Noble‐Metal Nanocrystals with Controlled Facets for Electrocatalysis

Noble‐metal nanocrystals (NCs) show excellent catalytic performance for many important electrocatalysis reactions. The crystallographic properties of the facets by which the NCs are bound, closely associated with the shape of the NCs, have a profound influence on the electrocatalytic function of the NCs. To develop an efficient strategy for the synthesis of NCs with controlled facets as well as compositions, understanding of the growth mechanism of the NCs and their interaction with the chemical species involved in NC synthesis is quite important. Furthermore, understanding the facet‐dependent catalytic properties of noble‐metal NCs and the corresponding mechanisms for various electrocatalysis reactions will allow for the rational design of robust electrocatalysts. In this review, we summarize recently developed synthesis strategies for the preparation of mono‐ and bimetallic noble‐metal NCs by classifying them by the type of facets through which they are enclosed and discuss the electrocatalytic applications of noble‐metal NCs with controlled facets, especially for reactions associated with fuel‐cell applications, such as the oxygen reduction reaction and fuel (methanol, ethanol, and formic acid) oxidation reactions.     

Luminescent Noble Metal Nanoclusters as an Emerging Optical Probe for Sensor Development

In the past few years, highly luminescent noble metal nanoclusters (e.g., Au and Ag NCs or Au/Ag NCs in short) have emerged as a class of promising optical probes for the construction of high‐performance optical sensors because of their ultrasmall size (<2 nm), strong luminescence, good photostability, excellent biocompatibility, and unique metal‐core@ligand‐shell structure. In this Focus Review, we briefly summarize the common syntheses for water‐soluble highly‐luminescent thiolate‐ and protein‐protected Au/Ag NCs and their interesting luminescence properties, highlight recent progress in their use as optical sensors with an emphasis on the mechanisms underlying their selectivity, and finally discuss approaches to improving their sensitivity. The scope of the works surveyed is confined to highly luminescent thiolate‐ and protein‐protected Au/Ag NCs.     

Direct Conversion of Bulk Metals to Size‐Tailored,Monodisperse Spherical Non‐Coinage‐Metal Nanocrystals

Monodisperse non‐noble metal nanocrystals (NCs) that are highly uniform in shapes and particle size are much desired in various advanced applications, and are commonly prepared by either thermal decomposition or reduction, where reactive organometallic precursors or/and strong reducing agents are mandatory; however, these are usually toxic, costly, or suffer a lack of availability. Bulk Group 12 metals can now be converted into ligand‐protected, highly crystalline, monodisperse spherical metal NCs with precisely controlled sizes without using any precursors and reducers. The method is based on low‐power NIR‐laser‐induced size‐selective layer‐by‐layer surface vaporization. The monodisperse Cd NCs show pronounced deep‐UV (DUV) localized surface plasmon resonance making them highly competitive DUV‐plasmonic materials. This approach will promote appreciably the emergence of a wide range of monodisperse technically important non‐coinage metal NCs with compelling functionalities.     

Direct Conversion of Bulk Metals to Size‐Tailored,Monodisperse Spherical Non‐Coinage‐Metal Nanocrystals

Monodisperse non‐noble metal nanocrystals (NCs) that are highly uniform in shapes and particle size are much desired in various advanced applications, and are commonly prepared by either thermal decomposition or reduction, where reactive organometallic precursors or/and strong reducing agents are mandatory; however, these are usually toxic, costly, or suffer a lack of availability. Bulk Group 12 metals can now be converted into ligand‐protected, highly crystalline, monodisperse spherical metal NCs with precisely controlled sizes without using any precursors and reducers. The method is based on low‐power NIR‐laser‐induced size‐selective layer‐by‐layer surface vaporization. The monodisperse Cd NCs show pronounced deep‐UV (DUV) localized surface plasmon resonance making them highly competitive DUV‐plasmonic materials. This approach will promote appreciably the emergence of a wide range of monodisperse technically important non‐coinage metal NCs with compelling functionalities.     

