Low-Temperature Synthesis of Flower-Shaped CuO Nanostructures

Mohammad Vaseem,† Ahmad Umar,† Sang Hoon Kim, and Yoon-Bong Hahn*. School of Semiconductor and Chemical Engineering and BK21 Centre for ...
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J. Phys. Chem. C 2008, 112, 5729-5735

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Low-Temperature Synthesis of Flower-Shaped CuO Nanostructures by Solution Process: Formation Mechanism and Structural Properties Mohammad Vaseem,† Ahmad Umar,† Sang Hoon Kim, and Yoon-Bong Hahn* School of Semiconductor and Chemical Engineering and BK21 Centre for Future Energy, Materials and DeVices, Chonbuk National UniVersity, Chonju 561-756, South Korea ReceiVed: October 26, 2007; In Final Form: January 30, 2008

Flower-shaped CuO nanostructures have been prepared by the simple solution process at 100 °C using copper nitrate, NaOH, and hexamethylenetetramine (HMTA) for 3 h without the use of any complex reagents. The morphological investigations by field emission scanning electron microscope (FESEM) revealed that the flowershaped nanostructures are monodispersed in large quantity and exhibit the nanocrystalline nature with monoclinic structure. The flower-shaped morphologies are strongly dependent on the concentration of HMTA, presence or absence of NaOH and HMTA, and reaction time. The possible growth mechanism for the formation of flower-shaped CuO products was also discussed in detail.

Introduction CuO is an important p-type transition-metal-oxide semiconductor, having a narrow band gap of (Eg )1.2 eV) and exhibiting a versatile range of applications such as fabrication of electrical, optical, and photovoltaic devices;1,2 gas sensing;3,4 heterogeneous catalysis;5 magnetic storage media;6 field-emission (FE) emitters;7 fabrication of solar cells;8,9 lithium ion electrode materials;10,11 and so forth. It also possesses complex magnetic phases and forms the basis for several high-Tc superconductors and materials with high magnetoresistance.12 Moreover, it can be used to prepare a variety of organic-inorganic nanostructured composites with unique characteristics, which includes high thermal and electrical conductivity, high mechanical strength, high-temperature durability, and so on.13,14 Due to versatile properties and wide applications, hitherto, various kinds of CuO nanostructures have already been synthesized using variety of fabrication techniques, which includes hydrothermal method,15 sol-gel technique,16 gas-phase oxidation,4 micro-emulsion,17 and so forth, as the solution process presents an easy, lowenergy, low-temperature, and cost-effective approach to obtain the products with good yield. Therefore, until now, a variety of CuO nanostructures by solution method have also been reported in literature such as nanorods, nanowires, nanotubes, nanosheets, nanoribbons, and so forth.2-17 Other than 1D morphologies of CuO structures, some complex structures of CuO are also reported in the literature. Recently, Liu et al. reported the formation of honeycombs and flower-like assemblies of CuO onto copper foil using (NH4)2S2O8, Na2WO4, Na2MoO4, and NaOH via hydrothermal process at 160 °C in 24 h.18 Lu et al. demonstrated the synthesis of nanoplatelets, leaflets, and nanowires of CuO by a two-step reaction process, in which the first step follows the synthesis of Cu(OH)2, whereas the next step was the formation of CuO, using KOH, NH3 CuSO4, and poly(acrylic) acid (PAA) in sealed vessel at 180 °C for more than 24 h.19 Liu et al. successfully synthesized the CuO dandelions prepared by mesoscale organization of CuO nanoribbons using * To whom correspondence should be addressed. E-mail: ybhahn@ chonbuk.ac.kr. † Authors contributed equally to this work.

