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Evolution of nickel sulfide hollow spheres through topotactic transformation† Chengzhen Wei,ab Qingyi Lu,*a Jing Sunab and Feng Gao*b In this study, a topotactic transformation route was proposed to synthesize single-crystalline b-NiS hollow spheres with uniform phase and morphology evolving from polycrystalline a-NiS hollow spheres. Uniform polycrystalline a-NiS hollow spheres were firstly prepared with thiourea and glutathione as sulfur sources under hydrothermal conditions through the Kirkendall effect. By increasing the reaction temperature the
Received 1st July 2013 Accepted 5th September 2013
polycrystalline a-NiS hollow spheres were transformed to uniform b-NiS hollow spheres. The b-NiS crystals obtained through the topotactic transformation route not only have unchanged morphology of hollow spheres but are also single-crystalline in nature. The as-prepared NiS hollow spheres display a good
DOI: 10.1039/c3nr03371f
ability to remove the organic pollutant Congo red from water, which makes them have application
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potential in water treatment.
Introduction Many ceramics and minerals display complicated phase diagrams with different stoichiometries and phase structures, which poses the question as to how to tune their crystal phase and morphology in solution syntheses.1–4 As is known, the crystal phase of nanocrystals is sensitive to the synthetic conditions. Synthetic control of the shape and crystal phase of nanocrystals is considered from an industrial perspective to be a powerful way for regulating material properties to suit practical applications.5–8 Nickel sulde is a chemically simple binary compound that has diversied phase diagrams with many thermodynamically stable crystal structures and stoichiometries including a-Ni3+xS2, b-Ni3S2, Ni7S6, Ni9S8, a-NiS, b-NiS, Ni3S4 and NiS2, which makes it an attractive and somewhat complicated model to investigate nanocrystal shape and phase polymorphism.1,9 Since its rst full investigation by Kullerud and Yund in 1962,10 this relatively complex nickel sulde system has been intensively studied by many research groups through different methods.11–16 However, a survey of all the reported literature reveals that different phases and morphologies of nickel suldes sometimes co-exist; therefore, obtaining a pure phase of nickel suldes with a uniform morphology still remains a big challenge.9 NiS with the simplest stoichiometry exists in two phases: the high-temperature phase a-NiS a
State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China. E-mail: qylu@nju. edu.cn
b
Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: XRD patterns; SEM images and TEM images. See DOI: 10.1039/c3nr03371f
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(hexagonal) and the low-temperature phase b-NiS (rhombohedral).1 The a-b phase transformation takes place between 282 and 379 C. Both hexagonal and rhombohedral NiS exhibit attractive electrical and catalytic properties and can be used in many elds such as in IR detectors, as cathode materials in rechargeable lithium batteries and as hydride sulfurization catalysts.16,17 The ability to synthesize NiS nanocrystals with controlled shape and phase would enable a variety of sizedependent physical properties to be explored. Hollow micro- and nano-sized structures have attracted increasing attention because of their wide promising applications in sensors, catalysis, drug carrier systems and lithium-ion batteries.18–21 Some progress has been made and several effective methods have been developed for the synthesis of hollow structured materials, such as the so and hard template route, Ostwald ripening, the Kirkendall effect, and chemical etching.22–27 However, although some hollow polyhedrons which were prepared by etching single-crystalline polyhedrons through a top-down method are single-crystalline, almost all the reported hollow spheres are polycrystalline and few studies have been reported on the synthesis of single-crystalline hollow spheres.28–31 Herein, a topotactic transformation route was proposed to synthesize single-crystalline b-NiS hollow spheres with uniform phase and morphology. Topotactic transformation was developed on the basis of the topological chemical method for the synthesis of crystals with special morphologies and applications.32 Through reaction designs, crystals of certain shapes are obtained rst and then under specic conditions the crystals would transform to other crystals without changing the spatial morphology but changing the crystal structures.33–37 This topological transformation process can not only increase the predictability of the produced materials' space structures but also make the assembly more
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Paper diversied to construct special structures. In this study, uniform polycrystalline a-NiS hollow spheres were prepared under hydrothermal conditions with thiourea and glutathione as sulfur sources through the Kirkendall effect. Then the obtained polycrystalline a-NiS hollow spheres were transformed to uniform b-NiS hollow spheres by increasing the reaction temperature. The b-NiS crystals obtained through the topotactic transformation route not only have unchanged morphology of hollow spheres but are also single-crystalline in nature. The as-prepared NiS hollow spheres display a good ability to remove the organic pollutant Congo red from water, which makes them have application potential in water treatment.
Nanoscale absorbance measurements spectrophotometer.
using
the
Hitachi
U-3900
Results and discussion The a-NiS hollow spheres were prepared by hydrothermally treating the mixture solution of nickel acetate, thiourea and glutathione at 160 C for 16 h. Fig. 1 shows the product's XRD pattern consistent with the standard parameters of JCPDS card 02-1280, demonstrating the formation of a-NiS crystals. The morphology and phase structure of the as-prepared sample are shown in Fig. 2. From the SEM images in Fig. 2a and b, it can be
Experimental Materials synthesis For the synthesis of polycrystalline a-NiS hollow spheres, 0.16 g of nickel(II) acetate tetrahydrate and 0.05 g of thiourea were dissolved in 10 mL of water under magnetic stirring. Then 0.05 g of glutathione was added to the mixture solution and further stirred for 10 min. Aer that the mixture solution was transferred into a Teon lined stainless steel autoclave, sealed and heated at 160 C for 12 h in an electric oven. The system was naturally cooled to room temperature. The nal products were collected and washed with distilled water and absolute alcohol several times, followed by drying in an oven at 60 C for further characterization. For the synthesis of single-crystalline b-NiS hollow spheres, two routes have been applied. One is that right aer the mixture solution was hydrothermally treated at 160 C for 12 h, the temperature was raised to 200 C and the solution was further treated for 16 more hours under hydrothermal conditions. The other one is that the mixture solution was directly treated at 200 C for 12 h under hydrothermal conditions.
Fig. 1
XRD pattern of the sample prepared at 160 C for 12 h.
