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Effects of modifiers on the hydrophobicity of SiO2 films from nano-husk ash
Accepted Manuscript Title: Effects of modifiers on the hydrophobicity of SiO2 films from nano-husk ash Author: Kejing Xu Qingwen Sun Yanqing Guo Shuhua Dong PII: DOI: Reference:
S0169-4332(13)00671-5 http://dx.doi.org/doi:10.1016/j.apsusc.2013.03.173 APSUSC 25463
To appear in:
APSUSC
Received date: Revised date: Accepted date:
25-9-2012 28-3-2013 29-3-2013
Please cite this article as: K. Xu, Q. Sun, Y. Guo, S. Dong, Effects of modifiers on the hydrophobicity of SiO2 films from nano-husk ash, Applied Surface Science (2013), http://dx.doi.org/10.1016/j.apsusc.2013.03.173 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.
*Highlights (for review)
Highlights ► Nano-husk ashes were prepared by burning rice-husk with self-propagating method.
►The super-hydrophobic SiO2 films were prepared from nano-husk ashes with modifiers. ► The contact angle of the SiO2 film modified with modifiers was more
ip t
than 160 º. ►The modifications of modifiers on SiO2 films were studied
Ac
ce pt
ed
M
an
us
cr
systematically.
Page 1 of 18
Graphical Abstract (for review)
cr
ip t
Graphical Abstract
with HSO
Ac
ce pt
ed
M
an
us
Without any modifier
1
Page 2 of 18
Effects of modifiers on the hydrophobicity of SiO2 films from nano-husk ash Kejing Xu, Qingwen Sun, Yanqing Guo and Shuhua Dong School of Materials Science and Engineering, Shandong University of Technology, 255049, China
Abstract: Nano-husk ashes were prepared by burning rice-husk with
ip t
self-propagating method. The super-hydrophobic SiO2 films from nano-husk ash were prepared by sol-gel method using hydroxy silicone oil (HSO), hexamethyldisilazane
cr
(HMS), or methyl triethoxysilane (MTS) as modifiers. The effects of modifiers on the hydrophobic property of SiO2 films were studied, and the performances were
us
characterized by the XRD, UV–Vis, BET, EDS, SEM, IR, and Contact Angle Analizer. The results showed that the contact angle of SiO2 films was more than 160 º when
an
volume ratio of the modifiers to silicon sodium solution (SSS) was 0.15. The mechanism of modifiers on SiO2 surfaces is a graft copolymerization. The hydrophobic groups in the modifiers replace the hydroxy groups on SiO2 surfaces and
M
make SiO2 surfaces present super-hydrophobicity.
1. Introduction
ed
Key words: super-hydrophobicity; SiO2 film; modifier; contact angle; nano-husk ash
ce pt
The surfaces with a water contact angle of higher than 150ºare known as super-hydrophobic ones, such as lotus leaf surfaces with a water contact angle of 160.4±0.7ºand only a roll angle of 1.9º[1-3]. Super-hydrophobic surfaces not only have oneself-cleaning function, but also anti-current conduction, anti-corrosion,
Ac
waterproof, antifogging, anti-frost, mould proof, anti-adhesion, and anti-pollution, etc. Therefore, they have a broad prospect of application in building, clothing and spinning, liquid transportation, biomedicine, commodity packaging, transport facility, and microanalysis [4-7]. Generally, super-hydrophobic surfaces are mainly produced in two ways. One is to create a rough structure on a hydrophobic surface, and the other is to modify a rough surface with low surface-energy materials [8-12]. So, the construction of rough surfaces on low surface-energy materials and the modification of low surface-energy materials on rough surface structures are essential ways for developing bionic super hydrophobic coating. In recent years, many studies for
1
Page 3 of 18
super-hydrophobic materials have been reported. Huang et al [13] prepared a lotus leaf-like Cu-Fe nano-rod film with a water contact angle of 156.5土2.1° on the copper alloy surface by sol-gel method using dodecafluorooctatriethoxysilane (FOS-12) as a modifier. Vogelaar et al [14] constructed a lotus leaf-like super-hydrophobic surface with a contact angle of 167° and a roll angle of 0.5°. Tian et al [15] prepared a SiO2
ip t
film with a contact angle of 158° by sol-gel- method, phase separation and self-assembly techniques. Hu et al [16] prepared a PMHS-SiO2 film with a contact
cr
angle of 163°and a roll angle of 3-5° by sol-gel method. Di et al [17] prepared a lotus
leaf-like hydrophobic coating with a contact angle of 152° and a roll angle of 5° using
us
γ-3-aminopropyltriethoxysilane (KH550) as a coupling agent. Xu et al [18] prepared a lotus leaf-like hydrophobic SiO2 film by sol-gel method using tetraethoxysilane and
an
methyl triethoxysilane as precursors.
