High sensitivity and ultra-low detection limit of chlorine gas sensor based on In2O3 nanosheets by a simple template method

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High sensitivity and ultra-low detection limit of chlorine gas sensor based on In2O3 nanosheets by a simple template method Jiangwei Maa, Huiqing Fana,b,c,*, Weiming Zhanga, Jianan Suia, Chao Wanga, Mingchang Zhanga, Nan Zhaoa, Arun Kumar Yadava, Weijia Wanga,*, Wenqiang Dongb, Shuren Wangc a 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 b Institute of Culture and Heritage, Northwestern Polytechnical University, Xi’an, 710072, China c International Joint Research Laboratory of Henan Province for Underground Space, Development and Disaster Prevention, Henan Polytechnic University, Jiaozuo 454003, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Melamine-formaldehyde Sponge In2O3nanosheets Gas sensor Chorine

Chlorine (Cl2) gas sensors with outstanding gas response and ultra-low detection limit by an easy and economical fabrication are increasingly in demand in practical application. In this work, In2O3 nanosheets with excellent gas sensing properties for Cl2 have been successfully synthesized using melamine-formaldehyde (MF) sponge as the template via a simple soak-drying-calcination approach. The gas response (Rgas/Rair) of In2O3 nanosheets sensor is up to 2353.4 toward 3 ppm Cl2 at 200 °C, which is about 158 times compared with In2O3 particles (14.9). The selectivity to Cl2 reaches up to 254.3 against other interferential gases. Additionally, the sensor based on In2O3 nanosheets displays extremely low detection limit of 0.037 ppb. Such excellent gas sensing performance is primarily ascribed to the unique sheet structure and abundant oxygen vacancies which are formed during the removal of MF sponge template. Therefore, the In2O3 nanosheets sensor shows great potential in Cl2 detection.

1. Introduction Chlorine (Cl2) with strong irritating odor is closely associated with our lives, such as in disinfectants, medicines, plastics, and so on. In industrial production, the real-time monitoring Cl2 is quite necessary because of its harmful nature. [1] Cl2 gas can quickly react with water and generate corrosive acid. Even dilute Cl2 would cause ocular discomfort to human. Inhalation of Cl2 also results in severe damage to the respiratory system due to the formation of chlorhydric acid. Cl2 with concentration higher than 1000 ppm (ppm) becomes fatal to human body. [2] Furthermore, Cl2 is one of the major elementary substance that destroys the ozone layer. Thus, Cl2 gas sensors with sensitive response and ultra-low detection limit should be developed and applied in safe handling, environment monitoring and leakage detection. [3,4] There are many techniques to detect Cl2, for example, gas chromatography [5], optical gas sensor [6] and impedance gas sensors [7,8]. Compared with other technologies, the economical impedance gas sensors, especially the impedance-semiconductor sensors with good stability have attracted much attention. Based on ZnO [9,10], WO3 [11,12], SnO2 [13,14], In2O3 [15,16] and their derivatives [17,18], Cl2 sensors are widely researched in various methods with diverse

⁎

morphologies. However, it is still a challenge to fabricate Cl2 sensors with outstanding sensitivity and low detection limit (ppb level) by a simple method. For example, Li et al. developed mesoporous In2O3 structures via a hydrothermal process and the gas response was merely 12 towards 5 ppm Cl2, indicating low sensitivity. [19] Dang fabricated SnO2 nanowire sensor with excellent responses to Cl2 (57 to 50 ppb), while the performed chemical vapor deposition (CVD) process is highly costly. [12] Navale et al. prepared ZnO nanoparticles by a solid-state strategy. However, the sensitivity towards 5 ppm Cl2 is just 3. [10] Generally, the template method as a simple and cost-efficient strategy is frequently used to prepare gas sensors. For instance, hollow SnO2 spheres were fabricated by carbon sphere templated way and showed high sensitivity to trimethylamine and ethanol. [20] Hollow NiO hemispheres were constructed using polymeric colloidal as the template and were used as high-performance ethanol gas sensors [21]. In2O3 microtubules sensor for HCHO detection was reported using cotton fibers as templates. [22] However, there are few researches about Cl2 sensors with excellent gas response and low detection limit via template method. In this work, we fabricated In2O3 nanosheets using melamine-formaldehyde (MF) sponge as the template through a simple soak-drying-

