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Accurate hydrogen leakage detection is a major requirement for the safe and widespread integration of this fuel in modern energy production devices, such as fuel cells. Quasi-1D nanowires of seven different metal oxides (CuO, WO3, Nb-added WO3, SnO2, ZnO, α-Bi2O3, NiO) were integrated into a conductometric sensor array to evaluate the hydrogen-sensing performances in the presence of interfering gaseous compounds, namely carbon monoxide, nitrogen dioxide, methane, acetone, and ethanol, at different operating temperatures (200–400 °C). Principal component analysis (PCA) was applied to data extracted from the array, demonstrating the ability to discriminate hydrogen over other interferent compounds. Moreover, a reduced array formed by only five sensors is proposed. This compact array may be easily implementable into artificial olfaction systems used in real hydrogen detection applications. [ABSTRACT FROM AUTHOR]

Accurate hydrogen leakage detection is a major requirement for the safe and widespread integration of this fuel in modern energy production devices, such as fuel cells. Quasi-1D nanowires of seven different metal oxides (CuO, WO3, Nb-added WO3, SnO2, ZnO, α-Bi2O3, NiO) were integrated into a conductometric sensor array to evaluate the hydrogen-sensing performances in the presence of interfering gaseous compounds, namely carbon monoxide, nitrogen dioxide, methane, acetone, and ethanol, at different operating temperatures (200–400 °C). Principal component analysis (PCA) was applied to data extracted from the array, demonstrating the ability to discriminate hydrogen over other interferent compounds. Moreover, a reduced array formed by only five sensors is proposed. This compact array may be easily implementable into artificial olfaction systems used in real hydrogen detection applications.

The search for a highly selective ethanol sensor is still an open challenge. Metal oxides still face selectivity issues when it comes to gas sensing especially in case of ethanol. Keeping this in sight, in this work we explore the sensing properties of WO 3 nanowires grown via the aerosol assisted chemical vapour deposition and decorated with CeO 2 nanoparticles by drop casting. The nanocomposites were characterized by X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray, photoluminescence, Raman and X-ray photoelectron spectroscopies. Upon exposure to different gas analytes, the electrical characterization showed that sensors responded well to ethanol both under dry and humid conditions (RH= 50%, 25 °C), though the response towards ethanol decreased in humid atmospheres. The long-term stability was studied, and a gas sensing mechanism is introduced and discussed. [Display omitted] • Tungsten trioxide nanostructures were grown via AACVD and decorated with CeO 2 nanoparticles by drop casting. • Their response to NO 2 , ethanol, H 2 , and CO at different operating temperatures was studied. • An ultrasensitive response towards low concentrations of ethanol was demonstrated. • Nanomaterials were tested under dry and humid conditions, showing excellent long-term stability. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Synthesis of TiO 2 -NiWO 4 /Ni 5 TiO 7 film based catalysts. • Clarification of experimental conditions on the synthesis of TiO 2 -TiWO 4 /Ni 5 TiO 7 catalysts. • High efficiency and long-term stability of TiO 2 -NiWO 4 /Ni 5 TiO 7 catalysts towards tar removal. Abstract One-dimensional nickel catalysts are more promising than nickel powders for catalytic tar removal from biomass gasification, originating from their unique morphologies and higher specific surfaces. Herein, we demonstrate the application of a Ti supported multilayer system, namely a TiO 2 /NiWO 4 -Ni 5 TiO 7 film, for catalytic tar removal. This film is synthesized via plasma electrolytic oxidation (PEO) combined with subsequently impregnation and annealing. The surface morphology and the size of as-synthesized Ni 5 TiO 7 nanowires are determined mainly by used electrolytes during PEO processes. For catalytic tar removal, a fixed bed reactor is utilized and naphthalene is chosen as a model tar. This TiO 2 /NiWO 4 -Ni 5 TiO 7 film exhibits higher efficiency than thermal cracking as well as long-term stability towards catalytic steam reformation of naphthalene. A naphthalene conversion rate of 63% is achieved at 800 °C with a short residence time of 0.34 s and a high tar load of 75 g m N −3. Such a TiO 2 -NiWO 4 /Ni 5 TiO 7 film is thus promising for future tar removal from biomass gasification in the industry. [ABSTRACT FROM AUTHOR]

