Nano Kirkendall Effect Related to Nanocrystallinity of Metal Nanocrystals

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Nano Kirkendall Effect Related to Nanocrystallinity of Metal Nanocrystals: Influence of the Outward and Inward Atomic Diffusion on the Final Nanoparticle Structure Zhijie Yang,†,‡ Nailiang Yang,†,‡ 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 § CEA/IRAMIS, CEA Saclay, 91191, Gif-sur-Yvette, France ‡

ABSTRACT: The Kirkendall effect is a classical phenomenon in materials science, and it is referred to as a nonreciprocal interdiffusion process through an interface of two metals with strikingly different atomic diffusivities, leading to a formation of vacancies called Kirkendall voids. The nanoscale Kirkendall effect has been vastly applied in the fabrication of hollow nanostructures after the first report on the synthesis of Co-based hollow nanocrystals. In this Feature Article, we briefly start with an introduction on the Kirkendall effect concept, followed by the general synthetic strategy toward the production of hollow Kirkendall voids. The overall synthetic strategies are based on the design of diffusion couples at the nanoscale, and then, we discuss the factors that govern the formation of Kirkendall voids at the nanoscale, from the viewpoint of the nanoparticle size, nanoparticle crystallinity, and nanoparticle environment. We conclude with a summary and perspectives on the design of hollow nanostructures governed by the Kirkendall effect.

1. INTRODUCTION Since the popularization of nanosciences and nanotechnology about two decades ago, nanostructured materials have attracted steadily ascending attention due to their unique size- and shape-dependent properties.1−5 Over the past decade, inorganic hollow nanostructures with designed fashion, controlled pore volume, and shell thickness have been fascinating due to their high surface to volume ratio and large pore volume, making them promising materials for technological applications including drug delivery systems, energy storage, and catalysis.6−8 Since the first report by Caruso and Mohwald on the colloidal-template method for the fabrication of inorganic hollow structures in 1998, a plethora of methods have been applied for engineering those hollow nanostructures. Among those methods, a template-mediated synthesis has been demonstrated to be a versatile and effective approach.9 A traditional colloidal templating method usually requires several procedures, including surface modification of the template, coating the surface of the colloidal template with desired materials, and a post-treatment of template removal. The size of the hollow interior obtained via such a colloidal-template approach is usually confined by the size of the template. In any case, the sizes of the colloids, such as silica, carbon, or polymer beads, are over hundreds of nanometers, which may veil the intrinsic properties of hollow nanostructures with size in the regime of below 100 nm. In recent years, a new self-templated method, which can overcome the size limitation of the colloidal template, has emerged, employing old physical principles, such as Ostwald ripening, oriented attachment, and the Kirkendall effect.10−13 Considerable nicely written reviews on this field are © XXXX American Chemical Society

available, concerning the general concept and synthesis of hollow nanostructures by the nanoscale Kirkendall effect.14−17 Here, we begin with a brief review on the recent progress of the production of hollow nanostructures (mainly metal oxides) governed by the nanoscale Kirkendall effect. Then, we discuss the various factors governing the formation of Kirkendall voids at the nanoscale, such as the particle size, nanoparticle crystallinity, particle environment, etc. Numerous chemical reactions or microstructural changes in solids take place through solid-state diffusion.18 In 1942, Ernest Kirkendall investigated interdiffusion of copper and zinc in brass at elevated temperatures, observing a net mass transport accompanied by a noticeable volume reduction in brass. He concluded that the reduction in volume was evident because of the faster movement for zinc species out of brass than the influx of copper species; thus, vacancy-assisted hopping was proposed as the main fashion of atomic transport. In 1947, Smigelkas and Kirkendall reported the experiment under carefully controlled conditions and demonstrated that the difference between the diffusion rates of copper and zinc in brass led to the formation of voids in the species at high temperatures. Although the deterioration of alloyed materials occurs due to the formation of Kirkendall voids, chemists further applied this destructive effect for engineering nanovoids inside nanomaterials. In 2004, Alivisatos’s group employed cobalt nanocrystals as a starting material to demonstrate that hollow nanocrystals of cobalt Received: June 23, 2015 Revised: July 13, 2015

A

DOI: 10.1021/acs.jpcc.5b06000 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

concentration. When the core M and shell E react to produce MxEy, rin(t) and rout(t) denote the position of the inner and outer boundaries at time t. Then, the outward growth rate of product is described as

oxides and chalcogenides can be successfully produced through the reaction of cobalt colloidal solution with oxygen (either sulfur or selenium) at elevated temperatures.10 This is the first explicit example discussed on the Kirkendall effect coming to nanoscale. In nanoscale objects, where there is little space for dislocations, and lattice shift in spherical objects may lead to large stresses, Kirkendall voiding takes place, which is not the case for the bulk phase materials. Later on, this principle has been demonstrated to be a general procedure for the production of hollow nanostructures, even an extension to cylindrical nanotubes.19,20

drout nMDM ΔCMrin(t ) = dt rout(t )[rout(t ) − rin(t )]

