Orientation Of Cations Research Articles

Adsorption of Imidazolium‐Based Ionic Liquid On Pt(111) Surface Studied Using Density Functional Theory

AbstractThe strength and nature of adsorption of imidazolium‐based tetraflouroborate ionic liquid (IL) on platinum surface has been investigated via first principle‐based density functional theory method. Adsorption of both IL cation and IL ion‐pair as a function of increasing alkyl chain length is taken into consideration. Three different orientations of ionic liquid cations are found to be stable with higher adsorption energy noticed for the alkyl chain parallel to the platinum surface. The anions are found to stabilize the IL cation orientation where the alkyl chain is oriented perpendicular the platinum surface. These results are further corroborated by the charge transfer analysis and electron density difference maps. The significant charge transfer between the ionic liquid pair and the surface indicates electrochemical applications for systems involving electrolytes and metal surface, where electrostatic interactions play a major role. The results of this investigation can be helpful for further analysis of electrode–electrolyte systems as well as the development of force field parameters for these systems.

Investigation of structural, electronic, and optical properties of halide perovskites with different amide cations: A first-principles approach

Hybrid halide perovskites (CH3NH3PbI3) have emerged as an appealing candidate in photovoltaic devices due to their low-cost fabrication and remarkable optical and electronic properties. However, the commercial applications of these perovskites are limited by their instability due to the rapid rotation and orientation of methylammonium cation (CH3NH3+), and interaction with PbI2 lattice at high temperatures. Since CH3NH3+ can deteriorate structural stability of MAPbI3 perovskites, it is possible to use molecular cation design and substitution as a mechanism to enhance the stability and remove this limitation. We here investigate the effect of replacing this cation with other organic cations (HCOHNH2PbI3 (FPbI3), CH3COHNH2PbI3 (AcPbI3), and NH2COHNH2PbI3 (UPbI3)) in order to enhance the performance of the perovskites, while also keeping the band gap energy (Eg) in an appropriate range for solar cell applications. The calculations are performed using the full potential linearized augmented plane wave method, providing lattice parameters and the formation energy of the proposed perovskites. Our results indicate that AcPbI3 perovskite outperforms the other structures in terms of various optical parameters, such as the absorption coefficient, optical conductivity, and reflectance. The optical and electronic band gaps of the AcPbI3 structure and its resonance form (Ac'PbI3) are found to be in agreement with each other, proposing them as optically tunable hybrid perovskites in emerging solar cells.

Automated design of hybrid halide perovskite monolayers for band gap engineering

The recent experimental confirmation of perovskite monolayers has sparked ongoing efforts in their prediction and synthesis, showcasing their flexible tunable band gap and potential in advanced functional devices. Although large-scale computational designs have been successfully performed for inorganic perovskite monolayers, the complexity introduced by organic cations hinders the same approaches applied to the hybrid halide perovskite monolayers. To address this challenge, we have proposed a high-throughput first-principles computational workflow that automates the design of hybrid halide perovskite monolayers. We strategically reduce the complexity of the configurations by analyzing the orientation of cations and the structural symmetry. Over 400 hybrid halide perovskite monolayers have been designed, and their structures and fundamental properties are stored in the database. Correlation analyses show a strong correlation between band gaps and metal-halogen-metal bond angles or metal-halogen bond lengths, consistent with prior studies for bulk and layered perovskites. The underlying physics that the band gap is modulated by the antibonding in the metal-halogen bond makes band gap engineering of hybrid halide perovskite monolayers feasible. Accordingly, initial research on lateral heterojunctions and solar cells has been conducted to explore the potential practical applications of the designed hybrid halide perovskite monolayers. Our study lays the foundation for further exploration of hybrid halide perovskite monolayers and highlights promising opportunities for their potential applications in electronic and optical devices.

Regulating structural stability and photoelectrical properties of FAPbI3via formamidine cation orientation.

