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

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

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

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

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.


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

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

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

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


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

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

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

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

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


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

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

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


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

0.28
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

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