Over the last decades, there have been numerous studies in catalysts converting solar and electrical energy into chemical energy. Since most of the chemical reactions take place on the catalyst surface, nanoparticles have been employed as efficient catalysts due to their large surface area. In particular, chemical and catalytic properties of the nanoparticles are dramatically changed by their morphology,structural design suitable for the wanted catalytic reaction is essential for improving catalytic performancesIn this study, we figured out the design principle suitable for photo- and electrocatalytic reactions and developed synthetic methods by combining various synthetic strategies. In Chapter I, we introduced background knowledges and synthetic methods of structurally well-defined nanoparticles suitable for photocatalytic hydrogen evolution and electrochemical carbon dioxide reduction. In chapter II, we synthesized a double-shell hollow metal-semiconductor nanostructure, which showed high stability. The resulting double-shell hollow nanostructure was employed as photocatalysts for hydrogen evolution reactions and exhibited high stability and efficiency superior to those of commercial catalysts. In Chapter III, metal nanoparticles were used as electrocatalysts for carbon dioxide reduction reactions. The Au nanostars with high index surface facets were successfully synthesized by a simple overgrowth process. The resulting nanostars exhibited high CO selectivity and efficiency. Similarly, the branched CuO nanoparticles, which were synthesized through the oxidation of the $Cu_2O$ nanocubes, showed enormous ethylene selectivity and stability higher than those of any other catalysts reported thus far. The correlation between the nanostructures and their catalytic properties is expected to contribute the improvement and commercialization of energy conversion processes using nanocatalysts in the near future.