SnO2-based Gas Sensors Research Articles

A Self-doping Surface Effect and its Influence on the Sensor Performance of Undoped SnO2 Based Gas Sensors

The variation of the calcination temperature in the fabrication procedure of sol-gel SnO2 thick film gas sensors leads to very different sensing characteristics [1]. In the present studies, the conduction mechanism and electrical characteristics of the surface of undoped SnO2 based sensing materials - calcined at different temperatures - are investigated by performing DC electrical resistance and work function changes measurements. The results show for the first time the existence of an intrinsic surface band bending for undoped SnO2 (calcination temperature 450°C). Because an intrinsic surface band bending has a big impact on the conduction mechanism and the concentration of surface species, which have an influence on the electrical resistance, the sensing performance is dramatically changed. The effect is similar to 1 at.% Ni surface loading, which showed a huge intrinsic surface band bending and an enhanced sensing performance in comparison to the undoped base material [2].

Fabrication of a SnO2-based acetone gas sensor enhanced by molecular imprinting.

This work presents a new route to design a highly sensitive SnO2–based sensor for acetone gas enhanced by the molecular imprinting technique. Unassisted and acetone-assisted thermal synthesis methods are used to synthesis SnO2 nanomaterials. The prepared SnO2 nanomaterials have been characterized by X-ray powder diffraction, scanning electron microscopy and N2 adsorption−desorption. Four types of SnO2 films were obtained by mixing pure deionized water and liquid acetone with the two types of as-prepared powders, respectively. The acetone gas sensing properties of sensors coated by these films were evaluated. Testing results reveal that the sensor coated by the film fabricated by mixing liquid acetone with the SnO2 nanomaterial synthesized by the acetone-assisted thermal method exhibits the best acetone gas sensing performance. The sensor is optimized for the smooth adsorption and desorption of acetone gas thanks to the participation of acetone both in the procedure of synthesis of the SnO2 nanomaterial and the device fabrication, which results in a distinct response–recovery behavior.

Influence of Dispersion of NiO on Di(propylene glycol)Methyl Ether Gas Sensing Properties of SnO2-Based Gas Sensor

Influence of Dispersion of NiO on Di(propylene glycol)Methyl Ether Gas Sensing Properties of SnO2-Based Gas Sensor

Catalytically Active Filters Deposited by SILD Method for Inhibiting Sensitivity to Ozone of SnO2-Based Conductometric Gas Sensors

Research has shown that successive ionic layer deposition (SILD) technology can be used for improvement operating characteristics of solid state conductometric gas sensors. This technology gives possibility to deposit nanolayers of noble metals such as Pd, Ag, SnO2-Au nanocomposites, and oxides of transition metals such as Cu, Fe, Mn, Co, which can play the role of catalytically active filters for ozone.

Effects of nanoadditives on stability of Pt/SnO2 as a sensing material for detection of CO

The effects of dopants and annealing temperature on the long term stability of SnO2-based gas sensors for detection of CO were investigated by doping Pt/SnO2 nanoparticles with MoO3, CeO2, Sm2O3 and SiO2. The sensing materials including pure SnO2 and SnO2 doped with 5.0 and 10.0wt% of nanoadditives were synthesized by a sol-gel method, then they were impregnated with 1.0wt% platinum. Accelerated stability tests were performed by exposing the fresh powders and the fabricated sensors to air with 30% relative humidity at 600°C for 15 days. Gas sensing measurements to 100ppm CO at 250°C, were carried out on the fabricated sensors on the 5th, 9th, 12th and 15th days in addition to the fresh point. MoO3, CeO2 and SiO2 additives were found to be effective in stabilizing the performance of Pt/SnO2 sensor, while Sm2O3 did not have the same effect. Increasing the annealing temperature, in the synthesis process, led to a decrease in sensitivity decline during the exposure period.

Spay Pyrolyzed Nanostructured Tin Dioxide Thin Films Doped by Cobalt: Correlation between Structural and Gas Sensing Characteristics

Effect of Co-doping on gas sensing, electrophysical and structural properties of the SnO2films deposited by spray pyrolysis has been studied. It is found that the influence of Co-doping on parameters of the SnO2-based gas sensors depends on the concentration of doping additives and could be accompanied by either improvement of sensor parameters at low levels of doping (CCo< 2-4 %) or degradation of the gas sensor operation characteristics while the concentration of additives exceeds 2-4%. An explanation of observed effects is given.

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.

