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Nanocomposite films prepared from stabilized aqueous SiO2 sols
NOC-16869; No of Pages 5 Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Nanocomposite films prepared from stabilized aqueous SiO2 sols Alfonz Plško a,⁎, Jana Pagáčová b, Jana Šulcová b, Barbora Bieliková a, Miriama Tomagová a, Katarína Michálková a, Anna Rodová a a b
Alexander Dubček University of Trenčín, Študentská 2, 911 50 Trenčín, Slovakia Faculty of Industrial Technologies, Alexander Dubček University of Trenčín, I. Krasku 1809/34, 020 01 Púchov, Slovakia
a r t i c l e
i n f o
Article history: Received 30 September 2013 Received in revised form 7 January 2014 Available online xxxx Keywords: Nanocomposite films; Silica; Sol-gel; AFM; Surface properties
a b s t r a c t Using sol-gel method, the films containing appropriate nanoparticles have a wide range of application as protective, dielectric and superhydrophobic layers. The system “stabilized aqueous SiO2 sol–acetic acid–isopropyl alcohol” was used to study the possibilities of preparation of nanocomposite SiO2 films and their properties. The properties of surface of prepared films, such as morphology, topography and adhesion forces, were studied using atomic force microscopy (AFM). The contact angle of water and diiodomethane was measured to specify the polar and disperse components of surface tension. The changes in observed properties, mainly the difference of samples X4, X5 and X6, are explained on the basis of changes in stabilization process of sols used for films preparation. For the given observed properties, the determining process is the change from electrostatic to steric stabilization of studied sols. © 2014 Published by Elsevier B.V.
1. Introduction The films based on SiO2 are still the subject of the investigation due to their excellent properties, such as chemical durability, low refractive index, excellent dielectric and sorption properties and many others. For film preparation, the sol-gel method is often used because it enables to obtain required film properties by easy change of initial preparation conditions [1–3]. These films are useful for applications in various fields relating to superhydrophobics [4,5], optics [6], environment [7], etc. Roughness, surface tension and adhesion are the surface properties, which are required for the given applications where the surface of prepared films seems to be determining factor [8–10]. For mentioned purposes, the nanocomposite films are commonly used, and they are prepared from sols which are formed by the dispersed particles with size from tenths to hundreds nanometres. The Stöber method is the most known method for preparation of the monodispersive sols in the medium of alcohol by base catalysed hydrolysis of TEOS [11–13]. Aqueous sols of SiO2 can be also prepared by direct hydrolysis of TEOS in aqueous medium [14,15]. One of the most used ways for preparation of aqueous SiO2 sols is the hydrolysis of aqueous solutions of sodium silicates in acid medium or the utilization of acid ion exchangers. The typical feature of SiO2 particles in aqueous sols is that there are ≡Si–OH groups on their surface. The concentration of these groups on surface and degree of their ionization are the determining factor for the stability of sols, i.e., agglomeration and aggregation of sols [16,17].
⁎ Corresponding author. E-mail address: [email protected] (A. Plško).
There are two processes of sol stabilization known—electrostatic and steric stabilization. The electrostatic stabilization is connected with the formation of electric double layer on the surface of sol particles. The first layer is formed by quite strong bound ions from dispersive medium. The given ions have the opposite polarity in comparison to charge of particles surface. In the dependence on ion radius and size of their ion charge, the charge of particles surface is compensated. The second layer is formed by co-ions, which compensate the charge of the first layer. This second layer is not bound strongly, and it is sometimes also called the diffusion layer. The particles repulse each other or link together in dependence on compensation degree of charge of the given particles in sol. The properties of electric double layer are closely connected with polarity of dispersive medium, pH, concentration of electrolytes and temperature. During steric stabilization, the surfactants are linked to surface of sol particles. On the basis of property of particle surface, i.e., lyophobic or lyophilic character of surface, the surfactant is linked to surface of particle by its corresponding part. It leads to the occurrence of the layer around the particle, and this layer causes that there is not any agglomeration of particles. The combination of two mentioned processes of stabilization occurs very often, and in relation to SiO2 sols, the organic acids can be used for the given combined stabilization [18,19]. Both processes of stabilization avoid to agglomeration of particles in sol [1,2]. The steric stabilization is also used for functionalization of sol particles [20]. These sols are used for preparation of nanocomposite films with required surface properties [21–23]. The polar and dispersion components of surface free energy (SFE), adhesion forces and morphology and rms-roughness of surface are determining surface properties for mentioned applications of SiO2 nanocomposite films. The polar component of SFE gives the information about the surface polarity. The dispersion component of SFE gives the
0022-3093/$ – see front matter © 2014 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.045
Please cite this article as: A. Plško, et al., Nanocomposite films prepared from stabilized aqueous SiO2 sols, J. Non-Cryst. Solids (2014), http:// dx.doi.org/10.1016/j.jnoncrysol.2014.01.045
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A. Plško et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
Table 1 Molar composition of prepared sols. Sample
Molar ratio
X1 X2 X3 X4 X5 X6 X7
x(SiO2)
x(AA)
x(IPA)
x(H2O)
0.0028 0.0046 0.0058 0.0067 0.0074 0.0079 0.0083
0.1938 0.1577 0.1329 0.1148 0.1011 0.0903 0.0816
0.5048 0.3520 0.2472 0.1709 0.1129 0.0672 0.0304
0.2986 0.4857 0.6141 0.7076 0.7786 0.8346 0.8797
information about roughness of surface or its porosity [24,25]. The morphology, topography and rms-roughness of surface are investigated in nano-level with AFM, which also enables to determine the surface stiffness and adhesion forces [26–28]. These properties enable to characterize the surface and look for the correlation between surface properties and properties required for application of nanocomposite SiO2 films in mentioned fields [21–23]. At the utilization of aqueous SiO2 sols for preparation of nanocomposite films, the problem is that aqueous sols are not able to make substrate surfaces wet. Therefore, it is necessary to investigate the systems of aqueous sols with solvents that allow wettability of substrates. In our work, the system “stabilized aqueous SiO2 sol–acetic acid–isopropyl alcohol” was used to study the possibilities of preparation of
X1
X2
X3
X4
X5
X6
X7
Please cite this article as: A. Plško, et al., Nanocomposite films prepared from stabilized aqueous SiO2 sols, J. Non-Cryst. Solids (2014), http:// dx.doi.org/10.1016/j.jnoncrysol.2014.01.045
A. Plško et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
3
nanocomposite SiO2 films. The properties of prepared films, such as morphology and surface properties, were studied.
