Maximizing the utilization efficiency of atoms

The utilization efficiency of atoms refers to the fraction of atoms positioned on the outmost surface relative to the total number of atoms in the whole material; therefore, it is directly proportional to the specific surface area of the material. Traditionally, in heterogeneous catalysis, the utilization efficiency of atoms is increased by reducing the size of the particles.^{Reference Xia and Yang20} For example, by reducing the edge length of a Pt nanocube from 20 to 3 nm, the utilization efficiency will be increased from 6 to 39%, while the specific surface area is increased from 14 to 93 m²/g. This explains why the current Pt/C catalysts used in PEMFCs are typically based upon Pt particles of only 3–5 nm in diameter. While reducing the size of catalytic particles is effective in increasing the utilization efficiency of atoms, the “quality” of the atoms on the surface may be compromised as the increased proportions of atoms at corners and edges will result in lower catalytic activity due to the strong binding of oxygen to those low-coordination atoms.^{Reference Xia and Yang20} Additionally, the reduction in contact area between the Pt nanoparticles and the carbon support tends to make the catalytic system less durable due to particle migration, sintering, and detachment.^{Reference Dai, Lu, Cao, Campbell and Xia21}

An alternative approach to increasing the utilization efficiency of atoms is based upon the development of nanosheets with a thickness approaching a few layers of atoms.^{Reference Xia and Yang20} One can easily understand this concept by considering the structure of graphene.^{Reference Allen, Tung and Kaner22} All of the carbon atoms in a graphene sheet are positioned on the surface, giving a utilization efficiency as high as 100%. For a nanosheet consisting of two layers of metal atoms, the utilization efficiency of atoms can still be kept at 100%. It was possible to hammer Au and other precious metals into ultrathin sheets (e.g., roughly 500 atoms in thickness) even by ancient goldsmiths, but not into nanosheets of only a few atoms in thickness. In a series of recent developments, however, nanosheets as thin as two atomic layers have been successfully synthesized for precious metals such as Pt, Pd, Rh, and Ru, by choosing the right combination of precursor, capping ligand, and solvent.^{Reference Yin, Liu, Pan, Walsh and Yang23–Reference Kong, Xu, Zhang, Dai, Oliaee, Li, Zeng, Wu and Peng26} These results point toward a new strategy for increasing the utilization efficiency of precious metals.

Learning from paper origami

Most of the atoms in nanosheets may not be active toward catalytic reactions because they are inevitably blocked by long-chain ligands necessary for both passivation and stabilization. It is almost impossible to drop-cast nanosheets on a catalytic support in a manner that will allow full exposure of both sides of each nanosheet. Additionally, nanosheets easily crumble, fold, and stack during sample preparation, making their practical use a major challenge (Figure 1a).

Figure 1.

Schematic illustration showing how the strategy of “paper origami” can be utilized to fold 2D sheets into 3D objects to solve issues such as easy stacking and poor dispersion on a supporting substrate.

These technical problems can be solved by “folding” a metal nanosheet into a three-dimensional, hollow structure, a trick that has its origin in paper origami (Figure 1b). In the folding process, the wall thickness of the resultant hollow structure can be kept essentially the same as the original sheet. The exceedingly high utilization efficiency of atoms should be largely retained while the folded structures can be easily dispersed on a catalytic support, just like conventional catalytic particles. While the concept of “folding” is intriguing, it is almost impossible in reality to implement it. An exception is graphene, which has been demonstrated to roll up to generate carbon nanotubes and related structures.^{Reference Kosynkin, Higginbotham, Sinitskii, Lomeda, Dimiev, Price and Tour27}

Alternatively, the same hollow structures can be readily fabricated through template-directed synthesis. The atoms of another metal of interest can be conformally deposited on the surface of a nanocrystal template in a layer-by-layer fashion to obtain core–shell nanocrystals. Removal of the template then leaves behind a hollow nanocrystal whose shape and physical dimensions are essentially identical to those of the original template.^{Reference Zhao, Wang, Yang, Gilroy, Qin and Xia15–Reference Zhang, Roling, Wang, Vara, Chi, Liu, Choi, Park, Herron, Xie, Mavrikakis and Xia17} Such hollow nanostructures are also known as nanocages due to the generation of small holes in the walls during the removal of the core by chemical etching. Conceptually, a nanocage can be considered as a “folded” nanosheet, but with small holes in the walls. If the walls are thinned to ca.1 nm, the utilization efficiency of atoms can reach 40%, and the surface structure can be engineered to yield optimal catalytic activity and selectivity.

