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Hydrothermal synthesis of cobalt-basic-carbonate nanobelts
Journal of Alloys and Compounds 461 (2008) 574–578
Hydrothermal synthesis of cobalt-basic-carbonate nanobelts Ruijin Yu b , Pingfang Tao a , Xiaosong Zhou a , Yueping Fang a,∗ a
School of Chemistry and Chemical Engineering, Guangxi Normal University, Guilin, Guangxi 541004, China b School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China Received 21 June 2007; accepted 14 July 2007 Available online 19 July 2007
Abstract Cobalt-basic-carbonate nanobelts were synthesized via hydrothermal process by hexamethylenetetramine (HMT) hydrolysis. The obtained nanobelts were with ∼100–200 nm in width and more than 10 m in length. The microstructure and composition of the nanobelts were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope and Fourier transform infrared spectra. 1D array of spinel Co3 O4 nanoparticles have been obtained by the thermal conversion of monodispersed nanobelts. Its magnetic property was also studied. © 2007 Elsevier B.V. All rights reserved. Keywords: Cobalt-basic-carbonate; Nanobelts; Magnetic properties
1. Introduction Owing to the fact that the size and shape of inorganic materials are important factors in determining their electrical, optical, and other properties, many efforts have been devoted to achieving rational control over the size, shape, dimensionality, and complexity of nanocrystals [1–7]. Cobalt oxides are active catalysts for oxidation reactions, as well as functional materials for oxide electrodes in electrochemical applications [8,9]. Among the various hydroxide-salt precursors for the cobalt oxides, hydroxide carbonates are more desirable, because there are no toxic product gases during their decompositions. The preparation of the metal-basic-carbonates usually involves co-precipitation of metal salts with an alkaline carbonate at constant pH in the range of 7–9 [10–13]. In recent years, many efforts were devoted for preparation of these compounds with highly dispersed cationic species, and investigations of their thermal decomposition behaviors. However, little attention has been paid to their dimensional and morphological controls and fabrication of nanomaterials from these compounds [14–16]. In this paper, we report the preparation of cobalt-basic-carbonate
∗
Corresponding author. Tel.: +86 773 5821167; fax: +86 773 5803930. E-mail address: [email protected] (Y. Fang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.07.041
nanobelts by hexamethylenetetramine (HMT) hydrolysis via a facile hydrothermal process at a relatively lower temperature without using any surfactants. In addition, the thermal conversion of monodispersed nanobelts of cobalt-basic-carbonate compounds to the one-dimensional (1D) arrays of spinel Co3 O4 nanoparticles has been achieved and its magnetic property was studied. 2. Experimental 2.1. Material synthesis In a typical procedure, 300 mg of hexamethylenetetramine (HMT) was dissolved in 24 ml of distilled water, and 1.5 mmol of cobalt dichloride (CoCl2 ) was added with stirring. Then this resulting transparent solution was transferred into a Teflon-lined autoclave (25 ml), heated to 100 ◦ C and maintained at this temperature for 24 h. After the hydrothermal treatment, the precipitate was collected and rinsed with distilled water and ethanol, then dried in air for further characterization. The sample was further heat-treated in static air. The calcination temperatures were set above 350 ◦ C. In this experiment, around 500 mg of as-prepared samples was calcined at the selected temperatures for 5 h with the heating rate fixed at 2 ◦ C min−1 .
2.2. Material characterization The obtained products were directly subjected to scanning electron microscopy characterizations (SEM, JEOL JSM-6300F at an accelerating voltage of 5–15 kV), powder-X-ray diffraction (XRD, Philips PW-1830 and Bruker AXS D8 ADVANCE X-ray diffractometer). For the transmission electron micro-
R. Yu et al. / Journal of Alloys and Compounds 461 (2008) 574–578 scopic (TEM, JEOL 2010F microscopes operated at an accelerating voltage of 200 kV) observations, the product was sonicated in ethanol for 20 min and the suspension was dropped onto a carbon-coated Cu grid, followed by evaporation of the solvent in the ambient environment. Fourier transform infrared spectra (FT-IR) were taken on a Bruker Vector-22 FT-IR spectrometer, which was measured from 400 to 4000 cm−1 at room temperature. Magnetic properties were examined with Quantum Design MPMS XL-7.