Rice husks and straws are representative agricultural wastes in the world. The annual yield of rice husks was about 127 million tons in 2008 [19]. Rice husk is
M
composed of 20% ash, 38% cellulose, 22% lignin, 18% pentose, and 2% of other organic components. The silica (SiO2) content of the ash with highly porous,
ed
lightweight and high surface area is more than 92% [20]. Most reports using rice husk ash describe its use as catalyst support for metals [21,22]. Some researchers had also used direct hydrothermal synthesis [23] and gasification process [24] to obtain rice
ce pt
husk ash as MCM-41 for catalytic reaction. It also has been used as a highly reactive pozzolanic material, leading to a significant improvement on strength and durability of normal concretes [25,26]. In this paper, a lotus leaf-like hydrophobic SiO2 film was prepared by modified sol-gel method using self-made nano-rice husk ash as a
Ac
precursor. This preparing process has the advantages of low preparation cost, simplicity, and the utilization of waste materials. 2. Experimental
2.1 Preparation of nano-rice hull ash Low temperature rice hull ash were prepared by burning rice-husk with self-propagating method at 500~600ºC for 4h in an air furnace, crushing by a roller and sieving with a 200 mesh screen. The mean particles size of rice hull ash is about 50 nm, the SiO2 content in the rice hull ash prepared in this work was 92.30%, and the 2
Page 4 of 18
specific surface area measured was 156.23 m2/g [27]. 2.2 Preparation of silicon-sodium solution (SSS) The silicon-sodium solution was obtained by boiling 100 ml 1M NaOH solution with 10g rice husk ash and removing precipitated impurities after cooling and
ip t
adjusting the pH to 3 with H2SO4. The SiO2 content of the sample prepared from silicon-sodium solution was more than 98%, and the detailed composition was shown
cr
in Table 1.
us
2.3 Preparation of nano-SiO2 films
The solution A was obtained by mixing an appropriate amount of absolute alcohol, ammonia water (28 wt%), and deionized water (volume ratio: 25:2:3). The
an
solution B was obtained by mixing an appropriate amount of absolute alcohol, silicon-sodium solution, and modifiers with the volume ratio of 1:1:(0.05~0.25). The
M
nano-rice husk ash sols were obtained by the following steps: (1) the solution A was dropped into the solution B; (2) being heated for 2 h at 60℃, and (3) being aged for
ed
48 h at room temperature. The super-hydrophobic nano-rice husk ash films were obtained by coating the glass slides and heating for 1 h at 90~100℃.
ce pt
2.4 Characterization of performances
The microstructures and particles sizes of samples were measured by a Sirion200 SEM. The specific surface areas of the samples were determined by a SORP-MAX BET meter. The compositions were analyzed by a ZXS100e 5700 X-Ray fluorescence
Ac
analyzer, and the elements and functional groups of the samples were analyzed by Nicolet 5700 Fourier transform infrared (FTIR) spectrometer. The water angles of the samples were determined by a JY-82 contact angle meter. 3. Results and discussion 3.1 Effects of modifiers on the properties of SiO2 films The relation curves of the dosages of modifiers and the contact angles of SiO2 films are shown in Fig.1. As seen, for the three modifiers, the contact angles increase gradually with the increase of additive amounts of modifiers, and their contact angles 3
Page 5 of 18
are largest when the additive amount is 0.15. Then, the contact angles decrease with the increase of additive amounts. Therefore, the best additive amount is 0.15 and the largest contact angle is 166.30º, belonging to super-hydrophobic film. Fig.2 shows the water contact angles before and after modified. The SEM micrographs of the samples without modified and modified with HSO
ip t
are shown in Fig.3. The film morphologies modified with the other two modifiers
were close to HSO and omitted. As seen, compared with the sample without modified,
cr
the SEM micrograph of sample modified with HSO showed the clear boundaries of the particles almost without agglomeration, and presented many pores on any particle.
us
3.2 Analysis for the formation mechanism of super-hydrophobic films
Fig.4 exhibits the FTIR spectra of SiO2 particles without modified and modified
an
with HMS. As seen, without modified, the broad peak at 3420 cm 1 is the dissymmetric stretching vibration band of O-H, the peak at about 1635 cm 1 is the
M
flexural vibration band of absorbed water H-OH, the strong peak at 1093 cm 1 and the short one at 955 cm 1 are the dissymmetric stretching vibration bands of Si-O-Si, the
ed
peaks at 804 cm 1 and 618.9 cm 1 are the symmetric stretching vibration bands of Si-O. Based on the natures of the functional groups, it reveals that the SiO2 without
ce pt
modified has not hydrophobicity.
