**2.4. Metal/semiconductor yolk/shell nanocrystals**

With a unique structure, metal-semiconductor yolk/shell nanocrystals play an important role in photocatalysis and other fields [56–63]. As photocatalysts, metal-semiconductor yolk/shell nanocrystals process some advantages, such as stronger absorption, higher catalysts selectiv‐ ity, and higher quantum yield. Nowadays, different kinds of metal-semiconductor yolk/shell nanoarchitecture have been designed to enhance their photocatalysis properties. Following it are several methods for preparing metal-semiconductor yolk/shell nanocrystal.

## *2.4.1. Template Free for formation of metal/semiconductor yolk–shell nanocrystals*

Following this method, metal/semiconductor yolk–shell nanocrystals are synthesized directly. During the process, no any nanostructure is used as template or core–shell nanocrystals forming during the process are taken as self-template [56, 64, 65]. For example, Pt–CeO2 yolk– shell nanoparticles can be synthesized by hydrothermal method, although the mechanism may involve several factors [64]. Pt nanoparticles and CeO2 precursors are put into the autoclave and heated at 90ºC in a period. The authors illustrate that the ratio of CeCl3 concentration and Pt colloid solution is the main factor to formation of yolk–shell nanoparticles. Only can Pt– CeO2 core–shell nanoparticles be obtained when a larger CeCl3/Pt ratio is employed. With a smaller ratio, the looser shell of Pt–CeO2 yolk–shell nanoparticles can be obtained. The yolk– shell nanoparticles enhance the properties of photocatalysis of hydride of Pt and CeO2.

Au–Cu2O yolk–shell nanocrystals are synthesized by prolonging the reaction time after formation of Au–Cu2O core–shell nanocrystals [65]. Cu2O shell grows on the surface of Au nanoparticles to form Au–Cu2O core–shell nanoparticles. As the reaction time increasing, Au core and Cu2O shell spare forming yolk–shell nanostructure. The longer the reaction time, the more obvious the yolk–shell nanostructure. Furthermore, the thickness of Cu2O shell can be tuned with Au colloid/Cu2+ ratio. As the authors showed in the articles, the distance between Au core and Cu2O shell and the thickness of Cu2O shell have a strong influence on the optical properties of Au–Cu2O yolk–shell nanoparticles (Figure 14). It means that the method for preparation of Au–Cu2O with tunable yolk–shell nanoparticle provides a route to tune the optical properties which is of importance for photocatalysis.

**Figure 13.** Large-scale TEM images of shape evolutions in Au/CdSe and Au/CdTe hybrid nanostructures with ∼5 nm sized Au. (A) Concentric core/shell of Au/CdSe; (B, C) heterodimer of Au/ CdSe; (D) heterodimer of Au/CdTe. The in‐ set diagrams highlight the phase separation-induced Au/CdSe and Au/CdTe morphologies. Scale bar: 20 nm. Adapted

Based on Figure 5, the metal/semiconductor core/shell NCs can further lead to fine tuning of plasmon–exciton coupling, different hydrogen photocatalytic performance, and enhanced

With a unique structure, metal-semiconductor yolk/shell nanocrystals play an important role in photocatalysis and other fields [56–63]. As photocatalysts, metal-semiconductor yolk/shell nanocrystals process some advantages, such as stronger absorption, higher catalysts selectiv‐ ity, and higher quantum yield. Nowadays, different kinds of metal-semiconductor yolk/shell nanoarchitecture have been designed to enhance their photocatalysis properties. Following it

Following this method, metal/semiconductor yolk–shell nanocrystals are synthesized directly. During the process, no any nanostructure is used as template or core–shell nanocrystals forming during the process are taken as self-template [56, 64, 65]. For example, Pt–CeO2 yolk– shell nanoparticles can be synthesized by hydrothermal method, although the mechanism may involve several factors [64]. Pt nanoparticles and CeO2 precursors are put into the autoclave and heated at 90ºC in a period. The authors illustrate that the ratio of CeCl3 concentration and

are several methods for preparing metal-semiconductor yolk/shell nanocrystal.

*2.4.1. Template Free for formation of metal/semiconductor yolk–shell nanocrystals*

from Zhao et al. [32] with permission; copyright Wiley-VCH Verlag GmbH & Co. KGaA.

photovoltaic, electrical properties.

**2.4. Metal/semiconductor yolk/shell nanocrystals**

310 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 14.** (A) Schematic illustration of formation of Au–Cu2O yolk–shell nanocrystals. (B–M) TEM images of prepared Au–Cu2O yolk–shell nanoparticles. (N) Photograph of Au–Cu2O during hollow process. (O) Their optical properties collected by experiment and simulation. Copyright: American Chemistry Society, 2011.

Co–SiO2 yolk–shell nanoparticles are obtained by reduction of CoO–SiO2 core–shell nanopar‐ ticles. Park et al. coat silica on CoO nanoparticles to form CoO–SiO2 nanoparticles then reduce CoO–SiO2 nanoparticles with hydrogen to form Co–SiO2 yolk–shell nanoparticles [66].

With template free method, some metal cation is introduced into hollow nanocrystals then reduced forming metal nanoparticles in the cavities of hollow structure [67, 68]. Cu–SiO2 yolk– shell nanocrystal is synthesized by reducing Cu2+ cation in cavity of SiO2 hollow particle and the size of Cu core can be tuned by multiple reduction cycles (Figure 15A–D) [67]. Besides that, yolk–shell nanocrystals can be employed because they also have cavities for reduction of metal cation. By this method, multiplied cores or cores with different materials can be synthesized. Au–SiO2 yolk–shell with tunable core is prepared by reducing HAuCl4 in the cavity of SiO2– SiO2 yolk–shell nanostructure (Figure 15F–I) [68]. On account of that, metal cation can be introduced into the cavities of hollow nanoparticles or yolk–shell nanoparticles, and other kinds of metal cation can be introduced into the cavities to react with metal core forming a new metal core or alloy core. For instance, Ag–SiO2 yolk–shell nanoparticles can be obtained by displacing Cu with Ag+ from Cu–SiO2 yolk–shell nanoparticles (Figure 15E) [67]. Moreover, because the cavity of yolk–shell can be taken as reactor, the synthesis way can be applied to form other yolk–shell nanostructures.

**Figure 15.** TEM images of various yolk–shell nanoparticles prepared with different way. (A) SiO2 hollow nanosphere, (B–D) Cu–SiO2 yolk–shell nanoparticles, and (E) Ag–SiO2 yolk–shell prepared by replacing the Cu core of Cu–SiO2 yolk–shell nanoparticles with Ag+; scale bar: 100 nm [67]. Copyright: Royal Society of Chemistry, 2004. (F) SiO2–SiO2 yolk–shell nanoparticles, (G-I) Au–SiO2 yolk–shell nanoparticles with different sizes Au core; scale bar: 50 nm [68]. Copyright: Wiley-VCH, 2010. (J) Au–CeO2 yolk–shell nanoparticles [69]. Copyright: RSC, 2012. (K-M) Pt–CeO2 yolk– shell nanoparticles [64]. Copyright: RSC, 2011.

Although some of mechanism of template free of yolk–shell nanostructure formation is unclear, it provides the method to design and prepare metal/semiconductor yolk–shell nanocrystals.