3. Results and discussion The morphology, component and structure of the as-prepared sample were examined with SEM, XRD and FT-IR. A low magnification SEM image is shown in Fig. 1A. Numerous bendy and curled nanobelts are formed with average width of 200 nm and more than several tens micrometers in length. The XRD pattern in Fig. 1B gives clear evidence that the as-prepared sample nanostructures are composed of the crystalline cobalt-basiccarbonate phase, Co(CO3 )0.35 (OH)1.1 Cl0.2 0.74H2 O (JCPDS
575
No.38-0547). No obvious peaks of impurities were seen in this pattern. Our FTIR investigation also affirms the formation of the cobalt-basic-carbonate phase. Fig. 1C presents the FTIR spectra of the as-prepared sample. The strong peaks at 3502–3505 cm−1 are assigned to the stretching vibration of the O–H group of molecular water and of hydrogen-bound O–H groups [17,18], noting that the peak at 1620–1632 cm−1 is due to the bending mode of water molecules [10]. The presence of CO3 2− in the sample is evidenced by its vibration bands from middle to lower wavenumbers which suggests the presence of a mono- or poly-dentate carbonate ligand [19,20]. The bands observed at 1501–1505 cm−1 are assigned to stretching vibration ν(OCO2 ). The sharp peak present in the spectra at 829–832 cm−1 is attributed to δ(CO3 ), and the rest of the minor bands at 1760–1700 and 1066–1072, 748–754, and 687–692 cm−1 can be assigned to ν(C O), δ(OCO), and ρ(OCO), respectively [10], while the bands at 937–964 cm−1
Fig. 1. (A) SEM image, (B) XRD and (C) FTIR patterns of as-prepared sample.
576
R. Yu et al. / Journal of Alloys and Compounds 461 (2008) 574–578
Fig. 2. TEM and HRTEM images of as-prepared nanobelts: (A) low magnification TEM image and (B) single nanobelt and inset is SAED pattern recorded from [0 0 1] zone axis.
are ascribed to ν(M–OH) bending modes while the band at 510–517 cm−1 is ascribed to ρw (MOH). More detailed nanostructural information about the asprepared sample is obtained from transmission electron microscopy (TEM) studies. Fig. 2A shows the low magnification TEM image of the nanobelts with the width of ∼100–200 nm. A typical nanobelt with the width of ∼200 nm is shown in Fig. 2B. The selected area electron diffraction (SAED) pattern (inset in Fig. 2B) recorded from this nanobelt can be indexed as an orthorhombic cobalt-hydroxide-carbonate single crystal recorded from the [0 0 1] zone axis. These nanobelts are single-crystalline as revealed by HRTEM analysis (Fig. 2C). A typical HRTEM image (Fig. 2C) taken from the nanobelt in Fig. 2A exposes clear lattice fringes of {2 0 0} planes parallel to the nanobelt axis with a d spacing of 0.499 nm. Energy dispersive X-ray spectroscopy (EDS) analyses show that there are Co, Cl, C and O elements recorded from the nanobelt in Fig. 2A and indicate that the nanobelt should be composed of the crystalline cobalt-basic-carbonate phase, Co(CO3 )0.35 (OH)1.1 Cl0.2 0.74H2 O. The reactions during the hydrothermal
treatment are likely to be as follows: CH2 6 N4 + 10H2 O → 6HCHO + 4NH3 ·H2 O
(1)
HCHO + O2 + 2NH3 ·H2 O → CO3 2− + 2[NH4 ]+
(2)
Co2+ + xOH− + yCl− + 0.5(2 − x − y)CO3 2− + nH2 O → Co(OH)x Cly (CO3 )0.5(2−x−y) nH2 O
(3)
Fig. 3 reports the TGA and DTG curves for the as-prepared sample, which decompose at several steps over 250 to 336 ◦ C. The net weight loss of the as-prepared sample in this stage was 22.3%, which was close to the expected value of 24% calculated for the change of Co(CO3 )0.35 Cl0.20 (OH)1.10 to Co3 O4 . Upon the calcination at 350 ◦ C, the cobalt-basic-carbonate compounds prepared in this work were converted into the Co3 O4 . Displayed in Fig. 4, for example, the as-prepared sample was transformed completely to the cubic phase spinelle Co3 O4 , as indicated by the XRD patterns (Fig. 4A). The TEM images (Fig. 4B) indicate that after the calcinations, the pristine cobalt-basic-carbonate
R. Yu et al. / Journal of Alloys and Compounds 461 (2008) 574–578
Fig. 3. TG-DTG pattern of as-prepared nanobelts.
