Nghiên cứu cải thiện khả năng quang xúc tác của g-C3N4 biến tính với kim loại (Fe, Co, Mg, Ag) và oxit bán dẫn (TiO2, ZnO)

MINISTRY OF EDUCATION AND TRAINING
HANOI NATIONAL UNIVERSITY OF EDUCATION

-----------------------------------

LAM THI HANG

INVESTIGATING THE ENHANCEMENT OF
PHOTOCATALYTIC PERFORMANCE OF g-C3N4
MODIFIED WITH METALS (Fe, Co, Mg, Ag) AND

SEMICONDUCTING OXIDES (TIO2, ZnO)

Specialization: Solid State Physics
Code: 9.44.07.04

SUMMARY OF THE DOCTORY OF PHYSICS

Hanoi, 2024

The project was completed at:
HANOI NATIONAL UNIVERSITY OF EDUCATION

Science instructor:

1: Prof. Dr. Nguyen Van Minh
2: Assoc. Prof. Dr. Do Danh Bich

Review 1: Assoc. Prof. Du Thi Xuan Thao – Phenikaa University

Review 2: Assoc. Prof. Nguyen Dinh Lam – VNU University of

Engineering and Technology

Review 3: Assoc. Prof. Pham Van Hai - Hanoi National University
of Education

The thesis has been defended before the School-level Thesis Judging
Committee meeting at Hanoi National University of Education
on 2024

Thesis can be found at the library:
- National Library, Hanoi
- Library of Hanoi National University of Education

1

PREAMBLE
In the last few decades, on planet Earth, the rapidly growing textile, dyeing, tanning,
organic chemical and petrochemical industries have contributed significantly to organic
pollution of water resources. Organic toxins often released from these industries are
pesticides, herbicides, organic dyes, etc., which mix directly with clean water and pollute
water sources. Synthetic organic dyes used in the textile, leather and paper industries are
highly toxic, mutagenic, carcinogenic and seriously affect aquatic ecosystems and have the
potential to cause health problems. serious problems related to human health. Nowadays, the
treatment of environmental pollution, especially the treatment of water pollution, has
become a hot and concerned issue worldwide and the treatment of polluted water is a major
persistent challenge. by scientists around the world. Therefore, in the field of water
treatment, researchers have constantly made efforts and persistently discovered modern and
effective technologies to remove toxic organic substances from polluted water. In particular,
the technology of decomposing toxic organic substances by photocatalysis is a widely used
environmentally benign technique, using clean energy sources (natural light) to decompose

substances. organic pollutants into non-toxic or less toxic products and thus effectively
overcome environmental pollution. However, photocatalytic water treatment also faces
some challenges because its effectiveness depends on many different factors such as the
type of catalyst, wavelength of light, and bandgap of the substance catalysis.
Using semiconductor materials as catalysts in the process of treating water pollution
is a highly appreciated idea in the green chemistry industry (researching chemicals to treat
environmental pollution). Some popular types of materials that are currently being
researched include metal oxides (TiO2, ZnO, WO3 ...), ferroelectric materials with ABO3
perovskite structure (BiFeO3, BaTiO3, SrTiO3), ABO4 semiconductor compounds (ZnWO4,
SnWO4) …. However, most of these materials have a large band gap (> 3.2 eV), so they
almost only absorb light in the ultraviolet region, accounting for about 4% of the solar
spectrum. Currently, finding semiconductor materials with small band gaps is a topic that
attracts great attention from research groups around the world with the goal of taking
advantage of sunlight sources in applications. photocatalysis, helping to expand application
scale, reduce costs and increase convenience. Besides, narrow band semiconductor materials
also have great potential in the field of energy conversion or clean fuel production such as
Hydrogen and Oxygen. To meet the goal of using sunlight, semiconductor materials need
to meet a number of requirements such as: (i) band gap less than 3.2 eV (380nm); (ii) large
contact surface area and (iii) small electron and hole recombination rate.
Recently, the material g-C3N4, a non-metallic organic semiconductor with unique
electronic structure and optical properties with a small band gap (on the order of 2.7 eV),
has received attention. Extensive research by scientists around the world. The g-C3N4
material possesses a number of superior physical properties such as high hardness, non-
toxicity, chemical and temperature stability in different environmental conditions, large
specific surface area, and high efficiency. relatively high quantum and biocompatible,...
Therefore, this material has potential applications in a number of fields such as photoelectric
conversion, temperature sensing, chemical sensing, biomedicine, and especially in the field

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of photocatalysis to extract H2 fuel from water, decompose CO2 gas and clean organic
pollution in the water environment.