copper nitrate, ammonia, NaOH, and NaNO3 in Teflon coated autoclave at 180 °C in 24 h.20 It was observed from the previous reports that to obtain complex nanostructures of CuO, generally high temperature, pressure, high pH of the solution with long reaction time are needed. Therefore, it is required to develop a simple and effective method to synthesized complex CuO nanostructures in large-quantity at low-temperature and short time. In this paper, we report a successful synthesis and characterization of monodispersed flower-shaped CuO nanostructures by solution process at 100 °C simply by using copper nitrate, NaOH, and hexamethylenetetramine (HMTA) for 3 h without the use of any complex apparatus and reagents. Moreover, several experiments have been performed by varying several reaction parameters, to determine the detailed possible growth mechanism of the as-synthesized CuO flower-shaped structures. Experimental Section Flower-shaped CuO nanostructures composed of thin triangularshaped leaves were synthesized by simple solution method at low-temperature (100 °C) using the copper nitrate [Cu(NO3)2‚ 3H2O] (Sigma Aldrich), hexamethylenetetramine [HMTA; C6H12N4] (Sigma Aldrich) and sodium hydroxide [NaOH]. All of the chemicals were used as-received without further purifications. In a typical synthesis process, 0.1 M (1.208 g) copper nitrate solution, made in 50 mL deionized water was mixed with the 0.05 M aqueous solution of hexamethylenetetramine (50 mL) under stirring at room temperature. Moreover, few drops of NaOH were added to adjust the pH ) 6.0 of the solution. The obtained solution was then heated and refluxed with continuous stirring at 100 °C for 3 h in a three-necked refluxing pot. During refluxing, temperature of the solution was controlled by inserting manually adjustable thermocouple in the refluxing pot. After refluxing under continuous stirring, black colored precipitates were obtained which were washed with methanol several times and dried at room-temperature. Highyield (∼90%) of the product was found which was determined from the initial amounts of reactants used in the reaction. In addition, to determine the detailed growth process of the flowershaped CuO structures, the effects of reaction time, concentration

10.1021/jp710358j CCC: $40.75 © 2008 American Chemical Society Published on Web 03/20/2008

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Figure 1. Low (a) and high (b) magnification FESEM images; (c) low and (d) high magnification TEM images; (e) XRD pattern and (f) EDS spectrum of the flower-shaped CuO nanostructures grown via the solution process by using 0.1 M Cu(NO3)2 and 0.05 M HMTA at 100 °C for 3 h (pH ) 6.0).

of HMTA, presence or absence of NaOH and HMTA have also been examined. The pH values and temperature for the time and HMTA concentration dependent experiments were kept constant at 6.0 and 100 °C, respectively. The structural properties of as-grown flower-shaped CuO nanostructures were examined by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) equipped with selected area electron diffraction (SAED). The crystal phase and crystallinity were analyzed by X-ray diffractometer (XRD) measured with Cu-KR radiations (λ ) 1.54178Å) in the range of 20-70° with 8°/min scanning speed. The quality and composition of the synthesized flower-shaped CuO nanostruc-

tures were characterized by the Fourier transform infrared (FTIR) spectroscopy in the range of 400-4000 cm-1. Results and Discussion Structural Characterizations of Flower-Shaped CuO Nanostructures. The flower-shaped CuO nanostructures, synthesized by the simple solution process using aqueous solutions of copper nitrate, HMTA and NaOH at 100 °C (pH ) 6.0), were observed from FESEM images (Figure 1). The lowmagnification image exhibits that the flower-shaped nanostructures are grown in large-quantity (a). The clear view of flowershaped structures reveals that the flowers are consists of many triangular shaped petals (b). The diameters of the petals are

Synthesis of Flower-Shaped CuO Nanostructures

Figure 2. FTIR spectrum of the flower-shaped CuO nanostructures grown via the solution process by using 0.1 M Cu(NO3)2 and 0.05 M HMTA at 100 °C for 3 hours (pH ) 6.0).