Materials characterization The crystallographic information of the samples was characterized by using X-ray diffraction (XRD) on a Shimadzu XRD6000 powder X-ray diffractometer with Cu Ka radiation (l ¼ ˚ Morphologies of the samples were investigated on a 1.5418 A). Hitachi S-4800 eld-emission scanning electron microscope (FE-SEM) at an acceleration voltage of 10.0 kV. The morphologies and structures of the samples were further investigated by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and the corresponding selected area electron diffraction (SAED), operated on a JEOL JEM-2100 with an acceleration voltage of 200 kV. Removal of Congo red 75 mg of the sample (a-NiS or b-NiS hollow spheres) was mixed with 150 mL of aqueous solution of Congo red with a concentration of 100 mg L1 in a beaker (capacity ca. 250 mL). The suspension was stirred at room temperature. At given time intervals, a series of aqueous solution were taken out and separated through centrifugation (10 000 rpm min1) for
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Fig. 2 (a and b) SEM images; (c–e) TEM images, SAED pattern (inset in Fig. 1e) and (f) HRTEM image of the sample prepared at 160 C for 12 h.
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Nanoscale clearly seen that the sample is composed of a large number of spheres. Almost all the spheres have an aperture, which indicates that the as-prepared spheres are hollow inside. The average diameter of these hollow spheres is about 2.5 mm. TEM analyses were further performed to get more information about the structure. A typical TEM image of the sample is shown in Fig. 2c, which conrms that the sample consists of hollow spheres with an aperture. It can also be observed that the hollow a-NiS spheres are composed of nanosized particles as revealed by the TEM image with a relatively large magnication shown in Fig. 2e. The corresponding SAED pattern (inset in Fig. 2e) shows intense diffraction rings, which can be indexed to (100), (101), and (102) planes of a-NiS. An HRTEM image (Fig. 2f) displays several sets of lattice fringes with a spacing of 0.26 nm, corresponding to the (101) planes of a-NiS crystals. Both the SAED pattern and the HRTEM image conrm that the obtained a-NiS hollow spheres are polycrystalline. To understand the formation process of a-NiS hollow spheres, samples prepared under hydrothermal conditions for different reaction times were collected and investigated by SEM,
Fig. 3
Paper TEM and XRD. At the early stage of reaction (0.5 h), solid spheres are obtained as shown by SEM and TEM images in Fig. 3a and b. The corresponding XRD pattern demonstrates that they are mainly amorphous or poorly crystallized (Fig. S1a, ESI†). Based on the EDS and EDX characterizations (Fig. S2†), it can be found that besides Ni and S, the solid spheres also contain a great number of C and O, which suggests that the solid spheres might be a complex compound of Ni and organic compounds. With the increase of the reaction time to 2 h, some broad diffraction peaks appear in the XRD pattern (Fig. S1b†), which can be identied as a-NiS. SEM and TEM investigations demonstrate that the obtained product is still composed of spheres but with a core–shell structure (Fig. 3c and d). On further increasing the reaction time to 4 or 8 h, the obtained a-NiS samples have core–shell structures with the cores becoming smaller and smaller (Fig. 3e–h). Based on the above morphology evolution in the time-dependent experiments, it is believed that the Kirkendall effect would explain well the formation of a-NiS hollow spheres. At an early stage of the reaction, the amorphous solid spheres form. Since the
SEM and TEM images of the samples prepared at 160 C for different reaction times: (a and b) 0.5 h; (c and d) 2 h; (e and f) 4 h; and (g and h) 8 h.
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amorphous solid spheres might be unstable at high temperature, under the hydrothermal conditions they would be transformed to a-NiS crystals. This process might begin on the surface of amorphous solid spheres to form a core–shell structure with amorphous cores. As the reaction continues, more and more amorphous cores would move to the surface and be transformed to a-NiS and nally, aer prolonging the reaction time to 12 h, the amorphous cores are consumed completely and perfect NiS hollow spheres are formed. Nickel sulde has diversied phase diagrams with many thermodynamically stable crystal structures and stoichiometries and different nickel suldes would mutually transform under certain conditions. In this study, it is found that under hydrothermal conditions a-NiS can be transformed to b-NiS by increasing the hydrothermal temperature from 160 C to 200 C although a-NiS (hexagonal) is the high-temperature phase and b-NiS (rhombohedral) is the low-temperature phase. Aer a-NiS hollow spheres form at 160 C, by directly increasing the reaction temperature to 200 C and further treating under hydrothermal conditions for 16 h, the transformation from a-NiS to b-NiS occurs and pure b-NiS crystals can be nally obtained, which can be conrmed by XRD patterns in Fig. 4. Fig. S3† and Fig. 5 display SEM and TEM images of the samples prepared by further treating the solution at 200 C for 12 h and 16 h aer a-NiS hollow spheres form, which reveal that during the transformation process, the hollow spherical morphology is maintained and the obtained b-NiS crystals are also hollow spheres with an aperture. However, SAED and HRTEM characterizations indicate that unlike the polycrystalline a-NiS spheres, these b-NiS spheres are single-crystalline in nature. The SAED pattern shown in Fig. 5f displays only a set of diffraction spots. The two diffraction spots with an angle of about 112 can be indexed to 01) and (410) of rhombohedral b-NiS, demonstrating the (1 single crystalline nature of the hollow sphere. The SAED pattern also indicates that there are many defects in the crystal which might be due to the unevenness of the spherical surface. An HRTEM image of the hollow sphere is presented in Fig. 5g, only Fig. 5 (a and b) SEM images; (c–e) TEM images, (f) SAED pattern and (g) HRTEM image of the sample prepared at 160 C for 12 h and then at 200 C for 16 h.
Fig. 4 XRD patterns of the samples prepared at 160 C for 12 h and then at 200 C for (a) 12 h and (b) 16 h.
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displaying a set of lattice fringes. The spacing between two adjacent lattice planes is about 0.29 nm, which is consistent 01) lattice plane of b-NiS. It is impossible to get the with the (1 HRTEM image of the whole hollow sphere. In order to further conrm the single crystallinity of the hollow spheres, HRTEM images from different parts of the hollow sphere are presented in Fig. S4.† In each of the HRTEM images, only one set of lattice fringes can be observed, which is totally different from the HRTEM image of the shell of the a-NiS hollow sphere, indirectly conrming the single crystallinity of the hollow b-NiS sphere. The topotactic transformation process is designed to transform crystals with a certain shape to other crystals without changing the spatial morphology but the crystal structures. It can not only increase the predictability of the produced materials' space structures but also make it possible to form special structures.
