Role of Surface Ligands in the Nanoparticle Assemblies: A Case

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Role of Surface Ligands in the Nanoparticle Assemblies: A Case Study of Regularly Shaped Colloidal Crystals Composed of Sodium Rare Earth Fluoride Wei Feng, Ling-Dong Sun,* and Chun-Hua Yan* Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, Peking University, Beijing 100871, China

bS Supporting Information ABSTRACT: Assembly of nanoparticles is a promising route to fabricate devices from nanomaterials. Colloidal crystals are welldefined three-dimensional assemblies of nanoparticles with longrange ordered structures and crystalline symmetries. Here, we use a solvent evaporation induced assembly method to obtain colloidal crystals composed of polyhedral sodium rare earth fluoride nanoparticles. The building blocks exhibit the same crystalline orientation in each colloidal crystal as indicated in electron diffraction patterns. The driving force of the oriented assembly is ascribed to the facetselected capping of oleic acid molecules on {110} facets of the nanoparticles, and the favorable coordination behavior of OA molecules is explained by the steric hindrance determined adsorption based on the studies of the surface atomic structure of nanocrystals and molecular mechanics simulation of OA molecules. The capping ligands also provide hydrophobic interactions between nanoparticles and further direct the oriented assembly process to construct a face-centered cubic structure. These results not only provide a new type of building block for colloidal crystals, but also clarify the important role of surface ligands, which determine the packed structure and orientations of nanoparticles in the assemblies.

1. INTRODUCTION Assembly of nanoparticles (NPs) into superlattices is currently of great interest for nanotechnology.1-4 Colloidal crystals composed of NPs as building blocks are regarded as threedimensional (3D) superlattices with long-range ordered structure. The structures of these assemblies are important because they may influence the properties of colloidal crystals,5 and it is proven to be determined by the charge, size, and shape of NPs, as well as the interactions between building blocks.6 Furthermore, the crystalline orientations of building blocks are also important to the properties of assemblies. As an example, in the assembly of tetragonal FePt NPs, ordered-orientations along [001] are preferred for data storage and permanent magnetic applications, because FePt NPs possess uniaxial anisotropy along the [001] direction with extremely high magnetocrystalline anisotropy constant.7,8 Orientation-ordered assemblies have been achieved with the geometry restriction for NPs with specific shapes9-12 or oriented interactions such as dipole-dipole forces.13,14 Another possible factor influencing the orientation of building blocks are the surface capping ligands adsorbed on the surface of NPs. These ligands will direct the assembly process by their intermolecular van der Waals interactions.15-20 These mentioned factors are generally considered for investigation of the oriented assembly of building blocks, but it is hard to tell which is dominant and crucial to the final packing structures. For example, r 2011 American Chemical Society

dipole-dipole interactions, magnetic interactions, and induced dipolar interactions exist in the assemblies of quantum dots, ironbased magnetic NPs, and noble metal NPs, respectively, together with the interaction between surface capping ligands adsorbed on each of these NPs.12,16,21 To address the individual factors influencing assembly behavior, sodium rare earth fluoride NPs with weakened dipoledipole interactions and weak paramagnetism are selected as a kind of suitable building block model to investigate the role of surface capping ligands. The recently reported synthesis method succeeds in obtaining ideal NPs with polyhedra shape to avoid geometry restriction in the assembly and with narrow size dispersity to achieve desired colloidal crystals.22-28 Having benefited from the transparency and extremely low phonon energy, these sodium rare earth fluorides are regarded as among the best upconversion emission matrices to convert near-IR photons into visible ones, by codoping Yb and Er ions as sensitizer and activators, respectively.29-32 Furthermore, integration of NaREF4:Yb, Er (RE = Y, Gd) NPs into desired assemblies is a necessary process to construct applicable devices for the most traditional application areas such as laser crystals, Received: November 29, 2010 Revised: February 4, 2011 Published: February 28, 2011 3343

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near-IR detectors, 3-D display components, and even solar cells.33-36 In the present work, we construct well-defined colloidal crystals composed of cubic-phased NaREF4 NPs by the solvent evaporation induced assembly method. Confirmed by electron diffraction characterization, all of the NPs have the same crystallography orientation in each colloidal crystal, as induced by the facet-selected capping of oleic acid (OA) molecules on the surface of NPs, and further leads to a face-centered cubic (fcc) close packed structure. On the basis of the surface atomic structure models of NPs and molecular mechanics simulation of coordinated OA molecules, the facet-selected capping of OA molecules is attributed to a synergistic effect of surface atomic geometry and steric hindrance.

