A Density Gradient Strategy for Preparation of Broken In2O3

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Functional Inorganic Materials and Devices

A Density Gradient Strategy for Preparation of Broken In2O3 Microtubes with Remarkably Selective Detection of Triethylamine Vapor Wei Yang, Liang Feng, Saihuan He, Lingyue Liu, and Shantang Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09375 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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ACS Applied Materials & Interfaces

A Density Gradient Strategy for Preparation of Broken In2O3 Microtubes with Remarkably Selective Detection of Triethylamine Vapor Wei Yang,† Liang Feng,‡ Saihuan He,† Lingyue Liu,† Shantang Liu*,† †

Key Lab for Green Chemical Process (Ministry of Education), School of Chemistry and

Environmental Engineering, Wuhan Institute of Technology, Wuhan 430205, P. R. China. ‡

Key Lab of Separation Science for Analytical Chemistry, Dalian Institute of Chemical Physics,

Chinese Academy of Sciences (CAS), Dalian 116023, P. R. China. *Corresponding author: (E-mail) [email protected].

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ABSTRACT: Tubule-like structured metal oxides, combined with macroscale pores onto their surfaces, can fast facilitate gas-accessible diffusion into the sensing channels, thus leading a promoted utilization ratio of sensing layers. However, it generally remains a challenge for developing a reliable approach to prepare them. Herein, this contribution describes a density gradient strategy for obtaining broken In2O3 microtubes from the In2O3 products prepared using a chemical conversion method. These In2O3 microtubes hold a diameter about 1.5 µm with many broken regions and massive ultra-fine nanopores onto their surfaces. When employed as a sensing element for detection of triethylamine (TEA) vapor, these broken In2O3 microtubes exhibited a significant response toward TEA at 1-100 ppm, and the lowest detected concentration can reach 0.1 ppm. In addition, an excellent selectivity of the sensor to TEA was also displayed, though upon exposure of other interfering vapors, including ammonia, methanol, ethanol, isopropanol, acetone, toluene, and hydrogen. Such promoted sensing performances toward TEA were ascribed to the broken configuration (superior gas permeability, high utilization ratio), 1D configuration with less agglomerations, and low bond energy for C-N in TEA molecule.

KEYWORDS: macroscale pores; In2O3 microtubes; sensing element; triethylamine vapor; significant response; excellent selectivity

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1. INTRODUCTION Based on the fundamental understanding of gas-sensing mechanism for most metal oxides, high surface area and wide porosity are advantageous in achieving high sensing response: 1,2 (i) high surface area can accelerate the oxygen adsorption and surface reactions between chemical molecules with chemisorbed oxygen; (ii) open porosity is favorable for gas-accessible diffusion and promoting utilization ratio of sensing layers. The performance of this gas sensor is mainly associated with the metal oxides’ morphologies,3 unlike optical sensor,4 colorimetric sensor,5 and magnetic relaxation switching (MRS) sensor.6 In numerous kinds of morphology-distinct metal oxides, the tubule-like micro/nanostructures have attracted enormous interest and been demonstrated to be promising sensing materials in detection of various gas species (including ammonia,7 hydrogen sulfide,8,9 hydrogen,10 ethanol,11,12 formaldehyde,13,14 and nitrogen oxides15), due to their large surface-to-volume ratios and excellent gas permeability.3,8,13 Up to now, many strategies have been extensively employed to synthesize oxides with tubular architectures, including

template-assisted

approaches

(eg.

carbon

nanotube,7

reactive-template,11

biotemplates,13 nanoporous alumina,16 etc), and electrospinning method.8-10,14,15,17,18 Particularly, compared with those regular micro/nanotubes, the broken tubule-like structures may possess stronger gas permeation behavior and create more reactive sites for gas adsorption, thus leading an advance in gas-sensing properties.7,10 Unfortunately, it is still an extreme difficulty to obtain broken configuration in tubule-like metal oxides using the above-mentioned approaches. Vapor-reaction methods can synthesize some interesting and unconventional 1D tube-like micro/nanostructures with macroporous geometry in certain circumstances.19,20 However, these desired tubule-like structures in the as-obtained products are impurity accompanied with the other morphologies. It is a complicated task to obtain uniform tubular morphology through optimizing synthesis parameters. In addition, these tube-like and other morphologies in the as3



