REViEWS

Crystal-phase and surface-structure engineering of ruthenium nanocrystals

Ming Zhao 1 and Younan Xia 1,2,3 ✉ Abstract | Metal nanocrystals with controlled shapes or surface structures have received increasing attention, owing to their desirable properties for applications ranging from catalysis to photonics, energy and biomedicine. Most studies, however, have been limited to nanocrystals with the same crystal phase as the bulk material. Engineering the phase of metal nanocrystals while simultaneously attaining shape-controlled synthesis has recently emerged as a new frontier of research. Here, we use Ru as an example to evaluate recent progress in the synthesis of metal nanocrystals featuring different crystal phases and well-controlled shapes. We first discuss synthetic strategies for controlling the crystal phase and shape of Ru nanocrystals, with a focus on new mechanistic insights. We then highlight the major factors that affect the packing of Ru atoms and, thus, the crystal phase, followed by an examination of the thermal stability of Ru nanocrystals in terms of both crystal phase and shape. Next, we showcase the successful implementation of these Ru nanocrystals in various catalytic applications. Finally , we end with a discussion of the challenges and opportunities in the field, including leveraging the lessons learned from Ru to engineer the crystal phase and surface structure of other metals.

The past two decades have witnessed the development of metal nanocrystals7–11. Unlike shape control, which of new colloidal metal nanocrystals that are finding only involves surface atoms or a small portion of inter- widespread use in applications such as catalysis, energy nal atoms (such as twin boundaries and stacking faults), conversion and storage, photonics, electronics and bio- any modulation of the crystal structure requires the medicine1–5. However, in the case of noble metals, their rearrangement of all atoms to form a new lattice. As extreme scarcity in the Earth’s crust (typically at levels of such, the physical and chemical attributes of the result- parts per billion) and ever-increasing price have quickly ant nanocrystals can be substantially altered to enhance created a barrier to their large-scale use. To enable the their properties and performance. For example, when widespread use of noble metals in catalytic applications, Ru is crystallized in a body-centred cubic (bcc) structure, it is necessary to substantially reduce the amount of computational simulations show that the metal becomes metal needed by improving the catalytic performance. ferromagnetic, whereas bulk Ru in the hexagonal One effective strategy relies on shape-controlled synthe- close-packed (hcp) phase is paramagnetic12. sis, which enables the engineering of facets and, thus, of For a bulk solid material, the total energy is domi- 1School of Chemistry and the surface structure of nanocrystals1–5. For instance, a nated by the bulk energy, which is extremely difficult Biochemistry, Georgia systematic study of the facet-dependent activity of Pd to overcome to rearrange the atoms. When moving to Institute of Technology, nanocrystals towards formic-acid oxidation indicated nanocrystals, the surface energy plays a more impor- Atlanta, GA, USA. that the well-defined {100} facets of Pd nanocubes were tant role, making it feasible to attain nanocrystals with 2The Wallace H. Coulter Department of Biomedical 3.4 times more active than the poorly defined surface atomic packing different from that of the bulk mate- 6 13 Engineering, Georgia Institute of a commercial Pd/C catalyst . When {100} facets rial . For example, it was demonstrated that Pd nano- of Technology and Emory were combined with twin boundaries in Pd bipyrami- cubes can undergo a phase transition from face-centred University, Atlanta, GA, USA. dal nanocrystals, the specific activity showed a 5.2-fold cubic (fcc) to face-centred tetragonal (fct) at pressures 3School of Chemical and 6 (ref.14) enhancement relative to the commercial Pd/C catalyst . above 24.8 GPa . It was also reported that fcc-Pt3Co Biomolecular Engineering, Shape-controlled synthesis has been accomplished for nanocrystals with a random distribution of the Pt and Georgia Institute of Technology, Atlanta, GA, USA. almost all noble metals, with notable examples including Co atoms could be converted into Pt3−xCo@Ptx core– (refs1–6) ✉e-mail: younan.xia@ Ag, Au, Pd, Pt and Rh . shell nanocrystals after annealing at 700 °C under H2 bme.gatech.edu In addition to shape control, people also search for atmosphere for 2 h. The Pt and Co atoms in the core https://doi.org/10.1038/ new control knobs, such as the crystal phase or struc- took a simple cubic packing for the generation of a s41578-020-0183-3 ture, that can be used to further enhance the properties L12 structure, and the core–shell nanocrystals showed

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Pd@fcc-Ru core–shell icosahedra and Pd@fcc-Ru core–shell nanocubes and fcc-Ru fcc-Ru icosahedral nanocages covered cubic nanocages enclosed by {100} facets21 by {111} facets and twin boundaries23 Pd@fcc-Ru core–frame cuboctahedra and fcc-Ru nanoframes with Discovery of fcc Pd@fcc-Ru core–frame octahedra and fcc-Ru 4H/fcc Au@Ru core–branch nanowires a cuboctahedral shape50 structure in Ru nanoframes with an octahedral shape25 and 4H/fcc-Ru nanotubes46, 48 nanocrystals24 Rh@fcc-Ru core–shell Pd@fcc-Ru core–shell octahedra octahedra covered by Synthesis of pure fcc-Ru and fcc-Ru octahedral nanocages {111} facets and a shell nanocrystals with tunable sizes17 enclosed by {111} facets22 thickness of 4.5 nm (REF.39)

2010 2012 2013 2016 2017 2018 2019

hcp-Ru nanoplates with a hcp-Ru nanocrystals with hcp-Ru Au@Ru fcc-core–hcp-branch hcp-Ru nanobranches 32 triangular shape29 an hourglass shape30 nanochains25, 26 nanocrystals53

hcp-Ru nanocrystals with hcp-Ru nanowires Pd@Ru fcc-core–hcp-branch a capped-column with a polycrystalline nanocrystals54 morphology29 structure 28

Fig. 1 | Timeline showing key accomplishments in engineering the crystal and surface structures of Ru nanocrystals. 4H, hexagonal Bravais with four minimal repeating atomic layers (ABCB); fcc, face-centred cubic; hcp, hexagonal close-packed.

substantial enhancements in both activity and durability (where H represents the hexagonal Bravais lattice and towards the oxygen-reduction reaction15. In addition to 4 stands for the number of minimal repeating atomic lay- the application of external stimuli (such as high pressure ers, ABCB), and that the deposited metals all possess the and/or high temperature), wet-chemical synthesis under same 4H phase20. Seed-mediated growth has also been ambient pressure and at relatively low temperature has applied to the facile synthesis of Ru nanocrystals with an also been explored to induce phase transitions in nano- fcc lattice and controlled surface structures, including the crystals. Under the right conditions, new crystal phases type of facet and the presence of twin boundaries21–23. can be produced and preserved during the homogene- Using Ru as an example, in this Review, we discuss ous nucleation process. Using this strategy, nanocrys- the recent progress in developing noble-metal nano- tals featuring new crystal phases have been achieved crystals with both new crystal structures and controlled for a number of noble metals, including hcp-Rh and shapes (Fig. 1). We choose to focus on Ru because it is fcc-Ru (refs16,17). one of the very few metals that have been prepared as Despite the variation in crystal phase, the prod- nanocrystals purely in the hcp or fcc phase7,8,17. Metals ucts reported in those studies typically showed poorly are well known for their limited polymorphism and, defined shapes or morphologies, so that the bene- essentially, all of them crystallize in one of the three fits expected from shape control tended to vanish. crystal structures stable under atmospheric pressure: bcc, Regarding shape-controlled synthesis, essentially all hcp and fcc. All noble metals form the fcc phase except studies reported so far only dealt with noble metals that Ru and Os, which favour hcp. Ru can also exist in the fcc take an fcc structure1–6. In recent years, it was reported phase under atmospheric pressure when prepared as that small domains featuring hcp or other metastable nanoparticles17,24. Unlike for other noble metals such phases could emerge in fcc noble-metal nanocrystals as Ag, Au, Pd and Pt, there are only a limited number of as a consequence of stacking faults4,6. However, the publications on the synthesis of Ru nanocrystals. Most percentage of the new phase in the whole sample is of the products obtained through homogeneous nuclea- very small, typically below 5%. This predicament has tion are poorly defined in shape and of very small sizes, motivated people to develop new approaches to simul- typically below 5 nm (refs24–27). There are only a few taneously control the crystal phase and shape of nano- reports on the homogeneous nucleation and growth crystals. In one method, one-pot synthesis was replaced of hcp-Ru nanocrystals in the shape of plates, discs, by seed-mediated growth, in which preformed seeds prisms or pyramids28–32. This can be attributed to the are introduced to direct the nucleation and growth of unique properties of Ru: the Ru3+/Ru pair has a much a metal of interest18,19. Relative to homogeneous nuclea- lower reduction potential than other noble metals, tion, heterogeneous nucleation has a lower energy bar- which makes it relatively difficult to produce Ru atoms rier and it allows one to finely manoeuvre the growth from a precursor through chemical reduction33; and Ru pattern. In particular, when nanocrystals with the thermodynamically favours the hcp structure, which, desired crystal phase and shape are leveraged as seeds, together with the very high cohesive and surface ener- one can simultaneously replicate both their crystal gies of Ru relative to those of other noble metals, makes phase and surface structure in the deposited metal18,19. it unfavourable to create and/or enlarge facets other For example, it was reported that Ir-based, Rh-based, than {0001}34. Thus, it is challenging to control the crys- Ru-based and Cu-based nanoribbons can be obtained tal phase of Ru while processing it as nanocrystals with through epitaxial growth on 4H-Au nanostructures well-controlled surface structures.

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We first review strategies for controlling the crystal stru­ phase and shape from the experimental and computational cture and shape of Ru nanocrystals, with a focus on the perspectives. Afterwards, we highlight the use of Ru nano- mechanistic understanding of various synthetic protocols. crystals in an array of catalytic applications, with a focus We also discuss the key factors responsible for the determi- on performance enhancement by engineering both crystal nation of the crystal structure. We then evaluate the ther- phase and surface structure. Finally, we comment on some mal stability of Ru nanocrystals in terms of both crystal of the remaining challenges and discuss future directions.

