Summary of thesis in materials science: 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.

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

MINISTRY OF EDUCATION

AND TRAINING

VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY

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

The thesis could be found at:

- National Library of Vietnam

- Library of Graduate University of Science and Technology

- Library of Institute of Science Materials

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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

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 Cu 2 O 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

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

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 carriers separations and the electrode stability increases 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 H 2 PRODUCTION

USING Cu 2 O 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 Cu 2 O thin film and protective layers

2.1.1 Synthesis of p-type and pn-type Cu 2 O films

a Fabrication of p-type Cu 2 O (p-Cu 2 O) 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

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 Cu 2 O on p-type Cu 2 O electrode – forming pn-Cu 2 O

homojunction

The solution used to

fabricate n-type Cu2O

comprised of 0.02 M

Cu(CH3COO)2 and 0.08

Figure 2.2 Synthesis curves of

p-Cu2O (a) and p-Cu2O thin film on FTO

(b)

Figure 2.6 Synthesis curves of n-Cu2O

on p-Cu2O (a) and pn-Cu2O thin film (b)

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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 TiO 2 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)

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-Cu 2 O WITH n-Cu 2 O, n-TiO 2 AND n-CdS PROTECTIVE LAYERS 3.1 Characteristics of p-Cu 2 O and pn-Cu 2 O electrodes

3.1.1 Morphology, structure of p-Cu 2 O and pn-Cu 2 O 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

Figure 2.11 The schematic of the process of transferring graphene (a)

and photograph of Cu2O electrode coated with PPMA/Graphene (b)

The X-ray diffractogram of

p-Cu2O 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, 42,55o, 61,60o, 73,75o và 77,45o

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

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

Figure 0.1 SEM image of the surface

and cross-section of p-Cu2O

Figure 0.4 XRD of the

p-Cu2O and pn-Cu2O

Figure 0.6 XPS spectrum of p-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),

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 0.8 I – V (a) and I – t (b) characteristic

curves of p-Cu2O and pn-Cu2O

Figure 0.9 I – t curves of

p-Cu2O and pn-Cu2O after two chopped - light cycles

Figure 0.7 Absorption spectrum (a), band gaps

(b) of p-Cu2O and pn-Cu2O

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

Table 0.1 The parameters of the I – V and I – t characteristic curves

measurements of p-Cu2O and pn-Cu2O

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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 TiO 2 semiconductor layer

3.2.1 Micromorphology, structure of the TiO 2 covering on p-Cu 2 O

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

pn-Cu2O films coated with

TiO2 are shown on the

(Figure 3.17)

concentration and crystallinity of

TiO2 and Cu2O, the samples 50

nm-TiO2/p-Cu2O and 50

nm-TiO2/pn-Cu2O were annealed at

demonstrated in the X-ray diffractogram (Figure 3.20)

3.2.2 The effect of the thickness and annealing temperature of the TiO 2 layer on the photo and photoelectrochemical properties of

Cu 2 O 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 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/p-

Cu2O samples at 350oC to investigate the effect of the TiO2 layer thickness

Figure 0.19 SEM images of 50nm-TiO2

/p-Cu2O annealed at different temperatures