Controlling the Formation and Structure of Nanoparticle Superlattices

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Controlling the Formation and Structure of Nanoparticle Superlattices through Surface Ligand Behavior Marco A. L. Cordeiro,*,† Edson R. Leite,‡ and Eric A. Stach† †

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States Department of Chemistry, Federal University of Sao Carlos, 13565-905 Sao Carlos, SP Brazil

‡

S Supporting Information *

ABSTRACT: The tailoring of nanoparticle superlattices is fundamental to the design of novel nanostructured materials and devices. To obtain specific collective properties of these nanoparticle superlattices, reliable protocols for their selfassembly are required. This study provides insight into the selfassembly process by using oleate-covered CeO2 nanoparticles (cubic and polyhedral shapes) through the correlation of experimental and theoretical investigations. The self-assembly of CeO2 nanoparticles is controlled by tuning the colloid deposition parameters (temperature and evaporation rate), and the ordered structures so obtained were correlated to the Gibbs free energy variation of the system. The analysis of the interparticle force contributions for each structure showed the importance of both the effective ligand mean size and its Flory− Huggins parameter in determining the total potential energies. Additionally, the roles of ligand solubility and effective mean size were used to understand the formation of specific superlattice phases as a function of temperature and ligand accommodation in the arrangement. Furthermore, the face-to-face interactions between nanoparticles were correlated to the type of exposed crystallographic facet in each particle.

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INTRODUCTION The hierarchical assembly of nanoparticle (NP) building blocks into ordered superstructures is one of the most promising techniques for producing new functional materials.1,2 Although several technologies have emerged that utilize individual NPs, superlattices can offer further unique physical and chemical properties, including new optical,3,4 magnetic,5 catalytic,6 electronic, and photonic7 devices and materials. In general, the structural ordering of NPs is created by the bottom-up approach of self-assembly (SA), which is based on the spontaneous organization of particles during the drying of an NP colloid. The spontaneity of this process is related to the decrease in the Gibbs free energy of the system, which can be promoted either directly through interparticle forces (van der Waals, electrostatic, magnetic, molecular, and entropic) and/or indirectly (e.g., magnetic field, templates, etc.).8,9 Although indirect SA provides interesting results, direct assembly excludes additional complex variants by dealing only with the NP’s building blocks and thermodynamic parameters.9 In fact, the understanding of the mechanisms of self-assembly is not trivial, and the interparticle interactions can be quite complex because of their interdependence.8 Inasmuch as the properties of supperlattice structures are dependent on their architecture, the NP stacking process plays a fundamental role. Usually, isotropic particles have a tendency to organize in face-centered cubic (fcc) or hexagonal close packing (hcp) arrangements, both of which have the same packing factor (∼0.74) and coordination number (12).10 Thus, © XXXX American Chemical Society

there is only a small difference in the free energy between these structures (on the order of ∼10−3kT) for each particle, with a slight bias toward the fcc structure.11,12 On the other hand, when anisotropic NPs pass through an SA process, more intricate arrangements can be achieved as a result of the interparticle forces involved,10,13,14 and lower packing factors can be obtained as a result of the particle shape complexity and interactions.10 Several studies have shown that it is possible to create superlattice phase diagrams, which can be used to describe reliable synthesis protocols. Generally, one varies either the temperature or evaporation rate of the colloid during the SA process or one selects different NP shapes or sizes when constructing superlattice phase maps of this type.8 In this study, we describe the arrangement of metal oxide NPs during the SA process, correlating the temperature, drying rate, NP shape, and ligand solubility with the formation of structures and phases. For this purpose, CeO2 NPs were synthesized by a two-phase hydrothermal method in a microwave oven. On the basis of the amount of surfactant included during the reaction, two different shapes (cubes and polyhedral) were produced. By controlling the SA parameters, it was possible to attain different assemblies, as described below. Hence, this study provides a fundamental understanding of how Received: August 13, 2016 Revised: September 23, 2016

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DOI: 10.1021/acs.langmuir.6b03026 Langmuir XXXX, XXX, XXX−XXX

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RESULTS Figure 1 shows TEM images of the polyhedral (Figure 1a,b) and cubic CeO2 NPs (Figure 1c,d) synthesized in this study.

the control of simple physiochemical parameters affects the resulting superlattice morphologies.

