J Nanopart Res (2015) 17:108 DOI 10.1007/s11051-015-2919-3

RESEARCH PAPER

Surface energy-driven growth of crystalline PbS octahedra and dendrites in the presence of cyclodextrin–surfactant supramolecular complexes Pradip Kumar • Whi Dong Kim • Seokwon Lee Dennis T. Lee • Kangtaek Lee • Doh C. Lee

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Received: 4 September 2014 / Accepted: 17 February 2015 / Published online: 25 February 2015 Ó Springer Science+Business Media Dordrecht 2015

Abstract PbS crystals of cubic, octahedral, and dendritic shapes are synthesized in an aqueous solution that contains supramolecular complexes of b-cyclodextrin (CD) and hexadecyltrimethylammonium bromide (CTAB). The morphology of the PbS crystals depends on the concentration of CD or CTAB in the reaction solution; for example, the branched dendritic structures evolve with an appropriate molar ratio of CD/CTAB supramolecular complexes and reaction time. Regardless of the CD/CTAB molar ratios, octahedral PbS crystals are observed at all compositions of CD/CTAB for the reaction times of 1–5 h, while self-assembled branched/dendritic structures are obtained only for CD/ CTAB molar ratios of 0.5, 1, and 2 after a prolonged reaction, e.g., for 24–48 h. Systematic investigation

P. Kumar
W. D. Kim
S. Lee
D. T. Lee
D. C. Lee (&) Department of Chemical and Biomolecular Engineering, KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea e-mail: [email protected] P. Kumar e-mail: [email protected] P. Kumar Center for Materials Architecturing, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea K. Lee Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul, Korea

reveals that both reaction time and CD/CTAB molar ratio are responsible for self-assembled branched/dendritic structures of octahedral crystals. Keywords PbS
Cyclodextrin/CTAB supramolecular complex
Crystals
Octahedral
Branched/dendritic structures

Introduction The architectural control of inorganic nano/microcrystals is enabled through the interplay between interfacial energy of the exposed surface facets of the crystals and surfactant molecules (Hwang et al. 2011; Yin and Alivisatos 2005). The scheme has allowed for anisotropic growth of crystals whose unit cells are isotropic, e.g., cubic zinc blende (Jung et al. 2012; Koh et al. 2010; Lee et al. 2011; Tang et al. 2006). Despite the significant progress in colloidal synthesis of anisotropic nanocrystals (Kim et al. 2014a, b), identifying a reaction parameter combination for a desired morphology in a given material system has yet relied on a more-or-less combinatorial search. Pb chalcogenide is a class of materials whose stock value has gone up as their relatively large Bohr exciton radius and symmetry in electron and hole effective masses have highlighted the promise of the materials for their use in photovoltaics, photodetectors, and optical switches (Choudhury et al. 2005; Ellingson et al. 2005; Fu and Tsang 2012; McDonald et al. 2005;

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Woo et al. 2014). In particular, PbS nanocrystals (NCs) have been a crux in the field of optoelectronic applications (Talapin et al. 2010). Various colloidal approaches have become available for the sizecontrolled PbS NCs. In the past few years, the focus has expanded to control of the morphology of PbS crystals with various shapes including spheres, rods, octahedra, plates, multipods, flower-like shapes, dendritic, and star-shaped structures, whether in the dimensions of nanoscale or micrometer scale (Lee et al. 2014; Li et al. 2010; Qiao et al. 2007; Qiu et al. 2011; Quan et al. 2008; Querejeta-Ferna´ndez et al. 2012; Wang et al. 2000; Wang et al. 2003, 2008, 2011; Zhao and Qi 2006). Breakdown of the symmetry in morphology may result in asymmetric electron–hole wavefunction overlap integral, leading to interesting photophysical properties, e.g., improved carrier multiplication efficiency, compared to their isotropic NC counterpart (Padilha et al. 2013). PbS crystals can be synthesized via various approaches such as hydrothermal, solvothermal, microwave, electrodeposition, and surfactant-assisted methods to obtain the different shapes (Lee et al. 2014; Qiao et al. 2007; Qiu et al. 2011; Quan et al. 2008; Querejeta-Ferna´ndez et al. 2012; Wang et al. 2003, 2008, 2011; Zhao and Qi 2006). Yet, because of the isotropy of the PbS crystal unit cell, the development of a facile and effective method for creating shape-controlled PbS structures is far from complete. Successful synthesis of PbS crystals of varying shapes requires understanding and control of surface energies of the crystal facets. Cyclodextrins (CDs), as a class of water-soluble and nontoxic cyclic oligosaccharides with a hydrophobic cavity and hydrophilic rims formed by hydroxyl groups, have been extensively investigated in host– guest chemistry for construction of versatile supramolecular aggregations owing to their special hydrophobic cavities (Szejtli 1982). CD is a well-known host of a multitude of small molecule guests (e.g., H2O, NH3, NH4?, and C6H6) via host–guest interaction. Figure 1 shows the molecular structure of b-CD and its inclusion complex with hexadecyltrimethylammonium bromide (CTAB). Recently, CDs have been extensively used in the surface functionalization of novel metal gold nanoparticles: for example, CDs drive the formation of nanoparticle assemblies via host–guest interactions (Huang et al. 2009, 2010). However, there are only a few reports for the use of CD/surfactant

