Topological Transformations of CoreâShell Precursors to

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Topological Transformations of Core-Shell Precursors to Hierarchically Hollow Assemblages of Copper Silicate Nanotubes Guowu Zhan, and Hua Chun Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11808 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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ACS Applied Materials & Interfaces

Topological Transformations of Core−Shell Precursors to Hierarchically Hollow Assemblages of Copper Silicate Nanotubes Guowu Zhan and Hua Chun Zeng* Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ABSTRACT: Functional hollow materials have attracted extensive research attention due to their promising prospects for catalysis.

Herein, we report an alternative synthesis of hierarchically hollow structured materials directly from core-shell structured templates, based on confined chemical reactions between the solid matters of core and shell under hydrothermal conditions. More specifically, we have developed a novel and facile strategy to transform core-shell structured Cu2O@mSiO2 (m = mesoporous) to tubular copper silicate assemblages (TCSA). Depending on the original shapes of Cu2O, TCSA can be tailored as spherical or cubic assemblages with stacking copper silicate nanotubes (inner diameter: 4.5 nm, thickness: 0.8 nm, length: ca. 96 nm) in the shell. Moreover, by utilizing the residual reductive Cu(I) (ca. 10 at% of total surface copper) on TCSA support, in-situ generations of Pd nanoparticles (~4.5 nm) and Au nanoparticles (~5.8 nm) were successfully achieved based on the spontaneous galvanic replacement reactions. Two integrated nanocatalysts (viz., Pd/TCSA and Au/TCSA) have been prepared with this approach. As an example, the Pd/TCSA exhibits excellent activity and recyclability for Suzuki-Miyaura cross-coupling reactions. KEYWORDS: integrated nanocatalysts, hierarchical structures, copper silicates, hollow, Suzuki-Miyaura reactions

1. INTRODUCTION It has been well conceived that new structures are pivotal for fundamental advances in exploiting new properties a material will have, since the structure dictates function.1-2 Particularly, core-shell structured materials (refer to Scheme 1, structures 1 and 2, differed with channel in shells) have attracted intense interest due to their distinguishing properties for broad technological applications, such as drug delivery, optics, electronics, absorbents, catalysis, biomedical imaging, environmental remediation, and beyond.3-10 Accordingly, versatile core-shell structured composites, including inorganic, organic, and inorganic/organic hybrid ones, have been designed and used over the past two decades. Among the possible inorganic shells, many research endeavors have been paid on growing sol-gel derived silica on a variety of colloidal materials. Insulation or confinement of materials with a surface phase of silica can have new technological advantages, such as high biocompatibility, controllable porosity, enhanced suspension stability, optical transparency, easy design and synthesis, and advanced fabrication (usually by means of Stöber process) and functionalization.11-12 Conceptually, core-shell structured materials can be used as sacrificial templates to create hollow interiors by (i) selective removal of core material (Scheme 1, structure 3) or (ii) elaborately etching inner part of the shell (Scheme 1, structure 4).13-14 In this regard, void space enclosed by porous shell can be functionalized as a nanoreactor for catalytic reaction or as a nanocontainer for enhanced loading of guest molecules in controlled release application.15-17 Therefore, in recent years, a massive work has been devoted to the synthesis of hollow materials based on selectively etching cores in the core-shell structured templates (or precursors) by choosing proper etching agents. Typically, such etchants include NaOH, Na2CO3, NaBH4, ammonia solution, and HF solution, which are particularly effective for hollowing silica-based core-shell structured

materials.18-21 In most of the reported systems, the cores were merely used as sacrificial templates for creating interior spaces and the shell parts were utilized only as a protective coating layer to retain the original boundary. Another concern arises from the fact that pore size distribution in silica shell derived from such etching method is usually randomly distributed, together with the relatively low surface area and pore volume.22 This is due to that the porosity of silica is based on reversible hydrolysis and condensation of alkoxides, e.g., pore size can be expanded beyond 10 nm by the Si–O bond breakage, while it can be reduced to zero by the Si–O bond reformation.23 To the best of our knowledge, up to now, there is no reported case which attempts to fabricate hollow materials via direct chemical interactions just between solid matters in core and shell (viz., allowing reaction of A (core) + B (shell) → C (hollow product) to take place). Such a synthesis is challenging, considering that the solvent-insoluble solid matters are difficult to be activated. However, as we believe, it is greatly important because it allows simultaneous composition transformation (forming a new phase) and morphology evolution in a single step (viz., one-pot synthesis). Our group has proposed an inside-out preinstallation-infusion method for the targeted synthesis of Keggin heteropoly acids within mesoporous silica hollow spheres.15 In that process, the encapsulated MoO2 cores were oxidized to movable Mo6+ ions and then infused to mesoporous silica shells, which formed heptamolybdate species (e.g., Mo7O246–) uniformly dispersed on the mesopore surfaces of silica, and also generated void space at the structure center (Scheme 1, transformation from structure 2 to structure 5). Afterward, an analogous strategy called “dissolution and entrapment” was developed to convert core-shell structure to void-shell structure, in which the cores (i.e., polymers) were firstly dissolved and the fragments were then incorporated into the mesoporous shell.24-26 However, the mesoporous shells in the above studies might suffer from the risk of pore blocking or structural damage due to

