preparation of solvent-dispersible nano-silica powder …tkjse/19-4/03-c10501_0048.pdf ·...

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

Inorganic nanoparticles are widely used to fabricate

organic�inorganic composites with enhanced mechani-

cal, thermal, optical, etc., properties suited to various ap-

plications [1�16]. The performances of the composites

are, however, dependent upon the size, size distribution,

and how uniform the particles disperse in the organic ma-

trix. For example, the inorganic domain for a hard coat-

ing, such as that applied on lenses or glasses, generally

has to be less than ~100 nm to avoid deterioration of op-

tical clarity [16].

For nano-silica derived from the sol-gel process, par-

ticle aggregation occurs naturally due to the presence of

active �OH groups on the particle surface. These �OH

groups tend to form hydrogen bonds or undergo conden-

sation reactions mutually to yield Si�O�Si linkages be-

tween neighboring particles. Hence, as the solvent of the

sol is removed, such as to form powdery products, large

irreversible aggregates (secondary particles) will form,

which are no longer dispersible in the original solvent.

To prevent aggregation of nanoparticles, it is generally

necessary to deactivate the �OH groups on the particle

surface. Physical means such as incorporation of chelat-

ing agents and surfactants, and various chemical modifi-

cation approaches are commonly adopted to achieve this

purpose. For example, surfactants can serve as a nano-

reactor or template for syntheses of independent nano-

particles that are encapsulated in the micelles of surfac-

tant molecules [17,18]. On the other hand, the amount

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

*Corresponding author. E-mail: [email protected]

of surface �OH can be reduced by reaction with a modi-

fier, such as those bearing RSi�X, R�OH, or R�NCO

species on the molecule [19�23]. For example, by bond-

ing with both 3-(trimethoxysilyl)propyl methacrylate

(MSMA) and trimethylethoxysilane (TMES) on nano-

silica, Huang et al. were able to prepare a paste-like ma-

terial consisting of ~98% nano-silica and 2% solvent,

which remained stable and dispersible over a prolonged

storage period (> 6 months) [23].

Dried silica powders have been utilized in a number

of industrial applications, such as fillers in filter films,

matrix of a catalyst, reinforcing component for powder

coatings, etc. However, it is often noted that the sizes of

the silica clusters in the sample can be rather large (> 500

nm) in these cases, due to serious particle�particle ag-

gregation, which may downgrade the quality of the pro-

ducts. It is, therefore, of great interest to prepare nano-

silica particles that do not aggregate during drying, and

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

The silica sol was prepared by hydrolysis and con-

densation of TEOS in the presence of water/IPA solu-

tions, as shown previously [14,23]. Briefly, TEOS was

mixed with IPA to form a homogeneous solution. Then,

HCl(aq) (pH 1.2) was added to this solution under con-

tinuous agitation. The molar ratio of TEOS:H2O:IPA was

set to be 1:4:1.16. The reaction was allowed to proceed

for 3 h, cf. Scheme 1(a). Using dynamic light scattering

method, it was found that with an extended period of

storage (typically one week), aggregation of the SiO2

particles occurred in the sols [23]. For this reason, the

�OH groups on the SiO2 particles were end-capped by

reaction with the capping agent TMES, which is a mono-

functional ethoxylsilane, cf. Scheme 1(b). Appropriate

amounts of TMES, IPA, and HCl(aq) (pH 0.6) were slowly

added into the as-prepared SiO2 sol under vigorous agi-

tation. After reaction for 3 h at room temperature, TMES-

capped silica (TSiO2) was obtained. The compositions

of various chemical species for this reaction are listed in

402 Chao-Ching Chang et al.

Scheme 1. Schematic representation of the paths for synthesisof TMES modified SiO2.

Table 1. The “R” values in the table stand for the mole

ratio of TMES/(TMES + TEOS). Subsequently, vacuum

distillation was applied at 50 �C to remove the volatile

species such as various alcohols and water in the TSiO2

sol. After 1 h of vacuum operation, weight of the sample

approached constant (c.f., Figure 1), and the product ap-

peared as a white powdery solid.

