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Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 16304
www.rsc.org/materials FEATURE ARTICLE
A review on self-cleaning coatings
V. Anand Ganesh,* Hemant Kumar Raut, A. Sreekumaran Nair and Seeram Ramakrishna*
Received 3rd June 2011, Accepted 13th July 2011
DOI: 10.1039/c1jm12523k
Published on 02 September 2011 on http://pubs.rsc.org | doi:10.1039/C1JM12523K

This review article summarizes the key areas of self-cleaning coatings, primarily focusing on various
materials that are widely used in recent research and also in commercial applications. The scope of this
article orbits around hydrophobic and hydrophilic coatings, their working mechanism, fabrication
techniques that enable the development of such coatings, various functions like Anti-icing, Electro-
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wetting, Surface switchability and the areas where selfcleaning technology can be implemented.
Moreover, different characterization techniques and material testing feasibilities are also analyzed and
discussed. Though several companies have commercialized a few products based on self-cleaning
coating technology, much potential still remains in this field.

1. Introduction on around the world to develop highly efficient and durable self-
cleaning coating surfaces with enhanced optical qualities. Apart
Many technologies existing in today’s world have been derived from the wide range of applications, this technology also offers
from nature. Self-cleaning technology is one amongst them. Many various benefits, which include reduction in maintenance cost,
surfaces in nature exhibit self-cleaning properties. The wings of elimination of tedious manual effort and also reduction in the time
butterflies1 and the leaves of plants, such as cabbage and lotus, are spent in cleaning work.
a few examples. Because of the extensive range of applications, Self-cleaning coatings are broadly classified into two major
from window glass cleaning, solar panel cleaning and cements to categories: hydrophilic and hydrophobic. Both of the categories
textiles, this technology received a great deal of attention during clean themselves by the action of water. In a hydrophilic coating,
the late 20th century and now numerous research works are going the water is made to spread (sheeting of water) over the surfaces,
which carries away the dirt and other impurities, whereas in the
hydrophobic technique, the water droplets slide and roll over the
Healthcare and Energy Materials Laboratories, National University of
Singapore, 2 Engineering Drive 3, Singapore. E-mail: anandganesh@nus.
surfaces thereby cleaning them. However, the hydrophilic coat-
edu.sg; seeram@nus.edu.sg; Tel: +065 6516 8596 ings using suitable metal oxides have an additional property of

Mr. Anand Ganesh Venkatesan Mr. Hemant Kumar Raut is

is pursuing Doctoral Degree at currently pursuing Doctoral
the National University of Sin- Degree at the National Univer-
gapore. His present research sity of Singapore (NUS). His
work is on self-cleaning hydro- current research work includes
philic and hydrophobic coatings fabrication and characterization
on glass and solar modules at of anti-reflective coatings on
Professor Seeram Ram- glass and solar modules at the
akrishna’s Healthcare and Healthcare and Energy Mate-
Energy Materials (HEM) rials (HEM) Laboratory,
Laboratory, NUS. He gradu- NUS. With a Bachelors in
ated with a first class honours Mechanical Engineering, his
degree in Mechatronics Engi- research interests lay in the field
V: Anand Ganesh neering from Anna University, Hemant Kumar Raut of nanotechnology, photonic
Chennai. His research interests nanostructures (biomimetic),
include the development of interferential antireflective
photochromic and self-cleaning coatings, biomimetic nano- coatings, solar energy and coating technology. He has experience
structures, surface modification and solar energy. in the field of industrial component designing and manufacturing,
steel processing and finite element modeling.

16304 | J. Mater. Chem., 2011, 21, 16304–16322 This journal is ª The Royal Society of Chemistry 2011
 View Online

chemically breaking down the complex dirt deposits by sunlight-

assisted cleaning mechanism. This review will discuss the mate-
rials, processes, mechanisms and characterization involved in
self-cleaning coatings. Further, the review will highlight the
challenges still to be met along with recent innovations in this
direction.

2. Self-cleaning effect
The self-cleaning phenomenon is related to the surface contact
angle. It is the angle formed at the three phase boundary (solid/
liquid/vapour) between the surfaces of the liquid drop to the
surface of the solid. In general, if the contact angle is <90
the
Published on 02 September 2011 on http://pubs.rsc.org | doi:10.1039/C1JM12523K

solid surface is termed as a hydrophilic surface. When the contact

angle (CA) is >90
, the surface is defined as a hydrophobic
surface. Similarly, a surface with a water contact angle
approaching zero is classified as ultra (super) hydrophilic and
a surface with a contact angle >150
is usually categorized as
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ultra (super) hydrophobic (Fig. 1).

3. Hydrophobic and superhydrophobic coatings

3.1. Nature’s lead
Nature is the source of inspiration for many researchers around
the world to develop aesthetic self-cleaning functional systems.
The lotus flower is referred to the symbol of purity in Asian Fig. 1 A schematic representation of hydrophilic, hydrophobic and
religions. Ward et al.,2 first observed this phenomenon and ultra (super) hydrophobic surfaces.
described the fact that, although the lotus leaf rises from muddy
water, it is clean and remains untouched by dirt and other
pollutants. The mystery behind this mechanism was unfolded These findings led the way for the fabrication of various biomi-
after the invention of the SEM in mid 1960s.3 Studies made using metic superhydrophobic surfaces inspired by nature. The results
SEM revealed that the surfaces, which appear to be macro- of the research work conducted by Guo et al.7 disclosed that
scopically smooth, exhibit microscopic roughness on different there are two major types of surface microstructures in plant
scale lengths.4–6 These surfaces, along with the presence of leaves with superhydrophobicity: (i) hierarchical micro and
epicuticular wax crystalloids, make the leaves superhydrophobic. nanostructures, (ii) unitary micro-line structures. This revelation

Dr A. Sreekumaran Nair is Dr Seeram Ramakrishna,

currently a research fellow at the FREng, FNAE, FAAAS is
Healthcare and Energy Mate- a Professor of Materials Engi-
rials (HEM) Laboratory of neering and Director of HEM
National University of Singa- Labs (http://serve.me.nus.edu.
pore. He graduated with a PhD sg/seeram_ramakrishna/) at the
in Chemistry from Indian Insti- National University of Singa-
tute of Technology (IIT) pore. He is an acknowledged
Madras (2006) and subse- global leader for his pioneering
quently became a JSPS post- work on science and engineering
doctoral fellow (2006–2008) in of nanofibers (http://
Japan. His research interests researchanalytics.thomsonreuters.
include development of materials com/m/pdfs/grr-materialscience.
A: Sreekumaran Nair for energy conversion and Seeram Ramakrishna pdf). He authored five books
storage, catalysts for solar and over four hundred peer
hydrogen, nanotechnology- reviewed international journal
based environmental remediation and probing charge transport papers, which attracted over 12 000 citations with an h-index of
mechanism in monolayer-protected clusters and 3-D superlattices. 52. Web of Science places him among the top one percent of
materials scientists worldwide. He is an elected international fellow
of major professional societies in Singapore, ASEAN, India, UK
and USA.

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paved way for the development of several synthetic methods to

mimic the natural superhydrophobic surfaces.

