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

Major Code

GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY ……………..*****……………. LE VAN HOANG FABRICATING RESEARCH AND PHOTOCATALYTIC, ELECTRICAL-PHOTOCATALYTIC PROPERTIES OF Cu2O WITH NANOSTRUCTURE COVERING LAYERS : Materials for optics, optoelectronics and photonics : 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

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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, electro-

photocatalytic 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 photo- corrosion. 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 p- Cu2O) 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, electro-

photocatalytic properties of the fabricated electrodes.

+ Investigate the mechanism of the photocatalysis, electron and

hole mobilities within Cu2O photocathode. Research item

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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 p-

Cu2O, 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 pn- Cu2O homojunction thus improving the photoelectrochemical characteristics such as photocurrent onset potential Vonset, charge increases carriers separations and the electrode stability

considerably.

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 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

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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 2O 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

was increased to 12 by a

NaOH 20 M solution. Figure 2.2. Synthesis curves of p- Cu2O (a) and p-Cu2O thin film on FTO (b)

The temperature of the electrochemical solution was kept constant at 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

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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 pn- Cu2O 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.

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With the passed

charge density of 1 C/cm2, the thickness of the Cu2O film was determined by SEM

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

measurement to be in the range of

1,4 – 1,5 m (Figure 3.1b).

The X-ray diffractogram of p-

the

Cu2O and pn-Cu2O shows fabricated Cu2O is a single crystal without impurities such as Cu or

Figure 0.4. XRD of the p- Cu2O and pn-Cu2O

CuO (Figure 3.4). The diffraction peaks at 2 values: 29,70o, 36,70o, 42,55o, 61,60o, 73,75o và 77,45o 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.

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3.1.2 Photo and photoelectrochemical properties of p-Cu2O and pn- Cu2O electrodes

Figure 3.7a

indicates that p-

Cu2O and pn- Cu2O electrodes photon absorb

with wavelength

shorter than 640

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

absorbance

increases in the

range of photon

wavelength from

300 nm to 560

nm. The band

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

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

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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 p-

Cu2O 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

Sample Vonset jmax Current density after 2 cycles of chopped – light (V) j180s ρ 180s (%) j’ j’/j jmax jtrap 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

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 pn- Cu2O 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

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to make pn

n-Cu2O was deposited successfully on p-Cu2O 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 different films with

values of X.

The crystal structure

Figure 0.13. SEM images of p-Cu2O coated with TiO2 at different thicknesses of the p-Cu2O and pn- Cu2O films coated with TiO2 are shown on the diffractogram X-ray

(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 oC the Ar in 30 minutes in Figure 0.17. XRD patterns of Cu2O with a 50 nm TiO2 layer 11

environment. The

micromorphology of

the

50nm-TiO2/p- Cu2O samples with annealing different

temperatures are

shown in Figure

3.19. The crystal

structures of the

samples after being

Figure 0.19. SEM images of 50nm-TiO2/p- Cu2O annealed at different temperatures 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/p- Cu2O 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 rate of annealed at different temperatures, decrease the

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/p- Cu2O samples at 350oC to investigate the effect of the TiO2 layer thickness.

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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

jmax Sample Vonset (V) j180s ρ 180s (%)

Current density after 2 chopped – light cycles jmax jtrap j j’ j’/j 0.55 1.60 0.27 0.00 0.27 0.10 0.37 0.04 1.25 p-Cu2O 0.55 1.05 0.28 0.05 0.23 0.12 0.52 0.02 7.15 50-p 50-p-300oC 0.50 0.56 0.40 0.00 0.40 0.20 0.50 0.12 30.00 50-p-350oC 0.58 0.84 0.88 0.37 0.51 0.51 1.00 0.28 34.10 50-p-400oC 0.56 1.10 0.87 0.43 0.44 0.33 0.75 0.15 17.24 50-p-450oC 0.57 1.30 1.30 0.50 0.80 0.53 0.66 0.27 20.77 0.68 1.25 0.64 0.10 0.54 0.41 0.76 0.14 11.20 pn-Cu2O 50-pn 0.70 1.21 1.12 0.40 0.72 0.42 0.58 0.12 10.72 50-pn-300oC 0.50 0.80 0.82 0.24 0.58 0.50 0.86 0.15 18.29 50-pn-350oC 0.53 0.75 1.06 0.29 0.77 0.70 0.91 0.13 12.27 50-pn-400oC 0.55 0.86 1.30 0.80 0.50 0.50 1.00 1.18 90.80 50-pn-450oC 0.55 1.16 1.36 0.40 0.96 0.55 0.57 0.23 16.91

The 50 nm-

TiO2/pn-Cu2O sample annealed at 400oC yields a maximum current

of

density 1.3 mA/cm2. After 2 light chopped –

cycles, the

photocurrent

density was steady

(j’/j = 1) and

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

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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 result,

with

TiO2 coated p-Cu2O, optimized the

annealing

is

the

temperature 350oC, 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

TiO2/p-Cu2O- 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/pn- Cu2O-400oC electrode has the current density jmax of roughly 1.3 mA/cm2, which retains 91% after 180s of activity measurement.

