Inorganic shell nanostructures to enhance performance andstability of metal nanoparticles in catalytic applications

Inhee Choi, Hyeon Kyeong Lee, Gyoung Woo Lee, Jiyull Kim, Ji Bong Joo*

Received: 7 November 2018 / Revised: 4 December 2018 / Accepted: 3 January 2019 / Published online: 1 March 2019

� The Nonferrous Metals Society of China and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract In this article, we review the recent progress and

our research activity on the synthesis of inorganic shell

nanostructures to enhance the catalytic performance and

stability of metal nanoparticles in catalytic applications.

First, we introduce general synthetic strategies for the

fabrication of inorganic nanoscale shell layers, including

template-assisted sol-gel coating, hydrothermal (or

solvothermal) synthesis and the self-templating process.

We also discuss recent examples of metal nanoparticles

(NPs) with nanoscale shell layers, namely core–shell, yolk–

shell and multiple NPs-embedded nanoscale shell. We then

discuss the performance and stability of metal particles in

practical catalytic applications. Finally, we conclude with a

summary and perspective on the further progress of inor-

ganic nanostructure with nanoscale shell layers for cat-

alytic applications.

Keywords Nanostructures; Inorganic shell; Metal

nanoparticle; Stabilization; Performance enhancement;

Catalyst

1 Introduction

It is well known that catalysts can reduce the activation

energy of a chemical reaction, resulting in acceleration of

the rate of chemical reaction. Recent works elucidate that

reaction pathways are highly influenced by the surface

properties of catalysts, resulting in the ability to precisely

tune selectivity by using well-controlled catalysts [1–4].

Catalysts can be either homogeneous or heterogeneous,

depending on whether they are in the same phase as

reactant and product, or not. Heterogeneous catalysts,

which are generally solid materials, can play with liquid

and/or gas phase reactants. Since heterogeneous catalysts

can be easily separated and recycled from the reaction

media, a lot of practical processes, including petrochemi-

cals, semiconductor, energy and environment, consist of

heterogeneous catalytic processes.

Heterogeneous catalysts usually consist of active sites

with the support materials [5, 6]. To achieve high activity

of heterogeneous catalysts, active sites should be highly

dispersed on the surface of support materials [7–9]. Thus,

people usually synthesized the metal nanoparticles sup-

ported on inorganic support materials through a couple of

synthetic methods. Traditionally, heterogeneous metal-

supported catalysts were synthesized by impregnation, ion

exchange and precipitation methods. As the synthetic

method has continuously advanced, catalytic materials that

have various nanostructures, including nanoparticles,

nanocolloids, ordered nanoporous materials and

nanocomposites, have been synthesized and studied in

terms of both fundamental science and practical engi-

neering [10–15].

Nanostructured materials have attracted much attention

in a variety of application fields, due to their extraordinary

characteristics and enhanced performance. Over the last

few decades, as the synthetic chemistry for synthesizing the

tailored nanomaterials has developed, well-defined and

complicated nanostructures have been successfully

I. Choi

Department of Life Science, University of Seoul, Seoul 02504,

Republic of Korea

H. K. Lee, G. W. Lee, J. Kim, J. B. Joo*

Division of Chemical Engineering, Konkuk University, Seoul

05029, Republic of Korea

e-mail: jbjoo@konkuk.ac.kr

123

Rare Met. (2020) 39(7):767–783 RARE METALShttps://doi.org/10.1007/s12598-019-01203-8 www.editorialmanager.com/rmet

synthesized [16–23]. Among various nanostructures, there

has been increasing interest in nanostructures consisting of

nanoscale shell layers that can have controllable physico-

chemical properties, such as composition, crystalline

characteristics, thickness and porosity [24–26]. When some

active materials are formed as nanoscale layers, they can

provide advantageous characteristics, including relatively

large surface area-to-volume ratio per unit mass, improved

diffusion of molecules to active site and facile surface

reactions in many chemical reactions [27–30].

Various types of nanostructures consisting of nanoscale

shell layers have been studied and suggested for not only

fundamental study, but also from the viewpoint of practical

application [18, 24, 31]. By adapting the well-controlled

sol-gel reaction followed by posttreatment, tailored meso-

porous shell layers, including SiO2, TiO2 and ZrO2, have

been easily synthesized [32–38]. Other synthetic methods,

such as hydrothermal or solvothermal synthesis, allow the

production of transition metal-oxide nanoshells, such as

CuOx and FeOx [39, 40]. Similar to the sol-gel chemistry of

the SiO2 shell, resorcinol–formaldehyde (R–F) resin was

also used for the formation of microporous R–F polymer

shells that can be converted to carbon shell through the

following pyrolysis process [41]. It is also reported that the

precursor monomer of carbonizable polymer, such as

dopamine, can produce microporous shell layer on the

surface of metal-oxide particles [42].

These kinds of nanostructured shell layers can play an

important role in metal-supported composites when used as

catalysts in chemical reactions. Nanoscale shells can pro-

vide physical barriers to prevent the sintering of metal NPs

and can isolate each metal NP individually, resulting in

high catalytic activity [30, 32, 33, 43]. The formation of

nanoscale shell with well-controlled nanopore can allow

the selective diffusion of reactant chemicals that induce

selective catalytic reaction. In addition, when the core

portion is empty, the nanoscale shell can produce locally

homogenous void space, resulting in confined chemical

reaction in nanoreactors.

In this article, we intend to summarize not only our

research results, but also recent progress in the fabrication

of inorganic shell nanostructures and performance

enhancement in terms of the activity and stability of metal

catalysts. We first introduce the general synthetic concepts

for fabricating inorganic nanoscale shell layer. Then, we

demonstrate several examples of nanostructured catalysts

that consist of metal nanoparticles with nanoscale shell

layers. We also discuss the enhancement of activity and

stability of the metal–nanoscale shell catalyst in catalytic

applications. Finally, we conclude with a summary and our

perspective on the continuous progress with the challenges

of nanostructured materials in the catalysis field.

2 General synthetic concepts

2.1 Template-assisted sol-gel synthesis

Since several pioneering works were reported in the early

2000s, there have been numerous synthetic technologies

reported on the formation of nanostructured shell layers on

the surface of substrate [44–46]. One of the most common

methods for synthesizing the nanostructured shell layer is

template-assisted sol-gel synthesis (Fig. 1). In fact, tem-

plate-assisted sol-gel synthesis has been the most widely

investigated method during the past decade for depositing

nanostructured shell layers on the surface of colloidal

particles. The principle of deposition of shell layers is

straightforward and involves several sequential steps: (1)

preparing a template substrate that can provide a surface

for the deposition of the nanoscale shell layer; (2)

depositing the nanostructured shell layer through sol-gel

coating of the precursor with the controlled reaction

kinetics to form the core–shell nanostructure; and (3)

posttreatment, including crystallization, the introduction of

porosity and/or the selective removal of substrate for target

applications.

Xia and coworkers reported pioneering works in which

they synthesized polystyrene@TiO2 core–shell and

void@TiO2 hollow nanostructure by infiltrating the

hydrolyzed TiO2 precursor, followed by sequential depo-

sition of nanoscale shell on the surface of closed packed PS

(polystyrene) particles, fast evaporation of solvent and

selective removal of PS particle [46]. Liz-Marzan and

coworkers achieved a thin TiO2 shell layer on simultane-

ously formed Ag nanoparticle [44]. Yu et al. demonstrated

solid core/mesoporous silica shell particles that have per-

pendicular mesopore channels by employing Cn-TAB

(n = 12–18) chemicals as pore forming agents [47]. They

regulated mesopore orientation and pore diameter by

adapting different alkyl chain length of Cn-TAB and

changing the synthetic parameters.

