Dye pollution
Current status
Driven by economic prosperity, the textile industry flourishes as a dominant force As its core component, dyes play a crucial role, but their discharge poses significant environmental challenges Approximately 700,000 tonnes of dyes are produced annually, with over 25% released into the environment due to inefficient processes The resilience of dyes to various agents and the complexity of their multi-ring structures hinder their biodegradation Consequently, dye-contaminated water sources become highly toxic and carcinogenic, posing health risks to humans and ecosystems Exposure to dyes has been linked to dermatitis, allergic conjunctivitis, liver cancer, and increased mutagenicity potential Among the diverse dye types, thiazine holds prominence due to its exceptional color brilliance and tinctorial strength.
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
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
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]
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
4 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
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]
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
5 such as catalyst doses, pH of the effluent, and dye concentrations, and light source intensity [13] b Influences of factors
In photocatalysis, the catalyst dose, pH level, and dye concentration significantly impact the process Determining the appropriate catalyst dose is crucial to ensure sufficient active sites while maintaining light availability for each particle The pH level affects the interaction between dyes and particles, influencing treatment efficiency Additionally, variations in dye concentration due to wash-off require the photocatalysis system to achieve consistent degradation performance across varying concentrations Therefore, these three factors are chosen as key parameters to optimize the photocatalysis process.
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.
Energy crisis
Current status
As fossil fuels diminish and energy demand skyrockets, the global energy crisis necessitates exploration of alternative energy sources Additionally, the transition to cleaner energy is crucial for environmental preservation and development Unfortunately, 1.2 billion people in developing nations lack electricity access, and current energy consumption contributes to 60% of greenhouse gas emissions, highlighting the urgent need for sustainable energy solutions.
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].
Hydrogen peroxide
Hydrogen peroxide (H2O2) is a highly valued environmentally friendly oxidant due to its ability to oxidize inorganic and organic substrates under mild conditions Unlike other oxidants, H2O2 produces minimal waste, eliminating the need for additional separation processes Its versatility extends to various industries, including hair dyeing, food processing, and environmental remediation Additionally, H2O2 plays a significant role in energy generation, releasing energy that can be harnessed for use.
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.
Production methods
There have been various production methods reported for H2O2 as summarized in Figure 1.2
A novel method of H2O2 production is the utilization of an H2/O2 fuel cell system The fuel cell method for the production of H2O2 is defined as an electrochemical process in which H2O2 is produced via the reduction of O2 at the thee phase boundary
7 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]
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 H2O2 production is quite high due to the constant replacement of catalyst as well as the low selectivity of the alkyl anthraquinol
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
8 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
Selecting an appropriate sacrificial agent is essential for photocatalysis processes since different agents offer distinct pathways for hydrogen peroxide (H2O2) generation and dose requirements The catalyst dose positively influences H2O2 production, but excessive amounts can diminish energy absorption per particle High-intensity light sources facilitate H2O2 photoproduction, but consider the trade-off between enhanced production and heat generation that can degrade H2O2 Moreover, higher-intensity light sources consume more power Therefore, optimizing these parameters is crucial for maximizing H2O2 production via photocatalysis Notably, ZnO is a widely recognized base material for constructing efficient photocatalysts for various redox-based applications.
[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.
Zinc oxide
Structure
ZnO, belonging to the II-VI semiconductor group, is a widely recognized semiconducting material Its crystal structure is categorized into the P63mc space group ZnO has three polymorphs: wurtzite, zinc blende, and rocksalt Wurtzite is the most common naturally occurring structure due to its high stability Zinc blende can only be obtained by growing the crystal on a cubic substrate, while the rocksalt structure requires high pressure.
Figure 1.3 Polymorphs of ZnO (a) wurtzite, (b) zinc blende, and (c) rocksalt [24]
Properties
ZnO has unique characteristics such as natural abundance, good photocatalysis, broad wide band-gap in the near-UV spectral region (3.32 eV), and a large free-exciton binding energy (60 meV) Owing to its good photocatalytic activity, and low toxicity, it can be potentially utilized for environmental restoration, photoproduction of H2O2, hydrogen evolution, and antimicrobial agent Moreover, TiO2, demonstrating a similar band-gap energy (3.2 eV) to that of ZnO, has been predicted to have a similar photocatalytic activity to which of ZnO However, the use of TiO2 for this purpose has been proved to be less economical than ZnO as the production cost of TiO2 is higher than ZnO The general property of ZnO depends on the size as well as the shape of the nanocrystals ZnO can take the structure of a zero-, one-, two-, or three-dimensional structure [25] Nanorods, nanowires, nanotubes, and nanoneedles are examples of the one-dimensional structure of ZnO crystal Meanwhile, two- and three-dimensional arrays of ZnO nanocrystals include nanosheets, -pellets, -flowers, -dandelions, and - snowflakes Having numerous approaches to synthesize, ZnO with desirable shape and size can be achieved These approaches can be divided into three main groups namely, mechanical, chemical, and biological as demonstrated in Figure 1.4
Figure 1.4 Synthesis routes of ZnO [26]
Synthesis
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
11 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
12 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
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
Metal alkoxides are highly sought after for metal oxide synthesis due to their strong affinity for nucleophilic reagents like water Their capacity to form homogeneous solutions with other metallic derivatives further enhances their utility in metal oxide synthesis In particular, for synthesizing ZnO nanostructures, both inorganic and organic salts such as zinc nitrate and zinc acetate have been effectively employed.
13 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
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
15 involvement of various perilous and hazardous chemicals that can impose various health risks
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
ZnO formation pathways involve both intracellular and extracellular mechanisms The intracellular pathway requires bacteria to internalize Zn2+ ions, which is a complex and time-consuming process In contrast, extracellular formation is more common and influenced by factors like pH and electrokinetic potential Fungi have gained attention for ZnO synthesis due to their high cell wall binding and protein secretion, resulting in higher productivity compared to bacterial approaches Fungi's tolerance to process parameters also suggests the industrial potential of this synthesis method.
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
18 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
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
The depletion of fossil fuels and rising energy consumption have escalated the energy crisis, driving governments worldwide to explore alternative energy sources Photocatalysis has emerged as a promising solution, offering an efficient way to generate and store energy To harness the full potential of photocatalysis, designing and constructing efficient photocatalysts is crucial Additionally, integrating a heterostructure system as a counter electrode can enhance the power efficiency of solar light, making photocatalytic reactions more practical for real-world applications.
19 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/cm 2 , 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
The photocatalytic activity of ZnO towards methylene blue (MB) dye degradation was found to be ~165 nm, significantly higher than samples prepared without a germanium layer This enhanced activity is attributed to the effective separation of charge carriers and increased specific surface area Additionally, ZnO effectively removes various dyes from aqueous media, demonstrating its potential for wastewater treatment applications.
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
20 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.
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
21 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 ZnOTiO2 with 2 w.t% Au loading in water-methanol mixture displayed a
The heterostructure of ZnO and TiO2 with the addition of Au exhibited remarkable hydrogen (H2) production, yielding 9.13 mmol/g after 5 hours of exposure to a 254 nm Pen-Ray lamp This result surpassed that of ZnO-TiO2 by sixfold Additionally, MZ-30, a heterostructure of ZnO and 30 wt% MoS2, demonstrated significant H2 production, establishing the potential of these composites for efficient photocatalytic systems.
ZnO-based immobilized particulate systems are highly sought after due to their potential for high H2 production rates However, the scalability of suspension particle systems on an industrial scale presents significant challenges Therefore, ZnO@ZnS core@shell nanorod-decorated systems are being developed as a promising alternative with a high production rate of 235 μmol/gãh This system overcomes the scalability limitations of suspension particle systems, making it a viable option for industrial-scale H2 production.
Ni foam-based photocatalyst, prepared by Chang and his co-workers, reveals an
Hydrogen production efficiency is enhanced by the availability of electrons and trapping holes through sacrificial agents In efforts to improve the economic viability of H2 production, methods without sacrificial agents are explored One approach is ZnO/GeO2, which yielded 0.01 and 0.002 mmol/s of hydrogen under UV-light, resulting in a 0.24% solar-to-hydrogen conversion efficiency Active sites play a crucial role in H2 formation, and electron trapping sites and other modifiers like Ag, Au, Al, and Pt enhance hydrogen production Rabell et al demonstrated hydrogen evolution using Al-doped ZnO, achieving a 0.26% hydrogen production rate with 5% Al content The presence of Al creates active sites, evidenced by increased turnover frequency values.
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/m 2 ãh 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 H2O2 productions 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.
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
Heterojunction Metal doping Non-metal doping
Heterojunction can be understood as the combination of two semiconductors with different bands alignment Based on the position of the band alignments, heterojunction can be can be divided into three types namely, type-I (straddling gap), type-II (staggered gap), and type-III (broken gap) [82] In the case of type I- heterojunction, the CB of semiconductor 1 is higher than the CB of semiconductor 2, meanwhile, the contrary is obtained for the VB of semiconductors 1 and 2 The photoexcited electron and holes are going to accommodate the valance band and CB of semiconductor 2, respectively [83] As a result, a high rate of electron-hole pairs recombination is obtained for this heterojunction, which is undesirable for a photocatalyst as demonstrated in Figure 1.13
As for type II heterojunction, both the VB and CB of semiconductor 1 are higher than the respective bands of semiconductor 2 Under illumination, the excited e in the
CB of semiconductor 1 is transferred to which of semiconductor 2, while the generated h + of semiconductor 2 migrates to the VB of semiconductor 1, thus, creating a spatial separation for electron-hole pairs, subsequently, enhancing the photocatalytic activity of the heterojunction photocatalyst as shown in Figure 1.14 [84]
Figure 1.14 Type II heterojunction Meanwhile, a distinct alignment between the bands of semiconductors 1 and 2 is observed for type-III as demonstrated in Figure 1.15
Due to significant band differences, semiconductors 1 and 2 form distinct materials without electron or hole transfer, indicating no synergistic effect in retarding recombination Heterojunctions can adjust electron pathways and energy gaps, but the optical structure remains unchanged Therefore, alternative modifications like doping are explored to alter the nature of the material.
