General Oriented Synthesis of Precise Carbon-Confined Nanostructures by Low-Pressure Vapor Superassembly and Controlled Pyrolysis

Enhancing mechanical and photocatalytic properties by surface microstructure regulation of TiO2 nanofiber membranes

In this work, TiO₂ nanofiber membrane (NFM) with a complete surface microstructure was prepared through regulating the surface microstructure of TiO₂ NFM by doping Zr. The crystal structures and morphological analyses indicated that the nanofiber membranes were consisted by disordered accumulation of Zr-doped TiO₂ nanofibers with a crack-free surface, small grain size and high aspect ratio. When the doping amount of Zr was 0.8 mL, the tensile strength of the doped membranes reached 1.27 MPa, which was 60.7% higher than that of pure TiO₂ NFM. The photocatalytic performance of Zr-doped TiO₂ NFM was evaluated by the degradation performance of Methylene orange (MO) under simulated sunlight irradiation. Compared with the undoped TiO₂ NFM, the 0.8-Zr/TiO₂ NFM presented a higher catalytic degradation efficiency (improved by 69.6%), and the photocatalytic performance maintained stable after five circulating. It was found that the doping of Zr ions effectively limited the surface crack size and grain size of TiO₂ nanofibers, thereby improving the tensile strength, and enhanced the surface area effect and carrier transfer efficiency of TiO₂ NFM. On the other hand, a narrow band-gap was obtained by doping a small amount of Zr ions, which expanded the visible light response range to improve the photocatalytic performance of TiO₂ nanofibers.

Hydrolysis-Promoted Building Block Assembly: Structure Transformation from Wheel and Ship to Cage

Accurately controlling the hydrolysis of metal ions can not only yield the desired structure of metal hydroxide clusters but also provide a deeper understanding of the formation process of natural hydroxide minerals. However, the capture of hydrolysis intermediates remains a significant challenge, and metal hydroxide clusters are mainly obtained by employing adventitious hydrolysis. In this study, we realized a hierarchical building block assembly from Y³⁺ ions to large Y₁₂, Y₃₄, and Y₆₀ clusters by controlling the hydrolysis process of lanthanide ions under different pH conditions. Single-crystal structural analysis showed that the Y₁₂ wheel, Y₃₄ ship, and Y₆₀ sodalite cage contain 4, 12, and 24 cubane-like [Y₄(μ₃-OH)₄]⁸⁺ units, respectively. The structure of the Y₆₀ cluster can be attributed to two Y₃₄ clusters or six Y₁₂ clusters linked by vertices. These clusters can be synthesized through the hydrolysis of Y³⁺ under different pH conditions, and Y₆₀ can be prepared from the obtained Y₁₂ or Y₃₄ crystals by the simple addition of Y³⁺ ions. The capture and conversion of the intermediates of lanthanide series hydroxide clusters, Y₁₂ or Y₃₄, during the assembly from Y³⁺ ions to Y₆₀ can facilitate an understanding of the formation process of high-nuclearity lanthanide clusters.

Advances in Understanding the Electrocatalytic Reconstruction Chemistry of Coordination Compounds

Coordination compounds including mainstream metal-organic frameworks and Prussian blue analogues receive extensive researches when they directly serve as electrocatalysts. Their reconstruction phenomena, that are closely associated with actual contributions and intrinsic catalytic mechanisms, are expected to be well summarized. Here, the recent advances in understanding reconstruction chemistry of coordination compounds are reviewed, including their main classifications and structural properties, reconstruction phenomena in electrocatalysis (e.g., oxygen/hydrogen evolution reaction, CO₂ reduction), influence factors of reconstruction parameters (e.g., reconstruction rate and reconstruction degree), and reconstruction-performance correlation. It is outlined that the reconstruction processes are influenced by electronic structure of coordination compounds, pH and temperature of testing solution, and applied potentials. The characterization techniques reflecting the evolution information before and after catalysis are also introduced for reconstruction-related mechanistic study. Finally, some challenges and outlooks on reconstruction investigations of coordination compounds are proposed, and the necessity of studying and understanding of these themes under actual working conditions of devices is highlighted.

