One-Dimensional Nanostructures from Layered Manganese Oxide

One-Dimensional Nanostructures from Layered Manganese Oxide Odair P. Ferreira,* Larissa Otubo, Ricardo Romano, and Oswaldo L. Alves* LQES - Laborato´ rio de Quı´mica do Estado So´ lido, Instituto de Quı´mica, UniVersidade Estadual de Campinas - UNICAMP, P.O. Box 6154, 13083-970, Campinas, SP, Brazil

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 2 601-606

ReceiVed July 20, 2005; ReVised Manuscript ReceiVed NoVember 7, 2005

ABSTRACT: In this paper, we report the preparation of one-dimensional (1D) nanostructures from layered compounds (2D). γ-MnOOH nanorods were successfully prepared from platelike layered Na-birnessite by simple hydro(solvo)thermal treatment at low temperature. It was observed that the reaction medium plays an important role in the phase and morphology modifications. The γ-MnOOH was thermally treated under different conditions of atmosphere (inert or oxidizing) and temperature, which led to several phases of manganese oxide nanorods, such as MnO2 (pyrollusite), Mn2O3 (bixbyite), and Mn3O4 (hausmannite). Indeed, the achievement of such phase tuning was possible because of the outstanding structural flexibility of manganese oxides. Considering the simplicity, reproducibility, and high yield of these methods, they may be employed for low-cost large-scale production of one-dimensional manganese oxide nanostructures. Introduction Nanoscale one-dimensional (1D) structures, such as nanorods, nanobelts, nanowires, and nanotubes, have attracted much attention because of their unique electronic, optical, and mechanical properties that differ drastically from their bulk counterparts.1-4 The novel properties of these nanostructures could be attributed to combinations of size, dimensionality, and morphology effects. 1D systems present the smallest dimensionality useful for efficient transport of electrons and optical excitation and are thus expected to play an essential role in the integration of nanoscale devices. Many other nanotechnological applications are expected for 1D systems.5-7 Studies on 1D materials have been increasing constantly, and a great number of 1D nanostructures are already described, including metals, oxides, and metal chalcogenides.3,4 The strategies explored for growing 1D nanostructures include template-assisted,8,9 vapor-liquid-solid growth,4 reverse micelle media,10,11 evaporation method,12 single-source route,13,14 hydro(solvo)thermal route,15-17 among others. Hydro(solvo)thermal and surfactant-assisted hydro(solvo)thermal methods have been widely used to prepare 1D nanostructures1,3,14,18-22 because they allow production of large quantities of nanoparticles at low cost. In addition, chemicals or precursors are also an important issue for the final cost of the products. Thus, a rational choice of the method and chemicals is essential for producing nanoparticles presenting appropriate properties and cost for applications in nanotechnology. Several studies point out that layered materials could form nanotubes or nanowires/nanorods, depending on the reaction conditions.23 Examples include graphite to carbon nanotubes,24 MS2 (M ) Mo and W) to MS2 nanotubes,25,26 V2O5 to VOx nanotubes,19,27 Na2Ti3O7 to TiO2 nanowires,28 and Ni(OH)2 to Ni(OH)2 nanorods.29 Then, it would be very interesting if 1D nanostructures could be also obtained from layered minerals or their analogues. In this work, we use the layered manganese oxide (Na-birnessite) as a starting material to obtain 1D nanostructures. Na-birnessite (Na-bir) is a mineral that can be found in great abundance as the major phase in manganese minerals from deep-sea nodules and in soil of some regions. * To whom correspondence should be addressed. Tel: +55-19-37883394. Fax: +55-19-3788-3023. E-mail: [email protected] (O.P.F.) or [email protected] (O.L.A.).

