Impact of the Silica Surface Nanoconfinement on the Microstructure of

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Impact of the Silica Surface Nanoconfinement on the Microstructure of Alkoxysilane Layers Grafted by Supercritical Carbon Dioxide Diane Rebiscoul, Susan Sananes Israel, Samuel Tardif, Vincent Larrey, André Ayral, and Francois Rieutord J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01967 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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The Journal of Physical Chemistry

Impact of the Silica Surface Nanoconfinement on the Microstructure of Alkoxysilane Layers Grafted by Supercritical Carbon Dioxide Diane Rébiscoul†~, Susan Sananes Israel†, Samuel Tardif‡, Vincent Larrey#, André Ayral , Francois Rieutord‡ §

† CEA, ICSM – UMR 5257 CEA-CNRS-UM-ENSCM, 30207 Bagnols-sur-Cèze Cedex, France ‡

Univ. Grenoble Alpes, CEA, IRIG-MEM, F-38000 Grenoble, France

# Univ.

§

Grenoble Alpes, CEA, LETI, F-38000 Grenoble, France

Institut Européen des Membranes, IEM, UMR-5635, Université de Montpellier, ENSCM,

CNRS, Place Eugène Bataillon, 34095 Montpellier cedex 5, France ~Corresponding

author: [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT The impact of nanoconfinement on the microstructure of alkoxysilane layers grafted by supercritical CO2 was determined using model system made of silica nanochannels, i.e. planar silica surface spaced of few nanometers. Two type of silica nanochannels of 3 and 5 nm gap size were grafted with alkoxysilanes having different head groups (thiol, amine, and iodo) using the same protocol than in our previous study on open flat silica surface (materials, T, P). Using the same characterization technique, X-ray reflectivity, we have directly compared the results obtained on open planar silica surface with confined ones. We show that the microstructure of grafted layers obtained on non-confined silica flat surface are not directly transposable on confined silica planar surface. In our experimental conditions, on flat surface the microstructure of the grafted layer is only driven by the alkoxysilane molecule while in confined planar silica surface, the microstructure is driven by both, the confinement size and the nature of the molecule. This may be explained by the modification of the molecules transport and the change of supercritical CO2 properties in nanoconfined media.


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INTRODUCTION Surface functionalization is intensively used in various technological fields, such as for microelectronics1-4, decontamination5-8, medical9, catalysis10-11 or biosensor applications12-13. The functionalization is often performed on silica having surfaces with various geometries: planar, convex and concave surfaces, as usually found in nanoobjects and nanopores. The most commonly studied solvent for surface functionalization and in particular for the grafting of alkoxysilane molecules on silica surfaces is supercritical CO2

14-16.

The physicochemical properties of CO2 in

supercritical state (T=31 ºC and P=74 bar), i.e. zero surface tension, high diffusivity and low density, increase the grafting efficiency and lower the duration and temperature of the process in mesoporous media, with respect to classical organic solvents 17. Moreover, it is easily recyclable, non-flammable and nontoxic and thus, considered as a green solvent. In nanoconfinement, a media of few nanometers, it is expected that the alkoxysilanes are constrained by the low free space available for their diffusion, their hydrolysis and their condensation with the surface silanols. Thus, if the molecules can be grafted at the surface, they would change their organization compared to the one obtained on a non-confined planar surface. It has been shown that this organization depends on the alkyl chain length and the surface curvature 18.

Thus, this should be the case with concave surface, spheres or cylinders, such as in cylindrical

mesopores of SBA-15 and MCM-41 silica, the most frequent materials studied

14-15, 17, 19-20.

Yet,

no direct comparison of the morphology and the structure of the grafted molecules as a function of the confinement has been reported so far. The aim of this study is to provide a comprehensive picture of the impact of nanoconfinement on the microstructure of grafted molecules. Our approach was to compare open planar model systems with confined ones, in the same conditions (materials, T, P). We have already 3 ACS Paragon Plus Environment


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investigated the microstructure of ultrathin layers prepared by grafting of soluble alkoxysilanes having different head groups (thiol, amine and iodo) in supercritical carbon dioxide (SC CO2) on open, flat silicon oxide surface

2-3, 16.

The results showed that the grafting temperature and the

nature of the head group strongly affect the microstructure of the grafted layers. Dense monolayers were obtained with 3-(mercaptopropyl)trimethoxysilane (MPTMS) at 60ºC, polycondensed layers were always prepared with [3-(aminoethylamino)propyl]trimethoxysilane (AEAPTMS), and dense bilayer were obtained with 3-(iodopropyl)triethoxysilane (IPTES) at 120ºC. We now performed the same study in model confined systems, using silica nanochannels. In confined medium, the characterizations of the grafted layers are generally performed using nitrogen adsorption-desorption, infrared spectroscopy, thermogravimetric analyses, nuclear magnetic resonance and elemental analysis

15, 17, 19-20.

