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High sensitivity and selectivity chlorine gas sensors based on 3D open porous SnO2 synthesized by solid-state method
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High sensitivity and selectivity chlorine gas sensors based on 3D open porous SnO2 synthesized by solid-state method Weiming Zhang, Qiang Li*, Chao Wang, Jiangwei Ma, Chao Wang, Haijun Peng, Yun Wen, Huiqing Fan State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
ARTICLE INFO
ABSTRACT
Keywords: Solid-state method 3D open porous SnO2 Cl2 Gas sensing
A simple solid-state reaction method was employed to synthesize the three-dimensional open porous SnO2 (3D OPSnO2) by grinding the mixture of tin tetrachloride pentahydrate, sodium hydroxide and sodium chloride. Sodium chloride worked as a template to build the 3D open porous structure. The morphological feature was three-dimensional open porous with diameters about 300–500 nm and the edge of the 3D open porous were composed of numerous SnO2 nanoparticles with grain size around 5.8 nm. The chlorine (Cl2) gas sensing properties of 3D OPSnO2 and bulk SnO2 (B–SnO2) sensors were systematically investigated. Gas response of the 3D OP-SnO2 sensor was 792.85 to 5 ppm Cl2 at 160 °C, which was 61 times higher than that of B–SnO2. Such outstanding gas sensing performance was mainly ascribed to the small grain size, which resulted in the change of conductivity mechanism and the decreased mobility of electrons. Thereby, the resistance of OP-SnO2 increased dramatically. Moreover, the unique structure and abundant oxygen vacancies also contributed to the excellent gas sensing performance, because they can elevate the specific surface area and provide sufficient Cl2 adsorption sites.
1. Introduction Among various toxic gases, chlorine (Cl2) is an important gas because it is widely used in production of commodities, such as paper products, dye stuffs and many other consumer products [1]. Cl2, however, is a dangerous gas to humans’ respiratory system, fatal at 1000 ppm concentration [2,3]. Owning to such negative impacts to human life, numerous methods have been reported for the Cl2 gas detection, such as optical, solid state potentiometric and electrochemical methods [4–7]. However, several problems such as high cost, nonportability and poor sensitivity to target analytes hinder its practical application. Therefore, it is urgent to develop rapid and accurate detectors of Cl2 gas at ppb level. Tin dioxide (SnO2), a n-type oxide, is a promising sensing material owning to its high mobility of conduction electrons (160 cm2/V·s) and chemical stability [8,9]. To obtain highly sensitive gas sensors, SnO2 with different morphologies such as nanomembranes, thin films, nanosheets, microflowers, raspberry-like and nanotubes have been prepared [10–15]. However, two main problems, methodology and property, should be considered. That is to say, sensors with excellent gas performance are commonly costly or produced in an extremely complex process while facile and cost-effective methods tend to provide sensors
*
with limitations, such as unsatisfactory sensitivity or selectivity. Suematsu et al. used stanice acid gel as a precursor and produced monodispersed SnO2 nanoparticles to obtain improved gas sensing performance but the preparation process is time consuming and complicated [16]. Choi et al. have synthesized SnO2 nanosheets via an aqueous process under moderate conditions and the assembled sensors only show high selectivity towards alkene gases [17]. Therefore, it is significant to develop a simple method, which can improve the overall sensing properties. Li et al. synthesized the In2O3/SnO2 gas sensor by solid-state reaction but it did not show excellent gas sensitivity at ppb level [18]. Chen et al. fabricated 3D open porous g-C3N4 by a two-step process for photocatalytic hydrogen evolution [19]. Inspired by the works, the salt-assisted synthesis of three-dimensional open porous SnO2 (3D OP-SnO2) by solid-state reaction is conducted. Solid-state reaction synthesis is particularly attractive due to its simplicity, low cost, capability of producing nanomaterials with adjustable grain sizes and controllable micromorphology [20,21]. With this approach, 3D OP-SnO2 sensors with high sensitivity, superior selectivity, lower limit of detection (LOD), and long-term stability towards Cl2 are synthesized. The excellent properties of 3D OP-SnO2 sensors likely originate from the decrease in grain size (D), especially for D ≤ 2 L (thickness of the depletion layer) [22,23]. The mechanism
Corresponding author. E-mail address: [email protected] (Q. Li).
https://doi.org/10.1016/j.ceramint.2019.07.036 Received 20 February 2019; Received in revised form 6 June 2019; Accepted 2 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Weiming Zhang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.07.036
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for the gas sensing of the 3D OP-SnO2 is discussed in detail. 2. Experimental section 2.1. Synthesis of SnO2 The starting materials were analytical grade SnCl4·5H2O, NaOH and NaCl. In a typical process, 3.51 g SnCl4·5H2O, 1.60 g NaOH and 2.86 g NaCl were mixed in an agate mortar and ground for 50 min at room temperature, respectively. Then the precursor was obtained after drying the mixture at 80 °C for 12 h. Subsequent calcination of the precursor was performed at 500 °C for 2 h in air. Finally, the calcined powder was dispersed, washed with distilled water and ethanol several times and dried at 80 °C. For comparison, bulk SnO2 (B–SnO2) sample was obtained by direct calcination of SnCl4·5H2O at 500 °C for 2 h in air. 2.2. Characterization Fig. 1. X-ray diffraction patterns of B–SnO2 and 3D OP-SnO2.
The obtained samples were analyzed by X-ray diffraction (XRD; D8 Advance, Bruker, Germany) at 20–80° using Cu-Kα radiation (λ = 1.5406 Å), 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). The specific surface area of all samples was obtained by N2 adsorptiondesorption method (TriStar II 3020, Micromeritics Ltd, Shanghai, China). The chemical state was tested by high-resolution X-ray photoelectron spectroscopy (XPS; VG ESCALA-B220i-XL, Thermo-Scientific, Surrey, UK) with a focused monochromatized Al Kα radiation (E = 1486.6eV).
