Catalysis-based Cataluminescent and Conductometric Gas Sensors: Sensing Nanomaterials, Mechanism, Applications and Perspectives

Gas environment detection has become more urgent and significant, for both industrial manufacturing and environment monitoring. Gas sensors based on a catalytically-sensing mechanism are one of the most important types of devices for gas detection, and have been of great interest during the past decades. However, even though many efforts have contributed to this area, some great challenges still remain, such as the development of sensitively and selectively sensing catalysts. In this review, two representative catalysis-based gas sensors, cataluminescent and conductometric sensors, the basis of optical and electric signal acquisition, respectively, are summarized comprehensively. The current challenges have been presented. Recent research progress on the working mechanism, sensing nanomaterials, and applications reported by our group and some other researchers have been discussed systematically. The future trends and prospects of the catalysis-based gas sensors have also been presented.


Introduction
Gas sensors based on a catalytically-sensing mechanism are one of the most important types of devices for gas environment detection.In the past decades, gas sensors have received broad attention due to their potential applications in industrial gas measurements, indoor air detection, coal gas monitoring, breath diagnosis, etc. [1][2][3][4].Among the many catalysis-based gas sensors, cataluminescent and conductometric sensors, which rely on optical signal and current signal acquisition, respectively, are representative [5][6][7][8].Sensing material is commonly the key part of the gas sensor.Therefore, the development of sensors mainly focuses on exploring high performance sensing materials during the past years.
Since Breysse et al. [9] first reported that the catalytic oxidation of carbon monoxide on the surface of thoria produced a chemiluminescent emission, and established the concept of cataluminescence, cataluminescence excited during the interaction between gases and catalyst surfaces has been

Challenges for Catalysis-Based Gas Sensors
Depending on the development of hi-tech industrial manufacturing and the increasing demand of high quality gas environment in our daily life, gas detection based on sensors has become more significant, accompanied with a strict requirement for high sensing performance [19,20].For example, as gas components are complicated, good selectivity is desired; in order to monitor gases at a trace concentration, ultra-high sensitivity with a low detection limit is necessary.To meet these requirements, catalysis-based gas sensors still have some great challenges: (i) Developing small sized sensing materials is significant to achieve a high reactivity for catalytic reaction with target molecules.In recent years, nanomaterials have shown remarkably enhanced catalytic performance.Currently, a general preparation route that can apply extensive sensing nanomaterials is highly demanded.(ii) Dealing with multi-component gases, two potential strategies could be considered: (a) improving the selectivity of gas sensor, which only shows response to a specific target gas; and (b) fabricating sensor array for multi-channel signal analysis simultaneously.The responding behaviors of different sensing materials toward different gases are not the same, making it possible to distinguish each target by combining with some algorithms such as principal component analysis (PCA) [21,22].(iii) Acquiring weak sensing signals including cataluminescence and electric current is difficult when detecting trace concentration gases.In many cases, as the gas concentration reduces, sensing signal produced by sensors becomes poor [23,24]; thus, an efficient approach to acquire those signals and distinguish valuable data from noise is important for gas detection.
(iv) In order to reduce cost and simplify the operation procedures, a stable sensing performance of catalysis-based gas sensors is essential.However, commonly, high sensitivity would be in opposition to good long-term stability.Obtaining acceptable sensitivity and stability simultaneously remains a challenge.To achieve this, fabricating multi-functional structure and composite would be a potential strategy.For example, porous single-crystalline nanostructure is benefit for good gas adsorption, high reactivity, and structural stability.(v) The catalytic mechanism of some sensing materials is not yet clear, which greatly restricts the design of novel high-performance sensing structures.

Sensing Mechanism and Features
When oxygen molecules adsorb onto sensing material, the electron (e − ) in valence bond will be excited to conduction bond under a certain external energy stimuli, such as a heating treatment.At the same time, a cavity (h + ) forms in the valence bond, as shown in Figure 1.Such electron specie possesses a high capability of oxidization, which enables it to react with some target gas molecules represented as R. When gas molecules go through the surface of sensing materials, catalytic redox reaction happens, resulting in the formation of excited state intermediates.These intermediates go back to basic state, leading to the release of photon, and thus a chemiluminescence can be detected during this process.The chemiluminescence wavelength is determined by the energy structure and forbidden band of sensing materials [25,26].The relative humidity in air is generally associated with the formation of chemisorbed hydroxyl groups, which may influence the surface catalysis during detection [27,28].
Catalysts 2016, 6, 210 3 of 22 (v) The catalytic mechanism of some sensing materials is not yet clear, which greatly restricts the design of novel high-performance sensing structures.

Sensing Mechanism and Features
When oxygen molecules adsorb onto sensing material, the electron (e − ) in valence bond will be excited to conduction bond under a certain external energy stimuli, such as a heating treatment.At the same time, a cavity (h + ) forms in the valence bond, as shown in Figure 1.Such electron specie possesses a high capability of oxidization, which enables it to react with some target gas molecules represented as R. When gas molecules go through the surface of sensing materials, catalytic redox reaction happens, resulting in the formation of excited state intermediates.These intermediates go back to basic state, leading to the release of photon, and thus a chemiluminescence can be detected during this process.The chemiluminescence wavelength is determined by the energy structure and forbidden band of sensing materials [25,26].The relative humidity in air is generally associated with the formation of chemisorbed hydroxyl groups, which may influence the surface catalysis during detection [27,28].In recent years, nanomaterials and nanotechnologies have been developed rapidly, providing some new opportunities for fabricating high-performance cataluminescent gas sensors [29,30].Thus, a series of cataluminescent gas sensors for specific applications can be designed on the basis of nanomaterials modulation.Those nanomaterials-based cataluminescent gas sensors would possess the following features: (i) External light resource is not necessary for gas detection.The influence factors to the sensing behaviors of cataluminescent gas sensors are reduced, which enhances the stability and accuracy.(ii) The luminescence is produced through the catalytic reactions on the interface between target gas and sensing nanomaterials, enabling a good reversibility.(iii) The selectivity of cataluminescent gas sensors can be significantly improved by specific modifications on sensing nanomaterials.(iv) Quantitative measurements are promising using the linear relationship of cataluminescent intensity versus gas concentration.Those attractive characteristics of cataluminescent gas sensors based on nanomaterials have attracted broad attention.Some recent research progresses on cataluminescent gas sensors for detecting different gases is presented below.