So far, g-C3N4 materials with diverse morphologies such as nanosheets, nanowires,
porous nanostructures and thin films have been researched and manufactured using different
technological processes such as vapor phase deposition ( CVD and PVD), solvothermal, and
pyrolysis from C- and N-rich precursors, etc. Unlike metal-containing semiconductor
photocatalysts, g-C3N4 can be easily synthesized by thermal polymerization from C and N
rich precursors such as dicyanamide, cyanamide, melamine and urea . However, research
shows that g-C3N4 material also has low quantum efficiency due to the high electron-hole
recombination rate; The absorption edge is at about 460 nm, so it only absorbs the blue light
region of the solar spectrum. Besides, g-C3N4 particles tend to cluster together, reducing the
specific surface area, leading to reduced photocatalytic efficiency. Recently, research on
modifying g-C3N4 materials to increase the lifetime of electron-hole pairs, reduce the band
gap energy and increase the specific surface area is the top priority solution for the research
of g-C3N4 materials.

Some basic measures to improve quantum efficiency and promote photocatalytic
activity of g-C3N4 materials include: (i) controlling surface morphology, creating thin
nanoleaf structures, porous structures or quantum dots, quantum wires, ... to increase the
specific surface area; (ii) combine the material with some other semiconductors to increase
the lifetime of the electron-hole pair, while reducing the band gap of the material; (iii)
coating the g-C3N4 surface with some metal nanoparticles that act as electron reservoirs (Pt,
Ag or Au nanoparticles); (iv) doping non-metal elements (P, S, O), transition metals (Fe,
Cu, Zn) to reduce the band gap while creating an electron capture center from the g-C3N4
crystal.

In Vietnam, research direction based on g-C3N4 materials is still quite new. Currently,
the material g-C3N4 has been initially deployed in the research group of Professor. Dr. Vo
Vien belongs to Quy Nhon University. The research team focuses on the technology of

manufacturing g-C3N4 material from melamine precursor and doping some non-metallic
elements (O, S) to enhance photocatalytic activity under visible light. of material g-C3N4. In
addition, the group also developed composite materials between g-C3N4 and GaN-ZnO or
Ta2O5. Research results show that the photocatalytic activity of composite materials
increases significantly compared to that of the component materials. The research team's
results supported two PhD students to successfully defend their PhD thesis in Chemistry. In
2018, the research group of Prof. Dr. Nguyen Ngoc Ha - Department of Chemistry, Hanoi
University of Education received funding from the National Foundation for Science and
Technology Development (Nafosted) for material research. nano composite materials based
on g-C3N4 and diatomite to effectively treat reactive dyes. In 2022, the PhD thesis of author
Dang Thi Ngoc Hoa of Hue University also researched the synthesis of g-C3N4 composite
for application in electrochemistry and photocatalysis. The author focuses on researching
composite materials such as ZIF-67/g-C3N4, ZIF-67/Fe2O3/g-C3N4, TiO2/g-C3N4 with
precursors for making g-C3N4 is melamine and focuses on photocatalytic decomposition of
Methylene Blue (MB), Diclofenac (DCF), Auramine O (AO).

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To our knowledge, apart from the above research groups, g-C3N4 material has not yet
been researched or widely announced in Vietnam. In this thesis, we choose to research the
production of g-C3N4 material from urea precursor using simple pyrolysis method, this is a
cheap chemical, easy to find, friendly and process research. Manufacturing technology to
achieve thin, well-crystallized foil samples, suitable for laboratory conditions of the
Department of Physics, Hanoi University of Education. From there, a good quality sample
was selected to conduct "Investigating the enhancement of photocatalytic
performance of g-C3N4 modified with metals (Fe, Co, Mg, Ag)". These metals are
cheap, chemically simple, have good conductivity and have been shown to have good
results in improving the photocatalytic ability of g-C3N4. Besides, we also chose to modify
with "semiconducting oxides (TiO2, ZnO)" because these are two promising photocatalytic
materials for environmental applications with outstanding properties such as: Good

photocatalytic properties, low cost, easy to manufacture and non-toxic.

Objectives of the thesis: (i) Research the influence of sample manufacturing
conditions on the structure, physical properties and photocatalytic ability of g-C3N4 material,
from which to select methods and conditions. Suitable technological conditions to
manufacture g-C3N4 thin-leaf material with good nano-crystalline size . (ii) Improve the
photocatalytic ability of g-C3N4 base material by modifying with metal elements (Fe, Co,
Mg, Ag) and combining materials with semiconducting oxides (TiO2, ZnO) to reduce the
band gap while creating an electron capture center, increasing the lifetime of the electron-
hole pair . From there, evaluate the influence of the concentration of modified metals as well
as the percentage of combined samples on the photocatalytic ability of g-C3N4 material.

Research subjects:
- Nano sheet material g-C3N4.
- Nanomaterial g-C3N4 modified with metals Fe, Co, Mg, Ag.
- Nanomaterial g-C3N4 combined with semiconductors TiO2 , ZnO.
Research Methods: The thesis is based on experimental methods, the sample is
manufactured mainly by polymerization through pyrolysis of N-rich organic precursors. A
number of manufacturing technologies are applied to synthesize the material. materials such
as pyrolysis in a noble gas environment, pyrolysis in an air environment.

