A Facile Soft-Chemical Synthesis of Cube-Shaped Mesoporous CuO

May 7, 2014 - ABSTRACT: Mesoporous cube-shaped CuO with a multishell microcarpet-like ..... 500 °C (Figure 5) treated samples also exhibited cube-sha...
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A Facile Soft-Chemical Synthesis of Cube-Shaped Mesoporous CuO with Microcarpet-Like Interior Sourav Ghosh, Mouni Roy, and Milan Kanti Naskar* Sol−Gel Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata 700 032, India S Supporting Information *

ABSTRACT: Mesoporous cube-shaped CuO with a multishell microcarpet-like patterned interior was synthesized via a facile aqueous-based process using copper nitrate, oxalic acid, and phosphoric acid in the presence of triblock copolymers (TBCs), L64, P123, and F68 at 80 °C/2 h followed by calcination at 300−500 °C. The obtained products were characterized by differential thermal analysis, thermogravimetry, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, N2 adsorption−desorption, field emission scanning electron microscopy, and transmission electron microscopy. A possible mechanism for the formation of microcarpet-like patterned interiors in the presence of TBCs as surfactants was illustrated.

1. INTRODUCTION Self-organization of nano-, meso-, and microscale level building components has attracted significant interest in material synthesis and device fabrications.1 Effectively controlling the architecture, size, morphology, and pattern of the selfassembled nanomaterials is important due to their morphology-dependent properties and applications.2 A number of selforganizing strategies, based on different driving forces like direct chemical bonding,3 van der Waals interactions,4 solvent adhesion and capillary effect,5 hydophilicity and hydrophobicity,1 electrostatic,6 and magnetic interactions, have been employed in various organization schemes.7 Mesoporous transition metal oxides have attracted great interest in recent times because of their unique properties and applications.8 Chichao et al. have synthesized mesoporous Fe2O3 by thermal decomposition of oxalates.9 Copper oxides, in particular, are important functional materials to be used as catalyst, gas sensors, superconductors, lithium-ion batteries, etc.10 Synthesis of mesoporous transition metal oxides, particularly copper oxides, is more difficult than mesoporous silica and alumina. However, there are few reports on the synthesis of mesoporous copper oxides. Yu et al.11 have prepared porous Cu2O hollow microspheres using sodium dodecyl sulfate (SDS) as a soft template accompanied by gas bubble process. Lai et al.12 synthesized mesoporous CuO utilizing highly ordered mesoporous silica SBA-15 as the template for a highly ordered mesoporous carbon CMK-3 following a nanocasting method. Recently, Yuan et al.13 have synthesized sponge-like mesoporous CuO ribbon clusters on Ni foam by the hydrothermal method at 95 °C for 10 h. Mesoporous CuO for their high surface area, uniform pore size distribution, well-defined shape selectivity, and low cost makes them ideal as heterogeneous catalysts. Self-assembly of CuO © 2014 American Chemical Society

nanostructures with controlled organization of primary building units into different morphologies such as ellipsoids, microflowers dandelions, microworms, microurchins, and hollow microstructures have demanded in new technological applications.14 Xu et al. studied the conversion of Cu(OH)2 nanowires to CuO nanoleaves by a hierarchical-oriented attachment mechanism.15 Hierarchical CuO nano-/microstructures with well-defined morphology have been synthesized by many chemical techniques.16 Hollow micro-/nanostructures are of potential interest in emerging areas of technology in the fields of catalysis, sensor, lithium-ion batteries, biomedical applications, etc.17 Synthesis of hollow structures with a multishell interior is a great challenge in terms of their technological importance. Xu and Wang synthesized multishelled Cu2O hollow spheres using CTAB-assisted multilamellar vesicles.18 Vanadium oxide hollow microspheres with multilevel interiors have been prepared by Pan et al.19 There are reports on the synthesis of Cu2O microcubes.20 Zhao et al. prepared cubeshaped Cu2O by the hydrothermal method.21 However, cubeshaped pure CuO is rarely reported in the literature. Therefore, fabrication of cube-shaped mesoporous CuO with patterned interiors through a simple synthesis procedure is highly desirable. In this communication, we report a facile soft-chemical synthesis of mesoporous CuO with a self-organized multishell patterning of carpet-like morphology via a simple process using copper nitrate, oxalic acid, and phosphoric acid in the presence of triblock copolymers (TBCs), L64, P123, and F68, at 80 °C/ 2 h followed by calcination at 300−500 °C. To the best of our Received: February 20, 2014 Revised: May 5, 2014 Published: May 7, 2014 2977

