Search

This study demonstrates a novel one-dimensional core-shell structure based on the coating of silver nanowires (Ag NWs) with a layer of titanium oxide (TiO2) nanoparticles. This approach for generating core-shell structures is facile and straightforward, utilizing a sol-gel method followed by the crystallization of TiO2 using a simple open-air hydrothermal method. The Ag nanowires are ˜10 μm in length and have a diameter of 100 nm, and their TiO2 polycrystalline shell is 10–15 nm in thickness. These novel structures offer large surface area and high stability, which are qualities that favor gas sensing performance. Gas sensing tests have demonstrated that the generated Ag NWs@TiO2 core-shell nanocomposites exhibit better sensing properties (response, selectivity, optimized working temperature, minimum concentration, and response and recovery time) when compared to sensors containing pure TiO2 nanoparticles. The mechanism of sensing enhancement can be attributed to the Schottky barrier that exists at the interface between the Ag NWs and the TiO2. The Ag core has an excellent conductive property for electronic transfer and further accelerates the oxygen ionization and surface redox reactions. These results may shed light on the design and construction of TiO2-based nanocomposites for gas sensor applications.

It is a big challenge to apply p-type metal oxide semiconductors for gas sensing. This study presents an efficient and rapid approach to detect triethylamine (TEA) by newly generated p-type Co3O4 nanosheets with rich oxygen vacancies. These porous Co3O4 nanosheets were synthesized via a simple but efficient hydrothermal method, followed by annealing process. The defect concentration, highly related to the morphology of the obtained sensing materials, was readily adjustable by controlling annealing temperature. It was found that the presence of abundant oxygen vacancies and the large specific surface area have synergistically promoted gas sensing performance using Co3O4 nanosheets in detecting TEA with a sensitive response (Sg/a = 124.36 for 100 ppm TEA) at an operating temperature (200 °C). Furthermore, the Co3O4 nanosheets have exhibited excellent selectivity, repeatability and stability, probably attributed to the formation of mesoporous structures and appropriate oxygen vacancy concentrations. This work may offer an effective path for designing p-type metal oxide semiconductor nanomaterials for gas sensing applications by controlling oxygen vacancies and particle morphologies.

This study reports a facile but efficient synthesis method for the preparation of titanium oxide coated vanadium oxide (V2O5@TiO2) core-shell nanostructures in water bath at mild temperatures (≤100 °C). This method is featured as short reaction time, no requirement of high temperature calcination (>500 °C) for TiO2 crystallization, easily tunable TiO2 shell thickness, high yield, and good reproducibility. The as-prepared V2O5@TiO2 nanocomposites can exhibit a large surface area, and good stability. The gas-sensing properties of V2O5@TiO2 core-shell nanoparticles were investigated, it was found that the core-shell materials exhibit superior sensing performance (high response, good selectivity, and short response time) toward ammonia than pure TiO2 or V2O5 sensor materials. Besides experimental studies, theoretical simulations using Density Functional Theory (DFT) method were conducted to fundamentally understand the adsorption behavior of different target gases, such as ammonia, ethanol, methanol, acetone and n-butylamine on the anatase TiO2 (101) surface. The results show that the ammonia is of the lowest adsorption energy (−1.04 eV), which can help explain why the V2O5@TiO2 core-shell sensor exhibits excellent response toward ammonia. These findings may bring a new insight into the designing of TiO2-based nanocomposites with diverse functions; meanwhile, provide a better understanding on interactions, at atomic scale, between the TiO2 crystalline surface and the reducing gases.

