2
DESCRIPTION
NIOBIUM NITRIDE AND METHOD FOR PRODUCING SAME, NIOBIUM
NITRIDE-CONTAINING FILM AND METHOD FOR PRODUCING SAME,
5 SEMICONDUCTOR, SEMICONDUCTOR DEVICE, PHOTOCATALYST,
HYDROGEN GENERATION DEVICE, AND ENERGY SYSTEM
TECHNICAL FIELD
[0001] The present invention relates to a niobium nitride and a method for
10 producing the niobium nitride, a niobium nitride-containing film and a method for
producing the niobium nitride-containing film, a semiconductor, a semiconductor
device, a photocatalyst suitable for water decomposition reaction, a hydrogen
generation device including the photocatalyst, and an energy system including the
hydrogen generation device.
15
BACKGROUND ART
[0002] Conventionally, water is decomposed into hydrogen and oxygen by
irradiating a semiconductor material serving as a photocatalyst with light.
[0003] For example, Patent Literature 1 discloses a method in which an n-type
20 semiconductor electrode and a counter electrode are disposed in an electrolytic
solution and the surface of the n-type semiconductor electrode is irradiated with
light to obtain hydrogen and oxygen from the surfaces of these electrodes. Patent
Literature 1 describes the use of a TiO2 electrode, a ZnO electrode, or the like as the
n-type semiconductor electrode.
25 [0004] Patent Literature 2 discloses a gas generator including a metal electrode
and a nitride semiconductor electrode that are connected to each other and are
disposed in a solvent. Patent Literature 2 describes the use of a nitride of a Group
13 element such as indium, gallium or aluminum for the nitride semiconductor
electrode.
30 [0005] These conventional semiconductor electrodes have a problem of low
hydrogen generation efficiency in water decomposition reaction by sunlight
irradiation. The reason for this is as follows. Semiconductor materials such as
TiO2 and ZnO can absorb only short wavelength light, that is, light having a
wavelength of approximately 400 nm or less. Therefore, only a very small fraction
35 of the total sunlight can be utilized. For example, when TiO2 is used, only about
4.7% of sunlight can be utilized. Furthermore, in view of a theoretical loss of the
absorbed light due to heat loss, the sunlight utilization efficiency is only about 1.7%.
3
[0006] Patent Literature 3 discloses a photocatalyst containing an orthorhombic
tantalum nitride as a semiconductor material capable of absorbing longer
wavelength visible light. Patent Literature 3 also reports that tantalum nitride
Ta 3N5 can absorb light having a wavelength of 600 nm or less. However, the light
5 having a wavelength of 600 nm or less accounts for only about 16% of the total
sunlight. Furthermore, in view of a theoretical heat loss, the utilization efficiency
is only about 6%.
CITATION LIST
10 Patent Literature
[0007] Patent Literature 1
Patent Literature 2
Patent Literature 3
15 SUMMARY OF INVENTION
Technical Problem
[0008] In order to decompose water by irradiating a semiconductor material with
light, 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 need to be located at
20 levels between which the oxidation-reduction potential of water (the level of oxygen
evolution and the level of hydrogen evolution) is present. Therefore, the
requirements for a semiconductor material that can be practically used for water
decomposition are that: the semiconductor material must be able to absorb longer
wavelength light (the semiconductor material must have a smaller band gap); the
25 band edges of the semiconductor material must be located at levels between which
the oxidation-reduction potential of water is present; and the semiconductor
material must be stable in water under light irradiation. However, semiconductor
materials that meet all of these requirements have not been found.
[0009] Thus, it is an object of the present invention to provide a substance that can
30 be used as a semiconductor material capable of absorbing longer wavelength light
(having a smaller band gap), having band edges at levels between which the
oxidation-reduction potential of water is present, and having high stability in water
under light irradiation.
35 Solution to Problem
[0010] The present invention provides a niobium nitride which has a composition
represented by the composition formula Nb3N5 and in which the constituent
JP 51(1976)-123779 A
JP 2003-024764 A
JP 2002-233769 A
4
element Nb has a valence of substantially +5.
Advantageous Effects of Invention
[0011] The niobium nitride of the present invention is a novel substance, and can
5 be used as a semiconductor material capable of absorbing longer wavelength light
(having a smaller band gap), having band edges at levels between which the
oxidation-reduction potential of water is present, and having high stability in water
under light irradiation.
10 BRIEF DESCRIPTION OF DRAWINGS
[0010] FIG. 1 is a conceptual diagram illustrating the energy levels of the niobium
nitride of the present invention and conventional semiconductor materials.
FIG. 2 shows TG-DTA (Thermogravimetry-Differential Thermal Analysis)
data of pentakis(dimethylamino)niobium, which is an example of a starting
15 material used in a method for producing a niobium nitride according to a first
embodiment of the present invention.
FIG. 3 is a schematic diagram showing an example of an apparatus for
carrying out the method for producing a niobium nitride according to the first
embodiment of the present invention.
20 FIG. 4 is a diagram illustrating a Nb3N5 synthesis mechanism in a method
for producing Nb3N5 according to a second embodiment of the present invention.
FIG. 5 is a schematic diagram showing an example of an apparatus for
carrying out a method for producing a niobium nitride-containing film according to
a third embodiment of the present invention.
25 FIG. 6 is a diagram illustrating a Nb3N5-containing film synthesis
mechanism in a method for producing a Nb3N5-containing film according to a fourth
embodiment of the present invention.
FIG. 7 is a diagram of the density of states distribution of Nb3N5.
FIG. 8 is a schematic diagram showing an example of a hydrogen generation
30 device according to a sixth embodiment of the present invention.
FIG. 9 is a schematic cross-sectional diagram showing another example of
the hydrogen generation device according to the sixth embodiment of the present
invention.
FIG. 10 is a schematic cross-sectional diagram showing still another
35 example of the hydrogen generation device according to the sixth embodiment of the
present invention.
FIG. 11 is a schematic diagram showing an example of an energy system
5
according to a seventh embodiment of the present invention.
FIG. 12 is a schematic diagram showing a more specific example of the
energy system according to the seventh embodiment of the present invention.
FIG. 13 shows an X-ray photoelectron spectrum of Nb3N5 according to
5 Example 1-1 of the present invention.
FIG. 14 shows an X-ray absorption near-edge structure (XANES) spectrum
of Nb3N5 according to Example 1-1 of the present invention.
FIG. 15 shows an ultraviolet-visible absorption spectrum of a
Nb3N5-containing film according to Example 1-2 of the present invention.
10 FIG. 16 shows a time course of the amount of hydrogen generated using
Nb3N5 according to Example 1-3 of the present invention.
FIG. 17 shows a time course of the amount of oxygen generated using Nb3N5
according to Example 1-4 of the present invention.
FIG. 18 shows ultraviolet-visible absorption spectra of films 1A to 1D
15 according to Example 2-1 of the present invention.
FIG. 19 shows X-ray photoelectron spectra of Nb 3d in the films 1A to 1D
according to Example 2-1.
DESCRIPTION OF EMBODIMENTS
20 [0013] In order to decompose water efficiently using sunlight so as to generate
hydrogen, a material used as a photocatalyst needs to be a semiconductor material
capable of absorbing relatively long wavelength visible light (having a narrow band
gap width), having band edges (the level of the top of the valence band and the level
of the bottom of the conduction band) located at levels between which the oxidation
25 potential and reduction potential of water are present, and being stable in an
aqueous solution under light irradiation.
[0014] How large the band gap should be to achieve a hydrogen generation
efficiency comparable to that of a now commonly used Si-based solar cell is
discussed below. In the case of a semiconductor material capable of absorbing light
30 having a wavelength of 700 nm or less, about 48% of the total sunlight can be
utilized. In view of a theoretical loss of this fraction of light due to heat loss, the
hydrogen generation efficiency is about 25%. This is the value obtained on the
assumption that the quantum efficiency is 100%. Therefore, if the semiconductor
material is incorporated into a device, other losses due to a decrease in the quantum
35 efficiency, reflection and scattering of light on the glass surface, absorption of light
by water, etc. need to be considered. Also taking into consideration these
efficiencies (quantum efficiency: 90%, efficiency due to device design factors such as
6
reflection and scattering: 90%), it can be presumed that the semiconductor material
with a band gap of 700 nm achieves a hydrogen generation efficiency of about 20%
at most. When the hydrogen generation efficiency is low, the installation area
naturally must be increased to generate a required amount of hydrogen. The
5 increase in the installation area not only causes an increase in cost but also makes
it difficult to install the device, unlike a solar cell, in a place with a limited area,
such as the roof of a detached house. The estimated power generation efficiency
that a simple type (not a tandem type) Si-based solar cell can achieve is about 20%.
Therefore, a semiconductor material with a band gap of 700 nm or more needs to be
10 used to obtain an efficiency equal to or higher than that of the solar cell. In
addition, since the water decomposition voltage is about 1.23 V, a semiconductor
material with a band gap smaller than 1.23 V (1010 nm or more in terms of
wavelength) cannot be used to decompose water in principle. Therefore, it is
desired to find a semiconductor material having a band gap width of 1.23 to 1.77 eV
15 (700 nm to 1010 nm in terms of wavelength, the same applies hereinafter).
[0015] Here, the valence band of a common oxide mainly consists of oxygen 2p
orbitals, and the level of the top of the valence band of the oxide is usually located at
about +3V (relative to NHE) (left of FIG. 1). On the other hand, the valence band
of a nitride mainly consists of nitrogen 2p orbitals. Therefore, the level of the top of
20 the valence band of the nitride is usually located on the negative side of that of the
top of the valence band of the oxide (center of FIG. 1). Therefore, as disclosed in
Patent Literature 3, the use of a nitride makes it possible to obtain a semiconductor
material having a smaller band gap than a material obtained using an oxide.
Tantalum nitride (Ta3N5) is one of the nitride semiconductor materials which are
25 reported to have a photocatalytic function of decomposing water to generate
hydrogen and can absorb the longest wavelength light among these materials, but
the wavelength of the light that Ta 3N5 can absorb is about 600 nm at most. This is
smaller than the wavelength of 700 nm, at which the efficiency equal to or higher
than that of a solar cell can be obtained.
