Concave Silica Nanosphere with a Functionalized Open-Mouthed

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Concave Silica Nanosphere with a Functionalized Open-Mouthed Cavity as Highly Active and Durable Catalytic Nanoreactor Jin Goo Kim,†,‡,§ Amit Kumar,†,‡,§ Seung Jin Lee,†,‡ Junghoon Kim,‡ Dong-Gyu Lee,†,‡ Taewan Kwon,†,‡ Seung Hwan Cho,‡ and In Su Lee*,†,‡ †

National Creative Research Initiative Center for Nanospace-Confined Chemical Reactions (NCCR), Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea ‡ Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea S Supporting Information *

ABSTRACT: Despite increasingly intensive research into catalytic hollow nanoreactors, most of the work has focused on the enclosed cavity structure, and attempts to use the openmouthed cavity have not been made so far, most likely due to the lack of methodologies for producing and functionalizing such a structure. This paper reports a synthetic strategy toward open-mouthed cavity-based nanoreactors, which renders the SiO2 nanosphere with a concave surface and also immobilizes catalytic nanocrystals (NCs) specifically inside the concave region. By putting the Janus silica-encapsulated manganese oxide (MnO) nanoparticle, with its highly off-centered core@shell structure, through the thermal hollow-conversion process, the edge-touching MnO nanoparticle was transformed into a hollow hemispherical manganese silicate layer with an opening to the outside, thus producing the bitten apple-like structure, conc-(Ni/ HMS)@SiO2, with an open-mouthed cavity on the SiO2 nanosphere. The galvanic replacement reaction occurring on the manganese-silicate layer of the conc-(Ni/HMS)@SiO2 afforded the site-specific immobilization of catalytic Pt NCs on the preformed concave interior surface, signifying the possible postsynthetic functionalization of an open-mouthed cavity which could be adapted for the development of a nanoreactor system. The newly developed nanoreactor, Pt@conc-SiO2, carrying tiny catalytic Pt NCs inside the semiexposed and also semiprotected pocket-like space, exhibited an increased reaction rate and a more extended range of applicable substrates in catalyzing the reduction of nitroarene compounds, compared with the enclosed cavity-based analogue, while preserving the high immobilization stability of Pt nanocatalysts during the recycling process.

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INTRODUCTION Hollow-structured nanoparticles (NPs), bearing just a few tens of nanometer-sized voids at the interior, have been attracting great interest on account of their distinct characteristics that are advantageous in a variety of biomedical,1,2 catalytic,2−5 and energy-related applications.4,6 In particular, when catalytic species are incorporated inside the hollow space, they are considered attractive candidates for the nanoreactor system that selectively and sustainably catalyzes the transformation of substrate molecules in a well-isolated and protected environment even without the assistance of organic surfactants; this makes up for the drawbacks of conventional nanoparticle-based catalysts, such as decline of activity in harsh reaction conditions or during the recycling process, interference by the surfacecapping ligands, and insufficient transition-state selectivity by protruding exposed surface atoms.3−5,7−9 Most efforts so far have been devoted to fabricate and functionalize an enclosed nanocavity, which is completely buried in a porous and inert shell material, proving its high effectiveness particularly in the aspect of selectivity for the size of the substrate molecules10−12 and maintenance of its high activity even under harsh operational conditions or during the recycling process.13−17 However, the highly limited accessibility to the closed cavity, © 2017 American Chemical Society

which is allowed by the slow diffusion process only through small pores or channels in the shell matrix, impedes the overall rate of the catalysis and narrows down the pool of applicable substrates to very small-sized molecules.18 In addition, this also hampers the postsynthetic modification of the preformed cavity, which is required for adjusting and optimizing the catalytic performance according to target applications.10,19,20 In this regard, the employment of an open-mouthed cavity, whose openness can provide unobstructed accessibility to the included catalysts for a much broader range of substrates, might be an effective approach to an advanced nanoreactor system with improved performance and higher adaptability, which overcomes the weaknesses of those based on the enclosed cavity. However, contrary to the intensive research so far with the enclosed cavity, attempts to use the open-mouthed cavity have never been made for the nanoreactor application, which is most likely due to the lack of synthetic methodologies to produce such a structure. While several metal nanocrystals (NCs) of Au, Pt, Pd, Ni, and their alloys were recently Received: May 31, 2017 Revised: August 29, 2017 Published: September 6, 2017 7785

