International RILEM Symposium on Photocatalysis, Environment and Construction Materials
8-9 October 2007, Florence, Italy
SURFACE ENGINEERING ON NATURAL STONE THROUGH TiO2
PHOTOCATALYTIC COATINGS
Giancarlo Potenza (1), Antonio Licciulli (1), Daniela Diso (1), Sergio Franza (2),
Angela Calia (3), Mariateresa Lettieri (3) and Giuseppe Ciccarella (1)
(1) Department of Innovation Engineering, University of Salento, Via Arnesano, Lecce, I- 73100, Italy
(2) Salentec SRL, via dell’esercito 8, 73020 Cavallino Le, Italy
(3) IBAM-CNR, Prov.le Lecce Monteroni, 73100 Lecce, Italy
The application of semiconductor photocatalytic films on natural stone has been
investigated for surface protection and selfcleaning. Sol Gel and hydrothermal processes
were used to synthesize TiO2 sols with enhanced photocatalytic activity and without the need
of thermal curing of the coated surface. The stone was a local (apulian) carbonatic
sedimentary and porous stone. Films and powders prepared from the TiO2 sols were studied
using x-ray diffraction to evaluate the microstructural evolution and identify rutile and
anatase phases. A morphological and physical characterization was carried on the coated
stone to establish the coating adhesion and the changes of aspect, colour and hydric
behaviour. The photocatalytic activity was evaluated by dye degradation rate under UV
irradiation measuring the color change with a colorimeter.
The hydrothermal process proved to be effective for obtaining photocatalytic surfaces with
selfcleaning and antipollution properties. With no need of high temperature post cure
treatments the hydrothermal TiO2 nanostructured sols look ideal candidate for coating
application on architectural materials including natural stone.
1.
INTRODUCTION
Titania is considered best photocatalytic material, for the degradation of environmental
pollutants, because is nontoxic, has high catalysis efficiency, low band-gap energy and long1
term stability . The activity of the TiO2 particles is widely influenced by crystal structure
(anatase or rutile), presence of doping elements, specific surface area, pore size
23
distribution , . Amorphous titania particles have negligible photocatalytic activity, after a
crystallization process the activity is enhanced and is higher for anatase nonocrystals than for
rutile. Another typical feaure of the amorphous TiO2 is its lower surface area, so the powders
might have a lower possibility to get efficiently in contact with pollutants. For this reason the
deposition on a mesoporous substrate is an important factor to improve the photocatalytic
4
activity of amorphous titania . Different methods have been used to synthesize titania sol or
nanoparticles: the most important is sol–gel method
studied as well:-precipitation
10,11,12,13
5,6,7,8,9
but other methods have been
and chemical vapour deposition
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14
.
International RILEM Symposium on Photocatalysis, Environment and Construction Materials
8-9 October 2007, Florence, Italy
In this paper the synthesis os TiO2 sols obtained by a hydrothermal process will be
described.
The sol–gel method is used, for the titania sol production, but causes the formation of
amorphous TiO2 particles, so that a thermal crystallization process is necessary and lead to
problems like particle agglomeration, grain growth, low surface area and phase
transformation from anatase to rutile, which all decrease the photocatalytic activity of
15
titania . The potential advantage of the hydrothermal process is to allow in liquid particle
crystallization and therefore the opportunity to have a sol made of crystallized TiO2
nanoparticles ready to be deposited on the substrate without any additional thermal process
16,17.
.
The TiO2 sols have been applied by spray coating on a local sedimentary cabonatic stone
named “Pietra di Lecce”. This stone is having high porosity (40%) and surface area therefore
is easily corroded by chemical biological and physical agents. TiO2 treatments have been
applied with the aim of having a self cleaning surface and air cleaning activity.
2.
EXPERIMENTAL
Aqueous TiO2 sols were prepared through sol-gel techniques starting from TPOT
(tetrapropyl orthotitanate, Sigma-Aldrich 97%) as ceramic precursor. Figure 1 shows the
preparation procedure for TiO2 sol hydrothermally crystallized. The biidrate oxalic acid
(Carlo Erba 99,8%) is dissolved in deionized water and then the TPOT is added dropwise.
Immediately a white precipitate is formed that is dissolved by warm stirring in about 2 hours.
After this process a TiO2 amorphous sol is obtained.
TPOT + H2O
(COOH)2
stirring for 24 h
amorphous TiO2 sol
hydrothermal reaction
125°C, 3,5 bar
deposition
drying at 60°C for 6 h
Figure 1: schematic diagram of the TiO2 sol hydrothermal preparation .
