Nanowires and their morphology induced catalytic properties (original) (raw)

Overall, this thesis aims to investigate the mechanism behind one dimensional nanostructure synthesis and catalytic performance, which hopes to contribute towards optimizing of catalytic process. More specifically, we planned to exploit the recent advances in one dimensional nanostructures synthesis, which we limits the scope to nanowires synthesis, and investigate the growth mechanism of the nanowires (NWs) synthesis if it was still not well understood. Next, with this understanding in synthesis mechanism, we hoped to create novel NWs structures, especially hybrid NWs structures, and apply them for photocatalysis and electrocatalysis. Besides that, we hoped to observe and identified specific advantages in catalysis arises from specific nanostructure, or in other words, morphology induced properties. TiO 2 nanowires had outstanding performance as light absorbing materials, and its large scale synthesis had been reported. However, there were not enough study done on the large scale synthesis mechanism, resulting in poor length and diameter control. We had demonstrated that large scale TiO 2 NWs synthesized via molten salt method adopts seed-assisted growth mode, with ripening as the main growth mechanism in chapter 2. Rutile NPs not only able act as seed to control the diameter of the resulting NWs, but also able to act as feedstock, where the selectivity comes from its own size. We had also identified the key to grow hybrid TiO 2 NWs was to control the rate of ligand adsorption and TiO 2 deposition in Chapter 3. We had tried various methods to increase ligand adsorption speed and decrease TiO 2 deposition rate. However, all the attempts had failed to control the lateral growth or the NWs obtained was not TiO 2. As we had exhausted all possible means but was unsuccessful to obtain the desired nanostructure, the project was halted. Ultrathin metal nanowires array was also an interesting system with great potential in catalysis. Although its synthesis mechanism is well studied, there had not been enough research conducted on the catalysis part. We have Abstract ii demonstrated that by simply changing the morphology of electrocatalyst from nanoparticles to aligned nanowire arrays, the catalytic activity can be improved dramatically by providing more electrochemical active surface for electrocatalysis, one-dimensional channels for improved mass transport and better conductivity in Chapter 4. Our approach provides a new and simple means to enhance the electrocatalytic activity, reduce the size of electrode for miniaturization of portable devices and improve the effectiveness of existing and emerging electrochemical technologies. We had proposed an EOR mechanism on Pd surface in Chapter 5, where OHions act as the main inhibitor that poison the Pd surface. We had discussed in details of the processes that might be happening in different part of the EOR CV. This understanding had helped us to identified the limiting factors of EOR mechanism and thus possible to suggest methods on improving the EOR activity and stability in DEFCs devices. A part from that, we had discovered that reservoir plays important role in determining the performance of Pd in EOR. The reservoir effect on our long and dense Au@Pd NWs array structure, proven to be a morphology induced property, might also be a contributing factor for its high performance in EOR. In addition, we had also shown that the present of two Pd facets on our Au@Pd NWs might be responsible for different ethanol oxidation product formation as proposed in our hypothesis through the studies conducted in Chapter 6. Pd (200) and (220) facets each have different affinity towards OHspecies, which results in different ethanol oxidation pathway on each facet, where Pd (200) facets responsible for the formation of acetic acid, and Pd (220) forms acetaldehyde. It was also discovered that some ligands bind specifically to one of the facets, which might served as a means to control ethanol oxidation product selectivity in the future. I would like to express my gratitude towards my supervisor, Professor Chen Hongyu, co-supervisor, Professor Liu Bin and my mentor, Professor Zhao Yanli. I would like to thanks Prof. Chen for inspired my interest in research and for all your teachings, guidance and advice for the past 7 years. I would like to show my gratefulness towards Prof. Liu for his patient teaching and guidance throughout my PhD studies. I also like to express my thankfulness to Prof. Zhao for all the care and resources he had provided for the past four years. I gratefully acknowledge the funding from the National Research Foundation (NRF), Prime Minister's Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program for supporting my 4 years course of PhD studies. I would also like to thanks my school, Interdisciplinary Graduate School for providing such a wonderful opportunity for me to experience multidisciplinary research.