From these analyses, the thickness of the

grafted layer, the grafting density, and the condensation ratio can be determined indirectly. However, the same characterization technique has never been used both on planar surface and on confined planar surfaces to allow a direct comparison of the grafted layer properties. Moreover, the grafting/filling profile in the nanoconfinement has never been directly measured. In our previous study, X-ray reflectometry was used to characterize grafted layers on large flat silica surfaces16. We used the same technique in transmission mode to probe grafted layers on two parallel and opposite flat surfaces spaced of 3 and 5 nm, i.e. in silica nanochannels. From the obtained results, the impact of the alkoxysilane and of the confinement on the grafting density and on microstructure of the grafted layer on millimeter distance were determined.


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METHODS Materials Nanochannels fabrication Nanochannels were fabricated using lithography and direct wafer bonding technologies. After the growth of thermal silica of 3 and 5 nm thick on the surface of a first 200 mm diameter silicon wafer, the samples were patterned (lithography and etching) in order to obtain a nanochannels network with a series of grooves having a period of 250 nm (alternation of silica nanopillars and nanochannels of 250 nm width) as described in Figure 1. With a second lithography and etching step, we prepare a second silicon substrate with large (4 cm2) and deep (50 µm) cavities that allow an easy fluid injection in the nanochannels. After hydrophilic surface preparation, we performed direct bonding with no additional material to join the two wafers together. Simple notch alignment was needed to obtain this structure as nanochannels are etched all over the first wafer. Finally, high temperature annealing at 1100°C was performed to strengthen the bonding assembly. Samples were then cutted from the two bonded wafers in order to obtain samples of 5 x 30 mm.



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Figure 1 : Description and atomic force microscopy images of the 3 and 5 nm grooves used to form nanochannels. Nanochannels grafting Based on the same grafting protocol as used on silicon substrates 16, MPTMS, AEAPTMS and IPTES were grafted in nanochannels samples using SC CO2 process. MPTMS and AEAPTMS were provided by Sigma Aldrich and IPTES was obtained by the halogen exchange of (3chloroproyl)triethoxysilane and NaI according to the procedure described in the literature 21. Table S1 summarizes some physicochemical properties of these three molecules. Before the grafting process and in order to obtain hydroxyl groups on the silica surface of the nanochannels, the samples were placed in 10 wt% nitric acid solution under reflux for 2 h and rinsed with ultrapure water. This protocol results in a hydrated silicon oxide layer of approximately 7 to 9 Å thick on top of the silicon wafer (as observed with X-ray reflectometry 16). The SC CO2 grafting process was then performed using a SEPAREX supercritical fluid extractor (See Figure S1). The activated nanochannels samples were placed in the stainless-steel reactor in an oven at the temperature indicated in Table 1. The samples were first dried at 60°C and 6 ACS Paragon Plus Environment

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90 bar in static mode (no SC CO2 flow) during 15 minutes to remove the free water in the empty part of the nanochannels. Then, the process described in Figure S2 was applied in dynamic (with SC CO2 flow) or static mode depending on the alkoxysilane selected. In dynamic mode, SC CO2 was flowed at 30 g/min and a pressure of 100 bar was maintained during all the process. After 5 minutes, a solution containing 1 wt% of the organic molecule (MPTMS or IPTES) diluted in acetone was mixed with the SC CO2 at 1.75 g.min-1 and flowed in the reactor for 3h30. Due to the gelation of AEAPTMS in contact with SC CO2, a 1 mL glass vial of AEAPTMS was placed directly in the reactor before the beginning of the process. Finally, samples were rinsed with SC CO2 for 5 minutes. At the end of the process, samples were stored under vacuum. The references of the samples and the process conditions are summarized in Table 1. Table 1 : References and experimental conditions used during the SC CO2 grafting process described in Figure S2. Sample reference Alkoxysilane

Mode

T (°C)

MPTMS

MPTMS

Dynamic

60

AEAPTMS

AEAPTMS

Static

60

IPTES

IPTES

Dynamic

100

X-ray reflectivity characterization Samples were characterized using hard X-ray reflectivity at 27 keV (λ=0.4592 Å) on BM32 at the European Synchrotron Radiation Facilities (Grenoble, France). Hard X-rays were required to cross the silicon wafer from the edge of the sample as described in Figure 2 (a) 22. In order to determine the homogeneity of the alkoxysilane grafting, analyses were performed at 3 distances from the entrance of the nanochannels: 1, 5 and 10 mm (Figure 2 (b)). Due to the high energy of 7 ACS Paragon Plus Environment


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X-ray, the beam has no interaction with the grafted alkoxysilanes in the nanochannels. Thus, the samples were not degraded during the analysis. 23.