Three-Dimensional Carbon-Coated LiFePO4 Cathode with Improved Li-Ion Battery Performance

LiFePO4 (LFPO)has great potential as the cathode material for lithium-ion batteries; it has a high theoretical capacity (170 m·A·h·g−1), high safety, low toxicity and good economic benefits. However, low conductivity and a low diffusion rate inhibit its future development. To overcome these weaknesses, three-dimensional carbon-coated LiFePO4 that incorporates a high capacity, superior conductivity and low volume expansion enables faster electron transport channels. The use of Cetyltrimethyl Ammonium Bromid (CTAB) modification only requires a simple water bath and sintering, without the need to add a carbon source in the LFPO synthesis process. In this way, the electrode shows excellent reversible capacity, as high as 159.8 m·A·h·g−1 at 2 C, superior rate capability with 97.3 m·A·h·g−1 at 5 C and good cycling ability, preserving ~84.2% capacity after 500 cycles. By increasing the ion transport rate and enhancing the structural stability of LFPO nanoparticles, the LFPO-positive electrode achieves excellent initial capacity and cycle life through cost-effective and easy-to-implement carbon coating. This simple three-dimensional carbon-coated LiFePO4 provides a new and simple idea for obtaining comprehensive and high-performance electrode materials in the field of lithium cathode materials.

A "MOFs plus ZIFs" Strategy toward Ultrafine Co Nanodots Confined into Superficial N-Doped Carbon Nanowires for Efficient Oxygen Reduction

N-doped carbon-confined transition metal nanocatalysts display efficient oxygen reduction reaction (ORR) performance comparable to commercial Pt/C electrocatalysts because of their efficient charge transfer from metal atoms to active N sites. However, the sheathed active sites inside the electrocatalysts and relatively large-size confined metal particles greatly restrict their activity improvement. Here, we develop a facile and efficient "MOFs plus ZIFs" synthesis strategy to successfully construct ultrafine sub-5 nm Co nanodots confined into superficial N-doped carbon nanowires (Co@C@NC) via a well-designed synthesis process. The unique synthesis mechanism is based on low-pressure vapor superassembly of thin zeolitic imidazolate framework (ZIF) coatings on metal-organic framework substrates. During the successive pyrolysis, the preferential formation of the robust N-doped carbon shell from the ZIF-67 shell keeps the core morphology without shrinkage and limits the growth of Co nanodots. Benefiting from this architecture with accessible and rich active N sites on the surface, stable carbon confined architecture, and large surface area, the Co@C@NC exhibits excellent ORR performance, catching up to commercial Pt/C. Density functional theory demonstrates that the confined Co nanodots efficiently enhance the charge density of superficial active N sites by interfacial charge transfer, thus accelerating the ORR process.

Universal Approach to Fabricating Graphene-Supported Single-Atom Catalysts from Doped ZnO Solid Solutions

Single-atom catalysts (SACs) have attracted widespread interest for many catalytic applications because of their distinguishing properties. However, general and scalable synthesis of efficient SACs remains significantly challenging, which limits their applications. Here we report an efficient and universal approach to fabricating a series of high-content metal atoms anchored into hollow nitrogen-doped graphene frameworks (M-N-Grs; M represents Fe, Co, Ni, Cu, etc.) at gram-scale. The highly compatible doped ZnO templates, acting as the dispersants of targeted metal heteroatoms, can react with the incoming gaseous organic ligands to form doped metal-organic framework thin shells, whose composition determines the heteroatom species and contents in M-N-Grs. We achieved over 1.2 atom % (5.85 wt %) metal loading content, superior oxygen reduction activity over commercial Pt/C catalyst, and a very high diffusion-limiting current (6.82 mA cm^-2). Both experimental analyses and theoretical calculations reveal the oxygen reduction activity sequence of M-N-Grs. Additionally, the superior performance in Fe-N-Gr is mainly attributed to its unique electron structure, rich exposed active sites, and robust hollow framework. This synthesis strategy will stimulate the rapid development of SACs for diverse energy-related fields.