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Nanoscale In this case, the transformation from a-NiS to b-NiS is conned in the shell of the hollow spheres, resulting in maintenance of the hollow structure. During the process, the close matching of crystal lattices between the two phases of NiS might be responsible for the transformation from polycrystalline a-NiS to single-crystalline b-NiS spheres. A schematic illustration of the crystal lattice matching between hexagonal a-NiS and rhombohedral b-NiS is shown in Fig. S5,† from which it can be seen that (101) lattice planes of a-NiS and (021) lattice planes of b-NiS have similar d-spacings and Ni ions have similar spatial organizations. The close matching indicates the minimal reorganization of a-NiS in the transformation process. With the increase of the hydrothermal temperature, Ni and S atoms begin to reorganize and the small a-NiS nanoparticles in the shell of a-NiS hollow spheres transform and grow together to form single-crystalline b-NiS hollow spheres. The topotactic transformation process from polycrystalline a-NiS to single-crystalline b-NiS can occur more easily when the mixture of solution of nickel acetate, thiourea and glutathione is treated at 200 C under hydrothermal conditions. Fig. S6† displays XRD patterns of the samples prepared at 200 C for different reaction times. At the early state (0.5 h), the obtained product was amorphous. With increasing reaction time, a-NiS crystals formed and then they transformed to b-NiS. Aer treating the mixture solution at 200 C for 12 h, pure b-NiS was obtained. Fig. S7† shows its XRD pattern, in which all the diffraction peaks can be indexed to rhombohedral b-NiS (JCPDS card no. 86-2281) and no impurities are detected, indicating the high purity of the as-synthesized b-NiS and complete transformation from a-NiS. Fig. 6 and 7 show TEM and SEM images of the samples prepared at 200 C for different reaction times. It can be deduced that the formation process at 200 C is similar to that at 160 C except for instant transformation from a-NiS to b-NiS. The amorphous solid microspheres rstly form at the initial stage (Fig. 6a and b). As the hydrothermal reaction proceeds, a-NiS crystallizes on the surface of amorphous solid microspheres to form core–shell structures (Fig. 6c and d). The freshly formed a-NiS crystals can very easily transform to b-NiS at 200 C and a-NiS and b-NiS are co-existent in the core–shell structures (Fig. 6e–j). On further increasing the reaction time, the amorphous core completely transforms to a-NiS and then to b-NiS to form a perfect hollow structure. The whole process only needs 12 h at 200 C under hydrothermal conditions. From SEM images in Fig. 7a and b, the present b-NiS sample is composed of a large number of hollow spheres with a diameter of about 2.5 mm. TEM images shown in Fig. 7c and d conrm the hollow nature of the spheres by displaying contrast difference between the pale center and the dark edge. Similar to the sample presented in Fig. 5, the SAED pattern taken from a hollow sphere (inset in Fig. 7d) also shows only a set of diffraction spots, demonstrating the singlecrystalline nature of the b-NiS hollow spheres. It is worth noting that the addition of glutathione plays an important role in the preparation of the hollow a-NiS and b-NiS spheres. In the reaction system, it can not only supply the sulfur source but also control the morphology and phase composition of the as-prepared samples. Without the addition of
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Fig. 6 TEM images of the samples prepared at 200 C for different reaction times: (a and b) 0.5 h; (c and d) 1 h; (e and f) 2 h; (g and h) 4 h and (i and j) 8 h.
glutathione, only mixed phases of b-NiS and Ni7S6 irregular precipitates can be obtained at 200 C, indicating that the sulfur source is not enough for obtaining pure b-NiS. By increasing the amount of thiourea to substitute for glutathione, the pure phase b-NiS can be obtained. However, the morphology is irregular instead of hollow spherical as displayed by the SEM image in Fig. S8a.† Similar results can be obtained when the reaction is carried out at 160 C and only pure a-NiS irregular crystals (Fig. S8b†) can be synthesized without the addition of glutathione. The transformation process from a-NiS irregular crystals to b-NiS can also be observed by directly increasing the hydrothermal temperature from 160 C to 200 C as XRD patterns in Fig. S9† conrm. On the other hand, glutathione can also act as the only sulfur source for the synthesis of NiS. When 0.05 g of glutathione was used as the only sulfur source under hydrothermal conditions at 160 C for 12 h, a-NiS can be
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Fig. 7 (a and b) SEM images; (c–d) TEM images and SAED pattern (inset in Fig. 4d) of the sample prepared at 200 C for 12 h.
obtained (XRD pattern shown in Fig. S10a†). The SEM image shown in Fig. S10b† conrms that besides some hollow spheres there are also many irregular nanoparticles. When the reaction was carried out under hydrothermal conditions at 200 C for 12 h with glutathione as the only sulfur source, Ni9S8 hollow spheres and irregular nanoparticles were obtained as shown in Fig. S10c and d.† These results reveal that glutathione has important effects on the formation of hollow a-NiS and b-NiS spheres. The as-prepared NiS samples were used to investigate their possible applications in water purication treatment. In our study, Congo red, a commonly used dye in the textile industry, was chosen as a typical organic water pollutant. Fig. 8a and b display the absorption spectra of an aqueous solution of 100 mg L1 Congo red in the presence of 75 mg of the as-synthesized hollow a-NiS and b-NiS spheres. It can be found that both the a-NiS (SEM images shown in Fig. 2) and b-NiS (SEM images shown in Fig. 5) hollow spheres exhibit good adsorption ability for Congo red dye. Fig. 8c depicts curves of the adsorption extent of Congo red as a function of contact time for four different NiS samples. It reveals that hollow a-NiS and b-NiS spheres show better adsorption ability than irregular aNiS and b-NiS crystals. IR characterizations of Congo red and the hollow spheres before and aer adsorption were carried out to conrm the adsorption of Congo red on the spheres. From the IR spectra shown in Fig. S11,† it can be seen that aer the experiment, the IR spectrum of hollow NiS spheres shows the characteristic peaks of Congo red, suggesting the adsorption of the organic pollutant on the surface of NiS hollow spheres. The hollow a-NiS spheres, b-NiS spheres, and irregular a-NiS and b-NiS crystals have specic surface areas of 15.61 m2 g1, 14.81 m2 g1, 13.77 m2 g1 and 11.97 m2 g1, respectively. Therefore, the difference in the removal ability of hollow NiS and irregular NiS might result from their different morphologies and specic surface areas. These results obviously show that the a-NiS and b-NiS hollow spheres can signicantly remove Congo red, which might have application potential in wastewater treatment.
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Fig. 8 (a) Absorption spectra of an aqueous solution of 100 mg L1 Congo red in the presence of 75 mg of hollow b-NiS spheres; (b) absorption spectra of an aqueous solution of 100 mg L1 Congo red in the presence of 75 mg of hollow aNiS spheres; (c) curves of the adsorption extent of Congo red as a function of contact time for different NiS samples.
