The Science and Art of Carving Metal Nanocrystals Aleksey Ruditskiy† and Younan Xia*,†,‡,§ †
School of Chemistry and Biochemistry and ‡School of Chemical and Biomolecular Engineering, 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 ABSTRACT: Oxidative etching is a powerful tool for carving out new designs in metal nanocrystals. In this issue of ACS Nano, Jin et al. demonstrate how this tool can be applied to the fabrication of Pd nanoframes by carefully balancing the rates of etching and growth during the excavation of solid nanocrystals. In this Perspective, we offer a brief overview on the evolution of oxidative etching as an alternative route to the facile synthesis of well-controlled metal nanocrystals, as well as an outlook into the future directions of the field.
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Oxidative Etching and Nanocrystal Synthesis. Because metal nanocrystals are typically synthesized in an oxygenated environment, either due to convenience or necessity, oxidative etching is inevitable.5 Oxidative etching is an electrochemical process by which metal atoms are converted to cations, followed by their stabilization through complexation with ligands. Although we only discuss metal nanocrystals in this Perspective, it should be emphasized that oxidative etching is also involved in the synthesis of other types of nanomaterials, including those made of carbon6,7 and metal oxides.8 The most common example of oxidative etching can be found in rust formation when steel is brought into contact with both air and water. In this case, the Fe0 atoms are oxidized into Fe2+/Fe3+ ions, while oxygen molecules are reduced into oxide/hydroxide ions. The presence of an electrolyte, such as halide ions, greatly accelerates the rate of oxidation by providing a salt bridge for the shuttled electrons. Like all corrosion processes, oxidative etching is characterized by the pitting phenomenon, a type of localized corrosion that creates discrete holes in a metal surface. This process is self-driving and capable of causing extensive damage to metal-based infrastructure, such as bridges and pipework, unless discovered and addressed. The critical role of oxidative etching in the formation of metal nanocrystals was initially discovered during a polyol synthesis of Ag nanocrystals.9 When combined with oxygen from air, the presence of Cl− impurity could cause significant changes to the morphology and purity of the final products. In particular, seeds lined with twin defects appeared in the initial
etal nanocrystals have fascinated scientists since Faraday’s time,1 but they have received steadily growing interest over the past few decades owing to their unique position as a bridge between atomic species and bulk materials, as well as their fascinating properties and applications. Like all other types of nanomaterials, the properties of metal nanocrystals show strong correlations to their physical parameters, including elemental composition, size, geometric shape, faceting (i.e., the arrangement of atoms on the surface), and internal structure (e.g., solid, hollow, or porous; convex or concave).2 As demonstrated by many research groups, all of these parameters can be controlled during synthesis to engineer the physicochemical properties of metal nanocrystals and thereby optimize their performance in a variety of applications. Solution-phase synthesis has emerged as the most powerful route to the production of metal nanocrystals with the quality, quantity, and reproducibility necessary for meaningful study of structure−property relationships and further exploration of applications.3 In a typical synthesis, a salt precursor is reduced or decomposed to generate atomic species, followed by the evolution of those species into nuclei, seeds, and nanocrystals. It is generally accepted that a nanocrystal is formed through the continuous deposition of atomic species onto the surface of a growing seed under the guidance and dictation of both thermodynamic and kinetic factors.4 By carefully manipulating these factors, we have witnessed the successful synthesis of nanocrystals with controlled properties, including those with well-defined shapes or facets, twin structures, and/or morphologies. © 2017 American Chemical Society