Synthesis of nanocages via galvanic replacement

As an electrochemical process, galvanic replacement involves the oxidation of one metal, known as a sacrificial template, by the ions of another metal with a higher reduction potential (Figure 2a). Upon contact, the atoms in the template are spontaneously oxidized and dissolved into the solution phase, while the ions of the second metal are reduced to the elemental state and conformally deposited onto the template. In a single step, one can generate hollow nanostructures with porous walls within a few minutes. Since our first report in 2002 for a system involving Ag nanocubes and HAuCl₄,^{Reference Sun and Xia28} this simple and versatile method has been applied to many different elements for the fabrication of a wide variety of nanostructures characterized by hollow interiors and porous walls.^{Reference Xia, Wang, Ruditskiy and Xia29} The only requirement is to choose the right electrochemical pairs to attain a favorable difference in reduction potential.

Figure 2.

(a) Schematic illustration showing the morphological and structural changes at different stages of the galvanic replacement reaction between a Ag nanocube and PtCl₄²⁻ in an aqueous solution. Step 1 shows the deposition of Pt atoms on the Ag template at the expense of oxidation of Ag atoms, generating a hollow interior; step 2 reveals the formation of a Pt-Ag alloy nanocage with a cubic shape. Transmission electron microscope (TEM) images of (b) the Ag nanocubes that served as a template for the formation of (c) Pt-Ag nanocages through the galvanic replacement reaction with PtCl₄²⁻ precursor. The inset shows the nanocages at a higher magnification. (d) High-angle annular dark field-scanning TEM image of a single Pt-Ag nanocage. Energy dispersion x-ray mapping of (e) Ag, (f) Pt, and (g) Ag + Pt. (d–g) Scale bar = 5 nm. (b–g) Reprinted with permission from Reference Reference Yang, Roling, Vara, Elnabawy, Zhao, Hood, Bao, Mavrikakis and Xia32. © 2016 American Chemical Society.

For the synthesis of Au-based nanocages, one can simply titrate an aqueous solution of HAuCl₄ into a suspension of Ag nanocrystals in water at a controlled temperature. Depending on the exact morphology of the Ag nanocrystals, holes can be created either on the side faces or at the corner sites. For example, when Ag nanocubes with slightly truncated corners are employed, the resultant nanocages tend to have small openings at the corner sites due to the site selectivity associated with the Ag dissolution and Au deposition processes.^{Reference Chen, McLellan, Siekkinen, Xiong, Li and Xia30} Since the redox potentials of both PtCl₄^2–/Pt and PdCl₄^2–/Pd (0.76 V and 0.59 V, respectively, versus the standard hydrogen electrode [SHE]) are more positive than that of the AgCl/Ag pair (0.22 V versus SHE), Pt- and Pd-based nanocages can also be prepared using a similar galvanic replacement reaction with Ag nanocrystals.^{Reference Chen, Wiley, McLellan, Xiong, Li and Xia31} Because the number of Au, Pt, or Pd atoms deposited is stoichiometrically related to the number of Ag atoms oxidized from the template, one cannot separately adjust the composition and wall thickness of the resultant nanocages. In addition, if the galvanic replacement reaction involves the imbalanced removal and deposition of atoms (in the case of Ag template, three Ag atoms are removed for every Au atom deposited when HAuCl₄ is employed as the precursor), the inevitable generation of vacancies makes it difficult for the deposited atoms to faithfully replicate the surface structure on the template. As a result, the nanocages are often characterized by a nonuniform wall thickness and a rough surface.

