S1

Supplementary Information

Thin film transfer for the fabrication of tantalum nitride photoelectrodes

with controllable layered structures for water splitting

Chizhong Wang, Takashi Hisatomi, Tsutomu Minegishi, Mamiko Nakabayashi, Naoya

Shibata, Masao Katayama, and Kazunari Domen*

Electronic Supplementary Material (ESI) for Chemical Science.This journal is © The Royal Society of Chemistry 2016

S2

Experimental Section

Monochromatic irradiation (full width at half wavelength: 10 nm) from a Xe lamp (MAX-302,

Asahi Spectra) was used to measure the incident photon-to-current conversion efficiency (IPCE).

The IPCE values were calculated using the equation:

IPCE = 1240 × Ilight / (λ × P) × 100% (1)

where λ (nm) is the wavelength of the monochromatic irradiation, Ilight (mA cm−2) is the

photocurrent density, and P (mW cm−2) is the incident photon flux for the monochromatic

irradiation. The flat band potential for Ta3N5 photoanode in a 0.5 M KPi electrolyte was

determined by the Mott−Schottky (M−S) method using a potentiostat-frequency response analyser

(METEK, VersaSTAT3-200) at a frequency of 1000 Hz and an AC amplitude of 10 mV. After

fitting the M−S plots, the flat band potential was derived from the intersection with the potential

axis in each plot.

S3

Fig. S1. (A) Cross-sectional SEM images and (B) XRD patterns for Ta3N5/Ta/Ti on Si substrates

with Ta3N5 film thicknesses of (a) 570, (b) 1120, and (c) 1620 nm.

10 20 30 40 50 60

1120 nm

570 nm

1620 nm

Inte

nsity (

a.u

)

2 (degree)

Ta3N

5

Si

Si

Ta3N5Ta

Ti

Si

Ta3N5

Ta

Ti

Si

Ta3N5

Ta

Ti

2 μm2 μm2 μm

(a) (b) (c)

(B)

(A)

S4

Fig. S2. XPS spectra of Ta3N5/Si (black lines), Ta3N5/Ta/Ti after the thin film transfer process (red

lines), and transferred Ta3N5/Ta/Ti after surface etching by a HF/HNO3/H2O (1:2:7, v/v) solution

(blue lines): (a) Ta 4f, (b) Si 2p, (c) N 1s, and (d) O 1s.

36 34 32 30 28 26 24 22 20

Ta loss feature

Ta 4f7/2

Ta 4f5/2

Inte

nsity (

a.u

.)

Binding Energy (eV)104 102 100 98 96 94 92 90

Si 2p1/2

Inte

nsity (

a.u

.)

Binding Energy (eV)

Si 2p3/2

402 400 398 396 394 392 390

Inte

nsity (

a.u

.)

Binding Energy (eV)

N 1s

536 534 532 530 528 526 524

Inte

nsity (

a.u

.)

Binding Energy (eV)

O 1s

(a)

(d)

(b)

(c)

S5

Fig. S3. TEM FFT diffraction patterns of Ta3N5 near the surface of a Ta3N5/Ta/Ti photoelectrode.

Ta3N5

S6

Fig. S4. Mott−Schottky (M−S) plot obtained from Ta3N5(570 nm)/Ta/Ti without Co(OH)x

cocatalyst. The linearly fitted M−S plot indicates that the flat band potential for the Ta3N5/Ta/Ti is

0.02 V vs. RHE, which is consistent with the previously reported value for Ta3N5 photoanodes.1, 2

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.20

1

2

3

4

Potential (V vs. RHE)

C-2 (

10

13 c

m4 F

-2)

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.20

1

2

3

4

S7

Fig. S5. IPCE spectrum for the Co(OH)x/Ta3N5(570 nm)/Ta/Ti photoanode measured at 1.23 V vs.

RHE.

400 450 500 550 600 6500

10

20

30

40

@ 1.23 V vs. RHE

IPC

E (

%)

Wavelength (nm)

S8

Fig. S6. Top-view SEM images of a Co(OH)x/Ta3N5(570 nm)/Ta/Ti photoanodes (a) before and (b)

after a static potential measurement at 1.23 V vs. RHE for 20 min.

1 μm1 μm

(a) (b)

S9

Fig. S7. Schematic of the procedure used to prepare Ta3N5 films modified with various interfacial

layers between the Ta3N5 and Ta layers. (a) Ta3N5/MNx/Ta/Ti (M = Nb, Ti). A metallic Nb (or Ti)

film is initially deposited on the Ta/Si by sputtering. After oxidation (700 °C for 2 h) and

nitridation (900 °C for 2 h in a 100 sccm NH3 gas flow) of the M/Ta/Si (M = Nb, Ti) sample, a

layer of NbNx (or TiNx) is formed on top of the Ta3N5 thin film on the Si substrate. (b)

Ta3N5/CdS/Ta/Ti. A thin layer of CdS (approximately 60 nm) is deposited on top of the

as-prepared Ta3N5/Si sample using chemical bath deposition (CBD), employing a previously

reported method.3 After depositing the modified contact layers on the Ta3N5/Si samples, the Ta3N5

photoelectrodes are prepared by following the identical transfer procedure shown in Fig. 1 in the

main text.

M/Ta/Si MNx/Ta3N5/Si Ti/Ta/MNx/Ta3N5/Si

Ta3N5/Si CdS/Ta3N5/Si Ti/Ta/CdS/Ta3N5/Si

Sputtering

CBD

(a)

(b)

Ti/Ta/MNx/Ta3N5

Ti/Ta/CdS/Ta3N5

M = Nb, Ti

S10

Fig. S8. Average photocurrent densities (n = 4, ±σ) of Ta3N5/Ta/Ti and Ta3N5/NbNx/Ta/Ti

photoelectrodes at different electrode potentials.

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.30

1

2

3

4

Cu

rre

nt

De

nsity (

mA

cm

-2)

Potential (V vs. RHE)

Ta3N

5/NbN

x/Ta/Ti

Ta3N

5/Ta/Ti

S11

Table S1 Work functions of various polycrystalline metals.4

Metal (Polycrystalline) Work function (eV)

Ta 4.25

Nb 4.30

Ti 4.33

Zr 4.05

Mg 3.66

S12

References

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Asahina, M. Shima, D. Cha, L. Zhao, K. Takanabe, J. Kubota and K. Domen, Nat. Commun.

2013, 4, 2566.

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Solid Films, 2002, 403, 204-211.

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