U.S. patent application number 10/657037 was filed with the patent office on 2005-03-10 for photo-electrolytic catalyst systems and method for hydrogen production from water.
Invention is credited to Jang, Bor Z..
Application Number | 20050051439 10/657037 |
Document ID | / |
Family ID | 34226483 |
Filed Date | 2005-03-10 |
United States Patent
Application |
20050051439 |
Kind Code |
A1 |
Jang, Bor Z. |
March 10, 2005 |
Photo-electrolytic catalyst systems and method for hydrogen
production from water
Abstract
A photo-electrolytic catalyst system which comprises two
materials: (a) a semiconductor material with a non-zero energy gap
Eg which, in response to an incident radiation having an energy
greater than Eg, generates electron-hole pairs as charge carriers;
and (b) a facilitating material in electronic contact with the
semiconductor material to facilitate separation of the
radiation-generated electrons from the holes to reduce the
probability of charge carrier recombinations The catalyst makes use
of both majority and minority charge carriers to promote
photo-electrolysis reactions for producing hydrogen directly from
water or an aqueous electrolyte at higher rates and improved
efficiencies.
Inventors: |
Jang, Bor Z.; (Fargo,
ND) |
Correspondence
Address: |
Bor Z. Jang
2902, 28 AVE, S.W.
FARGO
ND
58103
US
|
Family ID: |
34226483 |
Appl. No.: |
10/657037 |
Filed: |
September 8, 2003 |
Current U.S.
Class: |
205/639 ;
204/242; 205/637 |
Current CPC
Class: |
C25B 1/55 20210101; B01J
37/347 20130101; Y02E 60/36 20130101; B01J 35/0033 20130101; B01J
27/14 20130101; B82Y 30/00 20130101 |
Class at
Publication: |
205/639 ;
205/637; 204/242 |
International
Class: |
C25C 007/00; C25B
009/00 |
Claims
1. A photo-electrolytic catalyst system for hydrogen production
from water, said catalyst system comprising: (a) a first
semiconductor material with a non-zero energy gap Eg.sub.1 which,
in response to an incident radiation having an energy greater than
Eg.sub.1, generates electron-hole pairs as charge carriers; and (b)
at least a first facilitating material in electronic contact with
said semiconductor material to facilitate separation of the
radiation-generated electrons from the holes to reduce the
probability of charge carrier recombination.
2. The catalyst system as defined in claim 1, wherein both said
first semiconductor material and said first facilitating material
have at least one dimension being nanometer-scaled, smaller than or
equal to 100 nm.
3. The catalyst system as defined in claim 1, wherein at least one
of said semiconductor material and facilitating material is
porous.
4. The catalyst system as defined in claim 1, 2, or 3, wherein said
first facilitating material comprises an electron-drawing atom,
molecule, or ion.
5. The catalyst system as defined in claim 1, 2, or 3, wherein said
first semiconductor material comprises an element or compound
selected from the group consisting of group IV semiconductors,
III-V compounds, II-VI compounds, mixed crystals of II-VI
compounds, mixed crystals of III-V compounds, I-III-V.sub.2
compounds, I-IV-V.sub.2 compounds, ZMO compounds (where Z=an
alkaline or alkali metal and M=a transition metal or rare earth
metal element), oxides, phosphides, arsenides, sulfides, selenides,
tellurides, chalcogenides, chalcopyrites and combinations
thereof
6. The catalyst system as defined in claim 1, 2, or 3, wherein said
first semiconductor material has an energy band gap greater than
1.6 eV.
7. The catalyst system as defined in claim 1, 2, or 3, wherein said
first facilitating material comprises an element selected from
Group VI and Group VII of the Periodic Table of Elements.
8. The catalyst system as defined in claim 1, 2, or 3, wherein said
first facilitating material comprises a transition metal element or
a rare earth metal element.
9. The catalyst system as defined in claim 1, 2, or 3, wherein said
facilitating material comprises an element selected from the group
consisting of Fe, Mn, Co, Ni, Cr, and Ti.
10. The catalyst system as defined in claim 1, wherein said first
semiconductor material and/or said facilitating material has a
dimension smaller than 1 .mu.m.
11. The catalyst system as defined in claim 1, wherein said first
semiconductor material and/or said facilitating material is a
nano-scaled material with a dimension smaller than 100
nanometers.
12. The catalyst system as defined in claim 1, 2, or 3, further
comprising a second semiconductor material with an energy gap
Eg.sub.2 different from Eg.sub.1, wherein said second semiconductor
material is in electronic contact with said first facilitating
material.
13. The catalyst system as defined in claim 1, 2, or 3, wherein
said first semiconductor material is of n-type and said catalyst
system further comprises a second semiconductor material of p-type
in electronic contact with said first facilitating material.
14. The catalyst system as defined in claim 1, 2, or 3, further
comprising a second semiconductor material in electronic contact
with said first semiconductor material, wherein said second
semiconductor material has an energy gap Eg.sub.2 different from
Eg.sub.1.
15. The catalyst system as defined in claim 14, further comprising
at least a third semiconductor material, wherein said first, second
and third semiconductor materials are connected in series.
16. The catalyst system as defined in claim 14, further comprising
a second facilitating material in electronic contact with said
second semiconductor material.
