A Simple Absorbent Cotton Biotemplate to Fabricate SnO2 Porous

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A Simple Absorbent Cotton Biotemplate to Fabricate SnO2 Porous Microtubules and their Gas Sensing Properties for Chlorine Jiangwei Ma, Huiqing Fan, Xiaohu Ren, Chao Wang, Hailin Tian, Guangzhi Dong, and Weijia Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02235 • Publication Date (Web): 23 Nov 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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ACS Sustainable Chemistry & Engineering

A Simple Absorbent Cotton Biotemplate to Fabricate SnO2 Porous Microtubules and their Gas Sensing Properties for Chlorine

Jiangwei Ma, Huiqing Fan*, Xiaohu Ren, Chao Wang, Hailin Tian, Guangzhi Dong, Weijia Wang,

State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, No. 127 Youyixi Road, Beilin District, Xi’an 710072, China *Corresponding author, E-mail: [email protected]

ABSTRACT SnO2 porous microtubules (PMs) were synthesized by a two-step immersioncalcination method, which the absorbent cotton is used as biotemplate. This method is simple, eco-friendly and cost-efficient. The absorbent cotton as biotemplate not only supported the formation of microtubule-like structures, but also provided a hypoxia atmosphere to introduce oxygen vacancies in the calcination process. The as-synthesized SnO2 PMs maintain the morphology of absorbent cotton, which are long, curly and twisted. The gas sensing property of the SnO2 PMs sensor was systematically investigated for detection of chlorine (Cl2). Based on such porous microtubules, the sensor exhibited excellent sensitivity and selectivity to Cl2 at 200 °C. Compared with SnO2 particles, the gas response (Rgas / Rair) of the SnO2 PMs sensor to 10 ppm Cl2 at 200 °C was increased about 100 times. Finally, the enhanced gas sensing performance was associated with the hollow morphology of SnO2 PMs and the formation of abundant oxygen vacancies due to the decomposition of absorbent cotton.

KEYWORDS: gas sensor, SnO2, porous microtubules, absorbent cotton, 1

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biotemplate, Cl2

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INTRODUCTION Chlorine (Cl2), a greenish yellow hazardous gas with pungent smell, is extensively used in water purification, production of paper products, medicines, antiseptics, plastics and many other consumer products.1 In those industries, Cl2 should be real-time detected because it can react with body moisture to form corrosive acids. The immediately dangerous to life and health (IDLH) concentration of Cl2 is 10 ppm and the occupational exposure limit (OEL) is 0.5 ppm.2 Hence, a highly efficient and cost effective Cl2 gas sensor should be investigated at low concentration range for immediately monitoring. There are many metal oxide semiconductors which have been reported for the Cl2 gas detection, such as In2O3,3 ZnO,4 SnO25and WO36. Among different metal oxides, SnO2, an n-type semiconductor, is one of the most interesting prospects due to its inexpensiveness, less toxicity and availability of various morphologies. Studies indicated that the structure features of materials could dramatically affect their gas sensing properties.7 Until now, various SnO2 structures with different morphologies have been reported for gas sensing, such as nanoparticles,8 nanorods,9 nanotubes,10 nanosheets,11 microspheres,12 honeycomb-like structures,13 mesoporous structures14 and so on. From the previous research, the porous and hollow structures have received much attention for improving of gas sensors performance due to quick gas diffusion. For instance, Liu studied that porous SnO2 nanotubes utilizing carbon nanotube as template exhibited good gas sensing activity to ethanol at 200 °C and Cao reported porous SnO2 nanotubes using MnO2 nanorods as the sacrificial template displayed the sensitivity for alcohols.15,16 However, those templates are cost, inefficient and environmentally-unfriendly. Therefore, it is significant to find simple and cost-effective templates to synthesize SnO2 with porous and hollow structure. Compared with other templates, biotemplate is cheap and environmentally benign.17 Recently, there are many reports about SnO2 material fabricated with biotemplate for gas sensing test. Zhang reported that single porous SnO2 microtubes synthesized using Papilio maacki wings as template could detect 0.8 ppm HCHO at 3