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Figure 1. XRD patterns of the (a) as-prepared and oxide products calcined at (b) 300 °C, (c) 400 °C and, (d) 500 °C, each obtained (i) in the absence of TBC and in the presence of (ii) L64, (iii) P123, and (iv) F68.

knowledge, it is the first report on the formation of a multishell carpet-like patterned structure in cube-shaped mesoporous CuO. This study provides new insights into the design and synthesis of the pattern structure of cube-shaped materials, which are of significant interest for applications in catalysis, sensor, lithium-ion batteries, photovoltaic cells, dye-sensitized solar cells, etc.

were confirmed by Fourier transform infrared (FTIR) (Nicolet 5PC, Nicolet Analytical Instruments, Madison, WI) with a KBr pellet at a resolution of 4 cm−1. The Raman spectrum was recorded with a RENISHAW spectrometer with 514 nm radiation from an argon laser at room temperature. X-ray diffraction (XRD) studies of the samples were performed by Philips X’Pert Pro PW 3050/60 powder diffractometer using Ni-filtered Cu−Kα radiation (λ = 0.15418 nm) operated at 40 kV and 30 mA. The crystallite size (d) of CuO was determined by XRD peak analysis based on Scherrer’s equation: d = 0.9λ/B cos θ, where, λ is the wavelength of Cu−Kα, B is the full width at half-maximum intensity peak (fwhm) in radian, and θ is the angle of the largest peak. Nitrogen adsorption−desorption measurements were conducted at 77 K with a Quantachrome (ASIQ MP) instrument. The samples were outgassed in a vacuum at 250 °C for 4 h prior to the measurement. The surface area was obtained using the Brunauer− Emmet−Teller (BET) method within the relative pressure (P/Po) range of 0.05−0.20, and the pore size distribution was calculated by the Barret−Joyner−Halenda (BJH) method. The nitrogen adsorption volume at the relative pressure (P/Po) of 0.99 was used to determine the pore volume. The morphology of the powders was examined by field emission scanning electron microscopy, FESEM with Zeiss, Supra 35VP instrument operating with an accelerating voltage of 10 kV, and transmission electron microscopy, TEM using a Tecnai G2 30ST (FEI) instrument operating at 300 kV.

2. EXPERIMENTAL SECTION Synthesis. All reagents were analytical grade and used without further purification. In a typical experiment, 4 mmol of each TBC (L64, P123, and F68) was dissolved in 70 mL of Millipore water under stirring at 80 °C for 2 h followed by the addition of 20 mmol of Cu(NO3)2·3H2O solution in 70 mL of DI water. A total of 60 mL of 12 mmol of oxalic acid solution in water containing 2 mL of phosphoric acid was slowly added into the former solution under vigorous stirring at 80 °C. The molar composition of Cu(NO3)2· 3H2O/oxalic acid/TBC was maintained as 1:0.6:0.2. The stirring was continued for 15 min to obtain a bluish-green viscous mass (viscosity 14 mPa s). The hot viscous material was quenched in ice−water followed by centrifugation (8000 rpm) and washing (thrice) with acetone. It was then dried at 60 °C for 24 h followed by calcination at 300°, 400°, and 500 °C with a heating rate of 1 °C min−1 and 2 h dwell time at each temperature. The same experiment was performed in the absence of TBC. Characterization. The thermal behaviors of the uncalcined (asprepared) samples were studied by thermogravimetry (TG) and differential thermal analysis (DTA) (Netzsch STA 449C, Germany) from room temperature to 600 °C in air atmosphere at the heating rate of 10 °C/min. The characteristic vibration bands of the products

3. RESULTS AND DISCUSSION For the samples synthesized at 80 oC/2 h, Figure 1 shows the XRD patterns of the (a) as-prepared and oxide products calcined at (b) 300 °C, (c) 400 °C, and, (d) 500 °C, each obtained (i) in the absence of TBC and in the presence of (ii) 2978

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Figure 2. Raman spectra of (a) as-prepared, and (b) 500 °C-treated samples: (i) in the absence of TBC and in the presence of (ii) L64, (iii) P123, and (iv) F68.

