1

Supplementary Information

Photocatalytic generation of hydrogen by core-shell WO3/BiVO4

nanorods with ultimate water splitting efficiency

Yuriy Pihosh,1, §, * Ivan Turkevych,2, 4, § Kazuma Mawatari,1 Jin Uemura,1 Yutaka Kazoe,1 Sonya Kosar,1, 3

Kikuo Makita,2 Takeyoshi Sugaya,2 Takuya Matsui,2 Daisuke Fujita,4 Masahiro Tosa,4 Michio Kondo,2 and

Takehiko Kitamori1,*

1Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo,

Tokyo 113-8656, Japan

2National Institute of Advanced Industrial Science and Technology (AIST), AIST Central 2-13, Tsukuba, Ibaraki

305-0047, Japan

3Chernivtsy National University, Institute of Physics, Engineering and Computer Science, Storozhynetska 101,

Chernivtsy, 58000 Ukraine

4National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

----------------------------------------------------------------------------------------------------------------------------------------------------

* Corresponding authors email (kitamori@icl.t.u-tokyo.ac.jp; pihosh@icl.t.u-tokyo.ac.jp) § These authors contributed equally.

2

Fabrication of WO3-NRs/BiVO4+CoPi heterojunction photoanodes. At first, we deposited a compact

ITO film with the thickness of about 150 nm on fused silica substrates (5x5cm) at normal incidence angle (α=0º)

from the ITO target (99.999%, Furuchi Chem. Co.) in the Ar:O2 (15:0.3 SCCM) mixture and working pressure

of 0.6 Pa. Then, the stage was turned toward the Pt target (99.999%, Furuchi Chem. Co.) to deposit a thin Pt film

(~50 nm) in Ar atmosphere at the working pressure of 2 Pa. In the next step, the Pt film was encapsulated by the

deposition of the second ITO layer (150 nm), as described above. The ITO/Pt/ITO stack has a low sheet

resistance of ~3-4 Ω/☐ due to the encapsulated Pt layer, which simultaneously acts as a back reflector. Since the

Pt layer was encapsulated and had no contact with the electrolyte, it did not participate in the electrochemical

reaction and thus in future could be substituted by a less expensive metal, such as Ag or Al. After the deposition

of the ITO/Pt/ITO stack we set the sample over the third magnetron with W target (99.99%, Advantec Co.) and

changed the stage position to the GLAD regime with α=85º to the substrate normal. The WO3-NRs were

deposited in the GLAD regime by reactive sputtering in the O2:Ar (9.6 : 11 SCCM) mixture and low working

pressure of 0.3 Pa with the constant speed of substrate rotation of 45 rpm. The fabrication of WO3-NRs was

finalized by annealing in air at 575 °C for 4.5 h.

The precursor solution for the electrodepositon of BiVO4 was prepared by dissolving 10 mM of Bi(NO3)3 in

a solution of 35 mM VOSO4 adjusted to pH = 0.5 with HNO3. The Bi(III) is soluble at pH < 2, however no film

can be formed in such acidic solution. Therefore, at first the pH of the electrolyte was raised to 5.1 by 2 M

sodium acetate solution and then stabilized at pH = 4.7 by adding a few drops of concentrated HNO3, since

V(IV) starts to precipitate at pH > 5.

The electrodeposition of BiVO4 was conducted at potentiostatic conditions in the two electrode

configuration with the bias of 0.21 V applied between ITO/Pt/ITO/WO3-NRs as a working electrode and a Pt

mesh as a counter electrode. The deposition of amorphous BiVO4 was carried out at 55 °C by varying the

deposition time from 35 to 270 s. All freshly prepared samples were rinsed with distilled water, dried in the N2

steam and then annealed in air at 500 °C for 2 hours to convert the amorphous layer into a crystalline monoclinic

BiVO4.

The Co-Pi OER co-catalyst was deposited on the surface of BiVO4 from a solution of 0.15 M cobalt nitrate

in 0.1 M potassium phosphate buffer by a photo-assisted electrodeposition under 1 sun AM1.5G illumination.

The sample was biased vs a counter Pt mesh electrode at galvanostatic conditions to keep the photocurrent at

~10 μA cm-2. The optimized deposition time was found to be 500 s. The resulting photoanodes were rinsed with

distilled water and dried under a gentle N2 flow. The photoanodes based on flat film WO3/BiVO4 heterojunction

were prepared by using the same fabrication procedure, but without the GLAD regime. All chemicals were

purchased from Wako.