The molecular formula of hexamethyldisilazane is (CH3 )3SiNHSi(CH3 )3 and contains CH3 -，NH- and SiN- and other groups. Seen from Fig.4, modified with HMS, the stretching vibration bands of O-H and H-OH at 3421 cm 1 and 1635 cm 1
Ac
obviously decrease, and the CH3 -, NH- and SiN- groups present at 2953 cm 1 , 1635 cm 1 , and 956 cm 1 , respectively. These hydrophobic groups substitute for most O-H and H-OH groups, therefore, the SiO2 particles have hydrophobicity. The FTIR spectra of SiO 2 particles modified with MTS and HSO are shown in Fig.5. As seen, the both spectra are similar because of their same hydrolysis groups. The stretching vibration bands of O-H and H-OH at 3421 cm 1 and 1635 cm 1 also obviously decrease, and the CH3- groups present at 2953 cm 1 , the SiCH3groups are at 802.8 cm-1 and 1262.6 cm-1, SiO- groups are at 466.2 cm-1, 953.5 cm-1 and 1092.3 cm-1. Similarly, these hydrophobic groups substitute for most O-H and 4
Page 6 of 18
H-OH groups, therefore, the hydrophobicity of SiO2 particles greatly increase. White carbon black produces silicic acid at acid medium, and the small molecular silicic acid forms spherical grains with ruleless branch chain structures via dewatering condensation reaction, and these particles were known as primary grains. The three-dimensional aggregates are formed between primary grains via
ip t
surface contacting and chain linking, and known as secondary grains. Then, these three-dimensional aggregates further decompose into scattered SiO2 aggregations
（1）
M
an
us
cr
via hydrogen bonding of branch chains.
（2）
ce pt
ed
Moreover, HSO hydrolyzes into micromolecules under acid condition:
Hydrolysis products, hydroxide radicals and SiO2 aggregations generate surface hydroxide radical reactions. Micromolecular organic matters with methyl are grafted
Ac
on the surfaces of SiO2. So, the SiO2 surfaces become hydrophobe from hydrophile.
（3）
The Modification mechanism of HSO on SiO2 films is shown in Fig.6. The terminal hydroxyls with reaction activity produce condensation reactions with hydroxyls or alkoxy compounds, ≡Si-OH + H-Si≡ → ≡Si-O-Si≡ + H2, which forms hydrophobic groups and results in the increase of the contact angle. 5
Page 7 of 18
The modification of HMS on SiO2 is also a grafting reaction [28]. HMS first hydrolyzes into trimethylsilanol in acid medium (reaction (4)). Secondly, the trimethylsilanol and the surface hydroxyls in silica gels generate the dehydration between molecules to form grafting products of silica gels (reaction (5)). Simultaneously, the trimethylsilanol self also generates the dehydration between molecules to form HMS (reaction (6)). Thus, the SiO2 surface becomes
us
cr
ip t
hydrophobicity from primary hydrophilcity.
(4)
M
an
(5)
(6)
ed
MTS can be hydrolyzed under the excessive ammonia water ([NH3H2O] > 0.5 mol/L). The hydrolysates can further condense with the silanol groups of the silica
ce pt
particles to introduce the methyl groups on the surface. The growth of such modified silica particles can be explained by monomer addition mechanism, where after an initial burst of nucleation, growth occurs through the addition of hydrolyzed monomers to the particle surface. Because the hydrolyzed monomers of MTS were
Ac
added after the formation of the silica particles, the surface of the particles therefore was covered with methyl groups [29,30]. Fig.7 shows the modification of MTS on SiO2 [31].
（7）
（8）
6
Page 8 of 18
（9） 4. Conclusion The SiO2 film with the water contact angle of 50.09 ºwas obtained by sol-gel
ip t
method using nano-rice husk ash prepared via burning rice-husk with self-propagating method. The super-hydrophobic SiO2 film with more than 160 ºcontact angle was
cr
obtained with HSO ( or HMS, or MTS) as a modifier and the best dosage of modifiers
of 0.15 ( volume ratio to SSS). The hydroxyls of HSO with reaction activity generate
us
condensation reaction with compounds contained hydroxyls or alkoxyls, ≡Si-OH +
an
H-Si≡ →≡Si- O-Si≡ + H2, which form hydrophobic groups, and result in the increase of contact angles. The modification of HMS or MTS is a grafting reaction, the
M
hydrophobic groups in the modifiers replace the hydroxys on SiO2 surface and make SiO2 surface present super-hydrophobicity.
ed
Acknowledgments
This work was granted by the natural fund of Shandong Province (No.Y2008F50 and ZR2010EM045). The authors wish to thank Shandong University of Technology
ce pt
and Shandong ceramic basic complex materials research center for financial supports and our colleagues for their supports and helps. References
Ac
[1] W. Barthlott, C. Neinhuis. Planta, 202 (1997) l. [2] P. Ball. Nature. 400 (1999) 507. [3] K. Tadanaga, N. Katata, T. Minami. J. Am. Chem. Soc. 80 (1997) 3213. [4] A.R.Venkateswara, M.M. Kulkarni, D.P. Amalnerkar. J. Non-Crystalline Solids. 330 (2003)187. [5] A.R.Venkateswara, M.M. Kulkani, S. D. Bhagat. Colloid and Interface Sci. 85 (2005) 413. [6] M. Khayet. Appl. Surf. Sci. 238 (2004) 269. [7] B. Zhao, J. S. Moore, D. J. Beebe. Science. 291 (2001)1023. [8] T. Onda, S. Shibuichi, N. Satoh, K. Tsujii. Langmuir. 12 (1996) 2125. 7
Page 9 of 18
[9] L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, D. Zhu. Adv. Mater. 14 (2002) 1857. [10] H.Y. Erbil, A.L. Demirel, Y. Avci, O. Mert. Science. 299 (2003) 1377. [11] A.V. Rao, M.M. Kulkarni, D.P. Amalnerkar, T. Seth. J.Non-Cryst. Solids. 330 (2003)187.