This study has developed a simple but effective synthesis method to deposit bimetal Au-Pd nanoparticles onto WO3 hierarchical bundle nanowires through an in-situ redox reaction in a reducing aqueous medium. Hydrothermal method was used for the synthesis of WO3 hierarchical bundles ultrathin nanowire subunits, and the mechanisms controlling the segregation behavior and growth based on Na2SO4, citric acid, and ethylene glycol are discussed. It was found that the Pd-WO3 and/or Au-Pd-WO3 nanocomposites decorated with Au 2.4 wt% and Pd 0.48 wt% showed exemplary sensitivity (S) for 50 ppm n-butanol as S = 91 at 200 °C, approximately 14 and 1.4 times better than pristine WO3 (S = 5.7) and Pd-WO3 (S = 59), respectively. Moreover, the Pd-WO3 sensing results demonstrated that the Pd decoration could significantly reduce the operating temperature, similarly to the Au-Pd-WO3 sensor, which is particularly effective for n-butanol gas among common volatile organic compounds (VOCs). The analysis demonstrates that the bimetal Au-Pd decoration have resulted a synergistic effect in boosting the sensitivity of the WO3-based sensor toward n-butanol gas. The underlying sensing mechanism of such an Au-Pd-WO3 hybrid nanostructure is further discussed, based on chemical and electronic sensitizations in promoting the spillover of oxygen species and rectifying Schottky barriers at the interface of noble metal(s) and semiconductor oxide (e.g., WO3). This work may be beneficial for designing a unique Au-Pd-WO3 sensor with great potential to develop oxide-based sensors for detecting other hazardous gases at low temperature.

The detection and monitoring of organic amines commonly found in food and medical industries (e.g. n-butylamine) has attracted increasing interests as they can induce adverse health effects to humans even at low concentrations. In this study, we have demonstrated the facile synthesis of platinum (Pt)-decorated iron vanadate (FeVO4) nanorods through a combined hydrothermal-polyol method, for highly sensitive and selective detection of toxic n-butylamine vapor with fast response/recovery time. The pure and Pt-decorated FeVO4 nanorods were successfully characterized using X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopes (TEM), X-ray photoelectron spectroscopy (XPS), and nitrogen (N2) adsorption-desorption isotherms. The as-synthesized FeVO4 nanorods were highly porous, exhibiting pore diameters in the range of 2–20 nm, with a specific surface area of 21.4 m2/g. Following surface modification, small platinum (Pt) nanoparticles with varying sizes of 2–5 nm were uniformly loaded on the surface of these nanorods. The gas-sensing results reveal that the Pt-decorated FeVO4 nanorods exhibited three times higher response as well as three times faster response/recovery time, along with 5 times higher selectivity toward n-butylamine compared to the pure FeVO4 nanorods, under a relatively low optimum operating temperature of 240 °C. The enhanced sensing performance of the Pt-decorated FeVO4 nanorods is attributed to a few factors, including: (i) the larger surface area exhibited by the Pt-decorated FeVO4 nanorods compared to the pure FeVO4 nanorods (∼1.5 times larger), which may provide more sites for the adsorption of both oxygen and gas molecules, (ii) the increase in surface oxygen content of the FeVO4 nanorods after the Pt modification (based on XPS analysis), which may lead to the formation of more oxygen ions to react with the n-butylamine gas during the sensing process to produce more electrons, thus leading to improved conductivity, and (iii) the electronic and chemical sensitization induced by the deposited Pt nanoparticles. These findings will develop the potential of metal vanadate nanocomposites as highly sensitive and selective gas-sensing materials for detecting organic amines.

