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Unravelling the Formation Mechanism of Dendritic Fibrous Nanosilica Ayan Maity, Avik Das, Debasis Sen, Subhasish Mazumder, and Vivek Polshettiwar Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02996 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017
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Ayan Maitya, Avik Dasb, Debasis Senb, S. Mazumderb, Vivek Polshettiwara* aDepartment
of Chemical Sciences, Tata Institute of Fundamental Research (TIFR), Mumbai, India. *Email:
[email protected] b
Solid State Physics Division, Bhabha Atomic Research Centre (BARC), Mumbai 400 085, India
ABSTRACT: We studied the formation mechanism of dendritic fibrous nanosilica (DFNS) that involves several intriguing dynamical steps. Through electron microscopy and real-time small-angle X-ray scattering studies, it has been demonstrated that the structural evolution of bicontinuous microemulsion droplets (BMDs) and their subsequent coalescence, yielding nano-reactor template, is responsible for to the formation of complex DFNS morphology. The role of co-surfactant has been found to be quite crucial, which allowed the understanding of this intricate mechanism involving the complex interplay of self-assembly, dynamics of BMDs formation and coalescence. The role of BMDs in formation of DFNS has not been reported so far and present work allows a deeper molecular-level understanding of DFNS formation. INTRODUCTION Morphology-controlled nanomaterials play crucial roles in the development of technologies that address challenges in various fields including energy, environment and health. They have wide applications in materials science as exceptional building blocks for the fabrication of an assortment of valuable materials.1-5 Since last two decades, silica-based nanomaterials have drawn significant attention in research due to their vast applications.6-13 One notable recent invention is our dendritic fibrous nanosilica (DFNS), which is also known as KCC-1.14-17 This material possesses a unique fibrous morphology, which is unlike the tubular porous structure of various conventional silica materials. It is worth mentioning that DFNS has showed exceptional activities in a range of fields, such as catalysis, gas capture, solar energy harvesting, energy storage, sensors, and biomedical applications. The decipherment of actual molecular mechanism regarding formation of such complex nanostructure is not only crucial from scientific point of view but also is an essential technological requirement in today’s cutting edge of materials science. The field of nanomaterial synthesis, with controllable size, shape and morphology, has exploded in last two decades with a large number of unique morphology-controlled nanomaterials; however, understanding of their formation mechanism remains inadequate till date. Although few exceptional studies have been conducted to achieve detailed insights into formation mechanisms,18-27 the formation mechanisms of complex morphology-controlled nanomaterials have not yet been investigated. In general, the formation of various nanomaterials has been explained on the basis of various physicochemical processes, such as nucleation and growth, self-assembly, surfactant-templated growth, and structure-directing agent-assisted growth.18-27 However, for a nanostructure with a complex morphology the formation mechanism involves a complex interplay between these conventional processes, but decoding the actual mechanism remains a challenge.14-17, 28-30 In this work, we have unveiled detailed insight on the formation mechanism of dendritic fibrous nanosilica. We provide the mechanistic details of DFNS formation, which includes molecular level information and structural details of the complex organization of the surfactant and co-surfactant as well as the formation of bicontinuous
microemulsion droplets (BMDs) coalescence In-situ scattering investigation, which includes probing of molecular level information and structural details of the complex organization of the surfactant and co-surfactant, indicate the formation of BMDs has an important role to play towards the formation of DFNS. Although the use of droplets and coalescence has shown to have a dramatic effect on organic transformations and catalysis,30-33 the coalescence of droplets (BMDs) has not been reported to explain the formation of nanomaterials. EXPERIMENTAL SECTION DFNS Synthesis and Characterizations. In a typical synthesis, CTAB (2 g, 0.005 moles) and urea (2.4 g, 0.04 moles) were mixed in water (100 ml) at room temperature (RT) and stirred in a roundbottom flask for 30 min. TEOS (4.7 g, 0.023 moles) in cyclohexane (100 ml) was then added dropwise and stirred for 1 h at RT. The cosurfactant (alcohol, 0.055 moles) was then added dropwise, and the reaction mixture was further stirred for 30 min at RT. The reaction mixture was then heated to 82 °C using an oil bath and then kept at this temperature for 12 h under stirring. After cooling, the solid DFNS was isolated by centrifugation, washed with water and ethanol (3 times each) and dried at 80 °C for 12 h. DFNS was then calcined at 550 °C for 6 h to remove the remaining surfactants and co-surfactants. This process and obtained DFNS were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), confocal imaging, and small-angle X-ray scattering (SAXS). SAXS Studies. In-situ small-angle X-ray scattering measurements were carried out on different BMDs samples using a laboratory based SAXS facility with sample to detector distance of nearly ~ 1 m. The microemulsion was taken in 1 mm quartz capillary tube at different reaction time points and was placed in X-ray beam of 400 μm in diameter. Radial averaged scattering data was obtained. The accessible wave vector transfer (q) range of the SAXS measurement was ~ 0.1 to 2.5 nm-1 .The scattered intensities were measured as a function of wave vector transfer where 2θ is the scattering angle and λ is the wavelength of the radiation used (Cu K α 0.154 nm). The recorded scattering profiles were corrected for background and transmission.
