Flame synthesis of tungsten oxide nanostructures on diverse substrates

One-dimensional (1D) tungsten oxide nanostructures show great potential for applications in the areas of batteries, photoelectrochemical water-splitting, electrochromic devices, catalysts and gas sensors. 1D tungsten oxide nanostructures are currently synthesized by physical or chemical vapor deposition, which are limited by low temperatures, the need for vacuum conditions, frequently expensive catalysts, and difficulty in scaling up for mass-production. These limitations, however, can be overcome by flame synthesis. Here, using a co-flow multi-element diffusion burner, we demonstrate the atmospheric, catalyst-free, rapid, mild and scalable flame synthesis of diverse, quasi-aligned, large density, and crystalline tungsten oxide nanostructures on a variety of substrates. Specifically, under fuel-rich conditions, monoclinic 1D W₁₈O₄₉ nanowires and nanotubes were grown on tungsten, iron, steel and fluorinated tin oxide (FTO) substrates, with controlled diameters ranging from 10 to 400 nm and axial growth rates ranging from 2 to 60 μm/h. Monoclinic 1D WO₃ nanowires and nanotubes were grown, instead, on silicon and silicon dioxide substrates. Under fuel-lean conditions, diverse WO₃ nanostructures, including monoclinic 1D nanowires, cubic 2D nanobelts and monoclinic 3D nanocones were grown on tungsten and FTO substrates. The success of this versatile flame synthesis method is attributed to the large tunability of several synthesis parameters, including the flame stoichiometry, the tungsten source and growth substrate temperatures, the tungsten oxide vapor concentration, and the material of the growth substrate. This flame synthesis method can be extended to synthesize other 1D transition metal oxides as well, enabling many large-scale electronic and energy conversion applications.     

Flame synthesis of WO3 nanotubes and nanowires for efficient photoelectrochemical water-splitting

Tungsten trioxide (WO₃) is a technologically important material for photoelectrochemical (PEC) water-splitting for the solar production of hydrogen fuel from water. For PEC water-splitting, high aspect ratio WO₃ nanostructures such as nanowires (NWs) and nanotubes (NTs) are superior to planar WO₃ films because they orthogonalize the directions of light absorption (along the long axis) and charge transport (across the short radius), leading to both efficient light absorption and charge carrier collection. However, PEC water-splitting requires the growth of WO₃ on delicate transparent conducting oxide (TCO) substrates that cannot tolerate high temperature processing. To date, the large-scale, rapid, economical synthesis of high aspect ratio WO₃ nanostructures on these delicate TCO substrates remains a major challenge. Previously, we synthesized WO₃ NW arrays by a rapid, atmospheric and scalable flame vapor deposition (FVD) method, in which a flame oxidizes and evaporates tungsten metal to produce tungsten oxide vapors that condense onto a colder substrate in the form of NWs. Nevertheless, at substrate temperatures low enough to ensure the health of the TCO, the growth of WO₃ NW arrays was non-uniform and sparse due to limitations of the experimental design. Herein, we significantly improve the FVD design to grow uniform and densely packed WO₃ nanostructures on TCO substrates, thereby enabling the application of these WO₃ nanostructures to PEC water-splitting. The morphology of the nanostructures varied from densely packed multi-shell NTs and single-shell NTs to NWs as we increased the substrate temperature in the range 530–700 °C. Importantly, the WO₃ NTs synthesized by FVD had higher areal number density and longer length than state-of-the-art WO₃ NW photoanodes grown by chemical vapor deposition and hydrothermal methods, resulting in stronger light absorption and superior PEC water-splitting performance. Thus, in addition to being scalable, rapid and economical, the FVD method also synthesizes materials of high quality.     

