A Family of Mesocubes - American Chemical Society

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A Family of Mesocubes Sai Karthik Addu,‡ Jian Zhu,‡ K. Y. Simon Ng, and Da Deng* Department of Chemical Engineering and Materials Science, Wayne State University, 5050 Anthony Wayne Drive, Detroit, Michigan 48202, United States S Supporting Information *

ABSTRACT: It is challenging to develop a general universal procedure to fabricate mesoscale cubic structures on a large scale with different nanoscale building units. It is always desirable to tune the chemical compositions within confined arrangements without damaging the mesostructures to provide the desired physiochemical properties required by various devices/applications. Herein, we report the successful design and facile preparation of a family of mesocubes with different compositions, including (a) ZnSn(OH)6, (b) evenly distributed Zn2SnO4 and SnO2 nanoparticles, (c) hollow cubes of SnO2 nanoparticles, (d) high-ordered nanoparticles of Zn2SnO4&Sn@C; (e) SnO2@C core−shell subunits, (f) SnO2@C nanoparticle aggregates enclosed with oxidized carbon sheath, and (g) C nanobubbles, as building units, all, except ZnSn(OH)6, with the same confined arrangements of nanoparticles as building units inside the same framework of cubic mesostructures. This family of mesocubes will provide a rich pool of materials with different functional properties to meet demands in different applications and offer opportunities to evaluate fundamentals of structure−property−performance relationships. On the basis of the best of our knowledge, this family of facilely prepared mesocubes with unique combination of microsize cubes and compositions was reported for the first time, especially the carbon mesocubes formed by aggregation of carbon nanobubbles as the building subunits. Additionally, we demonstrated, for the first time, that two family members of mesocubes of Zn2SnO4&SnO2 and Zn2SnO4&Sn@C can be used as anode materials in lithium ion batteries with impressive high packing densities and superior rate performance.

1. INTRODUCTION The design and fabrication of nanoscale functional materials to explore fundamentals of morphology-dependent properties and numerous advanced applications of nanomaterials have been attracting much attention in the past 2 decades. Nanoscale materials are promising to achieve paradigm shifts in many fields, such as catalysis,1 drug delivery,2 energy storage,3−5 solar cells,6,7 absorption,8 photonics,9 chemical sensors,10 and reactors in confined space on the nanoscale.11 The ability to rationally design and facilely fabricate nanoscale materials will enable the wide adoption of nanomaterials in many fields, achieving tremendous positive impacts. Nanomaterials are typically prepared by template-assisted methods with multiple steps involved, hydrothermal methods under high temperature and high pressure conditions, top-down ball-milling with high energy, pyrolysis under high temperature, and chemical vapor deposition. However, it is still challenging to develop a general universal procedure to fabricate nanomaterials on a large scale with the same confined arrangements on the mesoscale, but © XXXX American Chemical Society

with different chemical compositions and properties to meet demanding requirements of various applications. Another challenging issue for nanoscale materials is the low tapped density associated with the small particle size and large surface area, which is not desirable for certain applications. For example, nanomaterials have been extensively explored for advanced lithium ion batteries to achieve high specific energy (by mass), but the critical issue of energy density (by volume) due to the low tapped density of nanomaterials is rarely addressed. Low tapped density could prevent the production of compact batteries, which is not acceptable for mobile electronic devices and electric vehicles with limited space. In fact, the tapped density of nanomaterials could be 1 order of magnitude lower as compared to those in the bulk state. This critical issue of low tapped density is most vividly illustrated by the following Received: May 1, 2014 Revised: July 1, 2014

A

dx.doi.org/10.1021/cm501580y | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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Figure 1. Schematic of the idea and procedure to prepare the family of mesocubes with different compositions and building substructures: (a) mesocube of ZnSn(OH)6 as the starting family member; (b) cube of evenly distributed Zn2SnO4 and SnO2 nanoparticles obtained by annealing a; (c) hollow cubes of SnO2 nanoparticles aggregates obtained by selective etching b in 1 M HCl; (d) cube of high-ordered nanoparticles of Zn2SnO4&Sn@C aggregates obtained by CVD treatment of b under C2H2; (e) cube of SnO2@C core−shell subunits obtained by etching d in 2 M HCl; (f) cube of SnO2@C nanoparticle aggregates enclosed with an oxidized carbon sheath obtained by oxidizing and etching d under concentrated HNO3; and (g) cube with C nanobubbles as building units obtained by completely removing Zn and Sn elements in d with concentrated HCl. Preparation of Mesocubes of Evenly Distributed Zn2SnO4 and SnO 2 Nanoparticle Aggregates. Mesocubes of Zn2SnO4&SnO2 nanoparticle aggregates were prepared by calcinating mesocubes of ZnSn(OH)6. To tune the size of the building units of evenly distributed Zn2SnO4 and SnO2 nanoparticle, mesocubes of ZnSn(OH)6 were calcinated at different temperatures of 650 and 800 °C, at different ramping rates of 1 and 20 °C/min, respectively. Preparation of Hollow Mesocubes of SnO2 Nanoparticle Aggregates. Typically, 50 mg of mesocubes of Zn2SnO4&SnO2 nanoparticle aggregates obtained with calcination at 650 °C described above were dispersed in 40 mL of 1.0 M HCl and kept for 24 h at room temperature with stirring to etch off the Zn ions and core part of the cube. The white product was collected by centrifugation, washed with deionized water several times until the solution became neutral, and then washed with ethanol and dried at 60 °C. Preparation of Mesocubes of High-Order Zn2SnO4&Sn@C. Typically, the porous mesocubes of evenly distributed Zn2SnO4 and SnO2 nanoparticle aggregates prepared by calcination at 800 °C described above were placed into a ceramic crucible and heated to 650 °C in a quartz tube furnace with ramping rate of 20 °C/min under Ar flow. The chemical vapor deposition (CVD) process was carried out at 650 °C for 1 h with a flow of 100 sccm of mixture gas (10% acetylene with argon as the balance). The tube furnace was purged with argon for at least 1 h to remove oxygen before CVD and cooled down naturally under argon after CVD. SnO2 was reduced to metallic Sn by acetylene under CVD conditions but not Zn2SnO4, and the black color indicated a carbon coating. Preparation of Mesocubes with SnO 2 @C Core−Shell Subunits. Typically, 25 mg of mesocubes of Zn2SnO4&Sn@C obtained above was dispersed in 20 mL of 2 M HCl for 2 days to selectively etch off Zn ions and metallic Sn. The as-treated sample still black in color was collected by centrifugation, washed with deionized water several times until the filtrate became neutral, and then washed with ethanol. Preparation of Solid Mesocubes of SnO2@Oxidized C Sheath Nanoparticle Aggregates. Typically, concentrated nitric acid (65 wt %, 15 M) was used to etch off Zn ions and oxidize metallic Sn particles into SnO2 and oxidize the C sheath from the precursor mesocubes of Zn2SnO4&Sn@C. Preparation of Mesocubes of C Nanobubbles as Building Units. The mesocubes of carbon were obtained by completely removing Zn and Sn enclosed in the mesocubes of Zn2SnO4&Sn@C by washing Zn2SnO4&Sn@C with concentrated HCl (12 M) etching for 2 days. The black, acid-etched product was washed thoroughly and collected.