Abstract One-dimensional metal oxide (MO) micro-wires and nano-wires (MOWs) can be excellent functional units for integrated and transparent electronics. However, MOWs produced using conventional synthesis methods are short, uncontrollable, and randomly-distributed, so they cannot be easily used to fabricate high-density transistor arrays with precisely-controlled MOW-channels. Here, we describe a large-scale direct-printed universal nanoscale MOW electronics which includes highly-aligned, digitally-controlled and arbitrarily-long MOW arrays and various nanoscale applications of MOW field-effect transistors (FETs), neuromorphic synaptic transistors, and gas sensors. Broad classes of pristine, doped and alloyed MOWs are fabricated, so we demonstrated all-MOWFETs composed of conducting indium oxide (In 2 O 3) wires and semiconducting indium zinc oxide (IZO) wires; the devices show a high carrier mobility μ ~17.67 cm2 V−1 s−1, comparable to μ of MO thin-film FETs. MOW synaptic transistors show presynaptic signals dependent postsynaptic behaviors similar to biological synaptic responses; which can be promising nano-electronic units of high-density neuromorphic devices. We also demonstrated MOW gas sensors which show high response to NO 2 gas. Our direct-printed, large-scale, and individually-controlled MOW electronics would be a promising approach in development of industrially-viable MOW electronics and open new horizons for precisely-controlled inorganic MOW electronics and nanoscale printed electronics. Graphical abstract A direct-printed nanoscale metal-oxide-wire (MOW) electronics is demonstrated with highly-aligned, arbitrarily-long and continuous MOW arrays. fx1 Highlights • A highly-aligned, arbitrarily-long, and continuous metal oxide nanowire (MOW) arrays are printed on large area. • Broad classes (pristine, doped and alloyed) of MOWs are demonstrated and characterized. • Universal nanoscale applications (transistors and gas sensors) proves the wide applicability of the direct-printed MOWs. [ABSTRACT FROM AUTHOR]

• The strong dependence of the magnetic properties of Mn nanowires on the step-edge geometry of the vicinal gold surfaces Au(3 3 2) and Au(3 2 2) was found. • Revealed the emergence of two exchange mechanisms (direct and indirect exchange) between manganese atoms, which are dominated either on Au(3 2 2) or on Au(3 3 2). • Found the different magnetic ground states for Mn nanowires on Au(3 3 2) and Au(3 2 2): antiferromagnetic ground state on Au(3 3 2) surface and ferromagnetic ground state on Au(3 2 2) surface. • Found the formation of stable magnetic oxide Mn nanowires on vicinal gold surfaces. • Found the increase of exchange interaction in oxide Mn nanowire compared to pure Mn nanowire due to strong interaction between Mn and O atoms. The development of quantum computing or magnetic storage systems requires the search for magnetic nanostructures with multiple magnetic configurations with controllable quantum spin states. The study of the magnetic properties of metal oxide nanowires is of particular importance for the development of these technological implementations. The present work is devoted to an ab initio study of the magnetic properties of Mn and Mn oxide (MnO) nanowires on vicinal Au surfaces with different step edge geometries. The existence of ferromagnetic and antiferromagnetic configurations of Mn and MnO nanowires on vicinal Au surfaces was found. The effect of oxidations and surface geometry on the appearance of ferromagnetic or antiferromagnetic properties of the nanowire was revealed. The ground state of pure Mn nanowires was shown to be dependent on the surface structure. At the same time, the ground state of oxide Mn nanowires is antiferromagnetic on all studied vicinal surfaces and did not depend on the surface structure. We observed an increase in the exchange interaction energy in nanowires under surface oxidation. The exchange interaction between manganese atoms in MnO nanowires is much stronger than in pure Mn nanowires. [ABSTRACT FROM AUTHOR]