Its general chemical formula is RyMnO2‚xH2O, y e 0.7 where R ) Na+ or K+.30 The layers are formed by sharing edges and corners of MnO6 octahedrons, and the interlayer distance nears 0.7 nm, this region being occupied by Na+ or K+ and water molecules. Manganese oxides and oxyhydroxides have been the target of many studies because of their outstanding structural flexibility combined with their physical and chemical properties, which provide potential applications as ion or molecular sieves, catalysts, cathode materials of secondary rechargeable batteries, and electrochromic and new magnetic materials.31-33 Thus, the preparation of manganese oxides and oxyhydroxides as 1D nanostructures may lead to some novel and unexpected properties. Several methods have been described in the literature to prepare MnOOH nanorods/nanowires;34-40 however, few studies are concentrated on synthetic routes of manganese oxyhydroxide nanowires/nanorods starting from mineral analogues. Stable nanoparticles of manganese oxide are difficult to prepare because of their strong tendency to precipitate or coagulate during synthesis. Notwithstanding, one may find in the literature some methods that succeed in preparing MnO2, Mn2O3, and Mn3O4 nanorods/nanowires.41-46 Generally, those methods lead to only one manganese oxide phase as the final product. So, it would be of great value if a unique starting precursor were able to generate several phases of manganese oxide nanorods in a controlled way. In this work, we report how the reaction media influences the structure and morphology of the product obtained from the hydro(solvo)thermal treatment of synthetic Na-bir. Taking advantage of the numerous possible phases of the manganese oxide, we also describe simple routes for preparing MnO2 (pyrolusite), Mn2O3 (byxbyite), and Mn3O4 (hausmannite) nanorods using MnOOH nanorods as the starting material. The resulting phase may be selected adjusting parameters such as temperature and oxidizing/inert atmosphere. Experimental Section Chemicals. All chemicals (reagent grade, Synth or Aldrich) were used as received, without further purification processes. Hydro(solvo)thermal Treatment of Na-bir. Na-bir was synthesized according to the procedure described by Xia et al.47 In the hydro(solvo)thermal treatments, 1.5 g of Na-bir was added to 100 mL of 0.3 mol L-1 dodecylamine aqueous solution (pH adjusted to 7 with hydrochloric

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Figure 1. XRD patterns of (a) Na-bir, (b) Na-bir partially intercalated with dodecylamine, (c) after autoclave treatment of Na-bir in a dodecylamine aqueous solution, and (d) after autoclave treatment of Na-bir in a dodecylamine ethanolic solution. (O) Na-bir; (b) dodecylamine intercalated Na-bir; (+) MnOOH (manganite); (*) γ-Mn2O3 or Mn3O4 (hausmannite). acid) or to 100 mL of 0.3 mol L-1 dodecylamine ethanolic solution (initial pH equal to 6). The suspensions were kept under stirring for 48 h at room temperature. In both cases, the suspensions were treated in a Teflon-lined autoclave at 170 °C for 120 h. The solid products were centrifuged and washed several times with ethanol to remove amine excess and dried under dynamic vacuum. For comparison, 1.5 g of Na-bir was added to 100 mL of distilled water and treated under the same conditions as described above. Thermal Annealing. The solid obtained after the hydrothermal treatment in the dodecylamine aqueous solution was annealed at 200700 °C in a tubular furnace (Barnstead/Thermolyne, model 21130) under static air or argon flow using heating rate of 10 °C min-1 and kept at a defined temperature for 1 h. Techniques. The chemical analyses were performed in the Na-bir dissolved in concentrated HCl. Mn determination was carried out by inductively coupled plasma optical emission spectrometry (ICPOES), using a Perking-Elmer, model Optima 3000 DV, and Na was determined through flame photometry, using an Analyzer, model 910. The average oxidation state (AOS) of manganese in Na-bir was determined employing the method developed by Xia et al.47 The synthesized products were characterized by transmission electron microscopy (TEM) using a Carl Zeiss CEM-902 microscope operating at 80 kV, scanning electron microscopy (SEM) using a JEOL JSM 6360LV microscope, X-ray powder diffraction (XRD) on a Shimadzu XRD6000 diffractometer using Cu KR radiation, FTIR spectroscopy using a Bomen FTLA2000 spectrometer and using the KBr pellet technique, and DTA-TGA analysis on a TA model SDT Q600 with a heating rate of 10 °C min-1 under air or argon flows of 100 mL min-1.