The exposed surface facets play an important role in determining the gas-sensing performance of nanostructured materials. This study reports the facile hydrothermal synthesis of zinc oxide nanoflakes with exposed {1 0 1¯ 0} facets, as confirmed by the high resolution transmission electron microscopy (HRTEM) and the corresponding selected area electron diffraction (SAED) analysis. The gas-sensing properties of the ZnO nanoflake sensor were investigated toward toxic n-butylamine, an important marker compound in food and medical industries. The pure ZnO nanoflake sensor exhibits a response of 23.9–50 ppm of n-butylamine at an optimum operating temperature of 300 °C. Density Functional Theory (DFT) simulations were used to study the adsorption behavior of n-butylamine on the ZnO(1 0 1¯ 0) surface. The results show that n-butylamine chemically adsorb on the ZnO(1 0 1¯ 0) surface through the formation of a bond between the nitrogen atom of the n-butylamine (C4H11N) and the surface Zn atom of ZnO. To further improve the gas-sensing properties, the as-prepared ZnO nanoflakes were subsequently loaded with three different quantities of Au (1.37, 2.82, and 5.41 wt% Au). The gas-sensing measurements indicate that the Au nanoparticle-decorated ZnO nanoflakes display superior sensing performance to non-modified ZnO nanoflakes by exhibiting 4–6 times higher response and an improved selectivity toward n-butylamine gas, along a decreased optimum operating temperature of 240 °C. Moreover, the response and recovery properties of the ZnO nanoflake sensor are improved by a factor of 1.5–2.5 depending on the Au loading. The enhanced sensing performance of the Au nanoparticle-decorated ZnO nanoflakes to n-butylamine gas can be attributed to the excellent catalytic activity of Au nanoparticles (NPs) which promotes a greater adsorption of oxygen molecules on the surface of ZnO and the presence of multiple electron depletion layers, specifically at the surface of ZnO and at the ZnO/Au interface, which greatly increases their conductivity upon exposure to the gas.

This study demonstrates the synthesis of self-assembled hexagonal WO3 nanowire bundles, with both ordered and random structures in the presence of SO42− ions, by a simple but efficient solvent-thermal method. The as-synthesized products are composed of hierarchical bundles with assembled ultrathin nanowire subunits. The growth and assembly mechanisms, regarding WO3 nanowire bundles, may be elucidated through oriented attachment or Ostwald ripening growth, based on a synergistic effect from both oxalic acid and ethylene glycol in dehydrating WO3·xH2O to WO3 nanocrystals and in controlling segregation behavior, along with high surface areas (18.4 m2 g-1 for ordered structures while 11.6 m2 g-1 disordered ones), which makes WO3 bundles great potential for gas sensing. To further enhance gas sensing, gold nanoparticles were chosen to deposit on the surface of WO3 nanowire bundles by acting with amino groups linked with WO3 bundles in the HAuCl4 solution with further oxidation. The Au-WO3 composites were examined towards various reducing gases, with exemplary sensitivities of (S = 127, 82), (S = 72, 52) towards n-butanol and acetone, respectively. The sensing mechanisms of such WO3 bundle structures are further understood in detail. This work will make the unique WO3 bundle nanostructures highly potential for gas sensing applications.

Tungsten oxides (stoichiometric and non-stoichiometric) have gained extensive research interests because of promising physicochemical properties and diverse applications. Ultrathin walled tungsten oxide (WO3) nanotubes have been fabricated by hydrothermal route in the presence of both K2SO4 and citric acid under mild conditions. WO3 nanocrystals randomly assembled into ultrathin layered structures with highly peaked end which are then interwoven into nanotubes (diameter of 10−15 nm, wall thickness of 1−2 nm, width of 21 nm), because of Ostwald Ripening growth kinetics. Non-stoichiometric WO3-x nanowires (diameter 10−15 nm) were also synthesized using K2SO4 in the absence of organic ligand. The Brunauer–Emmett–Teller (BET) surface area for h-WO3 nanotubes was 71.95 m2 g−1 higher than WO3-x nanowires (51.83 m2 g−1), WO3 nanowires (42.70 m2 g−1) and nanorods (24.60 m2 g−1). Photoluminescence (PL) spectra show that the h-WO3 nanotubes and WO3-x nanowires have more crystal defects and oxygen vacancies. Time-dependent experiments were conducted and the plausible growth mechanism of h-WO3 nanotubes was proposed. The WO3 nanostructures with different morphologies were used as gas sensors that could detect acetone and ethanol with superior sensing performance (Ra/Rg for WO3 nanotubes = 32 & 26, respectively) under mild conditions, notably attributed to the ultrathin wall with high surface areas, crystal defects and oxygen vacancies.