17. The catalyst system as defined in claim 1, 2, or 3, further
comprising a second facilitating material in electronic contact
with said first semiconductor material.
18. The catalyst system as defined in claim 17, wherein said first
facilitating material comprises a reduction catalyst and said
second facilitating material comprises an oxidation catalyst.
19. A method for converting optical energy into chemical energy to
drive a chemical reaction for producing hydrogen gas from an
aqueous electrolyte, said method comprising: (A) suspending
discrete photo-electrolytic catalysts in said electrolyte; and (B)
illuminating said catalysts with optical energy to produce hydrogen
gas; wherein said catalysts comprises (a) a first semiconductor
material with a non-zero energy gap Eg.sub.1 which, in response to
optical energy, generates electron-hole pairs as charge carriers;
and (b) at least a first facilitating material in electronic
contact with said semiconductor material to facilitate separation
of the optical energy-generated electrons from the holes to reduce
the probability of charge carrier recombination.
20. The method as defined in claim 19, further comprising operating
means to collect said hydrogen gas produced.
21. The method as defined in claim 19, wherein both said first
semiconductor material and said first facilitating material have at
least one dimension being nanometer-scaled, smaller than or equal
to 100 nm.
22. The method as defined in claim 19, wherein at least one of said
semiconductor material and facilitating material is porous.
23. The method as defined in claim 19, wherein said optical energy
is provided by solar radiation.
24. The method as defined in claim 19, 20, 21, 22 or 23, wherein
said catalysts further comprise a second semiconductor material
with an energy gap Eg.sub.2 different from Eg.sub.1, wherein said
second semiconductor material is in electronic contact with said
first facilitating material.
25. The method as defined in claim 19, 20, 21, 22 or 23, wherein
said first semiconductor material is of n-type and said catalysts
further comprises a second semiconductor material of p-type in
electronic contact with said first facilitating material.
26. The method as defined in claim 19, 20, 21, 22 or 23, wherein
said catalysts further comprise a second semiconductor material in
electronic contact with said first semiconductor material, wherein
said second semiconductor material has an energy gap Eg.sub.2
different from Eg.sub.1.
27. The method as defined in claim 24, wherein said catalysts
further comprise at least a third semiconductor material and said
first, second and third semiconductor materials are connected in
series.
28. The method as defined in claim 24, wherein said catalysts
further comprise a second facilitating material in electronic
contact with said second semiconductor material.
29. The method as defined in claim 19, 20, 21, 22 or 23, wherein
said catalysts further comprise a second facilitating material in
electronic contact with said first semiconductor material.
30. The method as defined in claim 29, wherein said first
facilitating material comprises a reduction catalyst and said
second facilitating material comprises an oxidation catalyst.
31. The method as defined in claim 19, 20, 21, 22 or 23, wherein
said first facilitating material comprises an electron-drawing
atom, molecule, or ion.
32. The method as defined in claim 19, 20, 21, 22 or 23, wherein
said first semiconductor material comprises an element or compound
selected from the group consisting of group IV semiconductors, II-V
compounds, II-VI compounds, mixed crystals of II-VI compounds,
mixed crystals of III-V compounds, I-III-V.sub.2 compounds,
II-IV-V.sub.2 compounds, ZMO compounds (where Z=an alkaline or
alkali metal and M=a transition metal or rare earth metal element),
oxides, phosphides, arsenides, sulfides, selenides, tellurides,
chalcogenides, chalcopyrites and combinations thereof.
33. The method as defined in claim 19, 20, 21, 22 or 23, wherein
said first semiconductor material has an energy band gap greater
than 1.6 eV.
34. The method as defined in claim 19, 20, 21, 22 or 23, wherein
said first facilitating material comprises an element selected from
Group VI and Group VII of the Periodic Table of Elements.
35. The method as defined in claim 19, 20, 21, 22 or 23, wherein
said first facilitating material comprises a transition metal
element or a rare earth metal element.
36. The method as defined in claim 19, 20, 21, 22 or 23, wherein
said facilitating material comprises an element selected from the
group consisting of Fe, Mn, Co, Ni, Cr, and Ti.
Description
FIELD OF THE INVENTION
[0001] This invention provides a photo-electrolytic catalyst system
for water splitting to produce hydrogen and oxygen gases. This
invention also provides a method of using an improved
semiconductor-based photo-electrolytic catalyst to promote solar
energy conversion of water or aqueous electrolyte into
hydrogen.
BACKGROUND OF THE INVENTION
[0002] A fuel cell converts chemical energy into electrical energy
and some thermal energy by means of a chemical reaction between
hydrogen-containing fuel and oxygen. As compared to other energy
sources, fuel cells provide advantages that include low pollution,
high efficiency, high energy density and simple fuel recharge. Fuel
cells can be used in electrochemical engines, portable power
supplies for various microelectronic and communication devices,
standby power supply facilities, power generating systems, etc.
Further, fuel cells utilize renewable resources and provide an
alternative to burning fossil fuels to generate power.
[0003] For fuel cell applications, hydrogen is the "ultimate fuel."
Hydrogen is practically inexhaustible since it is the most
plentiful element in the universe (over 95% of all matter).