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EXPERIMENTAL SECTION

Synthesis. The CeO2 NPs were created using the synthesis protocol of previous studies,15 adapted to microwave oven heating, instead of furnace heating. Using microwave irradiation has the advantage of increasing the reaction kinetics, giving better yields and the possibility of good control over both NP size and shape.16 In a typical nanoparticle synthesis procedure for polyhedral NPs, 25 mL of a 0.02 mol/L cerium(III) nitrate (Aldrich, 99.99%) aqueous solution was transferred to a 100 mL Teflon-lined autoclave with a ceramic pressure jacket. Then, 25 mL of toluene (Tedia, HPLC grade), 1.5 mL of oleic acid (OA) (Aldrich, 90%), and 0.25 mL of tert-butylamine (Aldrich, ≥99.5%) were added under ambient conditions without stirring, respectively. The sealed autoclave was then heated at a rate of 50 °C/min to 180 °C in a microwave oven (Synthos 3000, Anton Paar, 1500 W), kept at this temperature for 3 h, and then cooled to room temperature naturally. After the reaction, a two-phase system was obtained, and the brown supernatant was removed and centrifuged once to remove the solid impurities. Subsequently, NPs were precipitated by the addition of ethanol and submitted to cycles of ultrasonication, centrifugation, and solvent exchange. This procedure was performed eight times in order to remove as much free OA from the solution as possible. The purified nanoparticles could then be easily dispersed in nonpolar solvents (e.g., toluene, hexane, and chloroform). For the SA experiments, the NPs were dispersed in toluene. For the synthesis of the cubic CeO2 NPs, the same previous synthetic and purification procedures were performed, with the exception being the greater amount of OA added (4 mL). All materials used in the synthesis procedure described were used as received. More details about the synthesis are given in the schematic diagram of Figure S1 in the Supporting Information (SI). SA and Sample Preparation. The NP shape factor, evaporation ratio, and temperature were studied as the SA variables. For the evaporation study, two different, simple systems were produced. The first was set up by simple drop casting of the NP colloid onto a 3−5nm-thick lacey carbon substrate supported on standard TEM nickel grids and left to dry at room temperature (RT): this is hereafter called the normal mode. The second sample system was also created by drop casting the NP colloid on the same kind of TEM grids. However, immediately after deposition, another TEM grid was placed over the system to delay the solvent evaporation (Figure S2). Then, the sample was left to dry for 1 h at RT, and the bottom grid (where the colloid was originally dropped) was used for characterization. This last approach of delaying solvent evaporation is named the sandwich (sdw) grid method. To study the influence of the temperature on the previous experimental setups, the SA process was performed at two additional temperatures. One was realized by placing the NP colloid in a cold bath at 269.15 K and then drop casting, as before. For the lowest analysis temperature, another approach was taken. Instead of cooling the NP colloids, the temperature of the TEM grid was lowered. By using a flask filled with liquid nitrogen (N2(liq), 77.35 K), the TEM grids were cooled by immersion for 5 min. Immediately after removing the TEM grid from the N2(liq), the same NP colloid deposition method was performed. More details about the SA procedure are given in the schematics of Figure S2. Additionally, even though the deposition occurred at 77.35 K, all samples prepared via this method will be referred to by the temperature of 178 K (the freezing point of the toluene). Additional inferences on the temperature variation of the CeO2 colloid during the deposition are found in the SI and Figure S3. Characterization. The resulting samples were characterized by transmission electron microscopy (TEM), high-resolution electron microscopy (HRTEM), selected-area diffraction (SAED), and high annular dark field−scanning transmission electron microscopy (HAADF-STEM). A JEOL 2100F field emission transmission electron microscope operated at 200 kV was used for these experiments.

Figure 1. Typical TEM images of the as-synthesized NPs: (a, b) Polyhedral CeO2 NPs. (c, d) Cubic CeO2 NPs.

