This paper reviews recent studies on the preparation of two-dimensional (2D) metal nanostructures, particularly nanosheets. As metal often exists in the high-symmetry crystal phase, such as face centered cubic structures, reducing the symmetry is often needed for the formation of low-dimensional nanostructures. Recent advances in characterization and theory allow for a deeper understanding of the formation of 2D nanostructures. This Review firstly describes the relevant theoretical framework to help the experimentalists understand chemical driving forces for the synthesis of 2D metal nanostructures, followed by examples on the shape control of different metals. Recent applications of 2D metal nanostructures, including catalysis, bioimaging, plasmonics, and sensing, are discussed. We end the Review with a summary and outlook of the challenges and opportunities in the design, synthesis, and application of 2D metal nanostructures.

Two-Dimensional Metal Nanostructures: From Theoretical Understanding to Experiment.

Pd‐based metallenes, atomically thin layers composed primarily of under‐coordinated Pd atoms, have emerged as the newest members in the family of two‐dimensional (2D) nanomaterials. Moreover, the unique physiochemical properties, high intrinsic activity associated with metallenes coupled with the ease of applying chemical modifications result in great potential in catalyst engineering for fuel cell reactions. Especially in recent years, interest in Pd‐based metallenes is growing, as evidenced by surge in available literatures. Herein, we have reviewed the recent findings achieved in Pd‐based metallenes in fuel cells by highlighting the technologies available for deriving metallenes and manifesting the modification strategies for designing them to better suit the application demand. Moreover, we also discuss the perspective insights of Pd‐based metallenes for fuel cells regarding the surfactant‐free synthesis method, strain engineering, constructing high‐entropy alloy, and so on.

Pd‐Based Metallenes for Fuel Cell Reactions

Galvanic replacement reaction (GRR) has gained considerable interest as a facile and versatile synthetic method for modulating compositions, morphologies, and corresponding physicochemical properties of metallic nanoparticles. Thus far, extensive knowledge of GRR on monometallic templates has been accumulated, backed with ample experimental data and computational modeling and validation. The GRR templates have recently been extended to other materials such as alloys, oxides, sulfides, and liquid metals. These new materials have demonstrated potential applications in electrochemical energy conversion systems, which have been relatively unexplored for GRR-originated materials. In this review, the recent findings in GRR on these new template materials are introduced, pointing to the incredible versatility of the GRR methodology in diversifying the catalytic materials classes. We further discuss the remaining critical issues and future research directions of GRRs to fully exploit the potential of GRR in spearheading future advances in electrocatalytic energy conversion and other important applications. 

Galvanic replacement reaction to prepare catalytic materials

The development of efficient and stable Pd-based electrocatalysts is extremely important to facilitate the development of catalysts for polyol oxidation reactions. To synthesize Pd-based catalysts with excellent catalytic performance, a series of PdAg porous nanowires (PdAg PNWs) with different elemental ratios was constructed by facile synthesis using a seed-mediated method. The synthesized PdAg PNWs have a rough surface and a porous one-dimensional structure, which optimize the specific surface area and surface area of catalysts, thereby providing more active sites for catalysts. PdAg PNWs benefited from the geometric effect of porous nanowires and the synergy between Pd and Ag, showing excellent catalysis (8243.0 and 4137.0 mA mgPd-1) for the ethylene glycol oxidation reaction (EGOR) and glycerol oxidation reaction (GOR). Among them, the optimal Pd62Ag38 PNWs show the highest catalytic activity (6.0 times and 3.9 times higher than Pd/C) and stability compared with Pd57Ag43 PNWs, Pd51Ag49 PNWs, and Pd/C for EGOR and GOR. At the same time, this porous one-dimensional structure also endows PdAg PNWs with faster electron transfer capabilities than Pd/C. This work will likely provide an effective strategy for constructing cost-effective catalysts.

Simple Synthesis of PdAg Porous Nanowires as Effective Catalysts for Polyol Oxidation Reaction.

Gold nanoparticles represent an important class of functional nanomaterials for optoelectronics, biomedical applications, and catalysis. Therefore, controllable synthesis of nanoparticles with specified size and shape is important. Though reduction of gold ions is quite a simple process and may be performed with many different protocols, the reproducibility of the results and transfer of protocols between independent research groups remains a challenging task. Machine learning analysis based on statistical approaches is hardly applicable to the published data, since most of the researchers report only successful syntheses. In this work, we apply uniform sampling of the reaction parameter space. The concentrations of gold precursor, reducing agent, and surfactant were varied via an improved Latin hypercube sampling, and each run was performed under in situ UV–vis control. Based on the resulting set of optical spectra, we address the relevant chemical questions about nanoparticle formation, their shape, and period of growth. Our work demonstrates a data driven approach applied to the space of reaction parameters in a limited available set of experiments. 

Machine Learning Analysis of Reaction Parameters in UV-Mediated Synthesis of Gold Nanoparticles

Synthesis of Branched Au-PdAg Hybrid Nanosheets by Controlled Reduction in a Galvanic Replacement Reaction

Noble metal nanocrystals enclosed with curved surfaces are of great benefit for applications in electrocatalysis since the atomic steps and kinks on these facets have higher chemical activity. Herein, we report the fabrication of PdAg nanoshells with tunable thickness in the range of 5-13 nm and a unique concave cubic morphology, as well as the exploration of their applications for ethanol oxidation reaction (EOR) in alkaline media. The success of current work relies on the conformal deposition of PdAg on concave Au nanocubes, where the controlled reaction kinetics and proper chosen capping agent are both crucial for the growth mode. When loaded on carbon black and working as electrocatalysts, they exhibited superb electrochemical activity (e.g., 600.21 mA mg-1 in mass activity and 19.57 A m-2 in specific activity), together with improved EOR kinetics and long-term durability, as compared to Au@Pd nanoparticles and commercial Pd/C. The current work offers a feasible strategy to produce PdAg bimetallic nanocrystals with concave surface and validates their promising application as fuel cell catalysts, which could be extended to morphology engineering of other noble-metal nanocrystals for a broad range of applications.

Construction of Concave PdAg Nanoshells with Limited Thickness for Efficient Electroxidation of Ethanol

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Synthesis of Branched Au-PdAg Hybrid Nanosheets by Controlled Reduction in a Galvanic Replacement Reaction

Construction of Concave PdAg Nanoshells with Limited Thickness for Efficient Electroxidation of Ethanol