Figure 2b–g shows transmission electron microscope (TEM) images of the template based upon Ag nanocubes and the nanocages obtained through the galvanic replacement route, as well as the energy dispersion x-ray mapping of an individual nanocage.^{Reference Yang, Roling, Vara, Elnabawy, Zhao, Hood, Bao, Mavrikakis and Xia32} The nanocages are made of an alloy due to interdiffusion between Pt and Ag, which tends to be accelerated in the presence of vacancies caused by the 1:2 stoichiometric ratio between the number of the deposited Pt atoms and number of the dissolved Ag atoms. The nanocages did not show any sign of collapsing when the wall thickness was reduced to a few nanometers. It was found that the density of pores created in the walls of a nanocage increased with the amount of the PtCl₄^2– precursor added. Interestingly, the surface roughness of the resultant nanocages is dependent on the reaction temperature, a phenomenon that can be ascribed to the coalescence of atomic vacancies during the galvanic replacement and dealloying processes.^{Reference Xia, Wang, Ruditskiy and Xia29} Altogether, by introducing an aqueous PtCl₄^2– solution dropwise into a reaction mixture containing Ag nanocubes at 40°C, we were able to obtain Pt-Ag alloy nanocages with a smooth surface and in high yield. The nanocages displayed a single-crystal structure essentially identical to that of the initial Ag nanocubes, enclosed by {100} facets.

Synthesis of nanocages via template-directed deposition and then etching

We have also developed a different approach to the production of metal nanocages by combining template-directed deposition with selective etching. While this approach is applicable to many different combinations of metals, we have mainly focused on Pt and Pd. As illustrated in Figure 3, a typical synthesis starts from the conformal deposition of Pt atoms on Pd nanocrystals with well-controlled surface structures in a layer-by-layer fashion, followed by selective removal of the Pd cores through chemical etching.^{Reference Zhao, Wang, Yang, Gilroy, Qin and Xia15–Reference Zhang, Roling, Wang, Vara, Chi, Liu, Choi, Park, Herron, Xie, Mavrikakis and Xia17} Nanocages with well-defined surface structures and porous walls of controlled thickness down to a few atomic layers (or below 1 nm) can be readily obtained.

Figure 3.

Schematic illustrations showing the fabrication of nanocages enclosed by {111} facets (top row), {100} facets (middle row), and {111} facets plus twin boundaries (bottom row), respectively, through template-directed, conformal deposition, followed by selective etching of the template. The twin boundaries on the icosahedral nanocrystals are marked by red lines.

With regard to the template, we have mainly focused on Pd nanocrystals because of the ability to control their synthesis in terms of diverse morphologies, tunable sizes, and creation of different internal defect structures.^{Reference Jin, Liu, Zhang, Xie, Liu and Xia33–Reference Huang, Wang, Ruditskiy, Peng, Zhao, Zhang, Liu, Ye and Xia35} Additionally, the relatively small (0.77%) lattice mismatch between Pt and Pd enables epitaxial deposition of Pt atoms on a Pd template for the formation of Pd@Pt_nL core–shell nanocrystals (“nL” denotes the number of atomic layers in the shell). Afterward, different etchants such as FeCl₃ and HNO₃ can be employed to selectively remove the Pd cores, producing Pt-based nanocages characterized by hollow interiors, ultrathin and porous walls, and well-defined facet structures.^17–19 One of the major challenges for such a synthesis is to overcome the relatively high Pt–Pt bond energy (307 kJ•mol⁻¹), which tends to favor island growth rather than layer-by-layer deposition for the Pt atoms.^{Reference Xia, Xie, Liu, Peng, Lu, Wang, Kim and Xia36} Another challenge is to generate a Pt shell with an appropriate thickness while retaining a sufficiently large number of Pd channels for the selective removal of the Pd core through chemical etching. Both challenges were successfully addressed by elevating the reaction temperature to simultaneously accelerate the surface diffusion of Pt adatoms and facilitate the interdiffusion between Pd and Pt.^{Reference Zhang, Roling, Wang, Vara, Chi, Liu, Choi, Park, Herron, Xie, Mavrikakis and Xia17}

Figure 4a–d shows TEM images of the Pt-based nanocages derived from Pd nanocrystals with cubic, octahedral, icosahedral, and decahedral shapes, respectively.^{Reference Zhang, Roling, Wang, Vara, Chi, Liu, Choi, Park, Herron, Xie, Mavrikakis and Xia17–Reference He, He, Wang, Lin, Yin, Hong, Wu and Li19,Reference Wang, Vara, Luo, Huang, Ruditskiy, Park, Bao, Liu, Howe, Chi, Xie and Xia37} These nanocages were rich in Pt (>80 wt%). Significantly, by employing different Pd polyhedral nanocrystals as the templates, we were able to precisely control the atomic structures on the surface, including the presence of {100} and {111} facets, as well as twin boundaries. The synthetic method was also extended to other types of Pd templates such as nanorods and nanoplates for the generation of Pt-based nanotubes, prism-like nanocages, and nanorings.^{Reference Huang, Ruditskiy, Choi, Zhang, Liu, Ye and Xia38,Reference Wang, Luo, Huang, Chi, Howe, Xie and Xia39} Figure 4e shows a TEM image of a typical sample of Pt-based nanotubes, whose side faces and ends were covered by {100} and {111} facets, respectively, in addition to the penta-twinned structure.

