U.S. patent application number 13/943160 was filed with the patent office on 2014-01-16 for dual absorber electrodes.
The applicant listed for this patent is The Trustees of Boston College. Invention is credited to Chun Du, Matthew T. Mayer, Dunwei Wang.
Application Number | 20140014501 13/943160 |
Document ID | / |
Family ID | 49913022 |
Filed Date | 2014-01-16 |
United States Patent
Application |
20140014501 |
Kind Code |
A1 |
Wang; Dunwei ; et
al. |
January 16, 2014 |
Dual Absorber Electrodes
Abstract
Dual absorber electrodes are disclosed. In some embodiments, a
dual absorber electrode includes a first absorber material, such as
silicon, having a first bandgap, and a second absorber material,
such as hematite, deposited on a surface of the first absorber
material, the second absorber material having a second bandgap
larger than the first bandgap of the first absorber. In some
embodiments, the dual absorber electrodes of the present embodiment
may be utilized in an electrolytic cell for water splitting.
Inventors: |
Wang; Dunwei; (Newton
Highlands, MA) ; Mayer; Matthew T.; (Brighton,
MA) ; Du; Chun; (Brighton, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Trustees of Boston College |
Chestnut Hill |
MA |
US |
|
|
Family ID: |
49913022 |
Appl. No.: |
13/943160 |
Filed: |
July 16, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61672252 |
Jul 16, 2012 |
|
|
|
Current U.S.
Class: |
204/252 ;
204/290.01; 427/78 |
Current CPC
Class: |
Y02E 60/36 20130101;
C25B 11/0421 20130101; C25B 11/00 20130101; C25B 11/0405 20130101;
C25B 1/003 20130101; Y02E 60/366 20130101; C25B 1/10 20130101 |
Class at
Publication: |
204/252 ;
204/290.01; 427/78 |
International
Class: |
C25B 1/00 20060101
C25B001/00; C25B 11/00 20060101 C25B011/00; C25B 11/04 20060101
C25B011/04; C25B 1/10 20060101 C25B001/10 |
Goverment Interests
STATEMENT OF GOVERNMENT SUPPORT
[0002] This invention was made with Government Support under
Contract Number DMR1055762 awarded by the National Science
Foundation. The Government has certain rights in the invention.
Claims
1. A dual absorber electrode comprising a first absorber material
having a first bandgap; and a second absorber material deposited on
a surface of the first absorber material, the second absorber
material having a second bandgap larger than the first bandgap of
the first absorber.
2. The electrode of claim 1 wherein the first absorber material is
a silicon nanostructure and the second absorber material is a
hematite thin film deposited on the surface of the silicon
nanostructure.
3. The electrode of claim 1 further comprising a uniform interface
between the first absorber material and the second absorber
material with low defect densities and low impurity levels.
4. The electrode of claim 1 wherein the second absorber material is
deposited over the first absorber material in a conformal
fashion.
5. The electrode of claim 1 wherein the second absorber material is
a thin film conformally deposited on the surface of the first
absorber material.
6. The electrode of claim 1 wherein the first absorber material is
a photovoltaic junction.
7. The electrode of claim 1 wherein the second absorber material is
a photovoltaic junction.
8. The electrode of claim 1 further comprising a catalyzing member
in electrical contact with the first absorber material for
catalyzing hydrogen generation.
9. The electrode of claim 8 wherein the first absorber material is
a silicon nanostructure and the second absorber material is a
hematite thin film deposited on the surface of the silicon
nanostructure.
10. The electrode of claim 8 wherein a uniform interface is formed
between the first absorber material and the second absorber
material with low defect densities and low impurity levels.
11. The electrode of claim 8 wherein the second absorber material
is a photovoltaic junction.
12. A device for splitting water to generate hydrogen and oxygen
comprising: a first compartment having a first electrode, the
electrode comprising a first absorber material having a first
bandgap, and a second absorber material deposited on a surface of
the first absorber material, the second absorber material having a
second bandgap larger than the first bandgap of the first absorber;
a second compartment having a second electrode for catalyzing
hydrogen generation; and a semi-permeable membrane separating the
first compartment and the second compartment.
13. The device of claim 12 wherein the first absorber material is a
silicon nanostructure and the second absorber material is a
hematite thin film deposited on the surface of the silicon
nanostructure.
14. The device of claim 12 wherein a uniform interface is formed
between the first absorber material and the second absorber
material with low defect densities and low impurity levels.
15. The device of claim 12 wherein the second absorber material is
deposited over the first absorber material in a conformal
fashion.
16. The device of claim 12 wherein the second absorber material is
a thin film conformally deposited on the surface of the first
absorber material.
17. The device of claim 12 wherein the first absorber material is a
photovoltaic junction.
18. The device of claim 12 wherein the second absorber material is
a photovoltaic junction.
19. A method of fabricating an electrode comprising: obtaining a
silicon nanostructure; forming by vapor deposition a layer of
hematite on a surface of the silicon nanostructure by exposing the
silicon nanostructure to gas precursors of hematite; and repeating
the vapor-depositing to form additional layers of hematite on the
surface of the silicon nanostructure.
20. The method of claim 19 wherein the silicon nanostructure is a
photovoltaic junction.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of and priority to U.S.
Provisional Application No. 61/672,252, filed on Jul. 16, 2012, and
which is incorporated herein by reference in its entirety.
FIELD
[0003] The embodiments disclosed herein relate to photoelectric
electrodes for use in water splitting.
BACKGROUND
[0004] Photoelectrochemical (PEC) water splitting offers the
capability of harvesting the energy in solar radiation and
transferring it directly to chemical bonds for easy storage,
transport, and use in the form of hydrogen. Among the various
considerations of a PEC system, the choice of photoelectrode
materials is especially important because their properties, such as
optical absorption characteristics and chemical stability,
determine the system's performance. These materials should absorb
light broadly, be inexpensive, and be resistant to photo
corrosion.
