DESCRIPTION
SEMICONDUCTOR MATERIAL, OPTICAL HYDROGEN GENERATING DEVICE
USING SAME, AND METHOD OF PRODUCING HYDROGEN
5
TECHNICAL FIELD
[0001] The present invention relates to a semiconductor material having a
photocatalytic ability suitable for water decomposition by light irradiation and to an
optical hydrogen generating device using the semiconductor material. The present
10 invention also relates to a method of producing hydrogen using the semiconductor
material.
BACKGROUND ART
[0002] It is conventional to decompose water and collect hydrogen and oxygen by
15 irradiating a semiconductor material functioning as a photocatalyst with light.
[0003] For example, Patent Literature 1 discloses a method in which a n-type
semiconductor electrode and a counter electrode are disposed in an electrolyte
solution, and the surface of the n-type semiconductor electrode is irradiated with
light to collect hydrogen and oxygen from the surfaces of the two electrodes.
20 Patent Literature 1 describes using a TiO2 electrode, a ZnO electrode, a CdS
electrode or the like as the n-type semiconductor electrode.
[0004] Patent Literature 2 discloses a gas generator including a metal electrode
and a nitride semiconductor electrode that are connected together, the two
electrodes being placed in a solvent. A nitride of a Group 13 element such as
25 indium, gallium, or aluminum is used for the nitride semiconductor electrode.
[0005] Such conventional semiconductor electrodes have a problem of low hydrogen
generation efficiency in water decomposition reaction induced by irradiation with
sunlight. This is because the wavelength of light absorbable by the semiconductor
materials such as TiO2 and ZnO is short; that is, these semiconductor materials can
30 only absorb light having a wavelength of approximately 400 nm or less, so that the
proportion of utilizable light in the total sunlight is very small and about 4.7% in
the case of TiO2. Furthermore, considering a loss of absorbed light due to a
theoretical heat loss, the utilization efficiency of sunlight is about 1.7%.
[0006] TaON, Ta3N5, and Ag3VO4 have been reported as semiconductor materials
35 that can absorb longer-wavelength visible light. However, even for these
semiconductor materials, the wavelength of absorbable light is at most about 500 to
600 nm. In the case of TaON capable of absorbing light having a wavelength of 500
3
nm or less, the proportion of utilizable light in the total sunlight is about 19%.
However, considering a theoretical heat loss, the utilization efficiency is no more
than about 8%.
[0007] Meanwhile, Patent Literature 3 has recently reported that LaTaON2 is
capable of absorbing visible light having a wavelength of up to 650 nm. Thi5 s
means that LaTaON2 is capable of absorbing the longest wavelength light among
the semiconductor materials that have been hitherto reported to be capable of
decomposing water. In the case of LaTaON2 capable of absorbing light having a
wavelength of 650 nm or less, the proportion of utilizable light in the total sunlight
10 is about 41%. However, considering a theoretical heat loss, the utilization
efficiency is no more than about 20%.
[0008] Compound semiconductor materials containing Se, Te, or the like, and
particular sulfides (such as CdS, ZnS, Ga2S2, In2S3, ZnIn2S4, ZnTe, ZnSe, CuAlSe2,
and CuInS2), are also capable of absorbing light having a relatively long wavelength.
15 However, these materials are poor in stability in water and are impractical for
water decomposition reaction.
[0009] Patent Literature 4 discloses using a Group 5 element-containing
carbonitride as an electrode active material for an oxygen-reduction electrode used
as a positive electrode of a solid polymer fuel cell. However, Patent Literature 4
20 does not disclose the technical idea of using a Group 5 element-containing
carbonitride as a semiconductor material functioning as a photocatalyst
(photocatalytic material). In addition, the carbonitride of Patent Literature 4 is a
mixture of a carbonitride with an oxide or the like, and is used in a different form
from a photocatalytic material which is generally used in the form of a single-phase
25 highly-crystalline material in view of quantum efficiency.
CITATION LIST
Patent Literature
[0010] Patent Literature 1: JP S51(1976)-123779 A
30 Patent Literature 2: JP 2003-24764 A
Patent Literature 3: JP 4107792 B
Patent Literature 4: JP 2008-108594 A
SUMMARY OF INVENTION
35 Technical Problem
[0011] In order for water to be decomposed by irradiation of a semiconductor
material with light, the oxidation-reduction potentials of water (the hydrogen
4
evolution level and the oxygen evolution level) need to lie between the band edges
(the level of the top of the valence band and the level of the bottom of the conduction
band) of the semiconductor material. Therefore, the requirements for the
semiconductor material that can be practically used for water decomposition are
that the semiconductor material be capable of absorbing light in a wide wavelengt5 h
region (have a small band gap), have band edges between which the
oxidation-reduction potentials of water lie, and remain stable in water under light
irradiation. However, any semiconductor material that meets all the requirements
has not been discovered thus far.
10 [0012] The following will discuss how small the band gap needs to be in order to
achieve hydrogen generation efficiency comparable to that achieved in Si solar cells
which are commonly known in the present time. Assuming a semiconductor
material capable of absorbing light having a wavelength of 700 nm or less, then the
proportion of utilizable light in the total sunlight is about 48%. Considering a
15 theoretical loss due to a heat loss, the hydrogen generation efficiency is about 25%.
This value is based on the assumption of a quantum efficiency of 100%. Therefore,
when the semiconductor is used for a device, the following losses need to be further
taken into consideration: a loss corresponding to a decrease in quantum efficiency; a
loss due to reflection and scattering at a glass surface; and a loss due to light
20 absorption by water. Considering the efficiencies in terms of the losses (quantum
efficiency: 90%, efficiency depending on device design factors such as reflection and
scattering: 90%), it can be estimated that hydrogen generation efficiency of up to
about 20% is achieved in the case of a semiconductor material having a band gap
corresponding to an absorption edge wavelength of 700 nm. When the hydrogen
25 generation efficiency is low, the installation area required for generating a
necessary amount of hydrogen naturally increases, thus leading to an increase in
cost. In addition, it becomes difficult to install the device, for example, on a
limited-area roof of a single-family house in a similar manner to that for solar cells.
The power generation efficiency assumed to be achievable by simple-type Si solar
30 cells (not of the tandem type) is about 20%. Therefore, in order to obtain the same
or higher level of efficiency than that achieved by solar cells, a semiconductor
material having a band gap corresponding to an absorption edge wavelength of 700
nm or more is needed. Furthermore, semiconductor materials that have a smaller
band gap (corresponding to an absorption edge wavelength of 1008 nm or more) are
35 incapable of decomposing water in principle since the water decomposition potential
is about 1.23 V at ordinary temperature. Therefore, it is desired to discover a
semiconductor material having a band gap corresponding to an absorption edge
5
wavelength ranging between 700 nm and 1008 nm (1.23 eV and 1.77 eV).
[0013] It is therefore an object of the present invention to provide a semiconductor
material that has a band gap corresponding to an absorption edge wavelength of
1008 nm or less and that has the longest possible bandgap wavelength.
5
Solution to Problem
[0014] The present invention provides a semiconductor material including an
oxynitride containing at least one element selected from Group 4 elements and
Group 5 elements. In the oxynitride, part of at least one selected from oxygen and
10 nitrogen is substituted with carbon.
Advantageous Effects of Invention
[0015] The present invention can provide a semiconductor material that has a band
gap corresponding to an absorption edge wavelength of 1008 nm or less and that
15 has the longest possible bandgap wavelength.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a conceptual diagram of the energy levels of a semiconductor
material of an embodiment of the present invention and a conventional
20 photocatalytic material.
FIG. 2A shows a material in which an oxygen site of NbON is substituted
with carbon, and FIG. 2B shows a material in which NbON is doped with carbon.
FIGS. 3A to 3D show the electronic density of states of Ta-containing
materials determined by first-principles calculations.
25 FIGS. 4A to 4D show the electronic density of states of Nb-containing
materials determined by first-principles calculations.
FIGS. 5A to 5F show the electronic density of states of Nb-containing
materials determined by first-principles calculations.
FIGS. 6A to 6F shows the electronic density of states of Nb-containing
30 materials determined by first-principles calculations.
FIGS. 7A to 7F show the electronic density of states of Nb-containing
materials determined by first-principles calculations.
FIG. 8 shows a result of thin film X-ray diffraction measurement on a
Ta-containing material.
35 FIG. 9 shows a result of thin film X-ray diffraction measurement on a
Ta-containing material.
FIG. 10 shows results of SIMS (Secondary Ion Mass Spectrometry)
6
measurement on Ta-containing materials.
FIG. 11 shows the optical absorption characteristics of Ta-containing
materials.
FIG. 12 shows results of SIMS measurement on Nb-containing materials.