DOI: 10.1021/acs.chemmater.7b02235 Chem. Mater. 2017, 29, 7785−7793

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Chemistry of Materials Scheme 1. Synthetic Strategy for a Concave Silica Nanosphere with a Functionalized Open-Mouthed Cavity

fabricated to have a concave surface curvature,20−26 the structure with the open-mouthed cavity was rarely synthesized from oxide-nanoparticles, such as silica and silicate, which are best fitted for forming the protective shell of the nanoreactor.27−30 More to the point, there have not been any reports on the site-specific functionalization method of selectively decorating the open-mouthed cavity with catalytic species, which is essential to develop a catalytic nanoreactor system. Given this context, this paper presents a synthetic methodology toward an open-mouthed cavity-based nanoreactor, which renders the silica nanosphere of the concave surface structure and also immobilizes the catalytic NCs specifically inside the newly generated concave region (Scheme 1). In order to devise this methodology, we exploited the recently discovered solid-state hollow-conversion process of an MnO NP embedded inside the silica nanosphere, which had produced an interior cavity enclosed by a manganese silicate shell.31,32 The idea behind our strategy was that the use of an MnO NP with asymmetric silica encapsulation for this conversion process might leave a hollow crater on the silica surface, whose interior is semiexposed to the outside through the open entrance. It was also envisioned that the manganese silicate phase at the resultant concave surface might be employed to deposit noble-metal NCs via any possible redoxreactions involving reductive Mn2+ ions, which would enable the specific decoration of the preformed hollow interior with various catalytic species.10,33,34 Herein, we report the fabrication of the hollow silica nanoreactor with an open-mouthed nanocavity, immobilizing a high density of catalytic metal NCs on the concave interior surface, which could be realized by taking the above strategy involving the nanoscale hollowing of MnO/SiO2 system and the galvanic replacement reaction templated by the manganese silicate surface. The fabricated silica nanoreactor, containing catalytic Pt NCs inside the openmouthed cavity, exhibited an increased reaction rate and a more extended range of applicable substrates in catalyzing the reduction of nitroarene compounds, compared with the enclosed cavity-based analogue, while preserving the high immobilization stability of Pt catalysts during the recycling process. This paper therefore highlights those superior performances of the newly developed catalytic nanoreactor system, which are ascribed by the semiexposed and also semiprotected pocket-like nature of the open-mouthed cavity.

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HCl (Samchun Chem.), NaOH (Samchun Chem.), Hydroxylamine (Aldrich), Na2PtCl4·xH2O (Strem), and Na2PdCl4·xH2O (Strem) were all used as purchased without any purification. Analyses of transmission electron microscopy (TEM) were conducted with JEOL JEM-2100 and JEM-ARM200F. Scanning electron microscopy (SEM) was carried out with MERLIN (Carl Zeiss). The contents of the metal elements in the NPs were measured by inductive coupled plasma atomic emission spectrometry (ICP-AES) using Direct Reading Echelle ICP (LEEMAN). UV−vis spectroscopy was carried out with a JASCO V-650 UV−vis spectrophotometer. Synthesis of MnO@(SiO2/M2+) (M = Ni, Co, Cu) Nanospheres with Different Core@Shell Structures. The asy-MnO@(SiO2/ M2+), in which an MnO NP is asymmetrically encapsulated by a SiO2 shell, was synthesized through modifying the reverse microemulsion technique.31 Igepal CO-520 (0.6 mL) was dispersed in 20 mL of cyclohexane to generate a homogeneous reverse microemulsion suspension. A cyclohexane suspension of the oleic acid-coated MnO nanoparticle (10 mg), which was prepared by the thermal decomposition method, was then added to the suspension with continuous stirring. When the suspension became transparent after 15 min, aqueous solutions (30 mg/mL, 0.1 mL) of M2+ salts, such as Ni(NO3)2, CoCl2, or Cu(NO3)2, and a NH4OH solution (28−30%, 0.13 mL) were successively injected into the suspension with 15 min intervals. Lastly, a mixture of TEOS (0.16 mL) and TSD (0.04 mL) was added to the reaction suspension to initiate the silica formation and stirred continuously for 40 h at room temperature. By adding 3.0 mL of MeOH and centrifuging the collected MeOH layer, the solids of the resulting silica nanoshere encapsulating an MnO NP were isolated from the reaction suspension. The asy-MnO@(SiO2/M2+) NPs were then purified by repeating the washing with EtOH and distilled water and then lyophilized. The ece-MnO@(SiO2/M2+) and coc-MnO@(SiO2/M2+) were synthesized by injecting a mixture of TEOS (0.18 mL) and TSD (0.02 mL) and pure TEOS (0.40 mL), respectively, as a silica precursor to the above-described silica growing suspension instead of a mixture of TEOS (0.16 mL) and TSD (0.04 mL). Synthesis of conc-(M/HMS)@SiO2 (M = Ni, Co, Cu) through the Thermal Hollow Conversion Process. The powders of asyMnO@(SiO2/M2+) (M = Ni, Co, Cu) were placed in a tube-type furnace and heated to 600 °C with a 5 °C/min heating rate and then annealed for 15 h under a flow of gaseous mixture (4% H2 in Ar). Three dimensional off-centered distances of the MnO NP and the cavity from the center of the silica nanosphere were estimated from the distribution of their off-center displacement in the plane-projected TEM images using the previously reported calculation method.35 Galvanic Replacement Reaction with the conc-(Ni/HMS)@ SiO2; Immobilization of Pt NCs on the Concave Surface. conc(Ni/HMS)@SiO2 (2.0 mg) were immersed in 3.0 mL of an aqueous solution containing Na2PtCl4, (1.5 mg/mL) at pH 4.3 (adjusted by the addition of diluted HCl), and reacted at 50 °C for 2 h with constant stirring. After being cooled to rt, the resulting Pt@conc-SiO2 was isolated by centrifugation and purified by repeating the dispersion in an aqueous suspension and centrifugation three times. To investigate the Pt growth progress of the reaction, the samples for the TEM and EDS analyses were obtained from the reaction solution at 30 min, 1, 1.5, and 2 h of the reaction time period. Control Experiments for the Galvanic Replacement Reaction Templated by the conc-(Ni/HMS)@SiO2. First, Mn-free conc-

