From Winter Snowflakes to Spring Blossoms: Manipulating the Growth

Oct 20, 2007 - and pressures), and shorter reaction time. In addition, temporal growth of the dendrites can be readily followed and manipulated in...
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CRYSTAL GROWTH & DESIGN

From Winter Snowflakes to Spring Blossoms: Manipulating the Growth of Copper Sulfide Dendrites Wen Pei Lim,†,‡ Hong Yee Low,‡ and Wee Shong Chin*,† Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543, and Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602

2007 VOL. 7, NO. 12 2429–2435

ReceiVed June 30, 2006; ReVised Manuscript ReceiVed May 20, 2007

ABSTRACT: A simple and facile method for the preparation of copper sulfide dendritic structures in high yield under mild conditions is described. This new approach encompasses many advantages over the conventional solvothermal preparations of dendrites in terms of product quality (better morphology control, good monodispersity, and high yield), reaction conditions (lower temperatures and pressures), and shorter reaction time. In addition, temporal growth of the dendrites can be readily followed and manipulated in this preparation via time-resolved microscopic analysis. The crystalline nature of the dendrites was confirmed using high-resolution transmission electron microscopy and selected area electron diffraction. Several factors, including the reaction temperature, time, relative concentration between the reactants, and amine chain length, were identified to be important and investigated in detail. It was demonstrated that the dendritic structures are favored at high ethylenediamine (EDA) and low tributylphosphite (TBPT) concentrations in general. Unprecedented multilayer crystal growth originating from the dendritic core was observed through varying the ratios of the reagents. On the basis of these carefully controlled experiments, a plausible formation mechanism of the dendrites was suggested and discussed. To the best of our knowledge, this is the first report on making crystalline semiconductor dendrites with such full control and tunability of morphology and size. Introduction Although monodispersed synthesis of copper sulfide nanorods and nanoplatelets was recently reported,1 a general method of producing copper sulfide in various morphologies and different phases has not been explored. Copper sulfides are known to exist in many different phase compositions because of slight variation in their stoichiometry. Nonstoichiometric CuxS, x ) 1–2, have a wide range of well-established and prospective applications, e.g., as p-type semiconductors,2 solar cells,3 superionic materials,4 and in many chemical sensing applications.5 Sakamoto et al. have recently demonstrated that CuxS can be potentially used as nanometer-scale switches,6 as it is a mixed Cu ionic/electronic conductor. Because of its unique optical and electrical properties, copper sulfide is also widely applied as thin films2,7 and composite materials.3,8 Dendritic structures are an attractive type of supramolecular structures. They are generally observed in nonequilibrium growth systems and provide a natural framework for the study of hierarchical organization.9 Earlier insights on the mechanism of dendrite formation have been derived mainly from the studies of metallic systems.9c It is of interest to verify these mechanisms for the growth of semiconductor dendrites, as such information will be useful for the recent interest on morphology-controlled synthesis. In the past few years, different methods have been used for preparing dendritic structures, most of which were based on the solvothermal method.10–14 Although the solvothermal method is versatile in generating dendritic structures (judging from the diversity of materials that have been reported), the process necessarily involves heating under pressure for long hours in autoclaves, and this renders the systematic study of the growth mechanism difficult. Therefore, the development of a mild and more controlled method for creating such novel architectures will be of general interest. * Corresponding author. Tel: 65-6516-8031. Fax: 65-6779-1691. E-mail: [email protected]. † National University of Singapore. ‡ Institute of Materials Research and Engineering.

