Corrosion behaviour of silica hybrid coatings produced from basic catalysed particulate sols by...

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Surface & Coatings Technology 191 (2005) 228–235

Corrosion behaviour of silica hybrid coatings produced from basic

catalysed particulate sols by dipping and EPD

Y. Castro, B. Ferrari*, R. Moreno, A. Duran

Instituto de Ceramica y Vidrio (CSIC), Campus de Cantoblanco, c/ Kelsen, 28049 Madrid, Spain

Received 23 October 2003; accepted in revised form 3 March 2004

Available online 30 April 2004

Abstract

This paper compares the corrosion behaviour of silica hybrid coatings produced from basic catalysed sols by dipping and electrophoretic

deposition (EPD) onto stainless steel AISI 304. Particulate silica sols were prepared by basic catalysis with ethyltriethoxysilane (TEOS),

methyltriethoxysilane (MTES) and sodium hydroxide. To improve the stability, the starting sol was diluted with ethanol. The maximum

defect-free sintered thickness obtained by dipping was 3.5 Am while EPD allowed us to obtain coatings as thick as 12 Am. The protective

behaviour against corrosion and the corrosion kinetics of the produced films were studied through potentiodynamic and polarisation

resistance (PR) measurements in aggressive media (sea water and HCl). The corrosion resistance of coatings produced by dipping increases if

concentrated solutions are used and the coating are thicker than 2 Am. However, the EPD process allows producing protective coatings using

also diluted solutions, stable with time. The corrosion resistance of the steel is increased by two orders of magnitude for dip-coatings and by

four orders of magnitude for coatings produced by EPD. Moreover, silica hybrid coatings present an excellent resistance not only in sea water

but in HCl 1N media. AFM observations suggest that coatings produced by EPD are homogeneous and defect-free.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Immersion test; Electrophoretic deposition; Sol–gel/coatings; Silicon oxide

1. Introduction

Coatings manufacturing is a major feature of the sol–gel

method. Thin films may be easily prepared by dip-coating

but thicker coats are necessary for some applications as

increasing the corrosion resistance of metals. In these cases,

other deposition processes as the electrophoretic deposition

technique (EPD) can be used. A few studies concerning the

use of EPD for sol–gel systems have been published, and

most of them report the deposition from aqueous suspensions

of nanoparticles prepared by sol–gel [1,2], or commercially

available colloidal suspensions [3]. However, only little

information is available dealing with the deposition of

precursor sols prepared by sol–gel synthesis. In basic con-

ditions, the use of alkaline pHs leads to the clustering of

alkoxide chains which behave as dense particles dispersed in

the alcohol generated during synthesis or further dilution [4].

0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2004.03.001

* Corresponding author. Tel.: +34-917-355-870; fax: +34-917-355-

843.

E-mail address: [email protected] (B. Ferrari).

Much effort has been devoted to study the corrosion

protection of metals (stainless steel, aluminium and alumin-

ium alloys, copper, carbon steel, galvanized steel, etc.) by

sol–gel coatings, but published results are often contradic-

tory. Some papers have evaluated the protective behaviour

of the coating considering the variation of corrosion poten-

tial, a parameter not relevant for measuring the corrosion

behaviour [5]. Inorganic coatings are adequate barriers

against oxidation but show a poor behaviour in electrolytic

media due to the presence of micro-cracks or defects [6,7].

On the other hand, hybrid organic–inorganic coatings have

demonstrated to give an adequate protection to metals. The

incorporation of organic groups makes it possible to in-

crease ductility and thickness (f2 Am) and to reduce the

defects and micro-cracking, thus enhancing the electrolytic

corrosion behaviour [8]. The use of thicker coatings should

allow increasing the corrosion resistance provided the coat-

ings are dense and defect-free.

Previous works described the use of EPD process to

obtain thick silica coatings onto stainless steel using either

acid-catalysed sols containing colloidal particles, or basic-

catalysed particulate sols [9–11].

Y. Castro et al. / Surface & Coatings Technology 191 (2005) 228–235 229

The aim of this work has been to prepare thick silica

hybrid coatings using basic-catalysed particulate sols de-

posited by dipping and EPD in order to increase the

corrosion resistance of stainless steel substrates. The stabil-

ity of the sols and the processing parameters of dipping and

EPD were studied. The corrosion resistance of the coatings

has been evaluated through electrochemical methods (linear

voltametry and polarisation resistance) as a function of

different processing parameters: viscosity and concentration

of the sol, final thickness of the coating, applied current

density, and deposition time.