Herein, we report the catalyst assisted growth of TiO2 one-dimensional (1D) nanowires (NWs) on alumina substrates by the thermal oxidation technique. RF magnetron sputtering was used to deposit a thin Ti metallic layer on the alumina substrate, followed by an Au catalytic layer on the Ti metallic one. Thermal oxidation was carried out in an oxygen deficient environment. The optimal thermal growth temperature was 700 °C, in a mixture environment composed by Ar and O2. As a comparison, Ti films were also oxidized without the presence of the Au catalyst. However, without the Au catalyst, no growth of nanowires was observed. Furthermore, the effect of the oxidation temperature and the film thickness were also investigated. SEM, TEM, and EDX studies demonstrated the presence of Au nanoparticles on top of the NWs, indicating that the Au catalyst drove the growth process. Raman spectroscopy revealed the Rutile crystalline phase of TiO2 NWs. Gas testing measurements were carried out in the presence of a relative humidity of 40%, showing a reversible response to ethanol and H2 at various concentrations. Thanks to the moderate temperature and the easiness of the process, the presented synthesis technique is suitable to grow TiO2 NWs for many different applications.

Artificially programming a sequence of organic-metal oxide multilayers (superlattices) by using atomic layer deposition (ALD) is a fascinating and challenging issue in material chemistry. However, the complex chemical reactions between ALD precursors and organic layer surfaces have limited their applications for various material combinations. Here, we demonstrate the impact of interfacial molecular compatibility on the formation of organic-metal oxide superlattices using ALD. The effects of both organic and inorganic compositions on the metal oxide layer formation processes onto self-assembled monolayers (SAM) were examined by using scanning transmission electron microscopy, in situ quartz crystal microbalance measurements, and Fourier-transformed infrared spectroscopy. These series of experiments reveal that the terminal group of organic SAM molecules must satisfy two conflicting requirements, the first of which is to promptly react with ALD precursors and the second is not to bind strongly to the bottom metal oxide layers to avoid undesired SAM conformations. OH-terminated phosphate aliphatic molecules, which we have synthesized, were identified as one of the best candidates for such a purpose. Molecular compatibility between metal oxide precursors and the -OHs must be properly considered to form superlattices. In addition, it is also important to form densely packed and all-trans-like SAMs to maximize the surface density of reactive -OHs on the SAMs. Based on these design strategies for organic-metal oxide superlattices, we have successfully fabricated various superlattices composed of metal oxides (Al-, Hf-, Mg-, Sn-, Ti-, and Zr oxides) and their multilayered structures.

Tungsten oxide nanowires were synthetized by thermal oxidation in vacuum, using a custom tubular furnace, adding niobium in the growth process to evaluate the influence of niobium in the conductometric response towards hydrogen gas. Samples were characterized by XRD and Raman spectroscopy, to confirm the crystalline structure of the material. The conductometric response of fabricated sensors was evaluated towards hydrogen gas. The addition of small percentage of niobium during the synthesis enhance the hydrogen sensing performance of the devices. [ABSTRACT FROM AUTHOR]

Vapor-liquid-solid (VLS) growth process of single crystalline metal oxide nanowires has proven the excellent ability to tailor the nanostructures. However, the VLS process of metal oxides in general requires relatively high growth temperatures, which essentially limits the application range. Here we propose a rational concept to reduce the growth temperature in VLS growth process of various metal oxide nanowires. Molecular dynamics (MD) simulation theoretically predicts that it is possible to reduce the growth temperature in VLS process of metal oxide nanowires by precisely controlling the vapor flux. This concept is based on the temperature dependent "material flux window" that the appropriate vapor flux for VLS process of nanowire growth decreases with decreasing the growth temperature. Experimentally, we found the applicability of this concept for reducing the growth temperature of VLS processes for various metal oxides including MgO, SnO2, and ZnO. In addition, we show the successful applications of this concept to VLS nanowire growths of metal oxides onto tin-doped indium oxide (ITO) glass and polyimide (PI) substrates, which require relatively low growth temperatures. [ABSTRACT FROM AUTHOR]

The article looks at study regarding solution processed in Nanowire field effect transistors (NWFETs) for conventional bottom gate top contacts (BGTC structure) with a multiple nanowire network. Topics discussed include utilization of electrospinning which produce a scalable continuous nanowire network, use of annealing process to remove the polymer matrix, bilayer structure comprising insulating zirconium oxide and electrons can pass through several insulating oxides and semiconducting buffer.

