U.S. patent application number 13/630296 was filed with the patent office on 2013-01-31 for titanium dioxide coating method.
This patent application is currently assigned to NATIONAL TSING HUA UNIVERSITY. The applicant listed for this patent is NATIONAL TSING HUA UNIVERSITY. Invention is credited to Kuo-Hsin CHANG, Huan-Ching HSU, Chi-Chang HU, Ching-Chun HUANG.
Application Number | 20130026042 13/630296 |
Document ID | / |
Family ID | 47596338 |
Filed Date | 2013-01-31 |
United States Patent
Application |
20130026042 |
Kind Code |
A1 |
HU; Chi-Chang ; et
al. |
January 31, 2013 |
TITANIUM DIOXIDE COATING METHOD
Abstract
A titanium dioxide coating method is disclosed. An electrolyte
containing Ti.sup.3+, an oxidant, and at least one of
NO.sub.3.sup.- and NO.sub.2.sup.- is provided for an
electrodeposition device, wherein the oxidant is configured for
essentially oxidizing Ti.sup.3+ into Ti.sup.4+. A substrate is
immersed into the electrolyte and electrically connected to the
electrodeposition device. A cathodic current is applied to the
substrate via the electrodeposition device for reduction of
NO.sub.2.sup.- or NO.sub.3.sup.-. A titanium dioxide film is thus
formed on the surface of the substrate. The thickness, porosity,
and morphology of the titanium dioxide film can be controlled by
varying the electroplating parameters, and relatively uniform
deposits on various substrates of complex shapes can be obtained by
use of low cost instruments. The resultant structure of Ti.sup.4+
species oxidized from Ti.sup.3+ by the oxidant can be used to
control the deposition rate of TiO.sub.2.
Inventors: |
HU; Chi-Chang; (Hsinchu,
TW) ; HUANG; Ching-Chun; (Hsinchu, TW) ;
CHANG; Kuo-Hsin; (Hsinchu, TW) ; HSU; Huan-Ching;
(Hsinchu, TW) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NATIONAL TSING HUA UNIVERSITY; |
Hsinchu |
|
TW |
|
|
Assignee: |
NATIONAL TSING HUA
UNIVERSITY
Hsinchu
TW
|
Family ID: |
47596338 |
Appl. No.: |
13/630296 |
Filed: |
September 28, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12505936 |
Jul 20, 2009 |
|
|
|
13630296 |
|
|
|
|
Current U.S.
Class: |
205/96 ; 205/106;
205/333 |
Current CPC
Class: |
C25D 3/54 20130101; C01G
23/047 20130101; C25D 5/50 20130101; C01P 2006/14 20130101; C01P
2006/40 20130101; H01G 9/2031 20130101; C25D 5/18 20130101; C25D
9/08 20130101; H01M 14/005 20130101; C01P 2004/04 20130101; C01P
2004/03 20130101 |
Class at
Publication: |
205/96 ; 205/333;
205/106 |
International
Class: |
C25D 9/08 20060101
C25D009/08; C25D 5/50 20060101 C25D005/50; C25D 5/18 20060101
C25D005/18 |
Foreign Application Data
Date |
Code |
Application Number |
May 12, 2009 |
TW |
098115705 |
Claims
1. A titanium dioxide coating method comprising: providing an
electrolyte with a pH value .ltoreq.2 and containing Ti.sup.3+, an
oxidant, and at least one of NO.sub.3.sup.- and NO.sub.2.sup.- for
an electrodeposition device, wherein the oxidant is configured for
essentially oxidizing Ti.sup.3+ into Ti.sup.4+; immersing a
substrate into the electrolyte; electrically connecting the
substrate to the electrodeposition device; and applying a cathodic
current to the substrate via the electrodeposition device whereby
NO.sub.3.sup.- or NO.sub.2.sup.- is reduced to generate extensive
OH.sup.- for forming a titanium dioxide film on the surface of the
substrate.
2. The method as claimed in claim 1, wherein the oxidant is a weak
oxidant which is unable to further oxidize Ti.sup.4+ to
Ti.sup.6+.
3. The method as claimed in claim 2, wherein the weak oxidant
comprises NO.sub.3.sup.- or NO.sub.2.sup.-.
4. The method as claimed in claim 2, wherein the weak oxidant
comprises S.sub.2O.sub.8.sup.2-, ClO.sub.4.sup.-, ClO.sup.-,
BrO.sub.4.sup.-, BrO.sup.-, IO.sub.4.sup.- or IO.sup.-.
