Ultra-sensitive ethanol gas sensors based on nanosheet-assembled hierarchical ZnO-In2O3 heterostructures

Journal Pre-proof Ultra-sensitive ethanol gas sensors based on nanosheet-assembled hierarchical ZnO-In2 O3 heterostructures Kun Zhang (Investigation) (Visualization) (Writing - original draft), Shuaiwei Qin (Resources) (Investigation) (Visualization) (Writing original draft), Pinggui Tang (Conceptualization) (Supervision) (Writing - original draft) (Writing - review and editing), Yongjun Feng (Conceptualization) (Supervision), Dianqing Li (Conceptualization) (Supervision) (Writing - original draft) (Funding acquisition)

PII:

S0304-3894(20)30179-5

DOI:

https://doi.org/10.1016/j.jhazmat.2020.122191

Reference:

HAZMAT 122191

To appear in:

Journal of Hazardous Materials

Received Date:

21 October 2019

Revised Date:

23 January 2020

Accepted Date:

24 January 2020

Please cite this article as: Zhang K, Qin S, Tang P, Feng Y, Li D, Ultra-sensitive ethanol gas sensors based on nanosheet-assembled hierarchical ZnO-In2 O3 heterostructures, Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122191

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Ultra-sensitive ethanol gas sensors based on nanosheet-assembled hierarchical ZnO-In2O3 heterostructures Kun Zhang, Shuaiwei Qin, Pinggui Tang,* Yongjun Feng, and Dianqing Li* State Key Laboratory of Chemical Resource Engineering, and Beijing Engineering Center for

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Hierarchical Catalysts, Beijing University of Chemical Technology, Beijing 100029, P.R. China

Corresponding Author

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*E-mail: [email protected],

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*E-mail: [email protected].

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Graphical abstract

Highlights

A novel strategy to construct 3D structures assembled by 2D mesoporous nanosheets was

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proposed.

The unique architectures were obtained by thermal conversion of 3D hierarchical mixed metal

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glycerolate.

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Numerous heterojunction interfaces with intimate contacts were formed in the 2D nanosheets.

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The 3D ZnO/In2O3 heterostructures showed a response of as high as 170 to 50 ppm ethanol.

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The 3D ZnO/In2O3 heterostructures exhibited excellent selectivity, repeatability, and stability.

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ABSTRACT: Developing efficient sensing materials with super sensitivity and selectivity is imperative to fabricate high-performance gas sensors for satisfying future needs. Herein, we report the preparation of ultrathin nanosheet-assembled 3D hierarchical ZnO/In2O3 heterostructures for the sensitive and selective detection of ethanol by sintering the 3D hierarchical Zn/In glycerolate precursors consisting of ultrathin nanosheets synthesized through a facile solvothermal method. The obtained

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ZnO/In2O3 heterostructures were carefully characterized by XRD, SEM, HRTEM, BET and XPS. The results showed that the 20%ZnO/In2O3 heterostructure is built up by many ultrathin nanosheets composed of intimately connected ZnO and In2O3 nanoparticles and have a specific surface area as

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high as 137.1 m2 g-1. Because of the unique hierarchical structure, abundant mesoporous and

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formation of ZnO-In2O3 n-n heterojunctions, the 20%ZnO/In2O3 heterostructure based sensor was ultra-sensitive to ethanol gas at 240 °C and exhibited a response as high as 170 toward 50 ppm of

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ethanol, which is about 3.3 times higher than that of pure In2O3 based sensor. Moreover, the sensor based on 20%ZnO/In2O3 heterostructure has virtues of excellent selectivity, good long-term stability

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and moderate response and recovery speed (35/46 s) toward ethanol. Therefore, the ultrathin

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nanosheet-assembled 3D hierarchical heterostructures are promising materials for fabricating high-

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performance gas sensors.

Keywords: ZnO/In2O3 heterostructure; Hierarchical; Heterojunctions; Nanosheet; Gas sensor

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1. Introduction More and more harmful volatile organic compoundous (VOCs), such as ethanol, methanol, acetone, toluene and xylene etc, are discharged to the air with the rapid development of the economy and society, which significantly threatens the health of humans. Among these VOCs, ethanol, as an important chemical material, is widely used for medical treatment, industrial chemistry process, alcoholic drinks and food industry. However, it is easy to volatilize into air, and it will cause

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uncomfortable symptoms like respiratory irritation, narcosis and impaired preception once the air contains amounts of ethanol. In addition, the alcohol levels in the breaths of individuals who have drinks are considerably higher than the levels of people who haven’t drink. Therefore, the detection

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of ethanol in air and in the breaths of motorists is of significance to ensure the health of humanbeings

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and avoid drunk driving for safety. Accordingly, it is necessary to real-time monitor the levels of ethanol in air and in the breaths of motorists accurately [1, 2], which is now accomplished by

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chemiresistor gas sensors duo to their merits of easy preparation, facile integration, portability and moderate sensitivity. Among the gas sensing materials, metal oxides semiconductors (MOS) with

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tunable properties such as adjustable anion deficiencies and mixed positive ion valences have

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received vast attention because of their low cost, high stability and relatively high sensitivity [3, 4]. Although enormous progresses have been achieved, the present MOS based gas sensors still suffer

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from the shortcomings of high operation temperature, limited sensitivity and selectivity [5-8]. It is thus requisite to make more efforts to develop highly sensitive and selective sensing materials for the detection of ethanol at relatively low working temperature. During the past three decades, a mass of n-type MOS such as In2O3 [9, 10], ZnO [11-14], SnO2 [15, 16], SnO [17], Fe2O3 [18], TiO2 [19, 20], WO3 [21, 22], et al. and p-type MOS like Co3O4 [23, 4

