Response to oxygen and chemical properties of SnO2 thin-film gas sensors

The measurements of the response—in terms of the conductance changes—to oxygen adsorption of tin dioxide (SnO₂) thin-film-based gas sensors were performed. The sensing SnO₂ layers were obtained by means of the rheotaxial growth and thermal oxidation (RGTO) method. The sensor responses were measured under a dry gas flow containing oxygen in nitrogen, within the range of temperature from 25 to 540 °C. For comparison, similar studies were performed for a commercial SnO₂ thick-film (TGS 812) gas sensor.The in-depth profiles of the chemical composition of the RGTO SnO₂ layers were determined from the scanning Auger microprobe experiment. The changes in concentration ratios [O]/[Sn] and [C]/[Sn] from the near-surface region towards the grain bulk were shown.     

Ionic‐Activated Chemiresistive Gas Sensors for Room‐Temperature Operation

The development of high performance gas sensors that operate at room temperature has attracted considerable attention. Unfortunately, the conventional mechanism of chemiresistive sensors is restricted at room temperature by insufficient reaction energy with target molecules. Herein, novel strategy for room temperature gas sensors is reported using an ionic‐activated sensing mechanism. The investigation reveals that a hydroxide layer is developed by the applied voltages on the SnO₂ surface in the presence of humidity, leading to increased electrical conductivity. Surprisingly, the experimental results indicate ideal sensing behavior at room temperature for NO₂ detection with sub‐parts‐per‐trillion (132.3 ppt) detection and fast recovery (25.7 s) to 5 ppm NO₂ under humid conditions. The ionic‐activated sensing mechanism is proposed as a cascade process involving the formation of ionic conduction, reaction with a target gas, and demonstrates the novelty of the approach. It is believed that the results presented will open new pathways as a promising method for room temperature gas sensors.     

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.     

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.     

Experimental studies of O2-SnO2 surface interaction using powder, thick films and monocrystalline thin films

Surface properties of solids and the interactions between molecules and solid surfaces are important for many technical applications. They also involve a range of physical and chemical phenomena of fundamental scientific interest. The importance of oxygen chemistry at SnO₂ surfaces follows from the fact that SnO₂ is used as an active material in gas sensor applications. The operation principle of these sensors is usually based on measurable conductance response of the material, which is understood in terms of reactions of gas molecules with different oxygen species adsorbed onto the surface. The role of the lattice oxygen, but in particular, the bridging oxygen atoms on SnO₂ surfaces, is also active. Detailed understanding of the reaction mechanisms of various oxygen species at SnO₂ surfaces is important, as it offers a way to improve the sensitivity and selectivity of the sensors.Oxygen adsorption-desorption kinetics at the SnO₂ surface is studied experimentally using O₂-temperature-programmed desorption (TPD) method together with conductance measurements in the case of SnO₂ powder and polycrystalline thick films made from the powder. In addition, CO-TPD is studied and the transient behaviour of various oxygen species is considered. Molecular beam epitaxy (MBE) was also used to fabricate polycrystalline and monocrystalline thin films with the SnO₂(101) face on single crystal sapphire substrate. Simultaneous surface potential and conductance measurements during heating and cooling in different ambient atmospheres were used to characterize the monocrystalline SnO₂(101) surface after various surface treatments.     

Structural,morphological, optical and gas sensing properties of pure and Ce doped SnO<Subscript>2</Subscript> thin films prepared by jet nebulizer spray pyrolysis (JNSP) technique

Pure and cerium (Ce) doped tin oxide (SnO₂) thin films are prepared on glass substrates by jet nebulizer spray pyrolysis technique at 450 °C. The synthesized films are characterized by X-ray diffraction (XRD), scanning electron microscopy, energy dispersive analysis X-ray, ultra violet visible spectrometer (UV–Vis) and stylus profilometer. Crystalline structure, crystallite size, lattice parameters, texture coefficient and stacking fault of the SnO₂ thin films have been determined using X-ray diffractometer. The XRD results indicate that the films are grown with (110) plane preferred orientation. The surface morphology, elemental analysis and film thickness of the SnO₂ films are analyzed and discussed. Optical band gap energy are calculated with transmittance data obtained from UV–Visible spectra. Optical characterization reveals that the band gap energy is found decreased from 3.49 to 2.68 eV. Pure and Ce doped SnO₂ thin film gas sensors are fabricated and their gas sensing properties are tested for various gases maintained at different temperature between 150 and 250 °C. The 10 wt% Ce doped SnO₂ sensor shows good selectivity towards ethanol (at operating temperature 250 °C). The influence of Ce concentration and operating temperature on the sensor performance is discussed. The better sensing ability for ethanol is observed compared with methanol, acetone, ammonia, and 2-methoxy ethanol gases.     

