cerium ftir spectra

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RESEARCH PAPER Release studies of corrosion inhibitors from cerium titanium oxide nanocontainers Evaggelos D. Mekeridis Ioannis A. Kartsonakis George S. Pappas George C. Kordas Received: 19 March 2010 / Accepted: 7 July 2010 / Published online: 23 July 2010 Ó Springer Science+Business Media B.V. 2010 Abstract Cerium titanium oxide nanocontainers were synthesized through a two-step process and then loaded with corrosion inhibitors 2-mercaptobenzothi- azole (2-MB) and 8-hydroxyquinoline (8-HQ). First, polystyrene nanospheres (PS) were produced using polymerization in suspension. Second, the PS spheres were coated via the sol–gel method to form a cerium titanium oxide layer. Finally, the nanocontainers were made by calcination of the coated PS nanospheres. The size of the containers was 180 ± 10 nm as deter- mined by Scanning Electron Microscopy (SEM). X-Ray Diffraction Analysis (XRD) showed that the nanocontainers consist of anatase and cerianite crystalline phases. The presence and loading of the inhibitors in the nanocontainers was confirmed with Fourier Transform Infrared Spectroscopy (FT–IR) and Thermo Gravimetric Analysis (TGA), respectively. TGA revealed the amount of 10.43 and 4.61% w/w for 2-MB and 8-HQ in the nanocontainers, respectively. Furthermore, the release kinetics of the inhibitors from the nanocontainers was studied in corrosive environ- ment using electrochemical impedance spectroscopy (EIS) in the presence of aluminum alloys 2024-T3 (AA2024-T3). Keywords Nanocontainers Corrosion Inhibitors Synthesis Drug delivery Introduction Nanosized materials have been a subject of intensive investigations in variety of topics from optics and electronics to biotechnology and medicine (Zheng et al. 2006; Ding et al. 2006; Pappas et al. 2008). Materials such as nanoparticles, nanospheres, and micelles can be used as drug delivery and drug- controlled release systems. Hollow nanocontainers are of great interest because of their ability to encapsulate substances in their hollow inner cavities and release them at a later stage (Hu et al. 2005). Recently, it has been recognized that the nanocon- tainers loaded with corrosion inhibitors when incor- porated into coatings provide additional protection of metal alloys, such as AA2024-T3, from corrosion. AA2024-T3 is mainly used in aeronautical applica- tions. The methods of delivery of the inhibitors to the metal surface can influence the efficiency of the inhibiting action (Lamaka et al. 2007). According to Raps et al. (2009) and Khramov et al. (2004) the addition of corrosion inhibitors in sol–gel coatings in one hand can improve corrosion protection, on the other hand may deteriorate the barrier properties of the film. Encapsulating corrosion inhibitors in nanocon- tainers and then added them to the protective coating system is an advantageous method to unite the barrier properties of the coatings with the active action E. D. Mekeridis I. A. Kartsonakis G. S. Pappas G. C. Kordas (&) Sol-Gel Laboratory, Institute of Materials Science, NCSR ‘‘DEMOKRITOS’’, 153 10 Agia Paraskevi Attikis, Greece e-mail: [email protected] 123 J Nanopart Res (2011) 13:541–554 DOI 10.1007/s11051-010-0044-x

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Page 1: Cerium FTIR Spectra

RESEARCH PAPER

Release studies of corrosion inhibitors from ceriumtitanium oxide nanocontainers

Evaggelos D. Mekeridis • Ioannis A. Kartsonakis •

George S. Pappas • George C. Kordas

Received: 19 March 2010 / Accepted: 7 July 2010 / Published online: 23 July 2010

� Springer Science+Business Media B.V. 2010

Abstract Cerium titanium oxide nanocontainers

were synthesized through a two-step process and then

loaded with corrosion inhibitors 2-mercaptobenzothi-

azole (2-MB) and 8-hydroxyquinoline (8-HQ). First,

polystyrene nanospheres (PS) were produced using

polymerization in suspension. Second, the PS spheres

were coated via the sol–gel method to form a cerium

titanium oxide layer. Finally, the nanocontainers were

made by calcination of the coated PS nanospheres.

The size of the containers was 180 ± 10 nm as deter-

mined by Scanning Electron Microscopy (SEM).

X-Ray Diffraction Analysis (XRD) showed that

the nanocontainers consist of anatase and cerianite

crystalline phases. The presence and loading of the

inhibitors in the nanocontainers was confirmed with

Fourier Transform Infrared Spectroscopy (FT–IR) and

Thermo Gravimetric Analysis (TGA), respectively.

TGA revealed the amount of 10.43 and 4.61% w/w for

2-MB and 8-HQ in the nanocontainers, respectively.

Furthermore, the release kinetics of the inhibitors from

the nanocontainers was studied in corrosive environ-

ment using electrochemical impedance spectroscopy

(EIS) in the presence of aluminum alloys 2024-T3

(AA2024-T3).

