Solvothermal Synthesis of CoO, Co3O4, Ni(OH)2 ... - ACS Publications

Dec 4, 2008 - CoO, Co3O4, Ni(OH)2, and Mg(OH)2 nanotubes were synthesized by solvothermal treatment of corresponding colloidal hydroxide. According ...
1 downloads 0 Views 2MB Size
Download PDF
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 1–6

Communications Solvothermal Synthesis of CoO, Co3O4, Ni(OH)2 and Mg(OH)2 Nanotubes Linhai Zhuo, Jiechao Ge, Lihua Cao, and Bo Tang* College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal UniVersity, Engineering Research Center of Pesticide and Medicine Intermediate Clean Production and Key Laboratory of Molecular and Nano Probes, Ministry of Education, Jinan, 250014, P. R. China

ABSTRACT: In this paper, CoO, Co3O4, Ni(OH)2, and Mg(OH)2 nanotubes were synthesized by solvothermal treatment of corresponding colloidal hydroxide. These nanotubes were characterized by powder X-ray diffraction (XRD), selected area electron diffraction (SAED), and transmission electron microscopy (TEM). According to the time-dependent morphology evolution, it is likely that the growth is governed by a solution-solid process. Advantages of this method include that it is a simple and general process without the need for a catalyst, surfactant, or template, which makes it low cost, and that the raw materials are readily available. On the basis of the above results, other metal hydroxides with layered structure are therefore potentially capable of forming nanotubes. Inorganic nanotubes constitute an important family of nanostructures with interesting properties and potential applications, such as gas storage, energy conversion, fluid transportation, catalysis, electronics, optoelectronics, drug release, and sensing.1 Several different synthetic strategies have been explored for generating inorganic nanotubes, for example, template growth, direct synthesis from the vapor phase, precursors-assisted pyrolysis, sulfurization, misfit rolling, and solvothermal or hydrothermal processes.1d Because of their great chemical flexibility and synthetic tenability, solvothermal or hydrothermal processes have emerged as a powerful method for the fabrication of inorganic nanotubes. Many different kinds of nanotubes have been synthesized by solvothermal or hydrothermal method, such as VOX,2 Bi,3 CeO2-X,4 CeO2,5 Fe2O3,6 TiO2,7 and rare earth compounds.8 However, it always needs different routes to synthesize nanotubes of different materials using this method. So, it is significant to seek some general methods to synthesize nanomaterials, which could make the synthesis of nanomterials more rational and controllable.9 CoO, Co3O4, Ni(OH)2, and Mg(OH)2 are basic inorganic materials and have been found to be widely used in many fields. CoO, crystallizing in the rock salt structure, is antiferromagnetic (TN_298 K) and electrically insulating. In particular, CoO nanocrystals are significant owing to their potential applications based on magnetic, catalytic and gas-sensing properties.10 Co3O4 is used in applications in many fields, such as heterogeneous catalysts, electrochromic devices, solid-state sensors, energy storage as intercalation compounds, and rotatable magnets.11 Ni(OH)2 has also received increasing attention in recent years on account of its application in alkaline secondary batteries and also as a precursor of catalyst.12 Mg(OH)2 is used widely as a medical and industrial product. In recent research, it was found that Mg(OH)2 can be used as a starting material for controlling the synthesis of nanoscaled MgO, and the crystalline size and morphological features of Mg(OH)2 can be retained well.13 * Corresponding author. E-mail: [email protected].