Figure 2 : (a) Scheme of the X-ray reflectivity analysis principle of the silica nanochannels. The beam enters the sample from the edge and is reflected at the various interfaces of the nanochannels. (b) Location of the X-ray reflectivity measurements on the sample. After geometrical corrections related to the beam footprint over the sample (50 µm-high beam over a 5 mm-wide sample), background subtraction and normalization to the incident beam, the reflectivity curves were plotted in the standard R(q).q4 vs. q mode to remove the Fresnel decay, where q is the wavevector transfer (q = 4πsin()/λ with  the incident angle). Large fringes associated with the interference between the waves reflected at both bottom and top surfaces of the channels are visible in the curves. The fringe period is directly related to the thickness of the SiO2 nanochannels. X-ray reflectivity modelling In order to obtain the electron density profile ρe(𝑧), the X-ray reflectivity data were fitted with a two-box symmetric electron density profile as described in Figure 3 .


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Figure 3 : Description of the parameters used in the model including two parts in the electron density profile. In this model, the symmetric electron density profile is described as the sum of the error functions of two symmetric electron density profiles (1):

ρe(𝑧) = ― 𝛿

(

(

1 + erf ((𝑧 + 𝐿/2)/ 2/𝜎 2

)+𝛿(

1 + erf ((𝑧 ― 𝐿/2)/ 2/𝜎 2

) ― 𝑑(

1 + erf ((𝑧 ― 𝑡/2)/ 2/𝑟

)

2

1 + erf ((𝑧 + 𝑡/2)/ 2/𝑟 2

)+𝑑 (1)

With L and t the widths of the gap, δ and d the depths of the gap, σ and r the roughnesses of the electron density profiles 1 and 2 respectively. The reflected amplitudes 𝑎1 and 𝑎2 associated with the electron density profiles 1 and 2 are the following ones (2)(3) :

[

𝑖𝑞𝐿

[

𝑖𝑞𝑡

𝑎1(𝑞) ∝ 𝛿 ―exp( ―

𝑎2(𝑞) ∝ 𝑑 ―exp( ―

𝑖𝑞𝐿

]

(

q2𝜎2

]

(

q2𝑟2

2 ) + exp ( 2 ) exp ―

𝑖𝑞𝑡

2 ) + exp ( 2 ) exp ―

)

(2)

)

(3)

2

2

can be also written as (4) and (5): 9 ACS Paragon Plus Environment


(

q2𝜎2

(

q2𝑟2

𝑎1(𝑞) ∝ 𝛿[2𝑖 𝑠𝑖𝑛(𝑞𝐿/2)]exp ―

𝑎2(𝑞) ∝ 𝑑[2𝑖 𝑠𝑖𝑛(𝑞𝑡/2)] exp ―

)

(4)

)

(5)

2

2

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Knowing that 𝑅(𝑞) = |𝑎(𝑞)2|, with 𝑎(𝑞) = 𝑎1(𝑞) + 𝑎2(𝑞) , the reflectivity can be explicitly written as a function of the parameters L, t , δ , d, σ and r as (6) :

𝑅(𝑞)𝑞4

[

(

= (4𝜋𝑟𝑒)2 2𝛿sin (𝑞𝐿)exp ―

) +2𝑑𝑠𝑖𝑛(𝑞𝑡)exp ( ― )]

q2𝜎2

q2r2

2

2

2

(6)

where re is the classical electron radius. Using this last equation (6), the experimental X-ray reflectivity curves were fitted adjusting L, t, δ, d, σ and r parameters which were used to calculated ρe(𝑧) (1). The electron density ρe(𝑧) is given with an uncertainty of 0.01 e.A-3.

RESULTS Electron density profiles The experimental and simulated X-ray reflectivity curves of the samples before and after grafting processes are presented in the Figure 4.


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-7 1.21x10

-7 1.21x10

3 nm MPTMS

8x10-8

Rq4

Rq4

8x10-8

empty 1 mm 5 mm 10 mm

5 nm MPTMS


4x10-8

4x10-8

0 0.0

0.2

0.4

0.6

0 0.0

0.8

0.2

0.4

0.6

0.8

q (A-1)

q (A-1) -7 1.21x10

1.21x10-7

3 nm AEAPTMS


5 nm AEAPTMS


8x10-8

Rq4

Rq4

8x10-8

4x10-8

4x10-8

0 0.0

0.2

0.4

0.6

0 0.0

0.8

0.2

-7 1.21x10

0.4

0.6

0.8

q (A-1)

q (A-1)

1.21x10-7

3 nm IPTMS


8x10-8

5 nm IPTMS


8x10-8

Rq4

Rq4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4x10-8

4x10-8

0 0.0

0.2

0.4

0.6

0.8

0 0.0

0.2

q (A-1)

0.4

0.6

0.8

q (A-1)

Figure 4 : Experimental (dots) and simulated (lines) X-ray reflectivity curves of the samples before and after SC CO2 grafting processes at 1, 5 and 10 mm from the entrance of the 3 and 5 nm nanochannels. After a grafting process with MPTMS, the intensity of the fringes of the X-ray reflectivity curves of the 3 nm nanochannels are only slightly modified. Nevertheless, these modifications are significant and correspond at minima to a variation of 0.01 e.A-3 in the center of the nanochannels 11 ACS Paragon Plus Environment


