U.S. patent application number 13/716049 was filed with the patent office on 2014-03-06 for method of tunning wettability of titanium dioxide layers against water.
This patent application is currently assigned to KAIST (Korea Advanced Institute of Science and Technology). The applicant listed for this patent is KAIST (Korea Advanced Institute of Science and Technology). Invention is credited to Young-keun Lee, Jeong-young Park, Trong-Nghia Van.
Application Number | 20140065362 13/716049 |
Document ID | / |
Family ID | 50187969 |
Filed Date | 2014-03-06 |
United States Patent
Application |
20140065362 |
Kind Code |
A1 |
Park; Jeong-young ; et
al. |
March 6, 2014 |
METHOD OF TUNNING WETTABILITY OF TITANIUM DIOXIDE LAYERS AGAINST
WATER
Abstract
The present invention relates to a method of tunning wettability
of titanium dioxide layers against water by nanostructuring the
titanium dioxide layers to increase a hydrophilicity of the
titanium dioxide layers, and also coating the nanostructured
titanium dioxide layers with silane layers to increase a
hydrophobicity of the titanium dioxide layers. The method of
tunning wettability of titanium dioxide layers against water
according to the present invention comprises: (a) step of forming
titanium dioxide layer on a substrate; (b) step of forming silica
particle layers on the upper part of the titanium dioxide layer;
(c) step of etching a surface of the laminate prepared in step (b);
and (d) step of removing the silica particle layer etched and
remained in the step (c).
Inventors: |
Park; Jeong-young; (Daejeon,
KR) ; Lee; Young-keun; (Daejeon, KR) ; Van;
Trong-Nghia; (Daejeon, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Technology); KAIST (Korea Advanced Institute of Science
and |
|
|
US |
|
|
Assignee: |
KAIST (Korea Advanced Institute of
Science and Technology)
Daejeon
KR
|
Family ID: |
50187969 |
Appl. No.: |
13/716049 |
Filed: |
December 14, 2012 |
Current U.S.
Class: |
428/141 ; 216/41;
216/51 |
Current CPC
Class: |
Y10T 428/24355 20150115;
B82Y 40/00 20130101; C23C 14/5873 20130101; C23C 18/1216 20130101;
B82Y 30/00 20130101; C23C 18/122 20130101; C23F 1/02 20130101; C23C
14/3464 20130101; C23C 14/083 20130101; C23C 18/1254 20130101 |
Class at
Publication: |
428/141 ; 216/41;
216/51 |
International
Class: |
C23F 1/02 20060101
C23F001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 5, 2012 |
KR |
10-2012-0098413 |
Claims
1. A method for forming a nanostructured titanium dioxide layer,
the method comprising: (a) step of forming a titanium dioxide layer
on a substrate; (b) step of forming a number of particle layers on
the upper part of the titanium dioxide layer as an etching mask;
(c) step of etching a surface of the titanium dioxide layer as an
etching mask to form a nanostructure on the surface of the titanium
dioxide layer by the etching; and (d) step of removing the residual
particle layer.
2. The method for forming a nanostructured titanium dioxide layer
according to claim 1, wherein the particle layer is a silica
particle.
3. The method for forming a nanostructured titanium dioxide layer
according to claim 2, wherein the titanium dioxide layer in the
step (a) is formed by a sol-gel method, sputtering method or
thermooxidation method.
4. The method for forming a nanostructured titanium dioxide layer
according to claim 2, wherein the silica particle layer in the step
(b) is formed by Langmuir-Blodgett process.
5. The method for forming a nanostructured titanium dioxide layer
according to claim 4, wherein the surface pressure of the silica
particle layer in the Langmuir-Blodgett process is 10.about.12
mN/m.
6. The method for forming a nanostructured titanium dioxide layer
according to claim 4, wherein the silica particle layer is a
self-assembling monolayer.
7. The method for forming a nanostructured titanium dioxide layer
according to claim 2, wherein the etching in the step (c) is
conducted by ICP etching method or reaction-ion etching method.
8. The method for forming a nanostructured titanium dioxide layer
according to claim 7, wherein the etching is conducted for 10-40
sec.
9. The method for forming a nanostructured titanium dioxide layer
according to claim 2, wherein after conducting the step (d) a
silane layer is formed on the surface of the nanostructured
titanium dioxide layer to increase the hydrophobicity.
10. The method for forming a nanostructured titanium dioxide layer
according to claim 9, wherein the silane layer is consisted of one
or more of silane being selected from the group consisting of
1H,1H,2H,2H-perfluorooctylchlorosilane, octadecyltrichlorosilane,
(tridecanfluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane,
3,3,3-trifluoropropyl-trichlorosilane, and
dodecyltrichlorosilane.
