Hydrothermal synthesis of novel SnO2 nanoflowers and their gas-sensing properties

Hydrothermal synthesis of novel SnO2 nanoflowers and their gas-sensing properties

Author's Accepted Manuscript Hydrothermal synthesis of novel SnO2 nanoflowers and their gas-sensing properties Mingyu Wu, Wen Zeng, Yanqiong Li www...

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Author's Accepted Manuscript

Hydrothermal synthesis of novel SnO2 nanoflowers and their gas-sensing properties Mingyu Wu, Wen Zeng, Yanqiong Li

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S0167-577X(13)00484-9 http://dx.doi.org/10.1016/j.matlet.2013.04.010 MLBLUE15136

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Materials Letters

Received date: 28 January 2013 Accepted date: 4 April 2013 Cite this article as: Mingyu Wu, Wen Zeng, Yanqiong Li, Hydrothermal synthesis of novel SnO2 nanoflowers and their gas-sensing properties, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2013.04.010 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 galley proof before it is published in its final citable 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.

Hydrothermal synthesis of novel SnO2 nanoflowers and their gas-sensing properties Mingyu Wu, Wen Zeng*, Yanqiong Li College of Materials Science and Engineering, Chongqing University, Chongqing China 400030

Abstract: Self-assembly of one-dimensional nanoscale building blocks into functional 2D or 3D complex superstructures has stimulated a great deal of interest. In current work, using hydrothermal method and reagent of hexamethylenetetramine (HMT), we synthesize the SnO2 3D hierarchical nanostructures with an average diameter 200-400 nm, which exhibit flower-like architectures assembled by numerous one-dimensional tetragonal prism nanorods. Further comparative studies demonstrate that the HMT provides nucleation sites for the assembling of the nanorods, which plays a crucial role in producing such unique flower-like architectures. Meantime, a novel growth mechanism is proposed in detail. In property, the prepared SnO2 nanoflowers show excellent gas-sensing performances to ethanol of 50 ppm at an optimal temperature as low as 250oC. Such unique architectures may open up an avenue to further enhance the gas-sensing performances of SnO2 nanostructures for future sensor application.

Keywords: SnO2; Crystal growth; Functional; Ethanol; Gas sensors

*

Corresponding authors. E-mail: [email protected]



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TEL: +86-023-65102465

1. Introduction As a direct wide band gap n-type semiconductor, tin oxide (SnO2)has received considerable attention in both fundamental studies and practical applications due to unique properties such as photocatalysis, gas-sensing etc.[1, 2]. Since many fundamental physical and chemical properties of semiconductor materials rely not only on the composition but also structure, phase, shape and size, synthesis of nano- or micro-sized materials with a tunable shape or morphology has been the subject of insensitive research [3-5]. Recently, numerous studies have demonstrated that materials with large surface area, efficient catalytic activity, and structural stability, such as three-dimensional (3D) nanostructures (e.g., nanospheres[6], nanopyramids[7], nanoaloes[8], and nanoflowers[9]), exhibit enhanced gas-sensing performance in comparison with conventional bulk materials

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nanorods/nanowires/nanosheets as building blocks into 3D complex superstructures. These 3D hierarchical nanostructures are considered to be the most effective and promising candidate due to their porous nanostructures composed by the adjacent building blocks. Therefore, a significant challenging work to develop facile, solution-based, and shape-controlled self-assembly routes for the formation of SnO2 complex architectures from 1D nanocrystals. In this paper, we report the facile synthesis of unique SnO2 nanoflowers assembled from nanorods via the HMT-assisted hydrothermal process. Based on comparative studies, the possible formation mechanism was discussed in detail. Finally, their gas-sensing properties were investigated too. The as-synthesized SnO2 nanoflowers exhibit a high gas-sensing activity for ethanol.