example: the tapped density of graphite nanoparticles (commercial) of 30−40 nm in size is 0.26 g/cm3, as compared to that of bulk graphite with density of 2.23 g/cm3, a difference of almost 9 times. Other issues associated with electrode materials at the nanoscale are the poor electrical properties of the electrode due to interparticle resistance and low Coulombic efficiency attributed to large surface area induced side reactions between the electrode and electrolyte. Here we reports a facile procedure to prepare a family of cubic mesostructures of nanoparticle aggregates confined in cubes on a large scale instead of simple random nanoparticles to simultaneously overcome all the issues discussed above, in particular, achieving electrode materials with high packing density for lithium ion batteries. This family of cubic mesostructures with various chemicals compositions and building substructures, including (a) ZnSn(OH)6, (b) evenly distributed Zn2SnO4 and SnO2 nanoparticles, (c) hollow cubes of SnO2 nanoparticles, (d) high-ordered nanoparticles of Zn2SnO4&Sn@C, (e) SnO2@C core−shell subunits, (f) SnO2@C nanoparticle aggregates enclosed with oxidized carbon sheath, and (g) cubes with C nanobubbles as building units, will provide a rich pool of materials with different chemical and physical properties to meet demands in different applications. The overall idea and procedure with experimental conditions involved in each step are illustrated in Figure 1. To the best of our knowledge, family members c−g above have never been reported before. We also demonstrated, for the first time, that family members of b and d can be used as anode materials in lithium ion batteries with very high packing density with improve performances in lithium storage.

2. EXPERIMENTAL SECTION Preparation of Mesocubes of ZnSn(OH)6. Mesocubes of ZnSn(OH)6 as the starting family member was synthesized through a room-temperature coprecipitation method. Typically, calculated amounts of SnCl4 and ZnCl2 were dissolved in 50 mL of ethanol under stirring, followed by the addition of 50 mL of an aqueous solution of NaOH (0.32 M) drop-by-drop in 5 min. The mixture was stirred for 1 h and kept at room temperature without stirring for another 23 h. The white precipitate was collected by centrifugation and washed with ethanol and water several times to remove residual ions. The ZnSn(OH)6 powder was dried in a conventional oven overnight. B



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Materials Characterization. Powder X-ray diffraction (XRD) was carried out with a Rigaku Smartlab X-ray diffractometer using Cu Kα radiation (λ = 0.154 18 nm). The morphologies of the products was characterized by field emission scanning electron microscopy (JSM7600 FE SEM, equipped with Pegasus Apex 2 integrated EDS, with accelerating voltage of 15 kV) and by transmission electron microscopy (JEOL 2010 TEM instrument, with accelerating voltage of 200 kV). Electrochemical Measurements. A homogeneous slurry was prepared by mixing 80 wt % of the as-prepared active materials, 10 wt % of conductivity enhancer (Super-P carbon black, Timcal), and 10 wt % of polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP). The slurry was then applied to copper disks as current collectors and dried in a vacuum oven at 80 °C for 24 h. Coin-type cells were assembled in an argon-filled glovebox using the coated copper disk as the working electrode, metallic lithium foil as the counter electrode, 1 M solution of LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1, v/v) as the electrolyte, and PP/PE/PP trilayer membrane (Celgard 2320) as the separator. The electrochemical cells were charged and discharged galvanostatically at room temperature in the voltage window of 0.005− 3 V on a MTI BST8-WA battery tester.

associated with metallic Sn appeared in Figure 2c after CVD treatment, indicating the successful reduction of SnO2 to Sn by acetylene under CVD conditions. The reduction of SnO2 to metallic Sn under such a CVD condition is well-documented.12−15 Mesocubes of ZnSn(OH)6. We identified zinc hydroxystannate [ZnSn(OH)6] as the starting family member for the rich chemistry itself as well as its derivatives offered. Zinc hydroxystannate can be easily prepared as cubic nanostructures on a large scale.16−19 Preparation of cubes about 100 nm in size at room temperature has been reported by Li et al. from Na2SnO3·3H2O and ZnCl220 and by Cao et al. by grinding.21 Hollow cubes around 500 nm were prepared by Wang et al. through a room temperature alkali-assisted dissolution process.19 Polyhedral microcrystals with core−shell structure with size around 1 μm were prepared by a room temperature NH3 bubble templating method.22 The thermal decomposition of ZnSn(OH)6 into Zn2SnO4 and SnO2 has been welldocumented.23−26 Those derivatives from decomposed ZnSn(OH)6 not only offer different chemical and physical properties but also can find important applications in gas sensor and lithium ion batteries. A size of around 1 μm was the largest among all the zinc hydroxystannate prepared at room temperature, to the best of our knowledge. Here, we prepared uniform ZnSn(OH)6 cubes with a size of ∼2 μm without any surfactants in the mixture of ethanol and water system at room temperature for the first time. We developed a simple coprecipitation method with a water and ethanol mixture as the solvent to provide the right conditions to generate large amounts of mesocubes of ZnSn(OH)6 at room temperature. The chemical composition was confirmed by XRD (Figure 2a). The morphology of the as-prepred mesocubes of ZnSn(OH)6 is revealed by FESEM at different magnifications in Figure 3a− c. All mesocubes of ZnSn(OH)6 are similar in size, as shown in the low-magnification FESEM image (Figure 3a). The highmagnification FESEM image shows the perfect cubic structure of ZnSn(OH)6, with flat surfaces and sharp edges (Figure 3b,c), and some nanoparticles adsorbed on the surface were observed. The cubic structure and the solid nature of the mesocubes were confirmed by TEM (inset of Figure 3a). The XRD (Figure 2a) with sharp diffraction peaks and the nearly perfect cubic structure from TEM and SEM characterization (Figure S2 in Supporting Information) all suggest that the meoscubes of ZnSn(OH)6 are highly crystalline. The EDS of ZnSn(OH)6 is shown in Figure 3d, and the atomic ratio of Zn:Sn is ∼1:1, as expected. Mesocubes of Distributed Zn2SnO4 and SnO2 Nanoparticle Aggregates. We successfully confined both Zn2SnO4 and SnO2 nanoparticles in mesocubes by simply calcinating mesocubes of ZnSn(OH)6. Both Zn2SnO4 and SnO2 nanoparticles are highly functional, attracting much attention recently. Zn2SnO4 nanoparticles can be used as transparent conducting oxide,27 in gas sensors,28 in conductive inks in inkjet printing,29 in dye-sensitized solar cells (DSSCs),26 and as anode materials in lithium ion batteries.30−32 Zn2SnO4 nanostructures were typically prepared by complex and energy intensive methods of hydrothermal, microwave-assisted hydrothermal and vapor transport approaches.18,25,29,33 On the other hand, SnO2 is a well-known wide band gap n-type semiconductor (3.6 eV). SnO2 nanostructures have been intensively explored to enhance its performances in many applications, in particular, gas sensors and lithium ion batteries.34−37 The cubic structure of the ZnSn(OH)6 precursor provides the template to generate

