U.S. patent application number 12/481174 was filed with the patent office on 2010-12-09 for method for synthesis of titanium dioxide nanotubes using ionic liquids.
This patent application is currently assigned to UT-BATTELLE, LLC. Invention is credited to Sheng Dai, Huimin Luo, Jun Qu.
Application Number | 20100311615 12/481174 |
Document ID | / |
Family ID | 43301167 |
Filed Date | 2010-12-09 |
United States Patent
Application |
20100311615 |
Kind Code |
A1 |
Qu; Jun ; et al. |
December 9, 2010 |
METHOD FOR SYNTHESIS OF TITANIUM DIOXIDE NANOTUBES USING IONIC
LIQUIDS
Abstract
The invention is directed to a method for producing titanium
dioxide nanotubes, the method comprising anodizing titanium metal
in contact with an electrolytic medium containing an ionic liquid.
The invention is also directed to the resulting titanium dioxide
nanotubes, as well as devices incorporating the nanotubes, such as
photovoltaic devices, hydrogen generation devices, and hydrogen
detection devices.
Inventors: |
Qu; Jun; (Knoxville, TN)
; Luo; Huimin; (Knoxville, TN) ; Dai; Sheng;
(Knoxville, TN) |
Correspondence
Address: |
Scully Scott Murphy & Presser PC
400 Garden City Plaza, Suite 300
Garden City
NY
11530
US
|
Assignee: |
UT-BATTELLE, LLC
Oak Ridge
TN
|
Family ID: |
43301167 |
Appl. No.: |
12/481174 |
Filed: |
June 9, 2009 |
Current U.S.
Class: |
506/22 ; 205/322;
205/50; 428/402; 977/700; 977/840 |
Current CPC
Class: |
Y10T 428/2982 20150115;
C25B 1/55 20210101; C25D 11/26 20130101; B82Y 30/00 20130101; B82Y
40/00 20130101; C25D 3/665 20130101 |
Class at
Publication: |
506/22 ; 205/322;
205/50; 428/402; 977/700; 977/840 |
International
Class: |
C40B 40/18 20060101
C40B040/18; C25D 11/34 20060101 C25D011/34; B32B 5/16 20060101
B32B005/16 |
Goverment Interests
[0001] This invention was made with government support under
Contract Number DE-AC05-00OR22725 between the United States
Department of Energy and UT-Battelle, LLC. The U.S. government has
certain rights in this invention.
Claims
1. A method for producing titanium dioxide nanotubes, the method
comprising anodizing titanium metal in contact with an electrolytic
medium comprised of an ionic liquid.
2. The method of claim 1, wherein the ionic liquid has the formula:
##STR00011## wherein R.sup.1 and R.sup.2 are each independently a
saturated or unsaturated, straight-chained, branched, or cyclic
hydrocarbon group having at least one carbon atom, and optionally
substituted with one or more oxygen, nitrogen, and/or fluorine
atoms; and X.sup.- is a counteranion.
3. The method of claim 2, wherein R.sup.1 and R.sup.2 each contain
up to six carbon atoms.
4. The method of claim 2, wherein R.sup.1 and R.sup.2 are
straight-chained, branched, or cyclic alkyl groups.
5. The method of claim 4, wherein said alkyl groups contain up to
six carbon atoms.
6. The method of claim 1, wherein said ionic liquid contains a
fluorine-containing counteranion.
7. The method of claim 2, wherein said counteranion is a
fluorine-containing anion.
8. The method of claim 6, wherein said fluorine-containing anion is
selected from the group consisting of BF.sub.4.sup.-,
PF.sub.6.sup.-, N[SO.sub.2CF.sub.3].sub.2.sup.-,
N[SO.sub.2CF.sub.2CF.sub.3].sub.2, and CF.sub.3SO.sub.3.sup.-.
9. The method of claim 1, wherein the electrolytic medium further
comprises an amount of water.
10. The method of claim 9, wherein said amount of water is at least
about 25 wt % by total weight of ionic liquid and water.
11. The method of claim 9, wherein said amount of water is at least
about 50 wt % by total weight of ionic liquid and water.
12. The method of claim 9, wherein said amount of water is at least
about 75 wt % by total weight of ionic liquid and water.
13. The method of claim 1, wherein said titanium dioxide nanotubes
possess an outer diameter of or less than 45 nm.
14. The method of claim 1, wherein said titanium dioxide nanotubes
possess an outer diameter of or less than 40 nm.
15. The method of claim 1, wherein said titanium dioxide nanotubes
possess an outer diameter of or less than 35 nm.
16. The method of claim 1, wherein said titanium metal is in the
form of a foil.
17. A composition comprising titanium dioxide nanotubes having an
outer diameter of or less than 45 nm.
18. A composition comprising titanium dioxide nanotubes having an
outer diameter of or less than 40 nm.
19. A composition comprising titanium dioxide nanotubes having an
outer diameter of or less than 35 nm.
20. The composition of claim 17, wherein said titanium dioxide
nanotubes are in the form of an ordered array arranged
substantially perpendicular on a substrate.
21. The composition of claim 17, wherein said titanium dioxide
nanotubes possess a length-to-diameter aspect ratio of at least
100.
22. A photovoltaic device containing the titanium dioxide nanotubes
of claim 17.
23. A photovoltaic device containing the titanium dioxide nanotubes
of claim 21.
24. A hydrogen generation device containing the titanium dioxide
nanotubes of claim 17.
25. A hydrogen generation device containing the titanium dioxide
nanotubes of claim 21.
Description
FIELD OF THE INVENTION
[0002] The present invention relates generally to methods for
anodization of titanium, and more particularly, to anodization
methods for the preparation of titanium dioxide nanotubes.
BACKGROUND OF THE INVENTION
[0003] The unique properties of titanium dioxide (TiO.sub.2)
nanotubes have resulted in their use in several emerging advanced
applications, such as their use as water photoelectrolysis
catalysts (i.e., for hydrogen generation), photovoltaic components
in dye-sensitized solar cells, and as hydrogen gas sensors. There
is ongoing interest in using TiO.sub.2 nanotubes for numerous other
purposes, including as electronic components, microfluidic devices,
nanofiltration devices, drug delivery devices, photocatalytic
devices, and tissue engineering components.
[0004] TiO.sub.2 nanotubes have been prepared by several methods,
including electrochemical oxidation (i.e., anodization),
hydrothermal synthesis, and template-assisted synthesis. The
anodization method typically produces the most highly ordered
nanotube structure and most promising photovoltaic properties, as
described in, for example, C. A. Grimes, J. Mater. Chem., 17,
1451-1457 (2007). The anodization process is also favored due to
its general simplicity and high controllability. The anodization
process generally involves anodizing titanium in an electrolytic
solution containing water or a polar organic solvent (e.g.,
ethylene glycol, formamide, N-methylformamide, or
dimethylsulfoxide) having dissolved therein a fluoride-containing
electrolyte, such as HF, KF, NaF, NH.sub.4F, or a
tetraalkylammonium fluoride (e.g., Bu.sub.4NF).
[0005] However, several difficulties remain in anodization
processes of the art. For example, the anodization processes of the
art are limited in their ability to produce TiO.sub.2 nanotubes
having improved photovoltaic (PV) properties, particularly those in
which an improved PV property results from a finer outer tube
diameter (e.g., of or less than 45 nm) and/or higher aspect ratio,
e.g., a length-to-diameter aspect ratio of at least 100. In
addition, the electrolyte solutions used in anodization processes
of the art are generally limited in their attainable electrical
conductivity, thereby imposing a high energy usage during
anodization. Furthermore, the polar organic solvents of the art
(which are gaining in popularity over aqueous solutions) tend to
possess one or more unfavorable properties, such as high
volatility, flammability, and/or toxicity.
[0006] Accordingly, there is a need for a process that can produce
TiO.sub.2 nanotubes having improved photovoltaic properties,
particularly those having finer outer tube diameters (e.g., of or
less than 45 nm) and/or higher aspect ratios, e.g., a
length-to-diameter aspect ratio of at least 100. There would be a
further advantage if such a process was capable of growing the
TiO.sub.2 nanotubes at substantially high growth rates (e.g., at
least 0.2 .mu.m/hr). There is also a need in the art for a process
that can produce TiO.sub.2 nanotubes with less energy usage,
preferably by virtue of an increased electrical conductivity of the
anodization medium. There is an additional need in the art for such
a process which also does not require solvents that are volatile,
flammable, toxic, or which present an environmental liability.
SUMMARY OF THE INVENTION
[0007] In one aspect, the invention is directed to a method for
producing titanium dioxide nanotubes wherein titanium metal is
anodized while in contact with an electrolytic medium containing an
ionic liquid. The method advantageously provides the capability of
producing TiO.sub.2 nanotubes of finer outer tube diameters (e.g.,
of or less than 45 nm) and/or higher aspect ratios (e.g.,
length-to-diameter aspect ratios of at least 100) wherein the
resulting nanotubes typically possess one or more improved PV
properties. The method is also capable of growing TiO.sub.2
nanotubes at substantially high growth rates (e.g., at least 0.2
.mu.m/hr). The method is also capable of producing TiO.sub.2
nanotubes with less energy usage by virtue of the higher electrical
conductivities of ionic liquids as compared to traditional
anodization media. Moreover, in comparison to organic solvent-based
media of the art, the ionic liquid-based anodization medium of the
invention has the advantage of being generally non-volatile,
non-flammable, non-toxic, environmentally non-hazardous.
[0008] In another aspect, the invention is directed to the
TiO.sub.2 nanotubes produced by the above process. The TiO.sub.2
nanotubes produced by the above process have been generally found
to possess superior or unique photovoltaic properties. The superior
or unique photovoltaic properties may be attributed to the unique
physical and/or compositional characteristics of the produced
TiO.sub.2 nanotubes. The unique physical characteristics can
include, for example, the finer outer tube diameters and higher
aspect ratios described above. The unique compositional
characteristics can include the predominance of one phase over
another (e.g., rutile, anatase, and brookite phases), as well as
the presence or absence of a dopant.
[0009] In yet other aspects, the invention is directed to a device
containing the titanium dioxide nanotubes described above. For
example, in one particular embodiment, the device is a photovoltaic
device while in another particular embodiment the device is a
hydrogen generation device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIGS. 1A, 1B. Top view SEM micrographs at 100,000.times.
magnification (FIG. 1A) and 200,000.times. magnification (FIG. 1B)
of highly ordered TiO.sub.2 nanotubes synthesized in the ionic
liquid 1-butyl-3-methylimidazolium tetrafluoroborate
(BMIM-BF.sub.4).
[0011] FIGS. 2A, 2B. Cross-sectional SEM micrographs at
50,000.times. magnification (FIG. 2A) and 100,000.times.
magnification (FIG. 2B) of highly ordered TiO.sub.2 nanotubes
synthesized in the ionic liquid 1-butyl-3-methylimidazolium
tetrafluoroborate (BMIM-BF.sub.4).
