Ultralow Density, Monolithic WS2, MoS2, and MoS2/Graphene Aerogels

Nanocellulose-based porous lightweight materials with flame retardant properties: A review

This review discusses the development and application of nanocellulose (NC)-aerogels, a sustainable and biodegradable biomaterial, with enhanced flame retardant (FR) properties. NC-aerogels combine the excellent physical and mechanical properties of NC with the low density and thermal conductivity of aerogels, making them promising for thermal insulation and other fields. However, the flammability of NC-aerogels limits their use in some applications, such as electromagnetic interference shielding, oil/water separation, and flame-resistant textiles. The review covers the design, fabrication, modification, and working mechanism of NC porous materials, focusing on how advanced technologies can impart FR properties into them. The review also evaluates the FR performance of NC-aerogels by employing widely recognized tests, such as the limited oxygen index, cone calorimeter, and UL-94. The review also explores the integration of innovative and eco-friendly materials, such as MXene, metal-organic frameworks, dopamine, lignin, and alginate, into NC-aerogels, to improve their FR performance and functionality. The review concludes by outlining the potential, challenges, and limitations of future research on FR NC-aerogels, identifying the obstacles and potential solutions, and understanding the current progress and gaps in the field.

Synergistic role of MoS2 in gelation-induced fabrication of graphene oxide films

Supporting materials for electrocatalysts must exhibit relative chemical inertness to facilitate unimpeded movement of gas, and demonstrate electrical conductivity to promote efficient electron transfer to the catalyst. Conventional catalyst electrodes, such as glassy carbon, carbon cloths, or Ni foam, are commonly employed. However, the challenge lies in the limited stability observed during testing due to the relatively weak adhesion between the catalyst and the electrode. Addressing this limitation is crucial for advancing the stability and performance of catalyst-electrode systems in various applications. Here, we suggest a novel fabrication method for a freestanding conducting film, accomplished through gelation, incorporating 1T-MoS2 and graphene oxide. 1T-MoS2 nanosheets play a crucial role in promoting the reduction of graphene oxide (GO) on the Zn foil. This contribution leads to accelerated film formation and enhanced electrical conductivity in the film. The synergistic effect also enhances the film's stability as catalyst supports. This study provides insights into the effective utilization of MoS2 and graphene oxide in the creating of advanced catalyst support systems with potential applications in diverse catalytic reaction.

Uncovering Key Factors in Graphene Aerogel-Based Electrocatalysts for Sustainable Hydrogen Production: An Unsupervised Machine Learning Approach

The application of principal component analysis (PCA) as an unsupervised learning method has been used in uncovering correlations among diverse features of aerogel-based electrocatalysts. This analytical approach facilitates a comprehensive exploration of catalytic activity, revealing intricate relationships with various physical and electrochemical properties. The first two principal components (PCs), collectively capturing nearly 70% of the total variance, attested the reliability and efficacy of PCA in unveiling meaningful patterns. This study challenges the conventional understanding that a material's reactivity is solely dictated by the quantity of catalyst loaded. Instead, it unveils a complex perspective, highlighting that reactivity is intricately influenced by the material's overall design and structure. The PCA bi-plot uncovers correlations between pH and Tafel slope, suggesting an interdependence between these variables and providing valuable insights into the complex interactions among physical and electrochemical properties. Tafel slope stands to be positively correlated with PC1 and PC2, showing an evident positive correlation with the pH. These findings showed that the pH can have a positive correlation with the Tafel slope, however, it does not necessarily reflect a direct positive correlation with the overpotential. The impact of pH on current density (j)and Tafel slope underscores the importance of adjusting pH to lower overpotential effectively, enhancing catalytic activity. Surface area (from 30 to 533 m2 g-1) emerges as a key physical property, inclusively inverse correlation with overpotential, indicating its direct role in lowering overpotential and increasing catalytic activity. The introduction of PC3, in conjunction with PC1, enriches the analysis by revealing consistent trends despite a slightly lower variance (60%). This reinforces the robustness of PCA in delineating distinct characteristics of graphene aerogels, affirming their potential implications in diverse electrocatalytic applications. In summary, PCA proves to be a valuable tool for unraveling complex relationships within aerogel-based electrocatalysts, extending insights beyond catalytic sites to emphasize the broader spectrum of material properties. This approach enhances comprehension of dataset intricacies and holds promise for guiding the development of more effective and versatile electrocatalytic materials.

Oxygenated Hydrocarbons from Catalytic Hydrogenation of Carbon Dioxide

Once fundamental difficulties such as active sites and selectivity are fully resolved, metal-free catalysts such as 3D graphene or carbon nanotubes (CNT) are very cost-effective substitutes for the expensive noble metals used for catalyzing CO2. A viable method for converting environmental wastes into useful energy storage or industrial wealth, and one which also addresses the environmental and energy problems brought on by emissions of CO2, is CO2 hydrogenation into hydrocarbon compounds. The creation of catalytic compounds and knowledge about the reaction mechanisms have received considerable attention. Numerous variables affect the catalytic process, including metal–support interaction, metal particle sizes, and promoters. CO2 hydrogenation into different hydrocarbon compounds like lower olefins, alcoholic composites, long-chain hydrocarbon composites, and fuels, in addition to other categories, have been explained in previous studies. With respect to catalyst design, photocatalytic activity, and the reaction mechanism, recent advances in obtaining oxygenated hydrocarbons from CO2 processing have been made both through experiments and through density functional theory (DFT) simulations. This review highlights the progress made in the use of three-dimensional (3D) nanomaterials and their compounds and methods for their synthesis in the process of hydrogenation of CO2. Recent advances in catalytic performance and the conversion mechanism for CO2 hydrogenation into hydrocarbons that have been made using both experiments and DFT simulations are also discussed. The development of 3D nanomaterials and metal catalysts supported on 3D nanomaterials is important for CO2 conversion because of their stability and the ability to continuously support the catalytic processes, in addition to the ability to reduce CO2 directly and hydrogenate it into oxygenated hydrocarbons.