EXPERIMENTAL SECTION

General Consideration. Any reagent including MnCl2·4H2O (Junsei), sodium oleate (TCI), 1-octadecene (Aldrich), Polyoxyethylene (5) nonylphenylether (Igepal CO-520, Aldrich), NH4OH (Samchun Chem.), tetraethyl orthosilicate (TEOS, Acros), N-[3(trimethoxysilyl)propyl] ethylenediamine (TSD, Aldrich), Ni(NO3)2· xH2O (Strem), CoCl2·xH2O (Kanto), NaBH4 (Samchun Chem.), 7786


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Chemistry of Materials (Ni/HMS)@SiO2 were obtained by immersing them in aqua regia and stirred for 12 h at rt, followed by centrifugation and washing with DI water. Further, Mn-free conc-Ni@SiO2 NPs (2 mg) were added to 1.0 mL of an aqueous solution of Na2PtCl4 (0.5 mg/mL) at pH 4.3 and reacted at 70 °C for 2 h with constant stirring. After being cooled to rt, the solids of the resulting NPs were retrieved by centrifugation. For another control experiment, MnCl2 (2 mg) was added to 1 mL of an aqueous suspension of Na2PtCl4 (0.5 mg/mL) and Mn-free conc-(Ni/ HMS)@SiO2 (2.0 mg) and reacted at 70 °C for 2 h at pH 4.3. Syntheses of Nanoreactors, Pt@conv-SiO2 and Pt@encl-SiO2, for the Comparative Study of Catalytic Performance. For the preparation of the Pt@conv-SiO2, the Ni-HMS, which contains an Ni NC inside the cavity of a hollow manganese silicate nanoshell, was synthesized by selective etching of the gastight silica shell at the outtermost layer of the coc-(Ni/HMS)@SiO2.31 Then the deposition of Pt NCs was conducted by treating the Ni-HMS NPs (1.0 mg) in an aqueous solution of Na2PtCl4, (1.0 mL, 2.0 mg/mL), at 70 °C for 2 h. After being cooled to rt, the resulting Pt@conv-SiO2 were isolated by centrifugation and then purified by repeating the dispersion in an aqueous suspension and centrifugation three times. For the synthesis of the Pt@encl-SiO2, we followed the procedures in our previous report.10 The HMON@h-SiO2, containing a thin Mn3O4 layer at the interior surface of the hollow silica nanosphere, was prepared through the previously reported procedure, in which the MnO@SiO2 NPs were treated with NH2OH solution. And the resulting HMON@h-SiO2 NPs were immersed in an aqueous solution of Na2PtCl4·3H2O (1.0 mL, 1 mg/mL) at pH 2.4 and 70 °C and reacted for 30 min with constant stirring, which led to the deposition of Pt NCs on the interior surface of the cavity of hollow silica nanoshell. After being cooled to rt, the resulting Pt@encl-SiO2 NPs were isolated by centrifugation and then purified by repeating the dispersion in an aqueous suspension and centrifugation three times. Evaluation of Catalytic Performance of Nanoreactors in the Hydrogenation Reaction of Nitroarenes. The catalytic reduction of three nitroarenes by NaBH4 was used as the model reaction to evaluate the catalytic activity of the three nanoreactors. In a typical experiment, 10 μL of aqueous dispersion of catalyst (containing 6 pmoles Pt) was added to the mixture of nitroarene substrate (1.2 mM) and sodium borohydride (0.48 M) in total 5 mL of distilled water. 100 μL reaction mixture was taken out from the reaction suspension every 5 min to check the UV−vis spectrum. The change of extinction intensity was directly estimated by calculating the ratio of extinction at each time point (Ct) relative to its initial value (C0). For recycling, the catalyst was retrieved by centrifugation of reaction mixture at 14 500 rpm for 10 min, and the supernatant liquid was removed followed by washing the precipitated catalyst with DI water.

