Spatially Confined Formation and Transformation of Nanocrystals

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Spatially Confined Formation and Transformation of Nanocrystals within Nanometer-Sized Reaction Media Amit Kumar, Ki-Wan Jeon, Nitee Kumari, and In Su Lee*

Acc. Chem. Res. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/16/18. For personal use only.

National Creative Research Initiative Center for Nanospace-Confined Chemical Reactions (NCCRs) and Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea CONSPECTUS: The extensive research performed in the past two decades has enabled the production of a range of colloidal nanocrystals, mostly through solution-based procedures that generate and transform nanostructures in bulk-phase solutions containing precursors and surfactants. However, the understanding and control of each nanocrystal (trans)formation step during the synthesis are still complicated because of the high complexity of this process, in which multiple diverse events such as nucleation, subsequent growth, attachment, and ripening occur simultaneously in bulk suspensions. Unlike well-established solution-based methods, solid-state reactions, which had been at the forefront of traditional inorganic materials chemistry, are quite rarely utilized in the realm of nanomaterials because of the high temperatures required for most solid-state reactions, as a result of which the clusters and NCs are prone to migrate through the bulk reaction medium and sinter together uncontrollably into large particles. We have been pursuing the “nanospace-confined approach” to explore the use of a variety of solid and hollow silica nanoparticles as either solid-state or solution-phase reaction media to carry out the syntheses and transformations of nanocrystals in a unique microenvironment, partitioning the reactants, intermediates, and transition states from the rest of the bulk reaction medium. Such nanoconfined systems have the potential not only to enable efficient and selective nanocrystal conversion chemistries but also to provide fundamental understanding pertaining to the synthetic evolution of nanostructures and transient mechanistic steps. The unique spaces with sizes of a few tens of nanometers inside nanoconfined systems offer the opportunity to observe and elucidate novel deconvoluted chemical phenomena that are impossible to investigate in bulk systems, and comprehensive understanding of nanoconfined chemistry can be implicated in explaining and controlling the macroscopic chemical behaviors. This Account describes our focused research on developing spatially confined platforms for nanocrystal syntheses and transformations, highlighting our diversity-oriented strategy, namely, the “postdecoration approach”, which results in the evolution of new nanocatalytic sites in a preformed cavity for diversifying and controlling their morphologies, number, density and combinations. We discuss key examples of the “nanoconfined solid-state conversion approach” that involve novel reactions of nanocrystals within thermally stable solid silica nanospheres to synthesize and transform complex hybrid nanocrystals with increased complexity and functionality. In addition, an enlightening discussion of the examples of nanocrystal syntheses and conversions in nanoconfined solutions inside enclosed and exposed cavities of silica nanospheres is included. Finally, the important applications of nanospace-confined systems in various fields are also briefly discussed.

1. INTRODUCTION Because of their enormous technological applications and fundamental scientific interest, the synthesis of nanocrystals (NCs) with rationally designed dimensions and morphologies has been an immensely active area of research. The exquisite nanoscale properties heavily depend on the architecture, intricacy, and composition of the NCs, and these parameters can be controlled by understanding the complex nanostructure growth mechanisms and their precise engineering. Among existing synthetic methods, top-down lithographic approaches can be used only for planar surfaces with relatively low throughput and with challenges in controlling the nanostructures with few-nanometer resolution.1 On the other hand, significant progress has occurred in the realm of bottom-up solution-based methods, generating nanostructures with variable dimensionality and complexity, but these methods often demand rigorous control of reaction conditions with the © XXXX American Chemical Society

use of complex surfactants or templates and encounter difficulty in synthesizing multicomponent NCs with incompatible interfaces.2 Methods based on NC conversion chemistry exploit the processes of chemical etching, galvanic chemistry, ion exchange, and the atomic-diffusion-mediated nanoscale Kirkendall effect, but full exploitation of such approaches is yet to be achieved.3,4 Particularly, the high-temperature synthesis of multimetallic NCs on exposed supports while maintaining the nano size is a challenging task because of the facile sintering of NCs.5 Recently a variety of hollow and confined nanostructures have been explored to serve various applications such as performing chemical reactions and growing metal NCs, benefiting from their well-protected confined environment, high surface area, controllable interior access, and Received: July 6, 2018

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DOI: 10.1021/acs.accounts.8b00338 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Summary of solid/hollow silica-confined solid-state and solution-based nanocrystal syntheses and transformations.

