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Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
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Direct in Situ Observation and Analysis of the Formation of Palladium Nanocrystals with High-Index Facets Wenpei Gao,*,† Yusheng Hou,‡ Zachary D. Hood,§ Xue Wang,∥ Karren More,⊥ Ruqian Wu,‡ Younan Xia,§,∥,# Xiaoqing Pan,*,†,‡ and Miaofang Chi*,⊥
Nano Lett. Downloaded from pubs.acs.org by 5.62.159.191 on 10/06/18. For personal use only.
†
Department of Materials Science and Engineering and ‡Department of Physics and Astronomy, University of California, Irvine, Irvine, California 92697, United States § School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ∥ The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, United States ⊥ The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States # School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *
ABSTRACT: Synthesizing concave-structured nanoparticles (NP) with high-index surfaces offers a viable method to significantly enhance the catalytic activity of NPs. Current approaches for fabricating concave NPs, however, are limited. Exploring novel synthesis methods requires a thorough understanding of the competing mechanisms that contribute to the evolution of surface structures during NP growth. Here, by tracking the evolution of Pd nanocubes into concave NPs at atomic scale using in situ liquid cell transmission electron microscopy, our study reveals that concave-structured Pd NPs can be formed by the cointroduction of surface capping agents and halogen ions. These two chemicals jointly create a new surface energy landscape of Pd NPs, leading to the morphological transformation. In particular, Pd atoms dissociate from the {100} surfaces with the aid of Cl− ions and preferentially redeposit to the corners and edges of the nanocubes when the capping agent polyvinylpyrrolidone is introduced, resulting in the formation of concave Pd nanocubes with distinctive high-index facets. Our work not only demonstrates a potential route for synthesizing NPs with well-defined high-index facets but also reveals the detailed atomic-scale kinetics during their formation, providing insight for future predictive synthesis. KEYWORDS: Liquid cell, in situ transmission electron microscopy, high index, catalyst, nanoparticle
T
concave surface formation during synthesis. Previous investigations detailing the formation of concave NPs mainly relied on ex situ characterization techniques, such as ex situ electron microscopy of the NPs collected at different growth stages.17,19,24,25 These experiments provided valuable and detailed information related to the atomic structures attained at various intermediate states; however, the underlying kinetic mechanisms controlling the morphological evolution cannot be conclusively resolved using such ex situ approaches, and thus limited insight can be provided toward the development of new synthesis pathways. During the synthetic process, the shape of nanocrystals evolves dynamically and often is accompanied by atomic diffusion, dissociation or redeposition.1,18,26 In general, the motion of surface atoms is determined by the chemical environment and temperature, whereas the resulting shape of
he synthesis of noble-metal nanocrystals with high-index facets represents a unique strategy for improving the performance of catalysts, as a high-index facet has a higher density of under-coordinated atoms, steps, edges, and kinks, that serve as preferred, highly active catalytic sites.1−5 Depending on the arrangement and distribution of surface facets, nanocrystals with high-index surface can be prepared with different morphologies. Among the various possible geometries, nanocrystals with concave surfaces have attracted special interest because of the negative curvature offering the prevalence of high-index facets.6−9 A number of synthesis strategies have been reported for generating concave nanocrystals, including those involving selective etching,10,11 reduction under surface capping,12−16 seeded growth,8,17,18 galvanic replacement,19,20 kinetic control through modifying the parameters under reaction conditions,9,17,18,21 or electrochemical methods.22,23 Nevertheless, controlled synthesis with atomic precision, which is crucial toward optimizing the catalytic performance, can only be realized based on a thorough understanding of the mechanisms and dynamics of © XXXX American Chemical Society
Received: July 19, 2018 Revised: September 22, 2018
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DOI: 10.1021/acs.nanolett.8b02953 Nano Lett. XXXX, XXX, XXX−XXX
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The liquid cell was assembled according to the procedure described in an earlier paper.29 The holder was loaded into the TEM for in situ observations of the nanocubes in solution. Figure 1a shows a series of representative bright-field (BF)
