Tungsten oxide nanowire growth by chemically induced strain

We have investigated the formation of tungsten oxide nanowires under different chemical vapor deposition (CVD) conditions. We find that exposure of oxidized tungsten films to hydrogen and methane at 900 degrees C leads to the formation of a dense array of typically 10 nm diameter nanowires. Structural and chemical analysis shows that the wires are crystalline WO3. We propose a chemically driven whisker growth mechanism in which interfacial strain associated with the formation of tungsten carbide stimulates nanowire growth. This might be a general concept, applicable also to other nanowire systems.     

Growth of large-area aligned molybdenum nanowires by high temperature chemical vapor deposition: synthesis, growth mechanism, and device application

Large-area aligned Mo nanowires have been grown on stainless steel substrates by high-temperature chemical vapor deposition with the use of Mo metal. The detailed physical and chemical growth processes regarding the formation of the nanowires have been investigated using mass spectroscopy, thermogravimetry, and differential scanning calorimetry analysis, as well as structure analysis by electron microscopy. In reference to Gibbs energy calculation, our study reveals that the growth relies on the decomposition of MoO2 vapors through condensation of its vapor at high substrate temperatures. The aligned growth is a result of competing growth with the nanowires normal to the substrate surface reaching the final growth front. The field emission measurement and the vacuum luminescent tube study show that the Mo nanowires have potential application as electron emitters.     

A simple hydrothermal method for the large-scale synthesis of single-crystal potassium tungsten bronze nanowires

The large-scale synthesis of single-crystal K(x)WO(3) tungsten bronze nanowires has been successfully realized by a hydrothermal method under mild conditions. Uniform K(0.33)WO(3) nanowires with diameters of 5-25 nm and lengths of up to several micrometers are obtained. It is found that the morphology and crystallographic forms of the final products are strongly dependent on the sulfate and citric acid, which may act as structure-directing and soft-reducing agent, respectively. Some other influential factors on the growth of tungsten bronze nanowires, such as temperature and reaction time, are also discussed. It is worth noting that other alkali metal tungsten bronzes such as (NH(4))(x)WO(3), Rb(x)WO(3), and Cs(x)WO(3) could also be selectively synthesized by a similar route. Thus, this novel and efficient method could provide a potential mild route to selectively synthesize various tungsten bronze on-dimensional nanomaterials.     

单分散铁氧体纳米颗粒的生长机制与成分偏聚的透射电子显微研究

理解纳米晶的生长机制对单分散纳米晶的可控合成至关重要。本文以热分解法制备的双金属铁氧体(钴铁氧和锰铁氧)纳米颗粒为例,利用透射电子显微镜(TEM)系统研究了铁氧体纳米晶的生长机制,揭示了由此造成的成分偏聚现象。对不同时间阶段的反应产物的分析结果表明,两步加热法(即先后在相对低的温度和相对高的温度下加热反应)是制备高质量的单分散铁氧体纳米晶的关键;通过控制低温反应阶段的时间可实现纳米晶的形核阶段和生长阶段的有效分离,从而有利于单分散纳米晶的合成。利用扫描透射电子显微镜(STEM)及电子能量损失谱(EELS)谱学成像技术分析,我们进一步发现了双金属铁氧体纳米晶中的成分偏聚现象,表明双金属铁氧体纳米晶在形核阶段主要形成富Fe的核芯,而在生长阶段则形成更富Co/Mn的双金属铁氧体壳层。这些结果对制备高质量的单分散铁氧体纳米晶具有重要的指导意义,同时也有助于正确理解热分解法制备的铁氧体纳米晶的表面成分和相关表面物理化学性质。     

Direct synthesis of gallium oxide tubes,nanowires, and nanopaintbrushes

We demonstrate bulk synthesis of highly crystalline beta-gallium oxide tubes, nanowires, and nanopaintbrushes using molten gallium and microwave plasma containing a mixture of monatomic oxygen and hydrogen. Gallium oxide nanowires were 20-100 nm thick and tens to hundreds of micrometers long. Transmission electron microscopy (TEM) revealed the nanowires to be highly crystalline and devoid of any structural defects. Results showed that multiple nucleation and growth of gallium oxide nanostructures could easily occur directly out of molten gallium exposed to an appropriate composition of hydrogen and oxygen in the gas phase. These gallium oxide nanostructures should be of particular interest for optoelectronic devices and catalytic applications.     