solution partitioning.6,7 Among these, “rattle” or “yolk−shell” nano/microstructures, in which small catalytic NCs are confined within a hollow porous shell, have been extensively studied.8−11 Although the reactions within confined solutions boast the ability to avoid the unwanted sintering of nanocatalysts, enhance the reaction rates, and provide poresize-dependent substrate selectivity, their conventional synthesis requires tedious multistep routes: first overcoating presynthesized metal NCs with a template material, then modifying them with the shell material, and finally removing the template, which risks damaging or modifying the sophisticated core metal NCs under harsh template removal conditions. To circumvent such limitations, the ship-in-a-bottle approach has been adopted for growing NCs in a minimal number of synthetic steps selectively inside hollow silica (SiO 2 ) microspheres without the use of complicated surfactants or sacrificial templates.12−16 The superiority of SiO2-confined systems over other exposed/confined-templatebased systems stems from their high thermal stability, chemical inertness, and controllable nanoscale porosity, which can effectively stabilize and confine thermally sensitive nanostructures (such as noble-metal nanoclusters/NCs and anisotropic morphologies) at high temperatures and concomitantly provide facile chemical transport toward catalytic sites.17−19 However, such advantageous methods further demand diversity-oriented and highly controlled engineering of hollow interiors and exteriors with a variety of metal NCs and support materials and control over nanostructural features such as shape, size, and porosity. On the other hand, the field of chemical metallurgy is enriched with a plethora of hightemperature (>300 °C) intermetallic solid-state reactions resulting unique unrestricted metal combinations and alloys. Such enormous synthetic possibilities are not otherwise feasible by conventional solution-based methods. Unfortu-

nately, the applications of solid-state reactions for the synthesis of diverse functional nanomaterials are rarely witnessed.20−24 In this Account, we present nanoconfined metal NC synthesis, growth mechanisms, and postsynthetic conversions using SiO2 nanospheres (10−50 nm) as versatile and highly functional confiners under solid-state and solution-based conditions (Figure 1). In the nanoconfined solid-state reactions, SiO2 nanospheres embedded with different metal ions and/or NCs are subjected to high temperature (>500 °C) redox conditions, enabling the synthesis/transformation of NCs with diverse morphologies, compositions, and interfaces through highly controlled processes such as NC migration, phase mixing/segregation, sintering, and hollowing within the isolated nanospace of the SiO2 medium. In the nanoconfined solution-based reactions, hollow/concave SiO2 nanospheres are installed with a single metal seed and additional functionalities that can effectively direct the growth of surfactant-free NCs of controllable size, morphology, and porosity. Among these reactions, the “nanoconfined postdecoration approach” facilitates the evolution of unique combinations of nanocatalysts and support materials within the preformed SiO2 cavity. The following sections will first introduce key examples of high-temperature nanoconfined solid-state conversions, then discuss the synthesis and postsynthetic conversions of NCs in nanoconfined solutions and compile their important applications, and finally offer concluding remarks with future challenges.

2. NANOCONFINED SOLID-STATE SYNTHESIS AND CHEMISTRY OF NANOCRYSTALS 2.1. Nanoconfined Solid-State Migration and Phase Segregation of Nanocrystals

Air annealing of silica nanospheres (∼30 nm) encapsulating Fe3O4 NCs and Pd2+ complexes (Fe3O4@SiO2/Pd2+) at 800 °C led to the formation of Fe3O4/PdO heterodimers (Fe3O4/ B


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Figure 2. (a) Solid-state synthesis and morphological modulations of AuPd@Fe3O4 hybrid nanocrystals. (b−e) (top) Time-course TEM and (bottom right) HRTEM images of FeAuPd@h-SiO2 and (bottom left) histograms of off-center distances of FeAuPd nanocrystals upon heating in a reductive environment. (f) HRTEM images of oxidative phase-segregated FeAuPd nanocrystals with different morphologies depending on the position of the FeAuPd nanocrystal. Adapted from ref 26. Copyright 2015 American Chemical Society.

