A Synthetic Protocol for Preparation of Binary Multi-shelled Hollow

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A Synthetic Protocol for Preparation of Binary Multi-shelled Hollow Spheres and Their Enhanced Oxidation Application Guowu Zhan, and Hua Chun Zeng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03875 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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Chemistry of Materials

A Synthetic Protocol for Preparation of Binary Multi-shelled Hollow Spheres and Their Enhanced Oxidation Application 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, and Cambridge Centre for Advanced Research in Energy Efficiency in Singapore, 1 Create Way, Singapore 138602 ABSTRACT: Multi-shelled hollow materials (MSHMs) with multifunctional compositions and multilevel interiors have received great attention by virtue of their hierarchical hollow structures and diverse applications. In this contribution, we present a new synthetic protocol to fabricate binary MSHMs with up to quadruple shells through a rapid thermal process. The binary MSHMs contain hexagonal ZnO (the primary phase) and monoclinic CuO (the secondary phase), wherein, ZnO phase was derived from the straightforward degradation of a coordination polymer (for example, dandelion-like amorphous ZIF-90 (amZIF-90)), and CuO phase was derived from the preloaded copper nanoparticles on amZIF-90 which appear to enhance the oxidative degradation of amZIF-90. This protocol relies on the metal nanoparticle aided degradation of amZIF-90 in which the catalytic reactivity of metal was found in the order of Au < Co < Pt < Cu. Therefore, calcining amZIF-90/Au or amZIF-90 alone only led to the formation of non-hollow products. The number of layers in binary CuO/ZnO could be controlled by adjusting copper loading, annealing temperature or heating ramp rate. Furthermore, the resultant binary CuO/ZnO MSHMs exhibited pronounced catalytic activity on the advanced oxidation process (AOP) toward dye waste-water treatment. This synthetic protocol may be employed for fabrication of other binary MSHMs from the enhanced oxidative degradation of ZIFs or their analogues.

1. INTRODUCTION Compared to the same sized solid counterparts, hollow structured materials have attracted enormous attention owing to low density, high surface-to-volume ratios, high shell permeability, and confined cavity as a carrier for loading guest molecules or as a nanoreactor for catalysis reactions.1 Particularly, the multi-shelled hollow materials (MSHMs) with multifunctional compositions and multilevel interiors have great potentials for diverse applications such as photocatalysts,2 solar cells,3 gas sensors,4 lithium-ion batteries,5 and so forth. It is believed that an MSHM is more robust than a single-shelled material or a double-shelled material due to the synergistic effect of the adjacent shells in toughening the shell.6 In general, solution-based wet synthesis and annealing-based dry synthesis are two heavily utilized strategies for constructing MSHMs.7 The former method is generally referring to sacrificing hard template synthesis,6 multilamellar vesicles-assisted soft template synthesis,8 Kirkendall based nonequilibrium diffusion process,9-10 or successive Ostwald ripening process,11 while the latter method is usually induced by discrepant rates of shell formation and core shrinkage caused by temperature gradient at radial direction during the annealing process.12 There are several previous investigations based on the dry method to prepare multi-shelled metal oxides,12-17 where the diverse precursors include Prussian blue,14 metal ions in sucrose matrix,5, 12 metal ions in PVP gel microsphere,15 metal carbonate,13 and coordination polymer,16-17 etc. In recent years, metal–organic frameworks (MOFs) or supramolecular coordination polymers with one-, two-, or three-dimensional networks have received tremendous attentions, due to their broad varieties and flexibilities in the network’s topology, geometry, size, and chemical functionality.18 Up to now, there have been more than 20,000 different kinds of MOFs reported and studied in the literature.19 Zeolitic imidazolate frameworks (ZIFs) belong to a subclass of the most commonly studied MOFs, which are useful for a broad range of applications, such as gas storage, membrane separation, sensor technology,

heterogeneous catalysis, etc.20-22 In the solution-based wet synthesis, various ZIFs have been exploited as sacrificial templates to create hollow cavities for a wide variety of applications.23-24 For instance, hollow ZIFs can be prepared by using core-shell structured ZIF-67@ZIF-8 as a template.25-26 The transformation of ZIF-67 rhombic dodecahedra into hollow structured layered double hydroxides can be realized,27 and it was reported that the process was accelerated with the addition of divalent metal ions (Mg2+, Ni2+, Co2+). Likewise, annealing-based dry synthesis is also an attractive strategy for preparing metal oxide with or without carbon/nitrogen doping via thermal conversion of ZIFs (in nitrogen or air atmosphere).28-29 Usually, the derived metal oxides have single-shelled hollow structure, yolk-shelled hollow structure, or non-hollow structure depending on the annealing conditions of the parent ZIFs. To the best of our knowledge, MSHMs derived from calcination of ZIFs or their analogues have never been reported. Moreover, the formation of single-component MSHMs has been well-studied. Less effort has been devoted to the preparation of hybrid MSHMs containing more than two phases within a particle, which would enable multifunctional materials for tandem reactions or induce synergistic effects for improved catalytic performance. In a molecular level, ZIFs are constructed by self-assembly of metal ions/clusters with diverse organic ligands, which have a chemical composition (with M–linker–M configuration, M = metal ion) similar to the above-mentioned precursors to prepare multi-shelled metal oxides based on the dry method. In addition, ZIFs own ultra-high internal surface areas, tunable pore size, and large pore volume, which are ideal support materials for loading the second phase material (e.g., metal nanoparticles). Therefore, in this work, we present a novel and facile strategy to prepare a hybrid MSHM with binary compositions of CuO/ZnO, based on copper-aided oxidative degradation of an amorphous (am) ZIF-90. As illustrated in Figure 1, firstly, the amZIF-90 submicron particles (model 4 in Figure 1) were fabricated with the assistance of surfactant, and then various metal nanoparticles (such as Cu,