and cantilever stiffness, respectively. Substituting, we can obtain the following:
2. Experimental
kc Z c ¼ −
The stabilized sol SiO2 (ssSiO2), isopropanol (p.a., IPA), acetic acid (99 wt. %, AA) and distilled water (H2O) were used for preparation of SiO2 sols. The sols were prepared by mixing of given volumes of ssSiO2, IPA and AA. The compositions in molar ratio of the prepared sols are shown in the Table 1. The stabilized monodisperse SiO2 aqua sol (~ 3 wt. % SiO2) was prepared from the aqueous solution of sodium silicate (ratio of SiO2: Na2O = 3: 1 and w(SiO2) = 0.05) by ion exchange on acidic ionexchanger Amberlite IR-120. Silica sols were stabilized with NaOH (0.5 wt. %) where the ratio of SiO2: Na2O was approximately 100: 1. The average size of SiO2 particles in xerogel, which was prepared from sol mentioned above, was 32.1 ± 3.2 nm [29]. The dip-coating technique was used for deposition of SiO2 films on microscope slide glasses. First, the substrates were cleaned with detergent then rinsed with water and distilled water and dried at 105 °C for 10 min. The substrates were immersed into hot mixture of hydrogen peroxide and ammonia (3:1) for 15 min, rinsed with distilled water and isopropanol, and cleaned ultrasonically in distilled water for 10 min. Then, the substrates were dried at 105 °C and cleaned in isopropanol vapours for 5 min. The cleaned substrates were stored in isopropanol until the deposition. Before deposition, they were rinsed with isopropanol and dried at 105 °C for 10 min. The substrates were withdrawn from the sol at speed 20 mm ⋅ min− 1 . The films were dried at 80 °C for 15 min, heated at 10 °C ⋅ min− 1 and annealed at 400 °C for 50 min, and then they were cooled freely. The prepared films were stored at room temperature and ambient relative humidity. The surface of SiO2 films was investigated with atomic force microscope NT-206 (Micro test Machines Belarus) operated in the contact mode in the air at room temperature and relative humidity ~55 %. The MikroMasch NSC11/AlBS cantilever with a spring force constant of 3 N ⋅ m− 1 (measurement of morphology and topography) and 48 N⋅ m−1 (measurement of adhesion force and stiffness) was used. The surface morphology and topography were evaluated by SurfaceXplorer 1.0.8.65 program. The adhesion force were calculated considering that the adhesion force Fad is a combination of electrostatic force Fel, the van der Waals force FvdW, the meniscus or capillarity force Fcap and the forces of chemical bonds or acid-base interaction Fchem (Fad = Fel + FvdW + Fcap + Fchem), and at ambient conditions, the water neck is formed between AFM tip and substrate due to capillary condensation and adsorption of thin water films at surfaces. Then the adhesion force can be calculated on the basis of difference between minimum and zero line of the force–displacement curve. The stiffness was calculated considering that when the tip is in contact with substrate, it can be written as follows [26–28]: D ¼ Zp þ Zc þ δ
kc ks Z ¼ −keff Z p kc þ ks p
ð2Þ
The slope of the force–displacement curve is a measure of the stiffness of sample (keff). If the sample is much stiffer than the cantilever, that is, for ks ≫ kc, then keff ≈ kc, whereas keff ≈ ks when ks ≪ kc, i.e., when the sample is much more compliant than the cantilever. The average values and standard deviations of adhesion force and stiffness were calculated on the basis of ten measurements for randomly chosen places of measured sample. The polar γps and the dispersion γds component of surface free energy (SFE) of prepared SiO2 films were calculated on the basis of Fowkes theory [24]. At first, the contact angle Θ for the solid is measured using the dispersion liquid. Then γds is calculated from the following equation: 2
d
γs ¼ 0:25γ l ð1 þ cosΘÞ
ð3Þ
where γl is SFE for dispersion liquid for which γl = γdl . Next, the contact angle Θp is measured using the liquid for which γl = γdl + γpl . Using the determined value of γds and Θp, the quantity of γps can be calculated from the following equation: p
γs ¼
2 d d 0:5 p 0:5γ l 1 þ cosΘp – γs γl =γ l
ð4Þ
The polar and dispersion components of SFE were calculated from measurement of contact angle using diiodomethane and distilled water. The diiodomethane was chosen as dispersion liquid (γl = γdl = 50.8 mJ⋅m−2), and water was chosen as the liquid with the dominant polar component (γdl = 21.8 mJ ⋅ m−2, γpl = 51.0 mJ ⋅ m−2) [24,25]. The measurement of contact angle was carried out by sessile drop technique. Ten drops of liquid with volume of 10 μl were dropped on each SiO2 film. From these measurements, the average value and standard deviation were calculated. 3. Results The morphology of SiO2 films prepared from X1 to X7 sols, as 2D and 3D AFM images in dependence on molar ratio of SiO2 is shows on Fig. 1. Based on 2D and 3D AFM images, there are bumps with almost circle circumference and flat top site on the film surfaces. Except for samples X4 and X6, the bumps seem to be arranged each to another freely. For sample X4, the bumps are in slope position. For film surface of sample X6, the bumps seem to be rising from flat plane.