(1)

where DM, ΔCM, and nM denote the diffusivity, concentration difference, and volume density of M in the MxEy layer, respectively. For the creation of hollow structures, a minimum condition should be satisfied

2. OXIDATION OF METAL NANOPARTICLES Thanks to wide availability of high quality metal nanoparticles (M), the Kirkendall effect can be applied to yield uniform sized hollow compound nanocrystals (MxEy).21−36 In most cases, the E can be elemental O, S, Se, Te, N, or P. A general synthesis route toward metal compound hollow nanocrystals mainly involves two consecutive steps: colloidal synthesis of high quality metal nanocrystals, and the subsequent reaction of the metal nanocrystals takes place to generate target hollow compound MxEy, shown in Figure 1. To date, a wealth of

y·DM |ΔCM| + 1 > rout(t )3 − rin(t )3 x·DE|ΔC E|

(2)

In addition to the well-established fabrication of MxEy singlecomponent hollow nanoparticles, the nanoscale Kirkendall effect can be further extended to the design of multicomponent complex structures for application in catalysis, drug delivery, nanoelectronics, and nano-optics. A generalized fabrication method is as follows: (i) colloidal synthesis of functional nanoparticles, normally chemically inert metal or alloys; (ii) colloidal coating an active metal shell on the existing nanoparticles, forming contacted heterogeneous core/shell structures; (iii) subsequent oxidation of the active metal shell to produce yolk/shell structures. For example, a potential nanocatalyst of Pt/CoO yolk/shell nanostructure was synthesized, involving the synthesis of Pt/Co core/shell structures and the subsequent transformation of Co to CoO hollow structures.10 Gao et al.37 employed the FePt/Co core/shell structures to engineer FePt/CoS2 yolk/shell nanoparticles, which reveals the potent cytotoxicity toward cancer cells. Hence, the same group reported FePt/Fe2O3 yolk/shell nanoparticles that exhibited high cytotoxicity originated from the FePt and strong magnetic resonance (MR) contrast enhancement due to the Fe2O3.38 Cobalt oxide nanowires with Au nanoparticle inclusions (Au−Co3O4 nanowires) were prepared via colloidal polymerization of ferromagnetic Au/Co core/shell structures. Other multicomponent yolk/shell structures, such as gold/iron oxide, carbon/metal oxide, Pt/ Co3O4, and Ag/Fe2O3, were rationally designed powered by the nanoscale Kirkendall effect.39−41 In addition, plasmonic/ magnetic bifunctional Ag/Fe3O4 dumbbell structures are prepared by using Fe nanoparticles (slightly oxidized) as seeds for the direct nucleation and growth of Ag nanoparticles on the Fe nanoparticle surface, during which the Fe nanoparticles were oxidized to form hollow Fe3O4 nanoparticles.42 In all cases, the resulting nanoparticles governed by the nanoscale Kirkendall effect that are processed in organic solution can be soluble in nonpolar organic solvent and can be freely manipulated. Such type of nanoparticles, either hollow or yolk/shell, can act as artificial atoms to self-organize into 2D or 3D superlattices, which may provide an efficient way to fabricate metamaterials that are not observed in nature. Furthermore, these nanoparticles can be used as one component to grow binary/ternary superlattices, constructing bifunctional metamaterials with collective physical or chemical properties. Co-assembly of Au nanoparticles and Fe/Fe3O4 core/shell nanoparticles into 3D quasi-ternary superlattices was revealed by a finely controlled growth process.43 By tuning the molar ratio of Au to Fe/Fe3O4, a series of binary structures by analogy to binary atomic compounds, such as NaCl, NiAs, AlB2,

Figure 1. Schematic illustration of the general evolution process with the generation of voids by the nanoscale Kirkendall effect.

well-defined metal nanocrystals have been applied as a sacrificed template to grow hollow compound MxEy, such as Mg, Al, Fe, Co, Ni, Cu, Zn, In, Ag, Cd, Sn, Pb, etc. The first explicit example of this process was shown by oxidation of Co single nanocrystals in organic solution. A detailed timedependent structure evolution, as illustrated in Figure 1, gives powerful proof in the formation of Kirkendall voids. Similarly, the synthesis of hollow iron oxide nanocrystals has been further explored by several groups for their fascinating magnetic properties. Peng et al.31 have reported a facile solution-phase synthesis of monodisperse hollow Fe3O4 nanoparticles by controlled oxidation of amorphous Fe−Fe3O4, using trimethylamine N-oxide as the oxygen transfer reagent. By annealing the nanoparticles in air at 300 °C for 8 h, the as-formed Fe3O4 hollow nanoparticles can be transformed to γ-Fe2O3 without significant morphological change.31 Furthermore, Fe nanoparticles with tunable size have been synthesized, by subsequent oxidation under O2 flow (20% v/v, 20 mL/min) to produce γ-Fe2O3 hollow nanoparticles.32 It is worth noting here that the different oxidation strategy toward oxidizing metal nanocrystals may result in different crystal structures of metal oxides. Thus, it is highly desirable to establish a controllable chemical reaction to predict the reliable outcome of metal oxide nanocrystals by the nanoscale Kirkendall effect. Although there have been a large number of experimental demonstrations on the nanoscale Kirkendall effect, little theoretical study has been carried out.14,36 A generalized model to describe the Kirkendall effect is a simple steady-state diffusion governed by Fick’s first law. The outward and inward mass transportation is induced by the difference in atom B


