Tổng hợp và nghiên cứu khả năng ứng dụng của hạt nano vàng phân bố trên vật liệu polyme carbon nitride = sythesis and application of polymeric carbon nitride decorated with gold nanoparticles

➢ Chapter 3: p-C 3N4 - synthesis, characterization, and application In this chapter, different synthesis routes under various conditions and synthesis treatment methods of p-C3N4 are pr

GIAP VAN HUNG

Hung.GV202683M@sis.hust.edu.vn

Master of Science in Chemistry

Supervisor 1: Dr Nguyen Thi Tuyet Mai Signature

Institute: Chemical Engineering Institute

Supervisor 2: Dr Norbert Steinfeldt Signature

Institute: Leibniz Institute for Catalysis (LIKAT)

HANOI, OCTOBER 2022

DECLARATION OF INDEPENDENCE

I hereby declare that I have prepared this thesis independently and under the

guidance of my supervisor, without using any support and resources other than the

RoHan Project funded by the German Academic Exchange Service (DAAD, No

57315854) and the Federal Ministry for Economic Cooperation and Development

(BMZ) inside the framework "SDG Bilateral Graduate school programme

Hung Giap Van

ACKNOWLEDGEMENTS

First, I would like to express my deepest gratitude to my co-supervisors Dr Norbert Steinfeldt (LIKAT) and Dr Nguyen Thi Tuyet Mai (HUST), for his help and support throughout my thesis work Without them supervision, I would not have been able to complete my thesis

Special thanks to the Department of Physical Chemistry, Institute of Chemical Engineering, Hanoi University of Science and Technology together with the research group of Prof Jennifer Stunk at Leibniz Institute of Catalysis (LIKAT Rostock) created favorable conditions for me to study, research and completed this thesis

Finally, I would like to thank my family, friends, and relatives has always accompanied, encouraged, and shared with me They are a great source of motivation for me to continue my research path

ABSTRACT

Photocatalytic hydrogen generation is increasingly perceived as a potential alternative to traditional hydrogen production method [1] Polymeric carbon nitride (p-C3N4) has been considered as a promising photocatalyst candidate due

to its low cost, chemical stability, and visible light reactivity However, the performance of pure p-C3N4 is limited due to the rapid recombination of photoinduced electron-hole pairs Therefore, a lot of methods were explored to improve the activity of p-C3N4

In this work, p-C3N4 was prepared in different atmospheres, treated with

H2O2 or ultrasound, and then investigated with Au deposition to optimize its photocatalytic performance Before testing in hydrogen evolution reaction (HER) with triethanolamine (TEOA) as sacrificial agent, the materials were characterized

by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FTIR), UV-vis diffuse reflectance spectroscopy (DRS), thermal gravity analysis (TGA), elemental analysis (EA), and photoluminescence spectroscopy (PL) The synthesized materials showed improved charge separation efficiency and H2 evolution performance The highly-improved performance is ascribed to the efficient transfer of photo-generated electrons among molecule p-

C3N4 and Au, which is supported by photoluminescence spectra and transient

photocurrent responses

TABLE OF CONTENTS

ACKNOWLEDGEMENTS 3

ABSTRACT 3

TABLE OF CONTENTS 4

LIST OF TABLES 6

LIST OF FIGURES 7

LIST OF ABBREVIATIONS 10

CHAPTER 1 INTRODUCTION 11

1.1 Background 11

1.2 Objectives and outline of the thesis 11

1.2.1 Thesis outline 11

1.2.2 Thesis Objectives 12

CHAPTER 2 LITERATURE REVIEW 13

2.1 Introduction 13

2.2 Graphitic Carbon Nitride (p-C3N4) 14

2.2.1 History 14

2.2.2 Synthesis of p-C3N4 15

2.2.3 Physical and Chemistry properties 21

2.2.4 Modifications 22

2.2.5 p-C3N4 based nanocomposites 25

2.2.6 Trends of photocatalytic development of p-C3N4 27

2.3 Plasmonic Photocatalysis 28

2.3.1 Fundamentals 29

2.3.2 Surface plasmon benefits for photocatalysis 30

2.3.3 Metal nanoparticle/semiconductor junction 32

2.3.4 Gold nanoparticles based on p-C3N4 33

CHAPTER 3 p-C3N4 - SYNTHESIS, CHARACTERIZATION, AND

APPLICATION 34

3.1 Experimental 34

3.1.1 Preparation of p-C3N4 34

3.1.2 Post-treatment of p-C3N4 35

3.1.3 Material characterization 36

3.1.4 Photocatalytic -HER reaction 37

3.1.5 Electrochemical characterization 38

3.2 Results and Discussion 38

3.2.1 Structural and thermal properties 38

3.2.2 Optical and electrochemical properties 47

3.2.3 Evaluation of photocatalytic activity toward H2 evolution under white light irradiation 50

CHAPTER 4 Au/p-C3N4 COMPOSITES – SYNTHESIS, CHARACTERIZATION, AND APPLICATION 52

4.1 Experimental 52

4.1.2 Material characterization 54

4.1.3 Photocatalytic and electrochemical measurement 54

4.2 Results and Discussion 55

4.2.1 Effect of the p-C3N4 support 55

4.2.2 Effect of Au loading 61

4.2.3 Effect of Au deposition method 66

4.2.4 Possible photocatalytic mechanism 71

CHAPTER 5 GENERAL CONCLUSIONS AND OUTLOOK 73

5.1 General conclusions 73

5.2 Outlook 73

REFERENCES 75

LIST OF TABLES

Table 1 Position of the (002) reflection of p-C3N4 support 39 Table 2 Summary of characteristic parameters determined based on N2 adsorption-desorption isotherms of different p-C3N4 samples 42 Table 3 Near surface composition of various p-C3N4 heterostructures 45 Table 4 Atomic ratio of C, H, N of various p-C3N4 obtained by elemental analysis 46 Table 5 Band gap values of various p-C3N4 calculated from Tauc's method 48 Table 6 Position of p-C3N4 (002) reflection and crystallite size of Au nanoparticles

of 2 wt.% Au/p-C3N4 samples 56 Table 7 Table of band gap energy of 2 wt.% Au/p-C3N4 calculated from Tauc's method 58 Table 8 Crystallite size, nano size of different wt.% Au based on p-C3N4 (air) 62 Table 9 Crystallite size and band gap of Au/p-C3N4 synthesized by chemical reduction method 68

LIST OF FIGURES

Figure 2 1 Schematic diagram of the synthesis of p-C3N4 from the thermal polymerization of melamine, cyanamide, dicyandiamide, urea and thiourea [46] 16 Figure 2 2 Postulated Condensation of Melamine [63] 17 Figure 2 3 Schematic of self-modification to promote oriented vacancies [78] 19 Figure 2 4 Using NH3 atmosphere in the synthesis of p-C3N4 [79] 20 Figure 2 5 a) The pyrolysis time-layer thickness of p-C3N4 [84] b) Synthesis pathway of structurally distorted p-C3N4 nanosheets [87] 21 Figure 2 6 Scheme of monolayer (a) crystalline and (b) amorphous monolayer of graphite carbon nitride [89] 22 Figure 2 7 a) The “hard” template approach combines the sol-gel/thermal condensation method [112] b) Different morphologies can be obtained by the

