(1) 太陽光水素製造を目指した可視光応答型半導体光触媒の開発

太陽光水素製造を目指した 可視光応答型半導体光触媒の開発

京都大学大学院工学研究科 阿部 竜

2018 1 30 2

CO

H

O H

(or H

O

) + 1/2O

CO

(or CO)

N

NH

地表における太陽光スペクトル（AM1.5）

Wavelength / nm

1.5

1.0

0.5

0.0

1200 1000

800 600

400 200

Visible light Infrared

UV

Sp e ct ra l I rr a d ia n ce / W m

nm

500 nm

実用化への課題：太陽光スペクトル（可視光）の有効利用

5 4 3 2 1 0

x1018

1200 1000

800 600

400 200

Wavelength / nm Ph o to n n u m b e r / 1 0

m

nm

s

(ΔG° = 237 kJ/mol) ca. 2%

ca. 16%

ca. 32%

0.05

3.1 eV 1.6 eV

5

水分解系における変換効率の現状

Honda Soltec

High (10%~) Middle (~ 3%) Low (~ 1 %)

~2%

e⁻ e⁻

H2

Visible light

e^!

h⁺ O2 External bias

H2O O₂ H⁺

H2 e⁻

e⁻

e⁻ h⁺

H⁺ H₂

H2O O₂

6

太陽光水素製造におけるコスト試算例

6

T. F. Jaramillo et al.,

Energy Environ. Sci., 2013, 6, 1983

Type 1: Single Bed Particle Suspension STH Efficiency 10%

Type 2: Dual Bed Particle Suspension STH Efficiency 5%

Type 3: Fixed Panel Array STH Efficiency 10%

Type 4: Tracking Concentrator Array STH Efficiency 15%

DOE Fuel Cell Tech. Program’s Target: $2.00 - 4.00 per kg H

7

水分解系における変換効率の現状

Honda Soltec

High (10%~) Middle (~ 3%) Low (~ 1 %)

~2%

e⁻ e⁻

H₂

Visible light

e^!

h⁺ O₂ External bias

H2O O2

H⁺ H2 e⁻

e⁻

e⁻ h⁺

H⁺ H2

H₂O O2

8

光触媒（光電極）を用いた水分解：何がポイントか？

Honda Soltec _{e− e−}

H₂

Visible light

e^!

h⁺ O₂ External bias

H2O O2

H⁺ H2 e⁻

e⁻

e⁻ h⁺

H⁺ H2

H₂O O2

1

9

人工光合成研究の歴史（概要）

各種複合酸化物による水分解

・層状化合物 K4Nb6O17（堂免）

・トンネル構造化合物 Na2Ti6O13（井上）

1990

1998

1969

TiO2光触媒粒子による水蒸気分解（佐藤）

RhO_x-SrTiO₃光触媒による水蒸気分解（Lehn）

NiOx-SrTiO₃光触媒粒子による水の分解（堂免）

1980

全て紫外光 TiO

光電極による水分解の発見（本多・藤嶋）

1986

10

なぜ可視光水分解は困難か？

H⁺ H₂

O₂ H₂O e⁻

e⁻

h⁺ h⁺ (H₂/H⁺)

(H₂O/O₂) 0 + 1.23

(SHE, pH=0)

ca. + 3.0

(B.G. > 3.0 eV) (B.G. < 3.0 eV)

2p 2p

CB

(or HOMO) (or LUMO)

(a) (b) (c)

なぜ可視光水分解は困難か？

/eV Scaife D. E. Scaife, Solar Energy, 25(1980) 41

d⁰ d¹⁰ d

B.G. < 3.0 eV

O-2p

O-2p

Potential / V vs. SHE

0 +1.0

+3.0

+2.0 -1.0

C.B.

