Supercritical CO2-assisted fabrication of advanced two-dimensional materials and their heterostructure

Advanced two-dimensional (2D) materials and their heterostructure can be fabricated with the assistance of supercritical carbon dioxide (SC CO₂). And this fabrication strategy will undoubtedly have a promising future in materials processing.

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Exfoliated 2D Layered and Nonlayered Metal Phosphorous Trichalcogenides Nanosheets as Promising Electrocatalysts for CO2 Reduction

Two-dimensional (2D) materials catalysts provide an atomic-scale view on a fascinating arena for understanding the mechanism of electrocatalytic carbon dioxide reduction (CO₂ ECR). Here, we successfully exfoliated both layered and nonlayered ultra-thin metal phosphorous trichalcogenides (MPCh₃) nanosheets via wet grinding exfoliation (WGE), and systematically investigated the mechanism of MPCh₃ as catalysts for CO₂ ECR. Unlike the layered CoPS₃ and NiPS₃ nanosheets, the active Sn atoms tend to be exposed on the surfaces of nonlayered SnPS₃ nanosheets. Correspondingly, the nonlayered SnPS₃ nanosheets exhibit clearly improved catalytic activity, showing formic acid selectivity up to 31.6 % with −7.51 mA cm⁻² at −0.65 V vs. RHE. The enhanced catalytic performance can be attributed to the formation of HCOO* via the first proton-electron pair addition on the SnPS₃ surface. These results provide a new avenue to understand the novel CO₂ ECR mechanism of Sn-based and MPCh₃-based catalysts.     

Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors

Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO₂ into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO₂ on TiO₂ and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO₂ reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.     

Improved synthesis of porous C2N for CO2 conversion

Since the initial introduction of porous C₂N-h2D materials by the Baek group in 2015, these materials have exhibited highly promising applications in fields such as semiconductor devices, heterogeneous catalysis, gas storage and separation, biomedicine, and more. However, much of the existing research on C₂N materials has been based on theoretical calculations due to the challenges associated with their synthesis. In this study, an enhanced synthesis method for porous C₂N materials has been successfully developed, involving the innovative and nonexplosive synthesis of hexaaminobenzene trihydrochloride (HAB·3HCl) as a crucial intermediate, as well as a time-efficient synthesis of C₂N. Rigorous structural characterizations have been conducted, including solid-state NMR analysis, among others. The resultant C₂N material has been effectively employed to improve the efficiency of CO₂ conversion reactions. This straightforward protocol for synthesizing C₂N materials is poised to stimulate further exploration and application of this promising 2D material in the near future.     

Recent progress on two-dimensional materials confining single atoms for CO2 photoreduction

Photoreduction of CO₂ into value-added products offers a promising approach to overcome both climate change and energy crisis. However, low conversion efficiency, poor product selectivity, and unclear mechanism limit the further advancement of CO₂ photoreduction. The development of two-dimensional (2D) materials and construction of single atom sites are two frontier research fields in catalysis. Combining the advantages of 2D materials and single atom sites is expected to make a breakthrough in CO₂ photoreduction. In this review, we summarized the design and application, proposed challenges and opportunities, and laid a foundation for further research and application of 2D materials confining single atoms (SACs@2D) for CO₂ photoreduction.     

Nanostructured heterogeneous catalysts for electrochemical reduction of CO2

Electrochemical reduction of CO₂ provides a sustainable solution to address the intermittent renewable electricity storage while recycling CO₂ to produce fuels and chemicals. Highly efficient catalytic materials and reaction systems are required to drive this process economically. This Review highlights the new trends in advancing the electrochemical reduction of CO₂ by developing and designing nanostructured heterogeneous catalysts. The activity, selectivity and reaction mechanism are significantly affected by the nano effects in nanostructured heterogeneous catalysts. In the future, energy efficiency and current density in electrochemical reduction of CO₂ need to be further improved to meet the requirements for practical applications.     

