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Building crystalline Sb2S3 nanowire dandelions with multiple crystal splitting motif
Materials Letters 67 (2012) 222–225
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Building crystalline Sb2S3 nanowire dandelions with multiple crystal splitting motif Gonghua Wang, Chin Li Cheung ⁎ Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588, USA
a r t i c l e
i n f o
Article history: Received 11 August 2011 Accepted 21 September 2011 Available online 29 September 2011 Keywords: Sb2S3 nanowire Stibnite Solvothermal process Template Assembly Crystal splitting
a b s t r a c t Crystalline dandelion-like antimony (III) sulfide (Sb2S3) nanowires were synthesized by a PEG-assisted solvothermal process. The orthorhombic crystal structure and dandelion-like multi-branched nanowire morphology were revealed by X-ray diffractometry (XRD) and scanning electron microscopy (SEM) respectively. High-resolution transmission electron microscopy (TEM) identified that the highly crystalline Sb2S3 nanowires grew along the [001] direction with individual wire diameter of 195 ± 52 nm. The band gap of the Sb2S3 nanowires was measured to be ca. 1.67 eV. A combination of PEG-templated assembly and crystal splitting mechanism was likely responsible for the growth of the observed nanowire dandelion structures. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Antimony (III) sulfide (Stibnite, Sb2S3) is an important member of the main group metal chalcogenides compounds. Due to its excellent photoconductivity and semiconducting properties, Sb2S3 has received wide attention for its potential applications in solar energy conversions [1], thermoelectric cooling technologies and optoelectronics in the visible and IR region [2,3]. It can also be applied as a starting material for the synthesis of complex chalcogenides with 3D open-framework structure, such as antimony sulfur iodide (SbSI) [4]. Sb2S3 has an orthorhombic lattice with a chain-like structure (SbxSy)n. Since the binding between these chains is considerably weaker than those within the chains, Sb2S3 tends to form one-dimensional structures, such as nanowires, nanorods and nanoribbons under selected growth conditions [511]. Proper solvothermal conditions may be applied for promoting the nucleation and the subsequent growth of Sb2S3 crystals towards desired morphologies, for instance, shuttle-like Sb2S3 nanorods were synthesized via a polyvinylpyrrolidone-assisted solvothermal approach [12]. While Sb2S3 is reported to be semiconducting, there exist contradictions in its reported band gap energy. One study suggests that this material is a p-type semiconductor with a calculated indirect band gap of 1.35 eV [13]. Others reported that this material has a direct band gap of 1.78− 2.2 eV, as well as a dielectric constant of 10.9−14.4 [14]. In this work,
⁎ Corresponding author. Tel.: + 1 402 472 5172; fax: + 1 402 472 9402. E-mail address: [email protected] (C.L. Cheung). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.074
we report a facile synthetic approach to grow dandelion-like Sb2S3 nanowires with a measured direct band gap of 1.67 eV. 2. Experimental The dandelion-like multi-branched Sb2S3 nanowire structures were prepared by a solvothermal method. In a typical synthesis, 0.18 g of antimony (III) chloride (SbCl3) and 3 g of polyethylene glycol with molecular weight of ca. 6 kDa (PEG-6000) were dissolved in 45 ml of ethylene glycol (EG), followed by the addition of 1.7 g of thiourea ((NH2)2CS). The light orange solution was transferred to a Teflon-lined stainless steel autoclave which was then kept at 180 °C in a convection oven for 7 h. and allowed to cool to room temperature. The product was filtered, washed with de-ionized water and dried under vacuum at 80 °C for 2 h. The crystal structure of the obtained samples was characterized by X-ray diffraction (XRD) with a Rigaku D/Max-B x-ray diffractometer. The weighted average wavelength of the Cu Kα radiation is 1.5417 Å. The diffraction pattern of the sample was compared and identified with JCPDS File No. 42–1393. The morphology of the Sb2S3 nanowires was examined by a Hitachi S4700 scanning electron microscope (SEM) operated at 5 kV. The crystal structure of the product was studied with transmission electron microscopy (TEM) on a Tecnai G 2 F20 S-Twin operated at 200 kV. The Raman spectrum of the product was recorded at room temperature on a Renishaw InVia Raman microscope equipped with a 514.5 nm Ar ion laser, a monochromator and a
