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Experimental characterization of the nanoparticle size effecton the mechanical stability of nanoparticle-based coatings

Wajdi Heni, Laurent Vonna, and Hamidou HaidaraNano Lett., Just Accepted Manuscript • DOI: 10.1021/nl503768r • Publication Date (Web): 15 Dec 2014

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Experimental characterization of the nanoparticle size effect on the

mechanical stability of nanoparticle-based coatings

Wajdi HENI, Laurent VONNA*, Hamidou HAIDARA

Institut de Science des Matériaux de Mulhouse (IS2M) CNRS - UMR 7361, Université de Haute

Alsace, 15 rue Jean Starcky BP2488, 68057 Mulhouse Cedex, France

*Corresponding author: laurent.vonna@uha.fr

We present an experimental investigation of the mechanical stability of silica nanoparticle-based

coatings as a function of the size of the nanoparticles. The coatings are built following a layer-by-layer

procedure, alternating positive and negative surface charges. The mechanical stability of the

multilayers is studied in water, on the basis of an ultrasonic cavitation test. The resistance of the

coating to cavitation is found to remarkably increase with decreasing the size of the nanoparticles,

indicating an increase of the cohesive energy density. The relative contribution of van der Waals and

electrical double-layer interactions to the stability of the multilayer is discussed toward their size

dependence.

Keywords: Nanoparticle, Nanoparticle-based coating, Nanoparticle assembly, Cohesion energy

density, Cavitation, ultrasonic cavitation test

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Thin films of nanoparticles (NPs) show functional properties that make them useful for numerous

applications including sensing, electronics or optics.1–3 While an increasing number of applications are

proposed in literature, the poor mechanical stability of these systems remains a major drawback for

their industrial use. The weak adhesive interactions between the NPs and between the NPs and the

substrate lead indeed to fragile coatings that may loss their functionalities upon friction or scratch for

example. Additionally, failure of the coating might lead to release and dissemination of NPs in the

environment which constitutes a major public health and environmental issue. Different approaches

were proposed to improve the mechanical stability of nanoparticle-based coatings such as calcination

or hydrothermal treatments,4,5 atomic layer deposition6 and covalent chemical bonding.7 Besides such

post-treatments, the size of the NP is an intrinsic parameter which strongly impacts the cohesion of the

nanoparticle-based coating. For particles that only interact through van der Waals interactions, the

interaction potential UvdW between two particles is expected to follow:8

( ) ( )( )

( )

+

−++

++

−+−=

2

22

2

2

222

42ln2

2

4

42

²4

12 Rd

RRd

Rd

R

RRd

RAU vdW (1)

with A the Hamaker constant corresponding to the interaction between two identical materials through

the appropriate medium, R the radius of the NPs, and d the distance between the two particles. This

expression can be simplified for interparticle distances smaller than the particle radius (d/R<<1),

leading to UvdW = -(AR)/(12d). The cohesive energy density of the assembly is thus given by EvdW =

(nUvdW)/(2Vmesh), with n the number of NPs interacting in the elementary mesh and Vmesh the volume of

the elementary mesh which scales as R3, both n and Vmesh being a function of the packing density of the

NP assembly. Whereas the van der Waals interaction potential per NP pair scales as R, the cohesive

energy density EvdW appears thus to scale as :

EvdW ∝ nA/(24R²d) (2)

Despite the strong dependence of the cohesive energy density on the size of the NP (∝ R-²), this

parameter appears to be rarely discussed in the literature (theoretically or experimentally).9,10 Such

dependence should however undoubtedly impact the cohesion or mechanical behaviour of the

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assembly regardless of any post-treatment or additional interactions that might be involved in the

cohesion of the assembly.