the particle at the thermodynamically stable state is defined by the differences in the surface energies of the various facets; for example, the equilibrium shape of a particle in vacuum follows the Wulff construction with the lowest total surface energy.27 As an accepted strategy, surface capping agents have often been used to vary the enclosing environment to tune the relative surface energy of different facets and to control the atomic diffusion during synthesis.16,28 By limiting the atomic diffusion from corners and edges to {001} surfaces during growth, nanocubes can be synthesized with {001} faces as the exposing surface.1,29,30 Therefore, it is crucial to elucidate the role of capping agents in changing the diffusion kinetics to better control the growth of NPs. Previous research detailing atomic diffusion on extended two-dimensional surfaces used scanning tunneling microscopy (STM) under ultrahigh vacuum31 or atomic force microscope (AFM).32 Because these studies typically focused on extended, flat surfaces, results can not necessarily be translated to NPs due to their three-dimensional (3D) morphology, including corners, edges, and faces. In environmental TEM, the atomic diffusion on NPs can be monitored in situ during the structural changes under thermal annealing or in reactive gaseous environments.30,33−36 However, quantitative studies revealing the surface diffusion processes on 3D NPs under simulated liquid synthesis environments are limited due to the technical challenges for characterizing surface structures in situ at an adequate spatial resolution in liquids. Recently, advancements related to in situ liquid cell electron microscopy have provided valuable insights regarding the synthesis mechanisms of NPs37 and structure development of materials,38,39 such as the formation of Pt NPs,40,41 Pt−Fe nanorods,42 core−shell structures,29,43,44 and the etching of shaped NPs.45−47 To date, however, neither the role of ligands on altering the NP surface energy landscape nor the synthesis of concave-structured NPs has been studied in situ with a sufficient spatial resolution to reveal the critical dynamic diffusion behavior of metal atoms on NP surfaces in a controlled liquid environment. Here, we report the in situ TEM observation and analysis showing the evolution of Pd nanocubes to concave nanocrystals at subnanometer spatial resolution; the dynamic evolution of the NP morphology and the associated atomic diffusion kinetics were tracked and quantified. More importantly, by systematically conducting a series of in situ experiments under several controlled conditions, we identified the key contributors controlling the formation of concave Pd nanocrystals. We found that the copresence of Cl− and electron beam triggered the formation of PdCl42− ions, which are prone to diffuse along the surface of nanocubes. The presence of 2-pyrrolidone (2P) molecules in PVP as the surface capping on the other hand changes the local surface energies, leading to diffusion of PdCl42− ions from the side faces to corners and edges of the nanocubes, followed by Pd atom redeposition, resulting in the formation of concave Pd NPs. This work not only demonstrates the dynamic morphological evolution of Pd nanocubes to concave Pd NPs but also provides a new synthesis route for the fabrication of concave Pd NPs with high-index facets. Results. For the liquid-cell TEM experiments, Pd nanocubes in deionized (DI) water were dispersed on a silicon chip with electron transparent silicon nitride (SiN) window; after the samples were dried, a drop of aqueous solution containing KCl and PVP was added and then covered with another chip.
Figure 1. Diffusion of Pd atoms from center of sides/facets to corners on the surfaces of Pd nanocube. (a) Sequential BF-TEM images of a Pd nanocube showing the morphological change with time during electron irradiation in liquid cell. The particle shape became concave as highlighted by the arrows. (b−d) Schematics show the formation of concave nanocubes at different times during electron beam irradiation (0−150 s).
TEM images that illustrate how the Pd nanocube evolved into a concave nanocube via atomic diffusion; the actual progression of this morphological evolution is provided in Video S1. The initial Pd nanocube had an edge length of 20 nm at t = 0 s. After exposure under electron beam irradiation (300 kV) for ∼90 s, the sides ({100} facets) of the nanocube began to display brighter contrast in the TEM image, as highlighted by the red dashed curves at t = 91.200 s. A further change evident in the image shows that the projected cube morphology began to exhibit materials extensions at the corners (t = 123.840 s), consistent with the accumulation of Pd atoms at the cube corners. The brighter contrast in the TEM images on the nanocube surfaces indicates further development of the concavity, which was simultaneously accompanied by continuing Pd accumulation at the corners. Thus, observations indicate that the Pd atoms continuously diffused from the surface facets to the cube corners during electron beam irradiation. As such, an octopod-like nanostructure was formed at the conclusion of the experiment (at t = 149.760 s). Although this new structure was highly curved (concave) on all of the {100} faces of the nanocube with enhanced Pd protrusions present at the eight cube corners, the overall volume of the nanocube was retained; the transformation process from a Pd nanocube to a concave Pd nanocube involved a mass redistribution primarily from the centers of the side faces to the corners with the potential Pd dissolution into the solution being limited. The morphological evolution is shown schematically in Figure 1b−d: the initial Pd nanocube (Figure 1b) transforms to an intermediate concave nanocube B
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Figure 2. Measurement of the diffusion of Pd atoms on the surface of a nanocube. (a) The vertical and horizontal distance between surfaces (black and red triangles) and diagonal distance between corners from bottom left to top right and from bottom right to top left (green and blue) during the shape evolution of a Pd nanocube (measured from a time-lapse series of BF-TEM images as shown in Figure 1a). (b) Contour schematic showing the evolution of the nanocube morphology as a function of time. The local curvature is shown in color. (c−e) The volume migrated from surface centers to corners (c), the migration rate (d), and the change of migration rate (e), calculated from the first and second derivatives of the curves in (c).