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RESULTS AND DISCUSSION To establish this mechanism, we investigated the effect of the alcohol chain length which are known to play critical role in the stability of the emulsion.34-36 Alcohols with varying chain length, C3 (1-propanol), C4 (1-butanol), C5 (1-pentanol), C6 (1-hexanol), C8 (1-octanol), C10 (1-decanol), C12 (1-dodecanol), C14 (1-tetradecanol), C16 (1-hexadecanol) and C18 (1-octadecanol), were selected as the co-surfactants to study their effect on dendritic morphology of DFNS. The samples are named DFNS-Cx, where x is the alcohol
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chain length. Figure 1 shows the SEM images and particle size distributions of the as-synthesised DFNS particles using different alcohols as the co-surfactants. TEM images are shown in Figure 2. Notably, all particles showed spherical shapes and dendritic morphologies with varying degrees of fiber density. However, a dramatic reduction in particle size was observed from approximately 450 nm to 50 nm when the co-surfactant chain length was changed from C5 to C6 (Figure 1). The particle size remained unchanged (~50 nm) for all other long chain alcohols. These observations indicate that the cosurfactant plays a key role in the formation mechanism of DFNS.
Figure 2. TEM images of DFNS synthesized using various alcohols as co-surfactants.
Figure 1. SEM images of the as-synthesised DFNS particles using different alcohols, a) C3, b) C4, c) C5, d) C6, e) C8, f) C10, g) C12, h) C14, i) C16, and j) C18, k) their particle size distributions. Additional SEM images in figure S5 and textural data in figure S6.
To gain insight on how the chain length of the alcohol dictates the particle size, we studied the emulsion stability of the lamellar phases after insertion of alcohols in organized surfactant molecules. Their stability was monitored with time, and we observed an increase in stability with an increase in C-chain length (Figure 3). The increase in stability was due to increase in the structural similarity between CTAB and the alcohol carbon chain (Figure S1). An increase in the hydrophobic-hydrophobic interactions and a reduction in the head group-head group repulsion of the CTAB quaternary ammonium by the insertion of the neutral alcoholic head group increased the stability of emulsions.34,35 Silicic acid generated by TEOS hydrolysis also interacts with the quaternary ammonium of CTAB molecule as well as with alcohol head group, stabilizing the emulsion further. These interactions were re-confirmed by replacing 1-octanol with n-octane as a co-surfactant, which reduced the stability of the resulting emulsion (Figure S2).
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Langmuir of “coalescence” of BMDs, where small bicontinuous microemulsion droplets merge into bigger droplets while keeping their dendritic internal structure intact, was investigated. Coalescence of BMDs will be depends on their stability. Thus, BMDs with alcohols up to a C5 chain are less stable (Figure 3) and can undergo coalescence to yield bigger BMDs and in turn yield larger DFNS particles. As the stability of the BMDs increases at a chain length of C6 and above (Figure 3), BMDs do not undergo coalescence and remain isolated, yielding smaller DFNS particles. Thus, coalescence step clearly explains the relationship between alcohol chain length, lamellar phase stability, and the presence of bigger droplet sizes.37,38
Figure 4. SEM and confocal images of the emulsions using C5 (a,b) and C6 (c,d) alcohols.