2.6 eV

O₂/H₂O

1.0 0.8 0.6 0.4 0.2 0

−0.2

−0.4

−0.6

−0.8

−1.0 _SrNbO

6

Sr0.5Ba0.5Nb2O6

Ba0.5La0.2Nb2O6

BaNb2O6

MgTi2O5

ZnNb2O6

Ba6Ti2Nb8O30

Ba0.8Ca0.2TiO3

LaNbO4

ZnO KNbO3

Nb2O5

BaSnO3

SnO2

CdIn2O4

In2O3

Ba0.67Ca0.33Nb2O6

CaTiO3

NiNbO6

NbO2

MgTiO3

SrTiO3

TiO2 TiNb2O7

WNb2O8

CrTi2O7

FeNbO4

Cd2SnO4

CdO FeTiO3

Fe2TiO5

Sr2FeNbO6

FeTaO4

FeTa2O6

Sr0.5Ba0.5TiO3

BaTiO3

CrNbO4

U3O8

Fe2O3

2.0 2.5 3.0 3.5 4.0

La2Ti2O7

WO3

3.1 eV

TiO₂ 水素生成不可能

/V vs. SHE

可視光水分解への戦略１：二段階励起機構

(e.g., TiO₂, SrTiO₃)

e

(e.g., WO₃, Fe₂O₃)

e

h

H

O

O

H

H

Potential (V vs. SHE)

+ 0

+1.23

(O₂/H₂O)

C.B.

O 2p V.B.

酸化力不足 (酸素生成不能・不安定)

e

h

h

e

CdS + nh

→ Cd

+ SO

etc.

13

可視光水分解への戦略２：混合アニオン系への展開

(e.g., TiO₂, SrTiO₃)

e

(e.g., WO₃, Fe₂O₃)

e

h

H

O

O

H

H

Potential (V vs. SHE)

+ 0

+1.23

(O₂/H₂O)

C.B.

O 2p V.B.

e

h

h

H

H

H

O O

O-2p N-2p, S-3p, X-np

Chem. Phys. Lett. 2001; Chem. Commun.2001;

J. Photochem. Photobiol. A 2001; Chem. Phys. Lett. 2002;

Chem. Phys. Lett. 2003; J. Photochem. Photobiol. A 2004;

J. Phys. Chem.B, 2005;Chem. Commun.2005; Chem. Lett. 2008;

Chem. Phys. Lett. 2008; Chem. Mater. 2009; Chem. Commun. 2009;

Langmuir2010; J. Am. Chem. Soc. 2010;ChemSusChem2011;

J. Phys. Chem.C, 2011;J. Photochem. Photobiol. C (Invited)2011;

Bull. Chem. Sos. Jpn (Award account) 2011; Energy Environ. Sci. 2012;

J. Am. Chem. Soc. 2013; Catal. Sci. Tech. 2015; J. Am. Chem. Soc. 2016 (Fe³⁺)

(Fe²⁺)

二段階可視光励起型水分解システム

研究の概要

Bi4NbO8Cl

可視光酸素生成用酸窒化物光アノード

Chem. Lett. 2005; J. Am. Chem. Soc. 2010;

Thin Solid Films 2010;Energy Environ. Sci. 2011;

J. Am. Chem. Soc. 2011;J. Am. Chem. Soc. 2012;

J. Am. Chem. Soc. 2013;ChemElectroChem, 2015;

Catal. Sci. Technol.2015;APL Materials,2015;

Topics in Catalysis 2016

e⁻ e⁻

H₂

Visible light

e^!

h⁺ O2

External bias

H₂O O2

H⁺ H2

e⁻ e⁻

TaON BaTaO₂N

NbON Ta₃N₅

二段階光励起型可視光水分解システム

15

Ox H⁺/H2

O2/H2O

h⁺ e⁻

Visible light

H₂O O₂ Potential

Ox/Red

O₂ evolution photocatalyst

h⁺

H⁺ H₂

Visible light

e⁻

H₂ evolution photocatalyst

+

様々な可視光応答型光触媒が利用可能に

酸素生成系：水素生成能を有さない酸化物（WO₃

など）

水素生成系：酸素生成能を有さない非酸化物・色素など

レドックスを用いた光触媒系（水素生成）における逆反応

16

+1.23 (H⁺/H₂)

(O₂/H₂O) (Ox/Red)

Red Ox

e⁻

H⁺ H₂

h⁺ e⁻

e⁻

hn

e⁻

h⁺ e⁻

e⁻

H₂O O₂

hn

Red

Red Ox

e

+ ^Ox

Ox

逆反応による水素生成の停止

17

(Reaction conditions)