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Co, Au, and Pt) were loaded onto amZIF-90 support respectively. In contrast, highly crystalline submicron ZIF-90 crystals in rhombic dodecahedral shape (model 3 in Figure 1) were obtained in the absence of the surfactant during the synthesis. Later, multi-shelled CuO/ZnO hollow spheres (model 6 in Figure 1) could be readily obtained from a fast thermal treatment of amZIF-90/Cu in an air atmosphere, in which the conversion of spherical amZIF-90 to ZnO phase was initiated by the Cu nanoparticles deposited on their interface. However, thermal treatment of pure amZIF-90 or amZIF-90/Au only led to non-hollow product. Then, the formation mechanism of this hybrid MSHM was discussed in detail based on several series of experiments. To show their applicability, finally, the as-prepared multi-shelled CuO/ZnO composite was examined in the destruction of organic dyes in a typical advanced oxidation process (AOP).

2. EXPERIMENTAL SECTION 2.1. Chemicals. The following chemicals were used as Figure 1. Schematic illustration of the preparation of crystalline ZIF-90, amZIF-90, received without further purification: amZIF-90/M, and their conversion to multi-shelled CuO/ZnO. (1) ICA in methanol imidazole-2-carboxaldehyde (ICA, 97%, Alfa Aesar), solvent, (2) Zn2+ in water solvent, (3) rhombic dodecahedral crystal of ZIF-90, (4) dandelion-like amZIF-90 prepared in the presence of CTAC, (5) amZIF-90 supported zinc nitrate hexahydrate (Aldrich, 98%), sodium metal nanoparticles, M = Cu, Au, Co, or Pt, (6) multi-shelled CuO/ZnO, (7) nanorod as n-dodecyl sulfate (99%, Alfa Aesar), building block for amZIF-90, (8) composite of nanorod/metal-nanoparticles, (9) ball cetyltrimethylammonium chloride solution (CTAC, 25 and sticks model of ZIF-90 viewed from the [100] direction, (10) spherical micelle of wt% in H2O, Aldrich), cetyltrimethylammonium CTAC (cross-sectional view), (11) entrapment of imidazole linker in the micelle of bromide (CTAB, 96%, Fluka), CTAC, (12) a rod-like micelle of CTAC with entrapped imidazole linker dodecyltrimethylammonium bromide (DTAB, 99.9%, (CTAC+ICA), and (13) Zn ions penetrate into the rod-like micelle of CTAC+ICA. Sigma), sodium n-dodecyl sulfate (SDS, 99%, Alfa Note that the depicted objects are not proportional to real dimensions. Aesar), polyvinylpyrrolidone (PVP, Aldrich, K-30), adjusted in order to investigate their effects on the morphology of cupric acetylacetonate (Cu(acac)2, 98%, Fluka), the products. tetrabutylammonium borohydride (R-NBH4, 98%, Sigma), hydrogen peroxide (Merck, 30%), methylene blue (MB, Merck), 2.5. Control Experiment (R-NBH4 Treated amZIF-90). 60 mg ethanol (99.99%, VWR Chemical), and methanol (99.99%, VWR of amZIF-90 powder was re-dispersed into 40 mL of methanol by Chemical). Deionized water was used for all experiments. sonication for 10 min. Then, 5 mL of R-NBH4 methanolic solution (250 mM) was added, and the mixture was stirred for 1.5 2.2. Synthesis of amZIF-90. Firstly, 1.25 mL of Zn(NO3)2 h at room temperature. Finally, the product was recovered by aqueous solution (0.5 M) was mixed with 100 mL of water, then centrifugation and washed with ethanol twice. The annealing 20 mL of ICA methanolic solution (0.25 M) was added to the treatment of the solid was similar to the aforementioned process mixture. The mixture was stirred for 2 min before the fast addition on amZIF-90/Cu sample. of 2 mL of CTAC aqueous solution (5 wt%). Usually, the mixture became turbid within 5 min and it was further stirred for 1 h at 2.6. Characterization Techniques. Morphologies of samples room temperature before recovering the product by centrifugation, were characterized by field-emission scanning electron o washing (with 40 mL of ethanol twice), and drying (at 60 C microscopy (FESEM, JSM-5600LV), transmission electron overnight). microscopy (TEM, JEM-2010) and high-resolution TEM (HRTEM, JEM-2100F). The crystallographic information was 2.3. Synthesis of amZIF-90/Cu. 60 mg of the obtained amZIF-90 analyzed by X-ray diffraction (XRD, Bruker D8 Advance) powder was re-dispersed into 40 mL of methanol by sonication equipped with a Cu Kα radiation source. The elemental mapping for 10 min. Then, 6 mL of Cu(acac)2 methanolic solution (10 mM) was done by energy-dispersive X-ray spectroscopy (EDX, Oxford was added under vigorous stirring. The mixture was continuously Instruments, Model 7426). Specific surface areas, pore volume, stirred at room temperature for 1 h, before the addition of 5 mL of and pore size of samples were determined using N2 physisorption R-NBH4 methanolic solution (250 mM, immediately used after isotherms at 77 K (Quantachrome NOVA-3000 system). The the preparation). The mixture was continuously stirred for another surface compositions of the samples were analyzed by X-ray 0.5 h. Finally, the product was recovered by centrifugation and photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical). washed with ethanol twice. Metal loadings in the samples were determined by inductively 2.4. Calcination of amZIF-90/Cu. The obtained solid was coupled plasma optical emission spectrometry (ICP-OES, Optima calcined in an electric box furnace with an open hole connecting 7300DV, Perkin Elmer). Thermogravimetric analysis (TGA) with laboratory air with the following heating routine: the studies were carried out on a thermobalance (TGA-2050, TA ramping rate of 20°C/min to 300°C and soaking at 300°C for 1 h, Instruments) with a gas flow (air or N2) rate of 20 mL/min and a after which the samples were cooled down naturally to room heating rate of 5°C/min. The organic functional groups in the temperature. The heating ramp rate (e.g., 1°C/min, 5°C/min, samples were characterized by Fourier transform infrared 20°C/min) and soaking temperature (from 250°C to 800°C) were spectroscopy (FTIR, Bio-Rad).


