A hydrogen gas sensor based on Pt/nanostructured WO3/SiC Schottky diode

Pt/nanostructured WO3/SiC Schottky diodes were fabricated and applied for hydrogen gas sensing applications. The nanostructured WO3 films were synthesized from tungsten coated SiC substrates via an acid-etching method using a 1.5 M HNO3 solution for 1 hr, 2 hrs and 3 hrs duration. Scanning electron microscopy of the developed films revealed platelet crystals with thicknesses in the order of 20-60 nm and lengths between 100-700 nm. X-ray diffraction analysis revealed that the rate of oxidation of tungsten increases as the duration of acid-etching increases. The devices were tested towards hydrogen gas balanced in air at different temperatures from 25°C to 200°C. At 200°C, voltage shifts of 0.45 V, 0.93 V and 2.37 V were recorded for devices acid-etched for 1 hr, 2 hrs and 3 hrs duration, respectively upon exposure to 1% hydrogen, under a constant forward bias current of 500 µA.

Low temperature CO sensitive nanostructured WO3 thin films doped with Fe

Nanostructured tungsten oxide thin film based gas sensors have been developed by thermal evaporation method to detect CO at low operating temperatures. The influence of Fe-doping and annealing heat treatment on microstructural and gas sensing properties of these films have been investigated. Fe was incorporated in WO3 film by co-evaporation and annealing was performed at 400oC for 2 hours in air. AFM analysis revealed a grain size of about 10-15 nm in all the films. GIXRD analysis showed that as-deposited films are amorphous and annealing at 400oC improved the crystallinity. Raman and XRD analysis indicated that Fe is incorporated in the WO3 matrix as a substitutional impurity, resulting in shorter O-W-O bonds and lattice cell parameters. Doping with Fe contributed significantly towards CO sensing performance of WO3 thin films. A good response to various concentrations (10-1000 ppm) of CO has been achieved with 400oC annealed Fe-doped WO3 film at a low operating temperature of 150oC.

Conductometric gas sensors based on nanostructured WO3 thin films

Nanostructured WO3 thin films have been prepared by thermal evaporation to detect hydrogen at low temperatures. The influence of heat treatment on the physical, chemical and electronic properties of these films has been investigated. The films were annealed at 400oC for 2 hours in air. AFM and TEM analysis revealed that the as-deposited WO3 film is high amorphous and made up of cluster of particles. Annealing at 400oC for 2 hours in air resulted in very fine grain size of the order of 5 nm and porous structure. GIXRD and Raman analysis revealed that annealing improved the crystallinity of WO3 film. Gas sensors based on annealed WO3 films have shown a high response towards various concentrations (10-10000 ppm) H2 at an operating temperature of 150oC. The improved sensing performance at low operating temperature is due to the optimum physical, chemical and electronic properties achieved in the WO3 film through annealing.

Ethanol sensitivity of thermally evaporated nanostructured WO3 thin films doped and implanted with Fe

Ethanol sensing performance of gas sensors made of Fe doped and Fe implanted nanostructured WO3 thin films prepared by a thermal evaporation technique was investigated. Three different types of nanostructured thin films, namely, pure WO3 thin films, iron-doped WO3 thin films by co-evaporation and Fe-implanted WO3 thin films have been synthesized. All the thin films have a film thickness of 300 nm. The physical, chemical and electronic properties of these films have been optimized by annealing heat treatment at 300ºC and 400ºC for 2 hours in air. Various analytical techniques were employed to characterize these films. Atomic Force Microscopy and Transmission Electron Microscopy revealed a very small grain size of the order 5-10 nm in as-deposited WO3 films, and annealing at 300ºC or 400ºC did not result in any significant change in grain size. This study has demonstrated enhanced sensing properties of WO3 thin film sensors towards ethanol at lower operating temperature, which was achieved by optimizing the physical, chemical and electronic properties of the WO3 film through Fe doping and annealing.