revealed that the addition of TSD in the precursor led to the asymmetric silica deposition around the MnO NP, creating the off-centered core@shell structure, and the degree of the core displacement increased with the portion of TSD in the precursor mixture (Figure 1c). While the reaction with pure

Figure 1. TEM images and off-center displacement ratio histograms for (a-c) coc-MnO@(SiO2/Ni2+), ece-MnO@(SiO2/Ni2+), and asyMnO@(SiO2/Ni2+) and (d−f) after annealing process under reductive environment at 600 °C to form coc-(Ni/HMS)@SiO2, ece-(Ni/HMS) @SiO2, and conc-(Ni/HMS)@SiO2; insets of TEM image in box c and f represent corresponding HRTEM and EDS elemental mapping for Mn (green) and Ni (red). (g) Histograms for MnO diameter in asyMnO@(SiO2/Ni2+) and cavity diameter in conc-(Ni/HMS)@SiO2. (h) SEM image of conc-(Ni/HMS)@SiO2. In all cases, for generating the histograms 100 particles were randomly selected, and distances were measured in TEM images.

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RESULTS AND DISCUSSION Syntheses of MnO@(SiO2/Ni2+) Nanospheres with Various Core@Shell Structures. Under the aforementioned strategy, this study began with the attempt to move the radial location of an MnO NP embedded in the Ni2+ incorporating SiO2 nanosphere and investigate the influence of the change in the starting core@shell structure on the shape and type of hollow interior to be generated through the thermal transformation of the MnO@(SiO2/Ni2+). The synthesis of the MnO@(SiO2/Ni2+) with an off-centered MnO core was attempted by adopting the asymmetric silica encapsulation method which was recently developed by exploiting the selfcatalyzed deposition of amino-silane-containing silica on one side of colloidal NP.27 Accordingly, a mixture of N-[3(trimethoxysilyl)propyl]ethylenediamine (TSD) and tetraethyl orthosilicate (TEOS) was injected, instead of the pure TEOS, into a water-in-cyclohexane microemulsion suspension of oleic acid-coated MnO NP which had been applied for the coating of an isotropic silica shell. Transmission electron microscopy (TEM) analyses of the isolated solids after a 40 h-reaction

TEOS generated the coc-MnO@(SiO2/Ni2+) nanosphere of the typical concentric structure, the use of 10% TSD mixture resulted in the formation of the eccentric core@shell structure, ece-MnO@(SiO2/Ni2+), in which an MnO NP is embedded obviously beside the center of the silica nanosphere (Figure 1a, b). On the basis of the distribution of the off-centered distances in the plane-projected TEM images, the MnO core of the eceMnO@(SiO2/Ni2+) was estimated to be displaced three dimensionally by 14 (±5) nm from the center of SiO2 nanosphere with 62 (±4) nm radius.35 When the TSD portion in the precursor raised up to 20%, the MnO NC was found to be further shifted outward with 15 (±4) nm of off-centered distance and therefore resulted in the formation of a Janus-type 7787


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Chemistry of Materials

Figure 2. Time course TEM images of asy-MnO@(SiO2/Ni2+) during annealing under the reductive environment at 500 °C for 0, 2, 5, and 10 h. Inset image of elemental mapping for Mn (green) and Ni (red) showing NiO/MnO mixed oxide phase.

