MỤC LỤC
Unfortunately, as dyes are resistant to light, temperature, water, detergents, chemicals, and other agents such as bleach and perspiration, dyes remain stable in the environment and can not be treated with conventional treatment methods. It has been extensively reported that the exposure to dyes causes dermatitis, allergic conjunctivitis, hepatocarcinoma, increase in mutagenic potentiality [3], [4].
Dyes are considered the heart of this industrial sector, however, the discharge of dye-containing effluent is deemed as one of the most polluting of the industry. It is estimated that there are over 10,000 synthesized types of dyes and pigments around the world with an annual production of 700,000 tonnes [1].
For any particulate photocatalysis reaction system for the degradation of dye, the pH plays a major role as the interaction between the dyes and the particle can either be beneficial or detrimental to the treatment result at a specific pH [14]. 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 the case of photocatalysis, light is required to activate the process, resultantly, employing excessive amount of catalyst would reduce the amount of energy provided by light for each particle in the system. Similarly, dye wash-off occurs frequently in dying process, therefore, the dye concentration in effluent may vary for each batch, meanwhile, the treatment result must always be similar to one another.
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].
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. Amongst the well-known photocatalysts, ZnO has long been considered a base material for constructing an efficient photocatalyst for various redox-based purposes [22], therefore, ZnO has been selected to be studied and the influences of sacrificial agent, its volumes and catalyst doses are also examined in this thesis.
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]. 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.
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.
Meanwhile, under the presence of hole scavengers, two •O2 may reach with each other to create H2O2. To achieve good doping as well as good crystallinity of the material, various synthesis methods are employed.
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. 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.
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. 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 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. 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.
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.
International researches on ZnO-C
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. 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 and materials
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Preparation of G. mangostana pericarp extract
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. TGA-DTG: The samples were measured at Ho Chi Minh City University of Education using TGA/SDTA 851 thermobalance, Mettler Toledo, USA with the heating rate of 10 oC/min and temperature range of 0 to 800 oC in 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.
Characterization of ZnO-C calcinated at favorable temperature and time
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. 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. COD: Post-catalysis solution was also examined at Navitek Food and Environmental Testing Joint Stock Company using a Sievers InnovOx ES, Veolia with a detection range of 0.05 to 50.000 ppm and a limit of detection of 50 ppb.
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. 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. 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.
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. 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.
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). 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]. 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].
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. 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 CO2 and between carbon and ZnO lattices, also act as electron trapping site to prolong the lifetime of the photogenerated pairs. Moreover, the oxygen vacancies formed from the interaction The mentioned activities explain the promoted MB photodegradation using the ZnO-C nanocatalyst compared to pristine ZnO [133], as well as reveal the superior importance of h+, taking part in major photodegradation steps, which is highly consistent with the attained results using radical scavengers.
Reusability of ZnO-C-700-1.0 for the photocatalytic removal of MB Table 3.1 compares the MB photodegradation capability of the ZnO-C-700-1.0 nanocomposite in this research and different materials in former studies. Overall, along with the previously mentioned results, the ZnO-C composite can be potentially deployed in future industrial procedures as a potent solution for treating dye-contaminated wastewater. Due to its pristine band structure of ZnO-C, the photocatalytic degradation of MB using the material was hindered as a result, ZnO-C could be coupled with other visible light active photocatalysts such as CuO and graphitic carbon nitride (g-C3N4) to enhance the visible light activity of the pristine ZnO.