SUMMARY
[0005] Dual absorber electrodes are disclosed. According to some
aspects illustrated herein, there is provided a dual absorber
electrode that includes a first absorber material having a first
bandgap; and a second absorber material deposited on a surface of
the first absorber material, the second absorber material having a
second bandgap larger than the first bandgap of the first
absorber.
[0006] According to some aspects illustrated herein, there is
provided a device for splitting water to generate hydrogen and
oxygen that includes a first compartment having a first electrode,
the electrode comprising a first absorber material having a first
bandgap, and a second absorber material deposited on a surface of
the first absorber material, the second absorber material having a
second bandgap larger than the first bandgap of the first absorber;
a second compartment having a second electrode for catalyzing
hydrogen generation; and a semi-permeable membrane separating the
first compartment and the second compartment.
[0007] According to some aspects illustrated herein, there is
provided a dual absorber electrode that includes a first electrode
comprising first absorber material and a second absorber material
having a larger band gap than the first absorber material; and a
second electrode for catalyzing hydrogen generation in electrical
contact with the first absorber material.
[0008] According to some aspects illustrated herein, there is
provided a method of fabricating an electrode that includes
obtaining a silicon nanostructure; forming by vapor deposition a
layer of hematite on a surface of the silicon nanostructure by
exposing the silicon nanostructure to gas precursors of hematite;
and repeating the vapor-depositing to form additional layers of
hematite on the surface of the silicon nanostructure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The presently disclosed embodiments will be further
explained with reference to the attached drawings, wherein like
structures are referred to by like numerals throughout the several
views. The drawings shown are not necessarily to scale, with
emphasis instead generally being placed upon illustrating the
principles of the presently disclosed embodiments.
[0010] FIG. 1 illustrates an embodiment dual absorber electrode of
the present disclosure.
[0011] FIG. 2 illustrates an embodiment electrolytic cell for water
splitting suitable for use with a dual absorber electrode of the
present disclosure.
[0012] FIG. 3 illustrates an embodiment dual absorber particle of
the present disclosure.
[0013] FIG. 4 illustrates an embodiment method using the dual
absorber particles presented in FIG. 3 for water splitting.
[0014] FIG. 5A and FIG. 5B present energy band schematics
(quasi-static equilibrium under solar illumination) and
photocurrent-voltage plots for water oxidation by Fe.sub.2O.sub.3
on Si nanowires or FTO (fluorine doped tin oxide).
[0015] FIG. 6A and FIG. 6B present microstructure of the
Si/Fe.sub.2O.sub.3 nanowire photoelectrodes. FIG. 6A presents a
scanning electron micrograph (SEM) showing the arrangement of
chemically-etched Si nanowires. FIG. 6B presents a transmission
electron micrograph (TEM) showing the crystalline quality of the
ALD-grown Fe.sub.2O.sub.3 film on the Si nanowire surface. Insets:
electron diffraction pattern (top right) and energy-dispersive
X-ray spectroscopy (EDS) line scans revealing the compositional
makeup across the Si/Fe.sub.2O.sub.3 interface (bottom).
[0016] FIG. 7 is a low-magnification transmission electron
microscope (TEM) image showing the uniform and conformal nature of
the ALD-deposited hematite film on a typical etched Si
nanowire.
[0017] FIG. 8A and FIG. 8B present results of SiNWs/hematite
photoanode testing. FIG. 8A presents a graph of photocurrent
measured over 3 h under AM1.5 illumination and 1.0 V.sub.RHE
applied bias. Approximately every 15 min, the illumination was
blocked for a few seconds to check the dark current, which remained
around zero for the duration of the test. FIG. 8B presents J-V
curves before and after this 3 h test, showing negligible change in
performance.
[0018] FIG. 9A, FIG. 9B and FIG. 9C present J-V curves for hematite
films on planar substrates under different illumination
conditions.
[0019] FIG. 10A and FIG. 10B present band diagrams at
pre-equilibrium and equilibrium in dark, respectively. At
equilibrium, band bending at each junction results due to the
equilibration of chemical potentials across the materials. Band
edge positions and Fermi levels are approximate, determined
theoretically and from literature reports, and the x-axis depth of
each material is not to scale. The nature of the band behavior at
the interface has been proposed in the literature.
[0020] FIG. 11A, FIG. 11B, FIG. 11C, and FIG. 11D demonstrate the
dual-absorber nature of hematite/Si nanowires photoanodes for water
oxidation. FIG. 11A presents J-V curves for a typical electrode
under different and combined monochromatic illumination suggests a
synergistic effect when combining low-energy and high-energy
photons, and representative band energy diagrams show the two cases
of independent excitation. FIG. 11B presents the photocurrent
responses under stepwise monochromatic chopped illumination in the
wavelength range 300-1000 nm, under applied bias of 1.0 V.sub.RHE,
reveal the characteristic responses of Fe.sub.2O.sub.3 and Si in
the device. In reference to FIG. 11C, in the short wavelength
range, photons are primarily absorbed by the Fe.sub.2O.sub.3, and
stable anodic photocurrents are observed. As shown in FIG. 11D, at
longer wavelengths, photons pass through Fe.sub.2O.sub.3 to excite
only Si, producing zero net current.
[0021] FIG. 12A and FIG. 12B present the photocurrent responses of
hematite on FTO under stepwise monochromatic chopped illumination
in the wavelength range 300-780 nm under applied bias of 1.45
V.sub.RHE.