ð1Þ
where D is the displacement between the tip and the substrate, Zp is the piezo-deflection, Z c is the cantilever deflection and δ is the deformation of sample. In contact, D = 0, and if the system is in equilibrium, also k s δ = k c Z c , where k s and k c are sample stiffness
Table 2 Topography characteristics of surface of nanocomposite SiO2 films.
Zmean (nm) rms-roughness (nm)
X1
X2
X3
X4
X5
X6
X7
41.3 12.9
34.6 10.4
36.4 11.6
27.9 10.2
26.4 9.6
12.2 4.4
30.4 9.2
Please cite this article as: A. Plško, et al., Nanocomposite films prepared from stabilized aqueous SiO2 sols, J. Non-Cryst. Solids (2014), http:// dx.doi.org/10.1016/j.jnoncrysol.2014.01.045
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A. Plško et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
X7, the values of dispersion and polar component of SFE are almost the same. 4. Discussion
The topography characteristics (Zmean and rms-roughness) of studied films are shown in Table 2. In comparison to other samples, the values of Zmean and rms-roughness for samples X4 and X5 are lower slightly and in the case of the sample X6, the given values are lower markedly. Fig. 2 shows the average values of the width of the bump's base in dependence on molar ratio of SiO2. The error bars reflect the standard deviations of average values. The width of the bump's base is the same for all samples, and it has a value of about 1.1 μm. However, according to noticeable higher values of standard deviations, samples X4, X5 and X6 have the higher interval of the width of the bump's base. The dependence of average value of adhesion force with its standard deviation on molar ratio of SiO2 is shown in Fig. 3. In comparison to other samples, the average values of adhesion force and their standard deviations are different markedly for samples X4, X5 and X6. The dependence of average value of stiffness with its standard deviation on molar ratio of SiO2 of prepared films is shown in Fig. 4. In comparison to other samples, there is also the little increase of stiffness for samples X4, X5 and X6. The values of standard deviations of stiffness are almost the same for all samples. Fig. 5 shows the average values and standard deviations of surface free energy and its polar and dispersion components in the dependence on molar ratio of SiO2. The average values of the surface free energy have the slight rising tendency for samples from X1 to X3, and then there is a low decrease of SFE, and it is practically constant for samples from X4 to X7. The same tendency can be also observed for the dependence of the average values of the dispersion component of SFE on molar ratio of SiO2. However, the dependence of the polar component of SFE on molar ratio of SiO2 has opposite tendency. For samples X4–
The observed dependencies for the properties of nanocomposite films prepared from sols in system “ssSiO2–AA–IPA–H2O” can be explained on the basis of the properties of these sols, and it is especially based on the change of stabilization process of studied sols as well as changes during preparation of films and their thermal treatment. First, it is necessary to point out that the studied sols were prepared from stabilized aqueous SiO2 sol in which the SiO2 particles are stabilized by Na+ ions, i.e., by electrostatic stabilization. Considering the change of composition from X7 sol to X1 sol, the ratio x(AA)/x(SiO2) increases from 9.8 to 69.2, and at the same time, the polarity of solvent decreases, and it is evident from the ratio x(H2O)/x(IPA), which decreases from value 28.9 (for X7 sol) to value 0.6 (for X1 sol). These changes cause that in X7 sol with its high polarity of dispersive medium and its low x (AA)/x(SiO2) ratio, the stabilization by Na+ ions, i.e., electrostatic stabilization, is dominant. The increase of x(AA)/x(SiO2) ratio and the decrease of polarity of dispersive medium cause that the molecule of acetic acid, which acts as surfactant, is implemented into the electric double layer, and it is connected with formation of combined stabilization of SiO2 particles in sol. Finally, this mentioned process of combined stabilization becomes dominant [1,2,16]. The given process can be used for explaining the stability of SiO2 sols in the studied system. The explanation of observed differences in the properties of films