The photoelectrochemical (PEC) characterizations of the photoanodes were conducted according to the

standard PEC characterization protocol in a potassium phosphate buffer solution (pH=7) by using

ALS/CHI (608D) potentiostat and a standard three-electrode method with a Pt counter electrode and an Ag/AgCl

3

reference electrode. Two electrode measurements were conducted by following the same protocol. The I-V

characteristics and the photocurrent-time (Jp-t) profiles were recorded under simulated solar light provided by a

solar simulator (PEC-L01, Peccel Co.). The light intensity was adjusted by using an NREL calibrated

photodetector.

The VRHE potential was calculated by using the Nernst equation:

𝑉𝑅𝑅𝑅 = 𝑉𝐴𝐴/𝐴𝐴𝐴𝐴 + 0.059 ∗ 𝑝𝑝 + 𝑉𝐴𝐴/𝐴𝐴𝐴𝐴0 ,

where 𝑉𝑅𝑅𝑅 is the converted potential vs RHE, 𝑉𝐴𝐴/𝐴𝐴𝐴𝐴 is the experimental potential measured against the

Ag/AgCl reference electrode, and 𝑉𝐴𝐴/𝐴𝐴𝐴𝐴0 is the standard potential of Ag/AgCl at 25° C (0.198 V).

The incident photon to current conversion efficiency (IPCE) was measured in the two-electrode

configuration at the constant bias of 1V vs Pt electrode from 300 to 650 nm by using a tunable light source

provided by a stabilized 500 W Xenon lamp combined with a computer-controlled double grating

monochromator. The whole system was purchased from JASCO Co.

The oxygen and hydrogen evolution were directly measured in an airtight 2-electrodes PEC cell connected

to a gas micro-chromatograph (Inficon 3000, EZ IQ). Prior to measurements, the PEC cell was evacuated and

filled with Ar to atmospheric pressure repeatedly to eliminate air in the cell. The photoelectrode was biased at

1V vs the Pt counter electrode in pH 7 potassium phosphate buffer solution and illuminated by a simulated

AM1.5 solar light. The gas probes were taken every 10 minutes by the gas micro-chromatograph.

Physical characterization. X-Ray Diffraction (XRD) measurements were performed by using Rigaku

RINT-2500 XRD analyzer. The morphology of the samples was observed in Scanning Electron Microscope

(SEM) JEOL JSM-7001J equipped with the EDS probe, which was used to analyze the elemental composition

profiles. Scanning Probe Microscopy (SPM) analyses were performed in a semi-contact mode with a silicon

cantilever for topography measurements. In order to spread resistance measurements, the ITO film underlying

the nanorods was electrically biased at -2.0 V, whereas the conductive cantilevers (BudgetSewnsors, ContE-G,

Cr/Pt, spring constant k=0.22 N/m, tip radius 25 nm or Multi75E-G, Cr/Pt, k=2.4 N/m, tip radius 25 nm) were

grounded.

4

Supplementary Figures

Supplementary Figure S1. Top and cross section SEM images of optimized WO3-NRs (a, b) and core-shell

WO3-NRs/BiVO4 nanostructures (c, d).

Supplementary Figure S2. Optimization of the WO3-NRs/BiVO4 photoanode. (a) Photocurrents measured at

1.23VRHE under the standard AM1.5 illumination vs total charge density that passed during the

electrodeposition of BiVO4 for WO3-NRs/BiVO4 samples (blue) together with photocurrent values for initial

WO3-NRs (green) and optimized WO3-NRs/BiVO4+CoPi (red) samples. SEM images of initial WO3-NRs

prepared by GLAD (b) and after electrodeposition of BiVO4 (c-i).

1 μm

1 μm

1 μm

1 μm

WO3-NRs

ITOPt

WO3-NRs/BiVO4

(a) (b)

(c) (d)

ITOPt

100 nm

100 nm

0.0 0.4 0.5 0.6 0.7 0.8

1

2

3

4

5

6

7

Phot

ocur

rent

, Jp (

mA

cm-2)

Total charge density (C cm-2)

WO3-NRs

WO3-NRs/BiVO4

WO3-NRs/BiVO4 +CoPi(a)

(b)

(c)

(d)

(c)(d)

(e)

(b) WO3-NRs

(f)(g)

(h)(i)

500nm

500nm

500nm (f)(e)500nm 500nm (g)

(h)

(i)

500nm

500nm

500nm

5

Supplementary Figure S3. SPM characterization of WO3-NRs and WO3-NRs/BiVO4 samples: (a, c)

topographies, (b, d) local current maps and (c, f, g) I-V characteristics measured at selected points.

Supplementary Figure S4. (a) SEM image of a single WO3 nanorod with a BiVO4 conformal layer and (b) W,

Bi and V elemental distributions across the nanorod measured by EDS.