FIG. 13 shows the optical absorption characteristics of Nb-containin5 g
materials.
FIG. 14 shows a result of thin film X-ray diffraction measurement on a
Nb-containing material.
FIG. 15 is a schematic cross-sectional view showing an example of an optical
10 hydrogen generating device of an embodiment of the present invention.
FIG. 16 is a schematic cross-sectional view showing another example of an
optical hydrogen generating device of an embodiment of the present invention.
DESCRIPTION OF EMBODIMENTS
15 [0017] The present inventors have succeeded in providing a novel semiconductor
material that has a band gap corresponding to an absorption edge wavelength of
1008 nm or less and that has the longest possible bandgap wavelength, the
semiconductor material capable of remaining stable in water under light irradiation.
Furthermore, using such a novel semiconductor material, the present inventors
20 have succeeded in providing a method capable of producing hydrogen by light
irradiation with high efficiency and a device capable of generating hydrogen by light
irradiation with high efficiency.
[0018] A first aspect of the present invention provides a semiconductor material
including an oxynitride containing at least one element selected from Group 4
25 elements and Group 5 elements. In the oxynitride, part of at least one selected
from oxygen and nitrogen is substituted with carbon. The semiconductor material
according to the first aspect has a band gap corresponding to an absorption edge
wavelength of 1008 nm or less, and also has a bandgap wavelength longer than
those of conventional semiconductor materials. In addition, when the at least one
30 element selected from the Group 4 elements and Group 5 elements, an element to
be substituted with carbon, the degree of substitution with carbon, and the like, are
appropriately selected, the semiconductor material according to the first aspect can
be obtained as a semiconductor material having band edges between which the
oxidation-reduction potentials of water lie, being capable of absorbing visible light
35 with a wavelength of 700 nm or more, and further being excellent in stability in
water (particularly, neutral or acidic water) during light irradiation. Therefore,
hydrogen can be generated more efficiently than ever before by decomposing water
7
by light irradiation of the semiconductor material of the present invention
immersed in a solution containing an electrolyte and water.
[0019] A second aspect of the present invention provides the semiconductor
material as set forth in the first aspect, the semiconductor material having a
single-phase structure. The semiconductor material according to the second aspec5 t
can exhibit higher charge separation efficiency.
[0020] A third aspect of the present invention provides the semiconductor material
as set forth in the first or second aspect, the semiconductor material having a
monoclinic crystal structure. For example, in the case where Nb is used as the
10 element selected from the Group 4 or Group 5 elements, part of O and/or N of NbON
is substituted with C. In this case, it is desirable that the part of O and/or N of
NbON be substituted with C, with the crystal structure of NbON maintained.
Since the semiconductor material according to the third aspect of the present
invention has a monoclinic crystal structure, the crystal structure can be
15 maintained before and after the substitution with C. In addition, the crystallinity
is desirably high since it is expected that the closer to a single crystal structure the
crystal structure is, the larger the increase in quantum efficiency is. However, also
in the case of a homogeneous amorphous structure, high quantum efficiency,
although being lower than that in the case of a single crystal structure, can be
20 obtained.
[0021] A fourth aspect of the present invention provides the semiconductor
material as set forth in any one of the first to third aspects, wherein the at least one
element selected from the Group 4 elements and the Group 5 elements is Nb. The
semiconductor material according to the fourth aspect makes it possible to absorb
25 visible light having a longer wavelength.
[0022] A fifth aspect of the present invention provides the semiconductor material
as set forth in any one of the first to fourth aspects, wherein the Group 5 element is
in a form having substantially a valence of 5. The semiconductor material
according to the fifth aspect can have a more evident band gap.
30 [0023] A sixth aspect of the present invention provides the semiconductor material
as set forth in any one of the first to fifth aspects, the semiconductor material
having a photocatalytic ability. The semiconductor material according to the sixth
aspect makes it possible to provide a photocatalyst capable of effective utilization of
sunlight.
35 [0024] A seventh aspect of the present invention provides a method of producing
hydrogen, the method comprising the step of immersing the semiconductor material
according to the first aspect in a solution containing an electrolyte and water, and
8
then irradiating the semiconductor material with light to decompose the water.
With the production method according to the seventh aspect, hydrogen can be
generated more efficiently than ever before.
[0025] An eighth aspect of the present invention provides an optical hydrogen
generating device including a container, an electrode including a photocatalyti5 c
material, and a counter electrode. The photocatalytic material includes a
semiconductor material according to the first aspect. With the hydrogen
generating device according to the eighth aspect, hydrogen can be generated more
efficiently than ever before.
10 [0026] Hereinafter, embodiments of the semiconductor material, the method of
producing hydrogen, and the hydrogen generating device according to the present
invention will be described in more detail.
[0027] Efficient water decomposition and hydrogen generation using sunlight
requires that, as shown in the band state diagram on the left part of FIG. 1, the
15 material used as a photocatalyst be a semiconductor material capable of absorbing
visible light having a relatively long wavelength (semiconductor material having a
small band gap), and the semiconductor material have band edges (the level of the
top of the valence band and the level of the bottom of the conduction band) between
which the hydrogen evolution level and the oxygen evolution level lie. In addition,
20 the semiconductor material is required to remain stable in water under light
irradiation.
[0028] The valence band of a common oxide is composed of the oxygen p orbitals.
Therefore, the valence band is usually located at a deep level (high potential) (the
right part of FIG. 1). On the other hand, the valence band of a nitride or oxynitride
25 is composed of the nitrogen p orbitals or hybrids of the oxygen and nitrogen p
orbitals. Therefore, the valence band is usually located at a shallow level (low
potential) (the center of FIG. 1) compared to the valence band of an oxide. Thus, as
disclosed in Patent Literature 3, the use of an oxynitride makes it possible to obtain
a photocatalytic material (semiconductor material) that has a smaller band gap
30 than a material obtained by the use of an oxide. In general, simple nitrides are
prone to oxidation. Therefore, there may be a case where a simple nitride is
disadvantageously oxidized when left in water under light irradiation for a long
period of time. Thus, an oxynitride is more desirable than a simple nitride in view
of stability. However, there are known only oxynitride materials that have a band
35 gap larger than the aforementioned desired band gap (corresponding to an
absorption edge wavelength of 700 nm). The band gap is at least about 1.91 eV
(corresponding to an absorption edge wavelength of 650 nm).
9
[0029] Under these circumstances, the present inventors have found, from results
of first-principles calculations, that the valence band composed of the carbon p
orbitals is located at a shallow level (low potential) even compared to the valence
bands of nitrides and oxynitrides. As a result of a further study, the present
inventors have found that a semiconductor material has a valence band composed o5 f
the carbon p orbitals and has a smaller band gap than conventional nitrides and
oxynitrides when the semiconductor material includes an oxynitride that contains
at least one element selected from the Group 4 elements and the Group 5 elements
and in which part of at least one selected from oxygen and nitrogen is substituted
10 with carbon. Many of simple carbides of the Group 4 or Group 5 elements have
metallic conductivity and do not have a band gap. Therefore, the present invention
requires that the semiconductor material include an oxynitride in which part of
oxygen and/or nitrogen is substituted with carbon.
[0030] FIGS. 2A and 2B diagrammatically show the difference between a material
15 in which an oxygen site of monoclinic NbON is substituted with carbon and a
material in which NbON is doped with carbon. FIG. 2A shows a form in which an
oxygen site of NbON is substituted with carbon. In this form, a carbon atom is
present instead of an oxygen atom previously present at the oxygen site. FIG. 2B
shows a form in which NbON is doped with carbon. In this form, a portion other
20 than Nb, oxygen, and nitrogen sites is doped with carbon, with the crystal structure
of NbON maintained. In the case where carbon is present not in the form of a
substituent occupying the position of oxygen and/or nitrogen constituting the
oxynitride but simply in the form of a dopant surrounded by the sites of the metal
element, oxygen, and nitrogen constituting the oxynitride, the carbon as a dopant
25 causes a defect. The defect acts as a recombination center of an electron and a hole
generated by photoexcitation, thus decreasing the quantum efficiency. Therefore, a
semiconductor material consisting of an oxynitride doped with carbon is not
preferable in the present invention. It is desirable that an oxygen site and/or a
nitrogen site of an oxynitride be substituted with carbon.
30 [0031] In the present invention, energy levels are described not as vacuum levels
often used in the semiconductor field but as electrochemical energy levels (FIG. 3
and the subsequent figures showing electronic densities of states determined by
quantum chemical calculations are represented by a concept based on vacuum levels,
but do not necessarily indicate absolute levels).
