Thesis final

Nitrogen adsorption-desorption experiments revealed that prepared hollow CuO microspheres have a specific surface area of ​​~12.184 m2/g. The recyclability test revealed that hollow CuO microspheres can be reused several times without losing the photocatalytic activity.

INTRODUCTION

Background information.…

Information about dyes…

Wastewater treatment methods…

One of the well-accepted mechanisms for the photocatalysis is discussed in the next section (Fig. 1). The wavelength of the photons that can be received by the semiconductor photocatalyst is dependent on the band gap energy. The effect of acid red dye loading on the degradation efficiency was also investigated.

Positive charge allows dye molecules to easily adsorb onto the surface of the photocatalyst with a negatively charged part. Experimental research on the relationship between the degradation rate of organic dyes and temperature has also been studied. The surface charge and aggregation of the catalyst particles are related to the pH of wastewater [62,4].

Each of these added ions causes some decrease in the degradation rate of the dye. Therefore, in the presence of Fe2+, hydroxyl radicals are easily converted into hydroxides, thus lowering their concentration, and consequently less degradation of the dye is seen. Improved photocatalytic performance of the graphene-V2O5 nanocomposite in the degradation of methylene blue dye under direct sunlight.

Investigation of the Reaction Pathway of OH Radicals Produced by Fenton Oxidation under Wastewater Treatment Conditions.

Fig. 1. Photocatalysis mechanism. Reprinted from [35]

Photototcatalysis mechanism

Influencing factors

• Band gap
• Band gap engineering
• Charge mobility and material stability
• Catalyst loading
• Dye loading
• Dye adsorption
• Light intensity
• Temperature
• pH of solution
• Additives…

Particle shape and size affect reaction rates because they change the size of the band gap and the diffusion length of electron-hole pairs from the bulk to the surface-active sites. The electronic transition between the bands of a semiconductor material and these states requires less energy compared to the transition between the bands. Electron injection from excited states of adsorbed dye molecules into the CB of a semiconductor material can induce photochemical reactions [46].

The interaction between a dye molecule and a photocatalyst particle is related to the amount of dye and the nature of the functional groups of the dye molecule. Photocatalysts with good adsorption properties simultaneously adsorb and degrade dye molecules by taking advantage of the synergy principle [65]. Each dye molecule has its own correlation with the change in pH of the environment [84].

Some studies have reported that there is significant adsorption of the dye on the catalyst under acidic pH, especially in the range of 1–3, which can be observed visually [86]. Under basic conditions, hydroxide ions (OH-) and holes can react to form hydroxyl radicals on the surface of the catalyst.

Fig. 2. Band gap and band edge positions of semiconductors. Reprinted from [45]

Overview of photocatalysts

The degree of degradation decreases due to the lack of oxygen, which is attributed to the recombination of charge carriers. The degradation reaction rate is a function of the fraction of adsorption sites occupied by dissolved oxygen, making it a limiting factor for the photocatalytic process [93]. ZnO has been widely used as a photocatalyst for wastewater decolorization due to its low cost, chemical stability and large light absorption spectrum [27].

However, ZnO cannot be used for photocatalytic degradation of pollutants because it dissolves in acidic solutions at low pH [58].

CuO photocatalyst…

Each technique has its impact on the band gap and surface characteristics of CuO, such as particle size, surface morphology, and specific surface area [45]. The high electron-hole pair recombination rate due to its CB edge potential that is more positive than the hydrogen redox potential causes the low photocatalytic activity of CuO [63]. Some solutions shown in previous reports, such as the formation of a heterojunction [115], modification with other elements or doping [116] and morphology control [117] are considered as suitable strategies to stimulate the photocatalytic activity of CuO.

These practical methods can help overcome the limitations of CuO and improve its performance by shaping one or more properties. It should be noted that hydrogen peroxide is often added to the reaction mixture to enhance the photocatalytic activity of CuO. The reason is that VBs of CuO are more negative than the potential needed to generate OH radicals, so they cannot generate OH radicals under sunlight illumination.

Several types of CuO nanostructures showed almost no degradation after 15 h of light irradiation in the absence of hydrogen peroxide [119]. Thus, CuO microspheres can accelerate the degradation of H2O2 and the formation of free radicals such as ·OH, ·OOH and ·O2-.

Fig. 4. Various CuO shapes. Reprinted from [114]

Literature review

The above tables compare the efficiency of various CuO structures available in the literature for the degradation of RB and MB by considering various factors such as dye concentration, volume ratio of photocatalyst to dye, amount of H2O2, degradation rate and time. Of the selected articles, the color degradation percentage ranged between 61 and 100%, while the analyzed degradation time was between 30 and 240 minutes. Only five articles provided detailed experimental data for CuO-assisted photocatalysis in the presence of hydrogen peroxide.

For the production of photocatalysts, the hydrothermal method was mainly used, where copper nitrate hydrates acted as a precursor material for copper oxide. By varying the pH of the solution by adding ammonia or citric acid, different structures of CuO were obtained. In the selected papers, X-ray diffractometer (XRD), energy-dispersive X-ray (EDX), scanning electron microscope (SEM), and transmission electron microscope (TEM) were used for photocatalyst characterization.