Doping can be defined as the introduction of impurities into the structure of the pristine photocatalyst without altering the crystallographic structure of the parent compounds for creating a photocatalyst with the goal of increasing the separation efficiency of photo-excited charges Doping allows energy states inside the band-gap of the semiconductor near the energy band of the dopant type New states near the CB and VB can be obtained by electron donor and acceptor dopants
The VB of pure zinc oxide is comprised of O 2s, O 2p, and Zn 3p states, meanwhile,
O 2p, Zn 4s, and Zn 3s states make up for the CB The metal doping of ZnO involves the substitution of metals into Zn sites of ZnO for narrowing the bandgap energies of the composite materials Various types of metals can be used for this purpose such as alkaline metals (Na, K, and Li), and transition metals (Fe, Cu, Au, Ag, and Pd) The doping of metals into ZnO structure introduces a deep acceptor level in the band-gap introduce in the deep acceptor levels near the CB of ZnO, subsequently, reducing the band-gap of the bulk material and shifting the absorption region from UV to visible light as demonstrated in Figure 1.16 The interstitial diffusion of the metal dopant may induce O vacancies, which act as an electron sink for the efficient separation of e and h + pairs [86] Additionally, non-metal element may be also be incorporated into ZnO lattice to alter and enhance the photocatalytic activity of the pristine material for a specific application
Figure 1.16 Influences of metal doping on the band structure of ZnO
In this strategy, the O atom of ZnO is expected to be substituted with another p block element Through the hybridization of nonmetal dopant orbitals and O 2p states of ZnO, the bandgap of the composite material by the increment in the upper edge of the
VB As a result, the substituted atoms must have lower electron negativity than O and a similar lattice size to O atoms In recent years, the incorporation of nonmetals such as nitrogen, sulfur, and carbon into ZnO matrices has caught the attention of researchers around the world as it has been considered as a more viable strategy to reduce the band-gap and enhance the visible light-driven photocatalytic activity [87] ZnO doping with non-metal atoms, such as phosphorus, sulfur, nitrogen, and carbon can lead to the formation of intermediate energy levels near the VB as demonstrated in Figure 1.17, extending the VB and lowering the bandgap of the nanocomposite, subsequently, enhancing the visible-light photocatalytic activity of the material Moreover, O vacancies may also form, which act as an e trapping site to prolong the life of e and h + pair
Figure 1.17 Influences of non-metal doping on band structure of ZnO
Amongst the non-metal doping elements, carbon is proved to be the most prominent due to its abundant source and doping efficiency [88] Weilai Yu et.al reported the formation of a vacant state that is slightly higher than the Fermi level can be achieved by doping nitrogen and carbon due to their electronically deficient.
Carbon-doped zinc oxide (ZnO-C)
Structure
The increase in vacant sites makes it easier for electrons at the Fermi level to jump to higher energy level regions under light stimulation due to the vacant sites induced by the intercalation of C The reduction in bandgap can be attributed to the expansion of the bandgap, resulting from the generation of vacant sites However, the same number of valence electrons of S and O leads to none of the aforementioned phenomena Carbonaceous nanostructures may form on top of the semiconductor, which is resulted from the anchoring of carbon on O sites, aside from being intercalated into the crystal structure of the semiconductor through the substitution of carbon
O site of ZnO is expected to be substituted with C as demonstrated in Figure 1.18
Figure 1.18 Wurtzite crystal structure of ZnO-C
Photocatalyst mechanism
Light irradiation with energy exceeding the bandgap of ZnO-C excites electrons from the valence band (VB) to the conduction band (CB), leaving behind holes in the VB This electron-hole pair generation is crucial for the photocatalytic activity of ZnO-C.
Figure 1.19 Photocatalyst mechanism of ZnO-C The doping of carbon into ZnO lattices leads to the formation of an intermediate level, subsequently reducing the band-gap of the material Moreover, the lifetime of the charge carriers is also extended due to the formation of electron traps formed from the intercalated carbon and oxygen vacancies The photocatalytic process of ZnO-C occurs as follows The excited e combines with O2, h + combines with H2O, creating
•O2 and •OH, respectively Then, generated •OH and •O2 radicals oxidize organic substances Therefore, organic pollutants are degraded into H2O, and CO2 via the photocatalytic activity of ZnO-C 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 To achieve good doping as well as good crystallinity of the material, various synthesis methods are employed.
Synthesis of ZnO-C
The synthesis of ZnO-C requires a zinc precursor such as zinc acetate, zinc chloride, and zinc nitrate The intercalation of carbon into ZnO lattices using the mechanical method, which is discussed in section 1.3.3.1, followed by calcination for the recovery of ZnO crystal structure has been proved to be viable ZnO-C can also be obtained through the hydrothermal, which is extensively explained in section 1.3.3.2, of a zinc precursor with a carbon substrate The use of other surfactants such as CTAB and glucose or the utilization of precursor organic zinc salt is often used as a carbon source for the synthesis of carbon-doped ZnO (ZnO-C) In recent years, bio-reductants and surfactants derived from underutilized biomass extract have attracted the attention of researchers around the world [89]
Biomass extracts have emerged as promising carbon sources for doping ZnO, enhancing its photocatalytic properties By utilizing Cinnamomum camphora leaves extract, researchers have synthesized self-doped carbon zinc oxide with a reduced bandgap from 3.22 eV to 3.0 eV The introduction of carbon into ZnO lattices creates defects and oxygen vacancies that act as electron sinks, inhibiting the recombination of electron-hole pairs Additionally, calcination parameters such as temperature and time influence the characteristics of the resulting ZnO-C composite, which can further optimize its photocatalytic performance.
1.4.3.2 Influences of calcination temperature and time on the characteristics and photoactivity of ZnO-C
With the increase in calcination temperature, excessive carbon source is removed in a hasten way Simultaneously, the crystallization of ZnO follows a similar phenomenon However, the substituted carbon has a weaker bonding than that of O in ZnO structure, therefore, elevated temperature may disrupt the intercalated carbon and induce defects in the crystallinity of ZnO-C, which is a parameter that greatly affects photocatalytic performance of the material In addition, thermal sintering also occurs at higher temperature Such phenomenon also affects the photoactivity of ZnO-C as the specific surface area of the particles system is reduced due to the agglomeration of the particles Additionally, O defect sites in ZnO-C are also generated under prolonged calcination Although defect sites work as electron stink to increase the spatial separation of h + and e , these sites also work as recombination site if there are excessive sites are available
Due to the introduction of green processes, the utilization of phytochemicals that present in the plant-based extract as the C source for doping this element onto ZnO may be considered as an efficient alternative way to modify the optical properties of ZnO in a greener and less-toxic way In addition, the influence of temperature and time on the characteristics and photoactivity of ZnO-C are studied in this thesis.
Garcinia mangostana pericarp
Current status
Garcinia mangostana (G mangostana) is known as a typical evergreen tree cultivated in tropical regions and its pericarp, which is often discarded once the flesh is consumed Moreover, the high demand in the consumption of this fruit due to its pleasant taste has led to the mass discharge of the pericarp It has been extensively reported that the phytocompounds present in the pericarp of this kind of fruit such as xanthone, benzophenone, anthocyanin, and phenolic compounds are highly valuable
[91] The phytochemicals found in the pericarp of the food not only provide biological activities such as anti-inflammatory, α-glucosidase inhibitory, and antimicrobial activities but also reduce and control the shape of metal oxide [92], [93] The phytochemicals in the pericarp that are responsible for such purpose are flavonoids, steroids, phenolic
Utilizing the natural properties of Garcinia mangostana pericarp extract, researchers discovered 32 compounds, including condensed tannins This extract serves as a sustainable carbon source to enhance the photocatalytic performance of ZnO Its extraction becomes crucial in harnessing the potential of G mangostana pericarp extract for advanced applications.