Selective BTEX sensor based on a SnO2/V2O5 composite

A series of SnO2/V2O5 composites were designed and developed for selectively sensing BTEX (benzene, toluene, ethylbenzene, and xylol) based on a catalytic oxidation reaction. These composites were prepared with a facile solid phase reaction using sol–gel synthesized SnO2 and commercial V2O5 as raw materials. XRD and TEM were employed to characterize these composites. The results revealed forming process of the SnO2/V2O5 composites. Gas sensing tests of side-heating sensors showed that the SnO2/V2O5 composites had a much better response to BTEX rather than other VOCs such as ethanol, acetone, methanal, methanol and acetic acid compared with pure SnO2. Among these composites, the one containing 10wt% V2O5 has the best selectivity and response (Ra/Rg) toward BTEX. At 270°C, an optimal working temperature, the values of gas response to 50ppm (parts per million) BTEX are more than 5 times, meanwhile the limit of detection is as low as 0.5ppm. The influence of V2O5 on BTEX sensing of SnO2-based gas sensor was also discussed.

Free Energy and Electronic Properties of Water Adsorption on the SnO2(110) Surface

A molecular understanding of the adsorption of water on SnO2 surfaces is crucial for several applications of this metal oxide, including catalysis and gas sensing. We have investigated water adsorption on the SnO2(110) surface using a combination of dynamic and static calculations to gain fundamental insight into the reaction mechanism at room temperature. The reaction dynamics are studied by following water adsorption and dissociation on the SnO2 surface with metadynamics calculations at low and high coverage. The electronic structure in the relevant isolated minima is investigated through Mulliken charge analysis and projected density of states analysis. Surface bridging oxygen (Obr) is found to play a decisive role in water adsorption forming rooted hydroxyl groups with the water H atoms. Bond formation with H significantly changes the electronic configuration of Obr and presumably leads to reduced band bending at the SnO2 surface. The free-energy estimation indicates that on a clean SnO2(110) surface at room temperature both associative and dissociative adsorption occur, with the latter being thermodynamically favored. Oxygen coverage strongly affects the ratio between associatively and dissociatively adsorbed H2O, favoring associative adsorption at high oxygen coverage (oxidized surface) and dissociative adsorption at low oxygen coverage (reduced surface). Electronic analyses of isolated surface minima show the existence of two different electron-transfer phenomena occurring at the surface, depending on the water adsorption mechanism. The relevance of these findings in explaining the changes in electric conductivity occurring in SnO2-based gas sensors upon water adsorption is discussed. Whereas associative adsorption leads to electron enrichment of the metal oxide surface, dissociative adsorption induces surface electron depletion. Both mechanisms are consistent with the electrical conductivity changes occurring upon interaction of SnO2 with water, causing cross sensitivity to the latter. The theoretical results form the basis for correlating the existing atomistic models with the experimental data and offer a coherent description of the reaction events on the surface at room temperature.

Ambivalent effect of Ni loading on gas sensing performance in SnO2 based gas sensor

The gas sensing characteristics of pure and 0.4–2.0at% Ni-loaded SnO2 nanoparticles have been measured in dry and humid atmospheres. Approximately the same response to 50ppm CO, response/recovery kinetics, and resistance in air regardless of wide range of humidity variation from dry to 80% r.h. have been accomplished by loading 1.0 and 2.0at% Ni to SnO2. The role of Ni related surface species in the decrease of humidity dependence of gas sensing characteristics has been elucidated by diffuse-reflectance Fourier transform IR spectroscopy. The work function values determined from the transient of sensor resistance and contact potential difference revealed that Ni loading to SnO2 determines the appearance of surface electron acceptors responsible for a significant upward energy bands bending even in N2 atmosphere (>0.5eV), and, ultimately, explains the significant increase of the sensors baseline resistance and the decrease of the sensor signals. In this way, the origins of the ambivalent effect of Ni loading are clarified and the way towards a rational optimization of the sensor performance opened.

Improvement of H2S Sensing Properties of SnO2-Based Thick Film Gas Sensors Promoted with MoO3 and NiO

The effects of the SnO2 pore size and metal oxide promoters on the sensing properties of SnO2-based thick film gas sensors were investigated to improve the detection of very low H2S concentrations (<1 ppm). SnO2 sensors and SnO2-based thick-film gas sensors promoted with NiO, ZnO, MoO3, CuO or Fe2O3 were prepared, and their sensing properties were examined in a flow system. The SnO2 materials were prepared by calcining SnO2 at 600, 800, 1,000 and 1,200 °C to give materials identified as SnO2(600), SnO2(800), SnO2(1000), and SnO2(1200), respectively. The Sn(12)Mo5Ni3 sensor, which was prepared by physically mixing 5 wt% MoO3 (Mo5), 3 wt% NiO (Ni3) and SnO2(1200) with a large pore size of 312 nm, exhibited a high sensor response of approximately 75% for the detection of 1 ppm H2S at 350 °C with excellent recovery properties. Unlike the SnO2 sensors, its response was maintained during multiple cycles without deactivation. This was attributed to the promoter effect of MoO3. In particular, the Sn(12)Mo5Ni3 sensor developed in this study showed twice the response of the Sn(6)Mo5Ni3 sensor, which was prepared by SnO2(600) with the smaller pore size than SnO2(1200). The excellent sensor response and recovery properties of Sn(12)Mo5Ni3 are believed to be due to the combined promoter effects of MoO3 and NiO and the diffusion effect of H2S as a result of the large pore size of SnO2.

