*3.6. Morphology*

The morphology of a photocatalyst also affects the performance, for the surface area and the surface active sites are influenced by the structure. Photocatalysts have different morphologies, including 3D and porous morphologies, nanosheets, nanorods, nanoflowers, and nanowires [14,19,25,32,34,47,77,103]. Recently, Amirav et al. studied tunable nanorod heterostructures. They demonstrated that a longer CdSe seeded rod was more active than a shorter rod of the same diameter, as the surface active sites were located further apart. However, nanorods with comparable rod lengths but smaller diameters will provide higher activity [102]. Panmand et al. reported that more structural defects and surface states are created on CdS decorated Bi2S3 nanowires. Figure 7 presents the morphology of a composite photocatalyst. The photogenerated charge carriers of photocatalysts with such defect energy levels can be effectively separated, leading to enhanced photocatalytic activity [104].

**Figure 7.** (**a**) HRTEM image; (**b**) SAED pattern of CdS decorated Bi2S3 nanowires; magnified HRTEM images of (**c**) Bi2S3 nanowire; (**d**) CdS nanoparticle [104].

In addition, a 2D morphology (such as a nanosheet) can help to improve the photocatalytic activity. Zhang et al. reported that ZnIn2S4/g-C3N4 heterojunction nanosheets demonstrated higher H2

production rates compared to single heterojunction components [101]. The contact of the components to form heterojunctions is very important [105]. A nanorod array structure with a small inter-rod distance will not easily form heterojunctions with close contact between different components. In contrast, a 2D heterostructure has enhanced photocatalytic performance because the van der Waals interaction of the 2D heterostructure junction between 2D metals (1T-MoS2) and the 2D semiconductor (O-g-C3N4) minimizes the Schottky barrier, thus improving the efficiency of charge transfer [106].

Moreover, the notable high hydrogen production rate and good stability achieved by mesoporous monoclinic CaIn2S4 with surface nanostructures implies the importance of structure in enhancing the H2 evolution rate. Ding et al. reported that monoclinic CaIn2S4 (m-CaIn2S4) exhibits a lower ability than cubic CaIn2S4 (c-CaIn2S4) to absorb visible light, but it also has better photocatalytic performance. The better performance results from the larger surface area, higher pore volume, more negative conduction band potential, and efficient separation of photoexcited carriers of m-CaIn2S4 [107].

Furthermore, a core–shell structure will also benefit the photocatalyst, as it may have an increased surface area and change the surface properties. For example, Chang et al. reported that the growth of Ag2S-ZnO@ZnS core–shell nanorods on metal wire mesh had modified the surface from hydrophobic to superhydrophilic. In addition, the H2 production activity also increased with the increasing thickness of the ZnS shell [32]. A similar trend was also evidenced in studies of NiCo2O4@ZnS and Fe3O4@ZnS core shell photocatalysts [72]. Table 11 presents the performances of photocatalysts with different structures.


**Table 11.** Photocatalytic activity of photocatalysts with different structures.

#### 3.6.1. Facet Effect

It has been reported that the photocatalysis reaction mainly occurs at the surface of the photocatalyst. The exposure of certain facets leads to greatly improved activity of the photocatalysts, known as the facet effect. Therefore, the preparation of photocatalysts with specific morphologies and structures is an important topic in the photocatalysis field. The facet effect has been observed for some oxide-based photocatalysts. Li et al. [109] reported that efficient separation of photoexcited electron–hole pairs can occur between different facets of photocatalytic nanomaterials. In comparison with their analogs with randomly distributed cocatalysts, selective deposition of a reduction cocatalyst and an oxidation cocatalyst on the {010} and {110} facets of BiVO4 leads to higher photocatalytic activity. Ohno et al. [110] found the effect of facets on the photocatalytic activity of TiO2 photocatalysts. For the rutile TiO2 nanomaterials, the {110} facet can act as an effective reduction site, and the {011} facet can offer a site for effective oxidation. TiO2 photocatalysts show high activity because of the synergistic effect between the {110} and {011} facets.

Similar results were also found for the sulfide-based photocatalysts. Song et al. found that, in comparison with 2-D Cu2MoS4 nanosheet with the exposed {001} facet, Cu2MoS4 nanotube with the exposed {010} facet exhibited effectively improved the performance for photocatalytic degradation and water splitting [111]. Shen et al. reported that the crystal facets of ZnIn2S4 with a 3D-hierarchical persimmon-like structure will influence the photocatalytic activity of ZnIn2S4. Extending the reaction time did not reveal any significant influences on the band gap or surface area of ZnIn2S4. Hence, the increase in the percentage of the {006} facet enhances hydrogen production [108]. The atomic structure of the {006} facet mainly consists of unsaturated metal cations. During the H2 generation reaction, the exposed unsaturated Zn and In cations of ZnIn2S4 will attract S2<sup>−</sup> and SO3 − anions, which may help the oxidation process and speed up the consumption of photogenerated holes (Figure 8). This impedes the electron–hole recombination process, leading to improved photocatalytic activity.

**Figure 8.** Schematic illustration of the hydrogen generation reaction on the {006} facets of the Pt loaded ZnIn2S4 photocatalysts. Figure adapted from reference [108].

#### 3.6.2. Light Trapping (Light Harvesting)

It is reported that properly patterned surface textures can lead to dramatically enhanced light absorption by the photocatalyst because of the light trapping effect [32]. Some textured structures including nanowire arrays [112], ordered mesoporous structures [113], micro-hole arrays [114], and hemisphere-array films [115,116]—are able to increase the light harvesting and photocatalytic performance of photocatalysts. Zhang et al. [112] reported that a 3D ZnO nanowire array–CdS sample exhibited substantial light-trapping enhancement in the visible light region. A schematic illustration of interface scattering when the light was irradiated on the surface of porous photocatalyst is provided in Figure 9 [116]. To enhance the light absorption efficiency, these surface textures allow multiple reflections and light scattering within the nanostructures. The incident light can travel through the cavities and decrease the optical loss. Efficient light trapping can be achieved by tuning the shape and roughness of the textured surface [117].

**Figure 9.** Schematic illustration of interface scattering due to nanograss decorated pore-array photocatalysts. Figure adapted from [116].