Insight of the photoluminescence of atomically precise bimetallic nanoclusters with free electrons

Noble-metal nanoclusters with emission properties are promising candidtes for cell imaging, biosensing, photo-therapy applications due to their ultra-small size, good photostability, and high biocompatibility. It is of great importance to realize the atomic precision of such metal nanoclusters (NCs) because they allow us to investigate the origin of fluorescence at the atomic level. However, compared to quantum dots (QDs) and organic dyes, noble metal nanoclusters usually suffered from low quantum yield, which significantly limited their applications. Doping with other metal atoms is an effective strategy to enhance the fluorescent properties of metal NCs. Therefore, bimetallic nanoclusters possess enhanced emission properties relative to their monometallic counterparts. Although a couple of reviews have existed for bimetallic nanocluster, few of them concern their emissive properties. In this context, the emissive properties of atomically precise bimetallic nanoclusters are summarized in this minireview. Synergistic effect, state of aggregation, assembly, and isomerism on the fluorescence of bimetallic clusters and structure-photoluminescence property correlations are also discussed.     

Construction of an Upconversion Nanoprobe with Few‐Atom Silver Nanoclusters as the Energy Acceptor

Herein we report that few‐atom silver nanoclusters (Ag NCs) can be effective energy acceptors for upconversion phosphors (UCPs). A luminescence resonance energy transfer (LRET) probe for biothiols was constructed by decorating UCPs with dithiol‐stabilized Ag NCs. Owing to the unique properties of ultrasmall NCs, properties which bridge the gap between those of small molecules and those of nanoparticles, the use of approximately 1.9 nm Ag NCs as energy acceptors endows the probe with high energy‐transfer efficiency, good biocompatibility, and flexibility. The UCP–Ag NC nanoprobe enables rapid and robust target assay in solutions. It was also uploaded into living cells and used to detect intracellular biothiol levels with high discrimination. Moreover, the probe shows transportability in vivo and can be used for tissue imaging. The facile growth of few‐atom metal NCs on diverse templates may enable the development of various nanoprobes combining UCPs and metal NCs.     

Graphene‐Oxide‐Modified Lanthanide Nanoprobes for Tumor‐Targeted Visible/NIR‐II Luminescence Imaging

The synthesis of hydrophilic lanthanide‐doped nanocrystals (Ln³⁺‐NCs) with molecular recognition ability for bioimaging currently remains a challenge. Herein, we present an effective strategy to circumvent this bottleneck by encapsulating Ln³⁺‐NCs in graphene oxide (NCs@GO). Monodisperse NCs@GO was prepared by optimizing GO size and core–shell structure of NaYF₄:Yb,Er@NaYF₄, thus combining the intense visible/near‐infrared II (NIR‐II) luminescence of NCs and the unique surface properties and biomedical functions of GO. Such nanostructures not only feature broad solvent dispersibility, efficient cell uptake, and excellent biocompatibility but also enable further modifications with various agents such as DNA, proteins, or nanoparticles without tedious procedures. Moreover, we demonstrate in proof‐of‐concept experiments that NCs@GO can realize simultaneous intracellular tracking and microRNA‐21 visualization, as well as highly sensitive in vivo tumor‐targeted NIR‐II imaging at 1525 nm.     

Metal–Organic Gels from Silver Nanoclusters with Aggregation‐Induced Emission and Fluorescence‐to‐Phosphorescence Switching

Luminescent metal nanoclusters (NCs) are emerging as a new class of functional materials that have rich physicochemical properties and wide potential applications. In recent years, it has been found that some metal NCs undergo aggregation‐induced emission (AIE) and an interesting fluorescence‐to‐phosphorescence (F‐P) switching in solutions. However, insights of both the AIE and the F‐P switching remain largely unknown. Now, gelation of water soluble, atomically precise Ag₉ NCs is achieved by the addition of antisolvent. Self‐assembly of Ag₉ NCs into entangled fibers was confirmed, during which AIE was observed together with an F‐P switching occurring within a narrow time scale. Structural evaluation indicates the fibers are highly ordered. The self‐assembly of Ag₉ NCs and their photoluminescent property are thermally reversible, making the metal–organic gels good candidates for luminescent ratiometric thermometers.     