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2.7. Advanced Oxidation Process (AOP). The destruction of methylene blue (MB) via an AOP was studied according to our previous reports.30-31 Firstly, 10 mg of catalyst was dispersed into 80 mL of MB aqueous solution (20 ppm). The suspension was magnetically stirred for 30 min to ensure adsorption equilibrium, before adding 2 mL of hydrogen peroxide (30%). The reaction was conducted at ambient temperature. Afterwards, the liquid samples (1 mL) were withdrawn regularly from the reactor. The catalysts were separated from the solution (after diluting 5-fold by H2O) by filtration via syringe filters (hydrophilic PTFE, 0.45 m) and the MB concentration in the liquid was quantitative determined by measuring its absorption at 664 nm with a Shimadzu UV-2450 spectrophotometer.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of amZIF-90. Monocrystalline ZIF-90 (ca. 1.4 m) in rhombic dodecahedral form can be synthesized by directly mixing zinc nitrate aqueous solution with ICA methanolic solution at room temperature for 2 h. However, it is intriguing to find that a special ZIF-90 product with hierarchical morphology was obtained when the surfactant (CTAC) was added in the synthetic solution under any other identical conditions. Figure 2. Characterizations of amZIF-90. (a-c) TEM images, (d) SEM image, (e) TEM image Quite interestingly, this product has a and the corresponding length statistics of the building blocks, (f) EDX elemental maps, (g) dandelion-like configuration (spherical diameter = XRD patterns of amorphous amZIF-90 and crystalline ZIF-90 (insets give ball-and-stick 960±350 nm; Figure S1a-c) comprised of crystal models of the two materials in the photograph, and the TEM image in bottom panel numerous radially distributed nanorods as its shows a rhombic dodecahedral crystal of ZIF-90), and (h) high-resolution XPS spectra (N 1s building blocks, as shown in the TEM and SEM region) of amZIF-90, crystalline ZIF-90, and ICA linker (insets explains the spectral images (Figure 2a-d). The nanorods are quite assignments of ICA and its primary building unit in ZIF-90 with highlighted nitrogen atoms). uniform with ca. 8 nm in diameter and 38±11 nm in length (see Figure 2e). EDX elemental maps indicate that zinc, and crystalline ZIF-90 are evidenced in XPS and FTIR, carbon, and nitrogen elements are evenly distributed in the whole respectively. As displayed in Figure 2h, two N 1s peaks at 398.4 sphere (Figure 2f). The as-prepared product was also eV and 400.2 eV are found in the spectrum of neutral ICA characterized by XRD to verify their composition. The XRD molecule, which can be ascribed to imine and secondary amine pattern shown in Figure 2g confirms the product possesses a groups, respectively.34 After the coordination with Zn2+ (both random network topology rather than the sodalite topology of samples), the imine group disappears, and a new peak at 399.3 eV crystal ZIF-90 (CCDC number 693596).32-33 Due to its amorphous is in good agreement with the tertiary amine. A small part of the nature, hereafter this product was referred to as amZIF-90 (am = secondary amine group is observed due to the presence of amorphous). In addition, the densification of amZIF-90 was uncoordinated ICA linkers (i.e., unprotonated) on the external visually verified by the volume change by comparing it with surface of particles. In addition, XPS spectra of both samples in C crystalline ZIF-90 (inset in Figure 2g). In Figure S1, the 1s, O 1s, and Zn 2p regions are almost identical (Figure S3). physisorption study with N2 shows a much lower porosity of Likewise, FTIR spectra of amZIF-90 and crystalline ZIF-90 are amZIF-90 (Brunauer–Emmett–Teller (BET) surface area of 93 also similar to those reported in the literature,33 as displayed in 2 3 m /g, and t-plot micropore volume of 0.004 cm /g) than its Figure S4. The metal−nitrogen stretching vibration (νZn-N) in 2 3 crystalline counterpart (1054 m /g and 0.470 cm /g, respectively). amZIF-90 sample exhibits a strong transmission band at 530 cm–1, The amZIF-90 sample shows Type IV isotherm characteristic, confirming the coordination of Zn with N. Moreover, the presence which is associated with capillary condensation in mesopores of aldehyde groups in amZIF-90 was proved by both XPS and occurring at the medium relative pressure range (P/P0 = 0.45–0.8). FTIR (1674 cm–1 (νC=O)). In light of these observations, it is The mesopores (ranged 3–10 nm) are attributed to the interspaces clearly suggested that the structure of amZIF-90 is indeed among the radial-arrayed nanorods in the assemblages of disordered (viz., without a long-range order; refer to the inset of amZIF-90. A detailed formation mechanism for the amZIF-90 Figure 2g) in the presence of CTAC surfactant. The structure will be discussed shortly. amorphization observed in amZIF-90 sample seems to be caused It is found that zinc element contents in both amZIF-90 and by the surface pressurization effect in nanoscale (i.e., the crystalline ZIF-90 were 25.5 wt% from TGA measurement formation of ZIF-90 nanorods), similar to the amorphization (Figure S2), very close to the theoretical value in Zn(C4H3N2O)2 phenomena induced by either pressure or temperature.32, 35-36 (i.e., 25.6 wt% of Zn in ZIF-90), which can be translated to a 3.2. Formation Mechanism of amZIF-90. The growth of molar ratio of zinc to ICA linker at 1:2 in both samples. amZIF-90 structure can be described to a self-assembly process, Furthermore, similar coordination environment (i.e., tetrahedral and the evolution process is elucidated in Figure 1 (from models Zn centers and ICA linkers) and functional groups in amZIF-90