L64, (iii) P123, and (iv) F68. The as-prepared samples exhibit the crystallization of copper oxalate hydrate (C2CuO4·nH2O) (JCPDS 21-297), while the monoclinic CuO phase (JCPDS 050661) appeared at 300, 400, and 500 °C. Crystallization behavior of the samples was investigated by changing the synthesis time at 80 °C for 1 h and 6 h. Figure S1a,b (Supporting Information (SI)) shows XRD patterns of the asprepared and 500 °C-treated samples, respectively synthesized at 80 °C/1 h, while for the samples prepared at 80 °C/6 h, the corresponding XRD patterns are shown in Figure S1c,d (SI). However, by changing the synthesis time, no significant change in crystallization behaviors was observed for both the asprepared and 500 °C-treated samples; i.e, C2CuO4·nH2O and CuO phases appeared in the as-prepared and 500 °C-treated samples, respectively. The DTA curves of the as-prepared samples show a sharp exothermic peak at around 300 °C indicating the removal of CO2 and H2O from oxalate, and surfactant molecules, accompanying by a sharp decrease of about 50% mass loss up to 330 °C as observed from the TG curve (Figure S2, SI). However, practically no mass loss was observed from 330 to 600 °C, which suggested that all the decomposable substances were removed before 330 °C. Figure S3 (SI) shows the FTIR spectra of (a) as-prepared and oxide samples calcined at (b) 300 °C, (c) 400 °C, and (d) 500 °C, each obtained (i) in the absence of TBC, and in the presence of (ii) L64, (iii) P123, and (iv) F68, synthesized at 80 °C/2 h. For the as-prepared sample (Figure S3a, SI), the oxalate group (C2O42−) was identified from the vibration bands at around 823, 1321, and 1370 cm−1, which were the characteristic bands of νs(C−O), δ(O−CO), and νs(C−C), respectively, accompanying by the characteristic bands of hydroxyl groups at around 1625 cm−1 (bending mode) and 3500 cm−1 (stretching mode). A strong band at around 510 cm−1 originated from the stretching vibration of Cu−O bond. Interestingly, for the calcined samples (Figure S3b,c,d, SI), a broad band at around 480−595 cm−1 was due to Cu−O stretching vibration mode. By changing the synthesis time at 80 °C for 1 h and 6 h, the effect of FTIR spectra of both the as-prepared (Figure S4a,c for 1 h and 6 h samples, respectively, SI) and calcined (500 °C) samples (Figure S4b,d for 1 h and 6 h samples, respectively, SI) was studied. However, variation of reaction times could not significantly influence the FTIR spectra of the samples; i.e., the positions of the characteristic vibrations could not shift noticeably. Figure 2 shows the Raman spectra of (a) as-prepared and (b) 500 °C-treated samples obtained from (i) in the absence of TBC and in the presence of (ii) L64, (iii) P123, and (iv) F68

each. In the as-prepared sample, the appearance of stronger bands at 1517 and 564 cm−1 accompanying by weak bands at 1488, 587, and 610 cm−1 corroborated with the presence of copper oxalate.22 In the calcined (at 500 °C) samples, three Raman peaks appeared at around 295, 343, and 628 cm−1 for all the samples. The peak at 295 cm−1 was assigned to the Ag mode, while the peaks at 343 and 628 cm−1 corresponded to Bg modes of phonon vibrations of CuO.23 It is to be mentioned that CuO has C62h space group with two molecules per primitive cell. Among the 12 zone-center optical phonon modes with symmetries 4Au + 5Bu + Ag + 2Bg, only three Ag + 2Bg modes are Raman active. Interestingly, in the presence of L64 and P123, each Raman peak shifted a little to lower wavenumbers, while a significant shifting of the peaks at around 283, 335, and 612 cm−1 for the sample prepared from F68 signified a relative decrease in particle size as compared to the other samples. Figure 3 shows the FESEM images of the as-prepared samples obtained: (a) in the absence of TBC and in the

Figure 3. FESEM images of the as-prepared samples: (a) in the absence of TBC and in the presence of (b) L64, (c) P123, and (d) F68. Insets show the high magnification images.

presence of (b) L64, (c) P123, and (d) F68. All the samples revealed cube-shaped morphology. The 300 °C (Figure 4) and 500 °C (Figure 5) treated samples also exhibited cube-shape morphology of CuO. It suggests that during the transformation from C2CuO4·nH2O to CuO upon calcination, the cube-shape morphology remained unchanged. The size of the particles was 2979

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Figure 4. FESEM images of the 300 °C-treated samples: (a) in the absence of TBC and in the presence of (b) L64, (c) P123, and (d) F68. Insets show the high magnification images.

