Supporting Information

Highly Ordered ZnO/ZnFe2O4 Inverse Opals with Binder-free Heterojunction Interfaces for High-performance Photoelectrochemical

5 Water Splitting Tingting Yang,a+ Jiawei Xue,b+ Hao Tan,b Anjian Xie,a Shikuo Li,a* Wensheng Yan,b Yuhua Shena*

[a] Lab of Clean Energy & Environmental Catalysis, School of Chemistry and Chemical Engineering, Anhui University, Hefei 230601, China

10 [b] National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, 230029, P.R. China[+] These authors contributed equally to this work.

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX

15 DOI: 10.1039/b000000x

Experimental Section

Synthesis of monodisperse polystyrene spheres

Monodisperse polystyrene (PS) latex spheres were synthesized by emulsifier-free emulsion copolymerization technique

20 according to the literature[1] using styrene as polymerized monomer, and potassium persulfate (KPS) as initiator.

Styrene (210 mL) was washed in a separatory funnel four times with 200 mL of 0.1 M NaOH, then four times with 200

mL of water. The synthetic process was modified in our laboratory. In a typical synthesis of monodisperse PS latex

spheres with 300 nm diameter was as follows: Polymerization was conducted in a 250 mL reaction flask by magnetic

stirring (350 rpm) under a nitrogen atmosphere. Firstly, 100 mL deionized water was added to a three-neck flask.

25 Nitrogen was bubbled in the flask to blow away air. After 30 min, 6.2 mL prewashed styrene was added to the flask.

Then, the reaction flask was heated to 75 °C, 0.1 g prepurified KPS was added. After the reaction of 24 h, the flask was

removed from the oil bath. The subsequently formed white latex was filtrated through glass filter to remove aggregates,

and centrifuged, washed with deionized water for several times, finally the precipitate dispersed into water for use.

Hydrogen and oxygen evolution measurements

30 The photogenerated hydrogen and oxygen by PEC water splitting was performed in an air-tight reactor connected to a

closed gas circulation system (Beijing China Education Au-light Co., Ltd). Prior to measurements, the cell

compartments were carefully sealed with rubber septa and glycerin to prevent any gas leakage and then Ar-purged for

1 h. During measurements, the as-prepared of ZnO/ZnFe2O4 sample used as photoanodes were biased at 1.23 V vs RHE

in a stirred aqueous solution of 0.1 M Na2SO4 under AM 1.5G simulated sunlight. The amounts of hydrogen or oxygen

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2017

2

were determined by gas chromatography (GC-3240) equipped with a thermal conductivity detector (TCD), and

ultrahigh purity argon (Linde, 99.9995%) as carrier gas. The Faradaic efficiency of the as-prepared of ZnO/ZnFe2O4

sample can be calculated from hydrogen and oxygen evolution

5 measurements by the following formula:

where z is the number of electron gain-loss (for example, 2H+ back to H2, z is 2), n is amount of substance of H2 and

O2 (mol), F is the Faraday constant (96500 C/mol), and Q is the total charge.

10

Figure S1. (a-d) Different magnification SEM images of PS template, the inset is the photo of the PS film.

100%% /Faradaic efficiency znF Q

3

Figure S2. SEM image of ZnO inverse opal prepared by removing the PS template.

Figure S3. SEM images of ZnO/ZnFe2O4 inverse opal samples prepared with different dip coatings (a, b) 30 cycles, and (c,

5 d) 120 cycles.

4

Figure S4. EDX spectra of the typical ZnO/ZnFe2O4 inverse opal sample prepared with 60 dip coating cycles.

1020 1030 1040 1050 1060

Zn 2p3/2

1045.6 eV

ZnO ZnO@ZnFe2O4

Inte

nsity

(a.u

.)

Binding energy (eV)

Zn 2p1/2

1022.4 eV

730 725 720 715 710 705

Fe 2p3/2

711.4 eV

Sat.718.7 eV

Inte

nsity

(a.u

.)

Binding energy (eV)

Fe 2p1/2

725.4 eV

1000 800 600 400 200 0

Zn 2

p 1/2

Zn 2

p 3/2

Fe 2

p 1/2

Fe

2p 3/

2

O 1

s

O K

LL

C 1s

Zn 3

s

Fe 3

s

Zn 3

d

Inte

nsity

(a.u

.)

Binding energy (eV)

ZnO ZnO@ZnFe2O4

(a)

(b) (c)

5 Figure S5. XPS survey spectra (a), Zn 2p (b), and (c) Fe 2p of the pristine ZnO and the typical ZnO/ZnFe2O4 inverse opal

prepared with 60 dip coating cycles.

5

735 730 725 720 715 710 705

Fe 2p3/2

Sat.Fe 2p1/2

Inte

nsity

(a.u

.)

Binding energy (eV)

ZnO@ZnFe2O4 (in situ) ZnO@ZnFe2O4 (deposite)

300 600 900 1200 1500Raman shift ( cm-1)

Inte

nsity

(a.u

.)

