Deposition of Transparent, Hydrophobic TiO2 Film for the Protection of Outdoor and Marine Cultural Heritage Assets

doi:10.4236/ad.2013.12003

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Archaeological Discovery

2013. Vol.1, No.2, 32-36

Published Online October 2013 in SciRes (http://www.scirp.org/journal/ad) http://dx.doi.org/10.4236/ad.2013.12002

Deposition of Transparent, Hydrophobic TiO2 Film for the

Protection of Outdoor and Marine Cultural Heritage Assets

Fabio Stranges1,2, Marianna Barberio1,2*, Pasquale Barone1,2, Andrea Abenante1,

Andrea Leuzzi1, Peppino Sapia1,2, Fang Xu1, Assunta Bonanno1

1Physics Department, University of Calabria, Cosenza, Italy

2Biology, Ecolgy and Earth Science Department, University of Calabria, Cosenza, Italy

Email: *[email protected]

Received June 28th, 2013; revised August 1st, 2013; accepted August 10th, 2013

Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the

original work is properly cited.

In this work we present two new methods to obtain TiO2 transparent coverage and to impart superhydro-

phobicity to stones and ceramics surface of monuments. The first method, adapted for small artifacts eas-

ily transportable in restoration laboratory, consists of a simple evaporation of Ti directly on ceramic sur-

face in a controlled oxygen atmosphere. The second method consents the coverage of large surface di-

rectly in situ. The TiO2 is evaporated on a salt surface with desired dimensions and then deposited on ce-

ramic surfaces. In both cases the dioxide layers are transparent, don’t damage the ceramic surfaces and are

easily removable. In fact, the dioxide layer can be removed simply by 30 minutes of laser ablation process.

Keywords: Marine Artifacts; Photocatalic Activity; Protection of Cultural Heritage

Introduction

In recent decades several scientific efforts have been made to

fabricate superhydrophobic surfaces by using numerous tech-

niques and methods, for example, plasma treatment (Manoudis,

2009; Chen, 1999; Coulson, 2000; Tserepi, 2006), photolitho-

graphy (Oner, 2000; Gao, 2006) and sol-gel (Shirtcliffe, 2003;

Mahltig, 2003; Hikita, 2005). Water repellent coatings can be

important in many applications including, for example, the

prevention of icing in cold weather, the promotion of self-

cleaning process induced by rainwater on outdoor surfaces, the

prevention of clotting in artificial blood vessels, the decrease of

corona activity developed in conductors of transmission lines

under rainy conditions, the production of waterproof and stain

resistent textiles, and the reduction of friction in water (Ma-

noudis, 2009; Gao, 2006; Zorba, 2008). One of the applications

of the superhydrophobic coatings is their use as surface protec-

tive barriers for the preservations and conservation of ceramic

and stone monuments. The most important degradation factor

of outdoor, immovable cultural heritage is the rainwater which

can causes stone deterioration through cycles of freezing and

thawing inside the pores of the stones or by intraporous cry-

stallization of the salts transferred by the water (Manoudis,

2009; Manoudis, 2008). For this reason, the application of hy-

drophobic coatings has been suggested for the surface pro-

tection of outdoor cultural heritage assets (Manoudis, 2009;

Manoudis, 2008). Moreover several monuments are collocated

in marine sites and cannot be removed from seabed, in these

conditions the corrosion caused by marine water and microor-

ganism can causes serious problems to conservation. Recently

Manoudis et al. described (Manoudis, 2009; Manoudis, 2008) a

simple method that can be used to impart superhydrophobicity

to different stones surfaces (used in restoration of the castle of

Prague). This method is based on the dispersion of nanoparticle

in a polymeric solution that can be sprayed on the substrate.

The resulting composite polymer-nanoparticle film exhibits

superhydrophobic properties.

However, this method cannot be applied in marine archeo-

logical sites (in underwater conditions the spray process is im-

possible and the dispersed particles can create environmental

pollution). Moreover, in marine archeological sites the cover-

age of ceramic and stone must have an antibacterial action to

inhibit the deterioration caused by the attack of marine micro-

organisms. In this optics, a surface coverage by TiO2 substrate,

with its phocatalytic oxidation properties (Liu, 2005; Naeem,

2010), can solve both the problems: idrophobicity and antibac-

terial activity.

Here we describe two simple methods to obtain TiO2 cover-

age applicable in air and in marine water conditions. The ob-

tained coverages are transparent (so it don’t modify the artistic

properties of monument) and hydrophobic, don’t damage the

ceramic surface and are easily removable by a simple cleaning

process of laser ablation (Stranges, 2013).

