Recent advancements in fabrication methods with good shape and size control have broadened the scope of applications of plasmonic nanoparticles. Studying the optical properties of these particles has been a very effective and noninvasive way of investigating and characterizing them. Modeling and computation of the optical response of noble metal nanoparticles and their assemblies have played a seminal role in plasmonics research, by understanding the physics behind their plasmonic behavior, in replicating experimental results, and also in designing further research. Due to its sustainable and earth-abundant nature and broad-band (UV to near IR) plasmonic behavior, aluminum has emerged as a strong contender in the field of applications of plasmonic nanoparticles. In the first part of this thesis, Finite-Difference Time-Domain (FDTD) is used to characterize the optical properties of selectively faceted novel Al{111} nanoparticles. The calculated spectra show agreement with experimental spectra. Further, multipole analysis and charge distribution plots are used to understand the spectra and plasmonic behavior of these particles at different energies. The second part of the thesis sheds light on one of the applications of Al nanoparticles. Here, an Al nanodisk on p-GaN substrate covered with a TiO2 layer is studied, that exhibits enhanced H2 generation efficiencies due to a combination of plasmon-enhanced processes. Hot electrons are generated in the illuminated Al nanodisk get injected into the conduction band of the TiO2 layer, and there are plasmon-induced directly excited hot carriers in the TiO2 layer. Subsequently, these carriers are transferred into water molecules adsorbed on the TiO2 surface, driving the H2 evolution. These properties were investigated using Finite Element Method (FEM) and FDTD.