U.S. patent application number 14/664341 was filed with the patent office on 2015-07-09 for nanostructures having crystalline and amorphous phases.
This patent application is currently assigned to THE REGENTS OF THE UNIVERSITY OF CALIFORNIA. The applicant listed for this patent is Xiaobo Chen, Samuel S. Mao. Invention is credited to Xiaobo Chen, Samuel S. Mao.
Application Number | 20150190785 14/664341 |
Document ID | / |
Family ID | 42729019 |
Filed Date | 2015-07-09 |
United States Patent
Application |
20150190785 |
Kind Code |
A1 |
Mao; Samuel S. ; et
al. |
July 9, 2015 |
NANOSTRUCTURES HAVING CRYSTALLINE AND AMORPHOUS PHASES
Abstract
The present invention includes a nanostructure, a method of
making thereof, and a method of photocatalysis. In one embodiment,
the nanostructure includes a crystalline phase and an amorphous
phase in contact with the crystalline phase. Each of the
crystalline and amorphous phases has at least one dimension on a
nanometer scale. In another embodiment, the nanostructure includes
a nanoparticle comprising a crystalline phase and an amorphous
phase. The amorphous phase is in a selected amount. In another
embodiment, the nanostructure includes crystalline titanium dioxide
and amorphous titanium dioxide in contact with the crystalline
titanium dioxide. Each of the crystalline and amorphous titanium
dioxide has at least one dimension on a nanometer scale.
Inventors: |
Mao; Samuel S.; (Foster
City, CA) ; Chen; Xiaobo; (Albany, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mao; Samuel S.
Chen; Xiaobo |
Foster City
Albany |
CA
CA |
US
US |
|
|
Assignee: |
THE REGENTS OF THE UNIVERSITY OF
CALIFORNIA
Oakland
CA
|
Family ID: |
42729019 |
Appl. No.: |
14/664341 |
Filed: |
March 20, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13256421 |
Feb 5, 2012 |
9018122 |
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PCT/US2010/026081 |
Mar 3, 2010 |
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14664341 |
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61159759 |
Mar 12, 2009 |
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61163856 |
Mar 26, 2009 |
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61180208 |
May 21, 2009 |
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Current U.S.
Class: |
204/157.52 ;
204/157.4; 502/350; 502/5 |
Current CPC
Class: |
B01J 37/033 20130101;
B01J 37/0215 20130101; B01J 37/349 20130101; C01G 23/047 20130101;
B01J 35/0006 20130101; C01P 2002/85 20130101; B82Y 30/00 20130101;
B01J 37/347 20130101; B01J 37/10 20130101; Y10S 977/762 20130101;
B01J 35/002 20130101; B01J 19/127 20130101; B01J 2219/1203
20130101; C01B 3/042 20130101; C02F 1/725 20130101; B01J 21/063
20130101; B01J 37/0221 20130101; C01P 2006/40 20130101; B01J 35/004
20130101; C01P 2002/72 20130101; C01P 2004/64 20130101; C02F 1/32
20130101; B01J 23/42 20130101; Y02E 60/36 20130101; C01P 2004/04
20130101; B01J 37/0072 20130101; B01J 35/02 20130101 |
International
Class: |
B01J 21/06 20060101
B01J021/06; B01J 37/02 20060101 B01J037/02; C01B 3/04 20060101
C01B003/04; B01J 37/00 20060101 B01J037/00; B01J 35/02 20060101
B01J035/02; B01J 19/12 20060101 B01J019/12; B01J 35/00 20060101
B01J035/00; B01J 37/34 20060101 B01J037/34 |
Goverment Interests
STATEMENT OF GOVERNMENT SUPPORT
[0002] This invention was made with government support under
Contract No. DE-ACO2-05CH11231 awarded by the U.S. Department of
Energy. The government has certain rights in this invention.
Claims
1. A method of making a nanostructure comprising: providing a
titanium dioxide (TiO.sub.2) crystalline core; and changing a
portion of the TiO.sub.2 crystalline core to a TiO.sub.2 amorphous
outer most shell in contact with and completely surrounding the
crystalline core, wherein the TiO.sub.2 amorphous outer most shell
further comprises a hydrogen dopant and has a composition different
than the TiO.sub.2 crystalline core.
2. The method of claim 1 further comprising synthesizing the
TiO.sub.2 crystalline core.
3. The method of claim 2 wherein synthesizing the TiO.sub.2
crystalline core comprises: providing a precursor; and processing
the precursor to produce the TiO.sub.2 crystalline core.
4. The method of claim 3 wherein the precursor is selected from the
group consisting of a gas, a liquid, a solution, a gel, and a
solid.
5. The method of claim 3 wherein processing the precursor comprises
a technique selected from the group consisting of a solution
chemistry technique, a sol-gel technique, a hydrothermal technique,
a solvothermal technique, a thermal technique, an electrochemistry
technique, a chemical vapor deposition technique, and a physical
vapor deposition technique.
6. The method of claim 3 wherein processing the precursor comprises
producing crystalline nanoparticles that are arranged in an
assembly of the crystalline nanoparticles.
7. The method of claim 3 wherein processing the precursor comprises
depositing crystalline nanoparticles onto a substrate.
8. The method of claim 1 wherein changing the portion of the
TiO.sub.2 crystalline core to an amorphous phase comprises
employing ions, atoms, or molecules to create disorder in a portion
of the crystalline nanostructure.
9. The method of claim 1 wherein changing the portion of the
TiO.sub.2 crystalline core to an amorphous phase comprises a
hydrogenation technique.
10. The method of claim 1 changing the portion of the TiO.sub.2
crystalline core to an amorphous phase comprises an ion bombardment
technique.
11. The method of claim 1 changing the portion of the TiO.sub.2
crystalline core to an amorphous phase comprises exposing the
crystalline nanostructure to a noble gas at elevated temperature
and pressure.
12. The method of claim 11 wherein the noble gas is selected from
the group consisting of helium, neon, argon, krypton, and
xenon.
13. A method of photocatalysis comprising: contacting a reactant
fluid to a nanoparticle comprising a titanium dioxide (TiO.sub.2)
crystalline core, and a TiO.sub.2 amorphous outer most shell in
contact with and completely surrounding the crystalline core,
wherein the TiO.sub.2 amorphous outer most shell further comprises
a hydrogen dopant and has a composition different than the
TiO.sub.2 crystalline core; and exposing the nanostructure to
light, thereby producing a reaction product from the reactant
fluid.
14. The method of claim 13 wherein the light comprises
sunlight.
15. The method of claim 13 wherein the light comprises simulated
sunlight.
16. The method of claim 13 wherein the reactant fluid comprises an
environmental contaminant, and exposing the nanostructure to the
sunlight causes at least some of the environmental contaminant to
decompose.
17. The method of claim 13 wherein, the fluid comprises water, and
exposing the nanostructure to the sunlight causes decomposition of
at least some of the water, thereby producing the reaction product
of hydrogen.
18. The method of claim 17 wherein the reactant fluid comprises a
solution of liquid water and a sacrificial agent.