Figure 3. Bulk emulsion stability using different alcohols at room temperature. Although the dramatic effect on the particle size and increase in stability of the emulsion, re-confirms the key role of the alcohol in the DFNS formation mechanism, it was not evident how the micelle stability affected the particle size. It could be that these lamellar phases form BMDs with the water and cyclohexane channels 37,38 and act as templates. Then the size of the BMDs dictates the size of the DFNS particles. We attempted to image the BMDs by confocal microscopy and SEM using liquid sample capsules (Figure 4 and figure S3). Although confocal imaging indicated the presence of droplets (Figure 4b,d, figure S3), they were polydispersed, varying from a few nm to hundreds of μm. SEM images of the emulsions, obtained using a liquid sample holder, also indicated the presence of poly-dispersed droplets (Figure 4a,c). However, it should be noted that the DFNS formed were monodispersed. These results are a strong mismatch to our hypothesis, where the BMD size was expected to decrease from 500 nm to 50 nm. To explain the correlation between the micelle stability, a decrease in particle size and the presence of poly-dispersed droplets, possibly
Coalescence step also explains the dramatic decrement in particle size (from ~ 500 nm to ~ 50 nm) when the C5 alcohol is replaced by the C6 alcohol. At first, it appears that the C5-BMDs undergo coalescence while the C6-BMDs do not possibly due to their dramatically high stability. However, lamellar phases formed by C6 are not dramatically stable. Instead, the stability of these BMDs increases gradually with an increase in the alcohol chain length (Figure 3). Therefore, we attribute this abrupt particle size decrease to the energy required for coalescence of the BMDs. For the C5-BMDs, a reaction temperature of 82 °C provided enough energy to overcome the repulsion between the BMDs and undergo coalescence to produce larger BMDs and hence larger DFNS particle sizes. However, this temperature (82 °C) did not provide enough energy for the coalescence of the more stable C6-BMDs, and hence, they did not undergo coalescence. To prove this hypothesis, we carried out the synthesis using C6-BMDs at 76, 82, 86 and 90 °C (Figure 5), and we observed an increase in the DFNS average particle size from approximately 50 nm to approximately 110 nm with an increase in reaction temperature. Notably, when the temperature is fixed, the particle size did not significantly increase even after a long reaction time (Figure 6). These observations confirm the presence of a coalescence process during the DFNS synthesis. However, accelerated nucleation and growth rates could contribute to the formation of bigger size DFNS.
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Figure 5. Effect of temperature on DFNS particle size using C6 alcohol, at a) 76, b) 82, c) 86 and d) 90 oC reaction temperature. Notably, the confocal imaging indicated that the droplets were greater in size up to microns (Figure 4). However, the BMDs should be maximum 500 nm in size (maximum size of synthesised DFNS). This indicates that BMDs aggregates and float in the dispersion phase (Figure 7).38 The claim that BMDs float in oil droplets (and not in water droplets) was based on the fact that water channel of BMDs were closed as no TEOS leak and silica formation were observed outside of the BMDs. It indicates that the surface of the BMDs have non-polar alkane chains (Figure 7a), and hence, the BMDs will float only in oil droplets. Additionally, the dispersion phase, which contains these floating droplets, was found to be water (Figure S4), indicating that these oil droplets were stabilized by alcohols with alkane tails inside of the oil droplets and polar heads at the surfaces, interacting with water dispersion phase (Figure 7a). Interestingly, we observed that the alcohol chain length also affected the stability of these macro-oil droplets (Figure 7a), which, in general, seemed to break during the silica growth step (as no aggregated DFNS particles were observed in the SEM images). However, when C14 and C16 alcohols were used, these oil droplets were stable even during silica growth, which forced the systematic aggregation of DFNS particles in spherical shapes (Figure 7b,c).
Figure 6. Effect of reaction time on DFNS particle size using C5, C6 and C8 alcohols.