30 [0016] Under these circumstances, the present inventors have found, from the
result of the first-principles (ab initio) calculation using a density functional theory,
that a niobium nitride Nb3N5 obtained by substituting all the Ta sites of Ta3N5 by
Nb is a semiconductor material having a band gap width of a value between 1.23
and 1.77 eV (700 nm to 1010 nm). This is because the Nb 4d orbitals that are the
35 primary components of the bottom of the conduction band of Nb3N5 are located at a
level on the electrochemically positive side of the Ta 5d orbitals that are the primary
components of the bottom of the conduction band of Ta3N5 (right of FIG. 1). As a
7
result of the calculation, the calculated band gap width of Ta 3N5 was 1.1 eV, while
the calculated band gap width of Nb3N5 was 0.8 eV, as shown in Table 1. One of
the features of the first-principles calculation using the density functional theory is
that the calculated band gap width is usually estimated to be smaller than the
5 actual band gap width. However, in this calculation, the comparative evaluation of
the band gap widths can be made with high accuracy, and the orbital components
that constitute respective bands (such as the valence band and the conduction band)
can also be determined with high accuracy. In Table 1, the calculated value of the
band gap width of Ta 3N5 is underestimated to be 52% of the measured value thereof.
10 The actual band gap width of Nb3N5 of the present invention is calculated by
applying this ratio to the calculated value of the band gap width of 0.8 eV using the
following formula 1. According to the following formula 1, the band gap width of
Nb3N5 is considered to be about 1.5 eV. This means that Nb3N5 has a capacity
enough to absorb light having a wavelength of 700 nm or more.
15 [0017] [Table 1]
Ta 3N5
Nb3N5
Band gap width
Calculated value
1.1
0.8
Measured value
2.1
1.6*
*Measured value of Nb3N5 synthesized and analyzed in Example 1-1
[0018]
0.8 eV/52% = 1.5 eV … (Formula 1)
[0019] The present inventors have also found, from the result of the first-principles
20 calculation, that niobium in the niobium nitride exhibits the properties of a
semiconductor with a band gap when niobium has a valence of +5, which is the
highest valence of niobium, and that niobium having a lower valence than the
highest valence has a higher electron density in the conduction band, and as a
result, it has no apparent band gap. Therefore, it is preferable that niobium in the
25 niobium nitride of the present invention have a valence of substantially +5
(preferably, a valence of +4.8 to +5). More specifically, in the niobium nitride of the
present invention, the conduction band mainly consists of Nb 4d orbitals.
Therefore, it is desirable that the niobium has a valence of +5, in which no electrons
are present in the 4d orbitals. The present inventors have found, from the
30 first-principles calculation, that a niobium nitride in which niobium has a valence of
3 exhibits metallic conductivity and has no band gap because electrons are present
in the Nb 4d orbitals of the conduction band. However, niobium may sometimes
have a valence of about +4.8 due to unavoidable defects in manufacture, and the
8
like. In this case, the defects act as a recombination center of photoexcited carriers
and slightly decrease the excitation efficiency and mobility of carriers, but do not
have a significant influence on the semiconductor properties. Therefore, in the
present invention, such a decrease in the valence of niobium due to unavoidable
5 defects in manufacture is acceptable as long as niobium has a valence of about +4.8
or higher. In other words, the phrase “niobium has a valence of substantially +5”
means that niobium having a valence of about +5 is acceptable as long as that
valence value does not have a significant influence on the semiconductor properties,
that is, niobium preferably has a valence of +4.8 to +5.
10 [0020] A first aspect of the present invention has been found as a result of the
above studies and provides a niobium nitride which has a composition represented
by the composition formula Nb3N5 and in which a constituent element Nb has a
valence of substantially +5. The niobium nitride according to the first aspect is a
novel substance, and can be used as a semiconductor material capable of absorbing
15 longer wavelength light (having a smaller band gap), having band edges at levels
between which the oxidation-reduction potential of water is present, and having
high stability in water under light irradiation. That is, the first aspect of the
present invention can provide a semiconductor material having better optical
semiconductor properties for obtaining hydrogen and oxygen through decomposition
20 of water than conventional semiconductor materials (having a smaller band gap and
a higher sunlight utilization efficiency than conventional semiconductor materials).
[0021] As a result of intensive studies, the present inventors have also provided a
method for producing the niobium nitride according to the first aspect and a
semiconductor containing such a substance. The present inventors have further
25 provided a film containing the niobium nitride according to the first aspect and a
method for producing the film. The present inventors have further provided a
semiconductor device, a photocatalyst, a hydrogen generation device, and an energy
system each utilizing such a semiconductor or film.
[0022] A second aspect of the present invention provides a semiconductor
30 containing the niobium nitride according to the first aspect. Since the
semiconductor according to the second aspect contains the niobium nitride
according to the first aspect, it can absorb longer wavelength light (has a smaller
band gap), has band edges at levels between which the oxidation-reduction potential
of water is present, and has high stability in water under light irradiation.
35 [0023] A third aspect of the present invention provides a semiconductor as set forth
in the second aspect, wherein the semiconductor is an optical semiconductor. The
semiconductor according to the third aspect not only can be used in water but also
9
can achieve a higher sunlight utilization efficiency than conventional optical
semiconductors. The “optical semiconductor” in the third aspect of the present
invention is defined as a material not only having an electric conductivity within the
range of electric conductivity values of semiconductors but also having a band
5 structure in which the conduction band is substantially empty of electrons.
[0024] A fourth aspect of the present invention provides a method for producing a
niobium nitride, including the step of nitriding an organic niobium compound by
reacting the organic niobium compound with a nitrogen compound gas. The
production method according to the fourth aspect makes it possible to synthesize the
10 niobium nitride according to the first aspect.
[0025] A fifth aspect of the present invention provides a method for producing a
niobium nitride as set forth in the fourth aspect, wherein the organic niobium
compound contains a compound represented by the composition formula Nb(NR2)5,
where R is an alkyl group having 1 to 3 carbon atoms. The production method
15 according to the fifth aspect makes it easier to synthesize the niobium nitride
according to the first aspect.
[0026] A sixth aspect of the present invention provides a method for producing a
niobium nitride as set forth in the fifth aspect, wherein the organic niobium
compound contains pentakis(dimethylamino)niobium (Nb(N(CH3)2)5). The
20 production method according to the sixth aspect makes it easier to synthesize the
niobium nitride according to the first aspect.
[0027] A seventh aspect of the present invention provides a method for producing a
niobium nitride as set forth in any one of the fourth to sixth aspects, wherein a
reaction temperature in the nitriding step is equal to or higher than a nitridation
25 onset temperature of the organic niobium compound and is lower than a reduction
onset temperature of Nb. The production method according to the seventh aspect
makes it possible to nitride the organic niobium compound while preventing
reduction of niobium, and thus makes it easier to synthesize the niobium nitride
according to the first aspect.
30 [0028] A eighth aspect of the present invention provides a method for producing a
niobium nitride as set forth in the seventh aspect, wherein the organic niobium
compound contains pentakis(dimethylamino)niobium (Nb(N(CH3)2)5), and the
reaction temperature in the nitriding step is 120°C to 250°C. The production
method according to the eighth aspect makes it possible, when the organic niobium
35 compound contains pentakis(dimethylamino)niobium, to nitride
pentakis(dimethylamino)niobium while preventing reduction of niobium, and thus
makes it easier to synthesize the niobium nitride according to the first aspect.
10
[0029] A ninth aspect of the present invention provides a method for producing a
niobium nitride as set forth in any one of the fourth to eighth aspects, wherein a
concentration of water and oxygen contained in the nitrogen compound gas is 10
ppm by volume or less. Some of the organic niobium compounds used as starting
5 materials are highly reactive with water and oxygen. Therefore, a high content of
water and oxygen in the synthesis system may cause oxidation of the organic
niobium compound during the synthesis process. The production method according
to the ninth aspect makes it possible to inhibit oxidation of the organic niobium
compound during the synthesis process, and thus makes it easier to synthesize the
10 niobium nitride according to the first aspect.
[0030] A tenth aspect of the present invention provides a method for producing a
niobium nitride as set forth in any one of the fourth to ninth aspects, wherein a flow
rate of the nitrogen compound gas used in the nitriding step is 0.10 m minute-1 to
10.0 m minute-1 as a linear flow rate. The production method according to the
15 tenth aspect makes it easier to synthesize the niobium nitride according to the first
aspect.
[0031] An eleventh aspect of the present invention provides a method for producing
a niobium nitride as set forth in any one of the fourth to tenth aspects, wherein the
nitrogen compound gas used in the nitriding step contains at least ammonia. The
20 production method according to the eleventh aspect makes it easier to synthesize
the niobium nitride according to the first aspect.
[0032] A twelfth aspect of the present invention provides a method for producing a
niobium nitride as set forth in the fourth aspect, wherein the organic niobium
compound contains a compound represented by the composition formula
25 R1N=Nb(NR2R3)3, where R1, R2 and R3 are each independently a hydrocarbon group,
and the nitrogen compound gas contains ammonia. The production method
according to the twelfth aspect makes it easier to synthesize the niobium nitride
according to the first aspect.
[0033] A thirteenth aspect of the present invention provides a method for producing
30 a niobium nitride as set forth in the twelfth aspect, wherein a reaction temperature
in the nitriding step is equal to or higher than a nitridation onset temperature of
the organic niobium compound and is lower than a reduction onset temperature of
Nb. The production method according to the thirteenth aspect makes it easier to
synthesize the niobium nitride according to the first aspect.
35 [0034] A fourteenth aspect of the present invention provides a method for
producing a niobium nitride as set forth in the twelfth aspect or the thirteenth
aspect, wherein R1 is a tertiary butyl group (-C(CH3)3), and R2 and R3 are each
11
independently a straight-chain alkyl group (n-CnH2n+1, where n is an integer of 1 or
more). Since R1 is a tertiary butyl group (-C(CH3)3), the resulting organic niobium
compound as a raw material is a liquid, thus is easy to handle, and easily undergoes
a homogeneous reaction, and further the reduction onset temperature of Nb is
5 increased. In addition, since R2 and R3 are each a straight-chain alkyl group, the
reduction onset temperature of Nb is increased. Therefore, the production method
according to the fourteenth aspect makes it easier to synthesize the niobium nitride
according to the first aspect.
[0035] A fifteenth aspect of the present invention provides a semiconductor device
10 including the semiconductor according to the second aspect or the third aspect.
Since the semiconductor device according to the fifteenth aspect includes the
semiconductor according to the second aspect or the third aspect, it can be used as a
device having a high sunlight utilization efficiency and being usable in water.
[0036] A sixteenth aspect of the present invention provides a photocatalyst
15 consisting of the semiconductor according to the third aspect. Since the
photocatalyst according to the sixteenth aspect consists of the semiconductor
serving as an optical semiconductor according to the third aspect, it not only can be
used in water but also can achieve a higher sunlight utilization efficiency than
conventional photocatalysts.
20 [0037] A seventeenth aspect of the present invention provides a hydrogen
generation device including: the photocatalyst according to the sixteenth aspect; an
aqueous solution containing an electrolyte and being in contact with the
photocatalyst; and a container containing the photocatalyst and the aqueous
solution, wherein hydrogen is generated through decomposition of water in the
25 aqueous solution by irradiation of the photocatalyst with light. The hydrogen
generation device according to the seventeenth aspect can achieve more efficient
generation of hydrogen than conventional devices.
