Ordered double transition-metal (DTM) MXenes are a subfamily of two-dimensional (2D) carbides, nitrides, and carbonitrides, predicted to outperform single-metal MXenes in hydrogen evolution reaction (HER) catalysis due to the synergistic effect of two metals and their nonmetal (X) sublattice tailoring (X = C, N), resulting in tunable electronic structures. However, all synthesized DTM MXenes to date contain only carbon in the X sublattice. Here, we report the synthesis of a series of out-of-plane ordered DTM carbonitride MXenes (o-MXenes), Mo2Ti(CN)2Tx and Mo2Ti2(CN)3Tx, to systematically investigate the role of carbon to nitrogen ratio. To determine the optimal nitrogen content, we first evaluated the HER activity of the Mo2TiC2-yNyTx MXenes with density functional theory calculations and identified that 0.3 to 0.6 mol of nitrogen give enhanced performance compared to the carbide. We next synthesized and characterized 11 carbonitride MXenes with varying their C:N ratios and found nitrogen incorporation enhances HER activity compared to their carbide counterparts. Among them, Mo2TiC2-yNyTx MXene with 0.6 mol of nitrogen (y = 0.6) achieved the best performance, with an overpotential of ∼155 mV at 10 mA/cm2 under acidic conditions, compared with ∼236 mV for Mo2TiC2Tx. Our experimental and computational findings indicate that carbonitrides with ∼25-30 atom % nitrogen outperform all other o-MXene counterparts, with the improved performance arising from nitrogen-induced modulation of the electronic structure. This study identifies nonmetal sublattice control as a critical frontier in optimizing MXenes for sustainable energy applications.

Surface groups are central to the properties of MXenes, yet their role in optical anisotropy remains largely unexplored. Here, we use a topochemical route to synthesize single crystals of stacked Ti3C2Cl2 and hybrid organic-inorganic MXenes (h-MXenes) with lateral sizes of 38-75 μm, rotational registry, and tunable interlayer spacing. Solid-state NMR spectroscopy shows that topochemical substitution generates mixed amido, imido, and hydride surface motifs, which modify the electronic structure of the Ti3C2 inorganic core. Imaging spectroscopic ellipsometry with micron-scale spatial resolution enables reconstruction of the complex dielectric tensor of individual multilayer crystals. Ti3C2Cl2 exhibits a type-II hyperbolicity above 930 nm, whereas h-MXenes do not display hyperbolicity within the measured 300-1700 nm window, instead showing reduced in-plane conductivity, suppressed out-of-plane light absorption, and a chain-length-dependent blue shift of a near-infrared absorption feature. These results demonstrate topochemical surface modification as a direct handle for engineering MXenes as surface-programmable optical media.

The electronic structure of materials contains the key features describing the energetic interaction of adsorbed species on their surfaces. The dimensionality of electronic feature space, however, is incredibly high. Primary features of the electronic structure must be identified to define models that are otherwise nonapplicable due to their complexity. In this paper, we discuss exemplary efforts as they pertain to surface science and heterogeneous catalysis describing electronic structure effects on adsorption. We discuss these examples under three categories: first principles-based physically informed models, correlations found from first principles data, and fully data-driven approaches. While there exists plenty of overlap between these categories, the primary philosophies of the three methodologies are compared and contrasted. Based on this discussion, a perspective on the path forward to continue advancing atomic understanding of chemisorption is presented. Specifically, the unique benefits and drawbacks of each philosophy are discussed, and the union of a collective effort from all fronts is proposed. While there are advantages and disadvantages of approaching a problem from either a first principles or data driven approach, the insights gained from all efforts have proven to be useful to the catalysis community in illuminating the unknown regarding atomic-scale catalytic reaction phenomena.

A fundamental understanding of nanocrystal morphology is essential for predicting and controlling materials behavior in experimentally unexplored growth environments. Machine learning interatomic potentials (MLIPs) offer a promising route to accelerate morphology prediction by enabling efficient sampling of large configurational space at approaching firstprinciples accuracy, but their practical use to reactive nanocrystal materials remains limited due to uncertain transferability and poorly defined training-data requirements. To address these challenges, we present the AutoNanoShape framework, which combines stepwise global and local sampling with iterative first-principles refinement to develop data-efficient MLIPs for oxide surface and nanocrystal morphology prediction. AutoNanoShape achieves surface free energy accuracies with an MAE of ∼0.1 J/m2, enabling prediction of nanocrystal morphologies for the mineral oxide systems studied here across growth environments ranging from ambient to extreme conditions. We further find that training on surfaces with higher adsorbate coverages transfers more effectively to surfaces with lower adsorbate coverages than the reverse. AutoNanoShape reduces the number of first-principles calculations by∼90% compared to brute-force sampling. The resulting workflow also provides practical guidance for dataset construction, including approximate training-set sizes for both de novo model development and fine-tuning, offering a general strategy for efficient MLIP development in reactive nanocrystal systems.

