Impact of Vacancy Defects on Electrochemical Nitrogen Reduction Reaction Performance of MXenes.

We investigated electrochemical nitrogen reduction reaction (eNRR) on MXenes consisting of the vacancy defects in the functional layer using density functional theory calculations. We considered Mo2C, W2C, Mo2N, and W2N MXenes with F, N, and O functionalization and investigated distal and alternative associative pathways. We analyzed these MXenes for eNRR based on N2 adsorption energy, NH3 desorption energy, NRR selectivity, and electrochemical limiting potential. While we find that most of the considered MXenes surfaces are more favorable for eNRR compared to hydrogen evolution, these surfaces also have strong NH3 binding (>-1.0 eV) and thus will be covered with NH3 during operating conditions. Amongst all considered MXenes, only W2NF2 is found to have a low NH3 desorption energy along with low eNRR overpotential and selectivity towards eNRR. The obtained eNRR overpotential and NH3 desorption energy on W2NF2 are superior to those reported for pristine W2N3 as well as functionalized MXenes.

Decreasing toxicity and increasing photoconversion efficiency by Sn-substitution of Pb in 5-ammonium valeric acid-based two-dimensional hybrid perovskite materials.

The toxicity of Pb in halide-based hybrid perovskite materials stands in the way of their more extensive use, despite their excellent optical properties, high stability and very good photoconversion efficiency. The presented work focuses on addressing the toxicity issues in 2D perovskites. We use 5-ammonium valeric acid (AVA) as an organic spacer and partially or completely eliminate Pb by Sn and apply first principles-based density functional theory (DFT) calculations to determine the properties of these systems. Structural insights are gained, which predict the major changes in the inorganic framework including the metal-halide bond length and the bridging angle between two octahedral configurations. The replacement of Pb by Sn leads to a drastic reduction of the electronic band gap from 1.84 to 1.04 eV. Increasing the Sn content results in Sn-I bonds being stronger than the Pb-I bonds, which entails strong s-p coupling. The calculated effective masses of excitons decrease by up to ∼23% in the case of lead-free perovskites, which can be attributed to the more dispersive band edges due to stronger s-p coupling. The reduction of the effective masses of the charge carriers and the electronic band gap results in high electrical conductivity for the AVA2(MA)Sn2I7 2D perovskite structure. The three structures compared, where AVA2(MA)XI7 (X = Pb2, PbSn, Sn2) exhibit excellent thermoelectric power factors, which suggests promising applications for heat energy conversion. Moving toward lead-free 2D perovskites, the real part of the dielectric constants enhances, which may limit the radiative recombination of charge carriers. Furthermore, reducing the bandgap values by the substitution of Sn results in a red-shift in the edge of the absorption coefficients. Using the spectroscopic limited maximum efficiency (SLME) model, the best efficiencies of 32.20 and 30.08% are achieved for the AVA2(MA)PbSnI7 and AVA2(MA)Sn2I7 structures. The comparison of all three structures demonstrates that lead-free 2D perovskites are very good candidates for highly efficient solar energy conversion.

Mobility driven thermoelectric and optical properties of two-dimensional halide-based hybrid perovskites: impact of organic cation rotation.

The pivotal impact of organic cation rotation may result in structural complexity in two-dimensional (2D) halide-based hybrid perovskites. The crucial role of the orientation of the organic cation (MA = CH3NH3+) in the 2D Ruddlesden-Popper phase (2DRP) is explored using density functional theory (DFT) calculations. Our results propose that the MA cation rotation imposes the structural distortion in the PbI6 network, which is further responsible for the changes in nature and value of the electronic bandgap, charge density and optical absorption. The spin-orbit coupling effect results in a wide range of Rashba splitting parameters being obtained from 0.04 to 0.278 eV Å. The simulated optical absorption spectra suggest that absorption edge for the alignment of the MA molecule along the X-axis (having unidirectional hydrogen bonds) is higher than that of the alignment of the MA cation in the z-direction. Furthermore, the unidirectional hydrogen bonds between the MA cation and Pb-I framework significantly help to achieve the highest mobility of charge carriers up to ∼1437 cm2 V-1 s-1. Such high mobility leads to supremacy in the thermoelectric transport properties, which are investigated for the first time with the rotation of the MA cation. The calculated thermoelectric power factor at room temperature shows exceptionally high values (up to 2.04 mW m-1 K-2), leading to desired applications in thermoelectric devices. The rotation of the MA cation might be utilized as a useful tool for variation in optical absorption and transport coefficients. Therefore, our results spark the idea to develop 2D perovskites for real-time perspective in solar and heat energy utilization.

Hybrid Structure of Ionic Liquid and TiO2 Nanoclusters for Efficient Hydrogen Evolution Reaction.