3. RESULTS AND DISCUSSIONS Figure 2 shows the XRD patterns of three important family members: (a) mesocubes of ZnSn(OH)6 as the starting

Figure 2. Selected representative XRD patterns of mesocubes to confirm the compositions of (a) ZnSn(OH)6, (b) Zn2SnO4&SnO2 nanoparticle aggregates obtained by calcinating ZnSn(OH)6, and (c) Zn2SnO4&Sn@C obtained by CVD treatment of Zn2SnO4&SnO2, with all peaks assigned.

materials, (b) mesocubes of evenly distributed Zn2SnO4&SnO2 nanoparticles obtained by annealing ZnSn(OH)6 mesocubes, and (c) mesocubes of high-ordered nanoparticles of Zn2SnO4&Sn@C aggregates obtained by CVD treatment of Zn2SnO4&SnO2 mesocubes. The XRD pattern of the starting family member obtained by coprecipitation can be assigned to primitive cubic ZnSn(OH)6 (JCPDS card no. 20-1455). No other peak was observed, indicating the purity of as-prepared ZnSn(OH)6 (Figure 2a). The XRD pattern of sample prepared by calcinating ZnSn(OH)6 precursor at 800 °C can be assigned to Zn2SnO4 with cubic crystal structure (JCPDS card no. 241470) and tetragonal rutile SnO2 (JCPDS card no. 41-1445) and no other peaks observed (Figure 2b). This XRD pattern suggests the thorough conversion of ZnSn(OH)6 into Zn2SnO4 and SnO2 under heat treatment. After the CVD treatment under acetylene, the composition of the products changed to a mixture of Zn2SnO4 and tetragonal tin (JCPDS card no. 040673) (Figure 2c). The distinguishable peak at around 2θ = 26° for SnO2(110) in Figure 2b disappeared in Figure 2c, and a characteristic couple of peaks at around 2θ = 30° and 32° C



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Figure 3. Mesocubes of ZnSn(OH)6: FESEM images at (a) low-magnification overall view showing a similar size distribution, and the inset shows a typical mesocube viewed under TEM; (b) high-magnification zoomed-in view of a few mesocubes showing a rough surface; and (c) zoomed-in view of a typical mesocube with the surface clearly revealed. (d) EDS of mesocubes of ZnSn(OH)6, and the atomic ratio of Zn:Sn is ∼1:1.

Figure 4. Mesocubes of distributed Zn2SnO4 and SnO2 nanoparticle aggregates. FESEM images of (a) low-magnification overall view showing the perfect preservation of cubic structure after calcination treatment of its precursor of ZnSn(OH)6 and (b) high-magnifcaition zoomed-in view more clearly showing the surface roughness and the aggregation of Zn2SnO4 and SnO2 nanoparticles and the porous nature. TEM images of (c) a single mesocube with light contrast around the edges, suggesting porous structure, where dashed lines outline the orentiation of the mesocube and (d) the zoomed-in view more clearly shows the aggregation of building nanounits. The samples were obtained by calcinating mesocubes of ZnSn(OH)6 at 800 °C. D



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Figure 5. Hollow mesocubes of SnO2 nanoparticle aggregates: SEM images of (a) low-magnification overall view showing the well-preserved mesocubes and (b) high-magnification of a few typical mesocubes and the broken shell highlighted by arrows that reveals their hollow nature; TEM images of (c) a few typical hollow mesocubes of SnO2 nanoparticle aggregates with clear contrast between the core and shell parts in each cube, which further confirmed that they are hollow and (d) high-magnification zoomed-in view of a corner of the mesocube that clearly shows the aggregation of SnO2 nanoparticles and shell thickness.

mesocubes can also be indirectly proved by the uniform coating of carbon, the complete reduction of distributed SnO2 nanoparticles in mesocubes into Sn, and the successful preparation of mesocubes of carbon bubbles, as will be discussed. The EDS results of as-prepared Zn2SnO4&SnO2 are shown in Figure S1a (Supporting Information), with an atomic ratio of Zn:Sn of ∼1:1. The atomic ratio of O (46.55 atom %) was decreased compared to the atomic ratio of O in ZnSn(OH)6 (77.26 atom %) due to removal of the water during the heating process and the conversion of ZnSn(OH)6 to Zn2SnO4 and SnO2.23,24,26,38 Hollow Mesocubes of SnO2 Nanoparticle Aggregates. We successfully synthesized hollow mesocubes of SnO2 nanoparticle aggregates by selectively removing all Zn2+ ions from the mesocubes of Zn2SnO4&SnO2 nanoparticle aggregates discussed above. The rational design was based on our understanding that that Zn2+ ions can be easly etched off by dilute hydrochloride acid.39 At the same time, SnO44− can be protonated and dehydrated to SnO2, and SnO2 is relatively stable, preserving the Sn. The possible reactions involved are

nanoparticles of Zn2SnO4 and SnO2 confined locally and distributed evenly. The thermal decomposition reaction should be26 2ZnSn(OH)6 → Zn2SnO4 + SnO2 + 6H 2O

(1)