[0038] An eighteenth aspect of the present invention provides an energy system
including: the hydrogen generation device according to the seventeenth aspect; a
30 fuel cell; and a line for supplying the hydrogen generated in the hydrogen
generation device to the fuel cell. The energy system according to the eighteenth
aspect can achieve more efficient generation of energy than conventional systems.
[0039] A nineteenth aspect of the present invention provides a niobium
nitride-containing film containing a niobium nitride which has a composition
35 represented by the composition formula Nb3N5 and in which a constituent element
Nb has a valence of substantially +5. The niobium nitride-containing film
according to the nineteenth aspect contains a niobium nitride which has a
12
composition represented by the composition formula Nb3N5 and in which a
constituent element Nb has a valence of substantially +5, like the niobium nitride
according to the first aspect. Therefore, the niobium nitride-containing film
according to the nineteenth aspect can absorb longer wavelength light (has a
5 smaller band gap), has band edges at levels between which the oxidation-reduction
potential of water is present, and has high stability in water under light irradiation.
[0040] A twentieth aspect of the present invention provides a method for producing
a niobium nitride-containing film containing a niobium nitride which has a
composition represented by the composition formula Nb3N5 and in which a
10 constituent element Nb has a valence of substantially +5, the method including the
steps of: (I) vaporizing an organic niobium compound; and (II) bringing the
vaporized organic niobium compound and a nitrogen compound gas into contact
with a heated substrate. The production method according to the twentieth aspect
makes it possible to produce the niobium nitride-containing film according to the
15 nineteenth aspect.
[0041] A twenty-first aspect of the present invention provides a method for
producing a niobium nitride-containing film as set forth in the twentieth aspect,
wherein the organic niobium compound contains a compound represented by the
composition formula Nb(NR2)5, where R is an alkyl group having 1 to 3 carbon
20 atoms, and the nitrogen compound gas contains ammonia. The production method
according to the twenty-first aspect makes it easier to produce the niobium
nitride-containing film according to the nineteenth aspect.
[0042] A twenty-second aspect of the present invention provides a method for
producing a niobium nitride-containing film as set forth in the twenty-first aspect,
25 wherein in the step (II), the substrate is heated to a temperature that is equal to or
higher than a nitridation onset temperature of the organic niobium compound and
is lower than a reduction onset temperature of Nb. The production method
according to the twenty-second aspect makes it easier to produce the niobium
nitride-containing film according to the nineteenth aspect.
30 [0043] A twenty-third aspect of the present invention provides a method for
producing a niobium nitride-containing film as set forth in the twentieth aspect or
the twenty-first aspect, wherein the organic niobium compound contains
pentakis(dimethylamino)niobium (Nb(N(CH3)2)5). The production method
according to the twenty-third aspect makes it easier to produce the niobium
35 nitride-containing film according to the nineteenth aspect.
[0044] A twenty-fourth aspect of the present invention provides a method for
producing a niobium nitride-containing film as set forth in the twentieth aspect,
13
wherein the organic niobium compound contains a compound represented by the
composition formula R1N=Nb(NR2R3)3, where R1, R2 and R3 are each independently
a hydrocarbon group, and the nitrogen compound gas contains ammonia. The
production method according to the twenty-fourth aspect makes it easier to produce
5 the niobium nitride-containing film according to the nineteenth aspect.
[0045] A twenty-fifth aspect of the present invention provides a method for
producing a niobium nitride-containing film as set forth in the twenty-fourth aspect,
wherein in the step (II), the substrate is heated to a temperature that is equal to or
higher than a nitridation onset temperature of the organic niobium compound and
10 is lower than a reduction onset temperature of Nb. The production method
according to the twenty-fifth aspect makes it easier to produce the niobium
nitride-containing film according to the nineteenth aspect.
[0046] A twenty-sixth aspect of the present invention provides a method for
producing a niobium nitride-containing film as set forth in the twenty-fourth aspect
15 or the twenty-fifth aspect, wherein R1 is a tertiary butyl group (-C(CH3)3), and R2
and R3 are each independently a straight-chain alkyl group (n-CnH2n+1, where n is
an integer of 1 or more). Since R1 is a tertiary butyl group (-C(CH3)3), the resulting
organic niobium compound as a raw material is a liquid, thus is easy to handle, is
easily vaporized, and easily undergoes a homogeneous reaction, and further the
20 reduction onset temperature of Nb is increased. In addition, since R2 and R3 are
each a straight-chain alkyl group, the reduction onset temperature of Nb is
increased. Therefore, the production method according to the twenty-sixth aspect
makes it easier to produce the niobium nitride-containing film according to the
nineteenth aspect.
25 [0047] A twenty-seventh aspect of the present invention provides a semiconductor
device including the niobium nitride-containing film according to the nineteenth
aspect. Since the semiconductor device according to the twenty-seventh aspect
includes the niobium nitride-containing film according to the nineteenth aspect, it
can be used as a device having a high sunlight utilization efficiency and being
30 usable in water.
[0048] A twenty-eighth aspect of the present invention provides a photocatalyst
consisting of the niobium nitride-containing film according to the nineteenth aspect.
Since the photocatalyst according to the twenty-eighth aspect consists of the
niobium nitride-containing film according to the nineteenth aspect, it not only can
35 be used in water but also can achieve a higher sunlight utilization efficiency than
conventional photocatalysts.
[0049] A twenty-ninth aspect of the present invention provides a hydrogen
14
generation device including: the photocatalyst according to the twenty-eighth
aspect; an aqueous solution containing an electrolyte and being in contact with the
photocatalyst; and a container containing the photocatalyst and the aqueous
solution, wherein hydrogen is generated through decomposition of water in the
5 aqueous solution by irradiation of the photocatalyst with light. The hydrogen
generation device according to the twenty-ninth aspect can achieve more efficient
generation of hydrogen than conventional devices.
[0050] A thirtieth aspect of the present invention provides an energy system
including: the hydrogen generation device according to the twenty-ninth aspect; a
10 fuel cell; and a line for supplying the hydrogen generated in the hydrogen
generation device to the fuel cell. The energy system according to the thirtieth
aspect can achieve more efficient generation of energy than conventional systems.
[0051] Hereinafter, embodiments of a niobium nitride which can be used as a
semiconductor material, which has a composition represented by the composition
15 formula Nb3N5, and in which the constituent element Nb has a valence of
substantially +5 and a method for producing the niobium nitride, embodiments of a
niobium nitride-containing film containing the niobium nitride and a method for
producing the niobium nitride-containing film, and further embodiments of a device,
etc. utilizing the niobium nitride or the niobium nitride-containing film are
20 described with reference to drawings and a table. The following embodiments are
merely examples, and the present invention is not limited to the following
embodiments.
[0052] (First Embodiment)
A niobium nitride according to a first embodiment of the present invention
25 has a composition represented by the composition formula Nb3N5, and a constituent
element Nb has a valence of substantially +5. This niobium nitride is synthesized
by the following method.
[0053] The niobium nitride of the present embodiment can be synthesized by a
method including the nitriding step of nitriding an organic niobium compound by
30 reacting the organic niobium compound with a nitrogen compound gas.
[0054] As the organic niobium compound, a compound having a composition
represented by the composition formula Nb(NR2)5, where R is an alkyl group having
1 to 3 carbon atoms, preferably 1 to 2 carbon atoms, is suitably used. Among such
compounds, pentakis(dimethylamino)niobium (Nb(N(CH3)2)5) is particularly
35 preferred. Preferably, the organic niobium compound as the starting material
contains Nb(NR2)5, and more preferably it consists of Nb(NR2)5.
[0055] As the nitrogen compound gas, for example, ammonia, nitrogen, hydrazine,
15
or the like can be used. Among these, a nitrogen compound gas containing at least
ammonia is preferably used.
[0056] In this reaction, the nitrogen compound gas acts as a nitriding reagent but it
also has properties as a reducing agent. Generally, synthesis of a nitride by
5 reaction of an inorganic niobium compound with a nitrogen compound gas requires
a temperature of at least 450°C or higher. However, in the above temperature
range, niobium is reduced due to the properties of the nitrogen compound gas as a
reducing agent, and NbN (Nb has a valence of +3) is synthesized as a niobium
nitride. On the other hand, in the present embodiment, an organic niobium
10 compound which is more reactive with the nitrogen compound gas than an inorganic
niobium compound is used as a starting material. Therefore, the nitridation
reaction is allowed to occur in a lower temperature range. That is, in the method of
the present embodiment, it is possible to set the reaction temperature in the
nitriding step to a temperature that is equal to or higher than a nitridation onset
15 temperature of the organic niobium compound used as a starting material and is
lower than a reduction onset temperature of Nb. The case where the organic
niobium compound used as the starting material contains
pentakis(dimethylamino)niobium Nb(N(CH3)2)5 is described as an example. FIG. 2
shows TG-DTA data of pentakis(dimethylamino)niobium in an ammonia
20 atmosphere. This data reveals that the temperature required for the onset of the
nitridation reaction of pentakis(dimethylamino)niobium is at least 120°C.
Preferably, the reaction temperature in the nitriding step is lower than the
reduction onset temperature of Nb. Therefore, in the case where
pentakis(dimethylamino)niobium is used, it is possible to nitride
25 pentakis(dimethylamino)niobium while preventing reduction of niobium by reacting
pentakis(dimethylamino)niobium with a nitrogen compound gas at a temperature in
the range of 120°C or higher and lower than the reduction onset temperature of Nb,
and preferably at a temperature in the range of 120°C or higher and 250°C or lower.
In this way, only the function as a nitriding reagent among the properties of the
30 nitrogen compound gas acts on pentakis(dimethylamino)niobium. As a result, the
nitrogen compound gas can nitride the niobium compound without reducing it.
Desirably, the linear flow rate of the nitrogen compound gas is, for example, 0.10 m
minute-1 to 10.0 m minute-1. Furthermore, the above-described nitriding step can
be repeated, if necessary.
35 [0057] Some of the organic niobium compounds used as starting materials are
highly reactive with water and oxygen. For example, the reactivity of
pentakis(dimethylamino)niobium with water and oxygen is very high. Therefore,
16
water and oxygen contained in the synthesis system may cause oxidation of the
organic niobium compound during the synthesis process. This needs to be avoided
during the synthesis of Nb3N5. Therefore, the concentration of water and oxygen
contained in the nitrogen compound gas and in an inert gas used for purging during
5 the synthesis process as needed is desirably 10 ppm by volume or less, and more
desirably 1 ppm by volume or less. The inert gas used may be not only a so-called
rare gas such as He, Ne, Ar, Kr, or Xe but also nitrogen gas or the like.