Figure 5. FESEM images of the 500 °C-treated samples: (a) in the absence of TBC, and in the presence of (b) L64, (c) P123, and (d) F68.

not uniform for all the samples. However, the particles obtained in the presence TBCs were less agglomerated compared to those obtained in the absence of TBCs. In this case, TBCs acted as surfactants to minimize the agglomeration of the particles. The interior shelled structures (indicated by arrows in Figure S5, SI)) of the CuO particles were observed in the broken cubes, resembling a crack surrounded by solid particle arrangement. The interior structures of the CuO cubes were further confirmed by TEM images. Figure 6a−i shows the TEM images (low and high magnified images) of the carpet-like multishell patterned structure of CuO obtained by using L64, P123, and F68, respectively, each calcined at 300 °C (Figure 6a,d,g), 400 °C (Figure 6b,e, h) and 500 °C (Figure 6c,f,i). A contrast difference between the hollow and dense solid parts of the microstructures was noticed. It is to be noted that in this case surfactants (TBCs) and calcination temperatures could not have any significant effect on the microstructure-based textural properties, i.e., number of shells, shell thickness, and the space between the shells. Interestingly, no such pattern structure was formed for the sample prepared in the absence of any TBC (Figure S6, SI). It indicates that the surfactant (TBCs) played a

Figure 6. TEM images of carpet-like patterned structure of CuO: (a) L64 at 300 °C, (b) L64 at 400 °C, (c) L64 at 500 °C, (d) P123 at 300 °C, (e) P123 at 400 °C, (f) P123 at 500 °C, (g) F68 at 300 °C, (h) F68 at 400 °C, (i) F68 at 500 °C.

significant role in the formation of pattern structures. However, the surfactant-assisted as-prepared samples showed also no such multishell patterned structure (Figure S7, SI). It suggests that heating of calcination had a major contribution for shell formation. The particle size of CuO was found to be about 8− 15 nm for all the samples as observed by TEM images (Figure S8, SI). The selected area electron diffraction (SAED) patterns (Figure S9, SI) for the samples calcined at 500 °C corresponded to the monoclinic CuO with polycrystalline nature of the sample, which was further confirmed by the corresponding HRTEM images (Figure 7). It reveals the interplanar spacing of 0.25 nm corresponding to the (002 and 1̅11) lattice planes of monoclinic CuO. The elemental analysis 2980

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size as shown in their FESEM images (Figures 8a−c and 9a−c for a mole ratio of 0.4 and 0.6, respectively, using L64, P123, and F68 each). Interestingly, the corresponding TEM images (Figures 8d−f and 9d−f for a mole ratio of 0.4 and 0.6, respectively, using L64, P123, and F68 each) also show the multishell pattern structure of the cubes. However, the space between the shells became noticeably larger with an increase in concentration of the surfactants as compared to those with surfactant: Cu2+ mole ratio of 0.2. During thermal treatment, with increased surfactant concentrations, more organic materials were decomposed rendering a larger void formation between the dense CuO particles (discussed shortly in probable mechanism). With an increase in the heating rate of calcination at 500 °C, bulginess of the cubes was observed, which looked like cushion-like morphology (Figures S11a−c and S12a−c for the heating rate of 3 °C min−1 and 5 °C min−1, respectively, using L64, P123, and F68 each). The bulginess of the cubes became more prominent for the heating rate of 5 °C min−1. The interior microstructures of the corresponding samples were shown in the TEM images (Figures S11d−f and S12d−f for the heating rate of 3 °C min−1 and 5 °C min−1, respectively, using L64, P123, and F68 each). It is clear that for the samples with the heating rate of 3 °C min−1 the shells were not in good contrast as compared to the samples calcined with the heating rate of 1 °C min−1. For the heating rate of 5 °C min−1, the multishell pattern structure was collapsed. Therefore, the heating rate of calcinations is a controlling factor for the multishell pattern structure. For shorter synthesis time (1 h) at 80 °C, a trend of cube formation of the particles was observed. FESEM images for 1 h synthesized samples show that the edge of most of the cubes was rounded (Figure S13a−c using L64, P123, and F68, respectively). It indicates that at the initial stage of reaction, the growth of the particles was still incomplete which started growing toward a symmetrical cubic shape with time. TEM images of the corresponding samples (Figure S13d−f using L64, P123, and F68, respectively) reveal that a faint shell (shown by arrow) appeared in nearly cube-shaped CuO particles indicating the initiation of shell formation. Interestingly, with an increase in the higher reaction time of 6 h,

