Film Gas Sensors Research Articles

Laser-induced periodic surface structures on ZnO thin film for high response NO2 detection

Femtosecond laser-induced periodic structures (LIPSS) have been processed on ZnO thin film gas sensor devices for nitrogen dioxide (NO2) detection. From the morphology point of view, the nanostructures have been identified as high spatial frequency LIPSS (HSFL) with an average period of 145 nm. Through Raman analysis, a decrease of the typical wurtzite ZnO structure is shown, with a possible increase of defects such as Zn interstitials. The response under NO2 is enhanced if compared with the only-annealed ZnO thin film for concentrations as low as 1 ppm, reaching 1 ppb of detection limit (LOD) for the sensors with LIPSS. The Zn interstitials defects could be the source of the adsorbed NO2 species increasing the sensitivity. Reproducible results have been measured during 11 weeks in a row.

Design and Construction of Nanostructure TiO2 Thin Film Gas Sensor Prepared by R.F Magnetron Sputtering Technique

In this research, Mn-doped TiO2 thin films were grown on glass, Si and OIT/glass substrates by R.F magnetron sputtering technique with thicknesses (250 nm) using TiO2:Mn target under Ar gas pressure and power of 100 Watt. Through the results of X-ray diffraction, the prepared thin films are of the polycrystallization type after the process of annealing at 600°C for two hour The average crystalline size were 145.32, 280.97 and 261.23 nm for (TiO2:Mn) thin film on glass, Si and OIT/glass substrates respectively, while the measured surface roughness is between 0.981nm and 1.14 nm. The fabricated (TiO2:Mn) thin film on glass sensors have high sensitivity for hydrogen( H2 reducing gas) compared to the sensitivity for hydrogen gas on Si and OIT/glass substrates at room temperature as well faster in response and recovery times. It was found that the sensing properties strongly depended on the structure characteristics, especially the surface area and morphology.

Preparing SnO₂/MWCNT Nanocomposite Catalysts via High Energy Ball Milling.

Ball milling method was used to fabricate successfully tin dioxide (SnO₂)/multi-walled carbon nanotubes nanocomposite materials using SnCl₂ ×2H₂O as precursor together with soda and salt as admixture. The as-prepared materials were characterized by transmission electron microscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, Raman microscopy, and X-ray diffraction techniques. Observations revealed that applying both soda and salt are advantageous for increasing dispersity of tin dioxide nanoparticles on the surface of carbon nanotubes. These multi-walled carbon nanotube-based composites are promising candidates as thick film gas sensors or catalysts. Results indicate that SnO₂/MWCNT composites can be achieved under solvent free dry conditions, too.

Electrodeposition-based in situ construction of a ZnO-ordered macroporous film gas sensor with enhanced sensitivity

A ZnO-ordered macroporous film sensor integrated with standing nanosheets was in situ constructed based on electrodeposition, and exhibited enhanced sensitivity to ethanol gas.

The Use of Copper Oxide Thin Films in Gas-Sensing Applications

In this work, the latest achievements in the field of copper oxide thin film gas sensors are presented and discussed. Several methods and deposition techniques are shown with their advantages and disadvantages for commercial applications. Recently, CuO thin film gas sensors have been studied to detect various compounds, such as: nitrogen oxides, carbon oxides, hydrogen sulfide, ammonia, as well as several volatile organic compounds in many different applications, e.g., agriculture. The CuO thin film gas sensors exhibited high 3-S parameters (sensitivity, selectivity, and stability). Furthermore, the possibility to function at room temperature with long-term stability was proven as well, which makes this material very attractive in gas-sensing applications, including exhaled breath analysis.

UV-light-activated H2S gas sensing by a TiO2 nanoparticulate thin film at room temperature

A UV-light-activated TiO2 thin film gas sensor composed of nanoparticles is reported for H2S detection at room temperature. TiO2 nanoparticulate thin films were fabricated by bar coating of a mixture of TiO2 powder and acetic acid. The morphology and structural properties of the films were examined by scanning electron microscopy, X-ray diffraction, photoluminescence spectroscopy, and absorption spectroscopy. The nil response to H2S was significantly enhanced by UV light irradiation in the response levels and response/recovery kinetics. The effect of the sensor conductance and light intensity towards sensor optimization was discussed. The sensor exhibited high gas selectivity, repeatability, and linearity for practical applications of the sensor, but the interference of humidity on the sensing process needs to be solved. The results exhibited a simple way to detect parts per million concentrations of H2S at room temperature.