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◀ Fig. 2 | Ru nanocrystals with hexagonal close-packed structure. Transmission in the shape of hourglasses were obtained after 70 h of electron microscopy images of irregular particles (panel a), chains of particles (panel b), reaction30 (Fig. 2f). The formation of such a morphol- polycrystalline wires (panel c), plates (panel d), capped columns (panel e) and hourglasses ogy involved the coalescence of partially crystalline, (panel ). Panel reprinted with permission from ref.24, ACS. Panel adapted with f a b rod-like nanoparticles, followed by crystallization and permission from ref.25, ACS. Panel c reprinted with permission from ref.28, ACS. overgrowth. Panels d and e adapted with permission from ref.29, ACS. Panel f reprinted with permission from ref.30, ACS. Ru nanocrystals with an fcc structure The packing of atoms in bulk Ru is well documented to Synthesis and characterization follow an hcp lattice. The Ru nanocrystals shown in Fig. 2 Ru nanocrystals with an hcp structure all share the same hcp structure. In 2010, Somorjai and In a typical wet-chemical method involving homogene- colleagues24 observed peaks of both hcp and fcc phases ous nucleation, a precursor is reduced or decomposed in the X-ray diffraction (XRD) pattern of a Ru sample. to atoms, which nucleate to generate seeds when their This discovery indicated that Ru nanocrystals can take concentration passes supersaturation. As the atoms are a metastable fcc structure in addition to the thermody- depleted to a level below the minimum supersaturation, namically favoured hcp phase24. The fcc packing was fur- nucleation ceases and the seeds grow into nanocrystals ther confirmed by imaging at the atomic scale31: Fig. 3a–c with gradually enlarged sizes. By optimizing reaction shows atomic-resolution scanning transmission electron parameters such as the type of reductant and temper- microscopy images of Ru nanocrystals with atomic ature, Ru nanocrystals with relatively uniform sizes but packings31 consistent with fcc, a mix of fcc and hcp, and poorly defined shapes could be obtained using the polyol hcp phases. These assignments were in agreement with method. The Ru nanocrystals in Fig. 2a were synthesized the fast Fourier transform patterns. Later on, a method with butanediol, serving as both the solvent and the for the facile synthesis of Ru nanocrystals featuring a reductant24. When switching to ethylene glycol and pure fcc phase was reported, using an appropriate pair introducing the precursor via hot injection rather than of precursor and polyol17. The purity of the fcc phase was pre-dissolution in the polyol, a larger number of Ru verified by XRD analysis17. seeds was generated, and the smaller nanocrystals In conjunction with the different crystal structures, tended to assemble into chains via an attachment Ru nanocrystals also differ in terms of coordination mechanism25,26 (Fig. 2b). Because the standard reduction number, bond length and lattice disorder35,36. The aver- potential of Ru3+/Ru is quite low (0.39 V vs the standard age coordination number of surface atoms (denoted

hydrogen electrode, SHE), reducing agents stronger than as CNsurface) increases with particle size for both fcc-Ru 27 (Fig. 3d) polyols would be advantageous . To this end, sodium and hcp-Ru nanocrystals . The CNsurface of hcp-Ru

borohydride (NaBH4) was used to facilitate the genera- nanocrystals is smaller than that of their fcc counterparts tion of Ru atoms in an aqueous system for the formation when the particle size is below 3.9 nm, but becomes of Ru polycrystalline nanowires in the presence of tem- larger for bigger particles35. Because the coordination plates based on polycarbonate membranes with different number of surface atoms strongly correlates with sur- pore sizes, followed by the dissolution of the membranes face energy and, hence, with the strength of interaction with dichloromethane28 (Fig. 2c). with adsorbate molecules, Ru nanocrystals are antic- Solvothermal methods, in which the solvent is used ipated to show size-dependent properties in various at temperatures above its boiling point to leverage the catalytic reactions. The Ru–Ru bond length for fcc-Ru increased autogenous pressure, have also been explored. and hcp-Ru also depends on particle size, slightly elon- The reactivity of the reactant is greatly enhanced. gating as the particle size decreases35 (Fig. 3e). The bond A hydrothermal route was developed for the synthesis of length in fcc-Ru nanocrystals is always shorter than in Ru nanoplates with a thickness as small as 2 nm (ref.29) hcp-Ru nanocrystals, indicating a stronger bond in the (Fig. 2d) , involving the reduction of RuCl3 with HCHO fcc phase. in water at 160 °C. Density functional theory (DFT) During synthesis, the arrangement of atoms might calculations suggested that the Ru nanoplates adopted not perfectly follow a certain lattice because of its high an anisotropic growth pattern to minimize the total sensitivity to the fluctuations in reaction conditions. As a surface free energy through the expression of the most result, atomic disorder tends to appear in the nanocrys- stable {0001} facets. When sodium oxalate was added, tals, and its extent strongly depends on crystal phase and capped columns with a flat terminal plate on each end particle size35. The structural-order parameter (derived were formed, which then self-assembled into strings in using reverse Monte Carlo modelling) of fcc-Ru nano- an end-to-end manner29 (Fig. 2e). Two growth steps were crystals decreases as the particle size increases (Fig. 3f). involved in the formation of the Ru-capped columns: the For diameters below 3 nm, the order parameter of fcc-Ru axial growth of the trunk owing to the selective adsorp- nanocrystals is bigger than that of hcp-Ru nanocrys- tion of oxalate species on the side faces, {10–10}, which tals, whereas the opposite trend is observed for parti- retarded growth on these faces; and the radial growth cle sizes above 3 nm. For most catalytic applications, of the end plates resulting from perturbations to the nanocrystals with atomic disorder are desired because reaction system, such as the decomposition of oxalate. the disordered atoms are typically low-coordinated A modified solvothermal method was also reported, and can induce surface strain37,38. These highly active

in which Ru(acac)3 was reduced in mesitylene (boiling sites can effectively reduce the activation-energy bar-

point = 165 °C) at 140 °C in a closed vessel filled with H2. rier for reactants and, thereby, boost the catalytic With the assistance of dodecylamine, Ru nanocrystals performance.

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a b c

d e f fcc fcc fcc 7.0 2.698 2.680 Bond length (Å) 2.0 hcp hcp hcp 6.5 2.696 1.5

2.676 ameter age a.u.) 5

er 6.0 2.694 1.0 Av 2.672 (×10 der par dination number 5.5 2.692 0.5 Bond length (Å) Or

coor 5.0 2.690 2.668 0.0 2.0 3.0 4.0 5.0 2.0 3.0 4.0 5.0 2.0 3.0 4.0 5.0 Diameter (nm) Diameter (nm) Diameter (nm) Fig. 3 | Ru nanocrystals with different atomic packing and, thus, distinctive physicochemical properties. a–c | Atomic- resolution scanning transmission electron microscopy images of Ru nanocrystals characterized by a face-centred cubic (fcc) structure (panel a), a mix of fcc and hexagonal close-packed (hcp) structures (panel b) and an hcp structure (panel c). The insets show the corresponding fast Fourier transform patterns. d–f | Average coordination number of surface atoms (panel d), Ru–Ru bond lengths (panel e) and order parameters (panel f) of fcc-Ru and hcp-Ru nanocrystals as a function of particle diameter. The order parameter quantifies the degree of atomic order in nanocrystals. Panels a–c adapted with permission from ref.31 (https://pubs.acs.org/doi/abs/10.1021/jacs.6b11291); further permissions related to the material excerpted should be directed to the ACS. Panels d–f adapted from ref.35, by permission of PCCP Owner Societies.

Non-hcp nanocrystals with controlled facets a non-hcp structure. During synthesis, the cubic seeds Synthesis of core–shell nanocrystals. The Ru nanocrys- gradually evolved into truncated cubes, cuboctahedra, tals with hcp, fcc and a mix of hcp and fcc structures we truncated octahedra and octahedra, with different ratios have discussed so far are poorly defined in terms of of {100} to {111} facets. The packing of Ru adatoms repli- shape or surface structure, owing to the intrinsic pref- cated well that of the underlying Rh atoms, taking on an erence for the hcp phase and the low surface energy of fcc structure thermodynamically unfavourable in bulk Ru its {0001} facets33. To address this issue, one can rely on (ref.39). In spite of this successful synthesis, the lack of high- the use of preformed nanocrystals (the so-called seeds) quality Rh seeds with other facets or internal structures to build a platform for the nucleation and growth of Ru creates an obstacle to controlling the surface structure atoms, a process referred to as heterogeneous nuclea- of Ru nanocrystals. tion18,19. The energy barrier to heterogeneous nucleation is To address this issue, Pd, which is also an fcc metal, always lower than that to homogeneous nucleation. was selected as a substitute for Rh as the template. Additionally, seed-mediated growth offers many other Although the lattice mismatch between Pd and Ru benefits, including high reproducibility between batches, is slightly bigger than that between Rh and Ru (1.8% tight control over size and shape, and easy tracking of the vs 0.5%), the use of Pd seeds offers many advantages. growth process18. Firstly, there are many well-established protocols for In the context of seed-mediated growth, the seeds the synthesis of high-quality and uniform Pd nano- play a pivotal role in determining the growth behaviour crystals with controlled sizes, facets and internal struc- of Ru atoms18. Its use for the synthesis of Ru-based octa- tures40. Secondly, Pd is much less resistant to oxidative hedral nanocrystals with well-defined {111} facets was etching than Ru, making it feasible to selectively etch recently explored39 (Fig. 4a). The success of this synthesis away Pd for the production of Ru hollow nanocrys- relied on the use of 4.5-nm Rh cubes as seeds to facilitate tals41. In particular, when the Ru shells are only a few the heterogeneous nucleation and growth of Ru atoms. atomic layers thick, the utilization efficiency of Ru Rh was chosen because it is similar to Ru in terms of atoms in the hollow nanocrystals can be substantially lattice constant and electronegativity, and it can resist increased. Unlike in the one-shot synthesis involving oxidative etching under harsh reaction conditions (such Rh seeds, the Ru(III) precursor was titrated into the as the hydrothermal environment). Additionally, Rh can growth solution containing Pd seeds via a programma- be readily prepared as cubes with an edge length below ble syringe pump. As such, the generation and, thus, 5 nm. The small dimensions are key to the achievement deposition rate of Ru atoms could be manipulated to of preferential surface diffusion over deposition for the ensure layer-by-layer growth. Using this strategy, Pd@ formation of a smooth surface. Most importantly, Rh, Ru core–shell nanocubes with well-defined {100} facets unlike bulk Ru, has an fcc structure, making it a prom- were attained (Fig. 4b), in which the Ru atoms were crys- ising template for generating Ru nanocrystals with tallized in an fcc structure21. The fcc packing of Ru atoms