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CCD detector. The band gap of the product was measured using diffuse reflectance UV–visible spectroscopy with a Perkin–Elmer Lambda 950 spectrometer equipped with a 150 mm integrating sphere. The spectrum was measured relative to Spectralon using a 4 nm slit and 1 nm sampling interval. 3. Results and discussions The morphology of the as-synthesized products was revealed by SEM to be dandelion-like nanowire structures (Fig. 1). The zoomedin image shows that the individual wires are mostly terminated with tapered tips (Fig. 1a inset). These nanowires were often found organized into extended sheaf-like structures (Fig. 1b). The zoomed-in image of the interior structure of a “sheaf” illustrates the multiple-branching organization of the wires (circles in Fig. 1b inset). These wires were often found to split from the lower branches multiple times. This multiple-branching phenomenon was spotted throughout the Sb2S3 nanowire sample, suggesting it to be the basic structure building motif for the formation of the macroscopic dandelion-like morphology. The phase identity of the product was verified to be orthorhombic Sb2S3 by XRD. The XRD pattern of the Sb2S3 nanowire product (Fig. 2a) matches up well with the standard diffraction pattern of orthorhombic Sb2S3 (JCPDS ID 042–1393). All diffraction peaks can be indexed accordingly with lattice constants a = 11.24 Å, b = 11.31 Å, c = 3.84 Å. The identity of Sb2S3 nanowire was also confirmed by Raman spectroscopy. Four major Raman peaks located at 188, 251, 371 and 450 cm − 1 (Fig. 2b) correspond to the characteristic Raman shifts for bulk crystalline Sb2S3 [15]. The crystal lattice structure of the Sb2S3 nanowires was further investigated by TEM. Fig. 3 presents a typical Sb2S3 nanowire with diameter of 115 nm and length of 6.1 μm. The high crystallinity of this
Fig. 1. SEM images of (a) the dandelion-like Sb2S3 nanowires and (b) Sb2S3 nanowire “sheafs.” The zoomed-in views are shown in the insets.
Fig. 2. (a) XRD pattern and (b) Raman spectrum of the Sb2S3 nanowire dandelions.
wire was revealed by the clear lattice fringes in the high resolution TEM images at its tip, center and edge (Fig. 3b–d). The zoomed-in image of the center of the wire shows the average inter-planar distances to be 0.56 nm and 0.36 nm, which correspond to the (200) and (130) crystal plane lattice distances of orthorhombic Sb2S3 (Fig. 3e). It also implies that the wire growth direction is along the [001] direction. A selected area electron diffraction (SAED) pattern at the center of this wire further indicates the well-defined crystallinity and orthorhombic structure of this Sb2S3 nanowire (Fig. 3f). Based on a statistical analysis of 30 nanowires, the average wire diameter was determined to be ca. 195±52 nm (data not shown). Based upon the experimental results, the growth mechanism of the Sb2S3 dandelion-like structure synthesized by our PEG-assisted solvothermal method probably involved two major steps: the PEGassisted assembly [16] and a multiple crystal splitting mechanism. In this synthesis, PEG played two important roles as a soft template and a surface stabilizer. In the beginning of the synthesis, the flexible PEG chains dissolved and entangled in ethylene glycol and formed an amorphous spherical template. The Sb precursor was then dissolved into PEG/EG to form Sb-PEG globules which served as a template for the following assemblies. Upon the release of HS - or S 2- ions from thiourea, an orange colloidal solution appeared, indicating the emergence of dispersed Sb2S3 nuclei in the super-saturated solution. During the solvothermal treatment, the nuclei continued to grow into small crystals through the solid-solution-solid process (Ostwald ripening). As the reaction proceeded, fresh Sb2S3 crystals gradually grew out from surface of the Sb-PEG globules at a fast growth rate in the [001] direction, leading to the pseudo assembled Sb2S3 nanowires in a dandelion-like morphology. The use of PEG was necessary for the growth of this morphology because only random submicrowires were obtained in the absence of PEG [5]. We hypothesize that PEG stabilized the growth of Sb2S3 nanowires via strong binding with the SH group dissociated from thiourea (SH-PEG-SH). Similar to Bi2S3[17], Sb2S3 exhibits a chain-like crystal structure and tends to perform fast crystal splitting during the crystal growth. In this synthesis, likely due to the strong bonding between
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Fig. 3. TEM images of a typical Sb2S3 nanowire. (a) Low-magnification TEM image of one Sb2S3 wire; high magnification image of (b) the tip, (c) the center and (d) the edge of the Sb2S3 wire; (e) the zoomed-in view of (c) and (f) the selected area diffraction pattern of the center of the studied Sb2S3 nanowire.
