Eric D. Branson
Candidate
Chemical Engineering
Department
This thesis is approved, and it is acceptable in quality
and form for publication on microfilm:
Approved by the Thesis Committee:
C. Jeff Brinker
, Chairperson
The University of New Mexico Department of Chemical
Engineering and Sandia National Laboratories
Darren Dunphy
The University of New Mexico Department of Chemical
Engineering
Chris Apblett
The University of New Mexico Department of Chemical
Engineering and Sandia National Laboratories
Accepted:
Dean, Graduate School
Date
i
SOL-GEL APPROACH TO UV/OZONE PATTERNABLE
SUPERHYDROPHOBIC COATINGS
BY
ERIC D. BRANSON
BS, CHEMISTRY, ARKANSAS TECH UNIVERSITY, 1996
BS, CHEMICAL ENGINEERING,
UNIVERSITY OF NEW MEXICO, 2002
THESIS
Submitted in Partial Fulfillment of the
Requirements for the Degree of
Master of Science
Chemical Engineering
The University of New Mexico
Albuquerque, New Mexico
August 2008
DEDICATION
This manuscript is dedicated to all the family and friends that helped me
along the path to this degree…..
.
iii
ACKNOWLEDGMENTS
First and foremost, I would like to express my sincere gratitude to
everyone who played a role in my long journey to finish this degree.
I would like to acknowledge Jeff Brinker, my advisor, for continually
prodding me to finish. It has been a long trip and I thank you for not getting too
discouraged with me along the way.
I would like to thank individually those that helped me the most along the
way with their guidance and expertise. David J. Kissel, thanks for everything
without you I would not have finished the research. Adam Cook you are a great
friend and your constant “have you finished your masters yet” statement at least
once a week, which was irritating, help keep me on track. Connie Stewart,
thanks for the chemistry lessons on ozone and just being there. The daily
Starbucks run were life-saving. Tim Boyle, thanks for the FTIR lesson along with
the harassment about finishing. Darren Dunphy, thanks for being on my
committee and for all your help in the lab and elsewhere. Ralf Koehn, I may lose
my clearance for thanking you here but thank you for all the GISAXS tutoring and
helping with the Cassie-Baxter / Feng calculations using the scattering curves.
Seema Singh, thank you for your constant support and helping me out with
everything.
I would also like to thank Bernd Smarsly for GISAXS assistance, Cynthia
Edney for the FTIR measurements, Steven Howell for the AFM measurements
and YingBing Jiang for the TEM/EELS measurements.
iv
Chris Apblett and Paul Clem, two of the best bosses anyone could ask for,
I can not thank you guys enough (NO, I am not just kissing up). Thanks for
allowing me to work on this thesis part time through-out the last year. I probable
would not have finished the writing without you guys. Brother Apblett, man your
dam persistent, thanks for taken me under your wing and getting me through this.
You were the driving force man, THANK YOU!
Finally I would like to thank Janelle, my wife. Your constant
encouragement and belief in me helped more than you know.
v
SOL-GEL APPROACH TO UV/OZONE PATTERNABLE
SUPERHYDROPHOBIC COATINGS
BY
ERIC D BRANSON
ABSTRACT OF THESIS
Submitted in Partial Fulfillment of the
Requirements for the Degree of
Master of Science
Chemical Engineering
The University of New Mexico
Albuquerque, New Mexico
August, 2008
SOL-GEL APPROACH TO UV/OZONE PATTERNABLE
SUPERHYDROPHOBIC COATINGS
by
Eric D. Branson
B.S., Chemistry, Arkansas Tech University, 1996
B.S., Chemical Engineering, University of New Mexico, 2002
M.S., Chemical Engineering, University of New Mexico, 2008
ABSTRACT
The ability to control wetting behavior is crucial to several emerging
scientific fields. This ability is possible with a patternable superhydrophobic
coating. Here we present a patterning technique for a superhydrophobic sol-gel
coating using ultraviolet light and ozone. The contact angle of the coating can be
continuously varied from 170° to 0° with micron size pattern resolution using an
ozone producing ultraviolet light. FTIR, EELS, GISAXS, RI, and AFM
measurements were all conducted to determine the extent of chemical and
structural changes occuring in the sol-gel matrix due to the ultraviolet light /
ozone patterning. Theoretical contact angles calculated from the Cassie-Baxter
and Feng equations were compared to the observed contact angles. Good
agreement between all values was observed for both models and the
experimentally measured values.
vii
TABLE OF CONTENTS
LIST OF FIGURES ..........................................................................................................X
LIST OF TABLES ....................................................................................................... XIV
1. INTRODUCTION TO SUPER-HYDROPHOBIC MATERIALS ...........................1
1.1 What is a Super-Hydrophobic material? ..................................................................1
1.2 Examples of Super-Hydrophobic materials in Nature .............................................2
1.3 Surface Wetting .......................................................................................................5
2. SOL-GEL APPROACH TO MAKING SH MATERIALS ....................................11
2.1 The Sol-gel process ...............................................................................................11
2.2 Surface chemistry alteration of sol-gel materials...................................................14
2.3 Characterization of fractal surfaces .......................................................................16
2.4 Other Routes to Superhydrophobic Surfaces .........................................................21
2.4.1 Plastic Transformed ......................................................................................23
2.4.2 Carbon Nanotube Forests ..............................................................................24
2.4.3 Polyelectrolyte Multilayers ...........................................................................26
2.4.4 Galvanic Cell Reaction .................................................................................28
2.4.5 Nanosphere Lithography...............................................................................30
2.4.6 Sol-gel Foams ...............................................................................................31
2.4.7 Alumina Films ..............................................................................................33
3.0 EXPERIMENTAL & RESULTS .............................................................................36
3.1 Making of gels .......................................................................................................36
3.1.1 TFPTMOS Gels ............................................................................................36
3.1.2 TEOS Gels ....................................................................................................36
viii
3.1.3 TMOS Gels ...................................................................................................37
3.1.4 Gel Washing..................................................................................................37
3.1.5 Gel Sonication ...............................................................................................37
3.2 Coating procedures ................................................................................................38
3.2.1 Spin Coating..................................................................................................38
3.2.2 Dip Coating ...................................................................................................39
3.2.3 Spray Coating................................................................................................40
3.3 Patterning ...............................................................................................................41
3.3.1 Contact angle change during UV/Ozone exposure .......................................42
3.3.1.1 Proposed mechanism......................................................................45
3.3.2 SH film variation...........................................................................................47
3.3.3 Contact angle reproducibility........................................................................49
3.3.4 Patterning types .............................................................................................51
3.4 Lithography reversal by silane treatment ...............................................................52
3.5 Chemical change caused by UV/Ozone exposure .................................................53
3.5 Structural change caused by UV/Ozone exposure .................................................56
3.6 Cassie-Baxter and Feng approximations ...............................................................61
4.0 CONCLUSION ..........................................................................................................65
REFERENCES .................................................................................................................68
APPENDIX A: GISAXS – SCATTERING CURVES ..................................................71
ix
LIST OF FIGURES
Figure 1: A) Schematic showing how a material’s contact angle is determined. B)
Varying contact angles from less than 90º to greater than 90º on a solid
substrate. .............................................................................................................. 2
Figure 2: A) Lotus leaf, B) Close-up of the lotus leaf’s hierarchical surface, C)
Lotus self-cleaning ability also known as the “lotus effect”. ................................ 3
Figure 3: A) Armor like shell of the Stenocara beetle, B) Close-up of the array of
bumps 0.5 – 1.55mm apart and on average 0.5mm in diameter. ......................... 4
Figure 4: Interfacial energies involved in wetting a smooth surface according to
Young’s Equation ................................................................................................. 6
Figure 5: Two models that show different wetting configurations, A) Wenzel
model – liquid stays in complete contact with the surface. B) Cassie model –
liquid only contacts the top of the surface peaks. ................................................. 7
Figure 6: Pictorial representation of the overall Sol-gel process ......................... 11
Figure 7: Mechanism for the silylation process of silanol groups with HMDS26 . 16
Figure 8: AFM roughness measurement showing areal fraction cross-section
images at varying depths. Image courtesy of Seema Singh. ............................. 18
Figure 9: Scattering intensities of aerogel films from the Advanced Photon
Source located within Argonne National Labs. Films coated from A) sol 28 days
old, B) sol 37 days old. ....................................................................................... 19
Figure 10: Small angle x-ray scattering (SAXS) curves of an aerogel thin film
showing a mass fractal dimension of 2.7 (SAXS data courtesy of Bernd Smarsly)
........................................................................................................................... 20
Figure 11: A) and B) SEM images of fractal AKD surface top view and cross
section, C) water droplet on a fractal AKD surface, CA=174°, D) water droplet on
a smooth AKD surface, CA=109°. ...................................................................... 22
x
Figure 12: A) water droplet CA = 104° on a smooth i-PP surface, B) water droplet
CA = 160° on an i-PP coated glass slide, C) SEM image of i-PP surface on a
glass slide at 30°C drying temperature, D) at 60°C drying temperature. ............ 24
Figure 13: A) Uncoated forest of carbon nanotubes, B) PTFE coated forest of
carbon nanotubes, C) droplet suspended on top of PTFE coated forest. ........... 26
Figure 14: SEM of PAH/PAA films after (A) single acid treatment (B) combined
acid treatment. C) SEM after silica nanoparticle deposition, D) CA=172 on
PAH/PAA film after nanoparticle deposition and CVD. ....................................... 28
Figure 15: Varying time submerged 2nM AgNO3 and 5M HF solution A) 30min B)
60min. C) 10min submerged time in 20nM AgNO3 and 5M HF solution. D)
contact angle as deposited CA=67°. E) contact angle after n-dodecanethiol
surface modification. ........................................................................................... 29
Figure 16: A) 400nm polystyrene beads with CA=135°, B) 360nm polystyrene
beads with CA=144°, C) 330nm polystyrene beads with CA=152°, D) 190nm
polystyrene beads with CA=168°. ....................................................................... 31
Figure 17: MTEOS foams gelled with varying concentrations of ammonia, a)
1.1M and b) 2.2M. c) PTEOS/MTEOS with 22M ammonia. All three gels were
heated to 300°C.................................................................................................. 32
Figure 18: SEM showing the surface of the aluminum oxide film after A) only heat
treated to 400° and B) after immersion in boiling water for 10min. ..................... 34
Figure 19: Cartoon of wetting lithography showing how UV/Ozone alters a films
CA based on the film’s exposure time to the light ............................................... 42
Figure 20: Top) Lithography of wetting of a TFPTMOS film, contact angles
achieved by varying amounts of UV/Ozone exposure. Bottom) TEOS film
contact angle versus UV treatment times using the Jelight’s UVO-cleaner model
342. .................................................................................................................... 43
Figure 21: Temperature profile of the UVO cleaner during patterning of TEOS
film. ..................................................................................................................... 44
xi
Figure 22: Spectra output of the uncoated mercury grip lamp used in the UVO
cleaner (data courtesy of Crystec Company, Altötting, Germany) ...................... 46
Figure 23: Reaction mechanism showing the formation and destruction of ozone
ultimately responsible for the contact angle change observed during lithography.
........................................................................................................................... 47
Figure 24: Wetting lithography dependence on silica precursor ......................... 48
Figure 25: Wetting lithography dependence on film thickness. ........................... 49
Figure 26: CA curves showing the effects that occur based on the amount of time
elapsed from one exposure to the next. ............................................................. 50
Figure 27: Three types of mask: A) solid prototype mask, B) in-house mask, and
C) Photronics chrome lithographic mask. Dip coated TFPTMOS film at varying
stages after UV/Ozone patterning: D) just after patterning, E) under water, F)
pattern visible when removed from water, G) water blown off pattern disappears.
