From Self-Assembled Cu(II) Coordination Polymer to Shape

Feb 27, 2009 - Synopsis. A Cu(II) coordination polymer has been used as a precursor as well as a sacrificial template to obtain mesostructured CuS...
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CRYSTAL GROWTH & DESIGN

From Self-Assembled Cu(II) Coordination Polymer to Shape-Controlled CuS Nanocrystals

2009 VOL. 9, NO. 5 2457–2463

Mangayarkarasi Nagarathinam, Jialin Chen, and Jagadese J. Vittal* Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543, Singapore ReceiVed December 16, 2008; ReVised Manuscript ReceiVed January 22, 2009

ABSTRACT: A hydrogen-bonded 1D coordination polymer, {[Cu(HSglu)(H2O)] · H2O}n, 1, has been used as a precursor as well as a sacrificial template to synthesize covellite CuS nanomaterials. A remarkable correlation between the repeating patterns of the metal atoms in the coordination polymer with the preliminary shape of the obtained CuS is revealed from a detailed investigation of the effect of experimental conditions on the final products. The formation mechanism of the final mesostructured product has been explained on the basis of the role of the chelating ligand and solvents in the self-assembly process. Introduction Fabrication of zero-, one-, two-, and three-dimensional (0D, 1D, 2D, and 3D) nanocrystalline materials has attracted remarkable attention because of the intriguing size-, shape-, spatialorientation-, and arrangement-dependent properties.1,2 In fact, many methodologies in synthesizing metal sufide nanomaterials, copper sulfide for example, have been very well explored and include solventless and solution thermolysis of single-source thiolate-derived precursors,3,4 sacrificial templating method,5 solution phase reactions,6 solvothermal method,7 ultrasonic and microwave irradiation,8 micelles and microemulsions,9 electrodeposition,10 and so on. Mostly in solution methods, simple metal salts have been used as a precursor in the presence of sulfurating ligands, and to achieve the required size and shape, different surfactants were judiciously chosen. From the above literatures of metal chalcogenides, we can understand that whatever the reactants added, the reaction pathway proceeds most likely through the formation of a coordination complex with respect to the ligating ability of the organic moieties (surfactant, solvent) with the metal ion and then transforms to the required nanomaterials depending on the reaction conditions and on the preferred directional growth of the expected materials.3-9 It is well-known that in addition to the role of coordinating ability, supramolecular noncovalent interactions have also contributed to the formation of the final architecture of the higher dimensional nanomaterials.7e Other than singlesource precursor methods, only in very few methods metal complexes have been utilized as one of the precursors.11 There are very few reports which utilize 1D, 2D, and 3D coordination polymers to synthesize metal oxides; zinc oxide and γ-manganese oxide using hydrothermal synthesis, cadmium oxide by thermal degradation, and microwave-assisted synthesis of nickel nanowires.12 The generation of ultrathin gold nanowires has been recently reported using 1D coordination polymeric structure of gold(I).13 Despite these advances, it is still a challenge to develop a synthetic approach which can predict the shape of the final products with respect to the reactants. Because the formation mechanism of the nanomaterials depends basically on the coordinating ability and the type of coordination complex formed, selection of a preformed complex-the coordination polymer with predetermined orderly arrangement of metal ions and organic ligands itself as a precursor-cum-template-will * Corresponding author. Fax: 65-6779-1691. Tel: 65-6516-2975. E-mail: [email protected].

