Colloidal Cu3N and Zn3N2 nanoparticles: from the single-source precursor approach to photocatalysis

Kadzutu-Sithole, Rudo
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Colloidal Cu3N and Zn3N2are one of the least studied nanocrystals in nanoscience. In general, much controversy exists on the optical properties of Cu3N nanoparticles as evidenced by the great differences in the reported band gaps of Cu3N that range from ~0.13 to ~2.06 eV and in the case of Zn3N2, band gaps range from 0.9 to 3.36 eV. Additionally, electrical properties vary due to great differences in percentage composition of Cu in different samples hence Cu3N has been reported to be a metallic, semiconducting and insulating material. Furthermore, studies on the thermal stability have described the nitride nanoparticles as stable, metastable and unstable. Aspects such as reaction kinetics, magnetic properties, controlled size and dimensionality of colloidal Cu3N and Zn3N2 nanoparticles are not yet well understood. Therefore, to better understand the chemistry of colloidal Cu3N and Zn3N2 nanomaterials more studies must be performed on the synthesis, characterization and measurement of properties of the colloidal metal nitride nanoparticles. Initially, 2-pyrrolecarbaldpropylimine ligand was prepared from a condensation reaction between 2-pyrrolecarboxaldehyde and propylamine. After characterization, the ligand was deprotonated and subjected to complexation by separately introducing copper and zinc salts. Complexation of mono-functional N, N pyrrolylaldimine Schiff-base ligand led to the formation of mononuclear Cu and Zn complexes, bis(2-pyrrolecarbaldpropyliminato) copper/zinc (II) complex. Three different methods were employed in the thermolysis of the Cu complex (PCC) and the Zn complex (PPZ) namely; the conventional colloidal method, microwave-assisted method and chemical vapor deposition (CVD). Zinc has a very high affinity for oxygen hence, despite the different synthetic methods that were employed to thermolyze the zinc complex, ZnO was always the resultant product. Changing the single-source precursor did not solve the problem until zinc powder was ammonolyzed using chemical vapor deposition. Herein, we showed for the first time that as-synthesized single-source precursor (PPC) can be thermolyzed in organic solvents to give colloidal Cu3N NPs. Of great importance was the introduction of the microwave-assisted method in the synthesis of Cu3N nanocrystals. A comparative study was then carried out on the colloidal synthesis of Cu3N nanoparticles using the as-synthesized Schiff-base complex and copper (II) nitrate trihydrate. In agreement with previous reports, thermolysis of Cu(NO3)2resulted in the formation of Cu3N nanocubes but the Cu complex produced spherical nanoparticles with a more blue shifted band gap than the nitrate compared to the bulk material. The study then proceeded to investigate the morphological and crystal phase transformations that occur during the colloidal synthesis of Cu3N at 260 °C using Cu(NO3)2as the single-source precursor in octadecylamine and hexadecylamine coordinating ligands. A mixture of densely populated ‘nuclei’ and developing cubes was observed in the transmission electron microscopic images of the samples that were obtained after 5 min reaction time. Nanocubes and less of the nuclei were present after 10 min but only the cubes were detected at 15 min. Near-perfectly shaped nanocubes self-assembled into a brick-like wall pattern in both the octadecylamine and hexadecylamine capped nanoparticles. The cubes started to disintegrate at 20 and 30 min in octadecylamine and hexadecylamine respectively to eventually yield Cu nanoparticles after 60 min. We were intrigued by the changes that were detected in the organic ligands. Fourier-transform infrared and nuclear magnetic resonance spectroscopies indicated the presence of a nitrile group in the Cu3N nanoparticles suggesting that the amine ligands had been deprotonated resulting in the formation of nitriles. A one-pot colloidal synthesis of optically active Cu3N, Cu2S and Cu9S5 nanoparticles was then demonstrated by sulfidation of the nitride nanoparticles using a small amount of dodecanethiol. Previous studies have shown that the thiol ligand can act as a spectator solvent, capping and reducing agent but in this study, its introduction into a hexadecylamine-Cu3N mixture led to the substitution of N atoms in the crystal structure of Cu3N by S atoms leading to the formation of Cu2S nanoparticles after 5 min of addition. With time, more S-rich nanoparticles were obtained as evidenced by the obtained powder X-ray diffractograms that matched with reported Cu9S5 patterns. X-ray photoelectron spectroscopy confirmed the substitution of N in Cu3N that led to the detection of Cu+ and S2-ions in Cu2S and ultimately Cu2+and (S2)2- species in Cu9S5 nanoparticles after 10 min of adding the thiol. Increasing reaction time did not lead to further sulfidation of the Cu9S5nanoparticles.The thesis culminates by testing the photocatalytic activities of the as-synthesized Cu3N nanocubes, Cu2S hexagons and Cu9S5 spherical to elongated nanoparticles with direct band gaps of 2.41, 2.75 and 2.36 eV respectively. Cu3N proved to be the best (89%) photocatalyst in degrading methyl orange whilst Cu9S5 nanoparticles were the best (79%) in degrading methylene blue within a period of 3 hrs. Considering the best catalyst in each azo-dye, kinetic studies using the Langmuir–Hinshelwood model suggested that the photocatalytic reaction followed 1st order and 2nd order reaction pathways in methyl orange and methylene blue for Cu3Nand Cu9S5 nanoparticles respectively. Finally, high-performance liquid chromatography was employed in order to detect the reaction products of degrading each dye
A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy, in the Faculty of Science Department of Chemistry, University of the Witwatersrand