Qiaofeng Yao

Luminescent Metal Nanoclusters with Aggregation-Induced Emission

Thiolate-protected metal nanoclusters (or thiolated metal NCs) have recently emerged as a promising class of functional materials because of their well-defined molecular structures and intriguing molecular-like properties. Recent developments in the NC field have aimed at exploring metal NCs as novel luminescent materials in the biomedical field because of their inherent biocompatibility and good photoluminescence (PL) properties. From the fundamental perspective, recent advances in the field have also aimed at addressing the fundamental aspects of PL properties of metal NCs, shedding some light on developing efficient strategies to prepare highly luminescent metal NCs. In this Perspective, we discuss the physical chemistry of a recently discovered aggregation-induced emission (AIE) phenomenon and show the significance of AIE in understanding the PL properties of thiolated metal NCs. We then explore the unique physicochemical properties of thiolated metal NCs with AIE characteristics and highlight some recent developments in synthesizing the AIE-type luminescent metal NCs. We finally discuss perspectives and directions for future development of the AIE-type luminescent metal NCs.

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.

Balancing the rate of cluster growth and etching for gram-scale synthesis of thiolate-protected Au(25) 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.

Hierarchical heterostructures of Ag nanoparticles decorated MnO2 nanowires as promising electrodes for supercapacitors

Coating the redox-active transition-metal oxides (e.g., MnO2) with a conductive metal layer is one efficient approach to improve the electrical conductivity of the oxide-based electrodes, which could largely boost the energy density and power density of supercapacitors. Here, we report a facile yet efficient method to uniformly decorate conductive silver (Ag) nanoparticles (∼10 nm) on MnO2 nanowires (width of ∼10–20 nm), which leads to a remarkable improvement of the electrical conductivity and the supercapacitive performance of MnO2-based electrodes. For instance, at a low scan rate of 10 mV s−1, the as-designed Ag/MnO2 hybrid electrode delivers a specific capacitance of 293 F g−1, which is twofold higher than that of the bare MnO2 electrode (∼130 F g−1). In addition, the highly conductive Ag nanoparticle layer can also improve the rate capability of the Ag/MnO2 nanowire electrode, delivering a high specific energy density and power density of 17.8 W h kg−1 and 5000 W kg−1, respectively, at a current density of 10 A g−1.

Introducing amphiphilicity to noble metal nanoclusters via phase-transfer driven ion-pairing reaction.

Amphiphilicity is a surface property that has yet to be explored for the noble metal nanoclusters (NCs). This article shows how amphiphilicity may be added to sub-2-nm metal NCs by patching hydrophilic NCs (e.g., Au25(MHA)18 NCs where MHA is 6-mercaptohexanoic acid) with hydrophobic cations (e.g., cetyltrimethylammonium ion, CTA(+)) to about half of a monolayer coverage. Specifically we demonstrate the preparation of amphiphilic Au25(MHA)18@xCTA NCs (x = 6-9 where x is the number of CTA(+) per NC) by the phase-transfer (PT) driven ion-paring reaction between CTA(+) and -COO(-) (derived from the deprotonation of the terminal carboxyl group of MHA). Due to the coexistence of flexible hydrophilic MHA and hydrophobic MHA···CTA ligands in comparable amounts on the NC surface, the Au25(MHA)18@xCTA NCs (x = 6-9) exhibit good amphiphilicity, which enabled them to dissolve in solvents with distinctly different polarities and to self-assemble like a molecular amphiphile. Consequently, the amphiphilic Au25(MHA)18@xCTA NCs (x = 6-9) could self-organize into stacked bilayers at the air-liquid interface, similar to the formation of lyotropic liquid crystalline phases by common surfactants. The good solubility and molecular-amphiphile-like self-assembly properties can significantly increase the utility of noble metal NCs in basic and applied research.

Hierarchical TiO2-B nanowire@α-Fe2O3 nanothorn core-branch arrays as superior electrodes for lithium-ion microbatteries

This paper reports a simple yet efficient method for the synthesis of hierarchical TiO2-B nanowire@α-Fe2O3 nanothorn core-branch arrays based on a stepwise hydrothermal approach. The as-fabricated hybrid arrays show impressive performance as a high-capacity anode for lithium-ion batteries. The key design in this study is a core-branch hybrid architecture, which not only provides large surface active sites for lithium ion insertion/extraction, but also enables fast charge transport owing to the reduced diffusion paths for both electrons and lithium ions. The peculiar combination of attributes of TiO2 (good structural stability) and Fe2O3 (large specific capacity) provides the hybrid array electrodes with several desirable electrochemical features: large reversible capacity (∼800 mA·h·g−1 for specific mass capacity and ∼750 μA·h·cm−2 for specific areal capacity), good cycling stability, and high rate capability. The impressive electrochemical performance, together with the facile synthesis procedure, may provide an efficient platform to integrate the TiO2 nanowire@Fe2O3 nanothorn core-branch arrays as a three-dimensional thin film electrode for lithium-ion microbatteries.

