Behind the buzz and beyond the hype:
Our Nanowerk-exclusive feature articles
Posted: Jan 11, 2011
Nanotechnology solutions for self-cleaning, dirt and water-repellent coatings
(Nanowerk Spotlight) A surface is not simply the physical division
between an object and its environment; it
fulfils a range of functions of its own which
often play a crucial part in product design.
Surfaces are supposed to feel good to the
touch and to look good for as long as possible,
be easy to maintain and not be spoiled
by dirt, water stains or fingermarks. Traditional
coating materials often do not stand
the test of the increased demands made
on materials today. In recent years however,
advances have been made using methods
ascribed to nanotechnology1. The
following gives a short overview of the basis
of these structures, the processes involved
in their manufacture, the areas
where they can be used, and the effect they
have on the environment and human
Self-cleaning I – the "Lotus-Effect"
The lotus plant (Nelumbo nucifera, Figure 1) is revered in Asia for its exceptional cleanness. Although it grows in muddy waters, its leaves always appear immaculately clean. The plants' leaves are superhydrophobic, i.e. drops of water roll off free of residue, taking any impurities with them.
Figure 1: The lotus plant (Nelumbo nucifera).Investigations into the surface using reflection electron microscopy (REM) have shown that the surface of the leaf is not especially even, which you might intuitively assume, but has instead a special, characteristic roughness (Figure 2): Systematically arranged, water-repellent, nano-size wax crystals form three-dimensional structures, similar to small nipples, which are no greater than a few nanometers or micrometers in size.
When combined with the waxes' water-repellent chemical properties, these structures make the lotus leaf extremely non-wettable, a state called ultrahydrophobia or super-hydrophobia, and they give it its self-cleaning properties. Dirt particles only sit on the tip of the wax crystals and as a result only a very small surface area comes into contact with the plant's surface. If water falls onto a leaf surface like this, the interplay of the surface tension and the low attraction force between the surfaces and the water produce a spherical water drop which only sits on the tips of the wax structures. If the leaf tips in the slightest, the water drop immediately rolls off, taking the dirt particles with it (Figure 3). As the gravitational pull between the dirt and the leaf's surface is very slight, i.e. it is smaller than that between the water and the dirt, even lipophilic impurities, such as soot for example, can be washed away.
In most cases this form of self-cleaning serves not so much to protect the plant from dirt as from pathogens (e.g. fungal spores, bacteria). Super-hydrophobia is a property which is not only found in the lotus plant however, it is present in around 300 other plant species as well. Insects too, such as dragonflies and butterflies have this property on the surface of their wings
Figure 2: Electron microscope photograph of the surface of a lotus flower leaf. The combination of surface roughness and water-repellent wax cristals gives it superhydrophobic properties.The principle of self-cleaning was discovered in 1973 by the botanist Wilhelm Barthlott and his team at the University of Bonn. As it is a purely physio-chemical effect that is not bound to a living system, Barthlott believed that its technical implementation should be possible. Industry however showed little interest initially, and so he produced his own technical surfaces with these properties, applied for a patent for his invention2 and registered the brand name "Lotus-Effect®".
The first commercial product made in collaboration with industry was a silicone resin house paint which has since become widely used and in which silicon nanoparticles form micro-structured surface3. Other "Lotus-Effect®"products include ceramic roof tiles, architectural glass and a spray for industrial use which makes surfaces temporarily non-wettable and self-cleaning4. Advances are also being made in the development of engineered fabrics that are selfcleaning.
Composite materials made up of nanoparticles in a coating matrix make it possible to manufacture the surface structure required for the "Lotus-Effect". These polyester fabrics are of particular interest in the manufacture of awnings, parasols, sails and tents5. Their self-cleaning properties have led the Institute for Textile Technology and Process Engineering (ITV) in Denkendorf, Germany to award them the quality label "Self-cleaning inspired by nature"6.
Manufacturing surfaces which feature a "Lotus-Effect®"is technically challenging and still poses certain problems. To date there are only a few low-cost processes which can produce the surface structures on a large scale.
