Luận văn thạc sĩ Kỹ thuật hóa học: Green synthesis of carbon-doped zinc oxide using garcinia mangostana pericarp extract for photocatalytic degradation of methylene blue and photoproduction of hydrogen peroxide applications

VIETNAM NATIONAL UNIVERSITY HO CHI MINH CITY HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

LE MINH HUONG

GREEN SYNTHESIS OF CARBON-DOPED ZINC OXIDE

USING GARCINIA MANGOSTANA

PERICARP EXTRACT FOR PHOTOCATALYTIC DEGRADATION OF METHYLENE BLUE AND

PHOTOPRODUCTION OF

HYDROGEN PEROXIDE APPLICATIONS

MASTER’S THESIS

HO CHI MINH CITY, July 2023

Major: Chemical Engineering Major code: 8.52.03.01

THIS THESIS IS COMPLETED AT

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY – VNU-HCM Supervisor: Assoc Prof Nguyen Huu Hieu, Ph.D

Vo Nguyen Dai Viet, Ph.D

Examiner 1: Assoc Prof Tran Ngoc Quyen, Ph.D

Examiner 2: Assoc Prof Nguyen Tuan Anh, Ph.D

This master’s thesis is defended at HCM City University of Technology – VNU- HCM

City on July 24, 2023

Master’s Thesis Committee:

1 Assoc Prof Nguyen Truong Son, Ph.D – Chairman of the thesis committee; 2 Assoc Prof Tran Ngoc Quyen, Ph.D – Reviewer 1;

3 Assoc Prof Nguyen Tuan Anh, Ph.D – Reviewer 2; 4 Ly Tan Nhiem, Ph.D – Spokesperson;

5 Pham Trong Liem Chau, Ph.D – Scientific secretary

Approval of the Chairman of Master’s Thesis Committee and Dean of Faculty of Chemical Engineering after the thesis being corrected (If any)

CHAIRMAN OF THESIS COMMITTEE

(Signature with full name)

HEAD OF FACULTY OF CHEMICAL ENGINEERING

(Signature with full name)

Ho Chi Minh City University of Technology – VNU-HCM

Faculty of Chemical Engineering

SOCIALIST REPUBLIC OF VIETNAM Independence – Freedom – Happiness

THESIS TASK ASSIGNMENT

Student’s name: Le Minh Huong ID: 2270006

Date of birth: 07/11/1999 Place of birth: Ho Chi Minh city Major: Chemical Engineering Code: 8.52.03.01

1 Title:

Title in Vietnamese: Tổng hợp xanh vật liệu kẽm oxit pha tạp cacbon từ dịch chiết vỏ măng

cụt ứng dụng để quang phân hủy xanh methylene và quang tổng hợp hydroperoxide

Title in English: Green synthesis of carbon-doped zinc oxide using Garcinia mangostana

pericarp extract for photocatalytic degradation of methylene blue and photoproduction of hydrogen peroxide applications

- Preparation of Garcinia mangostana pericarp extract;

- Influences of calcination conditions on the characteristics and MB photodegradation

performance of ZnO-C prepared from Garcinia mangostana pericarp extract calcinated at

various temperatures and times;

- Characterization of the suitable ZnO-C;

- Photodegradation of MB using ZnO-C, the related photocatalysis mechanism, reusability,

and recyclability of ZnO-C for the removal of MB;

- Photoproduction of H2O2 in the presence of ZnO-C, the corresponding photocatalysis mechanism, reusability, and recyclability of ZnO-C for the production of H2O2

3 Assignment date: 02/2023 4 Completion date: 07/2023 5 Supervisor:

Assoc Prof Nguyen Huu Hieu, Ph.D.; Vo Nguyen Dai Viet, Ph.D

SUPERVISOR

(Signature with full name)

NGUYEN HUU HIEU VO NGUYEN DAI VIET

Ho Chi Minh City, July 12, 2023 HEAD OF KEY LABORATORY

(Signature with full name)

NGUYEN HUU HIEU

HEAD OF FACULTY OF CHEMICAL ENGINEERING

(Signature with full name)

ACKNOWLEDGEMENTS

Ever since I was born, my parents, especially my mom, have cherished me and cheered me for every step that I took in my life Little by little, I always tried my best to achieve the goals that I have set Four years of studying as well as two years of researching has brought me tons of experience along with memorable moments at Ho Chi Minh City University of Technology (HCMUT) – Vietnam National University Ho Chi Minh City (VNU-HCM) Every time I met a dead end during my enrollment and research in this university, it has and always been my mom who gave me the courage and the strength to continue to pursue my dreams and goals

I would like to send my special thanks of gratitude to Assoc Prof Nguyen Huu Hieu, Ph.D and Vo Nguyen Dai Viet, Ph.D I have learned various skills and knowledge from them not only just academic knowledge but also their ways of thinking and problem-solving Their thoroughness has helped a lot during research Without Their dedication, my thesis would not be implemented in time

I would also like to send my appreciation to all the members of VNU-HCM, Key Laboratory of Chemical Engineering and Petroleum Processing (Key CEPP Lab), HCMUT – VNU-HCM I hope that one day I can return to this university to recall all the memorable moments that I have had during years of enrollment

Ho Chi Minh City, July 2023 Author

Le Minh Huong

ABSTRACT

In this study, carbon doped-zinc oxide (ZnO-C) was green synthesized using

Garcinia mangostana pericarp extract and calcination The influences of

calcination temperature and time on the characteristics and methylene blue (MB) photodegradation performance of ZnO-C The samples calcinated at various temperatures and times were characterized with scanning electron microscopy, thermal gravimetry analysis-differential thermal analysis, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction

The ZnO-C sample calcinated at favorable temperature and time was characterized via transmission electron microscopy, Raman spectroscopy, nitrogen adsorption-desorption isotherms, ultraviolet–visible diffuse reflectance spectroscopy, electrochemical impedance spectroscopy, cyclic voltammetry, and photoluminescence spectroscopy Simultaneously, the band structure of ZnO-C was also calculated

The photodegradation of MB using ZnO-C was assessed The individual influences of factors such as catalyst doses (40, 50, and 60 mg), pH (3, 5, 7, 9, and 11), and initial dye concentrations (5, 10, 15, 20, and 25 mg/L) were evaluated The photocatalysis mechanism of ZnO-C for MB was proposed via total organic carbon and chemical oxygen demand and radical scavenger results The reusability and recyclability of ZnO-C for this process were studied after 10 cycles were also evaluated

The photoproduction of H2O2 was also studied the impacts of electron donors (methanol and isopropanol), its volumes (2.5, 5.0, and 7.5 mL), and catalyst doses (2.5, 5.0, and 7.5 mg) on the photoproduction of hydrogen peroxide using ZnO-C prepared at favorable conditions was individually studied The mechanism of ZnO-C for the photoproduction of hydrogen peroxide was suggested based on the determined band structure of ZnO and redox potentials of radicals and hydrogen peroxide The reusability and recyclability of ZnO-C for this process were similarly studied after 10 cycles The works done in this thesis are summarized in Figure 1

