On the correlation between morphology and gas sensing properties of RGTO SnO2 thin films

In this paper, we present the results of studies on optimalisation of morphology of the SnO₂ thin films grown by RGTO technique for application as gas sensor structures. The Sn thin films were grown on Si(111) wafer and Al₂O₃ ceramic plate heated in the range 235-295 °C and subsequently oxidized in dry oxygen atmosphere at high temperature, up to 700 °C. Our studies confirmed that the highest surface coverage of Sn droplets can be reached for the substrate temperature of about 265 °C leading to the highest surface-to-volume ratio of SnO₂ thin films. It was in a good correlation to the optimal gas sensor response and sensor sensitivity of RGTO SnO₂ thin films to nitrogen dioxide NO₂.     

XPS study of the surface chemistry of L-CVD SnO2 thin films after oxidation

In this paper we present the results of XPS study of the surface chemistry of L-CVD SnO₂ thin films onto Si(100) before and after subsequent additional oxidation. Moreover, the ageing effect was also studied in order to check the influence of ambient oxidation. As-deposited L-CVD SnO₂ thin films exhibit evident nonstoichiometry with the relative concentration [O]/[Sn] equal to 1.29 ± 0.1. After in situ oxidation at high temperature (800 K) the relative concentration [O]/[Sn] increases to 1.95 ± 0.05 which corresponds to the almost stoichiometric SnO₂. Almost the same relative concentration [O]/[Sn] of L-CVD SnO₂ thin films has been obtained after long term exposure to air. The oxidation states of L-CVD SnO₂ thin films in both cases were confirmed by the shape analysis of corresponding XPS O1s and Sn3d_5/2 peaks using the decomposition procedure. For the as-deposited L-CVD SnO₂ thin films a mixture of SnO and SnO₂ was observed, while for the oxidized L-CVD SnO₂ thin films the domination of SnO₂ was determined.     

H2S sensors based on SnO2 films: RGTO verses RF sputtering

Gas sensing characteristics of SnO₂ thin films prepared by RF sputtering have been investigated and compared to that of RGTO (Rheotaxially Grown and Thermally Oxidized) films. Both the sensor films exhibited a highly selective response towards H₂S with RF sputtered film showing better response characteristics. RF sputtered and RGTO films exhibited a maximum response of 54 and 15 towards 10 ppm of H₂S at an optimum operating temperature of 150 and 250 °C, respectively. Sputtered films exhibited a linear response in the wide concentration range from 500 ppb to 500 ppm while RGTO films were found to saturate for concentrations above 100 ppm. XPS investigations revealed that the RGTO films are more sub–stoichiometric or oxygen deficient than the sputtered films. Raman studies further indicates that the surface of sputtered and RGTO films are characterized by the presence of oxygen deficiency attributed to the “bridging-type” and deeper “in-plane/sub-bridging” oxygen vacancies, respectively. The improved response kinetics of the RF sputtered films is attributed to the presence of bridging type oxygen vacancies that facilitates the charge transfer between the sensor surface and H₂S molecules.     

Study of structural, electrical, optical, thermoelectric and photoconductive properties of S and Al co-doped SnO2 semiconductor thin films prepared by spray pyrolysis

In this paper, the effect of S and Al concentrations on the structural, electrical, optical, thermoelectric and photoconductive properties of the films was studied. The [Al]/[Sn] and [S]/[Sn] atomic ratios in the spray solutions were varied from 10 at.% to 40 at.% and 0 to 50 at.%, respectively. X-ray diffraction analysis showed the formation of SnO₂ cassiterite phase as a main phase and the numerous sulfur phases including S, SnS, SnS₂ and Sn₂S₃ in SnO₂:Al films. Scanning electron microscopy studies showed that in the absence of S, increasing the Al content results in a smaller grain size and with the addition of S, the films appear to contain small cracks and nodules. The minimum resistance of 0.175 (kΩ/□) was obtained for S-doped SnO₂:Al (40 at.%) film with 20 at.% S-doping. From the Hall effect measurements, the majority carrier concentration was obtained in order of 10¹⁷-10¹⁸ cm^− 3. The thermoelectric measurements showed that majority carriers change from electrons to holes for S-doping in SnO₂:Al (40 at.%) thin films. The maximum Seebeck coefficient of + 774 μV/K (at T = 370 K) was obtained for S-doped SnO₂:Al (10 at.%) film with 50 at.% S-doping. The band gap values were obtained in the range of 3.8-4.2 eV. The S-doped SnO₂:Al (40 at.%) films have shown considerably photoconductivity more than S-doped SnO₂:Al (10 at.%) with increasing S-doping. The best photoconductive property was obtained for co-doped SnO₂ thin film with 40 at.% Al and 5 at.% S concentration in solution.     