The morphology of the mesocubes of Zn 2 SnO 4 &SnO 2 nanoparticle aggregates was revealed by FESEM and TEM (Figure 4). The low-magnification overall view shows that the cubic structures were well-preserved after thermal decomposition (Figure 4a). More details of the structures were revealed in the high-magnification FESEM image (Figure 4b). The surface of the mesocubes of Zn 2 SnO 4 and SnO 2 nanoparticle aggregates is porous, indicating successful conversion and formation of particle aggregates as compared to its precursor of solid ZnSn(OH)6 mesocubes. The mesocubes of mixed oxides were further characterized by TEM (Figure 4c). The light contrast and course edges clearly reveal those subunits of evenly distributed Zn2SnO4 and SnO2 nanoparticles. The high-magnification TEM image (Figure 4d) more clearly shows the subunits and their aggregation. There are evenly distributed nanoparticles in the size ranges of about 35 nm and 12 nm, which could be assigned to Zn2SnO4 and SnO2 nanoparticles, respectively. The sizes measured from high-magnification TEM agree with those estimated from XRD (Figure 2b). The nanoparticle building units are randomly distributed inside the mainframe of cubes, forming mesocubes of nanoparticle aggregates. The void spaces between nanoparticle aggregations are observed. Those porous structures could facilitate the diffusion of acetylene during CVD through the cubes on the nanoscale. The porosity through the

Zn2SnO4 + 4H+ → 2Zn 2 + + H4SnO4

(2)

H4SnO4 → SnO2 + 2H 2O

(3) 2+

Therefore, upon the complete removal of Zn ions, there should still be SnO2 nanoparticles remaining in the framework. The morphology of as-prepared hollow SnO2 mesocubes is revealed by SEM and TEM (Figure 5). The cubic structure was well-maintained, as revealed by low-magnification SEM (Figure 5a). It is interesting to note that the as obtained mesocubes of E



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Figure 6. Mesocubes of high-order Zn2SnO4&Sn@C nanoparticle aggregates: FESEM images of (a) low-magnification overall view and (b) highmagnificaton top-view of two typical mesocubes; TEM images of (c) a typical mesocubes with light contrast around the edges, indicating preserved porosity, and (d) high-magnificaton zoomed-in view more clearly showing the carbon sheath wrapping the Zn2SnO4&Sn nanoparticles. The thickness of the carbon shell is 5 nm.

SnO2 are hollow. This is clearly revealed by the highmagnification view of a few typical mesocubes with broken surfaces, and the holes highlighted by white arrows (Figure 5b). Addtionally, we observed that the surface of mesocubes of SnO2 was slightly deflated (Figure 5b), which could be possibly attributed to attack by acid etching, and that the void core generated could not support the shell as compared to the solid core before etching. The hollow structure was confirmed by TEM (Figure 5c), with a clear contrast between the core and the shell parts in each mesocube. The corner of a typical hollow mesocube is shown in a zoomed-in TEM image, and the thickness of the shell was about 280 nm (Figure 5d). The highmagnification image confirms the hollow nature of the mesocubes and that the building units of the shell are SnO2 nanoparticle aggregates. The hollow mesocubes were entirely formed by aggregated nanoparticles closely compacted, and high-order random packing of nanoparticles creates nanopores in the shell as well (Figure S3, Supporting Information). The formation of unique hollow cubes, instead of porous solid structures duplicated from its precursor of solid cubes, might be ascribed as the following: During the preparation of mesocubes of Zn2SnO4&SnO2 nanoparticle aggregates by heating ZnSn(OH)6, the heat was transferred from the shell to the core part of the cubes and the diffusion of water molecues in reversed in direction. Thus, the Zn2SnO4&SnO2 nanoparticle formed on the shell should have a larger grain size as compared to those nanoparticle aggregates formed in the core part. In other words, the smaller nanoparticles in the core part with larger surface energy could be more easily attacked by acid etching as compared to those bigger particles on the shell part.40

Otherwise stated, the shell will be more densely packed as compared to the core part, forming hollow structures after treatement. The complete removal of Zn2+ ions is confiremd by EDS analysis (Figure S1b, Supporting Information), where the Sn peaks are dominant without any distinguishable Zn peaks, in contrast to the EDS pattern of its precuror, Zn2SnO4&SnO2. Mesocubes of High-Order Zn2SnO4&Sn@C Nanoparticle Aggregates. Another family member of mesocubes of high-order Zn2SnO4&Sn@C were successfully prepared by CVD treatment of the mesocubes of distributed Zn2SnO4 and SnO2 nanoparticle aggregates as precursors. Here acetylene was selected to play dual roles: (1) as reducing agent to selectively reduce SnO2 to metallic Sn and (2) as carbon source to coat Zn2SnO4&Sn with carbon sheaths. The reduction of SnO2 to metallic Sn by acetylene is well-documented.14,15,41 The porous 3-D structures of precursor of mesocubes of distributed Zn2SnO4 and SnO2 nanoparticle aggregates provide 3-D channels for acetylene to diffuse through the inside of each cube and allows CVD to occur locally. In other words, the carbon sheaths can encapsulate Zn2SnO4&Sn nanoparticles through the cubes. The successful reduction of SnO2 to metallic Sn by acetylene was confirmed by XRD (Figure 2c). The results also suggest that Zn2SnO4 is highly stable under the CVD conditions. The morphology of the Zn2SnO4&Sn@C mesocubes was revealed by FESEM and TEM (Figure 6). The uniform cubic structure was well-preserved after the CVD process, as revealed by the low-magnification FESEM overall view (Figure 6a). The zoomed-in view of two typical mesocubes of Zn2SnO4&Sn@C (Figure 6b) reveals that there are structures like bubbles/broken bubbles formed on the cube, F



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Figure 7. Mesocubes of high-order Zn2SnO4&Sn@C nanoparticle aggregates analyzed by elemental mapping and EDS: (a) FESEM image of the two mesocubes selected for elemental mapping, (b−e) the corresponding elemental mapping of C, O, Sn, and Zn, and (f) EDS analysis revealing that the atomic ration Zn:Sn remains 1:1 as before CVD.