In situ drawing of high molecular weight poly(ethylene terephthalate) in subcritical and supercritical CO2

The effects of draw conditions were studied for initially amorphous melt‐spun poly(ethylene terephthalate) fibers in the presence of subcritical and supercritical (SC) CO₂. Both in situ and posttreatment mechanical behavior along with morphological characteristics were investigated. Fibers soaked in subcritical CO₂ could be drawn to 30% higher draw ratios (DRs) compared with fibers that were cold‐drawn. In situ force response measured with a custom apparatus showed that fibers in subcritical CO₂ had no measurable resistance to deformation until strain hardening occurred. In contrast, fibers drawn in SC CO₂ displayed a yield response, a significant decrease in ductility, and a significant difference in postyield behavior. Fibers drawn in subcritical CO₂ showed slightly lower tensile properties compared with cold‐drawn samples whereas fibers treated in SC CO₂ had much lower tensile properties because of the limited DR achieved. X‐ray diffraction studies indicated that CO₂ enhances the development of the crystalline phase compared with cold‐drawn samples. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1881–1891, 1999     

CO2‐Assisted Fabrication of Two‐Dimensional Amorphous Molybdenum Oxide Nanosheets for Enhanced Plasmon Resonances

As a remarkable class of plasmonic materials, two dimensional (2D) semiconductor compounds have attracted attention owing to their controlled manipulation of plasmon resonances in the visible light spectrum, which outperforms conventional noble metals. However, tuning of plasmonic resonances for 2D semiconductors remains challenging. Herein, we design a novel method to obtain amorphous molybdenum oxide (MoO₃) nanosheets, in which it combines the oxidation of MoS₂ and subsequent supercritical CO₂‐treatment, which is a crucial step for the achievement of amorphous structure of MoO₃. Upon illumination, hydrogen‐doped MoO₃ exhibits tuned surface plasmon resonances in the visible and near‐IR regions. Moreover, a unique behavior of the amorphous MoO₃ nanosheets has been found in an optical biosensing system; there is an optimum plasmon resonance after incubation with different BSA concentrations, suggesting a tunable plasmonic device in the near future.     

Supercritical CO2-induced New Chemical Bond of C−O−Si in Graphdiyne to Achieve Robust Room-Temperature Ferromagnetism

The realization of ferromagnetic ordering of two-dimensional (2D) carbon material graphdiyne (GDY) has attracted great attention due to its promising application in spin semiconductor devices. However, the absence of localized spins makes the pristine GDY intrinsically nonferromagnetic. Herein, we report the realization of robust room-temperature (RT) ferromagnetism (FM) with Curie temperature (T_C) up to 325 K for GDY Nanosheets (GDYNs) by supercritical CO₂ (SC CO₂). Experimental and theoretical calculations reveal that the new chemical bond of C−O−Si can be formed because of the unique effect of SC CO₂, which help to enhance the charge transfer and generates long-range ferromagnetic order. The RT saturation magnetization (M_S) reaches 1.125 emu/g, which is much higher than that of carbon-based materials reported up to now. Meanwhile, by changing the conditions of SC CO₂ such as pressure, ferromagnetic responses can be manipulated, which is great for potential spintronics applications of GDY.     

Carbon Electrode Materials for Advanced Potassium-Ion Storage

Tremendous progress has been made in the field of electrochemical energy storage devices that rely on potassium-ions as charge carriers due to their abundant resources and excellent ion transport properties. Nevertheless, future practical developments not only count on advanced electrode materials with superior electrochemical performance, but also on competitive costs of electrodes for scalable production. In the past few decades, advanced carbon materials have attracted great interest due to their low cost, high selectivity, and structural suitability and have been widely investigated as functional materials for potassium-ion storage. This article provides an up-to-date overview of this rapidly developing field, focusing on recent advanced and mechanistic understanding of carbon-based electrode materials for potassium-ion batteries. In addition, we also discuss recent achievements of dual-ion batteries and conversion-type K−X (X=O₂, CO₂, S, Se, I₂) batteries towards potential practical applications as high-voltage and high-power devices, and summarize carbon-based materials as the host for K-metal protection and possible directions for the development of potassium energy-related devices as well. Based on this, we bridge the gaps between various carbon-based functional materials structure and the related potassium-ion storage performance, especially provide guidance on carbon material design principles for next-generation potassium-ion storage devices.     

