Vacancies play a pivotal role in determining the physical and chemical properties of materials. Introducing vacancies into two-dimensional (2D) materials offers a promising strategy for developing high-performance electrode materials for electrochemical energy storage. Herein, a facile top-down strategy is employed to create V-based MXenes with tunable vacancy concentrations, achieved by designing the precursor (V1-xCrx)2AlC (x = 0.05, 0.1, 0.3) MAX phase and precisely controlling the etching process. Systematic investigations reveal that introducing a moderate concentration of Cr-induced vacancies significantly enhances both the capacitance and rate performance of V-based MXenes. Specifically, V1.9CTz achieves a capacitance of 760 F g-1, far exceeding the 420 F g-1 of vacancy-free V2CTz MXene. In contrast, an excessively high vacancy concentration leads to deteriorated electrochemical performance and compromised structural stability. This work illustrates that defect engineering is a powerful approach to tailor the electrochemical properties of MXenes, offering a framework for designing next-generation MXene-based energy storage systems.

Bulk molybdenum carbides catalyze the Fischer-Tropsch (FT) reaction but display low selectivity to liquid hydrocarbons. Understanding which structural features of Mo carbide-based catalysts correlate with a selectivity to C5+ products requires detailed structure-performance studies. Here, we investigate the effect of Mo vacancies (VMo) on the FT performance of the two-dimensional (2D) i-MXene Mo4/3CT x , where T x are the surface passivating groups. We show that the initially ordered VMo sites, constituting ca. one-third of all Mo sites in Mo4/3CT x , undergo clustering already at 250 degrees C under H2. Following VMo clustering, lattice carbon is released as CH4 and CO, creating carbon vacancies (VC), which also form in Mo4/3CT x under FT conditions at 330 degrees C. The presence of VC sites is associated with a high selectivity towards methane and C2-C4 alkanes and a reduced selectivity for higher, C5+, alkanes. Mo4/3CT x converts CO at a rate of ca. 60-80 mmolCO (gcat h)-1, which is comparable to that of the T x -free 2D-Mo2C and 2D-Mo2C1-x reference catalysts that only contain a Mo+2 (carbidic) electronic state. However, post-FT analysis of Mo4/3CT x shows that it contains Mo+2, Mo+4, and Mo+5 states, possibly because oxygen atoms populate the VC sites and do not block the Mo+2 active sites at the surface. Mo4/3CT x undergoes a phase transition to cubic alpha-MoC1-x at 400 degrees C under H2, which leads to a continuous deactivation under FT conditions, suggesting, more generally, that pretreatment protocols for MXenes should be carefully optimized to unlock their potential in thermal catalysis.

On-surface synthesis has revolutionized the construction of functional nanostructures. Thus far, however, the concept of polymer tacticity has remained unexplored in this context. Tacticity impacts polymer properties, which in turn governs polymer applications. An all-syn arrangement of substituents in isotactic polymers, for instance, may lead to highly crystalline, high-strength materials. Thus, specialized methods have been developed involving chiral catalysts or ligands to control polymer tacticity. In this work, we demonstrate that two-dimensional surface confinement of on-surface polymerization can be combined with preorganization of monomers to synthesize isotactic polymer chains in the absence of a chiral auxiliary. To establish the concept, we synthesized a corannulene-substituted terphenylene monomer capable of C-H<middle dot><middle dot><middle dot>pi bonding, aromatic stacking, and van der Waals interactions. The weak intermolecular interactions steer the monomer into organized double strands, with the corannulene substituents oriented only on one side of the strand on Ag(111). A subsequent Ullmann polymerization of the self-assembled monomers leads to the formation of double-stranded isotactic polyphenylenes. The stereoregularity further allows the duplex to arrange into higher-order crystalline domains. Finally, the synthetic polymer duplex can be mechanically unzipped into individual isotactic polymer chains through scanning tunneling microscopy tip manipulation.

