Ph.D. Thesis

Development of Z-scheme heterojunction Type II
photocatalysts for efficient degradation of pollutants
under solar light irradiation

Graduate School of Yeungnam University

Department of Chemical Engineering
Major in Chemical Engineering

NGUYEN VAN QUANG

Advisor: Prof. Jae-Jin Shim

August 2021


Ph.D. Thesis

Development of Z-scheme heterojunction Type II
photocatalysts for efficient degradation of pollutants
under solar light irradiation

Advisor: Prof. Jae-Jin Shim

Presented as Ph.D. Thesis

August 2021

Graduate School of Yeungnam University

Department of Chemical Engineering
Major in Chemical Engineering

NGUYEN VAN QUANG


Nguyen Van Quang’s Ph.D. Dissertation is
approved
Committee member Prof. Moonyong Lee,
Ph.D _____________
_____________
Committee member Prof. Jae-Jin Shim, Ph.D
_____________
Committee member Prof. Taeho Yoon, Ph.D
Committee member Prof. Jinwoo Lee, Ph.D
_____________
Committee member Prof. Dohyung Kang, Ph.D _____________

August 2021

Graduate School of Yeungnam University
THƯ VIỆN TRƯỜNG ĐẠI HỌC BÁCH KHOA – ĐẠI HỌC ĐÀ NẴNG.

Lưu hành nội bộ


Acknowledgments
Firstly, I would like to express gratitude from the bottom of my heart to my supervisor Prof.
Jae-Jin Shim for his guidance, sincere advice, and supports during my stay for combined Master
and Ph.D program at Nano Energy Materials and Processing lab (formerly, Supercritical Fluids

and Nano Processing Lab). I would also like to thank the committee members: Prof. Moonyong
Lee, Prof. Taeho Yoon, Prof. Jinwoo Lee (KAIST), and Prof. Dohyung Kang for useful and
valuable recommendation on my dissertation and final defense.
Secondly, I would like to give my gratitude to postdocs and research professors in
NanoEMAP lab: Dr. Hoa, Dr. Ranjith, Dr. Amr, Dr. Rabie, Dr. Manjiri, Dr. Shafi, Dr. Darshna,
Dr. Kathik, and Dr. Debananda for their sharing of knowledge and research experience. I would
like to express my thanks to the past and present lab members: Dr. Marjorie, Dr. Toan, Mr.
Mostafa, Mr. Ganesh, Ms. Tensangmu, Mr. Abebaw, Mr. Chinh, Mr Tue, Ms. Lamiel, Mr.
Sarmad, Mr. Olvianas, Mr. Umer, Mr. Jinho, who shared their study experience and helped me
in the life in Korea.
I am also grateful to the Graduate School of Yeungnam University, which provided me with
full scholarship for my combined Master and Ph.D course, and the BK21+ Program for the
financial support during my stay in Yeungnam University.
Further, I would like to thank my senior staff from Building Material Division, Faculty of
Bridge and Road engineering, and close friends (Mr. Quang Hung, Mr. Minh Hoang, Ms. Minh
Tri, Mr. Van Thanh, Ms. Thi Phuong, Mr. Manh Hung ...) from Vietnam and (Mr. Van Dung,
Mr. Van Nam, Ms. Vinh Quy) in Korea for their company and encouragement during my stay in
Korea. Special thanks as well to Prof. Doan Quang Vinh (Rector of University of Science and
Technology-The University of Danang), Dr. Huynh Phuong Nam (Head of Department of
Personal Management-The University of Danang), Dr. Cao Van Lam (Dean of Faculty of Bridge
and Road Engineering, University of Science and Technology-The University of Danang) for
their unwavering support from Vietnam towards the completion of my doctoral degree. To
사모님,

Prof. Shim’s wife, for her best regards to me and great Korean meals.
Lastly, I am very grateful to my lovely family for special care, love, encouragement, and
supports. I would like to thank my parents (Dieu Nguyen, Thi Be Nguyen), my siblings (Hung

i
THƯ VIỆN TRƯỜNG ĐẠI HỌC BÁCH KHOA – ĐẠI HỌC ĐÀ NẴNG.