........................................................................................................................... 52
Figure 28: UV/Ozone exposure reversal. Blue bars represent the CA of a TMOS
film that has been exposed to varying amounts of UV/Ozone. The red bars
represent the CA re-attained on the exposed film after a 15 min vapor silanation
treatment. ........................................................................................................... 53
Figure 29: Fourier Transform InfraRed Spectroscopy (FTIR) of TMOS films with
varying contact angles. ....................................................................................... 55
Figure 30: Electron Energy Loss Spectroscopy (EELS). Top three images are for
a TMOS film with a CA=167º while the bottom three images are for a TMOS film
with CA=0º. The images on the left are TEM pictures showing where the EELS
measurements occurred. The middle images show amounts of carbon atoms
present in the material. There are significant amounts of carbon present in the
CA=167º film while relatively none exists in the CA=0º film. ............................... 56
Figure 31: Mass fractal dimensions obtain via GISAXS. Top) log-log plot of the
scattering intensity versus the scattering vector. Bottom) Mass fractal dimensions
obtained from a UV/Ozone exposed TMOS film. ................................................ 58
xii
Figure 32: Atomic Force Microscopy (AFM) images. Image on the left is of an as
casted SH TEOS film with a CA=165 º. Image on the right is of a SH TEOS film
after exposure to UV/Ozone for 5min possessing a CA=0º. ............................... 59
Figure 33: Index of Refraction – Blue line shows the change in the refractive
index of a TMOS film contacted with water as a function of contact angle. Yellow
line shows the volume fraction of solids while the Sky Blue line shows the volume
fraction of vapor for the film; both values were obtained from the Lorentz-Lorenz
equation. The Pink line represents the RI of a film that has not interacted directly
with water. .......................................................................................................... 61
Figure 34: Cassie-Baxter and Feng approximations for theoretical contact angles
compared to experimental values obtained on a TMOS film. ............................. 64
xiii
LIST OF TABLES
Table I: Contact Angles based on Onda and Feng equations projecting the CA of
a fractal surface from the contact angle on a smooth surface. (Calculations done
with Ralf Koehn currently at Ludwig-Maximilians University in Munich, Germany).
........................................................................................................................... 21
Table II: Comparing the various coating methods to our sol-gel coating approach
........................................................................................................................... 35
Table III: Spin coating TEOS film thicknesses versus spin coating speeds in
revolutions per minute (RPM). ............................................................................ 39
Table IV: Dip coating TEOS film thicknesses versus coating withdraw speeds in
inches per minute (IPM). .................................................................................... 40
Table V: Values obtained from GISAXS and RI measurements. Black values are
experiment values Red values were obtained from GISAXS measurements while
the blue represents valued obtained from RI measurements. ............................ 62
xiv
1. INTRODUCTION TO SUPER-HYDROPHOBIC MATERIALS
1.1 What is a Super-Hydrophobic material?
When water contacts a surface, it can interact with that surface in several
ways. Considering only wetting, surfaces can be classified as either hydrophobic
(water-fearing) or hydrophilic (water-loving). Imagine a liquid droplet setting on a
solid surface. When the liquid molecules show a stronger attraction for each
other than to the solid surface, in this case the cohesive forces are stronger than
the adhesive forces and the liquid droplet will bead-up and not wet the solid
surface. Here the solid is classified as hydrophobic. Now, when the liquid
molecules show a stronger attraction for the solid surface than for each other,
here the adhesive forces are stronger than the cohesive forces and the liquid
droplet will wet the solid surface. Here the solid is termed hydrophilic.
A quantitative approach to determining if a material is hydrophobic or
hydrophilic is by measuring the material’s wettability.1 One way to measure
wettability is by measuring the material’s contact angle (the angle of contact that
a water droplet makes with the material’s surface), as shown in Figure 1A. The
contact angle that forms on a solid surface is the resultant of the balance of three
interfacial energies: the solid-liquid (  SL ), solid-vapor (  SV ), and liquid-vapor
(  LV ). When in balance they dictate the state pictured in Figure 1A. The
balance is captured in the Young’s Equation shown in Equation 1. If a material’s
contact angle is below 90° the material is defined to be wetting or hydrophilic. If
a material’s contact angle is above 90° the material is defined to be non-wetting
1
or hydrophobic. Super-hydrophobicity is defined as the condition when a
material’s water contact angle is greater than 150°. Figure 1B shows three
different contact angle equilibrium states on a solid substrate.
A)
 LV
 SV
B)
θ
CA<90º
 SL
Young’s Equation
Vapor
cos  
Liquid
 SV   SL
 LV
Solid
CA=90º
CA>90º
Solid
Figure 1: A) Schematic showing how a material’s contact angle is
determined. B) Varying contact angles from less than 90º to greater than 90º
on a solid substrate.
1.2 Examples of Super-Hydrophobic materials in Nature
The two most widely studied superhydrophobic (SH) surfaces in nature
are the Lotus leaf (Nelumbo nucifera) and a tenebrionid beetle from the genus
Stenocara. The lotus leaf, shown in Figure 2A, has become known as the
symbol of purity because of its self cleaning properties that allow it to maintain a
pristine condition while living in a very dirty environment. While doing scanning
electron microscope (SEM) studies on several thousands of plant species,
Barthlott and Neinhuis noticed that species having a smooth surface needed to
be cleaned before examination while species with a rough surface were almost
2
entirely clean.2 Their SEM results revealed that the rough plants possessed
epicuticular wax crystals in combination with papillae (minute projections on the
leaves’ surfaces), Figure 2B. This combination of wax and papillae provide
plants with the ability to be self-cleaning, Figure 2C. Their work has led to a
correlation between surface roughness, water repellency, and the removal of
particles from the plant surfaces.
Figure 2: A) Lotus leaf, B) Close-up of the lotus leaf’s hierarchical surface, C)
Lotus self-cleaning ability also known as the “lotus effect”. Images from
Wikipedia9 and the Nees Institute for Biodiversity of Plants10 located at the
University of self-cleaning ability also known as the “lotus effect”. Images from
Wikipedia9 and the Nees Institute for Biodiversity of Plants10 located at the
University of Bonn.
The self-cleaning behavior witnessed by Barthlott and Neinhuis has
become known as the “lotus effect”. When rain falls on the lotus leaves, air
becomes trapped between the droplets and the plants surface. This trapped air,
a consequence of the inherent surface roughness, causes the droplet to form
very high contact angles, greater than 160º, allowing the droplets to roll on the
surface of the leaves essentially cleaning the lotus as it goes. The “lotus effect”
is the most sought after superhydrophobic effect to date.3
The Stenocara beetle, shown in Figure 3A, an inhabitant of the Namib
Desert, lives in an environment that is completely barren with an annual rainfall
3
sporadically ranging from 5mm to 80mm.4,5 In comparison, Arizona, the driest
state in the US, averages 180mm annually. The beetle’s armor-appearing back
is made up of an array of bumps roughly 0.5mm in diameter and spaced 0.5 –
1.55mm apart, shown in Figure 3B. The bump’s peaks are smooth, flat, and
completely hydrophilic. The rest of the beetle’s back is made of wax deposits
that create a superhydrophobic surface similar to the lotus leaf.
A
B
Figure 3: A) Armor like shell of the Stenocara beetle, B) Close-up of the array of
bumps 0.5 – 1.55mm apart and on average 0.5mm in diameter.
Periodically, a dense fog layer covers the dunes of the Namib allowing the
beetle to collect essential water for survival. During these foggy mornings, the
beetles climb to the top of the dunes. At the top they turn their back towards the
wind while assuming a head-standing stance allowing the fog to pass over them.
Tiny water molecules in the fog begin collecting on the hydrophilic pads located
atop the superhydrophobic bumps on their backs. Once the water droplet’s size
becomes large enough (roughly 5mm in diameter) to overcome the binding
forces from the hydrophilic pads, the droplet will begin to role down the beetles
superhydrophobic back directly to their mouth allowing them to drink. 6
Nature possesses properties that have been evolving and perfected over
the course of several billion years, which explains why scientists use nature for
4
their inspiration. Biomimicry is the process of studying nature’s ideas and then
imitating these designs and processes artificially in engineered materials. This
thesis presents the study of the structure and patterning of hydrophobic and
hydrophilic materials in patterns similar to those found on the lotus and beetle,
and determining the properties of these biomimetic surfaces.
The materials of choice for this thesis were sol gel based, building upon
previous work in biomimicry and sol gel processing.7 Brinker et al. used sol-gel
processing to produce silica aerogel films at ambient pressure and temperature.
Until this work aerogels were only achievable using a supercritical drying
process, which is very expensive and potentially dangerous. This research
developed a technique in which the reactive terminal hydroxyl groups are
capped-off with organosilane ligands. This allows the film to ‘springback’ upon
drying as the drying stresses vanish. This ‘springback’ feature enabled highly
fractal silica films to be produced without using a critical pressure dryer and will
be discussed in more detail later. A similar sol-gel approach is the basis of this
body of work with the specific chemistry and material created to be discussed in
future sections.
1.3 Surface Wetting
Based on Young’s equation and the Cassie-Baxter equation, the
hydrophobicity of a material can be adjusted in two ways.1,8-13 The first way
involves changing the materials surface chemistry in order to alter its surface
energy. This process is termed the chemical method. The chemical method can
only increase the contact angle (CA) on a completely smooth material to about
5
115º 10 e.g. Teflon® the most commonly known hydrophobic surface has a
CA=110º. The second way incorporates roughness to a surface introducing nonwettable water-vapor interfaces. This is termed the geometrical method. In order
to achieve a superhydrophobic CA both methods must be employed together.
The wetting of a completely smooth homogeneous surface can be
quantified in terms of its intrinsic contact angle θ (i.e. the contact angle on an
ideally flat surface) by balancing the three interfacial energies involved in wetting.
This is defined by Young’s equation:
 SV   SL   LV cos
where
(1)
 SV ,  SL ,and  LV are the three different interfacial energies: solid-vapor,
solid-liquid, and liquid-vapor involved in the wetting of the solid by the liquid,
shown in Figure 4.
 LV
 SV
θ
 SL
Vapor
Liquid
Solid
Figure 4: Interfacial energies involved in wetting a smooth surface according
to Young’s Equation
Young’s Equation only applies to a flat homogeneous surface, while the
modeling becomes much more complex when the surface becomes
heterogeneous (chemically or topographically), which is of importance because it
6
is known that surface roughness has the greatest effect on a material’s
hydrophobicity. There are two accepted models explaining how surface
roughness affects the contact angle – the Wenzel model and the Cassie-Baxter
model.1,13 In the Wenzel model, a water droplet conforms to the surface
roughness and in the Cassie-Baxter model, the droplet sits completely on top of
the surface roughness. These situations are shown in Figure 5A and B,
respectively.
A)
B)
Figure 5: Two models that show different wetting configurations, A) Wenzel
model – liquid stays in complete contact with the surface. B) Cassie model –
liquid only contacts the top of the surface peaks.
The Wenzel model relies on the assumption that the liquid remains in
complete contact with the material surface. When the surface wetting reaches
thermodynamic equilibrium, a relationship between the contact angle and the
roughness is assumed as follows:
cos  *  r cos 
where
(2)
 * represents the material’s contact angle, r
is the roughness factor,
defined as the ratio of the actual surface area of the rough surface to the
geometrical projected area of the solid, and

7
is the intrinsic contact angle for a
completely smooth surface of the same material and can be calculated from
Young’s equation, given in Equation (1).
In 1944, A. D. Cassie and S. Baxter published a paper that modified
Wenzel’s equation to account for surface heterogeneity (See Equation 3).
cos *  f1 cos1  f 2 cos 2
where
f1
is the fraction of the surface with contact angle
fraction of surface with contact angle
surface,
f2
(3)
1
and
2 , depicted in Figure 5B.
would represent the areal fraction of porosity and
f 2 is the
For a porous
2  180
(water
does not wet air), reducing Equation (3) to
cos  *  f (cos   1)  1 .
(4)
For fractal surfaces where the droplet remains in contact with the surface
roughness, Wenzel’s equation was modified by Onda to describe the relationship
between contact angles on a rough fractal surface,  f , and on a smooth surface
of the same solid,  .9,14
L
cos  f   
l
L
where  
l
D2
cos 
(5)
D 2
is equivalent to the surface roughness factor, l and L are the
upper and lower limits of fractality established by the scattering curve, and D is
the mass fractal dimension. Both the scattering curve and mass fractal
dimension are discussed in more detail in chapter 2.
