Quantification, Exchange, and Removal of Surface Ligands on Noble-Metal Nanocrystals.

ConspectusSurface ligands are vital to the colloidal synthesis of noble-metal nanocrystals with well-controlled sizes and shapes for various applications. The surface ligands not only dictate the formation of nanocrystals with diverse shapes but also serve as a colloidal stabilizer to prevent the suspended nanocrystals from aggregation during their synthesis or storage. By leveraging the facet selectivity of some surface ligands, one can further control the sites for growth or galvanic replacement to transform presynthesized nanocrystals into complex structures that are otherwise difficult to fabricate using conventional methods. Furthermore, the presence of surface ligands on nanocrystals also facilitates their applications in areas such as sensing, imaging, nanomedicine, and self-assembly. Despite their popular use in enhancing the properties of nanocrystals and thus optimizing their performance in a wide variety of applications, it remains a major challenge to quantitatively determine the coverage density of ligand molecules, not to mention the difficulty of substituting or removing them without compromising the surface structure and aggregation state of the nanocrystals.In this Account, we recapitulate our efforts in developing methods capable of qualitatively or quantitatively measuring, exchanging, and removing the surface ligands adsorbed on noble-metal nanocrystals. We begin with an introduction to the typical interactions between ligand molecules and surface atoms, followed by a discussion of the Langmuir model that can be used to describe the adsorption of surface ligands. It is also emphasized that the adsorption process may become very complex in the case of a polymeric ligand due to the variations in binding configuration and chain conformation. We then highlight the capabilities of various spectroscopy methods to analyze the adsorbed ligands qualitatively or quantitatively. Specifically, surface-enhanced Raman scattering, Fourier transform infrared, and X-ray photoelectron spectroscopy are three examples of qualitative methods that can be used to confirm the absence or presence of a surface ligand. On the other hand, ultraviolet-visible spectroscopy and inductively coupled plasma mass spectrometry can be used for quantitative measurements. Additionally, the coverage density of a ligand can be derived by analyzing the morphological changes during nanocrystal growth. We then discuss how the ligands present on the surface of metal nanocrystals can be exchanged directly or indirectly to meet the requirements of different applications. The former can be done using a ligand with stronger binding, whereas the latter is achieved by introducing a sacrificial shell to the surface of the nanocrystals. Furthermore, we highlight three additional strategies besides simple washing to remove the surface ligands, including calcination, heating in a solution, and UV-ozone treatment. Finally, we showcase three applications of metal nanocrystals in nanomedicine, tumor targeting, and self-assembly by taking advantage of the diversity of surface ligands bearing different functional groups. We also offer perspectives on the challenges and opportunities in realizing the full potential of surface ligands.

Toward controllable and predictable synthesis of high-entropy alloy nanocrystals.

High-entropy alloy (HEA) nanocrystals have attracted extensive attention in catalysis. However, there are no effective strategies for synthesizing them in a controllable and predictable manner. With quinary HEA nanocrystals made of platinum-group metals as an example, we demonstrate that their structures with spatial compositions can be predicted by quantitatively knowing the reduction kinetics of metal precursors and entropy of mixing in the nanocrystals under dropwise addition of the mixing five-metal precursor solution. The time to reach a steady state for each precursor plays a pivotal role in determining the structures of HEA nanocrystals with homogeneous alloy and core-shell features. Compared to the commercial platinum/carbon and phase-separated counterparts, the dendritic HEA nanocrystals with a defect-rich surface show substantial enhancement in catalytic activity and durability toward both hydrogen evolution and oxidation. This quantitative study will lead to a paradigm shift in the design of HEA nanocrystals, pushing away from the trial-and-error approach.

Quantitative Analysis of Dynamic Subacromial Ultrasonography: Reliability and Influencing Factors.