prepared from sols X4, X5 and X6 can be expressed on the basis of the fact that there is no sufficient amount of acetic acid molecules around the particle in these sols. Therefore, the steric stabilization is not the determining factor, but the influence of electrostatic stabilization decreases because the polarity of the solvent and the concentration of ions in dispersive medium have decreasing tendency. This “transition” state causes that there is the agglomeration of particles during drying of prepared films leading to the more non-uniform and denser aggregates of SiO2 particles because in sols X4, X5 and X6, the stabilization of particles is smaller. During subsequent thermal treatment, the more compact and less porous films are formed, and in comparison with other samples, their values of Zmean and rms-roughness are slightly lower. Moreover, for these three samples, the values of stiffness are higher, and the decrease of the dispersion component of SFE indicates the lower porosity of these films. This “transition” state during the stabilization of studied sols also represents the decrease of the polar component of SFE from sample X7 to sample X3 and than this value is almost constant. The irregularity in adhesion forces on surfaces of films prepared from sols X4, X5 and X6 is also the result of mentioned “transition” state. After thermal treatment,
Please cite this article as: A. Plško, et al., Nanocomposite films prepared from stabilized aqueous SiO2 sols, J. Non-Cryst. Solids (2014), http:// dx.doi.org/10.1016/j.jnoncrysol.2014.01.045
A. Plško et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
the created aggregates have non-uniform surfaces with respect to presence of acetic acid and ≡Si–OH groups, and according to this mentioned fact, there is the formation of markedly different regions in term of adhesion forces in the nanometre ranges. The agglomeration of sol particles during drying of films leads to the formation of bumps with almost circle circumference. The particles link together in fluid layer. The feeding of linking particles from all sides is uniform, except of the top side where the feeding stops quite fast. The result of the mentioned process is connected with formation of bumps on the film surface (Fig. 1) and they consist of nanoparticles of initial sol. 5. Conclusion The stable sols in system “stabilized aqueous SiO2 sol–acetic acid– isopropyl alcohol” were prepared and used for preparation of nanocomposite films. In relation to studied nanocomposite films, the AFM technique was used for determination of morphology, topography, width of the bump's base, stiffness and adhesion force. Surface free energy and its polar and dispersion component were determined by sessile drop technique. The changes in observed properties, mainly the difference of samples X4, X5 and X6, are explained on the basis of changes in stabilization process of sols used for films preparation. For the given observed properties, the determining process is the change from electrostatic to steric stabilization of studied sols. Acknowledgements This work was supported by the Grant Agency of the Slovak Republic (grant no. 1/0559/11 and project no. APVV-0487-11).
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References [1] C.J. Brinker, G.W. Scherer, Sol-Gel Science—The Physics and Chemistry of Sol-Gel Processing, Academic Press, Boston, 1990. [2] S. Sakka (Ed.), Handbook of Sol-Gel Science and Technology—Processing, Characterization and Applications, Kluwer Academic Publishers, New York, 2004. [3] A. Plško, P. Exnar, Silikáty 33 (1989) 69–84. [4] Y.L. Wu, Z. Chen, X.T. Zeng, Appl. Surf. Sci. 254 (2008) 6952–6958. [5] K. Funakoshi, T. Nonami, J. Am. Ceram. Soc. 89 (2006) 2782–2786. [6] Y. Zhao, W. Ren, H. Cui, J. Colloid Interface Sci. 398 (2013) 7–12. [7] H. Choi, E. Stathatos, D.D. Dionysiou, Desalination 202 (2007) 199–206. [8] T.P. Niesen, M.R. De Guire, J. Electroceram. 6 (2001) 169–207. [9] K. Liu, Y. Tian, L. Jiang, Prog. Mater. Sci. 58 (2013) 503–564. [10] M. Ma, R.M. Hill, Curr. Opin. Colloid Interface Sci. 11 (2006) 193–202. [11] W. Stöber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 257–274. [12] D.L. Green, J.S. Lin, Yui-Fai Lam, M.Z.