Facile synthesis and gas sensing properties of tubular hierarchical ZnO self-assembled by porous nanosheets

Accepted Manuscript Title: Facile synthesis and gas sensing properties of tubular hierarchical ZnO self-assembled by porous nanosheets Author: Faying Fan Pinggui Tang Yuanyuan Wang Yongjun Feng Aifan Chen Ruixian Luo Dianqing Li PII: DOI: Reference:

S0925-4005(15)00372-X http://dx.doi.org/doi:10.1016/j.snb.2015.03.048 SNB 18241

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

30-8-2014 15-3-2015 22-3-2015

Please cite this article as: F. Fan, P. Tang, Y. Wang, Y. Feng, A. Chen, R. Luo, D. Li, Facile synthesis and gas sensing properties of tubular hierarchical ZnO self-assembled by porous nanosheets, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.03.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile synthesis and gas sensing properties of tubular

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hierarchical ZnO self-assembled by porous nanosheets

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Faying Fan, Pinggui Tang, Yuanyuan Wang, Yongjun Feng, Aifan Chen, Ruixian

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Luo, and Dianqing Li

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State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing

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100029, China

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Abstract: A three-dimensional hierarchical porous ZnO with tubular structure has been prepared by calcining a

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tubular hierarchical hydrozincite precursor. The precursor was prepared through in-situ growth of nanosheets on an

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as-synthesized template of zinc complex nanotubes at room temperature without using of organic solvent,

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surfactant agent, and foreign template. The obtained three-dimensional hierarchical porous ZnO was characterized

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by XRD, SEM, HRTEM, BET, and FT-IR. The results show that the three-dimensional hierarchical porous ZnO,

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having a specific surface area of 78 m2·g-1, consist of interleaving mesoporous nanosheets composed of ZnO

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nanoparticles. Also its gas sensing properties was investigated, and the three-dimensional hierarchical porous ZnO

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shows superior gas sensing performance toward ethanol because the widely open and porous structure offers

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efficient gas diffusion route and the large specific surface area and small particle size supply abundant active sites

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for the gas adsorption.

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semiconductors, and it is widely used as the material for gas sensor due to its advantages of high sensitivity,

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stability, and low-cost [2]. Especially, the ZnO-based gas sensor shows good sensing performance to H2S [3, 4],

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NOx [5], and ethanol [6]. Because the ethanol sensor is an essential device in our daily life to ensure traffic safety

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Key words: ZnO; tubular; hierarchical structures; gas sensing; porous

1. Introduction

Environmental problems associated with harmful air pollution severely threaten the health and safety of

human beings, and it is very important and urgent to develop techniques for gases detecting. Semiconductor-based gas sensor is one of the most attractive gas detecting instruments and have attracted numerous attention because of the advantages of convenience, low cost, and fast detection [1]. ZnO is one of the most attractive n-type

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Corresponding authors: Tel: +86 10 6443 6992; fax +86 10 6443 6992. E-mail address: [email protected], [email protected] 1

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and occupational protection [7, 8], it is important to prepare materials with superior ethanol sensing properties. Small size ZnO nanoparticles have exhibited excellent gas sensing properties due to their high specific

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surface area and activity, but the inevitably irregular aggregation of the nanoparticles hinders the diffusion of gases

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into the inner part of the secondary particles, which will eventually deteriorate and delay the response [9]. Recently,

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hierarchical structures composed by building blocks of low-dimensional nanomaterials, such as nanoparticles,

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nanorods, and nanosheets, have been attracted great attentions due to their combined advantages of primary

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nano-sized and secondary micron-sized structures [10]. Hierarchical structures composed by nanoparticles not only

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have high specific surface areas which will supply abundant active site for the target gases but also have rich-pore

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structures which will facilitate the rapid and efficient diffusion of gases in the materials, and these properties will

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enhance the gas sensing performance of the materials [11]. Therefore, it is of great interest to fabricate hierarchical

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structures assembled by nanoparticles to ensure excellent gas sensing performance.

Furthermore, tubular hierarchical structures have drawn great attention due to their large specific surface area,

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low density, and good surface permeability. Tubular hierarchical structures composed of nanosheets of TiO2

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[12-14], MnO2-NiO [15], Fe2O3 [16], MoS2 [17], and SnO2 [18] have been prepared and exhibited excellent

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performances in the fields of photocatalysis, solar cells, and Li-ion battery. For example, Lou et al. [16, 18]

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prepared carbon-coated SnO2 and Fe2O3 nanosheets-based tubular hierarchical structures by using carbon nanorods

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as the templates, and these tubular hierarchical structures exhibited high specific capacity and excellent cycling

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stability due to their low-dimensional nanosized building blocks, tubular structures, and carbon nanocoating. Xie et

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al. [13] prepared anatase tubular hierarchical structures with excellent photocatalysis activity by hydrothermal method. Arnab Kanti Giri et al. [19] prepared porous ZnO microtubes with excellent cholesterol sensing and catalytic performances by a simple method. As tubular ZnO hierarchical structures have high specific surface area and good surface permeability, the tubular ZnO hierarchical structures would have good gas sensing performance. However, the gas sensing performances of tubular ZnO hierarchical structures have not been reported. Hence, it is necessary to study the potential application of the ZnO tubular hierarchical structures in the gas sensing.

