Formation of Various Morphologies of Covellite Copper Sulfide

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Formation of Various Morphologies of Covellite Copper Sulfide Submicron Crystals by a Hydrothermal Method without Surfactant Ai-Miao Qin,† Yue-Ping Fang,† Huang-Dong Ou,† Han-Qin Liu,† and Cheng-Yong Su*,†,‡

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 3 855-860

School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China, and the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China Received July 29, 2004;

Revised Manuscript Received November 25, 2004

ABSTRACT: Covellite copper sulfide submicron crystals in the shapes of ball-like, rodlike, and chrysanthemumlike architectures congregated from nanoslices with thickness of 20 to 100 nm have been prepared by a hydrothermal method without using any surfactant and characterized by X-ray diffraction, energy-dispersive X-ray spectrometer (EDX), UV-vis optical absorption spectra, and electron-microscopy techniques. A systematic investigation has been carried out to understand the factors influencing the evolution of the crystal morphology which was found to be predominant by the reactant molar ratio (Tu:Cu(NO3)2, Tu ) thiourea), the hydrothermal reaction time, and the temperature. Possible crystal growth processes are also discussed. Introduction Transition metal chalcogenides, such as CuS, CdS, ZnS, and PbS, have attracted increasing attention in recent years due to their excellent physical and chemical properties.1-6 Copper sulfides are of great interest owing to their variations on stoichiometric composition, nanocrystal morphologies, complex structures, valence states, unique properties, and their potential applications in numerous fields.5,7-10 In the form of thin films,11 copper sulfide has been used in photothermal conversion, electrodes, solar cell devices, coatings for microwave shields, and solar control.12-15 As a member of the chalcogenides, covellite copper sulfide shows semiconductor or metallic conductivity and transforms into a superconductor at 1.6 K.10 The CuS nanoparticles containing ZnS and CdS components have been considered as promising luminescent materials in the Cd/CuxS solar cell.14 Recently, various morphologies of copper sulfide have been reported, such as nanoparticles,16 nanowires, nanorods,17 nanotubes, nanovesicles,18 nanodisks,19 millimeter-scale tubular crystals, and so on.20,21 Many techniques have been established to prepare copper sulfides in various forms, including microwave,22 electrosynthesis,23 and thermolysis;24 however, most of these synthetic methods involve templates or surfactants. On the other hand, nonstoichiometric copper sulfides were often obtained due to the existence of several stable and metastable species ranging between Cu2S (chalcocite) and CuS (covellite). In this paper, we report the preparation of a series of covellite copper sulfide submicron crystals via a simple, mild and effective hydrothermal method at a relatively lower temperature without using any surfactants. The effects on the formation of morphologies of copper sulfide crystals, including the molar ratio of the reactants, the * Corresponding author. Fax: +86-20-84115178. E-mail: cedc63@ zsu.edu.cn. † Sun Yat-Sen University. ‡ Chinese Academy of Sciences.

hydrothermal reaction temperature and time, copper sources, and pH values, have been systematically investigated. Experimental Section In a typical preparation, 2.0 mmol of copper nitrate (Cu(NO3)2‚3H2O) was dissolved in a mixed solution of 11 mL of ethanol (99.9%) and 22 mL of water, and then 2.5 mmol of thiourea (Tu) was added with stirring. When the resulting mixture turned clear, the solution was transferred to and sealed in a Teflon-lined stainless steel autoclave of 40 mL capacity. The autoclave was heated to 120 °C and maintained at this temperature for 12 h and then allowed to cool to room temperature naturally. The precipitate formed at the bottom was filtered off, washed with distilled water and absolute ethanol several times, and then dried at 60 °C for 4 h. The samples were characterized by XRD, SEM, TEM, ED, and EDX measurements. The XRD patterns were recorded on a D/ Max-IIIA diffractometer with Cu KR radiation (λ ) 1.54056 Å) at a scanning rate of 0.07° s-1 with 2θ ranging from 10° to 70°. Scanning electron microscopy (SEM) images were obtained using a JSM-6330F apparatus, operating at 15 kV. Transmission electron microscopy (TEM) and electron diffraction (ED) patterns were taken with a JEM-2010 instrument operating at 200 kV. The composition of copper sulfide was determined by energy-dispersive X-ray spectrometer (EDX, Oxford ISIS-300). A Shimadzu spectrophotometer (model 2501 PC) equipped with an integrating sphere was used to record the UV-vis diffusion reflection spectra of the samples, and the photoluminescence (PL) measurement was carried out with an F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source.