Table 1. Optimal Experimental Conditions products

raw materials

precipitator

reaction time (h)

CoO Co3O4 Ni(OH)2 Mg(OH)2

10 mL of 0.025 M Co(NO3)2 10 mL of 0.025 M Co(NO3)2 10 mL of 0.025 M Ni(NO3)2 20 mL of 0.15 M MgCl2

10 mL 0.1 M NH3 · H2O 10 mL 0.1 M NH3 · H2O 10 mL 0.2 M NH3 · H2O 10 mL 5 M NH3 · H2O

24 24 24 36

However, there are few reports about the fabrication of nanotubes of these materials. For the formation of Co3O4, Ni(OH)2, and Mg(OH)2 nanotubes, the existing several methods are mainly based on using templates, such as structure-directing agents or porous anodic alumina membranes.14 To the best of our knowledge, no studies have been reported on the preparation of CoO nanotubes. The main reason is that oxides are more ionic than their 2D metal dichalcogenide analogues and thus are more difficult to fold into nanotubes.1c,15 Therefore, the preparation of these nanotubes still remains challenging. For the formation of nanotubes, it has been demonstrated that graphite and inorganic materials with similar layered structures can be turned into tubular structures.1c Many metal hydroxides crystallize in a layered structure, for example, Co(OH)2, Ni(OH)2, and Mg(OH)2 all possess the similar layer structure as CdI2.16 This structure similarity with graphite makes the formation of these or their oxides nanotubes possible. In this paper, CoO, Co3O4, Ni(OH)2, and Mg(OH)2 nanotubes were synthesized using a simple and general solvothermal method. The growth process is also discussed based on time-dependent experiments. All chemicals were analytically pure and were used as received. In a typical procedure, corresponding raw material was dissolved in deionized water, and then NH3 · H2O was added to the solution slowly. After stirring for about 15 min, the precipitation was washed with deionized water by centrifugation for 5-7 times, then transferred into a 20 mL Teflon-lined autoclave, which was filled with solvents (water/methanol ) 1/1, V/V) up to 80% of the total volume. Expect for the formation of CoO, 0.3 g NaNO3 was added to the other three

10.1021/cg070482r CCC: $40.75  2009 American Chemical Society Published on Web 12/04/2008

2

Crystal Growth & Design, Vol. 9, No. 1, 2009

Figure 1. XRD patterns of the four products: (a) CoO, (b) Co3O4, (c) Ni(OH)2, (d) Mg(OH)2.

Figure 2. EDS spectrum of the obtained Mg(OH)2 product.

systems, which was agitated to make it solvable. And then these systems were sealed and heated at 250 °C for different time. Optimal experimental conditions are shown in Table 1. The final samples were collected by centrifugation, and washed with deionized water to remove any possible ionic remnants, and then dried at a vacuum for 5 h. Phase identification of the samples were carried out using a Bruker D8-advance X-ray powder diffraction (XRD) with CuKa radiation (λ ) 1.5418Å). Core-level X-ray photoelectron spectra (XPS) of the nanoparticles were recorded with a VSW Instrument using a monochromatized Mg KR source. Transmission electron microscope (TEM) images and selected-area electron diffraction (SAED) were obtained on a Hitachi H-800 transmission electron