“Soft” and “Hard” slide approaches based on p-C3N4 [97] 24 Figure 2 8 Schematic representation of the charge pairs in p-C3N4 is shown with decay lines, where A represents the electron acceptor while D is the donor[46] 27 Figure 2 9 Typical bandgap energy values of semiconductor photocatalysts and redox potentials of processes including H2O separation, CO2 reduction and pollutant decomposition (reaction carried out with pH = 7) [46] 28 Figure 2 10 Schematic diagram of plasmon oscillations after application of an electric field, and electron cloud formation [145] 30 Figure 2 11 Scheme of the electromagnetic field intensification on M NP [146] 31 Figure 2 12 Mechanism of hot electron injection from metal/plasmonic nanoparticles into the semiconductor conduction band [146] 32 Figure 3 1 Schematic overview about process synthesis of pure p-C3N4 34 Figure 3 2 The crucible used in the synthesis of p-C3N4 34 Figure 3 3 Experimental system treatment p-C3N4 by H2O2 solution (left), the p-

C3N4 mixture after treatment by H2O2 (right) 35 Figure 3 4 Treatment p-C3N4 using the BANDELIN SONOPULS HD 2070 ultrasonic transducer system 36 Figure 3 5 Scheme of the experimental set-up used for study of HER reaction with p-C3N4 catalysts under irradiation with white light 37 Figure 3 6 XRD patterns of various p-C3N4 samples 39 Figure 3 7 ATR-IR spectra of all various p-C3N4 40

Figure 3 8 N2 adsorption-desorption isotherms and corresponding pore size

distribution curves (inset) of various p-C3N4 heterostructures 41

Figure 3 9 The fully survey spectra of XPS spectra for various p-C3N4 heterostructures 42

Figure 3 10 XPS spectra of C1s, N1s, O1s of all samples 44

Figure 3 11 TGA-DSC analysis of p-C3N4 support 46

Figure 3 12 UV-vis DRS spectrum of various p-C3N4 samples 47

Figure 3 13 PL spectra of various p-C3N4 49

Figure 3 14.a) Transient photocurrent responses and b) EIS of various p-C3N4 under visible light irradiation 50

Figure 3 15 Temporal H2 evolution of p-C3N4 samples synthesized under (a) air (300 W Xenon lamp) or (b) Ar atmosphere (1000 W Xenon lamp) after in-situ Au deposition at irradiation with white light (Au amount 2 wt.% ) 51

Figure 4 1 Schematic illustration of synthetic Au/p-C3N4 by photodeposition method 52

Figure 4 2 Schematic synthesis of Au/p-C3N4 synthesis by chemical reduction method, using NaBH4 reducing agent 53

Figure 4 3 Schematic synthesis of Au/p-C3N4 synthesis by chemical reduction method, using NaBH4 reducing agent with surfactant: sodium citrate, PVP 54 Figure 4 4 LED arrays irradiating visible light (24 V, 1.12 A, 24 W) 55

Figure 4 5 XRD patterns of synthesized Au/p-C3N4 composites 56

Figure 4 6 (a) ATR-IR spectra and (b) UV-vis DRS spectra of the different 2wt.% Au/p-C3N4 57

Figure 4 7 PL spectra of 2 wt.% Au/p-C3N4 59

Figure 4 8 Transient photocurrent (a) and EIS (b) of Au/p-C3N4 under visible light irradiation 60

Figure 4 9 Time course of H2 evolution of Au/p-C3N4 (a) under white and (b) visible light irradiation 60

Figure 4 10 XRD patterns of Au/p-C3N4 (air) with different wt.% Au 62

Figure 4 11 SEM images of (a) 2wt.% Au and (b) 8 wt % Au based on p-C3N4 (air) 63

Figure 4 12 ATR-IR spectra of with different wt.% Au based on p-C3N4 (air) 64 Figure 4 13 (a) UV-vis spectra and (b) PL spectra of different wt.% Au based on p-C3N4 (air) 65

Figure 4 14 Time course of H2 evolution of Au/p-C3N4 (air) with different wt.% Au under white light 66

Figure 4 15 Characterizations (a) XRD, (b) ATR-IR, (c) PL and (d) UV-vis DRS

of different methods of Au/p-C3N4 synthesis 67 Figure 4 16 SEM images of Au/p-C3N4 synthesized by chemical reduction using (a) NaBH4 in dark, (b) NaBH4 + Sodium citrate and (c) NaBH4 + PVP 69 Figure 4 17 XPS spectra of various p-C3N4 and Au/p-C3N4: (a) Fully scanned spectra of various p-C3N4 compare with Au/p-C3N4, (b) C 1s, (c) N 1s, (d) O 1s and (e) Au 4f 70 Figure 4 18 Time course of H2 evolution of Au/p-C3N4 synthesized samples (by different methods) under white light 71 Figure 4 19 Schematic illustration of the mechanism H2 evolution reaction by Au/p-C3N4 composite from water in the presence of TEOA 72

CHAPTER 1 INTRODUCTION 1.1 Background

The growing energy demand is a current problem which requires urgent actions, thus finding renewable energy sources is essential [2, 3] Among the well-known energy sources (solar, wind, water, geothermal, biomass and nuclear etc.), hydrogen is a promising one given by its wide range applicability (e.g., as fuel) [4-7] Photocatalytic hydrogen generation has received considerable attention, and it

is a promising hydrogen production method [1]

To obtain the desired catalytic activity for H2 formation, the selection of a semiconductor photocatalyst is an important factor [8-12] The widely used application of p-C3N4 inphotocatalysis can be explained by its low cost, high stability, and visible light activity [13] However, the performance of pure p-C3N4

is limited due to the rapid recombination of photoinduced electron-hole pairs Therefore, a lot of methods were studied to improve the activity of p-C3N4 In this thesis, p-C3N4 was prepared in different atmospheres, treated with H2O2 or ultrasound, and then evaluated with Au deposition to optimize its photocatalytic performance The synthesized materials showed improved charge separation efficiency and H2 evolution performance

1.2 Objectives and outline of the thesis

1.2.1 Thesis outline

➢ Chapter 1: Introduction

In this chapter, we will introduce the topic of the thesis, including previous studies, thereby giving reasons for choosing the topic for this thesis

➢ Chapter 2: Literature Review

This chapter will show the knowledge about p-C3N4 from the history of development, research, to its photocatalytic potential This is followed by an introduction on plasmonic photocatalyst materials, presenting the potential metal nanoparticles with surface plasmon effects Previous works on plasmonic materials used in combination with p-C3N4 with photocatalytic applications are also presented, including Au and p-C3N4-based composites

➢ Chapter 3: p-C 3N4 - synthesis, characterization, and application

In this chapter, different synthesis routes (under various conditions) and synthesis treatment methods of p-C3N4 are presented In addition, basic material

post-characterization methods are applied to study the formation and properties of

p-C3N4 to obtain information on the structural, optical, and photocatalytic properties

of the obtained materials Then, the photocatalytic hydrogen production ability of the prepared materials is assessed

➢ Chapter 4: Au/p-C 3N4 composites – synthesis, characterization, and application

This chapter focuses on the influence of p-C3N4 support, wt.% Au loading, and Au deposition methods on H2 conversion capacity: (preparation of Au/p-

C3N4)

➢ Chapter 5: General conclusion and outlook

This chapter contains conclusions from the presented work and points out the advantages and limitations of the topic Finally, suggestions are given for future research

1.2.2 Thesis Objectives

The main objectives of this master thesis are:

➢ Synthesis of p-C3N4 from urea under different atmosphere, and treatment of the prepared p-C3N4 with H2O2 or ultrasound

post-➢ Fabrication of composite materials based on the combination of Au and

p-C3N4.