Advances in metal-organic framework coatings: versatile synthesis and broad applications

Metal-organic frameworks (MOFs) as a new kind of porous crystalline materials have attracted much interest in many applications due to their high porosity, diverse structures, and controllable chemical structures. However, the specific geometrical morphologies, limited functions and unsatisfactory performances of pure MOFs hinder their further applications. In recent years, an efficient approach to synthesize new composites to overcome the above issues has been achieved, by integrating MOF coatings with other functional materials, which have synergistic advantages in many potential applications, including batteries, supercapacitors, catalysis, gas storage and separation, sensors, drug delivery/cytoprotection and so on. Nevertheless, the systemic synthesis strategies and the relationships between their structures and application performances have not been reviewed comprehensively yet. This review emphasizes the recent advances in versatile synthesis strategies and broad applications of MOF coatings. A comprehensive discussion of the fundamental chemistry, classifications and functions of MOF coatings is provided first. Next, by modulating the different states (e.g. solid, liquid, and gas) of metal ion sources and organic ligands, the synthesis methods for MOF coatings on functional materials are systematically summarized. Then, many potential applications of MOF coatings are highlighted and their structure-property correlations are discussed. Finally, the opportunities and challenges for the future research of MOF coatings are proposed. This review on the deep understanding of MOF coatings will bring better directions into the rational design of high-performance MOF-based materials and open up new opportunities for MOF applications.

MOF-derived electrocatalysts for oxygen reduction, oxygen evolution and hydrogen evolution reactions

The morphology and composition design of MOF-derived carbon-based materials and their applications for electrocatalytic ORR, OER and HER are reviewed.

Recent progress on metal–organic framework-derived materials for sodium-ion battery anodes

Recent progress on MOF-derived materials, including carbon and metal oxides/sulfides/selenides/phosphides, as anode materials for sodium-ion batteries is summarized.

In situ Ga-alloying in germanium nano-twists by the inhibition of fractal growth with fast Li+-mobility

In this study, Ge0.90Ga0.10 nano-twists were prepared by an in situ Ga-alloying method to inhibit the fractal growth of Ge. The mobility of Li+ in the Ge0.90Ga0.10 nano-twists was two orders higher than that in Ge. This advantage promotes fast charging of Li-ion batteries with the rate capability of 819 mA h g-1 at 16 A g-1.

Phytic Acid-Assisted Formation of Hierarchical Porous CoP/C Nanoboxes for Enhanced Lithium Storage and Hydrogen Generation

Application of transition metal phosphides (TMPs) for electrochemical energy conversion and storage has great potential to alleviate the energy crisis. Although there are many methods to get TMPs, it is still immensely challenging to fabricate hierarchical porous TMPs with superior electrochemical performances by a simple, green, and secure approach. Herein, we report a facile method to synthesize the CoP/C nanoboxes by pyrolysis of phytic acid (PA) cross-linked Co complexes that are acquired from reaction of PA and ZIF-67. The PA can not only slowly etch ZIF-67 and gain a hollow structure but also act as a source of phosphorus to prepare CoP/C nanoboxes. The CoP/C nanoboxes deliver an ultrahigh specific capacity (868 mA h g^-1 at 100 mA g^-1) and excellent cycle stability (523 mA h g^-1 after 1000 cycles at 500 mA h g^-1) when used as anode materials for lithium-ion batteries. Moreover, when used as an electrocatalyst for hydrogen evolution reaction, the CoP/C nanoboxes exhibit ultralow overpotential, small Tafel slope, and excellent durability in acidic media. The method to produce CoP/C nanoboxes is easy and environmentally friendly and can be readily extended to design other TMPs/C nanocomposites.

Stable Voltage Cutoff Cycle Cathode with Tunable and Ordered Porous Structure for Li-O2 Batteries

Ordered porous RuO₂ materials with various pore structure parameters are prepared via a hard-template method and are used as the carbon-free cathodes for Li-O₂ batteries under the voltage cutoff cycle mode. The influences of pore structure parameters of porous RuO₂ on electrochemical performance are systematically studied. Results indicate that specific surface area and pore size determine the specific capacity and round-trip efficiency of Li-O₂ batteries. Too small pores cause pore blockage and hinder the diffusion pathways of Li⁺ and O₂ , thereby causing small specific capacity and high overpotentials. Too large pores weaken the mechanical property of porous RuO₂ , thereby causing the rapid decrease in capacity during electrochemical reaction. The Li-O₂ battery based on the RuO₂ cathode with an average pore size of 16 nm (RuO₂ -16) exhibits a high round-trip efficiency of ≈75.6% and an excellent cycling stability of up to 70 cycles at 100 mA g^-1 with a voltage window of 2.5-4.0 V. The superior performance of RuO₂ -16 can be attributed to its optimal pore structure parameters. Furthermore, the in situ differential electrochemical mass spectrometry test demonstrates that RuO₂ can effectively reduce parasitic reactions compared with carbon materials.