577
nanobelts were transformed into strings of Co3 O4 nanoparticles interconnected along the original longitudinal directions of the belts. The obtained diffraction patterns show the typical d-values of Co3 O4 (Fig. 4C), which are the same as the XRD results. It is confirmed that the Co3 O4 nanoparticles are completely formed after 5 h calcining at 350 ◦ C. The magnetic property of the Co3 O4 nanoparticles was subsequently studied by a Quantum Design MPMS XL-7 magnetometer. The magnetization as a function of applied field is measured at 2 K and shown in Fig. 5. The magnetization tends to be saturated at the high field as 50 kOe. As an antiferromagnetic material, bulk Co3 O4 should have linear relationship between the magnetization and applied field. In this work, the observed M–H curve reveals the anomalous phenomenon in nanosized Co3 O4 .
Fig. 4. (A) XRD pattern of the cubic phase spinelle Co3 O4 transformed from as-prepared nanobelts; (B) TEM image of spinelle Co3 O4 ; (C) ED ring of Co3 O4 nanoparticles.
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R. Yu et al. / Journal of Alloys and Compounds 461 (2008) 574–578
Acknowledgments This work was supported by the Support program for 100 Young and Middle-aged Disciplinary Leaders in Guangxi Higher Education Institutions and the NSF of Guangxi Zhuang Autonomous Region (0448032). References
Fig. 5. Magnetic properties of the spinelle Co3 O4 nanoparticles at 2 K.
4. Conclusion In summary, nanobelts of cobalt-basic-carbonate compounds with size and morphological controls have been synthesized by facile hydrothermal methods with the HMT hydrolysis. The samples obtained from the HMT and CoCl2 solution have monodispersed belt-like morphologies of ∼100–200 nm in width and more than 10 m in length. The cobalt-basiccarbonate nanobelts are grown along the [0 1 0] direction (orthorhombic phase). Upon the calcination at more than 300 ◦ C, the above nanobelts are transformed into cubic phase spinel oxide, Co3 O4 , giving away 1D arrays of Co3 O4 nanoparticles. The M–H curve at 2 K reveals the anomalous phenomenon in nanosized Co3 O4 .
[1] M. Li, H. Schnablegger, S. Mann, Nature 402 (1999) 393. [2] X.G. Peng, L. Manna, W.D. Yang, J. Wickham, E. Cher, A. Kadavanich, A.P. Alivisatos, Nature 404 (2000) 59. [3] Y. Huang, X.F. Duan, Q.Q. Wei, C.M. Lieber, Science 291 (2001) 630. [4] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. [5] H.T. Shi, L.M. Qi, J.M. Ma, H.M. Cheng, J. Am. Chem. Soc. 125 (2003) 3450. [6] H. C¨olfen, S. Mann, Angew. Chem. Int. Ed. 42 (2003) 2350. [7] Y.P. Fang, A.W. Xu, W.F. Dong, Small 1 (2005) 967. [8] M.A. Langell, M.D. Anderson, G.A. Carson, L. Peng, S. Smith, Phys. Rev. B 59 (1999) 4791. [9] T. Takada, S. Kasahara, K. Omata, M. Yamada, Nippon Kagaku Kaishi (1994) 793. [10] D.G. Klissurski, E.L. Uzunova, Chem. Mater. 3 (1991) 1060. [11] P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lojacono, G. Moretti, J. Mater. Chem. 1 (1991) 531. [12] P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lojacono, G. Moretti, J. Chem. Soc., Faraday Trans. 88 (1992) 311. [13] T. Baird, K.C. Campbell, P.J. Holliman, R.W. Hoyle, D. Stirling, B.P. Williams, M. Morris, J. Mater. Chem. 7 (1997) 319. [14] R. Xu, H.C. Zeng, J. Phys. Chem. B 107 (2003) 12643. [15] Z.H. Wang, X.Y. Chen, M. Zhang, Y.T. Qian, Solid State Sci. 7 (2005) 13. [16] T. Ishikawa, E. Matijevic, Colloid Polym. Sci. 269 (1991) 179. [17] S. Kannan, C.S. Swamy, J. Mater. Sci. Lett. 11 (1992) 1585. [18] Z.P. Xu, H.C. Zeng, Chem. Mater. 11 (1999) 67. [19] K. Nakamoto, Infrared and Raman Spectroscopy of Inorganic and Coordination Compounds, Wiley, New York, 1978. [20] G. Busca, V. Lorenzelli, Mater. Chem. Phys. 7 (1982) 89.