Enhanced H2S gas sensing properties of multiple-networked Pd-doped SnO2-core/ZnO-shell nanorod sensors

Pd-doped SnO₂-core/ZnO-shell nanorods were synthesized by using a three-step process: thermal evaporation of Sn powders in an oxygen atmosphere, atomic layer deposition of ZnO, and Pd diffusion followed by annealing. The sensitivity of the multiple networked SnO₂-core/ZnO-shell nanorod sensor to H₂S gas was found to be improved further significantly by Pd doping. The Pd-doped SnO₂-core/ZnO-shell nanorod sensor showed sensitivities of 6.4, 15.4, and 36.2% at H₂S concentrations of 20, 50, and 100 ppm at room temperature. The sensitivity of the nanorods was improved by more than 10 times at a H₂S concentration of 100 ppm. The sensitivity enhancement of the SnO₂-core/ZnO-shell nanorods by Pd doping may be attributed to the spillover effect, active reaction site generation, and the enhancement of chemisorption and dissociation of gas.     

Local surface morphology and chemistry of SnO2 thin films deposited by rheotaxial growth and thermal oxidation method for gas sensor application

In this paper experimental results of a comparative X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS) study of the crystalline structure, the local morphology, and the surface and in-depth chemistry of SnO₂ thin films obtained by Rheotaxial Growth and Thermal Oxidation (RGTO) method are presented. XRD rules out even a minor presence of a coexisting SnO phase. AFM and SEM show a fractal like morphology of nanograins (20 nm typical size) agglomerated in clusters of crystallites with a bimodal size distribution. XPS shows that the surface of the SnO₂ crystallites is slightly under-stoichiometric as expected from the oxygen deficient termination of their facets. Noteworthy, as evidenced by XPS depth profiles, there are no significant changes of the surface chemistry of the RGTO film with argon ion sputtering.     

Interaction of reducing gases with tin oxide films prepared by reactive evaporation techniques

SnO_2−x films were prepared by reactive thermal and e-beam evaporation of Sn on alumina substrates and by post deposition thermal treatment. X-ray diffraction measurements found that films are tin dioxide (SnO₂) phase with small amounts of SnO phase. The surface conductivity of films was measured in air and in presence of H₂S, H₂ and C₂H₅OH vapors at four sensor operating temperatures of 433-493 K. The resistance of SnO_2−x films decreases on exposure to H₂S but shows no change with hydrogen and ethanol. H₂S response decreases with rise in sensor temperature while both response and recovery times improve. H₂S signal enhances with increase in resistivity of SnO_2−x coatings. Our experiments conclude that increase in film conductance is due to chemical reaction between H₂S and SnO_2−x surface and there is little or no role of interaction of gas molecules with surface adsorbed charged oxygen species.     

Development of sensors based on CuO-doped SnO2 hollow spheres for ppb level H2S gas sensing

An effort has been made to develop a new kind of SnO₂–CuO gas sensor which could detect an extremely small amount of H₂S gas at relatively low working temperature. The sensor nanomaterials were prepared from SnO₂ hollow spheres (synthesized by employing carbon microspheres as temples) and Cu precursor by dipping method. The composition and structural characteristics of the as-prepared CuO-doped SnO₂ hollow spheres were studied by X-ray photoelectron spectroscopy, X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy. Gas-sensing properties of CuO-doped SnO₂ hollow sphere were also investigated. It was found that the sensor showed good selectivity and high sensitivity to H₂S gas. A ppb level detection limit was obtained with the sensor at the relatively low temperature of 35 °C. Such good performances are probably attributed to the hollow sphere nanostructures. Our results imply that materials with hollow sphere nanostructures are promising candidates for high-performance gas sensors.     