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Fig. 1 b-cyclodextrin molecular structure and its inclusion complex with CTAB

inclusion complex to induce assembly and surface functionalization of semiconductor nanocrystals (Depalo et al. 2006; Dorokhin et al. 2010; Hou et al. 2005). Thus, CD/surfactant supramolecular inclusion complex can be used as a structure-directing agent in the semiconductor nanomaterial fabrication because it can interact with solid surface and selectively adsorb on specific facets of the crystals to control crystal growth direction. In this work, we report the formation of octahedral, branched, and dendritic PbS structures in the aqueous solutions of b-CD/CTAB supramolecular inclusion complex. CTAB is a cationic surfactant, which has positive-charged head group and long aliphatic hydrophobic chain, and can form supramolecular inclusion complex with neutral b-CDs molecules via hydrophobic interaction. The effect of molar ratio of CD/CTAB inclusion complex and growth reaction time on the morphology of PbS structures is evaluated and discussed. CD/CTAB-assisted growth enables a facile, highly reproducible synthesis of octahedral PbS crystals and their self-assembled branched/dendrites structures.

Materials and methods All chemicals, such as CTAB, b-cyclodextrin (CD), lead acetate trihydrate (Pb(OAc)2
3H2O), thioacetamide (TAA), and acetic acid, were purchased from SigmaAldrich and used as received. In a typical procedure, 30 mL of deionized water, 5 mL of aqueous CTAB, and CD (0.05 M) were mixed in a round-bottom flask. Under constant stirring, 4 mL of 1 M aqueous acetic acid, 2 mL of 0.5 M aqueous lead acetate trihydrate, and 2 mL of aqueous 0.5 M thioacetamide were added to the

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mixture at room temperature. After mixing all the components, the mixture was heated up to 80 °C. This led to the formation of a dark brown colloidal solution, indicating the formation of PbS NCs. The colloidal suspension of NCs was collected at different reaction times (1, 5, 24, and 48 h) by centrifugation and washed with water. The obtained PbS products were used for further study. The morphology of the as-prepared PbS products was characterized by a scanning electron microscope (SEM, Nova230, 10 kV) and a transmission electron microscope (TEM, FEI Tecnai F20, 200 kV). The crystallinity of the product was examined using an X-ray diffractometer (XRD, D/MAX-RB (12KW)).

Results and discussion Figure 2 shows that all samples prepared with varying CD/CTAB ratios have octahedral PbS crystals after the reaction time of 1 h. PbS products are isotropic at all CD/CTAB ratios. The CD/CTAB inclusion complex does not seem to render the crystals growth anisotropic; however, the growth rate seems to be affected by the molar ratio of CD/CTAB. Average size of octahedral PbS crystals, analyzed from SEM images (Fig. 2), is found to increase from *100 to 170 nm as CD/CTAB molar ratio increases from 0 to 1. This size variation is most likely due to relatively weaker passivation of CD/CTAB complex on the (111) facets of PbS compared to CTAB alone. In general, the growth rate of nanocrystals depends strongly on the surface concentration of surfactant (Oh et al. 2010). Unpassivated crystal surfaces allow fast penetration of adatoms and accelerate crystal growth. The external diameter of b-CD is around 1.53 nm (Murthy and Geckeler 2001), which is four times larger than (111) lattice spacing of PbS (0.34 nm). Therefore, the bulky CD/CTAB complexes do not fully cover the (111) surface of PbS crystals during the PbS growth, while the less bulky CTAB by itself can adhere to the surface. Notably, as CD/CTAB ratio increases to as high as 2, small crystals are formed possibly as a result of random assembly of the CD/CTAB complex (Fig. 2d). Figure 2e shows a representative TEM image for the case of CD/CTAB molar ratio being 0, which confirms that uniform octahedral PbS crystals are indeed observed. Furthermore, the crystalline nature is confirmed by HRTEM (Fig. 2f). The lattice spacing is determined to be