Corresponding author. E-mail addresses: [email protected] (H. Fan), [email protected] (W. Wang).

https://doi.org/10.1016/j.snb.2019.127456 Received 6 September 2019; Received in revised form 6 November 2019; Accepted 21 November 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Jiangwei Ma, et al., Sensors & Actuators: B. Chemical, https://doi.org/10.1016/j.snb.2019.127456

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Fig. 1. (a) XRD patterns of In2O3 particles and nanosheets; (b) Rietveld refinement for the XRD patterns of In2O3 nanosheets, inset of (b) crystal structures of In2O3 (cubic bixbyite-type structure); XPS high-resolution spectra of O 1s in In2O3 particles (c) and In2O3 nanosheets (d).

2. Experimental section

determination of the phase structure. The scanning electron microscopy (SEM) was carried out using a FEI Helios G4 CX microscope for the morphologies of samples. Transmission electron microscopy (TEM) images were recorded with a FEI Talos F200X microscope. X-ray photoelectron spectroscopy (XPS) were analyzed by a Kratos Axis Supra Spectrometer. The electron paramagnetic resonance (EPR) spectra were employed by Bruker A300 spectrometer. The specific surface area with nitrogen adsorption-desorption was measured on a Micromeritics Corp TriStar II 3020 analyzer by Brunauer-Emmett-Teller (BET) method. The pore size and size distribution were calculated using the Barrett-JoynerHalenda (BJH) method. Fourier transform infrared (FT-IR) spectra were measured by Bruker’s VERTEX-70 Series spectrometer.

2.1. Synthesis

2.3. Fabrication of gas sensors

Indium (Ⅲ) nitrate hydrate (In(NO3)3·4.5H2O, AR, ≥ 99.5 %, Sinopharm Chemical Reagent Co., Shanghai) was used directly. Commercial melamine-formaldehyde (MF) sponge was provided by Peiyou Building Materials Co. Ltd, Shanghai. Before use, it was ultrasonically cleaned in the absolute ethanol for 30 min and was dried at 80 °C. In(NO3)3·4.5H2O (1 g) was added in 160 mL of distilled water to obtain a stable and clear solution. Then the MF sponge (2.4 g) was soaked in the solution for 3 h. Then MF sponge was brought out and placed in vacuum at 80 °C overnight. Finally, In2O3 nanosheets were obtained by calcined the resultant In(OH)3 nanosheets at 650 °C for 3 h at a ramping rate of 3 °C·min−1. In2O3 particles, as a comparison, were collected by direct calcination In(NO3)3·4.5H2O (1 g) at the same condition.

To fabricate the sensors, the mixture of the obtained In2O3 samples with moderate amount of terpineol was coated on a commercial Al2O3 tube with Pt wires and Au electrodes. The operational temperature of the gas sensor is controlled by a Ni-Cr alloy coil that crosses the tube. The sensors were desiccated by the infrared radiation for 10 min and were annealed at 300 °C for 2 h. Finally, the sensors required to age on the TS-60 platform for 7 d to achieve their stabilization. The gas-sensing property was performed on a gas sensing equipment (WS-30A, Weisheng Ltd., Zhengzhou, China) with a chamber of 18 L under a static condition. The relative humidity (RH) was adjusted by water vapor and was recorded by a commercially available hygrometer. The testing of gas sensing property is similar to our previous study [13]. The gas response (S) was identified as the resistance ratio (Ra/Rg for reducing gases, Rg/Ra for oxidizing gases), where Ra and Rg represent the sensor resistance in air and in tested gas, respectively. For evaluation of the sensor to distinguish the target gas, the selectivity (K) could be introduced as St/Si, where St and Si are the gas response of the target gas and the maximum response of the interference gases, respectively. [24] The response/recovery times were determined as the time to

calcination process. MF sponge with high porosity and absorption capacity is a perfect template and the imino groups on the surface provide abundant adsorption sites for cations. [23] In the calcination, the MF sponge would decompose and provide an inert atmosphere, in favor for the formation of oxygen vacancies. Compared with In2O3 particles synthesized without template, the gas response of In2O3 nanosheets is improved about 158 times toward 3 ppm Cl2 at 200 °C. As expected, the In2O3 nanosheets prepared under the current facile process display excellent sensitivity, high selectivity and low detection limit (0.037 ppb).