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infiltration or interpenetration of core phase. It is noted that, while the cores were removed or reduced in the above processes, neither composition nor morphology of the porous shells was seriously altered, since their structures were largely retained. Therefore, to push this field of research further, it would be highly desirable to produce hierarchically hollow materials through direct chemical reactions between the two solid phases of core and shell, and at the same time to tune the product morphology through pre-shaping core-shell structures. Herein, in the above context, we will present a methodic concept of using both core and shell phases as starting precursors to synthesize hierarchically tiered hollow materials. More specifically, we will report the development of tubular copper silicate assemblages (TCSA), and their derived catalysts (e.g., loading noble metal nanoparticles). In this research, core-shell structured Cu2O@mSiO2 (m = mesoporous) was used both as a template and as a source of reactants, and TCSA was then formed based on the chemical reactions between the transition metal oxide core (viz., Cu2O) and the mesoporous siliceous shell (viz., mSiO2). As depicted in Scheme 1, the preparative procedure of TCSA consists of three major steps as follows: (i) synthesis of spherical and cubic Cu2O cores (structures 6 and 10), (ii) conformal coating of the cores with uniform shells of mesoporous silica (structures 7 and 11), and (iii) transformation of core-shell structured Cu2O@mSiO2 to TCSA under one-pot hydrothermal treatments (structures 8 and 12). Considering the tiered porosities attained in the TCSA, which may have some advantages for heterogeneous catalysis applications, metal nanoparticles (e.g., Pd, Au) were further deposited on the TCSA support based on spontaneous galvanic replacement reactions between metal precursors and the residual Cu(I) ions (vide infra). Finally, the workability of the resultant integrated nanocatalysts (i.e., M/TCSA; structures 9 and 13) were tested using model reactions of Suzuki-Miyaura coupling.

2. EXPERIMENTAL SECTION 2.1. Materials. The following chemicals were used as received without further purification: tetraethylorthosilicate (TEOS, Sigma-Aldrich, 99%), gold (III) chloride trihydrate (Sigma-Aldrich, 99.9%), palladium (II) chloride (Sigma-Aldrich, 99.9%), copper (II) nitrate trihydrate (Merck, 99.5%), copper (II) chloride dihydrate (Sigma-Aldrich, 99%), ammonia solution

Scheme 1. Schematic flowchart of targeted synthesis of tubular copper silicate assemblages (TCSA) and the derived integrated nanocatalysts. Structures (1-5): core-shell starting materials and some analogues with hollow interiors, which are discussed in the introduction section. (6, 10) Cu2O core (illustrated in red color), (7, 11) Cu2O@mSiO2 (mesoporous silica layer was illustrated in red color), (8, 12) TCSA, (9, 13) metal/TCSA integrated nanocatalysts (yellow tiny spheres: metal nanoparticles).