Materials and Methods

All the experiments were conducted on several ceramic frag-

ments obtained from a “Carosello” (Gattuso, 2012), a structural

hollow element made of clay, placed in arches, in domes or

even and in the walls of buildings such as churches and houses,

with the function lighten the structures (the specific “carosello”

used in these experiments come from a site in Calabria: the

Sanctuary of “Madonna del Buonconsiglio” in san Giacomo di

Cerzeto, Cosenza, Italy dated back to 1844 ± 11). The frag-

*Corresponding author.

F. STRANGES ET AL.

ments have dimensions of about 1 cm × 2 cm and a thickness of

about 2 cm.

We implement two different TiO2 coverage process. One,

applicable to little fragments of ceramic artifacts, contemplates

a titanium evaporation in ultra high vacuum chamber, while the

second, applicable either in laboratory or in marine water, is a

process of deposition of TiO2 layer, previously evaporated on a

salt, on ceramic surface.

The direct evaporation process was conducted in an ultra

high vacuum chamber with a base pressure of 1 × 10−9 mbarr.

Ti was evaporated, in a controlled oxygen atmosphere (the

partial pressure of oxygen in UHV chamber is fixed at 5 × 10−6

mbar), by heating an outgassed Mo boat ﬁlled with Ti crystal

through a current of 60 A. The evaporation time ranged from 1

h to 7 h. The Titanium and oxygen atoms react on ceramic

surfaces forming a TiO2 layer (the formation of dioxide was

confirmed by XPS data) with a thickness which increases with

exposure time. Figure 1 shows samples obtained with exposure

times of 1, 2, 3 and 4 hours, the sample coloration is unchanged

after 1 h of exposure while becomes darker with time evapora-

tion, clearly indicating the formation of TiO2 layers with a

thickness greater than 30 mm (the dioxide coloration is strictly

related to layer thickness and only for dimensions greater than

30 microns the layer is colored and visible to the naked eye).

The deposition process includes two phases: a direct evapo-

ration on a water soluble salt surface (NaCl as example) using

the process in UHV previous described and then the deposition

of TiO2 layer on ceramic surface on archeological site (air or

marine water). The salt was deposed on Aluminum foil and

then inserted in UHV chamber where an evaporation process of

1 h deposes a TiO2 transparent layer. Following the TiO2 layer

is covered by a silicon adhesive layer (to favorite the ceramic

adhesion) and then sealed by aluminum foil to the application

on ceramic. The salt surface and the aluminum foil were eli-

minated after deposition simply dissolving the salt with

water. In this work we use as soluble salt the NaCl and the

Na2B4O7·10H2O obtaining the same results.

This process consent to prepare TiO2 layer with desired di-

mensions in a laboratory and the application on ceramic surface

directly in situ. Figure 1 shows the ceramic covered by TiO2

deposed via NaCl salt, the dioxide layer is transparent and not

modifies the sample coloration.

All the TiO2 covered sample are analyzed by X-ray photo-

electron spectroscopy (XPS), X-ray fluorescence spectroscopy

(XRF) and by contact angle measurements to testing the hy-

drophobicity properties.

XPS measurements were conducted in an ultra-high vacuum

(UHV) chamber equipped for standard surface analysis with a

pressure in the range of 10−9 torr. Non monochromatic Mg-K



X-ray (hν= 1253.64 eV) was used as excitation source. The

Exposure 1 hExposure 4 h

Exposure 2 h

Exposure 3 h

Deposition via NaCl

After Laser Ablation

Figure 1.

Images of “Carosello” fragments after Ti evaporation, after TiO2 evaporation via NaCl and after laser ab-

lation.

F. STRANGES ET AL.

XPS spectra were calibrated with the C1s peak of a pure carbon

sample (energy position 284.6 eV). All XPS spectra have been

corrected by the analyzer transmission factor and background

subtracted using the straight line subtraction mode. Moreover

the XPS data were fitted assuming a Gaussian distribution.

XRF measurements were conducted by the X-123 SDD ap-

paratus by Amptek (USA), equipped by a gold cathode and a

beryllium revelator, operating at fixed angle.

The contact angle images was obtained placing a small drop

of distilled water (5 l) on the ceramic surface with and with-

out dioxide coverage. The equilibrium contact angle was de-

termined by drop shape analysis using an optical contact meter.

The contact angle was measured several times for each sample

to ensure data robustness.

After characterization all the samples was subjected to a La-

ser Ablation (LA) cleaning process to verify the reversibility of

dioxide coverage and the possibility of future restoration on

covered artifacts. The laser ablation procedure was performed

irradiating for several minutes (15' or 30') each sample, im-

mersed in an acetone solution, by the output of the first har-

monic (1064 nm) of a Quanta-Giant series 710 Nd:YAG laser

operating at 10 Hz. The spot size of the laser beam on the sur-

face of carosello is about of (7 - 8) mm and the power of laser

is fixed at 500 mJ/pulse. The immersion in acetone is necessary

to prevent the oxidation and carbonization of surface induced

by LA in air (Naeem, 2010). Figure 1 shows the images of

ablated artifacts, it is clear that the TiO2 layer is completely

removed as also confirmed by the results of XPS, XRF and

contact angle measurements.