room temperature.18 Porous hierarchical SnO2 by grapefruit exocarp-biotemplate was synthesized to detect HCHO at 200 °C.19 The loofah sponge and eggshell membrane-biotemplate SnO2 was applied to tested ethanol at 250 °C.20 Xie studied SnO2 mesoporous microfibers using cottons as biotemplate by hydrothermal synthesis, were used in detecting formaldehyde at 150 °C.21 However, several biotemplated SnO2 have been fabricated for Cl2 gas sensor applications. For instance, Su prepared hierarchical porous SnO2 by Rape pollen grains-biotemplate, which exhibited good gas responses to Cl2 at 210 ℃.22 Therefore, identifying suitable biotemplate for synthesized SnO2 to detect Cl2 is interesting work. Absorbent cotton, owning excellent hydrophilic properties and low burning point, was utilized as biotemplate in our work. And successfully synthesized SnO2 with porous microtubules structure by a simple immersed-calcination method. To prove the importance of biotemplate, SnO2 particles were prepared by direct calcination of tin tetrachloride hydrate without using biotemplate. Although the optimized the working temperature is slightly increased, a short response time and a high sensitivity are realized in SnO2 PMs. Compared with SnO2 particles, the gas response of SnO2 PMs for 10 ppm Cl2 was increased above 100 times at 200 °C. Moreover, the as-prepared SnO2 PMs sensor exhibited excellent selectivity and low detection limit about 33 ppb.

EXPERIMENTAL SECTION Synthesis. SnCl4·5H2O was used as a starting material without further purification in the experiment (≥99.0%, Sinopharm Chemical Reagent Co., Shanghai, China). Absorbent cotton was a soft biotemplate during the synthesis (Jing Yanggan Co. Ltd., Shangdong, China). In the synthesis, SnCl4·5H2O (1.0 g, 2.9 mmol) was dissolved into deionized water (40mL) by ultrasound for 15 min. The absorbent cotton fibers (0.8g) were immersed into the above solution for 4h and then dried at 80 °C for 12 h. After that, the resultant degreasing cotton was heated in air up to 500 °C at a ramping rate of 3 °C min-1, and kept for 2 h. As formed products, SnO2 PMs were obtained. For 4


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comparison, SnO2 particles were obtained by directly calcination of SnCl4·5H2O (1.0 g, 2.9 mmol) at 500 °C for 2 h in air.

Characterization. The phase structures were characterized using the X-ray diffraction (XRD; D8 Advance, Bruker, Karlsruhe, Germany). The surface morphologies of obtained products were observed by a field emission scanning electron microscopy (FE-SEM; JSM-6710F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM, JEM-3010, JEOL, Tokyo, Japan). The chemical state analysis was carried out by using high-resolution X-ray photoelectron spectroscopy (XPS; VG ESCALA-B220i-XL, Thermo-Scientific, Surrey, UK). The chemical structure was investigated with Fourier transform infrared spectroscopy (FT-IR; TENSOR27, Bruker, Billerica, MA, USA). Photoluminescence (PL) spectra were obtained at room temperature on a fluorescence spectrophotometer (QM-4, PTI, Lawrenceville, NJ, USA). Nitrogen adsorption-desorption was performed by using Brunauer-Emmett-Teller (BET) method (V-Sorb 2800P, Gold APP Corp., Beijing, China). An AC impedance analyzer (CHI660E, CH Instruments, Shanghai, China) was used to study the gas-sensing properties at different temperatures.

