Synthesis Of Metal Oxide Nanowires Research Articles

One-Dimensional Nanostructured Oxide Chemoresistive Sensors.

Day by day, the demand for portable, low cost, and efficient chemical/gas-sensing devices is increasing due to worldwide industrial growth for various purposes such as environmental monitoring and health care. To fulfill this demand, nanostructured metal oxides can be used as active materials for chemical/gas sensors due to their high crystallinity, remarkable physical/chemical properties, ease of synthesis, and low cost. In particular, (1D) one-dimensional metal oxides nanostructures, such as nanowires, exhibit a fast response, selectivity, and stability due to their high surface-to-volume ratio, well-defined crystal orientations, controlled unidirectional electrical properties, and self-heating phenomenon. Moreover, with the availability of large-scale production methods for nanowire growth such as thermal oxidation and evaporation–condensation growth, the development of highly efficient, low cost, portable, and stable chemical sensing devices is possible. In the last two decades, tremendous advances have been achieved in 1D nanostructured gas sensors ever since the pioneering work by Comini on the development of a SnO2 nanobelt for gas sensor applications in 2002, which is one such example from which many researchers began to explore the field of 1D-nanostructure-based chemical/gas sensors. The Sensor Laboratory (University of Brescia) has made major contributions to the field of metal oxide nanowire chemical/gas-sensing devices. Over the years, different metal oxides such as SnO2, ZnO, WO3, NiO, CuO, and their heterostructures have been grown for their nanowire morphology and successfully integrated into chemoresistive gas-sensing devices. Hence in this invited feature article, Sensor Laboratory research on the synthesis of metal oxide nanowires and novel heterostructures and their characterization and gas-sensing performance during exposure to different gas analytes has been presented. Moreover, some new strategies such as branched-like nanowire heterostructures and core–shell nanowire structures adopted to enhance the performance of nanowire-based chemical sensor are presented in detail.

Resistive gas sensors based on metal-oxide nanowires

Gas sensors are indispensable for detecting harmful gases in the environment. The morphology of a gas sensor significantly affects its sensing performance. Among the various morphologies, one-dimensional nanowires (NWs) have numerous advantages, such as high surface area, small dimensions, high charge-carrier concentrations, facile synthesis, high crystallinity, and stability. These excellent properties make NWs promising for gas sensing. Resistive-type metal oxide-based gas sensors are widely used for monitoring various toxic gases and volatile organic compounds. In this tutorial, the synthesis of metal oxide NWs, the fabrication of gas sensors, and their sensing mechanisms are discussed. Different types of NW-based gas sensors, such as single NWs, branched NWs, noble metal-functionalized NWs, heterojunction NWs, self-heating NWs, ultraviolet-activated NWs, core–shell NWs, and electronic-nose-based NWs, are comprehensively presented. Finally, we discuss future directions with regard to the improvement and potential of these NW gas sensors. This tutorial aims to provide an overview of the fundamental principle and state-of-the-art technology, which is useful for researchers and students working in the field of resistive-type NW-based gas sensors.

Localized Temperature and Chemical Reaction Control in Nanoscale Space by Nanowire Array

We introduce a novel method for chemical reaction control with nanoscale spatial resolution based on localized heating by using a well-aligned nanowire array. Numerical and experimental analysis shows that each individual nanowire could be selectively and rapidly Joule heated for local and ultrafast temperature modulation in nanoscale space (e.g., maximum temperature gradient 2.2 K/nm at the nanowire edge; heating/cooling time < 2 μs). By taking advantage of this capability, several nanoscale chemical reactions such as polymer decomposition/cross-linking and direct and localized hydrothermal synthesis of metal oxide nanowires were demonstrated.

Extremely non-equilibrium oxygen plasma for direct synthesis of metal oxide nanowires on metallic substrates

A promising method for the synthesis of metal oxide nanowires is based on the application of the extremely non-equilibrium gaseous environment found in oxygen plasma created by some types of discharges. The kinetic temperature of neutral gas is kept close to the room temperature, the electron temperature is a few eV, the ionization fraction below 10−6 and the dissociation fraction close to 100%. Plasma with such characteristics is obtained using electrodeless high frequency discharges driven by radiofrequency or microwave generators. Plasma parameters such as the electron density and energy distribution function, the Debye length, the dissociation and ionization fractions, the density of negatively charged molecules, the ratio between the positively charged molecules and atoms and the distribution of atoms and molecules over excited states depend on discharge parameters. The most important discharge parameters are the generator power, frequency and coupling, the purity and pressure of working gas and the gas flow, the dimensions of the discharge chamber, the materials facing plasma, the residual atmosphere, and, usually very importantly though often neglected, the properties of the samples mounted into a discharge chamber. Proper construction of the experimental system for the synthesis of metal oxide nanowires allows for almost 100% dissociation fraction and thus extremely rapid growing of nanowires. The particularities of oxygen plasma as well as real-time monitoring of the dissociation fraction are elaborated in this contribution. The lack of reliable experimental results on characterization of extremely non-equilibrium oxygen plasma is stressed.