Objective: Current imaging methods used to examine patients with subacromial impingement syndrome (SIS) are limited by their semi-quantitative nature and their capability of capturing dynamic movements. This study aimed to develop a quantitative analytic model to assess subacromial motions using dynamic ultrasound and to examine their reliability and potential influencing factors. Method: We included 48 healthy volunteers and examined their subacromial motions with dynamic ultrasound imaging. The parameters were the minimal vertical acromiohumeral distance, rotation radius, and degrees of the humeral head. The generalized estimating equation (GEE) was used to investigate the impact of different shoulder laterality, postures, and motion phases on the outcome. Result: Using the data of the minimal vertical acromiohumeral distance, the intra-rater and inter-rater reliabilities (intra-class correlation coefficient) were determined as 0.94 and 0.88, respectively. In the GEE analysis, a decrease in the minimal vertical acromiohumeral distance was associated with the abduction phase and full-can posture, with a beta coefficient of -0.02 cm [95% confidence interval (CI), -0.03 to -0.01] and -0.07 cm (95% CI, -0.11 to -0.02), respectively. The abduction phase led to a decrease in the radius of humeral rotation and an increase in the angle of humeral rotation, with a beta coefficient of -1.28 cm (95% CI, -2.16 to -0.40) and 6.60° (95% CI, 3.54-9.67), respectively. A significant negative correlation was observed between the rotation angle and radius of the humeral head and between the rotation angle and the minimal vertical acromiohumeral distance. Conclusion: Quantitative analysis of dynamic ultrasound imaging enables the delineation of subacromial motion with good reliability. The vertical acromiohumeral distance is the lowest in the abduction phase and full-can posture, and the rotation angle of the humeral head has the potential to serve as a new parameter for the evaluation of SIS.

Understanding the Role of Poly(vinylpyrrolidone) in Stabilizing and Capping Colloidal Silver Nanocrystals.

The ligands anchored to the surface of metal nanocrystals play an important role in controlling their colloidal synthesis for a broad spectrum of applications, but it remains a daunting challenge to investigate the ligand-surface and ligand-solvent interactions at the molecular level. Here, we report the use of surface-enhanced Raman scattering (SERS) to extract structural information about the binding of poly(vinylpyrrolidone) (PVP) to Ag nanocubes as well as its conformational changes in response to solvent quality. When a PVP chain binds to the surface of a Ag nanocube through some of its carbonyl groups, the segments between adjacent binding sites are expelled into the solvent as loops. As a result, the carbonyl peak (νC═O) resolved in the SERS spectrum includes the contributions from those anchored to the surface and those residing on the loops, with their frequencies located at νC═O(Ag) and νC═O(free), respectively. While νC═O(Ag) remains at a fixed frequency due to the coordination between the carbonyl groups with Ag surface, the spectral position of νC═O(free) is dependent on the solvent. As the strength of hydrogen bonding between PVP and solvent increases, the peak position of νC═O(free) shifts toward lower frequencies. When exposed to bad and good solvents in an alternating manner, the PVP loops undergo conformational changes between collapsed and extended states, altering the separation between the free carbonyl groups and the Ag surface and thereby the intensity of the νC═O peak.

Noble-Metal Nanoframes and Their Catalytic Applications.

Noble-metal nanoframes consisting of interconnected, ultrathin ridges have received considerable attention in the field of heterogeneous catalysis. The enthusiasm arises from the high utilization efficiency of atoms for significantly reducing the material loading while enhancing the catalytic performance. In this review article, we offer a comprehensive assessment of research endeavors in the design and rational synthesis of noble-metal nanoframes for applications in catalysis. We start with a brief introduction to the unique characteristics of nanoframes, followed by a discussion of the synthetic strategies and their controls in terms of structure and composition. We then present case studies to elucidate mechanistic details behind the synthesis of mono-, bi-, and multimetallic nanoframes, as well as heterostructured and hybrid systems. We discuss their performance in electrocatalysis, thermal catalysis, and photocatalysis. Finally, we highlight recent progress in addressing the structural and compositional stability issues of nanoframes for the assurance of robustness in catalytic applications.

