Summary of thesis in materials science: Fabrication and photocatalytic, electrophotocatalytic properties of Cu2O with nano-structured covering layers

MINISTRY OF EDUCATION
AND TRAINING

VIETNAM ACADEMY OF
SCIENCE AND TECHNOLOGY

GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY

……………..*****…………….

LE VAN HOANG

FABRICATING RESEARCH AND PHOTOCATALYTIC,
ELECTRICAL-PHOTOCATALYTIC PROPERTIES OF
Cu2O WITH NANOSTRUCTURE COVERING LAYERS

Major : Materials for optics, optoelectronics and photonics
Code : 9.44.01.27

SUMMARY OF THESIS IN MATERIALS SCIENCE

HA NOI - 2019


The thesis was completed at:
Institute of Materials Science – Vietnam Academy of
Science and Technology

Supervisors:
1. Prof. Dr. Nguyen Quang Liem
2. Assoc. Prof. Dr. Ung Thi Dieu Thuy

Reviewer 1:
Reviewer 2:
Reviewer 3:

The dissertation will be defended at Graduate University of
Science and Technology, 18 Hoang Quoc Viet street, Hanoi.
Time: .............,.............., 2019
The thesis could be found at:
- National Library of Vietnam
- Library of Graduate University of Science and Technology
- Library of Institute of Science Materials


INTRODUCTION
With the increasing population and economic boom, the demand
for energy escalates everyday. However, the major source of energy,
fossil fuel, is depleting and its price is projected to rise. Therefore,
finding clean, renewable and e nvironmentally friendly energy
sources is an urgent and practical issue of the entire world, not just
any country.
One of those clean and limitless energy sources is solar energy.
The question is how can we convert this massive source into other
types of energy that can be stored, distributed and utilized on
demand. Besides solar cell, another method is to store solar energy in
the bond of H2 molecules through photoelectrochemical (PEC) cells,
also known as artificial leaf. This process is similar to the
photosynthesis in nature: using sunlight to split water into H2 và O2.
The photoelectrochemical cell has the cathode made of p-type
semiconductor and the anode made of n-type semiconductor.

Among p-type semiconductor cathodes,

Cu2O has been

researched extensively. Since Cu2O has a small band gap in the range
of 1.9 – 2.2 eV, it is efficient in absorbing visible light. The
maximum theoretical solar-to-hydrogen conversion efficiency of
Cu2O is approximately 18%. Moreover, Cu2O is neither expensive
nor toxic, and can be easily synthesized from abundant natural
compounds. Nonetheless, one major drawback of Cu2O, which limits
its usage in water splitting, is its susceptibility to photo-corrosion.
The standard redox potentials of the Cu2O/Cu and CuO/Cu2O
couples lie within Cu2O's band gap so the preferred thermodynamic
process of photogenerated electrons and holes are reducing Cu+ into

1


Cu0 and oxidizing Cu+ into Cu2+, respectively. Thus, there are groups
concentrating on improving the stability and photocurrent of Cu2O.
In Vietnam, there are not many researches on Cu2O, most of
which focus on synthesizing Cu2O nanoparticles for environmental
treatment or fabricating Cu2O thin film by CVD. The research on
Cu2O thin film synthesized by electrochemical method for the water
splitting process in PEC cells is still new. Therefore, we choose to
conduct the thesis "Fabrication and photocatalytic, electrophotocatalytic properties of Cu2O with nano-structured covering
layers".
Objective of the thesis
Successfully fabricate Cu2O thin film having good crystal
structure. Fabricate layers protecting Cu2O electrode from photocorrosion. Study the photocatalytic, electro-photocatalytic water

splitting properties of the Cu2O electrode.
To achieve the aforementioned goal, the specific research
contents have been conducted:
+ Research on fabricating p-type Cu2O thin film (denoted as pCu2O) and n-type Cu2O (n-Cu2O) to make pn-Cu2O homojunction by
electrochemical synthesis.
+ Study the role of protective layers and the influence of synthesis
parameters on the stability and water splitting efficiency of Cu2O
electrode, on the basis of scientific information obtained from
analysis of micromorphology, structure and photo, electrophotocatalytic properties of the fabricated electrodes.
+ Investigate the mechanism of the photocatalysis, electron and
hole mobilities within Cu2O photocathode.
Research item
2