[0022] FIG. 13A presents representative photocurrent-voltage plots
for Fe.sub.2O.sub.3/Si nanowire and Fe.sub.2O.sub.3/FTO
devices.
[0023] FIG. 13B presents representative plots for
Fe.sub.2O.sub.3/Si nanowire devices of different Si doping
characteristics. The details of the Si doping are tabulated
below.
[0024] While the above-identified drawings set forth presently
disclosed embodiments, other embodiments are also contemplated, as
noted in the discussion. This disclosure presents illustrative
embodiments by way of representation and not limitation. Numerous
other modifications and embodiments can be devised by those skilled
in the art which fall within the scope and spirit of the principles
of the presently disclosed embodiments.
DETAILED DESCRIPTION
[0025] In order to completely split water, 1.6V of potential is
needed. However, there are many photoanode materials with otherwise
desirable properties that cannot produce 1.6V of potential. For
example, research on using hematite (Fe2O3) to absorb solar light
and split water is moving at a slow pace despite the positive
prospect of hematite having the suitable bandgap and being low
cost, and the limiting factor has been the intrinsic physical and
chemical properties of this material. In particular, hematite
suffers from a number of challenges, including, short hole
diffusion distances, poor catalytic activities, and mismatch of
hematite band edge positions with the reduction and oxidation
potentials of water. A considerable voltage is needed to induce the
water-splitting reaction (fundamentally 1.23 V but upwards of 2 V
in practice), so a large band gap semiconductor would be desired.
However, the band gap defines the minimum energy a solar photon
must have to be absorbed, and thus larger bandgaps lead to
decreased light absorption and therefore lower efficiencies. In
actual photoelectrolysis cells, this limitation manifests as a
dependence on an external applied bias to achieve the necessary
voltage for water splitting. A primary goal is therefore to reduce
or eliminate this dependence and achieve unassisted water
splitting. To do this, the solar spectrum needs to be more
efficiently utilized and a greater photovoltage needs to be
generated.
[0026] The materials of the present disclosure address these
challenges by direct coupling of a metal oxide, such as hematite,
with smaller-bandgap materials, such as silicon, to create
multiple-absorber systems. The materials of the present disclosure
can utilize the portion of solar spectrum that cannot be absorbed
by hematite (X>600 nm), which accounts for more than half of the
total solar energy. Electrodes of the present disclosure can
collect red and near-infrared photons (600 nm<.lamda.<1100
nm) to provide extra photovoltages when hematite is interfaced with
Si nanowires. The resulting photoelectrodes exhibit a photocurrent
turn-on potential as low as 0.6 V.sub.RHE, where V.sub.RHE
represents the applied potential relative to the standard potential
of a reversible hydrogen electrode, or RHE. The combined materials
of the present disclosure thus directly enhance the photovoltage
obtainable on a single device. This results in a reduction in the
magnitude of applied voltage which is necessary to achieve water
splitting. Conventional approaches rely on external sources to
provide the entire additional voltage, including electronic power
supplies or photovoltaic modules. The improvement of the present
disclosure comes from the more efficient utilization of the solar
spectrum resulting from the use of two absorbers, a unique
demonstration for metal oxide devices.
[0027] It should be noted however that the electrodes of the
present can be utilized in other photovoltaic applications, not
just water photoelectrolysis. Solid-state photovoltaic or
photoelectrochemical cells in which the generated charge is
collected in electronic form, rather than water splitting, could be
designed by this same principle. The presently disclosed design can
also be used to supply charge to other electrochemical reactions
other than water splitting, including, but not limited to,
photosynthesis of other useful molecules or fuels. The materials of
the present disclosure may be used in photoelectrochemical
synthesis applications in which photo-generated charge is used to
drive chemical reactions, in photovoltaic cells to generate
electricity, and in solar filters designed to selectively block
light.
[0028] In reference to FIG. 1, electrodes 100 of the present
disclosure comprise two absorber materials 102, 104 of different
bandgap, wherere the absorbers are stacked together to allow the
light to pass through the largest-bandgap absorber first. In some
embodiments, the inner absorber 102 having a smaller band-gap may
be in the form of a nanowire, nanoparticle, nanorod, nanonet or
another nanostructure. In some embodiments, the higher bandgap
absorber may form at least a partial shell around the smaller
bandgap absorber. In some embodiments, the inner absorber material
102 of the presently disclosed electrodes comprises one or more
smaller bandgap materials such as, for example, Si, Ge, InP, GaAs,
and similar materials. The outer absorber 104 of the
presently-disclosed electrodes may comprise one or more
larger-bandgap materials which are designed to produce larger
photovoltages as a result of having multiple complimentary light
absorbers. Suitable larger-bandgap materials include, but are not
limited to Fe.sub.2O.sub.3, WO.sub.3, TiO.sub.2, SnO.sub.2, ZnO,
and other similar metal oxides. In some embodiments, the electrode
100 may also include an electrical contact 106 electrically
connected to the inner absorber maternal 102 to enable collection
of electrons generated inside the inner absorber material 102.
[0029] In some embodiments, the dual absorber electrodes of the
present disclosure include a Si nanowire core with hematite
deposited over at least a part of the Si nanowire in a conformal
fashion. When hematite is deposited on Si nanowires in a conformal
fashion, hematite and Si can be independently excited by photons of
different energies in the solar spectrum. Under simultaneous
excitation, a dual-absorber mechanism develops. Charge flow is
enabled only when both are excited in a synergistic manner, and the
photopotentials developed within the two materials contribute to
enhanced water splitting performance as evidenced by a cathodic
shift in the photocurrent onset potential. The onset potential of
0.6 V.sub.RHE represents one of the lowest reported for hematite
photoanode devices, and is achieved without the use of catalysts,
hematite doping, or surface treatments. Accordingly, the utility of
hematite can be improved using direct coupling with small-bandgap
materials to more efficiently utilize the solar spectrum and to
enhance the photovoltages attainable by a single device. The device
fabrication depends on the growth of high-quality thin films of
hematite made possible by the ALD technique. Furthermore, the fact
that the active materials are primarily composed of three of the
four most abundant elements in Earth's crust (O, Si, Fe) offers
promise that renewable energy harvesting by photoelectrochemical
water splitting remains an achievable goal.