Typical Cataluminescent Detection System
Commonly, a cataluminescent detection system contains gas input unit, reaction unit and measurement unit.As illustrated in Figure 2, on the surface of a heating rod, a layer of cataluminescent materials is coated, which is then placed in a quartz tube.The working temperature can be modulated by controlling the voltage of the quartz tube.The pump controls the gas flow rate and disperses the target gas thoroughly.When the target gas flows through the tube, the catalytic reaction occurs on the surface of sensing materials.The cataluminescent intensity can be monitored In recent years, nanomaterials and nanotechnologies have been developed rapidly, providing some new opportunities for fabricating high-performance cataluminescent gas sensors [29,30].Thus, a series of cataluminescent gas sensors for specific applications can be designed on the basis of nanomaterials modulation.Those nanomaterials-based cataluminescent gas sensors would possess the following features: (i) External light resource is not necessary for gas detection.The influence factors to the sensing behaviors of cataluminescent gas sensors are reduced, which enhances the stability and accuracy.(ii) The luminescence is produced through the catalytic reactions on the interface between target gas and sensing nanomaterials, enabling a good reversibility.(iii) The selectivity of cataluminescent gas sensors can be significantly improved by specific modifications on sensing nanomaterials.(iv) Quantitative measurements are promising using the linear relationship of cataluminescent intensity versus gas concentration.Those attractive characteristics of cataluminescent gas sensors based on nanomaterials have attracted broad attention.Some recent research progresses on cataluminescent gas sensors for detecting different gases is presented below.

Typical Cataluminescent Detection System
Commonly, a cataluminescent detection system contains gas input unit, reaction unit and measurement unit.As illustrated in Figure 2, on the surface of a heating rod, a layer of cataluminescent materials is coated, which is then placed in a quartz tube.The working temperature can be modulated by controlling the voltage of the quartz tube.The pump controls the gas flow rate and disperses the target gas thoroughly.When the target gas flows through the tube, the catalytic reaction occurs on the surface of sensing materials.The cataluminescent intensity can be monitored by a photomultiplier tube equipped on a luminescence analyzer, which is commonly a Biophysics Chemiluminescence (BPCL) luminescence analyzer.

Cataluminescent Gas Sensors for Detecting Multi-Component Volatile Organic Compounds (VOCs)
VOCs generally exist in air, and they can cause short-or long-term adverse health effects [32].Cataluminescent gas sensors have been demonstrated as highly efficient for VOC detection.Lv et al. [33] put forward a novel approach for the discrimination of gas compounds through a transient cataluminescence phenomenon on a flowerlike MgO.Using flowerlike MgO as sensing material, the cataluminescent gas sensor exhibits highly active, ultrafast, and characteristic responses toward many analytes, including isopropanol, methyl alcohol, tetrachloroethylene, ethyl alcohol, ethyl acetate, butanone, acetonitrile, methanoic acid, dichloromethane, epoxy chloropropane, etc., as shown in Figure 3.It is expected that for practical applications, it would be available to collect transient cataluminescence curves of multifarious compounds on different catalysts to establish a standard database, thus researchers could identify unknown samples by comparing their transient cataluminescence curves to standards.Zhang and co-workers showed a gas sensor for ketones detection based on cataluminescence generated on the surface of NaYF4:Er nanomaterials [34].When the Er 3+ doping concentration is increased to 20%, the sensors show good sensing characteristics toward ketones.Under the optimal experimental conditions for detecting acetone and butanone, the gas sensor exhibits fast responses (3 s) and relatively low working temperature (250 °C).The linear ranges of cataluminescence intensity versus concentration are 2.388 to 143.28 μg•mL −1 for acetone and 2.45 to 49.0 μg•mL −1 for butanone, with a detection limit (signal-to-noise ratio = 3) of 1.7 and 0.7 μg•mL −1 , respectively.VOCs generally exist in air, and they can cause short-or long-term adverse health effects [32].Cataluminescent gas sensors have been demonstrated as highly efficient for VOC detection.Lv et al. [33] put forward a novel approach for the discrimination of gas compounds through a transient cataluminescence phenomenon on a flowerlike MgO.Using flowerlike MgO as sensing material, the cataluminescent gas sensor exhibits highly active, ultrafast, and characteristic responses toward many analytes, including isopropanol, methyl alcohol, tetrachloroethylene, ethyl alcohol, ethyl acetate, butanone, acetonitrile, methanoic acid, dichloromethane, epoxy chloropropane, etc., as shown in Figure 3.It is expected that for practical applications, it would be available to collect transient cataluminescence curves of multifarious compounds on different catalysts to establish a standard database, thus researchers could identify unknown samples by comparing their transient cataluminescence curves to standards.Zhang and co-workers showed a gas sensor for ketones detection based on cataluminescence generated on the surface of NaYF 4 :Er nanomaterials [34].When the Er 3+ doping concentration is increased to 20%, the sensors show good sensing characteristics toward ketones.Under the optimal experimental conditions for detecting acetone and butanone, the gas sensor exhibits fast responses (3 s) and relatively low working temperature (250 • C).The linear ranges of cataluminescence intensity versus concentration are 2.388 to 143.28 µg•mL −1 for acetone and 2.45 to 49.0 µg•mL −1 for butanone, with a detection limit (signal-to-noise ratio = 3) of 1.7 and 0.7 µg•mL −1 , respectively.(5) ethyl acetate; (6) butanone; (7)  Previously, we have reported cataluminescent gas sensors based on La2O3 and cocoon-like Au/La2O3 nanomaterials for detecting tetrahydrofuran, acetone, ethanol, benzene, chloroform and chlorobenzene [35].La(OH)3 precursors are firstly synthesized through a simple hydrothermal methods.After calcination at a high temperature, La(OH)3 is converted to La2O3 on the basis of a dehydration reaction.The La2O3 shows a cocoon-like morphology after calcinations, as shown in Figure 4.In addition, Au/La2O3 is prepared by modifying Au nanoparticles onto La(OH)3, which present a highly interconnected structure.It is advantageous for gas-sensing applications due to the high surface-to-volume ratio.Previously, we have reported cataluminescent gas sensors based on La 2 O 3 and cocoon-like Au/La 2 O 3 nanomaterials for detecting tetrahydrofuran, acetone, ethanol, benzene, chloroform and chlorobenzene [35].La(OH) 3 precursors are firstly synthesized through a simple hydrothermal methods.After calcination at a high temperature, La(OH) 3 is converted to La 2 O 3 on the basis of a dehydration reaction.The La 2 O 3 shows a cocoon-like morphology after calcinations, as shown in Figure 4.In addition, Au/La 2 O 3 is prepared by modifying Au nanoparticles onto La(OH) 3 , which present a highly interconnected structure.It is advantageous for gas-sensing applications due to the high surface-to-volume ratio.[33].Copyright American Chemical Society, 2016.
Previously, we have reported cataluminescent gas sensors based on La2O3 and cocoon-like Au/La2O3 nanomaterials for detecting tetrahydrofuran, acetone, ethanol, benzene, chloroform and chlorobenzene [35].La(OH)3 precursors are firstly synthesized through a simple hydrothermal methods.After calcination at a high temperature, La(OH)3 is converted to La2O3 on the basis of a dehydration reaction.The La2O3 shows a cocoon-like morphology after calcinations, as shown in Figure 4.In addition, Au/La2O3 is prepared by modifying Au nanoparticles onto La(OH)3, which present a highly interconnected structure.It is advantageous for gas-sensing applications due to the high surface-to-volume ratio.The cocoon-like Au/La 2 O 3 -based sensors exhibit high responses to VOCs compared to the unmodified sensor under the same working conditions, as shown in Figure 5.The pristine La 2 O 3 sensors show weak responses to tetrahydrofuran, acetone, and ethanol, while there is no response to benzene, chloroform, and chlorobenzene.After functionalization with Au nanoparticles, the cataluminescence intensities are evidently enhanced.The relative intensities go up to ca. 2179, 1808, 733, 2395, 458 and 270 to tetrahydrofuran, acetone, ethanol, benzene, chloroform and chlorobenzene, respectively.In addition, the response/recovery times are quite short.The high cataluminescent performance might be ascribed to the functional Au nanoparticles, which provide extraordinary catalysis activity.Oxygen species dissociating from Au nanoparticles, which possess high catalytic abilities, would migrate to the surface of La 2 O 3 , resulting in the increase of oxygen species concentration [36], which is significant for achieving a high catalytic activity in terms of the cataluminescent mechanism.The Au/La 2 O 3 -based cataluminescent gas sensors present good cataluminescence performance such as high intensity and signal/noise ratio, and fast response/recovery to targets, making them can be promisingly applied for the detection of VOCs in environment.The cocoon-like Au/La2O3-based sensors exhibit high responses to VOCs compared to the unmodified sensor under the same working conditions, as shown in Figure 5.The pristine La2O3 sensors show weak responses to tetrahydrofuran, acetone, and ethanol, while there is no response to benzene, chloroform, and chlorobenzene.After functionalization with Au nanoparticles, the cataluminescence intensities are evidently enhanced.The relative intensities go up to ca. 2179, 1808, 733, 2395, 458 and 270 to tetrahydrofuran, acetone, ethanol, benzene, chloroform and chlorobenzene, respectively.In addition, the response/recovery times are quite short.The high cataluminescent performance might be ascribed to the functional Au nanoparticles, which provide extraordinary catalysis activity.Oxygen species dissociating from Au nanoparticles, which possess high catalytic abilities, would migrate to the surface of La2O3, resulting in the increase of oxygen species concentration [36], which is significant for achieving a high catalytic activity in terms of the cataluminescent mechanism.The Au/La2O3-based cataluminescent gas sensors present good cataluminescence performance such as high intensity and signal/noise ratio, and fast response/recovery to targets, making them can be promisingly applied for the detection of VOCs in environment.Even though the cataluminescent responses shown above are quite attractive, the selectivity and recognition of specific target gases still remain a challenge.With this in mind, some studies focus on the modification of sensing materials [37,38].Conversely, we have demonstrated a sensor array strategy for effective recognition of different gases.Twelve VOCs have been discriminated based on a cataluminescent patterns obtained by a 4 × 4 nanomaterial-based array (Figure 6) [39].The airflow is supplied by a pneumatic pump and a precision flow meter is employed for the measurement of the gas flow rate.A digital temperature controller is used to control the temperature of the ceramic Even though the cataluminescent responses shown above are quite attractive, the selectivity and recognition of specific target gases still remain a challenge.With this in mind, some studies focus on the modification of sensing materials [37,38].Conversely, we have demonstrated a sensor array strategy for effective recognition of different gases.Twelve VOCs have been discriminated based on a cataluminescent patterns obtained by a 4 × 4 nanomaterial-based array (Figure 6) [39].The airflow is supplied by a pneumatic pump and a precision flow meter is employed for the measurement of the gas flow rate.A digital temperature controller is used to control the temperature of the ceramic chip.The final cataluminescent patterns are recorded by a camera closely placed to the ceramic chip after inputting target gas for about 1 min.The results indicate that the patterns with a relatively weak brightness of spots for chloroform and toluene can be obtained at 190 • C, as shown in Figure 7.When the temperature is increased to 210 • C and 230 • C, the brightness of the spots is feasible to differentiate ether, acetone, chloroform, and toluene.This finding indicates different patterns can be obtained at different working temperature, whichcan distinguish each VOC gas.Copyright Elsevier, 2013.

