Efficient charge separation and rapid interfacial reaction kinetics are crucial factors that determine the efficiency of photocatalytic hydrogen evolution.Herein,a fascinating 2D heterojunction photocatalyst with superior photocatalytic hydrogen evolution performance-holey C3N4 nanosheets nested with TiO2 nanocrystals(denoted as HCN/TiO2)-is designed and fabricated via an in situ exfoliation and con-version strategy.The HCN/TiO2 is found to exhibit an ultrathin 2D heteroarchitecture with intimate inter-facial contact,highly porous structures and ultrasmall TiO2 nanocrystals,leading to drastically improved charge carrier separation,maximized active sites and the promotion of mass transport for photocatalysis.Consequently,the HCN/TiO2 delivers an impressive hydrogen production rate of 282.3 pmol h-1 per 10 mg under AM 1.5 illumination and an apparent quantum efficiency of 13.4％at a wavelength of 420 nm due to the synergetic enhancement of surface reactions and charge separation.The present work provides a promising strategy for developing high-performance 2D heterojunctions for clean energy applications.