Synthesis and gas sensing performance of NiO decorated SnO2 vertical-standing nanotubes composite thin films

In this work, a new type NiO decorated SnO2 vertical-standing nanotubes composite thin film was synthesized successfully via a facile hydrothermal method. The microstructure, physico-chemical surface state and morphology of the composite thin film were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), respectively. The results showed that the NiO decorated SnO2 vertical-standing nanotubes composite thin film with a coral-like microstructure was built of numerous 1D SnO2 nanotubes with the size of 200–300 nm in diameter and 1.5 μm in length; while NiO nanoplates with the size of 20 nm in diameter decorated on the surface of SnO2 nanotubes. Due to the enhanced adsorption of oxygen molecules, formation of p-NiO/n-SnO2 heterojunction and porous structure, the NiO/SnO2 thin film gas sensor with an Ni2+/Sn4+ molar ratio of 5% exhibited high response value, good selectivity and quick response characteristics to ethanol at the optimum working temperature of 250 ℃.

Synthesis of molybdenum diselenide nanosheets and its ethanol-sensing mechanism

Abstract Molybdenum diselenide (MoSe2) nanosheets thin film gas sensor was firstly fabricated and its sensing potential to ppm-level ethanol vapor at low operating temperature was investigated. Ultrathin MoSe2 nanosheets were prepared in large scale through a facile liquid-phase exfoliation method using low-boiling-temperature solvent. The exfoliated MoSe2 nanosheets exhibited high purity and crystallinity with few atomic layer thickness. Systematical gas sensing tests demonstrated that MoSe2 nanosheets based thin film could be utilized as ethanol gas sensor with linear response, quick recovery, and good repeatability at 90 °C. The sensing mechanism of MoSe2 toward ethanol was investigated based on first principle calculation. The adsorption behavior of ethanol molecules on MoSe2 surface was revealed in light of adsorption orientation, adsorption energy, charge transfer, projected electronic density of state, and molecular orbital. The calculation well matched with experimental results. It is found the quick and completed recovery of MoSe2 nanosheets sensor was benefited by the appropriate physical interaction between ethanol and MoSe2 surface. This finding offers a competitive option instead of conventional graphene sensor for ethanol gas detection at low temperature.

MEMS based tin oxide thin film gas sensor for diabetes mellitus applications

Breath analysis has gained interest of many researchers lately. Acetone has been proven to be the biomarker for diabetes mellitus. The research depicts a thin film MEMS based gas sensor for acetone gas detection. MEMS techniques have been followed to develop the gas sensor involving the processes like oxidation, lithography, sputtering for deposition of thin films and electrodes, etc. Tin oxide thin film has been incorporated for detecting different concentration levels of acetone gas, viz, 20, 10, 7, 5, 3, 1.5 ppm. Various characterization like X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), gas sensing characterization for recording resistance changes have been performed. XRD patterns reveal the formation of SnO2. AFM and SEM depict clear images of grain boundaries on the film. SnO2 thin films have been annealed at 300 °C to achieve an optimum grain size of 86.3 nm as depicted by AFM. Optimum operating temperature for sensing acetone gas has been computed to be 360 °C. Tin oxide can detect even a very low concentration of 1.5 ppm acetone gas with a good resistance response change of 30%. Apart from acetone gas, other interfering gases present in breath have also been tested. The sensor does not respond to them. The response time of the sensor for detection of acetone gas is approximately 3 min and the recovery time is approximately 4 min. A suitable instrumentation circuitry has also been designed for the sensor which displays the resistance value of the thin film before and after it comes in contact with acetone gas.

Fabrication and characterization of SnO2 nanorods for room temperature gas sensors

Highly sensitive large-scale tin oxide (SnO2) nanostructures were grown on a glass substrate by thermal evaporation of a mixture of anhydrous tin (II) chloride (SnCl2) and zinc chloride (ZnCl2) powders at 550°C in air. We demonstrate a single cell vapor deposition system to precisely control nanostructural morphology of SnO2 by changing the weight ratio of SnCl2 and ZnCl2 and growth temperature. The morphology and structural property of as-grown nanostructures were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The SEM images revealed that the SnO2 nanostructures with different densities, sizes, and shapes can be achieved by adjusting the weight ratio of SnCl2 and ZnCl2. A thin film gas sensor based on SnO2 nanostructures with diameter ∼20 nm and length ∼100 nm showed ∼85% sensitivity and 53 seconds of response time, whereas the nanorods with diameter ∼100 nm and length ∼ 1μm showed ∼50% sensitivity with 198 seconds response time. The nanostructured material with small size and shape showed better sensitivity on sensing at room temperature compared to previously reported SnO2 based sensors.