-C. Hu, D.W. Schaefer, M.T. Harris, J. Colloid Interface Sci. 266 (2003) 346–358. [13] F. Branda, B. Silvestri, G. Luciani, A. Costantini, F. Tescione, Colloids Surf. A Physicochem. Eng. Asp. 367 (2010) 12–16. [14] K. Bredereck, F. Effenberger, M. Tretter, J. Colloid Interface Sci. 360 (2011) 408–414. [15] H. Du, P.D. Hamilton, M.A. Reilly, A. d'Avignon, P. Biswas, N. Ravi, J. Colloid Interface Sci. 340 (2009) 202–208. [16] C.N.R. Rao, A. Müller, A.K. Cheetham, The Chemistry of Nanomaterials—Synthesis, Properties and Applications, Wiley-VCH Verlag, Weinheim, 2004. [17] H.E. Berga, W.O. Roberts, Colloidal Silica Fundamentals and Applications, CRC Taylor and Francis, Boca Raton, London, New York, 2006. [18] B. Karmakar, G. De, D. Ganguli, J. Non-Cryst. Solids 272 (2000) 119–126. [19] B. Karmakar, G. De, D. Kundu, D. Ganguli, J. Non-Cryst. Solids 135 (1991) 29–36. [20] D.P. Fasce, I.E. dell'Erba, R.J.J. Williams, Polymer 46 (2005) 6649–6656. [21] M. Iijima, M. Tsukada, H. Kamiya, J. Colloid Interface Sci. 307 (2007) 418–424. [22] S. Dirč, V. Tagliazucca, E. Callone, A. Quaranta, Mater. Chem. Phys. 126 (2011) 909–917. [23] I.A. Rahman, M. Jafarzadeh, C.S. Sipaut, Ceram. Int. 35 (2009) 1883–1888. [24] M. Żenkiewicz, J. Achiev. Mater. Manuf. Eng. 24 (2007) 137–145. [25] R.J. Good, C.J. van Oss, in: M.E. Schrader, G.I. Loeb (Eds.), Modern Approaches to Wettability, Plenum, New York, 1992, pp. 1–29. [26] H.J. Butt, B. Cappella, M. Kappl, Surf. Sci. Rep. 59 (2005) 1–152. [27] B. Cappella, G. Dietler, Surf. Sci. Rep. 34 (1999) 1–104. [28] P. Klapetek, I. Ohlidal, Ultramicroscopy 94 (2003) 19–29. [29] A. Rodová, A. Plško, Preparation of Ceramic Materials, Proceedings of the Xth International ConferenceTechnical University, Košice, 2013, pp. 52–54.
Please cite this article as: A. Plško, et al., Nanocomposite films prepared from stabilized aqueous SiO2 sols, J. Non-Cryst. Solids (2014), http:// dx.doi.org/10.1016/j.jnoncrysol.2014.01.045
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Nanocomposite films prepared from stabilized aqueous SiO2 sols Alfonz Plško a,⁎, Jana Pagáčová b, Jana Šulcová b, Barbora Bieliková a, Miriama Tomagová a, Katarína Michálková a, Anna Rodová a a b
Alexander Dubček University of Trenčín, Študentská 2, 911 50 Trenčín, Slovakia Faculty of Industrial Technologies, Alexander Dubček University of Trenčín, I. Krasku 1809/34, 020 01 Púchov, Slovakia
a r t i c l e
i n f o
Article history: Received 30 September 2013 Received in revised form 7 January 2014 Available online xxxx Keywords: Nanocomposite films; Silica; Sol-gel; AFM; Surface properties
a b s t r a c t Using sol-gel method, the films containing appropriate nanoparticles have a wide range of application as protective, dielectric and superhydrophobic layers. The system “stabilized aqueous SiO2 sol–acetic acid–isopropyl alcohol” was used to study the possibilities of preparation of nanocomposite SiO2 films and their properties. The properties of surface of prepared films, such as morphology, topography and adhesion forces, were studied using atomic force microscopy (AFM). The contact angle of water and diiodomethane was measured to specify the polar and disperse components of surface tension. The changes in observed properties, mainly the difference of samples X4, X5 and X6, are explained on the basis of changes in stabilization process of sols used for films preparation. For the given observed properties, the determining process is the change from electrostatic to steric stabilization of studied sols. © 2014 Published by Elsevier B.V.
1. Introduction The films based on SiO2 are still the subject of the investigation due to their excellent properties, such as chemical durability, low refractive index, excellent dielectric and sorption properties and many others. For film preparation, the sol-gel method is often used because it enables to obtain required film properties by easy change of initial preparation conditions [1–3]. These films are useful for applications in various fields relating to superhydrophobics [4,5], optics [6], environment [7], etc. Roughness, surface tension and adhesion are the surface properties, which are required for the given applications where the surface of prepared films seems to be determining factor [8–10]. For mentioned purposes, the nanocomposite films are commonly used, and they are prepared from sols which are formed by the dispersed particles with size from tenths to hundreds nanometres. The Stöber method is the most known method for preparation of the monodispersive sols in the medium of alcohol by base catalysed hydrolysis of TEOS [11–13]. Aqueous sols of SiO2 can be also prepared by direct hydrolysis of TEOS in aqueous medium [14,15]. One of the most used ways for preparation of aqueous SiO2 sols is the hydrolysis of aqueous solutions of sodium silicates in acid medium or the utilization of acid ion exchangers. The typical feature of SiO2 particles in aqueous sols is that there are ≡Si–OH groups on their surface. The concentration of these groups on surface and degree of their ionization are the determining factor for the stability of sols, i.e., agglomeration and aggregation of sols [16,17].
⁎ Corresponding author. E-mail address: [email protected] (A. Plško).
