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Solvothermal synthesis of nanostructured NiO, ZnO and Co3O4 microspheres
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Materials Letters 62 (2008) 1957 – 1960 www.elsevier.com/locate/matlet
Solvothermal synthesis of nanostructured NiO, ZnO and Co3O4 microspheres Elvin Beach ⁎, Samantha Brown, Krenar Shqau, Matthew Mottern, Zack Warchol, Patricia Morris Materials Science and Engineering Department, The Ohio State University, 2041 North College Road, Columbus, OH 43210, USA Received 15 September 2007; accepted 25 October 2007 Available online 30 October 2007
Abstract A new solvothermal technique for synthesizing nanostructured microspheres of nickel oxide (NiO), zinc oxide (ZnO) and tricobalt tetraoxide (Co3O4) using nitrate precursors, urea and sodium dodecyl sulfate as a surfactant is described. The microspheres were shown to exhibit a morphology resembling a collection of platelets with nanoscale thickness. The surface area of the microspheres ranges from 62.8(±0.3) to 192.9 (± 1.3) m2/g, which appears to correspond to the density of platelet packing in the microsphere. The interior structure of the microspheres was investigated with a dual beam focused ion beam-scanning electron microscope and shown not to develop a hollow center. © 2007 Elsevier B.V. All rights reserved. Keywords: NiO; Co3O4; ZnO; Nitrate; Solvothermal
1. Introduction Metal oxide materials, specifically materials with nanoscale features, have been the subject of numerous research efforts in fields such as gas sensors [1,2], fuel cells [3], solar cells [4,5] and electrodes for lithium ion batteries [6] to name a few. Taking gas sensors as an example, high surface area is desired for increased reaction sites and several recent studies have fabricated porous gas sensors using a polymer microsphere as the template. The polymer is burned out leaving a large central pore with a metal oxide nanoparticle shell remaining [7–10]. While effective, these methods are typically time consuming and require multiple surfactant layers. Template free synthesis of porous or high surface area metal oxides has also been reported for a variety of materials [9,11–13]. Typically these reactions involve significant amounts of sodium or chlorine in the precursor materials and chlorine ions are known to adsorb strongly to the surface of metal hydroxides [14] before they are converted to metal oxides and require an additional process step, such as dialysis, to remove them from the final product.
⁎ Corresponding author. Tel.: +1 614 292 7427; fax: +1 614 688 4949. E-mail address: [email protected] (E. Beach). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.050
The technique reported previously by Liu et. al. [11] used a NiCl2 precursor to produce NiO microspheres. This paper presents a new method using nitrate precursors to eliminate chlorine contamination for synthesis of nanostructured NiO, ZnO, and Co3O4 microspheres. 2. Experimental 2.1. Synthesis A solvothermal process was used to synthesize the microspheres. All reagents were purchased from Sigma-Aldrich (Milwaukee, WI) and used as-received. First, 2 mmol of the appropriate metal-nitrate precursor; Ni(NO3) 2·6H2O, Zn (NO3)2·6H2O, or Co(NO3)2·6H2O; was dissolved in 20 mL of deionized water–ethanol (1:1 volume ratio). To this solution 20 mmol of urea and 0.1 g sodium dodecyl sulfate (SDS) was subsequently added. The solution was sealed in a Teflon®-lined stainless steel autoclave and placed in a 110 °C oven for 15 h. Upon completion of the reaction, the product was centrifuged at 10,000 rpm for 5 min and washed with deionized water and ethanol. The resulting precipitate, in the form of a metal hydroxide, was dried at 100 °C for 1 h, and then heated at 250 °C for 2 h to convert the metal hydroxide to the metal oxide final product.
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E. Beach et al. / Materials Letters 62 (2008) 1957–1960
Fig. 1. SEM images from (a–b) the NIO microspheres, (c–d) ZnO microspheres and (e–f) Co3 O4 microspheres.