The influence of hydrogen on the growth of gallium catalyzed silicon oxide nanowires

In this paper, we report that amorphous silicon oxide nanowires can be grown in a large quantity by chemical vapor deposition with molten gallium as the catalyst in a flow of mixture of SiH₄, H₂ and N₂ at 600 °C. Meanwhile, when we grow these nanowires under the same conditions but without H₂, octopus-like silicon oxide nanostructures are obtained. The reasons and mechanisms for the growth of these nanowires and nanostructures are discussed. Blue light emission is observed from SiO_x nanowires, which can be attributed to defect centers of high oxygen deficiency. These SiO_x nanowires may find applications in nanodevices and reinforcing composites.     

Field-emission enhancement of molybdenum oxide nanowires with nanoprotrusions

The field-emission properties of molybdenum oxide nanowires grown on a silicon substrate and its emission performance in various vacuum gaps are reported in this article. A new kind of molybdenum oxides named nanowires with nanoscale protrusions on their surfaces were grown by thermal vapor deposition with a length of ~1 μm and an average diameter of ~50 nm. The morphology, structure, composition and chemical states of the prepared nanostructures were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). According to XRD, XPS, and TEM analyses, the synthesized samples were composed of MoO₂ nanowires formed over a thin layer of crystalline Mo₄O₁₁. TEM observation revealed that these nanowires have some nanoscale protrusion on their surface. These nanoprotrusions resulted in enhancement of field-emission properties of nanowires comprising nanoprotrusions. The turn-on emission field and the enhancement factor of this type of nanostructures were measured 0.2 V/μm and 42991 at the vacuum gap of 300 μm, respectively. These excellent emission properties are attributed to the special structure of the nanowires that have potential for utilizing in vacuum nanoelectronic and microelectronic applications.     

Indium oxide, tin oxide and indium tin oxide nanostructure growth by vapor deposition

Indium oxide, tin oxide and indium tin oxide nanowires have been grown by vapor deposition on Si and quartz substrates. Under the growth conditions used, pure SiO_x nanowires, a mixture of SiO_x and indium oxide, tin oxide or indium tin oxide nanostructures, or pure indium oxide, tin oxide or indium tin oxide nanostructures could be obtained at different substrate temperatures. The growth mechanism of the obtained nanostructures at different substrate temperatures is discussed. Optical and electrical properties of the deposited pure indium oxide, tin oxide or indium tin oxide nanostructures have been measured, and low sheet resistances on quartz substrates have been obtained for indium oxide and indium tin oxide nanostructures.     

ZnO–SnO2 branch–stem nanowires based on a two-step process: Synthesis and sensing capability

ZnO–SnO₂ branch–stem nanostructures were realized on a basis of a two-step process. In step 1, SnO₂-stem nanowires were synthesized. In step 2, ZnO-branch nanowires were successfully grown on the SnO₂-stem nanowires through a simple evaporation technique. We have pre-deposited thin Au layers on the surface of SnO₂ nanowire stems and subsequently evaporated Zn powders on the nanowires. The ZnO branches, which sprouted from the SnO₂ stems, had diameters in a range of 30–35 nm. As-synthesized branches were of single crystalline hexagonal ZnO structures. Since the branch tips were comprised of Au-containing nanoparticles, the Au-catalyzed vapor–liquid–solid growth mechanism was more likely to control the growth process of the ZnO branches. To test a potential use of ZnO–SnO₂ branch–stem nanostructures in chemical gas sensors, their sensing performances with respect to NO₂ gas were investigated, showing the promising potential in chemical gas sensors.     

Functional nanowires of tin oxide

Quasi-one-dimensional nanostructures of tin oxide were produced in controlled conditions through condensation from the vapor phase. The preparation was assisted by noble metal catalysts and uniform single-crystalline nanowires were produced. The nucleation of nanowires was achieved at 470 °C, owing to the vapor–liquid–solid growth mechanism activated by the catalytic Pt clusters. The peculiar microstructural properties of these semiconducting metal oxide nanostructures will be summarized. The high aspect ratio and the high degree of crystallinity achieved for the nanowires foresee their functional exploitation. PACS 61.46.Hk; 81.07.Bc; 81.16.Hc; 68.37.Lp     

