SnO2 heterojunction microstructures: Facile room temperature solid-state synthesis and enhanced Cl2 sensing performance

Sensors and Actuators B 185 (2013) 110–116

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

In2 O3 /SnO2 heterojunction microstructures: Facile room temperature solid-state synthesis and enhanced Cl2 sensing performance Pei Li, Huiqing Fan ∗ , Yu Cai State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China

a r t i c l e

i n f o

Article history: Received 29 September 2012 Received in revised form 30 March 2013 Accepted 3 May 2013 Available online 13 May 2013 Keywords: In2 O3 /SnO2 Room temperature solid-state synthesis Heterojunction microstructures Gas sensor

a b s t r a c t A facile room temperature solid-state reaction route was employed to synthesize the In2 O3 /SnO2 heterojunction microstructures by grinding indium nitrate hydrate, tin dioxide and sodium hydroxide with proper molar ratios together without any surfactant and template. Their morphological feature is characterized as self-assembled by irregular-shaped nanospheres, and it was observed that a large quantity of nanocrystals with the size of 20–50 nm were present on In2 O3 sphere surfaces. The introduction of a small quantity of SnO2 in the reaction system was played an important role in the size- and shapecontrol. Furthermore, gas sensing properties of them were characterized in detail. The sensor based on In2 O3 /SnO2 heterojunction microstructures exhibited a much higher response to Cl2 than the sensor based on pure In2 O3 nanostructures, and the In/Sn = 12:1 (molar ratio) sample showed the highest response with quick response-recovery behavior as well as good selectivity and stability. The largest surface area of the In2 O3 /SnO2 heterojunction microstructures was also clarified by the analysis of nitrogen adsorption–desorption test, which contributes the great enhancement of the Cl2 gas sensing properties. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor gas sensors have been investigated extensively for the purpose of practical applications, such as gas leak detectors and environmental monitoring, which are relevant for chemical industries, environmental protection, public safety, and human health [1]. In particular, semiconductor metal oxides are extensively used as gas sensing materials because of their unique advantages, such as high sensitivity, low cost, and easy synthesis [2]. Semiconducting In2 O3 with a wide direct bandgap of 3.75 eV has attracted much attention in recent years, owing to its distinctive optical, chemical, and electronic properties and its applications in solar cells [3], field-emission displays, lithium-ion batteries [4], nanoscale biosensors [5], gas sensors [6], optoelectronics [7], and photocatalysis [8]. According to the literature, In2 O3 micro- and nanostructures have been usually synthesized using various growth methods, including chemical vapor deposition (CVD) [9,10], pulsed laser deposition (PLD) [11,12], direct current sputtering (DC) [13], radio frequency sputtering (RF) [14], layer deposition (ALD) [15,16], thermal evaporation [4,17], electrospinning [18,19], sol–gel [20,21], coprecipitation [22,23], hydrothermal [8,24], microemulaion [25], etc. However, many of these preparative methods are limited in

the practical applications due to low yields and rigorous reaction conditions, including the need for high temperatures, long reaction times, toxic templates, complex equipment, especial solvents and additives. Therefore, it is of great necessity to develop simple and reliable synthetic methods for hierarchical composite nanostructures with designed chemical components and controlled morphologies, which strongly affect the properties of nanomaterials. Herein, we report a facile room temperature solid-state reaction route for the preparation of the In2 O3 /SnO2 heterojunction microstructures without any surfactant and template for the mass-productive synthesis of In2 O3 /SnO2 nanocomposites with appropriate In/Sn molar ratios. The obtained nanomaterials were analyzed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) characterization techniques. The gas sensing properties of the resulting materials have also been investigated. The introduction of a small quantity of SnO2 in the reaction system was played an important role in the size- and shape-control of In2 O3 /SnO2 heterojunction microstructures, which largely enhanced the gas sensing properties. 2. Experimental 2.1. Materials and synthesis

∗ Corresponding author. Tel.: +86 29 88494463; fax: +86 29 88492642. E-mail address: [email protected] (H. Fan). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.05.010

The chemical regents indium nitrate hydrate (In(NO3 )3 ·4.5H2 O), tin dioxide (SnO2 ) and sodium hydroxide (NaOH) were of