[0030] FIG. 2 shows an exemplary illustration of a device 1100 of
the present disclosure for use in water splitting. The device 1100
includes two compartments, 1110 and 1120, each of which can be used
for the half reactions of H.sub.2 and O.sub.2 generations. Solar
energy is harnessed to separate charges, which then transfer to the
redox pairs in the solutions to perform reactions. The appropriate
energy alignment can be enabled by material choices (p-type for
H.sub.2 and n-type for O.sub.2) and the adjustment of solution pH.
Highly conductive components ensure efficient charge transport,
thus completing the full reaction of H.sub.2O splitting. In an
embodiment, compartment 1110 is filled with an acidic solution, and
compartment 1120 is filled with a basic solution. Compartments 1110
and 1120 are separated by a semi-permeable membrane 1140 that only
allows ionic exchange to balance potential buildup. In an
embodiment, the semi-permeable membrane 1140 is a charge-mosaic
membrane (CMM). In the acidic compartment 1110, a p-type material
acts to produce H.sub.2 upon illumination. The device 1100 includes
electrodes 1115 and 1125. In some embodiments, one electrode may
comprises a dual absorber material of the present disclosure, such
as silicon core embedded in a hematite shell. In some embodiments,
the second electrode catalyzes hydrogen generation. The second
electrode may be made of platinum or another metal capable of
acting as a catalyst for hydrogen generation, supporting hydrogen
generation or both.
[0031] The electrodes 1115 and 1125 can be connected together by
external contacts 1150 to ensure charge balance. In the solution,
opposite charges flow through the semi-permeable membrane 1140 to
annihilate each other. Both the acidic and the basic solutions
should be periodically refreshed by adding more acids or bases to
maintain an appropriate chemical potential difference by
maintaining a preset pH difference.
[0032] In reference to FIG. 3, in some embodiments, the materials
of the present disclosure may be presented as particles 300
including a dual absorber material including an inner absorber 302
and an outer absorber 304, combined with a suitable catalyzing
member 306 such as platinum or another catalyst metal. The
catalyzing member 304 may essentially act as the second electrode
for photochemical water splitting. The inner absorber 302 may be in
electrical contact with the catalyzing member 306 to allow
electrons to travel from the inner absorber 302 to the second
electrode material.
[0033] In reference to FIG. 4, in some embodiments, a plurality of
particles 300 may be added to a container 310 containing water 312
to effectuate splitting of water 312 into hydrogen an oxygen atoms
upon absorption of light by the particles 300.
[0034] In some embodiments, an electrode of the present disclosure
may include two (or more) semiconductors which absorb in different
regions of the solar spectrum. In some embodiments, the outer
absorber, such as hematite, operates as a typical photoelectrode
for water photo-oxidation, while the inner absorber, such as the Si
nanowire, uses the energy in long-wavelength photons to further
increase the energy of electrons that will be ultimately utilized
for water photo-reduction. The net effect is that hematite-based
water splitting can be carried out at reduced external
potentials.
[0035] In some embodiments, the outer absorber material of the
present disclosure may include a photovoltaic junction, either a
p-n junction or a p-i-n junction. In some embodiments, the outer
absorber material of the present disclosure is formed by coupling
an n-doped hematite and a p-doped hematite to form a p-n hematite
junction. In some embodiments, the inner absorber material may
include a photovoltaic junction, either a p-n junction or a p-i-n
junction. In some embodiments, both the outer absorber material and
the inner absorber material include or are a photovoltaic
junction.
[0036] In some aspects, the present disclosure provides methods of
growing photovoltaic hematite on lower bandgap material. Various
gas phase deposition methods may be utilized to form photovoltaic
hematite junctions of the present disclosure, including, but not
limited to, atomic layer deposition, chemical vapor deposition,
pulse laser deposition, evaporation and solution synthesis approach
and similar methods. In some embodiments, a uniform interface is
formed between the outer absorber material and the inner absorber
material. In some embodiments, the outer absorber material is
conformal to the surface of the inner material absorber. In some
embodiments, the interface between absorbers has low defect
densities and low impurity levels. Metal oxide deposition
techniques such as hydrolysis, simple vapor phase deposition, may
typically fail to produce an interface that meets this requirement.
In some embodiments, the outer absorber material is deposited by
atomic layer deposition to form an ultra-thin film of the outer
absorber material one atomic layer at a time.
[0037] In some embodiments, the method of forming an electrode of
the present disclosure includes depositing multiple layers of
hematite (outer absorber material) on a surface of a silicon
nanostructure (inner absorber material). First, a silicon
nanostructure of a desired shape and size is obtained. Next, a
layer of the hematite may be deposited on the silicon nanostructure
by exposing the silicon nanostructure at a first temperature to a
pulse of iron precursor, followed by a pulse of oxygen precursor.
In some embodiments, the first temperature may be selected based on
the reaction temperature of precursors, such as, for example
between about 140 C and about 180 C, to enable formation of n-type
hematite from precursors on the substrate surface. In some
embodiments, the precursors may be maintained at a temperature from
120 C to 135 C to yield appreciable vapor pressure of precursor.
This step may be repeated until the layer of the hematite is of a
desired thickness. Subsequently, the electrode may be annealed at a
temperature sufficient to crystallize the hematite.