Nickel nanoparticle-activated MoS2 for efficient visible light photocatalytic hydrogen evolution

Direct sunlight-induced water splitting for photocatalytic hydrogen evolution is the dream for an ultimate clean energy source. So far, typical photocatalysts require complicated synthetic processes and barely work without additives or electrolytes. Here, we report the realization of a hydrogen evolution strategy with a novel Ni-Ag-MoS₂ ternary nanocatalyst under visible/sun light. Synthesized through an ultrasound-assisted wet method, the composite exhibits stable catalytic activity for long-term hydrogen production from both pure and natural water. A high efficiency of 73 μmol g^-1 W^-1 h^-1 is achieved with only a visible light source and the (MoS₂)₈₄Ag₁₀Ni₆ catalyst, matching the values of present additive-enriched photocatalysts. Verified by experimental characterizations and first-principles calculations, the enhanced photocatalytic ability is attributed to effective charge migration through the dangling bonds at the Ni-Ag-MoS₂ alloy interface and the activation of the MoS₂ basal planes.

Three-Dimensional Printed MoS2/Graphene Aerogel Electrodes for Hydrogen Evolution Reactions

In this work, we demonstrate the use of direct ink writing (DIW) technology to create 3D catalytic electrodes for electrochemical applications. Hybrid MoS₂/graphene aerogels are made by mixing commercially available MoS₂ and graphene oxide powders into a thixotropic, high concentration, viscous ink. A porous 3D structure of 2D graphene sheets and MoS₂ particles was created after post treatment by freeze-drying and reducing graphene oxide through annealing. The composition and morphology of the samples were fully characterized through XPS, BET, and SEM/EDS. The resulting 3D printed MoS₂/graphene aerogel electrodes had a remarkable electrochemically active surface area (>1700 cm²) and were able to achieve currents over 100 mA in acidic media. Notably, the catalytic activity of the MoS₂/graphene aerogel electrodes was maintained with minimal loss in surface area compared to the non-3D printed electrodes, suggesting that DIW can be a viable method of producing durable electrodes with a high surface area for water splitting. This demonstrates that 3D printing a MoS₂/graphene 3D porous network directly using our approach not only improves electrolyte dispersion and facilitates catalyst utilization but also provides multidimensional electron transport channels for improving electronic conductivity.

The electronic properties and water desalination performance of a photocatalytic TiO2/MoS2 nanocomposites bilayer membrane: a molecular dynamic simulation

Due to the rapid depletion of water resources, more interest is paid for the efficient desalination process in recent years. MoS₂ membrane aroused attention due to its high mechanical stability and electronic properties, which can sustain extra-large strains. In this study, the electronic properties and water desalination performance of TiO₂/MoS₂-hexagonal, and TiO₂/MoS₂-rhombohedral nanocomposites bilayer membranes were studied and simulated for the first time. The effect of TiO₂ in the performance of MoS₂ was observed in water desalination under the defined applied pressure ranging from 50 to 250 MPa with a 6.4 Å pore diameter. The membrane structure is created and optimized. The energy minimized for TiO₂ from - 19,596.4282 kcal/mol for the initial structure to - 19,605.1611 kcal/mol for the final structure. For TiO₂/MoS₂-hexagonal, the energy minimized from - 4955.54271 eV) to - 4955.62091 eV and TiO₂/MoS₂-rhombohedral from - 6042.26925 eV to - 6046.91835 eV. A molecular dynamic (MD) simulation was performed using Material Studio 2019 to study the electronic properties under 0-1 eV electric field using the CASTEP code. The results showed a better photocatalytic performance under the external electric field. The effect of external electric field significantly intensifies absorption in the visible range and achieved a high photocatalytic activity on TiO₂/MoS₂. TiO₂, TiO₂/MoS₂-hexagonal and TiO₂/MoS₂-rhombohedral nanocomposites bilayer membranes are simulated and evaluated for the water desalination using ReaxFF software. Both MoS₂ phases with TiO₂ have achieved a high salt rejection up to 97% (P-value = 0.0036, R² = 0.958), while TiO₂/MoS₂-rhombohedral achieved the highest permeability (6.0*10^-8 mm g cm^-2 s^-1 bar^-1) (P-value = 0.000296, R² = 0.972) under 250 MPa applied pressure.

Self-Assembled TMD Nanoparticles on N-Doped Carbon Nanostructures for Oxygen Reduction Reaction and Electrochemical Oxygen Sensing Thereof

Here, we report on a universal carbothermal reduction strategy for the synthesis of well-dispersed WS₂ nanoparticles (∼1.7 nm) supported on a N-doped carbon (N_xC) nanostructure and the electrocatalytic activity toward oxygen reduction reaction (ORR). Bulk WS₂ powder (2 μm) is the source for WS₂ nanoparticles, and dicyandiamide is the source for N_xC and carbothermal reduction. Interestingly, WS₂/N_xC serves the purpose of innovative and robust active sites for ORR through an efficient four-electron transfer process with excellent durability. Remarkably, WS₂/N_xC suppresses the peroxide generation due to the dominating inner-sphere electron transfer mechanism where the direct adsorption of the desolvated O₂ molecule on the electroactive centers takes place. The mass activity (at 0.4 and 0.85 V vs RHE) of WS₂/N_xC outperforms the previously reported transition metal based electrocatalysts. The study further establishes a correlation between the work function and the ORR activity. We have also exploited WS₂/N_xC for electrochemical oxygen sensing, and there exists a direct correlation between oxygen sensing and ORR as both depend on the oxygen adsorption ability. Finally, the carbothermal reduction strategy has been extended for the synthesis of other TMDs/N_xC such as MoS₂/N_xC, MoSe₂/N_xC, and WSe₂/N_xC.