Organic cations play a significant role in the structural stability and photoelectrical properties of organic-inorganic hybrid perovskites. The orientation of organic cations impacts its interaction with inorganic octahedrons [PbI]6-, subsequently modifying the atomic structure and electronic and optical properties of perovskite materials. However, it is still challenging to regulate the stability of perovskites with different orientations. In this work, density functional theory calculations were performed to investigate the effects of the formamidine cation (FA+) located at the angles of 0°, 45°, 90° and 180° (relative to the normal of the crystal plane) along the typical crystal directions ([001], [010], [110] and [111]) on the structural stability and photoelectrical properties of formamidine lead iodide (FAPbI3). The results show that when FA+ is located at 45° along the [111] direction, FAPbI3 achieves the highest stability and excellent photoelectrical properties. The energy evolution curves display that the system with the orientation of [111] has the minimum energy value, signifying stronger stability than the other orientations. Especially, when FA+ is located at 45° along the [111] direction. it exhibits a stronger hydrogen bond between H and I atoms, shorter Pb-I bond length and smaller [PbI]6- octahedral tilt bond angle. The band gap in the [110] direction changes from direct to indirect while FAPbI3 with other FA+ orientations still maintains the direct band structure located at the high symmetric R point. Furthermore, FA+ orientation drives the redshift of FAPbI3 towards the long wavelength region in the [111] crystal direction, which enhances the light absorption coefficient. This work can offer guidance in employing molecular regulation technology for the development of stable perovskite solar cells.

Stacking Arrangement and Orientation of Aromatic Cations Tune Bandgap and Charge Transport of 2D Organic-Inorganic Hybrid Perovskites.

Chemical modifications on aromatic spacers of 2D perovskites have been demonstrated to be an effective strategy to simultaneously improve optoelectronic properties and stability. However, its underlying mechanism is poorly understood. By using 2D phenyl-based perovskites ([C6 H5 (CH2 )m NH3 ]2 PbI4 ) as models, the authors have revealed how the chemical nature of aromatic cations tunes the bandgap and charge transport of 2D perovskites by utilizing sum-frequency generation vibrational spectroscopy to determine the stacking arrangement and orientation of aromatic cations. It is found that the antiparallel slip-stack arrangement of phenyl rings between adjacent layers induces an indirect band gap, resulting in anomalous carrier dynamics. Incorporation of the CH2 moiety causes stacking rearrangement of the phenyl ring and thus promotes an indirect to direct bandgap transition. In direct-bandgap perovskites, higher carrier mobility correlates with a larger orientation angle of the phenyl ring. Further optimizing the orientation angle by introducing a para-substituted element in a phenyl ring, higher carrier mobility is obtained. This work highlights the importance of leveraging stacking arrangement and orientation of the aromatic cations to tune the photophysical properties, which opens up an avenue for advancing high-performance 2D perovskites optoelectronics via molecular engineering.

Comparing the Cubic and Tetragonal Phases of MAPbI3 at Room Temperature.

Stability and maintenance of the crystal structure are the main drawbacks of the application of organic-inorganic perovskites in photovoltaic devices. The ΔT = 62 K robust shift of the structural phase transition observed here allows us to conduct a comprehensive study at room temperature of the tetragonal versus cubic phase on MAPbI3. The absence of the shift in the cubic transition for all-inorganic CsPbI3 samples confirms the importance of both orientation and dynamics of the organic cations. Our results provide a unique opportunity to evaluate the physical properties of both cubic and tetragonal phases of MAPbI3 at the same temperature, eliminating different phonon effects as possible causes for different properties. Besides higher electrical resistivity, the perovskite cubic phase presents a faster charge carrier lifetime than the tetragonal phase and partial PL quenching, pointing toward increased trap-assisted nonradiative recombination. The light absorption coefficient in the cubic phase is larger than the absorption in the tetragonal phase in the green region.

Facile Control of Ferroelectricity Driven by Ingenious Interaction Engineering.

