Paper & Patents

MAX phase is a family of ceramic compounds, typically known for their metallic properties. However, we show here that some of them may be narrow bandgap semiconductors. Using a series of first-principles calculations, we have investigated the electronic structures of 861 dynamically stable MAX phases. Notably, Sc2SC, Y2SC, Y2SeC, Sc3AuC2, and Y3AuC2 have been identified as semiconductors with band gaps ranging from 0.2 to 0.5 eV. Furthermore, we have assessed the thermodynamic stability of these systems by generating ternary phase diagrams utilizing evolutionary algorithm techniques. Their dynamic stabilities are confirmed by phonon calculations. Additionally, we have explored the potential thermoelectric efficiencies of these materials by combining Boltzmann transport theory with first-principles calculations. The relaxation times are estimated using scattering theory. The zT coefficients for the aforementioned systems fall within the range of 0.5 to 2.5 at temperatures spanning from 300 to 700 K, indicating their suitability for high-temperature thermoelectric applications.

Employing density functional theory, we delved into the comprehensive pathways for methane oxidation on the Pd single atom supported with CeO2(111) encompassing sequential methane dehydrogenation, O2 dissociation, and oxidation processes. The introduction of a Pd atom into CeO2(111) led to a reduction in the barrier for CH4 dissociation to 0.50 eV. The methane dehydrogenation proceeded through a series of reactions: CH4 → CH3 → CH2 → CH → C, with all dehydrogenation steps being exothermic except the CH3 → CH2 step. The O2 dissociation reaction (O2 → O* + O*) is thermodynamically exothermic, with a dissociation barrier of 2.12 eV over Pd@CeO2. Subsequently, the generation of CO2 via the C* + O* and CO* + O* reactions is characterized by thermodynamically exothermic processes, with reaction energies of −1.20 and −1.01 eV, respectively. On the other hand, water production occurs through O* + H (an exothermic reaction) and OH* + H (an endothermic reaction) with reaction energies of −0.80 and +0.64 eV, respectively. These findings offer valuable insights into the potential pathways for single-atom catalysis involving transition metals supported on CeO2(111) in methane oxidation for industrial application.

Slow kinetics related to oxygen evolution reactions (OERs) are currently the main obstacle in developing effective and extremely stable oxygen electrocatalysts for alkaline water electrolysis cells. Catalysts based on spinel-structured cobalt ferrites (CoFe2O4), which have remarkable catalytic activity and quick kinetics, are highly promising OER electrocatalysts. Here, sulfur-doped cobalt ferrites nanoarrays on iron foam (S-CoFe2O4/IF) are made using an effective and simple technique based on anion–exchange reactions in order to create effective OER catalysts. These S-CoFe2O4/IF catalysts demonstrate exceptional OER activity via a topotactical transformation, wherein their Co oxide undergoes continuous cyclic voltammetry scanning to evolve into active Co oxyhydroxide (CoOOH) nanoplates while the Fe stays stable within the Fe–O component. This sulfidation, as evidenced by its electronic density of states, can induce surface reconstruction, leading to the formation of active CoOOH under OER conditions by elevating the O 2p energy level as a result of cation substitution. When sulfidation is applied to the spinel-structured CoFe2O4/IF, the total catalytic activity is greatly increased. This leads to a smaller Tafel slope and overpotential (42 mV dec−1 and 286 mV) at 0.1 A cm−2 than when CoFe2O4/IF is used as the catalyst (51 mV dec−1 and 304 mV). The relationship between activity and structure in spinel structures is explained by this work.