Conclusions In summary, uniform single-crystalline b-NiS hollow spheres were prepared through a topotactic transformation route from uniform polycrystalline a-NiS hollow spheres, which were prepared under hydrothermal conditions with thiourea and
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Nanoscale glutathione as sulfur sources through the Kirkendall effect. The b-NiS crystals obtained through the topotactic transformation route not only have unchanged morphology of hollow spheres but are also single-crystalline in nature. The close matching of crystal lattices between the two phases of NiS was believed to be responsible for the topotactic transformation from polycrystalline a-NiS hollow spheres to single-crystalline b-NiS hollow spheres. The as-prepared NiS hollow spheres display a good adsorption ability to remove the organic pollutant Congo red from water, which makes them have application potentials in water treatment.
Acknowledgements This work was supported by the National Basic Research Program of China (Grant nos 2013CB922102 and 2011CB935800) and the National Natural Science Foundation of China (Grant nos 21071076, 51172106 and 21021062).
References 1 A. Ghezelbash, M. B. Sigman Jr and B. A. Korgel, Nano Lett., 2004, 4, 537. 2 V. F. Puntes, D. Zanchet, C. K. Erdonmez and A. P. Alivisatos, J. Am. Chem. Soc., 2002, 124, 12874. 3 D. P. Dinega and M. G. Bawendi, Angew. Chem., Int. Ed., 1999, 38, 1788. 4 A. Ghezelbash and B. A. Korgel, Langmuir, 2005, 21, 9451. 5 H. Jiang, T. Zhao, J. Ma, C. Y. Yan and C. Z. Li, Chem. Commun., 2011, 47, 1246. 6 F. Jiao and P. G. Bruce, Adv. Mater., 2007, 19, 657. 7 B. C. Jang, M. Y. Park, O. B. Chae, S. J. Park, Y. J. Kim, S. M. Oh, Y. Z. Piao and T. H. Hyeon, J. Am. Chem. Soc., 2012, 134, 15010. 8 R. W. Y. Man, A. R. C. Brown and M. O. Wolf, Angew. Chem., Int. Ed., 2012, 51, 11350. 9 S. L. Yang, H. B. Yao, M. R. Gao and S. H. Yu, CrystEngComm, 2009, 11, 1383. 10 G. Kullerud and R. A. Yund, J. Petrol., 1962, 3, 126. 11 S. H. Yu and M. Yoshimura, Adv. Funct. Mater., 2002, 12, 277. 12 Y. Hu, J. F. Chen, W. M. Chen and X. L. Li, Adv. Funct. Mater., 2004, 14, 383. 13 L. Z. Zhang, J. C. Yu, M. S. Mo, L. Wu, Q. Li and K. W. Kwong, J. Am. Chem. Soc., 2004, 126, 8116. 14 Y. Hu, J. F. Chen, W. M. Chen, X. H. Lin and X. L. Li, Adv. Mater., 2003, 15, 726. 15 Y. Hu, J. F. Chen, W. M. Chen and X. L. Li, Adv. Funct. Mater., 2004, 14, 383.
12230 | Nanoscale, 2013, 5, 12224–12230
Paper 16 F. Cao, R. X. Liu, L. Zhou, S. Y. Song, Y. Q. Lei, W. D. Shi, F. Y. Zhao and H. J. Zhang, J. Mater. Chem., 2010, 20, 1078. 17 Y. Wang, Q. S. Zhu, L. Tao and X. W. Su, J. Mater. Chem., 2011, 21, 9248. 18 X. Y. Lai, J. Li, B. A. Korgel, Z. H. Dong, Z. M. Li, F. B. Su, J. Du and D. Wang, Angew. Chem., Int. Ed., 2011, 50, 2738. 19 Y. M. Sui, W. Y. Fu, Y. Zeng, H. B. Yang, Y. Y. Zhang, H. Chen and Y. X. Li, Angew. Chem., Int. Ed., 2010, 49, 4282. 20 Z. Y. Jiang, Z. X. Xie, X. H. Zhang, S. C. Lin, T. Xu, S. Y. Xie, R. B. Huang and L. S. Zheng, Adv. Mater., 2004, 16, 904. 21 B. Wang, J. S. Chen, H. B. Wu, Z. Y. Wang and X. W. Lou, J. Am. Chem. Soc., 2011, 133, 17146. 22 J. Liu and D. F. Xue, Adv. Mater., 2008, 20, 2622. 23 Y. D. Yin, C. Erdonmez, A. Cabot, S. Hughes and A. P. Alivisatos, Adv. Funct. Mater., 2006, 16, 1389. 24 M. M. Titirici, M. Antonietti and A. Thomas, Chem. Mater., 2006, 18, 3808. 25 X. Wang, X. L. Wu, Y. G. Guo, Y. T. Zhong, X. Q. Cao, Y. Ma and J. N. Yao, Adv. Funct. Mater., 2010, 20, 1680. 26 H. L. Xu and W. Z. Wang, Angew. Chem., Int. Ed., 2007, 46, 1489. 27 C. H. Kuo and M. H. Huang, J. Am. Chem. Soc., 2008, 130, 12815. 28 Y. M. Sui, W. Y. Fu, Y. Zeng, H. B. Yang, Y. Y. Zhang, H. Chen, Y. X. Li, M. H. Li and G. T. Zou, Angew. Chem., Int. Ed., 2010, 49, 4282. 29 M. Hu, S. Furukawa, R. Ohtani, H. Sukegawa, Y. Nemoto, J. Reboul, S. Kitagawa and Y. Yamauchi, Angew. Chem., Int. Ed., 2012, 51, 984. 30 S. H. Jiao, L. F. Xu, K. Jiang and D. S. Xu, Adv. Mater., 2006, 18, 1174. 31 L. Zhang, L. Zhou, H. B. Wu, R. Xu and X. W. Lou, Angew. Chem., Int. Ed., 2012, 51, 7270. 32 M. D. Cohen, G. M. J. Schmidt and F. I. Sonntag, J. Chem. Soc., 1964, 2000. 33 R. E. Schaak and T. E. Mallouk, J. Am. Chem. Soc., 2000, 122, 2798. 34 L. S. Li, N. J. Sun, Y. Y. Huang, Y. Qin, N. N. Zhao, J. N. Gao, M. X. Li, H. H. Zhou and L. M. Qi, Adv. Funct. Mater., 2008, 18, 1194. 35 M. T. Buscaglia, C. Harnagea, M. Dapiaggi, V. Buscaglia, A. Pignolet and P. Nanni, Chem. Mater., 2009, 21, 5058. 36 B. Gates, B. Mayers, Y. Y. Wu, Y. G. Sun, B. Cattle, P. D. Yang and Y. N. Xia, Adv. Funct. Mater., 2002, 12, 679. 37 D. H. Son, S. M. Hughes, Y. D. Yin and A. P. Alivisatos, Science, 2004, 306, 1009.