Cataluminescent Gas Sensors for Detecting Alcohol
Zhang and co-workers have fabricated a cataluminescent gas sensor for alcohol detection on the basis of the chemiluminiscence characteristics between SrCO3 nanomaterials and alcohol [40].They have investigated the influences of temperature and gas flow rate to sensing performance.The sensing range is around 6 to 3750 ppm.The detection limit is as low as 2.1 ppm.Moreover, they demonstrated that the SrCO3-based chemiluminiscent gas sensor exhibited a selectivity to alcohol with no response to gasoline, ammonia, or hydrogen.Such a specific gas sensor can be potentially applied for breath diagnosis and industrial gas monitoring.Copyright Elsevier, 2013.

Cataluminescent Gas Sensors for Detecting Alcohol
Zhang and co-workers have fabricated a cataluminescent gas sensor for alcohol detection on the basis of the chemiluminiscence characteristics between SrCO3 nanomaterials and alcohol [40].They have investigated the influences of temperature and gas flow rate to sensing performance.The sensing range is around 6 to 3750 ppm.The detection limit is as low as 2.1 ppm.Moreover, they demonstrated that the SrCO3-based chemiluminiscent gas sensor exhibited a selectivity to alcohol with no response to gasoline, ammonia, or hydrogen.Such a specific gas sensor can be potentially applied for breath diagnosis and industrial gas monitoring.

