*2.3. Magnetic Materials-Based Composite Photocatalysts*

Magnetic nanomaterials have been used as the core for the preparation of magnetically separable photocatalysts. To improve the activity, photocatalyst nanomaterials should be highly dispersible in solution. Nonetheless, it will become more difficult to separate and reuse the nanoparticles of photocatalysts by centrifugation for repeated operation if such good dispersal stability is achieved. Hence, the introduction of magnetic nanomaterials in photocatalysts will enable efficient separation for the repeated use of photocatalysts. For instance, in studies conducted on NiCo2O4@ZnS and Fe3O4@ZnS core–shell nanoparticles, the recycled photocatalyst exhibited a good hydrogen evolution rate even after being recycled three times [72]. The ZnS shell deposited on a magnetic core decreases the magnetic saturation of core–shell microspheres. CoFe2O4@ZnS nanoparticles exhibit superparamagnetic properties where no residual magnetism is left after repeated use of the photocatalyst [73]. Figure 6 shows the dispersion and magnetic separation of calcinated CoFe2O4@ZnS-0.5h photocatalyst. The superparamagnetic property is quite important for photocatalytic H2 generation in practical operations. After the magnet is removed, there should be no residual magnetism so as to prevent the aggregation of recycled photocatalysts.

**Figure 6.** (**a**) Dispersion of CoFe2O4@ZnS-0.5h photocatalysts, (**b**) the separation of CoFe2O4@ZnS-0.5h photocatalyst by an external magnet. Figure adapted from [73].

#### **3. Experimental Parameters for Enhancing Photocatalytic Activity**

#### *3.1. Loading with Metal*

The majority of the prior research to improve the activity of photocatalysts involved the loading of noble and non-noble metals and the coupling of semiconductors. Then the photogenerated electrons could be transported to the noble metals or delocalized and transported between photocatalysts.

#### 3.1.1. Noble Metal Loading

The photocatalytic activity for the production of hydrogen is affected by the noble metal loaded on the photocatalyst. Noble metal co-catalysts can accelerate the transport of photoexcited charge and thus create H2 desorption sites. This will eventually lead to higher H2 generation activity [74]. Although the rate of H2 generation will increase with increasing noble metal loads, the loading amount will reach a maximum, above which further increases of a noble metal will not decrease the photocatalytic activity. The reduction is probably due to two factors: (i) excess noble metal will lead to shielding of the incident light; and (ii) a higher amount of noble metal decorated on the photocatalyst surface will causes light scattering of the samples, thus reducing the effective irradiation absorbed by the reaction suspension [75,76].

Zhou et al. studied the ternary heterojunction photocatalyst CdS/M/TiO2 (M = Ag, Au, Pd, Pt). Some photocatalysts enriched with the noble metals are better than pristine TiO2 and binary heterojunction photocatalysts. For example, the hydrogen evolution rate of photocatalyst loaded with Pd (CdS/Pd/TiO2) is 6.7 times higher than that of CdS/TiO2 [77].

The photocatalytic H2 generation activity also increases as greater amounts of Au are loaded onto ZnS flowers. When the Au load is less than 4%, the photocatalytic activity is able to reach 3306 μmol h<sup>−</sup>1g−1. Further increasing the Au load to 6% reduces the activity. Such improvement of photoactivity can be ascribed to the following reasons. First, in comparison with the conduction band minimum of ZnS, Au has a lower Fermi level. The photoexcited electrons can be transported to Au. Second, the Au(I) loaded on the ZnS lattice will extend the light absorption wavelength and enhance the light harvesting efficiency [78]. Moreover, when Au nanoparticles are incorporated on S,N-modified TiO2 (SNT), the Au particles promote the visible light-driven hydrogen production activity. Because of surface plasmon resonance, 3.5 nm Au particles deposited on TiO2 can increase light absorption. The amount of hydrogen generated by the 3Au-SNT is 9 times that of pure SNT [79].

In addition to Au, Pt is frequently incorporated in semiconductor photocatalysts. In a previous study by Yu et al., Pt was loaded onto a Cu2ZnSnS4 (CZTS) semiconductor with a maximum content of 1%, and the hydrogen production rate increased. However, the performance decreased when the Pt load was further increased, due to the optical shielding effect. The production of hydrogen by 1% Cu2ZnSnS4-Pt was 8 times higher than that of bare Cu2ZnSnS4. Intimate contact between CZTS and Pt

