Anisotropic Growth of One-Dimensional Carbides in Single-Walled Carbon Nanotubes with Strong Interaction for Catalysis

Advances in Liquid-Phase Assembly of Clusters into Single-Walled Carbon Nanotubes

Insight into the behaviors of molecules in confined space is highly desired for the deep understanding of the mechanism of chemical reactions in a microenvironment. Yet the direct access of molecular evolutions at atomic resolution in nanoconfinements is still challenging. Among various guests, atomically precise clusters with well-defined structures are better suited for monitoring the chemical and physical processes in nanochannels because of their visibility under electron microscopy and identical structures that ensure homogeneous interactions. Developing an efficient method for assembling clusters into a confined space is essential for advancing mechanisms of these processes. In this Perspective, we provide an overview of the assembly of clusters into single-walled carbon nanotubes (SWCNTs) in the liquid phase. We begin with the introduction of assembling methodologies, followed by a discussion of mechanisms of confined assembly in liquids. The host-guest interactions between clusters and nanotubes and the molecular reactions in nanochannels revealed by transmission electron microscopy are unveiled, and the cluster@SWCNT heterostructure-based emerging applications are highlighted. At the end, we discuss the challenges and opportunities and expound our outlook in this field.

Directional growth and reconstruction of ultrafine uranium oxide nanorods within single-walled carbon nanotubes

Understanding the atomic structures and dynamic evolution of uranium oxides is crucial for the reliable operation of fission reactors. Among them, U4O9-as an important intermediate in the oxidation of UO2 to UO2+x -plays an important role in the nucleation and conversion of uranium oxides. Herein, we realize the confined assembly of uranyl within SWCNTs in liquid phase and reveal the directional growth and reconstruction of U4O9 nanorods in nanochannels, enabled by in situ scanning transmission electron microscopy (STEM) e-beam stimulation. The nucleation and crystallization of U4O9 nanorods in nanochannels obey the "non-classical nucleation" mechanism and exhibit remarkably higher growth rate compared to those grown outside. The rapid growth process is found to be accompanied by the formation and elimination of U atom vacancies and strain, aiming to achieve the minimum interfacial energy. Eventually, the segments of U4O9 nanorods in SWCNTs merge into single-crystal U4O9 nanorods via structural reconstruction at the interfaces, and 79% of them exhibit anisotropic growth along the specific 〈11̄0〉 direction. These findings pave the way for tailoring the atomic structures and interfaces of uranium oxides during the synthesis process to help improve the mechanical properties and stability of fission reactors.

SiX2 (X = S, Se) Single Chains and (Si-Ge)X2 Quaternary Alloys

Layered or chain materials have received significant research attention owing to their interesting physical properties, which can dramatically change when the material is thinned from bulk (three-dimensional) to thin two-dimensional sheet or one-dimensional (1D) chain form. Materials with the stoichiometry AX2 with A = Si or Ge and X = S or Se form an especially intriguing semiconducting class. For example, bulk silicon dichalcogenides (SiX2) consist of 1D chains held together by van der Waals forces. Although this structural configuration has the potential to reveal interesting physical phenomena within the 1D limit, obtaining SiX2 single chains has been challenging. We here examine experimentally and theoretically SiX2 materials in the low chain number limit. Carbon nanotubes serve as growth templates and stabilize and protect the structures, and atomic-resolution scanning transmission electron microscopy directly identifies the atomic structure. Two distinct chain structures are observed for SiX2. SixGe1-xS2(1-y)Se2y quaternary alloy chains are also synthesized and characterized, demonstrating tunable semiconducting properties at the atomic-chain level. Density functional theory calculations reveal that the band gap of these alloy chains can be widely tuned through composition engineering. This work offers the possibilities for synthesizing and controlling semiconductor compositions at the single-chain limit to tailor material properties.