Construction of ferroelectric and optimization of macroscopic polarization has attracted tremendous attention for next generation light weight and flexible devices, which brings fundamental vitality for molecular ferroelectrics. However, effective molecular tailoring toward cations makes ferroelectric synthesis and modification relatively elaborate. Here, the study proposes a facile method to realize triggering and optimization of ferroelectricity. The experimental and theoretical investigation reveals that orientation and alignment of polar cations, dominated factors in molecular ferroelectrics, can be controlled by easily processed anionic modification. In one respect, ferroelectricity is induced by strengthened intermolecular interaction. Moreover, ≈50% of microscopic polarization enhancement (from 8.07 to 11.68µCcm-2 ) and doubling of equivalent polarization direction (from 4 to 8) are realized in resultant ferroelectric FEtQ2ZnBrI3 (FEQZBI, FEtQ = N-fluoroethyl-quinuclidine). The work offers a totally novel platform for control of ferroelectricity in organic-inorganic hybrid ferroelectrics and a deep insight of structure-property correlations.

Exploring the electrosorption and surface charge amplification at the ionic liquid/cavity interface: Influence of imidazolium alkyl chain length and the size of the spherical cavities of the porous electrode

The surface charge amplification (SCA) phenomenon at the ionic liquid/cavity interface has been investigated as a result of co-ion electrosorption due to increasing the alkyl chain length of imidazolium. Three types of ionic liquids with different imidazolium alkyl chain lengths inside the different-sized spherical cavities of porous electrodes have been studied using the coarse-grained model within the framework of classical density functional theory (CDFT). Our results show that selectivity occurs at lower potentials for ionic liquids with longer cation chain lengths in smaller cavities. The significant point is that at low negative surface potentials for asymmetric ionic liquids, the electrosorption of anions as co-ions occurs in some spatial regions that are made possible by the accidental orientation of cations in the first layer in the vicinity of the cavity surface. This is evidence of the SCA phenomenon for asymmetric ionic liquids at the ionic liquid/cavity interface. Moreover, the comparison of results, especially the normalized screening strength in the first layer indicates that the smaller cavities containing the ionic liquid with cations that have longer alkyl chains amplify the electrode surface charge more.

Insight into the stabilization mechanism of imidazole-based ionic liquids at the interface of the carbon nanotubes: A computational study

Ab initio calculations have been performed to systematically investigate the structures, binding energies, vibrational spectra, as well as the non-covalent interactions of imidazolium-based ionic liquids (ILs) at the interface of carbon nanotubes (CNTs). The effects of alkyl length of imidazolium cations, types of anions as well as diameters of CNTs on the relevant interfacial structures and properties have been explored in this study. Our calculations demonstrated that the interfacial imidazolium cations display two different orientations (cations paralleling and perpendicular to the CNTs surface) irrespective of the alkyl chain length of cations, types of anions and the diameters of CNTs. By comparison with the calculated binding energies between cations and the CNTs, the cations paralleling to the CNTs surface are thermodynamically much more stable due to the significant interfacial π-π stacking interactions. In addition, our results indicated that there is significantly difference for the infrared (IR) spectra of the imidazolium cations (between two orientations) on the CNTs surface. Therefore, with the IR spectra, the specific orientations of imidazolium cations on surface of CNTs can be better understood. Moreover, interaction region indicator (IRI) as well as symmetry-adapted perturbation theory (SAPT) analysis were employed to deep understanding of the non-covalent interactions between imidazolium-based ILs and CNTs. Our calculations explore that the paralleling cations is stabilized on the CNTs surface by the dispersion (especially for the π-π stacking) interactions. However, for the cations perpendicular to the CNTs surface, electrostatic, induction as well as the dispersion interactions are all very important. Our calculated results provide a molecular level understanding the stabilization mechanism of imidazole-based ILs at the interface of the CNTs and will be a bit favorable to clarify the existing debate on the stabilization mechanism from various experimental observations.