Results and Discussion Layered Manganese Oxide. On the basis of chemical analyses, the formula of Na-bir can be written as Na0.33MnO2‚ xH2O. The AOS of Mn in the synthesized Na-bir was determined as 3.67, which is in accordance to that usually observed for layered manganese oxides.30,48 Figure 1a shows the XRD pattern of Na-bir. One can observe a characteristic profile of a layered compound, and all the diffraction peaks can be indexed to the Na-bir phase (space group: C2/m - JCPDS 43-1456). Sharp and intense peaks indicate a well-crystallized solid. The interlayer distance

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Figure 2. FTIR spectra of (a) Na-bir, (b) Na-bir partially intercalated with dodecylamine, (c) after autoclave treatment in an dodecylamine aqueous solution, and (d) after autoclave treatment in an dodecylamine ethanolic solution.

estimated from the XRD pattern (7.2 Å) is in good agreement with that reported by other authors.30 Figure 2a shows the FTIR spectrum of the Na-bir. Also, in this case, one can observe a good correspondence to the spectra reported in the literature for Na-bir.49 The intense bands observed at 513 and 478 cm-1 are attributed to MnO6 vibrations, and the broad band at 3448 cm-1 are attributed to the O-H stretching mode of adsorbed and interlamellar water molecules. The SEM image of Na-bir in Figure 3a reveals a platelike morphology, typical of layered materials. On the basis of the results described for Na-bir, one can conclude that the desired layered precursor (Na-bir) was synthesized with high quality. Hydro(solvo)thermal Treatment of Na-bir. To evaluate the use of Na-bir as a precursor for 1D nanomaterials, we treated this layered compound in different reaction media. For this purpose, we used dodecylamine in aqueous and ethanolic solutions. In the first step of the experiment, Na-bir was suspended in a dodecylamine aqueous solution for 48 h at room temperature. XRD results (Figure 1b) pointed out that the interlamellar Na+ ions were partially exchanged by CH3-(CH2)11-NH3+ cations produced by the protonation of dodecylamine with HCl. The peak located at 3,4° (2θ), as well as the harmonics, corresponds to the intercalated phase. The interlayer distance, 25.6 Å, is very close to one reported in the literature for the same intercalation compound.48 FTIR spectrum (Figure 2b) also provides evidence for the presence of the amine in the solid since ν(C-H) bands appear near 2900 cm-1. In the second step of the experiment, the suspension was treated in an autoclave. The XRD pattern of the final product is shown in Figure 1c. The sample exhibits the typical signature of crystalline γ-MnOOH (manganite, monoclinic system, space group: P21/c, JCPDS 41-1379). The extremely narrow and intense diffraction peak at 26.2° (2θ) points out that an anisotropic growth of γ-MnOOH crystals takes place under hydrothermal conditions. One can also observe peaks indicating the presence of the Mn3O4 phase, but in reduced amount. In the case of the hydrothermal treatment of the Na-bir in deionized water using the same experimental conditions but in the absence of dodecylamine, only an increase in the crystallinity of the solid was verified. Since no reactions or phase transformations

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Figure 3. SEM images of (a) Na-bir and its products after autoclave treatment in (b) an dodecylamine aqueous solution, (c) deionized water, and (d) an ethanolic dodecylamine solution.

Figure 5. TG curves of the γ-MnOOH nanorods under (a) air and (b) argon flow.

Figure 4. TEM images of MnOOH nanorods.

were observed in this case, it indicates that the reduction from Mn4+ (in Na0.33MnO2) to Mn3+ (in MnOOH) is driven by dodecylamine. Figure 2c shows the FTIR spectrum of the product obtained after the hydrothermal treatment of the suspended birnessite in an aqueous dodecylamine solution. This spectrum shows the characteristic bands of the γ-MnOOH phase, which was noted elsewhere.34,37 It is worth mentioning that no dodecylamine bands were observed in the spectrum of the autoclave product (Figure 2c), indicating that a deintercalating process took place during the reaction. Besides structural changes, the treatment with a dodecylamine aqueous solution also led to morphological modifications. SEM images show an evolution from platelike (Figure 3a) to rod morphology (Figure 3b). The nanorods, found as single rods or bundles, are straight and present uniform diameters along the growth direction. Note that the platelike morphology was preserved for the sample treated in the absence of dodecylamine