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as presented in Figure S3. For the 5 nm nanochannels, the MPTMS grafting leads to similar reflectivity curves whatever the distance from the entrance of the nanochannels and a global decrease of the reflectivity signal probably due to the decrease of density contrast between the silicon and the nanochannels. Thus, for a size of confinement of 5 nm, the modification of nanochannels is significant and homogeneous along the nanochannels after the grafting process. Grafting the 5 nm nanochannels with AEAPTMS results in reflectivity curves that are similar and mainly modified at large q. This may be due to a modification of the interface after the grafting process. After a SC CO2 process with AEAPTMS and IPTES in the 3 nm nanochannels and with IPTES in the 5 nm nanochannels, the reflectivity curves are different depending on the distance from the nanochannels entrance. This indicates a non-homogeneous grafting along the nanochannels. The intensity of the reflectivity curves for IPTES grafted in 3 and 5 nm nanochannels measured at 5 and 1 mm respectively is much reduced, highlighting a low density contrast. This is also the case with 3nm nanochannels grafted with AEAPTMS at 1mm from the network entrance. In order to determine the distribution of the grafted molecules perpendicular to the surface, the reflectivity curves were fitted with equation (6) and the obtained parameters were used to calculate the electron density profiles presented in Figure S4. The electron density profiles of the non-grafted samples highlighting an electron density in the center of the nanochannels of 0.25 e.A-3 i.e. 50% lower than the electron density of dense silica ρe = 0.66 e.A-3, confirms the efficiency of SC CO2 drying process. The difference between the electron density profiles of the empty and of the grafted nanochannels (ρe(𝑧)) were calculated and are presented on Figure 5. The low contrast of density obtained in the nanochannels after the grafting of AEAPTMS and IPTES in the 3 nm nanochannels at 1 and 5 mm respectively and of IPTES in the 5 nm nanochannels at 1 mm cannot be attributed 12 ACS Paragon Plus Environment

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to the simple addition of alkoxysilane molecules (Table S1) but to a probable nanochannels collapse. For these analyses, the fitting results were not presented. This point will be discussed later.

Figure 5 : Difference profiles of electron densities ρe(𝒛) between the empty nanochannels and the nanochannels grafted by SC CO2 process with MPTMS, AEAPTMS and IPTES at 1, 5 and 10 mm from the entrance of the 3 and 5 nm nanochannels. Impact of nanoconfinement on the grafting density

As demonstrated in supplementary information SI1, the added electron densities in the nanochannels presented in Figure 5 are mainly due to the alkoxysilanes grafted at the silica surface of the Si wafer. Thus, the grafting densities of alkoxysilanes at the silica surface of the Si wafer 𝛼 *and 𝛼 were calculated determining two types of area under the curve from the ρe(𝑧) profile.

These areas 𝐴 ∗ and 𝐴 (5) were obtained on two distances which were respectively the layer 13 ACS Paragon Plus Environment


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thickness characterized in the same conditions by X-ray reflectivity on a large flat surfaces 16 (Table S2) and the molecule length lal calculated using ChemSketch software and indicated in Table S1. 𝛼 ∗ (𝑜𝑟 𝛼) =

2.𝐴 ∗ (𝑜𝑟 𝐴) 𝑛eal

(5)

with 𝑛eal the number of electrons in the condensed alkoxysilane (X(CH2)3SiO2(O-). The results are presented in the Table 2. Table 2 : Grafting densities of alkoxysilanes on the surface of the silica of the Si wafer calculated with the thickness of the layer obtained in the same conditions on silica surface measured by X-ray reflectivity (𝛼*) from 16 and calculated supposing the molecule length indicated in Table S1 (𝛼). nc : value not calculated since the alkoxysilane length is larger than ℎ𝑛/2. Theoretical values calculated from the molecule length lal and the electron density of the liquid alkoxysilane (Table SI) and values calculated from 16 are also indicated. The uncertainties are on average about ± 0.2 al.nm-2.

Alkoxysilane

MPTMS

AEAPTMS

Distance from the entrance of the nanochannels

𝛼 ∗ (al.nm-2)

3 nm

5 nm

1 mm

1.5

4.8

5 mm

1.0

4.7

10 mm

1.4

5.4

1 mm

-

1.6

5 mm

2.1

1.6

10 mm

2.9

2.1

𝛼 (al.nm-2)