Results and Discussion Synthesis and Characterization. It was found that there are a number of factors significantly affecting the growth of the copper sulfide crystals under the hydrothermal reaction conditions; therefore, the control of the final crystal morphology is largely dependent upon the choice of these factors. We selected the solvent system EtOH/H2O with a ratio of 1:2 v/v as the hydrother-

10.1021/cg049736o CCC: $30.25 © 2005 American Chemical Society Published on Web 03/02/2005

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Figure 2. EDX spectrum of as-prepared copper sulfide product with CR ) 1.25 at 120 °C for 12 h.

Figure 1. Powder X-ray diffraction patterns of as-prepared copper sulfide products with CR ) 1.25 at 120 °C for (a) 12 h; (b) 6 h; (c) 3 h; and (d) 1 h.

mal reaction medium, and thiourea (Tu) as the sulfur source. Various copper-containing salts, such as CuCl2, Cu(CH3COO)2, CuCO3, and Cu(NO3)2, have been utilized to provide copper(II) cations. Among these copper sources Cu(NO3)2 was found to be the best candidate to demonstrate the process of crystal growth and morphology evolution of the copper sulfide crystal. A systematic study was carried out to investigate various influencing factors, and the products have been characterized by means of XRD, SEM, TEM, ED, and EDX measurements. In general the structure and bulk phase of the products were determined by XRD analysis, and the morphology of the sample was examined by SEM and TEM measurements. Three main forms of morphology were observed under the present hydrothermal reaction conditions: ball-shaped, rod-shaped, and chrysanthemum-shaped architectures. Formation of these architectures is found to depend on the molar ratio of the reactants, reaction time, and temperature, which will be discussed in detail later. For all the final products, the formation of a pure covellite phase of CuS with a hexagonal structure has been confirmed by powder XRD analysis. A typical XRD pattern of the product prepared from reaction of Tu and Cu(NO3)2‚3H2O (CR ) 1.25) at 120 °C for 12 h is shown in Figure 1a. The cell parameters of this sample were calculated to be a ) 3.794 Å and c ) 16.342 Å, which are in good agreement with the values reported in the literature (JCPDS Card File No. 06-0464). All salient diffraction peaks of other products also closely matched the standard diffraction data of powder crystal CuS. EDX analysis of the sample composition shows that only Cu and S are present with the atomic ratio close to 1 (Cu:S ) 50.90:49.10 for the rod-shaped product obtained from the reaction with CR ) 1.25, reaction time 12 h and temperature 120 °C; Cu:S ) 50.76:49.33 for the ball-shaped product obtained from the same reaction as above; Cu:S ) 49.34:50.66 for the chrysanthemum-shaped product obtained from the reaction with CR ) 4, reaction time 12 h, and temperature 120 °C). A typical EDX spectrum is depicted in Figure 2.

Figure 3. UV-vis spectrum of as-prepared copper sulfide product with CR ) 1.25 at 120 °C for 12 h.

