Letter pubs.acs.org/JPCL
Conformation of Capping Ligands on Nanoplates: Facet-EdgeInduced Disorder and Self-Assembly-Related Ordering Revealed by Sum Frequency Generation Spectroscopy Hao Zhang,†,‡ Fujin Li,† Qingbo Xiao,† and Hongzhen Lin*,† †
i-LAB, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou 215123, P. R. China School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, P.R. China
‡
S Supporting Information *
ABSTRACT: Surface-curvature-amplified conformational disorder in alkyl capping ligands has been observed previously when the nanoparticle radii approach the ligand length. Herein, sum frequency generation studies on oleic-acid-capped nanoplates show that even on faceted surfaces with dimensions tens of times greater than the ligand length a significant proportion of gauche defects exist in the capping layer. The molecular disorder on the nanosized facets is attributed to a facet-edge effect, which is diminished when increasing the facet size or assembling the nanofacets side to side. This feature is further explored to probe the self-assembly dynamics of nanoplates.
ynthesizing nanostructures using “wet chemical” methods and organizing them to proper assemblies are the key steps in nanotechnology to build up functional systems via bottomup strategies. Such nanostructures usually possess organic molecules chemi- or physisorbed at their surfaces. Conformation of adsorbed molecules often strongly impacts the selfassembling behaviors as well as the physical, chemical, and biological properties of the nanostructures.1,2 Particular attention has been paid to surfactant-like ligands chemically anchored to nanoparticles.1−11 Previous studies on spherical/ quasi-spherical nanoparticles have demonstrated a surfacecurvature-amplified conformational disorder in the alkane capping ligands when the radii of the nanoparticles are comparable to the ligand chain length, namely, ∼2 nm.1,9 Research on anisotropic nanocrystals and their assemblies arouses increasing interest in understanding the status of ligands adsorbed on nanosized faceted surfaces. While much attention has been paid to the organic−inorganic binding mechanisms, only a few reports have concerned the conformations of capping ligands on nanofacets.2,4,11 There are some scientifically important yet often ignored questions in this regard: Is the conformational distribution homogeneous for the capping ligands on a nanosized facet? How large should a nanofacet be for the conformations of adsorbed molecules to resemble that on a macroscopic facet? What is the relationship between ligand conformation and the self-assembly manner of nanocrystals? Molecular dynamics simulations on CdS nanorods in n-hexane have revealed a dramatic change in rod−rod interactions upon the order−disorder transition of the capping
S
© XXXX American Chemical Society
ligands on side facets.2 In contrast with the availability of computing tools, experimental probing of ligand conformation on specific nanosized facets remains a challenging task. Vibrational SFG spectroscopy has been proved to be useful in studying the conformation of capping ligands on nanoparticle surfaces.1,9,12−19 As a second-order nonlinear optical process, SFG is electrically forbidden in systems with inversion symmetry, which results in its sensitivity to conformation of long alkyl chains: in all-trans conformation the CH2 groups are inversion-symmetrically arranged and show very weak SFG response, and hence the lone terminal CH3 group dominates the SFG spectra. When gauche defects are present, stretch modes at the defect position become “SFG-allowed”.20 Therefore, the relative intensity of stretch modes of CH3 with respect to that of CH2 can be regarded as a semiquantitative measure of the degree of conformational order of alkyl chains. Nanoplates (nanodisks) have attracted much research attention because of their promising optical, electronic, and catalytic properties. Meanwhile, they are good models for the study of nanosized organic/inorganic interfaces owing to their well-defined faceted surfaces, tunable size and shape, and capability to assemble into various superstructures.21−23 NaYF4 has been widely used as a matrix material in the preparation of lanthanide-doped upconversion nanophosphors, which show Received: April 6, 2015 Accepted: May 26, 2015