3.1.1. Plant leaves with hierarchical structure. Fig. 2 shows

SEM images of hierarchical structures present in three different
plant leaves. Fig. 2a and b are SEM images of lotus leaf at low
and high magnifications, which show the uniformly textured
surface with 3–10 mm sized flanges and valleys tinted with a 70–
100 nm sized wax like material (Fig. 2a). A lot of nanorod like
structures with an average diameter of about 50 nm are randomly Fig. 3 The SEM images of biomimetic superhydrophobic surfaces made
by replicating the lotus leaf’s surface structure with lotus leaf as the
distributed on the subsurface layer (Fig. 2b).
template. (a) The positive replica with poly(dimethylsiloxane) (PDMS),
This textured surface helps the lotus leaf to exhibit super-
and the inset is a visualization of the advancing and receding CAs on the
Published on 02 September 2011 on http://pubs.rsc.org | doi:10.1039/C1JM12523K

hydrophobic property. The water contact angle observed is surface.9 (b) A photopolymer replica with UV-nanoimprint lithography
around 162
(inset of Fig. 2b).8,9 Fig. 2c and d show the SEM and the inset is the magnified image.10
images of rice leaf. The top surface of the leaf posses the papillae
with an average diameter of about 5–8 mm and they are arranged
in one-dimensional order (Fig. 2c). The sub layer of the surface nanopins on its surface along with the formed microstructure.
consists of innumerable nanopins that are proportionally well The WCA observed in this leaf is around 159
(inset of Fig. 2f).
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distributed to enhance the air trapping mechanism in the surface Inspired by the enthralling hierarchical structures exhibited by
(Fig. 2d). The water contact angle (WCA) exhibited by this nature, Ji et al.10 successfully replicated a superhydrophobic
surface is 157
(inset of Fig. 2d). Like lotus and rice leaves, taro surface using lotus leaf as a template. He employed a nanoscale
leaf also shows superhydrophobicity (Fig. 2e and f). Compared casting technique instead of conventional microfabrication or
to the above two plant leaves, taro leaf possesses distinct chemical synthesis (Fig. 3a). The SEM image shows a surface
microstructures (10 mm) that are distributed in their corre- morphology very similar to lotus leaf with small papillae
sponding nest like caves (Fig. 2e). A higher magnification SEM protrusions of an average diameter of 6 mm. The space between
image (Fig. 2f) shows the presence of harmoniously distributed the microstructured valleys and protrusions are textured by
intricate nanostructures. Similarly, two other kinds of lotus leaf
replicas (PDMS replica and photo polymer replica) were fabri-
cated by polymer casting and UV nanoimprint lithography.11
Fig. 3b shows the SEM image of the morphology obtained.
Besides lotus leaf, other plant leaves can also be used as
a template for fabricating superhydrophobic surfaces. Taro leaf
is used as a template to produce a superhydrophobic surface with
polystyrene film.12 Though these intriguing structures can be
replicated as an artificial surface using lotus leaf as a template,
there are still limitations due to the size of the lotus leaf and the
complexity of the surface morphology. Accordingly, developing
other superficial techniques to fabricate superhydrophobic
surfaces is indispensable and still remains a challenge for
researchers.

3.1.2. Plant leaves with unitary structure. Fig. 4 shows the

SEM images of unitary structures exhibited by different plant
leaves. On the rear face of ramee leaf (Fig. 4a), uniformly
distributed slick fibers with a diameter of 1–2 mm can be seen
forming a unitary structure that is different from the surfaces of
the aforementioned plant leaves with hierarchical structures.
This unique structure is also found on the surfaces of Chinese
watermelon shown in Fig. 4c and d. Surprisingly, the surface
morphologies of ramee leaf and Chinese watermelon are similar
and both exhibit a WCA of 159
. This discovery clearly
explains that the hierarchical structure is not the only necessary
Fig. 2 SEM images of natural superhydrophobic surfaces with hierar-
condition to exhibit superhydrophobicity.
chical structures.6 (a) and (b) are the SEM images of lotus leaf with low
Ding and Shing et al.13 fabricated superhydrophobic surfaces
and high magnifications, respectively, and the inset of (b) is a water CA
on it with a value of about 162
; (c) and (d) are the SEM images of rice by electrostatic layer-by-layer deposition of electrospun nano-
leaf with low and high magnifications, respectively, and the inset of (d) is fibrous membranes (Fig. 5). By this method, cellulose acetate
a water CA on it with a value of about 157
; (e) and (f) are the SEM fibers were made to form a unitary structure that is similar to that
images of taro leaf with low and high magnifications, respectively, and of ramee leaf (Fig. 5a). These fibers showed a smooth surface
the inset of (f) is the water CA on it with a value of about 159
. (Fig. 5a) showing superhydrophobicity (inset of Fig. 5a). After

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3.2. Materials and mechanism to produce hydrophobic and

superhydrophobic coatings

Inspired by the superhydrophobic properties exhibited by

nature, researchers around the world started working on devel-
oping technologies to produce surfaces with extremely low
surface energies and also to control the morphology of
the surface on a micron and nanometre scale. This idea of
controlling surface morphology opens up many possibilities for
developing a variety of engineered surfaces.
Techniques to produce hydrophobic and superhydrophobic
surfaces can be broadly classified into two categories: a) making
a rough surface from a low surface energy material; b) modifying
Published on 02 September 2011 on http://pubs.rsc.org | doi:10.1039/C1JM12523K

a rough surface with a material of low surface energy.

3.2.1. Roughening the surface of low surface energy material

3.2.1.1. Silicones. PDMS (polydimethylsiloxane) belongs to
Fig. 4 SEM images of natural superhydrophobic surfaces with a unitary
a group of organosilicon compounds, commonly known as sili-
structure.6 (a) and (b) are the SEM images of the ramee leaf rear face with
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low and high magnifications, respectively, and the inset of (b) is a water cones. The intrinsic deformability and hydrophobic properties of
CA on it with a value of about 164
; (c) and (d) are the SEM images of the PDMS makes it a highly suitable material for producing super-
Chinese watermelon surface with low and high magnifications, respec- hydrophobic surfaces. Various methods are practised to produce
tively, and the inset of (d) is the water CA on it with a value of about 159
. superhydrophobic surfaces using PDMS. For example, Khor-
asani et al.15 did surface modification on PDMS using a CO2
pulsed laser as an excitation source to introduce peroxide groups
onto the PDMS surface (Fig. 6a and b). These peroxides are
capable of initiating graft polymerization of 2-hydroxyethyl
methacrylate (HEMA) onto the PDMS. The water contact angle
(WCA) of the treated PDMS was measured to be 175
. The
reason for such an increase in WCA was due to the porosity and
chain ordering on the surface of PDMS. Jin et al.16 used a PDMS
elastomer containing micro and nanocomposite structures to
produce superhydrophobic surfaces. They employed laser
etching to induce roughness on the PDMS surface. The surface
produced by this technique exhibited WCA as high as 160
and
sliding angle lower than 5
.
Ma et al.17 used an electrospinning technique to produce
superhydrophobic membranes. The electrospun fibres (Fig. 7)
made of a PS-PDMS block blended with a PS (polystyrene)
homopolymer reached a WCA of about 163
. The large WCA is
because of the combined effect of enrichment of the fiber surfaces
by the PDMS component and the surface roughness due to the
small diameter of the fibers (150–400 nm). Recently, Zhao et al.18
produced a superhydrophobic surface by a casting technique.
Fig. 5 SEM images of biomimetic superhydrophobic surface with
The process of casting a micellar solution of PS-PDMS in the
unitary structure similar to that of ramee leaf. (a) Cellulose acetate
fibrous membranes by electrospinning, showing a unitary structure and
presence of humid air resulted in a superhydrophobic surface
superhydrophilicity,13 (b) FAS-modified cellulose acetate fibrous (Fig. 8) with a WCA of about 163
.
membranes, showing superhydrophobicity13 (c) and (d) SEM images of
PPFEMA-coated fibers and corresponding droplet images (inset).14 3.2.1.2. Fluorocarbons. Fluorinated polymers are attracting
lots of interest these days because of their extremely low surface
energies. Roughening these polymers will result in super-
the fluoroalkylsilane (FAS) modification, the cellulose acetate hydrophobic surfaces. Zhang et al.19 achieved super-
exhibited superhydrophobicity with WCA 140
. Rutledge et al.14 hydrophobicity by stretching a Teflon (polytetrafluoroethylene)
coated electrospun materials consisting of either uniform or film. The superhydrophobic property achieved is due to the
beads-on-string fibers with a thin layer of hydrophobic poly presence of fibrous crystals with large fractions of void space on
(perfluoroalkyl ethyl methaacrylate) (PPFEMA) by an iCVD the surface. Shiu et al.20 produced a rough surface on Teflon by
(initiated Chemical Vapour Deposition) process. The experi- treating it with oxygen plasma. The WCA obtained by this
mental results showed that the bead-on-string morphology has technique was 168
. Because of the limited solubility, many
improved superhydrophobicity (WCA around 175
) when fluorinated materials have not been used directly but linked with
compared to the bead-free fibers (Fig. 5c and d). other rough materials to make superhydrophobic surfaces.