(Figure 1c) and finally assumes the octopod-like particle shape (Figure 1d). The dynamic shape evolution can be further quantified by tracking the changes in the distances between the side faces (vertical and horizontal) and between the corners (diagonal) of the Pd nanocube as a function of time, as plotted in Figure 2a. The edges of the evolving surfaces are determined using the dark contrast in the BF-TEM images (Figure 1a and Video S1), and the distances between side faces are referenced to the dimensions of the thinnest part of the curved surface. Although no measurable changes were observed for the diagonal lengths (corner−corner distances) in the early stages under electron beam exposure, a decrease in the distance between the side faces was evident immediately after t = 0 s, as shown by the data points (black triangles) in Figure 2a. At t = 70 s, the diagonal distance between corners increased, which was noticeably delayed by 70 s relative to the decrease in distance between side faces. After exposure for 100 s, the surface evolution continued at a higher rate and at t = 150 s, the total increase of the diagonal distance was more than 2−4 nm while the face−face length decreased by 4−6 nm. The acceleration of the morphological evolution after t = 100 s can be attributed to the inwardly curved side faces and the outwardly growing corners that combine to create concavity of the nanocube. In Figure 2b, the edge contours from the two-dimensional (2D) projection in the BF-TEM images (Figure 1a and Video S1)
are shown by outlines spaced at a time interval of 15 s (the contours are colored to display the changes in local curvature). These time-lapse contours clearly demonstrate that as the nanocube morphology evolved, the centers of the side faces curved inward, accompanied by the simultaneous lengthening of the corners, which again confirms the preferred Pd atom diffusion from the side faces to the cube corners. The contour plot also reveals that the change in surface curvature accelerated over time, which is most notably evidenced by the increased rate for the elongation of corner-to-corner distance. A possible reason for this behavior may be that the atomic diffusion rate from the center of the side faces to the cube corners becomes faster as the curvature develops; however, even if the Pd diffusion rate remains constant, the migration of a similar number of atoms from the center of the side faces should still result in a greater change in the concavity as a smaller number of atoms are needed to extend the corners when they become more acute. On the basis of the evolution of the particle morphology observed in situ, we inspected the Pd diffusion kinetics by estimating the migrating volume. In the 2D schematic shown in the inset of Figure 2c and Figure S1, the original nanocube is represented by a square; in this model, the morphological change observed in situ is demonstrated by y, the inward decrease of distance between the cube edge centers, and x, the outward increase of the cube diagonal length. To simplify the C
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Figure 3. Ex situ HAADF-STEM images of Pd particles after in situ study. (a) The pristine Pd nanocube. (b) The Pd nanocube after the initial atomic diffusion on the surface, forming an extended surface area of “pitting” in the outermost atomic layers on the side faces. (c) Pd nanocube with concave surface; white dashed line indicates the size of starting nanocube with ∼20 nm edge length. (d−f) High-magnification images of the top surface of the nanocubes shown in (a−c), respectively.