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prepared products are all identical chemical compounds from the aspects of chemical composition and crystal structures. Therefore, it is difficult to separate them by morphology using a chemical method. To investigate the inherent property of the single morphology, it is necessary to separate them. Fortunately, the densities of these distinct morphologies may be different.21,22 In this regard, the density gradient separation in the liquid phase through centrifugation will be expected to be an effective method to obtain single or pure morphology. In diverse kinds of semiconducting metal oxides, In2O3, can act as an important gas-sensitive materials because of the low electrical resistance, favorable stability and suitability to prepare various morphologies using different indium-derived precursors.23-26 Although considerable efforts have been dedicated to synthesize tubular In2O3 microstructures and investigate their sensing properties,7-9,13,14 seldom works so far are tried to prepare tubule-like In2O3 morphology with broken configuration. Triethylamine (TEA), with a volatile, toxic, inflammable and explosive properties, is always applied as an important raw materials in the industrial production. In addition, the decay of fishes and seashells can also given off gaseous TEA after death, and the TEA content is elevated as the prolonged decay stage.27 Thus, it is necessary for development of a highly selective and sensitive sensor for detecting trace TEA in industry process, fishery field and surrounding circumstances. In this paper, we develop a chemical conversion and subsequent centrifugation for preparation of the broken In2O3 microtubes. Furthermore, a sensing platform has been established using these broken In2O3 microtubes for detection of triethylamine (TEA) vapor. The sensor revealed a obvious response toward TEA with a detection concentration low to 0.1 ppm. Furthermore, an excellent selectivity toward TEA vapor for the sensor can be also achieved, though exposed in the other interfering gases (including ammonia, methanol, ethanol, isopropanol, acetone, toluene 4


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and hydrogen). This present work may provide an effective avenue for obtaining single or desired configuration in metal oxides-based sensing materials.

2. EXPERIMENTAL SECTION 2.1. Materials. Raw In2O3 material was purchased from Alfa Aesar with a metal basis of 99.99%. Liquid chemicals (including methanol, ethanol, isopropanol, acetone, toluene, and triethylamine) were obtained from Sinopharm Chemical Reagent Co. Ltd, China. High pure ammonia and hydrogen (99.9%) were acquired from Tianlong gas Co, Wuhan, China. All these reagents used in our experiment were analytical grade without further purification. 2.2. Preparation for Broken In2O3 Microtubes. The broken In2O3 microtubes were prepared through a chemical conversion and a subsequent centrifugation separation method according to the previous literature.28,29 In a detailed synthesis, 1 g of raw In2O3 material was thoroughly dispersed in distilled water to obtain a slurry. Afterwards, the slurry was covered onto a quartz boat (length ~5 cm, width ~2.8 cm, and thickness ~0.2 cm) and subsequent dried at 80 oC in an oven. Then, this quartz boat was transferred to the central heating zone of a tube furnace (Taisite, SRJX-2-13, Tianjin, China). Before this reaction, the quartz tube was washed using pure NH3 for 30 min with a flux of 100 mL/min to eliminate air in the reaction atmosphere. Subsequently, the reaction system was heated from room temperature to 690 °C in 30 min accompanied by a flow of 150 mL/min pure NH3 at normal pressure, and hold at this temperature for about 5 hours. A large amount of brown and fluffy products (InN) were harvested after naturally cooled down (Figure S1). The faint yellow products (In2O3) were obtained via heating InN in the quartz boat at 610 °C for another 4 hours under air atmosphere with a flux of 150 sccm. The broken In2O3 microtubes were obtained using a centrifugation separation strategy as described as following. Firstly, the as-prepared In2O3 was uniformly dispersed in ethylene glycol (EG) by ultrasonic 5