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(Merck, 25%), triethylamine (TEA, Fisher, 99%), cetyltrimethylammonium chloride (CTAC, Sigma-Aldrich, 25%), polyvinylpyrrolidone (PVP, Aldrich, K-30), phenylboronic acid (Alfa Aesar, 98+%), hexamethylbenzene (Alfa Aesar, 99%), L-ascorbic acid (Sigma-Aldrich, 99%), potassium carbonate (Merck, 99%), sodium hydroxide (Merck, 99%), diethylene glycol (DEG, Sigma-Aldrich, 99%), ethyl acetate (Merck, 99.5%), acetone (Fisher, 99.5 %), and ethanol (Fisher, 99.99%). Seven kinds of aryl halides were bought from Sigma-Aldrich. Deionized water was used for all experiments. 2.2. Synthesis of spherical Cu2O. 0.5 mmol of Cu(NO3)2, 1.0 g of PVP and 20 mL of DEG were poured into a two-necked glass flask. The mixture was then heated at 45°C for 1 h to fully dissolve the solutes. Afterward, the mixture was heated from 45°C to 192°C within 19 min, and color of the solution turned from blue to gray-orange gradually. The product was then cooled, washed with acetone and ethanol for three times. 2.3. Synthesis of cubic Cu2O. Cubic Cu2O crystals were prepared through ascorbic acid reduction route at room temperature. More specifically, 0.1 mmol of CuCl2 and 0.1 g of PVP were dissolved in 40 mL of water, followed by a dropwise addition (30 µL/s) of 2.5 mL of 0.2 M NaOH. Then, the solution was stirred magnetically for 5 min, followed by a dropwise addition (10 µL/s) of 2.5 mL of 0.1 M ascorbic acid. After the addition, the mixture was further stirred for 5 min. The product was recovered by centrifugation and then washed with ethanol twice. 2.4. Preparation of core-shell structured Cu2O@mSiO2. Briefly, 16 mg of spherical Cu2O particles were re-dispersed in a mixed solvent (25 mL of H2O and 20 mL of ethanol) by sonication for 30 min. Then 0.16 mL of CTAC was added to the mixture, and stirred for 10 min. Later, 90 µL of TEA (10 vol%, diluted by ethanol) was added to the above mixture and stirred thoroughly for another 10 min, followed by an addition 50 µL of TEOS. The resultant mixture was further stirred at room temperature for 14 h before separating the solid phase via centrifugation. The product (i.e., Cu2O@mSiO2) was washed twice with ethanol and dried in an oven at 60°C overnight. In the case of cubic Cu2O, the mixed solvent was composed of 10 mL of H2O and 30 mL of ethanol, and 150 µL of TEA was used when all other experimental parameters remained unchanged. 2.5. Preparation of TCSA. Typically, 32 mg of the obtained Cu2O@mSiO2 core-shell precursors were re-dispersed into 10 mL of water under sonication for 1 h. Then, the suspension was hydrothermally treated at 190°C for 11 h. During that treatment, the encapsulated Cu2O core would in-situ react with the silica shell to form TCSA material. Reaction products were then collected via centrifugation, followed by washing with ethanol twice, and drying at 60°C overnight. 2.6. Preparation of integrated nanocatalysts (M/TCSA). In this synthesis, 25 mg of the above TCSA powder was re-dispersed into 10 mL of ethanol under sonication for 0.5 h. Then the ethanolic suspension was rapidly added to a mixture of 70 mL of water, 120 µL of PdCl2 (112.7 mM), and 500 µL of PVP (22.6 g/L) under vigorous stirring. The mixture was further stirred for 1 h at room temperature. The color of the suspension turned to black color after ca. 2 min, indicating the formation of Pd nanoparticles. After the reaction, the products were collected via centrifugation, followed by washing with ethanol twice. Deposition of Au nanoparticles on TCSA was carried out in a similar manner, except that HAuCl4 was used, and PVP (22.6 g⋅L−1) solution was added at 40 s after adding ethanolic TCSA suspension. 2.7. Characterization techniques. Morphologies of samples were characterized by transmission electron microscopy (TEM,


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JEM-2010) and high-resolution TEM (HRTEM, JEM-2100F). The crystallographic information was analyzed by X-ray diffraction (XRD, Bruker D8 Advance) and selected area electron diffraction (SAED, JEM-2100F). Elemental analysis was carried out by energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments, Model 7426). Specific surface areas, pore volume, and pore size of some representative samples were determined using N2 physisorption isotherms at 77 K (Quantachrome NOVA-3000 system). Metal loading in catalyst samples was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 7300DV, Perkin Elmer). Surface compositions were analyzed with X-ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical), and molecular functional groups were also characterized by Fourier transform infrared spectroscopy (FTIR, Bio-Rad). 2.8. Suzuki–Miyaura cross-coupling reactions. Firstly, 15 mg of catalyst powder was dispersed in 5 mL of ethanol by sonication for 15 min. Then, the suspension together with aryl halide (0.3 mmol), phenylboronic acid (0.6 mmol), hexamethylbenzene (0.15 mmol, as the internal standard), K2CO3 (1 mmol, dissolved in 0.7 mL of H2O), and 10 mL of ethanol were placed in a 50 mL of glass reactor. The resultant mixture was stirred at 80°C in an ambient atmosphere for a Figure 1. TEM images of (a) Cu2O spheres, (b,c) Cu2O cubes, (e,f) spherical given time. After the reaction, the solid catalyst was Cu 2O@mSiO2, (g-i) cubic Cu2O@mSiO2. Insets in (a,c) are the corresponding SAED separated via centrifugation, and the solution was patterns taken at a single particle. (d) XRD patterns of spherical and cubic Cu2O extracted with ethyl acetate. The separated catalyst was samples together with the simulated data. washed with ethanol and water, respectively, dried, and reused. The extracts were analyzed by gas chromatograph (GC, hydrolysis of TEOS was conducted in a water-ethanol cosolvent, Agilent 7890A) and GC-mass spectrometry (GC: HP 6890 and which is essential to achieve a uniform coating layer of SiO2, mass selective detector: HP 5973). Internal calibration method rather than the freestanding SiO2 beads. This is because alcohols was used for quantitative analysis of the amounts of reactants could affect the hydrolysis rate of TEOS.15, 30 TEA was used to consumed (conversion) and products generated (selectivity). provide a basic environment for TEOS hydrolysis by considering

