**2. Fundamentals**

#### *2.1. Principles of Photocatalysis*

As defined by the International Union of Pure and Applied Chemistry (IUPAC), photocatalysis is "A catalytic reaction involving light absorption by substrate" [32]. When semiconductor photocatalysts are irradiated by light with the photon energy larger than their band gap (BG) energy, the electrons in the valence band (VB) will be excited into the conduction band (CB), leaving positive holes in the VB. Because photocatalytic reactions are reactions happening at the surface of photocatalysts, the photo-induced free charge carriers need first to diffuse into the active sites on the surface of photocatalysts before they can induce photocatalytic reactions [33–35]. The photocatalytic process is schematically presented in Figure 1. However, for a particular substrate, whether it can undergo chemical reactions on the semiconductor photocatalysts depends on the relative positions between its redox potentials and the band edges of semiconductor photocatalysts [36–38]. There are four possibilities as follows:


**Figure 1.** Schematic diagram for the photocatalytic process.

#### *2.2. Mechanisms for the Enhanced Properties of Metal/Semiconductor (M/S) Nanocomposite Photocatalysts*

As mentioned above, by coupling with different metals, the mechanisms for the enhanced properties of M/S nanocomposite photocatalysts can be divided into two categories, i.e., the enhanced charge separation at the metal-semiconductor interface and the enhanced visible light absorption due to the SPR of metals.

#### 2.2.1. Enhanced Charge Separation

The enhanced charge separation of M/S nanocomposite photocatalysts originates from the electron transfer across the metal–semiconductor interface [31,39–41]. When a semiconductor is coupled with a metal, which possesses a higher work function than that of the semiconductor (it is the most common case for M/S nanocomposite photocatalysts), then the electrons will flow from the semiconductor to the metal until their Fermi levels (i.e., EF,m and EF,s) are aligned, leading to the upward bending of the band edges in the semiconductor as revealed in Figure 2. Accompanied with the band bending, a Schottky barrier (i.e., φSB in Figure 2) forms at the metal–semiconductor interface [39]. As has been shown, the efficiency of electron transfer across the metal–semiconductor interface increases with the height of the Schottky barrier [42]. In addition, the metals in M/S nanocomposite photocatalysts not only act as the electron reservoir to enhance the charge separation of the semiconductors, but also provide active sites for reductive reactions, hence dramatically improving the properties of M/S nanocomposite photocatalysts [43].

**Figure 2.** Enhanced charge separation at the metal-semiconductor interface.

#### 2.2.2. Enhanced Visible Light Absorption

When coupled with metals such as Ag and Au, the SPR of metals could endow the M/S nanocomposite photocatalysts with enhanced absorption toward visible light [44,45]. However, the mechanism governing the SPR-enhanced visible light absorption of M/S nanocomposite photocatalysts is still unclear [28,29,46,47]. So far, there exist three non-mutually exclusive mechanisms for the explanation of the SPR-enhanced properties of M/S nanocomposite photocatalysts under visible light irradiation, i.e., (a) SPR-induced electron injection from metals to semiconductors; (b) charge separation induced by near-field electric field (NFEF); (c) scattering-enhanced light absorption.

#### Surface Plasmon Resonance (SPR)-Induced Electron Injection

Due to the SPR of metals such as Au and Ag, the energy of excited electrons in the metals can be excited to the range between 1.0 and 4.0 eV above their Fermi levels. Then, the energetic electrons will overcome the Schottky barrier at the metal–semiconductor interface and transfer to the CB of semiconductors, leaving energetic holes in the metals, as schematically shown in Figure 3a. Thus, the

hot electrons in the semiconductors will drive reduction reactions and the hot holes in the metals will drive oxidation reactions.

**Figure 3.** (**a**) SPR-induced electron injection; (**b**) Charge separation induced by NFEF; (**c**) Scattering-enhanced light absorption.

Charge Separation Induced by Near-Field Electric Field (NFEF)

Under visible light irradiation, the SPR-excited hot electrons in the metal particles of M/S nanocomposite photocatalysts will generate intense electric field in their proximity (i.e., NFEF), which could be up to 100–10000 times larger than the electric field of incident light. Since the formation rate of electron-hole pairs in the semiconductors of M/S nanocomposite photocatalysts is proportional to the intensity of the NFEF (more specifically, |E|2), the generation of electron-hole pairs in M/S nanocomposite photocatalysts can be significantly enhanced. The process of NFEF-induced charge separation is schematically shown in Figure 3b.

## Scattering-Enhanced Light Absorption

If the size of the metal particles in M/S nanocomposite photocatalysts is larger than 50 nm, the SPR-excited metal particles can efficiently scatter the incident light, leading to the increase of the path length of light through the M/S nanocomposite photocatalysts. Thus, the light absorption of M/S nanocomposite photocatalysts will be improved due to the increased path length of light, resulting in the enhancement of the properties of M/S nanocomposite photocatalysts. Figure 3c schematically reveals the scattering-enhanced photon absorption mechanism.

#### **3. Structures of M/S Nanocomposite Photocatalysts**

Based on the morphologies and synthetic approaches, the structures of M/S nanocomposite photocatalysts could be divided into six categories, i.e., the conventional structure, core-shell structure, yolk-shell structure, Janus structure, array structure and multi-junction structure, which are schematically shown in Figure 4.

**Figure 4.** (**a**) Conventional structure; (**b**) Core-shell structure; (**c**) Yolk-shell structure; (**d**) Janus structure; (**e**) Array structure; (**f**) Multi-junction structure.

#### *3.1. Conventional Structure*

The conventional structure of M/S nanocomposite photocatalysts refers to the structure, in which the combination of semiconductor nanoparticles and metal nanoparticles is not delicately controlled. So far, many approaches have been developed to synthesize M/S nanocomposite photocatalysts with conventional structure, which include photoreduction, impregnation, deposition-precipitation, chemical vapor deposition (CVD) etc. [48–51].