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Langmuir powder sample i.e., DFNS was mounted in between two X-ray transparent polyamide films. The scattered X-ray intensity was recorded by a 2D detector and then radial averaged data were corrected for background and transmission before processing for further analysis. The accessible wave vector transfer (q) range of the SAXS measurement was ~ 0.1 to 2.5 nm-1. The scattered intensities were measured as a function of wave vector transfer, q 4 sin
Where 2θ is the scattering angle and λ is the wavelength of the radiation used (Cu Kα 0.154 nm). SAXS study revealed the intrinsic morphological details of the emulsion droplets vis-à-vis DFNS powders. SAXS intensity is basically the product of two functions, namely P(q), called Form factor which reveals size and shape of the nanostructures and the other one is called Structure factor which unveils the positional correlation among the nanostructures. In mathematical form, SAXS intensity under local monodisperse approximation can be written as,41 I (q )
C
0
P (q, r )D(r )v 2 (r )S (q, r )dr
,Where
D(r) denotes the particle size (r) distribution function and v(r) being the volume of the particle. The scale factor ‘C’ depends on the product of number density of nano-particles and contrast factor (i.e., the square of the scattering length density difference between the particles and the matrix). In order to fit the SAXS data, three distinguished contributions were considered in the mathematical model. The total scattering intensity can be written as,
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I (q )
I j (q ) j
Figure 7. (a) BMDs aggregated in a macro-oil droplet dispersed in the water phase. SEM images of the DFNS using (b) C14 and (c) C16 alcohols as co-surfactants. Additional SEM images are given in the figure S7. To understand the dynamic formations steps and to gain deeper molecular level insight as well as initial kinetics of the process real time investigations under the exact synthetic conditions at different reaction time intervals, were carried out using small-angle X-ray scattering (Figure 8-10). SAXS seems to be the best suitable technique to study these very sensitive emulsion droplets without perturbing their internal structure and molecular arrangements of amphiphiles.39,40 SAXS, which maps the coherent X-ray scattering-length density fluctuation of the inherent structure into scattering vector (q) space, was also crucial for the determination of the thickness of the water channel in the microemulsion droplets. It is worthy to mention that SAXS and SEM are complementary in nature. While SEM provides, direct morphological evidence by providing local image, SAXS provides statistically averaged quantitative information. Thus, the combined usage of these complementary techniques help in unraveling crucial structural evolution. Small-angle X-ray scattering (SAXS) measurements were carried out on different BMDs and DFNS samples using a laboratory based SAXS facility with sample to detector distance of ~ 1 m. The emulsion at different reaction time points was taken in 1mm quartz capillary tube and placed in X-ray beam of 400 μm in diameter. The
1
,where
I1(q) and I2(q)were considered as scattering contribution originating from same kind of species, which are disc like planner ojects with multi-lamellar structures having para crystalline like odering. The form factor for lamellar phase can be written as, 2
Pplanar (q, R, L)
J (2qR ) sin(qL / 2) , where ‘R’ being the ra2 2 R 4 (1 1 ) L (qR ) 2 qR qL / 2
dius of the disc vis-à-vis lamellar phase and ‘L’ being the thickness. Two types of lamellar correlation were attributed to multi-layered micelles, namely straight portion and bending portion. The structure factor for lamellar correlation with para-crystalline ordering can be written as,42,43 S PT ( q , N , d , )
N 2
N k N 2
xk S k , PT
m q N k 1 with S k , PT N k 2 ( N k m ) cos(mqd )e 2 m 1 2 2
2
where, N: mean number of stacks, d: stacking separation, Δ: stacking disorder parameter. The discs were not perfectly aligned, rather they were randomly oriented with variable separation distance. In order to account such small variations in the bilayer separations in the structure, para-crystalline theory was considered. The total structure factor has been decoupled into polydisperse structure factor S k,PT(q)
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Langmuir having Gaussian distribution. This can be understood as the consequence of polydispersity in the size of the different stacks due to disorder. The standard deviation ‘σ’ for the Gaussian-weighted (xk) distribution is chosen as N for N 5 0.5( N 1)
xk
1 2 2
e
( N N )2 k 2 2
for N 5
, where N is the mean number of stacks and
Nk is one of the bilayers in the range (N-2σ) and (N+2σ). The data fitting is done based on non-linear least square method and the unknown parameters are estimated.The third scattering contribution i.e., I3(q)was from free micelles. The form factor Psphere(q,r) for a spherical particle with radius ‘r’ can be written as: sin( q r ) qr cos( q r ) Psphere ( q, r ) 3 ( q r )3
2
In present work, a normalized lognormal distribution is considered as size distribution in thickness of the discs, where r0 (median) and σ (polydispersity index) are two parameters of distribution and related to mean radius 𝑟𝐴𝑉 = 𝑟0 𝑒𝑥𝑝 (𝜎 2 /2).
