Chiral Surface and Geometry of Metal Nanocrystals

Chirality is a basic property of nature and has great importance in photonics, biochemistry, medicine, and catalysis. This importance has led to the emergence of the chiral inorganic nanostructure field in the last two decades, providing opportunities to control the chirality of light and biochemical reactions. While the facile production of 3D nanostructures has remained a major challenge, recent advances in nanocrystal synthesis have provided a new pathway for efficient control of chirality at the nanoscale by transferring molecular chirality to the geometry of nanocrystals. Interestingly, this discovery stems from a purely crystallographic outcome: chirality can be generated on high-Miller-index surfaces, even for highly symmetric metal crystals. This is the starting point herein, with an overview of the scientific history and a summary of the crystallographic definition. With the advance of nanomaterial synthesis technology, high-Miller-index planes can be selectively exposed on metallic nanoparticles. The enantioselective interaction of chiral molecules and high-Miller-index facets can break the mirror symmetry of the metal nanocrystals. Herein, the fundamental principle of chirality evolution is emphasized and it is shown how chiral surfaces can be directly correlated with chiral morphologies, thus serving as a guide for researchers in chiral catalysts, chiral plasmonics, chiral metamaterials, and photonic devices.

A Generalized Surface Chalcogenation Strategy for Boosting the Electrochemical N2 Fixation of Metal Nanocrystals

Electrocatalytic nitrogen reduction reaction (NRR) is a promising process relative to energy-intensive Haber-Bosch process. While conventional electrocatalysts underperform with sluggish paths, achieving dissociation of N₂ brings the key challenge for enhancing NRR. This study proposes an effective surface chalcogenation strategy to improve the NRR performance of pristine metal nanocrystals (NCs). Surprisingly, the NH₃ yield and Faraday efficiency (FE) (175.6 ± 23.6 mg h^-1 g^-1 _Rh and 13.3 ± 0.4%) of Rh-Se NCs is significantly enhanced by 16 and 15 times, respectively. Detailed investigations show that the superior activity and high FE are attributed to the effect of surface chalcogenation, which not only can decrease the apparent activation energy, but also inhibit the occurrence of the hydrogen evolution reaction (HER) process. Theoretical calculations reveal that the strong interface strain effect within core@shell system induces a critical redox inversion, resulting in a rather low valence state of Rh and Se surface sites. Such strong correlation indicates an efficient electron-transfer minimizing NRR barrier. Significantly, the surface chalcogenation strategy is general, which can extend to create other NRR metal electrocatalysts with enhanced performance. This strategy open a new avenue for future NH₃ production for breakthrough in the bottleneck of NRR.

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.

Encapsulating Copper Nanocrystals into Metal-Organic Frameworks for Cascade Reactions by Photothermal Catalysis

Composite materials with multifunctional properties usually possess synergetic effects in catalysis toward cascade reactions. In this work, a facile strategy to the encapsulation of octahedral Cu₂ O nanocrystals (NCs) by metal-organic frameworks (MOFs) is reported, and an oriented growth of MOF enclosures (namely, HKUST-1) around Cu₂ O NCs with desired feedstock ratio is achieved. The strategy defines the parameter range that precisely controls the etching rate of metal oxide and the MOF crystallization rate. Finally, the Cu@HKUST-1 composites with uniform morphology and controlled MOF thickness have been successfully fabricated after the reduction of Cu₂ O to Cu NCs in HKUST-1. The integration of Cu NCs properties with MOF advantages helps to create a multifunctional catalyst, which exhibits cooperative catalytic activity and improved recyclability toward the one-pot cascade reactions under mild conditions involving visible-light irradiation. The superior performance can be attributed to the plasmonic photothermal effect of Cu NCs, while HKUST-1 shell provides Lewis acid sites, substrates and H₂ enrichment, and stabilizes the Cu cores.

Light-Induced In Situ Transformation of Metal Clusters to Metal Nanocrystals for Photocatalysis

In situ transformation of glutathione-capped gold (Aux) clusters to gold (Au) nanocrystals under simulated solar light irradiation was achieved and utilized as a facile synthetic approach to rationally fabricate Aux/Au/TiO2 ternary and Au/TiO2 binary heterostructures. Synergistic interaction of Aux clusters and Au nanocrystals contributes to enhanced visible-light-driven photocatalysis.