microscope and a JEOL-2010 microscope with an accelerating voltage of 200 KV. Energy-dispersive spectroscopy (EDS) data were obtained from GENESIS X-ray energy-dispersive spectroscope (EDAX Inc., America). Thermogravimetric analysis (TGA) and differential canning calorimetric (DSC) measurement were conducted on TGA-7 and DSC-7 at a heating rate of 10 °C/min. The phase composition and structure of the products were examined by XRD. The XRD patterns of the synthesized samples confirmed the formation of cubic phase of CoO (Figure 1a, JCPDS card 9-402), cubic phase of Co3O4 (Figure 1b, JCPDS card 42-1467), hexagonal phase of Ni(OH)2 (Figure 1c, JCPDS card 14-117), hexagonal phase of Mg(OH)2 (Figure 1d, JCODS card 7-239). SAED analyses of CoO, Co3O4, and Ni(OH)2 nanotubes revealed that they were crystalline. But additional SAED analysis of these “Mg(OH)2” nanotubes showed that these nanotubes were amorphous. The composition of these nanotubes was characterized by EDS, revealing the coexistence of Mg and O in the sample at an atomic ratio close to 1:2 (as shown in Figure 2). Hydrogen was not detected because the EDS can only determine elements after Boron. TGA and DSC measurement were also carried out to analyze the thermal behavior and decomposition process of the obtained unknown sample. A pronounced weight loss step was found in the temperature range from 310 to 470 °C. When the product was heated to 600 °C, the weight loss observed was 29.6%, which is slightly lower than the theoretical value (30.8%) calculated from the thermal decomposition of magnesium hydroxides: Mg(OH)2 f MgO + H2O (see Figure S1 in the Supporting Information). This result is in agreement with the previous reports.17 According to the above results, these unknown nanotubes are assumed to be Mg(OH)2. The XRD patterns shown in Figure 1d arose from some crystalline Mg(OH)2 nanosheets coexisting with those nanotubes. Clearly, part of the difficulty in preparing pure CoO is the greater stability of Co3O4 and the readily reducibility of CoO to Co metal. In addition, nanocrystals of CoO are even more difficult to prepare because of the additional problems associated with surface oxidation.18 The surface information of the CoO nanotubes can be properly provided by XPS. There is no evidence for impurities. As shown in Figure 3, the spectrum of the sample shows main peaks in the regions of Co (2s, 2p) and O (1s, 2s), only the peaks of Co2p can be observed. The peak at 781.49 eV is due to Co 2p3/2 with the shakeup satellite at 786.1 eV, whereas the peak at 797.6 eV is due to Co 2p1/2 with the satellite peak at 802.7 eV. The presence of two peaks at 781.49 and 797.6 eV and a highly intense satellite is consistent with the presence of Co2+ in the high-spin (4F) state. The absence of a feature at 778.1 eV indicates the nonexistence of Co metal impurity.18

Figure 3. Room-temperature X-ray photoelectron spectrum of the CoO nanotubes.

Communications

Crystal Growth & Design, Vol. 9, No. 1, 2009 3

Figure 4. TEM images of (a) CoO nanotubes, (b) Co3O4 nanotubes, (c) Ni(OH)2 nanotubes, (d) Mg(OH)2 nanotubes.

Figure 5. HRTEM images of (a) CoO nanotube, (b) Co3O4 nanotube, (c) Ni(OH)2 nanotube.

The size and morphology of the products were examined by TEM. As shown in Figure 4, typical TEM images of these nanotubes display uniform nanotube morphologies with diameters of 10-20 nm and lengths up to several micrometers. The TEM images show that these nanotubes are straight and the surfaces of them are smooth. The morphology of the obtained CoO products was also measured by scanning electron microscopy (SEM), which shows that the products could be obtained in high yield (see Figure S2 in the Supporting Information). High-resolution TEM (HRTEM) images provide further insight into the morphologies and structure details of these nanotubes. As shown in Figure 5a, the outer diameter of the CoO nanotube is about 18 nm, and inter diameter is about 7 nm. The corresponding diffraction pattern is shown in the inset of Figure 5a, where the individual planes are indexed. It is interesting that extra spots can be found in the diffraction pattern with d ) 1/3(111), which has three times the distance of the (111) plane. This superstructure is suggested to be caused by ordered oxygen vacancies inside the CoO nanotubes during formation. Similar results have also been found and studied in previous work and other materials,19 for example, large d spacings corresponding to some superstructures are also observed in the HRTEM image of γ-Fe2O3 nanoparticle.19f Images b and c in Figure 5 are the HRTEM images and corresponding diffraction patterns of Co3O4 and Ni(OH)2 nanotubes, respectively. Large superstructures are also found in these nanotubes. Further experiments were also carried out to investigate these superstructures. When CoO nanotubes were annealed at 650 °C in air, the products were converted to Co3O4. TEM image shows that these nanotubes are made of irregular nanoparticles, of which the hollow cavity and the wall become indistinct (Figure 6a). As shown in Figure 6b, HRTEM images reveal that the ordered superstructure disappears during the conversion from CoO to Co3O4. The lattice fringe with a spacing of 0.28 nm is in good agreement with the spacing of the (220) plane of Co3O4. NiO nanotubes could also be obtained by calcination of Ni(OH)2 nanotubes at 500 °C for 1 h. As shown in Figure 6d, superstructures are also found in these nanotubes, but the spacing of the superstructure has changed from 0.70 to 0.72 nm during the phase conversion from Ni(OH)2 to NiO. When Ni(OH)2 and