increases the production of hydrogen [80]. Similarly, in CdS photocatalyst, the hydrogen evolution reaches a quantum efficiency of 51% and a maximum of 4800 μmol h−<sup>1</sup> when 0.65 wt % of Pt is loaded on CdS. However, the activity can be further increased by loadings of 0.3 wt % Pt and 0.13 wt % PdS on CdS. The co-loading of noble and non-noble metals onto pristine CdS promotes the splitting of H2S into H2 and S [81]. Likewise, the addition of Pt to CuS-TiO2 enhances the evolution of hydrogen because excited electrons from the CB of CuS or through CB of TiO2 can be transferred directly to Pt sites, resulting in the reduction of protons to hydrogen [82]. Moreover, loading Pt on novel CdxCuyZn1−x−yS also enhances the photocatalytic performance. The presence of 0.5 wt % Pt increases the H2 production rate to 557 μmol h<sup>−</sup>1, as compared to 350 μmol h−<sup>1</sup> produced by Cd0.1Cu0.01 Zn0.89S alone [83].

The presence of Cu in a photocatalyst facilitates carrier separation and also increases light absorption. As reported in a study of In and Cu co-doped ZnS photocatalysts by Kimi et al. [84], photocatalytic performance is strongly related to the amount of doped Cu. With the suitable amount of doped Cu (0.03), hydrogen evolution reaches a maximum that is 8 times higher than that of hydrogen produced by In(0.1)-ZnS photocatalyst. However, with the incorporation of more Cu, the photocatalytic activity becomes lower than that of single doped In(0.1)-ZnS. An excessive amount of Cu causes light scattering, and the excess Cu also acts as recombination sites to halt the photocatalytic reaction.

Although the incorporation of noble metals in photocatalysts will increase the rate of hydrogen production, different noble metals have various effects on the enhancement. When four different noble metals, Pt, Rh, Pd, and Ru, are decorated on CdS/TiO2 photocatalyst, the amount of photogenerated H2 by Pt loaded (640 μmol h<sup>−</sup>1) photocatalyst is the highest [85]. Table 7 shows the photocatalytic H2 generation for noble metal-loaded photocatalysts.


**Table 7.** Photocatalytic H2 generation for noble metal-loaded photocatalyst.

#### 3.1.2. Transition Metal Doping

As observed from a few recent studies, transition metals (TM) have begun attracting attention because doping with TM can significantly enhance the photocatalytic performance by efficiently promoting the separation process of photoexcited holes and electrons. For instance, Chen et al. developed an in-situ photodeposition method to load Co on CdS nanorods. That work reported a highest photocatalytic activity of 1299 μmol h−<sup>1</sup> with the optimum loading of 1.0 wt % [87]. In comparison with nickel and iron, cobalt has the ability to improve the rate of H2 generation. From the work done by Zhou et al., the enhancement of the activity of MoSx was found to be in the order of Co > Ni > Fe [88]. This improved performance results from the higher amount of doped Co and the capability of Co to activate the S-edge sites [89]. A quick screening technology has been reported to find out the optimized composition of the photocatalyst. M-ZnS based photocatalysts (M = Cr, Cu, Ni, Mo, and Ag) for photoelectrochemical water oxidation applications can be screened rapidly using scanning electrochemical microscopy (SECM) with an optical fiber by finding, the spot with the highest photocurrent among the photocatalyst arrays [90].

Ni doping can enhance the photocatalytic activity of H2 generation by increasing the absorption of light of the doped photocatalyst. In contrast, when the amount of Ni loaded on ZnS-graphene composites is increased, it will degenerate the crystalline property of the photocatalyst [91]. In addition, the incorporation of Ni on Cd1−xZnxS microsphere photocatalyst increases the rate of hydrogen production to 191 μmol h<sup>−</sup>1g−<sup>1</sup> when an optimum amount of 0.1 wt % Ni is loaded. The increment results from the decreased particle size and increased surface area of the photocatalyst [92]. Incorporating a metal such as Ni onto the photocatalyst can accelerate the process of transferring electrons to the surface and decrease the band gap, leading to increased photocatalytic activity. A similar trend of enhancement by Ni doping was also observed with stainless steel wire mesh@doped ZnS photocatalyst [93]. Pristine stainless steel wire mesh C60 has a hydrophobic surface (water contact angle = 103◦). In contrast, the water contact angle of ZnS decorated wire mesh photocatalyst C60S0.5 is 0◦. Effective contact between the sacrificial solution and the photocatalyst surface is very important because the photocatalyst is used for photocatalytic H2 generation in aqueous sacrificial solution. Improving the hydrophilicity of the photocatalyst will lead to increased activity. Table 8 lists the photocatalytic H2 production performances for transition metal doped photocatalysts.


**Table 8.** Photocatalytic hydrogen production performances for transition metal doped photocatalysts.