Thermally evaporated tungsten oxide (WO3) thin films for gas sensing applications

In this thesis, the author proposed and developed gas sensors made of nanostructured WO3 thin film by a thermal evaporation technique. This technique gives control over film thickness, grain size and purity. The device fabrication, nanostructured material synthesis, characterization and gas sensing performance have been undertaken. Three different types of nanostructured thin films, namely, pure WO3 thin films, iron-doped WO3 thin films by co-evaporation and Fe-implanted WO3 thin films have been synthesized. All the thin films have a film thickness of 300 nm. The physical, chemical and electronic properties of these films have been optimized by annealing heat treatment at 300ºC and 400ºC for 2 hours in air. Various analytical techniques were employed to characterize these films. Atomic Force Microscopy and Transmission Electron Microscopy revealed a very small grain size of the order 5-10 nm in as-deposited WO3 films, and annealing at 300ºC or 400ºC did not result in any significant change in grain size. X-ray diffraction (XRD) analysis revealed a highly amorphous structure of as-deposited films. Annealing at 300ºC for 2 hours in air did not improve crystallinity in these films. However, annealing at 400ºC for 2 hours in air significantly improved the crystallinity in pure and iron-doped WO3 thin films, whereas it only slightly improved the crystallinity of iron-implanted WO3 thin film as a result of implantation. Rutherford backscattered spectroscopy revealed an iron content of 0.5 at.% and 5.5 at.% in iron-doped and iron-implanted WO3 thin films, respectively. The RBS results have been confirmed using energy dispersive x-ray spectroscopy (EDX) during analysis of the films using transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) revealed significant lowering of W 4f7/2 binding energy in all films annealed at 400ºC as compared with the as-deposited and 300ºC annealed films. Lowering of W 4f7/2 is due to increase in number of oxygen vacancies in the films and is considered highly beneficial for gas sensing. Raman analysis revealed that 400ºC annealed films except the iron-implanted film are highly crystalline with significant number of O-W-O bonds, which was consistent with the XRD results. Additionally, XRD, XPS and Raman analyses showed no evidence of secondary peaks corresponding to compounds of iron due to iron doping or implantation. This provided an understanding that iron was incorporated in the host WO3 matrix rather than as a separate dispersed compound or as catalyst on the surface. WO3 thin film based gas sensors are known to operate efficiently in the temperature range 200ºC-500 ºC. In the present study, by optimizing the physical, chemical and electronic properties through heat treatment and doping, an optimum response to H2, ethanol and CO has been achieved at a low operating temperature of 150ºC. Pure WO3 thin film annealed at 400ºC showed the highest sensitivity towards H2 at 150ºC due to its very small grain size and porosity, coupled with high number of oxygen vacancies, whereas Fe-doped WO3 film annealed at 400ºC showed the highest sensitivity to ethanol at an operating temperature of 150ºC due to its crystallinity, increased number of oxygen vacancies and higher degree of crystal distortions attributed to Fe addition. Pure WO3 films are known to be insensitive to CO, but iron-doped WO3 thin film annealed at 300ºC and 400ºC showed an optimum response to CO at an operating temperature of 150ºC. This result is attributed to lattice distortions produced in WO3 host matrix as a result of iron incorporation as substitutional impurity. However, iron-implanted WO3 thin films did not show any promising response towards the tested gases as the film structure has been damaged due to implantation, and annealing at 300ºC or 400ºC was not sufficient to induce crystallinity in these films. This study has demonstrated enhanced sensing properties of WO3 thin film sensors towards CO at lower operating temperature, which was achieved by optimizing the physical, chemical and electronic properties of the WO3 film through Fe doping and annealing. This study can be further extended to systematically investigate the effects of different Fe concentrations (0.5 at.% to 10 at.%) on the sensing performance of WO3 thin film gas sensors towards CO.

Low temperature response of nanostructured tungsten oxide thin films toward hydrogen and ethanol

Semiconducting metal oxide based gas sensors usually operate in the temperature range 200–500 °C. In this paper, we present a new WO3 thin film based gas sensor for H2 and C2H5OH, operating at 150 °C. Nanostructured WO3 thin films were synthesized by thermal evaporation method. The properties of the as-deposited films were modified by annealing in air at 300 °C and 400 °C. Various analytical techniques such as AFM, TEM, XPS, XRD and Raman spectroscopy have been employed to characterize their properties. A clear indication from TEM and XRD analysis is that the as-deposited WO3 films are highly amorphous and no improvement is observed in the crystallinity of the films after annealing at 300 °C. Annealing at 400 °C significantly improved the crystalline properties of the films with the formation of about 5 nm grains. The films annealed at 300 °C show no response to C2H5OH (ethanol) and a little response to H2, with maximum response obtained at 280 °C. The films annealed at 400 °C show a very good response to H2 and a moderate response to C2H5OH (ethanol) at 150 °C. XPS analysis revealed that annealing of the WO3 thin films at 400 °C produces a significant change in stoichiometry, increasing the number of oxygen vacancies in the film, which is highly beneficial for gas sensing. Our results demonstrate that gas sensors with significant performance at low operating temperatures can be obtained by annealing the WO3 films at 400 °C and optimizing the crystallinity and nanostructure of the as-deposited films.