DETAILED DESCRIPTION OF THE INVENTION
[0012] In a first aspect, the invention is directed to a method for
producing titanium dioxide nanotubes (also referred to herein as
"nanotubes"). As used herein, the term "titanium dioxide" is used
interchangeably with "titanium oxide". Either of these terms are
meant herein to correspond generally to compositions within the
chemical formula TiO.sub.2-x wherein 0.ltoreq.x.ltoreq.1. In
different embodiments, the variable x can be, for example, 0, 0.1,
0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7,
0.75, 0.8, 0.85, 0.9, 0.95, 1, or a particular range between any
two of the foregoing values. The foregoing generic formula includes
the specific composition TiO.sub.2, i.e., when x is 0. However, as
used herein, the term "TiO.sub.2" or "titanium dioxide" or
"titanium oxide" is meant to include any one or all of the
compositions within the generic formula TiO.sub.2-x, unless
otherwise specified.
[0013] The titanium metal used can have any suitable shape and can
be either pure titanium metal, a titanium alloy, or a titanium
coating on a substrate. Pure titanium can be of any suitable purity
level, but in preferred embodiments, the pure titanium has a
composition of at least 99.0, 99.9, 99.99, or 99.999% titanium.
Some examples of titanium alloys include the numerous intermetallic
and non-intermetallic titanium-aluminum alloys (e.g., Ti-6Al-4V,
TiAl (e.g., .gamma.-TiAl), TiAl.sub.3, or Ti.sub.3Al). Some
substrates onto which titanium metal may be coated onto include
other metals (e.g., aluminum-, tin-, copper-, zinc-, iron-, or
cobalt-based metals or metal alloys), metal oxides (e.g., oxides of
silicon, aluminum, tin, niobium, and rare earth metals, such as
hafnium oxide), organic polymers (e.g., conductive organic
polymers), and hybrid organic-inorganic polymers. In a particular
embodiment, the titanium metal is a coating on a conducting oxide
or doped tin oxide substrate, such as a silicon-doped tin oxide,
fluorine-doped tin oxide (FTO), or indium-doped tin oxide (ITO)
substrate. To produce a coating of the titanium on a substrate,
titanium metal is typically sputtered onto a substrate. Typically,
the titanium metal is in the form of a sheet or foil having a
suitable thickness, and preferably, a thickness of or less than 1
mm, 0.5 mm, 0.1 mm, 0.05 mm, 0.01 mm, or 0.001 mm, or a range
between any two of these values. Alternatively, the foregoing
thicknesses describe the thickness of a titanium coating on a
substrate, such as a metal- or polymer-based substrate.
[0014] The method involves anodizing titanium metal in physical
contact with an electrolytic medium (i.e., anodic medium, or
anolyte) containing an ionic liquid. As generally known in the art,
the anodization process involves application of an anodic voltage
to the titanium metal while the titanium metal is in contact with
the electrolytic medium. By being in "contact" with the
electrolytic medium is generally meant that at least a portion of
the titanium metal physically contacts the electrolytic medium
during anodization. The TiO.sub.2 nanotubes produced herein are
those grown at the (titanium metal)-(electrolytic medium)
interface. The method described herein preferably permits growth of
the TiO.sub.2 nanotubes at an efficient growth rate, i.e., a growth
rate of at least 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6, 0.7,
0.8, 0.9, 1.0, 1.2, 1.5, 1.7, or 2.0 .mu.m/hr.
[0015] The anodic voltage (anodic potential) causes the oxidation
of surface titanium metal to titanium dioxide in order that
TiO.sub.2 nanotubes are produced. In different embodiments, the
anodic voltage is preferably at least, for example, 1V, 5V, 10V,
15V, 20V, 25V, 30V, 40V, 50V, 60V, 70V, or 80V, or a range between
any two of these values. The anodic potential value can be
alternately expressed as a current density. In different
embodiments, the current density is preferably at least, for
example, 1.0.times.10.sup.-5, 5.0.times.10.sup.-5,
1.0.times.10.sup.-4, 5.0.times.10.sup.-4, 1.0.times.10.sup.-3,
5.0.times.10.sup.-3, 1.0.times.10.sup.-2 A/cm.sup.2, or a range
between any two of these values.
[0016] In one embodiment, the anodic voltage and/or current density
is substantially constant, e.g., within 1, 0.5, or 0.1V. In another
embodiment, the anodic voltage and/or current density is varied.
The anodic voltage and/or current density can be varied by either
being increased, decreased, or a combination thereof. The anodic
voltage and/or current density can be varied in a substantially
continuous or gradual manner by a set rate (e.g., 0.1, 0.5, 1, or 5
V/min); or alternatively, in an abrupt or discontinuous manner
(e.g., a sudden change of 10V to 20V); or alternatively, by a
change in a gradual rate (e.g., from 0.2 V/min to 1.5 V/min); or
alternatively, by a combination of a gradual change and an abrupt
change (e.g., 0.5 V/min increase to a voltage of 5V followed by a
sudden change to 15V). Any of the embodiments exemplified above in
which the voltage varies may also include one or more periods
wherein the voltage remains substantially constant.
[0017] The anodic voltage or current density can be applied for a
suitable period of time. Generally, a period of time is selected to
obtain a TiO.sub.2 of a particular length, wherein it is understood
in the art that longer periods of anodization time result in longer
TiO.sub.2 nanotube lengths. The higher the growth rate, the less
time is required to obtain a TiO.sub.2 nanotube of a particular
length. Therefore, depending on the TiO.sub.2 nanotube lengths
desired, in different embodiments, the anodic voltage or current
density may be applied for a time period of at least or less than,
for example, 1 minute, 2 minutes, 3 minutes, 15 minutes, 30
minutes, 1 hour, 2 hours, 3 hours, 5 hours, 10 hours, or 20 hours,
or a range between any two of these values.
[0018] The anodization process can be conducted at any suitable
temperature. Typically, the process is conducted at about room
temperature, i.e., at about 15, 20, 25, or 30.degree. C., or a
range between any two of these temperatures. In other embodiments,
the process is conducted at an elevated temperature, e.g., at about
40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, or 150.degree. C.,
or a range between any two of these temperatures. In other
embodiments, the process is conducted at a reduced temperature,
e.g., at about -60, -50, -40, -30, -20, -10, -5, 0, 5, or
10.degree. C., or a range between any two of these temperatures. In
yet other embodiments, any of the foregoing exemplary temperatures
represent a minimum temperature; or any of the foregoing exemplary
temperatures represent a maximum temperature; or the process is
conducted within a range of temperatures wherein a minimum and
maximum temperature of the range are selected from any of the
exemplary temperatures given above (for example, -60 to 40.degree.
C., -10 to 50.degree. C., or 50 to 120.degree. C.). Furthermore, in
one embodiment, the temperature is substantially constant (e.g.,
within .+-.10, 5 or 1.degree. C.), whereas in another embodiment,
the temperature is varied. The temperature can be varied by either
being increased, decreased, or a combination thereof. The
temperature can be varied in a substantially continuous or gradual
manner by a set rate (e.g., 1, 5, 10, or 15.degree. C./min); or
alternatively, in an abrupt or discontinuous manner (e.g., a change
of at least 20.degree. C. within 1 minute); or alternatively, by a
change in a gradual rate (e.g., from 2.degree. C./min to 10.degree.
C./min); or alternatively, by a combination of a gradual change and
an abrupt change (e.g., a change from 4.degree. C./min to
25.degree. C./min). Any of the embodiments exemplified above in
which the temperature varies may also include one or more periods
wherein the temperature remains substantially constant.
[0019] The anodization process can be followed by an annealing
process (i.e., a post-annealing step). An annealing process can be
useful to, for example, modify the phase of the initially produced
TiO.sub.2 nanotubes, e.g., to convert initially amorphous-phase
TiO.sub.2 nanotubes to crystalline-phase TiO.sub.2 nanotubes. More
crystalline TiO.sub.2 nanotubes typically possess an improved
charge transport property, and therefore, are generally more
suitable for dye-sensitized solar cell (DSSC) applications. The
annealing process is generally conducted at a temperature of at
least 300.degree. C., 400.degree. C., 450.degree. C., 475.degree.
C., 500.degree. C, 525.degree. C., 550.degree. C., 575.degree. C.,
600.degree. C., 650.degree. C., 675.degree. C., or 700.degree. C.,
or a range governed by any two of these values. Typically, the
annealing process is conducted under an oxygen-containing
environment. However, other environments may be used to modify the
composition. For example, an inert atmosphere environment (e.g.,
nitrogen or argon) may be used to limit the amount of oxidation.
Alternatively, for example, a carbonaceous environment (e.g.,
CO.sub.2, CO, CH.sub.4, or CH.sub.2CH.sub.2) can be used to create
carbon-modified TiO.sub.2 nanotubes.
[0020] According to the inventive method, the electrolytic medium
contains one or more ionic liquids. As understood in the art, an
ionic liquid includes a cationic component and an anionic
(counteranionic or counterion) component. In some embodiments, the
counteranion is preferably structurally symmetrical. In other
embodiments, the counteranion is preferably structurally
asymmetrical.
[0021] The counteranion (X.sup.-) of the ionic liquid is any
counteranion which, when associated with the cationic component,
permits the resulting ionic compound to behave as an ionic liquid.
As known in the art, the composition and structure of the
counteranion strongly affects the properties (e.g., melting point,
volatility, stability, viscosity, hydrophobicity, and so on) of the
ionic liquid.
[0022] In one embodiment, the counteranion of the ionic liquid is
non-carbon-containing (i.e., inorganic). The inorganic counteranion
may, in one embodiment, lack fluorine atoms. Some examples of such
counteranions include chloride, bromide, iodide,
hexachlorophosphate (PCl.sub.6.sup.-), perchlorate, chlorate,
chlorite, perbromate, bromate, bromite, periodiate, iodate,
dicyanamide (i.e., N(CN).sub.2.sup.-) aluminum chlorides (e.g.,
Al.sub.2Cl.sub.7.sup.- and AlCl.sub.4.sup.-), aluminum bromides
(e.g., AlBr.sub.4.sup.-), nitrate, nitrite, sulfate, sulfite,
phosphate, phosphite, arsenate, antimonate, selenate, tellurate,
tungstate, molybdate, chromate, silicate, the borates (e.g.,
borate, diborate, triborate, tetraborate), anionic borane and
carborane clusters (e.g., B.sub.10H.sub.10.sup.2- and
B.sub.12H.sub.12.sup.2-), perrhenate, permanganate, ruthenate,
perruthenate, and the polyoxometallates. The inorganic counteranion
may, in another embodiment, include fluorine atoms. Some examples
of such counteranions include fluoride, hexachlorophosphate
(PF.sub.6.sup.-), tetrafluoroborate, aluminum fluorides (e.g.,
AlF.sub.4.sup.-), hexafluoroarsenate (AsF.sub.6.sup.-), and
hexafluoroantimonate (SbF6.sup.-).