varied from the base to the tips, i.e., show sharpened tips with the wider bases. These petals are connected to each other through their wider bases, rooted in one center and form the flower-like morphologies. The typical length of one petal is about 600-800 nm, while the diameters at their bases and tips are in the range of 150 ( 50 nm and 50 ( 20 nm, respectively. The full array of one flower-shaped structure is in the range of 2-3 µm. This is very interesting to observe that it seems that these nanostructures are formed by the accumulation of many layers and each layer contains several petals. In addition to this, the sizes of the petals are differing to each other from the upper portion to the lower one which makes beautiful flower-like structures. For the detailed structural characterizations, TEM and HRTEM were used. The TEM image of a single CuO flower-shaped structure (c) is consistent with the FESEM

J. Phys. Chem. C, Vol. 112, No. 15, 2008 5731 observations. This image confirms that the flower-shaped nanostructures are made by the accumulation of several triangular-shaped petals composed of thin CuO sheets. These triangular-shaped petals are connected with each other through their wider bases and made flower-like morphologies. It is also seen from the TEM image of flower-like structures that the sizes of petal are not uniform and small-sized petals are arranged at the upper portion of flower-like structures. High-crystalline nature of the thin petals composed in flower-like structures is revealed by the HRTEM image with straight and parallel lattice fringes (d). The spacing between two neighboring fringes is 0.27 nm corresponding to the distance of the [110] plane of the monoclinic CuO. The crystal quality and chemical composition of the as-synthesized products were observed using the X-ray diffraction (XRD) pattern (e) and energy dispersive spectroscopy (EDS) spectrum (f), respectively. It can be seen that all of the diffraction peaks in the XRD pattern are characteristics of monoclinic phase CuO (JCPDS 05-0661). The major peaks located at 2θ ) 35.6 ° and 38.8 ° indexed as (1h11)-(002) and(111)-(200) planes, respectively, are characteristics for the phase pure monoclinic CuO crystallites (e). Additionally, no side products such as Cu(OH)2 and Cu2O are detected from the pattern. The EDS spectrum demonstrates that the products are made of Cu and O only and the atomic ratio of Cu to O is approximately equal to 1:1 in the flower-shaped structures (f). The quality and composition of as-synthesized flower-shaped CuO nanostructures was further characterized by the Fourier transform infrared (FTIR) spectroscopy and shown in Figure 2. Several absorption bands have been observed from the obtained FTIR spectrum of flower-shaped CuO nanostructures.

Figure 3. FESEM images of time-dependant reactions of flower-shaped CuO nanostructures grown via the solution process by using 0.1 M Cu(NO3)2 and 0.05 M HMTA at 100 °C (pH ) 6.0). (a) 0.5 h; (b) 1 h; (c) 3 h; (d) 5 h; (e) 7 h; (f) 9 h; (g) 11 h; (h) 14 h; and (i) 17 h.

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Figure 4. FESEM images of HMTA concentration-dependant morphologies of flower-shaped CuO nanostructures by keeping other reaction parameters constant (0.1 M Cu(NO3)2 at 100 °C for 3 h (pH ) 6.0)). (a) 0.025 M; (b) 0.05 M; (c) 0.075 M; (d) 0.1 M; (e) 0.15 M; and (f) 0.20 M.

Several strong absorption bands have been observed from the spectrum, i.e., at 421, 520, and 598 cm-1, which confirmed the formation of monoclinic CuO phase.21 In addition to the strong absorption bands, a weak and a broad absorption bands at 1629 and 3350 cm-1 have also been obtained which were assigned to the existence of water molecules, respectively.21,22 The appearance of a weak and small absorption peak at 1385 cm-1 was due to the presence of CO32- which usually obtained in the spectrum when the FTIR samples were prepared in the air.22 In addition to all of the observed peaks, a very weak absorption band at 1465 cm-1 has also been seen in the spectrum which was attributed due to the presence of CH2 bending vibrations.21,22 Morphologies of Flower-Shaped CuO Nanostructures. To determine optimized reaction time for the best CuO flowerlike morphologies, various time-dependent experiments have been performed ranging from 0.5 to 17 h. Figure 3 shows the FESEM images of time dependent morphology of CuO nanostructures. During the early stage of the reaction (0.5 h), shuttlelike morphologies can be formed (a). But with increasing the reaction time (1 and 2 h), these shuttle-like morphologies are started to adhere and assemble each other in somewhat flowerlike morphologies (b). It was observed that 3 h was the best reaction time to obtain the best monodispersed flower-like morphologies (c), but a further increase in the reaction time blurred the flower-shaped structures. As the best flower formed, in our reactions in only 3 h hence, it is believed that with prolonged reaction time, the precipitation over the formed flowers increases, which fills the gaps between two adjacent petals (d, for 5 h). By further extending the reaction time (from 7 to 17 h), the precipitation between interstices of two petals