Figure 6. TEM and HRTEM images of (a, b) Co3O4 nanotubes obtained by anneal CoO at 650 °C and (c, d) NiO nanotubes obtained by anneal Ni(OH)2 at 500 °C.

Co3O4 nanotubes were annealed at a higher temperature (>650 °C), these superstructures were not found and most of these nanotubes collapsed. The formation of CoO or CO3O4 needs a dehydration or deoxidization process in our experiment. To investigate the details of conversion from hydroxide to oxide, we conducted an evolution of XRD pattern study during the formation of CoO and Co3O4. As marked by the black square (Figure 7), some similar patterns emerge, which means that there is a similar phase transformation in the first step. With a longer reaction time, the intensity of XRD patterns of CoO and CO3O4 increases, whereas that of the intermediates is decreased

4

Crystal Growth & Design, Vol. 9, No. 1, 2009

Figure 7. XRD pattern evolution for (a) CoO and (b) Co3O4 nanotubes after 1, 3, and 24 h.

Figure 8. TEM images showing different stages of growth for Co3O4 nanotubes, (a) 1 h, (b) 1.5 h, (c) a magnified image of the circled region inb; (d) 2 h.

Figure 9. TEM images showing different stages of growth for CoO nanotubes: (a) 2 h, (b) a magnified image of the circled region in a, (c) 1.5 h, Ni(OH)2 nanotubes, and (d) 1.5 h, Mg(OH)2 nanotubes.

gradually. This means that the formation of single cubic phase of CoO and CO3O4 needs longer reaction time. For a complete view of the formation process of these nanotubes, a detailed time-dependent morphology evolution study was conducted. These precursors were characterized with XRD, but no peaks were detected, indicating that the crystallization did not take place in this step. Figure 8a-d show TEM images of different solvothermal time during the formation of Co3O4 nanotubes. After 1 h, as shown in Figure 8a, many irregularly hexagonal nanosheets were obtained. After solvothermal processing for 1.5 h (Figure 8b), most of these sheets broke and some tubelike structures formed (Figure 8c, a magnified image of the black circled region in Figure 8b). After 2 h, large quantity of nanotubes appeared, the edge and the hollow cavity of the nanotube can be seen clearly (Figure 8d). After solvothermal treatment for 2 h during the growth of CoO, the product obtained contains some long sheets including a lot of nanotubes (Figure 9a), and some of these sheets are in the process of dissolving and reassembling (Figure 9b, a magnified image of the circled region in Figure 9a). During the formation of Ni(OH)2 and Mg(OH)2 nanotubes, similar process was also found (images c and d in Figure 9, indicated by black circles). It should be noted that the overall hexagonal morphology of these nanosheets with the adjacent edges of 120° as indicated

by arrows (Figure 8a, images c and d in Figure 9) is consistent with the hexagonal crystallographic characteristics of Co(OH)2, Ni(OH)2, and Mg(OH)2. To provide further insight into the formation process of these nanotubes, some representative TEM images were arrested during the conversion from nanosheets to nanotubes. Images a and b in Figure 10 are images of the fracted nanosheets formed during the formation of CoO nanotubes. HRTEM (Figure 10c) and SAED analyze of the portion indicated by the box paneled in Figure 10b reveal that these fracted nanosheets are amorphous. Under solvothermal treament, these formerly crystallized nanosheets became amorphous structures, which is consistent with a local dissolution. The coexistence of nanotubes and nanosheets could be seen from Figure 10d whose reaction time was not long enough. HRTEM analysis of the nanotube revealed that it was not crystallized, which could be considered being in the initial stage of forming nanotubes. The coexistence of nanotubes and nanosheets suggested a link between the nanotubes and these nanosheets. However, because the fracted nanosheets and the initial nanotubes were all amorphous, the rolling mechanism was not responsible for the conversion from nanosheets to nanotubes. Because there is no surfactant or template in the system, we believe that the formation of these