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Figure 2. Schematic illustration with high-resolution TEM (HRTEM) images of the evolution process of 7.2 nm Co nanocrystals: (A) amorphous Co nanoparticles, (B) hcp Co with large domains, (C and D) very large domains, (E) small hcp single crystals, (F) hcp single crystals with stacking faults, and (G) hcp single crystals. The scale bar is 3 nm.

ment, Pb/PbS core/shell nanoparticles with hollow interiors are produced, whereas Pb/Ag core/shell nanoparticles with solid interiors are produced when the original Pb particles react with Ag+ in solution. This result was supported by the relative diffusion rates of the materials: Pb atoms diffuse faster than S atoms but slower than Ag atoms. Similarly, hollow PbO nanoparticles cannot form through the oxidation of Pb nanoparticles, because the inward diffusion of the oxygen anions is faster than the outward diffusion of Pb cations at the interface between Pb and PbO. In the following part, we discuss the diffusion couple with a faster outward diffusion rate of core materials than that of shell materials, specifically such as particle size, nanoparticle crystallinity, and particle environment effect. 3.1. Comparison of Diffusion Ion Processes between Amorphous Nanoparticles and Their Single Domain Counterparts: Influence of the Nanocrystallinity. Diffusion of ions in the solid phase takes place because of the presence of defects in those solids. Point defects, such as vacancies and interstitial ions, are responsible for lattice diffusion. Diffusion also takes place along line and surface defects including grain boundaries, dislocations, inner and outer surfaces, etc. As diffusion along linear, planar, and surface defects is generally faster than diffusion within the lattice, they

and NaZn13, can be produced. However, less effort has been devoted to the study of the collective properties of such a type of metamaterials.

3. WHICH FACTOR CONTROLS THE NANOSCALE KIRKENDALL EFFECT? The above part demonstrates that the Kirkendall effect is a powerful tool in the nanoscale regime to produce hollow nanostructures. The prerequisite in the Kirkendall effect is to form the diffusion couple. At the nanoscale, core/shell heterogeneous structures are formed as a diffusion couple, such as oxidation of metallic material. In this section, we will discuss which factor determines the formation of Kirkendall voids during the interdiffusion process in heterogeneous nanostructures. Let us consider spherical core/shell nanoparticles as a diffusion model. Depending on which component (core or shell materials) has a higher diffusion rate, voids can be formed at the material interface with a faster diffusion rate. As a result, core/shell nanoparticles with either a hollow interior or a cracked shell will be produced. Wang et al.44 took the example with Pb nanoparticles as a starting material. When Pb nanoparticles react with sulfur vapor under an inert environC


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Figure 3. STEM-HAADF images of 2D ordered assemblies of the amorphous 7.2 nm nanoparticles after oxidation at 200 °C for 10 min: (A) low magnification; (B) high resolution; (C) STEM-EELS spectrum image.

Figure 4. STEM-HAADF images of 2D ordered assemblies of the hcp single domain 7.1 nm nanocrystals after oxidation at 200 °C for 10 min: (A) low-magnification; (B) high-resolution; (C) STEM-EELS spectrum image.

environment during the chemical synthesis. Lee and co-workers reported a solution chemistry method capable of producing Au nanoparticles in three distinctive crystal structures, single domain crystals, decahedral multiply twinned particles (MTPs), and icosahedral MTPs, by tuning the concentration of Au precursor in solution.42 The other one mainly focuses on the postsynthesis treatment, like thermal annealing, reversible ligand exchange, and crystallinity segregation by supercrystallization.48−52 Nearly amorphous Co nanoparticles (denoted as Coam) produced from reverse micelles at room temperature can be transformed to single domain hexagonal close-packed (hcp)

are leading to high diffusivity or easy diffusion paths. Crystallinity is nevertheless an important nanoparticle attribute. Compared to the imperfection-free single domain nanocrystals, the presence of crystallographic defects and their distribution in the nanoparticles has a nontrivial impact on the physical and chemical properties. However, the control of nanoparticle crystal structure (called nanocrystallinity) has received much less attention and remains an open question.45−47 To date, two principal strategies toward controlled crystallinity of nanoparticles have been utilized, and the first one concerns thermodynamic or kinetic control through a controlled reaction D


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Figure 5. Schematic illustration of the different synthetic strategies toward the various crystalline structures of Co nanoparticles: (A) Reverse micelle methods: produce amorphous phase nanoparticles, followed by a thermal annealing to achieve hcp single-crystalline phase. (B) Thermal decomposition methods: cobalt carbonyl precursors (Co2(CO)8) are decomposed by heating at 200 °C for 1 h. (C) Hot-Injection methods: Co2(CO)8 is injected in a hot solution containing trioctylphosphine oxide (TOPO) and oleic acid (OA).

