Mesoporous Silica Nanoparticles under Sintering Conditions: A

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Mesoporous Silica Nanoparticles under Sintering Conditions: A Quantitative Study Fanny Silencieux,†,‡,∥ Meryem Bouchoucha,†,§,∥ Olivier Mercier,†,‡ Stéphane Turgeon,⊥ Pascale Chevallier,†,∥,⊥ Freddy Kleitz,*,§,∥ and Marc-André Fortin*,†,‡,∥ †

Laboratoire des Biomatériaux pour l’Imagerie Médicale, Axe Médecine Régénératrice, Centre Hospitalier Universitaire de Québec, Québec, G1L 3L5, Canada ‡ Department of Mining, Metallurgy and Materials Engineering, Université Laval, G1V 0A6, Québec, Canada § Department of Chemistry, Université Laval, G1V 0A6, Québec, Canada ∥ Centre de Recherche sur les Matériaux Avancés (CERMA), Université Laval, G1V 0A6, Québec, Canada ⊥ Axe Médecine Régénératrice, Centre Hospitalier Universitaire de Québec, Québec, G1L 3L5, Canada S Supporting Information *

ABSTRACT: Thin films made of mesoporous silica nanoparticles (MSNs) are finding new applications in catalysis, optics, as well as in biomedicine. The fabrication of MSNs thin films requires a precise control over the deposition and sintering of MSNs on flat substrates. In this study, MSNs of narrow size distribution (150 nm) are synthesized, and then assembled onto flat silicon substrates, by means of a dip-coating process. Using concentrated MSN colloidal solutions (19.5 mg mL−1 SiO2), withdrawal speed of 0.01 mm s−1, and wellcontrolled atmospheric conditions (ambient temperature, ∼ 70% of relative humidity), monolayers are assembled under wellstructured compact patterns. The thin films are sintered up to 900 °C, and the evolution of the MSNs size distributions are compared to those of their pore volumes and densities. Particle size distributions of the sintered thin films were precisely fitted using a model specifically developed for asymmetric particle size distributions. With increasing temperature, there is first evidence of intraparticle reorganization/relaxation followed by intraparticle sintering followed by interparticle sintering. This study is the first to quantify the impact of sintering on MSNs assembled as thin films.

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INTRODUCTION Mesoporous silica nanoparticles (MSNs) are being integrated into an increasing number of applications and technologies, ranging from catalysis, optics, chromatography, gas sensing, coatings in the semiconductor industry and water-repellent surfaces to drug delivery. Several comprehensive reviews have been published, describing the synthesis, functionalization, characterization, physicochemical, and textural properties of MSNs.1−3 MSNs are materials that exhibit exceptionally high pore volumes (>60%) and surface areas, high thermal and chemical stabilities, as well as versatility in surface functionalization.4−6 MSN synthesis is based on the self-assembly of organic surfactants in aqueous media, which form micelles of different sizes, shapes, and arrangements. Then, organic silica precursors are added to the solution, and condense on the micelles to thereby form tridimensional networks. Finally, the surfactants are removed by means of calcination (or solvent extraction), leaving nanoparticles of various sizes (typically from 70 to 500 nm) and pore morphologies (2D or 3D). For a comprehensive description of MSN synthesis procedures and resulting particles (size, morphological, and chemical characteristics), please refer to Wu’s review.1 Among the different variants of MSNs, those featuring exceptionally high pore volumes and surface areas are particularly appealing for catalytic and biomedical drug elution © XXXX American Chemical Society

applications. This is precisely the case for particles of the MCM class, such as MCM-41 and MCM-48, showing, respectively, bidimensional (2D) and tridimensional (3D) pore structures. Such systems are characterized by porosity higher than 60%, as well as by a good control over nanoparticle size.7,8 Those characteristics have made MCM-41 and MCM-48 systems perfect candidates for the fabrication of MSNs compact thin films in biomedicine and catalysis. An increasing number of applications require MSNs to be attached onto substrates, to form well-ordered and compact arrays of porous nanoparticles. Thin films made of porous nanoparticles can exhibit improved diffusion properties and a highly accessible pore system, compared with mesoporous thin films built from silica precursors.9 Among the few nanoparticle deposition techniques that have been used to disperse silica nanoparticles on substrates with good control over the thickness are: electrostatic self-assembly,10 Langmuir−Blodgett deposition,11 spin coating,12,13 dip-coating,14 and layer-by-layer coating.15,16 Suspensions of MSNs showing a high fraction of individualized nanoparticles (rather than agglomerates) are highly desirable in each one of these processes, as defects (such Received: August 12, 2015 Revised: October 29, 2015