19. The method of claim 18 wherein the sacrificial agent comprises
an alcohol.
20. The method of claim 13 wherein the reactant fluid comprises a
gas.
21. The method of claim 13 wherein the reactant fluid comprises a
liquid.
22. The method of claim 22 further comprising contacting additional
nanoparticles and the reactant fluid.
23. The method of claim 13 wherein the nanoparticles form a porous
network of nanoparticles.
24. The method of claim 13 wherein the nanoparticle further
comprises transition metal particles on a surface of the
nanoparticle.
Description
RELATED APPLICATIONS
[0001] This application is a Divisional of U.S. application Ser.
No. 13/256,421, filed Feb. 5, 2012 and entitled Nanostructures
Having Crystalline and Amorphous Phases; which claims priority to
PCT Application PCT/US2010/026081, filed Mar. 3, 2010 and entitled
Nanostructures having crystalline and amorphous phases, which in
turn claims priority to U.S. Provisional Patent Application Ser.
No. 61/159,759, filed Mar. 12, 2009; 61/163,856, filed Mar. 26,
2009; and, 61/180,208, filed May 21, 2009, all entitled
Nanostructures having crystalline and amorphous phases; which are
hereby incorporated by reference in their entireties.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to the field of material
science and, more particularly, to the field of material science
which includes nanostructures. The present invention also relates
to the field of chemistry and, more particularly, to the field of
photocatalysis.
[0004] Material science has uncovered semiconductors with
electronic properties that are strikingly beneficial to a number of
desirable clean energy and environmental technologies based on
solar-driven photocatalysis. Of a range of materials that have been
investigated in pursuit of greater utilization of solar energy,
currently the most effective semiconductor for photocatalysis is
titanium dioxide (TiO.sub.2), which absorbs light from only the
ultraviolet (UV) portion of the solar spectrum. That is, TiO.sub.2
has a band gap that corresponds to the energy of a UV photon. As a
consequence, TiO.sub.2 can absorb but a small fraction of solar
radiation, leaving over 90% of the energy in the solar spectrum
essentially wasted. Narrowing the band gap of TiO.sub.2 is
therefore vital to achieve efficient absorption of sunlight, which
is true for all wide band gap semiconductors if they are to be used
in an energy conversion process driven by solar radiation. While
impurity doping is a well-established method of tuning the band
gap, its application to TiO.sub.2 has had only limited success.
[0005] Solar radiation is an energy resource that can be used to
produce electricity and clean fuel, or used in combination with
selected semiconductors to induce environmentally important
photocatalytic reactions such as air purification and water
de-contamination. Effectiveness of solar-driven photocatalysis is
determined to a great extent by the semiconductor's capability of
absorbing visible and infrared light, in addition to the
requirement of a large surface area that can facilitate a fast rate
of surface reactions. Nanostructured TiO.sub.2 has emerged as a
unique wide band gap semiconductor photocatalyst that plays a key
role in a variety of solar-driven clean energy and environmental
technologies (see, e.g., Gratzel, Photoelectrochemical cells,
Nature, 414, 338-344 (2001); Hoffmann et al., Environmental
applications of semiconductor photocatalysis, Chem. Rev., 95, 69-96
(1995); and Fujishima et al., TiO.sub.2 photocatalysts and related
surface phenomena, Surf Sci. Rpts, 63, 515-582 (2008)).
Nevertheless, despite decades of extensive research, the true
potential of TiO.sub.2 has not been realized, as the material
absorbs only in the UV portion of the solar spectrum.
[0006] To overcome limited absorption of solar radiation by
TiO.sub.2, extensive efforts have been made to vary its chemical
composition by adding controlled metal or non-metal impurities that
generate discrete donor or acceptor energy states in the band gap
(see., e.g., Asahi et al., Visible-light photocatalysis in
nitrogen-doped titanium oxides, Science, 293, 269-271 (2001)).
Through such impurity doping, the solar absorption characteristics
of TiO.sub.2 have been improved to some extent. For example, when
non-metal light-element dopants are introduced, absorption by
TiO.sub.2 can be modified as the result of electronic transitions
from the dopant 2p or 3p orbitals to the titanium 3d orbitals.
Nitrogen-doped TiO.sub.2 so far exhibits the best response to solar
radiation, but its absorption in the visible and infrared
wavelength portions of the solar spectrum remains inefficient. For
example, see Chen et al., The electronic origin of the
visible-light absorption properties of C-, N- and S-doped TiO2
nanomaterials, J. Am. Chem. Soc., 130, 5018-5019 (2008), which
reported that N-doped TiO.sub.2 has: (a) a band gap of 3.0 eV; (b)
an absorption spectrum that exhibits a decreasing absorbance from
415-550 nm (a shoulder) and a diminishing absorbance from 550-800
nm (a tail); and (c) no absorption above 800 nm (i.e. an absorption
edge of 800 nm).
SUMMARY OF THE INVENTION
[0007] The present invention includes a nanostructure, a method of
making the nanostructure, and a method of photocatalysis. According
to an embodiment, the nanostructure includes a crystalline phase
and an amorphous phase. The amorphous phase is in contact with the
crystalline phase. Each of the crystalline and amorphous phases has
at least one dimension on a nanometer scale. According to another
embodiment, the nanostructure includes a nanoparticle. The
nanoparticle includes a crystalline phase and an amorphous phase.
The amorphous phase is in a selected amount. According to yet
another embodiment, a nanostructure includes crystalline titanium
dioxide and amorphous titanium dioxide. The amorphous titanium
dioxide is in contact with the crystalline titanium dioxide. Each
of the crystalline and amorphous titanium dioxide has at least one
dimension on a nanometer scale.
[0008] According to an embodiment, the method of making the
nanostructure includes providing a crystalline nanostructure and
changing a portion of the crystalline nanostructure to an amorphous
phase. A remaining portion of the crystalline nanostructure and the
amorphous phase each has at least one dimension on a nanometer
scale.
[0009] According to an embodiment, the method of photo catalysis
includes contacting a reactant fluid to a nanostructure. The
nanostructure includes a crystalline phase and an amorphous phase
in contact with the crystalline phase. Each of the crystalline and
amorphous phases has at least one dimension on a nanometer scale.