Cataluminescent Gas Sensors for Detecting Alcohol
Zhang and co-workers have fabricated a cataluminescent gas sensor for alcohol detection on the basis of the chemiluminiscence characteristics between SrCO 3 nanomaterials and alcohol [40].They have investigated the influences of temperature and gas flow rate to sensing performance.The sensing range is around 6 to 3750 ppm.The detection limit is as low as 2.1 ppm.Moreover, they demonstrated that the SrCO 3 -based chemiluminiscent gas sensor exhibited a selectivity to alcohol with no response to gasoline, ammonia, or hydrogen.Such a specific gas sensor can be potentially applied for breath diagnosis and industrial gas monitoring.
However, those gas sensors still demand a high working temperature to activate cataluminescent reactions.For example, a temperature of about 470 • C is needed for detecting alcohol using a TiO 2 -based cataluminescent sensor.Such a high temperature would induce a strong luminescence basis, which reduces the signal/noise ratio severely.Moreover, high temperature consumes energy, which is disadvantage for portable instrument since the power of batteries is limited.Therefore, a low temperature cataluminescent gas sensor is highly required.Zhang et al. [41] reported that ZrO 2 nanomaterials exhibited a good cataluminescent property at a relatively low temperature of 195 • C. The fabricated sensor shows a high stability even after working for 100 h.At a wavelength of 460 ± 10 nm, the ZrO 2 cataluminescence sensor exhibits a detection limit of 0.6 ppm to alcohol, while the detecting linear range is from 1.6 to 160 ppm.
Zhang et al. have also figured out a cataluminescent mechanism on the basis of energy transfer [42].Once some metal ions such as Ho 3+ , Co 2+ , and Cu 2+ are introduced into sensing materials, cataluminescent would be quenched.Meanwhile, the active intermediates formed during catalytic reactions transfer to those doping ions.On the basis of this finding, they reported a Eu-doped ZrO 2 cataluminescent gas sensor, which shows a high sensing performance for alcohol detection.The sensitivity of Eu-doped ZrO 2 sensor is about 72 times the ZrO 2 sensor without doping.They demonstrated that such a great performance enhancement could be explained as follows: when alcohol molecules react with ZrO 2 , some of the active intermediates are absorbed by catalyst ZrO 2 ; once there are Eu 3+ dopants in the sensing nanomaterial, active intermediates would transfer to Eu 3+ , resulting in the improvement of sensitivity.Their findings also indicate that the main factor limiting the sensitivity of cataluminescent gas sensor based on pure catalyst may be the inevitable energy quenching of excited intermediates with the catalyst.This point has also been confirmed by Lu et al. [43].They indicated that the chemisorbed oxygen on the surface of catalysts was one of the essential factors for catalytic oxidization of gaseous molecules during the cataluminescent process.
In addition, Zhang's research group prepared a TiO 2 nanomaterial with a particle size of about 20 nm through a sol-gel route [44].They found that TiO 2 nanoparticles exhibit strong cataluminescent intensity once they react with alcohol and acetone.Their results show that the linear relationships are good in the concentration ranges of 40 to 400 ppm for alcohol and 20 to 200 ppm for acetone.Lv and co-workers reported ZnO nanoparticles could react with alcohol and produce luminescence [45].They fabricated a ZnO-based cataluminescent gas sensor for detecting alcohol.A good linear relationship between cataluminescent intensity and alcohol vapor concentration is obtained in the range of 1 to 100 ppm, while the detection limit is about 0.7 ppm.

Cataluminescent Gas Sensors for Benzene Detection
Benzene is a colorless and volatile liquid with a strong aroma.Benzene vapors released from building materials and some plastic applications used in our daily life can rise up the risk of many diseases to human.Monitoring benzene with cataluminescent gas sensors have been of great interest.It was reported that cataluminescent sensors showed a good linear relationship (Figure 8a) and a selectivity towards benzene from some gas interferences (Figure 8b), including propanol, butanol, formaldehyde, butanone, dimethylbenzene, methanol, and toluene under the same conditions at a concentration of about 800 ppm [46].There is no signal for propanol, dimethylbenzene and methanol, while the intensity to benzene is particularly high.Zhang et al. [47] have confirmed that the selectivity, sensitivity and lifetime of the cataluminescent sensors are greatly dependent on the following factors: the careful selection of cataluminescent reagents responding to the definite species, the method of immobilizing the reagents, and the substrates selected for reagents immobilization.

Cataluminescent Gas Sensors for Ether Detection
The low-cost and portable approach for ether detection is required because of the increasing attention on environmental monitoring, homeland security, agriculture, and medical applications [48][49][50].Ether is widely used both in laboratory and industry.However, continued exposure to ether may lead to respiratory failure or even death due to its anesthetic effect [51].Cataluminescent gas sensors have been developed for sensitive ether detection in the past few years.For example, Li et al. [31] synthesized some SiO2/Fe3O4 microspheres which showed a high cataluminescent performance toward ether, as shown in Figure 9.The results indicate that the SiO2/Fe3O4 microsphere-based sensors exhibit good cataluminescent properties including stable intensity, high signal/noise values, short response and recovery time.After optimization, the gas sensor possesses a linear detection range from 10 to 3000 ppm (r = 0.9967) and a detection limit of about 6.7 ppm (S/N = 3), which is much below the standard permitted concentration of ether in indoor air.

Cataluminescent Gas Sensors for Ether Detection
The low-cost and portable approach for ether detection is required because of the increasing attention on environmental monitoring, homeland security, agriculture, and medical applications [48][49][50].Ether is widely used both in laboratory and industry.However, continued exposure to ether may lead to respiratory failure or even death due to its anesthetic effect [51].Cataluminescent gas sensors have been developed for sensitive ether detection in the past few years.For example, Li et al. [31] synthesized some SiO 2 /Fe 3 O 4 microspheres which showed a high cataluminescent performance toward ether, as shown in Figure 9.The results indicate that the SiO 2 /Fe 3 O 4 microsphere-based sensors exhibit good cataluminescent properties including stable intensity, high signal/noise values, short response and recovery time.After optimization, the gas sensor possesses a linear detection range from 10 to 3000 ppm (r = 0.9967) and a detection limit of about 6.7 ppm (S/N = 3), which is much below the standard permitted concentration of ether in indoor air.
Lu and co-workers used mesoporous TiO 2 nanoparticles to detect diethyl ether with a 440 nm bandpass filter.The cataluminescent gas sensor shows high selectivity toward diethyl ether over other VOCs, such as acetaldehyde, acetone, butanone and butanol, etc.The selectivity is ascribed to the difference of cataluminescent emission profiles of various VOCs reaction intermediates.They confirmed that only diethyl ether exhibited a strong cataluminescent emission at about 440 nm.Wang et al. [52] reported a three-dimensional hierarchical CdO nanostructure with a bio-inspired morphology which showed outstanding cataluminescent properties such as stable intensity, high signal/noise values, short response and recovery time.The limits of detection toward acetone and diethyl ether are ca.6.5 and 6.7 ppm, respectively.In addition, a principal components analysis method was demonstrated to be able to recognize acetone and diethyl ether clearly.Lu and co-workers used mesoporous TiO2 nanoparticles to detect diethyl ether with a 440 nm bandpass filter.The cataluminescent gas sensor shows high selectivity toward diethyl ether over other VOCs, such as acetaldehyde, acetone, butanone and butanol, etc.The selectivity is ascribed to the difference of cataluminescent emission profiles of various VOCs reaction intermediates.They confirmed that only diethyl ether exhibited a strong cataluminescent emission at about 440 nm.Wang et al. [52] reported a three-dimensional hierarchical CdO nanostructure with a bio-inspired morphology which showed outstanding cataluminescent properties such as stable intensity, high signal/noise values, short response and recovery time.The limits of detection toward acetone and diethyl ether are ca.6.5 and 6.7 ppm, respectively.In addition, a principal components analysis method was demonstrated to be able to recognize acetone and diethyl ether clearly.