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DOI: 10.1021/acs.langmuir.5b02961 Langmuir XXXX, XXX, XXX−XXX

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after a treatment at temperatures above 500 °C, and might condense and form Si−O−Si bonds.22,23 This study aims to provide quantitative measurements of the sintering behavior of MSNs, both in terms of particle size, as well as textural properties. For this, MSNs of very well controlled particle size distribution were synthesized and suspended in water. Flat silicon substrates were dip-coated in this solution, to provide self-assembled MSNs thin films that were subsequently sintered in the temperature range 500−900 °C. An electron microscopy study was performed to quantify the size of the MSNs, and an analytical model was elaborated to fit the particle size distributions. The textural properties of MSNs upon sintering were measured by physisorption and Xray diffraction (XRD), and correlated with the particle size distributions. Eventually, we aimed at clearly revealing the sintering behavior of MSNs at low temperatures ( 4, which is always the case in the present study. The integral in eq 6 can be solved analytically. Unfortunately, the solution written in ref 32 is erroneous. The correct solution is therefore given explicitly below. One of the advantages of the analytical solution expressed in the present work lies in the fact that it involves only common functions and hence the fitting can be performed by most mathematical and many graphing programs. Equation 6 was solved analytically as follows: First, the integral ∫ u0[(3u′2du′)/(ν(u′ − 1) − u′3)] can be solved using the partial fractions method.33 Decomposing the bottom third degree polynomial into one of first degree having a negative real root u0 and one of second degree having two complex roots, one gets

u3 − ν(u − 1) = (u − u0)(u 2 + u0u − ν/u0)

(7)

as stated by Coughlan and Fortes.32 This holds if C


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Figure 1. MCM-48 MSNs visualized by TEM (a,b); related particle size distribution (c).

u03 (u0 − 1)

ν=

I=

(8)

3

−

ν + 2

2

D +

3

−

ν − 2

2

D

[D ≥ 0]

or for u = δ −π I= 2 2 Δ

I= [D < 0]

u

+

⎤ (u0 − 2) ⎛ u(u0 − 1)(u0 + u) ⎞ 2 ⎥ ⎟ + ln⎜1 − u I 0 ⎥⎦ 2 u0 2 ⎝ ⎠ (11)

⎛ 2 −Δ − (2u + u ) 2 −Δ + u ⎞ −1 0 0 ⎟ · ln⎜ 2 2 −Δ ⎝ −Δ + (2u + u0) 2 −Δ − u0 ⎠ ⎛ ⎞ −1 2u 2 −Δ = 2 tanh−1⎜ ⎟ −Δ ⎝ Δ + u0(2u + u0) ⎠ 2

[Δ < 0; l > u ≥ 0]

(12.a)

or for Δ = 0 (i.e., u0 = −3 ; ν = 6.75)

I=

− 2u [Δ = 0; 2 > u ≥ 0] u0(2u + u0)

⎡ ⎛ ⎞⎤ 2u 2 Δ ⎢π + tan−1⎜ ⎟⎥ ⎢⎣ ⎝ Δ + u0(2u + u0) ⎠⎥⎦

[Δ > 0; u > δ]

RESULTS MSNs of narrow and well-controlled size distribution were synthesized, and dispersed as a stable aqueous colloidal suspension. By using dip-coating, silicon substrates were immersed in MSNs suspensions and withdrawn at different speeds. Then, thermal treatments were applied to the coatings. The homogeneity of the coatings was visualized by scanning electron microscopy (SEM) before and after sintering at high temperatures (500−900 °C). 1. MSNs Synthesis and Characterization. In the present study, MCM-48 mesoporous silica nanoparticles were synthesized according to a well-established procedure24,25,34 resulting in spherical MSNs of narrow particle size distributions (Figure 1). In brief, the synthesis is performed in basic conditions, using surfactant as a template for silica condensation, and followed by a calcination step (as described in the Experimental Section). A white powder was collected; size distribution and morphology of the particles were assessed by transmission electron microscopy (TEM) images (Figure 1). As expected, the assynthesized MSNs were spherical with a size distribution centered at 154 nm ±30 nm (full width of peak at halfmaximum - fwhm). The characteristic diffraction peaks noticed in low-angle XRD profiles (Figure S2) were similar to those observed for MCM-48 type in other works.35,36 XRD patterns confirmed the Ia3̅d space group, which is a 3D interpenetrating network of chiral channels. The main peak (211), followed by less intense

