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Template-free synthesis of mesoporous single-crystal CuO particles with dumbbell-shaped morphology
Materials Letters 132 (2014) 98–101
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Template-free synthesis of mesoporous single-crystal CuO particles with dumbbell-shaped morphology Sourav Ghosh, Mouni Roy, Milan Kanti Naskar n Sol-Gel Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata 700032, India
art ic l e i nf o
a b s t r a c t
Article history: Received 18 September 2013 Accepted 9 June 2014 Available online 16 June 2014
Dumbbell-shaped CuO particles were synthesized by a rapid hydrothermal process at 150 1C/2 h via template-free route. Powder X-ray diffraction (PXRD) showed the crystallization of Cu2(OH)2CO3 in the as-prepared samples, and monoclinic CuO phase for 300 1C-treated samples. The BET surface area, total pore volume and average pore diameter of the samples were found to be 67.4 m2 g 1, 0.39 cm3 g 1, and 23 nm, respectively. Microstructural analysis indicated dumbbell-shaped morphology comprising of selfassembled microrod-like particles (length 2–10 μm, dia 200–300 nm). Single-crystalline nature of the particles was confirmed by selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) images. A tentative formation mechanism was illustrated. & 2014 Elsevier B.V. All rights reserved.
Keywords: Mesoporous CuO Self-assembly Hydrothermal Crystallization Porous material Microstructure
1. Introduction Synthesis of hierarchical micro- and/or nanomaterials, selfassembled building blocks, has attracted much attention due to their novel properties and potential applications in optoelectronics, magnetics and biology [1]. Morphology, shape and size of the particles play a significant role in determining physico-chemical properties of metal oxides [2]. Mesoporous transition metal oxides because of their large surface-to-volume ratio, abundant porous structures, and certain degree of size and shape selectivity are advantageous for many important applications [3]. Copper oxides, in particular are important functional materials to be used as catalyst, gas sensors, superconductors, lithium-ion-batteries, supercapacitors etc. [4,5]. Self-assembly of CuO nanostructures with controlled organization of primary building units into different morphologies like ellipsoids, microflowers dandelions, microworms, microurchins, and hollow microstructures have demanded in new technological applications [6]. Xu et al. [7] synthesized 2D CuO nanoleaves from 1D Cu(OH)2 nanowires via hierarchicaloriented attachment. Recently, we have synthesized polycrystalline hierarchical hollow CuO microspheres [8] by the hydrothermal method. Single-crystalline nanostructured CuO was prepared by different methods, like the thermal oxidation process [9], oxidation of Cu metal in the presence of formamide [10] and solution-based synthesis using sodium dodecyl benzenesulfonate as templating agent [11].
n
Corresponding author: Tel.: þ 91 33 2473 3496x3516; fax: þ91 33 2473 0957. E-mail address: [email protected] (M.K. Naskar).
http://dx.doi.org/10.1016/j.matlet.2014.06.045 0167-577X/& 2014 Elsevier B.V. All rights reserved.
Synthesis of mesoporous dumbbell-shaped CuO is important due to their fundamental shape-dependent properties and surface reactivity toward many applications. Singh et al. [12] reported that dumbbell shaped nickel nanoparticles are more advantageous than conventional nanoparticles or core–shell structures in terms of better magnetic properties. They suggested that dumbbell shaped particles comprised of two different surfaces providing different surface chemistry in one particle. Zhong et al. [13] proposed orientation aggregation mechanism for the synthesis of dumbbell-shaped manganese oxide via pyrolysis of manganese formate in trioctylamine/oleic acid medium. Dumbbell-shaped CuO was synthesized in the presence of organic surfactant (sodium dodecyl sulfate, SDS) and copper dodecyl sulfate (CU(DS)2) via their hydrophobic interactions [14]. Keeping the above views in mind, the objective of the present work was to design a simple synthesis approach to prepare morphologically controlled CuO particles with hierarchical structure and mesoporosity. In this work, to the best of our knowledge, we report for the first time the synthesis of dumbbell-shaped mesoporous single-crystal CuO by a rapid hydrothermal process via a facile template-free route in the presence of aqueous-based precursors.