PEG and sulfur atoms on the low energy Sb2S3 planes (e.g. (100) planes), the tightly bound PEG molecules probably inserted themselves between these Sb2S3 crystal planes in the Sb2S3 lattice during the growth process as “molecular wedge” and initiated the crystal splitting. Subsequent wire growth continued until the next crystal splitting started by the surface insertion of PEG molecules. Our observed multi-branching results indicated that the repeated crystal splitting motif was the major driving force for the latitudinal growth of Sb2S3 nanowires into the dandelion 3-D structures rather than other reported Sb2S3 double-sheaf structures [18]. The optical property of the Sb2S3 nanowire dandelions was studied by diffuse reflectance spectroscopy at room temperature (Fig. 4). The Tauc plot (inset) of Sb2S3 nanowire dandelions indicates that this material has a likely direct optical band gap of 1.67 eV. This
value is close to the reported band gap values 1.65 eV [12] and 1.66 eV [13] of Sb2S3. 4. Conclusion We report a facile synthetic route for the dandelion-like multibranched single crystalline Sb2S3 nanowires using a solvothermal process with low-cost precursors. The multi-branched morphology suggest that the growth of Sb2S3 nanowire dandelions likely resulted from both the PEG-assisted assembly process and multiple crystal splitting due to the strong interaction between PEG and low energy Sb2S3 planes. The measured value of the direct band gap of the Sb2S3 nanowires, 1.67 eV, is comparable to the previously reported literature data.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Fig. 4. Diffuse reflectance spectrum of the Sb2S3 nanowires. (R: Reflectance; F(R): Kubelka-Monk function; E: photon energy) (inset) Tauc plot of the spectrum.
Acknowledgments We thank Nebraska Center for Energy Sciences Research and DOD/ ARO (W911NF-10-2-0099) for their financial support, and Drs. David Diercks and Yongfeng Lu for their help with the microscopy work.
[17] [18]
Savadogo O, Mandal KC. Sol Energ Mat Sol Cells 1992;26:117–36. Savadogo O, Mandal KC. Electron Lett 1992;28:1682–3. Rajpure KY, Bhosale CH. Mater Chem Phys 2002;73:6–12. Parise JB. Science 1991;251:293–4. Zhu GQ, Liu P, Miao HY, Zhu JP, Bian XB, Liu Y, Chen B, Wang XB. Mater Res Bull 2008;43:2636–42. Lou WJ, Chen M, Wang XB, Liu WM. Chem Mater 2007;19:872–8. Yu Y, Wang RH, Chen Q, Peng LM. J Phys Chem B 2006;110:13415–9. Malakooti R, Cademartiri L, Migliori A, Ozin GA. J Mater Chem 2008;18:66–9. Yang J, Zeng JH, Yu SH, Yang L, Zhang YH, Qian YT. Chem Mater 2000;12:2924–9. Geng ZR, Wang MX, Yue GH, Yan PX. J Cryst Growth 2008;310:341–4. Hu HM, Mo MS, Yan BJ, Zhang XJ, Li QW, Yu WC, Qian YT. J Cryst Growth 2003;258:106–12. Zhang L, Chen L, Wan HQ, Zhou HD, Chen JM. Cryst Res Technol 2010;45:178–82. Sun M, Li DZ, Li WJ, Chen YB, Chen ZX, He YH, Fu XZ. J Phys Chem C 2008;112: 18076–81. Juarez BH, Ibisate M, Palacios JM, Lopez C. Adv Mater 2003;15:319–23. Juarez BH, Rubio S, Sanchez-Dehesa J, Lopez C. Adv Mater 2002;14:1486–90. Zhou XF, Zhao X, Zhang DY, Chen SY, Guo XF, Ding WP, Chen Y. Nanotechnology 2006;17:3806–11. Tang J, Alivisatos AP. Nano Lett 2006;6:2701–6. Pilapong C, Thongtem T, Thongtem S. J Alloy Compd 2010;507:L38–42.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Building crystalline Sb2S3 nanowire dandelions with multiple crystal splitting motif Gonghua Wang, Chin Li Cheung ⁎ Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588, USA
a r t i c l e
i n f o
Article history: Received 11 August 2011 Accepted 21 September 2011 Available online 29 September 2011 Keywords: Sb2S3 nanowire Stibnite Solvothermal process Template Assembly Crystal splitting