The UV-visible diffusion reflection spectra of the asprepared samples were recorded in the range of 250850 nm on a Shimadzu spectrophotometer, and a representative spectrum is shown in Figure 3. It shows that the samples absorb in the spectral region between 250 and 500 nm. In addition, a broad band extending into the near-IR region can be observed, which is characteristic for covellite CuS.25 Investigation on the photoluminescence revealed no emission in the range 400-800 nm, which is consistent with the results in the literature.9,26 Evolution of Morphologies of CuS Crystals. It is notable that three major factors play the most important roles to control the morphology of copper sulfide crystals: the reactant molar ratio of Tu:Cu(NO3)2 (CR), the hydrothermal reaction time, and temperature. These three factors were studied using the following processes. The concentration of Cu(NO3)2 was kept constant (2.0 mmol in 33 mL of a mixed solution of EtOH/H2O with a ratio of 1:2 v/v), while the addition of thiourea was varied to give a series of reactant molar ratios, CR ) 1, 1.25, 2, 3, 4, and 6. For each CR, the reaction was carried out with different reaction times, i.e., 1, 3, 6, 9, 12, and 24 h, at three different temperatures, 120, 150, and 180 °C, respectively. It was found that increasing the temperature most likely resulted in faster growth of the crystal size, but the appearance of the crystals was largely dependent on the molar ratio CR and reaction time. The reactant molar ratio was crucial for the final crystal shape formation while the reaction time con-

Covellite Copper Sulfide Submicron Crystals

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Figure 5. SEM and TEM images of copper sulfide prepared with CR ) 4 at 120 °C for 12 h: (a) SEM image of a hollow chrysanthemum-like aggregate; (b) TEM image of a nanoslice of CuS and SAED pattern in the inset.

Figure 4. SEM images of copper sulfide prepared with CR ) 1.25 at 120 °C of different hydrothermal reaction time: (a) 1 h; (b) 3 h; (c) 6 h; (d) 12 h (CR ) 2).

tributed greatly to the morphology evolution process as well. For simplicity, two distinct morphology evolution processes will be discussed with respect to two different CR ranges. Reactant Molar Ratio CR ) 1-2. For example, when the CR was equal to 1.25, apparent nanosticks were observed as the reaction solution was maintained at 120 °C for only 1 h. A representative SEM image of the sample thus obtained is shown in Figure 4a. The size of the nanosticks was in the order of tens of nanometers in diameter and tens of micrometers in length. However, the XRD pattern recorded for these sticks indicated that the product is not a single phase. As shown in Figure 1d, the XRD pattern displays a number of peaks corresponding to not only the covellite copper sulfide but also some unknown materials which are probably the coordination precursors (see discussion below). The product exhibits poor crystallinity, suggesting that it is a possible transient phase, in which the covellite copper sulfide was growing from the coordination precursors. In order to further understand the process of the crystal growth and shape evolution, the reaction was carried out for a longer time. As the hydrothermal reaction time was prolonged to 3 h, the nanosticks seemed to disappear while the ball-shaped architecture with a diameter of about 3 µm was observed as shown in Figure 4b. Detailed examination of the SEM image indicates that the ball-shaped architecture is composed of nanoslices, which may be evolved from the above observed nanosticks because many of the slices still keep the apparent sticklike appearance. XRD measurement denotes that the phase was turned into covellite CuS as shown in Figure 1c. When the reaction solution was kept at 120 °C for 6 h or longer, the welldefined slices became clearer and most of the ballshaped crystals grew further into rod-shaped architectures as depicted in Figure 4c. From the XRD patterns (Figure 1c to 1a) we can see that the crystallinity was improved with prolonging the reaction time, and the final product can be unambiguously assigned to hexagonal CuS structure. A similar crystal growth and