2.2. Characterization X-ray powder diffraction (XRD) was determined by a Shimadzu 7000 diffractometer equipped with Cu Kα radiation (λ =1.5406 Å) for 2

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introduced to observe the elemental distribution. As Fig. 3d shows, it is obvious that In and O element are uniformly dispersed on the sheetstructure. To further verify the completely removal of templates, FT-IR investigation was performed on the as-prepared In2O3 nanosheets. The prominent peaks of MF sponge are shown in Fig. 4a. The broad absorption peak around at 3394 cm−1 is ascribable to the stretching of the secondary amine (NeH). [23] The peak at 2985 cm−1 is assigned to CeH bonds (aliphatic) bending vibrations. [29] The peak at 1692 cm−1 is due to the bending vibration of NeH bonds. The bands at 1542 cm−1 is attributed to the stretching vibration of ring (C]N). The strong peaks at 1467 cm−1, 1328 cm−1, and weak peak at 980 cm−1 correspond to the CeH bending. [30] The peak around at 1143 cm−1 is ascribed to the CeO stretching of aliphatic. The peak at 813 cm−1 is from triazine ring bending. [29] Furthermore, no characteristic peaks of MF sponge show in the FT-IR spectra of In2O3 nanosheets, which indicates the MF sponge is totally removed after calcination. Fig. 4b represents room temperature EPR spectra of In2O3 samples. Both In2O3 particles and In2O3 nanosheets display a single Lorentzian line (g = 2.003), which is attributed to singly ionized oxygen vacancies in the In2O3 structure. [31,32] It can be seen that In2O3 nanosheets exhibit much higher intensity than In2O3 particles, indicating In2O3 nanosheets possess more oxygen vacancies. The result of EPR spectra is in accordance with the analysis of XPS. To investigate the internal micro-structures, nitrogen adsorptiondesorption measurement was adopted. Fig. 4c and d show the corresponding isotherms, pore size distributions and accumulated pore volume. The isotherms of the as-prepared samples display type V with an H3 loop by the IUPAC classification. [33] According to the BJH method, the average pore size and accumulated pore volume of the In2O3 particles are 87 Å and 0.025 cm3/g, while that of In2O3 nanosheets are 145 Å and 0.105 cm3/g (inset of Fig. 4c and d). It is generally known that the obtained mesoporous architectures facilitate mass transfer, which is favorable to gas diffusion. Moreover, the BET surface areas of In2O3 particles and nanosheets are 5.4 and 22.7 m2/g, respectively. Evidently, the In2O3 nanosheets possess much higher BET surface area, which is advantageous to adsorb more gases and then enhance gas sensing properties.

achieve 90 % of the total gas response. For each sensor, every test was repeated thrice at same conditions and the average value was used in the final results. 3. Results and discussion 3.1. Phase and morphology The phase structures of the obtained In2O3 samples are characterized by XRD in Fig. 1a. Obviously, all diffraction peaks are perfectly indexed to cubic In2O3 (JCPDS NO.061416) and no impure phases are observed. XRD Rietveld refinements for the In2O3 samples were performed using the Fullprof software package. Fig. S1 and 1b demonstrate the refinement plots of typical In2O3 particles and nanosheets, respectively. The crystal structure of In2O3 samples is a bixbyite-type structure with space group Ia-3 (206). As Fig. 1b shows, it is composed of two types of In and one type of O atoms. [25] The In atoms are located at two distinct polyhedral sites (the 8b and 24d sites), which are surrounded by six equidistant oxide neighbors and trigonal prismatic Inoxygen distances, respectively. The O ions occupy Wyckoff positions at 48e sites and are coordinated with four In3+ cations. [26] Table S1 exhibits the parameters about refined equilibrium lattice and position. The grain size is calculated by the Scherrer formula as follow,