Co nanocrystals by a subsequent thermal annealing at 250 °C for 60 min. Figure 2 shows the HRTEM images of nanocrystallinity transition of Co nanoparticles from the initial amorphous phase to the single domain hcp phase (denoted as Cohcp).51 Co nanoparticles with controlled crystallinity keeping other particle parameters constant (i.e., nanoparticle size, surface coating) provide an ideal model for studying the nanoscale Kirkendall effect in nanoparticles of differing nanocrystallinity. Very recently, our group demonstrated that the diffusion process of oxygen and cobalt atoms in Co nanoparticles is controlled by their nanocrystallinity.53 By annealing the amorphous-like Co nanoparticles made by reverse micelles method, it is possible to obtain single domain hcp Co nanocrystals, whereas the nanoparticle size remains the same, permitting one to study the influence of nanocrystallinity on the oxygen diffusion process with Co nanocrystals. After deposition of 7 nm colloidal Co nanoparticles differing by the nanocrystallinity on the TEM grids covered by the carbon films, the two samples were simultaneously placed in a modified Schlenk-line setup and were subjected to an oxygen flow and heated to 200 °C. After aging for 10 min, the heating system is removed and an argon flow is introduced in order to stop the intense oxidation process. Let us first consider the Co nanocrystals that are ordered with 2D superlattices. Figure 3 shows that after oxidation the 2D array remains ordered for amorphous Co nanoparticles, whereas the nanoparticles’ contrasts are no longer homogeneous and exhibit a core/shell structure. This core/shell structure was further revealed by high-resolution high-angle annular dark field-scanning TEM (HAADF-STEM) image in Figure 3A, where the inner part of the nanoparticles is in amorphous state, whereas the outer part shows some degree of crystallinity, and the lattice fringe spacing is determined to be 2.2 Å, which is in good agreement with the (200) plane of the

CoO cubic phase (Figure 3B). The local chemical maps of the sample from STEM-electron energy loss spectrum (EELS) measurement are shown in Figure 3C, in which the Co atom (blue) is present throughout the particles with an intense signal in their central region (Figure 3C), whereas the signal of the oxygen atom (red) is mainly intense at the shell region of the nanoparticles with a “hollow” structure. The superposition of the two maps reveals a blue center and a purple edge, in agreement with a Co/CoO core/shell that is claimed. The same measurement is performed on single domain hcp Co nanocrystals with the same nanocrystal size (7 nm). Amazingly, the result markedly differs (Figure 4). The STEM-HAADF image in Figure 4A shows a hollow structure inside the nanocrystal. Note that, throughout the nanocrystal, a single domain of CoO remains present, contrary to what was observed with amorphous nanoparticles, where multicrystal domains are observed (Figure 4B). Figure 4C shows the Co and O projected distributions extracted from an EELS spectrum image, and the Co (blue) and O (red) composite signal demonstrates that the hollow nanocrystals are homogeneous in their chemical distribution. The above result unambiguously demonstrates that the nanocrystallinity of Co nanocrystals plays an important role during the diffusion of Co and O atoms within the Co nanocrystals, as a result of whether or not Kirkendall voids can be generated inside the nanocrystals. As a matter of fact, metallic cobalt can crystallize in three different types of crystal structures: face-centered cubic (fcc), hcp, and ε-phase.54−57 The oxidation of Co with different crystalline structures at the nanoscale is still another open question, in addition to the nanocrystallinity we have elaborated above. Figure 5 shows the HRTEM images of 8 nm Co nanocrystals with various crystal structures, namely, fcc (Cofcc) and ε phase (Coε).58 Figure 5B shows that the nanoparticles are polycrystalline with many defects, and the critical domain size distribution is very large E