2. EXPERIMENTAL SECTION Chemicals. Rare-earth trifluoroacetates were prepared as reported;37 oleic acid (90%, Alpha), oleylamine (OM; >80%, Acros), 1-octadecene (ODE; >90%, Acros), trifluoroacetic acid (99%, Acros), CF3COONa (>97%, Acros), absolute ethanol, and cyclohexane were used as received. Synthesis of NaREF4 NPs. The synthesis of nanobuilding blocks is similar to the pyrolysis method reported.22,38 Different rare earth ions were induced by the corresponding RE(CF3COO)3 precursors. As a typical reaction, 0.78 mmol Y(CF3COO)3, 0.20 mmol Yb(CF3COO)3, 0.02 mmol Er(CF3COO)3, and 1.00 mmol CF3COONa were added into a three-necked flask containing 10 mmol OA, 10 mmol OM, and 20 mmol ODE. The solution was heated to 140 C under vacuum and kept for about 15 min to fully eliminate the species with low boiling point. Then, the solution was heated to 300 C quickly and reacted for 30 min under the protection of Ar. After the reaction, the flask was removed from the heating mantle, and left to cool down to room temperature. 50 mL alcohol was added to decrease the solubility of the products, and the precipitation was collected with centrifugation at 7500 rpm for 15 min. The products were dispersed in cyclohexane and reprecipitated with alcohol to remove the excess organic species surrounding the nanoparticles. The final products were dispersed and kept in cyclohexane solution. Assembly of NaREF4 NPs. Assemblies of NaREF4 NPs are obtained on a copper grid or silicon plate via a solvent evaporation method. A drop of cyclohexane solution of monodispersed NPs was deposited on the substrate and dried under ambient conditions to fully vaporize the solvent. The particle concentration of the started solution was tuned by quantitative dilution of the original solution. Structure Characterization of Assemblies. The copper grid was used for the transmission electron microscope (TEM) observations which are carried on a JEOL 200CX under the working voltage of 160 kV or a Philips Tecnai F30 FEG-TEM operated at 300 kV for selected area electron diffraction (SAED) and high resolution TEM (HRTEM) observations.

3. RESULTS AND DISCUSSION Various methods have been developed to assemble NPs,39-42 but the most convenient and successful route is direct evaporation of solvent,1,43 which will increase the concentration of dispersed NPs and finally form assemblies as precipitates. This method is adopted here to prepare three-dimensional assemblies of NaYF4:Yb,Er NPs. The evaporation process is carried out on a copper grid or silicon plate with fixed area here. Because the surface tensions of solutions with different concentrations are nearly the same, the volume of solution left on the substrate will

Figure 1. TEM images of assemblies composed of NaYF4:Yb,Er NPs obtained with starting concentration of 2 mmol L-1 (a), 5 mmol L-1 (b), and 100 mmol L-1 (c). (d) and (e) are the amplified TEM images of an individual colloidal crystal in (c).

Figure 2. TEM image (a) and selected area electron diffraction pattern (b) of one typical colloidal crystal.

stay the same. Therefore, the amount of NPs left on the substrate is determined by the concentration of the starting solution, which is used as the key factor here to adjust the dimensions of assemblies. The phase and upconversion luminescent spectra of asprepared NaYF4:Yb,Er NPs are confirmed as shown in Figure S1 (Supporting Information). The original solution of NPs has a concentration of about 100 mmol L-1 calculated from the amount of precursors. Varied assemblies with different dimensions are achieved by diluting the original solution. With the starting concentration of about 2 mmol L-1, two-dimensional (2D) assemblies are formed on the copper grid, and hexagonal close-packed symmetry is observed (Figure 1a). On increasing the concentration to 5 mmol L-1, a second layer forms upon the 2D assembly as shown in Figure 1b. When the starting concentration reaches or exceeds 50 mmol L-1, colloidal crystals with 3D ordered structures can be obtained. When the starting concentration is 100 mmol L-1 (Figure 1c,d,e), the colloidal crystals are well developed to show regular shape. These colloidal crystals are interesting to investigate the NPs’ packing behavior and structure. Because of the similarity among rare earth ions, this strategy is further extended to similar assemblies composed of smaller NaGdF4:Yb,Er or larger hexagonal-phased β-NaREF4 NPs, as shown in Figure S2 (Supporting Information). 3344

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Figure 3. Structure models for unit cell of cubic NaREF4 crystal (a), {100} (b), {111} (c), and {110} (d) facets (insets of b, c, and d are the corresponding projection images of the facet). Blue and purple spheres represent F ions and RE/Na ions, respectively.