3. Results and discussion 3.1. Crystalline structure and morphology X-ray diffraction (XRD) patterns are recorded to identify the crystal structure of B–SnO2 and 3D OP-SnO2, as shwown in Fig. 1. All peaks correspond to the tetragonal SnO2 (JCPDS 41–1445). No additional peaks appear in both samples, indicating the successful formation of pure phase SnO2. The grain sizes of B–SnO2 and 3D OP-SnO2 are calculated to be about 5.9 nm and 10.8 nm by Sherrer's equation [27]. It can be explained that the growth of the particles was inhibited because the by-product NaCl leaded to the formation of “walls” surrounding the nanoparticles to keep them from growing [28,29]. In addition, it is reported that the thickness of the depletion layer (L) of SnO2 is about 3 nm in air [23]. Therefore, for 3D OP-SnO2, the grain size is less than 2 L, especially in chlorine atmosphere, which means the entire SnO2 particles could be regarded as depletion layer. Thicker depletion layer leads to larger change in resistance, which further results in high response. The Scanning electron microscopy (SEM) and High resolution transmission electron microscopy (HRTEM) images of the as-synthesized SnO2 samples are shown in Fig. 2. As depicted in Fig. 2 (a) and (b), B–SnO2 consists of large irregular aggregates of nanoparticles with diameters around 9–13 nm. Fig. 2(c) and (d) show the typical threedimensional open porous microstructure of 3D OP-SnO2. The diameter of pores is 300–500 nm and the boundary with 10–20 nm in thickness is made up of relatively close arrangements of nanoparticles, which is beneficial for diffusion of gases. To give further insight into the structure of the samples, the HRTEM images associated with select area electron diffraction (SAED) pattern are showed in Fig. 2 (e) and (f). The grain size statistics result is evaluated to be around 5.8 nm, which is consistent with the value estimated from XRD. The SAED pattern is corresponding to the cubic SnO2 with strong diffraction rings resulted from (110), (101), (200), and (211) planes. Given that the 3D open porous structure can provide higher specific surface area, as confirmed by the N2 adsorption-desorption isotherms in Fig. 3. The BET surface area of 3D OP-SnO2 is calculated to be 102 m2/g, which is larger than that of B–SnO2 (89 m2/g). It is well-known that the morphology of powders, especially the grain size, has great influence on the sensor response [30]. The grain size, specific surface area, and peak pore radius of B–SnO2 and 3D OP-SnO2 are summarized in Table S1. Significant difference is found between the two sets of data. Thus, 3D porous structure is conducive to increasing surface area and gas adsorption sites.
2.3. Fabrication of sensors and measurement of gas sensing properties To fabricate the gas sensors [24], mixture of SnO2 powder and glycerin was ground to form a paste. Afterwards, the paste was coated onto an alumina tube which is consisted of Au electrode, Pt wire and a Ni–Cr alloy resistor (Fig. S1a). After drying under IR light for several minutes, the device was calcined at 350 °C for 2 h and then aged at 300 °C for one week in order to improve stability and repeatability. The gas sensing properties were measured by an 18 L gas response instrument (HW-30A, Hanwei Ltd, Zhengzhou, China) with the constant loop voltage (Vc) of 5 V. The output voltage (Vout) was the terminal voltage of the load resistor RL and the working temperature of gas sensor was adjusted by varying the heating voltage (Vh). The schematic diagrams of test electric circuit and a photograph image of the instrument used were illustrated Figs. S1b and c. Quantitative target gas or liquid was injected into the test chamber by a micro-syringe. The evaporator in the chamber was used to heat the liquid into gas. After test, the chamber was removed for test gases to diffuse away. Gas response (S) was defined as follows: Soxidizing gases = Rgas/Rair and Sreducing gases = Rair/Rgas. Where Rair (Rair= (5-Vair)/(Vair/RL)) and Rgas (Rgas = (5-Vgas)/(Vgas/RL)) were the stabilized resistances in air and in the mixture of test gas, respectively. Vair and Vgas were the average Vout in air and the mixture of test gas, respectively. Here, the concentration of target gas or liquid was calculated by the following formulas [25,26]:
Cg =
V1 V2
(1)
C1 =
22. 4 × × × V1 M × V2
(2)
where Cg, Cl, V1, V2, M, ρ and ϕ stand for the concentration of target gas and liquid, volume of target gas and chamber, molecular weight, density and volume fraction, respectively. 2
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Fig. 2. SEM images of (a, b) B–SnO2 and (c, d) 3D OP-SnO2. (e) and (f) The corresponding HRTEM image of 3D OP-SnO2. The inset in (e) is grain size distribution of 3D OP-SnO2. The inset in (f) is the SAED pattern.
3.2. Formation mechanism of 3D OP structure 3D OP-SnO2 is formed according to the following equation [21]: SnCl4·5H2O (s)+4NaOH (s)→Sn(OH)4 (s)+4NaCl (s)+5H2O (g)
(3)
Sn(OH)4 (s)→SnO2·H2O (s)+H2O (g)
(4)
SnO2·H2O (s)→SnO2 (s)+H2O (g)
(5)
When NaOH is added to the mixture and ground, Sn(OH)4 is obtained in the first step (Eq. (3)). Then the decomposition of Sn(OH)4 will take place to produce SnO2·H2O by further grinding (Eq. (4)), owning to the exothermic effect of reaction in Eq. (1). Finally, 3D OPSnO2 is acquired by calcination at 500 °C for 2 h (Eq. (5)). To study the formation mechanism of 3D OP-SnO2, productions of SnCl4·5H2O NaCl and NaOH ground at different times (5 min, 10 min and 30 min) are observed using SEM. A possible mechanism for the formation of 3D OP-SnO2 is proposed based on the evolution in the morphologies of intermediate products, as schematically illustrated in Fig. 5. The water-soluble NaCl with a face-centered cubic crystal structure is used as the template as reported previously [19,35]. When adding NaOH to the agate mortar and grinding for 5 min, the nucleation of Sn(OH)4 starts on the surface of added NaCl particles and Sn(OH)4 aggregates along the cubic crystal structure [21]. Thus, the Sn(OH)4 particles are assembled into a 3D structure, gradually. Simultaneously, the by-product NaCl produced in the reaction process (Eq. (1)) is considered to separate the newly formed Sn(OH)4 nanoparticles and inhibited their growth because it provides an effective driving force for the formation of small particle [29]. Sn(OH)4 further transforms to SnO2·H2O due to heat released during the reaction. The resulted 3D complex of SnO2·H2O/NaCl is then calcinated at 500 °C, which facilitates the formation of SnO2 on the surface of NaCl particles. The 3D OP-SnO2 can be easily obtained after removing NaCl by distilled water.