Edge Rich Ultrathin Layered MoS2 Nanostructures for Superior Visible Light Photocatalytic Activity

Nanostructures of layered 2D materials have been proven one of the significant recent trends for visible-light-driven photocatalysis because of their unique morphology, effective optical adsorption, and rich active sites. Herein, we synthesized ultrathin-layered MoS₂ nanoflowers and nanosheets with rich active sites by using a facile hydrothermal technique. The photocatalytic performance of the as-synthesized MoS₂ nanoflowers (NF) and nanosheets (NS) were investigated for the photodegradation of MB (methylene blue), MG (malachite Green), and RhB (rhodamine B) dye under visible light irradiations. Ultrathin-layered nanoflowers showed faster degradation (96% in 150 min) in RhB under visible light irradiation, probably due to a large number of active sites and high available surface area. The kinetic study demonstrated that the first-order kinetic model best explained the process of photodegradation. The MoS₂ nanoflowers catalysts has similar catalytic performance after four consecutive cyclic performances, demonstrating their good stability. The results showed that the MoS₂ nanoflowers have outstanding visible-light-driven photocatalytic activity and could be an effective catalyst for industrial wastewater treatment.

Direct fabrication and characterization of vertically stacked Graphene/h-BN/Graphene tunnel junctions

We have used a lithography free technique for the direct fabrication of vertically stacked two-dimensional (2D) material-based tunnel junctions and characterized by Raman, AFM, XPS. We fabricated Graphene/h-BN/Graphene devices by direct deposition of graphene (bottom layer), h-BN (insulating barrier) and graphene (top layer) sequentially using a plasma enhanced chemical vapor deposition on Si/SiO2 substrates. The thickness of the h-BN insulating layer was varied by tuning the plasma power and the deposition time. Samples were characterized by Raman, AFM, and XPS. The I-V data follows the barrier thickness dependent quantum tunneling behavior for equally doped graphene layers. The resonant tunneling behavior was observed at room temperature for oppositely doped graphene layers where hydrazine and ammonia were used for n-doping of one of the graphene layers. The resonance with negative differential conductance occurs when the band structures of the two electrodes are aligned. The doping effect of the resonant peak is observed for varying doping levels. The results are explained according to the Bardeen tunneling model.

Multi-functional 2D hybrid aerogels for gas absorption applications

Aerogels have attracted significant attention recently due to their ultra-light weight porous structure, mechanical robustness, high electrical conductivity, facile scalability and their use as gas and oil absorbers. Herein, we examine the multi-functional properties of hybrid aerogels consisting of reduced graphene oxide (rGO) integrated with hexagonal boron nitride (hBN) platelets. Using a freeze-drying approach, hybrid aerogels are fabricated by simple mixing with various volume fractions of hBN and rGO up to 0.5/0.5 ratio. The fabrication method is simple, cost effective, scalable and can be extended to other 2D materials combinations. The hybrid rGO/hBN aerogels (HAs) are mechanically robust and highly compressible with mechanical properties similar to those of the pure rGO aerogel. We show that the presence of hBN in the HAs enhances the gas absorption capacities of formaldehyde and water vapour up to ~ 7 and > 8 times, respectively, as compared to pure rGO aerogel. Moreover, the samples show good recoverability, making them highly efficient materials for gas absorption applications and for the protection of artefacts such as paintings in storage facilities. Finally, even in the presence of large quantity of insulating hBN, the HAs are electrically conductive, extending the potential application spectrum of the proposed hybrids to the field of electro-thermal actuators. The work proposed here paves the way for the design and production of novel 2D materials combinations with tailored multi-functionalities suited for a large variety of modern applications.

Immobilization of TiO2 Nanoparticles in Hydrogels Based on Poly(Methyl Acrylate) and Succinamide Acid for the Photodegradation of Organic Dyes

Hydrogels have excellent properties that make them ideally suited as host matrices for the immobilization of photoreactive materials such as TiO2 nanoparticles that serve as catalysts in the photodegradation of organic dyes, which is of great importance in practical water pollution treatment applications. However, the application of hydrogels for this purpose remains poorly studied. The present study addresses this issue by developing two types of hydrogels based on poly(methyl acrylate) and succinamide acid with embedded TiO2 nanoparticles for use as photocatalysts in the photodegradation of organic dyes. The results of the analysis demonstrate that the TiO2 nanoparticles are distributed uniformly in the hydrogel matrices, and the hydrogels maintain their original structures after use. The photodegradation efficiencies of the developed TiO2-hydrogels are demonstrated to be reasonably close to that of freely distributed TiO2 nanoparticles in solution for four different organic dyes. In addition, the results of degradation-regeneration cycling tests demonstrate that immobilizing the TiO2 nanoparticles into the hydrogels greatly reduces their loss during utilization, and the photocatalysts can be easily reused. In fact, the two TiO2-hydrogels retain reasonably high photocatalytic degradation performance after four degradation-regeneration cycles.

Recent Developments in Nanocellulose-Based Aerogels in Thermal Applications: A Review

Naturally derived nanocellulose (NC) is a renewable, biodegradable nanomaterial with high strength, low density, high surface area, and tunable surface chemistry, which allows its interaction with other polymers and nanomaterials in a controlled manner. In recent years, NC aerogel has gathered a lot of attention due to environmental concerns. This review presents recent developments of NC-based aerogels and their controlled interactions with other polymers and nanomaterials for thermal applications that include electronic devices, the apparel industry, superinsulating materials, and flame-retardant smart building materials. After going through the distinctive properties of NC aerogels, they are orderly categorized and discussed as thermally insulated, thermally conductive, and flame-retardant materials.