formation of the ece-(Ni/HMS)@SiO2 with an enclosed but eccentric hollow interior structure. When the asy-MnO@(SiO2/Ni2+) was put through the thermal hollowing process, the edge-touching MnO NP was converted into a hollow hemispherical manganese silicate layer of 8 (±1) nm thickness, which has an opening to the outside. As a result, this conversion produced a hemispherical open-mouthed cavity of 17 (±2) nm diameter with negative curvature on the SiO2 nanosphere, which had been targeted by our initial strategy. TEM, HRTEM, STEM-EDS, and scanning electron microscopy (SEM) images in Figure 1f and Figure 1h showed the bitten apple-like structure of the resultant conc-(Ni/HMS)@SiO2 with a concave surface which has a manganese silicate-coated interior surface and a 6 (±1) nm-sized Ni NC immobilized on the concave surface. The time-course TEM analyses showed that the porous NiO/MnO mixed-oxide NC emerged just upon attaining a temperature of 500 °C, which indicates the mixing of the MnO NP with SiO2-embbedded Ni2+ ions (Figure 2). The occurrence of the hollowing process was apparently observed after 2 h from the region close to the air interface. For the following 8 h (from 2 to 10 h), the diffusion of the MnO continuously proceeded from the edge toward the interior of the SiO2 nanosphere, which caused the evolution of a 7 (±2) nm-sized void space with an irregular shape or outline into the hemispherical cavity of expanded diameter of 15 (±3) nm. In the mean time, the metallic Ni species, which was in situ generated and helped the hollowing process by reducing the residual Mn3O4 phase into the convertible MnO, began to be segregated at 5 h and rendered all the resulting conc-(Ni/ HMS)@SiO2 at 10 h to bear a 5 (±1) nm-sized Ni NC on the concave surface. According to the XRD data shown in Figure S2, upon heat-induced hollowing and transformation of asyMnO@(SiO2/Ni2+) to conc-(Ni/HMS)@SiO2, characteristic XRD peaks corresponding to crystalline MnO were diminished due to the formation of an amorphous manganese silicate phase, and new peaks corresponding to Ni(0) emerged. Similar trends of XRD patterns were also observed for the already reported conversion of coc-MnO@(SiO2/Ni2+) to coc-(Ni/ HMS)@SiO2.31,32 To probe the differently exposed cavities comprising manganese-silicate layers in conc-(Ni/HMS)@SiO2 and coc-(Ni/HMS)@SiO2, we have utilized XPS as a surfacesensitive technique and diagnosed the intensities of Mn 2p signals. As shown in Figure S3, owing to the presence of thick silica shell, coc-(Ni/HMS)@SiO2 exhibited an XPS peak of very weak intensity for Mn 2p at 641.5 eV; however, the Mn 2p peak underwent substantial enhancement upon Ar plasma etching treatment of the sample. However, in the case of the exposed cavity [con-(Ni/HMS)@SiO2], the binding energy peak at 641.5 eV was present in relatively higher intensity even

structure of the asy-MnO@(SiO2/Ni2+) in which the MnO NC touched the air interface of 56 (±4) nm-sized SiO2 nanosphere. Element mapping analyses of the asy-MnO@(SiO2/Ni2+) using scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) showed that the embedded Ni2+ ions were distributed evenly over the entire region of the SiO2 matrix (inset of Figure 1c). We recorded and compared XPS spectra of asy-MnO@(SiO2/Ni2+) and cocMnO@(SiO2/Ni2+) (Figure S1 of the Supporting Information, SI). In the XPS survey spectrum of coc-MnO@(SiO2/Ni2+), no Mn 2p peak at the expected binding energy could be observed; whereas, etching the same sample with Ar plasma resulted in the emergence of a moderate intensity peak at 641.5 eV, hence confirming the presence of Mn2+ (of MnO) embedded in SiO2. However, the XPS survey spectrum of asy-MnO@(SiO2/Ni2+) contained an Mn 2p peak at 641.5 eV, and etching the sample with Ar plasma caused no substantial effect on the intensity of the Mn 2p peak. This XPS data validated the different locations of MnO cores inside the SiO2 nanospheres and supported the different morphologies observed in TEM. Generation of Open-Mouthed Cavity Structure through the Thermal Hollow Conversion Process. A series of MnO@(SiO2/Ni2+) samples with different core@shell structures were then annealed under a flow of Ar and 4% H2 at 600 °C for 15 h, which tended to induce their hollow transformation. All the resulting products showed the change of the MnO NP into the thermodynamically more stable manganese-silicate layer, which created a void space including a single Ni NC, confirming the intended hollow transformation as revealed by TEM and high-resolution TEM (HRTEM). What was most noticeable in these observations was that the type and location of the cavity in the transformed hollow nanostructure could be well templated by the position of the MnO NP in the starting MnO@(SiO2/Ni2+) (Figure 1d−f). Accordingly, when the coc-MnO@(SiO2/Ni2+) was annealed at high temperature, the radial diffusion of the central MnO NP toward the surrounding silica left a 15 (±2) nm-sized spherical void containing an Ni NC of 7 (±1) nm-size, which is fully encircled by a spherical and isopachous manganese-silicate shell. This conversion therefore produced the enclosed and concentric hollow structure of the coc-(Ni/HMS)@SiO2, which is consistent with the observation in our preceding study.31 The hollow conversion process with the eceMnO@(SiO2/Ni2+) also produced the formation of a similar manganese-silicate-enclosed cavity of 19 (±3) nm-size. But the transformed cavity was found to be located outside the center of the SiO2 nanosphere, with the nearly identical off-centered distance of 14 (±4) nm with those of the MnO NC in the starting ece-MnO@(SiO2/Ni2+), which thus resulted in the 7788


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Figure 3. (a−d) Low- and high-magnified TEM images of conc-(Ni/HMS)@SiO2 upon reaction with Pt(II) at different times of galvanic replacement and synthesis of final product Pt@conc-SiO2 after 2 h and corresponding HRTEM image showing the Pt(111) lattice plane of the resulting Pt NCs, EDS elemental mapping (blue color dots are for Pt element), and SEM image. (e) Plot for % of Pt and Mn in Si at different times of galvanic replacement reaction.