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Fig. 8 A SEM image of a PS-PDMS surface cast from a 5 mg mL1

solution in dimethylformamide (DMF) in humid air.18

Yabu and Shimomura21 produced a honeycomb-like micro-

porous transparent polymer film with 500 nm to 50 mm sized
pores by casting a polymer solution under humid conditions. In
this process, the fluorinated glass substrate was placed over the
substrate holder. A metal blade was fixed perpendicular to the
substrate and the gap between the blade and the substrate was
adjusted to about 100 mm. Fluorinated copolymer solution was
supplied between the blade and the substrate. Humid air (relative
humidity around 60% at room temperature) was supplied to the
solution surface with a flow velocity of 10 L min1. The forma-
Fig. 6 (a) A schematic illustration of laser induced graft polymeriza-
tion of the honeycomb-patterned film was observed by an optical
tion.15 (b) A SEM image of a PDMS surface treated with a CO2 pulsed microscope. The transparent honeycomb-patterned films
laser.15 produced by this method (Fig. 9a and b) exhibited super-
hydrophobic properties with a WCA of about 160
.

3.2.1.3. Organic materials. Although silicones and fluoro-

carbons are extensively used to produce superhydrophobic
surfaces, recent research works have proved that hydrophobicity
can be obtained using paraffinic hydrocarbons as well. Lu et al.22
proposed a simple and inexpensive method for forming a super-
hydrophobic coating using ‘‘low-density poly ethylene’’ (LDPE).
In this method, a highly porous superhydrophobic surface of PE
was produced by controlling the crystallization time and nucle-
ation rate (Fig. 10). A WCA of about 173
was obtained by this
method. Jiang et al.23 showed that a superhydrophobic film can
be obtained by electrostatic spinning and spraying of PS solution
in dimethylformamide (DMF). The surface obtained was
composed of porous microparticles and nanofibers (Fig. 11).
Recent research work showed that alkylketene,24 poly-
carbonate25 and polyamide26 also exhibit superhydrophobic
properties.

3.2.1.4. Inorganic materials. Superhydrophobic properties

Fig. 7 A SEM image of a PS-PDMS/PS electrospun fiber mat and the have been exhibited by a few inorganic materials as well. Recent
droplets on it.17 research work conducted on materials like ZnO and TiO2

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Fig. 11 A SEM image of the PS surface produced by electrostatic

spinning and spraying.23

resulted in the production of films with reversibly switchable

wettability. Feng et al.27 synthesized ZnO nanorods by a two-step
solution method (Fig. 12). XRD study showed that the ZnO
nanorod films were superhydrophobic due to the low surface
energy of the (001) plane of the nanorods on the surface of the
film. When the film is exposed to UV radiation, electron–hole
pairs were produced resulting in the adsorption of hydroxyl
group on the ZnO surface. Consequently, the superhydrophobic
property of the film is converted to superhydrophilic. Dark
storage of the UV irradiated film for a week made it super-
Fig. 9 (a) A schematic illustration of honeycomb patterned21 film hydrophobic again.
preparation. (b) The honeycomb-patterned film (top image and cross-
section) cast from a solution of the polymer, shown in the inset, under
3.2.2. Making a rough surface and modifying the surface with
humid conditions.21
material of low surface energy. This section primarily focuses on
various techniques reported in the past few years to fabricate
rough surfaces and subsequently modifying the surface chemistry
to produce superhydrophobic membranes.

Fig. 12 A SEM image of aligned ZnO nanorods prepared by a two-step

Fig. 10 A SEM image of the flower-like crystal structure of PE.22 solution approach.27

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3.2.2.1. Wet chemical reaction and hydrothermal reaction. oxidation pre-treatment and chemically modifying the surface
Wet chemical reaction is a straightforward technique that can with PDMSVT with spin coating.
effectively control the dimensionality and morphology of the The hydrothermal technique is a recently developed method
nanostructures (nanoparticles, nanowires and mesoporous that uses a ‘‘bottom up’’ route in efficiently fabricating functional
inorganics) produced.28–30 This method was widely used in the materials with different patterns and morphologies.37–40 Nano-
fabrication of biomimetic superhydrophobic surfaces on metal lamellate structures on titanium were produced by an in situ
substrates like copper, aluminium and steel. Jiang et al.31 used hydrothermal synthesis method (Fig. 14a).41 The obtained
chemical composition method to produce a superhydrophobic superhydrophilic surface is converted to a superhydrophobic
surface on copper substrate. The substrate was immersed into n- surface by chemical modification using PDMSVT (inset of
tetradecanoic acid solution for about a week, which resulted in Fig. 14a). Li et al.42 established a new technique in which they
surface modification of the substrate, which then exhibited used an inorganic precursor route to produce superhydrophobic
superhydrophobicity. Zhang et al.32 employed a surface rough- complex metal oxide monoliths by selective leaching of a self-
Published on 02 September 2011 on http://pubs.rsc.org | doi:10.1039/C1JM12523K

ness technique by etching polycrystalline metals with acidic or generated MgO sacrificial template from the sintered two phase
basic solution of fluoroalkylsilane. After treating with fluo- composites (Fig. 14b). A superhydrophobic surface with an array
roalkylsilane, the etched surfaces exhibited superhydrophobicity of spiral Co3O4 nanorods was produced by a hydrothermal
(Fig. 13a and b). method in which Co(N–O3)2$6H2O is used as a resource under
Superhydrophobic surfaces on nickel substrates were created basic conditions (Fig. 14c).43 In recent years, an array of zinc
by employing a wet chemical process in which mono- nanorods exhibiting superhydrophobicity was fabricated due to
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alkylphosponic acid reacts with Ni to produce flowery micro- its potential applications in short-wavelength lasing, gas sensors,
structures constituting a continuous slipcover.33 A stable catalysts and piezoelectric materials.44–46 For example, Hou
superhydrophobic surface is produced on a copper substrate by et al.46 synthesized superhydrophobic ZnO nanorod film on
using oxalic acid as a reaction reagent and then chemical modi- a zinc substrate by oxidizing zinc metal and subsequently
fication is done using poly(dimethysiloxane) vinyl terminated modifying the surface using n-octadecyl thiol (Fig. 14d). Both
(PDMSVT) (Fig. 13c).34 A layer of interconnected Cu(OH)2 these techniques are time saving and scalable. The flexibility and
nanowires was generated on a Cu plate by immersing it into the simplicity of these methods help in producing morphologies of
mixture of NaOH and K2S2O8 solution. After chemical modifi- reasonable shape and size.
cation with dodecanoic acid, the surface exhibited super-
hydrophobicity (Fig. 13d).35 Hao et al.36 fabricated a biomimetic
superhydrophobic surface on magnesium alloy by microarc