2.2. Characterization Scanning electron microscope (SEM) images were collected using a FEI Sirion field-emission gun SEM. All samples were coated with 15 nm of osmium using a Filgen OPC-80T osmium plasma coater prior to SEM imaging. Focused ion beam (FIB) cross-sections were made using a FEI DB-235 FIB-SEM. Cross-sectioning was done with a 30 kV ion beam with currents limited to 100 pA or less as described previously for crosssectioning hollow particles [15]. X-ray diffraction (XRD) patterns were collected using a Scintag XDS2000 θ-θ diffractometer. XRD patterns were collected at room temperature using a continuous scan over an angular range of 2θ = 20– 90° with step size of 0.03° and scan rate of 1°/min. Complete N2 adsorption/desorption isotherms were measured at 77 K with a Micromeritics ASAP 2020. The multi-point BET surface area was estimated for from the adsorption isotherm for relative pressures b0.35. The pore volume and pore size was estimated
from the desorption isotherm by performing the BJH analysis. Samples were degassed, prior to analysis, under vacuum at 250 °C for a period of at least 12 h prior to measurement. 3. Results and discussion SEM images showing the morphology and XRD patterns revealing the crystal structure of the microspheres are shown in Figs. 1 and 2, respectively. The NiO microstructure is made up of platelets of NiO randomly oriented throughout the microsphere. The XRD pattern reveals that the microsphere is pure NiO with the rock salt crystal structure. The broadness of the peaks indicates that the platelets are very thin and an average of the Debye–Scherrer analysis on the 5 peaks suggests that the thickness is on the order of 11 nm. The ZnO has a similar morphology to the NiO; however, the platelets appear to be more densely packed in the microspheres as shown in Fig. 1(c). The XRD pattern reveals that ZnO forms the hexagonal wurtzite phase and applying the Debye–Scherrer formula to its pattern shown in Fig. 2(b) indicates the platelet thickness
E. Beach et al. / Materials Letters 62 (2008) 1957–1960
Fig. 2. XRD pattern from the (a) NiO, (b) ZnO and (c) Co3O4 microspheres The identified peaks correspond to NiO from ICDD# 47-1049, ZnO from ICDD# 751533 and Co3O4 from ICDD# 43-1003.
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product without further washing or purification as compared to materials made with a chloride precursor. The Co3O4 microspheres developed a significantly different microstructure relative to the NiO and ZnO. While the XRD confirms that the microspheres are pure spinel Co3O4, the platelets that formed are appreciably thicker than those making up the NiO and ZnO microspheres. Additionally, several areas in the sample exhibit bundles of small fibrils, as shown in Fig. 1(f). The fibril bundles mainly appear to precipitate near the corners of the large platelets. These differences in the microstructure of the Co3O4 may be due to a different formation mechanism. The formation of NiO and ZnO is believed to be similar to a mechanism proposed by Peng et. al. [16] and Chen et. al. [17]. The urea is decomposed into CO2 and NH3 during heating in the autoclave [11]. The ammonia is hydrolyzed and complexes with the metal ions in solution to form Ni(OH)2 [18] or Zn(OH)2 which was confirmed by XRD (not shown). The presence of small CO2 bubbles provides heterogeneous nucleation sites and the subsequent accumulation of the metal-hydroxide platelets on the bubble surface. Several of the metalhydroxide covered CO2 bubbles coagulate in the autoclave to form a metal-hydroxide microsphere. The SDS also interacts with the surface of the metal-hydroxide platelets and appears to limit the coagulation and growth of the platelets providing the final porous microstructures. Cobalt does not seem to follow the same path since some must convert from a + 2 charge in the precursor to a + 3 charge in the product while zinc and nickel maintain the + 2 charge. The formation mechanism of the Co3O4 appears to be more complex due to the change in oxidation state and is beyond the scope of the discussion in this letter. The results of the surface area, pore volume, and pore size measurements are shown below in Table 1. The NiO showed a surface area N 3× that of the ZnO. It was suggested by Liu et. al. that the NiO microspheres, made using a similar process but with a NiCl2 precursor, were hollow and the resulting microstructure with numerous cavities was beneficial for small molecule adsorption and therefore a large surface area [11]. The interior of the microspheres synthesized in this work