Figure 1 Work done in this thesis

TÓM TẮT

Trong nghiên cứu này, kẽm oxit pha tạp cacbon (ZnO-C) được tổng hợp xanh từ dịch chiết vỏ măng cụt Ảnh hưởng của nhiệt độ nung và thời gian nung đến đặc trưng và hiệu suất xúc tác quang phân hủy xanh methylene (MB) của ZnO-C được khảo sát Đặc trưng của vật liệu ZnO-C nung ở nhiệt độ và thời gian khác nhau được khảo sát bằng các phương pháp phân tích như: Kính hiển vi điện tử quét, phổ tán xạ năng lượng tia X, phân tích nhiệt trọng lượng-nhiệt vi sai, quang phổ hồng ngoại biến đổi Fourier, và nhiễu xạ tia X

Đặc trưng của vật liệu ZnO-C nung ở nhiệt độ và thời gian phù hợp được khảo sát bằng kính hiển vi điện tử truyền qua, phổ Raman, đường đẳng nhiệt hấp phụ-giải hấp nitơ, quang phổ phản xạ khuếch tán tử ngoại khả kiến, phổ trở kháng điện hóa, vôn kế tuần hoàn,và quang phổ huỳnh quang Đồng thời, năng lượng vùng dẫn và vùng hóa trị của vật liệu ZnO-C cũng được xác định

Hiệu suất quang phân hủy của ZnO-C đối với xanh metylene (MB) được khảo sát Ảnh hưởng của lượng chất xúc tác (40, 50, và 60 mg), pH (3, 5, 7, 9, và 11), và nồng độ MB (5, 10, 15, 20, và 25 mg/L) lên hiệu suất quang phân hủy MB được khảo sát Cơ chế quang phân hủy xanh metylen của vật liệu ZnO-C đã được đề xuất thông qua tổng lượng carbon hữu cơ, nhu cầu oxy hóa học, và ảnh hưởng của các chất tự do đến hiệu suất quang phân hủy MB Khả năng thu hồi và tái sử dụng của vật liệu ZnO-C được khảo sát qua 10 chu kỳ tái sử dụng liên tiếp để đánh giả khả năng tái sử dụng của vật liệu ZnO-C

Quá trình quang tổng hợp hydroperoxide (H2O2) sử dụng vật liệu ZnO-C cũng được khảo sát Ảnh hưởng của chất cho điện tử (methanol và isopropanol), thể tích của chất cho điện tử (2,5, 5,0, và 7,5 mL), và lượng xúc tác (2,5, 5,0, và 7,5 mg) của vật liệu ZnO-C đối với quá trình quang tổng hợp H2O2 cũng được khảo sát để tìm ra điều kiện phù hợp Thêm đó, cơ chế xúc tác quang ZnO-C đối với quá trình quang tổng hợp hydroperoxide được đề xuất dựa trên thế năng lượng của vùng dẫn và vùng hóa trị của ZnO-C Khả năng thu hồi và tài sử dụng của vật liệu ZnO-C cho quá trình quang tổng hợp H2O2 qua 10 chu kỳ tái sử dụng cũng được khảo sát

COMMITMENT OF THE THESIS’ AUTHOR

I hereby declare that the work is originally implemented by the author and carried out under the instructions of Assoc Prof Nguyen Huu Hieu, Ph.D and Vo Nguyen Dai Viet, Ph.D in Ho Chi Minh City University of Technology – Vietnam National University Ho Chi Minh City

I confirm that this work is the result of my research and is solely my work All the contribution-related to this thesis have been fully acknowledged I affirm that any formulation, idea, research, reasoning, or analysis borrowed from a third party is correctly and accurately cited in both techniques and the author’s rights

The author takes full responsibility for the full work

Author

Le Minh Huong

1.4.3 Synthesis of ZnO-C 30

1.5 Garcinia mangostana pericarp 31

1.5.1 Current status 31

1.5.2 Extraction methods 32

1.6 Domestic and international research and status on ZnO-C 33

1.6.1 Domestic researches on ZnO-C 33

1.6.2 International researches on ZnO-C 34

1.7 Objectives, contents, methods, essentiality novelty, and contribution of thesis 35

2.1 Chemicals, materials, facilities, equipment, and work location 51

2.1.1 Chemicals and materials 51

2.1.2 Facilities 52

2.1.3 Equipment 52

2.1.4 Work location 53

2.2 Experiments 53

2.2.1 Preparation of G mangostana pericarp extract 53

2.2.2 Study on influences of calcination temperature and time on characteristics and photodegradation performance of ZnO-C toward MB 54

2.2.3 Characterization of ZnO-C calcinated at favorable temperature and time 57

2.2.4 Study on the photodegradation of methylene blue (MB), related mechanism, reusability, and recoverability of the catalyst 58

2.2.5 Study on the photoproduction of hydrogen peroxide (H2O2), corresponding mechanism, reusability, and recoverability of catalyst 61

3.1 Influences of calcination temperature and time on the characteristics and

photocatalytic removal of MB of ZnO-C 64

3.1.1 Characteristics 64

3.1.2 Photocatalytic degradation of MB 71

3.2 Characterization of ZnO-C calcinated at favorable temperature and time 733.3 Photocatalytic degradation of MB, related mechanism, reusability, and recoverability of the catalyst 78

3.3.1 Individual influences of factors 78

3.3.2 Photocatalysis mechanism 81

3.3.3 Reusability and recyclability of catalyst 83

3.4 Photoproduction of H2O2, corresponding mechanism, reusability, and recoverability of the catalyst 85

3.4.1 Individual influences of impacting factor 85

3.4.2 Photocatalysis mechanism 88

3.4.3 Reusability and recyclability of catalyst 89

LIST OF FIGURES

Figure 1.1 Methylene blue and its chemical structure 3

Figure 1.2 Production routes for H2O2 6

Figure 1.3 Polymorphs of ZnO (a) wurtzite, (b) zinc blende, and (c) rocksalt 9

Figure 1.4 Synthesis routes of ZnO 10

Figure 1.5 High-energy ball milling process 11

Figure 1.6 Sol-gel process 12

Figure 1.7 Teflon-lined stainless steel autoclaves 13

Figure 1.8 Chemical vapor deposition process 14

Figure 1.9 Intracellular and extracellular formation of metal oxide nanoparticles 15

Figure 1.10 Formation of ZnO using phytochemicals 17

Figure 1.11 Photocatalyst mechanism of ZnO 18

Figure 1.12 Modification method on ZnO 23

Figure 1.13 Type I heterojunction 24

Figure 1.14 Type II heterojunction 25

Figure 1.15 Type III heterojunction 25

Figure 1.16 Influences of metal doping on the band structure of ZnO 27

Figure 1.17 Influences of non-metal doping on band structure of ZnO 28

Figure 1.18 Wurtzite crystal structure of ZnO-C 29

Figure 1.19 Photocatalyst mechanism of ZnO-C 29

Figure 1.20 Principle of SEM 36

Figure 1.21 Fundamental principles of EDS 37

Figure 1.22 Principle of FTIR spectrometer 39

Figure 1.23 Fundamental principle of XRD 40

Figure 1.24 Principle of TEM 41

Figure 1.25 Principle of Raman spectrometer 42

Figure 1.26 Adsorption isotherm according to IUPAC classification 43

Figure 1.27 Principle of UV–Vis DRS 44

Figure 1.28 Description of EIS circuit and the redox reaction that occurs at the surface of a three-electrode system 45

