Effect of surface modification and medium on the rheological properties of silica nanoparticle suspensions

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Effect of surface modification and medium on the rheological properties of silica nanoparticle suspensions Shuangbing Li a,b, Jixiao Wang a,b,1, Song Zhao a,b, Wei Cai a,b, Zhi Wang a,b, Shichang Wang a,b a

Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Tianjin key Laboratory of Membrane Science and Desalination Technology, State key Laboratory of Chemical Engineering, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, PR China

b

art ic l e i nf o

a b s t r a c t

Article history: Received 11 January 2016 Received in revised form 27 January 2016 Accepted 28 January 2016

The unmodified and 3-(trimethoxysilyl) propyl methacrylate (TPM) modified silica nanoparticles were synthesized via the modified Stöber method. The structure and chemical composition of silica nanoparticles were determined by scanning electron microscope (SEM), X-ray diffraction (XRD) characterization, Fourier Transform Infrared Spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results demonstrate that silica nanoparticles with the diameter of 90 nm and amorphous state are successfully modified by TPM. The rheological test results demonstrated that surface modification of nanoparticles and medium exerted great influence on the rheological properties of silica particle suspension. Under high shear rate of about 250 s
1, the viscosity of suspension comprising unmodified silica nanoparticles and deionized water, TPM-modified silica nanoparticles and deionized water, TPM-modified silica nanoparticles and polyethylene glycol, is about 0.4 Pa.s, 0.1 Pa.s and 30 Pa.s, respectively. The results are significant for preparing homogenous and stable silica nanoparticle suspension, and meaningful for ceramic processing. & 2016 Published by Elsevier Ltd

Keywords: Silica nanoparticle Ceramic processing Surface modification Medium Rheological properties

1. Introduction Ceramics are attracting a great deal of interest frequently due to their advantages including great mechanic property, thermal stability and chemical stability [1,2]. Nanoceramics are ceramic materials comprised of particles in nanoscale [3]. These nanoceramics have more advanced properties because of the large specific surface area, high surface activity of nanomaterials [4], and show excellent performance in several applications like nanoceramics semiconductor sensors and biological bone or other biological structure [5,6]. The properties of ceramics are highly related to the characteristics of the materials used. Silica nanoparticles won enormous popularity in the field of nanoceramic due to their high strength, hardness and chemical stability [7]. As an environmentally friendly preparation method, slip casting is widely used in the manufacturing of ceramics [8–11]. In order to obtain ceramics with high quality and homogenous microstructure, homogenous and stable particle slurry with high solid volume fraction should be readily prepared in this technique [12]. Therefore, the rheological properties of the suspensions would exert great influence on the ceramic processing [13–18]. E-mail address: [email protected] (J. Wang). Tel.: þ86 022 27404533; fax: þ 86 022 27404496.

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Over the last few decades, the effects of particle size, particle size distribution, additives and pH value on the rheological properties of colloidal suspension have been investigated vastly [13,14,19– 21]. According to these reports, the additives adsorbed onto the surface of particles can change the rheological properties of colloidal suspension by changing the electrical properties of particles surface. As a result, the surface properties of particles can also affect the rheological properties of suspension [13,20]. However, the role of surface modification of particles on the rheological properties of silica nanoparticle suspension was rarely reported. Water is environment friendly and easily available. Therefore, it has been extensively used for preparing colloidal suspension during ceramic processing [10,11]. Some scientists reported that organic solvent can also be used for preparing ceramic slurry [22]. However, the effect of the medium on the rheological properties of silica nanoparticle suspension was also rarely reported. Shear thinning refers that the viscosity of suspension would decrease with increasing shear rate, while shear thickening refers that the viscosity of suspension increases with increasing shear rate above a critical value. The silica nanoparticle suspension can show shear thinning and shear thickening phenomenon under outside shearing [23–25]. In fact, the particles in suspension are in different state with the increase of shear rate [26]. In equilibrium state, the particles in suspension form weak networks by Brownian motion or interaction of particles [27]. When the suspension

http://dx.doi.org/10.1016/j.ceramint.2016.01.199 0272-8842/& 2016 Published by Elsevier Ltd

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is sheared by outside force, the networks would be destroyed and then the particles would be layered. In this case, the suspensions show shear thinning behavior [26,27]. But in some occasions, shear thickening will occur after shear thinning, resulting from the hydroclusters formed in suspension by outside shearing [26]. This work mainly focus on investigating the effect of surface modification of silica nanoparticles and medium on shear thinning and shear thickening behavior of suspension. The silica nanoparticles were synthesized by the modified Stöber method [28,29]. Then, the surface of silica nanoparticles was modified by 3-(trimethoxysilyl) propyl methacrylate (TPM). After that, the unmodified and the TPM-modified silica nanoparticles were characterized by SEM, XRD, FTIR and XPS techniques. The zeta potential of the suspensions comprising 1 wt% of unmodified and TPMmodified silica nanoparticles in deionized water was measured by Zeta potentiometer, respectively. Finally, the rheological properties of the suspensions comprising of unmodified silica nanoparticles and water (51:49, vol/vol), TPM-modified silica nanoparticles and water (51:49, vol/vol), TPM-modified silica nanoparticles and PEG200 (51:49, vol/vol) were characterized and evaluated, respectively.