Furthermore, hydrogen is an inherently low cost fuel. Hydrogen has
the highest energy density per unit weight of any chemical fuel and
is essentially non-polluting since the main by-product of "burning"
hydrogen is water. Thus, hydrogen can be a means of solving many of
the world's energy related problems, such as global warming,
pollution, strategic dependency on oil, etc. Hydrogen can be
produced by various processes which split water into hydrogen and
oxygen or which oxidize methanol or ethanol into hydrogen and other
molecules. The hydrogen can then be stored and transported in a
solid state form.
[0004] Hydrogen is produced as a co-product in various industrial
processes. For example, hydrogen is produced as a co-product in the
electrolysis of aqueous alkali metal halide brines to yield the
corresponding alkali metal hydroxide, the halogen, and hydrogen.
Hydrogen is also produced as a co-product in the electrolysis of
aqueous alkali metal sulfates to yield the alkali metal hydroxide,
oxygen, and hydrogen. In all of these industrial processes,
electrical power must be applied from an external power supply,
across an anode and a cathode of an electrolytic cell, to yield
oxygen or halogen at the anode, the alkali metal hydroxide at the
cathode, and hydrogen as a cathode co-product.
[0005] The photolysis of water into H.sub.2 and O.sub.2 using solar
radiation is an attractive method of producing hydrogen since it
involves renewable and non-polluting energy sources: water and
sunlight. The conversion of solar energy into chemical energy has
the advantage of easy energy storage (in the form of the
photo-generated fuel), as compared with solar energy conversion via
photovoltaic or photo-thermal processes. An important process for
accomplishing the decomposition of water into H.sub.2 and O.sub.2
using solar radiation is photo-electrolysis.
[0006] Photo-electrolytic decomposition of water using a pair of
n-type and p-type semiconductor electrodes electrically connected
by an electrolyte solution was disclosed by Tchernev in U.S. Pat.
No. 3,925,212 (Dec. 9, 1975). Nozik (U.S. Pat. No. 4,094,751, June
13, 1978) disclosed photochemical diodes which use light to drive
both endoergic and exoergic chemical reactions such that optical
energy is converted into chemical energy. Either Schottky-type
(rectifying metal-semiconductor junction) or p-n type diodes were
employed to convert water into hydrogen and oxygen gases. In a
typical photochemical diode, for instance, the p-type portion and
the n-type portion are intimately joined together through an ohmic
contact. Gratzel, et al. (U.S. Pat. No. 4,389,290, Jun. 21, 1983)
disclosed a photolytic system comprising a darkened half cell and
an illuminated half cell in which oxidation and reduction are made
to occur. Ayers (U.S. Pat. No. 4,466,869, Aug. 21, 1984) developed
a multi-layered photo-electrolytic device to produce hydrogen. This
complicated device includes a multiplicity of stacked photoelectric
or photovoltaic elements between a substrate and an electrode, a
counter-electrode, and an unbiased external circuit. Another
complicated, multi-layer photo-electrolytic device is disclosed by
Gordon (U.S. Pat. No. 4,650,554, Mar. 17, 1987).
[0007] Photolytic decomposition of water was also accomplished by
Khan, et al. (U.S. Pat. No. 4,889,604, Dec. 26, 1989) who used a
catalyst system that included a semiconductor of hexagonal crystal
structure loaded with a noble metal and a transition metal oxide.
The catalyst was suspended in ethylene diamine tetra-acetic acid
(EDTA) as a part of the catalyst system. The photo-catalytic method
for water decomposition disclosed by Sayama, et al. (U.S. Pat. No.
5,262,023, Nov. 16, 1993) involved bringing an aqueous solution of
carbonate into contact with a semiconductor carrying a metal or a
metal compound, and irradiating the aqueous solution with light.
The photo-catalyst used by Takaoka, et al. (U.S. Pat. No.
5,759,948, Jun. 2, 1998) was composed of titanium oxide particles
which have part or the whole of an iron compound supported thereon.
A series of photo-catalysts each consisting of a catalytically
active ingredient (e.g., Cs and CdS) and a supporting or promoting
ingredient (e.g., Ni, Co, Fe, and K.sub.4Nb.sub.6O.sub.17) were
proposed by Park and co-workers (U.S. Pat. No. 5,865,960, Feb. 2,
1999; U.S. Pat. No.6,017,425, Jun. 25, 2000; U.S. Pat. No.
6,077,497, Jun. 20, 2000; U.S. Pat. No. 6,300,274, Oct. 9, 2001;
U.S. Pat. No. 6,297,190, Oct. 2, 2001; U.S. application Ser. No.
09/735,605, filed Dec.14, 2000). Bipolar electrodes with a
semiconductor coating, titanium dioxide dosed with iron, was
disclosed by Haug, et al. (U.S. patent application Ser. No.
09/783,228, filed Feb. 14, 2001).
[0008] The above inventions have one or several of the following
drawbacks: (1) complicated junctions or multiple-layer structures,
(2) insufficient photolytic or photo-electrolytic efficiency, (3)
slow hydrogen production rate, (4) involving non-water liquid
ingredients (e.g., EDTA, acid, strong base, or other potentially
hazardous chemicals), and (5) photo-catalysts with unknown
operating mechanisms (hence, the resulting efficiency, yield, and
rate can only be improved by trial-and-error and cannot be readily
or effectively optimized).