Electronic transport in monolayer MoS2 is significantly constrained by several extrinsic factors despite showing good prospects as a transistor channel material. Our paper aims to unveil the underlying mechanisms of the electrical and magneto-transport in monolayer MoS2. In order to quantitatively interpret the magneto-transport behavior of monolayer MoS2 on different substrate materials, identify the underlying bottlenecks, and provide guidelines for subsequent improvements, we present a deep analysis of the magneto-transport properties in the diffusive limit. Our calculations are performed on suspended monolayer MoS2 and MoS2 on different substrate materials taking into account remote impurity and the intrinsic and extrinsic phonon scattering mechanisms. We calculate the crucial transport parameters such as the Hall mobility, the conductivity tensor elements, the Hall factor, and the magnetoresistance over a wide range of temperatures, carrier concentrations, and magnetic fields. The Hall factor being a key quantity for calculating the carrier concentration and drift mobility, we show that for suspended monolayer MoS2 at room temperature, the Hall factor value is around 1.43 for magnetic fields ranging from 0.001 to 1 Tesla, which deviates significantly from the usual value of unity. In contrast, the Hall factor for various substrates approaches the ideal value of unity and remains stable in response to the magnetic field and temperature. We also show that the MoS2 over an Al2O3 substrate is a good choice for the Hall effect detector. Moreover, the magnetoresistance increases with an increase in magnetic field strength for smaller magnetic fields before reaching saturation at higher magnetic fields. The presented theoretical model quantitatively captures the scaling of mobility and various magnetoresistance coefficients with temperature, carrier densities, and magnetic fields.

Janus MXenes, a new category of two-dimensional (2D) materials, show promising potential for advances in optoelectronics, spintronics, and nanoelectronics. Our theoretical investigations not only provide interesting insights but also highlight the promise of Janus MCrCT2 (M = Ti, Mo; T = O, F, OH) MXenes for future spintronic applications and highlight the need for their synthesis. Electronic structure analysis shows different metallic and semimetallic properties: MoCrCF2 exhibits metallic property, TiCrC(OH)2 and MoCrCO2 exhibit near semimetallicity with spin polarization values of 61 and 86%, respectively, while TiCrCO2 and TiCrCF2 are completely half-metallic with 100% spin polarization at the Fermi level. All studied Janus MXenes exhibit intrinsic ferromagnetism, which is mainly attributed to the chromium (Cr) atoms, as shown by the spin density difference plots. Among them, the TiCrCO2 monolayer stands out with the highest exchange constant and ferromagnetic transition temperature (Tc). Notably, the O-terminated Janus MXenes exhibit weak perpendicular magnetic anisotropy, in contrast to the in-plane anisotropy observed for F and OH-terminated MXenes, making them particularly interesting for future spintronic applications, which we further demonstrate with micromagnetic simulation which reveal distinct current-induced switching behaviors in these Janus MXenes with different surface terminations.

This study investigates the catalytic degradation of ground-level ozone on low-index stoichiometric and reduced CeO2 surfaces using first-principles calculations. The presence of oxygen vacancies on the surface enhances the interaction between ozone and the catalyst by serving as active sites for adsorption and decomposition. Our results suggest that the {111} surface has superior ozone decomposition performance due to unstable oxygen species resulting from reactions with catalysts. However, when water is present, it competes with ozone molecules for these active sites, resulting a reduced catalytic activity or water poisoning. A possible solution could be heat treatment that reduces the vacancy concentration, thereby increasing the available adsorption sites for ozone molecules while minimizing competitive adsorption by water molecules. These results suggest that controlling moisture content during operation is crucial for the efficient use of CeO2-based catalysts in industrial applications to reduce ground-level ozone pollution.

Topological materials have attracted significant interest in condensed matter physics for their unique topological properties leading to potential technological applications. Topological nodal line semimetals, a subclass of topological materials, exhibit symmetry-protected nodal lines, where band crossings occur along closed curves in the three-dimensional Brillouin zone. When the nodal lines are gapped out due to perturbation in the Hamiltonian, a large Berry curvature (BC) arises in the surrounding area of the gapped nodal line, leading to exotic anomalous transport responses. In this paper, we studied the Co2CrX (X=Ga, Ge) Heusler compounds that exhibit mirror symmetry-protected nodal line states below the Fermi level. The BC calculation yields anomalous Hall conductivity (AHC) of about 292 and 217 S/cm for Co2CrX (X=Ga, Ge), respectively, at the Fermi level, which increases by up to 400% at the nodal line energy level. We theoretically analyzed that 20% and 60% zinc (Zn) alloying in Co2CrX (X=Ga, Ge) effectively lowers the Fermi level by 50 meV and 330 meV, respectively, aligning with the protected crossings. Consequently, we identified Co2CrGe0.4Zn0.6 and Co2CrGa0.8Zn0.2 as compositions to achieve the significant AHC of 800 and 1300 S/cm, respectively. The explicit AHC calculation for these alloyed compositions is in good agreement with our predictions. Our findings highlight that chemical alloying is an efficient way to enhance AHC in nodal line hosting materials.