Figure 1.29 Three electrodes setup 45

Figure 1.30 Fundamental principles of PL 46

Figure 1.31 Principle of UVvis spectrometer 47

Figure 2.1 G mangostana fruit 52

Figure 2.2 Extraction procedure of G mangostana pericarp 53

Figure 2.3 Synthesis of ZnO-C procedure 54

Figure 2.4 Photodegradation of MB using ZnO-C 56

Figure 2.5 Influences for scavengers on the photodegradation of MB 59

Figure 2.6 Procedure for evaluating the reusability and recyclability of catalyst for the photodegradation of MB 60

Figure 2.7 Photoproduction of H2O2 using ZnO-C 61

Figure 2.8 Procedure for evaluating the reusability and recyclability of catalyst for the photoproduction of H2O2 63

Figure 3.1 SEM images of ZnO-C synthesized at different calcination temperatures: (a) un-calcinated, (b) 500, (c) 600, (d) 700, and (e) 800oC 64

Figure 3.2 (a) EDS spectrum and (b) elemental composition of ZnO-C synthesized at different calcination temperatures 65

Figure 3.3 (a) TGA and (b) DTA curves of ZnO-C in atmospheric and N2 65

Figure 3.4 SEM images of ZnO-C-700 synthesized at different calcination times: (a) 0.5, (b) 1.0, (c) 1.5, (d) 2.0, and (e) 2.5 h 66

Figure 3.5 (a) EDS spectrum and (b) elemental composition of ZnO-C synthesized at different calcination times 67

Figure 3.6 Elemental mapping of ZnO-C synthesized at different calcination temperatures: (a) 0, (b) 500, (c) 600, (d) 700, and (e) 800oC 68

Figure 3.7 Elemental mapping of ZnO-C-700 synthesized at different calcination times: (a) 0.5, (b) 1.0, (c) 1.5, (d) 2.0, and (e) 2.5 h 68

Figure 3.8 FTIR spectra of ZnO-C synthesized at different calcination (a) temperatures and (b) times 70

Figure 3.9 XRD patterns of various ZnO-C samples 71

Figure 3.10 (a) Effects of calcination temperatures on the photodegradation of MB and (b) the corresponding kinetic plot of MB degradation 72

Figure 3.11 (a) Effects of calcination times on the photodegradation of MB and (b) the corresponding kinetic plot of MB degradation 73Figure 3.12 SAED patterns of (a) ZnO-C-0 and (b) ZnO-C-700-1 and HR-TEM images of (a) and (b) ZnO-C-0 and (e) and (f) ZnO-C-700-1 73Figure 3.13 Raman spectra of uncalcinated ZnO-C and ZnO-C-700-1.0 74Figure 3.14 (a) N2 adsorption-desorption isotherms and (b) pore size distributions of ZnO-C-0 and ZnO-C-700-1.0 75Figure 3.15 (a) UV–Vis DRS spectra and (b) band-gap energies of uncalcinated ZnO-C and ZnO-C-700-1.0 76Figure 3.16 PL spectra of uncalcinated ZnO-C and ZnO-C-700-1.0 77Figure 3.17 (a) EIS and (b) CV curves of ZnO-C-700-1.0 under dark and light conditions 78Figure 3.18 (a) Effects of catalyst dose on MB photodegradation efficiency of ZnO-C-700-1.0 and (b) corresponding kinetic plots of MB degradation 79Figure 3.19 (a) Effects of pH on MB photodegradation efficiency of ZnO-C-700-1.0 and (b) corresponding kinetic plots of MB degradation 80Figure 3.20 (a) Effects of initial dye concentration on MB photodegradation efficiency of ZnO-C-700-1.0 and (b) corresponding kinetic plots of MB degradation 81Figure 3.21 TOC and COD removal efficiency of post-photocatalysis solution 81Figure 3.22 (a) Effects of radical scavengers on the photocatalytic degradation and (b) corresponding kinetic plots for the degradation of MB 82Figure 3.23 Reusability of ZnO-C-700-1.0 for the photocatalytic removal of MB 84Figure 3.24 Effects of different electron donors on the photoproduction of H2O2using ZnO-C-700-1.0 86Figure 3.25 Effects of electron donor doses on the photoproduction of H2O2 using ZnO-C-700-1.0 87Figure 3.26 Effects of photocatalyst doses on the photoproduction of H2O2 using ZnO-C-700-1.0 88Figure 3.27 Plausible mechanism for the photoproduction of H2O2 89Figure 3.28 Reusability of ZnO-C-700-1.0 for the photoproduction of H2O2 89

LIST OF TABLES

Table 1.1 Domestic researches related to ZnO 33

Table 1.2 International researches on ZnO-C 34

Table 2.1 Chemicals used in this thesis 51

Table 2.2 List of equipment 52

Table 2.3 Influences of calcination temperature on the synthesis of ZnO-C 55

Table 2.4 Influences of calcination time on the synthesis of ZnO-C 55

Table 2.5 Influences of catalyst dose on the photodegradation of MB 58

Table 2.6 Influences of pH on the photodegradation of MB 58

Table 2.7 Influences of initial dye concentration on the photodegradation of MB 59

Table 2.8 Influences of different types of donors on photoproduction of H2O2 62

Table 2.9 Influences of electron donor doses on photoproduction of H2O2 62

Table 2.10 Influences of catalyst on photoproduction of H2O2 62

Table 3.1 MB photodegradation performances of ZnO-C-700-1.0 and other materials 84

Table 3.2 Photocatalytic production of H2O2 using ZnO-C-700-1.0 and others 90

EIS Electrochemical impedance spectroscopy

FTIR Fourier-transform infrared spectroscopy

IUPAC International Union of Pure and Applied Chemistry

SAED Selected area electron diffraction

TOC Total organic carbon

UV–Vis DRS Ultraviolet–visible diffuse reflectance spectroscopy UVVis Ultraviolet–visible spectroscopy

INTRODUCTION

Amongst well-known semiconductors, zinc oxide (ZnO) attracts a great deal of interest owing to its suitable physicochemical properties such as excellent optical and electrical properties, good catalytic activity, biocompatibility, environmental sustainability, and high photosensitivity As a result, ZnO can be utilized as a potential star in various fields related to photocatalysis For example, the diverse utilizations of photocatalysts for CO2 reduction, hydrogen evolution, and H2O2 production as well as the photodegradation of various organic compounds such as dyes, phenolic compounds, pesticides, and antibiotics However, the photocatalytic activity of ZnO is unstable due to its poor visible absorption ability along with the rapid recombination of photogenerated electron-hole pairs and low photon energy-conversion efficiency, resulting in the limited practical implementation of these semiconductors As a result, ZnO crystal structure is tuned to obtain a semiconductor with a desirable band structure