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analytical grade and use without further purification. All the chemical reagents were purchased from Shanghai Chemical Co. In a typical procedure, In(NO3 )3 ·4.5H2 O (0.3819 g, 1.0 mmol) and different amount of SnO2 were blended together in an agate mortar and ground thoroughly for 15 min at room temperature. The In/Sn molar ratios were pure SnO2 , pure In2 O3 , 15:1, 12:1, 9:1, 7:1 and 5:1. Then, NaOH (0.1600 g, 4.0 mmol) was added to the mixture and ground for further 45 min. To remove unreacted reactants and by-products, the mixture was washed several times in turn with distilled water and absolute ethanol, followed by drying at 80 ◦ C for 12 h. The white precursors were calcined at 500 ◦ C for 2 h. We refer to these samples as S1, S2, S3, S4, S5, S6 and S7, representing pure SnO2 , pure In2 O3 , and the In/Sn molar ratios of 15:1, 12:1, 9:1, 7:1 and 5:1, respectively. 2.2. Characterization and gas sensor measurements The phase structure and purity of the as synthesized nanomaterials were examined by X-ray diffraction (XRD; X’pert PRO MPD, Philips, Eindhoven, The Netherlands) with Cu-K␣ radia˚ at 40 kV, 30 mA over the 2 range 20–80◦ . tion ( = 1.5406 A) The morphology of the obtained samples was investigated using field emission scanning electron microscopy (FE-SEM; JSM-6701F, JEOL, Tokyo, Japan), and high resolution transmission electron microscopy (HRTEM; Tecnai F30G2 , FEI, Hillsboro, OR, USA). Bet surface area (BET) parameters of In2 O3 microstructures were identified by full analysis of nitrogen adsorption–desorption tests (3H-2000PS4, Beishide Ltd, Beijing, China). The basic fabricated process is as follows [26]. The asobtained In2 O3 /SnO2 heterojunction microstructures were mixed and slightly grinded with adhesive in an agate mortar to form a gas sensing paste. The paste used as sensitive body was coated on an alumina tube with Au electrodes and platinum wires, dried under IR light for several minutes in air, and then sintered at 400 ◦ C for 1 h. A Ni–Cr alloy crossing alumina tube was used as a heating resistor which ensured both substrate heating and temperature controlling. In order to improve their stability and repeatability, the gas sensors were aged at 300 ◦ C for 240 h in air. The gas sensing properties were tested using a gas response instrument (HW-30A, Hanwei Ltd, Zhengzhou, China). The gas sensing properties of In2 O3 /SnO2 heterojunction microstructures tested in the glass test chamber, and the volume of test chamber is 15 L. In the measuring electric circuit of gas sensor, a load resistor is connected in series with a gas sensor. The circuit voltage Vc is 5 V, and the output voltage (Vout ) is the terminal voltage of the load resistor RL . The working temperature of sensor is adjusted by varying the heating voltage Vh . When a given amount of tested gas was injected into a chamber, the resistance of sensor changed. As a result, the output voltage changed. Gas response (S) is defined as follows: response = Rg /Ra . where Rg and Ra are the resistance values measured in oxidizing atmosphere and air, respectively. For each sample, three sensors were made by the same fabricated process, each sensor was tested three times in the gas sensing testing process, and the gas response values were the average value. 3. Results and discussion 3.1. Crystalline structure and morphology Fig. 1 shows the X-ray diffraction patterns of pure SnO2 , pure In2 O3 , and In2 O3 /SnO2 nanocomposite with different In/Sn molar ratios. All the strong and sharp diffraction peaks marked by triangle are consistent with the standard value of the In2 O3 phase (JCPDS No. 65-3710). The peaks marked by pentacle are attributed to

Fig. 1. XRD patterns of In2 O3 /SnO2 heterojunction microstructures.