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Evolution of nickel sulfide hollow spheres through topotactic transformation† Chengzhen Wei,ab Qingyi Lu,*a Jing Sunab and Feng Gao*b In this study, a topotactic transformation route was proposed to synthesize single-crystalline b-NiS hollow spheres with uniform phase and morphology evolving from polycrystalline a-NiS hollow spheres. Uniform polycrystalline a-NiS hollow spheres were firstly prepared with thiourea and glutathione as sulfur sources under hydrothermal conditions through the Kirkendall effect. By increasing the reaction temperature the
Received 1st July 2013 Accepted 5th September 2013
polycrystalline a-NiS hollow spheres were transformed to uniform b-NiS hollow spheres. The b-NiS crystals obtained through the topotactic transformation route not only have unchanged morphology of hollow spheres but are also single-crystalline in nature. The as-prepared NiS hollow spheres display a good
DOI: 10.1039/c3nr03371f
ability to remove the organic pollutant Congo red from water, which makes them have application
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potential in water treatment.
Introduction Many ceramics and minerals display complicated phase diagrams with different stoichiometries and phase structures, which poses the question as to how to tune their crystal phase and morphology in solution syntheses.1–4 As is known, the crystal phase of nanocrystals is sensitive to the synthetic conditions. Synthetic control of the shape and crystal phase of nanocrystals is considered from an industrial perspective to be a powerful way for regulating material properties to suit practical applications.5–8 Nickel sulde is a chemically simple binary compound that has diversied phase diagrams with many thermodynamically stable crystal structures and stoichiometries including a-Ni3+xS2, b-Ni3S2, Ni7S6, Ni9S8, a-NiS, b-NiS, Ni3S4 and NiS2, which makes it an attractive and somewhat complicated model to investigate nanocrystal shape and phase polymorphism.1,9 Since its rst full investigation by Kullerud and Yund in 1962,10 this relatively complex nickel sulde system has been intensively studied by many research groups through different methods.11–16 However, a survey of all the reported literature reveals that different phases and morphologies of nickel suldes sometimes co-exist; therefore, obtaining a pure phase of nickel suldes with a uniform morphology still remains a big challenge.9 NiS with the simplest stoichiometry exists in two phases: the high-temperature phase a-NiS a
State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China. E-mail: qylu@nju. edu.cn
b
Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: XRD patterns; SEM images and TEM images. See DOI: 10.1039/c3nr03371f
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(hexagonal) and the low-temperature phase b-NiS (rhombohedral).1 The a-b phase transformation takes place between 282 and 379 C. Both hexagonal and rhombohedral NiS exhibit attractive electrical and catalytic properties and can be used in many elds such as in IR detectors, as cathode materials in rechargeable lithium batteries and as hydride sulfurization catalysts.16,17 The ability to synthesize NiS nanocrystals with controlled shape and phase would enable a variety of sizedependent physical properties to be explored. Hollow micro- and nano-sized structures have attracted increasing attention because of their wide promising applications in sensors, catalysis, drug carrier systems and lithium-ion batteries.18–21 Some progress has been made and several effective methods have been developed for the synthesis of hollow structured materials, such as the so and hard template route, Ostwald ripening, the Kirkendall effect, and chemical etching.22–27 However, although some hollow polyhedrons which were prepared by etching single-crystalline polyhedrons through a top-down method are single-crystalline, almost all the reported hollow spheres are polycrystalline and few studies have been reported on the synthesis of single-crystalline hollow spheres.28–31 Herein, a topotactic transformation route was proposed to synthesize single-crystalline b-NiS hollow spheres with uniform phase and morphology. Topotactic transformation was developed on the basis of the topological chemical method for the synthesis of crystals with special morphologies and applications.32 Through reaction designs, crystals of certain shapes are obtained rst and then under specic conditions the crystals would transform to other crystals without changing the spatial morphology but changing the crystal structures.33–37 This topological transformation process can not only increase the predictability of the produced materials' space structures but also make the assembly more
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Paper diversied to construct special structures. In this study, uniform polycrystalline a-NiS hollow spheres were prepared under hydrothermal conditions with thiourea and glutathione as sulfur sources through the Kirkendall effect. Then the obtained polycrystalline a-NiS hollow spheres were transformed to uniform b-NiS hollow spheres by increasing the reaction temperature. The b-NiS crystals obtained through the topotactic transformation route not only have unchanged morphology of hollow spheres but are also single-crystalline in nature. The as-prepared NiS hollow spheres display a good ability to remove the organic pollutant Congo red from water, which makes them have application potential in water treatment.
Nanoscale absorbance measurements spectrophotometer.
using
the
Hitachi
U-3900
Results and discussion The a-NiS hollow spheres were prepared by hydrothermally treating the mixture solution of nickel acetate, thiourea and glutathione at 160 C for 16 h. Fig. 1 shows the product's XRD pattern consistent with the standard parameters of JCPDS card 02-1280, demonstrating the formation of a-NiS crystals. The morphology and phase structure of the as-prepared sample are shown in Fig. 2. From the SEM images in Fig. 2a and b, it can be
Experimental Materials synthesis For the synthesis of polycrystalline a-NiS hollow spheres, 0.16 g of nickel(II) acetate tetrahydrate and 0.05 g of thiourea were dissolved in 10 mL of water under magnetic stirring. Then 0.05 g of glutathione was added to the mixture solution and further stirred for 10 min. Aer that the mixture solution was transferred into a Teon lined stainless steel autoclave, sealed and heated at 160 C for 12 h in an electric oven. The system was naturally cooled to room temperature. The nal products were collected and washed with distilled water and absolute alcohol several times, followed by drying in an oven at 60 C for further characterization. For the synthesis of single-crystalline b-NiS hollow spheres, two routes have been applied. One is that right aer the mixture solution was hydrothermally treated at 160 C for 12 h, the temperature was raised to 200 C and the solution was further treated for 16 more hours under hydrothermal conditions. The other one is that the mixture solution was directly treated at 200 C for 12 h under hydrothermal conditions.
Fig. 1
XRD pattern of the sample prepared at 160 C for 12 h.
