UC Irvine

The interaction of light and matter is at the core of many of our day-to-day technologies and life processes. Upon excitation by photons, electrons (in a material) move into a higher energy state. This electronic energy is then harnessed into electricity, fuel, heat, and localized electric fields by various devices and chemical/biochemical systems. This thesis focuses on two such applications namely - the conversion of light into chemical fuels and localized electric fields, using nanoscale systems. The first section of this work focuses on the generation of chemical fuel – hydrogen from water upon illumination with sunlight – Photoelectrochemical (PEC) water splitting. An efficient, cost-effective, and stable photoanode is the bottleneck of this technology. Chapter 2 and 3 have been dedicated to the fabrication and crystal, morphological, chemical, optical, and photoelectrochemical characterization of nanostructured high surface area ternary metal oxide semiconductor photoanodes. Particular emphasis has been paid to the evaluation of β-Mn2V2O7 which was recently reported as a promising photoanode material for water oxidation. Other materials like BiOI, which is interesting for its internal electric fields for charge separation, and BiFO3 – for potential ferroelectricity enhanced charge separation have been covered in chapter 3. FeWO4 stands out as a promising candidate owing to its optimal bandgap and exceptional stability in both acidic and alkaline electrolytes. The second section of this thesis deals with developing nanoscale chemical systems for converting light into localized electric fields. A nm3 region of intense electric field is created due to plasmonic coupling when two gold nanoparticles come together to form a dimer. The hotspot generated in the gap can be utilized for single-molecule detection using surface-enhanced Raman spectroscopy (SERS). However, a simple, scalable, single-pot synthesis of gold nanocrystal dimers remains a challenge decades after the discovery of SERS. We propose a light-directed synthesis of gold nanocrystal dimers in chapter 4 with four key milestones, 1) gold nanocrystal synthesis, 2) their controlled aggregation using dithiols, 3) their passivation using thiol-ene click chemistry and silica shell growth, and 4) light directed aggregation arrest at the dimer stage.