Final Report 2011-2012
November 30th, 2012
Integrated design of new tools for the study and development of advanced technologies and materials
for architecture and construction
Part 2: Photocatalytic efficiency of different kinds of TiO2 and their possible fields of application
PhD. Student: M. Sc. Erika Iveth Cedillo González
Tutor: Prof.ssa Cristina Siligardi
Co-Tutor: Dott.ssa Monia Montorsi, PhD
1. Introduction
Nowadays, the development of building materials has been focused not only in the improvement of their technical
characteristics, but also in the achievement of self-cleaning features by coating them with different kinds of
photocatalytic TiO2 films [1]. Although much research has been carried out to produce and commercialize these
materials, there are still several practical limitations that make difficult their acceptance between the final consumers.
These include the employment of UV light to activate the self-cleaning property; the low adhesion of the films; their
deactivation and consequently the need of restoration through the cycle life; the use of substrates that affect the
photocatalytic activity (for example soda-lime glasses), the selection of the appropriate typology of TiO 2 film for
specific applications; the low availability of information regards the deposited quantity of TiO2 and the final efficiency
of the self-cleaning material, the efficiency at extreme environmental conditions (low temperature, high humidity) and
the resistance to the chemical abrasion caused by common domestic cleaning agents. Since all these limitations are key
factors in the commercialization and application of self-cleaning materials, this project was focused on several of them
with the aim of studying and developing solutions and technologies to apply self-cleaning glasses in the Italian building
industry.
The first objective of this research was the production of TiO 2 self-cleaning glasses with enhanced adhesion. Among the
advantages of getting a better adhesion there are an easier industrial production due to the development of simple,
effective and reliable coating procedures; the less need of restoration and a decrease in the maintenance costs;
environmental benefits like the protection of aquatic microorganisms [2] (avoid the washing-out of the TiO2
nanoparticles from the substrate) and the lower risk of health threats [3] due to the dispersion of the fallen nanoparticles
and their effect on the human health. In this work, roughness modifications of soda-lime glass substrates by chemical
treatments were made for increasing the adhesion, using a D-optimal design. Since this kind of substrates have the
disadvantage that they promote the Na + diffusion from the glass to the film at high temperatures (usually those at which
the TiO2 anatase phase is formed) [4], the chemical treatments were also useful to decrease the initial surface
concentration of Na+ in the glass and avoiding the deactivation of the films.
The effect of environmental factors like the humidity and temperature on the self-cleaning performance of
photocatalytic films is not yet clear in the literature, since they also depend of a high number of variables [5-13].
Although several reports describe the effect of humidity on the photocatalytic activity of TiO 2 [5-13], in all these studies
the contaminant was in gaseous phase, while the effect of humidity on the self-cleaning performance of TiO2 versus
solid contaminants has not been reported. On the other hand, the effect of the temperature on the self-cleaning activity
of TiO2 films has been poorly described [14]. To study the influence of these factors on the performance of TiO2-coated
materials, dense and mesoporous TiO2 films, as well as films made from commercial nanoparticles (Degussa P25 and
PARNASOS COLOROBBIA TiO2) were used with the aim to determine besides the effect of the environmental
variables, the effect of the kind of TiO2 film on the self-cleaning efficiency and propose a guide line to the selection of
the typology of TiO2 film in accordance with the specific application or the environment where it will be used.
The influence of the deposited quantity of TiO2 on its photocatalytic performance has not been clearly reported on the
scientific literature, and the comparison between existing studies is not simple because some important variables like
the kind of film, the light intensity and the organic sacrificial molecule are different from one study to another [15-21].
Moreover, the quantity of TiO2 deposited is not reported directly in most of the studies [15-21]. On the other hand,
although the initial efficiency of self-cleaning materials is a very important key factor for their real application in the
building industry, not only the initial efficiency but their efficiency through the life cycle becomes critical for their
commercialization. This is rarely reported in the wide existing literature about photocatalytic self-cleaning materials
[22, 23]. For this reason, the self-cleaning activity of TiO2-coated glasses was tested after chemical treatments that
simulated conditions at which the films would be normally exposed if they’re used in buildings and houses (water, acid
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rain and cleaning agents). For the study of the dependence of the efficiency with the deposited TiO 2 quantity and the
resistance to the chemical abrasion to cleaning agents, soda-lime glasses coated with the commercial PARNASOS
COLOROBBIA TiO2 were used. The reason was because this is the only product actually ready to commercialization
and available in the market (from those used in this work).
