Supporting Information

Iron Oxide Photoelectrode with Multidimensional Architecture forHighly Efficient Photoelectrochemical Water SplittingJin Soo Kang, Yoonsook Noh, Jin Kim, Hyelim Choi, Tae Hwa Jeon, Docheon Ahn,Jae-Yup Kim, Seung-Ho Yu, Hyeji Park, Jun-Ho Yum, Wonyong Choi, David C. Dunand,Heeman Choe,* and Yung-Eun Sung*

ange_201703326_sm_miscellaneous_information.pdf

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Experimental Section

Preparation of AFF Photoelectrodes and Their Applications in Photoelectrochemical

Water Spitting: Slurry was prepared by mixing deionized water with 14.1 vol% iron oxide

powder (mean particle size < 5 m, purity ≥ 99%, Sigma-Aldrich) and 4 wt% binder

(polyvinyl alcohol, Mm = 89,00098,000, purity 99+%, Sigma-Aldrich). In particular, slurry

dispersion was assisted by a stirring and sonication process. The slurry was then poured into a

Teflon mold (28 mm inner diameter, 70 mm height) placed onto a Cu rod cooled using liquid

nitrogen. The temperature of the top surface of the Cu rod was fixed at -15 ˚C and was

controlled by a heater. After freezing, the frozen slurry was dried at a temperature of -90 ˚C

and pressure of 5 mTorr for 48 h. The green body was reduced and sintered in a tube furnace

in a 5% H2–95% Ar gas mixture with a heating rate of 5 ˚C min-1. The reduction was carried

out by using a double step process at 300 ˚C for 2 h and then at 500 ˚C for 2 h, whereas

sintering was carried out by using a single step process at 950 ˚C for 14 h. The Fe foam was

cut and polished into a uniform thickness of 500 m, and was potentiostatically anodized

with the assistance of ultrasonication at 80 V for 5 min at 25 ˚C using an ethylene glycol

electrolyte containing 0.25 wt% NH4F and 2 vol% H2O. Anodized samples were carefully

washed with deionized water and ethanol, and were used as photoanodes after thermal

annealing at 500 ˚C for 4 h to increase crystallinity.

Characterization and Physical Measurements of Materials: XRD data was measured

using a Rigaku D-MAX2500-PC. SEM images were taken with Carl Zeiss AURIGA, and

TEM analysis was performed using JEOL JEM-2100F. Absorbance, transmittance, and

reflectance were measured by using a UV-Vis-NIR spectrophotometer (V-670, Jasco). Cell

performances were characterized using a solar simulator (Oriel) at AM 1.5G condition (light

intensity: 100 mW cm-2), which was adjusted by the Si reference cell certified by the National

Institute of Advanced Industrial Science and Technology (AIST, Japan). A potentiostat

Autolab PGSTAT128N was used for J-V measurements of AFFs, which were carried out in a

three-electrode system with a Pt mesh counter electrode and a Ag/AgCl reference electrode at

a constant scan rate of 50 mV s-1. Standard 1 M NaOH solution was used as the electrolyte

after 30 min of Ar purging. IPCE spectra were obtained by using a monochromator (Newport

74125). Sheet resistance was measured by a four-point probe (CMT-SP 2000N, AIT). GC

analysis was performed with Agilent HP6890A.

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Table S1. Electrolyte compositions used for the fabrication of nanostructured anodic iron

oxide displayed in Figure 2a (Electrolyte 1), 2b (Electrolyte 2), 2c (Electrolyte 3), and 2d

(Electrolyte 4).

Electrolyte 1 Electrolyte 2 Electrolyte 3 Electrolyte 4

NH4F (wt%) 0.125 0.250 0.500 1.000

H2O (vol%) 1.0 2.0 3.0 4.0

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Table S2. Parameters and reliability factors obtained by Rietveld refinements of XRD

patterns displayed in Figure 4a.

Before reaction After reaction

Phase hematite magnetite hematite magnetite

Weight fraction (%) 57.71 42.49 60.14 39.86

Molar fraction (%) 66.43 33.57 68.63 31.37

Lattice parameters (Å ) a: 5.03

c: 13.73 a: 8.40

a: 5.03

c: 13.73 a: 8.39

Reliability factors

Rp: 8.42

Rwp: 11.4

Rexp: 6.87

Rp: 8.13

Rwp: 10.9

Rexp: 6.83

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Figure S1. (a,b) High-magnification SEM images of iron oxide nanoflakes on the surface of

AFFs.

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Figure S2. (a) SEM image and elemental EDS mapping results for (b) oxygen and (c) iron at

the innermost region (near the center) of AFF.

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Figure S3. High-magnification SEM image of the AFF’s surface.

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Figure S4. (a) Spectral intensity of incident light in the photoelectrochemical measurements

displayed with the standard AM 1.5G spectrum. (b) Transmittance spectrum of the AM 1.5G

filter.

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Figure S5. J-V characteristic of the AFF photoanode with error bars.

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Figure S6. Transmittance spectrum of AFF in UV-visible light region and digital photograph

images of pristine Fe foam and AFF with spot-welded Fe rod in charge of electrical contact

during the anodization and photoelectrochemical water splitting.

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Figure S7. XPS (a) survey and (b) Fe 2p spectra of AFFs before and after

photoelectrochemical water splitting at 1.23 V vs. RHE for 8 h.

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Figure S8. High-magnification TEM images of AFF (a) before and (b-d) after the 8 h of

photoelectrochemical water splitting at 1.23 V vs. RHE.

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Figure S9. J-V characteristic of the AFF photoanode with Co oxygen evolution catalysts with

error bars.