Chem. Soc. 125 (2003) 3896. [13] Z. Huang, Y. Zhu, J. Zhang. J. Phys. Chem. C, 111 (2007) 6821.
ip t
[12] S. Minko, M. Muller, M. Motornov, M. Nitschke, K. Grundke, M. Stamm. J. Am.
cr
[14] L.Vogelaar, R.G.H.Lammertink, M.Wessling. Langmuir. 22 (2006) 3125.
[15] H.Tian, T.S.Yang, Y.Q. Chen. Chem. Industry and Engineering Progress. 27
us
(2008) 1435.
[16] X.J. Hu, L. Liu, Y.F. Luo, D.M. Jia. Chinese J Mater. Res. 24 (2010) 266.
an
[17] Z.Y.Di, J.P.He, J.H. Zhou. J. Inorg. Mater. 25 (2010) 765.
[18] G.L. XU, L.L. Deng, P.H. Pi, D.F. Zhen, X.F. Wen, and Z.R. Yang. Chinese J. Inorg. Chem. 26 (2010) 1810.
M
[19] J. Umeda, K. Kondoh. Industrial Crops and Products. 32 (2010) 539. [20] J. James, M.S. Rao, Am. Ceram. Soc. Bull. 65 (1986) 1177.
ed
[21] F. Adam, C.J. Hann. J. Colloid Interface Sci. 280 (2004) 55. [22] F. Adam, J. N. Appaturi, R. Thankappan, M.A.M. Nawi. Appl. Surf. Sci. 257 (2010) 811.
ce pt
[23] T.R. Gaydhankar, P.N. Joshi, P. Kalita, R. Kumar, J. Mol. Catal. A: Chem. 265 (2007) 306.
[24] S. Chiarakorn, T. Areerob, N. Grisdanurak. Sci. Technol. Adv. Mater. 8 (2007) 111.
Ac
[25] S. Chandrasekhar, K.G. Satyanarayana, P.N. Pramada, P. Raghavan, T.N. Gupta. J. Mater. Sci. 38 (2003) 3159.
[26] M.H. Zhang, R. Lastra, V.M. Malhotra. Cem. Concr. Res. 26 (6) (1996) 963. [27] K.J. Xu, J.T. Xi, Y.Q. Guo, S.H. Dong. Solid state sci. 14 (2012) 10. [28] J. Yu, Y. Liu, M. He, S.J. Lu, Z. Hu, and Z. Luo. Chinese J. Colloid & polymer. 27 (2009) 13. [29] S.L. Chen, P. Dong, G.H. Yang, J.J. Yang. Ind. Eng. Chem. Res. 35 (1996) 4487. [30] H. Boukari, G.G. Long, M.T. Harris. J. Colloid Interface Sci. 229 (2000) 129. [31] R. L. Liu, Y. Xu, D. Wu, Y.H. Sun, H.C. Gao, H.Z.Yuan and F. Den. J. Non-Cryst. Solids. 343 (2004) 61. 8
Page 10 of 18
SiO2
Al2O3
Fe2O3
CaO
MgO
KO
Content %
98.35
0. 04
0. 02
0.06
0.02
0.94
Na2O
0.57
Ac
ce pt
ed
M
an
us
cr
Component
ip t
Table 1 component analysis of purified rice husk ash
9
Page 11 of 18
ip t cr us an M
Fig 1 Relation curves of dosage of modifiers
Ac
ce pt
ed
and the contact angles of SiO2 films
10
Page 12 of 18
ip t cr us an
Modified with MTS
ce pt
ed
M
Without any modifier
Modified with HSO
Ac
Modified with HMS
Fig.2 Contact angle video figures of the samples
11
Page 13 of 18
ip t cr us an
Modified with HSO
M
Without any modifier
Ac
ce pt
ed
Fig. 3 SEM micrographs of SiO2 films
12
Page 14 of 18
ip t cr us an M ed
Ac
ce pt
Fig. 4 FTIR spectra of SiO2 particles modified without any modifier and with HMS
13
Page 15 of 18
ip t cr us an M ed
Ac
ce pt
Fig.5 FTIR spectra of SiO2 particles modified with MTS or HSO
14
Page 16 of 18
ip t cr us
Ac
ce pt
ed
M
an
Fig.6 Modification mechanism of HSO on SiO2 films
15
Page 17 of 18
ip t cr us an M ed
Ac
ce pt
Fig.7 Modification of MTS on SiO2
16
Page 18 of 18
S0169-4332(13)00671-5 http://dx.doi.org/doi:10.1016/j.apsusc.2013.03.173 APSUSC 25463
To appear in:
APSUSC
Received date: Revised date: Accepted date:
25-9-2012 28-3-2013 29-3-2013
Please cite this article as: K. Xu, Q. Sun, Y. Guo, S. Dong, Effects of modifiers on the hydrophobicity of SiO2 films from nano-husk ash, Applied Surface Science (2013), http://dx.doi.org/10.1016/j.apsusc.2013.03.173 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.