Pt(0.5 wt%)-SrTiO3(Cr-doped): 0.3 g 30 mM-NaI aqueous solution: 250 mL (pH = 6.5) 300 W Xe-lamp, L-42 cut-off filter

Time / h Am o u n t o f H

ev ol ved /

mo l

IO3– e^– (IO3–/I^–)

I^–

Potential (V vs. NHE)

pH=7 +0.67 -0.41 (H⁺/H2) H₂production over Pt-SrTiO₃(Cr-doped) photocatalyst

IO₃^– (IO₃^–/I^–)

I^–

Potential (V vs. NHE)

pH=7 +0.67 -0.41 (H⁺/H₂)

e

h

H⁺ H2

e^–

H⁺ H2

I^–

IO₃^– Re-reduction of IO₃

e

12 10 8 6 4 2 0

10 8 6 4 2 0

I⁻ 0.3 H₂

Pt-SrTiO

/Cr

0

+1.23 (H⁺/H₂)

(O₂/H₂O) (Ox/Red)

Red Ox

e⁻

H⁺ H₂

h⁺ e⁻

e⁻

H

production hn

e⁻

h⁺ e⁻

e⁻

H₂O O₂

O

production

hn

+

Red

Ox

e

レドックスを用いた光触媒系（酸素生成）における逆反応

18

Pt-BiVO₄ とIO₃^­ の組み合わせによる酸素生成

e

I^– IO₃^– e⁻

h⁺ H₂O

O₂

Pt-BiVO

hn

e

I^– IO₃^–

Potential (V vs. NHE)

pH=7 (IO3–/I^–)

+0.82 +0.67

(O₂/H₂O)

Am o u n t o f O

ev ol ved /

mo l

Time / h

O₂evolution over Pt-BiVO₄photocatalyst

In the presence of IO

(0.1 mM, 250 µmol)

30

20

10

0

20 15 10 5 0

without NaI with NaI (10 mM)

Pt-WO₃とIO₃^­の組み合わせによる選択的酸素生成反応

e

I^– IO₃^– e⁻

h⁺ H₂O

O₂

Pt-WO

hn

e

I^– IO₃^–

Potential (V vs. NHE)

pH=7 (IO₃^–/I^–)

+0.82 +0.67

(O2/H2O)

Am o u n t o f O

ev ol ved /

mo l

Time / h

O₂evolution over Pt-WO₃photocatalyst

In the presence of IO

(0.1 mM, 250 µmol)

400 300 200 100

0

100 80 60 40 20 0

IO

Expected amount of O

(375 µmol)

I

400 300 200 100

0

100 80 60 40 20 0

with NaI (10 mM)

WO₃上におけるIO₃^­とI^­の吸着特性

21 WO

e

e⁻

h⁺ H₂O

O₂

hn

e

(IO

/I

)

+0.82 +0.67 (O

/H

O)

IO₃⁻

IO₃⁻ I⁻ I⁻

I⁻ I⁻

H₂O O₂ e⁻

e⁻ h⁺

h⁺

IO

IO₃⁻

e

Concentration of anion / mM

Am o u n t o f a n io n s a d so rb e d /

mo l g

Adsorption of IO₃⁻andI⁻on WO₃

IO

I

6 5 4 3 2 1 0

20 15 10 5 0

Pt-BiVO₄ とIO₃^­ の組み合わせによる酸素生成

22 e

I^– IO₃^– e⁻

h⁺ H₂O

O₂

Pt-BiVO

hn

e

I^– IO₃^–

Potential (V vs. NHE)

pH=7 (IO₃^–/I^–)

+0.82 +0.67

(O2/H2O)

Am o u n t o f O

ev ol ved /

mo l

Time / h

O₂evolution over Pt-BiVO₄photocatalyst

In the presence of IO

(0.1 mM, 250 µmol)

30

20

10

0

20 15 10 5 0

without NaI with NaI (10 mM)

BiVO₄上におけるIO₃^­とI^­の吸着特性

23 BiVO

e

e⁻

h⁺ H₂O

O₂

hn

e

(IO

/I

)

+0.82 +0.67 (O

/H

O)

