S1

Electronic Supplementary Information

Two-Dimensional GeAs with Visible Range Band Gap

Chan Su Jung,‡a Doyeon Kim,‡a Seunghwan Cha,‡b Yoon Myung,c Fazel Shojaei,d Hafiz

Ghulam Abbas,e Jung Ah Lee,a Eun Hee Cha,b Jeunghee Park,*a and Hong Seok Kang*f

a Department of Chemistry, Korea University, Sejong 339-700, Korea; b Graduate School of

Green Energy Engineering, Hoseo University, Asan 336-795, Korea; c Department of

Nanotechnology and Advanced Engineering, Sejong University, Seoul 05006, Korea; d

Department of Bioactive Material Science, Jeonbuk National University, Chonju, Chonbuk

560-756, Korea; e Department of Nanoscience and Nanotechnology, Jeonbuk National

University, Chonju, Chonbuk 560-756, Korea; f Department of Nano and Advanced

Materials, College of Engineering, Jeonju University, Chonju, Chonbuk 560-759, Korea

Contents

Figure S1. XRD patterns.

Figure S2. Raman spectrum.

Figure S3. PEC data of n-Si NWs.

Figure S4. H2/O2 gas evolution data of PEC cells.

Figure S5. Nyquist plots of ML- and FL-GeAs.

Figure S6. Cyclovoltammetry data.

Figure S7. Mott–Schottky plots of of ML- and FL-GeAs.

Figure S8. Nyquist plots of n-Si and n-Si/p-GeAs.

Figure S9. Mott–Schottky plots of n-Si and n-Si/p-GeAs.

Figure S10. Calculated band diagrams of p-GeAs (1L-3L) and n-Si in vacuum.

Figure S11. SEM, XRD, and PEC data of n-ZnO and n-ZnO/p-GeAs NW array on ITO

substrates.

References

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2018

S2

10 20 30 40 50 60 70 80

bulk GeAs

(-2

01

)

(-2

02

)

(-6

03

)

(-1

00

5)

(-8

04

)

(-4

02

)

(-2

01

)

(-5

13

)(-

51

3)

(-6

03

)(-

60

3)

(-3

11

)(31

0)

(00

3)

(-4

02

)(-

40

2)

(-4

02

)

(40

0)

(-2

01

)

(40

0)

(-2

02

) JCPDS No. 11-0524

ML-GeAs

Inte

nsity (

arb

. units)

Degree (2)

FL-GeAs

(-2

01

)

Figure S1. XRD patterns of bulk GeAs, ML-GeAs, and FL-GeAs nanosheets. In the case of

FL-GeAs, the NMP suspension was filtered on the AAO membrane. The displayed reference

peaks are for monoclinic phase GeAs (JCPDS No. 11-0524, C2/m, a = 15.59 Å, b = 3.792 Å,

c = 9.49 Å, = 101.28). The sample peaks well matched those of the reference. The bulk

crystal shows only peaks originating from the layer stacking axis. The FL-GeAs nanosheets

also show the strong (-201) and (-402) peaks originating from the layer axis.

S3

160 180 200 220 240 260 280 300 320 340

309.9

ML-GeAs

277.0259.5176.9

274.8

176.3259.1 276.6

309.5

307.7

257.6

FL-GeAs

No

ma

lize

d I

nte

ns

ity

(a

rb.

un

its

)

Raman Shift (cm-1

)

Bulk GeAs

175.6

Figure S2. Raman spectra of bulk GeAs, ML-GeAs, and FL-GeAs nanosheets. Excitation at

the wavelength of 514 nm was provided from Ar-ion laser. The peaks are similar to those

reported by Barreteau et al and Yang et al.S1,S2

S4

0.0 0.5 1.0 1.5 2.00

1

2

3

4

0.0 0.5 1.0 1.5 2.0

Potential vs. RHE (V)Curr

ent

Den

sity (

mA

cm

-2)

n-Si NWs

E0 (=1.23 V)

Figure S3. Current density vs. potential (vs. RHE) for n-Si photoanodes measured in a 0.1 M

Na2SO4/0.25 M NaOH (pH 13) electrolyte under AM1.5G (100 mW cm–2) condition.

The bare Si NWs exhibited a photocurrent density of 1.5 mA cm–2 at the water oxidation

potential E0 (vertical line, 1.23 V), corresponding to the short-circuit current, JSC. The onset

potential or open-circuit voltage (VOC) was estimated to be about 0.35 V, which is higher than

that of p-GeAs/n-Si NWs. We observed significant photocurrent decrease during the first scan

owing to the fast oxidation.