11. In a structure that a control of wettability against water is
possible, the structure comprising: a substrate; and a titanium
dioxide layer formed on the substrate, wherein the titanium dioxide
layer has a surface on which a number of nanostructures in a cone
shape is formed.
12. The structure according to claim 11, further comprising a
silane layer that is coated on the upper part of the titanium
dioxide layer.
Description
CROSS-REFERENCES TO RELATED APPLICATION
[0001] The present application claims priority under 35 U.S.C.
119(a) to Korean application number 10-2012-0098413, filed on Sep.
5, 2012, in the Korean Intellectual Property Office, which is
incorporated by reference in its entirety as if set forth in
full.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field
[0003] The present invention relates to a method of tunning
wettability of titanium dioxide layers against water, and more
specifically a method of tunning wettability of titanium dioxide
layers against water by nanostructuring the titanium dioxide layers
to increase a hydrophilicity of the titanium dioxide layers, and
also coating the nanostructured titanium dioxide layers with silane
layers to increase a hydrophobicity of the titanium dioxide
layers.
[0004] 2. Related Art
[0005] The wettability means the extent of wetting easiness of any
liquid against a solid face, and if the wettability is good, the
surface tension of liquid is low and thus, the liquid is
well-spread on the solid surface, and if the wettability is bad,
the surface tension of the liquid is high and thus the liquid is
not well spread on the solid surface.
[0006] Titanium dioxide (TiO.sub.2) known as a nontoxic material is
used as many uses in various fields. In a photocell device,
TiO.sub.2 is a main component of a fuel-sensitive solar cell, and
n-type of TiO.sub.2 thin-film is used together with noble metals to
recover a hot electron in nanodiode. Also, TiO.sub.2 is used in a
hydrogen generation or water splitting, and is used as the support
for a catalyst reaction.
[0007] Since TiO.sub.2 has a photocatalyst property that it becomes
a super-hydrophilicity state when UV is irradiated and also becomes
a super-hydrophobicity upon bonding with the self-assembling
monolayer, and thus, is applied to self-cleaning. Therefore, a
number of studies are being made on the relation between a water
contact angle and a surface morphology for a wetting action on the
surface of the material.
[0008] A roughness is a major factor in the wettability control,
and was quantitatively defined by Wenzel and Cassie-Baxter's
equation. Thus, a plan or approach is sought to roughen the texture
of a surface by using various methods including an etching,
lithography, particle coating technology, etc. in order to create
or mimic superhydrophobic biosurfaces in nature.
[0009] An effective method for the nanostructuring includes a
colloid lithography using a self-assembling microsphere or
nanosphere. This self-assembling microsphere or nanosphere layer is
used as a mask or template, and is combined with the
nano-manufacturing techniques consisted of a deposition,
evaporation, etching, and the like to form an ordered
nanostructure. Silicon dioxide and polystyrene particles are the
conventional components forming self-assembling 2D colloid
monolayer prepared by various conventional methods such as
drop-casting, dip-coating, spin-coating, or more specific
technologies such as a confined convection assembly,
Langmuir-Blodgett trough or electric field. Such approach makes a
nano technology easy and can create a simple but an interesting
nanoscale structure in a relatively low cost.
[0010] The inventors of the present invention developed the method
for tuning the wettability of titanium dioxide layers against water
by making the hydrophilicity of titanium dioxide layers or the
hydrophobicity of titanium dioxide layers via the modification of
the surface roughness of titanium dioxide layers, i.e., the
roughness change of titanium dioxide layers, and completed the
present invention.
SUMMARY
[0011] It is an object of the present invention to provide a method
for tuning wettability of titanium dioxide layers against water via
a surface modification of titanium dioxide layers.
[0012] Other purpose of the present invention is to provide a
titanium dioxide structure comprising titanium dioxide layers in a
shape of nanocone array being uniformly arranged.
[0013] Another purpose of the present invention is to provide a
titanium dioxide structure having an improved hydrophobicity since
it comprises a silane layer.
[0014] Upon using the method of tuning wettability of titanium
dioxide layers against water, the hydrophilicity and hydrophobicity
of titanium dioxide layers can be easily controlled depending on
the circumstances.
[0015] Therefore, upon utilizing the present invention, the
titanium dioxide structures having an improved hydrophobicity as
well as the titanium dioxide structures having an improved
hydrophilicity can be prepared.
[0016] Titanium dioxide structures prepared in the present
invention can be manufactured in a large scale because the titanium
dioxide layers having an improved hydrophilicity or hydrophobicity
can be formed on the large scale of the substrate.
