Fabrication of Smart Hybrid Nanoreactors from Platinum

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Fabrication of Smart Hybrid Nanoreactors from Platinum Nanodendrites Encapsulating in Hyperbranched Polyglycerol Hollow Shells Xue Li,† Jia Le Li,† Yan Wang,‡ and Tao Cai*,† †

Key Laboratory of Biomedical Polymers of Ministry of Education, College of Chemistry and Molecular Science, Wuhan University, Wuhan, Hubei 430072, P. R. China ‡ Key Laboratory of Material Chemistry for Energy Conversion and Storage of Ministry of Education, Huazhong University of Science and Technology, Wuhan, Hubei 430074, P. R. China S Supporting Information *

ABSTRACT: Hybrid nanorattles by spatial inclusion catalytic nanocores into hollow shells as nanoreactors have stimulated tremendous interests in advanced technologies for the sustainable production and conversion of energy. However, the nanoshells fabricated by inorganic compositions usually lack of desirable elasticity, permeability, and responsiveness to meet the requirement of increasingly complex and sophisticated catalytic systems. In this study, we describe a new strategy to construct a smart hybrid nanoreactor with an ultrathin and flexible polymer shell based on in situ thiol−ene photopolymerization. The morphological transitions in the hyperbranched poly(glycerol-co-N,N-diallyl glycidyl amine) (HPGD) shells of the Pt@hHPGD nanorattles result in the precise tuning of efficient mass transfer to encapsulated Pt nanocores in response to external stimuli. The fabricated Pt@hHPGD yolk− shell nanoreactors exhibited highly enhanced activity, longevity, and recyclability in catalyzing the hydrogenation of pnitrophenol, which are associated with the synergistic combination of the flexible and dual-responsive HPGD nanoshell and the immobilized metal-based catalyst system. KEYWORDS: hybrid nanorattles, nanoreactors, anionic ring-opening multibranching polymerization, thiol−ene photopolymerization, stimuli-responsive

1. INTRODUCTION In recent years, extensive research has been stimulated to the design and fabrication of high-performance metal-based nanocatalysts with regard to improved activity, longevity, and recyclability for catalytic transformations in sustainable fine chemical industry and environmental protection.1−5 In comparison with the homogeneous catalysts, which can dissolve/disperse in reaction media to make all catalytic sites accessible to the reactants, heterogeneous catalysts, especially free metal nanocatalysts, are apt to self-agglomerate to minimize their surface energy, leading to serious destruction of active centers and undesirable catalytic inactivation.6−8 Another challenge lies in the creation of recyclable systems where nanocatalysts can be easily isolated from the products and reused through a simple rejuvenation process. To circumvent the aforementioned difficulties, researchers commonly adopt appropriate supports to fix, stabilize, or even activate such nanoparticles to preserve their nanoscale size.9−11 Because of the weak physical interaction between nanoparticles and support materials, the involved nanoparticles are inclined to depart from the supports to either agglomerate into large pieces or dissociate to the reaction mixture. To date, an upsurge © XXXX American Chemical Society

in construction of hierarchical nanostructures (e.g., core−shell, yolk−shell, or bimetallic) as nanoreactors provided a new visual angle and analysis framework for studying next generation nanocatalysts.12−19 One unique characteristic of these nanostructures is to limit the direct contact or interaction among metal nanoparticles and therefore maintain metal nanoparticles steady and active even under stringent reaction conditions, whereas bestowing the effective diffusion of reactants to access as well as products to leave the surface of the metal nanoparticles. In this fashion, metal nanoparticles can spatially be confined in the nanoreactors partitioned from the bulk solution without leakage and aggregation. Building such smart nanoreactors, in particular functional polymers other than inorganic analogues, can synthetically recreate three-dimensional architectures and easily endow the nanoscale shell with desirable elasticity, permeability, and responsiveness.20−24 With the promise to surmount the intrinsically deficient end functionalities available in linear Received: February 5, 2018 Accepted: May 17, 2018

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DOI: 10.1021/acsanm.8b00191 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Scheme 1. Synthesis of Hyperbranched Poly(glycerol-co-N,N-diallyl glycidyl amine) (HPGD) Random Copolymers by Anionic Ring-Opening Multibranching Polymerization and Fabrication of Pt@hHPGD Yolk−Shell Nanoreactors by Combined Sol-Gel Co-Condensation, Thiol−Ene Photopolymerization, and NaOH Etching

