Yucong Yan

Intermetallic Nanocrystals: Syntheses and Catalytic Applications

At the forefront of nanochemistry, there exists a research endeavor centered around intermetallic nanocrystals, which are unique in terms of long-range atomic ordering, well-defined stoichiometry, and controlled crystal structure. In contrast to alloy nanocrystals with no elemental ordering, it is challenging to synthesize intermetallic nanocrystals with a tight control over their size and shape. Here, recent progress in the synthesis of intermetallic nanocrystals with controllable sizes and well-defined shapes is highlighted. A simple analysis and some insights key to the selection of experimental conditions for generating intermetallic nanocrystals are presented, followed by examples to highlight the viable use of intermetallic nanocrystals as electrocatalysts or catalysts for various reactions, with a focus on the enhanced performance relative to their alloy counterparts that lack elemental ordering. Within the conclusion, perspectives on future developments in the context of synthetic control, structure-property relationships, and applications are discussed.

Size-controlled synthesis of Pd nanosheets for tunable plasmonic properties

Hexagonal Pd nanosheets with extremely thin thickness are synthesized in high yields by reducing the Pd salt precursor in N,N-dimethylformamide (DMF) containing tungsten hexacarbonyl (W(CO)6), citric acid (CA), and cetyltrimethylammonium bromide (CTAB). During the reaction, W(CO)6 spontaneously decomposes to W particles and CO, serving as reducing and capping agents in the synthesis of Pd nanosheets, respectively. We found that the use of CA and CTAB is both beneficial to promote the formation of hexagonal Pd nanosheets with stacking faults. In addition, the edge length of the Pd nanosheets can be simply tuned by varying the amount of CA fed in the synthesis. Such nanosheets can be further grown by subsequent seeded growth with as-preformed Pd nanosheets as the seeds. The Pd nanosheets with different edge lengths exhibit tunable localized surface plasmon resonance (LSPR) in the range of 820–1067 nm, and thus hold great potential in the field of plasmon-enhanced catalysis.

Tuning Surface Structure and Strain in Pd–Pt Core–Shell Nanocrystals for Enhanced Electrocatalytic Oxygen Reduction

Icosahedral, octahedral, and cubic Pd@Pt core-shell nanocrystals with two atomic Pt layers are epitaxially generated under thermodynamic control. Such icosahedra exhibit remarkably enhanced catalytic properties for oxygen reduction reaction compared to the octahedra and cubes as well as commercial Pt/C, which can be attributed to ligand and geometry effects, especially twin-induced strain effect that is revealed by geometrical phase analysis.

Single-crystalline Pd square nanoplates enclosed by {100} facets on reduced graphene oxide for formic acid electro-oxidation

Single-crystalline Pd square nanoplates enclosed by {100} facets were generated on reduced graphene oxide and exhibited the substantially enhanced properties for the formic acid oxidation reaction. The combination of carbonyl groups formed on the surface of annealed graphene oxide and Br- ions played important roles in this synthesis.

PdCu alloy nanodendrites with tunable composition as highly active electrocatalysts for methanol oxidation

Metal nanodendrites composed of highly branched arms have received great attention as electrocatalysts owing to their reasonably large surface area and the potential existence of low-coordinated sites in high densities. Although significant progress has been made in the synthesis of bimetallic nanodendrites, few works involve a system consisting of Pd and Cu, particularly in the case of alloyed nanodendrites. Here, we report a facile and powerful approach for the synthesis of PdCu alloy nanodendrites with tunable composition through varying the molar ratio of the Pd and Cu salt precursors. The key to achieving PdCu alloy nanodendrites is the use of W(CO)6, which serves as a strong reducing agent. In addition, variation in the molar ratio of the precursors, from Pd rich to Cu rich, leads to shape evolution of the PdCu alloy, moving from a polyhedral to a dendritic nanostructure. This result indicates that galvanic replacement between a Cu rich alloy and a Pd precursor also plays an important role in the formation of PdCu alloy nanodendrites. When used as electrocatalysts for the methanol oxidation reaction (MOR), PdCu alloy nanodendrites exhibit remarkably enhanced catalytic properties relative to commercial Pd/C. Specifically, Pd35Cu65 alloy nanodendrites show the highest specific activity and mass activity for the MOR, 9.3 and 7.6 times higher than that of commercial Pd/C, respectively. This enhancement can be attributed to their dendritic structure and a possible bifunctional mechanism between Pd and Cu.

Embedding Ultrafine and High-Content Pt Nanoparticles at Ceria Surface for Enhanced Thermal Stability

Ultrafine Pt nanoparticles loaded on ceria (CeO2) are promising nanostructured catalysts for many important reactions. However, such catalysts often suffer from thermal instability due to coarsening of Pt nanoparticles at elevated temperatures, especially for those with high Pt loading, which leads to severe deterioration of catalytic performances. Here, a facile strategy is developed to improve the thermal stability of ultrafine (1–2 nm)‐Pt/CeO2 catalysts with high Pt content (≈14 wt%) by partially embedding Pt nanoparticles at the surface of CeO2 through the redox reaction at the solid–solution interface. Ex situ heating studies demonstrate the significant increase in thermal stability of such embedded nanostructures compared to the conventional loaded catalysts. The microscopic pathways for interparticle coarsening of Pt embedded or loaded on CeO2 are further investigated by in situ electron microscopy at elevated temperatures. Their morphology and size evolution with heating temperature indicate that migration and coalescence of Pt nanoparticles are remarkably suppressed in the embedded structure up to about 450 °C, which may account for the improved thermal stability compared to the conventional loaded structure.