S5

0 10 20 30 40 50 600.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 10 20 30 40 50 600

5

10

H2

O2

Faradic Efficiency H2 O

2

Time (min)G

as E

vo

lution

(m

ol cm

-2)

40

60

80

Fa

rad

ic E

ffic

ien

cy (

%)

Cu

rren

t D

ensity (

mA

cm

-2)

Time (min)

Figure S4. (a) Stability of photocurrent and (b) H2 and O2 evolution under the applied potential

of 0.9 V vs. RHE.

The initial photocurrent decreased initially (10 min), but becomes steady over 50 min. The

2:1 stoichiometric ratio of H2:O2 confirmed the photocatalytic water-splitting reaction: 2H2O

(l) O2 (g) + 4H+ + 4e- at the anode, and 2H+ + 2e- H2 (g) at the cathode. Faradic efficiency

(FE) for the production of H2 and O2 was calculated by 𝐹𝐸𝐻2=

2 ×𝑁H2×96485

𝑄 and 𝐹𝐸O2

=

4 ×𝑁O2×96485

𝑄 , where 𝑁H2 and 𝑁O2

are the amounts (mol) of H2 and O2, respectively, and Q

is the total amount of generated charge in coulomb (photocurrent time). The obtained Faradic

efficiency was about 70%.

S6

0 1 2 3 4 5 6

0.0

0.5

1.0

1.5

2.0ML-GeAs

-Z''

(k

)

Z' (k)

FL-GeAs

Figure S5. Nyquist plots of ML-GeAs and FL-GeAs nanosheets for EIS experiments in the

range from 100 kHz to 0.1 Hz, at a representative potential of 1.23 V (vs. RHE) in a 0.1 M

Na2SO4/0.25 M NaOH electrolyte. The equivalent circuit is shown on the right.

Electrochemical impedance spectroscopy (EIS) measurements were carried out using a

frequency range of 105 to 0.1 Hz and an amplitude of 10 mV. The charge-transfer resistance

between the electrode and electrolyte, Rct, is a key parameter for characterizing the

semiconductor–electrolyte charge transfer process. A semicircle in the Nyquist plot at high

frequencies represents the charge transfer process, and Rct is reflected by the diameter of the

semicircle. The x- and y-axes are the real part (Z) and negative imaginary part (–Z) of the

impedance, respectively. The simulation of EIS spectra using an equivalent circuit model

yielded the Rct values. The fitting parameter Re represents the internal resistance of the

electrolyte, and CPE represents the constant-phase element. From Table S1, the exfoliation

decreased Rct by 50% (4.7 and 2.4 k for the ML- and FL-GeAs nanosheets, respectively).

Table S1. Impedance parameters for the equivalent circuit shown in Figure S5.

Sample Re () Rct (k) CPE 10-5 (F)

ML-GeAs 242 4.7 1.0

FL-GeAs 63.7 2.4 1.7

S7

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5-40

-30

-20

-10

0

10

20

30

1 V0.5 V

C

urr

en

t (

A)

Potential (V vs. Ag/Ag+)

ML-GeAs

FL-GeAs

Figure S6. Cyclic voltammetry (CV) data of ML-GeAs and FL-GeAs in 0.25 M

tetrabutylammonium hexafluorophosphate (TBAPF6)/acetonitrile electrolyte.

The CV data were measured using a GC electrode as the working electrode, a Pt-wire counter

electrode, and a Ag/Ag+ reference electrode (Ag/0.01 M AgNO3 in 0.1 M TBAPF6/acetonitrile

electrolyte). The CV data recorded at a scan rate of 20 mVs-1 show strong oxidation peak (EOx)

and reduction peak (ERed). In order to estimate the valence band (VB) and conduction band

(CB) positions of the materials, the energy level is calibrated using 1 mM ferrocene/0.25 M

TBAPF6/acetonitrile electrolyte. The oxidation/reduction reaction of ferrocene/ferrocenium

(Fc/Fc+) is known to be -4.8 eV below the vacuum level.S3 We measured the formal potential

of Fc/Fc+ as 0.085 V against a Ag/Ag+ reference electrode. The VB (EV) and CB (EC) energies

of the materials were calculated from the oxidation (EOx) and reduction (ERed) potentials: EV =

-(EOx+4.715) eV; EC = -(ERed+4.715) eV, where the onset potential values are relative to the

Ag/Ag+ reference electrode. For ML-GeAs nanosheets, the onset value of EOx peak is 0.5 V,

and the position of the VB edge was estimated to be EV = -5.2 eV from the vacuum level. The

FL-GeAs nanosheets exhibit the onset of EOx peak at 1 V, corresponding to EV = -5.7 eV from

the vacuum level. These values are consistent with the respective calculated values of EV = -

5.0 eV for NL= 4 and EV = -5.8 eV for NL =1.