5. The method as claimed in claim 2, wherein a ratio of Ti.sup.3+
to the weak oxidant is equal to/above the stoichiometric ratio.
6. The method as claimed in claim 1, wherein the oxidant is a
strong oxidant in a stoichiometric ratio to Ti.sup.3+.
7. The method as claimed in claim 6, wherein the strong oxidant
comprises H.sub.2O.sub.2 or O.sub.3.
8. The method as claimed in claim 1, wherein the pH value of the
electrolyte is less than 1.
9. The method as claimed in claim 1 further comprising a post
annealing step after forming the titanium dioxide film.
10. The method as claimed in claim 9, wherein the post annealing
step is carried out at about 100-800.degree. C.
11. The method as claimed in claim 1, wherein the cathodic current
is applied by galvanostatic (constant dc current), potentiostatic
(constant voltage), potentiodynamic, or galvanodynamic methods, or
in the pulse voltage or pulse current modes.
12. The method as claimed in claim 1, wherein OH.sup.- is generated
by reduction of NO.sub.3.sup.- or NO.sub.2.sup.- at the
cathode.
13. The method as claimed in claim 12, wherein TiO(OH).sub.2 is
generated from a reaction between Ti.sup.4+ and OH.sup.- and then
dehydrated to form TiO.sub.2.
14. The method as claimed in claim 13, wherein the generation of
OH.sup.- by NO.sub.3.sup.- or NO.sub.2.sup.- reduction at the
cathode is catalyzed by TiO(OH).sub.2 and TiO.sub.2.
15. The method as claimed in claim 1, wherein the Ti.sup.3+ is
obtained from dissolution of titanium.
Description
RELATED APPLICATIONS
[0001] This application is a Continuation-in-part of co-pending
application Ser. No. 12/505,936 filed on Jul. 20, 2009.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a titanium dioxide coating
method and the electrolyte used therein, and more particularly to
an electrodeposition method for coating titanium dioxide and the
electrolyte used therein.
[0004] 2. Description of the Prior Art
[0005] Titanium dioxide, also known as titania, is widely
recognized as an important electrode material in semiconductor
photo-electrochemistry. Among the three main crystalline phases:
anatase, rutile, and brookite TiO.sub.2, the anatase form
(A-TiO.sub.2) is the most popular photo-electrode because the
lowest unoccupied molecular orbital of dyes, such as N719, is very
close to the conduction band of A-TiO.sub.2.
[0006] In addition, A-TiO.sub.2 generally shows relatively high
reactivity and chemical stability under ultraviolet light
excitation for water and air purifications, photocatalysts, gas
sensors, electrochromic devices, and so on, further emphasizing its
practical importance.
[0007] Several techniques were proposed for fabricating TiO.sub.2,
such as sol-gel, chemical vapor deposition, hydrothermal,
electrospinning, anodizing, and electrodeposition.
[0008] Among these methods, cathodic deposition of TiO.sub.2
becomes attractive because electrochemical deposition provides the
advantages of controlling the thickness and morphology by varying
the electroplating parameters, relatively uniform deposits on
complex shapes, and use of low cost instrumentation.
[0009] Sotiropoulos et al. (Electrochimica Acta 51 (2006)
2076-2087) prepared TiO2 films from acidic aqueous solutions of
TiOSO.sub.4 and H.sub.2O.sub.2 by room temperature potentiostatic
cathodic electrosynthesis. However, Sotiropoulos taught that
TiOSO.sub.4 was oxidized to Ti.sup.6+ by using a strong oxidant
H.sub.2O.sub.2, which needs to be reduced to prepare the TiO.sub.2
film.
[0010] Kim et al. (Electrochimica Acta 50 (2005) 2713-2718) taught
a novel approach using TiCl.sub.3 or TiCl.sub.4 as the precursors
for the electrodeposition of TiO.sub.2 films. Kim mainly focused on
the advantage in using CTAB and the pH value of the solution is
roughly 3 in all the cases (Kim, p. 2714 Experimental section 2,
paragraph 2).
[0011] Both of Sotiropoulos and Kim did not achieve high yield of
titanium dioxide and it is now a current goal to develop a cathodic
deposition method for coating titanium dioxide with higher yield in
comparison with the prior arts.
