Investigation of chemically-induced hot electron and photocatalytic activity on plasmonic nanostructures

Abstract

The conversion of solar energy into electrical and chemical energy is the most environmentally friendly technology to achieve a sustainable energy economy in the 21st century. In this thesis, I investigated various factors involved in the conversion of solar energy into electrical and chemical energy. In the first study, the hot electrons that participated in converting chemical energy into electrical energy were examined by fabricating a metal oxide catalytic nanodiode and analyzing the correlation between catalytic activity and hot electron flow in a solid-liquid environment. In the second study, phosphorus-doped graphite carbon nitride photocatalysts were synthesized and utilized as a photoelectrode for photoelectrochemical hydrogen evolution reaction, showing that the doping and hollow structure of nanocatalysts can highly impact the conversion of solar energy into chemical energy. Finally, in the last study, the overall factors related to the conversion of solar energy, electrical energy, and chemical energy were thoroughly investigated using an antenna-reactor type plasmonic nanodiode to examine the role of plasmonic hot carriers in photoelectrochemical water splitting. First, Chapter 1 covers the behavior and detection of hot electrons generated on the surface of metal-oxide catalysts. Then, we discuss the localized surface plasmon resonance and introduce how plasmonic hot electrons are transported at the metal-semiconductor interface. Next, Chapter 2 describes the behavior of hot electrons at the metal-oxide interface in a solid-liquid environment by measuring the hot electrons generated during the hydrogen peroxide decomposition reaction using a Pt nanowire (NW)/Si Schottky catalytic nanodiode. The catalytic activity, chemicurrent, and chemicurrent yield Pt NW/Si nanodiodes were increased than that of Pt film/Si nanodiodes. We firstly showed the energy conversion and dissipation mechanism at the metal-oxide interface in the liquid phase using a Pt NW deposited catalytic nanodiode. In Chapter 3, a photoelectrochemical (PEC) H$_2$ evolution reaction was studied using the phosphorus-doped graphitic carbon nitride (g-C$_3$N$_4$) nanoarray structure. Mesoporous phosphorus-doped g-C$_3$N$_4$ nanocatalyst was synthesized using a spherical silica template and a trioctyl phosphine oxide material. The synthesized catalyst showed light absorption enhancement, faster charge separation and transfer, an enlarged surface area, and a 5.4-fold increase in photocurrent and over 95% Faradaic efficiency than the bare g-C$_3$N$_4$. This study proved that without using any co-catalyst, we can produce H$_2$ economically with a g-C$_3$N$_4$ electrode simply through the effects of phosphorus doping and nanoarray structure. In Chapter 4, to investigate the role of plasmonic hot carriers in PEC water splitting, an antenna-reactor type Pt/Ag/TiO$_2$ metal-semiconductor Schottky nanodiode was fabricated and used as a photoanode. In a system in which Pt/Ag/TiO$_2$ nanodiodes and PEC cells are combined, plasmonic hot carriers excited from plasmonic Ag were utilized for the O$_2$ evolution reaction. We confirmed that the plasmonic hot carriers enhanced the photocurrent and incident photon to current efficiency. This result was also supported by the finite difference time domain (FDTD) simulation and the Faradaic efficiency measured by the actual amount of gas produced. This study provides an understanding of the dynamics and mechanisms of plasmonic hot carriers in plasmon-assisted PEC water splitting.