第31回無機材料に関する最近の研究成果発表会
東海大学校友会館
January 27, 2014
表面ナノ構造を制御した半導体
光触媒による水の可視光完全分解
前田 和彦
東京工業大学 大学院理工学研究科 化学専攻
JSTさきがけ研究者『光エネルギーと物質変換』領域
Research background
Conventional energyproduction system
Fossil Fuel
(Depletion…)
Photocatalytic H2 production
from water and solar energy
Sun
O2
2H2
Combustion
O2
Combustion
H 2O
CO2, SOx, NOx
(Environmental issues!!)
Semiconductor
photocatalyst
Basic principle of water splitting on a heterogeneous
photocatalyst
H 2O
hν
Photocatalyst
V (vs. NHE)
(pH 0)
0
1
H2 +
O2 ΔG0 = 238 kJ/
2
mol
Conduction band (C.B.)
e–
H+/H2
H2
Band gap
+1.0
H+
hν
O2/H2O
+2.0
+3.0
O2
h+
Valence band (V.B.)
H2O
(Oxy)nitrides as water-splitting photocatalysts
…Production of H2 as a renewable energy carrier
H2O
Sunlight
Photocatalyst
1
H2 +
O2 (ΔG0 = 238 kJ/mol)
2
Visible
UV
Maeda & Domen, J. Phys. Chem. C 2007, 111, 7851.
BaNbO2N
0 .8
0 .6
Ta3N5
SrTaO2N
N o rm a liz e d F (R ∞ )
1 .0
0 .4
0 .2
0
300
LaTiO2N
TaON
Ta2O5
400
500
600
Wav elen g th / n m
700
800
•  Wide visible light absorption
•  Suitable band structure
•  Stable under irradiation
GaN–ZnO solid solution…the first “reproducible” example
of achieving the visible-light-driven overall water splitting
H 2O
hν (< 3 eV)
H2 +
1
O2 ΔG0=238 kJ/mol
2
GaN (Ga1–xZnx)(N1–xOx) ZnO
(x = 0.42)
Maeda et al., J. Am. Chem. Soc. 2005, 127, 8286.
Maeda et al., Nature 2006, 440, 295.
Maeda & Domen, Chem. Mater. (Review) 2010, 22, 612.
Strategy to develop an efficient photocatalyst
H+
H2 ・Construction of active sites
・Decreasing activation energy
O2
Cocatalyst
nanoparticle
(e.g. NiO, RuO2)
H2O
e–
hν > Eg
h+
Recombination
GaN:ZnO　particle
u 
u 
u 
u 
Crystallinity
Particle size
Composition
etc.
Development of a new cocatalyst that efficiently promotes
the overall water splitting on GaN:ZnO
Development of a new cocatalyst for water splitting
Conventional cocatalysts
NiO, RuO2, IrO2…“single” component metal-oxides
H2
H+
O2
NiO, RuO2,
or IrO2
H2O
e–
hν > Eg
h+
Recombination
Photocatalyst　particle
Effect of Cr co-loading on water splitting activity
Maeda et al., J. Catal. 2006, 243, 303.
λ > 300 nm
Cocatalyst
Activity / µmol h–1
Element
(oxide)
Loading
amount / wt%
H2
O2
None
-
0
0
Cr
1
0
0
Fe
1
0
0
Co
1
2.0
Ni
1.25
Cu
Cr coloading
amount
/ wt%
Activity / µmol h–1
H2
O2
1
73
36
0
1
48
24
126
57
0.125
685
336
1
2.0
0
1
585
292
Ru
1
71
27
0.1
181
84
Rh
1
50
1.6
1.5
3835
1988
Pd
1
1.0
0
0.1
205
96
Ag
1
0
0
1
11
2.3
Ir
1.5
9.3
3.1
0.1
41
17
Pt
1
0.9
0.4
1
775
357
Catalyst: 0.3 g, Reactant soln.: distilled water 370~400 mL, Reaction vessel: Inner
irradiation-type, Light source: 450 W high-pressure mercury lamp
Overall water splitting on Rh2–yCryO3-loaded
GaN:ZnO under visible light irradiation
Amount of evolved gases / mmol
Evac.