Communications

Crystal Growth & Design, Vol. 9, No. 1, 2009 5

superstructures are found in CoO, Co3O4, Ni(OH)2 nanotubes and are suggested to be caused by ordered oxygen vacancies during formation. The formation of these nanotubes has been proposed as a solution-solid process. On the basis of the above results, other metal hydroxides with layered structure are therefore potentially capable of forming nanotubes. In particular, the present method suggests that nanotubes of cubic materials can be fabricated by solvothermal treatment of a precursor material with a layered structure. Acknowledgment. This work was supported by National Basic Research Program of China (973 Program, 2007CB936000), National Natural Science Funds for Distinguished Young Scholar (20725518), Major Program of National Natural Science Foundation of China (90713019), and Important Project of Natural Science Foundation of Shandong Province (Z2006B09). Supporting Information Available: Additional images (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org.

References Figure 10. TEM images corresponding to smaples obtained at different growth stages of CoO nanotubes. (a) Representative image of one of the fracted nanosheets. (b) Magnified view of the edge of a fracted nanosheet. (c) HRTEM images of the portion indicated by the box in panel b. (d) HRTEM image of one of the nanotubes formed initially.

nanotubes was the self-reorganization of those amorphous structures converted from their primary crystallized nanosheets. On the basis of the above time-dependent morphological evolution evidence, the growth of these nanotubes can be divided into three steps: These amorphous precursors would form some crystal nanosheets during the first stage. These precursors are generally described as being insoluble in water and methanol at room temperature and atmospheric pressure. However, when these precursors were heated under solvothermal conditions for 0.5 h, they precursors were difficult to collect by centrifugation (4000 rpm/min). After being heated for about 1 h, most of these precursors could be collected by centrifugation easily. The above experimental results show that these precursors can be dissolved in solvents and then recrystallized under solvothermal conditions. This dissolution-recrystallization process is mainly related to changes in the temperature and pressure of the system.8c Under solvothermal conditions, when the temperature and pressure of the system increase, these precursors gradually dissolve in solvents. As the concentration of precursors increases beyond saturation, these precursors recrystallize and grow into nanoplates owing to the intrinsic lamellar structures of these materials.12a,20,21 Second, some of these nanosheets were dissolved again to form some small growth entities (such as atomistic species or cluster building blocks). At last, these small growth entities were reorganized to form nanotubes because of the solvothermal process providing sufficient energy. On the basis of the above observed morphologies of products in different stages of evolution, it was possible to interpret that the reactions follow a anisotropic growth-dissolution-reorganization mechanism.22 In summary, inorganic nanotubes including CoO, Co3O4, Ni(OH)2, and Mg(OH)2 have been fabricated by a facile solvothermal method. It is a simple and general process without the need for a catalyst, surfactant, or template, which is low cost and the raw materials are readily available. Some larger