3. RESULTS AND DISCUSSION 3.1. Synthesis of core materials. Firstly, we prepared two types of shape-controlled cuprite (Cu2O) particles. As shown in Figure 1a-c, both spherical and cubic Cu2O are nearly monodisperse. The average sizes were 124 ± 10 nm in spheres (diameter), and 64 ± 6 nm in cubes (edge length). XRD patterns of both samples were indexed to cuprite crystal structure (JCPDS card no. 71-3645) as shown in Figure 1d. Likewise, as revealed by SAED patterns (insets of Figure 1), the Cu2O nanocubes are single crystals with 6 {100} planes on their crystal facets, whereas the Cu2O spheres are polycrystalline, comprising numerous small crystallites.27-28 3.2. Mesoporous silica shell encapsulation. As mentioned previously, sol–gel derived silica is an excellent host material for various nanoparticles.29 Herein, core-shell structured Cu2O@mSiO2 can be directly prepared by conformal coating the above obtained Cu2O cores with mesoporous silica. As shown in the Figure 1e-i, despite the different shapes, both Cu2O cores can wrapped with a uniform coating of mesoporous silica and the resultant core-shell structure contains only one Cu2O particle in the center. The overall morphology of Cu2O@mSiO2 depends on the initial geometric shape of the core. Therefore, Cu2O@mSiO2 products with both spherical and cubic shapes were obtained. The thickness of mSiO2 shell was 22 nm and 15 nm for the spherical and cubic Cu2O@mSiO2 composites, respectively. In order to grow a uniform silica shell on Cu2O cores, certain preparative parameters such as amounts of TEOS, CTAC, TEA and water/ethanol volumetric ratio, were systematically optimized (see Figures S1−S3 in Supporting Information). For instance, the

its alkalinity. It should be noted that Cu2O cores would be quickly dissolved if conventional ammonia solution or triethanolamine solution was used as a catalyst for the sol–gel silica coating.31 However, the hydrolysis rate of TEOS increased substantially when a large amount of TEA was used, leading to the fast generation of silica nuclei. In such a case, multiple cores were encapsulated within the silica shell (Figure S3i). In general, a higher amount of TEOS produces a thicker silica coating layer, thus, the thickness of SiO2 shell can be tuned. For instance, the shell thickness can be controlled varying from 21 to 85 nm. The amount of CTAC is another important parameter affecting the growth of silica shell on Cu2O core. We found that silica cannot be coated on Cu2O in the absence of CTAC. Nevertheless, adding a large amount of CTAC is not helpful, as it causes aggregation of product. Furthermore, CTAC can serve as a soft template for forming mesopores in the shell similar to its function in the formation of MCM-41 type of mesoporous silica.32 N2 physisorption results show that the average pore size in Cu2O@mSiO2 is around 2.5 nm, along with a total surface area of 217 m2⋅g−1 and a total pore volume of 0.161 cm3⋅g−1. 3.3. Fabrication of TCSA from chemical reactions between core and shell phases. Owing to the presence of mesopores in the shell, Cu2O core is able to react with SiO2 shell efficiently under hydrothermal conditions. After the treatment of Cu2O@mSiO2, it was found that Cu2O core disappeared totally, leaving behind a vacant interior derived from the original core. Meanwhile, the mSiO2 shell was converted completely to a porous assemblage of nanotubes (i.e., tubular copper silicate assemblages (TCSA)), which will be further characterized in the next section), as illustrated in Figure 2a-f, and Figures S4, S5. The nanotubes are



