Preparation of Solvent-Dispersible Nano-Silica Powder by Sol-Gel Method

Journal of Applied Science and Engineering, Vol. 19, No. 4, pp. 401-408 (2016) DOI: 10.6180/jase.2016.19.4.03

Preparation of Solvent-dispersible Nano-silica Powder by Sol-gel Method

Chao-Ching Chang1,2, Jo-Hui Lin1 and Liao-Ping Cheng1,2*

1Department of Chemical and Materials Engineering, Tamkang University, Tamsui, Taiwan 251, R.O.C. 2Energy and Opto-Electronic Materials Research Center, Tamkang University, Tamsui, Taiwan 251, R.O.C.

Abstract

Solvent dispersible nano-silica powder was prepared by a dual-step sol-gel process: first, SiO2 nanoparticles were synthesized through acid-catalyzed hydrolysis and condensation of tetraethyl orthosilicate in 2-propanol aqueous solution. Then, the particles were surface-modified by means of

the capping agent trimethylethoxysilane (TMES). The formed product, termed TSiO2 nanopowder, was dispersible in many organic solvents, and the dispersibility was found to depend on the amounts of

TMES bounded to the SiO2 nanoparticles. FTIR spectra of TSiO2 samples confirm Si-O-Si linkage

being formed between TMES and SiO2 through the capping reaction. The sizes of TSiO2 dispersed in various solvents, as determined by dynamic light scattering (DLS), fell largely over the range 2-20 nm for solvents with solubility parameters of 16-29.6 MPa1/2. TEM imaging of the nanoparticles indicated that they were well separated with the largest identifiable size of ~10 nm, agreeing with the results obtained from DLS.

Key Words: Nanoparticles, Dispersible, Sol-gel, Silica

1. Introduction active -OH groups on the particle surface. These -OH groups tend to form hydrogen bonds or undergo conden- Inorganic nanoparticles are widely used to fabricate sation reactions mutually to yield Si-O-Si linkages be- organic-inorganic composites with enhanced mechani- tween neighboring particles. Hence, as the solvent of the cal, thermal, optical, etc., properties suited to various ap- sol is removed, such as to form powdery products, large plications [1-16]. The performances of the composites irreversible aggregates (secondary particles) will form, are, however, dependent upon the size, size distribution, which are no longer dispersible in the original solvent. and how uniform the particles disperse in the organic ma- To prevent aggregation of nanoparticles, it is generally trix. For example, the inorganic domain for a hard coat- necessary to deactivate the -OH groups on the particle ing, such as that applied on lenses or glasses, generally surface. Physical means such as incorporation of chelat- has to be less than ~100 nm to avoid deterioration of op- ing agents and surfactants, and various chemical modifi- tical clarity [16]. cation approaches are commonly adopted to achieve this For nano-silica derived from the sol-gel process, par- purpose. For example, surfactants can serve as a nano- ticle aggregation occurs naturally due to the presence of reactor or template for syntheses of independent nanoparticles that are encapsulated in the micelles of surfac- *Corresponding author. E-mail: [email protected] tant molecules [17,18]. On the other hand, the amount 402 Chao-Ching Chang et al. of surface -OH can be reduced by reaction with a modi- densation of TEOS in the presence of water/IPA solu- fier, such as those bearing RSi-X, R-OH, or R-NCO tions, as shown previously [14,23]. Briefly, TEOS was species on the molecule [19-23]. For example, by bond- mixed with IPA to form a homogeneous solution. Then, ing with both 3-(trimethoxysilyl)propyl methacrylate HCl(aq) (pH 1.2) was added to this solution under con-

(MSMA) and trimethylethoxysilane (TMES) on nano- tinuous agitation. The molar ratio of TEOS:H2O:IPAwas silica, Huang et al. were able to prepare a paste-like ma- set to be 1:4:1.16. The reaction was allowed to proceed terial consisting of ~98% nano-silica and 2% solvent, for 3 h, cf. Scheme 1(a). Using dynamic light scattering which remained stable and dispersible over a prolonged method, it was found that with an extended period of storage period (> 6 months) [23]. storage (typically one week), aggregation of the SiO2 Dried silica powders have been utilized in a number particles occurred in the sols [23]. For this reason, the of industrial applications, such as fillers in filter films, -OH groups on the SiO2 particles were end-capped by matrix of a catalyst, reinforcing component for powder reaction with the capping agent TMES, which is a mono- coatings, etc. However, it is often noted that the sizes of functional ethoxylsilane, cf. Scheme 1(b). Appropriate the silica clusters in the sample can be rather large (> 500 amounts of TMES, IPA, and HCl(aq) (pH 0.6) were slowly nm) in these cases, due to serious particle-particle ag- added into the as-prepared SiO2 sol under vigorous agi- gregation, which may downgrade the quality of the pro- tation. After reaction for 3 h at room temperature, TMES- ducts. It is, therefore, of great interest to prepare nano- capped silica (TSiO2) was obtained. The compositions silica particles that do not aggregate during drying, and of various chemical species for this reaction are listed in can easily be dispersed in organic solvents. In this research, TMES was employed as a capping agent to treat silica nanoparticles that were synthesized from an acid- catalyzed sol-gel process. As TMES is mono-functional, it reduces effectively the amount of -OH groups on the particle surface. Therefore, even after vacuum-dried, the obtained nano-silica powder (termed TSiO2) can still be dispersed in various organic solvents without changing significantly the average particle size (< 10 nm). The preparation and characterization of TSiO2 are detailed in the sections given below.