were investigated to determine if an explanation for the large discrepancy in surface area could be established. Cross-sections were prepared and imaged from ∼10NiO and ZnO microspheres, respectively, using the FIB-SEM. Micrographs of selected cross-sectioned particles are shown in Fig. 3. The images in Fig. 3 show that neither the NiO nor the ZnO can be considered hollow microspheres. There is significant, relatively homogeneous porosity throughout the interior of both types of microspheres. The SEM image shown in Fig. 3(d), a cross section through the center of the ZnO microsphere, revealed a region near the center where a slightly larger pore was found; however, it is not of significant size to label the microsphere as hollow. There are many possible causes for the difference in measured surface area including platelet surface crystallography the possibility of a closed pore structure. Further research to determine the reasons for the surface area discrepancy is underway; however, it is beyond the scope of this discussion. Future work with these materials including optimizing the synthesis time, temperatures, surfactant and metal precursor concentrations should lead to the development of optimized materials Table 1 The BET estimated surface area and the BJH estimated pore volume and pore size for the NiO, ZnO and Co3O4 microspheres
is approximately 11 nm, or similar in thickness to the NiO. In addition to the SEM imaging, energy dispersive X-ray spectra (EDS) using a 5 keV electron beam were collected from the NiO and ZnO and did not show any sodium or sulfur contamination on the surface; within the limits of EDS detection. It is expected that this process yields a highly pure final
Material
Surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
NiO ZnO Co3O4
192.9 ± 1.3 62.8 ± 0.3 123.5 ± 1.3
0.55 ± 0.03 0.23 ± 0.01 0.26 ± 0.01
2.9 ± 0.2 7.2 ± 0.4 3.6 ± 0.2
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E. Beach et al. / Materials Letters 62 (2008) 1957–1960
Fig. 3. FIB-SEM cross-sections through (a and b) a NiO microsphere, and (c and d) a ZnO microsphere.
with further increased surface area for testing in novel solid state gas sensors.
4. Conclusions Nanostructured NiO, ZnO and Co3O4 microspheres were synthesized using a new solvothermal technique. The morphology of each material varied significantly. NiO microspheres were composed of relatively large, thin and loosely associated platelets while the ZnO showed smaller more densely packed platelets. The Co3O4 was composed of thicker platelets with bundles of nano-fibrils protruding from the corner of some platelets and its formation likely proceeds by a more complex route. The NiO had the highest surface area, followed by the Co3O4 and ZnO was the lowest. FIB cross-sections revealed that the NiO and ZnO particles were not hollow and showed fairly homogenous porosity throughout. Acknowledgements This work is supported by the National Science Foundation under IGERT Grant no. 0221678 and funding from the Materials Science and Engineering Department at The Ohio State University. References [1] G. Blaser, T. Ruhl, C. Diehl, M. Ulrich, D. Kohl, Physica A: Statistical and Theoretical Physics 266 (1999) 218–223.
[2] G. Korotcenkov, Sensors and Actuators B 107 (2005) 209–232. [3] B. Levy, Journal of Electroceramics 1 (1997) 239–272. [4] J. Bandara, C.M. Divarathne, S.D. Nanayakkara, Solar Energy Materials and Solar Cells 81 (2004) 429–437. [5] J. Bandara, J.P. Yasomanee, Semiconductor Science and Technology 22 (2007) 20–24. [6] C.L. Liao, Y.H. Lee, S.T. Chang, K.Z. Fung, Journal of Power Sources 158 (2006) 1379–1385. [7] F. Caruso, R.A. Caruso, H. Mohwald, Science 282 (1998) 1111–1114. [8] C. Martinez, B. Hockey, C. Montgomery, S. Semancik, Langmuir 21 (2005) 7937–7944. [9] W.P. Shao, et al., Chemistry Letters 34 (2005) 556–557. [10] Z.Y. Zhong, Y.D. Yin, B. Gates, Y.N. Xia, Advanced Materials 12 (2000) 206–+. [11] J. Liu, et al., Materials Letters 60 (2006) 3601–3604. [12] B. Liu, H.C. Zeng, Journal of the American Chemical Society 126 (2004) 8124–8125. [13] B. Liu, H.C. Zeng, Journal of the American Chemical Society 126 (2004) 16744–16746. [14] J.R. Zhang, L. Gao, Materials Chemistry and Physics 87 (2004) 10–13. [15] E. Beach, M. Keefe, W. Heeschen, D. Rothe, Polymer 46 (2005) 11195–11197. [16] Q. Peng, Y.J. Dong, Y.D. Li, Angewandte Chemie-International Edition 42 (2003) 3027–3030. [17] X.Y. Chen, et al., Journal of Crystal Growth 263 (2004) 570–574. [18] G. Soler-Illia, M. Jobbagy, A.E. Regazzoni, M.A. Blesa, Chemistry of Materials 11 (1999) 3140–3146.