The nanostructure is exposed to light, which produces a reaction
product from the reactant fluid.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The present invention is described with respect to
particular exemplary embodiments thereof and reference is
accordingly made to the drawings in which:
[0011] FIG. 1 illustrates embodiments of nanoparticles of the
present invention;
[0012] FIG. 2 illustrates an embodiment of a nanostructure of the
present invention;
[0013] FIG. 3 illustrates an embodiment of a nanostructure of the
present invention;
[0014] FIG. 4 illustrates an embodiment of a nanostructure of the
present invention;
[0015] FIG. 5 illustrates an embodiment of a nanoparticle of the
present invention;
[0016] FIGS. 6A, 6B, and 6C illustrate band gaps for a bulk
semiconductor, a crystalline nanoparticle, and an embodiment of a
nanoparticle of the present invention, respectively;
[0017] FIG. 7A provides XRD (x-ray diffraction) spectra for a white
TiO.sub.2 and a black TiO.sub.2, the latter of which is an example
of the present invention;
[0018] FIG. 7B provides Raman spectra for a white TiO.sub.2 and a
black TiO.sub.2, the latter of which is an example of the present
invention;
[0019] FIG. 7C provides a HRTEM (high resolution transmission
electron microscopy) image of crystalline a TiO.sub.2
nanoparticle;
[0020] FIG. 7D provides a HRTEM image of a TiO.sub.2 nanoparticle
that is an example of the present invention;
[0021] FIGS. 8A and 8B provide Ti 2p and O 1s XPS (x-ray
photoelectron spectroscopy) data, respectively, for white and black
TiO.sub.2, the latter of which is an example of the present
invention;
[0022] FIG. 9A is a photo comparing white and black TiO.sub.2, the
latter of which is an example of the present invention;
[0023] FIG. 9B provides spectral reflectance for white and black
TiO.sub.2, the latter of which is an example of the present
invention;
[0024] FIG. 9C provides absorbance for white and black TiO.sub.2,
the latter of which is an example of the present invention,
[0025] FIG. 9D provides valence band XPS data for white and black
TiO.sub.2, the latter of which is an example of the present
invention;
[0026] FIG. 9E provides electronic energy band structure of
TiO.sub.2 nanostructures, in accordance with an example of the
present invention, and compares it to that of crystalline TiO.sub.2
nanostructures;
[0027] FIG. 10A provides absorption spectra over time during a
photocatalysis experiment in which a methylene blue solution was
placed in a container that included black TiO.sub.2 nanoparticles,
which are examples of the present invention, and that was
irradiated with simulated solar light;
[0028] FIG. 10B provides a comparison of solar-driven
photocatalytic activity of black TiO.sub.2 nanostructures, which
are examples of the present invention, against that of white
TiO.sub.2 nanostructures under similar experimental conditions;
[0029] FIG. 10C provides data from cycling tests of solar-driven
photocatalytic activity of disorder-engineered, black TiO.sub.2
nanoparticles, which are examples of the present invention;
[0030] FIG. 11 provides a comparison of solar-driven photocatalytic
activity of black TiO.sub.2 nanoparticles, which are examples of
the present invention, against that of white TiO.sub.2 nanocrystals
under similar experimental conditions; and
[0031] FIG. 12 provides data from tests of solar-driven
photocatalysis of water that produced hydrogen using black
TiO.sub.2 nanoparticles, which are examples of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0032] Embodiments of the present invention include a nanostructure
and a method of making the nanostructure. The nanostructure may be
a nanoparticle or a collection of nanoparticles.
[0033] An embodiment of a nanostructure of the present invention
includes a crystalline phase and an amorphous phase. The amorphous
phase is in contact with the crystalline phase. Each of the
crystalline and amorphous phases has at least one dimension on a
nanometer scale. As used herein, a nanometer scale according to
some embodiments is within a range of about 0.5 to 200 nm,
according to other embodiments is within a range of about 0.5 to
100 nm, and according to yet other embodiments is within a range of
about 0.5 nm to 1 .mu.m. The at least one dimension for each of the
crystalline and amorphous phases may be a length, a diameter, or a
thickness. For example, if the nanostructure is a nanoparticle
having a crystalline core and an amorphous shell, the at least one
dimension for the crystalline core is a diameter and the at least
one dimension for the amorphous shell is a thickness. In an
embodiment, the amorphous phase is in a selected amount. Such an
amount is selected during fabrication of the nanostructure. The
expressions "disorder-engineering" and "disorder-engineered" are
used herein to describe the process of making the nanostructure and
selecting the amount of the amorphous phase (i.e. the disorder) in
the nanostructure.
[0034] Embodiments of nanoparticles of the present invention are
illustrated in FIG. 1. The nanoparticles 100 include a branched
nanoparticle 102, a nanodisk 104, a nanorod 106, a nanospindle 108,
a nanowire 110, and a quantum dot 112. Each of the embodiments 100
includes a crystalline phase 114 and an amorphous phase 116. The
nanoparticles 100 may be produced as nanocrystals using a solution
chemistry approach, which is discussed more fully below. Portions
of the nanocrystals may then be changed to the amorphous phase 116
by exposing the nanocrystals to a gas at elevated temperature and
pressure, by selectively exposing a portion or portions of the
nanocrystals to an ion beam, or by some other suitable
technique.
[0035] It will be readily apparent to one skilled in the art that
other embodiments of nanoparticles beyond those shown in FIG. 1
fall within the scope of the present invention. For example, an
embodiment of a nanoparticle of the present invention may be a
quantum dot that includes a protruding member where such a
nanoparticle includes crystalline and amorphous phases.
[0036] Another embodiment of a nanostructure of the present
invention is illustrated in FIG. 2. The nanostructure 200 includes
nanoparticles 202 that are formed into a porous network 204. Each
of the nanoparticles 202 include a crystalline core 206 (i.e. a
crystalline phase) and an amorphous shell or partial shell 208
(i.e. an amorphous phase). Both the crystalline core 106 and the
amorphous shell 208 have dimensions on a nanometer scale. In an
embodiment, the nanoparticles 202 are TiO.sub.2 nanoparticles that
include H atoms within the amorphous shell 208. The nanostructure
202 may be made using a solution chemistry technique to produce a
nanostructure of crystalline nanoparticles in a porous network. The
amorphous phase may be produced by exposing the nanostructure of
the crystalline nanoparticles to a gas at an elevated temperature
and pressure or by some other suitable technique.
[0037] It will be readily apparent to one skilled in the art that
the nanostructure 200 is a plane of nanoparticles 202 in a cubic
structure of the nanoparticles 202. Further, it will be readily
apparent to one skilled in the art that other nanostructures that
form a porous network of nanoparticles are within the scope of the
present invention. For example, the nanoparticles 202 may be
arranged in a hexagonal close packed structure or some other
structure that may or may not have a repeating structural unit as
in the cubic or hexagonal close packed structures. Moreover, a
nanostructure that forms a porous network of the present invention
may be formed by any of nanoparticles that are within the scope of
the present invention.
[0038] Another embodiment of a nanostructure of the present
invention is illustrated in FIG. 3. The nanostructure 300 includes
nanoparticles 302 coupled to a substrate 304. The nanoparticles 302
include a crystalline phase 306 and an amorphous phase 308. The
nanoparticles 302 may be grown on the substrate 304 by a vapor
deposition technique such as CVD (chemical vapor deposition) or PVD
(physical vapor deposition).
[0039] Another embodiment of a nanostructure of the present
invention is illustrated in FIG. 4. The nanostructure 400 includes
nanoparticles 402 coupled to a substrate 404. The nanoparticles
include a crystalline phase 406 and an amorphous phase 408. The
nanoparticles 402 may be deposited on the substrate using a
micro-printing process or a lithography process.
