Spontaneous Formation of High-Index Planes in Gold Single Domain

Letter pubs.acs.org/NanoLett

Spontaneous Formation of High-Index Planes in Gold Single Domain Nanocrystal Superlattices Nicolas Goubet,†,‡ Jianhui Yang,†,‡ Pierre-Antoine Albouy,§ and Marie-Paule Pileni*,†,‡,∥ †

Sorbonne Universités, UPMC Univ Paris 06, UMR 8233, MONARIS, F-75005, Paris, France CNRS, UMR 8233, MONARIS, F-75005, Paris, France § Laboratoire de Physique des Solides, Université Paris-Sud, 91405 Orsay, France ∥ IRAMIS CEA Saclay, 91191 Gif sur Yvette, France ‡

S Supporting Information *

ABSTRACT: Crystals of nanocrystals, also called supracrystals and nanocrystal superlattices, are expected to exhibit specific properties that differ from both the corresponding bulk material and nanosized elementary units. In particular, their surfaces have a great potential as nanoscale interaction plateforms. However, control of the symmetry, compacity, and roughness of their surfaces remains an open question. Here, we describe the spontaneous formation of upper vicinal surfaces for supracrystals of Au nanocrystals grown on a sublayer of ordered Co nanocrystals. Stepped or kinked surfaces vicinal to the {100}, {110}, and {111} planes are observed to be extended on the micrometer range. The formation of such high-index planes is explained by a heteroepitaxial relationship between both Co and Au nanocrystal superlattice. KEYWORDS: Nanoparticle superlattice, high-index plane, vicinal surfaces, nanocrystallinity, gold nanoparticle, heteroepitaxy

A

procedures but such an atomic roughness was demonstrated to serve as an active site for catalysis and used as a template for self-assembly.22−24 Nanocrystals with high-index facets are also reported to have enhanced catalytic activities compared to Wulff shape nanocrystals.25 To the best of our knowledge, the observation of stable vicinal surfaces of supracrystals involving nanocrystals instead of atoms is not yet reported. In this Letter, spontaneous formation on micrometer scale of kinked and stepped high-index planes on nanocrystal superlattice surface is demonstrated. The nanocrystal superlattices studied here are grown from solutions containing two different metallic nanocrystals synthesized by different organometallic methods (see Experimental Section). The single crystal Au nanocrystals coated with dodecanethiol have an average diameter of 5.6 nm and present a polydispersity of 5%. The average size and distribution of polycrystalline Au nanocrystals are 5.4 nm and 6%, respectively. The single crystal (ε-phase) Co nanocrystals coated with oleic acid have an average diameter of 8.4 nm and a size distribution of 5% (Supporting Information Figure S1). Mixing them together with a nanocrystal concentration ratio Co/Au = 1:2 and evaporating the bicomponent solution on silicon leads to nanocrystal superlattices that are material segregated. Indeed, the chemical map in Figure 1 shows that Au

rtificial nanocrystal superlattices, which are also called supracrystals, are periodic arrangements of nanocrystals in three-dimensions.1−18 Polycrystalline nanocrystals of noble metals are nearly spherical and exhibit random orientation within the supracrystal. On the contrary, single-crystal nanocrystals are normally faceted, which may lead to their preferential orientations along specific crystallographic directions of the supracrystal as has been recently reported.8,13,14,19 Supracrystals grown with single-crystal nanocrystals are thus expected to exhibit different properties compared to their counterpart grown with polycrystalline nanocrystals; for instance, an increase in Young modulus has been recently measured in the case of Au nanocrystals.15 Such supracrystals made of Au single nanocrystals have a great potential to be used as plasmonic substrate and to manipulate the light at a local scale due to the intense electromagnetic fields between adjacent nanocrystals.16,17,20,21 However, the supracrystalline upper surface needs to be finely controlled. Only few studies have been reported on the formation of nanocrystal superlattices with desirable low index upper planes.8,9 At the atomic level, structure−reactivity relationship is an important aspect of crystalline surfaces. A way to enhance the reactivity of surfaces is to decrease the coordination of their building units. High Miller index crystallographic planes exhibit a lower surface density and thus higher surface energy. A minimization of their surface energy may result in the periodic formation of steps and facets with lower indices. Obtaining these vicinal surfaces usually requires artificial and tedious © 2014 American Chemical Society

