*3.7. Fabrication Method*

Photocatalysts can be prepared by numerous methods, including the coprecipitation, cation exchange, chemical bath deposition, hydrothermal, and solvothermal methods. The co-precipitation method may lead to differences between the obtained final element composition in solid solution and the stoichiometric ratio [118]. The hydrothermal method requires a great amount of time to prepare a well-crystallized solid solution [119]. In the thermolysis method, the products are treated under high temperature to achieve fast fabrication, high crystallinity, and photocatalytic performance of the solid solution [120].

Li et al. found that Zn1−xCdxS photocatalyst fabricated using the simple Zn-Cd-Tu complex thermolysis method showed better performance than did those synthesized by the coprecipitation and hydrothermal methods [118,121]. Such a thermolysis method is preferred for the following reasons: (i) the precursors can be well mixed and reacted to prepare Zn-Cd-Tu complex by the ultrasonication process; (ii) the loss of precursors can be prevented during the fabrication process; and (iii) Zn1−xCdxS with a tiny crystallite size can be prepared because thiourea releases S2<sup>−</sup> ions slowly and offers some N and C atoms as the pinning points in the Zn-Cd-Tu complex [122].

Zhang et al. reported that a particular fabrication method will enable a photocatalyst to perform better in photocatalytic activity. The photocatalysts Cd1−xZnxS are prepared by three different methods: thermal sulfuration, co-precipitation without thermal treatment, and co-precipitation with thermal treatment. The photocatalyst which is synthesized by the thermal sulfuration method has better performance because the fabrication method allows uneven distribution of S2<sup>−</sup> ions, thus leading to a charge gradient [123]. A similar trend is also evidenced in the work done by Park et al. on a ternary CdS/TiO2/Pt hybrid. CdS/(Pt-sgTiO2) has the highest hydrogen production because of the electron transfer from CdS to Pt through TiO2 [124].

#### *3.8. Crystal Size*

Hydrogen production activity is also influenced by the crystal size of the photocatalyst. Li et al. [125] studied the photocatalytic hydrogen production performances of size-selected CdS nanoparticles decorated with co-catalyst Pt nanoparticles. When the size of CdS nanoparticles decreases from 4.6 to 2.8 nm, the H2 generation quantum yield can increase from 11% to 17%. Such a dependence was observed because the driving force of photoinduced carrier transfer from CdS to vacant states of Pt nanoparticles is size-dependent. Baldovi et al. [126] prepared MoS2 quantum dots by laser ablation of MoS2 particles in suspension and investigated their photocatalytic hydrogen production performance. Two types of MoS2 nanoparticles exhibited higher activity than that of bulk MoS2. When the size of MoS2 nanoparticles was decreased from 15–25 nm to 5 nm, the photocatalytic hydrogen generation

performance was almost doubled. Holmes et al. [127] and Grigioni et al. [128] reported the dependence of the photocatalytic water splitting activity on the size of CdSe nanoparticles. The photocatalytic activity increases as the size of CdSe nanoparticles decreases. Figure 10 presents the UV–vis absorption spectra and quantum yields of CdSe quantum dots photocatalysts with various size. They also reported that the light harvesting capability and the conduction band energy should compromise to achieve maximal photocatalytic H2 generation activity. There is an optimal size of 2.8 nm to achieve maximal photocatalytic H2 generation activity considering the compromise among light harvesting, band structure, and charge separation.

**Figure 10.** UV–vis absorption spectra and quantum yields of CdSe quantum dot photocatalysts with various size**.** Figure adapted from [128].