[0058] The nitriding step can be carried out using, for example, an apparatus 100
as shown in FIG. 3.
10 [0059] The apparatus 100 includes a tube furnace 111, a tube 112 penetrating the
tube furnace 111, and a boat 113 placed in the tube 112. Nb3N5 (Nb3N5 powder) can
be synthesized by heating a raw material 101 set in the boat 113 in the tube 112, in
a gas flow 102 containing a nitrogen compound gas flowing through the tube 112.
According to this method, a material in which the main component of a complete
15 nitride is not NbN or the like but Nb3N5 (Nb3N5-containing material) can be
synthesized.
[0060] Any tube and any boat may be used as the tube 112 and the boat 113 in the
tube furnace 111 as long as they withstand the temperatures at which they are used
and the environments in which they are used. For example, alumina and quartz
20 are suitably used.
[0061] The desired niobium nitride, which has a composition represented by the
composition formula Nb3N5 and in which the constituent element Nb has a valence
of substantially +5, can be obtained by the method described above.
[0062] (Second Embodiment)
25 Another embodiment of the production method of the niobium nitride Nb3N5
of the present invention is described below.
[0063] The production method of the present embodiment includes the step of
heating an organic niobium compound containing a compound represented by the
composition formula R1N=Nb(NR2R3)3, where R1, R2 and R3 are each independently
30 a hydrocarbon group, in an atmosphere containing ammonia. This step can be
carried out using, for example, the apparatus 100 as shown in FIG. 3, as in the first
embodiment. Preferably, the organic niobium compound as the starting material
contains R1N=Nb(NR2R3)3, and more preferably it consists of R1N=Nb(NR2R3)3.
Here, the case where the organic niobium compound consists of R1N=Nb(NR2R3)3 is
35 described as an example.
[0064] In the present embodiment, Nb3N5 (Nb3N5 powder) can be synthesized by
heating a raw material (R1N=Nb(NR2R3)3) 101 set in the boat 113 in the tube 112, in
17
the gas flow 102 containing ammonia flowing through the tube 112. According to
this method, a material in which the main component of a complete nitride is not
NbN or the like but Nb3N5 (Nb3N5-containing material) can be synthesized.
[0065] In the present embodiment, it is preferable that the raw material
5 R1N=Nb(NR2R3)3 be heated at a temperature that is equal to or higher than a
nitridation onset temperature of the R1N=Nb(NR2R3)3 and is lower than a reduction
onset temperature of Nb. Heating of the raw material in this temperature range
makes it possible to synthesize a material in which the main component of a
complete nitride is not NbN or the like but Nb3N5, and thus makes it easier to
10 synthesize a Nb3N5-containing material (Nb3N5 powder). Here, the reduction onset
temperature of Nb can be determined by TG-DTA measurement or the like. In the
Nb3N5 production method of the present invention, it is possible to synthesize Nb3N5
even if the temperature for heating the raw material R1N=Nb(NR2R3)3 is higher
than the reduction onset temperature of Nb of the R1N=Nb(NR2R3)3. However, in a
15 compound obtained in this case, the content of components other than Nb3N5, such
as NbN, tends to be higher than that in a compound obtained by heating
R1N=Nb(NR2R3)3 at a temperature lower than the reduction onset temperature of
Nb.
[0066] In the R1N=Nb(NR2R3)3 used as the starting material, R1, R2, and R3 are
20 each independently a hydrocarbon group. Therefore, self-condensation reaction is
inhibited in the R1N=Nb(NR2R3)3. As R1, a branched-chain hydrocarbon group is
suitable because the resulting material is a liquid and thus is easy to handle and
easily undergoes a homogeneous reaction, and further the reduction onset
temperature of Nb is increased. In particular, a tertiary butyl group (-C(CH3)3) is
25 suitable. As R2 and R3, straight-chain hydrocarbon groups are suitable because the
reduction onset temperature of Nb is increased. Straight-chain alkyl groups
(n-CnH2n+1, where n is an integer of 1 or more) like -CH3 and C2H5 are suitable.
Since a too long carbon chain causes a decrease in the reduction onset temperature
of Nb, the carbon number is preferably 3 or less (n 3). Since the raw material in
30 which R1 is a tertiary butyl group (-C(CH3)3), and R2 and R3 are each independently
a straight-chain alkyl group (n-CnH2n+1, where n is an integer of 1 or more) has high
heat resistance, it can be synthesized at higher temperatures. As a result, highly
crystalline Nb3N5 having much better optical semiconductor properties can be
synthesized more easily. It should be noted, however, that the starting material
35 used in the present embodiment is highly reactive with oxygen and water.
Therefore, the content of oxygen and water in the starting material is desirably 1
mol ppm or less of the total amount of the starting material, and more desirably 0.1
18
mol ppm or less.
[0067] The inert gas used may be not only a so-called rare gas such as He, Ne, Ar,
Kr, or Xe but also nitrogen gas or the like. It is desirable to use a gas having a low
content of oxygen and water. Therefore, the content of oxygen and water in the
5 inert gas is desirably 10 ppm by volume or less, and more desirably 1 ppm by
volume or less.
[0068] Next, the Nb3N5 synthesis mechanism in the production method of the
present embodiment is described with reference to FIG. 4. Here, the case where
tertiary-butylimino tris-(ethylmethylamino)niobium (tBuN=Nb(NEtMe)3), where R1
10 is a tertiary butyl group, R2 is an ethyl group, and R3 is a methyl group, is used as a
raw material 101 is described as an example. A compound produced by adding
ammonia (NH3) to this raw material (R1N=Nb(NR2R3)3) 101 acts as an initiator and
causes addition polymerization of R1N=Nb(NR2R3)3. Next, NR2R3 reacts with
ammonia and then is condensation polymerized. Thus, a Nb3N5 powder is obtained.
15 Since the production method of the present embodiment has such a reaction scheme,
much less NbN is produced compared to complete nitridation of Nb2O5 with
ammonia.
[0069] The production method of the present embodiment makes it possible to
synthesize not NbN but Nb3N5 as a complete nitride of Nb. As a result, Nb3N5
20 having better optical semiconductor properties for generation of hydrogen and
oxygen by decomposition of water than conventional semiconductor materials
(Nb3N5 having a smaller band gap and better sunlight utilization efficiency than
conventional semiconductor materials) can be produced easily and inexpensively.
[0070] (Third Embodiment)
25 An embodiment of the production method of the niobium nitride
(Nb3N5)-containing film of the present invention is described below.
[0071] The production method of the present embodiment includes the steps of: (I)
vaporizing an organic niobium compound (for example Nb(NR2)5, where R is an
alkyl group having 1 to 3 carbon atoms (preferably 1 to 2 carbon atoms); and (II)
30 bringing the vaporized organic niobium compound and a nitrogen compound gas
into contact with a heated substrate. These steps can be carried out using, for
example, a MOCVD (Metal-Organic Chemical Vapor Deposition) apparatus 500 as
shown in FIG. 5. This method makes it possible to produce a Nb3N5-containing
film while suppressing the generation of NbN. In addition, this method makes it
35 possible to produce a Nb3N5-containing film fixed to a substrate by chemical
bonding. The Nb3N5-containing film obtained by the method of the present
embodiment allows Nb3N5 to exhibit its excellent optical semiconductor properties
19
because the Nb3N5-containing film is well fixed to the substrate.
[0072] The reaction mechanism between the organic niobium compound and the
nitrogen compound gas and the suitable reaction temperature in the above step (II)
are the same as in the first embodiment. The compound suitably used as the
5 organic niobium compound and the gas suitably used as the nitrogen compound gas
are also the same as in the first embodiment. The MOCVD apparatus 500 includes
a vaporizer 511, an inlet pipe 512, a reaction chamber 513, a shower head 514, and a
susceptor 515. The vaporizer 511 vaporizes the raw material. The reaction
chamber 513 serves as a chamber into which a source gas obtained by vaporizing
10 the raw material in the vaporizer 511 is supplied to grow a crystal on the treatment
surface of a substrate 521. The inlet pipe 512 supplies a gas such as a source gas
to the reaction chamber 513 from the vaporizer 511. The shower head 514 is
connected to the end of the inlet pipe 512. It is placed inside the reaction chamber
513 and injects a source gas, a reactant gas, etc. to the substrate 521 to grow a
15 crystal thereon. The susceptor 515 supports the substrate 521 on which the crystal
is to be grown and heats the substrate 521.
[0073] In the MOCVD apparatus 500, a reactant gas 503 containing ammonia is
mixed into an inert gas 502 containing the raw material 501 heated (desirably
heated at a temperature lower than the reduction onset temperature of Nb) and
20 vaporized in the vaporizer 511. The mixed gas is injected from the shower head
514 to the substrate 521 heated by the susceptor 515. The temperature of the
heated substrate 521 is preferably in the range of temperatures that are equal to or
higher than the nitridation onset temperature of the raw material 501 and lower
than the reduction onset temperature of Nb. Thus, Nb3N5 can be deposited in the
25 crystalline form on the substrate 521 so as to form a Nb3N5-containing film. The
inert gas 502 used here may be not only a so-called rare gas such as He, Ne, Ar, Kr,
or Xe but also nitrogen gas or the like. It is desirable to use a gas having a low
content of oxygen and water. Therefore, the content of oxygen and water in the
inert gas is desirably 10 ppm by volume or less, and more desirably 1 ppm by
30 volume or less.
[0074] It is desirable that the pipe wall of the MOCVD apparatus 500 be made of
stainless steel because water and oxygen are less likely to be adsorbed on or
desorbed from stainless steel. In order to prevent the attachment of the raw
material onto the pipe wall, a solution of the raw material and an organic solvent
35 may be vaporized. In this case, a nonaqueous solvent capable of dissolving the raw
material and having the vaporization properties similar to those of the raw material,
such as hydrocarbon, is suitable as the organic solvent. For example,
20
ethylcyclohexane is suitably used.
[0075] According to the production method of the present embodiment, a
Nb3N5-containing film having a very low content of NbN can be produced easily and
inexpensively. As a result, a Nb3N5-containing film having better optical
5 semiconductor properties for generation of hydrogen and oxygen by decomposition of
water than films made of conventional semiconductor materials (a
Nb3N5-containing film having a smaller band gap and better sunlight utilization
efficiency than conventional semiconductor materials) can be produced easily and
inexpensively.
10 [0076] (Fourth Embodiment)
Another embodiment of the production method of the niobium nitride
(Nb3N5)-containing film of the present invention is described below.