Carbothermal chemical vapor deposition route to Se one-dimensional nanostructures and their optical properties

A simple and practical carbothermal chemical vapor deposition route has been developed for the growth of trigonal phase selenium nanowires and nanoribbons. In detail, the mixture of active carbon and selenium was heated for the chemical reaction to occur, followed by thermal evaporation and decomposition into elemental selenium. The as-prepared sample was characterized by X-ray diffraction, transmission electron microscopy, high-resolution electron microscopy, UV-vis absorption, and photoluminescence. The results show that trigonal Se nanowires have uniform diameters ranging from 20 to 60 nm and grow along the [001] direction, with the same growth direction found for nanoribbons. Spectral measurements suggest a large blue shift and two types of electron transition activity. The influences of experimental conditions on morphologies and growth processes are also discussed. This synthetic method should be able to be extended to grow other one-dimensional chalcogens and chalcogenides nanostructures.     

Aqueous-solution growth of GaP and InP nanowires: a general route to phosphide, oxide, sulfide, and tungstate nanowires

A general synthetic route has been developed for the growth of metal phosphide, oxide, sulfide, and tungstate nanowires in aqueous solution. In detail, cetyltrimethylammonium cations (CTA(+)) can be combined with anionic inorganic species along a co-condensation mechanism to form lamellar inorganic-surfactant intercalated mesostructures, which serve as both microreactors and reactants for the growth of nanowires. For example, GaP, InP, gamma-MnO(2), ZnO, SnS(2), ZnS, CdWO(4), and ZnWO(4) nanowires have been grown by this route. To the best of our knowledge, this is the first time that the synthesis of GaP and InP nanowires in aqueous solution has been achieved. This strategy is expected to extend to grow nanowires of other materials in solution or by vapor transport routes, since the nanowire growth of any inorganic materials can be realized by selecting an appropriate reaction and its corresponding lamellar inorganic-surfactant precursors.     

Photochemistry in the Low-Temperature Processing of Metal Oxide Thin Films by Solution Methods

Photochemistry has emerged in the last few years as a powerful tool for the low-temperature processing of metal oxide thin films prepared by solution methods. Today, its implementation into the fabrication procedure makes possible the integration of amorphous semiconductors or functional crystalline oxides into flexible electronic systems at temperatures below 350 °C. In this review, the effects of UV irradiation at the different stages of the chemical solution deposition of metal oxide thin films are presented. These stages include from the synthesis of the precursor solution to the formation of the amorphous metal-oxygen network in the film and its subsequent crystallization into the oxide phase. Photochemical reactions that can be induced in both the solution deposited layer and the irradiation atmosphere are first described, highlighting the role of the potential reactive chemical species formed in the system under irradiation, such as free radicals or oxidizing compounds. Then, the photochemical effects of continuous UV light on the film are shown, focusing on the decomposition of the metal precursors, the condensation and densification of the metal-oxygen network, and the nucleation and growth of the crystalline oxide. All these processes are demonstrated to advance the formation and crystallization of the metal oxide thin film to an earlier stage, which is ultimately translated into a lower temperature range of fabrication. The reduced energy consumption of the process upon decreasing the processing temperature, and the prospect of using light instead of heat in the synthesis of inorganic materials, make photochemistry as a promising technique for a sustainable future ever more needed in our life.     

Growth and assembly of crystalline tungsten oxide nanostructures assisted by bioligation

Here, we present a systematic study on the influence of the bioligand deferoxamine mesylate on the crystallization and assembly behavior of tungsten oxide in a soft-chemistry process. Without deferoxamine mesylate, this approach yields pseudo-single crystalline tungstite nanoplatelets consisting of a large number of crystallographically almost perfectly aligned primary crystallites. In the presence of a constant amount of deferoxamine, the particle morphology drastically changes with temperature, ranging from wormlike organic-inorganic hybrid nanostructures to single-crystalline tungsten oxide nanowires, highlighting the role of the bioligand in controlling the crystal growth and assembly behavior. The nanowires have a uniform diameter of about 1.3 nm, an aspect ratio of more than 500, and the structural flexibility of tungsten oxide. The presented process is based on the combination of biomimetic construction principles with nonaqueous sol-gel chemistry, thus combining the advantages of both tools, excellent control over particle morphology and high crystallinity at low temperature.     