Materials characterization The crystallographic information of the samples was characterized by using X-ray diffraction (XRD) on a Shimadzu XRD6000 powder X-ray diffractometer with Cu Ka radiation (l ¼ ˚ Morphologies of the samples were investigated on a 1.5418 A). Hitachi S-4800 eld-emission scanning electron microscope (FE-SEM) at an acceleration voltage of 10.0 kV. The morphologies and structures of the samples were further investigated by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and the corresponding selected area electron diffraction (SAED), operated on a JEOL JEM-2100 with an acceleration voltage of 200 kV. Removal of Congo red 75 mg of the sample (a-NiS or b-NiS hollow spheres) was mixed with 150 mL of aqueous solution of Congo red with a concentration of 100 mg L1 in a beaker (capacity ca. 250 mL). The suspension was stirred at room temperature. At given time intervals, a series of aqueous solution were taken out and separated through centrifugation (10 000 rpm min1) for
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Fig. 2 (a and b) SEM images; (c–e) TEM images, SAED pattern (inset in Fig. 1e) and (f) HRTEM image of the sample prepared at 160 C for 12 h.
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Nanoscale clearly seen that the sample is composed of a large number of spheres. Almost all the spheres have an aperture, which indicates that the as-prepared spheres are hollow inside. The average diameter of these hollow spheres is about 2.5 mm. TEM analyses were further performed to get more information about the structure. A typical TEM image of the sample is shown in Fig. 2c, which conrms that the sample consists of hollow spheres with an aperture. It can also be observed that the hollow a-NiS spheres are composed of nanosized particles as revealed by the TEM image with a relatively large magnication shown in Fig. 2e. The corresponding SAED pattern (inset in Fig. 2e) shows intense diffraction rings, which can be indexed to (100), (101), and (102) planes of a-NiS. An HRTEM image (Fig. 2f) displays several sets of lattice fringes with a spacing of 0.26 nm, corresponding to the (101) planes of a-NiS crystals. Both the SAED pattern and the HRTEM image conrm that the obtained a-NiS hollow spheres are polycrystalline. To understand the formation process of a-NiS hollow spheres, samples prepared under hydrothermal conditions for different reaction times were collected and investigated by SEM,
Fig. 3
Paper TEM and XRD. At the early stage of reaction (0.5 h), solid spheres are obtained as shown by SEM and TEM images in Fig. 3a and b. The corresponding XRD pattern demonstrates that they are mainly amorphous or poorly crystallized (Fig. S1a, ESI†). Based on the EDS and EDX characterizations (Fig. S2†), it can be found that besides Ni and S, the solid spheres also contain a great number of C and O, which suggests that the solid spheres might be a complex compound of Ni and organic compounds. With the increase of the reaction time to 2 h, some broad diffraction peaks appear in the XRD pattern (Fig. S1b†), which can be identied as a-NiS. SEM and TEM investigations demonstrate that the obtained product is still composed of spheres but with a core–shell structure (Fig. 3c and d). On further increasing the reaction time to 4 or 8 h, the obtained a-NiS samples have core–shell structures with the cores becoming smaller and smaller (Fig. 3e–h). Based on the above morphology evolution in the time-dependent experiments, it is believed that the Kirkendall effect would explain well the formation of a-NiS hollow spheres. At an early stage of the reaction, the amorphous solid spheres form. Since the
SEM and TEM images of the samples prepared at 160 C for different reaction times: (a and b) 0.5 h; (c and d) 2 h; (e and f) 4 h; and (g and h) 8 h.
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amorphous solid spheres might be unstable at high temperature, under the hydrothermal conditions they would be transformed to a-NiS crystals. This process might begin on the surface of amorphous solid spheres to form a core–shell structure with amorphous cores. As the reaction continues, more and more amorphous cores would move to the surface and be transformed to a-NiS and nally, aer prolonging the reaction time to 12 h, the amorphous cores are consumed completely and perfect NiS hollow spheres are formed. Nickel sulde has diversied phase diagrams with many thermodynamically stable crystal structures and stoichiometries and different nickel suldes would mutually transform under certain conditions. In this study, it is found that under hydrothermal conditions a-NiS can be transformed to b-NiS by increasing the hydrothermal temperature from 160 C to 200 C although a-NiS (hexagonal) is the high-temperature phase and b-NiS (rhombohedral) is the low-temperature phase. Aer a-NiS hollow spheres form at 160 C, by directly increasing the reaction temperature to 200 C and further treating under hydrothermal conditions for 16 h, the transformation from a-NiS to b-NiS occurs and pure b-NiS crystals can be nally obtained, which can be conrmed by XRD patterns in Fig. 4. Fig. S3† and Fig. 5 display SEM and TEM images of the samples prepared by further treating the solution at 200 C for 12 h and 16 h aer a-NiS hollow spheres form, which reveal that during the transformation process, the hollow spherical morphology is maintained and the obtained b-NiS crystals are also hollow spheres with an aperture. However, SAED and HRTEM characterizations indicate that unlike the polycrystalline a-NiS spheres, these b-NiS spheres are single-crystalline in nature. The SAED pattern shown in Fig. 5f displays only a set of diffraction spots. The two diffraction spots with an angle of about 112 can be indexed to 01) and (410) of rhombohedral b-NiS, demonstrating the (1 single crystalline nature of the hollow sphere. The SAED pattern also indicates that there are many defects in the crystal which might be due to the unevenness of the spherical surface. An HRTEM image of the hollow sphere is presented in Fig. 5g, only Fig. 5 (a and b) SEM images; (c–e) TEM images, (f) SAED pattern and (g) HRTEM image of the sample prepared at 160 C for 12 h and then at 200 C for 16 h.
Fig. 4 XRD patterns of the samples prepared at 160 C for 12 h and then at 200 C for (a) 12 h and (b) 16 h.
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displaying a set of lattice fringes. The spacing between two adjacent lattice planes is about 0.29 nm, which is consistent 01) lattice plane of b-NiS. It is impossible to get the with the (1 HRTEM image of the whole hollow sphere. In order to further conrm the single crystallinity of the hollow spheres, HRTEM images from different parts of the hollow sphere are presented in Fig. S4.† In each of the HRTEM images, only one set of lattice fringes can be observed, which is totally different from the HRTEM image of the shell of the a-NiS hollow sphere, indirectly conrming the single crystallinity of the hollow b-NiS sphere. The topotactic transformation process is designed to transform crystals with a certain shape to other crystals without changing the spatial morphology but the crystal structures. It can not only increase the predictability of the produced materials' space structures but also make it possible to form special structures.