➢ Characterization of their structural, optical, and photocatalytical properties

➢ Evaluation of the photocatalytic ability of p-C3N4 and Au/p-C3N4

➢ Searching for correlation between material properties and the photocatalytic activity in the HER reaction

CHAPTER 2 LITERATURE REVIEW 2.1 Introduction

Finding sustainable solutions for the problem of excessive energy demand (oil, coal, natural gas ) is a current challenge [14-16] Solar energy is known as a popular and abundant source of clean energy, and its conversion into other forms

of energy is widely applied [17-20]

During illumination, light induces series of photocatalytic reactions The catalyst can store the energy in chemical bonds [21-24], thus providing potential perspective to convert light energy into chemical energy,

Currently, scientists have made important strides in the development of semiconductor photocatalysts, but however further developments and improvements are needed [25-30] Therefore, the challenge is to find a photocatalytic material that can absorb the maximum spectrum of sunlight, which has great significance in today's fuel science

In summary, there are 3 steps to generate H2 and O2 in photocatalytic water splitting reaction: (1) generation of charge carriers under irradiation, (2) migration and separation of charge carriers, (3) generation of H2 or O2 from the reaction between H2O and charge carriers (electrons, holes) [31, 32] The photocatalytic properties of the catalyst can affect the ability to convert solar energy The absorption of sunlight is promoted when the photocatalyst has a narrow band gap The relatively negative conduction band enhances the reduction capacity of water for the purpose of generating H2 and the relatively positive valence band improves the oxidation potential of water to produce O2 [33-36]

Therefore, the development of photocatalysts with suitable band gap, efficient photogenerated charge carrier separation ability, and large surface area has great significance in improving the solar spectral absorption of the catalyst [37-41]

In the structure of p-C3N4, the π-conjugate system is formed based on sp2

hybridization of C and N atoms, giving p-C3N4 a relatively low band gap value in the carbon nitride family [13, 42-47] Typically, p-C3N4 has a band gap of about 2.7 ~ 2.8 eV, which allows it to absorb visible light up to ~460 nm p-C3N4 was considered as a metal-free organic semiconducting catalyst, the structure of p-C3N4

contains the aromatic C−N heterocycles in the p-C3N4 network In addition to this, the thermal and chemical stability of the material has important implications in its photocatalytic applications [48]

However, p-C3N4 has also some drawbacks which restrictits practical application These limitations include a) a relatively high recombination rate of charge carriers, b) limited absorption in the wavelength range above 460 nm, and c) extremely low conductivity [49, 50] To improve and optimize its photocatalytic activity, many methodologies have been proposed and tested to modify and control bulk p-C3N4 including band gap, heterostructure formation, electronic structure, (0-3D) shape control of p-C3N4 [51] Incorporation of metallic (e.g., Ni and Co) and non-metallic (e.g., N, C, B and P) ions into the structure of p-C3N4 is the most common strategy today During the copolymerization process, the doping procedure leads to the modification of the electronic structure and energy band configuration in the matrix of p-C3N4, which in turn leads to a change in the absorption region [52, 53]

The control of layers and geometrical shapes of p-C3N4 can be achieved by using different synthesis methods, such as, mechanical, and chemical exfoliation method, hard and soft templating method, and supramolecular preorganization methods [54-56] In fact, enhancing charge separation performance is very important to improve the photocatalytic performance, the p-C3N4 heterostructure possesses a suitable structural band in the heterojunction interface, whichhas been widely studied

2.2 Graphitic Carbon Nitride (p-C3N4)

However, due to the lack of modern analytical methods he was unable to distinguish the structures of p-C3N4 and K3C6N7NCN In 1922, during the pyrolysis

of Hg(SCN)2, in the final step of the deamination of a carboxylic acid, Franklin

[58] Since he could not demonstrate the phase change in p-C3N4, therefore his experiments were not convincing, he proposed the structure of p-C3N4 as 2, 5, 8 triamino-tri-s-triazine (C126H21N175)

In this context, the tri-s-triazine (s-heptazine) unit was proposed by Pauling and Sturdivant, which assumed the formation of the basic structure of p-C3N4 from three coplanar rings, the results were confirmed in terms of XRD pattern Three years later, Lucas and Redemann proposed structures for melam, melem and melon after discovering many anomalies in Liebig and Franklin's proposal for melon composition Their report indicates that the "melon" has a graphite structure [59] From 1950-1980, a series of experiments performed by Finkelstein and colleagues confirmed different derivatives of p-C3N4 [60]

In 1989, Liu and Cohen studied and hypothesized that carbon nitride

(β-C3N4 , sp3) is a super-hard material, even harder than diamond, which raised the bar research works on p-C3N4 materials [61] In 1996, Teter and Hemley also presented a theoretical study proposing a triazine-based structure for p-C3N4 [62] However, in 2008, Wang, Antonietti et al., reported a very important analysis of the H2 and O2 conversion capacity of the p-C3N4 photocatalyst under visible irradiation with aid of an electron donor and acceptor [13]

Since then, research on p-C3N4 has been greatly increased, focusing on the importance of p-C3N4 in the field of photocatalytic materials research

2.2.2 Synthesis of p-C 3 N 4

There have been many published reports on the synthesis of p-C3N4 via physical vapor deposition (PVD), chemical vapor deposition (CVD), thermosetting method, condensation method, … method of thermal irradiation and solid state reaction [43] Among the above-mentioned routes, thermal condensation method is the most accepted and used, due to its simplicity and time efficiency Typically, inexpensive nitrogen-rich species are used as organic precursors for thermal polycondensation Fig 2.1 shows the molecular structures

of several inexpensive nitrogen species used as precursors

Figure 2 1 Schematic diagram of the synthesis of p-C 3 N 4 from the thermal polymerization of melamine, cyanamide, dicyandiamide, urea and thiourea [46]

During thermal decomposition, the polyaddition and polycondensation mechanistic pathways for each precursor are different, but they have one thing in common: both undergo melamine formation In Wang's report, upon pyrolysis of cyanide molecules between 200-235°C, the cyanamide molecules were condensed

to dicyandiamide and then to melamine (aromatic mass) [13] After 300°C, the

p-C3N4 structure was formed that had cross-species homology, independent of the initial precursor The thermal polycondensation mechanism of melamine is shown

in Figure 2 The pyrolysis of melamine to 320°C will lead to self-condensation, through which melam and melem structures are formed The temperature continues to increase, condensation continues and deamination also takes place, when the melem and melam structures are relatively stable, a melon structure will appear, at 390°C

When the temperature reaches 520°C, crystalline phases of p-C3N4 appear leading to the formation of tri-s-triazine and s-triazine, theoretically, at about 600°C, tri-s-triazine becomes unstable, beyond 700°C the polymer p-C3N4 will decompose [63] (Fig 2.2) Although the formation of melamine intermediates is controversial, the thermal polycondensation mechanism is still the most recognized and used

Figure 2 2 Postulated Condensation of Melamine [63]

With Wang's et al evidence for the formation of p-C3N4, besides explaining the mechanism of thermal condensation, this can be considered as a milestone for its photocatalytic applications Another promising feature of p-C3N4 is related to the fact that it is a metal-free semiconductor with implications for H2 conversion under visible light irradiation [13]

Several novel synthesis or modification methods of p-C3N4 synthesis have been mentioned to overcome the weaknesses in surface area, band gap and electrical conductivity of p-C3N4, such as liquid impregnation [64], copolymerization with different agent samples [65], cavernous and interlayer doping [66], and exfoliation layers [67] The proposed methods aim to modify and improve the physicochemical properties of p-C3N4, and its catalytic activity

a Influence of the precursors

To synthesize p-C3N4, many typical precursors have been studied and used

in polycondensation synthesis or pretreatment Martin et al., successfully synthesized p-C3N4 using urea, thiourea and dicyandiamide as precursors They showed that p-C3N4 synthesized from urea precursors produced higher amounts of

