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

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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 nano- particles 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 re- search, 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-pro- panol (IPA, 99.8%), and hydrochloric acid (37 % in wa- ter) 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 parti- cles, 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 pro- ducts.
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