35 [0032] The semiconductor material of the present embodiment is a semiconductor
material including an oxynitride containing at least one element selected from the
Group 4 elements and the Group 5 elements. In the oxynitride, part of at least one
10
selected from oxygen and nitrogen is substituted with carbon.
[0033] When a semiconductor material is used as a photocatalyst, holes and
electrons generated by light irradiation need to be quickly charge-separated. The
efficiency of the charge separation influences the quantum efficiency (the number of
electrons activated by excitation / the number of incident photons). Therefore, i5 n
the case of a hydrogen generating device using a photocatalytic material that is
capable of absorbing visible light but is poor in charge separation efficiency,
electrons and holes generated by photoexcitation are likely to be recombined, which
deteriorates the quantum efficiency and thereby results in a decrease in the
10 hydrogen generation efficiency. Examples of factors that hinder the charge
separation include structural defects of the photocatalytic material. Therefore,
from the standpoint of charge separation efficiency, it is desired that a single-phase
highly-crystalline semiconductor material be used as the photocatalytic material.
This is because, in general, the higher the crystallinity of a semiconductor is, the
15 less such defects are. However, in the case where the semiconductor material is a
single-phase material, there are not necessarily many defects even when the
material is in an amorphous form. In such a case, the material is allowed to be in
an amorphous form.
[0034] For example, in the case where the semiconductor material is a multiphase
20 mixture (such as the case where the material consists mostly of Nb2O5 and
additionally contains a slight amount of NbCN), when electrons and holes generated
by photoexcitation of Nb2O5 move in the presence of NbCN that does not have the
same electron orbital as that of Nb2O5, the interface between NbCN and Nb2O5 acts
as a recombination center of the electrons and holes, thus decreasing the quantum
25 efficiency. Therefore, in the present invention, the semiconductor material
preferably has a single-phase structure when used as a photocatalytic material for
the purpose of generating hydrogen by water decomposition. The semiconductor
material may contain a small amount of impurities or defects as long as the
single-phase structure is maintained. This is because an intrinsic semiconductor
30 free from any impurities is very difficult to produce, and because an intrinsic
semiconductor has such a low electron conductivity as to reduce the mobility of
electrons generated by photoexcitation, which results in a decrease in the quantum
efficiency. Therefore, defects that are so slight as to allow optimal control of the
Fermi level are acceptable as long as the single-phase structure is maintained. In
35 addition, a phase of a small amount of impurities (e.g., an oxidized coating or the
like) may be contained in the surface of a single-phase bulk. Even in the presence
of a surface impurity phase containing a small amount of impurities, the
11
photocatalytic function can be exhibited by quantum effect. The content of the
impurities is preferably 1 mol% or less. From the standpoint mentioned above, the
crystallinity of the semiconductor material is preferably as high as possible. With
respect to defects of the semiconductor, as described above, there is a trade-off
relationship between the electron conductivity and the rate of deactivation caus5 ed
as a result of the defects acting as recombination centers of electrons and holes
generated by photoexcitation. Therefore, the amount of defects is preferably 1
mol% or less. Since there is a trade-off relationship between the electron
conductivity and the rate of deactivation by recombination with respect to defects,
10 the semiconductor is desirably formed to be thin to the extent that the single-phase
structure and high crystallinity are maintained. That is, when the semiconductor
is formed as a semiconductor layer, the semiconductor layer desirably has a small
thickness. This is because a decrease in the thickness of the semiconductor layer
improves the charge separation efficiency. It has been experimentally discovered
15 that the thickness of the semiconductor layer is desirably 500 nm or less in the
present invention. However, an excessive decrease in the thickness of the
semiconductor layer leads to poor crystallization, and also reduces the amount of
light absorbed. Therefore, the thickness of the semiconductor layer is desirably 10
nm or more. That is, the thickness of the semiconductor layer is desirably 10 nm
20 or more and 500 nm or less. Furthermore, it is desirable that the semiconductor
layer having a thickness of 10 nm or more and 500 nm or less be designed to have a
large surface area. As described above, an increase in the thickness of the
semiconductor layer increases the amount of light absorbed but decreases the
charge separation efficiency. On the other hand, a decrease in the thickness of the
25 semiconductor layer decreases the amount of light absorbed but improves the
charge separation efficiency. There is a trade-off relationship between light
absorption and charge separation efficiency with respect to the thickness of the
semiconductor layer. Therefore, the semiconductor layer is desirably designed to
have a small thickness and a large surface area. This can be achieved by
30 appropriately adjusting the shape of a substrate on which the semiconductor layer
is provided. With a large surface area, the semiconductor layer can absorb light
having once been transmitted through or scattered at the semiconductor layer
during light irradiation.
[0035] TaON is known to be a semiconductor having a photocatalytic ability and
35 capable of absorbing light having a wavelength of 500 nm or less. No example of
synthesis of a single-phase NbON has been hitherto reported. The present
inventors have developed a new synthesis process to synthesize a single-phase
12
NbON, and have found that the single-phase NbON is a semiconductor having a
photocatalytic ability and capable of absorbing light having a wavelength of 600 nm
or less. In addition, it has been experimentally confirmed that both TaON and
NbON are a photocatalytic material capable of water decomposition, and it has also
been confirmed that the oxidation-reduction potentials of water lie between th5 e
valence band and the conduction band of both TaON and NbON.
[0036] The band gaps of materials resulting from substitution of oxygen and/or
nitrogen sites of TaON with carbon were calculated by first-principles calculations.
FIGS. 3A to 3D show the electronic density of states distribution (Density of State)
10 determined by first-principles calculations for TaON and materials resulting from
substitution of oxygen and/or nitrogen sites of TaON with carbon. For example, for
the case of TaON, the first-principles calculation was carried out on the assumption
that a unit lattice includes four Ta atoms, four oxygen atoms, and four nitrogen
atoms, and such unit lattices are arranged continuously to infinity under periodic
15 boundary conditions. Therefore, FIG. 3B shows the electronic density of states
distribution of a material obtained by substitution of one oxygen atom with a carbon
atom in the unit lattice. That is, FIG. 3B shows the electronic density of states
distribution of a material having four Ta atoms, three oxygen atoms, four nitrogen
atoms, and one carbon atom in the unit lattice and thus containing 8.3 at% (mol%)
20 of carbon. FIG. 3C shows the electronic density of states distribution of a material
obtained by substitution of one nitrogen atom with a carbon atom in the unit lattice.
That is, FIG. 3C shows the electronic density of states distribution of a material
having four Ta atoms, four oxygen atoms, three nitrogen atoms, and one carbon
atom in the unit lattice and thus containing 8.3 at% (mol%) of carbon. FIG. 3D
25 shows the electronic density of states distribution of a material obtained by
substitution of one oxygen atom and one nitrogen atom with two carbon atoms in
the unit lattice. That is, FIG. 3D shows the electronic density of states distribution
of a material having four Ta atoms, three oxygen atoms, three nitrogen atoms, and
two carbon atoms in the unit lattice and thus containing 16.7 at% (mol%) of carbon.
30 [0037] The calculation result obtained by the first-principles calculation for TaON
of FIG. 3A was that the band gap was 1.93 eV which corresponds to 642 nm. It is
general that band gaps determined as a result of first-principles calculations are
smaller than actual band gaps. From the fact that the actually-measured band gap
of TaON is 500 nm, it was understood that the band gap calculated by the
35 first-principles calculation is 0.78 times the actually-measured band gap. FIGS. 3A
to 3D are the calculation results for the cases of carbon substitution in TaON having
the same monoclinic crystal structure. Generally, results of first-principles
13
calculations for the same crystal structure show the same trend. In view of this,
the ratio between the calculated value and actual measured value of the band gap of
TaON of FIG. 3A was applied to the band gap calculation results of FIGS. 3B to 3D
to estimate the band gaps. As a result, it was found that substitution of oxygen
sites of TaON with carbon has the greatest effect in reducing the band ga5 p
(providing longer-wavelength visible light responsivity).