Extraction methods
Due to the robustness and unique structure of plant cell walls The diffusion of plant metabolites may be greatly hindered, therefore, sounds at a high frequency (greater than 20 kHz) are employed to destroy plant cell walls to enhance penetration and the contact between the extraction solvent Through the collapse of the cell wall, the extraction speed is greatly accelerated Furthermore, this extraction method has been reported to greatly enhance the extraction efficiency through the decrease in the amount of extraction solution needed However, long-term exposure to ultrasonication may induce excessive heat due to the use of high-energy waves and the generate free radicals, as a result, these may degrade the desirable phytochemicals of the extracts Another problem associated with this method is that only small sample can be applicable and this method is not highly replicable [94] Therefore, conventional methods are often employed over ultrasonic-assisted extraction
For this method, the entire part of the leaves, stem bark, or root bark is placed inside a container and the extraction solution is poured into the container until the solution completely submerges the plant materials The container is sealed for an extended period to promote the diffusion of the plant metabolites to the extraction solution As a result, this method is highly suitable for thermolabile metabolites However, due to the low diffusion of the phytocompounds to the extraction solution, such procedure is lengthy and quite a hassle Moreover, bacteria growth is often associated with such method due to its lengthy procedure [95] Therefore, extraction methods that is less time-consuming is employed over Maceration
In this method, the part of the plant used is often grinded into fine powder Similarly, the grounded parts are soaked in the cold or hot solvent to initiate the extraction The
33 presence of heat increases the diffusion speed of the phytocompounds and the menstruum This method is highly suitable for the extraction of bioactive constituents that are readily soluble The rapid nature of infusion may hinder the extraction of the metabolites of a plant, resultantly, other extraction methods are involved
Similarly, the grinded plant parts are placed in a container, followed by the addition of a hot solvent to commence the extraction process Due to the submerging of the plant parts and elevated temperature, the extraction process is hastened and more thorough, therefore, this method is considered as more popular method for the extraction of plant Moreover, the ease in the extraction process, as well as replicability, is the main strong point of decoction The solvent used for any process highly depends on the selectivity, safety, cost, reactivity, recovery, and viscosity of the solvent As a result, a mixture between water and ethanol is chosen as the mixture is a relatively safe one for humans with reasonable cost, high recovery, and low viscosity along with good selectivity for the phytocompounds in the G mangostana pericarp in this thesis [96].
Domestic and international research and status on ZnO-C
Domestic researches on ZnO-C
ZnO and its derivatives have been extensively researched in Vietnam as demonstrated in Table 1.1 However, the utilization of a biomass mass extract as a carbon source for band structure alteration of ZnO has not been reported
Table 1.1 Domestic researches related to ZnO
Year Study Organization Obtained results Ref
ZnO thin films for degradation of rhodamine B
Hanoi National University of Education
ZnO-ZrO2 for degradation of phenol
Complete degradation of phenol after
Year Study Organization Obtained results Ref
2021 Sn doped ZnO for degradation of MB
Hanoi University of Science and Technology
Removal efficiency of 92% after 150 min [97]
ZnO/graphitic carbon nitride for degradation of MB
Quy Nhon University Removal efficiency of
Zinc oxide doped titanium dioxide/reduced graphene oxide for the removal of MB
Key Laboratory of Chemical Engineering and Petroleum Processing (Key CEPP Lab)
International researches on ZnO-C
ZnO-C has been widely employed as a promising material for the degradation of various dyes The synthesis of ZnO-C involves the use of a carbon source, as summarized in Table 1.2 Despite numerous studies on the synthesis of ZnO-C, research on its utilization is still in its early stages, highlighting the need for further investigation into the potential applications of this material.
G mangostana pericarp extract as a carbon source and a phytoreductant for the synthesis of ZnO-C
Table 1.2 International researches on ZnO-C
Year Study Carbon sources Obtained results Ref
2019 C/ZnO hollow sphere for the removal of tetracycline
Removal efficiency of 75.4% after 320 min [101]
ZnO@C core/shell sphere for photodegradation of methyl orange
Carbon self-doped ZnO nanofibers for the removal of caffeine
Removal efficiency of 80.4% after 120 min [103]
Year Study Carbon sources Obtained results Ref
2022 ZnO/C microflowers for the removal of MB
Removal efficiency of 99% after 120 min [104]
Objectives, contents, methods, essentiality novelty, and contribution of
Objective
Successful synthesis of ZnO-C using G mangostana pericarp extract with high photocatalytic activity for the removal of MB and H2O2 photoproduction
Conclusion on the influences of calcination temperature and time on the characteristics and photodegradation of MB performance of ZnO-C prepared from
Summary of the characteristics of ZnO-C at favorable calcination temperature and time;
Photocatalytic removal of MB, related photocatalysis mechanism, reusability, and recyclability of ZnO-C are concluded;
Photoproduction of H2O2, corresponding photocatalytic mechanism, reusability, and recyclability of ZnO-C for this process are summarized.
Content
Content 1: Preparation of G mangostana pericarp extract;
Content 2: Synthesis and investigation on the influences of calcination temperature and time on the characteristic and photocatalytic degradation performance of ZnO-C using G mangostana pericarp for MB;
Content 3: Characterization of ZnO-C prepared in favorable calcination temperature and time;
Content 4: Investigation on the photodegradation of MB, photocatalysis mechanism, reusability, and recyclability of ZnO-C;
Content 5: Studying on the photoproduction of H2O2, photocatalysis mechanism, reusability, and recyclability of ZnO-C.
Research methods
ZnO-C is synthesized through two steps:
Step 1: G mangostana pericarp is extracted via decoction [105];
Step 2: ZnO-C is green synthesized using G mangostana pericarp extract followed by calcination [106]
Principle: SEM is used to produce high-resolution images of the sample surface by using a narrow electron beam to scan the surface of the sample The mechanism of SEM is shown in Figure 1.20
Scanning electron microscopy (SEM) employs an electron beam to scan the sample's surface, and the resulting interactions produce emitted radiation This radiation is recorded to form an image of the sample, revealing its surface features and composition.
37 electron beams, controlled by systems of magnetic glass, are scanned onto the material surface When the electrons hit the sample, various interactions between the generated electrons and the sample surface These interactions include secondary electrons, backscattered electrons, diffracted backscattered electrons, photons, visible light, and heat Depending on the structure of the sample, scattering effect is different, therefore, giving out different surface images
Application: SEM is utilized to study the morphologies of uncalcinated and calcinated ZnO-C samples at various temperatures and times
Energy-dispersive X-ray spectroscopy (EDS)
Principle: EDS is an analytical technique used for the elemental analysis of a sample The basic principle of EDX is demonstrated in Figure 1.21 A high-energy beam of charged particles such as electrons or protons, or a beam of X-rays, is focused into the sample being studied to initiate the interaction between the charged particles and the atoms within the samples
Figure 1.21 Fundamental principles of EDS
Once the charged particles hit an electron in an inner shell, this electron is ejected from its shell to leave behind a hole, subsequently, another electron from a higher- energy shell occupies the hole The refilling of the vacancy by a higher-energy level electron leads to the emission of X-rays As each element has its own unique atomic structure, allowing a unique set of peaks on its X-ray spectrum, the identification as well as the composition of each element can be identified The number and energy of the X-rays emitted from a specimen can be measured by an energy-dispersive X-ray detector
Application: EDS is used to identify the elemental composition of uncalcinated and calcinated ZnO-C samples at various calcination temperatures and times
Principle: TGA is a method of thermal analysis involving the measurement of the mass of the sample over the changes in temperature The alteration of mass of the sample provides various information about physical phenomena such as phase transition, absorption, desorption, and adsorption; as well as chemical phenomena such as chemisorption, thermal decomposition, and solid-gas reactions
Application: TGA is used to study thermal changes during the calcination of
Fourier transform infrared spectroscopy (FTIR)
FTIR spectroscopy utilizes the principle of infrared absorption, where functional groups within a compound selectively absorb specific wavelengths of infrared radiation By analyzing these absorption wavelengths, the presence of certain compounds or functional groups in a material can be determined FTIR spectrometers function by generating infrared signals that are encoded with all infrared frequencies These signals are rapidly measured and subsequently decoded using Fourier transformation, a mathematical technique that yields a spectrum representing the spectral information This spectrum can then be compared to reference data to identify the compounds or functional groups present in the analyzed material.
Figure 1.22 Principle of FTIR spectrometer
Application: FTIR is utilized to identify the functional groups of G Mangostana pericarp extract, uncalcinated, and calcinated ZnO-C samples at various temperatures and times
Crystals possess a highly ordered atomic arrangement X-ray diffraction analysis involves directing X-rays onto a crystal sample The interaction between the X-rays and the crystal lattice results in the scattering of the X-rays Most of the scattered X-rays interfere destructively, while those obeying Bragg's law constructively interfere This constructive interference produces diffraction peaks at specific angles, which can be detected and analyzed to determine the crystal structure and properties.
2d sinθ = nλ (1.1) where d is the interplanar spacing, θ and λ are the incident angle and wavelength of the X-ray beam, respectively, and n is an integer XRD patterns, generated through the recorded angle of incident X-rays and the detected intensity of diffracted X-rays, is a plot intensity against the scattering angle (2θ) of X-rays The crystal structure can be determined by referencing the diffraction peaks in the patterns to the available powder diffraction data The fundamental principle of XRD is displayed in Figure 1.23
Figure 1.23 Fundamental principle of XRD
Application: XRD is utilized to analyze the crystallinities of ZnO-C samples prepared at different calcination temperatures and times
Principle: An accelerated electron beam is focused on a sample Electrons are either backscattered or absorbed by the sample upon contact with the sample surface
A considerable fraction of the incident electrons can pass through the sample with a thickness of less than 100 nm By utilizing this phenomenon, the concept of TEM is proposed Furthermore, TEM utilizes electron source with much higher voltages, therefore, images generated in this manner tend to have higher magnification due to the smaller de Broglie wavelength of electrons The image contrast recorded by TEM analysis can be attributed mainly to the thickness, atomic number, and crystal orientation of the components that present in the sample, as a result, TEM can be used to record, identify, and verify the shape of ZnO nanocrystal as well as ZnO-based heterostructure photocatalyst The principle of TEM analysis is demonstrated in Figure 1.24
Application: TEM is used to analyze the morphologies of the sample calcinated at favorable temperature and time
Raman spectroscopy operates on the principle of inelastic photon scattering, termed Raman scattering A monochromatic light source, typically a laser within the visible, near-infrared, or near-ultraviolet spectrum (or occasionally X-rays), interacts with molecular vibrations, photons, or excitations in a sample This interaction causes a shift in the laser photons' energy, revealing information about the sample's vibrational modes The energy shift enables distinguishing between different atoms, facilitating the analysis of its structural composition The scattered light is collected using a lens and processed through a noise filter or spectrometer to obtain the Raman spectrum.