[0040] In FIGS. 3 and 4, the amorphous phase, 308 or 408, of the
nanostructure, 300 or 400, may be produced by changing portions of
crystalline nanoparticles to the amorphous phase, 308 or 408, by
exposing the crystalline nanoparticles to a gas at elevated
temperature and pressure, by selectively exposing the crystalline
nanoparticles to an ion beam, or by some other suitable
technique.
[0041] A cross-section of an embodiment of a nanoparticle of the
present invention is illustrated in FIG. 5. The nanoparticle 500
includes a crystalline core 502 and an amorphous shell 504. Atoms
506 within the crystalline core 502 form a lattice structure (i.e.
a crystalline structure) and atoms 508 within the amorphous shell
504 exhibit disorder. In an embodiment, the amorphous shell 504
includes dopants 510. In an embodiment, the nanoparticle 500 is a
TiO.sub.2 nanoparticle that may include H dopant atoms in the
amorphous shell 504. While an assembly of such TiO.sub.2
nanoparticles retains the benefits of the crystalline cores 502,
the amorphous phase (i.e. the disorder) of the nanoparticles
possibly with the addition of a dopant significantly enhance
optical absorption into the visible and infrared. It is believed
that the enhanced optical absorption is due to a combination of a
narrowing of a band gap for the crystalline TiO.sub.2 nanoparticles
due to the addition of the amorphous shell 504 and to band tails
introduced within the band gap by the amorphous phase of the shell
504.
[0042] FIGS. 6A, 6B, and 6C illustrate band gaps for a bulk
semiconductor, a crystalline nanoparticle of the semiconductor, and
a nanoparticle of the semiconductor in accordance with an
embodiment of the present invention (e.g., the nanoparticle 500),
respectively. In FIG. 6A, the band gap for the bulk semiconductor
is the energy gap between the valence band E.sub.v (i.e. an energy
of E.sub.v or lower) and the conduction band E.sub.C (i.e. an
energy of E.sub.C or higher). When the bulk material absorbs a
photon, an electron moves from the valence band E.sub.v to the
conduction band E.sub.C. In FIG. 6B, the crystalline nanoparticle
of the semiconductor exhibits a widening of the band gap due to
quantum confinement. In FIG. 6C, the nanoparticle of the
semiconductor in accordance with an embodiment of the present
invention exhibits a narrowing of the band gap and the introduction
of band tails 602. In terms of absorbance (i.e. a measure of
absorption of light at a particular wavelength) for a nanoparticle
500 of TiO.sub.2, the narrowing of the band gap moves an absorption
edge from the UV portion of the spectrum to the infrared portion of
the spectrum and the band tails move the absorption edge further
into the infrared portion of the spectrum. As used herein, the
absorption edge is a light wavelength (e.g., a UV, visible, or
infrared wavelength) where there is some absorbance just below the
wavelength and there is no absorbance just above the wavelength.
The band gap for the nanoparticle 500 of TiO.sub.2 is significantly
lower than what was available previously and the wavelength for the
absorption edge is significantly higher. This allows applications
of an assembly of the nanoparticles 500 of TiO.sub.2 in
photocatalysis and photovoltaics with greatly improved response
when exposed to solar radiation. Thus, the present invention
provides chemical properties (e.g., improved photocatalysis) and
physical properties (e.g., lower band gap) for a particular
material that were not previously available.
[0043] The nanoparticle 500 (FIG. 5) may have its band gap tuned to
a particular value by controlling the size of the nanoparticle 500
and a thickness of the amorphous shell 504. For example,
embodiments of nanoparticles 500 made of TiO.sub.2 may be produced
in which the band gap can be tuned to various values between that
of a nanocrystal of TiO.sub.2 (e.g., a band gap of 3.3 eV) and that
of a nanoparticle 500 of TiO.sub.2 that has a relatively narrow
band gap (e.g., a band gap of 1.5 eV or lower).
[0044] Applications for TiO.sub.2 nanostructures of the present
invention include energy and the environment. Energy applications
include solar driven photocatalysis of water (i.e. water splitting
to produce H.sub.2) and photovoltaics. An embodiment of a
photovoltaic cell of the present invention incorporates a TiO.sub.2
nanostructure or nanostructures of the present invention in the
photovoltaic cell taught in PCT Published Application No. WO
2009/012397, filed on Jan. 22, 2009, which is hereby incorporated
by reference. Environmental applications include solar driven
photocatalysis for abatement of water and air pollution.
[0045] An embodiment of a method of making a nanostructure of the
present invention includes providing a crystalline nanostructure
and changing a portion of the crystalline nanostructure to an
amorphous phase. A remaining portion of the crystalline
nanostructure and the amorphous phase each has a least one
dimension on a nanometer scale. As used herein, a crystalline
nanostructure may be a crystalline nanoparticle or an assembly of
crystalline nanoparticles. With regards to the latter, the assembly
of crystalline nanoparticles may be free-standing (e.g. a porous
assembly of TiO.sub.2 crystalline nanoparticles) or may be coupled
to a substrate. Also, while the nanoparticles of the assembly of
crystalline nanoparticles are crystalline, the assembly need not
order the crystalline nanoparticles so that their crystalline
nature extends beyond an individual nanoparticle. That is, the
assembly may exhibit a crystalline nature beyond an individual
crystalline nanoparticle or might exhibit disorder between adjacent
crystalline nanoparticles. Further, the crystalline nanoparticles
may exhibit a limited number of defects such as dislocations or
twinned lattices.
[0046] Changing the portion of the crystalline nanostructure to the
amorphous phase may include employing ions, atoms, or molecules to
create disorder in the portion of the crystalline nanostructure. In
an embodiment, changing the portion of the crystalline
nanostructure to the amorphous phase employs hydrogenation, which
includes placing the crystalline nanostructure in a H.sub.2
environment at elevated temperature and pressure to produce an
amorphous shell (or partial shell) surrounding (or partially
surrounding) a crystalline core. More generally in such an
embodiment, changing the portion of the crystalline nanostructure
to the amorphous phase includes exposing the crystalline
nanostructure to a gas at elevated temperature and pressure. The
gas may be selected from H.sub.2, He, Ar, some other noble gas, or
some other suitable gas. If the gas is H.sub.2, or some other gas
with atoms that form bonds with the material of the nanostructure,
dopants may remain in the amorphous phase of the nanostructure
after completion of the gas exposure process. If the gas is He, Ar,
or some other noble gas, the atoms are anticipated to migrate out
of the amorphous phase of the nanostructure.
[0047] In another embodiment, changing the portion of the
crystalline nanostructure to the amorphous phase employs ions to
bombard the portion of the crystalline nanostructure. The ion
bombardment may employ an ion source or an ion implanter.
Alternatively, the ion bombardment may include striking a plasma in
the vicinity of the nanostructure and applying a bias to the
nanostructure. Depending upon the choice of ions, dopants may
remain in the amorphous phase following the ion bombardment
process.