2. Methods
For the adhesion study, SAINT GOBAIN soda-lime glasses were used as substrates. The D-optimal experimental
design and the statistic analysis of the results were presented in the 1 st report (2010-12). Representative attacked glasses
were coated by dip coating with a film of PARNASOS COLOROBBIA TiO 2 (from here called “Nanosuspension B”).
To obtain the correlation between roughness and adhesion, the scratch critical loads (C.L.) of the coated glasses were
measured. The surface Na of the original, acid-treated and TiO2 coated glasses was measured by XPS. The experiments
were carried out in ultra-high-vacuum (UHV) at a base pressure of 10 -9 mbar. X-ray photoemission data were recorded
with a double pass Perkin Elmer PHI 15-255G cylindrical-mirror electron analyzer (CMA) operated at constant pass
energy. X-ray photoemission was carried out with non-monochromatic Mg Kα photons (hν = 1253.6 eV) from a
Vacuum Generators XR3 dual anode source operated at 15 kV, 18 mA. The effect of low (33%), medium (63%) and
high (75%) relative humidity and the effect of low (10°C), medium (20°C) and high (30°C) temperature on the selfcleaning performance of mesoporous TiO2 (Malfatti et al. 2006 [24]), dense TiO2 (Costacurta et al. 2010 [14]) Degussa
P25 TiO2 (2 wt.% in ethanol) and TiO2 Nanosuspension B versus stearic acid (SA) at 365 nm were tested using silicon
wafers and soda-lime substrates. In the case of the humidity effect, the films were kept for 12 hours at the selected
relative humidity before the stearic acid deposition. To determine the effect of the TiO 2 quantity on the self-cleaning
performance, soda-lime glasses coated with 1, 3 and 5 layers of TiO 2 Nanosuspension B were used. To evaluate the
effect of the chemical abrasion, TiO2 Nanosuspension B-coated glasses were treated at 32°C for 19h with deionized
water (rain), acid rain (pH 2-3, EPA method [25]), 5% isopropanol and 5% detergent solution (equivalents to
commercial glass-cleaner and general cleaner solutions [26, 27]). An “aging test” with boiling water for 1.5 hours was
also performed. The deposition of stearic acid over all the films was carried out by spin coating, using a 0.05g SA/20mL
EtOH solution and a 1500 rpm speed for 30s. The photoactivity of the samples in all the experiments reported here was
measured by following the decrease of the Fourier-transform infrared (FTIR) absorption peaks of the stretching
vibrations of the C-H and C=O bonds of the stearic acid molecule. The FTIR spectra were acquired in transmission
mode using an ALPHA BRUKER spectrophotometer, averaging 32 scans between 4000 and 1000 cm -1 with a spectral
resolution of 4 cm-1.
3. Results and Discussion
3.1. Correlation between glass roughness and adhesion
Table 1 shows the chemical treatment and the final roughness of representative glasses (highest, high, medium and low
roughness), as well as the critical scratch loads of the same glasses coated with Nanosuspension B (D-optimal design,
1st Report).
Table 1. Roughness and scratch adhesion of some representative attacked glasses.
a
Defined as the load needed to form defects in the film, i.e., the beginning of the visible rings formed by the
Rockwell tip.
As observed in Table 1, the coated reference glass shows a non-homogeneous low adhesion (C.L. = 4.5587  3.6840
N). In this sample, when using high loads the coating is detached from the surface. Regards the adhesion between very
rough surfaces and TiO2 coatings, sample 5 (highest roughness) present one of the lowest scratch adhesion, with a C.L.
value of 3.3467  1.5688 N. Its non-homogeneous rough surface does not improve the adhesion, since the scratch C.L.
of sample 5 is lowest that the scratch C.L. of the reference glass. Despite the C.L. value, this high roughness seems to
promote a slightly enhancement of the adhesion, as the coating remain attached to the substrate also when high loads
were applied, contrary to the reference glass. Experiment 12, which presents a non-homogeneous and rough surface
(high roughness group), has a relatively high adhesion (C.L. = 6.7027  5.3309 N), however, with a high standard
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deviation. Therefore, this chemical treatment is not convenient for the pre-treatment of nano-coated glass substrates.