Received: August 27, 2014 Revised: October 14, 2014 Published: October 22, 2014 6632

dx.doi.org/10.1021/nl503289a | Nano Lett. 2014, 14, 6632−6638

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nanocrystals are concentrated into polyhedral supracrystals (green) embedded in a film of Co nanocrystals (red). Au supracrystals exhibit different shapes, like truncated tetrahedron, distorted square, or rhombus-like shapes as shown in Supporting Information Figure S2. Figure 2 shows HRSEM pictures of polyhedral Au nanocrystal superlattice surfaces exhibiting complex periodical patterns. These observations are characteristic for vicinal surfaces. The crystallographic planes that build the terraces can be different from one polyhedral supracrystal to the other ones. The major observed vicinal surfaces are based on {111} terraces (Figure 2a). However, few surfaces are also observed with less compact {100} and {110} terraces that are usually unstable in atomic system, as shown in Figure 2b,c. Their indexations and microfacet notations are summarized in Table 1. Parallel linear steps are observed that correspond to one

Figure 1. (a) SEM picture of the deposition obtained by evaporating a binary colloidal solution mixture made of single crystalline Au nanocrystal and Co nanocrystal. (b) Energy dispersive X-ray chemical map showing the polyhedral Au nanocrystal superlattice (green) within the Co nanocrystal film (red). The map is made by using the Mα1 X-ray line for Au and Kα1 for Co.

Figure 2. (a) HRSEM pictures of the {221} nanocrystal superlattice plane that is vicinal to {111}. (b) HRSEM pictures of the {35 9 1} nanocrystal superlattice plane that is vicinal to {100}. (c) HRSEM pictures of the {430} nanocrystal superlattice plane that is vicinal to {110}. Each FFT patterns and modeled planes are insets in the corresponding HRSEM pictures. (d) Description of the stepped surfaces notation by their microfacets. (e) Description of the notation for kinked surfaces by their microfacets. 6633


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Table 1. Characteristics of the High-Index Planes Observed on the Au Nanocrystal Superlattice Surfaces Miller index

terraces

{221} {331} {430} {35 9 1} {312} {13 11 5}

{111} {111} {110} {100} {111} {111}

decomposition of Miller index n1{h1k1l1} × n2{h2k2l2} × {h3k3l3} ΛHRSEM (nm) 3(111) 2(111) 3(110) 26(100) 3(111) 8(111)

× × × × × ×

(111̅) (111̅) (100) 8(110) × (111) 2(100) × (11̅1) 4(111)̅ × (111̅ )

21 13 23 19 18 20

ΛRX (nm)

α (deg)

figures

22.1 16 26 20.8 15.9 18.1

15.8 22.0 8.1 14.5 22.2 19.4

2a Supporting Information S7 2c 2b, Supporting Information S3 Supporting Information S4,S8 Supporting Information S9

Figure 3. (a) SEM picture of the deposition obtained by evaporating a binary colloidal solution mixture made of polycrystalline Au nanocrystal and Co nanocrystal. (b) Energy dispersive X-ray chemical map showing the interpenetration of Au nanocrystal superlattice (green) within the Co nanocrystal film (red). The map is made by using the Mα1 X-ray line for Au and Kα1 for Co. (c) High magnification of Co and Au nanocrystal superlattice boundary. (d) Large area HRSEM picture of Co and polycrystalline Au nanocrystal films.

(hkl) vicinal surfaces containing (h1k1l1) terraces are usually produced by introducing a small convenient miscut angle α between the reference (h1k1l1) plane and the actual plane of cut. Here, due to the spontaneous formation of vicinal supracrystal surfaces, we prefer to assign this parameter to a misorientation angle. It is easily computed from the scalar product between normale to both surfaces (Table 1). It is related to the shortest distance from one terrace to the next Λ and the step height h by27