D=

0.89γ β cosθ

(1)

where D is the average grain size, γ is the wavelength of the Cu Kα radiation, θ is the maximum angle and β is the half-peak width of diffraction peak at 2θ. The mean crystallite size of In2O3 particles and In2O3 nanosheets is 11.2 and 14.6 nm, respectively. To collect detailed information about the chemical states and oxygen species, the prepared samples were tested by XPS measurements. The survey spectrum in Fig. S2a and b shows that In2O3 particles and nanosheets consist of In and O elements, while C element located at 284.6 eV is adventitious to calibrate. No any other elements are observed, which indicates their high purity and is in agreement with the result of XRD. In Fig. S2c and d, the high-resolution XPS spectra of In 3d display two symmetric peaks at about 444.2 and 451.6 eV, which are associated with the In 3d3/2 and In 3d5/2. [27] In Fig. 1c and d, the correspondingly magnified O 1s spectra reveal the existence of three types of oxygen species. The intense peak at 529.6 eV is corresponded to the lattice oxygen (Olat), the moderate peak at around 531.2 eV is associated with the oxygen vacancies (Ovac), and whereas the tiny peak centred at 532.1 eV represents chemisorbed oxygen species (Oche). [28] Evidently, the relative percentages of the Ovac component of In2O3 nanosheets is higher than In2O3 particles (Table S2), indicating abundant oxygen vacancies in In2O3 nanosheets. Fig. 2a shows the In2O3 particles are randomly aggregated. As shown in Fig. 2b, the MF sponge is an intrinsic three-dimensional (3D) porous crosslinked structure. After adsorbed on the surface of MF sponge, In3+ cations are seeded and then In(OH)3 is generated after drying. As the inset of Fig. 2b shows, the morphology of In(OH)3 on the surface of MF sponge is flake-like. From Fig. 2c, it can be noticed that the sheet-like morphology of In(OH)3 can be reserved in In2O3. A closer observation in Fig. 2d displays the thickness of the In2O3 nanosheets was about 38 nm. The surface is rough and composed of abundant nanoparticles. In Fig. 3a, the TEM image of In2O3 nanosheets obviously shows the surface consisted of numerous nanoparticles, which is conformed to the SEM observation. The high-resolution TEM (HRTEM) image displays the clear fringes (0.253 nm, 0.292 nm and 0.413 nm), which are well matched with the lattice spacing of In2O3 ((400), (222) and (211)) (Fig. 3b). The diffraction rings observed in selected-area electron diffraction pattern (SAED) can be indexed as (211), (222), (400), (332), (431), (440) and (622) planes of cubic phase In2O3 (Fig. 3c). Moreover, the energy-dispersive X-ray spectroscopy (EDS) elemental mapping was

3.2. Formation mechanism The preparation process of In2O3 nanosheets is illustrated in Fig. 5. First, MF sponge was soaked into the indium nitrate solution. During the immersion, In3+ cations were adsorbed on the surface of MF sponge because of its high adsorption capacity and abundant amine groups for electrostatic forces. [23] Next, the MF sponge-In3+ was taken out and put into the oven. In the process of drying, In3+ cations were seeded and then In(OH)3 nanosheets were slowly generated. More importantly, the MF sponge-In(OH)3 was calcined at suitable rate and temperature. In the calcination, the In2O3 nanosheets were obtained and maintained the sheet morphology of In(OH)3. 3.3. Gas-sensing properties To investigate the gas sensing performances of the obtained In2O3 samples, Cl2, a typical harmful gas, is employed as the target gas. As is well-known, the operating temperature could significantly influence the gas-sensing properties. Firstly, the gas responses of In2O3 particles and In2O3 nanosheets sensors were examined at 100–300 °C to survey the optimum working temperature, as Fig. 6 shows. It’s obvious that the sensors based on In2O3 samples show a similar volcano-shaped framework between sensitivity and working temperature. By GaussAmp fitting, it indicates the optimal operating temperatures both are 200 °C in Fig. 6b and d. The explanation of this correlation is introduced as follows. Below 100 °C, the adsorption is dominant effect on the gas sensing properties, while the process of desorption gradually becomes the 3

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Fig. 2. SEM images of (a) In2O3 particles; (b) In(OH)3 on the surface of MF sponge; and (c) and (d) In2O3 nanosheets.

