Modulating oxygen coverage of Ti3C2Tx MXenes to boost catalytic activity for HCOOH dehydrogenation

Electrochemical sensor for selective diquat detection based on samarium-stannate-nanoparticle-anchored titanium aluminum carbide MXene nanocomposites

The persistent presence of diquat (DQ) residue poses a significant threat to human health, underscoring the need to monitor DQ levels in agricultural samples. This is crucial for both precision and post-harvest agriculture, which require non-destructive, rapid, and cost-effective analytical methods using electrochemical detection. We developed a novel nanocomposite, composed of samarium stannate (Sm2Sn2O7) nanoparticles anchored on an MXene (Ti3Al(1-x)C2-OH(x); TAC-1), for the electrochemical quantification of DQ. A three-dimensional titanium aluminum carbide MXene surface was hydroxylated and functionalized with Sm2Sn2O7 nanoparticles using a combustion method. The resulting heterogeneous TAC-1-Sm2Sn2O7 composite contained active sites that enabled selective and sensitive DQ detection owing to the synergistic effects of the enhanced electrocatalytic sites and rapid charge transfer. Under real-world conditions, a TAC-1-Sm2Sn2O7-modified screen-printed carbon electrode sensor exhibited outstanding electrochemical activity toward DQ, with good recovery (98 %) from prepared samples. Hence, the designed sensor electrode is well suited for DQ monitoring.

Solid Intercalation and Exfoliation of Cl-Terminated Multilayered MXenes toward O-Functionalized Single Layers

Although ion intercalation is becoming a powerful strategy to produce expanded layered materials and atomic layers in aqueous or organic systems, it usually suffers from sluggish kinetics with a long intercalating time of several days. Here, we present a facile approach to produce O-functionalized single-layer MXenes by solid intercalation of Cl-terminated accordion-like MXenes in molten salts and subsequent exfoliation. The process involves the intercalation of metal cations (Li+, Na+ and K+) and anions (CO32-) in molten salts, resulting in substitution with -O surface groups and formation of gases (CO2). Such unique solid intercalation significantly expands multilayered MXenes in 30 min with an enhancement of interlayer spacing from 11.2 to 12.7 Å, facilitating their easy exfoliation to single layers. The resultant O-functionalized MXenes have a highly expanded structure and elevated permittivity, achieving a reflection loss value of -50.5 dB at a thickness of 1.35 mm for electromagnetic wave absorption.

Histidine-Based "Transfer Stations" at Carbon-Immobilized Metal Particles Enable Rapid Hydrogen Transfer for Efficient Formic Acid Dehydrogenation

The interaction of surface metal species with the solution plays a key role in engineering heterogeneous catalytic processes. Herein, we present the facile synthesis of L-histidine-coordinated PdAg nanoparticles (4.03 ± 0.08 nm) anchored on pristine carbon supports (denoted as PdAg-NH2/C) and their use for formic acid dehydrogenation (FAD). Significant acceleration of FAD related to the histidine is observed, and the enhancement mechanism is experimentally and theoretically investigated. The presence of L-histidine at metal sites promotes rapid binding of formic acid molecules due to acid-base interactions. The local enrichment of both protons and formate at the metal-solution interfaces promotes the subsequent formate decomposition and hydride transfer to the metal surface. The as-generated surface H species are more concentrated compared to the previously reported catalyst where the metal is loaded on an amino modified support, this enabling a significantly enhanced H2 production. The optimal Pd1Ag1-NH2/C catalyst exhibits a high turnover frequency (TOF) of 6493.5 h-1 at 333 K based on the total amount of Pd, together with an H2 selectivity of 100%. This study emphasizes the critical role of optimizing local transport pathways near catalytic centers chemically and further provides insights into the rational development of heterogeneous catalysts for FAD technologies.