Keywords Nanocontainers � Corrosion �Inhibitors � Synthesis � Drug delivery

Introduction

Nanosized materials have been a subject of intensive

investigations in variety of topics from optics and

electronics to biotechnology and medicine (Zheng

et al. 2006; Ding et al. 2006; Pappas et al. 2008).

Materials such as nanoparticles, nanospheres, and

micelles can be used as drug delivery and drug-

controlled release systems. Hollow nanocontainers

are of great interest because of their ability to

encapsulate substances in their hollow inner cavities

and release them at a later stage (Hu et al. 2005).

Recently, it has been recognized that the nanocon-

tainers loaded with corrosion inhibitors when incor-

porated into coatings provide additional protection of

metal alloys, such as AA2024-T3, from corrosion.

AA2024-T3 is mainly used in aeronautical applica-

tions. The methods of delivery of the inhibitors to the

metal surface can influence the efficiency of the

inhibiting action (Lamaka et al. 2007). According to

Raps et al. (2009) and Khramov et al. (2004) the

addition of corrosion inhibitors in sol–gel coatings in

one hand can improve corrosion protection, on the

other hand may deteriorate the barrier properties of the

film. Encapsulating corrosion inhibitors in nanocon-

tainers and then added them to the protective coating

system is an advantageous method to unite the barrier

properties of the coatings with the active action

E. D. Mekeridis � I. A. Kartsonakis �G. S. Pappas � G. C. Kordas (&)

Sol-Gel Laboratory, Institute of Materials Science, NCSR

‘‘DEMOKRITOS’’, 153 10 Agia Paraskevi Attikis,

Greece

e-mail: [email protected]

123

J Nanopart Res (2011) 13:541–554

DOI 10.1007/s11051-010-0044-x

Page 2: Cerium FTIR Spectra

of the corrosion inhibitors (Kartsonakis et al. 2007;

Zheludkevich et al. 2005a, 2007a).

Kartsonakis et al. synthesized hollow titania spheres

using cationic polystyrene lattices which were pre-

pared by polymerization in suspension of styrene using

2.20-azobis (2-methylpropionamidine) dihydrochlo-

ride (AMPA) as an initiator (Kartsonakis et al. 2008).

Zirconia and silica nanoparticles were used as reser-

voirs for the storage and prolonged release of corrosion

inhibitors in silica and silica–zirconia based coatings

by Zheludkevich et al. (2005a, 2007b) and found that

the nanoparticles reinforced the coating and released

inhibitors during contact with moisture. Shchukin et al.

presented halloysite nanotubes with inner voids loaded

by 2-MB and outer surfaces layer-by-layer covered

with polyelectrolyte multilayers as a mean to opti-

mize hybrid sol–gel films (Shchukin et al. 2008).

Cerium molybdate nanocontainers were synthesized

and loaded with corrosion inhibitors (8-HQ and 1-H-

benzosulfonic acid) by Kartsonakis and Kordas (2009)

Some of the most effective and environmental

friendly corrosion inhibitors for aluminum alloys are

derived from cerium salts. Nanostructured sol–gel

coatings doped with cerium ions were investigated as

pretreatments for AA2024-T3 (Zheludkevich et al.

2005b). Titania-containing organic–inorganic hybrid

sol–gel films have been developed by Poznyak et al.

as an alternative to chromate-based coatings for

corrosion protection of aluminum alloys (Poznyak

et al. 2008).

8-HQ and 2-MB compounds were studied as

corrosion inhibitors by Lamaka et al. (2007) for

AA2024-T3. They found that these inhibitors provide

anticorrosion protection for AA2024-T3 forming a thin

organic layer of insoluble complexes on the surface of

the alloy. Inhibiting action is the consequence of

suppression of dissolution of Mg, Al, and Cu from the

corrosion active intermetallic zones (Lamaka et al.

2007; Sanyal 1981; Zheludkevich et al. 2007c).

Yasakau et al. examined the addition of 8-HQ at

different stages of the synthesis process to understand

the role of possible interaction of the inhibitor with the

components of the sol–gel system (Yasakau et al.

2008). 2-MB was evaluated by Zheludkevich et al.

(2005c) as corrosion inhibitor for protection of

AA2024-T3 in neutral chloride solutions.

Otsuka-Yao-Matsuo et al. (2004) studied the

photocatalytic behavior of CeTiO4 and CeTi2O6

powders. Keomany et al. prepared thin films of

(CeO2)x–(TiO2)1-x by a sol–gel process involving

two alcoxides (Ce(OBuS)4 and Ti(OBun)4 in BuOH)

were studied by cyclic voltammetry in a lithium-

conducting polymer electrolyte in order to examine

the influence of the structure on the electrochemical

insertion in such films, which are suitable counter

electrode materials for lithium-based electrochromic

windows (Keomany 1995).