PdO@SiO2) via heat-induced oxidative coalescence of Pd2+ ions onto the Fe3O4 core, followed by their thermal transformation to either FePd alloy or Fe3O4/Pd hybrid NCs.25 Subjecting the multicomponent system containing an additional Au NC [Fe3O4/Au@SiO2/Pd2+] to a temperature higher than 700 °C in a reductive environment resulted in slow outward migration of an in situ-reduced FeAuPd alloy phase, leaving behind a cavity inside the silica (Figure 2).26 The subsequent oxidative annealing of FeAuPd@SiO2 NCs converted them to phase-segregated oxidized hybrid NCs with diverse morphologies such as spherical, dumbbell, threeball snowman, and mushroomlike structures, depending on the degree of migration of the parent alloy NC (Figure 2). Such migration of the alloy NCs within the silica matrix is based on the change of the amorphous silica phase to the thermodynamically more stable glass transition phase at high temperatures, which decreases the viscosity of the elastic silica medium; guided by the in situ-developed hollow silica mold, the subsequent oxidative process converts the alloy NCs back to phase-segregated hybrid NCs with diverse morphologies based on the extent of migration. The motivation of this study was to synthesize multicomponent heterostructures with wellcontrollable complex morphologies and interfaces and also to study the dynamic and chemical behavior of NCs on a SiO2 support under high-temperature catalytic conditions.

In a related study, silica nanospheres encapsulating Au NCs and M2+ (M = Pt, Cu) together [Au/M2+@SiO2] were synthesized and annealed at high temperatures under different gaseous environments (Figure 3). In the case of Au/Pt2+@ SiO2, the resultant Au/Pt2+ NCs were thermally converted into reduced AuPt alloy NCs in a reductive environment.27 Upon oxidative annealing, Au/Cu2+@SiO2 uniquely transformed to AuCu alloy NCs and then to different Au−CuO heterostructures.28 The distinct evolution pathway of Au/CuO heterodimers through the kinetically accessed and oxidatively phase-segregated Au@CuO core@shell NCs reveals the escaping motion of the encapsulated Au core toward the silica surface, which is more facilitated by a thicker CuO shell (Figure 3). The high thermal stability of the Au@CuO with a very thin CuO shell thickness is due to the insufficient compressive lattice strainknowledge about such composition-dependent structural transitions is crucial for optimizing high-temperature catalysis. 2.2. Nanoconfined Solid-State Synthesis of Diverse Metal, Metal Oxide, and Alloy NCs

Unlike the large-sized (>10 nm) NCs confined in SiO2, highly mobile clusterlike small NCs that usually appear at the early stage of thermal coalescence are challenging to contain within the silica medium, and investigating their high-temperature C


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Figure 3. (a) Transformation of a Au−Cu core@silica shell hybrid nanocrystal during oxidative phase segregation. (b−e) HRTEM images and (insets) STEM-EDS line-scan profiles of 250 °C-annealed (b) [email protected]@SiO2 and (c) [email protected]@SiO2 and 600 °C-annealed (d) [email protected]@ SiO2 and (e) [email protected]@SiO2. (f) Snapshots from a recorded video of [email protected]@SiO2 using in situ-heating TEM. Adapted from ref 28. Copyright 2017 American Chemical Society.

Figure 4. (a) Confined synthesis of a metal/metal oxide/alloy NC within a nanometer-sized aminosilane-modified SiO2 NP. (b−g) TEM images and (insets) STEM-EDS elemental maps (red = Pd) of Pd2+/SiO2(NH2)2 under different conditions. The distribution of Pd NC diameters is shown at the bottom of each panel. (h−m) (top) TEM images and (bottom) STEM-EDS line-scan profiles of alloy NCs in SiO2: (h−j) metal− Pd@h-SiO2 and (k−m) metal oxide−Pd@h-SiO2. Scale bars = 10 nm. Adapted from ref 29. Copyright 2017 American Chemical Society.

NCs, resulting in a diverse pool of metal and alloy NCs (Figure 4).

solid-state behavior and chemistry is difficult. To extend our solid-state high-temperature investigation to such transient and mobile NCs, we used silica nanospheres modified with porogenic aminosilanes as the reaction medium.29 Such reactivity of the aminated silica medium upon thermal treatment generates an internal porosity gradient as nanoscale obstacles that limit the nucleation, growth, and migrations of

2.3. Nanoconfined Solid-State Reversible Transformation between Solid and Hollow Nanostructures

Upon annealing in a reductive environment, a metal cation (Pt2+/Pd2+)-incorporating silica-encapsulated manganese oxide D