Fabrication of the gas sensor. The preparation of the Cl2 sensor is described in previous literature23 and is presented in Figure S1. In general, the samples were mixed with an appropriate amount of glycerin and ground to form a paste. Then the paste was coated onto a prefabricated alumina tube with gold electrodes and platinum wires. The Ni-Cr heating wire crossed the tube and was used to adjust the operating temperature. Subsequently, the gas sensor was dried in an infrared radiation light and sintered at 300 °C for 2 h. Finally, the six wires of gas sensor were welded onto pedestals and aged at 300 °C for one week to ensure the stabilization of sensors resistance. The gas-sensing properties were measured by a gas response instrument with an 18L testing chamber (WS-30A, Weisheng Ltd., Zhengzhou, China) at a relative humidity (15±5% RH) (Figure S2). A calculated amount of target gases or liquid was introduced into the chamber by a syringe. The evaporator installed in in the 5



chamber was used to transform the liquid into gas, while two fans make test gas homogeneous. After the test, the chamber was removed for test gases to diffuse away. To assess the gas sensing properties, the gas response (S) was introduced. It was defined as the resistance ratio in air and gas. Rgas/Rair was to oxidizing gas and Rair/Rgas to reducing gas, where Rgas and Rair were the resistance of gas sensor in the measured gas and in air, respectively. The response/recovery times were defined as the time to attain 90% of the total resistance change during the responding and recovering processes, respectively. The stability was studied by repeating detection of 10ppm Cl2 after aging the device at 200 °C for different time (3d, 6d, 9d, 30d). In this work, the sensors were placed on the aged-platform (TS-60, Weisheng Ltd., Zhengzhou, China) for aging. For accurate results, each test was carried out three times under identical conditions and took an average value.

RESULTS AND DISCUSSION Phase and morphology. The phase structure of as-synthesized products is characterized by XRD in Figure S3. The tetragonal structure of SnO2 particles and SnO2 PMs can be confirmed by the standard card (JCPDS NO. 71-0652) and no other representative diffraction peaks are detected. According Scherrer Formula24 (see supporting informaiton), the grain size of SnO2 particles and PMs is about 10.8nm and 11.2nm, respectively. Therefore, the nanocrystalline grains of SnO2 particles and SnO2 PMs are of little difference. As shown in Figure S4, SnO2 particles are irregularly aggregated to large bulks. From Figure 1a and b, SnO2 PMs are long, curly and microtubule-like structures, which maintains the morphology of absorbent cotton. The SEM of SnO2 particles and PMs deposited on sensor substrates are showed in Figure S5 and S6. The morphology of SnO2 PMs and particles remain unchanged after deposited on the surface of the substrates. The TEM images of SnO2 PMs (Figure 1c) further reveal the porous microtubules structure, which is consistent with the result of SEM. Clear lattice fringes with interplanar distances of 0.330 nm and 0.264 nm were observed from high 6


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resolution transmission electron microscopy (HRTEM) image (Figure 1d), corresponding to the lattice spacing of the (110) and (101) plane in SnO2, respectively. Moreover, the selected-area electron diffraction (SAED) pattern (inset of Figure 3d) shows the diffraction rings which could be indexed as the (110), (101), (200), (211), and (112) planes of tetragonal SnO2, respectively.25-27 TEM-EDX elemental mapping analysis (Figure 1e) was used to further characterize the elemental distribution of the products. Obviously, Sn and O signals are uniformly distributed over the chosen region.

Figure 1. SnO2 PMs: (a)and (b) SEM image; (c) TEM image; (d) HRTEM image and (inset) SAED pattern; (f) EDX elemental mapping of O (blue), Sn (red).

XPS investigation was performed to determine the surface compositions and chemical states. Figure 2a and d represent the full-range XPS survey spectra of SnO2 particles and SnO2 PMs. It indicated the as-prepared samples are of high purity, which is consistent with the analysis of XRD. The observation of C 1s peak at 284.6 eV in the survey scan is ascribed to adventitious carbon-based additives, which is used as reference for calibration. The Sn 3d region of XPS is derived from Sn-O bond in SnO2 7