In the present work SnO2 nanowires were synthetized by using vapour-liquid-solid technique (VLS) on alumina substrates, using different metal nanoparticles as catalyst for the growth. In particular, the effect of Au, Sn and Pd nanoparticles was exploited. The growth was performed simultaneously on all substrates, keeping the same conditions for the three different catalysts used. Nanowires were investigated by FE-SEM, HR-TEM, and XRD measurements, confirming the morphology and the crystalline structure of the material. Moreover, conductometric sensing devices were fabricated depositing electrodes on top of these materials, and chemical sensing performances were evaluated toward some typical air pollutants.

Tungsten oxide nanowires have been synthetized by a simple thermal oxidation route in an oxygen atmosphere, at low vacuum pressure. The morphology of the nanostructures was investigated by scanning electron microscope (SEM). Chemical sensors were fabricated using tungsten oxide nanowires as active layer, directly deposited on the transducer. Gas sensing performances were evaluated towards some target chemical species (CO, NO2, NH3, Acetone, Ethanol). Firstly, a temperature screening was performed in presence of a fixed gas concentration, to determine the optimal working temperature of the sensors. Then, calibration curves for some chemical species were estimated. The influence of relative humidity was taken into account. Fabricated devices seem very promising for the detection of ammonia and carbon monoxide. [ABSTRACT FROM AUTHOR]

Abstract: Hybrid metal oxide nanowires (NWs), with small characteristic diameter and large aspect ratio, can have unique and yet tunable chemical, optical and electrical properties by independently controlling the chemical compositions and morphologies of the individual components. Such hybrid NWs are promising building blocks in many applications, such as catalysis, sensors, batteries, solar cells and photoelectrochemical devices. However, these applications are hindered by the lack of scalable and economic methods for the synthesis of hybrid NWs. Here, we report a simple, scalable and new sol-flame method to synthesize various hybrid metal oxide NWs, including nanoparticle-shell decorated NWs (NP-shell@NW), NP-chain decorated NW (NP-chain@NW) and doped NWs. The sol-flame process first coats existing NWs with NPs or dopants precursors prepared by the sol–gel process, and then dissociates/oxidizes these precursors in flame. The sol-flame method uniquely combines the merits of the flame process (e.g., high temperature and fast heating rate) with low temperature sol–gel method (e.g., broad material choices and excellent chemical composition control). For both the NP-shell@NW and NP-chain@NW cases, the high temperature flame, compared to furnace, provides much faster heating rate and shorter duration for annealing, which evaporates and burns the precursor solvent rapidly, causing NPs to quickly nucleate around NWs without significant agglomeration. Hence, higher loading density of NPs with smaller sizes is decorated to the NWs, and the formed hybrid NP@NW exhibits significantly higher catalytic activity than that of the furnace-annealed sample. Similarly when using the sol-flame method to dope NWs, the high temperature flame enables rapid dopant diffusion and short annealing duration that maintains the morphology of the original materials and protects the delicate NW substrates from damage. We believe that the new sol-flame method can be applied to synthesize various 1-D hybrid metal oxide nanostructures, thereby impacting diverse application fields. [Copyright &y& Elsevier]

Abstract: A cheap nanofabrication process for titania (TiO2) polycrystalline nanowire array for gas sensing applications with lateral size ranging from 90 to 180nm, and gas sensing characterizations are presented. Alternatively to typical pattern transfer techniques for submicron fabrication, authors focused on a standard 365nm UV photolithographic process able to fabricate sol–gel nanostructured titania nanowires from a solid thin film. Main aim of present work is the experimental validation of enhanced gas sensing response of nanopatterned metal oxide thin film sensors. Two different kinds of gas sensor with nanopatterned sensitive area have been realized onto silicon substrates and tested towards different EtOH concentrations; experimental tests have been carried out with a contemporary output signals collection from a nanowires-based gas sensor and a second device with solid sensitive film without patterning, in order to validate effects of nanomachining on sensitive material response. [Copyright &y& Elsevier]