8
Feng et al. modified the Cassie-Baxter equation to incorporate the fractal
roughness factor and the fraction of the surface under the water droplet occupied
by the solid material,
f s , and vapor, f v , where f s  fv  1 . The modification
can be seen in Equation (6).15
L
cos  f  f s  
l
D2
cos   (1  f s )
(6)
Since the SH materials used in this study are fractal with hydrophobic
nanopores, it is expected water cannot wet the internal porosity. Therefore
equations 4 and 6 will be used to predict the measured contact angle from
measured values of
f v , D f , and  determined experimentally.
f s and f v in equations 4 and 6, in contact
The fraction of solid and vapor,
with the water droplet can be estimated by using the Lorentz-Lorenz relationship
n
n
2
f
2
f
where
  V n
n
 2
1
s
2
s
2
s


1
2
(7)
n f is the film refractive index, Vs is the volume fraction of solids, and ns
is the refractive index of the solid skeleton19. For a silica backbone
ns is equal
to 1.457 (value for fused silica) reducing equation 7 to
n
n
2
f
2
f
  0.2723V
 2
1
s
9
(8)
The volume fraction of solids,
the fraction of solid,
fraction,
Vs , of the SH film is then assumed to be equal to
f s , of the film in contact with the water droplet. The vapor
f v , is them found by subtracting f s from one.
10
2. SOL-GEL APPROACH TO MAKING SH MATERIALS
2.1 The Sol-gel process
The sol-gel process can be defined in a broad sense as the preparation of
ceramic materials by preparation of a sol, gelation of the sol, and removal of the
solvent.17 The sol-gel process is adaptable to producing bulk materials, fibers,
and thin films. Chemically, the process involves the transition of a hydrolyzed
colloidal sol into a condensed gel; the overall process can be seen in Figure 6.
B
C
D
“s
pr
ing
ba
ck
”
A
E
Figure 6: Pictorial representation of the overall Sol-gel process16
The colloidal sol (step A in Figure 6) is a suspension of particles in a liquid
medium where the particles are small enough (from 1-1000nm in diameter) that
gravitational forces are negligible and their interactions are dominated by van der
Waals attractions and surface chemistry. The precursors are typically inorganic
metal salts or metal alkoxides. Tetraethylorthosilicate, ( Si(OC2 H 5 )4 )
abbreviated TEOS, is the most widely studied metal alkoxide precursor and is the
one used predominately for the experiments presented here. The precursor
11
undergoes a hydrolysis reaction in the presence of water. Hydrolysis occurs by
the nucleophillic attack of the oxygen from the water on the silicon atom, as seen
in Equation (9):
Si(OR ) 4  H 2O  Si(OR )3  OH  ROH ,
(9)
where R is an alkyl group and ROH is an alcohol. The (─) represents a
chemical bond between the alkoxide, Si (OR ) 3 , and the hydroxyl, OH , forming a
monomer. Using TEOS as the precursor equation (9) becomes
Si(OC 2H 5 )4  H 2O  Si(OC2 H 5 )3  OH  C2 H 5OH ,
(10)
if the reaction is allowed to continue to completion then all the OR groups will be
replaced with all OH groups yielding a tetrafunctional monomer,
Si(OR)4  4H2O  Si(OH )4  4ROH .
(11)
After two or more of the hydrolyzed monomers are formed, they may bond with
each another via a condensation reaction in one of two ways: through a water
condensation reaction (Equation 12) or an alcohol condensation reaction
(Equation 13):
Si(OR )3  OH  Si(OR )3  OH  (OR )3 Si  O  Si(OR )3  H 2O
(12)
Si(OR )3  OR  Si(OR )3  OH  (OR )3 Si  O  Si(OR )3  ROH .
(13)
These condensation reactions can continue, resulting in monomers, dimers,
trimers, etc.; inevitably building larger and larger molecules, in the process of
polymerization.23 In Figure 6, polymerization of the sol starts in step B. When a
solid network of monomers extends completely throughout the sol fully enclosing
12
the liquid, a “gel” is formed; this crosslinked network of highly fractal monomers is
also termed the framework of the gel. Gelation is occurring in step C of Figure 6.
Although the sol is now termed a “gel” with a high viscosity, there are still
interactions occurring. Monomers and clusters of monomers that have not yet
attached to the backbone network of the sol will either bond to or interact with the
backbone or with other monomer clusters over time. The reactions occuring after
gelation are called “aging” of the gel. As aging occurs the gel undergoes
changes in its structure and properties. As additional bonds form and particle
interactions continue, a contraction of the gel network may occur, causing
shrinkage of the gel, termed “syneresis”.17
Conventionally at this point the gel is either ambient dried producing a
dense xerogel (shown in Figure 6 step E) or supercritically dried (shown in Figure
6 step D) producing a highly fractal aerogel. Supercritical drying involves
removal of the solvent above its critical point. This process is very costly,
requires high pressures, and only produces small amounts of aerogel.
Prakash et al developed a drying technique where one could obtain the
same highly fractal aerogel material achieved during supercritical drying through
a drying process occurring at ambient pressure and temperature. The shrinkage
of the silica network, shown in step E of Figure 6, is due to pressure gradients
that develop within the pores of the gel during solvent evaporation.
Condensation of the highly reactive hydroxyl groups keeps the gel in this
shrunken state. Prakash replaced these hydroxyl groups with nonreactive
organic groups which render the shrinkage reversible and allows the gel to
13
“springback” to its pre-dried highly fractal state. The hydroxyl groups are
replaced with organic groups during a surface modification step that occurs
during the aging of the gel. The gel is subjected to a pore fluid exchange
procedure where the solvent in the gel is replaced with a mixture of hexane and a
modifying reagent like hexamethyldisalizane (HMDS) or trimethylchlorosilane
(TMCS).
This is a very brief overview of the entire sol-gel process. A more
complete treatment of the process is presented in the literature. 17 In the following
sections; surface modification chemistry, characterization techniques, as well as
rival approaches to making superhydrophobic materials will be discussed.
2.2 Surface chemistry alteration of sol-gel materials
The absolutely unique and essential feature of this coating process is that
unlike conventional coating systems, which shrink continuously during drying to
produce low-porosity films, this coating springs back (expands) to a lower density
state during the final stage of drying. The film shrinks initially due to solvent loss
and development of drying stress, but at the final stage of drying, it expands to
nearly twice its fully shrunken thickness. The resulting structure is highly porous.
Typical values for the refractive index of the material range from 1.05 to 1.12 at a
wavelength of 600 nm (the refractive index of air is 1.00). This “springback”
effect is important because it creates the nanoscale roughness needed to
develop the superhydrophobic effect. It occurs because a hydrophobic silicon
dioxide nanostructured network forms during drying.
14
The springback effect is achievable only after replacement of the reactive
hydroxyl groups with non-reactive organic groups. To do this, the gel is allowed
to age for 48 hours after gelation. It then undergoes a series of solvent
exchanges. This is done because the pores within the gel network are filled with
either methanol (MeOH) or ethanol (EtOH). The modifying reagent, HMDS,
exchanges poorly with either of these polar solvents because of the hydroxyls
present, so the solvent within the pores is replaced with hexane. Once in hexane
the gel is ready to undergo surface silylation. For this the hexane is replaced
with a mixture of hexane and HMDS (up to 10 vol%) and allowed to react for 24hr
in a 50°C oven. The gel is washed again with hexane and then ethanol.
In the surface derivatization or silylation process, hydrogen from one of
the silanol groups is replaced by a trialkysilyl group. In the case of HMDS the
trialkysilyl group is trimethylsilyl, Si—(CH3)3. These silyl derivatives are more
stable than their parent compounds and are less polar, producing a lower surface
energy. The mechanism for this is shown in Figure 7.26 The oxygen atom in the
silanol group and the nitrogen atom in the HMDS have larger electro-negativities
and acquire electrons from the hydrogen and silicon atoms to which they are
bonded. This gives the nitrogen and oxygen a slightly negative charge. This
negative charge attracts the positively charged ions, (H+ ion to the nitrogen, H3CSi+ to the oxygen) resulting in the formation of a trimethylsiloxane.
15
Figure 7: Mechanism for the silylation process of silanol groups with HMDS26
At this point the gel can be reliquified using ultrasound. A
superhydrophobic coating can now be obtained immediately after coating the
reliquified gel on to a substrate via essentially any standard coating method
including spin-coating, dip-coating, aerosol spraying, and ink-jet printing.
2.3 Characterization of fractal surfaces
These aerogel fractal surfaces have been characterized using different
techniques, like atomic force microscopy (AFM), small angle x-ray scattering
(SAXS), and small angle neutron scattering (SANS).25
The AFM is a scanning probe microscope that maps out the topography of
a surface of a material with resolution in the sub-nanometer range. Scans can
be done in static mode (contact mode) or dynamic mode. In both modes, a
cantilever probe tip is brought into proximity of the sample’s surface. Before
16
contact with the surface, forces between the sample and the tip cause a
deflection of the cantilever. This deflection is measured using a laser diode and
an array of photodiodes. In the contact mode, a constant force between the tip
and the sample is maintained allowing the tip to follow along the sample’s
surface. In dynamic mode, the cantilever is externally oscillated at its resonance
frequency; tip-sample interactions cause this oscillation to change providing
information about the sample’s characteristics.
In this study the AFM measurements were preformed using a Digital
Instruments Nanoscope IIIa SPM controller in contact mode. Figure 8 shows a
surface scan of a SH material made using the sol-gel approach. The small
images located at the bottom of the surface scan represent cross-sectional
measurements showing the depth dependant areal fraction of the surface
roughness.
17
Figure 8: AFM roughness measurement showing areal fraction cross-section
images at varying depths. Image courtesy of Seema Singh.
Small angle scattering of x-rays (SAXS) and neutrons (SANS) are used to
determine structural characteristics of materials in the nanometer range. In both
techniques the incident beam is scattered by a sample and the scattering pattern
is analyzed to determine the material’s size, shape, and orientation. The two can
be used in conjunction due to the two techniques giving different length scale
information.
SAXS measurements were done to determine the fractality of the aerogel
material made by the sol-gel approach. Although the aerogel material is
amorphous and does not produce distinct reflection peaks in SAXS, one can still
use the scattering features at larger scattering vectors, s . Figure 9 shows
scattering intensities of aerogel films using the 8ID beam-line at the Advanced
18
Photon Source located within Argonne National Labs. As the aerogel ages over
time, the scattering features are changing, indicating that a change in the nature
of the scattering surface has occurred.
Figure 9: Scattering intensities of aerogel films from the Advanced Photon
Source located within Argonne National Labs. Films coated from A) sol 28
days old, B) sol 37 days old.
The sharp interfaces between the pores and the voids in the aerogel
material produce an asymptotic behavior of Porod’s law:
I ( s)  s 4
(14)
where I (s ) is the scattering intensity. By fitting a power law equation to the line
in the graph of I (s ) versus s on a log-log plot one can obtain the mass fractal
dimension via the slope of the line. Dimensions between 1 and 3 are said to be
mass fractals and those with dimensions between 3 and 4 are surface fractals. A
fluoroalkyl-SH film has a mass fractal dimension of 2.7, as shown with the plot in
Figure 10.
19
Figure 10: Small angle x-ray scattering (SAXS) curves of an aerogel thin film
showing a mass fractal dimension of 2.7 (SAXS data courtesy of Bernd Smarsly)
Using the information gained from the AFM and SAXS and equations
provided by Cassie-Baxter and Feng, et. al. (equations 4 and 6), a prediction can
be made of a fractal material’s contact angle (CA) based upon the CA of a
smooth surface of the same material. Table I shows the predicted CAs based
upon a range of smooth surface CAs. A smooth dense silica film possesses a
microscopic contact angle of 100°. Based upon this contact angle, the fraction of
solid and vapor present in the fractal obtained from the AFM data (Figure 8), and
 l
the surface roughness factor, L
D2
(Figure 10), obtained from the GISAXS
data; the aerogel material should posses a macroscopic contact angle between
157.7° and 164.3°.