where l = −(1/2)((−Δ)1/2 + u0) is an upper limit of validity and Δ = (u20(u0 + 3))/(1 − u0) is a discriminant, whose sign (+,− or 0) is used to select the appropriate solution to I in order to keep every intermediate results real and finite: For Δ < 0 (i.e., u0 < − 3 ; ν > 6.75) I=

−1 2 Δ

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⎛u − u⎞ − 3 ⎡⎢ 3u′2 du′ = (u0 − 1) ln⎜ 0 ⎟ 3 2u0 − 3 ⎢⎣ ν(u′ − 1) − u′ ⎝ u0 ⎠

[Δ ≤ 0; l > u ≥ 0] or [Δ > 0; u ≥ 0]

(12.c2)

(12.c3) Importantly, in the calculation algorithm of eq 6, whenever the integral has to be forced to zero because u is outside the valid limits, f(u) should altogether be forced to zero.

(10)

Solving the integral in terms of u0 to obtain the simplest expression:

∫0

[Δ > 0; u = δ]

or for u > δ

(9)

or for D < 0 ⎡1 ⎛ 1 27 ⎞⎤ ν u0 = −2 2 cos⎢ cos−1⎜ 2 ⎟⎥ ⎝ 2 ν ⎠⎦ 3 ⎣3

[Δ > 0; δ > u > 0] (12.c1)

The alternate relationship is found using the traditional method of calculating the roots of a third degree polynomial. Defining the discriminant D = (ν2/4) − (ν3/27), then for D ≥ 0: u0 =

⎛ ⎞ 2u 2 Δ −1 tan−1⎜ ⎟ u (2 u u ) Δ + + Δ ⎝ 0 0 ⎠

2

(12.b)

or finally, for Δ > 0 (i.e., u0 > − 3 or ν < 6.75), as most programs work within the traditional limits − π/2 < tan−1x < π/2 instead of the limits 0 < tan−1x < π required here, a new discriminant is required: δ = (2u0/ (u0 − 1)). Then for u < δ D


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Figure 2. (a) MCM-48 nitrogen sorption isotherm at −196 °C and (b) corresponding NLDFT pore size distribution. The pore volume is 1.01 cm3 g−1, and the specific surface area is 1268 m2 g−1.

Figure 3. Hydrodynamic size distribution of as-suspended solutions (in water).

Figure 4. (a) Schematic representation of the dip-coating process: flat silicon substrates were dip-coated in MSN suspension at different withdrawal speeds. SEM images were obtained for each condition: (b) 0.01 mm s−1 (dense thin films obtained), (c) 0.08 mm s−1, (d) 0.2 mm s−1, and (e) 0.8 mm s−1 (partial coverage for those three last conditions).

2a). These nanoparticles showed a high specific surface area (1268 m2 g−1; BET method), and a large total pore volume (1.01 cm3 g−1). The mean pore diameter was estimated at 3.5 nm (by the NLDFT method, represented in Figure 2b).

(220), (420), and (332), are characteristics of a highly ordered MCM-48.24,37,38 The nitrogen sorption measurements revealed type IV physisorption isotherms characteristic of uniform mesoporous channels with narrow, cylindrical pores (Figure E