Surface Capping Agents and Their Roles in Shape-Controlled Synthesis of Colloidal Metal Nanocrystals.

Surface capping agents have been extensively used to control the evolution of seeds into nanocrystals with diverse but well-controlled shapes. Here we offer a comprehensive review of these agents, with a focus on the mechanistic understanding of their roles in guiding the shape evolution of metal nanocrystals. We begin with a brief introduction to the early history of capping agents in electroplating and bulk crystal growth, followed by discussion of how they affect the thermodynamics and kinetics involved in a synthesis of metal nanocrystals. We then present representative examples to highlight the various capping agents, including their binding selectivity, molecular-level interaction with a metal surface, and impacts on the growth of metal nanocrystals. We also showcase progress in leveraging capping agents to generate nanocrystals with complex structures and/or enhance their catalytic properties. Finally, we discuss various strategies for the exchange or removal of capping agents, together with perspectives on future directions.

One-Pot Synthesis of Pd@PtnL Core-Shell Icosahedral Nanocrystals in High Throughput through a Quantitative Analysis of the Reduction Kinetics.

The rational design and implementation of a one-pot method is reported for the facile synthesis of Pd@PtnL (nL denotes the number of Pt atomic layers) core-shell icosahedral nanocrystals in a single step. The success of this method relies on the use of Na2 PdCl4 and Pt(acac)2 as the precursors to Pd and Pt atoms, respectively. Our quantitative analysis of the reduction kinetics indicates that the PdII and PtII precursors are sequentially reduced with a major gap between the two events. Specifically, the PdII precursor is reduced first, leading to the formation of Pd-based icosahedral seeds with a multiply-twinned structure. In contrast, the PtII precursor prefers to take a surface reduction pathway on the just-formed icosahedral seeds. As such, the otherwise extremely slow reduction of the PtII precursor can be dramatically accelerated through an autocatalytic process for the deposition of Pt atoms as a conformal shell on each Pd icosahedral core. Compared to the conventional approach of seed-mediated growth, the throughput for the one-pot synthesis of Pd@PtnL core-shell nanocrystals can be increased by more than 30-fold. When used as catalysts, the Pd@Pt4.5L core-shell icosahedral nanocrystals show specific and mass activities of 0.83 mA cm-2 and 0.39 A mgPt -1 , respectively, at 0.9 V toward oxygen reduction. The Pt-based nanocages derived from the core-shell nanocrystals also show enhanced specific (1.45 mA cm-2 ) and mass activities (0.75 A mgPt -1 ) at 0.9 V, which are 3.8 and 3.3 times greater than those of the commercial Pt/C, respectively.

Enabling Complete Ligand Exchange on the Surface of Gold Nanocrystals through the Deposition and Then Etching of Silver.

We report an indirect method for the effective replacement of ligands on the surface of Au nanocrystals with different morphologies. The method involves the deposition of an ultrathin layer of Ag to remove a strong capping agent such as cetyltrimethylammonium chloride (CTAC), followed by selective etching of the Ag layer in the presence of citrate ions as a stabilizer. Using multiple characterization techniques, we confirm that the surface of the Au nanocrystals is covered by citrate ions after the indirect ligand exchange process, and there is essentially no aggregation during the entire process. We also demonstrate that this method is effective in suppressing the toxicity of Au nanospheres by completely replacing the initially used CTAC with citrate.

Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants.