Iodide and charge migration at defective surfaces of methylammonium lead triiodide perovskites: The role of hydrogen bonding

Methylammonium lead triiodide perovskite (CH3NH3PbI3) solar cells have exhibited impressive potential in photovoltaic applications, encouraging further work to improve operating stability. Ionic migration has been proposed to be a possible cause of the current–voltage hysteresis which is responsible for the instability of the CH3NH3PbI3 solar cells. Motivated by the variation of the surface geometries of the CH3NH3PbI3 material when going from the pristine to defective surfaces with the iodine vacancies, we adopt the density functional theory method to study the iodide migration along different paths at the defective surfaces followed by employing the time-dependent density functional theory method to calculate the light-induced electronic states along the paths. We find that the iodide movement is largely tuned by the reorientation and rotation of the organic cations with the disruption and formation of the hydrogen bonding during the iodide migration. Besides, the migration of the light-induced charge carriers varies along the paths with different orientations of the organic cations. We attribute the complicated dynamics of the mobile iodide ions and the associated trap states to the hydrogen bonding at the surfaces, providing crucial guidance for improving the stability and charge carrier lifetime in the CH3NH3PbI3 and other metal halide perovskites.

Influence of cation shape asymmetry on the interfacial features and capacitance curve of ionic liquids inside the spherical cavity of the porous electrode as an ionic liquid-based supercapacitor

Two imidazolium-based ionic liquids with the same anions but different cations (one was considered as a dimer model while the other as a monomer one) have been compared in some interfacial phenomena such as overscreening, surface charge amplification, and crowding, besides the shape of the capacitance curve inside the spherical cavity of the porous electrode. The coarse-grained model in the framework of classical density functional theory, CDFT, has been used to investigate the different electric double layer structures of these two models through different behaviors for their mean electric potential, MEP, local volume charge density, LVCD, and normalized integrated charge density, NICD, at negative electrode potentials. Although both models illustrate charge reversal, CR, and crowding phenomena, respectively, at moderate and high electrode potentials, at low electrode potentials close to the potential of zero charge, PZC, the monomer model illustrates CR, while the dimer model shows surface charge amplification, SCA. This different behavior may be explained by the fact that anions, due to their small sizes, are positioned in the spatial region that is created by the orientations of dimer cations at the electrode surface. This leads to a negative LVCD and partial charge density in the first layer near the electrode. The normalized integrated charge density as a function of r/σ can be used as a criterion for determining the type of screening phenomenon with curvature. Our results show that regardless of the cavity size, the screening behavior of ionic liquids in different cavities is similar, although its intensity varies with size. Other consequences of cation shape asymmetry for dimer models are related to fewer stored net ion charges inside the cavity at the negative potential region and also an asymmetric shape for the capacitance curve. It has also been identified that the shape of the capacitance curve is a charge concentration effect rather than a geometric effect.

Counter Anion Effects on the Formation and Structural Transformations of Mo(vi)-Hydrazone Coordination Assemblies: Salts, Solvates, Co-Crystals, and Neutral Complexes

Complex salts [1H]X and [1H](XA)0.5·2MeOH, and co-crystals [1H]X·0.5VA (X = chloride or bromide, XA = chloranilate or bromanilate, VA = o-vanillin azine), comprising [MoO2(HL)(MeOH)]+ ([1H]+) cation (H2L = 3-methoxysalicylaldehyde isonicotinoyl hydrazone), were prepared either by solution-based synthesis or by mechanochemical synthesis. Whereas [1H]X salts were extremely sensitive to humidity, their stability could be reinforced by the azine incorporation into the complex network. Solvent-mediated transformations of [1H]X led to methanol co-ligand replacement and afforded complexes [MoO2(HL)X] (2Cl·MeOH, 2Cl, and 2Br·0.5MeCN). However, solvates [1H](XA)0.5·2MeOH, under the same conditions, gave stable complexes [1H](XA)0.5 in which methanol remained coordinated. The differences in the assembly’s behavior were attributed to the packing arrangements, the relative orientation of cations and anions, and interactions between them. Polymorph [MoO2(L)(MeOH)] (1), not attainable by other routes, was the only product when compounds [MoO2(HL)X] were treated with a weak base at low temperatures. Tetranuclear [MoO2(L)]4 and polynuclear [MoO2(L)]n (2) supramolecular isomers, concomitantly crystallized when the reaction was conducted solvothermally. All of the complexes were characterized using X-ray diffraction methods (SCXRD and PXRD), spectroscopic methods (ATR-IR and solution-state and solid-state MAS NMR), and elemental and thermal analyses. The cytotoxicity of the different types of compounds against THP-1 and HepG2 cells was also evaluated.