[0023] In another embodiment, the counteranion of the ionic liquid
is carbon-containing (i.e., organic). The organic counteranion may,
in one embodiment, lack fluorine atoms. Some examples of such
counteranions include carbonate, the carboxylates (e.g., formate,
acetate, propionate, butyrate, valerate, lactate, pyruvate,
oxalate, malonate, glutarate, adipate, decanoate, and the like),
the sulfonates (e.g., CH.sub.3SO.sub.3.sup.-,
CH.sub.3CH.sub.2SO.sub.3.sup.-,
CH.sub.3(CH.sub.2).sub.2SO.sub.3.sup.-, benzenesulfonate,
toluenesulfonate, dodecylbenzenesulfonate, and the like), the
alkoxides (e.g., methoxide, ethoxide, isopropoxide, and phenoxide),
the amides (e.g., dimethylamide and diisopropylamide), diketonates
(e.g., acetylacetonate), the organoborates (e.g.,
BR.sub.1R.sub.2R.sub.3R.sub.4.sup.-, wherein R.sup.1, R.sub.2,
R.sub.3, R.sub.4 are typically hydrocarbon groups containing 1 to 6
carbon atoms), the alkylsulfates (e.g., diethylsulfate),
alkylphosphates (e.g., ethylphosphate or diethylphosphate), and the
phosphinates (e.g., bis-(2,4,4-trimethylpentyl)phosphinate). The
organic counteranion may, in another embodiment, include fluorine
atoms. Some examples of such counteranions include the
fluorosulfonates (e.g., CF.sub.3SO.sub.3.sup.-,
CF.sub.3CF.sub.2SO.sub.3.sup.-,
CF.sub.3(CF.sub.2).sub.2SO.sub.3.sup.-,
CHF.sub.2CF.sub.2SO.sub.3.sup.-, and the like), the fluoroalkoxides
(e.g., CF.sub.3O.sup.-, CF.sub.3CH.sub.2O.sup.-,
CF.sub.3CF.sub.2O.sup.-, and pentafluorophenolate), the
fluorocarboxylates (e.g., trifluoroacetate and
pentafluoropropionate), and the fluorosulfonylimides (e.g.,
(CF.sub.3SO.sub.2).sub.2N.sup.-).
[0024] In a particular embodiment, the counteranion (X.sup.-) of
the ionic liquid has a formula within the general chemical
formula
##STR00001##
[0025] In formula (1) above, subscripts m and n are independently 0
or an integer of 1 or above. Subscript p is 0 or 1, provided that
when p is 0, the group --N--SO.sub.2--(CF.sub.2).sub.nCF.sub.3
subtended by p is replaced with an oxide atom connected to the
sulfur atom (S).
[0026] In one embodiment, subscript p is 1, so that formula (1)
reduces to the chemical formula:
##STR00002##
[0027] In one embodiment of formula (2), m and n are the same
number, thereby resulting in a symmetrical counteranion. In another
embodiment of formula (2), m and n are not the same number, thereby
resulting in an asymmetrical counteranion.
[0028] In a first set of embodiments of formula (2), m and n are
independently at least 0 and up to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
or 11. For example, in a particular embodiment, m is 0 while n is a
value of 0 or above (e.g., 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or
11). Some examples of such anions include
F.sub.3CSO.sub.2NSO.sub.2CF.sub.3(Tf.sub.2N.sup.-),
F.sub.3CSO.sub.2NSO.sub.2CF.sub.2CF.sub.3,
F.sub.3CSO.sub.2NSO.sub.2(CF.sub.2).sub.2CF.sub.3,
F.sub.3CSO.sub.2NSO.sub.2(CF.sub.2).sub.3CF.sub.3,
F.sub.3CSO.sub.2NSO.sub.2(CF.sub.2).sub.4CF.sub.3,
F.sub.3CSO.sub.2NSO.sub.2(CF.sub.2).sub.5CF.sub.3, and so on,
wherein it is understood that, in the foregoing examples, the
negative sign indicative of a negative charge (i.e., "-") in the
anion has been omitted for the sake of clarity.
[0029] In a second set of embodiments of formula (2), m and n are
independently at least 1 and up to 2, 3, 4, 5, 6, 7, 8, 9, 10, or
11. For example, in a particular embodiment, m is 1 while n is a
value of 1 or above (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11).
Some examples of such anions include
N[SO.sub.2CF.sub.2CF.sub.3].sub.2(i.e., "BETI.sup.-"),
F.sub.3CF.sub.2CSO.sub.2NSO.sub.2(CF.sub.2).sub.2CF.sub.3,
F.sub.3CF.sub.2CSO.sub.2NSO.sub.2(CF.sub.2).sub.3CF.sub.3,
F.sub.3CF.sub.2CSO.sub.2NSO.sub.2(CF.sub.2).sub.4CF.sub.3,
F.sub.3CF.sub.2CSO.sub.2NSO.sub.2(CF.sub.2).sub.5CF.sub.3, and so
on.
[0030] In a third set of embodiments of formula (2), m and n are
independently at least 2 and up to 3, 4, 5, 6, 7, 8, 9, 10, or 11.
For example, in a particular embodiment, m is 2 while n is a value
of 2 or above (e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11). Some
examples of such anions include
N[SO.sub.2(CF.sub.2).sub.2CF.sub.3].sub.2,
F.sub.3C(F.sub.2C).sub.2SO.sub.2NSO.sub.2(CF.sub.2).sub.3CF.sub.3,
F.sub.3C(F.sub.2C).sub.2SO.sub.2NSO.sub.2(CF.sub.2).sub.4CF.sub.3,
F.sub.3C(F.sub.2C).sub.2SO.sub.2NSO.sub.2(CF.sub.2).sub.5CF.sub.3,
and so on.
[0031] In a fourth set of embodiments of formula (2), m and n are
independently at least 3 and up to 4, 5, 6, 7, 8, 9, 10, or 11. For
example, in a particular embodiment, m is 3 while n is a value of 3
or above (e.g., 3, 4, 5, 6, 7, 8, 9, 10, or 11). Some examples of
such anions include N[SO.sub.2(CF.sub.2).sub.3CF.sub.3].sub.2,
F.sub.3C(F.sub.2C).sub.3SO.sub.2NSO.sub.2(CF.sub.2).sub.4CF.sub.3,
F.sub.3C(F.sub.2C).sub.3SO.sub.2NSO.sub.2(CF.sub.2).sub.5CF.sub.3,
[0032] In a fifth set of embodiments of formula (2), m and n are
independently at least 4 and up to 5, 6, 7, 8, 9, 10, or 11. For
example, in a particular embodiment, m is 4 while n is a value of 4
or above (e.g., 4, 5, 6, 7, 8, 9, 10, or 11). Some examples of such
anions include N[SO.sub.2(CF.sub.2).sub.4CF.sub.3].sub.2,
F.sub.3C(F.sub.2C).sub.4SO.sub.2NSO.sub.2(CF.sub.2).sub.5CF.sub.3,
F.sub.3C(F.sub.2C).sub.4SO.sub.2NSO.sub.2(CF.sub.2).sub.6CF.sub.3,
F.sub.3C(F.sub.2C).sub.4SO.sub.2NSO.sub.2(CF.sub.2).sub.7CF.sub.3,
F.sub.3C(F.sub.2C).sub.4SO.sub.2NSO.sub.2(CF.sub.2).sub.8CF.sub.3,
and so on.
[0033] In a sixth set of embodiments of formula (2), m and n are
independently at least 5 and up to 6, 7, 8, 9, 10, or 11. For
example, in a particular embodiment, m is 5 while n is a value of 5
or above (e.g., 5, 6, 7, 8, 9, 10, or 11). Some examples of such
anions include N[SO.sub.2(CF.sub.2).sub.5CF.sub.3].sub.2,
F.sub.3C(F.sub.2C).sub.5SO.sub.2NSO.sub.2(CF.sub.2).sub.6CF.sub.3,
F.sub.3C(F.sub.2C).sub.5SO.sub.2NSO.sub.2(CF.sub.2).sub.7CF.sub.3,
F.sub.3C(F.sub.2C).sub.5SO.sub.2NSO.sub.2(CF.sub.2).sub.8CF.sub.3,
F.sub.3C(F.sub.2C).sub.5SO.sub.2NSO.sub.2(CF.sub.2).sub.9CF.sub.3,
and so on.
[0034] In a seventh set of embodiments of formula (2), m and n are
independently at least 6 and up to 7, 8, 9, 10, or 11. For example,
in a particular embodiment, m is 6 while n is a value of 6 or above
(e.g., 6, 7, 8, 9, 10, or 11). Some examples of such anions include
N[SO.sub.2(CF.sub.2).sub.6CF.sub.3].sub.2,
F.sub.3C(F.sub.2C).sub.6SO.sub.2NSO.sub.2(CF.sub.2).sub.7CF.sub.3,
F.sub.3C(F.sub.2C).sub.6SO.sub.2NSO.sub.2(CF.sub.2).sub.8CF.sub.3,
F.sub.3C(F.sub.2C).sub.6SO.sub.2NSO.sub.2(CF.sub.2).sub.9CF.sub.3,
F.sub.3C(F.sub.2C).sub.6SO.sub.2NSO.sub.2(CF.sub.2).sub.10CF.sub.3,
and so on.
[0035] In other embodiments of formula (2), m abides by one or a
number of alternative conditions set forth in one of the foregoing
seven embodiments while n abides by one or a number of alternative
conditions set forth in another of the foregoing seven
embodiments.
[0036] In another embodiment, subscript p is 0, so that formula (1)
reduces to the chemical formula:
##STR00003##
[0037] In different exemplary embodiments of formula (3), m can be
0 or above (e.g., up to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11), 1 or
above (e.g., up to 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11), 2or above
(e.g., up to 3, 4, 5, 6, 7, 8, 9, 10, or 11), 3 or above (e.g., up
to 4, 5, 6, 7, 8, 9, 10, or 11), 4 or above (e.g., up to 5, 6, 7,
8, 9, 10, or 11), 5 or above (e.g., up to 6, 7, 8, 9, 10, or 11), 6
or above (e.g., up to 7, 8, 9, 10, or 11), 7 or above (e.g., up to
8, 9, 10, 11, or 12), 8 or above (e.g., up to 9, 10, 11, or 12), or
9 or above (e.g., up to 10, 11, 12, 13, 14, 15, or 16). Some
examples of such anions include F.sub.3CSO.sub.3.sup.-(i.e.,
"triflate" or "TfO.sup.-"), F.sub.3CF.sub.2CSO.sub.3.sup.-,
F.sub.3C(F.sub.2C).sub.2SO.sub.3.sup.-,
F.sub.3C(F.sub.2C).sub.3SO.sub.3.sup.-(i.e., "nonaflate" or
"NfO.sup.-"), F.sub.3C(F.sub.2C).sub.4SO.sub.3.sup.-,
F.sub.3C(F.sub.2C).sub.5SO.sub.3.sup.-,
F.sub.3C(F.sub.2C).sub.6SO.sub.3.sup.-,
F.sub.3C(F.sub.2C).sub.7SO.sub.3.sup.-,
F.sub.3C(F.sub.2C).sub.8SO.sub.3.sup.-,
F.sub.3C(F.sub.2C).sub.9SO.sub.3.sup.-,
F.sub.3C(F.sub.2C).sub.10SO.sub.3.sup.-,
F.sub.3C(F.sub.2C).sub.11SO.sub.3.sup.-, and so on.