There is a growing interest in controlling the synthesis of colloidal metal nanocrystals and thus tailoring their properties toward various applications. In this context, choosing an appropriate combination of reagents (e.g., salt precursor, reductant, capping agent, and stabilizer) plays a pivotal role in enabling the synthesis of metal nanocrystals with diversified sizes, shapes, and structures. Here we present a comprehensive review that highlights one of the key reagents for the synthesis of metal nanocrystals via chemical reduction: the reductants. We start with a brief introduction to the compounds commonly employed as reductants in the colloidal synthesis of metal nanocrystals by showing their oxidation half-reactions and the corresponding oxidation potentials. Then we offer specific examples pertaining to the controlled synthesis of metal nanocrystals, followed by some fundamental aspects covering the general mechanisms of metal ion reduction based on the Marcus Theory. Afterwards, we present a case-by-case discussion on a wide variety of reductants, including their major properties, reduction mechanisms, and additional effects on the final products. We illustrate these aspects by selecting key examples from the literature and paying close attention to the underlying mechanism in each case. At the end, we conclude by summarizing the highlights of the review and providing some perspectives on future directions.

Synthesis of Palladium Nanoscale Octahedra through a One-Pot, Dual-Reductant Route and Kinetic Analysis.

Shape-controlled synthesis of colloidal metal nanocrystals has traditionally relied on the use of an approach that involves the reduction of a metal precursor by a single reductant. Once the concentration of atoms surpasses supersaturation, they will undergo homogeneous nucleation to generate nuclei and then seeds, followed by further growth into nanocrystals. In general, it is a grand challenge to optimize such an approach because the kinetic requirement for nucleation tends to be drastically different from what is needed to guide the growth process. In this work, we overcome this difficulty by switching to a dual-reductant approach, in which both strong and weak reductants are added into the same reaction solution. By controlling their amounts to program the reduction kinetics, the strong reductant only regulates the homogeneous nucleation process to generate the desired seeds, and once consumed, the weak reductant takes over to control the growth pattern and thereby the shape of the resulting nanocrystals.

Autocatalytic surface reduction and its role in controlling seed-mediated growth of colloidal metal nanocrystals.

The growth of colloidal metal nanocrystals typically involves an autocatalytic process, in which the salt precursor adsorbs onto the surface of a growing nanocrystal, followed by chemical reduction to atoms for their incorporation into the nanocrystal. Despite its universal role in the synthesis of colloidal nanocrystals, it is still poorly understood and controlled in terms of kinetics. Through the use of well-defined nanocrystals as seeds, including those with different types of facets, sizes, and internal twin structure, here we quantitatively analyze the kinetics of autocatalytic surface reduction in an effort to control the evolution of nanocrystals into predictable shapes. Our kinetic measurements demonstrate that the activation energy barrier to autocatalytic surface reduction is highly dependent on both the type of facet and the presence of twin boundary, corresponding to distinctive growth patterns and products. Interestingly, the autocatalytic process is effective not only in eliminating homogeneous nucleation but also in activating and sustaining the growth of octahedral nanocrystals. This work represents a major step forward toward achieving a quantitative understanding and control of the autocatalytic process involved in the synthesis of colloidal metal nanocrystals.

Reduction rate as a quantitative knob for achieving deterministic synthesis of colloidal metal nanocrystals.

Despite the incredible developments made to the synthesis of colloidal metal nanocrystals, it is still challenging to produce them in a reproducible and predictable manner. This drawback can be attributed to the fact that the protocols continue to be built upon qualitative observations and empirical laws. Because of the vast number of intricately entangled experimental parameters in a synthesis, it is almost impossible to predict and control the outcome by knowingly alternating these parameters. In this Perspective article, we discuss the recent efforts in pushing nanocrystal synthesis towards a deterministic process based upon quantitative measurements. In particular, we focus on how the reduction rate of a salt precursor can be used as a quantitative knob for predicting and controlling the outcomes of both nucleation and growth. We begin with a brief introduction to the techniques that have been used to extract the kinetic information of a synthesis and then discuss how the reduction rate is correlated with the defect structure, shape/morphology, and elemental distribution of the resultant nanocrystals. We conclude by highlighting some of the recent advances related to in situ probing of nanocrystal synthesis, with an emphasis on the real-time, quantitative aspects with regard to both nucleation and growth.

