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Ultrathin In2O3 nanosheets with uniform mesopores for highly sensitive nitric oxide detection Xue Wang, Juan Su, Hui Chen, Guo-Dong Li, Zhifang Shi, Haifeng Zou, and Xiaoxin Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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ACS Applied Materials & Interfaces

Ultrathin In2O3 nanosheets with uniform mesopores for highly sensitive nitric oxide detection Xue Wang,a,‡ Juan Su,b,‡ Hui Chen,a Guo-Dong Li,a Zhifang Shi,b Haifeng Zou,c,* Xiaoxin Zoua,* a

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin

University, 2699 Qianjin Street, Changchun 130012, China. b

School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China.

c

College of Chemistry, Jilin University, Changchun 130012, China

‡

These authors contributed equally.

* Corresponding author. E-mail address: [email protected] (X. Zou); [email protected] (H. Zou)

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Abstract

Nitric oxide (NOx, including NO and NO2) is one of the most dangerous environmental toxins and pollutants, mainly origins from vehicle exhaust and industrial emission. The development of sensitive NOx gas sensors is quite urgent for human health and environmental friendly. Up to now, it still remains a great challenge to develop NOx gas sensor, which can satisfy multiple application demands for sensing performance (such as high response, low detection temperature and limit). In this work, ultrathin In2O3 nanosheets with uniform mesopores were successfully synthesized through a facile two-step synthetic method. This is a success due to not only the formation of 2D nanosheets with a ultrathin thickness of 3.7 nm based on non-layered compound, but also the template-free construction of uniform mesopores in ultrathin nanosheets. Sensor based on the as-obtained mesoporous In2O3 ultrathin nanosheets exhibits ultra-high NOx response (Rg/Ra = 213 to 10 ppm), short response time (ca. 4 s), and quite low detection limit (10 ppb) under relatively low operating temperature (120 oC), which well satisfy multiple application demands. The excellent sensing performance should be mainly attributed to the unique structural advantages of mesopores and 2D ultrathin nanosheets.

Keywords. In2O3; Ultrathin nanosheets; Mesopores; Nitric oxide; Gas sensing


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Introduction Nitric oxide (denoted as NOx, including NO and NO2) widely exists in human’s living and production, e.g. automotive exhaust, industrial combustion, by-product of industrial synthesis, etc. Notably, NOx is quite environmentally-unfriendly, due to not only its toxicity for human beings, animals and plants, but also its contribution to the photochemical smog and acid rain.1-3 Thus, it is increasingly important and urgent to produce highly sensitive and reliable NOx gas sensor capable of real-time monitoring due to the demand of public health, environmental and industrial safety. Furthermore, the detection of NOx with ultra-low concentration is also required in some biological or medical domain, e.g. diagnose of asthma is based on the ppb-level detection of NO.4,5 Semiconducting metal oxides are the most widely used and investigated gas sensors for years since their electrical conductivity may vary greatly as changing the surrounding gas atmosphere.6-11 Such oxides sensors also possess the advantages of high sensitivity, low cost, good stability, and portability. Up to now, various metal oxides, such as In2O3, ZnO and WO3, have been developed through different synthetic and fabrication methods for NOx gas sensing applications.12-27 It is still important and challenging to develop low-cost and high-performance sensors which can satisfy multiple application demands for NOx gas sensing, including high, fast, and stable response, low detection temperature and limit. Besides introducing expensive noble metals, the gas sensing performance of metal oxides sensor can also be improved by designing special structure/morphology14-21 or interface22-27. Materials with porous structure have good application in gas sensing, due to not only the resulting large surface area, but also the improvement of gas transfer.28,29 Among the achievements of porous sensing materials, porous metal oxide nanosheets have attracted increasing interests due to the further combined structural advantages of two-dimensional (2D) nanosheets, which always contribute to the exposure of particular crystallographic facet or large surface area with amount of active sites.30-32 Nevertheless, it is not easy to construct 2D nanosheets based on non-layered compounds (such as cubic Co3O4 and In2O3), due to their difficulties in bond-cleavage and anisotropic growth towards 2D