the change in rate for volume transfer was not significant. Because the liquid environment near the nanoparticle remained nearly constant during the experiment, the slight increase in the number of Pd atoms migrating from the side faces to the corners during the later stage most likely originated from a weaker binding energy of the low-coordinated Pd atoms on the newly formed curved surfaces associated with highindex planes. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging was performed to investigate the morphological evolution of Pd nanocubes at atomic scale. Nanocubes at different evolving stages were imaged and shown in Figure 3. The pristine Pd nanocube, shown in Figure 3a,d (and Figure S2), displays atomically flat and smooth {100} side faces. Figure 3b,e represents the initial stages of morphological evolution, evidenced by the extended missing atomic layer in the center of the {100} side faces indicated by the red arrows, with additional atoms on the two sides. As a result, the concave surfaces formed on the outermost {100} planes with the valley as shallow as one atomic layer. The change from the pristine nanocube to the initial evolution evidences that atoms can migrate from face centers to the two sides, and the shape evolution proceeds at the rate down to single atomic layer. Continuous migration of
model, we draw a straight line between the face center and the corner. Atoms inside the valley (in blue) are transferred from the face center to the corners (in yellow). In three-dimensions (3D), the volume of the valley is a cone defined by the radius of the base, r, and the height, y. From this geometry, the radius can be derived as
(
2 2
x+
2a
y x+ r=
a
) (1)
The volume of atoms being transferred to the corners is
ji y x + 2 a zyz zz 2 1 1 2 1 jjjj zz ·y v = S ·y = πr y = π jj j 3 3 3 jj x + 2 a zzzz (2) k { By incorporating the average x and y values provided in Figure 2a, the average volume of Pd atoms transferred per facet is calculated, as shown in Figure 2c. The diffusion process started with low Pd mass migration and increased to a relatively large volume after t = 120 s. The first derivative yields the rate of volume transfer, as plotted in Figure 2d. The rate of volume transfer indicates that the diffusion process accelerated with time. However, the second derivative (Figure 2e) implies that
(
)
2
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presence of Br−.45 As Cl− has a stronger oxidizing effect than Br−, the presence of Cl− from the KCl together with the oxidative agents from radiolysis could oxidize and dissolve Pd atoms into the solution and form PdCl42−, a process known as halogen etching.47 The formed PdCl42− can preferentially stay on or diffuse along the surfaces rather than diffuse away in the solution due to the blockage of PVP capping on the surface. Upon dissociation from the Pd surfaces, these free PdCl42− ions are also prone to be reduced by electron beam and then “redeposit” on the surface of the nanocubes, establishing a dynamic dissolution−redeposition process. Such reduction and redeposition phenomenon was also observed in liquid cell during the oxidative etching of Pd nanocubes by Br−.45 The location of ion redeposition however is dictated by the surface energy landscape on the NPs, that is, Pd migrates from a location with high surface energy to a site with low surface energy. Overall, the process can minimize the surface energy of the NP. A low electron beam dose is used in our experiments and its potential effect on Pd diffusion is little. It have been reported that electron beam may facilitate the diffusion of atoms; for example, the self-diffusion of Au atoms on Au NPs under electron beam has been reported in other TEM experiments,49 and the nanoparticle coalescence and sintering driven by electron beam in STEM was seen.50 In these reported experiments, however, a high electron dose rate was used. The electron−atom interactions, such as Rutherford back scattering and other inelastic scattering, may drive the atomic motion. In our experiment, however, a very low beam dose which is less than 100 e/Å2·s is used. The beam effect is largely suppressed.29 A minimized electron beam effect in our experiments was also proven by the fact that we did not observe any surface atomic diffusion during extensive imaging on the Pd NPs either in DI water in liquid cell or on regular TEM grid. Atomic diffusion in the form of Pd atoms driven by electron beam irradiation thus does not play a major role in our study. As discussed above, the Cl− ions provided by KCl in the solution plays a crucial role in activating the surface Pd atoms. This process can be expressed using the following reaction that oxidize Pd