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in the overlayers could be attributed to the small lattice reaching the minimum supersaturation for homogene- mismatch between Pd and fcc-Ru (1.8%, 3.89 vs 3.82 Å) ous nucleation22. Additionally, the reduction kinetics of and to the templating effect of the Pd seeds, forcing Ru Ru(III) ions in the following stage was slowed down to atoms to replicate their arrangement. The core–shell a level favourable for layer-by-layer growth. Using Pd nanocrystals expressed well-defined {100} facets as a octahedral seeds with different edge lengths, Pd@Ru result of epitaxial growth, and the Ru shell thickness core–shell octahedra with tuneable sizes in the range could be tuned from one monolayer to approximately 12–26 nm were obtained (Fig. 4c). Similar to the cubic six atomic layers. case, the Ru shells in the octahedral nanocrystals When the synthetic protocol was applied to Pd octa- adopted an fcc structure. Both fcc-{111} and hcp-{0001} hedral seeds covered by {111} facets, homogeneous facets were composed of close-packed atoms. The Ru nucleation became a major issue, owing to the lower atoms, however, preferred to follow the fcc-{111} stack- surface energy of Pd{111} facets than Pd{100} facets. ing rather than the hcp-{0001} stacking to lower the To mitigate this issue, KBr was added to promote the total surface energy by achieving smooth surfaces at heterogeneous nucleation of Ru atoms on the Pd octa- the edges of the fcc template. Otherwise, the formation of hedral seeds. A kinetic study suggested that the presence a large number of low-coordination atoms at the edges of KBr largely suppressed the reduction of Ru(III) ions would substantially increase the total surface energy of <5 h into the reaction, thus preventing Ru atoms from the nanocrystals.

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Fig. 4 | Ru-based core–shell nanocrystals. Transmission electron microscopy images of Rh@Ru core–shell octahedra prepared from Rh cubic seeds (panel a) and Pd@Ru core–shell nanocrystals prepared from Pd cubic (panel b), octahedral (panel c) and icosahedral seeds (panel d). The insets show the corresponding energy-dispersive X-ray spectroscopy mapping images, confirming the core–shell structure. Panel a adapted with permission from ref.39, ACS. Panel b adapted with permission from ref.21, ACS. Panel c adapted with permission from ref.22, ACS. Panel d adapted with permission from ref.23, ACS.

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Apart from single-crystal seeds, Pd icosahedra and Synthesis of hollow nanostructures. Because Pd is less nanoplates, characterized by multiple twin boundaries resistant to oxidative etching than Ru, it can be selectively and stacking faults, respectively, were also leveraged as removed from the core by subjecting Pd@Ru nanocrys- seeds for the deposition of Ru (refs23,42,43). Compared tals to an etchant, leading to the formation of Ru-based with {100} and {111} facets, twin boundaries and hollow nanocrystals, a new class of catalytic materials stacking faults are more favourable sites for Ru depo- known as nanocages41,47. The success of selective etch- sition, owing to the high surface free energy and/or ing relies on the incorporation of Pd into the Ru shells surface strain. After deposition, the strong interaction due to atomic inter-diffusion during Ru deposition, between Ru adatoms and the defect sites requires more which can form atom-wide channels for the removal kinetic energy for Ru adatoms to diffuse to other sites. of Pd cores. A typical etchant is based on the Fe3+/Fe2+ Fortunately, the protocol for the synthesis of Pd@Ru pair. To accelerate the removal of Pd from the core, core–shell octahedra could be extended to the pro- KBr was introduced to increase the difference in stan­ duction of Pd@Ru core–shell icosahedra23 (Fig. 4d). dard reduction potential between the etchant (Fe3+/Fe2+; 2− Additionally, a hydrothermal approach was developed 0.77 vs SHE) and Pd (PdCl4 /Pd; 0.59 vs SHE) through 2− to synthesize Pd@Ru core–shell nanoplates with tunea- ligand exchange (PdBr4 /Pd; 0.49 vs SHE). ble thicknesses. Both the twin boundaries and stacking Under the right conditions, the facets on the surface faults, as well as the fcc structure, could be replicated by of Pd seeds can be faithfully transferred to the resultant the Ru shells43. Ru nanocages. Both {100} and {111} facets, as well as It should be pointed out that the presence of Pd twin boundaries, were maintained in Ru cubic, octa- cores inevitably causes ligand and geometric effects on hedral and icosahedral nanocages, respectively21–23 the Ru shells as a result of the difference in electron- (Fig. 5a–c). The thickness of Ru walls was approximately egativity and lattice mismatch, respectively, of two dis- 1.3 nm, corresponding to six atomic layers. The presence similar atoms44,45. The ligand effect was demonstrated of pinholes on the surface allowed the exposure of the to be dominant within a thickness of three atomic layers internal surfaces of nanocages during catalytic reactions and to largely vanish beyond that distance, whereas the and, thus, substantially enhanced the utilization effi- geometric effect was reported to alter the d-band centre ciency of Ru atoms. The elemental composition analysis of the shell through a longer-range effect (usually over suggested that all of the three types of nanocages had Ru a distance of six atomic layers)44,45. For the Pd@fcc-Ru contents higher than 87.5 wt.%, verifying the dominance core–shell nanocrystals reported in the literature, the of Ru. In addition to the shapes, the fcc phase of the Ru shell thickness can be tuned from one to six atomic lay- shells in the core–shell nanocrystals was also finely ers, making it possible to leverage or avoid the ligand retained in the resultant nanocages21–23. and/or geometric effects. Thus far, the explicit roles of The Au template of the 4H/fcc Au@Ru branched ligand and geometric effects in the Pd@fcc-Ru system nanowires could also be selectively etched away through have not been systematically evaluated. Additionally, the use of Cu2+/Cu+, yielding Ru nanotubes characterized during synthesis, the Pd atoms can diffuse into the by ultrathin Ru sheaths and decorated with tiny Ru nano- Ru shell and become impurity atoms, whose impact rods48 (Fig. 5d). The Au to Ru atomic ratio in the nano- on the fcc crystallization of Ru is still unclear. Finally, tubes was determined as 1:99, demonstrating the effective although the template effect of the Pd core is believed to removal of Au. Compared with the Pd–Ru system, the be decisive in directing the fcc packing of the deposited Ru higher proportion of Ru in the nanotubes could be attrib- atoms, the critical thickness within which the Ru atoms uted to the larger lattice mismatch between 4H Au and can follow the fcc structure has yet to be determined. 4H Ru (7.4%), suppressing atomic inter-diffusion. Both Seeds with crystal structures other than fcc were the 4H and fcc structures were well retained in the Ru also investigated. In one study46, Au nanowires with nanotubes after selective removal of the Au template48. a mixed 4H/fcc structure (60% of 4H and 40% of fcc) Apart from nanocages and nanotubes, nanoframes were employed as template for the deposition of Ru. Ru represent another class of nanomaterials that have found atoms were deposited on both 4H and fcc-twin sites, giv- extensive use in catalytic applications49. Nanoframes ing rise to highly branched Au–Ru hybrid nanowires. feature the complete removal of side walls and are com- Compared with the fcc structure, the 4H-structured sur- posed of only edges and corners. In a catalytic reaction, faces were more preferable sites for the epitaxial growth the highly open structure of a nanoframe allows the of Ru atoms, with 83.3% of Ru nanorods on the 4H sites molecules to access all atoms on the surface, resulting and only 16.7% on the fcc-twin sites. The high activity in a substantial increase in atom-utilization efficiency. of the 4H phase for atom deposition was ascribed to the Additionally, there is a large number of catalytically periodic concave and convex surfaces derived from active sites on nanoframes, including low-coordination the ABCB stacking sequence of close-packed planes. atoms, steps and kinks. With regard to synthesis, nanof- Because of the large lattice mismatch between Au and rames differ from nanocages substantially in terms of Ru, the Ru atoms on the 4H/fcc-Au nanowires followed a the deposition and surface diffusion of atoms. The for- vertical island growth to generate nanorods, rather than mation of a core–frame structure requires preferential a layer-by-layer mode for the formation of a smooth sur- deposition over surface diffusion to confine the atoms to face46. Importantly, both the 4H and fcc structures of the certain sites, whereas the opposite is desired for the fab- underlying Au wires were well replicated by the epitax- rication of core–shell nanocrystals49. Currently, the syn- ially deposited Ru adatoms, leading to the formation of thesis of nanoframes is mainly based on two strategies: 4H/fcc Ru shells decorated by abundant nanorods. site-selected deposition, followed by chemical etching and

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Fig. 5 | Ru nanocages and nanotubes. a–c | Transmission electron microscopy (TEM) images of Ru nanocages with cubic (panel a), octahedral (panel b) and icosahedral shapes (panel c), which are enclosed by {100} facets, {111} facets and {111} facets plus twin boundaries, respectively. d | TEM image of 4H/face-centred cubic Ru-based nanotubes. The insets show scanning TEM images of individual nanostructures. Panel a adapted with permission from ref.21, ACS. Panel b adapted with permission from ref.22, ACS. Panel c adapted with permission from ref.23, ACS. Panel d adapted with permission from ref.48, Wiley-VCH.