IO₃⁻

IO₃⁻ I⁻ I⁻

I⁻ I⁻

H₂O O₂ e⁻

e⁻ h⁺

h⁺

IO

e

Concentration of anion / mM

Am o u n t o f a n io n s a d so rb e d /

mo l g

Adsorption of IO₃⁻andI⁻on BiVO₄

IO

I

5 4 3 2 1

0

20 15 10 5 0

Visible light induced water splitting with two-step photoexcitation

24

h⁺

h⁺ e⁻

Visible light

I

IO

H₂O O₂

H⁺ H2

Visible light

e⁻

O2 evolution photocatalyst

H2 evolution photocatalyst

PtO_x/WO₃

Pt-SrTiO :Cr

Pt-WO

3

NaI

PtOx/WO3 Pt/SrTiO3:Cr

NaI (+NaIO3)

Pt/SrTiO₃(Cr,doped)

R. Abe, K. Sayama et al. Chem. Commun.2001;

Chem. Phys. Lett. 2001;

J. Photochem. Photobiol. A:Chem.

The world’s first demonstration of visible light driven overall water splitting

Amount of gas evolved / µmol

Time / h 500

400

300

200

100

0

200 150 100 50 0

H₂

O₂

dark

evac.

Q.E. ~ 0.5% at 420 nm

水素生成用光触媒としてのオキシナイトライド

25

e

(e.g., WO₃, BiVO₄)

H

O

O

H

H

Potential (V vs. NHE)

+ 0

+1.23

(O₂/H₂O)

C.B.

O 2p V.B.

e

h

h

(e.g., TaON, BaTaO

N) O 2p + N 2p hybridization

e

h

NH₃処理によるオキシナイトライドの調製

26 Ta

O

Ba

Ta

O

NH

気流中 850~900℃

TaON, Ta

N

BaTaO

N

4

3

2

1

0

800 700 600 500 400 300

Wavelength / nm

K.M.

Ta

O

Ta₃N₅

Ta₃N₅ BaTaO₂N TaON

Ta

O

TaON BaTaO₂N

オキシナイトライドを水素生成系とする可視光水分解

h⁺

h⁺ e⁻

Visible light

I

IO

H₂O O₂

H⁺ H2

Visible light

e⁻

O2 evolution photocatalyst

H2 evolution photocatalyst

Time / h

Amount of gas evolved / µmol

200

150

100

50

0

20 15 10 5 0

H₂ O₂

Under visible light irradiation (l> 420 nm)

WO₃

TaON SrTiO₃(Cr-doped)

R. Abe et al.

Chem. Commun. 2001; Chem. Phys. Lett. 2001;

J. Phys. Chem. B, 2005; Chem. Commun. 2005;

Chem. Phys. Lett. 2008; Chem. Lett. 2008;

Chem. Mater. 2009; J. Am. Chem. Soc. 2010

Pt/TaON + Pt/WO

Collaborative works with Prof. Domen

オキシナイトライド光触媒による長波長利用

h⁺

h⁺ e⁻

Visible light

I

IO

H2O O2

H⁺ H2

Visible light

e⁻

O2 evolution photocatalyst

H2 evolution photocatalyst

WO₃ TaON

SrTiO₃/Cr-doped

CaTaO₂N BaTaO₂N TaON

R. Abe et al.

Chem. Commun. 2001; Chem. Phys. Lett. 2001;

J. Phys. Chem.B, 2005; Chem. Commun. 2005;

Chem. Phys. Lett. 2008; Chem. Mater. 2009;

Chem. Commun. 2009; Langmuir 2010;

J. Am. Chem. Soc. 2010; ChemSusChem 2011; etc.

Ta₃N₅

TaON

BaTaO₂N Visible light

Ta₃N₅

800 700 600 500 400 300

WO₃ UV

TiO₂

TiO₂ WO₃ TaON Ta₃N₅ BaTaO₂N

混合アニオン化合物の課題：安定性の欠如

29

O-2p

ex. TaON, BiOX

Potential

X-np VB

e^-

Red.

Ox.

