Ultrasonic Wave Assisted Preparation and Properties of SiO 2 Aerogels. by Fractional Modification. Haijian Chen 1,e

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1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Ultrasonic Wave Assisted Preparation and Properties of SiO 2 Aerogels by Fractional Modification Weiming Yao 1, a, Wenzhi Zheng 1,b, Hechao Pan 1,c, Zhenyue Chi 1,d, Haijian Chen 1,e 1 School of Chemical Science and Chemical Engineering, GuangZhou University, Guangdong Guangzhou , China a viming.yao@163.com, b wenzhizheng@126.com, c panhechao@163.com, d zhenyue306@163.com, e ephone_lee520@yahoo.com.cn Keywords: Hydrophobic modification; SiO 2 aerogels; Fractional modification; Ultrasonic wave. Abstract. In this study, hydrophobic SiO 2 aerogel was prepared through fractional hydrophobic modification and ambient pressure drying of the raw material, tetraethoxysilane (TEOS) under ultrasonic wave and using trimethyl chlorosilane as modifier and acid catalyst in the process of sol-gel and acid-base catalysis. SiO 2 aerogels were analyzed and compared using scanning electron microscopy (SEM), transmission electron microscope (TEM), Brunauer-Emmett-Teller analysis method, Fourier transform infrared spectroscopy, X-ray diffraction. The results indicate that by adopting the ultrasonic wave, the specific surface area of SiO 2 aerogels was m 2 /g. Pore distribution in micro-pore range was nm. Pore distribution in meso-pore range was nm. The hydrophobic SiO 2 aerogels with the hydrophobic angle at 160 displayed super hydrophobicity. Introduction SiO 2 aerogel is a new multi-pore amorphous nano-material [1]. The existence of a pore structure results in an extremely large specific surface area ( m 2 /g), with very high porosity (80%-99.8%) and very low density (1-500 kg/m 3 ) [2,3] for SiO 2 aerogels. The material is widely used in thermal insulation materials, catalysis, environmental protection, optics, electricity, and other fields [4]. Moreover SiO 2 aerogels hydrophobic modification has been a hot research [5]. Hydrophobic modification usually has two kinds of methods-copolymerization and surface derivative [6]. Chlorotrimethylsilane is a common SiO 2 aerogel hydrophobic modifier [7] and it is an effective method using ultrasound technology to prepare nano-materials [8]. In this paper, with tetraethoxysilane as the silicon source and trimethyl chlorosilane as the hydrophobic modifier, hydrophobic SiO 2 aerogels were prepared by combining fractional hydrophobic modification under ultrasonic wave in ambient drying process. Their structures and performances were characterized and contrasted to provide basic information for the application study of SiO 2 aerogels. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-18/05/16,09:19:32)

2 1038 Advanced Materials and Processes II Experiment Preparation of Material. Hydrophobic SiO 2 aerogels were prepared using the ultrasonic wave method by fractional modification. First, under ultrasonic wave, tetraethoxysilane (TEOS), ethanol (EtOH) and double distilled water were mixed at 1:3:0.5:6, with the ph value adjusted to 3-4 by dropping trimethyl chlorosilane into the mixed liquor. Next, hydrolysis was conducted for 20 min at 70 C with the ph value adjusted to 7-8 by adding ammonia into the mixed liquor. The surface of the prepared SiO 2 alcogels was modified for 24 h in the ethanol solution of trimethyl chlorosilane. Trimethyl chlorosilane surface modification of ethanol solution for 24 h. Then, cyclohexane was aged for 24 h and then dried at ladder-elevating temperature and ambient pressure for 24 h to prepare the hydrophobic silica aerogels. Instruments and Characterization Methods. BILON-1000D ultrasonic reactor provided by Shanghai Bilon Instrument Co., LTD and S-3700N Scanning Electron Microscope (SEM) provided by Hitachi Corporation in Japan were used to observe the morphology of the aerogels. The accelerating voltage was 25 kv. After metal spraying treatment on the surface, the pore structure and particle size distribution on the surface of the sample were observed at a certain magnification. JEM-2010HR Transmission electron microscope(tem) provided by Japanese Electronics Co., LTD were used to observe the pore structure and particle size distribution on the surface of the sample. TENSOR27 Fourier Transform Infrared Spectrometer (FTIR) provided by the BRUKER Corporation in France was adopted to determine FTIR spectra of the aerogels using KBr pellet pressuring method to analyze the chemical groups of SiO 2 aerogels. XD-3 type X-ray Diffractometer provided by Beijing Purkinje General Instrument Co., Ltd in China was used to determine the XRD spectra of the samples and confirm the phases of the samples with the Ka radiation excited by Cu target as the radiation source, setting pipe voltage to 35 kv, current at 20 ma, scanning range at o and scanning speed of 40/min. ASAP2000 Specific Surface Area Analyzer provided by Micromeritics Instrument Corporation in the United States was used to collect data of adsorption curves with N 2 as adsorption medium, with the sample pre-treated at 250 C and tested at the temperature of -196 C. The specific surface areas of hydrophobic gels were calculated by adopting the theory of Brunauer-Emmett-Teller (BET). The meso-pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) method. The micro-pore area was calculated using Horvath-Kawazoe method. The contact angles of water drop on the surface of the aerogels were determined using a contact angle tester with Ellipse Fitting method to confirm the hydrophobic degree of the aerogels. Results and Discussions Infrared Spectrum and Hydrophobic Performance Fig. 1. FTIR of silica aerogels Fig. 2. Picture of hydrophobic silica aerogels Fig. 1 displays the infrared spectrum of unmodified and hydrophobically modified SiO 2 aerogels, with the infrared spectrum of the Ultrasonic wave shake and hydrophobically using TMCS modified SiO 2 aerogel labeled Modified, while the infrared spectra of the unmodified SiO 2 aerogel