2. Experimental

The starting sol was prepared under basic catalytic con-

ditions following the procedure described by Jonschker et al.

[12], where ethyltriethoxysilane (TEOS) and methyltriethox-

ysilane (MTES) were mixed with NaOH. The molar ratio

TEOS/MTES/NaOH and H2O/MTES+TEOS were 0.3/1.2/

0.2 and 1.18, respectively, and the final concentration of

silica was 267 g/l. The temperature of the synthesis was

maintained below 25 jC in order to avoid gelation. The

synthesis was performed under stirring, reflux and with N2

flow, to maintain the sol refrigerated and to preserve it from

the contact with open atmosphere. The concentrated sol,

called NaSi, was diluted by adding ethanol up to SiO2

concentrations of 210, 200, 188, 150 and 85 g/l. The size

of the particles produced during the hydrolysis and conden-

sation of alkoxides is below 20 nm.

The stability of 267 and 188 g/l NaSi sols was evaluated

by rheological measurements in closed conditions at 5 jC,using a rotational rheometer (Haake, RS50, Germany) under

controlled rate conditions. The shear rate was increased

from 0 to 1000 s�1 in 5 min, with 1 min at the maximum

rate and decreasing again to 0 in 5 min.

Coatings were deposited by dipping from 267, 210 and

188 g/l sols onto glass slides and polished stainless steel

(AISI 304) substrates, varying the withdrawal rate from 9 to

53 cm/min. The glass substrates were cleaned in an ultra-

sonic bath with absolute ethanol for 15 min. The stainless

steel was unsoiled using an alkaline commercial solution

(P3Emalan5668: P3Emalan0469, Miele, Germany) for 5 min

and subsequently washed with deionised water at 60 and

25 jC.A double coating was prepared in a two-step process

using NaSi 188 g/l with intermediate heat treatment at 500

jC for 30 min, the same applied after the second coating

deposition.

EPD coatings were deposited in galvanostatic conditions

using a potentiostate–galvanostate (AMEL 551, Italy).

Stainless steel AISI 304 substrate was used as working

electrode and graphite as counter-electrode. An electropho-

retic cell was designed consisting on a methacrylate contain-

er with a trap of the same material, which prevents contact

with atmosphere. All the system was maintained at 5 jC.

EPD parameters were studied in the light of Hamaker

equation (Eq. 1), according to which the deposited mass (m)

depends linearly on particle concentration (C), electrode

area (S), electric field (E), deposition time (t), and electro-

phoretic mobility (lE) [13].

m ¼ CSE tlE ð1Þ

EPD kinetics was studied for a constant current density

of 0.6 mA/cm2 and deposition times up to 60 min. Coatings

with a sintered thickness of 7 Am were produced by varying

the current density from 0.6 to 3.3 mA/cm2 and the

deposition time up to 35 min.

Coatings produced by either dipping and EPD were dried

at room conditions and sintered at 500 jC for 30 min in air,

with cooling and heating rates of 8 jC/min.

The thickness of coatings was measured on glass sub-

strates with a profilometer (Talystep, Taylor Hobson, UK),

while that of coatings onto stainless steel were calculated by

gravimetry and confirmed by scanning electron microscopy

(Zeiss DSM 950). Atomic force microscopy (AFM) (Nano-

tech, Spain) was used to characterise the surface texture of

the coatings.

The protective behaviour against corrosion was studied

by potentiodynamic polarisation in 0.6 N NaCl, with a

testing area of 1 cm2, without stirring. The tests were

controlled by a potentiostatic–galvanostatic equipment

(AMEL 256, Italy), using a potential sweep rate of 166

AV/s and a potential range of �500 to 1500 mV. Before

starting the measurement, the potential of open circuit was

measured until no changes were registered (approximately

30 min). The corrosion kinetics was studied through

polarisation resistance [14,15] in sea water and 1 N HCl.

The potential was varied from �20 to +10 mV with

respect to the corrosion potential and the potential sweep

rate was fixed to 200 AV/min. The evolution of the

polarisation resistance of coatings produced by dipping

was measured for immersion times varying from 0 to 1300

h of dipping. Saturated calomel (SCE) was used as a

reference electrode and platinum as a counter-electrode in

both experiments.

