Supplementary Material:

Low temperature-controlled synthesis of hierarchical Cu2O/Cu(OH)2/CuO

nanostructures for energy applicationsPriyanka Maratheya, Sakshum Khannaa, Ranjan Patib, Indrajit Mukhopadhyaya,b,

Abhijit Raya,b*

a Department of Solar Energy, Solar Research and Development Centre, Pandit Deendayal Petroleum Universityb Solar Research and Development Centre, Pandit Deendayal Petroleum University

Etch

ant C

once

ntra

tion

: 1M

NaO

H

1μm

As-prepared Sample

1μm

Annealed Sample1μm

As-prepared Sample

1μm

Annealed Sample

Etch

ant C

once

ntra

tion

: 3M

NaO

H

Etch

ant C

once

ntra

tion

: 5M

NaO

H

As-prepared Sample

1μm

1μm

Annealed Sample

Reac

tion

tem

p. :

room

tem

pera

ture

As-prepared Sample

1μm

Annealed Sample

1μm

Reac

tion

tem

p. :

80o C

Annealed Sample

1μm

As-prepared Sample

1μm

Reac

tion

tem

p. :

60o C

As-prepared Sample

1μm

Annealed Sample

2μm

(a)

(b)

FIG.S1. Morphological Evolution of CuO/Cu2O electrodes at (a) room temperature with 1M,

3M, 5M NaOH and 0.2 M (NH4)2S2O8 etchant concentration (b) at room temperature and hot

alkali bath (60°C and 80°C) in 3M NaOH and 0.2 M (NH4)2S2O8 etchant concentration

(with a bath duration of 20 min)

Section – S1: Capacitance calculation using Galvanostatic charge discharge (GCD) curve

Specific capacitance is calculated from galvanostatic charge-discharge curve by using the

following equations,

C= I ∆t∆ V m (S1)

Where I, ∆t, ∆V and m are the response current, total discharge time, potential window and

mass of the active material, respectively.

The specific capacitance was calculated by normalizing capacitance value calculated from the

GCD curves, according to eq. (S1), by the mass of the active material (supporting

information, section S2). Non-ideal but almost symmetric charge-discharge profile of copper

oxide system confirms and suggests involvement of pseudocapacitive behavior but at the

same time proves it to be a promising material for supercapacitor/charge storage application.

The discharge-specific capacitance determined from GCD of HA-80 electrode was 27.09 F/g

at 12.34 A/g. We observed a significant difference in the capacitance value for RT-3M

annealed and HA-80 electrodes having nanorod and nanoflake morphology respectively. It

was observed that, HA-80 electrode having nanoflakes exhibited nearly 1.5 times higher

specific capacitance compared to RT-3M annealed electrode comprising nanorods. The

higher specific capacitance of interconnected CuO nanoflakes assembly may be mainly

because of the lower charge transport resistance, high surface area and high utilization of

active material. Specific capacitance obtained for HA-80 electrode was 27.09F/g which is

comparable to the specific capacitance of 26F/g as reported by Zhang et al for globular

CuO.1Also, it has been reported earlier 2 that due to strain developed in the metal oxides

during charge-discharge process cracking of the nanostructures on electrode is likely, leading

to poor long-term stability, however, no such cracking was observed in the developed

electrodes. Further efforts are in progress to develop a hierarchal CuO structure with better

charge storage performance.

(c)

FIG.S2. Galvanostatic Charge Discharge (GCD) profile of (a) HA-80 and at (b) RT-3M

annealed electrodes respectively. (c) Stability of the HA-80 based supercapacitor for 500

charging and discharging cycles.

Section – S2: Mass calculation of active material

(A)Mass of Cuprous oxide deposited on FTO (Cuprous oxide being source of copper

ions) –

Mass of Cu2O was calculated from charge time curve (obtained from Bulk

electrolysis) during electrodeposition using following relation:

Mass (g) = (Q

F∗n∗M .W ) (S2)

[assuming 100% charge utilization for the redox reaction]

where: Q – total charge

F – Faraday Constant

n – no. of e-transfer in the reaction

M.W – molecular weight

(B) Mass of Cupric oxide – Mass of cupric oxide synthesized over electrodeposited

cuprous oxide on FTO was estimated using Raman Spectroscopy. For this study

microRaman microscope (in Via, Renishaw, UK) was used with 532 nm laser source

under ambient conditions having laser power of 2.5 mW, a 50× magnification

objective, accumulation time of 30 sec and spectral range of 100-800 cm-1was selected

for all the measurements. We used this microscopic configuration to examine locally,

the composition of the sample. Raman spectra was measured on 10-20 different points

on the electrodeposited sample, HA-80 sample and RT-3M annealed sample,

characteristic Raman bands of Cu2O and CuO near to 147/219/412/515/626 cm-1 and

275-283/330/614-618 cm-1were observed3-6; area under the curve was calculated for

all the points and was averaged. Percentage phase conversion from copper(I) oxide to

copper(II) oxide after chemical etching for both the samples HA-80 sample and RT-

3M annealed sample with respect to the electrodeposited copper(I) oxide was

calculated and was found to be 24% and 10% for HA_80/Cu2O and RT-3M

annealed/Cu2O respectively.

100 200 300 400 500 600 700

618.

4161

4.18

330.

9632

8.2414

7.39

626.

71

515.

08

412.

32282.

8827

6.62

Inte

nsity

(a.u

.)

Raman shift (cm-1)

Cu2O HA-80 RT-3M Annealed

219.

45

FIG. S3. Raman spectra of electrodeposited Cu2O, HA-80 and at RT-3M annealed sample respectively

Section – S3: Optical and Photoelectrochemical (PEC) measurements

200 400 600 800 1000 1200

Abso

rban

ce (a.

u.)

W avelength (nm )

UV

Vis

ible

Spec

trum

IR

Cu2O

CuO /C u2O

FIG. S4 Absorbance spectra of electrodeposited Cu2O and HA-80 electrode

Conversion of applied potential in the PEC measurement:

The applied potential was initially measured with respect to the Ag/AgCl reference electrode,

and then converted to the reversible hydrogen electrode (RHE) scale using the following

equation:

E(RHE) = E(Ag/AgCl) + 0.197 + 0.059 pH (S3)

(a)

Electrochemical Workstation

H 2O O2

Vacuum Level (eV)

3

4

5

6

FTO Cu2 O CuO

VA

Working electrode

Reference electrode

Counter electrode

H2O H 2

(b) (c)

0.0 0.2 0.4 0.6

-0.6

-0.4

-0.2

0.0

I (CuO )ph= 0.6 m A/cm 2

Photo

curr

ent

(mA

/cm

2)

Potential (V ) vs RHE

Dark (Cu2O ) Light (Cu2O ) CuO dark CuO light

I (Cu2O )ph= 0.54 m A/cm 2

(d) (e)

FIG. S5. (a)Schematic of PEC measurement, (b) photo response of HA-80 and Cu2O

electrode (c) photo response of Cu2O and CuO electrode in 1M Na2SO4, (d) a comparative

photoresponses of RT-3M and HA-80 electrodes, (e) chronoamperometry plot showing

improved stability of HA-80 hierarchical CuO/Cu2O structure as compared to the

electrodeposited cuprous oxide.

FIG. S6. The Incident Photon-to-current Conversion Efficiency (IPCE) spectra of the

developed electrodes.

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