Materials were manufactured at the Department of Physics and Center for Nano
Science and Technology, Hanoi University of Education. Fabricated samples are analyzed
for crystal structure and physical properties using a number of techniques such as: X-ray
diffraction (XRD), scanning electron microscopy (SEM, FE-SEM), electron microscopy.
transmittance (TEM) and high resolution transmittance (HRTEM), infrared absorption
spectroscopy (FTIR), surface area and pore volume measurement (BET), UV-Vis
absorption spectroscopy, fluorescence spectroscopy (PL), photoelectron spectroscopy
(XPS), Raman scattering spectroscopy.

The fabricated samples were used to perform photocatalytic processes for
decomposing 10 ppm RhB solution. The concentration of remaining organic compounds
was measured indirectly through UV-Vis optical absorption spectroscopy.

4

In addition, the thesis also uses a number of software to exploit and analyze and
calculate physical parameters of materials from experimental data such as Origin, UniCell,
ImageJ, JCPDS standard card library.

Scientific and practical significance of the project: With the orientation of
researching and applying g-C3N4 materials in the field of photocatalysis, the thesis has built
a process for manufacturing g-C3N4 base materials using the A simple method is urea
pyrolysis. This is a cheap but highly effective method of using precursors. This contributes
to proposing a technological process for effectively manufacturing semiconductor materials
that can be applied in the field of treating some organic waste in the aquatic environment .
Modifying the material by doping metals and combining it with other semiconductors
increases the photocatalytic ability of the g-C3N4 base material. The material has good
photocatalytic ability to decompose some organic compounds such as RhB, oriented for
application in decomposing some toxic organic substances in wastewater samples in
domestic and craft villages; Actively contribute to the process of cleaning the living
environment.

The content of the thesis includes: Overview of g-C3N4 materials, experimental
techniques, research results and analysis of the effects of sample manufacturing conditions;
The influence of Fe, Co, Mg, Ag metals on the structure, optical properties of materials and
photocatalytic ability of g-C3N4 base materials; Results of studying the structure and
properties of g-C3N4 materials combined with semiconductor TiO2 and ZnO.

Layout of the thesis: The thesis is presented in 145 pages with 22 tables and 109

figures, including an introduction, 5 content chapters, a conclusion, a list of research works
and references. As follows:

Introduction: Introduces the reason for choosing the topic, the object and purpose of
the research, and the scientific significance of the thesis.

Chapter 1: Presents an overview of the structural properties, morphology, physical
properties and some research on photocatalytic orientation of g-C3N4 materials. The typical
properties of g-C3N4 materials are the basis for analyzing results on pure g-C3N4 and g-C3N4
model systems denatured with metals and g-C3N4 combination in chapters 3, 4 and 5.

Chapter 2: Presents methods and procedures for sample fabrication, process for
evaluating photocatalytic ability, principles of measurements used in analyzing material
properties used in the thesis.

Chapter 3: Research on the effects of technological conditions on the crystal
structure, physical properties and photocatalytic ability of g-C3N4 materials.

Chapter 4: Research on physical properties and photocatalytic ability of g-C3N4
material modified with metals Fe, Co, Mg, Ag.

Chapter 5: Research on physical properties and photocatalytic ability of g-C3N4
material combined with semiconductors TiO2, ZnO.

Conclusion: Presents the main results of the thesis.
The main results of the thesis have been published in 07 scientific works

(including 04 articles published in international specialized journals, 03 articles
published in domestic specialized journals).

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Chapter 1 OVERVIEW

1.1. Material g-C3N4

1.1.1. Structural properties
g-C3N4 crystal has a hexagonal structure, belonging to the P 6̅m2 space group .

According to the research results, the base unit cell of the g-C3N4 crystal has 56

atoms, including 32 N atoms and 24 C atoms.

a) b)

Figure 1.2 (a) Unit cell and (b) AB-type layered structure of the crystal of g-C3N4.
1.1.3. Surface morphology of g-C3N4 material

Figure 1.6 TEM images of g-C3N4 material from different precursors.
The produced g-C3N4 material is usually in the form of a porous material.
However, the pore volume, pore size distribution and specific surface area of g-C3N4
depend on the precursor and material fabrication method.
1.1.4. Optical properties of g-C3N4 materials

The g-C3N4 layer unit with the gh-heptazine structure is a semiconductor with an
indirect band gap. Accordingly, the band gap value is 2.76 eV with the valence band
maximum (VBM) at point Γ and conduction band minimum (CBM) located at point
M. Meanwhile, the band gap energy in real The experimental range is from 2.67 eV
to 2.95 eV.
1.1.5. Photocatalytic properties of g-C3N4 materials

The photocatalytic ability of g-C3N4 can be applied to treat organic pollutants such
as: Rhodamine B (RhB), Methylene Blue (MB), Methyl Orange (MO), Phenol, ...
1.2. Photocatalysis mechanism and application potential of g-C3N4 materials
1.2.1. Photocatalytic mechanism of g-C3N4 materials
1.2.2. Application potential of g-C3N4 materials
1.3. Some manufacturing methods of g-C3N4 materials
1.3.1. Sol-gel method
1.3.2. Hydrothermal method
1.3.3. Heat polymerization method
1.4. Some research directions to improve photocatalytic properties of g-C3N4
materials
1.4.1. Combination of g-C3N4 with other materials

g-C3N4 can be combined with many other semiconductors to create
heterosemiconductor materials such as: TiO2, WO3, ZnO, Ag2WO4... Studies show that
the modification of g-C3N4 materials by uniform combination aims to reduce the
recombination of electron-hole pairs in the material to enhance the photocatalytic ability.