[0017] Titanium dioxide layers that the hydrophilicity and
hydrophobicity are controlled according to the present invention
represent the photocatalyst property and self-cleaning property,
and thus, can be applied to a multifunctional transparent film.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The above and other aspects, features and other advantages
of the subject matter of the present disclosure will be more
clearly understood from the following detailed description taken in
conjunction with the accompanying drawings, in which:
[0019] FIG. 1 is a mimetic diagram of the procedure for controlling
wettability of titanium dioxide layers against water;
[0020] FIG. 2(a) represents .pi.-A isotherm of SiO.sub.2 bead
monolayer prepared by using Langmuir-Blodgett trough, and FIG. 2(b)
is a top view of SEM image of silica nanosphere monolayer;
[0021] FIG. 3 is the top and cross-sectional views of SEM image
over an etching time of sputtering method TiO.sub.2 layer, wherein
(a) is a photograph at the time of unetching, (b) is that at 10 sec
etching, (c) is that at 15 sec etching, (d) is that at 20 sec
etching, (e) is that at 25 sec etching and (f) is that at 30 sec
etching;
[0022] FIG. 4 is a graph representing the water contact angle and
etching time of TiO layer of (a) before and (b) after the
silanation by using 1H,1H,2H,2H-perfluorooctyltrichlorosilane;
[0023] FIG. 5 is the cross-sectional and top views of SEM image and
photoimage of water contact angle after silanation of 20 sec
etching TiO.sub.2 thin-film, wherein (a) is sol-gel method, (b) is
a sputtering method, (c) is a thermooxidation method TiO.sub.2
layer;
[0024] FIG. 6 represents inclined wall structure upon 20 sec and 30
sec etching of TiO.sub.2, wherein (a) and (c) are schematic
diagrams of the wall structure upon 20 sec and 30 sec etchings of
sputtering and thermooxidation TiO.sub.2 layer, and (b) and (d) are
SEM images upon 20 sec and 30 sec etchings of sputtering TiO.sub.2
layer; and
[0025] FIG. 7 represents XPS spectrum of etched TiO.sub.2 layer,
wherein (a) and (b) are Ti component and F component of 20 sec
etching sol-gel TiO.sub.2 layer, respectively, and (c) and (d) are
XPS spectrums of Ti component and F component of 40 sec etching
thermooxidation TiO.sub.2 layer.
DETAILED DESCRIPTION
[0026] Hereinafter, a method of tuning wettability of titanium
dioxide layers against water of the present invention will be
described in more detail.
[0027] FIG. 1 depicts a mimetic diagram of the procedure for
controlling wettability of titanium dioxide layers against
water.
[0028] At first, titanium dioxide layers are formed on the
substrate conventionally used in the art of this field. In the
present invention the titanium dioxide layers are formed by a
sol-gel method, sputtering method or thermooxidation method
conventionally used in the art of this field.
[0029] Next, a silica particle layer is formed on the upper part of
the titanium dioxide layer. The silica particle layer in the
present invention is preferable to be formed by the
Langmuir-Blodgett process.
[0030] Langmuir-Blodgett process is the method for forming the
ordered monolayer film by locating the material to be formed on the
surface between water and air in a thin-film and collecting it by a
foreign force.
[0031] The size of silica particle that can be used in the present
invention is 50.about.500 nm in diameter. If the size of the silica
particle is less than 50 nm, it is difficult to construct the
self-assembling monolayer, and also since the erosion of the silica
layer is so fast in the etching procedure to be conducted later
that there is a problem that the production of nanostructure of the
titanium dioxide layer is less. And, if the size of the silica
particle exceeds 500 nm, there is a problem that the erosion of the
silica layer is slowed down, and thus, the production efficiency of
nanostructure of titanium dioxide layer is decreased. The size of
the silica particle preferably used is 200.about.300 nm. More
preferable size of the silica particle is 210.about.250 nm.
[0032] And, the silica particle preferably used in the present
invention is a sphere.
[0033] The silica particle is synthesized by Stober method, and the
synthesized silica particle is washed and then is diluted with
methanol without drying in order to avoid an aggregation of the
silica particle to prepare a silica suspension for
Langmuir-Blodgett process. The silica suspension is
ultrasonic-stirred with sodium dodecyl sulfate under the heating to
introduce a long hydrocarbon tail into the silica particle. The
long hydrocarbon tails on the surface of this silica particle make
the silica particles act as the amphipathic material. The
ultrasonic treatment is conducted by adding chloroform to the
silica particle suspension to which the
hydrophilicity-hydrophobicity is imparted. The silica suspension
thus treated drops into Langmuir-Blodgett trough that the substrate
wherein titanium dioxide layer is formed is installed, and after
the predetermined time, hexagonal close-packed (HCP) silica
monolayer is transferred to titanium dioxide layer by raising the
substrate to form the silica particle layer on the titanium dioxide
layer.