[0077] In the present embodiment, the method for producing a Nb3N5-containing
film in the case where the organic niobium compound contains a compound
15 represented by the composition formula R1N=Nb(NR2R3)3, where R1, R2 and R3 are
each independently a hydrocarbon group and the nitrogen compound gas contains
ammonia, in the production method of the third embodiment, is described.
Here, in particular, the case where the organic niobium compound consists of
R1N=Nb(NR2R3)3 is described. That is, the production method of the present
20 embodiment includes the steps of: (I) vaporizing R1N=Nb(NR2R3)3, where R1, R2,
and R3 are each independently a hydrocarbon group); and (II) bringing the
vaporized R1N=Nb(NR2R3)3 and ammonia into contact with a heated substrate.
These steps can be carried out using, for example, the MOCVD apparatus 500 as
shown in FIG. 5 described in the third embodiment. This method makes it possible
25 to produce a Nb3N5-containing film while suppressing the generation of NbN. In
addition, this method makes it possible to produce a Nb3N5-containing film fixed to
a substrate by chemical bonding. The Nb3N5-containing film obtained by the
method of the present embodiment allows Nb3N5 to exhibit its excellent optical
semiconductor properties because the Nb3N5-containing film is well fixed to the
30 substrate.
[0078] It is preferable that, in the step (II), the substrate be heated to a
temperature that is equal to or higher than the nitridation onset temperature of
R1N=Nb(NR2R3)3 as a raw material and is lower than the reduction onset
temperature of Nb. Heating of the substrate in this range of temperatures makes
35 it possible to synthesize not NbN but Nb3N5 as a complete nitride of Nb. It is
preferable that, in the step (I), the raw material be vaporized at a temperature that
is lower than the reduction onset temperature of Nb of this raw material.
21
[0079] R1N=Nb(NR2R3)3 used as the starting material is the same as that described
in the second embodiment. Therefore, detailed description thereof is omitted here.
[0080] The configuration of the MOCVD apparatus 500 is as described in the third
embodiment. In this MOCVD apparatus 500, the reactant gas 503 containing
5 ammonia is mixed into the inert gas 502 containing the raw material
(R1N=Nb(NR2R3)3) 501 heated (desirably heated at a temperature lower than the
reduction onset temperature of Nb) and vaporized in the vaporizer 511. The mixed
gas is injected from the shower head 514 to the substrate 521 heated by the
susceptor 515. At this time, preferably, the substrate 521 is heated to a
10 temperature that is equal to or higher than the nitridation onset temperature of the
raw material 501 and is lower than the reduction onset temperature of Nb, as
described above. Thus, Nb3N5 can be deposited on the substrate 521 so as to form a
Nb3N5-containing film. The specific examples of the inert gas 502 used herein are
the same as those of the inert gas used in the second embodiment.
15 [0081] It is desirable that the pipe wall of the MOCVD apparatus 500 be made of
stainless steel because water and oxygen are less likely to be adsorbed on or
desorbed from stainless steel. In order to prevent the attachment of
R1N=Nb(NR2R3)3 on the pipe wall, a solution of the R1N=Nb(NR2R3)3 and an organic
solvent may be vaporized. In this case, a nonaqueous solvent capable of dissolving
20 R1N=Nb(NR2R3)3 and having the vaporization properties similar to those of
R1N=Nb(NR2R3)3, such as hydrocarbon, is suitable as the organic solvent. For
example, ethylcyclohexane is suitably used.
[0082] Next, the synthesis mechanism of the Nb3N5-containing film in the
production method of the present embodiment is described with reference to FIG. 6.
25 Here, the case where tertiary-butylimino tris-(ethylmethylamino)niobium
(tBuN=Nb(NEtMe)3), where R1 is a tertiary butyl group, R2 is an ethyl group, and
R3 is a methyl group, is used as the raw material 501 is described as an example.
A compound produced by adding this raw material (R1N=Nb(NR2R3)3) 501 to
hydroxyl groups on the surface of the substrate 521 acts as an initiator and causes
30 addition polymerization of R1N=Nb(NR2R3)3. Next, NR2R3 reacts with ammonia
and then is condensation polymerized. Thus, a Nb3N5-containing film is obtained.
Since the production method of the present embodiment has such a reaction scheme,
much less NbN is produced compared to complete nitridation of Nb2O5 with
ammonia.
35 [0083] According to the production method of the present embodiment, a
Nb3N5-containing film having a very low content of NbN can be produced. As a
result, a Nb3N5-containing film having better optical semiconductor properties for
22
generation of hydrogen and oxygen by decomposition of water than films made of
conventional semiconductor materials (a Nb3N5-containing film having a smaller
band gap and better sunlight utilization efficiency than conventional semiconductor
materials) can be produced easily and inexpensively.
5 [0084] (Fifth Embodiment)
In the fifth embodiment, an embodiment of the niobium nitride Nb3N5 of the
present invention as a photocatalyst is described.
[0085] The photocatalyst of the present embodiment consists of the niobium nitride
Nb3N5 described in the first embodiment. The niobium nitride Nb3N5 described in
10 the first and second embodiments is a semiconductor having a band gap and can be
used as a photocatalyst. Hereinafter, the reason for this is described.
[0086] FIG. 7 shows density of states distribution of Nb3N5 obtained by the
first-principles band calculation. It can be seen from this figure that Nb3N5 is a
semiconductor having a band structure in which the valence band mainly consists of
15 nitrogen 2p orbitals, the conduction band mainly consists of niobium 4d orbitals,
and a band gap exists between these two bands. The measured value of the band
gap width is 1.6 eV as described later in Example 1-1. This value is equivalent to
the energy of light having a wavelength of 780 nm. That is, when Nb3N5 is
irradiated with light having a wavelength of 780 nm or less, electrons in the valence
20 band absorb the light and are excited into the conduction band. Here, when a
reactive substrate is present in the vicinity of the surface of Nb3N5 and the
oxidation-reduction potential of the reactive substrate is located at a potential on
the positive side of the bottom of the conduction band of Nb3N5, the excited electrons
move from Nb3N5 to the reactive substrate and the reduction reaction of the reactive
25 substrate can proceed. Examples of the reactive substrate having such an
oxidation-reduction potential include water, proton, oxygen, metal ions such as
silver (I) ion and iron (III) ion, and iodide ion. On the other hand, holes are
generated in the valence band with the photoexcitation of electrons. Here, when a
reactive substrate is present in the vicinity of the surface of Nb3N5 and the
30 oxidation-reduction potential thereof is located at a potential on the negative side of
the top of the valence band of Nb3N5, holes move from Nb3N5 to the reactive
substrate and the oxidation reaction of the reactive substrate can proceed.
Examples of the reactive substrate having such an oxidation-reduction potential
include water, hydroxide ion, metal ions such as iron (II) ion, iodine ion, and an
35 organic compound. Such a phenomenon shows that Nb3N5 acts as a photocatalyst.
[0087] (Sixth Embodiment)
As described in the fifth embodiment, the niobium nitride Nb3N5 of the
23
present invention can absorb visible light and has band edges at levels between
which the oxidation-reduction potential of water is present. In addition, the
niobium nitride of the present invention has high stability in an aqueous solution
under light irradiation. Therefore, the use of the niobium nitride of the present
5 invention as a photocatalyst makes it possible to obtain a hydrogen generation
device in which hydrogen is generated through decomposition of water. Since such
a hydrogen generation device using the niobium nitride Nb3N5 as a photocatalyst is
very efficient in utilizing sunlight, hydrogen can be generated more efficiently than
conventional devices.
10 [0088] The hydrogen generation device of the present embodiment includes: a
photocatalyst consisting of a semiconductor containing a niobium nitride Nb3N5; an
aqueous solution containing an electrolyte and being in contact with the
photocatalyst; and a container containing the photocatalyt and the aqueous solution.
When the photocatalyst is irradiated with light, water in the aqueous solution is
15 decomposed and hydrogen is generated.
[0089] The configuration of the hydrogen generation device of the present
embodiment is, for example, a configuration in which a photocatalyst consisting of a
semiconductor (optical semiconductor) containing the niobium nitride Nb3N5 is
suspended or immersed in an aqueous solution containing an electrolyte and the
20 resulting solution is placed in a container. When this device is irradiated with light,
water is decomposed by the photocatalyst, and thus hydrogen can be generated
more efficiently than in the use of conventional photocatalysts.
[0090] An example of the hydrogen generation device of the present invention is
described below with reference to FIG. 8. A hydrogen generation device 800 of the
25 present embodiment includes an aqueous solution containing an electrolyte
(electrolytic solution) 810, in which a photocatalyst consisting of a semiconductor
(optical semiconductor) containing the niobium nitride Nb3N5 described in the first
and second embodiments is suspended, and a container 811 containing the
electrolytic solution 810. This hydrogen generation device 800 decomposes water
30 by irradiating the electrolytic solution 810 with light so as to generate hydrogen.
[0091] At least a portion (herein referred to as a light incident portion 812) of the
container 811 is made of a material that transmits light such as sunlight so as to
allow light such as sunlight to reach the inside of the container 811. The container
811 is further provided with an outlet 814 for discharging hydrogen and oxygen
35 generated in the container 811 and an inlet 813 for supplying water to be
decomposed into the container 811. The hydrogen generation device 800 further
includes a hydrogen separation membrane 815, an oxygen outlet 816, and a
24
5
10
15
20
25
30
hydrogen outlet 817. The hydrogen separation membrane 815 separates hydrogen
from the gas discharged from the outlet 814. The hydrogen thus separated is
discharged from the hydrogen outlet 817. After the hydrogen is separated, the
remaining oxygen is discharged from the oxygen outlet 816.
[0092] Next, the operation of the hydrogen generation device 800 of the present
embodiment is described with reference to FIG. 8.
[0093] The electrolytic solution 810 placed in the container 811 and containing the
photocatalyst suspended therein is irradiated with sunlight through the light
incident portion 812 of the container 811 in the hydrogen generation device 800. In
this case, electrons are generated in the conduction band of the Nb3N5-containing
material in the electrolytic solution 810 and holes are generated in the valence band
thereof. The holes thus generated decompose water and causes oxygen to be
generated according to the reaction formula (1) below. On the other hand, the
electrons causes hydrogen to be generated according to the reaction formula (2)
below.
[0094]
35
[0095] The oxygen and hydrogen thus generated are discharged from the outlet 814
and separated through the hydrogen separation membrane 815, and then oxygen is
discharged from the oxygen outlet 816 and hydrogen is discharged from the
hydrogen outlet 817, respectively. Water is supplied into the container 811 through
the inlet 813 to replenish the water used for decomposition.
[0096] Since the photocatalyst used in the present embodiment has excellent
optical semiconductor properties, the probability of recombination of holes and
electrons is low. Therefore, in the hydrogen generation device 800, the quantum
efficiency of the hydrogen evolution reaction by light irradiation can be increased.