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Herein, three-dimensional tubular hierarchical porous ZnO self-assembled with nanoparticles-based

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nanosheets has been prepared through a novel simple method. A precursor of hydrozincite tubular hierarchical

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structures was synthesized at room-temperature firstly, and then the three-dimensional hierarchical porous ZnO

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was obtained by calcining the precursor at appropriate temperature. No surfactant and foreign template are used in

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this route, and the operating conditions are mild. To the best of our knowledge, such a facile route has not been

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reported before for self-assembling nanoparticles into nanosheets-based tubular hierarchical structures. Also, the 2

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gas sensing performance of the obtained three-dimensional hierarchical porous ZnO was investigated by using

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ethanol as a probe gas, and the obtained three-dimensional hierarchical porous ZnO exhibits high sensitivity and

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selectivity to ethanol, which makes it a potential material for application in gas sensors.

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2. Experimental section

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2.1 Synthesis of three-dimensional hierarchical porous ZnO

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All reagents were analytical grade, and water used was deionized and decarbonated with an electrical

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conductivity less than 10−6 S·cm-1. In a typical synthesis, NH4HCO3 (3.95 g) and zinc acetate dehydrate (5.4875 g)

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were dissolved in 50 mL and 25 mL of water to form solutions, respectively. The zinc acetate dehydrate solution

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was then added dropwise at a rate of 2 mL·min-1 into the NH4HCO3 solution with intensively mechanical stirring,

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and the resulting precipitate was continuously aged for 4 h at room-temperature with vigorous stirring. The

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precursor was collected and washed with water for 5 times, and then dried at 80 °C for 12 h. The

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three-dimensional hierarchical porous ZnO was prepared by calcining the precursor at 300 °C and 400 °C for 2 h

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with a heating rate of 5 °C·min-1, and they were denoted as ZnO-300 and ZnO-400, respectively. The precursors

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with different aging time (0, 0.5, 2, 4, and 24 h) were collected, washed, and dried to investigate the formation

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process of the tubular hierarchical structures.

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ZnO nanorods were prepared following the same procedure in our previous work [20], and the preparation

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details has been presented in the supporting information.

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2.2 Characterization

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performed on a Hitachi H-800 transmission electron microscopy operated at 100 kV. The BET specific surface

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area and pore distribution of the samples which were degassed at 150 °C for 8 h were determined by N2

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adsorption/desorption method, which were carried out on a Micromeritics Surface Area & Porosity Gemini VII

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2390 system. FT-IR spectra were recorded on a Bruker Vector 22 with a resolution of 2 cm-1 and an accumulation

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of 32 scans (using KBr discs method, with a weight ratio of sample to KBr of 1:100).

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The X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-Ultima III X-ray powder

diffractometer with Ni-filtered Cu Kα radiation (λ = 0.15406 nm, operated at 45 kV and 40 mA). A scan speed of 5°·min-1 in 2θ degree was used to record the patterns of the ZnO samples and precursor. The morphology of the sample was investigated by using a scanning electron microscope (SEM; Zeiss Supra 55) with an accelerating voltage of 20 kV. High resolution transmission electron microscopy (HRTEM) images were recorded on JEOL JEM-2010 microscopy with an accelerating voltage of 200 kV. Transmission electron microscopy (TEM) was

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2.3 Measurements of gas sensing performances of ZnO samples Gas sensors were fabricated as follows: the as-prepared powders were mixed with ethanol to form a paste,

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which was coated onto an alumina tube (4 mm in length, 1.2 mm in external diameter, 0.8 mm in internal diameter,

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and previously attached with a pair of gold electrodes) to form a film. A Ni–Cr heating wire was inserted into the

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tube to serve as a heater. The gas sensors were aged at 200 °C for 7 days before the gas sensing measurement. The

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gas-sensing performances of the samples were determined under laboratory conditions (35 ± 5 RH%, 30 ± 5 °C)

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by using the CGS-8 intelligent gas sensing analysis system (Elite Tech Co., Ltd.). The schematic diagram of

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analysis system and experimental process were illustrated in the previous reports [21]. The gas response of the

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sensor was defined as Rg/Ra for oxidizing gas and Ra/Rg for reducing gas, where Ra and Rg were the resistances of

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the sensor in the air and target gas, respectively [22].