shape evolution process was observed for the CR ranging from 1 to 2. It was found that increasing the reaction temperature will get the crystal to grow faster and the slices to grow thicker, but will not significantly influence the shape evolution. A representative SEM image for the product prepared from the reaction of CR ) 2, reaction time 12 h, temperature 120 °C is depicted in Figure 4d, whose morphology closely resembles the final product discussed above with CR ) 1.25. Reactant Molar Ratio CR ) 3-6. On the contrary, when the concentration of Tu was further increased such that CR was equal to or more than 3, the crystal growth and morphology evolution process became significantly different. The chrysanthemum-shaped architecture with a hole in the middle was formed in a broad range of reaction time, from 1 to 24 h. As an example, the SEM image of the product prepared from the reaction of CR ) 4 with reaction time 12 h at 120 °C is shown in Figure 5a. This crystal morphology looks like a chrysanthemum flower with a diameter of a few micrometers. The chrysanthemum structure is assembled from thin nanoslices (petals) with thickness in the order of about 20 nm. A TEM image of an individual petal separated by sonication is shown in Figure 5b with the selected area electron diffraction (SAED) pattern as an inset. The SEAD pattern can be indexed to the hexagonal phase of covellite CuS viewed along the [001] zone axis, which is perpendicular to the slice plane. The good crystallinity of the product was also examined by the XRD pattern which confirms the hexagonal structure of CuS crystals. Furthermore, the well-recorded SEAD pattern implies that the individual nanoslices are single crystals. Although the hydrothermal reaction time does not appear to dramatically influence the shape of the copper sulfide crystals, the diameter of the hollow chrysanthemum-like copper sulfide grew larger when the hydrothermal reaction time increased. In contrast to the reactions with CR less than 3, the products of the reactions with CR more than 3 do not exhibit significant morphology change along with reaction time increasing. However, fewer products were obtained in the latter reactions within the same reaction time in comparison to the former reactions, suggesting a relatively slow crystal growth process which will be discussed below. XRD analysis indicated that covellite copper sulfide formed in 3 h reaction still incorporates some unknown materials as shown in Figure 6a. When the reaction time was prolonged to 6 h, relatively pure copper sulfide was obtained (Figure 6b), which is comparable with the products formed in the reactions of 12 and 24 h (Figure

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Figure 6. Powder X-ray diffraction pattern of as-prepared copper sulfide product with CR ) 4 at 120 °C for (a) 24 h; (b) 12 h; (c) 6 h; and (d) 3 h.

Figure 7. SEM images of copper sulfide prepared with CR ) 4 for 12 h at different hydrothermal reaction temperatures: (a) top left, 120 °C (b) top right, 150 °C; (c, d) bottom, 180 °C.

6c,d). From the XRD patterns depicted in Figure 6 we can see that the crystallinity was improved along the increasing reaction time, but not as significant as that in reactions with CR less than 3. On the other hand, the nanoslices congregating the chrysanthemum flowers grew thicker, from about 20 nm to 100 nm, with the hydrothermal reaction temperature increases from 120 °C to 180 °C as shown in Figure 7a to 7d. This finding suggests that the reaction temperature dictates the crystal size other than the shape; that is, the higher the reaction temperature is, the faster the crystal grows. A similar crystal growth and shape evolution process was found even when the concentration of Tu was high enough for CR to reach 6. Growth Process. It is notable that the copperthiourea system has already been utilized to generate copper sulfides in various forms and compositions,9,11b,18,27 and its chemistry is clearly complicated as a result of the ready redox between them in combination with the complex copper-sulfur stoichiometry. This attracted our

