Journal of Non-Crystalline Solids 405 (2014) 104 –115

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Journal of Non-Crystalline Solids

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Adjustment of the morphology of MCM-41 silica in basic solution

Qishu Qu a,⁎, Gang Zhou b, Yi Ding a, Shaojie Feng a, Zuli Gu b

a School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei 230601, Chinab School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China

⁎ Corresponding author.E-mail address: quqishu@qq.com (Q. Qu).

http://dx.doi.org/10.1016/j.jnoncrysol.2014.09.0120022-3093/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 July 2014Received in revised form 2 September 2014Accepted 10 September 2014Available online xxxx

Keywords:Morphology;Basic solution;MCM-41;Silica

MCM-41 silica particles with rich morphologies were prepared in a strong basic solution under a variety of con-ditions. It was found that themorphology of mesostructured silica wasmainly affected by the rate difference be-tween silica condensation and mesostructure formation. The effect of various parameters such as types andamounts of the cosolvents, degree of the dilution, amount of the surfactant, and pH value on the size of ratedifference was studied systematically. Depending on the rate difference between silica condensation andmesostructure formation, mesoporous MCM-41 particles with various morphologies could be achieved, includ-ing spheres, hexagonal rods, spiral, orange, and gyroid.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Mesoporous silica particleswith controllablemorphologies are of in-terest for various applications including catalysis, drug delivery, packingmaterials for chromatography, and other applications [1,2].Mesoporoussilica particleswith variousmorphologies were normally controlled andsynthesized in an acidic solution because of the relative slow hydrolysisand condensation rate of the silica species at low pH value [3–9]. In abasic solution, the high rate of silica condensation always leads to theformation of micron to submicron sized spherical-like particles [10,11]. Therefore, controlling the shape of mesoporous silica in the basicsolution is very difficult. Lin et al. once developed a “delayed neutraliza-tion” approach to synthesize mesoporous silica particles with a varietyof morphologies. However, the solution pH should be controlled verycarefully [12–14]. Since silica condensation rate could be reduced in adilute solution, mesostructure silica particles with various morphologieswere prepared in a basicmediumvia a dilute solution route [15]. The for-mation mechanism of these morphologies was attributed to the deposi-tion of self-assembled silicate surfactant rope-like micelles. Using thesamemethod, silica materials with a helical mesostructure were obtain-ed in the dilute basic solution [16–19]. Nevertheless, compared to thenumerous morphologies of mesostructure silica obtained in an acidicsolution, only limited morphologies have been prepared in the basicsolution. Furthermore, although a rich variety of morphologies havebeen prepared, only a fewworks reported preparingmesostructure silicawith continuously transformed morphologies via slightly changingthe initial experimental condition [20,21]. Therefore, explaining the

formation mechanism of these morphologies is difficult. For example,different formation mechanisms were proposed for the formation ofthe chiral helical mesostructure. Wang et al. believed that the formationof the helical mesostructure was the result of reducing the surface freeenergy while Han et al. thought that the formation of the helicalmesostructurewas due to the tubular shaped surfactant–silica precursoradopting a helical conformation to increase the entropy of surroundingspheres. Thus, it is reasonable to expect that a better understanding ofthe self-assembly behavior ofmesostructure silica can be achieved if par-ticles with successive changed morphologies can be prepared.

It was proposed by Yu et al. that the final morphology was deter-mined by the competition between the surface free energy (F) and thefree energy of mesostructure formation (ΔG) [22]. Induction time (de-fined as the time from addition of silica source to the appearance ofmilky-white precipitates in solution) was used to predict the relativesize between F andΔG. If F is predominant, spherical particles were ob-tained. If ΔG is dominant, mesostructure silica with various shapes canbe obtained. However, the relative size between F and ΔG decided bywhich factor was not mentioned. In addition, although parameters af-fecting the shapes of the mesostructure silica synthesized in the acidicsolution have been studied systematically [23–26], systematic study ofthe influence of various experimental parameters on the morphologyof mesostructure silica in the basic solution has not been achieved. Inthis work, the effect of different parameters such as the types andamounts of cosolvents, the degree of the dilution, the concentration ofthe surfactant and silica source on thefinalmorphology ofmesostructuresilica were investigated in the basic solution. Mesostructure silica parti-cleswith successively changed shapeswere prepared by carefully chang-ing the experimental conditions. It was found that themorphology of themesostructure silica was mainly affected by the rate difference betweensilica condensation and mesostructure formation while the relative size

105Q. Qu et al. / Journal of Non-Crystalline Solids 405 (2014) 104 –115

of the rate was affected uppermost by the types and concentrations ofcosolvents added into the reactant.