▼
2.0
H2
1.5
λ > 400 nm
1.0
Used catalyst: 3.7 mmol
Evolved gases: 16.2 mmol
O2
0.5
0
0
5
10
15
20
25
Reaction time / h
RuO2 data
30
Catalytic cycle!!
35
Maeda et al., Nature, 2006, 440, 295.
Catalyst: 0.3 g, Reactant soln.: H2SO4 aq. 370 mL (pH 4.5), Reaction vessel: Inner
irradiation-type, Light source: 450 W high-pressure mercury lamp with a NaNO2 aq. filter
TEM images of Rh-loaded GaN:ZnO
before and after photodeposition of Cr2O3
Maeda et al., Angew. Chem., Int. Ed. 2006, 45, 7806.
Maeda et al., J. Phys. Chem. C 2007, 111, 7554.
Maeda et al., Chem. Eur. J. 2010, 16, 7750.
Revealed by XAFS and XPS
Cr2O3 (shell)
Rh (core)
Rh
Photoreduction
of Cr6+
Rh/GaN:ZnO
(before Cr2O3 deposition)
Cr2O3/Rh/GaN:ZnO
Time course of overall water splitting
on core/shell-structured Cr2O3/Rh/GaN:ZnO λ > 400 nm
Amount of evolved gases / mmol
Rh
Cr2O3/Rh
0.8
0.8
Rh
0.8
Cr2O3/Rh
+
H2
0.6
0.6
0.4
0.4
0.2
0.2
0.2
0
0
0
0 1 2 3 4
Reaction time / h
0.6
O2
0 1 2 3 4
Reaction time / h
0.4
0 1 2 3 4
Reaction time / h
Catalyst: 0.15 g, Reactant soln.: pure H2O 370 mL, Reaction vessel: Pyrex inner irradiationtype, Light source: 450 W high-pressure Hg lamp with a NaNO2 aq. (2 M) filter
水分解光触媒における助触媒研究
1980年～
2006年～ (前田、堂免ら)
助触媒 (RuO2など)
助触媒
H2
H2
H+
H+
e–
光触媒
従来型
(単一の金属種)
Cr2O3シェル
e–
光触媒
Cr含有複合酸化物型
H2
コア
H+
e–
光触媒
異種接合型
(コア/シェル型)
水分解光触媒研究の分野に助触媒開発という一大研究領域を確立
Major problem in the nanoparticulate core/shell system
Photodeposition
Aggregated Rh
Attempts to have
Rh well-dispersed
•  Impregnation method
No positive effect!!
•  New method
5 nm
Adsorption
To increase the activity of Cr2O3/Rh/GaN:ZnO by
introduction of Rh nanoparticle core with higher dispersion
TEM images of GaN:ZnO modified with Rh/Cr2O3
(core/shell) nanoparticles by an adsorption method
Rh
Cr2O3
High-dispersion!!
Sakamoto et al., Nanoscale, 2009, 1, 106.
Maeda et al., Chem. Eur. J., 2010, 16, 7750.
Size: 1~3 nm
Size distribution of Rh nanoparticles adsorbed
on the surface of GaN:ZnO
Number of Particles / count
80
Average size of
200 particles
60
1.9 ± 0.6 nm
40
Photodeposition method
Ave. 7.6 nm (50 particles)
20
0
0
1
2
3
4
Particle size / nm
5
6
Effect of the amount of Rh on activity
Saturated
adsorption
0 .5 > 400 nm
λ
H2
400
0 .4
300
0 .3
O2
200
0 .2
100
0
0 .1
Increase in H2
evolution site
0
0 .5
1 .0
1 .5
2 .0
2 .5
3 .0
A m o u n t o f R h lo a d e d / w t %
R a te o f g a s e v o lu tio n / µm o l h
-­‐1
500
0
A m o u n t o f R h ad d ed / wt %
Catalyst: 0.15 g, Reactant soln.: pure H2O 400 mL, Reaction vessel: Pyrex inner irradiationtype, Light source: 450 W high-pressure Hg lamp with a NaNO2 aq. (2 M) filter
Comparison of activity
…Rh loading amount: 0.3~0.4 wt%
A m o u n t o f e v o lv e d g a s e s / µm o l
2500
Adsorption method
Photodeposition method
2000
H2
1500
High-dispersion of the core
component is important!!
