Supplementary Information

Correlation between Ru-O Hybridization and

Oxygen Evolution Reaction in Ruthenate Epitaxial

Thin Films

Sang A Lee, a,b* Jegon Lee, a* Seokjae Oh, a Suyoun Lee, c Jong-Seong Bae, d Won Chegal, e

Mangesh S. Diware, f Sungkyun Park, g Seo Hyoung Chang, h Taekjib Choi, i Woo Seok Choia**

a Department of Physics, Sungkyunkwan University, Suwon 16419, Korea

b Department of Physics, Pukyong National University, Busan 48513, Korea

c Electronic Materials Research Center, Korea Institute of Science and Technology, Seoul 02792,

Korea

d Busan Center, Korea Basic Science Institute, Busan 46742, Korea

e Advanced Instrumentation Institute, Korea Research Institute of Standards and Science,

Daejeon 34113, Korea

f CeNSCMR and Department of Physics and Astronomy, Seoul National University, Seoul,

08826, Korea

g Department of Physics, Pusan National University, Busan 46241, Korea

h Department of Physics, Chung-Ang University, Seoul 06974, Korea

Electronic Supplementary Material (ESI) for Sustainable Energy & Fuels.This journal is © The Royal Society of Chemistry 2019

i Hybrid Materials Research Center, Department of Nanotechnology and Advanced Materials

Engineering, Sejong University, Seoul 05006, Korea

Experimental details

Thin films growth and characterization. High-quality epitaxial CRO thin films were grown on

atomically flat single-crystalline STO and Nb:STO substrates (CRO on Nb:STO was used for

electrochemical experiments) via PLE at 700 °C. An excimer laser (248 nm, Lightmachinery,

IPEX 864) with a fixed laser fluence of ~1.52 J/cm2 and a repetition rate of 2 Hz was used. We

systematically controlled the oxygen partial pressure from 0.1 to 100 mTorr during the film growth

in order to modify the cation ratio in the thin films. All the thin films were structurally

characterized using XRD (Rigaku, Smartlab & Pilatus (2D) detector).

Chemical stoichiometry. The chemical stoichiometry of the CRO thin films was studied at room

temperature using an XPS system (Alpha+, Thermo Fisher Scientific, UK) with a monochromated

Al-Kα X-ray source (hν = 1486.6 eV). The step size was 0.1 eV at a pass energy of 50.0 eV and

400 μm spot size. An NEC 6SDH pelletron accelerator with 3.0 MeV energy was used for RBS

measurement. He+2 was used as the source gas and the tilting angle was set to 5°.

Optical properties. The optical properties of the CRO thin films were investigated using a highly

surface sensitive tool, i.e., a spectroscopic ellipsometer (VUV-VASE Gen-II model, J. A.

Woollam, Co., Inc.) at room temperature. The optical spectra were obtained at photon energies

between 0.5 and 8.5 eV at an incident angle of 60°. A two-layer model (CRO thin film on STO

substrate) was sufficient for obtaining physically reasonable dielectric functions that reproduced

the optical spectrum of CRO in the literature.

Electrochemical measurements. OER activity was measured using a potentiostat (Ivium

Technologies) with a three-electrode set-up comprising a 3M NaCl saturated Ag/AgCl reference

electrode, Pt mesh counter electrode, and high-quality epitaxial CRO thin film as the working

electrode. The electrolyte (1.0 M KOH solutions) was prepared by mixing de-ionized water and

KOH flakes (Sigma Aldrich). All polarization curves were obtained by sweeping from 0.2 to 1.7

V vs. a reversible hydrogen electrode (RHE) at a sweep rate of 10 mV s–1. The sweep direction

was set to the larger positive potential.

0.25 0.26

0.75

0.76

0.77

0.78

0.79

STO

0.1 mTorr1 mTorr30 mTorr

1/d 0

01(Å

-1)

100 mTorr

CRO

0.25 0.261/d100(Å

-1)0.25 0.26 0.25 0.26

Figure S1. Reciprocal space maps of the CaRuO3 thin films grown at P(O2) = 100, 30, 1, and 0.1

mTorr, near the (103) Bragg plane in the SrTiO3 substrate.