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Figure 5. (a) Reversible transformation between solid and hollow nanostructures. (b, d, f) TEM and (c, e, g) STEM images (with EDS elemental line profiling for Mn) of (b, c) MnO@SiO2/Pt(II), (d, e) Pt-HSNP, and (f, g) Mn3O4@SiO2/PtO. (h) TEM images showing the cyclical transformation between the structures with solid and hollow interiors during successive annealing at 500 °C with repeated switching of the flowing gas between air and hydrogen. Adapted from ref 30. Copyright 2013 American Chemical Society.

core@shell morphology was gradually transformed in to a hollow structure composed of a manganese silicate phase embedded with tiny metal NCs (HSNP). Furthermore, a unique reversible thermal transformation between the hollow structure of HSNP and the solid core@shell structure with a Mn3O4 core encapsulated in a silica shell incorporating Pd/ PdO NCs was noticed upon annealing in oxidative and reductive environments, respectively (Figure 5).30−32 The hollowing of the manganese oxide core results from the phase transition of initially separated MnO and SiO2 into a thermodynamically more stable silicate phase through outward diffusion of Mn2+ ions toward SiO2, and upon oxidation the hollow silicate nanospheres revert to the initial core@shell structure by the filling of the interior cavities with a segregated Mn3O4 phase.

3. NANOCONFINED SOLUTION-BASED SYNTHESIS AND CHEMISTRY OF NANOCRYSTALS 3.1. Directed Growth of Metal Nanocrystals Inside Hollow/Porous Silica

To control the number, morphology, and homogeneity of NCs in solution-based NC synthesis inside the hollow cavity, installation of a single seed as a metal-growth-directing agent at an appropriate site and sufficient permeability of the silica shell toward metal precursors are crucial. We started with an Fe3O4/ Au hybrid NC encapsulated in a silica nanosphere, from which the Fe3O4 component was exclusively dissolved through a NaBH4-mediated reductive process, simultaneously affording a hollow and porous silica nanoshell by hydrolysis. During this, Au grains coalesced to form an active and protected single Au seed, resulting in a rattle-type structure, namely, Au@h-SiO2 (Figure 6a).33 The dissolution of Fe3O4 is facilitated by Au grains present in the hybrid NCs, which probably mediate the electron transfer from BH 4 − to the adjacent Fe 3 O 4 .

Figure 6. (a−c) Schematic illustrations of the syntheses of Au@hSiO2 and AuPd@h-SiO2 nanoreactors and confined growth of Au, Ag, and Ni NCs. (d−g) TEM images of (left to right) Au@h-SiO2, AgNC@Au@h-SiO2, AuPd@h-SiO2 (inset: EDS elemental maps with yellow = Au and magenta = Pd), and NiNC@AuPd@h-SiO2. Adapted with permission from refs 33 and 35. Copyright 2010 and 2015, respectively, Royal Society of Chemistry. E


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after reductive annealing of PtNDs along with other metal precursors inside silica.36 Hollow silica-based nanoreactors can also be used for preferential confined growth of PdO NCs without any seed when the internal cavity is premodified with a metal-growthdirecting supramolecular ligand such as cucurbit[7]uril (CB[7]) (Figure 8).37

Furthermore, larger Ag NCs were exclusively grown inside Au@h-SiO2 by treatment with AgNO3 and N2H4 (Figure 6b).33 In that study, the Au seed and the porous shell were indispensable to spatially confine the synthesis of AgNCs inside Au@h-SiO2. Additionally, larger-sized plasmonic Au NCs were also grown within Au@h-SiO2 upon reaction of HAuCl4 with H2O2 (Figure 6b).34 Unlike noble-metal NCs (Au and Ag), the growth of non-noble metals is more complicated and challenging in the bulk solutions, so we designed a nanoreactor with a catalytically active Au/Pdheterojunction NC seed inside the silica nanoshell (Au/Pd-hSiO2) for growing non-noble Ni NCs via seed-catalyzed reduction of Ni2+−hydrazine complexes (Figure 6c).35 To access high yields of catalytic NCs of commercial utility, we developed an efficient and scalable method for surfactantfree platinum nanodendrites (PtNDs) (Figure 7).36 For the