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be ruled out because the nanocrystal size can also play an important role during the Kirkendall process as discussed below. For single domain 8.3 nm Cohcp nanocrystals, the TEM image shows that nanoparticles’ contrasts are no longer homogeneous and exhibit a core/shell structure, which differs from the result for 7.2 nm Cohcp nanocrystals with a hollow structure after oxidation treatment (shown in Figure 6D). However, for another single domain 8.1 nm Coε, the TEM image shows that hollow nanoparticles can be produced as a result of the Kirkendall effect (see Figure 6E), while the shell of the hollow structure is composed of several large CoO domains with their characteristic lattice fringe, indicating a polycrystalline structure induced by the surface strain. The above explicit examples may allow one to make a conclusion that atomic ordering of the Co nanoparticles is indispensible for the creation of a single Kirkendall void inside the nanocrystals. Hollow structures cannot be produced from amorphous phase Co nanoparticles in which Co atoms are randomly distributed. This makes the atoms diffuse chaotically and disorderedly; hence, the oxidation process is very difficult after formation of an initial oxide layer and the core/shell structure forms. In other words, atoms diffuse along the lattices within the crystallized nanoparticles, which makes the diffusion of oxygen and cobalt atoms directional. As a result, Kirkendall voids within the nanoparticles generate with the continuous atomic interdiffusion. 3.2. Influence of Nanoparticle Size and Nanocrystallinity in the Final Structure. The size-dependent properties are nontrivial at the nanoscale, such as quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles, and superparamagnetism in magnetic particles.59−61 The size-dependent nanoscale Kirkendall effect was also discussed during the oxidation of Ni nanoparticles in air.62 Three different sizes, 9, 26, and 96 nm, of Ni nanoparticles are used as starting material to study the size-dependent oxidation behavior. For the smaller sized nanoparticles (9 and 26 nm), a single void forms beneath the NiO shell, and the void grows by moving across the nanoparticle while conversion to NiO occurs opposite to the site where the void initially formed. For the larger 96 nm nanoparticles, multiple voids form and eventually result in the porous NiO nanoparticles. This was explained by the fact that the self-diffusion is not sufficient to cause voids to aggregate into a single void. Furthermore, the size-dependent Kirkendall effect was also studied by Nakamura et al.,63,64 during the oxidation of Al, Zn, and Cu in air under low temperature. Generally, low-temperature oxidation of metals shows that an initial thin oxide layer (around 3 nm) was symmetrically formed very rapidly initially, and the oxidation rate drops off to very low levels gradually. According to the Cabrera−Mott theory,65 the low-temperature oxidation theory is based on the assumption that oxygen atoms are absorbed on the oxide surface and that electrons can tunnel through the oxide layer to establish the equilibrium between the metal and adsorbed oxygen. This process creates an electric field driving the metal cations migrating across the oxide layer. The growth of the oxide layer of Al and Zn stops after the formation of the initial oxide layer, whereas the growth of the Cu2O layer continues until the complete consumption of Cu nanoparticles, forming Cu2O hollow nanostructures. As a result of the oxidation of smaller Al nanoparticles, the rapid outward migration of metal cations at the initial oxidation stage produces hollow Al-oxide nanoparticles. However, larger sized Al nanoparticles would not lead to the growth of hollow

and falls in the range from 1 to 5 nm. The corresponding electron diffraction (ED) pattern confirms its fcc structure. Figure 5C shows that the Co nanocrystals produced are perfect single crystals with a complex cubic structure associated with the ε phase as verified by the ED pattern. In order to further determine the role of crystal structure in the nanoscale Kirkendall effect, 8 nm Co nanoparticles with various crystal structures (Coam, Cofcc, Cohcp, and Coε) were simultaneously placed in a modified Schlenk-line setup and were subjected to the same oxidation treatment at 200 °C for 10 min. After oxidation, the TEM contrast is not homogeneous and displays a core/shell structure for 8.3 nm Coam, which is in good agreement with the result obtained from 7.2 nm Co nanoparticle with the same amorphous phase (Figure 6A). For

Figure 6. HRTEM images of 8 nm Co nanoparticles with various crystalline structures after oxidation: (A) Coam; (B, C) Cofcc; (D) Cohcp; (E) Coε.

8.1 nm Cofcc polycrystalline nanoparticles, a hollowing process indicative of the Kirkendall effect takes place and results in hollow nanoparticles. In addition, a small fraction of the nanoparticles still has separated core materials inside the shell, forming a yolk/shell structure (Figure 6B and C). Such a yolk/ shell structure is frequently observed during oxidizing metal nanoparticles, which is a typical intermediate toward the ultimate hollow structure of the nanoscale Kirkendall effect.10 Here, the presence of both hollow and yolk/shell structures indicates that the oxidation rate varies among different 8.1 nm Co nanoparticles, which could be attributed to the large distribution of crystal domain sizes in this sample. Nevertheless, the effect of the size distribution of the Co nanocrystals cannot F


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Figure 7. Size dependence of the Kirkendall effect with different crystallinities of Co nanoparticles.