lattice, which has been observed in most reports of SnO2.24,28,29 Firstly, discrimination between SnO and SnO2 in photoemission studies is complicated due to only a small shift in the Sn 3d core level binding energy. The peaks at 486.6±0.2 eV and 487.0±0.3 eV are assigned to Sn 3d derived from Sn2+ in SnO and Sn4+ in SnO2, respectively. In our work, the Sn 3d core level spectra located a binding energies of 487.1 eV is assigned to Sn4+ in SnO2. Secondly, for the O 1s core level, Jimeńez reported a shift in the binding energy of 1 eV for SnO compared to SnO2, i.e. 530.4 eV for SnO and 531.4 for SnO2.30 The binding energy of O 1s is located at 531.0 eV in this work. Therefore, the chemical state of Sn in this sensor is Sn4+. The Sn 3d high resolution spectrum in Figure 2b and e can be fitted into Sn 3d3/2 and 3d5/2, and the spin-orbit splitting is 8.4 eV, which is in agreement with the value of SnO2.31 In addition, the O 1s high-resolution XPS spectrum of SnO2 can be deconvoluted into three peaks in Figure 2c and f. The peaks at 531.0 and 532.2eV belong to the lattice oxygen ((O)latt) and the oxygen vacancies ((VÖ)surf) in SnO2, whereas the peak at about 533.3 eV can be attributed to the surface hydroxyl groups ((O)OH).32 Obviously, more oxygen vacancies are detected in SnO2 PMs (24.51%) relative to SnO2 particles (14.6%). The different of SnO2 PMs and particles is the use of biotemplate before calcination. During the calcination process, SnO2 PMs are forming and absorbent cotton is being removed simultaneously. With the removing of absorbent cotton biotemplate, oxygen is consumed and carbon dioxide releases, which provides a hypoxia atmosphere.33 It is generally known that the hypoxia or inert gas treatment result to the formation of oxygen vacancies in metal oxide semiconductors.34 Therefore, SnO2 PMs have abundant oxygen vacancies. To further prove the concentration of oxygen vacancies, the room-temperature photoluminescence (PL) spectrum was performed. As shown in Figure S7, there is a broad dominant peak centered at around 2.75 eV. This emission is ascribed to the electron transition mediated by defect levels. And it can be deconvoluted into two peaks, which was also observed in earlier research.35 The peaks at 2.5eV and 2.8 eV are ascribed to oxygen vacancies and structural defects, respectively. Usually, oxygen vacancies are considered to be the most common defects, which serve as radiative 8


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centers in luminescence processes.36 The peak area of oxygen vacancies of SnO2 particles and PMs is 8733.3 and 9570.5, respectively. Thus, the results further prove a higher concentration of oxygen vacancies in SnO2 PMs compared with particles, which is in agreement with the results of XPS.

Figure 2. XPS high-resolution spectra and fitted curves of survey spectra (a, d), Sn 3d (b, e), O 1s (d, f).

Figure S8 shows the FT-IR spectrum of absorbent cotton, SnO2 PMs and SnO2 particles. The absorption in the region from 3000 cm-1 to 3700 cm-1 is ascribable to the vibration of hydroxyl groups (O-H), which originated from the adsorbed water on the surface. For the absorbent cotton, the broader band is due to self-associated O-H groups.37 The band around at 2891 cm-1 corresponds to aliphatic C-H stretching. The absorption in 2358 cm-1 and 1652 cm-1 are attributed to O-H and C=O of carboxyl (COOH). The peak in the region 1321 cm-1 and 1028 cm-1 corresponds to C-O stretching or O-H deformation in aromatic and primary.38 The peak at 592 cm-1 is due to the stretching vibration of aromatic O-H bond.39 Therefore, there are hydroxyl and carboxyl groups on the surface of absorbent cotton. For SnO2 particles and SnO2 PMs, The peak at about 1638 cm-1 corresponds to the bending vibration of water molecules, trapped in the samples.40 The peak at 549 cm-1 is attributed to the stretching modes of 9