3.2. Nanoconfined Growth of Porous Metal Nanodendrites Inside Pore-Engineered Nanoreactors

The synthesis of PtNDs with tunable porosity and high surface area is crucial for maximizing their atomic activity and surface utilization efficiency, which in turn results in their high performance and cost effectiveness.38 Unfortunately, our earlier-utilized Au@h-SiO2 nanoreactors needed to be prepared by a complex multistep method starting from the sacrificial metal oxide NPs without any control over the porosity or morphology of the PtNDs.33 We recently introduced pore-engineered nanoreactors (PENRs) for the synthesis of PtNDs with tunable nanoscale porosity and having designer morphologies such as “yolk in shell” and “shell in shell” (Figure 9).39 PENRs consist of a single Au seed for siteselective initiation of metal growth, amine-functionalized sizecontrollable silica cavities/pores for guiding the metal growth, and a chemically robust nanoporous outer silica shell for containing and stabilizing the overall structure. After metal growth is completed, porous discrete morphologies of PtNDs can be conveniently isolated by hydrolytic removal of silica components.39 3.3. Nanoconfined Postsynthetic Surface Modification Strategy inside Hollow Silica

The vast applications of nanoreactors demand novel strategies to install a range of catalytic metal NCs and harness the reactivity of multiple metal−metal oxide heterocombinations and their interfaces within confined nanospace. Ultrasmall NCs supported on metal oxides are highly valuable catalytic platforms because such hybrid systems not only circumvent the deactivation and sintering of the catalytic species but also harness synergistic effects due to the unique metal−metal oxide heterojunctions. We developed a postsynthetic modification strategy to functionalize the preformed void spaces of the hollow silica nanoparticles with diverse catalytic NCs.40 First, we synthesized hollow silica nanospheres with a Mn3O4 internal layer (HMON@h-SiO2) from MnO−Mn3O4 encapsulated core−shell silica nanospheres by reductively etching the MnO core and simultaneously generating porosity in the

Figure 7. (a) Synthesis of platinum-based nanodendrites and hybrid nanocrystals. (b−f) TEM images of (b) Pt@h-SiO2, (c) Lf-PtNDs, and (d−f) hybrid NCs (d, Pt/Au; e, Pt/Ag; f, Pt/Pd). The insets showing EDS elemental maps (Pt = red, Au/Ag/Pd = green). Adapted with permission from ref 36. Copyright 2010 Wiley.

synthesis of PtNDs, Na2PtCl4 was reduced with L-ascorbic acid in the presence of Au@h-SiO2, resulting in the confined and size-controllable Au-seed-mediated autocatalytic growth of dendritic Pt structures exclusively inside the cavity. By this strategy, spherical Pt/Au alloy NCs could also be prepared

Figure 8. (a) Strategy for the seedless confined growth of PdO NCs within CB@h-SiO2. (b, c) TEM images of (CB-SiO2)@SiO2 and CB@hSiO2. (d) TEM-based time-course study of the growth of PdO NCs inside CB@h-SiO2. Adapted from ref 37. Copyright 2017 American Chemical Society. F


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Figure 9. (a) Strategy to modulate the porosity of PtNDs grown inside PENRs. (b) (top) Bright-field TEM images and (insets) STEM-EDS line profile analyses and (bottom) SEM (left) and HAADF-STEM (right) images of PtNDs grown in PENRs with different porosities (scale bars = 50 nm). Adapted from ref 39. Copyright 2018 American Chemical Society.

Figure 10. (a) Postsynthetic functionalization of the interior surface of a hollow silica nanoparticle. (b, c) TEM images of HMON@h-SiO2 and HMON/Pd@h-SiO2. (d) EDS elemental maps of HMON/Pd@h-SiO2. (e) Postsynthetic exchange of the interior support inside a hollow silica nanoparticle. (f−h) (top) TEM and (bottom) STEM-EDS element mapping images of (CeO2/Ms)@h-SiO2 (M = Pd, Ir, Rh). (i) Schematic illustration of Pt deposition on defective In2O3 confined inside h-SiO2. (j, k) TEM images and STEM-EDS elemental maps (Pt = magenta) of In2O3/Pts@h-SiO2. Adapted from refs 40, 43, and 44. Copyright 2013, 2016, and 2017, respectively, American Chemical Society.