-2 -1 0 1 2

-0.1

0.0

0.1

-1

0

1

-20

0

20

#2 #4 #6 #8

Curre

nt (n

A)

Voltage (V)

#1 #3 #5 #7

Curre

nt (n

A)

#1 #2 #3

Curre

nt (n

A)

WO3-NRs

WO3-NRs/BiVO4

(a) (b)WO3-NRs

(c) (d)

WO3-NRs/BiVO4(layer)

WO3-NRs/BiVO4(clusters)

(e)

(f)

(g)

0 200 400 600 800 1000 1200

Distance[nm]

200nm

W

Bi

V

(a)

(b)

6

Supplementary Figure S5. XRD spectra of (a) WO3-NRs/BiVO4, (b) WO3-NRs, and (c) ITO/Pt/ITO samples.

20 25 30 35 40

2Theta (degree)

Inte

nsity

(a.u

.)

BiVO4 WO3 ITO + Pt

(a)

(b)

(c)

7

Supplementary Figure S6. I-V characteristics of the optimized WO3-NRs (green), WO3-NRs/BiVO4 (blue) and

WO3-NRs/BiVO4+CoPi (red) samples measured in a three-electrode configuration.

Supplementary Figure S7. Comparison of I-V characteristics of the same optimized WO3-NRs/BiVO4+CoPi

photoanode measured by two-electrode (red) and three-electrode (blue) methods.

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.20

1

2

3

4

5

6

7

8

Phot

ocur

rent

, Jp (

mA

cm-2)

Potential, VRHE (V)

WO3-NRs/BiVO4+CoPi

WO3-NRs/BiVO4

WO3-NRsDark

1.23VRHE

0.0 0.5 1.0 1.5 2.00

1

2

3

4

5

6

7

80.0 0.5 1.0 1.5 2.0

Potential (V)

Phot

ocur

rent

, Jp (m

A cm

-2)

Potential (VRHE)

3 electrode method

2 electrode method

Dark

1.23VRHE

8

Supplementary Figure S8. Combined effect of light concentration and elevated temperature on the performance

of an optimized WO3-NRs/BiVO4 and WO3-NRs/BiVO4+CoPi photoanodes. (a) Dependence of the photocurrent

on light intensity measured at different temperatures (25 – 50 oC) under 1V bias vs Pt counter electrode. The

dashed line shows a theoretical photocurrent of 6.7 [mA]×Im [suns] with the intensity exponent m = 1, i.e.

without additional recombination losses. (b) The intensity exponent m calculated from experimental data for

WO3-NRs/BiVO4+CoPi photoanode by a linear fit of log-log plot of the photocurrent vs light intensity,

Lg(Jp) ~ m×Lg(I). (c) Dependence of the photocurrent on light intensity at different temperatures (25 – 50 oC)

under 1V bias vs Pt counter electrode measured in the electrolyte containing 0.5M H2O2 as the hole scavenger.

(d) Separation (Psep) and injection (Pinj) efficiencies vs light intensity for different temperatures calculated for the

potential 1V vs Pt counter electrode. The absorption efficiency was assumed to be 100% with the absorption

photocurrent JA = 7.5 mA cm-2.

20 25 30 35 40 45 50 550.5

0.6

0.7

0.8

0.9

1.0

Inten

sity e

xpon

ent

Temperature (oC)

0.5 1.0 1.5 2.0 2.5 3.0 3.52468

101214161820

WO3-NRs/BiVO4

WO3-NRs/BiVO4+CoPi

25 oC

50 oC

50 oC

30 oC35 oC40 oC45 oC

Phot

ocur

rent

, Jp (m

A cm

-2)

Light concentration (suns)

25 oC

(a)

(b)

0.5 1.0 1.5 2.0 2.5 3.0 3.52468

101214161820

WO3-NRs/BiVO4

25 oC 50 oC

WO3-NRs/BiVO4+CoPi 25 oC 50 oC

Phot

ocur

rent

, Jp (m

A cm

-2)

Light concentration (suns)

(c)

0.5 1.0 1.5 2.0 2.5 3.0 3.520

40

60

80

100

25oC, 50oC WO3-NRs/BiVO4

25oC, 50oC WO3-NRs/BiVO4+CoPi

25oC, 50oC WO3-NRs/BiVO4+CoPi25oC, 50oC WO3-NRs/BiVO4

P sep, P

inj ef

ficien

cy (%

)

Light concentration (suns)

(d)

Psep

Pinj

H2O(buffer)

H2O + H2O2(buffer)

9

Supplementary Figure S9. Demonstration of the PEC cell heating to 50-60 oC by concentrated solar light.