24], NiO [25], and so on have been intensively investigated and widely used as sensing materials for detecting inflammable, toxic and harmful gases. Among them, In2O3, as an important n-type direct band semiconductor with a wide band gap of 3.6 eV, has received great attention due to their unique chemical and physical properties which make it a promising sensing material for the detection of various gases. Many studies have demonstrated that the sensing properties of sensing materials are primarily determined by the structure and composition of sensing materials, and

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various methods have been put forward to optimize microstructure to offer abundant pores and high specific surface areas. To date, In2O3 with various microstructures, such as hollow nanospheres, nanosheets, nanorods, flower-like structure, et al., have been successfully constructed. For example,

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Zhang et al. revealed that mesoporous In2O3 composed of lots of nanoparticles exhibited a sensing

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response of 22 to 100 ppm of ethanol at 320 °C [26]. Sofia et al. found that the response of In2O3 nanoparticles prepared by a continuous hydrothermal route reached 18 to 20 ppm of ethanol at 300

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°C [27]. Sun et al. prepared mesoporous In2O3 spheres through a nanocasting method in conjunction, and found that the response of the synthesized In2O3 spheres to 100 ppm of ethanol reached 63.4 at

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the optimal temperature of 220 °C [28]. However, defects such as poor selectivity and dissatisfactory

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sensitivity of pristine In2O3 nanomaterials limit the application of In2O3 based gas sensor. Besides the structure optimization, composition tunings including lattice doping, surface

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modification and heterojunction fabrication are considered to be efficient methods for enhancing the selectivity and sensitivity MOS [29]. The fabrication of heterojunctions have been demonstrated to be one of the most effective strategy to significantly improve the sensing performance due to their adjustable compositions in a wide range and the synergistic effects (geometrical effects, chemical effects and electronic effects) in the composites [30, 31]. For example, Ma et al. found that the 5

response of ZnO-In2O3 heterojunctions to 100 ppm of formaldehyde reached 46.8 at 300 °C, which is higher than that of intrinsic In2O3 due to the formation of ZnO-In2O3 heterojunctions [3]. Singh et al. reported that In2O3-ZnO heterojunction nanowires showed obviously enhanced response to ethanol at 350 °C in comparison with pristine In2O3 nanowires [29]. Park et al. revealed the response of Bi2O3-decorated In2O3 nanorods to 200 of ppm ethanol reached 17.74 at 200 °C, which is about 4.9 times higher than that of pristine In2O3 [32]. Therefore, the gas sensing performance of In2O3

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can be obviously improved by forming heterojunction structure with other metal oxides. However, most of the reported heterostructures based on In2O3 are one-dimensional (1D) nanostructures and still not sensitive enough to ethanol, and there have been rare studies on three-dimensional (3D)

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structures assembled by mesoporous two-dimensional (2D) heterojunction nanosheets. Such unique

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architectures combine the advantages of open structure, numerous heterojunction interfaces with intimate contacts in the 2D nanosheets and abundant mesopores for the diffusion, adsorption,

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reaction and desorption of gas molecules, which could improve the sensitivity and accelerate the response and recovery speeds of the sensors. Nevertheless, to the best of our knowledge, ZnO/In2O3

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composites with the mentioned unique heterostructures have not been reported and used for

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fabricating high-performance gas sensors.

In this work, we successfully prepared ultrathin nanosheets assembled 3D hierarchical

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ZnO/In2O3 heterostructures via calcinating 3D hierarchical Zn/In glycerolate (Zn/In-gly) precursors composed of ultrathin nanosheets which were prepared through a facile hydrothermal method. The 3D hierarchical ZnO/In2O3 heterostructures showed a larger specific surface area (137.1 m2 g-1) in comparison with pristine In2O3 (84.3 m2 g-1). In addition, as a proof-of-concept, the as-prepared ultrathin nanosheets assembled 3D hierarchical ZnO/In2O3 heterostructures were used to fabricate 6

gas sensors. It was found that the obtained unique 3D hierarchical ZnO/In2O3 heterostructures with a Zn/In molar ratio of 1:5 exhibited a sensing response as high as 170 to 50 ppm ethanol at the optimal working temperature of 240 °C, which is about 3.3-fold of the response of pristine In2O3 (52). Moreover, the 3D hierarchical ZnO/In2O3 heterostructures also showed admirable selectivity, moderate response and recovery speed as well as excellent long-term stability. Therefore, the obtained ultrathin nanosheets assembled 3D hierarchical ZnO/In2O3 heterostructure is an

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outstanding sensing material platform for fabricating high-performance ethanol gas sensors. 2. Experimental section

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2.1. Materials

All the chemical reagents used in the preparation process of 3D hierarchical ZnO/In2O3

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heterostructure were of analytical grade and used without further purification. Indium nitrate

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(In(NO)3) was supplied by Shanghai Macklin Biochemical Co., Ltd; zinc nitrate (Zn(NO)3·6H2O), sodium salicylate, urea, ethanol, isopropanol and glycerol were obtained from Beijing Tongguang

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Fine Chemical Co., Ltd.

2.2. Synthesis of 3D hierarchical Zn/In-gly precursors

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A series of 3D hierarchical Zn/In-gly precursors assembled by nanosheets with different Zn/In

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molar ratios were prepared through a facile hydrothermal method. Typically, 0.236 g of In(NO3)3, 0.046 g of Zn(NO3)2·6H2O, 0.471 g urea and 0.063 g of sodium salicylate was mixed in 23.56 g of isopropanol and 10 g of glycerol under magnetic stirring for 0.5 h at room temperature. Then, the mixed solution was transferred into a Teflon-lined stainless-steel autoclave (100 ml) which was heated in an oven at 180 °C for 6 h. The resultant precipitate was dried at 80 °C for 10 h after 5

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cycles of washing and centrifugation steps with ethanol. The 3D hierarchical Zn/In-gly precursors with Zn/In molar ratios of 5%, 10% and 30% were also prepared according to the same method by only changing the amounts of Zn(NO3)2 to be 0.011, 0.023 and 0.069 g, respectively. Pure In-gly precursor was prepared as well by without adding Zn(NO3)2. The prepared 3D hierarchical Zn/Ingly precursors are denoted as xZn/In-gly (x = 5%, 10%, 20% and 30%) according to the theoretical Zn/In molar ratios.