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Page 4 of 10 place quite rapidly; even at 7 min of the reaction, the amZIF-90 assemblage was formed readily (Figure S5) in the solution.

Figure 3. Characterizations of multi-shelled CuO/ZnO prepared by annealing amZIF-90/Cu composite at 300oC for 1 h with a ramping rate of 20 oC/min. (a-c) TEM images, (d) EDX elemental maps, and (e-h) high resolution TEM (HRTEM) images. Insets in (e): the related FFT pattern and a cross-sectional view of the structure model, where the yellow-color frame indicates the region of the enlarged HRTEM image. Insets in (f): TEM image at low magnification showing the observed region of the enlarged HRTEM image. Insets in (g): the corresponding FFT pattern of on the CuO along [010] zone axis. Insets in (h): TEM image at low magnification showing the region of the enlarged HRTEM image. The interplanar spaces of 2.60 Å, 2.31 Å, 2.53 Å, 2.52 Å in (e-h), can be resolved which are attributed to ZnO (0002), CuO (200), CuO (002), and CuO (11-1), respectively.

10 to 13). Firstly, surfactant molecules of CTAC spontaneously formed small spherical micelles in aqueous solutions. The change of spherical micelles to rod-like micelles was found due to the entrapment of imidazole linker (viz., ICA molecules) via hydrophobic interaction from the chain (hydrocarbon tail). 37 This swelling effect is different from the salt-induced sphere-rod micelle transition.38 The elongated rod-like micelles then serve as soft-template for the rod-like building blocks, where Zn ions would penetrate into these micelles and coordinate with the entrapped ICA linkers to form the nanorod-like amZIF-90. In this regard, this reaction process was carried out within the rod-like micelles which could be viewed as a shape-defined “oil” phase and thus a soft template for the confined growth. Afterward, the resultant nanorods (which are hydrophobic) were further assembled into globular aggregates in order to minimize their surface tension in the aqueous bulk solution. In fact, the formation of such nanorod building blocks and their self-assembly took