H2 than p-C3N4 synthesized from thiourea and dicyandiamide The reason for this

is that the obtained surface area is larger than that of p-C3N4 synthesized from other precursors, and the degree of polymerization is also higher, thus improving charge mobility, although the light absorption region of p-C3N4 prepared from urea is lower [68]

Besides, Yuan et al., compared the synthesis of p-C3N4 from two different precursors, melamine, and urea, under the same synthesis conditions (580°C for 3 h) The comparison shows that the surface area of urea-derived p-C3N4 (39.5 m2.g- 1

) is higher than that of melamine-derived p-C3N4 (3.7 m2.g-1) Besides, the product

obtained using urea-derived precursors could effectively separate optical charges, and they also reduce CO2 in the photocatalytic reaction [69] The reason behind the effectiveness of urea-derived p-C3N4 is related to the creation of holes in its structure, which strengthens the active sites, reducing the diffusion lengths when transporting the charge, thereby reducing recombination rate [70]

As a result of pyrolysis of urea at 550°C for 3 h, Zhang obtained p-C3N4

with high porosity The high porosity can be attributed to two causes: first, due to the formation of NH3 at low temperature, second, due to the production of CO2 at high temperature The porous structure, together with the high surface area and improved pore volume, enhances the activity of the material [71] To improve the properties of p-C3N4, an effective strategy is to treat the precursors Acid solutions (HCl, H2SO4, HNO3) were selected andused to treat precursors Pretreatment with HCl with 1 step, results in different morphologies of p-C3N4, for example porous p-C3N4 with morphology similar to belt or ribbon or cable bundle, after treatment, surface area higher than that of bare p-C3N4 [72, 73] Then treating melamine with

H2SO4 resulted p-C3N4 with higher specific surface than bare p-C3N4 Moreover,

H2SO4 affected the condensation process, so the modification of specific surface has been reported [74] Through the rolling-up mechanism, during the heating of HNO3 protonated melamine is obtained, thus resulting tubular p-C3N4 with highly porous structure This is caused since the decomposition of self-assembled rod-like protonated melamine hinders the recombination rate [75]

Therefore, for primary materials containing different atoms than C, N, and

H, can be also used as precursors Efficient structural and morphological modifications were reported in the case of synthesized p-C3N4 [76]

b Influence of atmosphere

The nature of the reaction medium has a great influence on the structure of

p-C3N4 In the polycondensation synthesis, the control of gas flow rate has a direct effect on the saturation concentration of the furnace air, thereby creating structural defects The structure suffers also abnormal changes, creating defects on the p-

C3N4 structure Defects in the p-C3N4 structure were controlled aiming the increase

of the number of active sites in the material without destroying its structure [77]

➢ Oxidative atmosphere

The most common atmosphere for p-C3N4 synthesis is air Many studies have reported the conduction of synthesis in air Antonietti's work was the pioneer one

were also identified, such , as the repeating pattern and the resulting small surface area Therefore, the the effect of synthesis atmosphere on the peculiarities of the synthesized p-C3N4 has received a lot of attention from scientists

➢ Reductive atmosphere

Niu et al created vacancies and distortions when they have used H2 (after calcination) to synthesize p-C3N4.The enhancement of optical absorption region above 460 nm was reported in this case, which can be explained by the accessibility

of H2 into the melon interlayer region, thus reacting with the atom in the structural framework of p-C3N4, and creating holes as shown in Fig 2.3 On top of that, by excluding higher hydrogen bonding interactions, strain is promoted on the planar p-C3N4 configuration, forming lone pairs on the edge of the N atom in the tri-s-triazine based p-C3N4 units within the π-conjugate system This new position leads

to a new electron transition n →π *, allowing greater electron density in the conjugated system, facilitating electron migration to CB and initiation of the photocatalytic process [78]

Figure 2 3 Schematic of self-modification to promote oriented vacancies [78]

Yang et al reported on synthesis of p-C3N4 under partial NH3 pressure (Fig 2.4) The results show that the surface area is significantly improved, so the photocatalytic hydrogen evolution from water is also enhanced This improvement

is due to the high temperature decomposition of NH3 forming two types of active radicals NH2 and NH* These radicals can corrode the N-(C)3, C-N=C and C=C lattice sites, leading to the formation of new pores These pores can improve e-/h+pairs separation efficiency and longer charge lifetimes [79]

Figure 2 4 Using NH 3 atmosphere in the synthesis of p-C 3 N 4 [79]

➢ Inert atmosphere

Dong et al reported the synthesis of p-C3N4 in nitrogen atmosphere [80] Nitrogen gas treatment creates nitrogen vacancies, improves separation of charge carriers, thereby enhancing N2 fixation for better photocatalysis In another study, Maschmeyer et al showed that the new hydrogenated defects within the p-C3N4

nanosheet enhanced the photocatalytic performance [81]

➢ Vacuum condition

In a different strategy, Yuan and colleagues used vacuum in the synthesis

of p-C3N4 from melamine as a precursor, with an efficiency of 61% Through controlling the reaction parameters, the band structure, electronic properties and surface area p-C3N4 changed, leading to improved photocatalytic material efficiency [69]

Regardless to the applied medium, the defects and lattice disturbances in the structure of p-C3N4 led to a change in the midgap (the "band tail" state), which

in turn enhanced the excitation preference of e-/h+ pairs in the photocatalytic reaction [82] Alternatively, such changes can act as trapping sites while reducing the recombination rate of charge carriers [83]

c Influence of the time and temperature of reaction

Another strategy to adjust the structure of p-C3N4, consists in changing the duration, temperature, heating rate of the condensation process In a study investigating urea condensation duration from 0 to 3h at 500oC (Fig 2.5a), led to materials with specific surface areas ranging from 31-288 m2.g-1 [84] Besides, the porosity of the material increased but the thickness decreased (from 36 nm to 16 nm) This is related to a change in NH-NH2 binding in the structure of p-C3N4 The hydrogen bonds are oxidized under insufficient temperature, causing the p-C3N4

layers to break down gradually through this exfoliation pathway [85]

In another study, Yan et al., synthesized p-C3N4 using melamine as precursor at different reaction temperatures ranging from 500 to 580°C [86] Studies have shown that different heating rates not only change the C/N ratio but also affect the band gap value (2.8 - 2.75 eV)

Another study indicated that the nanoscale p-C3N4 samples synthesized at

550, 600, 625, 650, 675 and 700°C showed a blue-shift associated with the conversion π → π * and quantum size confinement Fig 2.5b shows the color change from light yellow to orange with increasing temperature which confirms

the different absorption in each material [87]

Figure 2 5 a) The pyrolysis time-layer thickness of p-C 3 N 4 [84] b) Synthesis pathway

of structurally distorted p-C 3 N 4 nanosheets [87]

2.2.3 Physical and Chemistry properties

p-C3N4 has 7 different crystal forms, from which 6 have 3D structure:

α-C3N4, β-C3N4, cubic-C3N4, pseudo-C3N4, p-h-triazine and p-h-heptazine The last form, the 2D-graphitic p-C3N4 is the most stable [46, 61, 88] In the structure of p-

C3N4, the graphite phase exhibits a chemical structure like graphene As theoretically predicted, p-C3N4 has a crystalline phase and an amorphous phase as shown in Fig 2.6 Normally, the p-C3N4 substructure l consists of a planar bond formed based on the hydrogen bond in the NH/NH2 group There is also a stable C-N bond because C-N is highly stable, so it can avoid bond breakage, even when

it is processed at high temperature, the order of the atoms is almost preserved In the case of amorphous structure, the crystallization of p-C3N4 takes place at high temperature because of the fact that the monomers tend to self-disorganize Two significant structural changes are proposed: (i) the breakage of hydrogen bonds (ii) the outward torsion of the melon units due to the large external motion of the NH2

groups in the p-C3N4 crystal plane original [89]