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rather uniform; each has an average inner diameter of 4.5 nm, a thickness of 0.8 nm and the length of ca. 96 nm (Figure 2c). Interestingly, all the nanotubes have an open-ended structure (Figure S4). It is worthy to note that the nanotubes were self-organized into spherical or cubic assemblages according to the original shapes of Cu2O cores. In particular, the resultant hollow cavities essentially retained geometric shapes of their biphasic precursors (i.e., spherical Cu2O@mSiO2 of cubical Cu2O@mSiO2). The average sizes of interior spaces in spheres and cages were 223 nm (diameter) and 96 nm (side length) respectively, which were both larger than those of the original Cu2O cores. Taking the thickness of the mSiO2 shell (Figure 1) and the dissolution rate of the core into consideration, nevertheless, one can easily note that the formation of the TCSA was largely commenced on the external surface regions of the mSiO2 shells, therefore reserving the geometric shapes of their biphasic precursors. Because of these topological transformations, two types of hierarchical hollow assemblages, tubes-in-a-sphere and tubes-in-a-cage, were obtained. The structures in FESEM images (Figure 2g,h) and high-angle annular dark-field scanning TEM (HAADF-STEM) image (Figure 2i and Figure S6) exhibit a rambutan-like appearance. Furthermore, the effects of hydrothermal time, temperature, water amount, etc. on the morphology evolution (from core-shell Figure 2. TEM images of (a-c) spherical TCSA, (d-f) cubic TCSA, (g-h) SEM images structured Cu2O@mSiO2 to hierarchically structured of spherical TCSA, (i) HAADF-STEM image of spherical TCSA. The dash lines nanotubes) were systematically addressed, and the shown in (b) and (f) indicate the spherical and cubic hollow cavities. See more TEM, results are summarized in Figures S7 and S8. Thermal HAADF-STEM images of TCSA products in Supporting Information. stability of the products under aerobic conditions was also examined. Quite surprisingly, the nanotube assemblages remained essentially intact even after heating at newly formed phase is copper silicate, since all the diffraction 800°C for 5 h (Figure S9). In view of all these properties (e.g., peaks can be clearly indexed to copper silicate (JCPDS card no. high specific surface area, large pore volume, and good thermal 03-0219).34 The clay-type structures of copper silicate are stability, etc.), therefore, the prepared TCSA could be an excellent composed of alternating sheets of silica tetrahedron and copper material for catalysis applications. oxide octahedron, which tend to roll up to generate a tubular structure during crystal growth.35-36 In addition, we found that 3.4. Characterizations of TCSA. Crystal structures of the TCSA product might contain a small phase of CuO if the hierarchically structured TCSA have been carefully analyzed by synthesis was carried out at a lower temperature (e.g., 160°C), as several characterization techniques. Firstly, EDX elemental maps indicated in XRD data (pattern 2, Figure 4a) and TEM images (Figure 5f-h). Also, Figure 4b gives the FTIR spectrum of the TCSA to confirm the presence of copper silicate. For instance, the characteristic peaks at 1029 and 800 cm−1 are assigned to the asymmetric and symmetric stretching vibrations of Si–O–Si, respectively. The peaks at 3434 and 1636 cm−1 are ascribed to O– H stretching and bending vibrations, respectively, which belong to the moisture bound to the surface. But, the small band centered around 673 cm−1 is caused by bending vibration of O–H in octahedral position shared by three Cu atoms.33 The peak at 500 cm−1 is attributed to the bending vibration of O–Si–O. Weak absorption peaks belonging to the methyl and methylene units were found at 2960, 2924 and 2856 cm−1 (see inset in Figure 4b), which originated from the residual CTAC molecules. And, the disappearance of peak at 630 cm-1, corresponding to the stretching vibration of Cu–O, confirms the total conversion of Cu2O phase to copper silicate phase.37-38 Textural properties of TCSA were studied by N2 adsorption–desorption at 77 K, as shown in Figure Figure 3. (a) HAADF-STEM image and the corresponding EDX elemental 4c. The isotherms follow a Langmuir type IV with a distinct maps of TCSA, (b) EDX spectrum of TCSA, where the C and Ni signals hysteresis loop. BET specific surface area was as high as 307 originate from the Ni grid (i.e., the sample holder). m2⋅g−1, and total pore volume was 0.872 cm3⋅g−1 (much higher than that of Cu2O@mSiO2). As revealed by the structural model (Figure 3a) show that copper, silicon and oxygen are (Scheme 1), a wide range of porosity (i.e., from macroscale to homogenously distributed on the whole structure, suggesting a mesoscale) in TCSA comes from the hollow cavity of the continuous phase. The elemental composition was analyzed by assemblage, the inner space of nanotube, and the inter-nanotube EDX spectrum (Figure 3b), revealing the atomic ratio of Cu to Si space. Indeed, the inset curve in Figure 4c shows the mesopores in of 0.94, which is close to the data achieved in copper silicate TCSA have a broad size distribution ranged between 2 nm to 40 materials.33 Indeed, XRD investigation (Figure 4a) confirms the nm. It is also noted that maximum intensity (i.e., the majority of