[0038] The ionic liquids of the invention are generally in liquid
form (i.e., fluids) at or below 100.degree. C., more preferably at
or below 50.degree. C., and even more preferably, at or below room
temperature (i.e., at or less than about 15, 20, 25, or 30.degree.
C.). In other embodiments, the ionic liquids are in liquid form at
or below 0.degree. C., -5.degree. C., -10.degree. C., -20.degree.
C., or -30.degree. C. Preferably, the ionic liquid possesses a
melting point which is at or below any of the temperatures given
above. Though the invention primarily contemplates ionic liquids
that are naturally fluids at or below room temperature, the
invention also contemplates ionic liquids that are solid or
semi-solid at about room temperature or above, but which can be
rendered liquids at a higher temperature by the application of
heat. The latter embodiment may be particularly suitable if the
anodization process is preferably conducted at a higher temperature
(i.e., above room temperature).
[0039] The density of the ionic liquid is generally above 1.2 g/mL
at an operating temperature of interest, and particularly at a
temperature within 20-30.degree. C. In different embodiments, the
density of the ionic liquid is preferably at least 1.2, 1.3, 1.4,
1.5, 1.6, 1.7, or 1.8 g/mL, or a particular range between any two
of these values.
[0040] The viscosity of the ionic liquid is preferably no more than
50,000 centipoise (50,000 cP) at an operating temperature of
interest, and particularly at a temperature within 20-30.degree. C.
In more preferred embodiments, the viscosity of the ionic liquid is
no more than about 25,000 cP, 10,000 cP, 5,000 cP, 2,000 cP, 1,000
cP, 800 cP, 700 cP, 600 cP, 500 cP, 400 cP, 300 cP, 200 cP, 100 cP,
or 50 cP. Alternatively, the viscosity of the ionic liquid may
preferably be within a particular range established between any two
of the foregoing exemplary values.
[0041] The conductivity of the ionic liquid is preferably at least
0.01 mS/cm (0.001 S/m) at an operating temperature of interest, and
particularly at a temperature within 20-30.degree. C. In different
embodiments, the conductivity of the ionic liquid may preferably be
at least 0.01, 0.05, 0.1, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0,
8.0, 9.0, 10.0, 11.0. or 12.0 mS/cm, or a particular range between
any two of the foregoing values.
[0042] In one embodiment, the ionic liquid is an imidazolium-based
ionic liquid having a formula within the general formula:
##STR00004##
[0043] In formula (4) above, R.sup.1 and R.sup.2 are each
independently a saturated or unsaturated, straight-chained,
branched, or cyclic hydrocarbon group having at least one carbon
atom, and X.sup.- is a counteranion, as described above. In one
embodiment, R.sup.1 and R.sup.2 are different in structure or
number of carbon atoms, whereas in another embodiment, R.sup.1 and
R.sup.2 are the same either in structure or number of carbon atoms.
In different embodiments, R.sup.1 and R.sup.2 each independently
have a minimum of at least one, two, three, four, five, six, seven,
or eight carbon atoms. In other embodiments, R.sup.1 and R.sup.2
each independently have a maximum of two, three, four, five, six,
seven, eight, nine, ten, eleven, twelve, thirteen, fourteen,
fifteen, sixteen, seventeen, or eighteen carbon atoms. In other
embodiments, R.sup.1 and R.sup.2 independently have a number of
carbon atoms within a range of carbon atoms established by a
combination of any of the exemplary minimum and maximum carbon
numbers given above.
[0044] In a first embodiment, one or both of R.sup.1 and R.sup.2
are saturated and straight-chained hydrocarbon groups (i.e.,
straight-chained alkyl groups). Some examples of straight-chained
alkyl groups include methyl, ethyl, n-propyl, n-butyl, n-pentyl,
n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl,
n-tridecyl, n-tetradecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl,
and n-octadecyl groups.
[0045] In a second embodiment, one or both of R.sup.1 and R.sup.2
are saturated and branched hydrocarbon groups (i.e., branched alkyl
groups). Some examples of branched alkyl groups include isopropyl,
isobutyl, sec-butyl, t-butyl, isopentyl, neopentyl, 2-methylpentyl,
3-methylpentyl, and the numerous C.sub.7, C.sub.8, C.sub.9,
C.sub.10, C.sub.11, C.sub.12, C.sub.13, C.sub.14, C.sub.15,
C.sub.16, C.sub.17, and C.sub.18 saturated and branched hydrocarbon
groups.
[0046] In a third embodiment, one or both of R.sup.1 and R.sup.2
are saturated and cyclic hydrocarbon groups (i.e., cycloalkyl
groups). Some examples of cycloalkyl groups include cyclopropyl,
cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, and
their methyl-, ethyl-, and propyl-substituted derivatives and/or
their methylene, dimethylene, trimethylene, and tetramethylene
cross-linked derivatives (as crosslinked to a nitrogen atom of the
imidazolium ring). The cycloalkyl group can also be a polycyclic
(e.g., bicyclic) group by either possessing a bond between two of
the ring groups (e.g., dicyclohexyl) or a shared (i.e., fused) side
(e.g., decalin and norbornane).
[0047] In a fourth embodiment, one or both of R.sup.1 and R.sup.2
are unsaturated and straight-chained hydrocarbon groups (i.e.,
straight-chained olefinic or alkenyl groups). Some examples of
straight-chained olefinic groups include vinyl, 2-propen-1-yl,
3-buten-1-yl, 2-buten-1-yl, butadienyl, 4-penten-1-yl,
3-penten-1-yl, 2-penten-1-yl, 2,4-pentadien-1-yl, 5-hexen-1-yl,
4-hexen-1-yl, 3-hexen-1-yl, 3,5-hexadien-1-yl,
1,3,5-hexatrien-1-yl, 6-hepten-1-yl, ethynyl, propargyl, and the
numerous C.sub.7, C.sub.8, C.sub.9, C.sub.10, C.sub.11, C.sub.12,
C.sub.13, C.sub.14, C.sub.15, C.sub.16, C.sub.17, and C.sub.18
unsaturated and straight-chained hydrocarbon groups.
[0048] In a fifth embodiment, one or both of R.sup.1 and R.sup.2
are unsaturated and branched hydrocarbon groups (i.e., branched
olefinic or alkenyl groups). Some branched olefinic groups include
2-propen-2-yl, 3-buten-2-yl, 3-buten-3-yl, 4-penten-2-yl,
4-penten-3-yl, 3-penten-2-yl, 3-penten-3-yl, 2,4-pentadien-3-yl,
and the numerous C.sub.6, C.sub.7, C.sub.8, C.sub.9, C.sub.10,
C.sub.11, C.sub.12, C.sub.13, C.sub.14, C.sub.15, C.sub.16,
C.sub.17, and C.sub.18 unsaturated and branched hydrocarbon
groups.
[0049] In a sixth embodiment, one or both of R.sup.1 and R.sup.2
are unsaturated and cyclic hydrocarbon groups. Some examples of
unsaturated and cyclic hydrocarbon groups include cyclopropenyl,
cyclobutenyl, cyclopentenyl, cyclopentadienyl, cyclohexenyl,
cyclohexadienyl, phenyl, benzyl, cycloheptenyl, cycloheptadienyl,
cyclooctenyl, cyclooctadienyl, cyclooctatetraenyl, and their
methyl-, ethyl-, and propyl-substituted derivatives and/or their
methylene, dimethylene, trimethylene, and tetramethylene
cross-linked derivatives (as crosslinked to a nitrogen atom of the
imidazolium ring). The unsaturated cyclic hydrocarbon group can
also be a polycyclic (e.g., bicyclic) group by either possessing a
bond between two of the ring groups (e.g., biphenyl) or a shared
(i.e., fused) side (e.g., naphthalene, anthracene, and
phenanthrene).
[0050] In one embodiment, as in the examples above, one or more of
the hydrocarbon groups are composed solely of carbon and hydrogen,
i.e., do not include one or more heteroatoms, such as oxygen or
nitrogen atoms. In another embodiment, one or more of the
hydrocarbon groups include one or more heteroatoms, such as one or
more oxygen, nitrogen, and/or fluorine atoms. Some examples of
oxygen-containing hydrocarbon groups include those possessing one
or more hydroxyl (OH) groups, carbonyl groups (e.g., ketone, ester,
amide, or urea functionalities), and/or carbon-oxygen-carbon
(ether) groups. In a particular embodiment, the oxygen-containing
hydrocarbon group includes two or more ether groups, such as a
polyalkyleneoxide group, such as a polyethyleneoxide group. Some
examples of nitrogen-containing hydrocarbon groups include those
possessing one or more primary amine groups, secondary amine
groups, tertiary amine groups, and/or quaternary amine groups,
wherein it is understood that a quaternary amine group necessarily
possesses a positive charge and requires a counteranion. Some
examples of fluorine-containing hydrocarbon groups (i.e.,
fluorocarbon groups) include the partially-substituted varieties
(e. g., fluoromethyl, difluoromethyl, 2-fluoroethyl,
2,2-difluoroethyl, 2,2,2-trifluoroethyl, and the like) and
perfluoro-substituted varieties (e.g., perfluoromethyl,
perfluoroethyl, perfluoropropyl, perfluorobutyl, and the like).