Figure 1.25 Principle of Raman spectrometer
Application: Raman spectroscopy is used to assess the crystallinity and carbon structure of ZnO-C calcinated at suitable temperature and time
Principle: Gas (primarily nitrogen) adsorption-desorption measurements, in which the amount of gas adsorbed on a sample at given pressure values is recorded, have usually been conducted The amount of gas is plotted against relative pressure, which is the ratio between absolute pressure and saturation pressure, to yield an adsorption isotherm of the material According to the International Union of Pure and Applied Chemistry (IUPAC), adsorption isotherms can be divided into six main types as demonstrated in Figure 1.26 Gas adsorption analysis can also provide information like specific surface area through the monolayer coverage of adsorbed gas, which is provided by the linear fitting of Brunauer–Emmett–Teller (BET) theory as displayed in
Equation (1.2) x n(1 − x) = 1 n m C+C − 1 n m C x (1.2) where x is the relative pressure, n and n m are the amount of gas adsorbed and monolayer, respectively, and C is the BET constant It is recommended that the equation should be applied in the relative pressure range between 0.05 and 0.3
Figure 1.26 Adsorption isotherm according to IUPAC classification [107]
Application: N2 gas adsorption is used to determine the specific surface area of ZnO-C calcinated at favorable temperature and time
UltravioletVisible diffuse reflectance spectroscopy (UVVis DRS)
In UV-Vis Diffuse Reflectance Spectroscopy (UV-DRS), the principle aligns with UV-Vis spectroscopy The distinction lies in measuring the relative change in light transmission through a solution in UV-Vis, while UV-DRS assesses the relative change in the quantity of reflected light from a surface This principle is illustrated in Figure 1.27, depicting the fundamental mechanism of UV-DRS.
Figure 1.27 Principle of UV–Vis DRS The bandgap energy of ZnO-C is calculated from the Kubelka–Munk model as shown in Equation (1.3) through the extrapolation of the linear part of the plot
Essentiality
There are many dye wastewater treatment methods such as adsorption, and flocculation However, these conventional effluent treatment methods either give out undesirable treatment results or produce secondary pollutants Furthermore, the multiring structure nature of textile dyes suggests that they are highly resistant to biological treatment processes, making them persist in water for a long time Therefore, newer, cleaner approaches such as membrane filtration, and photocatalysis have been made to resolve the drawbacks of conventional treatment methods Another desirable organic compound that is highly sought in value is H2O2 The production of H2O2 has long been a challenge due to the costly maintenance of conventional methods and environmental concerns As a result, a greener method has been proposed for such a purpose In recent years, photocatalysis has attracted many researchers around the world In particular, ZnO can be utilized as a photocatalyst for the removal of MB and the production of H2O2 However, the two major drawbacks of ZnO are the high electron-hole pair recombination and high band-gap energy, which translate to restricted UV-light region, accounting for 4 – 5% operation of the material To hinder such drawbacks, modification to the band structure of ZnO is required Amongst them doping carbon into ZnO has been proved to be a highly effective method Carbon sources from fine chemicals possess high environmental concern, therefore, environmentally sources carbon sources from plant extract, in particular,
G mangostana pericarp, is utilized to synthesize ZnO as well as to provide carbon for the doping purpose.
Novelty of thesis
ZnO has commonly been used as a photocatalyst, however, its large barge band-gap as well as rapid h + and e recombination hinders its actual applications in industrial processes As a result, it is desirable to reduce the band-gap energy of bulk ZnO and prolong the life of photoexcited electrons to enhance the visible light absorption ability of ZnO Therefore, modifications of ZnO, including, doping and heterojunction Amongst modification routes, non-metal doping, in particular, carbon has been proven to be one of the most prominent methods for altering the band-gap structure and hindering the recombination of h + and e pair for ZnO Carbon sources for
The development of eco-friendly ZnO-C composites has gained significant attention, particularly through the incorporation of phytochemicals Despite the extensive exploration of various phytochemicals, the potential of Garcinia mangostana pericarp extract for ZnO-C synthesis remains largely unexplored This study aims to investigate the feasibility of utilizing G mangostana pericarp extract as a natural precursor for the synthesis of ZnO-C composites, offering a promising avenue for greener and sustainable material synthesis.
Contribution of thesis
G mangostana or commonly known as purple mangosteen contains an inedible, deep reddish-purple-colored pericarp as it becomes ripe that has always been negligibly ignored In specific, this part consists abundant of flavonoids, phenolic, ascorbic acid, and condensed tannins, which are primary compounds tending to strongly reduce the bulk metal ions to the metal oxide The utilization of G mangostana pericarp for the synthesis of ZnO-C opens up a new way for the valorization of discarded biomass By utilizing low-cost sources of reducing agent, ZnO-C with good optical properties can be obtained in a greener method as well as the utilization of the material for the photoproduction of H2O2 to solve the current need for newer greener energy and for the removal of undesirable MB in aqueous media
Chemicals, materials, facilities, equipment, and work location
Chemicals and materials
The list of chemicals used in this thesis is summarized in Table 2.1
Table 2.1 Chemicals used in this thesis
No Chemical Formula State Purity Origin
1 Zinc acetate dihydrate Zn(CH 3 COO) 2 2H 2 O Solid 99% China
6 Methylene blue C 16 H 18 ClN 3 S Solid 99% China
9 Potassium iodide KI Solid 99% China
10 Formic acid HCOOH Solid 99% China
11 Hydrochloric acid HCl Liquid 37% China
12 Potassium hydroxide KOH Solid 99% China
13 Sodium hydroxide NaOH Solid 99% China
G mangostana fruits, as shown in Figure 2.1, were purchased from Ben Thanh market, District 1, Ho Chi Minh City, Vietnam in July 2022 The pericarp was retained after the flesh was consumed
Facilities
Beaker (100, 500, and 1000 mL), thermometer, measuring cylinder (100 and
500 mL), stirring rod, pipette (1.5 and 10 mL), micropipette, magnetic stirring bar, plastic centrifuge tube, and Pasteur pipette.
Equipment
List of equipment used in this thesis is listed in Table 2.2
Experiments
Preparation of G mangostana pericarp extract
The extraction procedure of G mangostana pericarp is demonstrated in Figure 2.2
Figure 2.2 Extraction procedure of G mangostana pericarp
Interpretation: The obtained G mangostana pericarp was thoroughly washed with double-distilled water several times and dried at 50 o C for 3 days Afterward, the dried peel was cut into small pieces and ground using a CGM-20 grinder Then, 10 g of the
54 obtained fine powder was weighed and added to 200 mL of EtOH and water mixture with a volumetric ratio of 1:1 at 90 o C for 1 h The collected mixture was vacuum filtered and stored at 20 o C for later use.
Study on influences of calcination temperature and time on characteristics and
Synthesis of ZnO-C using G mangostana pericarp extract is demonstrated in Figure 2.3
Figure 2.3 Synthesis of ZnO-C procedure
Interpretation: Synthesis of ZnO-C was adopted with modification from the previous study [106] Briefly, 29.5 g of Zn(CH3COO)2.2H2O was added into a 184 mL mixture of G mangostana extract and MeOH with a volume ratio of 2:1 under constant stirring 90 mL of 0.3 M KOH solution, which was prepared in a medium of water and MeOH with a ratio of 2:1, was added dropwise into the prepared solution The mixture was kept under constant stirring at 90 o C for 18 h The obtained solution was washed with distilled water until a neutral pH was achieved Then, the washed mixture was centrifuged and dried at 60 o C to obtain an orange-brownish solid Subsequently, the solid was ground and calcinated ZnO-C samples
The influences of calcination temperature and time on the characteristics and photocatalytic activity of ZnO-C for the degradation of MB were studied
Calcination temperature is adjusted, meanwhile, calcination time is fixed at 1.0 h as demonstrated in Table 2.3
Table 2.3 Influences of calcination temperature on the synthesis of ZnO-C
Calcination time is varied, while, the suitable calcination temperature is fixated according to the value found in section 2.2.2.1a, as demonstrated in Table 2.4
Table 2.4 Influences of calcination time on the synthesis of ZnO-C
2.2.2.2 Characterization of ZnO-C calcinated at various temperatures and times
ZnO-C samples calcinated at various temperatures and times are characterized by using SEM-EDS, TGA-DTG, FTIR, and XRD
SEM-EDS analysis was performed using a JMS-IT 200 scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (JEOL, Japan) The samples were examined under an acceleration voltage of 10 kV, a magnification of 10000×, and a resolution of 512 × 384 pixels at the German-Vietnamese Academy of Technology and Ho Chi Minh City University of Food Industry.
Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG) were performed using a TGA/SDTA 851 thermobalance (Mettler Toledo, USA) The samples were measured at Ho Chi Minh City University of Education with a heating rate of 10 °C/min and a temperature range of 0 to 800 °C under both atmospheric and N2 conditions.
FTIR: The samples were analyzed at the Vietnam National University Ho Chi Minh
City, Key CEPP Lab, Ho Chi Minh City University of Technology.by using Alpha-E, Bruker Optik GmbH, Ettlingen, Germany The material was mixed with KBr powder, then the mixture was pressed into a flat thin film before measurement Operating parameters are resolution 0.2 cm -1 , spectral accuracy of 0.1 %T, and wavenumber range of 4000 – 400 cm -1
XRD: The samples were measured at German-Vietnamese Technology Academy,
Ho Chi Minh City University of Food Industry on a XRD D8 Advance, Bruker, Germany Operating parameters were temperature 30 o C, Cu-Kα radiation with wavelength λ = 1,5406 nm, and recorded 2θ angles are between 0 and 80 o
2.2.2.3 Influences of calcination temperature and time on MB photodegradation performance of ZnO-C
The evaluation procedure for the photodegradation of MB using ZnO-C samples calcinated at different temperatures and times is displayed in Figure 2.4
Figure 2.4 Photodegradation of MB using ZnO-C
Interpretation: In short, 50 mg of ZnO-C was added to 50 mL of MB solution with an initial concentration of 10 mg/L in a 100 mL quartz beaker under constant stirring Afterward, the obtained mixture was illuminated using two 11 W UV lambs or a 35 W visible light lamb for 120 min After each 30 min, 4 mL of the sample were drawn and filtered through a 0.22 μm nylon syringe filter The residual concentration of MB was
57 monitored through the absorbance changes at 665 nm using the UVVis spectrophotometer, followed by the calculation of degradation kinetic.
Characterization of ZnO-C calcinated at favorable temperature and time
ZnO-C prepared at suitable calcination temperature and time is characterized by using Raman spectroscopy, N2 gas adsorption-desorption, UVVis DRS, EIS, CV, and
TEM: The images were taken at Institute for Materials Science, Microscopy Center,
Ha Noi, using a JEM-1010, JEOL, Japan with an operating voltage of 200 kV, LaB6 as a cathode, and point resolution of 0.23 nm
Raman spectroscopy: The samples were measured at the Institute for Nanotechnology - Viet Nam National University Ho Chi Minh City with LabRam micro-Raman system, Horiba, Japan at an excitation wavelength of 632 nm
N 2 adsorption-desorption: The samples were analyzed at Institute of Applied Materials Science - Vietnam Academy of Science and Technology with NOVAtouch
LX gas sorption analyzers, Quantachrome Instruments Specific surface areas of the samples were measured by utilizing N2 adsorption-desorption isotherms at 77.35 K and
UVVis DRS: the optical properties of the samples were measured on a Lambda
750S, PerkinElmer, at the Institute of Applied Materials Science
EIS-CV: The samples were measured at the Laboratory of Electro Chemistry, Ho
A glassy carbon electrode served as the working electrode in a three-electrode electrochemical cell The electrode was coated with a suspension of carbon black dispersed in ethanol A Pt wire and a calomel electrode were employed as the counter and reference electrodes, respectively This electrochemical setup was utilized in conjunction with a CH Instrument 760D electrochemical workstation from the USA.
PL: The samples are tested at Vietnam Academy of Science and Technology by using a Horiba Jobin Yvon Fluorolog spectrometer, Japan, at an excitation wavelength of 260 nm, in the range of 200 – 800 cm -1
Study on the photodegradation of methylene blue (MB), related mechanism, reusability, and recoverability of the catalyst
The influences of catalyst doses, pH, and initial MB concentration on the photodegradation of MB utilizing ZnO-C were investigated The procedure for studying the influences of impacting factors is similar to the one described in section 2.2.2.3 The pH of the solution was adjusted with 0.1 M NaOH and 0.1 M HCl using a digital pH meter a Catalyst dose
ZnO-C samples with suitable calcination temperature and time are selected from section 2.2.2.1b The catalyst dose is varied, meanwhile, the initial MB concentration and pH of the dye solution are fixed as demonstrated in Table 2.5
Table 2.5 Influences of catalyst dose on the photodegradation of MB
No Material Catalyst dose (mg) pH Initial MB concentration (mg/L)
ZnO-C sample used for these experiments remains the same as selected from section 2.2.2.1b The catalyst dose is fixated to the value found in 2.2.4.1a, while, the initial
MB concentration is varied and the pH of the dye solution remains constant as demonstrated in Table 2.6
Table 2.6 Influences of pH on the photodegradation of MB
No Material Catalyst dose (mg) pH Initial MB concentration (mg/L)
ZnO-C sample used for these experiments is selected from section 2.2.2.1b The catalyst dose is fixated to the value found in section 2.2.4.1a, while, pH of the dye solution remains constant, which is found in 2.2.4.1b, the initial MB concentration is changed as demonstrated in Table 2.7
Table 2.7 Influences of initial dye concentration on the photodegradation of MB
No Material Catalyst dose (mg) pH Initial MB concentration
The experiment procedure for studying the influences of scavengers on the photodegradation of MB using ZnO-C to study the photocatalysis mechanism is demonstrated in Figure 2.5
Figure 2.5 Influences for scavengers on the photodegradation of MB
Interpretation: In short, a determined dose of ZnO-C, which is obtained in 2.2.2.3b was added to 50 mL of MB solution with initial concentrations found in section 58 in a 100 mL quartz beaker under constant stirring The pH of the solution was kept at neutral Radical scavengers are added to identify the most influencing radicals
Afterward, the obtained mixture was illuminated using two 11 W UV lambs for 120 min After each 30 min, 4 mL of the sample were drawn and filtered through a 0.22 μm nylon syringe filter The residual concentration of MB was monitored through the absorbance changes at 665 nm using the UVVis spectrophotometer EDTA, p-
Benzoquinone, and isopropanol were deployed as scavengers to capture h + , •O2 , and
•OH, respectively b Determination of TOC and COD
The removal efficiency of zinc oxide-carbon (ZnO-C) was evaluated by analyzing total organic carbon (TOC) and chemical oxygen demand (COD) in the solution after photocatalysis This analysis confirmed the photocatalytic mechanism involved in the degradation of methylene blue (MB).
TOC: Post-catalysis solution was analyzed at Navitek Food and Environmental Testing Joint Stock Company Initially, samples were digested using H2SO4 and
K2Cr2O7 at 200 o C for 2 h, followed by the titration using ferrous ammonium sulfate and ferroin as the indicator
The post-catalysis solution was analyzed by Navitek Food and Environmental Testing Joint Stock Company utilizing a Sievers InnovOx ES, supplied by Veolia This equipment boasts a detection range spanning from 0.05 to 50.000 ppm, with a minimal detection limit set at 50 ppb.
2.2.4.3 Reusability and recyclability of catalyst
The evaluation of the reusability and recyclability of ZnO-C for the photodegradation of MB and photoproduction of H2O2 is displayed in Figure 2.6
Figure 2.6 Procedure for evaluating the reusability and recyclability of catalyst for the photodegradation of MB
Interpretation: After each photocatalysis cycle, the sample was centrifuged, washed using distilled water, dried, and reused for the following cycle The obtained
ZnO-C was utilized again for the photodegradation of MB The described procedure was repeated for 10 cycles.
Study on the photoproduction of hydrogen peroxide (H 2 O 2 ), corresponding mechanism, reusability, and recoverability of catalyst
The evaluation for the photoproduction of H2O2 using ZnO-C in the presence of a sacrificial agent is displayed in Figure 2.7
Figure 2.7 Photoproduction of H2O2 using ZnO-C
Interpretation: A determined amount of ZnO-C was also added into a 50 mL mixture of water and a sacrificial agent in a 100 mL quartz beaker with different ratios under constant stirring Two commercial 11 W UV lambs or a 35 W visible light lamb were utilized as an illuminated source for 180 min After each 30 min, 2 mL of the sample were drawn and filtered through a 0.22 μm nylon syringe filter
The influences of different types of sacrificial agents, their dose, and catalyst dosage are studied
ZnO-C sample used for these experiments is selected from section 2.2.2.1b The sacrificial agent is altered between methanol and isopropanol Meanwhile, the sacrificial agent and catalyst doses remain constant as demonstrated in Table 2.8
Table 2.8 Influences of different types of donors on photoproduction of H2O2
No Material Sacrificial agent Sacrificial agent dose
A ZnO-C sample was chosen from 2.2.2.1b for experimentation.* An appropriate sacrificial agent was acquired from section 2.2.5.2.* The quantity of sacrificial agent was varied while the amount of catalyst remained steady, as shown in Table 2.9.