Chiral Surface and Geometry of Metal Nanocrystals

Chirality is a basic property of nature and has great importance in photonics, biochemistry, medicine, and catalysis. This importance has led to the emergence of the chiral inorganic nanostructure field in the last two decades, providing opportunities to control the chirality of light and biochemical reactions. While the facile production of 3D nanostructures has remained a major challenge, recent advances in nanocrystal synthesis have provided a new pathway for efficient control of chirality at the nanoscale by transferring molecular chirality to the geometry of nanocrystals. Interestingly, this discovery stems from a purely crystallographic outcome: chirality can be generated on high-Miller-index surfaces, even for highly symmetric metal crystals. This is the starting point herein, with an overview of the scientific history and a summary of the crystallographic definition. With the advance of nanomaterial synthesis technology, high-Miller-index planes can be selectively exposed on metallic nanoparticles. The enantioselective interaction of chiral molecules and high-Miller-index facets can break the mirror symmetry of the metal nanocrystals. Herein, the fundamental principle of chirality evolution is emphasized and it is shown how chiral surfaces can be directly correlated with chiral morphologies, thus serving as a guide for researchers in chiral catalysts, chiral plasmonics, chiral metamaterials, and photonic devices.

High-yield synthesis of sub-10 nm Pt nanotetrahedra with bare ⟨111⟩ facets for efficient electrocatalytic applications

Unlike other shapes, the design of tetrahedral Pt nanocrystals (Pt-NTd), which have the largest number of Pt(111) surface atoms and highest catalytic activities toward the electron transfer reactions, has widely been considered a synthetic challenge due to their thermodynamic instability. Here, we show that, by inducing their nucleation on functionalized carbon, Pt NTds can be obtained with tunable sizes and high yields. The carbon support anchors the nanocrystals early and prevents their oriented attachment leading to nanowire formation. Therein, an in situ generated amine is crucial for stabilization of Pt-NTds, which can later be removed to expose the Pt(111) facets for higher catalytic efficiency. The bare nanocrystals exhibit much improved stability and electrocatalytic activity characteristic of Pt(111) toward oxygen reduction reaction (ORR) and methanol and formic acid oxidation reactions. For example, ∼90% of their activity was retained after 5000 potential cycles, while the ORR onset potential was recorded to be very high, 1.01 V vs reversible hydrogen electrode (RHE).

Stabilization of Exposed Metal Nanocrystals in High-Temperature Heterogeneous Catalysis

Colloidal metal nanocrystals with uniform sizes, shapes, compositions, and architectures are ideal building blocks for constructing heterogeneous catalysts with well-defined characteristics toward the investigation of accurate structure-property relationships and better understanding of catalytic mechanism. However, their applications in high-temperature heterogeneous catalysis are often restricted by the difficulty in maintaining the high metal dispersity and easy accessibility to active sites under harsh operating conditions. Here, a partial-oxide-coating strategy is proposed to stabilize metal nanocrystals against sintering and meanwhile enable an effective exposure of active sites. As a proof-of-concept, controlled partial silica coating of colloidally prepared Pd_0.82 Ni_0.18 nanocrystals with the size of 8 nm is demonstrated. This partially coated catalyst exhibits excellent activity, selectivity, and stability, outperforming its counterparts with fully coated and supported structures, in reverse water gas shift (RWGS) catalysis particularly at high operating temperatures. This study opens a new avenue for the exploration of colloidal metal nanocrystals in high-temperature heterogeneous catalysis.

Effect of Temperature on the Nucleation and Growth of Precious Metal Nanocrystals

Understanding the effect of physical parameters (e.g., temperature) on crystallisation dynamics is of paramount importance for the synthesis of nanocrystals of well-defined sizes and geometries. However, imaging nucleation and growth is an experimental challenge owing to the resolution required and the kinetics involved. Here, by using an aberration-corrected transmission electron microscope, we report the fabrication of precious metal nanocrystals from nuclei and the identification of the dynamics of their nucleation at three different temperatures (20, 50, and 100 °C). A fast, and apparently linear, acceleration of the growth rate is observed against increasing temperature (78.8, 117.7, and 176.5 pm min^-1 , respectively). This work appears to be the first direct observation of the effect of temperature on the nucleation and growth of metal nanocrystals.

Vibrational Properties of Pd Nanocubes

The atomic disorder and the vibrational properties of Pd nanocubes have been studied through a combined use of X-ray diffraction and molecular dynamics simulations. The latter show that the trend of the mean square relative displacement as a function of the radius of the coordination shells is characteristic of the nanoparticle shape and can be described by a combined model: A correlated Debye model for the thermal displacement and a parametric expression for the static disorder. This combined model, supplemented by results of line profile analysis of the diffraction patterns collected at different temperatures (100, 200, and 300 K) can explain the observed increase in the Debye–Waller coefficient, and shed light on the effect of the finite domain size and of the atomic disorder on the vibrational properties of metal nanocrystals.

