Solution Phase Syntheses of Nanoparticles and Nanowires
Authors:   , Lim Teck Hock 
Published: November 2021

<p>This thesis is concerned with the preparation of metal and semiconductor nanostructures in solution, specifically bismuth and indium metal nanoparticles, gallium nitride nanoparticles, indium phosphide nanowires and zinc phosphide nanoparticles. There were two aims: firstly to study if gallium nitride nanoparticles with improved crystallinity and size distribution could be synthesized and secondly to find and develop new methods to prepare crystalline indium phosphide nanowires and zinc phosphide nanoparticles using precursors that are safe and cheap. The crystallinity, structures, morphologies and chemical compositions of the nanostructures synthesized in this thesis were studied primarily by transmission electron microscopy (TEM), powder X-ray diffraction (PXRD) and energy dispersive X-ray spectrometry (EDS). For the synthesis of gallium nitride, two approaches were taken. The first revolves around the direct metathesis reaction between gallium trichloride and lithium nitride under ambient pressure. A range of solvents with different polarities has been tested and only in highly polar solvents crystalline nanostructures were produced. These crystalline nanostructures however are not of gallium nitride. The second approach involves thermally decomposing an organometallic precursor. Organometallic compounds [Ga2(NMe2)6] (compound 1) and [(Me3C)2Ga(u-NHNHCMe3)] (compound 2) were chosen from the literature as precursors. Compound 1 was synthesized in a very small yield together with by-products. Thermal decomposition of the mixture produced no nanoparticles. A compound (compound S2) which is structurally similar to compound 2 was successfully synthesized and was subjected to thermal decomposition in ammonia to produce crystalline monodispersed nanoparticles. However, these nanoparticles could not be confidently identified as gallium nitride. The outcome from the reaction of lithium borohydride and indium trichloride was found to be strongly solvent dependent. In toluene a white precipitate was obtained. Both in isobutylamine and N,N-diethylaniline indium metal nanoparticles were produced as black solutions. Only in isobutylamine, small monodispersed indium nanoparticles can be produced. The isobutylamine method was extended to prepare bismuth metal nanoparticles. However, the bismuth nanoparticles prepared were moderately polydispersed in size. Two new methods were developed to prepare indium phosphide nanowires from red phosphorus and phosphorus pentabromide via Solution-Liquid-Solid growth. Borohydride reagents are required in both methods to produce chemically active intermediates which further react to form indium phosphide nanowires in the presence of pre-synthesized indium metal or bismuth nanoparticles. The diameter of indium phosphide nanowires prepared from red phosphorus depends strongly on the reaction sequences. If indium metal nanoparticles are formed prior to the addition of red phosphorus, large nanowires (> 300 nm) are produced. Reversing the sequences, small nanowires (50-100 nm) are produced. Red phosphorus residue remains in the products regardless of the reaction sequences and is difficult to remove completely by chemical means. The reaction which employs phosphorus pentabromide as precursor proceeds via intermediates of hydrogen phosphide and indium metal to form indium phosphide. The reaction temperature dictates the crystallinity of the product and needs to be >170 oC to produce crystalline indium phosphide. The way hydrogen phosphide is introduced to the reaction and the presence or absence of pre-synthesized metal seeds together control the morphology of indium phosphide synthesized. The best set of conditions established in this thesis allows the preparation of indium phosphide in ~100% nanowire morphology. The hydrogen phosphide method was adapted to produce zinc phosphide nanoparticles. The choice of the reaction solvent was found to be most critical. Amorphous particles were produced in trioctylphosphine at as high as 330 oC whereas in oleylamine and N,Ndiethylaniline crystalline zinc phosphide (a-Zn3P2) nanoparticles were produced at ~200 oC. An overall conclusion is given in the last chapter comparing the methods developed in this thesis with literature methods paying particular foci on the level of hazard and the costs of the chemical reagents involved.</p>