S8

0.2 0.4 0.60

2

4

6

8

10

0.2 0.4 0.60

5

10

C-2 (

x10

11 F

-2 c

m4)

Potential vs. RHE (V)

20 Hz

10 Hz

5 Hz

C-2 (

x10

12 F

-2 c

m4)

0.58 V

20 Hz

10 Hz

5 Hz

0.57 V

Potential vs. RHE (V)

(a) (b)

Figure S7. Mott–Schottky plots for the (a) ML-GeAs and (b) FL-GeAs nanosheets (on GC

electrode) by impedance measurements in a 0.1 M Na2SO4/0.25 M NaOH electrolyte. The flat

band potentials are obtained from the intercepts of the extrapolated lines.

The flat band potentials (Efb) were investigated using Mott–Schottky (MS) plots, i.e., the

reciprocal of capacitance vs. the applied potential at 5, 10, and 20 Hz. These MS plots were

measured by anodically sweeping the potential with an AC amplitude of 10 mV. For both

materials, the linear plots with negative slope confirm the p-type character. The lines show the

linear regions of the MS plots, and the Efb values are obtained from the intercepts of the

extrapolated lines. The Efb values of ML- and FL-GeAs were determined to be nearly the same

to each other (0.57 and 0.58 V vs. RHE at 20 Hz, respectively). On the basis of the ratio of the

inverse slopes of the plots, the hole carrier density in FL-GeAs should be approximately ten

times lower than in ML-GeAs.

S9

0 10 20 30 40 500

5

10

15

20

0 1 2 3 4 5 60.0

0.5

1.0

1.5

2.0

GeAs/Si

-Z''

(k

)

Z' (k)

(a) (b)

Si

GeAs/Si

Si

Z' (k)

Figure S8. Nyquist plots of n-Si and n-Si/p-GeAs for EIS experiments in the range from 1

MHz to 0.1 Hz at 0 V (vs. RHE) under (a) dark and (b) the irradiation of AM1.5G 100 mW

cm-2 using Xe lamp. The equivalent circuit is shown in the inset and the fitting curves are

resprensent by the solid lines. The equivalent circuit is shown in the insets and the fitting curves

are resprensent by the solid lines.

Electrochemical impedance spectroscopy (EIS) measurements in a 0.1 M Na2SO4/0.25 M

NaOH electrolyte were carried out for the electrode by applying an AC voltage of 20 mV at a

bias voltage of 1.23 V (vs. RHE). The x- and y-axes are the real part (Z) and negative imaginary

part (–Z) of the impedance, respectively. In dark, a semicircle in the Nyquist plot at high

frequencies represents the charge transfer process, with the diameter of the semicircle reflecting

the charge-transfer resistance. The simulation of EIS spectra (fitted lines) using an equivalent

circuit model (inset) yielded the Rct values. The fitting parameter Re represents the internal

resistance of the electrolyte, and CPE represents the constant-phase element related to the

double-layer capacitance. The charge-transfer resistance (Rct) between the electrode and

electrolyte is 33.4 and 20.7 k, respectively, for n-Si and n-Si/p-GeAs. The Re value is about

400 . Under light irradiation, two semicircles appear at high frequency range for n-Si/p-GeAs.

For n-Si, the second circle also exists above the range of the plot. The simulation of EIS spectra

(fitted line) using an equivalent circuit model (inset) yielded the two Rct values (Rct1 and Rct2),

with corresponding CPE (CPE1 and CPE2). The sum of Rct1 and Rct2 is 2.3 k. The n-Si shows

Rct = 89.5 k in this range. The large Rct value of Si NWs confirmed the sluggish charge

transfer kinetics on the surface of Si. The smaller Rct value of n-Si/p-GeAs indicates the much

efficient charge transfer of photoelectrode and electrolyte interface.

S10

0.5 0.6 0.7 0.8 0.9 1.0 1.10

1

2

3

0.5 0.6 0.7 0.8 0.9 1.0 1.10

1

2

3

0.0 0.2 0.4 0.60.0

0.5

1.0

1.5

0.2 0.4 0.6 0.8 1.00

2

4

6

0.0 0.2 0.4 0.6 0.8 1.00

2

4

-0.2 0.0 0.2 0.4 0.60.0

0.5

1.0

1.5

(c) (d)

0.9 V

C-2 (

10

18 F

-2 c

m4)

Potential vs. RHE (V)

2 kHz

1 kHz

0.5 kHz

Si GeAs/Si

C-2 (

10

16 F

-2 c

m4)

2 kHz

1 kHz

0.5 kHz

0.55 V

Potential vs. RHE (V)

Si

C-2 (

10

12 F

-2 c

m4)

0 V

2 kHz

1 kHz

0.5 kHz

Potential vs. RHE (V)

(a) (b)

GeAs/Si

C-2 (

10

11 F

-2 c

m4)

2 kHz

1 kHz

0.5 kHz

Potential vs. RHE (V)

-0.2

5

Figure S9. Mott-Schottky plots at 0.5, 1, and 2 kHz for (a) n-Si and (b) p-GeAs/n-Si in the

dark, and (c) n-Si and (b) p-GeAs/n-Si under the irradiation of AM1.5G 100 mW cm-2 using

Xe lamp. The flat band potentials are obtained from the intercepts of the extrapolated lines.