SUMMARY OF THE INVENTION
[0012] The present invention is directed to provide an electrolytic
method for coating titanium dioxide to gain the advantages of
controlling the thickness, porosity, and morphology by varying the
electroplating parameters, relatively uniform deposits on various
substrates of complex shapes, and use of low cost
instrumentations.
[0013] The present invention is directed to a cathodic deposition
method for coating a titanium dioxide film with higher yield in
comparison with the prior arts.
[0014] According to one embodiment, the present invention provides
a titanium dioxide coating method, which includes following steps.
An electrolyte containing Ti.sup.3+, an oxidant and at least one of
NO.sub.3.sup.- and NO2.sup.- is provided for an electrodeposition
device, wherein the oxidant is configured for essentially oxidizing
Ti.sup.3+ into Ti.sup.4+. A substrate is immersed into the
electrolyte and electrically connected to the electrodeposition
device. A cathodic current from the electrodeposition device is
applied to the substrate for reducing NO.sub.2.sup.- or
NO.sub.3.sup.- to generate extensive OH.sup.- and to form titanium
dioxide film on the surface of the substrate.
[0015] Other advantages of the present invention will become
apparent from the following descriptions taken in conjunction with
the accompanying drawings wherein are set forth, by way of
illustration and example, certain embodiments of the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The foregoing aspects and many of the accompanying
advantages of this invention will become more readily appreciated
as the same becomes better understood by reference to the following
detailed description, when taken in conjunction with the
accompanying drawings, wherein:
[0017] FIG. 1 illustrates a flowchart of a titanium dioxide coating
method according to one embodiment of the present invention;
[0018] FIG. 2 illustrates LSV (linear sweep voltammetry) curves
according to one embodiment of the present invention;
[0019] FIG. 3A illustrates first and second scans of LSV curves
according to one embodiment of the present invention;
[0020] FIG. 3B illustrates the corresponding EQCM (electrochemical
quartz crystal microbalance) responses of the first and second
scans of LSV in FIG. 3A according to one embodiment of the present
invention;
[0021] FIG. 3C illustrates an enlarged view of curve 1 in FIG.
3B;
[0022] FIG. 3D illustrates a SEM image of titanium dioxide depth
for 3 cycles according to one embodiment of the present
invention.
[0023] FIGS. 4A and 4B illustrate SEM (Scanning Electron
Microscope) images according to one embodiment of the present
invention;
[0024] FIGS. 4C and 4D illustrate TEM (Transmission Electron
Microscope) images according to one embodiment of the present
invention;
[0025] FIGS. 4E and 4F illustrate depth profiles of XPS (X-ray
photoelectron spectra) according to one embodiment of the present
invention;
[0026] FIG. 5A illustrates the LSV curves according to one
embodiment of the present invention;
[0027] FIG. 5B illustrates the corresponding EQCM (electrochemical
quartz crystal microbalance) responses of the LSV curves in FIG. 5A
according to one embodiment of the present invention; and
[0028] FIG. 5C illustrates the dependence of TiO.sub.2 mass on the
cycle number of CV according to one embodiment of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0029] FIG. 1 illustrates a flowchart of a titanium dioxide coating
method including following steps. Beginning at step S1, an
electrolyte with pH values .ltoreq.2 and containing Ti.sup.3+, an
oxidant and at least one of NO.sub.3.sup.- and NO.sub.2.sup.-. The
Ti.sup.3+ is essentially oxidized into Ti.sup.4+ by the oxidant and
NO.sub.3.sup.-/NO.sub.2.sup.- is the OH.sup.- provider. This
electrolyte is provided for an electrodeposition device. Next, at
step S2, a substrate is then immersed into the electrolyte and at
step S3, the substrate is electrically connected to the
electrodeposition device. At step S4, a cathodic current is applied
on the substrate via the electrodeposition device for reducing
NO.sub.2.sup.- or NO.sub.3.sup.- to generate extensive OH.sup.- for
depositing TiO.sub.2 films on the surface of substrates. The
cathodic current can be applied by galvanostatic (constant dc
current), potentiostatic (constant voltage), potentiodynamic, or
galvanodynamic methods, or in the pulse voltage or pulse current
modes.
[0030] In one preferred embodiment, an electrolyte with pH values
<1 is provided for titanium dioxide deposition. Ti.sup.3+ may be
obtained from dissolution of titanium, for example by dissolving
with H.sub.2O.sub.2 and ammonia.