a b s t r a c t Crystalline dandelion-like antimony (III) sulfide (Sb2S3) nanowires were synthesized by a PEG-assisted solvothermal process. The orthorhombic crystal structure and dandelion-like multi-branched nanowire morphology were revealed by X-ray diffractometry (XRD) and scanning electron microscopy (SEM) respectively. High-resolution transmission electron microscopy (TEM) identified that the highly crystalline Sb2S3 nanowires grew along the [001] direction with individual wire diameter of 195 ± 52 nm. The band gap of the Sb2S3 nanowires was measured to be ca. 1.67 eV. A combination of PEG-templated assembly and crystal splitting mechanism was likely responsible for the growth of the observed nanowire dandelion structures. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Antimony (III) sulfide (Stibnite, Sb2S3) is an important member of the main group metal chalcogenides compounds. Due to its excellent photoconductivity and semiconducting properties, Sb2S3 has received wide attention for its potential applications in solar energy conversions [1], thermoelectric cooling technologies and optoelectronics in the visible and IR region [2,3]. It can also be applied as a starting material for the synthesis of complex chalcogenides with 3D open-framework structure, such as antimony sulfur iodide (SbSI) [4]. Sb2S3 has an orthorhombic lattice with a chain-like structure (SbxSy)n. Since the binding between these chains is considerably weaker than those within the chains, Sb2S3 tends to form one-dimensional structures, such as nanowires, nanorods and nanoribbons under selected growth conditions [511]. Proper solvothermal conditions may be applied for promoting the nucleation and the subsequent growth of Sb2S3 crystals towards desired morphologies, for instance, shuttle-like Sb2S3 nanorods were synthesized via a polyvinylpyrrolidone-assisted solvothermal approach [12]. While Sb2S3 is reported to be semiconducting, there exist contradictions in its reported band gap energy. One study suggests that this material is a p-type semiconductor with a calculated indirect band gap of 1.35 eV [13]. Others reported that this material has a direct band gap of 1.78− 2.2 eV, as well as a dielectric constant of 10.9−14.4 [14]. In this work,
⁎ Corresponding author. Tel.: + 1 402 472 5172; fax: + 1 402 472 9402. E-mail address: [email protected] (C.L. Cheung). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.074
we report a facile synthetic approach to grow dandelion-like Sb2S3 nanowires with a measured direct band gap of 1.67 eV. 2. Experimental The dandelion-like multi-branched Sb2S3 nanowire structures were prepared by a solvothermal method. In a typical synthesis, 0.18 g of antimony (III) chloride (SbCl3) and 3 g of polyethylene glycol with molecular weight of ca. 6 kDa (PEG-6000) were dissolved in 45 ml of ethylene glycol (EG), followed by the addition of 1.7 g of thiourea ((NH2)2CS). The light orange solution was transferred to a Teflon-lined stainless steel autoclave which was then kept at 180 °C in a convection oven for 7 h. and allowed to cool to room temperature. The product was filtered, washed with de-ionized water and dried under vacuum at 80 °C for 2 h. The crystal structure of the obtained samples was characterized by X-ray diffraction (XRD) with a Rigaku D/Max-B x-ray diffractometer. The weighted average wavelength of the Cu Kα radiation is 1.5417 Å. The diffraction pattern of the sample was compared and identified with JCPDS File No. 42–1393. The morphology of the Sb2S3 nanowires was examined by a Hitachi S4700 scanning electron microscope (SEM) operated at 5 kV. The crystal structure of the product was studied with transmission electron microscopy (TEM) on a Tecnai G 2 F20 S-Twin operated at 200 kV. The Raman spectrum of the product was recorded at room temperature on a Renishaw InVia Raman microscope equipped with a 514.5 nm Ar ion laser, a monochromator and a
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CCD detector. The band gap of the product was measured using diffuse reflectance UV–visible spectroscopy with a Perkin–Elmer Lambda 950 spectrometer equipped with a 150 mm integrating sphere. The spectrum was measured relative to Spectralon using a 4 nm slit and 1 nm sampling interval. 