NOx sensing properties of In2O3 nanoparticles prepared by metal organic chemical vapor deposition

In₂O₃ nanoparticles were deposited by low-temperature metal organic chemical vapor deposition. The response of 10-nm thick In₂O₃ particle containing layers to NO_x and O₂ gases is investigated. The lowest detectable NO_x concentration is 200 ppb and the sensor performance is strongly dependent on the gas partial pressure as well as on the operating temperature. The sensor response towards 200 ppm of NO_x is found to be above 10⁴. Furthermore, the cross-sensitivity against O₂ is very low, demonstrating that the In₂O₃ nanoparticles are very suitable for the selective NO_x detection.     

Impedancemetric gas sensor based on Pt and WO3 co-loaded TiO2 and ZrO2 as total NOx sensing materials

Pt-loaded metal oxides [WO₃/ZrO₂, MO_x/TiO₂ (MO_x = WO₃, MoO₃, V₂O₅), WO₃ and TiO₂] equipped with interdigital Au electrodes have been tested as a NO_x (NO and NO₂) gas sensor at 500 °C. The impedance value at 4 Hz was used as a sensing signal. Among the samples tested, Pt-WO₃/TiO₂ showed the highest sensor response magnitude to NO. The sensor was found to respond consistently and rapidly to change in concentration of NO and NO₂ in the oxygen rich and moist gas mixture at 500 °C. The 90% response and 90% recovery times were as short as less than 5–10 s. The impedance at 4 Hz of the present device was found to vary almost linearly with the logarithm of NO_x (NO or NO₂) concentration from 10 to 570 ppm. Pt-WO₃/TiO₂ showed responses to NO and NO₂ of the same algebraic sign and nearly the same magnitude, while Pt/WO₃ and WO₃/TiO₂ showed higher response to NO than NO₂. The impedance at 4 Hz in the presence of NO for Pt-WO₃/TiO₂ was almost equal at any O₂ concentration examined (1–99%), while in the case of Pt/WO₃ and WO₃/TiO₂ the impedance increased with the oxygen concentration. The features of Pt-WO₃/TiO₂ are favorable as a NO_x sensor that can monitor and control the NO_x concentration in automotive exhaust. The effect of WO₃ loading of Pt-WO₃/ZrO₂-based sensor is studied to discuss the role of surface W-OH sites on the NO_x sensing.     

Titania NOx sensors for exhaust monitoring

Oxide semiconductors have been examined to develop NO_x sensors for exhaust monitoring. Titania doped with trivalent elements, such as Al³⁺, Sc³⁺, Ga³⁺ or In³⁺, has a good sensitivity and selectivity to NO between 450 and 550 °C, and shows rapid response. A sensor probe for monitoring exhaust NO_x has been fabricated. Many kinds of interference gases, such as C₃H₆, CO and SO₂, have been found to have only a slight influence on the sensor response to NO. The influence of O₂ and H₂O is also negligible, except for the cases of 0% H₂O and fuel-rich conditions. In accordance with these results, the sensor probe operates satisfactority in the exhaust gas of various combustion conditions without interference from the various kinds of gas species in the exhaust gases.     

Sn1-xFexOy: a new material with high carbon monoxide sensitivity

The preparation method and the sensing properties (sensitivity and selectivity to interfering gases) towards carbon monoxide of the new ternary compound Sn_1-xFe_xO_y deposited in the form of thin films, are presented in this paper. The metal of the VIIIB group is introduced with concentrations in the range 0<x<25 at %. Thin films are sputtered using the RGTO (rhotaxial growth and thermal oxidation) technique. This technique consists of metal deposition onto a substrate maintained at a temperature higher than the metal melting point and metal oxidation by means of an annealing cycle in pure oxygen. Particular emphasis is given to the relations between some preparation parameters of the material, namely the atomic percentage ofiron or the annealing cycle, and to the sensor sensitivity towards CO and other interfering gases like C₂H₅OH, H₂ and NO_x diluted in dry air. A sensitivity S=(G_gas-G_air)/G_air=3.5 towards 10 ppm of CO has been measured: the kinetic characteristics of the sensors are also presented, together with the working mechanism.     