atoms led to further concavity in the center of the side faces, as shown in Figure 3c,f. The nanocube finally assumed an octopod-like morphology with rounded corners. The rounding was driven by thermodynamics to assume a configuration with a lower total surface energy. As Pd atoms migrated from surface faces to the corners, they might also move toward the edges. From the HAADF STEM image in Figure 3c, a weak contrast is observed on the exterior edges of the nanocube, which connects to the four corners (observed more clearly in Figure S3). If Pd atom migration only occurred from the center of the side faces toward the corners, the edges should also exhibit concavity rather than forming a relatively uniform “outline” around the particle, as highlighted by the blue dashed line in Figure 3c. Pd atom migration is therefore from the side faces to both corners and edges of the Pd nanocube with the majority migrating to the corners. Atomic-resolution STEM images were further implemented to analyze the enclosing surface facets of the developed concave NPs. This is achieved by analyzing both the surface atomic lattice structure and the relative angle of the surface curvature to the original {100} Pd surface planes in the NPs (Figures S4 and S5). It was determined that the final concave Pd nanocubes are primarily composed of {210} highindex facets that connect the center portion of {100} surface facets with the corners and edges. Discussion. In order to elucidate the origin of the morphological transformation observed on the Pd nanocubes during the in situ microscopy experiments, we investigated the role of each key experimental factor, including electron beam, Cl−, and PVP, by combining control experiments and theoretical calculations. Three sets of control experiments were performed, including two in situ liquid cell TEM experiments and one ex situ experiment. In one liquid cell TEM experiment, the liquid solution contained only Pd nanocubes in deionized (DI) water, that is, without the presence of PVP and Cl−; the liquid cell for the other experiment contained Pd nanocubes and PVP solvated in DI water, that is, without the presence of Cl−. The ex situ experiment kept the mixture of Pd nanocubes, PVP, and KCl for 72 h. The first experiment was designed to understand the synergistic effect of PVP and Cl− while the second one could provide important informant about the role of Cl−. The ex situ experiment on the other hand is to investigate the role of electron beam. Interestingly, no change in the Pd nanocube morphology was observed during the course of either experiment (Figure S6), which suggests that the copresence of each component, that is, Cl−, PVP, and electrons, is critical to the formation of the Pd concave nanocubes. The detailed role of each component in the morphological evolution of nanocaved particles is discussed below. The electron beam plays a critical role in the transformation of Pd nanocubes into concave NPs, because the formation of concave structure does not occur without electron beam irradiation. The primary role of electron beam in the formation of concaved NPs likely lies in two folds: creating oxidizing agents through the radiolysis of water48 and promoting ion deposition.49 The electron beam is known to generate multiple species in liquid cells through the radiolysis of H2O, including OH•, HO2•, O, and H2O2.48 These newly formed solubilized species could potentially act as oxidizing agents similar to dissolved oxygen.48 It has been reported that oxidative species produced by the electron beam lead to the dissolution of Pd NPs in the
Pd + 4Cl− − 2e− → [PdCl4]2 −
The oxidation agents are the radiolysis products of H2O under electron beam, such as OH•, H2O•, O, and H2O2, and so forth. This reaction dissociates surface Pd atoms from the particles and forms mobile [PdCl4]2− compound. This process is critical in the overall morphological transformation which can be clearly proved by that no surface changes were observed in the controlled in situ experiments, where the starting solution contained everything else (PVP and Pd particles in DI water) but Cl−. Although the synergistic oxidative effect from Cl− ions and ionized species from H2O can weaken the surface PdPd bonding on the NP and the newly formed [PdCl4 ] 2− compounds can diffuse and dissolve in the aqueous solution, without other capping agents it is less likely to reshape the NP into a controlled morphology. The result from Cl− ions and electron beam can either dissolve the Pd nanocube by oxidative etching or promote the rounding of the NPs due to the dissolution from corners. However, with PVP capping the resulting NPs exhibit a concave surface morphology. The formation of concave NP thus significantly relies on directed E
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Figure 4. DFT calculations for concave Pd nanocubes. (a) DFT-calculated dependence of surface energy of Pd(111) (black line and squares), Pd(210) (red line and circles), Pd(110) (green line and pentagons), and Pd(100) (blue line and triangles) on surface coverage ratio. Crossovers indicate the cover ratios for which Pd(210) has the same surface energy as Pd(110) at 0.011, Pd(100) at 0.020 and Pd(111) at 0.039. (b) Charge difference for Pd(111) (black line) and Pd(210) (red line). Zero reference of distance is set for Pd atoms at interface. Positions of different Pd layers, as well as oxygen and nitrogen atoms, along the z-axis are indicated by black (Pd(111)) and red (Pd(210)) balls. Bond lengths (in Å) of NPd1 and OPd2 and distribution of charge difference near interface in Pd(111) and Pd(210) are shown in (c) and (d), respectively. (e) The DOS of Pd atoms near surface of Pd(111) (black) and Pd(210) (red). The solid (dashed) lines show DOS of Pd slabs with (without) 2P molecules adsorbed. Fermi energy is set to zero and shown by vertical blue line. Energy window from −1.0 to 0.0 eV is highlighted by region in light cyan. In (b−e), one 2P molecule is adsorbed onto Pd(111) and Pd(210).