the Sn-O-Sn.41 It is notable that the special peaks corresponding to the absorbent cotton disappeared, such as those located at 2891, 2359, 1321, 1028 cm-1. Therefore, the absorbent cotton has been completely removed. As we know, the specific surface area is a vital factor for semiconductors used for gas sensing. Figure S9 shows the nitrogen adsorption-desorption isotherms and pore size distributions of SnO2 particles and SnO2 PMs. According to the IUPAC classification,42 the isotherms are type IV with an H1 loop. The derived BJH pore size distribution of SnO2 particles and PMs centered around 23 and 47 nm, which is attributed to the mesopores (inset of Figure S9). The BET surface area and the total pore volume of SnO2 particles and PMs are 9.1 m2·g-1, 0.062cm3·g-1 and 15.1 m2·g-1, 0.17cm3·g-1, respectively. The increase in surface area and total pore volume are ascribed to using absorbent cotton as biotemplete. The absorbent cotton fibber as template supports the formation of the porous microtubule-like structures. The porous structure can increase the surface-to-volume ratio, decrease the aggregation degree of the particles and further increases the effective surface area. Because it can provide more active site, the larger surface area could enhance the gas adsorption. Furthermore, the porous microtubules structure is in favor of gas diffusion.

Scheme 1. Schematic illustration the formation process of SnO2 PMs.

Formation mechanism. The synthetic strategy of SnO2 PMs is designed on the basis of adsorption and hydrolysis of Sn4+, as schematically illustrated in Scheme 1. 10


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In case of adsorption, the negative groups (hydroxyl and carboxyl) on the surface of absorbent cotton could interact with Sn4+ by electrostatic forces, hydrogen bonding interactions and covalent bonds.43 After hydrolysis and calcination, SnO2 PMs were obtained by complete removal of absorbent cotton. The synthesis scheme of the formation of SnO2 PMs using absorbent cotton as biotemplate is shown in Scheme 1.

Gas-sensing properties. Chlorine was used as the target gas to investigate the gas sensing characteristics of the fabricated SnO2 PMs sensor. For systematic analysis, working temperature is an important factor of impedance-semiconductor oxide sensors.44 Figure 3a presents the resistance changes of the SnO2 PMs sensors to Cl2 with a concentration of 10 ppm at different temperatures from 100 °C to 300 °C. Figure 3b indicates that the optimized operating temperature of SnO2 PMs is 200 °C. The resistance transient curves and gas responses of the SnO2 particles sensors at different temperature are presented in Figure S10. The optimized working temperature of SnO2 particles is 100 °C. Because the gas response could not reach the equilibrium in chlorine, the recovery times are too long to be accurately calculated. Though the optimized working temperature of SnO2 particles is lower than SnO2 PMs, its gas response is relatively low and the response and the recovery times are so long. The reason for the increased optimized operating temperature of SnO2 PMs will be discussed in the section of gas-sensing mechanism.

Figure 3. (a) Typical transient resistance versus time and (b) the response of SnO2 PMs sensor upon exposure to 10 ppm Cl2 at a function of different operating temperatures. 11



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To further demonstrate the optimized working temperature, complex AC impedance spectra were performed. Figure 4a and c show the impedance spectra of the SnO2 PMs sensor in air and in 10 ppm Cl2 at different temperature, respectively. The value of Z′ in the low frequency end of the semicircle could be used to tentatively evaluate the resistance.45 When operating at 100-200 °C, the intrinsic excitation plays dominant role resulting in resistance decreasing. In fact, the oxygen species on the surface depends on different temperature, which are O2 (80 °C), O2− (150 °C), O− (300-400 °C), and O2− (550 °C).46,47 Beyond 200 °C, the resistance increases indicating that the sensor is able to capture more oxygen. Therefore, SnO2 PMs sensors obtained the lowest resistance at 200 °C (Figure 4a). Figure 4b shows the variation of logarithmic conductivity (ln σac) with reciprocal temperature (1000/T). The nature of the variation is linear and follows the Arrhenius relationship: 47 σac = σ0 exp (-Ea / KbT)

