Solid-state reaction synthesized Pd-doped tin oxide thick film sensor for detection of H2, CO, LPG and CH4

This paper deals with the synthesis of tin oxide (SnO₂) nano-powders by a solid-state reaction technique. The synthesized powders have been characterized by simultaneous thermo gravimetric and differential thermal analysis (TG–DTA) and X-ray diffraction (XRD) techniques. Suitable calcination temperature is established by XRD and TG–DTA analysis. Thick film sensors have been developed from as-prepared undoped and palladium (Pd) doped (0.5 and 1 wt%) SnO₂ powders using screen printing technology for the detection of various pollutant gases such as, hydrogen (H₂), carbon monoxide (CO), liquefied petroleum gas (LPG) and methane (CH₄). The surface of the thick film sensor has been characterized by field emission scanning electron microscopy (FESEM). The sensing characteristics of thick films have been studied from the aspect of crystallite size of sensing material and microstructure of the thick film surface. It is found that SnO₂ doped with 1 % Pd exhibits the maximum sensitivity (79 %) towards CO gas along with fast response/recovery time (80 s, 197 s) and almost insensitive for H₂, LPG and CH₄.     

The effect of Au and Pt nanoclusters on the structural and hydrogen sensing properties of SnO2 thin films

Au and Pt nanoparticle modified SnO₂ thin films were prepared by the sol-gel method on glass substrates targeting sensing applications. Structural and morphological properties of these films were studied using X-ray Diffraction and Scanning Electron Microscopy. It was proved that the films crystallized in tetragonal rutile SnO₂ crystalline structure. Scanning Electron Microscopy observations showed that the metallic clusters' dimensions and geometry depend on the kind of the metal (Au or Pt) while SnO₂ films surface remains almost the same: nanostructured granular very smooth. Optical properties of the films were studied using UV-visible spectroscopy. The modified SnO₂ films were tested as hydrogen sensors. The response of SnO₂, SnO₂-Au and SnO₂-Pt thin films against hydrogen was investigated at different operating temperatures and for different gas concentrations. The addition of metal nanoparticles was found to decrease the detection limit and the operating temperature (from 180 °C to 85 °C), while increasing the sensing response signal.     

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.     

Effect of processing and palladium doping on the properties of tin oxide based thick film gas sensors

In the present work, solid-state reaction and sol–gel route derived pure tin oxide (SnO₂) powders have been used to develop the palladium (Pd)-doped SnO₂ thick film sensors for detection of liquefied petroleum gas (LPG). Efforts have been made to study the gas sensing characteristics i.e., sensor response, response/recovery time and repeatability of the thick film sensors. The response of the sensors has been investigated at different operating temperatures from 200 to 350 °C in order to optimise the operating temperature which yields the maximum response upon exposure to fixed concentration of LPG. The optimum temperature is kept constant to facilitate the gas sensing characteristics as a function of the various concentration (0.25–5 vol%) of LPG. The structural and microstructural properties of Pd-doped SnO₂ powder and developed sensors have been studied by performing X-ray diffraction and field emission electron microscopy measurements. The improvement in the response along with better response and recovery time have been correlated to the reduction in crystallite size of SnO₂ powder and morphology of printed sensor in thick film form. It is found that the thick film sensor developed by using sol–gel route derived SnO₂ powder with an optimum doping of 1 wt% Pd is extremely sensitive (86 %) to LPG at 350 °C.     

Liquid petroleum gas sensor based on SnO2/Pd composite films deposited on Si/SiO2 substrates

SnO₂/Pd composite films were synthesized by d.c. sputtering of a SnO₂ target followed by thermal evaporation of a thin layer of Pd on top of it. This structure, deposited on Si wafer with 300 μm SiO₂ on the top, was subjected to rapid thermal annealing at 573 K for 5 min for the incorporation of Pd in SnO₂. The films were characterized by microstructural, optical, FTIR and Raman studies. Liquid petroleum gas (LPG) sensing measurements were carried out on these films. Sensitivity of 72% was obtained at an operating temperature of ∼573 K. The response time for these sensors was found to be ∼27 s. Sensitivity was found to increase with grain growth at higher sensing temperatures. It could be observed that the selectivity for LPG is extremely good as compared to that of methane, hydrogen, CO₂ and C₂H₅OH.     