In addition, since the photocatalyst used in the present embodiment has a small
band gap, it is also responsive to visible light. As a result, the hydrogen generation
device 800 of the present embodiment can generate more hydrogen than devices
using conventional optical semiconductor materials.
[0097] Other configuration examples of the hydrogen generation device of the
present embodiment are described with reference to FIG. 9 and FIG. 10.
[0098] A hydrogen generation device 900 of FIG. 9 includes a container 990, a
photocatalyst electrode 920, a conductive substrate 910, and a counter electrode 930.
25
The photocatalyst electrode 920 includes the Nb3N5-containing film described in the
third and fourth embodiments. The container 990 has, in its upper part, two
openings 980 for collecting hydrogen and oxygen respectively. The container 990
has, in its lower part, two openings 980 serving as water inlets. An aqueous
5 solution containing an electrolyte (electrolytic solution) 960 is placed in the
container 990. The container 990 has a separator 940 between the photocatalyst
electrode 920 and the counter electrode 930 to separate a hydrogen generation
chamber from an oxygen generation chamber. The separator 940 has a function of
transmitting ions and separating a gas generated on the photocatalyst electrode 920
10 side from a gas generated on the counter electrode 930 side. A portion (a light
incident portion 950) of the container 990 that faces the surface of the photocatalyst
electrode 920 disposed inside the container 990 is made of a material that transmits
light such as sunlight. The conductive substrate 910 and the counter electrode 930
are electrically connected by a conducting wire 970. As used herein, the counter
15 electrode refers to an electrode that can exchange electrons with the photocatalyst
electrode without the electrolytic solution. Accordingly, in the present embodiment,
there is no particular limitation on the positional relationship, etc. of the counter
electrode 930 with the photocatalyst electrode 920 and the conductive substrate 910,
as long as the counter electrode 930 is connected electrically to the conductive
20 substrate 910 that supports the photocatalyst electrode 920. It should be noted
that since the Nb3N5-containing film used in the present embodiment is an n-type
semiconductor, the counter electrode 930 serves as an electrode that receives
electrons from the photocatalyst electrode 920 without the electrolytic solution 960.
[0099] Since the photocatalyst electrode 920 is a semiconductor having a band gap,
25 it usually has a lower conductivity than metals. In addition, recombination of
electrons and holes needs to be prevented as much as possible. Therefore, it is
preferable to reduce the thickness of the photocatalyst electrode 920. Thus, here,
the photocatalyst electrode 920 is formed as a thin film (with a thickness of about 50
to 500 nm) on the conductive substrate 910. Furthermore, it is preferable to
30 increase the surface area of the photocatalyst electrode 920 to increase the light
absorption efficiency.
[0100] It is preferable that the portion of the conductive substrate 910 that is not
covered with the photocatalyst electrode 920 be covered, for example, with an
insulating material such as resin. This covering prevents the conductive substrate
35 910 from being dissolved in the electrolytic solution 960.
[0101] It is preferable to use a material with a low overvoltage for the counter
electrode 930. For example, it is preferable to use a metal catalyst such as Pt, Au,
26
Ag, Fe, or Ni as the counter electrode 930 because the use thereof increases its
activity. Any electrolytic solution can be used for the electrolytic solution 960 as
long as it is a solution containing water. An acidic, neutral or basic solution can be
used.
5 [0102] Another hydrogen generation device 1000 shown in FIG. 10 also includes
the container 990, the photocatalyst electrode 920, the conductive substrate 910,
and the counter electrode 930 (in FIG. 10, the same or similar members are
designated by the same reference numerals as in FIG. 9). The container 990 has
four openings 980 and contains the electrolytic solution 960. The photocatalyst
10 electrode 920 is provided on one surface of the conductive substrate 910 and the
counter electrode 930 is provided on the other surface thereof. The photocatalyst
electrode 920 is formed as a thin film (with a thickness of about 50 to 500 nm). The
photocatalyst electrode 920 and the counter electrode 930 are electrically connected
by the conductive substrate 910. The inside of the container 990 is separated into
15 a photocatalyst electrode 920 side and a counter electrode 930 side by the separator
940 and the conductive substrate 910 to separate a hydrogen generation chamber
from an oxygen generation chamber. A portion (a light incident portion 950) of the
container 990 that faces the surface of the photocatalyst electrode 920 disposed
inside the container 990 is made of a material that transmits light such as sunlight.
20 [0103] Hydrogen and oxygen can be generated by irradiating the hydrogen
generation devices shown in FIG. 9 and FIG. 10 with light (for example, sunlight)
through the light incident portion 950. In particular, in these hydrogen generation
devices, longer wavelength light can be absorbed and thus hydrogen can be
generated more efficiently.
25 [0104] The operation of the hydrogen generation device 900 of the present
embodiment is described in more detail. When the photocatalyst electrode 920
disposed in the container 990 is irradiated with sunlight through the light incident
portion 950 of the container 990 in the hydrogen generation device 900, electrons
are generated in the conduction band and holes are generated in the valence band,
30 respectively, in the portion of the photocatalyst electrode 920 irradiated with light.
Since the Nb3N5-containing film that constitutes the photocatalyst electrode 920 is
an n-type semiconductor, the potential of the surface of the photocatalyst electrode
920 is higher than the potential of the inside thereof. Therefore, the holes
generated at this time move to the surface of the photocatalyst electrode 920 along
35 the band edge of the valence band. Thus, water is decomposed on the surface of
the photocatalyst electrode 920 according to the above reaction formula (1), so that
oxygen is generated. On the other hand, the electrons move, along the band edge
27
of the conduction band, from the surface near-field region of the photocatalyst
electrode 920 to the conductive substrate 910 through the inside of the photocatalyst
electrode 920. When the electrons reach the conductive substrate 910, they are
transferred, through the conducting wire 970, to the side of the counter electrode
5 930 connected electrically to the conductive substrate 910. Thus, hydrogen is
generated on the surface of the counter electrode 930 according to the above
reaction formula (2).
[0105] The oxygen thus generated is discharged from the opening 980 for collecting
oxygen on the photocatalyst electrode 920 side, and the hydrogen thus generated is
10 discharged from the opening 980 for collecting hydrogen on the counter electrode
930 side, respectively. Water is supplied into the container 990 through the
opening 980 serving as a water inlet to replenish the water used for decomposition.
[0106] Since the Nb3N5-containing film provided for the photocatalyst electrode 920
used in the present embodiment has excellent optical semiconductor properties, the
15 probability of recombination of holes and electrons is low. Furthermore, the
hydrogen generation devices 900 and 1000 of the present embodiment are each a
so-called photoelectrochemical cell using, as an electrode, a Nb3N5-containing film
serving as an optical semiconductor. Therefore, in the hydrogen generation devices
900 and 1000, efficient charge separation between holes and electrons is achieved,
20 and thus the quantum efficiency of the hydrogen evolution reaction by light
irradiation is increased. In addition, since Nb3N5 has a small band gap, it is also
responsive to visible light in sunlight. As a result, the hydrogen generation devices
900 and 1000 of the present embodiment can generate more hydrogen than devices
using conventional optical semiconductor materials. Moreover, in the hydrogen
25 generation devices 900 and 1000, hydrogen and oxygen can be generated separately,
and thus it is easy to collect hydrogen and oxygen separately.
[0107] (Seventh Embodiment)
An embodiment of the energy system of the present invention is described.
The energy system of the present embodiment is a system that uses the
30 photocatalyst described in the fifth embodiment. The energy system of the present
embodiment is a system in which hydrogen generated through decomposition of
water by irradiation of the photocatalyst with light is supplied to a fuel cell and
converted into electrical energy.
[0108] The energy system of the present embodiment includes such a hydrogen
35 generation device as described in the sixth embodiment. FIG. 11 shows a
schematic view of the energy system of the present embodiment that uses sunlight.
The energy system of the present invention is not limited to the configuration
28
shown in FIG. 11.
[0109] The energy system of the present embodiment includes such a hydrogen
generation device 1110 as described in the sixth embodiment, a fuel cell 1120, and a
line 1130 for supplying the hydrogen generated in the hydrogen generation device
5 1110 to the fuel cell 1120.
[0110] The hydrogen generation device 1110 is installed in a place 1140 filled with
sunlight such as on the roof, for example. When the hydrogen generation device
1110 is installed, it is desirable to orient the surface of the hydrogen generation
device 1110 on which the photocatalyst is provided to face the sun so that the
10 hydrogen generation device 1110 can efficiently receive sunlight. The hydrogen
obtained in the hydrogen generation device 1110 through decomposition of water
caused by the photocatalytic reaction is discharged out of the hydrogen generation
device 1110 through the line 1130 so as to be supplied to the fuel cell 1120.
[0111] The line 1130 may be provided, for example, with a hydrogen storage unit
15 for storing hydrogen, a dehumidifier for removing moisture in the hydrogen, a
compressor unit when hydrogen is required to be compressed for storage, etc.
[0112] FIG. 12 shows a more specific example of the energy system of the present
embodiment. An energy system 1200 shown in FIG. 12 includes a hydrogen
generation device 1110, a hydrogen storage 1210, a fuel cell 1120, and a storage
20 battery 1220.
[0113] The hydrogen storage 1210 is connected to the hydrogen generation device
1110 by a first pipe 1230 and stores hydrogen generated in the hydrogen generation
device 1110. The hydrogen storage 1210 can be composed of, for example, a
compressor for compressing hydrogen generated in the hydrogen generation device
25 1110 and a high-pressure hydrogen tank for storing the hydrogen compressed by the
compressor.
[0114] The fuel cell 1120 includes a power generator 1121 and a fuel cell controller
1122 for controlling the power generator 1121. The fuel cell 1120 is connected to
the hydrogen storage 1210 by a second pipe 1240, and converts the hydrogen stored
30 in the hydrogen storage 1210 into electricity and heat. The second pipe 1240 is
provided with a block valve 1250. For example, a polymer electrolyte fuel cell can
be used as the fuel cell 1120.
[0115] The positive electrode and the negative electrode of the storage battery 1220
respectively are connected electrically to the positive electrode and the negative
35 electrode of the power generator 1121 in the fuel cell 1120 by a first line 1260 and a
second line 1270. The storage battery 1220 is provided with a capacity meter 1280
for measuring the remaining capacity of the storage battery 1220. For example, a
29
lithium ion battery can be used as the storage battery 1220.
[0116] Next, the operation of the energy system 1200 is described.
[0117] The hydrogen generated by the operation of the hydrogen generation device
described in the sixth embodiment is supplied into the hydrogen storage 1210
5 through the opening 980 for collecting hydrogen (see FIG. 9 and FIG. 10) and the
first pipe 1230.