[0038] The methods and materials of the present disclosure are
described in the following Examples, which are set forth to aid in
the understanding of the disclosure, and should not be construed to
limit in any way the scope of the disclosure as defined in the
claims which follow thereafter. The following examples are put
forth so as to provide those of ordinary skill in the art with a
complete disclosure and description of how to make and use the
embodiments of the present disclosure, and are not intended to
limit the scope of what the inventors regard as their invention nor
are they intended to represent that the experiments below are all
or the only experiments performed. Efforts have been made to ensure
accuracy with respect to numbers used (e.g. amounts, temperature,
etc.) but some experimental errors and deviations should be
accounted for.
EXAMPLES
[0039] EXPERIMENTAL PROCEDURE: Fabrication of
.alpha.-Fe.sub.2O.sub.3/Si NW devices: Silicon nanowire (NW) arrays
were synthesized by an electroless chemical etching method
previously reported. A cleaned n-type Si (100) substrate (P-doped,
5-15 .OMEGA.cm, University Wafer) was treated in a 3:1 (v/v)
H.sub.2SO.sub.4/H.sub.2O.sub.2 at 90.degree. C. for 15 min. and
then rinsed thoroughly with DI water. The substrate was then
immersed, with polished side facing upwards, into a solution of 4.4
M HF and 0.02 M AgNO.sub.3 at 50.degree. C. A typical etching time
of 30 min. created NWs of approximately 8 .mu.m in length. The
substrate was then removed, rinsed in DI water, and immersed in
concentrated HNO.sub.3 for 15 min. to remove Ag from the Si
surfaces. Finally, the substrate was gently but thoroughly rinsed
in DI water and gently dried in an N.sub.2 stream.
[0040] The general technique for Fe.sub.2O.sub.3 deposition has
been previously reported. The as-prepared Si NW arrays (or other
substrates, including cleaned planar Si and cleaned fluorine-doped
tin oxide on glass) were transferred to an atomic layer deposition
system (ALD; Cambridge Nanotech, Savannah 100) and the deposition
chamber was evacuated to a base pressure of 0.36 Torr. Iron
tert-butoxide (heated to 125.degree. C.) and water (25.degree. C.)
served as the precursors for Fe.sub.2O.sub.3 and were pulsed
alternatingly into the deposition chamber (heated to 180.degree.
C.) with a 10 cm.sup.3 min.sup.-1 flow of N.sub.2 as carrier gas.
The specific pulse sequence was previously reported. Following
deposition, a 15 min. heat treatment at 500.degree. C. in O.sub.2
was applied to all Fe.sub.2O.sub.3 samples to complete the
synthesis of photoactive hematite.
[0041] Anode devices for photoelectrochemical testing were then
prepared. For Fe.sub.2O.sub.3/Si NW devices, a small area of fresh
Si was exposed on the NW array surface by scraping away the NWs
with a blade and roughing the Si with a diamond pen. For ohmic
contact to the n-type Si, a drop of Ga/In eutectic (Sigma Aldrich)
was used for contact between the Si and a copper wire. Then, the
contact area and the edges and backside of the substrate were
passivated by application of insulating epoxy. The resulting
exposed area of Fe.sub.2O.sub.3-coated Si NWs was defined as the
active area to which photocurrents were normalized for each
specimen.
[0042] Photoelectrochemical testing: The photoelectrochemical
behavior of all samples was tested using a potentiostat (CH
Instruments CHI604C) in a three-electrode configuration, with an
alkaline/mercury oxide reference electrode (Hg/HgO/1M NaOH; CH
Instruments) and a Pt wire counter electrode. An aqueous
electrolyte solution of 1.0 M NaOH (pH 13.6), bubbled with N.sub.2
gas, was used. In a typical J-V experiment, the voltage was swept
linearly from 0.6 to 1.6 V.sub.RHE at a rate of 10 mV s.sup.-1.
Illumination sources included an AM 1.5 solar simulator (100 mW
cm.sup.-2, Newport Oriel 96000), an ultraviolet lamp (365 nm
wavelength, UVP UVGL-55), and an infrared laser (980 nm wavelength,
2000 mW cm.sup.-2 power, AixiZ). For the monochromatic spectral
illumination shown in FIG. 3b, a monochromator (Newport Oriel
Cornerstone 260) was coupled to the solar simulator.
[0043] Structural characterization: A scanning electron microscope
(JEOL JSM6340F) and transmission electron microscope (JEOL
JEM2010F) were used for specimen imaging. The TEM specimen was
prepared by dispersing scraped Fe.sub.2O.sub.3/Si nanowires in
isopropyl alcohol and dropping them onto Cu grids with lacey C
coatings. Assuming a cylindrical cross-section, a Si nanowire
diameter of 50 nm, and a Fe.sub.2O.sub.3 thickness of 20 nm
(typical values), the specimen thickness at the Fe.sub.2O.sub.3/Si
interface (along the electron beam path) was approximately 75 nm.
An energy-dispersive X-ray spectrometer (Oxford Inca) was used in
conjunction with the TEM to determine material compositions. All
TEM and EDS data were obtained using a 200 keV accelerating
voltage.
RESULTS: FIG. 5A shows a Fe.sub.2O.sub.3/Si system that relies on
dual light absorbers for charge generation and transfer, with
Fe.sub.2O.sub.3 absorbing higher-energy photons (>2.1 eV) and Si
absorbing lower-energy photons. Holes generated in Fe.sub.2O.sub.3
transfer to the electrolyte to oxidize water and evolve O.sub.2
gas. Photoexcited electrons in the Fe.sub.2O.sub.3 conduction band
undergo favorable recombination with holes from Si at the junction.