As formed amorphous sol is placed in a teflon autoclave (Mars 5, CEM Corporation) and
processed at different dwells at 125°C and at 3,5 bar. The heating rate was 2.5°C/min. Two
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International RILEM Symposium on Photocatalysis, Environment and Construction Materials
8-9 October 2007, Florence, Italy
sols are obtained: HT01 (125°C, 3,5 bar for 2,5 minutes) and HT02 (125°C, 3,5 bar for 10
minutes).
The hydrothermal sol is deposited on the carbonatic stone (5x5x1 cm3) through spray gun
with 0,8 mm diameter nozzle.
The crystallographic phases of the hydrothermal TiO2 coatings were investigated by an Xray Diffractometer at room temperature (Philips PW1729) using Cu Kα radiation
18
(λ=1,5406Å). Crystallite size was calculated by Sherrer’s equation from the broadening of
the (101) reflection for anatase and the (110) reflection of rutile:
kλ
d=
,
β ⋅ cosϑ
where d is the crystallite size, k is a constant (0,9 assuming that the particles are spherical), λ
is the wavelength of the X-ray radiation, β=FWHM is the full width at half maximum
(obtained after correction for the instrumental broadening) and θ is the angle of diffraction.
19
The anatase/rutile proportions were measured by the Spurr and Myers method :
1
WA =
I
1 + 1,26 ⋅ R
IA
where WA is the weight share of anatase in the mixture, while IA and IR are the integrated
intensities of the (101) reflection of anatase and the (110) reflection of rutile.
The surface morphology of the coating was observed by SEM analyses (ESEM-FEI).
Photocatalytic activity of the coating was evaluated through methylen red (C15H15N3O2
reagent, Sigma-Aldrich) decomposition test. The stones were cleaned and dried in oven at
60°C for 6 hours, then were covered with 470μl of methyl red alcoholic solution with
7,5 ⋅ 10 −4 mol/l concentration. The samples were irradiated by ultraviolet rays with an
intensity of 3,7 mW/cm2 up to 6 hours and in this period the stones gradually turned to their
original colour. The degradation activity of the TiO2 coatings were calculated by e Minolta
CR 200 Colorimeter that gives the variation of colour ΔE = ΔL2 + Δa 2 + Δb 2 in the CIElab
space, where ΔL represents the change in brightness, Δa and Δb the changes in hue. For
each test a blanc surface was kept and all the experiments were repeated five times.
A flow type photoreactor was used to examine the NOx degradation capability fo the
carbonatic stone treated with TiO2 sols. The samples (5x5x1 cm3) were irradiated with Osram
Vitalux lamp. Dry air containing 0,6 ppm of NOx (45% NO2 e 55 % NO) was passed through
the 3 litres Pyrex reactor at a rate of 5 ± 10% l/min. The NOx concentration was monitored
with a chemiluminescent NOx analyzer (Monitor Labs, Model 8440). A commercial
photocatalytic sol (from Tioxoclean inc. USA) was used as the reference photocatalyst..
3.
RESULTS AND DISCUSSION
In table 1 the processing conditions, the average crystalline size and the phase
compositions are summarized of the TiO2 powders prepared by hydrothermal process sol and
compared with the properties of TiO2 powders prepared by a calcination process at high
temperature.
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International RILEM Symposium on Photocatalysis, Environment and Construction Materials
8-9 October 2007, Florence, Italy
XRD analyses on TiO2 powders obtained by a drying process at 60°C of the hydrothermal
sols. In figure 2 the diffraction patterns of HT01 and HT02 powders are reported along with a
comparison with an hydrothermal sol treated at higher temperature and pressure (HT03) and
TiO2 powders obtained by a calcinations process at 400°C.
4000
A
A = Anatase
R = Rutile
3500
R
Intensity (a.u.)
3000
RA
2500
A
R
2000
R
TiO2calcined 400°C
R
A
R
1500
HT03
HT02
1000
HT01
500
10
20
30
40
50
60
70
80
2θ [degrees]
Figure 2: XRD pattern of low temperature titania powders prepare by hydrothermal process
Figure 2 shows the XRD spectra the phase evolution is clearly seen from anatase to rutile
with the increasing in temperature and processing time. In fact sample HT03, processed at
185°C, 13,5 bar for 10 minutes, shows a higher 27,3° peak respect to the samples processed
at lower temperature. For the sample HT03 the amount of rutile is about 73% respect to
33,5% of the sample HT01.