Table 2.9 Influences of electron donor doses on photoproduction of H2O2
No Material Sacrificial agent Sacrificial agent dose
ZnO-C sample used for these experiments is selected from 2.2.2.1b The suitable sacrificial agent is obtained in section 2.2.5.2 The sacrificial agent dose is selected from section 2.2.5.3, while the amount of catalyst added is changed as demonstrated in Table 2.10
Table 2.10 Influences of catalyst on photoproduction of H2O2
No Material Sacrificial agent Sacrificial agent dose
2.2.5.5 Reusability and recyclability of the catalyst
The evaluation of the reusability and recyclability of ZnO-C for the photodegradation of MB and photoproduction of H2O2 is displayed in Figure 2.8
Figure 2.8 Procedure for evaluating the reusability and recyclability of catalyst for the photoproduction of H2O2
Interpretation: After each photocatalysis cycle, the sample was also centrifuged, washed using distilled water, dried, and reused for the following cycle The obtained ZnO-C was reused again for H2O2 production following a similar procedure
3.1 Influences of calcination temperature and time on the characteristics and photocatalytic removal of MB of ZnO-C
Characteristics
ZnO-C morphology varies under different calcination conditions Initially, ZnO-C appears as clumps due to the encapsulation of ZnO by G mangostana phytochemicals After calcination, phytocompounds burn off, resulting in discrete ZnO particles Calcination temperature influences particle size, as higher temperatures induce thermal sintering, leading to larger ZnO particles.
Figure 3.1 SEM images of ZnO-C synthesized at different calcination temperatures: (a) un-calcinated, (b) 500, (c) 600, (d) 700, and (e) 800 o C
The presence of the elements in ZnO-C samples calcinated at different calcinated conditions is revealed by EDS As shown in Figure 3.2a, there are a total of five elements, namely C, N, O, Zn, P, and S It is observed for all ZnO-C samples that the presence of heteroatoms like C, P, S, and N is resulted from the extract of
G mangostana [108] Meanwhile, the weak signal of N, S, and P for the calcinated samples can be attributed to the low amount of these elements in the structure of the
Calcination at high temperatures successfully doped non-metallic elements into the ZnO wurtzite lattice, as evidenced by their presence after the process Elemental mapping revealed the distribution of these introduced elements, with C, O, and Zn producing strong signals that concealed the weaker signals of the additional materials.
Figure 3.2 (a) EDS spectrum and (b) elemental composition of ZnO-C synthesized at different calcination temperatures
To further validate the phenomena observed in SEM and EDS results of ZnO-C calcinated at different temperatures, the thermal behaviors of ZnO-C were recorded using TGA and DTA in both atmospheric and N2 conditions, as demonstrated in Figure 3.3
Figure 3.3 (a) TGA and (b) DTA curves of ZnO-C in atmospheric and N2
In overall, three stages of thermal decomposition occurred for ZnO-C when the temperature increased from 0 to 800 o C At first, the desorption of bound moisture occurred at 64.08 o C as evidenced by a large endothermic peak The second stage of decomposition occurred at 336.13 and 445.43 o C for atmospheric and N2 conditions respectively Such occurrence can be explained by the decomposition of various phytochemicals, which is validated by the large exothermic peaks at the aforementioned temperature in DTA curve of ZnO-C It is observed that the second decomposition stage initiated in atmospheric conditions at a much lower temperature with a much higher exothermic peak as the combustion of organic matter could commence in atmospheric conditions easier than in pure N2 The last stage, in which only a small mass percentage of the sample was lost, occurs in both atmospheric and
N2 conditions when the temperature was raised over 700 o C The large exothermic peaks exhibited over these temperatures as well as the small mass loss imply that the decompositions of the intercalated C atoms might have occurred Therefore, the suitable calcination temperature of ZnO-C was selected to be 700 o C
Meanwhile, the effects of calcination times on the particle size are demonstrated in Figure 3.4
Figure 3.4 SEM images of ZnO-C-700 synthesized at different calcination times:
In specific, a similar phenomenon occurred with the samples that are calcinated for a prolonged time Moreover, it is heavily reported that prolonged calcination of
67 metallic oxide facilitates the formation of O vacancies in the crystalline structures
[109] The formed oxygen vacancies may be beneficial for photocatalysis-based processes as these vacancies offer additional sites for the hindering of the recombination of electron-hole pairs However, a phenomenon, termed as bottlenecking, can be observed in which the excessive oxygen vacancies may act as electron-hole recombination sites, in which hindering the photocatalytic performance of the catalyst Therefore, the influences of calcination time on the crystal structure of ZnO-C must be carefully studied
EDS spectrum was utilized to study the changes in elemental compositions of the samples calcinated at different times as demonstrated in Figure 3.5a Similar results are obtained when the calcination time was prolonged The presence of other heteroatoms such as C, N, P, and S indicates the intercalation of these atoms into ZnO lattices It is noteworthy that the composition C and O decreases for samples calcinated for a longer period as depicted in Figure 3.5b The former indicates that the formations of O vacancies, which are favorable for the photocatalysis process as these vacancies provide additional sites for e to accommodate, are facilitated
Elemental distribution in ZnO-C was assessed using elemental mapping, which revealed uniform distribution of non-metallic and metallic elements The overlap of C, other non-metallic atoms, Zn, and O suggests their close association within the material.
68 on each other along the profile of Zn and O, validating the successful introduction of the dopants onto ZnO crystal matrices
Figure 3.6 Elemental mapping of ZnO-C synthesized at different calcination temperatures: (a) 0, (b) 500, (c) 600, (d) 700, and (e) 800 o C
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 FTIR analysis was utilized to reveal the presence of the possible functional groups of the prepared samples as illustrated in Figure 3.8 The appearance of the band located at the region sub 600 cm -1 , relating to the vibration of metal oxide bonds for all ZnO-C samples revealed the successful formation of the metal oxide [110] A broad absorption band can be seen at 3425 cm -1 , which is assigned to the OH stretching
69 vibration of the hydroxyl groups, for all samples Meanwhile, the bands at 1754 and
1625 cm -1 are related to the stretching vibration of the unconjugated carbonyl (C=O) group Simultaneously, absorption bands of the extract centered at 1520, 1409, 1384, and 1050 cm -1 are attributed to the stretching vibration of conjugated carbonyl (C=C), bending of CH2 and CH3, and stretching vibration of CO [111] Furthermore, absorption bands of the extract could be detected at 825 and 774 cm -1 which are ascribed to the out-of-plane bending of HC=CH cis and trans [112] It is noteworthy that the bands located at 1625, 1516, 1382, and 1048 cm -1 are also found in the uncalcinated ZnO samples The appearances of these peaks confirm the attachment of phytochemicals such as phenols, flavonoids, xanthones, and anthocyanins onto ZnO for the stabilization and reduction of the metal oxide particles In addition, the absorption bands of oxygenated functional groups could not be found after being calcinated, validating the decomposition of the bioactive compounds into carbon Moreover, weak absorbance bands centered at 1625 cm 1 could be detected for all post-calcinated ZnO-C samples, indicating the presence of C=O in the structure of ZnO-C Besides, the elemental mapping of ZnO-C samples reveals the existence of other non-metallic dopants such as sulfur, nitrogen, and phosphorus as shown in Figure 3.6 and 3.7 However, no vibration bands related to these elements are observed for the FTIR spectra of analyzed samples The former can be translated to the faint signals, derived from the low contents of the heteroatoms as shown in Figure 3.2b and 3.5b
Figure 3.8 FTIR spectra of ZnO-C synthesized at different calcination (a) temperatures and (b) times XRD patterns were employed to evaluate the crystallinities of ZnO-C samples, prepared at different calcination temperatures and times, as presented in Figure 3.9 The presence of the peaks located at 32.6, 34.7, 36.56, 47.83, 56.70, 63.14, 66.63, 68.21, 69.32, 72.85, and 77.17 o , corresponding to the (100), (200), (101),
(102), (110), (103), (200), (112), (201), (004), and (202) crystal planes of ZnO (JCPDS 36-1451), revealed the formation of high-purity ZnO wurtzite crystals [113] Furthermore, the introduction of heteroatoms into ZnO lattices induced no changes to the crystal structure of ZnO as observed by the well-defined peaks of ZnO wurtzite crystal [114] According to Figure 3.9a, the increase in the intensities of the characteristic peaks of ZnO with calcination temperature can be attributed to the burn- off of the extracts Subsequently, the diffraction of a more well-defined ZnO crystal is expected to yield such result When the calcination temperature exceeded 700 o C, the distinctive peaks of ZnO exhibited lower intensities when compared to one calcinated at 700 o C As validated by the TGA and DTA results, the doped C heteroatoms are decomposed, leading to a disorientation of the crystal structure, subsequently, decreasing the crystallinity of ZnO Such results further support that ZnO-C should be calcinated at 700 o C As shown in Figure 3.9b, an increase in the intensities of the characteristic peaks of ZnO with the calcination time is observed By prolonging the calcination time of the studied sample, a more thorough burn-off of the phytocompounds was achieved, as a result, more and more ZnO crystals are exposed However, the differences in the intensities of the characteristic peaks of ZnO for the samples calcinated at 1.5 h or longer are negligible
ZnO-C-700-2.5 ZnO-C-700-2.0 ZnO-C-700-1.5 ZnO-C-700-1.0 ZnO-C-700-0.5
Figure 3.9 XRD patterns of various ZnO-C samples
Photocatalytic degradation of MB
The effects of calcination temperature on the MB photodegradation efficiency of surveyed materials are illustrated in Figure 3.10a and b The photoactivity of ZnO-C sample for MB degradation peaked when being calcinated at 700 o C Moreover, a decrease in the photoremoval of MB using ZnO-C-800 can be explained by its disoriented crystal structure due to the decomposition of the intercalated heteroatoms and large particle size As depicted in Figure 3.1a, an increase in calcination temperature leads to an increase in the particles size of ZnO-C, which is highly undesirable as photocatalysis is defined as an interfacial process, in which MB molecules came to contact with the surface of ZnO to initiate the photocatalysis process Although the ZnO-C sample calcinated at 700 o C has a much higher particle size than those calcinated at 500 and 600 o C, this sample reveals the highest photoactivity for
The ZnO-C-700 sample exhibited excellent photocatalytic performance due to its favorable crystal structure Samples calcined below 700°C had incomplete crystalline structures (Figure 3.9b) Kinetic studies revealed that 700°C was the optimal calcination temperature, resulting in the highest photodegradation rate constant (0.0459 min-1).