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Nguyen, Thi Nguyet Nguyen, Thi Nga Nguyen, Son Nguyen), my brothers and sisters-in-law
(Phu Trong Nguyen, Day Pham, Thi Nga Hoang, Thi Hang Vo) and all my nephews and nieces.

Thank you for beautiful memories with all of you in Korea.

Nguyen Van Quang
Yeungnam University
Republic of Korea
August 2021

THƯ VIỆN TRƯỜNG ĐẠI HỌC BÁCH KHOA – ĐẠI HỌC ĐÀ NẴNG.

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ii

THƯ VIỆN TRƯỜNG ĐẠI HỌC BÁCH KHOA – ĐẠI HỌC ĐÀ NẴNG.

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Abstract
Recently, the Z-scheme photocatalytic system has attracted more attention in
the photocatalyst designs for environmental remediation applications. Compared
with conventional designs of the photocatalysts, the Z-scheme-based design has

been selected as an ideal solution to improve the photocatalytic activity and
stability of the catalysts. This Z-scheme photocatalytic system not only reduces
the recombination of photoinduced electrons and holes but also maintains a
prominent redox ability. This dissertation builds up the story of various
photocatalytic systems from the simple construction of ZnO/graphene to the
Zscheme-based construction among semiconductors (TiO2, C-MoS2, BiPO4,
BiOCl, Ag/AgBr, BiOBr, and Bi2O3). For each photocatalytic system, the
synthesis method, characterization, and photocatalytic activities of the
photocatalysts in the degradation of organic pollutants, such as methylene blue
(MB), rhodamine B (RhB), and methyl orange (MO), tetracycline hydrochloride
(TCH), hydroxychloroquine (HCQ) in water are presented. In addition, the
mechanism of pollutant degradation at each photocatalytic system was also
discussed.

iii


In Chapter 2, the nanospherical ZnO/rGO composite synthesized by a twostep
method showed the high MB and RhB removal efficiencies and good
stability under UV light.
In Chapter 3, a Z-scheme heterojunction photocatalyst of C-MoS2/TiO2
nanocomposite was prepared by a two-step hydrothermal method (high
temperature and high pressure). The Z-scheme photocatalyst exhibited the
improvement of photocatalytic activity in the degradation of MB, RhB, and TCH
under solar light.
In Chapter 4, a facile and rapid microwave-assisted one-step method was
used to synthesize the Z-scheme ternary photocatalyst of silk cocoon-like BiPO4
on BiOBr/Bi2O3 nanosheets. The Z-scheme charge transfer mechanism among
three bismuth-based components resulted in a stronger enhancement of RhB
degradation under solar light. The photocatalyst also performed good

photocatalytic activity towards the TCH, HCQ, and MO pollutants under solar
light.
Following the same synthesis method as in Chapter 4, the Z-scheme ternary
photocatalyst of layer-structured BiOCl/BiOBr/Bi2O3 nanocomposite was
prepared and the excellent photocatalytic activity and good stability of the
catalyst were demonstrated in the degradation of RhB dye with high dye
iv


concentration under visible and solar light. Moreover, this Z-scheme
photocatalyst also showed good photocatalytic activity for TCH and MO
pollutants.
Chapter 6 presents the synthesis of Z-scheme quaternary photocatalyst of
Ag/AgBr@BiOBr/Bi2O3 nanocomposite using the same method as described in
Chapters 4 and 5. This quaternary photocatalyst is highly efficient in degrading
the MO dye with high removal efficiency and excellent stability under visible
light. The photocatalyst also showed the good photocatalytic activity for RhB
and TCH under different light sources: Light-emitting diode (LED), halogen
lamp (from an overhead projector), and simulated solar light.
In addition, the rapid microwave-assisted one-step method could be used in
the large-scale processes for synthesizing the good Z-scheme photocatalysts for
the field of environmental remediation because the method is facile, fast, and
controllable.

