Heterostructure Engineering of 2D Superlattice Materials for Electrocatalysis

Electronic Properties of CrB/Co2CO2 Superlattices by Multiple Descriptor-Based Machine Learning Combined with First-Principles

In recent times, newly unveiled 2D materials exhibiting exceptional characteristics, such as MBenes and MXenes, have gained widespread application across diverse domains, encompassing electronic devices, catalysis, energy storage, sensors, and various others. Nonetheless, numerous technical bottlenecks persist in the development of high-performance, structurally flexible, and adjustable electronic device materials. Research investigations have demonstrated that 2D van der Waals superlattices (vdW SLs) structures comprising materials exhibit exceptional electrical, mechanical, and optical properties. In this work, the advantages of both materials are combined and compose the vdW SLs structure of MBenes and MXenes, thus obtaining materials with excellent electronic properties. Furthermore, it integrates machine learning (ML) with first-principles methods to forecast the electrical properties of MBene/MXene superlattice materials. Initially, various configurations of MBene/MXene superlattice materials are explored, revealing that distinct stacking methods exert significant influence on the electronic structure of MBene/MXene materials. Specifically, the BABA-type stacking of CrB (layer A) and Co2CO2 MXene (layer B) is most stable configureation. Subsequently, multiple descriptors of the structure are constructed to predict the density of states  of vdW SLs through the employment of ML techniques. The best model achieves a mean absolute error (MAE) as low as 0.147 eV.

A Universal In-Situ Interfacial Growth Strategy for Various MXene-Based van der Waals Heterostructures with Uniform Heterointerfaces: The Efficient Conversion from 3D Composite to 2D Heterostructures

Two-dimensional (2D) van der Waals heterostructures endow individual 2D material with the novel functional structures, intriguing compositions, and fantastic interfaces, which efficiently provide a feasible route to overcome the intrinsic limitations of single 2D components and embrace the distinct features of different materials. However, the construction of 2D heterostructures with uniform heterointerfaces still poses significant challenges. Herein, a universal in-situ interfacial growth strategy is designed to controllably prepare a series of MXene-based tin selenides/sulfides with 2D van der Waals homogeneous heterostructures. Molten salt etching by-products that are usually recognized as undesirable impurities, are reasonably utilized by us to efficiently transform into different 2D nanostructures via in-situ interfacial growth. The obtained MXene-based 2D heterostructures present sandwiched structures and lamellar interlacing networks with uniform heterointerfaces, which demonstrate the efficient conversion from 3D composite to 2D heterostructures. Such 2D heterostructures significantly enhance charge transfer efficiency, chemical reversibility, and overall structural stability in the electrochemical process. Taking 2D-SnSe2/MXene anode as a representative, it delivers outstanding lithium storage performance with large reversible capacities and ultrahigh capacity retention of over 97% after numerous cycles at 0.2, 1.0, and 10.0 A g-1 current density, which suggests its tremendous application potential in lithium-ion batteries.

Confining N-Doped Carbon Dots into PtNi Aerogels Skeleton for Robust Electrocatalytic Methanol Oxidation and Oxygen Reduction

Noble metallic aerogels with the self-supported hierarchical structure and remarkable activity are promising for methanol fuel cells, but are limited by the severe poisoning and degradation of active sites during electrocatalysis. Herein, the highly stable electrocatalyst of N-doped carbon dots-PtNi (NCDs-PtNi) aerogels is proposed by confining NCDs with alloyed PtNi for methanol oxidation and oxygen reduction reactions. Comprehensive electrocatalytic measurements and theoretical investigations suggest the improvement in structure stability and regulation in electronic structure for better electrocatalytic durability when confining NCDs with PtNi aerogels. Notably, the NCDs-PtNi aerogels perform 12-fold higher activity than that of Pt/C and maintain 52% of their initial activity after 5000 cycles toward acidic methanol oxidation. The enhanced stability and activity of NCDs-PtNi aerogels are also evident for oxygen reduction reactions in different electrolytes. These results highlight the effectiveness of stabilizing metallic aerogels with NCDs, offering a feasible pathway to develop robust electrocatalysts for fuel cells.