[0031] The oxidants can be divided into two groups, strong and weak
oxidants. When the weak oxidants are employed, Ti.sup.3+ can only
be oxidized to Ti.sup.4+, even excess oxidants are added. When the
strong oxidants are employed, a stoichiometric ratio between
Ti.sup.3+ and oxidants is required to oxidize Ti.sup.3+ to
Ti.sup.4+ which cannot be further oxidized to Ti.sup.6+. Referring
to Table 1, weak oxidants that essentially oxidize Ti.sup.3+ into
Ti.sup.4+ are provided and include without limitations to
NO.sub.3.sup.-, NO.sub.2.sup.-, S.sub.2O.sub.8.sup.2-,
ClO.sub.4.sup.-, ClO.sup.-, BrO.sub.4.sup.-, BrO.sup.-,
IO.sub.4.sup.- or IO.sup.-. The strong stoichiometric oxidants
include without limitations to H.sub.2O.sub.2 or O.sub.3.
TABLE-US-00001 TABLE 1 Oxidants that essentially oxidize Ti.sup.3+
into Ti.sup.4+ Oxidant Ti.sup.3+.fwdarw.Ti.sup.4+
.sup.@Ti.sup.4+.fwdarw.Ti.sup.6+ Color NO.sub.3.sup.- Yes No
transparent NO.sub.2.sup.- Yes No transparent *XO.sub.4.sup.- Yes
No transparent to pale yellow.sup.% *XO.sup.- Yes No transparent to
pale yellow.sup.% S.sub.2O.sub.8.sup.- Yes No transparent to pale
yellow.sup.% H.sub.2O.sub.2 Yes.sup.# Yes Tangerine O.sub.3
Yes.sup.# Yes Tangerine *X represents Cl, Br, I
.sup.#Stoichiometric ratio .sup.@excess oxidant .sup.%turning pale
yellow when excess oxidant is present
[0032] The continuous reduction of NO.sub.2.sup.- or NO.sub.3.sup.-
to N.sub.2 and NH.sub.3 generates extensive OH.sup.-, and
effectively enhances the deposition of TiO.sub.2 films on the
surface of substrates.
[0033] In one embodiment, a post annealing step is further
performed after forming the titanium dioxide film on the surface of
the substrate, wherein the post annealing step is carried out at
about 100-800.degree. C.
[0034] The following descriptions of specific embodiments of the
present invention have been presented for purposes of illustrations
and description, and they are not intended to be exclusive or to
limit the present invention to the precise forms disclosed, and
obviously many modifications and variations are possible in light
of the above teaching. It is intended that the scope of the
invention can be defined by the Claims appended hereto and their
equivalents.
[0035] TiO.sub.2 particulates are cathodically deposited onto
graphite substrates from an electrolyte bath containing 0.47 M HCl,
25 mM TiCl.sub.3 and 75 mM NaNO.sub.3 in an electrodeposition
device according to an embodiment of the present invention. A
pretreatment procedure of graphite substrates may be performed and
the detailed description thereof is herein omitted.
[0036] According to one embodiment of the present invention, the
redox reaction between Ti.sup.3+ and NO.sub.3.sup.- during
preparation of the deposition solution is herein disclosed.
Nitrates, acting as the oxidizers, were reduced to NO.sub.2
(reddish-brown bubbles) when the transparent NaNO.sub.3 solution
was added into the purple TiCl.sub.3 solution. Since NO.sub.2
molecules are soluble in acidic aqueous media, they will
automatically convert into NO.sub.3.sup.- and NO. This statement is
supported by the observation that reddish-brown bubbles gradually
disappear within 30-40 seconds and the purple TiCl.sub.3 solution
in presence of Ti.sup.3+ is a colorless transparent solution
indicating the formation of TiO.sup.2+ (see equations 1 and 2)
Ti.sup.3++NO.sub.3-.fwdarw.TiO.sup.2++NO.sub.2 (1)
3NO.sub.2+H.sub.2O.fwdarw.2HNO.sub.3+NO (2)
[0037] Curves 1-5 in FIG. 2 correspond to the i-E responses
measured from various electrolytes. As can be seen from curves 1
and 2, reduction commences at potentials negative to -0.6 V and no
gas evolution is found at potentials positive to -0.6 V. However, a
rapid generation of many bubbles is clearly observed when
potentials are negative to -0.6 V, indicating H.sub.2 evolution. On
curves 3 and 4, reduction starts in the more positive potential
region, revealing the facile reduction of NaNO.sub.2. In addition,
minor gas evolution commences from 0.4 V to -0.4 V with a low
current density, while gas evolution ceases in the potential range
from -0.4 V to -1.2 V and occurs dramatically again at potentials
behind -1.2 V. The above results indicate that NO.sub.2.sup.- is
responsible for the reduction in the more positive potential region
with minor gas evolution, presumably due to the reduction of
NO.sub.2.sup.- into N.sub.2 molecules. Since gas evolution
temporarily disappears in the potential range from -0.4 V to -1.2
V. This result suggests a further reduction of N.sub.2 to
NH.sub.4.sup.+ in such a negative potential range (see equations 3
and 4).