From 16 Table S2

6.8

3.5

3 nm

5 nm

1.1

3.3

0.8

3.3

1.1

3.8

-

1.6

1.1

1.6

1.2

2.2

From Table S1

5.1

4.7


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IPTES

1 mm

nc

-

5 mm

nc

1.8

10 mm

nc

2.6

7.1 bilayer

0.4

-

-

1.3

2.1

1.8

3.9

Assuming that the density of silanols at the silica surface of the Si wafer is similar between all samples (around 5 OH.nm-² 24), whatever the alkoxysilane used during the grafting process, 𝛼 ∗ (𝛼) values are lower than the 𝛼 ∗ values calculated from 16 and 𝛼 values calculated from Table S1 (Table 2). This highlights the general impact of the confinement on the grafting efficiency: it limits the density of grafted molecules. However, the decrease of the confinement size does not affect 𝛼 ∗ (𝛼) in the same way. On flat silicon oxide surface, the head group drives the microsctructure of the grafted layers. In confined silicon oxide surfaces, 𝛼 ∗ (𝛼) varies (AEAPTMS and IPTES) or not (MPTMS) along the nanochannels. In this last case, with MPTMS, 𝛼 ∗ (𝛼) is lower in 3 nm than in 5 nm nanochannels. Impact of the alkoxysilane on the grafting efficiency and homogeneity The size of the confinement coupled with the nature of the alcoxysilane may be the main driver of the efficiency and homogeneity of the grafting process. This is illustrated on Figure 6.



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Figure 6 : Illustration of the impact of the confinement size and alkoxysilane nature on the electron density distribution in the 3 and 5 nm nanochannels and on open silica surface from16. The molecule lengths lal calculated using ChemSketch software are also indicated (Table S1). Grafting with MPTMS: case of dense monolayer On non-confined flat silica surface, at 60ºC and 100 bar, a SC CO2 process with MPTMS leads to the formation a dense monolayer having a 𝛼 ∗ of 6.8 MPTMS.nm-² 2-4. In confinement, 𝛼 ∗ (𝛼) is homogenous along the nanochannels, lower than 6.8 MPTMS.nm-², and decreases with the size of the confinement. While in 5 nm nanochannels, the grafting is homogenous along the sample and close to the one obtained on non-confined planar silica surface (𝛼 ∗ = 7 MPTMS.nm-² vs. 𝛼 ∗ = 5 MPTMS.nm-²), in 3 nm samples, 𝛼 ∗ (𝛼) is lower. Indeed, in 3 nm confinement size, the grafting of MTPMS may lead to the formation of hydrogen bonds between the face to face thiols functions

25-26

(6). Such bonds may perturb the MPTMS diffusion into the 3 nm nanochannels

during the SC CO2 process. This would lead to an incomplete filling of the grafted layer on the silica of the Si wafer and then a lower 𝛼 ∗ (𝛼).


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(6) In the 5 nm nanochannels, such hydrogen bonds could also explain the presence of supplementary sorbed MPTMS molecules in the center of the nanochannels (almost 2 layers). MPTMS molecules could also self-react before they reach the O-H surface groups, leading to disorganized and lowdensity layers. However, this was observed only for process temperature higher than 60°C and in porous media at 150°C and 290 bar 17. Grafting with AEAPTMS: case of polycondensed layer Grafting process with AEAPTMS at 60°C and 100 bar leads to the formation of polycondensed layer on a non-confined silica surface having a 𝛼 ∗ of 3.5 AEAPTMS.nm-². This is classically explained by the orientation of the amine head groups towards the silicon oxide surface or towards other hydroxylated molecules

27-28.

In confined medium, a decrease of 𝛼 ∗ (𝛼) and in

some case an excess of electron in the center of the nanochannels are observed. This means that less dense and polycondensed structure is obtained. In 5 nm nanochannels, this excess of electron in the center of the nanochannels decreases with the distance from the entrance. This may highlight a grafting process driven by the molecule transport through the nanochannels. In 3 nm confinement, the grafting is not homogeneous along the nanochannels probably due to a partial nanochannels clogging. This has been already observed in MCM-41 having a 3.8 mean pore size 15. Moreover, the nanochannels collapse observed at 1 mm could be the result of the stress created during the molecules polycondensation29 of face to face grafted molecules at the silica surface and the low ratio gap thickness (3 or 5 nm) on nanochannels width.



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Grafting with IPTES: case of bi-layer With IPTES, the CO2 SC grafting process leads to the formation of a bilayer presenting an interpenetration of two monolayers on a non-confined silica surface with a 𝛼 ∗ of 7.1 IPTES.nm-² 16. However, in 3 and 5 nm confinement, the bilayer structure is modified. For the 5 nm nanochannels, this is indicated by the profile of ρe(𝑧) displayed in Figure 5. It shows an increase of the electron density at the silica surface on a distance corresponding to the size of the IPTES. For the grafted 3 nm nanochannels, it was not possible to calculate 𝛼 ∗ with a bilayer thickness. Moreover, whatever the confinement size, with IPTES, the CO2 SC grafting process is not efficient since 𝛼 are lower than 3.9 IPTES.nm-² and irregular as attested by the 𝛼 values obtained along the nanochannels. The probable collapse of few zones in nanochannels (3 nm at 5 mm and 5 nm at 1 mm) may also be explained by the polycondensation of the hydrolyzed IPTES. Factors driving the microstructure of the grafted alkoxysilanes in confinement In these process conditions, these last results highlight that the morphology and the structure of the grafted molecule obtained in nanoconfinement are modified compared to the one obtained on flat silica surface. This is particularly the case for 3 nm nanochannels 18 ACS Paragon Plus Environment