Figure 7. HRTEM images of CuO: (a) in the absence of TBC and in the presence of (b) L64, (c) P123, and (d) F68.

of CuO particles studied by EDX analysis is shown in Figure S10 (SI). It indicated a Cu/O atomic ratio of about 1. The effect of microstructure-based textural properties was studied by varying the experimental conditions such as concentration of the surfactants, heating rate of calcination, and synthesis time. The molar concentration of Cu(NO3)2· 3H2O/oxalic acd/TBC was varied to 1:0.6:0.4 and 1:0.6:0.6; heating rate of calcination at 500 °C was changed to 3 °C min−1 and 5 °C min−1; and the synthesis time at 80 °C was varied to 1 h and 6 h. With increasing the mole ratio of surfactant: Cu2+ from 0.2 to 0.4 and 0.6, the cube-shaped morphology of CuO particles retained. However, the selfassembled smaller particles in the cubes aggregated to a bigger

Figure 8. FESEM (a−c for L64, P123, and F68, respectively) and TEM (d−f for L64, P123 and F68, respectively) images of cube-shaped CuO synthesized with surfactant/Cu2+ mole ratio of 0.4. 2981

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Figure 9. FESEM (a−c for L64, P123, and F68, respectively) and TEM (d−f for L64, P123, and F68, respectively) images of cube-shaped CuO synthesized with surfactant/Cu2+ mole ratio of 0.6.

Figure 10. Nitrogen adsorption−desorption isotherms of the samples: (a) in the absence of TBC and in the presence of (b) L64, (c) P123, and (d) F68, each calcined at 300 °C, 400 °C, and 500 °C. Insets show the pre-size distributions of the corresponding samples.

formation of mesopores among platelike particles.24 The BJH pore size distributions of the corresponding samples derived from desorption data of the isotherms are shown in the insets of Figure 10. Appearance of some peaks at ∼2 nm is the pseudopeak (artifact)25 caused by the BJH computing method. The broader PSDs were due to the formation of interparticle void spaces26 of CuO nanoparticles as well as the absence of regular shape and size of the pores in the samples.27 With an increase in the calcination temperature, BET surface area and total pore volume decreased (Table S1, SI). The average pore diameter was in the range of about 18−40 nm having no systematic change with an increase in calcination temperature. It was due to the formation of disorder and irregular pores.

the cube-shaped morphology of CuO was completely destroyed forming highly agglomerated particles (Figure S14a−c using L64, P123, and F68, respectively). It is evident from the corresponding TEM images (Figure S14d−f using L64, P123, and F68, respectively). It is to be pointed out that for the formation of multishell, an optimum reaction time is required. Figure 10 shows the nitrogen adsorption−desorption isotherm of the samples obtained: (a) in the absence of TBC and in the presence of (b) L64, (c) P123, and (d) F68, each calcined at 300 °C, 400 °C, and 500 °C. It displays type IV isotherm according to IUPAC classification, which indicated mesoporous characteristic of the sample. The appearance of type H-3 hysteresis loop at high relative pressures indicated the 2982

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(TBCs), an exothermic reaction occurs. As a result, a stepwise temperature gradient is generated along the radial direction forming a multishell interior structure. However, with prolonged heating, the temperature gradient decreases resulting in more adhesion force than contraction force (Fa > Fc).30 It causes reverses the direction of materials movement; i.e., the inner core shrinks outward, generating hollow cavity. It is noteworthy that slow heating of calcination is an important parameter for the uniformity of multishell interior structure.31 Interestingly, with an increase in surfactant concentration, the spacing between the shells is found to be larger compared to that with less concentration of the surfactants. With thermally induced oxidative decomposition32 of hydrated C2CuO4 containing surfactants, the inner viscoelastic gel materials shrink continuously with the loss of organic components. It becomes more prominent at higher surfactant concentration leading to the formation of larger spacing between the shells. Zhang et al. reported that for the formation of hierarchical multishell Fe(OH)3, alkali concentration and reaction temperature are important parameters to control the reaction/ diffusion kinetics.33 It is worth mentioning that experimental conditions like concentration of the surfactant, synthesis time, and heating rate of calcination are important parameters to obtain such a multishell pattern interior structure.

