Construction and Building Materials 84 (2015) 201–218
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Review
TiO2-based nanocoatings for preserving architectural stone surfaces:
An overview
Placido Munafò, Giovanni Battista Goffredo ⇑, Enrico Quagliarini
Department of Civil and Building Engineering and Architecture, Polytechnic University of Marche, via Brecce Bianche 12, 60131 Ancona, Italy
h i g h l i g h t s
Nanocoatings formed by titanium dioxide can be applied on architectural stones.
Physical properties and photocatalysis of TiO2 on stones have been investigated.
TiO2 nanoparticles can efficiently improve stone conservation and aspect over time.
Treated stones can lead to environmental benefits and reduce maintenance costs.
This review aims to provide a report on the current state of the art in this field.
a r t i c l e
i n f o
Article history:
Received 17 November 2014
Received in revised form 12 February 2015
Accepted 28 February 2015
Keywords:
Titanium dioxide
Stone conservation
Nanotechnological building materials
Self-cleaning treatments
De-polluting products
Biocidal effect
a b s t r a c t
Titanium dioxide has been recently used in its nanometric form to develop smart products and coatings
on several building components so as to better preserve their visual aspect, mainly by way of its very efficient photocatalytic function. The integration of further nanostructured materials with titanium dioxide
may enhance its features or add new properties to these products. The aim of this review is to provide a
report on the latest developments in a specific area of the maintenance of architectural surfaces: the use
of multifunctional (self-cleaning, de-polluting, biocidal) nanocoatings based on titanium dioxide on
architectural stone surfaces. The results of several studies concerning different products containing
TiO2 nanoparticles, potentially added with other nanometric elements, have been summarised and compared from several points of view focused on their compatibility with treated substrates and their effectiveness against diverse degrading agents (soil, pollution and microorganisms). From the discussed
works, the application of TiO2-based products on several architectural stones seems to be feasible and
valuable; however several features are in need of deeper analyses before real large-scale use. In addition,
possible future developments by scientific research may provide further increased performances and
additional features and functions to these treatments.
Ó 2015 Elsevier Ltd. All rights reserved.
Contents
1.
2.
3.
Architectural stones: conservation issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The titanium dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.
Photoinduced phenomena of ultrafine titanium dioxide and their applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Applications of TiO2 nanocoatings for architectural stone surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.
Aesthetical changes on treated stones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.
Wettability of treated stones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.
Photocatalytic activity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1.
Self-cleaning ability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2.
De-polluting effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3.
Biocidal efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.
Durability of the coatings on stones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author. Tel.: +39 071 2204380; fax: +39 071 2204378.
E-mail addresses: p.munafo@univpm.it (P. Munafò), g.b.goffredo@univpm.it (G.B. Goffredo), e.quagliarini@univpm.it (E. Quagliarini).
http://dx.doi.org/10.1016/j.conbuildmat.2015.02.083
0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
202
202
203
204
204
206
209
209
211
211
213
202
4.
5.
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
1. Architectural stones: conservation issues
The urban environment, and the air quality in particular, may
cause or accelerate the deterioration of several architectural elements, including for example claddings, altering both their aesthetical aspect and their physical–chemical properties. Stone is
among the most widely used building materials since ancient
times, but stone surfaces may deteriorate over time due to various
sources of damage. Stone decay in polluted urban environments is
related to several deteriorating actions as for example the increase
of polluting gases and fine particulates that may cause the disaggregation of stony materials; the surface recession of stones
caused, among others, by crystallization of soluble and insoluble
salts inside the porous network or the action of acid rains that give
rise to the so-called ‘‘black crusts’’ [1,2].
Most of these alterations involve the visual appearance of the
stone itself. The aesthetic variations of building stones may be
related to further damaging processes such as for example growth
of organisms, bird droppings, fire damage, salt efflorescence, building defects [3], design features [4], external factors as humidity and
irradiation [2], the characteristics of stones themselves [5,6].
Blackening of architectural surfaces is more intense at urban
than at rural sites [1,6–8], where anthropogenic activities are more
intense. This type of degradation is of particular importance when
it concerns the historical monuments. At the same time also for
modern buildings there is the need to keep the exterior surfaces
clean; the improvement of air quality in urban areas may sustain
this process.
Chromatic variations, or blackening, are related to the area covered by light absorbing particles and can be measured as a change
in light reflectance from materials [8]. Its perception by human eye
depends on the original colour and pattern of the background and
also on the proximity to unsoiled and clean surfaces [9]. According
to Bellan et al., soiling is detected by the totality of human observers once coverage by black particles has reached 12.0% surface
coverage but it may began to become visible to certain observers
at approximately 2.4% surface coverage when making an edgeto-edge comparison (i.e. comparing soiled and clean samples
placed directly adjacent to each other); while a 3.6% surface coverage is the smallest level of shading triggering off a detection with a
reliability superior to 90% on any background analysed in the study
using edge-to-edge comparison [9].
The aesthetic issue is not the only one afflicting building stones
that must be taken into account, since degradation processes may
cause the loss of the original stony material. Biotic factors, especially microorganisms, are among the most common sources of
stone damage, affecting their visual aspect and their surface characteristics. Microbial growth and biological compounds may have
positive effects on building materials, as for example biological
cleaning, biodegradation of pollutants, bioconsolidation; but also
negative ones, damaging exposed surfaces (i.e. biodeterioration)
[10–12]. The main factors influencing the development of biofilms
are the physical–chemical properties of affected substrates (such
as surface roughness, initial porosity, mineralogical nature, pH)
[10,13–18], the type of microorganism [18,19], the atmospheric
conditions (temperature, humidity, solar radiation) [18,20,21]
and the architectural features of surfaces [16].
Stony and other porous materials usually show very high level
of bioreceptivity, i.e. the ability to be colonised by living organisms
[10,14], especially by bacteria and algae [12,22], and it may be further increased by the deterioration from other sources as environmental pollution.
Buildings are cleaned for a variety of different reasons, including to slow down harmful processes and improve their aspect
[8]. Conservation treatments such as cleaning and the ageing of
protective coatings may cause themselves further changes in the
aspect of architectural surfaces [3], modifying (sometimes irreversibly) their original aspect. Some stone cleaning methods may
show several potential harmful consequences on substrates as
abrasive damages, increased water uptake, loss of detail, chemical
residues, salt formation, surface etching, bleaching, staining, variations in surface roughness, colour changes [3,23].
In order to avoid or at least reduce aesthetical and physical–
chemical deteriorations related to stone cleaning, the maintenance
actions should be minimised, considering their potential invasiveness and environmental impact. To reduce the frequency and possible negative effects of cleaning, it is important to prevent the
formation of soiling itself, blocking the actions of external agents:
de-polluting treatments applied on building stones may be a
promising method to both reduce pollutants rate and slow down
soiling formation on stones; protective biocidal products may limit
the attack by biological agents slowing their development reducing
the need for more invasive interventions.
Nanotechnology [24–26] can be helpful to apply on surfaces
smart solutions to resolve the aesthetical problems and to improve
the conservation of building stones over time. Nanotechnological
solutions have been already developed and used in several fields
(e.g. fibre-reinforced composites; sterilizing products for hospital
and medical applications; self-cleaning glasses; wear-resistant,
anticorrosive nanostructured coatings), including the construction
sector [27,28]. Furthermore nanotreatments can be applied over
ancient traditional materials, as stones and bricks, in order to realise smart surfaces able to develop new functions similarly to
freshly-built surfaces, giving them the ability to improve their
own conservation [29–38]. Among nanotechnological products
used in building industry, titanium dioxide is surely one of the
most investigated and used.
2. The titanium dioxide
Since the late 1960s, but especially starting from the early 90s,
many researchers focused their attention on a peculiar material
showing several interesting features: the titanium dioxide.
Titanium dioxide (TiO2 or titania) is the most occurring oxide of
titanium. It is a polymorphic compound in three main different
mineral forms: rutile, brookite, and anatase; rutile and anatase
show a tetragonal crystalline structure while brookite is orthorombic [39–41].
Titanium dioxide is already used in a very wide range of common applications. In the form of microscaled particles TiO2 is
bright and has a very high refractive index, among the highest
available in nature [42]. Furthermore it is easily available, relatively inexpensive, harmless and chemically stable. Thanks to these
qualities, titania is the most widely used white pigment – acting
also as an opacifier, a sunscreen and a thickener – in several fields
since ancient times: paints, coatings, inks, ceramic glazes, cosmetics, plastics, papers, textiles and even in medicines, cosmetics,
skin care products, toothpastes and food colouring [40,43–51].
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
Ultrafine TiO2, also known as TiO2 nanoparticles, having a mean
diameter between 5 and 50 nm, conserves many of the properties
of pigmentary titania but at the same time it differs greatly [42].
The optical behaviour is among the most evident changes: the
diameter of TiO2 nanoparticles is far smaller than visible wavelengths, so visible spectrum of light is virtually transmitted
through the material and TiO2 nanoparticles of the mineral phases
may appear transparent, while UV light is scattered and absorbed
[42]. Moreover, TiO2 nanoparticles may exhibit additional properties related to their relation with UV spectrum of light.
2.1. Photoinduced phenomena of ultrafine titanium dioxide and their
applications
One of the main characteristic of nanometric TiO2 is its ability to
activate further features by means of the exposure to light: the
photoinduced properties. Titanium dioxide is typically a n-type
semiconductor and its photoinduced abilities are mainly related
to the generation of charge carriers (free electrons and holes) by
interaction with photons having sufficient energy [40,52,53]: in
the case of TiO2 ultraviolet (UV) light, including the UV range of
sunlight, is necessary to its own activation. The photogenerated
energy-rich charge carriers are highly reactive radicals having
strong reducing and oxidising ability that may react with other
chemical species (electron acceptors/donors) entering into contact
with the semiconductor. This reactivity leads to photocatalysis, the
acceleration of a photochemical reaction by means of a catalyst
interacting with light of sufficient energy: during the photocatalytic process both oxidation from photogenerated holes and
reduction from photogenerated excited electrons occur simultaneously [47,48,52,54].
The photodegradation ability of TiO2 implies its potential use
for several purposes in different fields: environmental engineering,
organic photosynthesis, microbiocidal applications, photokilling of
tumour cells, solar energy conversion [40]. Nowadays, the greatest
interest in TiO2 photocatalysis concerns the environmental applications, and in particular the photodegradation of organic compounds into harmless inorganic substances.
Titanium dioxide exposed to ultraviolet illumination may
reveal another photoinduced phenomenon [43,44,47,55]:
superhydrophilicity. Titanium dioxide is normally hydrophobic,
anyway under UV light it becomes amphiphilic, i.e. both hydrophilic and oleophilic at the same time, till the almost complete flattening of water droplets or organic liquids on the surface of titania
film [47]. The photoinduced high wettability of TiO2 depends on
the characteristics of UV illumination in many respects: it activates
even under low-intensity UV light but it becomes more rapid at
higher UV irradiation values [43]; it persists after the end of exposure to UV irradiation [44,55,56]; it is reversible so during long
storage in the dark TiO2 reverts to its normal, hydrophobic state
[43,47,55] and then it reactivates under UV light; furthermore
the hydrophilicity enhances itself by repeated UV illumination
cycles [40]. In the same way of photocatalysis, the mechanism proposed to explain the photoinduced superhydrophilicity bases on
the production of excited electrons and holes by ultraviolet illumination: the oxygen vacancies on the TiO2 surface interact strongly
with water leading to its dissociative absorption and producing
OH radicals that make the surface hydrophilic.
Since photoinduced processes occur on titania surface, higher
specific surface area and porosity of nanoparticles entail greater
photoactivity [57]. Among the crystalline structures of TiO2, anatase shows the highest photocatalytic activity because of its characteristics. The crystallite size is increased by increasing
temperature, and it can change abruptly during the anatase–rutile
phase transformation because of the coalescence of small anatase
particles into larger rutile grains, usually accompanied by
203
considerable grain and pore growth [57–59]: so this dimensional
variation must be prevented in order to preserve the photoactivity
of titania particles.
Although both photocatalysis and superhydrophilicity are activated by ultraviolet illumination and they can occur simultaneously on the same surface, the photochemical processes
responsible for these phenomena are completely different and
independent [43,56]. The photocatalysis may deteriorate external
polluting compounds in contact with the titania surface into less
harmful substances leading to a de-polluting effect. It can even
bring to a disinfectant property through the destruction of biological materials. The hydrophilic effect may create a uniform water
film over flat surfaces and impede the development of water droplets (anti-fogging effect). The synergy of these two photoinduced
properties may enhance these effects or cause different functions
on treated surfaces: the biocidal property is enhanced by
hydrophilicity since it may prevent the direct contact between living organisms and treated surfaces through the creation of a uniform water film, while the combination of photodegradation of
aggressive substances, including dirt, and hydrophilic water film
makes the removal of external agents easier, so causing a selfcleaning effect. Especially with regard to stones, the photogenerated effects of titanium dioxide could bring to a better
preservation of treated substrates, limiting the onset of degrading
processes due to stain, biological attacks and pollution.
Titanium dioxide is one of the most important and common
photocatalyst because of its outstanding efficiency: along with
the properties showed in its microscaled form, titania nanoparticles show high photocatalytic activity even under weak solar
irradiation in comparison with other metal oxide photocatalyst
(TiO2 has the highest light conversion efficiency), and good compatibility with a large number of other materials (including biological ones) without making original performances got worse.
Nanoscaled titanium dioxide particles have been used in fibres,
clothes, leathers, lightings and sprays due to their self-cleaning
and antibacterial properties [44].
Anyway, the widest application of titania is in building industry.
The self-cleaning and de-polluting effects of titanium dioxide fit
perfectly the use in the extensive variety of building materials
[44,45,47,51,60–65]: exterior and interior tiles, glasses, paints,
coatings, wallpapers, paving materials, plastic films, window
blinds, metallic panels, air cleaners, air conditioners, purification
system for wastewater or pools and other building elements in
order to obtain sterilizing, anti-fogging, de-pollution and selfcleaning surfaces and systems. It is possible to apply TiO2 in a wide
range of ways, so as to obtain different treatments: surface coatings (by spray or brush), dip coatings, thin film by anodic oxidation,
addition in the bulk of composite materials.
The activation of photoinduced properties by mere solar light in
a catalytic process very similar to photosynthesis can reduce the
power consumption and the emission of greenhouse gases.
Furthermore, the synergistic effect with the simple rainfall to
obtain self-cleaning surfaces limits the number of cleaning actions
and consequently the water consumption and the use of chemical
products, obtaining a environmentally friendly solution [66,67].