Gradual hydrophobization of silica aerogel for controlled drug release

We report on the successful fine-tuning of silica aerogel hydrophobicity, through a gas-phase surface modification process. Aerogel hydrophobicity is a widely discussed matter, as it contributes to the aerogel's preservation and determines its functionality. Still, a general procedure for tuning the hydrophobicity, without affecting other aerogel properties was missing. In the developed procedure, silica aerogel was modified with trimethylchlorosilane vapor for varying durations, resulting in gradual hydrophobicity, determined by solid-state NMR and contact angle measurements. The generality of this post-synthesis treatment allows its application on a variety of aerogel materials, while having minimum effect on their porosity and transparency. We demonstrate the applicability of the gradual hydrophobization by tuning drug release rates from the silica aerogel. Two chlorhexidine salts - widely employed as antiseptic agents - were used as model drugs, one representing a soluble drug, and the other an insoluble drug; they were entrapped in silica aerogel, following hydrophobization to varying degrees. The drug release patterns showed that depending on the degree, hydrophobization can increase or decrease release kinetics, compared to the unmodified aerogel. This arises from the effect of the hydrophobic degree on pore structure, diffusional rates and wetting of the aerogel carrier. We suggest the use of the gradual hydrophobization process for other drug-aerogel systems, as well as for other aerogel applications, such as transparent insulation panels, contaminate sorbents or catalysis supports.

Printed aerogels: chemistry, processing, and applications

As an extraordinarily lightweight and porous functional nanomaterial family, aerogels have attracted considerable interest in academia and industry in recent decades. Despite the application scopes, the modest mechanical durability of aerogels makes their processing and operation challenging, in particular, for situations demanding intricate physical structures. "Bottom-up" additive manufacturing technology has the potential to address this drawback. Indeed, since the first report of 3D printed aerogels in 2015, a new interdisciplinary research area combining aerogel and printing technology has emerged to push the boundaries of structure and performance, further broadening their application scope. This review summarizes the state-of-the-art of printed aerogels and presents a comprehensive view of their developments in the past 5 years, and highlights the key near- and mid-term challenges.

Bimetallic Co-Based (CoM, M = Mo, Fe, Mn) Coatings for High-Efficiency Water Splitting

Bimetallic cobalt (Co)-based coatings were prepared by a facile, fast, and low-cost electroless deposition on a copper substrate (CoFe, CoMn, CoMo) and characterized by scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction analysis. Prepared coatings were thoroughly examined for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution (1 M potassium hydroxide, KOH) and their activity compared to that of Co and Ni coatings. All five coatings showed activity for both reactions, where CoMo and Co showed the highest activity for HER and OER, respectively. Namely, the highest HER current density was recorded at CoMo coating with low overpotential (61 mV) to reach a current density of 10 mA·cm⁻². The highest OER current density was recorded at Co coating with a low Tafel slope of 60 mV·dec⁻¹. Furthermore, these coatings proved to be stable under HER and OER polarization conditions.

The lightest solid meets the lightest gas: an overview of carbon aerogels and their composites for hydrogen related applications

Hydrogen, a renewable and recyclable energy, has been regarded as the best solution for global energy supply in the 21^st century. Hydrogen production, hydrogen storage and hydrogen sensing are three most important aspects for hydrogen economy. Interestingly, the lightest solid, carbon aerogels (CAs), has found wide applications in these aspects due to its unique characteristics including large specific surface area, hierarchical porous structure, high electrical conductivity, superb chemical stability, and low fabrication cost. Herein, various fabrication strategies of CAs are presented, and their applications in the three most important aspects are comprehensively reviewed. In addition, the challenges and prospects are also discussed. In the light of the recent progress in CAs for hydrogen-related applications, this review provides a comprehensive assessment on materials selection, synthesis, hydrogen adsorption characteristics of CAs and catalytic activity of CA-supported nanocatalysts, offering a strategic guide to build a close connection between CAs and hydrogen economy.

Surface Functionalization of Ti3C2Tx MXene with Highly Reliable Superhydrophobic Protection for Volatile Organic Compounds Sensing

Two-dimensional (2D) transition-metal carbides (Ti₃C₂T_x MXene) have received a great deal of attention for potential use in gas sensing showing the highest sensitivity among 2D materials and good gas selectivity. However, one of the long-standing challenges of the MXenes is their poor stability against hydration and oxidation in a humid environment, limiting their long-term storage and applications. Integration of an effective protection layer with MXenes shows promise for overcoming this major drawback. Herein, we demonstrate a surface functionalization strategy for Ti₃C₂T_x with fluoroalkylsilane (FOTS) molecules through surface treatment, providing not only a superhydrophobic surface, mechanical/environmental stability but also enhanced sensing performance. The experimental results show that high sensitivity, good repeatability, long-term stability, and selectivity and faster response/recovery property were achieved by the FOTS-functionalized when Ti₃C₂T_x was integrated into chemoresistive sensors sensitive to oxygen-containing volatile organic compounds (ethanol, acetone). FOTS functionalization provided protection to sensing response when the dynamic response of the Ti₃C₂T_x-F sensor to 30 ppm of ethanol was measured over in the 5 to 80% relative humidity range. Density functional theory simulations suggested that the strong adsorption energy of ethanol on Ti₃C₂T_x-F and the local structure deformation induced by ethanol adsorption, contributing to the gas-sensing enhancement. This study offers a facile and practical solution for developing highly reliable MXene based gas-sensing devices with response that is stable in air and in the presence of water.

Banana Leaflike C-Doped MoS2 Aerogels toward Excellent Microwave Absorption Performance

We describe the design and manufacturing method of a lightweight C-doped MoS₂ aerogel with a special regular banana leaflike microstructure used for high-performance microwave absorbers. The aerogel precursor was first fabricated by a self-assembly process between alginate (Alg) and ammonium thiomolybdate (ATM), where Alg as a template was assembled with ATM into regular banana leaflike architectures along the ice growth direction during oriented freezing. After pyrolysis at 900 °C, the C-doped MoS₂ aerogels maintained low densities and porous hierarchal banana leaflike structures, where the banana leaves ranged in diameter from about 2 to 5 μm with the growth of small branches. Benefitting from these features, the C-doped MoS₂ aerogel possessed excellent microwave absorption performance in the frequency range of 2-18 GHz. The minimum reflection loss (RL) reached -43 dB at 5.4 GHz with a matching thickness of 4 mm, and the effective microwave absorption band (RL < -10 dB) reached 4 GHz (14-18 GHz) at a thickness of 1.5 mm. Our findings also provide strategies for designing MoS₂ aerogel nanostructures for electronic devices, catalysis, and other potential applications.