2NO.sub.2.sup.-+4H.sub.2O+6e.fwdarw.N.sub.2+8OH.sup.- (3)
N.sub.2+8H.sub.2O+6e.fwdarw.2NH.sub.4.sup.++8OH.sup.- (4)
[0038] On curve 5, gas evolves gently at about -0.1 V, disappears
at ca. -0.4 V and, dramatically evolves again at potentials
negative to -1.2 V, which completely follows the gas
evolution-disappearance phenomena measured from the solution
containing NO.sub.2.sup.-. Based on equations 1 and 2, reduction of
NO.sub.3.sup.- in the designed deposition bath for generating
concentrated OH.sup.- at the vicinity of electrode surface is very
similar to the reduction of NO.sub.2.sup.- (see equation 5).
Accordingly, reduction of NO.sub.2.sup.- or NO.sub.3.sup.- is
concluded to be an effective step in promoting the deposition of
TiO(OH).sub.2 (see equation 6). The TiO(OH).sub.2 is then dehyrated
to form TiO.sub.2 (see equation 7).
2NO.sub.3.sup.-+6H.sub.2O+10e.fwdarw.N.sub.2+12OH.sup.- (5)
TiO.sup.2++2OH.sup.-+xH.sub.2O.fwdarw.TiO(OH).sub.2.xH.sub.2O
(6)
TiO(OH).sub.2.xH.sub.2O.fwdarw.TiO.sub.2+(x+1)H.sub.2O (7)
[0039] The mechanism proposed in this invention not only reasonably
interprets the gas evolution/disappearance phenomena but also
explains the slight increase in bath pH after the deposition, which
is different from the slight decrease in pH found in previous case
of NO.sub.3.sup.- reduction. Based on equations 3, 4, and 6,
OH.sup.- is mainly provided by the NO.sub.2.sup.- or NO.sub.3.sup.-
reduction and the consequent N.sub.2 reduction, resulting in the
generation of NH.sub.4.sup.+. As a result, a slight increase in pH
found in this formulated solution after TiO.sub.2 deposition is
reasonable because the OH.sup.-/ electron ratios for the reduction
of NO.sub.2.sup.-, NO.sub.3.sup.-, and N.sub.2 are equal to 4/3,
6/5, and 4/3, respectively, which are larger than the
proton/electron ratio (equal to 1) for oxygen evolution at the
anode. Moreover, the deposition rate in this formulated solution is
very fast, attributable to the massive generation of OH.sup.-, the
catalytic reduction of NO.sub.2.sup.- and NO.sub.3.sup.- by
TiO(OH).sub.2 and TiO.sub.2, and the guarantee of TiO.sup.2+
formation via the redox reaction between Ti.sup.3+ and oxidants
such as NO.sub.3.sup.-/NO.sub.2.sup.-.
[0040] FIG. 3A illustrates the first and second scans of LSV
(linear sweep voltammetry) curves and FIG. 3B illustrates the
corresponding EQCM (electrochemical quartz crystal microbalance)
responses of the first and second scans of LSV measured from the
designed solution in order to precisely obtain the onset potential
of deposition. A comparison of the i-E and mass-E responses
indicates that there is always an incubation period for N.sub.2
evolution in the positive potential range, e.g., from 0.2 to -0.7 V
and from 0.1 to -0.65 V for the first and second sweeps,
respectively. Although in the incubation range, NO.sub.2.sup.- and
NO.sub.3.sup.- start to be reduced to N.sub.2, no significant
increase in mass is observed. The slight weight gain in this
potential region is probably due to the
NO.sub.2.sup.-/NO.sub.3.sup.- adsorption at the cathode. Based on
the EQCM result, once the potential is negative enough to
generate/accumulate concentrated OH.sup.-, TiO.sup.2+ will combine
with OH.sup.- to form TiO.sub.2 and an obvious weight gain is
visible behind this onset potential of deposition (-0.85 and -0.65
V for the first and second scans, respectively). Also note the
positive shift in the onset potential of deposition during the
second scan. This phenomenon is probably due to the
electrocatalytic property of TiO(OH).sub.2 and TiO.sub.2 already
deposited onto the graphite surface during the first scan for
NO.sub.3.sup.-/NO.sub.2.sup.-/N.sub.2 reduction.