mesocubes of SnO2@C aggregates and carbon bubble aggregates derived from this mesocubes of high-order Zn2SnO4&Sn@C nanoparticle aggregates as discussed next also confirmed that there are carbon sheaths throughout the cube. Additionally, the EDS analysis shows that the atomic ration of Zn:Sn remains 1:1, like its precursor (Figure 7f), indicating there was no Sn loss during the CVD process, although its melting point (less than 232 °C) is significantly lower than the CVD temperature (650 °C). This again suggest that the carbon can completely encapsulate them. The carbon content is 30.52 atom % from EDS analysis (Figure 7f). Mesocubes with SnO2@C Core−Shell Subunits. Mesocubes with SnO2@C rattlelike core−shell as building subunits could be derived from mesocubes of Zn2SnO4&Sn@C by etching with dilute HCl solution. This rational design was based on our understanding that metallic Sn and Zn2+ ions can be easily etched off in HCl solution, which is well-documented, but carbon is stable. Meanwhile, it is understood that SnO44− can be protonated and dehydrated to SnO2 inside the carbon under dilute HCl, forming SnO2@C core−shell particles. The uniform cubic structure can be well-preserved after the acid etching, as shown in the low-magnification FESEM image (Figure 8a). The high-magnification FESEM in Figure 8b shows more details about the nanosphere subunits at the corner

which confirmed the formation of carbon sheath. Additionally, the porous structures remained with the surface still highly rough, which indicates that the carbon coating occurred locally on the nanoscale. Otherwise stated, mesoscale cubic structures were well-preserved with high-ordered Zn2SnO4&Sn@C nanoparticles as building subunits. This is further confirmed by TEM (Figure 6c). The light contrast around the edges and rough edge lines all suggest its high porosity and the aggregation of Zn2SnO4&Sn@C nanoparticles. This is further confirmed by high-magnification TEM clearly showing the details of the subunits (Figure 6d). The building subunits of Zn2SnO4&Sn@C nanoparticles are wrapped by carbon sheaths of about 5 nm in thickness or about 14 layers of graphenes forming the carbon shell. The arrangement and order of the aggregates were random, but there are visible layers of carbon shell wrapping those Zn2SnO4&Sn nanoparticles. The void between particles is filled with carbon. In other words, the aggregation is more packed as compared to its precursor (Figure S4, Supporting Information). Elemental mapping and EDS analysis further confirmed the presence and even distribution of elements (C, O, Sn, and Zn) in each mesocube (Figure 7). The uniform distribution of carbon indicates that the carbon formed throughout the whole cubes, not just on the surface of cubes, as expected. Other family members of G



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Figure 8. Mesocubes with SnO2@C core−shell subunits: FESEM images of (a) low-magnification overall view and (b) high-magnification cornerview of one typical mesocube; TEM images of (c) a typical mesocube with clear contrast between dark SnO2 nanoparticle and light carbon preserved and (d) high-magnification zoomed-in view that clearly shows the SnO2@C core−shell nanoparticles as the building subunits and the space between the dark SnO2 core and carbon shell; (e) FESEM image of the selected two mesocubes used for elemental mapping analysis and (f, g, and h) corresponding elemental mapping results for C, O and Sn, respectively.

units was the same as its precursors due to the preservation of carbon shells. As compared to its precursor of mesocube of Zn2SnO4&Sn@C, the mesocube of SnO2@C is much lighter in contrast under TEM through the cube, which suggests the removal of Zn2+ ions and metallic Sn. The high-magnification

of the mesocube (in the area marked by a red dash in Figure 8a). The edges are light in contrast, indicating the partial removal of core and that carbon bubbles remain at the edge and on the surface. This is further confirmed by TEM (Figure 8c). As expected, the overall aggregation of nanoparticle building H



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Figure 9. Mesocubes of SnO2 nanoparticle aggregates enclosed by oxidized C sheaths: (a) low-magnification overall view and (b) high-magnification view of a few typical mesocubes with highly rough surface; TEM images of (c) typical mesocubes with light contrast around the edges and a dark body and (d) high-magnification zoomed-in view that clearly shows the SnO2 nanoparticle and oxidized or damaged C sheaths with holes.

case, highly concentrated HNO3 was used instead of dilute HCl, to obtain more SnO2 and to oxidize and open the carbon sheath. The EDS analysis of the mesocube of SnO2@oxidized carbon nanoparticle aggregates showed a much higher content of Sn (51.3 atom %) and lower content of carbon (40.1 atom %) (Figure S1d,Supporting Information), as compared to the EDS of dilute HCl treated SnO2@C sample (Figure S1c, Supporting Information). Also, no peak for zinc can be observed, indicating the full removal of Zn2+ ions. The uniform cubic structure can be preserved after the treatment under concentrated HNO3, as revealed by low-magnification FESEM (Figure 9a). The high-magnification FESEM image of a few typical treated mesocubes shows more details and surface texture (Figure 9b). As compared to dilute HCl treated mesocubes (Figure 8b), the mesocubes of SnO2@oxidized carbon have coarse surfaces, which could be attributed to oxidation of Sn and oxidization and opening of carbon sheaths.40,42,44 The porous structure and aggregation of nanoparticles of the mesocube were further characterized by TEM (Figure 9c). As compared to the dilute HCl treated sample of SnO2@C mesocubes (Figure 8c), the concentrated HNO3 treated cube has more solid nanosubunits due to more SnO2 being encapsulated, which was added through the oxidization of metallic Sn into SnO2 by HNO3. A highmagnification TEM (Figure 9d) shows that the aggregated nanoparticles encapsulated within severely damaged carbon sheaths and holes were observed on the carbon sheaths. Additionally, the thickness of the carbon sheaths was significantly reduced to about 3 nm as compared to that of its precursor (Figure 6d). The mechanism of breaking and thinning of carbon sheaths should be similar to shortening and thinning of multiwalled carbon nanotubes by concentrated

TEM (Figure 8d) shows details about the SnO2@C core−shell nanosphere subunits, and there are spaces between the SnO2 core and carbon bubbles, forming a rattlelike structure. There are also carbon bubbles with the whole core part removed, forming hollow carbon bubbles, which could be attributed to the extract amount of HCl that can also remove SnO44− due to the formation of H2[SnCl6]. The EDS mapping of SnO2@C mesocubes in Figure 8e−h demonstrates the uniformity of carbon, oxygen, and tin on the mesocubes by the colors red, green, and yellow, respectively. The EDS results (Figure S1c, Supporting Information) show that carbon is dominant in the composition, with atomic ratio of 90.26%, while a small amount (3.72 atom %) of Sn element and O (6.03 atom %) exist in the mesocubes. Mesocubes of SnO2 Nanoparticle Aggregates Enclosed by Oxidized C Sheaths. Mesocubes of SnO 2 nanoparticle aggregates enclosed by oxidized C sheaths with holes were derived from mesocubes of Zn2SnO4&Sn@C. The rational design was based on our understanding that concentrated HNO3, as a strong oxidant, can oxidize Sn back to SnO2. The redox reaction between Sn and nitric acid can be ascribed as42 Sn + 4HNO3(concd) → SnO2 + 4NO2 ↑ +2H 2O