[0034] In the Langmuir-Blodgett process, the surface pressure of
the silica particle layer is preferably 10.about.12 mN/m. This is
to avoid the overlapping of the silica spheres. Preferred surface
pressure is 11 mN/m.
[0035] Through the Langmuir-Blodgett process, the silica particle
layer formed on the titanium dioxide layer is self-assembling
monolayer.
[0036] FIG. 2(a) represents the .pi.-A isotherm of SiO.sub.2 bead
monolayer prepared by using Langmuir-Blodgett trough. A phase
transfer of the silica monolayer is achieved by the increase of the
surface pressure, and this can be seen from the change of the
isotherm slope. Also, the cross-sectional view of FIG. 2(a) is SEM
cross-sectional image of HCP silica nanosphere monolayer obtained
when the surface pressure is 11 mN/m. FIG. 2(b) represents the top
view of SEM image of the silica nanosphere monolayer. As shown in
FIG. 2(b), the monolayer of the silica particle is uniform and
extended to a centimeter scale.
[0037] Then, the surfaces of the laminates of the substrate,
titanium dioxide layer and silica particle layer prepared in the
above step are etched.
[0038] In the method of tuning wettability of titanium dioxide
layers against water, the etching is carried out by Inductively
Coupled Plasma (ICP) etching method, plasma etching method or
reaction-ion etching method.
[0039] ICP etching method is preferably used in the present
invention, wherein the titanium dioxide layer on the substrate
having the silica particle monolayer is etched as CF.sub.4
(etchant), O.sub.2 and Ar at the room temperature. In the present
invention, the etching mode is an anisotropic mode, and the
vertical etching ratio is much larger than the horizontal etching
ratio. The etching procedure is carried out by a physical etching
that kinetic Ar plasma ions collide with the sample ions. A
chemical etching is primarily occurred by the reaction as
follows:
CF.sub.4=2F+CF.sub.2 (1)
TiO.sub.2+4F=TiF.sub.4+2O (2)
TiO.sub.2+2CF.sub.2=TiF.sub.4+2CO (3)
[0040] The predicted product of titanium dioxide layer is TiF.sub.4
having higher boiling point of 284.degree. C. at an atmospheric
pressure.
[0041] Generally, upon referring to Wenzel state, the etched
TiO.sub.2 thin-film represents a smaller contact angle than that of
the thin-films unetched. Wenzel equation is as follows:
cos .theta.a=r cos .theta. (4)
[0042] Wherein, .theta.a is an apparent contact angle of water drop
on the rough surface; .theta. represents an inherent contact angel
on the surface of liquid droplet; r is a roughness factor which is
defined as the ratio of the projected geometry to actual rough
surface region. If .theta. is less than 90.degree., the surface
will be more hydrophilic, and if .theta. is greater than
90.degree., the surface will be more hydrophobic.
[0043] The silica particle layer is etched during the etching
procedure, wherein the titanium dioxide layer forms nanocones on
the upper part based on the part that each of the silica particles
is faced as the silica particle layer is etched. The cones formed
from the uniform and ordered array.
[0044] The etching ratio of the silica is much faster than that of
the titanium dioxide, and the etching ratio of SiO.sub.2 particle
is also considered in the formation of nanocones.
[0045] The preferable etching time in the present invention is in
the range of from 10 to 40 sec. If the etching time is less than 10
sec, there is a problem that the etching efficiency of the silica
particle is lowered and thus, there the nanocone formation is
insufficient, and if the etching time exceeds 40 sec, there is a
problem that the etchings of the titanium dioxide layer as well as
the silica particle layer are severed and thus, the nanocones are
disappeared. The preferable etching time is in the range of from 15
to 25 sec.
[0046] Then, the silica particle layer etched and remained in the
stage is removed to form the nanostructured titanium dioxide
layer.
[0047] In the present invention, the titanium dioxide layer
constitutes the uniform and ordered nanocone array by removing
silica remained on the titanium dioxide layer and thus, the
nanostructuring thereof is completed.
[0048] In the present invention, the hydrophilicity of the titanium
dioxide layer nanostructured as above is increased by modifying the
surface. That is, the titanium dioxide layer nanostructured
according to the method of the present invention has a smaller
water contact angle compared to thin-film TiO.sub.2 layer.
[0049] On the other hand, the hydrophobicity of titanium dioxide
layer can be increased by forming the silane layer on the upper
part of the titanium dioxide layer nanostructured in the step.
[0050] The silane layer in the present invention is preferably
consisted of one or more of silane being selected from the group
consisting of 1H,1H,2H,2H-perfluorooctylchlorosilane,
octadecyltrichlorosilane,
(tridecanfluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane,
3,3,3-trifluoropropyl-trichlorosilane, dodecyltrichlorosilane.