formation of SnO2 phase (JCPDS No. 41-1445). With increasing of SnO2 concentration, the peak intensities of this phase increases due to the growth of crystallites and enhancement of crystallization. Fig. 2(a–d) and Supplemental Fig. I(a–c) shows the SEM images of pure SnO2 , pure In2 O3 and In2 O3 /SnO2 heterojunction microstructures with different In/Sn molar ratios. It indicated that In2 O3 /SnO2 heterojunction microstructures are formed by many nanoparticles and the SnO2 plays an important role in controlling the particle size and morphology. As depicted in Fig. 2(a and b), pure SnO2 consisted of large irregular aggregates of nanoparticles with the diameter 20–100 nm, while pure In2 O3 consisted of large irregular spheres, the diameter 300–500 nm. Upon introduction of SnO2 , the particle size decreased, as shown in Fig. 2(c and d) and Supplemental Fig. I(a–c). When the In/Sn molar ratio reached to 12:1 (S4), as shown in Fig. 2c, there were large quantity of nanocrystals with the size of 20–50 nm present on In2 O3 surfaces, while the diameters of In2 O3 were 80–100 nm. With further increase of SnO2 , as can been seen from Fig. 2d and Supplemental Fig. I(c), the particle size increased slightly, and its morphology became irregular, the nanoparticles aggregated to form some irregular bulks, which may destroy the morphology and affect the Cl2 sensing performance of In2 O3 . Supplementary data related to this article found, in the online version, at http://dx.doi.org/10.1016/j.snb.2013.05.010. To give further insight into the morphology and structure of the In2 O3 /SnO2 heterojunction microstructures, TEM, HRTEM and EDS images associated with SAED pattern are performed. Fig. 3a shows TEM image of the In/Sn molar ratio 12:1 (S4), the SnO2 nanoparticles present on In2 O3 surfaces can be seen clearly, the diameters of In2 O3 are evaluated to be around 80–100 nm, while the diameters of SnO2 are evaluated to be around 20–50 nm, which is consistent with the value estimated in SEM. EDS (inset of Fig. 3a) displays that the In2 O3 /SnO2 heterojunction microstructures are elementally composed of indium, tin and oxygen, besides Cu element that was introduced during the sample preparation process. Fig. 3b is a typical HRTEM image recorded from the corresponding marked area in Fig. 3a. For In2 O3 with cubic structure, the fast growth direction was along (1 1 1), which revealed that the growth of cube occurred under non dynamic equilibrium conditions. At the same time, it could be seen that the growth rate along three equivalent crystallographic directions [1 0 0], [0 1 0], and [0 0 1] was uniform [27–29]. The inset in Fig. 3b shows a SAED pattern taken from the corresponding area. The SAED of the nanoparticles is consistent with cubic In2 O3 with strong ring patterns due to (2 2 2), (4 0 0), (4 4 0), and (6 2 2) planes.

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Fig. 2. SEM images of In2 O3 /SnO2 heterojunction microstructures (a) S1, (b) S2, (c) S4 and (d) S5.

3.2. Determination of specific surface area and porosity Fig. 4a and Supplemental Fig. II(a–f) show the N2 adsorption–desorption isotherms of pure SnO2 , pure In2 O3 and In2 O3 /SnO2 heterojunction microstructures with different In/Sn molar ratios. According to IUPAC classification, the similar nitrogen adsorption–desorption isotherms of the seven samples can be classified as a type IV isotherm, with a hysteresis loop where desorption required definitively higher energy than adsorption. The isotherms of the In2 O3 /SnO2 heterojunction microstructures samples show a hystersis loop at a relative high pressure indicating the large surface area. The large increase in N2 adsorption at relative high pressure also confirms the

large surface area. The amount of N2 adsorbed for the S1, S2, S3, S4, S5, S6 and S7 samples were 82.42, 68.14, 123.05, 201.09, 170.94, 118.02, 105.54 cm3 /g, respectively. Obviously, the In2 O3 /SnO2 heterojunction microstructures sample (S4) has the highest N2 adsorbed amount, which indicating the largest surface area. Supplementary data related to this article found, in the online version, at http://dx.doi.org/10.1016/j.snb.2013.05.010. The surface area of pure SnO2 , pure In2 O3 and different In/Sn molar ratios samples (S3–S7) were shown in Fig. 4b. It indicated that the surface area parameters of S4 sample is the highest than other samples. The results agree well with SEM images, TEM images and the XRD spectra.

Fig. 3. (a) TEM and (b) HRTEM images of In2 O3 /SnO2 heterojunction microstructures (S4). The inset in (a) is an EDX spectroscopy of the corresponding marked area. The inset in (b) is a SAED pattern taken from the corresponding area.