Structural and optoelectronic properties of indium doped SnO2 thin films deposited by sol gel technique

Indium doped tin oxide (SnO₂:In) thin films were deposited on glass substrates by sol–gel dip coating technique. X-ray diffraction pattern of SnO₂:In thin films annealed at 500 °C showed tetragonal phase with preferred orientation in T (110) plane. The grain size of tin oxide (SnO₂) in SnO₂:In thin films are found to be 6 nm which makes them suitable for gas sensing applications. AFM studies showed an inhibition of grain growth with increase in indium concentration. The rms roughness value of SnO₂:In thin films are found to 1 % of film thickness which makes them suitable for optoelectronic applications. The film surface revealed a kurtosis values below 3 indicating relatively flat surface which make them favorable for the production of high-quality transparent conducting electrodes for organic light-emitting diodes and flexible displays. X-ray photoelectron spectroscopy gives Sn 3d, In 3d and O 1s spectra on SnO₂:In thin film which revealed the presence of oxygen vacancies in the SnO₂:In thin film. These SnO₂:In films acquire n-type conductivity for 0–3 mol% indium doping concentration and p type for 5 and 7 mol% indium doping concentration in SnO₂ films. An average transmittance of >80 % (in ultra-violet–Vis region) was observed for all the SnO₂:In films he In doped SnO₂ thin films demonstrated the tailoring of band gap values. Photoluminescence spectra of the films exhibited an increase in the emission intensity with increase in indium doping concentration which may be due structural defects or luminescent centers, such as nanocrystals and defects in the SnO₂.     

Sensor Properties of Pd-Doped SnO2 Films Deposited by Laser Ablation

Polycrystalline SnO₂ and SnO₂ films were surface-doped with palladium using laser ablation. The effect of the energy density of pulsed KrF laser radiation on the plasma generation process and Pd deposition rate was studied, and the depth profiles of Pd in the films were determined. The gas response of the Pd/SnO₂, SnO₂, and Pd/SnO₂ films was studied between 200 and 380°C using a mixture of 1 vol % H₂ with N₂. Surface doping with Pd was found to enhance the hydrogen sensitivity of SnO₂ by two orders of magnitude.     

Silicon doped SnO2 films for liquid petroleum gas sensor

Silicon doped SnO₂ films were synthesized by sputtering SnO₂ layer onto glass substrates with appropriate amount of silicon sputtered onto them. The bilayer structures were subjected to rapid thermal annealing for the incorporation of Si in SnO₂ matrix. The films thus obtained were characterized by measuring optical and microstructural properties. Liquid petroleum gas (LPG) sensing properties were also investigated. FTIR and Raman studies were also carried out on these films, both, before and after LPG exposure.     

Hydrogen sensing properties of Pd-doped SnO2 sputtered films with columnar nanostructures

Pd-doped SnO₂ sputtered films with columnar nanostructures were deposited using reactive magnetron sputtering at the substrate temperature of 300 °C and the discharge gas pressures of 1.5, 12, and 24 Pa. Structural characterization by means of X-ray diffraction and scanning electron microscopy shows that the films composed of columnar nanograins have a tetragonal SnO₂ structure. The films become porous as the discharge gas pressure increases. Gas sensing measurements demonstrate that the films show reversible response to H₂ gas. The sensitivity increases as the discharge gas pressure increases, and the operating temperature at which the sensitivity shows a maximum is lowered. The highest sensitivity defined by (R_a − R_g) / R_g, where R_a and R_g are the resistances before and after exposure to H₂, 84.3 is obtained for the Pd-doped film deposited at 24 Pa and 300 °C upon exposure to 1000 ppm H₂ gas at the operating temperature of 200 °C. The improved gas sensing properties were attributed to the porosity of columnar nanostructures and catalytic activities of Pd doping.     

CO sensing with SnO2 thick film sensors: role of oxygen and water vapour

The paper investigates the gas response of nanocrystalline SnO₂ based thick film sensors upon exposure to carbon monoxide (CO) in changing water vapour (H₂O) and oxygen (O₂) backgrounds. The sensing materials were undoped, Pt- and Pd-doped SnO₂. We found that in the absence of oxygen, the sensor signal (defined as the ratio between the resistance in the background gas, R₀ and the resistance in the presence of the target gases, R, namely R₀/R) have the highest values. These values are higher for doped materials than for the undoped ones. The presence of humidity increases dramatically the sensor signal of the doped materials. In the presence of oxygen, the sensor signal decreases significantly for all sensor materials. The results indicate that there is a competitive adsorption between O₂ and H₂O related surface species and, as a result, different sensing mechanisms can be observed for CO.     