3. Results and discussion

3.1. Characterisation of particulate sols and coatings

The rheological characterisation of particulate NaSi

sols 188 and 267 g/l shows that they behave as New-

tonian fluids, with initial viscosities of 4 and 8.5 mPa s,

respectively [16]. Table 1 shows the rheological parame-

ters of fresh 267, 210 and 188 g/l sols, and of 267 g/l sol

after 1 and 2 h ageing, as well as the thickness of the

coatings obtained at different withdrawal rates: 53 (max-

imum rate of the lab lift used), 40, and 10 cm/min. The

viscosity of NaSi 267 g/l increases from 8.5 to 20 mPa s

Fig. 1. EPD kinetics at current density of 0.6 mA/cm2 for NaSi 267, 200,

150 and 188 g/l.

Table 1

Rheological properties of NaSi sols and characteristics of the coatings

NaSi sol Ageing Viscosity Stability Withdrawal rate (cm/min)

concentration (h) 5 jC (mPa s) (h) 5 jC10 40 53

(g/l)Thickness (Am)

267 Fresh 8.5 3 2.8 3.5 4.3

1 h 13 3.0 –

2 h 20 3.2 – –

210 Fresh – 1.6 2.1 2.6

188 Fresh 4 96 0.9 1.9 2.2

Fig. 2. Polarisation curves in 0.6 N NaCl of uncoated AISI 304 and

coatings with thickness 1.3, 2.3 and 3.3 Am obtained by dipping using the

NaSi 267 g/l.

Y. Castro et al. / Surface & Coatings Technology 191 (2005) 228–235230

in 2 h, indicating that the structure of the as-prepared sol

(267 g/l) develops quickly towards gelation, even when

maintained at temperatures below 5 jC in tight condi-

tions. Dilution retards the gelling process and the 188 g/

l sol maintains a constant viscosity of f4 mPa s for

around 96 h.

Dip-coatings were prepared on glass slides from NaSi

267, 210 and 188 g/l at different withdrawal rates (Table

1), but the critical thickness was not reached at any

concentration. The maximum thickness obtained after

sintering for the 267 g/l sol was 4.3 Am, but the coatings

present defects, as drips and edge effects, associated to

the high withdrawal rate (53 cm/min). When this velocity

is limited to 35–40 cm/min the maximum defect-free

thickness attainable by dipping is 3.5 Am. The defects

associated to high withdrawal rates are also evident for

coatings obtained from the 210 and 188 g/l sols, which

present a maximum defect-free thickness of 2.1 and 1.9

Am, respectively.

To obtain homogeneous coatings thicker than 2 Am with

diluted NaSi 188 g/l, a two-step dipping process was used,

obtaining a two-layer coating of 2.5 Am.

EPD tests were performed on AISI 304 stainless steel

substrates using a current density of 0.6 mA/cm2 for

deposition times up to 60 min, using the concentrated

and diluted sols. Since anodic deposition takes place, no

H2 bubbling is observed. A critical thickness, defined as

the maximum crack-free sintered thickness attainable, of

12 Am was reached after deposition times of 5, 25, 30 and

60 min for sol concentrations of 267, 200, 188 and 150 g/

l, respectively. Fig. 1 plots the variation of weight per unit

area with deposition time, for sols with different concen-

tration. As expected, the coating grows faster with in-

creasing concentration. However, deposition kinetics

shows a change of slope for all the SiO2 concentrations;

the higher the concentration, the shorter is the time at

which this change takes place. The concentration of solids

is one of the most critical parameters of the EPD process,

since there is a local increase of particles concentration in

the vicinity of the working electrode which allows the

coating formation, according to the Nicholson–Sarkar

model [11]. On the other hand, the sol concentration is

critical for the gelation process, as concluded from the

rheological study.

3.2. Electrochemical characterisation of coatings produced

by dipping

The protective character of the hybrid silica coatings was

determined by electrochemical measurements. Fig. 2 shows

the polarisation curves of coatings of different thickness,

(1.3, 2.3 and 2.8 Am) prepared by dipping from NaSi 267 g/

l, compared with the uncoated steel. Table 2 summarises the

parameters extracted from polarisation curves: breakdown

potential (Ep), passive region (DE) and passive current

density (ipass). These characteristic parameters are indicated

for the uncoated steel in Fig. 2.