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2. Experimental

All chemicals are analytical grade and used as received without further purification. Typically, 20ml deionized water, 20ml absolute ethanol, 0.4g SnCl4·5H2O, 0.5g NaOH, and HMT (30 mM) were mixed with vigorous stirring. Then the mixture was transferred into a teflon-lined stainless steel autoclave (50ml), sealed and maintained at 180oC for 24h. After the heating treatment, autoclave was cooled to room temperature naturally. The product of SnO2 nanoflowers was obtained. The ingredients and microstructures of the as-synthesized products were characterized by X-ray diffraction (XRD) with a Rigaku D/Max-1200 diffractometry employing Cu K radiation operated at 30kV and 100mA and Nova 400 Nano FESEM, respectively. The gas sensing properties were measured using a computer-controlled sensing system (HW-30A, Hanwei Electronics Co. Ltd.) under the lad conditions. In this paper, the relative response(S) of the sensor was defined as S=Ra/Rg at reductive atmosphere, while as S=Rg/Ra at oxidative atmosphere, where Ra, Rg were resistance in air and tested gas, respectively. The response and recovery time were dened as the time for reaching 90% of the full response change of sensor from the maximum and dropping to 10% of the maximum, respectively. 3. Results and discussion Fig. 1(a) illustrates the typical diffraction pattern for the prepared sample, and all the peaks can be well indexed to the tetragonal rutile SnO2 structure (JCPDS card NO. 41-1445). No diffraction peaks from any other impurities were observed, indicating the high purity of the SnO2 products. Fig. 1(b) shows a FESEM image of the as-synthesized product. 3D SnO2 flower-like



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structures with an average diameter of 200-400 nm were clearly obtained. Furthermore, one can see the uniform SnO2 nanoflowers are actually composed of 1D tetragonal prism nanorods. To gain insight into formation mechanism of the nanoflowers and how morphology evolves with processing conditions, we have performed a straightforward comparative investigation by altering the concentration of HMT and treating at different temperatures for 24h. The sample prepared with no HMT shows irregular morphology (Fig. 2(a)). However, Once the HMT (10 mM) is added at 160 °C, rod-like structures turn up (Fig. 2(b)). As the temperature of solution is increased to 180 °C, one can see a larger amount of rod-like nanostructures distributed in the sample (Fig. 2(c)). When the concentration of HMT is increased to 20 mM, more and more nanorods form, these nanorods aggregate irregularly at 160 °C (Fig. 2(d)). When the temperature reaches 180 °C, flower-like microstructures obtain (Fig. 2(e)). Hence, we may infer that decreasing temperature, nuclei are liable to aggregate. Moreover, lower temperature leads to lower crystalline growth velocity. By increasing the concentration of HMT, many nucleation sites form, which as a result decreasing size of 3D structure. Figure 3 illustrates schematically a formation process of SnO2 nanoflowers based on the above experimental observations. It is known that the HMT, which acts as a nonionic surfactant, allow the formation of four hydrogen bonding interactions: N–H···O or O–H···N, O–H···O, C–H···O and N–H···N at low temperature. These hydrogen bonded structures may create a three dimensional framework by electrostatic interaction [10]. This makes the Sn(OH)62- able to accumulate on the three dimensional framework in solution. Under the current hydrothermal treatment, numerous tiny SnO2 crystals will therefore nucleate on the HMT framework and grow, which as a result forms many nucleation sites. It is reported that, in order of reducing energy, the



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surfaces form the sequence for SnO2 crystal (001)> (101)> (100)> (110) [11], which led to the preferential growth direction along [001]. However, the centrosymmetric structure of low-axial ratios (c/a = 0.673) increases the probability of anisotropic growth of the SnO2 crystal along [110] [12]. Finally, the SnO2 nanoflowers with many rod-like structures form via the oriented attachment process. The gas responses of the sensor made of nanoflowers was measured by exposing it to various gases, including oxidation and reduction gases (C2H6O, H2S, H2, C2H2, C3H6O, CO2, and C2H4) at 50 ppm as shown in Fig. 4(a). The decrease of the gas response can be attributed the decrease of the surface area available of the SnO2 nanostructures. Clearly, the SnO2 nanoflowers sensor exhibits obvious response to ethanol reaching a maximum value of 58.6 but no longer than 10 to other gases. Fig. 4 (b) shows gas-sensing response of the SnO2 nanoflowers to ethanol of 50 ppm as a function of the working temperature ranging from 100 to 500°C. The optimal sensitivity is estimated to be 63.1 at the operating temperature of 250 °C. The response changes of the gas sensor to different ethanol gas concentrations (5-100 ppm) are shown in Fig. 4(c). The sensor responses to 5, 10, 30, 50, and 100 ppm ethanol are 11.5, 21.3, 40.1, 63.1 and 76.5, respectively. These values are relatively higher compared with other SnO2 nanostructures [1, 4, 8, 9, 13, 14, 15]. The significantly high response obtained in this study is attributed to the larger specific surface area for electrons, oxygen and target gas molecules, and abundant channels for gas diffusion and mass transport. 4. Conclusions In summary, we successfully synthesized 3D SnO2 unique flower-like morphology via a simple HMT-assisted hydrothermal process. The SnO2 nanoflowers were formed by the