temperature in the forthcoming measurements. It’s worth noting that the gas responses of In2O3 nanosheets sensor (2353.4) is about 158 times higher than that of In2O3 particles sensor (14.9) toward 3 ppm Cl2 at 200 °C. The selectivity is another important metric in practical gas detection. [37] According to the literature research, the sensors based on In2O3 show good sensing properties to ethanol (CH3CH2OH) [38,39], formaldehyde (HCHO) [40,41], acetone (CH3COCH3) [42,43], nitrogen

important factor. [34,35] In other words, as the temperature increases from 100 to 200 °C, chlorine molecules are more and more energetic to adsorb on the surface of materials, which improves gas-sensing properties. Beyond 200 °C, the gas adsorption barrier increases at the high temperatures and the chlorine desorption is encouraged, resulting in reduction of the gas response [36]. Obviously, the maximum response of the In2O3 particles and In2O3 nanosheets sensors to 3 ppm Cl2 is obtained at 200 °C, which is used as the optimized operating

Fig. 3. In2O3 nanosheets: (a) TEM image; (d) HRTEM image; (c) SAED pattern; and (d) EDX elemental mapping of In (red), O (yellow). 4

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Fig. 4. (a) FT-IR spectra of MF sponge, In2O3 particles and In2O3 nanosheets; (b) EPR spectra; and nitrogen gas adsorption-desorption isotherms of (c) In2O3 particles and (d) In2O3 nanosheets, and the insert is the pore size distribution and accumulated pore volume.

for Cl2. As shown in Fig. 7d, the sensors based on In2O3 samples display high sensitivity to Cl2 and the selectivity of In2O3 nanosheets sensor is much superior to that of In2O3 particles sensors at 200 °C. Subsequently, the sensors based on In2O3 samples exposed to Cl2 with various concentrations at 200 °C are tested (Fig. 8a and Fig. S4). Evidently, the gas response rapidly increases as the concentration increasing from 0.5 ppm to 5 ppm. Additionally, it is noticeable that the In2O3 nanosheets sensor shows an obvious gas response (2.24) upon exposure to a lower concentration (0.5 ppm Cl2), which illustrates the In2O3 nanosheets sensor could be employed in the practical applications. Fig. 8b reveals the concentration dependent sensitivity of In2O3 nanosheets sensor. The detection limit (DL) is unable to directly measure because of the limitation of experimental equipment and only extrapolated according to the results. [46] In general, the gas response of semiconducting oxide sensor is usually empirically expressed as:

dioxide (NO2) [44,45], and so on. Besides, in the practical application, the interference gases to Cl2 is mainly hydrogen (H2) because H2 is byproduct in the Cl2 production via the electrolysis of sodium chloride. Thus, the gas responses of In2O3 samples sensors to various gases are investigated. The detected gases include 100 ppm CH3CH2OH, HCHO, CH3COCH3, H2, methanol (CH3OH), ammonia (NH3) and 10 ppm Cl2 and NO2. Fig. 7a and c represent the gas-response characteristics of In2O3 particles and In2O3 nanosheets sensors exposed to diverse detected gases at 200 °C, respectively. Moreover, the sensitivity of In2O3 nanosheets sensor toward various target gases at different working temperature is exhibited in Fig. 7b. It can be noticed that the sensitivity to Cl2 is supreme compared with the other gases at each operating temperature. The selectivity of In2O3 samples sensors is displayed in Fig. S3. At 200 °C, K(SCl2/SNH3) is 13.3 and 2306.9 for In2O3 particles and In2O3 nanosheets, respectively. Furthermore, the inset of Fig. S3b shows the selectivity of In2O3 nanosheets sensor at different working temperature and the maximum value (254.3) is reached at 200 °C, which means the In2O3 nanosheets sensor has an outstanding selectivity

S = 1 + Ag (Pg)β

Fig. 5. Schematic illustration of the formation process of In2O3 nanosheets. 5

(1)

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Fig. 6. Gas response transient curves and the sensitivity of (a, b) In2O3 particles sensor and (c, d) In2O3 nanosheets sensor toward 3 ppm of Cl2 at various working temperature. Fig. 7. Real-time response of (a) In2O3 nanosheets sensor and (c) In2O3 particles sensor exposed to 10 ppm of NO2, Cl2 and 100 ppm hydrogen, ammonia, acetone, methanol, ethanol and formaldehyde at 200 °C; (b) the sensitivity of In2O3 nanosheets sensor to various gases at different temperature; (d) radar chart of gas responses of In2O3 particles (red lines) and In2O3 nanosheets (black lines) sensors to various gases at 200 °C.