2. Materials and methods

2.1. Materials and synthesis

The experimentwasmodified from reference [38]. In a typical synthe-sis batch, 1.0 g of cetyltrimethylammonium chloride (CTAC) was dis-solved in 20 g of distilled water. Then 1.48 g of sodium metasilicate(Na2SiO3·9H2O) and 4.75 mL of formamide (HCONH2) were added at35 °Cwithmagnetic stirring, giving a clear solution. The resulting solutionof molar composition 1Na2SiO3·9H2O:0.6CTAC:23HCONH2:213H2O wasstirred for 5 min at 35 °C and aged in a water bath for 3 h in a quiescentstate. The solid product was recovered by filtration, washed with waterand dried at 60 °C. The surfactant was removed by calcination in air at550 °C for 5 h.

2.2. Characterization

All the samples were characterized by field emission scanning elec-tron microscopy (FESEM, Hitachi S-4800 II, 15 kV), transmission elec-tron microscopy (TEM, Philips Tecnai-12, 120 kV), high-resolutionTEM (HRTEM, Tecnai G2 F30 S-TWIN, 300 kV), and powder X-ray dif-fraction (XRD, D8 Advance, Cu Kα). Thermogravimetric analysis (TGA)was carried out on a Pyris 1 TGA thermoanalyzer (PerkinElmer) withnitrogen as the carrier gas at a heating rate of 10 °C min−1. Nitro-gen adsorption–desorption measurements were performed usinga Micromeritics ASAP 2010 instrument. The pore size distribution wascalculated from the adsorption branch of the sorption isotherms usingthe Barret–Joyner–Halenda (BJH) method.

3. Results and discussion

It iswell known that the formation of themesostructure silica can bedivided into three stages: (1) cooperative assembly to form surfactant/silica composite aggregates, (2) growth of these complexes into orderedarrangement through accretion or interaction, and (3)multiphase ener-gy competition after the phase separation [26–28]. The final morpholo-gy of themesostructure silicawas determined by the competition of theenergy between silica condensation and mesostructure formation. Nor-mally, particles trend to a spherical shape to minimize surface energy.

Silica

Surfactant

0 Volume of etha

0.7 1 2

Spherical Spherical Spherical Hexagonal

Fig. 1. Summary of the effect of the amount of et

Thus, spherical morphology is always being expected unless the rateof mesostructure formation is predominant [29]. However, no experi-mental results showed under which situation the rate of silica conden-sation or the rate of mesostructure formation would be the dominant.Yu et al. found that induction time could be used to predict the relativesize between F and ΔG. They drew a conclusion that ΔG was increasedwith decreasing induction time. Thus, any factors that shorten the in-duction time would favor for the formation of silica particles with rod-like or crystal-like structures [22]. However, in the basic solution, wefound that there was no direct relationship between induction timeand the morphology of the mesostructure silica. For example, the rateof mesostructure formation was found becoming dominant when asmall amount of ethanol was added to the initial reaction system(Fig. 1). As a result, the morphology of the mesostructure silica wasgradually changed from spherical particle to rope, then to peeledorange-like particle. However, the induction time was found increasedfrom ca. 5 minwithout ethanol to ca. 8 minwith 5mL of ethanol. It sug-gests that the relative size of induction time cannot be used to predictwhich rate, silica condensation or mesostructure formation, is predom-inant. Then, what on earth was the rate difference affected? In order toanswer this question, various factors affecting the process of the particleformation were investigated first.