One example is the application of "Lotus-Effect®"structures in house paints. Also, the required surface structures can either be created from hydrophobic polymers early on at the manufacturing stage or post-production by means of moulding, etching or applying a powder made from hydrophobic polymers or from nanoparticles (e.g. silicon dioxide). A further possibility is the subsequent hydrophobisation of a previously manufactured surface which has the required structures7.
Possible uses and limitations
The sensitivity of these surface structures to mechanical load prevents them from being used widely. Whereas the lotus plant's structure continuously renews itself, technological imitations do not have this capability. Consequently no clothing textiles with the "Lotus-Effect" have been produced – washing in a washing machine would destroy the surface structures.
Figure 3: As it rolls off, a drop picks up the dirt particles which are lying loosely on the leaf's surface, thereby cleaning it.Self-cleaning also requires running water, rain for example, hence surface structures with these properties are not practicable for use in most interiors. Also, "Lotus-Effect"surfaces have a slightly matt finish and therefore cannot be used on optical glass (e.g. spectacles). Tensides (e.g. soap or washingup liquid) reduce the surface tension of water thereby disrupting the formation of drops. If self-cleaning surfaces are treated with detergents containing tensides, the "Lotus-Effect ®"therefore breaks down and water will make the surface wet. The surface is not destroyed by this and is fully functional again after rinsing in clear water.
However, the fact of this makes "Lotus-Effect" surfaces largely unsuitable for use on ceramic sanitary facilities (e.g. washing basins, shower trays, bath tubs etc.). Several years of practical experience with house paints have shown that the self-cleaning capability is only partially successful. For example, the splash zone of the outside walls of houses (the base course) becomes dirty in spite of the "Lotus-Effect". Furthermore, it only takes a simple fingermark to remove the "Lotus-Effect"because fat cancels out super-hydrophobia8. There are still limits, therefore, to technology's ability to imitate the natural self-cleaning capability of the lotus leaf.
Dirt and water-repellent "Easy-to-Clean" surfaces
As the above demonstrates, there are still only a handful of practical applications available on the market which have the "Lotus-Effect®". Nevertheless, there are already several products with dirt-repellent and waterrepellent properties on sale which, the manufacturers claim, are based on nanotechnology or contain nanoparticles. These include, for example, ceramic sanitary facilities, spectacle lenses or textiles. Often the advertisements for these products make reference to the lotus plant, however the products do not exhibit self-cleaning properties. In other words, running water alone (rain) is not usually sufficient to clean this type of product. It is simply easier to clean since dirt does not adhere so well to the surface. Ideally, these surface coatings obviate the need for aggressive cleaning detergents.
In many cases these products have so-called
"Easy-to-Clean" surfaces which, unlike products
with the "Lotus-Effect", feature a smooth
rather than a rough surface and are not only
water-repellent but also fat-repellent (lipophobic)
9. Many commonly used materials,
such as glass for example, generally have
a slightly rough surface, even if they appear
smooth to the eye, and attract both water and
dirt. Unlike "Lotus-Effect"surfaces "Easy-to-
Clean" coatings are not made rougher, but
flattened by applying hydrophobic and oleophobic
chemicals. However, the contact angle
(see text box) between a water drop and
the surface is smaller than is the case of the
"Lotus-Effect" (<140°), i.e. materials treated
in this way are not super-hydrophobic.
Whereas the "Lotus-Effect" is based on physical
as well as chemical principles, "Easyto-
Clean" coatings have a purely chemical
basis. Outdoor use, for example in window
glass, is only practicable on particularly suitable
surfaces on which sufficient water lands.