[0038] Similarly, the band gaps of materials resulting from substitution of oxygen
and/or nitrogen sites of NbON with carbon were calculated by first-principles
calculations. FIGS. 4A to 4D show the electronic density of states distribution
10 (Density of State) determined by first-principles calculations for NbON and
materials resulting from substitution of oxygen and/or nitrogen sites of NbON with
carbon. For example, for the case of NbON, the first-principles calculation was
carried out on the assumption that a unit lattice includes four Nb atoms, four
oxygen atoms, and four nitrogen atoms, and such unit lattices are arranged
15 continuously to infinity under periodic boundary conditions. Therefore, FIG. 4B
shows the electronic density of states distribution of a material obtained by
substitution of one oxygen atom with a carbon atom in the unit lattice. That is,
FIG. 4B shows the electronic density of states distribution of a material having four
Nb atoms, three oxygen atoms, four nitrogen atoms, and one carbon atom in the
20 unit lattice and thus containing 8.3 at% (mol%) of carbon. FIG. 4C shows the
electronic density of states distribution of a material obtained by substitution of one
nitrogen atom with a carbon atom in the unit lattice. That is, FIG. 4C shows the
electronic density of states distribution of a material having four Nb atoms, four
oxygen atoms, three nitrogen atoms, and one carbon atom in the unit lattice and
25 thus containing 8.3 at% (mol%) of carbon. FIG. 4D shows the electronic density of
states distribution of a material obtained by substitution of one oxygen atom and
one nitrogen atom with two carbon atoms in the unit lattice. That is, FIG. 4D
shows the electronic density of states distribution of a material having four Nb
atoms, three oxygen atoms, three nitrogen atoms, and two carbon atoms in the unit
30 lattice and thus containing 16.7 at% (mol%) of carbon.
[0039] The calculation result obtained by the first-principles calculation for NbON
of FIG. 4A was that the band gap was 1.61 eV which corresponds to 770 nm. It is
general that band gaps determined as a result of first-principles calculations are
smaller than actual band gaps. From the fact that the actually-measured band gap
35 of NbON is 600 nm, it was understood that the band gap calculated by the
first-principles calculation is 0.78 times the actually-measured band gap as in the
case of TaON materials. FIGS. 4A to 4D are the calculation results for the cases of
14
carbon substitution in NbON having the same monoclinic crystal structure.
Generally, results of first-principles calculations for the same crystal structure show
the same trend. In view of this, the ratio between the calculated value and actual
measured value of the band gap of NbON of FIG. 4A was applied to the band gap
calculation results of FIGS. 4B to 4D to estimate the band gaps. As a result, it wa5 s
found that substitution of oxygen sites of NbON with carbon has the greatest effect
in reducing the band gap (providing longer-wavelength visible light responsivity).
Furthermore, it was found that, in the case of FIG. 4D, the material is converted
into a conductive material due to excessive reduction in the band gap. The Fermi
10 level (0 eV) is present below the top of the valence band levels, which is for both
TaON and NbON. Such a state indicates that a level empty of electrons is present
among the valence band levels. Such an electronic state is not preferable because a
photoexcited electron is likely to transit down to the empty electronic level in the
valence band, which increases the probability of recombination of the excited
15 electron and a hole.
[0040] In view of the above, the band gaps of materials resulting from substitution
of oxygen sites of NbON with varying amounts of carbon were calculated by
first-principles calculations. FIGS. 5A to 5F show the electronic density of states
distribution (Density of State) determined by first-principles calculations for NbON
20 and materials resulting from substitution of oxygen sites of NbON with carbon.
For the cases of FIGS. 4A to 4D, the calculations were performed by assuming a unit
lattice including four Nb atoms. For the cases of FIGS. 5A to 5F, however, the
calculations were made by assuming a unit lattice including eight or more Nb atoms,
in order to vary the amount of substitutional carbon. The calculations were carried
25 out on the assumption that such unit lattices are arranged continuously to infinity
under periodic boundary conditions. Therefore, FIG. 5B shows the electronic
density of states distribution of a material obtained by substitution of one oxygen
atom with a carbon atom in a unit lattice including eight Nb atoms. That is, the
material of FIG. 5B has eight Nb atoms, seven oxygen atoms, eight nitrogen atoms,
30 and one carbon atom in the unit lattice, and thus contains 4.2 at% (mol%) of carbon.
FIG. 5C shows the electronic density of states distribution of a material obtained by
substitution of three oxygen atoms with three carbon atoms in a unit lattice
including sixteen Nb atoms. That is, FIG. 5C shows the electronic density of states
distribution of a material having sixteen Nb atoms, thirteen oxygen atoms, sixteen
35 nitrogen atoms, and three carbon atoms in the unit lattice and thus containing 6.3
at% (mol%) of carbon. For reference, FIG. 5D shows the electronic density of states
distribution of the same material as shown in FIG. 4B, that is, a material obtained
15
by substitution of one oxygen atom with a carbon atom in a unit lattice including
four Nb atoms. That is, FIG. 5D shows the electronic density of states distribution
of a material having four Nb atoms, three oxygen atoms, four nitrogen atoms, and
one carbon atom in the unit lattice and thus containing 8.3 at% (mol%) of carbon.
FIG. 5E shows the electronic density of states distribution of a material obtained b5 y
substitution of one oxygen atom with one carbon atom in a unit lattice including
thirty-two Nb atoms. That is, FIG. 5E shows the electronic density of states
distribution of a material having thirty-two Nb atoms, thirty-one oxygen atoms,
thirty-two nitrogen atoms, and one carbon atom in the unit lattice and thus
10 containing 1.0 at% (mol%) of carbon. FIG. 5F shows the electronic density of states
distribution of a material obtained by substitution of one oxygen atom with one
carbon atom in a unit lattice including sixteen Nb atoms. That is, FIG. 5F shows
the electronic density of states distribution of a material having sixteen Nb atoms,
fifteen oxygen atoms, sixteen nitrogen atoms, and one carbon atom in the unit
15 lattice and thus containing 2.1 at% (mol%) of carbon.
[0041] The calculation result obtained by the first-principles calculation for NbON
of FIG. 5A was that the band gap was 1.61 eV which corresponds to 770 nm. It is
general that band gaps determined as a result of first-principles calculations are
smaller than actual band gaps. From the fact that the actually-measured band gap
20 of NbON is 600 nm, it was understood that the band gap calculated by the
first-principles calculation is 0.78 times the actually-measured band gap as in the
case of FIG. 4A. FIGS. 5A to 5F represent calculations for the cases of carbon
substitution in NbON having the same monoclinic crystal structure, and results of
first-principles calculations for the same crystal structure generally show the same
25 trend. In view of this, the ratio between the calculated value and actual measured
value of the band gap of NbON of FIG. 5A was applied to the band gap calculation
results of FIGS. 5B to 5F to estimate the band gaps. As a result, the following facts
were revealed. For the case where 8.3 at% (mol%) of carbon is contained (FIG. 5D),
although a band gap-like valley is observed in the electronic density of states, the
30 Fermi level is present in the valence band and below the first peak from the top of
the valence band, and therefore, the material has an extremely small band gap or is
close to a conductive material. For the case where 6.3 at% (mol%) of carbon is
contained (FIG. 5C), the material is obviously a conductive material since the Fermi
level (0 eV) is present among the conduction band levels. However, for the case
35 where the carbon content is 4.2 at% (mol%) or less (FIGS. 5B, 5E, and 5F), it was
found that the Fermi level (0 eV) is present at the top of the valence band. That is,
it was found that, in the case of substitution of carbon for oxygen sites of NbON, a
16
longer-wavelength visible light-responsive material can be obtained in a preferred
electronic state by adjusting the carbon content to 4.2 at% (mol%) or less.
[0042] It is general that band gaps determined by quantum chemical calculations
are smaller than actual band gaps. However, quantum chemical calculations allow
accurate determination of the trend of the electronic density of states distribution5 .
That is, an accurate calculation can be made, for example, for determination as to
whether a material has a band gap characteristic of a semiconductor and whether
the material is a conductive material.
[0043] Next, the band gaps of materials resulting from substitution of nitrogen
10 sites of NbON with varying amounts of carbon were calculated by first-principles
calculations. FIGS. 6A to 6F show the electronic density of states distribution
(Density of State) determined by first-principles calculations for NbON and
materials resulting from substitution of nitrogen sites of NbON with carbon. For
the cases of FIGS. 4A to 4D, the calculations were performed by assuming a unit
15 lattice including four Nb atoms. For the cases of FIGS. 6A to 6F, however, the
calculations were made by assuming a unit lattice including eight or more Nb atoms,
in order to vary the amount of substitutional carbon. The calculations were carried
out on the assumption that such unit lattices are arranged continuously to infinity
under periodic boundary conditions. Therefore, FIG. 6B shows the electronic
20 density of states distribution of a material obtained by substitution of one nitrogen
atom with a carbon atom in a unit lattice including eight Nb atoms. That is, FIG.