Conductometric gas sensors based on nanostrutured WO3 thin films

Nanostructured WO3 thin films have been prepared bythermal evaporation to detect hydrogen at low t emperatures. The influence of heat treatment on the physical, chemical and electronic properties of these films has been investigated. The films were annealed at 400oC for 2 hours in air. AFM and TEM analysis revealed that the as-deposited WO3 film is high amorphous and made up of cluster of particles. Annealing at 400oC for 2 hours in air resulted in very fine grain size of the order of 5 nm and porous structure. GIXRD and Raman analysis revealed that annealing improved the crystallinity of WO3 film. Gas sensors based on annealed WO3 films have shown a high response towards various concentrations (10-10000 ppm) H2 at an operating temperature of 150oC. The improved sensing performance at low operating temperature is due to the optimum physical, chemical and electronic properties achieved in the WO3 film through annealing. - See more at: http://dl4.globalstf.org/?wpsc-product=conductometric-gas-sensors-based-on-nanostructured-wo3-thin-films-2#sthash.IrfhlZ6H.dpuf

Pt/WO3 nanoplatelet/SiC Schottky diode based hydrogen gas sensor

This paper presents the fabrication and study of a Schottky diode based on Pt/WO3 nanoplatelet/SiC for H2 gas sensing applications. The nanostructured WO3 films were synthesized from tungsten (sputtered on SiC) via an acidetching method using a 1.5 M HNO3 solution. Scanning electron microscopy of the developed films revealed platelet crystals with thicknesses in the order of 20-60 nm and lengths between 100-700 nm. The current-voltage characteristic and dynamic response of the diodes were measured in the presence of air and 1% H2 gas balanced in air from 25 to 300°C. Upon exposure to 1% H2, voltage shifts of 0.64, 0.93 and 1.14 V were recorded at temperatures of 120, 200 and 300°C, respectively at a constant forward bias current of 500 μA.

Nanostructured thin-film WO3 photoanodes for solar water and sea-water splitting

This article charts the development of the use of thin films of nanoparticulate WO3 and how they have been used to overcome problems associated with other photocatalytic materials and bulk WO3. Current technology is described and the authors' views on the outlook for future development is suggested.

On the application of nanostructured electrodes prepared by Ti/TiO2/WO3 template : A case study of removing toxicity of indigo using visible irradiation

New assays with HepG2 cells indicate that Indigo Carmine (IC), a dye that is widely used as additive in many food and pharmaceutical industries exhibited cytotoxic effects. This work describes the development of a bicomponent nanostructured Ti/TiO2/WO3 electrode prepared by template method and investigates its efficiency in a photoelectrocatalytic method by using visible light irradiation and applied potential of 1V. After 2h of treatment there are reduction of 97% discoloration, 62% of mineralization and formation of three byproducts assigned as: 2-amine-5-sulfo-benzoic acid, 2,3-dioxo-14-indole-5-sulfonic acid, and 2-amino-α-oxo-5-sulfo-benzeneacetic acid were identified by HPLC-MS/MS. But, cytotoxicity was completely removed after 120min of treatment. © 2013 Elsevier Ltd.

Sensing properties of e-beam evaporated nanostructured pure and iron-doped tungsten oxide thin films

Gas sensing properties of nanostructured pure and iron-doped WO3 thin films are discussed. Electron beam evaporation technique has been used to obtain nanostructured thin films of WO3 and WO3:Fe with small grain size and porosity. Atomic force microscopy has been employed to study the microstructure. High sensitivity of both films towards NO2 is observed. Doping of the tungsten oxide film with Fe decreased the material resistance by a factor of about 30 when exposed to 5 ppm NO2. The high sensitivity is attributed to an improved microstructure of the films obtained through e-beam evaporation technique, and subsequent annealing at 300oC for 1 hour.

