Effect of Hexagonal WO3 Morphology on NH3 Sensing (original) (raw)
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Hydrothermal synthesis and NH 3 gas sensing property of WO 3 nanorods at low temperature
Advances in Natural Sciences: Nanoscience and Nanotechnology, 2015
One-dimensional self-assembled single-crystalline hexagonal tungsten trioxide (WO 3) nanostructures were synthesized by wet chemical-assisted hydrothermal processing at 120°C for 24 h using sodium tungstate and hydrochloric acid. Urchin-like hierarchical nanorods (petal size: ∼16 nm diameter and 110 nm length) were obtained. The samples were characterized by field emission scanning electron microscopy, transmission electron microscopy, energy dispersive xray spectroscopy and x-ray diffraction. Sensors based on WO 3 nanorods were fabricated by coating them on SiO 2 /Si substrate attached with Pt interdigitated electrodes. NH 3 gas-sensing properties of WO 3 nanorods were measured at different temperatures ranging from 50°C to 350°C and the response was evaluated as a function of ammonia gas concentration. The gassensing results reveal that WO 3 nanorods sensor exhibits high sensitivity and selectivity to NH 3 at low operating temperature (50°C). The maximum response reached at 50°C was 192 for 250 ppm NH 3 , with response and recovery times of 10 min and 2 min, respectively.
RSC Advances, 2014
We report on the surface interaction between NH 3 and WO 3 nanoparticles having different exposed surfaces or different porous structure, to identify the relative importance of exposed crystal surfaces, porous architecture, and specific surface area in the oxide sensing properties. WO 3 nanocrystals with tailored morphology and definite prominent surfaces were synthesized by hydrothermal reactions. In parallel, inverted opal macroporous WO 3 films have been prepared by a one-step sol-gel procedure, and WO 3 hierarchical layers have been obtained by an innovative one-step dual-templating strategy which leads to macropores and mesopores simultaneously. The performances of WO 3 samples in NH 3 sensing, indicate that high-energy surfaces result in a significant improvement of the electrical response. Enhanced porous structure and high surface area are not enough to produce high electrical response, while their synergistic combination with tailored crystal faceting appears effective. XPS survey performed on shape controlled WO 3 nanocrystals demonstrated that, upon interaction with NH 3 , oxidized nitrogen atoms represent the prevalent species on the surface of rectangular (WO 3-RE) nanocrystals with highly exposed high-energy {020} and {002} facets. Conversely, in the case of rectangular platelets (WO 3-SS) and square platelets (WO 3-RS) with very low surface area of high-energy surfaces, N-H surface groups are predominant. These results suggest that {020} and {002} crystal surfaces provide privileged reactive sites for ammonia oxidation and therefore they play a key role in driving the sensing properties of the WO 3 layers.
Fine-tuning of gas sensitivity by modification of nano-crystalline WO3 layer morphology
Sensors and Actuators B: Chemical, 2015
The effect of WO 3 nano-crystal characteristic size and layer morphology on gas sensitive properties was investigated in order to define the optimum preparation process. WO 3 layers were synthesized by hydrothermal acidic precipitation method using different chemicals and reactive sputtering as reference. Micro-hotplate based conductivity type devices were fabricated and the sensitivity on NH 3 up to 100 ppm was measured in the temperature range of 140-240 • C. The measurements revealed that the characteristic size of the WO 3 nano-crystal plays primary role, but layer morphology opens the way towards extended measuring range. The nano-rod structures operated at 220 • C exhibit the best sensing characteristics in terms of sensitivity and stability over wide range of relative humidity.
Procedia Engineering, 2011
Nanosturctured WO 3 films were synthesized via anodization at elevated temperature. Nitric acid (HNO 3 ) and sulfuric acid (H 2 SO 4 ) were employed for the first time. Different nanostructured morphologies were observed. The novel conductometric H 2 gas sensor has been developed based on these WO 3 films. The sensor with anodized WO 3 film in HNO 3 exhibited high sensitivity towards H 2 as low as 0.06% at relatively low operating temperature (120 º C).
Synthesis and Sensing Properties to NH 3 of Hexagonal WO 3 Metastable Nanopowders
Materials and Manufacturing Processes, 2007
WO 3 is an important kind of wide-bandgap semiconducting metal oxides, which has a very promising property in gas-detection behavior. It has several polymorphs with triclinic, monoclinic, orthorhombic structures being the stable forms of this oxide. However, by a method known as acid precipitation, new metastable open crystalline forms with hexagonal structure have been successfully synthesized. The nanopowders were characterized by SEM, TEM, and XRD, and their sensing response to reducing gases (NH 3 ) was measured and compared to monoclinic WO 3 , showing a much better sensing property of the hexagonal WO 3 .