O

H

O

CB

N

, X

30

安定性の高い混合アニオン化合物光触媒の開発

H. Fujito, H. Kunioku et al., J. Am. Chem. Soc., 2016, 138, 2082

Collaborative works with Prof. H. Kageyama

200 150 100 50

0

16 12 8 4 0

Time of irradiation / h

Amount of gas evolved / µmol

H₂

O2 Under visible light (λ > 410 nm)

O

O O

H

H⁺

h

e

H Ru/SrTiO

:Rh

h

e

Bi

NbO

Cl

Cl-3p O-2p

Bi-6p Nb-4d

Fe

Fe

Cl-3p O-2p _h

JST-CRERST D3

Bi₄NbO₈X (X=Cl, Br) の光吸収

31 1.0

0.8 0.6 0.4 0.2

0.0

600 550

500 450 400

1.0 0.8 0.6 0.4 0.2 0.0

450 400 350 300

BiOBr BiOCl

Bi₄NbO₈Cl Bi₄NbO₈Br

Normalized Kubelka-Munk

Wavelength / nm

3.0x10⁹

2.5

2.0

1.5

1.0

0.5

0.0 C–2 / F–2 cm4

-0.7 -0.6 -0.5 -0.4 -0.3

Potential (V vs Ag/AgCl) 0.1 M Na₂SO₄ aq.

(pH2.0)

Potential (V vs Ag/AgCl) BiOCl

BiOBr Bi4NbO8Cl

Bi4NbO8Br –0.63

–0.48–0.42 –0.56

Mott-Schottky plot

Lin, X. et al., J.Mater. Chem. 2007, 17, 2145

Bi

NbO

X Cl

Bi₄NbO₈X (X=Cl, Br) の推定バンドレベル

32

Bi

NbO

Cl BiOCl, Bi

NbO

Br 3.42 eV

Potential / V NHE, pH=2

0

+1.0

+3.0 +2.0 –1.0

H⁺/H2

O2/H2O –0.43

2.99

2.39 eV

–0.28

2.11

2.48 eV

–0.22

2.26

2.78 eV

–0.36

2.42 CB

VB

Cl-3p +

O-2p Br-4p

Bi

NbO

Bi

NbO

BiOCl BiOBr

O-2p+

3.0x10⁹

2.5

2.0

1.5

1.0

0.5

0.0 C–2 / F–2 cm4

-0.7 -0.6 -0.5 -0.4 -0.3

Potential (V vs Ag/AgCl) 0.1 M Na₂SO₄ aq.

(pH2.0)

Potential (V vs Ag/AgCl) BiOCl

BiOBr Bi4NbO8Cl

Bi4NbO8Br –0.63

–0.48–0.42 –0.56

Mott-Schottky plot

DFT計算（状態密度）

33 14

12 10 8 6 4 2 0

8 6 4 2 0 -2 -4 -6 14 12 10 8 6 4 2 0

8 6 4 2 0 -2 -4 -6

BiOCl

BiOBr

Energy / eV

Total Bi O Br 10

8 6 4 2

0

8 6 4 2 0 -2 -4 -6

Total Bi (total) O (total) Cl (total)

Total Bi O Cl 10

8 6 4 2

0

8 6 4 2 0 -2 -4 -6

Total Bi (total) O (total) Cl (total)

DOS (electrons / eV)DOS (electrons / eV)

CB

CB VB VB

80

60

40

20

0

-6 -4 -2 0 2 4 6

Energy / eV 80

60

40

20

0

-6 -4 -2 0 2 4 6

Total Bi OCl Nb

Bi

NbO

Br Bi

NbO

Cl

Total Bi OBr Nb

DOS (electrons / eV)DOS (electrons / eV)

DFT計算（状態密度：VB付近）

34 14

12 10 8 6 4 2 0

-6 -5 -4 -3 -2 -1 0 1

14 12 10 8 6 4 2 0

-6 -5 -4 -3 -2 -1 0 1

BiOCl

BiOBr

Energy / eV Total Bi O Br 10

8 6 4 2

0

8 6 4 2 0 -2 -4 -6

Total Bi (total) O (total) Cl (total) Total Bi O Cl 10

8 6 4 2

0

8 6 4 2 0 -2 -4 -6

Total Bi (total) O (total) Cl (total)

DOS (electrons / eV)

Cl3p (Br4p) O2p

DOS (electrons / eV)