[0051] The ionic liquids according to formula (4) can contain any
of the above-described imidazolium-based cationic components
associated (i.e., complexed) with any of the above-described
counteranions X.sup.-. Some examples of imidazolium-based ionic
liquids according to formula (4) include
[1,3-dimethylimidazolium].sup.+[X].sup.-,
[1-methyl-3-ethylimidazolium].sup.+[X].sup.-,
[1-methyl-3-n-propylimidazolium].sup.+[X].sup.-,
[1-methyl-3-isopropylimidazolium].sup.+[X].sup.-,
[1-methyl-3-n-butylimidazolium].sup.+[X].sup.-(i.e.,
BMIM.sup.+X.sup.-),
[1-methyl-3-isobutylimidazolium].sup.+[X].sup.-,
[1-methyl-3-sec-butylimidazolium].sup.+[X].sup.-,
[1-methyl-3-t-butylimidazolium].sup.+[X].sup.-,
[1,3-diethylimidazolium].sup.+[X].sup.-,
[1-ethyl-3-n-propylimidazolium].sup.+[X].sup.-,
[1-ethyl-3-isobutylimidazolium].sup.+[X].sup.-,
[1-ethyl-3-sec-butylimidazolium].sup.+[X].sup.-,
[1-ethyl-3-t-butylimidazolium].sup.-[X].sup.-,
[1,3-di-n-propylimidazolium].sup.+[X].sup.-,
[1-n-propyl-3-isopropylimidazolium].sup.+[X].sup.-,
[1-n-propyl-3-n-butylimidazolium].sup.+[X].sup.-,
[1-n-propyl-3-isobutylimidazolium].sup.+[X].sup.-,
[1-n-propyl-3-sec-butylimidazolium].sup.+[X].sup.-,
[1-n-propyl-3-t-butylimidazolium].sup.+[X].sup.-,
[1,3-diisopropylimidazolium].sup.+[X].sup.-,
[1-isopropyl-3-n-butylimidazolium].sup.+[X].sup.-,
[1-isopropyl-3-isobutylimidazolium].sup.+[X].sup.-,
[1-isopropyl-3-sec-butylimidazolium].sup.+[X].sup.-,
[1-isopropyl-3-t-butylimidazolium].sup.+[X].sup.-,
[1,3-di-n-butylimidazolium].sup.+[X].sup.-,
[1-n-butyl-3-isobutylimidazolium].sup.+[X].sup.-,
[1-n-butyl-3-sec-butylimidazolium].sup.+[X].sup.-,
[1-n-butyl-3-t-butylimidazolium].sup.+[X].sup.-,
[1,3-diisobutylimidazolium].sup.+[X].sup.-,
[1-isobutyl-3-sec-butylimidazolium].sup.+[X].sup.-,
[1-isobutyl-3-t-butylimidazolium].sup.+[X].sup.-,
[1,3-di-sec-butylimidazolium].sup.+[X].sup.-,
[1-sec-butyl-3-t-butylimidazolium].sup.+[X].sup.-,
[1,3-di-t-butylimidazolium].sup.+[X].sup.-,
[1-methyl-3-pentylimidazolium].sup.+[X].sup.-,
[1-methyl-3-hexylimidazolium].sup.+[X].sup.-,
[1-methyl-3-heplylimidazolium].sup.+[X].sup.-,
[1-methyl-3-octylimidazolium].sup.+[X].sup.-,
[1-methyl-3-decylimidazolium].sup.+[X].sup.-,
[1-methyl-3-dodecylimidazolium].sup.+[X].sup.-,
[1-methyl-3-tetradecylimidazolium].sup.+[X].sup.-,
[1-methyl-3-hexadecylimidazolium].sup.+[X].sup.-,
[1-methyl-3-octadecylimidazolium].sup.+[X].sup.-,
[1-(2-hydroxyethyl)-3-methylimidazolium].sup.+[X].sup.-,
[1-allyl-3-methylimidazolium].sup.+[X].sup.-, wherein [X].sup.- can
be any of the counteranions as described above. In a preferred
embodiment, the counteranion [X].sup.- is selected from one or more
of PF.sub.6.sup.-, BF.sub.4.sup.-, (CF.sub.3SO.sub.2).sub.2N.sup.-,
CF.sub.3SO.sub.3.sup.-, an organoborate (e.g.,
BR.sub.1BR.sub.2R.sub.3R.sub.4).sup.-, CH.sub.3CO.sub.2.sup.-,
CF.sub.3CO.sub.2.sup.-, NO.sub.3.sup.-, Br.sup.-, Cl.sup.-,
I.sup.-, Al.sub.2Cl.sub.7.sup.-, and AlCl.sub.4.sup.-. Particularly
preferred counteranions are those having a formula within the
formulas (1), (2), or (3).
[0052] In formula (4), any one or more of the hydrogen atoms of the
ring carbon atoms can be substituted with a hydrocarbon group. In a
particular embodiment, the 2-position of the imidazole ring is
substituted with a methyl group. Some examples of such ionic
liquids include [1-butyl-2,3-dimethylimidazolium].sup.+[X].sup.-
and [1-octyl-2,3-dimethylimidazolium].sup.+[X].sup.-.
[0053] In another embodiment, the ionic liquid is an
N-alkylpyridinium-based ionic liquid having a formula within the
general formula:
##STR00005##
[0054] In formula (5), R.sup.1 represents a hydrocarbon group (with
or without heteroatom substitution), such as any of the hydrocarbon
groups described above for R.sup.1 and R.sup.2 of formula (4), and
the counteranion X.sup.- can be any of the counteranions described
above. Some examples of N-alkylpyridinium-based ionic liquids
include N-methylpyridinium X.sup.-, N-ethylpyridinium X.sup.-,
N-n-propylpyridinium X.sup.-, N-isopropylpyridinium X.sup.-,
N-n-butylpyridinium X.sup.-, N-isobutylpyridinium X.sup.-,
N-sec-butylpyridinium X.sup.-, N-t-butylpyridinium X.sup.-,
N-n-pentylpyridinium X.sup.-, N-isopentylpyridinium X.sup.-,
N-neopentylpyridinium X.sup.-, N-n-hexylpyridinium X.sup.-,
N-n-heptylpyridinium X.sup.-, N-n-octylpyridinium X.sup.-,
N-n-nonylpyridinium X.sup.-, N-n-decylpyridinium X.sup.-,
N-n-undecylpyridinium X.sup.-, N-n-dodecylpyridinium X.sup.-,
N-n-tridecylpyridinium X.sup.-, N-n-tetradecylpyridinium X.sup.-,
N-n-pentadecylpyridinium X.sup.-, N-n-hexadecylpyridinium X.sup.-,
N-n-heptadecylpyridinium X.sup.-, N-n-octadecylpyridinium X.sup.-,
N-vinylpyridinium X.sup.-, N-allylpyridinium X.sup.-,
N-phenylpyridimium X.sup.-, N-(2-hydroxyethyl)pyridinium X.sup.-,
N-benzylpyridinium X.sup.-, and N-phenethylpyridinium X.sup.-,
wherein X.sup.- can be any of the counteranions described above,
including the preferred counteranions.
[0055] In formula (5), any one or more of the hydrogen atoms of the
ring carbon atoms can be substituted with a hydrocarbon group. Some
examples of such ionic liquids include N-methyl-4-methylpyridinium
X.sup.-, N-ethyl-4-methylpyridinium X.sup.-,
N-methyl-4-ethylpyridinium X.sup.-, and N-octyl-4-methylpyridinium
X.sup.-.
[0056] In another embodiment, the ionic liquid is an ammonium-based
ionic liquid having a formula within the general formula:
##STR00006##
[0057] In formula (6), R.sup.1, R.sup.2, R.sup.3, and R.sup.4
independently represent a hydrocarbon group (with or without
heteroatom substitution), such as those described above for R.sup.1
and R.sup.2 of formula (4), or a hydrogen atom, wherein at least
one of R.sup.1, R.sup.2, R.sup.3, and R.sup.4 represents a
hydrocarbon group (with or without heteroatom substitution), and
the counteranion X.sup.- can be any of the counteranions described
above, including the preferred counteranions. In one embodiment,
one of R.sup.1, R.sup.2, R.sup.3, and R.sup.4 is a hydrocarbon
group while the rest are hydrogen atoms. In another embodiment, two
of R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are hydrocarbon groups
while two are hydrogen atoms. In another embodiment, three of
R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are hydrocarbon groups while
one is a hydrogen atom. In another embodiment, all of R.sup.1,
R.sup.2, R.sup.3, and R.sup.4 are hydrocarbon groups. Some examples
of ammonium-based ionic liquids include
methylammonium.sup.+X.sup.-, dimethylammonium.sup.+X.sup.-,
trimethylammonium.sup.+X.sup.-, tetramethylammornum.sup.+X.sup.-,
ethylammonium.sup.+X.sup.-, ethyltrimethylammonium.sup.+X.sup.-,
diethylammonium.sup.+X.sup.-, triethylammonium.sup.+X.sup.-,
tetraethylammonium.sup.+X.sup.-, n-propylammonium.sup.+X.sup.-,
n-propyltrimethylammonium.sup.+X.sup.-,
isopropylammonium.sup.+X.sup.-, n-butylammonium.sup.+X.sup.-,
n-butyltrimethylammonium.sup.+X.sup.-,
n-butylmethylammonium.sup.+X.sup.-,
di-(n-butyl)dimethylammonium.sup.+X.sup.-,
tri-(n-butyl)methylammonium.sup.+X.sup.-,
n-pentylammonium.sup.+X.sup.-,
n-pentyltrimethylammonmum.sup.+X.sup.-,
tri-(n-pentyl)methylammonium.sup.+X.sup.-,
n-hexylammonium.sup.+X.sup.-,
n-hexyltrimethylammonium.sup.+X.sup.-,
tri-(n-hexyl)methylammonium.sup.+X.sup.-,
n-heptylammonium.sup.+X.sup.-,
n-heptyltrimethylammonium.sup.+X.sup.-,
tri-(n-heptyl)methylammonium.sup.+X.sup.-,
n-octylammonium.sup.+X.sup.-,
n-octyltrimethylammonium.sup.+X.sup.-,
tri-(n-octyl)methylammonium.sup.+X.sup.-, choline.sup.+X.sup.-,
2-hydroxyethylammonium.sup.+X.sup.-, allylammonium.sup.+X.sup.-,
allytrimethylammonium.sup.+X.sup.-,
[(2-methacryloxy)ethyl]trimethylammonium.sup.+X.sup.-, and
(4-vinylbenzyl)trimethylammonium.sup.+X -, wherein X.sup.- can be
any of the counteranions as described above. In a preferred
embodiment, the counteranion X.sup.- is selected from one or more
of PF.sub.6.sup.-, BF.sub.4.sup.-, (CF.sub.3SO.sub.2).sub.2N.sup.-,
CF.sub.3SO.sub.3.sup.-, an organoborate (e.g.,
BR.sub.1BR.sub.2R.sub.2R.sub.3R.sub.4).sup.-,
CH.sub.3CO.sub.2.sup.-, HCO.sub.2.sup.-, CF.sub.3CO.sub.2.sup.-,
NO.sub.3.sup.-, H.sub.2PO.sub.4.sup.-, Br.sup.-, Cl.sup.-, I.sup.-,
Al.sub.2Cl.sub.7.sup.-, and AlCl.sub.4.sup.-. Particularly
preferred counteranions are those having a formula within the
formulas (1), (2), or (3).