area of the corresponding supercell, E2P is the energy of the optimized 2P molecule in the gas phase, and N2P is the number of 2P molecules in the supercell. The surface energies therefore vary with the coverage of 2P molecules. Figure 4a shows the calculated surface energies of each facet as a function of the density of absorbed 2P molecules, η = N2P/Asurf. In this calculation, the long-range vdW correction is integrated (details in Experimental Section). Interestingly, the absorption of 2P molecules on Pd(210) lowers the surface energy of Pd(210) more rapidly than on Pd(110), Pd(100), and Pd(111) when η increases. This stems from the fact that the 2P molecules bind more tightly to the Pd surface atoms on Pd(210) than to Pd atoms on Pd(110), Pd(100), and Pd(111). As a result, Pd(210) appears more stable than all Pd(100), Pd(111) and Pd(110) when the coverage of 2P molecules becomes larger than 0.039 (0.011 for Pd(110), 0.020 for Pd(100)). In fact, the high concentration of PVP used in our experiments can well provide such coverage of 2P molecules on Pd surfaces, especially because the concentration of 2P molecules can be even higher due to the facilitated breakage of PVP chains under the irradiation of electron beam. Therefore, our DFT calculations explicate the experimental observations, namely, the formation of Pd(210) facets is preferred when Pd nanocubes are exposed in the PVP solution used in our experiments. The interaction of 2P molecules with Pd atoms on different surface facets is elucidated by comparing the charge transfer and density of state (DOS) of Pd(210) with that of Pd(111), as Pd(111) has the lowest surface energy among the facets of the pristine nanocube. Figure 4b shows the planar-averaged
diffusion and site-specific redeposition, which are realized by PVP. In the pristine Pd nanocube, there are majorly three types of facets as shown in Figure S2: Pd{100} as the side faces, Pd{110} as the facets on the edges, Pd{111} as the small facets on the corners. Pd{210} facets form during the evolution toward concave nanocube. To elucidate the role of PVP, we performed density functional theory (DFT) calculations to compare the surface energy of the involved surface facets, that is, Pd(100), Pd(110), Pd(111) and Pd(210) with and without the adsorption of PVP. The surface energy of each involved surface facet in a vacuum environment is first calculated for reference, as shown in Table S1. Consistent with previous DFT results,51 the surface energy follows the order of Pd(111) < Pd(100) < Pd(110) < Pd(210), either with or without van der Waals (vdW) corrections. Although Pd(111) is the most stable surface for Pd in a vacuum environment as we show here and being reported,51 the adsorption of PVP was found to modify the relative surface energy of different faces of the nanocube. The modified surface energy of Pd(100), Pd(110), Pd(111), and Pd(210) surfaces was calculated by considering the absorption of 2-pyrrolidone (2P) molecules, which is the repeating units of PVP chain. The detailed models used in these calculations are shown in Figures S7 and S8, where similar approaches have been reported in previous theoretical studies of Au nanocrystals grown in PVP solutions.52,53 The surface energies of Pd in the PVP environment are calculated as E2P surf = [Eslab+2P − N2PE2P − NEbulk]/Asurf, where Eslab+2P is the energy of the slab containing N Pd atoms with 2P molecules, Ebulk is the energy per atom of the bulk Pd, Asurf is the surface F
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Figure 5. Schematics summarizing different routes toward the formation of nanocubes with concave side faces. (a) A pristine Pd nanocube, (b) with available Cl− and oxidative species from the ionized product in water by electron beam, the surface Pd atom forms [PdCl4]2− compound, (c) the [PdCl4]2− compounds are reduced and redeposit on the corners and edges of the nanocube, (d) final concave nanocube after further Pd species migration. (e) Preferential overgrowth at the corners, (f) selective etching/dissolution from the side faces, and (g) self-diffusion of Pd atoms from the center of side faces to the corners and edges.