In the following, the mechanical stability of model nanoparticle-based coatings is studied as a function

of the NP size using the ultrasonic cavitation test. The mechanical properties of such thin nanoparticle-

based coatings are extremely difficult to accede with the techniques commonly in use, and are thus

rarely discussed in the literature. In only few cases indeed, local elastic modulus and hardness were

retrieved form nanoindentation measurements, or macroscopic wear behaviour from abrasion under

mechanical shear.4–6 This mechanical characterization becomes even more difficult with ultrathin

nanoparticle-based coatings (one to a few layers thick). We used here ultrasonic cavitation as an

alternative test to study the adhesion and cohesion of the thin nanoparticle-based coatings.11

In this

test, a cyclic pressure variation is generated in a liquid by an ultrasonic horn. The expansion phase

leads to the nucleation and the cavitation of air bubbles that finally collapse, and form microjets

impacting the surface of the sample placed in front of the ultrasonic horn. It was shown that the

collapse of the bubble creates microjets expelled with a speed up to 400 km.h-1, and pressure waves of

several hundreds to thousands of MPa.12–15

The local impacts produced by the collapse of

microbubbles near a surface finally lead to erosion of material. This approach is commonly used to

study material resistance or to assess for surface hardening for example.11,16–20 Resistance of bulk

materials to erosion induced by cavitation is usually evaluated through the volume loss, the mass loss,

the mean and maximum erosion depth, or the shape of the impact. This technique was also

successfully used to characterize the mechanical resistance of thin organic or metallic coatings

deposited on metals or polymers.21–25

At the difference with the nanoindentation or scratch tests that

may be used for characterizing the mechanical properties of thin nanoparticle-based coatings, the

ultrasonic cavitation refers here more likely to a cyclic solicitation , and therefore to a fatigue test.

In order to demonstrate the NP size-effect in the mechanical behaviour of nanoparticle-based coatings,

this ultrasonic cavitation test was applied to multilayers of silica NPs (for a detail of the test see

Supporting information). The multilayers of spherical silica NPs with nominal radius of 8.5 nm, 15 nm

and 25 nm (from Micromod Partikeltechnologie GmbH, Rostock, Germany) were deposited on silicon

substrates following a layer-by-layer procedure. Figure 1a shows the main steps for the layer-by-layer

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built-up of the NP multilayers. A silicon substrate is first cleaned in a piranha solution and

functionalized with aminopropyltrimethoxysilanes (APTMS, from Sigma Aldrich), by immersion

during 12 hours in a 2 mM toluene solution.

Figure 1. (a) Sketch of the layer-by-layer assembly protocol. (b) Transmission electron microscope images of

silica NPs of radius R = 25 nm. (c and d) Scanning electron microscope image of a NP multilayer built with

silica NPs of radius R = 25 nm (4 layers), top view and cross section respectively.

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It was then followed by a thorough rinse with toluene, and dried under nitrogen flow. The first layer of

NPs was deposited by immersion during two hours in a 25 mg/mL aqueous solution of silica NPs. The

NPs are functionalized with carboxyl (-COOH) head groups which allow the electrostatic grafting of

the NPs on the amine (-NH2) terminated substrate. In a third step, this first NPs layer is UV-cleaned

(254 nm) during 1 hour, to remove the grafted molecules bearing the carboxyl (-COOH) head groups.

The bare silica NPs (without the carboxyl head group) are then functionalized with amine (-NH2) head

groups, by immersion during 12 hours in a 2 mM APTMS solution in toluene. A second layer of

carboxyl (-COOH) functionalized NPs is absorbed through electrostatic interactions with amine (-

NH2) head groups that were beforehand grafted on the upper side of the NPs by exposition to UV-

light. Starting from this UV etching step, this procedure is reproduced until the desired thickness (i.e.

number of layers) is achieved, as depicted in Figure 1a. This approach allows here an easy control of

the assembly and especially the thickness of the coating. Figure 1b shows the transmission electronic

image of silica NPs (radius R = 25 µm), and Figures 1c and 1d are the top view and cross section

electron microscope images of a multilayer (4 layers) built with these NPs, following the above

described procedure. It is worth noting here that we do not expect the low dispersion in the

nanoparticle sizes (8.5 nm ± 1 nm, 15 nm ± 2 nm and 25 nm ± 3nm) to significantly impact the

assembly process and the resulting packing of the NP coatings. In addition, any packing disorder

which may arise from this dispersion would be similar for all three multilayer coatings.