ZnO-C-0 ZnO-C-500 ZnO-C-600 ZnO-C-700 ZnO-C-800
ZnO-C-0 k = 0.0195 ZnO-C-500 k = 0.0373 ZnO-C-600 k = 0.0363 ZnO-C-700 k = 0.0459 ZnO-C-800 k = 0.0231 ln(A/A 0)
Time (min) Figure 3.10 (a) Effects of calcination temperature on the photodegradation of MB and (b) the corresponding kinetic plot of MB degradation
On the other hand, the influences of calcination time on the synthesis of ZnO-C-700 in the MB eliminating performance also played a crucial role as shown in Figure 3.11a and b As can be seen, all samples showed a relatively small difference in results, acquiring over 90% of removal efficiency after 120 min To compare the performance of the materials, rate constants were employed Amongst the studied samples, ZnO-C-700-1.0 exhibited the highest rate constant of 0.0567 min -1 This confirms that prolonged calcination time would lead to an over-decomposition of carbon and an excessive formation of oxygen vacancies, hence, one hour of calcination resulted in the most favorable material structure, as suggested by the results of XRD patterns According to the aforementioned evaluations, the ZnO-C-700-1.0 was chosen as the most appropriate sample for further investigation
ZnO-C-700-0.5 ZnO-C-700-1.0 ZnO-C-700-1.5 ZnO-C-700-2.0 ZnO-C-700-2.5
ZnO-C-700-0.5 k = 0.0433 ZnO-C-700-1.0 k = 0.0567 ZnO-C-700-1.5 k = 0.0439 ZnO-C-700-2.0 k = 0.0459 ZnO-C-700-2.5 k = 0.0438
Figure 3.11 (a) Effects of calcination times on the photodegradation of MB and (b) the corresponding kinetic plot of MB degradation
Characterization of ZnO-C calcinated at favorable temperature and time 73 3.3 Photocatalytic degradation of MB, related mechanism, reusability, and
Individual influences of factors
The investigation of the MB photodegradation using the optimal ZnO-C-700-1.0 sample was contemporaneously accessed via the catalyst dose, pH level, and MB concentration In terms of the catalyst dose (Figure 3.18), it can be observed that the photodegradation yield is enhanced when the dose of the catalyst increases from 40 to
50 mg This can be explained that at this stage, raising the amount of catalyst would hold up more active sites for dye molecules to interact and participate in the degradation process, leading to the highest eliminating efficiency acquired at 50 mg of ZnO-C-700-1.0 (k = 0.0567 min -1 ) Conversely, an increase to 60 mg indicated a slight decrease in the degradation performance This phenomenon can be a result of the excessive presence of ZnO-C catalyst, which led to notable adsorption of MB onto its surface Another plausible explanation is that an excessive quantity of catalyst can cause an increase in the turbidity of the solution, reducing the light transmission to the
79 materials in water, hence, inducing an inferior efficiency of the photodegradation process [130] Therefore, 50 mg was the optimal catalyst dose for the experiment
Figure 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
Furthermore, the pH level in the media influenced the MB photodegradation performance of ZnO-C-700-1.0, as well As depicted in Figure 3.19, a higher efficiency was achieved as the pH level increased This can be ascribed to the contribution of radicals such as •OH and •O2 - in the photoelimination, which can be more easily formed by the oxidation of more OH - in higher pH values, resulting in better reaction yields [131] The calculated rate constants also implied higher reaction rates within more basic environments, especially pH 11 with the highest rate constant (0.0786 min -1 ) and MB elimination efficiency (~99.99%) after 120 min Nevertheless, it is noteworthy that a remarkably high pH medium can cause a decrease in the photodegradation performance due to the stronger attraction of cationic MB with OH - rather than interacting with the ZnO-C surface
Time (min) pH 3 pH 5 pH 7 pH 9 pH 11 pH 3 k = 0.0199 pH 5 k = 0.0388 pH 7 k = 0.0567 pH 9 k = 0.0729 pH 11 k = 0.0786 ln(A/A 0)
Figure 3.19 (a) Effects of pH on MB photodegradation efficiency of
ZnO-C-700-1.0 and (b) corresponding kinetic plots of MB degradation
The MB elimination was also significantly affected by the MB concentration (Figure 3.20) Accordingly, it is noteworthy that increasing the dye concentration from
5 to 10 mg/L holds up better eliminating yield whereas, within the range of
10 – 25 mg/L, a higher initial MB amount indicated a negative impact This phenomenon can be attributed to the limited number of active sites owned by a fixed amount of the ZnO-C-700-1.0 nanocomposite regardless of the amount of MB present in the solution Moreover, an excessive MB dose also posed additional competition of dye molecules for active sites as well as an introduction of shielding effects on the solution, which could remarkably hinder the photodegradation performance [132] Along with the highest rate constant of 0.0786 min -1 , the results suggest the appropriate photoelimination can be acquired at 10 mg/L of MB concentration
Time (min) Figure 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
Photocatalysis mechanism
The photodegradation efficiency of ZnO-C-700-1.0 was further validated by monitoring the total organic carbon (TOC) and chemical oxygen demand (COD) in the reaction solution Figure 3.21 depicts the results, demonstrating the significant reduction of organic pollutants during photocatalytic treatment.
Figure 3.21 TOC and COD removal efficiency of post-photocatalysis solution Theoretically, these parameters are used to indirectly determine the content of organic pollutants that exist in the surveyed wastewater sample When time increased, the TOC and COD of the mixture were significantly removed, both exceeding 90% after 240 min This phenomenon implied the successful and efficient degradation of
MB molecules under irradiation over time using the ZnO-C catalyst
The photodegradation mechanism by ZnO-C-700-1.0 nanocomposite was investigated by utilizing free radical scavengers to the solution with MB It has been suggested that radicals, including reactive oxygen radicals (ROS), are of importance in the photodegradation of organic dyes Herein, EDTA, p-BQ, and IPA were deployed to capture photogenerated holes (h + ), •O2 -, and •OH, respectively As can be observed in Figure 3.22a and b, upon the addition of three scavengers, the eliminating yields reduced noticeably, which affirms the participation of radicals in the photodegradation activity In specific, the presence of EDTA remarkably hindered the photodegradation performance of ZnO-C-700-1.0 ~82% of efficiency after 120 min compared to relatively similar figures of ~91% in the presence of IPA and p-BQ The calculated reaction rates also proved the inferior results attributed to the association of radical scavengers in contrast to the non-added sample (k = 0.0786 min -1 ), particularly EDTA with the lowest at 0.0148 min -1 This phenomenon has elucidated the engagement of free radicals, especially h + , in the photodegradation process of MB by ZnO-C material
None k = 0.0786 EDTA k = 0.0148 IPA k = 0.0219 p-BQ k = 0.0207 ln(A/A 0)
Time (min) Figure 3.22 (a) Effects of radical scavengers on the photocatalytic degradation and
(b) corresponding kinetic plots for the degradation of MB
Based on the acquired results, the photodegradation pathway of MB by ZnO-C catalyst can involve crucial participation of h + and ROS (particularly •OH and •O2 -) Initially, under light irradiation, ZnO nanoparticles were excited to form photogenerated electron-hole (e - -h + ) pairs Upon the association of carbon in ZnO-C, the generated electrons are transferred to an intermediate band, formed from the
83 introduction of heteroatom into ZnO lattice, whilst the holes remained structurally at ZnO This could hinder the recombination of h + and e - , contributing to the enhanced reactivity and thus, the better photocatalytic performance of ZnO-C After that, the formed h + could merge with OH - in the solution to create hydroxyl radical •OH
Also, •O2 - resulting from the interaction between e - and dissolved oxygen in water could further react with a water molecule to additionally release •OH Moreover, the formation of •OH can be further enhanced in alkaline condition due to the abundance of OH - groups Simultaneously, hydrogen peroxide can be formed due to the difference in the redox potential of O2/•O2 - (0.046 eV) and CB of ZnO-C-700-1.0 (0.39 eV) The formed H2O2 molecules undergo another oxidation to create two •OH radicals The presence of both radicals, along with h + , then initiated the conversion of MB to its degraded form, followed by a subsequent reduction to create inorganic compounds of
ZnO-C nanocatalysts enhance photodegradation through several mechanisms CO2 and carbon-ZnO lattice interactions create electron trapping sites, prolonging photogenerated pair lifetime Oxygen vacancies, formed by interactions with H2O, contribute to the process These activities lead to more efficient photodegradation compared to pristine ZnO, highlighting the crucial role of h+ in key photodegradation steps Radical scavenger experiments support this conclusion, reinforcing the importance of h+ in the photodegradation process.