Durability is a crucial requirement in heterogeneous catalysis; however, many catalysts suffer from severe deactivation in humid conditions due to water poisoning. Ozone, as a significant air pollutant, should be efficiently removed through catalytic decomposition, making it imperative to develop a water-tolerant monolithic catalyst for practical air purification. In this study, we present highly durable Pd/CeO2 monolithic catalysts resistant to water poisoning achieved through a simple heat treatment of the ceria support. The heat treatment controlled the ceria surface properties, including oxygen vacancy defects, surface oxygen, and basicity, thereby improving resistance to water poisoning. When Pd/CeO2 monolithic catalysts were used in bench-scale ozone decomposition under humid conditions, the catalyst heat-treated at 900 °C exhibited superior performance without experiencing deactivation due to water poisoning. Modulating the ceria surface properties plays a pivotal role in enhancing water resistance, and heat-treated Pd/CeO2 monolithic catalysts stand as a promising candidate for practical ozone decomposition in air purification applications.

Despite having favorable energetics and tunable optoelectronic properties, utilization of BaTiO3 (BTO) for photocatalytic reactions is limited by its absorption only in the ultraviolet region. To address this challenge, BTO is doped with iridium (Ir) to induce visible light absorption. The visible light-induced photocatalytic H2 generation efficiency is enhanced by 2 orders of magnitude on selective conversion of the Ir valence state from Ir 4+ to Ir 3+. To understand such intriguing behavior, valence state-dependent changes in the optoelectronic, structural, and surface properties and electronic band structure are comprehensively investigated. The effect of electron occupancy change between Ir 4+ (t 2g 5 e g 0) and Ir 3+ (t 2g 6 e g 0) and their energetic positions within the band gap is found to significantly influence H 2 generation. Besides this, converting Ir 4+ to Ir 3+ enhanced the photocathodic current and lowered the onset potential. Results aid in designing photocatalysts to efficiently use low-energy photons for enhancing solar H2 production in these emerging BTO-based photocatalysts. Collectively, the observations made in this work highlight the promising application of Ir 3+ :BTO in z-scheme photocatalysis.

In this work, we report the results of density functional theory (DFT) calculations on a van der Waals (VdW) heterostructure formed by vertically stacking single-layers of tungsten disulfide and graphene (WS2/graphene) for use as an anode material in lithium-ion batteries (LIBs). The electronic properties of the heterostructure reveal that the graphene layer improves the electronic conductivity of this hybrid system. Phonon calculations demonstrate that the WS2/graphene heterostructure is dynamically stable. Charge transfer from Li to the WS2/graphene heterostructure further enhances its metallic character. Moreover, the Li binding energy in this heterostructure is higher than that of the Li metal’s cohesive energy, significantly reducing the possibility of Li-dendrite formation in this WS2/graphene electrode. Ab initio molecular dynamics (AIMD) simulations of the lithiated WS2/graphene heterostructure show the system’s thermal stability. Additionally, we explore the effect of heteroatom doping (boron (B) and nitrogen (N)) on the graphene layer of the heterostructure and its impact on Li-adsorption ability. The results suggest that B-doping strengthens the Li-adsorption energy. Notably, the calculated open-circuit voltage (OCV) and Li-diffusion energy barrier further support the potential of this heterostructure as a promising anode material for LIBs.