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2.3. Preparation of 3D hierarchical ZnO/In2O3 heterostructures The ultrathin nanosheets assembled 3D hierarchical ZnO/In2O3 heterostructures were prepared by sintering the as-prepared 3D hierarchical Zn/In-gly precursors at 350 °C for 3 h in a furnace with

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heating rate of 5 °C min -1. The calcined products are labeled as xZnO/In2O3 (x = 5%, 10% 20% and

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30%), where x corresponds to the value in the xZn/In-gly precursors, respectively. Pure In2O3 was prepared by calcining the In-gly precursor at 350 °C through the same process above. 3D hierarchical

2.4. Characterization

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optimum calcination temperature.

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20%ZnO/In2O3 heterostructures annealed at 300 and 400 °C were also prepared to determine the

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The X-ray diffraction (XRD) analyses were conducted on a Rigaku D/MAX-Ultima Ⅲ diffractometer using Cu Kα radiation (λ = 1.5406 Å) with scanning angle 2θ ranging from 2 to 70°.

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Fourier transform infrared (FT-IR) spectra were recorded using the KBr pellets method on a Bruker Vector 22 infrared spectrophotometer with a resolution of 1 cm−1 in the range of 4000−400 cm−1. The morphology and microstructure of the precursors and their sintered products were investigated using a scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX) and a high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010) with an 8

accelerating voltage of 200 kV. The specific surface areas and pore size distributions of the powders were measured using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods according to nitrogen adsorption-desorption isotherms obtained on the Micromeritics Surface Area and Porosity Gemini VII 2390 system, respectively. The surface elemental composition of samples was investigated using a X–ray photoelectron spectrometer (XPS, ESCALAB 250) with Al Kα radiation. The Zn and In elemental contents were determined by

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ICPS−7500 inductively coupled plasma emission spectrometer (ICP−ES). Room temperature photoluminescence (PL) measurements were carried out on F-7000 fluorescence spectrophotometer (λ = 325 nm) to study the oxygen vacancy.

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2.5. Gas sensing test

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A homogeneous slurry, prepared first by dispersing the synthesized products in ethanol under constant grinding, was drop-coated on the surface of the ceramic tubes (4 mm in length, 1.2 mm and

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0.8 mm in external and internal diameter) with four Pt wires connected to a pair of gold electrodes located at each end, and a sensing film was formed on the surface of the tube after drying at room

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temperature for 1 h. Subsequently, the tube was connected to a holder after being aged at 250 °C for

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2 h in an oven, and a Ni−Cr alloy coil heater was inserted into the ceramic tube to control the working temperature of the gas sensor by adjusting the currents. A static CGS-8 gas sensing measurement

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system (Elite Technology Company Limited) with a test chamber of 20 L was adopted to study the sensing properties of the sensors under laboratory conditions (16-35 °C, 15-35 RH%). The fabricated sensors were put into the test chamber initially full of fresh air until the resistances reached their steady values. Subsequently, a given amount of liquid ethanol was injected into the chamber by a micro-injector to test the sensing characteristics of the sensors. The ethanol gas was removed by 9

opening the chamber when the responses of sensors reached their steady values. The response of the sensor is defined as Ra/Rg for the n-type MOS prepared in this work, where Ra and Rg denote electrical resistances of the sensor in air and in the target gases, respectively [33]. The response and recovery times were defined as the time taken by the sensor to achieve 90% of the total resistance change upon exposure to and removal from the target gases, respectively [34]. 3. Results and discussion

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3.1. Structure and morphology characteristics of samples The crystal phase and composition of the prepared Zn/In-gly and In-gly precursors were examined by XRD and FT-IR spectroscopy. As shown in Fig. 1a, the XRD patterns of the

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synthesized precursors exhibit the same characteristic peak of In glycerate at 2θ = 10.5°, which is

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similar to that of the previously reported XRD pattern of the indium glycerate complex [35, 36]. The FT-IR characterization was further carried out to determine the composition of the precursors. Fig.

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1b displays the FT-IR spectra of In-gly and 20%Zn/In-gly, which are similar to each other. The broad peak centered at 3400 cm-1 and that at 1450 cm-1 are attributed to the stretching vibration and in-

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plane deformation vibration of O-H groups, respectively, and the peaks located at 2856, 2923 and

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1380 cm-1 can be ascribed to C-H stretching vibration. The strong absorption peaks observed at 1058 and 1122 cm-1 are assigned to the stretching vibrations of C-O and C-C groups in glycerol molecules,

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respectively [36]. In addition, several strong peaks at 591, 711 and 817 cm-1 ascribed to the Zn-O and In-O vibrations are also observed in the spectra. These results demonstrate the formation of Zn/In and In glycerate complex [35, 37]. After annealing at 350 °C for 3 h, the diffraction peaks assigned to Zn/In-gly and In-gly precursors disappeared, and a series of new peaks appeared (Fig. 1c), indicating the complete 10

transformation of Zn/In and In glycerate complex to oxides. One observes that a series of peaks emerge at 2θ values of 21.5, 30.1, 35.5, 41.2, 45.7, 51.0, 56.0 and 60.7°, corresponding to the characteristic diffraction peaks of (211), (222), (400), (332), (431), (440), (611) and (622) lattice planes of In2O3 with a cubic structure (a = b = c = 10.118 Å), which match well with the standard PDF card of In2O3 (JCPDS no. 06-0416). Interestingly, the XRD patterns of the 3D hierarchical ZnO/In2O3 heterostructures are nearly same to that of pristine In2O3, and no featured diffraction