Fig. 3. N2 adsorption-desorption isotherms of B–SnO2 and 3D OP-SnO2. Inset is the BET surface area of two samples.
X-ray photoelectron spectroscopy (XPS) is carried out to investigate the surface element and chemical states of B–SnO2 and 3D OP-SnO2. Fig. 4 (a) and (d) represent the full-wide scanned spectra of B–SnO2 and 3D OP-SnO2, which displays the presence of Sn, O, and C without any other peaks of impurities. The C1s peak at 284.6 eV is used as reference for calibration. The Sn 3d high resolution XPS spectrum can be deconvoluted into two peaks as shown in Fig. 4 (b) and (e). The two peaks at 495.4 eV and 487.3 ± 0.2 eV correspond to Sn 3d3/2 and Sn 3d5/2 with a doublet separation of 8.5 eV, which is in agreement with the standard values of Sn4+ in SnO2 and higher than that of metallic Sn (484.8 eV) or Sn2+ (485.9 eV) [31–34]. The O 1s high resolution XPS spectrum is illustrated in Fig. 4(c) and (f), which can be decomposed into three types of O contributions. The band at 530.8 ± 0.1 eV corresponds to lattice oxygen (OL) from Sn–O bonds, and the binding energies at higher banding energy peak of 532.1 ± 0.1 eV and 532.3 eV ± 0.1 eV are attributed to oxygen vacancies (OV) and chemisorbed oxygen (OC), respectively. The relative percentages of OV and OC in 3D OP-SnO2 of 25.51% and 3.21% are higher than that in B–SnO2 of 15.23% and 0.32%, respectively.
3.3. Gas response of the samples It is well-known that the operating temperature is an important factor for the resistive sensors. To optimize the working temperature of the sensors, parallel experiments are carried out at different temperatures [36,37]. B–SnO2 and 3D OP-SnO2 sensors have been exposed to 5 ppm of Cl2 in range of 80 °C–300 °C as shown in Fig. 6. Both B–SnO2 and 3D OP-SnO2 sensors display the largest variation in resistance at 3
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Fig. 4. XPS spectra of B–SnO2 (a, b, c) and 3D OP-SnO2 (d, e, f): (a, d) fully scanned spectra, (b, e) Sn 3d, (c, f) O 1s.
160 °C as shown in Fig. 6(a) and (b), which means the best response can be achieved at about 160 °C in both samples. It is obvious that 3D OPSnO2 sensor shows higher sensitivity and shorter response time to Cl2 than B–SnO2 at 160 °C as shown in Fig. 6(c). The gas response exhibits a typical volcano-type dependence against temperature as illustrated in Fig. 6(d). This phenomenon may be attributed to the kinetics and thermodynamics of gas adsorption and desorption [38,39]. Meanwhile, both sensors show a maximum response towards Cl2 at 160 °C while beyond 160 °C, sensor response decreases due to desorption of Cl2 from the sensor surface. Therefore, the best operating temperature is determined to be 160 °C and the subsequent tests are carried out at this temperature. Selectivity is defined as the ability of the sensors to detect a specific gas in a mixture of several gases, which is another crucial parameter of gas sensors [40]. The gas response transient curves of the 3D OP-SnO2 sensor are given in Fig. 7(a) and Fig. 7 (b) for 5 ppm oxidative gases and 1000 ppm oxidative gases, respectively. Apparently, as shown in Fig. 7(c), the 3D OP-SnO2 sensor demonstrates an excellent response towards Cl2 than other tested gases. The selectivity coefficient can be calculated according to selectivity = Starget gas/So, where Starget gas and
So are the response to target gas and other non-target gases, respectively, as shown in Fig. 7(d) [41]. The response value of 3D OP-SnO2 sensor for Cl2 is about 389.09, 762.50, 24.03, 13.91, 273.45, 247.81and 27.25 times higher than acetone, xylene, hydrogen peroxide, nitrogen dioxide, methanol, formaldehyde and ethanol, respectively. The results reveal that the 3D OP-SnO2 sensor displays superior selectivity to Cl2 against other interferential gases. The variation in output voltage with Cl2 concentration is shown in Fig. 8 (a) and (c). The output voltage of the sensors decreases with increasing of the concentration of target gas from 0.5 to 8 ppm. Fig. 8 (b) and (d) present the concentration dependent Cl2 response. It indicates that the response of two sensors increases rapidly with the increase of Cl2 in the range of 0.5–8 ppm. The inset of Fig. 8 (b) and (d) show that the logarithm of the gas response (Log S) is increased with the logarithm of Cl2 concentration (Log C) in a linear relation. Correspondingly, the limit of detections (LOD) of B–SnO2 and 3D OP SnO2 are 0.695 and 0.00229 ppm, which can be calculated from the fitting curve. Humidity is one of the considerable parameters in gas detections, especially for Cl2, due to the fact that Cl2 molecules can easily react with water vapor (H2O) to form corrosive acids, which is given by the
Fig. 5. Schematic illustrations showing the formation mechanism of the 3D OP-SnO2. 4
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Fig. 6. Dynamic response-recovery curves to 5 ppm Cl2 at different working temperatures of (a) B–SnO2 and (b) 3D OP-SnO2. (c) Dynamic response curves and gas responses of B–SnO2 and 3D OP-SnO2 exposed to 5 ppm Cl2 at 160 °C. (d) Gas response of the B–SnO2 and 3D OP-SnO2 sensor exposed to 5 ppm Cl2 at different working temperatures.