Biodegradable elastomer for soft tissue engineering

In this work we present the synthesis of a biodegradable, elastomeric material with a wide range of mechanical properties. The synthesis of the material was done by condensation polymerization of malic acid and 1,12–dodecandiol. The synthesized materials have low Young’s modulus ranging from about 1 to 4 MPa and a high elongation at break of 25–737% depending on the crosslinking density of the system. The cell growth observed under microscope showed good proliferation at 3 days of culture indicating good biocompatibility and support of L929 cells growth. The fabrication of 3D scaffold from these materials using the super critical CO₂ foaming method was also attempted. This method of scaffold fabrication is appropriate for materials that are easily hydrolysable and it also has the advantage of being a solvent free process. These materials are generally soft, biocompatible and biodegradable making them suitable for tissue engineering of soft tissues that are elastic in nature like muscles and blood vessels.     

Co-electrolysis of CO_2 and H_2O in high-temperature solid oxide electrolysis cells: Recent advance in cathodes

Co-electrolysis of CO_2 and H_2O using high-temperature solid oxide electrolysis cells(SOECs) into valuable chemicals has attracted great attentions recently due to the high conversion and energy efficiency,which provides opportunities of reducing CO_2 emission, mitigating global warming and storing intermittent renewable energies. A single SOEC typically consists of an ion conducting electrolyte, an anode and a cathode where the co-electrolysis reaction takes place. The high operating temperature and difficult activated carbon-oxygen double-bond of CO_2 put forward strict requirements for SOEC cathode. Great efforts are being devoted to develop suitable cathode materials with high catalytic activity and excellent long-term stability for CO_2/H_2O electro-reduction. The so far cathode material development is the key point of this review and alternative strategies of high-performance cathode material preparation is proposed. Understanding the mechanism of CO_2/H_2O electro-reduction is beneficial to highly active cathode design and optimization. Thus the possible reaction mechanism is also discussed. Especially, a method in combination with electrochemical impedance spectroscopy(EIS) measurement, distribution functions of relaxation times(DRT) calculation, complex nonlinear least square(CNLS) fitting and operando ambient pressure X-ray photoelectron spectroscopy(APXPS) characterization is introduced to correctly disclose the reaction mechanism of CO_2/H_2O co-electrolysis. Finally, different reaction modes of the CO_2/H_2O coelectrolysis in SOECs are summarized to offer new strategies to enhance the CO_2 conversion. Otherwise,developing SOECs operating at 300-600 °C can integrate the electrochemical reduction and the Fischer-Tropsch reaction to convert the CO_2/H_2O into more valuable chemicals, which will be a new research direction in the future.     

New poly(silanes-siloxanes) via hydrosilation in supercritical CO2 and subsequent crosslinking

In order to obtain crosslinkable, low dielectric, low Tg materials, the hydrosilation polymerization was carried out in supercritical carbon dioxide (SC CO₂) using Karstedt's catalyst, and a series of new fluorine-containing poly(carbosilane/siloxane) were synthesized and characterized. The molecular weights of polymers obtained in SC CO₂ were notably greater than those obtained in benzene. Hydrolysis and thermal curing of these polymers leads to materials considerably higher molecular weight which retain their low Tg values.     

A 2D Graphitic-Polytriaminopyrimidine (g-PTAP)/Poly(ether-block-amide) Mixed Matrix Membrane for CO2 Separation

Poly(ether-block-amide)/g-PTAP mixed matrix membranes (MMMs) were developed by incorporating different wt.% (1–10%) of a novel 2D g-PTAP nanofiller and its effects on membrane structure and gas permeability were studied. The novel 2D material g-PTAP was synthesized and characterized by various analytical techniques including field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Raman spectroscopy. The fabricated MMMs were investigated to study the interaction and compatibility between Pebax and g-PTAP. The MMMs showed an effective integration of g-PTAP nanofiller into the Pebax matrix without affecting its thermal stability. Gas permeation experiments with MMMs showed improved CO₂ permeability and selectivity (CO₂/N₂) upon incorporation of g-PTAP in the Pebax polymer matrix. The maximum CO₂ permeability enhancement from 82.3 to 154.6 Barrer with highest CO₂/N₂ selectivity from 49.5 to 83.5 were found with 2.5 wt.% of nanofiller compared to neat Pebax membranes.     

