Electronic Supplementary Information for:

Electrodeposited Single-Crystalline PbCrO4 Microrods for

Photoelectrochemical Water Oxidation: Enhancement of

Minority Carrier Diffusion

Sung Ki Cho,*a Ramavi Akbar,b Jinseok Kang,a Won-Hee Leea and Hyun S. Parkb

aDepartment of Chemical Engineering, Kumoh National Institute of Technology, 61 Daehak-

ro, Gumi-si, Gyeongsangbuk-do 39177, Republic of Korea

bFuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seongbuk-gu,

Seoul 02792, Republic of Korea

*chosk@kumoh.ac.kr

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

0 20 40 60

0

10

20

[HCrO-4]0 (mM)

[X] (

mM

) HCrO4-

Cr2O72-

Figure S1. The change in the concentration of chromate (HCrO4-) and dichromate (Cr2O7

2-)

according to the total concentration of chromate species.

100 s

300 s

500 s

6000 s

1000 s

30 μm

30 μm

30 μm

30 μm

30 μm

1 μm

1 μm

1 μm

1 μm

1 μm

Figure S2. FESEM images of PbCrO4 electrodeposited for various deposition periods.

0.83 : 10 1.7 : 20

2.5 : 30 5.0 : 60

(Pb2+:Cr3+, mM)

(a)

0 250 500 750 1000 1250-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

i (m

A)

t (s)

Pb:Cr (mM) 0.83 : 10 1.7 : 20 2.5 : 30 5.0 : 60

(b)

20 μm

Figure S3. (a) Chronoamperograms on PbO2-deposited FTO in 0.1 M KNO3 solution

containing various amounts of Pb2+ and Cr3+ with the fixed ratio (Pb2+ : Cr3+ = 1 :12), and (b)

FESEM images of PbCrO4 microrods electrodeposited with various amounts of Pb2+ and Cr3+.

The deposition potential was 1.2 V vs Ag/AgCl and the deposition charge was 0.07 C (on

0.26 cm2).

10 20 30 40 50 60

Inte

nsity

(a.u

.)

2degree)

Pb2CrO5 (Phoenicochroite) PDF#00-028-0530

5:60

5:30

5:20

5:10

5:5

(Pb2+:Cr3+, mM)

(Pb2+:Cr3+, mM)

5:5 5:10 5:20

5:30 5:60

(a)

(b)

(c)

20 μm

1 μm

0.4 0.2 0.0

-0.6

-0.4

-0.2

0.0

i (m

A)

E (V vs Ag/AgCl)

Pb2+ : Cr3+ (mM) 5 : 60 5 : 30 5 : 20 5 : 10 5 : 5

Figure S4. (a) FESEM images, (b) X-ray diffraction patterns, and (c) photoresponses of

PbCrO4 microrods electrodeposited with various ratio between Pb2+ and Cr3+ in the

electrolyte. The deposition potential was 1.2 V vs Ag/AgCl and the deposition charge was

0.07 C (on 0.26 cm2). The photoresponse was measured in the photoelectrochemical cell

contained 0.1 M Na2SO4 solution with 0.1 M Na2SO3 as a hole scavenger. UV-vis light (100

mW/cm2) was illuminated on PbCrO4 microrods.

1.2 V

1.5 V

1.7 V

10 20 30 40 50 60

Inte

nsity

(a.u

.)2 (degree)

1.2 V

1.3 V

1.4 V

1.5 V

1.7 V(a) (b)

20 μm 2 μm

Figure S5. (a) FESEM images and (b) X-ray diffraction patterns of as-deposited PbCrO4 with

varying deposition potential (V vs Ag/AgCl). The diffraction peaks from PbO2 was not found

in all pattern, indicating that the deposit PbO2 was amorphous.

10 20 30 40 50 60

Inte

nsity

(a.u

.)

2 (degree)

Pb2CrO5 (Phoenicochroite) PDF#00-028-0530

1.2 V

1.3 V

1.4 V

1.5 V

1.7 V1.2 V 1.5 V 1.7 V

400 500 600 700

Abso

rban

ce (a

.u.)