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structure.33-36 Therefore, there are more difficulties to construct porous and 2D nanosheet structures simultaneously, and very few successful examples.37-42 Li et al. reported porous ZnO nanosheets with thickness of 20–40 nm for acetone detection.37 Sun et al. studied the gas sensing properties of porous SnO2 hierarchical nanosheets with thickness of ca. 15 nm.38 Dong et al. developed porous NiO nanosheets with micron-sized thickness for the gas sensing of volatile organic compound vapours.39 To our knowledge, there is still no reported gas sensing materials related to metal oxides ultrathin nanosheets (thickness < 5 nm) with uniform mesopores. Although the ultrathin thickness further increase the exposed surface active sites to adsorb target gas, ultrathin nanosheets are more difficult to be constructed due to the high surface energy and thus tend to tack and grow together.33,43,44 Herein, through a facile two-step synthetic method, we successfully developed ultrathin In2O3 nanosheets with uniform mesopores, which exhibit ultra-high response of 213 for the detection of 10 ppm NOx under relatively low temperature (120 oC) in ca. 4 s response time. Furthermore, a quite low detection limit down to 10 ppb and good sensing reproducibility are also realized. Sensor based on the as-obtained mesoporous In2O3 ultrathin nanosheets well satisfy multiple application demands for sensing performance, including high and fast response, low detection temperature and ultra-low detection limit. Both of the mesopores and ultrathin thickness of ca. 3.7 nm contribute to their excellent gas sensing performance towards NOx gas. The synthetic and gas sensing mechanism are also discussed in detail in this work.

Experimental Section Material synthesis. Indium alkoxides (In-Gly) as precursors were first synthesized following our previous work.45 After In(NO3)3•4.5H2O (0.30 g) was dissolved in isopropanol (30 mL), glycerol (10 g) was added in the mixture. Subsequently, they were transferred in a 50 mL Teflon-Lined autoclave and heated at 180 °C for 1 h. After cooling down to room temperature, the white precipitate was separated and washed twice by deionized water and ethanol, and finally dried at 80 °C for 12 h in an oven.


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In-Gly (0.2 g) was dispersed in 30 ml deionized water and transferred to 50 mL Teflon-Lined autoclave. After hydrothermal treatment under 50 °C for 1 h, the white solid was separated and washed by ethanol for three times, and then dried in an oven at 60 oC for 12 h. The as-obtained powder was denoted as In-OH. Subsequently, In-OH solids were calcinated in air at 300, 400, 500 and 600 oC for 2 h, and resulting In2O3-300, In2O3-400, In2O3-500 and In2O3-600 samples, respectively. By contrast, In-Gly was directly calcinated at 400 oC, and the as-prepared sample was denoted as pIn2O3-400. Material characterization. The powder X-ray diffraction (XRD) patterns were performed with a Rigaku D/Max 2550 X-ray diffractometer using CuKα radiation (λ = 1.5418 Å) operated at 200 mA and 50 kV. The transmission electron microscopy (TEM), high-resolution TEM (HRTEM) images and the selected area electron diffraction (SAED) patterns were performed with a Philips-FEI Tecnai G2S-Twin with a field emission gun operating at 200 kV. The scanning electron microscopic (SEM) images were taken on a JEOL JSM 6700F electron microscope. The N2 adsorption–desorption isotherms and pore size distribution were measured by a Micromeritics ASAP 2020M system. The surface area data were calculated based on Brunauer–Emmett–Teller (BET) model. The infrared (IR) spectra were performed with a Bruker IFS 66V/S FTIR spectrometer. The atomic force microscope (AFM) images were taken with a Nanoscope IIIa AFM Multimode camera (Digital Instruments, Santa Barbara, CA) under ambient conditions. Fabrication and measurement of gas sensor. The gas sensor was fabricated by pasting the viscous slurry, a mixture of sample and ethanol, onto a ceramic tube combined with a pair of Au electrodes and four Pt wires on two ends of the tube. A heater of Ni-Cr alloy coil is passing through the ceramic tube. The temperature of sensor surface is adjusted by tuning current. To enhance stability and repeatability, the as-fabricated sensors were aged at 200 °C for 12 h in air. Gas sensing tests were performed on a commercial CGS-8 Gas Sensing Measurement System (Beijing Elite Tech Company Limited) with a test chamber (∼ 1 L in volume). Ambient air with a relative humidity of ∼30% was