Modulating Interface of Ni-Embedded Hollow Porous Ti3C2Tx MXene Film Toward Efficient EMI Shielding

Since the explosive growth of state-of-the-art electronics and devices raises concerns about electromagnetic pollution, exploring novel and efficient electromagnetic interference (EMI) shielding materials is desirable and crucial. Ti3C2Tx MXenes hold significant EMI shielding potential due to their inherent characteristics, including lightweight, metal-like conductivities, unique layered structure, and facile processing. Nonetheless, it remains challenging to fabricate Ti3C2Tx MXenes-based EMI shielding materials with efficient shielding capability and low reflection. Herein, an interface modulating strategy is designed to fabricate Ni-embedded hollow porous Ti3C2Tx MXene film. Benefiting from this strategy, the impedance matching is enhanced and the magnetic loss is simultaneously introduced. The multiple reflections, Ohmic loss, magnetic loss, and interfacial polarization concurrently contribute to the EMI shielding mechanism of the film. Accordingly, the film delivers an impressive EMI shielding effectiveness (SET) of 70.7 dB at a thickness of ≈55 µm, whilst the average reflection effectiveness (SER) is only 17.4 dB. The specific EMI shielding effectiveness (SSE/t) is as high as 35126 dB∙cm2∙g-1. This study demonstrates a novel and effective routine for constructing EMI shielding materials with superior shielding capability and minimal reflection.

CO2-Mediated Hydrogen Energy Release-Storage Enabled by High-Dispersion Gold-Palladium Alloy Nanodots

Developing and fabricating a heterogeneous catalyst for efficient formic acid (FA) dehydrogenation coupled with CO2 hydrogenation back to FA is a promising approach to constructing a complete CO2-mediated hydrogen release-storage system, which remains challenging. Herein, a facile two-step strategy involving high-temperature pyrolysis and wet chemical reduction processes can synthesize efficient pyridinic-nitrogen-modified carbon-loaded gold-palladium alloy nanodots (AuPd alloy NDs). These NDs exhibit a prominent electron synergistic effect between Au and Pd components and tunable alloy-support interactions. The pyridinic-N dosage in carbon substrate improves the surface electron density of the alloy catalyst, thus regulating the chemical adsorption of FA molecules. Specifically, the engineered Au3Pd7/CN0.25 demonstrates an outstanding room-temperature FA dehydrogenation efficiency, achieving ≈100% conversion and an initial turnover frequency (TOF) of up to 9049 h-1. The versatile AuPd alloy NDs also show the ability to convert CO2, one of the products of FA dehydrogenation, into FA (formate) with a 90.8% yield under mild conditions. Moreover, in-depth insights into the unique alloyed microstructure, structure-activity relationship, key intermediates, and the alloy-driven five-step reaction mechanism involving the rate-determining step of C─H bond cleavage from critical *HCOO species via D-labeled isotope, in situ infrared spectroscopy, and theoretical calculations are investigated.

Proton-Coupled Electron Transfer on Cu2O/Ti3C2Tx MXene for Propane (C3H8) Synthesis from Electrochemical CO2 Reduction

Electrochemical CO2 reduction reaction (CO2RR) to produce value-added multi-carbon chemicals has been an appealing approach to achieving environmentally friendly carbon neutrality in recent years. Despite extensive research focusing on the use of CO2 to produce high-value chemicals like high-energy-density hydrocarbons, there have been few reports on the production of propane (C3H8), which requires carbon chain elongation and protonation. A rationally designed 0D/2D hybrid Cu2O anchored-Ti3C2Tx MXene catalyst (Cu2O/MXene) is demonstrated with efficient CO2RR activity in an aqueous electrolyte to produce C3H8. As a result, a significantly high Faradaic efficiency (FE) of 3.3% is achieved for the synthesis of C3H8 via the CO2RR with Cu2O/MXene, which is ≈26 times higher than that of Cu/MXene prepared by the same hydrothermal process without NH4OH solution. Based on in-situ attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) and density functional theory (DFT) calculations, it is proposed that the significant electrocatalytic conversion originated from the synergistic behavior of the Cu2O nanoparticles, which bound the *C2 intermediates, and the MXene that bound the *CO coupling to the C3 intermediate. The results disclose that the rationally designed MXene-based hybrid catalyst facilitates multi-carbon coupling as well as protonation, thereby manipulating the CO2RR pathway.