Intrinsic Capacitance of Molybdenum Disulfide

The metallic, 1T polymorph of molybdenum disulfide (MoS₂) is promising for next-generation supercapacitors due to its high theoretical surface area and density which lead to high volumetric capacitance. Despite this, there are few fundamental works examining the double-layer charging mechanisms at the MoS₂/electrolyte interface. This study examines the potential-dependent and frequency-dependent area-specific double-layer capacitance (C_a) of the 1T and 2H polymorphs of MoS₂ in aqueous and organic electrolytes. Furthermore, we investigate restacking effects and possible intercalation-like mechanisms in multilayer films. To minimize the uncertainties associated with porous electrodes, we carry out measurements using effectively nonporous monolayers of MoS₂ and contrast their behavior with reduced graphene oxide deposited layer-by-layer on atomically flat graphite single crystals using a modified, barrier-free Langmuir-Blodgett method. The metallic 1T polymorph of MoS₂ (C_a,1T = 14.9 μF/cm²) is shown to have over 10-fold the capacitance of the semiconducting 2H polymorph (C_a,2H = 1.35 μF/cm²) near the open circuit potential and under negative polarization in aqueous electrolyte. However, under positive polarization the capacitance is significantly reduced and behaves similarly to the 2H polymorph. The capacitance of 1T MoS₂ scales with layer number, even at high frequency, suggesting easy and rapid ion penetration between the restacked sheets. This model system allows us to determine capacitance limits for MoS₂ and suggest strategies to increase the energy density of devices made from this promising material.

TMDs beyond MoS2 for Electrochemical Energy Storage

Atomically thin sheets of two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted interest as high capacity electrode materials for electrochemical energy storage devices owing to their unique properties (high surface area, high strength and modulus, faster ion diffusion, and so on), which arise from their layered morphology and diversified chemistry. Nevertheless, low electronic conductivity, poor cycling stability, large structural changes during metal-ion insertion/extraction along with high cost of manufacture are challenges that require further research in order for TMDs to find use in commercial batteries and supercapacitors. Here, a systematic review of cutting-edge research focused on TMD materials beyond the widely studied molybdenum disulfide or MoS₂ electrode is reported. Accordingly, a critical overview of the recent progress concerning synthesis methods, physicochemical and electrochemical properties is given. Trends and opportunities that may contribute to state-of-the-art research are also discussed.

Effect of Ni Doping on the MoS2 Structure and Its Hydrogen Evolution Activity in Acid and Alkaline Electrolytes

We have investigated three-dimensional (3D) MoS2 nanoarchitectures doped with different amount of Ni to boost the hydrogen evolution reaction (HER) in alkaline environment, where this reaction is normally hindered. As a comparison, the activity in acidic media was also investigated to determine and compare the role of the Ni sites in both media. The doping of MoS2, especially at high loadings, can modify its structural and/or electronic properties, which can also affect the HER activity. The structural and electronic properties of the Ni doped 3D-MoS2 nanoarchitecture were studied by X-ray diffraction (XRD), Raman spectroscopy, scanning and transmission electronic microscopy (SEM; TEM), and X-ray photoemission Spectroscopy (XPS). XPS also allowed us to determine the Ni-based species formed as a function of the dopant loading. The HER activity of the materials was investigated by linear sweep voltammetry (LSV) in 0.5 M H2SO4 and 1.0 M KOH. By combining the physicochemical and electrochemical results, we concluded that the Ni sites have a different role in the HER mechanism and kinetics in acidic and in alkaline media. Thus, NiSx species are essential to promote HER in alkaline medium, whereas the Ni-Mo-S ones enhance the HER in acid medium.

An in-plane Co9S8@MoS2 heterostructure for the hydrogen evolution reaction in alkaline media

Transition metal sulfides have emerged as promising hydrogen evolution reaction (HER) electrocatalysts in acidic media due to high intrinsic activity. They exhibit inferior HER activity in alkaline media, however, owing to the sluggish water dissociation kinetics. Herein, in-plane MoS₂/Co₉S₈ heterostructures are in situ grown on three-dimensional carbon network substrates with interconnected hierarchical pores by one-step pyrolysis to enhance the alkaline HER activity. The experiment results reveal that the HER kinetics of MoS₂ is accelerated after the construction of heterostructures. The synthesized MoS₂/Co₉S₈ heterostructures anchored on a three-dimensional interconnected hierarchical pore carbon network exhibit a lower overpotential of 177 mV than MoS₂ (252 mV) at 10 mA cm^-2 for the HER in 1 M KOH. The enhanced catalytic performance is mainly attributed to the accelerated water dissociation kinetics on the interface of MoS₂ and Co₉S₈. In combination with DFT calculations, it is revealed that assembling the interface construction synergistically favors the chemisorption of protons and the cleavage of the O-H bonds of the H₂O molecule, thus accelerating the kinetics of the HER. Moreover, the three-dimensional interconnected hierarchical pore carbon (3DC) network structure is beneficial for the circulation of the electrolyte and H₂ spillover. This study demonstrates the present strategy as a facile route for fabricating efficient HER catalysts.