Otherwise water drops may dry in places and
leave dirt behind. Where there is only a very
small amount of water the drops may leave
behind a visible trace as they roll off9. "Easyto-
Clean" does not mean that a surface with
this treatment never needs to be cleaned, however the amount of cleaning required
compared with that of traditional products
can be reduced. Depending on the extent
and nature of the dirt, a firm jet of water may
suffice to remove it (e.g. in the case of glass
fronts), or it may be easier to remove by mechanical
New types of products based on chemical
nanotechnology are also being used to protect
the surfaces of construction materials,
for example to prevent damage caused by
water penetration or to protect outside walls
from mildew, moss, algae and dirt. Also, protective
"anti-graffiti" coatings make it easy
to remove undesirable "art", as spray paints
do not adhere to them. Even chewing gum
can be removed more easily from surfaces
coated in this way. Unlike the coatings which
used to be applied for these purposes, the
new silane-based products do not seal the
surface, hence moisture can escape10. The
protection provided is usually permanent and
is retained even after repeated cleaning. Steel
and glass are popular materials in architecture.
However their appearance can be
spoiled by, for example, fingermarks which
have lipophilic characteristics. New types of
"anti-fingerprint" nano-coatings counteract
this as they are fat-repellent and modify the
light refraction so that the marks remain invisible9.
Transparent hydrophobic and oleophobic
coatings of this kind can be manufactured
using sol-gel processes11. In simple terms,
the raw materials are so-called silanes12 with
a silicon base atom which can be modified
by adding certain chemical substances to it
(e.g. fluorine compounds) in order to retain
the required properties. Nanoscale particles
(colloids) are produced from the silanes by
means of chemical reactions in a solution.
The dispersion of particles is called the sol.
Depending on the type of sol chosen, the solution
evaporates either at room temperature
or when it is heated, and the sol becomes
a viscous gel because the particles
concatenate into a dense web due to their
high reactivity. Once it has dried out a compact
layer is formed. The sol-gel process was
developed as early as the 1930s, but for some
years now has been ascribed to chemical
nanotechnology. One of the pioneers in this
field is the Leibniz Institute for New Materials
(INM) in Saarbrücken, Germany13.
Figure 4: Diagram of an "Easy-to-Clean" surface.
The sol can be applied to a great variety of
surfaces using traditional industrial processes
such as dip coating, spraying or spin-coating.
The resulting coatings are only a few nanometers
thin and transparent – an important
advantage over other traditional coating
materials such as Teflon®, which has a
dark colour because of its graphite content.
These coatings can consequently also be
used on, for example, glass. Sol-gel processes
are already used in the motor vehicle industry
to produce water-repellent coatings
for windscreens or exterior mirrors14. In industrial
manufacturing processes the coating
is burned into the surfaces at high temperatures,
thereby increasing their durability.
As a sol can also dry out at room temperature
to form a firm layer, surfaces such
as window glass can also be treated postproduction
by the consumer or a commercial
service provider (e.g. cleaning companies).
A number of different liquids or sprays
for this purpose are commercially available.
These coatings however are not durable and
need to be renewed after a certain period
of time. Unfortunately, information regarding
the exact constituents of post-production
applications with "Easy-to-Clean" properties
which can be obtained in shops or via the
internet is scarce as the manufacturers often
claim the right of trade secrecy. It is therefore
usually impossible to establish whether
products are based on chemical nanotechnology
in each case.
Impregnating agents are also available
which make textiles and leather dirt and water-
repellent and which, manufacturers claim,
contain nanoparticles or are based on nanotechnology.
Often, however, traditional surfactant
substances are used, such as fluorocarbon
resin or silicon oils, which produce
a nanoscale impregnated layer. Based on the
current understanding of nanotechnology,
one-dimensional nanostructures would
probably also be categorised as such, however
it appears that many products do not
fulfil an essential criterion of nanotechnology
– i.e. that the material has new properties.
Nonetheless there are still impregnating
materials on the market which manufacturers
claim, with reference to the lotus plant,
produce surface roughness and hydrophobia
on materials by means of nanoparticles,
making them dirt and water-repellent.