The introduction of carbon to the crystal lattice of ZnO would form an intermediate energy level in the bandgap of the material as well as enhance the effective absorption of visible light Moreover, in comparison with N and S doping, C doping can separate significantly charge pairs the most effectively, offering a potentially viable technique to raise the quantum efficiency of ZnO-based photocatalysts The source of carbon for doping mainly derives from a carbon precursor such as glucose, surfactants,

and plant-based extracts Particularly, Garcinia mangostana is known as a typical

evergreen tree cultivated in tropical regions and its pericarp, which is often discarded once the flesh is consumed The extract from this fruit pericarp not only serves as a reductant and stabilizer for the synthesis process but also plays as the carbon doping source

for ZnO Therefore, the topic of this thesis is “Green synthesis of carbon-doped zinc

oxide using Garcinia mangostana pericarp extract for photocatalytic degradation of

methylene blue and photoproduction of hydrogen peroxide applications”

CHAPTER 1: OVERVIEW1.1 Dye pollution

1.1.1 Current status

Nowadays, the rapid economic growth has led to the explosive escalation of many many large industries Among them, textile industry is one of the most dominant industries Dyes are considered the heart of this industrial sector, however, the discharge of dye-containing effluent is deemed as one of the most polluting of the industry It is estimated that there are over 10,000 synthesized types of dyes and pigments around the world with an annual production of 700,000 tonnes [1] However, over 25 % of that amount presents in effluent due to the inefficiency of the dying process [2] Unfortunately, as dyes are resistant to light, temperature, water, detergents, chemicals, and other agents such as bleach and perspiration, dyes remain stable in the environment and can not be treated with conventional treatment methods Furthermore, the multi-ring structure of dyes makes the biodegradation of dyes even more less effective Water sources that are contaminated with dyes are deemed highly toxic and carcinogenic Consequently, the intake of polluted water greatly impacts human health as well as other organisms It has been extensively reported that the exposure to dyes causes dermatitis, allergic conjunctivitis, hepatocarcinoma, increase in mutagenic potentiality [3], [4] There are various types of dyes presenting around the world, for example, acid, basic, vat, and dispersive dyes In their midst, thiazine is one of the most important dyes due to its brilliance of color and high tinctorial strength

1.1.2 Thiazine dye

The chromophore system of thiazine dye consists of three organic compounds of the heterocyclic series, having molecular structures with a ring of four atoms of carbon and one each of nitrogen and sulfur They are mainly used for the coloration of clothes and the treatment of some illnesses Due to their wide range of applications, they are mass-produced and consumed around the world Amongst the pigments used in dye industry, methylene blue (MB) as demonstrated in Figure 1.1, a cationic dye, is considered as one of the most consumed dyes [5] Therefore, the complete removal of MB from the effluent is highly desirable

Figure 1.1 Methylene blue and its chemical structure

1.1.3 Removal methods

Many methods have been proposed and studied for the removal of MB from effluents such as adsorption, coagulation, oxidation, and flocculation [6] However, these conventional effluent treatment methods either give out undesirable treatment results or produce secondary pollutants It is proved that secondary pollutants can be even more harmful than parental pollutants [7] Furthermore, the multiring structure nature of textile dyes makes them highly resistant to biological treatment processes Therefore, newer, cleaner approaches such as advanced oxidation process, and photocatalysis have been made to resolve the drawbacks of conventional treatment methods

1.1.3.1 Adsorption

Adsorption is a commonly used physicochemical method for wastewater treatment with the advantages of high treatment efficiency, low investment cost as well, and great reusability The adsorption process involves various interactions between pollutants and the adsorbent surface The adsorption process is influenced by many factors such as the nature of the adsorbent, the chemical properties of the solution, and the nature of the adsorbate [8] However, the adsorption process requires prolonged exposure time and generates a large amount of waste after treatment, termed secondary sludge and the complete removal of adsorbate remains one of the biggest issues for this method [9]

1.1.3.2 Biodegradation

The fundamental principle of this method is based on the breakdown of organic compounds by microorganisms, in which the conversion of organic matter into inorganic products is carried out by mainly by yeasts, and anaerobic and aerobic

bacteria [10] In particular, the aromatic ring structure of the textile dye is used as a source of raw materials for the growth of fungi and bacteria This method can remove textile dye with high selectivity, and low cost and it is can be deemed as an environmentally friendly method as it does not produce any secondary pollution However, the prolonged treatment time of this method is mainly derived from the recalcitrant multi-ring structure of textile dye Another drawback of this method is it requires the cultivation of the environment of microorganisms in prior to the initialization of the treatment

1.1.3.3 Coagulation-flocculation

Coagulation-flocculation is a process that utilizes the destabilization of solid particles by increasing their surface charge, consequently, leading to the agglomeration of these particles to form larger particles This effluent-treating method consists of three steps, flocculation, coagulation, and sedimentation Coagulants are added under violent mixing, as a result, the charge of dispersed particles is either reduced or neutralized under the act of coagulants Flocculants are added under mild mixing conditions to promote the gathering of smaller particles Finally, the large particles are removed via sedimentation In general, coagulants are materials such as metal salts or polymers, while flocculants are polymers that can promote flocs agglomeration to create larger particles for easier separation Phytochemicals have also been reported as highly efficient coagulants specifically in dye removal from effluents [11]

1.1.3.4 Photocatalysis

a Overview

Photocatalysis is a method that uses a catalyst to change the kinetics of a chemical reaction when exposed to light [12] When compared with other advanced oxidation processes, photocatalysis is a greener method as it uses environmentally friendly materials, and the energy source is utilized from natural light which is renewable Photocatalysis methods are classified into two types based on the reactant state and photocatalyst state As photocatalysis generates no secondary pollutants, it has caught the attention of various researcher for environmental remediation Resultantly, such method is being implemented in an actual process to achieve a greener treatment of MB For the photocatalysis degradation of MB, various factors should be considered

such as catalyst doses, pH of the effluent, and dye concentrations, and light source intensity [13]

b Influences of factors

In any catalysis-based process, the dose plays a major role by providing sufficient active sites for the process In the case of photocatalysis, light is required to activate the process, resultantly, employing excessive amount of catalyst would reduce the amount of energy provided by light for each particle in the system Therefore, it is crucial to determine the appropriate amount of catalyst used for the reaction system In addition, pH level of the effluent may fluctuate each time it is discharged For any particulate photocatalysis reaction system for the degradation of dye, the pH plays a major role as the interaction between the dyes and the particle can either be beneficial or detrimental to the treatment result at a specific pH [14] Similarly, dye wash-off occurs frequently in dying process, therefore, the dye concentration in effluent may vary for each batch, meanwhile, the treatment result must always be similar to one another Therefore, the ability to complete degrade dye at various concentration should be also taken into consideration Due to the above reasons, catalyst dose, pH of the solution, and dye concentration are chosen as factors that influence the process in this thesis