galvanic replacement. A facile approach to the synthesis of was also demonstrated50. The formation of Pd–Ru core– Pd–Ru core–frame octahedra involves the selective depo­ frame nanocrystals in this work relied on the Br−-assisted sition of Ru atoms on Pd cuboctahedral seeds25 (Fig. 6a). galvanic replacement reaction between Ru(III) ions and Specifically, Ru atoms were deposited at the truncated Pd for the generation of Ru atoms50. The shape of the corners and edges because of their higher surface energies Pd seeds kept evolving during the synthesis, from cubes relative to those of the side faces. The Pd–Ru core–frame to truncated cubes in the initial stage as a result of oxi- octahedral nanocrystals were then subjected to chemical dative etching and, eventually, to cuboctahedra due to etching for the selective removal of Pd in the core, resulting Br−-assisted galvanic replacement. The as-synthesized in the production of Ru octahedral nanoframes (Fig. 6b). Pd–Ru nanocrystals were characterized by a core–frame The atomic ratio of Ru to Pd in the nanoframes was deter- structure and a cuboctahedral shape with concave facets mined as 4.5:1. Similar to the case of nanocage synthesis, on the surface (Fig. 6c). It should be pointed out that the 2− the packing of Ru atoms in both the core–frame nano- standard reduction potential of PdBr4 /Pd is 0.49 V vs crystals and nanoframes followed an fcc structure, owing SHE, whereas that of Ru3+/Ru is 0.39 V vs SHE, implying to the templating effect exerted by the Pd seeds25. that the galvanic replacement between Ru(III) ions and The use of a combination of galvanic replacement, Pd is not thermodynamically favourable under standard site-selected deposition and wet-chemical etching for conditions. However, the presence of Pd seeds facilitates the facile synthesis of Ru cuboctahedral nanoframes the reduction of Ru(III) ions, promoting heterogeneous

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nucleation and, thereby, underpotential galvanic replace- fcc-Ru nanocrystals17. The formation of fcc-Ru nanocrys- 42 ment . The Pd–Ru core–frame cuboctahedra were then tals typically involves the use of Ru(acac)3 and triethylene

subjected to an etchant containing FeCl3, KBr and HCl glycol as the precursor and solvent/reductant, respec- for the formation of Ru cuboctahedral nanoframes tively17. Further investigation indicates that the acetyl­ (Fig. 6d) . After selective etching of Pd, the atomic ratio acetonate ions from the Ru(acac)3 precursor are critical of Ru to Pd in the nanoframes was 14:1. By varying the to the nucleation and growth of fcc-Ru nanocrystals. The amount of Ru(III) precursor, the ridge thickness of the Ru strong binding of acetylacetonate ions to Ru atoms not nanoframes could be tuned in the range of 3–10 atomic only results in an elongated Ru–Ru bond length to match layers. Importantly, the as-synthesized Ru cuboctahedral that in fcc-{111} facets but also effectively stabilizes the nanoframes crystallized in an fcc structure instead of the fcc packing of Ru atoms51. conventional hcp structure intrinsic to bulk Ru (ref.50). With regard to the synthesis based on heterogeneous nucleation, the templating effect is believed to be deci- Factors affecting the packing of Ru atoms sive in directing the packing of Ru atoms13. In this case, The packing of Ru atoms is affected by a number of fac- the lattice mismatch between the template and Ru has tors. For the synthesis involving homogeneous nucle- been utilized to control the packing of Ru atoms in the ation, the selection of an appropriate pair of Ru(III) resultant nanocrystals. An epitaxial method for the con- precursor and solvent is considered the most critical trolled growth of fcc-Ru or hcp-Ru overlayers on Pd–Cu factor responsible for the formation of either hcp-Ru or alloy seeds with tuneable lattice constants was reported52.

a b

10 nm 20 nm

c d

50 nm 50 nm

Fig. 6 | Pd–Ru core–frame nanocrystals and Ru nanoframes. a,b | Transmission electron microscopy (TEM) images of Pd–Ru core–frame octahedra (panel a) and the corresponding Ru octahedral nanoframes (panel b). c,d | TEM images of Pd–Ru core–frame cuboctahedra with concave surfaces (panel c) and the corresponding Ru cuboctahedral nanoframes (panel d). The insets show scanning TEM images of individual nanostructures. Panels a and b adapted with permission from ref.25, ACS. Panels c and d adapted with permission from ref.50, ACS.

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By varying the composition of the Pd–Cu alloy seeds, reactions are typically operated at elevated tempera- the lattice mismatch between the seed and Ru overlayer tures. Thus, it is vital to have knowledge about the ther- could be tuned in the range of 0.1–5.1%. When Pd–Cu mal stability of both the crystal structure and shape of

seeds with an atomic ratio of 1:3 (denoted as PdCu3) Ru nanocrystals to assess their value in these applications. were used as seeds, the lattice mismatch could be as low Transmission electron microscopy images of Ru icosahe- as 0.54%. As such, the deposited Ru atoms followed the dral nanocages subjected to thermal stress are shown in packing of the underlying seed and were crystallized Fig. 7a–d. The sample largely maintained both the icosa- into the fcc phase. DFT calculations suggested that hedral shape and nanocage structure up to 300 °C (ref.23).

the binding energies of fcc-Ru on both the PdCu3(100) Upon heating to 350 °C and above, the ultrathin walls

and PdCu3(111) surfaces, two major facets exposed on in the nanocages became fragmented and eventually

the PdCu3 cuboctahedral seeds, were stronger than that evolved into solid nanoparticles. The thermal stability of

of hcp-Ru on the PdCu3(111) surface, which was instru- the fcc structure of the Ru icosahedral nanocages was also mental to the formation of fcc-Ru. By contrast, once the investigated using in situ XRD (Fig. 7e). The fcc structure lattice mismatch was increased to a value above 1.3%, could be retained up to 300 °C and then quickly trans- whether the template was dominated by Pd or Cu, formed to the hcp phase upon heating to 350 °C (ref.23). the epitaxial growth of Ru became impossible and the For Ru cuboctahedral nanoframes, both the frame mor- resultant Ru overlayers took an hcp structure. phology and fcc structure could be preserved well up to The impact of lattice mismatch on the growth behav- 300 °C (ref.50). The characteristic hollow structure and iour of Ru atoms was confirmed using Au icosahedra ultrathin walls and ridges of nanocages and nanoframes as seeds. A recent study reported that the Ru branches might compromise their thermal stability47. By contrast, in the as-synthesized Au–Ru core–branch nanocrys- when Rh-derived Ru octahedra with a shell thickness tals took an hcp structure, which was attributed to the of 4.5 nm were subjected to thermal stress, they could large lattice mismatch between the fcc Au(111) and hcp retain both the fcc structure and octahedral shape up Ru(0001) (9.3%, 2.36 Å vs 2.14 Å)53. Although both to 400 °C (ref.39), showing a 100-°C improvement over studies ascribed the formation of hcp-Ru on Pd–Cu both nanocages and nanoframes. Overall, the thermal or Au seeds to the large lattice mismatch, we note that stability of fcc-Ru nanocrystals still needs to be further a phase-locked epitaxial growth of 4H/fcc Ru on the improved to enhance their performance in catalytic 4H/fcc Au template could be achieved regardless of applications. the large lattice mismatch46. This inconsistency implies The stability of Ru nanocrystals with different crystal that there should be other parameters, in addition to structures, shapes and sizes was also evaluated by com- lattice mismatch, that affect the packing of Ru atoms. puting their cohesive energies based on DFT calcula- The reduction kinetics of Ru(III) ions could be tions56 (Fig. 7f). When the number of atoms (N) is small, another critical parameter for the packing of Ru atoms the cohesive energies of fcc-Ru and hcp-Ru nanocrystals on a template33. One piece of compelling evidence was are quite close. The small difference in cohesive energy found in the deposition of Ru atoms on Pd nanocubes21. explains why fcc-Ru nanocrystals with sizes below 6 nm By increasing the deposition rate of Ru atoms, their growth can be readily obtained through the optimization of behaviour was switched from layer-by-layer to layer- reaction parameters17. However, when N is increased, plus-island and further to island growth. Interestingly, the cohesive energy of hcp-Ru nanocrystals becomes in conjunction with the variation in growth mode, increasingly larger than that of fcc-Ru nanocrystals, the packing of Ru atoms was changed from fcc to mixed suggesting that hcp-Ru nanocrystals are more stable for fcc/hcp and further to hcp. This observation suggests large numbers of atoms. In the region of N < 103, the that the reduction kinetics of Ru(III) ions could have cohesive energy of icosahedra is the lowest among those a major impact on the packing of Ru atoms. A recent of the various types of fcc-Ru nanocrystals. The supe- study related to the synthesis of Pd–Ru core–branch rior stability of fcc-Ru icosahedra compared with fcc-Ru nanocrystals also reported that the deposited Ru atoms truncated octahedra and decahedra was attributed to on fcc-Pd cuboctahedral seeds could follow an hcp the higher coordination number of atoms, which means structure54. Based on the analysis of the angles between more energy is required to break the larger number of each branch, the authors claimed that the formation of bonds56. In spite of the high surface energy, the low cohe- core–branch nanocrystals could be attributed to the dep- sive energy in the twin boundary of an icosahedron also osition of Ru atoms on the {111} facets of Pd seeds. The contributes to the enhanced stability, given that the local long-alkyl-chain surfactants are believed to play a critical structure in the twin region is similar to the hcp-(0001) role in directing the ABAB rather than ABCABC pack- stacking (ABAB), which is more energetically stable ing of Ru atoms on the close-packed {111} facets. Thus, than the ABCABC packing. Unlike for Pd and Ag, the the packing of Ru atoms should not be rationalized using high density of twin boundaries in fcc-Ru icosahedra just one factor. It is likely determined by the synergistic is believed to be responsible for the enhanced stability effect of reduction kinetics, surfactant, lattice mismatch compared with that of its single-crystal counterparts56. and other factors yet to be uncovered. Catalytic applications Evaluation of thermal stability Catalysts based on Ru nanocrystals have been applied Ru nanocrystals are promising catalysts for an array of to a number of reactions, including Fischer–Tropsch

applications, including ammonia synthesis, CO2 meth- synthesis, hydrogenation/dehydrogenation, hydrogen- anation and Fischer–Tropsch synthesis55. Most of these oxidation reaction (HOR), water splitting, ammonia