The size of the colloidal crystals in Figure 1c ranged from 200 to 600 nm. Most of these colloidal crystals possess regular shapes, indicating good “crystallinity”. Upon close examination of a single colloidal crystal as shown in Figure 1e, there is a prominent truncated octahedral shape, which is common for crystals with cubic symmetry.44 Meanwhile, there is a monolayer of 2D assembly existing as background on the copper grid, indicating that the “crystallization” of colloidal crystals and the formation of 2D assemblies took place at different stages during the evaporation of solvent. The SAED pattern of a single colloidal crystal composed of NaYF4:Yb,Er NPs presents a single-crystal like pattern (Figure 2b), which indicates that all building blocks in one colloidal crystal have almost the same orientation. Previous research has proven that this kind of assembly can be obtained by choosing building blocks with regular shapes,9-12,45 in which NPs must pack with fixed orientation because of geometry limitations. For sphere-like NPs, colloidal crystals with specific orientation were obtained from semiconductor quantum dots,4 where the oriented packed structures are known as a result of dipolar interactions among NPs. However, in our case, sodium rare earth fluorides are insulators with weakened dipolar interactions, and the spherical shape cannot provide space limitation, so there must be other factors responsible for the oriented structures. Because the selective adsorption of surface molecules is widely studied for the morphology control of NPs,46-48 and proven to influence their assembly geometry,15-17,49 we considered these molecules to have the main effect in our case. On the basis of the type of reagents used in the synthesis and their coordination ability with rare earth ions, OA molecules are considered to be the main capping ligands here.38 To study the favorite facets for the adsorption of OA molecules, the surface atomic model was used to investigate the packing geometry of OA molecules. R-NaREF4 has a cubic fluorite structure as shown in Figure 3a. F ions, shown in Figure 3 as blue spheres, form a fcc

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Figure 4. Optimized structures with fixed distances of 2.7 Å (a) and 3.8 Å (b) between O atoms from two adjacent OA molecules.

close packing structure, while Na and RE ions, shown as purple spheres, fill randomly in all tetrahedral interstices. {100}, {111}, and {110} are the most common low-index facets for this type of crystal.44 The atomic structures of these facets are shown in Figure 3b,c,d. When OA molecules are coordinated to the surface metal ions, O atoms of OA molecules will take the place of F atoms on the surface for their high affinity with rare earth ions. So, the sites of F atoms determine the possible position of coordinated OA molecules. 2.7 Å and 3.8 Å are two typical distances between F sites in the atomic structures of cubic-phase NaREF4 NPs. We used the MM2 force field to model the optimized geometrical structure of coordinated OA molecules with these two specific distances. For an individual OA molecule as shown in Figure S3 (Supporting Information), two O atoms can occupy two F sites within 2.7 Å, which means that bidentate coordination geometry can be achieved. For two OA molecules, when we fixed the distance between O atoms from different OA molecules as 2.7 Å, the optimized structure is distorted (Figure 4a). Because only the parallel arrangement of OA chains will maximize the packing density and therefore offer the strongest interparticle interactions to direct the orientation of the assembly, this distorted structure is treated here as an unstable state, which means that, if one OA molecule is already settled on the surface, steric hindrance would prevent the nearby F sites from being occupied by O atoms from another OA molecule. If this distance is set as 3.8 Å, two OA molecules would form a stable parallel conformation as shown in Figure 4b. The above studies suggest that the surface F sites with distance of 3.8 Å can be replaced by O atoms from two different OA molecules without interchain steric repulsion. We use these critical distances to model the geometry and priority of OA molecules adsorbed on different low-indexed facets of NaREF4 NPs. For {100} facets, the distance between each pair of F sites is 2.7 Å. This means OA molecules can form 3345

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Figure 6. TEM images and SAED patterns (insets) of assemblies composed of R-NaGdF4:Yb,Er observed along Æ110æ (a) and Æ001æ (b) directions of NPs.

Scheme 1. OA-Directed Oriented Assemblies

Figure 5. Suggested structure models for {100} (a), {111} (b), and {110} (c) facets of cubic phase NaREF4 NPs after the occupation of surface F sites by O atoms from OA molecules (red spheres represent the occupied F sites). The insets of (a), (b), and image (d) are the corresponding images from the top view.

bidentate coordination structures. But when two F sites are replaced by one OA molecule, the other nearby sites will be empty due to the steric hindrance as shown in Figure 4a. Therefore, only 50% of surface F sites can be replaced by O atoms of OA in this case, to form an ordered OA layer with alternate replacement of F sites as shown in Figure 5a. In the case of {111} facets, there are two kinds of F sites marked as 1 and 2 in Figure 3c with the coordination numbers of 3 and 1, respectively. The distances between F sites of the same type are about 3.8 Å, which is too far to utilize two O atoms from one OA molecule. This monodentate coordinated structure leads to the weak OARE interaction with a floating alkyl tail. And the distance between the unused O atom and nearby F sites is less than 3.8 Å, which causes the occupation of the nearby F sites difficult by other OA molecules. As a result, OA molecules tend to replace 50% of surface F sites in group 1 or group 2 (Figure 5b). But for {110} facets, the distance between two adjacent F sites (denoted as 1 and 2 in Figure 3d) is 2.7 Å, where OA molecules act as bidentate ligands. The distance between sites 1 and 3 is 3.8 Å, which is also suitable for the adsorption of two adjacent OA molecules to form a parallel structure and displace all surface F sites (Figure 5c,d). Above all, OA molecules are favorable to cap on the {110} facets of NaREF4 NPs to replace all surface F sites. Because the main interaction between upconversion NPs originates from the van der Waals forces among hydrophobic chains of OA molecules, the facet-selected adsorption of OA molecules will try to form a parallel arrangement to minimize the Gibbs free energy of the system and direct the orientations of NPs in the assemblies. NPs will “touch” each other along OA chains along the {110} direction to construct the assembly with a coordination number of twelve. In this manner, the assembly finally forms the fcc packed structures as shown in Scheme 1 with all NPs packed in an ordered way toward an orientation-ordered colloidal crystal. The NPs are identical, but colors are used to differentiate different packing layers. The cell parameters of the super cell are a = b = c = (2)1/2d (d is the smallest distance