Moreover, the film begins to detach over the track, but is not completely removed from the substrate. The fact that a
high roughness does not improve the adhesion could be attributed to the combination of roughness and the increase of
the water contact angle of the attacked surfaces (1st Report). This combination often results in air pockets being trapped
between the solid and liquid (the composite solid-liquid-air interface), thus leading to a significant decrease in the solidliquid adhesion [28]. As shown in Table 1, medium values of roughness (3.2045  2.7200 N; Exp. 18) do not improve
the adhesion of TiO2 coatings (compared with the reference glass). As in the previous case, the coating is slightly
detached from the substrate. From Table 1, it is clear that Exp. 9 has the highest adhesion, since presents a critical load
of 9.1659  2.4068 N. This high value was correlated to its homogeneous smooth surface. Moreover, this sample shows
one of the lowest variability of the adhesion from the sample set. In Exp. 9, the film is not detached from the substrate.
3.2. Humidity effect
Figure 1 shows the effect of humidity on the self-cleaning performance of the mesoporous TiO2, dense TiO2, Degussa
P25 TiO2 and Nanosuspension B TiO2. As can be observed in Figure 1, the mesoporous, dense and Degussa P25 TiO 2
samples show the highest self-cleaning activity at medium values of humidity, that is, at 63% RH. On the other hand,
the Nanosuspension B-coated glass shows a major performance when working at low humidity (33%), although in
general this sample has a low efficiency.
Figure 1. Self-cleaning effect of the mesoporous TiO2, dense TiO2, Degussa P25 TiO2 and Nanosuspension B TiO2
films versus stearic acid at 33%, 63% and 75% RH.
To explain the low activity of the films at 33% RH and then its increase at 63% RH it has to be considered that when
exposed to moisture, the surface of TiO2 becomes hydroxylated as a result of dissociative chemisorption of water into
the Ti4+ sites [5-7]. These hydroxyl groups or water molecules behave as hole traps [29], forming surface adsorbed
hydroxyls radicals, according with equations 1 and 2 [7].
TiO2 + hv  e- + h+ (Eq. 1)
H2O + hv  OH + H+ (Eq. 2)
It is generally accepted that these hydroxyls are adsorption sites acting as the primary oxidants of organics [5-7] and
their presence is critical to the photocatalytic activity. Therefore, the low activity at 33% RH was correlated with the
low quantity of hydroxyl radicals available for the oxidation of the SA, while when increasing humidity the availability
of hydroxyl radicals is higher and the photocatalytic activity increases. The decrease of the activity at 75% RH was
correlated with the inhibition of the SA adsorption over the TiO2 samples due to the formation of a layer of physisorbed
water that hinder the access of the pollutant molecules to the adsorption sites (competition between H 2O and SA for the
adsorption sites) [6].
3.3. Temperature effect
Heat, generated either by the UV lamp or by an external source, influences the reaction pathways and the kinetics of the
organic degradation process. Figure 2 shows the effect of temperature on the self-cleaning performance versus stearic of
mesoporous TiO2, dense TiO2, Degussa P25 TiO2 and Nanosuspension B TiO2, tested at 63% RH.
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Figure 2. Self-cleaning effect of the mesoporous TiO2, dense TiO2, Degussa P25 TiO2 and Nanosuspension B TiO2
films versus stearic acid at 10°C, 20°C and 30°C.
In general, from Figure 2 it can be observed that all the samples present the better self-cleaning performance at 30 °C.
From these samples, the mesoporous TiO2 shows an acceptable efficiency in all the three tested temperatures, since in
all the cases more of the 50% of the original stearic acid is degraded. Dense TiO 2 degrades the 100% of the SA from 20
°C, while the Degussa P25 TiO2 and the Nanosuspension B samples present the better efficiency just at 30 °C. The
increase of the efficiency of the Nanosuspension B at 63% when increasing temperature (from 20 °C, Figure 1 to 30 °C,
Figure 2) could be due to the desorption of water from the photocatalyst surface. The resultant desorbed water also can
release additional oxidation sites, allowing more pollutant to be adsorbed, and hence resulting in the increased oxidation
rate [8].
3.4. Sodium effect
As mentioned previously, when using soda-lime glasses as substrates for the development of self-cleaning materials by
the deposition of a photocatalytic film, the migration of the Na+ ion from the substrate to the film is detrimental for the
photocatalytic activity. This problem can be avoided by pre-treating the glass substrates with acids to reduce the surface
sodium concentration, or by treating the “as-prepared” deposited films with acids to remove the Na + from the films [30].
Figure 3 shows the XPS spectra of the original and acid-treated glasses coated with the Nanosuspension B, and the selfcleaning performance of the “as-prepared”, treated with deionized water and with HCl 0.2M [30] coated glasses. The
photocatalytic tests were carried out using glass substrates pre-treated with CH3COOH 96% for 4h to (1) decrease
roughness and (2) decrease Na surface concentration (1 st Report).