(1) (a) Tremel, W. Angew. Chem., Int. Ed. 1999, 38, 2175. (b) Rao, C. N. R.; Nath, M. Dalton Trans 2003, 1. (c) Tenne, R. Angew. Chem., Int. Ed. 2003, 42, 5124. (d) Remsˇkar, M. AdV. Mater. 2004, 16, 1497. (e) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (f) Xiong, Y.; Mayers, B. T.; Xia, Y. Chem. Commun. 2005, 5013. (g) Goldberger, J.; Fan, R.; Yang, P. Acc. Chem. Res. 2006, 39, 239. (h) Xi, G.; Peng, Y.; Yu, W.; Qian, Y. Cryst Growth Des. 2005, 5, 325. (i) Zhang, S. Y.; Fang, C. X.; Tian, Y. P.; Zhu, K. R.; Jin, B. K.; Shen, Y. H.; Yang, J. X. Cryst. Growth Des. 2006, 6, 2809. (2) Niederberger, M.; Muhr, H. J.; Krumeich, F.; Bieri, F.; Gu¨nther, D.; Nesper, R. Chem. Mater. 2000, 12, 1995. (3) Li, Y.; Wang, J.; Deng, Z.; Wu, Y.; Sun, X.; Yu, D.; Yang, P. J. Am. Chem. Soc. 2001, 123, 9904. (4) Han, W. Q.; Wu, L.; Zhu, Y. J. Am. Chem. Soc. 2005, 127, 12814. (5) Tang, C. C.; Bando, Y.; Liu, B. D.; Golberg, D. AdV. Mater. 2005, 17, 3005. (6) Jia, C. J.; Sun, L. D.; Yang, Z. G.; You, L. P.; Luo, F.; Han, X. D.; Pang, Y. C.; Zhang, Z.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 4328. (7) (a) Chen, Q.; Zhou, W.; Du, G. H.; Peng, L. M. AdV. Mater. 2002, 14, 1208. (b) Du, G. H.; Chen, Q.; Che, R. C.; Yuan, Z. Y.; Peng, L. M. Appl. Phys. Lett. 2001, 79, 3702. (8) (a) Wang, X.; Li, Y. Chem.sEur. J. 2003, 9, 5627. (b) Xu, A. W.; Fang, Y. P.; You, L. P.; Liu, H. Q. J. Am. Chem. Soc. 2003, 125, 1494. (c) Fang, Y. P.; Xu, A. W.; You, L. P.; Song, R. Q.; Yu, J. C.; Zhang, H. X.; Li, Q.; Liu, H. Q. AdV. Funct. Mater. 2003, 13, 955. (d) Wang, X.; Sun, X. M.; Yu, D.; Zou, B. S.; Li, Y. AdV. Mater. 2003, 15, 1442. (9) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature 2005, 437, 121. (10) (a) Seo, W. S.; Shim, J. H.; Oh, S. J.; Lee, E. K.; Hur, N. H.; Park, J. T. J. Am. Chem. Soc. 2005, 127, 6188. (b) An, K.; Lee, N.; Park, J.; Kim, S. C.; Hwang, Y.; Park, J. G.; Kim, J. Y.; Park, J. H.; Han, M. J.; Yu, J.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 9753. (c) Lagunas, A.; Payeras, A. M.; Jimeno, C.; Perica´s, M. A. Chem. Commun. 2006, 1307. (11) (a) He, T.; Chen, D.; Jiao, X.; Wang, Y. AdV. Mater. 2006, 18, 1078. (b) Li, W. Y.; Xu, L. N.; Chen, J. AdV. Funct. Mater. 2005, 15, 851. (12) (a) Liang, Z. H.; Zhu, Y. J.; Hu, X. L. J. Phys. Chem. B 2004, 108, 3488. (b) Wang, D.; Song, C.; Hu, Z.; Fu, X. J. Phys. Chem. B 2005, 109, 1125. (c) Yang, D.; Wang, R.; Zhang, J.; Liu, Z. J. Phys. Chem. B 2004, 108, 7531. (13) (a) Li, Y.; Sui, M.; Ding, Y.; Zhang, G.; Zhuang, J.; Wang, C. AdV. Mater. 2000, 12, 818. (b) Ding, Y.; Zhang, G.; Wu, H.; Hai, B.; Wang, L.; Qian, Y. Chem. Mater. 2001, 13, 435. (14) (a) Shi, X.; Han, S.; Sanedrin, R. J.; Galvez, C.; Ho, D. G.; Hernandez, B.; Zhou, F.; Selke, M. Nano Lett. 2002, 2, 289. (b) Wang, R. M.; Liu, C. M.; Zhang, H. Z.; Chen, C. P.; Guo, L.; Xu, H. B.; Yang, S. H. Appl. Phys. Lett. 2004, 85, 2080. (c) Cai, F. S.; Zhang, G. Y.; Chen, J.; Gou, X. L.; Liu, H. K.; Dou, S. X. Angew. Chem., Int. Ed.