(1)

where σ0 is a constant, Ea is the activation energy of conduction, Kb is a Boltzmann constant and T is the absolute temperature. The magnitude of the activation energy EA and EB obtained are 0.464 and 0.223 eV in Figure 4b. The obtained activation energy is smaller than the band gap energy (Eg = ~3.50 eV), which corresponds to the energy for the transition of defects. As Figure 4c shown, the resistance of the SnO2 PMs sensor is dramatically increased in Cl2 atmosphere compared with that in air at operating temperatures. The gas response of the SnO2 PMs sensor at different temperatures is shown in Figure 4d. The tendency of the gas response changes is similar to the result of the gas response instruments at different temperatures, which further demonstrates the optimum operating temperature of the SnO2 PMs sensor. As the result shown, the SnO2 PMs sensor exhibits highest sensitivity at 200 °C which is applied to all subsequent measurements. In addition, to optimize the calcination temperature, SnO2 PMs were obtained at 450, 550, 600 °C and the analysis about different temperature is discussed in Supporting Information.

12


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Figure 4. (a) Complex AC impedance plots of the SnO2 PMs sensor in air at different temperatures; (b) The Arrhenius plots of conductivity for the SnO2 PMs; (c) complex AC impedance plots of the SnO2 PMs sensor exposed to 10ppm Cl2 at different temperatures; (d) the relationship between the resistance and the gas response from panel (a) and (c).

In order to investigate the effect of morphology of SnO2 on the gas-sensing properties, the representative dynamic gas responses of SnO2 particles sensor and SnO2 PMs sensor were measured under the same conditions. As Figure 5a shows, the gas response of SnO2 PMs (1214) is greatly enhanced compared to that of SnO2 particles (11). The reason for the enhancement of gas-sensing property may be the unique porous microtubules for increased surface area and rapid gas diffusion. Selectivity is another important parameter for the evaluation of gas sensor. Figure 5b and c show the gas response transient curves of SnO2 PMs exposed to 10 ppm nitrogen dioxide (NO2), nitric oxide (NO) and sulfur dioxide (SO2) and 100 ppm ammonia (NH3), acetone (CH3COCH3), formaldehyde (HCHO) and ethanol (CH3CH2OH). When exposed in the air, NO immediately converts NO2. Therefore, the performance of the sensor is similar for NO and NO2. As the result shown in 13



Figure 5d, the SnO2 PMs sensor exhibits excellent selectivity to Cl2 against the other tested gases.

Figure 5. (a) Dynamic responses curves of sensor and sensitivity response of SnO2 particles and SnO2 PMs sensors exposed to 10 ppm Cl2 at 200 °C; typical transient resistance versus time of SnO2 PMs sensor upon exposure to (b) 10 ppm NO2, Cl2 and (c) 10ppm SO2, 100ppm ethanol, acetone, formaldehyde, ammonia; (d) selectivity of the sensor measured at 200 °C to different gases

As we all know that Cl2 molecules could easily react with water vapor in the air. Thus, humidity is a considerable parameter for the activity of Cl2 gas sensors. The gas sensing performance for Cl2 in 10 ppm at 10-85% RH was measured in Figure S11a. As Figure S11b shows, the gas response is decreased when the relative humidity is added, particularly above 60%. Hence, the high relative humidity should be avoided for the Cl2 gas sensors. 14


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Figure 6. (a) The response of SnO2 PMs sensor as a function of gas concentrations measured at 200 °C, inset: transient resistance versus concentration; (b) Dual-logarithm of gas response (S) and gas concentration (C) for the SnO2 PMs sensor, inset: the fifth-order polynomial fit of ten data point of base line; (c) the response/recovery time (d) stability of SnO2 PMs sensor exposed to 10 ppm Cl2.