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used as reference and diluting gas. After a certain volume of NOx gas was injected into the chamber, gas sensor was put into the chamber, and then purged with air to recover the sensor. The sensor was tested at the temperature range from 60 to 180 °C and the NOx gas concentrations were in the variation from 10 ppb to 10 ppm. The response and recovery time of the sensor are defined as the time taken by the sensor to achieve 90% of the total resistance change. The sensor response was defined as S = Rg/Ra, where Rg and Ra were the sensor resistance in NOx gas and air, respectively.

Results and Discussion Synthesis of mesoporous In2O3 ultrathin nanosheets. The mesoporous In2O3 ultrathin nanosheets were successfully prepared by a two-step synthetic method. Figure 1 presents the schematic illustration for the formation of mesoporous In2O3 ultrathin nanosheets. The whole preparation process includes three steps: i) the solvothermal synthesis of Indium alkoxides (In-Gly) microspheres as precursors according to our previous reported work45; ii) the conversion of In-Gly into mesoporous Indium hydroxide (In-OH) ultrathin nanosheets; iii) the thermal transformation of In-OH into In2O3 mesoporous ultrathin nanosheets. Both of SEM (Figure S1) and TEM (Figure 1e) images revealed that the morphology of In-Gly precursor was uniform microspheres with an average diameter of 450 nm. In addition, In-Gly is a kind of amorphous solid since no characteristic peak was detected in its XRD pattern (Figure 2a). The transformation of In-Gly microspheres into In-OH was performed in a hydrothermal system at 50 oC for 1 h without deliberate additives. As shown in Figure 2b, the C-H stretching bands in the region of 2800 ~ 3000 cm-1 and the C-O stretching bands in the region of 1000 ~ 1400 cm-1 in the IR spectrum of InGly are associated with the groups in glycerol ligands coordinated with In3+ ions.46,47 After 1 h hydrothermal treatment, the C-H and C-O stretching bands decreased dramatically and some of them disappeared entirely in the IR spectrum of In-OH, indicating that most of the glycerol ligands have been removed during the conversion from In-Gly to In-OH. The XRD pattern of the as-prepared In-OH


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indicates the crystalline indium hydroxide with mixture phases: most of the peaks (* in Figure 2a) are assigned to the cubic In(OH)3 (PDF#85-1338), and a minor InO(OH) phase also can be identified (# in Figure 2a)48. From the high-resolution transmission electron microscopy (HRTEM) of In-OH, the lattice spacing of 0.39 nm corresponds to the (200) crystal planes of cubic In(OH)3 (Figure S2). Besides In(OH)3 and InO(OH) reflections, an additional unidentified reflection (marked by & in Figure 2a) was also observed. The above mentioned InO(OH) and unidentified reflections also appear in the previously reported work.48,49 The SEM and TEM images (Figure 1g) show that In-OH consists of uniformly dispersed ultrathin nanosheets, and the results of atomic force microscope (AFM, Figure S3) images indicate that the thickness of In-OH ultrathin nanosheets is ca. 3.5 nm. N2-adsorption measurements (Figure S4) reveal that In-OH ultrathin nanosheets possess porous structure with pore size centered at around 2.6 nm, which mainly comes from the mesopores in nanosheets (insert of Figure 1g). Other pores with larger size should be induced by the interspaces formed by the adjacent In-OH ultrathin nanosheets. The morphology evolvement from In-Gly to In-OH was also investigated. After 0.5 h hydrothermal treatment, some sheet-like subunits were formed on the surface of In-Gly microsphere (Figure 1f). When the reaction time further extended to 1 h, the In-Gly microspheres was totally consumed and converted into uniformly dispersed In-OH ultrathin nanosheets (Figure 1g). Based on the above results, it is presumable that In-Gly was transformed to In-OH through the hydrolysis reaction of In-Gly, a kind of metal alkoxide. Under the hydrothermal treatment at 50 oC, In-OH ultrathin nanosheets were gradually formed on the surface of In-Gly microspheres and subsequently dispersed into the reaction system. It is extremely challenging to form In-OH ultrathin nanosheets, whose main component is cubic-In(OH)3 that lack of intrinsic driving force for 2D anisotropic growth. The successful synthesis of In-OH ultrathin nanosheets in this work may be attributed to the coordination of glycerol around In3+ ions, which may conduce to separate and linearly arrange In3+ ions, and thus contribute to 2D anisotropic growth of In-OH through hydrolysis (Figure S5).33 The fragments of linearly arranged In3+ ions are very likely to be formed in In-Gly precursor: i) three coordinated glycerol tend to ACS Paragon Plus Environment