Iridium oxide (IrO2) is the state-of-the-art electrocatalyst for water oxidation in electrolyzers, yet it suffers from instability under operating conditions. Here, we combine first-principles modeling with in situ liquid-phase transmission electron microscopy and device-scale characterization to resolve the atomic-scale morphology and dissolution dynamics of IrO2 nanocrystals. Our computational Wulff constructions uniquely incorporate high-index facets, providing new insights into thermodynamic facet-dependent stability under operating conditions. Atomically resolved studies reveal multiple distinct collective dissolution pathways, including high-index facet formation, monolayer reconstruction, step-edge formation, and monolayer delamination on {110} surfaces. Device-scale studies confirm that electrochemical operation results in high-index facet formation. Ab initio molecular dynamics simulations further show that initial dissolution kinetics are facet-dependent. These findings highlight how combining in situ imaging with first-principles modeling reveals atomic-scale dynamics that influence material performance.

The ability of energy storage materials to host ions and electrons is fundamental to their applications in batteries and pseudocapacitors. While many transition metals can accommodate electrons, only a few elements, like oxygen and chalcogens, can host cations. Even in MXenes, oxygen surface terminations are believed to enable redox activity and proton storage. In this work, using inelastic neutron scattering (INS) and atomic-resolution secondary ion mass spectrometry (SIMS), we reveal that, in addition to the oxygen surface groups, intercalated protons are stored in the interstitial sites of the Ti 3 C 2 T x MXene nanosheet, forming C-H bonds. We further showed its effect on the crystal structure evolution using synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS) and pair distribution function (PDF). Energetics of the proton at various anion sites were assessed computationally. This work demonstrates the potential of carbon sublattices as proton storage sites, suggesting that 2D transition-metal carbides should be considered for proton-based energy storage.

Perovskite oxides doped with transition metals play a critical role in several contemporary applications, such as electro- and photocatalysis and the synthesis of coke- and sintering-resistant catalysts. In this study, we present a systematic investigation of the doping preferences and surface segregation trends of 4d transition metals in ATiO(3) (A = Ca, Sr, Ba) perovskites using first-principles density functional theory (DFT) calculations. We further consider the influence of different facets and terminations such as (001)-AO/BO2, (110)-ABO/O-2, and (111)-AO(3)/B, and the application of tensile and compressive strain on the segregation behavior of these dopants. Results indicate that doping and segregation behaviors depend strongly on the exposed facet and applied strain, but less so on the host perovskite oxide. Additionally, we find that early transition metals strongly prefer bulk and subsurface doping, while later metals, beginning with Ru, segregate to the surface. We rationalize and corroborate the computed trends against a comprehensive set of previously published experimental data, highlighting the critical role of facet, termination, and strain in determining doping and segregation behavior. The results serve as a suitable starting point for designing perovskite systems with tailored properties.

Oxide surfaces play a crucial role in large-scale applications, including catalysis and electronics. Despite their common use, their surface stability remains a subject of ongoing debate, particularly in computational studies where pristine bulk-terminated models are often used instead of reconstructed, defective, or compositionally modified structures. Here, we use ab initio thermodynamics with explicit phonon-derived vibrational free-energy contributions to evaluate SrTiO3 surface structures beyond pristine bulk-terminated models, including surface and subsurface defects, double-layer reconstructions, and oxygen coverages across the (001) and (110) facets. At 298 K, the pristine bulk-terminated (001)-AO surface remains stable over most of the allowed chemical-potential window, although Sr-related surface defects become competitive near the TiO2-rich boundary, while the (001)-BO2 termination favors defect-containing structures under selected Sr and O chemical-potential conditions. At higher temperatures, these defects become more dominant: at 800 K, the pristine bulk-terminated (001)-BO2 surface is no longer thermodynamically favored, while at 1300 K, the pristine bulk-terminated (001)-AO surface is stable only within a narrow chemical-potential window. In contrast, the SrO and TiO2 double-layer reconstructions remain outside the allowed thermodynamic stability window throughout the temperature range examined, indicating that their experimentally observed formation is likely governed by factors beyond the equilibrium thermodynamic framework used here. Moreover, the (110) facet favors defect-containing and oxygen-modified structures over pristine bulk-terminated surfaces across the conditions considered. Together, these findings help reconcile idealized computational models with experimentally observed SrTiO3 surface behavior, while complementing established experimental and theoretical reconstruction models by showing that once phonon-derived vibrational free energies are explicitly included, defect-containing and oxygen-modified surface structures become thermodynamically preferred under experimentally relevant thermal and chemical conditions.