Figure 4.

Transmission electron microscope (TEM) images of Pt-based nanocages with (a) cubic, (b) octahedral, (c) icosahedral, and (d) decahedral shapes, which were fabricated by templating with cubic, octahedral, icosahedral, and decahedral nanocrystals of Pd, respectively. The insets show high-angle annular dark-field scanning TEM images of individual nanocages. Scale bars = 4 nm. (e) TEM image of Pt-based nanotubes derived from penta-twinned nanorods of Pd. The inset shows an atomic-resolution scanning TEM image of the corner region of a nanotube. (f) TEM image of Au-based nanocages derived from Ag@Au_6L core–shell nanocubes, where the subscript 6L denotes six atomic layers in the shell. The inset shows a TEM image of an individual nanocage at a higher magnification. Scale bar = 15 nm. (a, b) Adapted with permission from Reference Reference Zhang, Roling, Wang, Vara, Chi, Liu, Choi, Park, Herron, Xie, Mavrikakis and Xia17. © 2015 AAAS. (c) Adapted with permission from Reference Reference Wang, Figueroa-Cosme, Yang, Luo, Liu, Xie and Xia18. © 2016 American Chemical Society. (d) Adapted with permission from Reference Reference Wang, Vara, Luo, Huang, Ruditskiy, Park, Bao, Liu, Howe, Chi, Xie and Xia37. © 2015 American Chemical Society. (e) Adapted with permission from Reference Reference Wang, Luo, Huang, Chi, Howe, Xie and Xia39. © 2017 American Chemical Society. (f) Adapted with permission from Reference Reference Sun, Kim, Gilroy, Liu, König and Qin40. © 2016 American Chemical Society.

In addition to Pd nanocrystals, templates made of less expensive metals such as Ag or Cu have also been explored in an effort to reduce the cost.^{Reference Sun, Kim, Gilroy, Liu, König and Qin40–Reference Aslam, Chavez and Linic42} However, these metals generally have lower reduction potentials than those of Pt²⁺/Pt or Au³⁺/Au, and thus the template is susceptible to destruction by the galvanic replacement reaction with PtCl₄²⁻ or AuCl₄⁻ precursor. To address this issue, the Qin group at the Georgia Institute of Technology introduced a strong reductant, such as ascorbic acid, together with the use of an alkaline environment, to promote the reduction of the Pt or Au precursor and thereby suppress the galvanic replacement reaction.^{Reference Sun, Kim, Gilroy, Liu, König and Qin40} When the resultant Ag@Pt_nL or Ag@Au_nL core–shell nanocrystals were subjected to chemical etching with aqueous H₂O₂, the Ag cores could be selectively removed to generate Pt- or Au-based nanocages. In both cases, nanocages of approximately 40 nm in edge length could be obtained even when the wall thickness was reduced to 2 nm. Figure 4f shows a TEM image of a typical sample of the Au-based nanocages that were derived from Ag@Au_6L core–shell nanocrystals.