Materials Letters 62 (2008) 1957 – 1960 www.elsevier.com/locate/matlet
Solvothermal synthesis of nanostructured NiO, ZnO and Co3O4 microspheres Elvin Beach ⁎, Samantha Brown, Krenar Shqau, Matthew Mottern, Zack Warchol, Patricia Morris Materials Science and Engineering Department, The Ohio State University, 2041 North College Road, Columbus, OH 43210, USA Received 15 September 2007; accepted 25 October 2007 Available online 30 October 2007
Abstract A new solvothermal technique for synthesizing nanostructured microspheres of nickel oxide (NiO), zinc oxide (ZnO) and tricobalt tetraoxide (Co3O4) using nitrate precursors, urea and sodium dodecyl sulfate as a surfactant is described. The microspheres were shown to exhibit a morphology resembling a collection of platelets with nanoscale thickness. The surface area of the microspheres ranges from 62.8(±0.3) to 192.9 (± 1.3) m2/g, which appears to correspond to the density of platelet packing in the microsphere. The interior structure of the microspheres was investigated with a dual beam focused ion beam-scanning electron microscope and shown not to develop a hollow center. © 2007 Elsevier B.V. All rights reserved. Keywords: NiO; Co3O4; ZnO; Nitrate; Solvothermal
1. Introduction Metal oxide materials, specifically materials with nanoscale features, have been the subject of numerous research efforts in fields such as gas sensors [1,2], fuel cells [3], solar cells [4,5] and electrodes for lithium ion batteries [6] to name a few. Taking gas sensors as an example, high surface area is desired for increased reaction sites and several recent studies have fabricated porous gas sensors using a polymer microsphere as the template. The polymer is burned out leaving a large central pore with a metal oxide nanoparticle shell remaining [7–10]. While effective, these methods are typically time consuming and require multiple surfactant layers. Template free synthesis of porous or high surface area metal oxides has also been reported for a variety of materials [9,11–13]. Typically these reactions involve significant amounts of sodium or chlorine in the precursor materials and chlorine ions are known to adsorb strongly to the surface of metal hydroxides [14] before they are converted to metal oxides and require an additional process step, such as dialysis, to remove them from the final product.
⁎ Corresponding author. Tel.: +1 614 292 7427; fax: +1 614 688 4949. E-mail address: [email protected] (E. Beach). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.050
The technique reported previously by Liu et. al. [11] used a NiCl2 precursor to produce NiO microspheres. This paper presents a new method using nitrate precursors to eliminate chlorine contamination for synthesis of nanostructured NiO, ZnO, and Co3O4 microspheres. 2. Experimental 2.1. Synthesis A solvothermal process was used to synthesize the microspheres. All reagents were purchased from Sigma-Aldrich (Milwaukee, WI) and used as-received. First, 2 mmol of the appropriate metal-nitrate precursor; Ni(NO3) 2·6H2O, Zn (NO3)2·6H2O, or Co(NO3)2·6H2O; was dissolved in 20 mL of deionized water–ethanol (1:1 volume ratio). To this solution 20 mmol of urea and 0.1 g sodium dodecyl sulfate (SDS) was subsequently added. The solution was sealed in a Teflon®-lined stainless steel autoclave and placed in a 110 °C oven for 15 h. Upon completion of the reaction, the product was centrifuged at 10,000 rpm for 5 min and washed with deionized water and ethanol. The resulting precipitate, in the form of a metal hydroxide, was dried at 100 °C for 1 h, and then heated at 250 °C for 2 h to convert the metal hydroxide to the metal oxide final product.
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E. Beach et al. / Materials Letters 62 (2008) 1957–1960
Fig. 1. SEM images from (a–b) the NIO microspheres, (c–d) ZnO microspheres and (e–f) Co3 O4 microspheres.