In this paper, we report an elegant way to direct the growth of CuxS crystals into dendritic structures in a mild method using difunctional amines. Using our approach, various dendritic CuxS structures can be rapidly generated in high yield and we could perform a series of time-resolved experiments to follow the crystal growth. “Seasonal changes” of the crystal growth from winter snowflakes to spring blossom structures have been observed. We elucidate in this paper the effect of relative concentration and chemical nature of the diamine on the final morphologies of the dendritic crystals. Experimental Section Materials and Synthesis. All starting materials were purchased from commercial sources and used without further purification. The copper (I) thiobenzoate (CuTB) precursor was first prepared according to the literature method.15 All procedures for the preparation of copper sulfide dendrites were carried out using standard techniques under a nitrogen atmosphere. Degassed ethylenediamine (EDA) (4 mL) was added under stirring to a degassed solution of CuTB (0.04 g) in tributylphosphite (TBPT) (0.5 mL). The flask was immersed into a hot oil bath (100/ 120 °C) for 10 min and a black precipitate was formed. Ethanol was added to the reaction mixture and the precipitate was centrifuged, washed thoroughly with ethanol, and dried in vacuum overnight. In the experiment, the reaction temperature, relative concentration between the reactants, and diamine chain length were varied. The molar ratio between the EDA and CuTB (denoted as [EDA]/[CuTB]) was varied from 75 to 900 and the molar ratio between TBPT and CuTB (denoted as [TBPT]/[CuTB]) was varied from 3 to 18. Control experiments using diamine with different chain lengths were carried out using the same procedure. Characterization. The powder X-ray diffraction (XRD) pattern was acquired using a Bruker GADDS D8 with Cu KR radiation (λ ) 0.151478 nm). Transmission electron microscopy (TEM) was performed on a Philips CM100 microscope operating at 100 kV, and highresolution transmission electron microscopy (HRTEM) was performed on a Philips CM300 FEG instrument with an acceleration voltage of 300 kV. One drop of the nanocrystals dispersed in ethanol solution was placed on a 200 mesh carbon-coated copper grid and the grid was dried in vacuum before analysis. The scanning electron microscopy (SEM) images were obtained by using a JEOL JSM6700 microscope, operating at 10 amps and 15 kV. The sample was coated with gold to

10.1021/cg0604125 CCC: $37.00  2007 American Chemical Society Published on Web 10/20/2007

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Figure 1. (a–c) Representative TEM, SAED, and HRTEM images of CuxS snowflakes produced at 100 °C for 10 min; (d, e) SEM images showing the flatness and texture of the CuxS snowflakes. improve contrast. Tapping mode atomic force microscopy (AFM) was performed with Digital Instrument Nanoscope III under air and at room temperature.

Results and Discussion (A) Morphology and Structural Characterization. We have shown previously that monodispersed Ag2S nanocubes and crystals with controlled dimensions can be obtained from an amine-assisted decomposition of AgTB precursor.16 Recently, the reaction was found to work similarly for the CuTB precursor.17 By further changing to difunctional amine such as ethylenediamine (EDA), we report here that the CuxS crystals are now growing into dendritic structures. Thus, snowflakelike structure was obtained when the CuTB precursor was decomposed in EDA at 100 °C. Representative TEM and SEM images (Figure 1) show clearly that each CuxS crystal has a unique flat snowflakelike structure with six symmetrical dendritic trunks extending radially from the center. Each trunk has two sides of branches growing outwards at ∼60° to the trunk and in the same plane. The length of the trunk and branches ranges from 1.5 to 5.0 µm and 0.1 to 2.0 µm, respectively, whereas the thickness of each arm is about 100–300 nm. These well-defined dendrites can be obtained as the exclusive form in high yield under our experimental conditions, although broken flakes with less than six trunks were occasionally observed. The good crystalline nature of the snowflakes was confirmed by HRTEM and selected area electron diffraction (SAED) measurements (plots b and c in Figure 1). The branching-out areas of the crystals, however, were too thick for HRTEM lattice analysis. We could not confirm if the branches grew from defect or twinning sites. The XRD patterns of the dendrites can be fitted to a mixture of roxbyite Cu1.75S and digenite Cu1.8S phases (Figure 2). It is well known that Cu2–xS crystals often exist in a variety of phases that mostly consist of closed-packed arrays of sulfur atoms with Cu atoms distributed differently throughout the interstices.18a,b Displacive transformation, in which the closed-packed sulfur matrix is maintained, is facilitated by the high mobility of Cu atoms.18c Thus, nonstoichiometric copper sulfide is often formed