*Highlights (for review)
Highlights ► Nano-husk ashes were prepared by burning rice-husk with self-propagating method.
►The super-hydrophobic SiO2 films were prepared from nano-husk ashes with modifiers. ► The contact angle of the SiO2 film modified with modifiers was more
ip t
than 160 º. ►The modifications of modifiers on SiO2 films were studied
Ac
ce pt
ed
M
an
us
cr
systematically.
Page 1 of 18
Graphical Abstract (for review)
cr
ip t
Graphical Abstract
with HSO
Ac
ce pt
ed
M
an
us
Without any modifier
1
Page 2 of 18
Effects of modifiers on the hydrophobicity of SiO2 films from nano-husk ash Kejing Xu, Qingwen Sun, Yanqing Guo and Shuhua Dong School of Materials Science and Engineering, Shandong University of Technology, 255049, China
Abstract: Nano-husk ashes were prepared by burning rice-husk with
ip t
self-propagating method. The super-hydrophobic SiO2 films from nano-husk ash were prepared by sol-gel method using hydroxy silicone oil (HSO), hexamethyldisilazane
cr
(HMS), or methyl triethoxysilane (MTS) as modifiers. The effects of modifiers on the hydrophobic property of SiO2 films were studied, and the performances were
us
characterized by the XRD, UV–Vis, BET, EDS, SEM, IR, and Contact Angle Analizer. The results showed that the contact angle of SiO2 films was more than 160 º when
an
volume ratio of the modifiers to silicon sodium solution (SSS) was 0.15. The mechanism of modifiers on SiO2 surfaces is a graft copolymerization. The hydrophobic groups in the modifiers replace the hydroxy groups on SiO2 surfaces and
M
make SiO2 surfaces present super-hydrophobicity.
1. Introduction
ed
Key words: super-hydrophobicity; SiO2 film; modifier; contact angle; nano-husk ash
ce pt
The surfaces with a water contact angle of higher than 150ºare known as super-hydrophobic ones, such as lotus leaf surfaces with a water contact angle of 160.4±0.7ºand only a roll angle of 1.9º[1-3]. Super-hydrophobic surfaces not only have oneself-cleaning function, but also anti-current conduction, anti-corrosion,
Ac
waterproof, antifogging, anti-frost, mould proof, anti-adhesion, and anti-pollution, etc. Therefore, they have a broad prospect of application in building, clothing and spinning, liquid transportation, biomedicine, commodity packaging, transport facility, and microanalysis [4-7]. Generally, super-hydrophobic surfaces are mainly produced in two ways. One is to create a rough structure on a hydrophobic surface, and the other is to modify a rough surface with low surface-energy materials [8-12]. So, the construction of rough surfaces on low surface-energy materials and the modification of low surface-energy materials on rough surface structures are essential ways for developing bionic super hydrophobic coating. In recent years, many studies for
1
Page 3 of 18
super-hydrophobic materials have been reported. Huang et al [13] prepared a lotus leaf-like Cu-Fe nano-rod film with a water contact angle of 156.5土2.1° on the copper alloy surface by sol-gel method using dodecafluorooctatriethoxysilane (FOS-12) as a modifier. Vogelaar et al [14] constructed a lotus leaf-like super-hydrophobic surface with a contact angle of 167° and a roll angle of 0.5°. Tian et al [15] prepared a SiO2
ip t
film with a contact angle of 158° by sol-gel- method, phase separation and self-assembly techniques. Hu et al [16] prepared a PMHS-SiO2 film with a contact
cr
angle of 163°and a roll angle of 3-5° by sol-gel method. Di et al [17] prepared a lotus
leaf-like hydrophobic coating with a contact angle of 152° and a roll angle of 5° using
us
γ-3-aminopropyltriethoxysilane (KH550) as a coupling agent. Xu et al [18] prepared a lotus leaf-like hydrophobic SiO2 film by sol-gel method using tetraethoxysilane and
an
methyl triethoxysilane as precursors.