[0048] According to an embodiment, the method of making the
nanostructure of the present invention includes synthesizing the
crystalline nanostructure, which may include providing a precursor
and processing the precursor to produce the crystalline
nanostructure. The precursor may be a gas, a liquid, a solution, a
gel, or a solid. The processing of the precursor may include
solution chemistry technique (e.g., a liquid, solution, or gel
precursor) or a vapor deposition technique (e.g., a gaseous,
liquid, or solid precursor). The solution chemistry technique may
be chosen from a sol-gel technique, a hydrothermal technique, a
solvothermal technique, a thermal technique, an electrochemistry
technique, or some other suitable solution chemistry technique. The
vapor deposition technique may be chosen from a physical vapor
deposition technique or a chemical vapor deposition technique.
Synthesizing the crystalline nanostructure may include or be
followed by an annealing technique, which may be a vacuum annealing
technique. Techniques for synthesizing crystalline nanostructures
are known to those skilled in the art (see, e.g., Chen et al.,
Titanium Dioxide Nanomaterials: Synthesis, Properties,
Modifications, and Applications, Chem. Rev. 107, 2891-2959 (2007);
and Burda et al., Chemistry and Properties of Nanocrystals of
Different Shapes, Chem. Rev. 105, 1025-1102 (2005)).
[0049] An embodiment of a method of photocatalysis of the present
invention includes contacting a reactant fluid to a nanostructure.
The nanostructure includes crystalline and amorphous phases. Each
of the crystalline and amorphous phases having at least one
dimension on a nanometer scale. The method further includes
exposing the nanostructure to light. In an embodiment, the light
includes sunlight. In another embodiment, the light includes
simulated sunlight (i.e. from a light source designed to emit light
having a spectrum similar to natural sunlight). The reactant fluid
may be a liquid, a gas, or a combination of a liquid and a gas.
[0050] In an embodiment, the reactant fluid includes an
environmental contaminant. In such an embodiment, the light
striking the nano structure causes at least some of the
environmental contaminant to decompose. The environmental
contaminant may be an organic contaminant such an organic chemical
(e.g., methylene blue), bacteria, a virus, or some other organic
contaminant.
[0051] In another embodiment, the reactant fluid includes water. In
such an embodiment, the light striking the nanostructure causes the
water to decompose, which produces hydrogen. In an embodiment, the
reactant fluid is a solution that includes the liquid water and a
sacrificial agent (e.g., an alcohol such as methanol).
[0052] In an embodiment, the nanostructure comprises a
nanoparticle. In such an embodiment, additional nanoparticles may
be added to the reactant fluid where each at least some of the
additional nanoparticles comprise the crystalline and amorphous
phases. In an embodiment, the nanostructure comprises a porous
nanostructure that includes the crystalline and amorphous phases.
In an embodiment, the nanostructure comprises titanium dioxide. In
an embodiment, the nanostructure includes transition metal
particles such as Pt particles on a surface of the nanostructure.
Such transition metal particles may have a size within the range of
about 0.5 to 500 nm.
DISCUSSION
[0053] TiO.sub.2 nanostructures of the present invention have been
produced that exhibit enhanced solar absorption. Such
disorder-engineered TiO.sub.2 nanostructures exhibit a substantial
and stable solar-driven photocatalytic activity, along with a
surprisingly large optical response to visible and near infrared
radiation, absorbing photons over much of the solar spectrum.
[0054] In its simplest form, a disorder-engineered TiO.sub.2
nanostructure includes two phases, one is crystalline TiO.sub.2
(e.g., a quantum dot or a nanocrystal), and the second is an
amorphous surface where disorder as well as a dopant are introduced
(e.g., the nanoparticle 500 of FIG. 5). While an ensemble of
nanoparticles of the present invention retains the benefits of
crystalline TiO.sub.2 quantum structures for photocatalytic
processes, the introduction of disorder at their surfaces, together
with dopant inclusion, significantly enhances optical absorption in
the visible and infrared portions of the solar spectrum. This is
based on the consideration that disorder in semiconductors yield
band tails. Thus the distribution of electronic energy states
differs from that of a single crystal, with donor-like states
emerging in the conduction band tail and the corresponding states
in the valence band tail. These extended energy states due to the
introduction of disorders, in addition to energy levels produced by
dopants, can become the dominant centers for optical excitation and
recombination. FIG. 6C illustrates electronic energy levels of
doped and disorder-engineered semiconductor nanoparticles that
contribute to optical absorption, as compared to those of the bulk
semiconductor in FIG. 6A and of the un-modified crystalline
semiconductor nanoparticles in FIG. 6B.
[0055] To introduce disorder into a TiO.sub.2 nanostructure with
simultaneous dopant addition, a porous network of TiO.sub.2
crystalline nanoparticles was produced in which each nanoparticle
was a few nanometers in diameter. This was followed by
hydrogenation that generated an amorphous, hydrogenated layer on
the surfaces of the nanoparticles. A significant shift in the
absorption spectrum of such disorder-engineered TiO.sub.2
nanostructures was observed. The absorption edge of the absorption
spectrum moved from the UV to near infrared after hydrogenation.
This was accompanied by a dramatic color change and a significantly
enhanced solar-driven photocatalytic activity. The band gap of the
resulting black TiO.sub.2 nanostructures is narrowed down to
approximately 1.54 eV, as compared to about 3.30 eV for the
un-modified nanocrystalline material.
[0056] TiO.sub.2 nanocrystals were prepared using a sol-gel process
through which an organic surfactant was employed as a template to
direct the formation of a porous network of the TiO.sub.2
nanocrystals. Calcination helped remove the organic template and
enhance crystallization of the TiO.sub.2, which was followed by
heating in vacuum and hydrogenation under high pressure H.sub.2
atmosphere at an elevated temperature.
[0057] TiO.sub.2 nanocrystals were prepared with a precursor
solution of titanium tetraisopropoxide (TTIP), ethanol,
hydrochloric acid (HCl), deionized water, and the organic template,
Pluronic F127. For example, the molar ratios of
TTIP/F127/HCl/H.sub.2O/ethanol can be 1:0.005:0.5:15:40. The range
of molar ratios for each component of the solution can be
1:(0-1):(0-1):(1-50):(1-100). TiO.sub.2 nanocrystals were prepared
with a precursor solution of titanium butoxide, ethanol,
hydrochloric acid (HCl), deionized water, and the organic template,
Pluronic F127. For example, the molar ratios of titanium
butoxide/F127/HCl/H.sub.2O/ethanol can be 1:0.005:0.5:15:40. The
range of molar ratios for each component of the solution can be
1:(0-1):(0-1):(1-50):(1-100). TiO.sub.2 nanocrystals were also
prepared with a precursor solution of titanium chloride, ethanol,
hydrochloric acid (HCl), deionized water, and the organic template,
Pluronic F127. For example, the molar ratios of titanium
choloride/F127/HCl/H.sub.2O/ethanol can be 1:0.005:0.5:15:40. The
range of molar ratios for each component of the solution can be
1:(0-1):(0-1):(1-50):(1-100). In these solutions, titanium
tetraisopropoxide, titanium butoxide, and titanium chloride can be
replaced by other titanium precursors, such as titanium
tetrapropoxide, isopropoxide, or other organo-metallic compounds
including titanium. Ethanol can be replaced by other types of
alcohol, such as methanol. Pluronic F127 can be replaced by other
types of organic surfactants, such as P123, Triton X100. HCl can be
replaced by other types of acid, such as HSO.sub.4, HNO.sub.3.