2.3 Characterization

Infrared absorption spectra of the TSiO2 were ob-

tained using a Fourier Transform Infrared Spectropho-

tometer (Nicolet MAGNA-IR spectrometer 550, USA).

An appropriate amount of the TSiO2 sol was dropped

onto a KBr disc, and then the solvent was evaporated at

25 �C in a vacuum oven. For all scans, the spectra were

collected over the wavenumber range of 400�4000 cm-1

with a resolution of 4 cm-1. TEM micrographs of the sil-

ica particles were taken using Hitachi H-7100, Japan.

The samples were prepared by dropping IPA-dispersed

TSiO2 on a standard copper grid, and then IPA was re-

moved by vacuum at room temperature. The size and size

distribution of silica particles in various sols were deter-

mined by the dynamic light scattering (DLS) method, us-

ing Malvern Zetasizer Nano ZS, at 25 �C.

3. Results and Discussion

3.1 Chemical Structure Analyses by FTIR

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

lysis and condensation of alkoxysilanes under acidic con-

dition. FTIR analyses for this reaction have been per-

formed previously by many authors and the results were

well documented [24�26]. Figure 2 shows the FTIR

spectra of the TSiO2 (R5) formed at various times during

the course of its synthetic reaction, Scheme 1(b). The

absorption band at 946 cm-1 corresponds to the stretch-

ing vibration of Si�OH groups on the particle, whose in-

tensity decreases significantly during the initial 30 min

and then gradually reaches a constant level for the re-

maining 2.5 h. The broad band around 3320 cm-1 is as-

signed to various �OH groups, e.g., those on SiO2 or wa-

ter [25]. This band follows a trend similar to that ob-

served for Si�OH. The Si�CH3 signal of TMES is lo-

cated at 851 cm-1 [24], which grows as the reaction pro-

ceeds. Based on the above observations, it is confident

to put that reaction between TMES and the hydroxyl

groups of SiO2 has occurred to form �Si�O�Si(CH3)3

species on the particle surface. Figure 3 compares the

Preparation of Solvent-dispersible Nano-silica Powder by Sol-gel Method 403

Figure 2. FTIR spectra of a modified SiO2 (R5) at varioustimes during its synthesis.

Table 1. Molar compositions of various species for the

capping reaction

Sample coad TMES IPA H2O Ra

R4 0.67 1.34 0.67 0.4

R5 1 2 1 0.5

R6 1.5 3 1.5 0.6

R7 2.33 4.66 2.33 0.7

a R = TMES/(TMES + TEOS).

Figure 1. Weight of sample during solvent removal by vac-uum distillation.

spectra of TSiO2 nanoparticles prepared with different

added amounts of TMES (R-value). Obviously, as the R-

value is raised, the intensity of the Si�CH3 band (with re-

spect to Si�OH) increases. That is, more Si�OH has been

converted to Si�O�Si(CH3)3 when more TMES is added.

However, it is noted that the degree of increment be-

comes less significant for higher R values. For example,

between R = 0.4 and 0.5, the intensity ratio of Si�CH3/

Si�OH changes significantly from 0.13 to 1.37. How-

ever, between R = 0.6 and 0.7, the ratio increases only

45% form 2.27 to 3.3. In other words, R = 0.7 is ap-

proaching the saturated dosage of TMES for the capping

reaction.

3.2 Particle Size from DLS and TEM

The particle sizes of various freshly synthesized sols

were measured by means of DLS. As an example, Figure

4(a) shows the size distribution profile of a representa-

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. For the cases of R � 0.4, considerable particle ag-

gregations are found to occur during the late-stage of sol-

vent removal, and eventually flaky monolithic samples

are obtained, which is no longer dispersible in common

solvents. That is, at these levels of TMES dosages, the

particles still have considerable amounts of �OH groups

on their surfaces, which condense with each other upon

contact to form irreversible covalent bonds. In contrast,

for the cases of R = 0.5�0.7, the vacuum-dried products

appear as white fine powders, and can readily be dis-

persed in many organic solvents. Table 2 lists the solu-

bility parameters of the tested solvents (dispersants) along

with the measured sizes of the re-dispersed TSiO2 nano-

particles (DLS). For R5 dispersed in IPA, the average

size of the particles is ~2.6 nm, essentially the same as it

is in the original synthesized sol, which confirms that


Figure 3. FTIR spectra of TSiO2 nanoparticles prepared withdifferent amounts of TMES (R-values) after 3 h ofreaction.