nanocrystal in height (Figure 2a,c). Their periodicity is kept on micrometer ranges (Figure 2a and Supporting Information Figures S3−6) and this is confirmed by the image Fourier transform that presents lines of sharp reflections (insets in Figure 2a,c). Given its {hkl} indexation, such a surface can be described by the {h1k1l1} and {h2k2l2} Miller indices for terraces and step microfacets, respectively (Figure 2d). The numbers n1 and n2 in Table 1 correspond to the vectorial decomposition of the (hkl) vector in terms of these two sets of indices. Figures 2b show supracrystal surfaces that are kinked instead of stepped. At the difference of stepped surfaces, the sharp reflections of the Fourier transform are no more organized in parallel lines (Figure 2b); this is due to the presence of a second type of step microfacet with index {h3k3l3} (inset Figure 2e). A third number n3 is introduced for the vicinal surface notation as the (hkl) vector is now decomposed on three sets of indices. In this way, the kinked nanocrystal superlattice surfaces are characterized by their chirality, as observed with atomic surfaces.26 The density of low-coordinated nanocrystals is higher in the case of chiral surfaces compared to the achiral ones. One has to notice that some surfaces could not be indexed due to their complex symmetry (Supporting Information Figures S5−6).

sin α =

h Λ

(1)

At this point of the study, we need to identify which factors induce the formation of such vicinal surfaces that usually do not form spontaneously in crystals made of atoms. Indeed, by treating the sample with nitric acid, the Co nanocrystals can be dissolved while Au supracrystal vicinal surfaces remain intact (Supporting Information Figure S10). This observation shows that Au nanocrystal superlattices with vicinal surfaces are stables. Note that in absence of Co nanocrystals Au supracrystals are characterized by low index facets as observed previously.15 This clearly indicates that Co nanocrystals are 6634


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Figure 4. (a) SEM pictures of cleaved nanocrystal superlattice sample with high-index plane with a zoom-in of the interface between Au and Co nanocrystal superlattice. The upper and bottom inset FFT pattern correspond to the Au and Co nanocrystal assemblies, respectively. (b) X-ray SAXS pattern of binary mixture between single crystal Au and Co supracrystal. (c) Boundary between high-index Au nanocrystal superlattice plane and Co nanocrystal film.

needed to produce vicinal surfaces. Note that when pure Au nanocrystal solution is deposited in the presence of a small excess of oleic acid only compact superlattice planes are observed (Supporting Information Figure S11). This confirmed that the Co nanocrystal coating agent does not have any influence on the growth of such vicinal surfaces. Moreover, by replacing 5.6 nm single domain Au nanocrystals by its corresponding polycrystalline one, the nanocrystal superlattices are also material segregated but the Au film reveals only low index surfaces (Figure 3). No polyhedral shapes with high symmetry surfaces are observed. Figure 3d shows that the upper surface of Au nanocrystal film is made of compact lowindex planes and present disorder at their boundaries with Co nanocrystal film (Figure 3c). The slight variation in polydispersity (5 and 6%) cannot explain such difference in self-assembly behavior. From these data it is concluded that Au polyhedral supracrystal with vicinal surfaces can be produced only if Au nanocrystals are in single crystalline phase. Hence, the nanocrystallinity of nanocrystals involved in such process play a key role. SEM top views pictures show that polyhedral supracrystals are always surrounded by Co nanocrystal films (Figure 1). In

order to check if the Au supracrystals grow in direct contact with the silicon substrate or on a film of Co nanocrystals, the sample has been cleaved. HRSEM pictures of the edge show the systematic presence of an underlying film of Co nanocrystals (Figure 4). Fourier transforms of HRSEM images reveals that the underlying Co nanocrystals are well ordered into supracrystals (Figure 4a). As shown in Figure 4b and Supporting Information Figure S13, the small-angle X-ray scattering (SAXS) confirms a face-centered packing with compact [111] orientation parallel to the substrate; HRSEM shows that this orientation is kept up to the upper layer (see Supporting Information Figure S14). This strongly suggests that Co nanocrystal superlattice template is necessary to induce vicinal surfaces. Cubic parameters deduced from the analysis of SAXS data for both face-centered cubic superlattices are determined aCo = 17.3 nm for Co and aAu = 10.7 nm for Au (Supporting Information Figure S13). The superlattice parameter misfit is ca. 38% thus no direct epitaxy between the most close-packed planes of the two fcc superlattices is possible. However, the average gap between adjacent Co nanocrystals is dinter(Co) = 3.8 nm, which is larger than the value found with pure Co nanocrystal deposition (3.0 nm) 6635


