Facile Low-Temperature synthesis of novel carbon nitrides for efficient conversion of carbon dioxide into Value-Added chemicals.

The interest in using carbon nitrides (CN) for CO2 conversion has stimulated extensive research on CN synthesis. Herein, we report the synthesis of two novel CN materials using low-cost commercially available precursors at low temperatures in a short duration of time. Two CN materials, one derived from 5-amino tetrazole (named 4NZ-CN) and the other derived from 3, 5-diamino-1, 2, 4-triazole (named 3NZ-CN) precursors, are prepared by refluxing these precursors for 2 h at 100 °C. 4NZ-CN and 3NZ-CN catalysts show higher surface areas (55.80 and 52.00 m2 g-1) and more basic sites (10.05 and 5.65 mmol g-1) than the conventional graphitic carbon nitride (g-C3N4) derived from melamine, for which the corresponding values are 9.20 m2 g-1 and 0.62 mmol g-1, respectively. In addition, both CN exhibit a 3-fold higher catalytic activity for CO2 cycloaddition to epoxides than g-C3N4. The structure-activity relationship was ascertained using a combination of experimental and computational studies, and a catalytic mechanism was proposed. This work provides a facile strategy for the synthesis of novel CN materials at relatively low temperatures, and the developed catalysts show remarkable performance in the conversion of CO2 to value-added chemicals.

Rashba and Dresselhaus effects in doped methylammonium lead halide perovskite MAPbI3.

Inorganic-organic lead halide perovskites, particularly methylammonium lead halide (MAPbI3) perovskite, have been regarded as promising materials for optoelectronics and spintronics. However, the practical applications of these perovskites are limited by lead toxicity and instability under air and pressure. This study investigates the substitution of Pb with Sn and Ge in cubic MAPbI3 perovskite. The properties of the resulting hybrid perovskites are compared using state-of-the-art first-principles-based methodologies, viz., density functional theory (DFT) with generalized gradient approximation (PBE) and hybrid functional (HSE06), in conjunction with spin-orbit coupling (SOC). Here, we mainly study the Rashba-Dresselhaus (RD) effect, which arises due to two major mechanisms: (i) the breaking of inversion symmetry (static and dynamic) and (ii) SOC, originating from the presence of heavy elements. We find significant spin-splitting effects in the conduction band minimum and valence band maximum for hybrid perovskites. To gain a deeper understanding of the observed spin-splitting, the spin textures are analyzed, and Rashba coefficients are calculated. We find that the Dresselhaus effect comes into play in substituted hybrid structures in addition to the usual Rashba effect observed in the pristine compound. Additionally, we observe that the strength of Rashba spin-splitting is substantially tuned by the application of uniaxial strain (±5%). Moreover, certain hybrid perovskites exhibit mechanical stability and ductility, making them potential candidates in perovskite-based optoelectronics and spintronics applications.

O-Vacancy Mediated Partially Inverted Ferrospinels for Enhanced Activity in the Sulfuric Acid Decomposition for Hydrogen Production.

Understanding the characterization of a tailored Co3O4 spinel with Fe3+ doping poses a challenge due to the surface state complexity in bifunctional catalysts with higher cation diversity. Doping with secondary metal results in a double spinel structure (a hybrid of normal and inverted spinels). This enhances the catalytic properties by generating more active oxygen vacancies. The cobalt-rich (FeCo2O4) hybrid spinel and iron-rich (CoFe2O4) inverted spinel are synthesized using a wet impregnation method, supported over oxidized SiC (SiC-Pretrt) for an improved metal-support interaction. FeCo2O4 on pretreated SiC exhibits the highest catalytic activity (90% conversion at 1173 K) and stability (over 100 h) in sulfuric acid decomposition of the iodine-sulfur process for hydrogen production. This improved performance is attributed to the high electronegativity of Co3+, oxygen vacancies, and strong metal-support interaction. The high electronegativity of Co3+ weakens the S-O bond in M-S-O, enhancing the catalytic activity of the spinels. These results are further corroborated by detailed characterization and density functional theory calculations.

Electrocatalytic study of the hydrogen evolution reaction on MoS2/BP and MoSSe/BP in acidic media.