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after the grafting process with all the alkoxysilane studied. For MPTMS, an incomplete surface coverage is obtained and for AEAPTMS and IPTMS, some of the nanochannels are partially clogged. Several factors may explain these results. The first factor is the available space vs. the size of the molecule. This can affect both the molecules diffusion and their organization at the silica surface. Even if this pore clogging is sometimes observed in SBA-15 and MCM-41 silica, it is probably more intense in our samples. Indeed, in the nanochannels, the molecules diffuse over a longer confinement (few mm) than in the cylinders of the SBA-15 and MCM-41 (few hundred nanometers)30. The second factor is the change of SC CO2 properties in nanoconfined medium. Indeed, the correlation length of the density fluctuation and the mean pore density are modified in nanoconfinement. As example, the mean pore density in 7.5 nm pores is higher than the bulk density. This may modify the transport and the solubility of the alkoxysilane in CO2 and then the grafting process. Third, the sorption of CO2 at the silica interface may change its density (the density of the liquid film is 1.5–2 times larger than that of the confined fluid)

31-32 33-34

and/or its solubilization in the 2 to 3 layers of



Page 20 of 28

interfacial water strongly linked to the silica surface. This can modify the silica surface properties and then the condensation of the alkoxysilanes.

CONCLUSION The use of the same characterization technique on non-confined and confined flat silica surfaces, have showed that the microstructure of grafted layers obtained on non-confined silica flat surface are not directly transposable on confined silica flat surface. In our experimental conditions, on flat surface the microstructure of the grafted layer is only driven by the alkoxysilane molecules while in confined silica surface, the microstructure is driven by both, the confinement size and the nature of the molecules. This may be explained by the modification of the molecules transport and the change of SC CO2 properties in nanoconfinement. It is also expected that the addition of a curvature to confined surface should also modify the grafting density and the organization of the alkoxysilane into the layer. Model silica systems allowing the use of X-ray reflectivity have to be fabricated to precisely determine the effect of silica surface curvature. Supplementary information -

Table S1. Physicochemical properties of the grafted molecules. Molecule length is calculated using ChemSketch software. ρeal is calculated for each molecule as ρ𝑒𝑎𝑙 = 𝜌𝑚 ∑𝑖𝑐𝑗𝑍𝑗

∑𝑖𝑐𝑗𝐴𝑗,

where cj is the amount of element j in the material, Zj is the atomic number, Aj is the

atomic mass and ρm (g.Å-3) is the mass density. -

Table S2. Thickness th and electron density e of the layer grafted obtained from the simulation of the X-ray reflectivity curves from REF. 20 ACS Paragon Plus Environment


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-

Figure S1: Experimental setup of the SC CO2 grafting process.

-

Figure S2. SC CO2 and solvent flow applied during the SC CO2 grafting process with MPTMS and IPTMS molecules.

-

Figure S3. Impact of the variation of the electron density in the center of the nanochannels

ecenter of ±0.014 e.Å-3 on X-ray reflectivity curves. -

Figure S4. Electron density profiles obtained from the model used for the fitting of the experimental X-ray reflectivity curves of the samples before and after SC CO2 grafting processes with MPTMS, AEAPTMS and IPTMS at 1, 5 and 10 mm from the entrance of the 3 and 5 nm nanochannels networks.

-

Figure S4. Example of the area determination taking into account the surface density of electron on a distance corresponding to the molecule length for 5 nm nanochannels network grafted with MPTMS at 1 mm from the nanochannels network entrance.

-

SI1. Origin of the electron density profile: grafted silica surface of Si wafer or/and grafted silica surface of SiO2 nanochannels

ACKNOWLEDGEMENTS The authors would like to thank Zouhir MEHREZ, Romain LAURENT, Frank FOURNEL and Claudine BRIDOUX for the nanochannels preparation. Access to ESRF BM32 beamtime is gratefully acknowledged. Markus BAUM and Rémi BOUBON are also thank for their help during 21 ACS Paragon Plus Environment


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the experiments at ESRF. Emmanuel LEJEUNE for the help during the preparation of IPTES. This work was supported by Université de Montpellier and the Commissariat à l’Energie Atomique et aux énergies alternatives. The authors declare no competing financial interest.


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REFERENCES

1.

Weibel, G. L.; Ober, C. K., An Overview of Supercritical CO2 Applications in

Microelectronics Processing. Microelectron. Eng. 2003, 65, 145-152. 2.