The role of doping effect on the response of SnO2-based thin film gas sensors: Analysis based on the results obtained for Co-doped SnO2 films deposited by spray pyrolysis

Effect of doping by cobalt on gas sensing, electrophysical and structural properties of the SnO2 films was studied. Undoped and doped SnO2 films were deposited by spray pyrolysis method on alumina, silicon and quartz substrates kept at 410–450°C. The film structure was controlled by XRD and SEM methods. Ozone and hydrogen were used as tested gases. It was found that the influence of doping by cobalt on the SnO2-based gas sensor parameters depends on the concentration of doping additives and could be accompanied by either improvement of sensor parameters at low level of doping (CCo<2–4%) or degradation of the gas sensor operation characteristics while concentration of additives exceeds 2–4%. It was given an explanation of observed effects. In particular, it was shown that structural changes caused by incorporation of cobalt in the SnO2 lattice was the main factor, controlling the change of the SnO2 gas sensing properties during the process of the SnO2 bulk doping.

The sensing behaviour of metal oxides (ZnO, CuO and Sm2O3) doped-SnO2 for detection of low concentrations of chlorinated volatile organic compounds

A SnO2-based semiconductor thick film gas sensor was fabricated for detection of chlorinated volatile organic compounds (VOCs) such as dichloromethane (DCM), trichloroethylene (TCE) and 2-chloroethylethylsulfide (CEES) at concentrations of 10, 20 and 10ppm respectively with temperatures ranging from 100 to 400°C. Other semiconductors such as ZnO, CuO in 5.0wt% and Sm2O3 in 1.0, 5.0 and 10.0wt% were added to SnO2 by a co-precipitation method. The samples were characterized by TEM, XRD and BET. The characterization results showed that presence of dopants causes an increase in surface area and a decrease in particle size of the sensing materials. It was found that SnO2–5.0wt% Sm2O3 has the highest response to DCM and CEES, specially at low temperatures. Also the response was quite high for this sensor even at a concentration as low as 0.1ppm. However, presence of the promoters suppressed the sensitivity to TCE.

One-Step Hydrothermal Synthesis of High-Performance Gas-Sensing Crystals CdIn2O4 with Octahedral Shape

Octahedrally shaped CdIn2O4 was synthesized in a one-step hydrothermal reaction without any surfactant. These octahedral particles used as an ethanol sensor revealed excellent gas response, which is comparable to the well-known SnO2-based gas sensors. The octahedral CdIn2O4 powder possesses the active {111} facets, which contribute to the higher gas-sensing performance of octahedral CdIn2O4 than that of the nanosized CdIn2O4 powder obtained by coprecipitation method. The XPS results demonstrated that the CdIn2O4 octahedra enclosed by {111} facets provided more oxygen vacancies, resulting in the enhanced performance. The calculation also showed higher surface energies for the {111} orientation, which confirms the mechanism of the enhanced gas-sensing performance of the octahedral particles. The as-formed crystals with controlled morphology provide insights for the designed strategy of high-performance gas-sensing materials, and this synthetic route provides flexibility and selectivity for the deliberate preparation of other functional materials.

Structural Stability and Performance of Noble Metal-Free SnO2-Based Gas Sensors.

The structural stability of pure SnO2 nanoparticles and highly sensitive SnO2-SiO2 nanocomposites (0–15 SiO2 wt%) has been investigated for conditions relevant to their utilization as chemoresistive gas sensors. Thermal stabilization by SiO2 co-synthesis has been investigated at up to 600 °C determining regimes of crystal size stability as a function of SiO2-content. For operation up to 400 °C, thermally stable crystal sizes of ca. 24 and 11 nm were identified for SnO2 nanoparticles and 1.4 wt% SnO2-SiO2 nanocomposites, respectively. The effect of crystal growth during operation (TO = 320 °C) on the sensor response to ethanol has been reported, revealing possible long-term destabilization mechanisms. In particular, crystal growth and sintering-neck formation were discussed with respect to their potential to change the sensor response and calibration. Furthermore, the effect of SiO2 cosynthesis on the cross-sensitivity to humidity of these noble metal-free SnO2-based gas sensors was assessed.