Fig. 8b shows the linear equation is a good fit to Cl2, indicating its reliability in actual applications. On the basis of the IUPAC definition, the signal is valid if the ratio of signal-to-noise is equal to 3. Thus, the theoretical detection limit could be calculated as: [49]

where Ag is the prefactor, Pg is partial pressure of the measured gas and is directly proportional to the concentration, and β is the exponent. [47] Therefore, after logarithm based on formula (1), logarithm gas response (log S) presents linear relation with logarithm gas concentration (log C). [48] Log S = Ag + β Log (C)

DL (ppb) = 3 (2)

rmsnoise k

(3)

where k is the slop of the linear calibration curve of sensitivity (S) vs. 6

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Fig. 8. In2O3 nanosheets sensor at 200 °C: (a) The gas response at the concentration range from 0.5 to 5 ppm, inset is the transient response; (b) dual-logarithm of gas response (S) and gas concentration (C), inset is ten data points of experimental data; (c) the response/ recovery time; (d) the sensitivity in various relative humidity and at different time toward 3 ppm of Cl2.

of Cl2 gas compared with other In2O3 sensors. Ahn et al. indicated that oxygen vacancies are active sites to preferentially adsorb oxidizing gas. [52] In our work, the good sensing performance of In2O3 nanosheets sensor may be ascribed to the abundant oxygen vacancies and high surface area. From XPS and PL analysis, the use of MF sponge-templated is benefit to generation of oxygen vacancies, which is different from other sensors. Furthermore, the In2O3 nanosheets sensor is more competitive for each evaluation criterion, which is attributed to the economical and simple fabrication.

low gas concentration (ppb), and rmsnoise is the sensor noise and is calculated by the root-mean-square deviation on baseline of Fig. 8a. We took ten data points in the base gas response unexposed to Cl2.

rmsnoise =

∑ (y −yi )2 N

(4)

where y, yi, N is the baseline data point, the average and the number of data points, respectively. [50] Therefore, the DL value of the In2O3 particles and In2O3 nanosheets sensors is estimated to be about 7 ppb and 0.037 ppb, respectively. Fig. 8c exhibits the response/recovery curves of In2O3 nanosheets sensor toward 3 ppm Cl2 at the optimal working temperature of 200 °C. The response and recovery times are 53 and 17 s, respectively. In general, the high concentration of gases requires the short response time. [51] Thus, the response time (53 s) is reasonable to the low concentration of Cl2 (3 ppm). The relatively fast response and recovery can be applied for the real-time chlorine monitoring. Furthermore, it is well-known that Cl2 gas is soluble and can easily react with water. Therefore, the Cl2 detection in different relative humidity is significant. The relative responses at laboratory conditions (25 % RH) to high humidity (82 % RH) with 3 ppm Cl2 is shown in Fig. 8d. It can be found that the sensitivity dramatically decreased, especially at the range 50 % RH. As factors for gas sensors, stability and service life are required to be investigated. In stability test, the sensors were placed on the aging platform and were kept at the optimal working temperature (200 °C) to simulate the practical applications. Fig. 8d displays the long-term stability test of In2O3 nanosheets sensor exposed to 3 ppm Cl2 for 28 days. Obviously, the gas response decreases dramatically after the first 5d. However, the In2O3 nanosheets sensor exhibits excellent stability from the 5th to the 28th day, during which the sensitivity could be maintained at about 520 to 3 ppm Cl2. This gas response is totally practical in real-time detection. [2] To assess performance of In2O3 nanosheets sensor, a comparison with other reported In2O3 sensors-based to 5 ppm Cl2 is summarized in Table S3. The assessment criteria are optimum operating temperature (T), sensitivity, method complexity, and cost. As we can see, the In2O3 nanosheets sensor displays excellent response to the same concertation