Platinum-based nanocages for fuel-cell applications

As a novel class of catalytic materials, Pt-based nanocages showed substantially enhanced activity and durability toward ORR, a reaction key to the operation of PEMFCs. Our initial studies mainly focused on the cubic and octahedral nanocages fabricated from templates based upon Pd cubic and octahedral nanocrystals with edge lengths of 18 and 19 nm, respectively.^{Reference Zhang, Roling, Wang, Vara, Chi, Liu, Choi, Park, Herron, Xie, Mavrikakis and Xia17} Our results confirmed that the octahedral nanocages enclosed by {111} facets were more active than the cubic counterparts encased by {100} facets. The specific and mass activities of the octahedral nanocages were 1.98 mA•cm⁻² and 0.75 A•mg_Pt⁻¹, respectively, whereas those of the cubic nanocages were about 0.81 mA•cm⁻² and 0.38 A•mg_Pt⁻¹, respectively. For icosahedral nanocages, the surface is covered by 20 {111} facets and 30 twin boundaries, and their specific and mass activities went up to 3.50 mA•cm⁻² and 1.98 A•mg_Pt⁻¹, respectively. These values were almost 10× greater than those of the current state-of-the-art Pt/C catalyst.^{Reference Wang, Figueroa-Cosme, Yang, Luo, Liu, Xie and Xia18} The nanocage-based ORR catalysts also showed excellent stability when subjected to accelerated durability test. After 5000 cycles in the potential range of 0.6–1.1 V versus SHE, for example, the mass activity of the cubic and octahedral nanocages only decreased by 18% and 22%, respectively. When the number of cycles was increased to 10,000, 32% and 36% drops from the initial values were observed for the cubic and octahedral nanocages, respectively. A more substantial drop of nearly 40% was observed for the icosahedral nanocages after 5000 cycles. Nevertheless, the mass activity of the Pt-based icosahedral nanocages was still approximately four times (0.77 A•mg_Pt⁻¹ versus 0.19 A•mg_Pt⁻¹) as high as that of the pristine Pt/C catalyst.

The catalytic activity of the Pt-Ag alloy nanocages for ORR prepared through galvanic replacement showed variations as a function of time due to the change in elemental composition.^{Reference Yang, Roling, Vara, Elnabawy, Zhao, Hood, Bao, Mavrikakis and Xia32} When the initial Pt₁₉Ag₈₁ nanocages were subjected to electrochemical cycling in the potential range of 0.6–1.1 V versus SHE, the composition changed to Pt₄₆Ag₅₄, Pt₅₆Ag₄₄, Pt₆₅Ag₃₅, and Pt₇₉Ag₂₁ after 5000, 10,000, 20,000, and 30,000 cycles, respectively, due to the dissolution of Ag during electrochemical cycling. After 30,000 cycles, the Pt-Ag nanocages still showed mass activity twice as great as that of the commercial Pt/C catalyst. Based on density functional theory (DFT) calculations, the (100) surface with a composition of Pt₅₀Ag₅₀ gave the greatest specific activity because of the formation of the most stable O₂ transition state, in agreement with the experimental result that showed peak activity for the Pt-Ag nanocages with elemental composition close to Pt₅₀Ag₅₀.

Gold-based nanocages for biomedical applications

Applying Au nanomaterials to cancer theranostics has been of great interest.^{Reference Faraday11} One can functionalize the Au surface with a variety of ligands through the established Au−S chemistry to present a well-defined and precisely engineered interface to interact with the biological system of interest.^{Reference Love, Estroff, Kriebel, Nuzzo and Whitesides43} One can systematically examine the effects of a given parameter on various biological processes, including cellular uptake, tissue penetration, blood circulation, biodistribution, tumor targeting, and body clearance. Other applications are based upon the unique optical properties of Au nanomaterials, including their strong absorption and scattering enabled by LSPR. The absorption cross-section of a Au nanostructure is typically 5−6 orders of magnitude greater than that of a dye molecule, making it a superb contrast agent for optical imaging modalities. More significantly, the LSPR peak of a Au nanostructure can be readily tuned into the NIR region (750–1200 nm) to meet the requirement for in vivo applications while its size, shape, and surface properties can be optimized to enhance the targeting efficacy. The high efficiency of photothermal heating intrinsic to Au nanostructures can also be leveraged to eradicate cancer cells and even ablate malignant tissue.^{Reference Chen, Glaus, Laforest, Zhang, Yang, Gidding, Welch and Xia44} Furthermore, Au nanostructures can serve as a carrier for the delivery of therapeutics, whose release at the diseased site can be controlled using NIR irradiation with good spatial and temporal resolutions.^{Reference Yavuz, Cheng, Chen, Cobley, Zhang, Rycenga, Xie, Kim, Song, Schwartz, Wang and Xia45–Reference Sun, Wang, Wang, Xu, Zhao, Vangveravong, Mach and Xia49}

