**3. Visible-light-driven Ag3PO4 photocatalysts**

A breakthrough was made in finding a novel semiconductor material, Ag3PO4, as an active visible-light-induced photocatalyst [4]. Ag3PO4 demonstrates extremely high capability for O2 evolution from H2O and organic dye decomposition under visible-light irradiation [120]. More importantly, this novel photocatalyst can achieve a quantum efficiency up to 90% at wavelengths longer than 420 nm, which is clearly higher than that reported previously by using semiconductor photocatalysis.

So far, various methods have been proposed to further enhance the photocatalytic activity of Ag3PO4 under visible-light irradiation. One approach is the synthesis of Ag3PO4 with various morphologies. This is because photocatalytic reactions are typically surface-based processes; thus, the photocatalytic efficiency is closely related to the morphology and microstructure of a photocatalyst [83]. Recently, some new morphologies of Ag3PO4 have been developed [120– 126]. For example, Bi et al. fabricated the single-crystalline Ag3PO4 rhombic dodecahedrons and cubes, and they found that both of these samples exhibited higher photocatalytic activity than the microsized spherical Ag3PO4 particles [120]. Liang et al. synthesized hierarchical Ag3PO4 porous microcubes with enhanced photocatalytic property [123]. Wang et al. reported the synthesis of Ag3PO4 tetrapod microcrystals, and they demonstrated that Ag3PO4 tetrapod showed higher photocatalytic activity than the microsized spherical Ag3PO4 particles [126].

Another approach is to couple Ag3PO4 with other semiconductors, carbon materials, or noble metals to improve the photocatalytic activity, such as Ag3PO4/TiO2 [127, 128], Ag3PO4/AgX (*X* = Cl, Br, I) [129], Ag3PO4/BiOCl [130], Ag3PO4/Fe3O4 [131], Ag3PO4/SnO2 [132], Ag3PO4/ carbon quantum dots [133], Ag3PO4/reduced graphite oxide sheets [134], and Ag3PO4/Ag composites [135–137].