3.1. Effect of the types of the cosolvent

It is believed that the presence of the cosolvent such as ethanol in thesolution can lead to less driving force for the formation of surfactantaggregates, resulting in the weak interaction between the surfactantand silica precursor [30]. In this work, ethanol was selected first toinvestigate its effects on the morphology (Fig. 1). As illustrated inFig. 1, MCM-41 silica particles with successively changed morphologiescould be prepared simply by adding different amounts of ethanol to thesolution. The evolution of physicochemical properties as a function ofvolume of ethanol in the reactant was shown in Table 1 and Supportinginformation (Figs. S1–4).

Fig. 2 shows the representative SEM images of themesoporous silicaparticles prepared using ethanol as cosolvent. Table 2 shows the physi-cochemical properties of calcined MCM-41 silica particles obtained byadding different volumes of ethanol in the reactant. When the volumeof ethanol increased from 0 to 1.0 mL, it can be seen that the structureofmonodispersed silica particles changed gradually from the accumula-tion of nano-spheres into the aggregation of hexagonal rods while the

nol in solution 4

Rod

5

Orange Twist

7 8

Spherical

hanol on the morphology of silica particles.

Table 1Evolution of physicochemical properties of calcined MCM-41 silica particles as a functionof volume of ethanol in the reactant.

Sample Volume ofethanol added(mL)

BET surfacearea(m2 g−1)

Porevolume(mL g−1)

Mean porediameter(nm)

d001

(nm)Mass loss(wt.%)

0 0 1244 0.96 3.1 4.1 40.721 1 2280 1.15 2.0 3.6 45.312 2 1425 0.72 2.0 3.4 43.973 3 2437 1.27 2.1 3.8 46.934 4 1378 0.72 2.1 3.7 44.425 5 1430 0.74 2.1 4.0 45.246 6 1450 0.77 2.1 3.4 41.867 7 1327 0.72 2.1 3.6 42.238 8 331 0.18 2.2 – 32.019 9 316 0.16 2.0 – 31.41

A

C

E

Fig. 2. Representative SEM (A–E) and HRTEM (F) images of mesoporous silica particles synth(B) 0.6, (C) 0.75, (D) 0. 8, (E, F) 1.0 mL. Themolar composition of the solutionswas 1Na2SiO3·9

106 Q. Qu et al. / Journal of Non-Crystalline Solids 405 (2014) 104 –115

spherical shape was kept. The spherical shape obtained in all these ex-periments suggests that silica condensation rate is always predominantwhen only a small amount of ethanol was added to the reactant. How-ever, the addition of ethanol during the synthesis would help to reducethe polarity of the solvent and thus retard the hydrolysis rates of silaneprecursors [31]. The appearance of rod-likematerials onto the surface ofthe silica spheres was the direct evidence that the rate of silica conden-sation decreased with increasing volume of ethanol (Fig. 2B), since theattempt of nano-spheres elongated into a rodlike structure would be-come apparent when silica condensation rate decreased slightly [32].These rod-like materials kept growing. When 0.75 mL of ethanol wasadded to the reactant, some of the rods partially got rid of the shackleoriginated from the demand of minimizing the surface area (Fig. 2C).The breakage of the perfect spherical structure suggests that the rateof mesostructure formation is greatly increased at this experimental

B

D

F

esized with same reactant ratio but using different amounts of ethanol as additive. (A) 0,H2O:0.6 cetyltrimethylammonium chloride (CTAC):22.9HCONH2:213H2O:(0–3.4) ethanol.

Table 2Physicochemical properties of calcinedMCM-41 silica particles obtained by adding differ-ent volumes of ethanol in the reactant.

Sample Volume ofethanol added(mL)

BET surfacearea(m2 g−1)

Porevolume(mL g−1)

Mean porediameter(nm)

d001

(nm)Mass loss(wt.%)

10 0.6 1446 0.85 2.4 3.7 43.8611 0.75 1506 0.83 2.2 3.8 44.1212 0.8 1528 0.86 2.1 3.7 44.35

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condition. The smooth surface of all the spheres completely disappearedand replaced by the radial stacking rods after adding 0.8 mL of ethanolto the reactant (Fig. 2D). All the rods grew into nearly the same sizeby adding 1.0 mL of ethanol to the reactant (Fig. 2E). These experi-mental results demonstrate that changing the relative size of therate difference between the rate of silica condensation andmesostructureformation plays the most important role in altering the morphology ofprepared particles. Furthermore, it is obviously that the balance betweenthe rate of silica condensation and the rate of mesostructure formationcould be changed easily by adding some polar solvent such as ethanolto the solution.