80

60

40

20

0

-6 -4 -2 0

80

60

40

20

0

-6 -4 -2 0

Energy / eV

Bi

NbO

Br

Bi

NbO

Cl

O Cl Nb

Total Bi O Br Nb

O2p

DOS (electrons / eV)DOS (electrons / eV)

各酸ハロゲン化物の推定バンド構造：酸化物との比較

–0.31

O-2p

+3.11

Cl-3p

BiOCl

–0.22

O-2p

+2.54

CB

BiOBr

Br-4p Cl-3p

O-2p –0.16

+2.23

Bi

NbO

Cl

Br-4p O-2p

–0.10

+2.38

Bi

NbO

Br

VB

Bi Bi

Bi Bi

O-2p

+3.0

ex. TiO

O-2p

+0.1

~ +2.5 Bi-6s

B.G.

2.4–2.5 eV

V-3d Bi-6p

1) Kudo, A. et al., J. Am. Chem. Soc. 1999, 121, 11459.

2) Cooper, J. K.,et al., Chem. Mater. 2014, 26, 5365.

Ti-3d

NHE, pH=0

DOS

ex. BiVO

Bi : [Xe] 4f

5d

6s

6p

Bi₄NbO₈ClにおけるBiのPDOS

4 3 2 1 0

-6 -5 -4 -3 -2 -1 0 1

4 3 2 1 0

-6 -5 -4 -3 -2 -1 0 1

Bi-6s

Bi-6p

PDOS (electrons / eV)PDOS (electrons / eV)

Energy / eV

Bi1 Bi2 Bi3 Bi4

Bi-6s Bi-6p

Bi1 Bi2 Bi3 Bi4

Revised lone pair model (by Walsh et al.)

37 4458 Chem. Soc. Rev.,2011,40, 4455–4463 This journal iscThe Royal Society of Chemistry 2011 it does not form in others. The diversity of crystal structures

adopted by lone pair systems is highlighted in Table 1. The concept of an electron lone pair as a chemically inert species remains popular due to its ability to explain the distorted structures often observed in these materials, but it is unsatis- factory in its generality.

2.2 Revised model

To understand the nature of lone pairs it is instructive to begin with a discussion of the interactions that occur in undistorted structures for these materials. A quantum mechanical analysis, at the level of density functional theory (DFT),^14,15of the electronic structure of PbO^16,17and SnO^18,19in the CsCl structure, the undistorted parent of the litharge structure they adopt, shows that thes²electrons are certainly not chemically inert. They interact strongly with the anionpstates in the valence band giving rise to bonding and anti-bonding states (Fig. 2), which appear at the bottom and top of the upper valence band, respectively. The filleds²electrons of the cation donotform a non-bonding electron pair. So why does this interaction lead to the formation of distorted structures?

The formation of a distortion in the lattice allows the unoccupied cationpstates to hybridize with the anti-bonding states, resulting in a stabilisation of the occupied electronic states (Fig. 2). In the absence of a crystal distortion, the interaction of the cationporbitals has no net stabilising effect;

the interaction is composed of both positive and negative wavefunction overlap, which is forbidden by the crystal symmetry.

However, by distorting the lattice, the interaction becomes symmetry allowed and the orbital stabilisation is accompanied by an asymmetric electron density that is projected into the structural void. The asymmetric electron density has the familiar lone pair distribution, but is in fact a stabilized anti- bonding interaction between the electronic states of both the cation and anion.

Subsequent studies comparing PbO with PbS²⁰and SnO with SnX {X = S, Se, Te}²¹have shown that computations

based on quantum mechanics not only predict if a directional lone pair will form, but also explainwhylone pairs form in some materials and not in others. The relative energy of the cationsand anionpstates is critical to the formation of stereochemically active lone pairs. Since the cationpstates interact with the anti-bonding levels, it is vital for the cation on-site hybridisation that the anti-bonding levels have a strong component of the cationsstates. If there is a substantial cation spresence, then the mixing of the cationpstates can result in a strong stabilisation of the anti-bonding state. If, however, the anti-bonding states have only a weak contribution from the cation sstates, then the stabilisation will be significantly weaker. The electronic stabilisation must compensate for the reduced coordination in the distorted structures, and hence materials in which the anti-bonding states have only a weak cationscomponent will not form stereochemically active lone pairs; this is the case for most metal chalcogenides as shown in Table 1b.