In the present study, cerium titanium oxide hollow

nanospheres were synthesized and characterized by

SEM, XRD, TGA, and FT-IR. After that, these

nanocontainers were loaded with corrosion inhibitors

2-MB and 8-HQ to produce an inhibitor delivery

system. Studies were made on the % w/w loading of

the inhibitors by heat treatments and FT-IR spectros-

copy. The release of the inhibitors in a corrosive

environment was tested via EIS. The results suggest

the use of these loaded nanocontainers into coatings

on metal alloys for corrosion protection of metals

used for automobiles, ships, and airplanes.

Experimental

Materials and reagents

All chemicals were of analytical reagent grade.

Titanium tetraisopropoxide (TTIP, Aldrich), polyvi-

nylpyrrolidone (PVP, average molecular weight:

55,000, Aldrich), cerium (III) acetylacetonate (Ce

(acac)3, Aldrich), sodium chloride (NaCl, Aldrich),

sodium hydroxide (Aldrich), 2,20-Azobis (2-methyl-

propionamidine) dihydrochloride (AMPA, Aldrich),

and absolute ethanol (Aldrich), were used without

further purification. Styrene (Aldrich) was double

distilled under reduced pressure prior to use.

Preparation of nanocontainers

Cerium titanium oxide hollow nanospheres were

synthesized through a three-step process. The first

step involves the preparation of positive charged

polystyrene nanospheres. Styrene was polymerized by

polymerization in suspension according to the condi-

tions shown in Table 1. The polymerization process is

described in our previous study (Kartsonakis et al.

2008). In order to eliminate the result of oxygen effect,

the reactions were made in nitrogen atmosphere.

542 J Nanopart Res (2011) 13:541–554

123

Page 3: Cerium FTIR Spectra

Polymerization lasted 12 h. The solution was centri-

fuged and the precipitate was washed with distilled

water. During the second step, the PS nanospheres

were coated via sol–gel method. Sol–gel coating were

prepared with controlled hydrolysis of the alcoholic

solution of TTIP jai Ce(acac)3 in the presence of PS

nanospheres, NaCl, and PVP (Table 2). PVP and NaCl

were added to the mixture reaction to prevent aggre-

gation of the core particles. The positive charged

polystyrene reacts with the negative charged product

of the hydrolysis of TTIP and Ce(acac)3. Monomers or

oligomers of hydrolyzed TTIP and Ce(acac)3 are

condensed on the surface of the polystyrene. Aging of

the solutions at 60 �C, centrifugation, and washing of

the coated nanospheres were followed. The formation

of hollow nanospheres was achieved after heat treat-

ments of the composites at 600 �C with heating rate

10 �C min-1, where the polystyrene cores were

burned off (Kartsonakis et al. 2007, 2008).

Encapsulation and release of inhibitors

The obtained cerium titanium nanocontainers were

loaded with the corrosion inhibitors 8-HQ and 2-MB.

The loading procedure included first the preparation

of a saturated solution of the inhibitor in acetone.

After that, an amount of cerium titanium oxide

nanocontainers was placed in a sealed container. The

nanocontainers were placed in a vacuum system to

draw out the air inside them. Then, the saturated

solution of the inhibitor in acetone was inserted in the

sealed container and the whole mixture was stirred at

room temperature for 12 h. Finally, the cerium

titanium oxide nanocontainers loaded with the inhib-

itor were collected through centrifugation and were

dried under vacuum overnight.

The release of 8-HQ and 2-MB from nanocon-

tainers was studied via EIS. A typical three electrode

cell was used in a Faraday cage. For this purpose,

solutions of 0.01, 0.05, and 0.1% w/v concentration

of nanocontainers loaded with inhibitors in a corro-

sive environment (0.05 M NaCl) were prepared.

Furthermore, solutions of pure inhibitors 8-HQ and

2-MB with the same concentration were also studied

for comparison reasons. Panels of AA2024-T3 were

used as the working electrode, a platinum sheet as the

counter while a saturated calomel electrode (SCE)

served as reference electrode. The panels had been

previously cleaned, under specific conditions. The

AA2024-T3 panel cleaning includes the insertion of it

into 2% w/w solution of NaOH for 3 min at 40 �C.

After that, the panel is rinsed with distilled water and

is inserted into 4.33 M solution of HNO3 for 1 min at

room temperature. Finally, it is rinsed with distilled

water. EIS measurements were taken after 3, 6, 24,

48, 72 h of exposure in 0.05 M NaCl solution. The

exposed geometric area was 2 cm2 for all the

experiments. All the samples were in vertical

position; the experiments were carried out at room

temperature. For every result, a minimum of three

repetition measurements were taken.