[0118] In generating power in the fuel cell 1120, the block valve 1250 is opened
according to signals from the fuel cell controller 1122, so that the hydrogen stored in
the hydrogen storage 1210 is supplied to the power generator 1121 of the fuel cell
10 1120 through the second pipe 1240.
[0119] The electricity generated in the power generator 1121 of the fuel cell 1120 is
stored in the storage battery 1220 through the first line 1260 and the second line
1270. The electricity stored in the storage battery 1220 is supplied to home and
business users through a third line 1290 and a fourth line 1300.
15 [0120] In the hydrogen generation device 1110, the quantum efficiency of the
hydrogen evolution reaction by light irradiation can be increased. Thus, the energy
system 1200 including this hydrogen generation device 1100 can supply electric
power efficiently.
20 EXAMPLES
[0121] Hereinafter, the present invention is described further in detail with
reference to examples. The following examples are merely exemplary and are not
intended to limit the present invention.
[0122]
25 In Example 1, an example in which a compound represented by the
composition formula Nb(NR2)5 was used as an organic niobium compound that is a
starting material is described.
[0123] (Example 1-1)
Example 1-1 of the present invention is specifically described.
30 [0124] As a starting material, pentakis(dimethylamino)niobium (Nb(N(CH3)2)5)
powder (1.0 g) was used. The starting material was maintained at room
temperature for 8 hours under a flow of ammonia gas (with a purity of 99.999% or
more) at a linear flow rate of 2.2 m minute-1. Subsequently, the temperature of the
starting material was raised to 160°C at a temperature rise rate of 5°C minute-1
35 under the ammonia gas flow at a linear flow rate of 2.2 m minute-1, and then
maintained at 160°C for 8 hours. Then, the temperature of the material was
lowered to room temperature at a temperature drop rate of 5°C minute-1. Thus,
30
Nb3N5 was obtained. The elemental composition of the synthesized Nb3N5 was
analyzed. As a result, the composition ratio (molar ratio) thereof was Nb/N =
3.0/4.9, which was almost equal to the theoretical composition ratio of Nb3N5, i.e.,
Nb/N = 3.0/5.0. From this analysis result, production of Nb3N5 powder was
5 confirmed.
[0125] FIG. 13 shows the X-ray photoelectron spectrum of the Nb3N5 powder
synthesized in this example. The X-ray photoelectron spectrum is obtained by
observing the kinetic energy of photoelectrons emitted from a sample under X-ray
irradiation. In the spectrum of the sample, the peak position of the sample shifts
10 due to a difference in the chemical state of a target element. Therefore,
information about the valence and binding state of the element can be obtained
from the shift. Generally speaking, the higher the valence of the element is, the
more the peak position in the spectrum shifts toward the high binding energy side.
Furthermore, the higher the electronegativity of an element bonded to the target
15 element is, the more the peak position shifts toward the high binding energy side.
In the spectrum of FIG. 13, the Nb 3d5/2 peak is located at a binding energy of
205.6 eV. This means that the Nb 3d5/2 peak is located between the peak of Nb2O5
(where niobium has a valence of +5) as a reference and the peak of NbN (where
niobium has a valence of +3). The Nb element of Nb3N5 is bonded to the N element,
20 while the Nb element of Nb2O5 is bonded to the O element. Presumably, since the
O element is more electronegative than the N element, the peak of Nb2O5 shifts
more toward the high binding energy side than that of Nb3N5, as described above.
In view of the above, the above peak position of Nb3N5 in the spectrum means that
Nb species contained in the sample has a valence of +5 and it is bonded to N.
25 [0126] X-ray absorption fine structure (XAFS) analysis of the Nb3N5 powder
produced in this example was performed. The XAFS spectrum is an absorption
spectrum resulting from the excitation of inner shell electrons of a target element
that occurs when the element is irradiated with X-rays. A region near the
absorption edge of the XAFS spectrum is called an X-ray absorption near edge
30 structure (XANES) spectrum. The XANES spectrum is sensitive to the valence of a
target element, and generally, the higher the valence of the element is, the more the
absorption edge position in the spectrum shifts toward the high energy side. FIG.
14 shows the XANES spectrum of the Nb3N5 produced in this example. It can be
seen from FIG. 14 that for respective Nb compounds used as references, the
35 absorption edge of the spectrum of a compound in which Nb has a higher valence
shifts more toward the high energy side. Here, the absorption edge of the spectrum
of the target Nb3N5 almost coincides with that of a niobium oxide (Nb2O5) in which
31
Nb species has a valence of +5. This fact also means that Nb species contained in
the sample has a valence of +5.
[0127] The above results show that the reduction of Nb in the Nb3N5 powder was
prevented due to the effect of a nitridation temperature lower than that of the
5 conventional nitridation and the state of Nb5+ was maintained before and after the
calcination under the ammonia gas flow.
[0128] (Example 1-2)
As a raw material, pentakis(dimethylamino)niobium (Nb(N(CH3)2)5) was
used.
10 [0129] Next, a Nb3N5-containing film was synthesized using the MOCVD
apparatus 500 shown in FIG. 5. An ethylcyclohexane solution of the raw material
501 at 6.76 × 10-6 Pam3 s-1 (0.04 sccm) was vaporized at 130°C in the vaporizer 511.
The nitrogen gas 502 was used as an inert gas. The ammonia 503 at 1.69 × 10-3
Pam3 s-1 (10 sccm) was mixed w i th a mixed gas at 1.69 × 10-1 Pam3 s-1 (1000 sccm)
15 containing the source gas (vaporized raw material 501) and the nitrogen gas 502.
The resulting gas mixture was injected for 12 hours from the shower head 514 to
the substrate 521 (ITO film (with a thickness of 150 nm) / glass substrate) heated at
150°C by the susceptor 515. Thus, a Nb3N5-containing film with a thickness of 140
nm was obtained.
20 [0130] FIG. 15 shows the ultraviolet-visible absorption spectrum of the
Nb3N5-containing film produced in this example. It was found from FIG. 15 that
the resulting material can absorb visible light having a wavelength up to 780 nm.
This demonstrated that the above material was a semiconductor w i th a band gap of
1.6 eV (the following formula 2). This measured value almost coincided w i th the
25 band gap value (1.5 eV) of Nb3N5 estimated based on the result of the
first-principles band calculation.
[0131] Band gap [eV] = 1240 / Absorption wavelength [eV] ... (Formula 2)
[0132] (Example 1-3)
Example 1-3 of the present invention is specifically described.
30 [0133] Nb3N5 synthesized in Example 1-1 was allowed to be loaded w i th platinum
(Pt) as a co-catalyst thereon using the following method. Nb3N5 was impregnated
with hexachloroplatinate (IV) (H2PtCl6) that is equivalent to 1 wt% of Nb3N5 in an
aqueous solution, followed by drying in a steam bath. Thereafter, it was subjected
to a hydrogen reduction treatment at 200°C for 2 hours. Thus, Pt-loaded Nb3N5
35 was obtained.
[0134] Pt-loaded Nb3N5 (0.20 g) was suspended in 200 mL of a 10 vol% methanol
aqueous solution. FIG. 16 shows a time course of the amount of hydrogen
32
generated under irradiation of this suspension with visible light having a
wavelength of 420 nm to 800 nm. Using a 300-W xenon lamp as the light source,
the light irradiation was performed through a cold mirror for blocking light other
than the light having a wavelength of 420 nm to 800 nm. As a result, hydrogen
5 was generated at a maximum generation rate of 0.24 μmol hour-1 as shown in FIG.
16. It was confirmed from this that Pt-loaded Nb3N5 had a photocatalytic function
for reducing protons in the methanol aqueous solution to hydrogen under
irradiation with visible light.
[0135] (Comparative Example 1-1)
10 Pt-loaded Nb3N5 (0.20 g) was suspended in 200 mL of a 10 vol% methanol
aqueous solution in the same manner as in Example 1-3. The amount of hydrogen
generated in this suspension placed in a dark place was measured. However, even
after 7 hours from the start, no hydrogen was detected. This proved that the
hydrogen evolution reaction in the suspension in Example 1-3 proceeded by the
15 function of Nb3N5 as a photocatalyst.
[0136] (Example 1-4)
Nb3N5 synthesized in Example 1-1 was allowed to be loaded w i th iridium
oxide (IrO2) as a co-catalyst using the following method. Nb3N5 was suspended in
an aqueous solution containing i r i d i um (IV)-oxide dihydrate (IrO2 2H2O) colloid that
20 is equivalent to 5 wt% of Nb3N5, so that the iridium (IV)-oxide dihydrate colloid was
adsorbed on the surface of the Nb3N5. Thereafter, it was filtered, washed w i t h
water, and then subjected to a calcination treatment at 150°C in vacuum. Thus,
IrO2-loaded Nb3N5 was obtained.
[0137] IrO2-loaded Nb3N5 (0.20 g) was suspended in 200 mL of a 0.01M (mol L- 1 )
25 silver nitrate aqueous solution. FIG. 17 shows a time course of the amount of
oxygen generated under irradiation of this suspension with ultraviolet light. The
light irradiation was performed using a 450-W high-pressure mercury lamp as the
light source. As a result, oxygen was generated at a maximum generation rate of
3.1 μmol hour-1 as shown in FIG. 17. It was confirmed from this that Nb3N5 had a
30 photocatalytic function for oxidizing water in the silver nitrate aqueous solution to
oxygen under irradiation with light.
[0138] (Comparative Example 1-2)
IrO2-loaded Nb3N5 (0.20 g) was suspended in 200 mL of a 0.01M (mol L-1)
silver nitrate aqueous solution in the same manner as in Example 1-4. The
35 amount of oxygen generated in this suspension placed in a dark place was measured.
However, even after 3.5 hours from the start, no oxygen was detected. This proved
that the oxygen evolution reaction in the suspension in Example 4 proceeded by the
33
function of Nb3N5 as a photocatalyst.
[0139]
In Example 2, an example in which a compound represented by the
composition formula R1N=Nb(NR2R3)3 was used as an organic niobium compound
5 that is a starting material is described.
[0140] (Example 2-1)

As a raw material (R1N=Nb(NR2R3)3), tertiarybutylimino
tris-(ethylmethylamino)niobium ((CH3)3CN=Nb(N(C2H5)CH3)3) was used. The
10 reduction onset temperature of Nb in this raw material was determined based on
the TG-DTA data thereof. As a result, the reduction temperature of Nb in this raw
material was about 303°C.
Next, a Nb3N5-containing film was synthesized using the MOCVD
apparatus 500 shown in FIG. 5. An ethylcyclohexane solution of the raw material
15 501 at 6.76 × 10-6 Pam3 s-1 (0.04 sccm) was vaporized at 150°C in the vaporizer 511.