[0058] The solution was kept at 0.degree. C. to 100.degree. C. for
a period from 1 to 72 hours, and then evaporated and dried at
60-200.degree. C. for 1 to 72 hours. The dried powders were
calcinated at a temperature in the range from 150 to 850.degree. C.
for 1 to 72 hours to remove the organic template and enhance
crystallization of TiO.sub.2. Both the temperature ramp rate and
the cooling rate can be in the range from 0.01 to 1.degree. C./min.
The resulting white-colored powders of TiO.sub.2 nanocrystals were
first placed under vacuum for 30 minutes to 24 hour at a
temperature of 20 to 500.degree. C. and then hydrogenated in a 200
millibar to 300 bar H.sub.2 atmosphere at 20 to 800.degree. C. for
1 hour to 30 days to produce TiO.sub.2 nanoparticles of the present
invention.
[0059] Disorder-engineered TiO.sub.2 nanoparticles have been made
having a visual appearance that varies from white to gray to black
by varying the hydrogenation parameters. For example, grey
TiO.sub.2 nanoparticles have been made with the hydrogenation
parameters of pressure, temperature, and time of 5 bar, 250.degree.
C., and 12 hours and black TiO.sub.2 nanoparticles have been made
with the hydrogenation parameters of pressure, temperature, and
time of 20 bar, 500.degree. C., and 24 hours.
[0060] The structures of the TiO.sub.2 nanocrystals before
hydrogenation and of the TiO.sub.2 nanoparticles (i.e. black
TiO.sub.2 nanoparticles) after hydrogenation were investigated with
x-ray diffraction (XRD), Raman spectroscopy, and scanning and
transmission electron microscopy (SEM and TEM); the results are
provided in FIGS. 7A-7D.
[0061] FIG. 7A provides XRD data for white and black TiO.sub.2
nanoparticles in which the latter is an example of the present
invention. Both the white and black TiO.sub.2 nanoparticles were
highly crystallized according to the strong XRD diffraction peaks.
The crystalline phase for both has an anatase structure with an
average crystal size around 8 nm. The crystal size from the XRD
pattern was calculated using the Scherrer formula,
D=0.9.lamda./.beta. cos .theta., where D is the crystal size,
.lamda. is the wavelength of the x-ray radiation (0.15418 nm for Cu
K.sub..alpha. radiation), .beta. is the full width at half maximum,
and .theta. is the diffraction angle. After the hydrogenation
process, the average size of the TiO.sub.2 nanoparticles remains
essentially unchanged.
[0062] FIG. 7B provides Raman spectroscopy data, which was used to
examine structural variations of the TiO.sub.2 nanoparticles after
disorder introduction through hydrogenation. The three polymorphs
of TiO.sub.2 belong to different space groups,
D.sub.4h.sup.19(I4.sub.1/amd) for anatase, D.sub.2h.sup.15(pbca)
for brookite, and D.sub.4h.sup.14(P4.sub.2/mnm) for rutile, which
have distinctive characteristics in Raman spectra. From the
irreducible presentation of the optical modes, the anatase,
brookite, and rutile phases have 6 (3E.sub.g+2B.sub.1g+A.sub.1g),
36 (9A.sub.1g+9B.sub.1g+9B.sub.2g+9B.sub.3g), and 4
(A.sub.1g+B.sub.1g+B.sub.2g+E.sub.g) Raman-active modes,
respectively. For anatase TiO.sub.2, the six Raman-active modes are
at 144, 197, 399, 515, 519 (superimposed with the 515 cm.sup.-1
band), and 639 cm.sup.-1, respectively. As shown in FIG. 7B, the
un-modified, white TiO.sub.2 nanocrystals displayed typical anatase
bands. Nevertheless, new bands at 246.9, 294.2, 352.9, 690.1,
765.5, 849.1 and 938.3 cm.sup.-1 emerged for black TiO.sub.2
nanoparticles that are examples of the present invention. Other
than the 246.9 cm.sup.-1 band that might be attributed to a
brookite-like structure, these new Raman bands cannot be assigned
to any of the three polymorphs of TiO.sub.2, which is indicative of
the formation of new structural features after disorder-engineering
through hydrogenation.
[0063] FIGS. 7C and 7D provide HRTEM (high resolution TEM) images
of white and black TiO.sub.2 nanoparticles in which the latter is
an example of the present invention. Electron microscopy
observations indicate that TiO.sub.2 nanoparticles formed a porous
network with the size of individual crystalline particles
approximately 8 nm. The TiO.sub.2 nanocrystals (i.e. nanoparticles
without disorder) were highly crystallized, as seen from the
well-resolved lattice features shown in the HRTEM image in FIG. 7C.
After hydrogenation, however, the surfaces of a TiO.sub.2
nanoparticle that is an example of the present invention exhibits
disorder, as shown in FIG. 7D. The disorder at the surface of the
nanoparticle may be seen around the edge of the nanoparticle while
the crystalline nature of most of the nanoparticle can be seen in
the central portion of the nanoparticle. The disorder becomes more
apparent when comparing FIG. 7D for the nanoparticle that is an
example of the present invention with the nanocrystal of FIG. 7C
that does not have disorder. In FIG. 7C it is clear that there is
no disorder around the edge of the nanocrystal. This disordered
outer layer resulting from hydrogenation in FIG. 7D, approximately
1 nm in thickness, is reflected by the many new vibrational modes
observed in the Raman spectra of the black TiO.sub.2 nanoparticles
discussed above relative to FIG. 7B.
[0064] FIGS. 8A and 8B provide x-ray photoelectron spectroscopy
(XPS) data, which was used to examine the change of surface
chemical bonding due to hydrogenation of TiO.sub.2 nanoparticles
that are examples of the present invention. The valence state of
the titanium atoms is Ti.sup.4+ for both the white and black
TiO.sub.2 nanoparticles, as shown in FIG. 8A. The Ti 2p XPS data
were almost identical, suggesting that titanium atoms have similar
bonding environment after hydrogenation. In the Ti 2p spectral
region, there are four peaks at 458.9, 464.4, 472.3, and 477.9 eV.
These peaks can be attributed to Ti 2p3/2, Ti 2p1/2 and their
satellite peaks with typical Ti.sup.4+ characteristic binding
energies. All Ti 2p signals are symmetric with no shoulders at the
lower energy sides, suggesting that defect concentration associated
with Ti.sup.3+ was minimal. In contrast, the O 1s XPS spectra of
the white and black TiO.sub.2 nanoparticles have drastic
differences, as shown in FIG. 8B, indicating a change of the
bonding environment for oxygen atoms after hydrogenation. The O 1s
XPS spectra can be resolved into two peaks at about 530.0 and 530.9
eV. The narrower peak at 530.0 eV has typical O.sup.2- binding
energy with TiO.sub.2 composition, and the broader peak at 530.9 eV
can be attributed to Ti--OH species. For un-modified, white
TiO.sub.2 nanocrystals, the O 1s peak is dominated at 530.0 eV,
typical for crystalline TiO.sub.2. However, for the black TiO.sub.2
nanoparticles, the dominant O 1s peak moved to a much wider band at
530.9 eV, suggesting that Ti--OH bonding distribute over an
environment with large disorders on nanoparticle surfaces.