In aqueous solutions, the assembly of oppositely charged particles, or the adhesion of charged particles

on oppositely charged substrates were discussed on the basis of the DLVO approach.26–29 We here

adopt the same approach, the assembly of our NPs multilayers being performed in an aqueous solution

where the suspended carboxyl (-COOH) functionalized NPs interact either with the amine (-NH2)

terminated substrate (1st layer) or with a previously adsorbed NPs layer, which upper side was

beforehand grafted with amine (-NH2) groups following Figure 1a. In addition, during the sonication

test which is performed in water, we assume that water can penetrate through the NPs assembly and

fills the interparticle space. Under these conditions (aqueous solvent assembly), the carboxylic acid (-

COOH) and the amine (-NH2) head groups are negatively and positively charged respectively,

considering the pH of the deionized water we used during both the assembly and the test (pH = 6.7 ±

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0.5).30

As a result, the cohesive energy of our multilayers is determined by this assembly step and the

related interparticle interactions, namely the van der Waals and electrical double-layer interactions, as

accounted for by the DLVO approach.26–29 The following gives an estimate of this cohesive energy

density of the NPs assembly considering both the van der Waals interaction potential UvdW and the

electrical double-layer interaction potential Uedl between the NPs.

The van der Waals interaction potential UvdW is given by equation 1, considering a Hamaker constant A

= 0.7 x 10-20

J for spherical silica particles interacting across water.31

For the NPs considered in this

work (R = 8.5 nm, 15 nm and 25 nm) and an interparticle distance d = 0.5 nm, the magnitude of the

attractive van der Waals interaction potential per NP pair is thus |UvdW | ≈ 10 x 10-21 J, 17 x 10-21 J and

29 x 10-21

J respectively (or 2.4kT, 4.3kT and 7.2kT at room temperature). An estimate of the cohesive

energy of the NP assembly arising from the van der Waals interactions is given by equation 2. In the

case of an ideal hexagonal compact assembly, the four NPs of an elementary mesh of volume Vmesh =

(4R/√2)3 interact each with twelve neighbours, leading to a cohesive energy density EvdW for this ideal

particle assembly:

EvdW = 0.09A/(R²d) (3)

The magnitude of the cohesive energy density for the NPs considered in this work (R = 8.5 nm, 15 nm

and 25 nm) is thus EvdW ≈ 17 kJ m-3, 5 kJ m-3 and 2 kJ m-3 respectively. Although these values are

obtained on the basis of an estimate of the interparticle distance and an optimal organization of the

particles, this result clearly shows the remarkable increase of the cohesion of the NP assembly with

reducing the size of the NPs, which is expected to arise from the sole van der Waals interactions.

The electrical double-layer interaction potential Uedl between two oppositely charged NPs of same

radius R can be approximated by:31,32

)exp(2

4 2

0

2

0 dRd

RU r

edl κεπε

−+

Φ−= (4)

with Φ0 the surface electrostatic potential of the NPs (considered as the same for the positively and

negatively charged NPs), ε0 and εr the vacuum and relative dielectric constant respectively, κ-1 the

Debye length, and d the distance between the particles. The electrical double-layer interaction is

strongly dependent on the electrolyte concentration, the increase of which decreases the Debye

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screening length, κ -1

. Indeed, considering NPs of radius R = 15 nm and an interparticle distance d =

0.5 nm, the electrical double-layer interaction potential |Uedl| ≈ 28 x 10-21 J for an electrolyte

concentration of 1 M, and raises up to |Uedl | ≈ 153 x 10-21 J for an electrolyte concentration of 10-3 M,

or 7kT and 38kT at room temperature, respectively (with ε0 = 8.85 10-12

F/m, εr = 80, Φ0 = 50 mV and

κ-1 = 0.3 nm). This simple calculation shows that the contribution of the electrical double-layer

interactions to the overall is not negligible and may overcome that of the van der Waals interactions,

depending on the electrolyte concentration. The layer-by-layer assembly of NPs applied here, leads to

coatings made of NPs layers of alternating surface charges. In the ideal case of a hexagonal compact

assembly of NPs, the hexagonal mesh of volume V’mesh = 24√2R3 contains three beads of same charge

each interacting with six neighbouring NPs of opposite charge (three in the two adjacent plans). The

cohesive energy density Eedl that arises from the attractive electrical double-layer interactions between

the oppositely charged NPs writes thus:

)exp()2(

53.0d

RRd

BEedl κ−

+= (5)

with B = 4πε0εrΦ0² (according to equation 4). For the NPs considered in this work (R = 8.5 nm, 15 nm

and 25 nm), and an interparticle distance d = 0.5 nm, the magnitude of the cohesive energy density

that arises from the sole electrical double-layer interactions is thus Eedl ≈ 15.1 kJ m-3

, 4.9 kJ m-3

and

1.7 kJ m-3

for an electrolyte concentration of 1M, and Eedl ≈ 75.3 kJ m-3

, 24.5 kJ m-3

and 8.8 kJ m-3

for

an electrolyte concentration of 0.001M, respectively.