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Nanoscale In this case, the transformation from a-NiS to b-NiS is conned in the shell of the hollow spheres, resulting in maintenance of the hollow structure. During the process, the close matching of crystal lattices between the two phases of NiS might be responsible for the transformation from polycrystalline a-NiS to single-crystalline b-NiS spheres. A schematic illustration of the crystal lattice matching between hexagonal a-NiS and rhombohedral b-NiS is shown in Fig. S5,† from which it can be seen that (101) lattice planes of a-NiS and (021) lattice planes of b-NiS have similar d-spacings and Ni ions have similar spatial organizations. The close matching indicates the minimal reorganization of a-NiS in the transformation process. With the increase of the hydrothermal temperature, Ni and S atoms begin to reorganize and the small a-NiS nanoparticles in the shell of a-NiS hollow spheres transform and grow together to form single-crystalline b-NiS hollow spheres. The topotactic transformation process from polycrystalline a-NiS to single-crystalline b-NiS can occur more easily when the mixture of solution of nickel acetate, thiourea and glutathione is treated at 200 C under hydrothermal conditions. Fig. S6† displays XRD patterns of the samples prepared at 200 C for different reaction times. At the early state (0.5 h), the obtained product was amorphous. With increasing reaction time, a-NiS crystals formed and then they transformed to b-NiS. Aer treating the mixture solution at 200 C for 12 h, pure b-NiS was obtained. Fig. S7† shows its XRD pattern, in which all the diffraction peaks can be indexed to rhombohedral b-NiS (JCPDS card no. 86-2281) and no impurities are detected, indicating the high purity of the as-synthesized b-NiS and complete transformation from a-NiS. Fig. 6 and 7 show TEM and SEM images of the samples prepared at 200 C for different reaction times. It can be deduced that the formation process at 200 C is similar to that at 160 C except for instant transformation from a-NiS to b-NiS. The amorphous solid microspheres rstly form at the initial stage (Fig. 6a and b). As the hydrothermal reaction proceeds, a-NiS crystallizes on the surface of amorphous solid microspheres to form core–shell structures (Fig. 6c and d). The freshly formed a-NiS crystals can very easily transform to b-NiS at 200 C and a-NiS and b-NiS are co-existent in the core–shell structures (Fig. 6e–j). On further increasing the reaction time, the amorphous core completely transforms to a-NiS and then to b-NiS to form a perfect hollow structure. The whole process only needs 12 h at 200 C under hydrothermal conditions. From SEM images in Fig. 7a and b, the present b-NiS sample is composed of a large number of hollow spheres with a diameter of about 2.5 mm. TEM images shown in Fig. 7c and d conrm the hollow nature of the spheres by displaying contrast difference between the pale center and the dark edge. Similar to the sample presented in Fig. 5, the SAED pattern taken from a hollow sphere (inset in Fig. 7d) also shows only a set of diffraction spots, demonstrating the singlecrystalline nature of the b-NiS hollow spheres. It is worth noting that the addition of glutathione plays an important role in the preparation of the hollow a-NiS and b-NiS spheres. In the reaction system, it can not only supply the sulfur source but also control the morphology and phase composition of the as-prepared samples. Without the addition of
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Fig. 6 TEM images of the samples prepared at 200 C for different reaction times: (a and b) 0.5 h; (c and d) 1 h; (e and f) 2 h; (g and h) 4 h and (i and j) 8 h.
glutathione, only mixed phases of b-NiS and Ni7S6 irregular precipitates can be obtained at 200 C, indicating that the sulfur source is not enough for obtaining pure b-NiS. By increasing the amount of thiourea to substitute for glutathione, the pure phase b-NiS can be obtained. However, the morphology is irregular instead of hollow spherical as displayed by the SEM image in Fig. S8a.† Similar results can be obtained when the reaction is carried out at 160 C and only pure a-NiS irregular crystals (Fig. S8b†) can be synthesized without the addition of glutathione. The transformation process from a-NiS irregular crystals to b-NiS can also be observed by directly increasing the hydrothermal temperature from 160 C to 200 C as XRD patterns in Fig. S9† conrm. On the other hand, glutathione can also act as the only sulfur source for the synthesis of NiS. When 0.05 g of glutathione was used as the only sulfur source under hydrothermal conditions at 160 C for 12 h, a-NiS can be
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Fig. 7 (a and b) SEM images; (c–d) TEM images and SAED pattern (inset in Fig. 4d) of the sample prepared at 200 C for 12 h.
obtained (XRD pattern shown in Fig. S10a†). The SEM image shown in Fig. S10b† conrms that besides some hollow spheres there are also many irregular nanoparticles. When the reaction was carried out under hydrothermal conditions at 200 C for 12 h with glutathione as the only sulfur source, Ni9S8 hollow spheres and irregular nanoparticles were obtained as shown in Fig. S10c and d.† These results reveal that glutathione has important effects on the formation of hollow a-NiS and b-NiS spheres. The as-prepared NiS samples were used to investigate their possible applications in water purication treatment. In our study, Congo red, a commonly used dye in the textile industry, was chosen as a typical organic water pollutant. Fig. 8a and b display the absorption spectra of an aqueous solution of 100 mg L1 Congo red in the presence of 75 mg of the as-synthesized hollow a-NiS and b-NiS spheres. It can be found that both the a-NiS (SEM images shown in Fig. 2) and b-NiS (SEM images shown in Fig. 5) hollow spheres exhibit good adsorption ability for Congo red dye. Fig. 8c depicts curves of the adsorption extent of Congo red as a function of contact time for four different NiS samples. It reveals that hollow a-NiS and b-NiS spheres show better adsorption ability than irregular aNiS and b-NiS crystals. IR characterizations of Congo red and the hollow spheres before and aer adsorption were carried out to conrm the adsorption of Congo red on the spheres. From the IR spectra shown in Fig. S11,† it can be seen that aer the experiment, the IR spectrum of hollow NiS spheres shows the characteristic peaks of Congo red, suggesting the adsorption of the organic pollutant on the surface of NiS hollow spheres. The hollow a-NiS spheres, b-NiS spheres, and irregular a-NiS and b-NiS crystals have specic surface areas of 15.61 m2 g1, 14.81 m2 g1, 13.77 m2 g1 and 11.97 m2 g1, respectively. Therefore, the difference in the removal ability of hollow NiS and irregular NiS might result from their different morphologies and specic surface areas. These results obviously show that the a-NiS and b-NiS hollow spheres can signicantly remove Congo red, which might have application potential in wastewater treatment.
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Fig. 8 (a) Absorption spectra of an aqueous solution of 100 mg L1 Congo red in the presence of 75 mg of hollow b-NiS spheres; (b) absorption spectra of an aqueous solution of 100 mg L1 Congo red in the presence of 75 mg of hollow aNiS spheres; (c) curves of the adsorption extent of Congo red as a function of contact time for different NiS samples.
