**3.1. Crystal growth design**

**2.4. TiO2-based heterostructure photocatalysts**

344 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Table 1.** Recent reports of diverse heterostructured photocatalysts.

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

using semiconductor photocatalysis.

**Type II**

**p–n**

**Homojunction**

The heterojunctions provide a facile way to enable the effective separation of photoexcited electron–hole pairs, thus to enhance the photocatalytic performance. Table 1 lists recent prominent heterostructured photocatalysts as well as their photocatalytic application.

**Z-scheme** CdS/Au/TiO1.96C0.04,[112] CdS/Au/TiO2,[113] ZnO/CdS,[114] CdS/Au/ZnO,[115]

According to different electronic energy levels and band gaps of photocatalysts, five kinds of semiconductor heterojunctions have been reported: straddling alignment (type I), staggered alignment (type II), Z-scheme system, homojunctions, and p–n heterojunctions [118, 119]. In those work, the dynamics of electron and hole were studied, including the band gap, the

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

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

electron affinity, and the work function of different semiconductor heterojunctions.

**Type I** CdS/ZnS,[85] Bi2S3/CdS,[86] V2O5/BiVO4[30]

**Heterostructured photocatalysts**

CdS/TiO2,[87–89] SrTiO3/TiO2,[90, 91] Fe2O3/TiO2,[92] ZnO/CdS,[93] AgIn5S8/TiO2,[94] Ag3VO4/TiO2,[95] ZnFe2O4/TiO2[96]

CuFe2O4/TiO2,[97] CuO/ZnO,[98] MoS2/CdS,[99, X] Ag2O/TiO2,[101] CuInSe2/TiO2,[102] TiO2/ZnO,[103] ZnFe2O4/TiO2,[104] NiO/ZnO[105]

Anatase/rutile TiO2,[106] α/β-Ga2O3,[31] p–n Cu2O,[107] α/γ-Bi2O3,[108] Co-doped TiO2/TiO2,[109] W-doped BiVO4,[110] Pt/n-Si/n+-Si/Ag[111]

CuO/TiO2,[116] CaFe2O4/WO4[117]

Manipulating the crystal structure will result in controlling the percentage of exposed facets on crystal surfaces and thus can lead to a dramatic change in reactivity, which has been widely investigated in sensing [138], electronics [139], magnetic memory devices [140], and catalysis [141]. It is widely accepted in catalysis that a higher surface energy leads to a more reactive surface. Therefore, the control of the exposed facets is one of the most available and efficient methods to obtain more active surface [142], which has been investigated recently to promote photocatalytic activity [83].

Herein, we controllably prepared Ag3PO4 crystals with various new morphologies (including branched, tetrapod, nanorod-shaped, and triangular-prism-shaped Ag3PO4 crystals) via a facile and efficient synthesis process in the solvent mixture of N,N dimethylformamide (DMF) and H2O at room temperature (Fig. 6). The results indicate that the branched Ag3PO4 sample shows highly enhanced photocatalytic activity compared with other as-prepared Ag3PO4 samples, and the BET-specific surface area makes a greater contribution to the enhanced photocatalytic activity of as-prepared Ag3PO4 crystals [143].