Hydrothermal synthesis of cobalt-basic-carbonate nanobelts Ruijin Yu b , Pingfang Tao a , Xiaosong Zhou a , Yueping Fang a,∗ a
School of Chemistry and Chemical Engineering, Guangxi Normal University, Guilin, Guangxi 541004, China b School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China Received 21 June 2007; accepted 14 July 2007 Available online 19 July 2007
Abstract Cobalt-basic-carbonate nanobelts were synthesized via hydrothermal process by hexamethylenetetramine (HMT) hydrolysis. The obtained nanobelts were with ∼100–200 nm in width and more than 10 m in length. The microstructure and composition of the nanobelts were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope and Fourier transform infrared spectra. 1D array of spinel Co3 O4 nanoparticles have been obtained by the thermal conversion of monodispersed nanobelts. Its magnetic property was also studied. © 2007 Elsevier B.V. All rights reserved. Keywords: Cobalt-basic-carbonate; Nanobelts; Magnetic properties
1. Introduction Owing to the fact that the size and shape of inorganic materials are important factors in determining their electrical, optical, and other properties, many efforts have been devoted to achieving rational control over the size, shape, dimensionality, and complexity of nanocrystals [1–7]. Cobalt oxides are active catalysts for oxidation reactions, as well as functional materials for oxide electrodes in electrochemical applications [8,9]. Among the various hydroxide-salt precursors for the cobalt oxides, hydroxide carbonates are more desirable, because there are no toxic product gases during their decompositions. The preparation of the metal-basic-carbonates usually involves co-precipitation of metal salts with an alkaline carbonate at constant pH in the range of 7–9 [10–13]. In recent years, many efforts were devoted for preparation of these compounds with highly dispersed cationic species, and investigations of their thermal decomposition behaviors. However, little attention has been paid to their dimensional and morphological controls and fabrication of nanomaterials from these compounds [14–16]. In this paper, we report the preparation of cobalt-basic-carbonate
∗
Corresponding author. Tel.: +86 773 5821167; fax: +86 773 5803930. E-mail address: [email protected] (Y. Fang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.07.041
nanobelts by hexamethylenetetramine (HMT) hydrolysis via a facile hydrothermal process at a relatively lower temperature without using any surfactants. In addition, the thermal conversion of monodispersed nanobelts of cobalt-basic-carbonate compounds to the one-dimensional (1D) arrays of spinel Co3 O4 nanoparticles has been achieved and its magnetic property was studied. 2. Experimental 2.1. Material synthesis In a typical procedure, 300 mg of hexamethylenetetramine (HMT) was dissolved in 24 ml of distilled water, and 1.5 mmol of cobalt dichloride (CoCl2 ) was added with stirring. Then this resulting transparent solution was transferred into a Teflon-lined autoclave (25 ml), heated to 100 ◦ C and maintained at this temperature for 24 h. After the hydrothermal treatment, the precipitate was collected and rinsed with distilled water and ethanol, then dried in air for further characterization. The sample was further heat-treated in static air. The calcination temperatures were set above 350 ◦ C. In this experiment, around 500 mg of as-prepared samples was calcined at the selected temperatures for 5 h with the heating rate fixed at 2 ◦ C min−1 .
2.2. Material characterization The obtained products were directly subjected to scanning electron microscopy characterizations (SEM, JEOL JSM-6300F at an accelerating voltage of 5–15 kV), powder-X-ray diffraction (XRD, Philips PW-1830 and Bruker AXS D8 ADVANCE X-ray diffractometer). For the transmission electron micro-
R. Yu et al. / Journal of Alloys and Compounds 461 (2008) 574–578 scopic (TEM, JEOL 2010F microscopes operated at an accelerating voltage of 200 kV) observations, the product was sonicated in ethanol for 20 min and the suspension was dropped onto a carbon-coated Cu grid, followed by evaporation of the solvent in the ambient environment. Fourier transform infrared spectra (FT-IR) were taken on a Bruker Vector-22 FT-IR spectrometer, which was measured from 400 to 4000 cm−1 at room temperature. Magnetic properties were examined with Quantum Design MPMS XL-7.