[0051] --CF.sub.3 group has a low surface energy. If the surface
energies of some functional groups are compared, they are as
follows:
--CH.sub.2>--CH.sub.3>--CF.sub.2>--CF.sub.2H>--CF.sub.3.
[0052] Since the functional group having the low surface energy as
above is formed on the titanium dioxide layer, the hydrophobicity
of the titanium dioxide layer is increased.
[0053] In particular, the silane layer is formed on the upper part
of the titanium dioxide layer nanostructured in the method of
tuning wettability of titanium dioxide layer against water of the
present invention, a sample in case of etching it for 15-25 sec
represents hyper-hydrophobicity.
[0054] As such, in the present invention the titanium dioxide layer
becomes hydrophilicity or hydrophobicity as the surface of titanium
dioxide layer is modified, the wettability of the layer against
water can be tuned.
[0055] If the method of tuning wettability of the titanium dioxide
layer against water is used, the titanium dioxide structure having
the titanium dioxide layer in a shape of nanocone array being
uniformly arranged can be prepared.
[0056] The titanium dioxide structures of the present invention are
characterized by comprising the titanium dioxide layer in the form
of nanocone array uniformly arranged on the substrate. The titanium
dioxide layer structures of the present invention increase the
hydrophilicity thereof by nanostructuring of the titanium dioxide
layer surface to the nanocone array shape.
[0057] Also, the silane layer can be coated on the upper part of
the titanium dioxide layer. Wherein the silane layer is preferably
consisted of one or more of silane selected from the group
consisting of 1H,1H,2H,2H-perfluorooctylchlorosilane,
octadecyltrichlorosilane,
(tridecanfluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane,
3,3,3-trifluoropropyl-trichlorosilane, dodecyltrichlorosilane.
[0058] In the titanium dioxide structure of the present invention,
if the silane layer is coated on the titanium dioxide layer, the
hydrophobicity thereof is increased.
[0059] Hereinafter, the method of tunning wettability of titanium
dioxide layers against water is reviewed in more detail through
following examples and analyses.
Formation of TiO.sub.2 Layer
[0060] 1.1 Sol-Gel TiO.sub.2 Thin-Film
[0061] TiO.sub.2 thin-film was synthesized by using ethanol (Merck,
99.8%), Titanium Tetraisopropoxide (TTIP) (Daejung Chemical, 98% or
more) and nitric acid (Daejung Chemical, 60.about.62%) in the
sol-gel method: 0.5 ml TTIP was mixed with 4.5 ml ethanol, and then
10 .mu.l nitric acid was added to the mixture to prepare TiO.sub.2
sol while inhibiting the precipitation of TiO.sub.2 powder. The
TiO.sub.2 sol was applied to ultrasonic treatment for 30 min, and
then white TiO.sub.2 powder precipitate was filtered. Sol-gel
TiO.sub.2 layer having 85 nm thickness was formed over the silicon
substrate by a spin coating (3000 rpm, 18 s) by using the filtrate
solution filtered as above.
[0062] 1.2 Sputtering TiO.sub.2 Thin-Film
[0063] TiO.sub.2 thin-film was prepared by vapor-depositing
TiO.sub.2 on the silicon substrate by using a multi-target
co-sputtering technique: The sputtering ratio of TiO.sub.2 and Ti
is maintained as 3:1. Then, the thin-film was heated in a
400.degree. C. furnace which is at an ambient air for 4 hrs. The
thickness of the sputtered thin-film was 250 nm.
[0064] 1.3 Thermooxidation TiO.sub.2 Thin-Film
[0065] 250 nm Ti layer was formed on the silicon wafer by using
E-beam evaporator, and then heated in a 500.degree. C. furnace
which is at an ambient air for 8 hours to oxidate the Ti layer to
form a thermooxidation TiO.sub.2 thin-film.
[0066] The following procedures were progressed by using three
types of TiO.sub.2 thin-film prepared in Example 1.
Synthesis of SiO.sub.2 Particle
[0067] Silica particles were synthesized by the Stober method by
using 20 ml ethanol, 3.29 ml distilled water, 0.55 ml nitric acid
(Daejung Chemicals, 25.about.28%) and 2.3 ml
tetraethylorthosilicate (TEOS) (Aldrich). The synthesized silica
particles (spheres) are mono-dispersed, and the mean size thereof
was 225 nm.