Rebiscoul, D.; Perrut, V.; Renault, O.; Rieutord, F.; Olivier, S.; Haumesser, P. H.,

Alkoxysilane Layers Deposited by SC CO2 Process on Silicon Oxide for Microelectronics Applications. J. Supercrit. Fluids 2009, 51, 287-294. 3.

Rebiscoul, D.; Perrut, V.; Morel, T.; Jayet, C.; Cubitt, R.; Haumesser, P. H., Alkoxysilane

Layers Compatible with Copper Deposition for Advanced Semiconductor Device Applications. Langmuir 2010, 26, 8981-8987. 4.

Rebiscoul, D.; Perrut, V.; Morel, T.; Jayet, C.; Cubitt, R.; Haumesser, P. H., Alkoxysilane

Layers Compatible with Cu Deposition: Towards New Diffusion Barriers? Microelectron. Eng. 2012, 92, 45-48. 5.

Makowski, P.; Deschanels, X.; Grandjean, A.; Meyer, D.; Toquer, G.; Goettmann, F.,

Mesoporous Materials in the Field of Nuclear Industry: Applications and Perspectives. New J. Chem. 2012, 36, 531-541. 6.

Viltuznik, B.; Kosak, A.; Zub, Y. L.; Lobnik, A., Removal of Pb(Ii) Ions from Aqueous

Systems Using Thiol-Functionalized Cobalt-Ferrite Magnetic Nanoparticles. J. of Sol-Gel Sci. Technol. 2013, 68, 365-373. 7.

Viltuznik, B.; Lobnik, A.; Kosak, A., The Removal of Hg(Ii) Ions from Aqueous Solutions

by Using Thiol-Functionalized Cobalt Ferrite Magnetic Nanoparticles. J. of Sol-Gel Sci. Technol. 2015, 74, 199-207.



8.

Page 24 of 28

Wang, S. G.; Wang, K. K.; Dai, C.; Shi, H. Z.; Li, J. L., Adsorption of Pb2+ on Amino-

Functionalized Core-Shell Magnetic Mesoporous Sba-15 Silica Composite. Chem. Eng. J. 2015, 262, 897-903. 9.

Bharti, C.; Nagaich, U.; Pal, A. K.; Gulati, N., Mesoporous Silica Nanoparticles in Target

Drug Delivery System: A Review. Int. J. Pharm. Invest. 2015, 5, 124-33. 10.

Yokoi, T.; Kubota, Y.; Tatsumi, T., Amino-Functionalized Mesoporous Silica as Base

Catalyst and Adsorbent. Appl. Catal., A 2012, 421-422, 14-37. 11.

Amonette, J. E.; Matyas, J., Functionalized Silica Aerogels for Gas-Phase Purification,

Sensing, and Catalysis: A Review. Microporous Mesoporous Mater. 2017, 250, 100-119. 12.

Sarkar, K.; Dhara, K.; Nandi, M.; Roy, P.; Bhaumik, A.; Banerjee, P., Selective Zinc(Ii)-

Ion Fluorescence Sensing by a Functionalized Mesoporous Material Covalently Grafted with a Fluorescent Chromophore and Consequent Biological Applications. Adv. Funct. Mater. 2009, 19, 223-234. 13.

Karimi, M.; Badiei, A.; Mohammadi Ziarani, G., A Single Hybrid Optical Sensor Based on

Nanoporous Silica Type SBA-15 for Detection of Pb2+ and I− in Aqueous Media. RSC Adv. 2015, 5, 36530-36539. 14.

Lopez-Aranguren, P.; Saurina, J.; Vega, L. F.; Domingo, C., Sorption of Tryalkoxysilane

in Low-Cost Porous Silicates Using a Supercritical CO2 Method. Microporous Mesoporous Mater. 2012, 148, 15-24. 15.

Lopez-Aranguren, P.; Fraile, J.; Vega, L. F.; Domingo, C., Regenerable Solid CO2 Sorbents

Prepared by Supercritical Grafting of Aminoalkoxysilane into Low-Cost Mesoporous Silica. J. Supercrit. Fluids 2014, 85, 68-80. 16.

Sananes Israel, S.; Rebiscoul, D.; Odorico, M.; Flaud, V.; Ayral, A., Surface Properties of

Alkoxysilane Layers Grafted in Supercritical Carbon Dioxide. Langmuir 2019, 35, 2792-2800. 24 ACS Paragon Plus Environment

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17.

Tenorio, M. J.; Morere, J.; Carnerero, C.; Torralvo, M. J.; Pando, C.; Cabanas, A., Thiol

Group Functionalization of Mesoporous SiO2 SBA-15 Using Supercritical CO2. Microporous Mesoporous Mater. 2018, 256, 147-154. 18.

Bernardoni, F.; Kouba, M.; Fadeev, A. Y., Effect of Curvature on the Packing and Ordering

of Organosilane Monolayers Supported on Solids. Chem. Mater. 2008, 20, 382-387. 19.