much increased and finally blurred flower-like morphologies were obtained (e-i) for 7, 9, 11, 14, and 17 h, respectively). Figure 4 shows the FESEM image of flower-shaped CuO nanostructures as a function of HMTA concentration. During the experiments, the HMTA concentration was varied from 0.025 to 0.2 M, but other variables such as temperature (100 °C), pH (6.0), time (3 h) and copper nitrate concentration (0.1 M) were kept constant. As the concentration of HMTA increased, blurring and filling (the spaces of two adjacent petals) in the flower-shaped nanostructures were observed. However, the appropriate value of HMTA concentration for the best monodisperse flower-shaped morphologies was obtained to be 0.05 M. In our experiments, an aqueous solution of 0.1 M Cu(NO3)2 was mixed with different concentrations of HMTA (0.025-0.2 M). It is important to note that in the course of mixing of copper nitrate and HMTA, there was no immediate precipitation, but the clear, light-blue colored solution of copper nitrate turned into turbid by the addition of HMTA. Moreover, to maintain the pH value at 6.0 for all the reactions, few drops of 1 M NaOH was included in the solution and interestingly, it was observed that an immediate blue colored precipitate for Cu(OH)2 was appeared. This observation suggests that initially Cu(NO3)2 reacted with NaOH and forms the blue precipitate of Cu(OH)2 according to this simple chemical reaction:

Cu(NO3)2.3H2O + 2NaOH f Cu(OH)2 + 2NaNO3 +3H2O

The formation of Cu(OH)2 is very important for the growth of CuO crystallites which initially serve as building blocks for the

Synthesis of Flower-Shaped CuO Nanostructures

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Figure 5. Low (a) and high (b) magnification FESEM images of the CuO structures grown in presence of NaOH only by using 0.1 M Cu(NO3)2 at 100 °C for 3 h (without HMTA). Low (a) and high (b) magnification images of the CuO structures grown in presence of HMTA only by using 0.1 M Cu(NO3)2 and 0.05 M HMTA at 100 °C for 3 h (without NaOH).

Figure 6. Low (a) and high (b) magnification TEM images of individual petal-like CuO structures grown for 3 h in the presence of HMTA only (without NaOH). Low (c) and (d) magnification images of some arranged petal like structures grown in 3 hrs in presence of HMTA only; their high-resolution TEM image (e) and corresponding SAED pattern (f).

formation of the final products and hence, at appropriate heating, Cu(OH)2 lead the formation of CuO crystallites according to the chemical reaction given below: ∆

Cu(OH)298 CuO + H2O At the early stage of the reaction, only a few drops of NaOH were added in the reaction, hence there were not enough OHions to produce Cu(OH)2 units. Therefore, it is believed that

during the reaction, HMTA provides two things, i.e., first it hydrolyzed and produced OH- ions (it is reported that at elevated temperature HMTA can be hydrolyzed in the distilled water and slowly generate the OH- ions) by the chemical reaction mentioned below:23

(CH2)6N4 + 6H2O f 6HCHO + 4NH3 NH3 + H2O f NH4+ + OH-

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Figure 7. Schematic growth mechanism for the formation of flower-shaped CuO nanostructures.