Fig. 14 The biomimetic superhydrophobic surfaces constructed by

hydrothermal reactions. (a) The shape of a water droplet (about 10 mg)
on the surface with nanolamellate structures of CaTiO3 (inset) by using
an in situ hydrothermal synthesis on titanium, showing a water CA of
about 160
(inset);41 (b) a typical SEM image of MgAl2O4 monolith
Fig. 13 SEM images of biomimetic superhydrophobic surfaces fabri- obtained through a novel single-source inorganic precursor route, and
cated by wet chemical reaction. (a) and (b) SEM images of the etched steel after chemical modification with n-octadecanoic acid, the surface shows
and copper alloy treated with fluoroalkylsilane, respectively, both superhydrophobicity (inset);42 (c) a SEM image of the spiral Co3O4
showing good superhydrophobicity (inset);32 (c) a SEM image of copper nanorod arrays on a glass slide, and after chemical modification, the
immersed in 0.5 wt% oxalic acid for 5–7 days and treated with PDMSVT, surface shows good superhydrophobicity with a water CA of about
showing superhydrophobicity (inset);34 (d) a SEM image of a copper plate 162
(Inset);43 (d) a SEM image of the prepared ZnO, overview of
immersed in an aqueous solution of 2.0 M NaOH and 0.1 M K2S2O8 for the cross section on zinc substrate, and after chemical modification, the
60 min, showing good superhydrophobic properties after dodecanoic surface shows superhydrophobic with a water CA of about 153

acid modification (inset).35 (inset).46

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3.2.2.2. Electrochemical deposition. Electrochemical deposi- array of nanopits and nanopillars on the surface of the material.
tion is widely used to develop biomimetic superhydrophobic When this surface was hydrophobized with octadecyltricholor-
surfaces since it is a versatile technique to prepare microscale and osilane, it exhibited superhydrophobic properties with a WCA
nanoscale structures.47–52 Bell et al.53 employed a galvanic reaching up to 164
(Fig. 16a and b).
deposition technique on metals to deposit metallic salts solution,
which resulted in the formation of superhydrophobic surface 3.2.2.4. Self-assembly and layer-by-layer (LBL) methods.
with WCA of about 173
(Fig. 15a). The surface produced can The self-assembly and layer-by-layer (LBL) assembly techniques
effortlessly float on a water surface similar to pond skaters are based on sequential adsorption of a substrate in solutions of
(Fig. 15b). Jiang et al.54 employed electrochemical deposition oppositely charged compounds. These techniques continues to be
method, inducing long chain fatty acids to produce micro and the most popular and well-established methods for the formation
nanoscale hierarchical-structured copper mesh that exhibited of multilayer thin films. Self-assembly and layer-by-layer depo-
superhydrophobicity and superoleophilicity (Fig. 15c). Super- sition are inexpensive techniques in which micro- and nanoscale
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hydrophobic 3D porous copper films were fabricated by using superhydrophobic structures can be easily fabricated with finely
hydrogen bubbles as the dynamic template for metal electrode- controlled surface morphologies.68–80 Jiang et al.68 fabricated
position.55 Since the films were electrodeposited and grew within conducting superhydrophobic rambutan-like surface with hollow
the interstitial spaces between the hydrogen bubbles, the pore spheres of aniline by self-assembly technique in the presence of
diameter and wall thickness of the porous copper films were perfluorooctane sulfonic acid (PFOSA). Arrays of carbon
successfully tailored by adjusting the concentration of the elec- nanotubes are constructed on the cotton substrate to replicate
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trodeposition electrolyte, as shown in Fig. 15d. The magnified the lotus leaf structure.69 In order to control the assembly of
SEM image (inset of Fig. 15d) clearly shows the porous structure carbon nanotubes on cotton fibers, these fibers are modified by
with numerous dendrites in different directions forming a strong treated carbon nanotubes as macro-initiators. Dong et al.,70
film. Luzinov et al.71 and Rotello et al.72 adapted a new method to
convert hydrophobic surfaces into superhydrophobic surfaces.
3.2.2.3. Lithography. Lithography is a conventional tech- In this method, a magnetic material (FePt) with varying degree of
nique used to create micro- and nanopatterns. Different litho- fluorinated ligands is deposited over the surface to exhibit
graphic techniques that are in practice are: a) photolithography, superhydrophobicity. Superhydrophobic films with dual-scaled
b) electron beam lithography, c) X-ray lithography, d) soft roughness were also produced by assembling silica micro and
lithography, and e) nanosphere lithography and so on.56–65 Notsu nanospheres by electrostatic adsorption technique (Fig. 17a).73
et al.66 used a photocatalytic lithography technique on composite Rawlett et al.74 synthesized superhydrophobic surface by
(gold) surfaces to fabricate superhydrophilic and super- employing the breath figure method (Fig. 17b) in which an array
hydrophobic patterns. Martines et al.67 employed the technique of microscale pores are produced in polymer matrices through
of electron beam lithography and plasma etching to produce an spontaneously assembly of water vapour condensation.
Lee et al.75 synthesized an adjustable dual-size roughness
surface consisting of raspberry-like particles by the assembly of
silica particles (Fig. 17c). Bionic superhydrophobic coatings were
produced by decorating silver nanoparticles on a monolayer

Fig. 15 Biomimetic superhydrophobic surfaces fabricated by electro-

chemical deposition. (a) A water drop (8 mm3) on a silver/heptadeca-
fluoro-1-decanethiol (HDFT) superhydrophobic surface deposited on
a copper substrate;53 (b) a metallic model ‘‘pond skater’’ (body length
28 mm) of copper legs treated with silver and HDFT;53 (c) a SEM image
of the deposited films on one copper mesh knitted by about 55 mm wires Fig. 16 The superhydrophobic surfaces produced by lithography tech-
as substrates, and the surface shows superhydrophobicity after chemical niques. (a) A SEM image of nanopits and nanopillars produced by
modification with n-dodecanoic acid;54 (d) a SEM image of porous electron beam lithography and plasma etching.67 (b) A SEM image of the
copper films created by electrochemical deposition at a 0.8 A cm2 nanopillars after hydrophobization. The base diameter of the pillars is
cathodic current density in 0.5 M H2SO4 and 0.1 M CuSO4 for 45 s.55 about 120 nm.67

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silicate multilayers on a silica-coated substrate (Fig. 19a). This

assembly was then treated with fluorine to exhibit super-
hydrophobicity. Zhai et al.80 used a LBL technique to mimic the
superhydrophobic behaviour of the lotus leaves by creating
a honeycomb-like structure of a polyelectrolyte (poly (allylamine
hydrochloride)/poly (acrylic acid)) (PAH/PAA) multilayer
surface coated with silica nanoparticles. This highly textured
multilayer surface is treated with semi-fluorinated silane to
achieve superhydrophobic properties (Fig. 19b).