An estimate of the cohesive energy density of the NP assembly, Ecoh, is given by summing the

cohesive energy arising respectively from the van der Waals and the electrical double-layer

interactions, EvdW and Eedl, established previously (equations 3 and 5), both of them showing the same

dependence on the NP radius R, ∝1/R². Figure 2a shows this cohesive energy density Ecoh as function

of the NP radius, for an interparticle distance d = 0.5 nm, and for both an electrolyte concentration of

1M and 0.001M (inset). As expected from equations 2 and 5, this figure shows the remarkable

increase of the cohesive energy density as the size of the NP decreases. It also shows the relative

contribution of the electrical double-layer interactions vs the van der Waals one. The inset of Figure 2a

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shows that for low electrolyte concentrations (< 0.001 M), it is the electrical double-layer interactions

which may contribute essentially to the stability of the NP assembly.

Figure 2. (a) van der Waals, electrical double-layer and total cohesive energy density Ecoh of a NP assembly as

function of NP radius, for an interparticle distance of 0.5 nm and an electrolyte concentration of 1 M. Inset: for

an electrolyte concentration of 10-3 M (b) van der Waals, electrical double-layer and total cohesive energy

density Ecoh of a NP assembly as function of interparticle distance, for a NP radius of 15 nm and an electrolyte

concentration of 1M. Inset: for an electrolyte concentration of 0.001 M.

The cohesive energy density Ecoh shown in Figure 2a was calculated for an arbitrary minimal

interparticle distance d = 0.5 nm, and an ideal hexagonal compact assembly of NPs. Figure 1 shows

however that the assembly is highly random although compact. Such a disordered assembly is

characterized by lower coordination number, larger volume mesh or inversely, smaller density as

compared to the ideal hexagonal compact assembly. As a consequence, the average interparticle

distance can be strongly affected by the packing of the NPs. The previous evaluation of the cohesive

energy density allows considering the impact of the packing on the multilayer stability, through the

interparticle distance d. Disordered assemblies should lead to a larger mean distance between the

particles and thus to lower cohesive energy as shown by the dependence of the energy densities EvdW

and Eedl on the interparticle distance d, EvdW ∝ 1/d (equation 2) and Eedl ∝ exp(-d) (equation 5)

respectively. Figure 2b shows the cohesive energy density Ecoh of a NP assembly, as a function of the

interparticle distance d, for a particle radius R = 15 nm, and for both an electrolyte concentration of

1M and 0.001M (inset). At high electrolyte concentrations (~ 1 M), the cohesive energy density of the

NP assembly appears to be strongly dependent on the interparticle distance. For lower electrolyte

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concentrations (< 0.001 M), and as already mentioned, it is the electrical double-layer interactions

which contribute essentially to the cohesive energy density. The decrease of the cohesive energy

density is here monotonic, and much less affected by the increase of the interparticle distance as

compared to higher electrolyte concentrations.

Whereas the van der Waals interactions can reasonably be estimated from equation 1, the

determination of the electrical double-layer interactions from equation 4, and the use of the DLVO

approach to estimate the cohesive energy of the NP assembly in water show severe limitations. Indeed,

the exact zeta potential of both interacting surfaces as well as the electrolyte concentrations are

difficult to measure. Equation 4 might also break down at small distances for which the Poisson-

Boltzmann equation at the origin of the electrical double-layer potential no longer describes the ionic

distribution between surfaces. Moreover, additional interactions between NPs at contact may exist

such as hydration forces or acid-base interactions between COOH and NH2 groups.33

Some authors

have reported discrepancies between theory and experiments attributed to adsorbed organic matter34

,

to the viscoelasticity of grafted octadecyltrichlorosilanes,35 or to hydrogen bonding,30 all of them being

omitted in the DLVO approach. Although not representative of the absolute value of the NP

interaction potentials, this above approach used to estimate the cohesive energy of the NP assembly

illustrates however its scaling with the NP size, especially its remarkable increase with decreasing the

size of the NP. Additionally the stability is shown to be strongly affected by the electrolyte

concentration as well as the packing density of the NP assembly.