Recently, we have reported a robust sol-gel coating

method for synthesizing mesoporous SiO2, TiO2, ZrO2 and

R–F polymer shell nanostructures on the surface of col-

loidal particles [32–37, 41]. In the case of TiO2 and ZrO2

shells, we used either TBOT (titanium tetrabutoxide) or

ZBOT (zirconium tetrabutoxide) as a precursor, hydroxy

propyl cellulose (HPC) as a surfactant and colloidal silica

sphere prepared by Stober method as a template in

ethanolic solution. The shell thickness of TiO2 and ZrO2

layers can be conveniently controlled through either mul-

tiple coating or controlling the synthetic parameters, such

as the concentration of surfactant and the amount of pre-

cursor (Fig. 2) [36, 37]. Figure 2a1 shows that the shell

thickness of TiO2 hollow particle is conveniently tuned by

repeating the cycles of the coating process. A single run of

123 Rare Met. (2020) 39(7):767–783

768 I. Choi et al.

TiO2 coating produces ca. 25 nm thickness. When repeat-

ing the coating step three and five times, the shell thickness

increases to ca. 50 and 75 nm, respectively (Fig. 2b1–e1).

In the case of ZrO2 hollow nanostructure, the shell thick-

ness is controlled by varying the synthetic parameters, such

as the amount of ZrO2 precursor chemical (Fig. 2a2). As

the amount of ZrO2 precursor (ZBOT) is increased from

0.2 to 1.4 ml, the shell thickness continuously increases

from ca. 21 to 62 nm (Fig. 2b2–g2). In addition, the crys-

talline properties of inorganic shell nanostructures can also

be tuned by using different strategies. We have developed

several synthetic methods for inducing the crystallinity on

Fig. 1 Schematic showing general synthetic strategy for synthesis of inorganic shell nanostructures

Fig. 2 a1 Schematic showing typical template-assisted synthesis of TiO2 hollow shell nanostructures and corresponding transmission electron

microscopy (TEM) images of TiO2 hollow shell nanostructures prepared using multiple sol-gel coating processes: b1 once, c1 three times, d1 five

times, and e1 a plot indicating relationship between number of multiple coatings and shell thickness of hollow TiO2 samples; a2 schematic

showing typical template-assisted synthesis of ZrO2 hollow shell nanostructures and corresponding TEM images of amorphous hollow ZrO2

particle, when different amounts of precursor used: b2 0.2 ml, c2 0.4 ml, d2 0.6 ml, e2 1.0 ml and f2 1.4 ml; g2 relationship between amount of

ZrO2 precursor and shell thickness of hollow ZrO2 samples. Adapted with permission from Refs. [36, 37]

123Rare Met. (2020) 39(7):767–783

Inorganic shell nanostructures to enhance performance and stability of metal nanoparticles 769

the TiO2 shell layer, in which the structure integrity is well

maintained at the same time. We have recently developed

‘‘silica-protected calcination,’’ ‘‘partial etching and recal-

cination’’ and ‘‘acid treatment followed by calcination’’

processes to produce the crystalline TiO2 shell nanostruc-

ture with well-defined hollow morphology [34–36]. The

detailed procedures and chemistry are found not only in our

research articles, but also in our recent review articles

[18, 24].

2.2 Template-assisted hydrothermal and solvothermal

synthesis

Hydrothermal and solvothermal syntheses are not only

simple and conventional, but also reliable synthetic pro-

cesses for preparing powder materials by using solution-

based media as a starting material. These synthetic meth-

ods are suitable for controlling the crystal growth and mass

production of solid materials in mild reaction conditions.

Hydrothermal and solvothermal synthetic approaches can

be employed for fabricating nanoscale shell layers, as well

as shell-based nanostructures. Lou et al. [48–50] demon-

strated template-assisted hydrothermal approaches for the

synthesis of SnO2 shell layer. They reported shell-by-shell

templating approaches under hydrothermal conditions and

successfully synthesized SnO2 shells on the surface of

colloidal silica and hollow SnO2 nanostructures, followed

by HF etching. Do et al. achieved the hollow multiple-shell

Pt-WO3/TiO2-Au nanostructure using sucrose-derived

carbon sphere as the sacrificial template under hydrother-

mal conditions (Fig. 3) [51]. They first synthesized carbon

sphere@Pt-WO3 core–shell particle through hydrothermal

synthesis by using sucrose, H2PtCl6 and Na2WO4. Then,

as-synthesized carbon sphere@Pt-WO3 was coated with

solvothermally pre-synthesized titanate nanodisk through

an electrostatic force-induced layer-by-layer deposition

method. They repeated the several cycles of electrostatic

force-induced layer-by-layer deposition to obtain the car-

bon sphere@Pt-WO3/titanium nanodisk core–shell nanos-

tructure and deposit Au precursor, followed by heat

treatment to obtain double-shell Pt-WO3/TiO2-Au nanos-

tructure (Fig. 3a) [51]. Figure 3b shows that hollow Pt-

WO3/TiO2-Au nanostructures revealed well-defined hollow

morphology of ca. 1 lm in average diameter. Au NPs with

an average size of 22 nm are evenly distributed on the

surface of the outer TiO2 layers (Fig. 3c, d).

We also demonstrated SiO2@carbon core–shell nanos-

tructure and hollow carbon sphere through the hydrother-

mal synthetic route (Fig. 4). The pre-synthesized silica

particles were treated with aluminum trichloride (AlCl3),

followed by calcination for the introduction of acidic sites,

which are catalytic sites for the acid-catalyzed polymer-

ization of carbon precursor. The aqueous mixture of AlCl3-

treated SiO2 and sucrose was charged in a stainless steel

autoclave and treated under 200 �C for 10 h to obtain sil-

ica–polymer composites. SiO2@carbon core–shell particles

and hollow carbon shell nanostructures were obtained by

the carbonization process and followed an etching process

for the removal of silica template, respectively (Fig. 4a)

[52]. Figure 4b clearly shows the aggregates of the

monodispersed carbon sphere. The high-magnification

images show that some of the carbon particles were par-

tially broken, and the inside space was empty, indicating

that they have hollow nanostructure. The average diameter

of carbon particles is estimated to be ca. 340 nm, which is

similar to that of silica template (Fig. 4c).

2.3 Self-templated or template-free synthesis

Another synthetic strategy for synthesizing inorganic

nanostructured materials having nanoscale shell layer is the

self-templated or template-free method. As one of the most

representative examples, Hu et al. [53] synthesized hollow

TiO2 shell nanostructure by heating the amorphous TiO2

solid sphere protected with polymer in diethylene glycol

(DEG) solution. DEG plays an important role as not only a

solvent, but also an etchant. When amorphous TiO2

microsphere in DEG media is heated in the presence of

protective polymer (poly acrylic acid, PAA), hollow par-

ticles with nanoscale shell layer are eventually produced.

The functional groups of PAA can be strongly intercon-

nected on the surface of solid TiO2 particles, resulting in it

acting not only as a cross-linker, but also a protective layer

to connect local surface, and allow TiO2 particles to be

maintained against rapid dissolution by hot solvent. Thus,

once DEG solvent penetrates the TiO2 particle and starts to

etch the TiO2, preferential dissolution of the core portion

can happen, resulting in hollow shell structure.

Recently, Zhang et al. [20, 29, 54] developed a simple

method for converting dense solid silica particle to hollow

shell counterparts by the self-templated method, which is

called ‘‘surface-protected etching.’’ A typical surface-pro-

tected etching process involves the following two steps: (1)

preparation of silica particle with a protective layer of

polymer chemicals, and (2) preferential dissolution of the

core portion of silica particle by using a base etchant under

well-controlled conditions (Fig. 5a). Upon

polyvinylpyrrolidone (PVP) protection as a protective layer

on the surface of silica particles, their stability is dramat-

ically enhanced against dissolution by base etching against

chemicals such as NaOH. The PVP, which cross-links the

subunit of silica surface by strong binding between surface

OH groups and carbonyl groups of the PVP, allows NaOH

molecules to diffuse into the silica particle and to dissolve

oxide-rich area [55]. It makes the outer portion of the silica

particle to retain its original particle dimension through the

123 Rare Met. (2020) 39(7):767–783

770 I. Choi et al.

multiple strong interactions between the surfaces of the

silica subunit and PVP molecules. Hence, unprotected core

particles of silica are gradually dissolved out, resulting in

nanostructured hollow shell layers. Figure 5b, c shows

TEM images, which reveal that after etching for ca. 1 h,

the original monodispersed solid particle becomes porous.