Self-cleaning II – photocatalytic nanotitanium dioxide (TiO2)
To date, photocatalytic self-cleaning15 is
probably the most wide-spread application
ascribed to nanotechnology in the construction
industry. There are already a great number
of buildings worldwide which have been
treated with it. The photocatalytic properties
of TiO2 were discovered as long ago as
1967 by Akira Fujishima, a scientist at the
University of Tokyo, and the phenomenon
became known as the "Honda-Fujishima Effect".
The first house with self-cleaning exterior
surfaces was Fujishima's own9.
Titanium dioxide is hydrophilic due to its high
surface energy, hence water does not form
drops on a surface coated with it, but a
sealed water film instead. A coating of this
kind hence behaves in exactly the opposite
manner to a surface whose self-cleaning
properties are based on the "Lotus-Effect".
It is also transparent and can therefore also
be used on glass. In addition, TiO2 is photocatalytic,
in other words in the presence of
water, oxygen radicals are produced under
UV light irradiation which in turn can decompose
organic material such as, for example,
fats, oils, soot or plant materials. TiO2 is especially
reactive in nanoform. It is not expended
during catalysis, with the result that
the effect is lasting. On self-cleaning surfaces
of this type organic dirt is dissolved in the
water film, decomposed, and the residue is
removed by the next heavy shower of rain.
This does not however mean that surfaces
of this kind must never be cleaned. The
amount of dirt is merely reduced so that it
does not need to be cleaned as often16. From
an engineering point of view coating organic
or polymer surfaces (e.g. plastics) with TiO2
is complicated because the carrier materials
themselves, like any other organic material,
are attacked and destroyed by oxidation.
In order to prevent this, inorganic barrier
layers need to be applied and the TiO2
nanoparticles given a surface coating in order
that the different layers adhere to each
Photocatalytic self-cleaning only works in outdoor
use because it requires UV light and
water to remove the residue. Ways of modifying
the properties of TiO2 are currently being
researched so that it becomes photocatalytic
by irradiation with visible light. This is
achieved by doping them with metal atoms
such as, for example, chromium, vanadium18,
tungsten19 or carbon20. The results of
earlier research have already been implemented
in commercial products such as photocatalytically
active interior paints which reduce
gaseous pollutants in the air, for example
to improve the air quality in enclosed office
spaces21. As photocatalysis can also kill
microorganisms, it is conceivable that antimicrobial
coatings may be produced in the
future, for example for use in clinical areas,
and this is now being researched22.
The ultra-thin water film formed by the hydrophilic
properties of the nanoscale titanium
dioxide on surfaces additionally prevents
glass from misting up since water droplets
cannot form. The "Anti-Fog" effect, as it is
known, is particularly beneficial, for example,
in glass surfaces in conservatories, exterior
mirrors on motor vehicles, or even in
bathroom mirrors or spectacles.
Photocatalytic coatings with nano-TiO2 are
usually applied at the same time as the material
itself is manufactured, using Chemical
Vapor Deposition (CVD23). The coating
is already in use not just in glass surfaces,
but also in plastics (PVC), sound-proofing
panels, tiles, roof tiles or concrete slabs.
However by this method it is not possible to
coat these materials post-production. In the
case of glazing, it is important to ensure that
silicon seals are not used as their oils seep
over the glass, destroying the hydrophilic surface
properties and leading to unsightly
Figure 5: Hydrophilic coating (right) with TiO2 on float glass.As well as coating by
means of CVD, sol-gel processes can also
be used to create the surface functionality,
for example in motor vehicle windscreens or
exterior mirrors. This latter method is far
more preferable for the manufacturers as it
is carried out at lower temperatures, takes
less time, and saves energy costs24. House
paints with photocatalytically active TiO2 nanoparticles
are also available, which not only
have self-cleaning properties25 but which
also reduce nitrogen oxide and ozone26.