SUMMARY OF THE INVENTION
[0009] This invention provides simple (non-complicated) but
effective photo-electrolytic catalyst systems which comprise two
materials: (a) a semiconductor material with a non-zero energy gap
Eg which, in response to an incident radiation having an energy
greater than Eg, generates electron-hole pairs as charge carriers;
and (b) a facilitating material in electronic contact with the
semiconductor material to facilitate separation of the
radiation-generated electrons from the holes to reduce the
probability of charge carrier recombinations. The catalysts make
use of both majority and minority charge carriers to promote
photo-electrolysis reactions for producing hydrogen at higher rates
and improved efficiencies.
[0010] The semiconductor material and/or the facilitating material
preferably has at least one dimension being smaller than one micron
(1 .mu.m) and, further preferably, smaller than or equal to 100 nm.
Either or both materials are preferably porous. These two features
(small dimensions and high porosity levels) more readily facilitate
the separation of the photon-generated electrons and holes and
provide larger material-water interface areas where redox reactions
occur, leading to more efficient and faster hydrogen
production.
[0011] The present invention also provides a method of using these
photo-electrolytic catalysts to generate hydrogen gas from
water.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 (A) Upward bending of energy band curves for an
n-type semiconductor and (B) Downward bending of energy band curves
for a p-type semiconductor.
[0013] FIG. 2 (A) Upward bending of energy band curves for an
n-type semiconductor at a rectifying contact interface and (B)
Downward bending of energy band curves for a p-type semiconductor
at a rectifying contact interface.
[0014] FIG. 3 (A) Upward bending of energy band curves for an
n-type semiconductor at a rectifying semiconductor-electrolyte
contact interface and downward bending at a
semiconductor-facilitating material interface and (B) Downward
bending of energy band curves for a p-type semiconductor at a
rectifying semiconductor-electrolyte contact interface and upward
bending at a semiconductor-facilitating material interface.
DETAILED DESCRIPTION OF THE INVENTION
[0015] A. Basic Operating Mechanisms of the Invented
Photo-electrolysis Catalyst Systems
[0016] In conventional photo-electrolysis, light is absorbed in
separate, discrete semiconducting electrodes in contact with an
electrolyte. Ideally, the absorbed light generates electron-hole
pairs within the electrodes which, hopefully, are subsequently
separated by the semiconductor-electrolyte junctions. At the
cathode and anode, electrons and holes are respectively injected
into the electrolyte, thereby inducing reduction and oxidation
reactions, respectively. Electrons and holes are known to be strong
reducing and oxidizing agents, respectively. Hence, an overall
photo-electrolysis reaction is achieved in two steps: (1) electrons
and holes are first generated by photo-excitation of electrons
across an energy band gap (Eg) of a semiconducting electrode, and
(2) the electrons and holes drive chemical reactions in either or
both electrodes. In such a photo-chemical process, the possibility
exists that this sequence can drive reactions at more favorable
energies than can either direct photolysis or direct electrolysis
acting independently.
[0017] In order for such a photo-electrolysis process to work
efficiently, the electrons and holes generated must be immediately
separated, at least tentatively, from each other to reduce or
eliminate electron-hole charge recombinations. In a conventional
Schottky-type photo-electrolysis cell (e.g., as disclosed in U.S.
Pat. No. 3,925,212), the semiconductor/ohmic contact forms one
electrode, while metal forms the second, or counter-electrode. The
two electrodes are separated by the electrolyte, being joined
externally by an electrical circuit to provide a path for hole and
electron transfer, possibly as a means of separating electrons from
holes. In a conventional p-n type photo-electrolysis cell, the
p-type semiconductor/ohmic contact forms one electrode, while the
n-type semiconductor/ohmic contact forms the other electrode.
Again, the two electrodes are separated by the electrolyte and are
joined externally by an electrical circuit to provide a path for
electron and hole transfer.
[0018] In an improved photo-electrolysis system (Nozik, U.S. Pat.
No. 4,094,751), either Schottky-type or p-n type photo-chemical
diodes were employed to convert water into hydrogen and oxygen
gases. Each diode comprises two portions, e.g., in the case of a
p-n type diode, the p-type portion provided with an ohmic contact
and the n-type portion also provided with an ohmic contact. The two
portions are intimately joined or bonded together through the ohmic
contacts. The conventional wisdom maintains that, for a p-n
photo-chemical diode, the ohmic contact permits recombination of
the photo-generated majority carriers in the respective regions of
the diode, and thereby allows the minority carriers to be injected
into the electrolyte to complete the current path. That is, the
photons absorbed in each portion of the p-n photochemical diode
create electron-hole pairs; the minority holes (from the n-type
portion) and the minority electrons (from the p-type portion) are
injected into the electrolyte. The majority electrons and holes
must recombine for current continuity to exist, and this can only
happen if ohmic contacts are sandwiched between the p- and n-type
semiconducting portions of the diode. Such a concept has a serious
drawback in that the majority charge carriers (that normally
outnumber the minority charge carriers by several orders of
magnitude) are all wasted, not given an opportunity to catalyze the
respective oxidation and reduction reactions in an aqueous
medium.