2. Materials and methods 2.1. Materials Tetraethoxysilane [TEOS, 98%, Aladdin], absolute ethanol [99.9%, Aladdin], ammonium hydroxide [25%, Aladdin], polyethylene glycol with molecular weight of 200 g mol
1 [PEG200, 98%, Aladdin], nitric acid [HNO3, 68%, Aladdin], 3-(trimethoxysilyl) propyl methacrylate [TPM, 97%, Aladdin] were directly used without any purification process. The deionized water with the conductivity of less than 15 μs cm
1 was produced by HITECH Laboratory Water Purification System. 2.2. Synthesis and characterization of silica nanoparticles Silica nanoparticles were synthesized by the modified Stöber method [28,29]. The details were listed as follows. Firstly, 400 mL of absolute ethanol, a certain amount of deionized water and ammonium hydroxide were mixed in a 1000 mL three-necked flask, which was put in a 30 °C thermostatic water-bath and stirred for 30 min. Then 5.20 g of tetraethoxysilane (TEOS) was added to the mixture, and the reaction was conducted for 3 h. In this process, polydisperse silica particle seeds with small diameter were obtained. Secondly, some deionized water was added into the flask, and 10 min later, 52.00 g of TEOS was added to the flask and the reaction was kept for another 3 h. After that, half of the reaction liquid was taken out and the unmodified silica nanoparticles were obtained after purification by centrifugation and washing. Meanwhile, a certain amount of TPM was added to the remaining reaction liquid and kept for other 12 h at 30 °C. The TPM-modified silica nanoparticles were obtained after purification. All those silica nanoparticles were dried in a vacuum oven at 80 °C for 24 h. The size and shape of silica nanoparticles were characterized by scanning electron microscope (SEM, Hitachi S-4800) operating at an accelerating voltage of 10.0 kv. The zeta potential of the suspensions comprising 1 wt% of unmodified and TPM-modified silica nanoparticles in deionized water was measured by Zeta potentiometer (Malven Nano ZS), respectively [9,10]. The pH of the suspensions was adjusted by nitric acid and ammonium hydroxide. The wide-angle X-ray diffraction (XRD) patterns of the particle sample were recorded using an X-ray diffractometer (Rigaku Multiflex, D/MAX 2500). The surface properties of silica nanoparticles were characterized by FTIR (Bio-Rad FTS 6000) and XPS (PHI-5000CESCA).

2.3. Preparation and characterization of silica nanoparticle suspension The suspension sample USW was prepared with unmodified silica nanoparticles and deionized water (51:49, vol/vol). In detail, a certain amount of unmodified silica nanoparticles were added into deionized water and mixed by grinding and ultrasonic dispersion alternatively for several times. Similarly, the suspension samples MSW comprising of TPM-modified silica nanoparticles and deionized water (51:49, vol/vol), and MSP comprising of TPMmodified silica nanoparticles and PEG200 (51:49, vol/vol) were prepared, respectively. The rheological characterization of the suspension samples (USW, MSW, MSP) was conducted at 25 °C by stress-controlled rheometer (Anton-Paar MCR302) with parallel plates having diameter of 25 mm. The distance between the two plates was 0.50 mm. The pretreatment details of suspension samples before measurement have been reported elsewhere [19].