From the images, it is possible to identify the resulting morphology of CeO2 polyhedral NPs, which have a prevalence of {111} and {200} exposed facets and a mean size of 6.8 nm (Figure S4). The experimental HRTEM image (Figure S4a) and its simulation (Figure S4b) showed a good fit with the input structure (Figure S4c,d). The synthesis with a larger amount of OA resulted in cubic CeO2 NPs as a result of the stabilization of the {200} facets by the OA15,17 and a diagonal mean size of 7.5 nm (Figure S5). The HRTEM image simulation (Figure S5b) of an experimental cubic NP (Figure S5a) implied a cubic-like NP having primarily {200} exposed facets and soft {220} edges and {111} vertices. The arrangements of the NPs were characterized by HAADF-STEM, which uses incoherent electrons that are scattered to a high angle (>50 mrad) for image formation. The resulting signal is strongly dependent on the average atomic number of the atoms, and the contrast easily distinguishes NP overlays of a few layers. This is because dynamical effects and defocus affect this image approach less than standard TEM bright-field image approaches. As a result of these features, this technique provides images that can be more suitably interpreted than (HR)TEM images. This can be seen clearly in Figures S6−S9, which compare the same area of these samples by both HAADF-STEM and HRTEM. Furthermore, this technique has the advantage of being able to resolve the organization of a few layers of regular nanoparticles without the need for a tilt series (as would be required in TEM bright-field mode). This is because their relative locations can be easily identified by the thickness/NP stacking contrast. The sequence of HAADF-STEM images in Figures 2 and 3 shows the SA of polyhedral and cubic CeO2 NPs, respectively, at different temperatures and using different methods. At room temperature (Figures 2a,b and 3a,b), it was not possible to B

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Figure 2. (a−f) HAADF-STEM images of SA polyhedral CeO2 NPs, according to the normal method (left column of images) and swd method (right column of images), and the temperatures of assembly (RT, 269.15 K, and 178 K: first, second, and third lines of images, respectively). The corresponding electron diffraction pattern is inserted in each image.

Figure 3. (a−f) HAADF-STEM images of SA cubic CeO2 NPs, according to the normal method (left column of images) and swd method (right column of images), and temperatures of assembly (RT, 269.15 K and 178 K: first, second, and third lines of images, respectively). The corresponding electron diffraction pattern is inserted in each image.

identify an ordered SA of NPs by any method, but the analysis of the assembly structure performed with the NPs colloid at 269.15 K resulted in the first evidence of an ordered arrangement of NPs. Figures 2c,d and 3c,d show the SA of polyhedral and cubic CeO2 NPs, respectively, at 269.15 K, with the noticeable presence of two main structures of fcc and hcp. Both of these structures can be distinguished from each other according to the NP packing (Figures S10 and S11), by a direct interpretation of the HAADF-STEM images. Inasmuch as SA is a spontaneous process, there is a decrease in the Gibbs free energy of the system. The variation of Gibbs free energy for the SA (ΔG(SA)) can be represent as a function of the enthalpy variation (ΔH(SA)), entropy variation (ΔS(SA)), and temperature (T) so that

commonly negative in the SA processes, in which smaller amounts of energetic microstates available are represented by the arrangements of the NPs superstructures. Consequently, the spontaneity of the SA process is guaranteed by a negative value of enthalpy variation (correlated to interparticle forces).19 Hence, the negative values of Gibbs free energy can be promoted by decreasing the term “− TΔS(SA)” by lowering the temperature. In our study, the SA process is promoted mainly by van der Waals forces, according to the balance between attractive and repulsive forces. At RT, only disorganized polyhedral and cubic NP structures were observed, independent of the assembly method used. This suggests that ordered SA structures were not thermodynamically favorable because the term “− TΔS(SA)” was not small enough for a spontaneous arrangement. Furthermore, the samples produced by delaying solvent evaporation (sdw method) reduced the possibility of hidden spontaneous thermodynamic SA by kinetic factors. On the other hand, when the deposition process was performed with the NP colloid at lower temperatures, SA was observed. In this case, the negative value for the variation of Gibbs free energy is reached because the negative value of ΔH(SA) exceeds the positive term − TΔS(SA), which was reduced by the lower

ΔG(SA) = ΔH(SA) − T ΔS(SA)

(1)