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peaks of ZnO were observed in the 3D hierarchical ZnO/In2O3 heterostructures, which could be arising from the low amounts and crystallinity of ZnO in the ZnO/In2O3 heterostructures. The crystallite sizes of pure In2O3, 10%ZnO/In2O3, 20%ZnO/In2O3 and 30%ZnO/In2O3 calculated via

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the Scherrer Formula are about 13.3, 12.4, 10.51, 9.9 and 9.4 nm, respectively, indicating that the

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crystallite sizes of ZnO/In2O3 heterojunctions are smaller compared with pure In2O3 and gradually decrease with the increase of ZnO content (0 to 30%) duo to the geometrical effects of

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heterojunctions.

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Fig. 1. (a) XRD patterns of In-gly and Zn/In-gly precursors, (b) FT-IR spectra of In-gly and 20%Zn/In-gly and (c)

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XRD patterns of In2O3 and ZnO/In2O3 heterostructures.

The morphology and microstructure of In-gly and Zn/In-gly precursors as well as their

corresponding oxides samples were investigated by SEM. Fig. 2 shows the SEM images of the synthesized In-gly and Zn/In-gly precursors. It can be seen that the In-gly exhibits a 3D hierarchical structure composed of many ultrathin nanosheets. The morphology of Zn/In-gly precursors is

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somewhat different from that of In-gly, but they all have 3D hierarchical structures assembled by numerous ultrathin nanosheets in common. Interestingly, the unique 3D hierarchical structures of In-gly and Zn/In-gly precursors entailed to pristine In2O3 and ZnO/In2O3 heterostructures after calcining at 350 °C for 3 h. As shown in Fig. 3, one clearly observes that all the oxides samples have

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the 3D hierarchical structures consisting of abundant ultrathin nanosheets.

Fig. 2. SEM images of In-gly (a, b), 5%Zn/In-gly (c, d), 10%Zn/In-gly (e, f), 20%Zn/In-gly (g, h) and 30%Zn/Ingly (i, j).

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Furthermore, the elemental distribution and detailed microstructure of 20%ZnO/In2O3 sample were studied by EDS analysis and HRTEM. It can be seen from Fig. 4a that Zn, O and In elements are uniformly distributed in the EDS mapping image, indicating that ZnO-In2O3 n-n heterojunctions may be formed. The ICP-ES tests showed that the Zn/In molar ratio in 20%ZnO/In2O3 is about 9.9%. Fig. 4b presents the HRTEM images of the 3D hierarchical 20%ZnO/In2O3 sample, in which a 3D hierarchical structure composed of a mass of ultrathin

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nanosheets was observed. It can be found that there are many pores on the

Fig. 3. SEM images of In2O3 (a, b), 5%ZnO/In2O3 (c, d), 10%ZnO/In2O3 (e, f), 20%ZnO/In2O3 (g, h) and 30%ZnO/In2O3 (i, j). 14

nanosheets and the nanosheets consist of nanoparticles. Clear fringes are observed in the further enlarged HRTEM images. The spacings of the lattice fringes of 0.252 and 0.292 nm are ascribed to the (400) and (222) planes of In2O3, respectively. The interplane spacings of 0.244 and 0.281 nm are severally assigned to (101) and (100) planes of the hexagonal wurtzite ZnO [38-40]. The intimate connections of In2O3 and ZnO nanoparticles

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unambiguously demonstrate the successful formation of ZnO-In2O3 n-n heterojunctions.

Fig. 4. (a) EDS mapping for Zn, O and In elemental analysis of the 20%ZnO/In2O3 sample; and (b) HRTEM images of 20%ZnO/In2O3 sample.

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The textural characteristic of the obtained pristine In2O3 and ZnO/In2O3 heterostructures were studied by N2 adsorption-desorption isotherm method, and the obtained isotherms are displayed in Fig. 5a. According to the IUPAC calssification, the isotherms can be classified as type Ⅲ isotherms with H1-type hysteresis loops, indicating the existence of mesopores in these samples. The specific surface areas calculated according to the BET method for the 5%ZnO/In2O3, 10%ZnO/In2O3, 20%ZnO/In2O3 and 30%ZnO/In2O3 heterostructures are about 85.2, 90.4, 137.1 and 128.8 m2 g-1,

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respectively, which are larger than that of pure In2O3 (84.3 m2 g-1). Obviously, the specific surface area of the 3D hierarchical ZnO/In2O3 heterostructures shows the highest value of 137.1 m2 g-1 as

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the Zn/In molar ratio reaches 20%. The higher specific surface area would

Fig. 5. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution curves of pure In2O3 and ZnO/In2O3 heterostructures.

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benefit the adsorption of more oxygen molecules, which can faciliate the improvement of gas sensing response. In addition, the pore size distribution curves determined by using the BJH method from the desorption isotherm are presented in Fig. 5b. The average pore size of the 3D hierarchical 5%ZnO/In2O3, 10%ZnO/In2O3, 20%ZnO/In2O3 and 30%ZnO/In2O3 samples were about 9.6, 9.9, 12 and 11.8 nm, respectively, which are smaller than that of pure In2O3 (12.6 nm), and their pore volumes are larger compared with pure In2O3. These results indicate that the 3D hierarchical

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ZnO/In2O3 heterostructures have richer mesopores than pristine In2O3, which could favor the diffusion of gas molecules.