The thinnest coating (1.3 Am) shows a cathodic current

density lower than that of the uncoated steel; but the passive

region, DE, is not well defined and the values of the

corrosion and the breakdown potentials are �0.082 and

0.108 V, respectively. This behaviour is associated to the

presence of defects such as micro-pores, which promote a

fast increase of the current density.

Fig. 4. Polarisation curves for dipping coatings obtained from 267, 210 and

188 g/l sols prepared with a similar thickness and compared with the

uncoated steel.

Table 2

Electrochemical parameters of dip-coatings prepared from NaSi 267 g/l

Coating thickness (Am) Ep (V) DE (V) ipass (A/cm2)

NaSi 1.3 Am 0.108 0.19 8.3�10�9

NaSi 2.3 Am 0.836 1.01 9.3�10�10

NaSi 2.8 Am 1.4 1.45 5.0�10�10

Uncoated steel 0.63 0.69 1.8�10�7


The decrease of the passive current density from 10�7 to

10�10 A/cm2 for coatings thicker than 2 Am indicates an

important enhancement of the corrosion resistance, confirm-

ing that the breakdown potentials are much higher than

those of the uncoated steel. The passive range increases with

the coating thickness, achieving 1 and 1.4 V for coatings

with 2.3 and 2.8 Am, this also indicating better corrosion

behaviour. Although breakdown potentials >1 V can be

associated to the gas evolution (mainly O2), promoted by

the water decomposition that takes place at pH 7 for

potentials around 1 V, according to the Pourbaix diagram

of the thermodynamic stability of H2O. These results show

that coatings thicker than 2 Am behave as physical barriers,

preventing the electrochemical degradation of the substrates.

The effect of sol ageing on the protective behaviour of

the dip-coatings was also evaluated. Fig. 3 shows the

polarisation curves for coatings of similar thickness (f3

Am) prepared from different aged sols, compared with the

uncoated steel. The passive region decreases from 1.4 to 0.3

V after 1 h ageing, and tends to zero after 2 h, while the

viscosity of the sol evolves from 8.5 mPa s to 13 and 20

mPa s. The poorer protecting behaviour with ageing is

directly related to the viscosity increase. Indeed, it has been

reported that ageing promotes growth and agglomeration of

particles in the sol leading to porous films [17].

The effect of sol dilution on the corrosion resistance was

evaluated on dip-coatings thicker than 2 Am. Fig. 4 shows

the electrochemical behaviour of dip-coatings obtained from

NaSi 267 g/l and diluted sols (monolayer from 210 g/l and

two-layer coating with 188 g/l) compared with the uncoated

Fig. 3. Polarisation curves of uncoated steel and coatings obtained by

dipping using the 267 g/l as a function of viscosities.

steel. The dilution induces remarkable changes in the

potentiostatic polarisation curves. Although all the coatings

have roughly the same thickness, the passive range clearly

decreases as dilution increases, thus indicating a faster

degradation of the film. This suggests that coating porosity

likely increases with dilution.

The origin of defects with ageing or dilution of NaSi sols

could be related to the structure of these particulate sols.

During ageing, the growth of nanoparticles leads to inter-

stitial pores in the gel structure. If the final thickness is low,

the electrolyte can cross the coating through the pores

reaching the substrate. When dilution increases, the longer

drying time and the lower volume fraction of particles

produce thinner and more porous coatings.

3.3. Electrochemical characterisation of coatings produced

by EPD

Fig. 5 shows the polarisation curves of 7 Am coatings

obtained by EPD from the 188 g/l sol, compared with the

Fig. 5. Polarisation curves for EPD coatings of 7 Am obtained at different

current densities from 188 g/l sol compared with the uncoated steel.