[0065] FIGS. 9A-9D provide visual, optical, and band gap data for
porous networks of black TiO.sub.2 nanoparticles, which are
examples of the present invention, and porous networks of white
TiO.sub.2 nanocrystals for comparison purposes.
[0066] FIG. 9A is a photograph of white porous networks of
TiO.sub.2 nanocrystals and porous networks of black TiO.sub.2
nanoparticles, which provides a visual color comparison before and
after hydrogenation. Measured with diffusive reflectance and
absorbance spectroscopy and shown in FIGS. 9B and 9C, the band gap
of un-modified, white TiO.sub.2 nanocrystals is approximately 3.30
eV, slightly greater than that of the bulk anatase TiO.sub.2 due to
quantum confinement-induced band gap enlargement. The un-modified,
white TiO.sub.2 nanocrystals exhibited an absorption edge of about
430 nm. After hydrogenation, the color of the disorder-engineered
TiO.sub.2 nanoparticles turned to black (FIG. 9A), with absorption
edge in the near infrared beyond 1200 nm (.about.1 eV). The optical
property of a material is a reflection of its intrinsic electronic
structure such as the transitions from an occupied electronic level
to an empty level; they are transitions from the valence band to
the conduction band in a pure semiconductor. When dopants and
disorders are introduced, additional extrinsic electronic levels
can be located in the band gap. The long-tail absorption of the
disorder-engineered TiO.sub.2 nanoparticles in the near infrared
wavelength regime can be assigned to optical transitions involved
with surface disorders that have corresponding features in the O 1s
XPS spectrum. An abrupt change in both the reflectance and
absorbance spectra at approximately 806.8 nm (1.54 eV) can be
assigned to the band gap transition of the disorder-engineered
TiO.sub.2 nanoparticles. This result suggests that the band gap of
the black TiO.sub.2 nanoparticles is significantly narrowed.
Meanwhile, no color change was observed for the disorder-engineered
TiO.sub.2 nanoparticles over a period of one year since they were
synthesized for the first time.
[0067] The density of states (DOS) of the valence band of TiO.sub.2
nanoparticles was also measured by XPS. FIG. 9D shows the valence
band XPS result of disorder-engineered TiO.sub.2 nanoparticles
through hydrogenation, as compared to that of the un-modified,
white nanocrystals. The white TiO.sub.2 nanocrystals display
typical valence band characteristics of TiO.sub.2 with the onset
energy at about 1.62 eV, in excellent agreement with the
literature. Since the band gap of TiO.sub.2 is 3.30 eV from the
optical absorption spectrum, the edge of its conduction band would
occur at the position of -1.68 eV. For the black TiO.sub.2
nanoparticles, the edge of the valence band moves toward the vacuum
level with the onset at -1.72 eV and the shoulder of the valence
band edge at -1.06 eV. Combined with the results from optical
measurements, the valence band XPS onset can be assigned to the
contribution from the disorder, while the XPS shoulder can be
assigned to the valence band edge. Thus, an illustration of the
electronic band structure of the black TiO.sub.2 nanoparticles can
be obtained as shown schematically in FIG. 9E. Compared to that of
a white TiO.sub.2 nanocrystal, the valence band edge of the black
TiO.sub.2 nanoparticle moves up 2.68 eV, and the conduction band
edge moves up 0.92 eV towards the vacuum level, with a band gap of
1.54 eV and disorder band tail states near the valence band
edge.
[0068] Solar-driven photocatalytic activity of the
disorder-engineered, black TiO.sub.2 nanoparticles was examined by
monitoring the change in optical absorption of a methylene blue
solution around 660 nm during its photocatalytic decomposition
process. Other than being a reference nitrogenous compound for
evaluating photocatalysts, methylene blue happens to be a common
water contaminant that threats the ecosystem as well as human
health. Solar-driven photocatalysis measurements were conducted by
irradiating the sample solutions with a Newport Oriel full spectrum
solar simulator, installed with an AM 1.5 air mass filter that
generates about 1 sun power. In a typical experiment, an amount of
0.15 mg black TiO.sub.2 nanoparticles was added into a 3.0 ml
methylene blue solution that has an optical density (O.D.) of
approximately 1.0. In monitoring the photocatalytic decomposition
process, optical absorption spectra of the methylene blue solution
were measured with a Varian Cary Bio50 UV-visible spectrometer
under aerobic conditions and corrected for methylene blue
degradation in the absence of any photocatalyst.
[0069] FIGS. 10A-10C provide results of these photocatalysis
experiments in which black TiO.sub.2 nanoparticles, which are
examples of the present invention, were placed in a methylene blue
solution and exposed to simulated solar radiation. FIG. 10A shows
variations of the absorption spectra of the methylene blue solution
over time upon adding the black TiO.sub.2 nanoparticles and
applying the simulated solar light. The spectral intensity
decreases rapidly with an increase of irradiation time, indicating
that the methylene blue molecules are rapidly breaking down. FIG.
10B compares solar-driven photocatalytic activity of the
disorder-engineered, black TiO.sub.2 nanoparticles against that of
the un-modified, white TiO.sub.2 nanocrystals under the same
testing conditions, in which the x axis is the irradiation time and
the y axis is the O.D. of the methylene blue solution.
Photo-degradation is complete approximately 8 minutes after
initiation of the simulated solar radiation when the photocatalysts
were black TiO.sub.2 nanoparticles, which represents a greater than
300% improvement over the case where white TiO.sub.2 nanocrystals
were used as the photocatalysts. This greater than 300% improvement
is shown FIG. 11. The photo-degradation for the black TiO.sub.2
nanoparticles is complete at 8 minutes while the photo-degradation
for white TiO.sub.2 nanocrystals is not yet complete at 50
minutes.
[0070] FIG. 10C shows the result of cycling tests of the superior
solar-driven photocatalytic activity of the disorder-engineered,
black TiO.sub.2 nanoparticles. Once the photocatalytic reaction of
a testing cycle was complete, the subsequent cycle was started
after an amount of concentrated methylene blue compound was added
to make the O.D. of the solution approximately 1.0. FIG. 10C plots
the first 8-minute data in each of eight consecutive
photo-degradation cycles, which shows that the black TiO.sub.2
nanoparticles do not exhibit reduction of their photocatalytic
activity under solar radiation, even after repeated photocatalysis
cycles.