Combination of nanoparticles with single-metal sites synergistically boosts co-catalyzed formic acid dehydrogenation

The development of hydrogen technologies is at the heart of a green economy. As prerequisite for implementation of hydrogen storage, active and stable catalysts for (de)hydrogenation reactions are needed. So far, the use of precious metals associated with expensive costs dominates in this area. Herein, we present a new class of lower-cost Co-based catalysts (Co-SAs/NPs@NC) in which highly distributed single-metal sites are synergistically combined with small defined nanoparticles allowing efficient formic acid dehydrogenation. The optimal material with atomically dispersed CoN2C2 units and encapsulated 7-8 nm nanoparticles achieves an excellent gas yield of 1403.8 mL·g-1·h-1 using propylene carbonate as solvent, with no activity loss after 5 cycles, which is 15 times higher than that of the commercial Pd/C. In situ analytic experiments show that Co-SAs/NPs@NC enhances the adsorption and activation of the key intermediate monodentate HCOO*, thereby facilitating the following C-H bond breaking, compared to related single metal atom and nanoparticle catalysts. Theoretical calculations show that the integration of cobalt nanoparticles elevates the d-band center of the Co single atoms as the active center, which consequently enhances the coupling of the carbonyl O of the HCOO* intermediate to the Co centers, thereby lowering the energy barrier.

Engineering Single-Atom Sites with the Irving-Williams Series for the Simultaneous Co-photocatalytic CO2 Reduction and CH3CHO Oxidation

The bonding effects between 3d transition-metal single sites and supports originate from crystal field stabilization energy (CFSE). The 3d transition-metal atoms of the spontaneous geometrical distortions, that is the Jahn-Teller effect, can alter CFSE, thereby leading to the Irving-Williams series. However, engineering single-atom sites (SASs) using the Irving-Williams series as an ideal guideline has not been reported to date. Herein, alkynyl-linked covalent phenanthroline frameworks (CPFs) with phenanthroline units are developed to anchor the desired 3d single metal ions from d5 to d10 (Mn2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+). The Irving-Williams series was employed to accurately predict the bonding effects between 3d transition-metal atoms and phenanthroline units. To verify this, theoretical calculations and experimental results reveal that Cu-SASs/CPFs exhibits higher stability and faster charge-transfer efficiency, far surpassing other metal-SASs/CPFs. As expected, Cu-SASs/CPFs demonstrates a high photoreduction of CO2-to-CO activity (~30.3 μmol ⋅ g-1 ⋅ h-1) and an exceptional photooxidation of CH3CHO-to-CH3COOH activity (~24.7 μmol ⋅ g-1 ⋅ h-1). Interestingly, the generated *O2 - is derived from the process of CO2 reduction, thereby triggering a CH3CHO oxidation reaction. This work provides a novel design concept for designing SASs by the Irving-Williams to regulate the catalytic performances.

Enhancing Hydrogen Evolution Reaction Activity of Palladium Catalyst by Immobilization on MXene Nanosheets

Efficient catalysts with minimal content of catalytically active noble metals are essential for the transition to the clean hydrogen economy. Catalyst supports that can immobilize and stabilize catalytic nanoparticles and facilitate the supply of electrons and reactants to the catalysts are needed. Being hydrophilic and more conductive compared with carbons, MXenes have shown promise as catalyst supports. However, the controlled assembly of their 2D sheets creates a challenge. This study established a lattice engineering approach to regulate the assembly of exfoliated Ti3C2Tx MXene nanosheets with guest cations of various sizes. The enlargement of guest cations led to a decreased interlayer interaction of MXene lamellae and increased surface accessibility, allowing intercalation of Pd nanoparticles. Stabilization of Pd nanoparticles between interlayer-expanded MXene nanosheets improved their electrocatalytic activity. The Pd-immobilized K+-intercalated MXene nanosheets (PdKMX) demonstrated exceptional electrocatalytic performance for the hydrogen evolution reaction with the lowest overpotential of 72 mV (@10 mA cm-2) and the highest turnover frequency of 1.122 s-1 (@ an overpotential of 100 mV), which were superior to those of the state-of-the-art Pd nanoparticle-based electrocatalysts. Weakening of the interlayer interaction during self-assembly with K+ ions led to fewer layers in lamellae and expansion of the MXene in the c direction during Pd anchoring, providing numerous surface-active sites and promoting mass transport. In situ spectroscopic analysis suggests that the effective interfacial electron injection from the Pd nanoparticles strongly immobilized on interlayer-expanded PdKMX may be responsible for the improved electrocatalytic performance.