Hydrogen energy has received significant attention in the renewable energy sector due to its high energy density and environmentally friendly nature. For the efficient hydrogen generation from water, the hydrogen evolution reaction (HER) has to be optimized, which requires a highly efficient electrocatalyst. In this work, a hybrid structure of the ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (C2mim TfO) and (TiO2)n nanoclusters with n = 2-12 has been investigated in the pursuit of new catalyst materials for effective HER. We have employed state-of-the-art density functional theory (DFT) computations to depict the HER catalytic performance of IL/(TiO2)n hybrid systems through Gibbs free energy (ΔG) and an exchange-current-based "volcano" plot. We have explored the effect of the TiO2 nanoclusters on the structural and electronic characteristics of the IL, calculating the adsorption energy, the energies of the highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO), the HOMO-LUMO band gap Eg, and the work function ϕ. The variation in size of the TiO2 nanocluster in the IL/(TiO2)n hybrid system was found to have a significant influence on the electronic properties. The obtained results suggest that the ΔG of the hydrogen adsorption is remarkably close to the ideal value (0 eV) for the IL/(TiO2)5 system, which also reflects from the volcano plot, suggesting that this complex is the best HER catalyst among the studied systems; it might be even better than the traditional Pt-based catalyst. Thus, the present work suggests ways for the experimental realization of low-cost and multifunctional IL-based hybrid catalysts for clean and renewable hydrogen energy production.

Rashba Splitting in Two Dimensional Hybrid Perovskite Materials for High Efficient Solar and Heat Energy Harvesting.

The physical properties of two-dimensional (2D) lead halide based hybrid perovskites are quite exciting and challenging. Further, the role of organic cations in 2D perovskites is still in a debate. We investigated layered (CH3(CH2)3NH3)2(CH3NH3)Pb2I7 2D Ruddlesden-Popper (2DRP) phase (M1) and 2D derivative of CH3NH3PbI3 (M2) using density functional theory. The spin orbit coupling mediates the significantly large Rashba splitting energy of 328.5 meV for M2, which is higher than earlier 2D hybrid perovskites. At the picosecond time scale, the dynamical Rashba effect was observed due to organic and inorganic cation dynamics. Two step absorption suggests an indirect optical gap of 2.38 and 2.15 eV for M1 and M2, respectively and solar performance depicts excellent power conversion efficiency of 14.92% and 19.75% for M1 and M2, respectively. For the first time, we explored the thermoelectric properties of 2D hybrid perovskites and perceived high power factor for p-type doping in M2. Our findings suggest that these novel 2D perovskites have the potential to be used in solar and heat energy harvesting.

Impact of alkyl chain length and water on the structure and properties of 1-alkyl-3-methylimidazolium chloride ionic liquids.

The influence of the length of the alkyl chain and water molecules on the hydrogen-bond interaction of the chloride anion and imidazolium-based cation of the ionic liquid (IL) Cnmim Cl (where n = 2, 4, 6, 8, and 10) was investigated by combining attenuated total internal reflection infrared (ATR-IR) spectroscopy and density functional theory (DFT) calculations. Here, for the first time, the conformational isomerism of the alkyl chain of Cnmim Cl (n = 2, 4, 6, 8, and 10) is identified by marker IR bands. The IR peak at 1470 cm-1 related to the alkyl chain vibration exhibits a significant perturbation in its intensity and further shows a red shift upon increasing alkyl chain length. This indeed might be a marker IR band for conformational isomerism and also an indication of the interaction of the alkyl chain with the chloride anion. Further, in the C-H vibration region of the IR spectra, a significant variation of the IR intensities was observed for the νs(CH2) and νas(CH2-CH3) modes at 2931 and 2976 cm-1, respectively. These bands can be considered as further markers for conformational isomerism of the alkyl chain. Moreover, the peak at 2976 cm-1 assigned to an alkyl chain vibration reveals the maximum red shift of 20 cm-1 for n = 10, which suggests charge redistribution among ion-pairs as a result of the alkyl chain variations. Noticeably, the C2-H vibration does not show any significant change of its wavenumber position, suggesting that the alkyl chain length does not interfere with the hydrogen bond interaction between C2-H and the Cl anion. This was also evident from the DFT-calculated bond strength between C2-H and Cl, which remains unchanged upon varying the alkyl chain length. In aqueous solutions, blue shifts of the v(C2-H) band by +65, +60, +67, +62 and +62 cm-1 for Cnmim Cl (n = 2, 4, 6, 8, and 10) are observed, respectively. These results point to a weakening of the hydrogen bond between cation and anion, which is also supported and validated by results of the solvent (water) effect obtained using the polarized continuum model (PCM) of the DFT calculations.

Halogenation of SiGe monolayers: robust changes in electronic and thermal transport.

Phonon and electronic transport of buckled structured SiGe monolayer and halogenated SiGe monolayers (X2-SiGe, X = F, Cl, and Br) are investigated for the first-time using ab initio density functional theory (DFT). The phonon calculations reveal complete dynamical stability of SiGe and fluorinated (F2-SiGe) monolayers in contrast to earlier reported works, where a small magnitude of imaginary frequency in SiGe monolayer near the zone centre of the Brillouin zone (BZ) is observed. The phonon calculations of chlorinated and brominated SiGe reveal no dynamical stability even with very high convergence parameters and better computational accuracy. The lower value of lattice thermal conductivity in the case of F2-SiGe is attributed to the strong phonon anharmonic scattering and larger contribution of the three phonon process to anharmonic scattering. The semimetallic nature of the SiGe monolayer turns to semiconducting after halogenation. We have also calculated the electron relaxation time to study their precise thermoelectric parameters. The enhancement of the Seebeck coefficient and reduction in lattice thermal conductivity in the SiGe monolayer is observed after halogenation which results in the improvement of the thermoelectric figure of merit (ZT). The room temperature figure of merit, ZT, which is 0.112 for the SiGe monolayer, enhances significantly to 0.737 after addition of fluorine atoms. Our study suggests that the halogenation of two-dimensional materials can improve their thermoelectric properties.