Boosting Electrochemical Reduction of Nitrate to Ammonia by Constructing Nitrate-Favored Active Cu-B Sites on SnS2

The electrochemical reduction of nitrate to ammonia is an effective method for mitigating nitrate pollution and generating ammonia. To design superior electrocatalysts, it is essential to construct a reaction site with high activity. Herein, a simple two-step method is applied to in situ reduce amorphous copper over boron-doped SnS2 nanosheets(denoted as aCu@B-SnS2-x. DFT calculations reveal the combination of amorphous copper and B-doping strategy can construct Cu-B active twins and introduce sulfur vacancies on the surface of the inert SnS2, the active twins can efficiently adsorb nitrate and forcibly separate oxygen atoms from nitrate under the assistance of the exposed Sn atom, leading to strong nitrate adsorption. Benefiting from this, aCu@B-SnS2-x exhibited an ultrahigh NH3 FE of 94.6% at -0.67 V versus RHE and the highest NH3 yield rate of 0.55 mmol h-1 mg-1 cat (9350 µg h-1 mg-1 cat) at -0.77 V versus RHE under alkaline conditions. Besides, aCu@B-SnS2-x is confirmed to remain active after various stability tests, suggesting the practicality of utilizing aCu@B-SnS2-x in industrial applications. This work highlights the feasibility of enhanced nitrate-to-ammonia conversion efficiency by combining the doping method and amorphous metal.

Spatially Confined Microcells: A Path toward TMD Catalyst Design

With the ability to maximize the exposure of nearly all active sites to reactions, two-dimensional transition metal dichalcogenide (TMD) has become a fascinating new class of materials for electrocatalysis. Recently, electrochemical microcells have been developed, and their unique spatial-confined capability enables understanding of catalytic behaviors at a single material level, significantly promoting this field. This Review provides an overview of the recent progress in microcell-based TMD electrocatalyst studies. We first introduced the structural characteristics of TMD materials and discussed their site engineering strategies for electrocatalysis. Later, we comprehensively described two distinct types of microcells: the window-confined on-chip electrochemical microcell (OCEM) and the droplet-confined scanning electrochemical cell microscopy (SECCM). Their setups, working principles, and instrumentation were elucidated in detail, respectively. Furthermore, we summarized recent advances of OCEM and SECCM obtained in TMD catalysts, such as active site identification and imaging, site monitoring, modulation of charge injection and transport, and electrostatic field gating. Finally, we discussed the current challenges and provided personal perspectives on electrochemical microcell research.

Macroscopic Superlattice Membranes Self-Assembled from Gold Nanobipyramids with Precisely Tunable Tip Arrangements for SERS

A persistent challenge in utilizing Au nanocrystals for surface-enhanced Raman spectroscopy (SERS) lies in achieving controllable superstructures that maximize SERS performance. Here, a novel strategy is proposed to enhance the SERS performance by precisely adjusting the tip arrangements of Au nanobipyramids (BPs) in two-dimensional (2D) superlattices (SLs). This is achieved through ligand-exchange of Au BPs, followed by liquid-air interfacial assembly, resulting in large-area, transferrable SL membranes. The key to controlling the arrangement of Au BPs in the SLs is the regulation of the amount of free ligands added during self-assembly, which allows for the precise formation of various configurations such as tilted SLs, tip-on-tip SLs, and tip-to-tip SLs. Among these configurations, tip-on-tip SLs exhibit the highest enhancement factor for SERS, reaching an impressive value of 1.95×108, with uniform and consistent SERS signals across a large area. The experimental findings are further corroborated by simulations using the finite element method. This study establishes an efficient method for engineering the microstructure of 2D SLs composed of Au BPs, highlighting the importance of fine-tuning the tip arrangements of Au BPs to regulate SERS performance.