Figure 4. Particle size distribution of a TSiO2 sol (R = 0.5) asdetermined by DLS.

Table 2. Sizea of TSiO2 particles dispersed in various

solvents (�: solubility parameter)

Dispersant

Name�

(cal/cm3)1/2

R4 R5 R6 R7

Decane 06.6 �b 7.3 18 85

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 � �

H2O 23.2 � � � �

a Determined by DLS.b

�: Non-dispersible.

the TMES moiety (�Si(CH3)3) on the particle surface has

effectively prohibited bond formation between neigh-

boring particles. As a result, even in the compacted pow-

der form, the nanoparticles are able to regain their sizes

simply by dispersing in IPA. Table 2 also indicates that

the R5 sample is dispersible in a relatively wide range of

organic solvents (specifically, polar and non-polar ones)

with particle size less than 10 nm. For all the tested sol-

vents, the solubility parameters fall over the range 6.6�

23.2 (cal/cm3)1/2. It is interesting to find that the mea-

sured particle size decreases with increasing solubility

parameter of the solvent. For instance, the particle dia-

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 for-

mation between residual �OH on the particle and the dis-

persant. 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-bu-

tanol) 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 (� = 6.6 (cal/cm3)1/2).

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 con-

densation 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 indi-

cated that the particles are of circular shape and well sep-

arated with size over the range 3�10 nm, consistent with

that obtained from DLS measurements.

As is recognized, SiO2 nanoparticles may either be

negatively or positively charged, depending on the acidity

of the sol. After modified by TMES, the charge density

on the surface changes because the polar Si�OH groups

have been partly replaced by Si�O�Si(CH3)3. Such ef-

fect is manifested in Figure 6 in terms of zeta potentials

of the particles (SiO2 and TSiO2) dispersed in sols with

addition of hydrochloric acid of different pH values. It

appears that increasing acidity causes shifting of the

zeta potential from negative to positive values for both

types of particles. As a result, the isoelectric point (pH

where zeta potential is identically zero) can be identi-


Figure 5. TEM image of TSiO2 particles with R = 0.5.

Figure 6. Zeta potential of SiO2 and TSiO2 dispersed in solsof different pH values.

fied to be pH 1.2 for TSiO2 and pH1.6 for SiO2. Further,

it is found that the zeta potential of TSiO2 is always

more negative than that of SiO2 over the tested pH range,

which implies that particle�particle repulsive forces

are stronger for TSiO2 than for SiO2. This effect explains

partially why TSiO2 particles are more stable and less

prone to aggregation in the sol.

4. Conclusions

SiO2 nanoparticles synthesized from acid-catalyzed

sol-gel process were surface-modified by reaction with

the capping agent TMES. The formed products, termed

TSiO2 nanopowder, exhibited different level of solvent

dispersibility, depending on the amounts of TMES at-

tached to the SiO2 surface. Successful capping was veri-

fied by FTIR spectra of SiO2 and TSiO2 samples, which

indicated increment of Si�O�Si linkage and decrease of

�OH groups after capping reaction. The sizes of TSiO2

particles dispersed in various solvents, as determined by

DLS, fell largely over the range 2�20 nm for solvents

with solubility parameters of 16�29.6 MPa1/2. TEM

imaging indicated that the particles were well separated

without any evidence of aggregation, and the largest iden-

tifiable particle size was ~10 nm, consistent with the

DLS data.

Acknowledgement

The authors wish to thank the Ministry of Science

and Technology for financial support (104-2632-E-032-

001).

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Manuscript Received: Apr. 20, 2016

Accepted: May 5, 2016


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