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homogeneously disperse around a In3+ ion; ii) the as-formed In-glycerol complexes tend to linearly arrange via the effect of hydrogen bonding between coordinated glycerol. The formation of mesopores in nanosheets should be attributed to the removal of glycerol ligands during hydrolysis process according to the results of IR analysis (Figure 2b). After calculation in air at 400 oC, In-OH was transformed to mesoporous In2O3 ultrathin nanosheets labelled as In2O3-400. The IR spectrum of In2O3-400 (Figure 2b) does not contain C-H and C-O stretching bands associated with the groups in glycerol ligands, which have been totally removed during calcination process. The XRD pattern of In2O3-400 (Figure 2a) shows that In-OH is completely transformed to In2O3 with cubic phase (PDF#060416) without any impurity. The lattice spacing of 0.29 nm in HRTEM image corresponds to the (222) crystal planes of cubic In2O3 (Figure 3a). The diffraction rings observed in the SAED pattern (Figure 3b) also agree with the above results. Both of SEM and TEM images (Figure 1h) reveal the ultrathin sheet-like morphology of In2O3-400. The atomic force microscopic (AFM) image and the corresponding height profiles in Figure 3c show the nanosheets’ thickness of ca. 3.7 nm, which is somewhat bigger than that of In-OH nanosheets. It should be noted that the conversion from In-OH to In2O3-400 nanosheets comes with the removal of organic components and water molecule. Therefore, the increased thickness of nanosheets may be attributed to the combination of nanosheets during the calculation. The N2 adsorption–desorption isotherms of In2O3-400 are typical type-IV and an H1-type hysteresis loop, indicating the porous structure in In2O3-400 (Figure 3d). The derived BJH pore-size distribution shows a pore-size distribution centered at around 3 nm (Figure 3e). As shown in Figure 4, the mesopores distributed evenly in nanosheets have uniform pore size around 3 nm, which is in agreement with the results of BJH pore-size distribution. In addition, other pores with larger size should be induced by the interspaces formed by the adjacent In2O3-400 ultrathin nanosheets. The BET surface area of porous In2O3-400 is calculated to be 92 m2/g. The increased pore size of mesopores in In2O3-400 nanosheets compared with that of In-OH should be mainly resulted from the removal of organic components and water molecule and the tightness of particle packing during calcination. ACS Paragon Plus Environment