3.2.2.5. Electrospinning technique. Electrospinning is

a dominant technique for synthesizing fine nanofibers. This
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technique is widely used by several research groups to provide

sufficient surface roughness for inducing superhydrophobicity.
Electrospinning a hydrophobic material will directly result in
superhydrophobicity. Ma et al.81 employed electrospinning and
chemical vapour deposition techniques to produce super-
hydrophobic surfaces. In this process, poly (caprolactone) (PCL)
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was first electrospun and then it was coated with a thin layer of
hydrophobic polymerized perfluoroalkyl ethyl methacrylate
Fig. 17 The biomimetic superhydrophobic surfaces fabricated by self-
(PPFEMA) by chemical vapor deposition (CVD) (Fig. 20). The
assembly methods. (a) SEM image of microsphere array assembled by
300 nm silica spheres, showing superhydrophobicity with a water CA of WCA obtained by this process was about 175
.
about 161
and the scale bars are 50 mm and 5 mm, respectively;73 (b)
a three-dimensional AFM image of silicone pillars formed on breath 3.2.2.6. Etching and chemical vapour deposition. Plasma
figure templates in a humidity chamber at 73% relative humidity;74 (c) etching processes and CVD have been extensively used with
a SEM image of a raspberry-like particulate film fabricated by assembling polymers to fabricate functional surfaces with different
one layer of 35 nm silica particles on a large silica particulate film morphologies.82–85 Engineered surfaces exhibiting hydrophilic
prepared using Langmuir–Blodgett (LB) deposition;75 (d) a SEM image and hydrophobic properties are synthesized by plasma-based
of the superhydrophobic coating fabricated by layer-by-layer method on
a flat glass substrate with a water CA of about 168
.76

array of polystyrene microspheres (Fig. 17d).76 Stable super-

hydrophobic surfaces are produced by deposition of nanosized
silica particles on a glass substrate with the formation of a self
assembled monolayer of dodecyltrichlorosilane on the surface of
the film.77 Ming et al.78 also synthesized raspberry-like particles
using amine groups. In this process, amine-functionalized silica
particles of size 70 nm and epoxy-functionalized silica particles of
size 700 nm were covalently grafted through the reaction between
epoxy and amine groups (Fig. 18a and b). The surface exhibited
superhydrophobicity after being modified with PDMS. The Fig. 19 The biomimetic superhydrophobic surfaces fabricated by layer-
WCA achieved by this technique was about 165
. by-layer methods. (a) A SEM image of the silica-coated surface, showing
Zhang et al.79 fabricated a superhydrophobic surface by LBL good superhydrophobicity after chemical modification;79 (b) Double-
deposition of poly(diallyldimethylammonium chloride)/sodium roughened superhydrophobic surfaces made by layer-by-layer80
assembly.

Fig. 18 The biomimetic superhydrophobic surfaces fabricated by layer-

by-layer methods: (a) preparation of superhydrophobic films based on
raspberry-like particles.78 (b) AFM 3D images for PDMS-covered epoxy- Fig. 20 A SEM image of electrospun nanofibers (a) before (b) after
based films containing raspberry-like particles.78 iCVD coating.81

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Fig. 23 SEM images at (a) low and (b) high magnifications of the copper
surface etched with a modified Livingston’s dislocation etchant for 24 h at
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ambient temperature.91

polycrystalline metals (aluminium, copper and zinc) to make

superhydrophobic surfaces. The principle behind this technique
Fig. 21 The biomimetic superhydrophobic surfaces constructed by was the use of dislocation etchant that dissolves the dislocation
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plasma etching. (a) A SEM image of the rough surface after 3 min of SF6 sites in the grain. The etched surfaces (Fig. 23) after treating with
etching, showing superhydrophobicity;86 (b) an AFM image of an O2- fluoroalkylsilane exhibited superhydrophobic properties. The
plasma treated PMMA sample;87 (c) an optical image showing the pulsed WCA obtained by this process is about 153
.
plasma deposited poly(glycidyl methacrylate) array reacted with 50 mm Chemical vapour deposition (CVD) is a competent technique
amino-polystyrene microspheres;88 (d) a SEM image of Si nanowires to produce micro and nano surface topographies on a macro-
grown on the Si islands with Au cluster on the tips of the nanowires scopic substrate.92–94 Yan et al.92 produced pyramid like micro
treated by plasma etching, the scale bar is 5 mm.89 structures through capillary driven self-assembly during the
evaporation of water from aligned CNTs wrapped by poly
techniques to obtain different surface topographies (Fig. 21a).86 (sodium 4-styrenesulfonate). The surface exhibited good super-
Gogolides et al.87 employed nano-rinse and a mass production hydrophobicity. Ci et al.93 constructed an array of vertically
amenable plasma process to fabricate superhydrophobic poly aligned large diameter double walled carbon nanotubes by
(methyl methacrylate) (PMMA) surfaces under low pressure a water assisted chemical vapor deposition process. The prepared
conditions in high density plasma reactor (Fig. 21b). Garrod surface exhibited a WCA of about 170
.
et al.88 analyzed the stenocara beetle’s back and replicated the
surface by employing a micro-condensation process using 3.2.2.7. Sol–gel method and polymerization reaction. The sol–
plasma chemical patterns. The micro textures are designed and gel method can be employed in the fabrication of super-
constructed over Si surfaces and they exhibited super- hydrophobic surfaces in all kinds of solid substrates.95–102 Huang
hydrophobic behavior with WCA of about 174
(Fig. 21d).89 et al.101 fabricated biomimetic superhydrophobic surfaces on
Teshima et al.90 produced a transparent superhydrophobic alloys of copper using hexamethylenetetramine and ethylene
surface by a novel method consisting of two dry processing glycol (Fig. 24a), a strong bidentate chelating agent to Cu2+ and
techniques. In this method, nanotexture was first formed on Fe2+ ions with a high stability constant, as the capping reagent.
a poly(ethylene terephthalate) (PET) substrate via selective Duan et al.102 produced ordered pore indium oxide array films
oxygen plasma etching followed by plasma enhanced chemical bya sol-dipping method using polystyrene colloidal monolayers
vapour deposition using tetramethylsilane as the precursor (Fig. 24b). It is found that the superhydrophobic properties
(Fig. 22). The surface fabricated by this process showed a WCA exhibited by the film can be controlled by increasing the pore size
greater than 150
. on the film. Shirtcliffe et al.103 used different proportions of
Qian and Shen91 developed a simple surface roughening (organo-triethoxysilane) methyltriethoxysilane (MTEOS) to
method using a chemical etching technique on three

Fig. 24 The biomimetic superhydrophobic surfaces constructed by sol–

Fig. 22 AFM images of the PET surfaces90 (a) treated with oxygen gel methods. (a) A SEM image of a nanorod film of Cu–ferrite formed by
plasma, (b) coated with FAS layer (low-temperature CVD) after the a sol–gel process;101 (b) a SEM image of indium oxide pore array films
oxygen plasma treatment and (c) coated with TMS layer (PECVD) after after removal of the PS sphere and annealing at 400
C, based on the
the oxygen plasma treatment. colloidal monolayer templates by the sol-dipping method.102

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Fig. 27 The superhydrophobic surfaces created by polymerization

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reactions. (a) A SEM image of the nanoporous anodic aluminium oxide

substrate fabricated by anodizing for 30 min at 155 V and wet-chemical
etching at 45
C for 30 min, the surface shows superhydrophobic after
chemical modification with the pSAMs of poly(TMSMA-rfluoroMA),
and the scale bar is 1 mm;108 (b) an AFM image of the hybrid film surface
Fig. 25 Superhydrophobic surfaces produced by the sol–gel method. and water CA on it.109
Top, phenolphthalein in water on MTEOS sol–gel foams heated to 390
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C (left) and 400
C (right). Centre, a SEM image of an unheated sol–gel
foam. Bottom, foam films on glass cover slips with (left) drop of water produced film exhibited very high superhydrophobicity. Choi
with brilliant blue G and (right) imbibed.103 et al.108 used a new random copolymer, poly(TMSMA-r-fluo-
roMA) [3-(trimethoxysilyl)propylmethacrylate] (TMSMA),
methacrylate (MA)] on oxide-based substrates. The WCA
obtained was about 163
(Fig. 27a). A water repellent surface
instigated from quincunx-shaped composite particles was con-
structed by utilizing the encapsulation and graft of silica particles
to control the surface tomography (Fig. 27b).109