(nm)

1.2 1.3 1.4 1.5 1.7 V

0.4 0.2 0.0

-0.15

-0.10

-0.05

0.00

1.2 V 1.3 V 1.4 V 1.5 V 1.7 V

j (m

A/cm

2 )

E (V vs Ag/AgCl)

(a) (b)

(c) (d)

20 μm

1 μm

Figure S6. (a) FESEM images, (b) X-ray diffraction patterns, (c) UV-vis absorbance spectra

and (d) photoresponses of PbCrO4 electrodeposited with various deposition potential

followed by after thermal annealing (550oC, 1hr). The deposition charge was 0.07 C (on 0.26

cm2). The photoresponse was measured in the photoelectrochemical cell contained 0.1 M

Na2SO4 solution with 0.1 M Na2SO3 as a hole scavenger. UV-vis light (100 mW/cm2) was

illuminated on PbCrO4 microrods.

0 250 500 750 1000-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

i (

mA)

t (s)

w/o SDS w/ SDS

0.4 0.2 0.0-0.8

-0.6

-0.4

-0.2

0.0

i (m

A)

E (V vs Ag/AgCl)

1mM SDS 0.1C 1mM SDS 0.2C 1mM SDS 0.3C

(a) (b)

(c) (d)

0.1 C 0.2 C

0.3 C 0.4 C

20 30 40 50

Inte

nsity

(a.u

.)

2 (degree)

(e)

0.4 CWith SDS

0 1 2 3 4 5 60.0

0.2

0.4

0.6

MicrorodNanorodNanorod (SDS)

j ph (m

A/cm

2 )

PbCrO4 amount (mol/cm2)

2 μm

Figure S7. (a) Chronoamperograms on PbO2-deposited FTO in 0.1 M KNO3 solution

containing 5 mM Pb2+, 60 mM Cr3+, and 1 mM SDS. (b) FESEM images, (c) XRD, and (d)

photoresponses of PbCrO4 rods electrodeposited with SDS. (e) A plot of the photocurrent (at

0.4 V vs Ag/AgCl) for sulfite oxidation on PbCrO4 microrod, nanorods, and nanorods grown

with SDS versus PbCrO4 amount per unit area.

1.5 2.0 2.5 3.0

(hv

)1/2

E (eV)

Figure S8. (αhν)1/2 vs. hν (E) (α: absorption coefficient) plot for the as-deposited PbCrO4

microrods. α was obtained from absorbance (A) value according to following equation.

𝛼 =‒ ln 10 ‒ 𝐴

𝑧

where z is the film thickness (5 μm in this study).

0.4 0.2 0.0

-0.6

-0.4

-0.2

0.0

j (m

A/cm

2 )

E (V vs Ag/AgCl)

PbCrO4 microrods PbCrO4 microrods

+ a-TiO2 passivation

Figure S9. Linear sweep voltammograms (scan rate: 20 mV/s) of PbCrO4 microrods with and

without amorphous TiO2 (a-TiO2) passivation layer, measured in 0.1 M Na2SO3/0.1 M

Na2SO4 aqueous solution with chopped light under UV-visible irradiation (full xenon lamp,

100 mW/cm2). a-TiO2 layer was anodically deposited selectively on the exposed FTO surface

in the solution composed of TiCl3 and HCl (pH 2.3) as described in Ref. [1].

[1] D. Eisenberg, H. S. Ahn, and A. J. Bard, J. Am. Chem. Soc. 2014, 136, 14011−14014.