6B shows the electronic density of states distribution of a material having eight Nb
atoms, eight oxygen atoms, seven nitrogen atoms, and one carbon atom in the unit
lattice and thus containing 4.2 at% (mol%) of carbon. FIG. 6C shows the electronic
25 density of states distribution of a material obtained by substitution of three nitrogen
atoms with three carbon atoms in a unit lattice including sixteen Nb atoms. That
is, the electronic density of states distribution of a material having sixteen Nb
atoms, sixteen oxygen atoms, thirteen nitrogen atoms, and three carbon atoms in
the unit lattice and thus containing 6.3 at% (mol%) of carbon is shown. For
30 reference, FIG. 6D shows the electronic density of states distribution of the same
material as shown in FIG. 4C, that is, a material obtained by substitution of one
nitrogen atom with a carbon atom in a unit lattice including four Nb atoms. That
is, FIG. 6D shows the electronic density of states distribution of a material having
four Nb atoms, four oxygen atoms, three nitrogen atoms, and one carbon atom in
35 the unit lattice and thus containing 8.3 at% (mol%) of carbon. FIG. 6E shows the
electronic density of states distribution of a material obtained by substitution of one
nitrogen atom with one carbon atom in a unit lattice including thirty-two Nb atoms.
17
That is, FIG. 6E shows the electronic density of states distribution of a material
having thirty-two Nb atoms, thirty-two oxygen atoms, thirty-one nitrogen atoms,
and one carbon atom in the unit lattice and thus containing 1.0 at% (mol%) of
carbon. FIG. 6F shows the electronic density of states distribution of a material
obtained by substitution of one nitrogen atom with one carbon atom in a unit lattic5 e
including sixteen Nb atoms. That is, FIG. 6F shows the electronic density of states
distribution of a material having sixteen Nb atoms, sixteen oxygen atoms, fifteen
nitrogen atoms, and one carbon atom in the unit lattice and thus containing 2.1 at%
(mol%) of carbon.
10 [0044] The calculation result obtained by the first-principles calculation for NbON
of FIG. 6A was that the band gap was 1.61 eV which corresponds to 770 nm. It is
general that band gaps determined as a result of first-principles calculations are
smaller than actual band gaps. From the fact that the actually-measured band gap
of NbON is 600 nm, it was understood that the band gap calculated by the
15 first-principles calculation is 0.78 times the actually-measured band gap as in the
case of FIG. 4A. FIGS. 6A to 6F represent calculations for the cases of carbon
substitution in NbON having the same monoclinic crystal structure, and results of
first-principles calculations for the same crystal structure generally show the same
trend. In view of this, the ratio between the calculated value and actual measured
20 value of the band gap of NbON of FIG. 6A was applied to the band gap calculation
results of FIGS. 6B to 6F to estimate the band gaps. As a result, it was found that,
in any case where nitrogen of NbON is substituted with carbon, the effect of
providing longer-wavelength visible light responsivity (increasing the bandgap
wavelength) is obtained but the Fermi level (0 eV) is present below the top of the
25 valence band levels. This state indicates that a level empty of electrons is present
among the valence band levels. Although the material is a semiconductor, such an
electronic state is not preferable because a photoexcited electron is likely to transit
down to the empty electronic level in the valence band, which increases the
probability of recombination of the excited electron and a hole. Thus, it was found
30 that an increase in the bandgap wavelength of the semiconductor material is
achieved in any case where oxygen and/or nitrogen sites of NbON are substituted
with carbon, and that when the semiconductor material is used as a photocatalytic
material, the material is more preferably one resulting from substitution of oxygen
sites with carbon.
35 [0045] Next, the band gaps of materials resulting from substitution of both oxygen
sites and nitrogen sites of NbON with varying amounts of carbon were calculated by
first-principles calculations. FIGS. 7A to 7F show the electronic density of states
18
distribution (Density of State) determined by first-principles calculations for NbON
and materials resulting from substitution of oxygen and nitrogen sites of NbON
with carbon. For the cases of FIGS. 4A to 4D, the calculations were performed by
assuming a unit lattice including four Nb atoms. For the cases of FIGS. 7A to 7F,
however, the calculations were made by assuming a unit lattice including eight 5 or
more Nb atoms, in order to vary the amount of substitutional carbon. The
calculations were carried out on the assumption that such unit lattices are arranged
continuously to infinity under periodic boundary conditions. Therefore, FIG. 7B
shows the electronic density of states distribution of a material obtained by
10 substitution of one oxygen atom and one nitrogen atom with two carbon atoms in a
unit lattice including sixteen Nb atoms. That is, FIG. 7B shows the electronic
density of states distribution of a material having sixteen Nb atoms, fifteen oxygen
atoms, fifteen nitrogen atoms, and two carbon atoms in the unit lattice and thus
containing 4.2 at% (mol%) of carbon. FIG. 7C shows the electronic density of states
15 distribution of a material obtained by substitution of one oxygen site and one
nitrogen site with two carbon atoms in a unit lattice including eight Nb atoms.
That is, FIG. 7C shows the electronic density of states distribution of a material
having eight Nb atoms, seven oxygen atoms, seven nitrogen atoms, and two carbon
atoms in the unit lattice and thus containing 8.3 at% (mol%) of carbon. For
20 reference, FIG. 7D shows the electronic density of states distribution of the same
material as shown in FIG. 3D, that is, a material obtained by substitution of one
oxygen site and one nitrogen site with two carbon atoms in a unit lattice including
four Nb atoms. That is, FIG. 7D shows the electronic density of states distribution
of a material having four Nb atoms, three oxygen atoms, three nitrogen atoms, and
25 two carbon atoms in the unit lattice and thus containing 16.7 at% (mol%) of carbon.
FIG. 7E shows the electronic density of states distribution of a material obtained by
substitution of one oxygen site and two nitrogen sites with three carbon atoms in a
unit lattice including sixteen Nb atoms. That is, FIG. 7E shows the electronic
density of states distribution of a material having sixteen Nb atoms, fifteen oxygen
30 atoms, fourteen nitrogen atoms, and three carbon atoms in the unit lattice and thus
containing 6.3 at% (mol%) of carbon. FIG. 7F shows the electronic density of states
distribution of a material obtained by substitution of two oxygen sites and one
nitrogen site with three carbon atoms in a unit lattice including sixteen Nb atoms.
That is, FIG. 7F shows the electronic density of states distribution of a material
35 having sixteen Nb atoms, fourteen oxygen atoms, fifteen nitrogen atoms, and three
carbon atoms in the unit lattice and thus containing 6.3 at% (mol%) of carbon.
[0046] The calculation result obtained by the first-principles calculation for NbON
19
of FIG. 7A was that the band gap was 1.61 eV which corresponds to 770 nm. It is
general that band gaps determined as a result of first-principles calculations are
smaller than actual band gaps. From the fact that the actually-measured band gap
of NbON is 600 nm, it was understood that the band gap calculated by the
first-principles calculation is 0.78 times the actually-measured band gap as in th5 e
case of FIG. 4A. FIGS. 7A to 7F represent calculations for the cases of carbon
substitution in NbON having the same monoclinic crystal structure, and results of
first-principles calculations for the same crystal structure generally show the same
trend. In view of this, the ratio between the calculated value and actual measured
10 value of the band gap of NbON of FIG. 7A was applied to the band gap calculation
results of FIGS. 7B to 7F to estimate the band gaps. From the results, it is
understood that the material is obviously a conductive material in the case where
16.7 at% (mol%) of carbon is contained (FIG. 7D). It was also found that the Fermi
level (0 eV) is present at the top of the valence band in the case where 8.3 at%
15 (mol%) of carbon is contained (FIG. 7C). That is, it was found that, in the case of
substitution of carbon for oxygen and nitrogen sites of NbON, a longer-wavelength
visible light-responsive material can be obtained in a preferred electronic state by
adjusting the carbon content to 8.3 at% (mol%) or less. In addition, it was found
that a longer-wavelength visible light-responsive material can be obtained in a
20 preferred electronic state also in the case where the ratio between oxygen and
nitrogen substituted with carbon is not 1:1 (FIG. 7E). However, the material of
FIG. 7F, although being a semiconductor, is not preferable because a state empty of
electrons is created in the valence band.
[0047] It is general that band gaps determined by quantum chemical calculations
25 are smaller than actual band gaps. However, quantum chemical calculations allow
accurate determination of the trend of the electronic density of states distribution.
That is, an accurate calculation can be made, for example, for determination as to
whether a material has a band gap characteristic of a semiconductor and whether
the material is a conductive material.