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(Supporting Information Figure S15).28 On the other side, the gap between adjacent Au nanocrystals is dinter(Au) = 2.0 nm. This relaxation process suggests that the superlattices adapt their structures to introduce coherence between them. The step period Λ of vicinal surfaces can be extracted from largest periods of Fourier transform patterns and these values are summarized in Table 1. The step height h is given by h = aAu

The nanocrystal superlattice parameters deduced from the SAXS pattern in Supporting Information Figure S16 from the binary mixture between polycrystalline Au nanocrystal and Co nanocrystal are aCo = 16.5 nm and aAu_poly = 10.9 nm, respectively. With similar sizes as studied above, the present Co and Au nanocrystal superlattices are compressed and expended, respectively, compared to the ones made with single crystal Au nanocrystals. This is in agreement with the relaxation process during the formation of the heteroepitaxy. Moreover, there is no presence of sharp Bragg reflections out of the plane on the SAXS pattern, which suggests the absence of polyhedral nanocrystal superlattices. It has been previously demonstrated for Au single crystal that their self-assemblies results to polyhedral superlattices for either growth in solution or on substrate.8,14,15,29 On the other hand, in case of Au polycrystalline nanoparticles with same size, the self-assembly process is absent in solution and it has to be forced by evaporation of the solvant and formed superlattice film without polyhedral shape.12,14,29 Such nanocrystallinity effect is related to the nanocrystal shape.8 These observations reveal that single crystals have better packing effeciency with the presence of the orientational ordering compared to the polycrystalline nanoparticles and cause stronger interactions between single crystals. In the present study, the main shape of Au single crystals is the truncated octahedron whereas for polycrystals is icosahedron and decahedron. This explains the fact that supracrystals display individual polyhedral shapes in case of single crystal instead of an interpenetrated film structure. This stronger interparticle interaction amplifies the segregation process between Co and Au without intermixing at the superlattice junction as for polycrystalline nanoparticles (Figure 3d). In this condition, the system has to find a way to accommodate the contact between supracrystals boundaries that is the heteroepitaxial growth between nanocrystal superlattice and stabilization of vicinal surfaces. Here we suggest a way to produce high-index surfaces with low symmetry of Au nanocrystal superlattices by a simple method. These planes exhibit well-defined terraces and steps with a periodicity of ca. 20 nm in most cases. These surfaces can be either stepped or kinked. Unlike bulk vicinal surfaces, the high-index supracrystal planes are spontaneously formed. This is explained by the confinement of the Au nanocrystal superlattices on top of an underlying Co supracrystal film; it is thus possible to template the growth of high-index planes that are normally thermodynamically unstable. Thanks to the stronger single nanocrystal interactions and large superlattice misfit between Au and Co nanocrystals (due to the difference in nanocrystal diameters and coating alkyl chain length), it is suggested that the Au vicinal surfaces are produced by a “soft” heteroepitaxial growth on compact planes of Co nanocrystal superlattice. These low symmetry surfaces have good potential for surface-enhanced Raman spectroscopy and template substrate due to their periodical roughness and high concentration of low-coordinated nanocrystals. The chirality of the kinked nanocrystal superlattice planes offers the opportunity to produce plasmonic substrates with artificial optical chirality. Experimental Section. Synthesis of ε-Co Nanocrystals. Co nanocrystals were made by thermal decomposition described in ref 30. A 0.1 g sample of trioctylphosphine oxide and 0.15 mL of oleic acid were dissolved in 15 mL of odichlorobenzene. The solution was heated to 180 °C under flowing nitrogen. A 0.54 g sanple of Co2(CO)8 was dissolved in

h12 + k12 + l12 q1

(2) 27

where q1 = 1 if h1,k1,l1 are all odds and = 2 otherwise. The step period ΛRX computed using eqs 1 and 2 is reported in Table 1. The agreement between both determinations is quite satisfying taking into account the spatial resolution of the HRSEM. As detailed above, X-ray diffraction indicates that Au supracrystals are growing on a dense (111) sublayer of Co nanocrystals. Such a plane can be viewed as parallel rows of nanocrystals extending along the [11̅0] and [112̅] directions separated by a distance of 10.3 and 5.9 nm, respectively (see Supporting Information), if one restricts to the two larger distances. Indeed, Table 1 shows that Λ is a multiple of either distance, taking into account the moderate precision of its measure. This agreement reveals an epitaxial relationship between the {111} surface of Co supracrystals and high-index planes of Au supracrystals. The HRSEM picture in Figure 4c corresponds to an area with a low Au nanocrystal concentration; this allows imaging the starting step of Au nanocrystal superlattice growth. One can see the alignment between the Au and Co nanocrystal superlattice planes, which is in good agreement with the heteroepitaxy growth mechanism. A schematic view of the heteroepitaxial growth of Au supracrystals with high-index plane onto the Co supracrystal compact {111} plane is proposed in Figure 5.