XRD provided the crystal structure and EDX analysis provided information on the elemental composition of the material. Organic dyes such as rhodamine B and methylene blue were mainly investigated because they are one of the most commonly used chemicals in industry.

Aims of the present study

To evaluate the photocatalytic activity of copper oxide, a UV-Vis spectrophotometer was mainly used to monitor the change in absorbance after a certain period of time, and in some studies the calculation of the rate constant was chosen. Such a photocatalyst can be used to photodegrade organic pollutants directly under sunlight, thereby reducing the cost of the photocatalytic process. In addition, the preparation of porous photocatalysts of larger size can potentially solve the problems associated with separation processes.

For example, large-sized porous photocatalysts have a high specific surface area suitable for the photocatalytic processes and can be rapidly precipitated in the solution phase. Traditional techniques for the synthesis of hollow CuO microspheres typically depend on hard/soft template-based strategies that require hard chemicals for template elimination [130,131]. Unfortunately, much less effort has been centered on creating uncomplicated protocols for the synthesis of CuO with a high specific surface area [132].

The goal of this research is to find a new potential application of CuO in photocatalytic dye degradation and to evaluate its structure-performance relationship for potential use on an industrial scale. CuO-based photocatalyst will be investigated regarding the effectiveness of the photocatalytic activities against the degradation of RB and MB dyes in the aqueous solutions under a solar simulator light irradiation.

METHODOLOGY

• Materials
• Synthesis
• CuO photocatalyst
• Dye solutions
• Sample Characterization
• Photocatalytic activity test

The crystalline phases of the samples were investigated by X-ray powder diffraction using a Rigaku SmartLab X-ray diffractometer (XRD) equipped with a Cu Kα radiation source. Autosorb iQ nitrogen porosimeter (Quantachrome Instruments) was used to determine the surface area and pore size distribution of CuO microspheres. The distance was optimized once to provide 1 solar irradiance using a standard silicon-based reference cell.

The photodegradation of RB and MB solutions versus irradiation time was monitored using UV-Vis (Genesys 50) spectrophotometer. Photocatalytic dye degradation was performed using the "all-in" method, where all reagents were mixed together prior to solar simulator light irradiation. The photocatalytic activities of the photocatalyst were evaluated by adding 2 mg of CuO microspheres to 5 mL of dye solution (1x10-5 M) in a beaker and then stirring the mixture at 600 rpm for 10 min in the dark to establish an adsorption-desorption equilibrium.

At the given irradiation time intervals, a part of the solution was immediately taken by means of a syringe and filtered to the cuvette with a 100 nm filter.

RESULTS & DISCUSSION

• Morphological analysis
• Compositional analysis
• Optical analysis
• BET and BJH analysis
• Photocatalytic activity analysis
• Comparison with commercial CuO
• One-pot recyclability tests
• Tests in acidic conditions
• Tests in basic conditions

The hollow structure of CuO is not clearly visible due to the high wall thickness. Where Atis is the absorbance of the dye at each time period min) and A0 is the initial absorbance of the dye solution. H2O2 alone can degrade rhodamine b and methylene blue, but the degradation rate was significantly increased when used with CuO microspheres.

After the first photocatalytic degradation reaction, the solution was taken by syringe, leaving CuO on the bottom of the beaker. It is worth noting that due to their size, these CuO microspheres can quickly settle to the bottom of the flask, making them easily reusable. Reusability analysis showed that CuO microspheres can be effectively reused at least twice without losing efficiency.

The pH of the solution was lowered by adding small drops of concentrated HCl, so that the total volume of the dye solution remained almost the same. The concentration of the dyes was converted from their absorbance values ​​using the constant extinction coefficient of each dye in pure water. The pH of the solution was increased by adding small drops of concentrated NaOH solution, so that the total volume of the dye solution remained almost unchanged.

The chromophores of the dye remain intact after light irradiation, thus reducing the percentage of dye degradation [133].

Fig. 7. TEM image of prepared CuO microspheres.

CONCLUSION

LIMITATIONS & FURTHER STUDIES

Kinetic studies of photocatalytic degradation of direct yellow 12 in the presence of Zno catalyst. Photocatalytic degradation of model textile dyes in wastewater using ZnO as a semiconductor catalyst. Journal of Hazardous Materials. Photocatalytic decolorization and degradation of dye solutions and wastewater in the presence of titanium dioxide. Journal of Hazardous Materials.

On the mechanism of Tio2-photocatalyzed degradation of aniline derivatives.Journal of Photochemistry and Photobiology A: Chemistry. Fundamental Principles And Application Of Heterogeneous Photocatalytic Degradation Of Dyes In Solution.Chemical Engineering Journal. Photocatalytic degradation of dyes and organic contaminants in water using nanocrystalline anatase and rutile Tio2.

Porous Cuo Hexapod Nanostructures: Precursor Fabrication, Characterization, and Visible Light-Induced Photocatalytic Degradation of Phenol. Photocatalytic studies of copper oxide nanostructures for the degradation of methylene blue under visible light. Journal of Molecular Structure.