Metal Oxide Nanostructures and Their Gas Sensing Properties: A Review
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paper_title: Measurements of volatile organic compounds at a suburban ground site (T1) in Mexico City during the MILAGRO 2006 campaign: measurement comparison, emission ratios, and source attribution

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Abstract. Volatile organic compound (VOC) mixing ratios were measured with two different instruments at the T1 ground site in Mexico City during the Megacity Initiative: Local and Global Research Observations (MILAGRO) campaign in March of 2006. A gas chromatograph with flame ionization detector (GC-FID) quantified 18 light alkanes, alkenes and acetylene while a proton-transfer-reaction ion-trap mass spectrometer (PIT-MS) quantified 12 VOC species including oxygenated VOCs (OVOCs) and aromatics. A GC separation system was used in conjunction with the PIT-MS (GC-PIT-MS) to evaluate PIT-MS measurements and to aid in the identification of unknown VOCs. The VOC measurements are also compared to simultaneous canister samples and to two independent proton-transfer-reaction mass spectrometers (PTR-MS) deployed on a mobile and an airborne platform during MILAGRO. VOC diurnal cycles demonstrate the large influence of vehicle traffic and liquid propane gas (LPG) emissions during the night and photochemical processing during the afternoon. Emission ratios for VOCs and OVOCs relative to CO are derived from early-morning measurements. Average emission ratios for non-oxygenated species relative to CO are on average a factor of ~2 higher than measured for US cities. Emission ratios for OVOCs are estimated and compared to literature values the northeastern US and to tunnel studies in California. Positive matrix factorization analysis (PMF) is used to provide insight into VOC sources and processing. Three PMF factors were distinguished by the analysis including the emissions from vehicles, the use of liquid propane gas and the production of secondary VOCs + long-lived species. Emission ratios to CO calculated from the results of PMF analysis are compared to emission ratios calculated directly from measurements. The total PIT-MS signal is summed to estimate the fraction of identified versus unidentified VOC species.
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paper_title: Semiconducting Metal Oxide Based Sensors for Selective Gas Pollutant Detection

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A review of some papers published in the last fifty years that focus on the semiconducting metal oxide (SMO) based sensors for the selective and sensitive detection of various environmental pollutants is presented.
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paper_title: Oxide Semiconductor Gas Sensors

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Semiconductor gas sensors utilize porous polycrystalline resistors made of semiconducting oxides. The working principle involves the receptor function played by the surface of each oxide grain and the transducer function played by each grain boundary. In addition, the utility factor of the sensing body also takes part in determining the gas response. Therefore, the concepts of sensor design are determined by considering each of these three key factors. The requirements are selection of a base oxide with high mobility of conduction electrons and satisfactory stability (transducer function), selection of a foreign receptor which enhances surface reactions or adsorption of target gas (receptor function), and fabrication of a highly porous, thin sensing body (utility factor). Recent progress in sensor design based on these factors is described.
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paper_title: Semiconducting metal oxide sensor array for the selective detection of combustion gases

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A sensor array consisting of discrete thick-film sensors based on various semiconductor metal oxides (SMO) has been designed and fabricated for flue gas analysis purposes. The selection of the sensitive materials for the array has been accomplished as a result of extensive studies of gas-sensitive properties of SMO. The thick-film sensors, prototypes and the array’s components, were fabricated on the basis of commercial sensor platforms. A drop-coating technique was used for metal oxide paste deposition followed by in situ drying and annealing of the deposited films in air by platinum heaters integrated into the platforms. We show the results obtained with a variety of thick-film metal oxide species and examine their sensitivities at different fixed operating temperatures (200‐400 8C). The feasibility of the electronic system consisted of SnO2, ZnO, WO3, CuO, and In2O3 sensors to discriminate and recognize various gaseous constituents of a combustion gas is demonstrated. Principal component analysis along with several classification schemes were used to identify nitrogen oxides, ammonia, sulfur dioxide, and other gaseous pollutants. # 2003 Elsevier Science B.V. All rights reserved.
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paper_title: An electronic nose based on solid state sensor arrays for low-cost indoor air quality monitoring applications

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The occurrence of illnesses related with poor ventilation has driven an increasing attention towards indoor air quality monitoring. In buildings equipped with climate control systems, the diseases related to the air quality can be significantly reduced if smart intervening procedures, aiming to control the concentration of pollutants in the indoor air, can be implemented in the heating, ventilation air conditioning unit. When reliable information about both the indoor and outdoor air quality is made available, the climate control system can provide the most appropriate amount of ventilation, ensuring safe and comfortable living conditions. In this paper, a dedicated, miniaturized, low-cost electronic nose based on state-of-the-art metal oxide sensors and signal processing techniques was developed. The proposed device is targeted to the quantification of carbon monoxide and nitrogen dioxide in mixtures with relative humidity and volatile organic compounds by using an optimized gas sensor array and highly effective pattern recognition techniques. The electronic nose was tested in an environment reproducing real operating conditions. Exploiting the unique response patterns of the different sensors in the array and the capability of a simple fuzzy-logic system it was possible to identify and discriminate concentrations as low as 20 ppb for NO2 and 5 ppm for CO in the test gas environment, allowing to reach the necessary sensitivity towards the target pollutants together with the selectivity towards the typical interfering gas species. © 2004 Elsevier B.V. All rights reserved.
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paper_title: Thermal Desorption-Gas Chromatography/Mass Spectrometric Analysis of Volatile Organic Compounds Emitted from Automobile Chair in Thermal Condition

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There are many volatile organic compounds emissions from car chair in thermal condition to pollute environment and affect the health of the people.A method for the detection of volatile organic compounds from emissions of automobile chair parts in the thermal condition by TD-GC/MS was developed.The automobile Chair sample was put into a special plastic bag of 2 m3,the volatile organic compounds were emitted in the thermal condition,then were collected with a Tenax adsorption tube,and were tested by thermal desorption-gas chromatography-mass spectrometry.The total numbers of identified components were 49 VOCs emitted from the automobile chair in the thermal condition,including alkenes(11,7.46%),aromatic compounds(13,6.02%),alcohols(5,10.33%),ketones(5,3.56%),esters(7,29.51%),aldehydes(2,0.50%),N-containing compounds(2,29.36%) and siloxane(4,5.51%).If the content of VOC emissions was converted into the concentrations in the automobile air,the total volatile organic compounds(TVOC)is 7.361 mg/m3,which was 12 times over the tolerable concentrations prescribed in the standard.
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paper_title: Metal Oxide Gas Sensors: Sensitivity and Influencing Factors

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Conductometric semiconducting metal oxide gas sensors have been widely used and investigated in the detection of gases. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity. In this brief review, attention will be focused on changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors mentioned above.
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paper_title: Characterization of the gas sensors based on polymer-coated resonant microcantilevers for the detection of volatile organic compounds.