Wet-Chemical Assembly of 2D Nanomaterials into Lightweight, Microtube-Shaped, and Macroscopic 3D Networks

Despite tremendous efforts toward fabrication of three-dimensional macrostructures of two-dimensional (2D) materials, the existing approaches still lack sufficient control over microscopic (morphology, porosity, pore size) and macroscopic (shape, size) properties of the resulting structures. In this work, a facile fabrication method for the wet-chemical assembly of carbon 2D nanomaterials into macroscopic networks of interconnected, hollow microtubes is introduced. As demonstrated for electrochemically exfoliated graphene, graphene oxide, and reduced graphene oxide, the approach allows for the preparation of highly porous (> 99.9%) and lightweight (<2 mg cm^-3) aeromaterials with tailored porosity and pore size as well as tailorable shape and size. The unique tubelike morphology with high aspect ratio enables ultralow-percolation-threshold graphene composites (0.03 S m^-1, 0.05 vol%) which even outperform most of the carbon nanotube-based composites, as well as highly conductive aeronetworks (8 S m^-1, 4 mg cm^-3). On top of that, long-term compression cycling of the aeronetworks demonstrates remarkable mechanical stability over 10 000 cycles, even though no chemical cross-linking is employed. The developed strategy could pave the way for fabrication of various macrostructures of 2D nanomaterials with defined shape, size, as well as micro- and nanostructure, crucial for numerous applications such as batteries, supercapacitors, and filters.

Graphene Nanoarchitectonics: Recent Advances in Graphene-Based Electrocatalysts for Hydrogen Evolution Reaction

Under the double pressures of both the energy crisis and environmental pollution, the exploitation and utilization of hydrogen, a clean and renewable power resource, has become an important trend in the development of sustainable energy-production and energy-consumption systems. In this regard, the electrocatalytic hydrogen evolution reaction (HER) provides an efficient and clean pathway for the mass production of hydrogen fuel and has motivated the design and construction of highly active HER electrocatalysts of an acceptable cost. In particular, graphene-based electrocatalysts commonly exhibit an enhanced HER performance owing to their distinctive structural merits, including a large surface area, high electrical conductivity, and good chemical stability. Considering the rapidly growing research enthusiasm for this topic over the last several years, herein, a panoramic review of recent advances in graphene-based electrocatalysts is presented, covering various advanced synthetic strategies, microstructural characterizations, and the applications of such materials in HER electrocatalysis. Lastly, future perspectives on the challenges and opportunities awaiting this emerging field are proposed and discussed.

Promoting Electrocatalysis upon Aerogels

Electrocatalysis plays a prominent role in renewable energy conversion and storage, enabling a number of sustainable processes for future technologies. There are generally three strategies to improve the efficiency (or activity) of the electrocatalysts: i) increasing the intrinsic activity of the catalyst itself, ii) improving the exposure of active sites, and iii) accelerating mass transfer during catalysis (both reactants and products). These strategies are not mutually exclusive and can ideally be addressed simultaneously, leading to the largest improvements in activity. Aerogels, as featured by large surface area, high porosity, and self-supportability, provide a platform that matches all the aforementioned criteria for the design of efficient electrocatalysts. The field of aerogel synthesis has seen much progress in recent years, mainly thanks to the rapid development of nanotechnology. Employing precursors with different properties enables the resulting aerogel with targeted catalytic properties and improved performances. Here, the design strategies of aerogel catalysts are demonstrated, and their performance for several electrochemical reactions is reviewed. The common principles that govern electrocatalysis are further discussed for each category of reactions, thus serving as a guide to the development of future aerogel electrocatalysts.

Structurally Controlled Cellular Architectures for High-Performance Ultra-Lightweight Materials

The design and synthesis of cellular structured materials are of both scientific and technological importance since they can impart remarkably improved material properties such as low density, high mechanical strength, and adjustable surface functionality compared to their bulk counterparts. Although reducing the density of porous structures would generally result in reductions in mechanical properties, this challenge can be addressed by introducing a structural hierarchy and using mechanically reinforced constituent materials. Thus, precise control over several design factors in structuring, including the type of constituent, symmetry of architectures, and dimension of the unit cells, is extremely important for maximizing the targeted performance. The feasibility of lightweight materials for advanced applications is broadly explored due to recent advances in synthetic approaches for different types of cellular architectures. Here, an overview of the development of lightweight cellular materials according to the structural interconnectivity and randomness of the internal pores is provided. Starting from a fundamental study on how material density is associated with mechanical performance, the resulting structural and mechanical properties of cellular materials are investigated for potential applications such as energy/mass absorption and electrical and thermal management. Finally, current challenges and perspectives on high-performance ultra-lightweight materials potentially implementable by well-controlled cellular architectures are discussed.

Graphene-WS2 heterostructures by a lithography free method: their electrical properties

We have developed a lithography free technique for the fabrication of two-dimensional (2D) material based heterostructures. We fabricated graphene-WS₂ heterostructured devices using a transmission electron microscope grid as a shadow mask and their electrical transport characteristics were studied by electrical and magneto transport measurements. Graphene was directly deposited on a Si/SiO₂ substrate by radio frequency plasma enhanced chemical vapor deposition. WS₂ was synthesized by first depositing WO₃ followed by sulfurization. The temperature dependence of the resistance and magnetoresistance are measured for graphene, WS₂, and graphene-WS₂ heterostructure. At low temperatures, the transport is found to follow the variable-range hopping (VRH) process, where logarithmic R exhibits a T ^-1/3 temperature dependence, an evidence for the 2D Mott VRH transport. The measured low-field magnetoresistance also exhibits a quadratic magnetic field dependence ∼B ², consistent with the 2D Mott VRH transport.

Nanomaterials in Advanced, High-Performance Aerogel Composites: A Review

Aerogels are one of the most interesting materials of the 21st century owing to their high porosity, low density, and large available surface area. Historically, aerogels have been used for highly efficient insulation and niche applications, such as interstellar particle capture. Recently, aerogels have made their way into the composite universe. By coupling nanomaterial with a variety of matrix materials, lightweight, high-performance composite aerogels have been developed for applications ranging from lithium-ion batteries to tissue engineering materials. In this paper, the current status of aerogel composites based on nanomaterials is reviewed and their application in environmental remediation, energy storage, controlled drug delivery, tissue engineering, and biosensing are discussed.