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a b

25 nm 25 nm

c d

25 nm 25 nm

ef−4.8 Truncated-octa-fcc 100 002 101 102 110 Deca-fcc hcp −5.2 Icosa-fcc 111 200

500 °C −5.6 450 °C gy (eV) 150 100 75

400 °C e ener 350 °C −6.0

Intensity (a.u.) −5.4

300 °C Cohesiv 250 °C 200 °C −6.4 150 °C −5.6 N=103 100 °C −6.8 35 40 45 50 55 60 65 70 75 80 0.00 0.10 0.20 0.30 2θ (degree) N–1/3 Fig. 7 | Thermal stability of Ru nanocrystals. a–d | Transmission electron microscopy images of Ru icosahedral nanocages heated to different temperatures for 1 h: 250 °C (panel a), 300 °C (panel b), 350 °C (panel c) and 400 °C (panel d). e | In situ X-ray diffraction patterns of face-centred cubic (fcc) Ru icosahedral nanocages measured under Ar atmosphere in the temperature range of 100–500 °C. The characteristic peaks of fcc and hexagonal close-packed (hcp) Ru are marked by red and blue dashed lines, respectively. f | Cohesive energies of Ru nanocrystals with different shapes as a function of N−1/3, where N represents the number of atoms in the particles. The inset shows an enlarged view at N−1/3 = 0.18–0.24. Panels a–e adapted with permission from ref.23, ACS. Panel f adapted with permission from ref.56, ACS.

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synthesis and CO oxidation55,57–60. Conventionally, opti- nanocrystals (Fig. 8d). The superior selectivity of the mizing the performance of a Ru catalyst was mainly fcc-Ru nanocrystals was attributed to a stronger binding based on tuning of size or promotion from support with the C atoms, reducing the production of undesired

owing to the lack of methods capable of producing products such as CH4. Ru nanocrystals with controlled shapes and/or crystal structures. As we discussed, Ru nanocrystals with differ- Hydrogenation and dehydrogenation ent shapes and crystal structures can now be prepared. Ru nanocrystals have been explored as catalysts for It is, thus, an exciting time to further improve the per- hydrogenation, and their performance is highly sen- formance of the conventional catalysts based on hcp-Ru sitive to the structure58,59. hcp-Ru and fcc-Ru nano- or even introduce new properties. In this section, we crystals were used as catalysts for the hydrogenation focus on the contributions from both crystal and sur- of 4-nitrochlororbenzene to unveil the dependence of face structures in enhancing the catalytic performance performance on crystal structure52. The fcc-Ru catalyst of Ru nanocrystals. showed 99% conversion of 4-nitrochlororbenzene in 1 h, whereas only 61% conversion was achieved with hcp-Ru. Fischer–Tropsch synthesis In addition to the crystal phase, the performance of a Fischer–Tropsch synthesis has received considerable Ru catalyst in a hydrogenation reaction is also strongly attention because it produces liquid hydrocarbons from dependent on the surface structure. The activity of 55,57 CO and H2 gases . Compared with conventional fcc-Ru cubic, octahedral and icosahedral nanocages Co-based and Fe-based catalysts, Ru nanocrystals are enclosed by {100}, {111} and {111} plus twin bounda- attractive because of their impressive performance for ries, respectively, was systematically investigated towards both gas-phase and aqueous-phase Fischer–Tropsch the reduction of 4-nitrophenol21–23 (Fig. 8e). Based on syntheses at low temperatures57. However, the activity pseudo-first-order kinetics, the derived rate constants and stability of Ru catalysts still need further improve- for the cubic and octahedral nanocages were very close. ment for them to become cost-effective and sustaina- This suggests that the {100} and {111} facets display sim- ble. To this end, DFT calculations were conducted on ilar activity in catalysing the reduction of 4-nitrophenol. Ru nanocrystals with different crystal structures to By contrast, when icosahedral nanocages were used, search for an effective catalyst towards CO dissociation, the derived rate constant was two times higher than the rate-determining step in Fischer–Tropsch synthe- those of the cubic and octahedral nanocages. Unlike sis57. It was found that the hcp-(0001) step B has the {111}-encased octahedral nanocages, the icosahedral lowest energy barrier to CO direct dissociation among nanocages have twin boundaries on the surface, which all the active surfaces of both hcp-Ru and fcc-Ru nano- greatly enhance the activity towards the reduction of crystals (Fig. 8a). However, the density of this active site 4-nitrophenol. is extremely low and can hardly contribute to the over- The crystal and surface structures of Ru catalysts all performance. fcc-Ru nanocrystals have four types of also significantly affect their performance towards active sites whose energy barriers to CO dissociation fall dehydrogenation reactions23,50. For hydrazine decom- into the range 1.12–1.20 eV (Fig. 8b), whereas only {11–21} position at room temperature, fcc-Ru nanocrystals

facets are available on hcp-Ru nanoparticles in this showed a 1.8-fold enhancement in terms of H2 selectiv- region (Fig. 8a). The availability of a high density of active ity relative to their hcp counterparts50 (Fig. 8f). Because sites on the surface of fcc-Ru nanocrystals could substan- the selectivity of a catalytic reaction only depends on the tially enhance their activity towards Fischer–Tropsch properties of the active sites, the enhanced performance synthesis relative to their hcp counterparts. was attributed to their distinct crystal phase resulting Directed by the computational results, Ru nanocrys- in the exposure of different active sites. The effect of tals with fcc and hcp structures and variable sizes were surface structure was also explored by testing fcc-Ru synthesized and tested as catalysts57. When the size of nanocages with cubic, octahedral and icosahedral hcp-Ru nanocrystals was increased from 1.9 nm to 6.8 nm, shapes23,50. The nanocages with {100} and {111} facets, the mass activity at 413 K dropped by 1.7 times, owing as well as twin boundaries, all showed greatly enhanced (Fig. 8c) (Fig. 8f) to the decrease in specific surface area . By con- H2 selectivity . In particular, the fcc-Ru icosahe- trast, Ru nanocrystals with a size of 6.8 nm, but with dral nanocages containing {111} facets and twin bound-

an fcc structure, had a mass activity 1.6 times higher aries displayed a H2 selectivity 3.4 times greater than than that of 1.9-nm hcp-Ru nanocrystals, despite the that of fcc-Ru nanocrystals with a spheroidal shape, drastic reduction in specific surface area. The enhance- confirming the role of surface structure in boosting ment was substantially amplified when the reaction the performance of Ru catalysts. One plausible mecha- temperature was elevated to 433 K, where the 6.8-nm nism for the hydrazine decomposition pathway on Ru fcc-Ru nanocrystals exhibited a mass activity 3.2 times catalysts starts from the dissociation of N–N bonds to

greater than that of the 1.9-nm hcp-Ru nanocrystals. form NH2 intermediates, followed by the scission of The enhanced performance of fcc-Ru nanocrystals N–H bonds to generate N and H atoms, which then 50 over hcp-Ru nanocrystals is in agreement with the combine to produce N2 and H2 molecules . The sur- computational results, confirming the existence of a faces of fcc-Ru nanocrystals were instrumental for the large number of active sites on the fcc-Ru nanocrystals. reduction of the activation-energy barrier to N–N dis- Additionally, the selectivity towards the most desired sociation, facilitating the decomposition of hydrazine

product, C5+, was as high as 81.3% for the fcc-Ru nano- molecules and giving rise to the enhanced selectivity 21–23 crystals, compared with only 55.4–66.7% for the hcp-Ru towards H2 generation .

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Water splitting towards OER in terms of stability over a broad range of Water splitting has received tremendous interest, pH. A study examining the OER performance of Pt, Ir owing to its ability to produce oxygen and hydrogen and hcp-Ru nanocrystals found that the OER activity gases in a clean and renewable manner60,61. The anodic decreases in the order hcp-Ru > Ir > Pt (ref.64). To improve oxygen-evolution reaction (OER) in water splitting has the performance of Ru, a facile route to the synthesis of been widely used in energy storage and conversion57,62,63. Ru nanocrystals with an unconventional fcc phase and Noble metals including Ir and Ru are the best catalysts covered by different facets was developed39. All these abhcp Ru fcc Ru 1.50 1.50 Direct pathway H-assisted pathway Direct pathway H-assisted pathway

1.35 1.35 ier (eV) ier (eV) r r 1.20 1.20 1.20 1.20 1.12 1.12

1.05 1.05 Dissociation bar Dissociation bar 0.90 0.90

(0001) (1121) (0001) (1012) (1011) (2021) (100) (111) (211) (110) (111) (311) (321) (221) Step B Step A Step B Step A

c d CH4 CO2 C2–4 C5+ Alcohols Aldehyde 40 100 fcc (6.8 nm) 35 hcp (6.8 nm) hcp (1.9 nm) 80 ) 30 –1 h

–1 25 Ru 60 mol

20 ity (wt.%) CO 15 40 (mol Selectiv ms

r 10 20 5

0 0 400 410 420 430 440 fcc-6.8 nm hcp-6.8 nm hcp-1.9 nm Temperature (K)

e f 60 3.5 hcp-Ru particles Cubic cages fcc-Ru particles 50 3.0 Octahedral cages fcc-Ru octahedral cages Icosahedral cages 2 fcc-Ru cubic cages 2.5 R = 0.9986 40 fcc-Ru icosahedral cages