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treatment for 30 min. After optimized centrifugation speed and time (Figure S2), the supernatant (In2O3 nanowires) and the precipitates (In2O3 microtubes) were separately collected using a microsyringe under 1500 rpm for 5 min. Finally, the two In2O3 products (microtubes and nanowires) were obtained by washing with ethanol for several times and then dried at 250 °C to remove residual organic solvent. The separation process of the two In2O3 products from the asprepared In2O3 products was schematically described in Figure 1. 2.3. Characterizations. XRD (X-ray diffraction) pattern was achieved on a diffractometer (Rigaku D/Max 2500) with a Cu Kα1 radiation source (0.15406 nm). The 2θ range was scanned at 20~80° with a slope of 5 °/min. A field-emission SEM (Nova NanoSEM 450, FEI) was applied to identify the microstructures under an accelerating voltage ~3 kV. In TEM measurement, the suspension consisted of dispersed product in ethanol via ultrasonication, was then dropped onto a copper grid. The TEM profiles were performed at a HR-TEM instrument (Technai G2 Spirit, FEI) with an accelerating voltage ~300 kV. The surface areas were tested on a surface area analyzer (Micromeritics ASAP 2020 V3.00H) and calculated using Brunauer– Emmett–Teller (BET) equation. Prior to this test, the products were all degassed under vacuum at 250 oC for 6 h to remove possible absorbed water and other organic vapor onto their surfaces. XPS (X-ray photoelectron spectroscopy) technique was adopted on the spectrometer analyzer (ESCALAB250, Thermo VG Co, USA) with a exciting radiation of Al Kα (1486.6 eV). 2.4. Fabrication and Test of the Sensor. The fabrication process for the gas sensor have been achieved according to our previous work.30 Firstly, a base was required for fixing a commercial ceramic tube (length ~4 mm, external diameter ~1.2 mm, and internal diameter ~0.8 mm) with a couple of gold electrodes and platinum lead-wires encolsed onto its each sides. Then, an indirect-heated sensing platform was formed via inserting a heater strip (Ni-Cr alloy) through the ceramic tube. Afterwards, a suspension was prepared by uniformly 6


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dispersing the In2O3 products in absolute ethanol. Each 5 µL of the suspension was coated on the surface of ceramic tube to form sensing channel between the two Au electrodes. The layer thickness of sensing body was varied by the suspension volume deposited onto the ceramic tube. For sensing test, these as-fabricated sensors were performed in a static method on a WS-30A test apparatus (Winsen electronics technology Co., Zhengzhou, China). In a typical measurement, the sensor was firstly linked in series with a fixed resistor (RF=100 kΩ) in a circuit with a constant voltage (Vc=5 V) (Figure S3). The voltage of RF was monitored as the output signal (Vout) in our test process. The sensor can work at various temperatures by changing heating voltage (Vh). Previous to sensing test, the sensors were all aged at a certain operating temperature for 24 hours in fresh air to improve their stability. Then, a desired amount of pure TEA liquid was injected onto the heater in testing chamber to completely form a vapor. When Vout achieved equilibrium statement, the tested gas was released from the testing chamber. The concentrations of the gases in testing chamber were all approximately calculated using a distribution method.31 The sensor response (S) was defined as the Ra/Rg ratio (Ra: sensor resistance in fresh air; Rg: sensor resistance in gas analytes), which can be converted from the measured Vout. The definition of the response and recovery times was Vout to complete 0.9-fold of the saturation in adsorption and desorption processes. In measurement process, the atmosphere in the test chamber was showed at room temperature (25 oC) and 50% RH, and constantly detected by thermometer and moisture tester in WS-30A system.

3. RESULTS and DISCUSSION 3.1. Microstructure Properties. To check the information about crystal structure and composition phase of the as-obtained In2O3 product, the XRD technique was achieved. Figure 2 illustrated XRD profiles of the In2O3 product before centrifugation separation (consisted of In2O3 microtubes and nanowires). The all detected peaks, such as (222), (400), (440) 7