Although the critical micelle concentration (CMC) of CTAC is around 1 mM at 25oC,39 we found that the CMC dramatically decreased in the presence of ICA molecules, because their strong van der Waals interactions favor micelle formation. With the participation of ICA, the micelle derived nanorod-aggregate products could be fabricated under a wide range of CTAC concentrations (from 0.16 mM to 13 mM), as shown in the TEM images of the samples prepared with different CTAC concentrations (Figure S6). In addition to CTAC, the surfactant-induced amorphization of ZIF-90 was also studied with other surfactants (Figures S7, S8), such as cationic surfactants (e.g., cetyltrimethylammonium bromide, CTAB, and dodecyltrimethylammonium bromide, DTAB), and anionic surfactant (e.g., sodium n-dodecyl sulfate, SDS). In terms of product morphology, both cationic surfactants generated amZIF-90 with similar rod-like building units and their assemblages, which however could not be obtained when anionic SDS was used. In terms of product purity, pure amZIF-90 was synthesized by using DTAB as surfactant, but a small amount of crystalline ZIF-90 was also produced when CTAB was used (Figure S8). And only monocrystalline ZIF-90 with rhombic dodecahedral shape was produced in the presence of nonionic polyvinylpyrrolidone (PVP), consistent with the previous reports.40 Furthermore, we found that the surfactant-induced amorphization was not effective for other ZIFs (such as ZIF-8 and ZIF-67), which is probably due to (i) hydrophobic feature of ICA ligand (Note that 2-methylimidazole linker for constructing ZIF-8 and ZIF-67 is hydrophilic.); and (ii) strong interactions between the ICA ligand (with carbonyl groups) and CTAC surfactant. These two factors would affect the micelle formation process. However, the addition sequence of the reactants (Figure S9) and the amount of methanol (Figure S10) were also found to be critical in this micelle-induced assembly. For instance, since ICA is readily dissolved in methanol, the addition of too much methanol will prevent the micelle formation and result in generating rhombic dodecahedral ZIF-90.

3.3. Synthesis and Annealing Treatment of amZIF-90/Cu Composite. Next, copper nanoparticles with an average size of 2.4±0.4 nm were introduced to amZIF-90 through an in-situ reduction method by using Cu(acac)2 as a copper source and tetrabutylammonium borohydride (R-NBH4) as a reducing agent. The copper nanoparticles were formed not only on the external surface of amZIF-90 but also across the entire structure of amZIF-90 support due to its mesoporous nature, as evidenced in the EDX elemental mappings and line-scanning (Figure S11). The metallic state of copper was also confirmed by XPS (Figure S12). The XRD pattern of amZIF-90/Cu is still quite similar to the original amZIF-90 sample (Figure S13), while the copper diffraction peaks are not detectable due to their small sizes and their high dispersity in amZIF-90 support. The reduction of aldehyde groups in ICA ligand to alcohol derivative was also observed during the formation of copper nanoparticles, consistent with the covalent functionalization of crystalline ZIF-90 with


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NaBH4 (Figure S12).33 Moreover, copper loading on the amZIF-90 support can be controlled by adjusting the amount of Cu(acac)2 in the synthetic solution. Herein, annealing experiments of amZIF-90/Cu were firstly conducted on the samples with a copper loading of 2.1 wt%.

Figure 4. Typical TEM images (at different magnifications) of the products by annealing amZIF-90/Cu (copper loading: 2.1 wt%) at different heating conditions. (a,b) 300oC for 1 h with a ramping rate of 1oC/min, (c-e) 300oC for 1 h with a ramping rate of 5oC/min, (f) 400oC for 1 h with a ramping rate of 1oC/min, and (g-i) 400oC for 1 h with a ramping rate of 5oC/min.

Intriguingly, amZIF-90/Cu composite could serve as a spherical precursor to fabricate a complex multi-shelled hollow product, binary oxide CuO/ZnO, which will be presented later (vide infra). As shown in the TEM and high-angle annular dark field scanning TEM (HAADF-STEM) images (Figure 3a-c), by calcining amZIF-90/Cu at 300oC for 1 h, well-defined quadruple-shelled hollow spheres were produced, and each porous shell was formed from tiny crystallites (size at 9–20 nm, and BET surface area at 21 m2/g; Figure S14). As compared with the original amZIF-90/Cu particles, particle sizes of the multi-shelled product become smaller with an average size of 670±270 nm. EDX elemental maps in Figure 3d confirm the homogenous mixture of copper and zinc elements in all the shells. By applying a high ramping rate (e.g., 20oC/min), a large temperature gradient (ΔT) is expected along the radial direction of amZIF-90/Cu. Due to this non-equilibrium rapid heat treatment and relatively large size of amZIF-90/Cu (960±350 nm), the decomposition of amZIF-90 and thus the crystallization of ZnO took place at the surface region firstly, which then led to the first disintegration of amZIF-90/Cu (i.e., formation of the first shell of metal oxides) because of an inward shrinkage of core upon a weight loss during the decomposition/crystallization. The formed oxide shell was relatively rigid, while the shrinking core (i.e., smaller sized amZIF-90/Cu) detached from it and entered the second disintegration process (i.e., the formation of the second shell of metal oxides), as described in Figure 1 (model 6). Also, the disintegration process generated a lot of outward gas flows,14 which also accelerated the detachment of the core from the shell, and promoted the formation of gaps between them. The repeated disintegration finally resulted in the multi-shelled hollow product,

but the gap space between the shells was gradually narrowed due to a smaller ΔT in the shrinking core. In contrast, only a yolk-shell product (viz., single-shelled hollow sphere, as shown in Figure 4a,b) could be obtained using a low ramping speed (1oC/min), which can be ascribed to the smaller ΔT.5, 15 Thus, we can morph the number of shells in the hollow structure by controlling the heating ramp rate (Figure 4) or calcining temperature (Figure 5). When the temperature was increased, ZnO and CuO crystallites became larger, with increasing crystallinity, as revealed by XRD characterization (Figure S15). Remarkably, these binary multi-shelled hollow structures can be maintained even at 600oC. However, a serious fusion of oxide crystallites was observed at 800oC, at which only a single-shelled product was obtained. Consistent with the oxide formation, our XPS analysis (Figure S16) indicates a dramatic decrease in nitrogen content (from ICA linkers). This element could be removed completely at 300 oC for 1 h.