PbCrO4 nanorod photoelectrode

PbCrO4 nanorod was synthesized via precipitation method by adding 20 mL of 0.25 M KCr-

2O7 aqueous solution slowly into 20 mL of 0.5 M Pb(NO3)2 stirred vigorously with magnetic

stirrer [Lee, H. C.; Cho, S. K.; Park, H. S.; Nam, K. M.; Bard, A. J., Visible Light

Photoelectrochemical Properties of PbCrO4, Pb2CrO5, and Pb5CrO8. The Journal of Physical

Chemistry C 2017, 121 (33), 17561-17568.]. Yellow powder was precipitated immediately

and it was washed with DI water several times. The precipitates were filtered and then dried

in the drying oven at 120oC overnight. The precipitate was confirmed as PbCrO4 in the form

of nanorod with the diameter of about 230 nm by SEM and XRD analyses (Fig. S9a). Dried

PbCrO4 powder was dispersed in ethylene glycol with the sonication and the suspension

solution was drop-casted on a FTO substrate followed by annealing at 500oC for 3 hr in air.

The prepared PbCrO4 nanorod photoelectrode was tested in the photoelectrochemical cell

(Fig. S9b and c). PbCrO4 nanorod showed a lower photoresponse for the sulfite oxidation and

a bigger charge transfer resistance in Nyquist plot obtained from EIS measurement in the

phosphate buffer solution.

0.4 0.2 0.0

-0.6

-0.4

-0.2

0.0

j (m

A/cm

2 )

E (V vs Ag/AgCl)

Microrod Nanorod

20 30 40 50

Inte

nsity

(a.u

.)

2 (degree)

1 μm

(a) (b)

0.0 0.8 1.6 2.40.0

-0.8

-1.6

-2.4

Z'' (

k)

Z' (k)

RΩRct

CPE

Rct, Microrod= 2887 Ω

Rct, Nanorod= 4.4×1011 Ω

(c)

Figure S10. (a) X-ray diffraction pattern of PbCrO4 nanorod on FTO substrate (inset: FESEM

image). (b) the current-potential curve for the photoelectrochemical oxidation of sulfite on

PbCrO4 nanorod and (c) the Nyquist plot obtained from impedance spectroscopy analysis

(1.23 V vs RHE, 105 ~ 0.1 Hz, 10 mV amplitude) in the phosphate buffer solution, under

irradiation of 100 mW/cm2 light with Xenon lamp.

Figure S11. Linear sweep voltammograms (scan rate: 20 mV/s) of electrodeposited PbCrO4

microrods (0.3 C), in 0.1 M Na2SO3/0.1 M Na2SO4 aqueous solution with chopped light

under UV-visible irradiation (full xenon lamp, 100 mW/cm2). At back-side illumination, the

light intensity was slightly reduced to 82.5 mW/cm2 due to the light absorption from the

substrate.

0.4 0.2 0.0

-0.6

-0.4

-0.2

0.0

j ph (m

A/cm

2 )

E (V vs Ag/AgCl)

Front-side illumination Back-side illumination

0.3 0.4 0.5 0.6 0.7 0.8

-2

-1

0

ln (1

-)

E1/2 (V1/2)

360 nm 400 nm 450 nm 500 nm

Figure S12. Plot of ln(1-η) on PbCrO4 microrod for sulfite oxidation reaction against V1/2 (V :

electrode potential) with varying of the wavelength of irradiated monochromatic light.

Marked region indicated of the disturbance of the measurement of the quantum efficiency

due to the sulfite oxidation on FTO surface.

Derivation of Equation (7)

External quantum efficiency, η for PbCrO4 microrods and nanorods are given by,

𝜂𝑚𝑖𝑐𝑟𝑜𝑟𝑜𝑑 =𝑗𝑝ℎ, 𝑚𝑖𝑐𝑟𝑜𝑟𝑜𝑑

𝑞Φ= 1 ‒

exp ( ‒ 𝛼𝑊)1 + 𝛼𝐿𝑚𝑖𝑐𝑟𝑜𝑟𝑜𝑑

𝜂𝑛𝑎𝑛𝑜𝑟𝑜𝑑 =𝑗𝑝ℎ, 𝑛𝑎𝑛𝑜𝑟𝑜𝑑

𝑞Φ= 1 ‒

exp ( ‒ 𝛼𝑊)1 + 𝛼𝐿𝑛𝑎𝑛𝑜𝑟𝑜𝑑

Assuming that α and W for both PbCrO4 microrods and nanorods are same, both equations