Table of Contents
Acknowledgments .......................................................................................... i
Abstract

.........................................................................................................


iii Table of Contents .........................................................................................
vi List of Figures .............................................................................................
v


viii List of Schemes ...........................................................................................
xvi List of Tables ............................................................................................
xviii
Chapter 1. Introduction ................................................................................. 1
1.1 Research Objectives .................................................................................. 4
1.2 Dissertation Outline ................................................................................... 5
Chapter 2. Solvent-driven morphology-controlled synthesis of highly
efficient long-life ZnO/graphene nanocomposite photocatalysts for the
practical degradation of organic wastewater under solar light .....................
7
2.1 Introduction .............................................................................................. 7
2.2 Experimental ......................................................................................... 10
2.3 Results and Discussions ......................................................................... 16
2.4 Conclusions ............................................................................................ 51
Chapter 3. Synthesis of (101)-faceted octahedral TiO2 wrapped with
MoS2/C as visible light driven Z-scheme photocatalyst for the degradation
of organic pollutants ........................................................................................ 54
3.1 Introduction ............................................................................................. 54
3.2 Experimental ........................................................................................... 57
3.3 Results and Discussions .......................................................................... 63
3.4 Conclusions ............................................................................................. 91
Chapter 4. Facile microwave-assisted synthesis of Z-scheme of silk
cocoon-like BiPO4 on BiOBr/Bi2O3 nanosheets for degradation of organic
pollutants under solar light ............................................................................. 92
4.1 Introduction ............................................................................................. 92

4.2 Experimental ........................................................................................... 95
4.3 Results and Discussions .......................................................................... 99
4.4 Conclusions ............................................................................................ 117
vi


Chapter 5. Facile microwave-assisted synthesis of Z-scheme photocatalyst
of layer-structured BiOCl/BiOBr/Bi2O3 nanocomposite for degradation of
organic pollutants under visible light ........................................................... 119
5.1 Introduction ........................................................................................... 119
5.2 Experimental ......................................................................................... 121
5.3 Results and Discussions ........................................................................ 126
5.4 Conclusions ........................................................................................... 151
Chapter 6. Facile microwave-assisted synthesis of sustainable Z-scheme
heterojunction photocatalyst of Ag/AgBr nanoparticles on BiOBr/Bi2O3
nanosheets for efficient degradation of organic pollutants under visible light
.......................................................................................................................... 153
6.1 Introduction ........................................................................................... 153
6.2 Experimental ......................................................................................... 156
6.3 Results and Discussions ........................................................................ 161
6.4 Conclusions ........................................................................................... 190
Chapter 7. Conclusions and Recommendations ...................................... 192
7.1 Conclusions ........................................................................................... 192
7.2 Recommendations ................................................................................. 194
Bibliography
...................................................................................................
196
Appendix
......................................................................................................... 210
요약

225

..................................................................................................................

vii


List of Figures
Figure 2.1

XRD pattern of (a) graphite, (b) GO, (c) ZnO seeds/RGO
before calcination under Ar gas, (d) RGO, (e) ZnO
seeds/RGO after calcination under Ar gas, (f) rZG, (g)
dZG, and (h) sZG. The magnified XRD pattern on the right
shows the (100), (002) and (101) crystalline planes of the
samples.

19

Figure 2.2

(a) FTIR spectra of GO, ZnO seed/RGO, rZG, dZG, and
sZG nanocomposites; (b) Raman spectra of ZnO/RGO
nanocomposites and GO and the magnified spectra of the
dashed area (ZnO); and (c) G and 2D band peak intensity
ratios between ZnO/RGO for the three ZG
nanocomposites and GO.

21


Figure 2.3

XPS spectra: (a) survey scan of sZG and GO and (b−d)
deconvoluted Zn 2p, O 1s, and C 1s spectra of the sZG
sample.

25

Figure 2.4

SEM images of ZnO/RGO samples: (a) ZnO seeds/RGO, (b)27
rZG, (c) dZG, and (d) sZG.

Figure 2.5

32
TEM (a1−c1) and HRTEM (a2−c2) images and the
corresponding SAED patterns (a3−c3) of ZnO/RGO
samples (rZG (a), dZG (b), and sZG (c)) and elemental
mapping (d) and EDX spectrum (e) of sZG.

viii


Figure 2.6

(a) Nitrogen adsorption-desorption isotherms and (b) poresize distribution of the three as-prepared ZnO/RGO
nanohybrids: rZG, dZG, and sZG.

34


Figure 2.7

(a) UV-vis diffuse-reflectance spectra, (b) Nyquist plots,
(c) photocurrent response, and (d) PL spectra excited at a
wavelength λ = 325 nm at room temperature of the
samples (sZG, dZG, rZG, and pure ZnO).