Figure 5. (a-f) Typical TEM images of the products by annealing amZIF-90/Cu (copper loading: 2.1 wt%) at different temperatures for 1 h with a ramping rate of 20oC/min.

3.4. The Enhanced Oxidative Degradation of amZIF-90 by Cu. On the contrary, calcining the pristine amZIF-90 (without copper nanoparticles) only resulted in non-hollow ZnO, rather than the multi-shelled CuO/ZnO (Figure S17, BET surface area of 7.8 m2/g), in which polycrystalline ZnO crystallites of size around 20 nm are aggregated in a large sphere (ca. 700 nm). Likewise, non-hollow ZnO product with a rhombic dodecahedral shape was obtained by calcining the monocrystalline ZIF-90 sample (Figure S18). Hence, it can be speculated that the preloaded copper nanoparticles have a catalytic effect on facilitating the oxidative decomposition of amZIF-90, which will be termed as enhanced oxidative degradation hereafter. The actual copper species in the rapid transformation of amZIF-90/Cu is difficult to identify at present, because metallic Cu nanoparticles were finally oxidized Cu2O or CuO by the ambient oxygen. At the same time, however, such oxides might be locally reduced by reacting with decomposition products of the organic phase (ICA) in the process.41 It is important to note that copper nanoparticles were loaded across the entire amZIF-90 assemblage (Figure S11), although the catalytic actions of copper or copper oxide are mainly operative at the external surface of a shrinking core during the decomposition/crystallization.42 To validate the enhanced oxidative degradation of ZIF-90 by copper, amZIF-90/Cu samples with several copper loadings (0, 1.1, 1.7, 2.1 wt%) were investigated. Evidently, only when the copper content increased up to 1.7 wt%, multi-shelled products could be obtained (Figure S19). The formation of multi-shelled structure can be explained by the Cu-accelerated thermal decomposition of amZIF-90, which



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enables the high shrinkage rate (r1) of amZIF-90 and thereby the large contraction force promotes the space formed between the layers. In addition, the existing CuO from the oxidation of copper nanoparticles facilitates the construction rate (r2) of the metal oxide shell during the calcining treatment. Our TGA investigation further provides direct evidence that the catalytic action of the copper nanoparticles on the degradation of amZIF-90. In Figure 6a, the temperatures at which the weight loss reaches 50 wt% (Tw50) decrease substantially from 454 to 450, 442, and 380oC, as the copper contents in amZIF-90/Cu increase from 0 (viz., pure amZIF-90) to 1.1, 1.7, and 2.1 wt%, respectively. Time-evolving weight losses of amZIF-90 and amZIF-90/Cu (2.1 wt%) upon different soaking temperatures (over the range of 300 to 400oC) under the same air flow were further compared in Figure 6b and Figure 6c, respectively, where temperature ramping from room temperature to the specific temperature was conducted under a N2 flow. Again, the catalytic effect of copper on the framework decomposition was clearly demonstrated. It has been reported that enhanced degradations of polymers (e.g., polypropylene, polyacrylonitrile, poly(ester)imide, poly(ethylene)imine, etc.) were observed in the presence of copper or copper oxide compounds,41-45 which may initiate the bond breaking and propagate chain scission. To the best of our knowledge, nonetheless, the current work is the first report of copper nanoparticle-aided oxidative decomposition of coordination polymers (MOFs or ZIFs).

Figure 6. (a) TGA profiles of amZIF-90/Cu with different copper loadings, the samples were heated under flowing air (20 mL/min) at a heating rate of 5 oC/min. (b,c) Time evolution of weight loss of amZIF-90 and amZIF-90/Cu (copper loading: 2.1 wt%) soaking at different temperatures under an air flow (20 mL/min), where temperature ramping from room temperature to the specific temperature was conducted under flowing N 2 (20 mL/min). And (d) TGA profiles of amZIF-90/M (M = Au, Co, Pt, or Cu); the samples were heated under an air atmosphere (20 mL/min) at a heating rate of 5oC/min. “Raw” represents the sample without immobilization of any metal nanoparticle (viz., pure amZIF-90).

We also carried out a control experiment by calcining the R-NBH4 treated amZIF-90 (i.e., without copper) to examine the role of reducing agent (refer to Experimental Section), which indicates that R-NBH4 alone did not alert the intrinsic decomposition behavior of amZIF-90 (Figures S20 and S21). On the other hand, when amZIF-90/Cu sample was calcined under Ar ambiance, instead of air, non-hollow spheres were obtained even at 400oC (Figure S22), further confirming the oxidative decomposition of amZIF-90. Transformation of amZIF-90 phase to multi-shelled ZnO phase was further examined by using other