Figure 2. Representative XRD patterns of the crystals prepared in (a) ethylenediamine, (b) 1,3-diaminopropane, and (c) 1,4-diaminobutane.

and has been utilized as superionic conductor or p-type semiconductor.2 We also studied the surface topography of the snowflakes with tapping-mode AFM. In Figure 3a, snowflake crystal with one broken arm is shown. Fine details of the trunks and branches can be clearly seen in this mode of measurement. Cross-sectional scans revealed that the thickness of each snowflake gradually increases from the branches toward the center of the trunks and also toward the center of the crystal core. The thickness of different dendritic trunks, as well as the thickness of their two side branches, is largely similar. The height of the crystal core and trunk tip of a 5 µm snowflake is ∼310 and ∼120 nm, respectively. The topography images suggest that the growth of these snowflake structures originates from the center. (B) Monitoring the Growth of Dendrites. To understand the formation of the CuxS dendritic crystals, we report in the following the influence of several parameters (reaction time,

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Figure 3. Representative AFM images and cross-sectional scans of a CuxS snowflake obtained at 100 °C.

temperature, relative concentration of EDA and TBPT) to the evolution of this unusual structure. Temporal morphological evolution at 100 °C was followed by SEM measurements, as shown in Figure 4. Formation of fairly uniform dendritelike crystals (∼1.5 µm) can be observed as soon as the solution was heated to ∼3 min. These grow further into larger and more refined structures, with well-defined six-armed snowflakes discernible after ∼10 min. Interestingly, the snowflakes gradually smooth out (i.e., the trunks and branches grow into similar thickness) upon prolonged heating at the same temperature (Figure 4d). Similar growth is observed at higher reaction temperature (120 °C), but smoothed snowflakes (with larger size and thickness) are observed after 5 min in this case (Figure 5b). This confirms that the formation of snowflake architectures is the result of a delicate kinetic process, as less refined or smoothen structures are readily produced at a slightly different rate of reaction. The eventual smoothening of the snowflake crystals is indicative of lower energy architecture, confirming that dendritic snowflake structure is a nonequilibrium system.9 It seems clear that the growth is initiated from the center of the crystals and extends outwards to form the trunks and later the branches of the snowflakes. These results suggest that the dendritic structures are grown through a diffusion-limited process from the crystal center rather than through an oriented attachment of nanoparticles.19 On the other hand, the formation of 2D dendritic structures in an anisotropic manner in this case also indicates a possible templating effect of the EDA ligands. In the following sections C and D, we will investigate the role of EDA in more details.

(C) Factors Contributing to the Formation of Dendrites. To investigate the role of EDA, we vary the [EDA]/[CuTB] ratio at constant [TBPT]/[CuTB] ratio and temperature. SEM images shown in Figure 6 ([TBPT]/[CuTB] ) 9) illustrate how the dendritic structures evolve with increasing EDA. At low EDA concentration ([EDA]/[CuTB] ) 75), only platelike particles with irregular shapes are obtained (Figure 6a). Mixtures of dendrites and irregular particles are observed when [EDA]/ [CuTB] is increased to 150 (Figure 6b). Well-defined snowflake structures are observed when the ratio is increased to 300 (Figure 1d) and 600 (Figure 6c). At even higher EDA content, multilayer growth becomes possible as shown by little spring blossoms at the crystal centers (Figures 6d–f). When the constant [TBPT]/[CuTB] ratio was decreased to 3 or increased to 18, it was found that an accompanying adjustment of the [EDA]/[CuTB] ratio for a well-defined dendrite formation was required (Figure 7). Thus, the size and shape of the crystals, and their subsequent evolution to form snowflakes, is controlled by the proportions of the three components as shown in Figure 8. Hence, our investigations reveal that snowflake formation is favored at high EDA and low TBPT concentration in general. The regularity of the snowflake morphology is, however, dependent on the delicate balance between the amounts of these components used experimentally (comparing Figures 7d and 1d). TBPT in this case acted as a coordinating ligand to the precursor and thus stabilizes the decomposition of CuTB in EDA.17 The role of EDA molecule in assisting the growth of dendrites is clearly established, although the exact molecular interaction is still unknown at this stage. The “spring blossoms” observed at

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Figure 4. SEM images showing the temporal evolution of the CuxS dendritic crystals prepared at 100 °C: (a) 3, (b) 5, (c) 10, and (d) 20 min. Note that the scale bar for a and b is 2 µm but that in c and d is 5 µm.