Rice husks and straws are representative agricultural wastes in the world. The annual yield of rice husks was about 127 million tons in 2008 [19]. Rice husk is
M
composed of 20% ash, 38% cellulose, 22% lignin, 18% pentose, and 2% of other organic components. The silica (SiO2) content of the ash with highly porous,
ed
lightweight and high surface area is more than 92% [20]. Most reports using rice husk ash describe its use as catalyst support for metals [21,22]. Some researchers had also used direct hydrothermal synthesis [23] and gasification process [24] to obtain rice
ce pt
husk ash as MCM-41 for catalytic reaction. It also has been used as a highly reactive pozzolanic material, leading to a significant improvement on strength and durability of normal concretes [25,26]. In this paper, a lotus leaf-like hydrophobic SiO2 film was prepared by modified sol-gel method using self-made nano-rice husk ash as a
Ac
precursor. This preparing process has the advantages of low preparation cost, simplicity, and the utilization of waste materials. 2. Experimental
2.1 Preparation of nano-rice hull ash Low temperature rice hull ash were prepared by burning rice-husk with self-propagating method at 500~600ºC for 4h in an air furnace, crushing by a roller and sieving with a 200 mesh screen. The mean particles size of rice hull ash is about 50 nm, the SiO2 content in the rice hull ash prepared in this work was 92.30%, and the 2
Page 4 of 18
specific surface area measured was 156.23 m2/g [27]. 2.2 Preparation of silicon-sodium solution (SSS) The silicon-sodium solution was obtained by boiling 100 ml 1M NaOH solution with 10g rice husk ash and removing precipitated impurities after cooling and
ip t
adjusting the pH to 3 with H2SO4. The SiO2 content of the sample prepared from silicon-sodium solution was more than 98%, and the detailed composition was shown
cr
in Table 1.
us
2.3 Preparation of nano-SiO2 films
The solution A was obtained by mixing an appropriate amount of absolute alcohol, ammonia water (28 wt%), and deionized water (volume ratio: 25:2:3). The
an
solution B was obtained by mixing an appropriate amount of absolute alcohol, silicon-sodium solution, and modifiers with the volume ratio of 1:1:(0.05~0.25). The
M
nano-rice husk ash sols were obtained by the following steps: (1) the solution A was dropped into the solution B; (2) being heated for 2 h at 60℃, and (3) being aged for
ed
48 h at room temperature. The super-hydrophobic nano-rice husk ash films were obtained by coating the glass slides and heating for 1 h at 90~100℃.
ce pt
2.4 Characterization of performances
The microstructures and particles sizes of samples were measured by a Sirion200 SEM. The specific surface areas of the samples were determined by a SORP-MAX BET meter. The compositions were analyzed by a ZXS100e 5700 X-Ray fluorescence
Ac
analyzer, and the elements and functional groups of the samples were analyzed by Nicolet 5700 Fourier transform infrared (FTIR) spectrometer. The water angles of the samples were determined by a JY-82 contact angle meter. 3. Results and discussion 3.1 Effects of modifiers on the properties of SiO2 films The relation curves of the dosages of modifiers and the contact angles of SiO2 films are shown in Fig.1. As seen, for the three modifiers, the contact angles increase gradually with the increase of additive amounts of modifiers, and their contact angles 3
Page 5 of 18
are largest when the additive amount is 0.15. Then, the contact angles decrease with the increase of additive amounts. Therefore, the best additive amount is 0.15 and the largest contact angle is 166.30º, belonging to super-hydrophobic film. Fig.2 shows the water contact angles before and after modified. The SEM micrographs of the samples without modified and modified with HSO
ip t
are shown in Fig.3. The film morphologies modified with the other two modifiers
were close to HSO and omitted. As seen, compared with the sample without modified,
cr
the SEM micrograph of sample modified with HSO showed the clear boundaries of the particles almost without agglomeration, and presented many pores on any particle.
us
3.2 Analysis for the formation mechanism of super-hydrophobic films
Fig.4 exhibits the FTIR spectra of SiO2 particles without modified and modified
an
with HMS. As seen, without modified, the broad peak at 3420 cm 1 is the dissymmetric stretching vibration band of O-H, the peak at about 1635 cm 1 is the
M
flexural vibration band of absorbed water H-OH, the strong peak at 1093 cm 1 and the short one at 955 cm 1 are the dissymmetric stretching vibration bands of Si-O-Si, the
ed
peaks at 804 cm 1 and 618.9 cm 1 are the symmetric stretching vibration bands of Si-O. Based on the natures of the functional groups, it reveals that the SiO2 without
ce pt
modified has not hydrophobicity.