Sanchez-Vicente, Y.; Pando, C.; Cortijo, M.; Cabanas, A., Chemical Surface Modification

of Mesoporous Silica Sba-15 with a Tertiary Aminosilane Using Supercritical Carbon Dioxide. Microporous Mesoporous Mater. 2014, 193, 145-153. 20.

Sanchez-Vicente, Y.; Stevens, L. A.; Pando, C.; Torralvo, M. J.; Snape, C. E.; Drage, T.

C.; Cabanas, A., A New Sustainable Route in Supercritical CO2 to Functionalize Silica Sba-15 with 3-Aminopropyltrimethoxysilane as Material for Carbon Capture. Chem. Eng. J. 2015, 264, 886898. 21.

Matsura, V. G., Y.; Larionova, J.; Guérin, C.; Caneschi, A.; Sangregorio, C.; Lancelle-

Beltran, E.; Mehdi, A.; Corriu, R. J. P. , Synthesis of Magnetic Silica-Based Nanocomposites Containing Fe3O4 Nanoparticles. J. Mater. Chem. 2004, 14. 22.

Rieutord, F.; Eymery, J.; Fournel, F.; Buttard, D.; Oeser, R.; Plantevin, O.; Moriceau, H.;

Aspar, B., High-Energy X-Ray Reflectivity of Buried Interfaces Created by Wafer Bonding. Phys. Rev. B 2001, 63. 23.

Fenter, P.; Eberhardt, A.; Liang, K. S.; Eisenberger, P., Epitaxy and Chainlength Dependent

Strain in Self-Assembled Monolayers. J. Chem. Phys. 1997, 106, 1600-1608. 24.

Brinker, C.;

Scherer, G. Sol-Gel Science: The Physics and Chemistry of Sol-Gel

Processing, Academic Press, Boston, 1990. 25.

Colebrook, L.; Tarbell, D. S., Evidence for Hydrogen Bonding in Thiols from Nmr

Measurements. Proc. Natl. Acad. Sci. U. S. A. 1961, 47, 993-996. 25 ACS Paragon Plus Environment


26.

Page 26 of 28

Marcus, S. H.; Miller, S. I., Hydrogen Bonding in Thiols. J. Am. Chem. Soc. 1966, 88,

3719-&. 27.

Horr, T. J.; Arora, P. S., Determination of the Acid-Base Properties for 3-Amino, 3-Chloro

and 3-Mercaptopropyltrimethoxysilane Coatings on Silica Surfaces by XPS. Colloids Surf., A 1997, 126, 113-121. 28.

Metwalli, E.; Haines, D.; Becker, O.; Conzone, S.; Pantano, C. G., Surface

Characterizations of Mono-, Di-, and Tri-Aminosilane Treated Glass Substrates. J. Colloid Interface Sci. 2006, 298, 825-31. 29.

Rusmirovic, J. D.; Trifkovic, K. T.; Bugarski, B.; Pavlovic, V. B.; Dzunuzovic, J.; Tomic,

M.; Marinkovic, A. D., High Performance Unsaturated Polyester Based Nanocomposites: Effect of Vinyl Modified Nanosilica on Mechanical Properties. EXPRESS Polym. Lett. 2016, 10, 139159. 30.

Ciccariello, S.; Melnichenko, Y. B.; He, L. L., Phase Behavior of Carbon Dioxide Confined

in Silica Aerogel in the Vicinity of the Bulk Critical Point. J. Phys. Chem. C 2011, 115, 2233622346. 31.

Melnichenko, Y. B.; Wignall, G. D.; Cole, D. R.; Frielinghaus, H.; Bulavin, L. A., Liquid-

Gas Critical Phenomena under Confinement: Small-Angle Neutron Scattering Studies of CO2 in Aerogel. J. Mol. Liq. 2005, 120, 7-9. 32.

Stefanopoulos, K. L.; Steriotis, T. A.; Katsaros, F. K.; Kanellopoulos, N. K.; Hannon, A.

C.; Ramsay, J. D. F.; Iop, Structural Study of Supercritical Carbon Dioxide Confined in Nanoporous Silica by in Situ Neutron Diffraction. J. Phys.: Conf. Ser. 2012, 340, 012046-012052. 33.

Gruszkiewicz, M. S.; Rother, G.; Wesolowski, D. J.; Cole, D. R.; Wallacher, D., Direct

Measurements of Pore Fluid Density by Vibrating Tube Densimetry. Langmuir 2012, 28, 50705078. 26 ACS Paragon Plus Environment

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


34.

Rother, G.; Krukowski, E. G.; Wallacher, D.; Grimm, N.; Bodnar, R. J.; Cole, D. R., Pore

Size Effects on the Sorption of Supercritical CO2 in Mesoporous CPG-10 Silica. J. Phys. Chem. C 2012, 116, 917-922.



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Impact of the Silica Surface Nanoconfinement on the Microstructure of

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