2. Experimental In a typical synthesis, 5 mmol Cu(NO3)2 and 25 mmol urea were dissolved into 50 mL deionized (DI) water under stirring. The mix solution was transferred into a 100 mL Teflon-lined autoclave, followed by a hydrothermal treatment at 150 1C for 2 h. After the
S. Ghosh et al. / Materials Letters 132 (2014) 98–101
reaction, the particles were collected by centrifugation and washing with DI water, and dried at 60 1C for 4 h. The dried as-prepared powders were calcined at 300 1C with a heating rate of 1 1C min 1 and dwell time of 2 h. X-ray diffraction (XRD) studies of the calcined powders were performed by Philips X'Pert Pro PW 3050/60 powder diffractometer using Ni-filtered Cu-Kα radiation (λ ¼0.15418 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 ¼0.9λ/Bcosθ, where, λ is the wavelength of Cu-Kα, B is the full width at half maximum intensity peak (FWHM) in radian and θ is
99
the angle of the largest peak. Nitrogen adsorption–desorption measurements were conducted at 77 K with a Quantachrome (ASIQ MP, USA) instrument. The powders were outgassed in vacuum at 250 1C 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,
300 250
0.0016 0.0012
200
0.0008
150
0.0004 0.0000
100
0
500
1000
50
10
20
30
40
50
60
70
0
0.0
0.2
0.4
0.6
0.8
1.0
p/po Fig. 1. (a) XRD patterns of the particles (i) as-prepared and (ii) calcined (300 1C), and (b) N2 adsorption and desorption isotherms of mesoporous CuO. Inset shows pore size distributions evaluated from the adsorption isotherm.
Fig. 2. FESEM images of dumbbell-shaped particles: (a and b) as-prepared and (c and d) calcined at 300 1C.
100
S. Ghosh et al. / Materials Letters 132 (2014) 98–101
SupraTM 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. For TEM study Mo grid was used.
3. Results and discussion The XRD pattern (Fig. 1a) shows the characteristic peaks of (i) asprepared and (ii) calcined (300 1C) samples. In the as-prepared sample, Cu2(OH)2CO3 crystalline phase (JCPDS 76-0660) was observed while monoclinic CuO phase (JCPDS 05-0661) was identified for 300 1C-treated sample. Fig. 1b shows the nitrogen adsorption–desorption isotherms of the CuO particles. 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 formation of mesopores between platelike particles. It is to be noted that the uptake of nitrogen increased steeply above the relative pressure of about 0.75. The BJH pore size distribution (PSD) derived from adsorption data of the isotherms is shown in the inset of Fig. 1b. The broader PSD was due to the formation of interparticle void spaces [15] of CuO nanoparticles as well as the absence of regular shape and size of the pores in the samples. The BET surface area, total pore volume and average pore diameter of the particles were found to be 67.4 m2 g 1, 0.39 cm3 g 1 and 23 nm, respectively.
Fig. 2 shows the FESEM microstructures of (a and b) as-prepared, and (c and d) 300 1C-treated sample. It is clear that dumbbell-like morphology remained unchanged upon calcination at 300 1C. Interestingly, it was noticed that self-assembly of rod-shaped (length 2–10 μm, diameter 200–300 nm) particles rendered dumbbell-shaped (5–20 μm) structure. The higher magnification image of the dumbbell (Fig. 2d) indicates that some plate-like particles were grown in a patterned form at both ends of the dumbbell. The TEM image (Fig. 3a) also confirms the rod-like particles of CuO. It indicates that nanometer sized primary particles (10–20 nm) self-assembled together forming rodshaped particles (inset in Fig. 3a). The interparticle voids were generated between the component subunits. The primary particle size measured by Scherrer's equation was found to be 13 nm which was in good agreement with the TEM observation. The microstructures were further studied by selected area electron diffraction (SAED) and high resolution TEM (HRTEM). The appearance of slightly elongated diffraction spots in the SAED pattern (Fig. 3b) of CuO micro-rods indicates single crystals, which can be indexed to the phase pure monoclinic CuO [10]. The elongated diffraction spots in the micro-rod indicated the presence of multiple nanodomains with a small misorientation. It caused a random alignment among nanocrystallites in the microrods. The lattice image in HRTEM (Fig. 3c) also confirms the single-crystal nature of CuO micro-rod with a lattice spacing of 0.252 nm for the (002) plane [11]. The TEM-EDS analysis (Fig. 3d) reveals the uniform
Fig. 3. (a) TEM of rod-like CuO derived from dumbbell shaped structure (b) SAED pattern, (c) HRTEM image, and (d) EDX pattern.