Among heterogeneous photocatalytic materials applied for the photodegradation of dyes, semiconductors, having energy band-gap lower than 3.5 eV, such as titanium dioxide (TiO2) and zinc oxide (ZnO), are often used due to their tunable band structure In addition to the environmental remediation, photocatalysis has also been extensively utilized for resolving another emerging problem, which is energy shortage

1.2 Energy crisis

1.2.1 Current status

The depletion of fossil fuels and the increase in demand for energy has caused an energy crisis around the world, as a result, alternative energy sources have been extensively researched and reported Moreover, the approaching of greener energy is another requirement for the development of energy carriers Currently, 1.2 billion people in less developed countries do not have access to electricity Moreover, the current energy consumption accounts for 60% of greenhouse emissions [15]

As a result, greener energy sources such as biomass-derived energy carriers, hydrogen, and H2O2 have been extensively researched Amongst them, H2O2 is one of the most prominent greener energy carriers as it has been reported that high-test peroxide can release up to 2.887 MJ of energy per 1 kg of hydrogen peroxide [16]

1.2.2 Hydrogen peroxide

Hydrogen peroxide (H2O2) is a powerful, environmentally friendly, oxidant that can oxidize both inorganic and organic substrates, under mild conditions Unlike stoichiometric oxidants, such as tert-butyl hydroperoxide, N2O, or permanganate which produce large amounts of waste that require additional separation processes to obtain the desirable resultant H2O2 is also applied in other industries such as hair dye, food, and environmental remediation The high sought value of H2O2 also derives from its application in the generation of energy To some extent, the energy released from H2O2 can be utilized for rocket propellant Due to its vast applications as well as having minimal environmental concern, H2O2 has been considered one of the high valued chemicals As a result, the improvement in the production of H2O2 is a must

Wet chemical process Anthroquinone

oxidation

Oxidation

Direct synthesis Electrolysis

Oxidation Photocatalysis

Hydrogen peroxide

between gaseous O2, solid cathode, and aqueous media By utilizing such system, explosion due to the interaction between H2 and O2 is eliminated Moreover, electric power can also be generated in conjunction with H2O2 due to fuel cell set up However, as promising as it may sound, the application of fuel cells for H2O2 production is far from being implemented in an actual set up [17], [18]

1.2.3.2 Oxidation

The auto-oxidation process involves the indirect oxidation of H2 to H2O2 For this method, an alkyl anthraquinol (usually 2- ethyl anthraquinol) is oxidized by air or oxygen to the corresponding quinone and H2O2 [19] Subsequently, the anthraquinone is then reduced back to anthraquinol or anthrahydroquinone using hydrogen under pressure in the presence of a hydrogenation catalyst Afterward, anthrahydroquinone undergoes further hydrogenation to create the 5,6,7,8-tetrahydroanthrahydroquinone The regenerations of the quinone compounds derive from the oxidation of anthrahydroquinone and 5,6,7,8-tetrahydroanthrahydroquinone Simultaneously, H2O2 is also produced during the generation process The auto-oxidation process offers a much safer process for the production of H2O2, avoiding the explosions induced by the interaction between H2 and O2 However, the operation cost for this route of H2O2production is quite high due to the constant replacement of catalyst as well as the low selectivity of the alkyl anthraquinol

1.2.3.3 Photocatalysis

a Overview

In recent years, the utilization of photocatalysts in the presence of a sacrificial such as ethanol, methanol, and isopropanol for the production of H2O2 has been extensively studied due to the sustainability that this route provides However, common bulk catalysts can only initiate the reactions in ultraviolet (UV) region Moreover, in order to efficiently produce H2O2, an appropriate band structure of a semiconductor is required [20] As a result, the main challenges that arise in this route mainly come from the designs of photocatalysts with the ability to harvest visible light region as well as having good band structures to promote the photoproduction of H2O2 Furthermore, photocatalysis offers advantage over other conventional method such as being environmentally friendly and safety In addition, the photocatalytic production

of H2O2 depends on various factors such as the nature of sacrificial agent, its volume, photocatalyst doses, reaction, and illumination source intensity

b Influences of factors

Each sacrificial agent offers a unique pathway for the generation of H2O2 for a photocatalyst as well as the dose used for such processes [21] Resultantly, determining the suitable sacrificial agent for the specified photocatalysis process is crucial for the process Simultaneously, the generation of H2O2 using photocatalysis generally increases with catalyst dose, however, excessive amount of catalyst may reduce the amount of energy received of each particle in the system Meanwhile, utilizing a light source with high intensity would be beneficial for the photoproduction of H2O2, however, it should be considered that the generation of H2O2 using photocatalysis is a two-way reaction Such usage of light source may produce heat, leading to the degradation of the produced H2O2 Moreover, the fact that light source with higher intensity also consumes more power should be taken into consideration As a result, it is crucial to study the influences of the affecting factors onto the photoproduction of H2O2 Amongst the well-known photocatalysts, ZnO has long been considered a base material for constructing an efficient photocatalyst for various redox-based purposes [22], therefore, ZnO has been selected to be studied and the influences of sacrificial agent, its volumes and catalyst doses are also examined in this thesis

1.3 Zinc oxide

1.3.1 Structure

ZnO, belonging to an II-VI semiconductor group, is deemed as one of the most popular semiconducting materials, and its crystal structure is categorized into a space group of P63mc [23] The polymorphs shared by ZnO are wurtzite, zinc blende, and rocksalt, as displayed in Figure 1.3 According to previous studies, wurtzite is the more common naturally occurring crystal structure due to its high stability Simultaneously, the zinc blende can only be obtained through the growth of the crystal on a cubic substrate Meanwhile, high pressure is required for rock salt structure

Figure 1.4 Synthesis routes of ZnO [26]

1.3.3 Synthesis

1.3.3.1 Mechanical approach

High-energy blending by ball milling is a simple and low-cost technique in which mechanical energy is used to grind down different powders A smaller and more uniform size powder is obtained through the collisions of the balls with the bulk powder According to the other studies, two main events occur during the operation of high-energy ball milling An increase in the average size of the particle could be observed due to cold welding, followed by the fragmentation of particles [27] Aside from the aforementioned events, the genesis of structural defects, mechanical alloying, and chemical reactions also take place due to the exerted mechanical energy [28] The high-energy ball milling process is demonstrated in Figure 1.5 By utilizing this method for the synthesis of nanostructure materials, small-sized particles with homogeneous crystallinity and little tendency to agglomerate can be obtained The crystallinities, which affect the properties of the crystalline structures and morphologies, can be tailored through the modulation of grinding parameters such as grinding time, speed, ball-to-powder ratio, and mill geometry [29] For this method, precursor zinc salt such as Zn(NO3)2, Zn(CH3COO)2, and ZnCl2 are often milled with carbonate salts such as Na2CO3 and (NH4)2CO3 [30] The local pressure and heat generated by the collision of