In one study, Au nanocages were exploited as a novel vector for drug delivery by leveraging their hollow interiors, porous walls, and strong absorption of light in the NIR region. Initially, we attached a thermoresponsive polymer brush to the outer surface of Au nanocages to control the release of an encapsulated drug using NIR irradiation.^{Reference Yavuz, Cheng, Chen, Cobley, Zhang, Rycenga, Xie, Kim, Song, Schwartz, Wang and Xia45} We then advanced the release system by switching to phase-change materials (PCMs) based upon fatty acids or alcohols (Figure 5).^{Reference Shen, Zhu, Huo, Yang, Xue and Xia46–Reference Sun, Wang, Wang, Xu, Zhao, Vangveravong, Mach and Xia49} As the PCM changes its physical state from solid to liquid at its melting point, the theranostic agent premixed with the PCM is released from the cavities. As long as the theranostic agent is miscible with the PCM, it can be readily loaded into the cavities of Au nanocages together with the PCM via diffusion. We could control the release profile by varying the irradiance of the NIR laser and the duration of exposure.^{Reference Shen, Zhu, Huo, Yang, Xue and Xia46} In an in vitro study, we further demonstrated that the surface of Au nanocages could be functionalized with a ligand to target the receptors overexpressed on breast cancer stem cells (CSCs), after their cavities had been filled with a mixture of Dox and PCM.^{Reference Sun, Wang, Wang, Xu, Zhao, Vangveravong, Mach and Xia49} The Au nanocages could target and effectively destruct CSCs through a combination of targeted photothermal heating and chemotherapy.

Figure 5.

Schematic illustration showing how phase-change material, Au nanocage, and photothermal heating can be integrated for controlled release. Adapted with permission from Reference Reference Shen, Zhu, Huo, Yang, Xue and Xia46. © 2017 Wiley. Note: NIR, near infrared; PCM, phase-change material.

To track and quantify Au nanocages in vivo, we recently developed a robust method for their radio labeling with Cu-64 atoms and imaging by positron emission tomography (PET).^{Reference Yang, Huo, Gilroy, Sun, Sultan, Luehmann, Detering, Li, Qin, Liu and Xia50,Reference Yang, Liu, Fu and Qin51} The optical properties of the Cu-doped Au nanocages resembled those of the conventional nanocages, displaying strong and tunable absorption in the NIR region. When radioactive ⁶⁴CuCl₂ was partially used as a source for Cu deposition, we obtained Au nanocages with well-controlled radioactivity for imaging by PET (Figure 6a). After PEGylation (surface functionalization with poly(ethylene glycol) [PEG]), the radioactive Au nanocages were found to accumulate effectively in the tumor through the enhanced permeation and retention effect (see Figure 6b–c). We also sectioned the tumor at 24 h post injection into 40-μm-thick slices for autoradiographic analysis and found that the nanocages were uniformly distributed throughout the tumor regardless of the animal model.

Figure 6.

(a) Linear correlation between the radioactivity of ⁶⁴Cu-doped Au nanocages (NCs) and the amount of ⁶⁴CuCl₂ co-titrated with HAuCl₄ into the reaction system. (b) Transverse positron emission tomography images of 40-nm PEGylated (surface functionalization with poly(ethylene glycol) [PEG]) ⁶⁴Cu-Au nanocages in (upper) 4T1 (orthotopic, with the 4T1 tumor cells implanted in the breast fat pad) and (lower) patient-derived xenograft (PDX, where the cancer cells were derived from the tumor tissue of a human patient and implanted in flank) mouse models at 24 h post tail vein injection (30 μCi). (c) Auto-radiographs showing the intratumoral distributions of the Au nanocages. The color bars represent the signal intensity of accumulated ⁶⁴Cu-doped Au nanocages. Adapted with permission from Reference Reference Yang, Huo, Gilroy, Sun, Sultan, Luehmann, Detering, Li, Qin, Liu and Xia50. © 2017 Wiley. Note: NIR, near infrared.