Acetic acid sensing of Mg-doped ZnO thin films fabricated by the sol–gel method

Acetic acid vapor thin film gas sensor was developed by synthesizing Mg-doped ZnO nanoparticles using a low cost and facile sol–gel route and were characterized using field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and photoluminescence analysis. Morphological characterizations showed the formation of well-defined and highly crystalline ZnO nanoparticles on Si(100)/SiO2 substrate. Gas sensing characterization of dip coated Mg-doped ZnO thin films were performed in temperature range of 150–400 °C at different acetic acid vapor concentrations. At 300 °C, the sensitivity for pure ZnO, Zn0.98Mg0.02O and Zn0.94Mg0.06O samples at concentration of 200 ppm of acetic acid were 124, 78 and 67%, respectively. The highest sensitivity for Zn0.96Mg0.04O sample was 136% at the same vapor concentration and temperature. It showed a fast response time and recovery time (145 and 110 s, respectively).

Highly transparent and flexible NO2 gas sensor film based on MoS2/rGO composites using soft lithographic patterning

The MoS2/GO hybrid composite thin film is prepared by a facile solution mixing process. To reduce the composite film spin-coated on a SiO2 (300 nm)/Si substrate, thermal annealing at 600 °C for 1 h in a N2 atmosphere is used. The thermally reduced GO (rGO) composite film is subjected to line patterning through a soft lithographic patterning method, and the patterned MoS2/rGO composite thin film is re-transferred onto a PET film from the Si substrate. The patterned MoS2/rGO composite thin film has a thickness of about 10 nm and the transmittance of the patterned MoS2/rGO composite thin film on the PET substrate is 93%. The NO2 gas sensor device is fabricated and its basic characteristics are systematically investigated. The patterned MoS2/rGO composite thin film gas sensor has a sensitivity at least four times higher than the pure rGO thin film gas sensor. In addition, the characteristics of the sensor device are also maintained at a bending radius of 14 mm. This transparent and flexible patterned MoS2/rGO composite thin film can be used as a highly sensitive gas sensor that will detect a concentration as low as 0.15 ppm of harmful gases such as NO2.

Extremely Flexible Indium‐Gallium‐Zinc Oxide (IGZO) Based Electronic Devices Placed on an Ultrathin Poly(Methyl Methacrylate) (PMMA) Substrate

AbstractThe flexibility of metal oxide‐based electronic devices is severely limited by the thickness of their substrate. To enhance the flexibility of semiconducting metal oxide‐based electronic devices, a new and simple way to fabricate indium‐gallium‐zinc oxide (IGZO)‐based electronic devices on an ultrathin (1.9 µm) poly(methyl methacrylate) (PMMA) substrate is introduced. The PMMA layer spin‐coated on an unmodified glass slide has no chemical interactions at the interface, resulting in weak adhesion. Therefore, the PMMA layer with the devices is readily peeled off the underlying glass slide without using any sacrificial layer. Thin film transistors (TFTs) and gas sensors are fabricated on a 1.9 µm thick PMMA substrate. The fabricated bottom‐gated IGZO TFTs exhibits excellent transistor performances with a mobility of 10.7 cm2 V−1 s−1, a threshold voltage of 8.4 V and an on/off current ratio of 5 × 105. The PMMA substrate having palladium (Pd)‐decorated IGZO H2 sensors is attached to the nonplanar substrates such as wrinkled hand glove and poly(vinyl chloride) (PVC) gas tubes, and the sensors demonstrate an excellent sensitivity of ≈106% at 5% H2 concentration at room temperature. Furthermore, both electronic devices show superior flexibility without any performance degradation even at a bending radius down to <1 mm.

CdIn2O4 nanoporous thin film gas-sensor for formaldehyde detection

This paper describes the preparation with a sol-gel method as well as formaldehyde (HCHO) gas sensing properties of CdIn2O4 nanoporous thin film. CdIn2O4 nanoporous thin film was prepared using poly(ethylene-co-butylene)-b-poly(ethylene oxide) diblock copolymers as template. The film was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscope (AFM), and x-ray photoelectron spectroscopy (XPS), respectively. The analyzed results indicate that the obtained thin film is highly crystalline and has pore-solid architectures after removal of the polymer template by calcination. As-prepared CdIn2O4 nanoporous thin film was used directly as sensing device to investigate the sensing properties in HCHO atmosphere. The home-built CdIn2O4 nanoporous thin film gas-sensor shows high response, selectivity, and long-time stability toward HCHO gas at low operating temperature (100 °C), making the thin film not only interesting for sensor devices, but also for a number of related electrochemical applications.