The formation of stereochemically active lone pairs (distorted crystal structures) is therefore dependent on the strength of the interaction between the cationsstates and the anionpstates, and hence on their relative energies. The closer they are in energy, the stronger the interaction and the more cation states are present in the upper valence band, leading to an active lone pair effect. In fact, the cationsstates in these materials are lower in energy than the anionpstates and hence the most robust (with respect to formation of undistorted structures) lone pairs are found for oxides such as PbO, SnO, Bi2O3, and Sb2O3,i.e.those with oxygen anions in which the pstates are relatively low in energy. The corresponding atomic orbital energies are summarised in Fig. 4b. In moving down group 16 from oxygen to sulfur, selenium and tellurium, the anionpstates become higher in energy: as calculated at the DFT level of theory²²the orbital energies are!9.0 eV (O);

!7.0 eV (S);!6.5 eV (Se);!5.9 eV (Te);!5.6 eV (Po).

It should be noted that the largest increase is observed between O 2pand S 3p, and this is where the transition from distorted to symmetric structures generally occurs. The interaction of the anion p states with the cation s states is reduced significantly for heavier anions, leading to weaker on-site hybridisation of the cationsandpstates and weaker lone pairs. For the case of PbS, the sulfur anion 3pstates are already too high in energy to maintain the stereochemically active lone pair due to the extremely low energy of the Pb 6sstates arising from relativistic effects, while for Sn the 5sstates are higher in energy and the lone pair is maintained for SnS and SnSe. Tellurium has too weak an interaction with Sn 5sto form a lone pair distortion in its ground-state structure.

Waghmareet al.²³explored the effect of anionp–cations overlap in the distortion of the rocksalt structured chalco- genides of Ge, Sn and Pb, which follow the same trends discussed above. Similar arguments can also be made for the case of bismuth, with Bi2O3 and Bi2S3 showing stereo- chemically active lone pairs,²⁴while Bi2Se3and Bi2Te3do not, and for antimony, with Sb2X3{X = O, S, Se} displaying stereochemically active lone pairs and Sb2Te3not. In fact for the layered crystal structure adopted by Sb2Te3, Bi2Se3and Bi2Te3, the anionic lone pair becomes dominant.²⁵ Fig. 2 Illustration of the orbital interactions that lead to lone pair

formation in PbO (upper panel) and the corresponding energy level diagram (lower panel).

Published on 13 June 2011. Downloaded by Kyoto Daigaku on 20/08/2016 06:54:03.

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Walsh, A. et al., Chem. Soc. Rev., 2011, 40, 4455-4463.

Bi 6s Bi 6p

Anti-bonding

Bonding

O 2p

O-2p Bonding

Pb

: [Xe] 4f

5d

6s

Bi

: [Xe] 4f

5d

6s

Revised lone pair model for Bi₄NbO₈Cl

38

-10

-5 0 5

E n e rg y / e V

Density of states

Bi-6s Bi-6p

Ta-5p O-2p Cl-3p

Bi-6s + O-2p Bi-6p

Bi-6s

O-2p

Kunioku et al., J. Mater. Chem. A, 2018, DOI: 10.1039/C7TA08619A Kato et al., J. Am. Chem. Soc. 2017, DOI:10.1021/jacs.7b11497

シレン­アウリビリアス系の特異なバンド構造

39 Scaife D. E. Scaife, Solar Energy, 25(1980) 41

d⁰ d¹⁰ d

WO₃

2.6 eV

Potential / V SHE, pH=0

0 +1.0

+3.0

Bi

NbO

Cl +2.0

-1.0

C.B.

V.B.