charge difference, ΔρPd+2P−ρPd−ρ2P, along the z-axis, where ρPd+2P (ρPd) and ρ2P are the charge densities of the Pd slab with (without) 2P molecules adsorbed and the 2P molecules, respectively. The charge redistribution mainly takes place at the interface between the Pd atoms and 2P molecules. Figure 4c,d shows the bond lengths of NPd1 and OPd2 and more details of Δρ in a vertical plane. The charge transfer is more obvious for Pd(210) than Pd(111) and the bond lengths of both NPd1 and OPd2 are shorter for Pd(210) than Pd(111). Furthermore, the adsorption of 2P molecules causes a more noticeable change in the DOS of Pd(210) from −1.0 eV to the Fermi level (Figure 4e). In contrast, the change in the DOS is minor for 2P adsorption on Pd(111) (Figure 4e). All these results explain that 2P molecules interact more strongly with Pd atoms on Pd(210) than on Pd(111) and stabilize the former as the surface coverage increases, resulting in the modification of surface energy landscape on nanocubes. Such lowered surface energy in Pd(210) serves as the driving force of the directed diffusion of [PdCl4]2− and site-specific deposition (at the corners). As a result, a concaved morphology forms, which is primarily enclosed with (210) and other higher index facets with lower surface energy. The mechanisms involved in the morphological evolution from nanocube to concaved NP are schematically summarized in Figure 5a−d. The Pd atoms on the surface are capped by 2P molecules from PVP at the beginning with available Cl− in the solution close to the surface. With the ionized species generated by radiolysis from electron beam irradiation, the surface Pd atoms are oxidized and form [PdCl4]2− compounds (Figure 5a,b). Such [PdCl4]2− compounds are prone to diffuse along the surface; with the change on surface energy induced by 2P molecules, the [PdCl4]2− compounds are favorably reduced and deposited again onto the corners and edges to form the new Pd(210) faces (Figure 5c). Eventually such process relocates the atoms from the center of the surfaces to the corners and edges as shown in Figure 5d. Previously reported seed-based methods for synthesizing concave noble-metal nanocubes mainly included selective
overgrowth at the cube corners and selective etching of the facets, as shown schematically in Figure 5e,f. Here, we demonstrate that concave Pd nanocubes can be fabricated by directing the Pd species to migrate from the face centers to the corners of the nanocubes in the presence of PVP and Cl−, which is assisted by electron irradiation (Figure 5g). It is worth noting that a small amount of Br− and Na2PdCl4 may exist on the surface of the initial Pd nanocubes in our in situ experiment due to the use of KBr and Na2PdCl4 in the synthesis procedure. The existence of Na2PdCl4 can provide extra oxidative effect from the Pd2+ species; because of the stronger adsorption and weaker oxidative effect comparing to Cl−, residual Br− ions can also lead to site-preferred halogen etching, if they are preferentially adsorbed on specific sites. However, on the control in situ experiment performed for Pd nanocubes in the presence of PVP solvated in DI water, no shape changes were observed for the Pd nanocubes (Figure S6), even with extended irradiation times. Therefore, the residual Br− ions and Pd2+ on the nanocube surface, if present, do not noticeably contribute to the shape evolution from nanocube to concave structures. In conclusion, we demonstrate the fabrication of concave Pd nanocubes by directing the Pd species to migrate from the face centers to the corners of the nanocubes in the presence of PVP and Cl−, which is assisted by electron irradiation. This dynamic formation process was visualized in situ at subnanometer resolution by using liquid cell electron microscopy. The key factors defining this atomic diffusion behavior were elucidated by our control experiments. Our results point to a potential synthesis approach that can be used to control the formation of catalysts with concave surfaces. Because the evolution of such concave nanostructures is dependent on both time and electron irradiation, the extent of the concavity developing on the surfaces can be readily controlled by adjusting the electron irradiation time and/or energy. Furthermore, changing the type and concentration of species in the aqueous environment (i.e., PVP, KCl) can alter the surface energy landscape, thereby presenting multiple variables for controlling G
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(VASP) at the level of the generalized gradient approximation (GGA).55−58 We treat Pd-4d5s, O-2s2p, N-2s2p, C-2s2p, and H-1s as valence states, and adopt the projector-augmented wave (PAW) pseudopotentials to represent the effect of their ionic cores.59,60 The PVP chain consists of repeating units of the 2-pyrrolidone (2P) molecule, as shown in Figure S7. The effect of PVP surface capping can thus be evaluated by comparing the surface energies of different facets under the adsorption of 2P molecules (Figure S8). The energy cutoff for the plane-wave expansion is 450 eV.58 In the calculations for the gas phase of 2P molecules, a cubic supercell with a length of 15 Å was used. To obtain reliable adsorption geometries of 2P molecules on Pd surfaces, the long-range van der Waals (vdW) correction is taken into account according to the method proposed by Klimes et al.,61,62 in the form of the nonlocal vdW function (optB86b-vdW). The positions of all Pd atoms, except those in the central Pd layers, are optimized with the criterion that the force on each atom becomes less than 0.01 eV/Å. In the present work, we used (3 × 3) supercells for Pd(100) and Pd(111), and a (2 × 1) supercell for Pd(210),51 which contain 99, 90, and 84 Pd atoms, respectively.