Conclusions In summary, uniform single-crystalline b-NiS hollow spheres were prepared through a topotactic transformation route from uniform polycrystalline a-NiS hollow spheres, which were prepared under hydrothermal conditions with thiourea and
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Nanoscale glutathione as sulfur sources through the Kirkendall effect. The b-NiS crystals obtained through the topotactic transformation route not only have unchanged morphology of hollow spheres but are also single-crystalline in nature. The close matching of crystal lattices between the two phases of NiS was believed to be responsible for the topotactic transformation from polycrystalline a-NiS hollow spheres to single-crystalline b-NiS hollow spheres. The as-prepared NiS hollow spheres display a good adsorption ability to remove the organic pollutant Congo red from water, which makes them have application potentials in water treatment.
Acknowledgements This work was supported by the National Basic Research Program of China (Grant nos 2013CB922102 and 2011CB935800) and the National Natural Science Foundation of China (Grant nos 21071076, 51172106 and 21021062).
References 1 A. Ghezelbash, M. B. Sigman Jr and B. A. Korgel, Nano Lett., 2004, 4, 537. 2 V. F. Puntes, D. Zanchet, C. K. Erdonmez and A. P. Alivisatos, J. Am. Chem. Soc., 2002, 124, 12874. 3 D. P. Dinega and M. G. Bawendi, Angew. Chem., Int. Ed., 1999, 38, 1788. 4 A. Ghezelbash and B. A. Korgel, Langmuir, 2005, 21, 9451. 5 H. Jiang, T. Zhao, J. Ma, C. Y. Yan and C. Z. Li, Chem. Commun., 2011, 47, 1246. 6 F. Jiao and P. G. Bruce, Adv. Mater., 2007, 19, 657. 7 B. C. Jang, M. Y. Park, O. B. Chae, S. J. Park, Y. J. Kim, S. M. Oh, Y. Z. Piao and T. H. Hyeon, J. Am. Chem. Soc., 2012, 134, 15010. 8 R. W. Y. Man, A. R. C. Brown and M. O. Wolf, Angew. Chem., Int. Ed., 2012, 51, 11350. 9 S. L. Yang, H. B. Yao, M. R. Gao and S. H. Yu, CrystEngComm, 2009, 11, 1383. 10 G. Kullerud and R. A. Yund, J. Petrol., 1962, 3, 126. 11 S. H. Yu and M. Yoshimura, Adv. Funct. Mater., 2002, 12, 277. 12 Y. Hu, J. F. Chen, W. M. Chen and X. L. Li, Adv. Funct. Mater., 2004, 14, 383. 13 L. Z. Zhang, J. C. Yu, M. S. Mo, L. Wu, Q. Li and K. W. Kwong, J. Am. Chem. Soc., 2004, 126, 8116. 14 Y. Hu, J. F. Chen, W. M. Chen, X. H. Lin and X. L. Li, Adv. Mater., 2003, 15, 726. 15 Y. Hu, J. F. Chen, W. M. Chen and X. L. Li, Adv. Funct. Mater., 2004, 14, 383.
12230 | Nanoscale, 2013, 5, 12224–12230
Paper 16 F. Cao, R. X. Liu, L. Zhou, S. Y. Song, Y. Q. Lei, W. D. Shi, F. Y. Zhao and H. J. Zhang, J. Mater. Chem., 2010, 20, 1078. 17 Y. Wang, Q. S. Zhu, L. Tao and X. W. Su, J. Mater. Chem., 2011, 21, 9248. 18 X. Y. Lai, J. Li, B. A. Korgel, Z. H. Dong, Z. M. Li, F. B. Su, J. Du and D. Wang, Angew. Chem., Int. Ed., 2011, 50, 2738. 19 Y. M. Sui, W. Y. Fu, Y. Zeng, H. B. Yang, Y. Y. Zhang, H. Chen and Y. X. Li, Angew. Chem., Int. Ed., 2010, 49, 4282. 20 Z. Y. Jiang, Z. X. Xie, X. H. Zhang, S. C. Lin, T. Xu, S. Y. Xie, R. B. Huang and L. S. Zheng, Adv. Mater., 2004, 16, 904. 21 B. Wang, J. S. Chen, H. B. Wu, Z. Y. Wang and X. W. Lou, J. Am. Chem. Soc., 2011, 133, 17146. 22 J. Liu and D. F. Xue, Adv. Mater., 2008, 20, 2622. 23 Y. D. Yin, C. Erdonmez, A. Cabot, S. Hughes and A. P. Alivisatos, Adv. Funct. Mater., 2006, 16, 1389. 24 M. M. Titirici, M. Antonietti and A. Thomas, Chem. Mater., 2006, 18, 3808. 25 X. Wang, X. L. Wu, Y. G. Guo, Y. T. Zhong, X. Q. Cao, Y. Ma and J. N. Yao, Adv. Funct. Mater., 2010, 20, 1680. 26 H. L. Xu and W. Z. Wang, Angew. Chem., Int. Ed., 2007, 46, 1489. 27 C. H. Kuo and M. H. Huang, J. Am. Chem. Soc., 2008, 130, 12815. 28 Y. M. Sui, W. Y. Fu, Y. Zeng, H. B. Yang, Y. Y. Zhang, H. Chen, Y. X. Li, M. H. Li and G. T. Zou, Angew. Chem., Int. Ed., 2010, 49, 4282. 29 M. Hu, S. Furukawa, R. Ohtani, H. Sukegawa, Y. Nemoto, J. Reboul, S. Kitagawa and Y. Yamauchi, Angew. Chem., Int. Ed., 2012, 51, 984. 30 S. H. Jiao, L. F. Xu, K. Jiang and D. S. Xu, Adv. Mater., 2006, 18, 1174. 31 L. Zhang, L. Zhou, H. B. Wu, R. Xu and X. W. Lou, Angew. Chem., Int. Ed., 2012, 51, 7270. 32 M. D. Cohen, G. M. J. Schmidt and F. I. Sonntag, J. Chem. Soc., 1964, 2000. 33 R. E. Schaak and T. E. Mallouk, J. Am. Chem. Soc., 2000, 122, 2798. 34 L. S. Li, N. J. Sun, Y. Y. Huang, Y. Qin, N. N. Zhao, J. N. Gao, M. X. Li, H. H. Zhou and L. M. Qi, Adv. Funct. Mater., 2008, 18, 1194. 35 M. T. Buscaglia, C. Harnagea, M. Dapiaggi, V. Buscaglia, A. Pignolet and P. Nanni, Chem. Mater., 2009, 21, 5058. 36 B. Gates, B. Mayers, Y. Y. Wu, Y. G. Sun, B. Cattle, P. D. Yang and Y. N. Xia, Adv. Funct. Mater., 2002, 12, 679. 37 D. H. Son, S. M. Hughes, Y. D. Yin and A. P. Alivisatos, Science, 2004, 306, 1009.
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