3.4. Gas-sensing mechanism It is widely acceptable that the gas-sensing mechanism of semiconducting oxide is the depletion theory. [53] For the reducing gases, the dsorption-oxidation-desorption processes can be explained the resistance change [54]. In air, oxygen is adsorbed on the material’s surfaces, which captures electrons to form different oxygen species (O− 2 , O− and O2−) relying on the operating temperature. [55] It results in the electron depletion layers on the material’s surface. When the sensor introduced to reductive gases, the negatively charged oxygen could oxidize these gas molecules, during which release electrons to the conducting region of semiconducting oxide. For example, the gas-sensing mechanism for CH3OH at 200 °C (mainly O− 2 ) is: O2gas ↔ O2ads O2ads + e ↔ 2CH3OH +

(4) O− 2 ads

3O− 2

(5)

↔ 2CO2 + 4H2O + 3e

−

(6)

where the subscripts gas and ads refer to free gas and adsorption, respectively. For Cl2 (oxidizing gas), however, the oxygen species could be oxidized by Cl2 owing to the stronger oxidizability of Cl2. Therefore, the reactions may be as follow: − − Cl2 + 4O− 2 ads ↔ 2Cl ads + 4O2 + 2e

Cl2 + 7

2O2−latt

↔

2Cl−latt

+ O2 + 2e

−

(7) (8)

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Fig. 9. Schematic illustration of sensing mechanism of the In2O3 nanosheets for reducing and oxidizing gases.

sensor with a typical sheet structure by an easy soak-drying-calcination method with MF sponge as the template. The formation mechanism of the sheet structure mainly is attributed to MF sponge’s high absorption capacity and abundant amine groups for electrostatic forces. In the gas sensing measurement, a comprehensive analysis revealed that the sensor based on In2O3 nanosheets shows significantly enhanced sensitivity (2353.4) compared with the In2O3 particles (14.9) to 3 ppm Cl2 at 200 °C. What’s more, the In2O3 nanosheets sensor exhibits low detection limit and excellent selectivity for Cl2. The formation of more active oxygen vacancies and larger BET surface area from special morphology were mainly responsible for the improved sensing performances. Hence, we believe that the soak-drying-calcination strategy could be promising to enhance the gas sensing performance of other semiconducting oxide-based sensors.

where the subscript latt indicates lattice. At high temperature, the lattice oxygen may be involved and reaction (8) occurs. But the declining resistance in reaction (7) and (8) is inconsistent with the result of the experiment, while the resistance increases after exposure to Cl2. In other words, reaction (7) and (8) are not primary to the gas-sensing mechanism of Cl2. On the basis of the oxidizability, Cl2 could be competitive with the oxygen to be directly adsorbed on the surface. In addition, it could also be adsorbed on oxygen vacancies of the material’s surface. The reactions may be that: Cl2 + 2e− ↔ 2Cl−surf Cl2 + 2Ovac + 2e− ↔ 2Cl− O

(9) (10)

where the subscript surf means surface. In reaction (9) and (10), the electrons are captured from the conducting region and the resistance increases. Therefore, the sensing mechanism of oxidizing gases is mainly dominated by reaction (9) and (10). [56,57] Thus, the sensing mechanism of the In2O3 nanosheets to reducing and oxidizing gases is displayed in Fig. 9. The oxidizing gases could directly absorb not just on the surface (reaction (9)) but also the oxygen vacancy (reaction (10)), while the reducing gases only react with the pre-absorbed oxygen species (reaction (6)). It could be considered as the direct adsorption of Cl2 is competitive with that of oxygen (reaction (5) vs reaction (9)) for providing a trap state at the different energy on the band edges of the material. [56] Hence, the response of oxidizing gases is higher than of reducing gases, which is consistent with the experimental results. In addition, the strong electron-attracting ability of Cl2 is beneficial to reaction (9) and reaction (10). Therefore, the In2O3 nanosheets sensor displays high selectivity for the detection of Cl2. In addition, the sensing materials with controllable nanostructure and high surface are in favor of enhanced gas sensing properties. [58–61] Compared with In2O3 particles, the large surface area, abundant oxygen vacancies and nanostructurization of In2O3 nanosheets enhance prominently the capability to react with the target gases and improve the gas sensing properties. This is dominating reason that the gas responses of In2O3 nanosheets sensor are superior to the In2O3 particles.