Instrumentation

The average nanocontainer size and the morphology of

the substrate after 72 h of exposition in NaCl, were

determined by SEM using a PHILIPS Quanta Inspect

(FEI Company) microscope with W (tungsten) filament

25 kV equipped with EDAX GENESIS (AMETEX

PROCESS & ANALYTICAL INSTRUMENTS). The

phase of the nanocontainers was examined by XRD

using a powder diffractometer (SIEMENS D-500

equipped with a Cu Ka lamp with wavelength

1.5418 A). Temperature treatments such as Thermo

Gravimetric Analysis (TGA) were made using a Perkin

Elmer (Pyris Diamand S II) analyzer at the heating rate

Table 1 The conditions used in the preparation of polystyrene

latex at 80 �C

Material Quantity (g)

Styrene 9.06

AMPA 1.3

Water 900

Nanospheres’ size (nm) 195 ± 10a

a Determined by scanning electron microscopy analysis

Table 2 Conditions of preparation of coated spheres

Material Quantity (g)

Ethanol (ml) 800

PVP (g) 8.0

NaCl 5 mM (ml) 20

Polystyrene (g) *9

TTIP (ml) 9.0

Nanospheres’ size (nm) 215 ± 10a

a Determined by scanning electron microscopy analysis

J Nanopart Res (2011) 13:541–554 543

123

Page 4: Cerium FTIR Spectra

of 10 �C min-1 in air. Fourier Transform Infrared

Spectroscopy (FT-IR) was made using a BRUKER

EQUINOX 55-S spectrometer. Nitrogen adsorption

experiments and pore size measurements were per-

formed using a volumetric static sorption apparatus

(Autosorb-1 MP, Quantachrome Instruments). The

release of inhibitors from nanocontainers was studied

via Impedance analyzer (Solartron Sl 1260 Impedance/

gain-phase analyzer) connected to a Solartron PGstat

(Solartron Sl 1470 Electrochemical interface).

Results and discussion

Scanning electron microscopy analysis

As shown in Fig. 1, the polymerization process leads

to polystyrene nanospheres with uniform size with an

average diameter of 195 ± 10 nm. Figure 2 shows

that after calcinations, the cerium titanium oxide

hollow nanospheres exhibit an average diameter of

180 ± 10 nm. The EDX analysis shows that tita-

nium, cerium, and oxygen constitute the spectrum of

the spheres. Gold appears due to the gold coating that

was applied to the spheres in order to be conductive

for the SEM analysis (Fig. 3).

FT-infrared spectroscopy analysis

Figure 4 shows the FT-IR spectrum of the nano-

spheres, before and after calcination. This spectrum

verifies the formation of inorganic shells and the

complete removal of the organic components. The

FT-IR spectrum in Fig. 4a of the nanospheres prior to

calcinations reveals well-defined bands of the phenyl

group (703, 750, 1445, 1494, and 3022 cm-1) in

polystyrene. The peak at 2,919 cm-1 is due to the

CH2 group. The peaks at 1590, 1150, and 1163 cm-1

are the band characteristic of PVP, indicating that

PVP has not been well removed during the experi-

mental process. It can be seen that the characteristic

peaks of polystyrene are missing from the spectrum

after calcination.

The spectra in Fig. 4b, is the one after calcination;

the band characteristic of the polystyrene latex have

been disappeared, indicating that polystyrene latex

has been well removed from the core/shell composite

particles by calcination at 600 �C.

Both FT-IR spectra for as-prepared and after

calcination samples show absorption peaks at the wave

number region between 400 and 1,000 cm-1. This

region contains bands typical of metal oxygen bonding.

The absorption peaks of TiO2 are at 470, 525, 540, 579,

690, 700, 790 cm-1 (Kartsonakis et al. 2008;

Zheludkevich et al. 2005c; Keomany 1995; Verma

et al. 2004; Mc Devitt and Baun 1964). For CeO2, the

characteristic peaks are at 425 cm-1, 525 cm-1,

540 cm-1 (Kartsonakis et al. 2007; Zheludkevich

et al. 2005c; Keomany 1995; Verma et al. 2004). It is

mentioned that the FTIR spectra after calcinationsFig. 1 SEM images of polystyrene nanospheres

Fig. 2 SEM images of cerium titanium oxide hollow

nanospheres

544 J Nanopart Res (2011) 13:541–554

123

Page 5: Cerium FTIR Spectra

depict broad band at the region between 400 and

1,000 cm-1 and the above peaks can be distinguished.

The broad bands in the range of 3,200–3,500 cm-1

and at 1,652 cm-1 correspond to stretching vibration

of O–H bond of the physically adsorbed water in the

sample (Verma et al. 2004).

Thermogravimetric and differential thermal

analysis

Figure 5 shows the TGA–DTA diagrams of cerium

titanium oxide nanocontainers. The first weight loss is

observed in the range of 30–150 oC which can be

attributed to desorption of physically adsorbed water

(free and physisorbed water) (Kartsonakis et al. 2007).

