In this paper, we report on a photocatalytic water-splitting device comprising a TiO2 photoanode combined with Se/TiO2 heterojunction photovoltaic (PV) devices. A Se/TiO2 PV device can supply its photo-induced open circuit voltage to a TiO2-Pt photoelectrochemical cell (PEC) in order to assist photoelectrolysis for H2 and O2 gas generation without external electrical sources. The open circuit voltage was increased from 0.65 V to approximately 3.2 V by the serial connection of five Se/TiO2 PV devices. Hydrogen and oxygen bubbles were successfully generated by only light illumination without using any wired external voltage source. A photoanode with sol–gel derived porous TiO2 films was used to increase the surface area to attain a stronger photocatalytic reaction. Microdot patterns of Pt were formed as the co-catalyst on a TiO2 photoanode to suppress the back reaction of the photogenerated carrier. A Pt anode was formed on a glass substrate. Parameters related to the photocatalytic reaction, such as atmospheric pressure, the type of chemical additive, and the area ratio of Pt co-catalyst patterns to the surface area of the TiO2 photoanode, were varied to generate a larger gas volume with the water-splitting device.