XPS analysis was carried out to study the valence band, surface compositions and chemical

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states of the elements of pure In2O3 and 20%ZnO/In2O3 heterostructure. The valence band (VB)

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spectra for ZnO, In2O3 and 20%ZnO/In2O3 displayed in Fig. S1 in the Supporting Information show that the valence band maximum values deduced from the VB spectra by linear fitting are about 2.14,

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1.74 and 1.62 eV for ZnO, In2O3 and 20%ZnO/In2O3, respectively. The VB value of 20%ZnO/In2O3 shift upward in comparison with pristine ZnO and In2O3, suggesting that n-n heterojunctions are

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formed between ZnO and In2O3 nanoparticles [41]. As shown in Fig. 6a, the peaks corresponding to

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In, O and C elements were clearly observed in the survey spectrum of pure In2O3. Meanwhile, the high-resolution spectra of In 3d and O 1s spin–orbits of pristine In2O3 are displayed in Fig. 6c and

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e, respectively. The two peaks located at 451.8 and 444.3 eV can be assigned to the In 3d3/2 and 3d5/2 spin–orbits, indicating the presence of In3+ oxidation state. The O 1s spectrum of pristine In2O3 can be fitted into three curves with major peaks located at 530.0 ± 0.2 eV, 531.2 ± 0.2 eV and 532.3 ± 0.2 eV, which can be assigned to lattice oxygen (Olat), deficient oxygen (Odef) and surface oxygen (Oabs), respectively. The full survey scan spectrum of the 20%ZnO/In2O3 heterostructure exhibited 17

Fig. 6b is similar to that of pure In2O3, but two weak peaks located at 1044.9 eV and 1021.4 eV appeared (Fig. 6d), which can be attributed to the 2p1/2 and 2p3/2 spin–orbits of Zn with 2+ oxidation state, respectively. In comparison with the In 3d spectrum of pristine In2O3, the In 3d3/2 and 3d5/2 spin-orbits of 20%ZnO/In2O3 slightly shift to lower binding energy of 451.6 and 444.1 eV (Fig. 6c), indicating that a small number of electrons might move from ZnO to In2O3. Fig. 7f presents the O 1s spectrum of 20%ZnO/In2O3, which can also be fitted into three curves with major peaks located

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at 530.0 ± 0.2 eV, 531.2 ± 0.2 eV and 532.3 ± 0.2 eV, corresponding to the Olat, Odef and Oabs,

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respectively. The Oabs is the most active

Fig. 6. XPS spectra of the In2O3 sample (a) and 20%ZnO/In2O3 sample (b), In 3d (c), enlarged spectra of Zn 2p (d), O 1s of pure In2O3 sample (e) and O 1s of 20%ZnO/In2O3 sample (f).

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among the three oxygen species, and it plays a remarkable role on the gas sensing response of MOS because it can react with the target gas molecules. Therefore, the amounts of Oabs on the surface of MOS directly determine their sensitivity. The calculated relative percentages of Olat, Odef and Oabs are about 53.0, 32.7 and 14.3 on the surface of pristine In2O3 and are approximately 50.0, 32.8 and 17.2 on the surface of 20%ZnO/In2O3, respectively, demonstrating that there are more active Oabs on the surface of 20%ZnO/In2O3 compare to pristine In2O3. Hence, the formation of ZnO-In2O3 n-n

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heterojunctions can promote the generation of active Oabs on the surface, which is undoubtedly in favor of improving the sensitivity of MOS. 3.2. Gas sensing properties

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The sensing properties of MOS are significantly relying on the operating temperature which

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determines the surface state, adsorption/desorption of gases and the chemical reactions on the surface. In order to explore the optimal operating temperatures of the pure In2O3 and 3D hierarchical

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ZnO/In2O3 heterostructures, the responses of the pristine In2O3 and ZnO/In2O3 heterostructures with different Zn/In molar ratios to 50 ppm of ethanol at a series of operating temperatures were detected.

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As shown in Fig. 7a, the responses of all the sensors increase with increasing operating temperature,

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and then decrease when the temperature is higher than the optimal one. The sensors based on In2O3, 5%ZnO/In2O3, 10%ZnO/In2O3, 20%ZnO/In2O3 and 30%ZnO/In2O3 show the highest responses of

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52, 80, 113, 170 and 40 at the optimal operating temperatures of 240, 210, 210, 240 and 180 °C, respectively. The disparate optimal working temperature between the samples can be ascribed to their different absorption and activation ability toward oxygen and ethanol molecules (Fig. S2 in the Supporting Information and Fig. 11 in the latter text) due to their diverse surface properties. Obviously, 20%ZnO/In2O3 heterojunction based sensor has the highest response, which is about 3.3 19

times higher than that of pure In2O3 based sensor, demonstrating that the formation of n-n heterojunctions with ZnO can effectively enhance the sensitivity of In2O3. Therefore, the optimal working temperature of 240 °C is adopted to explore the sensing properties of 20%ZnO/In2O3

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heterojunctions.

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Fig. 7. (a) The responses of different sensors to 50 ppm ethanol at different working temperatures; (b) the responses of 20%ZnO/In2O3 calcined at different temperature to 50 ppm of ethanol; (c) the responses of 20%ZnO/In2O3

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obtained from 20%Zn/In-gly precursors synthesized with and without sodium salicylate to 50 ppm of ethanol; and

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(d) the response and recovery curves of the In2O3 and 20%ZnO/In2O3 to 50 ppm of ethanol at 240 °C.

