This study demonstrated the capability of a novel method in controlling the structural and electrochemical properties of electrocatalysts, utilizing a pulsed-ultraviolet (UV) setup for the synthesis procedure. A hand-made reactor provided a new set of parameters. The variation of UVon and UVoff periods resulted in samples with a range of different structures, compositions, and activities. Graphene/Pt was prepared with varying forms of illumination pulse, and its hydrogen oxidation and oxygen reduction reaction performances were evaluated. Controlling the reduction degree of Pt ions on partially reduced graphene oxide was achieved by manipulating the setup design. The results revealed a dominant growth and agglomeration phase of Pt particles, mostly with metallic states, by increasing both UVon and U-off time spontaneously. Long UVon without adequate UVoff did not result in promising electrocatalytic activities. In other words, different structures, compositions, and activities of samples suggested that not just the illumination is the crucial factor, it is also the resting time or UVoff which determines the surface adsorption kinetics, nucleation sites, and growth mechanism of nanoparticles. Further chemical reduction by highly concentrated ascorbic acid was used to confirm the proposed mechanisms, which lead to samples even with more metallic Pt (Pt-0).