Environment and health
Self-cleaning or easy to clean surfaces can
reduce the amount of cleaning required. In
the case of industrial cleaning in particular
it can reduce labour costs and extend a material's
durability. Lower energy costs and less
use of cleaning detergents are expected to
be the primary environmental benefits. It was
in anticipation of such outcomes that the
German Federal Foundation for the Environment
(DBU) awarded Wilhelm Barthlott its
respected Environment Prize in 1999 for his
discovery of the "Lotus-Effect"27. However,
there are currently no reliable, quantitative
studies of actual potential environmental
benefits. As a rule, descriptions of products'
environmental benefits do not contain an
analysis or evaluation of the amount of resources
used and/or the energy consumption
involved in their manufacture. Future
evaluation should also include information
on the fate and behaviour of the materials
once they reach the end of their life cycle
Current risk assessments conclude that there
is very little probability of a harmful impact
on the environment or human health from
coatings in which nanoparticles are firmly
embedded in a coating matrix, as in the case
of "Easy-to-Clean" coatings. There have so
far also been no indications of environmental
or health risks from surfaces coated with
the "Lotus-Effect". A recent investigation into
the abrasion resistance of test structures
which had been coated with zinc oxide
nanoparticle layers showed no significant release
from the coating material29. It does appear
possible however, that nanoparticles are
released as a result of the effects of weathering
on the coating matrix, for example
where they consist of biodegradable materials.
An investigation by Kaegi et al. has
shown that house paints release very small
amounts of synthetic TiO2 particles, between
20 and 300 nm in size, as a consequence
of weathering, and that these can enter the
soil via rainwater drains30. The photocatalytic
activities of TiO2 produce free oxygen radicals
that are toxic for aquatic organisms if
such nanoparticles enter the waters they inhabit31.
To date, threshold levels remain unknown.
The release of particles into the environment
can however be reduced or prevented
if nanocoatings and nanomaterials
are designed accordingly32.
Although surface coatings which have nanomaterials
firmly embedded in a matrix are
currently believed to pose only a very slight
risk to the health of users and consumers,
the protection of those who work in the companies
which manufacture nanoparticulate
raw materials requires special attention. The
German State of Hesse's Ministry for Economic
Affairs, Transport and State Development
recently commissioned a study by the
Institute for Applied Ecology in Darmstadt
(Öko-Institut Darmstadt) which confirmed in
its action recommendations for the manufacture
and use of nanomaterials in the paints
and coatings industry that there are significant
gaps in knowledge and information in
respect of exposure data and human and
eco-toxicological impact. The report goes on
to recommend that companies should be
guided by the precautionary principle and
that preventing inhalation be given top priority.
Where this proves impossible, appropriate
effective protective measures must be
implemented (e.g. filters, extraction units, respiratory
masks etc.). The release of particles
into the environment must be kept to the minimum
Impregnating agents in propellant sprays
can pose a danger to consumers' health if
not properly used. This is independent, however,
of whether they contain traditional surfactant
substances or are, according to the
manufacturers, based on nanotechnology.
In 2006 approximately just over 100 people
had to have medical treatment for serious
respiratory difficulties after they had used
a so-called "nano-spray" to treat glass and
ceramic surfaces in bathrooms. The cause
was initially suspected to be nanoparticles.
However, chemical analyses showed that the
products did not contain nanoparticles.
"Nano" in the product descriptions referred
only to the nanoscale protective layer produced
by the spray. The probable cause of
the health problems was the combination of
the surfactant substances (fluorosilanes) with
the spray's other chemical components34.
Propellant sprays produce aerosols (tiny
liquid droplets) between a few dozen nanometers
to around a hundred micrometers in
size. If inhaled, the larger droplets can become
blocked in the nose and the upper part
of the respiratory tract. However, droplets
smaller than 10 micrometers can penetrate
deep into the lung, in certain circumstances
as far as the alveoli, which may collapse as
result of the effect of the surfactant substances
from the impregnating sprays35. Inhaling
the atomised spray from these types
of products must therefore be avoided at all
costs and their directions for use must be adhered
to precisely. Use in enclosed spaces
(such as a bathroom) is inadvisable under
any circumstances. Manufacturers should
adjust spray systems to ensure that droplets
no smaller than 10 micrometers cannot be
sprayed. As a general rule, it is more advisable
to use impregnating agents – whether
advertised as "nano" or not – which come in
manual atomisers without propellants (pump
sprays) or in liquid form (e.g. as a foam).