Sample Analysis

The static water contact angle was analyzed to qualitatively

assess the effectiveness of the surface coverage process and

their influence on surface wetting. The sessile drop technique

was employed. Indeed, the simple observation of a small liquid

drop on a solid substrate provides useful information concern-

ing the liquid-solid interaction and the hydrophobicity. The

obtained results for both processes are summarized in Figure 2.

The ceramic surface is completely hydrophilic and the water

TiO2 directly evaporated on ceramic

(evaporation times (h)) Contact angle Θ (˚)TiO

2 deposited with NaCl salt on ceramic Contact angle Θ (˚)

(7 h)

118

(6 h)

116 

After TiO2 deposition

(5 h)

119

(4 h)

112

(3 h)

111 

After 15' LA

(2 h)

107

(1 h) 

110

After 30' LA

Figure 2.

Contact angle after all treatments described in paper.

F. STRANGES ET AL.

drop is completely adsorbed. These properties of ceramic is the

origin of surface degradation in marine water or after exposure

to atmospheric wet. So, the coverage with hydrophobic layer

ensure the conservation of artifacts. However, the coverage

must be transparent to not change the artistic and historic value

of artifacts. For this we realize TiO2 coverage with different

thickness (and different coloration) verifying the hydrophobic-

ity for each obtained layer. Figure 2 shows that the contact

angle, and then the hydrophobicity, increases with evaporation

time indicating for all samples an angle greater than 90˚ (hy-

drophobicity limit). In particular we obtain a contact angle of

about 110˚ for transparent layer obtained by 1 h evaporation

and an angle of 96˚ for transparent layer deposed via NaCl.

These clearly indicate that in both process it is possible to ob-

tain hydrophobic and transparent coverages.

LA process remove the TiO2 layer and the surface goes back

to being hydrophilic. The contact angle, in fact, decreases to

40˚ after 15' of LA and until get to 0 after 30' of ablation. All

these results was also confirmed by XRF and XPS analysis.

XPS data in Figure 3 indicated that both process (direct

evaporation and deposition) induce the only presence of Tita-

nium and oxygen on ceramic sample without alter the sample

composition or introducing impurities (Mo from evaporator

boat, aluminum, sodium or chlorine from salt and deposition

support foil). In particular in both process the Ti 2p lines are

centered at 458.5 eV clearly indicated the only presence of

TiO2 structures on surface (without presence of isolated Ti

atoms) (NIST database).

XRF data in Figure 4(A) indicated that the amount of TiO2

increases with evoparation times indicating the formation of

layer with thickness gradually larger. Data in Figure 4(B) indi-

cates, moreover, the composition of deposed layer of TiO2 on

salt (curve a for NaCl and b for Na2B4O7), it is clear that only

TiO2 is present on salt surface without any presence of impuri-

ties. Curve c and d in Figure 4(B) indicates the surface compo-

sition after LA cleaning process (15' curve c and 30' curve d) it

is clear that the Ti present on surface is completely removed

and the sample spectrum is identical at this of pure ceramic

(curve e).

Both process are, so, able to cover the ceramic surface giving

to ceramic an hydrophobic characteristic which can protect the

artifact from corrosion caused by marine water or atmospheric

Figure 3.

XPS survey spectra for samples treated with different deposition proc-

ess.

Figure 4.

XRF spectra for sample s treated with different deposition process.

wet. The obtained substrate are moreover completely transpar-

ent and can be removed simply by laser ablation process with-

out change the ceramic properties.

Conclusion

In this work, we present two methods to cover ceramic arti-

fact surfaces by a TiO2 transparent layer which gives surface to

the properties of hydrophobicity which ensure a protection by

water corrosion in marine water or in moist. The first process is

a simple evaporation of Ti in a controlled oxygen atmosphere

directly on ceramic surfaces. This process is applicable on

small artifacts, and easily transportable in a restoration labora-

tory. The second method contemplates the evaporation of TiO2

on a salt surface (deposed on an aluminum foil) which can be

successively deposed on artifacts. These methods prefer to the

coverage of artifacts of different dimensions directly in situ,

without damages of ceramics.

Both the process leads to the formation of transparent layer

which don’t damage the surfaces and don’t introduce defects.

Moreover the coverage is completely reversible. The dioxide

layer can, in fact, be removed simply by a cleaning process of

F. STRANGES ET AL.

laser ablation (30' at 1064 nm, 500 mJ) without surface dam-

aging.

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