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3. Results and discussion

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3.1 Structure of the three-dimensional hierarchical porous ZnO and the precursor

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The crystallographic phase of the samples was characterized by powder XRD. Fig. 1 shows the XRD patterns

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of the precursor and three-dimensional hierarchical porous ZnO with tubular structure. All diffraction peaks of the

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as-obtained precursor can be ascribed to hydrozincite (Zn5(CO3)2(OH)6, JCPDS No. 19-1458). Both of the samples

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obtained by calcining the precursor at 300 °C and 400 °C are pure hexagonal zinc oxide without impurities, which

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are in good agreement with the JCPDS No. 36-1451. In comparison with ZnO-300, the sharper diffraction peaks of

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ZnO-400 indicate its better crystallinity and larger crystal size. The average crystalline size of sample calculated

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while the length is not uniform. The SEM image in Fig. 1b displays that these rod-like hierarchical structures are

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constructed by plenty of nanosheets with thickness of 5-16 nm, and these nanosheets interleaved with each other to

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form a net-like and widely open structure. No other morphology can be detected. After calcining at 300 °C for 2 h,

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the precursor was converted into zinc oxide without obvious change of the size and morphology of the hierarchical

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structures, while these nanosheets become rough. Further increasing the calcining temperature to 400 °C, the

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according to the Scherrer formula and the first four Bragg reflections is ca. 6.57 nm for ZnO-300, and 17.57 nm for ZnO-400, respectively.

3.2 Morphology of the three-dimensional hierarchical porous ZnO and the precursor Fig. 2 shows the SEM images of the three-dimensional hierarchical porous ZnO and the precursor. The

low-magnification SEM image of the precursor shown in Fig. 2a indicates that the sample contains a large number of rod-like hierarchical structures. The outer diameter of these hierarchical structures is in the range of 0.4-0.7 μm,

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thickness of the nanosheet increases, and the morphology and size of the rod-like hierarchical structures are still

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similar to those of the precursor and ZnO-300. Fig. 3a shows typical TEM image of the precursor. The contrast between the dark edge and pale center under

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the TEM image (Fig. 3a) indicates the hollow nature of the precursor, and the internal diameter of the hollow

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structure is in the range of 70-122 nm. Fig. 3b exhibits the HRTEM image of the ZnO-300. The internal diameter

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of the ZnO-300 is about 71-108 nm, which is similar to the precursor, indicating that the calcination at 300 °C has

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little influence on the size and morphology of the precursor. The HRTEM image (Fig. 3c) confirms that the

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nanosheets units are composed of nanoparticles, and there are many pores among these particles. The particle size

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distribution curve of ZnO-300 presented in the inset of Fig. 3c indicates that the diameter of these nanoparticles is

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in the range of 5-16 nm, and the average particle size is about 9.2 nm, which is close to the value calculated from

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the XRD data. The thickness of the nanosheets is similar to the diameter of the nanoparticles, demonstrating that

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those nanoparticles are connected one by one to form monolayered nanosheets. The HRTEM images and inset of

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the size distribution in Fig. 3d indicate that the diameter of nanoparticle for ZnO-400 significantly increased to

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28.7 nm. Previous reports have shown that the widely open and porous structures are significantly favourable to

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the gas sensing response. Therefore, the prepared three-dimensional hierarchical porous ZnO with tubular structure

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are expected to exhibit enhanced gas sensing performance in comparison with the ZnO nanorods. The XPS analysis is carried out to characterize the chemical composition of the three-dimensional

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hierarchical porous ZnO, and the obtained results are shown in Fig. 4. The two peaks at binding energies of 1021

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of ZnO-400 at the first and second peaks, but lower than that of ZnO-400 at the third peak. The chemisorbed

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oxygen could decrease the electron density in the bulk but increase the electron density on the surface. The amount

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of the chemisorbed oxygen on the surface of ZnO-300 is larger than that of ZnO-400, which could be resulted

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from the smaller particle size of ZnO-300. Hence, the electron density of ZnO-300 is lower in the bulk but higher

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on the surface than that of ZnO-400. As a result, the binding energy of O 1s of ZnO-300 is higher in the bulk but

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lower on the surface than that of ZnO-400 [26]. In addition, the relative area of the third peak to the first peak is

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eV and 1044 eV in the XPS spectra of Zn 2p correspond to the Zn2+ 2p3/2 and Zn2+ 2p1/2, respectively. The data of O 1s have been fitted to three peaks. The first peak at lower binding energy (530.2 eV for ZnO-300 and 530.15 eV for ZnO-400) corresponds to the O atoms in ZnO matrix. The second peak located at 531.4 eV for ZnO-300 and 531.35 eV for ZnO-400 can be assigned to Ox- ions (O- and O2- ions) caused by oxygen deficiency [23]. The third peak at 532.15 eV for ZnO-300 and 532.33 eV for ZnO-400 is usually attributed to the chemisorbed or dissociated oxygen or OH species on the surface of ZnO [23-25]. The binding energy of O 1s of ZnO-300 is higher than that

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sharply decreased from 0.594 for ZnO-300 to 0.205 for ZnO-400, suggesting that the amount of the chemisorbed

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oxygen species on the surface of ZnO-300 is much higher than that of ZnO-400 due to the smaller particle size of

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ZnO-300 [27].