Functional MXenes: Progress and Perspectives on Synthetic Strategies and Structure-Property Interplay for Next-Generation Technologies

MXenes are a class of 2D materials that include layered transition metal carbides, nitrides, and carbonitrides. Since their inception in 2011, they have garnered significant attention due to their diverse compositions, unique structures, and extraordinary properties, such as high specific surface areas and excellent electrical conductivity. This versatility has opened up immense potential in various fields, catalyzing a surge in MXene research and leading to note worthy advancements. This review offers an in-depth overview of the evolution of MXenes over the past 5 years, with an emphasis on synthetic strategies, structure-property relationships, and technological prospects. A classification scheme for MXene structures based on entropy is presented and an updated summary of the elemental constituents of the MXene family is provided, as documented in recent literature. Delving into the microscopic structure and synthesis routes, the intricate structure-property relationships are explored at the nano/micro level that dictate the macroscopic applications of MXenes. Through an extensive review of the latest representative works, the utilization of MXenes in energy, environmental, electronic, and biomedical fields is showcased, offering a glimpse into the current technological bottlenecks, such asstability, scalability, and device integration. Moreover, potential pathways for advancing MXenes toward next-generation technologies are highlighted.

Defect-driven nanostructuring of low-nuclearity Pt-Mo ensembles for continuous gas-phase formic acid dehydrogenation

Supported metal clusters comprising of well-tailored low-nuclearity heteroatoms have great potentials in catalysis owing to the maximized exposure of active sites and metal synergy. However, atomically precise design of these architectures is still challenging for the lack of practical approaches. Here, we report a defect-driven nanostructuring strategy through combining defect engineering of nitrogen-doped carbons and sequential metal depositions to prepare a series of Pt and Mo ensembles ranging from single atoms to sub-nanoclusters. When applied in continuous gas-phase decomposition of formic acid, the low-nuclearity ensembles with unique Pt3Mo1N3 configuration deliver high-purity hydrogen at full conversion with unexpected high activity of 0.62 molHCOOH molPt-1 s-1 and remarkable stability, significantly outperforming the previously reported catalysts. The remarkable performance is rationalized by a joint operando dual-beam Fourier transformed infrared spectroscopy and density functional theory modeling study, pointing to the Pt-Mo synergy in creating a new reaction path for consecutive HCOOH dissociations.

Enhancing Oxygen Evolution Reaction Performance in Prussian Blue Analogues: Triple-Play of Metal Exsolution, Hollow Interiors, and Anionic Regulation

Prussian blue analogs (PBAs) are promising catalysts for green hydrogen production. However, the rational design of high-performing PBAs is challenging, which requires an in-depth understanding of the catalytic mechanism. Here FeMn@CoNi core-shell PBAs are employed as precursors, together with Se powders, in low-temperature pyrolysis in an argon atmosphere. This synthesis method enables the partial dissociation of inner FeMn PBAs that results in hollow interiors, Ni nanoparticles (NPs) exsolution to the surface, and Se incorporation onto the PBA shell. The resulting material presents ultralow oxygen evolution reaction (OER) overpotential (184 mV at 10 mA cm-2 ) and low Tafel slope (43.4 mV dec-1 ), outperforming leading-edge PBA-based electrocatalysts. The mechanism responsible for such a high OER activity is revealed, assisted by density functional theory (DFT) calculations and the surface examination before and after the OER process. The exsolved Ni NPs are found to help turn the PBAs into Se-doped core-shell metal oxyhydroxides during the OER, in which the heterojunction with Ni and the Se incorporation are combined to improve the OER kinetics. This work shows that efficient OER catalysts could be developed by using a novel synthesis method backed up by a sound understanding and control of the catalytic pathway.

Atomically Dispersed Cobalt/Copper Dual-Metal Catalysts for Synergistically Boosting Hydrogen Generation from Formic Acid

The development of practical materials for (de)hydrogenation reactions is a prerequisite for the launch of a sustainable hydrogen economy. Herein, we present the design and construction of an atomically dispersed dual-metal site Co/Cu-N-C catalyst allowing significantly improved dehydrogenation of formic acid, which is available from carbon dioxide and green hydrogen. The active catalyst centers consist of specific CoCuN6 moieties with double-N-bridged adjacent metal-N4 clusters decorated on a nitrogen-doped carbon support. At optimal conditions the dehydrogenation performance of the nanostructured material (mass activity 77.7 L ⋅ gmetal -1  ⋅ h-1 ) is up to 40 times higher compared to commercial 5 % Pd/C. In situ spectroscopic and kinetic isotope effect experiments indicate that Co/Cu-N-C promoted formic acid dehydrogenation follows the so-called formate pathway with the C-H dissociation of HCOO* as the rate-determining step. Theoretical calculations reveal that Cu in the CoCuN6 moiety synergistically contributes to the adsorption of intermediate HCOO* and raises the d-band center of Co to favor HCOO* activation and thereby lower the reaction energy barrier.