At first glance, the SA process is thermodynamically discouraged by the entropic term. Despite the possibility of entropic ordering at a high volume fraction of particles,8 spontaneous processes usually have a positive entropy variation because of the promotion of more energetic microstates available at higher temperatures, resulting in a disordered arrangement of the NPs.8,18 However, the entropy variation is C

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consists of a simple cubic (sc) structure (Figure 4c) with the {200} face-to-face in the stacking. In addition to the identification of the structural identification of the self-assembled nanoparticles, the orientation of each NP in the arrangements (NP−NP orientations) could also be identified. For the polyhedral NPs, HRTEM analysis of groups of particles showed that the systems were predominantly oriented such that faces {200}−{111} were aligned (Figure S13). Likewise, the cubic NPs could be also analyzed according to their NP−NP orientation, and three patterns were observed. The first (type 01) is related to an fcc/ hcp (ABC/ABA stacking) structure and consists of an arrangement where layer B settles over layer A by the overlap of two NPs by their corners and one by its edge (Figure S14a− c)). The second orientation (type 02) also is found in the fcc/ hcp structures and consists of an arrangement where layer B settles over layer A by overlapping four NPs by their corners ((Figure S14d−f). The last cubic NP−NP orientation is related to the sc structure and consists of an orientation along the {200}−{200} exposed faces (Figure S14a).

temperature. On the basis of our experimental observation, all samples prepared at 269.15 K can undergo SA spontaneously, showing that kinetics are now dominant. In fact, the spontaneity of the SA process is noticeable in the NP structures in Figures 2d and 3d, produced at 265.15 K and by delaying the solvent evaporation. On the other hand, when the NPs were prepared at the same temperature but now by normal deposition, a nonordered structure in the polyhedral sample (Figure 2c) and in a great part of the cubic sample (Figure 3c) was obtained. This indicates that kinetics are important to the SA process. When the experiments were performed at an even lower temperature (178 K), the Gibbs free energy has even more negative values, and it is possible to promote mainly the hcp structure formation in both SA methods (Figures 2e,f and 3e,f). Moreover, although fcc and hcp correspond to the majority of phases during the organization in all ordered samples prepared at 269.15 K, and the hcp phase is observed at 179 K, small areas of the samples ( 0.5). When χ > 0.5, the osmotic contribution becomes negative (attractive) as a result of poor ligand solubility, resulting in nanoparticle aggregation. Because the agglomeration of NPs is not commonly seen at lower temperatures, we rule out the possibility of values between 0.5 and 1 for the Flory−Huggins parameter. Concomitant with solubility changes, the “effective” length of the ligand can also be modified with temperature. To test this hypothesis, contour maps of the total potential energy as a function of the Flory− Huggins parameter and the ligand distention were plotted, by fixing the interparticle distance (values from Figure 5a) for each system of NPs (Figures 5b and S16). In each case, it is possible to verify regions of negative values of potential energy. As an example of this, Figure 6 shows the contour map for cubic NPs (fcc/hcp structure) at 178 K, with dashed lines separating the positive and negative values of the total potential energy, assuming an empirical interparticle distance of 2.2 nm. It is possible to show that, when χ < 0.5, values of 1.1 nm or smaller in effective ligand size can yield self-assembly for this specific arrangement of NPs. When χ > 0.5, the ligand behavior becomes attractive and any effective ligand size can lead to negative values of the total potential energy. Interestingly, no matter what type of SA arrangement or NP shape, the positive−negative boundary of total potential energy for χ < 0.5 is always where the interparticle distance is equivalent to twice the ligand effective length. This is reasonable because repulsion takes place as soon as the ligands are in contact with each other (Figure S16). Additionally, assuming that the ligand has lower solubility at lower temperatures, it is possible to assume higher values of χ for lower temperatures, as in the samples prepared by the

Figure 6. Total potential energy of polyhedral NPs in fcc/hcp structures according to the ligand effective mean size and Flory− Huggins parameter, with a fixed interparticle distance of 2.17 nm and a deposition temperature of 178 K.