36

Figure 2.8

Photocatalytic degradation of a 10 mg L−1 MB solution
(a) and a 10 mg L−1 RhB solution (b) under UV
irradiation in the absence and presence of RGO, sZG,
dZG, and rZG (0.1 g L−1); and pseudo-first order kinetics
of the photodegradation of MB (c) and RhB (d) under UV
irradiation.

42

Figure 2.9

Photocatalytic degradation of (a) 10 mg L−1 MB aqueous
solution at various loading of sZG catalyst under UV
irradiation, (b) RhB aqueous solution at various initial
concentrations at a catalyst loading of 0.1 g L−1 under UV
irradiation, and (c) 10 mg L−1 MB aqueous solution in the
presence of the sZG and pure ZnO catalysts under UV
and solar light irradiation.


44

Figure 2.10

(a) Trapping experiments for sZG (0.1 g L−1) in a 10 mg
L−1 RhB dye solution and (b) photocatalytic stability of
sZG (0.5 g L−1) in a 10 mg L−1 MB dye solution under
UV irradiation, and (c) SEM images of sZG before the
first reaction and after 15 photodegradation cycles.

47

ix


Figure 3.1

XRD patterns of commercial P25 (a), K2Ti6O13 (b), spindle-64
like TiO2 (c), CMS (d), and CMST samples (e).

Figure 3.2

XPS spectra of the CMST sample: survey spectrum (a)
and deconvoluted high-resolution Mo 3d (b), S 2p (c), C
1s (d), Ti 2p (e), and O 1s (f) spectra.

66

Figure 3.3


Raman spectra of CMS (a1−a2) and CMST (b1−b2)
samples and the magnified images (a2−b2) of the
rectangular dashed areas in a1 and b1.

68

Figure 3.4

SEM images of P25 (a), bulk MoS2 (b), CMS (c), MST
(d), CMST (e), spindle-like TiO2 (f), and EDX elemental
mapping of CMST (g−m).

70

Figure 3.5

HR-TEM images (a−e) and SAED
CMoS2/octahedral-TiO2 (CMST) (f).

Figure 3.6

UV-vis diffuse reflectance spectra (a), PL spectra excited
at a wavelength λ = 325 nm at room temperature (b),
Nyquist plots (EIS response of electrode sample in the
dark as well as in the solar light) (c), and the photocurrent
response of the samples (commercial P25, CMS, MST,
and CMST) under solar light (d).

75


Figure 3.7

Photodegradation of MB (10 mg L−1) under solar light
using different samples, 0.2 g L−1 (a), variations of –
ln(C/Co) versus irradiation time (b), Effect of pH of 10 mg
L−1 MB-dye solution (0.4 g L−1) (c), and effect of the
catalyst dose and dyes on photocatalytic activity (d).

80

x

pattern

of72


Figure 3.8

Time-dependent UV-vis absorption spectra of 10 mg L−1 82
tetracycline hydrochloride (TCH) in the presence of CMST (a)
and photodegradation of TCH under 150 W Xe lamp with
different amounts of CMST catalyst (b).

Figure 3.9

Kinetic curve of RhB degradation over the CMST catalyst 85 in
the presence of different scavengers (a) and photocatalytic
stability of CMST in a 10 mg L−1 RhB dye under solar light (b)

and TEM images of CMST before and after five cycles (c).

Figure 4.1

XRD

patterns

of

samples:

BiOBr/Bi2O3

(S0), 100

BiPO4/BiOBr/Bi2O3 (Samples S1-S4) corresponding to 1, 3,
6, and 12 ml of 0.1 M NaH2PO4 solution.
Figure 4.2

FTIR (a) and Raman (b) spectra of BiPO4/BiOBr/Bi2O3 102
(sample S3).

Figure 4.3

SEM images of BiPO4/BiOBr/Bi2O3 (sample S3) and its 103
magnified image from rectangular area.

Figure 4.4


HR-TEM (a-b) and EDX mapping spectra (c-d) of 104
BiPO4/BiOBr/Bi2O3 (sample S3).

Figure 4.5

UV-vis diffuse reflectance spectra (a) and Tauc plots (b) 106 of
BiPO4, BiOBr, Bi2O3, and BiPO4/BiOBr/Bi2O3 samples.