Nano-structured Cu2O thin film and Cu2O thin film coated with
protective layers.
Research method
The thesis was conducted by experimental method. For each
research content, we have chosen the appropriate method.
Structure and content of the thesis
The thesis consists of 132 pages with 14 tables, 109 figures and
graphs and is divided into four chapters:
Chapter 1 presents the introduction to the photocatalytic water
splitting process.
Chapter 2 presents the experimental methods used in the thesis.
Chapter 3 presents the result of the research on fabricating pCu2O, pn-Cu2O thin films and Cu2O thin film coated with TiO2, CdS
protective layers.
Chapter 4 presents the obtained results on p-Cu2O and pn-Cu2O
electrodes coated with conducting protective layers: Au, Ti,

graphene.
The last part of the thesis lists the related publications and the
references.
New results obtained in the thesis


We have successfully fabricated p-Cu2O and pn-Cu2O thin films
on FTO substrate with high quantity and homogeneity by
electrochemical synthesis. With the n-Cu2O layer making pnCu2O homojunction thus improving the photoelectrochemical
characteristics such as photocurrent onset potential Vonset, charge
carriers separations and the electrode stability increases
considerably.

3




The thesis has investigated the influence of the thickness and
annealing temperature of Au and TiO2 protective layers on the
stability of the Cu2O electrode. In addition, the thesis has
proposed optimized thickness and annealing temperatures for
these 2 materials on p-Cu2O and pn-Cu2O electrodes.



The thesis is the first work to study the effect of the thickness of
CdS and Ti protective layers on the photocatalytic water
splitting process on Cu2O electrode. This research has shown
the very good charge carrier separation ability of the CdS/Cu2O

junction and the ability to support the charge transport, moving
charge carriers from Cu2O to the electrolyte solution of the Ti
layer.



The thesis has investigated the effect of graphene mono and
multilayer on the photocatalytic water splitting of Cu2O.
CHAPTER 1. THE PHOTOCATALYTIC WATER

SPLITTING PROCESS FOR CLEAN FUEL H2 PRODUCTION
USING Cu2O PHOTOCATHODE
In this chapter, we present the urgency of developing the clean
fuel H2. One of the solutions for synthesizing H2 is the process of
photocatalytic water splitting using PEC cells. We present in detail
the structure, operation principle and energy conversion efficiency
evaluation of the PEC cell. Cu2O is a material being used as the
photocathode for the PEC cell. This chapter also shows fundamental
physicochemical properties of Cu2O, several methods of fabricating
Cu2O thin film. However, Cu2O is susceptible to photocorrosion due
to its redox potential lying within the band gap. We present a few
measures to protect Cu2O photocathode such as using protective
layers made of metal, oxide as well as other compounds. The
4


introduction to researches on Cu2O and recent advances in utilizing
Cu2O as photocathode for PEC cells are also presented in this
chapter.
CHAPTER 2. EXPERIMENTAL METHODS IN THE THESIS

In this chapter, we present in detail the experimental processes
used in this thesis.
2.1. Fabrication of Cu2O thin film and protective layers
2.1.1. Synthesis of p-type and pn-type Cu 2 O films
a. Fabrication of p-type Cu2O (p-Cu2O) photoelectrode
The

FTO

substrate

was used as the working
electrode. The electrolyte
solution contains 0.4 M
CuSO4 and 3 M lactic
acid. The solution pH
Figure 2.2. Synthesis curves of pCu2O (a) and p-Cu2O thin film on FTO
NaOH 20 M solution.
(b)
The temperature of the electrochemical solution was kept constant at
was increased to 12 by a