Formation of Hexagonal Close Packed SiO.sub.2 Monolayer and the
Coupling with the Titanium Dioxide Layer
[0068] The silica particles prepared from Example 2 were washed
four times with ethanol, and then diluted with 20 ml methanol
(Daejung Chemicals, 94%) without drying to prepare the silica
suspension to avoid the aggregation of the silica particles and to
use Langmuir-Blodgett technique. 2 ml of silica suspension was
used. To the silica suspension 2.5 mg of sodium dodecyl sulfate
(Aldrich) was added, and they are ultrasonic dispersed while
heating for 60 min to impart the hydrophilic-hydrophobic property
to the silica spheres. Then, to the silica suspension 3 ml of
chloroform (Junsei, 99%) was added and the ultrasonic treatment was
applied to that for 60 min again. In Langmuir-Blodgett trough
having an initial water area of 300 cm.sup.2, the substrate that
TiO.sub.2 thin-film prepared from Example 1 was formed was
installed. To the Langmuir-Blodgett trough 250 .mu.l of the silica
suspension was dropped. Before the isotherm process, a waiting was
made for 20 min until the initial surface pressure was stabilized
so that all the solvents are vaporized. Thereafter, the
self-assembling silica monolayer was pressured with the barrier
speed of 20 cm.sup.2/min until the target surface pressure was 11
mN/m. The hexagonal close packed SiO.sub.2 monolayer was formed on
the upper part of titanium dioxide thin-film by elevating the
vertical substrate that the TiO.sub.2 thin-film was formed with the
speed of 1 mm/min and thus, transferring the hexagonal close packed
SiO.sub.2 monolayer to the upper part of the titanium dioxide
thin-film. The transferring ratio was about 1.
Etching
[0069] The laminate that the hexagonal close packed SiO.sub.2
monomolecular layer was formed on the titanium dioxide thin-film
prepared from Example 3 was etched by using the mixed gas of
CF.sub.4, O.sub.2 and Ar by ICP dry etching method at the room
temperature. After the etching procedure, the samples were washed
by stirring them in the distilled water with the sonicator during 5
min, and then washed them in pure ethanol by the same method to
remove any residual silica on the surfaces of the samples.
Silanization of Surface of TiO.sub.2 Layer
[0070] Unetching TiO.sub.2 thin-film sample and TiO.sub.2 thin-film
samples which completed the procedures until Example 4 were dipped
into n-hexane comprising 0.5%
1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFTS) (Aldrich) for 10
min to coat the surfaces of the thin-film samples with PFTS. Then,
they are carefully washed with hexane to remove any residual PFTS.
After drying the thin-film samples at an ambient air, they are
treated for 1 hour in 110.degree. C. vacuum oven.
Analysis
[0071] The shapes and thicknesses of the samples as prepared from
the Examples were measured by SEM (FE Type) XL30SFEG (PHILIPS) and
UHR-SEM Magellan 400(FEI). X-ray photoelectron spectroscopy (XPS)
spectrum was taken on Sigma Probe system (Thermo VG Scientific)
equipped with Al--K.alpha. X-ray source (1486.3 eV) and 0.47 eV
FWHM of energy resolution under UHV condition OF 10.sup.-10 Torr.
The XPS spectrum was fitted by using CASAXPS software. Water
contact angle was measured on the thin-film samples by using a
goniometer (Phoenix 300 plux, SEC)). In order to achieve the
immobilization of water drops, the water contact angle was measured
at 1 min after making water drops contact with the surfaces of the
thin-film samples.
(1) Comparison of Shape of Sputtering TiO.sub.2 Thin-Film Over the
Etching Time
[0072] FIG. 3 shows the top and cross-sectional views of Scanning
Electron Microscopy (SEM) image of sputtering TiO.sub.2 thin-film
sample. FIG. 3(a) shows the images at (a) unetching, (b) 10 sec
etching, (c) 15 sec etching, (d) 20 sec etching, (e) 25 sec etching
and (f) 30 sec etching. As the etching time increases, the silica
nanospheres were increasingly etched to disappear, in particular it
can be ascertained that 225 nm of SiO.sub.2 is mostly etched after
30 sec etching as shown in FIG. 3(f).
(2) Comparison of the Height of TiO.sub.2 Nanocone According to
Etching Time Change of Sol-Gel/Sputtering/Thermooxidation TiO.sub.2
Thin-Films
[0073] Table 1 suggests the height change of TiO.sub.2 nanocone
according to the etching time change of
sol-gel/sputtering/thermooxidation TiO.sub.2 thin-film.
[0074] After 20 sec etching, the height of sol-gel TiO.sub.2
nanocone was .about.104.8 nm, which was almost 2 times over that of
sputtering TiO.sub.2 nanocone being .about.59.7 nm and that of
thermooxidation TiO.sub.2 nanocone being .about.54.6 nm. Thus, it
could be seen that sol-gel TiO.sub.2 thin-film sample has more
effect on etching than other TiO.sub.2 thin-film samples.