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merge together, resulting in areas of concentrated dissolution when etching is applied. Another factor relevant to our discussion of surface energy inhomogeneity is the lattice strain effect.14 This phenomenon can be described as the distortion of a crystal lattice away from its ideal configuration, caused by tension at the interface between two different phases, such as facet and twin defect boundaries. Briefly, a twin defect can be defined as an atomic plane that disrupts the ideal stacking sequence of a crystal lattice, with the lattice mirrored across the defect plane. Lattice strain can be either expansive or compressive and is quantified as the percentage change in bond length for surface atoms relative to their ideal bulk value. The effect is felt strongest at the interface and diminishes rapidly as the distance from the interface increases. Strained features increase the surface free energy of a nanocrystal considerably, making it far more prone to oxidative etching compared to its low-strain counterparts. This is the very inhomogeneity that we exploited to create the aforementioned pure samples of Ag nanocrystals.9,10 Significantly, mild etching could be used to target nanocrystals possessing multiple twinned defects within their structure, while leaving behind the single-crystal or singly twinned structures. Finally, surface composition is a vital parameter in determining the etching behavior of a nanocrystal.15 The propensity of an element to dissolution is governed by its reduction potential. In a surface composed of two different metals, the metal with the lower reduction potential will be more readily dissolved upon exposure to an etchant. By tailoring the surface composition of a nanocrystal, we can selectively promote or inhibit the etching at certain sites. For instance, by depositing a metal with a higher reduction potential at the corners or edges of an existing nanocrystal, we can protect these normally vulnerable sites from dissolution while promoting the etching of the unprotected zones. The Carving Protocol. As illustrated in Figure 1, carving removes atoms from the lattice of a nanocrystal one by one,
stage of a synthesis but were dissolved as the reaction proceeded, with the final products dominated by Ag nanocrystals in the single-crystal structure. In addition, Ag nanocrystals with a singly twinned structure could be produced in relatively high purity by weakening the etching strength with the use of oxygen/Br− pair.10 Further studies established that oxidative etching is universally involved in the solution-phase synthesis of nanocrystals from essentially all metals, including Cu, Au, Pd, Pt, Ir, and Rh.5 To this end, it was demonstrated that oxidative etching can serve as a simple and effective method for manipulating the population of seeds and, therefore, nanocrystals with different twin structures, greatly improving the uniformity, purity, and diversity of metal nanocrystals. As demonstrated by Jin et al. in this issue of ACS Nano11 and many prior studies, oxidative etching can also serve as a powerful tool to carve out new designs from existing metal nanocrystals, further increasing their diversity and expanding their scope of application.
As demonstrated by Jin et al. in this issue of ACS Nano and many prior studies, oxidative etching can also serve as a powerful tool to carve out new designs from existing metal nanocrystals, further increasing their diversity and expanding their scope of application. Surface Inhomogeneity and Site-Selective Etching. The atoms that make up the surface of a nanocrystal are not all equal. In fact, there exists a considerable difference in terms of free energy, and thus reactivity, among the surface atoms depending on their locations within the topmost layer. As a general rule, the atoms with the highest free energy are the most susceptible to oxidative etching. This correlation can be employed to initiate and to direct an etching process to carve metal nanocrystals into a myriad of different designs. One of the most important parameters in determining the free energy of an atom is the number of bonds connecting that atom with its nearest neighbors. This is known as the coordination number (CN), with the free energy of an atom increasing as the CN decreases. Atoms located at the edges and vertices of a nanocrystal intrinsically possess lower CNs than those found within the faces. Likewise, the atoms found in different types of facets on the surface also differ in CN and thus in their relative free energies. For instance, the free energies of the ideal, low-index facets of a face-centered cubic nanocrystal follow the order of γ{110} > γ{100} > γ{111} when in vacuum. If the surfaces are passivated with capping agents, the energy hierarchy will be altered, as will the susceptibility of various surfaces toward oxidative etching.12 Crystallographic defects, such as lattice vacancies and grain boundaries, are also spots with high propensities for oxidative etching.13 A vacancy is characterized by the absence of an atom from the lattice, resulting in lower CNs for all of the atoms surrounding the vacancy, while a grain boundary appears due to misalignment at the intersection of multiple crystal grains. Interestingly, unlike most other surface features of a nanocrystal, these are not static and are capable of shifting around the surface in response to the increase in kinetic energy or temperature. During this process, the defects may migrate and
Figure 1. Schematic illustration showing the interplay between oxidative etching (red) and growth (green) in the synthesis of metal nanocrystals. In this example, the rate of etching exceeds the rate of growth.