Figure 4.6

Photodegradation of 10 mg L−1 RhB (catalyst loading of 108
0.5 g L−1) using different samples: BiPO4, BiOBr, So, S1,
S2, S3 and S4 represented for the BiPO4/BiOBr/Bi2O3
synthesized using 0, 1, 3, 6 and 12 ml of 0.1 M NaH2PO4

xi


solution, respectively (a); rate constants of
photodegrdation reaction in the presence of as-prepared
samples (b); Effect of catalyst loading (c) and pH (d) on
photodegradation of 10 mg L−1 RhB using (sample S3)
under solar light.
Figure 4.7

Time-dependent UV-vis absorption spectra of RhB, TCH, 110
HCQ, and MO in the presence of BiPO4/BiOBr/Bi2O3
(sample S3). (Conditions: Catalyst loading of 0.5 g L−1,
pollutant concentration of 10 mg L−1, solar light).


Figure 4.8

Trapping experiments of photocatalytic reactions under 112
solar light (10 mg L−1 RhB dye, catalyst loading of 0.5 g L−1).

Figure 4.9

Photocatalytic reusability in a 10 mg L−1 RhB dye 116 (catalyst
loading of 0.5 g L−1) (a); XRD patterns and SEM images of
BiPO4/BiOBr/Bi2O3 before and after seven cycles (b).

Figure 5.1

XRD patterns of the BiOCl/BiOBr/Bi2O3 (a), BiOCl (b), 127 and
BiOBr (c) samples.

Figure 5.2

FTIR

(a)

and

Raman

(b)

spectra


of

the 129

BiOCl/BiOBr/Bi2O3, BiOCl/Bi2O3, BiOCl, and BiOBr samples.
Figure 5.3

SEM images (a, d); EDX elemental mapping (b-c, e-f) and 130
EDX analysis
(g)
of
the
BiOCl/BiOBr/Bi2O3 nanocomposite.

xii


Figure 5.4

UV-vis diffuse reflectance spectra (a), Tauc plots (b), 133
Nyquist plots (c), and photocurrent response (d) of the samples
(BiOCl, BiOBr, BiOCl/Bi2O3, and

Figure 5.5

BiOCl/BiOBr/Bi2O3) under solar simulator.
Photodegradation of RhB (10 mg L−1) under OHP light 136
(catalyst loading of 0.3 g L−1) using different samples: So,
S1, S2, and S3 represented for the BiOCl/BiOBr/Bi2O3
synthesized using 0, 15, 30, and 75 mg of KCl,

respectively (a); rate constants of photodegrdation reaction
in the presence of as-prepared samples (b);
Photodegradation of RhB (10 mg L−1) with various
catalyst loading under fluorescent lamp (c) and solar
simulator (d).

Figure 5.6

Effect of RhB concentration (15, 20, 25, and 30 mg L−1) 141 (a),
various temperature (25, 35, and 40°C) (b), kinds of pollutant
(10 mg L−1 MO and TCH) and catalyst loading (0.1, 0.3, and 0.5
g L−1) (c), ions (NO3−, Cl−, SO42−, NH4+, and HCO3−) (d), and
pH of RhB solution (pH = 2, 4, 6, 8, and 9) (e) on photocatalytic
activity under OHP light.

Figure 5.7

Kinetic

curve

of

RhB

degradation

over 145

BiOCl/BiOBr/Bi2O3 catalyst in the presence of different

scavengers (a) and photocatalytic reusability of the
BiOCl/BiOBr/Bi2O3 in a 15 mg L−1 RhB dye under OHP
light (catalyst loading of 0.5 g L−1) (b); XRD patterns (c)

xiii


and SEM images (d) of the BiOCl/BiOBr/Bi2O3 before and
after seven cycles.
Figure 6.1

XRD patterns of the Ag/AgBr@BiOBr/Bi2O3 (a), 162
BiOBr/Bi2O3 (b), and AgBr (c) samples.

Figure 6.2

SEM and TEM images of the BiOBr/Bi2O3 (a, b) and 164
Ag/AgBr@BiOBr/Bi2O3 (c, d).

Figure 6.3

High-resolution TEM images (a-e) and SAED pattern (f) 165 of
the Ag/AgBr@BiOBr/Bi2O3 sample.