Hence, from these equations, it is apparent that the second step growth of Cu(OH)2 units was followed by the HMTA generated hydroxyl ions. Therefore, continuous supply of Cu2+ ions and OH- ions from the copper precursor and HMTA, respectively, leads to the continuous generation of Cu(OH)2 units, which finally converts into CuO crystallites and forms the flowershaped CuO nanostructures. In addition to this, it has also been observed that the concentration of OH- ions can significantly affect the nucleation and growth behaviors (for instance, the number of nuclei and concentrations of growth units) of the CuO crystals.24 Therefore, to get specific nanostructure, there should be an optimal value of OH- ions in the solution. In our synthesis, we vary the concentration of HMTA while keeping other reaction parameters constant, and from the experimental results, it is seen that the concentration of HMTA is very important for the best quality, monodisperse flower-shaped nanostructures. Therefore, the concentration of HMTA required for the best results, from experiment, was determined to be 0.05 M (Figure 4b). At lower concentrations of HMTA (0.025 M), the obtained flower-shaped structures were not well-developed, and it seems that they were in the initial stage of the growth (Figure 4a). Moreover, it is reported that higher OH- concentrations may create diffusion layers on certain surfaces of CuO nanostructures, which may produce additional growth anisotropy, allowing only energetically favorable crystallographic planes to grow.25 In our case, we also observed the same phenomena that by increasing the concentration of HMTA, the quantity of the OH- ions increases and overgrowth over the formed flower-shaped structures was observed. By increasing the concentration of HMTA from 0.05 M, the overgrowth increased and filled the vacant spaces of two adjacent petals and finally blunt-shaped flowers were obtained at higher HMTA concentration (c-f). From these results, one can conclude that the HMTA concentration is one of the most important factors

which affect the flower-shaped morphology in a prominent way. In addition to providing the OH- ions to the solution, second, HMTA also acts as an additive which can control the shapes of the nanostructures.26 Therefore, to clarify the exact role of hexamethylenetetramine, some more experiments in the absence and presence of HMTA have been done (with keeping constant all other reaction parameters such as temperature, pH, and time) and shown in Figure 5. Figure 5, parts a and b, shows the low and high magnification images of CuO structures prepared without the use of HMTA, but in the presence of NaOH at a maintained pH of 6. It can be seen from the images that the blunt-shaped petals are formed without HMTA. Some petals were also connected to each other in such a manner that that they were trying to form flower-like morphologies. Reversibly, it was seen from the experiments done in the presence of HMTA (without NaOH) that well-defined with sharp and clean tips shaped nanopetals of the flower-shaped nanostructures were obtained (c, d). Hence, from these observations, one can conclude that the HMTA in our case is acting as an effective shape-directing agent and helps to control the shape of the petals and finally flower-shaped structures. To understand more about the exact morphologies and crystal quality of the as-grown petals in presence of HMTA, the samples were examined with TEM, HRTEM, and their corresponding SAED patterns. Figure 6a shows the low magnification images of the petals grown in presence of HMTA. The TEM observation is fully consistent with the FESEM observation shown in Figure 5, parts c and d. These petals were grown in high yield and exhibiting sharp tips with the clear and smooth surfaces. The corresponding HRTEM image of the circled portion of the petal-like structure shown in Figure 6a, confirming that the grown petals were crystalline and exhibiting a spacing between two neighboring fringes of 0.27 nm, corresponding to the distance of the [110] plane of the monoclinic CuO. In addition to the individual petals, some