6

1.4.2. Modification of g-C3N4 with metallic elements

Modification of g-C3N4 material by doping elements Fe, Co, Mg, Cu, Na, K,
Zr, Mn... or coating metal nanoparticles such as Au, Ag, Pt on the surface material

surface g-C3N4 has been studied by several groups. These studies show that
modifying g-C3N4 material by uniform doping gives better photocatalytic ability than
pure g-C3N4 material.

Chapter 2 EXPERIENCE

2.1. Material manufacturing process

2.1.1. C fabrication of pure g-C3N4 material

2.1.2. Fabrication of g-C3N4 doped Fe/Co/Mg

2.1.3. Fabrication of g-C3N4 materials coated with Ag metal nanoparticles

2.1.4. The model systems are fabricated and studied in the thesis

Table 2.1 Symbols of the model systems used in the thesis

Pure g-C3N4 model system was made in Ar gas environment

Temper 450 o C 500 o C 550 o C 600 o C 650 o C
ature
change gCN(Ar450 gCN(Ar)500 gCN(Ar)550 gCN(Ar)600 gCN(Ar)650

Change 0.5 h 1.0 h 1.5 h 2.0 h 2.5 h
gCN(Ar)1.0h gCN(Ar)1.5h gCN(Ar)2.0h gCN(Ar)2.5h
time gCN(Ar)0.5h

Sample system g-C3N4 purified in air environment

Temper 450 o C 500 o C 550 o C 600 o C 650 o C
ature gCN-650
change gCN-450 gCN-500 gCN-550 gCN-600

Change 0.5 h 1.0 h 1.5 h 2.0 h 2.5 h
time gCN-0.5h gCN-1.0h gCN-1.5h gCN-2.0h gCN-2.5h

Model system g-C3N4 doped with Fe/Co/Mg metal

Doped 0% 3% 5% 7% 10%
with Fe g-C3N4 FeCN3 FeCN5 FeCN7 FeCN10

Doping 0% 7% 8% 10% 12%
Co g-C3N4 CoCN7 CoCN8 CoCN10 CoCN12

Mg 0% 7% 8% 10% 12%
doping g-C3N4 MgCN7 MgCN8 MgCN10 MgCN12

Model system g-C3N4 coated with Ag/Au metal

Coated 0.00M 0.005M 0.007M 0.01M 0.03M 0.05M 0.1M
with Ag g-C3N4 gCN/Ag
nanopar gCN/Ag gCN/Ag gCN/Ag gCN/Ag gCN/Ag
ticles 0.005M 0.007M 0.01M 0.03M 0.05M 0.1M

Au . 0.00M 0.001M 0.003M 0.005M 0.007M 0.009M
nanopar g-C3N4
gCN/Au gCN/Au gCN/Au gCN/Au gCN/Au
ticle 0.001M 0.003M 0.005M 0.007M 0.009M
coating

2.2. Photocatalytic testing of organic matter decomposition

2.3. Methods for investigating the physical properties of model systems

Measurements taken to analyze the properties of materials include: X-ray

diffraction measurements; Scanning electron microscopy measurement; High

resolution transmission and transmission electron microscopy; Raman scattering

7

spectroscopy; Infrared absorption spectrometry; Method of measuring absorption
spectroscopy; X-ray photoelectron spectroscopy method; Fluorescence spectroscopy
method; Differential thermal analysis method; Nitrogen adsorption-desorption
isotherm method.

Chapter 3 RESEARCH IN FABRICATION OF GRAPHITIC CARBON
NITRIDE MATERIALS g-C3N4

3.1. The g-C3N4 system was manufactured in an Ar atmosphere
3.1.1. Effect of calcination temperature
3.1.1.1. Crystal structure

Figure 3.1 (a) XRD diagram of g-C3N4 sample system made from Urea precursor in Ar
atmosphere at different temperatures ; (b) Change in crystal lattice constant according to

sample heating temperature.
The XRD pattern shows 3 diffraction peaks at the angle 2θ about 12.47°;
24.59°and 27.17°. The diffraction intensity increased sharply from the calcination
temperature of 450 oC to 550 oC and gradually decreased as the calcination
temperature continued to increase.
3.1.1.2. Surface morphology

Figure 3.2 SEM of g-C3N4 sample system fabricated from Urea in Ar atmosphere at varying
temperatures (a) 450, (b) 500, (c) 550 and (d) 600 C °for 2 time now.

8

Figure 3.2 shows that the sample calcined at a temperature of 450 °C has a
morphology similar to a large, uneven membrane with holes and many folds on the
surface.