(Figure 3c). Such results strongly suggest that the formation of nanorods is assisted by dodecylamine. TEM images of the as-prepared MnOOH nanorods are shown in Figure 4. The nanorods present diameters in the range of 20-50 nm and lengths of hundreds of nanometers. The images also show single rods and bundles, in agreement with the SEM results. When Na-bir was suspended in a dodecylamine ethanolic solution, amine intercalation did not occur. After the suspension was exposed to the solvothermal treatment, a new phase was obtained, as may be observed in the XRD pattern in Figure 1d. The low intensity broad peaks indicate a poorly crystallized material, so it was not possible to distinguish unambiguously the obtained phase since the peaks could be associated to either γ-Mn2O3 (tetragonal system, space group: I41/amd, JCPDS 180803) or Mn3O4 (hausmannite, tetragonal system, space group: I41/amd, JCPDS 24-0734). On this point, FTIR spectroscopy contributed to the distinction of the phase obtained, since the FTIR spectrum in Figure 2d shows bands at 670, 604, and 480 cm-1, which are typically attributed to γ-Mn2O3.50-52 Moreover,

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Figure 6. XRD patterns of (a) MnOOH and its product after thermal treatment under air at (b) 300, (c) 500, and (d) 700 °C and under argon flow at (e) 350 °C and (f) 600 °C. (*) MnOOH (manganite); (+) MnO2 (pyrollusite); (O) Mn2O3 (bixbyite); (0) Mn5O8; (b) γ-Mn2O3; (v) Mn3O4 (hausmannite).

the product obtained is constituted by irregular particles, as shown by the SEM in Figure 3d. To obtain 1D nanostructures from layered compounds, it is important to understand the formation mechanism involved. Our results clearly indicate that the reaction medium strongly affects the structure and morphology of the product. Therefore, the comprehension of such aspects may lead to the preparation of novel 1D nanostructures. Several studies on the preparation of MnOOH nanorods have been reported; however, the formation mechanism under hydrothermal conditions remains unclear since the mechanistical

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propositions are somehow conflicting. Sun et al.39 studied the formation of MnOOH whiskers synthesized in the presence of cetyltrimethylammonium bromide using MnSO4 and ethylamine. The authors suggested that whisker formation is assisted by the surfactant. In another work, Tian et al.40 obtained γ-MnOOH nanofibers performing an initial exfoliation of layered manganese oxide with tetramethylammonium hydroxide solution and then hydrothermally treating the suspension with cetyltrimethylammonium chloride. The authors suggested that the surfactant rod-shaped micelles guide the product morphology. On the other hand, γ-MnOOH nanorods have been also obtained in the absence of surfactants, via reduction of KMnO4 by KI,38 and toluene,36 as examples. The mentioned studies demonstrate the importance of understanding the role of the surfactant and the reducing agents on the mechanism of formation of nanorods. In the present work, dodecylamine showed to be essential to the formation of MnOOH nanorods, since they were not obtained in deionized water. We believe that dodecylamine in aqueous solution provides the reducing medium and also acts as a surfactant, assisting the formation of the nanorods. Thermal Annealing of the γ-MnOOH Nanorods. It is wellknown that metal hydroxides may be converted to the respective oxides by dehydration under thermal annealing.37-39,53 The annealing parameters (i.e., temperature, reaction time, and atmosphere) are fundamental to control the phase and morphology of the final product. Consequently, using γ-MnOOH nanorods as a starting material, one could obtain different manganese oxides phases, keeping the nanorod morphology. With this aim, we studied the thermal annealing of γ-MnOOH under air and argon. The thermal behavior of MnOOH was first investigated through TG experiments. The TG curves of MnOOH under air and argon are shown in Figure 5, panels a and b, respectively. It was possible to distinguish in both atmospheres three distinct mass loss steps over the 25-700 °C temperature range, but the

Figure 7. SEM images of MnOOH thermally treated under air at (a) 300, (b) 500, (c) 700 °C, and argon at (d) 600 °C.