Controlled growth behavior of chemical vapor deposited Ni nanostructures

Isolation of four distinct nanostructured Ni products is demonstrated in a well-controlled chemical vapor deposition process. These nanostructures include core–shell Ni–NiO nanowires, horizontally oriented nanowires, vertically oriented nanowires, and fully isometric cubic crystals all obtained upon an amorphous SiO₂|Si growth substrate from an identical metal halide precursor. Transmission electron microscopy indicates the horizontally- and vertically-oriented nanowire products to be high-quality single crystals with a preferred growth axis along the ?001? direction while the Ni–NiO core–shell nanowires are polycrystalline metal at the center and surrounded by an outer oxide. The differing crystal structures are reflected in the magnetic response of each nanowire type, as evidenced by magnetoresistance measurements. Detailed discussion of the formation mechanisms leading to each of the four nanostructured Ni products is presented along with a discussion of the general applicability of this non-epitaxial growth process to other material systems.     

InAs/GaAs和InAs/InxGa1-xAs/GaAs纳米线异质结构的生长研究

利用金辅助金属有机化学气相沉淀法(MOCVD)在GaAs(111)B衬底上分别制备了InAs/GaAs和InAs/In _{x Ga1-xAs/GaAs(0≤x≤1)纳米线异质结构.实验结果显示,直接生长在GaAs纳米线上的InAs纳米线生长方向杂乱或者沿着GaAs纳米线侧壁向衬底方向生长,生长的含有In x Ga1-xAs组分渐变缓冲段的InAs/In x Ga1-x关键词： 纳米线异质结构 xGa1-xAs')" href="#">InxGa1-xAs 组分渐变缓冲层 金属有机化学气相沉淀法}     

Synthesis and morphology of silicon oxide nanowires from a free jet activated by electron-beam plasma

Silicon oxide nanowires were synthesized from monosilane–argon–hydrogen mixture by the gas-jet electron-beam plasma chemical deposition method with simultaneous oxygen injection into the vacuum chamber. The synthesis was performed on monocrystalline silicon substrates covered with micron and nanometer tin catalyst particles. The nanowires are formed the via vapor–liquid–solid mechanism in the “catalyst-on-bottom” mode, in which many nanowires grow from one catalyst particle. The process of synthesizing nanowires on a substrate with catalyst consists of three stages: heating to synthesis temperature, hydrogen plasma treatment, and nanowire growth. In the substrate region corresponding to the jet axis, different structures are formed depending on the catalyst particle size. For catalyst particles under 100 nm, there are formed structures of chaotically oriented and interlaced bundles of silica nanowires. For catalyst particles of 0.3–1 micron, there are formed oriented arrays of cylindrically shaped nanowire bundles (“microropes”). Cocoon-like structures are formed for catalyst particles of more than 1 micron.We propose a model of nanowire growth by this method, which is based on nonuniform heating of a catalyst particle by a directed plasma flow. It was found that for synthesis of oriented microrope arrays the initial tin film thickness should be less than 100 nm and the synthesis process should include a hydrogen plasma treatment stage.     

PREPARATION OF NANOWIRES AND MICROARRAYS

We report on the synthesis of one-dimensional (1D) solid nanostructures and ordered microarrays consisting of nanowires and carbon nanotubes. For 1D nanostructures, several synthesis methods, such as, carbothermal reduction of silica xerogels containing carbon nanoparticles, chemical vapor deposition on mesoporous active carbon with transition metal catalyst nanoparticles inside the pores, and simple physical evaporation with the help of transition metal nanoparticles as catalysts, have been used. For microarrays, anodic alumina membranes (AAMs) with highly ordered honeycomb structure were used as templates, chemical vapor reaction inside the nanochannels, Sol-gel technique, and selective electrodeposition in the channels, have been used to prepare microarrays embedded in AAMs.     