silica shell.41 Further treatment of HMON@h-SiO2 with Na2PdCl4 resulted spontaneous deposition of small-sized Pd NCs exclusively inside the silica cavity as a result of the galvanic replacement reaction between Pd(II) and the Mn3O4 layer, affording HMON/Pd@h-SiO2 (Figure 10a−d).42 The strategy is easily extendable to the deposition of other catalytic metal NCs such as Pt, Ir, and Rh NCs. The utility of HMON/ Pd@h-SiO2 was shown by the efficient hydrolytic oxidation of hydrosilanes with extremely high turnover numbers (up to >198 000), high recyclability, and high selectivity, which

emerged from the confined presence of ultrafine catalytic NCs modified on the Mn3O4 support layer and the possible synergistic support−catalyst interaction.40 In another postsynthetic modification strategy, the catalyst-immobilizing Mn3O4 support layer inside the cavity of HMON/Pt@hSiO2 was replaced with more reactive CeO2 while preserving the size and chemical state of the catalysts (Pt, Pd, Ir, and Rh) (Figure 10e−h).43 This support-exchange reaction involved the bidirectional redox behavior of the Mn3O4 phase having mixed-valent states of Mn, which either sequentially or G


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Figure 11. (a) Strategy for the synthesis of the concave nanoreactor and growth of the dendritic Pt nanoshell. (b−d) TEM images of (left to right) MnO@asy-SiO2, MnFe2O4@asy-SiO2, and MnO/Fe3O4@asy-SiO2. (e−g) TEM images of the nanoparticles resulting from the Pt growth reaction on Au@con-SiO2 at increasing initial Na2PtCl4 concentrations. Inset of (g) gives the HAADF image and STEM-EDS line profile showing the distribution of Pt (pink) and Si (blue) of PtND@SiO2. (h) Scheme of the synthesis of a concave silica nanosphere with a catalystfunctionalized open-mouthed cavity. (i) TEM images of MnO@SiO2/Ni2+ with different off-center MnO cores. Inset gives STEM-EDS elemental maps (Mn = green, Si = red). (j) TEM images showing the galvanic replacement reaction and synthesis of Pt@con-SiO2 at different reaction times. Adapted with permission from refs 45 and 46. Copyright 2016 Royal Society of Chemistry and 2017 American Chemical Society, respectively.

Figure 12. (a) Strategy for decorating the surfaces of a hollow carbon nanosphere with catalytic metal nanocrystals. (b) TEM and STEM images and EDS elemental maps (Pt = green, Pd = yellow, Rh = red, Ir = magenta) of Pts@h-C@Pds, Pts@h-C@Rhs, and Pts@h-C@Irs. Adapted from ref 47. Copyright 2014 American Chemical Society.

better chemical transport for efficient metal NC growth, and the pocketlike nanospace simultaneously optimizes the accessibility to and protection of the immobilized NCs. First, Au-seed-containing concave SiO2 nanospheres (Au@conSiO2) were synthesized from Janus Au/Fe3O4@asy-SiO2 NPs by selective dissolution of the Fe3O4 core (Figure 11a).45 Next, Au@con-SiO2 was utilized to grow PtNDs: the nucleation of Pt originated at the Au seed, and the growth progressed by continuous budding of new Pt branches to form dendritic nanotendrils within the concave region. With the

simultaneously effect the reduction of noble-metal ions and oxidation of Ce3+ ions during the galvanic replacement reaction. Pt NCs can be spontaneously deposited on the heat-induced defective In2O3 surface inside hollow silica by a redox process involving the oxygen vacancies in the In2O3 lattice (Figure 10i−k).44 3.4. Growth of Metal Nanocrystals in Open-Mouthed Silica Cavities

Compared with the enclosed cavities, the open-mouthed cavities of concave nanoreactors can advantageously provide H


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Figure 13. (a) Size-selective cyanosilylation reaction using HMON@h-SiO2. (b) Synthesis of a magnetically recyclable hollow nanoreactor. (c) Nanoreactor with an open-mouthed catalytic cavity that demonstrates better reactivity than nanoreactors with enclosed cavities and better stability than exposed catalysts. (d) Confined growth of a plasmonic Au NC for glucose detection. (e) Saturated absorption spectra of a PBS suspension containing Au@h-SiO2, HAuCl4, GOx, glucose, and BSA protein. (f) Calibration curves for the saturated SPR absorbance as a function of glucose concentration obtained in the absence (solid line and circle symbols) and the presence (dashed line with square symbols) of BSA protein. (g) Synthesis of sf-HMON. (h) Comparison of MRI relaxivities of HMONs with different capping ligands and MRI contrasts of mouse brain using different HMONs. Adapted with permission from refs 34 and 50. Copyright 2012 Wiley and 2018 American Chemical Society, respectively.