As the etching time is elongated to 2.75 and 3.00 h, the

interior of the silica particle becomes more porous, and

upon continued etching, the hollow sphere can be produced

(Fig. 5d, e).

As another method, the nanostructured silica shell layer

can also be synthesized by spontaneous dissolution, fol-

lowed by the regrowth process. It is well known that

amorphous silica colloids dispersed in an aqueous solution

containing weak base chemicals, such as Na2CO3 or

NaBH4, can spontaneously change from solid particle to

nanostructured hollow shell [56, 57]. The dissolution of

silica colloids appears due to high pH resulting from

Na2CO3 or NaBH4, in which silica can be decomposed to

soluble silicate species. When the concentration of silicate

species is continuously increased, and the solution is

eventually supersaturated at a certain condition, silicate

species are preferably precipitated and re-deposited on the

surface. When both spontaneous dissolution and re-depo-

sition of silica particle occur at the same time, solid silica

particle can be converted to core–shell or yolk–shell

nanostructure and then finally ends with the formation of

nanoscale hollow shell.

Ostwald ripening is also one of the classical phenomena

in small solid dispersion or liquid sol, in which small

particles dissolve and regrow on the surface of larger ones.

Yang and Zeng [58] proposed the Ostwald ripening

mechanism for the formation of hollow titanium dioxide

shell nanostructures from TiF3 under hydrothermal condi-

tions. They also extended the Ostwald ripening phenomena

to the synthesis of hollow metal-doped TiO2 nanostructures

through a similar hydrothermal process [59]. Archer et al.

also carried out pioneering work in which they synthesized

a nanostructured SnO2 shell by an inside–out Ostwald

ripening under hydrothermal conditions [49, 60]. Spherical

CeO2 hollow particles can easily be synthesized through

the Ostwald ripening mechanism. Zhang et al. demon-

strated porous CeO2 hollow nanostructures that were syn-

thesized in mixed solvent conditions, including ethylene

glycol, acetic acid and water under solvothermal conditions

[61]. They suggested that hollow CeO2 shell nanostructure

can be formed by following several sequential steps during

solvothermal synthesis. At the first stage, CeO2 nanopar-

ticles are initially formed through the hydrolysis and oxi-

dation of CeO2 precursor under solvothermal conditions

Fig. 3 a Schemes of synthesis of hollow multiple-shell Pt-WO3/TiO2-Au nanostructure: (1) one-pot hydrothermal synthesis of Pt-WO3@carbon

spheres, (2) coating with titanate nanodisk using a layer-by-layer strategy, followed by Au loading (3) and (4) calcination and heat treatment; b,

c TEM images of hollow H:Pt-WO3/TiO2-Au; d high-resolution TEM image indicating Au nanoparticle on surface of H:Pt-WO3/TiO2-Au

sample. Adapted with permission from Ref. [51]

123Rare Met. (2020) 39(7):767–783

Inorganic shell nanostructures to enhance performance and stability of metal nanoparticles 771

(Fig. 6b). Once the primary CeO2 nanoparticles are

formed, they initially lead to aggregation of the solid par-

ticle (Fig. 6c). Then, the hollowing process can happen by

Ostwald ripening and self-assembly, resulting in meso-

porous CeO2 hollow shell nanostructures, when continu-

ously elongating the reaction times under hydrothermal

conditions (Fig. 6d, e). Cai et al. also synthesized the CeO2

hollow nanoshell through Ostwald ripening via a micro-

wave-assisted aqueous hydrothermal process [62]. The

ripening phenomena can be extended to the synthesis of

other inorganic oxide shell nanostructure, including

cuprous oxide (Cu2O), cobalt oxide (Co3O4) and many

others [39, 63, 64].

3 Metal–nanostructured shell catalyst

3.1 Single nanoparticle–nanoscale shell

During the past decade, there have been a lot of studies on

the synthesis and catalytic applications of single nanopar-

ticle-supported nanoscale shell structures, which are the so-

called core–shell or yolk–shell nanostructures. Most of

these efforts are devoted to synthesizing metal nanoparticle

(NP) core with inorganic oxide shell. In general, the single

metal NP@oxide core–shell or yolk–shell nanostructure is

generated through several synthetic approaches, as shown

in Fig. 7. Pre-synthesized metal NP can be coated with

oxide materials, such as SiO2, to construct metal@oxide

core–shell nanostructures. Metal@oxide core–shell nanos-

tructures can then be transformed to their yolk–shell

counterparts through either self-hollowing of the oxide

layer, or core etching of the metal nanoparticle. As another

approach, metal@oxide core–shell nanostructure can be

applied to the coating of other inorganic materials, such as

TiO2, ZrO2 or carbon, to obtain core–shell–shell nanos-

tructure. If intermediate oxide shell layer is rapidly dis-

solved out compared to the outer one, we could achieve

selective etching of the intermediate layer, resulting in

metal@oxide yolk–shell structure.

Since we have established the reliable coating procedure

of various materials on nanoscale colloidal particles, we

have successfully achieved the practical synthesis of

metal@oxide yolk–shell nanostructure, which can have

various combinations of core and shell materials. Figure 8

demonstrates a model system of yolk–shell nanostructure

that consists of Au nanoparticle as core, with either SiO2 or

TiO2 as shell. Colloidal Au NPs can be easily synthesized

by the citrate reduction method in hot water, and stabilized

by a surfactant, such as PVP. For fabricating Au@SiO2

yolk–shell nanostructure, we chose resorcinol–formalde-

hyde (R–F) resin material as the intermediate sacrificial

Fig. 4 a Schematic indicating template-assisted hydrothermal synthesis of hollow carbon sphere and corresponding b scanning electron

microscopy (SEM) images and c TEM image of hollow carbon sphere. Adapted with permission from Ref. [52]

123 Rare Met. (2020) 39(7):767–783

772 I. Choi et al.

layer. Recently, it has been reported that R–F polymer has

similar sol-gel chemistry compared with SiO2, resulting in

either the monodispersed R–F resin microspheres being

successfully synthesized or the coating of colloidal particle

with nanoscale R–F layer being easily carried out, though

an extension of the Stober method [41, 65]. Figure 8b

shows that Au NPs can be coated with uniform R–F layer

to produce Au@R–F core–shell structure. The thickness of

R–F layer can be conveniently tuned by controlling syn-

thetic parameters, such as the R–F precursor’s concentra-

tion, water-to-ethanol ratio and addition of surfactant

[25, 65]. After the coating of SiO2 layer on the surface of

Au@R–F core–shell particle through the modified Stober

method, Au@R–F@SiO2 core–shell–shell nanostructures

were obtained. When Au@R–F@SiO2 particles were cal-

cined under air conditions, well-defined Au@SiO2 yolk–

shell nanostructures were obtained, due to burning out of

the intermediate R–F layer (Fig. 8c). The intermediate R–F

layer can be replaced by other inorganic materials, such as

SiO2 for fabricating Au@other oxide (e.g., TiO2) yolk–

shell nanostructure (Fig. 8d). For synthesizing Au@TiO2

yolk–shell, PVP-stabilized Au NPs can first be coated with

SiO2 to form Au@SiO2 core–shell structure. Figure 8e

shows that Au NPs can be encapsulated by uniform SiO2

layer. Similar to R–F, the thickness of SiO2 layer can be

tuned by varying the synthetic conditions. When Au@SiO2

particles are then coated with TiO2 layer through the sol-

gel reaction of TiO2 precursor, such as titanium butoxide,

under controlled conditions, Au@SiO2@TiO2 core–shell–

shell nanostructures can be obtained. Since the dissolution

kinetics of SiO2 on base conditions is much faster than that

of TiO2, the intermediate SiO2 layer can be preferentially

dissolved out in aqueous NaOH solution, resulting in

Au@TiO2 yolk–shell nanostructure (Fig. 8f).

Consistent with other nanostructure, the physicochemi-

cal characteristics of single metal–shell nanostructure, such

as core size, void size, shell thickness and crystallinity, can

be controlled by varying the synthetic conditions. Recently,

Lee et al. [66] have reported the relationship between

geometrical parameters of Au@TiO2 yolk–shell particles

Fig. 5 a Schematic showing concept of ‘‘surface-protected etching’’

for transforming solid SiO2 particles into permeable hollow shells;

corresponding TEM images showing morphology of SiO2 particles

after etching by NaOH for b 0 h, c 1.00 h, d 2.75 h and e 3.00 h.