[0041] Referring to FIG. 3D, the present invention achieve ca. 20
.mu.m (5.4, 7.4 and 7.6 .mu.m for 3 cycles). The dashed lines in
FIG. 3D indicate the boundary between deposit and substrate as well
as the boundaries of TiO.sub.2 deposits between each CV cycle,
respectively. The catalytic effect of TiO(OH).sub.2 and TiO.sub.2
for the NO.sub.3.sup.-, NO.sub.2.sup.-, and N.sub.2 reduction is
also one of the main reasons why the present invention achieved a
much higher yield of titanium dioxide (in comparison to 4 .mu.m for
20 cycles for Kim et al.). In addition, the usage of weak oxidants,
such as NO.sub.3.sup.- and NO.sub.2.sup.- even in excess,
guarantees the formation of TiO.sup.2+, which is also one of the
main reasons why the present invention achieved a much higher yield
of titanium dioxide.
[0042] The electrodes were cleaned in an ultrasonic DI water bath
and dried under a cool air flow after cathodic deposition. After
cleaning and drying, some electrodes were annealed at 400.degree.
C. in air for 1 hr. The morphologies were examined by a FE-SEM
(Field-Emission Scanning Electron Microscope, FE-SEM). The EQCM
study was performed by an electrochemical analyzer, CHI 4051A in a
one-compartment cell. The microstructure and SAED (selected area
electron diffraction, SAED) patterns of as-deposited and annealed
TiO.sub.2 deposits were observed through a TEM (FEI E.O Tecnai F20
G2). The depth profiles of Ti and O were measured by an X-ray
photoelectron spectrometer (XPS, ULVAC-PHI Quantera SXM), employed
Al monochromator (hv=1486.69 eV) irradiation as the
photosource.
[0043] It is favorable to prepare porous A-TiO.sub.2 films by
combining cathodic deposition from this designed solution with
lower pH value and post-deposition annealing. As illustrated in
FIGS. 4A and 4B, TiO.sub.2 films before and after annealing are
porous and the particle size is roughly estimated to be 60-100 nm.
The porous nature of TiO.sub.2 films prepared in this invention is
probably due to the extensive tiny bubble evolution during the
deposition. The particulates are considered as aggregates of
TiO.sub.2 primary particles.
[0044] The average size for as-deposited TiO.sub.2 primary
particles is about 6 nm, which is enlarged by post-deposition
annealing (ca. 10 nm for TiO.sub.2 annealed at 400.degree. C.) from
FIGS. 4C and 4D. The lattice clearly visible in FIG. 4D and the
diffraction rings in its inset indicate the anatase structure which
is transformed from the amorphous, as-deposited TiO.sub.2 by
post-deposition annealing. FIGS. 4E and 4F illustrate the depth
profiles of Ti, O, and C for as-deposited and annealed samples.
Clearly, the atomic ratio of Ti/O is approximately constant (ca.
1/2) within the whole oxide matrix.
[0045] These results confirm the formation of TiO.sub.2 in the
as-prepared and annealed films. Accordingly, combining cathodic
deposition from this designed solution and post-deposition
annealing is favorable for preparation of porous A-TiO.sub.2
films.
[0046] The aforementioned embodiment exemplified the reaction from
the electrolyte solution containing Ti.sup.3++ NO.sub.3.sup.-;
however, the redox reaction between Ti.sup.3+ and NO.sub.2.sup.- in
an electrolyte solution can be used for depositing titanium dioxide
films, too (See Equations 3, 4, 6, and 8).