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interest to explore the potential growth mechanism of copper sulfide crystals under the present hydrothermal reaction conditions. Since the reaction system was initially established under the ordinary laboratory atmosphere before the reaction solution was sealed in a Teflon-lined stainless steel autoclave and then heated, it is obviously essential to examine what had happened at this stage. The literature survey clearly shows that the reaction of thiourea with copper(II) ion is versatile in the open atmosphere (in air). When thiourea acts as the sole ligand, copper(I) complexes with the general formula [Cux(Tu)y]x+ will be inevitably obtained in final. A review of the Cambridge Structure Database (CSD) revealed a variety of thiourea-copper(I) structures, including monomeric, dimeric, tetrameric, polymeric, or ring structures depending on the counteranions and reaction conditions used.28,29 The known structures with nitrate as counteranion28 are [Cu4(Tu)6](NO3)4 and [Cu4(Tu)9](NO3)4, giving the molar ration of Tu:Cu as 1.5:1 and 2.25:1, respectively. However, copper(II) complexes can also be formed in the presence of the second ligand. [Cu(Tu)(bipy)2](ClO4)2 (bipy ) 2,2′-bipyridyl),30 [Cu(Tu)(phen)2](ClO4)2‚H2O (phen ) 1,10-phenanthroline),31 and [Cu(Tu)L](ClO4)2 (L ) N,N′-tetramethylene-bis(2pyridinaldimine)30 represent three thiourea-copper(II) structures available from CSD. Therefore, addition of thiourea to a copper(II) salt solution evidently leads to reduction of Cu2+ to Cu+ through a redox process,29c,d but this redox process is apparently subject to subtle influence from the outer environment. On the basis of the above literature results, it is rational to expect that thiourea-copper(II) complexes will be formed on initial addition of thiourea to copper(II) salts solution. A subsequent redox process leads to conversion of thiourea-copper(II) complexes to thiourea-copper(I) complexes. Such conversion can be visually observed via a simple color change of the reaction solution. When thiourea was added to the copper(II) salt solution, which is typically blue, the color was changed to green, indicative of thiourea-copper(II) complex formation. Further addition of thiourea or just leaving the green solution standing for a longer time will cause the solution to become colorless, indicative of conversion into thiourea-copper(I) complexes. The literature study suggested that copper nitrate is unique because its thiourea-copper(II) complex is retained in solution for a remarkably long time compared to other copper(II) salts such as CuSO4, provided that there is no excess thiourea in the reaction mixture.29c Our experimental results indicated that the green color of the Cu(NO3)2Tu reaction solution with CR no more than 2 was persistently kept until the solution was transferred to an autoclave. But when the concentration of Tu was increased such that CR was no less than 3, the reaction solution rapidly became colorless, implying that excess thiourea significantly speeds up the redox process. On the basis of the above observations, one can speculate that different copper complexes were predominant in reactions with different reactant molar ratios before the hydrothermal reaction started. These complexes act as the coordination precursors which will determine the growth process of the copper sulfide crystals.

Covellite Copper Sulfide Submicron Crystals

When molar ratio CR was ranging between 1 and 2, thiourea-copper(II) coordination precursors were predominant in the reaction solution at the beginning. Under hydrothermal conditions, the following reactions may occur (termed as the copper(II) route):

Cu2+ + nTu + xH2O h [Cu(Tu)n(H2O)x]2+ NH2CSNH2 + 2H2O f 2NH3 + CO2 + H2S Cu2+ + H2S f CuS + 2 H+ By contrast, when molar ratio CR fell in the range between 3 and 6, thiourea-copper(I) coordination precursors were predominant in the reaction solution with excess Tu according to the known copper-thiourea structures discussed above. Under hydrothermal treatment, the following reactions may be expected (termed as the copper(I) route):

NH2CSNH2 + 2H2O f 2NH3 + CO2 + H2S Cu+ + nTu h [Cu(Tu)n]+ 2Cu+ + H2S f Cu2S + 2 H+ -e

Cu2S 98 CuS On the other hand, the different coordination precursors in each case may also directly act as copper ion providers to undergo complex reactions to form CuS without releasing free Cu2+ or Cu+. The coordinated Tu ligands may also be direct sulfur sources during decomposition of the coordination precursors under hydrothermal conditions. Although detailed mechanisms could not be realized with the present experimental results, the overall apparent routes may be indicated as follows (termed as the thiourea-copper decomposition route):

[Cu(Tu)n(H2O)x]2+ f CuS + (n - 1)Tu + (x - 2)H2O + 2NH4+ + CO2 [Cu(Tu)n]+ + 2H2O f CuS + (n - 1)Tu + 2NH4+ + CO2 Decomposition of thiourea in the presence of water at a certain temperature to offer sulfur species is a common phenomenon.32 In the copper(II) route, the growth of copper sulfide crystal may be controlled by the release process of sulfur species from Tu, as well as dissociation of thiourea-copper(II) precursors. Formation of CuS was accompanied by vanishing of coordination precursors at the initial stage of hydrothermal treatment, which accounts for the reason why the sticklike product of 1 h reaction is not a single phase as discussed above. Prolonging the reaction time resulted in complete transformation of the coordination precursor into CuS nanoslices as well as improvement of the crystallinity, and congregation of the CuS nanoslices into both ball- and rodlike architectures implies a potential relationship of the final product morphology with the intermediate sticklike species. On the contrary,