3.2.2.8. Other techniques. Besides the techniques cited above,

researchers around the world are working on several other
methods like texturing,110,111 electrospraying,112,113 and sand-
blasting,114 which were employed to fabricate superhydrophobic
surfaces in recent times. In year 2009, Zhang et al.110 fabricated
superhydrophobic surfaces by surface texturing the porous
silicon films with capillary stress (Fig. 28). Chien et al.111
synthesized stable superhydrophobic surface of a three tier
Fig. 26 A superhydrophobic surface produced by a sol–gel method. The
image in the left shows the transparency of the coating. The image on the
right is the AFM image of a sol–gel film containing 30 wt% colloidal
silica.104

produce sol–gel foams. These foams, when exposed to different

temperatures, exhibited binary switching between super-
hydrophilicity and superhydrophobicity (Fig. 25). Hikita et al.104
prepared sol–gel films by hydrolysis and condensation of
alkoxysilane compounds. Colloidal silica and fluoroalkylsilane
were used to control the surface energy and roughness of the film.
By optimizing the amount of colloidal silica and fluo-
roalkylsilane in the film, superhydrophobicity was created in it
(Fig. 26). Shang et al.105 described an easy method to fabricate
a transparent superhydrophobic film by modification of silica-
based gel films with a fluorinated silane.
Polymerization is another efficient method to tailor different
surface topographies for the fabrication of superhydrophobic Fig. 28 Surface morphologies and wettability of alkene-modified PSi
surfaces.106–109 Tiemblo et al.106 synthesized superhydrophobic films with different etching times: (a–c) 120 min, with multiscale struc-
aerosol 200 using methoxysilanes in toluene reflux with p-tolue- tures and (d) 10 min, with a smooth porous surface. The inset of (b) is the
nesulfonic acid as a catalyst. Zhong et al.107 fabricated a poly- magnification of a papillary tip; the inset of (c) and (d) show water drops
propylene film with a nanoribbon polyaniline structure. The on the corresponding surfaces.110

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roughened texture of microscale carbon fabrics decorated with understanding of the various factors affecting frost nucleation,
submicroscale silica (SiO2) spheres and carbon nanotubes with particularly the surface energy of the base surface. The experi-
low contact angle hysteresis. They found that the produced mental results showed that air at the cold surface should be
surface exhibited superhydrophobicity without any surface supersaturated to ensure frost nucleation. But the supersatura-
treatment. Burkarter et al.112 used an electrospray technique to tion degree is mainly dependent on the surface energy, which will
deposit polytetrafluoroethylene (PTFE) films on fluorine-doped in turn affect the initial frost nucleation. They concluded that
tin oxide coated glass slides. Liu et al.113 demonstrated an inex- cold substrates of lower surface energy require a higher super-
pensive technique for the fabrication of superhydrophobic saturation degree for nucleation than higher energy surfaces, and
surfaces with a crater-like structure on Ti6Al4V alloy substrate by surface roughness will also reduce the required supersaturation
means of sandblasting with SiO2 microparticles, which is a pure degree. As the extreme of low energy surfaces, superhydrophobic
physical process, and the surface compositions remain films are also considered as promising materials for alleviating
unchanged. It is believed that this method should be easily frost growth on cold substrates. Gao et al.121 used nanoparticle–
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applied to other metals and their alloys. polymer composites to fabricate anti-icing superhydrophobic
coatings that can prevent the formation of ice upon the impact of
supercooled water. The experimental results showed that the
4. Functions of hydrophobic surfaces anti-icing capability of these composites depends on the super-
Though lots of research works are centred around fabrication hydrophobicity and also on the size of the particles exposed on
techniques for superhydrophobic surfaces, in recent years, the surface.
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researchers started focusing on various functions and applica- Icing of water on superhydrophobic surfaces is a complicated
tions of these surfaces (Fig. 29). This review article will explain phenomenon and there are lots of factors like temperature,
only the primary functions of superhydrophobic surfaces, such contact area, surface roughness and surface thermodynamics, all
as anti-icing, oil repellency, and electrowetting and elucidate of which play a vital role in the occurrence of this phenomenon.
different research works carried out in these areas. Further research is needed to get a clear understanding on the
effect of these factors on icing.

4.1. Anti-icing
4.2. Electrowetting and other functions
In cold regions, layers of ice gets deposited on solid materials,
particularly on overhead transmission lines, which results in the Electrowetting on superhydrophobic surfaces122–135 is an inter-
mechanical failure of the system. Recent research works orbits esting phenomenon that has attracted much attention in recent
around the fabrication of superhydrophobic surfaces to reduce years. In the year 2004, for the first time Krupenkin et al.127
the accumulation of snow and to even eliminate the formation of demonstrated a technique for dynamic electric control over the
ice on solid surfaces.115–120 wetting behaviour of the liquid droplets on a superhydrophobic
Kulinich et al.117–119 investigated the adhesion strength of surface by etching an array of microscopic cylindrical nanoposts
artificially created glaze ice (similar to that accreted in nature) on into the surface of a silicon wafer. He found that the wetting
rough fluoropolymer-based superhydrophobic coatings with properties of the surface can be tuned from superhydrophobic
similar self-assembled monolayers. Glaze ice is prepared by behaviour to nearly complete wetting as a function of applied
spraying supercooled water microdroplets on the target voltage and liquid surface tension (Fig. 30a). McHale et al.128
substrates at a sub-zero temperature. Ice adhesion is evaluated by investigated the electrowetting on a patterned layer of SU-8
spinning the samples at constantly increasing speed until ice photo-resist with amorphous Teflon coating, finding that contact
delamination occurred. Na et al.120 gave a fundamental angle decreased from 152
to 114
after a cycle from 0 to 130 V
and back to 0 V (Fig. 30b).

Fig. 30 Optical images of the electrowetting of liquid droplets on

superhydrophobic surfaces with no reversible effect. (a) Four images
demonstrating electrically induced transitions between different wetting
states of a liquid droplet on the nanostructured substrate;127 (b) images of
a water droplet on a SU-8 patterned surface with a Teflon-AF under
Fig. 29 Functions of superhydrophobic surfaces. various applied voltage.128

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Besides the works mentioned above, lots of research work has

been carried out in this area, which may lead the way to the
designing of electrowetting systems at very low voltages with
potential applications in the field of lab-on-a-chip and also in
developing functional microfluidic devices.
Evaporation and condensation of water droplets on a solid
substrate was first explained in the year 1977. Picknett et al.136
proposed two kinds of models to explain the phenomenon of
evaporation of water droplets on the solid surface. The first
model is constant contact angle with diminishing contact area
and the second is constant area with diminishing contact angle.
Birdi et al.137 found that there are two possibilities for a volatile
Published on 02 September 2011 on http://pubs.rsc.org | doi:10.1039/C1JM12523K

liquid drop on low surface energy substrates: (1) the rate of

evaporation is linear and follows the constant contact area mode
for the initial CA < 90
. (2) The rate of evaporation is non linear
and follows the constant CA mode for CA > 90
. Inspired by this
Fig. 31 Upon irradiation of TiO2 by ultra band gap light, the semi-
work, research138–143 has been carried out in investigating the
conductor undergoes photo-excitation. The electron and the hole that
evaporation and condensation phenomenon of water droplets on
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result can follow one of several pathways: (a) electron–hole recombina-

