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Evaluation of Photocatalysis TiO2 Nano-particles as a Self-Cleaning to

Calcareous Stone Monuments Surface

Sayed M. Ahmed

1*, Sawsan S. Darwish

2, Nagib A. Elmarzugi

3, 4, Mohammad A. Al-Dosari

5,Mahmoud

A.Adam6 , Nadia A. Al-Mouallimi

7

1The grand Egyptian Museum, Ministry of AntiquitiesCairo,Egypt,Email:sayedmansour32@yahoo.com

2Conservation department, Faculty of Archaeology, Cairo University, Giza, Egypt.

3Faculty of Pharmacy, Tripoli University and BioNano Integration Research Group, Biotechnology

Research Center, LARST, Tripoli, Libya, P.O.Box:13100 Email: nelmarzugi@gmail.com. 4Research and Innovation Dept., Institute of Bioproduct Development, Universiti Teknologi Malaysia.

5 National Nanotechnology Research Center, King Abdulalziz City for Science and Technology (KACST),

Riyadh, Saudi Arabia. 6Conservation department, Faculty of Archaeology, Cairo University, Giza, Egypt.

7Saudi Nano Information Center, JEDDAH, Saudi Arabia.

ABSTRACT

Ancient Egypt calcareous stone monuments, especially which located in open areas

suffering from many deterioration factors, air pollution, soluble salts, RH/temperature,

and biodeterioration. These are the main causes of decay of stone building materials.

The development and application of self-cleaning treatments on historical and

architectural stone surfaces would be a significant improvement in conservation,

protection and maintenance of cultural heritage.

In this work, nanometric titanium dioxide has been conducted as a promising

photocatalytic material owing to its ability to catalyze the complete degradation of many

organic contaminants and environmental factors.

The obtained nano-TiO2coating were applied and brushed over the surfaces of travertine

(marble and limestone often used in historical and monumental buildings), at room

pressure and temperature. SEM, coupled with EDX microanalysis used in order to

obtain the detailed information on coating homogeneity, surface morphology before and

after aging, and penetration depth of the TiO2 within the samples. Activity of the coated

surface has been evaluated through UV-light exposure to evaluate photo-induced effects,

capillary water absorption and colorimetric measurements performed to evaluate the

optical appearance.

Results show TiO2 nanoparticles good candidate for coating applications on calcareous

stone, where self-cleaning photo-induced effects are well evident and no alteration of

the original features.

Key Words

PhotocatalysisTiO2, Coating, Calcareous stone, Self-cleaning, UV aging.

1. INTRODUCTION

Historic architectural stone surfaces, due to their prevalently outdoor location, are

generally subject to a complex series of weathering and decay [1]; these types of culture

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Heritage are suffering from various environmental factors such as fluctuation of

temperature/humidity, hazardous gases, and microbes, in addition to physical and

chemical factors.

Especially, the façades of the buildings are deteriorated with direct exposure by these

factors; deformation and disfiguration of superficial decoration and formation of black

crusts are often observed on the stones [2, 3].

During the last decades, there has been a strong impulse in developing innovative

building materials that could offer extra value in addition to outstanding mechanical

properties and work-ability. Photocatalytic oxidation has a strong potentiality as being

an effective process for removing and destroying low-level pollutants in the air [4].

Most recently, the area of interest is shifting into practical and technological

applications, like self-cleaning construction materials and antimicrobial photocatalytic

coatings to surface of historic stones [5].

TiO2 is considered the most promising photocatalytic material for the degradation of

environmental pollutants. It is nontoxic, highly efficient, and very stable under UV [6,

7], the TiO2 photoactivity is strongly influenced by the microstructure, presence and

concentration of doping elements, The specific surface area and particle size [8,9]. In

fact, the self-cleaning property and the transparency of nano-TiO2 based materials could

play a very important role for monuments, historical buildings and any other

architectural surfaces exposed to urban pollution. The TiO2- based coatings could

potentially allow an easier maintenance of the original colour and aspect of the

historical surfaces, thanks to the superhydrophilicity and photocatalytic properties of

this material [10, 11].

In this study, TiO2 nanoparticles dispersed in an aqueous colloidal suspension have been

applied directly on historic marble stone, in order to obtain a nanometric film over the

stone surface. The aim is to verify if this coating technology has self-cleaning and

hydrophobic features, suitable for the restoration of stone materials belonging to our

cultural heritage. For this purpose, laboratory experiments, capillary water absorption,

simulated solar radiation, and colorimetric measurements, would be performed. In

addition the penetration of the oxide within the stone materials was assessed by means

of SEM–EDS analyses.