Adapted with permission from Ref. [29]

Fig. 6 a Schematic indicating the formation of CeO2 hollow spheres

through Ostwald ripening process; TEM images indicating morphol-

ogy change of CeO2 particles obtained at different reaction time: b 2

h, c 4 h, d 6 h and e 8 h. Adapted with permission from Ref. [61]

123Rare Met. (2020) 39(7):767–783

Inorganic shell nanostructures to enhance performance and stability of metal nanoparticles 773

and photocatalytic activity. The geometrical parameters of

Au@TiO2 yolk–shell nanostructure, including metal size,

void space and TiO2 shell thickness, are systemically var-

ied by changing the synthetic parameters. Specifically, the

diameter of void space inside the TiO2 shell can be con-

trolled by the concentration of tetraethyl orthosilicate

(TEOS) used during the growth of silica layer. The thick-

ness of TiO2 shell layer can be varied by repeating the TiO2

coating step, and the size of Au nanoparticle can be con-

trolled by seed-growing the original trapped Au nanopar-

ticle. Figure 9 shows that the shell thickness is varied from

ca. 14 to 80 nm, void diameter is varied from ca. 50 to

350 nm, and the Au NP size is varied from ca. 18 to

100 nm, respectively.

3.2 Multiple nanoparticles–nanoscale shell

To obtain high catalytic activity, a heterogeneous catalyst

must have several requirements, which are well-defined

active sites with large active surface area, high dispersion,

large resistance to thermal sintering and stability for long-

term operation. Although single metal NP@oxide core–

shell nanostructures are well-defined structure and attract a

lot of attention, other shell-based catalysts, which consist

of a large number of metal nanoparticles with nanoscale

shell layer, are more useful for practical purposes. Yin and

coworkers suggested the concept of a core–satellite catalyst

that consists of a monolayer of metal nanocatalysts

immobilized on the surface of SiO2 core (Fig. 10a)

[54, 67]. A typical procedure for immobilizing metal

nanoparticles involves the surface modification of silica

particle with 3-aminopropyltriethoxysilane (3-APTES),

followed by the controlled attachment of metal NPs

through chemical adsorption between amine groups and

metal. The SiO2 surface modified with metal nanoparticles

is then overcoated with another layer of silica, as shown in

Fig. 10b. When metal nanoparticles are overcoated with

dense silica layer, since there is poor accessibility and slow

surface reaction, the layer should become porous, allowing

reactant molecules to reach the metal surface, and pro-

tecting metal nanoparticles from metal sintering during

heat-treatment or catalysis reactions. Thus, the surface-

protected etching is applied to convert the dense outer layer

into porous shell nanostructure. Finally, well-defined

Fe3O4@SiO2@multiple Au NPs@porous SiO2 nanostruc-

tures were obtained (Fig. 10c). The loading of Au

nanocatalyst can be well controlled by varying the amount

of Au NP added during the adsorption step, and the

porosity of the outer silica layer can be tuned by elongating

the etching time. As discussed in the previous section, due

to the surface protection of PVP, the thickness of outer

silica layer does not show any apparent change, until

severe etching occurs. Final Fe3O4@SiO2@multiple Au

Fig. 7 Schematic showing synthetic strategies for preparing single

metal nanoparticle-oxide shell (core–shell or yolk–shell) nanostruc-

tures by several approaches

Fig. 8 a Schematic indicating synthesis of Au@SiO2 yolk–shell

nanostructure and corresponding TEM images of b Au@R–F core–

shell and c Au@SiO2 yolk–shell nanostructure; d schematic indicat-

ing synthesis of Au@TiO2 yolk–shell nanostructure and correspond-

ing TEM images of e Au@SiO2 core–shell and f Au@TiO2 yolk–

shell nanostructure

123 Rare Met. (2020) 39(7):767–783

774 I. Choi et al.

NPs@porous SiO2 nanostructures showed advantageous

characteristics, in which Au NPs are highly dispersed in

porous silica layers. The porous silica layer not only allows

reactant molecules to access the Au NPs for favorable

surface reaction, but also provides a physical barrier to

prevent Au sintering. Thus, it showed enhanced catalytic

performance in terms of the stability of Au NPs and

reaction recyclability under multiple runs of liquid phase

4-nitrophenol reduction, discussed in the next section.

Similar to the previous case, we also demonstrated

thermally stable Au nanocatalysts encapsulated in meso-

porous layer, by introducing cetyl trimethylammonium

bromide (CTAB)-derived mesoporous silica on the surface

of Au-decorated Fe3O4@SiO2 particles [32]. Although the

surface-protected etching technique provides well-defined

mesoporous outer shell layer, the porosity must be care-

fully controlled by monitoring the degree of etching. If the

etching significantly proceeds without any monitoring in

solution, the outer layer is continuously etched out, and

finally disappears by complete dissolution. Sometimes the

surface-protected etching process is highly dependent on

the experimental conditions and individuals. To fabricate

reliable mesoporous outer shell, we thus carried out over-

coating Au NPs with CTAB-derived mesoporous silica,

followed by calcination [32]. Figure 11a shows the syn-

thetic procedure for preparing Fe3O4@SiO2-Au@mSiO2

sample. Hydrothermally synthesized Fe3O4 particles are

coated with silica layer to produce Fe3O4@SiO2 core–shell

structure. After Au nanoparticles are decorated on the

surface of APTES-functionalized Fe3O4@SiO2, CTAB-

derived mesoporous silica layer, which has a perpendicular

pore structure, was introduced. Direct calcination under air

conditions gives Fe3O4@SiO2-Au@mSiO2 core–shell–

shell nanostructure. Additional water treatment makes the

outer mesoporous silica layer more porous, resulting in

large surface area and high pore volume. This allows

reactant molecules not only to be more accessible to the

encapsulated Au NPs, but also to be easily reacted on the

surface of Au NPs. Figure 11b shows TEM and SEM

images of our Fe3O4@SiO2-Au@mSiO2 sample, indicating

well-defined core–shell nanostructure. In addition, evenly

distributed and encapsulated Au nanoparticles in the por-

ous silica layer can be observed without any aggregation.

Figure 11c shows Fe3O4@SiO2-Au@mSiO2 sample which

reveals continuous N2 adsorption in the range of * 0.5 P/

P0 (relative pressure), indicating the presence of mesopore,

while control sample Fe3O4@SiO2-Au showed negligible

adsorption uptake. Owing to the presence of well-

Fig. 9 Typical TEM images of Au@Void@TiO2 nanostructures. Shown are examples for three sets made, namely, for Z@Y@X samples with

varying TiO2 shell thickness (X nm), TiO2 shell inner void diameter (Y nm) and gold nanoparticle diameter (Z nm). Adapted with permission

from Ref. [66]

Fig. 10 a Schematic showing synthetic procedures for fabrication of

SiO2/Au/SiO2 catalysts; TEM images of SiO2/Au/SiO2 catalysts

b before and c after surface-protected etching. Adapted with

permission from Refs. [54]

123Rare Met. (2020) 39(7):767–783

Inorganic shell nanostructures to enhance performance and stability of metal nanoparticles 775

developed mesopore, Fe3O4@SiO2-Au@mSiO2 and water-

treated Fe3O4@SiO2-Au@mSiO2-H2O showed relatively

high surface area values of 508 and 398 m2�g-1, respec-

tively, while Fe3O4@SiO2-Au sample had a low surface

area value (13 m2�g-1). As discussed in the next section,

Fe3O4@SiO2-Au@mSiO2 samples having well-developed

mesoporosity show improved molecule diffusion and

enhanced reaction kinetics.

4 Catalytic applications

Metal-nanostructured shells have been used as novel

nanocatalysts in many chemical reactions, because during

chemical reactions, they have several beneficial effects.

Metal–nanoscale shell colloidal nanostructures can

enhance the accessibility of the reactant to the metal core,

because the thickness of the porous shell layer is in the

range of 10–100 nm, resulting in a short diffusion pathway.