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In addition, In-OH was also calcinated under 300 oC, 500 oC and 600 oC, resulting the samples denoted as In2O3-300, In2O3-500 and In2O3-600. The XRD patterns (Figure S6) of the as-prepared samples indicate that In2O3-300, In2O3-500 and In2O3-600 samples also possess cubic phase (PDF#060416), and the crystallinity of the samples gradually increased with increasing the calcination temperature. As shown in Figure S7, the SEM images indicate that In2O3-300 and In2O3-500 samples possess the morphology of nanosheets, while the morphology of In2O3-600 sample is agglomerated particles instead of nanosheets due to the excessive calcination temperature. In addition, the results of N2 adsorption–desorption measurement (Figure S8) reveal that In2O3-300, In2O3-500 and In2O3-600 samples possess porous structures, but higher calcination temperature leads to obvious loss of samples’ pores and BET surface area (Table S1), which may induced by the particles’ packing and nanosheets’ aggregation. By contrast, another In2O3 sample labelled as p-In2O3-400 was synthesized by directly calcinating In-Gly at 400 oC. The as-prepared p-In2O3-400 possesses the morphology of microsphere (Figure S7) with a small BET surface area of 23 m2/g (Table S1). In addition, the XRD pattern of pIn2O3-400 also shows a set of characteristic peaks of cubic In2O3 (Figure S6) with much lower intensity compared with that of In2O3-400. Therefore, an additional hydrothermal treatment of In-Gly before calcination, not only realizes the morphology evolvement, but also improves the crystallinity of In2O3 product which is beneficial for gas sensing performance. NOx gas sensing of mesoporous In2O3 ultrathin nanosheets. In2O3 as a kind of important gas sensing material has attracted increasing research interests.14-17,50-56 The as-prepared mesoporous In2O3 ultrathin nanosheets possess the structural features of both mesoporous and 2D ultrathin nanosheets, such as large surface area and rich pore channels, which may be extremely beneficial for their gas sensing application. Therefore, we investigated the sensing performance of the as-prepared materials towards NOx. It should be noted that operating temperature significantly influences the gas sensing performance of metal oxides sensor: under too low temperature, target gas is insufficiently activated to reactive with the surface of metal oxides sensor to realize high and quick response; while under too high ACS Paragon Plus Environment

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temperature, the decrease of sensor’s surface active sites or high desorption ability of target gas would led to low gas sensing response. Figure 5 shows the gas responses of In2O3-300, In2O3-400, In2O3-500, In2O3-600 and p-In2O3-400 towards 10 ppm NOx gas under different operating temperature. The response value is defined as the ratio of Rg/Ra, where Rg and Ra are the electrical resistance of the sensing material in NOx gas and atmospheric air. It reveals that 120 oC should be optimal operating temperature for all the as-prepared sensors, under which In2O3-400 based sensor exhibits the highest response as high as 213. Therefore, 120 oC was selected as operating temperature in the following tests. Figure 6 shows the gas responses and response/recovery time of In2O3-400 during its cyclic exposure to increasing NOx gas concentrations at 120 oC. When In2O3-400 was exposed in NOx gas with certain concentration, its resistance rose rapidly and reached a maximum value in a short time, even at the lowest detecting concentration (10 ppb). Furthermore, the as-prepared In2O3-400 sensor exhibits rapid and reversible response signal to a quite wide concentration range of NOx gas from 10 ppb to 10 ppm. As shown in Figure 6b, the response and recovery time of In2O3-400 were short throughout the exposure concentrations range of NOx gas. Especially in the NOx concentrations range from 2 to 10 ppm, the response/recovery time always remained within 12 s. The highest response value of 213, lowest response time of 4 s and recovery time of 9 s, were obtained after exposure to 10 ppm NOx gas. We also investigated the gas response and the response/recovery time of In2O3-300, In2O3-500, In2O3-600 and p-In2O3-400 during their cyclic exposure to increasing NOx gas concentrations at 120 oC (Figure S9). It is indicated that the above as-prepared sensors exhibit reversible response signal to NOx gas at the concentration range from 10 ppb to 10 ppm. While, their recovery time is very long in some NOx concentrations, such as In2O3-300 in 0.01 and 0.05 ppm NOx, In2O3-500 in 0.1 and 0.5 ppm NOx, and p-In2O3-400 in 0.05 ppm NOx. In addition, a bar graph (Figure 7) was applied for the comparison of response value among all as-fabricated sensors to NOx gas with different concentrations. Mesoporous In2O3 ultrathin nanosheets (In2O3-300, In2O3-400, and In2O3-500) exhibit higher responses than p-In2O3400 and In2O3-600 throughout the exposure concentrations range of NOx gas. In addition, In2O3-400 exhibits the highest response throughout the exposure concentrations range of NOx gas, especially far ACS Paragon Plus Environment