2. EXPERIMENTS

2.1. Preparation and application of the nano-coatings on the stone surface

Commercial product distributed by (Aldrich, Germany). It consists of a nanopowdered

TiO2 (anatase Crystalline phase with particle mean diameter of 7 to 20 nm) dispersed in

an aqueous suspension of water (The obtained aqueous colloidal suspension having 2 wt

% of TiO2 content). Dispersions were subsequently stirred vigorously for 20 min [12].

Carrara marble stone (with a porosity of about 1%) as one of the most type of the

calcareous stones commonly used for monuments, The stone specimens were cleaned

with a soft brush and washed with deionized water in order to remove dust deposits. The

specimens were squared blocks, and cut following the UNI10859:2000 [13], Then dried

in an oven at 105 ◦C for 24 h, and before the application of the treatments, the stones

were kept at 23±2 ºC, 50 ± 5% R.H. for 24 h, the application of the product on the stone

samples has been made by brushing at room pressure and temperature.

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2.2. Laboratory Tests and analyses

All the treated samples underwent laboratory procedures, several tests were carried out

in order to assess the photocatalytic efficiency of the coatings on the stone.

The tests performed include:

2.2.1. Morphological analysis of the stone samples

The morphological characterisation of untreated and treated stone surfaces has been

performed by scanning electron microscopy (SEM) Philips (XL30), equipped with EDX

micro-analytical system, in order to evaluate the penetration dept of the TiO2 within the

samples, also to characterize the homogeneity and distribution of the coating film after

drying.

2.2.2. Colorimetric measurements

Evaluation of colour changes of the stone samples induced by application of the nano-

coating was carried out by colorimetric measurements using a CM-2600d Kon-ica

Minolta spectrophotometer, to assess chromatic variations. Chromatic values are

expressed in the CIE L*a*b* space, where L* is the lightness/darkness coordinate, a*

the red/green coordi-nate (+a* indicating red and −a* green) and b* the yellow/blue

coordinate (+b* indicating yellow and −b* blue) [14].

2.2.3. UV aging test

In order to evaluate the photo degradation effect of the coating and the stability of the

hydrophobic property of the protective film, accelerated aging tests were performed

through light emitted by luminare C.T.S. Art lux 40 with 2 UV fluorescent tubes 5000

K, 45 cm long, 100 W, 220 V, with plexiglas protection screen, with a UV-A

component, whose UV intensity was 2w/cm2, the distance between samples and light

source is 20 cm, the samples were left under UV irradiation for 45 days [15], which was

demonstrated in Fig. 1 and 2 respectively.

2.2.4. Physical properties

This test was performed on 3 samples measuring 3 x 3 x 2 cm, in order to evaluate the

amount of water absorbed by a stone specimen per surface unit time, the UNI

10921:2001 norm[16], the weight measurements were taken before and after a treatment

and after aging tests to identify the density, porosity and water absorption (UNI

10859:2000, cultural heritage – natural and artificial stones – determination of water

absorption by capillarity) [17].

3. RESULTS AND DISCUSSISON

3.1. Application of nano-coating

Stone treatments might to ensure the homogenous distribution of the nano-coating on

the surface of the stones, avoiding accumulation on the surface that hide the original

colour characteristics. Further requirements of the protection treatments are a good

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Penetration within the stone, in order to realize the adhesion of the inconsistent

superficial layers with the unweathered stone beneath.

Immediately before the application of the products, the surface of the specimens was

brushed to remove any dust or other type of loose materials. Then, the substrates were

pre-wetted with pure water in order to allow a proper application [18].

3.2. SEM/EDX

Scanning electron microscopy observations were performed on treated, untreated and

treated aged samples; the SEM micrographs of the untreated sample (Fig. 3a) show the

homogeneous structure of the fine-grained calcite mineral, the volume and distribution

of the pores, while the presence of some voids and disintegration was noticed because

of dissolving and disappearance of binding materials. The SEM micrographs of the

surfaces obtained after TiO2 deposition (Fig. 3b) show that, the coating film formed by

an uniformly spread film on the marble surface, also show the coating film with the

homogeneous and compact distribution, and crack-free is generally observed compared

to the untreated ones.

After UV aging, as shown in (Fig. 3C) no significant changes were observed in the

morphology of the protective film, similar results are obtained for both samples before

and after solar radiation, which indicate that the coating film is stable under the effect of

the artificial UV aging. This is due to the high photoactivity and unique physical and

chemical properties of nano-TiO2.

In fig. 4, EDX spectra are reported in order to show the presence of nano-TiO2 locally

spread on the surface of the treated marble sample.

3.3. Colour variations and hydrophobic properties.

(a) Colour test.