In addition, a metal nanoparticle can have high stability

against thermal sintering or aggregation, due to the pro-

tective function of the shell layer. Yolk–shell particle can

Fig. 11 a Schematic showing synthetic procedures for preparing Fe3O4@SiO2-Au@mSiO2 samples in which outer silica layer has cylindrical

pore structures by coating with CTAB-derived mesoporous silica, followed by calcination; b TEM and SEM images of water-treated

Fe3O4@SiO2-Au@mSiO2-H2O; c N2 adsorption–desorption isotherms of Fe3O4@SiO2-Au, Fe3O4@SiO2-Au@mSiO2 and Fe3O4@SiO2-

Au@mSiO2-H2O. Adapted with permission from Ref. [32]

123 Rare Met. (2020) 39(7):767–783

776 I. Choi et al.

provide a locally homogeneous reaction environment in the

void space of each individual particle, resulting in the

minimization of interference of the neighboring particles.

Each individual particle having a single active metal,

meaning that the number of active sites is limited, allows

the relationship between each physiochemical property of

the particles and the catalytic performance to be systemi-

cally investigated. Here, we discuss some recent results on

the catalytic applications and performance enhancement of

metal NPs/nanostructured shell catalysts, including single

metal nanoparticle/oxide shell yolk–shell particle, meso-

porous SiO2 shell-based inorganic micelle and multiple

nanoparticles–nanoscale shell nanostructures.

With the aid of the synthetic strategies discussed in the

previous section, Au NPs@TiO2 yolk–shell nanostructures

have been successfully prepared [30]. Figure 12a shows

TEM image, in which Au@TiO2 yolk–shell nanostructure

consists of an Au NP particle individually encased in a

TiO2 shell with * 200 nm in diameter and * 20 nm in

thickness. Although the Au@TiO2 yolk–shell catalyst is

calcined at high temperature up to 775 K, it shows similar

dimension of Au NP core, and no change of the structural

integrity of the TiO2 shell (Fig. 12 b). In practical Au NPs

catalysis, one of the most severe problems is thermal sin-

tering during either high temperature calcination, or cat-

alytic reaction. Au NPs tend to sinter and grow into big

particles, resulting in losing the unique catalytic activity

that is observed in the original particles. Since small Au

nanoparticle is encapsulated and protected by TiO2 shell

layer in Au@TiO2 yolk–shell particles, there is no change

of dimension and shape of the Au NPs. However, the Au/

TiO2-P25 catalyst, in which pre-synthesized Au NPs are

supported on commercial P25-TiO2, displays significant

metal sintering and growth to large Au NPs under heat

treatment at 775 K (Fig. 12c, d). The diffusion and surface

reaction in Au@TiO2 yolk–shell nanocatalyst are evaluated

by conducting gas phase CO adsorption and CO oxidation.

CO molecules are clearly observed to be adsorbed on the

surface of Au NPs. This indicates that CO molecules can

diffuse inwards through the TiO2 shell. The catalytic

activity toward CO oxidation by using the Au@TiO2 yolk–

shell catalyst was also evaluated. It is considerably active

in promoting the oxidation of CO and demonstrates a

higher turn of frequency (TOF) value than the conventional

Au/TiO2-P25 catalyst (Fig. 12e). In addition to the above

example, we have also studied photocatalytic hydrogen

production by using Au@TiO2 yolk–shell nanostructures

that have different physicochemical characteristics, such as

core size, void size, shell thickness and TiO2 crystallinity

[66, 68]. It is generally known that the photocatalysis

activity of TiO2-based catalyst is significantly dependent

on the crystalline property. Indeed, the hydrogen produc-

tion performance of Au@TiO2 yolk–shell is highly

influenced by the crystallinity of the TiO2 shell. We have

been systemically studying and finding out the relationship

between photoluminescence lifetime decay, hydrogen

production rate and crystalline properties [68].

Recently, Zhang et al. [69] suggested the interesting

concept of nanoscale shell-based ‘‘inorganic micelle’’ cat-

alyst, which has different hydrophilic and hydrophobic

functional groups on the inner void space and outer surface

of particle, respectively. To achieve selective surface

functionalization, CTAB micelles were first explored as a

pore-blocking agent during the surface functionalization

step using a hydrophobic chemical and then removed, to

open the mesopore channel, enabling the remaining inner

void surface to be hydrophilic. Figure 13a shows the con-

cept of hollow shell or yolk–shell type ‘‘inorganic

micelle,’’ in which the SiO2 shells are modified to

hydrophobic–hydrophilic interfaces. When Au@SiO2

inorganic micelle was used as a catalyst in 4-nitrophenol

reduction in aqueous phase, it showed slower reaction

kinetics than its pristine hydrophilic Au@SiO2 counterpart.

This indicates that the hydrophobic outer surface of micelle

structure influences either the dispersion of Au@SiO2

micelle particle in aqueous reaction media or the diffusion

of reactant molecule. They also demonstrated that the

inorganic micelle particle can be used as highly efficient

catalysts in the liquid phase catalyst in organic solvent. The

bromination of alcohols was tested in organic solvents,

such as dichloromethane (CH2Cl2), because alkyl halides

are valuable chemicals in organic chemistry. With the help

of hollow SiO2 and Au@SiO2 yolk–shell inorganic

micelles, not only the rate of bromination of benzyl alcohol

and a-methylbenzyl alcohol is accelerated, but also over

90% yields are obtained (Fig. 13b, c).

Another example of the stabilization effect and catalytic

efficiency enhancement of using metal/nanostructured

oxide shell is multiple metal nanoparticles encapsulated in

porous silica layer, which is prepared by surface-protected

etching process. As mentioned before, mesoporous SiO2

framework can effectively stabilize the embedded Au NPs

and prevent the efficiency reduction in catalysis caused by

aggregation or detachment of nanoparticles. The structural

stability and catalytic activity of unprotected Fe3O4/SiO2/

Au nanocomposites are severely decreased by agglomer-

ating the Au NPs after six successive cycles of 4-nitro-

phenol reduction. In the first cycle of 4-nitrophenol

reduction, the unprotected Fe3O4/SiO2/Au catalyst showed

high activity in terms of kinetics and conversion, as all the

Au NPs on the core surface contribute to the catalysis. The

catalytic activity of unprotected Fe3O4/SiO2/Au catalyst is

continuously decreased during the six successive runs of

catalytic reaction, since Au NPs are gradually detached out,

and the active surface area of Au NPs is dramatically

decreased (Fig. 14a). However, Au catalysts-protected

123Rare Met. (2020) 39(7):767–783

Inorganic shell nanostructures to enhance performance and stability of metal nanoparticles 777

Fig. 12 TEM images of a, b Au@TiO2 catalyst and c, d 1 wt% Au/TiO2-P25 reference sample, all shown as prepared a, c and after calcination

at 775 K b, d, where thermal sintering of the nanoparticles in Au/TiO2-P25 catalysts is indicated by red circles; e time dependence of CO

coverage on Au (H) and CO2 partial pressure (P) during oxidation of CO with O2. The first and third panels were obtained by first introducing

26.66 kPa of CO into the cell and then adding 26.66 kPa of O2; while in the second and fourth panels, the sequence was reversed. Adapted with

permission from Ref. [30]

123 Rare Met. (2020) 39(7):767–783

778 I. Choi et al.

porous SiO2 shell (Fe3O4/SiO2/Au/porous SiO2) showed

high metal dispersion, without any loss of Au NPs and

deformation of structural integrity. Fe3O4/SiO2/Au/porous

SiO2 catalysts maintained their activity well for successive

cycles of chemical reaction, with a slight drop of 4-nitro-

phenol conversion (Fig. 14b).