Heterogeneous Interface Design for Enhanced Sodium Storage: Sb Quantum Dots Confined by Functional Carbon

Antimony (Sb) is considered a promising anode material for sodium-ion batteries due to its high specific capacity and moderate working potential. However, the non-negligible volume variation leads to the rapid decay of capacity, which hinders the practical application of Sb anode materials. Here, an economical and scalable route with high yield is proposed to obtain Sb ultrafine nanocrystals embedded in a porous carbon skeleton. Notably, the synergetic effect of the heterogeneous structure is maximized by inducing the interfacial coupling SbOC and creating buffering space for the volume effect of Sb. The high-entropy phase interface creates the doping site breaking the periodicity of atoms and alters the electronic structure, also bridging the slip of intergranular defects. Thus, the electronic conductivity and phase interface structural stability are reinforced. The mechanism of accelerating electron migration at the heterogeneous phase interface is visualized through the density functional theory method, and the mass/charge-transfer kinetics is analyzed via the calculation of surface-induced capacitive contribution.

Unveiling the Growth Mechanism of Ordered-Phase within Multimetallic Nanoplates

Tuning the crystal phase of alloy nanocrystals (NCs) offers an alternative way to improve their electrocatalytic performance, but, how heterometals diffuse and form ordered-phase remains unclear. Herein, for the first time, the mechanism for forming tetrametallic ordered-phase nanoplates (NPLs) is unraveled. The observations reveal that the intermetallic ordered-phase nucleates through crystallinity alteration of the seeds and then propagates by reentrant grooves. Notably, the reentrant grooves act as intermediate NCs for ordered-phase, eventually forming intermetallic PdCuIrCo NPLs. These NPLs substantially outperform for oxygen evolution reaction (221 mV at 10 mA cm-2) and hydrogen evolution reaction (19 mV at 10 mA cm-2) compared to commercial Ir/C and Pd/C catalysts in acidic media. For OER at 1.53 V versus RHE, the PdCuIrCo/C exhibits an enhanced mass activity of 9.8 A mg-1 Pd+Ir (about ten times higher) than Ir/C. For HER at -0. 2 V versus RHE, PdCuIrCo/C shows a remarkable mass activity of 1.06 A mg-1 Pd+Ir, which is three-fold relative to Pd/C. These improvements can be ascribed to the intermetallic ordered-structure with high-valence Ir sites and tensile-strain. This approach enabled the realization of a previously unobserved mechanism for ordered-phase NCs. Therefore, this strategy of making ordered-phase NPLs can be used in diverse heterogeneous catalysis.

Nanocrystal Materials for Resistive Memory and Artificial Synapses: Progress and Prospects

BACKGROUND

Resistive random-access memory (RRAM) is considered to be the most promising next-generation non-volatile memory because of its low cost, low energy consumption, and excellent data storage characteristics. However, the on/off (SET/RESET) voltages of RRAM are too random to replace the traditional memory. Nanocrystals (NCs) offer an appealing option for these applications since they combine excellent electronic/optical properties and structural stability and can address the requirements of low-cost, large-area, and solution-processed technologies. Therefore, the doping NCs in the function layer of RRAM are proposed to localize the electric field and guide conductance filaments (CFs) growth.

OBJECTIVE

The purpose of this article is to focus on a comprehensive and systematical survey of the NC materials, which are used to improve the performance of resistive memory (RM) and optoelectronic synaptic devices and review recent experimental advances in NC-based neuromorphic devices from artificial synapses to light-sensory synaptic platforms.

METHODS

Extensive information related to NCs for RRAM and artificial synapses and their associated patents were collected. This review aimed to highlight the unique electrical and optical features of metal and semiconductor NCs for designing future RRAM and artificial synapses.

RESULTS

It was demonstrated that doping NCs in the function layer of RRAM could not only improve the homogeneity of SET/RESET voltage but also reduce the threshold voltage. At the same time, it could still increase the retention time and provide the probability of mimicking the bio-synapse.

CONCLUSION

NC doping can significantly enhance the overall performance of RM devices, but there are still many problems to be solved. This review highlights the relevance of NCs for RM and artificial synapses and also provides a perspective on the opportunities, challenges, and potential future directions.