The second weight loss in the range of 150–230 oC can

be attributed to the chemisorbed water; the monolayer

of H2O molecules which directly interact with the

solid surface such as cerium and titanium cations and

hydroxyls and to the dehydroxylation (release of OH

from the structure) (Kartsonakis et al. 2007; Kartso-

nakis and Kordas 2009). It is observed from the TGA

diagram that polystyrene is burned off between 290

and 400 �C (the third sharp weight loss). The fourth

weight loss between 400 and 450 �C is attributed to

the burn off of polyvinylpyrrolidone (Jablonski et al.

2008). Hence, calcination at 600 �C in air removed the

polystyrene core particles completely.

DTA diagram shows an exothermic peak between

260 and 340 �C. This peak is due to the condensation

of hydroxyl groups. The exothermic peak between

365 and 420 �C is due to crystallization of amorphous

cerium and titanium oxides into crystalline (Raps

et al. 2009). This was confirmed by the XRD pattern

shown in Fig. 6 of the sample treated at 600 �C.

The sudden decrease of temperature at 400 �C is

due to the accuracy of the instrument and depends on

the heating rate (10 �C min-1) and the organic

content (PS) of the sample which are both very high.

X-ray diffraction analysis

Crystalline phases were identified according to the

JCPDS (Joint Committee on Powder Diffraction

Standards) file numbers 21-1271 and 43-1002 for

Fig. 3 EDX analysis of

cerium titanium oxide

hollow nanospheres

Fig. 4 FT-IR spectra of: a cerium titanium oxide nanospheres

(before calcination), b Cerium Titanium oxide hollow nano-

spheres (after calcination)

J Nanopart Res (2011) 13:541–554 545

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Page 6: Cerium FTIR Spectra

anatase and cerianite, respectively. Figure 6 shows

the XRD pattern of the sample after calcination. The

peaks at 2h = 25.5� (101), 38.1� (004), 48.2� (200),

55.15� (105), 55.05� (211), and 62.9� (204) represent

to tetragonal anatase. According to the JCPDS

Library, the above peaks are slightly moved to higher

values of 2h. The presence of cerium identified from

the main peak at 2h = 28.6� (111), which is charac-

teristic of cerianite, causes stress to the crystal

structure of anatase leading to the increase of peak

positions that mentioned above (Verma et al. 2007).

Porosity measurements

The samples of the empty nanocontainers were

degassed at 300 �C for 18 h before the measurement.

The specific area was calculated with the B.E.T

method in the range of relative pressure 0.05–0.3 P/P0

and was found to be 129 m2 g-1. The pore size

distribution was calculated through the B.J.H method

at desorption isotherm and the mean pore radius

found to be 1.6 nm with a pore volume 0.503 cc g-1.

The hysteresis through desorption (Fig. 8) is charac-

teristic for curves of type IV (IUPAC). This fact

indicates the presence of mesopores in the sample.

The observation of steps at the adsorption isotherm

clearly denotes the presence of different size of pores

in the sample. This result is both presented in the pore

size distribution diagram and in the axis of relative

pressure (through hysteresis that come near to 0.2)

(Figs. 7, 8).

The loaded nanocontainers with 8-HQ and 2-MB

were degassed at 25 �C for 18 h. The measurement of

the 2-MB-filled sample showed some interesting

results compared to the hollow nanospheres. The total

amount of the adsorbate is less in the filled sample. The

specific area decreased significantly from 129 m2 g-1

for the hollow nanospheres to 17 m2 g-1 for the

nanospheres filled with 2-MB. Also the pore size

distribution, calculated from desorption branch

through B.J.H method, showed a decrease in the pore

volume from 0.503 to 0.301 cc g-1 indicating the

filling of the pores and the hollow structure with the

inhibitor. The pore size distribution of the filled sample

did not show any pores with radius below 4 nm. The

measurement of the sample filled with 8-HQ was

impossible, due to evaporation of the inhibitor under

the preparation conditions (high vacuum, leak test of

the instrument).

Encapsulation and release of inhibitors

TGA diagrams of pure 8-HQ, 2-MB, and cerium

titanium oxide nanocontainers loaded with 8-HQ or

2-MB are shown in Fig. 9. Pure 8-HQ began to

degrade at 120 �C until 212 �C where no residue left.

The diagram of cerium titanium nanocontainers

loaded with 8-HQ shows a first weigh loss between

30 and 130 �C corresponding to acetone and physi-

cally adsorbed water (free and physisorbed water)

(Takeuchi et al. 2005), a second weight loss between

130 and 170 �C due to 8-HQ that is on the surface of

the nanocontainers and finally a third and forth weight

loss from 170 to 850 �C correspond to oxidative

degradation of encapsulated 8-HQ. Pure 2-MB is

completely burned off between 180 and 330 �C. The

diagram of loaded nanoconainers with 2-MB depicts a

sharp weight loss from 200 to 330 �C corresponding to

Fig. 5 TGA and DTA curves of cerium titanium oxide hollow

nanospheres

Fig. 6 XRD pattern of cerium titanium oxide hollow

nanospheres

546 J Nanopart Res (2011) 13:541–554

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Page 7: Cerium FTIR Spectra

the oxidative degradation of the inhibitor that is on the

surface of the nanocontainers, a second weight loss

between 330 and 850 �C due to the oxidative degra-

dation of the inhibitor that are enclosed into the

nanocontainers.

