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The effects of gas adsorption and high current on the field emission characteristics of ZnO nanoneedles grown directly from metal electrodes have been systematically investigated. Exposure of nanoneedles to H2, N2, or O2 gas degraded the field emission characteristics, with O2 having the strongest impact. Complete recovery of emission current following H2 and N2 treatment was accomplished by vacuum annealing at 450 °C. Meanwhile, changes induced by O2 adsorption were irreversible. First-principle calculations revealed electronic structure modifications through change of work function and charge density distribution upon gas exposure. It is suggested that the emission current degradation originates from shifts in the Fermi level caused by charge transfer from nanoneedle surface to gas adsorbates. Moreover, field emission degradation has been observed at high currents as a result of surface melting at the nanoneedles apex caused by resistive heating. [ABSTRACT FROM AUTHOR]

Semiconducting and piezoelectric zinc oxide has two important structure characteristics: the multiple and switchable growth directions: 〈01&1sline;0〉, 〈2&11sline;0〉, and 〈0001〉; and the {0001} polar surfaces. The fast growth directions create nanobelts of different crystallographic facets, and the polar surfaces result in bending of the nanobelt for minimizing the spontaneous polarization energy. A combination of these distinct growth characteristics results in a group of unique nanostructures, including several types of nanorings, nanobows, platelet circular structures, Y-shape split ribbons, and crossed ribbons. We present here the as-grown nanoarchitectures naturally created by combining some of the fundamental structure configurations of ZnO, which could be unique for many applications in nanotechnology. [ABSTRACT FROM AUTHOR]

The monolithic catalyst, namely the structured catalyst, is one of the important categories of catalysts used in various fields, especially in catalytic exhaust after-treatment. Despite its successful application in conventional wash-coated catalysts in both mobile and stationary catalytic converters, washcoat-based technologies are facing multi-fold challenges, including: (1) high Pt-group metals (PGM) material loading being required, driving the market prices; (2) less-than ideal distribution of washcoats in typically square-shaped channels associated with pressure drop sacrifice; and (3) far from clear correlations between macroscopic washcoat structures and their catalytic performance. To tackle these challenges, the well-defined nanostructure array (nano-array)-integrated structured catalysts which we invented and developed recently have been proven to be a promising class of cost-effective and efficient devices that may complement or substitute wash-coated catalysts. This new type of structured catalysts is composed of honeycomb-structured monoliths, whose channel surfaces are grown in situ with a nano-array forest made of traditional binary transition metal oxide support such as Al2O3, CeO2, Co3O4, MnO2, TiO2, and ZnO, or newer support materials including perovskite-type ABO3 structures, for example LaMnO3, LaCoO3, LaNiO, and LaFeO3. The integration strategy parts from the traditional washcoat technique. Instead, an in situ nanomaterial assembly method is utilized, such as a hydro (solva-) thermal synthesis approach, in order to create sound structure robustness, and increase ease and complex-shaped substrate adaptability. Specifically, the critical fabrication procedures for nano-array structured catalysts include deposition of seeding layer, in situ growth of nano-array, and loading of catalytic materials. The generic methodology utilization in both the magnetic stirring batch process and continuous flow reactor synthesis offers the nano-array catalysts with great potential to be scaled up readily and cost-effectively. The tunability of the structure and catalytic performance could be achieved through morphology and geometry adjustment and guest atoms and defect manipulation, as well as composite nano-array catalyst manufacture. Excellent stabilities under various conditions were also present compared to conventional wash-coated catalysts. [ABSTRACT FROM AUTHOR]