Cataluminescent Gas Sensors for Detecting Aldehyde
BaCO3 nanomaterials were reported to have a good cataluminescent response to acetaldehyde [53].BaCO3-based gas sensor gives high sensitivity and selectivity to acetaldehyde at a working temperature of about 225 °C.At a wavelength of 555 nm, the linear range of cataluminescent intensity versus concentration of acetaldehyde is 2 to 2000 ppm, with a detection limit of 0.5 ppm (signal-tonoise ratio = 3).Ignorable levels of interference are observed while cyclohexane, n-hexane, carbon tetrachloride, ammonia, methylbenzene, chloroform, benzene, formaldehyde, ethanol and carbon dioxide are passing through the cataluminescent gas sensor.
For acetaldehyde detection, a zeolite-based cataluminescence gas sensor was reported by Liu et al. [54].They found that the reactions between acetaldehyde and O atoms in the cages of large-pore

Cataluminescent Gas Sensors for Detecting Aldehyde
BaCO 3 nanomaterials were reported to have a good cataluminescent response to acetaldehyde [53].BaCO 3 -based gas sensor gives high sensitivity and selectivity to acetaldehyde at a working temperature of about 225 • C. At a wavelength of 555 nm, the linear range of cataluminescent intensity versus concentration of acetaldehyde is 2 to 2000 ppm, with a detection limit of 0.5 ppm (signal-to-noise ratio = 3).Ignorable levels of interference are observed while cyclohexane, n-hexane, carbon tetrachloride, ammonia, methylbenzene, chloroform, benzene, formaldehyde, ethanol and carbon dioxide are passing through the cataluminescent gas sensor.
For acetaldehyde detection, a zeolite-based cataluminescence gas sensor was reported by Liu et al. [54].They found that the reactions between acetaldehyde and O atoms in the cages of large-pore zeolites could result in a light emission.The zeolite-based cataluminescence sensor shows a linear response from 0.06 to 31.2 ppm toward acetaldehyde vapor.In addition, methanol, ethanol, isopropanol, methylbenzene, chloroform, dichlormethane and acetonitrile do not impact the cetaldehyde determination on CsNaY catalyst.More interesting, acetaldehyde vapor could be distinguished from some homologous series, including formaldehyde, cinnamaldehyde, glutaraldehyde and benzaldehyde.They considered that the mechanism could be due to the stereoselectivity of zeolite.
A sensitive cataluminescence-based gas sensor for the determination of formaldehyde using nanosized V 2 Ti 4 O 13 as a probe has been reported by Zhou et al. [55].The sensor exhibits a high selectivity to formaldehyde at 490 nm and a good activity at 370 • C. The linear range is 0.1 to 40 ppm (r = 0.9995), and the detection limit is as low as 0.06 ppm.In addition, the sensor is applied for detecting formaldehyde in artificial gas, showing rapid response, high selectivity, and long lifetime for practical environmental analysis.
3.3.6.Cataluminescent Gas Sensors for Detecting Inorganic Gas H 2 S Previously, a Fe 2 O 3 -based cataluminescent gas sensor was reported for H 2 S detection [56].When H 2 S gas passes through the surface of Fe 2 O 3 catalyst, it is catalytically oxidized to SO 2 by oxygen in air.Electronically excited SO 2 molecules are produced during the reaction, and the cataluminescence is generated when they return to basic state.On the basis of this mechanism, the Fe 2 O 3 -based cataluminescent gas sensor shows a high selectivity to H 2 S without response to 12 other kinds of gases such as hydrocarbon, alcohol, NO 2 , and so on.The response time is within 15 s while the recovery time is less than 120 s.The linear range of the sensor is in the range from 8 to 2000 ppm with a detection limit of 3 ppm.
Zhou et al. [57] reported a cataluminescent sensor for simultaneous detection of formaldehyde and H 2 S in air.The cataluminescence is emitted from the surface of Zn 3 SnLa 2 O 8 nanomaterials operating at a temperature of 275 • C. The limits of detection are 0.07 ppm for formaldehyde and 0.22 ppm for H 2 S, respectively.Furthermore, they found that some potential interferences, such as vapors of acetaldehyde, ethanol, benzene, toluene, ethylbenzene, ammonia, sulfur dioxide, carbon dioxide, nitric oxide and nitrogen dioxide, did not disturb the performance of the Zn 3 SnLa 2 O 8 sensor.
For the catalyst-based sensors indicated above, a brief summary about their gas-sensing performance is shown in Table 1.

Sensing Mechanism and Features
According to carrier type such as electron or cavity, semiconductor gas sensors can be classified into n-type (e.g., CdO and SnO 2 ) and p-type (e.g., NiO) sensors [59,60].Most gas sensors are n-type, in which electrons are carriers instead of cavities during sensing process.The introduction of the catalytically sensing mechanism given below is on the basis of n-type metal oxides sensors.Although the exact fundamental mechanism is still controversial, it is commonly accepted that the conductometric changes of the semiconductor gas sensors are caused by the surface catalysis reaction between metal oxides and gas molecules [61,62].
For the n-type semiconductor gas sensor, the sensing process is that oxygen gases are adsorbed on the surface of semiconductor in air.The adsorbed oxygen species capture electrons from the inner of semiconductor.The negative charges trapped in these oxygen species cause a carrier depletion layer and thus reduce the conductivity.When the sensor is exposed to reducing gases, gas molecules will be oxidized by the active oxygen species while the electrons trapped by the active oxygen species return to semiconductor, leading to a decrease of the potential barrier height and an increase of conductivity.There are different active oxygen species including molecular (O 2 − ) and atomic (O − , O 2− ) ions on the surface of semiconductors depending on the working temperature.Generally, the molecular form is dominant below 150 • C, while the atomic oxygen species are found above this temperature [63,64].

Conductometric Detection System
Generally, there are two types of conductometric sensor structures: directly heated sensors and indirectly heated ones.A directly heated sensor means that the heater contacts with the sensing materials, which might lack stability and add anti-interference ability.Thus, most current nanostructure-based gas sensors are indirectly heated sensors, which can be further divided into two device structures, i.e., cylindrical and planar layouts (Figure 10).Alumina ceramics (wafers or tubes) are generally used as substrates to support sensing films.In the ceramic tube-based sensors, a heating wire is placed inside the ceramic tube, while in the ceramic wafer-based sensors, a layer of heating paste is on the backside of ceramic wafer.Some silica wafers can also be used as the substrate, which is advantageous for manufacturing small sized gas sensors because of its compatibility with integrated circuits.Gas-sensing measurement is usually conducted within a closed chamber with a volume of about 1000 mL equipped with appropriate gas flow inlet and outlet, as shown in Figure 11.During a typical gas-sensing detection, a constant voltage is applied onto the pair of electrodes between sensing films, then the current is measured and recorded by an amperemeter connected with a computer.A certain concentration of target gas in air is introduced into the test chamber, while after measurement the target gas is released by inputting fresh air.The acquired current signal is used for further gas-sensing performance analysis.Gas-sensing measurement is usually conducted within a closed chamber with a volume of about 1000 mL equipped with appropriate gas flow inlet and outlet, as shown in Figure 11.During a typical gas-sensing detection, a constant voltage is applied onto the pair of electrodes between sensing films, then the current is measured and recorded by an amperemeter connected with a computer.A certain concentration of target gas in air is introduced into the test chamber, while after measurement the target gas is released by inputting fresh air.The acquired current signal is used for further gas-sensing performance analysis.
Gas-sensing measurement is usually conducted within a closed chamber with a volume of about 1000 mL equipped with appropriate gas flow inlet and outlet, as shown in Figure 11.During a typical gas-sensing detection, a constant voltage is applied onto the pair of electrodes between sensing films, then the current is measured and recorded by an amperemeter connected with a computer.A certain concentration of target gas in air is introduced into the test chamber, while after measurement the target gas is released by inputting fresh air.The acquired current signal is used for further gas-sensing performance analysis.