Molecular hydrogen (H2) production by the electrochemical hydrogen evolution reaction (HER) is being actively explored for non-precious metal-based electrocatalysts that are earth-abundant and low cost like MoS2. Although it is acid-stable, its applicability is limited by catalytically inactive basal planes, poor electrical transport and inefficient charge transfer at the interface. Therefore, the present work examines its bilayer van der Waals heterostructure (vdW HTS). The second constituent monolayer boron phosphide (BP) is advantageous as an electrode material owing to its chemical stability in both oxygen and water environments. Here, we have performed first-principles based calculations under the framework of density functional theory (DFT) for the HER in an electrochemical double layer model with the BP monolayer, MoS2/BP and MoSSe/BP vdW HTSs. The climbing image nudged elastic band method (CI-NEB) has been employed to determine the minimum energy pathways for Tafel and Heyrovsky reactions. The calculations reveal that the Tafel reaction shows no reaction barrier. Thereafter, for the Heyrovsky reaction, we obtained a low reaction barrier in the vdW HTSs as compared to that in the BP monolayer. Subsequently, we have observed no significant difference in the reaction profile of MoS2/BP and MoSSe/BP vdW HTSs in the case of 2 × 2 supercell configuration. However, in the case of 3 × 3 and 4 × 4 configurations, MoSSe/BP shows a feasible Heyrovsky reaction with no reaction barrier. The coverages with 1/4H+ concentration (conc.) deduced high coverage with low conc. and low coverage with high conc. to be apt for the HER via the Heyrovsky reaction path. Finally, on observing the activation barrier of the Heyrovsky pathway along with that of second H adsorption at the surface, the Heyrovsky path is expected to be favoured.

Exploring strong and weak topological states on isostructural substitutions in TlBiSe[Formula: see text].

Topological Insulators (TIs) are unique materials where insulating bulk hosts linearly dispersing surface states protected by the Time-Reversal Symmetry. These states lead to dissipationless current flow, which makes this class of materials highly promising for spintronic applications. Here, we predict TIs by employing state-of-the-art first-principles based methodologies, viz., density functional theory and many-body perturbation theory (G[Formula: see text]W[Formula: see text]) combined with spin-orbit coupling effects. For this, we take a well-known 3D TI, TlBiSe[Formula: see text] and perform complete substitution with suitable materials at different sites to check if the obtained isostructural materials exhibit topological properties. Subsequently, we scan these materials based on SOC-induced parity inversion at Time-Reversal Invariant Momenta. Later, to confirm the topological nature of selected materials, we plot their surface states along with calculation of Z[Formula: see text] invariants. Our results show that GaBiSe[Formula: see text] is a strong Topological Insulator, besides, we report six weak Topological Insulators, viz., PbBiSe[Formula: see text], SnBiSe[Formula: see text], SbBiSe[Formula: see text], Bi[Formula: see text]Se[Formula: see text], TlSnSe[Formula: see text] and PbSbSe[Formula: see text]. We have further verified that all the reported TIs are dynamically stable, showing all real phonon modes of vibration.

Vacancy-Ordered Double Perovskites Cs2BI6 (B = Pt, Pd, Te, Sn): An Emerging Class of Thermoelectric Materials.

Vacancy-ordered double perovskites (A2BX6), being one of the environmentally friendly and stable alternatives to lead halide perovskites, have garnered considerable research attention in the scientific community. However, their thermal transport has not been explored much, despite their potential applications. Here, we explore Cs2BI6 (B = Pt, Pd, Te, Sn) as potential thermoelectric materials using state-of-the-art first-principles-based methodologies, viz., density functional theory combined with many-body perturbation theory (G0W0) and spin-orbit coupling. The absence of polyhedral connectivity in vacancy-ordered perovskites gives rise to additional degrees of freedom, leading to lattice anharmonicity. The presence of anharmonic lattice dynamics leads to strong electron-phonon coupling, which is well-captured by the Fröhlich mesoscopic model. The lattice anharmonicity is further studied using ab initio molecular dynamics and the electron localization function. The maximum anharmonicity is observed in Cs2PtI6, followed by Cs2PdI6, Cs2TeI6, and Cs2SnI6. Also, the computed average thermoelectric figure of merit (zT) for Cs2PtI6, Cs2PdI6, Cs2TeI6, and Cs2SnI6 is 0.88, 0.85, 0.95, and 0.78, respectively, which reveals their promising renewable energy applications.

Synergistic Effect of Work Function and Acoustic Impedance Mismatch for Improved Thermoelectric Performance in GeTe-WC Composite.