Figure 3.3 (a) Nitrogen adsorption-desorption isotherm and (b) Barrett-Joyner-
Halenda (BJH) pore volume distribution curve of g-C3N4 material.

c) Chemical composition analysis
Figure 3.4a presents the composite XPS spectrum of sample gCN(Ar)550

showing characteristic peaks of elements C, N and O at energies of 288 eV, 400 eV and
533 eV.

Figure 3.4 X-ray photoelectron spectroscopy of g-C3N4 material fabricated in Ar
environment at 550 oC for 2 hours: (a) synthesized XPS spectrum and high-resolution XPS

spectrum of (b) N1s state, (c) ) C1s and (d)O1s
e) Absorption properties

9

All samples show absorption margins at about 450 nm. This absorption edge
corresponds to the region-to-band transition between the top of the valence band and
the bottom of the conduction band.

f) Luminescence properties

Figure 3.9 (a) Fluorescence emission spectra of the g-C3N4 material system calcined
for 2 h at different temperatures and (b) the component fluorescence emission peaks

of the gCN(Ar)500 sample .
The fluorescence intensity increased as the calcination temperature increased
from 450 oC to 500 oC then gradually decreased as the calcination temperature
continued to increase.

Figure 3.12 (a) Adsorption properties and photocatalytic activity of RhB decomposition of
the g-C3N4 model system calcination for 2 h at different temperatures and (b) RhB
decomposition rate over time.
When the calcination temperature is 550 oC, the decrease in RhB solution

concentration becomes very strong, completely decomposing 10 ppm RhB solution in
2 hours.
3.1.2. Effect of sample heating time

The results show that when keeping the
calcination temperature at 550 oC and increasing the
calcination time, the g-C3N4 crystals become more
ordered with a decreasing lattice constant. Large
calcination time changes the surface morphology of
the material, the g-C3N4 sheets become smaller,
thinner and more porous. Besides, the width of the
optical band gap also tends to decrease as the
heating time increases. The changes in structure,
morphology and physical properties greatly affect
the photocatalytic activity of g-C3N4 materials.

10

Sample g-C3N4 heated at 550 o C for 2 hours or more exhibits strong photocatalytic

activity, almost completely decomposing RhB after 2 hours of illumination with

Xenon lamp.

3.2. The g-C3N4 system is made in air

Similar to the model system calcined in the noble gas environment Ar, the

model system calcined in air was also researched, manufactured and investigated

when changing two factors: calcination temperature and calcination time. The first

system is the gCN-T system, in which the calcination time is kept constant for 2

hours and the sample calcination temperature is changed from 450 oC to 650 oC. The

second system is the gCN-t system, in which the calcination temperature is kept fixed

at 550 oC and the calcination time is changed from 0.5 hours to 2.5 hours. The

physical and photocatalytic properties of the materials are analyzed to come to a

conclusion which model system best suits the material's photocatalytic application

goals.

gCN sample heated at 550 oC

for 2 hours almost completely

decomposes RhB in 180 minutes of

illumination under Xenon lamp

light. (Figure 3.28a ). For the

sample system with different

calcination times (Figure 3.28b), the

results also show that the

photocatalytic ability depends on

the sample calcination time. The

photocatalytic ability of the samples

is in the order gCN-1.5 < gCN-2.5 < gCN-0.5 < gCN-1.0 < gCN-2.0.

In the two atmospheres of Ar gas and air, when Urea is heated at 550 oC for 2

hours, the material g-C3N4 exhibits the best photocatalytic performance. Therefore, in

the next research directions, we choose the calcination condition to make the g-C3N4.

material is 550 oC for 2 hours. g-C3N4 materials fabricated at a temperature of 550 oC

for 2 hours in an Ar noble gas atmosphere gave higher photocatalytic efficiency,

completely decomposing RhB solution in 120 minutes of lamp illumination. Xenon.

Meanwhile, the sample fabricated in air at the same conditions completely

decomposed RhB in 180 min.

Table 3.7 Compare the photocatalytic results of pure g-C3N4 of the thesis author with

Author some results published by other authors. Sun et al [93]
Thesis author Dong et al [28]

Photocatalytic 100% 100% Decompose 78.9
ability of pure g- decomposition of decomposition of % RhB 10 ppm
RhB 10 ppm after RhB 5 ppm after after 120 minutes
C3N4
180 minutes 300 minutes

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Chapter 4 INCREASING THE PHOTOCATATIC ABILITY OF g-C3N4
MATERIALS BY METAL DOPING

4.1. The g-C3N4 system doped with Fe metal
4.1.1. Structural properties

All samples did not exhibit any diffraction peaks of Fe crystals. The (101) and

(002) diffraction peaks shift slightly to the left as the Fe concentration increases
(Figure 4.2b). The lattice constants are calculated as (a = b = 4.97 Å, c = 6.47 ) and (a
= b = 4.98 Å, c = 6.48 ) for the doped samples, respectively FeCN3 and FeCN5. The
increase in crystal structure parameters shows a certain change of the g-C3N4 crystal
upon Fe doping, leading to a less dense structural pattern in the crystal lattice. This
change can be due to the alternating doping configuration of large radius Fe ions in
the g-C3N4 crystal by chemically bonding with the six unpaired electron-paired
nitrogen atoms as shown in Figure 4.2, leading to crystal lattice expansion.
4.1.2. Fluctuating nature