From Fig. 3, it can be found that the induction time changed from ca.5 min for sample 0 to ca. 5.5 min for sample 1 and 1 mL of ethanol wasadded to the reactant. The increased induction time indicates that thesilica condensation rate decreased with increasing volume of ethanolin the reactant which confirmed again that the morphology evolutionwas mainly induced by changing the balance between the rate of silicacondensation and the rate of mesostructure formation. Thermogravi-metric analysis shows that the mass loss of the particles prepared byusing 0.6 to 1mLof ethanolwas larger than the particles preparedwith-out ethanol (Tables 1, 2). This experimental result suggests that thesurfactant involved in forming the particles increased with increasingvolume of ethanol in the reactant. It is believed that spherical silica/surfactant composites were soon formed through a cooperativetemplatingmechanism [33] via an S+I− pathway [27] when silica sourceand surfactant were mixed. Therefore, the increased mass loss could beattributed to the properties of some ethanol molecules to incorporatethe micelles and adopt the role of cosurfactant [34]. The HRTEM image(Fig. 2F) shows that particles of sample 1 from the surface to corewere aggregated with loosely connected rods with ordered structure,consistent with the result of XRD (Fig. S1). BET surface area up to2280 m2 g−1 was obtained and it could be attributed to the existenceof micropores and through pores (Table 1). The easy accessibility of

0 2 4 6 8 10

3

6

9

Indu

ctio

n Ti

me

(min

)

Volume of ethanol (mL)

Fig. 3. Effect of the volume of ethanol added into the reactant on the induction time. Themolar composition of the solutions was 1Na2SiO3·9H2O:0.6CTAC:22.9HCONH2:213H2O:(0–34) ethanol.

the reactant to the internal surfaces and pores makes these particlespromising as catalyst supports or chromatographic stationary phasesfor separation. It should be noted that although numerous morphol-ogies of silica particles have been prepared, no similar structure asthat shown in sample 1 has been reported.

When a higher volume of ethanol was used (1.6–5.0 mL), the rate ofmesostructure formation became predominant (Fig. 4). Thus, spherescollapsed into particles accumulated with a few intertwined hexagonalor hexagonal-like silica rods by adding 1.8mL of ethanol to the reactant(Fig. 4A–C). When the volume of ethanol further increased from 3.0 to4.0 mL, the rods decreased their surface charge density by roundingtheir edges and corners and further curved into the shape of “S” to ac-commodate the new balance between the rate of polymerization of thesilicatemicelles and the rate ofmesostructure formation (Fig. 4D, E). Fur-thermore, “S” type particle twisted into a shape like a distorted “8” andthen split and grew into two perfect peeled orange-like particles, asshown in Fig. 5. These shape adjustments reflected the process of estab-lishing a new balance between the free energy of the mesostructure for-mation and the surface free energy of the colloidal particles [22,26]. Itshould be noted that the induction time is the longest when the synthe-sis of MCM-41was carried out by adding 5mL of ethanol to the reactant,as shown in Fig. 3. It suggests that the rate difference between the silicacondensation and mesostructure formation is the biggest at this condi-tion. Thus, it is not surprising that a variety of morphologies of MCM-41 particles were obtained at this condition (Fig. S5).

