Cobalt incorporated sulfur-doped graphitic carbon nitride with bismuth oxychloride (Co/S-gC(3)N(4)/BiOCl) heterojunction is prepared by an ultrasonically assisted hydrothermal treatment. The heterojunction materials have employed in photoelectrochemical (PEC) water splitting. The PEC activity and stability of the materials are promoted by constructing an interface between the visible light active semiconductor photocatalyst and cocatalysts. The photocurrent density of Co-9% S-gC(3)N(4)/BiOCl has attained 393.0 mu A cm(-2) at 1.23 V vs. RHE, which is 7-fold larger than BiOCl and similar to 3-fold higher than 9% S-gC(3)N(4)/BiOCl. The enhanced PEC activity can be attributed to the improved electron-hole charge separation and the boosted charge transfer is confirmed by photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) analysis. The fabricated Co/S-gC(3)N(4)/BiOCl nanohybrid material has exhibited high stability of up to 10,800 s (3 h) at 1.23 V vs. RHE during PEC water splitting reaction and the obtained photo-conversion efficiency is 3.7-fold greater than S-gC(3)N(4)/BiOCl and 17-fold higher than BiOCl. The FESEM and HRTEM images have revealed the formation of heterojunction interface between S-gC(3)N(4) and BiOCl and the elemental mapping has confirmed the presence of cobalt over S-gC(3)N(4)/BiOCl. The heterojunction interface has facilitated the photo-excited charge separation and transport across the electrode/electrolyte interface and also the flat-band potential, which is confirmed by Mott-Schottky analysis. (C) 2021 Elsevier Inc. All rights reserved.