reaction temperature, shelled structures of ZnO made of bunched ZnO NW arrays are prepared, grown out of metallic Zn microspheres through a wet‐chemical route in a closed Teflon reactor. In this process, ZnO NWs are nucleated and subsequently grown into NWs on the surfaces of the microspheres as well as in strong alkali solution under the condition of the pre‐existence of zincate (ZnO₂^2–) ions. At a higher temperature (200 °C), three different types of bunched ZnO NW or sub‐micrometer rodlike (SMR) aggregates are observed. At room temperature, however, the bunched ZnO NW arrays are found only to occur on the Zn microsphere surface, while double‐pyramid‐shaped or rhombus‐shaped ZnO particles are formed in solution. The ZnO NWs exhibit an ultrathin structure with a length of ca. 500 nm and a diameter of ca. 10 nm. The phenomenon may be well understood by the temperature‐dependent growth process involved in different nucleation sources. A growth mechanism has been proposed in which the degree of ZnO₂^2–saturation in the reaction solution plays a key role in controlling the nucleation and growth of the ZnO NWs or SMRs as well as in oxidizing the metallic Zn microspheres. Based on this consideration, ultrathin ZnO NW cluster arrays on the Zn microspheres are successfully obtained. Raman spectroscopy and photoluminescence measurements of the ultrathin ZnO NW cluster arrays have also been performed.     

Optically Functional Zeolites: Evaluation of UV and VUV Stimulated Photoluminescence Properties of Ce3+‐ and Tb3+‐doped Zeolite X

The optical properties of Tb³⁺/Ce³⁺ doped zeolites are elucidated with emphasis on ultraviolet (UV) and vacuum ultraviolet (VUV) excitation and luminescence. Ce³⁺ sensitized Tb³⁺ emission with quantum yields of 85 % may be obtained at 330 nm excitation. Low absorptivity at 254 nm due to low Ce³⁺ concentrations or low Ce³⁺/Tb³⁺ ratios, which are required for the suppression of UV components, restricts their applicability as phosphors for Hg‐based discharges, e.g., in conventional fluorescent lamps. Near band edge excitation at 172 nm revealed an immediate quantum yield of 50 % enabled by a zeolite → Ce³⁺ (5d¹) → Tb³⁺ (4f⁷5d¹) energy transfer channel, which may be exploited for the down‐conversion of the Xe₂ excimer emission.     

1D Tellurium Nanostructures: Photothermally Assisted Morphology‐Controlled Synthesis and Applications in Preparing Functional Nanoscale Materials

A facile visible‐light‐assisted solution‐phase approach has been successfully developed to synthesize trigonal Te 1D nanostructures. By varying the relative amount of H₂TeO₃ and water‐soluble polymers, wirelike, beltlike, tubular Te, and Te nanoparticle‐joined 1D aggregates, as well as a novel thorny 1D assembly of Te nanothreads can be synthesized on a large scale. The diameter of the Te nanowires can be modulated by controlling the nucleation and growth process through modulation of the pH value of the reaction mixture. It is believed that the light irradiation and thermal effect play a significant role in this photothermally assisted technique. We have shown that the Te nanowires can be used as a template to prepare Pt–Te nanochains, where the composition of Pt in the Pt–Te 1D products can be modulated by adjusting the ratio of the Te nanowires and Pt salts. Preliminary optical investigations reveal that blue–violet emission of Te nanowires can be enhanced by the formation of defects or dislocations in the Te region through the galvanic replacement reaction between Te nanowires and H₂PtCl₆. In addition, we demonstrate that Te 1D nanostructures can be utilized to prepare Te at carbon‐rich nanocables and carbonaceous nanotubes. Te–Pt at carbon‐rich nanocables can also be fabricated using Te–Pt nanochains as the template. These Pt–Te nanochains and carbonaceous nanostructures are expected to find wide applications in electrochemistry, catalysis, fuel cells, sensors, and other fields. Furthermore, the successful preparation of Te 1D nanostructures with abundant shapes, Pt–Te nanochains, and their carbonaceous composite nanomaterials will offer great opportunities to explore the dependence of novel properties of nanomaterials on their morphology and composition, regulate the photoconductivity of semiconductors, and also be essential for the manufacture of potential optoelectronic devices.     