2. Experimental

2.1 Materials Tetraethoxysilane (TEOS, > 98%) was purchased from Fluka. Trimethylethoxysilane (TMES, 97%), 2-propanol (IPA, 99.8%), and hydrochloric acid (37 % in water) were purchased from Aldrich. All materials were used as received.

2.2 Preparation of Surface Modified Nano-silica

Powder Scheme 1. Schematic representation of the paths for synthesis The silica sol was prepared by hydrolysis and con- of TMES modified SiO2. Preparation of Solvent-dispersible Nano-silica Powder by Sol-gel Method 403

Table 1. The “R” values in the table stand for the mole moved by vacuum at room temperature. The size and size ratio of TMES/(TMES + TEOS). Subsequently, vacuum distribution of silica particles in various sols were deter- distillation was applied at 50 °C to remove the volatile mined by the dynamic light scattering (DLS) method, us- species such as various alcohols and water in the TSiO2 ing Malvern Zetasizer Nano ZS, at 25 °C. sol. After 1 h of vacuum operation, weight of the sample approached constant (c.f., Figure 1), and the product ap- 3. Results and Discussion peared as a white powdery solid. 3.1 Chemical Structure Analyses by FTIR

2.3 Characterization Scheme 1(a) depicts the synthesis of SiO2 by hydro-

Infrared absorption spectra of the TSiO2 were ob- lysis and condensation of alkoxysilanes under acidic con- tained using a Fourier Transform Infrared Spectropho- dition. FTIR analyses for this reaction have been per- tometer (Nicolet MAGNA-IR spectrometer 550, USA). formed previously by many authors and the results were

An appropriate amount of the TSiO2 sol was dropped well documented [24-26]. Figure 2 shows the FTIR onto a KBr disc, and then the solvent was evaporated at spectra of the TSiO2 (R5) formed at various times during 25 °C in a vacuum oven. For all scans, the spectra were the course of its synthetic reaction, Scheme 1(b). The collected over the wavenumber range of 400-4000 cm-1 absorption band at 946 cm-1 corresponds to the stretch- with a resolution of 4 cm-1. TEM micrographs of the sil- ing vibration of Si-OH groups on the particle, whose in- ica particles were taken using Hitachi H-7100, Japan. tensity decreases significantly during the initial 30 min The samples were prepared by dropping IPA-dispersed and then gradually reaches a constant level for the re- -1 TSiO2 on a standard copper grid, and then IPA was re- maining 2.5 h. The broad band around 3320 cm is as-

signed to various -OH groups, e.g., those on SiO2 or wa- Table 1. Molar compositions of various species for the ter [25]. This band follows a trend similar to that ob- capping reaction served for Si-OH. The Si-CH3 signal of TMES is lo- a Sample coad TMES IPA H2OR cated at 851 cm-1 [24], which grows as the reaction pro- R4 0.67 1.34 0.67 0.4 ceeds. Based on the above observations, it is confident R5 1 2 1 0.5 to put that reaction between TMES and the hydroxyl R6 1.5 3 1.5 0.6 groups of SiO hasoccurredtoformºSi-O-Si(CH ) R7 2.33 4.66 2.33 0.7 2 3 3 species on the particle surface. Figure 3 compares the a R = TMES/(TMES + TEOS).

Figure 1. Weight of sample during solvent removal by vac- Figure 2. FTIR spectra of a modified SiO2 (R5) at various uum distillation. times during its synthesis. 404 Chao-Ching Chang et al.

tive TSiO2 sol (R5 in Table 1). The sizes of the particles in the sol fall over a narrow range of ca. 1.5-9 nm, with the maximum number fraction located at 2.7 nm, which

is close to that of SiO2 sol before end-capped with TMES [23]. Such is consistent with the fact that capping of the

-OH groups can halt the growth/joining of SiO2 particles, and thus maintain the particle size. Liquid species,