123 126 129

270

180

90

Inte

nsity

(arb

. unit

s)

= 0

P(O2) = 100 mTorr

123 126 129

30 mTorr

2 (degrees)123 126 129

1 mTorr

0.1 1 10 1003.80

3.82

3.84

3.86

(a)

c a = b

P(O2) (mTorr)

c (Å)

a & b

(Å)

(b)

3.88

3.90

3.92

3.94

Figure S2. (a) Off-axis x-ray diffraction scans for the tetragonal CaRuO3 thin films around (204)

Bragg reflections from the SrTiO3 substrate with φ = 0, 90, 180, and 270°. (b) Evolution of the

lattice parameters as a function of P(O2).

352 348 344

2p1/2

2p3/2

Inte

nsity

(arb

. unit

s)

100 mTorr 30 10 1 0.1

Ca 2p

290 285 280 275

Ru 3d

satellite

3d3/23d5/2

532 528

O 1s

CRO

Vo

(a) Ca 2p Ru 3d O 1s

Ca 2p Ru 3d O 1s

352 348 344

(b)

Inte

nsity

(arb

. unit

s)

2p3/22p1/2

290 285 280 275

Binding Energy (eV)

3d5/23d3/2

satellite532 528

CROV0

Figure S3. X-ray photoelectron spectroscopy for the Ca 2p core-level, Ru 3d core-level, and O 1s

spectra for CaRuO3 thin films grown at different P(O2) values (a) before and (b) after conducting

cyclic voltammetry measurements, respectively. We could not find noticeable change in the thin

film stoichiometry, even after the OER measurements.

Table S1. The atomic concentration determined from Rutherford backscattering spectroscopy.

P(O2)(mTorr) Ca Ru O Ca/Ru

100 1.03 0.93 3.04 1.11

10 0.99 1.02 2.99 0.97

1 1.12 0.88 2.93 1.37

23.0 23.5

Inte

nsity

(arb

.unit

s) 100mTorr_sub.FWHM: 0.0207

23.0 23.5

30mTorr_sub.FWHM: 0.0240

23.0 23.5

10mTorr_sub.FWHM: 0.0405

(degrees)23.0 23.5

1mTorr_sub.FWHM: 0.0240

23.0 23.5

0.1mTorr_sub.FWHM: 0.0225

23.0 23.5 24.0

(d)(c)(a)

(b)100mTorr_filmFWHM: 0.0213

Inte

nsity

(arb

.unit

s)

23.5 24.0

30mTorr_filmFWHM: 0.0253

23.5 24.0

10mTorr_filmFWHM: 0.0451

23.5 24.0

1mTorr_filmFWHM: 0.0266

23.5 24.0

0.1mTorr_filmFWHM: 0.0268

44 46 48 50

Inte

nsity

(arb

. unit

s)

CRO(

002)

*

2 (degrees)0.4 0.8 1.2 1.6

P(O2) (mTorr) 100 30 10 1 0.1

2 (degrees)

0 1 2 3-10

0

10

Height

(nm

)

Distance (m)

0 1 2 3-10

0

10

Heigh

t (nm

)

Distance (m)

0 1 2 3-10

0

10

Height

(nm

)

Distance (m)

Figure S4. X-ray diffraction in epitaxial CaRuO3 thin films grown on Nb:SrTiO3 substrates with

rocking curve measurements, showing the same crystalline quality as the thin films grown on

SrTiO3 substrates. Nb:SrTiO3 substrates were employed as the bottom electrode for

electrocatalytic measurements. (a) XRD θ-2θ scans for the epitaxially strained CaRuO3 thin films

grown at various P(O2) values around the (002) Bragg reflections from the SrTiO3 substrates

(denoted with *). Representative rocking curve measurements for the (002) Bragg refection from

(b) the CaRuO3 film with different P(O2) and the Nb-SrTiO3 substrate. (c) XRR results show that

CaRuO3 thin films with different P(O2) have thickness of ~30 ± 3 nm. (d) Topographic image and

line scans of the CaRuO3 films grown at 100, 10, and 0.1 mTorr. The scale bar is 3 μm.