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the crystal growth process in the copper(I) route is apparently much more complicated, but the control step most likely relates to the oxidation process of Cu+ to Cu2+. The crucial factors for the formation of final CuS also include the release process of sulfur species and dissociation of thiourea-copper(I) precursors. Therefore, it is understandable that the crystal growth rate is relatively slow in comparison to the former case as discussed before. More complex intermediate species may exist at the initial stage under hydrothermal treatment as shown in the third route (thiourea-copper decomposition route), which made it difficult to identify the unknown materials exhibiting in the XRD patterns in Figure 6a. Increasing the temperature may speed up the growth rate but could not alter the growth process, so the reaction temperature showed more influence on the crystal size than on the crystal shape as mentioned before. It must be borne in mind that the above proposed mechanisms may only represent the predominant process in hydrothermal treatments with different reactant molar ratio CR ranges. Each case should not be excluded completely from the other. The reactions in each case just outline the eventual results without considering other possible intermediate species. Especially for the copper(I) route, vigorous investigations have been carried out to elucidate the sophisticated redox process,25,33 and an exact description of all the possible reactions falls beyond the scope of this paper. However, steady conversion of chalcocite Cu2S into thermodynamically stable covellite CuS at elevated temperature has been evidently accomplished,25,33 which supports the possible mechanism of the crystal growth process in the copper(I) route. Similar investigation has also been applied to other copper sources, such as CuCl2, Cu(CH3COO)2, and CuCO3. It is worth pointing out that CuCl2 exhibited a similar crystal growth and morphology evolution process; however, the remaining copper salts could not give satisfying yields of covellite copper sulfides with the morphology resembling those from Cu(NO3)2 under the present hydrothermal conditions. This is probably due to the fact that Cu(NO3) and CuCl2 can form clear solutions after addition of thiourea and stirring for a reasonable time, while Cu(CH3COO)2 and CuCO3 give precipitates with more complicated color changes on addition of thiourea, indicating an unclear redox and coordination process. As discussed above, formation of different coordination precursors and release of Tu and Cu2+ (or Cu+) are crucial factors determining the crystal growth process. Very recently, Qian and co-worker20 prepared copper sulfide CuS with hollow spheres composed of nanoflakes under hydrothermal conditions from CuSO4 and thiourea at 180 °C for 24 h. This further verified our deduction of crystal growth process because it is known that thiourea-copper(I) complexes can be easily formed from the reaction of CuSO4 and thiourea.29 It is also notable that various nonstoichiometric copper sulfides have been obtained with hydrothermal methods. Xie and co-workers9 prepared Cu9S8 crystals via the reaction between [Cu(NH3)4]2+ and thiourea in aqueous ammonia medium. Subsequent oxidation of Cu9S8 by SnCl4 offered CuS while reduction of Cu9S8 by KBH4 gave Cu7S4. Zhao and co-workers18 reported

