Chemically modified g-C3N4 for the photocatalytic H-2 evolution from water was explored. Bulk g-C3N4 was treated in hot HNO3 aqueous solution to obtain the oxidized material (o-g-C3N4), tested in water containing glucose as model water-soluble sacrificial biomass, using Pt as co-catalyst, under simulated solar light. The behaviour of o-g-C3N4 was studied in relation with catalyst amount, Pt loading, glucose concentration. Results showed that H2 production is favoured by increasing glucose concentration up to 0.1 M and Pt loading up to 3 wt%, and it resulted strongly enhanced using small amount of o-g-C3N4 (0.25 g L-1). o-g-C3N4 possesses superior photocatalytic activity (similar to 26-fold higher) compared to pristine g-C3N4, with H2 evolution further improved by ultrasound-assisted exfoliation and evolution rates up to ca. 1370 mu mol h(-1) per gram of catalyst, with excellent reproducibility (RSD < 6%, n = 3). Significant production was observed also in river water and seawater, with results far better (up to ca. 2500 mu mol h(-1)) compared to commercial AEROXIDE (R) P25 TiO2 under natural solar light. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.