With increasing calcination temperatures, growth of the particles occurred as evidenced by their increased trend of the crystallite sizes (Table S2, SI). The decreased values of BET surface area and pore volumes was due to the increase in crystallite sizes of CuO particles. For different samples, it was obvious that crystallite sizes of TBCs derived samples were smaller compared to those prepared in the absence of TBC. The samples obtained from F68 revealed the crystallite sizes of 7.5, 9.8, and 12.8 nm for 300, 400, and 500 °C-treated samples, respectively; these values were smaller compared to the corresponding samples obtained from P123, L64, and in the absence of any TBCs (Table S2, SI). For all the samples, a correlation existed among crystallite size, BET surface area, and pore volume. It is clear that BET surface area and total pore volume were comparable for the samples obtained from L64 and P123, while those values are relatively high for F68 derived samples (Table S1, SI). A maximum BET surface area of 115 m2 g−1 with a pore volume of 0.61 cm3 g−1 was obtained for the 300 °C-treated sample prepared from F68. The physical properties of the TBCs (Table S3, SI) played an important role in the textural properties of CuO.28 The hydrophilicity (high HLB value and PEO/PPO ratio) of the TBCs increased in the order of P123 < L64 < F68, which enhanced more interactions with the copper oxalate through hydrogen bonding rendering higher BET surface area as well as pore volume of the samples. Instead of a higher HLB value of L64 compared with P123, their BET surface area and total pore volume were comparable. It was due to longer PEO and PPO chains of P123 compared with L64, which facilitated in increasing surface area and pore volume. It is worth mentioning that two effects, namely, hydrophilicity and PEO/PPO chain lengths of the TBCs, are important to influence the textural properties of the products. Figure 11 shows a schematic illustration for the fabrication of the multishell microcarpet-like interior of CuO cubes. The

4. CONCLUSION In conclusion, cube-shaped mesoporous CuO with a carpet-like multishell pattern structure has been synthesized by a facile soft-chemical method in the presence of different triblock copolymers (TBCs) as surfactants. The surfactant, F68, having higher hydrophilicity and polymeric chain length, rendered a smaller particle size, higher BET surface area, and pore volume of CuO. The formation mechanism of this interesting multishell structure could be illustrated by the combined effect of contraction (Fc) and adhesion (Fa) forces caused by nonequilibrium heat treatment accompanying by oxidative decomposition of the large amount of organic species. Formation of multishell patterned structure of CuO particles was governed by optimum experimental conditions in terms of concentration of the surfactant, synthesis time, and heating rate of calcination. Mesoporous CuO with multishell pattern structures could have promising applications in different fields of materials science. The present method of preparing a mesoporous patterned structure may be applicable for the synthesis of other transition metal oxides.

Figure 11. Schematic representation of proposed formation mechanism for the multishell pattern structure of CuO.

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ASSOCIATED CONTENT

S Supporting Information *

formation of multishell structure is based on the heterogeneous contraction caused by nonequilibrium heat treatment.29−31 At initial stage of calcinations, a temperature gradient (ΔT) is developed along the radial direction leading to the formation of the CuO shell at the surface surrounded by hydrated copper oxalate (C2CuO4·nH2O) cores. As a result two forces, i.e., contraction force (Fc) and adhesion force (Fa) of opposite direction act on the interface between CuO shell and hydrated C2CuO4 cores, which is termed as heterogeneous contraction.30,31 At the early stage of calcination, the higher temperature gradient (ΔT) renders more contraction force (Fc > Fa). Thus, with the inward contraction of inner core, the preformed outer shell is detached. With the oxidative decomposition32 of oxalate and the organic surfactants

XRD patterns, DTA and TGA data, FTIR spectra, FESEM images, TEM images, SAED patterns, EDX patterns, tables of textural properties and crystallite size of CuO and physical properties of triblock copolymers. 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]. Fax: +91 33 24730957. Notes

The authors declare no competing financial interest. 2983

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ACKNOWLEDGMENTS The authors acknowledge financial support from Department of Science and Technology (DST), Government of India (Project No. GAP0616). S.G. and M.R. are thankful to Council of Scientific and Industrial Research (CSIR) for their fellowship.