The photodegradation of various toxic compounds present in
the urban atmosphere may also reduce the air pollution in the
areas nearby to treated surfaces [42,68–77]. Photodecomposition
of pollutants is an important and deeply studied feature of TiO2based coatings and treatments for construction industry, even for
indoor use. The influence of several factors as different reactors,
artificial light sources, irradiation power, water vapour has been
analysed to establish the efficiency of photocatalytic oxidation of
many polluting compounds (as for example ethanol, butanol,
ethylene, formaldehyde, toluene) for indoor air purification
[42,68–70]. De-polluting performances were deeply monitored
204
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
for cementitious products, paintings, claddings, roofing, glasses
and paving materials [68,69,71,72,75–84]. Several investigations
were carried out even using commercial products and they
involved the use of chamber to simulate a real case scenario
[71,72]; parking area directly polluted by a car [71]; several in field
tests and pilot applications, especially for road materials (concrete
roads and pavements) monitored over time [73,74,84]; laboratory
tests inside reactor examining the decomposition of several polluting compounds (NOx, BTEX, toluene) [73–77]. Photodegradation of
pollutants was quite evident even in real case studies and the
dependence on a wide range of factors such as temperature, relative humidity, the kind of contact with polluted air, UV light irradiance, the type of TiO2 and its application procedure were observed
[73,74,77].
Even the photogenerated biocidal ability of TiO2 (TiO2 is not a
germicide in the dark [85]) may be very important in building
industry, as told before. Several biocidal treatments have been
used on architectural substrates [86], including TiO2-containing
products. The microbiocidal effect of TiO2 was firstly reported by
Matsunaga et al. [87], then photocatalytic killing by titania was
studied on a wide spectrum of organisms including viruses, bacteria, fungi, algae, and cancer cells [85]. Characteristics of target
microorganism, especially structural properties as the complexity
and thickness of cell envelope, influence their own reaction to
TiO2 photocatalysis: in particular, fungi are usually less subject to
photocatalytic inactivation than bacteria and viruses because of
their greater structural complexity [85,88]. The real mechanism
which results in microbial disinfection by TiO2 is still under
debate; anyway it is mainly connected with the ability to photogenerate radicals (O2 , HO2, OH) able to oxidise and mineralise
organic molecules [89], with hydroxyl radicals (OH) being the primary species responsible for the biological inactivation in the
photocatalytic process [90,91]. The radicals may destroy extracellular as well as intracellular targets by oxidation, especially
the cell membrane by lipid peroxidation and several organic molecules including DNA [92]. Usually oxidation damages cell wall at
the beginning then it involves cell membrane and inner part of
the cell allowing the free efflux of intracellular contents that eventually leads to cell death [93]. Furthermore, depending also on
their size, free TiO2 particles may also enter into membranedamaged cells and subsequently attack directly the intracellular
components thus accelerating cell death [92,93]. Biocidal efficiency
seems to be also related to pH values [91] and it can be enhanced
by the doping with other materials (e.g. silver, carbon, sulphur)
[94]. The biocidal action of TiO2 is well-known [43,95] and it was
tested, among others, for several building materials and systems
(concrete, mortars, bricks, wood, roofing tiles, photocatalytic reactors for treatment of wastewater, indoor antimicrobial control
reactors) [88,96–107].
The use of TiO2 in building industry has been particularly widespread since the 2000s, nonetheless applications on stones (both
ancient and modern) and other historic surfaces are still quite limited and mainly focused in the sector of academic research despite
the potential benefits of this type of treatments. From here on we
will provide a reasoned report on the current state of art in this
research field.
3. Applications of TiO2 nanocoatings for architectural stone
surfaces
The TiO2-based treatments were successfully used on several
types of building elements (even for commercial purposes) especially during the production processes, but TiO2 nanoparticles
can be applied even on preexisting surfaces. Stone is among the
most widely used building materials since ancient times mainly
thanks to its availability and high mechanical strength, so a better
conservation of their aspect and features can be a key factor for
preservation purposes.
Since most of the degradation processes begin from outer layers
of the stones, the development and application of transparent TiO2
surface treatments on historical and architectural stone surfaces
could lead to significant improvements in conservation and protection of Architectural Heritage – saving costs for maintenance –
without altering the original features of treated substrates.
Furthermore, preventive and lasting actions may be more effective
than invasive and repeated solutions, especially in aggressive
urban environment, reducing eventual cultural losses.
In order to evaluate both efficiency and compatibility of TiO2
treatments for the restoration of historical stone surfaces, but even
for contemporary stone elements, it is necessary to testify the fulfilment of several essential requirements:
(1) titania treatments should be applied without altering the
original aspect of stone;
(2) physical and chemical changes induced to treated surfaces
should not lead to harmful consequences for the stone substrate and the coating should be compatible with other conservative treatments;
(3) TiO2 coatings must achieve their main goals (self-cleaning,
de-polluting, biocidal activities) effectively;
(4) additionally, the characteristics of the nanocoating must be
stable over time to avoid the triggering of undesirable and
unexpected effects and the self-cleaning performance must
be durable to limit the need of further treatments.
The evaluation of these features is the key factor to establish the
feasibility of the use of TiO2-based treatments on architectural
stone surfaces. As nanotreatments and substrates mutually influence their own properties, efficiency and durability [108–115], it
is important to conduct studies specifically focused on the TiO2stones interactions. Despite its wide use in building industry, the
number of papers concerning the application of titanium dioxide
on stones is still rather limited.
Titanium dioxide nanoparticles can be obtained by different
methods, as for example chemical vapour deposition and precipitation procedures, but the sol–gel method is the most widely
employed by far due to its inexpensive equipment required, low
temperatures during the process and the homogeneous and highly
pure product produced.
Limestones, even in purely local forms, are by far the most
widely used substrate in the works analysed [116–128]. Other
examined stones (sometimes in the same studies together with
limestone substrate) are marbles [117,122,129,130] and dolostones [131]. Most of the selected substrates were light coloured,
as they are more visually affected by dirt, and therefore in greater
need of cleaning, and less subject to brightening effect due to
TiO2-based treatments. Only two methods for application were
used: spray coating [116,118–121,123–125,131] and brushing
[117,122,126–129], mostly because of their simplicity and their
compatibility with external stone surfaces.
In the following subsections the requirements of titania treatments previously listed will be analysed as they have been in the
considered works. For the sake of clarity and to permit simpler
comparison between all studies evaluated in this review, they will
be fully reported in all following tables; whether they contemplated the section of interest of the specific table or not.
3.1. Aesthetical changes on treated stones
As regards the first prerequisite on aesthetic variation, most of
the work has focused exclusively on chromatic variation related
205
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
to the application of the coating [117,118,122,127,129–131]. The
authors contributed to several investigations [119,121] in which,
as well as colour analysis, the variation of specular reflection
(gloss) between untreated and treated surfaces was evaluated by
portable glossmeter. The specular reflection of stone substrates
was not altered by nanocoatings [119,121].
Colour changes have always been expressed in the CIELAB system: a perceptually uniform colour space (the same change in colour space corresponds to same change in perceived colour)
mimicking the human vision. The CIELAB colour space is defined
by three different chromatic coordinates: L⁄ (lightness, range:
0–100 from black to diffuse white), a⁄ (negative values towards
green, positive values towards red) and b⁄ (negative values
towards blue, positive values towards yellow). Colour difference
DE⁄ is defined as the Euclidean distance between two different
points:
DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðL1 L2 Þ þ ða1 a2 Þ þ ðb1 b2 Þ
ð1Þ
Conventionally DE⁄ = 1 is considered the just noticeable difference (JND) perceivable by human eye, furthermore the minimum
surface coverage (3.6%) required to trigger the detection of dirt
by human eye between clean and soiled surfaces given in the study
by Bellan et al. previously reported corresponds to a DE⁄ of about 1
[9]. Anyway several authors reported different values of JND in a
more generic 2–3 units range [132–134]. Moreover, for the restoration of ancient buildings colour changes with DE⁄ < 5 are considered fully acceptable [135,136].
In almost all of the analysed works colour variations were
below or at least very close to human perception threshold.
Aqueous colloidal suspensions (sols) [117–119,121–124,126,
127,130,131] usually brought about lower colour changes in comparison with other hybrid products containing additives
[117,124,125,127,129–131] (Table 1). Their colour changes were
usually very close to JND [118,119,122,123,127], till totally negligible values [121], or at least lower than the highest colour variation
suitable in the field of restoration [126,130], so these treatments
Table 1
Application of coatings and aesthetical variations. Standard deviations values (±) are reported for studies that expressly stated them. In case different treatments were analysed in
the following listed works, only those containing TiO2 were considered in this table.
Limestones
Reference
(first author)
Stone
Type of product applied
Application procedure
Aesthetic changes
after coating
applicationa
Potenza [116]
Luvidi [117]
Pietra di Lecce
Red limestone
Spray coating
Brushing
–
DE⁄ = 3.32 ± 1.57
DE⁄ = 3.29 ± 1.24
DE⁄ = 2.94 ± 0.94
DE⁄ = 4.34 ± 0.80
1.85 6 DE⁄ 6 2.61
DE⁄ = 1.06
DE⁄ = 1.4
DE⁄ = 2.0
–
–
DE⁄ = 0.4
DE⁄ = 0.6
DE⁄ = 1.5
a.a.: DE⁄ = 1.4
DE⁄ = 1.8
a.a.: DE⁄ = 2.1
DE⁄ = 2.15
a.a.: DE⁄ = 1.48
DE⁄ = 2.50
a.a.: DE⁄ = 1.11
0.66 6 DE⁄ 6 1.59
1.72 6 DE⁄ 6 5.92
2.20 6 DE⁄ 6 4.60
Licciulli [118]
Pietra di Lecce
Quagliarini
[119]
Quagliarini
[120]
Quagliarini
[121]
La Russa [122]
Travertine
TiO2 sol (sol–gel)
Commercial product
TiO2 sol
Commercial product
TiO2 sol
TiO2 sols (sol–gel)
Commercial product
TiO2 sol (sol–gel)
Travertine
TiO2 sol (sol–gel)
Travertine
Commercial product
Limestone
Commercial product
Black limestone
Spray coating
Spray coating, single-layer
Spray coating, multilayer
Spray coating, single-layer
Spray coating, multilayer
Spray coating, single-layer
Spray coating, multilayer
Brushing, low amount
Brushing, high amount
Munafò [123]
Travertine
TiO2 sol (sol–gel)
Spray coating, single-layer
Spray coating, multilayer
Pinho [124]
Fossiliferous white limestone
Pinho [125]
Fossiliferous white limestone
Bergamonti
[126]
Modica stone
Comiso stone
Marbles
Bergamonti
[127]
Aflori [128]
Pietra di Lecce
Luvidi [117]
White marble
La Russa [122]
Carrara marble
Repedea (porous bioclasticoolitic
limestone)
Commercial products
TiO2–SiO2 sols
TiO2–SiO2 sols
TiO2 sol
TiO2 sol after SiO2 treatment of
surfaces
TiO2 sol
TiO2 sol after SiO2 treatment of
surfaces
TiO2 sols
TiO2 sol with gold nanoparticles
TiO2nanocomposite
TiO2–Ag nanoparticles
Commercial product
TiO2 sol
Commercial product
Spray coating until saturation
Spray coating until apparent
refusal
Brushing
DE⁄ = 3.57
DE⁄ = 2.93
Brushing
Brushing
Brushing
Brushing, low amount
Brushing, high amount
Dolostone
a
DE⁄ = 4.78
DE⁄ = 3.92
2.7 6 DE⁄ 6 2.8
DE⁄ = 4.9
–
–
DE⁄ = 1.55 ± 0.85
DE⁄ = 2.12 ± 0.23
DE⁄ = 1.1
a.a.: DE⁄ = 1.3
DE⁄ = 1.2
a.a.: DE⁄ = 1.3
DE⁄ = 3.14 ± 0.36
Kapridaki
[129]
Ruffolo [130]
Dionysos and Thasos marbles
TiO2–SiO2 sol
Brushing
Marble
TiO2 sols
TiO2 + Ag sols
–
3.36 6 DE⁄ 6 3.79
3.03 6 DE⁄ 6 25.08
Pinho [131]
Dolostone
Commercial product
TiO2 sol
Spray coating
DE⁄ = 1.05
DE⁄ = 6.96
After ageing (a.a.) results are also reported when available.
206
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
can be considered transparent and compatible for the use on historical stones. Hybrid sols caused higher and less uniform changes
related to the material used in their composition [117,124–
127,129–131] with the only exceptions of the studies by Pinho
and Mosquera and La Russa et al., where part of the newly developed TiO2–SiO2 nanocomposites [124,125] and the commercial
polymer [122] under analysis showed very low colour changes.
Colour differences between an aqueous commercial product
and a new TiO2 functionalized polisiloxane were monitored by
Luvidi et al.: both coatings altered the aspect of stones in negligible
way, anyway the commercial treatment generally showed lower
DE⁄ values. Red and black limestones used in the study together
with white marble were subject to stronger colour modification.
This behaviour is mostly connected to the original colour of stones,
as TiO2 particles usually cause a whitening effect and it can be less
noticeable on light coloured surfaces [117].
Two different mesoporous titania–silica nanocomposites
(UCATip and UCATiO), developed and studied by Pinho and
Mosquera to ensure self-cleaning property, good adherence of
the coatings to substrate and improved mechanical resistance of
the surface, were applied on limestone obtaining different treatments as a functions of the TiO2 percent quantity used in the sols
[124]. Both treatments caused higher colour changes than commercial, aqueous dispersions of TiO2 particles. The commercial
products showed negligible colour variations and greater mass
concentrations of titania increased their DE⁄ values. On the other
hand the nanocomposites produced colour differences mainly
between JND (UCATiP coatings) and the limit suitable for the use
in the field of restoration (UCATiO coatings); part of the latter
(UCATiO containing minor loading of titania) showed even greater
colour changes, just beyond the threshold for acceptability. As for
these products, higher amounts of titania particles in the hybrid
materials decreased colour changes in an evident way, since the
chromatic shift is mostly related to the other reagents used in
the syntheses of the sols [124]. The intensity of colour changes
was confirmed during further analyses on dolostone [131]: TiO2–
SiO2 nanocomposite caused a chromatic variation over the generally accepted value for monumental stones (DE⁄ 6 5) and greater
than a commercial photocatalytic product but far smaller than a
commercial consolidant used to improve mechanical qualities of
stone. In the end, using various TiO2 nanoparticles in different proportions, several TiO2–SiO2 treatments causing colour variations
fully acceptable for stone conservation were obtained [125].
Chromatic variations were very close to both silica material used
for comparison and the human perception threshold.
Bergamonti et al. [126] evaluated colour difference of TiO2 sol
applied directly or with a silica interlayer on two different limestones: chromatic variations were quite limited and fully compatible with restoration purposes, moreover the presence of the
preliminary SiO2 coating further limited colour changes. To
increase photoactivity of the nanocoatings, metal doping (gold
(Au) addition) was used in a subsequent study [127] on Pietra di
Lecce: colour changes were still limited but Au-doped titania
clearly caused higher variations, closing the gap with the limit
acceptable in the field of Architectural Heritage.