Lastly, photoexcited electrons in Si, which achieve more negative
energies than in Fe.sub.2O.sub.3 itself, travel via the external
circuit to perform the water reduction and generate H.sub.2
gas.
[0044] FIG. 5B provides photocurrent-voltage plots under simulated
solar illumination (AM 1.5, 100 mW cm.sup.-2) in 1.0 M NaOH aqueous
electrolyte (pH: 13.6; scan rate: 10 mV s.sup.-1) show that the
presence of the Fe.sub.2O.sub.3/Si junction leads to a significant
cathodic shift of the photocurrent onset potential and the nanowire
array morphology results in enhanced photocurrent (red), as
compared to Fe.sub.2O.sub.3 on FTO (black).
[0045] FIG. 5B compares the photocurrent density with applied
potential (J-V) plots of hematite photoelectrodes with and without
the second absorber of Si nanowires. Most obvious is the
significant shift of the curve toward the cathodic direction.
Because the two photoelectrodes were prepared by the same batch of
hematite growth, the possibilities of unintentional doping or other
phenomenological surface effects such as passivation or catalyst
decoration can be ruled out. A second artifact considered was
whether the current came from photo corrosion of Si, which would be
a result of unsuccessful coverage of Si nanowires by hematite.
Electron microscopy studies revealed that the hematite layer was
conformal, as shown in FIG. 6A, FIG. 6B and FIG. 7.
[0046] The crystalline nature of hematite was also confirmed by the
inset electron diffraction pattern in FIG. 6B. While n-type doping
by Si has been shown to enhance electron conductivity in hematite,
no appreciable Si concentration was detected in the hematite layer
of our Fe.sub.2O.sub.3/Si nanowire devices, suggesting negligible
outward diffusion of Si into hematite took place during the mild
post-growth annealing treatment (500.degree. C.).
[0047] In reference to FIG. 8A and FIG. 8B, sustained photocurrent
without decay was measured for up to 3 h of continued
photoelectrochemical reactions. The data presented in FIG. 8A and
FIG. 8B support the conclusion that photocurrent is not due to Si
oxidation and that the atomic layer deposition (ALD) coating of
hematite protects the device from degradation. FIG. 8B shows that
the photocurrent did not degrade over 3 h, but instead increased
slightly. Before and after J-V curves in FIG. 8B show that the
device performance was not significantly affected by the long
test.
[0048] If a significant portion of the measured current did come
from photo corrosion of Si, a drastic decay of photocurrent would
be expected. Taken as a whole, it was concluded that the cathodic
shift of the photocurrent as shown in FIG. 6B is indeed a result of
additional photovoltage produced at the Fe.sub.2O.sub.3/Si junction
by Si nanowires.
[0049] In the systems of the present disclosure, the overall
photocurrent may be limited by the lowest performing component
which, in the present case, is hematite. As such, unless light
absorption by hematite is significantly improved, a dramatic
increase of photocurrent is not expected.
[0050] FIG. 9A, FIG. 9B, and FIG. 9C present J-V curves for
hematite films on planar substrates under different illumination
conditions. FIG. 9A shows that a planar Si substrates yield
hematite devices with similarly low onset potentials (<0.8
V.sub.RHE) but smaller photocurrent magnitudes, when compared with
the Si nanowire based devices. The lower current is largely due to
the smaller surface area and poorer light absorption by planar
devices. It should be noted that the photocurrents were even lower
than those measured on planar FTO substrate by a factor of 2. The
difference may be attributed to how the substrate influences the
hematite growth. The variety of crystal surfaces on etched Si
nanowires may be preferred over planar Si (100) which may not be
ideal for ALD hematite growth.
[0051] FIG. 9B shows planar device behavior under monochromatic
illumination exhibits some differences from nanowire devices.
Primarily, the UV lamp illumination yields performance close to the
combined UV and IR illumination condition. This may be due to the
planar substrate leading to shorter photon path lengths in
Fe.sub.2O.sub.3 (i.e. the Fe.sub.2O.sub.3 film appears thinner to
photons passing through along paths normal to the film surface) and
therefore more UV photons pass through Fe.sub.2O.sub.3 to excite
the Si.
[0052] FIG. 9C presents data for hematite devices on FTO
substrates, which suggest that such devices respond only to the UV
illumination, with no difference caused by adding the high-power IR
laser illumination. This is expected for a single-absorber
(Fe.sub.2O.sub.3) system.
[0053] In reference to FIG. 9A, when hematite was grown on planar
Si substrates, low photocurrents were measured. The higher
photocurrent measured on the hematite/Si nanowires combination may
be a result of increased light absorption by hematite on a textured
substrate (Si nanowires), and may be due to the enhanced path
length for photon absorption and the increased surface area.
Dynamic electrolyte diffusion into and from the regions between
adjacent Si nanowires is poor, since an electrochemically active
effective surface-area-to-projected-area ratio of <2, which is
two orders of magnitude lower than the true surface area, has been
measured on Si nanowires prepared by the same method in the
Me.sub.2Fc/Me.sub.2Fc.sup.+ electrochemical system. The cathodic
shift of the J-V plots observed on the planar hematite/Si system,
as shown in FIG. 9A, was comparable to that shown in FIG. 5B.