Electrospray Ionization Mass Spectrometry: A Powerful Platform for Noble‐Metal Nanocluster Analysis

Electrospray ionization mass spectrometry (ESI‐MS) is an analytical technique that measures the mass of a sample through “soft” ionization. Recent years have witnessed a rapid growth of its application in noble‐metal nanocluster (NC) analysis. ESI‐MS is able to provide the mass of a noble‐metal NC analyte for the analysis of their composition (n, m, q values in a general formula [M_nL_m]^q), which is crucial in understanding their properties. This review attempts to present various developed techniques for the determination of the composition of noble metal NCs by ESI‐MS. Additionally, advanced applications that use ESI‐MS to further understand the reaction mechanism, complexation behavior, and structure of noble metal NCs are introduced. From the comprehensive applications of ESI‐MS on noble‐metal NCs, more possibilities in nanochemistry can be opened up by this powerful technique.     

Colloidal Self‐Assembly of Catalytic Copper Nanoclusters into Ultrathin Ribbons

Metal nanoclusters (NCs) with diameter below 2 nm are promising catalysts in oxygen reduction reactions (ORR). However, the high surface energy of ultra‐small clusters leads to structural instability, shedding doubt on practical applications. Herein, we demonstrate a self‐assembly method to improve the durability of catalytic metal NCs, employing copper NCs capped by 1‐dodecanethiol (DT) to form free‐standing ribbons in colloidal solution. By tuning the cooperation between the dipolar attraction between Cu NCs and the van der Waals attraction between DT, the thickness of ribbons is adjusted to a single NC scale. Such free‐standing ribbons exhibit excellent catalytic activity and durability in ORR.     

Pre‐Incubation of Auric Acid with DNA Is Unnecessary for the Formation of DNA‐Templated Gold Nanoclusters

The rationale for the preparation of DNA‐templated gold nanoclusters (DNA‐Au NCs) has not been well understood, thereby slowing down the advancement of the synthesis and applications of DNA‐Au NCs. The interaction between metal ions and the DNA template seems to be the key factor for the successful preparation of DNA‐templated metal nanoclusters. With the help of circular dichroism in this contribution, we put efforts into interrogating the necessity of pre‐incubation of HAuCl₄ with poly‐adenine template in the formation of Au NCs by citrate reduction. Our results revealed that the pre‐incubation of HAuCl₄ with poly‐adenine is not favorable for the formation of Au NCs, which is distinctly different from the formation process for silver nanoclusters. It is our hope that this study can provide guidance in the preparation of Au NCs with more DNA templates.     

Nanocrystal/Metal–Organic Framework Hybrids as Electrocatalytic Platforms for CO2 Conversion

The tunable chemistry linked to the organic/inorganic components in colloidal nanocrystals (NCs) and metal–organic frameworks (MOFs) offers a rich playground to advance the fundamental understanding of materials design for various applications. Herein, we combine these two classes of materials by synthesizing NC/MOF hybrids comprising Ag NCs that are in intimate contact with Al‐PMOF ([Al₂(OH)₂(TCPP)]) (tetrakis(4‐carboxyphenyl)porphyrin (TCPP)), to form Ag@Al‐PMOF. In our hybrids, the NCs are embedded in the MOF while still preserving electrical contact with a conductive substrate. This key feature allows the investigation of the Ag@Al‐PMOFs as electrocatalysts for the CO₂ reduction reaction (CO₂RR). We show that the pristine interface between the NCs and the MOFs accounts for electronic changes in the Ag, which suppress the hydrogen evolution reaction (HER) and promote the CO₂RR. We also demonstrate a minor contribution of mass‐transfer effects imposed by the porous MOF layer under the chosen testing conditions. Furthermore, we find an increased morphological stability of the Ag NCs when combined with the Al‐PMOF. The synthesis method is general and applicable to other metal NCs, thus revealing a new way to think about rationally tailored electrocatalytic materials to steer selectivity and improve stability.     