Particle-wire-tube mechanism for carbon nanotube evolution

The synthesis of carbon nanotubes (CNTs) has been proved to be greatly promoted by vapor metal catalysts, but the fast reaction feature and the required high-temperature environment involved in CNT evolution usually make it difficult for an insight into the evolution mechanism. Here, we successfully freeze the synthetic reaction at intermediary stages and observe the detailed morphologies and structures of the obtained intermediates and various objects related to carbon nanotubes. It is unveiled that there is a kindred evolution linkage among carbon nanoparticles, nanowires, and nanotubes in the vapor catalyst-involved synthetic processes: tiny carbon nanoparticles first form from a condensation of gaseous carbon species and then self-assemble into nanowires driven by an anisotropic interaction, and the nanowires finally develop into nanotubes, as a consequence of particle coalescence and structural crystallization. The function of metals is to promote the anisotropic interactions between the nanoparticles and the structural crystallization. An annealing transformation of carbon nanoparticles into nanotubes is also achieved, which gives further evidence for the evolution mechanism.     

化学气相沉积法制备氧化锡自组装纳米结构

采用化学气相沉积法在镀有5-10 nm厚金膜的SiO2衬底上, 通过控制生长条件, 实现了二氧化锡纳米结构的自组装生长, 成功制备出了莲花状和菊花状的二氧化锡自组装纳米结构. 利用扫描电子显微镜、X射线衍射等表征分析手段对样品的表面形貌、结构及成份进行表征和研究. 并在此基础上, 讨论了两种自组装纳米结构的生长机制.     

Phase separations of single-crystal nanowires grown by self-catalytic chemical vapor deposition method

The fundamentals of phase separations of single-crystal III-V nitride nanowires grown by self-catalytic chemical vapor deposition method have been studied. Experimental tools, such as high resolution transmission electron microscopy and scanning electron microscopy, have been used to characterize the nanowires. The study indicates that nanowires with diameters exceeding about 100 nm undergo phase transitions and/or crystal structure deterioration. The study highlights a relationship between the crystal structure and the kinetics of growth of nanowires.     

Phase-controlled growth of metastable Fe5Si3 nanowires by a vapor transport method

We report the synthesis of single-crystalline nanowires (NWs) of metastable Fe5Si3 phase via an iodide vapor transport method. Free-standing Fe5Si3 NWs are grown on a sapphire substrate placed on a Si wafer without the use of any catalyst. The typical size of the Fe5Si3 nanowires is 5-15 microm in length and 100-300 nm in diameter. Synthesis of the metastable phase is induced by composition-dependent nucleation from the gas-phase reaction. Depending on the concentration ratio of FeI2(g) to SiI4(g), different phases of iron silicides are formed. The growth of nanowires is facilitated by the initial nucleation of silicide particles on the substrate and further self-seeded growth of the NWs. The present work not only provides a method for the synthesis of metastable Fe5Si3 nanowires but also suggests that the phase controlled synthesis can be further optimized to produce other metal-rich silicide nanostructures for future spintronic devices.     

Synthesis and characterization of TiO2@C core-shell nanowires and nanowalls via chemical vapor deposition for potential large-scale production

TiO(2) nanowires and nanowalls core structures covered with carbon shell were selectively synthesized by a simple chemical vapor deposition (CVD) method using commercial titanium powder as the starting material. Morphology and structure of the products were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The core shell structure is composed of single crystalline rutile titanium dioxide wrapped by amorphous carbon shell. By adjusting the growth temperature, morphology of the products can be controlled from one-dimensional nanowires to two-dimensional nanowalls. While TiO(2)@C nanowires were a preferred structure at higher temperature, TiO(2)@C nanowalls dominated the final product at lower temperature. A growth mechanism was proposed based on the initial growth state of these nanostructures, in which solid-state diffusion of the elements involved in the reaction was assumed to play an essential role. The obtained TiO(2)@C core shell structures may find potential applications in various nanoscale realms such as optoelectronic, electronic and electrochemical nanodevices and the simple synthesis procedure promises large scale production and commercialization of the titanium oxide@carbon nanostructures.     