Sensing mechanism of SnO2 gas sensors

Studies have been made of the gas sensing properties of both steady and unsteady state SnO₂ thin film gas sensors in contact with CH₄ and H₂ in air from 400 to 500° C. The results suggest a new sensing mechanism model for SnO₂ semiconductor flammable gas sensors. This model is based on the following points: (i) Sensor conductivity is determined by the concentration of carrier electrons. (ii) Carrier concentration is controlled by surface unsaturated oxygen adsorption site concentration which is decided by the balance between oxygen adsorption and the surface reaction between oxygen adsorbate and flammable gases. (iii) The activation energy of the reaction is changed by the Fermi energy change for any change in sensor conductivity. This model explains all experimental results.     

The synthesis and photocatalytic properties of epitaxial SnO2-ZnS nanocomb heterostructure

A growth of SnO₂-ZnS heterostructure was realized by a rational two-step thermal evaporation method. Starting with Sn powder, the SnO₂ bicrystalline nanoribbons were grown on the silicon wafers. The secondary growth of ZnS on the former SnO₂ nanostructures correspondingly resulted in the SnO₂-ZnS nanocomb structures. We investigated the morphology, detailed structure by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). It revealed that the ZnS nanobranches with [001] growth direction are epitaxial grown from the (− 101) crystal plane of the SnO₂ trunks with a good lattice match. The nanocomb exhibited an effective photocatalytic activity.     

DRIFT studies of thick film un-doped and Pd-doped SnO2 sensors: temperature changes effect and CO detection mechanism in the presence of water vapour

Diffuse reflectance infrared Fourier transform measurements were performed on tin oxide based thick film gas sensors operated in normal working conditions. We characterised SnO₂ sensors at different temperatures between room temperature and 300 °C. The results show the presence of different surface OH groups as well as coordinated water on the SnO₂ sensor surface. Their intensity changes with temperature. During the temperature cycles the bands’ peak positions are reversibly changed but their intensity is not. CO measurements were performed at 300 °C at different humidity levels (0 and 50% r.h.) on un-doped and Pd-doped sensors. In the presence of CO we observed in the spectra: a decrease of the OH groups on the SnO₂ surfaces, the appearance of gaseous CO₂ and CO in the pores of the sensitive layer and an increase of hydrated protons and of the free charge concentration. The effects are dramatically influenced by the water vapour concentration, temperature, dopands (Pd) and can be correlated with simultaneously performed sensor resistance measurements.     

Fast response detection of H2S by CuO-doped SnO2 films prepared by electrodeposition and oxidization at low temperature

Fast response detection of H₂S by CuO-doped SnO₂ films prepared was prepared by a simple two-step process: electrodeposition from aqueous solutions of SnCl₂ and CuCl₂, and oxidization at 600 °C. The phase constitution and morphology of the CuO-doped SnO₂ films were characterized by X-ray diffraction and scanning electron microscopy. In all cases, a polycrystalline porous film of SnO₂ was the product, with the CuO deposited on the individual SnO₂ particles. Two types of CuO-doped SnO₂ films with different microstructures were obtained via control of oxidation time: nanosized CuO dotted island doped SnO₂ and ultra-uniform, porous, and thin CuO film coated SnO₂. The sensor response of the CuO doped SnO₂ films to H₂S gas at 50–300 ppm was investigated within the temperature range of 25–125 °C. Both of the CuO-doped SnO₂ films show fast response and recovery properties. The response time of the ultra-uniform, porous, and thin CuO coated SnO₂ to H₂S gas at 50 ppm was 34 s at 100 °C, and its corresponding recovery time was about 1/3 of the response time.     