1D Nanostructures from Layered Manganese Oxide

temperature and mass loss intervals are different for the last two steps. Figure 5a shows weight loss events in the following temperature ranges: (i) between 25 and 150 °C; (ii) 150 and 270 °C; and (iii) 550 and 620 °C. The first weight loss (0.35%) can be attributed to the removal of adsorbed water from the nanorods. The second (1.65%) is associated to the dehydroxylation of the MnOOH followed by oxidation of Mn3+ to Mn4+ with formation of MnO2. Formation of MnO2 was confirmed by carrying a thermal annealing of MnOOH in a tubular furnace under static air at 300 °C. The XRD patterns (Figure 6a,b) indicate the conversion of MnOOH to MnO2 (pyrollusite, space group: P42/ mnm - JCPDS 24-0735). Increasing the annealing temperature to 500 °C promotes the intensification and narrowing of the peaks (Figure 6c), suggesting only an increase of the crystallinity of MnO2. The effect of the annealing treatment on morphology can be observed in Figure 7. The SEM images of MnOOH treated at 300 and 500 °C differ just slightly from precursor morphology, indicating that MnO2 was also obtained with rodlike morphology (Figure 7a,b and insets). The third event of the TG curve (7.5%), between 550 and 620 °C (Figure 5a), could be attributed to the conversion of MnO2 to Mn2O3 with elimination of 0.5 mol of O2. This was confirmed by treating MnOOH at 700 °C under air. The XRD pattern (Figure 6d) of the product indicates Mn2O3 formation (bixbyte, cubic system, space group: Ia3, JCPDS 41-1442). The SEM images of this product (Figure 7c and inset) show that the precursor morphology was mainly preserved, although some particles were broken into smaller pieces. This result indicates that 700 °C corresponds to the approximate temperature limit at which MnOOH can be heated under an air atmosphere while preserving the nanorod morphology. In the TG analysis under argon flow (Figure 5b), we also observed three weight loss steps: (i) between 25 and 150 °C, (ii) 150 and 330 °C, and (iii) 400 and 560 °C. The former event (0.7%) was attributed to the removal of adsorbed water from the nanorods. The second one is associated with the dehydroxylation of MnOOH leading to Mn2O3. Although the observed weight loss (10.5%) is in good agreement to the calculated one (10.24%), the XRD pattern of the product obtained by thermal annealing of the MnOOH under argon flow at 350 °C indicates the formation of γ-Mn2O3 (tetragonal system, space group: I41/ amd, JCPDS-18-0803) and Mn5O8 (monoclinic system, space group: C2/m, JCPDS-39-1218) (Figure 6e). Gonza´lez et al.54 studied the thermal decomposition of MnOOH under nitrogen and also observed the formation of both Mn2O3 and Mn5O8. To identify the product obtained after the third mass loss event in Figure 5b (1.5%), MnOOH was treated under argon flow at 600 °C. All peaks of the XRD pattern of the product (Figure 6f) were indexed to Mn3O4 (hausmannite, space group: I41/ amd, JCPDS 24-0734). Therefore, the MnOOH could also be converted to a mixed valence manganese oxide. The SEM images of this product (Figure 7d and inset) indicate that the morphology of the precursor is preserved, and Mn3O4 nanorods were obtained. We suppose that Mn5O8 is an intermediate phase to Mn3O4. Conclusions In conclusion, we have successfully prepared 1D nanostructures from a layered compound. We demonstrated a simple method to obtain MnOOH nanorods from Na-bir controlling the reaction medium of hydro(solvo)thermal treatment. These nanorods present diameters in the range of 20 to 50 nm and