superhydrophobic surfaces. tion on the surface; (b) electron–hole recombination in the bulk reaction
Sticky and magnetic properties of the water droplets,144–146 of the semiconductor; (c) electron acceptor A is reduced by photo-
interaction between a water droplet and solid surfaces,147,148 the generated electrons; and (d) electron donor D is oxidized by photo-
Leidenfrost droplets149–151 and oil-repellency are the other func- generated holes.
tions that are the areas of attention for researchers in recent
times.
surface. On the surface, holes oxidise the organic molecules while
electrons combine with atmospheric oxygen to give superoxide
5. Hydrophilic photocatalytic coatings radicals, which in turn attack nearby organic molecules. This
Unlike hydrophobic/superhydrophobic surfaces that rely solely results in the cleaning of surfaces by conversion of organic
on the flow of water to clean the surface, hydrophilic coatings molecules into carbon dioxide and water at ambient temperature.
chemically break down dirt and other impurities when exposed This process is called cold combustion. An example is the total
to sunlight. This process is called ‘‘photocatalysis’’. The tech- decomposition of stearic acid [CH3(CH2)16CO2H] in the presence
nique is basically inspired from the photosynthesis process of the of atmospheric oxygen to CO2 and H2O on TiO2 surfaces
green leaves, which uses sunlight to drive the chemistry. (Fig. 32).152 A large number of organic pollutants can be broken
Although a few products that work on the principle of hydro- down by this technique, which includes aromatics, polymers,
philicity are commercialized, this field is far from attaining dyes and surfactants. Superhydrophilicity in TiO2 is also
maturity. Research works are under way in developing hydro- a light induced property in which the holes produced by the
philic self-cleaning coatings and there are regular publications in
this field.

5.1. Materials and mechanism to produce hydrophilic coatings

5.1.1. Titanium dioxide (TiO2). In the year 2001, ‘Pilkington
Glass’ commercialised the first self-cleaning coating for glass
windows that was made of a thin transparent layer of TiO2. The
TiO2 cleans the window in sunlight through two distinct prop-
erties. (1) photocatalysis (2) hydrophilicity. During photo-
catalysis, organic dirt and other impurities present on the
coatings are chemically broken down by absorption of sunlight.
Hydrophilicity causes water to form sheets by reducing the
contact angle and washes away the dirt. TiO2 is highly efficient at
photocatalyzing dirt in sunlight. It is non toxic, cheap, easy to
deposit in the form of a thin film, chemically inert in the absence
of light and can easily reach the state of superhydrophilicity. All
these properties made TiO2 a highly suitable material to produce
hydrophilic surfaces. In normal conditions, TiO2 absorbs light Fig. 32 Photocatalytic decomposition of stearic acid is monitored by
with energy equal to or greater than its band-gap energy, infrared spectroscopy. The two C–H stretching bands decrease in area
producing excited charge carriers: positively charged holes (h+) with irradiation, indicating that the surface is self-cleaning. The photo-
and negatively charged electrons (e) (Fig. 31). Though most of catalysis takes place on a nanocrystalline TiO2 film under ƛ ¼ 365 nm
these charges undergo recombination, few of them migrate to the irradiation.152

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photo-excitation process oxidize the lattice oxygen at the surface explain the increase in the photocatalytic activity in the presence
of the material. of higher oxidation state dopants. Phase-separated dopants have
This results in oxygen vacancies that can be filled by adsorbed also been experimented to improve the self-cleaning property in
water, resulting in surface hydroxide groups that make the which a pure phase of TiO2 contains a pure phase of a second
wetted surface more suitable than the dry surface, lowering the material. Recent studies have centered around the use of nano-
static contact angle152 to almost 0
after irradiation. The self- particles (use of metallic gold or platinum nanoparticles that can
cleaning properties of TiO2 are basically governed by the assist photocatalysis in TiO2) as a method of incorporating
absorption of ultra band-gap light and electron–hole pair a phase-separated dopant.155,156
generation. The band gap of bulk anatase TiO2 is 3.2 eV, cor-
responding to light of wavelength of 390 nm (near UV range). 5.1.4. Other materials. Though TiO2 has been the main focus
of study in self-cleaning applications, other materials like WO3,
5.1.2. Improving TiO2. TiO2 has become the most significant ZrO2, ZnO, CdS and polyoxometallates have been investigated
Published on 02 September 2011 on http://pubs.rsc.org | doi:10.1039/C1JM12523K

material for photocatalytic hydrophilic coatings. The photo- in recent years. However, none of the materials could surpass
catalytic activity of TiO2 decreases by a considerable amount TiO2, which uses only light to activate the process.
when it is deposited as a smooth, nanocrystalline film. But most
of the requirements in the optical and glazing industries involve
the use of a robust, nanocrystalline film. Therefore, a lot of 5.2. Mechanisms employed to produce hydrophilic coatings
research effort is going into improving the self-cleaning proper-
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ties of these nanocrystalline films. Recent research work proved Various mechanisms are employed to produce hydrophilic
that the photocatalytic activity of thick films is higher than thin surfaces using TiO2 and other inorganic metal oxides. In the year
TiO2 films. TiO2 coatings of 3 mm thickness produced by spin 1978, Harrop et al.157 reported the first hydrophilic protective
coating and annealing TiO2 paste was tested for photocatalysis coatings on glass substrate. In this method, he used vinyl tri-
against a 25 nm coating. The thicker coatings absorb near-UV cholosilane films on float glass that converts an olefinic bond into
light more strongly and the photocatalytic activity was very high, other carbon functional groups resulting in hydrophilic proper-
exhibiting quantum yields of around 0.15% while it was only ties. Though this work was not very effective, it paved the way for
0.04% for the thin film. This indicated that the thicker films the evolution of research work in superhydrophilic coatings.
absorb more light and thus generate more excited charge carriers, Jiaguo Yu et al.158 employed a sol–gel technique using alkoxide
which have a life time long enough to reach the surface to induce solutions containing polyethylene glycol (PEG) to fabricate
chemical reaction in the surface. But there is a limit to increasing superhydrophilic TiO2 coatings. Ding et al. employed a sol–gel
the thickness of the film. When all available UV light is absorbed, technique to fabricate TiO2-based nanocomposite hydrophilic
or the distance to the surface is very high so that the charge coatings by mixing TiO2 nanoparticles with a sol–gel derived
carriers have very little chance of reaching it before they silica sol and methoxysilane group-bearing styrene-co-acrylate
recombine, a still thicker film will not increase the photocatalytic (SA) oligomer, and curing with aminopropyltriethoxysilane at
activity. The properties like optical clarity and durability are very ambient temperature. The resulting surface exhibited excellent
poor for thicker films and these issues have to be addressed in self-cleaning properties.
future research. Zhang et al.159 reported self-cleaning particle coatings by using
a LbL assembly technique. A sub-monolayer of SiO2 particles
5.1.3. Improving TiO2 by doping. Doping of TiO2 is an was covered with TiO2 nanoparticles with the help of oppositely
effective technique to improve the photocatalytic activity and it charged polyelectrolytes to generate a low-refractive-index film
can also be easily incorporated into CVD or sol–gel processes.153 exhibiting superhydrophilicity. The same research group160
Based on the methods that are employed to deposit coatings, investigated further the possibility of creating the dual functions
dopants can exist as a single phase, mixed oxide or a separate of self-cleaning and antireflection in double-layered TiO2–SiO2
phase. Recent research work in this direction mainly focus on the films that consisted of a dense top layer of TiO2 and a porous
transition metal dopants. These dopants, when present as a metal bottom layer of SiO2. The films were prepared by LbL assembly
oxide, can be divided into two, based on their effects: (1) lower of SiO2 nanoparticles and titanate nanosheets with polycations.
oxidation state ones and (2) higher oxidation state ones.153,154 Yaghoubi et al.161 produced a self cleaning TiO2 coating on
Metals with higher oxidation states, like Mo5+, Nb5+ and W6+, a polycarbonate substrate by employing a chemical surface
increase the photocatalytic activity whereas metals with lower
oxidation states (<+4) like Fe3+, Co2+ and Ni2+ slow it down.
Park et al.154 used differential scanning calorimetry (DSC) to
study the low oxidation state dopants. He found that lower
oxidation state dopants caused crystallization to occur at around
20
C higher than the higher oxidation state dopants when they
are in the form of mixed oxides. A study made using X-ray
photoelectron spectroscopy (XPS) explained that the higher
oxidation state metal-doped films had a higher concentration of
hydroxyl groups adsorbed onto the surface than undoped and
lower oxidation state metal-doped TiO2. Since hydroxyl groups Fig. 33 The surface topography of a substrate before (a) and after
play a vital role in the process of photocatalysis, this could chemical treatment (b).161