There are two processes of sol stabilization known—electrostatic and steric stabilization. The electrostatic stabilization is connected with the formation of electric double layer on the surface of sol particles. The first layer is formed by quite strong bound ions from dispersive medium. The given ions have the opposite polarity in comparison to charge of particles surface. In the dependence on ion radius and size of their ion charge, the charge of particles surface is compensated. The second layer is formed by co-ions, which compensate the charge of the first layer. This second layer is not bound strongly, and it is sometimes also called the diffusion layer. The particles repulse each other or link together in dependence on compensation degree of charge of the given particles in sol. The properties of electric double layer are closely connected with polarity of dispersive medium, pH, concentration of electrolytes and temperature. During steric stabilization, the surfactants are linked to surface of sol particles. On the basis of property of particle surface, i.e., lyophobic or lyophilic character of surface, the surfactant is linked to surface of particle by its corresponding part. It leads to the occurrence of the layer around the particle, and this layer causes that there is not any agglomeration of particles. The combination of two mentioned processes of stabilization occurs very often, and in relation to SiO2 sols, the organic acids can be used for the given combined stabilization [18,19]. Both processes of stabilization avoid to agglomeration of particles in sol [1,2]. The steric stabilization is also used for functionalization of sol particles [20]. These sols are used for preparation of nanocomposite films with required surface properties [21–23]. The polar and dispersion components of surface free energy (SFE), adhesion forces and morphology and rms-roughness of surface are determining surface properties for mentioned applications of SiO2 nanocomposite films. The polar component of SFE gives the information about the surface polarity. The dispersion component of SFE gives the
0022-3093/$ – see front matter © 2014 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.045
Please cite this article as: A. Plško, et al., Nanocomposite films prepared from stabilized aqueous SiO2 sols, J. Non-Cryst. Solids (2014), http:// dx.doi.org/10.1016/j.jnoncrysol.2014.01.045
2
A. Plško et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
Table 1 Molar composition of prepared sols. Sample
Molar ratio
X1 X2 X3 X4 X5 X6 X7
x(SiO2)
x(AA)
x(IPA)
x(H2O)
0.0028 0.0046 0.0058 0.0067 0.0074 0.0079 0.0083
0.1938 0.1577 0.1329 0.1148 0.1011 0.0903 0.0816
0.5048 0.3520 0.2472 0.1709 0.1129 0.0672 0.0304
0.2986 0.4857 0.6141 0.7076 0.7786 0.8346 0.8797
information about roughness of surface or its porosity [24,25]. The morphology, topography and rms-roughness of surface are investigated in nano-level with AFM, which also enables to determine the surface stiffness and adhesion forces [26–28]. These properties enable to characterize the surface and look for the correlation between surface properties and properties required for application of nanocomposite SiO2 films in mentioned fields [21–23]. At the utilization of aqueous SiO2 sols for preparation of nanocomposite films, the problem is that aqueous sols are not able to make substrate surfaces wet. Therefore, it is necessary to investigate the systems of aqueous sols with solvents that allow wettability of substrates. In our work, the system “stabilized aqueous SiO2 sol–acetic acid–isopropyl alcohol” was used to study the possibilities of preparation of
X1
X2
X3
X4
X5
X6
X7
Please cite this article as: A. Plško, et al., Nanocomposite films prepared from stabilized aqueous SiO2 sols, J. Non-Cryst. Solids (2014), http:// dx.doi.org/10.1016/j.jnoncrysol.2014.01.045
A. Plško et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
3
nanocomposite SiO2 films. The properties of prepared films, such as morphology and surface properties, were studied.
and cantilever stiffness, respectively. Substituting, we can obtain the following:
2. Experimental
kc Z c ¼ −