20
Table I: Contact Angles based on Onda and Feng equations projecting the CA of
a fractal surface from the contact angle on a smooth surface. (Calculations done
with Ralf Koehn currently at Ludwig-Maximilians University in Munich, Germany).

f (Onda et al)
f (Feng et al)
l=0.032 L=0.2 D=2.7
(L/l)D-2 = 3.6
ƒs=0.4
ƒv=0.6
ƒs=0.3
ƒv=0.7
ƒs=0.2
ƒv=0.8
ƒs=0.1
ƒv=0.9
ƒs=0.05
ƒv=0.95
ƒs=0.01
ƒv=0.99
86
75.4
120.0
128.6
138.6
151.0
159.6
170.9
91
93.6
128.7
136.0
144.3
155.0
162.4
172.2
95
108.3
136.5
142.6
149.6
158.7
165.0
173.3
100
128.8
148.3
152.6
157.7
164.3
168.9
175.0
103
144.2
157.6
160.6
164.2
168.9
172.1
176.5
105
159.0
166.8
168.5
170.6
173.4
175.3
177.9
2.4 Other Routes to Superhydrophobic Surfaces
A
In 1996, Onda did the first significant work using a chemical approach to
make a superhydrophobic material.27 He did a study on fractal surfaces that he
created from an alkylketene dimer (AKD), synthesized from stearoyl chloride. A
100μm thick AKD film was obtained by dip-coating a glass slide into a melted
AKD solution at 90°C. Fractal films with a fractal dimension of 2.29 were
obtained once the AKD re-solidified at room temperature under dry nitrogen gas;
SEM images are shown in Figure 11A,B. By comparing the CA of the fractal
AKD surface with that of a smooth AKD, Onda demonstrated the effect that a
fractal surface has on the material’s contact angle, shown in Figure 11C,D.
21
Figure 11: A) and B) SEM images of fractal AKD surface top view and cross
section, C) water droplet on a fractal AKD surface, CA=174°, D) water droplet on
a smooth AKD surface, CA=109°.27
Since Onda’s work there have been several different routes employed to
make superhydrophobic surfaces with varying benefits, such as high contact
angle; simplicity in processing; inexpensive; optically transparency, and contact
angle tunability. Plastic transformation28, carbon nanotubes29, polyelectrolyte
multilayer surfaces30, galvanic cell reactions31, nanosphere lithography32, sol-gel
foams33, and sol-gel alumina thin film method34 are examples of a few different
fabrication methods.
22
2.4.1 Plastic Transformed
Eribil et al., showed that a simple plastic could be turned into a
superhydrophobic surface by changing the surface roughness of the plastic. 28 A
smooth piece of commercially available hydrophobic plastic, isotactic
polypropylene (i-PP), exhibits a contact angle of 104°, but when the material is
made porous it yields contact angles of 160°, shown in Figure 12A,B. Granular iPP polymer was first dissolved in a solution of p-xylene and then mixed with a
nonsolvent like methyl ethyl ketone (MEK). When mixing the dissolved i-PP / pxylene solution with a MEK, the nonsolvent acted as a polymer precipitator; i.e.
increased the nucleation rate and also increased the wettability of the polymer
solution. In a vacuum oven, the i-PP solution (i-PP, p-xylene, and MEK) at
130°C was coated dropwise onto the substrate, cooled rapidly to 70°C, and then
dried at a controlled temperature. Figures 12C,D show i-PP coatings fabricated
at different drying temperatures. This coating has been successfully produced
on several different types of substrates, such as glass, aluminum foil, stainless
steel, Teflon, and polypropylene.
In general this is a simple method. However, there are drawbacks that
limit this material’s usability. One is the required 130°C deposition temperature
in a vacuum oven. This limits the type of substrate that can be used and also
where the material can be deposited; meaning that without the use of a vacuum
oven the material can not be coated. The i-PP coating either has a contact angle
of 104° or 160° with no variation, meaning that there is no ability to pattern the
material.
23
A
C
B
D
Figure 12: A) water droplet CA = 104° on a smooth i-PP surface, B) water droplet
CA = 160° on an i-PP coated glass slide, C) SEM image of i-PP surface on a
glass slide at 30°C drying temperature, D) at 60°C drying temperature. 28
2.4.2 Carbon Nanotube Forests
Carbon nanotubes grown on silicon substrates and coated with a
hydrophobic polytetrafluoroethylene (PTFE) coating, also known as Teflon,
exhibit contact angles as high as 170°.29 The carbon nanotubes were grown on
silicon substrates by using plasma enhanced chemical vapor deposition
(PECVD) method. Nickel (Ni) catalyst islands were formed on a silicon substrate
by sintering a thin Ni film at 650°C. Carbon nanotubes were then grown from
these Ni islands in a DC plasma discharge of acetylene and ammonia. The
diameter and density of the tubes were controlled by the thickness of the initial Ni
catalyst islands; while the plasma deposition time controlled the tube heights. A
24
carbon nanotube forest grown by this method is shown in Figure 13A; with tube
diameters of 50nm and height of 2μm. A water droplet placed on the ‘as grown’
forest of carbon nanotubes, shown in Figure 13A, was immediately absorbed.
When the nanotube heights are increased to 10 - 15μm an initial contact angle of
161° was obtained, but within a few minutes the droplet was completed absorbed
into the nanotube forest.
In order for the nanotubes to maintain their super-hydrophobicity a PTFE
coating was applied through a hot filament chemical vapor deposition (HFCVD)
process. The ‘as grown’ carbon nanotube forests were placed into a deposition
chamber where hexafluoropropylene oxide (HFPO) and perfluorobutane-1sulfonyl fluoride gas were passed over an array of stainless steel filaments
heated to 500°C. The HFPO gas was thermally decomposed to form
difluorocarbene (CF2) radicals. These CF2 radicals were deposited onto the tops
of the carbon nanotubes where they polymerized into PTFE with the help of a
promoter gas, perfluorobutane-1-sulfonyl fluoride. The as coated PTFE carbon
nanotube forests, shown in Figure 13B, still maintained their ‘as grown’
individuality and were also able to maintain contact angles between 160° and
170°, as shown in Figure 13C.
This method provides a very interesting way to incorporate carbon
nanotubes into a superhydrophobic material. The nanotubes provide a very
rough surface which is important in creating a SH material, but several key
features in this approach limit its practicality. First, in order to grow the
nanotubes the substrate must undergo a PECVD step at 650°C. Second, in
25
order to obtain permanent contact angles between 160° and 170°, the nanotubes
and substrate must undergo another chemical deposition (HFCVD) at 500°C.
These two features alone render this method of limited utility, and as with the
plastic deformation method above, it is not possible to pattern the material in
such a way as to control the contact angle.
Figure 13: A) Uncoated forest of carbon nanotubes, B) PTFE coated forest of
carbon nanotubes, C) droplet suspended on top of PTFE coated forest. 29
2.4.3 Polyelectrolyte Multilayers
The fabrication of polyelectrolyte multilayers or layer-by-layer assembly
(LbL) was created by alternating the dipping of a substrate into an aqueous
polycation solution and an aqueous polyanion solution.30 During each dip, a
monolayer was deposited onto the substrate. This coating changed the net
26
surface charge so that when the substrate was dipped into the alternate solution,
the growing film electrostatically attracts a monolayer of the other solution. This
continuous alternation of solutions will result in a multilayed film. Figures 14A
and 14B show a multilayered film of 100.5 assembled bilayers produced from
poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) dipping
solutions that were further processed by immersions in low pH solutions. The
films were then subjected to nanoparticle deposition in order to obtain the surface
roughness required to achieve superhydrophobicity. The PAH/PAA acid
immersed films are heated to 180°C for 2hr for cross-linking. Once the film is
crossed-linked, 50nm SiO2 particles are deposited onto the film by alternately
dipping into an aqueous solution of PAH and an aqueous suspension of
negatively charged nanoparticles. When the desired bilayer height is achieved;
the film undergoes its final dipping into the nanoparticle suspension. The film
must then be subjected to a chemical vapor deposition (CVD) of (tridecafluoro1,1,2,2-tetrahydrooctyl)-1-trichlorosilane followed by a 2hr heating at 180°C. The
final film, shown in Figure 14C, gives contact angles of 172°, shown in Figure
14D.
Building films layer by layer (100+ coatings) and having separate coating
steps to impart the required surface roughness and low surface energy becomes
time consuming and doesn’t allow for film deposition in a manner other than dipcoating. The substrate material is limited in size because of the need to be dipcoated and is limited by the requirement to withstand two separate 180°C heating
steps. This technique also lacks the ability to alter the films CA after coating.
27
A
C
B
D
Figure 14: SEM of PAH/PAA films after (A) single acid treatment (B) combined
acid treatment. C) SEM after silica nanoparticle deposition, D) CA=172 on
PAH/PAA film after nanoparticle deposition and CVD.30
2.4.4 Galvanic Cell Reaction
The galvanic cell reaction is an irreversible chemical reaction that
generates electricity and is carried out by immersion of a silicon wafer into a
solution consisting of a metallic ion and hydrofluoric acid (HF).31 Shi, et. al. used
the galvanic cell reaction to create superhydrophobic surfaces by immersing a
silicon wafer into a solution of silver nitrate (AgNO3) and HF at 45°C in a dark
environment. They found that depending on the concentration of the AgNO3 in
the HF solution and the amount of overall time that the silicon wafer was
28
submerged in the AgNO3-HF solution, the silicon wafer becomes covered with
silver (Ag) nanostructures, shown in Figure 15A,B,C. After the AgNO3-HF
treatment, the silicon wafer possesses a contact angle of 67°, shown in Figure
15D. In order to achieve a superhydrophobic contact angle the silicon wafer
must undergo further surface modification; an ethanol solution of n-dodecanethiol
was used to provide the necessary low surface energy coating. After 24 hours of
soaking in the ethanol solution, the Ag nanostructured silicon wafer achieved
contact angles of about 154°, see Figure 15E.
A
C
B
D
E
Figure 15: Varying time submerged 2nM AgNO3 and 5M HF solution A) 30min B)
60min. C) 10min submerged time in 20nM AgNO3 and 5M HF solution. D)
contact angle as deposited CA=67°. E) contact angle after n-dodecanethiol
surface modification.31
Immersion into AgNO3 – HF solution limits the type of substrate that can
be used and possibly lowers overall optical clarity of the coating. This overall
coating process is greater than 24 hrs which is extremely long. Also this
technique does not allow for the CA to be changed after superhydrophobicity is
reached.
29
2.4.5 Nanosphere Lithography
Nanosphere Lithography uses polystyrene nanospheres as templates to
produce periodic nanosphere arrays over large areas.32 Monodispersed
polystyrene spheres in a liquid/surfactant solution deposited by spin coating were
self-assembled into a close-packed array. Using an oxygen plasma etching
technique, the polystyrene spheres’ size were then altered. The plasma etching
reduced the diameter of the spheres while maintaining the original spacing of the
close-packed array. Once the appropriate sphere size has been achieved, the
arrays undergo a low surface energy modification. In this instance the arrays
were coated with a thin gold layer and then subjected to an octadecanethiol
treatment. Figure 16A,B,C,D shows polystyrene arrays with varying oxygen
plasma etching times, also in the inset of each picture is the contact angle
measurement varying from 135° to 168°, showing that the material has a small
tunable contact angle zone.
Using this method, film deposition is limited to spin- or dip-coating and the
substrate material must be able to withstand the oxygen plasma treatment step.
An important feature of this procedure is the ability to tune the film’s contact
angle, which differs from the previous methods. Shiu, et. al. were able to control
the variation in contact angle from 135° to 168°, but this variability will only allow
the material to be used in other hydrophobic processes and limits its use in
hydrophilic applications. The material is not patternable and with deposition of
the gold film the coated material may lose optical clarity.