2.0 ity (% ) R2 = 0.9988 30 1.5 selectiv 2 2 20

1.0 R = 0.9977 H –ln (absorbance) 0.5 10 0.0 0 02468110 12 14 16 8 Types of Ru catalysts Time (min) Fig. 8 | Ru catalysts for Fischer–Tropsch synthesis, hydrogenation and dehydrogenation. a,b | Calculated barriers for CO dissociation on various types of Ru facets and step edges in hexagonal close-packed (hcp, panel a) and face-centred cubic (fcc, panel b) structures. Only the favourable reaction pathways are indicated: red for direct dissociation and blue for

H-assisted dissociation. c | Catalytic activities (measured through the mass-specific activity , rms) of 1.9-nm hcp-Ru, 6.8-nm hcp-Ru and 6.8-nm fcc-Ru nanoparticles towards Fischer–Tropsch synthesis as a function of temperature (3.0 MPa syngas

H2/CO = 2:1). d | The distribution of hydrocarbon products for 1.9-nm hcp-Ru, 6.8-nm hcp-Ru and 6.8-nm fcc-Ru nanoparticles at 433 K. e | Plots of the absorbance as a function of reaction time using Ru cubic, octahedral and icosahedral nanocages as 2 catalysts towards the reduction of 4-nitrophenol by NaBH4. R is the coefficient of determination used in regression analysis, with a value of 1 indicating perfect fitting of all points. f | Selectivity towards hydrogen generation for the decomposition of hydrazine catalysed by various types of Ru catalysts. Panels a–d are adapted with permission from ref.57, ACS. Panel e adapted with permission from ref.23, ACS. Panel f adapted with permission from refs23,50, ACS.

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ab 12 220 12 (mV)

9 –2 j )

200 specifi c

–1 8 F

(A (A 6 at 10 mA cm

180 F –1 specifi c ) j 3 4 potential

er 160

0 Ov 0 1.0 1.2 1.4 1.6 1.8 2.0 Types of Ru catalysts Potential (V vs RHE)

fcc-Ru octahedra fcc-Ru truncated cubes fcc-Ru particles hcp-Ru particles Pt/C

cd60 2.0 0

1.8 Exchange cur –2 50 (mV) ) –4 1.6 –2 –2 40

cm –6 1.4 re –8 at 10 mA cm 30 1.2 nt (mA ent (mA r –10 1.0 20

Cur –12 cm potential 0.8 er –2

–14 10 ) Ov 0.6 –16 0 0.00 –0.15 –0.10 –0.05 0.00 0.05 Types of catalysts Potential (V vs RHE)

4H/fcc-Au 4H/fcc-Ru NTs 4H/fcc-Au-Ru NWs Pt/C Ru/C

Fig. 9 | Ru catalysts for water splitting. a | Double-layer-capacitance-normalized polarization curves measured using −1 −2 different types of Ru catalysts in a 0.05 M H2SO4 solution at a scan rate of 6 mV s . b | Overpotentials at 10 mA cm (left) and specific activities (right) of various Ru catalysts towards the oxygen-evolution reaction. c,d | Polarization curves (panel c), overpotentials at 10 mA cm−2 (left, panel d) and exchange-current densities (right, panel d) of 4H/face-centred cubic (fcc)-Au nanowires (NWs), 4H/fcc-Ru nanotubes (NTs) and 4H/fcc-Au–Ru NWs, as well as commercial Pt/C and Ru/C

electrocatalysts, measured in 1.0 M KOH solution towards the hydrogen-evolution reaction. jspecific, specific current density; RHE, reversible hydrogen electrode. Panels a and b are reproduced with permission from ref.39, ACS. Panels c and d adapted with permission from ref.48, Wiley-VCH.

Ru-based catalysts showed lower overpotentials rela- photoelectron spectroscopy analyses of fcc-Ru octahedra, tive to the commercial Pt/C, indicating superior OER steps formed on the surface, owing to the formation of (Fig. 9a) 39 activity . In particular, the fcc-Ru nanocrystals Ru oxide . A study of the OER activity of Ru and RuO2 exhibited an overpotential 9 mV lower and specific thin-film electrodes found that metallic Ru was more activity 1.4 times greater than those of hcp-Ru nano- active than its oxide in catalysing OER66. However, the crystals (Fig. 9b), suggesting an enhancement in activity dissolution of Ru in both acidic and alkaline media was related to the fcc phase. For fcc-Ru nanocrystals with 2–3 orders of magnitude faster than that of Ru oxide, cre- well-defined {111} facets, the overpotential dropped a ating a barrier to the use of Ru catalysts for OER. Despite further 29 mV, whereas the specific activity displayed the superior performance of fcc-Ru {111} facets towards a 3.3-fold enhancement relative to fcc-Ru nanocrystals. OER, the underlying mechanism has yet to be elucidated. fcc-Ru truncated nanocubes showed superior perfor- Presumably, the trend could be rationalized by exam- mance compared with hcp-Ru and fcc-Ru nanocrystals, ining the adsorption of reactant and product molecules but were less active than their octahedral counterpart. on the surfaces of fcc-Ru and hcp-Ru and their derived

Because truncated cubes are mainly covered by {100} oxides, in addition to the Ru–RuOx interfacial effect. facets, whereas octahedra are enclosed by {111} facets, The hydrogen-evolution reaction (HER) in water it was concluded that fcc-Ru {111} facets are more active splitting has also drawn extensive attention, because

than fcc-Ru {100} facets in catalysing OER. It should the generated H2 could provide fuels for a variety of be pointed out that, during OER, Ru should be oxi- clean-energy devices, such as fuel cells67. Currently, the

dized and converted into Ru oxide, which is also con- major challenge for the production of H2 from water split- sidered one of the most active electrocatalysts towards ting lies in the exploration of cost-effective catalysts with OER65. According to the electron microscopy and X-ray outstanding activity and stability towards HER68. To this

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end, tremendous efforts have been devoted to the iden- been considered as an alternative to Pt for use in fuel tification of replacements for the current state-of-the-art cells, owing to its impressive HOR activity and relatively Pt catalysts69,70. Ru nanocrystals with hcp, fcc and mixed low price71,72. The HOR performance of Ru catalysts hcp/fcc structures were successfully synthesized and eval- could be remarkably enhanced by switching from the uated as catalysts towards HER31. The hcp/fcc-structured conventional hcp structure to an fcc phase, and engineer- Ru catalyst was found to exhibit a hydrogen generation ing the facets73. Using Pt nanocrystals as seeds, Pt@Ru rate 2.5 times greater than that of Pt in alkaline solutions core–shell tetrahedra with tuneable sizes from 5.9 nm and was among the most active HER electrocatalysts. to 8.3 nm were successfully synthesized. The Ru-based DFT calculations indicated that, although Pt showed an tetrahedra crystallized in the fcc structure and were optimal free energy for H adsorption, its energy barrier to mainly covered by {111} facets. When tested as catalysts − − the Volmer step (H2O + e + Ru → Ru–Hads + OH ; Ru–Hads for HOR, the fcc-Ru-based tetrahedra exhibited greatly denotes the adsorption of a H atom on a Ru site), which enhanced performance compared with the hcp-Ru/C represents the rate-determining step in water dissocia- catalyst (Fig. 10a). In particular, the Ru-based nanocrys- tion, was 1.8 and 2.3 times higher than that of hcp-Ru tals in the fcc phase and with a size of 8.3 nm showed and fcc-Ru nanocrystals, respectively31. In particular, 12-fold and 18-fold enhancements in terms of mass and the Ru nanocrystals in the fcc phase showed the lowest specific activities compared with the hcp-Ru/C catalyst, energy barrier and could make a major contribution to respectively. After electrochemical measurements, the improving the HER performance. nanocrystals still maintained the tetrahedral shape, indi- The 4H/fcc Au–Ru core–branch nanowires were also cating good stability during HOR. The enhanced perfor- explored as catalysts towards HER, and their performance mance of the Ru-based tetrahedra could be attributed to was compared with that of commercial Pt/C and Ru/C the fcc structure and the exposed {111} facets. catalysts46,48. The Au–Ru nanowires required an overpo- To support this argument, DFT calculations were tential 19 mV and 27 mV lower than that of the commer- conducted to investigate the structure–activity correla- cial Pt/C and Ru/C catalysts, respectively, to achieve a tions73. The authors calculated the free energy for oxy- current density of 10 mA cm−2 (Fig. 9c,d). After selectively gen adsorption at 0 V vs reversible hydrogen electrode + − removing the Au template, the resultant 4H/fcc-Ru nano- (ΔGO, corresponding to * + H2O → O* + 2H + 2e ; * tubes showed a further drop of 4 mV in terms of overpo- represents the active site for atom adsorption) and the tential to drive a current density of 10 mA cm−2. Different dissociated adsorption free energy of a hydrogen atom

from the hcp/fcc-structured Ru nanocrystals, the electro- on the surface (ΔGH, corresponding to * + 1/2H2 → H*) catalytic HER kinetics of the nanotubes is determined with and without the adsorption of oxygen on dif-

by the Tafel step (Ru–Hads + Ru–Hads → H2 + 2Ru), rather ferent Ru slabs. For a highly active HOR catalyst, the than by the Volmer step48. By extrapolating the Tafel adsorption and desorption of hydrogen atoms on an plots, the exchange-current density of the 4H/fcc-Ru O-passivated surface should be appropriately balanced, −2 nanotubes was derived as 1.81 mA cm , whereas those suggesting that a ΔGH value close to zero is desired. The