3. Results and discussion The morphology, component and structure of the as-prepared sample were examined with SEM, XRD and FT-IR. A low magnification SEM image is shown in Fig. 1A. Numerous bendy and curled nanobelts are formed with average width of 200 nm and more than several tens micrometers in length. The XRD pattern in Fig. 1B gives clear evidence that the as-prepared sample nanostructures are composed of the crystalline cobalt-basiccarbonate phase, Co(CO3 )0.35 (OH)1.1 Cl0.2 0.74H2 O (JCPDS
575
No.38-0547). No obvious peaks of impurities were seen in this pattern. Our FTIR investigation also affirms the formation of the cobalt-basic-carbonate phase. Fig. 1C presents the FTIR spectra of the as-prepared sample. The strong peaks at 3502–3505 cm−1 are assigned to the stretching vibration of the O–H group of molecular water and of hydrogen-bound O–H groups [17,18], noting that the peak at 1620–1632 cm−1 is due to the bending mode of water molecules [10]. The presence of CO3 2− in the sample is evidenced by its vibration bands from middle to lower wavenumbers which suggests the presence of a mono- or poly-dentate carbonate ligand [19,20]. The bands observed at 1501–1505 cm−1 are assigned to stretching vibration ν(OCO2 ). The sharp peak present in the spectra at 829–832 cm−1 is attributed to δ(CO3 ), and the rest of the minor bands at 1760–1700 and 1066–1072, 748–754, and 687–692 cm−1 can be assigned to ν(C O), δ(OCO), and ρ(OCO), respectively [10], while the bands at 937–964 cm−1
Fig. 1. (A) SEM image, (B) XRD and (C) FTIR patterns of as-prepared sample.
576
R. Yu et al. / Journal of Alloys and Compounds 461 (2008) 574–578
Fig. 2. TEM and HRTEM images of as-prepared nanobelts: (A) low magnification TEM image and (B) single nanobelt and inset is SAED pattern recorded from [0 0 1] zone axis.
are ascribed to ν(M–OH) bending modes while the band at 510–517 cm−1 is ascribed to ρw (MOH). More detailed nanostructural information about the asprepared sample is obtained from transmission electron microscopy (TEM) studies. Fig. 2A shows the low magnification TEM image of the nanobelts with the width of ∼100–200 nm. A typical nanobelt with the width of ∼200 nm is shown in Fig. 2B. The selected area electron diffraction (SAED) pattern (inset in Fig. 2B) recorded from this nanobelt can be indexed as an orthorhombic cobalt-hydroxide-carbonate single crystal recorded from the [0 0 1] zone axis. These nanobelts are single-crystalline as revealed by HRTEM analysis (Fig. 2C). A typical HRTEM image (Fig. 2C) taken from the nanobelt in Fig. 2A exposes clear lattice fringes of {2 0 0} planes parallel to the nanobelt axis with a d spacing of 0.499 nm. Energy dispersive X-ray spectroscopy (EDS) analyses show that there are Co, Cl, C and O elements recorded from the nanobelt in Fig. 2A and indicate that the nanobelt should be composed of the crystalline cobalt-basic-carbonate phase, Co(CO3 )0.35 (OH)1.1 Cl0.2 0.74H2 O. The reactions during the hydrothermal
treatment are likely to be as follows: CH2 6 N4 + 10H2 O → 6HCHO + 4NH3 ·H2 O
(1)
HCHO + O2 + 2NH3 ·H2 O → CO3 2− + 2[NH4 ]+
(2)
Co2+ + xOH− + yCl− + 0.5(2 − x − y)CO3 2− + nH2 O → Co(OH)x Cly (CO3 )0.5(2−x−y) nH2 O
(3)
Fig. 3 reports the TGA and DTG curves for the as-prepared sample, which decompose at several steps over 250 to 336 ◦ C. The net weight loss of the as-prepared sample in this stage was 22.3%, which was close to the expected value of 24% calculated for the change of Co(CO3 )0.35 Cl0.20 (OH)1.10 to Co3 O4 . Upon the calcination at 350 ◦ C, the cobalt-basic-carbonate compounds prepared in this work were converted into the Co3 O4 . Displayed in Fig. 4, for example, the as-prepared sample was transformed completely to the cubic phase spinelle Co3 O4 , as indicated by the XRD patterns (Fig. 4A). The TEM images (Fig. 4B) indicate that after the calcinations, the pristine cobalt-basic-carbonate
R. Yu et al. / Journal of Alloys and Compounds 461 (2008) 574–578
Fig. 3. TG-DTG pattern of as-prepared nanobelts.