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Besides the working temperature, the calcination temperature also has significant influence on the structure and surface states. The effect of calcination temperature on the sensing responses of 20%ZnO/In2O3 heterojunctions was investigated, and the obtained results are shown in Fig. 7b. It can be seen that the 20%ZnO/In2O3 heterojunctions prepared at calcination temperature of 350 °C exhibits the highest response to 50 ppm of ethanol (170), which is considerably larger than those of

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20%ZnO/In2O3 heterojunctions calcined at 300 and 400 °C. Hence, 350 °C can be regarded as the optimum calcination temperature for the preparation of 20%ZnO/In2O3 n-n heterojunctions for ethanol gas sensing. Moreover, we found that sodium salicylate plays a key role on improving the sensing response of 20%ZnO/In2O3 heterojunctions. As revealed in Fig. 7c, 20%ZnO/In2O3 heterojunction obtained from 20%Zn/In-gly precursor synthesized with sodium salicylate shows about 5 times higher sensing response than that without sodium salicylate to 50 ppm of ethanol,

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indicating that sodium salicylate have great impacts on the microstructure of 20%Zn/In-gly precursor.

In recent years, a series of In2O3 based gas sensors have been fabricated to detect ethanol gas,

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and their sensing properties are shown in Table 1. Most of the reported sensors showed a response

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of lower than 50 and 100 to 50 and 100 ppm of ethanol, respectively. For example, Zhang et al. found that the prepared mesoporous In2O3 nanostructures showed a response of 18 to 50 ppm of

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ethanol at 320 °C [26]; Liu et al. revealed that the response of In2O3/MoS2 nanocomposites reached 35.8 to 50 ppm ethanol at 260 °C [42]. Huang et al. prepared ZnO@In2O3 nanofibers through coaxial

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electrospinning method and reported that ZnO@In2O3 nanofibers displayed a response of 31.87 to

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100 ppm of ethanol [43]. In addition, porous In2O3-ZnO composite nanofibers prepared via the electrospinning method exhibited a response value of 98 to 100 ppm of ethanol at 350 °C [44].

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Obviously, the responses of the sensor based on the 3D hierarchical 20%ZnO/In2O3 heterojunctions to 50 and 100 ppm of ethanol are significantly higher than those of the sensors based on In2O3 sensing materials mentioned in Table 1. Besides, the optimal operating temperature of the 3D hierarchical 20%ZnO/In2O3 heterojunctions is 240 °C, which is lower than most of the reported optimal operating temperatures. 21

Table 1. Sensing Performance of Different In2O3 Based Materials

Sensing materials

Temperature

Concentration

(°C)

(ppm)

220

100

Response

Reference

ACS Appl. Mater. Interfaces mesoporous In2O3

63.4 2014, 6, 401 ACS Appl. Mater. Interfaces

Bi2O3-decorated In2O3 nanorods

200

200

17.74 2015, 7, 8138

260

100

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Sens. Actuators, B

In2O3/ZnS microspheres

11.7

2018, 264, 263 Langmuir

In2O3 nanoparticles

300

20

18

In2O3/MoS2 nanocomposites

ZnO@In2O3 nanofibers

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In2O3-ZnO nanowires

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20%ZnO/In2O3 heterojunctions

50

300

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NiO-In2O3 composite nanofibers

260

50

225

350

50

100

Appl. Surf. Sci.

18

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320

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mesoporous In2O3

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2012, 28, 1879

2016, 378, 443 Appl. Surf. Sci.

35.8 2018, 447, 49 J. Alloys Compd. 41 2017, 699, 567 Sens. Actuators, B

31.87 2018, 255, 2248 Sens. Actuators, B

100

98 2011, 160, 1346

240

50/100

170/300

This work

Fig. 7d and Fig. S3 in the Supporting Information display the dynamic response and recovery

curves of the pristine In2O3 and ZnO/In2O3 heterojunctions to 50 ppm ethanol at 240 °C, respectively. One sees that the resistances of In2O3 and ZnO/In2O3 heterojunctions based sensors

22

decreased dramatically when ethanol was injected into the chamber, suggesting that In2O3 and ZnO/In2O3 heterojunctions exhibit typical n-type behavior as previously reported. The response/recovery times of the 5%ZnO/In2O3, 10%ZnO/In2O3, 20%ZnO/In2O3 and 30%ZnO/In2O3 are about 48/60, 40/48, 35/46 and 60/54 s, respectively. Interestingly, the response and recovery times of 20%ZnO/In2O3 heterojunctions are smaller than those of pristine In2O3 (48 and 60 s, respectively), indicating that the formation of ZnO-In2O3 n-n heterojunctions with appropriate ratios

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can optimize the textural characteristic to faciliate the diffusion of gas molecules. The repeatability of the sensor based on 20%ZnO/In2O3 heterojunctions to 50 ppm of ethanol at 240 °C was investigated, which is shown in Fig. 8a. The stable and repeatable dynamic response

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and recovery curves demonstrate the excellent sensing repeatability of 20%ZnO/In2O3

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heterojunctions. The selectivity is another important criterion to evaluate the performance of gas sensors for practical applications. The response of the sensors based on pure In2O3 and

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20%ZnO/In2O3 heterojunctions to 50 ppm of different test gases including ethanol, acetone, toluene, xylene, methanol and formaldehyde were measured at 240 °C. As shown in Fig. 8b, the responses

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of 20%ZnO/In2O3 based sensor to these gases are all enhanced compared with the pristine In2O3

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based sensor, and the response to ethanol was the highest. The response of 20%ZnO/In2O3 heterojunctions based sensor to 50 ppm ethanol was as high as 170, which was about 3.4, 8.5, 10.6,

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10 and 28.3 times higher than those to acetone (50), toluene (20), xylene (16), methanol (17) and formaldehyde (6), respectively, indicating that 20%ZnO/In2O3 n-n heterojunctions has superior selectivity to ethanol. The responses of 20%ZnO/In2O3 n-n heterojunctions based sensor to ethanol gas with various concentrations was further investigated at 240 °C. Fig. 8c exhibits the dynamic response curves of 23

the sensors based on 20%ZnO/In2O3 heterojunctions to ethanol in the concentration range from 1-5