except for only a few yolk/shell structures in larger sized 9.4 nm nanocrystals. Meanwhile, the shell thickness of the oxide and the size of the cavity were increased with increasing size of Co nanocrystals. It clearly indicates that the Co atoms, in Coε phase, have a higher mobility than the oxygen ones, as a result of diffusing out of Co atoms and the accompanying vacancies were fluxed inward, concentrating at the interface of CoO. Hexagonal close-packed (hcp) structure is, as already mentioned, another single domain crystal phase for Co with a higher packing density than that of ε phase. From this atomic packing feature, we expect that the diffusion of Co atoms slows down compared to Coε and consequently the diffusivity of O atoms is higher relative to the slowed down Co diffusion in the atomically packed crystal. Consequently, three main differences between these two single crystals, Cohcp and Coε, have been observed: (i) The volume of the cavities is smaller with Cohcp compared to Coε nanocrystals. (ii) With a similar size (8.4 nm), core/shell (Co/CoO) nanocrystals and CoO hollow nanoparticles are produced with Cohcp and Coε nanocrystals, respectively. (iii) With both single domain nanocrystals, CoO shells are produced. However, in the low size range, a single domain shell is produced with Cohcp nanocrystals, whereas they are polycrystalline with their Coε counterparts. Face-centered cubic (fcc) structure is another well-known closest packing structure for bulk Co. Unfortunately, it is rather difficult to produce single domain fcc Co nanocrystals. The twinned boundaries always exist inside the nanocrystals. 57 Co fcc nanocrystals with various sizes are produced, and they are subjected to the same oxidation treatment. After oxidation, hollow nanoparticles are produced for the small sized Cofcc nanoparticles. The cavity size is larger for Cofcc, while the shell

nanoparticles, because the growth of the oxide layers stops after the formation of the initial oxidation layer. Very recently, our group systemically investigated the size and crystallinity dependence on the Kirkendall effect by using Co nanoparticles as a model (see Figure 7).66 The size of Co nanoparticles differing by their crystal structures, namely, Coam, Cofcc, Cohcp, and Coε, is effectively tuned from ∼4 to ∼10 nm. All the experiments were carried out under the same condition processed at 200 °C for 10 min under pure O2 flux. After oxidation, for Coam nanoparticles, solid particles with a dense core were produced with various diameters. With the precise observation, the oxidation products of small particles (4.9 nm), a high homogeneous contrast was observed corresponding to a pure CoO phase from the HRTEM studies.66 A mixture of homogeneous solid and core/shell nanoparticles was obtained with increasing Coam nanoparticle size to 6.9 nm. A further increase in the nanoparticle size to 8.1 nm reveals that the nanoparticles’ contrasts are no longer homogeneous and exhibit a core/shell structure with a shell thickness around 3.5 nm, where the shell is a polycrystalline CoO layer and the core is still amorphous Co. This transition from a solid to a core/shell structure is due to the initial oxide layer facilitating the passage of cobalt ions across the oxide layer, and after a certain thickness of oxide film formation, the ion transport becomes difficult to take place. Hence, without a stronger thermal activation, the oxide layer would increase quite slowly and the core/shell structure is formed. However, a significant difference was observed in the investigation on single crystalline Coε nanocrystals. For Coε nanocrystals with various nanocrystal sizes ranging from 5.5 to 9.4 nm, hollow nanoparticles were produced with the formation of a polycrystalline CoO shell, G


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Figure 8. HRTEM images of the isolated Co nanoparticles deposited on a TEM grid: (A) amorphous phase; (B) hcp phase; (C) STEM-EELS spectrum images of isolated amorphous Co nanoparticle after oxidation; (D) STEM-EELS spectrum images of isolated hcp Co nanocrystal after oxidation.

nanoparticles nor those of the corresponding bulk materials. By oxidizing Co nanoparticles, we recently have found that the nanoparticles ordered or not on a substrate have a nontrivial effect upon the oxygen diffusion process. For isolated 7 nm hcp nanocrystals, polycrystalline CoO solid nanoparticles would be produced after oxidation treatment (Figure 8B), whereas hexagonally ordered nanoparticles would produce single crystalline CoO nanoparticles as discussed above (Figure 4). Similarly, for 7.2 nm amorphous Co nanoparticles, an isolated nanoparticle can be fully oxidized and be transformed to polycrystalline CoO solid nanoparticles (Figure 8A), whereas partial oxidation takes place for a 2D ordered nanoparticle array (Figure 3). The above result can be explained as follows: when the nanoparticles are self-ordered, the interdigitation of the coating alkyl chains between the adjacent nanoparticles reduces the oxygen diffusion speed, compared to the isolated nanoparticles coated with the alkyl chain monolayer. Hence, it is necessary to point out that a faster diffusion of oxygen atoms through the hcp Co nanocrystals can result in the shrinkage of the hollow structure, leaving a solid structure. Thus, the Co and O interdiffusion through isolated hcp Co nanocrystals is not strictly termed as a Kirkendall effect despite the presence of the diffusion couple. The density of the surface protection layer can affect the Kirkendall process. Our result shows that the nanoparticles assembled into a compact 2D array effectively increase the density of the protection layer (alkyl chains). Furthermore, the density of the protection layer can also be tuned by controlling the solubility of ligand in the selected solvent. Hung et al.30 reported the synthesis of Cu nanoparticles in the presence of tetradecylphosphonic acid (TDPA). The as-synthesized Cu nanoparticles were dispersed either in hexane or in chloroform under ambient conditions in order to study the room temperature oxygen diffusion through Cu nanoparticles with