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treatment method to create hydrophilic groups on the poly- 6. Characterization techniques

carbonate substrate (Fig. 33).
Beobide et al.162 employed dip coating technique to produce Several characterization techniques are employed to analyze the
multi-functional coatings consisting of two-layer stacks with surface morphology and to compute the water contact angle of
a mesoporous SiO2 layer and a dense/mesoporous TiO2 layer. hydrophilic and hydrophobic surfaces. This paper also details
This coating exhibited both anti-reflective and self-cleaning a few characterization techniques that are widely used in the
properties (Fig. 34). Xu et al.163 employed a LbL dip coating analysis of materials, contact angle and surface chemistry.
method using a TiO2 sol and a methanol solution of NH4F as Contact angle measurements play a pivotal role in the charac-
precursors to fabricate transparent, visible light activated C–N– terization of hydrophilic and hydrophobic coatings. Dodiuk
F co-doped TiO2 films exhibiting superhydrophilicity. The WCA et al. used this technique (contact angle Goniometer) to compute
of these films were 2.3–3.1
(Fig. 35). Bhatia et al.164 employed the contact angle and sliding angle of hydrophobic surface made
a nano surface texturing technique to induce superhydrophilicity of polycarbonate (PC) (Fig. 37).
Atomic force microscopy (AFM) topography is a technique
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on a glass substrate. In this process a thin layer of nickel is

deposited on the glass substrates, followed by annealing to create widely used in surface roughness measurements of the self-
Ni (nickel) nanoparticles. These Ni nanoparticles were used as an cleaning coatings. Teshima et al. used this technique to analyze
etch mask to pattern the glass substrates and removed after the roughness of the superhydrophobic surface produced by
etching by nitric acid rinse. The resulting glass surface exhibited etching using PET (Polyethylene terephthalate). Environmental
excellent self cleaning properties (Fig. 36). ellipsometric porosimetry (EEP), grazing incidence X-ray
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Fujishima’s group165–170 and a few other groups171,172 did novel analyzes at low and wide angles (GI-SAXS and GI-WAXS),
works in the area of superhydrophilicity and photocatalysis. electronic and nearfield microcopies, field-emission scanning
Recently, Akira et al.158 developed hydrophobic/super- electric microscopy (FE-SEM), transmission electron micros-
hydrophilic patterns by a new fabrication technique consisting of copy (TEM), Fourier-transform infrared spectroscopy (FTIR),
five steps: (1) photocatalytic reduction of Ag+ to Ag (nucleation), UV-visible transmittance, X-ray photoelectron spectroscopy
(2) electroless Cu deposition, (3) oxidation of Cu to CuO, (4) (XPS) and differential scanning calorimetry (DSC) are a few
deposition of a self-assembled monolayer (SAM), and (5) pho- other techniques that are widely used in characterizing hydro-
tocatalytic decomposition of selected areas of the SAM. A philic and hydrophobic coatings.
hydrophobic/superhydrophilic pattern with 500 mm2 hydrophilic
areas was obtained in this process. The same group also fabri-
7. Applications of self-cleaning coatings
cated a SiO2/TiO2 bilayer film with self-cleaning and antireflec-
tion properties by employing sol–gel and dip coating techniques. Self-cleaning coatings find applications in diversified fields.
Gu et al.173 produced TiO2 nanofibers with diameters of 200– Potential application sectors include the textile industry (self-
550 nm by high temperature calcinations of the as-electrospun cleaning clothing), automobile industry (self-cleaning windshield
tetrabutyl titanate (Ti(OC4H9)4)/polystyrene (PS) composite glass, car bodies and mirrors), optical industry (cameras, sensors,
fibers prepared by sol–gel processing and electrospinning tech- lenses and telescopes), marine industry (anticorrosion protec-
niques. The fiber films exhibited extremely stable super- tion) and aerospace industry (non sticky surfaces). Self-cleaning
amphiphilicity and self-cleaning properties. coatings can also be used in windows (window coatings), solar
Though many research works have been carried out; this modules (self-cleaning coatings for solar modules) and in paints
review article within its scope has highlighted only a few novel (exterior paints with self-cleaning properties).
and important works conducted in the area of hydrophilic Because of the potential applications of self-cleaning coatings,
surface fabrication. many companies have already been attracted to this technology

Fig. 34 (a) An AFM top view image of a mesoporous SiO2 coating. (b) An AFM top view image of a mesoporous TiO2 coating.162

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Fig. 35 AFM 3D images of the surface of (a) C–TiO2 film; (b) C–N–F–TiO2-0.5 film; (c) C–N–F–TiO2-1 film; (d) C–N–F–TiO2-2 film.163

Fig. 37 Sessile drops for static contact angle measurements of: (a)
uncoated PC and PC coated with (b) 0.5 wt% silica 1.5 wt% FPOSS
Fig. 36 SEM images of nickel nanoparticles on glass substrates after
(mixed) and (c) 0.5 wt% silica 1.5 wt% FPOSS (two-layer coating).
rapid thermal annealing for 5 min at (a) 600
C, and (b) 650
C
temperatures.164

and they have commercialized a few products. The Pilkington

8. Other areas of focus
group of companies commercialized the first self-cleaning coated
float glass product called Pilkington Activ (http://www. Though many research works have been successful in the
pilkington.com/). Self-cleaning paints have been commercial- laboratory, they fail miserably when converted into real time
ized by a German-based company named Lotusan (http://www. products due to many reasons. ‘‘Peeling off’’ of the coatings
lotusan.de/) and they are now commonly available in Europe. from substrates is a major concern that has to be addressed
Cardinal Glass Industries (Neat Glass) (http://www. while developing self-cleaning membranes for windows. The
cardinalcorp.com/), Saint-Gobain (SGG Aqua Clean) (http:// coating should not have any micro cracks/scratches on its
www.saint-gobain.com/en) and PPG Industries (http://www. surface. The membrane should be deposited onto the substrate
ppg.com/en/Pages/default.aspx) are a few other companies without any air gaps. The behaviour of water droplets on the
working on this technology. surface of the coating is one other area that has to be focused as

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Fig. 38 A flowchart explaining the summary of various materials and fabrication procedures.

well. The coating produced should not allow the water droplets Acknowledgements
to penetrate through it. These are other factors that require
further research and development to make self-cleaning coatings V.A.G and H.K.R. thank National University of Singapore for
more viable for commercialization. The flowchart (Fig. 38) graduate research fellowship. A.S.N and S.R. thank National
below gives a brief summary of various materials and fabrica- Research Foundation, Singapore for partially supporting the
tion procedures involved in the synthesis of self-cleaning program (Grant number: NRF 2007 EWT-CERP 01-0531).
coatings.
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