MXenes are a rapidly expanding family of 2D materials known for their unique tunability, yet studies of their edges remain sparse compared to their better-characterized basal planes. Herein, we use density functional theory (DFT) to computationally investigate the structural and chemical properties of Ti2CTx nanoribbons and 2D-nanoparticles. Our findings reveal that under-coordinated edge atoms exhibit stability and reactivity distinct from that forming the basal planes, driven by edge symmetry and environmental chemical potentials. Specifically, the constructed stability diagrams and Wulff constructions illustrate how 2D-nanoparticle morphologies and edge terminations evolve under different hydrogen chemical potentials. Reducing conditions favor fluorine terminations, while oxidizing environments stabilize oxygen-terminated edges. Hydrogen adsorption analysis highlights unique edge-specific chemistries, with certain terminations achieving hydrogen evolution reaction (HER) overpotentials comparable to those of the previously identified basal planes of Mo2C MXene. Notably, our study identifies the {010} and {110} edges as highly active catalytic sites under specific conditions, emphasizing the role of edge under-coordination in dictating catalytic behavior. These results underscore the potential of MXene edges for tailoring properties beyond the basal plane, providing pathways for designing next-generation materials for catalysis, energy, and environmental applications.

The escalating demands of industrialization and development underscore the necessity for an efficient and scalable carbon capture and storage (CCS) methodology. Mineral carbonation of MgO presents itself as a promising solution due to its considerable theoretical capacity for CO2 adsorption. However, the sluggish kinetics of the carbonation process pose a significant challenge. Consequently, a comprehensive understanding of the structural and chemical alterations occurring during carbonation is imperative for material design. In this study, we conduct a thorough structural and chemical investigation of the MgO (sourced from different mine tailings) carbonation process using electron microscopic techniques. Our findings demonstrate that treating MgO with polar solvents enhances its degree of carbonation significantly, offering a promising avenue for improvement. Moreover, we observe a particle size dependency in MgO carbonation and note that the inclusion of additional materials, such as Si-based compounds, further accelerates the carbonation. Density functional theory (DFT) calculations provide insight into surface functionalization as a result of solvent treatment and its mechanistic effect on the origin of the enhanced carbonation of polar solvent-treated MgO, revealing a stronger interaction between CO2 and the treated MgO (100) surface as compared to the non-polar solvent treated surfaces. These discoveries showcase an alternative approach for enhancing MgO carbonation, thereby offering a potential method for sequestering atmospheric CO2 more effectively using mine waste rich in MgO.

In compositionally complex materials, there is controversy on the effect of enthalpy versus entropy on the structure and short-range ordering in so-called high-entropy materials. To help address this controversy, we synthesized and probed 40 M4AlC3 layered carbide phases containing two to nine metals and found that short-range ordering from enthalpy was present until the entropy increased enough to achieve complete disordering of the transition metals in their atomic planes. We transformed all of these layered carbide phases into two-dimensional (2D) sheets and showed the effects of the order versus disorder on their surface properties and electronic behavior. This study suggests the key effect that the competition between enthalpy and entropy has on short-range order in multicompositional materials.

Rapid and accurate detection of molecular species with a high degree of selectivity and sensitivity constitutes the ultimate goal of designing sensors for various applications, from studying nutrients in soil-water systems to assessing physiological conditions in human health. With recent progress, two-dimensional (2D) transition metal carbides/nitrides have shown great potential for applications such as energy storage and electromagnetic interference shielding. However, the fundamental electrochemical studies and subsequent applications of MXenes for molecular sensing are still in their infancy. Here, we use 2D sheets of titanium carbide (Ti3C2Tx) MXene for electrochemical detection of phosphate, a key molecule for sustainability of life on earth, and major contributor to environmental pollution. Ti3C2Tx MXene sensors were demonstrated to be highly selective towards phosphate with a sensing range from 1 µM to 350 µM and a limit of detection (LOD) of 1.31 µM. This work lays the foundation for molecular sensing using Ti3C2Tx MXene in complex environments. Thiba Nagaraja and colleagues report atomically thin two-dimensional Ti3C2Tx MXene material as an electrochemical phosphate sensor. This study lays the groundwork to further understand the properties of Ti3C2Tx MXene for development of in-field electrochemical phosphate sensors.