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REFERENCES

(1) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (2) Nie, Z.; Petukhova, A.; Kumacheva, E. Nat. Nanotechnol. 2010, 5, 15. (3) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (4) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Nature 2004, 43, 30. (5) Clark, T. D.; Ferrigno, R.; Tien, J.; Paul, K. E.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 5419. (6) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042. (7) Chen, H. M.; Liu, R.-S.; Li, H. L.; Zeng, H. C. Angew. Chem., Int. Ed. 2006, 45, 2713. (8) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.; Salbeck, J.; Spreitzer, H.; Gratzel, M. Nature 1998, 395, 583. (9) Yu, C.; Dong, X.; Guo, L.; Li, J.; Qin, F.; Zhang, L.; Shi, J.; Yan, D. J. Phys. Chem. C 2008, 112, 13378. (10) Basu, M.; Sinha, A. K.; Pradhan, M.; Sarkar, S.; Pal, A.; Pal, T. Chem. Commun. 2010, 46, 8785. (11) Yu, Y.; Zhang, L.; Wang, J.; Yang, Z.; Long, M.; Hu, N.; Zhang, Y. Nanoscale Res. Lett. 2012, 7, 347. (12) Lai, X.; Li, X.; Geng, W.; Tu, J.; Li, J.; Qiu, S. Angew. Chem., Int. Ed. 2007, 119, 752. (13) Yuan, Y. F.; Pei, Y. B.; Fang, J.; Zhu, H. L.; Yang, J. L.; Guo, S. Y. Mater. Lett. 2013, 91, 279. (14) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (15) Xu, H.; Wang, W.; Zhu, W.; Zhou, L.; Ruan, M. Cryst. Growth Des. 2007, 7, 2720. (16) Ghosh, S.; Naskar, M. K. RSC Adv. 2013, 3, 13728. (17) Lou, X. W.; Archer, L. A.; Yang, Z. Adv. Mater. 2008, 20, 3987. (18) Xu, H.; Wang, W. Angew. Chem., Int. Ed. 2007, 46, 1489. (19) Pan, A.; Wu, H. B.; Yu, L.; Lou, X. W. Angew. Chem., Int. Ed. 2013, 52, 2226. (20) Zhang, D.-F.; Zhang, H.; Guo, L.; Zheng, K.; Han, X.-D.; Zhang, Z. J. Mater. Chem. 2009, 19, 5220. (21) Zhao, H. Y.; Wang, Y. F.; Zeng, J. H. Cryst. Growth Des. 2008, 8, 3731. (22) Castro, K.; Sarmiento, A.; Martinez-Arkarazo, I.; Madariaga, J. M.; Fernandez, L. A. Anal. Chem. 2008, 80, 4103. (23) Wang, X.; Xi, G.; Xiong, S.; Liu, Y.; Xi, B.; Yu, W.; Qian, Y. Cryst. Growth Des. 2007, 7, 930. (24) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (25) Janssen, A. H.; Koster, A. J.; de Jong, K. P. J. Phys. Chem. B 2002, 106, 11905. (26) Yu, J.; Wang, J.; Li, Z.; Li, L.; Liu, Q.; Zhang, M.; Liu, L. Cryst. Growth Des. 2012, 12, 2872. (27) Ghosh, S.; Naskar, M. K. RSC Adv. 2013, 3, 4207. (28) Ghosh, S.; Naskar, M. K. J. Am. Ceram. Soc. 2014, 97, 100. (29) Guan, J.; Mou, F.; Sun, Z.; Shi, W. Chem. Commum. 2010, 46, 6605. (30) Zhou, L.; Zhao, D.; Lou, X. W. Adv. Mater. 2012, 24, 745. (31) Zhang, G.; Yu, L.; Wu, H. B.; Hoster, H. E.; Lou, X. W. Adv. Mater. 2012, 24, 4609. (32) Zhang, L.; Wu, H. B.; Madhavi, S.; Hng, H. H.; Lou, X. W. J. Am. Chem. Soc. 2012, 134, 17388. (33) Zhang, L.; Wu, H. B.; Lou, X. W. J. Am. Chem. Soc. 2013, 135, 10664.

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