2.2. Characterization Scanning electron microscope (SEM) images were collected using a FEI Sirion field-emission gun SEM. All samples were coated with 15 nm of osmium using a Filgen OPC-80T osmium plasma coater prior to SEM imaging. Focused ion beam (FIB) cross-sections were made using a FEI DB-235 FIB-SEM. Cross-sectioning was done with a 30 kV ion beam with currents limited to 100 pA or less as described previously for crosssectioning hollow particles [15]. X-ray diffraction (XRD) patterns were collected using a Scintag XDS2000 θ-θ diffractometer. XRD patterns were collected at room temperature using a continuous scan over an angular range of 2θ = 20– 90° with step size of 0.03° and scan rate of 1°/min. Complete N2 adsorption/desorption isotherms were measured at 77 K with a Micromeritics ASAP 2020. The multi-point BET surface area was estimated for from the adsorption isotherm for relative pressures b0.35. The pore volume and pore size was estimated
from the desorption isotherm by performing the BJH analysis. Samples were degassed, prior to analysis, under vacuum at 250 °C for a period of at least 12 h prior to measurement. 3. Results and discussion SEM images showing the morphology and XRD patterns revealing the crystal structure of the microspheres are shown in Figs. 1 and 2, respectively. The NiO microstructure is made up of platelets of NiO randomly oriented throughout the microsphere. The XRD pattern reveals that the microsphere is pure NiO with the rock salt crystal structure. The broadness of the peaks indicates that the platelets are very thin and an average of the Debye–Scherrer analysis on the 5 peaks suggests that the thickness is on the order of 11 nm. The ZnO has a similar morphology to the NiO; however, the platelets appear to be more densely packed in the microspheres as shown in Fig. 1(c). The XRD pattern reveals that ZnO forms the hexagonal wurtzite phase and applying the Debye–Scherrer formula to its pattern shown in Fig. 2(b) indicates the platelet thickness
E. Beach et al. / Materials Letters 62 (2008) 1957–1960
Fig. 2. XRD pattern from the (a) NiO, (b) ZnO and (c) Co3O4 microspheres The identified peaks correspond to NiO from ICDD# 47-1049, ZnO from ICDD# 751533 and Co3O4 from ICDD# 43-1003.
1959
product without further washing or purification as compared to materials made with a chloride precursor. The Co3O4 microspheres developed a significantly different microstructure relative to the NiO and ZnO. While the XRD confirms that the microspheres are pure spinel Co3O4, the platelets that formed are appreciably thicker than those making up the NiO and ZnO microspheres. Additionally, several areas in the sample exhibit bundles of small fibrils, as shown in Fig. 1(f). The fibril bundles mainly appear to precipitate near the corners of the large platelets. These differences in the microstructure of the Co3O4 may be due to a different formation mechanism. The formation of NiO and ZnO is believed to be similar to a mechanism proposed by Peng et. al. [16] and Chen et. al. [17]. The urea is decomposed into CO2 and NH3 during heating in the autoclave [11]. The ammonia is hydrolyzed and complexes with the metal ions in solution to form Ni(OH)2 [18] or Zn(OH)2 which was confirmed by XRD (not shown). The presence of small CO2 bubbles provides heterogeneous nucleation sites and the subsequent accumulation of the metal-hydroxide platelets on the bubble surface. Several of the metalhydroxide covered CO2 bubbles coagulate in the autoclave to form a metal-hydroxide microsphere. The SDS also interacts with the surface of the metal-hydroxide platelets and appears to limit the coagulation and growth of the platelets providing the final porous microstructures. Cobalt does not seem to follow the same path since some must convert from a + 2 charge in the precursor to a + 3 charge in the product while zinc and nickel maintain the + 2 charge. The formation mechanism of the Co3O4 appears to be more complex due to the change in oxidation state and is beyond the scope of the discussion in this letter. The results of the surface area, pore volume, and pore size measurements are shown below in Table 1. The NiO showed a surface area N 3× that of the ZnO. It was suggested by Liu et. al. that the NiO microspheres, made using a similar process but with a NiCl2 precursor, were hollow and the resulting microstructure with numerous cavities was beneficial for small molecule adsorption and therefore a large surface area [11]. The interior of the microspheres synthesized in this