following reaction: 2Cl2 + H2O → HCl + HClO. The gas sensing performance for Cl2 in 5 ppm at 20–87% relative humidity (RH) is given in Fig. 9(a). The gas response in Cl2 atmosphere almost linearly decreases with increased RH, as shown in Fig. 9(b), which means that high RH should be avoided in the detection of Cl2. Fig. 9(c) shows the response dynamic curve of the 3D OP-SnO2 sensor in 5 ppm Cl2, illustrating the
response and recovery time of 3 s and 27 s, respectively. The corresponding temperature-dependent response and recovery times are compiled in Fig. 9 (d). The response time of the sensor tends to be stable (3–15s) at various temperature which suggests that the response time is mainly controlled by chemical reaction [42,43]. But the recovery time is dependent on the temperature. This is related to the Knudsen
Fig. 7. (a) Gas response transient curves of 3D OP-SnO2 exposed to 5 ppm H2O2, NO2, Cl2 and (b) 1000 ppm acetone, xylene, methanol, formaldehyde, ethanol. (c) Selectivity and (d) selectivity coefficient of 3D OP-SnO2 exposed to different gases at 160 °C. 5
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Fig. 8. (a) and (c) Dynamic response curves of B–SnO2 and 3D OP-SnO2. (b) and (d) Gas response of B–SnO2 and 3D OP-SnO2 at the concentration range from 0.5 to 8 ppm in Cl2. Inset of (b) and (d) are Dual-logarithm of gas response (S) and gas concentration (C) for B–SnO2 and 3D OP-SnO2.
Fig. 9. (a) Dynamic response curves and (b) gas response of the 3D OP-SnO2 sensor in 5 ppm Cl2 at different humidity conditions. (c) Response/recovery time of 3D OP-SnO2 sensor in 5 ppm Cl2. (d) Response/recovery time of 3D OP-SnO2 sensor at different working temperatures.
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optimal temperature (160 °C) [48]. Thus, the reaction in Eq (8) takes place. However the reacted amount is quite low since the oxidizing ability of Cl2 is stronger than O2 and only 3% adsorbed oxygen on surface is detected in XPS analysis. Furthermore, oxygen vacancies ((VO¨ )surf) that has a more significant impact on the gas sensitivity by comparing and analyzing the data in Fig. S2, Fig. S3 and Figs. 4, Fig. 6. It is proposed that Cl2 molecules can be adsorbed on the surface to form (Cl-)surf or adsorbed on (VO¨ )surf to form (ClO–)surf as reactions (9)–(10). Fig. 11(a) presents reactions on the surface of 3D OP-SnO2 sensor. Compared to that in air, the thickness of depletion layer further increases in Cl2 due to the fact that besides reaction (7), reactions (8)–(10) also proceed. Therefore, electrons are extracted, trapped and consumed. The following reactions may occur [49,50]:
O2 + e
Cl2 + O2
Cl2 + 2(VO
diffusion, which can be described using following equation [44,45]:
2RT M
ads
+e
2Clads + O2
Cl2 + 2e− ⇋ 2(Cl−)surf
Fig. 10. The stability of 3D OP-SnO2 sensor in 5 ppm Cl2.
4R DK = 3
(7)
(O2 )surf
¨)
surf
+ 2e
(8) (9)
2(Cl O )surf
(10)
where the subscripts of surf and ads represent surface and adsorption, respectively. (VO¨ )surf stands for oxygen vacancies on the surface. It is well-known that the conduction zone and depletion layer exhibit different conduction behaviors. The resistance of SnO2 sensor is mainly determined by the resistive depletion layer contacts formed between the particles. Thus, equivalent circuits of SnO2 gas sensor can be explained as serial connections between semiconducting conduction zones (Rconduction zone) and resistive interparticle contacts (Rdepletion layer) in Fig. 11(b). Based on the above analytical results, the remarkable gas-sensing performances towards Cl2 of 3D OP-SnO2 sensor may be ascribed to three factors. First, the responses are greatly enhanced owing to the reduction of grain size. When D < 2 L, conductance is limited by grain control [51–53], as illustrated in Fig. 11(c). The whole grain is considered to be an electron depletion layer with high resistance, which means that the migration of electrons is greatly hindered. As a result, the resistance is further increased. However, conductance is influenced by grain boundary control (known as Schottk barrier at grain boundaries) for B–SnO2 sensor due to D≫2 L. Second, larger specific surface areas and oxygen vacancies are favorable for gases adsorption owing to abundant active sites. Lastly, thanks to the unique morphology with three-dimensional open porous nanostructure, the gas molecules can diffuse more effectively.
(6)
where Dk, r, R, T and M denote the Knudsen diffusion coefficient, pore radius, the gas constant, temperature and molecular weight of diffusion gas, respectively. The molecule weight of Cl2 is higher than other tested gases, which makes the diffusion of Cl2 on the surface of sensing layer slower than other gaseous molecules at the same operation temperature. To explore the time stability of as-prepared sensors, the gas sensing response is measured over a period of 50 days as shown in Fig. 10. It can be seen that up to 86% of the gas response can be maintained even after 30 days, indicating relatively high time stability. A comparison between the sensor and literature reports on Cl2 sensing properties is summarized and presented in Table S2. It is noteworthy that the sensing properties of 3D OP-SnO2 in our work including response, response time and LOD exhibited good competitiveness. 3.4. Gas sensing mechanism Essentially, the sensing mechanism is based on the variation of resistance, which can be ascribed to the chemisorption, subsequent reactions and desorption on the semiconducting oxide [46,47]. For SnO2, a typical n-type semiconductor metal oxide, its sensing mechanism to Cl2 can be illustrated as Fig. 11. In general, oxygen species are adsorbed on the surface of SnO2 when exposed to air, and turned into chemical adsorbed oxygen. The adsorbed oxygen species consume electrons and produce superoxide ions (O2-), which is known to be dominant at
4. Conclusions A three-dimensional open porous SnO2 was successfully synthesized by a simple solid-state method. Gas-sensing properties of the 3D OP-
Fig. 11. Schematic diagram of (a) very small depletion layer (high conductivity) in air and depletion layer thickness increases in Cl2 (b) equivalent circuit of n-type oxide semiconductors (c) three mechanisms of grain size dependence of conductance in semiconductor gas sensing materials. 7
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SnO2 sensor towards Cl2 was investigated and compared with B–SnO2, the best performances in terms of sensitivity, selectivity and LOD were found in 3D OP-SnO2 sensor. The response value to 5 ppm Cl2 of 3D OPSnO2 was 61 times greater than that of the contrast sample and the selectivity coefficient could reach 230 at least for partial reducing gases. Moreover, the LOD of Cl2 gas was obtained to be as low as 2.29 ppb. The improvement of sensing properties was attributed to decrease in grain size, high specific surface area and increase in the concentration of oxygen vacancy.