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Fig. 4. (a) N2 adsorption–desorption curves of In2 O3 /SnO2 heterojunction microstructures (S4), and (b) surface area of S1–S7.

Fig. 5. Schematic illustrating the formation mechanism of In2 O3 /SnO2 heterojunction microstructures.

3.3. Formation mechanism In2 O3 is formed according to the following equation: In(NO3 )3 + 3NaOH → In(OH)3 + 3NaNO3

(1)

In(OH)3 → InOOH + H2 O

(2)

2InOOH → In2 O3 + H2 O

(3)

When NaOH was added to the mixture and ground, In(OH)3 was obtained in the first step of the solid-state reaction (Eq. (1)); After calcination of corresponding solid-state reaction precursors, In2 O3 crystals were obtained. During the calcination process, In(OH)3 can be dehydrated to form InOOH at even low temperature (Eq. (2)). As the temperature increased, InOOH dehydrated, then In2 O3 obtained (Eq. (3)). We proposed a plausible mechanism for the formation of In2 O3 /SnO2 heterojunction microstructures, as schematically illustrated in Fig. 5. When adding NaOH to the agate mortar for grinding 15 min, the reactant became to viscous state, indicating that the NaOH was dissolved and formed the cladding on the mixture of In(NO3 )3 ·4.5H2 O and SnO2 . Then, thanks to the mechanical force by grinding, the original large block crushed into smaller particles which can be regarded as many minor reaction units. With further grinding, In(OH)3 /SnO2 heterojunction microstructures was formed, SnO2 nanocrystals were present on In(OH)3 nanoparticles surfaces. Finally, after calcination of In(OH)3 /SnO2 heterojunction microstructures precursors, In2 O3 /SnO2 heterojunction microstructures were obtained.

ranging from 5 to 100 ppm at 260 ◦ C. The results show that the response of each sensors increased with an increase in the concentration of Cl2 . Furthermore, it can be found that the sensors based on In2 O3 /SnO2 heterojunction microstructures to each Cl2 concentration exhibit much higher response than those based on pure SnO2 nanostructures (S1) and sample of pure In2 O3 structures (S2). The appropriate addition of SnO2 would be beneficial to the improvement of gas sensing properties, but superabundant addition may reduce the available adsorption sites and worsen the gas sensing properties, the In/Sn molar ratios 12:1 (S4) sensor has the highest response. These seven samples can be characterized by a typical Freundlich isotherm curve [30]. The Freundlich isotherm equation can be empirically represented as  = KCb , where K = ˛RTna ,

3.4. Gas sensing properties Fig. 6 displays the plots of gas response versus the gas concentration when the sensors based on the samples with different In/Sn molar ratios (S1–S7) were exposed to Cl2 with the concentration

Fig. 6. Gas response of the sensors based on samples with different amount of SnO2 added exposed to Cl2 at concentrations ranging from 5 to 100 ppm at 260 ◦ C, with an insert showing a linear fitting for log S vs. log C (C is the Cl2 concentration, S = Rg /Ra ). The error bars means the standard deviation between the three times in the gas sensing testing of all the three sensors made by the same fabricated process.

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Fig. 7. Gas response of sensor as a function of sample (a) Typical response and recovery curve of sensors based on In2 O3 /SnO2 heterojunction microstructures (S4) exposed to 50 ppm Cl2 at different working temperatures. (b) Temperature dependence of the sensor gas response to 50 ppm Cl2 . The error bars in (b) means the standard deviation between the three times in the gas sensing testing of all the three sensors made by the same fabricated process.

Fig. 8. Gas response of In2 O3 /SnO2 heterojunction microstructures (S4) exposed to Cl2 at (a) concentrations ranging from 5 to 100 ppm at 260 ◦ C and (b) 50 ppm.