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3.3 The formation process of the tubular hierarchical precursor The precursor with different aging time were collected and characterized to study the formation process of the

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hierarchical structures of the precursor. Fig. 5a and 5b show that the precursor without aging is composed of

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spheres with the diameter of several micrometers. The magnified SEM indicates that these spheres are constructed

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by nanorods with smooth surface, and the TEM image (the inset in Fig. 5b) indicates that the nanorods have a

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hollow structure. The spheres still exist after aging for 0.5 h (Fig. 5c and d), and several small nanosheets are

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present at the end side of the nanotubes. When the aging time reaches to 2 h, the sphere is broken and a mixture of

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nanotubes and nanosheets-based hierarchical structures is formed (Fig. 5e and Fig. 5f). All nanotubes are

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transformed into nanosheets-based tubular hierarchical structures when the precursor was aged for 4 h (see Fig. 2a,

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Fig. 2b and Fig. 3a). As the aging time proceeded to 24 h, the precursor still shows tubular hierarchical structures

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without obvious changes of morphology and size that could be observed (Fig. 5g and 5h). The internal diameter of

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the zinc carbonate hydroxide hydrate at 4 h (70-122) and 24 h (72-105 nm) is similar to the diameter of nanotubes

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of the precursor without aging (70-139 nm), indicating that the nanosheets grow in situ on the surface of the

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nanotubes to form the tubular hierarchical structures.

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The crystal structure of precursors with different aging time was characterized by XRD (Fig. 6a). The product

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spectroscopy was further used to investigate the composition of the precursors with different aging time, and the

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obtained results are shown in Fig. 6b. All the precursors have the same absorption bands, and the bands observed

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in the range of 3400-3600 cm-1 are indexed to hydroxyl groups and water. The strong bands centered at 1546 and

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1391 cm-1 are ascribed to the C=O group [28], and the bands centered at 1046 and 834 cm-1 are assigned to the

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CO32- group [29]. The FT-IR analysis together with the XRD data indicate that increasing the aging time did not

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change the constituents but increased the purity and crystallinity of the precursor.

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without aging is a mixture. The diffraction peaks located at 13.1, 16.5, 24.3, 28.4, 30.6, 32.9 and 36.2° can be index to the hydrozincite (Zn5(CO3)2(OH)6, JCPDS No. 19-1458), while the diffraction peaks at 7.7 and 17.8° cannot be indexed to any know materials containing zinc in the JCPDS database. When the aging time was increased to 2 h, the intensity of the diffraction peaks of hydrozincite increased, while the intensity of the diffraction peaks for impurities gradually decrease. Further increasing the aging time to 4 h or longer, the diffraction peaks for impurities disappeared, and only the diffraction peaks of hydrozincite were observed. FT-IR

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The scheme model of the growth process is shown in Fig. 7. The nanotubes of zinc complex were formed at

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first, and then the nanosheets grew onto the surface of the nanotubes. With the increasing of the aging time, the

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amount of the nanosheets increased, and they were standing perpendicularly on the nanotube while intersect with

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each other and forming a highly open structure. The three-dimensional hierarchical porous ZnO were then

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obtained by calcining the precursor at appropriate temperature, and the smooth nanosheets were converted into

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porous nanosheets.

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It is also found that the concentration of NH4HCO3 is very crucial for the formation of the tubular hierarchical

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structures. Fig. 8 shows the SEM images of samples prepared with different concentration of NH4HCO3. When the

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concentration was decreased to 0.5 M, nanofibers were formed at first, but these nanofibers were transferred into

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nanoflakes after aging. When the concentration was increased to 1.5 M, nanofibers were also formed firstly, while

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samples with bamboo leaf shape instead of tubular hierarchical structure were gradually formed after aging for a

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certain time.

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3.4 Textural analysis of three-dimensional hierarchical porous ZnO

Fig. 9 shows the N2 adsorption-desorption isotherms and pore size distribution curves of three-dimensional

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hierarchical porous ZnO. The hysteresis features of the ZnO-300 should be classified as IV type isotherms,

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suggesting the presence of abundant mesopores [30, 31]. The ZnO-400 has III type isotherms, and the abrupt

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increase in the curve at high relative pressure (above 0.9) is due to the presence of macropores in ZnO-400 [30].

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Both of the tubular hierarchical ZnO have type H3 hysteresis loop, which does not show any limiting adsorption at

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distribution, and both of them have smaller pores in the region of 1-3 nm due to the presence of slit-shaped pores.

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The sizes of the larger pores of ZnO-300 are narrowly distributed in the range of 3-50 nm, and the most probable

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pore size is 8.3 nm. The size of the larger pores of ZnO-400 with the most probable pore size of 24 nm ranges from

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8 nm to 100 nm. In a word, ZnO-300 has a higher specific surface area and smaller pore size than ZnO-400

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because of its smaller nanoparticle size [33].

