Molecular oxygen performs a vital role in the photocatalytic activity of anatase TiO2, but there is a little experimental insight into the role of molecular oxygen on the synthesis of TiO2 based photocatalysts. Herein, we have shown that Ni(OH)(2)/TiO2 prepared in the presence and absence of molecular oxygen results in significantly varied hydrogen production activity. The sample synthesized in the presence of O-2 and N-2 produced 6624 mu mol/h/g and 4468 mu mol/h/g of hydrogen under direct solar light exposure. Additionally, the samples prepared in the presence of light irradiation produced 8289 mu mol/h/g of hydrogen, a 72 fold jump in hydrogen production compared to TiO2. XPS, FTIR, Raman, and ESR measurements were carried out to investigate the underlined mechanism for such variation in the photocatalytic activity. Our results suggest that the presence of molecular oxygen during Ni(OH)(2)/TiO2 synthesis causes the formation of terminal OH and reduced the oxygen vacancies on the surface of TiO2, which can significantly alter the H-2 production. Also, the reusability of the photocatalysts is greatly affected by the synthesis conditions, namely the presence of light and molecular oxygen. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.