2. EXPERIMENTAL SECTION

poly(ethylene glycol) (PEG) analogues, hyperbranched polyglycerol (HPG) with peripheral diols gains the advantages over PEG in a large variety of sophisticated applications once appropriate synthetic protocols have been developed.25,26 In view of a relatively low monomer concentration in the course of polymerization and immediate opening of oxirane rings, anionic ring-opening multibranching polymerization using slow monomer addition of glycidol with other oxirane comonomers has been explored for introducing tailored functionalities into HPG scaffolds with a broad range of accessible molecular weights, narrow molecular weight distributions, high solubility, thermal stability, chemical resistance, and biocompatibility.27,28 The incorporation of HPGs to inorganic-polymer hybrid nanocatalysts obviously offers great prospect for the generation of smart nanoreactors with abundant functional groups. The association of flexible HPGD polymers with Pt nanoparticles opens up exciting perspectives to obtain innovative yolk−shell nanoreactors that ideally integrate the dual responsiveness of the polymers with catalytic properties of the noble metal nanoparticles. In this contribution, we explored the synthetic fashion for the fabrication of narrowly dispersed hybrid nanorattles, enclosing a movable Pt nanodendrite in the inner cavity of flexible and dual-responsive hyperbranched polymer shell (Pt@hHPGD hybrid nanorattles, HPGD = hyperbranched poly(glycerol-coN,N-diallyl glycidyl amine, “h” refers to “hollow”)). A three-step strategy for the fabrication of yolk−shell nanoreactors is described in Scheme 1. It was envisioned that if this process is properly taking place, it could be a versatile and efficient method for decorating the exterior shells of the preformed rattle nanostructures with desirable elasticity, permeability and responsiveness, which are more advantageous for catalytic performances than other nanostructures.

2.1. Materials. The following chemicals were purchased from Sigma-Aldrich Chem. Co. and used as received, unless otherwise stated: (3-mercaptopropyl)-trimethoxysilane (MTS, 95%), 2,2-dimethoxy-2-phenylacetophenone (DMPA, 99%), ethylene glycol dimethacrylate (EGDMA, 98%), sodium methoxide (NaOCH3, 95%), sodium hydroxide (NaOH, ≥ 98%), 1,1,1-tris(hydroxymethyl)ethane (TME, 99%), 1,4-dioxane (99.8%) and glycidol (GLY, 96%). GLY and 1,4dioxane were purified by distillation under reduced pressure prior to use. N,N-diallyl glycidyl amine (DAGA) was prepared following a procedure described elsewhere.25,29 The synthesis of Pt@SiO2-MTS was performed following the previously reported method (see the Supporting Information for details).30,31 2.2. Synthesis of Hyperbranched Poly(glycerol-co-N,Ndiallyl glycidyl amine) (HPGD) Random Copolymers via Anionic Ring-Opening Multibranching Polymerization (AROMP). The synthesis of hyperbranched poly(glycerol-co-N,Ndiallyl glycidyl amine) (HPGD) random copolymers was achieved in a straightforward anionic ring-opening multibranching polymerization (AROMP) of GLY and DAGA in 1,4-dioxane, using TME as the initiator and NaOCH3 as the catalyst (Scheme 1).25,29,32 In the course of AROMP, TME (181 mg, 1.51 mmol) and 0.3 equiv. of NaOCH3 (24.3 mg, 0.45 mmol) were sequentially dissolved in anhydrous methanol (2 mL). Stirring at room temperature for 2 h and evacuation at 80 °C overnight afforded the partially deprotonated sodium alkoxide. The formed initiator salt was dispersed in anhydrous 1,4dioxane (20 mL), sonicated for 0.5 h and degassed by bubbling argon for 0.5 h. The flask was then sealed with a rubber stopper and the reaction was allowed to proceed under continuous stirring at 90 °C. A mixed solution of freshly distilled, argon-purged GLY (M1, 8.00 mL, 120.8 mmol) and DAGA (M2, 4.62 g, 30.2 mmol) in an equal volume of anhydrous 1,4-dioxane was slowly syringed to the reaction mixture over a period of 24 h. After dosing, AROMP was conducted for another 12 h for thorough polymerization. Afterward, the reaction flask was quenched in an icy water bath and methanol was added to protonate the active chain ends. Unreacted monomers and solvents were removed by distillation and the crude product was purified by repeated precipitation from DMF into a 10-fold excess of cold diethyl ether and finally dried in vacuo, to afford a highly viscous liquid. Yield: 82%. [M1]/[M2]/[I] = 80:20:1; Mn,NMR = 7400 g/mol. 1H NMR B