2D MXenes polar catalysts for multi-renewable energy harvesting applications

The synchronous harvesting and conversion of multiple renewable energy sources for chemical fuel production and environmental remediation in a single system is a holy grail in sustainable energy technologies. However, it is challenging to develop advanced energy harvesters that satisfy different working mechanisms. Here, we theoretically and experimentally disclose the use of MXene materials as versatile catalysts for multi-energy utilization. Ti₃C₂T_X MXene shows remarkable catalytic performance for organic pollutant decomposition and H₂ production. It outperforms most reported catalysts under the stimulation of light, thermal, and mechanical energy. Moreover, the synergistic effects of piezo-thermal and piezo-photothermal catalysis further improve the performance when using Ti₃C₂T_X. A mechanistic study reveals that hydroxyl and superoxide radicals are produced on the Ti₃C₂T_X under diverse energy stimulation. Furthermore, similar multi-functionality is realized in Ti₂CT_X, V₂CT_X, and Nb₂CT_X MXene materials. This work is anticipated to open a new avenue for multisource renewable energy harvesting using MXene materials.

Acidic CO2-to-HCOOH electrolysis with industrial-level current on phase engineered tin sulfide

Acidic CO₂-to-HCOOH electrolysis represents a sustainable route for value-added CO₂ transformations. However, competing hydrogen evolution reaction (HER) in acid remains a great challenge for selective CO₂-to-HCOOH production, especially in industrial-level current densities. Main group metal sulfides derived S-doped metals have demonstrated enhanced CO₂-to-HCOOH selectivity in alkaline and neutral media by suppressing HER and tuning CO₂ reduction intermediates. Yet stabilizing these derived sulfur dopants on metal surfaces at large reductive potentials for industrial-level HCOOH production is still challenging in acidic medium. Herein, we report a phase-engineered tin sulfide pre-catalyst (π-SnS) with uniform rhombic dodecahedron structure that can derive metallic Sn catalyst with stabilized sulfur dopants for selective acidic CO₂-to-HCOOH electrolysis at industrial-level current densities. In situ characterizations and theoretical calculations reveal the π-SnS has stronger intrinsic Sn-S binding strength than the conventional phase, facilitating the stabilization of residual sulfur species in the Sn subsurface. These dopants effectively modulate the CO₂RR intermediates coverage in acidic medium by enhancing *OCHO intermediate adsorption and weakening *H binding. As a result, the derived catalyst (Sn(S)-H) demonstrates significantly high Faradaic efficiency (92.15 %) and carbon efficiency (36.43 %) to HCOOH at industrial current densities (up to -1 A cm^-2) in acidic medium.

Weakly aligned Ti3C2Tx MXene liquid crystals: measuring residual dipolar coupling in multiple co-solvent systems

Residual Dipolar Coupling (RDC), acquired relying on weakly alignment media, is highly valuable for the structural elucidation of organic molecules. Arising from the striking features of no background signals and low critical concentrations, two-dimensional (2D) liquid crystals (LCs) show the clear advantages of acting as alignment media to measure RDCs. So far, creating multisolvent compatible 2D LC media through a simple and versatile method is still formidably challenging. Herein, we report the rapid creation of aligned media based on the Ti₃C₂T_x MXene, which self-aligned in multiple co-solvents including CH₃OH-H₂O, DMSO-H₂O, DMF-H₂O, and acetone-H₂O. We demonstrated the applicability of these aligned media for the RDC measurement of small organic molecules with different polarities and solubilities. Notably, Ti₃C₂T_x MXene LCs without chemical modification enabled RDC measurements on aromatic molecules. The straightforward preparation of Ti₃C₂T_x media and its compatibility with multiple solvents will push RDC measurement as a routine methodology for structural elucidation. It may also facilitate the investigation of solvation effects on conformational dynamics.