Fig. 6. Polarisation curves for EPD coatings obtained at current density of 1

mA/cm2 for NaSi 188 g/l as a function of thickness.


uncoated steel. In order to reach the same final thickness,

the deposition time was shorter as the current density

increased. For current densities of 0.6, 1.5 and 3.3 mA/

cm2, deposition times of 35, 3 and 1 min, respectively, were

needed. The poor definition of the corrosion potentials is

due to the noise registered by the equipment as they are

close to the detection limits (10�12 A). Table 3 shows the

deposition time and the passivity current density (ipass)

determined from the polarisation curves. Significant differ-

ences appear among coatings of similar thickness prepared

at increasing current densities for decreasing deposition

times. The presence of the coating improves the corrosion

resistance of the steel for EPD current densities V1.5 mA/

cm2. The passive current density falls down in nearly four

orders of magnitude, from 10�7 to 10�11 A/cm2 and Epitt for

coated samples exceeds 1 V, much higher than Epitt of the

uncoated steel, suggesting a high coating density and the

absence of defects.

The coating obtained applying a higher current density

(3.3 mA/cm2) shows a worse electrochemical behaviour,

with a small passive range and lower pitting potential,

suggesting the presence of defects and pores that allow

the electrolyte diffusion, thus promoting the failure of the

protective barrier. The basic character of the sol implies

that the particles are negatively charged and thus the

deposition takes place at the anode. So the stainless steel

is under corrosion during the coating process. When a

high current density is applied, Fe2+ ions formed by

substrate oxidation can migrate or diffuse through the

coating [18].

The voltametric tests allow determining the maximum

EPD current density that can be applied for obtaining

defect-free coatings. The optimum conditions are reached

for a current density of 1.5 mA/cm2 where a 7 Am coating is

obtained in 3 min.

Another parameter that determines the corrosion behav-

iour is the coating thickness. Coatings with thickness

between 1 and 8 Am were prepared by EPD at a current

density of 1 mA/cm2 for deposition times up to 35 min

using the diluted 188 g/l sol. Fig. 6 shows the electrochem-

ical behaviour of these coatings. There is a minimum

thickness (f2.8 Am) for the coating to be protective, the

corrosion current density decreasing four orders of magni-

tude from 2.6 10�7 to 4 10�11 A/cm2.

Table 3

Electrochemical parameters of 7 Am coatings prepared by EPD from NaSi

188 g/l

Samples Deposition ipass (A/cm2)

i (mA/cm2)time (min)

0.8 16 9.0�10�11

1 9 7.0�10�11

1.5 3 9.7�10�11

3.3 1 1.1�10�10

Uncoated AISI 304 – 1.8�10�7

The corrosion kinetics was evaluated through the mea-

surement of the polarisation resistance (PR) evolution in two

aggressive electrolytic media: 0.6 N NaCl, simulating the sea

water conditions, and strongly acid 1 N HCl (pH=0.5). Fig.

7a and b shows the evolution of the PR with time in both

electrolytic media for a 7 Am EPD coating compared with the

uncoated steel. The PR of the uncoated steel immersed in

NaCl maintains constant and equal to 106 V�cm2 during the

Fig. 7. Polarisation resistance versus immersion time for uncoated steel and

a 7 Am coating obtained by EPD with the 188 g/l sol in 0.6 N NaCl (a) and

1 N HCl (b).


entire test (5000 h) due to the uniform dissolution of the

metal. The dashed line indicates that the PR of the coated

steel maintains over 1012 V�cm2, the detection limit of the

equipment, for more than 2 months of immersion, confirm-

ing that no defects appeared in the coating during this time,

and verifying that the coating acts as a stable protective

barrier, against the corrosion of the metal substrate.

The corrosion behaviour of EPD coatings was also

studied in a much more aggressive corrosion medium (1

N HCl), Fig. 7b. In this case, the PR of the uncoated steel

falls down to 103 V�cm2 in less than 1 h, indicating the

rapid degradation of the substrate in this medium. Contrari-

ly, PR of the EPD-coated steel maintains over the detection

limit of the equipment for at least 200 h of immersion, then

decreasing slowly up to reach the PR of the uncoated steel

after 1000 h.

The stability of these coatings in acid media without

appearance of defects in strong aggressive conditions shows

the excellent behaviour of the hybrid EPD silica coatings.

3.4. Comparison of dipping and EPD coatings

In order to compare the efficiency of the two processes

for producing protective coatings, Fig. 8 shows the polar-

isation curves of 2.8 Am-thick coatings obtained by dipping

from NaSi 267 g/l and by EPD using the 188 g/l sol. The

electrochemical behaviour improves for both coating pro-

cedures. The passivation current density decreases by three

orders of magnitude for the dip-coating and by four orders

of magnitude for the EPD coating, compared to the uncoat-

ed steel, confirming that EPD coatings present an increased

protective character against corrosion. This better behaviour

could be associated with the improved packing behaviour of

the particles moving in an electric field, this likely leading

to a higher density of the EPD coatings.