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re

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ppm and 5-200 ppm. One sees that the responses of the sensor steadily increase along with

Fig. 8. (a) The response repeatability curve of 20%ZnO/In2O3 to 50 ppm ethanol at 240 °C; (b) the responses of

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In2O3 and 20%ZnO/In2O3 sensors to 50 ppm ethanol and other typical air pollutants (acetone, toluene, methanol,

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formaldehyde and xylene); (c) the responses of the 20%ZnO/In2O3 sensor to different concentrations ethanol at

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240 °C; (d) linear relationship of 20%ZnO/In2O3 to 1-200 ppm ethanol at 240 °C.

the increasing of ethanol concentration. The responses to 1, 2, 5, 10, 20, 50, 100 and 200 ppm of ethanol are about 2.5, 5.5, 16, 30, 62, 170, 300 and 500, respectively. It is worth noting that the sensor still shows a response of 2.5 to ethanol even when the concentration is as low as 1 ppm, demonstrating that 20%ZnO/In2O3 heterojunctions can be used to detect ethanol with low

24

concentration. The relationship of the response with the concentration of ethanol is further demonstrated in Fig. 8d. It can be found that the responses nearly increase linearly when the concentration of ethanol increases from 1 to 200 ppm, and the coefficient of determination (R2) is 0.985, indicating that 20%ZnO/In2O3 heterojunctions based sensors can detect ethanol gas in a wide concentration range. The humidity also significantly influences the sensing response of MOS materials, and high

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humidity will seriously deteriorate the sensitivity. Fig. 9a shows the responses of the sensor based on 20%ZnO/In2O3 to 50 ppm of ethanol with relative humidity in the range of 15-70%. It can be found that the response was as high as 150 when the humidity increased to 55% and still retained to

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be 120 as the humidity further increased to 70%, indicating that the 20%ZnO/In2O3 based sensor

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can be used to detect ethanol at high humidity conditions. Furthermore, the long-term stability is one of the most important parameters of sensing performance for the practical application.

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Therefore, the long-term stability of the 20%ZnO/In2O3 heterojunctions based sensor to 50 ppm ethanol at 240 °C was examined, and the obtained results are exhibited in Fig. 9. It can be seen that

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the responses of the 20%ZnO/In2O3 heterojunctions based sensor nearly maintained at around 160

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in the 50 days test, suggesting that the 20%ZnO/In2O3 heterojunctions based sensor has wonderful long-term stability toward ethanol. Therefore, the sensor based on 20%ZnO/In2O3 heterojunctions

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exhibits the advantages of excellent repeatability, remarkable selectivity to ethanol, wide detection range and super long-term stability, which are beneficial for its practical application in the field of gas sensors.

25

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Fig. 9. The responses of 20%ZnO/In2O3 to 50 ppm of ethanol with different relative humidity (a) and stability

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3.2. Gas sensing mechanism

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curve of 20%ZnO/In2O3 to 50 ppm of ethanol at 240 °C (b).

The fundamental sensing mechanism of MOS materials has been widely discussed in previous

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literature, which involves the surface reactions between the target gas and adsorbed oxygen species

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thereby leading to changes in their electrical resistance. When the sensors are exposed to air at 240 °C, O2 molecules would be adsorbed onto the surface of pristine In2O3 and 20%ZnO/In2O3 heterojunctions and will capture electrons from their conduction bands, giving rise to more active chemisorbed oxygen species of O2−, O− and O2− as well as an electron depletion layer (EDL) near the surfaces of In2O3 and ZnO nanoparticles [45, 46]. As a result, the resistances of pristine In2O3

26

and 20%ZnO/In2O3 heterojunctions will increase. These processes are shown in the following equations (1)-(4) [47]. O2(g) → O2(ads)

(1)

O2(ads) + e− → O2−(ads)

(2)

O2−(ads) + e− → 2O−(ads)

(3)

O−(ads) + e− → O2−(ads)

(4)

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Due to the higher Fermi level of ZnO than In2O3 [10, 48], the free electrons in the conduction band of ZnO will transfer to the conduction band of In2O3 until their Fermi levels reach equilibrium (Fig. 10a), bringing about higher electron concentration in the conduction band of In2O3

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nanoparticles in the 20%ZnO/In2O3 heterojunctions, which is beneficial to the adsorption of O2

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molecules and formation of active chemisorbed oxygen species on the surface of 20%ZnO/In2O3 heterojunctions. In addition, the smaller crystalline size of In2O3 nanoparticles and much larger

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specific surface area of the 20%ZnO/In2O3 heterojunctions can offer more active sites for the adsorption of O2 molecules, resulting in more absorbed oxygen species on the surface of

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20%ZnO/In2O3 heterojunctions compared to pristine In2O3. Furthermore, it is reported that the

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oxygen vacancy has remarkable effect on the sensing performance of MOS materials [49]. The photoluminescence spectra of In2O3 and ZnO/In2O3 heterostructures displayed in Fig. S4 in the

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Supporting Information demonstrate that the intensity of the emission peak at 655 nm increases with the increasement of ZnO amounts first and then decreases, and the intensity reaches the maximum when the ZnO amount is 20%. According to previous report, the visible emission is arising from the recombination of singly ionized oxygen vacancy with the photo-generated hole in MOS [49]. The higher emission intensity of 20%ZnO/In2O3 heterostructure suggests that there are more oxygen 27

vacancies in 20%ZnO/In2O3 than other samples, which can facilitate the adsorption of O2 molecules and formation of active chemisorbed oxygen species. Therefore, the thickness of EDL near the surface of In2O3 nanoparticles in the 20%ZnO/In2O3 heterojunctions will be larger than that of pristine In2O3 because of the formation of more chemisorbed oxygen species due to the electronic effect, geometrical effect and more oxygen vacancies mentioned above (Fig. 10b). The sensing process of the obtained materials can be considered as surface-control model because their particle