30
A
B
C
D
Figure 16: A) 400nm polystyrene beads with CA=135°, B) 360nm polystyrene
beads with CA=144°, C) 330nm polystyrene beads with CA=152°, D) 190nm
polystyrene beads with CA=168°.32
2.4.6 Sol-gel Foams
Sol-gel foams are formed during a sol-gel phase separation process, as
described earlier.33 A methyltrimethoxysilane, hydrochloric acid (HCl), and water
mixture was allowed to hydrolyze for one hour at 22°C. The hydrolyzed mixture
was then catalyzed with varying concentrations of an ammonia solution for
gelation. A phase separation occurred simultaneously with gelation creating
pores within the gel that varied from hundreds of nanometers to tens of microns
in diameter. The gels were then left to age for 20 hours followed by air drying for
three days. Finally, a heat treatment step was performed where the gels were
heated to 300°C for further cross-linking and oxidation. The resulting foams were
31
then cut from the gel using a razor blade and the internal regions were then
measured for a contact angle. The contact angles ranged between 150° and
160°. Figure 17a,b,c shows the internal framework of foams made with differing
concentrations of ammonia. When heated to 400°C or greater the foams
became hydrophilic. However, when subjected to a chemical vapor treatment of
chlorotrimethylsilane (TMSCl) or 2-(3-methylbutyl) dimethylsilylchloride
(PDMSCl)
the contact angles could be restored to values close to their original
non-heated values.
Figure 17: MTEOS foams gelled with varying concentrations of ammonia, a)
1.1M and b) 2.2M. c) PTEOS/MTEOS with 22M ammonia. All three gels were
heated to 300°C.33
The foams were fabricated in a similar fashion as the material featured in
this study using sol-gel chemistry. Unlike our process which will be described in
detail in the next chapter, Shirtcliffe, et. al. uses the bulk gel made as a result of
the sol-gel process as the final material.33 The bulk gel was formed in a mold
and must be removed from the mold and further modified on a surface in order to
impart SH properties to the material. This substrate-less material is therefore
limited in its practical uses. While this process does allow for CA alteration, the
method does not lend itself to patterning.
32
2.4.7 Alumina Films
Alumina thin films made by the sol-gel method also have shown
superhydrophobic properties.34 The alumina films are made from a mixture of
aluminum-tri-sec-butoxide, isopropyl alcohol, ethyl acetoacetate, and water.
Soda lime glass plates were dip-coated into the aluminum mixture and heated to
400°C for 10min; obtaining a porous aluminum oxide (Al2O3) thin film. The film
then underwent a 10min immersion into boiling water. This immersion allowed
for the formation of flowerlike structures (boemite crystals) on the top of the Al 2O3
films, shown in Figure 18A,B. Once the films were heated again to 400°C for
10min, they underwent a silane surface modification step. A partially hydrolyzed
fluoroalkylsilane (FAS), in this case heptadecafluorodecyltrimethoxysilane, was
used to lower the surface free energy of the Al2O3 film ultimately achieving
contact angles between 160° and 165°. When heated to 500ºC these films
become superhydrophilic with CA < 5º.
These SH surfaces can be made patternable by UV irradiation
combined with deposition of a titanium oxide (TiO2) layer between the Al2O3 layer
and the FAS layer. The TiO2 layer was deposited using a mixture of titanium nbutoxide, ethanol, acetyl acetate and water in a 1:160:1:4 molar ratio. Once the
Al2O3 layer was coated with the TiO2 solution, it was heated to either 350°C or
500°C and then coated with the FAS. Contact angles as high as 160° were
obtained, after UV irradiation from a high pressure mercury lamp the contact
angle of these films reduced to below 5°. By using a photomask these
hydrophilic areas may be patterned within the superhydrophobic regions.
33
A
B
Figure 18: SEM showing the surface of the aluminum oxide film after A) only heat
treated to 400° and B) after immersion in boiling water for 10min. 34
Unlike any of the processes detailed above, this process allows for the
material to be patterned via UV irradiation although this requires and extra
process step with the addition of the TiO2 layer. The drawback to this process is
the required 400ºC heating and the immersion into boiling water limiting the type
of usable substrate.
As stated earlier, the inventors of the above techniques believe that their
particular method is quick, cheap, easy, and can be used to coat various
materials. Table I shows a brief summation of the methods reviewed above
along with a summary of the method that was used in this work for comparison.
As can be seen, the method of choice for this work has applicability
in being flexible for deposition method and substrate, and allows for a tunable
contact angle along with a patternable surface to control the amount of
hydrophobicity exhibited by the surface. Since this capability to pattern the
material is the centerpiece of this thesis, the next section will cover the patterning
34
and a secondary vapor silylation treatment step of the aerogel films made using
the sol-gel process.
Table II: Comparing the various coating methods to our sol-gel coating approach
Coating Method
Contact
Angle (deg)
Substrate
Pretreatment
Deposition
Surface Modification
Tunable
CA
Plastic Transformation
160
unknown
dropwise / 130ºC
vacuum oven
rapid cooling from
130º to 70º
No
No
No
Carbon Nanotube
Forest
170
Ni catalyst islands IN
PECVD @ 650ºC
plasma discharge
acetylene/ammonia
HFCVD coating @ 500ºC
No
No
No
Polyelectrolyte
Multilayers
172
acidic soakings
100+ dip coatings
CVD the 180ºC / 2hrs
No
Yes
No
Galvanic Cell Reaction
154
dodecanethiol soaking
overnight
No
No
No
Nanosphere
Lithography
168
Sol-Gel Foam
ultrasonic washings in
AgNO3 / HF immersion
acetone and ethanol
Patternable Transparent
unknown
spin coating
Oxygen Plasma /
Ag deposition /
octadecanethiol rinse
132º - 170º
No
No
160
no substrate
no deposition
heated to 300ºC
No
No
No
Sol-Gel Alumina
160
Al2O3 heated to 400º /
immersion in boiling
water
dip coating
heptadecafluorodecyltrimethoxisilane
5º - 160º
Yes
No
Coating 1 (10/912,576)
172
acidic soakings
10 - 100+ dip coatings
w/ Si particles 0.2 - 20
microns in diameter
CVD the 180ºC / 2hrs
No
Yes
No
Coating 2
(PCT/AU2004/000462)
165
unknown
spray, spin, dip with
hexane solvent
Room temp or 150ºC
heating to crosslink
polymer strands
No
No
Depends on
size of
particulate
material
Our Sol-Gel Coating
172
None
spray, spin, dip
None
0º - 170º
Yes
Yes
35
3.0 EXPERIMENTAL & RESULTS
Building upon past work by Sai Prakash (master thesis, University of New
Mexico, awarded December 1995), the concepts gained from his work with the
tetraethylorthosilicate sol will be employed throughout this work.7
3.1 Making of gels
Three different silica precursors were used in this study; a fluorinated
silane, 3,3,3-trifluoropropyl-trimethoxysilane (TFPTMOS) and two unfluorinated
silanes, tetraethylorthosilicate (TEOS) and tetramethylorthosilicate (TMOS). All
techniques produced superhydrophobic materials. The fluorinated gels were
made in a one-step base catalyzed procedure while the unfluorinated gels were
made with a two-step acid base catalyzed procedure.7,35,36
3.1.1 TFPTMOS Gels
The fluorinated gels were made with TFPTMOS, trimethoxysilane
(TMOS), methanol (MeOH), water (H2O), and ammonium hydroxide (NH4OH) in
a 0.33:1:41.56:5.85:0.003 molar ratio. This mixture was stirred for one hour and
aged for 96hr at 50°C.35
3.1.2 TEOS Gels
The TEOS gels were made with an initial mixture of TEOS, ethanol
(EtOH), H2O, and hydrochloric acid (HCl) in a 1:4:1:0.007 molar ratio (B2
solution)7 that was allowed to react at 60°C while stirring for 1.5hr and then
stored overnight in a freezer. A portion of the B2 solution was added to EtOH in
a 1:4 volume ratio. This mixture was then catalyzed with 0.5N NH4OH solution.
36
To accelerate gelation and promote siloxane condensation, the gel was aged at
50°C for 48 hours.
3.1.3 TMOS Gels
The details for these gels can be found in David Kissel’s master’s thesis
entitled “Mechanical Property Characterization of Solgel Derived Nanomaterials
Using an Acoustic Wave Technique”; University of New Mexico, awarded
December 2007. The TMOS gels were made with H2O, TMOS, MeOH, HCl in a
molar ratio of 68.2:1.0:3.1:3.75x10-3. Gellation occurred between four and five
days at 50°C.36
3.1.4 Gel Washing
After aging, all gels were subjected to a pore fluid-exchange procedure
using 6% HMDS or TMCS in hexane to allow for surface modification. First, the
gel is washed with two ethanol washes in approximately two hours followed by
two hexane washes in approximately two hours. The gel is then washed with a
6vol% mixture of HMDS in hexane over a 24 hour period.7 Finally the gel is
washed again in excess hexane twice in approximately two hours and then
excess ethanol over a two hour period. At this point the gel is ready for
sonication.
3.1.5 Gel Sonication
After the gel is washed, it is sonicated in order to reliquify the gel. In the
sonication step, a weight percent mixture of gel and EtOH or MeOH are
sonicated in a special glass container that allows for recirculation of the sol on a
37
600watt VirSonic ultrasonic cell disrupter sonicator (VirTis; Gardiner, NY;
Virsonic Digital 600) equipped with a 0.5inch titanium disrupter probe at a power
setting 65watts for three sets of 30min. During sonication, the container is
placed in an ice bath to prevent solvent evaporation; ethanol was added back
into the sol if evaporation did occur. Once reliquified the sols are passed through
a 1μm syringe filter to remove the large sized clusters that did not break-up
during sonication. Once filtered, the sol is ready for the coating method of
choice: spin-, dip-, or spray-coating. Until coating occurs the sols are placed in a
freezer for storage.
3.2 Coating procedures
Approximately 30min before coating, the sol is removed from the freezer
and allowed to equilibrate with room temperature. After warming, the sol can be
coated in various ways: spin, spray, or dip. These three coating techniques allow
for thin films with thicknesses varying from tens of nanometers to several microns
to be coated on virtually any substrate.
3.2.1 Spin Coating
Spin coating depositions limit the type of substrates that can be used and
the overall film thicknesses. For this work, a Headway spin coater was used
(Headway Research, Inc.; Garland, TX; model EC101). In order to obtain a
completely uniform coating a circular substrate is ideal. Square and rectangular
substrates can also be used as long as the substrate is small enough to fit within
the spin-coater’s 6 inch diameter catch basin. However edge effects, such as
38
thickness variation or striation will occur at the corner of the films on a noncircular substrate. Table III gives TEOS film thicknesses versus coating speeds
in revolutions per minute (RPM). In spin coating as the coating speed increases
the thickness decreases. The thickness measurements were obtained using an
ellipsometer (J.A. Woollam Co., Inc.; Lincoln, NE; model EC110) equipped with
100W mercury lamp. Films with thickness greater than the values obtained in
Table III can be obtained with a multiple coat process.
Table III: Spin coating TEOS film thicknesses versus spin coating speeds in
revolutions per minute (RPM).
Coating
Speed
(rpm)
Thickness
(Å)
Refractive
Index
500
2368
1.14
1000
1767
1.12
1500
1551
1.12
2000
1347
1.12
3.2.2 Dip Coating
Dip coating, like spin coating, also limits the type of substrates that can be
used and overall film thickness. The substrate shape can vary but must be able
to be completely submerged in the sol bath and depending on the amount of sol
present the sample size must vary accordingly; i.e. the more sol available the
larger the substrate that can be used and vice-versa. Regardless of substrate
shape, edge effects will occur along the sides and bottom of the film. Table IV,
shows TEOS film thicknesses obtained with varying withdraw rates in inches per
39
minute (IPM). Unlike spin coating, film thicknesses during dip coating increase
as the coating speed increases.
Table IV: Dip coating TEOS film thicknesses versus coating withdraw speeds in
inches per minute (IPM).