Applications of Catalysis-Based Conductometric Semiconductor Gas Sensors
Although semiconductor gas sensors have been widely used in portable gas detection systems, the sensing performance is significantly influenced by the morphology and structure of sensing materials, resulting in a great obstacle for gas sensors based on bulk materials or dense films to achieve highly-sensitive properties [65][66][67].Conductometric gas sensors based on nanomaterials are promising to improve gas-sensing properties including sensitivity, selectivity and response/recovery speeds due to their large specific surface area and high catalysis activity [68][69][70].Here, some nanomaterial-based semiconductor gas sensors are summarized to show the gas-sensing enhancement effect.Different from the classification method of previous cataluminescent gas sensors by following the target gases, conductometric sensors are introduced on the basis of different types of sensing materials.

Noble Metal Nanoparticle Enhanced Catalytically Sensing Effect
Noble metal nanoparticles commonly possess high catalysis properties.They have been used widely to modify metal oxide semiconductors to enhance the gas-sensing performance [71,72].Up to now, significant efforts have been made to improve the sensitivity and selectivity to VOCs by doping noble metals in metal oxides [73].As one of the most popular sensing metal oxides, ZnO has been investigated in the past decades because of its excellent electronic and optical properties [74,75].

Applications of Catalysis-Based Conductometric Semiconductor Gas Sensors
Although semiconductor gas sensors have been widely used in portable gas detection systems, the sensing performance is significantly influenced by the morphology and structure of sensing materials, resulting in a great obstacle for gas sensors based on bulk materials or dense films to achieve highly-sensitive properties [65][66][67].Conductometric gas sensors based on nanomaterials are promising to improve gas-sensing properties including sensitivity, selectivity and response/recovery speeds due to their large specific surface area and high catalysis activity [68][69][70].Here, some nanomaterial-based semiconductor gas sensors are summarized to show the gas-sensing enhancement effect.Different from the classification method of previous cataluminescent gas sensors by following the target gases, conductometric sensors are introduced on the basis of different types of sensing materials.

Noble Metal Nanoparticle Enhanced Catalytically Sensing Effect
Noble metal nanoparticles commonly possess high catalysis properties.They have been used widely to modify metal oxide semiconductors to enhance the gas-sensing performance [71,72].Up to now, significant efforts have been made to improve the sensitivity and selectivity to VOCs by doping noble metals in metal oxides [73].As one of the most popular sensing metal oxides, ZnO has been investigated in the past decades because of its excellent electronic and optical properties [74,75].Various ZnO nanostructures such as nanowires [76][77][78], nanorods [79,80], nanosheets [81] and nanospheres [82] have been developed.In particular, porous single-crystalline ZnO nanosheets have been synthesized, which not only exhibit highly sensitive performance but also possess significantly long-term stability.Therefore, surface modification of noble metals on those porous single-crystalline ZnO nanosheets would be promising.We have reported Ag nanoparticles-modified ultra-thin porous single-crystalline ZnO nanosheets, which exhibit high sensitivity to ethanol with a detection limit of 1 ppb [83,84].Similarly, Pt nanoparticles are decorated to porous single-crystalline ZnO nanosheets, which show good response to low concentrations of chlorobenzene from 10 to 500 ppb [85].
Hierarchical structures may provide more spaces for gas diffusion into the deep place of the sensing film [86][87][88].For example, Mohammad et al. [89] prepared a hierarchical structure based on ZnO nanowire or nanodisk which possessed high surface area to volume ratio and an increased proportion of exposed active planes.Those hierarchical ZnO structures showed significantly improved sensitivity and fast response to acetone compared to some other mono-morphological ZnO.Therefore, assembling the porous single-crystalline ZnO nanosheets into flower-like hierarchical structures may further improve the sensing properties because three-dimension (3D) hierarchical structures can provide more spaces for gas diffusion [90,91].Thus, Au nanoparticles are used to modify flower-like hierarchical ZnO structures assembled by porous single-crystalline nanosheets, which is super-sensitive to acetone with a detection limit of 0.5 ppb [92].Pt nanoparticles are also modified onto the surfaces of In 2 O 3 hierarchical structures, which could exhibit excellent responses to CO at room temperature [93].
Primarily, there are two sensitization mechanisms generally considered for noble metal modification, i.e., the electronic sensitization caused by interfacial electronic redistribution and the chemical sensitization caused by interfacial atom transport [94].In the electronic sensitization mechanism, electrons are extracted from the oxide (ZnO) to the noble metal nanoparticles due to the work function difference and enriched there, which results in an increase in the depth of the electron depletion layer in the oxide.Because the potential barrier height between neighboring grains is elevated, the noble metal decorated oxide sensor will become more sensitive to the change in environmental atmosphere.Ag nanoparticles are mostly considered to function as electronic sensitization mechanism.However, Au and Pt nanoparticles mostly enhance the sensing performance through its catalytic activity by a chemical sensitization mechanism, which is also known as the spillover effect and has been well-established in the catalysis process of metal-supported catalysts.In this mechanism, the noble metal may provide more active sites for the adsorption of molecular oxygen and target gases and facilitate the dissociation of these molecules due to their highly catalytic or conductive nature.In air, the oxygen anions adsorbed on noble metals will diffuse into the surface region of oxides near the noble metals.In the sensing process, the target gases are activated by the noble metals as well, and then spill over and are oxidized by the adsorbed oxygen anions near the noble metals.
Jang et al. have reported vertically ordered SnO 2 nanorods decorated with Au nanoparticles, which could detect sub-ppb levels of volatile reducing gases with a fast response speed due to the sensitization effects of the Au nanoparticles, as shown in Figure 12 [95].It has been confirmed that Au nanoparticles residing in the bamboo joint-like spaces enabled the SnO 2 nanobamboos to detect sub-ppb level of target gases by maximizing the enhancement of the depletion region.In addition, they reported a facile method for the utilization of metal decoration on both the inner and outer surfaces of hollow metal oxide nanostructure.Close-packed SnO 2 nanodome arrays decorated with Au nanoparticles are fabricated by soft-template method and self-agglomeration of an Au film [96].The Au-decorated sensor shows a response 18 times higher than that of the bare SnO 2 nanodome arrays to C 2 H 5 OH at 300 • C. Through the general method, some other noble metals such as Pd and Ag can also be decorated onto sensing materials which exhibit much enhanced gas-sensing performance [97,98].