Figure 5. SEM images showing the temporal evolution of the CuxS dendritic crystals formed at 120 °C: (a) 3, (b) 5, and (c) 10 min.

the crystal core at high EDA content further suggests the role of EDA in directing the nucleation/crystallization at the crystal centers. In our effort to understand the molecular influence of EDA on this growth process, we have repeated the preparation using diamines with longer chain length. When EDA was replaced with 1,3-diaminopropane, only poorly defined dendritic structures were obtained. On the other hand, faceted CuxS particles of ∼260 nm sizes were produced instead when 1,4-diaminobu-

tane was used (Figure 9). Because longer chain diamines have higher conformational freedom while EDA is well-known to be a strong chelating agent, we could postulate that the coordinating ability of EDA would have an influence on the growth of the snowflakes. On the other hand, XRD patterns of the CuxS crystals prepared from different diamines revealed slightly different mixtures of phases formed (Figure 2). Although a mixture of roxbyite and digenite has been detected for the dendrites, it appears that the

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Figure 6. Representative SEM images of CuxS snowflakes prepared with constant [TBPT]/[CuTB] ratio (9) and different [EDA]/[CuTB] ratios at 100 °C for 10 min. [EDA]/[CuTB] ) (a) 75, (b) 150, (c) 600, and (d) 900. (e, f) Zoom-in images of d showing the blossomlike crystal core.

Figure 7. Representative SEM images of CuxS crystals formed at 100 °C for 10 min, with (a) [TBPT]/[CuTB] ) 18, [EDA]/[CuTB] ) 300; (b) [TBPT]/[CuTB] ) 18, [EDA]/[CuTB] ) 600; (c) [TBPT]/[CuTB] ) 3, [EDA]/[CuTB] ) 150; and (d) [TBPT]/[CuTB] ) 3, [EDA]/[CuTB] ) 300.

digenite phase in the mixtures reduces as the chain length of the diamine increases such that only roxbyite phase was obtained for the faceted particles produced with 1,4-diaminobutane. It has been reported that crystal phase can be controlled by the kinetic or thermodynamic formation of different seeds at different temperatures,20 or by controlling the injection rate and temperature modulation during preparation.21 Although there has been no clear indication in the literature on which of the two CuxS phases is more stable thermodynamically, we

believe that the roxbyite phase is metastable at small crystallite size (see the Supporting Information). Formation of nonthermodynamically stable phase has often been reported for nanoscale samples,20–24 e.g., the synthesis of TiO2 consistently resulted in anatase nanoparticles, which will transform to the stable rutile phase upon reaching a critical size.22 More recently, Barnard et al. has reported the importance of surface chemistry in effecting the critical size for anatase-to-rutile transition.23 There has also been an earlier report on different phase

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kinetics; when supersaturation is increased sufficiently, particle diffusion starts to dominate the crystal growth leading to dendritic growth. Because we have found that amine will catalyze the decomposition of MTB precursors (M ) Ag, Pb, Cu, Cd, In, etc.) in general,16,17,27 saturation of monomers is expected at high amine content. The crystal formation reaction will become kinetically driven at high monomer content and thus nonequilibrium dendritic structures will be produced.