Coating
Thickness Refractive
Speed
(Å)
Index
(inches/min)
1
5
10
15
643.8
915.1
1559.8
1738.5
1.1
1.1
1.1
1.1
3.2.3 Spray Coating
Spray coating allows for the widest variety of substrates that can be
coated; encompassing virtually any type and size. It also uses the least amount
of sol of any of the coating processes. One downfall to this technique is the
ability to control film uniformity and thickness. Thicknesses can vary from a few
nanometers to microns and no two coatings will be the same, either in thickness
or uniformity unless the process involves an automated spray coating system,
like robocasting. Because of this non-uniformity it is difficult to obtain thickness
measurements using an ellipsometer, therefore another technique must be used
to determine the film’s thickness such as a scanning electron microscope (SEM)
or a profilometer.
40
3.3 Patterning
Patterning of the SH material was done with Jelight Company Inc’s, UV
Ozone-cleaner (UVO) model 342. The cartoon in Figure 19 shows a simplistic
interpretation of how the patterning works. A substrate coated with one of the
three SH sols, described above, is placed film-side-up into the UVO sample tray
and is exposed to the UV light. It is thought that the UV light in combination with
the ozone being generated replaces methyl groups with hydroxyl groups within
the film thereby controlling the value of θ in the Cassie-Baxter and Feng
equations 4 and 6. This proposed mechanism will be discussed later. A mask
can be placed in contact with the film to specify the desired areas where this
replacement will occur. The contact angle of the masked regions remains
unchanged, while the contact angle of the exposed regions is progressively
reduced depending on the exposure time of the films to the UV/Ozone. This will
be discussed in more detail later.
41
Patterned UV exposure
t1=10’s sec
t2=2-3min
t3>3min
Spatial gray scale control of contact angle
CA~165º
CA~130º
CA~92º
H2O
unexposed
H2O
t1
CA<45º
H2O
H2O
t2
t3
Figure 19: Cartoon of wetting lithography showing how UV/Ozone alters a films
CA based on the film’s exposure time to the light
3.3.1 Contact angle change during UV/Ozone exposure
A series of TEOS samples were placed in the UVO cleaner’s 6.5” x 6.5”
sample tray six millimeters below the mercury lamp to determine how the contact
angle changes with time; the results are shown in Figure 20. As can been seen,
the wetting angle undergoes a period of incubation during which little change in
contact angle is seen, followed by a region of fairly rapid change in contact angle
with additional exposure. At long time, completely hydrophilic surfaces are
obtained, and further exposure to these surfaces does not incur any further
changes in the contact angle.
42
Lithography of Wetting
UV Treatment Times
Contact Angle (deg)
160
Super-Hydrophobic
120
80
40
Super-Hydrophilic
0
0
200
400
600
800
Time (sec)
Figure 20: Top) Lithography of wetting of a TFPTMOS film, contact angles
achieved by varying amounts of UV/Ozone exposure. Bottom) TEOS film
contact angle versus UV treatment times using the Jelight’s UVO-cleaner model
342.
Several factors were checked to determine the cause for the change in
wetting angle: temperature, UV wavelengths, ozone only, and atmosphere. A
thermocouple was placed inside the UVO cleaner to determine the overall
temperature increase of the sample chamber. Figure 21 shows that after ten
minutes the maximum temperature the sample chamber reaches is 75°C. This
temperature increase does not cause a change the contact angle of the film. A
side study showed that heating to 450°C in air produced no change in the film’s
43
contact angle, so this low temperature exposure has no effect on the contact
angle.
80
70
Temperature (˚C)
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
9
10
Time (min)
Figure 21: Temperature profile of the UVO cleaner during patterning of TEOS
film.
The low pressure uncoated mercury grid lamp, located inside the UVO
cleaner, produces two main wavelengths of UV simultaneously: 184.9nm and
253.7nm. The spectral output of the lamp is shown in Figure 22. SH samples
with a CA>160° were placed under three different monochromatic wavelengths of
light (245nm, 302nm, 365nm) to determine if a change in wetting angle occur.
After 15min exposures at each wavelength there was no change in the films
contact angles, indicating that monochromatic UV light, by itself, cannot cause
the change in contact angle observed.
Due to the fact that a UV/Ozone cleaner is being utilized as the UV
source, ozone alone was tested to determine its role in contact angle alteration.
A SH TEOS film was placed in a small chamber attached to an ozone generator.
44
At maximum ozone generation the film’s CA changed 3° in a 60min exposure.
An exposure of greater than 18hr yielded a CA change of only 20°. These
results show that ozone alone can not account for the CA changes that occur
during the UV/Ozone process. It must therefore be that a combination of UV and
ozone exposure is responsible for the observed change.
A second test to determine if ozone plays a role in the alteration of the
contact angle was to replace the ambient air atmosphere with nitrogen, without
the presence of oxygen ozone can not be generated. The UVO sample chamber
was purged with nitrogen for one hour to displace the oxygen within the sample
chamber. After an exposure of three minutes to the UV/Ozone, the TMOS film
CA changed from 165° to 144°, a 21° overall change. This change is
considerably less than the 100° change in contact angle observed with the same
TMOS film at ambient room conditions. A longer nitrogen purge period was done
overnight (time >12hr) resulting in the same 21° contact angle change. This
change in contact angle indicates that complete chamber displacement of
oxygen with nitrogen did not occur, since some oxygen must have remained in
the system to create the small change in contact angle that was observed.
3.3.1.1 Proposed mechanism
The spectral output of the low pressure uncoated mercury grid lamp used
in the UVO cleaner is shown in Figure 22. This type of lamp emits two main
wavelengths of light, 184.9nm and 253.7nm. The 184.9nm wavelength is
ultimately responsible for the formation of the ozone via the reaction mechanism
45
shown in Figure 23. Atmospheric oxygen is decomposed to form activated
atomic oxygen. The activated oxygen quickly reacts with atmospheric oxygen to
form ozone. The 253.7nm wavelength then decomposes the ozone producing
more atmospheric oxygen and activated oxygen.
Figure 22: Spectra output of the uncoated mercury grip lamp used in the UVO
cleaner (data courtesy of Crystec Company, Altötting, Germany)
During this continuous cycle of ozone formation and destruction, the
253.7nm light is also absorbed by methyl groups on the SH film. The terminating
methyl groups become energized and react readily with the activated oxygen
atoms producing simple volatile molecules like CO, CO2, H2O, etc. replacing the
methyl groups with hydroxyl groups. These hydroxyl terminated groups have a
higher surface free energy than the methyl groups resulting in a greater attraction
to the water and thus a decrease in the contact angle.
46
O2 + hv184.9nm → 2 O*
O* + O2 → O3
Ozone
Formation
O3 + hv253.7nm → O2 + O*
O* + O3 → 2O2
Ozone
Destruction
Surface
Reaction
Si—CH3 + hv253.7nm → Si—[CH3]* + 4 O*
Si—[CH3]* → Si—OH + CO2 + H2O
Figure 23: Reaction mechanism showing the formation and destruction of ozone
ultimately responsible for the contact angle change observed during lithography.
3.3.2 SH film variation
The UV/Ozone mechanism was tested to determine if the observed
contact angle changes were material specific. Contact angles were measured
after varying times of UV/Ozone exposure. All three precursor materials
(TFPTMOS, TEOS, and TMOS) showed a reduction in the films’ CA from the
effect of the UV/Ozone exposures. See Figure 24 for the results. The TMOS
film appeared to have a quicker response to the UV/Ozone followed by the TEOS
and TFPTMOS films. In addition, the wait time between UV/Ozone treatments
may not have been held constant, the typical wait time between exposures is
5min. This could also account for the variability within the CA changes and is
discussed later in Section 3.3.3.
47
UV/Ozone Treatment of Various Materials
180
TMOS
160
TFPTMOS
B2
Contact Angle (deg)
140
120
100
80
60
40
20
0
0
50
100
150
200
250
300
350
400
450
Time (sec)
Figure 24: Wetting lithography dependence on silica precursor
The UV/Ozone mechanism also was tested to determine if film thickness
affected the wetting angle change. Figure 25 shows data collected from three
TMOS films with thicknesses of 243nm, 589nm, and 967nm. As shown there is
very little effect in overall contact angle change with UV/Ozone exposures less
than 120sec. As UV/Ozone times become greater than 120sec there appears to
be a slight dependence on film thickness. The 243nm film obtains a 0º contact
angle first followed by the 589nm film and then the 967nm film.
48
180
243nm
160
589nm
967nm
Contact Angle (deg)
140
120
100
80
60
40
20
0
0
50
100
150
200
250
Time (sec)
Figure 25: Wetting lithography dependence on film thickness.
3.3.3 Contact angle reproducibility
Three TMOS films with exactly the same film thickness and starting CA
were compared to determine the ability to reproduce the wetting angle curves
similar to the one shown in Figure 20bottom. The only variation in the test was
the amount of wait time between UV/Ozone exposures: 5sec, 5min, and
overnight. For example the 5sec wait time; the first sample is exposed to
UV/Ozone for 30sec and is removed from the UVO cleaner. The next sample is
loaded into the chamber, 5sec after the last exposure the next sample is exposed
for 60sec, then removed from the chamber and the next sample is loaded. Five
seconds after the last exposure the next sample is exposed. Figure 26 shows
the results of the test. There is little difference in the contact angle values for
UV/Ozone exposures less than 120sec. After the 150sec sample a significant
49
difference begins to appear with the three CA values being 129º, 115º, and 95º.
The overall time it took the overnight wait sample to reach the 0º threshold was
300sec. This time is 120sec greater that the 5sec wait sample and 90sec greater
than the 5min wait sample.
180
160
Contact Angle (deg)
140
120
100
80
60
5min wait
Overnight wait
40
5sec wait
20
0
0
50
100
150
200
250
300
Time (sec)
Figure 26: CA curves showing the effects that occur based on the amount of time
elapsed from one exposure to the next.
This variation in the exposure time to reach a 0º CA is believed to be due
to the residual ozone present within the UVO sample chamber. The half-life of
ozone in air at 25ºC based solely on thermal decomposition is three days.37 This
does not take into account contaminants in the air that will react readily with the
ozone. Considering these contaminants the average half-life of ozone is typically
between 15-30min.37 One possible explanation for this effect is that shorter wait
times between exposures result in new samples receiving a slightly higher
50
exposure to activated ozone complexes, resulting in the decreased incubation
time before the hydrophobic to hydrophilic transition is initiated.
3.3.4 Patterning types
Three types of mask were employed in patterning the SH films presented:
a solid prototype, a photolithographic mask made in-house, and a
photolithographic mask made by a professional mask house (Photronics,
Somewhere, Some State). Figure 27A,B,C shows examples of all three masks.
Since it is exposure to UV combined with exposure to ozone which is proposed
as a mechanism, areas transparent to the UV wavelengths will become
hydrophilic while regions not exposed to the UV remain superhydrophobic. The
in-house made mask (Figure 27B) is a piece of nontransparent tape over a
quartz wafer with the desired pattern cut out exposing the quartz slide. Quartz
must be used to allow the correct wavelengths of light to reach the sample.
Figure 27D shows a TFPTMOS film after it was patterned with this mask; the
pattern is not visible on the dry film. In Figure 27E, the film is submerged under
water; close inspection reveals the patterned image. Figure 27F shows the film
immediately after removal from the water revealing the Sandia National
Laboratories thunderbird emblem. The emblem is completely visible due to
adhered water in the hydrophilic regions of the pattern. When the water was
blown off, the emblem completely disappears as shown in Figure 27G.
51
A
D
B
E
C
F
G
Figure 27: Three types of mask: A) solid prototype mask, B) in-house mask, and
C) Photronics chrome lithographic mask. Dip coated TFPTMOS film at varying
stages after UV/Ozone patterning: D) just after patterning, E) under water, F)
pattern visible when removed from water, G) water blown off pattern disappears.
3.4 Lithography reversal by silane treatment
The lower contact angles obtained after UV/Ozone exposure are
reversible allowing for the original contact angle to be re-attained almost
completely. A post-vapor silanation treatment will reverse the UV/Ozone
degradation of the methyl groups. Superhydrophobic films with original contact
angles greater than 160° prior to UV/Ozone treatment and CAs as low as 0° after
treatment have been restored to CAs up to 155°, this can be seen in Figure 28.