When more ethanol was used, the induction time decreased withincreasing volume of ethanol (Fig. 3). Meanwhile, the reversed morphol-ogy evolution process occurred. The orange segments unfolded (Fig. 4G),twisted (Fig. 4H), and aggregated into spheres again (Fig. 4I, J). As can beseen clearly from Fig. 4I and J, the surfacemorphology of the spheres pre-pared by using high volume of ethanol (≥8.0 mL) was far different withthose obtainedwithout ethanol. The surface of these sphereswas coveredwith hemispherical raspberry-like particles. Thermogravimetry resultslisted in Table 1 show that the mass loss of the particles prepared byadding 8 and 9mL of ethanol to the reactantwas only 32 and 31%, respec-tively, which was far less than other MCM-41 particles. It indicates thatthese two kinds of spherical particles were constructed in accordancewithdifferentmechanisms.Weproposed that suchmorphology changingis the result of the cosolvent instead of cosurfactant behavior of theethanol when the ethanol concentration is high. When a large amountof ethanol was added to the synthesis mixture, a process similar to thewell-known Stöber approach [35] occurred. In this process, the high con-centration of ethanolwouldmainly act as a cosolvent producing sphericalparticles. Furthermore, the high amount of ethanol could disrupt the self-assembly of surfactant micelles due to the decrease of the surfactant ag-gregates for the weak solvophobic effect [36]. As a result, the amount ofsurfactant incorporated in the formation of silica/surfactant compositesdecreased. Consequently, spherical particles with low surface area wereobtained. Corresponding BET surface area as low as ca. 320 m2 g−1 listedin Table 1 confirmed this speculation. HRTEM shows that these particlespossess a wormhole-like mesostructure (Fig. S4). Such wormhole-likemesostructure was also confirmed by powder X-ray diffraction patternshown in Fig. S1.

The effect of other solvents with different polarities on themorphol-ogy ofMCM-41particleswas studied further (Table 3). Itwas found thatthe morphologies of silica particles were strongly affected by the polar-ities of the cosolvents added to the solution (Figs. S6–12).

When the polarity of the cosolvent used for the preparation is small-er than that of methanol, the morphology evolution of thus preparedmesoporous silica particles would undergo similar process as that ofusing ethanol as the cosolvent. Fig. S6 shows the morphology evolutionof mesoporous silica particles prepared by using different volumes ofethyl ether as cosolvent. It can be seen that with increasing amount ofethyl ether in the solution, themorphologies of silica particles graduallychanged from perfect spherical particles with smooth surface to spher-ical particles full of sulcus, to spherical particles aggregatedwith rodlike

108 Q. Qu et al. / Journal of Non-Crystalline Solids 405 (2014) 104 –115

particles, to single rodwith hexagonal structure, to curved rod, to peeledorange-like particle, and finally to sphere again. However, when the po-larity of the cosolvent is bigger than that of methanol, the morphologychange of silica particles is unobvious regardless of the amount of thecosolvent added into the solution. Fig. S12 shows that when ethyleneglycol up to 0.21 mol was added in the solution, only spherical particleswith small furrows on the surface could be obtained even though the po-larity of ethylene glycol is only slightly larger than that of methanol.

The strong effect of the types of cosolvents on themorphology of themesostructure silica is probably due to the fact that the formation of anymesoporous silica particles has to undergo the process of forming thesurfactant/silica composite firstly. Since the existence of the cosolventwould affect the state of the surfactant, it would alter the magnitudeof the interaction between the surfactant and silica source. Consequent-ly, the relative size of the rate difference would be changed, which wasfavored for the mesostructure formation.

A

C

E

Fig. 4.Representative SEM images ofmesoporous silica particles synthesized using (A) 1.6, (B) 1composition of the solutions was 1Na2SiO3·9H2O:0.6CTAC:22.9HCONH2:213H2O:(5.4–30.6) et

3.2. Effect of dilution

Different amounts of water were added to the solution to test theeffect of the dilution on themorphology ofmesostructure silica since di-lute solution route has been proven to be an effective method to controlthemorphology [15]. Fig. 6A shows the SEM image of sample 0. It can befound that thus prepared particles having spherical shape with smoothsurface and the particles were formed by the aggregation of silica nano-particles. The formation of the spherical particles suggests that silicacondensation rate is predominant in this reaction system. Fig. 6Bshows that the surface of the spheres exhibited a furrow or sulcus-likemorphology when 90 mL of H2O was used. It indicates that the rate dif-ference is reduced by the dilution process although both the rate of silicacondensation and mesostructure formation are decreased simulta-neously. When more water was used, the formation of the specificmesostructure becomes even more distinct. More rope-like particles

B

D

F

.8, (C) 2, (D), 3, (E) 4, (F) 5, (G) 6, (H) 7, (I) 8, and (J) 9mL of ethanol as additive. Themolarhanol.