(4)

Note that metastannic acid (H2SnO3) may be generated as the intermediate under highly concentrated HNO3 but could easily dehydrate to form SnO2 during the drying process.43 Meanwhile, concentrated HNO3 has been widely used to oxidize and functionalize carbon and open carbon nanotubes. Under concentrated HNO3, amorphous carbon could form and graphene layers could be cut open, providing additional defective sites and increasing the carbon reactivity.44 In this I



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Figure 10. Mesocubes of C nanobubbles: (a) low-magnification overall view and (b) high-magnification view of a few typical mesocubes; TEM images of (c) a typical mesocubes of carbon nanobubbles as building subunits and (d) high-magnification zoom-in view clearly shows only carbon bubbles as the building subunits and the complete removal of both Zn2SnO4 and Sn; (e) FESEM image of one selected mesocube used for elemental mapping analysis and (f) the corresponding elemental carbon map.

HNO3.44 The opening of carbon sheaths could provide additional reactivity sites for the SnO2 nanoparticles encapsulated, and this should be beneficial to certain applications, e.g., sensors and photocatalysis. Mesocubes of Carbon Nanobubbles. We also developed the first general procedure to produce mesocubes of carbon nanobubbles using the mesocubes of Zn2SnO4&Sn@C as precursor. This method, in principle, could be applied to prepare carbon bubble aggregates in different geometries starting from given ZnSn(OH)6 template with various shapes instead of a cube, such as spheres or polyhedrons. Here, instead of dilute HCl, concentrated HCl was used to completely remove both Zn2SnO4 and Sn inside the precursor. We understand that, under concentrated HCl, the H4 SnO 4 generated according to eq 2 will undergo the following reaction:45,46 H4SnO4 + 6HCl → H 2[SnCl 6] + 4H 2O

Therefore, both Zn and Sn can be completely removed from mesocubes of Zn2SnO4&Sn@C, which left behind carbon only under concentrated HCl. The as-prepared mesocubes of carbon bubbles were characterized by FESEM and TEM (Figure 10). The low-magnification FESEM shows that the all-cubic structures were well-preserved after the acid etching in concentrated HCl (Figure 10a). The high-magnification FESEM image of a few typical carbon mesocubes indicates that there was no collapse observed, with the cubic outlines well-maintained. The good structure stability suggests that there are carbon bubbles packed inside the cube to support the structure. The complete removal of both Zn2SnO4 and Sn is more clearly revealed by TEM (Figure 10c). Compared to the mesocube of Zn2SnO4&Sn@C precursor (Figure 6c), the acidetched mesocube is much lighter in contrast under TEM, with the size and shape well-preserved. The TEM image under high magnification (Figure 10d) reveals some bubblelike hollow

(5) J



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Figure 11. Electrochemical performances of mesocubes of Zn2SnO4&SnO2 and Zn2SnO4&Sn@C: (a) charge−discharge profiles of the first two cycles of mesocubes of Zn2SnO4&SnO2; (b) corresponding differential capacity profiles (dQ/dV vs V) of part a; (c) charge−discharge profiles of the first two cycles of mesocubes of Zn2SnO4&Sn@C; (d) corresponding differential capacity profiles (dQ/dV vs V) of part c; (e) cycling performances of both mesocubes Zn2SnO4&SnO2 and Zn2SnO4&Sn@C at different currents of 50 and 100 mA/g.

transport into the mesocubes and deposited evenly through the mesocubes. Application of Mesocubes in LIBs with High Packing Density. To demonstrate the tremendous potentials of this family of mesocubes, we selected two members, mesocubes of Zn2SnO4&SnO2 and Zn2SnO4&Sn@C, to evaluate in this preliminary investigation, although all the members are electrochemically active in reversible lithium storage and other applications. Both nanoscale Zn2SnO4 and SnO2 have been extensively explored as carbon-alternative anode materials for lithium ion batteries (LIBs).18,39,47−53 However, the synergistic effect of evenly distributed Zn2SnO4 and SnO2 confined in a cube on reversible lithium storage has been rarely studied. In our case, although the mesocubes are in microscale, they are still electrochemically active, due to the fact that the nanoparticles of Zn2SnO4&SnO2 as the building units and the porous structure could facilitate the diffusion of Li ions. In other words, the salient advantages of nanoparticles, such as short Li+ diffusion paths and high rate performance, are not lost even they are packed into mesocubes. Similarly, metallic Sn-

structures, which were the hollow carbon structures obtained by removing the core of Zn2SnO4&Sn. The overall surface distribution of the carbon bubbles is almost the same as its precursor of mesocubes of high-order Zn2SnO4&Sn@C, and the light contrast of the TEM image also suggests that the wrapped cores of Zn2SnO4&Sn were almost completely removed (Figure S5, Supporting Information). This observation again demonstrates that the carbon was uniformly coated on all the subunits after CVD treatment, not just coated on the outside surfaces of the mesocubes. EDS of the carbon mesocubes (Figure S1e, Supporting Information) shows that all the peaks associated with the elements of Sn and Zn are not distinguishable. It is more evidence of the feasibility of this method, and almost all of the Zn2SnO4 and Sn inside the carbon was removed. The elemental mapping of the carbon mesocube in Figure 10f reveals the uniform distribution of carbon on the whole mesocube. The uniform coating of carbon on all nanosubunits, even at the core part of the mesocubes, is more evidence to demonstrate that the Zn2SnO4&SnO2 mesocubes were porous so that the acetylene vapor can K