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higher response than those of other as-prepared sensors in the NOx concentrations range from 1 to 10 ppm. As shown in Figure 8, the sensing reproducibility of In2O3-400 can be confirmed by the 15 cycles of response-recovery towards 10 ppm NOx gas at 120 oC, which shows no obvious loss of response value. As shown in Figure 9, a model is used to explain the sensing mechanism of the as-prepared In2O3 ultrathin nanosheets towards NOx gas. The sensing process is closely related to the chemisorption of oxygen or NOx on the surface or pore wall of In2O3, which can be summarized as following: i) When In2O3 nanosheets are exposure to air, oxygen molecules (O2) in air are chemisorbed on the surface or pore wall of In2O3 nanosheets. The free electrons from the conduction band or donor level of In2O3 would be easily trapped by O2 to form chemisorbed oxygen or oxygen ions (O2-/O-), leading to the formation of a depletion layer. The depletion layer gives rise to a potential barrier and thus a highresistance state. ii) When In2O3 nanosheets were exposure to NOx gas, NOx molecules tend to further trap the free electrons from the conduction band or donor level of In2O3 to form NOx-/NOx+1- throughout nanosheets’ surface and pore wall, due to the high electron affinity of NOx molecules. The as-formed NOx-/NOx+1- may exist three forms: NO2- resulted from the chemisorption of NO2 or the reaction of NO and chemisorbed O2-/O-; NO- resulted from the chemical adsorption of NO; NO3- resulted from the reaction of NO/NO2 and chemisorbed O2-/O-. The chemisorbed NOx-/NOx+1- as electron donor decrease the electron density and increase the thickness of depletion regions and the resistance. Based on the above sensing mechanism, the sensing performance of In2O3 sensor depends on surface chemisorption which are most related to its nanostructure. Therefore, the as-prepared mesoporous In2O3 ultrathin nanosheets exhibit excellent sensing performance mainly due to their structure features. Firstly, the transport path is shortened attributed to ultrathin nanosheet structures, which greatly improve the sensitivity of the as-fabricated sensor. Secondly, the uniform mesopores in In2O3 ultrathin nanosheets are beneficial for gas diffusion. Thirdly, both of the mesoporous and 2D ultrathin structures increase the exposed surface active sites for chemisorption of NOx gas, contributing to the ultra-high response. Although In2O3-300 has larger BET surface area than In2O3-400 (Table S1), but relatively poorer ACS Paragon Plus Environment

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crystallinity leads to lower response toward NOx gas (Figure S6). Therefore, among the as-prepared mesoporous In2O3 ultrathin nanosheets, In2O3-400 exhibits the highest sensing response towards NOx gas due to its relatively large surface area and high crystallinity.

Conclusions In summary, we presented the synthesis of mesoporous and ultrathin In2O3 nanosheets via a twostep synthetic method. Benefit from the mesoporous and 2D ultrathin structures, the resulting In2O3 nanosheets exhibited excellent gas sensing performance towards NOx gas. An ultra-high response (Rg/Ra = 213) with short response time (ca. 4 s) was observed towards 10 ppm NOx at a relatively low temperature of 120 oC. Furthermore, the mesoporous In2O3 ultrathin nanosheets were also proved to possess a quite low detection limit (10 ppb) and excellent gas sensing stability towards NOx gas. Such sensing performance well satisfies multiple application demands for NOx detection. This work would provide some insights to develop advanced functional materials by fabricating ultrathin nanosheets with uniform mesopores based on non-layered compounds, and also promote the development and application of reliable NOx gas sensor with excellent performance.

Acknowledgements. X.Z. acknowledges the financial support from the NSFC 21401066, Jilin Province Science and Technology Development Plan 20150520003JH and 20170101141JC, and Science and Technology Research Program of Education Department of Jilin Province [2016] No. 410. H.Z. acknowledges the NSFC 51272085 and 21671078, and the Opening Research Funds Projects of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University (2016-06). J.S. acknowledges the financial support from the NSFC 21403140. G.-D.L, acknowledges the financial support from the NSFC 21371070 and Jilin Province Science and Technology Development Plan 20160101291JC.