Thermal Stability of Metal Nanocrystals: An Investigation of the Surface and Bulk Reconstructions of Pd Concave Icosahedra.

Despite the remarkable success in controlling the synthesis of metal nanocrystals, it still remains a grand challenge to stabilize and preserve the shapes or internal structures of metastable kinetic products. In this work, we address this issue by systematically investigating the surface and bulk reconstructions experienced by a Pd concave icosahedron when subjected to heating up to 600 °C in vacuum. We used in situ high-resolution transmission electron microscopy to identify the equilibration pathways of this far-from-equilibrium structure. We were able to capture key structural transformations occurring during the thermal annealing process, which were mechanistically rationalized by implementing self-consistent plane-wave density functional theory (DFT) calculations. Specifically, the concave icosahedron was found to evolve into a regular icosahedron via surface reconstruction in the range of 200-400 °C, and then transform into a pseudospherical crystalline structure through bulk reconstruction when further heated to 600 °C. The mechanistic understanding may lead to the development of strategies for enhancing the thermal stability of metal nanocrystals.

Toward a Quantitative Understanding of the Reduction Pathways of a Salt Precursor in the Synthesis of Metal Nanocrystals.

Despite the pivotal role played by the reduction of a salt precursor in the synthesis of metal nanocrystals, it is still unclear how the precursor is reduced. The precursor can be reduced to an atom in the solution phase, followed by its deposition onto the surface of a growing nanocrystal. Alternatively, the precursor can adsorb onto the surface of a growing nanocrystal, followed by reduction through an autocatalytic process. With Pd as an example, here we demonstrate that the pathway has a correlation with the reduction kinetics involved. Our quantitative analyses of the reduction kinetics of PdCl42- and PdBr42- by ascorbic acid at room temperature in the absence and presence of Pd nanocubes, respectively, suggest that PdCl42- was reduced in the solution phase while PdBr42- was reduced on the surface of a growing nanocrystal. Our results also demonstrate that the reduction pathway of PdBr42- by ascorbic acid could be switched from surface to solution by raising the reaction temperature.

Quantitative Analysis of the Reduction Kinetics Responsible for the One-Pot Synthesis of Pd-Pt Bimetallic Nanocrystals with Different Structures.

We report a quantitative understanding of the reduction kinetics responsible for the formation of Pd-Pt bimetallic nanocrystals with two distinctive structures. The syntheses involve the use of KBr to manipulate the reaction kinetics by influencing the redox potentials of metal precursor ions via ligand exchange. In the absence of KBr, the ratio between the initial reduction rates of PdCl4(2-) and PtCl4(2-) was about 10.0, leading to the formation of Pd@Pt octahedra with a core-shell structure. In the presence of 63 mM KBr, the products became Pd-Pt alloy nanocrystals. In this case, the ratio between the initial reduction rates of the two precursors dropped to 2.4 because of ligand exchange and, thus, the formation of PdBr4(2-) and PtBr4(2-). The alloy nanocrystals took a cubic shape owing to the selective capping effect of Br(-) ions toward the {100} facets. Relative to the alloy nanocubes, the Pd@Pt core-shell octahedra showed substantial enhancement in both catalytic activity and durability toward the oxygen reduction reaction (ORR). Specifically, the specific (1.51 mA cm(-2)) and mass (1.05 A mg(-1) Pt) activities of the core-shell octahedra were enhanced by about four- and three-fold relative to the alloy nanocubes (0.39 mA cm(-2) and 0.34 A mg(-1) Pt, respectively). Even after 20000 cycles of accelerated durability test, the core-shell octahedra still exhibited a mass activity of 0.68 A mg(-1) Pt, twice that of a pristine commercial Pt/C catalyst.

Extremely high efficient nanoreactor with Au@ZnO catalyst for photocatalysis.