[0058] In another embodiment, the ionic liquid is a
phosphonium-based ionic liquid having a formula within the general
formula:
##STR00007##
[0059] In formula (7), R.sup.1, R.sup.2, R.sup.3, and R.sub.4
independently represent a hydrocarbon group (with or without
heteroatom substitution), such as those described above for R.sup.1
and R.sup.2 of formula (4), and the counteranion X.sup.- can be any
of the counteranions described above, including the preferred
counteranions. Some examples of phosphonium-based ionic liquids
include butyltrimethylphosphonium.sup.+X.sup.-,
dibutyldimethylphosphonium.sup.+X.sup.-,
tributylmethylphosphonium.sup.+X.sup.-,
butyltriethylphosphonium.sup.+X.sup.-,
dibutyldiethylphosphonium.sup.+X.sup.-,
tributylethylphosphonium.sup.+X.sup.-,
tetrabutylphosphonium.sup.+X.sup.-,
triisobutylmethylphosphonium.sup.+X.sup.-,
tributylhexylphosphonium.sup.+X.sup.-,
tributylheptylphosphonium.sup.+X.sup.-,
tributyloctylphosphonium.sup.+X.sup.-,
tributyldecylphosphonium.sup.+X.sup.-,
tributyldodecylphosphonium.sup.+X.sup.-,
tributyltetradecylphosphonium.sup.+X.sup.-,
tributylhexadecylphosphonium.sup.+X.sup.-,
hexyltrimethylphosphonium.sup.+X.sup.-,
dihexyldimethylphosphonium.sup.+X.sup.-,
trihexylmethylphosphonium.sup.+X.sup.-,
hexyltriethylphosphonium.sup.+X.sup.-,
tetrahexylphosphonium.sup.+X.sup.-,
trihexyloctylphosphonium.sup.+X.sup.-,
trihexyldecylphosphonium.sup.+X.sup.-,
trihexyldodecylphosphonium.sup.+X.sup.-,
trihexyltetradecylphosphonium.sup.+X.sup.-,
trihexylhexadecylphosphonium.sup.+X.sup.-,
octyltrimethylphosphonium.sup.+X.sup.-,
dioctyldimethylphosphonium.sup.+X.sup.-,
trioctylmethylphosphonium.sup.+X.sup.-,
octyltriethylphosphonium.sup.+X.sup.-,
tetraoctylphosphonium.sup.+X.sup.-,
trioctyldecylphosphonium.sup.+X.sup.-,
trioctyldodecylphosphonium.sup.+X.sup.-,
trioctyltetradecylphosphonium.sup.+X.sup.-,
trioctylhexadecylphosphonium.sup.+X.sup.-,
tridecylmethylphosphonium.sup.+X.sup.-,
tetradecylphosphonium.sup.+X.sup.-, wherein X.sup.- can be any of
the counteranions described above, including the preferred
counteranions.
[0060] In another embodiment, the ionic liquid is a
piperidinium-based ionic liquid having a formula within the general
formula:
##STR00008##
[0061] In formula (8), R.sup.1 and R.sup.2 independently represent
a hydrocarbon group (with or without heteroatom substitution), such
as those described above for R.sup.1 and R.sup.2 of formula (4),
and the counteranion X.sup.- can be any of the counteranions
described above, including the preferred counteranions. Some
examples of piperidinium-based ionic liquids include
1,1-dimethylpiperidinium.sup.+X.sup.-,
1-methyl-1-ethylpiperidinium.sup.+X.sup.-,
1-methyl-1-propylpiperidinium.sup.+X.sup.-,
1-methyl-1-butylpiperidinium.sup.+X.sup.-,
1-methyl-1-isobutylpiperidinium.sup.+X.sup.-,
1-methyl-1-pentylpiperidinium.sup.+X.sup.-,
1-methyl-1-hexylpiperidinium.sup.+X.sup.-,
1-methyl-1-heptylpiperidinium.sup.+X.sup.-,
1-methyl-1-octylpiperidinium.sup.+X.sup.-,
1-methyl-1-decylpiperidinium.sup.+X.sup.-,
1-methyl-1-dodecylpiperidinium.sup.+X.sup.-,
1-methyl-1-tetradecylpiperidinium.sup.+X.sup.-,
1-methyl-1-hexadecylpiperidinium.sup.+X.sup.-,
1-methyl-1-octadecylpiperidinium.sup.+X.sup.-,
1,1-diethylpiperidinium.sup.+X.sup.-,
1,1-dipropylpiperidinium.sup.+X.sup.-,
1,1-dibutylpiperidinium.sup.+X.sup.-, and
1,1-diisobutylpiperidinium.sup.+X.sup.-, wherein X.sup.- can be any
of the counteranions described above, including the preferred
counteranions.
[0062] In another embodimnent, the ionic liquid is a
pyrrolidinium-based ionic liquid having a formula within the
general formula:
##STR00009##
[0063] In formula (9), R.sup.1 and R.sup.2 independently represent
a hydrocarbon group (with or without heteroatom substitution), such
as those described above for R.sup.1 and R.sup.2 of formula (4),
and the counteranion X.sup.- can be any of the counteranions
described above, including the preferred counteranions. Some
examples of pyrrolidinium-based ionic liquids include
1,1-dimethylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-ethylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-propylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-butylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-isobutylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-pentylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-hexylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-heptylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-octylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-decylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-dodecylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-tetradecylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-hexadecylpyrrolidinium.sup.+X.sup.-,
1-methyl-1-octadecylpyrrolidinium.sup.+X.sup.-,
1,1-diethylpyrrolidinium.sup.+X.sup.-,
1,1-dipropylpyrrolidinium.sup.+X.sup.-,
1,1-dibutylpyrrolidinium.sup.+X.sup.-, and
1,1-diisobutylpyrrolidinium.sup.+X.sup.-, wherein X.sup.- can be
any of the counteranions described above, including the preferred
counteranions.
[0064] In another embodiment, the ionic liquid is a sulfonium-based
ionic liquid having a formula within the general formula:
##STR00010##
[0065] In formula (10), R.sup.1, R.sup.2, and R.sup.3 independently
represent a hydrocarbon group (with or without heteroatom
substitution), such as those described above for R.sup.1 and
R.sup.2 of formula (4), and the counteranion X.sup.- can be any of
the counteranions described above, including the preferred
counteranions. Some examples of sulfonium-based ionic liquids
include trimethylsulfonium.sup.+X.sup.-,
dimethylethylsulfonium.sup.+X.sup.-,
diethylmethylsulfonium.sup.+X.sup.-,
triethylsulfonium.sup.+X.sup.-,
dimethylpropylsulfonium.sup.+X.sup.-,
dipropylmethylsulfonium.sup.+X.sup.-,
tripropylsulfonium.sup.+X.sup.-,
dimethylbutylsulfonium.sup.+X.sup.-,
dibutylmethylsulfonium.sup.+X.sup.-,
tributylsulfonium.sup.+X.sup.-,
dimethylhexylsulfonium.sup.+X.sup.-,
dihexylmethylsulfonium.sup.+X.sup.-,
trihexylsulfonium.sup.+X.sup.-,
dimethyloctylsulfonium.sup.+X.sup.-,
dioctylmethylsulfonium.sup.+X.sup.-, and
trioctylsulfonium.sup.+X.sup.-, wherein X.sup.- can be any of the
counteranions described above, including the preferred
counteranions.
[0066] The ionic liquid can also be composed of several other types
of cationic components, such as the many nitrogen- or
sulfur-containing ring systems not mentioned above. Some of these
other ring systems include, for example, cationic derivatives of
piperazine, pyrazine, pyrrole, thiophene, thiazine, phenothiazine,
morpholine, 1,4-thioxane, and 1,4-dithiane ring systems, all of
which are contemplated herein as cationic components of an ionic
liquid.
[0067] The ionic liquid can be of any suitable purity level.
Preferably, the ionic liquid has a purity at least or greater than
95%, 96%, 97%, 98%, 99%, 99.5%, or 99.9%. The ionic liquid is
preferably substantially devoid of salt byproducts (e.g.,
LiNO.sub.3) that are typically produced during synthesis of the
ionic liquid. In preferred embodiments, it is desirable that the
ionic liquid contains less than 1% by weight of salt byproducts,
and more preferably, less than 0.5%, 0.1%, 0.01%, or even 0.001% by
weight of salt byproducts.
[0068] In one embodiment, the electrolytic medium contains solely
one or a combination of ionic liquids. For example, the
electrolytic medium can be substantially devoid of any non-ionic
liquid compounds or materials, such as any solvents and electrolyte
salts.
[0069] In another embodiment, the electrolytic medium contains one
or more ionic liquids in admixture with one or more non-ionic
liquids. The one or more non-ionic liquids may function as, for
example, a solvent for the ionic liquid or other admixed compounds,
or as a reactive material which is consumed in the preparation of
the TiO.sub.2 nanotubes, or as a property modifier of the
electrolytic medium (e.g., to adjust viscosity or volatility), or
as a process modifier.
[0070] In one embodiment, the one or more non-ionic liquids include
a polar protic liquid. Some examples of polar protic non-ionic
liquids include water, the alcohols (e.g., methanol, ethanol,
isopropanol, n-butanol, t-butanol, the pentanols, hexanols,
octanols, or the like), diols (e.g., ethylene glycol, diethylene
glycol, triethylene glycol), and protic amines (e.g.,
ethylenediamine, ethanolamine, diethanolamine, and
triethanolamine).
[0071] In another embodiment, the one or more non-ionic liquids
include a polar non-protic liquid. Some examples of polar
non-protic non-ionic liquids include the nitriles (e.g.,
acetonitrile, propionitrile), sulfoxides (e.g., dimethylsulfoxide),
amides (e.g., dimethylformamide, N,N-dimethylacetamide),
organochlorides (e.g., methylene chloride, chloroform,
1,1,-trichloroethane), ketones (e.g., acetone, 2-butanone),
dialkylcarbonates (e.g., ethylene carbonate, dimethylcarbonate,
diethylcarbonate), organoethers (e.g., diethyl ether,
tetrahydrofuran, and dioxane), HMPA, NMP, and DMPU.
[0072] In yet another embodiment, the one or more non-ionic liquids
include a non-polar liquid. Some examples of non-polar liquids
include the liquid hydrocarbons, such as a pentane, hexane,
beptane, octane, pentene, hexene, heptene, octene, benzene,
toluene, or xylene.
[0073] In different embodiments, the non-ionic liquid may be
included in an amount of, or at least, or less than, for example,
0.1 wt %, 0.5 wt %, 10 wt %, 20 wt %, 30 wt %, 40 wt %, 50 wt %, 60
wt %, 70 wt %, 80 wt %, 90 wt % by total weight of the ionic liquid
and non-ionic liquid. The non-ionic liquid may also be preferably
present within a range established by any two of the foregoing
values.
[0074] In some embodiments, one or more of any of the foregoing
classes of non-ionic liquids, or specific non-ionic liquids, is
excluded. For example, in some embodiments, it may be preferable to
exclude non-ionic liquids having a boiling point over 25.degree.
C., 50.degree. C., or 100.degree. C. In other embodiments, it may
be preferable to exclude non-ionic liquids having a boiling point
under 25.degree. C., 50.degree. C., or 100.degree. C. In yet other
embodiments, it may preferable to include a non-ionic liquid in
which the ionic liquid is substantially soluble, or partially
soluble, or substantially insoluble (e.g., as separate phases). In
a particular embodiment, non-ionic liquids are not present, i.e.,
excluded.
[0075] In one embodiment, the electrolytic medium (i.e., ionic
liquid therein) is substantially devoid of water. By being
"substantially devoid" of water is meant that the ionic liquid
contains less than 0.5%, 0.1%, or 0.01% by weight of water (i.e.,
wt %) with respect to the total weight of ionic liquid and water.
In a particular embodiment, the ionic liquid is completely devoid
of water (i.e., dry) such that any water present is less than 0.001
wt %, or undetectable.
