RTD 2013SHINE

Electronic Structural Origin of Efficiency Enhancement from Underlayer in Hematite Photoanode

1 Laboratory for High Performance Ceramics, EMPA-Swiss Federal Laboratories for Materials Science & Technology, Switzerland2 Laboratory for Photonic and Interfaces, Ecole Polytechnique Federal de Lausanne, Switzerland3 Department of Chemistry, University of Basel, Switzerland

Yelin Hu1,2, Florent Boudoire1,3, Matthew Mayer2, Michael Graetzel2, Artur Braun1

Hematite (α-Fe2O3) is an important photoanode material capable of high quantum conversion efficiency in the oxygen evolution reactionupon water splitting. However, one primary limitation for the application of ultrathin hematite layer is the slow transportation of photoexci--ted electrons to an external circuit, increasing efficency losses during operation. It has been shown that these losses can be limited byimplementation of metal oxide interfacial layers, such as Nb2O5, SnO2, SiOX and SnO2, between conducting substrate and ultrathin hem--atite layer. However, the detailed mechanism for electrode activation is still unknown. In the present study, synchrotron-based X-rayspectroscopy is used to study the electronic structure origins of interfacail enhancement in metal oxide/hematite photoanodes.

Introduction

Preparation & Photoelectrochemical Analysis

1 10 100 1000 10000 1000000

-20

-40

-60

-80

thet

a

Frequency (Hz)

FTO-Fe2O3

dark, 0V

light, 0.3V

16 18 20 22 24 26 28 300

-5

-10

-15

-20

-25

晻Z'' (Ω

)

Z' (Ω)

FTO-Fe2O3

dark, 0V

light, 0.3V

晻 500 1000 1500 2000 2500 3000 3500 4000晻

-1000

-2000

-3000

-4000

-5000

Z'' (Ω

)晻

Z' (W)

Ultrathin Hematite: ~ 20 nm by spray pyrolysis

0.4 0.6 0.8 1.0 1.2 1.4 1.6

0

2

4

6

RIn (Ω

)

Potential vs RHE (V)

SnO2-Fe2O3

FTO-Fe2O3

Nb2O5-Fe2O3

RIn

0.4 0.6 0.8 1.0 1.2 1.4 1.60.0000

0.0002

0.0004

0.0006

Ctr

ap (F

cm

-2)

Potential vs RHE (V)

SnO2-Fe2O3

FTO-Fe2O3

Nb2O5-Fe2O3

Ctrap

540 538 536 534 532 530 528

CPS

(-)

Binding Energy (eV)

SnO2-Fe2O3

SnO2

540 538 536 534 532 530

CPS

(-)

Binding Energy (eV)

Nb2O5-Fe2O3

Nb2O5

540 538 536 534 532 530 528

CPS

(-)

Binding Energy (eV)

FTO-Fe2O3

FTO

0.5

O2-/O

H- (

-)

RS

RIn

CIn

Cbulk

Rtrap

Rct,trap

Ctrap

FTO Fe2O3 H2O

light

SnO2: ~ 3 nm, Nb2O5: ~ 5 nm by ALD

SnO2-Fe2O3 Nb2O5-Fe2O3 FTO-Fe2O3

Onset (VRHE) 0.88 0.97 1.06

Onset potential shift to cathodic direction with SnO2 and Nb2O5 underlayer.The EIS feature at high frequency relates to charge transfter on substrate-hematite interface. Onemore Randle Circuit added for EIS analysis.Interfacial-charge-transfer resistance decreases after implementation of SnO2 and Nb2O5 underlayer.

Kelvin probe suggests increased interface band bending by underlayer, improving charge transferon substrate-hematite interface.

SnO2-Fe2O3 Nb2O5-Fe2O3 FTO-Fe2O3

∆φ (eV) 0.92 0.83 0.79

∆φ by Kelvin probe spectroscopy

Hematite Samples for Interface Analysis

500 400 300 200 100 00

50000

100000

150000

200000

Sn MNN

Fe 3p

Sn 3d3/2

CPS

(-)

Binding Energy (eV)

Sn 3d5/2

Sn 4d

O 1s

C 1s

Si 2s

Si 2p

FTOFTO-Fe2O3

XPS & NEXAFS studied in ALS BL 9.3.2

Hematite with gradient thickness prepared to identifysubstrate-Fe2O3 interface

O 1s XPS & NEXAFS

1.08

1.81.53SnO2

SnO2-Fe2O3

Nb2O5

Nb2O5-Fe2O3FTO FTO-Fe2O3

O 1s core level XPSCrystal lattice O increases when hematite deposited on the substrates, especially for the ones withmetal oxide underlayers, suggesting that SnO2 and Nb2O5 improve crystallinity of ultrathin hematite.It matches SEM results.

520 525 530 535 540 545 550

晻N

EXA

FS A

bsor

ptio

n

Photon Energy (eV)

SnO2-Fe2O3

SnO2

Fe 3d t2g eg

O2-

OH-

O2-

OH-

O2-

OH-

524 525 526 527 528 529 530 531 532

晻

NEX

AFS

Abs

orpt

ion

Photon Energy (eV)

SnO2-Fe2O3

Nb2O5-Fe2O3

FTO-Fe2O3

t2g eg

Unoccupied states filledwith underlayers

Crystal-field splitting enhancedwith underlayers

O k edge NEXAFS

FTO-Fe2O3SnO2-Fe2O3

FWHM of fitted t2g peak 0.9258

Nb2O5-Fe2O3

1.56 1.6

Unoccupied O p states below the lowest unoccupied Fe d orbitals (features around 526.5 eV) are presented by FWHM of t2g peak, and it shows that they are eliminated with metal oxide underlayer,suggesting reduced displacement of Fe3+ ions in the corundum structure and improved crystallinity.Features around 528.3 eV represent crystal-field splitting of hematite. The broadening of Fe 3d-O 2phybridized states reduced with metal oxide underlayer, leading to improved p-d orbital hybridization.

Conclusion

Interface of metal oxide/ultrathin hematite was studied. Two metal oxides chosen: SnO2 & Nb2O5

Impedance spectroscopy suggests improved interfacial charge transfer on substrate/hematite inter--face. It may be attributed to stronger band bending on interface.

O 1s XPS shows improved crystallinity and reduced oxygen vacancy with implantation of underlayer

Metal oxide underlayers have been shown to improve the degree of p-d orbital hybridization.

0.8 1.0 1.2 1.4 1.60.0

0.5

1.0

1.5

2.0

2.5

Phot

ocur

rent

Den

sity

(mA

/cm

2 )

Potential vs RHE (V)

SnO2-Fe2O3

Nb2O5-Fe2O3

FTO-Fe2O3