[0019] Contrary to the teachings of the prior art and in accordance
with the present invention, photo-electrolytic catalyst systems are
provided which comprise two materials: (a) a semiconductor material
with a non-zero energy gap Eg which, in response to an incident
radiation having an energy greater than Eg, generates electron-hole
pairs as charge carriers; and (b) a facilitating material in
electronic contact with the semiconductor material to facilitate
separation of the radiation-generated electrons from the holes to
reduce the probability of charge carrier recombinations. Such a new
photo-electrolysis catalyst system, preferably in the form of a
fine powder or a micron- or sub-micron-scale entity, may simply be
suspended or dispersed in an aqueous electrolyte. Preferably, the
facilitating material has a high surface-to-volume ratio, thus
providing a large surface area whereon majority charge carriers can
tentatively reside prior to catalyzing respective reduction and
oxidation reactions at the facilitating material-electrolyte
(water) interfaces. In addition to the majority charge carriers
being capable of catalyzing the chemical reactions, minority charge
carriers are capable of being directly injected into the aqueous
medium to complete the current path as well as catalyzing the redox
reactions.
[0020] The need to have a high surface-to-volume ratio can be
achieved by making the catalyst system in small scales or of high
porosity. The facilitating material and/or the semiconductor
material preferably are (is) micron-scaled and further preferably
nano-scaled (<100 nm). The facilitating material is preferably
micro-porous or nano-porous, with a large number of micron- or
nano-scale pores to accommodate water molecules and to allow the
resulting hydrogen and oxygen molecules to escape. These large
surface areas provide large facilitating material-water interfaces
where reduction/oxidation reactions occur. The present invention
also eliminates the need for both large planar electrode systems
and external circuitry.
[0021] The hole-electron separation-facilitating material
preferably has an attribute in that it can tentatively retain
majority charge carriers (e.g., electrons) therein or thereon to
give the retained charge carriers improved chances of encountering
reactant species and reacting therewith (e.g.,
2H.sup.++2e.sup.-.fwdarw.H.sub.2 or 2H.sub.2O+2e.sup.-.fwdarw.2OH.-
sup.-+H.sub.2). For an n-type semiconductor, the facilitating
material may preferably comprise a reduction catalyst such as
platinum (Pt). For a p-type semiconductor, the facilitating
material may preferably comprise an oxidation catalyst such as
ruthenium oxide (RuO.sub.2). Although these features are preferred,
they are not necessary requirements. For an intrinsic
semiconductor, either type or both types of catalysts may be used
as a facilitating material.
[0022] B. The Constituent Semiconductor Material
[0023] For the purpose of clearly defining the claims, the term
semiconductor refers to a single element-type material such as
silicon (Si) or a multiple-element material such as gallium
arsenide (GaAs) provided that it has non-zero energy band gap (Eg).
A multiple-element semiconductor typically contains at least one
metal element. In the present context, any conventionally defined
"electrical insulator" such as oxides, nitrides and carbides
(despite having a large energy gap, e.g., Eg greater than 3.0 eV)
will be considered as a semiconductor. A "metal" refers to an
element of Groups 2 through 13, inclusive, plus selected elements
in Groups 14 and 15 of the periodic table. Thus, the term "metal"
broadly refers to the following elements:
[0024] Group 2 or IIA: beryllium (Be), magnesium (Mg), calcium
(Ca), strontium (Sr), barium (Ba), and radium (Ra).
[0025] Groups 3-12: transition metals (Groups MB, IVB, VB, VIB,
VIIB, VIII, IB, and IIB), including scandium (Sc), yttrium (Y),
titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium
(Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W),
manganese (Mn), technetium (Tc), hafnium (Hf), vanadium (V),
niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo),
tungsten (W), manganese (Mn), technetium (Tc), rhenium (Re), iron
(Fe), ruthenium (Ru), osmium (Os). cobalt (Co), rhodium (Rh),
iridium (Ir), nickel (Ni), palladium (Pd), platinum (Pt), copper
(Cu), silver (Ag), gold (Au), zinc (Zn), cadmium (Cd), and mercury
(Hg).
[0026] Group 13 or IIIA: boron (B), aluminum (Al), gallium (Ga),
indium (In), and thallium (TI).
[0027] Lanthanides: lanthanum (La), cerium (Ce), praseodymium (Pr),
neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu),
gadolinium (Gd), terbium (Th), dysprosium (Dy), holmium (Ho),
erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).
[0028] Group 14 or IVA: germanium (Ge), tin (Sn), and lead
(Pb).
[0029] Group 15 or VA: antimony (Sn) and bismuth (Bi).
[0030] For photo-electrolysis catalyst applications, the metal is
preferably selected from the group consisting of aluminum,
antimony, bismuth, boron, cadmium, copper, iron, gallium,
germanium, indium, lead, titanium, tin, and zinc. Preferably a
counter ion element is selected from the group consisting of
hydrogen, oxygen, carbon, nitrogen, chlorine, fluorine, boron,
iodine, sulfuir, phosphorus, arsenic, selenium, tellurium and
combinations thereof These elements may be used to react with a
metal to form a compound or ceramic of hydride, oxide, carbide,
nitride, chloride, fluoride, boride, iodide, sulfide, phosphide,
arsenide, selenide, and telluride, and combinations thereof The
range of compound semiconductor or ceramic materials that can be
used as a semiconductor includes, but not limited to, I-VII, II-VI,
and III-V compounds. All of these materials can be selectively
doped with electron-donors or electron acceptors to produce n-type
or p-type semiconductors. Pure, un-doped semiconductors are
referred to as intrinsic semiconductors. Table 2 shows some of the
preferred semiconductors for use in the invented photo-electrolysis
catalyst.