Comparing the TGA diagrams, it is observed that

pure 8-HQ is burned off at higher temperatures in the

samples of cerium titanium nanocontainers loaded

with this inhibitor. This retardation (roughly 200 �C

higher than pure inhibitor) is attributed to the

protection provided by the shell of nanocontainers.

This result indicates that 8-HQ is encapsulated into

nanocontainers. The same analysis can be made for

the nanocontainers loaded with 2-MB. The burn off

2-MB between 330 and 850 �C corresponds to

inhibitor that is inside the shell of the nanocontainers.

The weight losses observed by the TGA measure-

ments were used to determine the amount of inhibitors

loaded into the nanocontainers. First, we consider the

sample of nanocontainers loaded with 8-HQ. We take

the weight of the sample at 170 �C (G1) and at 850 �C

(G2), we then subtract G2 from G1 (DG = G1 - G2)

and last we divide DG by G1 to obtain the ratio mass

loss, rm = DG*100/G1 due to 8-HQ. In the case of

Ce–Ti nanocontainers loaded with 8-HQ, rm is about

4.37% w/w. In the case of Ce–Ti nanocontainers

loaded with 2-MB, we take G1 at 330 �C because the

2-MB burns off at this temperature. Above this

temperature, the weight loss corresponds to mass

encapsulated in the nanocontainer. G2 is taken at

900 �C. We estimate rm about 25.36% w/w for the

Ce–Ti nanocontainers loaded with 2-MB.

The release of the inhibitors was studied via EIS. If

the inhibitors are released from the nanocontainers,

they should provide corrosion protection of the

AA2024-T3 panels. This can be observed by EIS.

The first experiment was accomplished using pure

inhibitors in the solution (e.g. 2-MB 0.1% w/v in a salt

solution) in order to clarify the extend of corrosion

protection attributed to the inhibitors (Figs. 11, 13).

The second experiment was carried out using filled

nanocontainers with inhibitors (8-HQ Fig. 11, 2-MB

Fig. 13) in order to observe the effect of protection due

to the inhibitor release from the nanocontainers. The

third experiment was done using different amounts of

loaded nanocontainers in the solution (e.g. 0.01, 0.05,

and 0.1% w/v) shown in Figs. 10 and 12 for 8-HQ and

2-MB, respectively. One can perceive an influence of

the concentration on the corrosion protection attrib-

uted to the release of the inhibitors from the

nanocontainers. The best results against corrosion

were obtained for 0.1% w/v, and correspond to the

Fig. 7 BJH pore

distribution of cerium

titanium oxide hollow

nanospheres

Fig. 8 Isotherms of cerium titanium oxide hollow nanospheres

and loaded nanospheres with 2-MB

J Nanopart Res (2011) 13:541–554 547

123

Page 8: Cerium FTIR Spectra

protection provided by the same concentration of pure

inhibitor (0.1% w/v of 8-HQ and 2-MB). According to

TGA measurements, the concentration of 2-MB in

0.1% w/v nanocontainers is 86.1 mM and the con-

centration of 8-HQ is 17.2 mM. The impedance at low

frequencies corresponds to the polarization resistance

of the AA2024-T3 electrode and, therefore, can be

used to estimate the corrosion protection (Lamaka

et al. 2007). In the low frequency region, it can be seen

that the total value of impedance is about one order of

magnitude higher for the specimens immersed in the

NaCl solution containing the nanocontainers loaded

with inhibitors (Figs. 10, 11, 12, 13). It can be clearly

seen that both chemical compounds worked as

Fig. 9 TGA curves of:

pure 8-HQ, pure 2-MB, and

cerium titanium oxide

nanocontainers loaded with

8-HQ and 2-MB

Fig. 10 Bode diagrams of AA2024 after 72 h of immersion in

0.05 M NaCl with 0.01, 0.05, 0.1% w/v of nanocontainers

loaded with 8-HQ and without nanocontainers

Fig. 11 Bode diagrams of AA2024 after 72 h of immersion in

0.05 M NaCl with a 0.1% w/v of nanocontainers loaded with

8-HQ, b pure inhibitor 8-HQ, and c bare AA2024

548 J Nanopart Res (2011) 13:541–554

123

Page 9: Cerium FTIR Spectra

corrosion inhibitors comparing to the solution without

loaded nanocontainers. Two time constants revealed at

5 and 0.01 Hz for the solution with nanocontainers.

The higher frequency time constant can be assigned to

the capacitance of the double layer on the surface of

the alloy. The low frequency time constant is related

to a diffusion limitation of the corrosion process

(Lamaka et al. 2007).

Instead of capacitances, Constant Phase Elements

(CPE) were used in all fittings procedures because the

phase angle of the capacitor is different from -90�.