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Langmuir 2. Preparation of Aqueous Suspensions of MSNs for the Dip-Coating Process. As-synthesized MSNs were suspended in nanopure water at various mass concentrations (4, 10, 20, 30, 40 mg mL−1). After sedimentation, the supernatant was sampled and analyzed by dynamic light scattering (DLS) to confirm the presence of a well-dispersed, nonaggregated colloidal solution of MCM-48 MSNs (Figure 3), optimal for the dip-coating process.12 The concentration of Si in the suspensions was measured by neutron activation analysis, which confirmed the high concentration of MSNs suspended in the aqueous solutions. In fact, between 42% and 65% of the initial concentration of MSNs remained in suspension (Table S1). The highest concentration (40 mg mL−1 MSNs (SiO2) initially suspended; 9.1 mg Si mL−1 final concentration) was retained for the dip-coating procedures. As-suspended MCM48 MSNs had a hydrodynamic diameter of (176.1 nm ±49.4 fwhm) (Figure 3.b). The colloidal stability of this suspension was assessed by DLS throughout the course of the dip-coating procedures (Figure S3). Neither aggregation, nor evidence of MSNs sedimentation was found in the course of the procedure. 3. MSNs Monolayer by Dip-Coating Process. Coupons from single-side polished silicon wafers were thoroughly cleaned prior to use for dip-coating in a suspension of MSNs (Figure 4a). Four different withdrawal speeds were used (0.01, 0.008, 0.2, and 0.8 mm s−1) in order to evaluate the influence of this parameter on the homogeneity and thickness of MSNs deposition. During the process, both the immersion speed (1 mm s−1) and the immersion duration (4 s) were kept constant. SEM images of the resulting thin films (Figure 4b−e) evidenced the necessity of performing dip-coating depositions at slow withdrawal speeds (0.01 mm s−1) in order to achieve highly homogeneous coverage (Figure 4b). At higher withdrawal speeds, MSNs did not cover all of the ground substrate, and aggregates were also clearly visible (Figure 4c−e). Furthermore, Figure 5a,b confirmed the homogeneity of MSNs monolayers deposited at 0.01 mm s−1 withdrawal speed. At this analytical condition, the point of contact between the particles and the substrate is clearly visible (images taken at 74° tilt angle). At the slowest withdrawal speed, no sign of aggregation was found. The silicon substrates covered with MSNs using a withdrawal speed of 0.01 mm s−1 are referred to as “MSNs monolayers” in the following text. 4. Particle Size Distribution of MSN Monolayers Sintered on Silicon Substrates. The MSN monolayers were sintered at increasing temperatures (from 500 °C up to 900 °C), and imaged in SEM (Figure 6). At each condition, the MSN monolayer pattern was clearly preserved. By comparison with the nonsintered MSN monolayers (Figure 6a), sintered substrates exhibited increasing patterns of particle grouping, and segregated levels of spaces between particle groups (particularly evident at 900 °C; Figure 6f). Particle size distributions were generated by Clemex Vision Software, and fitted using the modified Lifshitz, Slyozov, and Wagner (LSW) equation as described in the Experimental Section. The resulting function f(u), presented in the Experimental Section, was fitted to the six distributions shown in Figure 6. All correlation coefficients reached r2 ≥ 0.097. Each one of the fits showed skewness and curvature well correlated with the whole range of the particle size distribution. Qualitatively, the fittings were initially satisfactory for four of the distributions but for two, corresponding to the heat treatments at 500 °C and at 600 °C, the fitted curves emphasized a double peak distribution. In fact, the particle size

Figure 5. SEM images (74° tilt angle) of flat silicon substrates dipcoated in MSNs prior to sintering (a and b; 0.01 mm s−1 withdrawal speed). MSNs were assembled in the form of monolayers, and images a and b were taken on purpose at the edge of a disruption of such monolayers. After sintering at 900 °C (c and d), necks are clearly visible at the points of contact between substrate and particles, as well as between the particles.