Iron Precursor Decomposition in the Magnesium Combustion Flame: A New Approach for the Synthesis of Particulate Metal Oxide Nanocomposites

Powders of Fe–Mg–O nanocomposite particles have been grown using a novel chemical vapor synthesis approach that employs the decomposition of a metalorganic precursor inside the metal combustion flame. After annealing in controlled gas atmospheres composition distribution functions, structure and phase stability of the obtained magnesiowüstite nanoparticles are measured with a combination of techniques such as inductively coupled plasma‐optical emission spectroscopy, energy dispersive X‐ray spectroscopy, X‐ray diffraction, and scanning and transmission electron microscopy. Complementary Mössbauer spectroscopy measurements reveal that depending on Fe loading and temperature of annealing either metastable and superparamagnetic solid solutions of Fe³⁺ ions in periclase (MgO) or phase separated mixtures of MgO and ferrimagnetic magnesioferrite (MgFe₂O₄) nanoparticles can be obtained. The described combustion technique represents a novel concept for the production of mixed metal oxide nanoparticles. Adressing the impact of selected annealing protocols, this study underlines the great potential of vapor phase grown non‐equilibrium solids, where thermal processing provides means to trigger phase separation and, concomitantly, the emergence of new magnetic properties.     

Evolution of ZnO nanostructures by non-catalytic growth process on steel alloy substrate: Structural and optical properties

Various kind of ZnO nanostructures such as nanowires, nanonails and nanocombs were synthesized by the thermal evaporation process onto the steel alloy substrate without the use of metal catalyst or any additives. Detailed structural characterizations indicated that the grown products possess a single crystallinity with the wurtzite hexagonal crystal structure. Presence of strong optical-phonon E₂ mode, in all the cases, presents the good crystallinity with the wurtzite hexagonal phase for the deposited products. Additionally, appearance of dominated, strong and sharp UV emission in the room-temperature photoluminescence spectra confirmed the good optical properties for the grown nanostructures. A vapor–solid growth mechanism has been proposed for the growth of the nanostructures.     

Study of the morphological evolution of ZnO nanostructures on various sapphire substrates

Zinc oxide (ZnO) nanostructures were grown on A-, C- and R-plane sapphires by metal organic chemical vapor deposition (MOCVD) technique. The shape of nanostructures was greatly influenced by the underlying sapphire substrate. Vertical aligned nanowires were observed on A- and C-plane sapphires, whereas the nanopencils were grown on R-plane sapphire. A correlation between the morphological and optical properties of the nanostructures has been established, where the morphological and structural characteristics are responsible for the evolution of optical properties. The nanowires, grown on C-plane sapphires, have shown superior optical properties. Comparatively higher photo-induced wettability transition has also been observed for ZnO nanostructures on R-plane sapphire.     

Structures of nanowires with Zn-ZnO:CuO junctions for detecting ethanol vapors

The synthesis of ZnO-ZnO:CuO structures in the form of overlapping layers of nanowires of pure and copper oxide-doped zinc oxide is described. These structures are tested as ethanol vapor sensors. The following two-stage method is used to form ZnO:CuO nanowires. At the first stage, ZnO nanowires are formed by chemical deposition from a solution. At the second stage, arrays of ZnO nanowires are coated with a copper-containing layer. The CuO content on the surface of ZnO nanowires is changed by varying the number of immersions in a Cu(NO₃)₂ solution. The formed structures are studied by scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray analysis. The interaction of the grown sensor structures with ethanol vapors is analyzed by measuring the potential difference between the layers of pure zinc oxide and copper oxide-modified zinc oxide in the temperature range 190–300°C. The response of the sensor is investigated at various ethanol vapor concentrations and detection temperatures.     

Atomic engineering of oxide nanostructure superlattices

Low-dimensional nanoscale oxide-metal hybrid structures have been fabricated by decoration of a vicinal noble metal surface, Pd(1,1,17), by 1-D and 2-D Mn-oxide nanowires and nanostripes. STM and LEED demonstrate that highly ordered superlattices of Mn-oxide nanostructures can be obtained. The coupling of the oxide nanophases to the metal steps and terraces leads to a mesoscopic stabilization of the step–terrace morphology thus creating nearly perfect nanopatterned oxide systems.     