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and (622) with high intensity, can be indexed to a body-centered cubic phase In2O3 structure (JCPDS card, no. 06-0416). Furthermore, it was not clearly observed the diffraction peaks associated with the other indium-derived substances (including metallic In and InN). This result indicated the acquisition of pure In2O3 product, or the possible impurity possessed a extreme concentration below the limit of detection in XRD instrument. As a proof of concept, the In2O3 microtubes and nanowires shared the identical composition phase and crystal structure. The morphologies and microstructures of the In2O3 product before and after centrifugation separation were also observed by FESEM, as showed in Figure 3. From this SEM image (Figure 3a) for the In2O3 materials before centrifugation treatment, it was clearly seen that the In2O3 materials were mainly consisted of two kinds of distinct morphologies (microtubes and nanowires). Significantly, these microtubes were mixed with the nanowires at a nonuniform distribution, thus leading a great challenge for studying the inherent sensing properties of single In2O3 microtubes. The specific information about In2O3 morphology was further exhibited in Figure 3b using an magnified SEM image. From this image, these In2O3 microtubes possessed broken geometry, even split into some sheet-like architectures. Notably, the nanowires in the mixture were all very short with about several micrometers in length. Especically, the length of some nanowires were only several hundreds of nanometers. After centrifugation separation, the In2O3 microtubes were obtained, as showed in Figure 3(c-d). From the image in Figure 3c, almost no nanowires could be obviously observed, declaring a relatively high purity of In2O3 microtubes. Surface characteristic of these In2O3 microtubes was further checked by an high-magnification SEM image in Figure 3d. From this figure, it was clearly observed that these microtubes hold many broken proportion, thus generating plenty of pores (as indicated by red arrows), which were favorable for gas-specific diffusion 8


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and create more active sites for molecular adsorption. Furthermore, the SEM image (inset in Figure 3d) showed some individual In2O3 microtubes were about 1.5 µm in diameter. Additionally, the microtube surfaces were not very smooth with bulges of In2O3 crystals. The broken mechanism of the In2O3 microtubes was interesting and explained herein. Due to these In2O3 microtubes derived from the InN precursor, thus we were focused on the hole formation in the InN microtubes. Certainly, the shape and location for these holes would be not obviously varied during the chemical conversion process from InN to In2O3. Observed from Figure 3d, the major holes formed at the creases of two walls, not at one flat wall. This appearance was similar with the previous reports.19,20 In general, these dislocation areas will exhibit a higher surface area. As known to us all, the regions are less stable when possess a higher surface area, and the defect will be formed at dislocations, which was reported for group III nitrides.32 It is probably inclined to decomposition for these microtubes with these structural defects at such a high temperature (690 oC in our case), thus resulting in many holes onto their surfaces.19 To obtain further structural imformation of the In2O3 microtubes, TEM technique was adopted. Figure 4a presented a typical TEM image of a representative In2O3 microtubes. It could be obviously seen that a highly dense and ultrafine nanopores were distributed onto the microtube surfaces (displayed using red rings). A magnified TEM observation was further demonstrated these nanopores with around several nanometers in size, as revealed in Figure 4b. For TEM image at a higher resolution (Figure 4c), the clearly resolved lattice fringe of the In2O3 microtubes was measured to about ~0.29 nm, and related to the (222) planes in cubic phase In2O3, which was the preferred orientation for most vapor-synthesized In2O3 materials due to its lower surface energy.33,34 Furthermore, the In2O3 microtubes possessed a

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polycrystalline, which was proved by a series of concentric rings in the relevant selected area electron diffraction (SAED) pattern, as inserted in Figure 4d. To investigate the BET surface area and porosity of the In2O3 products, the N2 adsorptiondesorption isotherms and pore distribution were analyzed (Figure 5). The surface area for In2O3 microtubes and nanowires were calculated to be 58.05 and 34.14 m2/g, respectively. It was apparent that the In2O3 microtubs possessed a higher BET surface area, which could offer more gas adsorption sites and was favorable for enhancement in sensing performances. And the In2O3 microtubs also exhibited more higher BET surface area than that of conventional In2O3 nanotubes (~34.9 m2/g) prepared using a electrospun technique.8 Furthermore, the pore size distribution of these two In2O3 products was also presented (inset of Figure 5). For the In2O3 microtubes, the pore ranged around 1.4~19 nm, and 42.6~89.6 nm, which were attributed to the pores distributed onto the microtube surfaces. While for the In2O3 nanowires, a wide range was centered at ~13 nm implied by the size distribution curve, which may be corresponding to the small pores onto In2O3 nanowires. To further obtain surface composition and chemical states of the as-obtained In2O3 products, element analysis was conducted using an XPS technique, as shown in Figure 6. The presence of In, O and C in the In2O3 microtubes was declared by the XPS survey in Figure 6a. The peaks for other element can not be detected, verifying a highly pure In2O3 microtubes, which were also demonstrated by our XRD results. Furthermore, in Figure 6b, two strong peaks with binding energy at 452.1 eV (In 3d3/2) and 444.6 eV (In 3d5/2) were presented in the In 3d XPS spectra of In2O3 microtubes. The separation degree between two peaks was 7.5 eV, which implied that In3+ was principal state for In element in this product.35 Figure 6(c-d) separately displayed the O 1s spectrum of the In2O3 microtubes and nanwires. The O 1s can be deconvoluted as three peaks in various chemical states: OOH, OV, and OL.36 OOH located at 10