Identifying the Active Sites in MoSi2@MoO3 Heterojunctions for Enhanced Hydrogen Evolution

Developing Two-dimensional (2D) Mo-based heterogeneous nanomaterials is of great significance for energy conversion, especially in alkaline hydrogen evolution reaction (HER), however, it remains a challenge to identify the active sites at the interface due to the structure complexity. Herein, the real active sites are systematically explored during the HER process in varied Mo-based 2D materials by theoretical computational and magnetron sputtering approaches first to filtrate the candidates, then successfully combined the MoSi2 and MoO3 together through Oxygen doping to construct heterojunctions. Benefiting from the synergistic effects between the MoSi2 and MoO3, the obtained MoSi2@MoO3 exhibits an unprecedented overpotential of 72 mV at a current density of 10 mA cm-2. Density functional theory calculations uncover the different Gibbs free energy of hydrogen adsorption (ΔGH*) values achieved at the interfaces with different sites as adsorption sites. The results can facilitate the optimization of heterojunction electrocatalyst design principles for the Mo-based 2D materials.

Plasma-Engineered CeOx Nanosheet Array with Nitrogen-Doping and Porous Architecture for Efficient Electrocatalysis

Surface engineering has been proved efficient and universally applicable in improving the performance of CeO2 in various fields. However, previous approaches have typically required high-temperature calcination or tedious procedures, which makes discovery of a moderate and facile modification approach for CeO2 an attractive subject. In this paper, porous CeO2 nanosheets with effective nitrogen-doping were synthesized via a low-temperature NH3/Ar plasma treatment and exhibited boosted hydrogen evolution reaction performance with low overpotential (65 mV) and long-term stability. The mechanism of the elevated performance was investigated by introducing Ar-plasma-treated CeO2 with no nitrogen-doping as the control group, which revealed the dominant role of nitrogen-doping by providing abundant active sites and improving charge transfer characteristics. This work illuminates further investigations into the surface engineering methodologies boosted by plasma and the relative mechanism of the structure-activity relationship.

Titanium-Based Superlattice with Fe(III)-Regulable Bandgap and Performance for Optimal and Synergistic Sonodynamic-Chemotherapy Guided by Magnetic Resonance Imaging

Superlattices have considerable potential as sonosensitizers for cancer therapy because of their flexible and tunable band gaps, although they have not yet been reported. In this study, a Ti-based organic-inorganic superlattice with good electron-hole separation was synthesized, which consisted of orderly layered superlattices of 2,2'-bipyridine-5,5'-dicarboxylic acid (BPDC) and Ti-O layers. In addition, the superlattice was coordinated with Fe(III) and encapsulated doxorubicin (DOX) to prepare Ti-BPDC@Fe@DOX@PEG (TFDP) after biocompatibility modification. TFDP can realize the simultaneous generation of reactive oxygen species and release of DOX under ultrasound irradiation. Moreover, adjusting the Fe(III) content can effectively modulate the band gap of the superlattice and increase the efficiency of sonodynamic therapy (SDT). The mechanisms underlying this modulation were explored. TFDP with Fe(III) can also be used as a contrast agent for magnetic resonance imaging (MRI). Both in vitro and in vivo experiments demonstrated the ability of TFDP to precisely treat cancer using MRI-guided SDT/chemotherapy. This study expands the applications of superlattices as sonosensitizers with flexible and tailored modifications and indicates that superlattices are promising for precise and customized treatments.

Interfacial engineering of transition metal dichalcogenide/carbon heterostructures for electrochemical energy applications

To support the global goal of carbon neutrality, numerous efforts have been devoted to the advancement of electrochemical energy conversion (EEC) and electrochemical energy storage (EES) technologies. For these technologies, transition metal dichalcogenide/carbon (TMDC/C) heterostructures have emerged as promising candidates for both electrode materials and electrocatalysts over the past decade, due to their complementary advantages. It is worth noting that interfacial properties play a crucial role in establishing the overall electrochemical characteristics of TMDC/C heterostructures. However, despite the significant scientific contribution in this area, a systematic understanding of TMDC/C heterostructures' interfacial engineering is currently lacking. This literature review aims to focus on three types of interfacial engineering, namely interfacial orientation engineering, interfacial stacking engineering, and interfacial doping engineering, of TMDC/C heterostructures for their potential applications in EES and EEC devices. To accomplish this goal, a combination of experimental and theoretical approaches was used to allow the analysis and summary of the fundamental electrochemical properties and preparation strategies of TMDC/C heterostructures. Moreover, this review highlights the design and utilization of the interfacial engineering of TMDC/C heterostructures for specific EES and EEC devices. Finally, the challenges and opportunities of using interfacial engineering of TMDC/C heterostructures in practical EES and EEC devices are outlined. We expect that this review will effectively guide readers in their understanding, design, and application of interfacial engineering of TMDC/C heterostructures.