50oC. To create the Cu2O film, a potential of + 0,2 V vs. RHE was
applied on the FTO electrode. The thickness of the Cu2O film was
controlled by fixing the charge density at 1 C/cm2.
b. Fabrication of n-type Cu2O on p-type Cu2O electrode – forming
pn-Cu2O
homojunction
The solution used to
fabricate

n-type

Cu2O

comprised of 0.02 M
Cu(CH3COO)2 and 0.08

Figure 2.6. Synthesis curves of n-Cu2O
on p-Cu2O (a) and pn-Cu2O thin film (b)
5


M CH3COOH. The solution pH was raised to 4,9. The solution
temperature was kept at 65oC. The n-type Cu2O (n-Cu2O) film was
synthesized by applying a potential of +0,52 V vs. RHE. The charge
density passed through FTO and p-Cu2O working electrodes was
fixed at 0.45 C/cm2.
2.1.2. Electron beam evaporation to deposit TiO2 layer
We coated TiO2 layers with different thicknesses on p-Cu2O and
pn-Cu2O electrodes by the electron beam evaporation method. The
source material Ti3O5 used for evaporation was of 99,9% purity. The
thickness of TiO2 layers on Cu2O was controlled at 10 nm, 20 nm, 50
nm and 100 nm.
2.1.3. Chemical bath deposition of CdS layer
We synthesized the CdS layer by the chemical bath deposition
method from the precursor solution of 0,036 M Cd(CH3COO)2 and
0,035 M (NH2)2CS. The thickness of the CdS layer was controlled
by varying the deposition time (from 30 to 300s) on Cu2O electrode
at 75oC. We continued to deposit a 10 nm layer of Ti on the

CdS/Cu2O film by thermal evaporation. The electrodes were then
annealed in Ar environment at 400oC in 30 minutes.
2.1.4. Sputtering Au film
We used the radio frequency magnetron sputtering method to coat
a Au layer on p-Cu2O and pn-Cu2O electrodes. We varied the
sputtering duration (60s, 100s, 200s and 300s) to fabricate Au layers
with different thicknesses on Cu2O electrode.
2.1.5. Thermal evaporation to deposit Ti layer
We use the thermal evaporation method to deposit Ti layers with
different thicknesses on p-Cu2O and pn-Cu2O electrodes. The Ti
source for evaporation was of 99,9% purity. The thickness of Ti
6


coating layers on Cu2O was controlled at 5nm, 10nm, 15nm và 20
nm. After depositing Ti on Cu2O, the sample was annealed in Ar
environment to increase the interaction between the Ti protective
layer and the light absorber layer. The annealing temperature was
400oC and the time was 30 minutes.
2.1.5. Monolayer graphene coating
The Cu2O electrode was coated with graphene by transferring
monolayer graphene on Cu substrate on Cu2O electrode (Figure
2.11a).

Figure 2.11. The schematic of the process of transferring graphene (a)
and photograph of Cu2O electrode coated with PPMA/Graphene (b)
Repeating the above process with monolayer graphene yield
multilayer graphene coated electrode. We denote the p-Cu2O and pnCu2O electrodes with graphene coating as X Gr/p-Cu2O and X
Gr/pn-Cu2O, with X being the number of coated graphene layers,
respectively.

CHAPTER 3. RESULT OF THE FABRICATION OF p-Cu2O
WITH n-Cu2O, n-TiO2 AND n-CdS PROTECTIVE LAYERS
3.1. Characteristics of p-Cu2O and pn-Cu2O electrodes
3.1.1. Morphology, structure of p-Cu2O and pn-Cu2O electrodes
Figure 3.1a shows that p-Cu2O has a cubic structure, the size of
the edges is approximately 1 – 1,5 m. The fabricated p-Cu2O film is
homogeneous.

7


With
charge

the

passed

density

of

1

2

C/cm , the thickness of
the

Cu2O

determined

film

was

by

SEM

Figure 0.1. SEM image of the surface
and cross-section of p-Cu2O

cross-section

measurement to be in the range of
1,4 – 1,5 m (Figure 3.1b).
The X-ray diffractogram of pCu2O and pn-Cu2O shows the
fabricated Cu2O is a single crystal
without impurities such as Cu or
CuO (Figure 3.4). The diffraction
peaks at 2 values: 29,70o, 36,70o,
o

o

42,55 , 61,60 , 73,75

o

và 77,45

o

Figure 0.4. XRD of the pCu2O and pn-Cu2O

match with the crystal planes (110), (111), (200), (220), (311) and
(222).
Figure
3.6

is

the

XPS spectra
of

p-Cu2O

film. On the
XPS

Figure 0.6. XPS spectrum of p-Cu2O

spectrum of
Cu2p, the peak of the binding energy of the electron pair Cu2p3/2 at
934 eV and Cu2p1/2 correspond to the Cu2+ ion. Moreover, there
exist satellite peaks of Cu2p3/2 and Cu2p1/2 at 942.25 eV and