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based materials are also considered as an attractive lithiumstorage material, with a theoretical capacity of 990 mA h g−1 (or Li4.4Sn).54,55 However, Sn-based materials suffer from poor cyclability issues caused by volume changes during charging and discharging. There are two main strategies to address the poor cyclability issue: (1) to prepare materials on the nanoscale and (2) to prepare Sn-based material/carbon composites. For example, Zn2SnO4@C,18,31 SnO2@C, and Sn@C55−59 have demonstrated improved performance in LIBs. The encapsulating of Zn2SnO4&Sn in thin carbon sheaths and packing into a cube on the mesoscale for lithium storage have not been reported. Another issue associated with nanoscale materials widely reported in literature is low tapped density, which makes it difficult to improve the packing density of electrodes. Our preliminary results show that both mesocubes of Zn2SnO4&SnO2 and Zn2SnO4&Sn@C can be highly useful as both high-energy and high-packing-density anode materials. The results of preliminary investigation of the electrochemical performances of both mesocubes of Zn2SnO4&SnO2 and Zn2SnO4&Sn@C are summarized in Figure 11. In the first cycle discharge profiles of both mesocubes of Zn2SnO4&SnO2 (Figure 11a) and Zn2SnO4&Sn@C (Figure 11c), plateaus around 0.5 V (vs Li/Li+) are observed, which can be attributed to the lithium insertion into Zn2SnO4 and subsequent formation of alloy with Sn or Zn.32,60,61 This provide additional electrochemical evidence of the presence of Zn2SnO4 in both mesocubes. The first cycle irreversible capacity losses (ICLs) for mecosubes of Zn2SnO4&SnO2 and Zn2SnO4&Sn@C are 39.6% and 24.9%, respectively. This first cycle ICLs can be attributed to irreversible reduction of SnO2 and Zn2SnO4 and formation of solid electrolyte interphase (SEI) in the former, while only irreversible reduction of Zn2SnO4 and formation of SEI in the later as SnO2 has been chemically reduced to metallic Sn under CVD. The difference in electrochemical reactions involved in the two mesocubes could explain the difference in the first cycle ICL observed. From the second cycle onward, the charge−discharge profiles are highly overlapped, indicating the same electrochemical reactions. Given all the active materials involved in the two mesocubes, the possible electrochemical reactions are32,62 Zn2SnO4 + 8Li+ + 8e− → 2Zn + Sn + 4Li 2O

(6)

SnO2 + 4Li+ + 4e− → Sn + 2Li 2O

(7)

Sn + x Li+ + x e− ↔ Sn + LixSn

(8)

Zn + y Li+ + ye− ↔ Li yZn 6C + Li+ + e− ↔ LiC6

(0 ≤ x ≤ 4.4)

(y ≤ 1)

Keeping in mind the different compositions of the two mesocubes, unique peaks associated with SnO2 were observed in mesocubes of Zn2SnO4&SnO2 (Figure 11b), and unique peaks associated with metallic Sn and carbon sheaths were observed in mesocubes of Zn2SnO4&Sn@C (Figure 11d). For Zn2SnO4&SnO2, a stronger cathodic peak for SnO2 reduction at about 0.85 V can be attributed to the reaction of SnO2 with lithium ions and the formation of Sn and Li2O (Figure 11b), according to eq 7, which was not observed in Zn2SnO4&Sn@C (Figure 11d), indicating the presence and absence of SnO2 in the former and latter, respectivlely, as expected. For Zn2SnO4&Sn@C, the unique cathodic peak at about 0.6 V could be assigned to the alloy of Li and Sn and the formation of LixSn (Figure 11d), according to eq 8, while the oxidation peaks between 0.4 and 0.8 V in the charging cycle could be assigned to dealloying reactions of LixSn.14 Additionally, there were no irreversible metallic Sn surface reaction peaks between 1.05 and 1.55 V observed in the first discharge cycle, suggesting that no metallic Sn was exposed to electrolyte, which provides more evidence to show that all the metallic Sn was encapsulated by carbon phase. In fact, the broad satellite peak around 0.9 V could be assigned to the formation of SEI on the carbon surface due to decomposition of electrolyte. The capacity vs cycle number plots for the two mesocubes of Zn2SnO4&SnO2 and Zn2SnO4&Sn@C are shown in Figure 11e. The specific capacities of 448 and 542 mAh/g were obtained for mesocubes of Zn 2 SnO 4 &SnO 2 and Zn2SnO4&Sn@C, respectively, after 20 cycles tested at a current of 50 mA/g. When the currents were doubled to 100 mA/g, there was no noticeable fading in capacities observed (392 and 512 mA h/g for Zn2SnO4&SnO2 and Zn2SnO4&Sn@ C, respectively), which indicates that the materials may have good rate performance. Zn2SnO4&Sn@C even shows an improved rate performance, for it only had very small fading of 30 mA h/g, smaller than the fading of 56 mA h/g for Zn2SnO4&SnO2 when charge/discharge currents were doubled. After 35 cycles, capacities of 250 and 370 mA h g−1 were still maintained for mesocubes of Zn 2 SnO 4 &SnO 2 and Zn2SnO4&Sn@C, respectively. Although the initial capacity of the former is higher, the carbon coating could significantly improve the cycling performance of the latter. Carbon coating could improve electrode conductivity and buffer volume variation, which are beneficial to cycling performance. It is particularly interesting to highlight that the mesocubes of both Zn2SnO4&SnO2 and Zn2SnO4&Sn@C nanoparticle aggregates have high tapped densities. Figure 12 illustrates the volume occupied by the same weight of mesocubes as compared to commercial TiO2 (AEROXIDE TiO2 P25). The tapped densities for Zn2SnO4&SnO2 and Zn2SnO4&Sn@C were estimated to be 1.14 and 0.98 g/cm3, respectively, which are much higher than the tapped density of commercial TiO2 at 0.13 g/cm3. The significant high tapped densities of both mesocubes of Zn2SnO4&SnO2 and Zn2SnO4&Sn@C can be attributed to close-compaction of cubic structures on the mesoscale. High packing density is highly desirable to achieve useful batteries and dramatically reduce the volume taken up by battery systems for various applications. To illustrate the significance of high packing density, the capacity densities (by volume) were estimated on the basis of specific capacities (by mass). The specific capacities of mesocubes of Zn2SnO4&SnO2 and Zn2SnO4&Sn@C at the 35th cycle are 250 and 370 mA h g−1, and we assume that commercial TiO2 has a theoretical specific capacity of 168 mA h g−1. We find the capacity densities

(9) (10)

To better interpret the reactions involved in the two mesocubes during the charge/discharge cycles, differential capacity profiles (dQ/dV vs V) were plotted (Figure 11b,d). For both mesocubes with the presence of the common component Zn2SnO4, the cathodic peaks at about 0.45 and 0.14 V for the first discharge processes are observed, which can be attributed to eq 6 and forward eqs 8 9, respectively. The anodic peaks at ∼0.6 and 1.34 V for the first charge process can be attributed to the backward eqs 8 and 9 and a partially reversible reaction in eq 6. For the second cycle, the cathodic peak at 0.45 V disappeared and another dominant peak at ∼1 V was observed, indicating different lithium insertion reactions.32,60,61 L



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concentrate HNO3 and concentrate HCl, respectively, and EM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ‡

S.A. and J.Z. contributed equally.

Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We thank the Lumigen Instrument Center, Wayne State University, Detroit, MI.

(1) Zaera, F. ChemSusChem 2013, 6, 1797. (2) Biju, V. Chem. Soc. Rev. 2014, 43, 744. (3) Zhang, Q. F.; Uchaker, E.; Candelaria, S. L.; Cao, G. Z. Chem. Soc. Rev. 2013, 42, 3127. (4) Wang, H. L.; Dai, H. J. Chem. Soc. Rev. 2013, 42, 3088. (5) Wagemaker, M.; Mulder, F. M. Acc. Chem. Res. 2013, 46, 1206. (6) Chen, G. Y.; Seo, J.; Yang, C. H.; Prasad, P. N. Chem. Soc. Rev. 2013, 42, 8304. (7) Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Chem. Soc. Rev. 2013, 42, 2824. (8) Ma, J.; Zhu, Z. L.; Chen, B.; Yang, M. X.; Zhou, H. M.; Li, C.; Yu, F.; Chen, J. H. J. Mater. Chem. A 2013, 1, 4662. (9) Ding, C. Q.; Zhu, A. W.; Tian, Y. Acc. Chem. Res. 2014, 47, 20. (10) Potyrailo, R.; Naik, R. R. Annu. Rev. Mater. Res. 2013, 43, 307. (11) Lin, C. H.; Liu, X. Y.; Wu, S. H.; Liu, K. H.; Mou, C. Y. J. Phys. Chem. Lett. 2011, 2, 2984. (12) Lee, S. H.; Mathews, M.; Toghiani, H.; Wipf, D. O.; Pittman, J. C. U. Chem. Mater. 2009, 21, 2306. (13) Deng, D.; Kim, M. G.; Lee, J. Y.; Cho, J. Energy Environ. Sci. 2009, 2, 818. (14) Deng, D.; Lee, J. Y. J. Mater. Chem. 2010, 20, 8045. (15) Deng, D.; Lee, J. Y. ACS Nano 2009, 3, 1723. (16) Yin, J.; Gao, F.; Wei, C.; Lu, Q. Inorg. Chem. 2012, 51, 10990. (17) Cai, P.; Ma, D.-K.; Liu, Q.-C.; Zhou, S.-M.; Chen, W.; Huang, S.-M. J. Mater. Chem. A 2013, 1, 5217. (18) Zhao, Y.; Huang, Y.; Wang, Q.; Wang, K.; Zong, M.; Wang, L.; Sun, X. Ceram. Int. 2014, 40, 2275. (19) Wang, L.; Tang, K.; Liu, Z.; Wang, D.; Sheng, J.; Cheng, W. J. Mater. Chem. 2011, 21, 4352. (20) Wei, W.; Yang, S.; Zhou, H.; Lieberwirth, I.; Feng, X.; Müllen, K. Adv. Mater. 2013, 25, 2909. (21) Cao, Y.; Jia, D.; Zhou, J.; Sun, Y. Eur. J. Inorg. Chem. 2009, 2009, 4105. (22) Fan, H.; Ai, S.; Ju, P. CrystEngComm 2011, 13, 113. (23) Jena, H.; Kutty, K. V. G.; Kutty, T. R. N. Mater. Chem. Phys. 2004, 88, 167. (24) Stambolova, I.; Toneva, A.; Blaskov, V.; Radev, D.; Tsvetanova, Y.; Vassilev, S.; Peshev, P. J. Alloys Compd. 2005, 391, L1. (25) Wrobel, G.; Piech, M.; Dardona, S.; Ding, Y.; Gao, P.-X. Cryst. Growth Des. 2009, 9, 4456. (26) Liu, R.; Du, W.; Chen, Q.; Gao, F.; Wei, C.; Sun, J.; Lu, Q. RSC Adv. 2013, 3, 2893. (27) Young, D. L.; Moutinho, H.; Yan, Y.; Coutts, T. J. J. Appl. Phys. 2002, 92, 310. (28) Chen, Z.; Cao, M.; Hu, C. J. Phys. Chem. C 2011, 115, 5522. (29) Lehnen, T.; Zopes, D.; Mathur, S. J. Mater. Chem. 2012, 22, 17732. (30) Cherian, C. T.; Zheng, M.; Reddy, M. V.; Chowdari, B. V. R.; Sow, C. H. ACS Appl. Mater. Interfaces 2013, 5, 6054.

Figure 12. Optical image to compare the same weight of (a) mesocubes of Zn2SnO4&SnO2 nanoparticle aggregates, (b) mesocubes of Zn2SnO4&Sn@C nanoparticle aggregates, and (c) commercial TiO2 nanoparticles (AEROXIDE, P25). The tapped densities are estimated to be 1.14, 0.98, and 0.13 g/cm3 for a, b, and c, respectively.

for mesocubes of Zn2SnO4&SnO2 and Zn2SnO4&Sn@C and P25 TiO2 to be 285, 363, and 22 mA h cm−3, respectively. Otherwise stated, capacity densities could be about 13 and 17 times higher than that of commercial TiO2, even based on nonoptimized mesocubes.

4. CONCLUSIONS In summary, we reports a facile procedure to prepare a family of cubic mesostructures of nanoparticle aggregates confined in cubes on a large scale to simultaneously overcome issues of difficulty in synthesis, tuning compositions, and properties, in particular, achieving electrode materials with high packing density for LIBs. This family of cubic mesostructures with various chemical compositions and building substructures will provide a rich pool of materials with different chemical and physical properties to meet demands in different applications. The chemical and physical properties of materials were tuned by altering the compositions of the mesocubes while the same cubic structures were preserved, ranging from semiconductors, including Zn2SnO4 and SnO2, to conductive materials, including Sn and carbon. The carbon mesocubes formed by aggregation of carbon nanobubbles as the building subunits were reported. We also demonstrated, for the first time, that two family members of mesocubes, Zn2SnO4&SnO2 and Zn2SnO4&Sn@C, can be used as anode materials in lithium ion batteries with very high packing densities. It is our ongoing effort to explore applications for all members of this family of mesocubes, and the results will be updated once available.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

EDS results of Zn2SnO4&SnO2 mesocubes, hollow SnO2 mesocubes, and mesocubes prepared from Zn2SnO4&Sn@C, including SnO2@carbon, SnO2@ oxidized carbon and carbon bubble aggregates, obtained by treated with dilute HCl, M



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A Family of Mesocubes - American Chemical Society

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