typically beginning at the surface. In most instances, however, carving is counteracted by the deposition of atoms back onto the structure (i.e., regrowth) in the presence of a reducing agent in the system. By biasing the equilibrium between the two, one can convert an existing nanocrystal into a larger nanocrystal through the buildup of new atomic layers, transform the shape or morphology of a nanocrystal by shuffling around the atoms, or hollow out a solid nanocrystal for the formation of a hollow/ porous structure. The most commonly used protocol involves the use of oxygen from air in conjunction with a halide ion, such as Cl−, Br−, or I−. Upon the application of this etchant, the metal nanocrystal surface acts as a cathode for the reduction of the 24
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oxygen at the expense of the anodic oxidation and dissolution of the metal atoms, with the halide ions acting as a charge carrier and a coordination ligand. Another example is based upon Fe3+ ions, which are readily reduced into Fe2+ by drawing electrons from the constituent atoms of a metal nanocrystal. This approach has been employed to create hollow Pt structures with superior catalytic properties by selectively dissolving the Pd cores from Pd@Pt core−shell nanocrystals.16,17 It has also been demonstrated that an etchant consisting of NH4OH and H2O2 can selectively target the {111} facets on single-crystal Ag nanocrystals, generating complex structures such as octopods.18 Furthermore, it has been shown that oxygen alone can cause PtNi3 nanocrystals to hollow out through Ni dealloying for their transformation into Pt3Ni nanoframes.19 Galvanic replacement is another form of oxidative etching that can result in both structural and compositional changes in a metal nanocrystal.20 This process occurs when the atoms within the metal nanocrystal possess a lower reduction potential than the metal ions added as an etchant. The result is the oxidation and dissolution of the initial nanocrystal (i.e., playing the role as a sacrificial template) coupled with the simultaneous reduction and deposition of the etchant ions on the surface. This approach has been widely explored for the fabrication of hollow nanostructures, such as bimetallic Au−Ag and Pd−Ag nanocages and nanoframes with potential applications in drug delivery and catalysis.20,21 Furthermore, it has been reported that galvanic replacement could be coupled with a diffusion process, such as the Kirkendall effect, to generate even more complex structures, such as hollow Au−Ag nanoboxes with multiple nested walls.22
Figure 2. Transmission electron microscopy images of the Pd nanoframes obtained by excavating solid nanocrystals with different shapes: (a) octahedra, (b) cuboctahedra, (c) nanocubes, and (d) concave nanocubes. Reprinted from ref 11. Copyright 2016 American Chemical Society.
I− etchant pair) and regrowth (powered by formaldehyde) at the edges and vertices of the template nanocrystal, while promoting etching at the faces. Over time, this arrangement converts the solid nanocrystal into a nanoframe. As mentioned previously, the edges and vertices are highly susceptible to etching due to their relatively higher free energies. However, these same differences encourage the deposition of newly formed atoms for the purpose of passivating and thus stabilizing the high-energy sites. The newly created octahedral nanoframes displayed enhanced catalytic properties toward the electrooxidation of formic acid when benchmarked against solid octahedral counterparts, likely due to the predominance of low CN atoms on the nanoframe surfaces. To summarize the impact of etching control on the formation of metal nanocrystals, let us take a look at the possible new designs that can be carved out from a single initial shape. Figure 3 shows a partial list of products that can be obtained by carving a metal nanocube through oxidative etching.25 By carving out atoms from the vertices, one obtains a cuboctahedron as the intermediate product. Continuing with this etching mode will yield an octahedron. In comparison, excavating the enlarged corner facets at the previous step will produce a concave cuboctahedron. Subjecting the nanocube to galvanic replacement will result in a nanocage, followed by an open nanoframe after further dissolution. Excavating from the faces of a nanocube will produce a concave cube, followed by a nanoframe as etching is continued. On the other hand, if the edges of the concave nanocube are targeted, the resultant shape will be a multipronged octopod. All in all, at least seven distinct shapes can be carved out from a simple nanocube by controlling the oxidative etching process.
In this issue of ACS Nano, Jin et al. showcase the power of coupling oxidative etching with co-reduction by reporting a facile approach to the synthesis of Pd nanoframes from nanocrystals with a number of initial morphologies. Coupling oxidative etching with coreduction provides an effective approach for modulating the equilibrium between etching and regrowth, thereby creating nanostructures with controllable properties. For instance, adding a reducing agent during the overgrowth of Au on Ag nanocubes successfully suppresses galvanic etching, producing Ag@Au core−shell structures with notable chemical stability and strong surfaceenhanced Raman scattering activity.23 Additionally, there is a report on the etching/regrowth-based conversion of Pd nanocubes into octahedra, while modulating the size of the final octahedra by controlling the reduction strength.24 In this system, the atoms that were dissolved during the etching process were subsequently reduced and deposited back onto the surface of the nanocrystals. In this issue of ACS Nano, Jin et al. showcase the power of coupling oxidative etching with co-reduction by reporting a facile approach to the synthesis of Pd nanoframes from nanocrystals with a number of initial morphologies.11 Figure 2 shows transmission electron microscopy (TEM) images of some typical examples. The success of this approach lies in achieving a balance between etching (supplied by the oxygen/ 25
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facilitate oxidative etching. For example, the work by Ling et al. has shown that galvanic replacement can be initiated under dry conditions by using an elastomer stamp infused with etchant metal ions.28 The development of dry synthetic methods should cut down on waste and help push nanocrystal science out of the idealized confines of the academic lab and into the world at large.