Monitoring and control of the gaseous combustion process are critically important in advanced energy systems such as power plants, gas turbines, and automotive engines. However, very limited gas sensing solutions are available in the market for such applications due to the inherent high temperature of the combustion gaseous atmosphere. In this study, we fabricated and demonstrated high-performance metal oxide based nanorod array sensors assisted with ultra-violet (UV) illumination for in situ and real-time high-temperature gas detection. Without UV-illumination, it was found that surface decoration of either 5 nm LSFO or 1 nm Pt nanoparticles can enhance the sensitivity over CO at 500 °C by an order of magnitude. Under the 254 nm UV illumination, the CO gas-sensing performance of Ga2O3-based nanorod array sensors was further enhanced with the sensitivity boosted by 125% and the response time reduced by 30% for the La0.8Sr0.2FeO3(LSFO)- decorated sample. The UV-enhanced detection of CO might be due to the increased population of photo-induced electron-hole pairs, whereas for LSFO-decorated nanorod array sensor under UV illumination, the enhancement is through a combination of the sensitizing effect and photocurrent effect. [ABSTRACT FROM AUTHOR]

Composite nanoarchitectures represent a class of nanostructured entities that integrates various dissimilar nanoscale building blocks including nanoparticles, nanowires, and nanofilms toward realizing multifunctional characteristics. A broad array of composite nanoarchitectures can be designed and fabricated, involving generic materials such as metal, ceramics, and polymers in nanoscale form. In this review, we will highlight the latest progress on composite nanostructures in our research group, particularly on various metal oxides including binary semiconductors, ABO3-type perovskites, A2BO4 spinels and quaternary dielectric hydroxyl metal oxides (AB(OH)6) with diverse application potential. Through a generic template strategy in conjunction with various synthetic approaches--such as hydrothermal decomposition, colloidal deposition, physical sputtering, thermal decomposition and thermal oxidation, semiconductor oxide alloy nanowires, metal oxide/perovskite (spinel) composite nanowires, stannate based nanocompostes, as well as semiconductor heterojunction--arrays and networks have been self-assembled in large scale and are being developed as promising classes of composite nanoarchitectures, which may open a new array of advanced nanotechnologies in solid state lighting, solar absorption, photocatalysis and battery, auto-emission control, and chemical sensing. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) ZnO–CuO core-shell nanorod arrays have been synthesized by a three-step process on silicon (100) substrates. A hydrothermal method was used to grow 3D ZnO nanorod arrays, followed by deposition of Cu nanofilm using sputtering, which was oxidized subsequently at 400 °C to form CuO shell surrounding ZnO nanorod core. The control over oxygen flow and pressure during the Cu nanofilm oxidation was found to improve the uniformity and intactness of CuO shell surrounding ZnO nanorod core. With higher oxygen flow, more conformal CuO thin film coating was achieved on the 3D ZnO nanorods array as a result of more sufficient oxygen access at the bottom portion of ZnO nanorods array. Higher pressure during thermal oxidation favored the formation of non-conformal ZnO–CuO core-shell nanorods and impurity Zn2SiO4possibly at the interface of ZnO and Si substrate. UV-vis absorption spectroscopy revealed higher absorption efficiency in the visible region in ZnO–CuO core-shell nanorods than in pure ZnO nanorods. The fabricated ZnO–CuO core-shell nanorods could be useful nanoscale building blocks in solar cells and light emitting devices. [ABSTRACT FROM AUTHOR]

Amorphous silica nanowires generally grow in either a metal-catalyzed vapour–liquid–solid (VLS) process or a solid–liquid–solid (SLS) process. So far, ordered alignment of impurity-free SiOx(1 < x< 2) nanowires has not been achieved. In this study, without using metal catalysts, laterally aligned amorphous silica nanowire arrays have been successfully synthesized on a Si(100) substrate surface at ∼700–1100 °C. These amorphous silica nanowires have diameters of ∼100–350 nm and lengths of ∼1–20 μm, which are uniformly decorated with ∼4–11 nm SiO nanocrystals as a result of possible thermal decomposition of amorphous silica at high temperature. A carbon-assisted and lattice strain driven mechanism might be responsible for the lateral alignment of these SiO nanocrystal decorated amorphous silica nanowires. [ABSTRACT FROM AUTHOR]