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synthesis and assembly of nanowire by digenite phase copper sulfide (Cu9S5) nanoparticles and nanotube and nanovesicles by djurleite phase copper sulfide (Cu1.94S) nanoparticles via an organic amine-assisted hydrothermal process. The common feature of their work lies in that these reactions were carried out in a basic medium, implying that pH value plays an important role in the process of forming copper sulfides with different compositions. Our preliminary experimental results indicated that covellite copper sulfide crystals in form of nanoslices can only be formed in the pH value range between 2 and 7, in agreement with the literature results that conversion from chalcocite Cu2S into covellite CuS was easily achieved at low pH values.25,33 Conclusion We have demonstrated that a viable and effective hydrothermal procedure is established to prepare phase pure covellite copper sulfide submicron crystals at relatively lower temperature without using any surfactants. Three main morphologies of ball-shaped, rodshaped, and chrysanthemum-shaped architectures have been achieved, all of which are composed of CuS nanoslices witha thickness of 20 to 100 nm. The crystal growth and morphology evolution process have been elucidated, and various influencing factors have been systematically investigated by means of XRD, SEM, TEM, ED, EDX, and so on. The reactant molar ratio (CR), hydrothermal reaction time, and temperature represent the most important factors for the morphology evolution, and the coordination precursors play an important role to determine the crystal growth. Two distinct crystal growth processes have been proposed which contribute to formation of different crystal morphologies. Acknowledgment. The financial support of NNSF of China, NSF of Guangdong Province, and the RFDP of Higher Education are greatly appreciated. References (1) Yang, P.; Wu, Y.; Fan, R. Int. J. Nanosci. 2002, 1, 1. (2) (a) Xia, Y.; Yang, P.; Sun Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim F.; Yan, H. Adv. Mater. 2003, 15, 353. (b) Ge, J. P.; Li, Y. D. Adv. Funct. Mater. 2004, 14, 157. (c) Yu, S.H.; Yoshimura, M. Adv. Mater. 2002, 14, 296. (3) Dloczik, L.; Engelhardt, R.; Ernst, K.; Lux-Steiner, M. C.; Ko¨nenkamp, R. Sens. Actuators, B 2002, 84, 33. (4) Chen, X.; Xu, H.; Xu, N.; Zhao, F.; Lin, W.; Lin, G.; Fu, Y.; Huang, Z.; Wang, H.; Wu, M. Inorg. Chem. 2003, 42, 3100. (5) Osakada, K.; Taniguchi, A.; Kubota, E.; Dev, S.; Tanaka, K.; Kubota, K.; Yamamoto, T. Chem. Mater. 1992, 4, 562. (6) Meldrum, C. F.; Flath, J.; Knoll, W. Langmuir 1997, 13, 2033. (7) Wang, S.; Yang, S. Chem. Phys. Lett. 2000, 322, 567. (8) Nair, M. T. S; Nair, P. K. Semicond. Sci. Technol. 1989, 4, 191. (9) Jiang, X.; Xie, Y.; Lu, J.; He, W.; Zhu, L.; Qian, Y. J. Mater. Chem. 2000, 10, 2193.