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High sensitivity and selectivity chlorine gas sensors based on 3D open porous SnO2 synthesized by solid-state method Weiming Zhang, Qiang Li*, Chao Wang, Jiangwei Ma, Chao Wang, Haijun Peng, Yun Wen, Huiqing Fan State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
ARTICLE INFO
ABSTRACT
Keywords: Solid-state method 3D open porous SnO2 Cl2 Gas sensing
A simple solid-state reaction method was employed to synthesize the three-dimensional open porous SnO2 (3D OPSnO2) by grinding the mixture of tin tetrachloride pentahydrate, sodium hydroxide and sodium chloride. Sodium chloride worked as a template to build the 3D open porous structure. The morphological feature was three-dimensional open porous with diameters about 300–500 nm and the edge of the 3D open porous were composed of numerous SnO2 nanoparticles with grain size around 5.8 nm. The chlorine (Cl2) gas sensing properties of 3D OPSnO2 and bulk SnO2 (B–SnO2) sensors were systematically investigated. Gas response of the 3D OP-SnO2 sensor was 792.85 to 5 ppm Cl2 at 160 °C, which was 61 times higher than that of B–SnO2. Such outstanding gas sensing performance was mainly ascribed to the small grain size, which resulted in the change of conductivity mechanism and the decreased mobility of electrons. Thereby, the resistance of OP-SnO2 increased dramatically. Moreover, the unique structure and abundant oxygen vacancies also contributed to the excellent gas sensing performance, because they can elevate the specific surface area and provide sufficient Cl2 adsorption sites.
1. Introduction Among various toxic gases, chlorine (Cl2) is an important gas because it is widely used in production of commodities, such as paper products, dye stuffs and many other consumer products [1]. Cl2, however, is a dangerous gas to humans’ respiratory system, fatal at 1000 ppm concentration [2,3]. Owning to such negative impacts to human life, numerous methods have been reported for the Cl2 gas detection, such as optical, solid state potentiometric and electrochemical methods [4–7]. However, several problems such as high cost, nonportability and poor sensitivity to target analytes hinder its practical application. Therefore, it is urgent to develop rapid and accurate detectors of Cl2 gas at ppb level. Tin dioxide (SnO2), a n-type oxide, is a promising sensing material owning to its high mobility of conduction electrons (160 cm2/V·s) and chemical stability [8,9]. To obtain highly sensitive gas sensors, SnO2 with different morphologies such as nanomembranes, thin films, nanosheets, microflowers, raspberry-like and nanotubes have been prepared [10–15]. However, two main problems, methodology and property, should be considered. That is to say, sensors with excellent gas performance are commonly costly or produced in an extremely complex process while facile and cost-effective methods tend to provide sensors
*
with limitations, such as unsatisfactory sensitivity or selectivity. Suematsu et al. used stanice acid gel as a precursor and produced monodispersed SnO2 nanoparticles to obtain improved gas sensing performance but the preparation process is time consuming and complicated [16]. Choi et al. have synthesized SnO2 nanosheets via an aqueous process under moderate conditions and the assembled sensors only show high selectivity towards alkene gases [17]. Therefore, it is significant to develop a simple method, which can improve the overall sensing properties. Li et al. synthesized the In2O3/SnO2 gas sensor by solid-state reaction but it did not show excellent gas sensitivity at ppb level [18]. Chen et al. fabricated 3D open porous g-C3N4 by a two-step process for photocatalytic hydrogen evolution [19]. Inspired by the works, the salt-assisted synthesis of three-dimensional open porous SnO2 (3D OP-SnO2) by solid-state reaction is conducted. Solid-state reaction synthesis is particularly attractive due to its simplicity, low cost, capability of producing nanomaterials with adjustable grain sizes and controllable micromorphology [20,21]. With this approach, 3D OP-SnO2 sensors with high sensitivity, superior selectivity, lower limit of detection (LOD), and long-term stability towards Cl2 are synthesized. The excellent properties of 3D OP-SnO2 sensors likely originate from the decrease in grain size (D), especially for D ≤ 2 L (thickness of the depletion layer) [22,23]. The mechanism
Corresponding author. E-mail address: [email protected] (Q. Li).
https://doi.org/10.1016/j.ceramint.2019.07.036 Received 20 February 2019; Received in revised form 6 June 2019; Accepted 2 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Weiming Zhang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.07.036
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for the gas sensing of the 3D OP-SnO2 is discussed in detail. 2. Experimental section 2.1. Synthesis of SnO2 The starting materials were analytical grade SnCl4·5H2O, NaOH and NaCl. In a typical process, 3.51 g SnCl4·5H2O, 1.60 g NaOH and 2.86 g NaCl were mixed in an agate mortar and ground for 50 min at room temperature, respectively. Then the precursor was obtained after drying the mixture at 80 °C for 12 h. Subsequent calcination of the precursor was performed at 500 °C for 2 h in air. Finally, the calcined powder was dispersed, washed with distilled water and ethanol several times and dried at 80 °C. For comparison, bulk SnO2 (B–SnO2) sample was obtained by direct calcination of SnCl4·5H2O at 500 °C for 2 h in air. 2.2. Characterization Fig. 1. X-ray diffraction patterns of B–SnO2 and 3D OP-SnO2.
The obtained samples were analyzed by X-ray diffraction (XRD; D8 Advance, Bruker, Germany) at 20–80° using Cu-Kα radiation (λ = 1.5406 Å), 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). The specific surface area of all samples was obtained by N2 adsorptiondesorption method (TriStar II 3020, Micromeritics Ltd, Shanghai, China). The chemical state was tested by high-resolution X-ray photoelectron spectroscopy (XPS; VG ESCALA-B220i-XL, Thermo-Scientific, Surrey, UK) with a focused monochromatized Al Kα radiation (E = 1486.6eV).