Our group also demonstrated the stabilization effect of

mesoporous oxide shell on metal nanoparticle catalysts. As

Fig. 13 a Schematics of hollow SiO2 micelle and Au@SiO2 micelles; catalytic activities for bromination of b benzyl alcohol and c a-

methylbenzyl alcohol over various catalysts. Adapted with permission from Ref. [69]

Fig. 14 Catalytic conversion results of 4-nitrophenol over a Fe3O4/SiO2/Au catalyst and b Fe3O4/SiO2/Au/porous SiO2 catalyst as a function of

reaction time in six successive cycles. Adapted with permission from Ref. [67]

123Rare Met. (2020) 39(7):767–783

Inorganic shell nanostructures to enhance performance and stability of metal nanoparticles 779

previously discussed, we have demonstrated Au NPs

encapsulated in mesoporous oxide layer, which is prepared

by either SiO2 overcoating followed by base etching or the

formation of CTAB-derived mesoporous SiO2 layer fol-

lowed by calcination. As expected, the Fe3O4@SiO2-Au

catalyst that has exposed Au NP showed the highest

reaction kinetics (k is ca. 0.223 min-1) in the first run of

catalytic reaction. Fe3O4@SiO2-Au@nSiO2, which is pre-

pared by overcoating with nonporous silica layer, showed

the lowest rate constant (k is ca. 0.00034 min-1).

Fe3O4@SiO2-Au@mSiO2 and water-etched Fe3O4@SiO2-

Au@mSiO2-H2O samples showed reasonable k value, such

as 0.0771 and 0.104 min-1, respectively. We also evalu-

ated the stability and recyclability of each catalyst by

conducting multiple runs of catalytic reaction. While

Fe3O4@SiO2-Au catalyst showed the highest conversion in

the first cycle, the value rapidly decreased and finally

reached\ 10% in the successive five reaction runs

(Fig. 15a). Since the reactant could not diffuse inward to

the active metals due to the dense silica layer, the

Fe3O4@SiO2-Au@nSiO2 displayed negligible activity

during five reaction runs (Fig. 15b). Fe3O4@SiO2-

Au@mSiO2 and Fe3O4@SiO2-Au@mSiO2-H2O samples

showed stable conversion values during five successive

runs (Fig. 15c, d). The Fe3O4@SiO2-Au@mSiO2-H2O

sample showed the best performance in the recycling test,

in terms of reaction kinetics and stability. It should be

noted that introducing a mesoporous SiO2 followed by

calcination and water treatment provides a large amount of

active Au NPs encapsulated porous layer without metal

sintering. The porous layer allows facile molecule diffusion

and retains Au NPs without detachment during multiple

reaction runs, resulting in the most stable recyclability with

enhanced activity, while exposed Au NPs in Fe3O4@SiO2-

Fig. 15 Cycling experimental results of different catalysts: a Fe3O4@SiO2-Au, b Fe3O4@SiO2-Au@nSiO2, c Fe3O4@SiO2-Au@mSiO2 and

d water-treated Fe3O4@SiO2-Au@mSiO2-H2O (insets being TEM images of each catalyst). Adapted with permission from Ref. [32]

123 Rare Met. (2020) 39(7):767–783

780 I. Choi et al.

Au sample is easily detached out by destabilization of Au-

APTES interaction during the 4-NP reduction, resulting in

the continuous decrease in the number of active Au,

leading to low conversion.

5 Summary and outlook

During the past decades, extensive progress has been made

on the synthesis, property control and practical applications

of nanostructured materials, as synthetic chemistry and

characterization tools have been continuously developed.

In the past few years, we have studied the synthesis and

catalytic applications of novel metal-oxide shell nanos-

tructures. In this mini-review, we briefly review our

research results and the recent progress in metal-inorganic

shell nanostructures for catalytic applications. First, we

introduce the general synthetic methodology for preparing

nanostructured inorganic shell layer, including template-

assisted sol-gel chemistry, template-assisted hydrothermal

synthesis and the self-templating process. We also discuss

two representative concepts of metal NPs/inorganic shell

nanostructures, namely core (or yolk)–shell and multiple

NPs encapsulated in nanoscale shell. Finally, we discuss

the stability and performance enhancement of metal-inor-

ganic shell nanostructures in practical catalysis.

The examples discussed in this article represent not only

our efforts, but also recent progress in the engineering of

inorganic shell nanostructures for designing highly active

and sustainably stable catalysts. Although there have been

pioneering works and advanced studies in this research

field, extensive research works are still necessary to

address many important challenges in practical catalysis

industries. The first challenge is that chemical reaction over

metal NPs/nanoscale shell nanostructures is limited within

simple model reactions. Even though we discussed simple

catalytic applications, such as CO oxidation, bromination

of benzyl alcohol in organic solvent and 4-nitrophenol

reduction, the use of inorganic shell nanostructure catalysts

should be extended to other important catalytic reactions,

such as hydrogenation, partial oxidation and condensation.

Recently, there have been several pioneering works, in

which more-complicated nanostructured catalysts have

been fabricated and used as a catalyst for other important

chemical reactions, such as the Suzuki coupling reaction,

epoxidation and olefin hydrogenation [37, 70–72]. We

believe that these efforts will have positive effects on the

use and extension of nanostructured catalysts in the prac-

tical catalysis field. The second challenge is that inorganic

shell nanostructure catalysts should have not only the

improved activity and sustainable stability, but also highly

tunable selectivity on a targeted chemical reaction. One

intuitive approach for improving selectivity is that metal

nanoparticles can be fabricated with well-controlled shape

and introduced to shell nanostructures. There have been

well-known examples, including shape-controlled Pt, Pd

and other nanocrystals, which have demonstrated signifi-

cantly improved selectivity during chemical reactions

[2, 3, 10]. As another synthetic approach, some other

functional groups, such as acidic site or base site, can be

introduced to the surface of nanostructured shell [37],

resulting in improved selectivity toward targeted reactions,

such as dehydration, isomerization and condensation. One

drawback to be overcome is the need to develop suit-

able synthetic methods either to encapsulate shape-con-

trolled nanoparticles with exposed active surface or to

selectively introduce other active sites on the targeted

surface. In addition, there are also the important issues of

how to maintain the shape of metal nanoparticle and sta-

bility of active site during posttreatment and catalysis. The

final challenge is the mass production of inorganic shell

nanostructure catalysts. Although inorganic nanostructured

catalysts showed unique catalytic properties at laboratory

scale, the production amount of the catalyst is limited to the

scale of several milligrams to grams. Mass production is

still one of the most challengeable works. Once suit-

able synthetic processes for preparing large amounts of

nanostructured catalysts are developed, and are tested in

either bench, pilot or commercial scale reactor, it is

believed that nanostructured catalysts will contribute to

solving many drawbacks in practical catalysis industries.

Acknowledgements This work is supported by the Korea Institute of

Energy Technology Evaluation and Planning (KETEP) and the

Ministry of Trade, Industry and Energy (MOTIE, No.

20174010201490). This work is also financially supported by the

Korea Environment Industry & Technology Institute (KEITI) through

‘‘The Chemical Accident Prevention Technology Development Pro-

ject’’ granted by the Korea Ministry of Environment (MOE, No.

2017001960004).

References

[1] Lee H, Habas SE, Kweskin S, Butcher D, Somorjai GA, Yang P.

Morphological control of catalytically active platinum

nanocrystals. Angew Chem Int Ed. 2006;45(46):7824.

[2] Bratlie KM, Lee H, Komvopoulos K, Yang P, Somorjai GA.

Platinum nanoparticle shape effects on benzene hydrogenation

selectivity. Nano Lett. 2007;7(10):3097.

[3] Lee I, Delbecq F, Morales R, Albiter MA, Zaera F. Tuning

selectivity in catalysis by controlling particle shape. Nat Mater.

2009;8:132.

[4] Lee I, Zaera F. Catalytic conversion of olefins on supported

cubic platinum nanoparticles: selectivity of (100) versus (111)

surfaces. J Catal. 2010;269(2):359.

[5] Kim P, Joo JB, Kim H, Kim W, Kim Y, Song IK, Yi J.

Preparation of mesoporous Ni–alumina catalyst by one-step

sol–gel method: control of textural properties and catalytic

application to the hydrodechlorination of o-dichlorobenzene.

Catal Lett. 2005;104(3):181.