Interplay of H2, water vapor and oxygenat the surface of SnO2 based gas sensors – An operando investigation utilizing deuterated gases

The surface reactivity toward hydrogen and water vapor of three different (in grain size and amount of OH groups on the surface, different synthesis route) undoped tin dioxide materials (SnO2 Ac, SnO2Cl, SnO2 Cl K) was investigated at 300°C by applying simultaneous resistance measurements and DRIFTS (diffuse reflectance infrared spectroscopy). The changes of the surface species, i.e. OH groups, were examined due to exposure to hydrogen H2/deuterium D2, watervapor H2O/deuterated water vaporD2O and carbon monoxide CO. A possibility to minimize IR related disturbing phenomena in the absorbance spectra is the use of deuterated gases (D2, D2O): everything which deals with OH groups/water related species is shifted in the IR spectrum. We were able to show that the reaction with those reducing gases can be traced back to the reaction with adsorbed oxygen. There was no evidence for a direct reaction of the test gas with OH groups or in case of H2, a direct reaction with (lattice) oxygen to hydroxyl groups.

Gas sensor application of Ag nanoclusters synthesized by SILD method

The potential for successive ionic layer deposition (SILD) technology to be used in the surface modification of SnO2 films using Ag nanoclusters was analyzed in this paper. It was found that the surface modification by SILD of Ag nanoclusters can be used to improve both the sensitivity and response rate of SnO2-based gas sensors to CO and H2. Models showing the promotional role of Ag additives were then discussed. It was also established that the presence of Ag clusters on the surface of SnO2 depresses the sensor response to ozone. This effect can be used to improve the selectivity of sensors of reducing gases aimed for operation in atmosphere containing oxidizing gases.

An Au clusters related spill-over sensitization mechanism in SnO2-based gas sensors identified by operando HERFD-XAS, work function changes, DC resistance and catalytic conversion studies

The role of Au additives in SnO(2)-based thick film gas sensors was investigated by a combination of operando investigation techniques, namely spectroscopic high energy resolved fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) and simultaneous DC resistance and work function change measurements. The results have shown that the Au is present in the form of small metallic particles at the surface of the host metal oxide without changing its bulk or surface electronic properties. The sensitization effect of Au can therefore be attributed to the "spill-over effect", meaning that the Au particles enrich the surface of the active metal oxide with oxygen species which consequently react with reducing gases such as CO and H(2). This is in contrast to the effect of Pd and Pt promoters which were found to be distributed at an atomic level on the surface and in the bulk of the supporting sensing material and therefore have a tremendous effect on its bulk and surface electronic properties.

New Model for a Pd-Doped SnO2-Based CO Gas Sensor and Catalyst Studied by Online in-Situ X-ray Photoelectron Spectroscopy

The functional mechanism of oxide-based gas sensors and the gas catalysis reaction are controversial topics. The traditional static photoelectron data collection technique is obviously not satisfied for this study. However, after appropriate remodification of the data collection system we propose a new, online XPS in-situ data collection technique for overcoming this difficulty. This online photoelectron collection technique was employed for the first time on this kind of study with SnO2-based gas sensors. The results we found are as follows: (1) For rutile SnO2 in powder form, its O1s peak of 531.8 eV would come from the lattice oxygen in the surface area, not from chemisorbed oxygen as proposed conventionally; (2) we observed that in the adsorptive process of CO onto Pd-doped SnO2, Pd, CO, and lattice oxygen in the surface area of SnO2 could form an intermediate product [Pd(CO)4]O4, from which comes a unified new model for illustrating the mechanism of this kind gas sensor, on one hand, and the oxidation catalysis process, on the other; (3) we noticed a transient magnetic phenomenon during the adsorption process of CO onto SnO2. This could open a door for studying spin sensors.

A novel tin oxide-based recoverable thick film SO 2 gas sensor promoted with magnesium and vanadium oxides

An SnO 2-based thick film gas sensor was developed to detect sulfur dioxide (SO 2) gas at the ppm level. The SnO 2-based sensors were prepared by mixing tin oxide with various promoters such as V 2O 5, MoO 3, Sb 2O 3, Al 2O 3, CeO 2, and MgO. Their sensor responses and recovery behaviors were investigated in a flow system. It was found that the SnO 2-based gas sensor promoted by simultaneous mixing of both 5 wt% MgO and 2 wt% V 2O 5, showed not only a 44% sensor response for the detection of SO 2 at the 1 ppm level, but also good repeatability by means of thermal treatment in air. In particular, this sensor showed a response of approximately 20% even at the level of 0.1 ppm (100 ppb). The sensor recovery properties, as well as the sensor response, could be enhanced by the simultaneous addition of both MgO and V 2O 5 promoters as compared with the SnO 2 sensor. These results are due to the promoting effects of MgO and V 2O 5 on response and recovery properties of the SnO 2-based sensor, respectively.