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metal nanoparticles (e.g., Au, Pt, Co), as summarized in Figures S23-25. Obviously, Pt could also assist the decomposition of amZIF-90 to multi-shelled products, but this is not the case using Au and Co (i.e., there is no comparable catalytic effect). Indeed, our TGA result (Figure 6d) is also consistent with this observation; the descending trend of Tw50 follows: Au (463oC) > Co (429oC) > Pt (409oC) > Cu (380oC). Other research groups have pioneered work in using polymers adsorbed with metal ions as precursors for the preparation of MSHMs.5, 12-17 In the current work, however, we found that the adsorption of divalent copper ions (Cu2+ from Cu(NO3)2 or Cu(acac)2) on amZIF-90 only led to the formation of single-shelled hollow spheres (Figure S26), and even non-hollow product was generated from the calcination of amZIF-90 adsorbed with Zn2+ ions, as revealed in Figure S27. This is due to that the divalent copper or zinc ions do not show the aforementioned catalytic effects (as seen from the TGA curves in Figure S21, Tw50 of the three samples are all around 441oC). Although the metal salts will decompose to form metal oxide during the calcining, a slower disintegration rate leads to a smaller contraction force, as compared to the enhanced oxidative degradation due to the preloaded copper nanoparticles. On the basis of the above investigation, therefore, it appears that metallic state of copper is more likely responsible for the observed rapid degradation of amZIF-90, although its actual catalytic activity remains to be further confirmed, as mentioned earlier. Compared to previous methods (with precursor systems of polymers and adsorbed metal ions), there are two major differences of our present work for creating multi-shelled hollow structures: (i) combinations of metal ions and organic components are achieved by coordination in MOFs (i.e., a very homogenous distribution and high loading of metal ions); and (ii) the catalytic copper nanoparticles enhance the oxidative degradation of ZIF-90 frameworks. XRD patterns (Figure S15) of the well-developed multi-shelled products (gray powders, Figure S28) can be indexed to binary hexagonal ZnO (JCPDS No. 71-6424) and monoclinic CuO (JCPDS No. 48-1548) without any other impurities present, while the pure ZnO was obtained from the calcination of amZIF-90. It was reported that the most common facets for wurtzite ZnO are {0001}, {2-1-10}, and {01-10} (refer to Figure S29). The former one is with polar surfaces, whereas the latter two belong to non-polar surfaces with lower surface energies.46-47 Interestingly, we found that the ZnO crystallites generated from calcining amZIF-90/Cu tend to maximize the areas of the polar {0001} facets; high-resolution TEM (HRTEM) images taken around the topmost hollow shell at different locations confirm that the {0001} facets of ZnO are predominantly exposed surfaces (Figure 3e,f). For instance, HRTEM images show that most lattice fringes viewed perpendicularly to the radial direction of a sphere have a spacing of 0.26 nm which corresponds well to the inter-plane space of the ZnO(0002).48 It has been known that the different exposed facets of ZnO are very important for their catalytic activity and gas sensing ability;49-51 and the ZnO {0001} serves best for the methanol formation.49-51 Furthermore, HRTEM images in Figure 3g,h reveal an intimate connectivity between CuO and ZnO phases. The close contacts of CuO to ZnO in the shell can also be verified from our EDX analysis (Figure S30). Such phasic connectivity is attributed to the fact that Cu-catalyzed decomposition of amZIF-90 nanorods takes place primarily at the interfaces between them. Moreover, the multi-shelled CuO/ZnO composite with abundant crystal planes of ZnO {0001} could be further transformed to Cu/ZnO composite by in-situ reduction under H2/N2 (10%/90%) at 250°C for 2 h, which retains the multi-shell configuration in the reduced state (viz., Cu/ZnO, Figure S31). The reduction of CuO to metallic copper was also affirmed by both XRD (Figure S15) and XPS (Figure S32). The