distributions obtained from the SEM images clearly point to a swelling of the particles occurring in the temperature range 500−600 °C. To approximate the intermediate distributions at 500 °C and at 600 °C (Figure 4a,b), a linear combination of curves was fitted, having the same two shape parameters for the two conditions. The resulting fits are qualitatively satisfactory, and, mathematically, they bring the correlation coefficients to r2 ≥ 0.098. The mean particle diameter increases from 153 to 163 nm, up to 164 nm at 500, 600, and 700 °C, respectively. In this temperature range, particles evolve from one type of population before sintering, to a complete transformation (to a second type of population) after treatment at 700 °C. With increasing temperature, the particle distribution widens toward higher mean diameter values, then narrows again at 700 °C. At higher temperatures, the distribution widens again but toward decreased values of diameter, and with only one apparent subdistribution. Mean diameters of 146 and 140 nm are measured at 800 and 900 °C, respectively. An increasingly negative skewness is noted on the high temperature distributions (ν = 5.54). Together with the presence of necks between the particles, and a much higher calculated density (Table 1), negative skewness possibly indicates that favorable conditions for Ostwald ripening to occur are reached at 900 °C. Below 900 °C, intraparticular diffusion of the atoms within the highly porous framework seems to be the main mechanism of “particle ripening”, and this leads to particle densification first. The Ostwald ripening mechanism, i.e., the growth of large particles at the expense of smaller ones, is likely to occur mainly in the presence of dense, interconnected particles (>900 °C), and this phenomenon should be reflected as increased negative skewness on particle size distributions. 5. Evolution of the Textural Properties of MSNs upon Sintering. To correlate the evolution of the particle size distributions with evidence of densification and loss of pore volume, bulk MSNs were sintered in air (same procedure as for F


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Figure 6. SEM images of MSNs layers (0.01 mm s−1) (a) before sintering, (b) after sintering at 500 °C, (c) 600 °C, (d) 700 °C, (e) 800 °C, and (f) 900 °C. Particle size distributions are provided, and fits were performed according to the equation adapted from the LSW theory (see Experimental Section).

Table 1. Variations in the Characteristics of MSNs after Sintering at Increasing Temperaturesa sintering temperature [°C]

average MSNs diameter, D [nm]

MSNs volume variation (%)

specific surface area (Ss) [m2 g−1]

pore volume (Vp) [cm3 g−1]

calculated density ρNP [g cm−3]

density variation (%)

nonsintered 500 600 700 800 900

152 153 163 164 146 140

+1.2 + 22.2 + 25.8 - 11.2 - 21.9

1268 1428

1.01 1.11

0.68 0.64

−5.9

1477

0.81

0.79

16.2

833

0.43

1.13

66

a

Average diameter (D) extracted from SEM visualisations (Figure 5); specific surface and pore volume data extracted from physisorption measurements on bulk MSNs; the density (ρNP) was calculated from the pore volume Vp (see Experimental Section).

occurred at 900 °C, where the (220), (321), (400), (420), and (332) peaks were no longer noticeable. This effect is due to the densification of the particles, as confirmed by the decrease in pore volume (0.68 cm3 g−1) from 500 °C, and of pore diameter from 700 °C (Figure 7, Table 1). The pore size was not influenced by sintering at 500 °C (∼3.5 nm), but decreased at higher temperature: 3.2 nm at 700 °C and 2.6 nm at 900 °C. In the same way, the pore volume decreased by a factor 2 when sintered at 900 °C (Table 1: from 1.01 cm3 g−1 to 0.43 cm3 g−1). Finally, the specific surface increased until 700 °C and then decreased.

the MSNs monolayers), and measured by low-angle XRD as well as physisorption for their physical properties. Figure 7a reveals the evolution of pore structure (low-angle XRD measurements) submitted to increasing sintering temperatures. Figure 7b shows the evolution of pore size values, extracted from physisorption results, whereas Table 1 summarizes the variations in the characteristics of MSNs after sintering. The density (ρNP) was calculated from the pore volume (Vp), as detailed in the Experimental Section. First, it is evident from Figure 7a that sintering changes the mesopore order even at moderate sintering temperatures (500 °C). The (211) peak, corresponding to an interplanar distance of 3 nm, was shifted to higher angles in accordance with the pore diameter decrease. A drastic difference in pore structure G


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Figure 7. (a) Evolution of MSNs pore mesostructure (low-angle XRD) submitted to increasing sintering temperatures; (b) evolution of pore size (physisorption results). Those measurements were performed on bulk MSNs.