between NPs along the Æ110æ direction), and the orientation of the superlattice is the same as NaREF4 NPs (as shown in the inset of Scheme 1). Æ110æ and Æ001æ are two typical directions of an fcc structure to study the ordered 2D projections.1,14 Along the Æ110æ direction, the projection will be an elongated hexagonal structure. The distance from the centered NP to each of the four nearest NPs is (3)1/2d/2, and the distance between NPs along the elongated axis is d. In the projection along the Æ001æ direction, a tetragonal structure can be found with (2)1/2d/2 as the distance between adjacent NPs. As proof, these two kinds of assembly projections were found from TEM images as shown in Figure 6. The multilayer assemblies are used here because they have a suitable thickness to get TEM images. The diffraction spots of the (1-1-1) and (1-11) facets in Figure 6a are much brighter than others, indicating a zone axis of Æ110æ, which is in accordance with the assembly model in Scheme 1. This elongated hexagonal view is the commonly observed one which is in accordance with the results as shown in Figure 1 and Figure 2, where the directions along adjacent particles and elongated axis are Æ111æ and Æ200æ, respectively. The projection distance between NPs along the Æ111æ and Æ200æ directions are 8.8 and 10 nm, respectively, which are in good agreement with the calculated results as (3)1/2d/2 and d. The other type of tetragonal projection is also found with bright diffraction spots of (2-20) and (220) facets (insets of Figure 6b) with a zone axis of Æ001æ. The measured projection 3346

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Langmuir distance is 6.9 nm in this image, which is also in accordance with the predicted one as (2)1/2d/2.

4. CONCLUSION In summary, we presented here the solvent evaporation induced assembly of NaREF4 NPs. By tuning the concentration of NPs in the starting solution, 3D colloidal crystals with regular shape and fcc packed structure can be obtained. NPs are proven to have the same crystalline orientation in each colloidal crystal by electron diffraction analysis. The driving force for the assembly of NaREF4 NPs is mainly ascribed to the van der Waals interactions between OA molecules which are selectively attached on specific {110} facets of cubic phase NPs. The selectivity originates from the surface atomic structure of NPs and steric hindrance between OA molecules. The results are beneficial for understanding the orientation of NPs in assemblies, and can be extended to other building block systems. ’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental details, additional figures and structure models. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: þ86-10-6275-4179; e-mail: [email protected] (Ling-Dong Sun) and [email protected] (Chun-Hua Yan).

’ ACKNOWLEDGMENT This work is supported by the NSFC (Nos. 20821091 and 20971005). ’ REFERENCES (1) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335–1338. (2) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989–1992. (3) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664–670. (4) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. Rev. Phys. Chem. 1998, 49, 371–404. (5) Zeng, H.; Li, J.; Liu, J. P.; Wang, Z. L.; Sun, S. H. Nature 2002, 420, 395–398. (6) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55–59. (7) Thiele, J. U.; Folks, L.; Toney, M. F.; Weller, D. K. J. Appl. Phys. 1998, 84, 5686–5692. (8) Shima, T.; Takanashi, K.; Takahashi, Y. K.; Hono, K. Appl. Phys. Lett. 2004, 85, 2571–2573. (9) Song, Q.; Ding, Y.; Wang, Z. L.; Zhang, Z. J. J. Phys. Chem. B 2006, 110, 25547–25550. (10) Zhang, Y. W.; Sun, X.; Si, R.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2005, 127, 3260–3261. (11) Chen, M.; Kim, J.; Liu, J. P.; Fan, H. Y.; Sun, S. H. J. Am. Chem. Soc. 2006, 128, 7132–7133. (12) Zeng, H.; Rice, P. M.; Wang, S. X.; Sun, S. H. J. Am. Chem. Soc. 2004, 126, 11458–11459. (13) Srivastava, S.; Kotov, N. A. Soft Matter 2009, 5, 1146–1156. (14) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545–610. (15) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904–13910.

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