From the XRD patterns the influence of the hydrothermal process on the crystallization of
TiO2 powder can be clearly evaluated, the characteristic peaks of the anatase and rutile
structure are evident and sharpened significantly. The effect of the reaction time was well
evident in the anatase/rutile weight fraction but there were no significant changes in the
average crystal sizes, instead the increasing of temperature up to 185°C led to the formation
of polycrystalline TiO2 sol mainly composed by rutile phase.
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International RILEM Symposium on Photocatalysis, Environment and Construction Materials
8-9 October 2007, Florence, Italy
Table 1: Average crystalline size and phase composition of low-temperature TiO2 powders
Particles size, nm (±10%)
Sample
Processing conditions
HT01
125°C; 3,5 bar; 2,5 minutes
3,2
HT02
125°C; 3,5 bar; 10 minutes
3,75
HT03
185°C; 13,5 bar; 10 minutes
5,76
calcined TiO2
400°C; 1 bar; 2 hours
8,32
XRD
TEM
5,4 (12*)
Phase
composition of
synthesized
samples
Anatase 66,5%
Rutile-33,5%
Anatase 53,5%
Rutile-46,5%
Anatase 27%
Rutile-73%
Anatase-80%
Rutile 20%
*size of aggregates
In figure 3 are reported SEM images of the hydrothermal TiO2 sol deposited on the
carbonatic stone.
There were no significant change in the morphology with the surface treatment. Only 0,55
decrease of capillary rise and 10% decrease in permeability of the carbonatic stone.
A
B
B
A
Figure 3: SEM images of no covered stone (figure 3A) and TiO2, hydrothermally crystallized
for 2.5 minute, deposited on the carbonatic stone (figure 3B)
The photocatalytic activity of the carbonatic stone covered by TiO2 was evaluated through
methyl red degradation test and NOx reduction test. In figure 5 is shown the chemical
structure of the methyl red molecule. The starting colour parameters were taken through the
Minolta CR 200 Colorimeter and their evolution in the time was estimated. In figure 4 the
trend of ΔE is reported as function of the radiating time for the sample HT01, HT02 and a
commercial TiO2 sol (Tioxoclean). From the graph is well evident that after 9 hours of
irradiation the coating of HT01 sol was able to degrade almost all the dye and to turn the
stone to its original colour (horizontal line). The in house prepared sols were more active
than the commercial one and the HT01 was significantly higher than HT02.
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International RILEM Symposium on Photocatalysis, Environment and Construction Materials
8-9 October 2007, Florence, Italy
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The experimental apparatus for the NOx control was the same as used by Takeuchi . NO
is converted to HNO3 with photooxidation by way of NO2.
1
hυ
NO + O2 ⎯⎯→
NO2
2
1
hυ
2 NO2 + O2 + H 2 O ⎯⎯→
2 HNO3
2
(Eq.1)
(Eq.2)
The starting NOx concentration, before entering in the reactor (Ca), and at the exit of the
photochemical reactor (Cb) are measured thereactor. The percent of photoconversion is
C − Cb
⋅ 100 for the different irradiation time. The photocatalytic activity is
calculated as: a
Ca
higher in HT01 than the HT02 were the rutile phase content is higher.
16
14
original colour
HT01
HT02
TIOXO
not covered
stone
12
ΔE
10
8
6
4
2
0
0
2
4
6
A
8
B
time (hours)
Figure 4: ΔE variation during the irradiation time for stones covered with different sols and
the sample used for the colour removal determination after 2 (figure 4A) and 8 hours (figure
4B) of UV irradiation.
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International RILEM Symposium on Photocatalysis, Environment and Construction Materials
8-9 October 2007, Florence, Italy
100
HT01
NOx removal(%)
80
HT02
60
40
20
0
0
10
20
30
40
50
60
Time (min)
Figure 5: NOx removal from TiO2 coated carbonate stone from a starting Nox concentration
of 0,6 ppm
4.
CONCLUSIONS
The process developed using sol-gel aided by hydrothermal treatment was effective for
the preparation of crystallised titania nanoparticles suspended in water.
These sols can be used for the surface engineering of porous carbonatic stone. Under
controlled processing condition the coating are effective for obtaining photocatalytic surfaces
with selfcleaning and antipollution properties with no need of post curing at high
temperature. During the microwave hydrothermal process both rutile and anatase are formed.
Rutile is prevailing with prolonged heating in autoclave. Selfcleaning tests and NOx removal
prove that anatase phase is more active so that short heating at relatively low temperature in
autoclave are effective to crystallise an active anatase. The spray coating process was a
practical, cheap and easy way to TiO2 coating without significantly changing the morphology
and permeability of the porous carbonatic stone.
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