Influence of surface Pd doping on gas sensing characteristics of SnO2 thin films deposited by spray pirolysis

In this article the analysis of steady state and transient gas sensing characteristics of undoped and Pd surface doped SnO₂ films, deposited by spray pyrolysis, is described. The influence of parameters such as air humidity (2-50% RH), operation temperature (25-500 °C) and Pd surface concentration (0-1% ML Pd) on gas response to CO and H₂ (0.1-0.5%), response time, shape of sensitivity S(T) curves and activation energy of τ(1/kT) dependencies are discussed. A mechanism based on a chemisorption model is proposed to explain how Pd influences the gas sensing characteristics of SnO₂ films.     

Characterizing individual SnO2 nanobelt field-effect transistors and their intrinsic responses to hydrogen and ambient gases

The intrinsic electrical properties of individual single-crystalline tin dioxide nanobelts, synthesized via catalyst-free physical vapor deposition, were studied and correlated to the surface oxygen deficiency with the presence of various ambient gases, especially hydrogen. Four-terminal field-effect transistor (FET) devices based on individual SnO₂ nanobelts were fabricated with SiO₂/Si as back gate and RuO₂/Au as contacts. Four-probe I–V measurements verify channel-limited transistor characteristics and ensure that the hydrogen gas sensing reflect electrical modification of the nanobelt channel. The demonstrated results of the intrinsic SnO₂ nanobelt based hydrogen sensor operating at room temperature provide useful information on the synthesis of room temperature chemo-resistive gas sensors with good sensitivity and stability. To evaluate the impact of surface gas composition on the electrical properties of SnO₂ nanobelts, their temperature-dependent resistivity (ρ), effective carrier mobility (μ_eff) and effective carrier concentration (n_e) were determined under different oxygen concentrations.     

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.     

Pt-Pd Nanoalloys Functionalized Mesoporous SnO2 Spheres: Tailored Synthesis,Sensing Mechanism,and Device Integration

Methane (CH₄), as the vital energy resource and industrial chemicals, is highly flammable and explosive for concentrations above the explosive limit, triggering potential risks to personal and production safety. Therefore, exploiting smart gas sensors for real-time monitoring of CH₄ becomes extremely important. Herein, the Pt-Pd nanoalloy functionalized mesoporous SnO₂ microspheres (Pt-Pd/SnO₂) were synthesized, which show uniform diameter (≈500 nm), high surface area (40.9–56.5 m² g⁻¹), and large mesopore size (8.8–15.8 nm). The highly dispersed Pt-Pd nanoalloys are confined in the mesopores of SnO₂, causing the generation ofoxygen defects and increasing the carrier concentration of sensitive materials. The representative Pt₁-Pd₄/SnO₂ exhibits superior CH₄ sensing performance with ultrahigh response (R_a/R_g = 21.33 to 3000 ppm), fast response/recovery speed (4/9 s), as well as outstanding stability. Spectroscopic analyses imply that such an excellent CH₄ sensing process involves the fast conversion of CH₄ into formic acid and CO intermediates, and finally into CO₂. Density functional theory (DFT) calculations reveal that the attractive covalent bonding interaction and rapid electron transfer between the Pt-Pd nanoalloys and SnO₂ support, dramatically promote the orbital hybridization of Pd₄ sites and adsorbed CH₄ molecules, enhancing the catalytic activation of CH₄ over the sensing layer.     

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.     

Effect of Au and NiO catalysts on the NO<Subscript>2</Subscript> sensing properties of nanocrystalline SnO<Subscript>2</Subscript>

Nanocrystalline tin dioxide has been synthesized, and its surface has been modified with Au and NiO. Their distributions in the nanocrystalline tin dioxide have been examined by X-ray diffraction and transmission electron microscopy. The NO₂ sensing properties of the materials have been studied in the range 100–1000 ppb. Both gold and nickel enhance the NO₂ response of SnO₂. Codoping with Au and NiO markedly enhances its sensing response and, in addition, lowers the peak response temperature. The observed effect of NO₂ concentration in dry air on the sensing response of the SnO₂〈Au, NiO〉 nanocomposite can be understood in terms of the sequence of processes that take place on the SnO₂ surface upon nitrogen dioxide adsorption in the presence of chemisorbed oxygen.     

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.     

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.     