thickness is similar compared with that of Cohcp nanocrystals. A yolk/shell structure was produced for larger sized 9.5 nm Cofcc nanocrystals. Furthermore, we find that both the shell thickness and cavity size of the oxide Cofcc increase with an increase of the nanocrystal size. The resulting cavity size compared to the initial size of the oxidized nanoparticles is slightly larger compared to Cohcp nanoparticles but smaller than the Coε ones. That is because Cofcc nanoparticles are not single domain, and the presence of defects inside crystalline lattices yields a faster O diffusion rate through Co lattices. Also, we find a critical CoO shell thickness for the transition between hollow nanoparticles to either core/shell or yolk/shell structure exists with a value of 3.2 nm. After the formation of certain thick oxide layers, the migration velocity of Co and O atoms becomes too difficult to diffuse through its oxide, which suppressed the further oxidation, leaving a core at the center. Hence, we pointed out that the size effect for Co nanoparticles is related to (i) the crystallinity of nanoparticles (amorphous, polycrystalline, or single domain); (ii) the crystal phase of nanocrystals (epsilon or hcp); and (iii) the critical thickness of the oxide layer. 3.3. Nanoparticle Environment Effect. Apart from the above-mentioned that nanoparticle size and nanocrystallinity have a nontrivial effect on the Kirkendall diffusion process, the reaction environment further discussed plays a crucial role during the diffusion process. It is needed to mention that the nanoparticles discussed are coated with a hydrophobic alkyl chain. Hence, the nanoparticle environment includes nanoparticle surface coating and reaction medium. Uniform nanoparticles coated with alkyl chains tend to self-order into a periodically ordered array upon evaporation of the carrier solvent. Over the past decade, a large number of collective physical properties of such assemblies of nanoparticles have been discovered.67−69 They are neither those of the isolated H


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Figure 9. (A) Schematic illustration of a copper grid with deposition of Co nanoparticles. (B) TEM image of hcp Co nanocrystals with electron irradiation (zone 2 in part A). (C) Representative TEM image of Co nanocrystals after oxidation corresponding to zone 2 in part A. HAADF-STEM images of hcp single-domain Co without electron irradiation (D) and with electron irradiation (E), respectively, and elemental maps taken from the rectangle part for cobalt (green), using Co L-edge; for oxygen (red), using O K-edge; the RGB maps by superposition of cobalt and oxygen maps. (F) HAADF-STEM images of oxidized hcp Co nanocrystals with electron irradiation.

Figure 10. STEM-EELS images of hcp Co single domain crystals without (A) or with (B) electron beam irradiation.

In addition, the surface oxidation layer of the metal nanoparticles can also have an impact on the nanoscale Kirkendall effect. TEM is widely used to observe particle morphology, and the evolution of particle morphology observed from TEM usually can be affected as main evidence of the nanoscale Kirkendall effect. However, our recent studies revealed that the TEM beam irradiation changes the nanoparticle environment after their exposure to beam irradiation.70 When an ensemble of Co nanoparticles with single crystalline structures is exposed to the TEM beam prior to the oxidation

different reaction media. Hollow Cu2O nanoparticles were produced in chloroform medium, whereas Cu/Cu2O core/shell nanoparticles were obtained when the Cu nanoparticles were dispersed in hexane. The solubility experiments showed that TDPA is relatively more soluble in chloroform than in hexane. It indicates that the ability of Cu nanoparticles to dissolve oxygen is larger in chloroform than that in hexane. As a result, the surface coating of TDPA can affect the formation of Kirkendall voids in different solvents. I


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catalysis, drug delivery, energy storage, devices, etc., are expected to be markedly enhanced. We suggested that the nanoscale Kirkendall effect might be a general method to form various core/shell and hollow nanostructures under controlled conditions. Furthermore, this can be applied to design multicomponent nanostructures, because combining multiple components within individual nanoparticles is an efficient way to control chemical and physical properties to produce advanced biomedicine and efficient catalysts.

treatment, the subsequent oxidation experiments show that the irradiated Co nanoparticles behave differently compared to unirradiated nanoparticles. Detailed STEM-EELS images are shown in Figure 9, from which elemental distributions and atomic migration can be observed. For the unirradiated Co nanoparticles, the Co and O maps showed a similar distribution, a centered cavity can be observed from the overlapped RGB images, whereas the Co and O maps behaved differently for the irradiated nanoparticles, where the Co signal was enhanced in the center and the O signal was richer at the periphery, forming a core/shell structure from the overlapped RGB image. The irradiated Co nanoparticles behave differently during the oxygen diffusing process, probably because a Co/ CoxOy core/shell structure is formed during the process of electron beam irradiation, and this formed CoxOy shell would hamper the further oxygen diffusion process (see Figure 10), according to the Cabrera−Mott mechanism for oxidation of metals. Although it is generally accepted that a metal/metal oxide core/shell nanostructure is initially formed at the early stage of the Kirkendall process, transformation from the core/ shell to hollow nanostructure takes place via the continuous interdiffusion process. In other words, a break occurs after the formation of an initial oxide layer that hampers the metal/ oxygen interdiffision process, having an impact on the formation of Kirkendall voids within the nanoparticles.