3. Results and discussion 3.1. Characterization of shape and phase structure of silica nanoparticles The particle size and shape have been characterized by SEM and the results are shown in Fig. 1(a) and (b). It can be seen that the silica nanoparticles are spherical with diameter of about 90 nm. The unmodified silica particles are severely aggregated, while the TPM-modified silica particles show well dispersion. Therefore, the surface properties of particles can greatly affect the disperse state of particles. The phase composition of silica nanoparticles can be deduced from XRD patterns. As can be seen from Fig. 1(c), each of the XRD patterns with only one broad peak in the range of 20–35° means the two samples are both in amorphous state [30]. Hence, the modification of the particles did not change the phase structure. The zeta potential of the suspensions comprising 1 wt% of unmodified and TPM-modified silica nanoparticles was characterized, respectively. The results shown in Fig. 1(d) indicate that the zeta potential value of both suspensions decreases with decreasing pH. The main reason for this phenomenon is the addition of acid would suppress the ionization of H2SiO3 (H2SiO32HSiO3
þ H þ ). Additionally, the value of zeta potential of TPM-modified silica nanoparticle suspension is slightly lower than that of unmodified silica nanoparticle suspension. This phenomenon may result from the decrease of Si–OH groups after grafting TPM onto the surface of particles, which can also suppress the ionization of H2SiO3. 3.2. Characterization of surface properties of silica nanoparticles The surface properties of unmodified and TPM-modified silica nanoparticles were characterized by FTIR. As can be seen from Fig. 2, the characteristic peaks for silica appeared at 3451 cm
1, 939 cm
1 and 808 cm
1, corresponding to O – H, Si–O–Si and Si– OH, respectively [31]. The intensity of the peak at 3451 cm
1 decreased with the modification of the surface of particles, which indicated that the – OH groups of silica nanoparticles were reacted with TPM. The schematic diagram of the reaction between silica particles and TPM can be seen from Fig. 3. The peaks at 1705 cm
1 and 1637 cm
1 emerging in the spectrum of TPM-modified silica nanoparticles could be assigned to the stretching vibrations of C =O and C =C [31–33]. These two new peaks also demonstrated that the surface of particles was successfully modified by TPM. The XPS characterization was conducted to further investigate the chemical properties of the unmodified silica particles and TPM-

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Fig. 1. SEM, XRD and Zeta potential characterization of silica nanoparticle samples: (a) SEM image of unmodified silica, (b) SEM image of modified silica, (c) XRD patterns of unmodified silica and modified silica, (d) Zeta potential as a function of pH of suspensions comprising 1 wt% of unmodified silica and TPM-modified silica nanoparticles in deionized water.

modified silica particles. As can be seen from Fig. 4, the XPS spectra showed the peaks of Si 2p, Si 2s, C KLL, C 1s, O KLL, O 1s and O 2s. The C: Si of unmodified silica particles is about 0.56:1, indicating that the – Si(OCH2CH3)3 is not absolutely hydrolyzed. The C: Si of TPM-modified silica particles is about 1.06:1, which is nearly two times that of unmodified silica particles. The O: Si of TPM-modified silica particles is slightly lower than that of unmodified silica particles, which is due to the dealcoholization reaction between TPM and –OH group. The increase in the ratio of element C and the

decrease in the ratio of element O demonstrated that the TPM has reacted with the – OH group of silica particles. Fig. 5(a) and (b) shows the XPS spectra of C 1s for unmodified silica particles and TPM-modified silica particles. Compared with unmodified silica particles, the appearance of peak at 288.3 eV corresponding to C=O further indicated that TPM has been successfully grafted onto the surface of silica particles [34,35]. The grafting ratio of TPM can be defined as the ratio of the number of TPM molecule to the number of silica molecule. Therefore, the grafting ratio of TPM can be calculated as follows [36].

Fig. 2. FTIR spectra of unmodified and TPM modified silica nanoparticles.

Fig. 3. Schematic diagram of the reaction of silica particles with TPM.

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Fig. 4. XPS spectra of unmodified silica and TPM-modified silica particles.

Fig. 5. Deconvolution of XPS spectra: (a) C1s of unmodified silica particles, (b) C1s of TPM-modified silica particles and (c) Si 2p of TPM-modified silica particles.

USW

10

3

10

2

10

1

10

0

10

104

4

TSW

MSW MSP

103 Viscosity /Pa

Viscosity /Pa.s

10

102 101 100

-1

10

-1

10

0

1

10

Shear rate /s

10

10-1

2

10-1

100

101

102

-1

-1

Shear rate /s

Fig. 6. Viscosity as a function of shear rate of suspension samples unmodified and TPM-modified silica particles in water.

Fig. 7. Viscosity as a function of shear rate of suspension samples TPM modified silica particles in water and PEG200.

r = ASi − C /Asilica × 100%

7185.5  100%¼ 8%. Therefore, the grafting ratio of TPM is about 8%.

(1)

In Eq. (1), r is the grafting ratio of TPM, ASi-C is the peak area of Si–C for TPM, Asilica is the peak area of silica. In order to well know the grafting ratio of TPM, the XPS spectrum of Si 2p for TPMmodified silica particles was shown in Fig. 5(c). The peaks at 102.4 eV and 103.5 eV correspond to the binding energy of Si–C for TPM and the binding energy silica, respectively [37,38]. The peak area of Si–C and silica is 577.6 and 7185.5, respectively. As a result, the grafting ratio of TPM can be calculated as: r ¼ 577.6/

3.3. Characterization of rheological properties of suspension The ceramic processing and the quality of ceramics have much relation with the rheological properties of colloidal suspension. The low viscosity and shear thinning phenomenon under high shear rate are significant for the ceramic processing [39]. Thus, the rheological properties of suspensions, including unmodified silica

