Finite Size Effect of Proton

Conductivity of Sol-Gel Derived

Amorphous Aluminosilicate Thin Film

Y. Aokia, H. Harada, H. Habazakia, T. Kunitakeb

a Faculty of Engineering, Hokkaido Univ.

b Institute of Physical and Chemical Research, RIKEN

1. Background

2. Thickness dependency of proton conductivity of a-Al0.1Si0.9Ox thin films with various Al/Si composition

3. Rondom resistor network model of conductivity scaling in a-Al0.1Si0.9Ox thin films

4. Mechanism of finite size effect of the proton conductivity

5. Summary.

ContentsContents

For intermediate temperature fuel cellFor intermediate temperature fuel cell

0.000 0.001 0.002 0.003

0

-1

-2

-3

-4

-5

Log (

σ/

S c

m-1

)

T-1 / K-1

T. Norby, Solid State Ionics 125 (1999) 1.

Target

Amorphous Al0.1Si0.9Ox

Thin filmThin film

(Y. Aoki, et al, Electrochem. Solid State Lett., 2008, 11, P13)

100 200

T/°C400

1. Thermally and chemically stable, and proton conductivity by the presence of Brønsted acid site on the anionic framework

2. Amorphous materials rather suitable for fabricating gas-tight thin electrolyte due to the non-granular form

regime-iii

iiiiv

Size enhancement of proton conductivitySize enhancement of proton conductivity

No percolationPercolationScalingSaturating

ζ

Maximum cluster

d >> ζd ~ ζλ < d < ζd < λ

In aperiodic phase, there is a

random distribution of

protonic pathway because of

structural inhomogeneity

Fast

Slow

Conduction is peroclative!

Y. Aoki et al., JACS, 133, 3471, 2011.; PCCP, 14, 2735, 2012

250°C,

Al0.1Si0.9Ox

Objective

We conducted here a survey of the proton conductivity

and the microstructures for the amorphous

aluminosilicate thin films by changing the Al/Si

composition in order to clear the structure giving

inhomogeneity of conductivity in nm length scale.

Synthesis of AlnSi1-nOx thin film by multiple spin-castSynthesis of AlnSi1-nOx thin film by multiple spin-cast

Hydrolysis

OH

OH

OHOH

OH

OH

OH

OH

Pt

Precursor sol:Al(OsBu)3 + Si(OEt)4 in 1-PrOH with Al/Si = 1/9;30 or 100 mM

Deposition

450°C

10 min

3 times

Al0.1Si0.9Ox

thin film(20-1400 nm)

Cooling

Pt/Sisubstrate

Anodized at 0.5 V (SCE) in 5 M H2SO4

at 40°C

O

MO

OO

OOO

O

O

OOO

O

OM

M

M

O

R

R

R

R

R

ROH

MOH

OHO

OOO

O

O

OHOO

OH

OHM

M

M

OH

Humidified, hot air

3000 rpm20 sec

2 , 5nm /

one spin.

Layer-by-layer spin-cast process is a stepwise film deposition technique from a precursor solution and enables the fabrication of metal oxide film with nm thickness precision.

REPEAT several times

Morphology of a-AlnSi1-nOx thin filmsMorphology of a-AlnSi1-nOx thin filmsIn this study, the a-AlxSi1-xO2-α nanofilms were fabricated on ITO substrate by the surface sol-gel process in a mode of spin-coating.

Substrate(Al plate) 50 nm

2 nm

Void-free, densely-packed layer of amorphous oxide

Al0.1Si0.9Ox film(100 nmd)

Al/Si (by EDX)Notific.

7 / 93

10 / 90

20 / 80

29 / 71

46 / 54

Al7Si93

Al10Si90

Al20Si80

Al30Si70

Al45Si55

Conductivity across filmConductivity across film

0.1 kHz

2 kHz

in dry air: obs.: calc.

Rs

Qox

Rox

Qi

220°C

250°C

ITO layer

Al20Si80

100 nm

Pt

Solartron

Pt

a-Al0.1Si0.9Ox film (120 nmd)a-Al0.1Si0.9Ox film (120 nmd)

50

40

30

20

10

0

cell

volt

age

at

400

/ m

V

- 1.6 - 1.2 - 0.8 - 0.4 0.0

ln([H2]Pt/ [H2]Pd)

theoretical Al45Si55 Al7Si93

[ ][ ] H

Pd

H

Pt

Pd

Pt

F

RT

H

H

F

RTEMF

γ

γ

2ln

22

2 −−=

∆ = 29.0

∆ = 25.0

∆ = 29.4

Pd

Al m

Si n

Ox

Pt 100%

H2

20 ~ 80%

H2

/ Ar

Al7Si93

Al45Si55

Thickness-dependent σ of a-AlnSi1-nOx filmThickness-dependent σ of a-AlnSi1-nOx film

Al7Si93 Al20Si80

Al30Si70 Al45Si55

Al7Si93

Al10Si90

Al20Si80

Al45Si55

Al30Si70

Low Al/Si films reveal the Low Al/Si films reveal the

thickness dependent thickness dependent

conductivityconductivity

250°C,Dry air

0.7 eV

0.8 eV

0.8 eV0.9 eV

Random resistor network calculationRandom resistor network calculation

R1

R2

n

1V

electrode

200 x 200 x n resistor network

R1: poor conductive matrixR2: ion-conductive matrixλ: size of matrixF2: fraction of R2 (<0.249)

G[i][j] = 1/R1 or 1/R2

ii-1 i+1

i-n

i+n

i+nm

i-nm

G[i][i-1]

G[i][i-nm]

G[i][i+1]

G[i][i-n]

G[i][i+n]

G[i][i+nm]

V(i-1)

V(i-nm)

V(i-n)

V(i+1)

V(i+nm)

V(i+n)

V(i)

y z

x

Kirchhoff’s law at each lattice point

( ) 0=−∑j

jiij VVG

G[i][i-1] [V(i-1)-V(i)] + G[i][i+1] [V(i+1)-V(i)]

+ G[i][i-n] [V(i-n)-V(i)] + G[i][i+n] [V(i+n)-V(i)]

+ G[i][i-nm] [V(i-nm)-V(i)] + G[i][i+nm] [V(i+nm)-V(i)] = 0

Kirkpatrick, S. Rev. Mod. Phys. 1973, 45, 574.