Synthesis and Characterization of ZnO Nanowires

Zinc oxide is a wide bandgap (3.37 eV) semiconductor with a hexagonal wurtzite crystal structure. ZnO prepared in nanowire form may be used as a nanosized ultraviolet light-emitting source. In this study, ZnO nanowires were prepared by vapor-phase transport of Zn vapor onto gold-coated silicon substrates in a tube furnace heated to 900 ?C. Gold serves as a catalyst to capture Zn vapor during nanowire growth. Size control of ZnO nanowires has been achieved by varying the gold film thickness…     

A review of methods for synthesis of nanostructured metals with emphasis on iron compounds

Synthesis of metal nanoparticles with specific properties is a newly established research area attracting a great deal of attention. Several methods have been put forward for synthesis of these materials, namely chemical vapor condensation, arc discharge, hydrogen plasma—metal reaction, and laser pyrolysis in the vapor phase, microemulsion, hydrothermal, sol-gel, sonochemical, and microbial processes taking place in the liquid phase, and ball milling carried out in the solid phase. The properties of metal nanoparticles depend largely on their synthesis procedures. In this paper the fundamentals, advantages, and disadvantages of each synthesis method are discussed.     

Chemically assisted release of transition metals in graphite vaporizers for atomic spectrometry

The processes associated with the vaporization of microgram samples and modifiers in a graphite tube ET AAS were investigated by the example of transition metals. The vapor absorption spectra and vaporization behavior of μg-amounts Cd, Zn, Cu, Ag, Au, Ni, Co, Fe, Mn and Cr were studied using the UV spectrometer with CCD detector, coupled with a continuum radiation source. The pyrocoated, Ta or W lined tubes, with Ar or He as internal gases, and filter furnace were employed in the comparative experiments. It was found that the kinetics of atomic vapor release changed depending on the specific metal–substrate–gas combination; fast vaporization at the beginning was followed by slower ‘tailing.’ The absorption continuum, overlapped by black body radiation at longer wavelengths, accompanied the fast vaporization mode for all metals, except Cd and Zn. The highest intensity of the continuum was observed in the pyrocoated tube with Ar. For Cu and Ag the molecular bands overlapped the absorption continuum; the continuum and bands were suppressed in the filter furnace. It is concluded that the exothermal interaction of sample vapor with the material of the tube causes the energy evolution in the gas phase. The emitted heat is dispersed near the tube wall in the protective gas and partially transferred back to the surface of the sample, thus facilitating the vaporization. The increased vapor flow causes over-saturation and gas-phase condensation in the absorption volume at some distance from the wall, where the gas temperature is not affected by the reaction. The condensation is accompanied by the release of phase transition energy via black body radiation and atomic emission. The particles of condensate and molecular clusters cause the scattering of light and molecular absorption; slow decomposition of the products of the sample vapor–substrate reaction produces the ‘tailing’ of atomic absorption signal. The interaction of graphite with metal vapor or oxygen, formed in the decomposition of metal oxide, is the most probable source of chemical energy, which facilitates the vaporization. Intensity of the process depends on chemical properties of the sample and substrate and efficiency of mass and heat transfer by the protective gas. The discussed mechanism of chemically assisted vapor release signifies the energy exchange between all participants of the vaporization process in ET AAS including the matrix, modifier, purge gas and analyte. The finding contributes in the ET AAS theory regarding the mechanisms of vaporization and mass transfer in the presence of matrix and modifiers.     

A green route for microwave synthesis of sodium tungsten bronzes NaxWO3 (0<x<1)

A green route has been developed for microwave synthesis of sodium tungsten bronzes Na_xWO₃ (0<x<1) from Na₂WO₄, WO₃ and tungsten powder. The hybrid microwave synthesis was carried out in argon atmosphere using CuO powder as the heating medium. Tungsten powder is used as the reducing agent instead of the alkali metal iodides previously used for the microwave synthesis of oxide bronzes. The prepared samples were characterized by powder X-ray diffraction, energy-dispersive X-ray analysis and scanning electron microscopy, and their phase constitutions, crystal structures and morphologies are in consistence with that in the literature. This synthesis method is simple, green and atom economic, and promising for preparation of other oxide bronzes and related compounds.     