a = ˛exp(−E/kT), ˛ and R are constants, C is the Cl2 concentration. Consequently, the logarithm Freundlich equation can be modified as: log S = log B + b × log C, where S = Rg /Ra . The inset of Fig. 6 shows well linear fits for log S vs. log C for all samples. It is observed that the correlation coefficient b of all sensors firstly increases and then decreases, and its values are 0.6331, 0.6400, 1.5098, 1.9440, 1.8017, 1.2562 and 1.0318, respectively. Among them, the S4 sensor has the maximum b and the highest response to Cl2 concentration. Therefore, the following study of the selectivity was focused on the S4 sensor. Considering the response of gas sensor is greatly influenced by operating temperature, parallel experiments were carried out in the range of 80–300 ◦ C to optimize the proper working temperature of the sensor. Fig. 7a shows the relation between the resistance and the operating temperature of the In2 O3 /SnO2 heterojunction microstructures sensor (S4). The resistance in air decreased with increase of temperature in the range of 100–260 ◦ C, while the resistance in Cl2 atmosphere increased, and the relative resistance difference value increased. According to Fig. 7a, the resistance in air is nearly independent of the operating temperature in the range of 260–300 ◦ C, which means that the sensor based on In2 O3 /SnO2 heterojunction microstructures is expected to exhibit excellent thermal stability over a relatively broad operating temperature range. This feature is of great significance especially for applications in areas with temperature fluctuations. If the temperature continues to increase (300 ◦ C), a dynamic equilibrium occurs between the adsorption and desorption of Cl2 on the surface of the sensor material, leading to a constant resistance in air, but a decrease resistance in Cl2 , probably because of intrinsic defects that became responsible for the conductance of the sensor at higher temperature, and the increased mobility of the charge carriers [31].

Moreover, it can be seen from Fig. 7b that the response increased with the operating temperature and reached a maximum value of 1034.3 ± 67.7 (50 ppm Cl2 ) at 260 ◦ C. When the operating temperature increased further (300 ◦ C), the response value decreased. Therefore, the best operating temperature was determined to be 260 ◦ C for the subsequent detections of the In2 O3 /SnO2 heterojunction microstructures. Fig. 8a illustrates the typical response recovery characteristics of the sensor based on the In/Sn molar ratio 12:1 sample (S4) to Cl2

Fig. 9. Stability of In2 O3 /SnO2 heterojunction microstructures based (S4) sensor to Cl2 with a concentration of 50 ppm at 260 ◦ C. The error bars means the standard deviation between the three times in the gas sensing testing of all the three sensors made by the same fabricated process.

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Fig. 10. (a) Schematic diagram of the proposed mechanism of In2 O3 /SnO2 heterojunction microstructures, and (b) AC impedance spectroscopy of sensors based on S1, S2, S3, S4 (Inserted figure), S5, S6 and S7, respectively.

with concentrations of 5, 10, 30, 50, and 100 ppm at 260 ◦ C. When exposed to 5 ppm Cl2 , the response was about 5.2, indicating that high gas response can be achieved in detecting low concentration Cl2 using In2 O3 /SnO2 heterojunction microstructures as sensing material. With an increase in the concentration of Cl2 , the response increased dramatically. Furthermore, the sensor showed a quick response and short recovery time. When exposed to 50 ppm Cl2 , the response and recovery time (defined as the time required reaching 90% of the final equilibrium value) is 2 s and 9 s, respectively, as shown in Fig. 8b. Fig. 9 displays the stability of the In2 O3 /SnO2 heterojunction microstructures based (S4) sensor to Cl2 with a concentration of 50 ppm at 260 ◦ C. The results show that the gas response was decreased over time. But the response was still very high even after 30 days went by. 3.5. Gas sensing mechanism In2 O3 is n-type semiconductive oxides which conduction electrons come primarily from point defects (oxygen vacancies and interstitial metal atoms). For metal oxide gas sensors, the change of resistance is mainly caused by the adsorption and desorption of gas molecules on the surface of the sensing structure. We propose an analogous model for the In2 O3 /SnO2 heterojunction microstructures, as schematically shown in Fig. 10a. First, oxygen species were adsorbed on the surface of particles in the air, and then were ionized into O− ads or O2− ads by capturing free electrons from the particles, thus leading to the formation of thick space charge layer and increasing of surface band bending. The doped SnO2 has a lower band gap and work function (Eg = 3.59 eV) than that of In2 O3 (Eg = 3.75 eV), electrons from SnO2 are transported toward In2 O3 , leading to the formation of an accumulation layer at the SnO2 –In2 O3 interface. Upon exposure to Cl2 , Cl2 is adsorbed by the heterojunction microstructures and electrons are released from the heterojunction between In2 O3 and SnO2 , Cl2 acts as an electron acceptor in the reaction, resulting in the increase of the resistance of the sensor. On the other hand, the trapped electrons are released to the heterojunction between In2 O3 and SnO2 by Cl2 after stopping the supply of Cl2 , leading to a decrease of the resistance. The enhanced sensing performance of the In2 O3 /SnO2 heterojunction microstructures also can be ascribed to its larger BET surface area. As the In2 O3 /SnO2 heterojunction microstructures sample have the largest BET surface area (39.90 m2 /g), (while for pure In2 O3 was only 6.34 m2 /g), the sensor can absorb more Cl2 , the resistance increase and the resistance decrease became more notable, which can enhance its sensing performance. The larger BET surface area increase the accessible surface area and facilitate the