[0076] In another embodiment, the ionic liquid contains an amount
of water. In different embodiments, the amount of water can
preferably be at, less than, or at least 1 wt %, 2 wt %, 5 wt %, 10
wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt %, 35 wt %, 40 wt %, 45 wt
%, 50 wt %, 55 wt %, 60 wt %, 65 wt %, 70 wt %, 75 wt %, 80 wt %,
85 wt %, 90 wt %, 95 wt %, or 97 wt % of water by total weight of
the ionic liquid and water. The water may also be preferably
present within a range established by any two of the foregoing
values.
[0077] In one embodiment, the ionic liquid is admixed with one or
more electrolyte salts or acids. The acid can be a strong acid
(e.g., the mineral acids) or weak acid (e.g., organoacid, such as a
carboxylic or phosphoric acid). Some examples of electrolyte salts
or acids include the fluoride-containing electrolytes (e.g., HF,
KF, LiF, NaF, NH.sub.4F, or a tetraalkyammonium fluoride, such as
Bu.sub.4NF), chloride-containing electrolytes (e.g., HCl, KCl,
NaCl, NH.sub.4Cl, or a tetraalklyammonium chloride, such as
Et.sub.4NCl), bromide-containing salts (e.g., HBr, KBr, NaBr,
NH.sub.4Br, or a tetraalklyammonium bromide, such as Bu.sub.4NBr),
nitrate salts (e.g., LiNO.sub.3, NaNO.sub.3, KNO.sub.3, and
Mg(NO.sub.3).sub.2), nitrite salts, phosphate salts, phosphite
salts, alkylphosphate salts, phosphinate salts, sulfate salts,
alkylsulfate salts, carboxylate salts (e.g., sodium glycolate,
sodium acetate, sodium propionate, potassium oxalate), carbonate
salts, bicarbonate salts, perchlorate salts, chlorate salts,
perbromate salts, and bromate salts.
[0078] The electrolytic medium may also include one or more
surfactants. The surfactants can be included to, for example,
modify or adjust the growth characteristics of the TiO.sub.2
nanotubes during growth.
[0079] In one embodiment, the one or more surfactants includes an
ionic surfactant, which can be either an anionic, cationic, or
zwitterionic surfactant. Some examples of anionic surfactants
include the fluorinated and non-fluorinated carboxylates (e.g.,
perfluorooctanoates, perfluorodecanoates, perfluorotetradecanoates,
octanoates, decanoates, tetradecanoates, fatty acid salts), the
fluorinated and non-fluorinated sulfonates (e.g.,
perfluorooctanesulfonates, perfluorodecanesulfonates,
octanesulfonates, decanesulfonates, alkyl benzene sulfonate), the
fluorinated and non-fluorinated sulfate salts (e.g., dodecyl
sulfates, lauryl sulfates, sodium lauryl ether sulfate,
perfluorododecyl sulfate, and other alkyl and perfluoroalkyl
sulfate salts). The majority of cationic surfactants contain a
positively charged nitrogen atom, such as found in the quaternary
ammonium surfactants, e.g., the alkyltrimethylammonium salts
wherein the alkyl group typically possesses at least four carbon
atoms and up to 14, 16, 18, 20, 22, 24, or 26 carbon atoms. Some
examples of cationic surfactants include the quaternary ammonium
surfactants (e.g., cetyl trimethylammonium bromide, benzalkonium
chloride, and benzethonium chloride), the pyridinium surfactants
(e.g., cetylpyridinium chloride), and the polyethoxylated amine
surfactants (e.g., polyethoxylated tallow amine). Some examples of
zwitterionic surfactants include the betaines (e.g., dodecyl
betaine, cocamidopropyl betaine) and the glycinates. Some examples
of non-ionic surfactants include the alkyl polyethyleneoxides,
alkylphenol polyethyleneoxides, copolymers of polyethyleneoxide and
polypropyleneoxide (e.g., poloxamers and poloxamines), alkyl
polyglucosides (e.g., octyl glucoside, decyl maltoside), fatty
alcohols, (e.g., cetyl alcohol, oleyl alcohol), fatty amides (e.g.,
cocamide MEA, cocamide DEA), and polysorbates (e.g., polysorbate
20, polysorbate 40, polysorbate 60, polysorbate 80).
[0080] In another aspect, the invention is directed to TiO.sub.2
nanotubes produced according to the method described above. The
produced nanotubes can possess any suitable phase of TiO.sub.2. In
some embodiments, the TiO.sub.2 nanotubes are predominantly of a
particular phase, such as anatase, rutile, or brookite. The phase
can also be described in terms of a crystallographic type of
lattice, such as an orthorhombic or tetragonal lattice. In other
embodiments, the nanotubes possess a mixture of phases, or are
amorphous. The nanotubes may also be transformed from one phase
partially or predominantly into another phase. The nanotubes can be
made to undergo a phase conversion by any suitable method known in
the art, such as by subjecting the nanotubes to a temperature that
induces a phase conversion, e.g., a post-annealing step conducted
at a temperature of about 450.degree. C. to induce a phase change
from amorphous to anatase. The nanotubes may also be grown under
suitable conditions of, for example, temperature, anodization
voltage, water concentration, and choice of ionic liquid, such that
one or more phases are favored, or that one or more phases are
disfavored.
[0081] In addition, the TiO.sub.2 nanotubes can include one or more
dopant species by suitable doping procedures conducted either
during the growth process (i.e., during anodization as an in situ
doping process) or after the TiO.sub.2 nanotubes have been produced
(i.e., as a post-doping process). Some examples of dopants include
the alkali metals (e.g., Li.sup.+, Na.sup.+, K.sup.+), alkaline
earth metals (Mg.sup.2+, Ca.sup.2+, Sr.sup.2+, Ba.sup.2+),
transition metals (e.g., V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, W, and
the noble metals), main group metals (e.g., B, Al, C, Si, N, P, As,
Sb, F), and the rare earths (e.g., Ce, Nd, Eu). An in situ doping
process may be conducted by, for example, including the dopant
species in the electrolytic medium during the growth process. In
particular, a metal dopant (e.g., Fe, Cu, and the like) can be
incorporated in the nanotubes by being included in the titanium
substrate, i.e., as an alloy, such as a Cu--Ti or Cu--Fe alloy. A
post-doping process may be conducted by, for example, subjecting
the TiO.sub.2 nanotubes to a plasma etching, electron or ion
bombardment, sputtering, CVD, or ion implantation; alternatively,
the TiO.sub.2 nanotubes may be exposed to a dopant-containing gas
(e.g., NH.sub.3, SiH.sub.4, CH.sub.4, H.sub.2, or F.sub.2),
typically at an elevated temperature; or alternatively, the
TiO.sub.2 nanotubes may be subjected to a solution processing step
wherein the solution contains the desired dopant species (e.g.,
ammonium or nitrate ions for nitrogen doping).
[0082] The TiO.sub.2 nanotubes may also be suitably
surface-functionalized or coated. For example, the TiO.sub.2
nanotubes may be surface-functionalized by one or more reactive
siloxane molecules which contain a functional group (e.g.,
hydroxyl, amine, ionic, or hydrophobic group). Alternatively, or in
addition, the TiO.sub.2 nanotubes can be coated (e.g., by a CVD
process) with a metal or metal alloy (e.g., Ni, Pd, Pt, Ag, Au, Nb,
W, or an alloy of these) or a main group element (e.g., B, C, N, P,
or F) or a compound or material containing one or more of these
elements. The surface functionalization or coating may serve any
suitable function, such as to modify the photovoltaic properties,
electron transport properties, catalytic properties, or interface
properties.
[0083] The TiO.sub.2 nanotubes can have any suitable outer
diameter. For example, in different embodiments, the TiO.sub.2
nanotubes may have an outer diameter of about, at least, or less
than 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 60 nm, 70 nm, 80 nm,
90 nm, 100 nm, 120 nm, 150 nm, 200 nm, or 250 nm, or a range
between any two of these values. In preferred embodiments, the
TiO.sub.2 nanotubes have an outer diameter of or less than 45 nm,
40 nm, 35 nm, 30 nm, or 25 nm.
[0084] The TiO.sub.2 nanotubes can have any suitable pore (i.e.,
inner) diameter. For example, in different embodiments, the
TiO.sub.2 nanotubes may have a pore diameter of about, at least, or
less than 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45
nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 120 nm, 150 mn, or
200 nm, or a range between any two of these values. The TiO.sub.2
nanotubes can possess any of the foregoing pore diameters in
combination with any of the above outer diameters, wherein it is
understood that the pore diameter is less than the outer diameter,
and the difference in pore diameter and outer diameter generally
corresponds to the wall thickness.
[0085] The TiO.sub.2 nanotubes can have any suitable wall
thickness. For example, in different embodiments, the TiO.sub.2
nanotubes may have a wall thickness of about, at least, or less
than 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 10 nm, 12 nm, 15 nm, 18 nm, 20
nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, or 50 nm, or a range between
any two of these values.
[0086] The TiO.sub.2 nanotubes can have any suitable length. A
desired length of the nanotube can generally be attained by growing
the nanotube for a suitable period of time at a particular growth
rate. In different embodiments, the TiO.sub.2 nanotubes may have a
length of about, at least, or less than, for example, 100 nm, 150
nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 500 nm, 600 nm, 700 nm,
800 nm, 900 nm, 1 .mu.m, 2 .mu.m, 5 .mu.m, 10 .mu.m, 15 .mu.m, 20
.mu.m, 25 .mu.m, 30 .mu.m, 40 .mu.m, 50 .mu.m, 60 .mu.m, 70 .mu.m,
80 .mu.m, 90 .mu.m, 100 .mu.m, or 120 .mu.m, or a range between any
two of these values. The corresponding length-to-diameter aspect
ratio (i.e., "aspect ratio") can be, for example, at least about 5,
10, 20, 30, 40, 50, 100, 150, 200, 250, 500, 600, 700, 800, 900,
1000, or a range between any two of these values.
[0087] As used herein particularly in describing the outer
diameter, pore size, wall thickness, and length of the nanotubes,
the term "about" generally indicates within .+-.0.5, 1, 2, 5, or
10% of the indicated value (e.g., 40 nm.+-.2%, which indicates
40.+-.0.8 nm or 39.2-40.8 nm). Moreover, an outer diameter of about
40 nm can indicate either a measurement error for a physical
characteristic of a single nanotube or a variation or average in a
physical characteristic across several nanotubes.
[0088] When applied as a photovoltaic component, the produced
TiO.sub.2 nanotubes preferably possess recombination
characteristics that result in sufficient photoconversion
efficiencies. Preferably, the TiO.sub.2 nanotubes possess a
photoconversion efficiency of at least, for example, 4%, 5%, 6%,
7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18, 19%, 20%,
21%, or 22%. The TiO.sub.2 nanotubes also preferably possess a
specific resistance of or less than 1.0.times.10.sup.-2 .OMEGA.cm,
and more preferably, of or less than 1.0.times.10.sup.-3 .OMEGA.cm
or 1.0.times.10.sup.-4 .OMEGA.cm.
