Highly Efficient Gas Sensor Using a Hollow SnO2 Microfiber for

Letter pubs.acs.org/acssensors

Highly Efficient Gas Sensor Using a Hollow SnO2 Microfiber for Triethylamine Detection Yihui Zou,†,# Shuai Chen,‡,# Jin Sun,† Jingquan Liu,† Yanke Che,∥ Xianghong Liu,§ Jun Zhang,*,§ and Dongjiang Yang*,†,⊥ †

Collaborative Innovation Centre for Marine Biomass Fibers, Materials and Textiles of Shandong Province, School of Environmental Science and Engineering, Qingdao University, Qingdao 266071, P. R. China ‡ State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China § College of Physics, Qingdao University, Qingdao 266071, P. R. China ∥ Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Beijing 100190, P. R China ⊥ Key Laboratory of Coal Science and Technology of the Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, P. R. China S Supporting Information *

ABSTRACT: Triethylamine (TEA) gas sensors having excellent response and selectivity are in great demand to monitor the real environment. In this work, we have successfully prepared a hollow SnO2 microfiber by a unique sustainable biomass conversion strategy and shown that the microfiber can be used in a high-performance gas sensor. The sensor based on the hollow SnO2 microfiber shows a quick response/recovery toward triethylamine. The response of the hollow SnO2 microfiber is up to 49.5 when the concentration of TEA gas is 100 ppm. The limit of detection is as low as 2 ppm. Furthermore, the sensor has a relatively low optimal operation temperature of 270 °C, which is lower than those of many other reported sensors. The excellent sensing properties are largely attributed to the high sensitivity provided by SnO2 and the good permeability and conductivity of the one-dimensional hollow structure. Thus, the hollow SnO2 microfiber using sustainable biomass as a template is a significant strategy for a unique TEA gas sensor. KEYWORDS: alginate, hollow SnO2 microfiber, triethylamine gas sensor, high selectivity, fast response/recovery

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SnO2 is rather complicated and requires a high cost, which limits its practical application. One-dimensional (1D) structures have an enhanced surfaceto-volume ratio, which facilitates the rapid and efficient adsorption of gas molecules.16 Meanwhile, 1D carbon materials can improve the conductivity of the sensor materials. The carbon could also favor the chemical stability of the materials, which would ensure remarkable sensing properties over a wide temperature range. In addition, a hollow micro/nanostructure could provide high permeability of the TEA gas, allowing easy diffusion of gas molecules in the sensing layer.17 Thus, it is expected that 1D hollow SnO2 microfibers combining the merits of the 1D morphology and hollow structure could exhibit high performance in TEA detection. In this work, we synthesized a hollow SnO2 microfiber using alginate (sustainable seaweed) fiber as a template. The Sn atoms were introduced by a facile ion-exchange process in

riethylamine (TEA) sensors have drawn much attention for wide application in chemical industry. TEA is explosive and can cause health problems such as head discomfort and lung problems if the concentration is higher than 10 ppm in air.1 Traditional methods for TEA detection include a gas tube detection method and gel chromatography. However, their further application is hindered by the high cost and tedious testing procedure.2,3 Metal oxide semiconductors such as ZnO,4 SnO2,5 In2O3,6 and Fe2O37 are increasingly in demand for gas detection with low power consumption and fast response. In particular, gas sensors based on SnO2 exhibit high response to various noxious gases, such as NO,8 NO2,9 CO,10 H2S,11 and volatile harmful vapor.12 Nevertheless, gas sensors for TEA detection based on SnO2 have been reported very little. Wang et al.13 have synthesized SnO2 nanorods for use as TEA gas sensors, which showed a response of 30 to 1000 ppm TEA at 250 °C. However, their sensing response is not positive and needs further improvement. Recently, Ju et al.14 used NiO/SnO2 hollow spheres for TEA detection with a response of 48.6 to 10 ppm TEA at 220 °C. However, the loading process of NiO on © XXXX American Chemical Society

Received: April 25, 2017 Accepted: June 27, 2017 Published: June 27, 2017 A

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Figure 1. Scheme illustrating the procedure for the fabrication of the hollow SnO2 microfiber from sustainable alginate fiber.

