Narrow band-gap (NBG) Ag2S nanocrystals (NCs) attaching on the surface of wide band-gap (WBG) Ag8- W4O16 nanorods were prepared by employing a facile in situ anion exchange method with the reaction between S2- and WO42-, and the photocatalytic activity was evaluated by the photocatalytic decolorization of methyl orange solution under visible-light irradiation. It was found that in situ anion exchange could uniformly deposit Ag2S NCs on the surface of Ag8W4O16 nanorods, controllably adjust the size, distribution and amount of Ag2S NCs, and solidly connect Ag2S NCs to the Ag8W4O16 nanorods via the replacement of S2- in the solution with lattice WO42- on the Ag8W4O16 surface. The photocatalytic results indicated that the as-prepared Ag2S/Ag8W4018 composite photocatalysts exhibited obviously higher activity compared with the pure Ag8W4O18 and N-TiO2 photocatalysts. On the basis of band structures of Ag2S and Ag8W4O16 semiconductors and the quantum size effect of Ag2S NCs, a possible photocatalytic mechanism about the Ag2S nanocrystal-sensitized Ag8W4O18 nanorods was proposed to account for the effective visible-light photocatalytic activities. This present work may provide some insight into the design of novel and high-efficiency NBG semiconductor NCs coupled with WBG semiconductor composite photocatalysts. (c) 2014 Elsevier Inc. All rights reserved.