Novel fabrication method of diverse one-dimensional Pt/ZnO hybrid nanostructures and itssensor application

A novel low-temperature, solution-phase method for the facile fabrication of a variety ofone-dimensional (1D) metal/metal oxide hybrid nanostructures has been developed. Thismethod is based on the wet chemical synthesis of metal oxide nanowires, followed by thesurface coating of metal nanoparticles on metal oxide nanowire templates via reduction ofmetal ions along with controlled etching of metal oxide nanowires at the core, all in alow-temperature liquid environment. As a proof-of-concept, we applied this method to thefabrication of various 1D Pt/ZnO hybrid nanostructures including Pt nanoparticle-coatedZnO nanowires/nanotubes and Pt nanotubes on silicon and polymer substrates. Thediverse morphology tuning is attributed to the control of pH in the solution with differentmetal precursor concentrations and amounts of reducing agent. The change ofmorphology, crystalline structure, and composition of various 1D Pt/ZnO hybridnanostructures was observed by SEM, TEM (HRTEM), XRD and ICP-AES,respectively. Further, we have demonstrated a highly sensitive strain sensor (gauge factor = 15) with a Pt nanotube film fabricated by the developed method on a flexible polymersubstrate.

From nucleation to nanowires: a single-step process in reactive plasmas

This feature article introduces a deterministic approach for the rapid, single-step, direct synthesis of metal oxide nanowires. This approach is based on the exposure of thin metal samples to reactive oxygen plasmas and does not require any intervening processing or external substrate heating. The critical roles of the reactive oxygen plasmas, surface processes, and plasma-surface interactions that enable this growth are critically examined by using a deterministic viewpoint. The essentials of the experimental procedures and reactor design are presented and related to the key process requirements. The nucleation and growth kinetics is discussed for typical solid-liquid-solid and vapor-solid-solid mechanisms related to the synthesis of the oxide nanowires of metals with low (Ga, Cd) and high (Fe) melting points, respectively. Numerical simulations are focused on the possibility to predict the nanowire nucleation points through the interaction of the plasma radicals and ions with the nanoscale morphological features on the surface, as well as to control the localized 'hot spots' that in turn determine the nanowire size and shape. This generic approach can be applied to virtually any oxide nanoscale system and further confirms the applicability of the plasma nanoscience approaches for deterministic nanoscale synthesis and processing.

A Scalable Method for the Synthesis of Metal Oxide Nanowires

We report a novel, scaleable and versatile method for large scale synthesis of tungsten trioxide nanowires and their arrays on a variety of substrates including amorphous quartz, fluorinated tin oxide, etc. The synthesis concept uses the chemical vapor transport of metal oxide vapor phase species onto substrates using air or oxygen flow over hot-filament sources. The hot-filaments were designed to provide uniform heating and gas phase composition over large substrate areas. The results show that the nucleation densities could be varied from 106/cm2 to 1010/cm2 with decreasing substrate temperature. A thermodynamic model for determining the nucleation density and nanowire diameter is presented. The analysis indicates that the nucleation for nanowires occurs through condensation of sub-oxide, WO2 species. This is in contrast to WO3 species condensation into WO3 solid phase for nanoparticle formation. The dispersion behavior of as- synthesized nanowires in aqueous and organic solvents is also studied.

A Scalable Method for the Synthesis of Metal Oxide Nanowires

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Ethylene glycol-mediated synthesis of metal oxide nanowires

A simple and convenient method has been demonstrated for large-scale synthesis of metal oxide (including TiO2, SnO2, In2O3, and PbO) nanowires with diameters around 50 nm and lengths up to 30 µm. In a typical procedure, tetraalkoxyltitanium, Ti(OR)4 (with R = –C2H5, –iso-C3H7, or –n-C4H9), was added to ethylene glycol and heated to 170 °C for 2 h under vigorous stirring. The alkoxide was transformed into a chain-like, glycolate complex that subsequently crystallized into uniform nanowires. Similarly, nanowires made of tin glycolate were synthesized by refluxing SnC2O4·2H2O in ethylene glycol at 195 °C for 2 h, and nanowires consisting of indium and lead glycolates were prepared by adding In(OOCC7H15)(OiPr)2 and Pb(CH3COO)2 to ethylene glycol, followed by heating at 170 °C for 2 h. The nanowires could be readily collected as precipitates after the reaction solutions had been cooled down to room temperature. By calcining at elevated temperatures, each glycolate precursor could be transformed into the corresponding metal oxide without changing the wire-like morphology. Electron microscopic and XRD powder diffraction studies were used to characterize the morphology, crystallinity, and structure of these nanowires before and after calcination at various temperatures. A plausible mechanism was also proposed to account for the one-dimensional growth of such nanostructures in a highly isotropic medium. This mechanism was supported by XRD, FT-IR, solid state 13C-NMR, and TGA measurements. As a demonstration of potential applications, the polycrystalline nanowires made of SnO2 were used as functional components to fabricate sensors that could detect combustible gases (CO and H2) with greatly enhanced sensitivity under ambient conditions.

The synthesis of metal oxide nanowires by directly heating metal samples inappropriate oxygen atmospheres

ZnO, MgO, and GeO2nanowires were successfully synthesized by simply heating the desired metal powderto a temperature above its melting point in a flow of mixed gases (20% O2,80% Ar, with the total flow rate of 120 sccm). Transmission electron microscopyobservations show that as-synthesized products are exclusively nanowires,structurally uniform and single crystalline. The same technique was used tofabricate arrays of ZnO nanowires on silicon substrates, which would be ofparticular interest for direct integration in the current silicon-technology-basedoptoelectronic devices. Based on our experimental results, a metal self-catalyticgrowth mechanism was proposed and described conceptually. Because of theabsence of impurities such as transition metal or noble metal throughout thewhole growth process, the intrinsic properties of the resulting metal oxidenanowires could be expressed and utilized. And with in-depth understanding ofthe growth mechanism, our method could be efficient and controllable inextension to many other low-melting-point metals, such as Al, In, and Sn, for thesynthesis of corresponding metal oxide nanostructures.