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lengths of several hundreds of nanometers. The results obtained in this work indicate that 1D nanostructures could be obtained from layered minerals, although further investigations should be performed to have a better understanding of the formation mechanisms involved. In addition, MnOOH nanorods were thermally treated under different conditions (oxidizing or inert atmosphere and temperature) leading to the production of MnO2, Mn2O3, and Mn3O4 nanorods. The preparation of several phases was possible because of the outstanding structural flexibility of manganese oxides. The thermal treatment of MnOOH nanorods at 300 and 700 °C under air led to the formation of MnO2 (pyrollusite) and Mn2O3 (bixbyite) nanorods, respectively. On the other hand, the thermal treatment of MnOOH nanorods at 600 °C under argon resulted in Mn3O4 nanorods. These manganese oxides and oxyhydroxide nanorods prepared in this work may provide novel properties offering more applications in nanotechnology. In addition, considering the simplicity and reproducibility of the procedures and the high yields of the nanorods, these methods could be used on a large scale for low cost production of 1D nanostructures. Acknowledgment. The authors O.P.F., L.O., and R.R. acknowledge financial support from the Brazilian agencies CNPq and CAPES. The authors are indebted to Dr. Carlos A. P. Leite for assistance with the TEM images. This is a contribution of the Millenium Institute of Complex Materials. References (1) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (2) Law, M.; Goldberger, J.; Yang, P. D. Annu. ReV. Mater. Res. 2004, 34, 83. (3) Rao, C. N. R.; Deepak, F. L.; Gundiah, G.; Govindaraj, A. Prog. Solid State Chem. 2003, 31, 5. (4) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (5) Korneva, G.; Ye, H.; Gogotsi, Y.; Halverson, D.; Friedman, G.; Bradley, J. C.; Kornev, K. G. Nano Lett. 2005, 5, 879. (6) Xu, N. S.; Huq, S. E. Mater. Sci. Eng., R. 2005, 48, 47. (7) Wang, Z. L. Annu. ReV. Phys. Chem. 2004, 55, 159. (8) Wei, J. Q.; Jiang, B.; Li, Y. H.; Xu, C. L.; Wu, D. H.; Wei, B. Q. J. Mater. Chem. 2002, 12, 3121. (9) Martin, B. R.; Dermody, D. J.; Reiss, B. D.; Fang, M. M.; Lyon, L. A.; Natan, M. J.; Mallouk, T. E. AdV. Mater. 1999, 11, 1021. (10) Shi, H. T.; Qi, L. M.; Ma, J. M.; Wu, N. Z. AdV. Funct. Mater. 2005, 15, 442. (11) Pinna, N.; Willinger, M.; Weiss, K.; Urban, J. Schlogl, R. Nano Lett. 2003, 3, 1131. (12) Ye, C.; Meng, G.; Jiang, Z.; Wang, Y.; Wang, G.; Zhang. L. J. Am. Chem. Soc. 2002, 124, 15180. (13) Jun, Y. W.; Lee, S. M.; Kang, N. J.; Cheon, J. Am. Chem. Soc. 2001, 123, 5150. (14) Xie, G.; Qio, Z. P.; Zeng, M. H.; Chen, X. M.; Gao, S. L. Cryst. Growth Des. 2004, 3, 513. (15) Zhang, Z.; Blom, D. A.; Gai, Z.; Thompson, J. R.; Shen, J.; Dai, S. J. Am. Chem. Soc. 2003, 125, 7528. (16) Yang, J.; Zeng, J. H.; Yu, S. H.; Yang, L.; Zhou, G. E.; Qian, Y. T. Chem. Mater. 2000, 12, 3259. (17) Hu, H. M.; Mo, M. S.; Yang, B. J.; Zhang, X. J.; Li, Q. W.; Yu, W. C.; Qian, Y. T. J. Cryst. Growth 2003, 258, 106. (18) Fang, Y. P.; Xu, A. W.; Qin, A. M.; Yu, R. J. Cryst. Growth Des. 2005, 5, 1221. (19) Niederberger, M.; Muhr, H. J.; Krumeich, F.; Bieri, F.; Gu¨nther, D.; Nesper, R. Chem. Mater. 2000, 12, 1995. (20) Sun, X.; Li, Y. Chem. Eur. J. 2003, 9, 2229. (21) Bu, W. B.; Hua, Z. L.; Zhang, L. X.; Chen, H. R.; Huang, W. M.; Shi, J. L. J. Mater. Res. 2004, 19, 2807. (22) Zhang, L. Z.; Yu, J. C.; Xu, A. W.; Li, Q.; Kwong, K. W.; Wu, L. Chem. Commun. 2003, 23, 2910. (23) Tenne, R.; Homyonfer, M.; Feldman, Y. Chem. Mater. 1998, 10, 3225. (24) Iijima, S. Nature 1991, 354, 56. (25) Tenne, R.; Genut, M. M.; Hodes, G. Nature 1992, 360, 444. (26) Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. Science 1995, 267, 222.