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0.30 nm, which is close to (200) lattice spacing of bulk PbS, hinting the single-crystalline octahedral PbS and preferential crystal growth in the {100} direction. Figure 3a shows the XRD patterns of octahedral PbS crystals for the reaction time of 1 h. All diffraction peaks are indexed to be a face-centered cubic (fcc) structure of PbS (a = 0.5931 nm, JCPDS # 00-0050592). It is worth noting that the intensity ratio of (111) to (200) peaks (in current work) is much higher (e.g., 5.94 for CD/CTAB = 0) than that in the bulk PbS (0.95), indicating that our PbS crystals have a large portion of {111} facets (Fig. 3b). In order to investigate the effect of reaction time, we examined the PbS crystals grown for longer reaction times. Figure 4a–d shows the XRD patterns and SEM images of octahedral PbS with the variation of CD/ CTAB molar ratios for the reaction time of 5 h. The intensity ratio of (111) to (200) peaks is very high (*6.2–33) in all samples as obtained for the reaction time of 1 h (see Fig. 3b), which indicates that octahedral PbS morphology remains unchanged up to the reaction time of 5 h. Inset of each figure shows the corresponding SEM image of octahedral PbS crystals. SEM image analysis shows that average crystal size increased from *200 to 350 nm with the increase of CD/CTAB molar ratios from 0 to 2. The trend suggests that the reaction time affects only crystal size, but not morphology, of PbS crystals in the initial reaction stage. Figure 5a–d shows the XRD patterns and corresponding SEM images of the as-obtained PbS products after reaction for 24 h. For the CD/CTAB molar ratio of 0 (Fig. 5a), the intensity ratio of (111) in relation to (200) peaks is higher (*3.7) as found for shorter reaction time of 1–5 h. In contrast, the ratio is low (\2) for the CD/CTAB ratios of 0.5, 1, and 2. These XRD patterns are well supported by corresponding SEM images of PbS crystals. For the molar ratio of 0 (i.e., only CTAB), there is no change in octahedral PbS morphology, while for the molar ratios from 0.5 to 2, some PbS crystals start turning into branched structures (inset of Fig. 5b–d). These results indicate that molar ratios of CD/CTAB and reaction growth time could change the morphology of the PbS product. The effect of reaction time and molar ratios on the growth of PbS crystals becomes more evident after a reaction time of 48 h. In the absence of CDs molecules, i.e., CD/CTAB = 0, octahedral PbS crystals with the size of 200 nm to 1 lm is observed (Fig. 6a). In contrast, the presence of CD/CTAB complex (for molar ratios of

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Fig. 2 a-d SEM images of octahedral PbS crystals synthesized with varying molar ratios of CD/CTAB: a 0, b 0.5, c 1, and d 2. e, f TEM and HRTEM images of PbS crystals (CD/CTAB = 0) at reaction time of 1 h

0.5, 1, and 2), branched and dendritic structures are clearly observed (Fig. 6b–d). These SEM observations of branched/dendritic structures are well supported by observed XRD patterns of same PbS products (Fig. 7). For molar ratio of 0, the intensity ratio of (111) to (200) peaks is *4, hinting the formation of the octahedral PbS similarly to the reaction time of 1–24 h (see Fig. 3b). However, the ratio approaches unity, in the presence of

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CD/CTAB inclusion complex (for molar ratios of 0.5, 1, and 2) (Fig. 7b–d). The peak ratio changes indicate the transmorph of PbS crystals from octahedral to branched/dendritic structures. Above XRD and SEM analysis of PbS crystals confirmed that the composition of CD/CTAB inclusion complex plays an important role in growth and self-assembly of crystals and their final morphology (Fig. 8).