Ultrathin Rh Nanosheets with Rich Grain Boundaries for Efficient Hydrogen Oxidation Electrocatalysis

Two-dimensional (2D) Pt-group ultrathin nanosheets (NSs) are promising advanced electrocatalysts for energy-related catalytic reactions. However, improving the electrocatalytic activity of 2D Pt-group NSs through the addition of abundant grain boundaries (GBs) and understanding the underlying formation mechanism remain significant challenges. Herein, we report the controllable synthesis of a series of Rh-based nanocrystals (e.g., Rh nanoparticles, Rh NSs, and Rh NSs with GBs) through a CO-mediated kinetic control synthesis route. In light of the 2D NSs' structural advantages and GB modification, the Rh NSs with rich GBs exhibit an enhanced electrocatalytic activity compared to pure Rh NSs and commercial Pt/C toward the hydrogen oxidation reaction (HOR) in alkaline media. Both experimental results and theoretical computations corroborate that the GBs in the Rh NSs have the capacity to ameliorate the adsorption free energy of reaction intermediates during the HOR, thus resulting in outstanding HOR catalytic performance. Our work offers novel perspectives in the realm of developing sophisticated 2D Pt-group metal electrocatalysts with rich GBs for the energy conversion field.

Interlayer electronic coupling regulates the performance of FeN MXenes and Fe2B2 MBenes as high-performance Li- and Al-ion batteries

When two-dimensional (2D) materials are stacked into van der Waals structures, interlayer electronic coupling can induce excellent properties in energy storage materials. Here, we investigate the interlayer coupling of the FeN/Fe2B2 heterojunction as an anode material, which is constructed using vertically planar FeN and puckered Fe2B2 nanosheets. These structures were searched by the CALYPSO method and computed by density functional theory calculations. The stabilities of the FeN monolayer, Fe2B2 monolayer, and FeN/Fe2B2 heterojunction were tested in terms of dynamics, mechanics, and thermodynamics, respectively. These structures have good performances as anode materials, including the capacities of the FeN (Fe2B2) monolayer of 9207 mA h g-1 (2713 mA h g-1) and 3069 mA h g-1 (1005 mA h g-1) for Al and Li, respectively. The stable FeN/Fe2B2 heterojunction shows extremely low diffusion barriers of 0.01 eV, a high Al ion capacity of 4254 mA h g-1, and relatively low voltages. Hess's law revealed that the interlayer electronic coupling impacts the adsorption process of the FeN layer in the FeN/Fe2B2 heterojunction, which decreases the pz orbital of the N atom for the heterojunction. The unequal distribution of electrons between the layers results in interlayer polarization; the value of interlayer polarization was quantitatively calculated to be 0.64 pC m-1. The presence of adsorbed Li and Al atoms between the layers helps maintain the original structure and prevents the interlayer sliding from damaging the heterojunction. These findings offer insights for understanding the structural and electronic properties of the FeN/Fe2B2 heterojunction, which provides crucial information for rational design and advanced synthesis of novel electrode materials.

Self-assembled c-oriented Ni(OH)2 films for enhanced electrocatalytic activity towards the urea oxidation reaction

This study investigates the electrocatalytic properties of the transparent c-oriented Ni(OH)2 films self-assembled from colloidal 2D Ni(OH)2 nanosheets for urea oxidation. The synthesis process yields highly uniform close-packed superlattices with a dominant c-axis orientation. The self-assembled c-oriented Ni(OH)2 films exhibit advantageous electrocatalytic performance in urea oxidation, presenting significantly lower overpotentials and higher current densities compared to randomly distributed Ni(OH)2 particles. In-depth in situ impedance analysis and Raman spectroscopy demonstrate that the c-oriented Ni(OH)2 films possess a higher propensity for a Ni valence transition from +2 to +3 during the urea oxidation process. This finding provides crucial insights into the catalytic behavior and electronic transformations of c-oriented Ni(OH)2 films, shedding light on their superior electrocatalytic activity for urea oxidation. Overall, this study advances our understanding of urea electrooxidation mechanisms and contributes to the design of efficient urea electrocatalysts.

