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
SunO2

2H2Combustion

O2Combustion

H 2O

CO2, SOx, NOx(Environmental issues!!)

Semiconductor
photocatalyst

Basic principle of water splitting on a heterogeneous
photocatalyst
H 2Ohν	

Photocatalyst
V (vs. NHE)
(pH 0)

0

H2 +1
O2 ΔG0 = 238 kJ/
2molConduction band (C.B.)

e–H+/H2H2Band gap+1.0H+hνO2/H2O+2.0+3.0O2h+
Valence band (V.B.)

H2O

(Oxy)nitrides as water-splitting photocatalysts…Production of H2 as a renewable energy carrier
1H2 +O2 (ΔG0 = 238 kJ/mol)
H2O
2Photocatalyst	

Sunlight	Visible	  UVMaeda & Domen, J. Phys. Chem. C 2007, 111, 7851.

BaNbO2N0 .80 .6

Ta3N5SrTaO2N	  N o rm a liz e d 	  F (R ∞ )1 .00 .40 .20300

LaTiO2NTaON
Ta2O5400

500

600

Wav elen g th 	  /	  n m700800• Wide visible light absorption• Suitable band structure• Stable under irradiation

GaN–ZnO solid solution…the first “reproducible” exampleof 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 > Egh+

Recombination

GaN:ZnOparticleu Crystallinityu Particle sizeu Compositionu 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

H2H+O2

NiO, RuO2,or IrO2

H2O

e–hν > Egh+

Recombination

Photocatalystparticle

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)

Loadingamount / wt%

H2O2

None-00Cr

100Fe

100Co

12.0

Ni

1.25

Cu

Cr coloading
amount/ wt%

Activity / µmol h–1H2O2

173

360

148

24

126

57

0.125

685

336

12.001585

292

Ru

171

270.1

181

84

Rh

150

1.61.53835

1988

Pd

1

1.000.1

205

96

Ag

1001112.3

Ir1.59.3

3.10.1

4117Pt10.90.4

1775357

Catalyst: 0.3 g, Reactant soln.: distilled water 370~400 mL, Reaction vessel: Innerirradiation-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.
▼

2.0H21.5

λ > 400 nm

1.0

Used catalyst: 3.7 mmol
Evolved gases: 16.2 mmol
O2

0.500510152025
Reaction time / hRuO2 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

Photoreductionof 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

0000 1 2 3 4Reaction time / h

0.6

O20 1 2 3 4Reaction time / h

0.40 1 2 3 4Reaction time / hCatalyst: 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 nmAdsorption

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
Cr2O3High-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 adsorbedon the surface of GaN:ZnO

Number of Particles / count80

Average size of200 particles

60

1.9 ± 0.6 nm

40

Photodeposition method
Ave. 7.6 nm (50 particles)

20001234Particle size / nm

56Effect of the amount of Rh on activity	  Saturatedadsorption	

0 .5 > 400 nm
λH2	

4000 .4

3000 .3

O2	2000 .2

10000 .1

Increase in H2evolution site	
00 .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-­‐1500

0A 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 l2500	  Adsorption method
Photodeposition method	

2000

H2	1500

High-dispersion of the corecomponent is important!!

1000

O2	H2	500
0

λ > 400 nm

O2	01

23
R eac tio n 	  tim e	  /	  h45Catalyst: 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
01.5$nm

3.8$nm

6.6$nmCatalyst: 0.15 g, Reactant soln.: H2SO4 aq. 400 mL (pH 4.5), Reaction vessel: Pyrex innerirradiation-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
H2H+e–C.B.

H2 evolution
cocatalyst

GaN:ZnOparticlehν > Eg

V.B.

h+
O2 evolution
cocatalyst

O2H2O

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 lVisible light water splitting…Effect of coloading Mn3O4160	  	  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
2000246810R eac tio n 	  tim e	  /	  h

12Catalyst: 0.1 g of each, Reactant solution: distilled water 100 mL, Top-irradiation type witha 300 W Xe lamp and a cutoff filter	Summaryn Precise control of Rh core size in Rh/Cr2O3 nanoparticles

ü Successful introduction of size-controlled Rh nanoparticles onto
the surface of GaN:ZnO photocatalystü 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 shellfunctions as a selective permeable membrane.

Modification of surface structure in nano-scale is highly importantfor enhancing water-splitting activity with visible light!	

Acknowledgementu Prof. K. Domen…The Univ. of Tokyo
The bossu 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 projectu 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