[0054] In reference to FIG. 10A and FIG. 10B, in combination with
FIG. 5A, the band diagrams predict that the magnitude of cathodic
shift should depend on how much photovoltage can be produced by Si
nanowires, which in turn is determined by the extent of band
bending within Si. One way to test this prediction is to change the
position of the Fermi level within Si by, for instance, using Si
with different doping levels or doping type or both. When the Fermi
level is closer to the valence band edge (such as in p-type Si),
the photovoltage obtained on Si would be smaller, resulting in a
lower value in the cathodic shift. Combined with the fact that an
appreciable band-bending depth is necessary for charge separation,
a lightly n-doped Si is desired for the dual absorber system to
exhibit the most significant cathodic shift. This indeed was the
case. As shown in FIG. 13B, Si n-doping levels of 10.sup.14,
10.sup.16, and 10.sup.18 cm.sup.-3 were examined which led to
typical onset potentials of approximately 0.6, 0.8, and 0.9
V.sub.RHE, respectively, whereas using p-type Si (10.sup.15
cm.sup.3) resulted in no cathodic shift as compared to hematite/FTO
(fluorine doped tin oxide) devices.
[0055] In some embodiments, the dual absorber is designed such that
both photon-to-charge conversion processes take place in a
concerted fashion. To this end, a set of experiments was carried
out to prove the dual absorber nature of these devices using
monochromatic light for excitation. First, an ultraviolet (UV) lamp
was used to illuminate the device with a wavelength of 365 nm at a
relatively low power (3 mW cm.sup.-2). These UV photons have
sufficient energy to excite hematite and therefore most of them are
absorbed within hematite itself, rather than penetrating through to
the Si. The resulting J-V curve and representative band diagram are
shown in FIG. 11A (purple). When hematite alone is excited,
photogenerated electrons flow towards Si where they encounter a
barrier which prevents their flow. This results in charge
accumulation but no net current flow through the device until a
sufficient anodic bias is applied to allow electrons to tunnel
through to the Si conduction band. The resulting photocurrent onset
potential of .about.1.0 V.sub.RHE reflects the expected performance
for a hematite-only device without significant contribution from
Si.
[0056] Conversely, when an infrared (IR) laser of 980 nm wavelength
was used as the lone light source, the photons had insufficient
energy to excite hematite and instead passed through to be absorbed
by Si. Despite the high power of the IR illumination (laser power
.about.2000 mW cm.sup.-2), no appreciable photocurrent was observed
until a considerable anodic potential was applied, with only small
photocurrents emerging when biased above 1.2 V.sub.RHE (FIG. 11A,
red). This may be due to the fact that photoexcited holes in Si
cannot be annihilated by electrons from the hematite conduction
band, and furthermore they do not have sufficient energy to inject
into hematite and perform water oxidation without large anodic
bias.
[0057] However, combining the two light sources for simultaneous
illumination of the hematite/Si nanowires device created a
synergistic effect that resulted in full development of the
photopotential and cathodic shift of the onset potential. The J-V
curve (FIG. 11A, blue) shows that, in fact, two combined
single-wavelength light sources are capable of producing a curve
shape similar to that obtained under full solar spectrum
illumination as presented in FIG. 5B, albeit with a lower
photocurrent magnitude due to the bottleneck in hematite caused by
using low-power UV illumination. For the case of dual excitation,
the band diagram in FIG. 5A portrays the cathodic shift of the
bands of each n-type material under illumination and the electronic
current flow that is produced. The synergy produced by combining
the UV and IR illumination sources illustrates the dual-absorber
nature of the hematite/Si nanowires photoelectrodes. This clearly
contrasts the single-absorber mechanism of typical hematite/FTO
devices, wherein only the UV illumination elicited a photocurrent
response (FIG. 9C). It is also different from when Si is the lone
active photoanode material, with hematite acting in a passivating
or catalytic role.
[0058] As further evidence of the device response to photon energy,
the wavelength-dependent photocurrent was measured using
monochromatic light in the wavelength range 300-1000 nm under a 1.0
V.sub.RHE applied bias. The illumination was achieved by passing
simulated solar light (AM 1.5 spectrum; intensity adjusted to 100
mW cm.sup.-2) through a monochromator. Depicted in FIG. 11B, the
photocurrent response reflected the behavior predicted by the J-V
curves and band diagrams in FIG. 11A. In the short-wavelength range
(300-580 nm) where the primary absorber is hematite, appreciable
anodic photocurrent was observed. Since the photocurrent was
nonzero, the photoexcited electrons in hematite were reaching the
external circuit, meaning that under this applied anodic bias
photoexcited electrons from the hematite conduction band can inject
into Si to produce a stable photocurrent. In stark contrast, no net
current flow was observed in the long-wavelength region (580-1000
nm). Also different from the photocurrents measured under
short-wavelength photon illumination was the apparent noise level,
that in the long-wavelength region being significantly higher. It
is possible that this "noisy" current is due to random collection
of photogenerated electrons and holes, both from Si. This may occur
because the photogenerated hole transfer through hematite into
water is forbidden due to the lack of short-wavelength photons and
the large energy barrier to the hematite valence band.
[0059] FIG. 11B shows how the photocurrent levels change and the
unique transient behaviors. To better present these features,
magnified views of currents in two spectral regions, 320-400 nm and
700-780 nm are re-plotted as FIG. 11C and FIG. 11D, respectively.
In FIG. 11C, it can be seen that the photocurrent first went up
with the increasing wavelength. This trend tracks the abundance of
photons in the solar spectrum within this region. Further
increasing the photon wavelength beyond 440 nm, however, resulted
in a lower photocurrent. This may be due to poorer light absorption
by hematite in the longer wavelength regions. The net photocurrent
was diminished at 580 nm and beyond, as has been previously
discussed.
[0060] The second feature concerns the obvious transient
phenomenon, which is manifested in the form of current spikes when
light was switched on and off. To understand the nature of these
transient spikes, it is necessary to clarify that the anodic
photocurrent of an electrode is a measure of how fast electrons are
collected. An anodic spike in the chronoamperometry plot would
indicate a surge of electrons and is often explained by charging
and discharging effect of trap states. For the hematite/Si nanowire
system, these transient spikes may be originated from two sources,
trap states at the hematite/electrolyte interface or those at the
hematite/Si interface.