The stabilized sol SiO2 (ssSiO2), isopropanol (p.a., IPA), acetic acid (99 wt. %, AA) and distilled water (H2O) were used for preparation of SiO2 sols. The sols were prepared by mixing of given volumes of ssSiO2, IPA and AA. The compositions in molar ratio of the prepared sols are shown in the Table 1. The stabilized monodisperse SiO2 aqua sol (~ 3 wt. % SiO2) was prepared from the aqueous solution of sodium silicate (ratio of SiO2: Na2O = 3: 1 and w(SiO2) = 0.05) by ion exchange on acidic ionexchanger Amberlite IR-120. Silica sols were stabilized with NaOH (0.5 wt. %) where the ratio of SiO2: Na2O was approximately 100: 1. The average size of SiO2 particles in xerogel, which was prepared from sol mentioned above, was 32.1 ± 3.2 nm [29]. The dip-coating technique was used for deposition of SiO2 films on microscope slide glasses. First, the substrates were cleaned with detergent then rinsed with water and distilled water and dried at 105 °C for 10 min. The substrates were immersed into hot mixture of hydrogen peroxide and ammonia (3:1) for 15 min, rinsed with distilled water and isopropanol, and cleaned ultrasonically in distilled water for 10 min. Then, the substrates were dried at 105 °C and cleaned in isopropanol vapours for 5 min. The cleaned substrates were stored in isopropanol until the deposition. Before deposition, they were rinsed with isopropanol and dried at 105 °C for 10 min. The substrates were withdrawn from the sol at speed 20 mm ⋅ min− 1 . The films were dried at 80 °C for 15 min, heated at 10 °C ⋅ min− 1 and annealed at 400 °C for 50 min, and then they were cooled freely. The prepared films were stored at room temperature and ambient relative humidity. The surface of SiO2 films was investigated with atomic force microscope NT-206 (Micro test Machines Belarus) operated in the contact mode in the air at room temperature and relative humidity ~55 %. The MikroMasch NSC11/AlBS cantilever with a spring force constant of 3 N ⋅ m− 1 (measurement of morphology and topography) and 48 N⋅ m−1 (measurement of adhesion force and stiffness) was used. The surface morphology and topography were evaluated by SurfaceXplorer 1.0.8.65 program. The adhesion force were calculated considering that the adhesion force Fad is a combination of electrostatic force Fel, the van der Waals force FvdW, the meniscus or capillarity force Fcap and the forces of chemical bonds or acid-base interaction Fchem (Fad = Fel + FvdW + Fcap + Fchem), and at ambient conditions, the water neck is formed between AFM tip and substrate due to capillary condensation and adsorption of thin water films at surfaces. Then the adhesion force can be calculated on the basis of difference between minimum and zero line of the force–displacement curve. The stiffness was calculated considering that when the tip is in contact with substrate, it can be written as follows [26–28]: D ¼ Zp þ Zc þ δ
kc ks Z ¼ −keff Z p kc þ ks p
ð2Þ
The slope of the force–displacement curve is a measure of the stiffness of sample (keff). If the sample is much stiffer than the cantilever, that is, for ks ≫ kc, then keff ≈ kc, whereas keff ≈ ks when ks ≪ kc, i.e., when the sample is much more compliant than the cantilever. The average values and standard deviations of adhesion force and stiffness were calculated on the basis of ten measurements for randomly chosen places of measured sample. The polar γps and the dispersion γds component of surface free energy (SFE) of prepared SiO2 films were calculated on the basis of Fowkes theory [24]. At first, the contact angle Θ for the solid is measured using the dispersion liquid. Then γds is calculated from the following equation: 2
d
γs ¼ 0:25γ l ð1 þ cosΘÞ
ð3Þ
where γl is SFE for dispersion liquid for which γl = γdl . Next, the contact angle Θp is measured using the liquid for which γl = γdl + γpl . Using the determined value of γds and Θp, the quantity of γps can be calculated from the following equation: p
γs ¼
2 d d 0:5 p 0:5γ l 1 þ cosΘp – γs γl =γ l
ð4Þ
The polar and dispersion components of SFE were calculated from measurement of contact angle using diiodomethane and distilled water. The diiodomethane was chosen as dispersion liquid (γl = γdl = 50.8 mJ⋅m−2), and water was chosen as the liquid with the dominant polar component (γdl = 21.8 mJ ⋅ m−2, γpl = 51.0 mJ ⋅ m−2) [24,25]. The measurement of contact angle was carried out by sessile drop technique. Ten drops of liquid with volume of 10 μl were dropped on each SiO2 film. From these measurements, the average value and standard deviation were calculated. 3. Results The morphology of SiO2 films prepared from X1 to X7 sols, as 2D and 3D AFM images in dependence on molar ratio of SiO2 is shows on Fig. 1. Based on 2D and 3D AFM images, there are bumps with almost circle circumference and flat top site on the film surfaces. Except for samples X4 and X6, the bumps seem to be arranged each to another freely. For sample X4, the bumps are in slope position. For film surface of sample X6, the bumps seem to be rising from flat plane.
ð1Þ
where D is the displacement between the tip and the substrate, Zp is the piezo-deflection, Z c is the cantilever deflection and δ is the deformation of sample. In contact, D = 0, and if the system is in equilibrium, also k s δ = k c Z c , where k s and k c are sample stiffness
Table 2 Topography characteristics of surface of nanocomposite SiO2 films.