The SnO2 PMs sensor not only exhibits an excellent selectivity but also an ultra-low limit of detection. The gas responses and transient curves of the SnO2 PMs sensor to different concentrations of Cl2 at 200 °C are shown in Figure 6a. The results indicate that the responses increase sharply at the concentration ranged from 0.5 to 10 ppm. It can be seen that the SnO2 PMs sensor reveals the concentration dependence on Cl2 gas in Figure 6b. For the limits of our experimental setup the Cl2 gas detection limit (DL) could not be measured but be extrapolated from the experimental data.48 Generally, the gas response of the semiconducting oxide can be empirically represented as following formula: S = 1 + Ag•(Pg)β

(2)

where Ag is a prefactor, Pg is the gas partial pressure and β is the response exponent 15



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on Pg.49 As Figure 6b shows, the data could be fitted linearly between logarithm gas response (log S) and logarithm gas concentration (log C) for Cl2 gas. According to the IUPAC definition, when the signal-to-noise ratio equals 3, the signal is considered to be valid. Therefore, the theoretical detection limit was calculated using the following equation: 𝑟𝑚𝑠𝑛𝑜𝑖𝑠𝑒

DL(ppb) = 3 𝑠𝑙𝑜𝑝 𝑒

(3)

where rmsmoise is the sensor noise, which can be calculated using the variation in the gas response at baseline using the root-mean-square deviation, and slope is extrapolated from the linear calibration curve as shown in Figure 6b. Finally the detection limit of the SnO2 PMs sensor is calculated to be 33 ppb. The response/recovery times are mainly dependent on the kinetics of gas diffusion and reactions between the measured gas molecules and the surface of the sensor. From Figure 6c, the response and recovery times of the SnO2 PMs sensor are 13 s and 9 s in 10 ppm Cl2 gas at 200 °C, respectively. For the SnO2 PMs sensor, the structure of porous microtubules was favorable to a superior free volume and effortless diffusion of Cl2 gas molecules in and out of the sensing material.50 It is renowned that the stability is also a key parameter for gas sensors. The long-term stability test of SnO2 PMs sensor for 10 ppm Cl2 gases at 200 °C is carried out by six experimental cycles at different time, and the results are presented in Figure 6d. The result reveals that the gas response decreased only about 22% after exposing to air at 200 °C for one month, indicating that SnO2 PMs are a potential candidate material for Cl2 gas sensors. Table S1 shows the comparison with other reported sensing materials for Cl2 gas. It can be seen that the SnO2 PMs sensor in this study reveals the good gas performance to Cl2. The SnO2 PMs sensor shows the well competitiveness in Cl2 sensitivity among all these compounds in the list, which ascribes to the simple and cost-efficient method.

Gas-sensing mechanism. The generally accepted gas sensing mechanism for metal oxide is determined using surface-gas interaction model, which the adsorption 16


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and desorption of target gas molecules on the surface of the sensing material mainly leads to the change of resistance. For SnO2, conduction electrons primarily originate from point defects, such as oxygen vacancies and interstitial metal atoms. To Cl2 gas, the following reactions may occur: 50,51 Cl2 + 2e- ⇋ 2(Cl-)surf Cl2 + 2(VÖ)surf + 2e- ⇋ 2(Cl- O)surf

(4) (5)

Cl2 + 2(O2-)ads ⇋ 2(Cl-)ads + 2O2 + 2e-

(6)

Cl2 + 2(O2-)latt ⇋ 2(Cl-)latt + O2 + 2e-

(7)

where, the subscripts surf, ads and latt denote surface, adsorb and lattice, respectively. For eq 4 and eq 5, Cl2 molecules are directly adsorbed on the surface and oxygen vacancy at the surface ((VÖ)surf) of oxide sensors, respectively. For eq 6 and eq 7, Cl2 interacts with chemisorbed oxygen (mainly (O2-)ads and lattice oxygen (O2-)latt), respectively.52 Because the electrons are captured and released, eq 4 and eq 5 increase the resistance, whereas eq 6 and eq 7 would result in the resistance declined. In our study, the resistance of SnO2 PMs sensors obviously increased in the Cl2 atmosphere. Thus, the gas sensing mechanism is related to eq 4 and eq 5, as Figure 7 shows. For reducing gases (ammonia, acetone, formaldehyde and ethanol), they can react with the adsorbed oxygen53 on the sensor surface, which is an electron-liberating process: 54 O2 + e- → (O- 2)ads (100-300 °C) CH3CH2OH + 3(O- 2)ads → 2CO2 + 3H2O + 3e-