We fabricated a photocatalytic Au@ZnO@PC (polycarbonate) nanoreactor composed of monolayered Au nanoparticles chemisorbed on conformal ZnO nanochannel arrays within the PC membrane. A commercial PC membrane was used as the template for deposition of a ZnO shell into the pores by atomic layer deposition (ALD). Thioctic acid (TA) with sufficient steric stabilization was used as a molecular linker for functionalization of Au nanoparticles in a diameter of 10 nm. High coverage of Au nanoparticles anchored on the inner wall of ZnO nanochannels greatly improved the photocatalytic activity for degradation of Rhodamine B. The membrane nanoreactor achieved 63% degradation of Rhodamine B within only 26.88 ms of effective reaction time owing to its superior mass transfer efficiency based on Damköhler number analysis. Mass transfer limitation can be eliminated in the present study due to extremely large surface-to-volume ratio of the membrane nanoreactor.

High density unaggregated Au nanoparticles on ZnO nanorod arrays function as efficient and recyclable photocatalysts for environmental purification.

Photodegradation of organic pollutants in aqueous solution is a promising method for environmental purification. Photocatalysts capable of promoting this reaction are often composed of noble metal nanoparticles deposited on a semiconductor. Unfortunately, the separation of these semiconductor-metal nanopowders from the treated water is very difficult and energy consumptive, so their usefulness in practical applications is limited. Here, a precisely controlled synthesis of a large-scale and highly efficient photocatalyst composed of monolayered Au nanoparticles (AuNPs) chemically bound to vertically aligned ZnO nanorod arrays (ZNA) through a bifunctional surface molecular linker is demonstrated. Thioctic acid with sufficient steric stabilization is used as a molecular linker. High density unaggregated AuNPs bonding on entire surfaces of ZNA are successfully prepared on a conductive film/substrate, allowing easy recovery and reuse of the photocatalysts. Surprisingly, the ZNA-AuNPs heterostructures exhibit a photodegradation rate 8.1 times higher than that recorded for the bare ZNA under UV irradiation. High density AuNPs, dispersed perfectly on the ZNA surfaces, significantly improve the separation of the photogenerated electron-hole pairs, enlarge the reaction space, and consequently enhance the photocatalytic property for degradation of chemical pollutants. Photoelectron, photoluminescence and photoconductive measurements confirm the discussion on the charge carrier separation and photocatalytic experimental data. The demonstrated higher photodegradation rates demonstrated indicate that the ZNA-AuNPs heterostructures are candidates for the next-generation photocatalysts, replacing the conventional slurry photocatalysts.

Prominent electric properties of BiFeO3 shells sputtered on ZnO-nanorod cores with LaNiO3 buffer layers.

In this work, template-assisted methods were adopted to grow BiFeO3 (BFO)-nanorod arrays on substrates. Well-aligned ZnO-nanorod arrays (ZNAs) grown hydrothermally were chosen as positive templates. It was found that perovskite BFO could not be radio frequency (RF)-magnetron sputtered directly on a ZNA at elevated temperatures. Only amorphous BFO was obtained. However, polycrystalline BFO shells could be fabricated by RF-magnetron sputtering on ZNA templates by the introduction of LaNiO3 (LNO) buffer layers. The LNO buffer layer deposited on the ZNA by RF-magnetron sputtering was demonstrated to improve the adhesion and crystallization of the sequentially sputtered BFO shells. The electrical properties were evaluated by conductive atomic force microscopy and piezoresponse force microscopy. Bulk-limited Poole-Frenkel emission dominates the conduction of BFO shells at positive bias, while barrier-limited Schottky emission accounts for the conduction at negative bias due to the interface between the Pt/Ir-coated tip and the BFO. The piezoelectric coefficient (d33) was estimated to be ∼32.93 pm V(-1) and a polarization of 133 µC cm(-2) was derived. These values are higher than those reported previously for BFO films.