962.25 eV corresponding to Cu2+ in CuO or Cu(OH)2.
8


3.1.2 Photo and photoelectrochemical properties of p-Cu2O and pnCu2O electrodes
Figure 3.7a
indicates that pCu2O

and

pn-

Cu2O electrodes
absorb

photon

with wavelength
shorter than 640
nm,

the

Figure 0.7. Absorption spectrum (a), band gaps
(b) of p-Cu2O and pn-Cu2O

absorbance
increases in the
range of photon
wavelength from

300 nm to 560
nm.

The

band

gaps of p-Cu2O
and

Figure 0.8. I – V (a) and I – t (b) characteristic
curves of p-Cu2O and pn-Cu2O

pn-Cu2O

were calculated to be 1.85 – 1.90 eV (Figure 3.7b).
Figure 3.9a shows that p-Cu2O
has Vonset  +0.55 V (vs. RHE), pnCu2O has Vonset  +0,68 V. Thus,
making pn homojunction has had
positive effect, shifting the Vonset
0.13 V to the anodic side. The
maximum photocurrent density jmax
at 0 V vs. RHE if p-Cu2O is
9

Figure 0.9. I – t curves of pCu2O and pn-Cu2O after two
chopped - light cycles


approximately 1.6 mA/cm2, 1.3 that of pn-Cu2O (1.25 mA/cm2).

However, Figure 3.9b shows that the maximum current density of pCu2O mostly contributed to the photoelectrochemical corrosion
process. After the I – V measurement, at the first cycle of stability
test, the maximum of the p-Cu2O electrode is jmax = 0.17 mA/cm2
(meaning that 89.37% of p-Cu2O was corroded after the I – V
measurement). Meanwhile, the jmax value of pn-Cu2O is 0.64
mA/cm2, corresponding to 51,2% corrosion. The measured results
are indicated in Table 3.1 and Figure 3.9.
Table 0.1. The parameters of the I – V and I – t characteristic curves
measurements of p-Cu2O and pn-Cu2O
Current density after 2 cycles j180s ρ 180s
of chopped – light
(%)
jmax jtrap j
j’ j’/j
p-Cu2O 0.55 1.60 0.17 0.00 0.17 0.15 0.88 0.02 1.25
pn-Cu2O 0.68 1.25 0.64 0.10 0.54 0.41 0.76 0.14 11.20
Sample Vonset jmax
(V)

The corrosion rate of p-Cu2O electron after 2 cycles of turning the
light on – off (chopped – light) is determined from the ratio j’/j.
Here, j and j’ are respectively steady current density in the 1st and
2nd chopped – light cycles. Table 3.1 shows j’/j of p-Cu2O and pnCu2O are respectively 0.88 and 0.76. Therefore, the corrosion rate of
p-Cu2O electrode is higher than that of pn-Cu2O. The p-Cu2O
electrode has trap current density jtrap = 0 mA/cm2 demonstrating
that photogenerated carriers, after moving to the electrode's surface,
will participate in the corrosion reaction.
Conclusion: We have fabricated p-Cu2O electrode with p-Cu2O
having cubic structure, film thickness of roughly 1.4 m by the
electrochemical deposition method. Also by this method, a layer of

10


n-Cu2O was deposited successfully on p-Cu2O to make pn
homojunction. This method of synthesizing p-Cu2O and pn-Cu2O
electrodes has high reproducibility. The p-Cu2O and pn-Cu2O films
fabricated are single crystal which preferably orient on the (111)
plane. The band gap of p-Cu2O and pn-Cu2O is in the range of 1.85 –
1.90 eV. The pn-Cu2O homojunction helps increase the Vonset of the
electrode, the charge separation under illumination and thus,
increases the electrode's stability.
3.2. TiO2 semiconductor layer
3.2.1. Micromorphology, structure of the TiO2 covering on p-Cu2O
Figure 3.13 indicates
the micromorphology of
the X nm-TiO2/p-Cu2O
films

with

different

values of X.
The crystal structure
of the p-Cu2O and pnCu2O films coated with
TiO2 are shown on the
X-ray

diffractogram

Figure 0.13. SEM images of p-Cu2O
coated with TiO2 at different thicknesses

(Figure 3.17).
To

increase

the

doping

concentration and crystallinity of
TiO2 and Cu2O, the samples 50
nm-TiO2/p-Cu2O

and

50

nm-

TiO2/pn-Cu2O were annealed at
temperatures from 300 oC đến 450
o

C in 30 minutes in the Ar
11

Figure 0.17. XRD patterns of

Cu2O with a 50 nm TiO2 layer


environment.