Role of surface properties of MoO3-doped SnO2 thin films on NO2 gas sensing

The role of surface morphology of MoO₃-doped SnO₂ thin film on the gas sensing properties is analyzed. SnO₂ thin films doped with 1, 3, 5 and 10 wt% MoO₃ are prepared by sol-gel spin coating process. Structural and morphological properties are studied using glancing angle X-ray diffractometer, atomic force microscopy, transmission electron microscopy and high resolution transmission electron microscopy. Energy dispersive X-ray analysis and X-ray photoelectron spectroscopy studies are used for chemical analysis. A good correlation is found between the characteristics of the surface and gas sensing properties of these films. MoO₃ addition is found responsible for increase in acidic nature of films which in turn increases their sensitivity and selectivity towards NO₂ gas.     

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.     

The gas sensitivity of nano TiO2-SnO2 film doped with Cd(II) ion

The nanocomposite oxide (0.2TiO₂-0.8SnO₂) doped with Cd²⁺ powder have been prepared and characterized by XRD and their gas-sensing sensitivity were characterized using gas sensing measurement. Experimental results show that, bicomponent nano anatase TiO₂ and rutile SnO₂ particulate thick film doped with Cd²⁺ behaves with good sensitivity to formaldehyde gas of 200 ppm in the air, and the optimum sensing temperature was reduced from 360 °C to 320 °C compared with the undoped Cd²⁺ thick film. The gas sensing thick films doped with Cd²⁺ also show good selectivity to formaldehyde among benzene, toluene, xylene and ammonia as disturbed gas and could be effectively used as an indoor formaldehyde sensor.     

Gas sensing selectivity of oxygen-regulated SnO2 films with different microstructure and texture

The selectivity of gas sensing materials is increasingly important for their applications. The oxygen-regulated SnO₂ films with (110) and (101) preferred orientation were obtained through magnetron sputtering, followed by annealing treatment. Their micro-structure, surface morphology and gas response were investigated by advanced structural characterization and property measurement. The results showed that the as-prepared (110)-oriented SnO₂ film was oxygen-rich and had more adsorption sites while the as-prepared (101)-oriented SnO₂ film was oxygen-poor and more sensitive to de-oxidation. H₂ gas sensitivity, response speed, selectivity between H₂ and CO of the (110)-orientated SnO₂ film was superior to that of the (101)-orientated SnO₂ film. After treated at high temperature and high vacuum, the reduction of gas-sensing properties of the annealed (110) SnO₂ film was much more than that of the annealed (101) SnO₂ film. The lattice oxygen was responsible for the difference in gas-sensing response between (110) and (101)-oriented SnO₂ films under oxygen regulation. This work indicated the gas-sensing selectivity of the different crystal planes in SnO₂ film, providing a significant reference for design and extension of the related materials.     

X-ray absorption near-edge spectroscopy investigation of the oxidation state of Pd species in nanoporous SnO2 gas sensors for methane detection

We report the correlation of the aging of Pd-doped SnO₂ methane sensors with the change of the oxidation state of Pd. Mesoporous SnO₂ doped with palladium species was prepared and exposed to different gas mixtures at high temperature (600 °C) to simulate long term usage. After each exposure step a fraction of the sample was cooled down to “freeze” the current oxidation state of Pd which was then analyzed by X-ray Absorption Near-Edge Spectroscopy (XANES) using the 'white line' (i.e. the absorption peak corresponding to the transition from the 2p_3/2 core level to unoccupied 4 d states) intensity of the L(III) edge as a probe for the oxidation state. The Pd oxidation state correlates with the response of the resistive SnO₂ sensor to methane gas, as determined by measuring the gas response to different concentrations of methane. Samples treated with 5000 ppm methane in air show a significant reduction of Pd(II) to Pd(0), depending clearly on the carrier gas (synthetic air, pure nitrogen) and on the temperature (600 °C vs. 300 °C).     

Synthesis of SnO2/Pd composite films by PVD route for a liquid petroleum gas sensor

Synthesis of SnO₂/Pd composite films was carried out by depositing antimony doped SnO₂ films by magnetron sputtering technique and evaporating a thin layer of palladium on the top of it. This bi-layer structure was subjected to rapid thermal annealing for the incorporation of Pd in SnO₂. The films thus obtained were characterized by measuring electrical, optical and microstructural properties. Liquid petroleum gas-sensing properties were also investigated.     