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AUTHOR INFORMATION

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*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

4. CONCLUSIONS AND OUTLOOK The nanoscale Kirkendall effect, based on atomic interdiffusion through the interface between the heterogeneous solids, has been demonstrated to be a powerful tool for the fabrication of hollow-based nanostructures. Here, we have summarized the strategy used for the nanofabrication guided by the nanoscale Kirkendall effect mainly from the oxidation of metal nanocrystals. The overall strategy can be summarized as the fabrication of core/shell nanostructures with a core material having a faster diffusion rate than that of the shell. After formation of the core/shell nanostructures, it is essential to know which factor would determine the formation of Kirkendall voids. In this article, several factors, such as nanoparticle size, nanoparticle crystallinity, and nanoparticle environment, have been specifically discussed. They appear to play an important role during the process of atomic interdiffusion within core/shell nanostructures, despite the core having a faster mobility. This research area is emerging. In the fundamental point of view, we need to have a deep understanding on the process involved in the nano-Kirkendall effects. Some, not trivial, models are needed to carefully explain the atomic outward and inward diffusion. This Feature Article mainly concentrates on the diffusion of oxygen atoms within metal nanoparticles. The influence of nanocrystallinity on spherical nanocrystals remains an open question. Here we provide some examples. However, this has to be highly developed and understood. We need to know if the data presented here can be generalized to other metal nanocrystals and then other nanomaterials. Less is known on the Kirkendall effect of the shaped nanocrystals. Some preliminary results indicate that the shape of the nanocrystals is retained after the Kirkendall process. This has to be confirmed and explained through theoretical simulations. Nowadays the core/shell or hollow nanocrystals have been produced for either isolated or 2D self-assemblies. From our knowledge, none is known concerning the 3D superlattices. In the latter case, the great potentialities of these nanocrystals in

Zhijie Yang, born November 7, 1984, obtained his BSc and MSc in chemistry from Shandong University in 2007 and 2010. In 2014, he received his PhD in chemistry from the Pierre and Marie Curie University under the supervision of Prof. Marie-Paule Pileni on nanocrystallinity effects in Co nanoparticles. He is currently a postdoctoral researcher at Pierre and Marie Curie University. His research interests mainly focused on the nanocrystallinity control of colloidal nanocrystals and the chemical transformations of nanocrystals with differing nanocrystallinity. His current research is devoted to the self-assembly of binary nanocrystal superlattices, and also the physical properties of these new materials.

Nailiang Yang received his BS degree in Material Chemistry from Sun Yat-sen University, China, in 2007. He then obtained his Doctor degree from University of Chinese Academy of Sciences in 2013 under the supervision of Professor Dan Wang. Because of his effort in photoelectric conversion with semiconductor and graphene, he received the award of CAS Excellent Doctoral Dissertations. After J


Feature Article


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graduation, he joined Professor Pileni’s group as a postdoctoral fellow at Université Pierre et Marie Curie, France. His research fields extended to critical synthesis and assembly of metallic nanoparticles, and also the intrinsic properties related to nanocrystallinity.

Professor Marie-Paule Pileni is a senior scientific researcher at the Commisariat à l’Energie Atomique et aux Energies Alternatives (CEA) - “Saclay” and a Distinguished Professor at University Pierre et Marie Curie (UPMC). Her research has been highly interdisciplinary over her entire scientific career, with major scientific breakthroughs in understanding mechanisms in colloidal solutions, modification of enzymes, formation of various assemblies, at mesoscopic scale, in 1D, 2D, and 3D superlattices. She is a member (1999−present) and chair (2004−2010) of Institut Universitaire de France (IUF), which favors the development of high quality research and interdisciplinary projects among French universities. She received the Langmuir award of the American Chemical Society, the lecture award of the Japanese Chemical Society, the research award of the Alexander von Humboldt Foundation in Germany, the Descartes-Huygens prize of the Royal Netherlands Academy of Arts, Science and Emila Valori prize from the French Academy of Sciences, European Research Council (ERC), Advanced Grant Award (2010), Catalán-Sabatier Lectureship award from the Royal Society of Chemistry of Spain, Life Achievement award from Journal Colloid Interface Science, and Chalmers Lecture award.

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ACKNOWLEDGMENTS The research leading to these results has been supported by an Advanced Grant of the European Research Council under Grant 267129. We thank Dr. Imad Arfaoui and Dr. Nicolas Schaeffer for the correction of the language. Z.Y. thanks the China Scholarship Council for financial support.

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Nano Kirkendall Effect Related to Nanocrystallinity of Metal Nanocrystals

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