Synthesis and characterization of magnetic gold nanomaterials and their application in the reduction of methylene blue

Abstract

This thesis reports the synthesis and full characterization of phosphine-stabilized gold nanomagnetic materials. These nanomagnetic materials were modified on the surface for the treatment of wastewater from the textile industry. It was subsequently used as catalysts for the reduction, removal and recyclability studies of methylene blue dyes in an aqueous solution. It is commonly known that dyes from the textile industry are toxic to the environment through the wastewater stream and the reduction and/or removal of these dyes are a crucial study needed to keep our water safe and clean for human usage. The synthesis of these nanomagnetic materials involved the making of naked superparamagnetic iron oxide nanoparticles (SPIONs) and this nanoparticle (EJL1) was prepared using the commonly used co-precipitation method. The metal precursor used for these naked SPIONs involved the mixing of two iron precursors (FeCl2.4H2O and FeCl3.6H2O) in aqueous solution, with an addition of ammonium hydroxide (NH4OH) solution to form a dark black precipitate, which indicates the formation of magnetite (Fe3O4), referred to as naked SPIONs in this thesis, due to its superparamagnetic properties. This was fully characterized using ultraviolet-visible spectroscopy (UV-Vis), Fourier-transform infrared spectroscopy (FT-IR), powder X-Ray diffraction (PXRD) and high-resolution transmission electron microscopy (HRTEM) techniques. From the FTIR. it could be confirmed that the Fe3O4 was obtained. The purity of the naked SPIONs was confirmed by using PXRD, as their diffraction pattern matched those of standard magnetite samples. The FT-IR spectrum showed the absorbance peaks that are characteristic of magnetite. The HRTEM confirmed an average particle size of 18.7 ± 5.6 nm. The SPIONs was then modified by using chloroauric acid (HAuCl4.3H2O), which was reduced onto the SPIONs surface using two different reducing agents, namely sodium borohydride (NaBH4) and sodium citrate (Na3C6H5O7). There were six catalysts successfully synthesized (EJL2-EJL7) using the direct gold-coating method. These were fully characterized using powder X-Ray diffraction (PXRD), ultraviolet visible spectroscopy (UV-Vis), Fourier-transform infrared spectroscopy (FT-IR), and high-resolution transmission electron microscopy (HRTEM), techniques. The ultraviolet visible spectroscopy clearly showed the difference in SPR peaks between the naked SPIONs and AuNPs. This proves that the reduction of these catalysts was successfully employed. The PXRD concluded that the gold-coated nanoparticles were seen to have gold diffraction peaks which was confirmed by PXRD. A well-known problem facing gold-coated nanoparticles (AuNPs) is their tendency to agglomerate, hence using a stabilizer on the AuNP surface to prevent agglomeration was important. Two phosphine ligands, 1,3,5-triaza-7-phosphaadamantane and triphenylphosphine, were used as stabilizer to produce phosphine-stabilized gold-coated SPIONs. Twelve phosphine-stabilized gold-coated SPIONs were successfully synthesized (EJL8 to EJL19). These were characterized using inductively coupled plasma spectroscopy (ICP), Fourier-transform infrared spectroscopy (FT-IR), high-resolution transmission electron microscopy (HRTEM) and ultraviolet visible spectroscopy (UV-Vis), techniques. From the FT-IR spectra it can be concluded that these ligands were successfully attached to the surface of the gold coated SPIONs as there was presence of -CH, C=C and C-N stretching frequencies which were not present on the naked SPIONs. From HRTEM, the TEM micrographs obtained showed that the gold coated SPIONs was not agglomerated, therefore the phosphine ligands stabilizer resolved this issue. All the nanomagnetic materials used in this research had three different magnetite (Fe3O4) to gold (Au) metal ratios, namely 1:10, 1:50 and 1:100 respectively. These catalysts were used for methylene blue (MB) dye reduction and removal from aqueous solution. The kinetics of this reaction using our range of nanomaterials are discussed and helped us found that the adsorbents not only reduce the MB, but also remove it completely from the aqueous solution when extracting the nanomaterial by applying an external magnetic field. The MB was desorbed from the nanoparticle surface by acetonitrile, after which the adsorbent could easily be recycled up to 5 times. Furthermore, since these catalysts have magnetic properties, they can be removed by applying an external magnetic field, making catalyst recovery and recyclability much easier.