The impedance of the CPE depends on frequency

according to the following equation (Kartsonakis

et al. 2007; Zheludkevich et al. 2005b). The imped-

ance of the CPE depends on frequency according to

the following equation

1

Z¼ QðjxÞn ð1Þ

where Z is the impedance, Q a parameter equals to (1/

|Z|) at x = 1 rad s-1, x is the frequency and n B 1 a

power coefficient calculated as ratio of phase angle at

maximum of corresponding time constant to -90�.

The capacitance of the inhibitor is calculated by the

following equation:

Cinh ¼ QinhðxmaxÞðninh�1Þ ð2Þ

xmax is the frequency at which the imaginary

impedance reaches a maximum for the respective

time constant.

Rsol is the resistance of the solution, Rox is the

resistance of the native oxide layer, Rinh is resistance

of the inhibitor layer and Rpol is the polarization

resistance. Cox and nox are the parameters of constant

phase element (CPE) describing the capacitance of

the oxide layer, Cinh and ninh are the parameters of the

CPE describing the capacitance of the inhibitor layer.

Cdl and ndl are the parameters of CPE which

characterize the capacitance of the double layer

capacitance (Lamaka et al. 2007).

For the system with the nanocontainers, two time

constants are observed at high frequencies which

can be attributed to the native aluminum oxide layer

(at about 1 Hz) and to the layer of adsorbed

inhibitor (at about 50 Hz). In the case of 2-MB,

the time constant attributed to the presence of the

adsorbed layer is better observed at 72 h in NaCl

with 0.05 and 0.1% w/v nanocontainers. In the case

of 8-HQ, a wide time constant is observed consisted

of the two phases that are mentioned above.

Figure 14 shows the equivalent circuit used to fit

the experimental data. Tables 3 and 4 summarize

the parameters obtained after fitting.

Figures 15, 16, and 17 present the evolution of the

capacitance of the inhibitor layer as a function of the

time for different nanocontainer concentrations. One

Fig. 13 Bode diagrams of AA2024 after 72 h of immersion in

0.05 M NaCl with a 0.1% w/v of nanocontainers loaded with

2-MB, b pure inhibitor 2-MB, and c bare AA2024

Fig. 12 Bode diagrams of AA2024 after 72 h of immersion in

0.05 M NaCl with 0.01, 0.05, 0.1% w/v of nanocontainers

loaded with 2-MB and without nanocontainers

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Page 10: Cerium FTIR Spectra

can perceive a dependence of the capacitance from

the thickness of the layer that is formed from the

dielectric constant and the resistance of the inhibitor

after exposure of AA2024-T3 in 0.05 M NaCl

solution with nanocontainers (Lamaka et al. 2007).

It is observed that each inhibitor affects the time

evolution and the extend of the capacitance in a

different fashion. 2-MB loaded nanocontainers exhibit

much higher values than 8-HQ loaded nanocontainers

(Figs. 15, 16, 17). Furthermore, the parameters of

capacitance and resistance of 8-HQ do not change

significantly as the time goes by compared to 2-MB.

This may be attributed to the fast formation of a stable

layer of the inhibitor on the aluminum surface. A

prolonged decrease of the capacitance is observed for

the solution with 0.1% w/w concentration of nano-

containers which can be attributed to the increase of

thickness of the inhibitor layer. Both inhibitors form a

dense layer on the surface of the aluminum alloy

2024-T3.

Figures 15, 16, and 17 present the direct effect of

the concentration of the 8-HQ-loaded nanocontainers

to the resistance of the aluminum oxide layer, Rox.

Low values of Rox are obtained for concentration

0.01% w/w. The values of Rox at 0.1% w/w are one

order of magnitude higher than for concentration of

0.01% w/w. In the case of 2-MB loaded nanocon-

tainers, these shifts are also of an order of magnitude

with much higher absolute values, though. One can

highlight at this position, that the curves of the theta

versus frequency match for the inhibitor solution and

loaded nanocontainer solution. This observation is a

strong support of the assumption of the release of the

inhibitors from the nanocontainers. This result dem-

onstrates that the inhibitors are not trapped perma-

nently in the nanocontainers.

SEM images and optical observation

Figures 18 and 19 show an optical visualization of

the protection of the AA2024-T3 surface provided

by the loaded nanocontainers after exposure for 72 h

in 0.05 M NaCl solution. Figure 18 demonstrates

the degradation of the bare sample exposed in the

corrosive environment without the presence of

nanocontainers. One can perceive a fully corroded

sample with many pits. On the other hand, the

addition of the nanocontainers has decreased the

number of the pits on the surface of AA2024-T3.

This result is in agreement with the respective EIS

measurements.