(8) (9)

For oxidizing gases, they are not only absorbed on the surface (eq 4) but also oxygen vacancy (eq 10). Furthermore, the strongly oxidizing gases can be considered to adsorb competitively with oxygen (eq 4 vs eq 8) and provide a trap state at a different energy with respect to the band edges of the material.51 Meanwhile, Cl2 is stronger oxidative than NO, NO2 and SO2, which the electron capturing ability is better. In another word, Cl2 molecules more easily occur eq 4 and eq 5. Therefore, SnO2 PMs exhibits good selectivity to Cl2. Compared with the SnO2 particles, the gas response of the SnO2 PMs sensor was dramatically increased to Cl2. The reason maybe that the porous microtubules structure of SnO2 PMs increases the effective surface area and is in favor of gas 17



diffusion, resulting to the gas adsorption. Additionally, there are a higher concentration of oxygen vacancies in SnO2 PMs than SnO2 particles, which is the result of XPS and PL. As above analysis, the oxygen vacancies could contribute to eq 5 and further made the gas response increased. The gas sensing mechanism of SnO2 particles may be mainly dominated by eq 4. At lower temperature (100 °C) could restrain gas adsorption and increase gas desorption, resulting in a decreased gas response decreased.55 For SnO2 PMs, it obtained the lowest resistance at 200 °C. When the temperature reached 200 °C, charge carriers participate in the electronic conductivity and not necessary to overcome the potential barrier.46,56 Therefore, the SnO2 PMs sensor reaches a maximum response at 200 °C. Above all, the SnO2 PMs sensor using degreasing cotton as biotemplate reveals the excellent gas response and response/recovery time for Cl2. Owing to its simple fabricating process, inexpensiveness and high stability, the present approach demonstrates a potential method for porous microtubules gas sensors synthesis. In addition, the SnO2 PMs sensor is applicable to rapid and accurate detection of Cl2.

Figure 7. Schematic illustration of the sensing mechanism of the SnO2 PMs for Cl2. 18


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CONCLUSIONS In summary, we report a fabrication of sensitive and selective detection of Cl2 gas sensor based on SnO2 porous microtubules structure. By a simple immersioncalcination method, the SnO2 PMs were synthesized with degreasing cotton as biotemplate. The SnO2 PMs sensor exhibits impressive sensitivity (638.3 to 5 ppm), excellent selectivity, fast response/recovery time (13 s/9 s) and low detection limit (33 ppb) to Cl2 at 200 °C. In addition, the sensing mechanism was explicitly deduced in detail. The porous microtubules structure increases the effective surface area and is in favor of gas diffusion, resulting to the gas adsorption. The formation of oxygen vacancies is beneficial from the removing process of absorbent cotton biotemplate. The enhanced sensing properties could be attributed to the porous microtubules structure and abundant oxygen vacancies of SnO2 PMs.

Supporting Information Schematic diagram of the test circuit, XRD patterns, Scherrer Formula, SEM images, XPS

survey

spectra,

PL

spectra,

FT-IR

spectra,

BET

results,

nitrogen

adsorption-desorption isotherms and pore size distribution, transient resistance response, gas response at different humidity, table for comparison with other Cl2 sensors and the analysis about calcined at different temperature.

ACKNOWLEDGMENTS This work is supported by the National Nature Science Foundation (51672220), the National Defense Science Foundation (32102060303), the Xi'an Science and Technology Foundation (2017040CG-CF024, 2017086CGRC049-XBGY005), the Shaanxi Provincial Science Foundation (2017KW-018), the NPU Gaofeng Project (17GH020824) of China, and the Fundamental Research Funds for the Central Universities (31020170QD084). We would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for XRD and SEM experiments. 19



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For Table of Contents Use Only: Abstract Graphic:

Synopsis: The absorbent cotton-biotemplate SnO2 porous microtubules exhibited excellent selectivity and low detection limit to detect chlorine.

26


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A Simple Absorbent Cotton Biotemplate to Fabricate SnO2 Porous

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