Herein we report on the facile, one-pot, scalable, room-temperature (RT) synthesis of quantum-confined, 1D lepidocrocite (1DL) titanate nanofilaments (NFs)-by reacting titanium oxysulfate with tetramethylammonium hydroxide, both cheap and ubiquitous precursors-for times that are significantly shorter than previously reported synthesis methods that required 80 degrees C and, in some cases, days of reaction time. Samples are characterized by X-ray diffraction, Raman and X-ray photoelectron spectroscopies, small angle X-ray scattering, SAXS, scanning and transmission electron microscopy, revealing no significant differences from the higher-temperature synthesis methods, except in the fact that the lengths of the NFs varied. A parallelepiped model, one lepidocrocite layer thick, is used to fit the SAXS patterns of colloidal suspensions, CSs. The ribbons made from material reacted for 4 h are approximate to 33 nm long and 2.7 nm wide; after 12 h reaction time their lengths are >100 nm and their widths are approximate to 3.0 nm. This is the first diffraction-based evidence that the ribbons actually lengthen with time. Powders made using this method can be dried and re-dispersed in water. The resulting CS adsorbed more rhodamine 6G than any previous 1DL-based CS tested. From Tauc plots, we obtain bandgap energies of approximate to 4.0 eV, which is only weakly dependent on reaction time. This work establishes a simple, one-pot, RT, atmospheric-pressure, inexpensive and highly scalable method to synthesize quantum-confined titanate-based nanomaterials optimized for photocatalytic, environmental, and optoelectronic applications among others.

The persistence of antibiotics such as tetracycline in aquatic systems poses severe environmental and health risks by potential antimicrobial resistance. To address this, a hybrid photo-electro-Fenton oxidation system based on MXene-derived electrodes was developed for efficient tetracycline degradation. The integration of photo, electro, and Fenton processes synergistically enhances hydroxyl radical (center dot OH) generation and charge-carrier separation, ensuring superior removal efficiency. The cathode was synthesized via a Schiff base formation method, which facilitates functionalization of Ti3C2Tx MXene with ferrocene, as confirmed by X-ray diffraction, X-ray photoelectron spectroscopy, and UV-visible spectrometry. The Ti3C2-TiO2 photoanode was fabricated by electrochemical oxidation of Ti3C2Tx. The photocatalytic properties of anatase TiO2, when combined with Ti3C2Tx, create a Schottky junction that significantly improves charge separation, thereby enhancing the photo-electrocatalytic activity of the system. The hybrid photo-electro-Fenton (PEF) system demonstrates a substantial enhancement in tetracycline removal efficiency (similar to 90%) compared to unmodified Ti3C2Tx-based electrodes (similar to 46%). Furthermore, the Ti3C2-TiO2 Schottky photoanode showed enhanced removal efficiency over a commercial P25-based photoanode for photocatalytic degradation. Through this hybrid PEF oxidation system, the removal efficiencies achieved above 90% in neutral and acidic pH, indicating significant efficacy for the advanced oxidation process. The transformation products formed during the PEF process were analyzed with liquid chromatography coupled with high-resolution mass spectrometry, showing breakdown of tetracycline and decreasing ecotoxicity with increasing treatment time. Moreover, the MXene-derived electrode system demonstrates stability and consistent degradation performance over numerous cycles, making it a promising material for environmental remediation applications.

Two-dimensional transition metal carbo-chalcogenides (TMCCs) represent a novel class of layered materials with tunable electronic structures and high chemical versatility, making them promising candidates for energy and environmental applications. In this work, the structural and thermochemical properties of a series of TMCCs are investigated. High-temperature oxidative solution calorimetry, conducted at 800 degrees C in molten sodium molybdate (3Na(2)O & centerdot;4MoO(3)), was used to determine the enthalpies of oxidation and formation from the elements at 25 degrees C, enabling a comprehensive thermodynamic assessment. Comparison of these enthalpies reveals distinct stability trends across the series. Cu0.67Nb2S2C exhibits the most exothermic oxidation enthalpy, indicating a strong thermodynamic driving force for oxidation and reduced resistance to oxidative degradation. Its positive enthalpy of formation from the elements suggests lower thermodynamic stabilization relative to the other compositions, consistent with the energetic influence of Cu and residual Fe within the Nb-S-C framework. In contrast, delaminated Nb2S2C exhibits poor oxidative stability but a highly exothermic enthalpy of formation, consistent with a defect-rich, metastable structural state rather than enhanced lattice stability. These findings establish correlations between crystal structure and energetic stability, providing insights into the design of robust TMCC-based materials for advanced energy and environmental technologies.