Balancing the Rate of Cluster Growth and Etching for Gram‐Scale Synthesis of Thiolate‐Protected Au25 Nanoclusters with Atomic Precision

We report a NaOH‐mediated NaBH₄ reduction method for the synthesis of mono‐, bi‐, and tri‐thiolate‐protected Au₂₅ nanoclusters (NCs) with precise control of both the Au core and thiolate ligand surface. The key strategy is to use NaOH to tune the formation kinetics of Au NCs, i.e., reduce the reduction ability of NaBH₄ and accelerate the etching ability of free thiolate ligands, leading to a well‐balanced reversible reaction for rapid formation of thermodynamically favorable Au₂₅ NCs. This protocol is facile, rapid (≤3 h), versatile (applicable for various thiolate ligands), and highly scalable (>1 g Au NCs). In addition, bi‐ and tri‐thiolate‐protected Au₂₅ NCs with adjustable ratios of hetero‐thiolate ligands were easily obtained. Such ligand precision in molecular ratios, spatial distribution and uniformity resulted in richly diverse surface landscapes on the Au NCs consisting of multiple functional groups such as carboxyl, amine, and hydroxy. Analysis based on NMR spectroscopy revealed that the hetero‐ligands on the NCs are well distributed with no ligand segregation. The unprecedented synthesis of multi‐thiolate‐protected Au₂₅ NCs may further promote the practical applications of functional metal NCs.     

Metal‐Enhanced Fluorescence of CdTe Nanocrystals in Aqueous Solution

Metal‐enhanced fluorescence of semiconductor nanocrystals (NCs) is investigated. There is very little attention paid to the metal‐enhanced fluorescence in aqueous solution, which has great potential applications in bioscience. In this work, we directly observe metal‐enhanced fluorescence of CdTe NC solution by simply mixing CdTe NCs and Au nanoparticles, both of which are negatively charged. In order to study this kind of photoluminescence enhancement in aqueous solutions, we propose a calibration method, which takes into account the light attenuation in solutions. After consideration of the light weakening in transmission, the maximal PL enhancement is about 3 times as large as the ones without Au NPs. Some factors related to the enhanced magnitude of fluorescence, for instance, the concentration and the molar feed ratio of CdTe NCs and Au NPs, are studied in detail. Furthermore, the decreased lifetimes of CdTe NCs induced by Au NPs are also obtained, which are in accord with the enhancement of the photoluminescence.     

Electrochemical Synthesis of Tetrahexahedral Rhodium Nanocrystals with Extraordinarily High Surface Energy and High Electrocatalytic Activity

Noble metal nanocrystals (NCs) enclosed with high‐index facets hold a high catalytic activity thanks to the high density of low‐coordinated step atoms that they exposed on their surface. Shape‐control synthesis of the metal NCs with high‐index facets presents a big challenge owing to the high surface energy of the NCs, and the shape control for metal Rh is even more difficult because of its extraordinarily high surface energy in comparison with Pt, Pd, and Au. The successful synthesis is presented of tetrahexahedral Rh NCs (THH Rh NCs) enclosed by {830} high‐index facets through the dynamic oxygen adsorption/desorption mediated by square‐wave potential. The results demonstrate that the THH Rh NCs exhibit greatly enhanced catalytic activity over commercial Rh black catalyst for the electrooxidation of ethanol and CO.     

具特定晶面且大比表面积贵金属纳米晶催化基元的构筑

贵金属纳米晶在电催化等领域具有广泛应用. 其催化活性往往与纳米晶体的表面结构直接相关,而催化剂的贵金属原子利用率与比表面积密切相关. 因小尺寸纳米晶难以保留特定的晶面,而具有特定表面的纳米晶通常结晶成尺寸较大、比表面积比较小的晶体,调控纳米晶的尺寸和表面结构两种策略似乎相互矛盾. 如何可控合成同时具有特定表面结构和大比表面积的贵金属纳米晶具有重要的意义. 本综述从形貌调控角度详细介绍提高贵金属纳米晶原子利用率的方法策略;总结调控单贵金属及其合金同时具有特定晶面和大比表面积的研究现状;最后,对纳米晶的形貌调控领域未来的发展趋势提出展望.     

Excavated Cubic Platinum–Tin Alloy Nanocrystals Constructed from Ultrathin Nanosheets with Enhanced Electrocatalytic Activity

Excavated polyhedral noble‐metal materials that were built by the orderly assembly of ultrathin nanosheets have both large surface areas and well‐defined facets, and therefore could be promising candidates for diverse important applications. In this work, excavated cubic Pt–Sn alloy nanocrystals (NCs) with {110} facets were constructed from twelve nanosheets by a simple co‐reduction method with the assistance of the surface regulator polyvinylpyrrolidone. The specific surface area of the excavated cubic Pt–Sn NCs is comparable to that of commercial Pt black despite their larger particle size. The excavated cubic Pt–Sn NCs exhibited superior electrocatalytic activity in terms of both the specific area current density and the mass current density towards methanol oxidation.     