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3.5 The gas sensing performances of the ZnO samples

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the higher side of the pressure, indicating the presence of slit-shaped pores [32]. The ZnO-300 also shows the type H1 hysteresis loop, which is associated with the mesoporous structures and/or the aggregates of nanoparticles with a narrow size distribution. The specific surface area obtained by using the Brunauer-Emmett-Teller (BET) method for the ZnO-300 and ZnO-400 are 78 and 29 m2·g-1, respectively. The large specific surface area of ZnO-300 can be attributed to the highly open structure, monolayered nanoparticles-aggregated porous nanosheets, and the small nanoparticles size. The pore size distribution curves show that both of the samples have a bimodal pore size

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In this work, the prepared three-dimensional hierarchical porous ZnO have high specific surface area and

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highly open, porous, flaky, and tubular structures, which will grant ZnO materials with good gas sensing

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performances. Fig. 10 shows the gas sensing response of ZnO samples towards 50 ppm ethanol as a function of

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operating temperature. With the increase of the operating temperature, the sensing response of all the gas sensors

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increases at first and reaches maximum value at 250 °C and then the gas sensing response decreases. Thus, the

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optimum operating temperature for all the gas sensors is 250 °C. Because sufficient thermal energy is essential to

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overcome the activation energy barrier, the amount of the chemically adsorbed gases increases with the increase of

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the operating temperature, and the gas sensing response increases [34]. However, the gas desorption become

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dominant with the further increase of the operating temperature, and the gas sensing response decreases. The gas

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sensing response reaches maximum when the amount of the adsorbed gases gets to maximum at the optimum

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operating temperature [35]. Fig. 10 also indicates that the sensing responses of 1-D ZnO nanorods (the structures

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and morphology have been presented in the supporting information), ZnO-300, ZnO-400, and commercial ZnO

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towards 50 ppm ethanol are 21.5, 37.5, 33, and 17.5, respectively. Both of the tubular ZnO hierarchical structures

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exhibit higher sensing response than 1-D and commercial ZnO.

The gas sensing response is defined as the resistance change of materials in air circumstance and detected gas.

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The oxygen molecules in air is chemisorbed onto the surface of ZnO and form O2-, O2- and O- by capturing

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electrons from the conduction band, resulting in a high resistance. When the gas sensor is exposed to the reducing

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gases, such as ethanol, H2, and CO, the formed O2-, O2- and O- will react with the reducing gases. As a result, the

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captured electrons are released back into the materials, and the resistance is reduced. The mechanism can be explained by the following chemical reactions [36, 37]:

O2( ads )  e   O2  ( ads )

(1)

O2( ads )  e  2O( ads)

(2)

CH 3CH 2 OH ( ads )  6O  ( ads )  2CO2 ( g )  3 H 2 O( l )  6e 

(3)

The gas sensing response of the materials generally correlates to two processes: diffusion and chemisorption

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of gases [33, 38]. The diffusion of gas depends on the efficient access in the materials. As depicted in the Fig.11,

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the open-ended nanosheets of the tubular hierarchical structures could offer direct pathways for the transportation

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of gases from the out enviroment into the inner part of the materials [9]. Besides, as the pore size of the materials

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is larger than the diameter of the ethanol (0.44 nm), the ethanol molecular could freely pass into and out of pores

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of ZnO materials and can easily contact with the inner surface of the porous ZnO [11]. Therefore, 8

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three-dimensional hierarchical porous ZnO (ZnO-300 and ZnO-400) shows higher gas sensing response than 1-D

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and commercial ZnO. In addition, the specific surface area of the material is a key factor for the adsorption of gas,

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because the higher specific surface area will supply abundant surface active sites for the gas adsorption [38] [39].

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The specific surface area of ZnO-300 is significantly larger than that of ZnO-400, thus ZnO-300 shows higher

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sensing response to ethanol than ZnO-400. Besides, the grain size is also crucial for the gas sensing response of the

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materials. Depletion layer was formed on the surface of ZnO when the electrons of the conduction band were

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captured by the chemisorbed oxygen. The thickness of the depletion layers decreases as the reducing gas was

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injected into the system, and the resistance of the material decreases. When the grain size of semiconducting metal

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oxide is closed to their corresponding 2 Ld (thickness of the depletion layer, ca. 15 nm for ZnO), the nanoparticles

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is almost completely depleted, and the gas sensing response will sharply increase with the decrease of the

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nanoparticles [39]. On the other hand, a significant fraction of the zinc and oxygen atoms are surface atoms for the

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small-size nanoparticles, and these surface atoms can participate in surface reactions. Therefore, the small particle

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is favorable to the chemosorption of gases, which will increases the gas sensing response [11, 23]. In a word, the

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superior gas sensing response of the tubular hierarchical ZnO-300 is due to the small nanoparticle size, large

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specific surface area and efficient gas diffusion access.

Gas sensing tests were carried out at the working temperature of 250 °C by varying the gas concentration. Fig.

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12A shows the real time response curves of ZnO-300 toward ethanol with different concentrations. The response

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of ZnO-300 was maintained unchanged under the air atmosphere (its response is 1). However, the response

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12B, the response and recovery time for ZnO-300 towards 50 ppm of ethanol at the operating temperature of 250

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°C is 14 s and 8 s, respectively.