Rational Design of Carbon-Based 2D Nanostructures for Enhanced Photocatalytic CO2 Reduction: A Dimensionality Perspective

Photocatalytic CO₂ reduction is a revolutionary approach to solve imminent energy and environmental issues by replicating the ingenuity of nature. The past decade has witnessed an impetus in the rise of two-dimensional (2D) structure materials as advanced nanomaterials to boost photocatalytic activities. In particular, the use of 2D carbon-based materials is deemed as highly favorable, not only as a green material choice, but also due to their exceptional physicochemical and electrical properties. This Review article presents a diverse range of alterations and compositions derived from 2D carbon-based nanomaterials, mainly graphene and graphitic carbon nitride (g-C₃N₄), which have remarkably ameliorated the photocatalytic CO₂ performance. Herein, the rational design of the photocatalyst systems with consideration of the aspect of dimensionality and the resultant heterostructures at the interface are systematically analyzed to elucidate an insightful perspective on this pacey subject. Finally, a conclusion and outlook on the limitations and prospects of the cutting-edge research field are highlighted.     

Single-Layer 2D Ni−BDC MOF Obtained in Supercritical CO2-Assisted Aqueous Solution

The development of an efficient strategy for fabricating two-dimensional metal-organic framework (MOF) nanosheets with high yield and high stability is desirable. Herein, we demonstrate for the first time that large, single-layer 2D nickel-benzene dicarboxylate (Ni−BDC) MOF nanosheets can be fabricated with the assistance of supercritical (SC) CO₂ in a pure aqueous system. Detailed experimental evidence reveals that the SC CO₂ molecule can exchange with the lattice-coordinated H₂O molecules, side-on coordinate with the metal Ni¹ sites on the Ni−BDC surface, and finally break the interlayer hydrogen bond to exfoliate the bulk Ni−BDC into a 2D MOF. More importantly, a thin SC CO₂ layer building up at the water−Ni−BDC interfaces can transform the pristine hydrophilic interface into a super-hydrophobic one. This super-hydrophobic layer at the water-MOF interface can effectively prevent dissociation, thus promoting the stability of Ni−BDC in aqueous system.     

Efficient strategies for boosting the performance of 2D graphitic carbon nitride nanomaterials during photoreduction of carbon dioxide to energy-rich chemicals

Nowadays, the alarming growing interest in providing a solution to increasing concentration of atmospheric carbon dioxide (CO₂) and the associated pollution has attracted global attention. The consequential effects of CO₂ are detrimental to the environment owing to the continuous depletion of carbon-emitting fossil fuels. Photocatalytic CO₂ reduction (CO₂R) to valuable chemicals and fuels is one the promising alternative option to mitigate the global menace instigated by CO₂ emissions. If the strategies for enhancing the CO₂R are unavailable, inefficient, or inappropriate, then efficiency conversion CO₂ to valuable products can become problematic. In that case, the emission of CO₂ results in synchronizing upsurge in the global-mean air surface temperature on the earth and sea levels from 1980 to 2100. This study presents different strategies for boosting the photocatalytic performance of 2D graphitic carbon nitride (g-C₃N₄) for CO₂R reaction. The first part consists of the fundamental principles of photocatalysis. The second part presents some answers to the question: what governs the mechanism of photocatalytic CO₂R? The existing literature lack comprehensive information about the strategical influence of available reactor designs on the photoactivity of 2D g-C₃N₄ for CO₂ conversion to energy-rich chemicals and ways to improve them as discussed in this study. This was then followed by strategies about the synthetic methods for enhancing photocatalytic CO₂R over 2D g-C₃N₄ materials before the discussion of the strategies for enhancing the CO₂ photoreduction on the 2D g-C₃N₄ nanomaterials. Some groups of g-C₃N₄ nanomaterials for photoreduction of CO₂R were also discussed. Unlike the previous reviews in the field, the present study presents some innovation to bridge the knowledge gaps of the previous reviews and corresponding insight thereof. For future breakthroughs, this study also explains some problems with the interpretation in the field. We also highlight insights into innovation on exclusion and inclusion criteria about the performance metrics and present some queries, concerns, and problems with the previous studies. The concluding part consists of research outlooks, including commonly overlooked challenges and future perspectives for ensuring highly efficient strategies, applications of 2D g-C₃N₄ photocatalysts, and CO₂ conversion to meet industrial scale expectations. The present study hypothesized that considering the current technological age, the experiment should go beyond presenting only illustration and analysis about the band energy, but the detailed explanation/information about the pathways of the various products formed using molecular dynamics system and artificial intelligence aspects should be combined in the future studies.     