of the commercial Pt/C and Ru/C catalysts were calcu- ΔGO values of fcc-(111) and hcp-(0001) surfaces were lated to be 0.92 mA cm–2 and 0.82 mA cm−2, respectively. positive, whereas those of fcc-(100), hcp-(10–11) and After 10,000 rounds of potential cycling in the range of hcp-(10–10) surfaces were negative (Fig. 10b), indicating 0.03–0.04 vs reversible hydrogen electrode, the nanotubes that the high-coordination atoms on the close-packed maintained both their morphology and the 4H/fcc phase. surfaces were relatively inert in terms of oxygen adsorp- The enhancement in HER performance for 4H/fcc-Ru tion. Similarly, hydrogen atoms were found to absorb nanotubes can be ascribed to several factors: the large more strongly on the more open and unpassivated specific surface area arising from the porous and hierar- surfaces than on fcc-(111) and hcp-(0001). Despite the

chical structure of the nanotubes; the presence of a large strong absorption of hydrogen atoms, the ΔGH values number of highly active sites such as steps, kinks, defects of fcc-(100), hcp-(10–11) and hcp-(10–10) surfaces were and phase boundaries that periodically occur in 4H and much more positive than those of the fcc-(111) and fcc phases; and the weakened charge-transfer resistance hcp-(0001) surfaces when O-passivation was involved compared with conventional nanocrystals, which facil- in the surface absorption (Fig. 10c). Because both H and itates electron transport during the HER process for O atoms preferentially occupy the same active sites, the nanotubes with ultrathin walls48. inertness of fcc-(111) and hcp-(0001) towards oxygen absorption make them promising surfaces for balanc- Hydrogen-oxidation reaction ing the adsorption and desorption of H atoms, giving Hydrogen-based fuel cells can directly convert the rise to the enhanced performance73. Compared with hcp- 67 chemical energy in H2 into electricity . Their charac- (0001), the fcc-(111) surface showed a drop of 0.031 eV

teristic high efficiency and zero emissions (with H2O as in ΔGH (0.114 eV vs 0.145 eV), confirming the role of fcc the only by-product) make them the most promising structure in increasing the HOR activity. candidate for the replacement of fossil fuels as power sources, as they would avoid thermal-conversion losses Ammonia synthesis. Ammonia, a reduced form of while remediating many of the environmental issues67. nitrogen, has played a pivotal role in the production One of the greatest challenges in the commercializa- of foods over the past century and significantly affected tion of this technique lies in the high cost of Pt cata- human life. Currently, the industrial production of lysts, which are essential for catalysing both the anodic ammonia is based on the Haber–Bosch process, which oxygen-reduction reaction and cathodic HOR. Ru has requires high pressures and temperatures and, thus,

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ab fcc-Ru /C 8.3 nm fcc-Ru7.4 nm/C 0.4 4 fcc-Ru5.9 nm/C hcp-Ru/C O absorption 0.2 (eV) 0.0 O G )

2 Δ –0.2 disc –2 c O-passivated surface cm 0.4

0 ) –2 0.2

j (mA 0.3 cm (eV)

0.0 0.2 H G (mA 0.1 Δ –0.2 –2

specific 0.0 –0.4 j Unpassivated surface –0.6 0 20 40 60 80 ) fcc-(111) fcc-(100) (0001) -(10–11 E (mV vs RHE) hcp- hcp hcp-(10–10) de 1.5 hcp-(0001) N–N TS 175 1.0 fcc-(111) fcc-(100) 0.5 cage-(100) 170 N2(g) hcp 0.0 165

–0.5 (°C) gy (eV) 50 T –1.0 Ener N2* 160 –1.5 2N* fcc 155 –2.0

–2.5 Optimal N* binding 150 2 3456 Size (nm)

Fig. 10 | Ru catalysts for hydrogen-oxidation reaction, N2 reduction and CO oxidation. a | Hydrogen-oxidation reaction

polarization curves recorded over various Ru-based catalysts in 0.1 M HClO4 aqueous solution saturated with H2. The inset

shows the specific activities of all samples. b,c | The free energy to form absorbed oxygen (ΔGO) at 0 V vs reversible hydrogen

electrode (RHE, panel b) and the dissociation energy of hydrogen (ΔGH) on the bare and oxygen-passivated surfaces (panel c)

of various Ru slabs. d | Potential-energy surfaces for N2 dissociation on four model surfaces, hexagonal close-packed (hcp)-(0001), face-centred cubic (fcc)-(111), fcc-(100) and cage-(100), which is a model representative of the ultrathin,

hollow-cage structure. All states are referenced to gas-phase N2, denoted as N2(g). The * represents adsorbed states (with 2N* being the energy of two adsorbed N* atoms at infinite separation) and N–N TS represents the transition-state energy

of the N–N bond-breaking event. e | Size dependence of the temperature for 50% conversion of CO to CO2 (T50) catalysed by 73 fcc and hcp Ru nanocrystals. j, current density; jspecific, specific current density. Panels a–c adapted with permission from ref. , ACS. Panel d adapted with permission from ref.21, ACS. Panel e adapted with permission from ref.17, ACS.

is extremely energy-consuming74. To mitigate these after the initial testing, in contrast to the pristine sample, issues, KBR developed a new catalyst based on Ru in which contained a large portion of nanocrystals below the 1970s75. Compared with the Fe-based catalyst used 1 nm in size. DFT calculations76 revealed that the most

in the Haber–Bosch process, the Ru catalyst exhibited a active B5 sites for N2 dissociation, the rate-determining 20-fold enhancement in terms of activity. Moreover, the step in ammonia synthesis, only existed on particles with Ru catalyst could be operated at high ammonia concen- sizes bigger than 1 nm, and their proportion was dramat-

trations and over a wide range of H2 to N2 ratios, making ically increased at a size of 2 nm. When the particle size it very attractive for industrial use75. However, as a pre- was further increased, both the proportion of the active

cious metal, Ru has a high price tag and extremely low B5 sites and the specific surface area were decreased, abundance in the Earth’s crust. To make use of this pre- compromising the catalytic activity76. cious metal affordably and sustainably, a lot of effort In addition to the particle size, it was recently reported has been devoted to improving the performance of Ru that both the crystal and surface structures of Ru nano- catalysts to reduce their loading in a practical catalyst. crystals can also be tuned, offering new opportunities for The activity of Ru catalysts for ammonia synthesis the optimization of their performance towards ammonia strongly depends on the particle size76. When Ru nano- synthesis. The facile synthesis of fcc-Ru nanocages with crystals with a broad size range of 0–3 nm were tested as different surface structures was reported, and DFT cal- catalysts, their activity exhibited a substantial increase culations were conducted to systematically assess their

in the initial stage of testing and then started to degrade. activities towards N2 reduction for ammonia synthe- Electron-microscopy characterization indicated that sis21–23 (Fig. 10d). Although both hcp-(0001) and fcc-(111)

all the nanocrystals were bigger than 1 nm (1–4.5 nm) surfaces are closely packed, N2 molecules were found to

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bind more strongly to fcc-(111) by 0.1 eV. As a conse- by 8 times when the particle size was increased from quence, the activation-energy barrier to N–N dissocia- 2 nm to 6 nm (ref.24). The variation in catalytic activity tion was reduced by 0.24 eV, suggesting that the fcc-(111) was found to be associated with the oxidation of Ru nano- 81,82 surface is more active towards N2 dissociation. The vol- crystals under net-oxidation-reaction conditions . For cano plot for ammonia synthesis has predicted that an large Ru nanocrystals, only a small extent of oxidation

ideal catalyst for ammonia synthesis should bind atomic was detected after reaction. The thin RuO2 film formed

N approximately 0.2–0.5 eV stronger than hcp-Ru nan- on the surface resulted in a Ru@RuO2 core–shell struc- oparticles77. The phase transition from hcp to fcc moves ture, which was demonstrated to be extraordinarily active the energy of the adsorbed N into the optimal region of towards CO oxidation82. By contrast, small Ru nanocrys- binding (Fig. 10d), favouring a more efficient competition tals underwent severe oxidation, resulting in the gen-

of N–N dissociation against N2 desorption. The fcc-Ru eration of a thick RuO2 overlayer on the surface, which nanocrystals also exhibited facet-dependent activity. roughened the particle surface and became catalytically Despite the slightly weakened binding to atomic N by inactive for CO oxidation81,82. It should be pointed out 0.06 eV (undesirable), the fcc-(100) surface showed a fur- that the increase in activity could not be exclusively attrib- ther drop of 0.08 eV relative to the fcc-(111) surface for uted to the enlarged particle size because the crystal struc- the activation-energy barrier to N–N dissociation. The ture of the Ru nanocrystals also changed when the size

absorbed N2 molecules were also stabilized by 0.24 eV was altered. Whereas 2-nm Ru nanocrystals took an hcp on fcc-(100) in contrast to fcc-(111), resulting in a more phase, the XRD pattern of 6-nm nanocrystals indicated preferential N–N-dissociation process over surface deso- the presence of both hcp-Ru and fcc-Ru (ref.24). rption. Moreover, the activity of the {100} facets on fcc-Ru To examine the effects of particle size and crys- nanocrystals could be further enhanced by switching tal structure on the catalytic activity, Ru nanocrystals from the conventional solid nanocrystals to nanocages with controlled sizes and a pure hcp or fcc phase were with a hollow interior. On the basis of computational successfully synthesized and applied to CO oxidation 21 17 results , the interatomic distance within each layer is (CO to O2 ratio = 1:1) under net-neutral conditions . reduced by 0.017 Å for the cubic nanocages relative to When hcp-Ru nanocrystals were used as a catalyst, the

their solid counterparts (2.685 Å vs 2.702 Å), which has temperature required for 50% conversion of CO into CO2