[0075] Also, upon 30 sec etching, the height of sol-gel TiO.sub.2
nanocones was greatly decreased to .about.47.2 nm, whereas the
heights of sputtering and thermooxidation TiO.sub.2 nanocones were
89.8 nm and 72.3 nm, respectively. That is, if SiO.sub.2 sphere
mask is disappeared, since the upper part of titanium dioxide
nancone is not shielded, it could be seen that the surface part
having a more etching ratio is faster.
[0076] In case of sol-gel TiO.sub.2 thin-film, upon etching for 30
sec or more, most of the titanium dioxides are etched to expose the
silicon substrate beneath the titanium dioxide layer. Eventually,
upon etching for 40 sec, all of TiO.sub.2 layers were removed.
[0077] Further, if the sputtering and thermooxidation TiO.sub.2
thin-film were etched for 40 sec, it could be ascertained that
there was a decrease in the height of titanium dioxide nanocone
than that of 30 sec etching. Also, it could be ascertained that the
thermooxidation TiO.sub.2 thin-film endures the etching better than
the sputtering TiO.sub.2 thin-film.
TABLE-US-00001 TABLE 1 Height of titanium dioxide nanocone (nm)
Etching time Thermooxidation (s) So-gel TiO.sub.2 Sputtering
TiO.sub.2 TiO.sub.2 10 54.6 33.2 30.3 20 104.8 59.7 54.6 30 47.2
89.8 72.3 40 -- 76.8 70.4
(3) Relation Between the Contact Angle and the Etching Time Before
and after the Silanation of the Surface of TiO.sub.2 Layer
[0078] FIG. 4 depicts a graph representing the relation between the
etching time and the water contact angle of TiO.sub.2 layer before
(a) and after (b) the silanation by using
1H,1H,2H,2H-perfluorooctyltrichlorosilane.
[0079] FIG. 4(a) shows the relation between the etching time and
the water contact angle of TiO.sub.2 layer not silanated on the
surface, and represented the features that the contact angle is
decreased as the etching time is increased overall of sol-gel,
sputtering and thermooxidation TiO.sub.2 thin-film samples. That
is, as the etching is progressed, it could be ascertained that the
hydrophilicity of titanium dioxide layer was increased by the
nanostructuring of titanium dioxide layer. In case of sputtering
TiO.sub.2 thin-film sample, the value was represented that the
water contact angle was slightly increased upon 30 sec etching, but
this may be caused from a contamination of sample surface.
[0080] FIG. 4(b) represents the relation between the etching time
and the contact angle after coating the silane layer on TiO.sub.2
layer surface, and wherein the unetching, sol-gel, sputtering and
thermooxidation TiO.sub.2 thin-film samples represented the water
contact angle of nearly 100.degree.. In case of the samples etched
during 10 sec and 20 sec, it represented that the water contact
angle was rapidly increased in sol-gel TiO.sub.2 thin-film sample,
and the angle was moderately increased in sputtering and
thermooxidation TiO.sub.2 sample, and these coincide with the
height growth of titanium dioxide of Table 1.
(4) Comparison of Shape and Contact after the Silanation of 20 Sec
Etching Sol-Gel/Sputtering/Thermooxidation TiO.sub.2 Thin-Films
[0081] FIG. 5 is the cross-sectional and top views of SEM image and
photoimage of water contact angle after the silanation of 20 sec
etching TiO.sub.2 thin film, wherein (a) is a sol-gel method, (b)
is a sputtering method, (c) is a thermooxidation method TiO.sub.2
layer. Sol-gel TiO.sub.2 thin-film had the contact angel of
155.degree. upon 20 sec etching and represented the
super-hydrophobicity, and the contact angles of sputtering
TiO.sub.2 thin-film and thermo-oxidation TiO.sub.2 thin-film were
130.degree. and 138.degree., respectively.
(5) Relation Between the Water Repellency and the Geometrical
Structure of Titanium Dioxide Nancone
[0082] The lower surface energy of --CF.sub.3 group together with
air trapped beneath the water drops due to the geometrical
structure of titanium dioxide nanocone increases the water
repellency. This result coincides with Cassie and Baxter equation,
and the wetting of heterogeneous surface is represented by the
following equation:
cos .theta.a=.SIGMA.fi cos .theta.i (5)
[0083] Wherein .theta.a is an apparent contact angle of water drop,
fi is an area part of the constitutional component i, and the
contact angle of i on the plane is .theta.i. Wherein the sample to
be used can be considered as 2-component system consisted of
titanium dioxide and air, having the water contact angle of
180.degree. in air (non-wetting). Since .SIGMA.fi=1, the equation
(5) is as follows:
cos .theta.a=f1(cos .theta.1+1)-1 (6)
[0084] Equation (6) proved that when f1 is sufficiently low, since
the apparent contact angle (.theta.a) reaches to nearly 180.degree.
and thus cos .theta.a is close to -1, if a larger number of air is
present below the water drop, it becomes more hydrophobic surface.