ZnO@ZnFe2O4 (in situ) ZnO@ZnFe2O4 (deposite)

(b)(a)

Figure S6. Raman spectra (a), and Fe 2p XPS spectra (b) of the different ZnO/ZnFe2O4 samples.

The Raman spectra of different ZnO/ZnFe2O4 samples were characterized as shown in Figure S6a. The peak at 437

cm–1 was assigned to the normal E2H mode of ZnO. The peak at 379 cm–1 arise from the A1T modes, respectively, whilst

5 the mode at 593 cm–1 correspond to E1L, respectively.[2] The other peaks at 220, 288, 402, 488, 602 cm-1 can match the

characteristic vibrations of Fe2O3.[3] The intensity of the peaks in the mixed samples were weaker than that of the in

situ transformed samples, suggesting ZnFe2O4 coating on the surface of ZnO in the transformed samples. Figure S6b

exhibited the Fe 2p XPS spectra of different ZnO/ZnFe2O4 samples. It can be seen that the Fe 2p1/2 peak obviously

shifted to a lower energy level compared to the ZnFe2O4 deposited on ZnO composite sample, confirming the presence

10 of strong interfacial interactions in the in situ transformed ZnO/ZnFe2O4 sample. It is in agreement with the results of

Figure 1d. The strong interfacial interactions are in favor of electron fluently transferring through their interface.

400 500 600 700 800

2

4

6

Abso

rban

ce E

nhan

cem

ent

Wavelength (nm)

ZnO ZnO@ZnFe2O4

1.6 2.0 2.4 2.8 3.2

ZnO ZnO@ZnFe2O4

(h

)2 (a.u

.)

Photon energy (eV)

(a) (b)

Figure S7. (a) Plots of (h)2 versus photo energy (eV), and (b) Absorbance enhancement factor curves of pristine ZnO

and ZnO/ZnFe2O4 inverse opal prepared with 60 dip coating cycles.

15

6

300 400 500 600 700 800

1E16

1E17

1E18

Wavelength (nm)

Phot

on fl

ux (

phot

ons

m-2 s

-1 n

m-1)

0

2

4

6

ZnO@ZnFe2O4

Inte

grea

ted

curre

nt d

ensit

y (m

A cm

-2)

300 400 500 600 700 8001E15

1E16

1E17

Wavelength (nm)Phot

on fl

ux (

phot

ons

m-2 s

-1 n

m-1)

0.0

0.4

0.8

1.2 ZnO

Inte

grea

ted

curre

nt d

ensit

y (m

A cm

-2)

(b)

(a)

Figure S8. Integrated current densities for the pristine ZnO (a) and typical ZnO/ZnFe2O4 prepared with 60 dip coating

cycles (b) inverse opal samples as a function of wavelength, by integrating their UV−vis absorption spectra with a standard

AM 1.5G solar spectrum (ASTM G-173-03).

50 50 100 150 200 250

0.0

0.4

0.8

1.2

1.6

Curre

nt d

ensit

y (m

A/cm

2 )

Time (s)

ZnO 30 cycles 90 cycles 120 cycles

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.0

0.5

1.0

1.5

2.0

Curre

nt D

ensit

y (m

A/cm

2 )

Potential vs RHE

Dark ZnO 30 cycles 90 cycles 120 cycles

(a) (b)

Figure S9. J-V curves under AM 1.5 G simulated sunlight at a scan rate of 10 mV s-1 in 0.1 M Na2SO4 solution (a), and

Amperometric I−t curves under chopped light irradiation with on/off interval of 20 s at 1.23 V vs RHE (b) of ZnO/ZnFe2O4

inverse opal samples prepared with different dip coating cycles.

7

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.030.050.070.11

0.170.25

1.341.351.381.41

10 h8 h6 h4 h2 h 0 h

ZnO ZnO@ZnFe2O4

Curre

nt d

ensit

y (m

A/cm

2 )

1.41 1.41

Figure S10. The photocurrent retention ratio of the pristine ZnO and the typical ZnO/ZnFe2O4 inverse opal prepared with

60 dip coating cycles under continuous AM 1.5G illumination at 1.23 V vs RHE for 10 h.

k eV

Element Weight% Atomic%

O K 26.21 57.14Fe K 13.26 8.21Zn K 60.53 34.65

500 nm

(a) (b)

5 Figure S11. (a) SEM image, and (b) EDX spectrum of the typical ZnO/ZnFe2O4 inverse opal prepared with 60 dip coating

cycles under continuous AM 1.5G illumination at 1.23 V vs RHE for 10 h.

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

20

40

60

80

100

Fara

daic

effc

ienc

y(%

)

Irradiation Time (h)

H2

O2

(b)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

20

40

60 H2

O2

Amou

nt o

f evo

lved

gase

s (

mol

)

Irradiation Time (h)

(a)

Figure S12. (a) H2 and O2 evolution curves measured at 1.23 V vs. RHE under AM 1.5G, and (b) Faradaic efficiency

curves of the generated H2 and O2 from the as-prepared typical ZnO/ZnFe2O4 inverse opal electrode.