SnO2 thin-films prepared by a spray-gel pyrolysis: Influence of sol properties on film morphologies

Nanostructured tin oxide films were prepared by depositing different sols using the so-called spray-gel pyrolysis process. SnO₂ suspensions (sols) were obtained from tin (IV) tert-amyloxide (Sn(t-OAm)₄) or tin (IV) chloride pentahydrate (SnCl₄·5H₂O) precursors, and stabilized with ammonia or tetraethylammonium hydroxide (TEA-OH). Xerogels from the different sols were obtained by solvent evaporation under controlled humidity.The Relative Gelling Volumes (RGV) of these sols strongly depended on the type of precursor. Xerogels obtained from inorganic salts gelled faster, while, as determined by thermal gravimetric analysis, occluding a significant amount of volatile compounds. Infrared spectroscopic analysis was performed on raw and annealed xerogels (300, 500 °C, 1 h). Annealing removed water and ammonium or alkyl ammonium chloride, increasing the number of Sn-O-Sn bonds.SnO₂ films were prepared by spraying the sols for 60 min onto glass and alumina substrates at 130 °C. The films obtained from all the sols were amorphous or displayed a very small grain size, and crystallized after annealing at 400 °C or 500 °C in air for 2 h. X-ray diffraction analysis showed the presence of the cassiterite structure and line broadening indicated a polycrystalline material with a grain size in the nanometer range. Results obtained from Scanning Electron Microscopy analysis demonstrated a strong dependence of the film morphology on the RGV of the sols. Films obtained from Sn(t-OAm)₄ showed a highly textured morphology based on fiber-shape bridges, whereas the films obtained from SnCl₄·5H₂O had a smoother surface formed by “O-ring” shaped domains.Lastly, the performance of these films as gas sensor devices was tested. The conductance (sensor) response for ethanol as a target analyte was of the same order of magnitude for the three kinds of films. However, the response of the highly textured films was more stable with shorter response times.     

Gas sensing properties of lead doped tin oxide thick films

The present investigation deals with the fabrication of liquid petroleum gas (LPG) sensor materials based on semiconducting oxide SnO₂. The gas sensor materials have been prepared by conventional solid-state route. The effect of Pb incorporation, operating temperature, morphology, and sensitivity is discussed using the results of X-ray diffraction (XRD), along with sensing performance. Out of various sensor compositions, Pb doped SnO₂ sintered at 1000 °C for 2 h has shown high sensitivity towards LPG at an operating temperature of 150 °C. Different characterization techniques have been employed, such as surface area analyzer, X-ray diffraction (XRD), to study the formation of SnO₂, surface area and crystallite size, respectively. The results suggested the possibility of utilizing the sensor element for the detection of LPG.     

Effect of gas type,pressure and temperature on the electrical characteristics of Al-doped SnO2 thin films deposited by RGTO method for gas sensor application

In this paper, we present the experimental results of Al-doped SnO₂ thin films obtained by Rheotaxial Growth and Thermal Oxidation (RGTO) method. The effect of gas type (synthetic air, CO, NO₂ and H₂), pressure (10⁻⁴, 1 and 100 mbar) and temperature (in the range 300–650 K), on the electrical properties of Al-doped SnO₂ thin films were considered. The change of the work function of Al-doped SnO₂ thin films as a function of exposure time to synthetic air, CO (150 ppm), H₂ (1000 ppm) and NO₂ (80 ppm) was discussed under different pressures of 10⁻⁴, 1 and 100 mbar. The effect of ambient temperature at 303,373 and 433 K on the work function difference was investigated. The results reveal that the sensitivity of reaction to the gases was improved to high ambient temperature. The time and temperature dependent of electrical properties of the Al-doped SnO₂ films were studied using four probe method. The Al-doped SnO₂ films demonstrate negative temperature coefficient (NTC) characteristics of resistance in the higher temperature range as well as positive temperature coefficient (PTC) characteristics of resistance in the lower temperature range. The best sensitivity and the optimum work temperature were also considered.     

Influence of light on the adsorption processes during interaction between reducer gases and a SnO2 film

The influence of the light of a low-power light-emitting diode on the adsorption processes in SnO₂ sensor layers of test structures of gas sensors has been analyzed. It is established that the optical activation of SnO₂ surface induces an additional peak of gas sensitivity at temperatures that are lower than the temperature of maximum gas sensitivity of the sensor without illumination. The results indicate that there are two mechanisms of light stimulation of gas adsorption.     

