Controlling the Deposition of Pd on Au Nanocages: Outer Surface Only versus Both Outer and Inner Surfaces

Plasmonic and catalytic Au NBP@AgPd nanoframes for highly efficient photocatalytic reactions

Heterogeneous metal nanostructures with excellent plasmonic performance and catalytic activity are urgently needed to realize efficient light-driven catalysis. Herein, we demonstrate the preparation of hollow Au nanobipyramid (NBP)@AgPd nanostructures by employing Au NBP@Ag nanorods as templates. The products could transform from Au NBP@AgPd nanoframes to nanocages, along with the redshift and broadening of the plasmon wavelength. Particularly, the plasmon intensity of these nanostructures remained considerable among the shape evolution process. Based on the selective absorption of CTAB, the Ag atoms on the side surfaces of the Au NBP@Ag nanorods were employed as the sacrificial templates to reduce Pd atoms through galvanic replacement. The reduced Pd and Ag atoms produced through the reduction reaction were preferably co-deposited on the corners and edges at the early stage and later deposited directly on the defect sites of the side facets, as more Ag atoms were released. The discontinued distribution of the Pd atoms gives an opportunity to etch away the Ag atoms in the cores, leading to the formation of hollow Au NBP@AgPd nanostructures after the etching process. It is worth noting that the deposition of the ultrathin AgPd nanoframe had little influence on the plasmonic properties of Au NBPs, as verified by electrodynamic simulations. The Au NBP@AgPd nanoframe showed great photocatalytic activity toward Suzuki coupling reactions under laser irradiation. Taken together, these results suggest that the hot electrons successfully transfer from Au NBP to the AgPd nanoframes to participate in the photocatalytic reactions. This study affords a promising route for the synthesis of anisotropic bimetallic nanostructures with excellent plasmonic performances.

Shape-Controlled Synthesis of Platinum-Based Nanocrystals and Their Electrocatalytic Applications in Fuel Cells

To achieve environmentally benign energy conversion with the carbon neutrality target via electrochemical reactions, the innovation of electrocatalysts plays a vital role in the enablement of renewable resources. Nowadays, Pt-based nanocrystals (NCs) have been identified as one class of the most promising candidates to efficiently catalyze both the half-reactions in hydrogen- and hydrocarbon-based fuel cells. Here, we thoroughly discuss the key achievement in developing shape-controlled Pt and Pt-based NCs, and their electrochemical applications in fuel cells. We begin with a mechanistic discussion on how the morphology can be precisely controlled in a colloidal system, followed by highlighting the advanced development of shape-controlled Pt, Pt-alloy, Pt-based core@shell NCs, Pt-based nanocages, and Pt-based intermetallic compounds. We then select some case studies on models of typical reactions (oxygen reduction reaction at the cathode and small molecular oxidation reaction at the anode) that are enhanced by the shape-controlled Pt-based nanocatalysts. Finally, we provide an outlook on the potential challenges of shape-controlled nanocatalysts and envision their perspective with suggestions.

Colloidal Synthesis of Metal Nanocrystals: From Asymmetrical Growth to Symmetry Breaking

Nanocrystals offer a unique platform for tailoring the physicochemical properties of solid materials to enhance their performances in various applications. While most work on controlling their shapes revolves around symmetrical growth, the introduction of asymmetrical growth and thus symmetry breaking has also emerged as a powerful route to enrich metal nanocrystals with new shapes and complex morphologies as well as unprecedented properties and functionalities. The success of this route critically relies on our ability to lift the confinement on symmetry by the underlying unit cell of the crystal structure and/or the initial seed in a systematic manner. This Review aims to provide an account of recent progress in understanding and controlling asymmetrical growth and symmetry breaking in a colloidal synthesis of noble-metal nanocrystals. With a touch on both the nucleation and growth steps, we discuss a number of methods capable of generating seeds with diverse symmetry while achieving asymmetrical growth for mono-, bi-, and multimetallic systems. We then showcase a variety of symmetry-broken nanocrystals that have been reported, together with insights into their growth mechanisms. We also highlight their properties and applications and conclude with perspectives on future directions in developing this class of nanomaterials. It is hoped that the concepts and existing challenges outlined in this Review will drive further research into understanding and controlling the symmetry breaking process.

Synthesis of Pd nanorod arrays on Au nanoframes for excellent ethanol electrooxidation

Au-Pd hollow nanostructures have attracted a lot of attention because of their excellent ethanol electrooxidation performance. Herein, we report a facile preparation of Au nanoframe@Pd array electrocatalysts in the presence of cetylpyridinium chloride. The reduced Pd atoms were directed to mainly deposit on the surface of the Au nanoframes in the form of rods, leading to the formation of Au nanoframe@Pd arrays with a super-large specific surface area. The red shift and damping of the plasmon peak were ascribed to the deposition of the Pd arrays on the surface of the Au nanoframes and nanobipyramids, which was verified by electrodynamic simulations. Surfactants, temperature and reaction time determine the growth process and thereby the architecture of the obtained Au-Pd hollow nanostructures. Compared with the Au nanoframe@Pd nanostructures and Au nanobipyramid@Pd arrays, the Au nanoframe@Pd arrays exhibit an enhanced electrocatalytic performance towards ethanol electrooxidation due to an abundance of catalytic active sites. The Au NF@Pd arrays display 4.1 times higher specific activity and 13.7 times higher mass activity than the commercial Pd/C electrocatalyst. Moreover, the nanostructure shows improved stability towards the ethanol oxidation reaction. This study enriches the manufacturing technology to increase the active sites of noble metal nanocatalysts and promotes the development of direct ethanol fuel cells.