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Aleksey Ruditskiy: 0000-0002-1146-827X Younan Xia: 0000-0003-2431-7048 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS Some of the research discussed in this Perspective has been supported in part by the National Science Foundation (DMR 1506018) and startup funds from Georgia Tech. Figure 3. Partial list of possible products that can be obtained by carving a metal nanocube along different crystallographic directions.
REFERENCES (1) Faraday, M. The Bakerian Lecture: Experimental Relations of Gold (and Other Metals) to Light. Philos. Trans. R. Soc. London 1857, 147, 145−181. (2) Ruditskiy, A.; Peng, H.-C.; Xia, Y. Shape-Controlled Metal Nanocrystals for Heterogeneous Catalysis. Annu. Rev. Chem. Biomol. Eng. 2016, 7, 327−348. (3) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angew. Chem., Int. Ed. 2009, 48, 60−103. (4) Xia, Y.; Xia, X.; Peng, H.-C. Shape-Controlled Synthesis of Colloidal Metal Nanocrystals: Thermodynamic versus Kinetic Products. J. Am. Chem. Soc. 2015, 137, 7947−7966. (5) Zheng, Y.; Zeng, J.; Ruditskiy, A.; Liu, M.; Xia, Y. Oxidative Etching and Its Role in Manipulating the Nucleation and Growth of Noble-Metal Nanocrystals. Chem. Mater. 2014, 26, 22−33. (6) Moon, C.-Y.; Kim, Y.-S.; Lee, E.-C.; Jin, Y.-G.; Chang, K. Mechanism for Oxidative Etching in Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 155401. (7) Yoshimoto, M.; Yoshida, K.; Maruta, H.; Hishitani, Y.; Koinuma, H.; Nishio, S.; Kakihana, M.; Tachibana, T. Epitaxial Diamond Growth on Sapphire in an Oxidizing Environment. Nature 1999, 399, 340− 342. (8) Sui, Y.; Fu, W.; Zeng, Y.; Yang, H.; Zhang, Y.; Chen, H.; Li, Y.; Li, M.; Zou, G. Synthesis of Cu2O Nanoframes and Nanocages by Selective Oxidative Etching at Room Temperature. Angew. Chem., Int. Ed. 2010, 49, 4282−4285. (9) Wiley, B.; Herricks, T.; Sun, Y.; Xia, Y. Polyol Synthesis of Silver Nanoparticles: Use of Chloride and Oxygen To Promote the Formation of Single-Crystal, Truncated Cubes and Tetrahedrons. Nano Lett. 2004, 4, 1733−1739. (10) Wiley, B. J.; Xiong, Y.; Li, Z.-Y.; Yin, Y.; Xia, Y. Right Bipyramids of Silver: A New Shape Derived from Single Twinned Seeds. Nano Lett. 2006, 6, 765−768. (11) Wang, Z.; Wang, H.; Zhang, Z.; Yang, G.; He, T.; Yin, Y.; Jin, M. Synthesis of Pd Nanoframes by Excavating Solid Nanocrystals for Enhanced Catalytic Properties. ACS Nano 2016, DOI: 10.1021/ acsnano.6b06491. (12) Ortiz, N.; Skrabalak, S. E. On the Dual Roles of Ligands in the Synthesis of Colloidal Metal Nanostructures. Langmuir 2014, 30, 6649−6659. (13) Hines, M. A. In Search of Perfection: Understanding the Highly Defect-Selective Chemistry of Anisotropic Etching. Annu. Rev. Phys. Chem. 2003, 54, 29−56.
OUTLOOK AND CHALLENGES Oxidative etching has proven to be a powerful companion to the growth process in our quest to develop metal nanocrystals, enabling us to control their size, shape, morphology, and structure simultaneously. While the results of the many reported syntheses are apparent, the mechanistic details of the etching processes may still need to be resolved. Frequently, hypotheses are formed based only on the observed initial components and final products, without any information with regard to the intermediate structures and their temporal transformation. The development of in situ TEM techniques will enable researchers to gain those crucial insights. Recently, Alivisatos et al. performed just such a set of experiments by observing the in situ oxidative etching of various Au nanocrystals inside a graphene pocket.26 Notably, they observed a non-equilibrium intermediate during the etching of a Au nanocube before the intermediate was etched away to generate the expected spherical particle. Further development in microscopy techniques may shed new light on what we used to think of as well-understood systems.