Solvent Dependent Shape and Magnetic Properties of Doped ZnO Nanostructures

This study reports on a new solution phase synthesis leading to cobalt and manganese doped ZnO which have been theoretically predicted ferromagnetic at room temperature. The solvothermal synthesis involving the reaction of zinc and cobalt acetate or manganese oleate with benzyl alcohol leads to pure inorganic nanoparticles that are diluted magnetic semiconductors. The addition of an inert solvent, that is used in order to control the amount of benzyl alcohol, drastically influences the particles morphology and strongly affects the magnetic behaviors. Cobalt doped particles are paramagnetic or ferromagnetic depending on the synthesis conditions. In order to exclude the formation of secondary phases and/or metal clusters and to understand the role of the solvent on the magnetic properties, the local structure of Co²⁺ and Mn²⁺ in the wurtzite ZnO matrix were characterized by XRD, UV‐visible diffuse reflectance and electron paramagnetic resonance.     

Co‐synthesis of ZnO–CuO Nanostructures by Directly Heating Brass in Air

ZnO–CuO nanostructures have been simultaneously synthesized by directly heating a CuZn alloy (brass) on a hotplate in ambient conditions. Depending on the Zn concentrations in the brasses, the dominant products transition from CuO nanowires to ZnO nanostructures. By changing the growth temperature and local Zn contents, 1D ZnO nanowires/nanoflakes, 2D ZnO nanosheets, and complicated 3D ZnO networks are obtained. Electron microscopy studies show that the as‐synthesized ZnO nanoflakes and nanosheets are single crystalline. Based on “self‐catalytic” growth, a tip‐growth mechanism for ZnO nanostructures is discussed, in which the Cu in brass plays an important role to confine the lateral growth of ZnO. Finally, the electron field emission from the ZnO–CuO hybrid systems is tested for the demonstration of potential applications.     

Synthesis and Optical Properties of Europium‐Doped ZnS: Long‐Lasting Phosphorescence from Aligned Nanowires

Quasi‐aligned Eu²⁺‐doped wurtzite ZnS nanowires on Au‐coated Si wafers have been successfully synthesized by a vapor deposition method under a weakly reducing atmosphere. Compared with the undoped counterpart, incorporation of the dopant gives a modulated composition and crystal structure, which leads to a preferred growth of the nanowires along the [01 0] direction and a high density of defects in the nanowire hosts. The ion doping causes intense fluorescence and persistent phosphorescence in ZnS nanowires. The dopant Eu²⁺ ions form an isoelectronic acceptor level and yield a high density of bound excitons, which contribute to the appearance of the radiative recombination emission of the bound excitons and resonant Raman scattering at higher pumping intensity. Co‐dopant Cl^– ions can serve not only as donors, producing a donor–acceptor pair transition with the Eu²⁺ acceptor level, but can also form trap levels together with other defects, capture the photoionization electrons of Eu²⁺, and yield long‐lasting (about 4 min), green phosphorescence. With decreasing synthesis time, the existence of more surface states in the nanowires forms a higher density of trap centers and changes the crystal‐field strength around Eu²⁺. As a result, not only have an enhanced Eu²⁺ 4f⁶5d¹–4f⁷ intra‐ion transition and a prolonged afterglow time been more effectively observed (by decreasing the nanowires' diameters), but also the Eu²⁺ related emissions are shifted to shorter wavelengths.     

Size‐ and Shape‐Control of Crystalline Zinc Oxide Nanoparticles: A New Organometallic Synthetic Method

A novel organometallic synthetic method has been developed for the preparation of crystalline ZnO nanoparticles of controlled size and shape. Isotropic nanoparticles with a mean size between 3 and 6 nm and nanorods with a mean diameter of 3–4 nm and length up to 120 nm have been obtained in this way. This synthetic method takes advantage of the exothermic reaction of the precursor Zn(c‐C₆H₁₁)₂ ( 1 ) toward moisture and air and involves the presence of long‐alkyl‐chain amines as stabilizing ligands. The influence of the different experimental parameters (concentration, solvent, nature of the ligand, time, and temperature) on the size and shape of the ZnO nanoparticles has been studied, together with the mechanism of their formation, by NMR spectroscopy, transmission electron microscopy, and X‐ray diffraction techniques. The nanoparticles prepared in this way can be dissolved in most of the common organic solvents, forming colloidal solutions. The surface state of the nanoparticles as well as the possibility of forming luminescent solutions from which regular monolayers can be deposited are also reported.     