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Supporting Information Available. SEM image of In-Gly; HRTEM and AFM images of In-OH; N2 adsorption–desorption isotherms and pore size distribution of In-OH, In2O3-300, In2O3-500, and In2O3-600; schematic representation for the composition of In-Gly precursor; XRD patterns, BET surface area and the response towards 10 ppm NOx at 120 oC of In2O3-300, In2O3-400, In2O3-500, In2O3-600 and p-In2O3-400; SEM images, gas response and the response/recovery time towards NOx at 120 oC of In2O3-300, In2O3-500, In2O3-600 and p-In2O3-400; This material is available free of charge via the Internet at http://pubs.acs.org.


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Figure captions Figure 1. (a-d) Schematic representation for the formation of mesoporous In2O3 ultrathin nanosheets from In-Gly microspheres; (e, f, insert images of g and h) TEM and (g, h) SEM images of the samples corresponding to a, b, c and d, respectively; 0 h, 0.5 h, and 1 h are hydrothermal treatment time. Figure 2. (a) The powder X-ray diffraction (XRD) patterns and (b) infrared (IR) spectra of In-Gly, InOH, and In2O3-400; the region of C-H and C-O stretching bands are marked in yellow. Figure 3. (a) High-resolution transmission electron microscopy (HRTEM) image, (b) selected area electron diffraction (SAED) patterns, (c) atomic force microscope (AFM) images, (d) N2 adsorption– desorption isotherms and (e) pore size distribution of In2O3-400. Figure 4. (a) TEM and (b, c) HRTEM images of In2O3-400. The HRTEM images correspond to the parts marked in (a). In2O3-400 ultrathin nanosheets possess uniform mesopores with pore size centered at around 3 nm. Figure 5. Response comparison of sensors based on In2O3-300, In2O3-400, In2O3-500, In2O3-600 and pIn2O3-400 during exposure to 10 ppm NOx gas under different temperatures. Figure 6. (a) The gas response and (b) response/recovery time of In2O3-400 during its cyclic exposure to increasing NOx gas concentrations at 120 oC. Figure 7. Response comparison of sensors during exposure to NOx gas with different concentrations. Figure 8. 15 cycles of response-recovery of In2O3-400 towards 10 ppm NOx gas at 120 oC. Figure 9. Sensing mechanism of the as-prepared mesoporous In2O3 ultrathin nanosheets towards NOx gas.


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Figure 1.


21


Figure 2.

(b)

25

30

35 40 o 45 2 Theta ( )

50

55

4000

In-O

OH

In-O

C-O

OH

3000

2000

In-O

C-O

OH

C-H

In-Gly OH

*

In-OH

OH

Intensity (a.u.)

(440)

(431) (400)

*

#(211) (420) #(022)

(332)

*& * * *

In-OH

In-Gly 20

In2O3-400

In2O3-400

(222)

(400) (411) (013)

(200)

*

(211)

(222)

(a) Intensity (a.u.) (211) (200)

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-11000

Wavenumber (cm )


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Figure 3


23


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Figure 4


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Figure 5

210

In2O3-300

180

In2O3-400

Response (Rg/Ra)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In2O3-500

150

In2O3-600

120

p-In2O3-400

90 60 30 0

60

90

120 150 o Temperature ( C)

180


25


Figure 6 (a) 10

10ppm

100ppb

0.10

5ppm

50ppb

8

Resistance (MΩ)

0.08

10ppb

0.06

6

2ppm 0.04 0 5 10 15 20 25 30 35 40 45

4

1ppm

2

50ppb

0

10ppb 0

20

500ppb

100ppb 40

60 80 Time (min)

(b)200

100

120

In2O3-400 response recovery

150 Time (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

50

0

2 0.5 0.01 0.05 0.1 1 Concentration (ppm)

5

10


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Figure 7 210 180 Response (Rg/Ra)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In2O3-300

In2O3-600

In2O3-400

p-In2O3-400

In2O3-500

150 2

120 90 1 60 30 0

0

0.01

0.01

0.05

0.05

0.1

0.1 2 0.5 1 Concentration (ppm)

5

10


27


Figure 8

10 ppm NOX

14 12

Resistance(MΩ )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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off

10 8 6 4 2 0

on

-2

0

20

40

60

80

100 120 140 160 180 200 220 Time (min)


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Figure 9


29


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