When the conc-(Ni/HMS)@SiO2 was immersed in an aqueous solution of Na2PtCl4 with constant stirring at 50 °C, the brown color of the suspension gradually turned darker, generating the nearly black suspension within the initial 2 h. In the time-course TEM analyses with samples isolated from the reaction suspension at different time periods, nucleation and deposition of tiny Pt NCs began to emerge at 0.5 h at the concave region of some of the conc-(Ni/HMS)@SiO2. As the reaction proceeded, increasing numbers of Pt NCs were generated exclusively on the manganese-silicate layer at the interior surface of the hollow cavity, while slightly raising their sizes from 1.2 (±0.2) nm to 2.0 (±0.3) nm over 1 to 2 h, thus developing the high density coverage of entire area of the hemispherical concave surface with deposited Pt NCs (Figure 3). In the meantime, the Ni NC immobilized inside the cavity, which was readily oxidized into NiO phase in an aqueous suspension, did not show any significant change, indicative of the involvement in Pt formation, in the size, shape, or location (Figure S6). Another observation during the reaction was that the Mn contents, measured by the EDS analyses, decreased progressively along with the increase of Pt contents, implying that the Pt growth proceeded at the expense of Mn2+ of the manganese silicate layer (Figure 3e). Galvanic conversion of conc-(Ni/HMS)@SiO2 to Pt@conc-SiO2 was also confirmed by the presence of broad Pt peak around 40° in XRD spectrum (Figure S2) and Pt 4f peak in the XPS spectrum (Figure S7). Experiments at room temperature and at lower pH conditions resulted in reduced reactions rates, and thus produced only a quite slight amount of Pt NCs during a given reaction time period, which is the trend observed in the previous reaction with the Mn3O4 NC (Figure S8).34 When the conc-Ni@SiO2, prepared by releasing all Mn2+ ions from the conc-HMS@SiO2

before Ar plasma etching of the sample (Figure S3). These XPS analyses support the results obtained from TEM about the different locations of cavities in coc-(Ni/HMS)@SiO2 and con(Ni/HMS)@SiO2. We have also measured the surface properties (surface area, porous volume, and pore size) of coc-(Ni/HMS)@SiO2 and conc-(Ni/HMS)@SiO2 by N2adsorption−desorption based Brunauer−Emmett−Teller (BET) analysis (Table S1 and Figure S4). When the above thermal hollowing strategy was performed with asy-MnO@(SiO2/Co2+) and asy-MnO@(SiO2/Cu2+), containing other first-row transition metal ions besides the Ni2+, their conversions also generated analogous open-mouthed cavity structures of conc-(Co/HMS)@SiO2 and conc-(Cu/HMS)@ SiO2, bearing a Co and a Cu NCs, respectively, most likely through a similar mechanism as described above for the formation of the conc-(Ni/HMS)@SiO2 (Figure S5). Compared with these, the hollow conversion reaction with the asyMnO@SiO2, without any transition metal ions embedded in the SiO2 matrix, showed a lack of reproducibility, therefore often remaining as the initial Janus-type core@shell structure, presumably due to the impediment of the unmanageable inclusion of the unconvertible Mn3O4 phase. Site-Specific Decoration of the Open-Mouthed Cavity with Catalytic Pt NCs through the Manganese-Silicate Templating Galvanic Replacement Reaction. With the intention of the functionalization of the newly created conc(Ni/HMS)@SiO2 for the nanorector application, we investigated the possible deposition of catalytic noble metal NCs on the manganese-silicate-coated interior surface inside the openmouthed cavity, which was inspired by our finding on the galvanic replacement reaction templated by Mn2+-containing metal-oxide NCs such as Mn3O4, MnFe2O4, and [email protected],34 7789


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Figure 4. (a) Conversion yields (%) and rate constants (k) for the reduction of different substituted nitroarenes under pseudo-first-order kinetics for three different nanocatalysts. (b) Comparison of conversion yields (%) for the reduction of 4-nitroaniline using recycled catalysts. (c) Pictorial representation for depicting rationale behind superiority of Pt@conc-SiO2 over Pt@encl-SiO2, and Pt@conv-SiO2.