Declaration of Competing Interest None. Acknowledgments This work has been supported by the NPU Fundamental Research Funds for the Central Universities (3102019GHXM002 and 3102019MS0401), the National Nature Science Foundation (51672220, 51902259 and 51902258), the SKLSP Project (2019-TZ-04) of China, the National Defense Science Foundation (32102060303), and the HPU Open-end Fund of International Joint Research Laboratory of Henan Province for Underground Space Development and Disaster Prevention. We would also like to thank the Analytical & Testing Center of Northwestern Polytechnical University for SEM, XPS and TEM test. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.127456. References [1] C. Winder, The toxicology of chlorine, Environ. Res. 85 (2001) 105–114. [2] C.W. White, J.G. Martin, Chlorine gas inhalation: human clinical evidence of toxicity and experience in animal models, Proc. Am. Thorac. Soc. 7 (2010) 257–263. [3] T. Miyata, H. Tomohiro, M. Tadatsugu, High sensitivity chlorine gas sensors using

4. Conclusions In summary, we successfully fabricated the ultrasensitive Cl2 gas 8

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Vinu, Recent progress on the sensing of pathogenic bacteria using advanced nanostructures, Bull. Chem. Soc. Jpn. 92 (2018) 216–244. [61] Z. Li, H. Li, Z. Wu, M. Wang, J. Luo, H. Torun, P. Hu, C. Yang, M. Grundmann, X. Liu, Y. Fu, Advances in designs and mechanisms of semiconducting metal oxide nanostructures for high-precision gas sensors operated at room temperature, Mater. Horiz. 6 (2019) 470–506. Jiangwei Ma is currently a Ph.D. candidate under the supervision of Prof. Huiqing Fan at the school of materials science and engineering, Northwestern Polytechnical University. Her research interests include the synthesis of functional materials and their applications in gas sensors. Huiqing Fan is a professor now at the school of materials science and engineering, Northwestern Polytechnical University, China. He is interested in the ceramic materials for dielectric and piezoelectric applications as well as nanocrystalline materials for gas

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J. Ma, et al.

Nan Zhao is currently a Ph.D. candidate under the supervision of Prof. Huiqing Fan at the school of materials science and engineering, Northwestern Polytechnical University. His research interests include the synthesis of nanomaterials and their applications in electrochemical energy storages.

sensors, photo catalysts and thin-layer devices Weiming Zhang received his B.S. degree from Xi'an University of Technology in 2017, and he is now an M.S. candidate at Northwestern Polytechnical University. His current research focuses on gas sensors.

Arun Kumar Yadav is currently a postdoctoral research scholar under the supervision of Prof. Huiqing Fan at the school of materials science and engineering, Northwestern Polytechnical University, China. His research interests include the synthesis of the novel lead-free ceramic materials for the high energy storage density, actuators and sensors applications.

Jianan Sui received her B.S. degree from Northeast Forestry University in 2016, and she is now an M.S. candidate at Northwestern Polytechnical University under the supervision of Prof. Huiqing Fan. Her current research focuses on lead-free electromechanical ceramic materials.

Weijia Wang obtained her Ph.D. at the department of physik, Technische Universität München. Her research interests are solar cells and photocatalysis.

Chao Wang received his B.S. degree from Northwestern Polytechnical University in 2016, and he is now an M.S. candidate at Northwestern Polytechnical University under the supervision of Prof. Huiqing Fan. His current research focuses on nanostructured materials for photocatalysis.

Wenqiang Dong is a professor at institute of culture and heritage, Northwestern Polytechnical University, China.

Mingchang Zhang is currently a Ph.D. candidate under the supervision of Prof. Huiqing Fan at the school of materials science and engineering, Northwestern Polytechnical University. His research interests include the synthesis of nanomaterials and their applications in electrochemical energy storages.

Shuren Wang is a professor at international joint research laboratory of Henan province for underground space development and disaster prevention, Henan Polytechnic University, China.

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