Hydrogen Bonds and In situ Photoinduced Metallic Bi0/Ni0 Accelerating Z-Scheme Charge Transfer of BiOBr@NiFe-LDH for Highly Efficient Photocatalysis

For heterojunction system, the lack of stable interfacial driving force and definite charge transfer channel makes the charge separation and transfer efficiency unsatisfactory. The photoreaction mechanism occurring at the interface also receives less attention. Herein, a 2D/2D Z-scheme junction BiOBr@NiFe-LDH with large-area contact featured by short interface hydrogen bonds and strong interfacial electric field (IEF) is synthesized, and in situ photoinduced metallic species assisting charge transfer mechanism is demonstrated. The hydrogen bonds between O atoms from BiOBr and H atoms from NiFe-LDH induce a significant interfacial charge redistribution, establishing a robust IEF. Notably, during photocatalytic reaction, Bi0 and Ni0 are in situ performed in heterojunction, which separately act as electron transport mediator and electron trap to further accelerate charge transfer efficiency up to 71.2 %. Theoretical calculations further demonstrate that the existence of Bi0 strengthens the IEF. Therefore, high-speed spatial charge separation is realized in Bi0/BiOBr@Ni0/NiFe-LDH, leading to a prominent photocatalytic activity with a tetracycline removal ratio of 88.3 % within 7 min under visible-light irradiation and the presence of persulfate, far exceeding majority of photocatalysts reported previously. This study provides valid insights for designing hydrogen bonding heterojunction systems, and advances mechanistic understanding on in situ photoreaction at interfaces.

Biomimetic Chiral Nanomaterials with Selective Catalysis Activity

Chiral nanomaterials with unique chiral configurations and biocompatible ligands have been booming over the past decade for their interesting chiroptical effect, unique catalytical activity, and related bioapplications. The catalytic activity and selectivity of chiral nanomaterials have emerged as important topics, that can be potentially controlled and optimized by the rational biochemical design of nanomaterials. In this review, chiral nanomaterials synthesis, composition, and catalytic performances of different biohybrid chiral nanomaterials are discussed. The construction of chiral nanomaterials with multiscale chiral geometries along with the underlying principles for enhancing chiroptical responses are highlighted. Various biochemical approaches to regulate the selectivity and catalytic activity of chiral nanomaterials for biocatalysis are also summarized. Furthermore, attention is paid to specific chiral ligands, materials compositions, structure characteristics, and so on for introducing selective catalytic activities of representative chiral nanomaterials, with emphasis on substrates including small molecules, biological macromolecule, and in-site catalysis in living systems. Promising progress has also been emphasized in chiral nanomaterials featuring structural versatility and improved chiral responses that gave rise to unprecedented chances to utilize light for biocatalytic applications. In summary, the challenges, future trends, and prospects associated with chiral nanomaterials for catalysis are comprehensively proposed.

Polyoxometalate-Structured Materials: Molecular Fundamentals and Electrocatalytic Roles in Energy Conversion

Polyoxometalates (POMs), a kind of molecular metal oxide cluster with unique physical-chemical properties, have made essential contributions to creating efficient and robust electrocatalysts in renewable energy systems. Due to the fundamental advantages of POMs, such as the diversity of molecular structures and large numbers of redox active sites, numerous efforts have been devoted to extending their application areas. Up to now, various strategies of assembling POM molecules into superstructures, supporting POMs on heterogeneous substrates, and POMs-derived metal compounds have been developed for synthesizing electrocatalysts. From a multidisciplinary perspective, the latest advances in creating POM-structured materials with a unique focus on their molecular fundamentals, electrocatalytic roles, and the recent breakthroughs of POMs and POM-derived electrocatalysts, are systematically summarized. Notably, this paper focuses on exposing the current states, essences, and mechanisms of how POM-structured materials influence their electrocatalytic activities and discloses the critical requirements for future developments. The future challenges, objectives, comparisons, and perspectives for creating POM-structured materials are also systematically discussed. It is anticipated that this review will offer a substantial impact on stimulating interdisciplinary efforts for the prosperities and widespread utilizations of POM-structured materials in electrocatalysis.