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nanoparticles in water (USW), TPM-modified silica nanoparticles in water (MSW) and TPM-modified silica nanoparticles in PEG200 (MSP) were investigated, respectively. Fig. 6 shows the viscosity of suspension of samples USW and MSW as a function of shear rate. It can be seen that the viscosity of each sample decreases with increasing shear rate, indicating shear thinning occurs. The lowest viscosity of USW and MSW is about 0.4 Pa s and 0.1 Pa s under 250 s
1, respectively. The reason for this phenomenon is that the weak network formed by particles has been destroyed by outside shearing and then the particles are layered in suspension [11,19,26,27]. Fig. 6 also indicates that the viscosity of suspension sample USW is larger than that of MSW, which could be explained by the large attractive force among unmodified silica nanoparticles [27]. Generally, there are large amount of Si–OH on the surface of silica nanoparticles. As a result, the nanoparticles interact with each other by hydrogen bond. While the quantity of Si–OH would decrease a lot after TPM

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modification, which decreases the interaction of particles and weakens the intensity of network formed by particles. As a result, the viscosity of suspension decreases with modification of particle surface. Fig. 7 shows the viscosity of suspension samples MSW and MSP as a function of shear rate. It can be seen that the viscosity of the suspension comprising TPM-modified silica particles and water decreases throughout with increasing shear rate with only shear thinning occurring. While the viscosity of the suspension comprising TPM-modified silica particles and PEG200 decreases with increasing of shear rate from 0.1 s
1 to about 15 s
1 and then increases with increasing shear rate from about 15 s
1 to 250 s
1 with shear thickening occurring. The lowest viscosity of suspension MSW is about 0.1 Pa s at the shear rate of about 250 s
1. The viscosity of suspension MSP under the critical shear rate is about 7 Pa s and the maximum value of viscosity of suspension MSP during shear thickening reaches to 30 Pa s. The reason for shear

Fig. 8. The schematic diagram of disperse state of unmodified and TPM-modified silica particles in deionized water and PEG200.

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thinning phenomenon can be explained as above-mentioned. The shear thickening phenomenon after shear thinning can be explained by hydrodynamic lubrication theory [26,40]. The most essential cause for this phenomenon is the medium changed the interaction of particles. When the surface of silica particles was modified by TPM, the surface of nanoparticles would be covered by the methacrylate propyl group, and Si–OH, which is schematically described in Fig. 8. The TPM-modified silica nanoparticles can adsorb a thin layer of PEG200 onto the surface of particles by the interaction of the backbone of PEG200 and Si–OH [41]. As a result, the particles are separated by a layer of medium with a few nanometers thickness. The layer of medium on the surface of particles can provide a repulsion force to prevent the aggregation of particles [20,41,42]. While the nanoparticles in deionized water can contact with each other and aggregate together due to the large amount of hydrogen bond between particles. As a result, the suspension comprising TPM-modified silica nanoparticles and water only shows shear thinning phenomenon under outside shearing [27]. For the suspension sample MSP, under low shear rate, the weak network formed by TPM-modified silica particles would be destroyed by outside shearing with shear thinning occurring. In this regime, the viscosity of suspension MSP is much lower than that of suspension MSW, which is probably because the network formed by silica nanoparticles is disturbed by the repulsion force between nanoparticles [27]. As the shear rate further increases to exceed a critical value, the shear force would overcome the repulsion force, inducing the formation of the hydroclusters in suspension [26,40]. As a result, the viscosity increases with increasing shear rate with shear thickening occurring. These results demonstrated that medium also has great influence on the rheological properties of silica nanoparticle suspension. The results of rheological characterization indicate that choosing proper medium and modifying the surface properties of particles are effective methods to obtain stable silica nanoparticle suspension, which is significant for preparing nanoceramics with high quality.

4. Conclusions In this work, the surface of silica nanoparticles has been successfully modified by TPM. The surface modification of particles and medium can greatly affect the rheological properties of silica nanoparticle suspensions. The suspensions comprising of unmodified silica particles or TPM-modified silica and water only show shear thinning phenomenon. The suspension comprising of TPM-modified silica particles and PEG200 shows shear thickening phenomenon after shear thinning under high shear rate. Additionally, under high shear rate, the viscosity of the suspension comprising of TPM modified silica particles and water is the lowest. Thus, the viscosity of suspension can be adjusted through modifying the surface properties of particles or choosing proper medium, which would be beneficial for ceramic processing.

Acknowledgments This work was supported by Public Science and Technology Research Funds Projects of Ocean (No. 201405013-5), the Program of Introducing Talents of Discipline to Universities (No. B06006), National Natural Science Foundation of China (20836006), National High Technology Research and Development Program of (2012AA03A611), and the project State key Laboratory of Chemical Engineering (SKL-ChE-12T12).

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Please cite this article as: S. Li, et al., Effect of surface modification and medium on the rheological properties of silica nanoparticle suspensions, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.01.199i