**3.2. Metal/semiconductor heterodimer nanocrystals: the role of metal**

Integration with metal is a commonly used configuration to improve the photocatalytic hydrogen evolution performance of a semiconductor. The metal may play a variety of roles in the enhancement of photocatalytic performance. In the following sections, we will focus on the role of metal as cocatalyst and the plasmonic effect of noble metals.

## *3.2.1. The cocatalyst role*

Since the work by Kraeutler and Bard in 1978 loading Pt on the surface of TiO2 [88], the loading of metal nanoparticles onto different semiconductor photocatalysts has been regarded as a popular strategy to improve the photocatalytic performance in photocatalytic water splitting. Besides Pt, other metal cocatalysts, including Pd, Rh, Ru, Ir, Ag, Ni, Co, etc., have been recognized as efficient cocatalysts for photocatalytic hydrogen evolution [89–96]. The proc‐ esses of charge transfer between metal cocatalyst and host semiconductor photocatalyst are described in Figure 21A.

The metal cocatalysts mainly play two roles in the improvement of photocatalytic perform‐ ance. One is to assist in electron–hole separation through the formation of Schottky barrier between the metal cocatalyst and the light-harvesting semiconductor. A Schottky barrier is a type of junction resulting from the intimate contact of a metal with a semiconductor (Figure 21B). The metal cocatalyst with larger work function, that is, lower Fermi energy level, should more readily trap the photogenerated electrons migrated to the surface of the host semicon‐ ductor photocatalyst. Meanwhile, the photogenerated holes stay at the host photocatalyst and

photogenerated electrons and holes, resulting poor photocatalytic activity. Therefore, improv‐ ing the crystalline quality will decrease the amount of defects and improve photocatalytic activity. If the particle size becomes small, the distance that photogenerated electrons and holes have to migrate to reaction sites on the surface becomes short, and this results in a decrease in the recombination probability. Also, the fabrication of junction structure has been recognized

The surface catalytic reaction is a successive step of charge separation. The important points for this step are surface character (active sites) and quantity (surface area). Even if the photo‐ generated electrons and holes possess thermodynamically sufficient potentials for water splitting, they will have to recombine with each other if the active sites for redox reactions do not exist on the surface. Cocatalysts such as Pt, NiO, and RuO2 are usually loaded to introduce

The processes of the photocatalytic reaction on a semiconductor photocatalyst involve light absorption, charge separation, carrier migration, and surface catalytic reactions. Therefore, developing band gap engineering to narrow down the band gap of semiconductor materials for absorbing broader spectrum of solar energy and materials engineering to tune the physical properties (crystal structure, crystallinity, and particle size) for gaining efficient charge separation and migration and creating enough active sites are the key problems of improving

Integration with metal is a commonly used configuration to improve the photocatalytic hydrogen evolution performance of a semiconductor. The metal may play a variety of roles in the enhancement of photocatalytic performance. In the following sections, we will focus on

Since the work by Kraeutler and Bard in 1978 loading Pt on the surface of TiO2 [88], the loading of metal nanoparticles onto different semiconductor photocatalysts has been regarded as a popular strategy to improve the photocatalytic performance in photocatalytic water splitting. Besides Pt, other metal cocatalysts, including Pd, Rh, Ru, Ir, Ag, Ni, Co, etc., have been recognized as efficient cocatalysts for photocatalytic hydrogen evolution [89–96]. The proc‐ esses of charge transfer between metal cocatalyst and host semiconductor photocatalyst are

The metal cocatalysts mainly play two roles in the improvement of photocatalytic perform‐ ance. One is to assist in electron–hole separation through the formation of Schottky barrier between the metal cocatalyst and the light-harvesting semiconductor. A Schottky barrier is a type of junction resulting from the intimate contact of a metal with a semiconductor (Figure 21B). The metal cocatalyst with larger work function, that is, lower Fermi energy level, should more readily trap the photogenerated electrons migrated to the surface of the host semicon‐ ductor photocatalyst. Meanwhile, the photogenerated holes stay at the host photocatalyst and

as an effective strategy to avoid charge recombination in semiconductors.

active sites for H2 evolution [87], which will be discussed later.

318 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**3.2. Metal/semiconductor heterodimer nanocrystals: the role of metal**

the role of metal as cocatalyst and the plasmonic effect of noble metals.

photocatalytic hydrogen evolution efficiencies.

*3.2.1. The cocatalyst role*

described in Figure 21A.

**Figure 21.** (A) Processes of charge transfer between host photocatalyst and cocatalyst, taking Pt as the example of coca‐ talyst [86]. Copyright: America Chemistry Society, 2010. (B) Schematic of Schottky barrier [90]. Copyright: Royal Soci‐ ety of Chemistry, 2010. (C) Two general steps for proton reduction reaction [89]. Copyright: America Chemistry Society, 2013.

migrate to its surface. This retards the possibility of electron–hole recombination and enhances efficient separation of the photogenerated electrons and holes. It improved the overall photocatalytic activity of the water splitting because it helps to promote charge separation, which in return reduces both bulk and surface electron/hole recombination.

The other role of metal cocatalysts is to serve as the reaction sites to catalyze the proton reduction to H2. Proton reduction course on cocatalysts goes through at least two steps: a discharge step and catalytic step (Figure 21C) [89]. Most semiconductor photocatalyst, particularly oxides, lack surface H2 evolution sites rather than O2 evolution sites since their CB levels are not sufficiently negative for electrons to reduce water to produce H2 without catalytic assistance whereas their VB levels are usually positive enough for holes to oxidize water and form O2 even in the absence of cocatalyst [92]. Metal cocatalysts could lower the activation energy or overpotential for H2 evolution reaction on the surface of semiconductors. It also accelerates the surface chemical reaction by inhibiting the backward reaction.

Overall, the role played by the metal cocatalysts dispersed on the surface of the semiconductor photocatalysts is extremely important. Many factors can affect the capability of H2 evolution cocatalysts in the semiconductor-based photocatalytic water splitting, such as cocatalyst loading amount and its particle size and structure [89, 94]. For example, there is a volcano-type trend between the loading amount of a given cocatalyst and the photocatalytic activity (regardless of the synthesis method, photocatalyst type, and loaded cocatalyst) [94, 95]. At the same loading amount, metal cocatalysts with smaller size and high dispersion display higher catalytic activity due to their larger specific surface area and more active sites [96].