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DOI: 10.1021/acs.jpclett.5b00717 J. Phys. Chem. Lett. 2015, 6, 2170−2176
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The Journal of Physical Chemistry Letters promising potentials in biosensing and imaging.24−26 The luminescence properties of such nanophosphors are often dramatically influenced by their surface chemistry;27 however, a clear understanding about the geometrical organization of capping ligands on the nanosurfaces is still lacking. In this work, we have investigated the conformation of oleic acid (OA) molecules chemisorbed on NaYF4 nanoplates using SFG spectroscopy. The hydrophobic nanoplates can be readily trapped onto water surface (water/vapor interface) and selfassemble into 2D arrays. This allows the particular faceted surfaces of the nanoplates to lie in about the same plane and respond collectively to the probing light. The experimental details are described in the Supporting Information. The nanoplates studied here are of regular hexagon shape with a side length of 40 ± 10 nm. Each nanoplate can be viewed as having one top facet, one bottom facet, and six side facets (β phase of NaYF4, hexagonal crystal system). The originally centrosymmetric nanosystem becomes asymmetric when being adsorbed onto water surface. Considering the arrangement of the nanoplates and the dielectric nature of NaYF4, it is reasonable to speculate that the detected SFG signal mainly comes from the capping ligands on the top and bottom facets. The ligand chains extended to the air (on top facets) are expected to dominate the SFG resonance, while those on the bottom facets cause destructive but minor interference without changing the spectral shape.28,29 A small fraction of the ligands on the side facets, along the three-phase interface, may also make a slight contribution. The SFG spectra in the CH-stretch region recorded for the nanoplates on water surface are displayed in Figure 1a. The observed peaks can be readily assigned as CH2 symmetric stretch (d+) at ∼2850 cm−1, CH2 antisymmetric stretch (d−) at ∼2906 cm−1, CH3 symmetric stretch (r+) at ∼2880 cm−1, and its Fermi resonance (r+-FR) at ∼2944 cm−1.19,20,30 As can be seen, the CH3 transition bands are just a bit stronger than those of CH2 at low surface concentration of the nanoplates but overshadow the latter at relatively high concentrations. For clarification, the SFG spectra are fitted to Lorentzian line shapes using the reported method1 (the fitting parameters are listed in the Supporting Information), and the ratio of the CH3 (r+) to CH2 (d+) mode intensity (named “order ratio” hereafter) is plotted against the surface concentration (Figure 1b). It has been theoretically demonstrated that an average of one gauche defect in the alkyl chain can give rise to the CH2 stretch intensity similar in magnitude to the CH3 stretch.31 The order ratio of ∼1.3 obtained at low surface concentration (at which the nanoplates are not or are only slightly assembled) implies the existence of gauche defects in a remarkable fraction of the capping ligands. As the surface concentration increases, the order ratio is first enlarged and gets up to a plateau of ∼5.5 at the end, corresponding to a disorder−order transition of the alkyl chains in company with self-assembly of the nanoplates. Note that OA has a double bond along its chain, which is in cis conformation and is much more rigid than the saturated alkyl moieties. This introduces a “systematic kink” to the chain. Nevertheless, increase in r+/d+ intensity ratio is undoubtedly a sign of chain ordering. For comparison, the change in surface tension is also recorded while varying the surface concentration of the nanoplates. Because the nanoplates are insoluble in water and most of them are located on the water surface, the decrease in surface tension directly reflects the increase in surface coverage. The surface tension reaches a constant level as the coverage
Figure 1. (a) SFG spectra (SSP) of OA-capped NaYF4 nanoplates (side length 40 ± 10 nm) on water surface at different surface coverage rates. The spectra are offset for a clear view. (b) Ratio of CH3 to CH2 mode intensities (r+/d+, black squares) and surface tension (red circles) as a function of surface concentration of the nanoplates. The surface coverage rates are estimated according to the surface tension isotherm. Coverage rates larger than 100% correspond to multilayers.