Sol-gel TiO.sub.2 thin-film samples have the contact angle of
155.degree. and become super-hydrophobic whereas the sputtering and
thermooxidation TiO.sub.2 thin-film samples represent considerably
strengthen water contact angle compared to the plane sample and 10
sec etched sample.
[0085] In case of the sample etched longer than 20 sec, although
the sputtering method and thermooxidation method TiO.sub.2
nanocones reached to the highest height and have higher air area
higher below the water drop, the decrease in the water contact
angle of three types of titanium dioxides due to the bigger space
between the nanocones was found. In order to explain such certain
circumstance, the geometric surface structure should be
considered.
[0086] Sputtering and thermooxidation titanium dioxide nanocone
array formed after 20 sec etching represents the structure of
inclined side wall. As described above, the solid phase, liquid
phase and gas phase complex interface as the air pocket trapped
below the water drop further represents the super-hydrophobicity.
Consequently, meniscus along the inclined side wall is allowed to
form and maintain the complex interface, based on Laplace pressure.
Younghao Xiu et al. suggested the role and mechanism of Laplace
pressure. The relation between the Laplace pressure and the
inclined angle is given by the following equation:
.DELTA. p = p - p 0 = .gamma. cos ( .theta. - .alpha. ) R 0 + h tan
.alpha. ##EQU00001##
[0087] Wherein .DELTA.p, p, and p0 represent the Laplace pressure,
the pressure and atmospheric pressure of the liquid side of the
meniscus, respectively; .gamma. represents the surface tension of
water; .theta. represents Young's water contact angle on the
surface; .alpha. represents the inclined angle; R.sub.0 represents
a half of the width between bottom sides of two adjacent side
walls; h represents the distance between the meniscus and the
floor. The equation represents the dependency of the inclined angle
to the Laplace pressure. If the inclined angle is small, Laplace
pressure is high, a larger amount of air is trapped beneath the
water drop.
[0088] Upon etching for more than 30 sec, SiO.sub.2 nanosphere
monolayer on the titanium dioxide layer surface is completely
etched to disappear. If all the silica particles are removed, the
upper part of titanium dioxide is not protected any more, and the
etching is occurred more rapidly at a higher part of the surface.
Thus, after 30 sec etching, nanocone sides are inclined more than
after 20 sec, and the water contact angle of 30 sec etched
TiO.sub.2 thin-film samples is smaller. Also, due to the inherent
roughness of sputtering method and thermooxidation method
TiO.sub.2, the pyramid-shape structure of the upper part of the
nanocone may be the factor increasing the water contact angle of
the samples. On the other hand, the pyramid-shape structure can be
replaced with a stepped structure on the surface of 30 sec etching
nanocone. The explanation on this is shown in FIG. 6, and can be
used in explaining that the water contact angle of the sputtering
method and thermooxidation TiO.sub.2 etched for 40 sec is more
lowered. Due to insufficient thickness of sol-gel TiO.sub.2
thin-films, the water contact angle on the sol-gel TiO.sub.2 film
etched for 30 sec with the free-titanium dioxide on each of silicon
cones is considerably reduced, and this is also explained similarly
to the sputtering and thermooxidation TiO.sub.2 films. The high
inclined angle of the silicon cones also explains the reduced water
contact angle of 30 sec etched sol-gel TiO.sub.2 thin-film
sample.
(6) Confirmation of Etching Product Upon Etching of the Titanium
Dioxide Layer
[0089] XPS spectrum of etched TiO.sub.2 layer is decipted in FIG.
7. (a) and (b) are Ti component and F component of 20 sec etching
sol-gel TiO.sub.2 layer, respectively, and (c) and (d) are Ti
component and F component of 40 sec etching thermo-oxidation
TiO.sub.2 layer, respectively.
[0090] According to this, TiF.sub.4 might be not formed practically
in the etching process; fluorine atoms can physically adsorb on
surface, or occupy and substitute the positions of oxygen atoms,
behave as dopants in TiO.sub.2 structures. The fluorine dopants are
able to improve photocatalytic activity of titanium dioxide.
[0091] Although the concrete Examples of the present invention are
given as above, the present invention is not limited to the above
and can be variously modified within the technical scope of the
present invention and this modification belongs to the claims of
the present invention, as indicated below.
* * * * *