[0089] In a particular embodiment, the invention is directed to an
array of nanotubes attached to a substrate. The substrate is
typically the substrate on which the nanotubes have been grown,
i.e., titanium metal. However, the original substrate of titanium
metal can be modified by numerous methods known in the art. For
example, the titanium metal substrate can be overlayed with a
coating of another metal (e.g., gold, platinum, nickel, or zinc), a
metal oxide, or a polymeric material. The titanium metal substrate
may alternatively be selectively removed (e.g., by acid etching,
cutting, or melting) and wholly or partially replaced with another
substrate.
[0090] The nanotube array can have a particular orientation with
respect to the substrate surface. For example, in one embodiment,
it is preferred for the longitudinal dimension of the nanotubes to
be oriented either completely perpendicular to the surface (i.e.,
precisely 90.degree.), or substantially perpendicular to the
surface, e.g., 90.+-.10.degree. (i.e., 80.degree. to -80.degree.),
90.+-.5.degree., 90.+-.2.degree., or 90.+-.1.degree. with respect
to the surface. In another embodiment, it may be preferred for all
or a portion of nanotubes to have a longitudinal dimension oriented
obliquely to the surface within a range of angles, e.g., 45.degree.
to -45.degree., 60.degree. to -60.degree., or 70.degree. to
-70.degree., with respect to the surface. In yet another
embodiment, it is preferred for the longitudinal dimension of the
nanotubes to be oriented either completely aligned (i.e., parallel)
with the surface (i.e., precisely 0.degree.), or substantially
aligned to the surface, e.g., 0.+-.10.degree., 0.+-.5.degree.,
0.+-.2.degree., or 0.+-.1.degree. with respect to the surface. A
non-perpendicular orientation of the nanotubes can be accomplished
by, for example, applying an anisotropic force on the nanotubes
during growth, either continuously (e.g., by pressing with a
counter-surface or applying a directional flow pressure of anolyte)
or in intervals (e.g., by sound wave burst or cavitation).
[0091] The nanotubes in the array can also possess a degree of
uniformity. The uniformity can be in any desired property, such as
the outer tube diameter, pore diameter, wall thickness, tube to
tube spacing, length, orientation with respect to the substrate,
aspect ratio, and/or one or more non-physical (i.e., property)
attributes. In a particular embodiment, the nanotubes in the array
are substantially uniform in one or more aspects. Typically, by
being substantially uniform is meant that the nanotubes show no
more than 15% or 10%, and more preferably, no more than 5%, 2%, 1%,
0.5%, or 0.1% deviation in one or more attributes of the nanotubes.
In a particular embodiment, the nanotubes possess an ordered
arrangement with each other. The ordered arrangement can be, for
example, a hexagonal close packed or cubic arrangement.
[0092] In another aspect, the invention is directed to a
photovoltaic device containing the TiO.sub.2 nanotubes described
above. The photovoltaic device can be any such device in which the
TiO.sub.2 nanotubes function as light absorbers, charge
transporters, or both, in converting light (typically sunlight)
into electrical energy.
[0093] In a particular embodiment, the TiO.sub.2 nanotubes are
incorporated in a dye-sensitized solar cell (DSSC) device. As known
in the art, a DSSC device separates the light absorption and charge
transport processes of a solar cell device by including a dye, also
known as a dye sensitizer, which becomes electronically excited
after absorbing a photon, whereafter the electronically excited dye
injects an electron into a semiconductor oxide capable of charge
transport. In the inventive DSSC device described herein, the
TiO.sub.2 nanotubes function as improved charge transporting
materials which receive electrons from the dye. Typically, the DSSC
contains an electrode (i.e., working electrode or photoanode) and a
counterelectrode in contact with an electrolyte layer sandwiched
therebetween, wherein the electrolyte layer commonly contains the
charge transport material (herein, the TiO.sub.2 nanotubes), a dye,
a liquid medium, and typically, an oxidation-reduction pair. The
electrolyte layer may also contain electrically conductive
particles, such as carbon nanotubes, carbon fibers, carbon black,
or the like. The electrolyte layer may also contain oxide
semiconductor particles having a photovoltaic property, or
alternatively, be in contact with an oxide semiconductor film
having a photovoltaic property. Some examples of suitable oxide
semiconductor particles or films include those composed of
Nb.sub.2O.sub.5, In.sub.2O.sub.3, BaTiO.sub.3, SrTiO.sub.3, ZnO,
ITO, Bi.sub.2O.sub.3, SnO.sub.2, Ho.sub.2O.sub.3, ZrO.sub.2,
Ta.sub.2O.sub.5, Al.sub.2O.sub.3, La.sub.2O.sub.3, Sr.sub.2O.sub.5,
TiO.sub.2, CeO.sub.2, and Y.sub.2O.sub.3. The particles are
typically nanoparticles, which typically possess a size within the
range of 1-200 nm, 1-100 nm, 5-200 nm, 5-100 nm, 10-200 nm, 10-100
nm, or 10-50 nm.
[0094] In the DSSC device, the TiO.sub.2 nanotubes are in
electrical contact with, and more typically, attached to, the
electrode. The electrode and counterelectrode are commonly
constructed of, for example, a transparent base material (e.g.,
PET, PEN, PC, or PES) coated with one or more conductive layers
(e.g., ITO, SnO.sub.2, FTO, Au, Pt, or a carbon-based, e.g.,
graphene material). The dye can be any suitable dye known in the
art, such as an organic dye (e.g., eosin, rhodamine, melocyanine,
coumarin) or an organometallic dye (e.g., ruthenium or iron
complexes containing at least one bipyridine or terpyridine
ligand). The liquid medium can include any suitable solvent (e.g.,
acetonitrile, propionitrile, ethylene carbonate, diethyl carbonate,
DMSO), gelling agent (e.g., a polymer), electrolyte, or ionic
liquid. The oxidation-reduction pair can be any of the
oxidation-reduction pairs known in the art, including, for example,
iodine/iodide, bromine/bromide, chlorine/chloride, or polyhalide
ions, such as I.sub.3.sup.-, I.sub.5.sup.-, I.sub.7.sup.-,
Br.sub.3.sup.-, Br.sub.2I.sup.-, and the like.
[0095] In another aspect, the invention is directed to a hydrogen
generation device containing the TiO.sub.2 nanotubes described
above. The hydrogen generation device can be any such device in
which the TiO.sub.2 nanotubes function to photoelectrolytically
react with water to form hydrogen. Typically, the electromagnetic
radiation used by the TiO.sub.2 nanotubes in the photoelectrolysis
of water is solar radiation, or a component thereof (e.g., within
the ultraviolet spectrum). The TiO.sub.2 nanotubes may also be
chemically configured to modify, expand, or narrow the light
absorption range, such as to expand photoelectrolysis capability
into visible light by use of a suitable modifier, such as CdS. In
the device, the TiO.sub.2 nanotubes are typically part of a
photoanode as describe above, which, when acted upon by solar
radiation, causes the photoelectrolysis of water to produce
hydrogen. The photoanode is in electrical communication with a
cathode, such as Pt. Typically, a liquid medium (e.g.,
electrolyte), as described above, is included to permit electron
flow between the electrodes. The device can also be configured as a
single photoelectrode system (i.e., wherein only the photoanode is
photoactive), bi-photoelectrode system (i.e., wherein both the
photoanode and counterelectrode are photoactive), hybrid
photoelectrode system (i.e., wherein photovoltaic components, such
as Si semiconductors, are included), or dye-sensitized
photoelectrode system (i.e., wherein a dye is included to promote
electron transport). The device preferably possesses a photocurrent
density of at least 0.25, 0.5, 1.0, 2.0, 2.5, 3.0, or 3.5
mA/cm.sup.2 at an appropriate voltage (e.g., 0.2V vs. Ag/AgCl). The
rate of hydrogen evolution is preferably at least 1 L/h, and more
preferably, at least 1, 2, 3, 5, 7, 10, 11, 12, or 15 L/h for a
photoanode having an area of, for example, about or less than 0.5,
1, 1.5, or 2 m.sup.2.
[0096] In yet another aspect, the invention is directed to a
hydrogen gas sensor containing the TiO.sub.2 nanotubes described
above. The hydrogen gas sensor can be any such device in which a
hydrogen gas-dependent shift in eletrical resistance of the
TiO.sub.2 nanotubes is used as a basis for detecting the presence
of hydrogen gas. The change in electrical resistance of the
TiO.sub.2 nanotubes from a hydrogen-less environment to a
hydrogen-containing environment can be, for example, about or at
least 50%, 100%, 200%, 500%, 1000%, 5000%, 10,000%, 50,000%,
100,000%, 200,000%, 300,000%, 400,000%, or 500,000%, for a change
in concentration of hydrogen gas of or less than, for example, 10
ppm, 20 ppm, 50 ppm, 100 ppm, 200 ppm, 300 ppm, 400 ppm, or 500
ppm, or a range therein.
[0097] Examples have been set forth below for the purpose of
illustration and to describe certain specific embodiments of the
invention. However, the scope of this invention is not to be in any
way limited by the examples set forth herein.
EXAMPLE 1
Preparation of TiO.sub.2 Nanotubes
[0098] The ionic liquid 1-butyl-3-methylimidazolium
tetrafluoroborate (BMIM-BF.sub.4) was mixed with deionized water in
a BMIM-BF.sub.4: water weight ratio of 0.276: 1, and the resulting
fluid used as the electrolyte. The anodization process was
conducted on a 0.5 mm thick pure titanium foil (purchased from
McMaster-Carr) using a two-electrode DC power supply with a voltage
output range of 0-40 V. The titanium foil surface was used
as-received (milled) and cleaned with acetone followed by an
ethanol rinse before the synthesis. The titanium foil was connected
as the working electrode and a piece of platinum mesh was used as
the counter electrode. The synthesis was at room temperature, i.e.,
about 21.degree. C. A constant potential of 10 volts was applied
during the synthesis. The time-dependent anodization current was
monitored using a multimeter. The current was about 2 mA at the
beginning of the synthesis and gradually decreased and stabilized
at a level of 0.5 mA for 80 minutes before being quickly increased
to above 20 mA, at which point the synthesis was stopped.
EXAMPLE 2
Microscopic Analysis of the TiO.sub.2 Nanotubes
[0099] The as-anodized titanium surface prepared according to
Example 1 above was analyzed by scanning electron microscopy (SEM).
Before imaging, the surface was ultrasonically cleaned in acetone
for 10 minutes during which time the top oxide layer was spalled
off to expose the nanotubes underneath.
[0100] FIGS. 1A and 1B show 100,000.times. and 200,000.times.
magnified, respectively, top view SEM micrographs of the surface.
As shown, the SEM images show a thin, dense layer containing a
highly ordered array of TiO.sub.2 nanotubes covering the
as-anodized titanium surface. The nanotubes are shown to have a
pore size of 25-35 nm and an outer tube diameter of 40-55 nm.
[0101] FIGS. 2A and 2B show 50,000.times. and 100,000.times.
magnified, respectively, cross-sectional SEM micrographs of the
surface. As shown, the nanotubes have a length of 250-350 nm.
[0102] While there have been shown and described what are at
present considered the preferred embodiments of the invention,
those skilled in the art may make various changes and modifications
which remain within the scope of the invention defined by the
appended claims.
* * * * *