A TiO2–SiO2 nanocomposite developed and analysed by
Kapridaki et al. showed limited colour changes on marble [129],
anyway the lowest aesthetical variations due to a hybrid product
was reported by La Russa et al. [122], where colour changes were
totally unnoticeable.
The work of Ruffolo et al. focused on both undoped and silverdoped titania coatings to contrast the degradation by marine fouling of underwater archaeological sites [130]. Silver nanoparticles
were scarcely used for historical stones because they can form silver agglomerations and cause high colour variations [128]. To
ensure better adherence to selected stone substrate (marble) the
sols were dispersed in a binder (acetone solution of ParaloidB72).
Pure titania showed colour changes just above the human eye
possibility of detection, independent on the titania loading used
in the product. Ag-doped titania caused slightly higher modifications in low concentrations, but the visual appearance was greatly
conditioned by elevated presence of the metallic compound, as
chromatic differences increased dramatically with increasing AgTiO2 content, up to a totally unacceptable level (DE⁄ > 25) in the
case of Ag-TiO2/binder ratio equal to 1/1: a ratio of 1/10 was suggested as the upper limit to avoid aesthetical nuisances on treated
stones.
In conclusion, colour changes due to the application of the TiO2based nanocoatings were partly dependant on the characteristics
of treated stones, especially their original colour: the whitening
effect of titania altered more the aesthetic aspect of dark-coloured
stones [117]. The amount of titania applied on stone affected chromatic variations, but not in a strictly proportional way [119,121–
123]. Furthermore, colour changes were not clearly dependant on
the type of application (spray-coating or brushing) used, considering that both methods showed very wide ranges of DE⁄ values,
usually barely noticeable to naked eye and mostly under the suitable limit for restoration uses. The presence of functionalised additives usually led to higher chromatic variations, but the changes
can be associated with other parts of the coating as binder.
Finally, the aspect of stones after the application of nanocoatings seems to be constant over time, as seen at the end of accelerated ageing processes [122,123].
3.2. Wettability of treated stones
Water, and water penetration in particular, is among the most
influential and therefore potentially harmful abiotic factors of
decay for stones and other porous materials. Moreover, it is a carrying agent of several biological compounds and aggressive
microorganisms that can deteriorate stones further.
As a consequence the permeability of treated stones is the most
studied modification of original properties of substrates in order to
check the compatibility of TiO2-based treatments with stones. To
analyse the consequences of titania coatings on wettability of
treated stones, several analyses were performed: contact angle
[117,118,120–125,129], water absorption by capillarity [116,118,
120–122,124–127,129], water vapour permeability [116,118,129]
and nebulised water absorption [120,121,123]. Despite the importance of UV illumination to activate the photoinduced hydrophilicity of titania and the presence of previous studies about the
relationship between UV light and wettability on several TiO2-treated substrates, the vast majority of the tests on stones were carried
out without direct sources of UV light. A summary of the results is
reported in Table 2.
Contact angle (CA) measurements were not always possible
since the rapid absorption of water drops by limestones even
before treatment [117,118,122]. Pure titania sols may have lower
contact angle values with respect to untreated stones even without
exposure to UV light required to trigger the hydrophilic behaviour
of the TiO2 [117,121,124]. A commercial aqueous TiO2 sol containing residual diethylene glycol from synthesis decreased CA values
in the most evident way, leading to a behaviour quite close to
superhydrophilicity [121]. The results were not always consistent,
since evident increase of original contact angles, little variations
(even positive) or no variation at all by pure TiO2 sol in the absence
of UV light were equally reported [120,124]. The photoinduced
hydrophilicity of TiO2 coatings during the exposure to UV light
having an irradiance value of 20 W/m2 (more intense than the
usual irradiation used during the experiments carried out on other
substrates) was clearly confirmed by a research team including the
authors: CA values were at least halved compared to untreated
207
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
Table 2
Behaviour of coated stones towards water. In case different treatments were analysed in the following listed works, only those containing TiO2 were considered in this table.
Much of the results listed here were estimated from graphical data in articles and not expressly stated, so they should be considered approximate.
Limestones
Reference (first
author)
Stone
Type of product, type of application
Wettability tests
Results compared
to untreated (UT)
surfacesa
Potenza [116]
Pietra di Lecce
TiO2 sol, spray coating
DCI = 0.55
DP = 10%
Luvidi [117]
Red limestone
Commercial product, brushing
TiO2 sol, brushing
Commercial product, brushing
TiO2 sol, brushing
TiO2 sols, spray coating
Commercial product, brushing, low
amount
Capillary rise
Water vapour
permeability
Contact angle
Contact angle
Contact angle
Contact angle
Contact angle
Capillary rise
Water vapour
permeability
Contact angle
Capillary rise
Water vapour
permeability
–
–
Contact angle
Contact angle under UV
light
Capillary rise
Water absorption
Water absorption under
UV light
Contact angle
Contact angle under UV
light
Capillary rise
Water absorption
Water absorption under
UV light
Contact angle
Capillary rise
Water absorption
Water absorption under
UV light
Contact angle
Capillary rise
Water absorption
Water absorption under
UV light
Contact angle
Capillary rise
Commercial product, brushing, high
amount
Contact angle
Capillary rise
TiO2 sol, spray coating, single-layer
Water absorption
TiO2 sol, spray coating, multilayer
Water absorption under
UV light
Water absorption
Black limestone
Licciulli [118]
Pietra di Lecce
Commercial product, spray coating
Quagliarini [119]
Travertine
Quagliarini [120]
Travertine
TiO2 sol, spray coating, single-layer
TiO2 sol, spray coating, multilayer
TiO2 sol, spray coating, single-layer
TiO2 sol, spray coating, multilayer
Quagliarini [121]
Travertine
Commercial product, spray coating,
single-layer
Commercial product, spray coating,
multilayer
La Russa [122]
Munafò [123]
Pinho [124]
Limestone
Travertine
Fossiliferous white limestone
Pinho [125]
Fossiliferous white limestone
Bergamonti [126]
Modica stone
Comiso stone
Bergamonti [127]
Pietra di Lecce
Commercial products
TiO2–SiO2 sols, spray coating until
saturation
TiO2–SiO2 sols, spray coating until
apparent refusal
TiO2 sol, brushing
TiO2 sol brushed after SiO2 treatment of
surfaces
TiO2 sol, brushing
TiO2 sol brushed after SiO2 treatment of
surfaces
TiO2 sols, brushing
TiO2 sol with gold nanoparticles,
–b
–b
–b
–b
–c
DQ 0% DCI = 0%
10% 6 DP 6 7%
–c
DQ = 0% DCI = 0%
DP = 6%
–
–
Dh = +32%
Dh = 56%
DQ 0%
DQ = 18%
DQ = 54%
Dh = 0%
Dh = 65%
DQ = +13%
DQ = +13%
DQ = 45%
Dh = 72%
DQ = 8%
DQ = +28%
DQ = +25%
Dh = 82%
DQ = +8%
DQ = +29%
DQ = +6%
Water absorption under
UV light
Contact angle
Capillary rise
Contact angle
Capillary rise
Contact angle
Capillary rise
Capillary rise
Capillary rise
–d
DQ = 87%
a.a.: DQ = 94%
–d
DQ = 91%
a.a.: DQ = 94%
DQ = +28%
a.a.: DQ = 3%
DQ = 26%
a.a.: DQ = 9%
DQ = 0%
a.a.: DQ = +18%
DQ = +6%
a.a.: DQ = +24%
–62% 6 Dh 6 +15%
–
+44% 6 Dh 6 +98%
97% 6 DTWU 6 96%
+30% 6 Dh 6 +98%
99% 6 DTWU 6 –94%
DRCI = +6%
DRCI = +5%
Capillary rise
Capillary rise
DRCI = 3%
DRCI = +28%
Capillary rise
Capillary rise
25% 6 DRCI 6
DRCI = 25%
15%
(continued on next page)
208
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
Table 2 (continued)
Marbles
Reference (first
author)
Stone
Type of product, type of application
Wettability tests
Results compared
to untreated (UT)
surfacesa
Aflori [128]
Repedea (porous bioclasticoolitic
limestone)
brushing
TiO2nanocomposite, brushing
TiO2–Ag nanoparticles, brushing
–
–
–
–
Luvidi [117]
White marble
La Russa [122]
Carrara marble
Contact angle
Contact angle
Contact angle
Dh = 54%
Dh = +56%
Dh = +163%
a.a.: D h 0%
DQ = 26%
a.a.: DQ = 10%
Dh = +181%
a.a.: D h 0%
DQ = 26%
a.a.: DQ = 15%
Dh = +46%
DWCA = 88%
DP = 34%
Commercial product, brushing
TiO2 sol, brushing
Commercial product, brushing, low
amount
Capillary rise
Commercial product, brushing, high
amount
Contact angle
Capillary rise
Dolostone
Kapridaki [129]
Dionysos and Thasos marbles
TiO2–SiO2 sol, brushing
Ruffolo [130]
Marble
TiO2 sols
TiO2 + Ag sols
Contact angle
Capillary rise
Water vapour
permeability
–
–
Pinho [131]
Dolostone
Commercial product, spray coating
TiO2 sol, spray coating
–
–
–
–
–
–
a
Capillary rise percentage results may refer to maximum amount of absorbed water Q (kg/m2), capillarity index CI (the ratio between the area underneath the absorption
curve and the amount of water absorbed at the final time of the test times the whole duration of the test), relative capillarity index RCI (the ratio between the amount of water
absorbed by treated samples for the whole duration of the test and the amount absorbed by untreated ones), total water uptake TWU (the percentage ratio between the final
weight at the end of the test and the original dry constant weight) or water capillary absorption coefficient WCA (kg/m2 h); according to available data. After ageing (a.a.)
results are also reported when available.
b
It was impossible to perform the analysis on untreated stones.
c
It was impossible to perform the analysis on both treated and untreated stones.
d
The contact angle of untreated surfaces was considered equal to 0°.
surfaces [120]. On the other hand, hydrophilicity of titania under
the same light exposure seemed not durable in the long run. A preliminary analysis [123] demonstrated that the impact of TiO2 on
wettability of treated stones under UV illumination was clearly
reduced at the end of accelerated ageing, as well as its photoactivity. The effect of photoexcited titania nanoparticles on wettability
was evident at time zero, whereas after artificial weathering the
amount of absorbed water was practically unaltered regardless to
the presence of UV light [123]. The evaluation of water absorption
by porous and irregular surfaces – as stones may be – may depend
on several parameters, including environmental conditions, so the
increase of case studies and the use of different analyses are necessary to better define the influence of the combination of TiO2 films
and UV rays on treated substrates. Nanocomposites showed
different behaviours connected to the materials used in their structure: most hybrid coatings were intentionally hydrophobic
[117,122,124,125,129] to reduce the risk of greater water absorption by treated substrates. Even for hybrid products, the contact
with water was deeply influenced by the content of titania used
in the sol production, as contact angles decreased with increasing
percentage of TiO2 in the sols [124,125]: the behaviour remained
hydrophobic anyway, with values ranging to the minimal increase
observed among hydrophobic products till the double of those of
untreated stones [124,125]. On the other hand, the size of TiO2 particles used in the sols seemed to be not so important for final
results [125]. On marbles, Luvidi et al. and Kapridaki et al. noted
similar and more average behaviour [117,129]. La Russa et al. stated the greatest increase of contact angle values for marble surfaces (they were almost tripled in comparison with untreated
stone); treated limestones showed even higher CA values but it
was not possible to compare reasonably the results with untreated
case since the original contact angle was considered equal to 0°
due to the absorption by limestone [122]. The results were absolutely unconnected with the total amount of sols deposited over
treated surfaces [120–122]. Artificial ageing through simulated
solar radiation may alter further the contact between liquid drops
and coated substrates. As for hydrophobic coatings studied by La
Russa et al., treated marbles completely lost their water repellency
and the CA values were very close to those of uncoated surfaces,
while the reduction of hydrophobicity of treated limestones was
far less evident (variations between 22% and 27%) [122].
Quagliarini et al. reported the complete loss of photoinduced
hydrophilicity of TiO2 sols previously noted [120] after ageing:
anyway there were still differences with respect to untreated surfaces because CAs were higher and more homogeneous [123].
As for water absorption by capillarity, different parameters
were taken into account (see Table 2 for further details). It is
important to notice that standard tests to evaluate capillary rise
do not allow the exposure to external light sources, so photogenerated hydrophilicity of TiO2 nanoparticles cannot be activated and
the changes observed are not ascribable to this feature. Anyway,
nanocoatings alter surface morphology of treated stones and consequently modify their water uptake even without the presence
of a real hydrophilic effect, as clearly shown by the results.
Capillary rise was not influenced in a negative way by titania treatments. Aqueous TiO2 sols usually caused negligible variances
[116,120,121,126,127] or no differences at all [118,120,126] in
comparison with untreated surfaces. The same (or very similar)
aqueous TiO2 sols may manifest slightly different performances,
within a 10–15% spread of absolute variation, depending on various parameters: the amount of titania deposited on stone
[120,121], the synthesis process used to obtain the product [127],
the specific stone under treatment [126]. In order to avoid higher
water absorption by treated surfaces, composite treatments were
usually hydrophobic: total water uptake by both limestones and
marbles was reduced in an evident way [122,127] till negligible
values of water uptake [124,125,129]. The presence of SiO2 nanoparticles in the composites was clearly the main cause of the
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
reduction in capillary rise. The use of gold nanoparticles increased
the hydrophobic behaviour on limestone, but results were consistent to those of aqueous TiO2 sols [127]. The results can be influenced by original properties of stone substrates, especially
surface roughness and porosity, and the heterogeneity of stone
samples themselves [122,126]. The results were also partly dependant on the kind of treated stone. In contrast to what was seen for
the pure TiO2 coatings, Bergamonti et al. [126] reported different
behaviours between two diverse limestones previously treated
with SiO2 sol, although having similar porosity: the absorption
by Modica stones was not influenced by treatments, while Comiso
stones showed a noticeable increase after the application of the
TiO2 coating. Furthermore, the polymeric TiO2-containing product
analysed by La Russa et al. influenced the capillary rise of treated
surfaces in very different ways: water absorption by marble was
almost unaltered in both results and kinetic, on the other hand
limestone absorbed a highly reduced quantity of water. The results
were very homogeneous and independent of the amount of product applied over stone. Finally, it is important to notice that different type of stones brought to very dissimilar values of water
absorption by capillarity: usually limestones absorbed much more
water than marbles (up to 1.000 times more) and there were very
evident differences between diverse limestones too.
In order to evaluate the effect of photoinduced hydrophilicity
on treated surfaces, the authors and others developed a test to
measure water absorption under UV illumination (UV irradiation
value: 20 W/m2), spraying a settled amount of nebulised water
on almost vertical surfaces exposed to UV lamps [120,121,123].