6

(15) (16)

(17)

(18) (19)

Crystal Growth & Design, Vol. 9, No. 1, 2009 2004, 43, 4212. (d) Fan, W.; Sun, S.; You, L.; Cao, G.; Song, X.; Zhang, W.; Yu, H. J. Mater. Chem. 2003, 13, 3062. Rosenfeld Hacohen, Y.; Popovitz-Biro, R.; Prior, Y.; Gemming, S.; Seifert, G.; Tenne, R. Phys. Chem. Chem. Phys. 2003, 5, 1644. (a) Oswald, H. R.; Asper, R.; in lieth, R. M. A., Eds. Preparation and Crystal Growth of Materials with Layered Structures;, D. Reidel Publishing: Dordrecht, The Netherlands, 1977; Vol. 1, p 71. (b) Wells, A. F. Structural Inorganic Chemistry; Clarendon Press: Oxford, U.K., 1985. (c) Greenwood, N. N.; Earnshaw, A.; Chemistry of the Elements, 2nd ed.; Butterworth: London, 1997. (d) Li, X. L.; Liu, J. F.; Li, Y. D. Mater. Chem. Phys. 2003, 80, 222. (a) Wang, J. A.; Novaro, O.; Bokhimi, X.; Lo’pez, T.; Go’mez, R.; Navarrete, J.; Llanos, M. E.; Lo’pez-Salinas, E. Mater. Lett. 1998, 35, 317. (b) Ardizzone, S.; Bianchi, C. L.; Fadoni, M.; Vercelli, B. Appl. Surf. Sci. 1997, 119, 253. Ghosh, M.; Sampathkumaran, E. V.; Rao, C. N. R. Chem. Mater. 2005, 17, 2348. (a) Chueh, Y. L.; Lai, M. W.; Liang, J. Q.; Chou, L. J.; Wang, Z. L. AdV. Funct. Mater. 2006, 16, 2243. (b) Dimoulas, A.; Travlos, A.;

Vellianitis, G.; Boukos, N.; Argyropoulos, K. J. Appl. Phys. 2001, 90, 4224. (c) Wang, Z. L.; Yin, J. S.; Jiang, Y. D.; Zhang, J. Appl. Phys. Lett. 1997, 70, 3362. (d) Travlos, A.; Boukos, N.; Apostolopoulos, G.; Dimoulas, A. Appl. Phys. Lett. 2003, 82, 4053. (e) Yang, G. Y.; Dickey, E. C.; Randall, C. A.; Randall, M. S.; Mann, L. A. J. Appl. Phys. 2003, 94, 5990. (f) Xie, H. Y.; Zuo, C.; Liu, Y.; Zhang, Z. L.; Pang, D. W.; Li, X. L.; Gong, J. P.; Dickimson, C.; Zhou, W. small 2005, 1, 506. (20) Yu, J. C.; Xu, A.; Zhang, L.; Song, R.; Wu, L. J. Phys. Chem. B 2004, 108, 64. (21) (a) Hou, Y.; Kondoh, H.; Shimojo, M.; Kogure, T.; Ohta, T. J. Phys. Chem. B 2005, 109, 19094. (b) Sampanthar, J. T.; Zeng, H. C. J. Am. Chem. Soc. 2002, 124, 6668. (22) (a) Lu, J.; Xie, Y.; Xu, F.; Zhu, L. J. Mater. Chem. 2002, 12, 2755. (b) Chen, M.; Xie, Y.; Lu, J.; Xiong, Y.; Zhang, S.; Qian, Y.; Liu, X. J. Mater. Chem. 2002, 12, 748.

CG070482R