A Single-Step Route to Single-Crystal Molybdenum Disulphide (MoS2) Monolayer domains

We report a simple, single-cycle synthetic method for forming highly-crystalline, micron-sized monolayer domains of phase-pure MoS₂. This method combines liquid chemistry with discrete, layer-by-layer deposition from a novel Mo precursor. Single-crystalline MoS₂ with domain sizes up to 100 μm have been obtained and characterised by optical and electron microscopy as well as Raman and photoluminescence spectroscopy.

Polymeric Graphene Bulk Materials with a 3D Cross-Linked Monolithic Graphene Network

Although many great potential applications are proposed for graphene, till now none are yet realized as a stellar application. The most challenging issue for such practical applications is to figure out how to prepare graphene bulk materials while maintaining the unique two-dimensional (2D) structure and the many excellent properties of graphene sheets. Herein, such polymeric graphene bulk materials containing three-dimensional (3D) cross-linked networks with graphene sheets as the building unit are reviewed. The theoretical research on various proposed structures of graphene bulk materials is summarized first. Then, the synthesis or fabrication of these graphene materials is described, which comprises mainly two approaches: chemical vapor deposition and cross-linking using graphene oxide directly. Finally, some exotic and exciting potential applications of these graphene bulk materials are presented.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

This review describes an in-depth overview and knowledge on the variety of synthetic strategies for forming metal sulfides and their potential use to achieve effective hydrogen generation and beyond.

Electrostatically Charged MoS2/Graphene Oxide Hybrid Composites for Excellent Electrochemical Energy Storage Devices

We demonstrate, for the first time, a new method of fabricating hybrid MoS₂/poly(ethyleneimine)-modified graphene oxide (PEI-GO) composites assembled through electrostatically charged interaction between the negatively charged MoS₂ nanosheets and positively charged PEI-GO in an aqueous solution. The GO can not only improve the electronic conductivity of the MoS₂/PEI-GO composites, leading to an excellent charge-transfer network, but also hamper the restacking of MoS₂ nanosheets. The composition ratios between MoS₂ and PEI-GO were also optimized with the highest specific capacitance of 153.9 F g^-1 where 96.0% of the initial specific capacitance remains after 6800 cycles. The specific capacitance of only 117.5 F g^-1 was observed for the pure MoS₂ nanosheets, and 68.2% of the initial specific capacitance was achieved after 5000 cycles. The excellent electrochemical performance of the hybrid MoS₂/PEI-GO composites was demonstrated by establishing an asymmetric supercapacitor with a MoS₂/PEI-GO-based negative electrode and an activated-carbon positive electrode. The asymmetric supercapacitor provided a maximum capacitance of 42.9 F g^-1, and 93.1% of the initial capacitance was maintained after 8000 cycles. Furthermore, a MoS₂/PEI-GO//activated-carbon asymmetric supercapacitor delivered an energy density of 19.3 W h kg^-1 and a power density of 4500 W kg^-1, indicating the potential of the hybrid MoS₂/PEI-GO composites in electrochemical energy storage applications.

Mechanism Investigation of High-Performance Li-Polysulfide Batteries Enabled by Tungsten Disulfide Nanopetals

Understanding the reaction kinetics and mechanism of Li-polysulfide batteries is critical in designing advanced host materials for improved performance. However, up to now, the reaction mechanism within the Li-polysulfide batteries is still unclear. Herein, we study the reaction mechanism of a high-performance Li-polysulfide battery by in situ X-ray diffraction (XRD) and density functional theory (DFT) calculations based on a multifunctional host material composed of WS₂ nanopetals embedded in rGO-CNT (WS₂-rGO-CNT) aerogel. The WS₂ nanopetal serves as a "catalytic center" to chemically bond the polysulfides and accelerate the polysulfide redox reactions, and the 3D porous rGO-CNT scaffold provides fast and efficient e^-/Li⁺ transportation. Thus, the resulting WS₂-rGO-CNT aerogel accommodating the polysulfide catholyte enables a stable cycling performance, excellent rate capability (614 mAh g^-1 at 2 C), and a high areal capacity (6.6 mAh cm^-2 at 0.5 C). In situ XRD results reveal that the Li₂S starts to form at an early stage of discharge (at a depth of 25% of the lower voltage plateau) during the discharge process, and β-S₈ nucleation begins before the upper voltage plateau during the recharge process, which are different from the conventional Li-S battery. Moreover, the WS₂ itself could be lithiated/delithiated during the cycling, making the lithiated WS₂ (Li _xWS₂, 0 ≤ x ≤ 0.3) a real host material for Li-polysulfide batteries. DFT calculations suggest that Li _xWS₂ (0 ≤ x ≤ 0.3) exhibits moderate binding/anchoring interactions toward polysulfides with adsorption energies of 0.51-1.4 eV. Our work reveals the reaction mechanism of the Li-polysulfide batteries and indicates that the lithiated host plays an important role in trapping the polysulfides.

Cobalt sulfide aerogel prepared by anion exchange method with enhanced pseudocapacitive and water oxidation performances

This work introduces the anion exchange method into the sol-gel process for the first time to prepare a metal sulfide aerogel. A porous Co₉S₈ aerogel with a high surface area (274.2 m² g^-1) and large pore volume (0.87 cm³ g^-1) has been successfully prepared by exchanging cobalt citrate wet gel in thioacetamide and subsequently drying in supercritical ethanol. Such a Co₉S₈ aerogel shows enhanced supercapacitive performance and catalytic activity toward oxygen evolution reaction (OER) compared to its oxide aerogel counterpart. High specific capacitance (950 F g^-1 at 1 A g^-1), good rate capability (74.3% capacitance retention from 1 to 20 A g^-1) and low onset overpotential for OER (220 mV) were observed. The results demonstrated here have implications in preparing various sulfide chalcogels.