This study reports facile hydrothermal strategies for the synthesis of novel ternary α-Fe 2 O 3 –ZnO–Au nanocomposites under mild conditions, through further surface coating of ZnO and Au nanoparticles (NPs) on α-Fe 2 O 3 nanorods. The ternary α-Fe 2 O 3 –ZnO–Au nanocomposites are found to show (1) higher sensitivity/responses ( S ) of 113 and 57 toward 100-ppm n -butanol and acetone, respectively compared to single α-Fe 2 O 3 ( S = 11.7, 9.1 for n -butanol, acetone) and binary α-Fe 2 O 3 -ZnO ( S = 54.4, 28 for n -butanol, acetone) sensing materials, and (2) lower optimum operating temperature, i.e., 225 °C. The enhanced sensitivity could be attributed to the chemical sensitization effect induced by the Au NPs, and the existence of conjugated depletion layers in the nanocomposites which promote a greater drop in resistance upon exposure to the gas. These results will be useful for future design of iron oxide-based ternary nanocomposites as gas-sensing materials with high sensitivity, selectivity and stability.

One-dimensional silver vanadate (Ag2V4O11) nanostructures have been prepared by a facile hydrothermal method with no any additives under mild conditions. The pertinent variables for generating such nanostructures were investigated, and found that the composition and crystallinity of Ag2V4O11 could be controlled, e.g., by adjusting solution pH (pH = 1) benefiting for the crystallinity of Ag2V4O11 nanobelts. These nanostructures have high surface-to-volume ratios and could be explored for gas sensing materials. The gas sensing performances of such nanomaterials were tested, and found that the Ag2V4O11 nanobelts showed a relatively high sensitivity towards amine(s), with a low detection limit (5 ppm) and quite good selectivity of amines to ammonia at an optimized working temperature of ∼260 °C. The possible sensing mechanism was finally discussed. This study may offer a promising approach to prepare sensing materials towards detecting and monitoring organic amines for practice applications.

This study represents that a facile and efficient hydrothermal method has been developed for the synthesis of sodium vanadium oxide (Na2V6O16·3H2O) nanobelts with lengths varying from hundreds of nanometres up to a few micrometres, and widths ranging from 80 to 250 nm. These Na2V6O16·3H2O nanobelts transform into Na1.08V3O8 nanorods by calcination at 350 ◦ C for 3 h and this results in a slight shrinkage. This material has been characterized and examined in terms of its gas sensing properties and it was found that the Na1.08V3O8 nanorods exhibit a good sensitivity towards alcohol(s), and acetone at the optimized operating temperature of 260 ◦C. The as-prepared Na1.08V3O8 nanorods displayed a higher sensitivity towards ethanol than pure V2O5 nanobelts and metal oxide-coated/doped V2O5 nanobelts, as a result of the existence of extra Na+ ions and the decreased electronegativity of V ions that can enhance the gas molecule adsorption and hence the sensing performance. Density functional theory (DFT) simulation has also been carried out to understand the sensing mechanism. This study would be useful for developing sodium vanadium oxide nanostructures as potential gas sensor materials. © 2014 Published by Elsevier B.V.

Zinc oxide single crystals with a high percentage of exposed { 4 2 ¯ 2 ¯ 3 ¯ } reactive facet were prepared by a facile hydrothermal route. X-ray diffraction, field emission scanning electron microscopy, and high resolution transmission microscopy confirmed the faceted single crystal structure. Through the density functional theory (DFT) calculations, it is validated that the { 4 2 ¯ 2 ¯ 3 ¯ } surface of ZnO crystals has high surface energy. When used as a sensing material in gas sensors, ZnO crystals with dominating exposed { 4 2 ¯ 2 ¯ 3 ¯ } planes exhibited a superior sensitivity toward toxic and flammable gases.