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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-Cu2O- 400oC samples

Sample Vonset (V) jmax j180s ρ 180s (%) Current density after 2 chopped – light cycles jmax jtrap j j’ j’/j

10-p +0.58 1.02 0.71 0.20 0.51 0.20 0.39 0.04 5.63 20-p +0.56 1.30 0.66 0.09 0.57 0.36 0.63 0.12 8.18 3.10 +0.58 0.84 0.88 0.37 0.51 0.51 1.00 0.28 50-p 100-p +0.58 0.93 0.86 0.30 0.56 0.23 0.41 0.10 11.63 10-pn +0.46 0.47 0.70 0.30 0.40 0.57 1.40 0.60 85.72 20-pn +0.47 0.73 0.93 0.25 0.68 0.68 1.00 0.45 48.39 50-pn +0.55 0.86 1.30 0.80 0.50 0.50 1.00 1.18 90.80 100-pn +0.47 0.44 1.09 0.81 0.27 0.27 1.00 1.29 118.34

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.

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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

3.33b). Figure 0.32. XRD pattern of p-Cu2O after coating CdS

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 p-

Cu2O 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

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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

interacting with H+, thus slowing down from

between n-CdS and p-Cu2O (very high jtrap). However, this very thick n-CdS layer coats uniformly on the surface of Cu2O, preventing Cu2O the 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

jmax Sample Vonset (V) j 180s ρ 180s (%) Current density after 2 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 98.75 30 s-p +0.51 1.03 1.63 0.82 0.81 0.70 0.86 0.54 66.87 60 s-p +0.49 1.19 1.65 0.85 0.80 0.65 0.81 0.50 69.70 120 s-p +0.38 0.70 0.57 0.25 0.33 0.27 0.81 0.19 66.67 180 s-p +0.49 0.48 1.30 0.86 0.44 0.40 0.91 0.92 29.23 300 s-p +0.49 0.68 2.44 1.87 0.57 0.55 0.97 1.95 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

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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 the X-ray for different sputtering durations. On with Au

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.

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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 (Figure and pn-Cu2O electrodes 4.17, blue and purple curve). In this Figure 0.17. I – t curves of Cu2O and Au coated Cu2O in the 1st on – off cycle case, we have observed a positive

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/p-

and 20nm-

Cu2O Ti/pn-Cu2O electrodes after and before

thermal annealing.

The composition

and structure of Ti

coated Cu2O electrode was analyzed by X-

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

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

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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

j180s ρ 180s Sample Vonset (V)

jmax Current density after 2 chopped – light cycles jmax jtrap j j’ j’/j +0.55 1.60 0.17 0.00 0.17 0.15 0.88 0.02 1.25

p-Cu2O 5nm-Ti/p +0.56 1.75 0.70 0.23 0.47 0.42 0.90 0.27 38.57 10nm-Ti/p +0.54 1.63 0.56 0.15 0.41 0.40 0.97 0.22 39.29 15nm-Ti/p +0.53 1.40 0.73 0.31 0.42 0.32 0.76 0.28 38.36 20nm-Ti/p +0.57 1.30 1.20 0.59 0.61 0.48 0.79 0.22 18.33 pn-Cu2O +0.68 1.25 0.64 0.10 0.54 0.41 0.76 0.14 11.20 5nm-Ti/pn +0.54 1.60 1.65 0.49 1.16 0.91 0.78 0.45 27.27 10nm-Ti/pn +0.53 0.82 1.10 0.20 0.90 0.69 0.77 0.42 38.18 15nm-Ti/pn +0.52 1.00 1.14 0.29 0.85 0.76 0.89 0.38 33.33 20nm-Ti/pn +0.55 1.36 0.50 0.05 0.45 0.45 1.00 0.40 29.42

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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.

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By analysis of Raman

spectrum of the electrode, it can

be proven that graphene layers

exist on top of the Cu2O layer the Raman (Figure 4.29). On

Figure 0.9. Raman spectrum of 3-Gr/p-Cu2O

spectrum, we have observed 2 peaks at 1580 cm-1 (G-band) and 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

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the p-Cu2O sample shows 2 reduction peaks of Cu2O at +0.27 V and +0.07 V vs. RHE. These reduction events are related to the

photocorrosion of Cu2O to create Cu. With the sample with monolayer graphene, the peak at +0.27 V is still observable.

However, for samples with 2 and 3 layers of graphene, these peaks

cannot be observed. This result show that the samples coated with 2

and 3 layers of graphene has higher p-Cu2O electrode protection.