have same exponential term, and therefore, we can write,

(1 ‒ 𝜂𝑚𝑖𝑐𝑟𝑜𝑟𝑜𝑑)(1 + 𝛼𝐿𝑚𝑖𝑐𝑟𝑜𝑟𝑜𝑑) = (1 ‒ 𝜂𝑛𝑎𝑛𝑜𝑟𝑜𝑑)(1 + 𝛼𝐿𝑛𝑎𝑛𝑜𝑟𝑜𝑑)

And it can be rearranged to,

1𝛼

+ 𝐿𝑚𝑖𝑐𝑟𝑜𝑟𝑜𝑑

1𝛼

+ 𝐿𝑛𝑎𝑛𝑜𝑟𝑜𝑑

=1 ‒ 𝜂𝑛𝑎𝑛𝑜𝑟𝑜𝑑

1 ‒ 𝜂𝑚𝑖𝑐𝑟𝑜𝑟𝑜𝑑

Finally, we obtain,

𝐿𝑚𝑖𝑐𝑟𝑜𝑟𝑜𝑑 = ( 1 ‒ 𝜂𝑛𝑎𝑛𝑜𝑟𝑜𝑑

1 ‒ 𝜂𝑚𝑖𝑐𝑟𝑜𝑟𝑜𝑑)𝐿𝑛𝑎𝑛𝑜𝑟𝑜𝑑 +

1𝛼( 1 ‒ 𝜂𝑛𝑎𝑛𝑜𝑟𝑜𝑑

1 ‒ 𝜂𝑚𝑖𝑐𝑟𝑜𝑟𝑜𝑑‒ 1)

400 500 600 700

Microrod Nanorod

Abso

rban

ce (a

.u.)

(nm)

(a) (b)

-0.8 -0.4 0.0 0.4 0.80.0

0.6

1.2

1.8

2.4 Microrod Nanorod

1/C2 (x

1011

F-2)

E (V vs Ag/AgCl)

Figure S13. (a) UV-vis absorbance spectra and (b) Mott-Schottky plots (measured in pH 7 0.2

M phosphate solution with 10 mV AC amplitude and 800 Hz AC frequency. The film

thickness was about 5 µm which is equivalent to 4 µmol/cm2 of PbCrO4.

Figure S14. The current-potential curve for the photoelectrochemical oxidation of sulfite on

the as-deposited and post-annealed PbCrO4 under irradiation of 100 mW/cm2 light with

xenon lamp.

0.4 0.2 0.0

-0.15

-0.10

-0.05

0.00

j (

mA/

cm2 )

E (V vs Ag/AgCl)

Asdep. Annealed

2 3 4 50.80

0.85

0.90

0.95

1.00

I T(L)

L=d/a

(a) (b)

0.50 0.25 0.00-1.5

-1.0

-0.5

0.0

0.5

i (nA

)E (V vs. Ag/AgCl)

Before approach After approach

iT,∞ = 4nFCDa

Figure S15. (a) Approach curves for Pt tip (radius, a = 5 µm) and (b) cyclic voltammograms

before and after approach to PbCrO4-electrodeposited (1.2 V vs. Ag/AgCl, 20000 s) FTO

substrate. All curves was measured with 1 mM ferrocenemethanol (D = 6.7×10-6 cm2/s) as an

electroactive species in 0.1 M Na2SO4 aqueous solution. For approach curve, tip potential

was held at 0.5 V (vs. Ag/AgCl) while the substrate was left to the open circuit. The approach

speed was 0.5 μm/s and the theoretical curve (---) was obtained from following equation for

the negative feedback on the insulating substrate1;

𝐼𝑇(𝐿) =𝑖𝑇

𝑖𝑇,∞= [0.292 +

1.515𝐿

+ 0.6553𝑒𝑥𝑝( ‒ 2.4035/𝐿)] ‒ 1

The experimental approach curve was slightly deviated from the theoretical one, which might

be due to imperfect insulating behavior of PbCrO4 microrods under dark condition. For cyclic

voltammogram, the measured steady-state current was 1.31×10-9 A, which was almost equal

to the theoretical one (iT,∞ = 4nFCDa = 1.3×10-9 A) and it decreased after the approach due to

the negative feedback effect.