S. Ghosh et al. / Materials Letters 132 (2014) 98–101
101
Fig. 4. Schematic representation of the proposed formation mechanism for dumbbell shaped morphology of CuO.
distribution of copper and oxygen in the particles with their atomic concentration ratio of about 1. Fig. 4 shows a tentative formation mechanism. Under hydrothermal reaction condition at 150 1C, urea decomposed slowly with the formation of NH3 and CO2. In aqueous medium, copper nitrate interacted with CO2 forming Cu2(OH)2CO3. The Cu2(OH)2CO3 under heat treatment at 300 1C was converted to CuO. The following probable reactions are described: CO(NH2)2 þxH2O-2NH3 þ CO2 þ (x 1)H2O
(1)
2Cu(NO3)2 þCO2 þ3H2O-Cu2(OH)2CO3 þ4HNO3
(2)
Cu2(OH)2CO3-2CuO þCO2 þH2O
(3)
It is mentioned worthy that the nuclei of Cu2(OH)2CO3 were formed first in the presence of decomposable urea. With the progress of reaction time, these nuclei oriented into rod shaped particles following “oriented attachment” mechanism [16]. In this growth mechanism, spontaneous self-organization of the smaller particles took place with a common crystallographic orientation. The rod-like particles further self-assembled into dumbbellshaped morphology through stepwise orientation and aggregation of a large number of nanoparticles into single-crystalline architectures in three dimensions. 4. Conclusions Mesoporous single-crystal CuO with dumbbell-shaped morphology was obtained by a simple and rapid hydrothermal process at 150 1C/2 h without using any organic templates. The hierarchical architecture of the synthesized particles could have significant affect on their catalytic, electronic and magnetic properties. It could find potential applications in the field of catalysis, magnetic storage media, field emission (FE) emitter, solar cell etc. The present method
has technological impact in terms of morphologically controlled synthesis by a simple process.
Acknowledgment The authors would like to thank the Director of this Institute for his kind permission to publish this paper. They also acknowledge the help rendered by Material Characterization division for material characterizations. The authors (S. Ghosh and M. Roy) are thankful to CSIR for their fellowship. The work was funded by DST-SERB Project, Government of India (No. GAP 0616).
References [1] Dinsmore AD, Hsu MF, Nikolaides MG, Marquez M, Bausch AR, Weitz DA. Science 2002;298:1006–9. [2] Sun S, Sun Y, Zhang X, Zhang H, Song X, Yang Z. CrystEngComm 2013;15:5275–82. [3] Bach U, Lupo D, Comte P, Moser JE, Weissortel F, Salbeck J, et al. Nature 1998;395:583–5. [4] Qiu G, Dharmarathna S, Zhang Y, Opembe N, Huang H, Suib SL. J Phys Chem C 2012;116:468–77. [5] Yuan YF, Pei YB, Fang J, Zhu HL, Yang JL, Guo SY. Mater Lett 2013;91:279–82. [6] Liu B, Zeng HC. J Am Chem Soc 2004;126:8124–5. [7] Xu H, Wang W, Zhu W, Zhou L, Ruan M. Cryst Growth Des 2007;7:2720–4. [8] Ghosh S, Naskar MK. RSC Adv 2013;3:13728–33. [9] Cheng S-L, Chen M-F. Nanoscale Res Lett 2012;7:119–25. [10] Zhang Z, Sun H, Shao X, Li D, Yu H, Han M. Adv Mater 2005;17:42–7. [11] Wang X, Xi G, Xiong S, Liu Y, Xi B, Yu W, et al. Cryst Growth Des 2007;7:930–4. [12] Singh MK, Agarwal A, Gopal R, Swarnkar RK, Kotnala RK. J Mater Chem 2011;21:11074–9. [13] Zhong X, Xie R, Sun L, Lieberwirth I, Knoll W. J Phys Chem B 2006;110:2–4. [14] Wang H, Shen Q, Li X, Liu F. Langmuir 2009;25:3152–8. [15] Yu J, Wang J, Li Z, Li L, Liu Q, Zhang M, et al. Cryst Growth Des 2012;12:2872–6. [16] Penn RL, Banfield JF. Science 1998;281:969–71.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Template-free synthesis of mesoporous single-crystal CuO particles with dumbbell-shaped morphology Sourav Ghosh, Mouni Roy, Milan Kanti Naskar n Sol-Gel Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata 700032, India
art ic l e i nf o
a b s t r a c t
Article history: Received 18 September 2013 Accepted 9 June 2014 Available online 16 June 2014
Dumbbell-shaped CuO particles were synthesized by a rapid hydrothermal process at 150 1C/2 h via template-free route. Powder X-ray diffraction (PXRD) showed the crystallization of Cu2(OH)2CO3 in the as-prepared samples, and monoclinic CuO phase for 300 1C-treated samples. The BET surface area, total pore volume and average pore diameter of the samples were found to be 67.4 m2 g 1, 0.39 cm3 g 1, and 23 nm, respectively. Microstructural analysis indicated dumbbell-shaped morphology comprising of selfassembled microrod-like particles (length 2–10 μm, dia 200–300 nm). Single-crystalline nature of the particles was confirmed by selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) images. A tentative formation mechanism was illustrated. & 2014 Elsevier B.V. All rights reserved.