G H

I J

Fig. 4 (continued).

a b

c d

Fig. 5. Representative SEM images of the morphology evolution of silica rods. Experimental conditions: samples (a) and (b) were prepared by adding 4.2 mL of ethanol to the initial re-actant; sample (c) was obtained by adding 4.4 mL of ethanol to the reactant; sample (d) was obtained by adding 5.0 mL of ethanol to the reactant.

109Q. Qu et al. / Journal of Non-Crystalline Solids 405 (2014) 104 –115

Table 3Physical properties of cosolvents.

Cosolvent Formula Polar Dielectric constant (ε, 25 °C)

Ethyl ether H5C2OC2H5 4.34Ethyl acetate CH3C=OOC2H5 6.02Tetrahydrofuran CH2CH2OCH2CH2 7.58Acetone CH3C=OCH3 × 20.7Ethanol CH3CH2OH × 24.3Methanol CH3OH × 32.7Ethylene glycol HOCH2CH2OH × 37.0Ethylene glycerol CH2OHCHOHCH2OH × 42.5

110 Q. Qu et al. / Journal of Non-Crystalline Solids 405 (2014) 104 –115

appeared from the surface of the spherical particles (Fig. 6C, D), fused to-gether (Fig. 6E), and grow into big ropes with hexagonal shape (Fig. 6F,G). Finally, the spherical particles collapsed and single rope-like particlesappeared (Fig. 6H). When molar ratio of H2O/Na2SiO3·9H2O exceeded6400, only silica sol can be obtained. From the above results, we candraw a conclusion that the dilution method is not effective in the casethat the silica condensation rate is predominant in an initial reaction sys-tem. However, if the rate of silica condensation is smaller than the rate ofmesostructure formation in an initial reaction system, the dilution meth-od would help in formatting a rich variety of morphologies includinghexagons with different lengths, thicknesses and degree of curvatures,gyroid-, doughnut-, and single-crystal-like shapes (Fig. S13).

3.3. Effect of the concentration of the reactants

Since the rate of silica condensation could be reduced by decreasingthe concentration of silica source, its effect on themorphology ofMCM-41 silica particleswas expected to bemore remarkable than the effect ofdilution. However, to our surprise, no similar morphology evolution asthat shown in Fig. 6 could be obtained when only the amount of silicasourcewas decreased. It can be found from Fig. 7 that spherical particleswithmore andmore big pits instead of sulcus-likemorphologywere ob-tained when the concentration of Na2SiO3·9H2O decreased (Fig. 7A–D).When the concentration of Na2SiO3·9H2O was down to 86.3 mM,spherical particles were emerged into the silica gel (Fig. 7E) and disap-peared completely when the concentration of Na2SiO3·9H2O was fur-ther down to 72.0 mM (Fig. 7F). In contrast, sample g was preparedby changing the concentration of the CTAC used for preparing sampled from 128 to 76.7mM. It can be seen from Fig. 7G that perfect sphericalparticles with smooth surface appeared again when the concentrationof CTACwas reduced. Considering that the interaction between the sur-factant and the silica precursor in sample dwas bigger than that in sam-ple g, spherical particles with smooth surface should be obtained insample d. Why the reversed result was obtained? In order to answerthis question, the effect of the surfactant concentration on themorphol-ogy of silica particles was further investigated. Fig. S14 shows that thesame morphology evolution as that shown in Fig. 6 was achieved bysimply decreasing the concentration of CTAC. Fig. S14f shows that theshape of the silica particleswas affected by the CTAC even if the concen-tration of CTAC was lower than its cmc. It indicates that the surfactantplays a more important role in controlling the morphology of mesopo-rous silica particles than that of silica source. Now back to the question,according to the results of these experiments, we can draw a conclusionthat the effect of the concentration of silica source on themorphology isrelatively small in the case when the concentration of the surfactant ishigh enough. The high concentration of the surfactant ensures thestrong interaction between the surfactant and the silica precursor. Thestrong interaction means that the rate of silica condensation is pre-ferred. In this case, the changing of the amount of silica source wouldonly affect the number of silica nuclei used for constructing the parti-cles. In the process of particle formation directed by the surfactant,just like the process of constructing a high building that the skeletonof the building was created firstly, the silica skeleton with three dimen-sional network-like structures was formed firstly by loosely connecting