which the M2+ cations were fixed in a novel “egg box” with coordination by α-L-guluronate (G) blocks, which are negatively charged in the alginate macromolecule.18,19 When the Sn−alginate fibers were heated in air, the metal ions tended to aggregate on the surface of the fiber, and heterogeneous contraction of the fiber caused by the nonequilibrium heat treatment and carbon combustion led to the formation of a hollow structure. As expected, the response of the hollow SnO2 microfiber can reach 49.5 when the concentration of TEA gas is 100 ppm. As shown in Figure 1a, we first extracted sodium alginate from seaweed. We then used the wet-spinning method to prepare calcium alginate microfibers, which were immersed in 1 M HCl aqueous solution in order to achieve thorough Ca2+/H+ ion exchange. The obtained protonated alginate fibers were immersed into the SnCl2 solutions to form Sn−alginate fibers (Sn-AFs). As shown in Figure S1, the diffraction peak at 2θ = 20.89° was ascribed to immobilization of the Sn2+ cations through coordination with four G-blocks, which is known as the “egg box”.20 The Sn-AFs were then calcined at 500 °C in air to obtain the hollow SnO2 microfibers. As presented in Figure 1b, at the initial stage, a few pores appeared in the inner solid core as a result of dehydration in the fiber-forming material.21 As the temperature increased, the carbonaceous component in the inner core was consumed completely, and some Sn2+ migrate to the surface of the fiber and are oxidized to SnO2. Then the hollow SnO2 microfiber comes into being. Figure 2a displays the powder X-ray diffraction (PXRD) pattern corresponding to the hollow SnO2 microfiber. The XRD pattern is indexed finely with the standard card (PDF 97003-9173), and it also displays that there are no impurities in

Figure 2. (a) XRD pattern for the hollow SnO2 microfiber. (b−d) High-resolution (b) C 1s, (c) Sn 3d, and (d) O 1s XPS spectra of the hollow SnO2 microfiber.

the hollow SnO2 microfiber, indicating that Sn-AF can be completely transformed into the SnO2 microfiber with excellent crystallinity and purity.22,23 To further investigate the composition and chemical state of the atoms in the samples, X-ray photoelectron spectroscopy (XPS) was performed. The XPS spectrum proves that there are tin, oxygen, and carbon in the hollow SnO2 microfiber (in Figure 2b,d). Figure S2 shows the XPS survey spectrum of the SnO2 microfiber, which B

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200 and 300 °C, abundant pores were produced along the whole fiber, since most of the organic moieties of the alginate fiber precursors decomposed during the pyrolysis process. Complete combustion of alginate-derived carbonaceous material led to the hollow structure of the SnO2 microfiber at 500 °C. This hollow SnO2 microfiber displayed a large Brunauer− Emmett−Teller (BET) specific surface area of 31.59 m2g−1 (Figure S7a). The average Barret−Joyner−Halenda (BJH) pore diameter results demonstrate that the pores in the samples were mostly distributed around 9.61 nm (Figure S7b). The abundant loose interspaces in the microfibers provide multiple effective channels for the diffusion of TEA gas and create more passageways for the interchange of electrons in the surface reaction. The representative transmission electron microscopy (TEM) images show that the SnO2 microfibers are composed of NPs with diameters of about 10 nm (Figure 3c). The highresolution TEM (HRTEM) images (Figure 3d) present two values of the d spacing, 0.236 and 0.167 nm, corresponding to (200) and (222) lattice fringes, respectively, according to the standard card (PDF 97-003-9173). To analyze the composition of the hollow SnO2 microfiber, we performed energy-dispersive X-ray spectroscopy (EDS) and obtained the elemental mapping. As shown in Figure 3e−h, O, Sn, and C are distributed on a single fiber uniformly. The content of carbon was estimated by thermogravimetric analysis (TGA), as shown in Figure S8. The SnO2 microfibers showed a weight loss of 4.1%, which corresponds to the carbon content existing in the samples. The sensing properties of hollow SnO2 microfiber sensors toward 100 ppm TEA gas were tested over a wide range of temperatures from 160 to 380 °C (Figure 4a). The highest response was 49.5 when the temperature was 270 °C, which is defined as the optimal working temperature for the SnO2 microfiber sensor. The increase in response with increasing temperature is ascribed to the fact that the activation energy barrier between TEA and the adsorbed oxygen species such as O2−, O−, and O2− ions becomes higher.14 Afterward, the response gradually decreases as the temperature increases further. This is mainly attributed to the dynamic balance of TEA adsorption and desorption on the sensors. Under the same equilibrium pressure, the equilibrium adsorption capacity decreases with increasing temperature.27 Figure 4b shows the response curve of the hollow SnO2 microfiber at varying concentrations of TEA gas between 2 and 1000 ppm at 270 °C. The sensitivity of the SnO2 microfiber sensor reached 1.01 when the TEA concentration was 2 ppm and then increased as the TEA concentration gradually increased. The response of the sensor was 84.7 for a TEA concentration of 1000 ppm. The hollow SnO2 microfiber sensor displayed very fast response and recovery sensing characteristics toward TEA, as shown in Figure 4c. The response and recovery times were 14 and 12 s, respectively, which are much shorter than those of other reported TEA gas sensors based on metal oxides (see Table S1). The response increased with increasing TEA concentration and reached 84.3 at a concentration of 1000 ppm. Furthermore, the sensor also showed a distinct response to TEA concentrations as low as 2 ppm, indicating that the hollow SnO2 microfiber has a low detection limit at the parts per million level for TEA detection. The stability of the hollow SnO2 microfiber sensor over 1200 s was also measured, and the results are shown in Figure 4d.