577
nanobelts were transformed into strings of Co3 O4 nanoparticles interconnected along the original longitudinal directions of the belts. The obtained diffraction patterns show the typical d-values of Co3 O4 (Fig. 4C), which are the same as the XRD results. It is confirmed that the Co3 O4 nanoparticles are completely formed after 5 h calcining at 350 ◦ C. The magnetic property of the Co3 O4 nanoparticles was subsequently studied by a Quantum Design MPMS XL-7 magnetometer. The magnetization as a function of applied field is measured at 2 K and shown in Fig. 5. The magnetization tends to be saturated at the high field as 50 kOe. As an antiferromagnetic material, bulk Co3 O4 should have linear relationship between the magnetization and applied field. In this work, the observed M–H curve reveals the anomalous phenomenon in nanosized Co3 O4 .
Fig. 4. (A) XRD pattern of the cubic phase spinelle Co3 O4 transformed from as-prepared nanobelts; (B) TEM image of spinelle Co3 O4 ; (C) ED ring of Co3 O4 nanoparticles.
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R. Yu et al. / Journal of Alloys and Compounds 461 (2008) 574–578
Acknowledgments This work was supported by the Support program for 100 Young and Middle-aged Disciplinary Leaders in Guangxi Higher Education Institutions and the NSF of Guangxi Zhuang Autonomous Region (0448032). References
Fig. 5. Magnetic properties of the spinelle Co3 O4 nanoparticles at 2 K.
4. Conclusion In summary, nanobelts of cobalt-basic-carbonate compounds with size and morphological controls have been synthesized by facile hydrothermal methods with the HMT hydrolysis. The samples obtained from the HMT and CoCl2 solution have monodispersed belt-like morphologies of ∼100–200 nm in width and more than 10 m in length. The cobalt-basiccarbonate nanobelts are grown along the [0 1 0] direction (orthorhombic phase). Upon the calcination at more than 300 ◦ C, the above nanobelts are transformed into cubic phase spinel oxide, Co3 O4 , giving away 1D arrays of Co3 O4 nanoparticles. The M–H curve at 2 K reveals the anomalous phenomenon in nanosized Co3 O4 .
[1] M. Li, H. Schnablegger, S. Mann, Nature 402 (1999) 393. [2] X.G. Peng, L. Manna, W.D. Yang, J. Wickham, E. Cher, A. Kadavanich, A.P. Alivisatos, Nature 404 (2000) 59. [3] Y. Huang, X.F. Duan, Q.Q. Wei, C.M. Lieber, Science 291 (2001) 630. [4] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. [5] H.T. Shi, L.M. Qi, J.M. Ma, H.M. Cheng, J. Am. Chem. Soc. 125 (2003) 3450. [6] H. C¨olfen, S. Mann, Angew. Chem. Int. Ed. 42 (2003) 2350. [7] Y.P. Fang, A.W. Xu, W.F. Dong, Small 1 (2005) 967. [8] M.A. Langell, M.D. Anderson, G.A. Carson, L. Peng, S. Smith, Phys. Rev. B 59 (1999) 4791. [9] T. Takada, S. Kasahara, K. Omata, M. Yamada, Nippon Kagaku Kaishi (1994) 793. [10] D.G. Klissurski, E.L. Uzunova, Chem. Mater. 3 (1991) 1060. [11] P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lojacono, G. Moretti, J. Mater. Chem. 1 (1991) 531. [12] P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lojacono, G. Moretti, J. Chem. Soc., Faraday Trans. 88 (1992) 311. [13] T. Baird, K.C. Campbell, P.J. Holliman, R.W. Hoyle, D. Stirling, B.P. Williams, M. Morris, J. Mater. Chem. 7 (1997) 319. [14] R. Xu, H.C. Zeng, J. Phys. Chem. B 107 (2003) 12643. [15] Z.H. Wang, X.Y. Chen, M. Zhang, Y.T. Qian, Solid State Sci. 7 (2005) 13. [16] T. Ishikawa, E. Matijevic, Colloid Polym. Sci. 269 (1991) 179. [17] S. Kannan, C.S. Swamy, J. Mater. Sci. Lett. 11 (1992) 1585. [18] Z.P. Xu, H.C. Zeng, Chem. Mater. 11 (1999) 67. [19] K. Nakamoto, Infrared and Raman Spectroscopy of Inorganic and Coordination Compounds, Wiley, New York, 1978. [20] G. Busca, V. Lorenzelli, Mater. Chem. Phys. 7 (1982) 89.