High-performance solid-electrolyte SOx sensor using MgO-stabilized zirconia tube and Li2SO4---CaSO4---SiO2 auxiliary phase

Solid-electrolyte-based electrochemical SO_x sensors fabricated with MgO-stabilized zirconia and Li₂SO₄---CaSO₄---SiO₂ (4:4:2 in molar ratio) exhibit fairly good sensing characteristics for 2–200 ppm SO₂ in air at 600–750 °C, with the e.m.f. responses following the Nernst equation for the two-electron reduction of SO₂. The 90% response and 90% recovery times to 20 ppm SO₂ are 10 s and 7 min at 650 °C, and 10 s and 3 min at 700 °C, respectively. It is further found that the sensor exhibits excellent selectivity to SO_x in the coexistence of CO₂ and NO_x, and good long-term stability. The sensor is simple in structure, easy to prepare, and quite tough chemically and mechanically. These features should ensure practical use for this SO_x sensor.     

Characterization of ozone sensors based on WO3 reactively sputtered films: influence of O2 concentration in the sputtering gas, and working temperature

We report on electrical responses of tungsten oxide thin film ozone sensors based on a tungsten trioxide (WO₃)/tin oxide (SiO₂)/Si structure with interdigitated Pt electrodes. The influence of O₂ concentration in the sputtering gas and working temperature of the sensor are investigated. Sensitivity to ozone increases with O₂ content in the sputtering gas. It reaches its highest value for sensors fabricated with 50% O₂. For these sensors, the best ozone sensitivity and shortest response and recovery times are obtained at a working temperature of 523 K. Ozone sensitivity is compared to other ozone sensors.     

Gas-sensing property and mechanism of CaxLa1−xFeO3 ceramics

LaFEO₃ and Ca_xLa_1−xFeO₃ ceramic powders have been prepared by the coprecipitation method from La(NO₃)₃, Fe(NO₃)₃ and Ca(NO₃)₂ aqueous solutions. The orthorhombic perovskite phases of LaFeO₃ and Ca_xLa_1−xFeO₃ are characterized by X-ray diffraction patterns. The sensors fabricated with those powders have high sensitivity to alcohol. Partial substitution of La³⁺ in LaFeO₃ with Ca²⁺ can enhance the sensitivity of the materials to reducing gases. The resistance of an LaFeO₃ sensor in air, vacuum and alcohol-containing air has been measured. Complex impedance spectroscopy has been used to try and analyse the gas-sensing mechanism. According to the experimental results, it can be deduced that the surface adsorptive and lattice oxygen govern the sensing properties of LaFeO₃ and Ca_xLa_1−xFeO₃ ceramics.     

PECVD-SiOxNy films for large area self-sustained grids applications

In this work we study the structural properties and mechanical stress of silicon oxynitride (SiO_xN_y) films obtained by plasma enhanced chemical vapor deposition (PECVD) technique at low temperatures (320 °C) and report the feasibility of using this material for the fabrication of large area self-sustained grids. The films were obtained at different deposition conditions, varying the gas flow ratio between the precursor gases (N₂O and SiH₄) and maintaining all the other deposition parameters constant. The films were characterized by ellipsometry, by Fourier transform infrared (FT-IR) spectroscopy and by optically levered laser technique to measure the total mechanical stress. The results demonstrate that for appropriated deposition conditions, it is possible to obtain SiO_xN_y with very low mechanical stress, a necessary condition for the fabrication of mechanically stable thick films (up to 10 μm). Since this material (SiO_xN_y) is very resistant to KOH wet chemical etching it can be utilized to fabricate, by silicon substrate bulk micromachining, very large self-sustained grids and membranes, with areas up to 1 cm² and with thickness in the 2–6 μm range. These results allied with the compatibility of the PECVD SiO_xN_y films deposition with the standard silicon based microelectronic processing technology makes this material promising for micro electro mechanical system (MEMS) fabrication.     