Hydrophilic treatments clearly decreased water absorption values
under UV light in comparison with both untreated stone and treated surfaces before the exposure to UV light. The results were
much more homogeneous with respect to untreated surfaces, since
the characteristics of the coatings prevailed over the original properties of the substrate [120], but this behaviour was not fully confirmed during further analyses [121,123] and it was not durable
over time [123]. Anyway, water uptake, a possible harmful source
of damage for stones, was not increased and there were not evident
differences between treated and untreated surfaces.
Capillary water absorption may remain unaltered over time or
be deeply conditioned by ageing. The hydrophilicity of TiO2 coatings disappears at end of accelerated weathering processes [123];
on the other hand, the durability of hydrophobic property seems
to be strictly related to the substrate. The high porosity of limestones leads to greater penetration of the coating into substrate
and higher quantity of product applied in long-term uses, as a consequence the behaviour towards water is more stable during time,
while hydrophobic coatings over marbles tend to lose their feature
over time [122].
With regard to water vapour permeability, hydrophilic titania
coatings caused almost no change in comparison with untreated
stones [116,118], while hydrophobic coatings led to more evident
decrease, in any case within the acceptable ranges of hydrophobic
products applied to monument surfaces [129].
3.3. Photocatalytic activity
3.3.1. Self-cleaning ability
Self-cleaning activity of titania coatings was evaluated by the
use of different test methods. All analysed procedures shared the
application of artificial dyes to stain the samples before the
exposure to UV illumination and the evaluation of the photodegradation of applied stains by means of colourimetry. The tests
differed in the type (methyl red [116,118], rhodamine B
[117,119,121,123], methylene blue [122,124,125,131] and methyl
orange [126,127,129]), concentration and quantity of dye used;
the intensity of UV irradiance and its wavelength range; the total
209
time of exposure to light sources. Moreover, because of the chromatic heterogeneity of both stains and original substrates, three
different chromatic parameters related to CIELAB colour space
were considered to assess the self-cleaning ability: total colour
change DE⁄, variation of red coordinate Da⁄, chromatic change
DC based on the combination of colorimetric coordinates a⁄ and
b⁄ (Table 3). Consequently, direct comparison between the different results can only be merely qualitative in nature. As for example, different concentrations of the staining agent in the same
dye brought to different values of self-cleaning efficiency by the
same coating [126]. Hydrophobic treatments may strongly reduce
the absorption of dyes by stones thus altering the level of selfcleaning activity evaluated by colourimetry [117,124]. Because of
the possible heterogeneity of applied stains, it is better to consider
the original colour of stones before staining [121,123,126,127] in
order to estimate the (percentage) decolouration of dye related
to initial, unaltered condition. To better evaluate the photoactivity
of titania, in some works self-cleaning tests were supported measuring the oxidation of a dye (methyl-orange, MeO) added directly
to TiO2 sols [126,127,129]. Aqueous solutions of MeO were mixed
with aqueous TiO2 sols and exposed to UV light: photo-oxidation
was evaluated by means of spectrometry [126,127] or visual observation [129]. The optical absorption of visible light by MeO dyes
was clearly reduced and it was decoloured faster and in a more evident way compared with MeO solutions under UV light without
TiO2 nanoparticles, up to total decolouration in a few hours.
Anyway, these results are merely approximate since they not
describe the self-cleaning efficiency of TiO2 coatings after the
application on the selected substrates.
The final photodecolouration of stains by TiO2 treatments at the
end of the tests generally ranged from minimum values close to
55–60% [118,119,121,122,125] till almost complete removal of
dye
and
restoration
to
original
colour
of
stones
[116,121,123,124,127] (Table 3). Independently of the final effectiveness at the end of the self-cleaning tests, the photodecolouration was evidently faster at beginning (during the first hours of
exposure to UV light) with limited exceptions [116–118,124–
126], then a significant reduction in the rate of degradation was
clearly observed. In most cases, the applied dyes were partly
decomposed by simple exposure to UV light even on untreated
stones, but the decolouration caused exclusively by UV irradiation
was more constant over time and clearly lower at the end of the
tests compared with the efficiency of photocatalytic coatings. The
variable kinetic of self-cleaning activity may be related to the
porosity of the coating/substrate system, since low porosity
induced less absorption of dye in the substrate limiting its presence on the outer layers of the coatings, where it can be easily
degraded by UV light and photocatalysis of TiO2 [124,125].
Generally speaking, best results were obtained on limestones, but
the direct comparison by La Russa et al. pointed out better behaviour by the same coating applied on marble rather than on limestone [122].
Higher anatase loadings might increase self-cleaning efficiency,
but the effect was not proportional to the amount of TiO2 used
[116,119,123]. The relation between TiO2 loadings used in the
diverse products applied on stones and their self-cleaning efficiency is reported in Fig. 1. In some cases, the increased titania content did not lead to any significant enhancement of photoactivity
[121,122,124], so the use of higher TiO2 loading in the sols or the
application of greater amounts of product on stones can be unnecessary. A too much high amount of titania can even be detrimental: it can bring to irregular or cracked films without increasing the
self-cleaning activity [118]. Pinho and Mosquera [125] observed
that various TiO2–SiO2 nanocomposites containing a 10%w/v content of TiO2 lost great part of the self-cleaning effectiveness exhibited by similar treatments with lower TiO2 amount (1% and 4%,
210
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
Table 3
Photoactivity of the coatings: self-cleaning efficiency. In case different treatments were analysed in the following listed works, only those containing TiO2 were considered in this
table. Much of the results listed here were estimated from graphical data in articles and they were not expressly stated, so they should be considered approximate.
Limestones
Reference (first
author)
Stone
Type of product, type of application
Self-cleaning test procedure
(staining agent, UV
irradiance, duration)
Decolouration of stainsa
Potenza [116]
Luvidi [117]
Pietra di Lecce
Red limestone
TiO2 sol, spray coating
Commercial product, brushing
TiO2 sol, brushing
Commercial product, brushing
TiO2 sol, brushing
TiO2 sols, spray coating
Commercial product, spray coating
TiO2 sol, spray coating, single-layer
TiO2 sol, spray coating, multilayer
TiO2 sol, spray coating, single-layer
TiO2 sol, spray coating, multilayer
Commercial product, spray coating,
single-layer
Commercial product, spray coating,
multilayer
Commercial product, brushing, low
amount
Commercial product, brushing, high
amount
TiO2 sol, spray coating, single-layer
Methyl-red, 37 W/m2, 9 h
–
–
–
–
Methyl-red, 37 W/m2, 6.5 h
73% 6 DE⁄ 6 86%
–
–
–
–
61% 6 Da⁄ 6 65%
Da⁄ = 60%
Da⁄ = 62%
Da⁄ = 72%
–
–
Da⁄ = 79%
Black limestone
Licciulli [118]
Pietra di Lecce
Quagliarini [119]
Travertine
Quagliarini [120]
Travertine
Quagliarini [121]
Travertine
La Russa [122]
Munafò [123]
Limestone
Travertine
RhB, 4 W/m2, 26 h
–
–
RhB, 4 W/m2, 26 h
Da⁄ = 80%
Blue methylene, 20 W/m2,
5 days
DE⁄ = 66%
RhB, 4 W/m2, 24 h
TiO2 sol, spray coating, multilayer
Pinho [124]
Fossiliferous white limestone
Pinho [125]
Fossiliferous white limestone
Bergamonti
[126]
Modica stone
Commercial products
TiO2–SiO2 sols, spray coating until
saturation
TiO2–SiO2 sols, spray coating until
apparent refusal
TiO2 sol, brushing
TiO2 sol brushed after SiO2 treatment
of surfaces
Comiso stone
TiO2 sol, brushing
TiO2 sol brushed after SiO2 treatment
of surfaces
Marbles
Dolostone
Bergamonti
[127]
Pietra di Lecce
Aflori [128]
Repedea (porous
bioclasticoolitic limestone)
Luvidi [117]
White marble
La Russa [122]
Carrara marble
TiO2 sols, brushing
TiO2 sol with gold nanoparticles,
brushing
TiO2 nanocomposite, brushing
TiO2–Ag nanoparticles, brushing
Commercial product, brushing
TiO2 sol, brushing
Commercial product, brushing, low
amount
Commercial product, brushing, high
amount
TiO2–SiO2 sol, brushing
Kapridaki [129]
Dionysos and Thasos marbles
Ruffolo [130]
Marble
TiO2 sols
TiO2 + Ag sols
Pinho [131]
Dolostone
Commercial product, spray coating
TiO2 sol, spray coating
DE⁄ = 58%
Methylene blue, 800+ h
Da⁄ = 72%
a.a: 16% 6 Da⁄ 6 21%
Da⁄ = 75%
a.a: 18% 6 Da⁄ 6 33%
83% 6 DE⁄ 6 90%
45% 6 DE⁄ 6 76%
Methylene blue, 1000+ h
32% 6 DE⁄ 6 79%
Methyl orange 0.001 kmol/
m3, 18 h
Methyl orange 0.1 kmol/m3,
18 h
Methyl orange 0.001 kmol/
m3, 18 h
Methyl orange 0.1 kmol/m3,
18 h
Methyl orange 0.001 kmol/
m3, 18 h
Methyl orange 0.1 kmol/m3,
18 h
Methyl orange 0.001 kmol/
m3, 18 h
Methyl orange 0.1 kmol/m3,
18 h
Methyl orange, 12 h
DC⁄ = 0.95
–
–
–
–
RhB, 20 h
Da⁄ = 63%
Da⁄ = 36%
DE⁄ = 65%
Blue methylene, 20 W/m2,
5 days
DC⁄ = 0.60
DC⁄ = 0.95
DC⁄ = 0.60
DC⁄ = 0.85
DC⁄ = 0.60
DC⁄ = 0.90
DC⁄ = 0.50
DC⁄ = 0.90
DC⁄ = 0.90
DE⁄ = 75%
Methylene blue, 0.001 kmol/
m3, 24 h
–
–
DE⁄ = 24.26 (1.97 UT
efficiency)b
–
–
Methylene blue, 800+ h
DE⁄ = 78%
DE⁄ = 84%
a
Photodecolouration of stains under UV light may be measured through different parameters used to evaluate the self-cleaning efficiency and the chromatic change: DE⁄,
Da⁄, DC⁄. See full articles for further details. After ageing (a.a.) results were also reported when available.
b
The results were compared to the behaviour of untreated (UT) surfaces since the absolute performance of the coating was not reported.
being the latter the most efficient). The final photodecolouration
by coatings containing a high loading of TiO2 was even inferior to
that by a silica product used as basis for comparison [125].
Furthermore, the highest amount of titania worsened the
adherence to the stone substrate and the increase of mechanical
strength given to stone by SiO2 nanoparticles measured in the
cases with lower TiO2 quantity. The self-cleaning ability of the
coatings was deeply dependant even on the type of TiO2 particle
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
211
change due to photocatalytic activity; anyway the colour variation
during self-cleaning test was almost double compared with
untreated surfaces under the same UV light [129].
The substrate itself, because of its aesthetical properties as well
as other physical–chemical characteristics, can deeply influence
the self-cleaning performance, at least from a chromatic point of
view: that is another reason why it is important to consider the
original aspect of stone before the beginning of the test in order
to estimate the overall colour perceived and the visual effect of
self-cleaning referred to initial conditions. Furthermore, to better
evaluate the effect of stone substrates, the authors used TiO2 treatments on alumina samples as a reference towards coated limestones: the decolouration on alumina was clearly less efficient,
slower and more constant over time. Moreover, differences
between untreated and treated alumina surfaces were more limited [123].
Fig. 1. Self-cleaning efficiencies (percentual photodecolouration of applied stains
measured by colourimetry) of the products analysed in the selected works as a
function of the TiO2 amounts used in the sols. Names of products listed here refer to
the original classifications given in related articles. The same base product could
present different percent amounts of TiO2 in its composition and so it could be
presented more than once.
used in the synthesis of the nanocomposites [125]. A third but less
significant parameter influencing the photoactivity was the size of
titania particles used: larger and spherical TiO2 particles within a
mesoporous SiO2 matrix worked in a more efficient way compared
with smaller crystals usually used in TiO2-based products [125].
The presence of additives – mainly silica (SiO2) and metallic elements – to integrate other functions into photoactive coatings may
influence the self-cleaning ability. The photocatalyst developed by
Luvidi et al. – containing a polysiloxane – was investigated only on
white marble surface using rhodamine B as reference staining
agent: its photodecolouration was among the lowest recorded
(inferior to 40% of original colour of stain) and clearly minor than
that by commercial aqueous sol used for comparative purposes
[117]. TiO2/SiO2 hybrid products against methylene blue may
clearly show lower and slower self-cleaning effect in comparison
with pure TiO2 nanoparticles dispersed in distilled water [124],
but the use of a nonionic surfactant (n-octylamine) in the formulation may lead to overall performances better than simple aqueous
sols [124,125,131]. Silica interlayer between coatings and stone did
not alter significantly the self-cleaning performance [126].
Similarly, metal doping in order to enhance photoactivity and
response to visible light of TiO2-based coating [54] may cause
negligible differences in the self-cleaning efficiency, as it was
tested for Ag-doped titania in the decolouration of methyl orange
[127]. Anyway, because of several factors (as for example preparation method, dopant concentration and distribution of dopants)
conditioning the characteristics of coatings and the variety of
methods and procedures to testify their photoactivity, univocal
conclusions about doped TiO2 are difficult to make [54]. The analysis by Kapridaki et al. on a TiO2–SiO2 nanocomposite stated the
results as absolute decolouration DE⁄ without any reference to
original colour of stains and the value of proportional chromatic
3.3.2. De-polluting effect
The degradation of pollutants through photocatalysis is one of
the most studied feature of TiO2 used for building materials, on
the other hand the analysis of de-polluting ability of TiO2 on stones
is still scarcely investigated and the available literature is consequently limited.
Nitrogen oxides NOx (nitric oxide NO and nitrogen dioxide NO2)
were used to simulate the air pollution of urban atmosphere
[116,118,119,121]. The tests were performed putting treated samples inside a reaction chamber under UV light exposure by means
of similar flow-through methods, measuring the concentration of
polluting agents entering the reactor and at its exit. Different procedures and parameters (light intensity, ratio between NO and NO2
components, NOx flow rate, test schedule) were used, so the results
cannot be considered homogeneously. Only limestones were used
in these tests.
The degradation under UV light was very evident. It ranged
from a minimum value of 25% [121] to almost complete removal
of pollutants [116,118] (Table 4). The amount of sol sprayed over
stone surfaces, the concentration of titania in the sol and the type
of nanoparticles used (anatase, rutile or both of them) might lead
to different abatement values of gases [119,121] or influence only
the kinetic of degradation without altering final results [116,118].