Simple replacement reaction for the preparation of ternary Fe(1-x)PtRu(x) nanocrystals with superior catalytic activity in methanol oxidation reaction

The finding of new metal alloyed nanocrystals (NCs) with high catalytic activity and low cost to replace PtRu NCs is a critical step toward the commercialization of fuel cells. In this work, a simple cation replacement reaction was utilized to synthesize a new type of ternary Fe(1-x)PtRu(x) NCs from binary FePt NCs. The detailed structural transformation from binary FePt NCs to ternary Fe(1-x)PtRu(x) NCs was analyzed by X-ray absorption spectroscopy (XAS). Ternary Fe(35)Pt(40)Ru(25), Fe(31)Pt(40)Ru(29), and Fe(17)Pt(40)Ru(43) NCs exhibit superior catalytic ability to withstand CO poisoning in methanol oxidation reaction (MOR) than do binary NCs (FePt and J-M PtRu). Also, the Fe(31)Pt(40)Ru(29) NCs had the highest alloying extent and the lowest onset potential among the ternary NCs. Furthermore, the origin for the superior CO resistance of ternary Fe(1-x)PtRu(x) NCs was investigated by determining the adsorption energy of CO on the NCs' surfaces and the charge transfer from Fe/Ru to Pt using a simulation based on density functional theory. The simulation results suggested that by introducing a new metal into binary PtRu/PtFe NCs, the anti-CO poisoning ability of ternary Fe(1-x)PtRu(x) NCs was greatly enhanced because the bonding of CO-Pt on the NCs' surface was weakened. Overall, our experimental and simulation results have indicated a simple route for the discovery of new metal alloyed catalysts with superior anti-CO poisoning ability and low usage of Pt and Ru for fuel cell applications.     

Formation of a Highly Reactive Cobalt Nanocluster Crystal within a Highly Negatively Charged Porous Coordination Cage

Earth‐abundant first‐row transition‐metal nanoclusters (NCs) have been extensively investigated as catalysts. However, their catalytic activity is relatively low compared with noble metal NCs. Enhanced catalytic activity of cobalt NCs can be achieved by encapsulating Co NCs in soluble porous coordination cages (PCCs). Two cages, PCC‐2a and 2b, possess almost identical cavity in shape and size, while PCC‐2a has five times more net charges than PCC‐2b. Co²⁺ cations were accumulated in PCC‐2a and reduced to ultra‐small Co NCs in situ, while for PCC‐2b, only bulky Co particles were formed. As a result, Co NCs@PCC‐2a accomplished the highest catalytic activity in the hydrolysis of ammonium borane among all the first‐row transition‐metals NCs. Based on these results, it is envisioned that confining in the charged porous coordination cage could be a novel route for the synthesis of ultra‐small NCs with extraordinary properties.     

A Facile Phosphine‐Free Method for Synthesizing PbSe Nanocrystals with Strong Optical Limiting Effects

PbSe semiconductor nanocrystals (NCs) have attracted ever‐growing interest owing to both their fundamental physics and potential applications in a diverse range of fields such as optoelectronic devices and nonlinear optics. The current fabrication strategy for colloidal PbSe NCs, however, frequently involves acutely toxic reagents and tedious reaction procedures, and is plagued by products with poorly controlled size and morphology. Herein, we report a facile, low‐cost, and phosphine‐free method for synthesizing PbSe NCs, which provides highly uniform NCs with tunable mid‐IR absorption, and they are promising for bio‐related applications. These high quality NCs were obtained by the reaction of elemental Se and PbCl₂ in oleylamine as both the ligand and reaction medium. The high flexibility and reproducibility of the method reported in this study allows us to synthesize monodispersed PbSe NCs with well‐controlled size and morphology. In addition, these products show strong optical limiting effects, and thus hold potential for developing nonlinear optical devices.