Transcript 固液界面の構造とダイナミクス

Electrode Dynamics at
Platinum-Water Interface
Osamu Sugino
ISSP, University of Tokyo
Metal/water interface
• Hydrophibic/hydrophobic
– wet/repel
• Redox reaction
– rusting
• Catalysis
– fuel cell reaction
– electrolysis
Response to external field: water
• Large dipole moment
– free rotation
– screening er=78!
• H-Bond network
– 0.2eV (90% ionic, 10% covalent)
– retardation of ~ ps
– H3O+ diffusion (Grothus)
+0.35
−0.7
Response to ext. field: interface
• H-bond network disturbed
– water-metal interaction ~0.5eV
• Contact layer formed
– less mobile but not icy
– dipole layer
• potential drop: bias voltage
• inner Helmholtz layer
V
Response to ext. field: reaction
• Large field and dense surface charge
• Chemical reaction (redox)
– electron transfer
– reactive species formed
e−


e−
e−




First-Principles MD simulation
• Electrode dynamics @ anode in acid
e−
H

e−
e−
H


H+


H
H+






H+

reservoir
pH=0~1
Modeling
•MD (classical nuclei and adiabatic electrons)
•32 H2O + 36 Pt
•Direct simulation of ~10 ps
•DFT for electrons
•Bias up to ~ −0.8 V vs. SHE
e−
To apply bias












•Put excess e−
•Water screens within several ps
•analyze the contact layer
•see the reaction H3O++ e− H(ad)+H2O
Effective Screening Medium
M.Otani. and O.S., PRB 73, 115407 (‘06)
Embed interface slab in classical medium
DFT
metal
+
+
+
+
+
Continuum theory
+
water
－
+
－
+
+
－
+
+
－
+
+
－
+
－
+
－
water: er=78
ions: Poisson-Boltzmann
er
Continuum theory
DFT
－
+
metal water
+
－
+
+
+
+
 I ( r ),  e ( r )
－
+
－
+
－
－
+
－
－
+
+
+
－
+
－
+
+
 c (r )
er
－
e r (r )
•Kohn-Sham

1
2
 
2
m
( r )  V H ( r )  V XC   e ( r ) 
m
( r )  e m
•Poisson
 e r (r ) 

 V H ( r )     e ( r )   I ( r )   c ( r ) 
 4

•Poisson-Boltzmann continuum
 c ( r )   c V H ( r ), e r ,  ,  c ( 0 ) 
m
(r )
Large-scale simulation
• Supercomputers
• Simplest ESM modeling
– Capacitor model
– Classical ions (electrolyte ions) not included
Pt(111)/water interface
bulk water
Contact layer
Pt
Oxygen distribution function
bulk water
Contact layer
Pt
Contact layer formation
1 e− / 40 Pt
1 e− / 12 Pt
Distribution function f(z)
water density larger by 20 %
Top view
• last 2 ps
2D H-bond network
Summary of the structure
• Contact layer
– One molecular layer thick (~3 Å)
– ‘Bulky’ water: z > 3Å
– Water density depends on the bias
• H-bond network
– 2D network at the contact layer
• Screening of water (εr~10)
– Surface electrons are densely induced
H3O+ accepts an electron
Reaction H3O++e-H(ad)+H2O
Population
Red: positive
Blue: negative
relative to
charge
in the bulk
Adiabatic picture on charge transfer
Adiabatic picture on charge transfer
Level crossing
H3O+ LUMO
5d
V
Orbital energy
Total energy
Restructuring afterwards
“Reorganization”
After H adsorption
H2O with O-down
appears but
unfavorable
electrostatically
Reorientation
hampered by Hbond network
Jumping reorientation motion
0.0ps
1.8ps
H/Pt(111) at aqueous condition
Migrates almost freely (1.7 ps)
Summary
• New first-principles simulation of the
biased metal/water interface
• Microscopic details on Helmholtz layer and
reaction dynamics
• Water assists the reaction on Pt
• A step towards microscopic understanding
of electrochemistry
Thank you!
Acknowledgment
ES and ISSP Supercomputers
Collaborators
Minoru Otani (ISSP)
T. Ikeshoji (AIST), Y. Morikawa and I. Hamada
(Osaka U.), Y. Okamoto (NEC)
H/Pt(111) at vacuum
Kallen et al. PRB (2001)
H is trapped at on-top site
DOS projected to the H3O+ orbital
Transfer from 5d band to this orbital
遷移金属と水の相互作用(UHV)
ロジウム/水 相互作用
IRAS等による構造決定
(吉信研)
水の吸着エネルギーDFT計算
By S. Meng PRB (2004)
遷移金属/水界面＝接触層形成
V=−0.23V vs Vpzc
酸素up構造
V=+0.52V vs Vpzc
酸素down構造
M.F.Toney Nature (’94)
目的
• 電位がかかった金属/水界面の構造
– 水和構造の解明
• 接触層と水素結合網の形成
– 電気二重層の解明
• 電位と水の応答
• 高速な化学反応（化学・電気エネルギー変換）
– 水素発生、酸素発生のメカニズム
– なぜ白金か？水の役割は？
第一原理計算
液体水＝分子動力学計算
長い緩和時間→数ps
32 H2O + H
ＣＰＵ1-2週間＝1ps
•3 layer of Pt(111)
•12 Pt for each layer
3 2 3
電場をかける＝表面に過剰電子を配置
Put excess electrons
Water
conduction band
Pt
Water
valence band