The

micromorphology of
the

50nm-TiO2/p-

Cu2O samples with
different

annealing

temperatures
shown
3.19.

are

in

Figure

The

crystal

structures

of

the

samples after being
annealed at different
temperatures

Figure 0.19. SEM images of 50nm-TiO2/pCu2O annealed at different temperatures

are

demonstrated in the X-ray diffractogram (Figure 3.20).
3.2.2. The effect of the thickness and annealing temperature of the
TiO2 layer on the photo and photoelectrochemical properties of
Cu2O electrode
The photoelectrochemical characterization result of 50nm-TiO2/pCu2O and 50nm-TiO2/pn-Cu2O electrodes are shown in Figure 3.23
and Table 3.2. All the samples, after being coated with TiO2 and
annealed

at

different

temperatures,

decrease

the

rate

of

photocorrosion on the electrode. The annealing process decrease the
potential barrier between the 2 materials and the amount of Ti3+ ions.
Though increasing the annealing temperature helps increasing the
maximum current density, the trap current density and the electrode
corrosion rate also increase. We decided to anneal the X nm-TiO2/pCu2O samples at 350oC to investigate the effect of the TiO2 layer
thickness.

12


Table 0.1. The parameters of the I – V characterization and the
stability test of the 50 nm-TiO2/p-Cu2O and 50 nm-TiO2/pn-Cu2O
electrodes annealed at different temperatures
Sample

p-Cu2O
50-p
50-p-300oC
50-p-350oC
50-p-400oC
50-p-450oC
pn-Cu2O
50-pn

50-pn-300oC
50-pn-350oC
50-pn-400oC
50-pn-450oC
The

50

Vonset jmax
(V)
0.55
0.55
0.50
0.58
0.56
0.57
0.68
0.70
0.50
0.53
0.55
0.55

1.60
1.05
0.56
0.84
1.10
1.30
1.25

1.21
0.80
0.75
0.86
1.16

Current density after 2
chopped – light cycles
jmax jtrap j j’ j’/j
0.27 0.00 0.27 0.10 0.37
0.28 0.05 0.23 0.12 0.52
0.40 0.00 0.40 0.20 0.50
0.88 0.37 0.51 0.51 1.00
0.87 0.43 0.44 0.33 0.75
1.30 0.50 0.80 0.53 0.66
0.64 0.10 0.54 0.41 0.76
1.12 0.40 0.72 0.42 0.58
0.82 0.24 0.58 0.50 0.86
1.06 0.29 0.77 0.70 0.91
1.30 0.80 0.50 0.50 1.00
1.36 0.40 0.96 0.55 0.57

j180s ρ 180s
(%)
0.04
0.02
0.12
0.28
0.15
0.27

0.14
0.12
0.15
0.13
1.18
0.23

1.25
7.15
30.00
34.10
17.24
20.77
11.20
10.72
18.29
12.27
90.80
16.91

nm-

TiO2/pn-Cu2O
sample

annealed

o

at 400 C yields a

maximum current
density

of

1.3

2

mA/cm . After 2
chopped – light
cycles,

the

photocurrent
density was steady
(j’/j = 1) and

Figure 0.2. I – t curve of 50 nm-TiO2/pCu2O (a, b) and 50 nm-TiO2/pn-Cu2O (c, d)
annealing at different temperature
13


after 3 minutes of the stability test, the current density only show
9.2% reduction. Therefore, we kept the annealing temperature at
400oC and investigate the influence of TiO2 film thickness on the
photocatalytic activity and stability of pn-Cu2O. The result of I – V
characterization and electrode stability are indicated in Figure 3.24c,
d and Table 3.3. We have investigated the photoelectrochemical