work were investigated to determine if an explanation for the large discrepancy in surface area could be established. Cross-sections were prepared and imaged from ∼10NiO and ZnO microspheres, respectively, using the FIB-SEM. Micrographs of selected cross-sectioned particles are shown in Fig. 3. The images in Fig. 3 show that neither the NiO nor the ZnO can be considered hollow microspheres. There is significant, relatively homogeneous porosity throughout the interior of both types of microspheres. The SEM image shown in Fig. 3(d), a cross section through the center of the ZnO microsphere, revealed a region near the center where a slightly larger pore was found; however, it is not of significant size to label the microsphere as hollow. There are many possible causes for the difference in measured surface area including platelet surface crystallography the possibility of a closed pore structure. Further research to determine the reasons for the surface area discrepancy is underway; however, it is beyond the scope of this discussion. Future work with these materials including optimizing the synthesis time, temperatures, surfactant and metal precursor concentrations should lead to the development of optimized materials Table 1 The BET estimated surface area and the BJH estimated pore volume and pore size for the NiO, ZnO and Co3O4 microspheres
is approximately 11 nm, or similar in thickness to the NiO. In addition to the SEM imaging, energy dispersive X-ray spectra (EDS) using a 5 keV electron beam were collected from the NiO and ZnO and did not show any sodium or sulfur contamination on the surface; within the limits of EDS detection. It is expected that this process yields a highly pure final
Material
Surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
NiO ZnO Co3O4
192.9 ± 1.3 62.8 ± 0.3 123.5 ± 1.3
0.55 ± 0.03 0.23 ± 0.01 0.26 ± 0.01
2.9 ± 0.2 7.2 ± 0.4 3.6 ± 0.2
1960
E. Beach et al. / Materials Letters 62 (2008) 1957–1960
Fig. 3. FIB-SEM cross-sections through (a and b) a NiO microsphere, and (c and d) a ZnO microsphere.
with further increased surface area for testing in novel solid state gas sensors.
4. Conclusions Nanostructured NiO, ZnO and Co3O4 microspheres were synthesized using a new solvothermal technique. The morphology of each material varied significantly. NiO microspheres were composed of relatively large, thin and loosely associated platelets while the ZnO showed smaller more densely packed platelets. The Co3O4 was composed of thicker platelets with bundles of nano-fibrils protruding from the corner of some platelets and its formation likely proceeds by a more complex route. The NiO had the highest surface area, followed by the Co3O4 and ZnO was the lowest. FIB cross-sections revealed that the NiO and ZnO particles were not hollow and showed fairly homogenous porosity throughout. Acknowledgements This work is supported by the National Science Foundation under IGERT Grant no. 0221678 and funding from the Materials Science and Engineering Department at The Ohio State University. References [1] G. Blaser, T. Ruhl, C. Diehl, M. Ulrich, D. Kohl, Physica A: Statistical and Theoretical Physics 266 (1999) 218–223.
[2] G. Korotcenkov, Sensors and Actuators B 107 (2005) 209–232. [3] B. Levy, Journal of Electroceramics 1 (1997) 239–272. [4] J. Bandara, C.M. Divarathne, S.D. Nanayakkara, Solar Energy Materials and Solar Cells 81 (2004) 429–437. [5] J. Bandara, J.P. Yasomanee, Semiconductor Science and Technology 22 (2007) 20–24. [6] C.L. Liao, Y.H. Lee, S.T. Chang, K.Z. Fung, Journal of Power Sources 158 (2006) 1379–1385. [7] F. Caruso, R.A. Caruso, H. Mohwald, Science 282 (1998) 1111–1114. [8] C. Martinez, B. Hockey, C. Montgomery, S. Semancik, Langmuir 21 (2005) 7937–7944. [9] W.P. Shao, et al., Chemistry Letters 34 (2005) 556–557. [10] Z.Y. Zhong, Y.D. Yin, B. Gates, Y.N. Xia, Advanced Materials 12 (2000) 206–+. [11] J. Liu, et al., Materials Letters 60 (2006) 3601–3604. [12] B. Liu, H.C. Zeng, Journal of the American Chemical Society 126 (2004) 8124–8125. [13] B. Liu, H.C. Zeng, Journal of the American Chemical Society 126 (2004) 16744–16746. [14] J.R. Zhang, L. Gao, Materials Chemistry and Physics 87 (2004) 10–13. [15] E. Beach, M. Keefe, W. Heeschen, D. Rothe, Polymer 46 (2005) 11195–11197. [16] Q. Peng, Y.J. Dong, Y.D. Li, Angewandte Chemie-International Edition 42 (2003) 3027–3030. [17] X.Y. Chen, et al., Journal of Crystal Growth 263 (2004) 570–574. [18] G. Soler-Illia, M. Jobbagy, A.E. Regazzoni, M.A. Blesa, Chemistry of Materials 11 (1999) 3140–3146.