123Rare Met. (2020) 39(7):767–783

Inorganic shell nanostructures to enhance performance and stability of metal nanoparticles 781

[6] Kim P, Kim H, Joo JB, Kim W, Song IK, Yi J. Effect of nickel

precursor on the catalytic performance of Ni/Al2O3 catalysts in

the hydrodechlorination of 1,1,2-trichloroethane. J Mol Catal A

Chem. 2006;256(1–2):178.

[7] Kim P, Joo JB, Kim W, Kim J, Song IK, Yi J. Preparation of

highly dispersed Pt catalyst using sodium alkoxide as a reducing

agent and its application to the methanol electro-oxidation.

J Mol Catal A Chem. 2007;263(1–2):15.

[8] Joo JB, Kim P, Kim W, Yi J. Preparation and application of

mesocellular carbon foams to catalyst support in methanol

electro-oxidation. Catal Today. 2008;131(1–4):219.

[9] Kim P, Joo JB, Kim W, Kim J, Song IK, Yi J. NaBH4-assisted

ethylene glycol reduction for preparation of carbon-supported Pt

catalyst for methanol electro-oxidation. J Power Sources. 2006;

160(2):987.

[10] Kim C, Lee H. Shape effect of Pt nanocrystals on electrocat-

alytic hydrogenation. Catal Commun. 2009;11(1):7.

[11] Jia CJ, Schuth F. Colloidal metal nanoparticles as a component

of designed catalyst. Phys Chem Chem Phys. 2011;13:2457.

[12] Mahmoud MA, Tabor CE, El-Sayed MA, Ding Y, Wang ZL. A

new catalytically active colloidal platinum nanocatalyst: the

multiarmed nanostar single crystal. J Am Chem Soc. 2008;

130(14):4590.

[13] Taguchi A, Schuth F. Ordered mesoporous materials in cataly-

sis. Micropor Mesopor Mat. 2005;77(1):1.

[14] Li W, Wu Z, Wang J, Elzatahry AA, Zhao D. A perspective on

mesoporous TiO2 materials. Chem Mater. 2014;26(1):287.

[15] Davidson M, Ji Y, Leong GJ, Kovach NC, Trewyn BG, Richards

RM. Hybrid mesoporous silica/noble-metal nanoparticle mate-

rials—synthesis and catalytic applications. ACS Appl Nano

Mater. 2018;1(9):4386.

[16] Zhang Q, Joo JB, Lu Z, Dahl M, Oliveira D, Ye M, Yin Y.

Self-assembly and photocatalysis of mesoporous TiO2

nanocrystal clusters. Nano Res. 2011;4(1):103.

[17] Park J, Song H. Metal@Silica yolk–shell nanostructures as

versatile bifunctional nanocatalysts. Nano Res. 2011;4(1):33.

[18] Zhang Q, Lee I, Joo JB, Zaera F, Yin Y. Core–shell nanos-

tructured catalysts. Acc Chem Res. 2012;46(8):1816.

[19] Chen D, Cao L, Huang F, Imperia P, Cheng YB, Caruso RA.

Synthesis of monodisperse mesoporous titania beads with con-

trollable diameter, high surface areas, and variable pore diam-

eters (14–23 nm). J Am Chem Soc. 2010;132(12):4438.

[20] Zhang Q, Ge J, Goebl J, Hu Y, Lu Z, Yin Y. Rattle-type silica

colloidal particles prepared by a surface-protected etching pro-

cess. Nano Res. 2009;2(7):583.

[21] Yun HJ, Lee H, Joo JB, Kim W, Yi J. Influence of aspect ratio of

TiO2 nanorods on the photocatalytic decomposition of formic

acid. J Phys Chem C. 2009;113(8):3050.

[22] Yin Y, Rioux RM, Erdonmez CK, Hughes S, Somorjai GA,

Alivisatos AP. Formation of hollow nanocrystals through the

nanoscale kirkendall effect. Science. 2004;304(5671):711.

[23] Yang Z, Niu Z, Lu Y, Hu Z, Han CC. Templated synthesis of

inorganic hollow spheres with a tunable cavity size onto

core–shell gel particles. Angew Chem Int Ed. 2003;42(17):1943.

[24] Joo JB, Dahl M, Li N, Zaera F, Yin Y. Tailored synthesis of

mesoporous TiO2 hollow nanostructures for catalytic applica-

tions. Energ Environ Sci. 2013;6:2082.

[25] Joo JB, Liu H, Lee YJ, Dahl M, Yu H, Zaera F, Yin Y. Tailored

synthesis of C@TiO2 yolk–shell nanostructures for highly effi-

cient photocatalysis. Catal Today. 2016;264(15):261.

[26] Moon GD, Joo JB, Dahl M, Jung H, Yin Y. Nitridation and

layered assembly of hollow TiO2 shells for electrochemical

energy storage. Adv Funct Mater. 2014;24(6):848.

[27] Liang X, Li J, Joo JB, Gutierrez A, Tillekaratne A, Lee I, Yin Y,

Zaera F. Diffusion through the shells of yolk–shell and

core–shell nanostructures in the liquid phase. Angew Chem Int

Ed. 2012;51(32):8034.

[28] Li J, Liang X, Joo JB, Lee I, Yin Y, Zaera F. Mass transport

across the porous oxide shells of core–shell and yolk–shell

nanostructures in liquid phase. J Phys Chem C. 2013;117(39):

20043.

[29] Zhang Q, Zhang T, Ge J, Yin Y. Permeable silica shell through

surface-protected etching. Nano Lett. 2008;8(9):2867.

[30] Lee I, Joo JB, Yin Y, Zaera F. A Yolk@Shell nanoarchitecture

for Au/TiO2 catalysts. Angew Chem Int Ed. 2011;50(43):10208.

[31] Joo JB, Zhang Q, Dahl M, Zaera F, Yin Y. Synthesis, crys-

tallinity control, and photocatalysis of nanostructured titanium

dioxide shells. J Mater Res. 2013;28(3):362.

[32] Lee H, Jeong U, Kim Y, Joo JB. Magnetically-separable and

thermally-stable Au nanoparticles encapsulated in mesoporous

silica for catalytic applications. Top Catal. 2017;60(9–11):763.

[33] Jeong U, Joo JB, Kim Y. Au nanoparticle-embedded

SiO2–Au@SiO2 catalysts with improved catalytic activity,

enhanced stability to metal sintering and excellent recyclability.

RSC Adv. 2015;5:55608.

[34] Joo JB, Lee I, Dahl M, Moon GD, Zaera F, Yin Y. Controllable

synthesis of mesoporous TiO2 hollow shells: toward an efficient

photocatalyst. Adv Funct Mater. 2013;23(34):4246.

[35] Joo JB, Zhang Q, Dahl M, Lee I, Goebl J, Zaera F, Yin Y.

Control of the nanoscale crystallinity in mesoporous TiO2 shells

for enhanced photocatalytic activity. Energ Environ Sci. 2012;5:

6321.

[36] Joo JB, Zhang Q, Lee I, Dahl M, Zaera F, Yin Y. Mesoporous

anatase titania hollow nanostructures though silica-protected

calcination. Adv Funct Mater. 2012;22(1):166.

[37] Joo JB, Vu A, Zhang Q, Dahl M, Gu M, Zaera F, Yin Y. A

sulfated ZrO2 hollow nanostructure as an acid catalyst in the

dehydration of fructose to 5-hydroxymethylfurfural. Chem-

SusChem. 2013;6(10):2001.

[38] Lou XW, Yuan C, Zhang Q, Archer LA. Platinum-functional-

ized octahedral silica nanocages: synthesis and characterization.

Angew Chem Int Ed. 2006;45(23):3825.

[39] Teo JJ, Chang Y, Zeng HC. Fabrications of hollow nanocubes of

Cu2O and Cu via reductive self-assembly of CuO nanocrystals.

Langmuir. 2006;22(17):7369.

[40] Cao SW, Zhu YJ, Cheng GF, Huang YH. Preparation and

photocatalytic property of a-Fe2O3 hollow core/shell hierarchi-

cal nanostructures. J Phys Chem Solids. 2010;71(12):1680.

[41] Li N, Zhang Q, Liu J, Joo JB, Lee A, Gan Y, Yin Y. Sol–gel

coating of inorganic nanostructures with resorcinol–formalde-

hyde resin. Chem Commun. 2013;49:5135.