Figure 5. Scheme of the heteroepitaxial growth of a Au nanocrystal superlattice on a [111] oriented Co superlattice serving as substrate.

The relaxation process in the nanocrystal superlattice can be more pronounced compared to bulk material due to the softness of nanocrystals coated by organic molecules. The relation between the supracrystal is assimilated to a “soft” heteroepitaxy. In order to find the optimal ratio between Au and Co nanocrystals, depositions with [Co]/[Au] ratios ranging from 2:1 to 1:5 are investigated. The SEM pictures and chemical maps (Supporting Information Figure S12) reveals that ratio 1:1 also provides some vicinal surfaces but with smaller correlation distances compared to the ratio 1:2, which is optimal. In addition to a large superlattice misfit, a key parameter for the formation of vicinal planes and heteroepitaxial growth is the nanocrystallinity of the Au nanocrystal. 6636


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Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, P.R. China.

3 mL of o-dichlorobenzene under vigorous stirring, operating in a glovebox under nitrogen atmosphere. The solution was sucked by a syringe and rapidly injected into the hot solution containing trioctylphosphine oxide and oleic acid. The brown solution immediately turned black indicating the formation of Co nanocrystals. The reaction was held at 180 °C for 20 min and then cooled to room temperature. Synthesis of Au Nanocrystals with Selective Nanocrystallinity. Au nanocrystals were synthesized using methods described in ref 31. In the case of nanocrystals 5 nm in diameter, 0.124 g of AuPPh3Cl was dissolved at 100 °C in 25 mL of toluene under nitrogen protection. 500 μL of dodecanethiol was added under vigorous stirring to this solution. A second solution was prepared by dissolution of 0.434 g tert-butylamine−borane complex in 2 mL of toluene and heated at the same temperature. Both solutions were mixed together after total dissolution of all the products. The formation of Au NCs was revealed by a change from colorless to bright brown and then dark red. Au nanocrystals were washed by precipitation in ethanol and redispersion in toluene. A postsynthesis step was used to select the single crystal fraction within the colloidal solution. For this purpose, the colloidal solution was placed in a container with a large opening and left under solvent saturated atmosphere. After 1 week, the solution was centrifuged to recover the aggregated single crystal Au nanocrystals whereas the polycrystals remains in solution. The black precipitate was redispersed in hexane and became the single crystal colloidal solution while the supernatant became the polycrystalline nanocrystal solution.12,14 Nanocrystal Superlattices Preparation. A solution of Au and Co nanocrystals with fixed nanocrystal concentration (6.7 × 1017 part/L) was mixed with the desired stoichiometry. The resulting solution was poured on a silicon substrate and left to evaporate under nitrogen. Electron Microscopy Characterization. We used a Hitachi SU-70 field emission scanning electron microscope at 15 kV for HRSEM images. Elemental maps were obtained by EDS in the SEM operating at 20 kV (Oxford X-Max). Pattern of vicinal surface were modeled with Vesta software. Small-Angle X-ray Scattering. The grazing incidence smallangle scattering patterns were acquired with a homemade system. The X-ray generator was a rotating copper anode operated with a small-size focus (0.1 × 0.1 mm2) at 40 kV and 20 mA. The optics were a double-bounce parabolic multilayergraded monochromator. The samples on silicon substrate were mounted on a rotation stage and the diffraction patterns were recorded on a photostimulable imaging plate. The image readout was performed with a STORM 820 Molecular Dynamics scanner.

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Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. N.C. and J.Y. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the advanced grant of the European Research Council under n° 267129.

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ASSOCIATED CONTENT

S Supporting Information *

Nanocrystal size distributions, additional HRSEM pictures, and structural characterizations. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

(J.Y.) Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Shaanxi Key 6637


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Spontaneous Formation of High-Index Planes in Gold Single Domain

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