3. Results and discussions The morphology of the as-synthesized products was revealed by SEM to be dandelion-like nanowire structures (Fig. 1). The zoomedin image shows that the individual wires are mostly terminated with tapered tips (Fig. 1a inset). These nanowires were often found organized into extended sheaf-like structures (Fig. 1b). The zoomed-in image of the interior structure of a “sheaf” illustrates the multiple-branching organization of the wires (circles in Fig. 1b inset). These wires were often found to split from the lower branches multiple times. This multiple-branching phenomenon was spotted throughout the Sb2S3 nanowire sample, suggesting it to be the basic structure building motif for the formation of the macroscopic dandelion-like morphology. The phase identity of the product was verified to be orthorhombic Sb2S3 by XRD. The XRD pattern of the Sb2S3 nanowire product (Fig. 2a) matches up well with the standard diffraction pattern of orthorhombic Sb2S3 (JCPDS ID 042–1393). All diffraction peaks can be indexed accordingly with lattice constants a = 11.24 Å, b = 11.31 Å, c = 3.84 Å. The identity of Sb2S3 nanowire was also confirmed by Raman spectroscopy. Four major Raman peaks located at 188, 251, 371 and 450 cm − 1 (Fig. 2b) correspond to the characteristic Raman shifts for bulk crystalline Sb2S3 [15]. The crystal lattice structure of the Sb2S3 nanowires was further investigated by TEM. Fig. 3 presents a typical Sb2S3 nanowire with diameter of 115 nm and length of 6.1 μm. The high crystallinity of this
Fig. 1. SEM images of (a) the dandelion-like Sb2S3 nanowires and (b) Sb2S3 nanowire “sheafs.” The zoomed-in views are shown in the insets.
Fig. 2. (a) XRD pattern and (b) Raman spectrum of the Sb2S3 nanowire dandelions.
wire was revealed by the clear lattice fringes in the high resolution TEM images at its tip, center and edge (Fig. 3b–d). The zoomed-in image of the center of the wire shows the average inter-planar distances to be 0.56 nm and 0.36 nm, which correspond to the (200) and (130) crystal plane lattice distances of orthorhombic Sb2S3 (Fig. 3e). It also implies that the wire growth direction is along the [001] direction. A selected area electron diffraction (SAED) pattern at the center of this wire further indicates the well-defined crystallinity and orthorhombic structure of this Sb2S3 nanowire (Fig. 3f). Based on a statistical analysis of 30 nanowires, the average wire diameter was determined to be ca. 195±52 nm (data not shown). Based upon the experimental results, the growth mechanism of the Sb2S3 dandelion-like structure synthesized by our PEG-assisted solvothermal method probably involved two major steps: the PEGassisted assembly [16] and a multiple crystal splitting mechanism. In this synthesis, PEG played two important roles as a soft template and a surface stabilizer. In the beginning of the synthesis, the flexible PEG chains dissolved and entangled in ethylene glycol and formed an amorphous spherical template. The Sb precursor was then dissolved into PEG/EG to form Sb-PEG globules which served as a template for the following assemblies. Upon the release of HS - or S 2- ions from thiourea, an orange colloidal solution appeared, indicating the emergence of dispersed Sb2S3 nuclei in the super-saturated solution. During the solvothermal treatment, the nuclei continued to grow into small crystals through the solid-solution-solid process (Ostwald ripening). As the reaction proceeded, fresh Sb2S3 crystals gradually grew out from surface of the Sb-PEG globules at a fast growth rate in the [001] direction, leading to the pseudo assembled Sb2S3 nanowires in a dandelion-like morphology. The use of PEG was necessary for the growth of this morphology because only random submicrowires were obtained in the absence of PEG [5]. We hypothesize that PEG stabilized the growth of Sb2S3 nanowires via strong binding with the SH group dissociated from thiourea (SH-PEG-SH). Similar to Bi2S3[17], Sb2S3 exhibits a chain-like crystal structure and tends to perform fast crystal splitting during the crystal growth. In this synthesis, likely due to the strong bonding between
224
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Fig. 3. TEM images of a typical Sb2S3 nanowire. (a) Low-magnification TEM image of one Sb2S3 wire; high magnification image of (b) the tip, (c) the center and (d) the edge of the Sb2S3 wire; (e) the zoomed-in view of (c) and (f) the selected area diffraction pattern of the center of the studied Sb2S3 nanowire.