H⁺/H2

O2/H2O

2. eV

2.23

Band gap / eV 1.0

0.8 0.6 0.4 0.2 0

−0.2

−0.4

−0.6

−0.8

−1.0

Flat band potential / V vs. SHE

SrNbO6

Sr0.5Ba0.5Nb2O6

Ba0.5La0.2Nb2O6

BaNb2O6

MgTi2O5

ZnNb2O6

Ba6Ti2Nb8O30

Ba0.8Ca0.2TiO3

LaNbO4

ZnO KNbO3

Nb2O5

BaSnO3

SnO2

CdIn2O4

In2O3

Ba0.67Ca0.33Nb2O6

CaTiO3

NiNbO6

NbO2

MgTiO3

SrTiO3

TiO2 TiNb2O7

WNb2O8

CrTi2O7

FeNbO4

Cd2SnO4

CdO FeTiO3

Fe2TiO5

Sr2FeNbO6

FeTaO4

FeTa2O6

Sr0.5Ba0.5TiO3

BaTiO3

CrNbO

4

U3O8

Fe2O3

2.0 2.5 3.0 3.5 4.0

La2Ti2O7

WO3

Bi

NbO

Cl

O-2p

O-2p

Photocatalytic H ₂ or O ₂ evolution on Bi ₄ NbO ₈ Cl

40

UV light (λ > 300 nm) Visible light (λ > 400 nm) 2.0

1.5

1.0

0.5

0.0 Amount of H2 evolved / µmol

18 15 12 9 6 3 0

Time of photoirradiation / h Potential / V

vs. SHE, pH=2

0 +1.0

+3.0 Bi₄NbO₈Cl +2.0

-1.0

C.B.

V.B.

H⁺/H₂

O₂/H₂O 2. eV

–0.28

2.11 ^Amo

unt of evolved O2/ µmol Amount of evolved H2/ µmol

Irradiation time / h Irradiation time / h

H₂evolution from MeOHaq. O₂evolution from Fe³⁺or Ag⁺aq.

40

30

20

10

0

8 6 4 2 0

Under visible light (l> 410 nm)

AgNO₃aq.

FeCl3aq.

光触媒活性評価〜Fe³⁺を電子受容体とする酸素生成反応〜

41 70 60 50 40 30 20

50

40 30 20 10 0

Amount of gas evolved/µ mol

8 6

4 2

0

Time/h BiOBr

Bi₄NbO₈Br

Am o u n t o f O

ev ol ved / μm o l

Time / h 2θ / degree (Cu Kα)

Bi₄NbO₈Cl Bi₄NbO₈Cl (After)

Bi₄NbO₈Br Bi₄NbO₈Br(After)

BiOBr BiOBr (After)

In te n si ty / a . u .

λ > 400 nm 5 mM FeCl₃aq.

FeCl₃ ( )

Photocatalyst 0.1 g 5 mM FeCl₃aq. 250 mL

(pH2.4)

BiOBr BiOCl

Bi₄NbO₈ClをO₂生成系とするZスキーム可視光水分解

42 e^-

h⁺

H⁺ H₂ e^-

h⁺

Fe³⁺

Fe²⁺

H₂O O₂

Bi

NbO

Cl

Ru/SrTiO

:Rh

Am o u n t o f O

ev ol ved / μm o l

Time / h H

O

Bi₄NbO₈Cl 0.1 g, Ru/SrTiO₃:Rh 0.15 g FeCl₃2mM (pH 2.4)

2 mM FeCl₃aq.

λ > 400 nm

Bi

NbO

Cl Z

200

150

100

50

0

16 12 8

4 0

Without redox

1) Konta, R.,et al. J. Phys. Chem. B2004, 108, 8992-8995.

H. Fujito, H. Kunioku et al., J. Am. Chem. Soc., 2016, 138, 2082

可視光水分解に向けた新たな材料設計指針

O-2p

ex. TaON, BiOX

Potential

X-np VB

e^-

Red.

Ox.

O

H

O

CB

N

, X

Sillen-Aurivillius

Cl-3p O-2p

CB

VB

e^-

O

H

O

ex. Bi

NbO

X

Red.

Ox.

Bi-6s

Financial Supports

•Nippon Sheet Glass foundation for Materials Science and Engineering

•JST-CREST

Acknowledgements

Lab. Members

• Dr. Masanobu Higashi

• Dr. Osamu Tomita

• Dr. Akinobu Nakada

• Dr. Hironobu Kunioku

• Mr. Hajime Suzuki

Kyoto University

• Prof. Hiroshi Kageyama

• Mr. Daichi Kato