6Ti.sup.3++2NO.sub.231
+2H.sub.2O.fwdarw.6TiO.sup.2++N.sub.2+4H.sup.+ (8)
[0047] FIGS. 5A and 5B show the typical LSV and .DELTA.m-E curves
measured at 25 mV s.sup.-1 from 0 to -1.6 V (vs. Ag/AgCl) in
diluted baths A and B, respectively. Bath A is defined as a
deposition solution containing 30 mM H.sub.2O.sub.2, 60 mM
TiCl.sub.3, and 75 mM NaNO.sub.3. Bath B is defined as a deposition
solution containing 60 mM TiCl.sub.3 and 135 mM NaNO.sub.3. In FIG.
5A, the onset potential of reduction on both i-E curves is the
same, -0.47 V, which is reasonably due to the same reaction,
NO.sub.3.sup.- reduction on the EQCM electrode. The reduction
currents on curve 1 are always higher than that on curve 2 at any
specified potentials negative to -0.47 V although the concentration
of NO.sub.3 in both baths should be the same under the assumption
that most NO.sub.2 gases generated in bath B are not dissolved in
the deposition bath. Accordingly, the formation of certain
Ti.sup.4' hydroxyl species (e.g.,
##STR00001##
in bath A is favorable for the NO.sub.3.sup.- reduction.
[0048] Referring to FIG. 5B, the mass of TiO.sub.2 increases
sharply from 0 to 70 ng in the potential region between -0.71 and
-0.8 V and then, a gradual increase to 145 ng at potentials
negative to -0.8 V on curve 1. On curve 2, significant increase in
mass commences at ca. -0.68 V and then, a shoulder is found between
-0.68 and -0.9 V. After that, a sharp increase in mass occurs from
-0.9 to -1.0 V and a gradual increase from 70 to 130 ng at
potentials negative to -1.0 V. Clearly, the TiO.sub.2 deposition
rate in bath A is obviously higher than that in bath B,
attributable to the formation of Ti.sup.4+ hydroxyl species
containing bridged OH groups in the solution. Such Ti.sup.4+
hydroxyl species (with olation) need fewer OH.sup.- to form the
polymeric oxy-hydroxyl Ti precipitates which will be converted to
TiO.sub.2 through dehydration. Accordingly, the formation of
Ti.sup.4+ hydroxyl dimmers containing bridged OH groups favors the
cathodic deposition of TiO.sub.2.
[0049] Referring to FIG. 5C, Lines 1 and 2 show the dependence of
TiO.sub.2 mass on the cycle number of CV between 0 and -1.6 V from
baths A and B, respectively. Clearly, the dependence of TiO.sub.2
mass on the cycle number of CV from both deposition baths is
linear. However, the slope of curve 1 is obviously higher than that
of curve 2, revealing that the deposition solution containing
H.sub.2O.sub.2 is more favorable for the cathodic deposition of
TiO.sub.2 in comparison with that containing NO.sub.3.sup.- only.
Hence, the resultant structure of Ti.sup.4+ species oxidized from
Ti.sup.3+ by the oxidant determines the deposition rate of
TiO.sub.2.
[0050] To sum up, a titanium dioxide coating method according to
the present invention includes a cathodic deposition using an
electrolytic solution containing Ti.sup.3+, an oxidant, and at
least one of NO.sub.3.sup.- and NO.sub.2.sup.-, and a
post-deposition annealing process, which is favorable for preparing
porous A-TiO.sub.2 films. The redox reaction between Ti.sup.3+ and
oxidant to form Ti.sup.4+ prior to cathodic deposition effectively
promotes the TiO.sub.2 deposition. The resultant structure of
Ti.sup.4+ species oxidized from Ti.sup.3+ by the oxidant determines
the deposition rate of TiO.sub.2. The continuous reduction of
NO.sub.2.sup.- or NO.sub.3.sup.- to N.sub.2 and NH.sub.3 generates
extensive OH.sup.- and effectively enhances the deposition of
TiO.sub.2 for forming a TiO.sub.2 film at the substrate
surface.
[0051] The porous, anatase structure of annealed TiO.sub.2,
examined by FE-SEM, TEM, and SAED analyses is expected to be good
for the dye-sensitized solar cell (DSSC) application. In addition,
A-TiO.sub.2 may be applicable for water and air purifications,
photocatalysts, gas sensors, electrochromic devices, and so on.
[0052] While the invention is susceptible to various modifications
and alternative forms, a specific example thereof has been shown in
the drawings and is herein described in detail. It should be
understood, however, that the invention is not to be limited to the
particular form disclosed, but to the contrary, the invention is to
cover all modifications, equivalents, and alternatives falling
within the spirit and scope of the appended claims.
* * * * *