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pores) around an equivalent sphere size of 4.1 nm, is quite close to the inner diameter of the nanotubes (4.5 nm). As displayed in Figure 4d, two Cu species are present in TCSA sample, one with a Cu 2p3/2 binding energy of 932.9 eV and the other with a Cu 2p3/2 binding energy of 935.7 eV, which are consistent with observed Cu(I) and Cu(II) species, respectively. The other three signals at 940.1, 942.4, and 944.4 eV are shakeup satellites due to the presence of Cu(II).33, 39 Our XPS peak deconvolution analysis indicates that the surface copper was comprised of 90% Cu2+ and 10% Cu+. The containing of copper (I) ions can be attributed to two reasons: (i) the physical adsorption of free Cu(I) ions from the dissociation of Cu2O,40 and (ii) the chemically bonded Cu(I) ions in copper silicate framework, since the Cu(I) ions can also be partially coordinated with oxygen atoms (from the tetrahedral [Si2O5]2− layer) on the surface, which is different with the dominant coordinately saturated copper (II) ions. 3.5. Further confirmation on the formation mechanism of TCSA. Apparently, the topological transformations of Cu2O@mSiO2 precursor to TCSA Figure 4. (a) XRD patterns of TCSA samples prepared at hydrothermal temperature of should be induced by an evacuation followed by 160°C (pattern 2) and 180°C (pattern 1), where the marked peaks belong to CuO phase. JCPDS card no. 03-0219 is shown for reference (blue vertical lines in bottom). infusion of Cu2O core into mSiO2 shell, which is named (b) Representative FTIR spectrum of TCSA sample. (c) N2 adsorption−desorption as an “inside-to-outside diffusion-reaction” mechanism. isotherms of TCSA (blue line) and Cu2O@mSiO2 (green line), inset shows pore size To prove this, pure Cu2O alone, in the form of spheres distribution of TCSA (by using the Barrett–Joyner–Halenda (BJH) method). (d) High or cubes, was employed and treated under the same resolution Cu 2p3/2 XPS spectra of TCSA sample. B.E. = binding energy. hydrothermal condition (190°C for 11 h). In this process, the oxidation of Cu2O spontaneously occurred to form a of Cu2O core. For instance, the cubic Cu2O core, which is single thermodynamically more stable CuO phase; the products from crystalline, would release copper ions at a slower rate, compared this conversion all in a common morphology of platelet stacks (TEM images, Figure 5a-c; and XRD Figure 5d).41 Under the hydrothermal condition, Cu(I) ions of Cu2O could be oxidized by the dissolved ambient oxygen. In the absence of SiO2 phase, the Cu2O (Figure 1a-c) would release Cu2+ ions in the whole solution upon this oxidation, which led to a CuO product in different morphology (Figure 5a-c). In another control experiment, Stöber silica spheres and Cu2O spheres were physically mixed and the mixture was treated under similar hydrothermal condition (190°C for 11 h). In this case, we found that unorganized tubular copper silicate (Figure 5e) was produced instead of the organized assemblages (viz., TCSA). This experiment further confirms that chemical reactions indeed happened between Cu2O phase and silica phase. It also suggests that the core-shell configuration of Cu2O phase and silica phase prompts the self-assembly of copper silicate nanotubes, which is more advantageous over the physical mixture of the individual components. On the other hand, it has been demonstrated that water can be used as a mild etchant to silica by breaking the Si–O–Si networks and producing water-soluble silicon hydroxide species such as Si(OH)4, SiO(OH)3−, etc.18, 42 Related to the present study, copper ions can react with the generated silicon hydroxide species, thereby forming copper silicate product (TCSA). This mechanism can be illustrated schematically in Figure 6. As shown, the Figure 5. TEM images of the products prepared from (a) hydrothermally treated mesopores in silica shell serve as channels for Cu2+ ions spherical Cu2O in water (at 190°C for 11 h), and (b,c) hydrothermally treated cubic to diffuse, while the silicon hydroxide species were Cu2O in water (at 190°C for 11 h). (d) XRD patterns of the products shown in (a) and mainly produced on the external surface regions of the (b), along with the standard XRD pattern of monoclinic CuO (JCPDS card no. channels which were most exposed to the aqueous 48-1548). (e) TEM image of product from the hydrothermally treated physical mixture solution. Accordingly, the formation of TCSA (via of Cu2O and SiO2 (190°C for 11 h). (f-h) TEM images of comparison product via reaction between the migrating Cu2+ ions and the silicon different hydrothermal treatment conditions, see the detailed information in Figures hydroxide species) occurred largely on the external S7, S8. (i) TEM image of the product from calcination of Cu2O@mSiO2 in air, where the red arrow indicates an interior void. See more TEM images in Supporting surface regions of mSiO2 shells. The transport rate of Information. Cu2+ ions seemed to be dependent on the dissolution rate



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depending on the original shape of core phase. Moreover, our method enables the TCSA material as reductive support which can be readily used for in-situ deposition of Pd or Au nanoparticles without using any other additional reducing agents, as reported below.

Figure 6. Schematic illustration of the topological transformation of Cu2O@mSiO2 by chemical reaction between the solid matters in core and shell, where d is the diameter of the voids in TCSA, and d' is the size of original Cu2O core. The tiny navy color and yellow color spheres within the dotted circle represent the movable Cu2+ ions and silicon hydroxide species, respectively.

to the spherical polycrystalline Cu2O core which has a faster dissolution rate. Therefore, the resultant interior space of spherical TCSA is much larger than that of cubical TCSA, as depicted in the difference between d and d’ (Figure 6) in each case. In fact, the depletion of Cu2O cores was highly dependent on the amount of water, the hydrothermal temperature and hydrothermal time (Figure 5f-h). Partially etched cores were found at low hydrothermal temperatures ( 20 mL), all solids (viz., Cu2O cores and silica shells) would be etched completely, producing a clear blue aqueous solution (Cu2+). Meanwhile, the hydrothermal mixture had an initial pH value of 7.14, and the pH after reaction slightly decreased to 6.05, indicating that acid etching is not the reason for the topological transformation of Cu2O@mSiO2. We also carried out solid-state experiments (without using water as a transport medium), where the core-shell configuration of Cu2O@mSiO2 maintained its initial shape even being heated at 500°C for 5 h, but its core was transformed to CuO due to oxidation by air (Figure 5i and Figures S10, S11). Also, together with the removal of organics (e.g., PVP), Ostwald ripening of Cu2O/CuO proceeding in the confined space results in the formation of a central space in the core structure (Figure 5i, indicated by the red arrow).43 All these findings suggest that water plays as an important media in activating the reactions between Cu2O core and silica shell. Nanotube-based hierarchical structures have been observed previously by several groups who use silica spheres or silica fibers as templates, copper nitrate or copper acetate as copper ion sources, and ammonia or sodium hydroxide solution as alkaline sources.33-34, 44-46 These methods can be categorized as a class of “outside-to-inside etching” processes. However, the shape uniformity of products is still a concern, and usually the resultant hollow assemblages are heavily aggregated. In contrast, our currently proposed “inside-to-outside diffusion-reaction” method for fabrication of TCSA represents an easier synthesis, and the assemblages of nanotubes can be facilely fabricated in different shapes