606 Crystal Growth & Design, Vol. 6, No. 2, 2006 (27) Souza, A. G.; Ferreira, O. P.; Santos, E. J. G.; Mendes, J.; Alves, O. L. Nano Lett. 2004, 4, 2099. (28) Wei, M. D.; Konishi, Y.; Zhou, H. S.; Sugihara, H.; Arakawa, H. Chem. Phys. Lett. 2004, 400, 231. (29) Liang, J. H.; Li, Y. D. Chem. Lett. 2003, 32, 1126. (30) Luo, J.; Suib, S. L. J. Phys. Chem. B 1997, 101, 10403. (31) Thackeray, M. M. Prog. Solid State Chem. 1997, 25, 1. (32) Brock, S. L.; Duan, N. G.; Tian, Z. R.; Giraldo, O.; Zhou, H.; Suib, S. L. Chem. Mater. 1998, 10, 2619. (33) Feng, Q.; Kanoh, H.; Ooi, K. J. Mater. Chem. 1999, 9, 319. (34) Zhang, W.; Yang, Z.; Liu, Y.; Tang, S.; Han, X.; Chen, M. J. Cryst. Growth 2004, 263, 394. (35) Yuan, Z. Y.; Ren, T. Z.; Du, G. H.; Su, B. L. Appl. Phys. A 2005, 80, 743. (36) Zhang, Y. C.; Qiao, T.; Hu, X. Y. J. Solid State Chem. 2004, 177, 4093. (37) Sharma, P. K.; Whittingham, M. S. Mater. Lett. 2001, 48, 319. (38) Xi, G.; Peng, Y.; Zhu, Y.; Xu, L.; Zhang, W.; Yu, W.; Qian, Y. Mater. Res. Bull. 2004, 39, 1641. (39) Sun, X.; Ma, C.; Wang, Y.; Li, H. Inorg. Chem. Commun. 2002, 5, 747. (40) Tian, Z.; Feng, Q.; Sumida, N.; Makita, Y.; Ooi, K. Chem. Lett. 2004, 33, 952. (41) Wang, X.; Li Y. J. Am. Chem. Soc. 2002, 124, 2880. (42) Wang, X.; Li Y. Chem. Eur. J. 2003, 9, 300.

Ferreira et al. (43) Yuan, Z. Y.; Ren, T. Z.; Du, G.; Su, B. L. Chem. Phys. Lett. 2004, 389, 83. (44) Liu, Y.; Liu, Z.; Wang, G. Appl. Phys. A 2003, 76, 1117. (45) Wang, W.; Xu, C.; Wang, G.; Liu, Y.; Zheng, C. AdV. Mater. 2002, 14, 837. (46) Seo, J. W.; Jun, Y. W.; Ko, S. J.; Cheon, J. J. Chem. Phys. B 2005, 109, 5389. (47) Xia, G. G.; Tong, W.; Tolentino, E. N.; Duan, N.; Brock, S. L.; Wang, J.; Suib, S. L. Chem. Mater. 2001, 13, 1585. (48) Wortham, E.; Bonnet, B.; Jones, D. J.; Rozie`re, J.; Burns, G. R. J. Mater. Chem. 2004, 14, 121. (49) Luo, J.; Huang, A.; Park, S. H.; Suib, S. L.; O’Young, C. L. Chem. Mater. 1998, 10, 1561. (50) He, W.; Zhang, Y.; Zhang, X.; Wang, H.; Yan, H. J. Cryst. Growth 2003, 252, 285. (51) Gillot, B.; Guendouzi, M. E.; Laarj, M. Mater. Chem. Phys. 2001, 70, 54. (52) Al Sagheer, F. A.; Hasan, M. A.; Pasupulety, L.; Zaki, M. I. J. Mater. Sci. Lett. 1999, 18, 209. (53) Du, G. H.; Yuan, Z. Y.; Van Tendeloo, G. Appl. Phys. Lett. 2005, 86, 63113. (54) Gonza´lez, C.; Gutie´rrez, J. I.; Gonza´lez-Velasco, J. R.; Cid, A.; Arranz, A.; Arranz, J. F. J. Therm. Anal. 1996, 47, 93.

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