PEG and sulfur atoms on the low energy Sb2S3 planes (e.g. (100) planes), the tightly bound PEG molecules probably inserted themselves between these Sb2S3 crystal planes in the Sb2S3 lattice during the growth process as “molecular wedge” and initiated the crystal splitting. Subsequent wire growth continued until the next crystal splitting started by the surface insertion of PEG molecules. Our observed multi-branching results indicated that the repeated crystal splitting motif was the major driving force for the latitudinal growth of Sb2S3 nanowires into the dandelion 3-D structures rather than other reported Sb2S3 double-sheaf structures [18]. The optical property of the Sb2S3 nanowire dandelions was studied by diffuse reflectance spectroscopy at room temperature (Fig. 4). The Tauc plot (inset) of Sb2S3 nanowire dandelions indicates that this material has a likely direct optical band gap of 1.67 eV. This
value is close to the reported band gap values 1.65 eV [12] and 1.66 eV [13] of Sb2S3. 4. Conclusion We report a facile synthetic route for the dandelion-like multibranched single crystalline Sb2S3 nanowires using a solvothermal process with low-cost precursors. The multi-branched morphology suggest that the growth of Sb2S3 nanowire dandelions likely resulted from both the PEG-assisted assembly process and multiple crystal splitting due to the strong interaction between PEG and low energy Sb2S3 planes. The measured value of the direct band gap of the Sb2S3 nanowires, 1.67 eV, is comparable to the previously reported literature data.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Fig. 4. Diffuse reflectance spectrum of the Sb2S3 nanowires. (R: Reflectance; F(R): Kubelka-Monk function; E: photon energy) (inset) Tauc plot of the spectrum.
Acknowledgments We thank Nebraska Center for Energy Sciences Research and DOD/ ARO (W911NF-10-2-0099) for their financial support, and Drs. David Diercks and Yongfeng Lu for their help with the microscopy work.
[17] [18]
Savadogo O, Mandal KC. Sol Energ Mat Sol Cells 1992;26:117–36. Savadogo O, Mandal KC. Electron Lett 1992;28:1682–3. Rajpure KY, Bhosale CH. Mater Chem Phys 2002;73:6–12. Parise JB. Science 1991;251:293–4. Zhu GQ, Liu P, Miao HY, Zhu JP, Bian XB, Liu Y, Chen B, Wang XB. Mater Res Bull 2008;43:2636–42. Lou WJ, Chen M, Wang XB, Liu WM. Chem Mater 2007;19:872–8. Yu Y, Wang RH, Chen Q, Peng LM. J Phys Chem B 2006;110:13415–9. Malakooti R, Cademartiri L, Migliori A, Ozin GA. J Mater Chem 2008;18:66–9. Yang J, Zeng JH, Yu SH, Yang L, Zhang YH, Qian YT. Chem Mater 2000;12:2924–9. Geng ZR, Wang MX, Yue GH, Yan PX. J Cryst Growth 2008;310:341–4. Hu HM, Mo MS, Yan BJ, Zhang XJ, Li QW, Yu WC, Qian YT. J Cryst Growth 2003;258:106–12. Zhang L, Chen L, Wan HQ, Zhou HD, Chen JM. Cryst Res Technol 2010;45:178–82. Sun M, Li DZ, Li WJ, Chen YB, Chen ZX, He YH, Fu XZ. J Phys Chem C 2008;112: 18076–81. Juarez BH, Ibisate M, Palacios JM, Lopez C. Adv Mater 2003;15:319–23. Juarez BH, Rubio S, Sanchez-Dehesa J, Lopez C. Adv Mater 2002;14:1486–90. Zhou XF, Zhao X, Zhang DY, Chen SY, Guo XF, Ding WP, Chen Y. Nanotechnology 2006;17:3806–11. Tang J, Alivisatos AP. Nano Lett 2006;6:2701–6. Pilapong C, Thongtem T, Thongtem S. J Alloy Compd 2010;507:L38–42.