NASICON-based solid-electrolyte cell as transducer for enzyme sensors

Enzyme sensors for glucose and uric acid have been developed based on a solid-electrolyte cell using NASICON (Na₃Zr₂Si₂PO₁₂). These potentiometric devices respond reversibly to glucose and uric acid over a concentration range of two orders of magnitude with sensitivities of −54 and −52 mV/decade, respectively. The sensors can be used for a batch-type as well as a flow-through-type measuring system. Primarily the sensors respond to the H₂O₂ that is produced by the enzymatic reactions. The role of the three-phase region, electrolyte solution, sensing electrode metal and NASICON has been investigated. The liquid electrolyte/sensing electrode metal interface is found to work as a potential-determining as well as a rate-determining interface for the enzyme sensor.     

Potentiometric CO2 gas sensor with lithium phosphorous oxynitride electrolyte

Potentiometric cell, Au/LiCoO₂ 5 m/o Co₃O₄/Li_2.88PO_3.73N_0.14/Li₂CO₃/Au, has been fabricated and investigated for monitoring CO₂ gas. A LiCoO₂–Co₃O₄ mixture was used as the solid-state reference electrode instead of a reference gas. The idea is to keep the lithium activity constant on the reference side using thermodynamic equilibrium at a given temperature. The thermodynamic stability of the reference electrode was studied from the phase stability diagram of Li–Co–C–O system. The Gibb’s free energy of formation of LiCoO₂ was estimated at 500°C from the measured value of the cell emf. The sensors showed good reversibility and fast response toward changing CO₂ concentrations from 200 to 3000 ppm. The emf values were found to follow a logarithmic Nernstian behavior in the 400–500°C temperature range. CH₄ gas did not show any interference effect. Humidity and CO gas decreased the emf values of the sensor slightly. NO and NO₂ gases affect this sensor significantly at low temperatures. However, increased operating temperature seems to reduce the interference.     

Selective NH3 gas sensor based on Langmuir-Blodgett polypyrrole film

Polypyrrole thin films have been deposited onto a glass substrate by the Langmuir-Blodgett technique to fabricate a selective ammonia (NH₃) gas sensor. The d.c. electrical resistance of the sensing elements is found to exhibit a specific increase upon exposure to different gases such as NH₃, CO, CH₄, H₂ in N₂ and pure O₂. The polypyrrole thin-film detector shows a considerable increase of resistance when exposed to NH₃ in N₂, and negligible response when exposed to comparable concentrations of interfering gases such as CO, CH₄, H₂ in N₂ and pure O₂. The calibration curve for NH₃ in N₂ at room temperature is measured in the concentration range from 0.01 to 1%. The relative change of the electrical resistance is about 10% for the lower detectable limit of 100 ppm of NH₃ in N₂. The sensitivity of the Langmuir-Blodgett polypyrrole towards ammonia is considerably higher than that of the electrochemical polypyrrole. The fast rise time and the high sensitivity of the detector are reported as a function of number of the polypyrrole layers. Long-term aging tests of the selective NH₃ gas sensor are performed.     

Manufacture and characterization of sol–gel V1−x−yWxSiyO2 films for uncooled thermal detectors