ZnO Nanotetrapods: Controlled Vapor‐Phase Synthesis and Application for Humidity Sensing

A systematic study on controlled synthesis of ZnO nanotetrapods by combining metal‐vapor transport, oxidative nucleation/growth, fast‐flow quenching, and water‐assisted cleaning is reported. The technique developed in this work makes possible the fabrication of ZnO nanotetrapods with different morphologies, with arm diameters down to 17 nm, and with arm lengths ranging from 50 nm up to a few micrometers. The octa‐twin model is verified for the growth of the ZnO nanotetrapods. Photoluminescence (PL) studies indicate a higher level of surface and subsurface oxygen vacancies for smaller ZnO nanotetrapods. The ZnO nanotetrapods are first used for the fabrication of resistor‐type humidity sensors, which show high sensitivity, quick response/recovery, long lifetime, and a wide range of humidity response. These favorite characteristics of the humidity sensors are ascribed to the unique morphology of the nanotetrapods, which can create a film with faceted pores and large internal surfaces.     

Growth of Bulk ZnO Single Crystals via a Novel Hydrothermal Oxidative Pressure‐Relief Route

A novel hydrothermal oxidative pressure‐relief (HOPR) route has been successfully developed for the growth of high‐quality bulk ZnO single crystals, using metallic zinc and H₂O₂ as the raw materials, at 400 °C for 20 h in an alkali solvent. X‐ray powder diffraction reveals the ZnO crystals have a wurtzite structure. Two typical morphologies of perfect hexagonal pyramidal and hexagonal prismatic ZnO single crystals, and bidirectional adhesive crystals, are identified by scanning electron microscopy analysis. The average size of the single crystals is ～ 1.0 mm in length and ～ 0.2 mm in diameter. Short hexagonal prismatic, novel polygonal layer‐like, and nanowire ZnO crystals are also obtained by altering the reaction conditions, such as the reaction time and the speed of pressure release. The growth mechanism is a spontaneous nucleation and self‐growth process. This novel HOPR route gives an alternative choice for obtaining well‐crystallized ZnO bulk crystals from solution.     

Lateral Epitaxial Overgrowth of ZnO in Water at 90 °C

Lateral epitaxial overgrowth (LEO) of ZnO has been demonstrated in water at 90 °C. The process starts with hydrothermal epitaxial growth of ZnO(0001) on MgAl₂O₄(111), followed by channel stamping of photoresist to define “growth windows”. LEO films grow in zinc‐precursor solutions at pH 10.9; sodium citrate addition controls out‐of‐plane growth. Transmission electron microscopy indicates threading dislocation reductions from ～ 2 × 10¹⁰ to < 2 × 10⁸ cm^–2 from the window to the wing regions. Microphotoluminescence and Hall‐effect measurements indicate improved material quality. Wing tilt was observed. Double LEO demonstrates the possibility of complete dislocation reduction.     

铟掺杂ZnO体单晶的生长及其性质

研究了In掺杂n型zno体单晶的化学气相传输法生长和材料性质.利用霍尔效应、x射线光电子能谱、光吸收谱、喇曼散射、阴极荧光谱等手段对晶体的特性和缺陷进行r分析.掺In后容易获得浓度为1018～lO19cm-3的n型ZnO单晶,掺人杂质的激活效率很高.随着掺杂浓度的提高,znO单晶的带边吸收和电学性质等发生明显的变化.分析了掺In-ZnO单晶的缺陷及其对材料性质的影响.     

Growth of Wurtzite ZnS Micrometer‐Sized Diskettes and Nanoribbon Arrays with Improved Luminescence

We report on the synthesis of wurtzite ZnS micrometer‐sized diskettes (including those lined up with ZnS nanowires) and ZnS nanoribbon arrays. Using ZnS powder as a source material, a vapor–solid growth based on a two‐stage temperature‐controllable thermal evaporation and condensation process is realized. Significant enhancement of luminescence compared to the ZnS source material is observed from these ZnS micro‐ and nanometer‐sized structures. The structures may serve as ideal model systems in the nano‐ to micrometer range for studying the optical and electronic properties of ZnS material. They can also be treated as prospective building blocks of two‐ and/or three‐dimensional arrays and are promising candidates for fabricating novel electronic and optoelectronic devices.     

基于ZnO纳米结构的H2、NH3气体敏感性研究

通过简单的水热法在金电极上刺备了氧化锌纳米棒,从而制成了气体传感器。研究了氧化锌纳米棒的结构和特性,发现用该材料制成的气体传感器时于500ppm的NH3和H2在150℃下有较灵敏的反应,并探讨了产生传感效应的机理。该传感器制备方法简单、廉价、环保,适合大批量生产,有望应用于工业生产和日常生活的气体探测。