characteristics of the p-Cu2O and pn-Cu2O electrodes coated with
TiO2 thin film of different thickness and annealed at different
temperatures.
As indicated
by the
with

result,
TiO2

coated p-Cu2O,
the

optimized

annealing
temperature
o

350 C,

is
the

oprimized
thickness is 50
nm. The 50 nm- Figure 0.3. I – t and I – t curves of p-Cu2O (a, b)
and pn-Cu2O (c, d) coverd different thickness of
TiO2/p-Cu2OTiO2
350 oC electrode

has the current density jmax at approximately 0.9 mA/cm2, which
retains 34% after 180s of activity measurement. With TiO2 coated
pn-Cu2O, the optimized annealing temperature is 400 oC, the TiO2
thickness is in the range of 50 nm – 100 nm. The 50 nm-TiO2/pnCu2O-400oC electrode has the current density jmax of roughly 1.3
mA/cm2, which retains 91% after 180s of activity measurement.
14


Table 0.2. The parameters of the I – V characterization and the
stability test of the X nm-TiO2/p-Cu2O-350oC, X nm-TiO2/pn-Cu2O400oC samples
Current density after 2 j180s ρ 180s
chopped – light cycles
(%)
jmax jtrap j j’ j’/j
1.02 0.71 0.20 0.51 0.20 0.39 0.04 5.63
1.30 0.66 0.09 0.57 0.36 0.63 0.12 8.18
0.84 0.88 0.37 0.51 0.51 1.00 0.28 3.10
0.93 0.86 0.30 0.56 0.23 0.41 0.10 11.63
0.47 0.70 0.30 0.40 0.57 1.40 0.60 85.72
0.73 0.93 0.25 0.68 0.68 1.00 0.45 48.39
0.86 1.30 0.80 0.50 0.50 1.00 1.18 90.80
0.44 1.09 0.81 0.27 0.27 1.00 1.29 118.34

Sample Vonset jmax
(V)
10-p
20-p
50-p
100-p
10-pn

20-pn
50-pn
100-pn

+0.58
+0.56
+0.58
+0.58
+0.46
+0.47
+0.55
+0.47

3.3. The CdS layer
3.3.1. Morphology and structure of the CdS covered Cu2O electrode

Figure 0.4. SEM images of p-Cu2O samples coated with CdS
at different times
The micromorphology of the p-Cu2O eletrodes after n-CdS
deposition at different times is shown in Figure 3.28.

15


The chemical composition and
crystal structure of the sample are
characterized by X-ray diffraction
(Figure 3.32), X-ray photoelectron
spectroscopy (Figure 3.33a) and
Raman

spectroscopy

(Figure
Figure 0.32. XRD pattern of
p-Cu2O after coating CdS

3.33b).

Figure 0.33. EDX spectrum (a) and Raman spectrum of the
300s-CdS/p-Cu2O electrode (b)
3.3.2. photoelectrochemical properties of CdS protected Cu2O
The photoelectrochemical measurement results of CdS coated pCu2O electrodes are shown in Figure 3.34 and Table 3.4.

Figure 0.34. I – V (a) and I – t (b) curves of CdS coated p-Cu2O
16


The Cu2O electrodes coated with CdS shows noticeable charge
carrier separation due to the pn heterojunction. Because the CdS
layer is thick, the generated electrons are trapped at the interface
between n-CdS and p-Cu2O (very high jtrap). However, this very
thick n-CdS layer coats uniformly on the surface of Cu2O, preventing
H+, thus slowing down the

Cu2O from interacting with

photoelectrochemical corrosion process. After 180s of I – t
measurement, 20% of the 300s-CdS/Cu2O sample was corroded.
Table

0.3.