Efficient separation of CO2/CH4 by ionic liquids confined in graphene oxide: A molecular dynamics simulation

Ionic liquids (ILs)/graphene oxide (GO) membranes have been regarded as a prospective alternative in CO2 separation. However, the correlation between the microstructure of ILs/GO and CO2 separation performance is unclear. In this work, the dynamic properties and interactions for CO2/CH4 in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][TF2N]) confined between two GO sheets with different layer spacings were studied using molecular dynamics simulation. The number density and angular orientation of cations suggest that there is a dense cation adsorption layer near GO with the imidazole rings and alkyl side chains mostly paralleled to GO surface. The strong interaction of GO-cations weakens the interaction of cations-anions, facilitating the adsorption of CO2. The overlapping distribution regions of the number density of CO2/CH4 with the cations and anions reveal that CO2/CH4 mainly distribute around the ILs area of the ILs/GO membrane, which improves the gas selectivity. The RDFs results of CO2/CH4-ILs indicate that the confined ILs/GO system is more favorable for capturing gases than bulk ILs. The stronger interaction of CO2-anions/cations and the faster diffusion of CO2 than CH4 reflect the high solubility selectivity and diffusion selectivity for CO2/CH4 in ILs/GO membrane. In addition, it was found that decreasing the layer spacing would increase the solubility selectivity, but could decrease the diffusion selectivity. However, solubility selectivity plays a dominant role, thus 2 nm is the optimal layer spacing. Furthermore, the low viscosity ILs were found to be beneficial to improve the diffusion selectivity. Finally, [Bmim][TF2N]/GO membrane is predicted to possess superior CO2/CH4 separation performance than [Bmim][PF6]/GO and [Bmim][BF4]/GO membranes.

Electron Dynamics in Hybrid Perovskites Reveal the Role of Organic Cations on the Screening of Local Charges.

The large tolerance of hybrid perovksites to the trapping of electrons by defects is a key asset in photovoltaic applications. Here, the ionic surface terminations of CH3NH3PbI3 are employed as a testbed to study the effect of electrostatic fields on the dynamics of excited carriers. We characterize the transition across the tetragonal to orthorhombic phase. The observed type II band offset and drift of the excited electrons highlight the important role that organic cations have on the screening of local electrostatic fields. When the orientation of organic cations is frozen in the orthorhombic phase, the positively charged termination induces a massive accumulation of excited electrons at the surface of the sample. Conversely, no electron accumulation is observed in the tetragonal phase. We conclude that the local fields cannot penetrate in the sample when the polarizability of freely moving cations boosts the dielectric constant up to ε = 120.

The Role of Position and Orientation of Organic Fa Central Cation in Physical Properties of Formamidinium Lead Chloride Perovskite

The Role of Position and Orientation of Organic Fa Central Cation in Physical Properties of Formamidinium Lead Chloride Perovskite

Role of organic cation orientation in formamidine based perovskite materials

The rotation of organic cations is considered to be an important reason for the dynamic changes in stability and photoelectric properties of organic perovskites. However, the specific effect of organic cations rotation on formamidine based perovskite is still unknown. In our work, first-principles calculations based on density functional theory are used to examine the effect of the rotation of formamidine cations in FAPbI3 and FA0.875Cs0.125PbI3. We have comprehensively calculated the structure, electronic and optical properties of them. We found a coupling effect between formamidine cations rotation and cesium atom. This coupling effect changes the inclination angle of octahedron to regulate electron distribution, band gaps, and optical absorption. Hence, changing the cation orientation and substitution atom is a feasible way to dynamically adjust the energy band, dielectric constant and absorption edge of perovskite. Preparing perovskite with tunable properties is just around the corner through this way.