Notes and References
1 Veith, M. and Schubert, M., 2009, Innovativ
und Smart – Funktionale Oberflächen schaffen
hohen Nutzen, Lippe Wissen & Wirtschaft,
2 European Patent EP 0772514, 15.2.96, Selfcleansing
surfaces of objects and methods for
the production thereof.
14 Aegerter, M.A., et al., 2008, Coatings made by sol-gel and chemical nanotechnology, J Sol-Gel Sci technol (47), 203-236.
15 Photocatalytic self-cleaning is the property of
surfaces coated with titanium dioxide (TiO2)
nanoparticles. Titanium dioxide (TiO2) is a semiconductor;
electron hole pairs generate light
if the energy of the photons is greater than the
band gap Eg (inner photoelectrical effect). The
electrons or holes can diffuse to the surface in
titanium dioxide and produce radicals that
break down organic substances. The holes in
particular have a strong oxidising effect; OH
radicals are produced from water, as organic
substances are broken down. The end products
are often CO2 and water. The band gap
Eg is .2 eV for anatas, the form of TiO2 that is
most efficient for photocatalysis (ca. 3.0 eV for
the less efficient rutil crystal structure). Since
this energy corresponds to a wavelength of ca.
390 nm, only ultraviolet light is effective. The
super-hydrophilic properties of the surfaces are
created by oxygen gaps on the TiO2 surface.
It is here that the OH groups are bonded to
allow a good netting with water.
16 Fujishima, A. and Zhang, X., 2006, Titanium dioxide photocatalysis: present situation and future approaches, Comptes Rendus Chimie 9 (5-6), 750-760.
17 Quilitz, M. et al., 2008, Innovative Schichtsysteme – Neue Entwicklungen der chemischen Nanotechnologie, Journal für Oberflächentechnik 7, 8-10.
22 In chemical vapour deposition (CVD) at least
two volatile compounds are fed with a gas flow
into a reaction chamber containing the object
to be coated. The addition of reaction energy
causes a chemical reaction to take place on
the surface of the object. If the displacement
of a layer on the surface is to be preferred to
the formation of solid particles in the gas phase,
the CVD must take place at very low pressures.
23 Aegerter, M.A. et al., 2008, Coatings made
by sol-gel and chemical nanotechnology, J Sol-
Gel Sci technol (47), 203-236.
28 Vorbau, M. et al., 2009, Method for the characterization
of the abrasion induced nanoparticle
release into air from surface coatings,
Aerosol Science 40, 209-217.
29 Kaegi, R. et al., 2008, Synthetic TiO2 nanoparticle emission from exterior facades into the aquatic environment, Environmental Pollution 156, 233-239.
30 Battin, T.J., et al., 2009, Nanostructured TiO2:
Transport Behavior and Effects on Aquatic Microbial
Communities under Environmental
Conditions, Environmental Science & Technology
31 Kölbel, M., 2008, Steckt der Teufel im Teilchen? Über die Risiken der Nanotechnologie, in: S. Leydecker (Endnote 9), 44-49.
32 Hermann, A. et al., 2009, Sichere Verwendung
von Nanomaterialien in der Lack- und Farbenbranche
– Ein Betriebsleitfaden, commission
by Verkehr und Landesentwicklung Hessisches
Ministerium für Wirtschaft: HA Hessen Agentur
35 Muntwyler, R., 2004, Nicht nur Leder, auch die
Lunge wird imprägniert, K-Tipp, 16-17 www.ktipp.ch/downloadfile/1020525.
By NanoTrust, Austrian Academy of Sciences. NanoTrust Dossiers are published irregularly and contain the research results of the Institute of Technology Assessment in the framework of its research project NanoTrust. The Dossiers are made available to the public exclusively on epub.oeaw.ac.at/ita/nanotrust-dossiers.