The flat band potentials (Efb) in a 0.1 M Na2SO4/0.25 M NaOH electrolyte were investigated

using Mott–Schottky (MS) plots. Under dark, the Efb values of n-Si and p-GeAs/n-Si were

determined as 0.9 and 0.55 V (vs. RHE), respectively. Under light irradiation, the Efb values of

n-Si and p-GeAs/n-Si were determined as 0 and -0.25 V (vs. RHE), respectively. This result

means that the Efb position is negatively shifted by about 0.3 V relative to that of n-Si. This

shift is consistent with the cathodic shift of the onset potentials in the PEC cell. Therefore, the

cathodic shift of the onset potential upon the FL-GeAs deposition is attributed to that of the Efb

position. The discrepancy between the Efb and the photocurrent onset is likely due to the

overpotential losses and voltage drop in the circuit.

S11

Figure S10. Band diagrams of the p-GeAs (1L-3L) and n-Si in vacuum before contact, and

both diagrams are based on our calculation. The positions of CBM (EV), VBM (EC), and Fermi

level (EFi, EFp, and EFn) versus the vacuum level are shown with the potentials of HER and

OER at pH 0 and 13 (left and right ends).

S12

(a)

20 30 40 50 60 70 80 0.0 0.5 1.0 1.5

0.0

0.2

0.4

0.6

0.8

Zn

O(0

04

)

Zn

O(1

03

)Zn

O(0

02

)Z

nO

(00

2)

Zn

O(1

00

)G

eA

s(-

40

2)

(-4

02

)

Monoclinic GeAs (11-0524)

ZnO

GeAs-ZnO

2 (Degree)

Inte

nsity (

arb

. un

its)

(10

0) (0

02

)

(10

1)

(10

2)

(11

0)

(10

3)

(20

0)

(11

2)

(20

1)

(00

4)

(20

2)

Hexagonal ZnO (80-0074)

p-GeAs/n-ZnO (Dark)n-ZnO (Dark)

n-ZnO (Light)

GeAs-5000 rpm dropcasting (ZOW-29)

C

urr

ent D

ensity (

mA

cm

-2)

Potential vs. RHE (V)

p-GeAs/n-ZnO (Light)

(b) (c)

Figure S11. (a) SEM image of vertically aligned ZnO nanowire array (deposited FL-GeAs) on

the ITO substrates. (b) XRD patterns of ZnO and GeAs/ZnO NW array. The displayed

reference peaks are for ZnO (JCPDS No. 80-0074, a = 3.253 Å and c = 5.215 Å), and

monoclinic phase GeAs (JCPDS No. 11-0524, C2/m, a = 15.59 Å, b = 3.792 Å, c = 9.49 Å,

= 101.28). (c) Current density vs. potential (vs. RHE) for ZnO NW and p-GeAs/n-ZnO NW

array photoanodes measured in a 0.1 M Na2SO4 (pH 6.2) electrolyte under AM1.5G (100 mW

cm–2) condition. Scan rate is 20 mV s–1.

Vertically ZnO NW array was grown on ITO substrates using chemical vapor deposition of

Zn powders under oxygen flow at 500 C. The length of ZnO NW is 10 m, and the diameter

is 100 nm. The FL-GeAs nanosheets were deposited using the same method as that used for Si

NWs. The GaAs/ZnO show the strongest (-402) peak of GeAs in the XRD pattern, confirming

S13

that the deposition of FL-GeAs was successful. With the FL-GeAs deposition, the photocurrent

density at 1.23 V was increased from 0.23 to 0.6 mA cm–2.

S14

References

S1. C. Barreteau, B. Michon, C. Besnard and E. Giannini, High-Pressure Melt Growth and

Transport Properties of SiP, SiAs, GeP, and GeAs 2D Layered Semiconductors. J. Crystal

Growth, 2016, 443, 75-80.

S2. S. Yang, Y. Yang, M. Wu, C. Hu, W. Shen, Y. Gong, L. Huang, C. Jiang, Y. Zhang and P.

M. Ajayan, Adv. Funct. Mater., 2018, 1707379.

S3. R. R. Gagné, C. A. Koval and G. C. Lisensky, Ferrocene as an Internal Standard for

Electrochemical Measurements. Inorg. Chem., 1980, 19, 2854-2855.