[42] Liu R, Mahurin SM, Li C, Unocic RR, Idrobo JC, Gao H,

Pennycook SJ, Dai S. Dopamine as a carbon source: the con-

trolled synthesis of hollow carbon spheres and yolk-structured

carbon nanocomposites. Angew Chem Int Ed. 2011;50(30):

6799.

[43] Lee I, Joo JB, Yin Y, Zaera F. Au@Void@TiO2 yolk–shell

nanostructures as catalysts for the promotion of oxidation

reactions at cryogenic temperatures. Surf Sci. 2016;648:150.

[44] Pastoriza-Santos I, Koktysh DS, Mamedov AA, Giersig M,

Kotov NA, Liz-Marzan LM. One-pot synthesis of Ag@TiO2

core–shell nanoparticles and their layer-by-layer assembly.

Langmuir. 2000;16(6):2731.

[45] Mayya KS, Gittins DI, Dibaj AM, Caruso F. Nanotubes prepared

by templating sacrificial nickel nanorods. Nano Lett. 2001;

1(12):727.

[46] Zhong Z, Yin Y, Gates B, Xia Y. Preparation of mesoscale

hollow spheres of TiO2 and SnO2 by templating against crys-

talline arrays of polystyrene beads. Adv Mater. 2000;12(3):206.

[47] Yoon SB, Kim JY, Kim JH, Park YJ, Yoon KR, Park SK, Yu JS.

Synthesis of monodisperse spherical silica particles with solid

123 Rare Met. (2020) 39(7):767–783

782 I. Choi et al.

core and mesoporous shell: mesopore channels perpendicular to

the surface. J Mater Chem. 2007;17:1758.

[48] Lou XW, Yuan C, Archer LA. Shell-by-shell synthesis of tin

oxide hollow colloids with nanoarchitectured walls: cavity size

tuning and functionalization. Small. 2007;3(2):261.

[49] Chen JS, Archer LA, Lou XW. SnO2 hollow structures and TiO2

nanosheets for lithium-ion batteries. J Mater Chem. 2011;21:

9912.

[50] Lou XW, Yuan C, Archer LA. Double-walled SnO2 nano-co-

coons with movable magnetic cores. Adv Mater. 2007;19(20):

3328.

[51] Nguyen CC, Vu NN, Do TO. Efficient hollow double-shell

photocatalysts for the degradation of organic pollutants under

visible light and in darkness. J Mater Chem A. 2016;4:4413.

[52] Joo JB, Kim P, Kim W, Kim J, Kim ND, Yi J. Simple prepa-

ration of hollow carbon sphere via templating method. Curr

Appl Phys. 2008;8(6):814.

[53] Hu Y, Ge J, Sun Y, Zhang T, Yin Y. A self-templated approach

to TiO2 microcapsules. Nano Lett. 2007;7(6):1832.

[54] Zhang Q, Lee I, Ge J, Zaera F, Yin Y. Surface-protected etching

of mesoporous oxide shells for the stabilization of metal

nanocatalysts. Adv Funct Mater. 2010;20(14):2201.

[55] Pirzada T, Arvidson SA, Saquing CD, Shah SS, Khan SA.

Hybrid carbon silica nanofibers through sol–gel electrospinning.

Langmuir. 2014;30(51):15504.

[56] Zhang T, Ge J, Hu Y, Zhang Q, Aloni S, Yin Y. Formation of

hollow silica colloids through a spontaneous dissolution-re-

growth process. Angew Chem Int Ed. 2008;47(31):5806.

[57] Fang X, Chen C, Liu Z, Liu P, Zheng N. A cationic surfactant

assisted selective etching strategy to hollow mesoporous silica

spheres. Nanoscale. 2011;3:1632.

[58] Yang HG, Zeng HC. Preparation of hollow anatase TiO2

nanospheres via ostwald ripening. J Phys Chem B. 2004;

108(11):3492.

[59] Li J, Zeng HC. Hollowing Sn-doped TiO2 nanospheres via

ostwald ripening. J Am Chem Soc. 2007;129(51):15839.

[60] Lou XW, Wang Y, Yuan C, Lee JY, Archer LA. Template-free

synthesis of SnO2 hollow nanostructures with high lithium

storage capacity. Adv Mater. 2006;18(17):2325.

[61] Xu Y, Zhang Y, Zhou Y, Xiang S, Wang Q, Zhang C, Sheng X.

CeO2 hollow nanospheres synthesized by a one pot tem-

plate-free hydrothermal method and their application as catalyst

support. RSC Adv. 2015;5:58237.

[62] Cao CY, Cui ZM, Chen CQ, Song WG, Cai W. Ceria hollow

nanospheres produced by a template-free microwave-assisted

hydrothermal method for heavy metal ion removal and catalysis.

J Phys Chem C. 2010;114(21):9865.

[63] Chang Y, Teo JJ, Zeng HC. Formation of colloidal CuO

nanocrystallites and their spherical aggregation and reductive

transformation to hollow Cu2O nanospheres. Langmuir. 2004;

21(3):1074.

[64] Liu B, Zeng HC. Symmetric and asymmetric ostwald ripening in

the fabrication of homogeneous core–shell semiconductors.

Small. 2005;1(5):566.

[65] Liu J, Qiao SZ, Liu H, Chen J, Orpe A, Zhao D, Lu GQ.

Extension of the Stober method to the preparation of monodis-

perse resorcinol-formaldehyde resin polymer and carbon

spheres. Angew Chem Int Ed. 2011;50(26):5947.

[66] Lee YJ, Joo JB, Yin Y, Zaera F. Evaluation of the effective

photoexcitation distances in the photocatalytic production of H2

from water using Au@Void@TiO2 yolk–shell nanostructures.

ACS Energy Lett. 2016;1(1):52.

[67] Ge J, Zhang Q, Zhang T, Yin Y. Core-satellite nanocomposite

catalysts protected by a porous silica shell: controllable reac-

tivity, high stability, and magnetic recyclability. Angew Chem

Int Ed. 2008;47(46):8924.

[68] Dillon RJ, Joo JB, Zaera F, Yin Y, Bardeen CJ. Correlating the

excited state relaxation dynamics as measured by photolumi-

nescence and transient absorption with the photocatalytic

activity of Au@TiO2 core–shell nanostructures. Phys Chem

Chem Phys. 2013;15:1488.

[69] Zhang Q, Shu XZ, Lucas JM, Toste FD, Somorjai GA, Alivi-

satos AP. Inorganic micelles as efficient and recyclable micellar

catalysts. Nano Lett. 2014;14(1):379.

[70] Kim M, Park JC, Kim A, Park KH, Song H. Porosity control of

Pd@SiO2 yolk–shell nanocatalysts by the formation of nickel

phyllosilicate and its influence on Suzuki coupling reactions.

Langmuir. 2012;28(15):6441.

[71] Nabid MR, Bide Y, Abuali M. Copper core silver shell

nanoparticle–yolk/shell Fe3O4@chitosan-derived carbon

nanoparticle composite as an efficient catalyst for catalytic

epoxidation in water. RSC Adv. 2014;4:35844.

[72] Li X, Zhang W, Zhang L, Yang H. Pd nanoparticles confined in

fluoro-functionalized yolk–shell-structured silica for olefin

hydrogenation in water. Chin J Catal. 2013;34(6):1192.

Ji Bong Joo received his Ph.D.

degree in Chemical and Bio-

logical Engineering from Seoul

National University in 2009. He

then moved to the University of

California, Riverside, where he

was a postdoctoral fellow under

the supervision of Prof. Zaera

and Prof. Yin. In 2014, he came

back to Korea as a senior sci-

entist at the Korea Institute of

Energy Research (KIER). In

2016, he joined the faculty at

the Department of Chemical

Engineering at the Konkuk

University. His research interests are in the synthesis of nanostruc-

tured catalysts and their application in photocatalysis, heterogeneous

catalysis and electrochemical catalysis.

123Rare Met. (2020) 39(7):767–783

Inorganic shell nanostructures to enhance performance and stability of metal nanoparticles 783