| This thesis is dedicated to exploring the potential applications of plasmonic metal nanoparticles and understanding their fundamental enhancement mechanisms. Photocatalysis and solar energy conversion are the two main topics investigated in this work. In Chapter 2, we demonstrate the growth of a variety of carbonaceous materials by plasmonic heating. When the metal nanoparticles are irradiated with a laser near their plasmon resonant frequency, a localized region at high temperature is achieved due to the ohmic losses at the plasmon resonance. In this plasmon resonant chemical vapor deposition (PRCVD) process, high temperatures are created at the surface of the plasmonic nanoparticles to trigger the dissociation of gaseous precursors, (e.g. carbon monoxide), which results in the deposition of amorphous carbon, graphitic carbon, and carbon nanotubes. The formation of iron oxide nanocrystals is also observed at the beginning of the reaction due to a trace amount of iron pentacarbonyl in the CO feed gas. The growth of iron oxide at the surface of gold nanoparticles forms a new type of composite catalyst, Au nanoparticle/Fe2O3, which also catalyzes the growth of carbon nanotubes through this plasmon excitation process. The real time temperature dependence and sequential growth of different carbonaceous and metal oxide materials are monitored and characterized by Raman spectroscopy and infrared spectroscopy. Additionally, pre-defined microstructure geometries of crystalline iron oxide and carbon nanotubes are demonstrated by rastering the focused laser spot during the growth process in a controlled fashion.;In Chapter 3, the concentrations of gas phase reaction products are observed in real time using mass spectrometry, which is used to evaluate the performance of Au nanoparticle/Fe2O3 composite catalysts. This new plasmonic composite catalyst exhibits an excellent catalytic ability in the CO oxidation reaction, which exceeds that of the Au nanoparticles and Fe 2O3, alone. This indicates that this reaction is not driven solely by thermal (plasmonic) heating of the gold nanoparticles, but relies intimately on the interaction of these two materials. This hybrid plasmonic nanoparticle catalyst and PRCVD method open up new possibilities in the local chemistry, enabling new growth pathways of materials, not possible using standard CVD methods with uniform heating.;In addition to photocatalysis, we also explored plasmonic enhancement of solar energy conversion. In Chapter 4, plasmonic gold nanoparticles are incorporated into dye sensitized solar cells (DSSCs) by electron beam deposition. Increased photocurrents are observed due to the thin layer of plasmonic gold, which results in a 45 % increase in the cell's power conversion efficiency. This enhancement is attributed to the strong plasmon-induced electric field from the presence of gold nanoparticles, as indicated by electromagnetic finite-difference-time-domain (FDTD) simulations. Additionally, the photoluminescence spectra of the TPBP dye molecule rule out the mechanism of plasmon energy transfer through a Forester resonance process. Another potential application of plasmonic nanoparticles that we have explored is solar fuel production. In Chapter 5, we demonstrate an enhancement of solar methane production by the reduction of aqueous carbon dioxide (CO2) in the visible wavelength range. The underlying enhancement mechanisms of the Au nanoparticle/TiO2-catalyzed photoreduction of CO2are investigated by irradiating several different sample configurations with a wide range of wavelengths. Based on these results, we attribute the plasmonmic enhancement to the local electric field enhancement. However, several questions remain open and further studies will be required in order to obtain a deeper and more quantitative understanding of the plasmonic enhancement process. Some future directions include exploring the dependence of the doping concentration in the TiO2, the chemical state of the catalytic surface, and how the plasmonic nanoparticles perform in co-catalyst systems (e.g., TiO2-Fe2O3). Chapter 6 describes a side project carried out at the beginning of my graduate work. In this Chapter, we report a novel method for creating three-dimensional carbon nanotube structures from dense, vertically-grown carbon nanotube forests. The minimum power density for burning carbon nanotubes is also determined in this study. |