Anyway, the differences were not proportional to the amount of
product used to cover the stone substrates. The NO abatement both
started and ended together with UV illumination [119,121], so the
effect on outdoor surfaces can be noticed only under natural daylight. The de-polluting efficiencies of treated stones obtained under
laboratory conditions may be considered comparable to the NOx
abatement values noted in real world setting, since in the latter
case the reduction of the concentrations of NOx was within a 25–
70% range, depending mainly on the outdoor conditions [73,74,84].
3.3.3. Biocidal efficiency
The effectiveness of TiO2-based coatings specifically on stones
against biological attacks was studied only recently. Even for biocidal efficiency, a variety of test methods were used, different in procedures, microorganisms growth on surfaces, schedules, types of
measurements: the outcomes of these studies are summarised in
Table 4.
The growth of Aspergillus niger (a fungal specie) over marble and
limestone in outdoor conditions was monitored visually by La
Russa et al. [122]. The inhibition in cell growth on both lithotypes
by titania was clearly visible; limestones were more subject to fungal growth because of their surface characteristics (higher roughness and porosity). Greater amounts of titania deposited over
stones did not improve the efficiency of the biocidal feature. It is
212
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
Table 4
Photoactivity of the coatings: de-polluting and biocidal actions. In case different treatments were analysed in the following listed works, only those containing TiO2 were
considered in this table. Much of the results listed here were estimated from graphical data in articles and they were not expressly stated, so they should be considered
approximate.
Limestones
Reference (first
author)
Stone
Type of product, type of
application
Test procedure (pollutant or
biological agent)
De-polluting/biocidal
efficiencya
Potenza [116]
Luvidi [117]
Pietra di Lecce
Red limestone
TiO2 sol, spray coating
Commercial product, brushing
TiO2 sol, brushing
Commercial product, brushing
TiO2 sol, brushing
TiO2 sols, spray coating
Commercial product, spray coating
TiO2 sol, spray coating, single-layer
TiO2 sol, spray coating, multilayer
TiO2 sol, spray coating, single-layer
TiO2 sol, spray coating, multilayer
Commercial product, spray
coating, single-layer
Commercial product, spray
coating, multilayer
Commercial product, brushing, low
amount
Commercial product, brushing,
high amount
TiO2 sol, spray coating, single-layer
TiO2 sol, spray coating, multilayer
TiO2–SiO2 sols, spray coating until
saturation
TiO2–SiO2 sols, spray coating until
apparent refusal
TiO2 sol, brushing
TiO2 sol brushed after SiO2
treatment of surfaces
TiO2 sol, brushing
TiO2 sol brushed after SiO2
treatment of surfaces
TiO2 sols, brushing
TiO2 sol with gold nanoparticles,
brushing
Ag nanocomposite, brushing
NOx, 0.6 ppm (de-polluting)
–
–
–
–
NOx, 0.6 ppm, 30 W/m2, 5 l/min
(de-polluting)
NOx, 0.6 ppm, 20 W/m2, 1.5 l/min
(de-polluting)
–
–
NOx, 0.6 ppm, 20 W/m2, 1.5 l/min
(de-polluting)
Degradation = 90%
–
–
–
–
82% 6 degradation 6 85%
Degradation = 87%
Degradation = 36%
Degradation = 52%
–
–
Degradation = 28%
Black limestone
Licciulli [118]
Pietra di Lecce
Quagliarini
[119]
Quagliarini
[120]
Quagliarini
[121]
Travertine
La Russa [122]
Travertine
Travertine
Limestone
Munafò [123]
Travertine
Pinho [124]
Fossiliferous white limestone
Pinho [125]
Fossiliferous white limestone
Bergamonti
[126]
Modica stone
Comiso stone
Marbles
Bergamonti
[127]
Pietra di Lecce
Aflori [128]
Repedea (porous
bioclasticoolitic limestone)
Luvidi [117]
White marble
La Russa [122]
Carrara marble
Dionysos and Thasos marbles
Ruffolo [130]
Marble
TiO2 sols
Dolostone
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Escherichia coli (biocidal)
Candida albicans (biocidal)
Inhibition zone = 12 mm
Inhibition
zone = 12.5 mm
Inhibition zone = 13 mm
Inhibition zone = 15 mm
–
Kapridaki [129]
Aspergillus niger, observation after
5 days (biocidal)
–
–
No colonisation
No colonisation
Biofilm degradation under UV light
(biocidal)
Stenotrophomonas maltophilia
(biocidal)
Micrococcus sp. (biocidal)
Stenotrophomonas maltophilia
(biocidal)
Micrococcus sp. (biocidal)
Commercial product, spray coating
TiO2 sol, spray coating
Colonisation’s start
Colonisation’s start
Escherichia coli (biocidal)
Candida albicans (biocidal)
Commercial product, brushing
TiO2 sol, brushing
Commercial product, brushing, low
amount
Commercial product, brushing,
high amount
TiO2–SiO2 sol, brushing
Pinho [131]
Aspergillus niger, observation after
5 days (biocidal)
TiO2–Ag nanoparticles, brushing
TiO2 + Ag sols
Dolostone
Degradation = 38%
–
–
DE⁄ = 20.51 (2.44 UT
efficiency)b
0% 6 bacterial
survival 6 55%
0% 6 bacterial
survival 6 19%
Bacterial survival = 0%
Bacterial survival = 0%
–
–
a
The results are reported according to several parameters (the concentration of pollutants, visual appearance of biological colonisation, dimensions of the zone within the
microbial growth is impeded, amount of bacteria survived on the original) separately considered in the papers.
b
The results were compared to the behaviour of untreated (UT) surfaces since the absolute performance of the coating was not reported.
important to notice that this work analysed a product constituted
by simple nanopowdered TiO2 dispersed in an aqueous suspension
of an acrylic polymer without any specific biocidal additive, so
even titania nanoparticles alone are able to slow down the enlargement of fungal colonies.
Kapridaki and Maravelaki established the efficiency of a TiO2–
SiO2 nanocomposite treatments (containing an organosilane, the
hydroxyl-terminatedpolydimethylsiloxane PDMS) by the use of
biologically decayed marble after the identification of microorganisms (bacteria) on stone surfaces [129]. The samples were treated
and then exposed to UV light together with untreated specimens
for 24 h; the effect of coating was assessed by colourimetry. The
total colour change at the end of the test related to the original
appearance of biofilm on marbles, i.e. the visual deterioration of
pre-existing biological materials, was more than double in the case
of treated surfaces.
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
Silver nanoparticles were specifically used to develop biocidal
treatments for monumental stones, since the well-known antibacterial ability of this element and the increase of photoactivity of
titania procured by noble metals: hybrid nanocomposites containing Ag nanoparticles with or without TiO2 were prepared [128].
Antibacterial (Escherichia coli) and antifungal (Candida albicans)
features of titania/silver treatments were evaluated measuring
the dimensions of inhibition zones in the agar diffusion test. Both
compounds showed good inhibitory activities, but the presence
of TiO2 improved the biocidal effect of silver nanoparticles even
without UV illumination, since Ag-doped titania may present better photoactivity in the visible light domain. Simple Ag nanoparticles showed the same efficiency against both biological agents,
while the maximum effectiveness of TiO2/Ag compound was
reached against fungal growth. Moreover, it seemed that TiO2/Ag
nanocomposite modify the stone mineralogical structure in less
extent.
Antibacterial activity of TiO2, Ag and Ag-doped TiO2 was further
evaluated specifically for the preservation of submerged archaeological artefacts by means of two different tests [130]: inoculation
in plates holding different concentrations of analysed products
exposed to UV and solar lamps; and SEM examination of marble
slabs immerged in marine water containing marine bacteria under
natural daylight. Titania treatments clearly hindered bacterial
colonisation and the efficiency was inversely proportional to the
amount of TiO2 used: highest concentration in the plate (0.1%)
led to worst results whereas the lowest one (0.01%) fully degraded
microbial colonisation (no explanation was given for this phenomenon in the study). Silver nanoparticles and TiO2/Ag nanocomposites caused the complete removal of bacteria independently of
their concentration. Moreover, the results were partly dependant
on the resistance of the bacterial strains separately inoculated in
the test plates (Stenotrophomonasmaltophilia or Micrococcus) used:
the former showed higher survival rate with the high-TiO2 containing photocatalyst. SEM analysis of biofouling on submerged marble
slabs showed first steps of colonisation by microorganisms; on the
contrary treated surfaces showed no formation at all of spontaneous colonisation after 72 h of simulated immersion in sea water.
The antibacterial ability of TiO2-based products on stones was fully
confirmed.
As for the influences of the analysed substrates on the biocidal
activity, it seemed by optical observation that antifungal feature
was more efficient on marble rather than on limestone [122]; anyway even untreated marble was less subjected to fungal colonisation in comparison with limestone (mainly because of their
intrinsic characteristics), so there is a need for further analyses to
establish the contribution of stones on final performance.
3.4. Durability of the coatings on stones
Currently, the less and most recently studied aspect of titania
coatings applied on stones is their resistance to weathering over
time. The durability of TiO2 on diverse substrates has been recently
investigated [137–145] but researches evaluating the efficiency of
TiO2 nanotreatments on architectural stones through time are still
very limited.
Several factors may lead to a decrease in photoactivity of TiO2.
The main reason causing the deactivation of photocatalytic materials is the production during photogenerated reactions of by-products and intermediates absorbed by active sites; other decreases of
photoactivity may depend on the photopolymerization of several
species on the TiO2 surface or on the accumulation of several
materials (as oxidised inorganic forms or fouling) covering the surface and blocking pores [70].
The effect of ageing was artificially simulated in different ways.
Durability of the coatings was tested in terms of changes of their
213
original properties (as reported in previous sections), resistance
and effectiveness over time. Part of the procedures used to put
under stress TiO2 photocatalysts were based on test methods conceived to assess the resistance of other type of materials as adhesives [124,125,131] or stone substrates themselves [126,131] and
not directly of coating-like treatments. Other ageing processes
simulated the natural conditions related to possible real case applications in order to assess the efficiency of the coatings used outdoor [122,123].
Peeling test was performed to assess the adherence of TiO2
hybrid coatings to the stone substrate [124,125,131]. The quantity
of removed material was estimated by weighing and by SEM/EDX
analysis. Simple aqueous TiO2 sols applied on stones were partially
detached by peeling, whereas the adhesion of titania nanoparticles
embedded in a silica matrix obtained by the addition of a nonionic
surfactant (n-octylamine) was much stronger [124,131], providing
long-term wear resistance. On the other hand, untreated surfaces
showed the highest loss of material, so the presence of TiO2 coatings with no additives partly improved the mechanical characteristics of the stone surfaces. Too much high titania content in the
sols (10%) usually led to more porous and detachable coatings
[125], so the ratio between TiO2 and SiO2 components must be well
balanced in order to obtain the better results.
Mesoporous TiO2/SiO2 nanocomposite clearly showed high
durability as consolidant products [131]. Treated dolostones were
artificially aged through cyclical salt crystallization test: homogeneous, crack-free TiO2/SiO2 coating improved impressively the
mechanical resistance of very friable dolostone and left them
almost unaltered till the end of the weathering procedure (30
cycles), while untreated stones fully disintegrated after 4 ageing
cycles and stones treated with a commercial consolidant lose about
90% of their mass after 15 cycles. Similar analysis was performed
on TiO2 coatings applied directly on limestones or after the deposition of a silica interlayer: there was no evident difference in the
mass variations between coated and untreated surfaces [127].
La Russa et al. simulated the ageing due to outdoor environment by means of prolonged exposure to artificial solar light
[122]. After ageing, only the wettability of hybrid coatings was
analysed: the hydrophobicity due to the polymer dispersed in an
aqueous suspension with TiO2 was totally lost on marble surfaces
and there were not evident differences compared to untreated surfaces both in kinetic and in final values of absorbed water; whereas
hybrid titania coatings on limestone preserved their original feature (very low water absorption) till the end of the accelerated ageing procedure. Different behaviours may be justified by the great
difference of TiO2 quantity applied on stones, its penetration depth
and the different original porosity of analysed substrates.
Till the present time, durability of self-cleaning activity of titania during accelerated weathering was evaluated only by the
authors [123]. Treated (aqueous TiO2 sol applied in two different
amounts) and untreated travertines were artificially aged by
means of diverse combinations of strong UV light exposure,
repeated staining and water rainfall. Photoactivity and
hydrophilicity of the coatings were deeply analysed during ageing
and at its end. The behaviour towards water was fully altered and
there was no presence at all of a hydrophilic effect at the end of the
ageing: anyway, water absorption was very close to that of
untreated stones, so there were no harmful effects. Furthermore,
photoactivity of the coatings drastically decreased. Simple exposure to strong UV illumination (about 14 months of average real
exposure in the Mediterranean Basin) clearly reduced the selfcleaning ability of the coatings. Different natures of stressing conditions deeply influenced the self-cleaning efficiency over time
causing different performances. The prolonged UV irradiation
was the main factor affecting the efficiency of titania. The effect
of repeated staining decreased further the photoactivity, while
214
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
simulated rainfall partly removed the applied stains and the reaction residues of repeated photocatalytic tests barely improving the
self-cleaning effectiveness of treated surfaces. The amount of titania applied on stones was almost non influential for the results,
altering especially the degrading kinetic of photoactivity but not
the final values: anyway multilayer treatment endured better
under the less stressful ageing (mere UV light exposure without
repeated staining and water stream) and a slower and more constant decrease of its self-cleaning effectiveness was noted for all
weathering conditions.
4. Discussion
Several works about compatibility with various stone properties, photocatalytic activity and durability of titania nanocoatings
were analysed. Because of the presence of several parameters
and performances to be considered (transparency, wettability,
self-cleaning ability, de-polluting efficiency, biocidal effect) it is
not really possible to establish the best product analysed overall.
Furthermore each property of the coatings has been often analysed
in different ways depending on the study taken into account. The
following discussion is provided in an attempt to evaluate the analysed coatings overall according to the various aspects involved in
the research.
Most of the coatings almost did not alter the visual appearance
of treated stones. Usually hybrid coatings showed higher aesthetical changes dependant on the amount of TiO2 nanoparticles used in
the hybrid product [124,131] or on the presence of metallic additive (Ag) [130]. The commercial product analysed by the authors
and others [121] applied on travertine showed the lowest aesthetic
variation by far (colour changes were barely measurable by instrument). The results were scarcely related to the type of application
(brushing or spray-coating) and the amount of product deposited.
On average marbles underwent minor colour changes, anyway this
behaviour may be more related to the original lighter colour of
white marble surfaces rather than their other physical–chemical
properties, since the brightening effect of TiO2 particles has less
effect on light surfaces. Colour variations seem not to increase over
time. It is important to notice that the differences between the
majority of the treatments are very little so they can be considered
almost negligible.