3.6. Fabrication of integrated nanocatalysts based on TCSA support. To further functionalize TCSA material, we prepared M/TCSA (where M = Pd, Au) supported catalysts through metal deposition on TCSA support. As mentioned earlier, there were some residual Cu(I) ions (ca. 10 at% of total surface copper) on TCSA. The driving force for Pd deposition on TCSA support was caused by spontaneous galvanic replacement between PdCl42− and Cu+ ions, on account of the fact that the standard reduction potential of Cu2+/Cu+ (0.16 V vs. the standard hydrogen electrode (SHE)) is lower than that of Pd2+/Pd (0.92 V vs. SHE).47-48 In Figure 7a-d, the as-synthesized Pd nanoparticles with a uniform size of ca. 4.5 nm were deposited on the tubular support. Moreover, the resultant Pd/TCSA well maintained the original assembly morphology. High-resolution TEM image (Figure 7e) shows a single Pd nanoparticle, in which a lattice fringe of 2.3 Å corresponds to the d spacing of (111) planes.49 In order to achieve an efficient deposition of Pd nanoparticles on TCSA support, the preparation conditions including reaction temperature, PVP concentration, and the amount of Pd precursor were systematically investigated (refer to Figures S12−S14). Several key findings can be drawn as follows. It was found that PVP molecule can prevent the aggregation of Pd nanoparticles by coordination interactions between its carbonyl groups and surface Pd atoms.50 In our syntheses, varying amounts of PVP were tested in order to control the size of Pd nanoparticles. We found that too much PVP would inhibit the reduction of PdCl42−, resulting in a low metal loading. Thus, an appropriate concentration of PVP is very important in controlling both size and loading efficiency of Pd nanoparticles. In addition, Ostwald ripening obviously occurred at elevated reaction temperatures (e.g., 60°C),51 leading

Figure 7. (a-e) TEM and HRTEM images of Pd loading on the spherical TCSA support. (f) High resolution Pd 3d XPS spectra of Pd/TCSA catalyst. B.E. = binding energy. (g) TEM image of Pd loading on cubic TCSA support. (h,i) TEM images of Au loading on spherical TCSA support. Inset in (d) gives a structural model. And insets in (b, g, i) are statistics of the particle sizes (horizontal axis: particle size (nm); vertical axis: relative frequency (%)).


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to an increase in metal particle size (Figure S12). On the contrary, the reduction was too slow in a water-ice bath, resulting in a low loading content of Pd. Therefore, room temperature seems to be optimal for the fabrication of Pd/TCSA nanocatalysts. Furthermore, Figure 7f shows the XPS spectra corresponding to Pd 3d5/2 and Pd 3d3/2 doublet pair in our Pd/TCSA sample. The binding energy at 335.5 eV corresponds to 3d5/2 component of Pd0, further testifying to the presence of metallic Pd. Meanwhile, the disappearing of Cu(I) XPS signal after metal deposition also confirms the spontaneous galvanic replacement reaction between Pd2+ and Cu+ (refer to Figure S15). Therefore, it is anticipated that as-obtained Pd nanoparticles were strongly adhesive on TCSA support, which would be important to the longevity of catalyst. Furthermore, BET specific surface area and total pore volume of Pd/TCSA decrease to 191 m2⋅g−1 and 0.554 cm3⋅g−1, respectively, after loading Pd nanoparticles on TCSA. Likewise, Pd nanoparticles with an average size of 4.0 nm could also be immobilized on cubic TCSA support, as displayed in Figure 7g. In a similar way, Au nanoparticles with a larger size of ca. 5.8 nm could be deposited on TCSA support as a result of a greater difference in the reduction potentials between Au3+/Au pair (1.52 V vs. SHE) and Cu2+/Cu+ pair (0.16 V vs. SHE), as shown in Figure 7h,i and Figure S16.47, 52 Therefore, the versatility of the current preparative methodology for integrated nanocatalysts is once again demonstrated. 3.7. Catalytic applications of Pd/TCSA for Suzuki–Miyaura cross-coupling reactions. The catalytic activity of Pd/TCSA integrated nanocatalysts (with Pd loading of 1.96 wt%) was evaluated in Suzuki–Miyaura cross-coupling reactions. In this reaction, organoboron reagents are employed as coupling partner with aryl halides to construct carbon–carbon bonds, which is very important in pharmaceutical and fine chemicals industries.49, 53-55 As anticipated, a negligible amount of cross-coupling product (1.2% yield) was formed in 3 h in the blank experiment under any other identical conditions but without using catalysts. In comparison, Pd/TCSA catalysts were very active for the Suzuki–Miyaura coupling of iodobenzene and phenylboronic acid (Table 1). The yield of biphenyl increased from 72.7% to 100% as the reaction time increasing from 5 min to 60 min. Besides, a wide range of other substrates with a variety of functional groups was also examined. No matter electron-donating group (entries 2 and 3) or electron-withdrawing group (entries 4 and 5) substitutions were employed, all the reactions proceeded extraordinarily well and led to very high yields of their corresponding biaryls product, with reaction times ranging from 15 min to 2 h. For example, when reacting with phenylboronic acid, 1-iodo-4-nitrobenzene afforded a 100% yield for 4-nitrobiphenyl just within 15 min. The turnover frequencies (TOFs) of Pd/TCSA catalysts in different reactions were determined with respect to biaryls production. A very high TOF of 434 h−1 was achieved, when 4’-iodoacetophenone or 1-iodo-4-nitrobenzene was used as substrates to react with phenylboronic acid. The TOF values are comparable or better than those of other Pd catalysts reported in the literature, as listed in Table S5. In addition, we found that aryl bromides were less effective when reacting with phenylboronic acid (entries 6 and 7), as compared with aryl iodides. For instance, the yield of biphenyl was only 39.6%, and the corresponding TOF was 37 h−1 when employing bromobenzene and phenylboronic acid at 80°C for 3 h. So more Pd/TCSA catalyst was required to achieve a higher yield (entry 6 (note b: using 50 mg of catalyst).56 In this study, spent catalysts were collected after reaction without any post-treatment such as calcination or reduction. Encouragingly, the catalysts could maintain their catalytic activities even after 6 recycle runs, showing a high total turnover number (TON) of 651 that is vital for practical applications. As indicated from TEM images of the spent catalysts (Figure S18),