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Fig. 3 a XRD patterns of PbS crystals synthesized with different CD/CTAB ratios for the reaction time of 1 h. b Intensity peak ratio of I111/1200 as a function of CD/CTAB molar ratio at different times

Fig. 4 a–d XRD patterns of PbS crystals at different molar ratios 0, 0.5, 1, and 2, respectively, for the reaction time of 5 h. Inset shows the corresponding SEM images of octahedral nanocrystals. Scale bar = 100 nm

In order to clarify the role of CD/CTAB inclusion complex in the formation of octahedral and branched/dendritic PbS structures and to understand their formation mechanism, we performed a control experiment under identical experimental conditions, whereas we used only CDs instead of CTAB or CD/ CTAB supramolecular inclusion complexes. The XRD pattern of the as-obtained PbS products showed very low intensity ratio of (111) in relation to (200) peaks

(0.24) than the bulk PbS (0.95), indicating that our products are {100}-rich. Indeed, SEM investigation reveals that the PbS crystals are mostly cubic with six {100} facets. Based on above experimental findings, we propose a growth mechanism for obtained PbS crystals. Growth mechanisms of octahedral and other PbS structures, such as cubic, stars, and dendritic, have been investigated in previous reports (Lee et al. 2014,

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Fig. 5 a–d XRD patterns of PbS crystals at different molar ratios of 0, 0.5, 1, and 2, respectively, for the reaction time of 24 h. Inset shows the corresponding SEM images of PbS crystals

Qiu et al. 2011; Quan et al. 2008; Querejeta-Ferna´ndez et al. 2012; Wang et al. 2003, 2008, 2011; Zhao and Qi 2006). It is widely accepted that truncated octahedron nuclei is the building unit of different shaped PbS nanocrystals and relative growth rates of six (100) and eight (111) facets are responsible for the formation of various nanoscale geometries. In addition, it is well known that surface energies associated with different crystallographic planes are usually different and a general sequence can be elucidated as c{111} \ c{100} \ c{110} (Wang 2000). The shape of NCs is profoundly affected by the ratio between the growth rates along the {100} and {111} directions. When no capping agents are used, the extremely fast growth along the eight equivalent {111} directions of the tetradecahedral nuclei results in the formation of cubic-shaped particles. If the growth perpendicular to {100} facet is preferred, such faces eventually disappear and resulting morphologies are eight-faced octahedral with {111} facets. Figure 9 illustrates a possible growth mechanism for cubic and octahedral crystals and their structural evolution into branched/dendritic structures. In Fig. 9a, growth of cubic-shaped crystal is presented in the presence of only CD molecules. The CD molecule used in this work is neutral b-CD, which cannot selectively

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adsorb on any facets of octahedral nuclei, and crystal growth will favor the {111} direction, resulting in the cubic-shaped crystals. This result suggests that the presence of only CDs molecules has no impact on the crystal growth. In contrast, CTAB is a cationic surfactant, which can selectively stabilize the {111} faces of octahedral nuclei, containing Pb or S only. Since their ionic head groups can strongly interact with the charged {111} faces rather than the uncharged {100} faces, which contain mixed Pb/S, the interaction between Pb and –N(CH3)? 3 ions can significantly elevate the activation energy of the {111} faces. This would consequently result in a preferential growth at the {100} facets in comparison to the {111} facets, resulted in the formation of octahedral crystals. Figure 9b summarizes the formation mechanism of octahedral PbS crystals: the effect of reaction time and CD/CTAB inclusion complex on the final morphology. At the initial reaction time of 1 h, octahedral PbS crystals (as shown in Fig. 2) are observed for all compositions of CD/CTAB and no change in their morphology up to the reaction time of 5 h (as shown in Fig. 4). However, in the longer reaction time of 24–48 h, CD/CTAB composition significantly influences the final morphology of PbS structures. With only CTAB (CD/CTAB = 0) as a capping agent, there is no change in the morphology of

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Fig. 6 SEM images of PbS products at different CD/CTAB molar ratios for the reaction time of 48 h

octahedral PbS crystals, but in the presence of CD/ CTAB inclusion complex, octahedral crystals begin to cluster into six-branched/dendritic structures (Fig. 6b– d). The six-branched structures corroborate the idea that octahedral crystals serve as seeds for the assembly process (Querejeta-Ferna´ndez et al. 2012). Further, the formation of branched/dendrites structure also can be explained by the inherent anisotropy of crystal structure or crystal surface reactivity as the driving force for the particle assembly (Jones 2004; Kim et al. 2014a, b; Tang et al. 2000, 2006). In principle, neighboring particles prefer to bind on the same crystal facets, e.g., {111} binds to another {111}. Such orientation would allow them to minimize the interface energy (Harfenist et al. 1997; Lu et al. 2004). In addition, the axial anisotropy associated with the presence of electrical dipoles in the crystals is inherent to many nanoscale materials and implicated in self-assembly processes (Shanbhag and Kotov 2006; Shim and Guyot-Sionnest 1999; Wang 1998). Therefore, nanocrystals are driven to attach to the end of growing patterns when they have