Biomass incorporates carbon captured from the atmosphere and can serve as a renewable feedstock for producing valuable chemicals and fuels. Here we look at how electrochemical approaches can impact biomass valorization, focusing on identifying chemical transformations that leverage renewable electricity and feedstocks to produce valorized products via electro-privileged transformations. First, we recommend that the field should explore widening the spectrum of platform chemicals derived from bio-feedstocks, thus offering pathways to molecules that have historically been derived from petroleum. Second, we identify opportunities in electrocatalytic production of energy-dense fuels from biomass that utilize water as the hydrogen source and renewable electricity as the driving force. Finally, we look at the potential in electrochemical depolymerization to preserve key functional groups in raw feedstocks that would otherwise be lost during harsh pre-treatments in traditional depolymerization routes. On the basis of these priorities, we suggest a roadmap for the integration of biomass and electrochemistry and offer milestones required to tap further into the potential of electrochemical biomass valorization. Biomass is a renewable source of carbon that can be exploited to produce valuable chemicals and fuels. This Perspective discusses the electrochemical valorization of biomass, identifying specific chemical transformations in which the approach can excel.

Fundamental understanding of the interaction between atoms and molecules with the surfaces of oxides including semiconducting oxides is crucial for the development of several thermo-, photo-, and electro-, catalytic reactions as well as any application where surfaces are exposed to an environment beyond vacuum. While previous studies have postulated material features (descriptors) that to some extent suggest the adsorption energy trends on semiconducting oxides, a physics based model to describe the interaction of atoms and molecules with the surfaces of these materials is still lacking. In this study, we perform a series of controlled in-silico experiments involving doping of quintessential semiconducting oxides (SrTiO3, SrZrO3, and TiO2) to identify the perturbation by the dopant to the electronic structure of the host oxide and its resultant effect on the adsorption energies of simple atoms and molecules. We identify that a combination of three surface features: unique surface resonance states of the host-metal and lattice oxygen atoms of the terminating surface oxide layer as well as the gap states dominated by the introduced dopants contribute to the adsorption energy in a concerted fashion. We find that this intricate interplay between on the one hand host-metal and on the other hand oxygen surface resonance states with the adsorbate, respectively, results in a deviation from the well-established adsorbate scaling relations seen for NHx(x=0-2) and CHx(x=0-3) but not OHx and SHx. Through this lens, we develop a physics based adsorption model hitherto referred as the Generalized Concerted Coupling model (GCC-model). The introduced model provides a physical understanding with an associated electronic structure descriptor rooted in the unique surface resonances that accurately captures the adsorption energy trends on doped semiconducting oxides. This paves the way for the atomistic design of doped semiconducting oxides for different catalytic applications, including sustainable energy applications such as electrochemical water-splitting.

The escalating demands of industrialization and development underscore the necessity for an efficient and scalable Carbon Capture and Storage (CCS) methodology. Mineral carbonation of MgO presents itself as a promising solution due to its considerable theoretical capacity for CO2 adsorption. However, the sluggish kinetics of the carbonation process pose a significant challenge. Consequently, a comprehensive understanding of the structural and chemical alterations occurring during carbonation is imperative for material design. In this study, we conduct a thorough structural and chemical investigation of the MgO (sourced from different mine tailings) carbonation process using electron microscopic techniques. Our findings demonstrate that treating MgO with polar solvents enhances its degree of carbonation significantly, offering a promising avenue for improvement. Moreover, we observe a particle size dependency in MgO carbonation and note that the inclusion of additional materials, such as Si-based compounds, further accelerates the carbonation. Density functional theory (DFT) calculations provide insight into surface functionalization as a result of solvent treatment and its mechanistic effect on the origin of the enhanced carbonation of polar solvent-treated MgO, revealing a stronger interaction between CO2 and the treated MgO (100) surface as compared to the non-polar solvent treated surfaces. These discoveries showcase an alternative approach for enhancing MgO carbonation, thereby offering a potential method for sequestering atmospheric CO2 more effectively using mine waste rich in MgO.