Strain-Stabilized Metastable Face-Centered Tetragonal Gold Overlayer for Efficient CO2 Electroreduction

Synthesis of the unconventional phase of noble metal nanocrystals may create new opportunities in exploring intriguing physicochemical properties but remains challenging. In the research field of thin film growth, the interface strain offers a general driving force to stabilize the metastable phase of epitaxial film. Herein we extend this concept to the field of noble metal nanocrystals and report the solution synthesis of metastable face-centered tetragonal Au that has not been discovered before. The successful synthesis relies on the formation of intermetallic AuCu₃@Au core-shell structure, where the interface strain stabilizes the metastable fct Au overlayer. Compared with the face-centered cubic Au counterpart, the metastable fct Au shows greatly improved catalytic activity toward CO₂ reduction to CO. The density functional theory calculations and spectroscopic studies reveal that the metastable fct Au upshifts the d-band center, which lowers the energy barrier of key intermediate COOH* formation and thus facilitates the reaction kinetics.

Gold nanocages for effective photothermal conversion and related applications

Gold nanocages are highly effective in converting light to heat, making them versatile for an array of photothermal applications.

High-Density Pd Nanorod Arrays on Au Nanocrystals for High-Performance Ethanol Electrooxidation

In the synthesis of Au/Pd bimetallic nanocrystals, a layer-by-layer growth is favored, owing to the low bonding energy between Pd atoms ( E_Pd-Pd) in comparison with E_Au-Pd, resulting in homogeneous core/shell nanostructures. Herein, we demonstrate designed synthetic tactics to unconventional Au/Pd heterostructures through a deposition-dominant growth pathway of the newly reduced Pd atoms, which break the intrinsically favored layer-by-layer growth. Pd thus grows on Au seeds in a heterogeneous nucleation manner. The resulting anisotropic Pd nanorods array on the two basal facets and three side facets of the Au triangular seeds in a high density to form 2D/1D Au/Pd heterostructures. It is noticed that Pd nanorods align in an extremely high order. They grow almost in a row with the base of the rod located overlapped on the Au surface. This versatile approach has been also applied to other Au nanocrystal seeds, involving hexagonal nanoplates, circular nanodisks, nanorods, and nanobipyramids. Furthermore, the 2D/1D Au/Pd heterostructures exhibit an enhanced electrocatalytic performance toward ethanol oxidation in alkaline condition, owing to their unique structure and the exposure of Au. We believe that our synthetic strategy is highly valuable for the construction of multimetallic nanostructures with desired architectures and thus intriguing properties.

Conversion of bimetallic PtNi3 nanopolyhedra to ternary PtNiSn nanoframes by galvanic replacement reaction

Hollow multimetallic PtNiSn nanoparticles (NPs) were formed from solid Ni-core/Pt-frame NPs by the galvanic replacement reaction (GRR) of Ni by Sn. The GRR was performed by adding SnCl4·5H2O dissolved in ethylene glycol into the PtNi3 NPs containing suspension. The reaction yielded nanoframes with a hollow interior, having Pt-rich edges covered with a thin, incomplete Sn layer. They were investigated using transmission electron microscopy (TEM), energy dispersion X-ray spectroscopy (EDS) and X-ray diffraction (XRD). EDS analysis showed that the GRR rate could be modified by changing the solvent and the concentration of tin ions. Indeed, compared to water, ethylene glycol was found to facilitate the reduction of tin chloride and to affect nickel dissolution. TEM analysis revealed that the galvanic replacement of nickel and tin involves two different mechanisms. The first one consists of nickel oxidation followed by reduction of tin ions. In the second mechanism, oxidation of nickel and reduction of tin ions occur simultaneously.

Shape-Controlled Synthesis of Colloidal Metal Nanocrystals by Replicating the Surface Atomic Structure on the Seed

Controlling the surface structure of metal nanocrystals while maximizing the utilization efficiency of the atoms is a subject of great importance. An emerging strategy that has captured the attention of many research groups involves the conformal deposition of one metal as an ultrathin shell (typically 1-6 atomic layers) onto the surface of a seed made of another metal and covered by a set of well-defined facets. This approach forces the deposited metal to faithfully replicate the surface atomic structure of the seed while at the same time serving to minimize the usage of the deposited metal. Here, the recent progress in this area is discussed and analyzed by focusing on the synthetic and mechanistic requisites necessary for achieving surface atomic replication of precious metals. Other related methods are discussed, including the one-pot synthesis, electrochemical deposition, and skin-layer formation through thermal annealing. To close, some of the synergies that arise when the thickness of the deposited shell is decreased controllably down to a few atomic layers are highlighted, along with how the control of thickness can be used to uncover the optimal physicochemical properties necessary for boosting the performance toward a range of catalytic reactions.