Investigation of Sm1−xCaxFe0.7Co0.3O3 films (x = 0.0–0.3) prepared by modified sol–gel spin-coating method for carbon monoxide and propane gas sensing

Sm1−xCaxFe0.7Co0.3O3 thin film gas sensors with different calcium concentrations (x = 0.0–0.3) were prepared using a modified sol–gel spin-coating method and then annealed at 650 °C for 2 h in air. The thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and electrical resistance measurements at concentrations from 1 to 200 ppm for carbon monoxide (CO) and from 1 to 500 ppm for propane (C3H8) in the temperature range of 25–300 °C for both gases. The resulting XRD data reveal that a pure phase with perovskite structure was obtained for each substitution in x and corresponds to the orthorhombic system with a Pbnm space group. Topography and roughness of the coating were obtained by AFM; roughness results of the sample do not significantly affect the response of the sensor, but the Ca2+ content does. Morphology of the nanoparticles and the thickness of each doping in calcium were observed by SEM, and the thickness was around 300 nm. The gas-sensing characteristics of the Sm–Ca–Fe–Co–O system were investigated in CO and C3H8 gases. It was found that the electrical resistance and gas-sensing characteristics were influenced by the substitution of Sm2+ by Ca2+ ions. When x was greater than or equal to 0.2 and CO concentration was 200 ppm and operating temperature was 300 °C, there was a higher response of the sensor, reaching up to 100%. For operating temperatures below 100 °C, the response of the Sm1−xCaxFe0.7Co0.3O3-based sensors to CO or C3H8 was < 5% regardless of the degree of substitution of Ca. On the other hand, at temperatures above 100 °C, the response depended on the gas to be sensed. In addition, the partial substitution of Ca in the structure increased the response of the sensor in relation to CO and C3H8 concentrations. It is suggested that this is due to the process of electric valence compensation.

Effect of Substrates on Structural Properties of Pure Anatase Phase Titanium Dioxide Thin Films Prepared By Mist Chemical Vapor Deposition

Titanium dioxide (TiO2) thin film has attracted considerable interest as one of promising sensing materials applying for various thin-film gas sensors because of its outstanding sensing properties, chemical stability and electrical properties, etc. [1]. Among the three phases of TiO2 films, anatase phase TiO2 has better sensitivity to H2 and volatile organic compound gases than rutile phase TiO2 [2,3]. However, there were still issues on synthesizing the stable pure anatase phase TiO2 using different methods. Compared with other reported methods, the mist chemical vapor deposition (mist CVD) [4] is an alternative method to synthesize TiO2 thin films, by which the growth of oxide thin film can be precisely controlled according to our previously research [5]. In order to investigate the effect of substrates on structural properties of TiO2 films fabricated by mist CVD method, four kinds of substrates were selected for comparison: glass, quartz glass, gallium doped zinc oxide (GZO) film and p-type silicon. During the mist CVD process, a solution of mixed precursors was prepared by dissolving titanium tetraisopropanolate in ethanol. The mist droplets were generated from the solution by ultrasonic transducer and transferred to the reaction chamber by compressed air. The substrates were set in the reaction chamber, which was heated and kept at 400 ˚C during the deposition. The structural properties of TiO2 thin films were investigated by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscope, atomic force microscope. The optical properties of TiO2 thin films were investigated by transmittance spectroscopy. According to the morphological analysis, TiO2 films with uniform surface were observed on the different substrates. Examining the XRD patterns of TiO2 films, the following diffraction peaks (101), (004), (200), (211), (201), (204) and (215) were observed, which corresponded to the diffraction peaks of anatase phase TiO2. However, it was found that the intensity of dominated (101) peak increased with the order of TiO2 films on quartz glass, GZO, silicon and glass. The results of TiO2 films by Raman spectroscopy measurement were also agreed with that of XRD measurement. In summary, the pure anatase phase TiO2 thin films could be synthesized by mist CVD method. The obtained anatase phase TiO2 films have high potential to be applied to thin film gas sensors, photocatalyst and dye-sensitized solar cells. The growth mechanism of TiO2 films on the different substrates will be revealed in the paper. References M. V. Baryshnikova, L. A. Filatov, A.S. Petrov, and S. E. Alexandrov, Chemical Vapor Deposition, 21, 327-333 (2015).M. Epifani, A. Helwig, J. Arbiol, R. Díaz, L. Francioso, P. Siciliano, G. Mueller, and J. R. Morante, Sensors and Actuators B, 130, 599-608 (2008).E. Sennik, N. Kilinc, Z. and Z. Ozturk, Journal of Alloys and Compounds, 616, 89-96 (2014).T. Kawaharamura, K. Mori, H. Orita, T. Shirahata, S. Fujita, and T. Hirao, Japanese Journal of Applied Physics, 52, 035501 (2013).X. Li, C. Li, T. Kawaharamura, D. Wang, N. Nitta, M. Furuta, H. Furuta, and A. Hatta, Transactions of the Materials Research Society of Japan, 39[2], 161-164 (2014).