Figure 3. XPS spectra of the original and acid-treated glasses coated with the Nanosuspension B, and the selfcleaning performance of the “as-prepared”, treated with deionized water and with HCl 0.2M coated samples.
From Figure 3 it is observed that the best efficiency is obtained when the as-prepared film is treated with HCl
0.2M/24h, although a simple treatment of the as-prepared film with deionized water also results in a high efficiency of
the film, which oxidizes almost the 100% of the original SA in four hours. Figure 3 also shows that although the Na
surface concentration of the original substrate was reduced by the acid treatment, this method by itself is not enough to
promote a high efficiency of the coated-glass. This effect can be attributed not only at the surface sodium from the glass
but also to the sodium from the Nanosuspension B (Relative atomic concentrations from XPS: Original glass 3%, Acidtreated glass 1%, Nanosuspension B-coated original glass 17%, Nanosuspension B-coated acid-treated glass 13%).
3.5. Chemical resistance
Figure 4 shows the effect of chemical treatments of the as-prepared Nanosuspension B TiO2-coated glass on its selfcleaning performance.
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Figure 4. Self-cleaning performance of the Nanosuspension B TiO2-coated glass versus stearic acid after its
chemical treatment with deionized water, acid rain, 5% isopropanol, 5% detergent solution and boiling water.
From a general point of view, the Nanosuspension B presents a good chemical resistance (that is, the TiO 2 particles
seem not to be removed from the substrates since they still present photocatalytic activity after the treatments) to the
rain (water), boiling water (aging test) and cleaning agents (isopropanol 5% and detergent solution). Indeed, these
chemical treatments promote an enhancement of the photocatalytic activity of the glasses. This can be attributed to the
following facts: (i) the surface of the TiO2 films becomes well hydrated (surface hudroxyls) after the treatments or (ii)
the treatments promote the elimination of residues from the solvents or surfactants used to stabilize the TiO2
Nanosuspension, since these tests were realized with the “as-prepared” coated-glasses, without any thermal treatment at
high temperature to remove these residues (normally, the thermal treatment is used to generate the anatase TiO2 phase
on the film, but since this Nanosuspension already contains the TiO 2 in the anatase form, thermal treatments were not
realized). The treatment with acid rain also increase the efficiency of the films probably due to the remotion of the Na
from the films.
3.6. TiO2 quantity
Figure 5 shows the effect of the number of deposited layers of Nanosuspension B TiO 2 in the self-cleaning performance
of the coated glasses versus stearic acid. From this figure is observed that the number of deposited layers has not a great
influence on the self-cleaning performance of the coated-glasses, indeed, the glass-coated with only one layer of TiO2
presents a slightly better efficiency.
Figure 5: Effect of the number of deposited layers of Nanosuspension B TiO2 in its self-cleaning performance
versus stearic acid.
4. Findings
Low roughness of the glass substrates promotes an enhancement of the adhesion of the Nanosuspension TiO 2 films. In a
general point of view, at medium values of relative humidity (63%) all the tested films present a better efficiency,
except the Nanosuspension B film, which works slightly better at 33% RH. From this preliminary study, it was
observed that mesoporous TiO2 films can be used at low, medium and high humidities, with efficiencies of 60%, 80%
and 70% of SA degradation, respectively. Furthermore, dense TiO 2 films can be used also at these three values of
humidity, although the efficiency at 75% RH is around 50% of SA degradation. Degussa P25 and Nanosuspension B
samples presented low efficiencies at all the tested humidities. In general, at 30°C all the tested film presented a better
efficiency and the mesoporous and dense samples were those which presented the better performances. The mesoporous
TiO2 can be used at 10°C, 20°C and 30°C, since it presents in all the cases efficiency higher than 60%. On the other
hand, dense films works better at 20 and 30 °C, where degrade the 100% of the original SA. P25 and Nanosuspension B
have better performance at 30°C. Regards the Nanosuspension B-coated glasses, these improve their performance when
treated with HCl 0.2M, deionized water, cleaning agents and acid rain. The chemical resistance of these glasses is good
and the TiO2 particles seem not to be removed from the substrates since they still present photocatalytic activity after
the treatments. The number of layers of deposited TiO 2 do not greatly influences the performance of the coated glasses.
5. Future work
The study of the temperature effect will be continued to test lower temperatures, since this effect has not been reported
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in literature. The effect of the chemical treatments with water, cleaning agents and acid rain of the Nanosuspension B
coated glasses will be extended with the aim to understand the mechanism by which the efficiency is enhanced.
6. References
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