Low-temperature hydrothermal synthesis of WO3 nanorods and their sensing properties for NO2
Journal of Materials Chemistry, 2012
Tungsten trioxide (WO 3 ) nanorods with an aspect ratio of $50 have been successfully synthesized by hydrothermal reaction at a low temperature of 100 C. The crystal structure, morphology evolution and thermal stability of the products are characterized in detail by XRD, FESEM, FTIR, and TG/ DTA techniques. The diameter evolution and distribution of WO 3 nanorods strongly depend on hydrothermal temperature and time. Hydrothermal conditions of 100 C and 24 h ensure the formation of well-defined WO 3 nanorods. The transition of the crystal structure from monoclinic WO 3 to hexagonal WO 3 occurs after calcination at 400 C. The appropriate calcination conditions of the WO 3 nanorods are defined to be 600 C and 4 h for gas-sensing applications. Response measurements reveal that the WO 3 sensor operating at 200 C exhibits high sensitivity to ppm-level NO 2 and small crosssensing to CO and CH 4 , which makes this kind of sensor a competitive candidate for NO 2 -sensing applications. Moreover, impedance measurements indicate that a conductivity mechanism of the sensor is mainly dependent on the grain boundaries of WO 3 nanorods. A possible adsorption and reaction model is proposed to illustrate the gas-sensing mechanism. † Electronic supplementary information (ESI) available: Gas-sensing test apparatus; TEM, HRTEM and SAED images of as-synthesized nanorods; and FESEM images of the WO 3 nanorods after calcination at different temperatures. See
SENSING OF AMMONIA, DIMETHYL-AND TRIMETHYLAMINE USING WO3 NANOMATERIALS DEPOSITED ON MICROHOTPLATES
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WO 3 nanoparticles obtained by sol-gel and WO 3 nanostructures obtained from hard nanotemplates with/without added Cr or Cu have been drop-deposited onto microhotplates. The gas sensing properties towards ammonia, dimethyl-and trimethylamine have been tested in both, steady and pulsed, modes. The measured electrical response will be discussed as a function of the synthesis method, catalytic additive and measuring procedure and the mechanisms taking place at the surface of the nanostructured WO 3
Current Applied Physics, 2011
The large-scale nanowire-like (NW) structure of tungsten oxide is synthesized by the deposition of tungsten metal on the substrate of porous single-wall carbon nanotubes (SWCNTs) film, followed by thermal oxidation process. The morphology and crystallinity of the synthesized materials are analyzed by SEM, TEM, XRD, and Raman spectroscopy. Results showed that tungsten oxide NWs deposited on SWCNTs have a porous structure with an average diameter of about 70 nm and a length of up to micrometers. The NH 3 gas-sensing properties of tungsten NWs were measured at different temperatures. A maximum response of 9.7e1500 ppm at 250 C with rapid response and recovery times of 7 and 8 s are found, respectively. In addition, the gas sensing mechanism of fabricated NWs is also discussed in term of surface resistivity and barrier height model.
Synthesis, FTIR studies and sensor properties of WO3 powders
Current Opinion in Solid State and Materials Science, 2007
Several synthetic approaches were used to obtain nano-sized porous and nonporous monoclinic WO 3 (m-WO 3) powders. All of these methods begin with a standard preparative method where H 2 WO 4 is first generated by passing a Na 2 WO 4 solution through a cationexchange resin. It is shown that high surface area particles are produced by dripping the H 2 WO 4 exiting from the ion-exchange column into a solution containing oxalate and acetate exchange ligands or alternatively, into a water-in-oil (W/O)-based emulsion. Porous materials are produced using surfactant-templating architectures. The surface properties were investigated by IR spectroscopic studies during thermal evacuation and the use of chemical probes. The nature of the surface depends on the initial evacuation temperature of the WO 3 surface as this alters the relative number of the Lewis and Brønsted acid sites along with the amount of adsorbed water. Infrared studies of the adsorption of various molecules on the powders led to a new size-selective approach to improve selectivity in semiconducting metal oxide (SMO) sensors. The key for achieving high selectivity is based on using a dual sensor configuration where the response on a porous WO 3 powder sensor was compared to the response on a nonporous WO 3 powder sensor. Detection selectivity between methanol and dimethyl methylphosphonate (DMMP) is obtained because the access of a gas molecule in the interior pore structure of WO 3 is sizedependent leading to a size-dependent magnitude change in the conductivity of the SMO sensor.