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result indicates that the MSHMs can be further functionalized or transformed into other materials for different applications. 3.5. Application of the Binary CuO/ZnO MSHM. Transition metal oxides have been proved as efficient Fenton-like catalysts to decolorize and decompose organic pollutants in wastewater via AOPs as they could initiate the formation of hydroxyl radicals from H2O2 decomposition.52-54 Herein, the CuO/ZnO based MSHMs were used for MB destruction in neutral water via AOPs, and the destruction efficiencies were compared among three different catalyst samples, such as ZnO (derived from pure amZIF-90 calcination at 400oC for 1 h with a ramping rate of 20oC/min), yolk-shelled CuO/ZnO (derived from amZIF-90/Cu (copper loading: 2.1 wt%) calcination at 400oC for 1 h with a ramping rate of 1oC/min), and multi-shelled CuO/ZnO (derived from amZIF-90/Cu (copper loading: 2.1 wt%) calcination at 400oC for 1 h with a ramping rate of 20oC/min). Firstly, the pure amZIF-90 showed very little activity in MB decomposition, but about 40% of the initial MB was adsorbed within 30 min (Figure 7a), indicating that amZIF-90 acts as an excellent adsorbent instead of a good catalyst in AOP. By contrast, it was found that the three metal oxide samples showed a small adsorption amount of MB on the catalyst surface, which only slightly affects the total MB concentration in solution (referring to the color symbols in Figure 7a). As shown in Figure 7c, in the absence of catalyst, there is no appreciable destruction of MB after adding H2O2. ZnO alone also showed poor MB degradation performance. Interestingly, CuO/ZnO based systems can substantially enhance the destruction rate of MB, where the decomposition rate showed the following order: ZnO < yolk-shelled CuO/ZnO < multi-shelled CuO/ZnO. For instance, the destruction efficiencies of MB over ZnO, yolk-shelled CuO/ZnO, and multi-shelled CuO/ZnO reached 26%, 59%, and 80%, respectively, after 2 h. The enhanced activity of CuO/ZnO system is due to the unpaired electrons in the metal oxides, which prompt the generation of hydroxyl radicals from H2O2.52 It was reported that supported CuO nanoparticles could serve as catalysts for the oxidative degradation of organic dye with hydrogen peroxide.55 Indeed, as shown in Figure S34, CuO nanoparticles were more effective than ZnO nanoparticles in the AOP. Furthermore, the morphology of CuO/ZnO system also affects the catalytic performance. With the same amount of CuO in the system, the activity of the multi-shelled CuO/ZnO was better than the yolk-shelled CuO/ZnO, which could be ascribed to the full exposure of metal oxide surface (higher efficiency of active site usage)56 and the close proximity between CuO and ZnO phases. In particular, it is understandable that our multi-shelled CuO/ZnO is highly porous, molecules in solution could penetrate through the pores of the metal oxide shells, and thereby the destruction reactions could take place in its interior (viz., on both sides of each shell). In this regard, our multi-shelled CuO/ZnO catalyst also works like numerous nanoscale reactors inside the MB solution, ensuring reactants to have high collision opportunities and/or sufficient retention time with the catalyst once they get into such reactors. Moreover, our multi-shelled CuO/ZnO was also used to decolorize Rhodamine 6G (a widely studied dye). As reported in Figure S35, the catalyst exhibits a high activity in the destruction reaction under similar evaluation conditions. In addition, the recyclability of the catalyst for MB destruction was studied in four consecutive runs (Figure S36). Interestingly, the catalyst was quite stable during the 4 repetitive cycles of AOP, and the slight decrease in destruction efficiency was largely due to the loss of solid catalyst during the repeated experiments.

4. CONCLUSION

In summary, the present research provides a new protocol for the preparation of binary multi-shelled hollow materials (MSHMs) based on a metal nanoparticle aided oxidative degradation of amorphous ZIF-90 (amZIF-90). For instance, with the assistance of copper nanoparticles, the amZIF-90 could serve as a spherical precursor to prepare multi-shelled CuO/ZnO hollow structures with rapid thermal processing. Since amZIF-90 (with Zn–ICA–Zn configuration) is a representative example of coordination polymers (general form M–linker–M), and copper nanoparticles have a catalytic effect on facilitating the oxidative decomposition of the coordination polymer, therefore, this protocol will allow for a general strategy for preparing many other binary MSHMs with the substrate phase derived from the coordination polymer and the second phase derived from the metal nanoparticles. In addition, the following findings can be drawn from this work. (1) The surface pressurization effect in nanoscale caused by CTAC surfactant is critical for the amorphization of ZIF-90 framework. (2) The preloaded copper nanoparticles on amZIF-90 nanorods facilitate the framework decomposition which leads to the formation of multi-shelled hollow products by increasing both shrinkage rate of amZIF-90 (r1) and shell formation rate of metal oxide (r2). (3) The efficiency of metal-aided oxidative decomposition was found in the order of Au < Co < Pt < Cu, which was consistent with the observed trend of Tw50: Au (463oC) > Co (429oC) > Pt (409oC) > Cu (380oC). Besides Cu nanoparticles, Pt nanoparticles could also assist the decomposition of amZIF-90 to multi-shelled products. (4) Ramping rate and soaking temperature are two important factors in amZIF-90/Cu transformation process affecting the morphology of hollow products. (5) Furthermore, the multi-shelled CuO/ZnO composite could be used as an excellent catalyst for dye destruction owing to low mass transfer resistance, higher efficiency of active site usage, and the confined interior for effective catalytic reaction.

Figure 7. (a) The plot of MB concentrations during the adsorption region (-30 to 0 min) and destruction region (0 to 120 min) by using pure amZIF-90 as a catalyst, and the color symbols represent the MB concentrations at the initial time (i.e., before adding H2O2) by using three different metal oxides as catalysts (see the structural models). (b) UV-vis absorption spectra of MB as a function of time during the AOP by using the multi-shelled CuO/ZnO as a catalyst. See other UV-vis spectra data in Figure S33. And (c) destruction efficiency of MB vs. reaction time over different catalyst samples. Blue color bars: blank experiment without any catalyst; orange color bars: ZnO catalyst; green color bars: yolk-shelled CuO/ZnO catalyst; yellow color bars: multi-shelled CuO/ZnO catalyst.



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■ ASSOCIATED CONTENT Supporting Information Additional experimental results of the studied samples, including Figures S1 to S36. This material is available free of charge via the Internet at http://pubs.acs.org.

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

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

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS We gratefully acknowledge the financial support by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program.

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A Synthetic Protocol for Preparation of Binary Multi-shelled Hollow

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