Herein, we report the catalyst assisted growth of TiO2 one-dimensional (1D) nanowires (NWs) on alumina substrates by the thermal oxidation technique. RF magnetron sputtering was used to deposit a thin Ti metallic layer on the alumina substrate, followed by an Au catalytic layer on the Ti metallic one. Thermal oxidation was carried out in an oxygen deficient environment. The optimal thermal growth temperature was 700 °C, in a mixture environment composed by Ar and O2. As a comparison, Ti films were also oxidized without the presence of the Au catalyst. However, without the Au catalyst, no growth of nanowires was observed. Furthermore, the effect of the oxidation temperature and the film thickness were also investigated. SEM, TEM, and EDX studies demonstrated the presence of Au nanoparticles on top of the NWs, indicating that the Au catalyst drove the growth process. Raman spectroscopy revealed the Rutile crystalline phase of TiO2 NWs. Gas testing measurements were carried out in the presence of a relative humidity of 40%, showing a reversible response to ethanol and H2 at various concentrations. Thanks to the moderate temperature and the easiness of the process, the presented synthesis technique is suitable to grow TiO2 NWs for many different applications. [ABSTRACT FROM AUTHOR]

An impurity doping in semiconductors is an important irreversible process of manipulating the electrical properties of advanced electron devices. Here, we report an unusual reversible dopant activation/deactivation phenomenon, which emerges at an interface between indium tin oxide (ITO) and single-crystalline oxide channel. We found that the interface electrical resistance between ITO electrodes and single-crystalline oxide nanowire channel can be repeatedly switched between a metallic state and a near-insulative state by applying thermal treatments in air or vacuum. Interestingly, this electrical switching phenomenon disappears when the oxide nanowire changes from the single-crystalline structure to the lithography-defined polycrystalline structure. Atmosphere-controlled annealing experiments reveal that atmospheric oxygen induces repeatable change in the interfacial electrical resistance. Systematic investigations on metal cation species and channel crystallinity demonstrate that the observed electrical switching is related to an interface-specific reversible Sn-dopant activation/deactivation of ITO electrode in contact with a single-crystalline oxide channel.

Surface molecular transformations on nanoscale metal oxides are inherently complex, and directing those reaction pathways is still challenging but important for designing their various applications, including molecular sensing, catalysts, and others. Here, a rational strategy to direct a reaction pathway of volatile carbonyl compounds (nonanal: biomarker) on single-crystalline ZnO nanowire surfaces via molecular modification is demonstrated. The introduction of a methylphosphonic acid modification on the ZnO nanowire surface significantly alters the surface reaction pathway of nonanal via suppressing the detrimental aldol condensation reaction. This is directed by intentionally decreasing the probability of two neighboring molecular activations on the nanowire surface. Spectrometric measurements reveal the correlation between the suppression of the aldol condensation surface reaction and the improvement in the sensor performance. This tailored surface reaction pathway effectively reduces the operating temperature from 200 to 100 °C while maintaining the sensitivity. This is because the aldol condensation product (( E )-2-heptyl-2-undecenal) requires a higher temperature to desorb from the surface. Thus, the proposed facile strategy offers an interesting approach not only for the rational design of metal oxide sensors for numerous volatile carbonyl compounds but also for tailoring various surface reaction pathways on complex nanoscale metal oxides.

Semiconductor photoelectrodes for photoelectrochemical water splitting are at the heart of the area of renewable energy. Among various semiconductors as photoelectrode materials, metal oxide nanowires have attracted exceptional attention due to their favorable band-edge positions, wide distribution of bandgaps, low cost, and outstanding stability. In this study, we develop a general strategy for the preparation of semiconductor-oxide-nanowire photoanodes. Various metal oxide nanowires are grown directly from and on the metal or alloy substrates by thermal oxidation at ultra-low pressure using the corresponding metal or alloy foil as substrates. These metal oxide nanowires and their underlying conductive substrates are used directly as integrated photoanodes, which effectively avoids the detachment of the active light-harvesting materials from the substrates during the photoelectrochemical test. For example, a water photo-oxidation current density of the iron oxide photoanode reaches 0.71 mA/cm2 over a period of 20 h under 100 mW cm−2 illumination at the potential of 1.23 V versus the reversible hydrogen electrode. The successful preparation of these semiconductor-oxide-nanowire photoanodes have important implications for further development of photoelectrodes and highly efficient solar water-splitting devices. Image 1 • A general strategy for the preparation of semiconductor-oxide-nanowire photoanodes. • Various metal oxide nanowires are grown from and on the metal or alloy substrates. • The integrated photoanodes are composed of nanowires and the conductive substrates. • Photoelectrochemical performance of the integrated photoanodes. [ABSTRACT FROM AUTHOR]