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ACS Applied Nano Materials (DMSO-d6, δ, ppm, TMS): 5.70−5.90 (2H × m, −CHCH2), 4.95− 5.15 (4H × m, −CHCH2), 4.35−4.84 (H × (l+3), -OH), 3.25−3.85 (H × (5l+3m+6), -OCH-, -OCH2-), 3.02−3.22 (4H × m, −N−CH2− CHCH2), 2.33−2.55 (2H × m, −N−CH2−CH−O−), 0.87 (3H, −CH3), where the letters l and m refer to the degree of polymerization of GLY and DAGA monomers in HPGD random copolymers, respectively. 2.3. Synthesis of Pt@SiO2@HPGD Core−Shell−Shell Nanospheres via Thiol−Ene Photopolymerization. The HPGD polymer shells were covalently attached to the surface of silica template layers using thiol−ene photopolymerization.33,34 The stock aqueous solution of Pt@SiO2-MTS nanospheres, as-prepared by the protocol in Supporting Information, were centrifuged and redispersed into DMF solution (30 mL, 1 mg/mL) comprised of HPGD (Mn,NMR = 7400 g/mol, 296 mg, 0.04 mmol) and EGDMA (188 μL, 1 mmol) by ultrasonication for 0.5 h. Subsequently, DMPA (10.3 mg, 0.04 mmol) was dosed to the reaction flask. The mixture was flushed with dry argon for 20 min and then irradiated by a 365 nm UV lamp at ambient temperature for 2 h. The obtained black suspensions were purified by extraction several times with DMF and absolute ethanol and finally redispersed into 20 mL of ultrapure water for further use. 2.4. Synthesis of Pt@hHPGD Yolk−Shell Nanoreactors by NaOH Etching. The synthetic method for the creation of vacant space between catalytic nanocore and tunable polymeric shell is based on a compositional difference, by which silica template layer of Pt@ SiO2@HPGD nanospheres was selectively etched away whereas the HPGD shell remains intact. About 40 mg of Pt@SiO2@HPGD nanospheres were introduced into an etching agent, 10 mL of 10 wt % NaOH. After 48 h etching reaction at room temperature, the excessive NaOH and Na2SiO3 were removed from the raw product by five centrifugation-redispersion runs in absolute water and ethanol. Furthermore, the hybrid nanorattles were dialyzed against ultrapure water for another week (MWCO 1000 g/mol). Finally, the Pt@ hHPGD yolk−shell nanoreactors were centrifuged, redispersed into 20 mL of ultrapure water and stored at ambient temperature for further use. 2.5. Confined Catalytic Hydrogenation of 4-Nitrophenol in Pt@hHPGD Yolk−Shell Nanoreactors. As a model reaction, the hydrogenation of 4-nitrophenol by NaBH4 was conducted to evaluate the catalytic performances of Pt@hHPGD yolk−shell nanoreactors.35,36 In a typical procedure, the Pt@hHPGD hybrid nanorattles was homogeneously dispersed in deionized water (1.0 mL, 10 mg/ mL) by ultrasonication and dosed to 4-nitrophenol aqueous solution (1 mL, 0.15 mM), followed by vigorously stirring for 10 min at ambient temperature. Around 1.5 mL of a freshly prepared, ice-cold NaBH4 (0.01 M) aqueous solution was swiftly dosed to the glass vial, which was stirred until the deep yellow solution completely fade. The resulting solution was transferred into a quartz cuvette to collect UV− visible absorption spectra. The transformation of absorption at 400 and 300 nm, respectively, was used to quantify reaction conversion. After each reaction run, the nanocatalysts were recycled by facile centrifugation and reused shortly without other treatments for another run. The process was repeated ten cycles to investigate the stability of the nanocatalyst. 2.6. Instrumentation. All nuclear magnetic resonance (NMR) spectra were acquired on a Bruker ARX 300 MHz NMR spectrometer. The chemical shifts were given in ppm and internally referenced to residual proton signals of the deuterated solvent peak, δ = 2.50 for DMSO-d6. Gel permeation chromatography (GPC) measurements were performed with an Agilent 1260 system using DMF (containing 0.25 g/L of LiBr) as the eluent at a flow rate of 1.0 mL/min. A refractive index detector, 5 μm guard column, three PL gel columns (two Agilent Mixed-C 5 μm columns and one Agilent Mixed-D 5 μm column) and narrow molecular weight poly(ethylene glycol) standards were used. Transmission electron microscopy (TEM) measurements were conducted on a JEOL JEM-2100F microscope operated at 200 kV. Surface compositions of the nanoparticles were detected by X-ray photoelectron spectroscopy (XPS) measurements on a Kratos AXIS Ultra DLD spectrometer sourcing with a monochromatized Al Kα Xray source (1468.71 eV photons). The hydrodynamic diameter of

nanoparticles was measured form the dynamic laser scattering (DLS) analysis conducting on a Brookhaven 90 plus laser light scattering spectrometer (θ = 90°). At least five measurements were performed to obtain the average values.