galvanic replacement reaction, the metal NCs could be flexibly functionalized on the interior surfaces as well as separately on the internal and external surfaces, and the remaining manganese oxide core can be conveniently removed by acid treatment (Figure 12). Compared with the case of HMON-hSiO2, such clearly differentiated metal NC deposition on the external carbon surfaces of MnO@C may be due to the much lower reduction potential of MnO(s)/Mn3+(aq) compared with Mn3O4(s)/Mn3+(aq), leading to the facile deposition of Pt on the outer carbon surface via the transfer of electrons from the MnO cores through the conductive carbon shell.

direction of silica-embedded amines, the metal growth propagated two-dimensionally along the curvature of the silica sphere. When the surface of Au@con-SiO2 was covered with an amine-deficient silica layer, the growth of PtNDs was restricted only to the concave region. Our second strategy toward an open-mouthed cavity-based nanoreactor uses Janus MnO NCs asymmetrically encapsulated in Ni2+-incorporated silica nanospheres (asy-MnO@SiO2/Ni2+), which upon annealing in a reductive environment form con-(Ni/HMS)@ SiO2 having a concave cavity composed of a manganese− silicate layer and containing a Ni NC (Figure 11h).46 Finally, postsynthetic functionalization of the open cavity with Pt NCs was accomplished by a galvanic replacement reaction templated by the Mn2+-containing manganese silicate layer, resulting Pt@con-SiO2.

4. APPLICATIONS OF NANOSPACE-CONFINED SYSTEMS AND SYNTHESIZED NCS A variety of hollow and porous silica nanospheres equipped with complex NCs and supports are ideal candidates for catalytic applications. Inside these nanoreactors, highly active, surfactant-free, selectively accessible catalytic surfaces and relatively higher compartmentalized reactant concentrations provide kinetically favorable settings, and at the same time, by means of the pore size and gatekeeper functionalities of the silica shell, the substrate selectivity can also be controlled. For example, HMON@h-SiO2 having an internal layer of Lewis acidic hollow Mn3O4 catalytic surface efficiently carried out the cyanosilylation reaction selectively on small-sized aromatic

3.5. Site-Specific Synthesis of Metal Nanocrystals on Hollow Carbon Nanospheres

Hollow carbon nanospheres (HCNSs) are interesting morphologies for functionalization with catalytic NCs for various applications.24 We exploited our postsynthetic galvanic replacement chemistry on manganese oxide-encapsulating carbon shells (MnO@C) for depositing a variety of noblemetal NCs (Pt, Pd, Ir, and Rh) selectively on the outer surface and rendering an interior porous MnO core.47 In addition, by rearrangment of the sequence of the carbon shell coating and I


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Accounts of Chemical Research Table 1. Applied Nanoreactors and Their Key Structural Features and Functional Advantages

on the fully exposed supports.46 The H2O2-mediated growth of plasmonic Au NCs inside Au@h-SiO2 was utilized as a tool to quantitatively monitor glucose even in a complex medium containing interfering bovine serum albumin (BSA) protein (Figure 13d−f), suggesting potential applications of these nanoreactors in biosensing.34 HMONs capped with carboxylate-anchored ligands exhibit a significant increase in the magnetization value with improved in vivo T1 magnetic resonance imaging (MRI) efficacy in mouse brain (Figure 13g,h).49,50 The key structural features and advantages of different nanoreactors are summarized in Table 1.

aldehydes, which are permeable through the nanopores (∼1 nm pore size, as measured by N2 sorption isotherms) in the silica shell. The shell permeability could also be tuned by alkylation of the silica pores, where the reaction rates and yields of the products were affected by both the molecular shape and dimensions of the aldehyde substrates (Figure 13a), rendering the dual advantages of substrate size selectivity and high catalytic reactivity.41 As shown in Figure 13b, we also synthesized an Fe3O4/hollow Mn3O4 heterodimer encapsulated in a porous (∼1 nm pore size, as measured by N2 sorption isotherms) silica shell (HMON-Fe3O4@h-SiO2), which gives the additional practical advantage of easy magnetic separation of the nanoreactors and their recyclability without the loss of catalytic reactivity even after 10 successive cycles.48 To overcome the slow diffusion of reactant/product molecules through nanoporous enclosed cavity, which usually hampers the diffusion of large-sized and negatively charged substrates, we invented open-mouthed cavity-based nanoreactors (Pt@ con-SiO2) that provide unobstructed access to the catalyst and keep the catalytic NCs well-protected (Figure 13c). Control experiments suggested that Pt@con-SiO2 exhibited superior reactivity compared with the nanoreactors having an enclosed cavity and enhanced robustness over the catalysts supported