there was no or negligible change in morphology and size of both Pd nanoparticles and TCSA support. It should be pointed out that, in many reports in literature, severe aggregation of Pd metal during Suzuki–Miyaura coupling reactions is an issue hindering the recyclability of Pd nanoparticles.57 Table 1. Suzuki–Miyaura cross-coupling reactions catalyzed by our Pd/TCSA integrated nanocatalysts.

Entry

1

R

–H

X

I

Time (min)

Yield (%)

5

72.7

15

88.2

30

96.0

60

100

TOF a (h−1)

383

2

–CH3

I

120

96.8

308

3

–OCH3

I

60

100

390

4

–COCH3

I

15

100

434

5

–NO2

I

15

100

434

6

–H

Br

180

39.6, 77.3 b

37

7

–OCH3

Br

180

38.9

36

Run 1

100

109 d

Run 2

100

217 d

Run 3

100

326 d

Run 4

100

434 d

Run 5

100

543 d

Run 6

100

651 d

8c

–H

I

a

TOF is defined as the amount of biaryl product formed per metal site per hour measured at 15 min (entries 1 to 5), but in the cases using aryl bromides, the time was based on 60 min (entries 6 and 7). ICP test showed that Pd loading in catalysts was 1.96 wt%. b 50 mg of catalyst was used. c Recovered catalyst was used in iterative cycles under the same reaction conditions (reaction time 60 min per run); see the graphic data in Figure S19. d The data listed are total turnover numbers (TON; entry 8), which was calculated as (total mole of biaryl product)/(mole of Pd metal). See more time-dependent catalytic data in Tables S1−S4. Typical GC-MS spectra of the reaction solutions were shown in Figure S17.

4. CONCLUSION In summary, we have developed a simple, effective methodology to prepare tubular copper silicate assemblages (TCSA) via topological transformations from core-shell structured Cu2O@ mSiO2. Both composition and structure of the solid template are fully transformed during the one-pot hydrothermal reaction through our proposed “inside-to-outside diffusion-reaction” mechanism. Morphologically, the design-made TCSA are composed of numerous nanotubes assembling on their cubic or spherical shell with a large interior space. Therefore, this approach provides a new way for utilization of core-shell materials for the fabrication of highly hollow nanostructures with hierarchical meso- and macropores. Importantly, the as-obtained



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TCSA could be readily used as support material for deposition of Pd (or Au) nanoparticles based on the spontaneous galvanic replacements (between Pd (or Au) metal precursors and the residual Cu(I) ions on TCSA surface) without the use of auxiliary reducing agents. Benefitting from its structural merits of large void space, high specific surface area, and high thermal stability, the as-designed Pd/TCSA integrated nanocatalyst exhibits excellent catalytic activities in Suzuki-Miyaura coupling reactions, which could be reused six times without loss of catalytic activity.

■ ASSOCIATED CONTENT Supporting Information Additional experimental results of the studied samples by using HAADF-STEM, TEM, XRD, XPS, and more catalytic results in Suzuki–Miyaura cross-coupling reactions, including Tables S1 to S5, and Figures S1 to S19. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID Hua Chun Zeng: 0000-0002-0215-7760 Guowu Zhan: 0000-0002-6337-3758

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program. We also like to thank Mr. Bowen Li for kind help on some experiments.

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Topological Transformations of CoreâShell Precursors to

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