3. RESULTS AND DISCUSSION 3.1. Synthesis of HPGD Random Copolymers. In view of a relatively low monomer concentration in the course of polymerization and immediate opening of oxirane rings, anionic ring-opening multibranching polymerization (AROMP) using slow monomer addition of glycidol (GLY) with other oxirane comonomers has been explored for introducing tailored functionalities into hyperbranched polyglycerol (HPG) scaffolds with a broad range of accessible molecular weights, narrow molecular weight distributions, high solubility, thermal stability, chemical resistance and biocompatibility.25−30 However, the limited options for characterization of the grafted polymer brushes and the incompatibility with highly basic catalysts eventually lead to the failure in direct AROMP from silica-mediated nanoparticles. To optimize desirable functionalities and maximize the structural advantages, narrowly distributed hyperbranched poly(glycerol-co-N,N-diallyl glycidyl amine) (HPGD) random copolymers, prepared a priori via AROMP of GLY and N,N-diallyl glycidyl amine (DAGA) in the presence of sodium methoxide as the catalyst and 1,1,1tris(hydroxymethyl)ethane (TME) as the initiator, was then linked to silica-mediated nanoparticles through thiol−ene photopolymerization, as described in Scheme 1. GLY is an established multibranching monomer for AROMP, whereas the selection of DAGA lies in their well-preserved pH and temperature sensitivity and sufficient supply of grafting sites.25,29 The representative structure of HPGD random copolymer was analyzed by 1H NMR spectroscopy as illustrated in Figure S1. All the chemical shifts were assigned on the basis of analogy with previously reported data.25−30 It is worth noting that the resonances attributed to the characteristic signals of methyl protons (3H, i) adjacent to the vinyl groups from the isomerized DAGA units, are not discernible in the 1H NMR spectrum of HPGD. Owing to the lower interaction of smaller and better solvated sodium counterion with π-orbitals of the double bond, the involvement of sodium methoxide catalysts effectively suppressed the isomerization of terminal allyl moieties in the DAGA units.29 The molecular weight (Mn,NMR) and composition of the HPGD random copolymers can be calculated from 1H NMR spectroscopy data, by comparison of the integrated areas of methine protons and methylene protons at δ = 3.25−3.85 ppm (5H, b and c) adjacent to the diols or ether groups in the polyether backbone, to those of methylene protons (2H, f) at δ = 3.02−3.22 ppm adjacent to vinyl groups in the DAGA branches, to those of methyl protons (3H, a) at δ = 0.87 ppm in the TME initiators, as described in Table S1.29,30 A fixed [GLY]/[TME] molar feed ratio of 80:1 were selected for the synthesis of HPGD random copolymers to achieve a balance between appropriate temperature sensitivity and ample grafting sites (vinyl groups from DAGA component). With the increase in molar feed ratio of [DAGA] to [TME] from 10 to 60, the number-average molecular weight (Mn,GPC) of the HPGD random copolymers increases from 6000 to 11000 g/mol, as determined from gel permeation chromatography (GPC). The GPC measurements showed a monomodal molecular weight distribution with polydispersity index (PDI) of 1.16−1.28, confirming excellent C


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and enclosed by smooth silica nanoshells in narrow distribution, indicating the uniform deposition of silica on the surface of the Pt nanocores. The Fourier transform infrared (FTIR) spectrum of the Pt@ SiO2-MTS nanospheres is displayed in Figure S3a. The formation of Pt@SiO2-MTS nanospheres is accompanied by the signature of stretching vibration of Si−O−Si bonds at around 1,098 cm−1 and thiol groups at around 2,550 cm−1. The X-ray photoelectron spectroscopy (XPS) wide-scan, C 1s and S 2p core-level spectra of the Pt@SiO2-MTS nanospheres are demonstrated in Figures 2a, e, and e′.39,40 Concomitantly, the Pt 4f signal has been completely erased in the XPS wide-scan spectrum owing to the compact and thick distribution of SiO2MTS shells. The curve-fitted XPS C 1s core-level spectrum (Figure 2e) splits into two peak components of respective binding energies (BEs) at 284.6 eV for the -C-C/-CH−/−C-Si species and 285.6 eV for the -C-S species.39,40 The S 2p corelevel spectrum (Figure 2e′) is composed of a spin−orbit split doublet (S 2p3/2 and S 2p1/2) with respective BEs at 163.1 and 164.3 eV and spectral component area ratio approximates 2:1, which is in accordance with the covalently bonded sulfur species. 3.3. Preparation of Pt@SiO2@HPGD Core−Shell−Shell Nanospheres. The peripheral thiol groups on the SiO2-MTS shell allow facile inclusion of Pt@SiO2-MTS nanospheres into the HPGD scaffolds via in situ thiol−ene photopolymerization in a “grafting from” strategy.41,42 The TEM images of Figure 1c, d, and d′ depict a solid Pt@SiO2-MTS core−shell nanosphere of deeper contrast surrounded by spherical polymer frameworks of differential contrast, leading to an exquisite core− shell−shell nanocomposite. The molecular weights of HPGD random copolymers can be used to control the grafting efficiency and cross-linking density of in situ polymerization, as well as adjust the shell thickness of the resulting nanospheres. The average size, SDI and shell thickness of Pt@SiO2@HPGD core−shell−shell nanospheres are tabulated in Table 1. The changes in surface composition of the exterior corona of the nanospheres can also be investigated by XPS measurements. The Pt@SiO2@HPGD-3 nanospheres with shell thickness comparable to the 8 nm probing depth of XPS technique in organic matrices led to the signal elimination of characteristic S and Si elements in the XPS wide-scan spectrum (Figure 2b). The C 1s core-level spectrum (Figure 2f) can be curve-fitted with three peak components having BEs at 284.6, 285.6, and 286.2 eV, attributable to the respective -C-C/-CH-, -C-S/-C-N and -C-O species.39 Concomitantly, the -C-O peaks, associated with the abundant ether repeat units in the crosslinked HPGD domain, have taken dominant position, in reference to the -C-C/-CH- peaks. Differing from that of the starting Pt@SiO2-MTS nanospheres, no remaining signals of the thiol groups is visible in the FTIR spectrum of Pt@SiO2@ HPGD-3 nanospheres, suggesting the successful inclusion of Pt@SiO2-MTS nanospheres into the HPGD frameworks (Figure S3b). 3.4. Preparation of Pt@hHPGD Yolk−Shell Nanoreactors. Despite of tedious procedure, template-assisted methods have been widely recognized as the most representative and versatile approach toward hollow nanostructures, which involve the manipulation of the distinct physical/ chemical properties between the removable templates and the shell components. The silica fillings of the Pt@SiO2@HPGD core−shell−shell nanospheres could be etched away under basic conditions, forming interior cavity between the Pt