Large scale hybrid nanowires consisting of Zn2SnO4periodic nanowires and Ag2O nanoparticles on both nanowire surfaces and tips have been successfully synthesized at 650 °C using a unique one-step silver oxide catalyzed vapor-solid-solid (VSS) growth process. The single crystal Zn2SnO4nanowires mainly grew along the [113] direction with three major morphologies including straight linear, L-shape, and radiative sparse. Two-dimensional nucleation and periodic ledge growth processes were proposed to be responsible for the Zn2SnO4periodical nanowire formation. These Ag2O/Zn2SnO4hybrid nanowires were responsive to ethanol at ∼150 °C upon ∼150 ppm ethanol pulses, with one order of magnitude electrical conductivity decrease. This surprising decrease might be triggered by ethanol pulses associated with Ag2O nanoparticles on nanowire surfaces and electrode-nanowire film interfaces, leading to a catalytic activation of ambient oxygen detection. Ar plasma treatment on the nanowire surface inversely leads to conductivity increase upon ethanol pulses, which suggests a successful plasma removal of catalytic Ag2O and oxygen ions, therefore enabling the detection of ethanol molecules, instead of ambient oxygen. This reversible catalytic ambient ethanol/oxygen detection mechanism enabled by the hybrid nanowire configurations could provide a new path for designing smart gas detection devices compatible with multiple-transient-gas detection. [ABSTRACT FROM AUTHOR]

Solution-processed Mg-alloyed ZnO nanowire arrays have been achieved recently without using high temperature annealing process. By introducing thermal annealing processes in oxygen-rich ambient condition, the UV near-band-edge (NBE) emission was surprisingly mitigated until disappeared with annealing temperature increasing from 400 to 900 °C. As the annealing temperature increased, intensity of UV peak decreased while intensity of visible peak (490–520 nm) increased. The structure evolution upon thermal annealing was revealed to be responsible for these abnormal photoluminescence property variations, where unusual (Zn,Mg)1.7SiO4 epitaxially evolved on ZnMgO nanowires surface and contributed to the quenching of UV NBE emission. The structure evolution induced UV-NBE quenching and nanoscale localized alloying in semiconductor ZnMgO nanowires could bring up opportunities in catalysis, optoelectronics, spintronics, and sensors. [ABSTRACT FROM AUTHOR]

Polar surface dominated ZnO nanopropeller arrays were synthesized by a two-step high temperature solid-vapor deposition process. The axis of the nanopropellers is a straight nanowire along the c axis and enclosed by {2110} surfaces, which grew first; the sixfold symmetric nanoblades are later formed along the crystallographic equivalent a axes (<2110>) perpendicular to the nanowire; and the array is formed by epitaxial growth of nanoblades on the nanowire. The top surface of the nanoblade is the Zn terminated +c plane, showing surface steps and possible secondary growth of nanowires due to higher self-catalytic activity, while the back surface is the oxygen-terminated -c plane, which is smooth and inert. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The electronic band structure of a solid state semiconductor determines many of its physical and chemical characteristics such as electrical, optical, physicochemical, and catalytic activity. Alteration or modification of the band structure could lead to significant changes in these physical and chemical characteristics, therefore we introduce new mechanisms of creating novel solid state materials with interesting properties. Over the past three decades, research on band structure engineering has allowed development of various methods to modify the band structure of engineered materials. Compared to bulk counterparts, nanostructures generally exhibit higher band structure modulation capabilities due to the quantum confinement effect, prominent surface effect, and higher strain limit. In this review we will discuss various band structure engineering strategies in semiconductor nanowires and other related nanostructures, mostly focusing on metal oxide systems. Several important strategies of band structure modulation are discussed in detail, such as doping, alloying, straining, interface and core–shell nanostructuring. [ABSTRACT FROM AUTHOR]