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significantly increased as the resistance sharply decreased once the ethanol was injected, and then the response retained to 1 with the elimination of the ethanol. When the sensor is exposed to the air, oxygen molecules are adsorbed onto the surface of the sensing materials and ionized by electrons from the conduction band of materials to form chemisorbed oxygen species. When the sensor is exposed to the ethanol, the ethanol molecules will react with the chemisorbed oxygen species and the resistance of the material will decrease. The response and recovery time is defined as the time taken for the response reaching 90% of the final equilibrium value. As shown in Fig.

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Fig. 12C presents the gas sensing response of ZnO-300 to ethanol with different concentrations at the

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operating temperature of 250 °C. The responses are about 160, 136.1, 84.8, 36.6, 12.5, 5.3, 3.2, and 1.6 to ethanol

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with concentration of 500, 200, 100, 50, 20, 10, 5, and 2 ppm, respectively. The sensing response sharply increases

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with the increasing of the concentration of ethanol when the concentration of ethanol is below 200 ppm, and the 9

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response slowly increases when the concentration is above 200 ppm. The sensing response increases slowly when

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the concentration of ethanol is in the range of 200 to 500 ppm, indicating that the adsorption of ethanol gradually

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approaches saturated. The relatively high detection concentration indicates the high saturated adsorption amount of

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gas on the surface and the high specific surface area of the materials [40]. Furthermore, the response dependence on

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the gas concentration can be well fitted by calibration curves (as shown in Fig. 12D). The correlation coefficient

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(R2) obtained by the linear fitting is 0.9941 in the range of 2-100 ppm. The good fitness indicates that the sensor

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can be further developed to analyze the concentration of ethanol.

The gas sensing selectivity is another important parameter for gas sensor. The gas sensing response of

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ZnO-300 to different detected gases at 250 °C are presented in Fig. 13. The results show that the response of all the

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ZnO sensors to ethanol is higher than that to methanol, dimethylbenzene, hexamethylene, and isopropanol,

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indicating that the ZnO sensor has higher selectivity to ethanol. The gas sensing response is related to the

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adsorption and reaction of the detected gases on the surface of materials, and the adsorption of the detected gases

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on the surface of ZnO depends on the nature of the gases, such as the polarity, molecular weight, and structure of

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the gas molecule. According to the gas absorption and surface reaction, the alcohol with larger molecule can be

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easier adsorbed and release more electrons. So the sensors show higher response to ethanol than to methanol.

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However, the adsorption of isopropanol onto the materials will be significantly hindered due to its steric effect, so

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the response to isopropanol is lower than that to ethanol [41]. The C-H bonds in methyl group of dimethylbenzene

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are relatively easier to be broken and oxidated than that of hexamethylene, so the gas sensing response to

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particle size and large specific surface area of ZnO-300 will supply abundant surface active site for the ethanol

26

adsorption and reaction.

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dimethylbenzene is higher than that to hexamethylene. The bond strength of C-H and C-O are 414 and 360 kJ/mol, so CH3OH, C2H5OH, and isopropanol can be decomposed much easily than dimethylbenzene (C8H10) and hexamethylene (C6H12). Hence, the response to ethanol is highest among the five gases [41-43]. Besides, the response of ZnO-300 to ethanol are much higher than those responses of other three ZnO samples, because the ZnO-300 has a superior porous structure, large specific surface area, and small particle size as we discussed in above. The superior porous structure of ZnO-300 will facilitate the efficient ethanol diffusion, and the small

27

Table 1 lists the gas sensing data of the sensor based on ZnO towards ethanol reported in the literature. It

28

shows that the gas sensor in this work exhibits lower operating temperature, higher sensing response, faster

29

recovery to ethanol than most of the others. The relatively low operating temperature of ZnO in this is due to its

30

highly-open three-dimensional hierarchical porous structures, which could provide numerous connective channels 10

Page 10 of 32

for the diffusion of gas [31]. The low operating temperature could avoid the growth of nano-crystallite and prolong

2

the usage of sensors, which favors their applications in practice [44]. Hence, the superior gas sensing performance

3

of the ZnO-300 makes it a potential material for the fabrication of ethanol sensor.

4

4. Conclusions

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Three-dimensional hierarchical porous ZnO have been prepared by a two-step method. A precursor of

6

hydrozincite with tubular hierarchical structure composed of nanosheets has been prepared firstly by a simple

7

precipitation method at room-temperature in the absence of foreign templates and surfactant agent, and then the

8

three-dimensional hierarchical porous ZnO with a specific surface area of 78 m2·g-1 were obtained by calcining the

9

precursor at 300 °C. The prepared tubular three-dimensional hierarchical porous ZnO were composed of

10

nanoparticles-based nanosheets with an average particle size of about 9.2 nm, and the outer and inner diameters of

11

the tubular hierarchical structures are 0.4-0.7 μm and 71-108 nm, respectively. The gas sensing performances of

12

the prepared ZnO samples have also been investigated, and the results indicate that the prepared three-dimensional

13

hierarchical porous ZnO exhibit low operating temperature, high and fast response, and good selectivity to ethanol.

