The development of cost-effective, functional materials that can be efficiently used for sustainable energy generation is highly desirable. Herein, a new molecular precursor of bismuth (tris(selenobenzoato)bismuth-(III), [Bi(SeOCPh)(3)]), has been used to prepare selectively Bi or Bi2Se3 nanosheets via a colloidal route by the judicious control of the reaction parameters. The Bi formation mechanism was investigated, and it was observed that the trioctylphosphine (TOP) plays a crucial role in the formation of Bi. Employing the vapor deposition method resulted in the formation of exclusively Bi2Se3 films at different temperatures. The synthesized nanomaterials and films were characterized by p-XRD, TEM, Raman, SEM, EDX, AFM, XPS, and UV-vis spectroscopy. A minimum sheet thickness of 3.6 nm (i.e., a thickness of 8-9 layers) was observed for bismuth, whereas a thickness of 4 nm (i.e., a thickness of 4 layers) was observed for Bi2Se3 nanosheets. XPS showed surface oxidation of both materials and indicated an uncapped surface of Bi, whereas Bi2Se3 had a capping layer of oleylamine, resulting in reduced surface oxidation. The potential of Bi and Bi2Se3 nanosheets was tested for overall water-splitting application. The OER and HER catalytic performances of Bi2Se3 indicate overpotentials of 385 mV at 10 mA cm(-2) and 220 mV, with Tafel slopes of 122 and 178 mV dec(-1), respectively. In comparison, Bi showed a much lower OER activity (506 mV at 10 mA cm(-2)) but a slightly better HER (214 mV at 10 mA cm(-2)) performance. Similarly, Bi2Se3 nanosheets were observed to exhibit cathodic photocurrent in photoelectrocatalytic activity, which indicated their p-type behavior.