V_1−x−yW_xSi_yO₂ films for uncooled thermal detectors were coated on sodium-free glass slides with sol–gel process, followed by the calcination under a reducing atmosphere (Ar/H₂ 5%). The V_1−x−yW_xSi_yO₂ films as prepared inherit various phase transition temperatures ranging from 20 to 70 °C depending on the dopant concentrations and the fabrication conditions. Compared to the hysteresis loop of plain VO₂ films, a rather steep loop was obtained with the addition of tungsten components, while a relaxed hysteresis loop with the tight bandwidth was contributed by Si dopants. Furthermore, the films with switching temperature close to room temperature were fabricated to one-element bolometers to characterize their figures of merit. Results showed that the V_0.905W_0.02Si_0.075O₂ film presented a satisfactory responsivity of 2600 V/W and detectivity of 9 × 10⁶ cm  Hz^1/2/W with chopper frequencies ranging from 30 to 60 Hz at room temperature. It was proposed that with appropriate amount of silicon and tungsten dopants mixed in the VO₂, the film would characterize both a relaxed hysteresis loop and a fair TCR value, which effectively reduced the magnitude of noise equivalent power without compromising its performance in detectivity and responsivity.     

EPR and DRS evidence for NO2 sensing in Al-doped ZnO

Zinc oxide (ZnO) is a well-known semiconducting multifunctional material wherein properties right from the morphology to gas sensitivity can be tailor-made by doping or surface modification. Aluminum (Al)-incorporated porous zinc oxide (Al:ZnO) exhibits good response towards NO₂ at low-operating temperature. The NO₂ gas concentration as low as 20 ppm exhibits S = 17% for 5 wt.% Al-incorporated ZnO. The NO₂ response increases with operating temperature and concentration and reaches to its maximum at 300 °C without any interference from other gases such as SO₃, HCl, LPG and alcohol. Physico-chemical characterization likes differential thermogravimetric analysis (TG-DTA) electron paramagnetic resonance (EPR) and diffused reflectance spectroscopy (DRS) have been used to understand the sensing behavior for pure and Al-incorporated ZnO. The TG-DTA depicts formation of ZnO phase at 287 °C. The EPR study reveals distinct variation for O⁻ (g = 2.003) and Zn interstitial (g = 1.98) defect sites in pure and Al:ZnO. The DRS studies elucidate signature of adsorbed NO_x species in aluminium-incorporated zinc oxide indicating its tendency to adsorb these species even at low temperatures. This paper is an attempt to correlate the gas sensing behavior with the physico-chemical studies such as EPR and DRS.     

Trace detection of nitroaromatic compounds with layer-by-layer assembled {SBA/PSS}n/PDDA modified electrode

The {SBA/PSS}_n/PDDA films modified electrode was prepared by layer-by-layer (LBL) assembly with mesoporous SiO₂ (SBA), poly(sodium 4-styrene-sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDDA) in this paper. SBA is a large pore-size mesoporous material with highly ordered hexagonally arranged mesochannels and high thermal stability etc. The electrochemical characteristics of the {SBA/PSS}_n/PDDA films have been studied by electrochemical impedance spectroscopy in 0.1 M KCl solution containing 5.0 mM Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ at the formal potential of 0.230 V. The ultratrace nitroaromatic compounds (NACs) such as TNT, TNB, DNT and DNB were determined by differential pulse voltammetry (DPV) measurement. The sensitivities for NACs determination with {SBA/PSS}_n/PDDA modified electrode were dependent on the number of layers, pH and ionic strength of electrolyte, based on which a set of optimized conditions for film fabrication was inferred. The current responses were linear with NACs ranging from 10⁻⁹ to 10⁻⁷ mol/l. The results showed that the {SBA/PSS}_n/PDDA modified electrode established a new way for fast, simple and sensitive analysis of NACs.     

Effect of surface modification on NO2 sensing properties of SnO2 varistor-type sensors

NO₂ sensing properties of SnO₂-based varistor-type sensors have been investigated in the temperature range of 400-650°C and in the NO₂ concentration range of 15–30 ppm. Pure SnO₂ exhibited a weak nonlinear I–V characteristic in air, but clear nonlinearity in NO₂ at 450°C. The breakdown voltage of SnO₂ shifted to a high electric field upon exposure to NO₂ and the magnitude of the shift was well correlated with NO₂ concentration. Thus, SnO₂ exhibited some sensitivity to NO₂ as a varistor-type sensor. When SnO₂ particles coated with a SiO₂ thin film were used as a raw material for fabricating a varistor, the breakdown voltage in air was approximately the double that of pure SnO₂ and the sensitivity to 15 ppm NO₂ was enhanced slightly. However, the sensitivity to 30 ppm NO₂ decreased. The Cr₂O₃-loading on SnO₂ also led to an increase in the breakdown voltage in air, but the Cr₂O₃ addition was not effective for promoting the NO₂ sensitivity under the present experimental conditions.     