3. Results and discussion 3.1. Crystalline structure and morphology X-ray diffraction (XRD) patterns are recorded to identify the crystal structure of B–SnO2 and 3D OP-SnO2, as shwown in Fig. 1. All peaks correspond to the tetragonal SnO2 (JCPDS 41–1445). No additional peaks appear in both samples, indicating the successful formation of pure phase SnO2. The grain sizes of B–SnO2 and 3D OP-SnO2 are calculated to be about 5.9 nm and 10.8 nm by Sherrer's equation [27]. It can be explained that the growth of the particles was inhibited because the by-product NaCl leaded to the formation of “walls” surrounding the nanoparticles to keep them from growing [28,29]. In addition, it is reported that the thickness of the depletion layer (L) of SnO2 is about 3 nm in air [23]. Therefore, for 3D OP-SnO2, the grain size is less than 2 L, especially in chlorine atmosphere, which means the entire SnO2 particles could be regarded as depletion layer. Thicker depletion layer leads to larger change in resistance, which further results in high response. The Scanning electron microscopy (SEM) and High resolution transmission electron microscopy (HRTEM) images of the as-synthesized SnO2 samples are shown in Fig. 2. As depicted in Fig. 2 (a) and (b), B–SnO2 consists of large irregular aggregates of nanoparticles with diameters around 9–13 nm. Fig. 2(c) and (d) show the typical threedimensional open porous microstructure of 3D OP-SnO2. The diameter of pores is 300–500 nm and the boundary with 10–20 nm in thickness is made up of relatively close arrangements of nanoparticles, which is beneficial for diffusion of gases. To give further insight into the structure of the samples, the HRTEM images associated with select area electron diffraction (SAED) pattern are showed in Fig. 2 (e) and (f). The grain size statistics result is evaluated to be around 5.8 nm, which is consistent with the value estimated from XRD. The SAED pattern is corresponding to the cubic SnO2 with strong diffraction rings resulted from (110), (101), (200), and (211) planes. Given that the 3D open porous structure can provide higher specific surface area, as confirmed by the N2 adsorption-desorption isotherms in Fig. 3. The BET surface area of 3D OP-SnO2 is calculated to be 102 m2/g, which is larger than that of B–SnO2 (89 m2/g). It is well-known that the morphology of powders, especially the grain size, has great influence on the sensor response [30]. The grain size, specific surface area, and peak pore radius of B–SnO2 and 3D OP-SnO2 are summarized in Table S1. Significant difference is found between the two sets of data. Thus, 3D porous structure is conducive to increasing surface area and gas adsorption sites.
2.3. Fabrication of sensors and measurement of gas sensing properties To fabricate the gas sensors [24], mixture of SnO2 powder and glycerin was ground to form a paste. Afterwards, the paste was coated onto an alumina tube which is consisted of Au electrode, Pt wire and a Ni–Cr alloy resistor (Fig. S1a). After drying under IR light for several minutes, the device was calcined at 350 °C for 2 h and then aged at 300 °C for one week in order to improve stability and repeatability. The gas sensing properties were measured by an 18 L gas response instrument (HW-30A, Hanwei Ltd, Zhengzhou, China) with the constant loop voltage (Vc) of 5 V. The output voltage (Vout) was the terminal voltage of the load resistor RL and the working temperature of gas sensor was adjusted by varying the heating voltage (Vh). The schematic diagrams of test electric circuit and a photograph image of the instrument used were illustrated Figs. S1b and c. Quantitative target gas or liquid was injected into the test chamber by a micro-syringe. The evaporator in the chamber was used to heat the liquid into gas. After test, the chamber was removed for test gases to diffuse away. Gas response (S) was defined as follows: Soxidizing gases = Rgas/Rair and Sreducing gases = Rair/Rgas. Where Rair (Rair= (5-Vair)/(Vair/RL)) and Rgas (Rgas = (5-Vgas)/(Vgas/RL)) were the stabilized resistances in air and in the mixture of test gas, respectively. Vair and Vgas were the average Vout in air and the mixture of test gas, respectively. Here, the concentration of target gas or liquid was calculated by the following formulas [25,26]:
Cg =
V1 V2
(1)
C1 =
22. 4 × × × V1 M × V2
(2)
where Cg, Cl, V1, V2, M, ρ and ϕ stand for the concentration of target gas and liquid, volume of target gas and chamber, molecular weight, density and volume fraction, respectively. 2
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Fig. 2. SEM images of (a, b) B–SnO2 and (c, d) 3D OP-SnO2. (e) and (f) The corresponding HRTEM image of 3D OP-SnO2. The inset in (e) is grain size distribution of 3D OP-SnO2. The inset in (f) is the SAED pattern.
3.2. Formation mechanism of 3D OP structure 3D OP-SnO2 is formed according to the following equation [21]: SnCl4·5H2O (s)+4NaOH (s)→Sn(OH)4 (s)+4NaCl (s)+5H2O (g)
(3)
Sn(OH)4 (s)→SnO2·H2O (s)+H2O (g)
(4)
SnO2·H2O (s)→SnO2 (s)+H2O (g)
(5)
When NaOH is added to the mixture and ground, Sn(OH)4 is obtained in the first step (Eq. (3)). Then the decomposition of Sn(OH)4 will take place to produce SnO2·H2O by further grinding (Eq. (4)), owning to the exothermic effect of reaction in Eq. (1). Finally, 3D OPSnO2 is acquired by calcination at 500 °C for 2 h (Eq. (5)). To study the formation mechanism of 3D OP-SnO2, productions of SnCl4·5H2O NaCl and NaOH ground at different times (5 min, 10 min and 30 min) are observed using SEM. A possible mechanism for the formation of 3D OP-SnO2 is proposed based on the evolution in the morphologies of intermediate products, as schematically illustrated in Fig. 5. The water-soluble NaCl with a face-centered cubic crystal structure is used as the template as reported previously [19,35]. When adding NaOH to the agate mortar and grinding for 5 min, the nucleation of Sn(OH)4 starts on the surface of added NaCl particles and Sn(OH)4 aggregates along the cubic crystal structure [21]. Thus, the Sn(OH)4 particles are assembled into a 3D structure, gradually. Simultaneously, the by-product NaCl produced in the reaction process (Eq. (1)) is considered to separate the newly formed Sn(OH)4 nanoparticles and inhibited their growth because it provides an effective driving force for the formation of small particle [29]. Sn(OH)4 further transforms to SnO2·H2O due to heat released during the reaction. The resulted 3D complex of SnO2·H2O/NaCl is then calcinated at 500 °C, which facilitates the formation of SnO2 on the surface of NaCl particles. The 3D OP-SnO2 can be easily obtained after removing NaCl by distilled water.