Electrocatalysis of polysulfide conversion by sulfur-deficient MoS2 nanoflakes for lithium–sulfur batteries

Lithium–sulfur batteries are promising next-generation energy storage devices due to their high energy density and low material cost. Efficient conversion of lithium polysulfides to lithium sulfide (during discharge) and to sulfur (during recharge) is a performance-determining factor for lithium–sulfur batteries. Here we show that MoS2−x/reduced graphene oxide (MoS2−x/rGO) can be used to catalyze the polysulfide reactions to improve the battery performance. It was confirmed, through microstructural characterization of the materials, that sulfur deficiencies on the surface participated in the polysulfide reactions and significantly enhanced the polysulfide conversion kinetics. The fast conversion of soluble polysulfides decreased their accumulation in the sulfur cathode and their loss from the cathode by diffusion. Hence in the presence of a small amount of MoS2−x/rGO (4 wt% of the cathode mass), high rate (8C) performance of the sulfur cathode was improved from a capacity of 161.1 mA h g−1 to 826.5 mA h g−1. In addition, MoS2−x/rGO also enhanced the cycle stability of the sulfur cathode from a capacity fade rate of 0.373% per cycle (over 150 cycles) to 0.083% per cycle (over 600 cycles) at a typical 0.5C rate. These results provide direct experimental evidence for the catalytic role of MoS2−x/rGO in promoting the polysulfide conversion kinetics in the sulfur cathode.

Stellated Ag-Pt bimetallic nanoparticles: An effective platform for catalytic activity tuning

The usefulness of Pt-based nanomaterials for catalysis can be greatly enhanced by coupling morphology engineering to the strategic presence of a second or even third metal. Here we demonstrate the design and preparation of stellated Ag-Pt bimetallic nanoparticles where significant activity difference between the methanol oxidation reaction (MOR) and the oxygen reduction reaction (ORR) may be realized by relegating Ag to the core or by hollowing out the core. In particular the stellated Pt surface, with an abundance of steps, edges, corner atoms, and {111} facets, is highly effective for the ORR but is ineffective for MOR. MOR activity is only observed in the presence of a Ag core through electronic coupling to the stellated Pt shell. The bimetallic Ag-Pt stellates therefore demonstrate the feasibility of tuning a Pt surface for two very different structure sensitive catalytic reactions. Stellated bimetallics may therefore be an effective platform for highly tunable catalyst designs.

Fast Synthesis of Thiolated Au25 Nanoclusters via Protection–Deprotection Method

This letter reports a new synthesis strategy for atomically precise Au nanoclusters (NCs) by using a protection-deprotection method. The key in our synthesis strategy is to introduce a surfactant molecule to protect thiolate-Au(I) complexes during their reduction. The protecting layer provides a good steric hindrance and controls the formation rate of thiolated Au NCs, which leads to the direct formation of atomically precise Au NCs inside the protecting layer. The protecting layer was then removed from the surface of thiolated Au NCs to bring back the original functional groups on the NCs. The protection-deprotection method is simple and facile and can synthesize high-purity thiolated Au25 NCs within 10 min. Our synthesis protocol is fairly generic and can be easily extended to prepare Au25 NCs protected by other thiolate ligands.

Two‐Phase Synthesis of Small Thiolate‐Protected Au15 and Au18 Nanoclusters

Thiolate-protected Au NCs are often synthesized as a mixture and have to be separated by high resolution techniques such as solubility-based fractionation (SF), [ 3 ] polyacrylamide gel electrophoresis (PAGE) [ 4 ] and size-exclusion chromatography (SEC). [ 5 ] Stable thiolate-protected Au NCs with discrete core sizes (or magic sizes) of n = 15, 18, 25, 38, 55, 68, 102, 130, 144, and 187 have been obtained this way. [ 3b , 3d , 4d , 5c , 6 ]