Pulsed laser deposition of Y2O3 : Eu thin film phosphors

Thin films of Y₂O₃ : Eu cathodoluminescent (CL) phosphors were deposited using pulsed laser deposition using deposition temperature between 250°C and 800°C, O₂ pressures between residual vacuum (2×10⁻⁵ Torr) and 6 Torr, and post annealing up to 1200° for 1 h in air. The CL efficiency of the best thin film was about one third that of the starting powder. The brightness and efficiency of the thin films improved as the deposition temperature, O₂ pressure and post annealing temperature were increased, except that O₂ pressures above 600 mTorr did not significantly improve the CL properties. At deposition temperatures >600°C, the surface morphology changed from a smooth film to a nodular deposit for O₂ pressures >200 mTorr, with nodule dimensions ≈100 nm. Simultaneously, the CL properties improved dramatically because of enhanced optical scattering out of the thin film. Optical scattering was discussed in terms of anomalous diffraction. The CL properties also improved dramatically with high temperature post annealing. This effect was interpreted in terms of improved crystallinity and activation of the Eu. The low brightness and efficiency of thin films versus powder was affected by depletion of the Eu in the thin films owing to the deposition process.     

Material properties and the influence of metallic catalysts at the surface of highly dense SnO2 films

We present an approach to optimize the specific response to gases by using specially prepared nanosized platinum on highly dense sputtered polycrystalline SnO₂. Structural and morphological analyses of the SnO₂ and platinum thin films were performed. Gas measurements were carried out with single chip thin-film SnO₂ sensor arrays on silicon substrates. Pt nanoclusters covering the sensitive layer significantly affect the O₃, CO and NO₂ sensitivities and the corresponding dynamic response.     

The comparative XPS and PYS studies of SnO2 thin films prepared by L-CVD technique and exposed to oxygen and hydrogen

The electronic properties of the space charge layer of the tin dioxide thin films, prepared by the laser-induced chemical vapour deposition (L-CVD), have been studied using X-ray photoelectron spectroscopy (XPS) and photoemission yield spectroscopy (PYS). Based on the analysis of Sn3d_5/2 XPS peak, the influence of exposition to molecular oxygen O₂ and hydrogen H₂ on the stoichiometry of the L-CVD deposited SnO₂ thin films, as well as the interface Fermi level position in the band gap, have been determined and compared to the variation of the work function determined from the threshold of the ex situ recorded photoemission yield spectra. The observed changes of the interface Fermi level position and the work function upon adsorption/desorption of O₂ and H₂ were attributed to decrease/increase of the concentration of oxygen vacancies in the near surface region.     

Plasma planarization for sensor applications

Filling trenches in silicon using phosphosilicate glass (PSG) provides many possibilities for novel device structures for sensors and actuators. This paper describes a plasma planarization technique that provides fully planarized PSG filled silicon trenches for sensor applications. The technique consists of planarizing the substrate using two photoresist layers and plasma etching-back. The lower resist layer is the AZ5214 image reversal resist, which is patterned and then thermally cured. The upper resist layer is a global HPR204 coating. The plasma etching-back is carried out using CHF₃/C₂F ₆ gas mixture with an O₂ addition. It is shown that by using the image reversal photoresist approach, fully planarized surface coating can be obtained without resorting to an additional mask. By adding 25 sccm (14%) O₂ into the 137 sccm CHF₃+18 sccm C₂F₆ gas mixture, the etch rates for the photoresist and PSG can be matched. Process optimization for the two layer resist coating and plasma etching is discussed