While these values are close to the original CAs for the film, the original values
were not achieved due to either an irreversible chemical or irreversible
52
structural/geometrical change that occurred in the film either during UV/Ozone
patterning or exposure to water after patterning.
Figure 28: UV/Ozone exposure reversal. Blue bars represent the CA of a TMOS
film that has been exposed to varying amounts of UV/Ozone. The red bars
represent the CA re-attained on the exposed film after a 15 min vapor silanation
treatment.
3.5 Chemical change caused by UV/Ozone exposure
During UV/Ozonation the terminated methyl groups are replaced with
hydroxyl groups. The extent of replacement varies with exposure to the
UV/Ozone. Two forms of spectroscopy were used to measure the extent of
methyl group replacement: Fourier Transform InfraRed Spectroscopy (FTIR) and
Electron Energy Loss Spectroscopy (EELS).
FTIR is a spectroscopy technique that passes infrared radiation through a
sample material. Some of the infrared radiation is absorbed by the sample and
53
some of it passes through or is transmitted. This resulting spectrum represents
the molecular adsorption and transmission, creating a molecular fingerprint of the
sample. Like a human fingerprint no two molecular structures produce the same
spectrum. Figure 29 shows FTIR spectrums of six TMOS films with varying
UV/Ozone exposures from 0 to 5min. The broad stretch shown in each spectra
between 2900 and 3000 cm-1 indicates the present of aliphatic moieties. As the
UV/Ozone exposure time increases from 0 to 5min, the area under this stretch
decreases as determined by comparison to the unchanged stretch at 1507 cm-1
(not shown). This decrease in area corresponds to a decrease in C—H bonding
which agrees well with the decrease in contact angle observed experimentally.
This aliphatic stretch decrease also agrees with the proposed UV/Ozone
mechanism that shows a replacement occurring between terminated methyl
groups with hydroxyl groups. Due to the methyl groups replacement with
hydroxyl groups, one would expect to see an increase in the Si—OH groups,
broad stretch between 3200 and 3600cm-1, as the C—H stretch decreases. It is
unclear why this increase is not present.
54
0.03
0.02
0.01
0
Absorbance
0.010.02-
0 min.
1 min.
0.03-
2 min.
0.04-
3 min.
0.05-
4 min.
5 min.
0.060.070.080.090.13700
3600
3500
3400
3300
3200
3100
3000
2900
2800
2700
-1
Wavenumber (cm )
Figure 29: Fourier Transform InfraRed Spectroscopy (FTIR) of TMOS films with
varying contact angles.
EELS is a spectroscopy technique typically used in conjunction with
Transmission Electron Microscopy (TEM). During TEM, the atomic electrons
within a sample are bombarded with free electrons.38 Inelastic collisions occur
causing a energy loss in the free electrons. By using an electron spectrometer
the energy loss is measured to determine what atomic electrons caused the loss.
EELS is a very good technique for measuring low atomic number species, i.e.
carbon. Figure 30 shows EELS data collected from two TMOS films, one with
CA=167º and one with CA=0º. The 167º film shows relatively large amounts of
carbon atoms present while the 0º film shows none. This correlates well to the
theory that surface methyl groups (containing carbon) are replaced by hydroxyl
groups during the UV/Ozone treatments.
55
C
TEM
O
CA=167º
C
TEM
O
CA=0º
Figure 30: Electron Energy Loss Spectroscopy (EELS). Top three images are for
a TMOS film with a CA=167º while the bottom three images are for a TMOS film
with CA=0º. The images on the left are TEM pictures showing where the EELS
measurements occurred. The middle images show amounts of carbon atoms
present in the material. There are significant amounts of carbon present in the
CA=167º film while relatively none exists in the CA=0º film.
3.5 Structural change caused by UV/Ozone exposure
Three measurements were employed to determine if any structural
changes occurred to the films during UV/Ozone exposure: Grazing Incidence
Small Angle X-Ray Scattering (GISAXS), Atomic Force Microscopy (AFM), and
Refractive Index (RI) measurements.
GISAXS is an x-ray technique (similar to SAXS) used to determine surface
and thin film properties. An x-ray beam at a very small incident angle is directed
at the film. The x-rays are scattered by electron density changes within the film.
56
These scattered x-rays are recorded on a 2D detector where the scattering
intensities are recorded. A log-log plot of the scattering intensities, I(s), versus
the scattering vector, s, yields the information about the mass fractal of the film,
shown in Figure 31 (top). The slope of the linear portion of the plot yields the
mass fractal dimension, (D). Figure 31 (bottom) shows the mass fractal
dimensions obtained for TMOS films exposed to the UV/Ozone for varying times.
All GISAXS measurements were done at Argonne National Laboratory’s
Advanced Photon Source, beam-line 8ID.
As can be seen the mass fractal dimension increases as a function of
UV/Ozone exposure. This indicates shrinkage of the film’s porosity due possibly
to the condensation of the film’s silicon-dioxide network.
57
1000
100
I(s)
D
10
1
0.1
Mass Fractal Dimension
l
s [nm-1]
L
-1
-1.2
-1.4
-1.6
-1.8
-2
-2.2
-2.4
-2.6
-2.8
-3
0
50
100
150
200
UV/Ozone exposure (sec)
Figure 31: Mass fractal dimensions obtain via GISAXS. Top) log-log plot of the
scattering intensity versus the scattering vector. Bottom) Mass fractal dimensions
obtained from a UV/Ozone exposed TMOS film.
AFM measurements were done to determine if the film’s surface
roughness changed as a result of UV/Ozone exposure. Figure 32 shows scans
of two TMOS films, one with a CA=165º and the other with a CA=0º. The RMS
roughness values for the films are 76.9nm and 75.9nm, respectively. Since this
is within the margin of error, it indicates that there is no change in the films
surface roughness due to the UV/Ozone exposures. All AFM measurements
58
were done by Stephen Howell at Sandia National Labs using a MLCT-AUNM-A
tip, a spring constant of 0.05N/m with the scan mode set to jump mode in an
ambient environment. Since GISAXS data indicated a possible densification of
the SiO2 network, but the AFM data did not indicate a change in the surface
roughness, these data seem to indicate that bulk properties are somehow related
to the irreversible loss in contact angle observed due to fraction of the solid in
contact with the liquid droplet,
fv .
CA=165° (10um x10um scan)
CA=0° (10um x10um scan)
RMS roughness: 76.9 nm
RMS roughness: 75.9 nm
2.0µm
2.0µm
Figure 32: Atomic Force Microscopy (AFM) images. Image on the left is of an as
casted SH TEOS film with a CA=165 º. Image on the right is of a SH TEOS film
after exposure to UV/Ozone for 5min possessing a CA=0º.
The Index of Refraction (RI) of a material is a ratio of the velocity of light
through a reference material (usually light in a vacuum and is equal to 1) to the
velocity of light though the sample material. Figure 33 shows how the RI varies
with varying amounts of UV/Ozone exposures. A TMOS film with a CA=165º and
a RI=1.054 was exposed to UV/Ozone for varying amounts of time obtaining
contact angles of 151.3º, 129.8º, 115.6º, 71º, and 0º. After each exposure, the
59
film’s RI was measured immediately after exposure and no change was
observed; remaining at 1.054. This indicates that the UV/Ozone causes no
change in the films thickness or in porosity. A second TMOS film was treated in
the same manner except for after UV/Ozone exposure the film was placed in
contact with water followed by measurement of the RI. As can be seen in Figure
33, the RI increases as contact angle decreased. This indicates that once the
film is placed in contact with water and condensation of the silica network occurs
causing a decrease in film porosity. Plugging these values in to the LorentzLorenz equation, we obtained a volume fraction of vapor (fv) and solid (fs) for a
film with a RI of 1.078 as fs=0.214 and fv=0.786 and for a RI of 1.238 as fs=0.621
and fv=0.379. These large changes in the volume fractions of the two phases
indicate that a change in the bulk film porosity has occurred as a result of water
interaction, which is in agreement with the data observed from both GISAXS and
AFM indicating a bulk change.
60
1.26
0.9
1.24
0.8
0.7
1.2
0.6
RI w/H2O
RI w/o H2O
Fraction Solids
Fraction Vapor
1.18
1.16
0.5
0.4
1.14
Volume Fraction
Refractive Index @ 600nm
1.22
0.3
1.12
0.2
1.1
0.1
1.08
1.06
0
20
40
60
80
100
120
140
160
0
180
Contact Angle (deg)
Figure 33: Index of Refraction – Blue line shows the change in the refractive
index of a TMOS film contacted with water as a function of contact angle.
Yellow line shows the volume fraction of solids while the Sky Blue line shows
the volume fraction of vapor for the film; both values were obtained from the
Lorentz-Lorenz equation. The Pink line represents the RI of a film that has not
interacted directly with water.
3.6 Cassie-Baxter and Feng approximations
Using the values obtained from the GISAXS and RI measurements,
theoretical contact angles were calculated to determine a relationship between
projected contact angles from the Cassie-Baxter and Feng equations (equations
4 and 6, respectively) to the contact angle obtained experimentally. Table V
shows all values obtained from the above experiments. The CA rough is the
observed macroscopic CA change on our SH film at varying UV/Ozone
61
exposures, while the CAsmooth is the observed microscopic CA change on a
TMCS treated microscope slide at varying UV/Ozone exposures. The D and
(L/l)D-2 values were obtained from the log-log plots of the scattering intensities
versus the scattering vector. The fs and fv values were calculated using the RI
measurements.
Table V: Values obtained from GISAXS and RI measurements. Black values are
experiment values Red values were obtained from GISAXS measurements while
the blue represents valued obtained from RI measurements.
Exp Time (sec)
0
60
120
150
180
210
CAexperimental
164
143
136
129
120
91
CAsmooth
100
61.5
30.8
30.3
20.8
10.6
D
2.35
2.36
1.7
1.94
1.94
1.77
(L/l)^D-2
2.1330 2.1797 0.5224 0.8782 0.8782 0.6079
fs
0.2138 0.2167 0.2079 0.2282 0.2809 0.6209
fv
0.7862 0.7833 0.7921 0.7718 0.7191 0.3791
Solving each equation, the Cassie-Baxter and the Feng, with the values
presented in Table V, we were able to obtain theoretical values that closely
matched our observed values as shown in Figure 34.
The contact angles predicted by the Cassie-Baxter model correspond well
with the experimentally observed CA values. Although the projected CA values
for the films exposed to water after UV/Ozone treatments are lower than the
observed values, the curve trends nicely with the experimental values. The CA
approximations made for the film that was not exposed to water correspond well
to observed results until the values past the 180sec point. At this point the
62
theoretical curve continues horizontally and begins to deviate from
experimentally obtained contact angles.
The Feng CA approximation also correlates well with the observed
experimental values. The initial and 60sec values are both lower than the
experimental values. After 120sec both the approximated CAs and experimental
CAs correlate nicely with each other. Because of the limited time and unknown
results, multiple repeats of the same sample were not studied. Since only one
set of data points were obtained these points may be an outlier. Similar to the
CB predictions, the curve for the Feng approximation of the film not exposed to
water continues horizontally after the 180sec data point; again indicating a
divergence between the theoretical and experimental contact angles.
63
180
160
140
Contact Angle (deg)
120
100
80
60
Experimental CA
CB prediction
Feng prediction
40
CB - no water
Feng - no water
20
0
0
50
100
150
200
UV/Ozone exposure time (sec)
Figure 34: Cassie-Baxter and Feng approximations for theoretical contact angles
compared to experimental values obtained on a TMOS film.
64
4.0 CONCLUSION
Nature’s ability to control wetting has inspired theories of its potential in
both technological and day to day applications. The importance of this control
has been realized by scientists worldwide and has led to the creation of many
superhydrophobic materials. Very few of these materials possess the ability to
alter its contact angle, and the majority that do can only change contact angles
between superhydrophobic and superhydrophilic. The limited few that can vary
their contact angle between 170º and 0º require extensive and harmful chemical
processes resulting in its limited manufacturability. The study presented here
yields a process that creates a simple SH coating with a noninvasive lithographic
technique enabling the control of the materials contact angle with micron level
precision.