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DISCUSSION Many emerging applications of MSNs (optics, self-cleaning and/or hydrophobic surfaces, and biomedicine) require their assembly in the form of monolayers. Therefore, porous coatings of MSNs on flat substrates have been developed using different deposition techniques: spin coating,12,13 dip-coating,14 layer-bylayer coating,15,16,39,40 and Langmuir−Blodgett deposition.11 A relatively simple process such as dip-coating is often preferred because it can be upscaled. Most studies that have reported on the deposition of MSNs on substrates by dip-coating or layerby layer coating (two parent techniques) have evidenced the deposition of multilayers of MSNs.13,15,16 Compared with all of the previously reported methodologies, the dip-coating approach, reported in the present work, enables the selfassembly of well-ordered MSN monolayers without the use of polymers or surfactants that could interfere with the sintering process. To obtain homogeneous well-assembled monolayers, it is of prime importance to start with nonagglomerated colloidal suspensions of monodisperse and individual MSNs. In the present study, well-defined spherical particles of a narrow size distribution were obtained (Figure 1), similar in size, morphology and textural properties, to previously reported MSN-48 structures.24,25 Such particles, adequately dispersed as individual colloids in water suspension (e.g., Figure 3, Figure S3), promote the self-assembly of uniform, compact and wellordered MSNs layers. Another key point to obtain homogeneous and well-assembled monolayers is the dip-coating parameters such as the withdrawal speed or the atmosphere. In the present study, using a withdrawal speed of 0.01 mm s−1 allowed the self-assembly of MSNs monolayers (Figure 4b) whereas higher withdrawal speeds would lead to partially covered substrates (Figure 4c−e). The fixation of nanoparticles to substrates can be achieved either by using heterogeneous chemical binding reactions,41 electrostatic charges through addition of a polycation or polyanion,16 or by sintering the nanoparticles onto a substrate of relatively similar nature. The main advantage of sintering is the possibility to avoid the introduction of contaminants into the silica framework, while enabling an optimal cohesion of materials structure. Above a certain temperature, the diffusion of atoms along the surfaces of MSNs is facilitated, therefore promoting the occurrence of necks binding adjacent particles together, as well as binding particles to the substrate. At higher temperatures and upon densification of silica, volume diffusion

is expected to become the predominant mechanism. Indeed, the transition temperatures at which Si and O atoms begin to diffuse along surfaces, then in the volume for denser silica, are essential to understand the sintering of MSNs. The presence of necks is one of the key indications confirming the occurrence of surface diffusion. Previous studies performed on the sintering of MSNs have evidenced the presence of large necks between MSNs (560 nm mean diameter) sintered at 1050 °C (compacts),17 whereas sintering of a hierarchical assembly of small (50 nm) and large (500, 1000, 1500 nm) silica particles, sintered at 450 °C and visualized in SEM, did not reveal any evidence of neck formation.11 Until now, the exact sintering conditions allowing the establishment of necks between MSNs, while avoiding the collapse of the mesopore structure, have not been comprehensively investigated. Our results demonstrate that necks are already well established for samples sintered at 900 °C (Figure 5d). However, such a high sintering temperature also promotes the shrinking of MSNs (Figure 6f, Table 1) and the occurrence of crevices between clusters of particles (Figure 6f). At 700 °C, a moderate inflection in pore volume is noticed compared to the results obtained at 600 °C (Table 1). Overall, such results indicate the necessity to sinter MSN monolayers at temperatures below 700 °C, in order to preserve the optimal mesopore volume and structure, while securing the anchoring of MSNs both to the silicon substrates and to other particles. To our knowledge, this is the first time a comprehensive textural analysis has been performed to reveal the evolution of pore volume, specific surface area, and density of MSNs upon sintering in the 500−900 °C range. Another reason for sintering at lower temperatures than 900 °C is the decrease of silanol groups (Si−OH) in MSNs, occurring at such high temperatures.22 In fact, the presence of silanol groups at the surface of adjacent silica nanoparticles, is crucial to establish necks between the silica particles upon sintering (as schematized in Figure 8). MSNs are made of amorphous silica, where silanol groups are present not only at the surface, but also inside the particle structure.22,42 Upon heat treatments at increasing temperatures (400−700 °C), the number of −OH groups decreases sharply.21,23 This fact was also confirmed in the present study, by comparing the 29Si NMR profiles of MSNs sintered at 600 °C, to that of untreated MSNs (Figure 9). Both materials were submitted to a preliminary hydration treatment, followed by drying in the same conditions as for the dip-coated surfaces. H