allow us to understand the mechanism of formation of the corresponding nanostructures. Herein, the hydrogen-bonded one-dimensional zigzag coordination polymer {[Cu(HSglu)(H2O)] · H2O}n, 1, has been used to generate copper sulfide nanocrystals using solvothermal reactions and the probable correlation between the arrangement of metal ions present in the coordination polymer and the shape of the resulting nanomaterials will be discussed. Experimental Section All the chemicals and solvents used were commercially available and used as received. The ligand H3Sglu and the coordination polymer {[Cu(HSglu)(H2O)] · H2O}n, 1, were synthesized according to the reported procedure and on characterization the data matched exactly with the reported results.14 Hydrothermal Reaction. For a typical reaction, the precursor solid 1 (50.0 mg, 0.143 mmol) was treated with thiourea (54.3 mg, 0.713 mmol) in 25 mL of H2O (precursor: 5 sulfur source) in Teflon-lined stainless steel autoclave. The autoclave was then placed in oven at the desired temperature and time. The obtained product was washed several times with water and ethanol and dried under vacuo. The reaction conditions were varied to explore the effect of different reaction parameters on the size and morphology of the product. Effect of Time with Water As Solvent. The above procedure was followed at constant molar ratio (1:5) and constant reaction temperature of 100 °C, but the reaction time was varied at 8, 12, and 48 h. Effect of Temperature with Water As Solvent. The typical procedure was followed at constant molar ratio (1:5) and constant reaction time of 24 h but with varying the temperature from 100 to 130, 160, and 190 °C. Effect of Molar Ratio of Precursor with Water As Solvent. In this procedure, 1:3, 1:5, and 1:10 precursor:thiourea molar ratios were used, but constant reaction time (24 h) and constant reaction temperature (100 °C) were maintained. All the above reactions with the same reaction conditions were carried out using ethylene glycol as solvent. Instruments and Characterization. 1H NMR spectrum was recorded on a Bruker ACF 300 MHz instrument with TMS as internal reference at 25 °C. The identity and the phase of the products were verified by the X-Ray powder diffraction using a D5005 Siemens X-Ray diffractometer with the Cu KR line as the irradiation source (λ ) 1.5417 Å). All diffraction data were acquired in the reflection mode with a step size of 0.016°, conducted at 40 kV and 20 mA, in the 2θ range 2-60°. The sample was either packed tightly on the sample holder or dispersed in ethanol and transferred to a piece of small glass slide and fixed on the holder after the sample was dried. The surface morphology and the distribution of the product were analyzed using FESEM in which the images were acquired using the JEOL JSM-6700F fieldemission scanning electron microscope. The samples were placed over

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Investigation of the effect of time with water as solvent revealed that on varying the reaction time for 8, 12, and 48 h resulted only in the formation of the pure hexagonal covellite phase of CuS (JCPDS 00-006-0464) as confirmed from the PXRD. The stability of the coordination polymer under these reaction conditions (100 °C and 1:5 molar ratio) up to 6 h could be seen from the isolation of unreacted blue crystalline compound, 1, in addition to the CuS of hexagonal phase. In light of this fact, this may be interpreted that the coordination polymer acts as a template in the formation of shape-controlled nanomaterials; i.e., the Cu2+ ions on the surface of the coordination polymer react with the S2- ions generated from thiourea at high temperature (eq 1 below)7e to form CuS, and the next layer starts reacting once the CuS in the surface of the crystals got peeled off to the solution. The reaction could be represented by the following equations Figure 1. XRPD pattern of CuS produced at 100 °C, 24 h with 1:5 molar ratio of coordination polymer to thiourea and the bar graph of the standard from JCPDS data carbon tape attached on the metal holder and coated with a thin layer of platinum using JEOL JFC-1600 auto fine coater to increase their conductivity. On the other hand, EDX was acquired using another model, JEOL JSM-6701F field emission scanning electron microscope. Similar sample preparation was performed for EDX except that no coating of the platinum was required. The size and morphology of the products were determined from HRTEM images which were taken using the JEOL 2010 and 3010 transmission electron microscopes. A drop of the sample dispersed in ethanol was placed on the carbon-coated copper grid of mesh size 300 before it was moved to the sample chamber of the transmission electron microscope for viewing. As for the SAED images, they were captured during the acquisition process of TEM images.