control over the polymerization via slow monomer addition technique. The observed Mn,GPC of HPGD random copolymers are found to be slightly lower than anticipated, which probably arises out of the lower hydrodynamic volume of hyperbranched polymers with compact structure than that of linear standards and the interaction of peripheral terminal groups with the GPC column fillers.25,29,30 3.2. Preparation of Pt Nanodendrites and Pt@SiO2MTS Core−Shell Nanospheres. Because of the high cost of noble metal catalysts, the controlled construction of monodisperse platinum nanodendrites (Pt NDs) with diameter greater than 5 nm and easy to scale-up synthetic protocol is of crucial importance in exploring highly active and recyclable nanocatalysts for the fine chemicals industry and environmental protection.28 A one-step seed-mediated growth strategy for the synthesis of water-dispersed Pt NDs within the range of 12−28 nm in size were developed by reduction of chloroplatinic acid solution with L-ascorbic acid for further Pt deposition on different feed molar ratio of seeds.37,38 Transmission electron microscopy (TEM) images reveal that the grape-like Pt NDs were composed of a certain amount of nanocrystallites (3−5 nm in size) and had narrow size distribution index (SDI) less than 1.1 (Figure 1a and Figure S2).

Figure 1. TEM images of the (a) Pt-2, (b) Pt@SiO2-MTS, (c) Pt@ SiO2@HPGD-3, (d, d′) Pt@SiO2@HPGD-4, (e, e′) Pt@hHPGD-3 and (f, f′) Pt@hHPGD-4 nanoparticles. All scale bars are 100 nm.

Typical synthesis of hollow nanostructures involves the sequential growth of desirable shells with the assistance of sacrificial templates. A solid silica shell was constructed on the Pt ND surfaces to form Pt@SiO2-MTS core−shell nanostructures by the sol−gel co-condensation of tetraethyl orthosilicate (TEOS) and (3-mercaptopropyl) trimethoxysilane (MTS). The SiO2-MTS coating adopts a dual role both as initiation sites and sacrificial template.31,33,34 The TEM images of the Pt@SiO2-MTS core−shell nanospheres with a shell thickness of about 22 nm is visualized in Figure 1b. Pt NDs were centered D


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Figure 2. XPS wide-scan, C 1s, N 1s, S 2p, and Pt 4f core-level spectra of the (a, e, e′) Pt@SiO2-MTS, (b, f, f′) Pt@SiO2@HPGD-3, (c, g, h) Pt@ hHPGD-3, and (d) Pt@hHPGD-4 nanoparticles.