To experimentally evaluate the cohesive energy of NP assemblies, and to demonstrate its dependency

on the NP size, the cavitation test was applied to NPs multilayers of same thickness (≈ 200 nm), built

with NPs of radius R = 8.5 nm (14 layers), R = 15 nm (7 layers) and R = 25 nm (5 layers),

respectively. Representative optical microscope images and electron microscope images of the

cavitation damages induced in a 200 nm thick multilayer of silica NPs of R = 8.5 nm are shown in

Figure 3. These damages are induced after a 35 sec ultrasonic test at 50 W. We note that the main

damages are induced at the center of the projected area of the sonotrode tip positioned above the

sample as shown in Figure 3a. In this test configuration, the density of cavitating bubbles is indeed the

highest in the middle of the sample and decreases radially from the center toward the border of the

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exposed surface of the sample.22,25,36

The stress resulting from the multiple impacts of the imploding

bubbles is thus the highest in the middle of the projected area of the sonotrode tip. In the case

presented in Figure 3, the multilayer at the center of the sonotrode projected area is totally removed

from the substrate (zone 1 in Figures 3c and d), and the bare silicon is observed with only traces of the

multilayer. Further away from the center of this zone, the area covered by the multilayer increases

(zone 2 in Figures 3c and e). Finally, at the border of the exposed area, the NP multilayer appears

clearly, with holes resulting from cavitation (zone 3 in Figures 3c and f). This damage pattern is

representative of the erosion process usually observed for bulk materials or thin coatings which results

from the multiple impacts of cavitating bubbles.13,25,37,38

Figure 3. (a) Optical microscope image of the cavitation damages induced in a NP multilayer (200 nm thick)

built with NPs of radius R = 8.5 nm (14 layers), after an ultrasonic test of 35 sec at 50 W. (b) Thresholded image

of (a) showing the central damaged spot inside the projected area of the sonotrode tip (dashed circle). (c) Sketch

of the test configuration showing the distribution of the cavitation induced stress on the sample. (d) Scanning

electron microscope images of the cavitation damages induced in a NP multilayer (200 nm thick) built with NPs

of radius R = 8.5 nm (14 layers), after an ultrasonic test of 35 sec at 50 W, in zone 1 of the sample, (e) in zone 2

of the sample and (f) in zone 3 of the sample. The light grey areas correspond to the bare silicon substrate

whereas the darker areas correspond to the multilayer.

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In the following, an experimental estimate of the relative cohesive energy density of the NP assembly

is given on the basis of the surface area of multilayer removed by ultrasonic cavitation. For this,

optical microscope images of the sample after each sequence of ultrasonic testing were thresholded

and the surface area of the observed bare silicon substrate normalized by the area of the sonotrode tip

(Figure 3b). The evolution of the surface fraction of multilayer removed from the silicon substrate

after stepwise ultrasonic testing of the sample is shown in Figure 4 for three NP multilayers of

identical thickness (≈ 200 nm), built with NPs of radius R = 8.5 nm (14 layers), R = 15 nm (7 layers)

and R = 25 nm (5 layers), respectively.

Figure 4. Surface fraction of multilayer removed from the substrate as function of ultrasonic testing time, at 50

W, for three multilayers of same thickness (200 nm) built with NPs of radius R = 8.5 nm (14 layers), R = 15 nm

(7 layers) and R = 25 nm (5 layers), respectively. (Error bars are estimated from the scattering of the data

obtained from two independent samples)

In all three cases, the surface fraction of multilayer removed from the substrate progressively increases

until reaching a plateau. The plateau corresponds here to a threshold beyond which the local stress

induced by the cavitation pressure is not large enough to remove the NPs from the substrate. The

surface fraction of eroded multilayer at the plateau is found to increase with the size of the NPs, at ~

8% for the NPs of radius R = 8.5 nm, ~ 30% for the NPs of radius R = 15 nm, and at ~ 90% for the

NPs of radius R = 25 nm respectively. High plateau values are associated with a fast increase of the