The intensity of all absorption peaks increased as the Fe content increased.
Magnification of the FTIR absorption peaks (Figure 4.3b) shows a slight shift of the
814 cm−1 peak toward higher wavenumbers as the Fe content increases.
Magnification of the FTIR absorption peaks (Figure 4.3b) shows a slight shift of the
814 cm-1 peak toward higher wavenumbers as the Fe content increases, to 812.1, 813,
813, and 813.9 cm-1 for the g-C3N4, FeCN5
and FeCN7 samples. Meanwhile, the peaks at
1240 cm-¹ and 1320 cm-¹ almost do not change
position. This further shows that the influence
of Fe impurity on the g-C3N4 lattice structure ,
although very small, leads to a slight
expansion of the benzene ring as observed in
the XRD analysis.
4.1.3. Nitrogen BET adsorption - desorption
spectroscopy results

The BET surface areas are 91, 100, 132 and
104 m2/g for g-C3N4, FeCN5, FeCN7 and
FeCN10, respectively. This result shows that
the specific surface area increases slightly when

doping Fe into the g-C3N4 crystal lattice . This
result shows that the specific surface area

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increases slightly when doping Fe into the g-C3N4 crystal lattice. Because a large specific

surface area is beneficial for photocatalytic activity, we predict that the FeCN7 sample

with the largest BET surface area will have high photocatalytic activity. Figure 4.4b also

shows that the average pore size of all samples is about 35-40 nm.

4.1.4. Optical absorption properties

The absorbance of this tail gradually increases with increasing Fe content,

which can be reasonably explained by the incorporation of Fe into the g-C3N4 lattice ,

leading to the formation of impurity energy levels in the restricted area.

Table 4.2 Band gap energy values of g-C3N4 samples doped with Fe with different

concentrations.

Sample gC 3 N 4 FeCN3 FeCN5 FeCN7 FeCN10

Eg ( eV) 2.92 2.83 2.8 2.81 2.8

4.1.5. Luminescent properties

It is clear that the PL

intensity of the Fe-doped g-C3N4

sample is significantly reduced

compared to that of the pure g-

C3N4 nanoparticles. Because the

luminescence intensity reflects

the recombination rate of

electron-hole pairs, the lower the

PL intensity, the slower the

recombination rate. The sharp decrease in PL intensity indirectly shows that the

recombination rate of electron-hole pairs is low, which is necessary for improving

photocatalytic performance. The reason for reducing the recombination rate of
electron-hole pairs may be due to the presence of Fe2+/Fe³+ ion in the g-C3N4 crystal

lattice, which acts as an electron capture center. When excited, the electron receives

energy from a photon and jumps from the top of the valence band to the bottom of the

conduction band to become a free electron. Then, the electron easily moves to the
impurity level of Fe3+ due to its location in the forbidden band. As a result, the

lifetime of the electron-hole pair increases, which is beneficial for the photocatalytic

process.

4.1.6. Chemical composition analysis

Figure 4.7 XPS spectra of Fe-doped g-C3N4 samples with different
concentrations.

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The pure g-C3N4 material exhibits characteristic peaks of C, N and O at 284 eV,
397 eV and 532 eV respectively while the FeCN7 sample also shows a sharp XPS
peak at 710 eV energy.
4.1.7. Photocatalytic properties of RhB . degradation

The photocatalytic performance was significantly improved for all Fe-doped g-C3N4
samples. The RhB decomposition rate gradually increases with the doping Fe content,
reaching the highest value for FeCN7 and then decreasing when increasing the Fe
content to 10%. RhB solution was almost completely decomposed after 30 minutes for
sample FeCN7 while sample FeCN5 needed about 50 minutes and sample FeCN10
needed about 60 minutes. A first-order kinetic model is used to determine the
photocatalytic reaction rate, ln(Co /C) = kt, where the rate constant k is calculated from
the slope of the linear relationship of plot of ln(Co/C) versus reaction time (Figure
4.8b). The FeCN7 sample exhibits the greatest reaction rate constant (k~0.117), which
is about 10 times larger than that of pure g-C3N4 (k~0.012). The order of samples with
strong to weak photocatalytic ability is FeCN7, FeCN5, FeCN10, FeCN3, g-C3N4. The

photocatalytic ability of the FeCN7 sample has good stability, the photodegradation
rate of RhB is about 95% after three cycles of reuse.
4.2. Model system g-C3N4 doped with Co metal
4.2.1. Structural properties