the resultant morphologies. Hence, this work offers new opportunities for the controlled and predictive synthesis of functional NPs. This work also demonstrates the dual function of liquid cell TEM: not only assisting the direction-specific atom diffusion through dynamic association and dissociation with halide ions, but also directly revealing synthesis mechanisms through in situ observation of atom diffusion dynamics at the sub-nm scale. Such in situ imaging allows the dynamic structural and chemical evolutions to be probed at a high spatial resolution as a function of time and synthesis variables, providing direct structural models for theory to predict functionalities of intermediate phases. Combining such in situ observations with first-principle calculations will provide opportunities to explore metastable intermediate phases that are challenging to be studied by ex situ techniques. Furthermore, taking advantage of the flexibility of tuning electron beam condition during in situ experiments may also serve as a facile method to explore directed synthesis with real time adaptive control. Experimental Section. Synthesis of Pd Nanocubes. The Pd nanocubes were synthesized in a similar way to that described in a previous report.54 A mixture of PVP (105 mg, Aldrich, Mw = 55 000), ascorbic acid (60 mg, Aldrich), and KBr (600 mg, Aldrich, >99% trace metal basis) were dissolved with magnetic stirring in a 20 mL glass vial containing 8.00 mL of DI water. The contents were then heated to 80 °C for 10 min under air. Next, 3.00 mL of an aqueous solution containing 57.0 mg of Na2PdCl4 (Aldrich, ≥99.99% trace metal basis) was added using a glass pipet. The glass vial was then capped and allowed to proceed at 80 °C for 3 h. The final product was collected by centrifugation and washed with DI water ten times. S/TEM Characterization, in Situ Liquid Cell TEM Studies, and Ex Situ Studies. In situ liquid cell TEM experiments were performed on an FEI Titan S/TEM operated at 300 kV using a SiN chip-based liquid cell holder (Protochips Poseidon 510). Before the in situ experiment, a small volume (1.5 μL) of the Pd nanocube suspension (2 mg/mL) was added on to the bottom chip, after drying an aqueous solution of PVP (9.5 mg/ mL, Aldrich) and/or KCl (0.01 M, Aldrich) was then added to the same chip. The bottom chip with the liquid layer was then covered by the top chip. This assembled cell was then loaded in the TEM holder with two O-rings for vacuum sealing. During the operation, an electron beam with a spot size of 200 nm diameter was used, with a current density of 2 pA/cm2 (as measured on the screen). TEM images were recorded using a Gatan OneView camera. The entire process was recorded by acquiring a series of BF-TEM images at a speed of 25 frame per second (fps) and 4k × 4k pixels per frame. The sequential TEM images were stacked and aligned to improve the signal/ noise ratio for further analysis. During the course of the experiments, the nanocubes only moved a few nanometers in distance and small in-plane rotations were occasionally observed. These movements were corrected in our distance measurements. We estimate that any small error due to movement of the nanocubes in solution within the liquid cell does not exceed three pixels or more than 0.4 nm in all the distance measurements. We also adjust the objective focus before acquiring each TEM image. High-resolution STEM images were acquired using an FEI Titan STEM with a CEOS Cs probe corrector, operated at 300 kV. Density Functional Theory Calculations. DFT calculations were performed with the Vienna Ab Initio Simulation Package
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ASSOCIATED CONTENT
S Supporting Information *
Supporting Information Available: Video S1 The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b02953. Density functional theory calculation for the surface energy of involved surface facets; Figures S1−S9; Table S1 (PDF) Video S1: Morphological evolution of a series of representative bright-field TEM images that illustrate how the Pd nanocube evolved into a concave nanocube via atomic diffusion (AVI)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (M.C). *E-mail:
[email protected] (X.P.). *E-mail:
[email protected] (W.G.). ORCID
Zachary D. Hood: 0000-0002-5720-4392 Younan Xia: 0000-0003-2431-7048 Miaofang Chi: 0000-0003-0764-1567 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS W.G was supported by the Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Grant DE-SC0014430, and partially by National Science Foundation (NSF) under Grant DMR1506535 and DMR-1629270. Research was supported in part by the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory (ORNL), which is a U.S. Department of Energy (DOE), Office of Science User Facility (W.G. and M.C.). Y.H. was supported by DOE-BES (Grant DE-FG0205ER46237) and computing allocation by NERSC. Research supported in part by the Fuel Cell Technologies Office, U.S. DOE-EERE (K.M.). As a visiting student, X.W. was partially H
DOI: 10.1021/acs.nanolett.8b02953 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
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supported by the China Scholarship Council. Z.D.H. acknowledges a Graduate Research Fellowship award from the National Science Foundation (DGE-1650044) and the Georgia Tech-ORNL Fellowship.
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