Keywords: Mesoporous CuO Self-assembly Hydrothermal Crystallization Porous material Microstructure
1. Introduction Synthesis of hierarchical micro- and/or nanomaterials, selfassembled building blocks, has attracted much attention due to their novel properties and potential applications in optoelectronics, magnetics and biology [1]. Morphology, shape and size of the particles play a significant role in determining physico-chemical properties of metal oxides [2]. Mesoporous transition metal oxides because of their large surface-to-volume ratio, abundant porous structures, and certain degree of size and shape selectivity are advantageous for many important applications [3]. Copper oxides, in particular are important functional materials to be used as catalyst, gas sensors, superconductors, lithium-ion-batteries, supercapacitors etc. [4,5]. Self-assembly of CuO nanostructures with controlled organization of primary building units into different morphologies like ellipsoids, microflowers dandelions, microworms, microurchins, and hollow microstructures have demanded in new technological applications [6]. Xu et al. [7] synthesized 2D CuO nanoleaves from 1D Cu(OH)2 nanowires via hierarchicaloriented attachment. Recently, we have synthesized polycrystalline hierarchical hollow CuO microspheres [8] by the hydrothermal method. Single-crystalline nanostructured CuO was prepared by different methods, like the thermal oxidation process [9], oxidation of Cu metal in the presence of formamide [10] and solution-based synthesis using sodium dodecyl benzenesulfonate as templating agent [11].
n
Corresponding author: Tel.: þ 91 33 2473 3496x3516; fax: þ91 33 2473 0957. E-mail address: [email protected] (M.K. Naskar).
http://dx.doi.org/10.1016/j.matlet.2014.06.045 0167-577X/& 2014 Elsevier B.V. All rights reserved.
Synthesis of mesoporous dumbbell-shaped CuO is important due to their fundamental shape-dependent properties and surface reactivity toward many applications. Singh et al. [12] reported that dumbbell shaped nickel nanoparticles are more advantageous than conventional nanoparticles or core–shell structures in terms of better magnetic properties. They suggested that dumbbell shaped particles comprised of two different surfaces providing different surface chemistry in one particle. Zhong et al. [13] proposed orientation aggregation mechanism for the synthesis of dumbbell-shaped manganese oxide via pyrolysis of manganese formate in trioctylamine/oleic acid medium. Dumbbell-shaped CuO was synthesized in the presence of organic surfactant (sodium dodecyl sulfate, SDS) and copper dodecyl sulfate (CU(DS)2) via their hydrophobic interactions [14]. Keeping the above views in mind, the objective of the present work was to design a simple synthesis approach to prepare morphologically controlled CuO particles with hierarchical structure and mesoporosity. In this work, to the best of our knowledge, we report for the first time the synthesis of dumbbell-shaped mesoporous single-crystal CuO by a rapid hydrothermal process via a facile template-free route in the presence of aqueous-based precursors.
