Mimicking the natural photosynthesis, artificial Z-scheme photocatalysis enables more efficient utilization of solar energy for sustainable chemical fuel production. Herein, a direct Z-scheme g-C3N4/WO3 photocatalyst with host-guest architecture is rationally designed, demonstrating significantly enhanced activities of photocatalytic H-2 production. Unprecedented atomic-scale imaging of both the in-plane and interlayer structures in g-C3N4 revealed the well-defined interfaces in such architecture, where the 2D g-C-3 N-4 layers stand vertically on the flat facets of WO3 nanocuboids. Through both experimental and theoretical investigations, mechanistic insights regarding the direct Z-scheme electron transfer from WO3 to g-C-3 N-4 were obtained. The Z-scheme electron transfer was driven by the internal electric field at the interfacial junction, defined by the covalent W-O-N-(C)(2) interaction. Under simultaneous light excitation, this atomically defined junction induces a rapid electron injection from WO3 to inhibit the fast recombination kinetics within g-C-3 N-4 and prolong the charge carrier lifetime of g-C-3 N-4, thereby liberating more excited electrons with high reducing power for H-2 production. (C) 2017 Elsevier B.V. All rights reserved.