Supplementary Figure S10. Schematic illustration of the GaAs/InGaAsP double-junction PV cell structure and

assembly of the PEC-PV tandem cell. (a) and (b) show IPCE spectra of the PV cell and the PEC-PV tandem,

respectively. (c) I-V characteristics of the PV cell measured under normal incident light at 1 sun (red), and in

parallel to the incident light in the tandem assembly without the photoanode at 1 sun (blue), at 3 suns (green) and

at dark conditions (black).

0 20 40 60 80 100 12020

30

40

50

60

Cell t

empe

ratu

re (0 C)

Time (min)

Thermocouple

Photoanode Pt

Concentratedsolar light

Under 3 suns illumination

PV cell structure

400 600 800 1000 1200 1400

0

20

40

60

80

100

IPCE

(%)

Wavelenght (nm)

(b)

PV(top cell)

PV(bottom cell)

PEC

PEC-PV tandem

400 600 800 1000 1200 1400

0

20

40

60

80

100

IPCE

(%)

Wavelenght [nm]

(a)

PV(top cell)

PV(bottom cell)

PV cell

-0.5 0.0 0.5 1.0 1.510-5

10-4

10-3

10-2

10-1

100

101

102

Abso

lute

cur

rent

den

sity

(mA

cm-2)

Voltage (V)

dark

⊥, 1 sun Jsc=12.11 mAcm-2

Voc = 1.33V

(c)

, 3 sun

, 1 sun

GaAs(top cell, Eg = 1.42 eV)

InP(substrate)

Top grid

Back contact

InGaAsP(botom cell, Eg = 1.0 eV)

Pd Pd

H2O

O2

45o H2

H2O

Jp

PEC-PV tandem

Pt

10

Supplementary Figure S11. Photograph of the assembled PEC-PV tandem cell.

Supplementary Figure S12. Schematic illustration of a possible water splitting panel with O2 and H2 collecting

tranches that are fitted with PEC-PV tandems, Pt counter electrodes and hemispherical light concentrators on the

top of the panel module.

PV cell

Photoanode

Pt counter electrode

PV-PEC tandem

Micro-gap hydrophobic H2 / O2 separation channels

Contact

Pt PECPV O2H2

11

Supplementary Figure S13. I-V characteristics of the photoanodes based on core-shell WO3-NRs/BiVO4+CoPi

nanostructures (a) and WO3/BiVO4+CoPi flat films (b) measured at 90o and 45o with respect to the incident light.

(c,d) Cross section SEM images of the core-shell and flat film photoanodes, respectively.

0.0 0.2 0.4 0.6 0.8 1.0 1.20

2

4

6

8

Phot

ocur

rent

, Jp (m

A cm

-2)

Potential (V)

at 90o

at 45o

0.0 0.2 0.4 0.6 0.8 1.0 1.20

2

4

6

8

Ph

otoc

urre

nt, J

p (m

A cm

-2)

Potential (V)

at 90o

at 45o

WO3-NRs/BiVO4+CoPi WO3-flat film/BiVO4+CoPi (a) (b)

1 μm

(c)

ITOPt

Core-shell WO3-NRs/BiVO4+CoPi

1 μm

WO3- flat film

BiVO4+CoPi - flat film

(d)

ITOPt

12

Supplementary Figure S14. I-V characteristics of the photoanodes based on core-shell WO3-NRs/BiVO4+CoPi

nanostructures measured under concentrated light of 3 suns in the standard electrolyte and with hole scavenger

0.5M H2O2, and calculated Psep and Pinj efficiencies at 25 oC (a,c) and 50 oC (b,d), respectively. The absorption

efficiency was assumed to be 100% with the absorption photocurrent JA = 7.5 mA cm-2.

0.0 0.2 0.4 0.6 0.8 1.0 1.202468

10121416182022

Pho

tocu

rrent

, Jp (m

A cm

-2)

Potential (V)

3 Suns, 25oC H2O+ H2O2 (buffer) H2O (buffer)

0.0 0.2 0.4 0.6 0.8 1.0 1.202468

10121416182022

Pho

tocu

rrent

, Jp (m

A cm

-2)

Potential (V)

3 Suns, 50oC H2O+H2O2 (buffer) H2O (buffer)

0.0 0.2 0.4 0.6 0.8 1.0 1.20

20

40

60

80

100

P sep,

P inj e

fficie

ncy

(%)

Potential (V)

3 Suns, 25 oC Psep

Pinj

0.0 0.2 0.4 0.6 0.8 1.0 1.20

20

40

60

80

100

P sep,

P inj e

fficie

ncy

(%)

Potential (V)

3 Suns, 50oC Psep, Pinj

(a) (b)

(c) (d)