Modulation of Surface Ti-O Species in 2D-Ti3C2TX MXene for Developing a Highly Efficient Electrocatalyst for Hydrogen Evolution and Methanol Oxidation Reactions

Developing cost-effective and earth-abundant noble-metal-free electrocatalysts is imperative for the imminent electrochemical society. Two-dimensional Ti₃C₂T_X (MXene) exhibits tunable properties with high electrical conductivity and a large specific surface area, which improve its electrochemical performance. Herein, the low-temperature annealing method is used to enrich MXene with a maximum number of Ti-O terminals without formation of titanium dioxide (TiO₂) under neutral pH conditions. MXene annealed at 200 °C is found to have a large number of Ti-O termination groups, resulting in a large electrochemically active surface area and increased active sites (-O termination groups) and hence excellent electrocatalytic performance compared to other samples as well as previous reported work. The optimized sample is found to show the lowest overpotential value of 0.07 V at 10 mA cm^-2 and a Tafel slope of 0.15 V dec^-1 toward the hydrogen evolution reaction (HER), whereas for the methanol oxidation reaction (MOR), the current density is 18.08 mA cm^-2, and the onset potential is -0.51 V. In addition, it also shows long-term stability and durability toward HER as well as MOR.

Boosting Charge Transfer Efficiency by Nanofragment MXene for Efficient Photoelectrochemical Water Splitting of NiFe(OH)x Co-Catalyzed Hematite

The use of oxygen evolution co-catalysts (OECs) with hematite photoanodes has received much attention because of the potential to reduce surface charge recombination. However, the low surface charge transfer and bulk charge separation rate of hematite are not improved by decorating with OECs, and the intrinsic drawbacks of hematite still limit efficient photoelectrochemical (PEC) water splitting. Here, we successfully overcame the sluggish oxygen evolution reaction performance of hematite for water splitting by inserting zero-dimensional (0D) nanofragmented MXene (NFMX) as a hole transport material between the hematite and the OEC. The 0D NFMX was fabricated from two-dimensional (2D) MXene sheets and deposited onto the surface of a three-dimensional (3D) hematite photoanode via a centrifuge-assisted method without altering the inherent performance of the 2D MXene sheets. Among many OECs, NiFe(OH)x was selected as the OEC to improve hematite PEC performance in our system because of its efficient charge transport behavior and high stability. Because of the great synergy between NFMX and NiFe(OH)x, NiFe(OH)x/NFMX/Fe2O3 achieved a maximum photocurrent density of 3.09 mA cm-2 at 1.23 VRHE, which is 2.78-fold higher than that of α-Fe2O3 (1.11 mA cm-2). Furthermore, the poor stability of MXene in an aqueous solution for water splitting was resolved by uniformly coating it with NiFe(OH)x, after which it showed outstanding stability for 60 h at 1.23 VRHE. This study demonstrates the successful use of NFMX as a hole transport material combined with an OEC for highly efficient water splitting.

Rapid Ferroelectric-Photoexcited Bacteria-Killing of Bi4Ti3O12/Ti3C2T x Nanofiber Membranes

In this study, an antibacterial nanofiber membrane [polyvinylidene fluoride/Bi4Ti3O12/Ti3C2T x (PVDF/BTO/Ti3C2T x )] is fabricated using an electrostatic spinning process, in which the self-assembled BTO/Ti3C2T x heterojunction is incorporated into the PVDF matrix. Benefiting from the internal electric field induced by the spontaneously ferroelectric polarization of BTO, the photoexcited electrons and holes are driven to move in the opposite direction inside BTO, and the electrons are transferred to Ti3C2T x across the Schottky interface. Thus, directed charge separation and transfer are realized through the cooperation of the two components. The recombination of electron-hole pairs is maximumly inhibited, which notably improves the yield of reactive oxygen species by enhancing photocatalytic activity. Furthermore, the nanofiber membrane with an optimal doping ratio exhibits outstanding visible light absorption and photothermal conversion performance. Ultimately, photothermal effect and ferroelectric polarization enhanced photocatalysis endow the nanofiber membrane with the ability to kill 99.61% ± 0.28% Staphylococcus aureus and 99.71% ± 0.16% Escherichia coli under 20 min of light irradiation. This study brings new insights into the design of intelligent antibacterial textiles through a ferroelectric polarization strategy.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s42765-022-00234-8.