rate approaches unity (full monolayer coverage). When the added amount of nanoplates exceeds that needed for full monolayer coverage (coverage rate >100%), multilayers (faceto-face assemblies) start to form, which hardly affect the surface tension and the order ratio anymore. As shown in Figure 1b, the evolution of the order ratio (r+/d+) with increasing surface concentration matches well with the decreasing trends of the surface tension. It means that the capping ligands tend to become more and more ordered until the water surface is fully covered by the nanoplates. This phenomenon is very similar to that observed for surfactant Langmuir film on water surface: surfactant molecules adopt disordered “liquid-state” conformations (resulting in low order ratios) when their surface density is low, while at high surface densities the trans extended conformation dominates for maximization of intermolecular van der Waals attractions and correspondingly higher order ratios are obtained.20 On the other hand, because the capping ligands are chemically bound to the nanoplate facets, the local surface density of the ligands is expected to be sufficiently high and independent of the surface coverage of the nanoplates on water. (The OA/Ln3+ ratio was about 64:1 for preparation of the nanoplates. Details of the coordination chemistry can be 2171
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The Journal of Physical Chemistry Letters found in the literature.32,33) From this point of view, the hydrocarbon chains anchored to a planar nanofacet should always adopt ordered conformations whether or not the nanofacet is associated with other facets. The relatively disordered conformation of the capping ligands observed on nonassembled nanoplates suggests that the nanosized facet cannot be simply treated like a macroscopic crystal facet, even though its size (hexagon side length of ∼40 nm) is much larger than the ligand chain length (∼2 nm for OA). The major difference between the studied nanoplate surface and a macroscopic facet lies in two aspects. First, the orientation of a nanoplate may fluctuate as a result of instant imbalance of forces from surrounding water molecules. Second, the limited size of a nanofacet may make the facet edges relatively more significant and unable to be neglected as usually done for a macroscopic facet. The former factor, that is, Brownian-type waving of the nanoplates, may slightly broaden the orientation distribution of the adsorbed ligands and reduce the SFG coherence among them, whereas it should not affect the density of gauche defects in the capping layer. The effect of orientation fluctuation can be further ruled out by comparing the SFG spectra of nonassembled and assembled nanoplates on flat CaF2 substrates (Supporting Information). Previously reported simulations show that the order ratio (r+/d+) is not only related to the fraction of defected chains but also dependent on the molecular tilt angle.31 To cause a dramatic decrease in the order ratio without introducing more defects, however, would require a significant variation of the average tilt angle, which is unlikely in our case. Indeed, only a small change has been observed in the average orientation of the terminal methyl groups during the self-assembly of the nanoplates (Supporting Information). The ordering of ligands on assembled nanoplates also excludes the possibility of disordered distribution of underlying metal ions.9 Therefore, the observed gauche defects are most likely brought by the facet edges. Considering that the edges between the top facet and the side facets are relatively sharp (a dihedral angle of 90°), the ligands anchored to the edge atoms (ions) should have much free volume to accommodate conformational defects; however, although the facet is only nanoscale in size, the proportion of edge atoms in relative to those in the middle of the facet is still so small that the edge ligands alone cannot account for the number of gauche defects we observed. One has to assume that the free volume provided by the edge curvature can be “shared” by the ligand chains located in the inner region of the nanofacet (Figure 2). Indeed, while the docking groups (−COOH) are immobilized on the faceted surface, the flexible alkyl chains, provided that the capping density is high enough, would have a tendency to stay away from each other for minimization of interchain steric repulsions (at the sacrifice of some extent of van der Waals attraction, on the other hand). In other words, the chains in the middle of the facets tend to push those near and at the edges to occupy the available free space as much as possible until they reach a thermal equilibrium. It should be noted that the additional free volume may not be equally shared by all of the capping ligands. The closer a chain to the facet edge, the more volume it should have. The ligands on the edges are expected to have jog-type defects, while kink defects are more probable for those at the inner region of the facet (Figure 2). Supposing that a chain at the very edge of a facet can extend outward until it becomes parallel to the facet, a simple approximation of the ratio of the additional free volume
Figure 2. Schematic representation (a cross-sectional view) of the facet-edge effect on conformations of capping ligands and the diminishing of edge effect upon side-to-side assembly of the nanoplates. Only a part of the top facet and side facet is displayed for two nanoplates.