Atomic force microscopy was used to characterise the

surface texture of the coatings, since it was not possible to

detect neither defects nor pores in the transparent glass-like

coatings by scanning electron microscopy. Fig. 9 shows the

Fig. 8. Polarisation curves of uncoated steel and coatings with a thickness

of 2.8 Am obtained by dipping and EPD using the 267 and 188 g/l sols,

respectively.

Fig. 9. AFM images of coatings obtained by dipping from 267 g/l sol (a),

188 g/l sol (b), and 188 g/l with double dipping (c).

Fig. 10. AFM images of coating obtained by EPD applying a current

density of 1.5 mA/cm2 for 3 min using the NaSi 188 g/l.


AFM images of the topography of the dip-coatings obtained

from NaSi 267 g/l sol (a), and 188 g/l (b) and the two-layer

coating (c) with thicknesses of 2.3, 1.9 and 2.5 Am, respec-

tively. AFM indicates that the dip-coating from fresh con-

centrated sol presents a homogeneous, pore-free and dense

surface (Fig. 9a), while the coatings prepared from the

diluted sol present a less smooth surface with apparent

porosity regions (Fig. 9b). This effect is highlighted in the

two-step dipping process (Fig. 9c). As explained above, a

higher content of solvent could lead to porous structures

because the solidification process occurs faster than conden-

sation reactions, likely originating a lower packing density

[19]. AFM observations suggest that dip-coatings deposited

from diluted particulate sols are not dense enough to prevent

the contact between the substrate and the electrolyte.

On the other hand, the topographic analysis of the coat-

ings obtained by EPD applying 1.5 mA/cm2 for 3 min from

a diluted 188 g/l sol illustrate a dense and homogeneous

texture (Fig. 10). Electrochemical measurements and AFM

analysis show that dense coatings are obtained by EPD even

using the diluted sol. This represents a clear advantage

respecting to dip-coating method, since EPD allows the

dilution of the sol, which largely increases the sol stability

and simplifies processing.

4. Conclusions

Homogenous and cracks-free hybrid silica coatings have

been obtained by dipping and by electrophoretic deposition

(EPD) from a base catalysed sol (NaSi) with initial SiO2

concentration of 267, and 210 and 188 g/l after dilution with

ethanol.

Homogeneous defect-free coatings with a maximum

thickness of 3.5 Am were prepared by dipping. Thicker

coatings have been obtained by EPD, where a critical

thickness around 12 Am was reached after sintering at

500 jC.Coatings produced by dipping improve the corrosion

resistance of the steel when surpass 2 Am. For achieving

this thickness, it is necessary to use the concentrated sol,

which is highly unstable and ages in a few hours, thus

leading to non-homogeneous or defective coating.

EPD allows producing thicker coatings with enhanced

corrosion resistance, even when the sol is previously dilut-

ed. This facilitates processing and extends the stability of

the sol over much longer times.

The protective character of EPD coatings depends on the

electrical conditions imposed during forming. Current den-

sities higher than 1.5 mA/cm2 lead to corrosion of the metal

substrate used as electrode, which has a deleterious effect on

the coating properties. The measurement of corrosion ki-

netics from polarisation resistance tests demonstrates that a

7 Am-thick SiO2 coating obtained by EPD from 188g/l sol

acts as an effective barrier against corrosion in sea water and

1 N HCl.

AFM studies suggest a negative effect of dilution on

dipping, while the surface texture and the packing density of

EPD coatings appear uniform even after dilution.

Acknowledgements

This work has been supported by projects MAT2003-

05902-C02-01 (CICYT, Spain) and EC BRITE

Programme (BE97-5111) in collaboration with INM

(Germany), Miele (Germany), Corus (The Netherlands),

Ferro (France) and ABB (France). Dr. Mennig and Dr.

Niegisch (INM) are gratefully acknowledged for sol

processing.

Dr. Ferrari acknowledges CSIC and the European Social

Foundation for its financial support. Authors acknowledge

to the New Microscopies Laboratory (UAM) for their

collaborations to the performance of AFM studies.

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