水の分極と遮蔽
Water
conduction band


Water
valence band

Pt
イオン分布の変化→コンデンサモデル
Water
conduction band


Water
valence band

Pt
conductor
Capacitor model to mimic
role of the ions in solution
Effective Screening
Medium method
er=
M.Otani. and O.S., PRB 73, 115407 (‘06)
Embed slab in
dielectric continuum
Non-repeated slab embedded in a
dielectric continuum
Poisson equation:
Kohn-Sham equation:
Total energy expression
水の構造
• 負の電位を印加（負の表面電荷）
– 接触層の形成は？
– 水素結合網の形成は？
• ESM-FPMD (STATE)シミュレーション
Contact water layer
−0.04 e/Pt
−0.5 V
15
15
0 .2 5 < n < 0 .6 9
H d is trib u tio n
hydrogen
10
5
0
O d is tru b u tio n
0 .2 5 < n < 0 .6 9
oxygen
10
5
0
0
2
4
z (A )
6
8
0
2
4
z (A )
6
8
2D H-bond network in the
contact layer
−0.08 e/Pt
−1.0 V
化学反応性のシミュレーション
• ヒドロニウムイオンの導入
• 表面からの引力
• 接触層へ到達
• 電子移動&プロトン移動反応→水素吸着
– H3O++e−→H2O+H(ad)
• 水素の表面拡散→会合脱離
– 2H(ad)→H2
Snapshot
0.0ps
~
0.5ps
~
1.4ps
~1.5ps
Reaction intermediate
Excess charge & Dipole moment & Pt-H distance
Reaction intermediate
4-fold coordinated H3+!
Reaction intermediate
The Volmer step
Electronic structure
How does the electron transfer?
DPopulation (isosurface)
−0.70
Population analysis
+0.35
： Population
Electron deficit
Excess electrons
DOS projected to the H3O+ orbital
Transfer from 5d band to this orbital
After the reaction
Water-assisted efficient diffusion of H
水が反応を促進している
1. Proton-relay via H-bond network
• H+ efficiently reaches the contact layer
and the reaction site
2. Polarization of water (ε=10-20)
• Large surface electron density prompts
reduction reactions
3. Water-assisted fast surface diffusion
これからの課題
• ESMの改良
– イオンによる遮蔽効果
• 酸素極での反応
– 多数の経路
• 白金の特異性
– 卑金属、酸化物
• 非断熱計算
– TDDFT
• 大規模化・超並列化＝metal O(N)法
http://www.lsbu.ac.uk/w
ater/hbond.html
Electrode Dynamics
• Non-equilibrium response of water to
– existence of metal surface
– application of bias potential