Rational design of highly efficient MXene-based catalysts for the water-gas-shift reaction

Water molecules linked by hydrogen bonds are responsible for the high efficiency of bi-functional catalysts for the water-gas-shift (WGS) reaction because water can act as a proton transfer medium. Herein, we propose an associative pathway for the WGS reaction assisted by water to realize hydrogen production. Based on this pathway, we show by first-principles calculations that a large family of oxygen-terminated two-dimensional transition metal carbides and nitrides (MXenes) deposited on Au clusters are promising catalysts for the WGS reaction. Remarkably, the rate-determining barriers for *CO → *COOH on Au/M_n+1X_nO₂ are in the range from 0.15 eV to 0.39 eV, indicating that WGS can occur at much lower temperatures. Furthermore, a comprehensive microkinetic model is constructed to describe the turnover frequencies (TOF) for the product under the steady-state conditions. More importantly, there is a perfect linear scaling relationship between the rate-determining barriers of the WGS and the free energy of the adsorbed hydrogen. Besides, the potential energy diagrams for CO reforming reveal that the F terminations introduced in experiments have only a slight influence on the catalytic performance of the oxygen-terminated MXenes. Our work not only opens a new avenue towards the WGS reaction but also provides many ideal catalysts for hydrogen production.

Enhanced uranium photoreduction on Ti3C2Tx MXene by modulation of surface functional groups and deposition of plasmonic metal nanoparticles

Photocatalytic reduction of soluble hexavalent uranium (U(VI)) is a novel and efficient avenue to enriching U(VI), where the free U(VI) is firstly bound on the surface of photocatalysts and then reduced to insoluble tetravalent uranium (U(IV)) by photoelectrons. Therefore, constructing the efficient U(VI) binding sites on photocatalysts is an efficient strategy to boost catalytic activity toward U(VI) photoreduction. Herein, we successfully constructed an efficient catalyst for U(VI) photoreduction by depositing Ag nanoparticles on Ti₃C₂T_x MXene with abundant U(VI) binding sites (Ag/Ti₃C₂T_x-O). Impressively, the U(VI) extracting mass over Ag/Ti₃C₂T_x-O under light reached up to 1257.6 mg/g in 120 min, which was almost 11 times as high as that without light. Further mechanistic studies indicated that the U(VI) binding sites on Ti₃C₂T_x MXene in Ag/Ti₃C₂T_x-O were beneficial to the reduction of U(VI) by significantly decreasing its reduction potential. More importantly, hot electrons generated by Ag nanoparticles were transferred into the binding sites to easily reduce the bound U(VI), resulting in the remarkable performance of Ag/Ti₃C₂T_x-O during U(VI) enrichment.

Single-Atomic Ruthenium Active Sites on Ti3 C2 MXene with Oxygen-Terminated Surface Synchronize Enhanced Activity and Selectivity for Electrocatalytic Nitrogen Reduction to Ammonia

Downsizing the catalyst to atom scale offers an effective way to maximize the atom utilization efficiency for electrocatalytic nitrogen reduction reaction (NRR). Herein, single-atomic ruthenium (Ru) anchored on a chemically activated Ti₃ C₂ with O-terminated groups (Ti₃ C₂ O) was designed to catalyze the NRR process. The catalyst achieved a superior activity and selectivity with ammonia yield rate of 27.56 μg h^-1  mg^-1 and faradaic efficiency of 23.3 % at a low potential of -0.20 V versus the reversible hydrogen electrode. According to the atomic resolution images from aberration-corrected scanning transmission electron microscopy, Ru sites on Ti₃ C₂ O achieved good dispersion on atomic scale. X-ray photoelectron spectroscopy analysis further demonstrated that the O-termination groups were successfully activated. Density functional theory calculations combined with experiments revealed that single Ru sites binding to four oxygen were the main reaction centers that permitted the hydrogenation of *NNH₂ to *NHNH₂ in a novel distal/alternating hybrid path while reducing the energy barrier of the potential-limiting step to 0.78 eV from 0.96 eV in the distal path alone or 1.18 eV in the alternating path alone, thereby significantly promoting the NRR dynamics.