The nitrogen gas 502 was used as an inert gas. The ammonia 503 at rates of 0
Pam3 s-1 (0 sccm), 1.69 × 10-4 Pam3 s-1 (1 sccm), 8.45 × 10-4 Pam3 s-1 (5 sccm), and
1.69 × 10-3 Pam3 s-1 (10 sccm) each were mixed w i th a mixed gas at 1.69 × 10-1
Pam3 s-1 (1000 sccm) containing the source gas (vaporized raw material 501) and
20 the nitrogen gas 502. The resulting gas mixture was injected for 6 hours from the
shower head 514 to the substrate 521 (ITO film (with a thickness of 150 nm) / glass
substrate) heated at 300°C by the susceptor 515. Thus, a film 1A (ammonia: 0
sccm), a film 1B (ammonia: 1 sccm), a film 1C (ammonia: 5 sccm), and a film 1D
(ammonia: 10 sccm) each w i th a thickness of 200 nm were obtained.
25 [0141]
FIG. 18 shows the ultraviolet-visible absorption spectra of the films 1A to
1D. The absorption edges of 1A to 1D are considered to be about 780 nm from the
ultraviolet-visible absorption spectra thereof. FIG. 19 shows the X-ray
photoelectron spectra of Nb 3d in the films 1A to 1D and the Nb3N5 powder (referred
30 to as “1P”) synthesized in Example 1-1. It can be seen from FIG. 19 that the peak
at 205.6 eV on the low energy side, which is considered to be the peak of Nb3N5,
increases as the amount of ammonia increases (from 1A to 1D). As a result, it was
confirmed that the films 1B to 1D contained Nb3N5, that is, they were
Nb3N5-containing films.
35 [0142] In this example, the ammonia flow rates during the synthesis of the
Nb3N5-containing films were 0 to 1.69 × 10-3 Pam3 s-1 (0, 1, 5, and 10 sccm). It
was confirmed that the Nb3N5-containing film was also synthesized when the
34
ammonia flow rate was higher than the above rates, although the film forming rate
was lower.
[0143] (Example 2-2)
A Nb3N5 powder was synthesized using the apparatus 100 shown in FIG. 3.
5 The raw material 101 was set in the boat 113 in the quartz tube 112 with an inner
diameter of 25 mm in the tube furnace 111. As the raw material 101, the same
tertiary-butylimino tris-(ethylmethylamino)niobium ((CH3)3CN=Nb(N(C2H5)CH3)3)
was used as in Example 2-1. This raw material 101 was heated at 300°C for 4
hours under an ammonia flow at 1.69 × 10-1 Pam3 s-1 (1000 sccm). Thus, a powder
10 2 was obtained.
[0144] This powder 2 was subjected to elemental analysis, and it was found that
the powder 2 consisted only of Nb and N, and the ratio of the number of Nb atoms
and that of N atoms (Nb/N) was 3.0/4.9. This result indicated that Nb3N5 was
produced. The X-ray photoelectron spectrum of the powder 2 was compared w i th
15 the X-ray photoelectron spectrum of the Nb3N5 powder of Example 1-1. As a result,
it was confirmed that these data almost coincided w i th each other. It was thus
confirmed that the powder 2 was a Nb3N5-containing material.
[0145] In this example, the ammonia flow rate during the synthesis of the
Nb3N5-containing material was 1.69 × 10-1 Pam3 s-1 (1000 sccm). It was confirmed
20 that the Nb3N5-containing material was also synthesized when the ammonia flow
rate was in the range of 10 sccm to 1000 sccm, although the production rate was low.
It was confirmed that the Nb3N5-containing material was also synthesized when the
ammonia flow rate was 1000 sccm or higher, although the production rate was
unchanged.
25 [0146] (Example 2-3)
As a raw material (R1N=Nb(NR2R3)3), tertiarybutylimino
tris-(diethylamino)niobium ((CH3)3CN=Nb(N(C2H5)2)3) was used. The reduction
onset temperature of Nb in this raw material was determined based on the TG-DTA
data thereof, in the same manner as in Example 2-1. As a result, the reduction
30 onset temperature of Nb in this raw material was about 410°C.
[0147] Next, a Nb3N5-containing film was synthesized using the MOCVD
apparatus 500 shown in FIG. 5. An ethylcyclohexane solution of the raw material
501 at 6.76 × 10-6 Pam3 s-1 (0.04 sccm) was vaporized at 150°C in the vaporizer 511.
The nitrogen gas 502 was used as an inert gas. The ammonia 503 at 1.69 × 10-3
35 Pam3 s-1 (10 sccm) was mixed w i th a mixed gas at 2.87 × 10-1 Pam3 s-1 (1700 sccm)
containing the source gas (vaporized raw material 501) and the nitrogen gas 502.
The resulting gas mixture was injected for 6 hours from the shower head 514 to the
35
substrate 521 (ITO film (with a thickness of 150 nm) / glass substrate) heated at
400°C by the susceptor 515. Thus, a film 3 w i th a thickness of 300 nm was
obtained.
[0148] The X-ray photoelectron spectrum of Nb 3d in this film 3 was measured.
5 The peak of Nb 3d was observed at 205.6 eV on the low energy side. It was
confirmed from this result that the film 3 was a Nb3N5-containing film.
[0149] (Example 2-4)
As a raw material (R1N=Nb(NR2R3)3), tertiarybutylimino
tris-(dimethylamino)niobium ((CH3)3CN=Nb(N(CH3)2)3) was used. The reduction
10 onset temperature of Nb in this raw material was determined based on the TG-DTA
data thereof, in the same manner as in Example 2-1. As a result, the reduction
onset temperature of Nb in this raw material was about 250°C.
[0150] Next, a Nb3N5-containing film was synthesized using the MOCVD
apparatus 500 shown in FIG. 5. An ethylcyclohexane solution of the raw material
15 501 at 6.76 × 10-6 Pam3 s-1 (0.04 sccm) was vaporized at 150°C in the vaporizer 511.
The nitrogen gas 502 was used as an inert gas. The ammonia 503 at 1.69 × 10-3
Pam3 s-1 (10 sccm) was mixed w i th a mixed gas at 2.87 × 10-1 Pam3 s-1 (1700 sccm)
containing the source gas (vaporized raw material 501) and the nitrogen gas 502.
The resulting gas mixture was injected for 6 hours from the shower head 514 to the
20 substrate 521 (ITO film (with a thickness of 150 nm) / glass substrate) heated at
240°C by the susceptor 515. Thus, a film 4 w i th a thickness of 500 nm was
obtained.
[0151] The X-ray photoelectron spectrum of Nb 3d in this film 4 was measured.
The peak of Nb 3d was observed at 205.6 eV. It was confirmed from this result that
25 the film 4 was a Nb3N5-containing film.
[0152] (Example 2-5)

As an example of the hydrogen generation device of the present invention, a
hydrogen generation device having the same configuration as that of the hydrogen
30 generation device 900 shown in FIG. 9 was fabricated. The configuration of the
hydrogen generation device of the present embodiment is described w i th reference
to FIG. 9. As shown in FIG. 9, the hydrogen generation device 900 of Example 2-5
included a rectangular glass container 990 with openings in the upper part, a
photocatalyst electrode 920, a conductive substrate 910, and a counter electrode 930.
35 The glass container 990 contained 1 mol L - 1 of H2SO4 aqueous solution as the
electrolytic solution 960. As thephotocatalyst electrode 920, a 1-cm-square
electrode, which was fabricated in Example 2-1, in which the 200-nm-thick film 1D
36
was provided on the substrate 521 (ITO film (with a thickness of 150 nm) / glass
substrate) (corresponding to the conductive substrate 910), was used. The
photocatalyst electrode 920 was disposed so that the surface thereof faced the light
incident surface 950 of the glass container 990. A platinum plate was used as the
5 counter electrode 930. The conductive substrate 910 and the counter electrode 930
were electrically connected by the conducting wire 970. The current flowing
between the photocatalyst electrode 920 and the counter electrode 930 was
measured with an ammeter.
[0153]
10 A solar simulator manufactured by SERIC Ltd. was used to apply simulated
sunlight. The surface of the photocatalyst electrode 920 was irradiated with light
at an intensity of 1 kW/m2 through the light incident portion 950 of the hydrogen
generation device 900 of Example 2-5. The gas generated on the surface of the
counter electrode 930 was collected for 60 minutes, and the components of the
15 collected gas were analyzed and the amount of the gas generated was measured by
gas chromatography. The photocurrent flowing between the photocatalyst
electrode 920 and the counter electrode 930 was measured with an ammeter. The
apparent quantum efficiency was calculated using the amount of the gas generated
in the counter electrode 930. About 59 μL of oxygen was generated from the
20 photocatalyst electrode 920, and about 121 μL of hydrogen was generated from the
counter electrode 930. About 0.19 mA photocurrent was observed, and thus the
calculated apparent quantum efficiency was about 2%.
[0154] It was confirmed from these results that the Nb3N5-containing film used in
this example had optical semiconductor properties for generation of hydrogen and
25 oxygen by decomposition of water under light irradiation.
INDUSTRIAL APPLICABILITY
[0155] Since the Nb3N5 of the present invention can be used as a semiconductor
material, high efficient hydrogen generation can be achieved by utilizing sunlight.
30 Hydrogen thus obtained can be used as a fuel for fuel cells, for example.
37
CLAIMS
1. A photocatalyst consisting of an optical semiconductor containing a niobium
nitride which has a composition represented by the composition formula NboNs and
in which a constituent element Nb has a valence of substantially +5.
2. A hydrogen generation device comprising:
the photocatalyst according to claim l;
an aqueous solution containing an electrolyte and being in contact with the
photocatalyst; and
a container containing the photocatalyst and the aqueous solution, wherein
hydrogen is generated through decomposition of water in the aqueous
solution by irradiation of the photocatalyst with light.
3. An energy system comprising:
the hydrogen generation device according to claim 2;
a fuel cell; and
a line for supplying the hydrogen generated in the hydrogen generation
device to the fuel cell.
4. A photocatalyst consisting of a niobium nitride-containing film containing a
niobium nitride which has a composition represented by the composition formula
NbsNs and in which a constituent element Nb has a valence of substantially +5.
5. A hydrogen generation device comprising:
the photocatalyst according to claim 4;
an aqueous solution containing an electrolyte and being in contact with the
photocatalyst; and
a container containing the photocatalyst and the aqueous solution, wherein
hydrogen is generated through decomposition of water in the aqueous
solution by irradiation of the photocatalyst with light.
6. An energy system comprising:
the hydrogen generation device according to claim 5;
a fuel cell; and
a line for supplying the hydrogen generated in the hydrogen generation
device to the fuel cell.