Synthesis of ZnOPhysical approach

Biological approachChemical

Sol-gelHydrothermalChemical reductionChemical vapor

Plant extractMicroorganismBiotechnologyMilling

SonochemicalArc plasma

the balls are the main driving force for the formation of ZnCO3 The post-milled powder is then calcinated to obtain ZnO In another study, bulk ZnO or Zn powder can also be used as the zinc precursor to obtain ZnO nanostructures [31]–[33] The use of pristine Zn as a precursor, followed by the calcination in O to attain the desirable product, has also been reported [34]

Figure 1.5 High-energy ball milling process [35]

In recent years, sonochemical method has been proven to be a useful technique for the synthesis of novel materials with interesting properties The technique is based on a physical phenomenon: acoustic cavitation, in which the attractive forces of the molecules in the liquid phase are overwhelmed by mechanical activation Under ultrasonication, small oscillating bubbles are formed from the rapid compression and expansion of the liquid The bubbles violently collapse after reaching an unstable size, generating localized hotspots with extreme conditions As a result, this phenomenon has been exploited for the synthesis of various nanomaterials, including ZnO

Another commonly used physical method is arc plasma, which is based on electrical arc discharge synthesis For scalable processes, well-dispersed nanomaterials are obtained from bulk material through evaporation and condensation This approach, utilizing thermodynamic nonequilibrium conditions, allows the synthesis of nanostructure with more fascination morphological properties Moreover, the nonequilibrium conditions allow chemical reactions to take place at a much lower

temperature, making it plausible for applications in which the substrates have low thermal stability Morphological properties of the materials can be tuned through the adjustment of the process parameters such as the volumetric flow rates of the center, sheath, carrier, oxidative gases, heating rate, and powder feeding rate [36] A wide variety of zinc precursors can be used for this method ranging from metallic zinc to inorganic and organic zinc salt The use of metallic vapor has been proved to be quite useful for the synthesis of metal oxide, nonetheless, the utilization of metallic precursor is uneconomical as well as undesirable quantities of product [37]

The mechanical approaches offer a wide variety of advantages such as simple, low-cost, and industrial scalability However, the operating parameters of these approaches are hard to be tuned Therefore, chemical approaches with their ease to tailor the synthesis conditions have been considered over the mechanical approaches

1.3.3.2 Chemical approach

Sol-gel method is considered as one of the most effective methods used for the nano-synthesis of different materials In this method, a colloidal suspension called sol is the result of the hydrolysis of precursors The polymerization of this mixture leads to the formation leads the formation of liquid sols into a solid gel, which is subsequently further treated to obtain the desirable products The sol-gel process can be summarized in Figure 1.6

Figure 1.6 Sol-gel process [38]

Metal alkoxides are often used for the synthesis of metal oxide due to their strong interaction with nucleophilic reagents such as water Furthermore, their ability to form a homogeneous solution with other presenting metallic derivatives is another reason why these agents are often used for the synthesis of metal oxide [39] For the synthesis of ZnO nanostructure, both inorganic and organic salt such as zinc nitrate and zinc

acetate could be used as a precursor [40], [41] The nanostructure and size of ZnO, which affects the properties of the synthesized material, can be tailored through the alteration of synthesis parameters such as gel aging time, precursor concentration, pH, and annealing temperature [42]

The hydrothermal method utilizes a special closed reaction vessel in which crystal growth is performed called an autoclave as demonstrated in Figure 1.7

Figure 1.7 Teflon-lined stainless steel autoclaves

Through the heating of the reaction system, high-temperature and high-pressure is achieved through the vapor pressure generated by itself For this synthesis route, the crystal growth occurs as followings: First, the hydrothermal medium, carrying dissolved reactants in the form of ions or molecular group, enter the solution The temperature gradient between the upper and lower section of the vessels leads to the separation of the ions and molecules In this stage, the nutritions in the medium continue to dissolve in the high-temperature region Meanwhile, the low-temperature region acts both as accommodation places for the ions and molecules and as sites for the deposition of these ions and molecules for the growth of seed crystals At the growth interface, the ions and molecules are adsorbed, decomposed, and desorbed constantly for the growing of desirable crystals Through the alternation of the operating parameter such as temperature, time, and hydrothermal medium, desirable parameters of the crystals could be obtained [43] For this route, precursor zinc salt solution such as zinc nitrate and Zn(CH3COO)2 is often dissolved in water as a medium in the presence of another aqueous base solution such as NaOH and KOH [44], [45]

Head bolt Filter pad Cover Teflon cover Steel cap Teflon liner Solvent Solid reagent Shell

Bottom

Chemical vapor deposition, one of the well-known chemical approaches, is a technique in which substances in the vapor phase are condensed on the surface of a substrate to generate solid phase material The process of this method is displayed in Figure 1.8

Figure 1.8 Chemical vapor deposition process [46]

Chemical vapor deposition can be utilized either under atmospheric pressure or in low-pressure conditions Chemical vapor deposition exploited in atmospheric conditions offers a high deposition rate but features low purity and poor uniformity resultants as a trade-off Meanwhile, low-pressure chemical vapor deposition requires a higher temperature and give outs product with higher uniformity and purity at a slower pace than the atmospheric condition It is found that ZnO nanostructures can be grown using chemical vapor deposition by utilizing either ZnO or metallic Zn or a mixture of the two If metallic Zn is used, a supplement of O2 is required in the gas stream Deposition temperature, pressure, flow rate, gas composition, deposition, and chamber geometry are examples of the factors that affect the synthesis of metal oxides As a result, these parameters can be controlled to obtain the desirable shape and size of ZnO nanostructure The conventional approaches used for the synthesis of metal NPs are quite expensive and some are even hazardous to the environment due to the

involvement of various perilous and hazardous chemicals that can impose various health risks

1.3.3.3 Biological approach

In recent years, biological approaches have been proved to be a green method for the synthesis of metal oxide with several advantages over the conventional method such as low-cost and ease of fabrication [47] Due to the sustainability of the method, it is also considered as a plausible alternative for the synthesis of nano-sized metal oxide structures In this approach, microorganisms and plant extracts are often utilized The biosynthesis of metal oxide nanoparticles by using organisms may occur either in an extracellular or an intracellular environment It is believed that metallic ions that are taken into the microorganism cells are reduced by proteins and enzymes produced within the cells of bacteria for the intracellular process In the case of extracellular synthesis, studies suggest that the enzymes and proteins secreted by the microorganisms during the incubation time can function as both a stabilizing and a reducing agent for the synthesis of metal oxide nanostructure The synthesis of metal oxides following this route is summarized in Figure 1.9

Figure 1.9 Intracellular and extracellular formation of metal oxide nanoparticles