1.6 1.2 0.8 0.4

-2

-1

0

1

j (m

A/cm

2 )

E (V vs. Ag/AgCl)

1st 2nd

Figure S16. Cyclic voltammograms (scan rate 20 mV/s) on PbCrO4 microrods, which was

electrodeposited at 1.2 V vs. Ag/AgCl for 10000 s.

Rough estimation of faradaic efficiency for oxygen evolution on PbCrO4 from SECM

measurement

The oxidation reaction on PbCrO4 microrod substrate would increase the concentration of

oxygen near the substrate surface, which would be associated with the magnitude of the tip

current. Due to a huge difference of the size of the surface area and the relevant current

between the substrate (0.16 cm2, 23~60 μA) and the tip (7.8×10-8 cm2, < 20 nA), the

concentration of oxygen near the PbCrO4 substrate would not be significantly interfered by

the oxygen reduction on the tip, and therefore, it could be theoretically calculated as follows.

Assuming that the concentration gradient within the diffusion layer is linear (diffusion layer

approximation), and oxygen was not initially present in the electrolyte (C(x, 0) = 0 for all x),

the current on the substrate (isub) is,

(1)

𝑖𝑠𝑢𝑏

𝑛𝐹𝐴= 𝐷

𝐶(𝑥 = 0)(𝑡)

𝛿(𝑡)

where A is the substrate area (0.16 cm2), D is a diffusion coefficient of oxygen (2×10-5 cm2/s),

C(x=0)(t) is a time-dependent concentration of oxygen on the substrate, and δ(t) is a time-

dependent diffusion layer thickness. At any time, the current flow causes a generation of

oxygen, where the amount of oxygen electrochemically generated is almost equal to the mole

of oxygen in the volume of the diffusion layer (A×δ(t));

(2)𝐶(𝑥 = 0)(𝑡)

𝐴𝛿(𝑡)2

=𝑡

∫0

𝑖𝑠𝑢𝑏(𝑡)

𝑛𝐹𝑑𝑡

The combination of Eqs. (1) and (2) give us,

(3)𝐶(𝑥 = 0)(𝑡) ∙

𝐴2(𝑛𝐹𝐴

𝑖𝑠𝑢𝑏∙ 𝐷𝐶(𝑥 = 0)(𝑡)) =

𝑡

∫0

𝑖𝑠𝑢𝑏(𝑡)

𝑛𝐹𝑑𝑡

And therefore, C(x=0)(t) is,

(4)

𝐶(𝑥 = 0)(𝑡) =1

𝑛𝐹𝐴2𝐷

∙ 𝑖𝑠𝑢𝑏(𝑡) ∙𝑡

∫0

𝑖𝑠𝑢𝑏(𝑡)𝑑𝑡

Since we assume the liner diffusion layer, the concentration of oxygen at x=11 μm, where the

tip was positioned, would be,

(5)

𝐶(𝑥 = 11 𝜇𝑚)(𝑡) =1

𝑛𝐹𝐴2𝐷

∙ 𝑖𝑠𝑢𝑏(𝑡) ∙𝑡

∫0

𝑖𝑠𝑢𝑏(𝑡)𝑑𝑡 ‒𝑖𝑠𝑢𝑏(𝑡)

𝑛𝐹𝐴 (11 𝜇𝑚𝐷 )

Since the diffusion of oxygen to UME tip reaches steady-state in a quite short time (< 1 s),

the tip current (itip = 4nFCDr) was obtained from the following equation;

(6)

𝑖𝑡𝑖𝑝 =4𝐷𝑟

𝐴 { 2𝐷

∙ 𝑖𝑠𝑢𝑏(𝑡) ∙𝑡

∫0

𝑖𝑠𝑢𝑏(𝑡)𝑑𝑡 ‒ (11 𝜇𝑚𝐷 )𝑖𝑠𝑢𝑏(𝑡)}

where r is a radius of UME disk (5 μm in this study).