marginal effects on the binding energies of N2 molecules (T50) increased with particle size, indicating that the activ- and atomic N. However, the slightly constricted atomic ity would drop as the nanoparticle size increased (Fig. 10e). layers are predicted to lower the activation-energy barrier The opposite trend was observed for fcc-Ru nanocrystals, to N–N dissociation by 0.08 eV, giving rise to a further where the activity was enhanced with increasing parti- improvement of activity towards ammonia synthesis. cle size. In particular, when the particle size was above Despite their promising performance, the character- 3 nm, Ru nanocrystals with an fcc structure were always istic hollow structure and ultrathin walls of nanocages more active than their hcp counterparts. On the basis of make them vulnerable to fragmentation under harsh reverse Monte Carlo modelling, the authors argued that conditions, such as elevated temperatures and high the size-dependent and crystal-phase-dependent per- pressures21–23,50. Given that the Haber–Bosch process on formance of the Ru nanocrystals could be rationalized Fe-based catalysts is typically operated at high tempera- by the order parameter35,80, which captures the degree tures in excess of 400 °C, one may wonder about the fea- of atomic order in nanocrystals. The order parameter of sibility of using these nanocages in practical applications, fcc-Ru nanocrystals decreases for larger particles (Fig. 3f), because both the fcc phase and well-defined facets could which could be anticorrelated to their CO-oxidation only be retained up to 300 °C. Fortunately, the superior performance. Additionally, the order parameters as a activity of Ru nanocages would allow one to conduct the function of particle size for hcp-Ru and fcc-Ru nano- synthesis under relatively mild conditions. For example, crystals intersect at a size of roughly 3 nm, above which the reactor designed by KBR can efficiently produce the fcc particles show a lower order parameter than hcp ammonia at 286 °C when the catalyst is based on Ru particles35. This observation is consistent with the trend (ref.75). Additionally, an electrochemical method con- shown in Fig. 10e, in which the CO performance of ducted at room temperature and under ambient pres- hcp-Ru and fcc-Ru nanocrystals displays an intersection sure has recently emerged as an alternative for ammonia at approximately 3 nm. The authors concluded that the synthesis78,79. Thus far, the dependence of the activity of enhanced performance of fcc-Ru nanocrystals towards Ru nanocrystals towards ammonia synthesis on crystal CO oxidation could be attributed to the decrease in phase and surface structure has not been experimentally atomic ordering with increasing particle size. explored, but it deserves to be studied in the future. Conclusions and outlook CO oxidation We used Ru as an example to highlight the progress in CO oxidation, one of the most extensively studied reac- controlling both the crystal phase and surface structure tions in heterogeneous catalysis, is becoming increasingly of metal nanocrystals. In addition to the conventional

important in the context of air cleaning, H2 purification hcp phase, Ru nanocrystals in fcc, 4H and mixed 4H/fcc and treatment of automobile exhaust gases17,24,80. Among phases have been successfully synthesized. In conjunc- all the noble-metal catalysts, Ru has shown remarkable tion with the creation of new crystal phases, a variety of 80 performance . The CO oxidation (CO to O2 ratio = 1:2.5) nanostructures featuring variations in surface structure, was studied on Ru nanocrystals with sizes in the range including different types of facets and twin boundaries, 2–6 nm, revealing that the activity at 240 °C was increased have also been achieved. We also discussed the major

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factors affecting the crystallization of Ru atoms and inter-diffusion between the template and the Ru shell examined the thermal stability of Ru nanocrystals in under thermal stress, and the need for Earth-abundant terms of both crystal phase and surface structure. When materials as the cores to achieve cost-effective catalysts71. used as catalysts, the performance of Ru nanocrystals Moving forward, one cannot just rely on a trial- has been found to strongly correlate with the crystal and-error approach or on the qualitative explanations phase, facet type and twin structure. Importantly, Ru of the synthesis that are commonly found in the liter- nanocrystals featuring unconventional crystal phases ature. Instead, a quantitative and more comprehensive and well-controlled facets showed enhanced perfor- understanding of the mechanism underlying the syn- mance towards various catalytic reactions, opening a thesis is urgently needed, which will allow researchers to new avenue for achieving cost-effective and sustainable conduct the synthesis in a predictable fashion84,85. Once applications of this precious metal. the quantitative parameters, such as reduction rate, are In spite of the impressive progress, the simultaneous well correlated to the crystal phase and/or surface struc- engineering of the crystal phase and surface structure ture of Ru nanocrystals, it will be possible to produce of Ru nanocrystals is still in the early stages of develop- the desired Ru nanocrystals by selecting reagents that ment. In particular, most of the studies rely on the use meet the requirements and even to predict the charac- of a second metal as the template for the replication of ter of the products without conducting a synthesis. For both crystal and surface structures. The involvement example, a strong correlation exists between the initial of a second metal inevitably introduces some impurity reduction rate of Pd(II) ions and the twin structure of Pd atoms into the Ru shell due to inter-diffusion, which may nanocrystals. As such, single-crystal, multiply twinned not be desired for some reactions. Also, some key points or stacking-fault-lined nanocrystals could be deter- in the reported research need to be further validated, ministically obtained by simply controlling the initial including the explicit roles played by template, reduc- reduction rate of a Pd(II) precursor85. By leveraging this tion kinetics and reagents in determining the packing of correlation, a rational route to the one-pot synthesis of Ru atoms; the critical thickness within which the depos- Cu bipyramids with the assistance of Pd was designed86. ited Ru atoms can still follow the crystal structure of the In this route, the reduction rate of the Pd(II) precursor template; and the origin of the enhancement in catalytic was intentionally tuned to a range favourable for the performance. To address these issues, experimental formation of Pd seeds with a single twin in the initial and computational endeavours should be integrated to stage, followed by the growth of Cu atoms on the seeds achieve mechanistic insights down to the atomic and/or to produce right-bipyramidal nanocrystals86. molecular level. Once the explicit role played by each Another direction is to extend the success of engi- factor in affecting the packing of Ru atoms have been neering both the crystal phase and surface structure resolved, it will be feasible to talk about rational synthe- of Ru nanocrystals to other metals. Researchers have sis of Ru nanocrystals with tight control over both the successfully leveraged the use of surfactants, reduction crystal phase and surface structure. kinetics and/or templates to produce metal nanocrystals It is worth noting that the as-synthesized Ru hol- with different crystal phases, including hcp-Rh ultrathin low nanocrystals can only retain their fcc structure and nanosheets, fcc-Co nanocrystals and 4H/fcc-Ag nano- shapes at temperatures below 300 °C (refs23,50). There ribbons87–89. It should be pointed out that the surface is a pressing need to improve their thermal stability to structures of these metal nanocrystals remain poorly extend their use to more catalytic applications. Although defined owing to the lack of shape control. Currently, the thermal-stability issue could be mitigated by pro- shape-controlled synthesis has been mainly accom- ducing Ru nanocrystals with thick shells, the increased plished for fcc metals, particularly noble metals such shell thickness would compromise their mass activity39, as Au, Ag, Pd, Pt and Rh. Although prior studies have which is one of the major concerns in using precious paved the way for engineering both the crystal phase metals. As an alternative, the core–shell, core–frame and surface structure of metals, it remains challenging to and core–sheath nanocrystals that served as precursors move to other systems owing to the lack of appropriate to the Ru hollow nanocrystals could provide the same capping agents and a good understanding of the kinetic function in terms of catalysis71. Despite the blocked effect. We hope that this Review will provide researchers inner surface, the presence of the core could serve as a with the necessary insights into the rational synthesis of physical support to enhance the thermal stability of the Ru nanocrystals with control over both crystal phase and Ru shells83. Moreover, the presence of the core allows one surface structure. We anticipate that the rationale under- to reduce the thickness of Ru shells down to one mon- lying the case of Ru will inspire researchers to explore olayer, by which the utilization efficiency of Ru atoms more opportunities in other materials. could be increased to 100%. Other issues include sur- face reconstruction and composition changes owing to Published online 18 March 2020

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81. Aßmann, J. et al. Understanding the structural 85. Wang, Y., Peng, H. C., Liu, J., Huang, C. Z. & Acknowledgements deactivation of ruthenium catalysts on an Xia, Y. Use of reduction rate as a quantitative knob This work was supported, in part, by research grants from the atomic scale under both oxidizing and reducing for controlling the twin structure and shape of National Science Foundation, including DMR-1505400, CHE- conditions. Angew. Chem. Int. Ed. 44, 917–920 palladium nanocrystals. Nano Lett. 15, 1445–1450 1505441 and CHE-1804970. It was also supported by (2005). (2015). start-up funds from the Georgia Institute of Technology. 82. Qadir, K. et al. Intrinsic relation between catalytic 86. Lyu, Z. et al. A rationally designed route to the one-pot activity of CO oxidation on Ru nanoparticles and synthesis of right bipyramidal nanocrystals of copper. Author contributions Ru oxides uncovered with ambient pressure XPS. Chem. Mater. 30, 6469–6477 (2018). The authors contributed equally to all aspects of the article. Nano Lett. 12, 5761–5768 (2012). 87. Duan, H. et al. Ultrathin rhodium nanosheets. 83. Wang, X. et al. Palladium–platinum core–shell Nat. Commun. 5, 3093 (2014). Competing interests icosahedra with substantially enhanced activity and 88. Osorio-Cantillo, C., Santiago-Miranda, A. N., The authors declare no competing interests. durability towards oxygen reduction. Nat. Commun. Perales-Perez, O. & Xin, Y. Size- and phase-controlled 6, 7594 (2015). synthesis of cobalt nanoparticles for potential Publisher’s note 84. Yang, T. H., Gilroy, K. D. & Xia, Y. Reduction rate biomedical applications. J. Appl. Phys. 111, 07B324 Springer Nature remains neutral with regard to jurisdictional as a quantitative knob for achieving deterministic (2012). claims in published maps and institutional affiliations. synthesis of colloidal metal nanocrystals. Chem. Sci. 89. Fan, Z. et al. Stabilization of 4H hexagonal phase in 8, 6730–6749 (2017). gold nanoribbons. Nat. Commun. 6, 7684 (2015). © Springer Nature Limited 2020

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