2. Experimental In a typical synthesis, 5 mmol Cu(NO3)2 and 25 mmol urea were dissolved into 50 mL deionized (DI) water under stirring. The mix solution was transferred into a 100 mL Teflon-lined autoclave, followed by a hydrothermal treatment at 150 1C for 2 h. After the
S. Ghosh et al. / Materials Letters 132 (2014) 98–101
reaction, the particles were collected by centrifugation and washing with DI water, and dried at 60 1C for 4 h. The dried as-prepared powders were calcined at 300 1C with a heating rate of 1 1C min 1 and dwell time of 2 h. X-ray diffraction (XRD) studies of the calcined powders were performed by Philips X'Pert Pro PW 3050/60 powder diffractometer using Ni-filtered Cu-Kα radiation (λ ¼0.15418 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 ¼0.9λ/Bcosθ, where, λ is the wavelength of Cu-Kα, B is the full width at half maximum intensity peak (FWHM) in radian and θ is
99
the angle of the largest peak. Nitrogen adsorption–desorption measurements were conducted at 77 K with a Quantachrome (ASIQ MP, USA) instrument. The powders were outgassed in vacuum at 250 1C 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,
300 250
0.0016 0.0012
200
0.0008
150
0.0004 0.0000
100
0
500
1000
50
10
20
30
40
50
60
70
0
0.0
0.2
0.4
0.6
0.8
1.0
p/po Fig. 1. (a) XRD patterns of the particles (i) as-prepared and (ii) calcined (300 1C), and (b) N2 adsorption and desorption isotherms of mesoporous CuO. Inset shows pore size distributions evaluated from the adsorption isotherm.
Fig. 2. FESEM images of dumbbell-shaped particles: (a and b) as-prepared and (c and d) calcined at 300 1C.
100
S. Ghosh et al. / Materials Letters 132 (2014) 98–101
SupraTM 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. For TEM study Mo grid was used.
3. Results and discussion The XRD pattern (Fig. 1a) shows the characteristic peaks of (i) asprepared and (ii) calcined (300 1C) samples. In the as-prepared sample, Cu2(OH)2CO3 crystalline phase (JCPDS 76-0660) was observed while monoclinic CuO phase (JCPDS 05-0661) was identified for 300 1C-treated sample. Fig. 1b shows the nitrogen adsorption–desorption isotherms of the CuO particles. 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 formation of mesopores between platelike particles. It is to be noted that the uptake of nitrogen increased steeply above the relative pressure of about 0.75. The BJH pore size distribution (PSD) derived from adsorption data of the isotherms is shown in the inset of Fig. 1b. The broader PSD was due to the formation of interparticle void spaces [15] of CuO nanoparticles as well as the absence of regular shape and size of the pores in the samples. The BET surface area, total pore volume and average pore diameter of the particles were found to be 67.4 m2 g 1, 0.39 cm3 g 1 and 23 nm, respectively.
Fig. 2 shows the FESEM microstructures of (a and b) as-prepared, and (c and d) 300 1C-treated sample. It is clear that dumbbell-like morphology remained unchanged upon calcination at 300 1C. Interestingly, it was noticed that self-assembly of rod-shaped (length 2–10 μm, diameter 200–300 nm) particles rendered dumbbell-shaped (5–20 μm) structure. The higher magnification image of the dumbbell (Fig. 2d) indicates that some plate-like particles were grown in a patterned form at both ends of the dumbbell. The TEM image (Fig. 3a) also confirms the rod-like particles of CuO. It indicates that nanometer sized primary particles (10–20 nm) self-assembled together forming rodshaped particles (inset in Fig. 3a). The interparticle voids were generated between the component subunits. The primary particle size measured by Scherrer's equation was found to be 13 nm which was in good agreement with the TEM observation. The microstructures were further studied by selected area electron diffraction (SAED) and high resolution TEM (HRTEM). The appearance of slightly elongated diffraction spots in the SAED pattern (Fig. 3b) of CuO micro-rods indicates single crystals, which can be indexed to the phase pure monoclinic CuO [10]. The elongated diffraction spots in the micro-rod indicated the presence of multiple nanodomains with a small misorientation. It caused a random alignment among nanocrystallites in the microrods. The lattice image in HRTEM (Fig. 3c) also confirms the single-crystal nature of CuO micro-rod with a lattice spacing of 0.252 nm for the (002) plane [11]. The TEM-EDS analysis (Fig. 3d) reveals the uniform
Fig. 3. (a) TEM of rod-like CuO derived from dumbbell shaped structure (b) SAED pattern, (c) HRTEM image, and (d) EDX pattern.