Moreover, it has been proved that bacteria can internalize Zn2+ ions for the intracellular synthesis of ZnO nanostructure to occur [48] However, the extracellular formation of ZnO is widely more acceptable due to the complexity as well as the lengthy procedure of the intracellular processes [49] Other factors such as the solution pH and the electrokinetic potential of the bacteria also play a role in the formation of ZnO Another microorganism that has caught the attention of researchers throughout the world for the synthesis of ZnO is fungi Owing to their high cell wall binding and elevated protein secretion capability, the genesis of nanostructure ZnO in terms of productivity using such microorganism is much higher than that generated from bacteria [50] Moreover, the resistance of fungi to process parameters such as temperature, pressure, and agitation rate indicates the industrial scalability of the approach for the synthesis of ZnO

According to other studies, the biosynthesis of nanoparticles using microorganisms is generally tedious as this method often deals with cell culture preparation, incubation period can be quite tedious and time-consuming [51] The phytogenic green synthesis of that is comparatively simple and fast The plant extracts are obtained as followings: First, the obtained plant parts are washed with distilled water, followed by the drying of these parts The obtained dried parts are then ground and mixed with other solvents such as water, ethanol, and methanol at a low temperature, not exceeding 80oC, to attain the extract The ease green synthesis of ZnO nanostructure involves mixing the plant extracts prepared from plants with high phytochemicals content and a zinc salt that act as a zinc precursor The phytochemicals of the plant extract act as reducing, capping, and stabilizing agents in the synthesis of ZnO nanostructure, therefore, the addition of other surfactants is not needed According to previous studies, the interactions between Zn2+ ions and other phytochemicals lead to the formation of Znphyto complexes as demonstrated in Figure 1.10

Figure 1.10 Formation of ZnO using phytochemicals [52]

The obtained mixture is then calcinated to obtain ZnO nanostructures The morphologies of the ZnO nanostructure can be tuned by changing various conditions such as pH, temperature, and zinc precursor concentration Therefore, large-scale production of metal oxide with the advantage of being economical, simple, and green can be implemented Different synthesis methods may induce different shapes, defects and crystallinities on ZnO, the photocatalysis mechanism remains similar with the differences

1.3.4 Photocatalyst mechanism

The absorption of a photon leads to the generation of an electron (e) and hole (h+), which is one of the compulsory factors for the photocatalytic process When being illuminated with a photon with energy that is higher or equal to the bandgap of material, e and h+ are generated, then e moves to the conduction band (CB) meanwhile h+remains at the valence band (VB) as shown in Figure 1.11 The photocatalytic process occurs as follows The excited e combines with O2, h+ combines with H2O, creating superoxide (•O2) and hydroxide (•OH) radicals, respectively Then, generated •OH and •O2 radicals oxidize organic substances Therefore, organic pollutants are degraded into H2O, CO2, and less harmful inorganic substances via the photocatalytic activity of catalyst material Meanwhile, under the presence of hole scavengers, two •O2 may reach with each other to create H2O2 Moreover, two H+ ions may reach with

dissolved oxygen and two photoexcited e to create H2O2 Due to the generation of various reactive oxygen radicals, ZnO and its derivatives have been employed in various redox-based applications

Figure 1.11 Photocatalyst mechanism of ZnO

1.3.5 Applications

There have been extensive reports on the utilization of ZnO and its derivatives as a photocatalyst for environmental treatment [53] In recent years, the application of ZnO-based material with high photoactivity has expanded to hydrogen production, organic photocatalysis synthesis, energy storage, and energy generation The vast application of ZnO opens up a new era for sustainable development and green production

1.3.5.1 Energy generation and storage

In recent years, energy crisis has become a concern for governments around the world due to the depletion of fossil fuels and increasing energy demands Photocatalysis has opened up an additional pathway for energy generation and storage Therefore, the design and construction of an efficient photocatalyst is the central task for the real-world implementation of photocatalytic reactions The power efficiency of solar light may be increased by utilizing another heterostructure system as a counter

electrode Kuppu et al prepared NiO@ZnO modified TiO2-CsPbI3 photoanode for Perovskite solar cell They reported that TiO2-CsPbI3, NiO-modified TiO2-CsPbI3, ZnO-modified TiO2-CsPbI3, and NiO@ZnO modified TiO2-CsPbI3 photo-anodes displayed a conversion efficiency of 6.66, 7.21, 7.86, and 8.73%, respectively Meanwhile, an inverse Perovskite cell was also assembled for NiO@ZnO modified TiO2-CsPbI3 photo-anodes and demonstrated a power conversion efficiency of 8.03% [54] Currently, the emergence of photo-supercapacitor, in which photoenergy can be stored and discharged simultaneously, has caught the attention of researchers around the world Altaf et al outlined the utilization of g-C3N4/ZnO nanowires as photo-supercapacitor They revealed that the synthesized heterostructure g-C3N4/ZnO nanowires-based photo-supercapacitor showed excellent cycling stability over 25,000 charge/discharge cycles with exceptional capacitance retention and Coulombic efficiency, which is a parameter that represents the discharge capability of a supercapacitor, of 90.2 and 99.9%, respectively, using lithiated Nafion membrane as the electrolyte and separator It is noteworthy that the energy density of the reported heterostructure increased by 21.5-fold with UV-illumination, provided by EMTO-TERA with a light intensity of 8.8 mW/cm2, and reached 11 Wh/kg [55]

Aside from the generation of electric energy from solar light, ZnO has been proven to have a photothermal effect Zhang et al prepared germanium-plated ZnO nanorod arrays for photothermal conversion To evaluate the conversion efficiency of the prepared germanium-plated ZnO nanorod arrays, they introduced a parameter termed energy storage ratio The parameter obtained for germanium-plated ZnO nanorod arrays, which is prepared from hexamethylenetetramine and zinc nitrate hexahydrate precursor solution (60 mmol/L) in the ratio of 1:1 with a germanium layer thickness of 165 nm, is two-fold higher sample prepared without the deposition of germanium layer under the simulated sunlight irradiation [56] Another problem that ZnO can tackle is the removal of various dyes in aqueous media

1.3.5.2 Photocatalytic degradation of dyes

In recent years, the explosive growth of industries has led to the mass discharge of various polluting agents such as recalcitrant organic dyes, antibiotics, pesticides, and heavy metal ions Due to their various impacts on humans, other aqueous critters, and

the environment, it is highly desirable to remove these agents Photocatalysis has caught the attention of various researchers around the world as this process has long been considered a greener approach for the removal of the aforementioned pollutants Amongst the reported catalyst for such purpose, ZnO is often employed due to its ease of preparation and economics It is reported that pristine ZnO prepared with the biological method exhibits a high photocatalytic performance toward various dyes [57]–[60] It is noteworthy that the turbidity induced by the color of the dye can greatly hinder the photocatalytic performance of ZnO and its derivatives [60], [61] Moreover, the removal performance of ZnO, prepared with biological approaches, is comparable to the one that has been modified via doping or heterojunction to improve the photocatalytic activity Meanwhile, the difference in removal efficiencies of dyes can be correlated to the recalcitrant nature of a specific dye Overall, the results for the photocatalytic removal of dyes in aqueous media reveal a potential application of photocatalysis in water remediation for dye effluent Yashni et al synthesized ZnO

NPs using Citrus sinensis peel extract for the degradation of congo red in aqueous

media with a maximum removal rate of 96% They revealed that the bio-inspired ZnO NPs are highly applicable on a larger scale as they revealed that the production of ZnO with a particle size less than 100 nm costs 20.25 USD per kg, which is much lower than commercial ZnO that comes at the price of 40-100 USD/kg Moreover, they stated that the remediation of dye effluent containing using biosynthesized ZnO NPs via photocatalysis is much more economical due to the requirement of less cost factor when compared to other conventional treatment methods In this study, the annual turnover for bio-inspired ZnO of 901,038.18 USD per year is confirmed when utilizing the preparation route that is proposed by Yashni and his co-workers [62] Aside from the treatment of dyes and generation of energy, photocatalysis has also been utilized to produce energy carriers.