Direct and Oriented Conversion of CO2 into Value-Added Aromatics

The oriented conversion of CO₂ into target high-value chemicals is an effective way to reduce carbon emissions, but still presents a challenge. In this communication, we report the oriented conversion of CO₂ into value-added aromatics, especially para-xylene, in a single pass by combining core–shell structured Zn-doped H-ZSM-5 (Zn-ZSM-5@SiO₂) and a Cr₂O₃ component. Through precise regulation of the acidity of Zn-ZSM-5@SiO₂, high para-xylene selectivity (38.7 % in the total products) at a CO₂ conversion of 22.1 % was achieved. Furthermore, a CO₂-assisted effect in the synthesis of aromatics during the tandem process has been clarified through a control experiment. The CO₂ reactant can act as a hydrogen acceptor to accelerate the dehydrogenation of alkenes, intermediates in the synthesis of aromatics, thereby increasing the driving force towards aromatics in the tandem reaction process.     

Heterogeneous Water Oxidation: Surface Activity versus Amorphization Activation in Cobalt Phosphate Catalysts

Is water oxidation catalyzed at the surface or within the bulk volume of solid oxide materials? This question is addressed for cobalt phosphate catalysts deposited on inert electrodes, namely crystallites of pakhomovskyite (Co₃(PO₄)₂?8 H₂O, Pak) and phosphate‐containing Co oxide (CoCat). X‐ray spectroscopy reveals that oxidizing potentials transform the crystalline Pak slowly (5–8 h) but completely into the amorphous CoCat. Electrochemical analysis supports high‐TOF surface activity in Pak, whereas its amorphization results in dominating volume activity of the thereby formed CoCat material. In the directly electrodeposited CoCat, volume catalysis prevails, but not at very low levels of the amorphous material, implying high‐TOF catalysis at surface sites. A complete picture of heterogeneous water oxidation requires insight in catalysis at the electrolyte‐exposed “outer surface”, within a hydrated, amorphous volume phase, and modes and kinetics of restructuring upon operation.     

Enhanced Charge Transfer Process and Photocatalytic Activity over a Phosphonate-based MOF via Amorphization Strategy

Recently, amorphous materials have gained great attention as an emerging kind of functional material, and their characteristics such as isotropy, absence of grain boundaries, and abundant defects are very likely to outrun the disadvantages of crystalline counterparts, such as low conductivity, and ultimately lead to improved charge transfer efficiency. Herein, we investigated the effect of amorphization on the charge transfer process and photocatalytic performance with a phosphonate-based metal–organic framework (FePPA) as the research object. Comprehensive experimental results suggest that compared to crystalline FePPA, amorphous FePPA has more distorted metal nodes, which affects the electron distribution and consequently improves the photogenerated charge separation efficiency. Meanwhile, the distorted metal nodes in amorphous FePPA also greatly promote the adsorption and activation of O₂. Hence, amorphous FePPA exhibits a better performance of photocatalytic C(sp³)−H bond activation for selective oxidation of toluene to benzaldehyde. This work illustrates the advantages of amorphous MOFs in the charge transfer process, which is conducive to the further development of high performance MOFs-based photocatalysts.