The molecular formula of hexamethyldisilazane is (CH3 )3SiNHSi(CH3 )3 and contains CH3 -，NH- and SiN- and other groups. Seen from Fig.4, modified with HMS, the stretching vibration bands of O-H and H-OH at 3421 cm 1 and 1635 cm 1
Ac
obviously decrease, and the CH3 -, NH- and SiN- groups present at 2953 cm 1 , 1635 cm 1 , and 956 cm 1 , respectively. These hydrophobic groups substitute for most O-H and H-OH groups, therefore, the SiO2 particles have hydrophobicity. The FTIR spectra of SiO 2 particles modified with MTS and HSO are shown in Fig.5. As seen, the both spectra are similar because of their same hydrolysis groups. The stretching vibration bands of O-H and H-OH at 3421 cm 1 and 1635 cm 1 also obviously decrease, and the CH3- groups present at 2953 cm 1 , the SiCH3groups are at 802.8 cm-1 and 1262.6 cm-1, SiO- groups are at 466.2 cm-1, 953.5 cm-1 and 1092.3 cm-1. Similarly, these hydrophobic groups substitute for most O-H and 4
Page 6 of 18
H-OH groups, therefore, the hydrophobicity of SiO2 particles greatly increase. White carbon black produces silicic acid at acid medium, and the small molecular silicic acid forms spherical grains with ruleless branch chain structures via dewatering condensation reaction, and these particles were known as primary grains. The three-dimensional aggregates are formed between primary grains via
ip t
surface contacting and chain linking, and known as secondary grains. Then, these three-dimensional aggregates further decompose into scattered SiO2 aggregations
（1）
M
an
us
cr
via hydrogen bonding of branch chains.
（2）
ce pt
ed
Moreover, HSO hydrolyzes into micromolecules under acid condition:
Hydrolysis products, hydroxide radicals and SiO2 aggregations generate surface hydroxide radical reactions. Micromolecular organic matters with methyl are grafted
Ac
on the surfaces of SiO2. So, the SiO2 surfaces become hydrophobe from hydrophile.
（3）
The Modification mechanism of HSO on SiO2 films is shown in Fig.6. The terminal hydroxyls with reaction activity produce condensation reactions with hydroxyls or alkoxy compounds, ≡Si-OH + H-Si≡ → ≡Si-O-Si≡ + H2, which forms hydrophobic groups and results in the increase of the contact angle. 5
Page 7 of 18
The modification of HMS on SiO2 is also a grafting reaction [28]. HMS first hydrolyzes into trimethylsilanol in acid medium (reaction (4)). Secondly, the trimethylsilanol and the surface hydroxyls in silica gels generate the dehydration between molecules to form grafting products of silica gels (reaction (5)). Simultaneously, the trimethylsilanol self also generates the dehydration between molecules to form HMS (reaction (6)). Thus, the SiO2 surface becomes
us
cr
ip t
hydrophobicity from primary hydrophilcity.
(4)
M
an
(5)
(6)
ed
MTS can be hydrolyzed under the excessive ammonia water ([NH3H2O] > 0.5 mol/L). The hydrolysates can further condense with the silanol groups of the silica
ce pt
particles to introduce the methyl groups on the surface. The growth of such modified silica particles can be explained by monomer addition mechanism, where after an initial burst of nucleation, growth occurs through the addition of hydrolyzed monomers to the particle surface. Because the hydrolyzed monomers of MTS were
Ac
added after the formation of the silica particles, the surface of the particles therefore was covered with methyl groups [29,30]. Fig.7 shows the modification of MTS on SiO2 [31].
（7）
（8）
6
Page 8 of 18
（9） 4. Conclusion The SiO2 film with the water contact angle of 50.09 ºwas obtained by sol-gel
ip t
method using nano-rice husk ash prepared via burning rice-husk with self-propagating method. The super-hydrophobic SiO2 film with more than 160 ºcontact angle was
cr
obtained with HSO ( or HMS, or MTS) as a modifier and the best dosage of modifiers
of 0.15 ( volume ratio to SSS). The hydroxyls of HSO with reaction activity generate
us
condensation reaction with compounds contained hydroxyls or alkoxyls, ≡Si-OH +
an
H-Si≡ →≡Si- O-Si≡ + H2, which form hydrophobic groups, and result in the increase of contact angles. The modification of HMS or MTS is a grafting reaction, the
M
hydrophobic groups in the modifiers replace the hydroxys on SiO2 surface and make SiO2 surface present super-hydrophobicity.
ed
Acknowledgments
This work was granted by the natural fund of Shandong Province (No.Y2008F50 and ZR2010EM045). The authors wish to thank Shandong University of Technology
ce pt
and Shandong ceramic basic complex materials research center for financial supports and our colleagues for their supports and helps. References
Ac
[1] W. Barthlott, C. Neinhuis. Planta, 202 (1997) l. [2] P. Ball. Nature. 400 (1999) 507. [3] K. Tadanaga, N. Katata, T. Minami. J. Am. Chem. Soc. 80 (1997) 3213. [4] A.R.Venkateswara, M.M. Kulkarni, D.P. Amalnerkar. J. Non-Crystalline Solids. 330 (2003)187. [5] A.R.Venkateswara, M.M. Kulkani, S. D. Bhagat. Colloid and Interface Sci. 85 (2005) 413. [6] M. Khayet. Appl. Surf. Sci. 238 (2004) 269. [7] B. Zhao, J. S. Moore, D. J. Beebe. Science. 291 (2001)1023. [8] T. Onda, S. Shibuichi, N. Satoh, K. Tsujii. Langmuir. 12 (1996) 2125. 7
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[9] L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, D. Zhu. Adv. Mater. 14 (2002) 1857. [10] H.Y. Erbil, A.L. Demirel, Y. Avci, O. Mert. Science. 299 (2003) 1377. [11] A.V. Rao, M.M. Kulkarni, D.P. Amalnerkar, T. Seth. J.Non-Cryst. Solids. 330 (2003)187.