Significant efforts have been devoted to investigating the oxidation of MXenes in various environments. However, the underlying mechanism of MXene oxidation and its dependence on the electrode potential remain poorly understood. Here we show the oxidation behavior of MXenes under the working conditions of electrochemical processes in terms of kinetics and thermodynamics by using constant-potential ab initio simulations. The theoretical results indicate that the potential effects can be attributed to the nucleophilic attack of water molecules on metal atoms, similar to that taking place in the Oxygen Evolution Reaction. Building upon these findings, we deduced the oxidation potential of the common MXenes, and proposed antioxidant strategies for MXene. Finally, we demonstrated that MBenes, the boron analogs of MXenes, may undergo a similar nucleophilic attack in water and inferred that molecule-induced Walden inversion is widely present in material reconstructions. This work contributes to a fundamental understanding MXene stability at the atomic level, and promotes the transition in materials discovery from trial-and-error synthesis to rational design. MXenes are two-dimensional materials with promising applications, but their oxidation mechanisms under electrochemical conditions are poorly understood. Here, the authors use theoretical simulations to investigate how the oxidation of MXenes is influenced by the electrode potential.

Heterogeneous catalysts consisting of supported metallic nanoparticles typically derive exceptional catalytic activity from their large proportion of undercoordinated surface sites which promote adsorption of reactant molecules. Simultaneously, these high energy surface configurations are unstable, leading to nanoparticle growth or degradation and eventually a loss of catalytic activity. Surface morphology of catalytic nanoparticles is paramount to catalytic activity, selectivity, and degradation rates, however it is well-known that harsh reaction conditions can cause the surface structure to change. Still, limited research has focused on understanding the link between nanoparticle surface facets and degradation rates or mechanisms. Here, we study a model Au supported catalyst system over a range of temperatures using a combination of in situ transmission electron microscopy, kinetic Monte Carlo simulations, and density functional theory calculations to establish an atomistic picture of how variations in surface structures and atomic coordination environments lead to shifting evolution mechanisms as a function of temperature. By combining experimental results, which yield direct observation of dynamic shape changes and particle sublimation rates, with computational techniques, which enable understanding the fundamental thermodynamics and kinetics of nanoparticle evolution, we illustrate a two-step evolution mechanism in which mobile adatoms form through desorption from low-coordination facets and subsequently sublimate off the particle surface. By understanding the role of temperature in the competition between surface diffusion and sublimation, we are able to show how individual atomic movements lead to particle scale morphological changes and rationalize why sublimation rates vary between particles in a system of nearly identical nanoparticles.

MXenes are 2D materials with great potential in various applications. However, the degradation of MXenes in humid environments has become a main obstacle in their practical use. Here we combine deep neural networks and an active learning scheme to develop a neural network potential (NNP) for aqueous MXene systems with ab initio precision but low cost. The oxidation behaviors of super large aqueous MXene systems are investigated systematically at nanosecond timescales for the first time. The oxidation process of MXenes is clearly displayed at the atomic level. Free protons and oxides greatly inhibit subsequent oxidation reactions, leading to the degree of oxidation of MXenes to exponentially decay with time, which is consistent with the oxidation rate of MXenes measured experimentally. Importantly, this computational study represents the first exploration of the kinetic process of oxidation of super-sized aqueous MXene systems. It opens a promising avenue for the future development of effective protection strategies aimed at controlling the stability of MXenes.

The two-dimensional (2D) metallic phase of MoS2, 1T-MoS2, has extraordinary electrical conductivity in contrast to the common 2D semiconducting phase, 2H-MoS2. However, the thermodynamic instabilities of 1T-MoS2 hinder its application. In this work, we investigate the possibilities of stabilizing 1T-MoS2 through heterostructure design using first-principles calculations. We found that MXene-based heterostructures could hamper phase transitions from 1T-MoS2 to 2H-MoS2 enabled by a larger phase transition kinetic energy barrier. Based on this finding, we propose a general and effective strategy for stabilizing 1T-MoS2, that is, building heterostructures using 1T-MoS2 and oxygen-functionalized MXenes. Besides, we have also observed that due to the occurrence of electron transfer in the heterostructure, 1T-MoS2 in the heterostructure exhibits improved hydrogen adsorption free energy and more active sites compared to the monolayer 1T-MoS2. These findings provide guidance for promoting and developing 1T-MoS2 for practical applications. In addition, the proposed heterostructure design strategy could inspire the study of phase transition behaviors and electrochemical properties of materials using interfaces.