Figure 8a depicts the SAXS profiles for the emulsions with different alcohol chain lengths and shows the in-situ scattering profiles of the emulsion as a function of reaction time points. The broad peaks observed in each intensity profile at a relative peak position ratio of 1:2 were attributed to the lamellar correlation of the micelles in the BMDs.39,40 To fit the in situ SAXS profiles, we considered disc-like planar objects featuring a multi-lamellar positional correlation with a paracrystalline ordering (Figure 9). We attribute these to small-angle scattering contributions from two different components of the multilayered micelles, namely, the nearly straight portion and sharp bending portion. It is evident that only one lamellar contribution cannot fit the experimental data and consideration of two lamellar contributions with different positional correlation explains well the scattering data. It is also crucial to note that the relative heights of the lamellar peaks (at q* and 2q*) were such that the fittings of SAXS profiles were only adequate by considering at least two types of positional correlation.
Figure 8. Small angle X-ray scattering intensities of (a) the BMDs at different alcohol chain lengths, (b) the water channel widths, and (c) the sheet thicknesses obtained from the SAXS study of the respective DFNS powders.
-1
D r
2 ln r r 0 exp 2σ 2 2πσ 2 r 2
1
Scattering Intensity (cm )
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Experimental One lamellar Fit Two lamellar Fit
1
10
0
10
-1
10
-2
10
0.1
-1
q (nm )
1
Figure 9. Fitting of SAXS profile by considering one and two types of positional correlation, for DFNS-C6 emulsion. It is interesting to note that the positions of the lamellar correlation peaks gradually shifted toward lower q with an increase in C-chain of alcohols, indicating an effect of the alcohol chain length on the lamellar phases of the BMDs. It should be noted that, although the
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peaks shifted to lower q, they maintained the same relative position ratio of 1:2, which ensures that the positional correlation is a superposition of two lamellar components of the same species. We derived the thicknesses of the water channels, at the straight portion as well as the bending portion, from the SAXS data positional correlation of the lamellar phases in the BMDs (Figure 10). A dramatic decrease in the water channel width from 9.8 nm to 3.3 nm was observed when the C5 surfactant was replaced by the C6 co-surfactant. However, a
similar change did not occur from C6 to C8 (2.9 nm). To compare the inherent structure of the final products, i.e., DFNS prepared using these alcohols with that of the respective BMDs, we studied the SAXS of the DFNS powders as well. Notably, italso showed a narrowing of the fibre sheet thickness at C6 from 8 nm to 2 nm (Figure 8c), which is in good agreement with the water channel width in the BMDs, as silica is growing in the water channel (and hence, its width dictates the fibre sheet thickness).
Figure 10. Small-angle X-ray scattering intensities as function of reaction time point (top) and estimated water channel width at different reaction time points (bottom), for C5 and C6 emulsion. Time points: CTAB and urea mixed in water at room temperature (RT) for 30 minutes (t0), cyclohexane and TEOS were added to above solution and stirred for 1 h at RT (t1), co-surfactant (alcohol) was added to above solution and stirred for 30 minutes at RT (t2), above reaction mixture was heated to 82 0C under stirring and reaction time was 0 (t3), 15 (t4), 30 (t5), 45 (t6), 60 (t7), 75 (t8), 90 (t9), 150 (t10), 210 (t11), 270 (t12) and 330 minutes (t13). Figure 8b and c depict the estimated width of the water channels (at the straight portion) and of the silica sheet, respectively, for BMDs and DFNS. These observations match well with those mentioned earlier, i.e. a decrease in particle size at C6 (Figure 1). The decrease
in water channel width confirms an increase in the stability of the lamellar phases of the CTAB-alcohol, possibly causing strain on the water channel to narrow down, in addition to a reduced size of the respective BMDs. A confirmation that the alcohol chain length is the