Figure 2 6 Scheme of monolayer (a) crystalline and (b) amorphous monolayer of

graphite carbon nitride [89]

C3N4, respectively The ordered p-C3N4 properties are important because they determine the photocatalytic activity By the time, the guest molecules on the surface of the catalyst are oriented in the nanopores, so the incorporation of the catalytic cofactors in the porous mass takes place In general, the principle begins when the p-C3N4 precursor is impregnated with a sample of neutral silica, resulting

in the dry powder being calcined and condensed into a p-C3N4 lattice confined to

a silica mold Then, the silica sample was removed with NH4HF2 in HF solutions

to obtain only ordered neutral p-C3N4

• Soft-template

To simplify the synthesis, a “greener” approach was taken targeting the synthesis of porous p-C3N4 micro and nanostructures [96] This methodology uses

latter mentioned reagents are considered as amphoteric organic molecules, which are capable to form colloidal micelles and to cause the growth of precursors around them and finally conformational orientation [97] as presented in Fig 2.7b Typical soft-structuring agents used as ionic liquids [98, 99], Triton X-100 [100, 101], bubbles [102-104], and biomolecules [103, 104]

• Supramolecular

When supramolecular combinations are formed, process of self-assembly

of the template will occur via the hydrogen bonds, which lead to the adoption of molecules orientation to the medium in the absence of an optimized external template [105] The role of hydrogen bonding is enormous in the formation of the ordered molecular building blocks, leading to an aggregate with reversibility, specificity directionality [106, 107] By approaching simple organic units of triazine building block units, we can synthesize 3D p-C3N4 structures, namely nanoparticles, nanotubes, and nanosheets Basically, the adjustment rationally the ratio of monomers, precipitation temperature, and pairs of hydrogen bonding donor-acceptor crystallization solvent will be the factors to choose the most dominant form [107] For photocatalysts, crystallinity plays an important role in the directional flow of electrons

• Template-free

Free-template synthesis involves the use of a conventional and heat-assisted (hydrogen) coagulation cluster [108] to produce porous p-C3N4 Also known as non-simulation approach, it is a versatile method to obtain different morphologies, such as nanorods, quantum dots, “seaweed” architectures, microspheres, nanofibers, and others [109-111] In comparison to the other methods, such as hard slides using silica as sacrificial templates, this method does not require steps after slide removal, so it does not use hazardous fluorinating agents (HF), NH4HF2) In addition, no post-treatment is required as when conventional surfactants are used

to remove completely the carbon or any foreign matter from the soft mold molecules The free sample approach meets the urgent need for a simpler and environmentally friendly way to effectively expand the p-C3N4 application field

Gu et al reported free-sample synthesis to obtain p-C3N4 microspheres with enhanced hydrogen generation For example, a simple cooling method was used with a post-heating step to obtain hierarchical microspheres with nanoscale surfaces [111] In another study, nano p-C3N4 was synthesized by a simple dissolution-regeneration step with HNO3, followed by a calcination step These

materials show enhanced photoactivity on methylene blue (MB) degradation [110]

Figure 2 7 a) The “hard” template approach combines the sol-gel/thermal condensation method [112] b) Different morphologies can be obtained by the “Soft”

and “Hard” slide approaches based on p-C 3 N 4 [97]

b Copolymerization

The modification process in the polymerization is essentially molecular doping, from which the intrinsic π-conjugation system of p-C3N4 is also modified, creating a certain change in electrical and optical properties, band structure, and thus improve the photocatalytic efficiency In addition, a major drawback that needs to be improved and overcome is the recombination rate of charge carriers, besides, inadequate sunlight absorption and low surface area, which causes inhibits the photocatalytic (electrical) functions [65, 113] Therefore, it is expected to modify the π polymer conjugate system (intrinsic property) by adding aromatic groups suitable for the p-C3N4 structure or organic additives to the g structure p-

C3N4 in the synthesis of copolymers [114-117]

c Exfoliation

Exfoliation is the process of using mechanical, physical or chemical force

to separate the layers of p-C3N4 into individual layers, which then form the same stacked mass Currently, there are three common methods for p-C3N4 exfoliation:

will decrease and the specific surface area will tend to increase This phenomenon occurs due to the presence of weak van der Waals forces between the p-C3N4

layers Similar to graphite materials, efforts have been made to apply the same exfoliating principle to large amounts of p-C3N4 The goal here is to reduce the macroscopic 3D p-C3N4 structure to the microscopic 2D structure Two-dimensional (2D) NSs are a potential structure due to their exceptional electronic, optical and biocompatible properties compared with bulk materials [118] In particular, the 2D anisotropy of p-C3N4 possesses high electron mobility, favorable band potential, large surface area, and more flexible active sites, reducing surface defects , enhanced the charge carriers lifetime [67]

d Treatment by H 2 O 2

In addition, enhancement of the photocatalytic performance of pure p-C3N4

can also be achieved by treating it with H2O2 H2O2 treatment leads to the formation

of oxygen functional groups [220, 223] or surface-active species [223] The oxygen functional groups entering the structure of p-C3N4 are assumed to enhance light absorption and increase electron transfer rate [220], induce electron traps [221], and inducing intrinsic electronic and band structure modulation [222] The results of treating p-C3N4 with H2O2 were investigated with the blocking of the surface sites responsible for the adsorption of the substrates [223]

2.2.5 p-C 3 N 4 based nanocomposites

a Carbon-based p-C 3 N 4 – Metal-free heterojunction

Various forms of heterojunction including p-C3N4 have been reported, thanks to the flexibility of carbon-based materials, which can bind p-C3N4 via surface functional groups by chemical or physical interactions Depending on the selection of carbon-based materials with different physicochemical properties from p-C3N4, functionally different materials can be obtained for photocatalytic application, for example, isotype p-C3N4/p-C3N4 (changes precursors), graphene/p-C3N4, carbon nanotubes (CNT)/p-C3N4, fullerenes-C60/p-C3N4 and carbon-based p-C3N4 materials [119-123]

b p-C 3 N 4 /Metal oxide

Various heterojunctions based on the conjugation of p-C3N4 with SC metal oxides have been studied and applied Among these TiO2 is considered as the most suitable candidate to combine with p-C3N4 Sereval TiO2/p-C3N4 heterojunctions have been reported, for example p-C3N4/P25 was synthesized with different mass ratios, the optimization showed 84/16 ratio for efficiency highest for MB decline

[124] In another study, Muñoz-Batista and coworkers synthesized p-C3N4/TiO2

with different p-C3N4 weight ratios ranging from 0.25-4 wt.% The results indicate that the samples 0.5-1 wt.% p-C3N4 increased photoactivity and selectivity, which could be explained by the close contact between both SCs, thereby enhancing charge carrier separation [125] Other metal oxides were also investigated and studied and combined with p-C3N4 such as ZnO [126], WO3 [127-129], Cu2O [130], Fe2O3 [131, 132], etc

The original idea was to take the advantage of the difference in the band gap of the SCs, thereby light absorption properties, and band energy potentials, to enabling an efficient and optimal coupling through type II heterojunctions In this context, ZnO and p-C3N4 hybrid material were prepared by a simple monolayer method with 5-fold enhanced photocurrent under visible light, besides the photocatalytic produced H2 is also enhanced about 3.5 times under UV light This improvement is based on better separation of photo-holes from ZnO to the p-C3N4

[133]

c p-C 3 N 4 /Metal organic Frameworks (MOFs)