Synthesis of Flower-Shaped CuO Nanostructures initially arranged petals for the possible formation of flowershaped structures were also seen in the TEM samples and demonstrated in Figure 6, parts c and d. From these TEM observations, one can certainly conclude that initially individual petals were formed which arranged in a proper reaction environment for the final formation of flower-shaped structures. The corresponding HRTEM (e) and SAED pattern (f) obtained from circled portion of the arranged petals shown in Figure 6d confirmed that the formed structures are crystalline and possessing the monoclinic phase of CuO. Growth Mechanism for the Formation of Flower-Shaped CuO Nanostructures. From the observed results, one can predict the possible growth mechanism for the formation of flower-shaped CuO nanostructures. Figure 7 shows a schematic for the step by step formation of flower-shaped structures. As demonstrated in Figure 5, parts c and d, when the copper nitrate was mixed only with HMTA (without NaOH) (a), a turbid solution was obtained which was the building units (probably Cu2+ions, obtained from the Cu(NO3)2) for the formation of final product (b). After refluxing the said turbid solution at 100 °C for more than 1 h, the color of the solution was changed to blue due to the formation of Cu(OH)2 nuclei (due to the origination of OH- ions by the hydrolyzation of HMTA which produces OH- ions) and aggregation of nuclei (c). The formation of Cu(OH)2 nuclei in this process was not fast and the precipitation starts when HMTA gets a proper environment (temperature and time) to hydrolyze itself for the generation of OH- ions, which react with copper nitrate for the formation of Cu(OH)2 nuclei. With continuous reaction at prolonged reaction time, the Cu(OH)2 converted into CuO crystals. The formed CuO small crystals were arranged with time and form petallike structures (d). This is worthwhile to notice that by using only HMTA, we did not have any flower-like morphology and only individual or few arranged petal-like structures were obtained. By contrast, when we used NaOH with HMTA, the individual petal started to aggregate and flower-like morphologies were obtained. Interestingly, it was seen that when we used NaOH with HMTA, the solution was immediately changed from transparent to blue due to the instant formation of Cu(OH)2 nuclei (e) (as NaOH instantly produces and provides OH- ions for the formation of Cu(OH)2 nuclei). These formed Cu(OH)2 nuclei were transferred to the CuO via the simple chemical reaction discussed above: Cu(OH)2 fCuO + H2O. With prolonged reaction times, the initially formed CuO nuclei were assembled and form individual petals and finally flowerlike morphologies (f) and (g), respectively. It is known that NaOH is a strong electrolyte and it may neutralize the surface charges of the CuO and affect the aggregation.27 Interestingly, it was also observed from our systematic results that NaOH played an important role in aggregating the individual petals. Moreover, it is also reported that the flower-like or spherical assemblies of CuO have strong binding between individual petals due to electrostatic attraction.27 Therefore, it is expected that the electrostatic attraction also play an important role in assembling of the individual petal for the formation of flowerlike morphologies. However, more studies are needed to obtain more conclusive evidence for the growth of these structures. Conclusions In conclusion, we successfully synthesized flower-shaped CuO nanostructures via simple solution process at 100 °C, by