Colour alteration recorded before and after coating application and after artifial UV

aging. The aim of this analysis is to determine if the nanopowders can induce an

increase off colour variation between the treated, untreated and the treated aged

surfaces. The colour modification (ΔE*) was calculated using the following relation:

(ΔE*) =

Where ΔL*, Δa* and Δb* represent the difference between the value of each chromatic

coordinate in treated samples and the value in untreated ones.

Such parameter is important for aesthetic reasons, since a coating should not induce

ΔE* greater than 5 [19], in order to preserve the original colour of surfaces.

After treatment and aging, negligible colour variations were observed, (Colour change

with ΔE* less than 5 is conventionally not visible to the naked eye). The average color

change caused by TiO2 deposition was ΔE* = 2.17 and 3.06. This value is fully

compatible with their use for architectural heritage, and more specifically, with the

requirements of the maintenance field. The chromatic variation can be considered

noticeable by human eye; the obtained data are fully listed in Table 1.

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(b)Physical properties measurements.

Water is one of the most important abiotic factors of decay in porous materials [20].

Once it penetrates into the pores by capillary force, water carries out its deteriorating

effect through the chemical dissolution of the carbonatic component of the stone,

through physical phenomena such as freezing/thawing cycles, salt crystallization and

through microorganisms growth. For this reason, the hydrophobic property of the nano-

coating products was tested by capillary absorption [21].

Density, porosity and capillarity water absorption was measured in order to assess the

decrease of wettability, analyses were carried out on both untreated and freshly treated

samples and after accelerated aging by UV radiation, to simulate the coating behaviour

after a certain period of solar irradiation, which may lead to a decreased water

resistance, due to alterations in the coating film.

This test was performed on 3 specimens, samples weight were taken after being dried in

oven at 105◦C for 24 hours, then the samples were immersed in water bath for 24 hours

and weighed again, then measure the samples dimensions (height, width, length), to

determine the samples volume [22]. The physical properties of marble samples before

and after treatment are shown in Fig. 5, 6 and 7 respectively.

It is evident, from the physical measurements, that the treated samples are higher in

their bulk density; the efficiency of the nano-coating in formation of a protective layer

appears from the reduction in water absorption and porosity, which can be referred to

the penetration of the nanoparticles into voids and pores also the photocatalytic and self-

cleaning activities of TiO2 nanoparticles.

The hydrophobicity of the treated surfaces was tested again after UV aging, treated

surfaces seem to be not affected by solar radiation, no significant difference was observed

in the behaviour of the samples after aging, which can be referred to the photocatalytic

activity of TiO2 nano coating and it's stable under UV irradiation. The completed data

are listed in Table 2.

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Fig. 1: The samples exposed to the UV irradiation.

Fig. 2: The samples inside box under UV source.

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Fig. 3: SEM images (2000×) of Marble surfaces to show the Morphology of the

coatings: untreated (a), Treated (b) and after UV Aging (c).

A

b

C

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Fig.4: EDX spot of marble surface after application of TiO2 nano- coating.

Table 1: Color measurements for treated and aged samples.

Δ (treated and untreated samples) Δ (UV aged and untreated samples)

ΔL* Δa* Δb* ΔE* ΔL* Δa* Δb* ΔE*

2.03 0.41 0.66 2.17 2.89 0.41 0.92 3.06

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Table 2: Values of physical properties and hydrophobic measurements after

coating application and aging.

TiO2nano-particles Cons (2%) Density

gm/cm3

Porosity

%

Water

absorption

%

Un treated sample 2.656 0.09 0.11

Treated sample 2.788 0.07 0.08

Treated sample after UV aging 2.788 0.07 0.08

2.55

2.6

2.65

2.7

2.75

2.8

Treated sampleafter UV aging

Treated sample Un treated sample

Density (g cm3)

Fig. 5: Density of stone specimens before and after aging.

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Fig. 6: Porosity of stone specimens before and after aging.

Fig. 7: Capillarity water absorption of stone specimens before and after aging.

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CONCLUSION

In this study photocatalytic, hydrophobic and self-cleaning properties of an organic-

TiO2 coating have been tested. It is well evident that the TiO2 coatings can effectively

able to accelerate the degradation process of the dye under UV exposure. Hydrophobic

measurements have been performed before and after treatment, as well as after solar

radiation, in order to simulate the aging of the coating. Results have shown good water

repellence after treatments and after aging, colour changes on the coated and uncoated

stone showed negligible variations before and after the application, and after artificial

aging.

Acknowledgments The authors acknowledge the valuable assistance given by Professor Merfat H. Khalil,

the national center for housing and building research, and Prof. Yansuri Matsuda,

(JICA), for their valuable technical support.

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