The position of the (002) diffraction peak shifts slightly toward the smaller 2 theta
angle when doped with Co. The (002) diffraction peak shifts toward the small 2 theta
angle, demonstrating the decrease of the value 𝑑in the formula 2𝑑𝑠𝑖𝑛𝜃 = 𝑛𝜆. From
there, it can be inferred that the lattice constant 𝑐increases slightly with this peak
shift. This can be explained by the large radius Co2+ ions filling the interstitial space
between the heptazine units, causing a slight stretch of the lattice, leading to an
increase in the lattice constant 𝑎, 𝑏, 𝑐.
4.2.2. Vibration properties

g-C3N4 sample and the Co-doped samples both exhibit scattering peaks at the

14

same positions. The analysis results of the peak position of 706 cm-1 corresponding to
the oscillations of heptazine units are shown in Figure 4.12b, showing that the peak
position of the Co-doped samples has a slight shift towards the wave number. smaller
than the pure sample. This observation can be attributed to the presence of Co
impurities interspersing the vacancies between the heptazine units in the g-C3N4
lattice that affects the oscillations of these units. With different impurity
concentrations, the effect is also different. This is consistent with the results of X-ray
diffraction analysis.
4.2.3. Chemical composition analysis

Figure 4.15 shows typical XPS spectra of the C1s and N1s states. The C1s peak of pure g-
C3N4 is analyzed into 3 peaks at positions with binding energies of: 283.0 eV representing the

C-C bond, 284.6 eV representing the C=N bond and 286.2 eV characterize the N-C=N bond.
Regarding the characteristic peak C1s of sample CoCN10, we found that the peak is also
decomposed into 3 component peaks at 281.6; 283.1 and 285.0 eV. However, based on the
intensity and shape of the spectral peak, we determine that the 285.0 eV peak corresponds to the
C=N bond. Accordingly, the XPS peak tends to shift slightly toward higher binding energy
(horizontal arrow). Similarly, the characteristic XPS spectrum of the N1s state in Figure 4.15b
also shows a separation into three component peaks corresponding to C-N=C bonds at 395.9;
N-(C)3 at 397.2 and C-H-N at 399.2 eV. These peaks also tend to shift slightly toward higher
binding energies when doped with Co. The characteristic peak of the Co2p state includes two
component peaks at 780.7 eV and 794.7 eV corresponding to two energy states with
different spin levels Co2p3/2 and Co2p1/2 . These are the binding energy levels corresponding
to the ionic state of the Co impurity that exists in the CoCN10 sample.
4.2.4. Optical properties

15

Research shows that Co doping does not change the optical absorption properties

of the g-C3N4 material. The fluorescence peak intensity of Co-doped samples tends to

decrease gradually compared with pure samples. Detailed analysis of the position of

the component fluorescence peaks (Figure 4.18b) of the 2 samples g-C3N4 and

CoCN10 revealed a slight shift of the fluorescence peak towards the low wavelength

for the P3 and P4 peaks . This is relatively consistent with the observation in the UV-

vis spectrum, that the Co impurity does not significantly affect the energy band

structure of the g-C3N4 material, however, changes the shape of the absorption base.

The gradual decrease in fluorescence intensity indirectly reflects the reduced amount

of electron-hole recombination, which is favorable for photocatalysis. Sample

CoCN10 has the lowest fluorescence intensity, promising for the best photocatalytic

performance.

4.2.5. Test of

photocatalytic activity

The order of

samples with

photocatalytic ability

from strong to weak is

CoCN10 > CoCN12 >

CoCN8 > CoCN7 > g-

C3N4 and is the same

for both cases using

different light sources.

4.3 . Sample system g-

C3N4 metal doped Magnesium Mg

4.3.2. Analysis of the chemical

composition on the surface

Characteristic XPS spectrum

analysis of the Mg2p state is

presented in Figure 4.21, showing

the existence of the impurity

element Co in the Co2+ state on the

surface of the g-C3N4 crystal.

4.3.3. Test of photocatalytic ability

Observing Figure 4.23 it was

found that the photocatalytic

treatment results under sunlight

and Xenon lamp light were

almost the same. Sample

MgCN10 has the best

photocatalytic performance; RhB

solution is almost completely

decomposed after 60 minutes of

illumination with the catalyst

16

MgCN10. While at the same time, the catalyst, the sample g-C3N4, only decomposed
more than 50% of the RhB solution of the same concentration. The order of samples
with catalytic ability from strong to weak is MgCN10, MgCN8, MgCN12, MgCN7,
g-C3N4.
4.4. Results of studying the properties of Ag-coated g-C3N4 materials
4.4.1. Research results on structural properties and grain morphology of g-C3N4
coated with Ag
Figure 4.24a presents the
diffraction pattern (XRD) of
the fabricated Ag-coated g-
C3N4 samples. The results show
that, g-C3N4 and g-C3N4
nanosheets coated with Ag
clusters with different Ag+

concentrations exhibit similar
diffraction peaks. The pure g-
C3N4 sample exhibits three
distinct diffraction peaks at
about 13.00; 24.93 and 27.65o correspond to the diffraction planes (100), (101) and
(002) of the hexagonal phase of the graphite carbon nitride crystal (JCPDS tag
number 87-1526). It can be seen that the XRD intensity of Ag-coated g-C3N4 samples
gradually decreases as the Ag+ concentration in the initial solution increases . In
addition, the (002) peak position has a slight shift toward the larger 2θ angle. To
clearly observe the shift of the (002) peak position, we normalized this peak intensity
and then fit it using a Gaussian function. Figure 4.24b shows the fitting curves of the
(002) peak of the fabricated samples and the displacement of the peak position can be
clearly seen. These observations indicate that the Ag clusters had some influence on
the crystal structure of g-C3N4. However, the XRD patterns of the samples also
showed that no diffraction peak corresponding to Ag crystals was observed in all Ag-
coated g-C3N4 samples.