Single crystalline nanowires composed of semiconducting metal oxides formed via a vapor-liquid-solid (VLS) process exhibit an electrical conductivity even without an intentional carrier doping, although these stoichiometric metal oxides are ideally insulators. Suppressing this unintentional doping effect has been a challenging issue not only for metal oxide nanowires but also for various nanostructured metal oxides toward their semiconductor applications. Here we demonstrate that a pure VLS crystal growth, which occurs only at liquid-solid (LS) interface, substantially suppresses an unintentional doping of single crystalline SnO 2 nanowires. By strictly tailoring the crystal growth interface of VLS process, we found the gigantic difference of electrical conduction (up to 7 orders of magnitude) between nanowires formed only at LS interface and those formed at both LS and vapor-solid (VS) interfaces. On the basis of investigations with spatially resolved single nanowire electrical measurements, plane-view electron energy-loss spectroscopy, and molecular dynamics simulations, we reveal the gigantic suppression of unintentional carrier doping only for the crystal grown at LS interface due to the higher annealing effect at LS interface compared with that grown at VS interface. These implications will be a foundation to design the semiconducting properties of various nanostructured metal oxides.

Vapor-liquid-solid (VLS) growth process of single crystalline metal oxide nanowires has proven the excellent ability to tailor the nanostructures. However, the VLS process of metal oxides in general requires relatively high growth temperatures, which essentially limits the application range. Here we propose a rational concept to reduce the growth temperature in VLS growth process of various metal oxide nanowires. Molecular dynamics (MD) simulation theoretically predicts that it is possible to reduce the growth temperature in VLS process of metal oxide nanowires by precisely controlling the vapor flux. This concept is based on the temperature dependent "material flux window" that the appropriate vapor flux for VLS process of nanowire growth decreases with decreasing the growth temperature. Experimentally, we found the applicability of this concept for reducing the growth temperature of VLS processes for various metal oxides including MgO, SnO 2 , and ZnO. In addition, we show the successful applications of this concept to VLS nanowire growths of metal oxides onto tin-doped indium oxide (ITO) glass and polyimide (PI) substrates, which require relatively low growth temperatures.

Metal oxide nanowires hold great promise for various device applications due to their unique and robust physical properties in air and/or water and also due to their abundance on Earth. Vapor-liquid-solid (VLS) growth of metal oxide nanowires offers the high controllability of their diameters and spatial positions. In addition, VLS growth has applicability to axial and/or radial heterostructures, which are not attainable by other nanowire growth methods. However, material species available for the VLS growth of metal oxide nanowires are substantially limited even though the variety of material species, which has fascinating physical properties, is the most interesting feature of metal oxides. Here we demonstrate a rational design for the VLS growth of various metal oxide nanowires, based on the "material flux window". This material flux window describes the concept of VLS nanowire growth within a limited material flux range, where nucleation preferentially occurs only at a liquid-solid interface. Although the material flux was previously thought to affect primarily the growth rate, we experimentally and theoretically demonstrate that the material flux is the important experimental variable for the VLS growth of metal oxide nanowires. On the basis of the material flux window concept, we discover novel metal oxide nanowires, composed of MnO, CaO, Sm2O3, NiO, and Eu2O3, which were previously impossible to form via the VLS route. The newly grown NiO nanowires exhibited stable memristive properties superior to conventional polycrystalline devices due to the single crystallinity. Thus, this VLS design route offers a useful guideline for the discovery of single crystalline nanowires that are composed of functional metal oxide materials.