Figure 8. Effect of CuTB/EDA/TBPT mole fractions on the CuxS snowflake formation. Compositions richer in CuTB are limited by the solubility of CuTB in TBPT.

formation through the steric effects of the stabilizers during crystal growth.24 Thus, we believe that the chelating/templating effect of EDA molecule onto the surface of the growing crystals is probably playing a major role here. (D) Role of EDA and the Reaction Mechanism. The formation of dendritic structures is usually explained by a diffusion-limited monomer-cluster aggregation (DLMCA) model.9 This model anticipates that, because of their Brownian trajectories, monomers generated far from a central cluster cannot penetrate deeply into a cluster without intercepting a cluster arm. The arms thus effectively screen the interior from the flux of incoming monomers, hence the monomers stick irreversibly at first contact with the exterior site to form dendrites. In the case of our CuxS dendrites, however, the simple DLMCA model cannot account for the increasing thickness toward the center as observed in our cross-sectional AFM scans (Figure 3), the formation of 2D structures, as well as the multilayer growth into spring blossoms at high EDA content (Figure 6). Interestingly, the formation of 2D and 3D dendritic platinum structures in micellar surfactant and liposome solutions, respectively, has been reported and their formation was attributed to the surfactant-templated growth.25 Thus, in order to explain the differential and multilayer growths, we propose to modify the aggregation model by taking into account a saturated content of monomers present around the growing clusters. The factor of supersaturation has already been known to influence the overall complexity of snow crystals.26 Thus, when supersaturation is low, the snow crystal growth is dominated by attachment

The growth of anisotropic crystals has been reported in systems containing organic polyamine with N-chelation properties such as triethylenetetramine and EDA, but the exact mechanism and the role of the polyamine are still unknown.28 Some reports suggested that initial coordination of EDA on the metal cation forms a molecular template that favors the subsequent growth of anisotropic structures.28d–g In our preparation, it is speculated that the role of EDA is more complicated because a molecular precursor is used here. We have found also that although dendrite formation was observed when the CuTB–TBPT–EDA homogenous mixture was heated to 100 °C, no dendrites were formed when we inject CuTB–TBPT solution into EDA at 100 °C. This supports our postulation that initial coordination of EDA with the CuTB precursor has occurred. Indeed, addition of EDA to the CuTB–TBPT solution produced a new absorption band at 620 nm. This initial coordination of EDA is believed to have increased the decomposition rate of the precursor and hence a higher concentration of monomers. This further explained the difference in the crystal phase and morphology of the crystals obtained from diamine with longer chain lengths. Thus, we believe that the role of diamine EDA in this preparation is several-fold: (i) coordinate to the CuTB precursor prior to decomposition, (ii) catalyze and lower the decomposition onset of the precursor, and (iii) serve as a shape-directing templating group for the growing crystals. The exact nature of role (iii) is still unclear at the moment, but we have successfully induced the growth of PbS dendrites using similar approach.27 It is interesting to note that reactions with diamines of more than two carbon units (1,3-diaminopropane and 1,4-diaminobutane in this case) do not produce unique dendritic shapes. We believe the origin of this nonequilibrium architecture is derived from a kinetically controlled reaction, hence factors that influence the rate of monomer generation: temperature, stability

Figure 9. Representative SEM images of the crystals prepared with (a) 1,3-diaminopropane and (b) 1,4-diaminobutane; in both cases, crystals were prepared at 100 °C, [diamine]/[CuTB] ) 600, [TBPT]/[CuTB] ) 9, and a reaction time of 10 min.

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of precursor, relative amount of the catalytic reagent, etc., are parameters that can be optimized to achieve good morphology control. Conclusion We report here a new and simple approach to prepare CuxS dendrites under mild conditions. The successful preparation of these dendrites exemplifies the exquisite shape control that can be achieved through identifying the appropriate surface capping and careful tuning of the growth conditions. This new approach encompasses many advantages over earlier reported solvothermal preparations of dendrites, certainly in terms of better morphology control, good monodispersity, and high yield of the crystals obtained. Using this simple method, shape evolution of the dendrites under different conditions can be followed readily to provide more insights to the general understanding of the formation mechanism. Supporting Information Available: Representative XRD patterns of the roxbyite Cu1.75S nanoplates with approximate diameters and thicknesses of (a) 62 and 16 nm, (b) 34 and 13 nm, and (c) 23 and 10 nm, respectively. The simulated diffraction pattern from the JCPDS databases is plotted for comparison. This material is available free of charge via the Internet at http://pubs.acs.org.

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