5. CONCLUSION The strategies showcased above encompass the synthesis and manipulation of sizes, shapes, compositions, and versatile postsynthetic transformations of NCs in nanosized silicaconfined reaction volumes. The use of a solid-state silica medium containing metal precursors or NCs affords platforms to investigate novel heat-induced processes providing libraries of surfactant-free NCs and designer precursors to build highly engineered and functional nanomaterials. Leveraging the confinement effect and highly viscous rubbery state of silica at high temperatures allows the solid-state reactions to be J


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Accounts of Chemical Research conveniently captured at multiple stages, and complex heterostructures can be isolated as synthetic snapshots. The knowledge about the mechanistic steps and indirect effects of metal additives on heat-induced solid-state nanoscale conversions provides opportunities to improve upon macroscale commercial metallurgy. A variety of hybrid nanostructures can be conveniently synthesized with controllable morphologies and porosity without the use of complicated surfactants and reaction conditions in scalable reaction volumes. The diversityoriented postsynthetic approach installs silica- or carbon-based cavities with an arbitrary choice of catalytic functionalities and designer conjugation with metal oxide supports for “on demand” optimization of catalytic performance. Many challenges regarding the thermal solid-state nanoconfined NC synthesis and conversion strategies still remain to be solved in the future, such as controlling the temperatureinduced fast outward migration of NCs in the silica medium; preserving, synthesizing, or controlling the thermodynamically less stable anisotropic NC morphologies; synthesizing alloy NCs using less miscible metal precursors; controlling the elasticity and viscosity of the silica medium; tuning the chemical functionality of SiO2; and discovering new multifunctional and thermally stable nanoconfinement medium materials (other than SiO2). In addition, the nanoconfined solution-based methods still remain to be applied to the discovery of novel chemistries of nanomaterials having a variety of dimensions, shapes, compositions, and intricacies, such as nanosized coordination polymers, metal−organic frameworks, biomolecules, and their hybrids. The research in this field holds promise for the future development of new confined systems produced from a variety of materials, resulting in highly functional nanomaterials and customizable platforms for new properties and applications in biomedical science, sustainable energy conversion, and highly efficient catalysis.

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His current research is focused on the synthesis and modification of hollow metal and metal oxide nanoparticles with interior cavities and finding their applications in catalysis and biomedical fields.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (NRF-2016R1A3B1907559 to I.S.L.).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (I.S.L.). ORCID

In Su Lee: 0000-0002-2588-1444 Notes

The authors declare no competing financial interest. Biographies Amit Kumar received his Ph.D. in 2009 from Indian Institute of Technology, Kanpur in India and now is a research assistant professor at POSTECH in South Korea. His research is focused on the synthesis and versatile applications of nanoreactors. Ki-Wan Jeon received his Ph.D. from Arizona State University in the United States in 2013 and now is a research assistant professor at POSTECH. His research is focused on the chemistry of nanoconfined nanocrystals. Nitee Kumari received her Ph.D. in 2010 from Indian Institute of Technology, Kanpur. Now she is a postdoc at POSTECH. Her research is focused on nanoreactors for organic reactions. In Su Lee received his Ph.D. in 2000 from Seoul National University. He worked at Kyung Hee University as assistant and associate professor from 2006 to 2011, after which he moved to POSTECH and joined the Department of Chemistry as an associate professor. K


Article

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Accounts of Chemical Research (50) Lee, J. H.; Kumari, N.; Kim, S. M.; Kim, S.; Jeon, K.-W.; Im, G. H.; Jang, M.-S.; Lee, W. J.; Lee, J. H.; Lee, I. S. Anchoring ligandeffect on bright contrast-enhancing property of hollow Mn3O4 nanoparticle in T1-weighted magnetic resonance imaging. Chem. Mater. 2018, 30, 4056−4064.

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