The products presented diverse behaviours towards water. The
outcomes of the works analysed were very different even because
of the heterogeneity of test methods used. The possible presence of
additives in their composition was clearly fundamental to determine the wettability of treated surfaces. Coatings containing only
TiO2 nanoparticles usually entailed scarce variations of wettability
and sometimes clear hydrophilicity without lead to a greater
absorption of water. Nanocomposites were usually hydrophobic
because of the characteristics of additives used: this behaviour
was very evident in some cases till almost complete repel of water
and negligible absorption by limestone [122,124,125]. A comparative analysis [122] pointed out that treated limestone assumes a
more hydrophobic feature and that characteristic is more durable
over time than on marble.
The self-cleaning ability of the coatings was fully confirmed. As
most of the photodecolouration of stains is due to TiO2, the presence of nanoadditives or other further treatments was not necessary to increase the efficiency of the products. The highest
photodecolouration values were obtained by simple TiO2 sols
[116,121,124], while comparative analyses between simple and
composite products usually showed higher results by the former
or scarce differences. Photodecolouration of soil was not directly
influenced by the specific type of the substrate.
The multifunctionality of the coatings was analysed considering
their de-polluting and biocidal effects. De-polluting effect of TiO2
was clearly noted but it showed very different results according
to the test procedures used: anyway the pure TiO2 coatings analysed by Potenza et al. and Licciulli et al. clearly showed the best
performances by far [116,118]. The photocatalytic degradation of
pollutants was not strictly related to the self-cleaning efficiency,
as the best de-pollution products were not the most effective
against soil. The biocidal activity of both simple and hybrid products was very evident: it seems that the most part of the sterilizing
activity was due to metallic additives which usually prevented the
start of colonisation by biological agents; anyway a synergic,
improving effect between TiO2 and nanomaterials was also noted.
A direct comparison between the results is rather complicated due
to the heterogeneity of methods employed, including the presence
and use of diverse microorganisms. The biocidal ability of the coatings seems to be more efficient against bacteria, but further analyses are necessary to establish the real effectiveness against other
microorganisms. The impact of the substrate nature on both depolluting and biocidal properties deserves further investigation,
since it was scarcely evaluated in real comparative tests.
The photoactivity of TiO2 nanocoatings over time with regard to
the different associated aspects and functions has been still poorly
investigated, so it is too early to assess the actual longevity of this
type of treatments in many respects.
Furthermore, TiO2 coatings (TiO2–SiO2 nanocomposites) have
been used to improve the mechanical strength of stones and their
resistance to weathering along with self-cleaning feature
[124,125,131]: the performances of TiO2-containing products as
consolidants were very good indeed but the results were not discussed in this paper as this field of the research is still rather
limited.
5. Conclusions
In this paper, a review about the application of titania photocatalytic nanocoatings on architectural stone surfaces was presented.
The main aim of these treatments is to obtain multifunctional
(mainly self-cleaning) stone surfaces activated by solar light in
order to limit the maintenance actions and their costs: this feature
can be even more important for Architectural Heritage – that
requires more maintenance actions – than for common building
surfaces.
In conclusion, several factors indicate that TiO2-based products
are an ideal solution to use in the field of preservation of architectural surfaces. First of all, titanium dioxide is considered the most
efficient photocatalyst readily available, so it is possible to use TiO2
products for large-scale purposes. From the results discussed
above, studied coatings proved compatible with stone substrates
and clearly efficient in several respects, so their application on
building stones seems to be feasible and useful. The main reason
for their extensive use on architectural surfaces is their preventive
activity that impedes the beginning of aesthetic problems and nuisances and the evolution of degrading processes (mainly biological
or water-related) on treated surfaces: this hindering feature may
increase the effectiveness and reduce the number and costs of
maintenance interventions. In addition, these surface treatments
can be easily used directly on pre-existing stone substrates: all
analysed products were simply applied by brushing or spraycoating. Furthermore, it is important to notice that the hybrid
nanomaterials may realise multifunctional coatings exhibiting in
the same time several properties (especially self-cleaning, biocidal
and mechanical strengthening) typical of diverse products, thus
simplifying and improving the conservative procedures.
The analyses carried on up to now still lack a more complete
approach. Several multidisciplinary features are in need to be
investigated deeper before large-scale applications on architectural
stones:
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
(1) the lifespan, both of coatings themselves and of their
efficiency;
(2) the relations between nanotreatments and substrates,
including the influence of the application procedures used;
(3) the risk assessment of environmental impact and health
issues mainly related to the release of nanoparticles during
the deposition and the chronic exposure to outdoor ageing
factors, considering the different phases of the nanotreatments (production, application, long-term use and their possible, final removal and disposal);
(4) the possibility to increase the efficiency of the products evaluating several parameters, as for example: the size of nanoparticles, the titania loading in the sols, the use of doping
materials to enhance the photoactivity, the reactivity of
TiO2 to visible light, and other features;
(5) the integration of new functions still in development, as well
as the increase of mechanical resistance;
(6) the cost benefit analysis, especially evaluating the abovequoted features and direct comparison between
photocatalysis and traditional (or other) cleaning methods
and restorative procedures.
References
[1] Ozga I, Ghedini N, Giosuè C, Sabbioni C, Tittarelli F, Bonazza A. Assessment of
air pollutant sources in the deposit on monuments by multivariate analysis.
Sci
Total
Environ
2014;490:776–84.
http://dx.doi.org/10.1016/
j.scitotenv.2014.05.084.
[2] Urosevic M, Yebra-Rodríguez A, Sebastián-Pardo E, Cardell C. Black soiling of
an architectural limestone during two-year term exposure to urban air in the
city of Granada (S Spain). Sci Total Environ 2012;414:564–75. http://
dx.doi.org/10.1016/j.scitotenv.2011.11.028.
[3] Grossi CM, Brimblecombe P, Esbert RM, Alonso FJ. Color changes in
architectural limestones from pollution and cleaning. Color Res Appl
2007;32:320–31. http://dx.doi.org/10.1002/col.20322.
[4] Chew MYL. Tan PP. Facade staining arising from design features. Constr Build
Mater 2003;17:181–7.
[5] Creighton PJ, Lioy PJ, Haynie FH, Lemmons TJ, Miller JL, Gerhart J. Soiling by
atmospheric aerosols in an urban industrial area. J Air Waste Manag Assoc
1990;40:1285–9. http://dx.doi.org/10.1080/10473289.1990.10466783.
[6] Grossi CM, Esbert RM, Díaz-Pache F, Alonso FJ. Soiling of building stones in
urban environments. Build Environ 2003;38:147–59. http://dx.doi.org/
10.1016/S0360-1323(02)00017-3.
[7] Pio CA, Ramos MM, Duarte AC. Atmospheric aerosol and soiling of external
surfaces in an urban environment. Atmos Environ 1998;32:1979–89. http://
dx.doi.org/10.1016/S1352-2310(97)00507-4.
[8] Brimblecombe P, Grossi CM. Aesthetic thresholds and blackening of stone
buildings. Sci Total Environ 2005;349:175–89. http://dx.doi.org/10.1016/
j.scitotenv.2005.01.009.
[9] Bellan LM, Salmon LG, Cass GR. A study on the human ability to detect soot
deposition onto works of art. Environ Sci Technol 2000;34:1946–52. http://
dx.doi.org/10.1021/es990769f.
[10] Guillitte O. Bioreceptivity: a new concept for building ecology studies. Sci
http://dx.doi.org/10.1016/0048Total
Environ
1995;167:215–20.
9697(95)04582-L.
[11] De Belie N. Microorganisms versus stony materials: a love–hate relationship.
Mater Struct 2010;43:1191–202. http://dx.doi.org/10.1617/s11527-0109654-0.
[12] Gaylarde C, Ribas Silva M, Warscheid T. Microbial impact on building
materials: an overview. Mater Struct Constr 2003;36:342–52. http://
dx.doi.org/10.1617/13867.
[13] Guillitte O, Dreesen R. Laboratory chamber studies and petrographical
analysis as bioreceptivity assessment tools of building materials. Sci Total
Environ 1995;167:365–74. http://dx.doi.org/10.1016/0048-9697(95)04596S.
[14] Miller AZ, Sanmartín P, Pereira-Pardo L, Dionísio A, Saiz-Jimenez C, Macedo
MF, et al. Bioreceptivity of building stones: a review. Sci Total Environ
2012;426:1–12. http://dx.doi.org/10.1016/j.scitotenv.2012.03.026.
[15] Tran TH, Govin A, Guyonnet R, Grosseau P, Lors C, Garcia-Diaz E, et al.
Influence of the intrinsic characteristics of mortars on biofouling by
Klebsormidium flaccidum. Int Biodeterior Biodegrad 2012;70:31–9. http://
dx.doi.org/10.1016/j.ibiod.2011.10.017.
[16] Barberousse H, Ruot B, Yéprémian C, Boulon G. An assessment of façade
coatings against colonisation by aerial algae and cyanobacteria. Build Environ
2007;42:2555–61. http://dx.doi.org/10.1016/j.buildenv.2006.07.031.
[17] Dubosc A, Escadeillas G, Blanc PJ. Characterization of biological stains on
external concrete walls and influence of concrete as underlying material. Cem
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
215
http://dx.doi.org/10.1016/S0008Concr
Res
2001;31:1613–7.
8846(01)00613-5.
Escadeillas G, Bertron A, Blanc P, Dubosc A. Accelerated testing of biological
stain growth on external concrete walls. Part 1: development of the growth
tests. Mater Struct 2007;40:1061–71. http://dx.doi.org/10.1617/s11527-0069205-x.
Irving TE, Allen DG. Species and material considerations in the formation and
development of microalgal biofilms. Appl Microbiol Biotechnol
2011;92:283–94. http://dx.doi.org/10.1007/s00253-011-3341-0.
Ortega-Calvo JJ, Ariño X, Hernandez-Marine M, Saiz-Jimenez C. Factors
affecting the weathering and colonization of monuments by phototrophic
microorganisms. Sci Total Environ 1995;167:329–41. http://dx.doi.org/
10.1016/0048-9697(95)04593-P.
Jain A, Bhadauria S, Kumar V, Chauhan RS. Biodeterioration of sandstone
under the influence of different humidity levels in laboratory conditions.
Build
Environ
2009;44:1276–84.
http://dx.doi.org/10.1016/
j.buildenv.2008.09.019.
Gaylarde CC, Gaylarde PM. A comparative study of the major microbial
biomass of biofilms on exteriors of buildings in Europe and Latin America. Int
Biodeterior
Biodegrad
2005;55:131–9.
http://dx.doi.org/10.1016/
j.ibiod.2004.10.001.
Young ME, Urquhart DCM, Laing RA. Maintenance and repair issues for stone
cleaned sandstone and granite building façades. Build Environ
2003;38:1125–31. http://dx.doi.org/10.1016/S0360-1323(03)00084-2.
Feynman RP. There’s plenty of room at the bottom. Eng Sci 1960;23:22–36.
Taniguchi N. On the basic concept of nanotechnology. Proc Int Conf Prod Eng
Tokyo II Jpn Soc Precis Eng 1974:18–23.
Drexler KE. Molecular engineering: an approach to the development of
general capabilities for molecular manipulation. Proc Natl Acad Sci
1981;78:5275–8. http://dx.doi.org/10.1073/pnas.78.9.5275.
Zhu W, Bartos PJM, Porro A. Application of nanotechnology in construction.
Mater Struct 2004;37:649–58. http://dx.doi.org/10.1617/14234.
Pacheco-Torgal F, Jalali S. Nanotechnology: advantages and drawbacks in the
field of construction and building materials. Constr Build Mater
2011;25:582–90. http://dx.doi.org/10.1016/j.conbuildmat.2010.07.009.
Karatasios I, Katsiotis MS, Likodimos V, Kontos AI, Papavassiliou G, Falaras P,
et al. Photo-induced carbonation of lime-TiO2 mortars. Appl Catal B Environ
2010;95:78–86. http://dx.doi.org/10.1016/j.apcatb.2009.12.011.
Mosquera MJ, De los Santos DM, Montes A, Valdez-Castro L. New
nanomaterials for consolidating stone. Langmuir 2008;24:2772–8. http://
dx.doi.org/10.1021/la703652y.
Ocak Y, Sofuoglu A, Tihminlioglu F, Böke H. Protection of marble surfaces by
using biodegradable polymers as coating agent. Prog Org Coat
2009;66:213–20. http://dx.doi.org/10.1016/j.porgcoat.2009.07.007.
Mosquera MJ, de los Santos DM, Rivas T. Surfactant-synthesized ormosils
with application to stone restoration. Langmuir 2010;26:6737–45. http://
dx.doi.org/10.1021/la9040979.
Illescas JF, Mosquera MJ. Surfactant-synthesized PDMS/silica nanomaterials
improve robustness and stain resistance of carbonate stone. J Phys Chem C
2011;115:14624–34. http://dx.doi.org/10.1021/jp203524p.
De Ferri L, Lottici PP, Lorenzi A, Montenero A, Salvioli-Mariani E. Study of
silica nanoparticles – polysiloxane hydrophobic treatments for stone-based
monument protection. J Cult Herit 2011;12:356–63. http://dx.doi.org/
10.1016/j.culher.2011.02.006.
Ruffolo SA, La Russa MF, Aloise P, Belfiore CM, Macchia A, Pezzino A, et al.
Efficacy of nanolime in restoration procedures of salt weathered limestone
rock. Appl Phys A 2014;114:753–8. http://dx.doi.org/10.1007/s00339-0137982-y.
Licchelli M, Malagodi M, Weththimuni M, Zanchi C. Nanoparticles for
conservation of bio-calcarenite stone. Appl Phys A 2014;114:673–83.
http://dx.doi.org/10.1007/s00339-013-7973-z.
Kiele E, Lukseniene J, Griguceviciene A, Selskis A, Senvaitiene J, Ramanauskas
R, et al. Methyl-modified hybrid organic–inorganic coatings for the
conservation of copper. J Cult Herit 2014;15:242–9. http://dx.doi.org/
10.1016/j.culher.2013.06.002.
Franzoni E, Fregni A, Gabrielli R, Graziani G, Sassoni E. Compatibility of
photocatalytic TiO2-based finishing for renders in architectural restoration: a
preliminary study. Build Environ 2014;80:125–35. http://dx.doi.org/10.1016/
j.buildenv.2014.05.027.
Diebold U. The surface science of titanium dioxide. Surf Sci Rep
2003;48:53–229. http://dx.doi.org/10.1016/S0167-5729(02)00100-0.
Carp O. Photoinduced reactivity of titanium dioxide. Prog Solid State Chem
2004;32:33–177. http://dx.doi.org/10.1016/j.progsolidstchem.2004.08.001.
Di Paola A, Cufalo G, Addamo M, Bellardita M, Campostrini R, Ischia M, et al.
Photocatalytic activity of nanocrystalline TiO2 (brookite, rutile and brookitebased) powders prepared by thermohydrolysis of TiCl4 in aqueous chloride
solutions. Colloids Surf Physicochem Eng Asp 2008;317:366–76. http://
dx.doi.org/10.1016/j.colsurfa.2007.11.005.