Small Size Effect on Catalysis-Based Conductometric Gas Sensors
The small size effect of metal oxides refers to the effect that when the particle size decreases the sensitivity of the sensor increase.Xu et al. have explained the phenomena by a semiquantitative model [99].Three different cases can be distinguished according to the relationship between particle size (D) and the width of depletion layer (L) which is produced around the surface of crystallites.When the particle size is less than double of the width of its depletion layer, the sensor becomes highly sensitive to ambient gas molecules.However, small sized nanoparticles are inclined to aggregate because of their high surface energy.
To resolve this issue, SnO2/graphene nanocomposites were prepared by a wet chemical method [100].The formation process of SnO2/graphene nanocomposites is illustrated in Figure 13.There are many functional groups on graphene including hydroxyl and epoxy groups on the planes, as well as carboxyl groups at the edges, which acts as anchor sites to enable stannic ions attaching onto the graphene [101,102].Since graphene are two-dimensional, the size of SnO2 colloids can be quite small on the basis of the steric hindrance effect [103].The SnO2/graphene nanocomposites exhibit an improved sensitivity compared to traditional SnO2.Trace benzene is detected using the SnO2/graphene.The detection limit is as low as 5 ppb.Besides, owing to the π-conjugate with graphene, the responses to benzene are higher than to ethanol.Similar findings are also confirmed for the SnO2/MWCNT nanocomposites, which are highly sensitive to persistent organic pollutants (POPs) [104].According to the report, the gas sensing properties of SnO2/graphene nanocomposites are better than that of SnO2/MWCNT nanocomposites because that graphene could make more abundant exposure of the sheet surface to the environment [103].

Small Size Effect on Catalysis-Based Conductometric Gas Sensors
The small size effect of metal oxides refers to the effect that when the particle size decreases the sensitivity of the sensor increase.Xu et al. have explained the phenomena by a semiquantitative model [99].Three different cases can be distinguished according to the relationship between particle size (D) and the width of depletion layer (L) which is produced around the surface of crystallites.When the particle size is less than double of the width of its depletion layer, the sensor becomes highly sensitive to ambient gas molecules.However, small sized nanoparticles are inclined to aggregate because of their high surface energy.
To resolve this issue, SnO 2 /graphene nanocomposites were prepared by a wet chemical method [100].The formation process of SnO 2 /graphene nanocomposites is illustrated in Figure 13.There are many functional groups on graphene including hydroxyl and epoxy groups on the planes, as well as carboxyl groups at the edges, which acts as anchor sites to enable stannic ions attaching onto the graphene [101,102].Since graphene are two-dimensional, the size of SnO 2 colloids can be quite small on the basis of the steric hindrance effect [103].The SnO 2 /graphene nanocomposites exhibit an improved sensitivity compared to traditional SnO 2 .Trace benzene is detected using the SnO 2 /graphene.The detection limit is as low as 5 ppb.Besides, owing to the π-conjugate with graphene, the responses to benzene are higher than to ethanol.Similar findings are also confirmed for the SnO 2 /MWCNT nanocomposites, which are highly sensitive to persistent organic pollutants (POPs) [104].According to the report, the gas sensing properties of SnO 2 /graphene nanocomposites are better than that of SnO 2 /MWCNT nanocomposites because that graphene could make more abundant exposure of the sheet surface to the environment [103].

Combination of the Catalytic Effects of Noble Metal Nanoparticles and Small Size
Graphene has been used to prepare nanocomposite with SnO2 previously, in which the SnO2 nanoparticles are uniform, small and well dispersed [105].It is found that by combining with the πconjugate function of graphene, the SnO2/graphene nanocomposites exhibit a highly sensitive performance to benzene.However, the SnO2/graphene shows low sensitivity to other VOCs.For some specific applications, noble metal is modified onto the SnO2/graphene nanocomposites.It was reported that Ag/SnO2/Graphene ternary nanocomposites were sensitive to acetone [106].Figure 14 shows the response comparison between Ag/SnO2/graphene and SnO2/graphene nanocomposites to 150 ppm of acetone.The response of the Ag/SnO2/graphene is much higher than that of the SnO2/graphene.Besides, NO2 sensing is also observed from the Ag/SnO2/Graphene ternary nanocomposites at room temperature, in which the response time and recovery time are much shorter than that of SnO2/Graphene binary nanocomposites [107].The mechanism is ascribed to the catalysis activity enhancement by Ag nanoparticles which can accelerate the catalysis rate on the surface reactions.During the sensing process, oxygen molecules would react preferentially with Ag nanoparticles forming oxygen anions which transfer to SnO2 nanoparticles.

Summary and Perspective
Catalysis plays a significant role for monitoring gas environment by gas sensors.A review on the recent progresses of two representative catalysis-based gas sensors, cataluminescent and conductometric sensors, is presented.It has been indicated that some great challenges remain for these sensors, including the development of small sized sensing materials to achieve a high reactivity for catalytic reaction, improving the selectivity which show only response to specific target gas, multi- Graphene has been used to prepare nanocomposite with SnO 2 previously, in which the SnO 2 nanoparticles are uniform, small and well dispersed [105].It is found that by combining with the π-conjugate function of graphene, the SnO 2 /graphene nanocomposites exhibit a highly sensitive performance to benzene.However, the SnO 2 /graphene shows low sensitivity to other VOCs.For some specific applications, noble metal is modified onto the SnO 2 /graphene nanocomposites.It was reported that Ag/SnO 2 /Graphene ternary nanocomposites were sensitive to acetone [106].Figure 14 shows the response comparison between Ag/SnO 2 /graphene and SnO 2 /graphene nanocomposites to 150 ppm of acetone.The response of the Ag/SnO 2 /graphene is much higher than that of the SnO 2 /graphene.Besides, NO 2 sensing is also observed from the Ag/SnO 2 /Graphene ternary nanocomposites at room temperature, in which the response time and recovery time are much shorter than that of SnO 2 /Graphene binary nanocomposites [107].The mechanism is ascribed to the catalysis activity enhancement by Ag nanoparticles which can accelerate the catalysis rate on the surface reactions.During the sensing process, oxygen molecules would react preferentially with Ag nanoparticles forming oxygen anions which transfer to SnO 2 nanoparticles.Graphene has been used to prepare nanocomposite with SnO2 previously, in which the SnO2 nanoparticles are uniform, small and well dispersed [105].It is found that by combining with the πconjugate function of graphene, the SnO2/graphene nanocomposites exhibit a highly sensitive performance to benzene.However, the SnO2/graphene shows low sensitivity to other VOCs.For some specific applications, noble metal is modified onto the SnO2/graphene nanocomposites.It was reported that Ag/SnO2/Graphene ternary nanocomposites were sensitive to acetone [106].Figure 14 shows the response comparison between Ag/SnO2/graphene and SnO2/graphene nanocomposites to 150 ppm of acetone.The response of the Ag/SnO2/graphene is much higher than that of the SnO2/graphene.Besides, NO2 sensing is also observed from the Ag/SnO2/Graphene ternary nanocomposites at room temperature, in which the response time and recovery time are much shorter than that of SnO2/Graphene binary nanocomposites [107].The mechanism is ascribed to the catalysis activity enhancement by Ag nanoparticles which can accelerate the catalysis rate on the surface reactions.During the sensing process, oxygen molecules would react preferentially with Ag nanoparticles forming oxygen anions which transfer to SnO2 nanoparticles.

