The synthesis, characterization and photocatalytic activity of various morphologies and sizes of ZnO photocatalysts
Date
2018
Authors
Nkabinde, Siyabonga Sipho
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Abstract
The increasing abundance of industries, together with the high human population density has prompted the pollution of the hydrosphere with organic and inorganic matter at a very high rate. To triumph over the problems caused by water pollution, and to comply with strict environmental regulations, researchers have been focusing on the development of new or improvement of existing water purification processes. One such process is known as Advanced Oxidation Processes (AOPs), and is based on using light quanta in conjunction with a photocatalyst (TiO2, ZnO, CdS, etc.) in order to degrade organic pollutants in an aqueous medium. An important factor in AOPs is the type of the photocatalyst being used as it controls the rate at which dyes are degraded when exposed to light quanta with energy higher or equals to its band gap energy. The photocatalytic activity of a photocatalyst is controlled by its properties such as surface area, crystallinity and morphology. These properties can be controlled by varying reaction parameters such as reaction time, type of precursor, and pH used when it is being synthesized. Furthermore, the photocatalytic activity also depends on operational parameters under which the photocatalyst is used. The operational parameters include factors such as the photocatalyst concentration, concentration of organic compounds in solution, pH at which the dye exists in nature, and light intensity. Zinc oxide nanoparticles were synthesized was synthesized using microwave assisted heating method. Microwave assisted heating method was chosen over conventional colloidal method due to its ability to heat reaction mixtures homogeneously and very short reaction times. The synthesis of ZnO nanoparticles using a co-precipitation method between Zn(CH3COO)•2H2O and NH3/NaOH via microwave assisted heating and their photocatalytic activity investigated. The crystalline structure, morphology and optical properties of as-synthesized ZnO were characterized by Powder X-ray diffraction (PXRD), Transmission Electron Microscopy (TEM) Scanning Electron Microscopy (SEM) and Ultra-violet Visible (UV-Vis) and Photoluminescence Spectrophotometer. ZnO was first synthesized using cetyltrimethylammonium bromide (CTAB) as capping agent and ammonia (NH3) as precipitating agent in order to assess its photocatalytic activity in the degradation of Rhodamine B. Rhodamine B was used as a model dye as it is frequently used in industry. Rod like ZnO nanoparticles were produced with an average length of 608 nm and a width of 205 nm.
The operational parameters to be used during the course of the research were determined by performing photocatalyst concentration, dye concentration, light intensity, and pH studies. The extent of direct hydrolysis of the Rhodamine B dye under UV light without the photocatalyst was first measured to eliminate the possible contribution from the undesired variables to the overall efficiency. It was observed that using only light without a photocatalyst or vice versa could not degrade the dye, a combination of the two was needed for dye degradation. The ZnO photocatalyst was found to efficiently degrade the Rhodamine B dye at photocatalyst concentration of 160 ppm, light intensity of 210 W and at pH 8.
Morphological diversity was achieved by varying reaction parameters such as pH of the precursor solution and changing the type of zinc metal salts. Varying the pH changed the growth nature of ZnO forming pseudo-spherical nanoparticles at pH 7, bullet-like nanoparticles at pH 10 and rod-like nanoparticles at high pH of 14. The photocatalytic degradation experiments revealed that ZnO nanoparticles with different morphologies degraded Rhodamine B at different rates. The pseudo-spherical, bullet-, and rod-like nanoparticles degraded the dye at 150, 180, and 210 min, respectively. The difference in the rate of degradation was attributed to surface area differences and proportion of exposed polar facets (i.e. [0001], [000-1]) on the surface of the different morphologies.
Different sizes of the ZnO nanoparticles were prepared by varying the reaction time length, with short reaction time producing smaller particles than the longer reaction time. The photocatalytic activity of the nanoparticles was examined for the photocatalytic degradation of Rhodamine B as the test dye in aqueous solution under solar irradiation. The size of the nanoparticles was found to be highly dependent on the reaction time. Particle size influenced the photocatalytic activity with the smaller sized nanoparticles being more efficient in degrading Rhodamine B than the larger nanoparticles. The reason for the high catalytic activity was attributed to higher surface area. The as-synthesized ZnO photocatalyst showed good photocatalytic stability and can be reused four times with only gradual loss of activity. Thus, it is an efficient photocatalytic material for degrading contaminated coloured wastewater for reuse in textile industries under mild conditions.
The syntheses of ZnO nanoparticles using different precursors Zn(NO3)2•6H2O, Zn(CH3COO)2•2H2O, ZnCl2, and ZnSO4.H2O, resulted in different structural, optical and photocatalytic activity. The difference in the properties of the nanoparticles synthesized in this study was attributed to the fact that the counter anions (i.e. NO3-, Cl-, SO42-, and CH3COO-) in the zinc metal salts are capable of coordinating to the crystal planes of ZnO differently. The different coordinating abilities of the counter anions resulted in dissimilar rates of growth and different morphologies. The BET surface area measurements were found to be 13.25, 12.34 12.02, and 1.24 m2g-1 for the NO3-, CH3COO-, Cl-, and SO42- counter anions, respectively. The difference in surface area indicated that the binding abilities of the counter anions to the decreased in the order NO3- > CH3COO- > SO42- > Cl-, with the NO3- anion binding more strongly to the crystal planes (i.e. [0001] and [000-1]) of ZnO and hence hindering rapid growth of the nanoparticles. Photocatalytic degradation studies indicated that the nanoparticles with the highest surface area were more active than those with lower surface area.
Description
A dissertation submitted in fulfilment of the requirements for the degree of Master of science in the School of Chemistry, Faculty of Science University of the Witwatersrand ,
March 2018
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Citation
Nkabinde, Siyabonga Sipho, (2018) The synthesis, characterization and photocatalytic activity of various morphologies and sizes of ZnO photocatalyst, University of the Witwatersrand, Johannesburg, https://hdl.handle.net/10539/25762.