30 [0048] Considering all the matters discussed above, it can be determined which of
substitution of oxygen sites with carbon and substitution of nitrogen sites with
carbon is more advantageous. That is, either substitution of oxygen sites with
carbon or substitution of nitrogen sites with carbon has the effect of providing
longer-wavelength visible light responsivity, and random substitution of both
35 oxygen sites and nitrogen sites with carbon also has the effect of providing
longer-wavelength visible light responsivity. However, it was found that
preferential substitution of oxygen sites with carbon produces a greater effect of
20
providing longer-wavelength visible light responsivity with a smaller amount of
substitutional carbon. Also, it was found that the material is converted into a
conductive material having no semiconductor properties when the substitutional
carbon amount is too large. Furthermore, it was found that the larger the
substitutional carbon amount is, the smaller the band gap is, as long as th5 e
substitutional carbon amount is within a predetermined range. In addition, it was
found that, in the case of substitution of oxygen sites with carbon, the effect of
providing longer-wavelength visible light responsivity is markedly exhibited when
the proportion of substitutional carbon is 4.2 at% or less. It was also found that, in
10 the case of substitution of nitrogen sites with carbon or random substitution of
oxygen sites and nitrogen sites with carbon, the effect of providing
longer-wavelength visible light responsivity is exhibited when the proportion of
substitutional carbon is 8.3 at% or less.
[0049] Since the conduction band is composed of the outermost d orbitals of the
15 metal element Ta or Nb which are empty of electrons, the levels of the d orbitals of
Ta or Nb are not changed even when oxygen and/or nitrogen sites are substituted
with carbon. Therefore, it was found that the band gap-reducing effect provided by
substitution of oxygen and/or nitrogen sites with carbon is an effect obtained as a
result of the valence band levels being displaced. That is, it was found that control
20 of the amount of carbon substituting for oxygen and nitrogen allows control of the
magnitude of the band gap and at the same time allows control of the valence band
levels. If the semiconductor material is used as a photocatalyst for water
decomposition by photocatalyst light irradiation, the oxygen evolution overpotential
in the water decomposition can be freely set by controlling the amount of carbon
25 substituting for oxygen and nitrogen sites since the semiconductor material is a
n-type semiconductor. Generally, in water decomposition reaction, the oxygen
evolution overpotential is larger than the hydrogen evolution overpotential.
Therefore, it was found that the capability of controlling the oxygen evolution
overpotential is effective in device design.
30 [0050] As described above, the semiconductor material of the present embodiment
is capable of absorbing visible light, and has band edges between which the
oxidation-reduction potentials of water lie. Furthermore, the semiconductor
material of the present embodiment is excellent in stability in water during light
irradiation. Therefore, when the semiconductor material of the present
35 embodiment is immersed in water containing an electrolyte and is irradiated with
sunlight to decompose the water, hydrogen can be generated more efficiently than
ever before.
21
[0051] According to the present embodiment, it is also possible to implement a
hydrogen production method including the step of irradiating the semiconductor
material of the present embodiment immersed in a solution containing an
electrolyte and water with light so as to decompose the water.
[0052] The production method of the present embodiment can be carried ou5 t
similarly to commonly-known methods (see Patent Literature 1 and 2, for example)
by replacing a commonly-known photocatalytic material with the above-described
photocatalytic material. Specific examples include a method using the
below-described optical hydrogen generating device of the present embodiment.
10 [0053] With the production method of the present embodiment, hydrogen can be
generated with high efficiency.
[0054] The optical hydrogen generating device of the present embodiment includes
a container, an electrode containing a photocatalytic material, and a counter
electrode. The photocatalytic material includes the above-described semiconductor
15 material of the present embodiment. Examples of the configuration of the
hydrogen generating device of the present embodiment are shown in FIG. 15 and
FIG. 16.
[0055] When the semiconductor material in the form of a bulk or powder is
irradiated with light in water, even if the water is decomposed to generate hydrogen
20 and oxygen, most of the hydrogen and oxygen generated are almost instantly
recombined into water. Therefore, it is preferable that hydrogen and oxygen be
generated separately from each other. Thus, it is preferable that the photocatalytic
material be formed into an electrode, a separate counter electrode electrically
connected to the electrode be provided, and a chamber for hydrogen generation and
25 a chamber for oxygen generation be separated. In some cases, a configuration may
be employed in which hydrogen is generated from one surface of a single electrode
and oxygen is generated from the opposite surface of the single electrode.
[0056] An optical hydrogen generating device of FIG. 15 includes a container 9, a
photocatalytic electrode 2, a conductive substrate 1, and a counter electrode 3. The
30 container 9 has, in its upper part, two openings 8 for respectively collecting
hydrogen and oxygen. Also, the container 9 has, in its lower part, two openings 8
serving as feed water inlets. A solution 6 containing an electrolyte and water is
held in the container 9. In order to separate a chamber for hydrogen generation
and a chamber for oxygen generation from each other, the container 9 has a
35 separator 4 between the photocatalytic electrode 2 and the counter electrode 3.
The separator 4 has a function of allowing ion permeation and separating a gas
generated on the photocatalytic electrode 2 side from a gas generated on the counter
22
electrode 3 side. The portion (light incident portion 5) of the container 9 that faces
the surface of the photocatalytic electrode 2 disposed in the container 9 is made of a
material that allows transmission of light such as sunlight. The photocatalytic
electrode 2 and the counter electrode 3 are electrically connected by a conducting
wire 75 .
[0057] The photocatalytic electrode 2 is a semiconductor having a band gap, and
thus generally has a lower conductivity than metals etc. In addition,
recombination of electrons and holes needs to be prevented as much as possible.
Therefore, the photocatalytic electrode 2 preferably has a small thickness. Thus, in
10 this embodiment, the photocatalytic electrode 2 is formed as a thin film (with a
thickness of about 50 to 500 nm) on the conductive substrate 1. Also, in order to
increase the light absorption efficiency, the photocatalytic electrode 2 preferably has
a large surface area.
[0058] The photocatalytic electrode 2 preferably has high crystallinity. When the
15 electrode is flat and smooth, its crystal orientation is preferably in the thickness
direction of the electrode. When the electrode is not flat or smooth, its crystal
orientation is preferably in a direction parallel to the movement direction of
electrons or holes generated by photoexcitation.
[0059] Another optical hydrogen generating device shown in FIG. 16 also includes
20 the container 9, the photocatalytic electrode 2, the conductive substrate 1, and the
counter electrode 3 (the same members as those of FIG. 15 are denoted by the same
reference numerals in FIG. 16). The container 9 has the four openings 8, and the
solution 6 containing an electrolyte and water is held in the container 9. The
photocatalytic electrode 2 is provided on one surface of the conductive substrate 1,
25 and the counter electrode 3 is provided on the other surface. The photocatalytic
electrode 2 is formed as a thin film (with a thickness of about 50 to 500 nm). The
photocatalytic electrode 2 and the counter electrode 3 are electrically connected by
the conductive substrate 1. In order to separate a chamber for hydrogen
generation and a chamber for oxygen generation from each other, the inside of the
30 container 9 is divided into a photocatalytic electrode 2-side section and a counter
electrode 3-side section by the separator 4 and the conductive substrate 1. The
portion (light incident portion 5) of the container 9 that faces the surface of the
photocatalytic electrode 2 disposed in the container 9 is made of a material that
allows transmission of light such as sunlight.
35 [0060] Hydrogen and oxygen can be generated by irradiating the optical hydrogen
generating device shown in FIG. 15 or FIG. 16 with light (e.g., sunlight) through the
light incident portion 5. In particular, the optical hydrogen generating device is
23
capable of absorbing light in a wide wavelength region, and thus can generate
hydrogen with high efficiency.
EXAMPLES
[0061] Hereinafter, the present invention will be described in detail with referenc5 e
to examples and comparative examples. However, the present invention is not
limited to these examples.
[0062]
A thin film of TaCNO (a semiconductor in which oxygen or nitrogen sites of
10 TaON are substituted with carbon) and a semiconductor thin film of TaON for
comparison were formed on quartz substrates by reactive sputtering. The sputter
deposition conditions are shown in Table 1.
[0063] [Table 1]
Type of
Film Substrate Target
Sputtering
output
(W)
Flow rate of
Ar
(Pa•m3/s)
Flow rate of
O2
(Pa•m3/s)
Flow rate of
N2
(Pa•m3/s)
Total
pressure
(Pa)
Substrate
temperature
(°C)
Deposition
time
(hr)
Film thickness
(nm)
TaON Quartz TaN 30 6.1 × 10−4
(3.6 sccm)
8.5 × 10−5
(0.5 sccm)
2.4 × 10−3
(14.3 sccm) 0.5 800 8 250
TaCNO Quartz TaC 30 6.1 × 10−4
(3.6 sccm)
5.9×10−5
(0.35 sccm)
2.4×10−3
(14.3 sccm) 0.5 800 8 150
15 [0064] FIG. 8 shows a thin film X-ray diffraction pattern of the TaON thin film
fabricated by reactive sputtering using oxygen and nitrogen gases and using TaN as
a sputtering target (starting material). It was confirmed that an almost
single-phase TaON thin film was obtained, except for a halo peak of quartz of the
substrate. FIG. 9 is a thin film X-ray diffraction pattern of the TaCNO thin film (a
20 thin film made of a material in which oxygen or nitrogen sites of TaON are
substituted with carbon) fabricated by reactive sputtering using oxygen and
nitrogen gases and using TaC as a sputtering target. Similarly to the above, it was
confirmed that an almost single-phase TaON thin film was obtained, except for a
halo peak of quartz of the substrate. TaC was used as a target for the TaCNO.