As it was mentioned, the action of the nanocon-

tainers is based on the formation of chelate com-

plexes on the aluminum surface that are difficult

to be dissolved. This can be proved by the EIS

measurements with the appearance of two time

Fig. 14 Equivalent circuit used for fitting experimental EIS

spectra

Table 3 Calculated values for EIS data obtained at different

immersion times for NaCl solution with 0.01, 0.05 and 0.1% w/v

Ce–Ti oxide nanocontainers loaded with 2-MB

Parameter 0.01% 0.05% 0.1%

Rsol (X cm2) 124 132 130

Rinh (X cm2) 3,106 1,541 1,677

Qinh (S cm-2) 1.01E-4 5.84E-5 7.25E-5

Ninh 0.760 0.810 0.820

Cinh (F cm-2) 6.49E-5 3.61E-5 4.23E-5

Rox (X cm2) 1,310 5,612 8,307

Rpol (X cm2) 1,747 3,866 8,322

Table 4 Calculated values for EIS data obtained at different

immersion times for NaCl solution with 0.01, 0.05 and 0.1% w/v

Ce–Ti oxide nanocontainers loaded with 8-HQ

Parameter 0.01% 0.05% 0.1%

Rsol (X cm2) 141.4 145.8 131.4

Rinh (X cm2) 552.6 305.7 69.16

Qinh (S cm-2) 5.53E-4 5.67E-4 9.84E-5

Ninh 0.790 0.720 0.780

Cinh (F cm-2) 3.75E-4 2.45E-5 7.64E-5

Rox (X cm2) 554.8 351 1796

Rpol (X cm2) 1,015 1,414 8,204

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Fig. 15 Evolution of capacitance and resistance of inhibitor film after exposure for 72 h in 0.05 M NaCl solution containing Ce–Ti

nanocontainers loaded with 2-MB

Fig. 16 Evolution of resistance of aluminum oxide layer after exposure for 72 h in 0.05 M NaCl solution containing Ce–Ti

nanocontainers loaded with: 2-MB, 8-HQ

Fig. 17 Evolution of capacitance and resistance of inhibitor film after exposure for 72 h in 0.05 M NaCl solution containing Ce–Ti

nanocontainers loaded with 8-HQ

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constants for the solutions with the nanocontainers

assigned to the aluminum oxide layer and to the

adsorbed layer of the corrosion inhibitor or to the

products of the interaction of Al3?, Mg2?, or Cu2?

that are formed through the initial stages of

corrosion. The layer that is formed by the chelate

complexes blocks the penetration of chloride ions to

the native oxide layer leaving it intact (Lamaka

et al. 2007; Yasakau et al. 2008).

The formation of the chelate complexes on the

surface of aluminum and to the active regions of

2024-T3 alloy stops the evolution of corrosion and is

the cause of locally active protection Fig. 19. The

totally corroded AA2024-T3 is presented in Fig. 20. On

the other hand, the formation of the inhibitor layer can

be observed using SEM as shown in Fig. 21 and Fig. 22.Fig. 18 Visual photograph of AA2024-T3 sample after

exposure for 72 h in 0.05 M NaCl

Fig. 19 Visual

photographs of AA2024-T3

panel after exposure for

72 h in 0.05 M NaCl with

a 0.01%, b 0.05%, c 0.1%

w/v of nanocontainers

loaded with 8-HQ and

d 0.01%, e 0.05%, f 0.1%

w/v of nanocontainers

loaded with 2-MB

Fig. 20 SEM images of

AA2024-T3 panel after

exposure for 72 h in 0.05 M

NaCl solution without

nanocontainers

a magnification 1,000,

b magnification 50,000 and

c EDX analysis

552 J Nanopart Res (2011) 13:541–554

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Page 13: Cerium FTIR Spectra

Conclusion

Cerium titanium hollow nanocontainers were

synthesized. Their size was characterized by SEM

measurements and was 170 ± 10 nm. XRD analysis

showed that the nanospheres consist of anatase and

cerianite crystalline phases. Thermal treatments with

TGA and DTA proved that hollow nanospheres are

produced due to burn off of polystyrene cores. The

synthesized nanocontainers have different size of

pores. Moreover, these nanocontainers were loaded

with the corrosion inhibitors 8-HQ and 2-MB.

Thermal treatments with TGA and DTA proved

that nanocontainers were loaded with 4.37% w/w of

8-HQ and 25.36% w/w of 2-MB, respectively. Fur-

thermore, the introduction of nanocontainers loaded

with inhibitor to a corrosive environment to which an

AA2024-T3 is exposed; shows that as the concen-

tration of loaded nanocontainers is increased, a more

effective protective layer is formed on the surface of

the metal alloy, through the release of the inhibitor

from the nanocontainers. These nanocontainers can

be used in a vast field of implementations such as

additives in corrosion protective coatings.

Acknowledgments This project was supported by FP7

Collaborative Project ‘‘MUST’’. The abbreviation ‘‘MUST’’

stands for ‘‘Multi-Level Protection of Materials for Vehicles by

‘‘SMART’’ Nanocontainers’’ (EC Grant Agreement Number

NMP3-LA-2008-214261).

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