3 Advanced Materials Research Vols labeled Unmodified. The high absorption peaks at 1080 cm -1 and 462 cm -1 were the characteristic absorption spectral bands of the anti-symmetry stretching vibration, symmetry stretching vibration, as well as the planar rocking and blending vibrations of the Si-O-Si that comprise the framework of SiO 2 aerogels. There were no obvious changes in the absorption peaks before and after modification. Moreover, the peak area at 1080 cm -1 was relatively large, indicating that the silica framework was the main component before and after modification. Fig. 2 shows that the ultrasonic wave shake of the hydrophobic SiO 2 aerogels prepared by gradual modification using TMCS reached to 160 o, indicating a good hydrophobic performance. N 2 Adsorption-desorption Isotherms, Surface Morphology and Analysis on Pore Distribution and Microstructure Fig. 3. SEM photograph of silica aerogels Fig. 4. TEM photograph of silica aerogels Figs. 3 to 4 illustrate SEM photograph and TEM photograph of silica aerogel modified with the infrared spectrum of the Ultrasonic wave shake and hydrophobically using TMCS. The SEM and TEM photograph show that the Microscopic structure of SiO 2 aerogels is composed of nano spherical particles which are between 30 nm and 100 nm. They also have numerous pores and are of a typical Nanoporous material. Fig. 5. Nitrogen adsorption hydrophilic SiO 2 aerogels prepared by using ultrasonic wave Fig. 6. Nitrogen adsorption hydrophilic SiO 2 aerogels prepared by using ultrasonic wave Fig.7. Nitrogen adsorption hydrophilic SiO 2 aerogels prepared by using ultrasonic wave Figs. 5, 6 and 7 are the pore distribution in micro-pore range, pore distribution in meso-pore range, Nitrogen adsorption isotherms of SiO 2 aerogels prepared with the three methods. In the diagrams, Fig. 5 are the hydrophobic SiO 2 aerogels prepared using ultrasonic wave, Fig. 6 are the hydrophilic SiO 2 aerogels prepared using ultrasonic wave, and Fig. 7 are the hydrophilic SiO 2 aerogels prepared with the water bath heating method. The specific surface area of hydrophobic SiO 2 aerogels prepared by fractional hydrophobic modification and ultrasonic wave was m 2 /g, which showed that under the conditions of ultrasonic dispersion the SiO 2 aerogels were connected to -Si(CH 3 ) 3, the three methyls of which formed a hydrophobic structure with an umbrella shape. This umbrella-shaped hydrophobic structure connected to the SiO 2 would link to each other and form a hydrophobic membrane which means more conducive to the dispersion of aerogels particles to further improve the specific surface area of aerogels.

4 1040 Advanced Materials and Processes II The pore size of the micro-pores and meso-pores of SiO 2 aerogels prepared using the three methods could be seen from Figs. 5, 6 and 7. Pore distribation in meso-pores ranged between 2.0 nm and 20 nm, mostly between 2 nm and 4.5 nm. Pore distribution in micro-pore ranged from 0.5 nm to 1 nm. The Phase Determination of the Materials Fig. 8 are the XRD curves of the hydrophilic SiO 2 aerogels prepared by water bath heating (a), the hydrophilic SiO 2 aerogels prepared by ultralline wave (b), and the hydrophobic SiO 2 aerogels prepared by ultralline wave (c). None of the three curves exhibit obvious characteristic crystalline diffraction peak clearly. There was only an obvious non-crystalline dispersion peak in the range of 2θ 20-25, and the peak was relatively low, indicating that the SiO 2 aerogels prepared by water bath heating method and prepared by ultralline wave heating method were disordered amorphous silica. Fig. 8. X-Ray diffraction for silica nanometer porous material Conclusion Under ultrasonic wave and using trimethy chlorosilane as modifier, hydrophobic SiO 2 aerogel pepared through fractional hydrophobic modification and ambient pressure drying demonstrates super hydrophobic performance with ranged micro-pores, meso-pores and a specific surface area of m 2 /g. Moreover, the SiO 2 aerogel prepared hereby still remains as disordered amorphous silica. References [1] F. Shi, L.J. Wang: Materials Letters. (2006) 60: [2] J.J. Zhu, J. Yao, X.M. Lv, J.J. Ding, F.H. Du, J.M. Xie: Journal of The Chinese Ceramic Society. (2009) 37: [3] H.E. Rassy, P. Buisson, B. Bouali, A. Perrard, A.C. Pierre: Langmuir. (2003) 19: [4] N. Leventis, S. Mulik, X.J. Wang: Journal of Non-Crystalline Solids. (2008) 354: [5] P.B. Sarawade, J.K. Kim, H.K. Kim, H.T. Kim: Applied Surface Science. (2007) 254: [6] B. Zhou, J. Shen, Y.H. Wu, G.M. Wu, X.Y. Ni: Materials Science and Engineering C. (2007) 27: [7] H. Wang, S.M. Liu, X.S. Cheng, S. Anze, P.A. Liu, L.K. Zeng: Journal of the Chinese Ceramic Society. (2009) 37: [8] R.E. Challis, F. Blarel, M.E. Unwin, J. Paul, X.N. Guo: IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control. (2009) 56:

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