14

The superior gas sensing performance of the three-dimensional hierarchical porous ZnO is essentially attributed to

15

its highly open and porous structure, high specific surface area, and small nanoparticle size.

17

Acknowledgment

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19 20 21 22 23 24 25 26 27 28 29 30 31

This work was supported by the National Natural Science Foundation of China, the Fundamental Research

Funds for the Central Universities (YS1406), and the State Key Laboratory of Chemical Resource Engineering.

References

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[47] D. Wang, S. Du, X. Zhou, B. Wang, J. Ma, P. Sun, Y. Sun, G. Lu, Template-free synthesis and gas sensing properties of hierarchical hollow ZnO microspheres, CrystEngComm, 15 (2013), pp. 7438-7442. [48] G. Lu, X. Wang, J. Liu, S. Qiu, C. He, B. Li, W. Liu, One-pot synthesis and gas sensing properties of ZnO mesoporous architectures, Sens. Actuators, B, 184 (2013), pp. 85-92. [49] Y. Cai, H. Fan, M. Xu, Q. Li, C. Long, Fast economical synthesis of Fe-doped ZnO hierarchical nanostructures and their high gas-sensing performance, CrystEngComm, 15 (2013), pp. 7339-7345. [50] R. Zou, G. He, K. Xu, Q. Liu, Z. Zhang, J. Hu, ZnO nanorods on reduced graphene sheets with excellent field

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emission, gas sensor and photocatalytic properties, J. Mater. Chem. A, 1 (2013), pp. 8445-8452.

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Table 1. Comparison of the sensing properties of different ZnO material toward ethanol.

Concentration

approaches

(ppm)

Temperature

Sensor

(ºC)

response

Microwave ZnO core-shell

hydrothermal

300

500

300

171

100

300

28.9

100

275

8

100

460

26.1

100

370

50

270

method ZnO added MoO3 ZnO nanowires

Chemical solution route

Hierarchical hollow

Hydrothermal

ZnO microspheres

synthesis

ZnO mesoporous

Solvothermal

architectures

approach Economical solution

Fe-doped ZnO

combustion

ZnO nanorods on Three-dimensional hierarchical porous ZnO

Hydrothermal route Precipitation

50

methods

250

recovery time

References

(s) 1 and 27

[45]

75 and 75

[46]

[11]

1 and 19

[47]

6 and 38

[48]

45

10 and 6

[49]

27.5

9 and 18

[50]

14 and 8

This work

36.6

M

2

15)

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synthesis reduced graphene

31 (and

200 ( and 50)

Response and

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Ethanol

Synthesis

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Materials

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Biographies

2

Dianqing Li received his MS degree in 1989 at Beijing Institute of Chemical Technology and PhD degree in 2001

3

at Tianjin University. His research interests are the development and application of functional inorganic materials.

4

He is currently a professor in the State Key Laboratory of Chemical Resource Engineering at Beijing University of

5

Chemical Technology (BUCT).

6

Faying Fan received her bachelor’s degree from BUCT and is currently pursuing a Ph.D degree at BUCT. Her

7

research project involves sensing materials and environmentally friendly catalytic materials.

8

Pinggui Tang received his PhD degree in 2011 at BUCT. He is a lecturer in the College of Science in BUCT. His

9

research is focused on intercalation chemistry of layered double hydroxides and inorganic functional materials.

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Yongjun Feng obtained his Ph.D. in 2007 at the Inorganic Materials Laboratory at Blaise Pascal University

11

(Clermont-Ferrand, France) and was a postdoctoral fellow at the University of Poitiers (France) for Prof. N.

12

Alonso-Vante. Now, he has been working as associate professor in Applied Chemistry at BUCT. Now he focuses

13

on developing layered functional materials, novel catalyst support materials and non-platinum metal

14

electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs).

15

Yuanyuan Wang received her bachelor’s degree in 2014 at BUCT.

16

Ruixian Luo was a postdoctoral research associate at the Department of Chemical Engineering in Purdue

17

University in 1985–1987. She was a visiting professor in Case Western Reserve University (CWRU) in 1988, 1994,

18

1996 and 1998. She is now pursuing academic research into chemical sensors and mentoring graduate students at

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BUCT.

Aifan Chen was a visiting professor at Purdue University and CWRU in 1986–1988. He is a professor in the College of Science at BUCT. His research interests include the development of sensing materials and chemical sensors.

16

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Figures captions

2

Fig. 1 XRD patterns of three-dimensional hierarchical porous ZnO with tubular structure and the precursor.

3

Vertical lines on the bottom represent JCPDS PDF card No. 36-1451 and No. 19-1458 for hexaganol ZnO and

4

hydrozincite, respectively.

5

Fig. 2 SEM images of (a, b) the precursor, (c) ZnO-300, and (d) ZnO-400.