10 H2 and O2 evolution reactions of the as-prepared typical ZnO/ZnFe2O4 inverse opal electrode were measured as

shown in Figure S12. The as-prepared typical ZnO/ZnFe2O4 inverse opal generated 62.2 mol cm-2 and 29.5 mol

8

cm-2 H2 and O2, respectively, at 1.23 V vs. RHE under AM 1.5G for 4 h. The ratio of the H2 and O2 was near the

stoichiometric value of 2.0. The obtained Faradaic efficiencies of 85.3% and 77.6% as shown in Figure S12b

determined by the measurement of the evolved H2 and O2 gas, respectively, suggesting that the photocurrent is indeed

aroused by the oxygen evolution reaction and hydrogen evolution reaction.

5-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2

0.0

0.9

1.8

2.7

3.6 ZnO ZnO@ZnFe2O4

1/C2 10

10(F

-2cm

4 )

Potential ( V) vs Ag/AgCl

Figure S13. Mott–Schottky plots measured in the DC potential range from -0.4 V to 0.4 V vs. RHE at a frequency of 10

kHz of the pristine ZnO and the typical ZnO/ZnFe2O4 inverse opal prepared with 60 dip coating cycles in the dark.

10

15

20

9

Based on the above HRTEM results of the ZnO@ZnFe2O4 heterojunction interfaces, ZnO (100) surface and ZnFe2O4

(111) surface were modelled for computational simulations. For a ZnO unit cell, the space group is P 63 M C, with

lattice parameter = = 90, = 120 and a = b = 3.25Å, c = 5.21 Å. For ZnFe2O4 unit cell, the space group is F D -

3 M, with lattice parameter = = = 90 and a = b = c = 8.35 Å. To minimize the lattice mismatch, a ( × 2) 3

5 ZnFe2O4 (111) slab with a thickness of six atom layers which contains 8 Zn atoms, 16 Fe atoms and 32 O atoms were

built to match a (4 × 4) ZnO (100) slab with a thickness of four layers which contains 32 Zn atoms and 32 O atoms.

All of the calculations were been performed using the computational software Materials Studio. A Hubbard-like,

localized term was added to the local density approximation (LDA), which was called (LDA+U), and was used to

describe the exchange-correlation effects and electron ion interactions, respectively. The value of U was set to 8.0 and

10 6.8 eV for Zn and Fe, respectively. The vacuum slab perpendicular to the surface models was 15 Å, which was enough

to separate the interaction between periodic images. We used a 4 × 5 × 1 Monkhorst-Pack k-point mesh for geometry

optimization and a 4 × 4 × 1 mesh to calculate its density of states. The cutoff energy for planewave was been chosen

to be 340 eV. In the geometry optimization process, the energy change, maximum force, maximum stress and maximum

displacement tolerance values were set to 1 × 10-5 eV per atom, 0.03 eV Å-1, 0.05 GPa, and 0.001 Å, respectively.

15

Figure S14. (a) Side-view geometry of the ZnO@ZnFe2O4 interface model, (b) Top-view geometry of the ZnO (100)

surface model, and (c) Top-view geometry of the ZnFe2O4 (111) surface model. The gray, pink, and red spheres represent

Zn, Fe, and O atoms, respectively.

20

10

Table S1. Comparison of the ZnO/ZnFe2O4 inverse opal electrode in this work with previously reported ZnFe2O4-based

photonaodes toward PEC water splitting.

Samples Photocurrent

@1.23V(vs. RHE)

/mA cm-2

Maximum

photoconversion

Efficiency %

Stability (vs.

RHE)

Electrolyte Reference

TiO2/ZnFe2O4 nanotrees

0.85 0.31 N.A. 1 M KOH 4

ZnFe2O4 films 0.1 0.021 N.A. 0.1 M NaOH 5

SrTiO3/ZnFe2O4 films 0.188 N.A. 3 h at 1.23 V 1 M NaOH 6

Ag/ZnO/ZnFe2O4 porous films

0.013 N.A. N.A. 0.5 M Na2SO4 7

ZnO/TiO2/FeOOH nanowires

1.2 0.36 2h at 1.1 V 0.5 M Na2SO4 8

Au/ZnFe2O4/ZnO nanorods

0.85 0.35 N.A. 0.1 M Na2SO4 9

ZnFe2O4 nanorods 0.35 N.A. 3h at 1.23 V 1 M NaOH 10

Al-treated Fe2O3/ZnFe2O4

0.38 N.A. N.A. 1 M NaOH 11

ZnFe2O4 nanorods 0.25 N.A. 3h at 1.23 V 1 M NaOH 12

ZnO@ZnFe2O4 inverse opal networks

1.4 0.81 loss 4.9% after 10h at 1.23 V

0.1 M Na2SO4 This work

These electrodes were tested under AM 1.5 G simulated sunlight (100 mW cm-2).

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