Phase composition of selenized Cu2ZnSnSe4 thin films determined by X-ray diffraction and Raman spectroscopy

Thin layers of Sn onto Cu-Zn alloy with different component ratios were processed at different temperatures. Scrupulous comparative analyses were performed by room temperature Raman spectroscopy and X-ray-diffractometry. An excess of tin on the surface results in isothermal selenization at 450 °C in the hexagonal residuals of unstable SnSe₂ in the well-crystallized Stannite — Cu₂ZnSnSe₄. In similar selenization conditions, copper-rich layers as precursors result in the Stannite phase with micro-immersions of CuSe. Low-temperature photoluminescence spectra of selenized films indicated to two Gaussian shaped bands at 0.81 and 1.16 eV.     

Transparent conducting properties of Ni doped zinc oxide thin films prepared by a facile spray pyrolysis technique using perfume atomizer

Undoped and Ni doped zinc oxide (Ni–ZnO) thin films were prepared by a facile spray pyrolysis technique using perfume atomizer from aqueous solution of anhydrous zinc acetate (Zn(CH₃COOH)₂ and hexahydrated nickel chloride (NiCl₂·6H₂O) as sources of zinc and nickel, respectively. The films were deposited onto the amorphous glass substrates kept at (450 °C). The effect of the [Ni]/[Zn] ratio on the structural, morphological, optical and electrical properties of Ni doped ZnO thin film was studied. It was found from X-ray diffraction (XRD) analysis that both the undoped and Ni doped ZnO films were crystallized in the hexagonal structure with a preferred orientation of the crystallites along the [002] direction perpendicular to the substrate. The scanning electron microscopy (SEM) images showed a relatively dense surface structure composed of crystallites in the spherical form whose average size decreases when the [Ni]/[Zn] ratio increases. The optical study showed that all the films were highly transparent. The optical transmittance in the visible region varied between 75 and 85%, depending on the dopant concentrations. The variation of the band gap versus the [Ni]/[Zn] ratio showed that the energy gap decreases from 2.95 to 2.72 eV as the [Ni]/[Zn] ratio increases from 0 to 0.02 and then increases to reach 3.22 eV for [Ni]/[Zn] = 0.04. The films obtained with the [Ni]/[Zn] ratio = 0.02 showed minimum resistivity of 2 × 10⁻³ Ω cm at room temperature.     

Fabrication of resistive CO gas sensor based on SnO2 nanopowders via low frequency AC electrophoretic deposition

A resistive CO gas sensor has been fabricated using AC electrophoretic deposition (ACEPD) technique. SnO₂ thick films are deposited by applying low frequency (0.01–1,000 Hz) AC electric field to a stable suspension of SnO₂ nanoparticles in acetyl acetone. A carbon film base electrode is used as deposit substrate. Effect of CO gas exposure on conductivity of the SnO₂ film at 300 °C is investigated. Results show that the sensor is sensitive and its response is repeatable. This work shows that ACEPD can be used as an easy and cheap technique for fabrication of electronic devices such as ceramic gas sensors.     

Nanocrystalline ITO-Sn2S3 transparent thin films for photoconductive sensor applications

Nanocrystalline indium tin oxide (ITO) film containing 5 wt% Sn was prepared on glass substrate by the spray pyrolysis technique at a substrate temperature of 500 °C. In order to enhance the photosensitivity of ITO, thiourea (CS(NH₂)₂ was added to the precursor to obtain the [S]/[In] proportion of 0.1, 0.2, 0.4 and 0.6. The X-ray diffraction patterns showed that beside the bixbyite structure of ITO, the characteristic peaks corresponding to Sn₂S₃ appeared in XRD profiles recorded for the films with [S]/[In] = 0.1 and 0.2. In addition, sulfur additive caused a considerable decline in crystallinity quality. The optical properties of the films were studied using transmittance measurements in the wavelength range 300–1,000 nm. As a result, ITO and ITO-Sn₂S₃ thin films were prepared with resistivity of 3.06–3.7 × 10^?4 Ω cm and a transmittance of 88–91 % at the wavelength of 550 nm. Moreover, the electrical resistances of ITO and ITO-Sn₂S₃ films as a function of time were measured in darkness and under illumination of light in the visible range. The photoresistance results revealed that the ITO-Sn₂S₃ film with [S]/[In] = 0.2 was efficiently sensitive to visible light for photoconductive sensor applications, besides being high conductive and transparent.