*2.4.2. Using template for metal-semiconductor yolk–shell nanocrystals*

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

because the cavity of yolk–shell can be taken as reactor, the synthesis way can be applied to

**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–

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

from Cu–SiO2 yolk–shell nanoparticles (Figure 15E) [67]. Moreover,

by displacing Cu with Ag+

form other yolk–shell nanostructures.

312 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

shell nanoparticles [64]. Copyright: RSC, 2011.

nanocrystals.

Template employed for synthesizing metal-semiconductor yolk–shell nanocrystals can be conclude into two kinds of templates: soft template and hard template.

**Using soft template for metal-semiconductor yolk**–**shell nanocrystals:** By soft template protocol, several kinds of chemical are usually employed as soft template, such as biomolec‐ ular, polymers, surfactants, or microemulsions [56, 70]. In most of case, the large molecular would form microemulsions in solution and precursors react on the boundary of microemul‐ sions. Therefore, sizes or morphologies of yolk–shell nanoparticles could be tuned by tuning cell of microemulsions [71–73]. For metal semiconductor yolk–shell nanoparticles, metal core is reduced in the cavities of microemulsions then semiconductor shell grows on the boundary of microemulsions. For instance, Priebe and Fromm synthesize the Ag–SiO2 yolk–shell nanoparticles in cyclohexane and Igepal CO-520 system [72]. Ag+ is introduced into cavity of microemulsions and reduced to nanoparticles. Subsequently, silica precursor is introduced into the system and form the shell on the boundary of microemulsions (Figure 16). Besides, because Ag+ is reduced in the cavities, the size of Ag core can be tuned by using the series of concentration of AgNO4 solution.

**Figure 16.** (A) Schematic for formation of Ag–SiO2 yolk–shell nanoparticles and (B–I) TEM images of Ag–SiO2 yolk– shell nanoparticles with various Ag core size; scale bar: 500 nm, Copyright: Wiley-VCH, 2014.

Metal cores can also be introduced the cavities of microemulsions and then the shell grows to form metal/semiconductor yolk–shell nanoparticles directly [73]. Hyunjoon Song and his coworkers introduce Pd nanoparticles into the system of hexanol and igepal CO-630, followed by adding tetraethyl orthosilicate (TMOS) and octadecyltrimethoxysilane (C18TMS) for shell growth (Figure 17). Their protocol provides a possible method for preparing metal/semicon‐ ductor yolk–shell nanoparticles with other metal core and semiconductor shell.

**Figure 17.** (A)Schematic for preparing Pb–SiO2 yolk–shell nanoparticles and TEM images of Pb cores (B), Pb–SiO2 core– shell nanoparticles (C), and Pb–SiO2 yolk–shell nanoparticles(D). Copyright: ACS, 2011.

**Using hard template for metal/semiconductor yolk–shell nanostructure:** By hard template method, shell coating on core is synthesized as middle layer and subsequently removed or etched. The material of middle shell includes SiO2, carbon, polymer, and so on. For example, Au–TiO2, Au–ZrO2, Au–SnO2, and Au–SiO2 yolk–shell nanoparticles are synthesized by coating semiconductor shell on Au–oxides and subsequently etching oxides layer (Figure 18) [74–77]. In most of case, SiO2 is employed as hard template to form the middle layer and etched by NaOH, HF, or other reagents.

For example, Au–ZrO2 yolk–shell nanoparticles can also be synthesized by etching SiO2 layer of Au–SiO2–ZrO2 core–shell nanoparticles (Figure 18A) [74]. It is reported that such Au– ZrO2 yolk–shell nanostructure is stable at high temperature and can be used as catalyst for the oxidation of CO. Taking SiO2 as template, SnO2 hollow nanosphere, and Au–SnO2 yolk–shell nanoparticles can be obtained (Figure 18B). Lou et al. show their work on preparation of Au– SnO2 yolk–shell nanoparticles, etching the SiO2 layer with HF and they found that controlling the size of SiO2 template cage-like and double layer shell of Au–SnO2 can be obtained [75]. The method can also be used for synthesis of yolk–shell with other hybrid material shell [78].