The

parameters

of

the

photoelectrochemical

characterization of CdS coated p-Cu2O
Sample Vonset
(V)

jmax

p-Cu2O
30 s-p
60 s-p
120 s-p
180 s-p
300 s-p

1.60
1.03
1.19
0.70

0.48
0.68

+0.55
+0.51
+0.49
+0.38
+0.49
+0.49

Current density after 2 j 180s
chopped – light cycles
jmax jtrap j j’ j’/j
0.17 0.00 0.17 0.15 0.88 0.02
1.63 0.82 0.81 0.70 0.86 0.54
1.65 0.85 0.80 0.65 0.81 0.50
0.57 0.25 0.33 0.27 0.81 0.19
1.30 0.86 0.44 0.40 0.91 0.92
2.44 1.87 0.57 0.55 0.97 1.95

ρ 180s
(%)
98.75
66.87
69.70
66.67
29.23
20.08

We have studied the effect of CdS deposition time on the

photoelectrochemical characteristic and stability of the Cu2O
electrode. The 300s deposition time, corresponding to a CdS
thickness of 600nm, shows the highest current density  2.4 mA/
cm2. This electrode also possess the highest stability. Only 20% of
the activity is lost after 180s of photocatalytic stability measurement.
CHAPTER 4. THE INFLUENCE OF CONDUCTIVE LAYERS
ON THE PHOTOELECTROCHEMICAL CHARACTERISTIC
OF THE Cu2O ELECTRODE

17


4.1. H+ reduction catalytic activity of Au NPs and Au coated
Cu2O electrode

Figure 0.3. Au protective mechanism on p-Cu2O (a) and pn-Cu2O (b)
4.1.1. H+ reduction catalytic activity of Au NPs
4.1.2. Morphology and structure of Au coated Cu2O electrodes
The Au layer was chosen for 2 purposes: conducting protective
layer and catalyst for the hydrogen evolution reaction (Figure 4.3).
The electrodes with different Au layer thicknesses are denoted as
Xnm-Au/p-Cu2O and Xnm-Au/pn-Cu2O, with Xnm being the
thickness of the Au layer. The Au coated electrodes annealed 30
minutes in the Ar environment at different temperatures are denoted
as Xnm-Au/p-Cu2O-YoC, with YoC being the annealing temperature.
Figure 4.2 is the SEM images of the pn-Cu2O electrode coated
with Au for different sputtering durations. On the X-ray

Figure 0.6. SEM iamges of Au
coated pn-Cu2O electrode with

different sputtering times

Figure 0.9. XRD pattern of Au
coated Cu2O electrode before
and after PEC measurement
18


diffractogram, there appears diffraction peaks of Au at 2 values of
38,25o, 44,50o and 64,75o, corresponding to the crystal planes (111),
(200) and (220) of Au (Figure 4.7).
4.1.3. The photo and PEC properties of Au coated Cu2O electrodes

Figure 0.5. I-V characteristic curve and stability of p-Cu2O (a, b) and
pn-Cu2O (c, d) electrodes coated with Au at different thicknesses
Among the Au coated p-Cu2O electrodes, the one with 100 nm
Au coating has the highest stability. The electrodes with thinner Au
coating show higher current density. However, the thin Au layer is
not enough to protect the Cu2O electrode from photocorrosion. The
high photocurrent density is mostly contributed by the electrode
corrosion process. After 3 minutes of stability test, the remaining
photocurrent density is 30% of the initial photocurrent density. With
Au coated pn-Cu2O electrodes, the 200nm-Au/pn-Cu2O has the
highest current density and stability of approximately 0.76 mA/ cm2.
After 3 minutes of stability test, the remaining current density is 50%
of the initial current density.
19


Figure 4.17 illustrate the current

density versus time curve after 2
chopped – light cycles at 0 V vs.
RHE at 1 Sun illumination. The
electron accumulation is better seen
at the Au/electrolyte interface when
coating the Au layer on the p-Cu2O
and pn-Cu2O electrodes (Figure
4.17, blue and purple curve). In this
case, we have observed a positive

Figure 0.17. I – t curves of
Cu2O and Au coated Cu2O
in the 1st on – off cycle

current when the light was turned off. This has proven that the
photogenerated electrons have been trapped inside the Au coating.
Therefore, the Au layer has an important contribution as a catalyst
and protective layer for Cu2O photoelectrode.
4.2. Ti protective layer
4.2.1. Morphology, structure of the Ti coated Cu2O electrode
Figure 4.19 is SEM
images of 20nm-Ti/pCu2O

and

20nm-

Ti/pn-Cu2O electrodes
before

and

after

thermal annealing.
The

composition

and structure of Ti
coated Cu2O electrode
was analyzed by Xray diffraction (Figure

Figure 0.6. SEM images of Ti coated
Cu2O phủ Ti before and after annealing

4.21), X-ray photoelectron spectroscopy and Raman spectroscopy.
20


In the XPS spectrum (Figure 4.24), the characteristic region of Ti
2p in the 20 nm-Ti/p-Cu2O electrode shows the peaks 2p3/2 at 458
eV and 2p1/2 at 463,76 eV, corresponding to TiO2.