Figure 4.25 TEM images of pure samples g-C3N4 (ab) and g-C3N4 /Ag 0.01M (cd).
The inset of figure (c) shows the Ag nanocluster diameter histogram.

17

Figures 4.25c and 4.25d demonstrate the presence of densely distributed and
uniformly distributed small sphere-like Ag NPs decorated on the surface of g-C3N4
nanosheets. The inset of Figure 4.25c shows a histogram of Ag NP diameters with
particle sizes ranging from 3 to 5 nm (average diameter is 4 nm).
4.4.2. Results of X-ray photoelectron spectroscopy (XPS) research

Figure 4.26 XPS spectrum (a) and XPS spectrum of C1s (b), N1s (c) and Ag3d (d) atoms of
samples g-C3N4 and g-C3N4 /Ag 0.01M.

Elemental Ag was detected in a 0.01M gCN/Ag sample at a binding energy of
about 368 eV.
4.4.3. Research results on optical properties

g-C3N4 nanosheets pure
show the absorption edge at
about 430 nm and the
absorption edges of the g-C3N4
samples coated with Ag
nanoparticles shifted slightly
to longer wavelength,
indicating a narrowing band
gap . Accordingly, the
bandgap energy estimated
using the graph of Tauc (inner part of Figure 4.27a) for the indirect semiconductor
was reduced from 2.88 eV for the 4 crystalline g-C3N4 nanosheet purity down to 2.82
eV for gCN/Ag 0.01M sample. Furthermore, Figure 4.27a also exhibits an increase in
absorbance around 400 nm (arrows pointing upwards) for Ag-coated g-C3N4 samples,
which can be assigned to the absorption due to plasmon resonance surface (SPR) of
Ag NPs.

18

4.4.4. Results of research on photocatalytic ability

g-C3N4 samples showed

enhancement in both

adsorption and

photocatalytic efficiency

significantly. Although

after 120 min of

irradiation using pure g-

C3N4 nanoplates About

25% RhB still existed, but

some Ag-coated samples

such as gCN/Ag 0.01M,

gCN/Ag 0.03M and

gCN/Ag 0.1M decomposed almost 100% RhB after only 60 minutes. The 0.01M

gCN/Ag sample exhibited the strongest photocatalytic activity in the decomposition

of RhB, as evidenced by the largest slope of the C/Co curve. The order of samples

with increasing photocatalytic efficiency is g-C3N4, gCN/Ag0.005M, gCN/Ag0.007M,

gCN/Ag0.05M, gCN/Ag0.1M, gCN/Ag0.03M, gCN/Ag0.01M and gCN/Ag0.01M.

Figure 4.29b shows the obvious change of RhB concentration in the UV-vis

absorption spectrum of the 0.01M gCN/Ag heterodimer as a function of time. After

50 min of Xenon irradiation, the 554 nm absorption peak of RhB not only

disappeared completely but also changed from 554 nm to 530 nm, demonstrating the

decomposition of the conjugated RhB structure.

From the photocatalytic

results of RhB decomposition

of the obtained material

systems, we compared the

ability to decompose RhB of

pure g-C3N4 samples, FeCN7,

CoCN10, MgCN10 and

gCN/Ag 0.01M. as shown in

Figure 4.30. The results

showed that, after only 30

minutes of irradiation, FeCN7

samples decomposed 100% of RhB, CoCN10, MgCN10 and gCN/Ag 0.01M samples

decomposed about 75, 82 and 91%, respectively, while g-C3N4 samples decomposed

g-C3N4 can only decompose about 52% of RhB. The first-order kinetic model is also

used to determine the photocatalytic reaction rate of the above samples , ln(C o /C) = kt,

where the rate constant k is calculated from the slope of the bond. linear relationship of

the graph ln(Co/C) compared to reaction time (Figure 4.30b). The complete

decomposition time of the 10 ppm RhB solution and the k rate of pure g-C3N4, FeCN7,

CoCN10, MgCN10 and 0.01M gCN/Ag samples are listed in Table 4.5. Table 4.5

shows that the material g-C3N4 doped with 7% Fe (FeCN7) has a reaction rate of

0.117 , 9.75 times higher than pure g-C3N4 (k = 0.012 ) . Samples of g-C3N4 doped

with Co, Mg 10% (CoCN10, MgCN10) and samples of g-C3N4 coated with 0.01M Ag