Berkemeister, F. et al., Phys. Rev. B 2007, 76, 024205.

Solving the large sparse matrix of

40000n × 40000n.

Fitting by RRN modelFitting by RRN model

R1: low-resistance pathR2: high-resistance pathλ : unit length of pathF2: fraction of R2

R1

R2

n

1V

electrode

200

200

Al10Si90

Al7Si93

250°C,Dry air

Al7Si93 Al10Si90

R1 / Ω 3.3×1012 6.3×1010

R2 / Ω 2.6×1010 2.2×108

λ / nm 30 40

FR2 0.011 0.092

Length of the conduction path unit is related to some meso-structure.

HRTEM of a-AlnSi1-nOx thin filmsHRTEM of a-AlnSi1-nOx thin films

The condensed domains (dark) with 5-20 nm (~λ)

distributed in the uncondensed matrix (bright).

The condensed domains connected each other to form

consecutive network throughout the film

Al7Si93

Al10Si90

Al20Si80

Al30Si70 Al45Si55

Fcon = 0.18

~ FR2

Fcon = 0.29

Fcon = 0.47 Fcon = 0.53

> Pc (0.254)

Dark Bright

12.1 ± 3.0 11.0 ± 2.0

Difference in density

due to fluctuation of

glass network

Dark Bright

35.5 ± 2.5 30.3 ± 3.9

Al7Si93 uncountable

Al10Si90 0.18

Al20Si80 0.29

Al30Si70 0.47

Al45Si55 0.53

Al/Si < 10

filmF

con

Pc = 0.249 Al/Si > 20

Highly-conducting path

can be identical to the

condensed glassy

matrix

Percolation mechanismPercolation mechanism

Size scaling

Percolation

Condensed glass domain

SummarySummary

The amorphous AlnSi1-nOx thin films prepared by sol-gel

process reveal the percolative proton conductivity because of formation of the conductive high-density glass domain and the poor-conductive low-density domain by the density fluctuation of glass network.

The bulk conductivity of these films is critical to the

concentration and the conductance of the conductive, condensed microdomains. The concentration is proportional to the Al/Si molar ratio and is beyond critical threshold at Al/Si > 20/80, however the conductance is decreased with increasing the Al/Si ratio by the structural modification of an aluminosilicate glass framework.

AcknowledgementsAcknowledgements

This work was financially supported by the Grant-in-Aid of JSPS for Scientific

Research on Young Scientists (B) and by the Global COE Program (Project

No. B01: Catalysis as the Basis for Innovation in Materials Science) from the

Ministry of Education, Culture, Sports, Science and Technology, Japan.

Ionic conduction in amorphous MxSi1-xO2-α nanofilmsIonic conduction in amorphous MxSi1-xO2-α nanofilms

10-2

10-1

100

101

102

103

104

105

106

RA

S / Ω

cm

2

3.02.82.62.42.22.01.81.61.4

103 T

-1 / K

-1

Ge

La

Ti

Zr

Ta

Ce

Hf

Al

W

VSi

400 300 200 100ºC

Ba

FeSn

0.15 Ω cm2

• The strong-acid compounds, M = Al, Hf, Zr and Ti, show the ASR < 0.15 Ω cm2 at T < 400°C in dry air.

• Al0.12Si0.88O1.91 achieve the ASR < 0.15 Ω cm2 at around 300°C.

• Proton conduction in silicate films increases in proportional to the acid strength.Al < Hf < Zr ≈ Ti < Fe ≈ Ta < Sn ≈ Ce < W ≈ La < V ≈Ge < Ba ≈ Si

300°C

in dry air

Aoki, Y. et al. Adv. Mater. 2008, 20, 4387.

Conductivity scaling behaviorConductivity scaling behavior

Hf0.1Si0.9Ox@300°CHf0.1Si0.9Ox@300°C

Al0.1Si0.9Ox@250°C

(power-law scaling)

Al0.1Si0.9Ox@250°C

(power-law scaling)

Hf0.25Si0.75Ox@300°CHf0.25Si0.75Ox@300°C

TDS of HDO in D2O-treated filmTDS of HDO in D2O-treated film

Adsorbed

water

50 nm

500°C

120 nm

300 nm

SiOx

(300 nm)

300°C

cv

Thermally-stable

protonic carriers

of anhydrous

conductivity

Pretreatment: 200°C 6h in D2O/air10-6 Pa, 30 K min-1.

Pretreatment: 200°C 6h in D2O/air10-6 Pa, 30 K min-1.

The protonic

carrier in a-AlSiOx

films is not silanol

group

Conduction pathConduction path

The network made of cross-linking of

Brønsted acid centers may allow the fast

proton transport so as to be the high

conductive pathway.

(i) Local segrgation of silica moiety

and (ii) termination by OH group

disturb the development of pathway

(acid network), lowering the

conductivity.

Chemical diffusion constantChemical diffusion constant