deposition with TEM grids after immersion in N2(liq). In these samples, the colloids freeze instantly after drop casting, but in few seconds (less than 5 s), they melt and dry noticeably. During the melting, NPs can get closer to a condition of colloidal instability (χ ≅ 0.5) or even pass through it (χ > 0.5) (according to Figures 6 and S16). This situation where colloid destabilization is most prevalent indicates the shortest interparticle distance possible in ordered SA of NPs. As a result of these rules, the distance between particles can be analyzed on the basis of the ligand behavior. Although the samples prepared at RT have larger interparticle distances, resulting from almost fully outstretched ligands, it is found that there are groups of NPs with cores nearly in contact, in the samples prepared at low temperatures (Figures 7 and 8). This

Figure 7. Polyhedral CeO2 NPs prepared at (a) RT and (b) 178 K and (c, d) illustrations of ligand behavior. F

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added to the stacking, extra net forces can be summed among the NPs in the bottom layer (Figure S17b). If the net forces are attractive, then the increment from the upper layers can intensify the contraction of the oleate groups and smaller values of interpaticle distance can be found in arrangements of NP multilayers compared to monolayers. Hence, it is possible to infer that interparticle distances in multilayer arrangement are smaller when compared to those in monolayers at RT as a result of extra attraction forces. On the other hand, there is a limit to which the interparticle distance can contract for a specific NP layer. When the full NP coordination number is reached (12), additional layers will not contribute additional forces in that specific NP layer. This behavior is equivalent to that seem in crystalline materials, where lattice parameters closer to the surface relax in response to the absence of available atoms to which to bond.46 The last characteristic of the NP arrangements that the previous inference about ligand behavior can help explain is the NP orientation in each ordered arrangement. At higher temperatures (RT and 269.15 K), the effective ligand mean size is larger than at lower temperatures (Figure 7c). Because the forces among ligands are basically repulsive, NPs in face-toface orientations can be less influential than edge-on-edge interactions at shorter distances (d < 2l). Additionally, the ABC stacking type could provide vacancies in the arrangement, allowing for ligand accommodation. This occurs because the NPs in layer C can settle over a vacancy in layer B in such a way that it does not face an NP in layer A, whereas in structure ABA any NP in layer A will have an NP facing it through the vacancy of layer B. When the ligands have lower solubility (smaller effective mean size, Figure 8c), the ligand accommodation is not so critical and hcp (ABA) and sc structures can occur more easily. However, it may not be possible to exclude dipole−dipole interactions in the system. As described previously, the synthesized CeO2 particles have {200} and {111} exposed facets in the polyhedral NPs and mainly {200} in the cubic NPs. In CeO2 materials, whereas {111} and {220} facets have the lowest surface energy (nonpolar surfaces types I and II according to the classification of Tasker, respectively),47,48 {200} facets have the highest surface energy because of their surface polarity (polar surface type III),48 as a result of their possible terminations with cerium or oxygen ions.49 Despite this, the exposed {200} facets in the NPs produced in this study primarily consist of cerium ions15,17 because the oleate groups can bind to only this cation and dipoles of the same nature in {200} faces could be present.15 To compensate for this charge instability, CeO2 NPs can structurally change at the nanoscale through the slight rearrangement of {300} faces by nanofaceting and the reduction of Ce ions.50,51 Furthermore, these preferential exposed faces in each CeO2 particle type can grant a shape-dependence factor and may contribute to the face-toface assembly. In general, cubically shaped particles undergo to a face-to-face assembly, maximizing the interactions at the faces,52 resulting in close-packed-like structures.51 This situation is seen at the lowest temperature by the presence of sc structures (i.e., {200}−{200} face interactions). Likewise, polyhedral CeO2 NP also show face-to-face assembly, but the interactions between the facets is dependent on the exposed face. Figure S13a shows a typical two-layer CeO2 polyhedral, and the inserted FFTs imply the prevalence of {200}−{111} interactions rather than {111}−{111} or {200}−{200} interactions. As described before, {200} faces can present

Figure 8. Cubic CeO2 NPs prepared at (a) RT and (b) 178 K and (c, d) illustrations of ligand behavior.