Figure 6.4

XPS spectra: survey spectrum (a) and deconvoluted high- 168
resolution spectra of Ag 3d (b), Bi 4f (c), Br 3d (d), and O 1s (e)
of Ag/AgBr@BiOBr/Bi2O3 catalyst.


Figure 6.5

UV-vis diffuse reflectance spectra (a), Tauc plots for 171
estimating the bandgap (b), PL spectra excited at a wavelength
λ = 325 nm at room temperature (c), Nyquist plots (d), and
photocurrent responses (e) of the as-prepared samples under
solar light.

Figure 6.6

Time-dependent UV-vis absorption spectra of TCH in the 175
presence of Ag/AgBr@BiOBr/Bi2O3 (sample S3) (a);
Photodegradation curves of TCH (b), variations of – ln(C/Co)
versus irradiation time (c), and degradation rate constants in the
presence of different catalysts (d). (Conditions: 0.5 g L−1, 10 mg
L−1 TCH, 150 W Xe lampsolar simulator).

Figure 6.7

Photocatalytic degradation of TCH, RhB, and MO under 178
LED light (a); OHP light (b); Photocatalytic degradation
xiv


of TCH under three different light sources (LED, OHP,
and solar simulator) (c); illustration of the photocatalytic
reaction system with MO dye under OHP light (d) using
Ag/AgBr@BiOBr/Bi2O3 catalyst (Conditions: Room
temperature, initial pollutant concentrations of 10 mg L−1,
catalyst loading of 0.3 g L−1).


Figure 6.8

Effect of the MO concentration (a) (catalyst loading of 0.5 181
g L−1); catalyst loading (b) (10 mg L−1 MO); pH of RhB solution
(c) (catalyst loading of 0.3 g L−1, 10 mg L−1 RhB); and operating
temperature (d) (catalyst loading of 0.3 g L−1, 10 mg L−1 RhB)
on the degradation efficiency under

Figure 6.9

OHP light.
Kinetic curve of RhB degradation in the presence of 184
different scavengers (a) and photocatalytic stability of the
catalyst in a 10 mg L−1 MO dye under OHP light (b) and TEM
images (c) and XRD pattern (d) of catalyst before and after nine
cycles.

List of Schemes

xv


Scheme 2.1

Schematic illustration of the synthesis of the ZnO/RGO 12
nanocomposites with three different ZnO morphologies.

Scheme 2.2


Mechanism of the photodegradation of dyes and
illustration of electron transfer between the RGO sheets
and ZnO under UV light in the presence of the ZnO/RGO
catalyst.

Scheme 3.1

Schematic illustration of the synthesis
CMoS2/octahedral TiO2 nanocomposite.

Scheme 3.2

Mechanism of the photodegradation of organic pollutants
and illustration of electron transfer under solar light in the
presence of the C-MoS2/TiO2 catalyst.

88

Scheme 4.1

Schematic
of

97

Scheme 4.2

Mechanism of the photodegradation and illustration of
charge transfer of photogenerated electrons and holes
under visible light in the presence of BiPO4/BiOBr/Bi2O3

catalyst.

Scheme 5.1

Schematic illustration of the
BiOCl/BiOBr/Bi2O3 nanocomposite.

of

50

the 59

illustration
of
the
synthesis
the BiPO4/BiOBr/Bi2O3 catalyst.

synthesis

of

114

the122

Scheme 5.2

Scheme 6.1


148
Mechanism of the photodegradation of organic pollutants
and illustration of electron transfer under light irradiation
in the presence of the BiOCl/BiOBr/Bi2O3 catalyst.
Schematic illustration
of
the
synthesis
of
the

158 Ag/AgBr@BiOBr/Bi2O3 catalyst.

xvi


Scheme 6.2

Mechanism of the photodegradation and illustration of

186

the charge transfer of the photogenerated electrons and holes
under visible light

in

Ag/AgBr@BiOBr/Bi2O3 catalyst.