relative to the columnar volume can be written as (see the Supporting Information for details) π
6 × 4 L2a + ΔV = 3 3 2 V aL 2
2π 3 L 3
≈ 1.8x + 0.8x 2 (1)
where L refers to the chain length of the capping ligand, a is the side length of the hexagon facet, and x = L/a. Taking L = 2 nm and a = 40 nm in our case, the additional volume is ∼10% of the columnar volume. This value denotes what percent of volume is “added” to each of the ligands on average. It seems not so big but can be critical for the generation of gauche defects in our case. When the nanoplates are assembled side to side, the edge effect will be diminished, with the “additional” volume being crowded by ligand chains from neighboring nanoplates (Figure 2). Actually, aggregation/assembly (or gelation) accompanied ligand ordering has been theoretically predicted or experimentally observed in a variety of nanosystems.6−8,15,34−36 Herein we indicate that for the ordering to occur the ligands on different nanosurfaces do not have to intercalate into each other as often suggested.11,37 In certain situations, a coplanar and compact arrangement of a number of nanofacets can lead to ligand ordering on all of the facets. The remaining question is to see whether the 10% addition of volume is adequate to result in the substantial decrease in the order ratio (r+/d+) from 5.5 to 1.3. According to the literature, the volume created by a kink (a gauche−trans−gauche sequence) defected molecule is only slightly higher than the volume of an all-trans molecule.38 It is worth mentioning that about one defect per chain is enough for the order ratio to drop to a level around 1.3.31 Most of the defected molecules may still preserve a rather high degree of orderness, being nearly all-trans (one or two gauche sites per molecule) and well-packed with each other. Although the proportion of defected chains among all the capping molecules is high, the number of defect sites would be only 10−20% of the total number of carbon−carbon single bonds. (A kink defect involves two gauche sites, while one OA molecule has 14 C−C bonds.) This estimation of gauche percentages is commensurate with previous discussions and molecular simulations at room temperature for long alkyl ligands on other substrates.39,40 Moreover, the trans−gauche isomerization of different chains often displays a cooperative effect,41−44 during which the chain−chain distances are almost unaltered. In such a case, one merely needs to consider the variation in the volume of the isomerized segments but not the 2172
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The Journal of Physical Chemistry Letters cylindrical rotational volume of the entire chain. Studies on lipid monolayers and bilayers have illustrated that trans−gauche transition is accompanied by a volume change of only a few percent.,39,40,45 From this viewpoint, the 10% volume difference in the present case is sufficient to lead to a significant fraction of defected chains. This is particularly true when the packing density of the molecular chains is high. (Comparison of the present results to the SFG data for OA Langmuir monolayer further confirms the high capping density of OA on the nanoplates. See the Supporting Information for details.) Because of the low-energy barrier and small volume change, kink-type disordering is readily to occur and can even be observed at molecular areas as low as 0.19 nm2.38 Defect-free state, corresponding to a high order ratio (r+/d+), can only be obtained at sufficiently high packing density. A slight increase in the available volume per molecule would boost the kink deformations and make the order ratio to drop abruptly. Note that only the conformational transitions along C−C single bonds need to be considered under our measurement conditions; the rigid cis CC double bond is not included when we talk of “all-trans”. It is interesting to investigate the average orientation angle of the terminal CH3 groups, which reflects the tilt angle of the molecular chains. Methyl group belongs to C3v symmetry, and its orientation is characterized by the angle between its symmetric axis and the surface normal. According to the facet-edge model, at the nonassembled state of the nanoplates, a minor fraction of the capping ligands, that is, those adsorbed on the edge, would possess relatively large tilt angle. The majority of OA chains just need to slightly tilt (4 to 5°) for the average volume per chain to increase by 10%. In consistence with our expectation, the average tilt angle of the terminal methyl group is found to be close to zero (4.6 ± 4.6°) on wellassembled nanoplates while a little bit larger (12.8 ± 3.6°) on nonassembled ones. (See section 4 in the Supporting Information.) A surface-curvature-amplified conformational disorder has been depicted for alkane ligands on relatively small spherical nanoparticles.1,9 From these reports, one can see that the order ratio (r+/d+) becomes no more sensitive to the particle size when the nanoparticle radius exceeds the ligand chain length. The maximum order ratio shown therein is