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around 532 eV, corresponding to the hydroxyl ions onto the surfaces. OV was corresponded to the oxygen vacancy nearly at 531 eV. OL standed for the lattice oxygen at approximate 530 eV. For the In2O3 material, it was believed that OOH and OL possessed no contribution to the sensing response, whereas OV had great influence on gas-sensing property. The more content of OV, the more oxygen can be absorbed onto the material surface. It can be obviously observed that the content of OV for the In2O3 microtubes was more higher than that of the In2O3 nanowires, which was responsible for enhancement in sensing performces of the In2O3 microtubes. 3.2. Triethylamine (TEA) Sensing Properties. As a chemiresistor-type sensor, its gassensing performance was strongly depended on the operated temperature. Thus, the optimum operating temperature of this as-fabricated gas sensor using these broken In2O3 microtubes was firstly determined under ambient humidity of 50%. As a comparison, the TEA sensing of the as-obtained In2O3 nanowires at various operating temperature was also displayed in Figure 7. From this figure, the sensor responses of In2O3 microtubes and nanowires toward 100 ppm TEA vapor increased as the elevated temperature from 100 to 300 oC, whereas gradually decreased with further promoting temperature from 350 oC. Notably, the In2O3 microtubes exhibited a higher TEA sensitive response than that of the In2O3 nanowires at identical operating temperature, which was ascribed to the specific structure features (such as hollow and broken architectures). Specifically, these broken and hollow surfaces in the In2O3 microtubes can be effectively advantageous for gas diffusion, thus improving the utilization ratio of sensing layers and sensing interactions. The higher sensor response towards TEA for In2O3 microtubes than that of In2O3 nanowires was also supported by the result of BET surface area and OV content (Figure 5 and Figure 6), respectively. In a sensing event, the gas molecules should possess sufficient activity to break the energy barrier to interact with oxygen 11



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species onto the surface.37 However, as further promoting the operated temperature, the gas adsorption ability would be reduced, leading a decrease in sensor response.38 To obtain high sensor response (Ra/Rg), the operating temperature of the In2O3 microtubes-based sensor was optimized at 300 °C. In addition, the influence of sensing layer thickness on sensor response was also investigated (Figure S4). This result implied that an appropriate thickness of the sensing film was required to obtain high sensor response.28,39 Moreover, the effect of the relative humidity (RH) on sensor response was also evaluated (Figure S5). As described for In2O3-based sensor in previous report,40 the sensor response based on the broken In2O3 microtubes was also sharply reduced as the promoted relative humidity. This may be explained that water molecules can occupy the active site onto the In2O3 surface, leading a low sensor response.41 To check real-time detection ability, a dynamic response characteristic of our broken In2O3 microtube sensor towards various concentration of TEA vapor was tested at 300 oC (Figure 8a). It was obviously seen that the sensor responses promoted rapidly as the increased concentration of TEA vapor, and sharply reduced after the removal of TEA. Remarkably, the sensor cannot reach a response equilibrium at a certain peroid. Therefore, a response event at each TEA concentration was recorded for 2 min for further sensing measurement and comparsion. In addition, the enlarged response transient for this sensor towards 1 ppm TEA was also inserted in Figure 8a. An obvious response was exhibited for detection of TEA vapor low to 1 ppm, which was far below the threshold exposure value (10 ppm) established by NIOSHA (the National Institute for Occupational Safety & Health Administration).42 Also, the response/recovery times of this sensor at various concentrations of TEA vapor was displayed in Figure 8b. The response times were 88, 62, 20, 15, 10, 12 and 12 s when upon exposure to TEA gas at 1-100 ppm. When TEA concentration was higher than 10 ppm, the response times were less than 15 s, whereas the recovery time was in range from 216 to 650 s. Meanwhile, the recovery times 12