Creating highly confined chemical environments partitioned from the bulk solution, nanoreactors have been broken through in the replacement of conventional heterogeneous catalysts.13−19 The complexity of the Pt@hHPGD hybrid nanorattles in both structure and composition might endow the nanocatalysts with protective capability and enhanced dispersity in aqueous media for catalytic applications. Although the catalytic reduction of p-nitrophenol (p-NP) by borohydride ions is thermodynamically feasible, the kinetic restriction by the strong repelling among the intermediate p-nitrophenolate and borohydride ions usually requires the involvement of noble metal nanoparticles for electron transfer. Upon dosing excessive NaBH4 reducing agents to the reaction mixture, the catalytic hydrogenation of p-NP to p-aminophenol (p-AP) can be regarded as a pseudo-first-order reaction, accompanied by the decrease of specific absorption peak of the reactant (p-NP) at 400 nm and a concomitant increase in the absorption peak of the product (p-AP) at 300 nm, as recorded by time-related changes in UV−vis spectroscopy in Figure 3A. No catalytic reduction of p-NP was observed in the presence of any types of Pt@SiO2 and Pt@SiO2@HPGD nanospheres since all the reactants can hardly penetrate through the dense silica layer. A linear correlation was fitted between -ln[Ct/C0] and reaction time t in Figure 3B and the apparent reaction rate ka was calculated to be 3.0 × 10−3 s−1, which is comparable to the starting naked Pt-2 nanodendrites (3.4 × 10−3 s−1) and most of previously reported cases.35,36 As expected, with the increase of shell thickness from 0 to 16 nm, ka decreases from 3.4 × 10−3 to 2.6 × 10−3 s−1 (Figure S5). It is rather remarkable that the flexible HPGD shell held the capability to regulate the interfacial behavior of the hybrid nanorattles and generated more open channels than the analogues surrounded by rigid constituents (such as aromatic polymers or inorganic nanoparticles). The nanoreactors can be recycled by facile

nanocore and HPGD shell. The mechanical stability is of paramount importance in the design of nanocatalysts. The yolk−shell nanostructure remains intact after being repetitively subjected to various organic solvents and acid/base extraction, centrifugation, and catalytic reactions, as demonstrated by the TEM images in Figure 1e, e′, f, and f′. The HPGD shell with a thickness of 13 nm holds ample strength to fully protect the interior cavity (Figure 1f, f′). Further compressing the HPGD shell thickness to approximate 8 nm will give rise to the creation of wrinkled and concave yolk−shell nanostructures, as shown in Figures 1e and 1e′.20,21 The phenomenon can be attributed to the comparatively small molecular weight and the reduced degree of cross-linking in the HPGD domains. It can be further reflected by the signal recapture of Pt element in the wide-scan and Pt 4f core-level spectra of Pt@hHPGD-3 hybrid nanorattles (Figure 2c, h). For comparison purpose, no metal signals (Pt 4f7/2 and Pt 4f5/2 in the range of 65−80 eV from the inner Pt nanocores) are detected in the XPS wide scan spectrum of Pt@hHPGD-4 hybrid nanorattles (Figure 2d), which result from the coverage of a much thicker HPGD coating layer with thickness around 13 nm. Figure S4 describes the respective thermogravimetric ananlysis (TGA) curves of the HPGD-3 random copolymers (curve a), Pt@hHPGD-3 (curve b) and Pt@hHPGD-4 hybrid nanorattles (curve c). The onset of thermal decomposition temperatures for HPGD copolymers were around 300 °C. The weight content of HPGD shell in the Pt@hHPGD-3 hybrid nanorattles, as derived from the weight residuals at 700 °C of these three samples, is 72.0 wt %, which is smaller than that of 83.2 wt % in the Pt@hHPGD-4 hybrid nanorattles, consistent with the fact that the latter hybrid nanorattles possess a thicker exterior shell. Further decrease in the shell thickness of the Pt@hHPGD hybrid nanorattles to 3 nm failed to form stable yolk−shell nanostructures when preparation under similar protocols. E


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The shell thicknesses of the single shell and double shell were derived from the TEM images.

LCST (30−35 °C), the HPGD branches undergo a phase transition to a hydrophobic state and adopt a compact conformation and become less open structure to the diffusion rate through the shell. The Arrhenius-mode increase in the ka with reaction temperature is partly offset by the decrease of diffusion rate, leading to a drastic drop of ka. A gradual acceleration of ka is observed as the diffusion of reactants will no more suppressed by the completely collapsed HPGD polymer shells once the reaction temperature exceeds 35 °C. The amine pendant groups in the HPGD shells also endow Pt@hHPGD-3 hybrid nanorattles with pH sensitivity. The results can be reflected by the decrease of corresponding Dh upon raising the solution pH, with the most rapid change being observed in solution pH between 6 and 8 (Figure 3D). For the catalytic reduction of p-NP by NaBH4, the depletion of pnitrophenolate ions under acidic conditions (pH 8.0) because the contribution of mass transport from the fully collapsed HPDG branches will has negligible effects on the overall ka (Figure 3D). Similar trends on the change of hydrodynamic diameter and ka have been found when reversely decreasing the reaction temperature or solution pH during the hydrogenation of p-NP adopting the Pt@hHPGD-3 yolk−shell nanoreactors as catalysts, confirming excellent reversibility of temperature and pH responsiveness of as-obtained nanoreactors (Figure S7). Thus, the HPGD polymer shells with dual responsiveness on the Pt@hHPGD hybrid nanorattles can perform as “nanopistons” to regulate the diffusion of reactants and the corresponding ka toward the catalytic hydrogenation.