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Figure 9. 29Si MAS NMR profiles (59.60 MHz) of nonsintered and sintered (600 °C) MSNs.

phenomenon necessarily affects the nanoparticle diameter at the beginning of the densification process. Between 700 and 800 °C, particle size distributions show only one population (Figure 6d,e), and both pore volume and pore radius decrease significantly (Table 1 and Figure 7). The density increases for sintering temperatures over 700 °C (Table 1). The mechanism occurring in this temperature range involves an intraparticle diffusion/densification process. The evolution of the particle size distributions could be fitted by an approach based on the Brown-Coughan and Fortes (BCF) equation (Experimental Section, eq 6). The decrease in the mean pore diameter and the disappearance of the pore structure (Figure 7) indicates a collapse of the pores caused both by diffusion of Si and O, as well as by hydroxyl condensation (Figure 8). Very few studies have reported on the characterization of the textural properties of MSNs upon sintering. Saito et al. measured the densification and pore structure changes for sintered compact assemblies of MSNs,17 and found that sintering below 800 °C preserved the mesoporous structure. Above this temperature, a sharp increase in the bulk density and a strong decrease in the specific surface area were observed. Collapse of the pores was observed around 1000 °C, followed by a strong densification of the silica material (around 1200 °C). No evidence of bonding between MSNs was reported for the samples sintered at 1050 °C and below, which is different from the observations made in the present study. Indeed, at 900 °C, the condition for which a drastic increase of density was observed, bridges between the particles were also noticed, revealing interparticle sintering. Volume decrease tends to form packs of trigonal structures separated by crevices (Figure 6). Hence, observation of the behavior of MSNs upon sintering by using both thin layers and bulk sintered powders provides comprehensive data to understand the densification and pore structure evolution of MSNs upon sintering.

They appeared to be highly hydroxylated; however, a sharp decrease in Q2 and Q3 groups was observed between nonsintered and sintered samples. In brief, the hydroxyl groups at the surface of the particles condense, water is released, and oxo bridges (Si−O−Si) allow for more consolidation and atom diffusion to occur within the silica structure. At a sintering temperature of 500 °C, a slight increase in particle diameter is noticed (Table 1, Figure 6), that can be attributed to the initiation of Si and O rearrangement due to −OH condensation into Si−O−Si (Table 1). The strong decrease in silanol groups in the range 400−700 °C, is also reflected in the endothermic effect appearing on TGADSC profiles in the same temperature range (Figure S4). Between 500 and 600 °C, strong particle swelling occurs (more than 10% of the initial diameter). Close inspection of the SEM images (Figure 6c) shows the presence of particles of slightly heterogeneous morphologies. This unexpected increase in particle size may be attributed to a rearrangement of the SiO2 framework, possibly in the presence of H2O from further Si−OH condensation reactions, which under these conditions translates into nanoparticle swelling. Anumol et al.43 also reported that pore collapse under sintering begins at the outermost layer of particles. This phenomenon forms a solid shell toward which the silica of the internal walls of the MSN structure diffuses, forming a partially hollow shell. This

CONCLUSION Spherical mesoporous silica nanoparticles MSNs (MCM-48type) of narrow particle size distribution were successfully synthesized, characterized, dispersed in water, and used to assemble thin films of MSNs by the dip-coating process. Monolayers of compact-assembled MSNs were only obtained for low withdrawal speed of 0.01 mm s−1. Very slow withdrawal speeds are necessary to reach uniform, compact, and flat coatings. The potential of sintering as an efficient means of attaching the thin film to the substrate was explored. The mesopore structure and the total pore volume were preserved at 500 °C. The increase of MSNs size observed at 600 °C, without a decrease in pore size, was explained by the swelling induced by rearrangement of the silica structure. From 700 °C, a decrease in particle size and a loss of pore volume was observed, leading to a densification of the nanoparticles, explained by intraparticle sintering. An analytical fit based on the LSW theory, was developed to fit the nanoparticle distributions obtained after sintering. At 900 °C, evidence of interparticle sintering was observed in the form of bridges. In order to keep the pore characteristics of MCM-48 (high pore volume, high specific surface and interconnected pores), sintering should be kept at

Mesoporous Silica Nanoparticles under Sintering Conditions: A

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