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were below 200 s with TEA concentration at 2-10 ppm. It can be concluded that both absorption and desorption of TEA gas were severely favored by the huge density of nanopores onto the In2O3 microtubes (Figure 4b).43 Moreover, after fine linear fitting, these sensor response showed an excellent linear feature versus TEA concentration ranged 1-100 ppm, and the R2 (correlation coefficient) for this fitting curve was ~0.989, as shown in Figure 8c. To further evaluate detection ability to low concentration of TEA vapor, a dynamic response transient of this sensor towards 0.1 ppm TEA was tested in Figure 8d. A clear response (~1.9) can be observed, indicating that the sensor can detect TEA as concentration low to 0.1 ppm. In addition, the response time for this sensor towards 0.1 ppm was 103 s, which was more longer than that of 1-100 ppm. This result implied that the absorption process may be seriously influenced by the porous configuration in the broken In2O3 microtubes, especially when exposed in a low concentarion of TEA. Furthermore, a favorable TEA sensor should show particular ability to identify TEA vapor in the complicated atmosphere. To illustrate the selectivity of this as-fabricated sensor using broken In2O3 microtubes against other interference gases, the cross-response to other gases (including ammonia, methanol, ethanol, isopropanol, acetone, toluene, and hydrogen) at 100 ppm was further investigated in Figure 9. It was clearly observed that our sensor presented the highest sensor response towards TEA against the others. The ratios of sensor responses between the TEA gas and the other gases at 100 ppm were also calculated and drew (Inset in Figure 9). It could be acknowleged that the higher response ratio (STEA/Sother gas), the sensor showed more appreciable selectivity towards TEA vapor. The highest and lowest response ratios were 52.56 and 4.89, which were all far above 3. Such result indicated that this sensor displayed an satisfactory selectivity toward TEA vapor.

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To investigate the reliability of sensor response, the fifteen reversible cycles consisted of response and recovery processes for our sensor to 10 ppm TEA vapor was further measured at 300 oC (Figure 10a). It was confirmed that the sensor presented a relatively repeatable characteristic at each cycle. The sensor responses towards 10 ppm TEA vapor for each cycle were 11.17, 11.81, 11.51, 11.42, 11.52, 12, 12.16, 11.52, 11.99, 11.49, 11.32, 12.09, 11.55, 11.66, and 11.27, respectively. The average value was calculated as 11.63 ± 0.31, and the variation was less than 3%. Furthermore, the sensor stability was also examined for two week plotted using the actual resistance changes in clean air and 10 ppm TEA vapor, respectively. As displayed in Figure 10b, the sensor resistances varied and fluctuated at a extremely narrow range at 470~490 kΩ and 40~44 kΩ, when separately exposed to clean air and 10 ppm TEA vapor. Both of the maximal deviations in sensor resistance can be controlled lower to 5%. Moreover, the corresponding responses toward 10 ppm TEA were also showed that this sensor possessed a relatively high stability during a two-week measurements. Such results testified that the sensor possessed a reliable response and favorable stability when detection of TEA vapor. In addition, the post-sensing In2O3 microtubes were also checked using XRD, SEM and nitrogen isotherm technique. It was observed that the In2O3 products after sensing measurements can maintain the chemical composition, crystal structures, and essential tubular morphology with 1D architecture and less aggregation (Figure S6). Furthermore, the BET surface area was shrunked not very much (about 10%) in this process (Figure S7). These results also proved that the microtubular In2O3 can possess a relatively high stability against heat treatment in the sensing events. A comparison of the sensor response to TEA vapor between our sensor and previous reports was concluded in Table 1.44-49 It was noteworthy that our fabricated sensor herein