Results and Discussion Hydrothermal reaction of the insoluble coordination polymer, 1, and the sulfurating ligand, thiourea, with a 1:5 molar ratio in water at 100 °C for 24 h in an autoclave resulted in the formation of black product. All the peaks in the powder X-ray diffraction pattern (PXRD) of the sample have been indexed to covellite, the hexagonal phase of CuS (JCPDS 00-006-0464) as shown in Figure 1. The absence of peaks corresponding to the precursor 1, copper oxide, or other phases of copper sulfide indicates the purity of the product. The quantitative analysis of the EDX spectrum reveals that the atomic ratio of Cu to S in the samples is nearly equal to 1.1:1 (see Figure S1 in the Supporting Information), which is very close to our expected stoichiometric ratio of CuS. The typical FESEM image of the as synthesized product given in Figure 2a reveals the formation of hierarchical 3D flowerlike morphology through self-assembly of nanoplates. The thickness of the plate-like petals constituting the flower is at around 25.6 ( 4 nm. The presence of microspheres with some sharp plate-like edges as seen in the TEM image (Figure 2b) indicates that the 3D flowerlike nanomaterials have densely packed arrangement of petals. The average size of flower is at around 1.25 ( 0.2 µm. The interplanar spacing of the parallel fringes in the HRTEM image (Figure 2c) is 0.304 nm, which agrees well with the (102) plane of the hexagonal CuS. Though the flowerlike mesostructure is an assembly of many petals, the regular hexagonal array of diffraction spots in the SAED pattern indicates that the petals are of single-crystalline nature (inset in Figure. 2b). The effect of reaction conditions have been studied by varying the reaction time, reaction temperature, and molar ratio of the starting materials to monitor the growth process and the respective change of size and morphology of CuS.

2NH2CSNH2+3H2O f H2S + 2NH+ 4 +CO3

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[Cu(HSglu)(H2O)]·H2O + H2S f CuS + H3Sglu + 2H2O (2) Equation 2 illustrates the displacement of the ligands by the sulfide ions to form CuS.7e The displacement of the tridentate ligand by the sulfide ions appears to occur spontaneously under hydrothermal conditions with increase in the molar ratio of sulfide ions accompanied by the change of pH of the medium. The kinetic and thermodynamic stability of CuS is likely to favor this reaction. Hence it is pertinent to discuss the probable correlation between the arrangement of metal ions present in the coordination polymer and the shape of the nanomaterials generated. The FESEM and TEM images of the products obtained by varying the reaction time reveal that it forms small size particles of less than 100 nm (see Figure S2 in the Supporting Information) in 8 h and on increasing the reaction time to 12 h, the growth and self-assembly results in the formation of a mixture of big plates and flowerlike morphology as shown in Figure 3. The big plates are in turn composed of self-assembly of many small plates. The average size of the flowerlike particles is 1 ( 0.1 µm. The fingerlike patterns seen on the TEM image (Figure 3b) indicate the presence of plate-like nanomaterials and that the flowers are not well grown. The interplanar spacing 0.304 nm shown in the inset of Figure 3c corresponds to the (102) plane of the CuS and the single-crystalline nature of the plate is visible from the SAED pattern shown as an inset in Figure. 3b. The products formed at 24 h (Figure 2) as discussed earlier, indicate that on further increasing the time from 12 to 24 h, the plates in the self-assembled large plates start growing and consequently the breakage in the plate resulting in hierarchical 3D flowerlike morphology. At 48 h, prolonged heating results in the disintegration of the flower-like mesostructure to plates with the size of 25 ( 5 nm as seen in TEM (see Figure S3 in the Supporting Information). This also demonstrates that reaction for 24 h is the optimum condition to synthesize CuS with 3D flowerlike morphology. Investigation of the effect of temperature with water as solvent revealed that on increasing the reaction temperature from 100 to 130 and 160 °C with a 1:5 coordination polymer:thiourea molar ratio at 24 h also resulted in the formation of pure hexagonal covellite phase of CuS (Figure 4). The improvement in the crystallinity is confirmed by the increase in the intensity of the peaks as the reaction temperature changes from 130 to 160 °C. FESEM image of the samples at 130 °C shows the

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Figure 2. FESEM, TEM images at different magnifications of CuS produced via hydrothermal reaction at 100 °C with 1:thiourea molar ratio of 1:5 for 24 h. SAED pattern is given as inset in the TEM image in (b) and the magnified HRTEM is given as an inset in HRTEM image (c).

Figure 3. FESEM and TEM images of CuS produced via hydrothermal reaction at 100 °C with 1:thiourea molar ratio of 1:5 for 12 h. SAED pattern is given as inset in the TEM image in (b) and the magnified HRTEM is given as an inset in HRTEM image in (c).