At least seven distinct shapes can be carved out from a simple nanocube by controlling the oxidative etching process. Developing etching methods that do not rely on the relative free energies of surface atoms for targeting would significantly increase the usefulness of oxidative etching. Recently, Qin et al. demonstrated that a H2O2 etchant can be produced by enzymes affixed to the surface of a Ag nanocube.27 Further development of methods for attaching such moieties to specific sites of the surface, such as edges or particular facets, as well as controlling the local etchant concentration, would provide a more precise and powerful tool for carving at the nanoscale. Finally, it may be prudent to move beyond wet chemistry as a means to 26
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(14) Sneed, B. T.; Young, A. P.; Tsung, C.-K. Building Up Strain in Colloidal Metal Nanoparticle Catalysts. Nanoscale 2015, 7, 12248− 12265. (15) Gilroy, K. D.; Ruditskiy, A.; Peng, H.-C.; Qin, D.; Xia, Y. Bimetallic Nanocrystals: Syntheses, Properties, and Applications. Chem. Rev. 2016, 116, 10414−10472. (16) Zhang, L.; Roling, L. T.; Wang, X.; Vara, M.; Chi, M.; Liu, J.; Choi, S.-I.; Park, J.; Herron, J. A.; Xie, Z.; Mavrikakis, M.; Xia, Y. Platinum-Based Nanocages with Subnanometer-Thick Walls and WellDefined, Controllable Facets. Science 2015, 349, 412−416. (17) Wang, X.; Figueroa-Cosme, L.; Yang, X.; Luo, M.; Liu, J.; Xie, Z.-X.; Xia, Y. Pt-Based Icosahedral Nanocages: Using a Combination of {111} Facets, Twin Defects, and Ultrathin Walls to Greatly Enhance their Activity toward Oxygen Reduction. Nano Lett. 2016, 16, 1467− 1471. (18) Mulvihill, M. J.; Ling, X. Y.; Henzie, J.; Yang, P. Anisotropic Etching of Silver Nanoparticles for Plasmonic Structures Capable of Single-Particle SERS. J. Am. Chem. Soc. 2010, 132, 268−274. (19) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J. A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.; Stamenkovic, V. R. Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces. Science 2014, 343, 1339−1343. (20) Xia, X.; Wang, Y.; Ruditskiy, A.; Xia, Y. 25th Anniversary Article: Galvanic Replacement: A Simple and Versatile Route to Hollow Nanostructures with Tunable and Well-Controlled Properties. Adv. Mater. 2013, 25, 6313−6333. (21) Yang, X.; Roling, L.; Vara, M.; Elnabawy, A.; Zhao, M.; Hood, Z.; Bao, S.; Mavrikakis, M.; Xia, Y. Synthesis and Characterization of Pt-Ag Alloy Nanocages with Enhanced Activity and Durability toward Oxygen Reduction. Nano Lett. 2016, 16, 6644−6649. (22) Gonzalez, E.; Arbiol, J.; Puntes, V. F. Carving at the Nanoscale: Sequential Galvanic Exchange and Kirkendall Growth at Room Temperature. Science 2011, 334, 1377−1380. (23) Yang, Y.; Liu, J.; Fu, Z.-W.; Qin, D. Galvanic Replacement-Free Deposition of Au on Ag for Core−Shell Nanocubes with Enhanced Chemical Stability and SERS Activity. J. Am. Chem. Soc. 2014, 136, 8153−8156. (24) Liu, M.; Zheng, Y.; Zhang, L.; Guo, L.; Xia, Y. Transformation of Pd Nanocubes into Octahedra with Controlled Sizes by Maneuvering the Rates of Etching and Regrowth. J. Am. Chem. Soc. 2013, 135, 11752−11755. (25) Zhang, H.; Jin, M.; Xia, Y. Noble-Metal Nanocrystals with Concave Surfaces: Synthesis and Applications. Angew. Chem., Int. Ed. 2012, 51, 7656−7673. (26) Ye, X.; Jones, M. R.; Frechette, L. B.; Chen, Q.; Powers, A. S.; Ercius, P.; Dunn, G.; Rotskoff, G. M.; Nguyen, S. C.; Adiga, V. P.; Zettl, A.; Rabani, E.; Geissler, P. L.; Alivisatos, A. P. Single-Particle Mapping of Nonequilibrium Nanocrystal Transformations. Science 2016, 354, 874−877. (27) Wang, C.-W.; Sun, X.; Chang, H.-T.; Qin, D. Generation of Enzymatic Hydrogen Peroxide To Accelerate the Etching of Silver Nanocrystals with Selectivity. Chem. Mater. 2016, 28, 7519−7527. (28) Zhang, Q.; Lee, Y. H.; Phang, I. Y.; Pedireddy, S.; Tjiu, W. W.; Ling, X. Y. Bimetallic Platonic Janus Nanocrystals. Langmuir 2013, 29, 12844−12851.
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