Growth and characterization of manganese silicide nanostructures by thermal chemical vapor deposition for magnetic properties and hydrogen evolution reaction / Najwa Hamzan

Najwa , Hamzan (2024) Growth and characterization of manganese silicide nanostructures by thermal chemical vapor deposition for magnetic properties and hydrogen evolution reaction / Najwa Hamzan. PhD thesis, Universiti Malaya.

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Abstract

The growth of manganese silicide nanorods and nanowires using vapor transport growth was studied in this work. The manganese silicide nanorods and nanowires were grown on SiO2/c-Si and c-Si substrates using a thermal chemical vapor deposition vacuum system. The study starts with substrate temperature's effect on growth and nanorods' structural properties. The manganese silicide nanorods (Mn5Si3) were grown on SiO2/c-Si substrate by varying reaction temperatures at 750 to 1 000 °C. These variations produced different Mn5Si3 nanorods tops, such as flat-tip and diamond tops. Increasing the reaction temperature increased the delivery of Mn vapor to the substrate and resulted in temperature-controlled phase formation. The growth mechanism followed a direct vapor transport process. Next, with the same growth parameters (850 °C and 6.20 mbar), studies on different substrates (SiO2/c-Si and c-Si) have been carried out. In contrast to SiO2/c-Si, which showed the formation of Mn5Si3 nanorods, the Mn4Si7 nanowires with irregular grains were grown on the c-Si substrate. The effect of reaction pressure on the nanowires' growth and structure properties has also been studied. Low pressures (0.03 and 0.12 mbar) showed the formation of high-density nanowires, while at high pressures (0.52 to 6.20 mbar), nanowires with less density and uniformity grew. In the last part of this work, the studies on morphology and structural properties of the nanowires have been done at lowest pressure (0.12 mbar). As Mn powder was spread flat over a c-Si (111) substrate, the nanowires could grow on top of the bottom substrate. Based on EDX the spectra confirmed the formation of MnSi and Mn4Si7 nanowires on top and bottom substrates. The magnetic measurements indicated that the Mn5Si3 nanorods were ferromagnetic. In addition, with the same growth parameters (850 °C and 6.20 mbar), Mn5Si3 nanorods exhibited ferromagnetic solid properties, while Mn4Si7 nanowires showed a weak ferromagnetic property at low temperature (4 K) and paramagnetic at room temperature (300 K). The M-H curves of MnSi nanowires grown on 0.12 mbar reveal ferromagnetic properties at 4 K and paramagnetic at 300 K. As for nanowires grown at different pressures, M-H curve reveals nanowires grown at low pressures (0.03 and 0.12 mbar) exhibit paramagnetic behavior at room temperature and ferromagnetic at 4 K while nanowires at higher pressure (6.20 mbar) show a very weak magnetic hysteresis loop have been observed at 4 K and paramagnetic properties at 300 K. Lastly, the electrochemical hydrogen evolution reaction (HER) of manganese silicide nanowires also been studied. The as-synthesized MnSi nanowire at 0.12 mbar showed the best electrocatalytic performance towards HER with an overpotential of 0.39 V and the smallest Tafel slope of 95 mV dec−1.

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