Three-dimensional (3D) graphene-based composites have drawn increasing attention in energy applications due to their unique structures and properties. However, the assembly of semiconductor nanoparticles (NPs) in 3D graphene structure is usually a complicated and time-consuming process. Here, we demonstrate a strategy of fast assembling Ag3PO4 NPs within 3D graphene aerogels (GAs) with a facile in situ ion filtration-precipitation method, in which the sequential filtration of Ag+ and PO43- solutions through the porous 3D graphene structure enables the fast assembly of well-dispersed Ag3PO4 NPs throughout the 3D GAs. The 3D GAs not only prevent the agglomeration of Ag3PO4 NPs due to the porous and interconnected microstructure but also promote the separation of photo-generated charges in Ag3PO4 due to the excellent electrical conductivity of reduced graphene oxide (rGO) that composes 3D GAs. As a result, the 3D Ag3PO4/GAs composite shows a considerable enhancement in both the activity and stability in photocatalytic oxygen evolution from water splitting when compared with pristine Ag3PO4. In addition to Ag3PO4 NPs, the in situ ion filtration-precipitation method is also feasible for fast assembling other semiconductor NPs in 3D graphene structures (C) 2016 Elsevier B.V. All rights reserved.