Room temperature facile synthesis of CuO nanostructures by resistive heating

CuO porous layer and nanowires were in situ grown on Cu wires by a very simple catalyst-free thermal oxidation process based on resistive heating of pure metal wires at ambient conditions. The morphological, compositional and structural characterization of the obtained samples revealed that the nanowires are monoclinic single and bi-crystalline structures with mean diameters of 90–300 nm and typical length in the range 1–5 μm. The effects of the applied voltage values and treatment time on the morphology of the metal substrate and on the size of the nanowires were investigated. Different from the vapor–solid (V–S) mechanism, the growth of nanowires is found to be based on the Cu ion diffusion.     

Morphology,chemical composition,and electrical characteristics of hybrid (Ni-C) nanocomposite structures grown on the van der Waals GaSe(0001) surface

The morphology and chemical composition of metal (Ni), carbon, and composite (Ni-C) nanostructures grown on oxidized and unoxidized (0001) surfaces of a layered GaSe crystal by electron beam vacuum evaporation of the material from a liquid ion source in an electric field have been investigated using atomic force microscopy and X-ray photoelectron spectroscopy. It has been demonstrated that this technology makes it possible to grow nanostructures with different morphologies depending on the growth mode and substrate surface state. Dense homogeneous arrays of nickel nanoparticles (Ni@C) (with geometrical sizes of ～1–15 nm and a lateral density of higher than 10¹⁰ cm^?2) encapsulated into carbon shells, as well as carbon layers (with a thickness of the order of several nanometers), are grown on the unoxidized van der Waals GaSe(0001) surface, whereas Ni-C composite nanostructures are grown on the oxidized surface. The formation of oxide nanostructures on the van der Waals surface and their chemical composition have been examined. Vertical hybrid Au/Ni/(Ni-C)/n-Ga₂O₃(Ni@C)/p-GaSe structures grown on the GaSe(0001) surface contain Ni@C nanoparticles embedded in the wide-band-gap n-Ga₂O₃ oxide. The current-voltage characteristics of these structures at temperatures close to T = 300 K exhibit specific features of the Coulomb blockade effect.     

Temperature-controlled catalytic growth of one-dimensional gallium nitride nanostructures using a gallium organometallic precursor

Catalytic growth of 1-D GaN nanostructures is achieved at temperatures from 550 to 850 °C using NH₃ and gallium acetylacetonate. Structural characterization of the 1-D GaN nanostructures by HRTEM shows that straight GaN nanowires, needle-like nanowires (nanoneedles), and bamboo-shoot-like nanoneedles are synthesized at 750, 650, and 550 °C, respectively. In addition to selecting a proper catalyst, providing sufficient precursors has been demonstrated to be a crucial factor for the low-temperature growth of 1-D GaN nanostructures via the VLS mechanism. Possible mechanisms for forming nanoneedles at low temperatures are proposed. PACS 61.46.+w; 68.65.–k; 81.07.–b     

Comparison between ZnO nanowires grown by chemical vapor deposition and hydrothermal synthesis

Vertically aligned zinc oxide nanowires (NWs) were synthesized by two different techniques: chemical vapor deposition (CVD) and hydrothermal synthesis. To compare the effects of different growth conditions, both F-doped SnO₂ (FTO) coated-glass and silicon wafers were used as substrates. Before NWs growth, all the substrates were covered with a ZnO seed layer film obtained with the same procedure, which acts as nucleation site for the subsequent growth of the nanowires both during CVD and hydrothermal synthesis. We studied the influence of the two synthesis techniques and the growth duration on the final morphology, orientation, and density of the ZnO NWs using electron microscopy and X-ray diffraction, while the NWs optical quality was addressed by UV–Vis spectroscopy. By discussing advantages and disadvantages of both synthesis methods, we finally show that the application purpose often drives the choice of the NWs growth process and the substrate to be used.