Two types of out-of-substrate Ag–Ag 2 O nanoplates were grown on a ZnO substrate through a surfactantless photochemical method. First, the in situ photochemically synthesized Ag–Ag 2 O nanoparticles further crystallized into nanoplate-like superstructures with rough surfaces and ragged edges. The nanoparticle-mediated crystallization process was governed by a layer-by-layer crystallization mechanism. Our study should help fundamentally understand the formation mechanism of hierarchical nanoparticle superstructures. Under continuous UV illumination, the hundreds of nanometer-sized rough nanoplates (i.e., the nanoplate-like superstructures of nanoparticles) can be transformed into large smooth nanoplates with sizes of up to several micrometers. The out-of-substrate Ag–Ag 2 O nanoplates/ZnO heterostructures are potentially promising for photocatalytic applications.