Zmean (nm) rms-roughness (nm)
X1
X2
X3
X4
X5
X6
X7
41.3 12.9
34.6 10.4
36.4 11.6
27.9 10.2
26.4 9.6
12.2 4.4
30.4 9.2
Please cite this article as: A. Plško, et al., Nanocomposite films prepared from stabilized aqueous SiO2 sols, J. Non-Cryst. Solids (2014), http:// dx.doi.org/10.1016/j.jnoncrysol.2014.01.045
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A. Plško et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
X7, the values of dispersion and polar component of SFE are almost the same. 4. Discussion
The topography characteristics (Zmean and rms-roughness) of studied films are shown in Table 2. In comparison to other samples, the values of Zmean and rms-roughness for samples X4 and X5 are lower slightly and in the case of the sample X6, the given values are lower markedly. Fig. 2 shows the average values of the width of the bump's base in dependence on molar ratio of SiO2. The error bars reflect the standard deviations of average values. The width of the bump's base is the same for all samples, and it has a value of about 1.1 μm. However, according to noticeable higher values of standard deviations, samples X4, X5 and X6 have the higher interval of the width of the bump's base. The dependence of average value of adhesion force with its standard deviation on molar ratio of SiO2 is shown in Fig. 3. In comparison to other samples, the average values of adhesion force and their standard deviations are different markedly for samples X4, X5 and X6. The dependence of average value of stiffness with its standard deviation on molar ratio of SiO2 of prepared films is shown in Fig. 4. In comparison to other samples, there is also the little increase of stiffness for samples X4, X5 and X6. The values of standard deviations of stiffness are almost the same for all samples. Fig. 5 shows the average values and standard deviations of surface free energy and its polar and dispersion components in the dependence on molar ratio of SiO2. The average values of the surface free energy have the slight rising tendency for samples from X1 to X3, and then there is a low decrease of SFE, and it is practically constant for samples from X4 to X7. The same tendency can be also observed for the dependence of the average values of the dispersion component of SFE on molar ratio of SiO2. However, the dependence of the polar component of SFE on molar ratio of SiO2 has opposite tendency. For samples X4–
The observed dependencies for the properties of nanocomposite films prepared from sols in system “ssSiO2–AA–IPA–H2O” can be explained on the basis of the properties of these sols, and it is especially based on the change of stabilization process of studied sols as well as changes during preparation of films and their thermal treatment. First, it is necessary to point out that the studied sols were prepared from stabilized aqueous SiO2 sol in which the SiO2 particles are stabilized by Na+ ions, i.e., by electrostatic stabilization. Considering the change of composition from X7 sol to X1 sol, the ratio x(AA)/x(SiO2) increases from 9.8 to 69.2, and at the same time, the polarity of solvent decreases, and it is evident from the ratio x(H2O)/x(IPA), which decreases from value 28.9 (for X7 sol) to value 0.6 (for X1 sol). These changes cause that in X7 sol with its high polarity of dispersive medium and its low x (AA)/x(SiO2) ratio, the stabilization by Na+ ions, i.e., electrostatic stabilization, is dominant. The increase of x(AA)/x(SiO2) ratio and the decrease of polarity of dispersive medium cause that the molecule of acetic acid, which acts as surfactant, is implemented into the electric double layer, and it is connected with formation of combined stabilization of SiO2 particles in sol. Finally, this mentioned process of combined stabilization becomes dominant [1,2,16]. The given process can be used for explaining the stability of SiO2 sols in the studied system. The explanation of observed differences in the properties of films prepared from sols X4, X5 and X6 can be expressed on the basis of the fact that there is no sufficient amount of acetic acid molecules around the particle in these sols. Therefore, the steric stabilization is not the determining factor, but the influence of electrostatic stabilization decreases because the polarity of the solvent and the concentration of ions in dispersive medium have decreasing tendency. This “transition” state causes that there is the agglomeration of particles during drying of prepared films leading to the more non-uniform and denser aggregates of SiO2 particles because in sols X4, X5 and X6, the stabilization of particles is smaller. During subsequent thermal treatment, the more compact and less porous films are formed, and in comparison with other samples, their values of Zmean and rms-roughness are slightly lower. Moreover, for these three samples, the values of stiffness are higher, and the decrease of the dispersion component of SFE indicates the lower porosity of these films. This “transition” state during the stabilization of studied sols also represents the decrease of the polar component of SFE from sample X7 to sample X3 and than this value is almost constant. The irregularity in adhesion forces on surfaces of films prepared from sols X4, X5 and X6 is also the result of mentioned “transition” state. After thermal treatment,
Please cite this article as: A. Plško, et al., Nanocomposite films prepared from stabilized aqueous SiO2 sols, J. Non-Cryst. Solids (2014), http:// dx.doi.org/10.1016/j.jnoncrysol.2014.01.045
A. Plško et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
the created aggregates have non-uniform surfaces with respect to presence of acetic acid and ≡Si–OH groups, and according to this mentioned fact, there is the formation of markedly different regions in term of adhesion forces in the nanometre ranges. The agglomeration of sol particles during drying of films leads to the formation of bumps with almost circle circumference. The particles link together in fluid layer. The feeding of linking particles from all sides is uniform, except of the top side where the feeding stops quite fast. The result of the mentioned process is connected with formation of bumps on the film surface (Fig. 1) and they consist of nanoparticles of initial sol. 5. Conclusion The stable sols in system “stabilized aqueous SiO2 sol–acetic acid– isopropyl alcohol” were prepared and used for preparation of nanocomposite films. In relation to studied nanocomposite films, the AFM technique was used for determination of morphology, topography, width of the bump's base, stiffness and adhesion force. Surface free energy and its polar and dispersion component were determined by sessile drop technique. The changes in observed properties, mainly the difference of samples X4, X5 and X6, are explained on the basis of changes in stabilization process of sols used for films preparation. For the given observed properties, the determining process is the change from electrostatic to steric stabilization of studied sols. Acknowledgements This work was supported by the Grant Agency of the Slovak Republic (grant no. 1/0559/11 and project no. APVV-0487-11).
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Please cite this article as: A. Plško, et al., Nanocomposite films prepared from stabilized aqueous SiO2 sols, J. Non-Cryst. Solids (2014), http:// dx.doi.org/10.1016/j.jnoncrysol.2014.01.045