Allen NS, Edge M, Verran J, Stratton J, Maltby J, Bygott C. Photocatalytic titania
based
surfaces:
environmental
benefits.
Polym
Degrad
Stab
http://dx.doi.org/10.1016/
2008;93:1632–46.
j.polymdegradstab.2008.04.015.
Fujishima A, Rao TN, Tryk DA. Titanium dioxide photocatalysis. J Photochem
Photobiol C Photochem Rev 2000;1:1–21. http://dx.doi.org/10.1016/S13895567(00)00002-2.
216
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
[44] Fujishima A, Zhang X. Titanium dioxide photocatalysis: present situation and
future approaches. Comptes Rendus Chim 2006;9:750–60. http://dx.doi.org/
10.1016/j.crci.2005.02.055.
[45] Chen J, Poon C. Photocatalytic construction and building materials: from
fundamentals to applications. Build Environ 2009;44:1899–906. http://
dx.doi.org/10.1016/j.buildenv.2009.01.002.
[46] Brunella MF, Diamanti MV, Pedeferri MP, Di Fonzo F, Casari CS, Li Bassi A.
Photocatalytic behavior of different titanium dioxide layers. Thin Solid Films
2007;515:6309–13. http://dx.doi.org/10.1016/j.tsf.2006.11.194.
[47] Fujishima A, Zhang X, Tryk D. TiO2 photocatalysis and related surface
phenomena. Surf Sci Rep 2008;63:515–82. http://dx.doi.org/10.1016/
j.surfrep.2008.10.001.
[48] Fujishima A, Honda K. Electrochemical photolysis of water at a
semiconductor electrode. Nature 1972;238:37–8. http://dx.doi.org/10.1038/
238037a0.
[49] Hashimoto K, Irie H, Fujishima A. TiO2 photocatalysis: a historical overview
and future prospects. Jpn J Appl Phys 2005;44:8269. http://dx.doi.org/
10.1143/JJAP.44.8269.
[50] Tobaldi DM, Tucci A, Camera-Roda G, Baldi G, Esposito L. Photocatalytic
activity for exposed building materials. J Eur Ceram Soc 2008;28:2645–52.
http://dx.doi.org/10.1016/j.jeurceramsoc.2008.03.032.
[51] Guo S, Wu Z, Zhao W. TiO2-based building materials: above and beyond
traditional applications. Chin Sci Bull 2009;54:1137–42. http://dx.doi.org/
10.1007/s11434-009-0063-0.
[52] Pelaez M, Nolan NT, Pillai SC, Seery MK, Falaras P, Kontos AG, et al. A review
on the visible light active titanium dioxide photocatalysts for environmental
applications. Appl Catal B Environ 2012;125:331–49. http://dx.doi.org/
10.1016/j.apcatb.2012.05.036.
[53] Nakata K, Fujishima A. TiO2 photocatalysis: design and applications. J
Photochem Photobiol C Photochem Rev 2012;13:169–89. http://dx.doi.org/
10.1016/j.jphotochemrev.2012.06.001.
[54] Devi LG, Kavitha R. A review on non metal ion doped titania for the
photocatalytic degradation of organic pollutants under UV/solar light: role of
photogenerated charge carrier dynamics in enhancing the activity. Appl Catal
http://dx.doi.org/10.1016/
B
Environ
2013;140–141:559–87.
j.apcatb.2013.04.035.
[55] Wang R, Hashimoto K, Fujishima A, Chikuni M, Kojima E, Kitamura A, et al.
Light-induced amphiphilic surfaces. Nature 1997;388:431–2. http://
dx.doi.org/10.1038/41233.
[56] Fujishima A, Rao TN, Tryk D. TiO2 photocatalysts and diamond electrodes.
Electrochim. Acta 2000;45:4683–90. http://dx.doi.org/10.1016/S00134686(00)00620-4.
[57] Habibpanah AA, Pourhashem S, Sarpoolaky H. Preparation and
characterization of photocatalytic titania–alumina composite membranes
by sol–gel methods. J Eur Ceram Soc 2011;31:2867–75. http://dx.doi.org/
10.1016/j.jeurceramsoc.2011.06.014.
[58] Zhang H, Banfield JF. Thermodynamic analysis of phase stability of
nanocrystalline titania. J Mater Chem 1998;8:2073–6. http://dx.doi.org/
10.1039/A802619J.
[59] Zhang H, Banfield JF. Understanding polymorphic phase transformation
behavior during growth of nanocrystalline aggregates: insights from TiO2. J
Phys Chem B 2000;104:3481–7. http://dx.doi.org/10.1021/jp000499j.
[60] Zhao X, Zhao Q, Yu J, Liu B. Development of multifunctional photoactive selfcleaning glasses. J Non-Cryst Solids 2008;354:1424–30. http://dx.doi.org/
10.1016/j.jnoncrysol.2006.10.093.
[61] Chen J, Kou S, Poon C. Photocatalytic cement-based materials: comparison of
nitrogen oxides and toluene removal potentials and evaluation of selfcleaning performance. Build Environ 2011;46:1827–33. http://dx.doi.org/
10.1016/j.buildenv.2011.03.004.
[62] Ruot B, Plassais A, Olive F, Guillot L, Bonafous L. TiO2-containing cement
pastes and mortars: measurements of the photocatalytic efficiency using a
rhodamine B-based colourimetric test. Sol Energy 2009;83:1794–801. http://
dx.doi.org/10.1016/j.solener.2009.05.017.
[63] De Niederhãusern S, Bondi M, Bondioli F. Self-cleaning and antibacteric
ceramic tile surface. Int J Appl Ceram Technol 2013;10:949–56. http://
dx.doi.org/10.1111/j.1744-7402.2012.02801.x.
[64] Bondioli F, Taurino R, Ferrari AM. Functionalization of ceramic tile surface by
sol–gel technique. J Colloid Interface Sci 2009;334:195–201. http://
dx.doi.org/10.1016/j.jcis.2009.02.054.
[65] Folli A, Pade C, Hansen TB, De Marco T, Macphee DE. TiO2 photocatalysis in
cementitious systems: insights into self-cleaning and depollution chemistry.
http://dx.doi.org/10.1016/
Cem
Concr
Res
2012;42:539–48.
j.cemconres.2011.12.001.
[66] Diamanti MV, Ormellese M, Pedeferri M. Characterization of photocatalytic
and superhydrophilic properties of mortars containing titanium dioxide. Cem
Concr
Res
2008;38:1349–53.
http://dx.doi.org/10.1016/
j.cemconres.2008.07.003.
[67] Cassar L. Photocatalysis of cementitious materials: clean buildings and clean
air. Mrs Bull 2004;29:328–31. http://dx.doi.org/10.1557/mrs2004.99.
[68] Zhao J, Yang X. Photocatalytic oxidation for indoor air purification: a
literature review. Build Environ 2003;38:645–54. http://dx.doi.org/10.1016/
S0360-1323(02)00212-3.
[69] Wang S, Ang HM, Tade MO. Volatile organic compounds in indoor
environment and photocatalytic oxidation: state of the art. Environ Int
2007;33:694–705. http://dx.doi.org/10.1016/j.envint.2007.02.011.
[70] Mo J, Zhang Y, Xu Q, Lamson JJ, Zhao R. Photocatalytic purification of volatile
organic compounds in indoor air: a literature review. Atmos Environ
2009;43:2229–46. http://dx.doi.org/10.1016/j.atmosenv.2009.01.034.
[71] Maggos T, Bartzis JG, Liakou M, Gobin C. Photocatalytic degradation of NOx
gases using TiO2-containing paint: a real scale study. J Hazard Mater
2007;146:668–73. http://dx.doi.org/10.1016/j.jhazmat.2007.04.079.
[72] Maggos T, Bartzis JG, Leva P, Kotzias D. Application of photocatalytic
technology for NOx removal. Appl Phys A 2007;89:81–4. http://dx.doi.org/
10.1007/s00339-007-4033-6.
[73] Beeldens A. Air purification by road materials: results of the test project in
Antwerp. In: Baglioni P, Cassar L, editors. Proc Int RILEM Symp Photocatal
Environ Constr Mater. Bagneux: Rilem; 2007. p. 187–94.
[74] Cassar L, Beeldens A, Pimpinelli N, Guerrini GL. Photocatalysis of cementitious
materials. In: Baglioni P, Cassar L, editors. Proc Int RILEM Symp Photocatal
Environ Constr Mater. Bagneux: Rilem; 2007. p. 131–45.
[75] Demeestere K, Dewulf J, De Witte B, Beeldens A, Van Langenhove H.
Heterogeneous photocatalytic removal of toluene from air on building
materials enriched with TiO2. Build Environ 2008;43:406–14. http://
dx.doi.org/10.1016/j.buildenv.2007.01.016.
[76] Hüsken G, Hunger M, Brouwers HJ. Comparative study on cementitious
products containing titanium dioxide as photo-catalyst. In: Baglioni P, Cassar
L, editors. Proc Int RILEM Symp Photocatal Environ Constr
Mater. Bagneux: Rilem; 2007. p. 147–54.
[77] Hüsken G, Hunger M, Brouwers HJH. Experimental study of photocatalytic
concrete products for air purification. Build Environ 2009;44:2463–74. http://
dx.doi.org/10.1016/j.buildenv.2009.04.010.
[78] de Richter R, Caillol S. Fighting global warming: The potential of
photocatalysis against CO2, CH4, N2O, CFCs, tropospheric O3, BC and other
major contributors to climate change. J Photochem Photobiol C Photochem
Rev 2011;12:1–19. http://dx.doi.org/10.1016/j.jphotochemrev.2011.05.002.
[79] Guo M-Z, Poon C-S. Photocatalytic NO removal of concrete surface layers
intermixed with TiO2. Build Environ 2013;70:102–9. http://dx.doi.org/
10.1016/j.buildenv.2013.08.017.
[80] Kawakami M, Furumura T, Tokushige H. NOx removal effects and physical
properties of cement mortar incorporating titanium dioxide powder. In:
Baglioni P, Cassar L, editors. Proc Int RILEM Symp Photocatal Environ Constr
Mater. Bagneux: Rilem; 2007. p. 163–70.
[81] O’Keeffe C, Gannon P, Gilson P, Kafizas A, Parkin IP, Binions R. Air purification
by heterogeneous photocatalytic oxidation with multi-doped thin film
titanium dioxide. Thin Solid Films 2013;537:131–6. http://dx.doi.org/
10.1016/j.tsf.2013.04.016.
[82] Strini A, Sanson A, Mercadelli E, Sangiorgi A, Schiavi L. Low irradiance
photocatalytic degradation of toluene in air by screen-printed titanium
dioxide layers. Thin Solid Films 2013;545:537–42. http://dx.doi.org/10.1016/
j.tsf.2013.08.032.
[83] Xiao G, Huang A, Su H, Tan T. The activity of acrylic-silicon/nano-TiO2 films
for the visible-light degradation of formaldehyde and NO2. Build Environ
2013;65:215–21. http://dx.doi.org/10.1016/j.buildenv.2013.04.014.
[84] Guerrini GL, Peccati E. Photocatalytic cementitious roads for depollution. In:
Baglioni P, Cassar L, editors. Proc Int RILEM Symp Photocatal Environ Constr
Mater. Bagneux: Rilem; 2007. p. 179–86.
[85] Seven O, Dindar B, Aydemir S, Metin D, Ozinel M, Icli S. Solar photocatalytic
disinfection of a group of bacteria and fungi aqueous suspensions with TiO2,
ZnO and Sahara desert dust. J Photochem Photobiol Chem 2004;165:103–7.
http://dx.doi.org/10.1016/j.jphotochem.2004.03.005.
[86] De Muynck W, Maury-Ramirez A, De Belie N, Verstraete W. Evaluation of
strategies to prevent algal fouling on white architectural and cellular
concrete. Int Biodeterior Biodegrad 2009;63:679–89. http://dx.doi.org/
10.1016/j.ibiod.2009.04.007.
[87] Matsunaga T, Tomoda R, Nakajima T, Wake H. Photoelectrochemical
sterilization of microbial cells by semiconductor powders. FEMS Microbiol
Lett 1985;29:211–4. http://dx.doi.org/10.1111/j.1574-6968.1985.tb00864.x.
[88] Chen F, Yang X, Wu Q. Antifungal capability of TiO2 coated film on moist
wood.
Build
Environ
2009;44:1088–93.
http://dx.doi.org/10.1016/
j.buildenv.2008.07.018.
[89] Sichel C, de Cara M, Tello J, Blanco J, Fernández-Ibáñez P. Solar photocatalytic
disinfection of agricultural pathogenic fungi: Fusarium species. Appl Catal B
Environ 2007;74:152–60. http://dx.doi.org/10.1016/j.apcatb.2007.02.005.
[90] Cho M, Chung H, Choi W, Yoon J. Linear correlation between inactivation of
E. coli and OH radical concentration in TiO2 photocatalytic disinfection. Water
Res 2004;38:1069–77. http://dx.doi.org/10.1016/j.watres.2003.10.029.
[91] Koizumi Y, Taya M. Kinetic evaluation of biocidal activity of titanium dioxide
against phage MS2 considering interaction between the phage and
photocatalyst particles. Biochem Eng J 2002;12:107–16. http://dx.doi.org/
10.1016/S1369-703X(02)00046-3.
[92] Dalrymple OK, Stefanakos E, Trotz MA, Yogi Goswami D. A review of the
mechanisms and modeling of photocatalytic disinfection. Appl Catal B
Environ 2010;98:27–38. http://dx.doi.org/10.1016/j.apcatb.2010.05.001.
[93] Huang Z, Maness P-C, Blake DM, Wolfrum EJ, Smolinski SL, Jacoby WA.
Bactericidal mode of titanium dioxide photocatalysis. J Photochem Photobiol
Chem 2000;130:163–70. http://dx.doi.org/10.1016/S1010-6030(99)00205-1.
[94] Hamal DB, Haggstrom JA, Marchin GL, Ikenberry MA, Hohn K, Klabunde KJ. A
multifunctional biocide/sporocide and photocatalyst based on titanium
dioxide (TiO2) codoped with silver, carbon, and sulfur. Langmuir
2010;26:2805–10. http://dx.doi.org/10.1021/la902844r.
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
[95] Hartmann NB, Von der Kammer F, Hofmann T, Baalousha M, Ottofuelling S,
Baun A. Algal testing of titanium dioxide nanoparticles—testing
considerations, inhibitory effects and modification of cadmium
bioavailability. Toxicology 2010;269:190–7. http://dx.doi.org/10.1016/
j.tox.2009.08.008.
[96] Linkous CA, Carter GJ, Locuson DB, Ouellette AJ, Slattery DK, Smitha LA.