Chem. Soc. 125 (2003) 3896. [13] Z. Huang, Y. Zhu, J. Zhang. J. Phys. Chem. C, 111 (2007) 6821.
ip t
[12] S. Minko, M. Muller, M. Motornov, M. Nitschke, K. Grundke, M. Stamm. J. Am.
cr
[14] L.Vogelaar, R.G.H.Lammertink, M.Wessling. Langmuir. 22 (2006) 3125.
[15] H.Tian, T.S.Yang, Y.Q. Chen. Chem. Industry and Engineering Progress. 27
us
(2008) 1435.
[16] X.J. Hu, L. Liu, Y.F. Luo, D.M. Jia. Chinese J Mater. Res. 24 (2010) 266.
an
[17] Z.Y.Di, J.P.He, J.H. Zhou. J. Inorg. Mater. 25 (2010) 765.
[18] G.L. XU, L.L. Deng, P.H. Pi, D.F. Zhen, X.F. Wen, and Z.R. Yang. Chinese J. Inorg. Chem. 26 (2010) 1810.
M
[19] J. Umeda, K. Kondoh. Industrial Crops and Products. 32 (2010) 539. [20] J. James, M.S. Rao, Am. Ceram. Soc. Bull. 65 (1986) 1177.
ed
[21] F. Adam, C.J. Hann. J. Colloid Interface Sci. 280 (2004) 55. [22] F. Adam, J. N. Appaturi, R. Thankappan, M.A.M. Nawi. Appl. Surf. Sci. 257 (2010) 811.
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[23] T.R. Gaydhankar, P.N. Joshi, P. Kalita, R. Kumar, J. Mol. Catal. A: Chem. 265 (2007) 306.
[24] S. Chiarakorn, T. Areerob, N. Grisdanurak. Sci. Technol. Adv. Mater. 8 (2007) 111.
Ac
[25] S. Chandrasekhar, K.G. Satyanarayana, P.N. Pramada, P. Raghavan, T.N. Gupta. J. Mater. Sci. 38 (2003) 3159.
[26] M.H. Zhang, R. Lastra, V.M. Malhotra. Cem. Concr. Res. 26 (6) (1996) 963. [27] K.J. Xu, J.T. Xi, Y.Q. Guo, S.H. Dong. Solid state sci. 14 (2012) 10. [28] J. Yu, Y. Liu, M. He, S.J. Lu, Z. Hu, and Z. Luo. Chinese J. Colloid & polymer. 27 (2009) 13. [29] S.L. Chen, P. Dong, G.H. Yang, J.J. Yang. Ind. Eng. Chem. Res. 35 (1996) 4487. [30] H. Boukari, G.G. Long, M.T. Harris. J. Colloid Interface Sci. 229 (2000) 129. [31] R. L. Liu, Y. Xu, D. Wu, Y.H. Sun, H.C. Gao, H.Z.Yuan and F. Den. J. Non-Cryst. Solids. 343 (2004) 61. 8
Page 10 of 18
SiO2
Al2O3
Fe2O3
CaO
MgO
KO
Content %
98.35
0. 04
0. 02
0.06
0.02
0.94
Na2O
0.57
Ac
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ed
M
an
us
cr
Component
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Table 1 component analysis of purified rice husk ash
9
Page 11 of 18
ip t cr us an M
Fig 1 Relation curves of dosage of modifiers
Ac
ce pt
ed
and the contact angles of SiO2 films
10
Page 12 of 18
ip t cr us an
Modified with MTS
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ed
M
Without any modifier
Modified with HSO
Ac
Modified with HMS
Fig.2 Contact angle video figures of the samples
11
Page 13 of 18
ip t cr us an
Modified with HSO
M
Without any modifier
Ac
ce pt
ed
Fig. 3 SEM micrographs of SiO2 films
12
Page 14 of 18
ip t cr us an M ed
Ac
ce pt
Fig. 4 FTIR spectra of SiO2 particles modified without any modifier and with HMS
13
Page 15 of 18
ip t cr us an M ed
Ac
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Fig.5 FTIR spectra of SiO2 particles modified with MTS or HSO
14
Page 16 of 18
ip t cr us
Ac
ce pt
ed
M
an
Fig.6 Modification mechanism of HSO on SiO2 films
15
Page 17 of 18
ip t cr us an M ed
Ac
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Fig.7 Modification of MTS on SiO2
16
Page 18 of 18