0.01 0.1 1 10 100 P (O2) (mTorr)

Solution

Desolved Ca atom Desolved Ru atom

1 M KOH in solution

0

1

2

3

4

5

6

7

8

Atom

ic co

ncen

tratio

n (p

pm)

Figure S5. Atomic concentration of dissolved elements in the KOH solution after conducting

cyclic voltammetry measurements obtained by inductively coupled plasma-optical emission

spectrometry. The result indicates that the OER measurement does not strongly affect the

concentration of dissolved species in the solution. In particular, no noticeable Ru dissolution was

observed during the OER measurements.

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6-100

0

100

200

300 100 mTorr 30 10 1 0.1

Curre

nt d

ensit

y(A

/cm2 )

Potential(V vs. RHE)

P(O2) =

Figure S6. Cyclic voltammetry measurement for the CaRuO3 thin films with various P(O2). The current densities are normalized by the geometric surface area. (Full scaled figure 4 (a))

P(O2)(mTorr)

RF (roughnessfactor)

Overpotential (V) @ J (150μA cm-2)

Specificcurrent density (μA cm-2)

@ η= 0.17 (V)

Tafelslope(mVdecade-1)

100 1.000 - 90.76 174.26

30 1.000 0.183 132.19 122.46

10 1.001 0.147 203.20 120.24

1 1.070 0.150 188.54 158.37

0.1 1.073 0.156 175.74 162.70

Table S2. The surface roughness factor (RF), overpotential value at 150 µA/cm2, specific current

density at 0.17 V for overpotential, and Tafel slopes of epitaxial CaRuO3 thin films grown at

different P(O2).

0 2 4 6 8 10 12 140

1

2

3

4

5

6

7

8

Experimental data Simulation

P(O2) = 100 mTorr 10 0.1

-Z'' (

kcm

2

Z' (kcm20.1 1 10 100

10

12

14

16

R1(b)

(c)

R 2 (kcm

2 )

P(O2) (mTorr)

In onset potential( 1.35 V vs. RHE)

(a)

C1

R2C2

Figure S7. (a) Nyquist plot of electrochemical impedance spectra of the CaRuO3 thin films with

different P(O2). (b) Equivalent circuit in the impedance simulation and (c) the value of R2 in

equivalent circuit as a function of P(O2).

1.4 1.6 1.8 2.0

1.30

1.32

1.34

1.36

162.70 mV s-1

158.37 mV s-1

120.24 mV s-1122.46 mV s-1

Pote

ntial

(V vs

. RHE

)

Log(Current density(A/cm2))

P (O2) (mTorr) = 100 30 10 1 0.1

174.26 mV s-1

Figure S8. Tafel plots of epitaxial CaRuO3 thin films grown at different P(O2) values. All the

films are normalized to the catalyst surface area.

54

56

58

60

0.1 1 10 1000.12

0.15

0.18

0.21

Over

pote

ntial

(V)

120

140

160

180(b)

Tafe

l slop

e (m

V de

cade

-1)

P(O2) (mTorr)

(a)

WS (

eV2 )

1.00 1.05 1.10 1.15 1.200.12

0.15

0.18

0.21

Ca/Ru ratio

Over

pote

ntial

(V)

55

56

57

58

59

60

WS (

eV2 )

Figure S9. OER catalytic activity with modified electronic structure in CaRuO3 thin films. (a)

Overpotential around 100 μA/cm2, Tafel slopes, and spectral weight of the CaRuO3 thin film at the

B peak as a function of P(O2). The OER activity shows the same trend as the charge transfer

transition. (b) Overpotential and charge transfer transition for O 2p → Ru 4d eg as a function of

the cation ratio. The relationship between overpotential and WsB also show the same tendency,

indicating that the OER activity is enhanced according to the decreased hybridization strength.