M5X4, the newest and thickest structures in the MXene family, shows promise as mechanically robust nanomaterials. However, the essential role of oxide in their synthesis is poorly understood, which poses a challenge for discovering new M5AX4 MAX phase precursors. One possibility is that oxygen dissolves into the carbon sublattice, forming stable oxycarbide layers within the MAX phase. Herein, we investigate the layer-by-layer elemental composition of three M5AX4 compositions: Ti2.5Ta2.5AlC4, Ti2.675Nb2.325AlC4, and Mo4VAlC4. By modeling the structural stability of each composition, we investigate the possible stabilizing role of oxygen. To guide future application of M5X4 MXenes, we also calculate the electronic structure and mechanical properties of the parent M5AX4 MAX phases. This work clarifies the role of oxygen incorporation into MAX phases and its implications for the synthesis and potential applications of their MXene derivatives.

Nickel-iron layered double hydroxides (NiFe-LDH) show excellent activity, their poor conductivity limits practical implementation in electrolysers. Previous reports have shown that combining nickel-iron layered double hydroxide (NiFe-LDH) materials with MXenes significantly increases the oxygen evolution reaction (OER) activity, however, the rationale behind this is not clear. Herein, we report operando X-ray absorption spectroscopy (XAS) of V2CTx MXene-enhanced NiFe-LDH catalysts, revealing important insights into MXene-hydroxide synergy. Operando X-ray absorption reveals a two-step vanadium transformation: V2CTx initially oxidizes during hydrothermal synthesis, acting as a reducing agent that promotes ordered Fe2NiO4 formation, then undergoes further oxidation under OER conditions, to form mixed-valence V3+/4+/5+ oxide species that establish transient electronic coupling with NiFe active sites. Finally, the NiFe@V2CTx composites were tested in an Anion Exchange Membrane (AEM) electrolyzer over 144 h of continuous operation at 500 and 1000 mA cm-2, with NF25 achieving degradation rates as low as 0.76 mV h-1 at 1000 mA cm-2. Postmortem tests reveal that V2CTx undergoes progressive dissolution during operation, yet the structurally modified NiFe electrodes retain superior activity relative to the unmodified reference throughout the full test duration. These findings demonstrate that V2CTx functions beyond passive conductive support as an active electronic participant whose structural legacy sustains durable performance even after vanadium leaching.

The development of aqueous ammonium-ion batteries (AAIBs) requires electrode materials that combine high NH4 + storage capacity with rapid and reversible ion transport. Herein, a metal-vacancy MXene/polyaniline (Mo4/3CTz/PANI) composite is reported, in which the pseudocapacitive response is synergistically activated by introducing 0.1 m H2SO4 into 1 m (NH4)2SO4 electrolyte. This proton-assisted modulation enables rapid and reversible NH4 +/H3O+ co-intercalation, in contrast to the negligible ion insertion observed in the absence of H2SO4. Combined experimental and density-functional theory (DFT) analyses reveal that proton doping significantly improves the electronic conductivity of PANI and induces a reversible Mo6+/Mo5+ redox transition during cycling, which dynamically modulates the NH4 + adsorption energy (from -4.155 to -4.567 eV), thus facilitating both intercalation and deintercalation of NH4 +. As a result, the composite achieves a high specific capacity of 245 mAh g-1 at 0.1 A g-1, with excellent capacity retention of 84.2% after 11,000 cycles at 1.0 A g-1. Furthermore, the MnO2/CNTs||M:P = 5:1 full cell delivers a high energy density of 81.6 Wh kg-1 and a power density of 16 000 W kg-1. This work highlights a promising strategy for advancing MXene-based electrodes via proton-enhanced ion storage mechanisms, paving the way for high-performance AAIBs.