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exhibited the highest sensor response compared with those reported sensor. Therefore, our sensor may be as a potential candidate in detection of TEA vapor. 3.3. Disscussion of the Sensitive Mechanism. The remarkable enhancement in sensing performance of the broken In2O3 microtubes can be schematically described by an electron depletion layer model in Figure 11. As a classic n-type semiconductor oxide (In2O3), its gassensing process is believed the interaction between the tested gases and the ionized oxygen species (mainly O- at 300 oC in our case) onto its surfaces.50 Such interactions promote electron transfer of the In2O3 material, thus leading a formation of electron depletion layers. And the sensor resistances are strongly affected by the thickness of electron depletion layers.51 When this In2O3 sensor is exposed in air, the oxygen molecules can be directly adsorbed onto the In2O3 surface. Subsequently, the adsorbed oxygen molecules can be conversed to the ionized oxygen species via capturing free electrons from the conductor band in In2O3. Consequently, an electron depletion layers are formed and the sensor resistor is raised. After upon exposure of TEA vapor, the free electrons can be released back to the In2O3 by the interactions between the TEA and O-, probably described as following reaction:48,52 2(C2H5)3N + 43O- → 15H2O + 12CO2 + 2NO2 + 43eThis significantly reduces the thickness of the electron depletion layer, thus making a depressed variation of the sensor resistor. The enhancement of TEA sensing performances for the broken In2O3 microtubes may be attributed to the following two aspects from In2O3 microstructure and TEA feature. In one hand, the broken In2O3 microtubes possess a high permeation for gas diffusion, thus the electron depletion layers can be formed both onto the outer and inner surfaces.53 In addition, the wall thickness of the broken In2O3 microtubes (~30 nm) is much lower than the diameter 15



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of the In2O3 nanowires (70~80 nm), whereas is near the twice Debye length (2δ, δ=1~10 for most metal oxides).51,54 Thus, the whole grain of the broken In2O3 microtubes is depleted at a releatively high degree compared with that of the In2O3 nanowires. This means that the utilization ratio of the broken In2O3 microtubes is promoted, which can be supported by our experiment results in BET surface area and OV content (Figure 5 and 6). Furthermore, the ID architecture in the broken In2O3 microtubes may hold a low tendency to form aggregates and accumulation, which is favorable for obtaining stable sensor response, especically in a high operating temperature.7 Therefore, the as-fabricated sensor using these broken In2O3 microtubes displayed a remarkable repeatability and stablity (Figure 10). On the other hand, the sensing responses are importantly impacted by the bond energy of tested gas molecules. The chemical bond is easier to break with the lower bond energy. In this case, the bond energy for C-N (307 kJ/mol) is the most lowest than that of N-H (386 kJ/mol), C-H (411 kJ/mol), C-C (345 kJ/mol), C-O (361 kJ/mol), O-H (458.8 kJ/mol), C=O (798.9 kJ/mol) and H-H (432 kJ/mol),.55,56 Apparently, TEA molecule is the activest one as far as bond energy in these eight gas species, leading the highest sensor response (Figure 9). Although the actual sensing process is tremendously complex and may be involved in many aspects, it is mainly responsible for the improved sensing behavior towards TEA that are favorable gas diffusion, high utilization ratio, and 1D configuration of broken In2O3 microtubes, and active C-N in TEA molecule.

4. CONCLUSIONS In a whole, the broken In2O3 microtubes have been successfully prepared via a route consisting of a chemical conversion and subsequent centrifugation separation method. The microstructural analysis illustrated that these In2O3 microtubes hold many broken regions and numerous ultra-fine nanopores onto their surfaces. The as-fabricated sensor using these broken 16


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In2O3 microtubes exhibited a reliable response toward TEA at a concentration lower to 0.1 ppm, which is all far below the threshold exposure value (10 ppm) established by NIOSHA. Moreover, an impressive selectivity of the sensor toward TEA was also achieved, though exposed in other interferential vapors (including ammonia, methanol, ethanol, isopropanol, acetone, toluene and hydrogen). In addition, the sensor implied a remarkable repeatability and stability toward TEA gas. This excellent sensing property toward TEA vapor can be attributed to the broken configuration in In2O3 microtubes and the high active C-N in TEA molecule.