Defect Engineering for Modulating the Trap States in 2D Photoconductors

Defect-induced trap states are essential in determining the performance of semiconductor photodetectors. The de-trap time of carriers from a deep trap can be prolonged by several orders of magnitude as compared to shallow traps, resulting in additional decay/response time of the device. Here, it is demonstrated that the trap states in 2D ReS2 can be efficiently modulated by defect engineering through molecule decoration. The deep traps that greatly prolong the response time can be mostly filled by protoporphyrin molecules. At the same time, carrier recombination and shallow traps in-turn play dominant roles in determining the decay time of the device, which can be several orders of magnitude faster than the as-prepared device. Moreover, the specific detectivity of the device is enhanced (as high as ≈1.89 × 1013 Jones) due to the significant reduction of  the dark current through charge transfer between ReS2 and molecules. Defect engineering of trap states therefore provides a solution to achieve photodetectors with both high responsivity and fast response.

A porous nickel cyclotetraphosphate nanosheet as a new acid-stable electrocatalyst for efficient hydrogen evolution

The stability of non-precious metal-based electrocatalysts for the acidic hydrogen evolution reaction (HER) is of great importance. Here, we have used nickel cyclotetraphosphate (Ni2P4O12) nanosheet arrays as a HER electrocatalyst for the first time. The Ni2P4O12 arrays were obtained through a facile low-temperature phosphorylation process and possess superior HER catalytic activities and stability in acid. The Ni2P4O12 delivers a small overpotential of 131.8 mV at -10 mA cm-2 and a low Tafel slope of 47.8 mV dec-1 in 0.5 M H2SO4, comparable to most of the non-precious metal-based catalysts. Importantly, the Ni2P4O12 shows a negligible potential change (6.5 mV) over 80 000 s continuous testing in acid. The remarkable catalytic performances of Ni2P4O12 are mainly attributed to the inductive effect of P4O124- and its polymer-like structure, promoting it as a potential acid-stable HER electrocatalyst.

Novel MOF shell-derived surface modification of Li-rich layered oxide cathode for enhanced lithium storage

Li-rich layered oxide materials have attracted increasing attention because of their high specific capacity (>250 mAh g^-1). However, these materials typically suffer from poor cycling stability and low rate performance. Herein, we propose a facile and novel metal-organic-framework (MOF) shell-derived surface modification strategy to construct NiCo nanodots decorated (∼5 nm in diameter) carbon-confined Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ nanoparticles (LLO@C&NiCo). The MOF shell is firstly formed on the surface of as-prepared Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ nanoparticles via low-pressure vapor superassembly and then is in situ converted to the NiCo nanodots decorated carbon shell after subsequent controlled pyrolysis. The obtained LLO@C&NiCo cathode exhibits enhanced cycling and rate capability with a capacity retention of 95% after 100 cycles at 0.4 C and a high capacity of 159 mAh g^-1 at 5 C, respectively, compared with those of LLO (75% and 105 mAh g^-1). The electrochemical impedance spectroscopy and selected area electron diffraction analyses after cycling demonstrate that the thin C&NiCo shell can endow LLO with high electronic conductivity and structural stability, indicating the undesired formation of the spinel phase initiated from the particle surface is efficiently suppressed. Therefore, this presented strategy may open a new avenue on the design of high-performance electrode materials for energy storage.

Pyrolysis of metal–organic framework (CuBTC) decorated filter paper as a low-cost and highly active catalyst for the reduction of 4-nitrophenol

The fabrication of noble metal free catalysts with excellent performance and high stability by a simple, efficient, general and low-cost approach remains an urgent task for solving the problem of resource shortage.