Figure 4. XRPD pattern of CuS produced via hydrothermal reaction at different temperatures for 24 h and the bar graphs of the standards from JCPDS data.

presence of particles of two different sizes (see Figure S4 in the Supporting Information). CuS nanomaterials mostly of spherical morphology consisting of small plate-like single-crystal nanoparticles of regular size are isolated on increasing the temperature to 160 °C as shown in Figure 5. The size of the

particle is around 0.628 µm ( 0.2 µm, which is comparatively smaller than that obtained at 100 °C. This might be due to the rate of diffusion and nucleation, which occurs at a faster rate, or the fact that the particles break down to smaller pieces. The SAED pattern for the samples obtained at 160 °C is single crystalline, as high temperature would normally improve the crystallinity of the products. On increasing the reaction temperature to 190 °C, covellite CuS is contaminated with digenite phase of Cu2S as shown in Figure 4. Overall, temperature is absolutely an important factor in determining not only the phase composition but also the size and morphology of the copper sulfide. By changing the molar ratio of precursor to thiourea from 1:3 to 1:5 and 1:10, we could observe the cubic phase of Cu2S, digenite (JCPDS 00-002-1284), in addition to the CuS only at a 1:10 molar ratio (see Figure S5 in the Supporting Information). This might be due to the partial reduction of Cu2+ to Cu+ during the reaction carried at a high concentration of thiourea. On reducing the molar ratio of the precursor to thiourea from 1:5 to 1:3, we observed an interesting complete petal bedlike morphology where small plates self-assemble to form large plates as shown in Figure 6. The thickness of the small plate is 30.2 ( 8 nm. At a molar ratio of 1:10, the high concentration of sulfide source results in an increase in the degree of saturation.15 The behavior of nucleation of CuS is susceptible to the degree of saturation because of its small value of solubility product. This induces the occurrence of nucleation burst with large number of nuclei seeds being formed.6 Though different phases has been observed, formation of many nanosized-rod shaped

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Figure 5. (a, b) FESEM at different magnification and (c) TEM images of CuS produced via hydrothermal reaction with molar ratio of Cu(Hsglu) to thiourea being 1:5 for 24 h at 160 °C.

Figure 6. FESEM images of CuS produced via hydrothermal reaction at 100 °C for 24 h with 1:3 and 1:10 Cu(Hsglu):thiourea molar ratios.

Figure 7. FESEM and TEM images of CuS produced using EG as solvent at 100 °C with a 1:5 Cu(Hsglu):thiourea molar ratio for 24 h. SAED pattern is given as an inset in the TEM image in (b) and the magnified HRTEM is given as an inset in the HRTEM image in (c)

particles of length (41.8 ( 8 nm) and width (16 ( 4 nm) are noted (see Figure S6 in the Supporting Information). Investigation of the Effect of Solvent, Ethylene Glycol (EG), With Time. The black product obtained from EG reaction for 24 h shows the formation of CuS of rice-ball morphology; CuS is formed because of the self-assembly of little rods (“rice grain”) as shown in Figure 7 (see Figures S7 and S8 in the Supporting Information). The size of the CuS (0.495 ( 0.09 µm) is smaller than the CuS produced from water as solvent. This might be due to the more viscous nature of EG, which in consequence would have slowed down the diffusion of ions and assisted the formation of new nucleus before they start to grow at the surface of the nuclei.5a,16,17 Disintegration of the particles has been observed by increasing the reaction time to 48 h, as noted before in water. As investigated, reaction conditions in EG at 100 °C with a 1:5 molar ratio for 24 h would be the optimum condition to

obtain rice-ball morphology. The effect of solvent definitely plays a role in the hierarchical assembly of CuS, hence the shape of the overall morphology could be controlled by varying the viscosity of the solvent, which affects the nucleation time and growth period.17 The formation of one single pure hexagonal covellite phase even at high molar ratio 1:10 and at high temperature lead us to conclude that solvent plays an influencing role in determining the phase. The particles of roughly spherical structure and irregular shape have been produced at precursor molar ratios of 1:3 and 1:10, respectively (see Figures S9 and S10 in the Supporting Information). Formation Mechanism of the CuS with Different Morphology. The formation of flowerlike morphology can be rationalized on the basis of the above investigations on varying the reaction through the stages as shown in Scheme 1. On increasing the reaction time, rods or platelike structures self-