Like other metal oxide and nonmetallic heterojunction systems, MOFs also act as photocatalysts and generate photoelectric charge carriers under UV irradiation [134] A prominent feature of MOFs is their large specific surface area, void space, pore size and light trapping capacity which are adjustable and controllable [135, 136] There were several MOFs associated with p-C3N4 For example, Fu et al reported a work on the MIL-125(Ti)/p-C3N4 hybrid nanocomposite synthesized through solvothermal method at 150°C The resulting material exhibits interesting properties such as high surface area and thermal stability, mesoporous structure, and improved light absorption in the UV region [137]

d Intercalation of non-metal or metallic NPs on p-C 3 N 4

Due to the excellent electronic and optical properties of metallic, metallic or bimetallic elements, their combination with p-C3N4 has received much attention [138] This combination is a well-known approach within the band gap technique The most optimal is to prioritize the use of metals and non-metals with

non-a smnon-all bnon-and gnon-ap of non-about 2.0 eV to obtnon-ain the highest photon energy in the UV region from sunlight, thereby increasing the formation and the lifetime of the charge carriers in photocatalytic oxidation [35]

However, the average band gap of p-C3N4 (2.7 eV) is limited to absorbing surface light wavelengths below 460 nm, thereby limiting the photocatalytic ability

of p-C3N4 Therefore, to improve the absorption of p-C3N4 in the visible region, doping p-C3N4 with metals and non-metals is the most appropriate and satisfactory approach

2.2.6 Trends of photocatalytic development of p-C 3 N 4

Among the photocatalysts (ZnO, WO3, CdS, …) p-C3N4 is a typical catalyst that can react in the visible light region, The band gap energy of p-C3N4 will in void region covered by the filled valence band and the vacant conduction band After being excited by the visible light, the photogenerated charge carriers are transported to the surface of p-C3N4 and are involved in various catalytic oxidation and reduction processes (Fig 2.8) [46]

Figure 2 8 Schematic representation of the charge pairs in p-C 3 N 4 is shown with decay

lines, where A represents the electron acceptor while D is the donor[46]

In the strategy of improving the photocatalytic performance of materials, optimal separation of the carriers is the most efficient route During the recombination of light generated charge carriers, the excited electrons will be transferred back to the VB, in association with fluorescensce, and heat release In this case, those excited electrons can react with the adsorbed species on the surface

of p-C3N4 [46] Therefore, the study of reducing the recombination rate of charge carrier has received lot of attention in order to improve the photocatalytic activity

of materials Normally, the synthesis of heterostructure photocatalysts is based on the combination of p-C3N4 with semiconductor materials

In case of semiconductor nanocomposites heterojunction material based on p-C3N4, utilizing different combinations of material and preparation conditions can enhance or reduce the conduction and valence band values of semiconductor materials Fig 2.9 depicts the typical bandgap structures of different semiconductor

photocatalysts, and the relevant redox potential values estimated in CO2 reduction, water splitting, and pollution photodegradation [46]

Figure 2 9 Typical bandgap energy values of semiconductor photocatalysts and redox potentials of processes including H 2 O separation, CO 2 reduction and pollutant

decomposition (reaction carried out with pH = 7) [46]

2.2.7 Hydrogen Evolution Reaction (HER) for p-C 3 N 4

Hydrogen evolution reaction (HER) is being regarded as a fundamental and practical interest in scientific technology today This has attracted more and more attention in the past decades due to the growing demand for renewable energy and the possibility of using fuel cells as green devices in energy conversion Similar to other evolutionary reactions, HER requires a significant energy potential, and therefore it is important to find the appropriate electrocatalyst to maximize the efficiency of the photocatalysis Noble metals such as Pt, Ru and Pd are ideal HER electrocatalysts in terms of overvoltage However, their high cost and scarcity make them impractical options, and the quest to find inexpensive electronic catalysts is indeed an area of active research p-C3N4 is an earth-abundant semiconductor photocatalyst that benefits large-scale industrial application of H2

production, so it becomes a promising candidate due to Wide light absorption range, structural stability, and low cost In general, the H2 photocatalysis on p-

C3N4 can be divided into three processes, including light absorption under light irradiation to generate electron-hole pairs, contact charge separation followed, the surface reduction reaction to produce H2

2.3 Plasmonic Photocatalysis

Photocatalysis based on plasmonic materials is an attractive and simple strategy attracting much attention Metal (M) nanoparticles (NPs) (e.g., Au, Ag, Pt) are dispersed on the surface of a semiconductor photocatalyst (e.g., TiO2, p-

C3N4, ZnO) In addition, there are many factors that influence the light-induced reactions and their efficiency These include, for example, the type of the NPs, size and morphology of the NPs, distance between the particles, metal loading, environmental conditions, pH and temperature [139]

2.3.1 Fundamentals

By definition, a plasmon is a collective oscillation of free electrons inside

M NP (metal nanoparaticles) The term ''plasmon'' refers to the similarity in the near-plasma-like behavior of the free electrons produced in M and produced after irradiating with an external electromagnetic radiation [140]

Plasmonic photocatalysis can contribute to a significant increase in photocatalytic efficiency in visible light applications In the 20 century, Paul Drude proposed a model of electrical conduction using the same principle of the kinetic theory of gases as applied to metals (nanometer scale) but for the behavior of electrons in solids, thereby reducing the permittivity of the dielectric M [141, 142] Plasmons are characterized by a few parameters including electron density (n), plasma or pulse frequency (ωp), elementary charge (e), permittivity M (ε0), and electron mass (p) as shown in equation below:

𝜔𝑝 = √𝑛 𝑒

2

𝑒0 𝑚According to Drude's model principle, oscillating clouds can be described

as how free electrons from an M particle resonate after an external electric field is applied The number of oscillations is calculated based on the number of plasma waves (fp) The plasma wave in the plasmon can be calculated according to the following equation:

𝑓𝑝 = 12𝜋 𝜔𝑝

In 1920, researchers classified plasmons as a waveform Through the application of Maxwell's equations, with certain limited conditions, we can quantify plasmons (plasma oscillations) [143] The Mie theory correctly describes the properties of an electrodynamic response for a spherical particle M [144]

The electron clouds of M will shift in a certain direction, along the external magnetic field, causing a lack of charge on one side and excess on the other However, the total mechanical energy is conserved and there is almost no energy dispersion The charge gradient caused by the external magnetic field creates a repulsive force, prompting the electron clouds to oscillate to resonate with the electric field, the displacement of which is illustrated in Fig 2.10

Figure 2 10 Schematic diagram of plasmon oscillations after application of an electric

field, and electron cloud formation [145]

2.3.2 Surface plasmon benefits for photocatalysis

The enhancement of surface plasmon orientation in the photocatalytic reaction mechanism mainly relies on two mechanisms: a) SC light absorption excitation and b) hot electron injection Two effects appear after the M NP surface

is irradiated with an electromagnetic wave, after which the M NP starts to resonate

In fact, there are many mechanisms of light absorption and energy transfer on M

NP when exposed to SC, but it is still imperative to know the specific location of the energy levels in the individual system: M NP and SC

The SC light absorption mechanism (Fig 2.11) consists of two effects The first effect shows the electromagnetic field strength in the surroundings of metal nanoparticles The second effect is the photoelectron diffusion on the SC surface

in close contact with the metal nanoparticle Once the electromagnetic energy from the light is incident on the surface of the metal nanoparticle, its vicinity enhances the resonance distance, which covers several nanometers Theoretically, the number of e-/h+ pairs is proportional to the square of the incident field (E-field2 =

e-/h+ pairs) It is also important to note that the photogenerated h+ (minority) remain

on the surface with a site of contact allowing them to react with species in solution before recombining with e-