Facile synthesis of PtCu3 alloy hexapods and hollow nanoframes as highly active electrocatalysts for methanol oxidation

PtCu alloy nanocrystals, especially for the PtCu3 component, hold great potential as catalysts in methanol oxidation reaction (MOR) owing to their promising catalytic properties arising from the bifunctional mechanism associated with Pt and Cu sites. However, Pt and Cu salt precursors are difficult to be co-reduced in a synthesis due to the large difference in their redox potential, because of which Pt or Cu nanocrystals are initially generated and then evolve into a PtCu alloy with remarkable variation of the PtCu component by underpotential deposition (UPD) and galvanic replacement, respectively. Here we report a facile method for the shape-controlled synthesis of PtCu3 alloy nanocrystals by co-reduction of Pt and Cu salt precursors using oleylamine (OAm) and N,N-dimethylformamide (DMF) as co-reducing agents at different temperatures. PtCu3 alloy hexapods with preferential overgrowth along the direction are obtained at 220 °C through co-reduction reaction accompanied by Cl− ion induced underpotential deposition (UPD). Meanwhile, galvanic replacement between PtCu3 alloy nanocrystals and the Pt precursor co-exists with overgrowth induced by co-reduction reaction at 190 °C, eventually resulting in PtCu3 hollow nanoframes. Both the PtCu3 alloy hexapods and hollow nanoframes exhibit remarkably enhanced catalytic properties in terms of activity and stability towards MOR relative to the commercial Pt/C. Specifically, the PtCu3 alloy hexapods and hollow nanoframes show the highest specific (2.96 mA cm−2) activity and mass activity (1.861 mA μgPt−1) for MOR, respectively, which are 13.5 and 24.2 times higher than those of the commercial Pt/C. This enhancement can be attributed to their unique structures and the possible bifunctional mechanism between Pt and Cu.

Core–shell and alloy integrating PdAu bimetallic nanoplates on reduced graphene oxide for efficient and stable hydrogen evolution catalysts

PdAu bimetallic nanoplates are synthesized by titrating HAuCl4 into an aqueous solution containing in situ generated Pd square nanoplates (PdSPs) on reduced graphene oxide (rGO) in the presence of ascorbic acid (AA), serving as a reducing agent, at different injection rates. At a high injection rate (e.g., 45 mL min−1), PdAu core–shell nanoplates on rGO are generated by strong galvanic replacement between PdSPs and the Au precursor, followed by reduction of the Pd precursor. In contrast, PdAu nanoplates with a core–shell and alloy integrating structure are obtained on rGO by co-reduction of the Au and Pd precursors by AA due to the inhibition of the abovementioned galvanic replacement at a slow injection rate (e.g., 0.5 mL min−1). The PdAu nanoplates with a core–shell and alloy integrating structure on rGO exhibit a substantially enhanced catalytic activity towards the hydrogen evolution reaction (HER) relative to the PdAu core–shell nanoplates and PdSPs on rGO, and is comparable to the commercial Pt/C. Significantly, these core–shell and alloy integrating nanoplates on rGO have much superior durability over Pt/C for catalyzing the HER under acidic conditions. The remarkable enhancement in activity and durability can be attributed to the cooperative function of the PdAu alloy and electron coupling between nanoplates and graphene.

Size-controlled synthesis of Au nanorings on Pd ultrathin nanoplates as efficient catalysts for hydrogenation

Au nanorings are of particular interest in catalysis owing to their fascinating properties, however, it still remains a tremendous challenge to generate such hollow nanostructures with small size. Here, we report the synthesis of Au nanorings with an outer diameter of less than 20 nm by a seed-mediated approach with Pd nanoplates as seeds in the presence of Ag+ ions. The incorporation of Ag+ ions substantially slows down the reduction rate of the Au precursor through an underpotential deposition (UPD) process, leading to the formation of small Au nanoparticles. These small Au nanoparticles then coalesced into nanorings on small Pd nanoplates. The outer diameter of the Au nanorings is tuned by varying the size of the Pd nanoplates. The smallest Au nanorings are evaluated as catalysts towards the hydrogenation of 4-nitrophenol, showing substantially enhanced catalytic activity relative to the Au nanoparticles with the same size. This enhancement in the catalytic activity can be attributed to the unique structural features including the large specific surface areas, convenient accessibility of both interior and exterior surfaces for the reactants, and existence of highly active sites in the interior part.

Enhanced oxygen reduction activity of Pt shells on PdCu truncated octahedra with different compositions

Pd@Pt core–shell nanocrystals with ultrathin Pt layers have received great attention as active and low Pt loading catalysts for oxygen reduction reaction (ORR). However, the reduction of Pd loading without compromising the catalytic performance is also highly desired since Pd is an expensive and scarce noble-metal. Here we report the epitaxial growth of ultrathin Pt shells on PdxCu truncated octahedra by a seed-mediated approach. The Pd/Cu atomic ratio (x) of the truncated octahedral seeds was tuned from 2, 1 to 0.5 by varying the feeding molar ratio of Pd to Cu precursors. When used as catalysts for ORR, these three PdxCu@Pt core–shell truncated octahedra exhibited substantially enhanced catalytic activities compared to commercial Pt/C. Specifically, Pd2Cu@Pt catalysts achieved the highest area-specific activity (0.46 mA cm−2) and mass activity (0.59 mA μgPt−1) at 0.9 V, which were 2.7 and 4.5 times higher than those of the commercial Pt/C. In addition, these PdxCu@Pt core–shell catalysts showed a similar durability with the commercial Pt/C after 10 000 cycles due to the dissolution of active Cu and Pd in the cores.