Interlayer Charge Transfer Induced Electrical Behavior Transition in 1D AgI@sSWCNT van der Waals Heterostructures

The emergence of one-dimensional van der Waals heterostructures (1D vdWHs) opens up potential fields with unique properties, but precise synthesis remains a challenge. The utilization of mixed conductive types of carbon nanotubes as templates has imposed restrictions on the investigation of the electrical behavior and interlayer interaction of 1D vdWHs. In this study, we efficiently encapsulated silver iodide in high-purity semiconducting single-walled carbon nanotubes (sSWCNTs), forming 1D AgI@sSWCNT vdWHs. We characterized the semiconductor-metal transition and increased the carrier concentration of individual AgI@sSWCNTs via sensitive dielectric force microscopy and confirmed the results through electrical device tests. The electrical behavior transition was attributed to an interlayer charge transfer, as demonstrated by Kelvin probe force microscopy. Furthermore, we showed that this method of synthesizing 1D heterostructures can be extended to other metal halides. This work opens the door for the further exploration of the electrical properties of 1D vdWHs.

Sorting of Cluster-Confined Metallic Single-Walled Carbon Nanotubes for Fabricating Atomically Vacant Uranium Oxide

Single-walled carbon nanotube (SWCNT) heterostructures have shown great potential in catalysis, magnetism, and nanofluidics, in which host SWCNTs with certain conductivity (metallic or semiconducting) are highly required. Herein, inspired by the large molecular weight and redox properties of polyoxometalate (POM) clusters, we reported the selective separation of POM encapsulated metallic SWCNTs (POM@m-SWCNTs) with a uniform diameter through density gradient ultracentrifugation (DGU). The confined POMs increased the SWCNT density and amplified the nanotubes' density difference, thus greatly lowering the centrifugal force (70,000g) of DGU. With this strategy, a series of POM@m-SWCNTs of ∼1.2 nm with high purity were sorted. The mechanism supported by theoretical and experimental evidence showed that the separation of m-SWCNTs depended on not only nanotube/cluster size but also the conductivity of SWCNTs. The smallest 1.2 nm m-SWCNT that can exactly accommodate a 0.9 nm-{Mo6} cluster exhibited the maximum electron transfer to inner clusters; thus, intertube π-π stacking of such m-SWCNTs was greatly loosened, leading to the preferential dispersion into individual ones and partitioning in the uppermost layer after DGU. As a proof-of-concept application, this sorting strategy was extended to separate heavy-element 238U-encapsulated m-SWCNTs. The phase-pure, tiny (1-2.5 nm) U4O9 crystals with atomic vacancy clusters were fabricated on m-SWCNTs through growth kinetic control. This work may provide a general way to construct desired actinide materials on specific SWCNTs.

Structural Diversity of Single-Walled Transition Metal Dichalcogenide Nanotubes Grown via Template Reaction

Monolayers of transition metal dichalcogenides (TMDs) are an ideal 2D platform for studying a wide variety of electronic properties and potential applications due to their chemical diversity. Similarly, single-walled TMD nanotubes (SW-TMDNTs)-seamless cylinders of rolled-up TMD monolayers-are 1D materials that can exhibit tunable electronic properties depending on both their chirality and composition. However, much less has been explored about their geometrical structures and chemical variations due to their instability under ambient conditions. Here, the structural diversity of SW-TMDNTs templated by boron nitride nanotubes (BNNTs) is reported. The outer surfaces and inner cavities of the BNNTs promote and stabilize the coaxial growth of SW-TMDNTs with various diameters, including few-nanometers-wide species. The chiral indices (n,m) of individual SW-MoS2 NTs are assigned by high-resolution transmission electron microscopy, and statistical analyses reveals a broad chirality distribution ranging from zigzag to armchair configurations. Furthermore, this methodology can be applied to the synthesis of various TMDNTs, such as selenides and alloyed Mo1- x Wx S2 . Comprehensive microscopic and spectroscopic analyses also suggest the partial formation of Janus MoS2(1- x ) Se2 x nanotubes. The BNNT-templated reaction provides a universal platform to characterize the chirality-dependent properties of 1D nanotubes with various electronic structures.