Figure 2 11 Scheme of the electromagnetic field intensification on M NP [146]

The hot electron injection mechanism has been widely studied and applied (Fig 2.12) Initially, the mechanism was formed with a bond accompanied by the vibrational energy of the electron cloud of M NP, involving the electron transfer mechanism from NP to SC [147] In the case of metal nanoparticles, the incident magnetic field has a direct effect on the oscillations of the electron cloud, surrounding the nanoparticle forms electron-rich regions, thus creating holes and hot electrons [148] When the material absorbs light, the lowest electron fraction transitions to an excited state into the highest hot electrons that can be transferred

to the CB of the SC by two possible pathways: direct transmission [149] and signal tunnel response signal [150, 151]

Precious metal nanoparticles have excellent electronic mobility, as well as efficient absorption, making them potential candidates for photocatalytic reactions [147] Hot h+ are generated in the metal nanoparticle, which migrate from the VB

of the SC into the metal nanoparticle This charge transfer occurs when the NP M oscillates and is in close contact with the missing positively charged SC It must

be emphasized that this possible transfer is only one hypothesis that can explain how to offset the generation of global hot charge inside metal nanoparticles

During resonance, the surface plasmon excitons interfere with the incident light, the unpaired electrons will be transferred to the excited state Through the process of charge transfer between electrons on the surface of the material, a certain amount of energy has been generated and transferred to the semiconductor [152, 153] The plasmon resonance appeared, promoting light absorption, improving the photocatalytic activity demonstrated by increasing the average path length of the photons or enhancing the local electric field generated by the knives motion on the semiconductor surface

Figure 2 12 Mechanism of hot electron injection from metal/plasmonic nanoparticles

into the semiconductor conduction band [146]

2.3.3 Metal nanoparticle/semiconductor junction

One could say that once the M/SC is in close contact, a junction will form The interaction between M NP and SC determines the quality of that junction The working function (ɸ) is defined as the energy difference between the Fermi level and the vacuum level, and represents the minimum energy required to transfer an

e- from a solid to a position in the vacuum outside the surface Its side allows exits

on M [154]

When using the M NP/n-type SC junction, three cases can occur:

(1) | ɸM | = | ɸSC|, Fermi energy levels of metal (EF, M) and SC (EF, SC) align before contact But after connecting, no exchange of charge is seen

(2) Ohmic junction, | ɸM | <| ɸSC|, after contact, M(e-) with interface has more energy than SC(e-) so M(e-) will propagate to SC in the direction of alignment EF, M/EF, SC

(3) Schottky junction, | ɸM | >| ɸSC|, after contact, SC(e-) on the interface has higher energy than points in M NP, then SC(e-) will move in Fermi direction

Theoretically, in the 3rd case (Schottky junction) M NP could act as an electric pump Thanks to the advantages of the electron generated from this case, the carrier separation is further enhanced when induced inside the assisted irradiated SC with M NP deposited on its surface This allows to reduce the direct recombination rate of the e-/h+ pairs and ensures that the photogenerated e- migrates from CBTiO2 towards the M NP [155, 156]

2.3.4 Gold nanoparticles based on p-C 3 N 4

Plasmonic photocatalysis has been studied using AuNPs mainly because AuNPs can absorb efficiently visible light It is concluded that gold (absolute potential Au= -5.1 eV) is very interesting for photocatalyst related applications due

to its different morphology, optical reactivity, and electronic structure their

uniqueness[157, 158] Metal nanostructures can act as antennas to convert light

into a local electric field, or act similarly to waveguides to precisely route light to

the desired location with high precision to nanometers [159]

The crystal structure of gold is in fact an octahedron with a face-centered cubic (fcc) Each structural unit contains a unit cell of the cube, the atoms in that unit will be arranged and attached to each corner and center of all faces in the cube

In the fcc crystal structure, 8-unit cells will be divided among equivalent atoms, while the radial atom will only share 2-unit cells Theoretically, the characteristic atomic radius of gold has been calculated and falls around 0.1442 nm [160]

The unique surface-localized plasmon resonance (LSPR) phenomenon in case of M NP, such as Au, Ag and Pt has received much attention towards the scientific society Based on the oscillation of electrons on the metal surface, the LSPR phenomenon was formed, thereby enhancing the light absorption in the visible region, improving the photocatalytic efficiency [161-164]

Photocatalytic activity of Au/p-C3N4 materials has also been studied and published, mainly applied for CO2 reduction reaction [165, 166], decomposition

of organic substances [166-169]

In another study, Au/p-C3N4 was also used to investigate the evolution of photocatalytic hydrogen [170] Single Au atoms on the 2D structure of p-C3N4

have been studied, the band gap is also narrowed in the presence of Au The

Au-Pt alloy was also loaded onto p-C3N4, it was observed that the LSPR effect was improved markedly compared with single loading of Au particles onto p-C3N4

[171]

CHAPTER 3 p-C3N4 - SYNTHESIS, CHARACTERIZATION, AND

APPLICATION 3.1 Experimental

The whole process of p-C3N4 synthesis is shown in the diagram below Fig 3.1

Figure 3 1 Schematic overview about process synthesis of pure p-C 3 N 4

3.1.1 Preparation of p-C 3 N 4

Initially, 6 g of urea were introduced into a porcelain crucible (55 mm high,

45 mm in diameter, as depicted in Fig 3.2) The crucible was placed in a furnace (air atmosphere), covered with a lid, and heated to 550°C at a constant speed (4°C/min) and calcined for 2 h The resulting yellow product was named p-C3N4

(air)

Figure 3 2 The crucible used in the synthesis of p-C 3 N 4

Likewise, 6 g of urea was also introduced into a porcelain crucible, which was placed into a furnace and heated up to 550°C (at a heating rate of 5°C/min) under Argon atmosphere and calcined for 2 h The resulting product has a slightly

3.1.2 Post-treatment of p-C 3 N 4

First, 50 ml of prepared H2O2 solution (30%) was added into a 100 ml glass flask, then stirred well on a magnetic stirrer(500 rpm) Next, 1 g p-C3N4 (air) was added to the mixture under continuous stirring Water cooling system was installed, and the temperature was maintained during the whole experiment at 75°C Stirring time is carried out for 6 hours The resulting mixture was centrifuged

at 9000 rpm for 30 min and the solid was washed with water 3 times to remove excess H2O2 The above process is repeated 2 times Finally, the obtained solid was dried overnight in at 80oC in an air-drying oven The resulting product is called p-C3N4 (air)/H2O2 Performing the same experiment with p-C3N4 (argon), the sample named p-C3N4 (argon)/H2O2 was obtained

Figure 3 3 Experimental system treatment p-C 3 N 4 by H 2 O 2 solution (left), the p-C 3 N 4

mixture after treatment by H 2 O 2 (right)

For ultrasonic vibration treatment, initially, 1 gr of pure p-C3N4 (air) was placed in a 400 ml beaker containing 200 ml of distilled water The beaker was put

in a basin of ice cold water Then the ultrasonic probe was immersed in the cooled suspension and the sample was irradiated with ultrasound for 3 hours A BANDELIN SONOPULS HD 2070 was used as the ultrasonic transducer The set amplitude was 50% One pulse lasted 1s followed by 5s pause

Finally, before being dried at 80oC in an air-drying oven, the mixture was centrifuged and washed 3 times with distilled water, at 9000 rpm centrifuge for 30 min The products obtained were p-C3N4 (air)/ultrasonic

Similarly for p-C3N4 (argon), we obtained the sample after ultrasonic treatment as p-C3N4 (argon)/ultrasound