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Scheme 1. Probable Formation Mechanism for the Nanomaterial CuS

assemble to form large plates and the plates in the self-assembled large plates then start to grow; in consequence, the plates break or pop out of the big plate to result in 3D flowerlike morphology. Though preformed Cu-thiolate, Cu-cystine complexes have been used to synthesize CuS, the orderly arrangement of metal and ligands in the crystal lattice of the starting materials have not been considered.3,4,7i Because we are dealing with the role of predetermined arrangement of metal atoms and ligands of the coordination polymer in the formation of obtained nanomaterials, it is relevant to discuss (i) the crystal packing of this compound briefly, (ii) whether it is possible to predict the shape of the nanomaterial from the coordination polymer used, and (iii) whether it is possible to achieve the expected theoretical shape through experiments or does the insoluble coordination polymer-the precursor-act as a template in the generation of nanomaterials. Each copper atom in the coordination polymer, 1, is bonded through phenolato oxygen atom, secondary amine nitrogen atom, carboxylate oxygen atom of the ligand and to one aqua ligand. The square pyramidal geometry of the metal atom is completed by the side chain carboxylate oxygen atom of neighboring ligand and leads to the formation of zigzag 1D coordination polymer with the nearest Cu · · · Cu distance of 6.098 Å (Figure 8). The hydrogen of the phenoxy oxygen and the aqua ligand of one chain forms hydrogen bond with the carboxylate oxygen of neighboring 1D polymeric chain results in 2D hydrogen-bonded sheetlike structure. The nearest Cu · · · Cu distance between two chains in the layer is 5.96 Å as shown in Figure 8.14 The Cu · · · Cu distance between the interlayer is 11.94 Å, as shown in structure A of Scheme 2. The thickness of the CuS plate formation depends on the size of the interlayer empty space for the sulfide ion to pass through and on the rate of diffusion of the nuclei. From Scheme 2, it may be understood that the

Figure 8. Portion of the {[Cu(Hsglu)(H2O)] · H2O}n, 1.

2D

sheetlike

structure

of

large interlayer distance (11.94 Å) and the steric hindrance by the ligands from both the layers provide fewer options for growth along the y direction and can support 2D growth along the x and z axes. Though many other factors also contribute to the shape of the nanomaterials, the possible structure that could be predicted from this coordination polymer on careful investigation of the packing pattern is 2D plate or rod or sheetlike structure (structure D in Scheme 2). Though different experimental conditions resulted in the isolation of various morphologies of CuS, platelike particles of smaller size, densely packed to less densely packed hierarchical 3D flowerlike, and petal bedlike morphologies are the major morphologies encountered in these reactions. So, the basic morphology obtained from our reaction is the platelike morpholgy and there exists only the variation in the thickness and the size of the plates. This could easily be accounted for by the concentration of the reactants, rate of nucleation, diffusion, and growth. For example, at 130 and 160 °C, the rate of release of sulfur from thiourea and the rate of diffusion of sulfide ions increase which lead to the faster reaction rate resulted in relatively small size of plates which self-assemble to form nanomaterials of spherical morphology as compared to the flowerlike particles at 100 °C.4 Reducing the molar ratio to 1:3 from 1:5 at the optimum temperature of 100 °C results in less sulfide source and hence the displacement of the ligands by the sulfide ions takes place at a slower rate, which assists in the formation of small plates at the earlier stage and then self-assembles to form a large plate with the petal bed. This also supports the formation of flowerlike morphology at a 1:5 molar ratio, where the nucleation rate is expected to be faster than 1:3; nucleation in combination with growth would have resulted in breaking of the petal bed and hence forming flowers. The formation of platelike structure as the basic morphology clearly demonstrates that insoluble coordination polymer acts as a template in the generation of nanomaterials. The isolation of unreacted crystalline coordination polymer in the reactions less than 8 h reiterates that the nucleation occurs on the surface of the insoluble coordination polymer which acts as a template. The isolation of thin crispy plate or sheetlike nanomaterials (TEM image in Figure S11 of the Supporting Information) from the solid-gas reaction, i.e., reaction of solid coordination polymer with H2S gas (solventless reaction) also indicates that the insoluble 2D coordination polymer acts as a self-sacrificial template in the generation of CuS. The SEM of the solid-gas reaction shows the presence of no well-defined morphology, which might be due to the aggregation of thin sheets. On using the Debye-Scherrer’s formula for the peaks in the respective XRPD, the grain size was determined to be 15.0 nm.18 It is also worth mentioning here that the coordinating and hydrogen-bonding ability of the ligand on the surface of the CuS nanoplates is believed to be responsible for the selfassembly process leading to the hierarchical shape in the later

