Synthesis of structure and composition-controlled metal and semiconductor nanomaterials and their catalytic properties from electrocatalysis to photocatalysis

Abstract

Nanostructured materials have received wide attention due to interesting chemical and physical properties, rather than bulk materials. This phenomenon is stemming from nanoscale dimensions, deviating from general behaviors in bulk materials. In particular, nanostructured materials with controlled shape and composition exhibit outstanding physical, electronic, optical, chemical, magnetic, and other properties that can be applied to various fields. In this regard, development of controlling nanostructured materials has attracted attention from researchers, especially in catalysis. However, synthesis of high qualified nanostructured materials with controllable composition, and structure has thus far not been well developed due to the complex formation and growth mechanism of individual materials. Therefore, the purpose of this dissertation is to develop the synthetic method for the composition and structure-controlled nanoparticles as a highly active heterogeneous catalyst, on the basis of understanding the physical, chemical, optical and catalytic properties of materials at the nanoscale. Furthermore, the developed synthetic method promotes the highly qualified nanostructured catalytic materials for their applications from fuel cell to photocatalysis. In chapter 2, nitrogen-doped Pt/C catalysts were synthesized and improved the electrocatalytic activity toward ORR. Nitrogen-doped carbon materials have recently proved to be effective catalytic platforms for the oxygen reduction reaction (ORR). Despite the recent synthetic advances for the preparation of nitrogen-incorporated carbon materials, the low-temperature and water-based synthesis of nitrogen-doped carbon materials has rarely been explored due to the difficulties in nitrogen-doping under such mild conditions. Here, nitrogen doped Pt/C (Pt/NC) catalysts are prepared using a facile, low temperature, aqueous-phase method. Hydrazine treatment of a Pt/C catalyst successfully yields Pt/NC with controlled nitrogen content. The as-prepared Pt/NC catalysts exhibit enhanced electrocatalytic activity and stability toward ORR in comparison to nitrogen-free Pt/C, and their ORR activities are highly dependent on the level of nitrogen-doping. The Pt/NC catalyst containing 2.0 at% nitrogen results in the largest improvement of ORR activity. In chapter 3, we report a universal sulfide-assisted synthesis strategy to prepare dumbbell-like M-Ag heterodimers (M = Pd, Au, Pt). Sulfide ions can give fine control over the reaction kinetics of Ag precursors, resulting in the anisotropic overgrowth of Ag to realize the dumbbell-like heterodimers irrespective of the surface facets or components of the M domain. The M-Ag heterodimers were facilely transformed to M-Ag2S heterodimers via a simple sulfidation reaction. This study provides a versatile approach to realizing not only metal-metal heterodimers but also semiconductor-metal heterodimers and will pave the way for designing heteronanostructures with unprecedented morphologies and functions. In chapter 4, Au nanord-CdS yolk-shell nanoparticles were synthesized and improved the photocatalytic activity toward hydrogen evolution. Solar-driven photocatalytic reactions have been of tremendous interest with the expectation of providing green energy in the near future. Photocatalysts capable of promoting this reaction are often composed of semiconductor. However, semiconductor photocatalysts have shown low effectiveness due to the limited range of light absorbance and the rapid recombination of photogenerated electron-hole pairs, restringing the practical applications. Here, we demonstrate that synthesis of Au nanorod-CdS yolk-shell nanoparticles (NPs) provides a unique approach to enhance efficient light harvesting by combining plasmonic metal nanocrystals and semiconductor in controlled structural configuration. For the first time, the Au nanorod-CdS yolk-shell NPs were successfully synthesized by sulfidation and following cation exchange reaction. Surprisingly, the efficient multiple reflections of incident light within the void in yolk-shell NPs and the localized surface plasmonic resonance (LSPR) property from the Au nanorod in yolk part create a synergy effect to promote efficient light harvesting, and thus the photocatalytic hydrogen production reaction. The high hydrogen yield observed makes the Au nanorod-CdS yolk-shell NPs promising materials for solar conversion.