The preparation of composite materials is a promising methodology for concurrent optimization of electrical and thermal transport properties for improved thermoelectric (TE) performance. This study demonstrates how the acoustic impedance mismatch (AIM) and the work function of components decouple the TE parameters to achieve enhanced TE performance of the (1-z)Ge0.87Mn0.05Sb0.08Te-(z)WC composite. The simultaneous increase in the electrical conductivity (σ) and Seebeck coefficient (α) with WC (tungsten carbide) volume fraction (z) results in an enhanced power factor (α2σ) in the composite. The rise in σ is attributed to the creation of favorable current paths through the WC phase located between grains of Ge0.87Mn0.05Sb0.08Te, which leads to increased carrier mobility in the composite. Detailed analysis of the obtained electrical properties was performed via Kelvin probe force microscopy (work function measurement) and atomic force microscopy techniques (spatial current distribution map and current-voltage (I-V) characteristics), which are further supported by density functional theory (DFT) calculations. Furthermore, the difference in elastic properties (i.e., sound velocity) between Ge0.87Mn0.05Sb0.08Te and WC results in a high AIM, and hence, a large interface thermal resistance (Rint) between the phases is achieved. The correlation between Rint and the Kapitza radius depicts a reduced phonon thermal conductivity (κph) of the composite, which is explained using the Bruggeman asymmetrical model. Moreover, the decrease in κph is further validated by phonon dispersion calculations that indicate the decrease in phonon group velocity in the composite. The simultaneous effect of enhanced α2σ and reduced κph results in a maximum figure of merit (zT) of 1.93 at 773 K for (1-z)Ge0.87Mn0.05Sb0.08Te-(z)WC composite for z = 0.010. It results in an average thermoelectric figure of merit (zTav) of 1.02 for a temperature difference (ΔT) of 473 K. This study shows promise to achieve higher zTav across a wide range of composite materials.

Origin of Rashba Spin Splitting and Strain Tunability in Ferroelectric Bulk CsPbF3.

Spin-orbit coupling (SOC) in conjunction with broken inversion symmetry acts as a key ingredient for several intriguing quantum phenomena, viz., Rashba-Dresselhaus (RD) effect. The coexistence of spontaneous polarization and the RD effect in ferroelectric (FE) materials enables the electrical control of spin degrees of freedom. Here, we explore the FE lead halide perovskite CsPbF3 as a potential candidate in the field of spintronics by employing state-of-the-art first-principles-based methodologies, viz., density functional theory (DFT) with semilocal and hybrid functional (HSE06) combined with SOC and many-body perturbation theory (G0W0). For a deeper understanding of the observed spin splitting, the spin textures are analyzed using the k.p model Hamiltonian. We find there is no out-of-plane spin component indicating that the Rashba splitting dominates over Dresselhaus splitting. We also observe that the strength of Rashba spin splitting can be substantially tuned on application of uniaxial strain (±5%). More interestingly, we notice reversible spin textures by switching the FE polarization in CsPbF3 perovskite, making it potent for perovskite-based spintronic applications.

Theoretical insights into C-H bond activation of methane by transition metal clusters: the role of anharmonic effects.

In heterogeneous catalysis, the determination of active phases has been a long-standing challenge, as materials' properties change under operational conditions (i.e. temperature (T) and pressure (p) in an atmosphere of reactive molecules). As a first step towards materials design for methane activation, we study the T and p dependence of the composition, structure, and stability of metal oxide clusters in a reactive atmosphere at thermodynamic equilibrium using a prototypical model catalyst having wide practical applications: free transition metal (Ni) clusters in a combined oxygen and methane atmosphere. A robust methodological approach is employed, where the starting point is systematic scanning of the potential energy surface (PES) to obtain the global minimum structures using a massively parallel cascade genetic algorithm (cGA) at the hybrid density functional level. The low energy clusters are further analyzed to estimate their thermodynamic stability at realistic T, p O2 and p CH4 using ab initio atomistic thermodynamics (aiAT). To incorporate the anharmonicity in the vibrational free energy contribution to the configurational entropy, we evaluate the excess free energy of the clusters numerically by a thermodynamic integration method with ab initio molecular dynamics (aiMD) simulation inputs. By analyzing a large dataset, we show that the conventional harmonic approximation miserably fails for this class of materials, and capturing the anharmonic effects on the vibration free energy contribution is indispensable. The latter has a significant impact on detecting the activation of the C-H bond, while the harmonic infrared spectrum fails to capture this, due to the wrong prediction of the vibrational modes.