Recent Advances in the Preparation and Application of Two-Dimensional Nanomaterials

Two-dimensional nanomaterials (2D NMs), consisting of atoms or a near-atomic thickness with infinite transverse dimensions, possess unique structures, excellent physical properties, and tunable surface chemistry. They exhibit significant potential for development in the fields of sensing, renewable energy, and catalysis. This paper presents a comprehensive overview of the latest research findings on the preparation and application of 2D NMs. First, the article introduces the common synthesis methods of 2D NMs from both "top-down" and "bottom-up" perspectives, including mechanical exfoliation, ultrasonic-assisted liquid-phase exfoliation, ion intercalation, chemical vapor deposition, and hydrothermal techniques. In terms of the applications of 2D NMs, this study focuses on their potential in gas sensing, lithium-ion batteries, photodetection, electromagnetic wave absorption, photocatalysis, and electrocatalysis. Additionally, based on existing research, the article looks forward to the future development trends and possible challenges of 2D NMs. The significance of this work lies in its systematic summary of the recent advancements in the preparation methods and applications of 2D NMs.

Preparation, properties and applications of two-dimensional superlattices

As a combination concept of a 2D material and a superlattice, two-dimensional superlattices (2DSs) have attracted increasing attention recently. The natural advantages of 2D materials in their properties, dimension, diversity and compatibility, and their gradually improved technologies for preparation and device fabrication serve as solid foundations for the development of 2DSs. Compared with the existing 2D materials and even their heterostructures, 2DSs relate to more materials and elaborate architectures, leading to novel systems with more degrees of freedom to modulate material properties at the nanoscale. Here, three typical types of 2DSs, including the component, strain-induced and moiré superlattices, are reviewed. The preparation methods, properties and state-of-the-art applications of each type are summarized. An outlook of the challenges and future developments is also presented. We hope that this work can provide a reference for the development of 2DS-related research.

Regeneration of Activated Sludge into SiO2-Decorated Heteroatom-Doped Porous Carbon as Advanced Electrodes for Li-S Batteries

The regeneration of harmful activated sludge into an energy source is an important strategy for municipal sludge treatment and recycling. Herein, SiO₂-modified N,S auto-doped porous carbon (NSC@SiO₂) with high conductivity (70 S m^-1) is successfully obtained through a simple calcination method of the activated sludge from wastewater treatment. Further, P-doped NSC@SiO₂ (NSPC@SiO₂) is designed to achieve a higher surface area (891 m² g^-1 vs 624 m² g^-1), a larger pore volume (0.87 cm³ g^-1 vs 0.08 cm³ g^-1), and more carbon defects. Due to its special structure, NSPC@SiO₂ is used as a sulfur host of lithium-sulfur batteries. The results of polysulfide adsorption experiments, S 2p X-ray photoelectron spectra (XPS), Li₂S nucleation experiments, polysulfide symmetric cells, measurement of the galvanostatic intermittent titration (GITT), polarization voltage difference, lithium-ion diffusion rate, and Tafel slope verified that NSPC@SiO₂ greatly improved the adsorption capacity of polysulfides, lowered the barrier to Li₂S formation and the internal resistances of cells, and accelerated Li⁺ ion diffusion and the reaction kinetics of polysulfide conversion, resulting in the excellent performance of polysulfide capture and superior rate performance and cyclic stability. By comparing NSPC@SiO₂ with NSC@SiO₂, a higher initial capacity (1377 mAh g^-1 vs 1150 mAh g^-1 at 0.1C), better rate capacity (912 mAh g^-1 vs 719 mAh g^-1 at 2C), and low capacity decay (0.094% per cycle within 200 cycles) are obtained. Our work provides direction for the treatment, disposal, and resource utilization of activated sludge.