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multilayer surface area removed from the substrate. Analysing results of Figure 4 more quantitatively,

a maximal rate of the multilayer surface fraction removed from the substrate can be estimated. These

rates are of ~ 0.17 % s-1 for the NPs of radius R = 8.5 nm, ~ 0.72 % s-1 for the NPs of radius R = 15

nm, and ~ 11.1 % s-1

for the NPs of radius R = 25 nm respectively. Additionally, the multilayer built

with the smallest NPs (R = 8.5 nm) clearly shows an induction time of ~ 25 sec during which the bare

substrate does not appear after ultrasonic cavitation. This induction time reduces to ~ 10 sec for the

intermediate size of NPs (R = 15 nm) and vanishes totally in the case of the larger NPs (R = 25 nm).

During this period, the cavitation effectively induces damages in the coating as assessed by electron

microscope observations, showing the presence of micropits similar to those observed in Figure 3f,

which depth is lower than the coating thickness. These micropits grow in size after additional steps of

ultrasonic application until reaching the substrate. Finally, the whole kinetics of the multilayer surface

fraction removed from the substrate after an ultrasonic test accounts for the resistance of the NP

multilayer to cavitation, including the induction step, the rate of multilayer removal, as well as the

plateau value. A relative low plateau level and low increasing rate (eventually associated to an

induction regime) both reveal high resistance of the system to cavitation. The results of Figure 4

finally show an increasing resistance to cavitation of the multilayers as the size of the NPs decreases.

This resistance to cavitation is directly related to the cohesive energy of the NP assembly that opposes

the local pressures associated to the imploding microbubbles, and demonstrates higher cohesion of the

multilayer as the size of the NPs decreases, as expected from the earlier discussion on the origin of the

stability of NP assemblies. However, as discussed above and illustrated in Figure 2b, the cohesive

energy density is also dependent on the packing of the NP assembly which may also be size

dependent. The observed experimental size dependence of the cohesion (Figure 4) thus requires that

we check which of the packing order or the explicit analytical dependence of the cohesion energy on

the particle size (Figure 2a) here controls and predominates the behaviour of the NPs coatings. We did

that through both the literature and complementary Atomic Force Microscopy (AFM)

characterizations of the coatings. Indeed, although very sparse, the existing literature on this issue,

either as simulation or experimentally observed results, all show that the packing disorder increases

with decreasing the size of the particles, for spherical and quasi-monodisperse particles covering the

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size range from nanometer to micrometer scale.39–41

The complementary AFM characterization we

performed (see Supporting Information) shows that the measured thickness of the coatings made of

large particles (R = 15 nm and R = 25 nm) closely matches the theoretical one estimated from the

number of layers, using the close-packed model. On the other hand, the same measured thickness for

the coating of the smaller size particle (R = 8.5 nm) was found to be larger than the theoretical one,

clearly indicating a more disordered (loosely packed) assembly. On the basis of these literature and

complementary characterization, one would expect the cohesive energy density of the coating to

decrease as the packing disorder (or the porosity) increases. Otherwise stated, one would expect the

cohesion of the coating to decrease as the size of the particle decreases which is exactly the opposite of

our analytical scaling through DLVO, and of our experimental findings. However we cannot exclude

some influence of this packing density that slightly modulates the cohesion of the particle assembly:

toward a slight decrease for small particles (less packed) or a slight increase for larger particles (more

packed).

We proposed earlier that the electrical double-layer interactions between the NPs play a non-negligible

role in the stability of the NP assemblies. To estimate the contribution of the electrical double-layer

interactions to the overall stability of the multilayers, the same cavitation tests were performed on

thermally treated multilayers. Samples were heated up to 400°C for 14 hours, under dried air.

Following this procedure, the grafted amine (-NH2) and carboxyl (-COOH) head groups should totally

be removed from the surface of the NPs.42–44

Figure 5a displays the XPS spectra (takeoff angle of 15°)

of the C 1s region of a carboxyl (-COOH) functionalized NP monolayer adsorbed on an amine (-NH2)

coated surface, before and after thermal annealing. The spectra before thermal annealing (top) shows

the characteristic peaks of the carboxyl group (O-C=O at 289.2 eV, C=O at 288 eV, C-O and C-N at