each of the surfactant/silica composite. After that, more silica nucleiwere deposited onto the preformed bones. However, during this pro-cess, if the number of silica nuclei was not large enough to guaranteethe growth of the silica particles, only a loosened structure instead ofa densely aggregated structure could be obtained. Therefore, someholes would be left inside the particles. The existence of these holes in-side the spherical particles made the mechanical stability of them de-creased and consequently induced the collapse of the silica shell. ASEM image of a broken silica sphere (Fig. 7D1) proved this conclusion.The arrow in Fig. 7D1 points out a pit on the particle surface. This pro-posal was recently confirmed by Hollamby et al. who use a methodnamed time-resolved small-angle neutron scattering to investigatethe growth of themesoporous silica particles [37].When the concentra-tion of the surfactant decreased, the rate of forming the silica skeletonsdecreased. Therefore, the gradually formed silica nuclei have enoughtime to deposit onto the surface of the silica bones. Thus, silica particleswith compact structure as that shown in Fig. 7G could be obtained. Sim-ilar experimental results were obtained when higher concentration ofNa2SiO3·9H2O and CTAC was used for the preparation (Fig. S15).

On the other hand, in the case of the surfactant at low concentra-tions, changing the concentration of both silica source and surfactantwould significantly affect the morphology of the silica particles (Figs.S16 and S17). Since the interaction between the surfactant and the silicaprecursor is weak, the small changes in the concentration of these reac-tantswouldmake a substantial contribution on changing themagnitudeof the interaction and which consequently caused the great change inparticle morphology.

3.4. Effect of the pH of the solution

The morphologies of mesostructure silica particles are also stronglyaffected by the pH value of the initial reaction solution. In this work,the pH value of the solution was tuned by the volume of the formamideadded into the solution. With the hydrolysis of the formamide, the pHvalue of the solution decreased gradually. Fig. 8 shows the effect of thevolume of the formamide on the morphology of the silica particles. Itcan be found that only shapeless particles could be obtained withoutusing any formamide (Fig. 8A). When 1 mL of HCONH2 was addedinto the solution, spherical silica particles were obtained. However,these particleswere very frangible and the fragments can be seen every-where in the SEM image (Fig. 8B). Since the rate of the pH change is pro-portional to the volumeof the formamide in the solution, the drop of thepH is small when only 1 mL of HCONH2 was used. Consequently, thecondensation rate of silica precursorwas slow resulting in the formationof some interspaces inside the particle. This experimental result provedour proposal again that the effect of the concentration of silica source onthe morphology of silica particles is insignificant in the case that silicacondensation rate is predominant. Interestingly, although this phenom-enon is unfavorable for the formation of spherical silica particles withcompact structure, it can be used for preparing the materials with hier-archical structure as shown in the inset of Fig. 8D prepared by using3 mL of HCONH2. When enough amount of formamide was added intothe solution, the pH value of the solutionwould drop quickly and inducegeneration of abundant silica nuclei for forming a densely aggregatestructure. Consequently, spherical silica particles with compact struc-ture were obtained (Fig. 8F–H).

Since ammonia was generated during the hydrolysis of the formam-ide and it is well known that ammonia was favored for the formation ofspherical silica particles [35], formamidewas changed into ethyl acetateto check whether the existence of NH3·H2O would affect the morphol-ogy of silica particles. It can be seen from Fig. 9 that spherical silica par-ticles could be obtained when ethyl acetate was used to adjust the pHvalue of the solution. However, the surfaces of the silica particles arenot smooth and there are someholes on the surface of the particles. Fur-thermore, spherical particles with unexpected ordered flower-like pat-ternswere appeared in someparticles' surface.When different amounts

A B

D

F E

C

G H

Fig. 6. Representative SEM images of mesostructure silica particles prepared by adding (A) 20, (B) 90, (C) 120, (D) 140, (E) 160, (F) 200, (G) 400, and (H) 600mLH2O. The composition ofthe solutions was 1Na2SiO3·9H2O:0.6CTAC:22.9HCONH2:(213–6400) H2O.