[0061] FIG. 12A and FIG. 12B present the photocurrent responses of
hematite on FTO under stepwise monochromatic chopped illumination
in the wavelength range 300-780 nm under applied bias of 1.45
V.sub.RHE. In reference to FIG. 12A, the hematite film is the only
contributor to photocurrent, and thus photocurrent is produced only
by wavelengths up to 580 nm. The dotted box denotes the area
detailed in FIG. 12B, where it can be seen that the transient
photocurrent behavior is similar to that of the Si
nanowires/hematite devices shown in FIGS. 11A-11D. Photocurrents in
FIG. 12B are higher than those of FIGS. 11A-11D because
FTO/hematite devices do not have the heterojunction barrier that Si
nanowires/hematite devices exhibit.
[0062] By performing control experiments on hematite/FTO substrates
under similar conditions (FIGS. 11A-11D versus FIGS. 12A-12B) it
was concluded that while the transient behaviors under UV and blue
illumination may be explained by the nature of the
hematite/electrolyte interface, those under red and near-infrared
illumination can only be explained by charging and discharging of
the hematite/Si interface. That is, when light is switched on,
rapid charge separation takes place within Si, electrons being
collected to produce an anodic photocurrent and holes moving to the
Si/hematite interface to be trapped there. If these trapped holes
recombine with photogenerated electrons from hematite, which would
take place under dual-illumination conditions as that shown in
FIGS. 1A-1B, a steady-state photocurrent will be measured; in the
absence of effective photocharge generation within hematite,
however, the initial photocurrent would quickly decay to the base
level (zero net current), resulting in a transient spike. When
light is switched off, annihilation of the initially trapped holes
requires back-electron transfer into Si, leading to a cathodic
photocurrent spike. This feature can be clearly observed in FIG.
11D.
[0063] Accordingly, the interfaces between Si and hematite are
preferable sites for photogenerated holes (from Si) and electrons
(from hematite) to recombine. Such a recombination enables forward
current flow and is critically important for the realization of the
dual-absorber-based "Z-scheme" as shown in FIG. 5A.
[0064] FIG. 13A presents representative photocurrent-voltage plots
for Fe.sub.2O.sub.3/Si nanowire (NW) and Fe.sub.2O.sub.3/FTO
devices. The top-performing Fe.sub.2O.sub.3/Si NW device (red) and
three other typical preparations (pink) show similar photocurrent
onset potentials as well as peak photocurrent densities. Three
Fe.sub.2O.sub.3/FTO devices (black and grey) show the range of
behaviors with sample variation. As can be seen from the plots, the
Fe.sub.2O.sub.3/Si NW samples showed less sample variation than the
Fe.sub.2O.sub.3/FTO samples. FIG. 13B presents representative plots
for Fe.sub.2O.sub.3/Si NW devices of different Si doping
characteristics. The details of the Si doping are tabulated in
Table I below.
TABLE-US-00001 TABLE 1 Silicon doping characteristics FIG. S6b
Resistivity Approx. doping color Dopant (.OMEGA. cm) level
(cm.sup.-3) Red P 5-15 10.sup.14 Green P 0.04-0.12 10.sup.16 Blue
Sb 0.007-0.020 10.sup.18 Pink B 8-20 10.sup.15
[0065] In an embodiment, a dual absorber electrode includes two
absorber materials of different bandgap coupled together and
stacked together so to allow the light to pass through the
largest-bandgap absorber first. In some embodiments, the dual
absorber electrodes of the present embodiment may be utilized in an
electrolytic cell for water splitting.
[0066] In some embodiments, a dual absorber particle includes a
first absorber material and a second absorber material having a
larger band gap than the first absorber material, and a suitable
second electrode material, such as platinum or another catalyst
metal, in electrical contact with the first absorber material.
[0067] In some embodiments, a dual absorber electrode includes a
first absorber material having a first bandgap; and a second
absorber material deposited on a surface of the first absorber
material, the second absorber material having a second bandgap
larger than the first bandgap of the first absorber.
[0068] In some embodiments, a device for splitting water to
generate hydrogen and oxygen includes a first compartment having a
first electrode, the electrode comprising a first absorber material
having a first bandgap, and a second absorber material deposited on
a surface of the first absorber material, the second absorber
material having a second bandgap larger than the first bandgap of
the first absorber; a second compartment having a second electrode
for catalyzing hydrogen generation; and a semi-permeable membrane
separating the first compartment and the second compartment.
[0069] In some embodiments, a dual absorber electrode that includes
a first electrode comprising first absorber material and a second
absorber material having a larger band gap than the first absorber
material; and a second electrode for catalyzing hydrogen generation
in electrical contact with the first absorber material.
[0070] In some embodiments, a method of fabricating an electrode
that includes obtaining a silicon nanostructure; forming by vapor
deposition a layer of hematite on a surface of the silicon
nanostructure by exposing the silicon nanostructure to gas
precursors of hematite; and repeating the vapor-depositing to form
additional layers of hematite on the surface of the silicon
nanostructure
[0071] All patents, patent applications, and published references
cited herein are hereby incorporated by reference in their
entirety. It will be appreciated that several of the
above-disclosed and other features and functions, or alternatives
thereof, may be desirably combined into many other different
systems or applications. Various presently unforeseen or
unanticipated alternatives, modifications, variations, or
improvements therein may be subsequently made by those skilled in
the art which are also intended to be encompassed by the following
claims.
* * * * *