1TABLE 1 Selected semiconductor materials. n-type p-type
Miscellaneous TiO.sub.2 Cu.sub.2O CoO CuS NiO
M.sub.xZn.sub.yO.sub.2 MTiO.sub.3 ZnO Fe.sub.2O.sub.3 WO.sub.3 Si,
Te, SiC Si, Ce, SiC (M is a transition metal element or rare-earth
metal element) II-VI compounds CdS CdTe CdSe ZnTe CdTe ZnSe III-V
compounds GaP GaP GaAs GaAs InP InAs AlAs AlAs AlSb AlSb GaSb GaSb
InP Mixed crystals of II-VI compounds Cd.sub.1-xZn.sub.xS Mixed
crystals of III-V compounds GaAs.sub.xP.sub.1-x GaInP.sub.2
Cu.sub.xIn.sub.yGa.sub.zSe.sub.2 GaIn.sub.1-xAs
Al.sub.xGa.sub.1-xAs Chalcopyrites Chalcopyrites Chalcogenides
Chalcogenides I-III-V.sub.2 compounds CuInS.sub.2 CuInS.sub.2
AgInSe.sub.2 CuGaS.sub.2 AgInS.sub.2 CuAlS.sub.2 CuInSe.sub.2
CuAlSe.sub.2 CuInSe.sub.2 II-IV-V.sub.2 compounds ZnSiP.sub.2
ZnSiAs.sub.2 CdSiP.sub.2 ZnGeP.sub.2 CdSnP.sub.2 ZnSnAs.sub.2
CdSnAs.sub.2 ZnSnP.sub.2 ZnSnSb.sub.2 CdSnP.sub.2 CdSnAs.sub.2 ZMO
compounds (Z = alkaline or alkali metal) K.sub.4Nb.sub.6O.sub.17
Na.sub.2Ti.sub.6O.sub.13 K.sub.2Ti.sub.6O.sub.13
BaTi.sub.4O.sub.9
[0031] C. Electron-Hole Separation-Facilitating Materials
[0032] If a semiconductor is in a physical contact with a metal, a
rectifying contact (Schottky junction) or an ohmic contact is
formed, depending on the type of metal used. Assume that the
surface of an n-type semiconductor has somehow been negatively
charged, as schematically shown in FIG. 1A. The negative charges
repel the free electrons that had been near the surface and leave
positively charged donor ions behind (e.g., As.sup.+). Any electron
which drifts toward the surface (i.e., moving in the negative
X-direction toward the surface) "feels" this repelling force. As a
consequence, the region near the surface has less free electrons
than the interior of the solid. This region is known as the
depletion layer or space charge region. The repelling force of an
external negative charge may be customarily represented by an
upward-bending energy band curve near the surface, FIG. 1A. This
implies that the electrons like to roll downhill. Similarly, if a
p-type semiconductor is somehow positively charged at the surface,
the positive charge carriers (holes) are repelled toward the inert
part of the material and the band edges are bent downward, FIG. 1B.
This represents a potential barrier for holes because the holes
have to drift upward in order to come closer toward the
surface.
[0033] Assume that a metal with a work function .phi..sub.M is
brought into contact with an n-type semiconductor with a work
function .phi..sub.n, where .phi..sub.M>.phi..sub.n, electrons
start to flow from the semiconductor "down" into the metal until
the Fermi energies of both materials are equal. As a consequence,
the metal will be charged negatively and a potential barrier is
formed just as shown in FIG. 1A. This means that the energy bands
in the bulk semiconductor are lowered by the amount
(.phi..sub.M-.phi..sub.n) with respect to a point A at the
metal-semiconductor interface, FIG. 2A Such a junction is a
rectifying contact. Contrarily, if .phi..sub.M<.phi..sub.n, the
contact will be an ohmic contact and the energy band curves near
the metal-semiconductor interface will be down-ward bending.
[0034] Similarly, if a p-type semiconductor (work function
.phi..sub.p) is brought into contact with a metal and
.phi..sub.M<.phi..sub.p, electrons diffuse from the metal into
the semiconductor and, therefore, the surface of the p-type
semiconductor is charged positively. A potential barrier
(.phi..sub.p>.phi..sub.M) is formed just as shown in FIG. 1B.
This situation is illustrated by a "downward" potential barrier
(for the hole), indicated in FIG. 2B. Contrarily, if
.phi..sub.M>.phi..sub.p, the contact will be an ohmic contact
and the energy band curves near the metal-semiconductor interface
will be up-ward bending. When a p-type semiconductor and an-n-type
semiconductor are brought into contact with each other, a
rectifying potential barrier is established.