ZnO-C + h𝜐 → h + (ZnO-C) + e - (ZnO-C) (3.1) h + (ZnO-C) + OH - → •OH (3.2) e - (ZnO-C) + O2 → •O2 - (3.3)
Reusability and recyclability of catalyst
The reusability and recyclability of the nanocomposite were studied after ten consecutive cycles of recovery as shown in Figure 3.23 It is noteworthy that insignificant changes in the photodegradation efficiency of ZnO-C-700-1.0 can be
After multiple cycles, the MB photoelimination yield exhibited stability, decreasing from 100% to 95% after the fifth cycle and 83% after the final run This decline indicates the exceptional reusability of ZnO-C-700-1.0, as its performance remained consistently high even after ten recovery cycles.
The ZnO-C-700-1.0 nanocomposite exhibited exceptional photocatalytic activity, achieving a remarkable 100% degradation of MB within 120 minutes, surpassing the average 90.55% achieved by other ZnO-based materials Notably, the visible light photoactivity of ZnO-C was limited due to its intrinsic band structure To overcome this limitation, ZnO-C can be combined with visible light active photocatalysts like CuO and g-C3N4 to enhance its overall photocatalytic performance.
Table 3.1 MB photodegradation performances of ZnO-C-700-1.0 and other materials
Photoproduction of H 2 O 2 , corresponding mechanism, reusability, and
Individual influences of impacting factor
The photocatalytic generation of H2O2 was indicated as a sustainable pathway to utilize the solar/light irradiation energy as well as produce a considerable source of this
86 multi-application substance The process incorporates photogenerated hole-electron pairs, making a redox reaction as oxygen gas molecules, adsorbed in the solution, were reduced to form hydrogen peroxide along with the contemporaneous oxidation of a sacrifice agent Herein, the photocatalytic H2O2 production using ZnO-C-700-1.0 nanocomposite was accessed via the presence of a sacrificial electron donor, its dose, and the amount of catalyst Regarding the examination of different electron donors, the experiment was conducted with 5 mL of two substances namely, isopropanol (IPA) and methanol (MeOH) to study their effects on the production rate of H2O2 As can be observed in Figure 3.24, both exhibited enhanced photogeneration of H2O2 upon prolonged reaction times In specific, the photoproduction of H2O2 using IPA was generally higher, particularly after the 120-min mark with a noticeable increase, reaching 8 mmol/L as compared to 3.5 mmol/L for MeOH after 180 min This phenomenon can be mainly attributed to the distinguished nature among agents with IPA being the better electron donor, which can facilitate the photocatalytic production of H2O2 [143] Therefore, IPA was selected as the suitable sacrificial compound
Figure 3.24 Effects of different electron donors on the photoproduction of H2O2 using ZnO-C-700-1.0
Turning to the volume of IPA, there also witnessed a noteworthy influence on the production rate of H2O2 as shown in Figure 3.25 Increasing the amount of IPA from 2.5 to 5.0 mL resulted in an increased product formation Thus, a further increase up to 7.5 mL caused an even more production, which is nearly 2.5-fold higher than that of 2.5 mL (11.5 versus 4.75 mmol/L, respectively, at 180 min) Since a higher dose of
IPA as an electron donor, was added to the reaction mixture, more electrons could be released and interact with reactants, as a result, the H2O2 production yield was additionally enhanced Conclusively, the volume of IPA was set at 7.5 mL
Time (min) Figure 3.25 Effects of electron donor doses on the photoproduction of H2O2 using ZnO-C-700-1.0
Furthermore, the amount of ZnO-C-700-1.0 composite played a requisite part in the reaction as well It is noteworthy that a higher dose of the catalyst would contain more active sites for all components to interact, which causes a higher formation rate Yet, the least amount at 5 mg indicated a superior H2O2 production yield with 38.75 mmol/L whilst increasing the catalyst dose to 10 or 15 mg posed a remarkably negative impact on the production of H2O2 (Figure 3.26) A plausible explanation for those results is that an over-raised amount of catalyst can lead to a decrease in the optical transparency of the solution In other words, an inferior illumination of the reaction mixture would result in an ineffective utilization of the photocatalyst and hence, a lower formation rate of the product Therefore, 5 mg was the appropriate amount of ZnO-C-700-1.0 for the investigation
Figure 3.26 Effects of photocatalyst doses on the photoproduction of H2O2 using ZnO-C-700-1.0
Photocatalysis mechanism
Photocatalytic H2O2 production using ZnO-C involves photogenerated electron-hole pairs excited by UV light The electrons captured by dissolved oxygen in the CB undergo reduction reactions to form superoxide radicals (•O2-) This electron capture is possible due to the redox potential of O2/•O2- being less negative than the CB potential of ZnO-C.
H2O are responsible for acting as electron donors, leading to the generation of H + Furthermore, the elimination of h + resulting from the participation of IPA also promotes the utilization of e – to reduce O2, ultimately enhancing the H2O2 production performance as shown in Equation (3.9) [144] Eventually, H2O2 is either indirectly formed via the reaction between radical •O2 - and H + and an electron from the CB or directly created through the interaction between dissolved oxygen and H + and two electrons from the CB as demonstrated in Equations (3.11) and (3.12) These reactions are spontaneous as the redox potentials of •O2 -/H2O2 and O2/H2O2 are 0.89 and 0.23 eV, respectively, which are also less negative than the CB of ZnO-C-700-1.0 [145]
Figure 3.27 Plausible mechanism for the photoproduction of H2O2
Reusability and recyclability of catalyst
The reusability and recyclability of a catalyst are crucial factors in efficient compound production Assessing these properties is essential to ensure sustainability and cost-effectiveness As shown in Figure 3.28, the reusability of ZnO-C-700-1.0 for photocatalytic production of H2O2 was tested after 10 recovery cycles By evaluating these properties, researchers can optimize catalyst performance and minimize environmental impact.
CycleFigure 3.28 Reusability of ZnO-C-700-1.0 for the photoproduction of H2O2
Based on the results, a considerable decrease in the H2O2 production efficiency occurred in the following cycles However, more than 27 μM/g⸳h of H2O2 can be effectively produced in the 10 th cycle, indicating good recyclability of the fabricated material The aforesaid decrease can be due to the loss of ZnO or carbonaceous components after the long-term reactions A material washout during the recovering process is also a plausible explanation for the attenuation in the H2O2 production rate
A comparison between ZnO-C-700-1.0 and other Zn-based composites regarding the photocatalytic production of H2O2 is summarized in Table 3.2
Table 3.2 Photocatalytic production of H2O2 using ZnO-C-700-1.0 and others
The data indicate that the synthesized ZnO-C sample can hold up a far greater H2O2 generation rate than others Interestingly, compared to the 10% ZnO/g-C3N4, despite less amount of catalyst used and being illuminated with a less-powered light source for a shorter duration, ZnO-C-700-1.0 exhibited a higher production yield (38.75 versus 13.29 mM/g⸳h, respectively), showing the excellent capability of the fabricated material In conclusion, the acquired results re-confirm the promising performances of ZnO-C in the application of photocatalytic producing H2O2
In this thesis, ZnO-C was successfully green synthesized using G mangostana pericarp extract The calcination temperature and time affect the crystallinity and defect in the crystal structure of ZnO In detail, the crystallinity of ZnO-C increases with calcination temperature, however, the crystallinity of ZnO-C is greatly hampered when surpassing a critical temperature of 700 o C Meanwhile, the defects of ZnO are induced when the calcination time is increased, however, excessive defect sites that work as recombination sites for photoexcited electrons and holes are formed when ZnO-C was calcinated longer than 1.0 h Such results play a role in the MB photodegradation performance of ZnO-C calcinated at 700 o C for 1 h (ZnO-C-700-1.0) The synthesized ZnO-C-700-1.0 with an average particle size of 2570 nm exhibits a band-gap of 2.63 eV, which is much lower than that of pristine ZnO (3.12 eV) due to the formation of an intermediate band created from the doping of carbon Moreover, the photoactivity of ZnO-C-700-1 as well as the ability to hinder electron-hole pairs recombination via carbon doping are enhanced due to the formation of an intermediate band from the doping of carbon Simultaneously, the photocatalyst can generate reactive oxygen species for redox-based processes due to its well-suited band structure
ZnO-C-700-1.0, containing 50 mg of ZnO, successfully removed nearly all MB solution (10 mg/L, pH 11) under UV irradiation The degradation involved the formation of h+, •O2-, and •OH radicals at specific bands in ZnO-C-700-1.0 These radicals effectively broke down MB into harmless products like CO2 and water Notably, ZnO-C-700-1.0 demonstrated exceptional reusability, maintaining 82.6% of its initial efficiency after ten photocatalytic degradation cycles This finding highlights the potential of ZnO-C-700-1.0 as an effective and sustainable solution for treating MB-contaminated wastewater.
Meanwhile, an outstanding H2O2 production rate of 38.75 mM/g⸳h was recorded in the presence of 7.5 mL of IPA and 5 mg of the catalyst calcinated favorable conditions Moreover, the plausible photocatalytic production of H2O2 mechanism is ascribed to the strong redox potentials of the bands of ZnO-C-700-1.0 The synthesized catalyst also shows good reusability and recyclability for the photoproduction of H2O2
92 after 10 cycles, maintaining 71.9% of the original performance at 27.88 mM/g⸳h after
In conclusion, the obtained materials show high photoactivity for environmental remedy as well as energy carrier production Such results also imply that the synthesized catalyst using G mangostana pericarp extract can be further implemented for an actual process
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