Summary and Perspective
Catalysis plays a significant role for monitoring gas environment by gas sensors.A review on the recent progresses of two representative catalysis-based gas sensors, cataluminescent and conductometric sensors, is presented.It has been indicated that some great challenges remain for these sensors, including the development of small sized sensing materials to achieve a high reactivity for catalytic reaction, improving the selectivity which show only response to specific target gas, multi-

Summary and Perspective
Catalysis plays a significant role for monitoring gas environment by gas sensors.A review on the recent progresses of two representative catalysis-based gas sensors, cataluminescent and conductometric sensors, is presented.It has been indicated that some great challenges remain for these sensors, including the development of small sized sensing materials to achieve a high reactivity for catalytic reaction, improving the selectivity which show only response to specific target gas, multi-channel signal analysis, weak sensing signals acquirement facing with trace concentration gases, a good long-term stability, and exploring the nature of sensing mechanisms.The cataluminescent gas sensors are summarized from their applications for detecting different gases, while the conductometric ones are introduced on the basis of catalysis effect by noble metal modification and small size.
To meet the increasing requirements on high-performance gas sensors, great efforts are catalysis-related investigations forward.On the one hand, sensitive and selective catalytically sensing nanomaterials are highly desired.Compared to single-component sensing materials, the doped or cooperated composites commonly show an enhanced sensing performance, suggesting that fabricating composites (such as graphene hybrid) would be a promising direction.On the other hand, regarding the detection system, catalysis-based sensor array such as electronic nose, instead of one single sensor, has shown the fascinating advantage of recognizing multi-component gases effectively.Finally, we expect that this review of some details of the typical catalysis-based gas sensors would provide some valuable opportunities for global researchers to develop high-performance sensors, and to explore related surface catalysis chemistries and physics.

Figure 4 .
Figure 4. (a) Scanning electron microscopy (SEM) image of the cocoon-like La 2 O 3 ; (b) SEM and (c) Transmission electron microscopy (TEM) images of Au/La 2 O 3 nanomaterials; and (d) X-ray diffraction (XRD) patterns of the La 2 O 3 and Au/La 2 O 3 nanostructures, reproduced with permission from [35].Copyright Royal Society of Chemistry, 2011.
Catalysts 2016, 6, 210 7 of 22chip.The final cataluminescent patterns are recorded by a camera closely placed to the ceramic chip after inputting target gas for about 1 min.The results indicate that the patterns with a relatively weak brightness of spots for chloroform and toluene can be obtained at 190 °C, as shown in Figure7.When the temperature is increased to 210 °C and 230 °C, the brightness of the spots is feasible to differentiate ether, acetone, chloroform, and toluene.This finding indicates different patterns can be obtained at different working temperature, whichcan distinguish each VOC gas.

Figure 7 .
Figure 7.The patterns of the sensor array at different working temperatures.The concentrations of the VOCs: 2000 ppm; flow rate of carrier gas: 240 mL•min −1 , reproduced with permission from [39].Copyright Elsevier, 2013.

Figure 7 .
Figure 7.The patterns of the sensor array at different working temperatures.The concentrations of the VOCs: 2000 ppm; flow rate of carrier gas: 240 mL•min −1 , reproduced with permission from [39].Copyright Elsevier, 2013.

Figure 7 .
Figure 7.The patterns of the sensor array at different working temperatures.The concentrations of the VOCs: 2000 ppm; flow rate of carrier gas: 240 mL•min −1 , reproduced with permission from [39].Copyright Elsevier, 2013.

Figure 8 .
Figure 8.(a) Sensing curves of the cataluminescent sensor to benzene at different concentrations ranging from 1 to 210 ppm.The inset shows the linear relationship between cataluminescence and gas concentration.(b) The selectivity towards benzene from gas interferences.Flow rate of carrier gas: 200 mL•min −1 ; temperature of sensor: 210 °C; gas concentration: 800 ppm; wavelength: 420 nm, reproduced with permission from [46].Copyright John Wiley & Sons, 2013.

Figure 8 .
Figure 8.(a) Sensing curves of the cataluminescent sensor to benzene at different concentrations ranging from 1 to 210 ppm.The inset shows the linear relationship between cataluminescence and gas concentration.(b) The selectivity towards benzene from gas interferences.Flow rate of carrier gas: 200 mL•min −1 ; temperature of sensor: 210 • C; gas concentration: 800 ppm; wavelength: 420 nm, reproduced with permission from [46].Copyright John Wiley & Sons, 2013.

Figure 11 .
Figure 11.Schematic diagram of the conductometric detection system.

Figure 11 .
Figure 11.Schematic diagram of the conductometric detection system.

Figure 12 .
Figure 12.(a) Schematics of the fabrication procedures of the SnO2 nanobamboos; (b) cross-sectional SEM image of the SnO2 nanobamboos; (c) responses of the SnO2 nanobamboos to 50 ppm of C2H5OH at different operating temperatures (inset in (c) shows the baseline resistance of the SnO2 nanobamboos as a function of temperature in air); (d) responses to 50 ppm C2H5OH, CH3COCH3, and C7H8 for plain SnO2 films, bare SnO2 nanorods, top-surface Au-decorated SnO2, and SnO2 nanobamboos at optimal temperature; and (e) response curves and 90% response times of the SnO2 nanobamboos to 50 ppm C2H5OH, CH3COCH3, and C7H8, reproduced with permission from [95].Copyright Royal Society of Chemistry, 2015.

Figure 12 .
Figure 12.(a) Schematics of the fabrication procedures of the SnO 2 nanobamboos; (b) cross-sectional SEM image of the SnO 2 nanobamboos; (c) responses of the SnO 2 nanobamboos to 50 ppm of C 2 H 5 OH at different operating temperatures (inset in (c) shows the baseline resistance of the SnO 2 nanobamboos as a function of temperature in air); (d) responses to 50 ppm C 2 H 5 OH, CH 3 COCH 3 , and C 7 H 8 for plain SnO 2 films, bare SnO 2 nanorods, top-surface Au-decorated SnO 2 , and SnO 2 nanobamboos at optimal temperature; and (e) response curves and 90% response times of the SnO 2 nanobamboos to 50 ppm C 2 H 5 OH, CH 3 COCH 3 , and C 7 H 8 , reproduced with permission from [95].Copyright Royal Society of Chemistry, 2015.

Table 1 .
The comparison on the sensing performance of gas sensors based on different catalysts.