1000
O2
H2
500
0
λ > 400 nm
O2
0
1
2
3
R eac tio n tim e / h
4
5
Catalyst: 0.15 g, Reactant soln.: pure H2O 400 mL, Reaction vessel: Pyrex inner irradiationtype, Light source: 450 W high-pressure Hg lamp with a NaNO2 aq. (2 M) filter
Effect of the size of Rh nanoparticles on activity
Ikeda et al., J. Phys. Chem. C 2013, 117, 2467.
Rate of gas evolution / µmol h
-1
λ > 400 nm
600
500
400
Smaller is better!
300
200
100
0
1.5$nm
3.8$nm
6.6$nm
Catalyst: 0.15 g, Reactant soln.: H2SO4 aq. 400 mL (pH 4.5), Reaction vessel: Pyrex inner
irradiation-type, Light source: 450 W high-pressure Hg lamp with a NaNO2 aq. (2 M) filter
Overall water splitting on a particulate photocatalyst
promoted by two different types of cocatalysts
H2
H+
e–
C.B.
H2 evolution
cocatalyst
GaN:ZnO
particle
hν > Eg
V.B.
h+
O2 evolution
cocatalyst
O2
H2O
Introduction of both H2 and O2 evolution cocatalysts to improve activity!
…But no successful example for constructing such a structure…
A m o u n t o f e v o lv e d g a s e s / µm o l
Visible light water splitting
…Effect of coloading Mn3O4
160
Mn 3 O 4 + R h/C r2 O 3 (H 2 )
140
Mn 3 O 4 + R h/C r2 O 3 (O 2 )
120
R h/C r2 O 3 (H 2 )
100
Mn 3 O 4 (H 2 )
Mn3O4 0.05 wt %
λ > 420 nm
R h/C r2 O 3 (O 2 )
Mn 3 O 4 (O 2 )
80
60
40
20
0
0
2
4
6
8
10
R eac tio n tim e / h
12
Catalyst: 0.1 g of each, Reactant solution: distilled water 100 mL, Top-irradiation type with
a 300 W Xe lamp and a cutoff filter
Summary
n  Precise control of Rh core size in Rh/Cr2O3 nanoparticles
ü  Successful introduction of size-controlled Rh nanoparticles onto
the surface of GaN:ZnO photocatalyst
ü  For application in the core component, smaller Rh works better.
ü  Loading another oxygen evolution cocatalyst of Mn3O4
nanoparticles further enhances the water-splitting activity.
n  Mechanism of H2 evolution on Rh/Cr2O3 nanoparticles
ü  The core hosts active sites for H2 formation, while the Cr2O3 shell
functions as a selective permeable membrane.
Modification of surface structure in nano-scale is highly important
for enhancing water-splitting activity with visible light!
Acknowledgement
u  Prof. K. Domen…The Univ. of Tokyo
The boss
u  Prof. T. Teranishi, T. Ikeda, T. Yoshinaga…Kyoto Univ. & Tsukuba Univ.
u  Dr. M. Yoshida, N. Sakamoto, A. Xiong…(former) students of our group
Collaboration on the core/shell cocatalyst project
u  Dr. D. Lu…Tokyo Institute of Technology
TEM observations
u  日本板硝子材料工学助成会, 日本学術振興会, 科学技術振興機構さきがけ
Funding support

Symposium B

プログラム（目次） 第 1 日目：8 月 3 日（水）