The accuracy of the analytical solution for itip was checked by comparing with the numerical

solution obtained from the electrochemical simulation using COMSOL Multiphysics v.5.2

software. Unfortunately, the simulation could not be directly utilized for the estimation of the

faradaic efficiency, since it could not take into account the transient boundary condition for

the substrate (isub(t)) in this study. Figure S1 shows the simulation domain and electrode

configurations (Fig. S1a), and the plots of the analytical and simulated itip along with the

SECM measurement time t at the constant isub (Fig. S1b).

30 60 900

5

10

15

20

isub = 50.6 uA

i tip (n

A)

t (s)

Analytical solution Numerical solution

isub = 23.3 uA

Max. itip ([O2]max = 1.27 mM)(a) (b)

Figure S17. (a) Simulation geometry of SECM experiments, and (b) the plot of analytical and

simulated itip along with time.

The values of itip obtained from both methods under constant isub condition were quite similar

though the magnitude of itip from the analytical solution was about 15~20% larger than that

from the simulation. Oxygen concentration near the electrodes and the corresponding itip

would be overestimated as the diffusion layer approximation has a margin of error of 12%. [1]

Additional error (~8%) would come from the reduced oxygen flux to the tip compared to

usual radial diffusion which corresponds to itip in Eq. 6; Oxygen flux outward from the

substrate (3.79 × 10-10 mol/cm2·s for 23.3 μA and 8.19 × 10-10 mol/cm2·s for 50.6 μA,

respectively.) was much lower than diffusion flux from bulk to the tip (3.3 × 10-8 mol/cm2·s

for 10 nA), though the total amount of oxygen generated on the substrate was much higher

than the amount consumed on the tip.

Despite considerable overestimation, the analytical itip the corresponding faradaic efficiency

(itip,measured / itip,analytical) can be roughly calculated for time-dependent isub during SECM

measurement. Following plot showed itip measured during SECM experiment compared to

the calculated one, and the corresponding faradaic efficiency.

30 60 900

5

10

15

itip (analytical) itip (measured) Faradaic efficiency

t (s)

i tip (n

A)

0.0

0.3

0.6Faradaic efficiency

Figure S18. The plot of the measured and analytical itip, and the corresponding faradaic

efficiency (itip,measured / itip,analytical) along with SECM measurement time.

As soon as the oxygen was evolved on PbCrO4 substrate, the concentration of oxygen near

the substrate began to increase, and it was collected on the tip. The theoretical tip current

increases continuously with the oxygen concentration which were still less than oxygen

solubility in water, 1.27 mM). On the other hands, the measured current started to decline in

50 s. Subsequently, the corresponding faradaic efficiency gradually decreased after reaching

about 70%, which might be due to the photocorrosion of PbCrO4 microrod. Note that the

faradaic efficiency might be underestimated due to the overestimation of itip, theoretical.

[1]. Bard, A.; Faulkner, L., Electrochemical Methods: Fundamentals and Applications. John

Wiley & Sons, Inc: 2001, p. 34.

Table S1. The amount of Pb in the buffer solution before and after the water oxidation. It was

measured with inductively coupled plasma atomic emission spectroscopy (ICP-AES, Varian 720-ES).

Sample Concentration (ppb)

1. Phosphate solution (before PEC) 17 ± 5

2. Phosphate solution after PEC on PbCrO4 microrods(1.23 V vs RHE, 1000 s, 1 sun) 16 ± 6

3. Phosphate solution after PEC on Co-Pi-decorated PbCrO4 microrods (1.23 V vs RHE, 1000 s, 1 sun) 16 ± 5