桥连氧化钨纳米线的可控合成及气敏性质

The rapid development of industrialization has resulted in severe environmental problems. A comprehensive assessment of air quality is urgently required all around the world. Among various technologies used in gas molecule detection, including Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, mass spectroscopy (MS), electrochemical sensors, and metal oxide semiconductor (MOS) gas sensors, MOS gas sensors possess the advantages of small dimension, low power consumption, high sensitivity, low production cost, and excellent silicon chip compatibility. MOS sensors hold great promise for future Internet of Things (IoT) sensors, which will have a profound impact on indoor and outdoor air quality monitoring. The development of nanotechnology has significantly enhanced the development of MOS gas sensors. Among various nanostructures like nanoparticles, nanosheets and nanowires, the emergence of quasi-one-dimensional (q1D) nanowires/nanorods/nanofibers, with unique q1D geometry (facilitating fast carrier transport) and large surface-to-volume ratio, potentially act as ideal sensing channels for MOS sensors with extremely small dimension, and good stability and sensitivity. These structures have thus been the focus of extensive research. Among the various MOS nanomaterials available, tungsten oxide (WO_3-x, 0 ≤ x < 1) nanowires feature the characteristic properties (multiple oxidation states, rich substoichiometric oxides with distinct properties, photo/electrochromism, (photo)catalytic properties, etc.), and unique q1D geometry (single-crystalline pathway for fast carrier transport, large surface-to-volume ratio, etc.). WO_3-x nanowires have broad applications in smart windows, energy conversation & storage, and gas sensing devices, and have thus become a focus of attention. In this paper, the fundamental properties of tungsten oxide, synthesis methods and growth mechanism of tungsten oxide nanowires are reviewed. Among various (vapor-liquid-solid (VLS), vapor-solid (VS) and thermal oxidation) growth methods, the thermal oxidation method enables an in situ integration of WO_3-x nanowires on predefined electrodes (so-called bridged nanowire devices) via the oxidation of lithographically patterned W film at relatively low growth temperature (~500 ℃) because of interfacial strain, defects and oxygen on the surface of the W film. The novel bridged nanowire-based sensor devices outperform traditional lateral nanowire devices in terms of larger exposure area, low power consumption via self-heating, and greater convenience in device processing. Recent progress in bridged WO_3-x nanowire devices and sensitive NO_x molecule detection under low power consumption have also been reviewed. Power consumption of as low as a few milliwatts was achieved, and the detection limit of NO₂ was reduced to 0.3 ppb (1 ppb = 1 × 10^-9, volume fraction). In situ formed bridged WO_3-x nanowire devices potentially satisfy the strict requirements of IoT sensors (small dimension, low power consumption, high integration, low cost, high sensitivity, and selectivity), and hold great promises for future IoT sensors.     

Understanding the Metal-Directed Growth of Single-Crystal M-TCNQF(4) Organic Nanowires with Time-Resolved, in Situ X-ray Diffraction and First-Principles Theoretical Studies

The deterministic growth of oriented crystalline organic nanowires (CONs) from the vapor-solid chemical reaction (VSCR) between small-molecule reactants and metal nanoparticles has been demonstrated in several studies to date; however, the growth mechanism has not yet been conclusively understood. Here, the VSCR growth of M-TCNQF(4) (where M is Cu- or Ag-) nanowires is investigated both experimentally and theoretically with time-resolved, in situ X-ray diffraction (XRD) and first-principles atomistic calculations, respectively, to understand how metals (M) direct the assembly of small molecules into CONs, and what determines the selectivity of a metal for an organic vapor reactant in the growth process. Analysis of the real-time growth kinetics data using a modified Avrami model indicates that the formation of CONs from VSCR follows a one-dimensional ion diffusion-controlled tip growth mechanism wherein metal ions diffuse from a metal film through the nanowire to its tip where they react with small molecules to continue growth. The experimental data and theoretical calculations indicate that the selectivity of different metals to induce nanowire growth depends strongly upon effective charge transfer between the organic molecules and the metal. Specifically, the experimental finding that Cu ions can exchange and replace Ag ions in Ag-TCNQF(4) to form Cu-TCNQF(4) nanowires is explained by the significantly stronger chemical bond between Cu and TCNQF(4) molecules than for Ag, due to the strong electronic contribution of Cu d-orbitals near the Fermi level. Understanding how to control the VSCR growth process may enable the synthesis of novel organic nanowires with axial or coaxial p/n junctions for organic nanoelectronics and solar energy harvesting.