xvii

the

presence

of


List of Tables
Table 2.1

Photocatalytic activity (catalyst loading of 0.1 g L‒1, dye

35

concentration of 10 mg L‒1, UV irradiation),
physicochemical,
Table 5.1

and optical

properties

ZnO/RGO nanocomposites
Comparison of the photocatalytic activity of the
BiOCl/BiOBr/Bi2O3 nanocomposite

xviii


of the
143


Chapter 1
Introduction
In recent years, water pollution has placed significant demands on water
treatment due to rapid population growth and economic development [1]. Various
physical and chemical techniques, such as adsorption, solvent extraction [2],
chemical

precipitation,

ion

exchange,

ultrafiltration,

reverse

osmosis,

electrochemical technologies [3], biodegradation [4], and photocatalysis [5‒7],
have commonly been used to remove pollutants from water. The adsorption
method allows rapid separation with high recoveries, but it does not remove the
pollutants completely. The physicochemical methods are useful for treating heavy
metals from wastewater but have significant drawbacks, such as high-energy
consumption, inconsistent removal effectiveness, and accumulation of harmful
sludge after treatment [8]. Although the biodegradation method based on the

metabolism of microorganisms can achieve complete removal of toxic organic
compounds, it has limitations, such as substrate inhibition at high concentrations
of toxic substrate, which in turn result in low biodegradation rates and high costs
[4,9].
On the other hand, the photocatalytic degradation method based on semiconductor
materials is an effective solution because of its low cost, easy operation, and high
efficiency [1].

1
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Photocatalysis includes photochemistry and catalysis, where light and catalyst
(semiconductors) are necessary conditions for chemical reactions to occur.
Semiconductors have bandgap with empty conduction band (CB) and filled
valence band (VB). As a photon with energy equal to or larger than bandgap
energy of semiconductors, the semiconductors can be activated and an electron
from the VB can jump to the CB. The generation of free electrons and holes is
essential for photocatalytic reactions. These photogenerated electrons and holes
are associated with simultaneous oxidation-reduction reactions for the degradation
of organic pollutants, which is different from other methods of wastewater
treatment. However, there are a number of challenges in photocatalysts and design
strategies to improve the photocatalytic activity properties such as the mismatch
between the bandgap of semiconductors and the solar spectrum, the inefficient
separation of electron-hole pairs, and the instability of the photocatalyst [10].
Therefore, various strategies have been employed to enhance the photocatalytic
activity of the photocatalyst under solar or visible illumination such as
semiconductor/carbon materials (graphene) [11], semiconductor/ semiconductor

heterojunction (type I, type II, and type III heterojunction)
[12], Z-scheme photocatalytic system and so on [10]. Although the
heterojunction-type photocatalytic system can achieve an efficient charge
separation by the migration of electrons in the CB and holes in the VB of two
semiconductors, the redox ability of photoinduced electrons and holes becomes
lower after the charge migration. Recently, the Z-scheme photocatalytic system

2
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has more attention because this photocatalytic system not only reduces the
recombination of electrons and holes but also retains prominent redox ability [13].
In this research, the researcher developed both synthesis methods and
photocatalytic materials to open up highly applicable industrial-scale potentials
including the fabrication capability of the photocatalyst and the application
potentials for wastewater treatment. Initially, the research focused on two common
wide bandgap semiconductor-based photocatalysts, namely, zinc oxide (ZnO) and
titanium dioxide (TiO2), and the photocatalysts were synthesized by a complicated
two-step approach to enhance the
photocatalytic activity under solar light. Finally, the research applied the rapid
microwave-assisted one-step route to synthesize bismuth-based photocatalysts to
not only enhance the photocatalyst activity under visible/solar light but also
improve the stability of photocatalyst. The research used organic pollutant
compounds as target contaminants such as methylene blue (MB), rhodamine B
(RhB),

and


methyl

orange

(MO),

tetracycline

hydrochloride

(TCH),

hydroxychloroquine (HCQ) in water to study the photocatalytic activity of the asprepared photocatalysts. The as-prepared samples were characterized by X-ray
diffraction (XRD), transmission electron microscopy (TEM), high-resolution
transmission electron microscopy (HR-TEM), Raman spectrometer, X-ray
photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Bio-Rad
Excalibur Series FTS 3000 spectrometer (FT-IR), BET physisorption analyzer,

3
THƯ VIỆN TRƯỜNG ĐẠI HỌC BÁCH KHOA – ĐẠI HỌC ĐÀ NẴNG.

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