25 Therefore, although the obtained thin film had a crystal system of a monoclinic
single-phase TaON, it is expected that a slight amount of carbon remained and
substituted for oxygen or nitrogen sites. In addition, since the sputtering was
performed at a high temperature of 800°C, it is generally thought that carbon
diffused quickly, and substituted for oxygen or nitrogen sites without being present
30 as a defect-causing dopant.
[0065] FIG. 10 shows results of SIMS analysis carried out for the TaON and
TaCNO thin films in the depth direction. The carbon content in the TaCNO was
1.5 to 1.0 at% (mol%), and the carbon content in the TaON was 0.5 to 0.3 at%
24
(mol%), since 1  1023 atoms/cm3 corresponds to about 100 at% (mol%). From this,
it was found that the carbon content was greater in the TaCNO than in the TaON
with a significant difference.
[0066] FIG. 11 shows results of measurements of the optical absorption
characteristics of the TaON thin film and the TaCNO thin film formed on quart5 z
substrates. Influence of interference patterns are seen in the curves. The tangent
lines to the optical absorption curves are those drawn by neglecting the interference
patterns. From the tangent lines, it was found that the bandgap wavelength of the
TaON was 500 nm and the bandgap wavelength of the TaCNO was 580 nm, and
10 thus that an 80 nm increase in bandgap wavelength is achieved by substitution of
oxygen or nitrogen sites of TaON with 1.5 to 1.0 at% (mol%) of carbon.
[0067] A film of TaON and a film of TaCNO were formed on glassy carbon
substrates by reactive sputtering under the same conditions as above, and the
resultant products were used as working electrodes. Using the glassy carbon
15 having conductivity as a current collector, the working electrodes were each
connected by a lead to a platinum electrode serving as a counter electrode. Each
pair of the working electrode and the counter electrode was immersed in a 0.1 M
sulfuric acid aqueous solution, and the wavelength dependence of photocurrent was
measured in a wavelength range of 900 nm to 300 nm by irradiating the TaON or
20 TaCNO electrode with xenon lamp light dispersed by a prism. The maximum
photocurrent was 2 μA/cm2 at a wavelength of 400 nm. It was found that a
photocurrent can be observed at a wavelength of 500 nm or less for the TaON and at
a wavelength of about 600 nm or less for the TaCNO. The fact that a photocurrent
was observed in the aqueous solution free from other substances than sulfuric acid
25 indicates that water decomposition reaction took place. In order to confirm that
these results were not due to dissolution reaction of the electrode, light irradiation
was performed continuously for two weeks. During this period, there was no
change in the photocurrent. Also, there was observed no change before and after
the test in the results of the thin film X-ray diffraction measurement on the TaON
30 and TaCNO. Oxides, nitrides, and oxynitrides of the Group 4 elements and Group
5 elements are more stable in acidic solutions than in alkaline solutions, and the
metal elements are less soluble in acidic solutions than in alkaline solutions.
Therefore, in view of durability, the semiconductor material of the present invention
is desirably used in a neutral or acidic aqueous solution.
35 [0068]
A thin film of NbCNO (a semiconductor material in which oxygen or
nitrogen sites of NbON are substituted with carbon) and a NbON thin film for
25
comparison were formed on quartz substrates by reactive sputtering. The sputter
deposition conditions are shown in Table 2.
[0069] [Table 2]
Type of
Film Substrate Target
Sputtering
output
(W)
Flow rate of
Ar
(Pa•m3/s)
Flow rate of
O2
(Pa•m3/s)
Flow rate of
N2
(Pa•m3/s)
Total
pressure
(Pa)
Substrate
temperature
(°C)
Deposition
time
(hr)
Film
thickness
(nm)
NbON Quartz NbN 30 6.1 × 10−4
(3.6 sccm)
6.8 × 10−5
(0.4 sccm)
2.4 × 10−-3
(14.3 sccm) 0.5 300 8 100
NbCNO Quartz NbC 30 6.1 × 10−4
(3.6 sccm)
5.1 × 10−5
(0.3 sccm)
2.4 × 10−3
(14.3 sccm) 0.5 300 8 120
[0070] For the case of NbCNO, the Nb oxide was easily formed when the substrat5 e
temperature was increased up to a temperature that allows sufficient crystallization.
Therefore, the substrate temperature was set to 300°C. Auger electron
spectroscopy analysis was carried out for the NbON thin film and the NbCNO thin
film in the depth direction. In both the NbON and NbCNO, the ratio of Nb :
10 oxygen : nitrogen was 33 to 36 at% : 33 to 35 at% : 32 to 34 at%. In the case of
ideal NbON, the ratio of Nb : oxygen : nitrogen must be 33.3 at% : 33.3 at% : 33.3
at%. Therefore, it was found that almost single-phase NbON and NbCNO thin
films were synthesized. Since analysis of trace carbon by Auger electron
spectroscopy is difficult, the quantification of carbon was performed by another type
15 of measurement.
[0071] FIG. 12 shows results of SIMS analysis carried out for the NbON and
NbCNO thin films in the depth direction. The carbon content in the NbCNO was
about 3.5 at% (mol%), and the carbon content in the NbON was about 0.25 at%
(mol%), since 1  1023 atoms/cm3 corresponds to about 100 at% (mol%). From this,
20 it was found that the carbon content is greater in NbCNO than in NbON with a
significant difference.
[0072] FIG. 13 shows results of measurements of the optical absorption
characteristics of the NbON thin film and the NbCNO thin film formed on quartz
substrates. Influence of interference patterns are seen in the curves. The tangent
25 lines to the optical absorption curves are those drawn by neglecting the interference
patterns. From the tangent lines, it was found that the bandgap wavelength of the
NbON was 600 nm and the bandgap wavelength of the NbCNO was 720 nm, and
thus that a 120 nm increase in bandgap wavelength is achieved by substitution of
oxygen or nitrogen sites of NbON with about 3.5 at% (mol%) of carbon.
30 [0073] A film of NbON and a film of NbCNO were formed on glassy carbon
substrates by reactive sputtering under the same conditions as above, and the
resultant products were used as working electrodes. Using the glassy carbon
having conductivity as a current collector, the working electrodes were each
26
connected by a lead to a platinum electrode serving as a counter electrode. Each
pair of the working electrode and the counter electrode was immersed in a 0.1 M
sulfuric acid aqueous solution, and the wavelength dependence of photocurrent was
measured in a wavelength range of 900 nm to 300 nm by irradiating the NbON or
NbCNO electrode with xenon lamp light dispersed by a prism. The maximu5 m
photocurrent was 11 μA/cm2 at a wavelength of 450 nm. It was found that a
photocurrent can be observed at a wavelength of 600 nm or less for NbON and at a
wavelength of about 720 nm or less for NbCNO. The fact that a photocurrent was
observed in the aqueous solution free from other substances than sulfuric acid
10 indicates that water decomposition reaction took place. In order to confirm that
these results were not due to dissolution reaction of the electrode, light irradiation
was performed continuously for two weeks. During this period, there was no
change in the photocurrent.
[0074]
15 In order to confirm that the main phase of the NbON and NbCNO described
in Example 2 was monoclinic NbON, a NbON thin film was fabricated on a quartz
substrate by reactive sputtering with a reduced sputtering output at a reduced
deposition rate. The sputter deposition conditions are shown in Table 3.
[0075] [Table 3]
Type of
Film Substrate Target
Sputtering
output
(W)
Flow rate of
Ar
(Pa•m3/s)
Flow rate of
O2
(Pa•m3/s)
Flow rate of
N2
(Pa•m3/s)
Total
pressure
(Pa)
Substrate
temperature
(°C)
Deposition
time
(hr)
Film
thickness
(nm)
NbON Quartz NbN 20 6.1 × 10−4
(3.6 sccm)
6.8 × 10−5
(0.4 sccm)
2.4 × 10−3
(14.3 sccm) 0.5 700 8 100
20
[0076] NbON is an unknown material, and no example of successful single-phase
synthesis of NbON has been reported in the past. Thus, reference data for X-ray
diffraction of NbON is not available. Therefore, the lattice constant of NbON was
calculated by assuming that NbON has the same monoclinic crystal structure as
25 TaON, placing a Nb atom at the same coordinate as Ta of TaON, and optimizing the
structure by a first-principles calculation. For confirmation, the lattice constant of
TaON was also calculated by optimizing the structure by a first-principles
calculation using the known crystal structure of TaON. The result was well
matched to the lattice constant previously reported in a X-ray diffraction database.