mass transport of gas in the material, which are in favor of their applications in fields such as catalyst and sensor. In order to observe clearly dielectric response of In2 O3 /SnO2 heterojunction microstructures, AC impedance spectroscopy of In2 O3 /SnO2 sensor with different amounts of SnO2 -added in the frequency range of 100 Hz to 10 MHz at 260 ◦ C (50 ppm Cl2 ) are shown in Fig. 10b. Upon the introduction of SnO2 , the diameter of semicircle of AC impedance spectroscopy enlarged, the impedance increased, too. The AC impedance spectroscopy of the sensor based on S4 shows the largest semicircle with a diameter about 200 k (the inserted figure in Fig. 10b) which is far larger than the value of sensors based on S1–S3, S5–S7. This is mainly due to the largest surface area of the S4 sample, which leading to the largest resistance. With the further increase of SnO2 , the surface area decrease, and the resistance declined. It agrees well with SEM images, N2 adsorption–desorption curve and the surface area bar graph. More details of the enhancing effect of the In2 O3 /SnO2 heterojunction microstructures on the sensing properties need further investigation. 4. Conclusions In summary, In2 O3 /SnO2 heterojunction microstructures were synthesized by a facile room temperature solid-state reaction route without any surfactant and template. SEM images showed the molar ratio of In/Sn effect profoundly on morphologies of In2 O3 /SnO2 nanocomposites and the In2 O3 /SnO2 heterojunction microstructures was obtained when the molar ratio of In/Sn reached to 12:1 (S4). A possible growth mechanism of the In2 O3 /SnO2 heterojunction microstructures has also been proposed. The gas sensing measurements demonstrated that the sensors based on In2 O3 /SnO2 heterojunction microstructures exhibited a much higher response to Cl2 than the sensor based on pure In2 O3 nanostructures and the In2 O3 /SnO2 homojunction microstructures based (S4) sensor had the highest response. This is possibly due to the large specific surface area (utility function) and narrow necks between individual architectures (transducer function) of the In2 O3 /SnO2 heterojunction microstructures. Therefore, it is expected that this facile route prepared the In2 O3 /SnO2 heterojunction microstructures would be a promising candidate for applications in Cl2 sensor. Acknowledgements This work was supported by the National Natural Science Foundation (51172187), the SPDRF (20116102130002, 20116102120016) and 111 Program of MOE (B08040), the Xi’an Science and Technology Foundation (CX1261-2, CX1261-3,

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CX12174), and the NPU Fundamental Research Foundation of China (NPU-FFR-JC201232).

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Biographies Pei Li received the MSc degree in 2010 from Henan Normal University. She is now a PhD candidate at Northwestern Polytechnical University. Her research interests include the synthesis of functional materials and their application in gas sensor. Huiqing Fan obtained his BSc in Physics, MSc in Electronic Engineering, and PhD in Electronic Materials Science from Xi’an Jiaotong University. In 2003, he was promoted to a professor in School of Materials Science and Engineering, Northwestern Polytechnical University. He has published over 250 peer-reviewed papers and holds 12 Chinese patents. His research interests include ferroelectric, piezoelectric, pyroelectric and photo-electronic ceramics in polycrystalline and single crystal form, as well as nanocrystalline materials for photo-catalysis as well as chemical sensors and thin-layer devices. Yu Cai received the BSc degree in 2011 from China University of Geoscience. He is a MSc degree student at Northwestern Polytechnical University. He is now working on characterization and fabrication of nano-materials for gas sensor.