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Scheme 2. (A) Packing Pattern of the Hydrogen-Bonded Coordination Polymer Showing (B) Two Layers Only with Cu Atoms; (C) Possible Directions for the Sulfur (yellow balls) Attack on the Cu(II) Atoms (single layer is shown for clarity); (D) Possible 2D Structure Arising from This Reaction

stage of the reaction.7a This could be explained by introducing a change in the pH of the reaction mixture by varying the molar ratio of the thiourea. The pH value at molar ratio of 1:5 is determined to be pH 5, which is believed to be the optimum pH to produce CuS with flowerlike morphology. As the concentration of thiourea increases from 1:3 to 1:10, the pH of the reaction mixture is found to decrease. The change in the pH of the solution could probably have disrupted the hydrogenbonding interactions that contribute to the self-assembly of the nanoparticles at higher dimensions.19 The optimum temperature 100 °C is thought to be playing an influential role in favoring self-assembly of these plates to higher dimensional hierarchical assembly in addition to the pH and the formation of final product depends on the rate of reaction and the suitable sulfur source, which is also confirmed by the solid-solution reaction at room temperature. The reaction of coordination polymer with aqueous solution of Na2S at different molar ratios (1:3, 1:5, 1:10) with different pH levels resulted in the isolation of the CuS nanoparticles of less than 30 nm with no well-defined morphology (for synthesis, see the Supporting Information). Varying the reaction conditions, such as increasing the reaction time and heating or cooling the reaction with Na2S as sulfur source, also resulted in similar products. The formation of nanoparticles might be due to the fast reaction rate (the reaction could be completed within 5 min) (TEM image in Figure S12 of the Supporting Information). It is worth mentioning here that synthesis of CuS from copper salt, glutamic acid, and thiourea under the same reaction conditions resulted in CuS but with a mixture of morphologies in low yield. This also demonstrates that isolation of a single morphology is very facile from the coordination polymer method. In summary, the shape of the obtained CuS could be

determined from the arrangement of metal atoms and ligands in the coordination polymer. The observed product formation on varying the temperature and molar ratio clearly reveals that the insoluble coordination polymer acts as a precursor-cumsacrificial template in the generation of the shape-controlled nanomaterials, only on the extreme conditions, it led to the smaller size particles that still consist mainly of small plates or rods. Conclusions The insoluble nature of a coordination polymer with predetermined arrangement of metal atoms and ligands has been used for the first time as a precursor as well as a sacrificial template to synthesize copper sulfide with predefined shape. The effect of several reaction parameters on the morphology and size of the CuS has been investigated and the optimum conditions for petal-bed-like, flowerlike, and rice-ball-like morphologies have been successfully achieved. The detailed investigation on varying the reaction parameters and the successful isolation of petal-bed-like morphology demonstrates once again that the orderly arrangement of metal atoms in the coordination polymer can be transferred to the nanomaterials by fine-tuning the reaction conditions. The self-assembly of the shape of the nanomaterials obtained at the initial growth to 3D flower and rice-ball-like morphologies support that the noncovalent hydrogen bonding interactions from the reduced Schiff base ligand derived from L-glutamic acid of the coordination polymer is believed to play an important role in achieving defined shape, which can successfully be controlled by the pH of the reaction mixture. It is believed that the careful investigation of the packing pattern of the coordination polymer in the

Self-Assembled Polymer to Shape-Controlled Nanocrystals

crystal lattice and judicious choice of the coordinating ligand would surely lead to synthesize nanomaterials with inexpensive, eco-friendly, simple synthetic procedure and with definite shape and size. Further work on the use of suitable coordination polymeric architecture to synthesize metal sulfides and selenides with predictable shapes is in progress.

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Acknowledgment. We thank NUS for their generous financial support (Grant R-143-000-371-112). Supporting Information Available: PXRD pattern, EDX analysis spectrum, SEM and TEM images (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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