Plasmonic gold nanoparticles (NPs) have interesting properties in both surface plasmon resonance and electrocatalytic reactions. However, the mechanism of local redox events at electrocatalytic Au NPs remains uncertain. This dissertation presents the mechanism study of catalyzed redox reactions at plasmonic nanoparticles by a spectroelectrochemistry methodology. Furthermore, the applications of plasmonic Au NPs are presented for fuel cell based reactions and solar water splitting. Firstly, planar and ultramicroelectrode (UME) of indium tin oxide (ITO) electrode is modified by Au nanoparticles (NPs) for dark-field scattering spectroelectrochemistry study of hydrazine oxidation. Photolithography, ion milling, plasma etching and sputtering system are utilized for ITO UME fabrication in the clean room. Dark-field scattering (DFS) study of single Au NPs shows light scattering signal decrease in the low overpotential region upon hydrazine oxidation because of the double layer charging and surface adsorbates. Strong light scattering can be obtained due to nitrogen bubble formation on the Au NP surface and bulk solution, which is accompanied by a decrease in oxidation current due to the deactivation of Au NPs. Au anodization is observed at high overpotential without hydrazine, such behavior is limited with the presence of > 50 mM hydrazine. Secondly, Au coated semi-transparent ITO electrode is further modified by Pt for electrocatalytic and spectroelectrochemistry study of methanol and formic acid oxidation. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) results confirm the Au-Pt core-shell structure. Enhanced catalytic activity is found for both methanol and formic acid oxidation with Au@Pt structure with respect to Au-ITO. The oxidation current density is more than 200 times greater than that on Au-ITO with Au@Pt(30) sample for both methanol and formic acid. Au NPs are anodized at the potential window of methanol oxidation at lower Pt coverage. Au anodization diminishes and is terminated with increasing Pt thicknesses. Finally, Au-incorporated hematite is screened by scanning photoelectrochemical microscopy (SPECM) to tailor and optimize its electronic properties for solar water oxidation under visible light irradiation. The pristine and Au-incorporated hematite materials are also characterized by SEM techniques. Au is found to exist in the form of embedded nanoparticles in the Au-hematite structures. Au-incorporated hematite exhibits enhancement in photocurrent for Au concentration up to 3% (atomic percent) and the performance drop is observed with 4 – 10% of Au. Subsequently, pristine and Au-incorporated hematite thin film photoanodes are fabricated by spin-coating method with optimal precursor concentrations of Au and Fe(NO3)3 to confirm the results of SPECM. The photoelectrochemical (PEC) response confirms that 3% Au inside hematite film is optimum for efficient water oxidation. Mott-Schottky analysis of the bulk samples confirms an improvement in charge carrier density for Au-incorporated hematite. The generation of oxygen from the substrates was quantitatively measured by surface generation tip collection mode of SECM.