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sizes are in the order of 10 nm, and their resistances are primarily determined by the adsorption of oxygen and target gas molecules. As shown in Fig. 11a, the Ra value of the sensor based on 20%ZnO/In2O3 heterojunctions increased by about 12.7 times from 15 MΩ at 110 °C to 190 MΩ at

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240 °C, while the Ra value of the pristine In2O3 based sensor only increased by about 5.3 times from

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0.7 to 3.7 MΩ when the temperature raised from 110 to 240 °C, further suggesting that more

comparison with pristine In2O3.

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chemisorbed oxygen species were formed on the surface of 20%ZnO/In2O3 heterojunctions in

When the ethanol gas was introduced, ethanol molecules will be adsorbed onto the surfaces of

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pristine In2O3 and 20%ZnO/In2O3 heterojunctions, and subsequently are oxidized by the reactive

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oxygen species, leading to the release of trapped electrons back into the conductance bands of In2O3 and 20%ZnO/In2O3 heterojunctions. It is reported that the surface oxygen species of O− is of

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predominance in the range of 150-300 °C. As the working temperature is 240 °C, the adsorbed ethanol molecules will be oxidized by reactive O− species according to Equation 5, and the captured electrons will be released back to the conductance bands of In2O3 and 20%ZnO/In2O3, giving rise to the decrease of the thickness of the EDL near the surface domains as well as their resistances. CH3CH2OH(ads) + 6O−(ads) → 2CO2(g) + 3H2O(g) + 6e− 28

(5)

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Fig. 10. The schematic diagram of the proposed sensing mechanism of In2O3 and 20%ZnO/In2O3.

Fig. 11. The resistance values of In2O3 and 20%ZnO/In2O3 in air (a) and in 50 ppm of ethanol (b).

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The resistance values of the sensors based on pristine In2O3 and 20%ZnO/In2O3 heterojunctions

in 50 ppm of ethanol are exhibited in Fig. 11b. One sees that the Rg value of the pristine In2O3 based sensor decreased by about 5.5-folds from 0.39 to 0.071 MΩ as the temperature increased from 110 to 240 °C. Meanwhile, the Rg value of the sensor based on 20%ZnO/In2O3 heterojunctions shrunk by about 9.5-folds, which is considerably larger than that of pristine In2O3 based sensor, suggesting 29

that much more absorbed O− species reacted with ethanol molecules on the surface of 20%ZnO/In2O3 heterojunctions and more electrons were released back to the conductance band in comparison with pristine In2O3. Consequently, the 20%ZnO/In2O3 heterojunctions based sensor displayed much higher sensing response than pristine In2O3 based sensor according to the definition of response.

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4. Conclusions

In summary, a facile strategy to fabricate ultrathin nanosheet-assembled 3D hierarchical ZnO/In2O3 heterostructures is developed. The 3D hierarchical Zn/In glycerolate precursors

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consisting of ultrathin nanosheets were synthesized first through solvothermal approach and

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subsequently sintered at 350 °C. The morphology and structural properties of 3D hierarchical ZnO/In2O3 heterostructures were characterized by various techniques, which demonstrates the

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successful formation of ZnO/In2O3 n-n heterojunctions and the 20%ZnO/In2O3 n-n heterojunctions had the highest specific surface area among the obtained samples. Sensing performance tests showed

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that 20%ZnO/In2O3 n-n heterojunctions based sensor had an ultrahigh response of 170 to 50 ppm of

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ethanol at the optimum operating temperature of 240 °C, which is about a 3.3-fold increase compared to that of the pure In2O3 based sensor. The moderate response and recovery speed (35/46

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s), excellent selectivity, good linear relationship between the response and the ethanol concentration in a wide range and super long-term stability toward ethanol demonstrate that the ultrathin nanosheet-assembled 3D hierarchical 20%ZnO/In2O3 heterostructure is a promising sensing material to fabricate high-performance sensors for the sensitive and selective detection of ethanol gas. The enhanced sensing performance can be attributed to the unique nanosheet-assembled 3D 30

hierarchical structure with smaller particle size, abundant mesopores, larger specific surface area and more oxygen vacancies as well as the formation of a large number of ZnO-In2O3 n-n heterojunctions in the 2D nanosheets, which bring the superiority of the geometrical and electronic effects into full play to facilitate the generation of more reactive chemisorbed oxygen species on the surface of 20%ZnO/In2O3 heterostructure. The strategy reported here can be facilely extended to prepare other similar 3D hierarchical heterostructures with outstanding sensing performance to

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VOCs gases.

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The authors declare no competing financial interest.

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Conflicts of interest

Credit authorship contribution statement

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Kun Zhang: Investigation, Visualization, Writing-original draft. Shuaiwei Qin: Resources, Investigation, Visualization, Writing-original draft. Pinggui Tang: Conceptualization, Supervision,

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Writing-original draft, review and editing. Yongjun Feng: Conceptualization, Supervision.

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Dianqing Li: Conceptualization, Supervision, Writing-original draft, Funding acquisition.

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Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgments The authors are grateful for the financial supports from the National Natural Science Foundations of China (U1507119, 21627813, 21521005), National Key Research and Development Program of China (2016YFB0301600) and the Fundamental Research Funds for the Central Universities (XK1802-6). Pinggui Tang particularly appreciates the aids of China Scholarship

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Council.

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