which was well identified by TEM, HRTEM, SEM, and STEMEDS element mapping images in Figure 3d. Evaluation of Catalytic Performance of the Nanoreactor System with Catalyst-Functionalized OpenMouthed Cavity. The identified structure of the resultant Pt@conc-SiO2, which carries a high density of supported Pt NCs in the semiprotected and also semiexposed pocket-like space, led us to expect any distinct and advantageous catalytic properties, compared to conventional nanoreactors mostly based on the enclosed cavities. In order to verify the effectiveness of the Pt@conc-SiO2, as a nanoreactor system, this study made a comparative investigation of its performance in the catalytic reduction of nitroarenes to aminoarenes36 with Pt@encl-SiO2 and Pt@conv-SiO2 which carry SiO2-supported Pt NCs on the internal and external surfaces of the hollow silica nanosphere, respectively (Figure S12). Synthetic protocols for Pt@encl-SiO2 and Pt@conv-SiO2 have been detailed in the Experimental Section. Textural characterization data (surface area, pore volume, and pore size) of Pt@conc-SiO2, Pt@enclSiO2, and Pt@conv-SiO2 catalysts were obtained using N2 absorption BET analysis (Figure S13 and Table S2). Three nitroarene substrates including 4-nitroaniline (NAE), 4-nitrophenol (NPL), and 4′-amino-5′-nitrobenzo-15-crown-5 (NCN) were subjected to the NaBH4-mediated reduction for 25 min at 25 °C using above-mentioned three catalysts (0.1 mol % based on the Pt content) and the progress of the reaction and % conversion yields were monitored using a UV− vis spectrophotometer (Figures S14, S15, and S16). In all of the cases, NaBH4 was purposefully used in large excess ([NaBH4]/ [Nitroarene] = 400) to keep the reaction kinetics pseudo first

in an acid solution, was treated in a Na2PtCl4 solution or a Na2PtCl4/MnCl2 mixed solution at 50 °C, for the purpose of control, any formation of metallic Pt species was not detected (Figure S9). Also, using an additional reducing agent such as NaBH4 and ethylene glycol in the presence of Na2PtCl4 resulted in nonspecific deposition of Pt NCs randomly on entire surface of silica sphere (Figure S10). This indicates that the manganese-silicate phase including Mn2+ ions in the cavity is responsible for the selective Pt deposition process inside the cavity. Another control experiment with the coc-(Ni/HMS)@ SiO2, which has the manganese-silicate layer at the inside of the enclosed cavity, did not generate any Pt deposition most likely due to the inaccessible interior cavity covered by the dense silica shell (Figure S11). By taking all of the above observations, the generation of Pt NCs at the concave surface of the concHMS@SiO2 can be interpreted by a galvanic replacement reaction which involves the reduction of the PtCl42− into Pt(s) and the concomitant release of oxidized Mn3+(aq) ions from the manganese silicate phase exposed at the readily accessible surface of the open-mouthed cavity. Accordingly, the galvanic replacement reaction templated by the manganese-silicate surface enabled the site-specific immobilization of tiny catalytic Pt NCs on the preformed concave interior surface of the conc(Ni/HMS)@SiO2, signifying the possible postsynthetic modification of the open-mouthed cavity which could be adapted for the development of a nanoreactor system. As a result, this could produce the targeted Pt@conc-SiO2, carrying a high density of dispersively supported Pt catalysts in the openmouthed cavity of the bitten-apple-shaped SiO2 nanosphere, 7790


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Figure 5. TEM images of (a) Pt@conc-SiO2, (b) Pt@conv-SiO2, and (c) Pt@encl-SiO2 before first use (left) and after third use (right). Histograms represent the size distribution of randomly selected 100 Pt NCs in corresponding TEM images.

reduction of NAE was also performed using a lower amount of the catalysts Pt@conc-SiO2, Pt@conv-SiO2, and Pt@encl-SiO2 (0.001 mol %), which resulted 66%, 12%, and 10% conversion respectively, within 2 h (Figure S17). In the case of Pt@concSiO2, after recycling the recovered catalyst two more times, the conversion yield was slightly lowered to 62%, validating the high stability of the catalyst; whereas, recycled Pt@conv-SiO2 performed poorly, showing decline in the conversion yield to 2% after third cycle. Pt@encl-SiO2 could only give 9%, and 10% after second and third recycling steps (Figure S17). Previously, different morphologies of silica such as fibrous nanosilica in combination with polyethylenimine as metal sequestering polymer have been used to obtain good stability and activity of supported metal nanocatalysts.37,38 We believe, in the case of Pt@conc-SiO2, catalytic NCs are protected and stabilized in the pocket-like surface cavity during the reaction and recycling process. The above catalytic activity investigation evidenced the superiority of the Pt@conc-SiO2 over typical Pt@conv-SiO2 and Pt@encl-SiO2 nanoreactors in carrying out the reactions with a wide range of substrates with high activity and durability.