286.6 eV). Figure 5b displays the XPS spectra (takeoff angle of 15°) of the N 1s region of a single NPs

layer adsorbed on the amine (-NH2) functionalized substrate, before and after thermal annealing. The

nitrogen signature arises from the amine terminated monolayer (-NH2) coating the silicon substrate

and in interaction with the carboxyl (-COOH) functionalized NPs, but also from the upper side of the

NPs on which amine (-NH2) groups have been grafted after UV cleaning and removing of the carboxyl

(-COOH) groups exposed on that upper side. Two peaks are observed at 399.4 eV and 401.3 eV

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corresponding respectively to NH2 and NH3+ groups, which presence is characteristic of an APTMS

monolayer.42,45 After thermal treatment the carboxyl (-COOH) and amine (-NH2) groups totally

disappear from the surface of the NPs as shown in Figures 5a and 5b.

Figure 5. AlKα XPS of the (a) C1s region of a carboxyl (-COOH) functionalized NP monolayer adsorbed

on an amine (-NH2) coated surface, before and after thermal annealing and, (b) N 1s region of a single

NPs layer adsorbed on the amine (-NH2) functionalized substrate, before and after thermal annealing.

The nitrogen signature arises here from the amine terminated monolayer (-NH2) coating the silicon

substrate and from the upper side of the NPs that was grafted with (-NH2) (step 4, Figure 1a).

Attractive electrical double-layer interactions should in this way totally disappear after such a thermal

annealing, and the NPs in the multilayer should thus interact only through van der Waals interactions,

affecting the bond strength between NPs and thus leading to a decrease of the mechanical stability of

the multilayer. Figure 6 shows the surface fraction of multilayer removed after a single step of

ultrasonic application of 5 sec at 50 W applied to thermally annealed multilayers (200 nm thick) built

with NPs of radius R = 8.5 nm, R = 15 nm and R = 25 nm, respectively. These surface fractions are

compared to those measured on the reference as-prepared multilayers (without thermal annealing) and

retrieved from the results shown in Figure 4. For the NPs of radius R = 8.5 nm, the test time is too

short to erode and remove the multilayer from the substrate, whether the sample was thermally

annealed or not. For the two other NP sizes, the thermally annealed samples show a higher surface

fraction of multilayer removed from the substrate. Whereas almost no erosion of the multilayer was

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observed after 5sec of ultrasonic application at 50 W for the as-prepared multilayer built with NPs of

15 nm radius, 5.6 % of monolayer was removed after thermal annealing. This surface fraction rises up

to 62.1 % for the thermally annealed multilayers built with NPs of 25 nm radius, whereas it only

reaches 46.9 % and for as-prepared samples. These behaviours seem to support and demonstrate the

role of the electrical double-layer interactions in the stability of the multilayer. It has to be noticed that

the same dependence of the multilayer stability on the size of the NP observed previously with as-

prepared samples, is here observed with thermally annealed samples. For a fixed ultrasonic testing

time and power, an increase of the surface fraction of multilayer removed from the substrate is indeed

observed with increasing the size of the NPs, revealing again a decrease of the cohesive energy density

with increasing the size of the NPs.

Figure 6. Surface fraction of NP multilayer removed from the substrate after an ultrasonic testing time of 5 sec

at 50 W, for multilayers built with NPs of radius R = 8.5 nm, R = 15 nm and R = 25 nm , after thermal annealing

and as-prepared.

In summary, we investigated the damages induced by ultrasonic cavitation in multilayers of silica NPs

as a function of the NP size. From the growth of the surface fraction of NP multilayer removed from

the substrate after stepwise ultrasonic testing, we found higher resistance of the NP multilayer to

cavitation with decreasing the size of the NPs. This resistance to cavitation demonstrates

experimentally for the first time, an increase of the cohesive energy with decreasing the size of NPs in

the case of NPs interacting through van der Waals and electrical double-layer interactions. Following

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the same experimental approach, we estimated the relative contribution of the electrical double-layer

interactions to the overall stability of such NP assemblies. Additionally, we demonstrate here that the

cavitation test usually applied to bulk materials is well suited to evaluate the mechanical stability of

NPs based coatings, showing high sensitivity to evaluate the influence of slight variations of the

interparticle interactions on the mechanical behaviour of such systems.

Supporting information Available: The ultrasonic test and thickness of the multilayers. This material

is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The authors would like to thank Dr. A. Ponche for his help with XPS

interpretation.

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