Qin et al. (10) Blachnik, R.; Mu¨ller, A. Thermochim. Acta 2000, 361, 31. (11) (a) Wang, S.; Wang, W.; Liu, Z. Mater. Sci. Eng. B 2003, 103, 184. (b) Nascu, C.; Pop, I.; Ionescu, V.; Bratu, I.; Indrea, E. Mater. Lett. 1997, 32, 73. (c) Madara´sz, J.; Okuya, M.; Kaneko, S. J. Eur. Ceram. Soc. 2001, 21, 2113. (12) Lindroos, S.; Arnold, A.; Leskela¨, M. Appl. Surf. Sci. 2000, 158, 75. (13) Erokhina, S.; Erokhin, V.; Nicolini, C. Langmuir 2003, 19, 766. (14) Reijnen, L.; Meester, B.; Goossens, A.; Schoonman, J. Chem. Vap. Deposition 2003, 9, 15. (15) Sˇ etkus, A.; Galdikas, A.; Mironas, A.; Sˇ imkiene, I.; Ancutiene, I.; Janickis, V.; Kacˇiulis, S.; Mattogno, G.; Ingo, G. M. Thin Solid Films 2001, 391, 275. (16) Dong, X.; Potter, D.; Erkey, C. Ind. Eng. Chem. Res. 2002, 41, 4489. (17) Lu, J.; Zhou, Y.; Chen, N.; Xie, Y. Chem. Lett. 2003, 32, 30. (18) Lu, Q.; Gao, F.; Zhao, D. Nano Lett. 2002, 2, 725. (19) Zhang, P.; Gao, L. J. Mater. Chem. 2003, 13, 2007. (20) Chen, X.; Wang, Z.; Wang, X.; Zhang, R.; Liu, X.; Lin, W.; Qian, Y. J. Cryst. Growth 2004, 263, 570. (21) Wang, C.; Tang, K.; Yang, Q.; Bin, H.; Shen, G.; Qian, Y. Chem. Lett. 2001, 30, 494. (22) (a) Zhang, Y.; Qiao, Z.; Chen, X. J. Solid State Chem. 2002, 167, 249. (b) Liao, X.; Chen, N.; Xu, S.; Yang, S.; Zhu, J. J. Cryst. Growth 2003, 252, 593. (23) Co´rdova, R.; Go´mez, H.; Schrebler, R.; Cury, P.; Orellana, M.; Grez, P.; Leinen, D.; Ramos-Banrado, J. R.; Rı´o, R. D. Langmuir 2002, 18, 8647. (24) Larsen, T. H.; Sigman, M.; Ghezeibash, A.; Doty, R. C.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 5638. (25) Ewen, J. S.; Franz, G.; Brett, A. S.; Thomas, W. H. Langmuir 1991, 7, 2917. (26) Isarov, A. V.; Chrysocjoos, J. Langmuir 1997, 13, 3142. (27) (a) Szymaszek, A.; Pajdowski, L.; Biernat, J. Electrochim. Acta 1980, 25, 985. (b) Krunks, M.; Leskela¨, T.; Mannonen, R.; Niinisto¨, L. J. Therm. Anal. Calorim. 1998, 53, 355. (c) Ugai, Ya. A.; Semenov, V. N.; Averbakh, E. M. Russ. J. Inorg. Chem. 1981, 26, 147. (d) Kore, R. H.; Kulkarni, J. S.; Haram, S. K. Chem. Mater. 2001, 13, 1789. (28) (a) Vranka, R. G.; Amma, E. L. J. Am. Chem. Soc. 1996, 88, 4270. (b) Griffith, E. H.; Hunt, G. W.; Amma, E. L. J. Chem. Soc., Chem. Commun. 1976, 432. (29) (a) Johnson, K.; Steed, J. W. J. Chem. Soc., Dalton Trans. 1998, 2601. (b) Taylor, I. F., Jr.; Weininger, M. S.; Amma, E. L. Inorg. Chem. 1974, 13, 2835. (b) Stocker, F. B.; Troester, M. A.; Britton, D. Inorg. Chem. 1996, 35, 3145. (c) Bott, R. C.; Bowmaker, G. A.; Davis, C. A.; Hope, G. A.; Jones, B. E. Inorg. Chem. 1998, 37, 651. (d) Bombica, P.; Mutikainen, I.; Krunks, M.; Leskela¨, T.; Madara´sz, J.; Niinisto¨, L. Inorg. Chim. Acta 2004, 357, 513. (e) Piro, O. E.; Piatti, R. C. E.; Bozan, A. E.; Saivarezza, R. C.; Arvia, A. J. Acta Crystallogr., Sect. B 2000, 56, 993. (30) Ferrari, M. B.; Corradi, A. B.; Fava, G. G.; Palmieri, C. G.; Nardelli, M.; Pelizzi, C. Acta Crystallogr., Sect. B 1973, 29, 1808. (31) Ferrari, M. B.; Fava, G. G.; Montenero, A. Cryst. Struct. Commun. 1975, 4, 577. (32) Kuang, D.; Xu, A.; Fang, Y.; Liu, H.; Frommen, C.; Fenske, D. Adv. Mater. 2003, 15, 1747. (33) (a) Brelle, M. C.; Torres-Martinez, C. L.; McNulty, J. C.; Mehra, R. K.; Zhang, J. Z. Pure Appl. Chem. 2000, 72, 101. (b) Drummond, K. M.; Grieser, F.; Healy, T. W.; Silvester, E. J.; Giersig, M. Langmuir 1999, 15, 6637. (c) Orphanou, M.; Leontidis, E.; Kyprianidou-Leodidou, T.; Koutsoukos, P.; Kyriacou, K. C. Langmuir 2004, 20, 5605.

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