Fig. 7 XRD patterns of PbS crystals for the reaction time of 48 h

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Fig. 8 XRD pattern of the as-synthesized PbS prepared only in the presence of cyclodextrin. Inset shows the corresponding SEM image of cubic PbS crystals

the largest dipole moment along the (100) axis and observed branched structure. Now a question arises: why do NCs self-assemble into branched/dendritic structures in the presence of CD/CTAB complex instead of simple larger mesocrystals which they self-assemble into in the presence of only CTAB? In order to explain the possible formation mechanism of observed branched/dendritic PbS structures, we explore the driving force of self-assembly by CD/CTAB complex system. It is well known that double-layered CTAB can stabilize the colloidal nanocrystals in polar solvents, since polar head group Fig. 9 Schematic illustration of possible mechanism of PbS crystal growth of truncated octahedral seeds containing six {100} and eight {111} crystallographic faces into a cubic crystals in the cyclodextrin solution and b octahedral crystals and their self-assembled dendritic nanostructures in the aqueous solution of CD/ CTAB inclusion complex

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of outer layer prevents aggregation of nanocrystal by electrostatic repulsive force (Nikoobakht and ElSayed 2001). In this sense, in the presence of only CTAB, the PbS seeds could assemble into larger octahedral crystals, assisted by passivated CTAB double layer. On the other hand, in the case of CD/ CTAB complex (CD/CTAB = 0.5 and 1), this double layer was cleaved as CTAB’s hydrophobic tail slides into CD cavity (Fig. 10) (Huang et al. 2010). At this moment, the hydrophobic chain of CTAB is exposed in the polar solvent due to the long chain length (2.2 nm) compared with CD cavity height (0.79 nm) (Mohanty et al. 2011). Therefore, we believe that the hydrophobic attraction between alkyl chains plays a key role for self-assembly. However, in the case of CD/CTAB = 2, this CTAB/2CD complex does not necessarily have the hydrophobic attraction for selfassembly, since two CDs effectively cover the alkyl chain of CTAB (Fig. 10c). The outer rim of CD is fully hydrophilic, and thus hydrophobic attraction was ruled out for self-assembly (Jiang et al. 2011). Jiang and coworkers recently demonstrated experimentally that surfactant/2CD complexes were assembled by hydrogen bond between CDs in the part of surfactant/2CD complex, while hydrophobic or electrostatic interaction was ruled out (Jiang et al. 2010). We speculate that the hydrogen bonds between CDs are responsible for the assembly of PbS NCs.

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Fig. 10 Schematic of CD/CTAB complex interaction with PbS crystal surface at different CD/CTAB molar ratios

Shape-controlled synthesis of inorganic NCs, e.g., quantum dots, has been extensively studied because of their facet-dependent physical and chemical properties including electrical conductivity, photocatalytic activity, and molecular absorption (Chiu et al. 2012; Huang et al. 2012; Kuo et al. 2011). For example, octahedral Cu2O NCs bounded by the (111) facets are highly conductive than Cu2O cubes enclosed by (100) faces. Rhombic dodecahedral gold NCs are much more catalytically active than gold cubes. Our approach to the synthesis of PbS crystals with fine shape control would translate to the growth of PbS and lead chalcogenide NCs of designed properties.

Conclusions In summary, we have demonstrated a simple hydrothermal approach for the synthesis of novel octahedral and branched/dendritic PbS crystals in the presence of CD/CTAB supramolecular complex. Molar ratio of CD/CTAB and reaction time are found to play a significant role in the formation of sizecontrolled octahedral crystals and their self-assembled branched/dendritic structures. The message underlying the experimental results is that the interaction between surfactant complex and nanocrystal surface relies on the charge distributions. For self-assembly of PbS nanocrystals, CD/CTAB complex plays a key role by hydrophobic attraction or hydrogen bond depends on the CD/CTAB ratio. On a related note, the controlled growth of octahedral and other shaped

PbS crystals can be exploited in their use in photocatalytic or (opto) electronic applications. Acknowledgments This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy in Korea. (No. 20133030011330 and No. 20133010011750). The work was also supported by the National Research Foundation (NRF) grant funded by the Korean government (Grants NRF-2014R1A2A2A01006739).

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