displays the presence of Sn, O, and C without any other peaks of impurities or byproducts. Figure 2b shows the highresolution C 1s spectra. The higher binding energy shown at 284.8 eV is ascribed to C−C, while the binding energy appearing at 285.8 eV corresponds to C−OH and C−O−C, indicating the existence of epoxide, hydroxyl, and carboxyl functionalities.24 For Sn, the binding energies at 486.5 and 495.0 eV exhibit two symmetric peaks, corresponding to Sn 3d5/2 and Sn 3d3/2, as shown in Figure 2c. The energy gap between the two symmetric peaks is 8.5 eV, which is in agreement with the standard values.25 The O 1s spectrum is shown in Figure 2d. Deconvolution gives a peak at 530.8 eV associated with O2− ions within the SnO2 lattice and a peak at 532.0 eV corresponding to O2− ions in the SnO2 matrix’s oxygen-deficient area.26 Therefore, the nanoparticles (NPs) distributed on the microfiber are SnO2. The morphology of the SnO2 microfibers was examined using field-emission scanning electron microscopy (FESEM), and the results are presented in Figure 3a,b. The SnO2 samples

Figure 3. (a) SEM image of hollow SnO2 microfibers in overall appearance. (b) Cross-section SEM image of a single hollow SnO2 microfiber. (c) TEM image of a typical fragment of a hollow SnO2 microfiber. (d) HRTEM image of the hollow SnO2 microfiber. (e) SEM image of a hollow SnO2 microfiber. (f−h) EDS maps of (f) O, (g) Sn, and (h) C in the microfiber shown in (e).

exhibited typical 1D hollow fibrous morphology with a diameter of ∼8 μm (Figure 3a). The thickness of the shell layer of the hollow SnO2 microfiber was about 500 nm (Figure S3). The space in the hollow channel can effectively shorten average path length for gas transport. The diameter of the original Ca-AFs was ∼9 μm (Figure S4), similar to that of ionexchanged Sn-AFs (∼9 μm; Figure S5). However, the morphology changed a lot when the microfibers were annealed in air. As shown in Figure S6, the SnO2 microfiber at 100 °C displayed a few pores compared with the inner solid core of SnAF shown in Figure S5, which are due to dehydration of the fiber-forming material. When the heating temperature rose to C

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Figure 5. Schematic of the hollow SnO2 microfiber gas sensing mechanism. (a) Oxygen is adsorbed on the surface of the SnO2 NPs and ionized to O−. (b) Gas is transported in the 1D hollow microfiber. (c) The trapped electrons are released back to the SnO2 NPs’ conduction band.

Figure 4. (a) Response of the hollow SnO2 microfiber sensor to 100 ppm TEA gas at different working temperatures. (b) Response to different concentrations of TEA gas at 270 °C. (c) Response/recovery curve of the hollow SnO2 microfiber sensor to 100 ppm TEA at 270 °C. (d) Stability of the hollow SnO2 microfiber sensor to 100 ppm TEA at 270 °C. (e) Gas sensing selectivity of the hollow SnO2 microfiber sensor to different gases at 270 °C at a concentration of 100 ppm.