30 Generally, crystal lattice constants can be calculated by first-principles calculations
with good accuracy. FIG. 14 shows a thin film X-ray diffraction pattern of the
NbON thin film fabricated by reactive sputtering using oxygen and nitrogen gases
and using NbN as a sputtering target. It was confirmed that an almost
single-phase NbON thin film was obtained, except for a halo peak of quartz of the
27
substrate.
[0077] A film of NbON was formed on a glassy carbon substrate by reactive
sputtering under the same conditions as above, and the resultant product was used
as a working electrode. Using the glassy carbon having conductivity as a current
collector, the working electrode was connected by a lead to a platinum electrod5 e
serving as a counter electrode. The working electrode and the counter electrode
were immersed in a 0.1 M sulfuric acid aqueous solution, and the wavelength
dependence of photocurrent was measured in a range of 900 nm to 300 nm by
irradiating the NbON electrode with xenon lamp light dispersed by a prism. The
10 maximum photocurrent was 20 μA/cm2 at a wavelength of 450 nm. It was found
that a photocurrent can be observed at a wavelength of 600 nm or less for NbON.
The fact that a photocurrent was observed in the aqueous solution free from other
substances than sulfuric acid indicates that water decomposition reaction took place.
In order to confirm that these results were not due to dissolution reaction of the
15 electrode, light irradiation was performed continuously for two weeks. During this
period, there was no change in the photocurrent.
[0078] In the present examples, reactive sputtering using TaC as a target was used
for fabrication of TaCNO, and reactive sputtering using NbC as a target was used
for fabrication of NbCNO. However, TaCNO and NbCNO may be fabricated by
20 other commonly-known thin film production methods such as sputtering, MOCVD,
and plasma CVD. Alternatively, a method may be employed in which carbon is
implanted into preliminarily-fabricated TaON thin film and NbON thin film by a
commonly-known method such as carbon ion implantation so as to obtain TaCNO
and NbCNO. In the case of simple ion implantation, carbon is present in the form
25 of a dopant. Therefore, for example, it is preferable that oxygen or nitrogen sites
be substituted with carbon by a method such as a thermal diffusion process
performed in a nitrogen atmosphere, an ammonia atmosphere or the like from
which impurities such as oxygen and water have been sufficiently removed. In the
thermal diffusion process, the shortest thermal treatment time settable by the
30 apparatus used may be sufficient as long as the temperature is increased up to a
minimum temperature required for crystallization of the material used.
[0079] In the semiconductor material, the carbon content is not particularly limited
as long as the semiconductor function is not impaired by change in the crystal
structure of the oxynitride. From a result of a first-principles calculation, it was
35 found that the preferred carbon content is 8.3 at% (mol%) or less, although
depending on which sites are substituted with carbon. From a result of a
first-principles calculations, it was found that the band gap can be adjusted by
28
controlling the amount of carbon substituting for oxygen or nitrogen to be 8.3 at% or
less. It was found that the electronic state in the d orbitals constituting the
conduction band is hardly affected by the carbon substitution. Furthermore, it was
found that the valence band levels can be controlled by controlling the amount of
carbon substituting for oxygen or nitrogen sites. For example, when a n-typ5 e
semiconductor having a photocatalytic function is used for water decomposition
reaction induced by sunlight, the difference between the top of the valence band
levels and the oxygen evolution level corresponds to an overpotential in an
electrochemical reaction such as water electrolysis. In water decomposition
10 reaction, oxygen evolution generally acts as a rate-limiting factor. Therefore, the
oxygen evolution overpotential can be controlled by controlling the substitutional
carbon amount and thereby controlling the valence band levels. When the oxygen
evolution overpotential is small (the top of the valence band levels is high), the
magnitude of the photocurrent per unit electrode area cannot be made larger.
15 Therefore, in the case of use in a device, the electrode surface area needs to be
increased to obtain a large amount of photocurrent. However, depending on the
electrode fabrication process, the increase in the surface area of the electrode may
be limited. Therefore, in order to increase sufficiently the photocurrent per
apparent unit electrode area when the electrode has a surface area limited by the
20 electrode fabrication process for the device, it is recommended to control the amount
of carbon substituting for oxygen or nitrogen sites. In the case where it is
advantageous to ensure an appropriate oxygen evolution overpotential in terms of
device configuration, band design may be performed for the valence band levels by
controlling the amount of substitutional carbon, even when the control of the
25 amount of carbon makes the valence band levels deeper, resulting in an increase in
the band gap and thereby slight deterioration in sunlight utilization efficiency.
[0080] In addition, from a result of a first-principles calculation, it was found that
the properties of a semiconductor having a band gap are exhibited when the Group
5 element is in a form having a valence of 5 which is the highest possible valence,
30 and that when the Group 5 element is in a form having a smaller valence than the
highest valence, electron density in the conduction band is increased, and thus no
evident band gap occurs. Therefore, in the semiconductor material of the present
invention, it is preferable that the Group 5 element be in a form having
substantially a valence of 5 (preferably a valence of 4.8 to 5). In addition, it is
35 desirable that the Group 4 element be in a form having substantially a valence of 4
(preferably 3.8 to 4). The reason for this is as follows. In the case of Nb, for
example, the conduction band is composed of the Nb d orbitals, and therefore, Nb is
29
desirably in the pentavalent form in which the d orbital is empty of electrons. By a
first-principles calculation, it was found that when Nb is in the trivalent form in
which electrons are present in the d orbital, metallic conductivity is exhibited and
no band gap occurs due to the presence of electrons in the conduction band.
However, there may be a case where the Group 5 element is in a form having 5 a
valence of about 4.8 because of an inevitable production defect or the like. In this
case, a defect level is formed due to the defect, a phenomenon in which a broad
absorption edge is observed occurs, and the efficiency of absorption for wavelengths
around the band gap wavelength is somewhat decreased. However, there is no
10 significant influence on the semiconductor properties. Therefore, in the present
invention, it is acceptable that the valence of the Group 5 element is decreased to
about 4.8 due to an inevitable production defect. In other words, “that the Group 5
element be in a form having substantially a valence of 5” means that the Group 5
element is allowed to have a valence close to but less than 5 as long as there is no
15 significant influence on the semiconductor properties, and that the Group 5 element
is preferably in a form having a valence of 4.8 to 5. Also, “that the Group 4 element
be in a form having substantially a valence of 4” means that the Group 4 element is
allowed to have a valence close to but less than 4 as long as there is no significant
influence on the semiconductor properties, and that the Group 4 element is
20 preferably in a form having a valence of 3.8 to 4.
[0081] In the present examples given above, oxygen or nitrogen sites of an
oxynitride of a Group 5 element are substituted with carbon. However, it was also
found that the effect of increasing the bandgap wavelength can be similarly
obtained also when oxygen or nitrogen sites of an oxynitride such as Zr2ON2 or
25 Ti2ON2 which includes a tetravalent central metal element selected from the Group
4 elements are substituted with carbon. Also in the case where oxygen or nitrogen
sites of Zr2ON2 or Ti2ON2 are substituted with carbon, the resultant material is
allowed to be in an amorphous form as long as the material has a single-phase
structure. When in a crystalline form, the material desirably has a cubic structure.
30
INDUSTRIAL APPLICABILITY
[0082] With the semiconductor material of the present invention, hydrogen can be
generated with high efficiency using sunlight. The obtained hydrogen can be used,
for example, as fuel for fuel cells.
35
30
WE CLAIM
1. A semiconductor material comprising an oxynitride containing at least one
element selected from Group 4 elements and Group 5 elements,
wherein part of at least one selected from oxygen and nitrogen is substitute5 d
with carbon in the oxynitride, and
wherein the semiconductor material has a single-phase structure.
2. The semiconductor material according to claim 1, having a monoclinic
10 crystal structure.
3. The semiconductor material according to claim 1, wherein the at least one
element selected from the Group 4 elements and the Group 5 elements is Nb.
15 4. The semiconductor material according to claim 1, wherein the Group 5
element is in a form having substantially a valence of 5.
5. The semiconductor material according to claim 1, having a photocatalytic
ability.
20
6. A method of producing hydrogen, comprising the step of immersing the
semiconductor material according to claim 1 in a solution containing an electrolyte
and water, and then irradiating the semiconductor material with light to decompose
the water.
25
7. An optical hydrogen generating device comprising a container, an electrode
including a photocatalytic material, and a counter electrode,
wherein the photocatalytic material includes the semiconductor material
according to claim 1.