J. Phys. Chem. C, Vol. 112, No. 15, 2008 5735 using copper nitrate, NaOH, and hexamethylenetetramine (HMTA) for 3 h. The detailed structural characterizations exhibited that the nanostructures are grown in large-quantity, possessing a nanocrystalline nature with monoclinic structure. Through extensive experiment, it was observed that the reaction time, concentration of HMTA, and presence or absence of NaOH and HMTA have strong impact on the morphologies of the as-synthesized flower-shaped CuO nanostructures. On the basis of the experimental results obtained by varying several reaction parameters, a plausible growth mechanism for the formation of flower-shaped CuO structures has also been proposed. Acknowledgment. This work was supported in part by the Brain Korea 21 project in 2007 and the Korea Research Foundation grant (KRF-2005-005-J07502) (MOEHRD). Authors wish to thank Mr. T. S. Bae and J. C. Lim, KBSI, Jeonju branch, and Mr. Jong-Gyun Kang, Centre for University Research Facility (CURF) for taking good quality FESEM and TEM images, respectively. References and Notes (1) Musa, A. Q.; Akomolafe, T.; Carter, M. J. Sol. Energy Mater. Sol. Cells 1998, 51, 305. (2) Ray, S. C. Sol. Energy Mater. Sol. Cells 2001, 68, 307. (3) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Taracon, J. M. Nature 2000, 407, 496. (4) Zhang, J. T.; Liu, J. F.; Peng, Q.; Wang, X.; Li, Y. D. Chem. Mater. 2006, 18, 867. (5) Reitz, J. B.; Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 11467. (6) Kumar, R. V.; Diamant, Y.; Gedanken, A. Chem. Mater. 2000, 12, 2301. (7) Zhu, Y. W.; Yu, T.; Cheong, F. C.; Xu, X. J.; Lim, C. T.; Tan, V. B. C.; Thong, J. T. L.; Sow, C. H. Nanotechnology 2005, 16, 88. (8) Xu, Y.; Chen, D.; Jiao, X. J. Phys. Chem. B 2005, 109, 13561. (9) Xu, J.; Xue, D. J. Phys. Chem. B 2005, 109, 17157. (10) Lanza, F.; Feduzi, R.; Fuger, J. J. J. Mater. Res. 1990, 5, 1739. (11) Gao, X. P.; Bao, J .L.; Pan, G. L.; Zhu, H. Y.; Huang, P. X.; Wu, F.; Song, D. Y. J. Phys. Chem. B 2004, 108, 5547. (12) Zheng, X. G.; Xu, C. N.; Tomokiyo, Y.; Tanaka, E.; Yamada, H.; Soejima, Y. Phys. ReV. Lett. 2000, 85, 5170. (13) Kumar, R. V.; Elgamiel, R.; Diamant, Y.; Gedanken, A.; Norwig, J. Langmuir 2001, 17, 1406. (14) Brookshier, M. A.; Chusuei, C. C.; Goodman, D. W. Langmuir 1999, 15, 2043. (15) Cao, M. H.; Hu, C. W.; Wang, Y. H.; Guo, Y. H.; Guo, C. X.; Wang, E. B. Chem. Commun. 2003, 15, 1884. (16) Armelao, L.; Barreca, D.; Bertapelle, M.; Bottaro, G.; Sada, C.; Tondello, E. Thin Solid Films 2003, 442, 48. (17) Zhang, H.; Zhang, X.; Li, H.; Qu, Z.; Fan, S.; Ji, M. Cryst. Growth Des. 2007, 7, 820. (18) Liu, Y.; Chu, Y.; Zhuo, Y.; Li, M.; Li, L.; Dong, L. Cryst. Growth Des. 2007, 7, 467. (19) Lu, C.; Qi, L.; Yang, J.; Zhang, D.; Wu, N.; Ma, J. J. Phys. Chem. B 2004, 108, 17825. (20) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (21) Wiedemann, H. G.; van Tets, A.; Giovanoli, R. Thermochim. Acta 1992, 203, 241. (22) (a) Liu, X. M.; Zhang, X. G.; Fu, S. Y. Mater. Res. Bull. 2006, 41, 620. (b) Nyquist, R. A.; Kagel, R. O. Infrared Spectra of Inorganic Compounds; Academic Press, Inc.: New York, London, 1971; p 220. (23) Govender, K.; Boyle, D. S.; Kenway, P. B.; O’Brien, P. J. Mater. Chem. 2004, 14, 2575. (24) Liu, J.; Huang, X.; Li, Y.; Suleiman, K. M.; He, X.; Sun, F. Cryst. Growth Des. 2006, 6, 1690. (25) Chang, Y.; Zeng, H. C. Cryst. Growth Des. 2004, 4, 397. (26) Wang, H.; Xie, C.; Zeng, D.; Yang, Z. J. Colloid Interface Sci. 2006, 297, 570. (27) Li, D.; Leung, Y. H.; Djurisic, A. B.; Liu, Z. T.; Xie, M. H.; Gao, J.; Chan, W. K. J. Cryst. Growth 2005, 282, 105.