S. Ghosh et al. / Materials Letters 132 (2014) 98–101
101
Fig. 4. Schematic representation of the proposed formation mechanism for dumbbell shaped morphology of CuO.
distribution of copper and oxygen in the particles with their atomic concentration ratio of about 1. Fig. 4 shows a tentative formation mechanism. Under hydrothermal reaction condition at 150 1C, urea decomposed slowly with the formation of NH3 and CO2. In aqueous medium, copper nitrate interacted with CO2 forming Cu2(OH)2CO3. The Cu2(OH)2CO3 under heat treatment at 300 1C was converted to CuO. The following probable reactions are described: CO(NH2)2 þxH2O-2NH3 þ CO2 þ (x 1)H2O
(1)
2Cu(NO3)2 þCO2 þ3H2O-Cu2(OH)2CO3 þ4HNO3
(2)
Cu2(OH)2CO3-2CuO þCO2 þH2O
(3)
It is mentioned worthy that the nuclei of Cu2(OH)2CO3 were formed first in the presence of decomposable urea. With the progress of reaction time, these nuclei oriented into rod shaped particles following “oriented attachment” mechanism [16]. In this growth mechanism, spontaneous self-organization of the smaller particles took place with a common crystallographic orientation. The rod-like particles further self-assembled into dumbbellshaped morphology through stepwise orientation and aggregation of a large number of nanoparticles into single-crystalline architectures in three dimensions. 4. Conclusions Mesoporous single-crystal CuO with dumbbell-shaped morphology was obtained by a simple and rapid hydrothermal process at 150 1C/2 h without using any organic templates. The hierarchical architecture of the synthesized particles could have significant affect on their catalytic, electronic and magnetic properties. It could find potential applications in the field of catalysis, magnetic storage media, field emission (FE) emitter, solar cell etc. The present method
has technological impact in terms of morphologically controlled synthesis by a simple process.
Acknowledgment The authors would like to thank the Director of this Institute for his kind permission to publish this paper. They also acknowledge the help rendered by Material Characterization division for material characterizations. The authors (S. Ghosh and M. Roy) are thankful to CSIR for their fellowship. The work was funded by DST-SERB Project, Government of India (No. GAP 0616).
References [1] Dinsmore AD, Hsu MF, Nikolaides MG, Marquez M, Bausch AR, Weitz DA. Science 2002;298:1006–9. [2] Sun S, Sun Y, Zhang X, Zhang H, Song X, Yang Z. CrystEngComm 2013;15:5275–82. [3] Bach U, Lupo D, Comte P, Moser JE, Weissortel F, Salbeck J, et al. Nature 1998;395:583–5. [4] Qiu G, Dharmarathna S, Zhang Y, Opembe N, Huang H, Suib SL. J Phys Chem C 2012;116:468–77. [5] Yuan YF, Pei YB, Fang J, Zhu HL, Yang JL, Guo SY. Mater Lett 2013;91:279–82. [6] Liu B, Zeng HC. J Am Chem Soc 2004;126:8124–5. [7] Xu H, Wang W, Zhu W, Zhou L, Ruan M. Cryst Growth Des 2007;7:2720–4. [8] Ghosh S, Naskar MK. RSC Adv 2013;3:13728–33. [9] Cheng S-L, Chen M-F. Nanoscale Res Lett 2012;7:119–25. [10] Zhang Z, Sun H, Shao X, Li D, Yu H, Han M. Adv Mater 2005;17:42–7. [11] Wang X, Xi G, Xiong S, Liu Y, Xi B, Yu W, et al. Cryst Growth Des 2007;7:930–4. [12] Singh MK, Agarwal A, Gopal R, Swarnkar RK, Kotnala RK. J Mater Chem 2011;21:11074–9. [13] Zhong X, Xie R, Sun L, Lieberwirth I, Knoll W. J Phys Chem B 2006;110:2–4. [14] Wang H, Shen Q, Li X, Liu F. Langmuir 2009;25:3152–8. [15] Yu J, Wang J, Li Z, Li L, Liu Q, Zhang M, et al. Cryst Growth Des 2012;12:2872–6. [16] Penn RL, Banfield JF. Science 1998;281:969–71.