Figure 0.7. XPS spectrum of Ti
2p3/2 of 20 nm-Ti/p-Cu2O

Figure 0.21. XRD pattern of Ti
coated Cu2O

4.2.2. The photoelectrochemical properties of the Ti coated Cu2O
electrode
Table 0.4. The parameters of the photoelectrochemical measurement
of the Ti coated Cu2O samples
Sample

Vonset jmax
(V)

Current density after 2 j180s ρ 180s
chopped – light cycles
jmax jtrap j j’ j’/j

p-Cu2O

+0.55 1.60 0.17 0.00 0.17 0.15 0.88 0.02 1.25

5nm-Ti/p
10nm-Ti/p
15nm-Ti/p
20nm-Ti/p
pn-Cu2O
5nm-Ti/pn
10nm-Ti/pn
15nm-Ti/pn
20nm-Ti/pn

+0.56 1.75 0.70
+0.54 1.63 0.56
+0.53 1.40 0.73

+0.57 1.30 1.20
+0.68 1.25 0.64
+0.54 1.60 1.65
+0.53 0.82 1.10
+0.52 1.00 1.14
+0.55 1.36 0.50

0.23
0.15
0.31
0.59
0.10
0.49
0.20
0.29
0.05
21

0.47 0.42 0.90
0.41 0.40 0.97
0.42 0.32 0.76
0.61 0.48 0.79
0.54 0.41 0.76
1.16 0.91 0.78
0.90 0.69 0.77
0.85 0.76 0.89
0.45 0.45 1.00

0.27
0.22

0.28
0.22
0.14
0.45
0.42
0.38
0.40

38.57
39.29
38.36
18.33
11.20
27.27
38.18
33.33
29.42


The parameters of the photoelectrochemical and I – V, I – t
measurements of the Ti coated Cu2O electrodes are indicated in
Table 4.4. The 5nm-Ti/p-Cu2O sample has 0.15 mA higher
maximum photocurrent density and 4 times the jmax value compared
to p-Cu2O, proving that the 5nm Ti coating has reduced the electrode
corrosion. The maximum photocurrent density decreases when
increasing the Ti coating thickness from 5 – 20 nm. In addition, jmax
and jtrap tend to rise. This phenomenon happens because when the
thickness of the Ti layer increases, the quantity of photogenerated
electrons trapped at the interface between Cu2O and Ti increases,
accelerating the self reduction process from Cu2O to Cu0 at the

interface between Cu2O and Ti and thus, the corrosion rate.
Therefore, for p-Cu2O, the optimized Ti coating thickness is
approximately 5 – 10 nm. The same conclusion can be drawn for the
pn-Cu2O electrode. Therefore, a 5 – 10 nm thick Ti coating on the
pn-Cu2O yields optimized charge separation and transport from the
light absorber to the interface with the electrolyte.
4.3. Graphene protective layers
4.3.1. Morphology, structure of graphene coated electrode

Figure 0.8. SEM images of graphene coated electrodes before and
after catalytic activity measurement
On the SEM images (Figures 4.28a, b), thin layers on p-Cu2O and
pn-Cu2O can be observed.
22


By

analysis

of

Raman

spectrum of the electrode, it can
be proven that graphene layers
exist on top of the Cu2O layer
(Figure 4.29). On the Raman
spectrum, we have observed 2
peaks at 1580 cm-1 (G-band) and Figure 0.9. Raman spectrum

of 3-Gr/p-Cu2O
2616 cm-1 (2D-band).
4.3.2. The PEC properties of graphene coated Cu2O electrodes

Figure 0.10. I – V characteristic and stability of the p-Cu2O (a, b)
and pn-Cu2O (c, d) electrodes coated with graphene
The

I–V

characteristics

and

the

parameters

of

the

photoelectrochemical measurement of graphene coated Cu2O
samples are indicated in Figure 4.32 and Table 4.5. The light LSV of
23