Deciphering the Orientation of the Aromatic Spacer Cation in Bilayer Perovskite Solar Cells through Spectroscopic Techniques.

Slowing the degradation of perovskite-based solar cells (PSCs) is of substantial interest. We engineered the surface by introducing a hydrophobic overlayer on a three-dimensional (3D) perovskite using fluorinated or nonfluorinated aryl ammonium cation spacers. The placement of a fluoroarene cation allows the formation of a bilayer structure, that is, layered/3D perovskites. By doing so, the surface hydrophobic character increases notably by the virtue of the perfluorinated benzene moiety. The fabricated devices thereof gave higher performance and longevity than control devices in addition to boosting reliability. The fluoro-phenethylammonium iodide (FPEAI)-based devices showed lower nonradiative carrier recombination. To decipher the orientation of the spacer cation in this bilayer structure, we probed the surface by polarization-modulated infrared reflection-absorption spectroscopy and noted substantial differences in the orientation due to the presence of fluorine substitution. We hypothesize that the stronger van der Waals interactions due to the higher electronegativity in FPEAI govern the orientation and performance enhancement and act as a barrier to moisture decomposition.

Two Ongoing Magnetic Transitions Originating from Spin-Peierls Dimerization and Cation Orientation Transformation in an S = 1/2 Spin Chain System

Two Ongoing Magnetic Transitions Originating from Spin-Peierls Dimerization and Cation Orientation Transformation in an <i>S</i> = 1/2 Spin Chain System

Theoretical study of the synthesis, characterization and hydrogen storage properties of a high-density hydrogen storage material: (CH3NH3)BH4

Inspired by both alkaline metal borohydrides and organic-inorganic hybrid perovskite, we predict a pair of complex structures of (CH3NH3)BH4 with tremendous high hydrogen capacity (21.27 wt.%). Through comparison and analysis of the electronic structures of alkali metal atoms, CH3NH3, NH4, and NH3BH3 molecule, it is concluded that similar spatial and electronic structures show the feasibility of synthesizing (CH3NH3)BH4 by a substitution reaction. Firstly, theoretical structures (S1 and S2 in P1) with stable configurations have been reconstructed by cation substitution followed by a series of restrictive structural optimizations, and both the lattice parameters and the position coordinate information of (CH3NH3)BH4 are obtained. Ignoring the relatively mobile hydrogen, the structural symmetries of S1 and S2 are I4mm and P4/nmm, respectively. X-ray diffraction characterizations of S1 and S2 are consistent with the experimental results. Secondly, the calculated elastic constants of (CH3NH3)BH4 (S1 and S2) with P1 symmetry indicate that angles α, β and γ oscillate at right angles due to the influence of the cation orientation. The calculated spatial dependence of bulk (B), Young's (E), and shear (G) modulus obviously show that the two P1 phases all have strong elastic anisotropy. Thirdly, the calculated electronic properties show that the protonic amine-H, hydridic borane-H, and neutral methane-H are widely distributed in (CH3NH3)BH4, which allow for weaving in a planar dihydrogen bonding network, which in turn influences the dehydrogenation reaction. Last and most important, we propose the following dehydrogenation process of (CH3NH3)BH4 via the intermediate compounds: 2(CH3NH3)BH4 → CH3NH2BH2NHCH3BH3+3H2. For each dehydrogenation step, the free energy change is negative, which means (CH3NH3)BH4 can decompose spontaneously, similar to ammonium borohydride, which is strongly related to the planar dihydrogen bonding network.

Novel (110) Double-Layered Guanidinium-Lead Iodide Perovskite Material: Crystal Structure, Electronic Structure, and Broad Luminescence

In this work, a new crystalline structure of two-dimensional (2D) Ruddlesden–Popper guanidinium lead iodide [C(NH2)3]2PbI4 with a particular organic cation orientation is proposed. Density function...