Because the photogenerated electrons move to the surface of

Cu2O then move to the graphene layer, slowing down the reduction of Cu2O to Cu0 on the electrode surface. This shows that the graphene layer coated on p-Cu2O slows the corrosion of Cu2O, thus increases the electrode stability. When coating 2 layers of graphene,

the resistance of the coating increases, accelerating the corrosion

process. However, when coating 3 layers of graphene, the current

density jmax rises after the I – V measurement. This fact clearly indicates that the electrons are trapped at the interface between p-

Cu2O and graphene. The value jmax is 7 times that of p-Cu2O. The value jtrap at the interface between p-Cu2O and graphene increases roughly 3 times compared to when monolayer and 2-layer graphene

were coated. The result can be explained by the fact that when

stacking graphene layers, the area of graphene islands on Cu2O increases. Therefore, the area of the interface between Cu2O and the electrolyte solution decreases. The stability of Cu2O is improved. However, when the graphene layers stack, the resistance of the

coating and the defects increases, increasing the number of electrons

trapped at the interface Cu2O and graphene.

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Table 0.5. The parameters of the photoelectrochemical measurement

of graphene coated Cu2O samples

Sample Vonset (V) j180s 180s (%)

jmax Current density after 2 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 1-Gr/p-Cu2O +0.56 1.14 0.65 0.17 0.48 0.43 0.90 0.27 41.54 2-Gr/p-Cu2O +0.51 1.72 0.50 0.12 0.38 0.32 0.85 0.19 38.00 3-Gr/p-Cu2O +0.51 1.13 1.35 0.46 0.89 0.68 0.77 0.27 20.00 pn-Cu2O +0.68 1.25 0.64 0.10 0.54 0.41 0.76 0.14 11.20 1-Gr/pn-Cu2O +0.52 1.03 0.55 0.00 0.55 0.37 0.67 0.25 45.46 2-Gr/pn-Cu2O +0.52 1.02 1.12 0.29 0.83 0.63 0.76 0.28 25.00 3-Gr/pn-Cu2O +0.52 1.25 1.13 0.15 0.98 0.67 0.68 0.34 30.09

CONCLUSION

With the aim of fabricating Cu2O thin film for electro – photocatalytic water splitting application, the thesis has concentrated

on synthesizing Cu2O thin film by electrochemical method. The fabricated film has high homogeneity, stability and can be made at

large scale. From these Cu2O thin films, we investigate the influence of protective layers coated on the electrode's photoelectrochemical

characteristics. From the obtained result, some conclusions can be

drawn:

1. We have successfully fabricated p-Cu2O and pn-Cu2O thin films on FTO substrate with high quantity and homogeneity by

homojunction improve help

electrochemical synthesis technique. With the n-Cu2O layer making photoelectrochemical pn-Cu2O characteristics such as photocurrent onset potential Vonset, charge carrier separation and the electrode's stability increase considerably.

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2. An n type semiconductor such as n-TiO2 and n-CdS coating helps improve the charge separate. However, the photo-electrons which

have been trapped at the interface between protective layer and Cu2O increase when increse the thickness of protective layers. For p-Cu2O, the optimized TiO2 thickness is 50 nm and annealed at 350oC. For pn-Cu2O, the optimized TiO2 thickness is 50 nm and annealed at 400oC. The best time deposition of CdS is 180 – 300s. 4. The annealing process helps to increase the linkage and reduce the

potential barrier between Cu2O and the Au, Ti and graphene conductive material. The thickner protective layer was deposited, the

more photo-electrons were trapped at the interface. The optimal

thickness and annealing temperature of the Au layer are 100-200nm and 400oC. A thin Ti layer 5-10nm has good support for the electronic separation and the movement of electrons from Cu2O to the surface between Ti and electrolyte solution. The graphene layers

coated on Cu2O electrodes increase the optical current density and the stability of electrodes.

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LIST OF PUBLICATIONS 1. Hoang V. Le, Ly T. Le, Phong D. Tran, Jong-San Chang, Ung Thi Dieu Thuy and Nguyen Quang Liem, “Hybrid amorphous

for better

MoSx-graphene protected Cu2O photocathode performance in H2 evolution”, International Journal of

Hydrogen Energy, available online 9 May 2019. (IF: 4.229)

2. Hoang V. Le, Phong D. Tran, Huy V. Mai, Thuy T.D. Ung,

Liem Q. Nguyen, “Gold protective layer decoration and pn

homojunction creation as novel strategies to improve

photocatalytic activity and stability of the H2-evolving copper (I) oxide photocathode”, International Journal of Hydrogen

Energy 43 (2018) 21209-21218. (IF: 4.229)

3. Hoang V Le, Thi Ly Le, Ung Thi Dieu Thuy, Phong D Tran, in engineering of viable hybrid “Current perspectives

photocathodes for solar hydrogen generation”, Advances in

Natural Sciences: Nanoscience and Nanotechnology 9 (2018)

023001 (13p).

4. Tien D. Tran, Mai T.T. Nguyen, Hoang V. Le, Duc N. Nguyen,

Quang D. Truong, Phong D. Tran, “Gold nanoparticles as an

outstanding catatyst for the hydrogen evolution reaction”, Chem.

Commun. 54 (2018) 3363-3366. (IF: 6.29)

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