Ruthenium-based nanocages for potential use in ammonia synthesis

Ruthenium has found widespread use in an array of catalytic applications, including ammonia synthesis. However, extending the template-directed synthesis to Ru is challenging because it has a hexagonal close-packed (hcp) lattice rather than the face-centered-cubic (fcc) structure taken by other precious metals such as Pd, Pt, Ag, Au, Rh, and Ir. Nevertheless, our group recently demonstrated the synthesis of Ru-based cubic, octahedral, and icosahedral nanocages with walls as thin as five atomic layers (Figure 7a).^{Reference Zhao, Figueroa-Cosme, Elnabawy, Vara, Yang, Roling, Chi, Mavrikakis and Xia52–Reference Zhao, Lang, Vara, Elnabawy, Gilroy, Hood, Zhou, Figueroa-Cosme, Chi and Xia54} More interestingly, during deposition, the Ru atoms were packed into an fcc structure, rather than hcp, as a result of the influence of the Pd template. When the Pd core was removed through chemical etching, the resultant Ru nanocage was able to maintain the fcc structure. These transformed icosahedral nanocages enclosed by multiple twin boundaries were able to retain their icosahedral shape and the fcc structure at temperatures as high as 300°C.^{Reference Zhao, Lang, Vara, Elnabawy, Gilroy, Hood, Zhou, Figueroa-Cosme, Chi and Xia54} DFT calculations suggested that the fcc-structured Ru nanocages, when doped with some Pd impurities, were catalytically more active than the hcp-structured Ru nanoparticles toward nitrogen reduction for ammonia synthesis.

Figure 7.

(a) Transmission electron microscope (TEM) image of the Ru-based icosahedral nanocages prepared by templating with Pd icosahedral nanocrystals. The inset shows a high-resolution TEM image of a nanocage. Scale bar = 5 nm. (b) X-ray diffraction patterns recorded from the Ru-based icosahedral nanocages upon in situ heating to different temperatures, indicating that the face-centered-cubic (fcc) structure could be retained up to 300°C. The characteristic peaks of the fcc and hexagonal close-packed structures were marked by blue and red dashed lines, respectively. Reprinted with permission from Reference Reference Zhao, Lang, Vara, Elnabawy, Gilroy, Hood, Zhou, Figueroa-Cosme, Chi and Xia54. © 2018 American Chemical Society.

Hollow versus core–shell nanocrystals

While we have mainly focused on nanocages in this article, the core–shell nanocrystals used to generate nanocages may also provide the same function in terms of catalysis.^{Reference Xie, Choi, Lu, Roling, Herron, Zhang, Park, Wang, Kim, Xie, Mavrikakis and Xia55–Reference Tan, Luo, Nan, Zou, Chen, Shu, Liu, Li, Song, Liao and Adzic58} Of course, blocking the inner surface of the shell will reduce the utilization efficiency of atoms and thus compromise the mass activity. However, the presence of the core, which serves as a support, allows one to reduce the shell thickness to one atomic layer, giving a utilization efficiency of 100%, without concerns about possible deformation of the shell.

In two recent studies, we demonstrated that the Pt shells in the Pd@Pt_2.7L core–shell icosahedra could be well retained even after heating to 300°C, while pores started to appear on the surface of Pt-based icosahedral nanocages at 200°C.^{Reference Wang, Choi, Roling, Luo, Ma, Zhang, Chi, Liu, Xie, Herron, Mavirkakis and Xia57,Reference Vara, Wang, Howe, Chi and Xia59} As another benefit of core–shell nanocrystals, the inclusion of a second metal with an electronegativity different from the metal in the shell can modulate the electronic structure of the surface and thereby enhance its catalytic activity. Additionally, the strain field caused by the lattice mismatch between the core and the shell can also give rise to an enhancement in catalytic activity.

Despite these advantages, there are a number of issues in applying core–shell nanocrystals to catalytic applications at elevated temperatures. First, the shell composition might undergo constant changes during the operation of a catalytic device due to diffusion at elevated temperatures. The in situ electron microscopy study suggested that the Pd@Pt_4L core–shell octahedra gradually evolve into Pd-Pt alloy octahedra under thermal stress, accompanied by compositional changes from Pt-rich to Pd-Pt alloys for the shell and the surface.^{Reference Vara, Wang, Howe, Chi and Xia59,Reference Vara, Roling, Wang, Elnabawy, Hood, Chi, Mavrikakis and Xia60} Second, the material used for the core has to be sufficiently abundant in order to achieve cost-effective and sustainable use of a precious metal. At the current stage of development, Pd is still the best choice of template material for the production of core–shell nanocrystals, but possible replacement candidates for Pd include Ag, Cu, Fe, Co, and Ni.