[0071] In addition to a superior photocatalytic capability in
decomposing methylene blue as an example water contaminant, the
disorder-engineered, black TiO.sub.2 nanoparticles exhibit
unprecedented efficiency and stability of photocatalytic hydrogen
production under sunlight. FIG. 12 shows measured quantities of
hydrogen gas as a function of time over a 22-day testing period of
solar hydrogen production with disorder-engineered, black TiO.sub.2
nanoparticles as the photocatalyst. Solar hydrogen experiments were
conducted under conditions simulating daily solar activities. The
full spectrum solar simulator was used as the excitation source,
which produces about one sun power at the sample consisting of
black TiO.sub.2 nanoparticles loaded with 0.6 wt % Pt, placed in a
Pyrex glass container filled with 1:1 water-methanol solution.
Methanol is commonly applied as the sacrificial agent for solar
water splitting that enhances trapping of photo-generated holes in
addition to enabling pure hydrogen gas release. Measurements were
conducted initially for 15 consecutive days; each day the sample
was irradiated for five hours, and then stored in darkness
overnight before testing the next day. Hydrogen generation was
measured using a Varian gas chromatograph and data were taken
approximately every hour during solar irradiation.
[0072] About 0.2.+-.0.02 mmol hydrogen gas can be generated under
one hour solar irradiation with about 0.02 grams of
disorder-engineered, black TiO.sub.2 nanoparticles, yielding a
hydrogen production rate of 10 mmol per hour per gram of
photocatalysts. This hydrogen production rate, measured under
simulated sunlight, is two orders of magnitude greater than the
yields of known semiconductor photocatalysts. After testing for 13
days, 30 ml pure water was added to compensate for the loss and
measurements continued for two additional days before the sample
was stored in darkness for two days (days 16 & 17) without
measurements. Experiments were resumed for five more days after the
two-day storage period. Throughout the testing cycles, the
disorder-engineered, black TiO.sub.2 nanocrystals exhibit superior
stability in addition to very high hydrogen production capability.
For comparison, unmodified white TiO.sub.2 nanocrystals were also
tested under the same experimental conditions and no hydrogen gas
was detected.
[0073] As the hydrogenation process was applied to realize
disorder-engineered TiO.sub.2 nanocrystals, we conducted
experiments to quantify the amount of hydrogen absorbed in black
TiO.sub.2. Measurements were conducted using a hydrogen storage
capacity testing system (Intelligent Gravimetric Analyzer, Hiden
Isochema), and we found that black TiO.sub.2 nanoparticles contain
about 0.25 wt. % of hydrogen. In solar hydrogen production
experiments, 20.0 mg of black TiO.sub.2 photocatalysts were used,
which produced 20 mmol (40 mg) of hydrogen over a period of 100
hours. Note that, 20.0 mg of black TiO.sub.2 photocatalysts only
contain about 0.05 mg of hydrogen, which is significantly smaller
than the amount of hydrogen generated (40 mg) in the solar hydrogen
production experiments. This result indicates that black TiO.sub.2
photocatalysts do not act as a hydrogen reservoir for solar-driven
hydrogen production; it is consistent with the observation that the
photocatalysts remain black after cycling experiments.
[0074] To evaluate what fraction of the solar hydrogen activity of
black TiO.sub.2 is due to the visible and infrared portion of the
solar spectrum, we measured photocatalytic hydrogen production by
placing an UV filter in front of the solar simulator that blocks
photons with wavelength shorter than 400 nm. While white TiO.sub.2
nanocrystals do not exhibit any visible and infrared photocatalytic
activity, hydrogen production was observed under visible and
infrared irradiation using black TiO.sub.2 as the photocatalyst.
The rate of hydrogen production under visible and infrared light
was measured to be about 0.1 mmol per hour per gram of
photocatalysts. Although small compared to hydrogen production
under simulated sunlight, this rate is consistent with the
microscopic structure of individual black TiO.sub.2 nanoparticles
in which, disorders are present at the surface layer (.about.1 nm)
while the majority of the volume remains single crystalline. On the
other hand, for UV as well as for visible and infrared
light-excited electrons, the disorder layer of black TiO.sub.2
nanoparticles offers much-desired trapping sites to suppress rapid
carrier recombination, thus creating a significantly higher
hydrogen production rate than what conventional photocatalysts can
achieve.
[0075] The realization of black TiO.sub.2 nanostructures, such as
black TiO.sub.2 nanoparticles and porous networks of such
particles, that absorb photons in the entire range of the solar
spectrum offers an unprecedented, nearly ideal photocatalyst that
has not only a substantially enhanced photocatalytic activity under
solar radiation but also the desired stability for practical
implementation. In fact, the black TiO.sub.2 nanostructures
irradiated constantly by fluorescent light at room temperature for
over a year exhibit essentially the same superior photocatalytic
activity. Applications of solar-driven photocatalysis for
decomposition of organic compounds or splitting of water, for
example, are highly scalable and economical, as compared to the
processes that primarily rely on UV radiation. One the other hand,
the concept of disorder engineering introduced in this report opens
a new route for manipulating optical absorption of semiconductor
nanostructures in general. It is anticipated that, through disorder
engineering along with proper doping, band gap energy is less a
limiting factor for many wide band gap materials that are
traditionally not considered for solar energy applications.
Whatever the future may be for these non-traditional
semiconductors, it is tantalizing to envision diverse clean energy
and environmental technologies enabled by the black TiO.sub.2,
achieved here with disorder engineering through hydrogenation.
REFERENCES
[0076] 1. Gratzel, M., Photoelectrochemical cells, Nature, 414,
338-344 (2001). [0077] 2. Hoffmann, M. R., Martin, S. T., Choi, W.,
& Bahnemann, D. W., Environmental applications of semiconductor
photocatalysis, Chem. Rev., 95, 69-96 (1995). [0078] 3. Fujishima,
A., Zhang, X. & Tryk, D. A., TiO.sub.2 photocatalysts and
related surface phenomena, Surf. Sci. Rpts, 63, 515-582 (2008)
[0079] 4. Asahi, R., Morikawa, T., Ohwaki, T, Aoki, K., & Taga,
Y., Visible-light photocatalysis in nitrogen-doped titanium oxides,
Science, 293, 269-271 (2001). [0080] 5. Chen, X. B. & Burda,
C., The electronic origin of the visible-light absorption
properties of C-, N- and S-doped TiO.sub.2 nanomaterials, J. Am.
Chem. Soc., 130, 5018-5019 (2008). [0081] 6. Chen, X. & Mao, S.
S., Titanium dioxide nanomaterials: Synthesis, properties,
modifications, and applications, Chem. Rev., 107, 2891-2959 (2007).
[0082] 7. Burda, C., Chen, X., Narayanan, R., & El-Sayad, M.
A., Chemistry and properties of nanocrystals of different shapes,
Chem. Rev., 105, 1025-1102 (2005). [0083] 8. PCT Published
Application No. WO 2009/012397, published on Jan. 22, 2009.
[0084] It is noted that as used herein and in the appended claims,
the singular forms "a", "and", and "the" include plural referents
unless the context clearly dictates otherwise.
[0085] The foregoing detailed description of the present invention
is provided for the purposes of illustration and is not intended to
be exhaustive or to limit the invention to the embodiments
disclosed. Accordingly, the scope of the present invention is
defined by the appended claims.
* * * * *