Superhydrophobic sol-gel materials were prepared by sonicating a
hydrophobic silica gel in ethanol. The surface of the gel was made hydrophobic
by replacement of the pendant hydroxyls with methyl groups. Inherent
‘springback’ of the silicon dioxide structure creates the nanoscale roughness
needed to develop the superhydrophobic effect.
UV/Ozone treatments were carried out on SH films, spin-coated or dipcoated to various thicknesses. It was found that the films’ contact angles
decreased with increased exposure times to UV/Ozone. This contact angle
change was also shown to be reversible with a post silylation treatment.
Two forms of spectroscopy were used to measure the extent of chemical
changes occurring during the UV/Ozone treatments: Fourier Transform Infrared
65
Spectroscopy (FTIR) and Electron Energy Loss Spectroscopy (EELS). The FTIR
measurements showed a reduction in C-H bonding as UV/Ozone exposure time
increased. EELS measurements confirmed FTIR measurements by showing a
decrease in carbon atoms with increased UV/ozone exposure
GISAXS measurements conducted at the Advanced Photon Source
revealed an increase in the overall mass fractal dimension of the bulk film
indicating a condensation of the silicon dioxide network. AFM measurements
indicated that no change in the surface roughness occurs during UV/Ozone
exposures. The RI measurements confirmed GISAXS measurements by
showing a collapse in the silicon dioxide network with an increase in the films
index of refraction.
Using GISAXS and RI measurements, theoretical contact angles obtained
from the Cassie-Baxter and Feng equations were compared to the observed
contact angles. Good agreement between all values was observed for both
models and the experimentally measured values.
A proposed UV/Ozone mechanism has UV light continuously forming and
destroying ozone creating atomic oxygen in the process. The SH film’s
terminated methyl groups also become excited during the UV exposure. The
excited methyl groups bond with the atomic oxygen to form less volatile
compounds and are replaced within the film with higher free energy hydroxyl
groups creating a reduction in the film’s contact angle.
This thesis describes a technique to photo-lithographically pattern the
wetting behavior on any surface from 0º to 170º. The technique has been proven
66
to allow for continuous optical adjustment of water contact angle throughout a
very wide range, allowing for this technique to be tailored to any application such
as water collection, protection of electronics, coating of airplanes, and collection
of biowarfare agents. Future applications for this technique are seemingly
limitless.
67
REFERENCES
1. Wenzel, R.N.; Resistance of solid surfaces to wetting by water; Industrial and
Engineering Chemistry, vol. 28 (8) p.988, 1936.
2. Barthlott, W. et al., Planta, vol. 202, p.1, 1997 Neinhuis, C. et al., Annals of
Botany vol.79, p.667, 1997.
3. Gu, Z., Uetsuka, H., Takahashi, K., Nakajima, R., Onishi, H., Fujishima, A.,
Sato, O.; Structural color and the lotus effect; Angewandte ChemieInternational Edition, vol. 42 (8), p.894, 2003.
4. Parker, A.R., Lawrence, C.R.; Water capture by a desert beetle; Nature vol.
414, p.33, 2006.
5. http://www.worldwildlife.org/wildworld/profiles/terrestrial/at/at1315_full.html
6. Lei, Z., et al; Patterned superhydrophobic surfaces towards a synthetic mimic
of the namib desert beetle; Nano Letters, vol. 6 (6), p.1213, 2006.
7. Prakash, S., Brinker, C., Hurd, A.; Silica aerogel films at ambient pressure;
Journal of Non-Crystalline Solids, vol. 190, p.264, 1995.
8. Nishino, T., Meguro, M., Nakamae, K., Matsushita, M., Yasukiyo, U.; The
lowest surface free energy based on –CF3 alignment; Langmuir, vol. 15,
p.4321, 1999.
9. Hazlett, R.; Fractal applications: wettability and contact angle; Journal of
Colloid and Interface Science, vol. 137, p.527, 1990.
10. Shang, H., Wang, Y., Limmer, S., Chou, T., Takahashi, K., Cao, G.; Optically
transparent superhydrophobic silica-based films; Thin Solid Films, Vol.472,
p.37, 2005.
11. Bartell, F., Shepard, J.; Surface roughness as related to hysteresis of contact
angles; Journal of Physical Chemistry, vol. 57, p.211, 1953.
12. Woodward, J., Gwin, H., Schwartz, D.; Contact angles on surfaces with
mesoscopic chemical heterogeneity; Langmuir, vol. 16, p.2957, 2000.
13. Cassie, R., Baxter, S.; Wettability of porous surfaces; Transactions of the
Faraday Society, vol.40, p546, 1944.
68
14. Onda, T., Shibuichi, S., Satoh, N., Tsujii, K.; Super-water-repellent fractal
surfaces; Langmuir, vol. 12 (9), p.2125, 1996.
15. Feng, L., Li, S., Li, H., Zhia, J., Song, Y., Jiang, L., Zhu, D.B.; Superhydrophobic surface of aligned polyacrylonitrile nanofibers; Angewandte
Chemie-International Edition, vol. 41 (7), p.1221, 2002.
16. http://www.llnl.gov/str/May05/pdfs/05_05.4.pdf.
17. Brinker, C.J., Scherer, G.W.; Sol-gel science: the physics and chemistry of
sol-gel processing; Academic Press, 1990.
18. Ostwald, W.; Z. Phys. Chem.; vol. 34, p.495, 1900.
19. Brinker, C.J.; Hydrolysis and condensation of silicates: effects on structure;
Journal of Non-Crystalline Solids, vol. 100, p.31, 1988.
20. Iler, R.; The chemistry of silica; Wiley, New York, 1979.
21. Aelion, R., Loebel, A., Eirich, F.; Hydrolysis of ethyl silicate; Journal of the
American Chemical Society, vol. 72 p.5705, 1950.
22. Vega, A., Scherer, G.; Study of structural evolution of silica gel using H and Si
NMR; Journal of Non-Crystalline Solids, vol. 111, p.153, 1989
23. Zerda, T., Artaki, I., Jonas, J.; Study of polymerization processes in acid and
base catalyzed silica sol-gels; Journal of Non-Crystalline Solids, vol. 81,
p.365, 1986.
24. Orcel, G., Hench, L., Artaki, I., Jonas, J.; Effect of formamide additive on the
chemistry of silica sol-gels II. Gel structure; Journal of Non-Crystalline Solids,
vol, 105, p.223, 1988.
25. Doshi, D., Shah, P., Singh, S., Branson, E., Malanoski, A., Watkins, E.,
Majewski, J., van Swol, F., Brinker, C.J., Investigating the interface of
superhydrophobic surfaces in contact with water; Langmuir v.21(17), p.7805,
2005.
26. Knapp, D.; Handbook of analytical derivatization reactions; John Wiley &
Sons, New York, 1979.
27. Onda, T., Shibuichi, S., Satoh, N., Tsujii, K.; Super-water-repellent fractal
surfaces; Langmuir vol. 12 (9), p.2125, 1996.
28. Eribil, H.Y., Demirel, A.L., Avci, Y., Mert, O.; Transformation of a simple
plastic into a superhydrophobic surface; Science vol. 299 (28), p.1377, 2003.
69
29. Lau, K., Bico, J., Teo, K., Chhowalla, M., Amaratunga, G., Milne, W.,
McKinley, G., Gleason, K.; Superhydrophobic Carbon Nanotube Forests;
Nano Letters, vol. 3 (12) p.17-1, 2003.
30. Zhai, L., Cebeci, F.C., Cohen, R.E., Rubner, M.F.; Stable superhydrophobic
coatings from polyelectrolyte multilayers; Nano Letters, vol. 4 (7), p.1349,
2004.
31. Shi F., Song, Y., Niu, J., Xia, X., Wang, Z., Zhang, X.; Facile method to
fabricate a large-scale superhydrophobic surface by galvanic cell reaction;
Chemistry of Materials, vol. 18, p.1365, 2006.
32. Shiu, J.Y., Kuo, C.W., Chen, P., Mou, C.Y.; Fabrication of tunable
superhydrophobic surfaces by nanosphere lithography; Chemistry of
Materials vol. 16 (4), 2004.
33. Shirtcliffe, N.J, McHale, G., Newton, M.I. and Perry, C.C.; Intrinsically
superhydrophobic organosilica sol-gel foams; Langmuir vol. 19, p 5626,
2003.
34. Tadanaga, K., Marinaga, J., Minami, T.; Formation of superhydrophobicsuperhydrophilic pattern of flowerlike alumina thin film by the sol-gel method;
Journal of Sol-Gel Science and Technology, vol. 19, p.211, 2000.
35. Branson, E., Shah, P., Singh, S., Brinker, C.; Patent-Application #11/104,917.
36. Kissel, D.; Mechanical Property Characterization of Solgel Derived
Nanomaterials Using an Acoustic Wave Technique; master thesis, University
of New Mexico, awarded December 2007.
37. www.lenntech.com/ozone/ozone-decomposition.htm
38. Hillier, J., Baker, R., Microanalysis by means of electrons; Journal of Applied
Physics, vol.15, p.663, 1944.
70
APPENDIX A: GISAXS – SCATTERING CURVES
GISAXS scattering curves of all samples plotted together to showing trends. L
and l values were fixed from this curve at L=0.27 and l=0.031.
Porod Plots from GISAXS Data for UV-Ozone Treated
Superhydrophobic Aerogels
CA165, 100C
log (I)
CA146, UVO
CA138, UVO
CA127, UVO
CA98, UVO
CA0, UVO
0.00
0.01
0.10
1.00
q (1/Angstrom)
Zoom of Porod regions showing the slope of the line:
Scattering curve for the as coated SH coating with CA=164°.
log (intensity)
lot#056501_CA=164 as coated
-1.6
-1.4
-1.2
-1
-0.8
log q (1/A)
71
-0.6
-0.4
-0.2
0
Scattering curve for SH film exposed to 1min UV/Ozone possessing a CA=146°.
log (intensity)
lot#056508_CA=146 UV-ozone 1 min
-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
log q (1/A)
Scattering curve for SH film exposed to 1.5min UV/Ozone possessing a
CA=138°.
log (intensity)
lot#056509_CA=138 UV-ozone 1.5 min
-1.8
-1.6
-1.4
-1.2
-1
-0.8
log q (1/A)
72
-0.6
-0.4
-0.2
0
Scattering curve for SH film exposed to 2min UV/Ozone possessing a CA=98°.
log (intensity)
lot#056510_CA=127 UV-ozone 2 min
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
log q (1/A)
Scattering curve for SH film exposed to 2.5min UV/Ozone.
log (intensity)
lot#056511_CA=98 UV-ozone 2.5 min
-1.8
-1.6
-1.4
-1.2
-1
-0.8
log q (1/A)
73
-0.6
-0.4
-0.2
0
Scattering curve for SH film exposed to 3.5min UV/Ozone.
log (intensity)
lot#056512_CA=98 UV-ozone 3.5 min
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
log q (1/A)
Comparision of all scattering curves together.
4.00E+00
3.50E+00
3.00E+00
CA=165
CA=146
log (intensity)
2.50E+00
CA=138
CA=127
2.00E+00
CA=98
CA=0
1.50E+00
1.00E+00
5.00E-01
0.00E+00
-1.8
-1.6
-1.4
-1.2
-1
-0.8
log q (1/A)
74
-0.6
-0.4
-0.2
0
0
Plot showing trend of mass fractal dimensions and contact angle change.
Affects of UV-Ozone Treatment on SH Aerogel Films
3
180
160
Fractal Dimension
2.5
140
2
120
100
1.5
1
80
Fractal Dimension Below
1 nm scale
60
Water Contact Angle
40
0.5
20
0
0
50
100
150
0
250
200
UV-O3 time (s)
75
Fractal Dimension Above
1 nm scale