Gas sensors based on doped-CNT/SnO2 composites for NO2 detection at room temperature

For the first time nitrogen or boron doped carbon nanotubes were added into a SnO₂ matrix to develop a new hybrid CNTs/SnO₂ gas sensors. The hybrid sensor is utilised to detect low ppb concentrations of NO₂ in air, by measuring resistance changes of thin CNTs/SnO₂ films. The tests are performed at room temperature. For comparison, pure SnO₂ and N or B-substituted CNT sensors are also examined. Comparative gas sensing results reveal that the CNTs/SnO₂ hybrid sensors exhibit much higher response towards NO₂, at least by a factor of 10, and good baseline recovery properties at room temperature than the blank SnO₂ and the N or B-substituted CNT sensors. This finding shows that doping SnO₂ with low quantity of CNTs doped with heteroatoms can dramatically improve sensitivity.     

SnO2:Cu films doped during spray pyrolysis deposition: The reasons of the gas sensing properties change

The influence of Cu doping on electrophysical, structural and gas sensing properties of the SnO₂ films deposited by spray pyrolysis was considered in this paper. It was shown that the addition of Cu in SnO₂ even in small concentrations was accompanied by strong changes in the SnO₂-based gas sensors performances. The reasons of observed changes were discussed. The conclusion was made that the decrease of response of heavy doped SnO₂:Cu-based gas sensors was mainly connected with both structural disordering of heavy doped SnO₂:Cu metal oxide, and the appearance of the fine dispersed phase formed in the SnO₂ matrix.     

Stabilizing MoS2 Nanosheets through SnO2 Nanocrystal Decoration for High‐Performance Gas Sensing in Air

The unique properties of MoS₂ nanosheets make them a promising candidate for high‐performance room temperature sensing. However, the properties of pristine MoS₂ nanosheets are strongly influenced by the significant adsorption of oxygen in an air environment, which leads to instability of the MoS₂ sensing device, and all sensing results on MoS₂ reported to date were exclusively obtained in an inert atmosphere. This significantly limits the practical sensor application of MoS₂ in an air environment. Herein, a novel nanohybrid of SnO₂ nanocrystal (NC)‐decorated crumpled MoS₂ nanosheet (MoS₂/SnO₂) and its exciting air‐stable property for room temperature sensing of NO₂ are reported. Interestingly, the SnO₂ NCs serve as strong p‐type dopants for MoS₂, leading to p‐type channels in the MoS₂ nanosheets. The SnO₂ NCs also significantly enhance the stability of MoS₂ nanosheets in dry air. As a result, unlike other MoS₂ sensors operated in an inert gas (e.g. N₂), the nanohybrids exhibit high sensitivity, excellent selectivity, and repeatability to NO₂ under a practical dry air environment. This work suggests that NC decoration significantly tunes the properties of MoS₂ nanosheets for various applications.     

Preparation and CO gas-sensing behavior of Au-doped SnO2 sensors

A study on the low-temperature CO gas sensors based on Au/SnO₂ thick film was reported. Au/SnO₂ powders, with different Au loading from 0.36 to 3.57 wt%, were prepared by a deposition-precipitation method. Thick films were fabricated from Au/SnO₂ powders. The Au/SnO₂ thick-film sensors exhibited high sensitivity to CO gas at relatively low operating temperature (83-210 °C). We also reported the effect of the Au loading in Au/SnO₂ on the CO gas sensing behavior. The optimal Au loading in as-prepared Au/SnO₂ was 2.86 wt%.     

Structural and gas-sensing properties of nanometre tin oxide prepared by PECVD

A low-temperature plasma-enhanced chemical vapour deposition (PECVD) technique has been employed to produce ultrafine tin oxide powders. The structural features and phase transition of this material have been characterized using differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction, infrared spectroscopy (IR) and transmission electron microscopy (TEM). The oxygen absorption behaviour and gassensing properties have been investigated by electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and electrical measurements. Thick film gas sensors made from such ultrafine SnO₂ powders yield better sensitivities than those of normal undoped SnO₂ gas sensors. A gas-sensing reaction mechanism is also proposed.     

Gas sensing characteristics of MEMS gas sensor arrays in binary mixed-gas system

MOS gas sensor arrays based on MEMS gas sensor platforms were developed for the detection of carbon monoxide (CO), nitrogen oxides (NO_x) and ammonia (NH₃), and their gas sensing characteristics in binary mixed-gas system were investigated. Three gas sensing materials with nano-sized particles for these target gases, Pd–SnO₂ for CO, In₂O₃ for NO_x and Ru–WO₃ for NH₃ were synthesized using a sol–gel method. All the sensors showed good properties for their target gases at the optimum points for micro-heater operation. From the experimental data in MEMS gas sensor arrays in a binary mixed system, the gas sensing behavior and sensor response in mixed gas systems were scrutinized. The gas sensing behaviors to the mixed gas systems suggested that specific adsorption and selective activation of adsorption sites might occur in gas mixtures and offer the priority for the adsorption of specific gas. Thorough analysis of the sensing performance of the sensor arrays will make it possible to discriminate the components in gas mixtures as well as their concentrations.