Figure 3 4 Treatment p-C 3 N 4 using the BANDELIN SONOPULS HD 2070 ultrasonic

transducer system

3.1.3 Material characterization

X-ray diffraction (XRD) powder patterns were recorded on a Panalytical X’Pert diffractometer equipped with a Xcelerator detector using automatic divergence slits and Cu Kα1 radiation (40 kV, 40 mA; λ = 0.15406 nm) The obtained intensities were converted from automatic to fixed divergence slits (0.25°) for further analysis Phase identification was done using the PDF-2 database of the International Center of Diffraction Data (ICDD)

X-ray photoelectron spectroscopy (XPS) was recorded on a photoelectron spectrometer (Multilab 2000, Thermo Fisher, USA) using Al Kα radiation as the excitation source All XP spectra are referenced to the C1s line at 284.6 eV

Attenuated total-reflectance infrared (ATR-IR) spectra were acquired using

a Bruker ALPHA FTIR spectrometer, resolution 4 cm-1, with each obtained mass spectra being performed 64 scans The powder sample is deposited directly on the ATR crystal without further pretreatment

Thermogravimetric analyses (TGA) and differential scanning calorimetry measurements (DSC) were performed in corundum crucibles in the temperature range of 25 to 600°C with a heating rate of 10 K min-1 in synthetic nitrogen atmosphere simultaneously on a NETZSCH STA 449 F5 Jupiter device (Germany)

Nitrogen adsorption–desorption isotherms were collected on a Micromeritics ASAP 2020, USA The Brunauer-EmmettTeller (BET) surface area and the pore size distribution were calculated from the adsorption and desorption

Elemental composition was obtained at the laboratory for elemental analysis at LIKAT Rostock by using atomic absorption spectroscopy (AAS) and a PerkinElmer AAS-Ananalyst 300 spectrometer The samples were prepared for analysis by using acid digestion with H2SO4/KHSO4

UV-vis spectra were measured in diffuse reflectance (DRS) with a spectrophotometer (Cary-5000, Agilent, USA) from 200 to 800 nm using BaSO4

as the reference material

Photoluminescence (PL) of the catalysts was determined on a fluorescence spectrophotometer (Agilent Technologies Inc., Mulgrave, Australia) at room temperature with an excitation wavelength of 350 nm

3.1.4 Photocatalytic -HER reaction

Photocatalytic experiments (mcat=25 mg) were performed in a small batch reactor at 25°C under irradiation with white or visible light The solvent (50 mL) was a mixture of TEOA and water (10v/90v) The amount of hydrogen formed was determined by GC analysis (Agilent 6890) using an Ar stream flowing through the reactor (see also Fig 3.5) Before the light was switched on, the aqueous TEOA solution and all tubes were flushed with Ar for 10 min to remove the air

Figure 3 5 Scheme of the experimental set-up used for study of HER reaction with

p-C 3 N 4 catalysts under irradiation with white light

After receiving the GC area from the GC system to calculate the amount of H2

produced in units of µmol.min.-1g-1, the following formula was used:

𝜏𝐻2 = 𝐹𝐴𝑟.𝑉𝑜𝑙% 𝐻2

100%.𝑉𝑚.𝑔𝑐𝑎𝑡 1000 (µmol.min-1.g-1)

In there:- 𝜏𝐻2 is the amount of H2 produced in µmol min-1.g-1 units

- F Ar is flow rate of Ar (ml·min-1)

- 𝑉𝑚 is molar volume at 25 °C, Vm=24.5 ml·mmol-1

- 𝑔cat is mass of catalyst used in the reaction (mg)

- 𝑉𝑜𝑙% 𝐻2 is volume percent of H2 in 𝑡ℎ𝑒 𝑔𝑎𝑠 𝑠𝑡𝑟𝑒𝑎𝑚

- 𝑉𝑜𝑙% 𝐻2 = 𝐴𝐻2

𝐺𝐶

𝑓 𝐻2

- 𝐴𝐻𝐺𝐶2 is GC area of H2 obtained from GC

- 𝑓𝐻2 is the calibration factor for H2

The amount of hydrogen per gram of catalyst at different reaction times (μmol·h 1

-) was obtained by integrating the hydrogen formation rate over time with the Origin 2022b tool

3.1.5 Electrochemical characterization

All photoelectrochemical measurements were monitored on a Zennium electrochemical workstation equipped with a PP211 CIMPS system (Zahner, Germany) with three electrodes The working electrode is prepared via a coating method Briefly, 20 mg sample was dispersed in a mixture of 100 µL Nafion solution (5 wt.%) and 900 µL isopropyl alcohol under ultrasonic for 10 min, then the dispersion was dropped on an ITO glass with active area of 1.5×1.5 cm2 and dried naturally in air Pt wire and Ag/AgCl (3M, NaCl) electrode were used as counter electrode and reference electrode, respectively The electrolyte was sodium sulfate (0.5 M) The photocurrent response curves were measured with the light on and off every 20 s As light source a 430 nm LED lamp (400 mW.cm-2) was used Electrochemical impedance spectroscopy (EIS) was carried out through

a potential static method in the frequency range of 0.01-100 KHz

3.2 Results and Discussion

3.2.1 Structural and thermal properties

a) Structure and morphology

X-ray diffraction was performed to explore the crystal structure, phase composition and purity of p-C3N4, as shown in Fig 3.6 The XRD patterns of the different p-C3N4 samples showed reflections at 2 = 13.1o and 27.3o,, respectively The small peak at 13.1o is assigned to the (100) plane and the strong peak at 27.3o

is assigned to the (002) plane of p-C3N4, these two peaks are two characteristic peaks of p-C3N4[172]

When treating p-C3N4 with H2O2, the XRD patterns showed a small shift of

from 27.2° to 27.6° for p-C3N4 (argon) The cause of this change can be traced to the removal of impurities that may not be present in the p-C3N4 structure, the formation of flat oxidized p-C3N4 layers [173], the introduction of heteroatomic oxygen [174], or the formation of oxygen-containing groups (hydroxyl and/or nitrogen oxide groups) [175]

Treatment of p-C3N4 by ultrasound, at a vertices of the plane (002) there is

a change in peak intensity of exfoliated p-C3N4, and there is a slight leftward shift

of the XRD peaks for p-C3N4 (002) compared with bulk p-C3N4 (Table 1), signaling the process of successful exfoliation, which indicates that the ultrasonic treatment increased the interlayer stacking distance, demonstrating that the layering process of p-C3N4 was successfully converted into 2D nanosheets Increase the number of active sites, good transfer of photogenesis charges, and improve photocatalytic efficiency [176-179]

Figure 3 6 XRD patterns of various p-C 3 N 4 samples Table 1 Position of the (002) reflection of p-C 3 N 4 support

Fig 3.7 shows FT-IR spectra of the as-prepared photo- catalysts in the wavelength range of 500- 3500 cm-1 Differences in band positions between the single samples were not observed The band at 810 cm-1 is characteristic of tri-s-triazine [180, 181] The bands located in the range of 1240-1632 cm-1 are assigned

to the typical stretching modes of C-N heterocyles [175] The wide band from 3000

to 3600 cm-1 is thought to originate from the stretching vibrational modes NH (primary) and NH2 (secondary), and OH [132, 181] suggests that amine functions persist in materials after hydrogen peroxide and ultrasonic treatment After treatment with H2O2 and Ultrasound, the main absorption bands of p-C3N4 were similar to those of C3N4, indicating that the main structure of the tris-s-triazine units in C3N4 was not destroyed

Figure 3 7 ATR-IR spectra of all various p-C 3 N 4

N2 sorption results are presented in Fig 3.8 All samples show type IV

isotherms with small hysteresis loops indicating that the obtained samples have many abundant pores with mesoporous and macrophage structures [182]