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self-assembly of numerous nanorods. Here, HMT was introduced as an assembling and structure-directing agent to controllable synthesis of SnO2 nanostructures and the concentration of HMT played a critical role in producing SnO2 morphologies. By the measurement of gas sensing properties, SnO2 nanoflowers showed the perfect performances toward the ethanol at low temperature, suggesting the potential applications as advanced gas sensing materials. Acknowledgements This work was supported in part by National Natural Science of China (51202302), Fundamental Research Funds for the Central Universities (No.CDJZR12110051), and the National Undergraduate Innovative Project of China (No.1110611007).

References [1] Khoang ND, Trung DD, Duy NV, Hoa ND, Hieu NV. Sens Actuators B 2012; 174: 594– 601 [2] Li HH, Meng FL, Liu JY, Sun YF, Jin Z, Kong LT, Hu YJ, Liu JH. Sens Actuators B 2012; 166-167: 519– 525 [3] Park JY, Choi SW, Kim SS. J Phys Chem C 2011; 115: 12774–12781 [4] Shi L, Lin HL. Langmuir 2011; 27: 3977–3981 [5] Wang B, Zhu LF, Yang YH, Xu NS, Yang GW. J Phys Chem C 2008; 112: 6643-6647 [6] Li ZP, Zhao QQ, Fan WL, Zhan JH. Nanoscale 2011; 3: 1646-1652 [7] Deshmukh RG, Badadhe SS, Vaishampayan MV, Mulla IS. Mater Lett 62 2008; 62: 4328-4331 [8] Mei L, Deng JW, Yin XM, Zhang M, Li QH, Zhang ED, Xu Z, Chen LB, Wang TH. Sens Actuators B 2012; 166–167: 7–11 [9] Wang H, Liang QQ, Wang WJ, An YR, Li JH, Guo L. Cryst Growth Des 2011; 11: 2942–2947 

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[10] Das S, Kim DY, Choi CM, Hahn YB. J Cryst Growth 2011; 314: 171–179 [11] Liu B, Zhang LH, Zhao H, Chen Y, Yang HQ. Sens Actuators B 2012; 173: 643– 651 [12] Kim DH, Kim WS, Lee SB, Hong SH. Sens Actuators B 2010; 147: 653–659 [13] Song F, Su HL, Han J, Lau WM, Moon WJ, Zhang D. J Phys Chem C 2012; 116: 10274 10281 [14] Wang HK, Fu F, Zhang FH, Wang HE, Kershaw SV, Xu JQ, Sun SG, Rogach AL. J Mater Chem 2012; 22: 2140–2148 [15] Zeng W, Miao B, Zhou Q, Lin LY, Physica E 2013; 47:116–121

Figure Captions Fig.1 (a) XRD patterns and (b) SEM image of the as-prepared SnO2 nanoflowers. Fig.2 SEM images of the samples prepared with various concentrations of HMT: (a) 0 mM, (b) 10 mM, (d) 20 mM at 160 °C, (c) 10 mM, (e) 20 mM at 180 °C. Fig.3 Schematic illustration of the formation processes of nanoflowers. Fig.4 (a) Response of the sensor made of nanoflowers to different tested gases. (b) Sensitivity of the sensor to the ethanol at temperatures from 100 to 500 °C. (c) Response of the sensor to different concentrations of ethanol.

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¾ Unique nanoflowers were firstly reported in the field of SnO2 nanostructures by hydrothermal process. ¾ A novel growth mechanism is proposed based on comparative studies. ¾ SnO2 nanoflowers showed the excellent properties toward the ethanol. ¾ Enhanced key gas sensing property meet basic needs for practical applications.



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Graphic Abstracts In this work, we prepare SnO2 nanoflowers through a simple HMT-assisted hydrothermal process, and report systematically their gas-sensing properties.

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