hollow SnO2 microfiber comes into contact with air, oxygen can be adsorbed onto the surface of the SnO2 NPs and capture the superfluous free electrons in the SnO2’s conduction band, causing the oxygen to be ionized to oxygen species such as O2−, O2−, and O−.29 Also, the electron depletion layer forms on the material’s surface as a result of the adsorption of negatively charged oxygen, as shown in Figure 5a. Moreover, it is considered that grain boundaries and grain junctions can inevitably react during the sensor response process and be called the active sites.34 The use of alginate fiber as the template can immobilize the Sn2+ ions into the “egg box”, and thus, the ion diffusion rate may be decreased during the initial process of annealing, preventing the particles from growing. As the temperature increases, the “egg box” structure is converted to a metal/carbon core/shell structure. The carbon shell can still efficiently prevent further aggregation of the SnO2 grains.24 Thus, we get smaller SnO2 NPs and more grain boundaries as the active sites. Meanwhile, on the basis of the effect of the particle size on n-type semiconducting gas sensors, when D < 2L (where D is the diameter of the sensor particle and L is the thickness of the depletion layer), the grains’ whole surface tends to become the depletion region, and then the conductance could be grain-controlled.35 The results of previous reports demonstrated that L is about 3 nm for SnO2 sensors.36 The crystallite diameter of the hollow SnO2 microfiber is distributed around 7.07 nm, as determined using the Scherrer formula (see the Supporting Information), indicating that the SnO2 crystallites in the hollow microfiber are depleted completely. This means that a majority of the Sn atoms are active and can participate in the gas response, offering more reaction points for oxygen species and TEA gas. Figure 5b shows the advantage of the 1D hollow structure, which ensures efficient diffusion

Obviously, the sensor response to 100 ppm TEA was almost unchanged, indicating good stability of the sensor. The selectivity of gas sensing is another significant factor in gas detection applications. Figure 4e shows the gas sensing response of the hollow SnO2 microfiber to different detected gases at 270 °C at the same concentration of 100 ppm. Obviously, the sensitivity to TEA gas is much higher compared with the response to the other five gases: almost 15 times higher than that of p-xylene, 7 times that of acetone, 4 times that of benzene and isopropanol, and 3 times that of ethanol, indicating excellent selectivity for TEA. Table S1 lists the gas sensing properties of various sensors for TEA gas detection reported in the recent literature. The TEA gas sensor based on the hollow SnO2 microfiber in this work shows better sensing sensibility, a lower working temperature, and much quicker response/recovery compared with most of the reported TEA gas sensors.14,28−33 The superior sensing properties are due to the unique 1D hollow structure, which can offer efficient channels for target gas diffusion.15 Meanwhile, the hollow SnO2 microfiber gas sensor exhibits a much lower working temperature. This could prevent the nanocrystallite growth and improve the durability of the sensor. That may favor this gas sensor’s practical application. In a word, the excellent gas sensing properties of hollow SnO2 microfiber make it a promising material for use in TEA sensors. On the basis of above results, the mechanism of hollow SnO2 microfiber gas sensing is summarized in Figure 5. When the D

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channels for the target gas. Then the conduction band of the SnO2 NPs regains the trapped electrons, making the resistance decrease. Meanwhile, the carbon could offer high electrical conductivity and enhance the electronic interaction between the SnO2 NPs25 and promote a better interconnection between the gas and the SnO2 NPs, which avoids the decrease of the response,37 as shown in Figure 5c. In summary, we have demonstrated a sustainable biomass process for making a TEA sensor with a typical hollow microfibrous structure. The particular egg-box structure efficiently immobilizes Sn2+ cations during the subsequent calcination process to synthesize the hollow SnO2 microfiber. The hollow SnO2 microfiber delivers a higher response than most of the other reported TEA sensors, relatively high stability, and outstanding selectivity for TEA gas. The high sensitivity based on SnO2 and the excellent permeability attached to the unique structural characteristics such as the 1D carbon morphology and porous hollow structure synergistically contribute to the outstanding response to TEA gas. This work will be a sensible approach for the fabrication of SnO2 gas sensors and highlights the potential of hollow SnO2 microfibers for future use in the gas sensor field.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.7b00276. Preparation of the hollow SnO 2 microfiber and fabrication of the microfiber-based gas sensor; XRD patterns of AF and Sn-AF; full-survey XPS spectrum of the hollow SnO2 micofiber; SEM images of Ca-AFs, SnAFs, and Sn-AFs after annealing at different temperatures; nitrogen adsorption−desorption isotherm, pore size distribution curve, and TGA curve for the hollow SnO2 microfiber; summary of sensing performances of various gas sensors for TEA (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jingquan Liu: 0000-0001-6178-8661 Yanke Che: 0000-0002-9671-3704 Dongjiang Yang: 0000-0002-9365-3726 Author Contributions #

Y.Z. and S.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (5147308, 51672143, and 21601098), the Shandong Provincial Science Foundation (ZR2015BQ006 and ZR2016EMB07), the Shenzhen Peacock Plan (No. 1208040050847074), and the Qingdao Applied Fundamental Research Project (16-5-1-92-jch and 17-1-1-81jch). E

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