Preparation and characterization of Pt loaded WO3 films suitable for gas sensing applications

This paper presents the preparation of nanostructured platinum (Pt) loaded tungsten oxide (WO3) thin films by radio frequency (RF) magnetron sputtering technique. Even though, Pt loading does not produce any phase change in WO3 lattice, it deteriorates the crystalline quality and induces defects on WO3 films. The Pt loading in WO3 has profound impact on structural and optical properties of the films by which the particle size, lattice strain and optical band gap energy are reduced. Nanoporous film with reduced particle size is obtained for 5 wt% Pt loaded WO3 sample which is crucial for gas sensors. Hence the sensing response of 5 wt% Pt loaded sample is tested towards carbon monoxide (CO) gas along with pure WO3 sample. The sensing response of Pt loaded sample is nearly 15 times higher than pure WO3 sample in non-humid ambience at an operating temperature 200 °C. This indicates the suitability of the prepared films for gas sensors. The sensing response of pure WO3 film depends on the humidity while the Pt loaded WO3 film shows stable response in both humid and non-humid ambiences.

Effect of Substrates on Structural Properties of Pure Anatase Phase Titanium Dioxide Thin Films Prepared by Mist Chemical Vapor Deposition

The mist chemical vapor deposition method was used to synthesize pure anatase phase titanium dioxide thin films on quartz glass, glass, gallium oxide doped zinc oxide film and p-type silicon wafer substrates. The effects of substrates on the structural and optical properties of titanium dioxide thin films were investigated. Titanium dioxide films deposited on all substrates were anatase phase and showed the same dominant (101) growth orientation. The crystallinity of titanium dioxide films grown on different substrates increased in an order of glass, quartz glass, p-type silicon and gallium oxide doped zinc oxide. The titanium dioxide thin film deposited on p-type silicon showed the highest percentage of exposed {001} facets. The morphological results showed that the titanium dioxide films with good uniformity were obtained on all of the substrates. The surface area of titanium dioxide films deposited on p-type silicon substrate was larger than that of titanium dioxide films deposited on quartz glass, glass and gallium oxide doped zinc oxide substrates. The transmittance of obtained titanium dioxide films on transparent substrates was greater than 75% in visible region. The obtained pure anatase phase titanium dioxide thin film on p-type silicon is expected to be applicable to thin film gas sensors.

The Detection Characterization of NO X Gas Using the MWCNT/ZnO Composite Film Gas Sensors by Heat Treatment

The Detection Characterization of NO X Gas Using the MWCNT/ZnO Composite Film Gas Sensors by Heat Treatment

Ce doped NiO nanoparticles as selective NO2 gas sensor

Metal oxide gas sensors are promising portable gas detection devices because of their advantages such as low cost, easy production and compact size. The performance of such sensors is strongly dependent on material properties such as morphology, structure and doping. In the present study, we report the effect of cerium (Ce) doping on nickel oxide (NiO) nano-structured thin film sensors towards various gases. Bare NiO and Ce doped NiO nanoparticles (Ce:NiO) were synthesized by sol-gel method. To understand the effect of Ce doping in nickel oxide, various molar percentages of Ce with respect to nickel were incorporated. The structure, phase, morphology and band-gap energy of as-synthesized nanoparticles were studied by XRD, SEM, EDAX and UV–vis spectroscopy. Thin film gas sensors of all the samples were prepared and subjected to various gases such as LPG, NH3, CH3COCH3 and NO2. A systematic and comparative study reveals an enhanced gas sensing performance of Ce:NiO sensors towards NO2 gas. The maximum sensitivity for NO2 gas is around 0.719% per ppm at moderate operating temperature of 150 °C for 0.5% Ce:NiO thin film gas sensor. The enhanced gas sensing performance for Ce:NiO is attributed to the distortion of crystal lattice caused by doping of Ce into NiO.