order, and the conversion rate was only dependent on the nitroarene substrate. The reaction rate constant (k/min−1) in each case was calculated by plotting the logarithm of the relative extinction intensity [ln(Ct/C0)] [at 380 nm for NAE, 400 nm for NPL, and 440 nm for NCN] versus the reaction time (t) under pseudo-first-order kinetics (Figure 4a). As shown in Figure 4a, Pt@conc-SiO2 as catalyst efficiently afforded >99% conversion of NAE to p-phenylenediamine within 25 min with reaction rate (k = 0.181 min−1) comparable to the Pt@conv-SiO2 (k = 0.128 min−1, %Y = 96) and much higher than in the case of Pt@encl-SiO2 (k = 0.041 min−1, %Y = 66). After confirming the efficient catalytic potency for small and neutral NAE substrates, we further extended the broader applicability of our Pt@conc-SiO2 nanoreactor system toward the reduction of NPL (anionic substrate) and NCN (bulky size substrate). Notably, Pt@conc-SiO2 efficiently converted NPL and NCN to the corresponding amino-derivatives with 91% (k = 0.099 min−1) and 95% (k = 0.111 min−1) conversion yields, respectively, within 25 min. As expected, Pt@conv-SiO2 resulted comparable conversion yields 81% (k = 0.069 min−1) and 79% (k = 0.60 min−1) in the case of NPL and NCN, respectively, due to the well-exposed catalytic-site accessibility. However, Pt@encl-SiO2 demonstrated very slow reaction rates and poor conversion yields for NPL (k = 0.060 min−1, %Y = 36) and NCN (k = 0.013 min−1, %Y = 26%). Table S3 in the SI consists of turn over number (TON) and turn over frequency (TOF) data estimated for the above-mentioned reactions. The above study validates the wide applicability of Pt@conc-SiO2 nanoreactor, particularly in cases where Pt@encl-SiO2 based nanoreactor design shows poor performance due to the limited approach of charged and bulky substrates toward catalytic active sites.10,12,31 After affirming the high activity and broad applicability of Pt@conc-SiO2 for catalytic reactions, we further tested its durability during the recycling process. After finishing the reaction, the catalysts were retrieved by centrifugation and were reused for the reduction of a fresh batch of NAE. As shown in Figure 4b, Pt@conc-SiO2 showed negligible loss of activity even after the third cycle. TEM analysis of the three times used Pt@conc-SiO2 showed insignificant change in Pt NCs density and size (Figure 5). As expected, Pt@encl-SiO2 was found to be equally durable showing almost consistent conversion yield after third run. However, in the case of Pt@ conv-SiO2, the conversion yield dropped from 96% (first use) to 18% after third use. TEM analysis of used Pt@conv-SiO2 revealed that the loss of activity is due to the increase in the size of Pt NCs from 1.2 (±0.2) nm to 6 (±1) nm, which could be due to the fully exposed surface of SiO2 support. Further,

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CONCLUSIONS In summary, by exploiting the solid-state hollow conversion process of the conc-MnO@(SiO2/Ni2+) with modulated core@ shell structure, we demonstrated a method of synthesizing a concave silica nanosphere, conc-(Ni/HMS)@SiO2, with the rarely explored open-mouthed cavity structure, which can constitute a new class of catalyst−support material. Moreover, the use of the galvanic replacement reaction occurring at the manganese-silicate layer also enabled the site-specific immobilization of a high density of catalytic NCs on the preformed concave interior surface of the conc-(Ni/HMS)@SiO2, producing an open-mouthed cavity-based nanoreactor system which has been unprecedented so far. This also represents the development of the postsynthetic functionalization protocol of the open-mouthed cavity-based nanoreactor, which facilitates diversifying and adjusting their catalytic parameters according to prespecified applications. Furthermore, through the comparative investigation with analogous systems, we also demonstrated the distinct and superior properties of the newly developed Pt@conc-SiO2 carrying catalytic Pt NCs inside the semiexposed and also semiprotected pocket-like space, in catalyzing the reduction reaction of nitroarene compounds, which include fast reaction rate, broad scope of applicable substrates, and high stability of immobilized catalysts during the recycling process. 7791


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02235. Additional TEM images, XRD-, XPS-, BET-data and reaction kinetics data (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Seung Hwan Cho: 0000-0001-5803-4922 In Su Lee: 0000-0002-2588-1444 Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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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) (I.S.L). We thank Mr. Taewon Jin and Prof. Ji Hoon Shim at POSTECH for their calculation of off-centered distances of core in the core@shell NPs.

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Concave Silica Nanosphere with a Functionalized Open-Mouthed

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