Structural and properties of nanocrystalline WO3/TiO2-based humidity sensors elements prepared by high energy activation

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Low tungsten content of nanostructured material supported on carbon for the degradation of phenol

A comparative study using different mass proportions of WO3/C (1%, 5%, 10% and 15%) for H2O2 electrogeneration and subsequent phenol degradation was performed. To include the influence of the carbon substrate and the preparation methods, all synthesis parameters were evaluated. The WO3/C materials were prepared by a modified polymeric precursor method (PPM) and the sol-gel method (SGM) on Vulcan XC 72R and Printex L6 carbon supports, verifying the most efficient metal/carbon proportion. The materials were physically characterized by X-ray diffraction (XRD) and by X-ray photoelectron spectroscopy (XPS) techniques. The XRD and the XPS techniques identified just one phase containing WO3 and elevated oxygen concentration on carbon with the presence of WO3. The oxygen reduction reaction (ORR), studied by the rotating ring-disk electrode technique, showed that WO3/C material with the lowest tungsten content (1% WO3/C), supported on Vulcan XC 72R and prepared by SGM, was the most promising electrocatalyst for H2O2 electrogeneration. This material was then analyzed using a gas diffusion electrode (GDE) and 585mgL-1 of H2O2 was produced in acid media. This GDE was employed as a working electrode in an electrochemical cell to promote phenol degradation by an advanced oxidative process. The most efficient method applied was the photo-electro-Fenton; this method allowed for 65% degradation and 11% mineralization of phenol during a 2-h period. Following 12h of exhaustive electrolysis using the photo-electro-Fenton method, the total degradation of phenol was observed after 4h and the mineralization of phenol approached 75% after 12h. © 2013 Elsevier B.V.

Soft-chemical synthesis, characterization and humidity sensing behavior of WO3/TiO2 nanopowders

Tungsten oxide/titania (WO3/TiO2) nanopowders were synthesized by the polymeric precursor method which varied the WO3 content between 0 and 10 mol%. The powders were thermally treated in a conventional furnace and their structural, microstructural and electric properties were evaluated by X-ray diffraction (XRD), Raman spectrometry, N 2 physisorption, NH3 chemisorption, temperature-programmed reduction (TPR), X-ray absorption near-edge spectroscopy (XANES) in situ XANES and extended X-ray absorption fine structure spectroscopy (EXAFS) and transmission electron microscopy (TEM). XRD and Raman spectrometry confirmed the homogeneous distribution of an amorphous WO3 phase in the TiO 2 matrix which stabilized the anatase phase through the generation of [TiO5·V0] or [TiO5·V 0] complex sites. Conventional TPR-H2 (temperature programmed reduction) along with XANES TPR-H2 and XANES TPR-EtOH showed that WO3/TiO2 sample reduction occurs through the formation of these complex clusters. Moreover, the addition of WO3 promoted an increase in the surface acidity of doped samples as revealed by NH3 chemisorption. The WO3/TiO2 bulk-ceramic samples were further used to estimate their potential application in a humidity sensor in the range of 15-85% relative humidity. Probable reasons that lead to the different humidity sensor responses of samples were given based on the structural and surface characterizations. Correlation between the sensing performance of the sensor and its structural features are also discussed. Although all samples responded as a humidity sensor, the W2T sample (2 mol% added WO3) excelled for sensitivity due to the increase in acid sites, optimum mean pore size and pore size distribution. © 2013 Elsevier B.V.

Corrosion studies on the plasma-sprayed nanostructured titanium dioxide coating

Purpose: The purpose of this paper is to report the resistance of plasma-sprayed titanium dioxide (TiO2) nanostructured coatings in a corrosive environment.----- Design/methodology/approach: Weight loss studies are performed according to ASTM G31 specifications in 3.5?wt% NaCl. Electrochemical polarization resistance measurements are made according to ASTM G59-91 specifications. Corrosion resistance in a humid and corrosive environment is determined by exposing the samples in a salt spray chamber for 100?h. Microstructural studies are carried out using an atomic force microscope and scanning electron microscope.----- Findings: The nanostructured TiO2 coatings offer good resistance to corrosion, as shown by the results of immersion, electrochemical and salt spray studies. The corrosion resistance of the coating is dictated primarily by the geometry of splat lamellae, density of unmelted nanoparticles, magnitude of porosity and surface homogeneity.----- Practical implications: The TiO2 nanostructured coatings show promising potential for use as abrasion, wear-resistant and thermal barrier coatings for service in harsh environments.----- Originality/value: The paper relates the corrosion resistance of nanostructured TiO2 coatings to their structure and surface morphology.