Photocatalytic inhibition of algae growth using TiO2, WO3, and cocatalyst
modifications. Environ Sci Technol 2000;34:4754–8. http://dx.doi.org/
10.1021/es001080+.
[97] Maury-Ramirez A, De Muynck W, Stevens R, Demeestere K, De Belie N.
Titanium dioxide based strategies to prevent algal fouling on cementitious
materials. Cem Concr Compos 2013;36:93–100. http://dx.doi.org/10.1016/
j.cemconcomp.2012.08.030.
[98] Fonseca AJ, Pina F, Macedo MF, Leal N, Romanowska-Deskins A, Laiz L, et al.
Anatase as an alternative application for preventing biodeterioration of
mortars: evaluation and comparison with other biocides. Int Biodeterior
Biodegrad 2010;64:388–96. http://dx.doi.org/10.1016/j.ibiod.2010.04.006.
[99] Zhang Z, MacMullen J, Dhakal HN, Radulovic J, Herodotou C, Totomis M, et al.
Biofouling resistance of titanium dioxide and zinc oxide nanoparticulate
silane/siloxane exterior facade treatments. Build Environ 2013;59:47–55.
http://dx.doi.org/10.1016/j.buildenv.2012.08.006.
[100] Graziani L, Quagliarini E, Osimani A, Aquilanti L, Clementi F, Yéprémian C,
et al. Evaluation of inhibitory effect of TiO2 nanocoatings against microalgal
growth on clay brick façades under weak UV exposure conditions. Build
Environ 2013;64:38–45. http://dx.doi.org/10.1016/j.buildenv.2013.03.003.
[101] De Filpo G, Palermo AM, Rachiele F, Nicoletta FP. Preventing fungal growth in
wood by titanium dioxide nanoparticles. Int Biodeterior Biodegrad
2013;85:217–22. http://dx.doi.org/10.1016/j.ibiod.2013.07.007.
[102] Gladis F, Schumann R. Influence of material properties and photocatalysis on
phototrophic growth in multi-year roof weathering. Int Biodeterior
Biodegrad 2011;65:36–44. http://dx.doi.org/10.1016/j.ibiod.2010.05.014.
[103] Matsunaga T, Okochi M. TiO2-mediated photochemical disinfection of
Escherichia coli using optical fibers. Environ Sci Technol 1995;29:501–5.
http://dx.doi.org/10.1021/es00002a028.
[104] Hong J, Ma H, Otaki M. Controlling algal growth in photo-dependent
decolorant sludge by photocatalysis. J Biosci Bioeng 2005;99:592–7. http://
dx.doi.org/10.1263/jbb.99.592.
[105] Peller JR, Whitman RL, Griffith S, Harris P, Peller C, Scalzitti J. TiO2 as a
photocatalyst for control of the aquatic invasive alga, Cladophora, under
natural and artificial light. J Photochem Photobiol Chem 2007;186:212–7.
http://dx.doi.org/10.1016/j.jphotochem.2006.08.009.
[106] Kim S-C, Lee D-K. Preparation of TiO2-coated hollow glass beads and their
application to the control of algal growth in eutrophic water. Microchem J
2005;80:227–32. http://dx.doi.org/10.1016/j.microc.2004.07.008.
[107] Chen F, Yang X, Mak HKC, Chan DWT. Photocatalytic oxidation for
antimicrobial control in built environment: a brief literature overview.
http://dx.doi.org/10.1016/
Build
Environ
2010;45:1747–54.
j.buildenv.2010.01.024.
[108] Yu J, Zhao X. Effect of substrates on the photocatalytic activity of nanometer
TiO2 thin films. Mater Res Bull 2000;35:1293–301. http://dx.doi.org/10.1016/
S0025-5408(00)00327-5.
[109] Ma Y, Qiu J, Cao Y, Guan Z, Yao J. Photocatalytic activity of TiO2 films grown
on different substrates. Chemosphere 2001;44:1087–92. http://dx.doi.org/
10.1016/S0045-6535(00)00360-X.
[110] Chen J, Poon C. Photocatalytic cementitious materials: influence of the
microstructure of cement paste on photocatalytic pollution degradation.
Environ Sci Technol 2009;43:8948–52. http://dx.doi.org/10.1021/es902359s.
[111] Lopez L, Daoud WA, Dutta D, Panther BC, Turney TW. Effect of substrate on
surface morphology and photocatalysis of large-scale TiO2 films. Appl Surf Sci
2013;265:162–8. http://dx.doi.org/10.1016/j.apsusc.2012.10.156.
[112] Stephan D, Wilhelm P, Schmidt M. Photocatalytic degradation of rhodamine B
on building materials—influence of substrate and environment. In: Baglioni P,
Cassar L, editors. Proc Int RILEM Symp Photocatal Environ Constr
Mater. Bagneux: Rilem; 2007. p. 299–306.
[113] Diamanti MV, Lollini F, Pedeferri MP, Bertolini L. Mutual interactions
between carbonation and titanium dioxide photoactivity in concrete. Build
Environ 2013;62:174–81. http://dx.doi.org/10.1016/j.buildenv.2013.01.023.
[114] Wu J-M, Zhang T-W. Photodegradation of rhodamine B in water assisted by
titania films prepared through a novel procedure. J Photochem Photobiol
Chem 2004;162:171–7. http://dx.doi.org/10.1016/S1010-6030(03)00345-9.
[115] Asiltürk M, Sayılkan F, Erdemoğlu S, Akarsu M, Sayılkan H, Erdemoğlu M,
et al. Characterization of the hydrothermally synthesized nano-TiO2
crystallite and the photocatalytic degradation of Rhodamine B. J Hazard
Mater 2006;129:164–70. http://dx.doi.org/10.1016/j.jhazmat.2005.08.027.
[116] Potenza G, Licciulli A, Diso D, Franza S, Calia A, Lettieri M. Surface engineering
on natural stone through TiO2 photocatalytic coating. In: Baglioni P, Cassar L,
editors.
Proc
Int
RILEM
Symp
Photocatal
Environ
Constr
Mater. Bagneux: Rilem; 2007. p. 315–22.
[117] Luvidi L, Laguzzi G, Gallese F, Mecchi AM, Sidoti G. Application of TiO2 based
coatings on stone surfaces of interest in the field of cultural heritage. In:
Ferrari A, editor. 4th Int Congr Sci Technol Safeguard Cult Herit Mediterr
Basin, vol. 2. Napoli: Grafica Elettronica; 2010. p. 495–500.
[118] Licciulli A, Calia A, Lettieri M, Diso D, Masieri M, Franza S, et al. Photocatalytic
TiO2 coatings on limestone. J Sol–Gel Sci Technol 2011;60:437–44. http://
dx.doi.org/10.1007/s10971-011-2574-9.
217
[119] Quagliarini E, Bondioli F, Goffredo GB, Licciulli A, Munafò P. Smart surfaces
for Architectural Heritage: preliminary results about the application of TiO2based coatings on travertine. J Cult Herit 2012;13:204–9. http://dx.doi.org/
10.1016/j.culher.2011.10.002.
[120] Quagliarini E, Bondioli F, Goffredo GB, Licciulli A, Munafò P. Self-cleaning
materials on Architectural Heritage: compatibility of photo-induced
hydrophilicity of TiO2 coatings on stone surfaces. J Cult Herit 2013;14:1–7.
http://dx.doi.org/10.1016/j.culher.2012.02.006.
[121] Quagliarini E, Bondioli F, Goffredo GB, Cordoni C, Munafò P. Self-cleaning and
de-polluting stone surfaces: TiO2 nanoparticles for limestone. Constr Build
Mater 2012;37:51–7. http://dx.doi.org/10.1016/j.conbuildmat.2012.07.006.
[122] La Russa MF, Ruffolo SA, Rovella N, Belfiore CM, Palermo AM, Guzzi MT, et al.
Multifunctional TiO2 coatings for Cultural Heritage. Prog Org Coat
2012;74:186–91. http://dx.doi.org/10.1016/j.porgcoat.2011.12.008.
[123] Munafò P, Quagliarini E, Goffredo GB, Bondioli F, Licciulli A. Durability of
nano-engineered TiO2 self-cleaning treatments on limestone. Constr Build
Mater
2014;65:218–31.
http://dx.doi.org/10.1016/
j.conbuildmat.2014.04.112.
[124] Pinho L, Mosquera MJ. Titania–silica nanocomposite photocatalysts with
application in stone self-cleaning. J Phys Chem C 2011;115:22851–62. http://
dx.doi.org/10.1021/jp2074623.
[125] Pinho L, Mosquera MJ. Photocatalytic activity of TiO2–SiO2 nanocomposites
applied to buildings: influence of particle size and loading. Appl Catal B
Environ
2013;134–135:205–21.
http://dx.doi.org/10.1016/
j.apcatb.2013.01.021.
[126] Bergamonti L, Alfieri I, Lorenzi A, Montenero A, Predieri G, Barone G, et al.
Nanocrystalline TiO2 by sol–gel: characterisation and photocatalytic activity
on Modica and Comiso stones. Appl Surf Sci 2013;282:165–73. http://
dx.doi.org/10.1016/j.apsusc.2013.05.095.
[127] Bergamonti L, Alfieri I, Franzò M, Lorenzi A, Montenero A, Predieri G, et al.
Synthesis and characterization of nanocrystalline TiO2 with application as
photoactive coating on stones. Environ Sci Pollut Res 2014;21:13264–77.
http://dx.doi.org/10.1007/s11356-013-2136-5.
[128] Aflori M, Simionescu B, Bordianu I-E, Sacarescu L, Varganici C-D, Doroftei F,
et al. Silsesquioxane-based hybrid nanocomposites with methacrylate units
containing titania and/or silver nanoparticles as antibacterial/antifungal
coatings for monumental stones. Mater Sci Eng B 2013;178:1339–46. http://
dx.doi.org/10.1016/j.mseb.2013.04.004.
[129] Kapridaki C, Maravelaki-Kalaitzaki P. TiO2–SiO2–PDMS nano-composite
hydrophobic coating with self-cleaning properties for marble protection.
http://dx.doi.org/10.1016/
Prog
Org
Coat
2013;76:400–10.
j.porgcoat.2012.10.006.
[130] Ruffolo SA, Macchia A, La Russa MF, Mazza L, Urzì C, De Leo F, et al. Marine
antifouling for underwater archaeological sites: TiO2 and Ag-doped TiO2. Int J
Photoenergy 2013;2013:1–6. http://dx.doi.org/10.1155/2013/251647.
[131] Pinho L, Elhaddad F, Facio DS, Mosquera MJ. A novel TiO2–SiO2
nanocomposite converts a very friable stone into a self-cleaning building
material. Appl Surf Sci 2013;275:389–96. http://dx.doi.org/10.1016/
j.apsusc.2012.10.142.
[132] Sharma G, Bala R. Digital color imaging handbook. CRC Press; 2002.
[133] Mahy M, Van Eycken L, Oosterlinck A. Evaluation of uniform color spaces
developed after the adoption of CIELAB and CIELUV. Color Res Appl
1994;19:105–21. http://dx.doi.org/10.1111/j.1520-6378.1994.tb00070.x.
[134] Sundqvist B, Morén T. The influence of wood polymers and extractives on
wood colour induced by hydrothermal treatment. Holz Als Roh Werkst
2002;60:375–6. http://dx.doi.org/10.1007/s00107-002-0320-2.
[135] Miliani C, Velo-Simpson ML, Scherer GW. Particle-modified consolidants: a
study on the effect of particles on sol–gel properties and consolidation
effectiveness. J Cult Herit 2007;8:1–6. http://dx.doi.org/10.1016/
j.culher.2006.10.002.
[136] García O, Malaga K. Definition of the procedure to determine the suitability
and durability of an anti-graffiti product for application on cultural heritage
porous materials. J Cult Herit 2012;13:77–82. http://dx.doi.org/10.1016/
j.culher.2011.07.004.
[137] Hassan MM, Dylla H, Mohammad LN, Rupnow T. Evaluation of the durability
of titanium dioxide photocatalyst coating for concrete pavement. Constr
http://dx.doi.org/10.1016/
Build
Mater
2010;24:1456–61.
j.conbuildmat.2010.01.009.
[138] Olabarrieta J, Zorita S, Peña I, Rioja N, Monzón O, Benguria P, et al. Aging of
photocatalytic coatings under a water flow: long run performance and TiO2
nanoparticles release. Appl Catal B Environ 2012;123–124:182–92. http://
dx.doi.org/10.1016/j.apcatb.2012.04.027.
[139] Daniotti B, Lupica Spagnolo S, Galliano R. The durability experimental
evaluation of photocatalytic cement-based materials. In: de Freitas VP,
Corvacho M, Lacasse M, editors. Proc Int Conf Durab Build Mater
Compon. Porto: FEUP edições; 2011. p. 225–32.
[140] Zhang M-H, Tanadi D, Li W. Effect on photocatalyst TiO2 on workability,
strength and self-cleaning efficiency of mortars for applications in tropical
environment. In: 35th Conf Our World Concr Struct. Singapore: CI Premier;
2010. p. 65–76.
[141] Krishnan P, Zhang M-H, Yu L, Feng H. Photocatalytic degradation of
particulate pollutants and self-cleaning performance of TiO2-containing
silicate coating and mortar. Constr Build Mater 2013;44:309–16. http://
dx.doi.org/10.1016/j.conbuildmat.2013.03.009.
218
P. Munafò et al. / Construction and Building Materials 84 (2015) 201–218
[142] Graziani L, Quagliarini E, Bondioli F, D’Orazio M. Durability of self-cleaning
TiO2 coatings on fired clay brick façades: Effects of UV exposure and wet &
dry cycles. Build Environ 2014;71:193–203. http://dx.doi.org/10.1016/
j.buildenv.2013.10.005.
[143] Guo M-Z, Ling T-C, Poon C-S. Nano-TiO2-based architectural mortar for NO
removal and bacteria inactivation: Influence of coating and weathering
conditions. Cem Concr Compos 2013;36:101–8. http://dx.doi.org/10.1016/
j.cemconcomp.2012.08.006.
[144] Maury-Ramirez A, Demeestere K, De Belie N. Photocatalytic activity of
titanium dioxide nanoparticle coatings applied on autoclaved aerated
concrete: effect of weathering on coating physical characteristics and
gaseous toluene removal. J Hazard Mater 2012;211–212:218–25. http://
dx.doi.org/10.1016/j.jhazmat.2011.12.037.
[145] Sciancalepore C, Bondioli F. Durability of SiO2–TiO2 photocatalytic coatings
on ceramic tiles. Int J Appl Ceram Technol, in press; doi:10.1111/ijac.12240.
Keep reading this paper — and 50 million others — with a free Academia account
Used by leading Academics
Kenneth Vecchio
University of California, San Diego
Petra Reinke
University of Virginia
Susan Trolier-McKinstry
Penn State University
Wolfgang Tremel
Johannes Gutenberg-Universität Mainz