such as methanol, ethanol, water, etc., in the TSiO2 sols can be removed by vacuum-distillation to yield solid products. For the cases of R £ 0.4, considerable particle ag- gregations are found to occur during the late-stage of solvent removal, and eventually flaky monolithic samples Figure 3. FTIR spectra of TSiO2 nanoparticles prepared with are obtained, which is no longer dispersible in common different amounts of TMES (R-values) after 3 h of reaction. solvents. That is, at these levels of TMES dosages, the particles still have considerable amounts of -OH groups spectra of TSiO2 nanoparticles prepared with different on their surfaces, which condense with each other upon added amounts of TMES (R-value). Obviously, as the R- contact to form irreversible covalent bonds. In contrast, value is raised, the intensity of the Si-CH3 band (with re- for the cases of R = 0.5-0.7, the vacuum-dried products spect to Si-OH) increases. That is, more Si-OH has been appear as white fine powders, and can readily be dis- converted to Si-O-Si(CH3)3 when more TMES is added. persed in many organic solvents. Table 2 lists the solu- However, it is noted that the degree of increment be- bility parameters of the tested solvents (dispersants) along comes less significant for higher R values. For example, with the measured sizes of the re-dispersed TSiO2 nano- between R = 0.4 and 0.5, the intensity ratio of Si-CH3/ particles (DLS). For R5 dispersed in IPA, the average Si-OH changes significantly from 0.13 to 1.37. How- size of the particles is ~2.6 nm, essentially the same as it ever, between R = 0.6 and 0.7, the ratio increases only is in the original synthesized sol, which confirms that 45% form 2.27 to 3.3. In other words, R = 0.7 is ap- a proaching the saturated dosage of TMES for the capping Table 2. Size of TSiO2 particles dispersed in various d reaction. solvents ( : solubility parameter) Dispersant 3.2 Particle Size from DLS and TEM d R4 R5 R6 R7 Name 3 1/2 The particle sizes of various freshly synthesized sols (cal/cm ) b were measured by means of DLS. As an example, Figure Decane 06.6 ´ 7.3 18 85 ´ ´´ 4(a) shows the size distribution profile of a representa- Hexane 007.24 6.4 Toluene 08.9 ´ 5.8 ´´ Acetone 009.77 ´ 6.4 ´´ Tetrahydrofyran 09.9 ´ 5.3 ´´ Dimethylsulfoxide 10.8 ´ 4.5 ´´ Butanol 11.4 ´ 4.8 6.1 6.9 Isopropanol 11.6 ´ 2.6 4.1 5.3 Ethanol 12.7 ´ 2.9 4.4 5.4 Methanol 14.5 ´ 2.9 ´´

H2O23.2´´´´ a Figure 4. Particle size distribution of a TSiO2 sol (R = 0.5) as Determined by DLS. determined by DLS. b ´: Non-dispersible. Preparation of Solvent-dispersible Nano-silica Powder by Sol-gel Method 405

the TMES moiety (-Si(CH3)3) on the particle surface has As is recognized, SiO2 nanoparticles may either be effectively prohibited bond formation between neigh- negatively or positively charged, depending on the acidity boring particles. As a result, even in the compacted pow- of the sol. After modified by TMES, the charge density der form, the nanoparticles are able to regain their sizes on the surface changes because the polar Si-OH groups simply by dispersing in IPA. Table 2 also indicates that have been partly replaced by Si-O-Si(CH3)3.Suchef- the R5 sample is dispersible in a relatively wide range of fect is manifested in Figure 6 in terms of zeta potentials organic solvents (specifically, polar and non-polar ones) of the particles (SiO2 and TSiO2) dispersed in sols with with particle size less than 10 nm. For all the tested sol- addition of hydrochloric acid of different pH values. It vents, the solubility parameters fall over the range 6.6- appears that increasing acidity causes shifting of the 23.2 (cal/cm3)1/2. It is interesting to find that the mea- zeta potential from negative to positive values for both sured particle size decreases with increasing solubility types of particles. As a result, the isoelectric point (pH parameter of the solvent. For instance, the particle dia- where zeta potential is identically zero) can be identi- meter rises to 7.3 nm, corresponding to an aggregation of ~3 particles, when it is dispersed in decane. This may be associated with the polarity and/or hydrogen bonds formation between residual -OH on the particle and the dispersant. The real causes, however, are sophisticate and beyond the scope of the present research. For the R = 0.6 and 0.7 cases, the formed nanoparticles are dispersible only in the three tested alcohols (ethanol, IPA, and 1-butanol) and decane, and the particle sizes are somewhat larger than R5 in the same alcohol. The fact that higher R values give rise to particles with smaller amounts of -OH groups on the particle surface is expected to play a role; as is evident, the decreased polarity has rendered the particles somewhat dispersible in non-polar solvent like decane (d = 6.6 (cal/cm3)1/2). Figure 5. TEM image of TSiO2 particles with R = 0.5. The unmodified SiO2 nanoparticles tend strongly to gather into large clusters upon dispersant removal during drying. Such is clearly demonstrated in TEM imaging of the particles, which involves a vacuum step for sample preparation [23]. As the solvents are gradually removed from the sol, particle-particle contacts become frequent and the interactions between them are enhanced. Even- tually aggregates are formed by hydrogen bonding or condensation between hydroxyl groups on their surface. By contrast, aggregation phenomenon is not evident for the

TSiO2 nanoparticles (R = 0.5–0.7) since their surface -OH amounts have been greatly reduced. Figure 5 indicated that the particles are of circular shape and well separated with size over the range 3-10 nm, consistent with Figure 6. Zeta potential of SiO2 and TSiO2 dispersed in sols that obtained from DLS measurements. of different pH values. 406 Chao-Ching Chang et al.

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