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cause of the narrowing of water channel was also visible from the systematic study of the evolution of the BMDs at different reaction time points. We observed a decrease in the water channel width as soon as the co-surfactant alcohol was added (Figure 10). The water channel width remained unchanged even after heating for a few hours, indicating the stability of these BMDs during the DFNS synthesis (Figure 10). The narrowing of the water channel is attributed to the swelling of the cyclohexane channel (oil portion of the BMDs) after the addition of alcohols. These observations from the SAXS study confirm the observations from our other experiments and allowed us to correlate the role of co-surfactants in stabilizing the BMDs and their coalescence to generate templates for DFNS formation. Thus, we revealed a five-step formation mechanism of DFNS using the process of coalescence of microemulsion droplets (Figure 11). In the first step, the CTAB molecules self-assemble to form lamellar phases, which then stabilized by the co-surfactant (alcohols) via an insertion step between two CTAB molecules (step-II). Then they form bicontinuous microemulsion droplets (BMD) with water and cyclohexane (oil) channels (step-III). These BMDs then agglomerate to minimize the energy of the system (step-IV) and if their stability is poor, then they will undergo a coalescence process to form bigger droplets (step-V). However, if they are stable, they will not undergo coalescence process. Within the BMDs, the silica precursor (TEOS) in the cyclohexane channel will diffuse to water interface and gets hydrolysed. Once their critical concentration is achieved in the water channel (required for super-saturation), nucleation is triggered, which is followed by a growth step. The size, shape and morphology of the formed silica is then dictated by the BMDs that are acting as a nanoreactor (template) for silica growth.
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self-assemble to form lyotropic liquid crystalline structure (hexagonal-rod shaped). Then the hydrolysis-polycondensation of the silica precursor molecules takes place around the polar head groups to from a silica network. Removal of the amphiphiles (surfactants) generates pores within the silica framework.44,45 In case of DFNS synthesis, although it also uses CTAB as the surfactant but due to use of two immiscible solvents (water and cyclohexane), and high concentration of CTAB,46 it forms BMDs having lamellar phases. Unlike micelles in case of MCM-41 and SBA-15, BMDs are acting as template (nano-reactor). TEOS molecules diffuses to the water channel from the oil channel of the lamellar phase and then undergoing hydrolysis. Once critical concentration of hydrolyzed TEOS molecules (silicic acid and silicates) is achieved, nucleation is triggered via their polycondensation, which is then followed by the growth step, forming silica fibers (sheets) of the thickness equivalent to water channel width. Since water channels are dendritically arranged in the spherical droplet of BMDs, DFNS produced had dendritic fibrous morphology. While tuning the size of MCM-41 and SBA-15 is difficult, in case of DFNS, by tuning the size of the BMDs, control over the DFNS size was possible. CONCLUSIONS We have demonstrated the use of coalescence of BMDs to establish the precise formation mechanism of nanomaterials, for DFNS. It followed self-assembly of CTBA and alcohols to form lamellar phases, which then formed bicontinuous microemulsion droplets. These BMDs underwent coalescence to from nano-reactor (a template), within which nucleation and growth steps took place to yield silica nanospheres with fibrous morphology. The effect of the co-surfactant chain length provided the insight on this intricate interplay of various processes. Confocal microscopy and SEM studies using liquid capsules as well as real time investigations using small-angle Xray scattering, confirms the hypothesis. The present work reveals the important role of the coalescence process during the formation of DFNS in particular, but also indicates the possibility of manifestation of such process even for other nanomaterials and thus needs be brought into scrutiny. We are attempting to understand this system further, using theoretical calculations, cryo-TEM and DLS studies. SUPPORTING INFORMATION Figures as referred in the MS, additional SEM and confocal images and textural properties data, are supplied as Supporting Information. ACKNOWLEDGMENTS We thank the Department of Atomic Energy (DAE), Government of India, for funding. We also acknowledge the use of EM and confocal microscopy facilities of TIFR, Mumbai. We sincerely thank B. Chalke, M. H. Kombrabail for their technical support.
REFERENCES Figure 11. Formation mechanism of DFNS via the coalescence of microemulsion droplets
1. 2.
DFNS formation mechanism follows different path than well-known MCM-41 and SBA-15 silicas. Synthesis of MCM-41 and SBA-15 requires liquid crystalline template of surfactant molecules, CTAB and P123 respectively. In both the synthesis, water is the single solvent used. In typical formation mechanism, amphiphilic molecules
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