shorter distance is possible only because of the drastic reduction of the ligand effective length and their lower solubility in the colloid. Consequently, a rough schematic of the ligand behavior is shown in Figures 7c and 8c for the NPs in RT and Figures 7d and 8d for the samples with a shorter effective ligand length. Interestingly, Goubet et al.19 have used the same equations to show that supercrystal formation is mainly driven by solvent-mediated interactions with the ligands and not solely by the van der Waals attraction between NPs cores, and further studies have emphasized the importance of these ligand−solvent interactions.43,44 In general, the application of these potential energy models does not highlight the changes in the ligand effective size, considering that the ligands are fully stretched during the SA process. This is because these traditional theories use concepts of polymer science in which ligands/polymers (large molecular size) are treated according to their radius of gyration and Flory radius and very small molecules can be treated according to their fully stretched size. This approximation is suitable for small molecules/ligands in most systems. However, in our experiments, the molecular length cannot be properly approximated as a result of several possible mechanisms of molecule modification (e.g., tilting, fold on itself, etc.). These then could pass through the process of a brush-to-mushroom transition35,45 (Figures 7 and 8), which is necessary to take into account when considering the interactions between NPs. Although the previous results explain the general variation of interparticle distances according to temperature, the data is not clear as to whether there is a slightly larger interparticle distance in the NP monolayers when compared to the ordered multilayers (Figure 5b), for those NPs prepared in the same manner. Nevertheless, a possible explanation can be obtained by the analysis of attractive forces for each isolated NP system. In most of the arrangements in this study, NP single layers have a coordination number of 6, resulting in a 6-fold symmetry of net forces in two dimensions (2D) (assuming an isotropic system of forces, Figure S17a). When additional NP layers are G

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ACKNOWLEDGMENTS This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science User Facility, at Brookhaven National Laboratory under contract no. DE-SC0012704. We thank Dr. Eli A. Sutter for suggesting the sandwich method described herein.

dipoles due to the Ce stabilization by ligands on the exposed facets, and this could minimize the attractive interactions between these faces due to Ce−Ce approaching in opposite facets. On the other hand, {200} facets are more likely to interact attractively with {111} facets because these planes consist of Ce and O ions (nonpolar surface), inasmuch as part of the face-to-face interactions are due to Ce−O approaching in opposite facets. Moreover, the particle−particle orientations of both the polyhedral and cubic particles are controlled by their exposed facets. The resulting superlattice occurs preferentially (>90%) by stacking NP layers at high density, independent of the particle shape and temperature. In other words, fcc and hcp superstructures grow in the ⟨111⟩ and ⟨001⟩ directions, respectively. Independent of the presence of dipole−dipole interactions and the complexity of the {200} facets, it is clear that the effective ligand mean size and its solubility play principal roles in determining the total potential energy in the arrangements by setting the interparticle distances.

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CONCLUSIONS In this study, experimental and theoretical studies of the selfassembly of oleate-covered cubic and polyhedral CeO2 NPs are reported. By tuning the colloid deposition parameters (temperature and evaporation rate), several ordered structures were formed, with their structures being correlated to the Gibbs free energy variation of the system. An analysis of the total potential energy (sum of all contributions to the interparticle forces) of each ordered structure showed that the effective NPs ligand size and its Flory−Huggins parameter (solubility) controlled the overall structure. When analyzing the effects of these parameters, a boundary location between negative (spontaneous) and positive (no spontaneous) potentials for the interparticle distances was found and was correlated to the data determined by electron microscopy characterization. Additionally, we showed that the behavior of ligand solubility and effective mean size can also support the formation of specific phases as a result of the effects of temperature and ligand accommodation in the arrangement. In addtion, the role of face-to-face interactions according to the NP shape factor was determined and correlated to the type of exposed crystallographic facet. These approaches could be extended to other systems of NPs in order to further understand the organic ligand behavior. These results also provide a promising avenue toward the construction of NP superlattice phase diagrams. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03026. Additional information about the synthesis and structural characterization of the CeO2 NPs, sample preparation, SA superlattice characterization, and energy potential calculations (PDF)

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*E-mail: [email protected]. Notes

The authors declare no competing financial interest. H

DOI: 10.1021/acs.langmuir.6b03026 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.6b03026 Langmuir XXXX, XXX, XXX−XXX