111Q. Qu et al. / Journal of Non-Crystalline Solids 405 (2014) 104 –115

A

C D

B

E F

G D1

Fig. 7. SEM images ofmesostructure silica prepared by using (A) 187, (B) 158, (C) 130, (D) 101, (E) 86.3, and (F) 72.0mMNa2SiO3·9H2O. The composition of the solutionswas (0.88–0.34)Na2SiO3·9H2O:0.6CTAC:22.9HCONH2:213H2O.

112 Q. Qu et al. / Journal of Non-Crystalline Solids 405 (2014) 104 –115

A B

C D

E F

G H

Fig. 8. SEM images of mesostructure silica prepared by adjusting the volume of formamide added into the solution. (A) 0, (B) 1, (C) 2, (D) 3, (E) 4, (F) 5, (G) 7, and (H) 9 mL. Other con-ditions are the same as in Fig. 6.

113Q. Qu et al. / Journal of Non-Crystalline Solids 405 (2014) 104 –115

114 Q. Qu et al. / Journal of Non-Crystalline Solids 405 (2014) 104 –115

of ethyl acetatewere added into the solution, silica particleswith similarshapes were obtained. In order to ensure that the special morphologywas induced by the lack of NH3·H2O in the solution, different amountsof NH3·H2Owere added into the solution. However, no noticeable mor-phology changes could be found (Fig. 9E, F). It indicates that the specialmorphology is induced by using ethyl acetate and the unique morphol-ogies can be attributed to the dual functions of ethyl acetate in theparticle formation. One function is to lower the pH value, the otheracted as a cosolvent to alter the interaction between silica source andthe surfactant.

When solution pH was adjusted by adding different amounts ofH2SO4 into the initial reaction system, only nanoparticles could be ob-tained. The reason for this experimental result can be attributed to thefast hydrolysis and condensation rate of silica source (Fig. S18). Thus,dropping the pH value slowly and homogeneously favors the formationof mesostructured silica with specific structure.

A

C

E

Fig. 9. SEM images ofmesoporous silica prepared by using (A) 3, (B) 4, (C) 4.75, and (D) 6mL eprepared by adding 2 and 4 mL NH3·H2O into sample c. Other conditions are the same as in Fi

4. Conclusions

Mesoporous MCM-41 silica particles with various morphologieswere obtained in the basic solution by changing the parameters affect-ing the magnitude of the rate difference between the surfactant andsilica source. Among all the parameters, the types and concentrationof cosolvents played a key role in regulating the magnitude of the ratedifference. Mesoporous MCM-41 with tunable morphologies such asspheres, rods, hexagonal rods, spirals, oranges, and gyroids could beprepared simply using different amounts of cosolvents. The effect of di-lution on the morphology of silica particles depended on which rate ispredominant in an initial reaction system. When silica condensationrate is dominant, the impact of dilution on the morphology changes isnot significant. When the rate of mesostructure formation is dominant,however, a rich variety of morphologies of mesostructure MCM-41 sili-ca particles could be obtained by using the dilutionmethod. The effect of

B

D

F

thyl acetate as additive to adjusting the pH value of the solution. Samples (E) and (F) wereg. 6.

115Q. Qu et al. / Journal of Non-Crystalline Solids 405 (2014) 104 –115

the concentration of silica source on the morphology of silica particleswas observable only in the situation when the silica condensation rateis slow. When silica condensation rate is fast, the internal structure in-stead of the morphology of the particle was affected by changing theamount of silica source. Particles with hierarchical structure were ob-tained when low amount of silica source was used because no enoughsilica nuclei were provided to fulfill the interstice in-between the silicaskeletons. Changing the pH value was found only altering the particlesize instead of changing the particle morphology.

Acknowledgment

This work was supported by NSFC (20975090).

Appendix A. Supplementary data

Small angel XRD patterns, N2 adsorption/desorption isotherms, TGand DTG curves, and HRTEM images of mesoporous silica particles pre-pared using various amounts of ethanol in the reactant. SEM images ofsilica particles with a variety of morphologies synthesized at differentmolar ratios. Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.jnoncrysol.2014.09.012.

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