centrifugation and reused for at least 10 successive cycles with average conversions in all cases exceeding 95% and negligible mass loss (Figure S6). Thus, the synthesized hybrid nanorattles can serve as a confined nanoreactor system for catalytic reactions. The hydrodynamic diameter (Dh) of the Pt@hHPGD-3 hybrid nanorattles displayed a temperature-responsive characteristic, with the most abrupt transformation being observed in temperature between 30 and 35 °C (Figure 3C), which is attributable to the interplay of the hydrophilic glycerol branches and the hydrophobic DAGA units in the HPGD shells. For this intermediate range of compositions, the modulation of the lower critical solution temperatures (LCST) over a wide temperature range by altering the DAGA comonomer ratio. The discontinuous coil-to-globule transition of the HPGD shell upon heating renders the dependence of reaction temperature on ka deviating from Arrhenius law, when Pt@hHPGD hybrid nanorattles were evaluated as nanoreactors for the reduction of p-NP by NaBH4 (Figure 3C).31,43 At reaction temperature lower than the LCST (≤30 °C), the HPGD branches are hydrophilic, fully swollen in water and assume an extended conformational state, and afford less steric hindrance to the diffusion of reactants through the HPGD corona. Consequently, a rise in reaction temperature leads to a monotonic acceleration of ka, as predicted by the Arrhenius equation. On the contrary, once the reaction temperature approaches the

4. CONCLUSIONS We have developed a new strategy for controlled construction of Pt@hHPGD hybrid nanorattles by inclusion of individual Pt nanodendrite into the flexible HPGD spherical framework by selective etching of the silica template in the Pt@SiO2@HPGD core−shell−shell nanospheres. The latter nanospheres were prepared a priori by sol−gel co-condensation of TEOS and MTS, followed by in situ thiol−ene photopolymerization of HPGD random copolymers. This synthetic protocol is applicable to a broad range of metal nanoparticles in nonagglomerated fashion and is capable of manipulating the spatial distribution of nanoparticles within the hollow polymer matrix. In view of the characterization of their flexible and stable yolk−shell nanostructures, the catalytic performances were explored in the hydrogenation of p-NP by NaBH4 in aqueous solution, and the yolk−shell nanoreactors were observed to have outstanding catalytic activity with pH and temperature sensitivities, high thermal stability and good recyclability. It can be envisioned that such Pt@hHPGD nanoreactors would display molecular sieving properties, which are therefore used to regulate substrate reactivity and selectivity in heterogeneous catalysis by manipulation the molecular traffic to and from the active nanocores. In the case of HPGD hollow shells exhibiting dual stimuli-responsive features, they allow the mass transport to entrapped catalytically active cargo to be triggered or creased by environmental signals. We believe the current synthetic protocols offer a wide variety of possibilities

Table 1. Average Size, Size Distribution Index, and Shell Thickness of the Platinum Nanodendrites, Platinum@Silica Core−Shell, and Platinum@Silica@Polymer Core−Shell− Shell Nanoparticles sample

Dnc (nm)

Dwc (nm)

SDIc

12 16 21 28 60 65

13 17 22 30 64 68

70

Pt-1 Pt-2 Pt-3 Pt-4 Pt@SiO2-MTSa Pt@SiO2@ HPGD-1b Pt@SiO2@ HPDG-2b Pt@SiO2@ HPDG-3b Pt@SiO2@ HPDG-4b Pt@SiO2@ HPDG-5b

shell thickness (nm)d

CVc (%)

1.08 1.06 1.05 1.07 1.05 1.06

22 22 + 3

18 23 19 21 11 13

73

1.04

22 + 5

10

76

79

1.04

22 + 8

10

85

88

1.03

22 + 13

9

92

96

1.02

22 + 16

7

a

The Pt@SiO2-MTS core−shell nanoparticles were prepared using the Pt-2 nanodendrites as seeds. bThe Pt@SiO2@HPGD core−shell−shell nanoparticles were prepared using the Pt@SiO2-MTS nanoparticles as precursors. cDn is the number-average diameter (nm), Dw is the weight-average diameter (nm), n is the number of nanoparticles, SDI is the size distribution index, and CV is the coefficient of variation or the ratio of standard deviation to the mean, as follows: k