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The gas sensors based on polymer-coated resonant microcantilevers for volatile organic compounds (VOCs) detection are investigated. A method to characterize the gas sensors through sensor calibration is proposed. The expressions for the estimation of the characteristic parameters are derived. The effect of the polymer coating location on the sensor's sensitivity is investigated and the formula to calculate the polymer-analyte partition coefficient without knowing the polymer coating features is presented for the first time. Three polymers: polyethyleneoxide (PEO), polyethylenevinylacetate (PEVA) and polyvinylalcohol (PVA) are used to perform the experiments. Six organic solvents: toluene, benzene, ethanol, acetone, hexane and octane are used as analytes. The response time, reversibility, hydrophilicity, sensitivity and selectivity of the polymer layers are discussed. According to the results, highly sensitive sensors for each of the analytes are proposed. Based on the characterization method, a convenient and flexible way to the construction of electric nose system by the polymer-coated resonant microcantilevers can be achieved.
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paper_title: Semiconductor gas sensors: dry synthesis and application.

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Since the development of the first chemoresistive metal oxide based gas sensors, transducers with innovative properties have been prepared by a variety of wet- and dry-deposition methods. Among these, direct assembly of nanostructured films from the gas phase promises simple fabrication and control and with the appropriate synthesis and deposition methods nm to μm thick films, can be prepared. Dense structures are achieved by tuning chemical or vapor deposition methods whereas particulate films are obtained by deposition of airborne, mono- or polydisperse, aggregated or agglomerated nanoparticles. Innovative materials in non-equilibrium or sub-stoichiometric states are captured by rapid cooling during their synthesis. This Review presents some of the most common chemical and vapor-deposition methods for the synthesis of semiconductor metal oxide based detectors for chemical gas sensors. In addition, the synthesis of highly porous films by novel aerosol methods is discussed. A direct comparison of structural and chemical properties with sensing performance is given.
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paper_title: Quantitative analysis of pesticide residue based on the dynamic response of a single SnO2 gas sensor

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Abstract A new rapid detecting method (called dynamic measurements) was reported to detect and distinguish the presence of two pesticide gases and their mixture in the ambient atmosphere. The method employed only a single SnO 2 -based gas sensor in a rectangular temperature mode to perform the qualitative analysis of pure pesticide gas and a binary gas mixture (acephate and trichlorphon) in air. Experimental results showed that a high selectivity of the sensor was achieved in the range of 250–300 °C and modulating frequency 20 mHz. The quantitative analysis between the pure pesticide gas and the mixture was performed by a fast Fourier transformation (FFT). The higher harmonics of the FFT characterized the nonlinear properties of the response at the sensor surface. The amplitudes of the higher harmonics exhibited characteristic changes which depended on the gas concentration and the kinetics of the gas species on the sensor surface.
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paper_title: New approach for the detection of organophosphorus pesticide in cabbage using SPME/SnO2 gas sensor: principle and preliminary experiment

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A new approach for fast detection of organophosphorus pesticide like dichlorvos in cabbage using solid phase microextraction (SPME) coupled with tin dioxide gas sensor was investigated. In comparison the experiments of guide sample of dichlorvos, this method which employed only one SnO2 gas sensor rather than an array of sensors in a rectangular temperature mode could perform the qualitative analysis. The quantitative analysis was performed using polar plots and peak area.
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paper_title: Rectangular mode of operation for detecting pesticide residue by using a single SnO2-based gas sensor

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Abstract A new rapid detecting method (called dynamic measurements) was reported to detect and distinguish the presence of two pesticide gases in the ambient atmosphere. The method employed only a single SnO 2 -based gas sensor in a rectangular temperature mode to perform the qualitative analysis of a binary gas mixture (acephate and trichlorphon) in air. Experimental results showed that high selectivity of the sensor achieved in the range of 250–300 °C and modulating frequency 20 mHz, one can easily observe the qualitative difference among the response to pure acephate and trichlorphon gases of the same concentration and to the mixture.
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paper_title: Surface processes in the detection of reducing gases with SnO2-based devices

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Abstract Single-crystal faces, thin evaporated and sputtered films and sintered specimens of SnO 2 were investigated in UHV. After exposure to hydrogen, water, arsine, carbon monoxide, methane, acetic acid and ethanol, a sensitive mass spectrometer recorded the desorbing species in thermal desorption spectroscopy and served for reactive scattering investigations. The surface conductivity was observed at the same time. The combination of the different results with published infrared spectroscopic data allows some conclusions to be drawn on reaction mechanisms, surface binding and the origin of conductance variations. It will be shown that even such a simple molecule as methane undergoes many reaction steps, some of which affect the observed conductance change. The investigation of more complex molecules, such as ethanol and acetic acid, supports the understanding of the methane reactions; the decomposition of methane on SnO 2 is accompanied by synthesis reactions yielding these products. Finally, some consequences and possible improvements of gas sensors are discussed.
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paper_title: Detecting Pesticide Residue by Using Modulating Temperature Over a Single SnO2-Based Gas Sensor

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Abstract : A new rapid detecting method (called dynamic measurements ) was reported to detect and distinguish the presence of two pesticide gases in the ambient atmosphere. The method employed only a single SnO 2 -based gas sensor in a rectangular temperature mode to perform the qualitative analysis of a binary gas mixture (acephate and trichlorphon) in air. Polar plots was used for quantitative analysis which the feature extraction was performed by FFT. Experimental results showed that high selectivity of the sensor achieved in the range of 250~300 0 C and modulating frequency 20mHz, one can easily observe the qualitative difference among the response to pure acephate and trichlorphon gases of the same concentration and to the mixture, and the concentration of pesticide gases can be obtained based on the changes of polar plots. Keywords: SnO 2 gas sensor; modulating temperature; Pesticide residue Introduction SnO2-based gas sensors had been extensively applied to gases detection [1]. Present well known advantages included their low costs and high sensitivities; well known disadvantages concerned their lack of stability and selectivity[2,3]. In practical applications, several attempts were usually made to
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paper_title: Conduction Model of Metal Oxide Gas Sensors

paper_content:

Tin dioxide is a widely used sensitive material for gas sensors. Many research and development groups in academia and industry are contributing to the increase of (basic) knowledge/(applied) know-how. However, from a systematic point of view the knowledge gaining process seems not to be coherent. One reason is the lack of a general applicable model which combines the basic principles with measurable sensor parameters.
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paper_title: Semiconductor gas sensors: dry synthesis and application.

paper_content:

Since the development of the first chemoresistive metal oxide based gas sensors, transducers with innovative properties have been prepared by a variety of wet- and dry-deposition methods. Among these, direct assembly of nanostructured films from the gas phase promises simple fabrication and control and with the appropriate synthesis and deposition methods nm to μm thick films, can be prepared. Dense structures are achieved by tuning chemical or vapor deposition methods whereas particulate films are obtained by deposition of airborne, mono- or polydisperse, aggregated or agglomerated nanoparticles. Innovative materials in non-equilibrium or sub-stoichiometric states are captured by rapid cooling during their synthesis. This Review presents some of the most common chemical and vapor-deposition methods for the synthesis of semiconductor metal oxide based detectors for chemical gas sensors. In addition, the synthesis of highly porous films by novel aerosol methods is discussed. A direct comparison of structural and chemical properties with sensing performance is given.
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paper_title: Study of Influencing Factors of Dynamic Measurements Based on SnO2 Gas Sensor

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The gas-sensing behaviour based on a dynamic measurement method of a single SnO2 gas sensor was investigated by comparison with the static measurement. The influencing factors of nonlinear response such as modulation temperature, duty ratio, heating waveform (rectangular, sinusoidal, saw-tooth, pulse, etc.) were also studied. Experimental data showed that temperature was the most essential factor because the changes of frequency and heating waveform could result in the changes of temperature essentially.
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paper_title: Gas sensing behavior of a single tin dioxide sensor under dynamic temperature modulation

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Abstract The gas sensing behavior of a single SnO 2 gas sensor was investigated based on a dynamic measurement method in which temperature was modulated by heating waveform and frequency, and the results were compared with those of static measurement. The heating waveforms studied were rectangular, sinusoidal, triangular, saw-tooth, pulse, trapezoidal, and etc. However, the most essential factor was the modulated temperature itself, because any change in heating waveform and frequency resulted in the changes in surface temperature of the sensor element.
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paper_title: Gas Sensors Based on Semiconducting Metal Oxide One-Dimensional Nanostructures

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This article provides a comprehensive review of recent (2008 and 2009) progress in gas sensors based on semiconducting metal oxide one-dimensional (1D) nanostructures. During last few years, gas sensors based on semiconducting oxide 1D nanostructures have been widely investigated. Additionally, modified or doped oxide nanowires/nanobelts have also been synthesized and used for gas sensor applications. Moreover, novel device structures such as electronic noses and low power consumption self-heated gas sensors have been invented and their gas sensing performance has also been evaluated. Finally, we also point out some challenges for future investigation and practical application.
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paper_title: Development of microstructure In/Pd-doped SnO2 sensor for low-level CO detection

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Abstract In/Pd-doped SnO 2 is synthesized via a sol–gel method and coated on a silicon substrate with Pt electrodes to fabricate a microstructure sensor. The sensor shows high response to CO with very low cross response to common interference gases at an operating temperature of 140 °C. Especially, the sensor can detect CO down to 1 ppm (the response value is about 3), and the response time and recovery time are about 15 and 20 s, respectively. These results make our sensor a good candidate in practical CO sensors.
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paper_title: Effect of deposition conditions on the microstructure and gas-sensing characteristics of Te thin films

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Abstract Effect of deposition temperature, post-deposition annealing and substrate crystallinity on the microstructure and gas sensitivity of tellurium thin films has been investigated. As the substrate temperature during deposition is increased, due to increased mobility of adatoms, microstructure of films gradually changes from a smooth amorphous nature at low temperatures to polycrystalline films showing dendritic growth at high temperatures. It is also seen that improvement in crystalline nature of substrate leads to enhancement in grain size of the deposited films. Post-deposition annealing is found to have much less influence on the film morphology compared to films deposited on heated substrates. For films deposited at same temperature (on different substrates), gas sensitivity improves with reduction in the grain size. This has been attributed to increase in effective surface area for gas–film interaction. On the contrary, for films deposited at different temperatures, increase in grain size (higher deposition temperature) leads to better sensitivity. This has been understood to arise from reduced intragrain defect density. Small grain size and reduced defect density are seen to be conducive to better gas sensitivity of the films.
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paper_title: Direct evidence of weak-link grain boundaries in a polycrystalline MgB2 superconductor

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The observation of voltage-flux oscillations due to the radio-frequency–superconducting quantum interference device (rf-SQUID) effect in polycrystalline bulk MgB2 superconductor is reported. This provides a direct evidence of the presence of Josephson junction-type weak-link grain boundaries in MgB2. Analysis of the SQUID modulation indicates that only a few grain boundaries are of weak-link type whereas others are strong links. Generation of even and odd harmonics are observed when the MgB2 sample is subjected to both ac and dc fields. This also indicates the presence of weak-link grain boundaries. Flux noise in the bulk rf-SQUID in MgB2 is studied and compared with the flux noise in bulk high-Tc rf-SQUID.
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paper_title: Morphology-dependent electric transport in textured ultrathin Al films grown on Si

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The thickness and temperature dependence of the electrical resistivity of the (111) textured ultrathin aluminum metal films grown on (111) Si substrates using molecular-beam epitaxy have been investigated. For films with thickness <50nm, the room-temperature value of resistivity—contrary to the predictions of existing theoretical models—is found to increase monotonically with thickness. In addition, the temperature dependence of these films exhibited a metal-to-insulator transition at ∼110K. The studies of films using atomic force microscopy, x-ray photoelectron spectroscopy, and x-ray diffraction revealed that the observed anomalous thickness and temperature dependence of resistivity arise due to the formation of two-dimensional network of Al islands, and the low-temperature electric transport of such films could be explained using variable range hopping conduction.
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paper_title: Examination of Au/SnO2 core-shell architecture nanoparticle for low temperature gas sensing applications

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Abstract Au/SnO 2 core-shell structure nanoparticles (NPs) were synthesized using two methods, microwave and conventional precipitation. In both cases, the size of the Au core was 12–18 nm and the thickness of the SnO 2 shell was 8–12 nm. The particle size of SnO 2 synthesized using the microwave and precipitation method was 3–5 nm and 1–2 nm, respectively. Upon heating to 400–600 °C, both particles maintained their core-shell morphology but the smaller SnO 2 particles prepared using the precipitation method were more sintered. The resistance changes in films of these particles were measured as a function of CO concentration. The Au/SnO 2 particles prepared using the microwave method showed higher sensor response than those prepared using the precipitation method, even providing a significant signal at testing temperatures approaching ambient conditions, thereby affording a new class of material for gas sensing. Both sets of core-shell particles showed higher sensor response than the SnO 2 nanoparticles. The role of the Au core as a catalyst in improving the adsorption and oxidation of CO gas is important for improving the low temperature response. In addition, the maintenance of the size of SnO 2 in the microwave method during sintering contributed to the higher response towards CO sensing.
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paper_title: Nanocomposites of sub-10 nm SnO2 nanoparticles and MWCNTs for detection of aldrin and DDT

paper_content:

The preparation of materials with size and porosity in the nanometre range is of technological importance for a wide range of sensing applications. Here, the ultrasensitive detection of Persistent Organic Pollutants (POPs), i.e., Aldrin and DDT, has been investigated using a sub-10 nm SnO2/MWCNT nanocomposite-based gas sensor. The features of prepared nanocomposites are carefully studied using SEM, TEM, and Raman spectroscopy. The sensing material shows a very attractive improved sensitivity compared to a conventional SnO2 sensor. One major advantage of this sensing material is the stable attachment between SnO2 nanoparticles and carbon nanotubes. Accordingly, facile preparation, high sensitivity, long-term stability, and nanoscale effects in this composite can create novel avenues and applications for fabricating important POPs sensors.
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paper_title: Detection of reducing gases by SnO2 thin films: an impedance spectroscopy study

paper_content:

Abstract Impedance spectroscopy studies were carried out on SnO 2 thin films after exposure to H 2 S and NH 3 gases in frequency range of 0.1 Hz to 1 MHz. The results showed that the impedance is mainly contributed by potential barrier at grain boundaries. Cole–cole plots showed a reduction in real part of impedance at low frequencies on exposure to both gases. Analysis of data revealed that this arises due to reduction in real part of grain boundary impedance and not due to negative capacitance (inductance) effect. The effect has been attributed to formation of ions on interaction of reducing gases and their drift to grain boundary region in the presence of applied field.
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paper_title: The effect of grain size on the sensitivity of nanocrystalline metal-oxide gas sensors

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The effect of grain size on the sensitivity of chemoresistive nanocrystalline metal-oxide gas sensors was evaluated by calculating the effective carrier concentration as a function of the surface state density for a typical sensing material, SnO2, with different grain sizes between 5 and 80 nm. This involved numerical computation of the charge balance equation (the electroneutrality condition) using approximated analytical solutions of Poisson’s equation for small spherical crystallites. The calculations demonstrate a steep decrease in the carrier concentration when the surface state density reaches a critical value that corresponds to a condition of fully depleted grains, namely, when nearly all the electrons are trapped at the surface. Assuming that the variations in the surface state density are induced by surface interactions with ambient gas molecules, these calculations enable us to simulate the response curves of nanocrystalline gas sensors. The simulations show that the conductivity increases line...
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paper_title: Receptor Function and Response of Semiconductor Gas Sensor

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Theoretical approaches to receptor function and response of semiconductor gas sensor are described, following the illustrations of some relevant key issues such as tunneling transport. Depletion in small semiconductor crystals is characterized by the occurrence of new type depletion (volume depletion) after conventional one (regional depletion), and inclusion of both types makes it possible to formulate the receptor function and response to oxygen (air base), oxidizing gas (nitrogen dioxide), and reducing gas (hydrogen). The equations derived theoretically using physical parameters of the semiconductor side and chemical parameters of the gases side appear to reproduce satisfactorily the sensing behavior to the aforementioned gases as well as the influence of changes in physical parameters such as grain size and donor density. Extension to the semiconductor crystals dispersed with surface electron-traps shows that the traps act as a sensitizer to promote sensor response.
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paper_title: Hydrothermal synthesis of hierarchical nanoparticle-decorated ZnO microdisks and the structure-enhanced acetylene sensing properties at high temperatures

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a b s t r a c t Nanoparticle-decorated ZnO microdisks showing a hierarchical structure were successfully synthesized through a citric acid-assisted hydrothermal process. The ZnO microdisks had a hexagonal wurtzite struc- ture with high crystallinity. Sensors based on these microdisks exhibited high response values, fast response-recovery, good selectivity and long-term stability to 1-4000 ppm acetylene at 420 ◦C. In addi- tion, even 1 ppm acetylene can be detected with high response (S = 7.9). The hierarchical structure can facilitate to fix the ZnO nanoparticles leading to a less aggregated configuration, which is expected to contribute much to the excellent acetylene sensing properties at high temperatures. © 2011 Elsevier B.V. All rights reserved.
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paper_title: Receptor function of small semiconductor crystals with clean and electron-traps dispersed surfaces

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Abstract Theoretical exploration has been conducted to formulate the receptor function and response of a semiconductor gas sensor using thin plate-like crystals. The receptor function can be formulated over a wide range of depletion from conventional one (regional depletion) to new type one (volume depletion) by focusing attention on how the surface density of adsorbed oxygen ions (O − ) is related with chemical moiety outside the crystals and physical moiety inside. The responses to gases thus derived are inversely proportional to the thickness of the plates used in the range of volume depletion. Electron-traps, if dispersed on the crystals, are shown to exert drastic effects in promoting sensitivity to gases.
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paper_title: Effect of grain boundaries on paraconductivity of YBa 2 Cu 3 O x

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Abstract To investigate the effect of grain boundaries on paraconductivity of YBa 2 Cu 3 O x , melt-textured and c -axis oriented thin films with controlled grain boundaries (superconducting transition width, Δ T , varying between 0.54 and 2.85 K) were prepared, and dc-conductivity has been measured as a function of temperature. In the logarithmic plots of excess-conductivity (Δ σ ) and reduced temperature ( ϵ ), starting from low values of ϵ , we have observed three different regions namely critical region, mean field region and short wave fluctuation region. A correlation is observed between the range of critical region and Δ T , which is found to increase with Δ T . While for Δ T values smaller than 2.5 K only static critical region is observed, for higher Δ T s both static and dynamic critical regions are observed. In the mean field region a crossover from 3D to 2D was observed for all the samples. At ϵ values larger than 0.24, the excess-conductivity decreased sharply as ϵ −3 , which suggested the existence of the short wave fluctuations.
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paper_title: Porous SnO2 nanospheres as sensitive gas sensors for volatile organic compounds detection

paper_content:

Porous SnO2 nanospheres with high surface areas have been synthesized through a solvothermal method in the absence of any templates. The structure and morphology of the resultant products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption–desorption technique. The as-prepared SnO2 porous nanospheres with the diameters ranging from 90–150 nm are composed of small nanocrystals with average sizes of less than 10 nm. Results demonstrated that the formation of porous SnO2 nanospheres is ascribed to etching the center part of the nanospheres. It was found that hydrochloric acid and NaClO played important roles in determining the final morphologies of the porous SnO2 nanospheres. The gas sensing properties of the as-prepared porous SnO2 nanospheres were investigated. By the comparative gas sensing tests, the porous SnO2 nanospheres exhibited a superior gas sensing performance toward ppb level 2-chloroethanol and formaldehyde vapor, implying promising applications in detecting toxic volatile organic compounds (VOCs).
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paper_title: Templating route for synthesizing mesoporous zeolites with improved catalytic properties