Fig. 3. N2 adsorption-desorption isotherms of B–SnO2 and 3D OP-SnO2. Inset is the BET surface area of two samples.
X-ray photoelectron spectroscopy (XPS) is carried out to investigate the surface element and chemical states of B–SnO2 and 3D OP-SnO2. Fig. 4 (a) and (d) represent the full-wide scanned spectra of B–SnO2 and 3D OP-SnO2, which displays the presence of Sn, O, and C without any other peaks of impurities. The C1s peak at 284.6 eV is used as reference for calibration. The Sn 3d high resolution XPS spectrum can be deconvoluted into two peaks as shown in Fig. 4 (b) and (e). The two peaks at 495.4 eV and 487.3 ± 0.2 eV correspond to Sn 3d3/2 and Sn 3d5/2 with a doublet separation of 8.5 eV, which is in agreement with the standard values of Sn4+ in SnO2 and higher than that of metallic Sn (484.8 eV) or Sn2+ (485.9 eV) [31–34]. The O 1s high resolution XPS spectrum is illustrated in Fig. 4(c) and (f), which can be decomposed into three types of O contributions. The band at 530.8 ± 0.1 eV corresponds to lattice oxygen (OL) from Sn–O bonds, and the binding energies at higher banding energy peak of 532.1 ± 0.1 eV and 532.3 eV ± 0.1 eV are attributed to oxygen vacancies (OV) and chemisorbed oxygen (OC), respectively. The relative percentages of OV and OC in 3D OP-SnO2 of 25.51% and 3.21% are higher than that in B–SnO2 of 15.23% and 0.32%, respectively.
3.3. Gas response of the samples It is well-known that the operating temperature is an important factor for the resistive sensors. To optimize the working temperature of the sensors, parallel experiments are carried out at different temperatures [36,37]. B–SnO2 and 3D OP-SnO2 sensors have been exposed to 5 ppm of Cl2 in range of 80 °C–300 °C as shown in Fig. 6. Both B–SnO2 and 3D OP-SnO2 sensors display the largest variation in resistance at 3
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Fig. 4. XPS spectra of B–SnO2 (a, b, c) and 3D OP-SnO2 (d, e, f): (a, d) fully scanned spectra, (b, e) Sn 3d, (c, f) O 1s.
160 °C as shown in Fig. 6(a) and (b), which means the best response can be achieved at about 160 °C in both samples. It is obvious that 3D OPSnO2 sensor shows higher sensitivity and shorter response time to Cl2 than B–SnO2 at 160 °C as shown in Fig. 6(c). The gas response exhibits a typical volcano-type dependence against temperature as illustrated in Fig. 6(d). This phenomenon may be attributed to the kinetics and thermodynamics of gas adsorption and desorption [38,39]. Meanwhile, both sensors show a maximum response towards Cl2 at 160 °C while beyond 160 °C, sensor response decreases due to desorption of Cl2 from the sensor surface. Therefore, the best operating temperature is determined to be 160 °C and the subsequent tests are carried out at this temperature. Selectivity is defined as the ability of the sensors to detect a specific gas in a mixture of several gases, which is another crucial parameter of gas sensors [40]. The gas response transient curves of the 3D OP-SnO2 sensor are given in Fig. 7(a) and Fig. 7 (b) for 5 ppm oxidative gases and 1000 ppm oxidative gases, respectively. Apparently, as shown in Fig. 7(c), the 3D OP-SnO2 sensor demonstrates an excellent response towards Cl2 than other tested gases. The selectivity coefficient can be calculated according to selectivity = Starget gas/So, where Starget gas and
So are the response to target gas and other non-target gases, respectively, as shown in Fig. 7(d) [41]. The response value of 3D OP-SnO2 sensor for Cl2 is about 389.09, 762.50, 24.03, 13.91, 273.45, 247.81and 27.25 times higher than acetone, xylene, hydrogen peroxide, nitrogen dioxide, methanol, formaldehyde and ethanol, respectively. The results reveal that the 3D OP-SnO2 sensor displays superior selectivity to Cl2 against other interferential gases. The variation in output voltage with Cl2 concentration is shown in Fig. 8 (a) and (c). The output voltage of the sensors decreases with increasing of the concentration of target gas from 0.5 to 8 ppm. Fig. 8 (b) and (d) present the concentration dependent Cl2 response. It indicates that the response of two sensors increases rapidly with the increase of Cl2 in the range of 0.5–8 ppm. The inset of Fig. 8 (b) and (d) show that the logarithm of the gas response (Log S) is increased with the logarithm of Cl2 concentration (Log C) in a linear relation. Correspondingly, the limit of detections (LOD) of B–SnO2 and 3D OP SnO2 are 0.695 and 0.00229 ppm, which can be calculated from the fitting curve. Humidity is one of the considerable parameters in gas detections, especially for Cl2, due to the fact that Cl2 molecules can easily react with water vapor (H2O) to form corrosive acids, which is given by the
Fig. 5. Schematic illustrations showing the formation mechanism of the 3D OP-SnO2. 4
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Fig. 6. Dynamic response-recovery curves to 5 ppm Cl2 at different working temperatures of (a) B–SnO2 and (b) 3D OP-SnO2. (c) Dynamic response curves and gas responses of B–SnO2 and 3D OP-SnO2 exposed to 5 ppm Cl2 at 160 °C. (d) Gas response of the B–SnO2 and 3D OP-SnO2 sensor exposed to 5 ppm Cl2 at different working temperatures.
following reaction: 2Cl2 + H2O → HCl + HClO. The gas sensing performance for Cl2 in 5 ppm at 20–87% relative humidity (RH) is given in Fig. 9(a). The gas response in Cl2 atmosphere almost linearly decreases with increased RH, as shown in Fig. 9(b), which means that high RH should be avoided in the detection of Cl2. Fig. 9(c) shows the response dynamic curve of the 3D OP-SnO2 sensor in 5 ppm Cl2, illustrating the
response and recovery time of 3 s and 27 s, respectively. The corresponding temperature-dependent response and recovery times are compiled in Fig. 9 (d). The response time of the sensor tends to be stable (3–15s) at various temperature which suggests that the response time is mainly controlled by chemical reaction [42,43]. But the recovery time is dependent on the temperature. This is related to the Knudsen
Fig. 7. (a) Gas response transient curves of 3D OP-SnO2 exposed to 5 ppm H2O2, NO2, Cl2 and (b) 1000 ppm acetone, xylene, methanol, formaldehyde, ethanol. (c) Selectivity and (d) selectivity coefficient of 3D OP-SnO2 exposed to different gases at 160 °C. 5
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Fig. 8. (a) and (c) Dynamic response curves of B–SnO2 and 3D OP-SnO2. (b) and (d) Gas response of B–SnO2 and 3D OP-SnO2 at the concentration range from 0.5 to 8 ppm in Cl2. Inset of (b) and (d) are Dual-logarithm of gas response (S) and gas concentration (C) for B–SnO2 and 3D OP-SnO2.