ASSOCIATED CONTENT Supporting Information The commercial In2O3 onto quartz boat before and after the ammonia treatment, XRD pattern of the product after ammonia treatment, SEM images of In2O3 products under various centrifugation speed and time, schematic diagram for the sensing measurement and the configuration of sensor, sensor response versus thickness of sensing layer, sensor response under different relative humidity, SEM image and XRD pattern of the In2O3 product after the sensing measurements, and a typical nitrogen adsorption/desorption isotherm and corresponding size distribution for the In2O3 microtubes after sensing measurements are listed. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (S. T. Liu.). ORCID 17



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Shantang Liu: 0000-0002-6403-8388 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work was supported through the National Natural Science Foundation of China (Grant No. 61603279, 21471120), and the scientific research program which is funded by Wuhan Institute of Technology.

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Figure 1. Schematic illustration for obtaining the single In2O3 morphology with broken microtubes and nanowires using a centrifugation separation strategy.

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Figure 2. XRD pattern of the as-obtained In2O3 product (consisted of In2O3 microtubes and nanowires) before centrifugation separation.

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Figure 3. SEM images of the as-obtained In2O3 products (a, b) before and (c, d) after centrifugation separation. Inset in d exhibited a magnified image of several In2O3 microtubes.

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Figure 4. (a) A typical TEM image of an individual In2O3 microtubes; (b) the enlarged magnification of the individual In2O3 microtubes in a; (c) HR-TEM image of the In2O3 microtubes; (d) SAED image of the In2O3 microtubes.

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Figure 5. Typical nitrogen adsorption-desorption isotherms and the corresponding pore-size distribution of the In2O3 microtubes (a) and nanowires (b), respectively.

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Figure 6. XPS spectra of survey spectrum (a); In 3d spectrum (b); O 1s spectrum (c) of the broken In2O3 microtubes; O 1s spectrum of the In2O3 nanowires (d).

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Figure 7. Sensor responses based on In2O3 microtubes and In2O3 nanowires to 100 ppm TEA vapor as function of operating temperature from 100 to 350 oC, respectively.

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Figure 8. (a) Dynamic response performance of the sensor based on the broken In2O3 microtubes exposed to TEA vapor at various concentrations ranging from 1 to 100 ppm. Inset: an enlarged response transient of the sensor to 1 ppm TEA; (b) response time and recovery time of the sensor at different concentrations of TEA vapor; (c) A linear fitting curve of the sensor response versus TEA concentration at 1-100 ppm; (d) an actual response transient of the sensor toward 0.1 ppm TEA vapor.

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Figure 9. Response and recovery characteristic of the sensor based on broken In2O3 microtubes towards 100 ppm of various gases. Inset: the response ratios for TEA to the other gases.

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Figure 10. (a) A real-time response curve of the sensor alternately exposed to 10 ppm TEA vapor and the clean air for fifteen cycles; (b) two week-stability of the sensor plotted using actual resistance variations in clean dry, 10 ppm TEA vapor, and the corresponding responses.

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Figure 11. Schematic diagram of TEA sensing mechanism using an electron depletion layer model.

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Table.1 Comparison of sensor response towards TEA gas in present work and other literatures. Sensing materials

TEA (ppm)

Operating temp (oC)

Response (Ra/Rg)

Refs

SnO2 spherical flowers

100

350

4

[44]

V2O5 hollow spheres

100

370

7.3

[45]

α-Fe2O3 microrods

100

275

11.8

[46]

CoFe2O4 nanocrystallines

100

190

8

[47]

TiO2/SnO2 nanosheets

100

260

52.3

[48]

Au@SnO2/Fe2O3 nanoneedles

100

300

39

[49]

broken In2O3 microtubes

100

300

72.5

this work

36


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Graphic for Manuscript

A density gradient strategy has been developed for preparation of broken In2O3 microtubes. The as-fabricated sensor using these broken In2O3 microtubes can detect TEA vapor at a concentration as low as 0.1 ppm.

37


A Density Gradient Strategy for Preparation of Broken In2O3

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