Layer-by-Layer Assembly-Based Heterointerfaces for Modulating the Electronic Properties of Ti3C2Tx MXene

Two-dimensional (2D) transition-metal carbides (MXenes) are emerging as promising materials for a wide range of applications owing to their intriguing electrical, optical, and optoelectronic properties. However, the modulation of metallic Ti₃C₂T_x MXene electronic properties is the key challenge to fabricate functional nanoelectronic devices. Here, we demonstrate a solution-processable route to fabricate Ti₃C₂T_x MXene/CuI nanoparticle heterointerfaces by employing a layer-by-layer assembly process. The charge transfer at the heterointerfacial assembly is monitored qualitatively from the quenched photoluminescence emission of CuI. The stable electrical conductivity and consistent Raman spectra of the 3-LBL assembly (three sequential stacks of CuI/MXene) signify the oxidation stability of Ti₃C₂T_x thin films even after exposure to the ambient environment for 2 months. Furthermore, the 3-LBL assembly exhibited a three-dimensional (3D) variable-range hopping-based electrical conduction in the temperature range 2 ≤ T < 100 K, contrary to the weak localized transport phenomenon in Ti₃C₂T_x MXene. The difference in charge transport mechanism is supported by distinct magnetoresistance (MR) of the Ti₃C₂T_x MXene (negative MR, -0.4%) and 3-LBL assembly (positive MR, 1.6%). Therefore, the modulated electrical transport and superior oxidation stability of the Ti₃C₂T_x MXene in the 3-LBL assembly have the potential to develop next-generation optoelectronic and memory devices.

Surface functionalized highly porous date seed derived activated carbon and MoS2 nanocomposites for hydrogenation of CO2 into formic acid

In recent years, substantial progress has been made towards developing effective catalysts for the hydrogenation of CO₂ into fuels. However, the quest for a robust catalyst with high activity and stability still remains challenging. In this study, we present a cost-effective catalyst composed of MoS₂ nanosheets and functionalized porous date seed-derived activated carbon (f-DSAC) for hydrogenation of CO₂ into formic acid (FA). As-fabricated MoS₂/f-DSAC catalysts were characterized by FE-SEM, XRD, Raman, FT-IR, BET, and CO₂-TPD analyses. At first, bicarbonate (HCO₃^-) was successfully converted into FA with a high yield of 88% at 200 °C for 180 min under 10 bar H₂ atmosphere. A possible reaction pathway for the conversion of HCO₃^- into FA is postulated. The catalyst has demonstrated high activity and long-term stability over five consecutive cycles. Additionally, MoS₂/f-DSAC catalyst was effectively used for the conversion of gaseous CO₂ into FA at 200 °C under 20 bar (CO₂/H₂ = 1:1) over 15 h. The catalyst exhibited a remarkable TOF of 510 h^-1 with very low activation energy of 12 kJ mol^-1, thus enhancing the catalytic conversion rate of CO₂ into FA. Thus, this work demonstrates the MoS₂/f-DSAC nanohybrid system as an efficient non-noble catalyst for converting CO₂ into fuels.

Modulation of the Electronic Properties of MXene (Ti3C2Tx) via Surface-Covalent Functionalization with Diazonium

The physical and chemical properties of MXenes are strongly dependent on surface terminations; thus, the tailoring of surface functional groups in two-dimensional transition-metal carbides (MXenes) may extend the applicability of these compelling materials to a wider set of fields. In this work, we demonstrate the chemical modification of Ti₃C₂T_x MXene via diazonium covalent chemistry and the subsequent effects on the electrical properties of MXene. The 4-nitrophenyl group was grafted onto the surface of MXene through a solid-liquid reaction, which was confirmed by various characterization methods, including X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, electron energy loss spectroscopy, atomic force microscopy, and transmission electron microscopy. The degree of modification of MXene is expediently tunable by adjusting the concentration of the diazonium salt solution. The work function of functionalized MXene is modifiable by regulating the quantity of grafted diazonium surface groups, with an adjustable range of around 0.6 eV. Further, in this study, the electrical properties of modified MXene are investigated through the fabrication of field-effect-transistor devices that utilize modified MXene as a channel material. It was demonstrated that with increasing concentration of 4-nitrophenyl groups grafted onto the surface the on/off current ratio of the modified MXene was improved to as much as 3.56, with a corresponding decrease in conductivity and mobility. The proposed approach of controlled modification of surface groups in Ti₃C₂T_x may imbue Ti₃C₂T_x with favorable electronic behaviors and demonstrate prospects for use in electronic field applications.