Dn =

i=1

=

k

k

k

∑ niDi /∑ niDw = ∑ niDi4 /∑ niDi3SDI = Dw /DnCV i=1

k ∑i = 1 (Di

i=1

− Dn)

k−1

i=1

2

/Dn

d

F


Article


Figure 3. (A) UV−vis spectroscopy monitored at different reaction times at 25 °C; (B) plot of −ln(Ct/C0) as a function of reaction time; influence of increasing (C) reaction temperature and (D) solution pH on the pseudo-first-order rate constant ka (black line, open circles) and hydrodynamic diameters Dh (red line, solid squares) during the catalytic hydrogenation of p-NP using the Pt@hHPGD-3 yolk−shell nanoreactors as catalysts. Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces. Science 2014, 343, 1339−1343. (2) Zhang, L.; Roling, L. T.; Wang, X.; Vara, M.; Chi, M. F.; Liu, J. Y.; Choi, S.; Park, J.; Herron, J. A.; Xie, Z. X.; Mavrikakis, M.; Xia, Y. N. Platinum-Based Nanocages with Subnanometer-Thick Walls and WellDefined, Controllable Facets. Science 2015, 349, 412−416. (3) Huang, X.; Zhao, Z.; Cao, L.; Chen, Y.; Zhu, E.; Lin, Z.; Li, M.; Yan, A.; Zettl, A.; Wang, Y. M.; Duan, X.; Mueller, T.; Huang, Y. HighPerformance Transition Metal-Doped Pt3Ni Octahedra for Oxygen Reduction Reaction. Science 2015, 348, 1230−1234. (4) Zhang, Y.; Cui, X.; Shi, F.; Deng, Y. Nano-Gold Catalysis in Fine Chemical Synthesis. Chem. Rev. 2012, 112, 2467−2505. (5) Chang, L. Y.; Barnard, A. S.; Gontard, L. C.; Dunin-Borkowski, R. E. Resolving the Structure of Active Sites on Platinum Catalytic Nanoparticles. Nano Lett. 2010, 10, 3073−3076. (6) Zeng, H. C. Integrated Nanocatalysts. Acc. Chem. Res. 2013, 46, 226−235. (7) Prieto, G.; Tuysuz, H.; Duyckaerts, N.; Knossalla, J.; Wang, G.H.; Schuth, F. Hollow Nano- and Microstructures as Catalysts. Chem. Rev. 2016, 116, 14056−14119. (8) Zhang, Q.; Lee, I.; Joo, J. B.; Zaera, F.; Yin, Y. Core-Shell Nanostructured Catalysts. Acc. Chem. Res. 2013, 46, 1816−1824. (9) Yang, H.; Bradley, S. J.; Chan, A.; Waterhouse, G. I. N.; Nann, T.; Kruger, P. E.; Telfer, S. G. Catalytically Active Bimetallic Nanoparticles Supported on Porous Carbon Capsules Derived from Metal-Organic Framework Composites. J. Am. Chem. Soc. 2016, 138, 11872−11881. (10) Petrosko, S. H.; Johnson, R.; White, H.; Mirkin, C. A. Nanoreactors: Small Spaces, Big Implications in Chemistry. J. Am. Chem. Soc. 2016, 138, 7443−7445. (11) Kim, S. M.; Jeon, M.; Kim, K. W.; Park, J.; Lee, I. S. Postsynthetic Functionalization of a Hollow Silica Nanoreactor with Manganese Oxide-Immobilized Metal Nanocrystals Inside the Cavity. J. Am. Chem. Soc. 2013, 135, 15714−15717. (12) Fang, X.; Liu, Z.; Hsieh, M.-F.; Chen, M.; Liu, P.; Chen, C.; Zheng, N. Hollow Mesoporous Aluminosilica Spheres with Perpendicular Pore Channels as Catalytic Nanoreactors. ACS Nano 2012, 6, 4434−4444. (13) Cai, T.; Zhang, B.; Chen, Y.; Wang, C.; Zhu, C. X.; Neoh, K. G.; Kang, E. T. Preparation and Unique Electrical Behaviors of

to engineer smart hybrid nanoreactors with various compositions.

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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/acsanm.8b00191. Experimental details on the synthesis and characterization of Pt nanodendrites, Pt@SiO2-MTS nanospheres, and HPGD copolymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yan Wang: 0000-0001-6569-5649 Tao Cai: 0000-0001-7200-5343 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (51503155, 51773156, and 51703167), the Natural Science Foundation of Jiangsu Province (BK20160384), the Natural Science Foundation of Hubei Province (2016CFB381), and Open Research Fund Program of Key Laboratory of Material Chemistry for Energy Conversion and Storage of Ministry of Education at Huazhong University of Science and Technology.

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Fabrication of Smart Hybrid Nanoreactors from Platinum

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