MIS gas sensors based on porous silicon with Pd and WO3/Pd electrodes

Pd and WO₃/Pd gate metal-oxide-semiconductor (MIS) gas sensitive structures based on porous silicon layers are studied by the high frequency C(V) method. The chemical compositions of composite WO₃/Pd electrodes are characterized by secondary-ion mass spectrometry (SIMS). The atomic force microscopy (AFM) was used for morphologic studies of WO₃/Pd films. As shown in the experiments, WO₃/Pd structures are more sensitive and selective to the adsorption of hydrogen sulphide compared to Pd gate. The analyses of kinetic characteristics allow us to determine the response and characteristic times for these structures. The response time of MIS-structures with thin composite WO₃/Pd electrodes (the thickness of Pd is about 50 nm with WO₃ clusters on its surface) is slower compared to the structures with Pd electrodes. Slower sensor responses of WO₃-based gas sensors may be associated with different mechanism of gas sensitivity of given structures. The enhanced sensitivity and selectivity to H₂S action of WO₃/Pd MIS-structures can also be explained by the chemical reaction that occurs at the catalytic active surface of gate electrodes. The possible mechanisms of enhanced sensitivity and selectivity to H₂S adsorption of MIS gas sensors with WO₃/Pd composite gate electrodes compared to pure Pd have been analyzed.     

SnO2–Au nanocomposite synthesized by successive ionic layer deposition method: Characterization and application in gas sensors

A possibility of synthesizing the SnO₂–Au nanocomposite by the successive ionic layer deposition (SILD) method is demonstrated in this article. It is shown that as a result of successive treatments in solutions of Sn(OH)_xF_yCl_z and HAuCl₄ the SnO₂–Au nanocomposite with a Sn/Au ratio varying from 1:1 to 6:1 can be formed on the surface of substrates. It is found that the value of this ratio depends on the concentration of F ions in solution. The gold in the indicated composite is in the metallic state. The growth of the SnO₂–Au composite takes place through the formation of 3-D precipitates, which form a continuous film after 13 deposition cycles. As a result, a layer with averaged thickness of up to 20 nm is formed on the surface. Nanocomposite films, even after treatment at an annealing temperature T_an ∼ 600 °C, have finely dispersed structure. The size of the Au clusters incorporated in the SnO₂ matrix is in the range from 3 to 15 nm. Gas sensing characteristics of SnO₂ films modified by SnO₂–Au nanocomposites are discussed as well. It is shown that surface modification by SnO₂–Au nanocomposites can be used for improving operating characteristics of conductometric SnO₂-based gas sensors.     

Kinetics-Driven Dual Hydrogen Spillover Effects for Ultrasensitive Hydrogen Sensing

Palladium (Pd)-modified metal oxide semiconductors (MOSs) gas sensors often exhibit unexpected hydrogen (H₂) sensing activity through a spillover effect. However, sluggish kinetics over a limited Pd-MOS surface seriously restrict the sensing process. Here, a hollow Pd-NiO/SnO₂ buffered nanocavity is engineered to kinetically drive the H₂ spillover over dual yolk-shell surface for the ultrasensitive H₂ sensing. This unique nanocavity is found and can induce more H₂ absorption and markedly improve kinetical H₂ ab/desorption rates. Meanwhile, the limited buffer-room allows the H₂ molecules to adequately spillover in the inside-layer surface and thus realize dual H₂ spillover effect. Ex situ XPS, in situ Raman, and density functional theory (DFT) analysis further confirm that the Pd species can effectively combine H₂ to form Pd-H bonds and then dissociate the hydrogen species to NiO/SnO₂ surface. The final Pd-NiO/SnO₂ sensors exhibit an ultrasensitive response (0.1–1000 ppm H₂) and low actual detection limit (100 ppb) at the operating temperature of 230 °C, which surpass that of most reported H₂ sensors.     

Gas-sensing properties of nanostructured SnO2-based sensor synthesized with different methods

Herein we report the preparation of SnO₂ nanomatierials by chemical precipitation, sol-gel and dissolution-pyrolysis. Furthermore, we studied their sensing properties. The composition, crystal structure and ceramic microstructure of the powders obtained are characterized by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The results show SnO₂ fabricated through the three methods has rutile structure and the sizes of spherical particles are below 30 nm. From the result, we can also know that the thick films deposited onto alumina substrates show different morphology, and which are fabricated by dissolution-pyrolysis has fibrous structure. We investigate the sensitivities, response and recovery times of the three sensors. The results of gas sensing measurement show that SnO₂-based sensor prepared by dissolution-pyrolysis method has high sensitivity, quick response and recovery behavior to the gases we studied. It also has wider range of working temperature that is from 25 to 400 °C compared with SnO₂-based sensor fabricated by the other two methods.