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Summary Zeolites with intracrystal mesopores have been paid much attention recently due to the combination of the advantages of mesoporous materials and zeolite crystals. This review describes the synthetic routes for preparation of mesoporous zeolites by various templates including nanostructured carbons, mesoscale cationic polymers, mesoscale organosilanes, and nanosized inorganic materials. The interaction between these mesoscale templates with silica or aluminosilicate species in the starting gels has been discussed. Furthermore, the excellent catalytic properties over these mesoporous zeolites are systemically summarized.
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paper_title: Grain size effects on gas sensitivity of porous SnO2-based elements

paper_content:

Effects of grain size on gas sensitivity (S) are investigated by using porous sintered SnO2 elements fabricated with pure SnO2, foreign oxide-stabilized SnO2, or impurity-doped SnO2. When the SnO2 crystallite size (D) is controlled to a size in the range 5–32 nm, S for H2, CO and i-C4H10 is found to increase steeply as D decreases to be comparable with or less than 2L (≈ 6 nm) is both pure and stabilized elements, where L is the depth of the space-charge layer. However, S for ethyl alcohol gas is found to be also affected by surface acid-base properties, being greatly promoted by basic oxides. It is found that the control of L by doping impurities (Al3+ or Sb5+) into the SnO2 lattice results in great changes in S even when D is the same. Thus Al-doped SnO2 shows high sensitivity with increasing L even at D above 20 nm, while Sb-doped SnO2 is insensitive in the whole D region. A model for the grain-size effects is proposed, in which the transducer function is operated by a mechanism of grain control, neck control or grain-boundary control, depending on D.
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paper_title: Hall measurement studies and an electrical conduction model of tin oxide ultrafine particle films

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The Hall effect has been measured for ultrafine particle films of tin oxide (particle size 50–200 A) in contact with reducing gases. Both the carrier concentration and mobility increased with an increasing concentration of reducing gas. As a result, conductivity, the product of the carrier concentration and mobility, shows a large gas concentration dependence. This means the ultrafine particle films are highly sensitive to gases. The variation of the carrier mobility with the gas concentration is interpreted in terms of a model in which the width of a long, thin channel, determined by particle size and Debye length, increases with a decrease in the number of O− ions desorbed by a chemical reaction with the reducing gases.
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paper_title: Conduction Model of Metal Oxide Gas Sensors

paper_content:

Tin dioxide is a widely used sensitive material for gas sensors. Many research and development groups in academia and industry are contributing to the increase of (basic) knowledge/(applied) know-how. However, from a systematic point of view the knowledge gaining process seems not to be coherent. One reason is the lack of a general applicable model which combines the basic principles with measurable sensor parameters.
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paper_title: Hydrogen sulfide gas sensing properties of thin films derived from SnO2 sols different in grain size

paper_content:

Abstract Sols of crystalline SnO 2 with different mean grain (crystallite) sizes between 6 and 16 nm were prepared by subjecting stannic acid gel to hydrothermal treatments under various conditions. Thin film sensor devices with different film thicknesses between 200 and 900 nm were fabricated to investigate sensing properties to H 2 S gas. It was found that the sensor response to H 2 S, defined as the ratio of resistance in air to that in H 2 S-containing air, was greatly enhanced with decreasing film thick and with increasing grain size up to 16 nm, as expected from the gas diffusion-reaction theory. The response was surprisingly large, exceeding 10 4 at 150 °C, for the device deposited with a 200 nm-thick film of SnO 2 12 nm in grain size.
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paper_title: Chemical control synthesis of nanocrystalline SnO2 by hydrothermal reaction

paper_content:

Abstract Nanocrystalline SnO 2 (3 nm) was synthesized by a Sn/H 2 O 2 /H 2 SO 4 hydrothermal oxidation at 150°C. Infrared spectrum (IR), Raman spectrum, X-ray diffraction (XRD), and transmission electron microscopy (TEM) were used to characterize the nanocrystalline SnO 2 .
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paper_title: Theory of gas-diffusion controlled sensitivity for thin film semiconductor gas sensor

paper_content:

Abstract Influences of gas transport phenomena on the sensitivity of a thin film semiconductor gas sensor were investigated theoretically. A diffusion equation was formulated by assuming that an inflammable gas (target gas) moves inside the film by Knudsen diffusion, while it reacts with the adsorbed oxygen following a first-order reaction kinetic. By solving this equation under steady-state conditions, the target gas concentration inside the film was derived as a function of depth ( x ) from the film surface, Knudsen diffusion coefficient ( D K ), rate constant ( k ) and film thickness ( L ). The gas concentration profile thus obtained allowed to estimate the gas sensitivity ( S ) defined as the resistance ratio ( R a / R g ), under the assumption that the sheet conductance of the film at depth x is linear to the gas concentration there with a proportionality constant (sensitivity coefficient), a . The derived equation shows that S decreases sigmoidally down to unity with an increase in L k/D K . Further by assuming that the temperature dependence of rate constant ( k ) and sensitivity coefficient ( a ) follows Arrenius type ones with respective activation energies, it was possible to derive a general expression of S involving temperature ( T ). The expression shows that, when the activation energies are selected properly, the S versus T correlation results in a volcano-shaped one, its height increasing with decreasing L . The dependence of S on L at constant T as well as on T at constant L can thus be simulated fairly well based on the equation.
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paper_title: Investigation of the Structural and Electrochemical Properties of Size-Controlled SnO2 Nanoparticles

paper_content:

Four types of size-controlled SnO2 nanoparticles were synthesized by a colloidal method to investigate the structural and electrochemical properties of various particle sizes. The samples were characterized by means of transmission electron microscopy (TEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and electrochemical measurements. TEM and XRD analyses showed that the samples were well dispersed with an average particle size of ∼20 nm (sample A), ∼11 nm (sample B), ∼5.6 nm (sample C), and ∼2.3 nm (sample D). XPS analyses of four size-controlled SnO2 samples confirmed that the chemical state of the Sn was Sn4+, as evidenced by the presence of SnO2, not Sn2+ related to the Sn(OH)2. The Sn NEXAFS spectra of the four samples indicated that the samples had a tetragonal rutile structure and showed no noticeable change in the oxidation state of tin ions with decreased particle size, in good agreement with the XPS analysis. The ele...
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paper_title: Grain size effects on gas sensitivity of porous SnO2-based elements

paper_content:

Effects of grain size on gas sensitivity (S) are investigated by using porous sintered SnO2 elements fabricated with pure SnO2, foreign oxide-stabilized SnO2, or impurity-doped SnO2. When the SnO2 crystallite size (D) is controlled to a size in the range 5–32 nm, S for H2, CO and i-C4H10 is found to increase steeply as D decreases to be comparable with or less than 2L (≈ 6 nm) is both pure and stabilized elements, where L is the depth of the space-charge layer. However, S for ethyl alcohol gas is found to be also affected by surface acid-base properties, being greatly promoted by basic oxides. It is found that the control of L by doping impurities (Al3+ or Sb5+) into the SnO2 lattice results in great changes in S even when D is the same. Thus Al-doped SnO2 shows high sensitivity with increasing L even at D above 20 nm, while Sb-doped SnO2 is insensitive in the whole D region. A model for the grain-size effects is proposed, in which the transducer function is operated by a mechanism of grain control, neck control or grain-boundary control, depending on D.
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paper_title: Functionalizing nanowires with catalytic nanoparticles for gas sensing application.

paper_content:

Metal oxide semiconducting nanowires are among the most promising materials systems for use as conductometric gas sensors. These systems function by converting surface chemical processes, often catalytic processes, into observable conductance variations in the nanowire. The surface properties, and hence the sensing properties of these devices can be altered dramatically improving the sensitivity and selectivity, by the deposition of catalytic metal nanoparticles on the nanowire's surface. This leads not only to promising sensor strategies but to a route for understanding some of the fundamental science occurring on these nanoparticles and at the metal/nanowire junction. In particular studying these systems can lead to a better understanding of the influence of the catalyst particle on the electronic structure of the nanowire and its electron transport. This report surveys results obtained so far in this area. In particular, the comparative sensing performance of single quasi-1D chemiresistors (i.e., nanowires or nanobelts) before and after surface decoration with noble metal catalyst particles show significant improvement in sensitivity toward oxidizing and reducing gases. Moreover, one finds that the sensing mechanism can depend dramatically on the degree of metal coverage of the nanowire.
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paper_title: The template-free synthesis of square-shaped SnO2 nanowires: the temperature effect and acetone gas sensors

paper_content:

Square-shaped single-crystalline SnO2 nanowires and their sphere-like hierarchical structures were synthesized successfully with a template-free hydrothermal approach. It was found that an intermediate phase—Na2Sn(OH)6—is first produced because it is slow to dissolve in ethanol/water media. The intermediate phase gradually decomposes and converts into SnO2 at temperatures higher than 200 °C. The reaction temperature also affects the microstructure of SnO2 nanomaterials. Uniform square-shaped SnO2 nanowires, which form sphere-like hierarchical structures in 100% structure yield, can be produced at 285 °C on a large scale. The diameter of the nanowires shows a decrease accompanying the increase of the reaction temperature. The temperature effect could be a result of the faster and oriented growth of SnO2 nanowires along their direction at higher temperature. Chemical sensors constructed with square-shaped SnO2 nanowires exhibit excellent stability, good sensitivity and selectivity, as well as a quick response and short recovery times under exposure to acetone gas in practical applications.
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paper_title: Fabrication of a SnO2 Nanowire Gas Sensor and Sensor Performance for Hydrogen

paper_content:

SnO2 nanowire gas sensors have been fabricated on Cd−Au comb-shaped interdigitating electrodes using thermal evaporation of the mixed powders of SnO2 and active carbon. The self-assembly grown sensors have excellent performance in sensor response to hydrogen concentration in the range of 10 to 1000 ppm. This high response is attributed to the large portion of undercoordinated atoms on the surface of the SnO2 nanowires. The influence of the Debye length of the nanowires and the gap between electrodes in the gas sensor response is examined and discussed.
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paper_title: Ethylene glycol-mediated synthesis of metal oxide nanowires

paper_content:

A simple and convenient method has been demonstrated for large-scale synthesis of metal oxide (including TiO2, SnO2, In2O3, and PbO) nanowires with diameters around 50 nm and lengths up to 30 µm. In a typical procedure, tetraalkoxyltitanium, Ti(OR)4 ::: (with R = ::: –C2H5, –iso-C3H7, or –n-C4H9), was added to ethylene glycol and heated to 170 °C for 2 h under vigorous stirring. The alkoxide was transformed into a chain-like, glycolate complex that subsequently crystallized into uniform nanowires. Similarly, nanowires made of tin glycolate were synthesized by refluxing SnC2O4·2H2O in ethylene glycol at 195 °C for 2 h, and nanowires consisting of indium and lead glycolates were prepared by adding In(OOCC7H15)(OiPr)2 and Pb(CH3COO)2 to ethylene glycol, followed by heating at 170 °C for 2 h. The nanowires could be readily collected as precipitates after the reaction solutions had been cooled down to room temperature. By calcining at elevated temperatures, each glycolate precursor could be transformed into the corresponding metal oxide without changing the wire-like morphology. Electron microscopic and XRD powder diffraction studies were used to characterize the morphology, crystallinity, and structure of these nanowires before and after calcination at various temperatures. A plausible mechanism was also proposed to account for the one-dimensional growth of such nanostructures in a highly isotropic medium. This mechanism was supported by XRD, FT-IR, solid state 13C-NMR, and TGA measurements. As a demonstration of potential applications, the polycrystalline nanowires made of SnO2 were used as functional components to fabricate sensors that could detect combustible gases (CO and H2) with greatly enhanced sensitivity under ambient conditions.
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paper_title: Highly porous CdO nanowires: preparation based on hydroxy- and carbonate-containing cadmium compound precursor nanowires, gas sensing and optical properties

paper_content:

Highly porous cadmium oxide (CdO) nanowires have been prepared by calcining the hydroxy- and carbonate-containing cadmium compound precursor nanowires. The large-scale precursor nanowires were synthesized through a hydrothermal method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize and analyze the as-synthesized precursor nanowires as well as the calcined products. It was revealed that the wire-like morphology of the precursor was fundamentally retained during the process of calcination and the CdO nanowires obtained were polycrystalline with highly porous structures. In order to illustrate the formation mechanism of the porous structures, the morphology and composition evolutions of the precursor nanowires under different stages of the calcining process were further investigated via SEM, x-ray diffraction (XRD) and infrared (IR) absorbance spectroscopy. Gas sensing has been explored for the sensor device fabricated with highly porous CdO nanowires, which demonstrates that it has good response owing to its special structures and great selectivity to NOx. Furthermore, the UV–visible and photoluminescence spectra of highly porous CdO nanowires have also been investigated.
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paper_title: Novel fabrication of an SnO2 nanowire gas sensor with high sensitivity

paper_content:

We fabricated a nanowire-based gas sensor using a simple method of growing SnO2 nanowires bridging the gap between two pre-patterned Au catalysts, in which the electrical contacts to the nanowires are self-assembled during the synthesis of the nanowires. The gas sensing capability of this network-structured gas sensor was demonstrated using a diluted NO2. The sensitivity, as a function of temperature, was highest at 200 °C and was determined to be 18 and 180 when the NO2 concentration was 0.5 and 5 ppm, respectively. Our sensor showed higher sensitivity compared to different types of sensors including SnO2 powder-based thin films, SnO2 coating on carbon nanotubes or single/multiple SnO2 nanobelts. The enhanced sensitivity was attributed to the additional modulation of the sensor resistance due to the potential barrier at nanowire/nanowire junctions as well as the surface depletion region of each nanowire.
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paper_title: The sensitivity of gas sensor based on single ZnO nanowire modulated by helium ion radiation

paper_content:

In this letter, we present a gas sensor using a single ZnO nanowire as a sensing unit. This ZnO nanowire-based sensor has quick and high sensitive response to H2S in air at room temperature. It has also been found that the gas sensitivity of the ZnO nanowires could be modulated and enhanced by He+ implantation at an appropriate dose. A possible explanation is given based on the modulation model of the depletion layer.
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paper_title: Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors

paper_content:

Based on the achievement of synthesis of ZnO nanowires in mass production, ZnO nanowires gas sensors were fabricated with microelectromechanical system technology and ethanol-sensing characteristics were investigated. The sensor exhibited high sensitivity and fast response to ethanol gas at a work temperature of 300 °C. Our results demonstrate the potential application of ZnO nanowires for fabricating highly sensitive gas sensors.
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paper_title: Gas sensing properties of defect-controlled ZnO-nanowire gas sensor

paper_content:

The effect of oxygen-vacancy-related defects on gas-sensing properties of ZnO-nanowire gas sensors was investigated. Gas sensors were fabricated by growing ZnO nanowires bridging the gap between two prepatterned Au catalysts. The sensor displayed fast response and recovery behavior with a maximum sensitivity to NO2 gas at 225 °C. Gas sensitivity was found to be linearly proportional to the photoluminescence intensity of oxygen-vacancy-related defects in both as-fabricated and defect-controlled gas sensors by postannealing in Ar and H2 atmosphere. This result agrees well with previous theoretical prediction that oxygen vacancies play a role of preferential adsorption sites for NO2 molecules.
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paper_title: Synergism between Cu and Zn sites in Cu/Zn catalysts for methanol synthesis

paper_content:

Abstract Three Cu-based catalysts prepared using RF plasma-sputtering (Cu/ZnO and Cu/Al 2 O 3 ) and coprecipitation (Cu/Zn/Al oxide) methods were employed for the investigation of the synergism between Cu and Zn sites. The static secondary ion mass spectrometry (SIMS) experiments indicate that, on both Cu/ZnO/Al 2 O 3 and Cu/ZnO catalysts, CO is adsorbed at Cu sites (SIMS peaks appear at 91 and 93 amu), and H is bound to ZnO sites (67, 69, 70, 71 and 81, 83, 84, 85 amu) when the catalyst surfaces are exposed to H 2 , CO and CO 2 . Methoxy, the intermediate species of methanol synthesis, is found to be bound at Zn sites (95, 97, 98, 99 amu). CO and CO 2 are found to induce Zn migration from sub-layers to the topmost layer while H 2 is heterolyzed easier due to the presence of Cu. The coexistence of ZnO with Cu could enhance the capability of Cu to adsorb CO species and itself to adsorb H 2 species. Al 2 O 3 shows no synergetic effect with Cu in this experiment. No CO and H 2 are detected on the Cu/Al 2 O 3 catalyst exposed to syngas. The synergetic effect between Cu and Zn in the course of methanol synthesis was discussed.
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paper_title: Influence of noble metals on the structural and catalytic properties of Ce-doped SnO2 systems

paper_content:

This paper discusses the influence of the presence of a second dopant (Pt or Pd) on the SnO 2·Ce matrix. The characteristics of the material and the catalytic activity of oxidizing methanol were evaluated. Two routes were used to prepare the SnO2·5 mol% Ce·1 mol% NM (NM = Pt or Pd). Transmission electronic microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and chromatographic assays were used to study the samples. Almost no changes in the morphology were detected when the samples were prepared by the impregnation route. In contrast, preparation by the chemical route (modified polymeric precursor (MPP) route) revealed crystallite growth and CeO2 segregation. No sample showed any metallic cluster, but all the samples containing noble metal displayed a very marked catalytic property, suggesting significant activity even at room temperature. The products formed were CO 2 and water, representing a competitive advantage from the standpoint of environmental protection. © 2003 Elsevier B.V. All rights reserved.
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paper_title: The effect of grain size on the sensitivity of nanocrystalline metal-oxide gas sensors

paper_content:

The effect of grain size on the sensitivity of chemoresistive nanocrystalline metal-oxide gas sensors was evaluated by calculating the effective carrier concentration as a function of the surface state density for a typical sensing material, SnO2, with different grain sizes between 5 and 80 nm. This involved numerical computation of the charge balance equation (the electroneutrality condition) using approximated analytical solutions of Poisson’s equation for small spherical crystallites. The calculations demonstrate a steep decrease in the carrier concentration when the surface state density reaches a critical value that corresponds to a condition of fully depleted grains, namely, when nearly all the electrons are trapped at the surface. Assuming that the variations in the surface state density are induced by surface interactions with ambient gas molecules, these calculations enable us to simulate the response curves of nanocrystalline gas sensors. The simulations show that the conductivity increases line...
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paper_title: Receptor Function and Response of Semiconductor Gas Sensor

paper_content:

Theoretical approaches to receptor function and response of semiconductor gas sensor are described, following the illustrations of some relevant key issues such as tunneling transport. Depletion in small semiconductor crystals is characterized by the occurrence of new type depletion (volume depletion) after conventional one (regional depletion), and inclusion of both types makes it possible to formulate the receptor function and response to oxygen (air base), oxidizing gas (nitrogen dioxide), and reducing gas (hydrogen). The equations derived theoretically using physical parameters of the semiconductor side and chemical parameters of the gases side appear to reproduce satisfactorily the sensing behavior to the aforementioned gases as well as the influence of changes in physical parameters such as grain size and donor density. Extension to the semiconductor crystals dispersed with surface electron-traps shows that the traps act as a sensitizer to promote sensor response.
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paper_title: Nanocrystalline Metal Oxides for Methane Sensors: Role of Noble Metals

paper_content:

Methane is an important gas for domestic and industrial applications and its source is mainly coalmines. Since methane is extremely inflammable in the coalmine atmosphere, it is essential to develop a reliable and relatively inexpensive chemical gas sensor to detect this inflammable gas below its explosion amount in air. The metal oxides have been proved to be potential materials for the development of commercial gas sensors. The functional properties of the metal oxide-based gas sensors can be improved not only by tailoring the crystal size of metal oxides but also by incorporating the noble metal catalyst on nanocrystalline metal oxide matrix. It was observed that the surface modification of nanocrystalline metal oxide thin films by noble metal sensitizers and the use of a noble metal catalytic contact as electrode reduce the operating temperatures appreciably and improve the sensing properties. This review article concentrates on the nanocrystalline metal oxide methane sensors and the role of noble metals on the sensing properties.
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paper_title: Ethanol sensors based on Pt-doped tin oxide nanopowders synthesised by gel-combustion

paper_content:

Abstract The ethanol sensing properties of SnO 2 powders prepared by a gel-combustion method have been investigated. SnO 2 powders with different average crystallite size, 〈 D 〉, ranging from 6 up to 100 nm, were synthesized by this process starting from metallic tin as raw material and citric acid and/or urea as fuel substances. The average crystallite size of the synthesized powders was found to depend on the nature and loading of the fuel substance used in the combustion process. Thick film sensors based on the gel-combustion SnO 2 powders, annealed at 600 °C, have shown good sensitivity to low concentrations of ethanol (50–200 ppm). The smaller particles have shown a higher sensitivity than the bigger ones. At the addition of 1 wt% Pt, a remarkable enhancement of the sensitivity to C 2 H 5 OH and response and recovery time was observed for the sample with smaller SnO 2 particles. Pt was instead found to suppress CO sensitivity, thus increasing the selectivity towards ethanol.
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paper_title: Nano-crystalline Cu-doped ZnO thin film gas sensor for CO

paper_content:

A Cu-doped ZnO (CZO) film was prepared on a glass substrate by co-sputtering using ZnO and Cu targets. The CZO film possessed a columnar structure consisting of small crystals with an average grain size of around 5 nm. The CO-sensing properties of the CZO film were tested at operating temperatures of 150, 300, 350 and 400 °C. It was found that CZO-based sensor exhibited the highest sensitivity to CO at 350 °C. An obvious change in resistance of the CZO film was also observed when the sensor was exposed to 6 ppm CO at 150 °C. The sensing mechanism of the nano-structured CZO sensor is discussed.
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paper_title: Inhibition of Crystallite Growth in the Sol-Gel Synthesis of Nanocrystalline Metal Oxides

paper_content:

Crystal growth upon firing of hydrous transition metal oxide gels can be effectively inhibited by replacing the surface hydroxyl group before firing with another functional group that does not condense and that can produce small, secondary-phase particles that restrict advancing of grain boundaries at elevated temperatures. Accordingly, fully crystallized SnO 2 , TiO 2 , and ZrO 2 materials with mean crystallite sizes of ∼20, 50, and 15 angstroms, respectively, were synthesized by replacing the hydroxyl group with methyl siloxyl before firing at 500°C. An ultrasensitive SnO 2 -based chemical sensor resulting from the microstructural miniaturization was demonstrated.
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paper_title: Grain size effects on gas sensitivity of porous SnO2-based elements

paper_content:

Effects of grain size on gas sensitivity (S) are investigated by using porous sintered SnO2 elements fabricated with pure SnO2, foreign oxide-stabilized SnO2, or impurity-doped SnO2. When the SnO2 crystallite size (D) is controlled to a size in the range 5–32 nm, S for H2, CO and i-C4H10 is found to increase steeply as D decreases to be comparable with or less than 2L (≈ 6 nm) is both pure and stabilized elements, where L is the depth of the space-charge layer. However, S for ethyl alcohol gas is found to be also affected by surface acid-base properties, being greatly promoted by basic oxides. It is found that the control of L by doping impurities (Al3+ or Sb5+) into the SnO2 lattice results in great changes in S even when D is the same. Thus Al-doped SnO2 shows high sensitivity with increasing L even at D above 20 nm, while Sb-doped SnO2 is insensitive in the whole D region. A model for the grain-size effects is proposed, in which the transducer function is operated by a mechanism of grain control, neck control or grain-boundary control, depending on D.
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paper_title: Dense doping of indium to coral-like SnO2 nanostructures through a plasma-assisted strategy for sensitive and selective detection of chlorobenzene

paper_content:

A plasma-assisted strategy for densely doping indium to SnO2 nanostructures for gas-sensing applications is reported. The morphology, structure, and composition of the as-prepared nanostructures were characterized by field emission scanning electronic microscopy (FESEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), and x-ray photoelectron spectrometry (XPS), respectively. The results show that the densities of hydroxyl and carboxyl groups of the coral-like SnO2/carbonaceous nanocomposites are remarkably improved by using a plasma treatment (PT), which enables them to adsorb a large quantity of indium ions and thereby enhance the doping. In gas-sensing measurements, it is found that the sensor is sensitive to chlorobenzene with a high response and short response and recovery times. Besides, the gas-sensing properties of the sensor based on the In-doped SnO2 with PT are greatly improved compared with sensors based on In-doped SnO2 without PT and pure SnO2. The enhanced doping and the special coral-like structure are demonstrated as the mechanism of improvement. The kinetic processes of gas adsorption and desorption are also investigated. Furthermore, it is revealed that chlorobenzene can be clearly identified from some gas references by using principal component analysis, exhibiting a good selectivity. Our findings not only provide a promising building block for developing a sensitive and selective gas sensor for environmental monitoring, but also demonstrate a novel plasma-assisted strategy which could be potentially developed as a general method for dense doping of nanomaterials.
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Title: Metal Oxide Nanostructures and Their Gas Sensing Properties: A Review

Section 1: Introduction
Description 1: This section introduces the importance of air quality, the necessity of gas sensors, and the advantages and challenges of metal oxide gas sensors.

Section 2: Gas Sensing Mechanism
Description 2: This section explains the basic working principles and mechanisms behind metal oxide gas sensors, particularly focusing on n-type metal oxides like SnO₂.

Section 3: Device Structure
Description 3: This section details the components and types of device structures used in metal oxide gas sensors, including conductometric, FET, and impedometric types.

Section 4: Nano Effect of Small Size of Metal Oxide Nanoparticles
Description 4: This section discusses the "small size effect" of metal oxide nanoparticles and its impact on sensor sensitivity and performance.

Section 5: Porous Film of Metal Oxides
Description 5: This section covers the differences between dense and porous metal oxide films and their implications for gas sensing.

Section 6: Porous Nanostructures of Metal Oxide and Their Gas Sensing Properties
Description 6: This section reviews various porous nanostructures like nanowires, nanotubes, and nanospheres, and their superior gas sensing properties.

Section 7: Porous Nanowires
Description 7: This section focuses on porous nanowires and their gas sensing advantages due to high surface-to-volume ratios and porous structures.

Section 8: Doping of the Metal Oxide Nanostructures in Nanoscale Levels
Description 8: This section discusses the effects of doping metal oxide nanostructures to enhance gas sensing capabilities.

Section 9: Conclusions and Perspectives
Description 9: This section provides a summary of the current state of metal oxide gas sensors and suggests future research directions and potential improvements.