Highly Self-Healable 3D Microsupercapacitor with MXene-Graphene Composite Aerogel

High-performance microsupercapacitors (MSCs) with three-dimensional (3D) structure provide an effective approach to improve the ability of energy storage. Because the electrodes with 3D structure are generally easily destroyed under mechanical deformation in practical applications, we fabricated a self-healable 3D MSC consisting of MXene (Ti₃C₂T _x)-graphene (reduced graphene oxide, rGO) composite aerogel electrode by wrapping it with a self-healing polyurethane as an outer shell. The MXene-rGO composite aerogel combining large specific surface area of rGO and high conductivity of the MXene can not only prevent the self-restacking of the lamella structure but also resist the poor oxidization of MXene to a degree. The MSC based on a 3D MXene-rGO aerogel delivers a large area specific capacitance of 34.6 mF cm^-2 at a scan rate of 1 mV s^-1 and an outstanding cycling performance with a capacitance retention up to 91% over 15 000 cycles. The 3D MSC presents an excellent self-healing ability with specific capacitance retention of 81.7% after the fifth healing. The preparation of this self-healable 3D MSC can provide a method for designing and manufacturing next-generation long-life multifunctional electronic devices further to meet the requirements of sustainable development.

Bioinspired Ultralight Inorganic Aerogel for Highly Efficient Air Filtration and Oil-Water Separation

Inorganic aerogels have been attracting great interest owing to their distinctive structures and properties. However, the practical applications of inorganic aerogels are greatly restricted by their high brittleness and high fabrication cost. Herein, inspired by the cancellous bone, we have developed a novel kind of hydroxyapatite (HAP) nanowire-based inorganic aerogel with excellent elasticity, which is highly porous (porosity ≈ 99.7%), ultralight (density 8.54 mg/cm³, which is about 0.854% of water density), and highly adiabatic (thermal conductivity 0.0387 W/m·K). Significantly, the as-prepared HAP nanowire aerogel can be used as the highly efficient air filter with high PM_2.5 filtration efficiency. In addition, the HAP nanowire aerogel is also an ideal candidate for continuous oil-water separation, which can be used as a smart switch to separate oil from water continuously. Compared with organic aerogels, the as-prepared HAP nanowire aerogel is biocompatible, environmentally friendly, and low-cost. Moreover, the synthetic method reported in this work can be scaled up for large-scale production of HAP nanowires, free from the use of organic solvents. Therefore, the as-prepared new kind of HAP nanowire aerogel is promising for the applications in various fields.

Synthesis, structure and applications of graphene-based 2D heterostructures

With the profuse amount of two-dimensional (2D) materials discovered and the improvements in their synthesis and handling, the field of 2D heterostructures has gained increased interest in recent years. Such heterostructures not only overcome the inherent limitations of each of the materials, but also allow the realization of novel properties by their proper combination. The physical and mechanical properties of graphene mean it has a prominent place in the area of 2D heterostructures. In this review, we will discuss the evolution and current state in the synthesis and applications of graphene-based 2D heterostructures. In addition to stacked and in-plane heterostructures with other 2D materials and their potential applications, we will also cover heterostructures realized with lower dimensionality materials, along with intercalation in few-layer graphene as a special case of a heterostructure. Finally, graphene heterostructures produced using liquid phase exfoliation techniques and their applications to energy storage will be reviewed.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS₂, MoSe₂, MoTe₂, WS₂ and WSe₂, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1-2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

Highly enhanced response of MoS2/porous silicon nanowire heterojunctions to NO2 at room temperature

Molybdenum disulfide/porous silicon nanowire (MoS₂/PSiNW) heterojunctions with different thicknesses as highly-responsive NO₂ gas sensors were obtained in the present study. Porous silicon nanowires were fabricated using metal-assisted chemical etching, and seeded with different thicknesses. After that, MoS₂ nanosheets were synthesized by sulfurization of direct-current (DC)-magnetic-sputtering Mo films on PSiNWs. Compared with the as-prepared PSiNWs and MoS₂, the MoS₂/PSiNW heterojunctions exhibited superior gas sensing properties with a low detection concentration of 1 ppm and a high response enhancement factor of ∼2.3 at room temperature. The enhancement of the gas sensitivity was attributed to the layered nanostructure, which induces more active sites for the absorption of NO₂, and modulation of the depletion layer width at the interface. Further, the effects of the deposition temperature in the chemical vapor deposition (CVD) process on the gas sensing properties were also discussed, and might be connected to the nucleation and growth of MoS₂ nanosheets. Our results indicate that MoS₂/PSiNW heterojunctions might be a good candidate for constructing high-performance NO₂ sensors for various applications.

Molybdenum disulfide/porous silicon nanowire (MoS₂/PSiNW) heterojunctions with different thicknesses as highly-responsive NO₂ gas sensors were obtained in the present study.

Recent advances in three-dimensional graphene based materials for catalysis applications

This review presents recent theoretical and experimental progress in the construction, properties, and catalytic applications of 3D graphene-based materials.

Multifunctional Cellular Materials Based on 2D Nanomaterials: Prospects and Challenges

Recent advances in emerging 2D nanomaterial-based cellular materials (2D-CMs) open up new opportunities for the development of next generation cellular solids with exceptional properties. Herein, an overview of the current research status of 2D-CMs is provided and their future opportunities are highlighted. First, the unique features of 2D nanomaterials are introduced to illustrate why these nanoscale building blocks are promising for the development of novel cellular materials and what the new features of 2D nanoscale building blocks can offer when compared to their 0D and 1D counterparts. An in-depth discussion on the structure-property relationships of 2D-CMs is then provided, and the remarkable functions that can be achieved by engineering their cellular architecture are highlighted. Additionally, the use of 2D-CMs to tackle key challenges in different practical applications is demonstrated. In conclusion, a personal perspective on the challenges and future research directions of 2D-CMs is given.