6

Fig. 3 HRTEM images of (a) the precursor, (b, c) ZnO-300, and (d) ZnO-400. Insets in (c) and (d) are the size

7

distributions of the ZnO nanoparticles.

8

Fig. 4 XPS spectra of Zn 2p and O 1s of ZnO-300 and ZnO-400.

9

Fig. 5 SEM images of precursors with aging time of (a, b) 0 h, (c, d) 0.5 h, (e, f) 2 h, and (g, h) 24 h.

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cr

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1

Fig. 6 (a) XRD patterns and (b) FT-IR spectra of precursors prepared with different aging time. Vertical lines on

11

the bottom in (a) represent JCPDS PDF card No. 19-1458 for hydrozincite.

12

Fig. 7 The scheme model of the formation process of three-dimensional hierarchical porous ZnO with tubular

13

structure.

14

Fig. 8 SEM images of precursors prepared with 0.5 M NH4HCO3 and aging time of (a) 0 h, (b) 4 h, and (c) 24 h,

15

and precursors prepared with 1.5 M NH4HCO3 and aging time of (d) 0 h, (e) 4 h, and (f) 24 h.

16

Fig. 9 N2 adsorption-desorption isotherms (left) and pore size distribution curves (right) of ZnO-300 and ZnO-400.

17

Fig. 10 Gas sensing response of ZnO samples to 50 ppm ethanol at different operating temperature.

18

Fig. 11 The scheme model of the diffusion of gases in the three-dimensional hierarchical porous ZnO with tubular

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10

structure.

Fig. 12 (A) Real time response curves of ZnO-300 dependence on the concentration of ethanol at 250 °C. (B) Gas

sensing response and recovery time of ZnO-300 to 50 ppm ethanol at 250 °C. (C) Gas sensing response and (D) linear fitting of the sensing response of ZnO-300 towards different concentration of ethanol. Fig. 13 Gas sensing selectivity of ZnO samples to different gases with the concentration of 50 ppm at 250 °C.

25 26 27

17

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Fig. 1 XRD patterns of three-dimensional hierarchical porous ZnO with tubular structure and the precursor.

3

Vertical lines on the bottom represent JCPDS PDF card No. 36-1451 and No. 19-1458 for hexaganol ZnO and

4

hydrozincite, respectively.

an

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Fig. 2 SEM images of (a, b) the precursor, (c) ZnO-300, and (d) ZnO-400.

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Fig. 3 HRTEM images of (a) the precursor, (b, c) ZnO-300, and (d) ZnO-400. Insets in (c) and (d) are the size

4

distributions of the ZnO nanoparticles.

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Fig. 4 XPS spectra of Zn 2p and O 1s of ZnO-300 and ZnO-400.

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Fig. 5 SEM images of precursors with aging time of (a, b) 0 h, (c, d) 0.5 h, (e, f) 2 h, and (g, h) 24 h.

4

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Fig. 6 (a) XRD patterns and (b) FT-IR spectra of precursors prepared with different aging time. Vertical lines on

3

the bottom in (a) represent JCPDS PDF card No. 19-1458 for hydrozincite.

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Fig. 7 The scheme model of the formation process of three-dimensional hierarchical porous ZnO with tubular

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structure.

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Fig. 8 SEM images of precursors prepared with 0.5 M NH4HCO3 and aging time of (a) 0 h, (b) 4 h, and (c) 24 h,

4

and precursors prepared with 1.5 M NH4HCO3 and aging time of (d) 0 h, (e) 4 h, and (f) 24 h.

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Fig. 9 N2 adsorption-desorption isotherms (left) and pore size distribution curves (right) of ZnO-300 and ZnO-400.

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Fig. 10 Gas sensing response of ZnO samples to 50 ppm ethanol at different operating temperature.

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1 Fig. 11 The scheme model of the diffusion of gases in the three-dimensional hierarchical porous ZnO with tubular

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Fig. 12 (A) Real time response curves of ZnO-300 dependence on the concentration of ethanol at 250 °C. (B) Gas

3

sensing response and recovery time of ZnO-300 to 50 ppm ethanol at 250 °C. (C) Gas sensing response and (D)

4

linear fitting of the sensing response of ZnO-300 towards different concentration of ethanol.

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Fig. 13 Gas sensing selectivity of ZnO samples to different gases with the concentration of 50 ppm at 250 °C.

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Graphical abstract

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2

3

Three-dimensional hierarchical porous ZnO with tubular structure assembled by porous nanosheets has been

5

prepared. The obtained ZnO have specific surface area of 78 m2·g-1, and shows superior gas sensing performance

6

toward ethanol.

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1

Highlights

2 3 Three-dimensional hierarchical porous ZnO with tubular structure has been prepared.

5 The tubular hierarchical ZnO has widely open and porous structure.

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4 The formation process of the tubular hierarchical porous ZnO has been investigated.

6 Three-dimensional hierarchical porous ZnO shows high sensing response to ethanol.

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