[0035] The interface between a liquid electrolyte (e.g., water) and
a semiconductor is similar to a metal-semiconductor junction and
can be an ohmic contact or a rectifying contact, depending upon the
relative magnitudes of the work functions as explained above, using
FIGS. 1A, 1B, 2A, and 2B. With an n-type ZnO (Eg=3.35 eV) as an
example, photo-chemical splitting of water at the water-ZnO
interface and the direct conversion of solar energy into chemical
energy may be represented as a process in the rectifying
metal-semiconductor junction (e.g., at interface A of FIG. 3A) or
in one half of a p-n junction. When solar radiation is absorbed in
the n-type ZnO, electron-hole pairs are created. The potential
gradient established at the depletion zone forces the electrons to
drift into the bulk of the semiconductor and likely become wasted
(not participating in any chemical reactions). However, the holes
drift to the surface of the semiconductor where they may combine
with HO.sup.- ions or react with
H.sub.2O(H.sub.2O+2h.sup.+.fwdarw.2H.sup.++1/2O.sub.2) and result
in the evolution of oxygen gas.
[0036] The present invention provides a facilitating material that
"attracts" the photo-generated electrons or promotes the
photo-generated electrons to sweep across the "bulk" of the n-type
semiconductor and across the facilitating material-semiconductor
interface (e.g., interface B of FIG. 3A) so that they have
opportunities to combine with H.sup.+ or react with H.sub.2O at the
facilitating material-electrolyte interface to produce hydrogen
(e.g., through 2H.sub.2O+2e.sup.-.fwdarw.2OH.sup.-+H.sub- .2).
Without the presence of this facilitating material, these electrons
would somehow recombine with the defects or holes in the bulk of
the semiconductor and would be wasted. In one preferred embodiment,
the semiconductor has one dimension smaller than 1 .mu.m and
further preferably has one dimension smaller than 100 nanometers
(nm). Preferably, the facilitating material also has a dimension
smaller than 1 .mu.m and, further preferably, smaller than 100 nm.
Alternatively or additionally, the facilitating material and/or the
semiconductor material is porous, being meso-porous (pore
size>10 .mu.m), micro-porous (0.1 .mu.m<pore size<10
.mu.m), and/or nano-porous (pore size<0.1 .mu.m). The small
dimension(s) of the semiconductor make it easier for the
photo-generated charges to readily sweep across the bulk of the
semiconductor (the "bulk" itself being so small) with reduced
chances of recombinations. A porous semiconductor means a larger
surface area to promote hole-water contacts. Both the small
dimension(s) and the pores of the facilitating material mean
greater interface areas between the facilitating material and water
where desirable hydrogen production reactions can occur.
[0037] In the case of a p-type semiconductor (e.g., an
acceptor-doped ZnSe), the corresponding process involves the
generation of hole-electron pairs by the absorption of solar
radiation. The electrons in this case are forced to drift to the
surface while the holes drift to the bulk of the semiconductor. At
the semiconductor-water interface (e.g., interface A of FIG. 3B),
the electrons are transferred to the electrolyte where they combine
with H.sup.+ and cause the evolution of hydrogen. Without the
assistance of a facilitating material, the holes would somehow
disappear in the bulk through undesired recombinations and would
not assist in the water splitting reactions. The facilitating
material "attracts" the photo-generated electrons or promotes the
photo-generated holes to sweep across the "bulk" of the p-type
semiconductor and across the facilitating material-semiconductor
interface (e.g., interface B of FIG. 3B) so that they have
opportunities to combine with OH.sup.- or react with H.sub.2O at
the facilitating material-electrolyte interface to produce oxygen
and hydrogen ion (e.g., through H.sub.2O+2h.sup.+.fwdarw.2-
H.sup.++1/2O.sub.2.Arrow-up bold.).
[0038] The overall photoelectro-catalytic splitting of water may be
conveniently expressed as:
hv-2h.sup.++2e.sup.- (h=Planck's constant, v=frequency of the
radiation) (1)
H.sub.2O+2h.sup.+.fwdarw.2H.sup.++1/2O.sub.2.Arrow-up bold. (2)
2H.sub.2O+2e.sup.-.fwdarw.2OH.sup.-+H.sub.2 .Arrow-up bold. (3)
[0039] However, from the perspective of reaction mechanisms, the
overall reaction may begin with electrolysis of water:
H.sub.2O+(.DELTA.E.apprxeq.1.23 eV).fwdarw.OH.sup.-+H.sup.+ (4)
[0040] The required water redox potential (.DELTA.E.apprxeq.1.23
eV) may come from the semiconductor, the facilitating material, or
a combination. The resulting ions of OH.sup.- and H.sup.+ may then
react with holes and electrons to produce oxygen and hydrogen,
respectively:
OH.sup.-+2h.sup.+.fwdarw.H.sup.++1/2O.sub.2.Arrow-up bold. (5)
2H.sup.++2e.sup.-.fwdarw.H.sub.2.Arrow-up bold. (6)
[0041] It is important to realize that both electrons and holes are
essential to completing the above loop of reactions (Eqn.(2)-(6)).
Even though one may be interested in collecting hydrogen only, the
production of oxygen is indispensable because the production of
oxygen (Eqn.(2) or Eqn.(5)) also results in the formation of
H.sup.+ ions as a co-product, which is needed for hydrogen
production to complete the loop. This is another reason why a
facilitating material is desired in the invented photo-electrolytic
catalyst system. Further, a small amount of energy is lost to heat
and other loss mechanisms and, hence, a potential greater than 1.23
eV (typically >1.6 eV) is required for electrolysis of water.
