Synthesis of metal nanostructure via chemical vapor deposition

Abstract

Because the nano-sized metals exhibit interesting chemical and physical properties that are not observed in bulk, they have been studied in various fields. When they are applied to surface-sensitive applications such as catalyst and detection, surface characters including surface cleanness or surface lattice are significantly important. Vapor phase synthesis has a strong advantage, which is a direct growth of nanostructures with ultraclean surface on a desired substrate, compared with wet-chemistry synthesis. Since the vapor phase synthesis of metal nanostructures usually requires a high temperature condition, however, there is a huge limitation of usable substrates. In this thesis, a chemical vapor deposition (CVD) method using metal halide as a precursor and the nanostructures grown by this method are showed. Furthermore, the reaction mechanism in this CVD system is revealed and various applications are showed. In chapter 1, a new CVD strategy for growing Au nanostructure is reported. In this way, AuCl was first used as a precursor for Au and it was revealed that the AuCl underwent a disproportionation reaction providing Au atoms. Importantly, this method requires only below 200 °C to synthesize Au nanostructures and the temperature condition is the lowest temperature compared with the previous reports. The Au nanoparticles were uniformly synthesized over the entire substrate without any aggregations and had polyhedral surface enclosed by a well arranged Au (111). The size of Au nanoparticle was controlled as the reactions temperature changed and the morphology of Au nanostructures was controlled to free-standing Au nanoplate as the distance between AuCl and substrate changed. In chapter 2, a novel CVD method to synthesize a very homogeneously alloyed AuAg nanoplate is reported. In this report, AgI was first used as Ag precursor and the mechanism by which the strong interaction between iodine and Au made possible to alloy Ag to Au selectively was revealed. AgI is vaporized as the temperature increased and the vaporized AgI was transferred by carrier gas and deposited to Au plate. As the temperature further increased, the deposited AgI began to decompose to Ag and I. Subsequently, the Ag was alloyed to Au. Importantly, the atomically flat surface and crystallinity of Au nanoplate was not damaged by the reaction. Furthermore, we observed the Ag contents while finely changing the temperature and nanoporous plate was prepared by selectively etching Ag. In chapter 3, the application studies using the prepared nanostructures are reported. First, the Au NPs synthesized by CVD method using AuCl were applied were applied as electro catalyst for methanol oxidation reaction (MOR). The CVD-AuNPs electrode exhibited the 33 times higher oxidation current density compared with the commercial Au NPs electrode. Besides, the value was about two times higher than the highest oxidation current density reported previously. We have suggested that the excellent catalytic activity is attributed to the following reasons: 1. the ultraclean surface without any organic contaminant, 2. the perfect contact between Au NPs and substrate (supporting electrode), and 3. the polyhedral surface enclosed by well-defined Au (111) lattice plane. Next, we achieved attomolar detection of miRNA marker for prostate cancer using Au nanowire-on-Au film sensor. With this sensor, the different miRNA marker that are miRNA141 and miRNA375 were selectively detected. Besides, extracellular miRNAs released from real cancer cells were successfully detected. Finally, the nanoporous plates prepared from alloy nanoplate synthesized by CVD method using AgI were applied to surface enhanced Raman spectroscopy (SERS). We optimized alloying condition to provide the smallest nanopore. The optimized nanoporous plates exhibited relative standard deviations of 4.7% and 5.9% for uniformity and reproducibility for the SERS signals, respectively.