Fig. 9. (a) Dynamic response curves and (b) gas response of the 3D OP-SnO2 sensor in 5 ppm Cl2 at different humidity conditions. (c) Response/recovery time of 3D OP-SnO2 sensor in 5 ppm Cl2. (d) Response/recovery time of 3D OP-SnO2 sensor at different working temperatures.
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optimal temperature (160 °C) [48]. Thus, the reaction in Eq (8) takes place. However the reacted amount is quite low since the oxidizing ability of Cl2 is stronger than O2 and only 3% adsorbed oxygen on surface is detected in XPS analysis. Furthermore, oxygen vacancies ((VO¨ )surf) that has a more significant impact on the gas sensitivity by comparing and analyzing the data in Fig. S2, Fig. S3 and Figs. 4, Fig. 6. It is proposed that Cl2 molecules can be adsorbed on the surface to form (Cl-)surf or adsorbed on (VO¨ )surf to form (ClO–)surf as reactions (9)–(10). Fig. 11(a) presents reactions on the surface of 3D OP-SnO2 sensor. Compared to that in air, the thickness of depletion layer further increases in Cl2 due to the fact that besides reaction (7), reactions (8)–(10) also proceed. Therefore, electrons are extracted, trapped and consumed. The following reactions may occur [49,50]:
O2 + e
Cl2 + O2
Cl2 + 2(VO
diffusion, which can be described using following equation [44,45]:
2RT M
ads
+e
2Clads + O2
Cl2 + 2e− ⇋ 2(Cl−)surf
Fig. 10. The stability of 3D OP-SnO2 sensor in 5 ppm Cl2.
4R DK = 3
(7)
(O2 )surf
¨)
surf
+ 2e
(8) (9)
2(Cl O )surf
(10)
where the subscripts of surf and ads represent surface and adsorption, respectively. (VO¨ )surf stands for oxygen vacancies on the surface. It is well-known that the conduction zone and depletion layer exhibit different conduction behaviors. The resistance of SnO2 sensor is mainly determined by the resistive depletion layer contacts formed between the particles. Thus, equivalent circuits of SnO2 gas sensor can be explained as serial connections between semiconducting conduction zones (Rconduction zone) and resistive interparticle contacts (Rdepletion layer) in Fig. 11(b). Based on the above analytical results, the remarkable gas-sensing performances towards Cl2 of 3D OP-SnO2 sensor may be ascribed to three factors. First, the responses are greatly enhanced owing to the reduction of grain size. When D < 2 L, conductance is limited by grain control [51–53], as illustrated in Fig. 11(c). The whole grain is considered to be an electron depletion layer with high resistance, which means that the migration of electrons is greatly hindered. As a result, the resistance is further increased. However, conductance is influenced by grain boundary control (known as Schottk barrier at grain boundaries) for B–SnO2 sensor due to D≫2 L. Second, larger specific surface areas and oxygen vacancies are favorable for gases adsorption owing to abundant active sites. Lastly, thanks to the unique morphology with three-dimensional open porous nanostructure, the gas molecules can diffuse more effectively.
(6)
where Dk, r, R, T and M denote the Knudsen diffusion coefficient, pore radius, the gas constant, temperature and molecular weight of diffusion gas, respectively. The molecule weight of Cl2 is higher than other tested gases, which makes the diffusion of Cl2 on the surface of sensing layer slower than other gaseous molecules at the same operation temperature. To explore the time stability of as-prepared sensors, the gas sensing response is measured over a period of 50 days as shown in Fig. 10. It can be seen that up to 86% of the gas response can be maintained even after 30 days, indicating relatively high time stability. A comparison between the sensor and literature reports on Cl2 sensing properties is summarized and presented in Table S2. It is noteworthy that the sensing properties of 3D OP-SnO2 in our work including response, response time and LOD exhibited good competitiveness. 3.4. Gas sensing mechanism Essentially, the sensing mechanism is based on the variation of resistance, which can be ascribed to the chemisorption, subsequent reactions and desorption on the semiconducting oxide [46,47]. For SnO2, a typical n-type semiconductor metal oxide, its sensing mechanism to Cl2 can be illustrated as Fig. 11. In general, oxygen species are adsorbed on the surface of SnO2 when exposed to air, and turned into chemical adsorbed oxygen. The adsorbed oxygen species consume electrons and produce superoxide ions (O2-), which is known to be dominant at
4. Conclusions A three-dimensional open porous SnO2 was successfully synthesized by a simple solid-state method. Gas-sensing properties of the 3D OP-
Fig. 11. Schematic diagram of (a) very small depletion layer (high conductivity) in air and depletion layer thickness increases in Cl2 (b) equivalent circuit of n-type oxide semiconductors (c) three mechanisms of grain size dependence of conductance in semiconductor gas sensing materials. 7
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SnO2 sensor towards Cl2 was investigated and compared with B–SnO2, the best performances in terms of sensitivity, selectivity and LOD were found in 3D OP-SnO2 sensor. The response value to 5 ppm Cl2 of 3D OPSnO2 was 61 times greater than that of the contrast sample and the selectivity coefficient could reach 230 at least for partial reducing gases. Moreover, the LOD of Cl2 gas was obtained to be as low as 2.29 ppb. The improvement of sensing properties was attributed to decrease in grain size, high specific surface area and increase in the concentration of oxygen vacancy.
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