Therefore, a semiconductor with a band gap greater than 1.6 eV is
preferred.
[0042] The facilitating material may include an element selected
from Group VI and Group VII of the Periodic Table of Elements. The
facilitating material may include a transition metal element or a
rare earth element. Preferably, the facilitating material may
include an element selected from the group consisting of Fe, Mn,
Co, Ni, Cr, and Ti. The above elements were found to constitute
very effective facilitating materials.
[0043] In order to make better use of the solar radiations, it is
advantageous to combine or mix two or three semiconductor materials
together that share with the same facilitating material to form a
hybrid photo-electrolytic catalyst system. The two or three
semiconductors are preferably of different energy band gaps,
covering both UV and visible wavelengths. A semiconductor material
or a hybrid combination of two or three semiconductor materials may
be in electronic contact with two or more facilitating materials.
Preferably, one facilitating material is an electron-drawing
material while another one is a hole-drawing material. Such a
combination could more effectively facilitate the separation of
electrons from holes immediately after they are photo-generated in
a semiconductor. When three or more semiconductor materials are
combined, they are preferably connected electrically in series. The
resulting solid state junctions at the micro- or nano-scale act to
reduce semiconductor-electrolyte interface voltage
requirements.
[0044] Another preferred embodiment of the present invention is a
method for producing hydrogen from water or an aqueous electrolyte.
The method comprises suspending photo-electrolytic catalysts as
illustrated above in an aqueous electrolyte (including, but not
limited to, pure water or sea water) and then illuminating the
catalysts with optical energy. The method may further include means
of collecting the hydrogen gas produced.
EXAMPLE 1
[0045] A micro-crystalline powder of GaP (average particle
size.apprxeq.0.7 .mu.m) was doped with sulfur to produce an n-type
semiconductor. A thin layer of platinum (Pt) was sputter-coated
over a portion of individual n-GaP particles to serve as a
facilitating material. When such a photo-electrolytic catalyst
system was suspended in an aqueous solution, hydrogen was
evolved.
EXAMPLE 2
[0046] Materials similar to those in Example 1 were used, but the
average particle size of GaP was slightly below 100 nm. When such a
photo-electrolytic catalyst system was suspended in the same
aqueous solution, hydrogen was much more vigorously evolved.
EXAMPLE 3
[0047] Several samples were prepared for this example. As in
Example 2, nano-scaled n-type GaP powder was used, but the
facilitating materials were Mn, Fe, Ni, and Co, respectively. When
such photo-electrolytic catalyst systems were suspended in the same
aqueous solution, hydrogen was vigorously evolved. The
solar-to-hydrogen power conversion efficiencies for these samples
were found to be in the following order: Mn.apprxeq.Fe>Ni>Co
(as facilitating materials).
EXAMPLE 4
[0048] Several samples were prepared for this example. As in
Example 3, nano-scaled p-type ZnTe powder was used and the
facilitating materials were Mn, Fe, Ni, and Co, respectively. When
such photo-electrolytic catalyst systems were suspended in the same
aqueous solution, hydrogen was vigorously evolved. The
solar-to-hydrogen power conversion efficiencies for these samples
were found to be in the following order: Mn.apprxeq.Fe>Ni>Co
(as facilitating materials). The efficiencies are slightly lower
than those of corresponding catalysts using n-tppe GaP as the
semiconductor material
EXAMPLE 5
[0049] Several samples were prepared for this example. Nano-scaled
n-type ZnO powder was used, but the facilitating materials were
RuO.sub.2, IrO.sub.2, and NiO, respectively. The three oxides are
well-known oxidation catalysts. When such photo-electrolytic
catalyst systems were suspended in the same aqueous solution,
hydrogen was vigorously evolved.
EXAMPLE 6
[0050] Three samples were prepared for this example. As in Example
2, nano-scaled n-type GaP powder was used, but the facilitating
materials, MnO, Mn.sub.2O.sub.3 and Mn.sub.3O.sub.4, respectively,
were deposited via reactive sputtering. When such
photo-electrolytic catalyst systems were suspended in the same
aqueous solution, hydrogen was more vigorously evolved in the
samples containing Mn.sub.3O.sub.4 or Mn.sub.2O.sub.3 than in that
containing MnO as the facilitating material. This may be understood
from the fact that polyvalent ions (such as Mn.sup.3+) are more
powerful electron-drawing materials than a divalent Mn.sup.2+.
Furthermore, with a polyvalent ion, reactions similar to the
following may occur:
4Mn.sup.3++2H.sub.2O.fwdarw.4Mn.sup.2++O+4H.sup.+ (7)
4Mn.sup.2++4H.sup.+.fwdarw.4Mn.sup.3++2H.sub.2 (8)
[0051] The present invention provides photo-electrolytic catalysts
that are efficient in un-biased water splitting for hydrogen
production. No external circuit or electric power is needed, the
only energy source is solar radiation or other forms of light. No
complex solid state microelectronic devices (such as multi-layer
thin-film junctions) are needed.
* * * * *