1.3.5.3 Energy carrier production

In recent years, the depletion of fossil fuels has caught the attention of various researchers around the world to look for an alternative energy source Simultaneously, the environmental concern for the utilization of conventional fossil fuels also inspires

the need for greener energy sources As a result, carbon-free energy carriers such as hydrogen peroxide, ammonia, and hydrogen are introduced and extensively studied

The production of hydrogen H2 using photocatalysis has long been extensively researched (H2) is regarded as the ultimate energy carrier due to its immense gravimetric energy density, having higher and lower heating values of 141.2 and 119.9 MJ/kg [63] The utilization of this gas for energy would significantly impede environmental concerns as carbonaceous species emissions are not generated Therefore, the production of H2 is highly sought to approach green and sustainable development Conventional hydrogen production such as sorption-enhanced steam reforming, coal gasification, and coal pyrolysis utilizes fossil energies for the production of hydrogen, which is highly undesirable as researchers around the world are finding a new energy source that can replace fossil fuels Aside from the limited source of fossil fuels, the generation of carbonaceous emission when this kind of fuel is employed should also be taken into consideration [64] As a result, the utilization of renewable resources such as light and water for the production of H2 is highly regarded

It is reported that the minimum photon energy thermodynamically required for water splitting is 1.23 eV, ca 1000 nm Therefore, the entire visible region could be employed for the production of H2 via water splitting The production of H2 by photocatalysis may be employed in suspension particle or immobilized particulate systems Ramírez-Ortega et al reported that the suspension particle system of ZnOTiO2 with 2 w.t% Au loading in water-methanol mixture displayed a H2 production of 9.13 mmol/g after 5 h under the irradiation of a 254 nm V Pen-Ray with an intensity of 4.4 mW/cm2 lamp The obtained results for the heterostructure of ZnO and TiO2 with the loading of Au demonstrated 6 times higher hydrogen production than ZnOTiO2 [65] In another study made by Hunge et al., MZ30, which is the heterostructure between ZnO and 30 w.t% MoS2, revealed a H2 production of 235 μmol/g·h [66] A suspension particle system may impose a high production rate for H2, however, the scalability of this kind of system on an industrial scale may pose serious challenges [67] Therefore, the development of ZnO-based immobilized particulate systems is highly desired ZnO@ZnS core@shell nanorod-decorated Ni foam-based photocatalyst, prepared by Chang and his co-workers, reveals an

outstanding hydrogen production of 5806 μmol/g⸳h using Na2SO3 as a sacrificial agent, 2 × 4 cm immobilized photocatalyst under the irradiation of 350 W xenon lamp with excellent and feasible recoverability [68] It is believed that the production of hydrogen involves the utilization of generated electrons, therefore, sacrificial agents are often employed to trap holes for the efficient production of H2 To further promote the economic aspect of H2 production, the production of H2 without the use of any sacrificial agent is highly recommended Rabell et al prepared ZnO/GeO2 for hydrogen evolution without the addition of a sacrificial agent They reported a hydrogen production of 0.01 and 0.002 mmol/s for the heterostructure catalyst and pristine ZnO under the irradiation of 254 nm UV-light Simultaneously, they affirmed that solar to hydrogen conversion of 0.24%, which is 6 times higher than pristine ZnO [69] It has been affirmed that the formation of H2 is associated with active sites of the photocatalyst, electron trapping sites as well as active sites provided by Ag, Au, Al, and Pt are also employed to increase the production of H2 [70]–[74] Rabell et al also conducted the evolution of hydrogen without the usage of sacrificial agents on Al-doped ZnO They reported that with 5% Al content in w.t%, the hydrogen production achieves a hydrogen production of 0.26% They also confirmed that the production of H2 is associated with active sites provided by Al through the increased turn over frequency value for ZnO/Al with higher heteroelement content [75] Aside from the formation of H2 in the aforementioned Water splitting using an immobilized photocatalyst for hydrogen production is also a promising way as oxygen (O2) is also generated Pan et al reported a decent overall water splitting performance of 2418.1 and 1185.9 μmol/m2·h for H2 and O2, respectively, using ZnO nanoarrays/LaCrO3 film heterojunction, prepared with two times of electrodeposition [76] Different schemes for the formation of H2 may occur at different pH levels It is reported that the formation of H2 is highly favorable in acidic conditions due to the requirement of additional energy for the generation of photons in alkaline conditions [74] Haddad et al confirmed the production of H2 is less favorable in alkaline pH using 5%CuO/ZnO heterostructure with the catalyst dose of 0.25 mg catalyst/mL solution and SO3 as a sacrificial agent [77]

In addition to H2, H2O2, a potent energy carrier, is considered one of the most important chemicals in the world due to its wide range of applications such as a bleaching agent, energy carrier, and disinfectant in wastewater treatment [78] Owing to its high energy density (2.1 MJ/kg for H2O2 with a concentration of 60%) and carbon-free energy-generating process, it is considered one of the alternative fuels to fossil fuels The photoproduction of H2O2 using ZnO and ZnO-based materials is highly scalable for industrial processes [79] Moreover, the photocatalytic H2O2productions of some ZnO-based materials under UV illumination are much higher than under sunlight or simulated irradiation Such low production of H2O2 can be mitigated by the utilization of higher energy-density light sources via UV irradiation [80], [81] Comparably, the usage of a high-power illumination source may induce a higher cost of operation along with a lower production rate of H2O2 Although UV irradiation may be limited to natural sources, it can be provided with a lower consumption cost with little impact on the environment Pristine ZnO offer a decent photocatalysis performance under UV irradiation, however, the aims of photocatalysis processes are shifting to visible light irradiation, therefore, ZnO is modified.

1.3.6 Modification

Due to the undesirable performance of bulk ZnO as well as the limited absorption in the visible light region, several modification routes such as doping with metal, non-metal, and heterojunction have been exploited to improve the photocatalytic performance of ZnO as demonstrated in Figure 1.12 Different modification approaches may alter the photocatalytic mechanism of bulk ZnO

Figure 1.12 Modification methods on ZnO

MODIFICATION APPROACHES