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2020 Volume 36 Issue 11
2020, 36(11): 190601
doi: 10.3866/PKU.WHXB201906019
Abstract:
It is important to determine the effects of misfit dislocations and other defects on the domain structure, ferroelectricity, conductivity, and other physical properties of ferroelectric thin films to understand their ferroelectric and piezoelectric behaviors. Much attention has been given to ferroelectric PbTiO3/SrTiO3 or PbZr0.2Ti0.8O3/SrTiO3 heterointerfaces, at which improper ferroelectricity, a spin-polarized two-dimensional electron gas, and other physical phenomena have been found. However, those heterointerfaces were all (001) planes, and there has been no experimental studies on the growth of (010) PbTiO3/SrTiO3 heterointerface due to the 6.4% misfit between two materials. In this study, we selected an atomically flat (010) PbTiO3/SrTiO3 heterointerface grown using a two-step hydrothermal method as the research subject, and this is the first experimental report on that interface. Interfacial dislocations can play a significant role in causing dramatic changes in the Curie temperature and polarization distribution near the dislocation cores, especially when the size of a ferroelectric thin film is scaled down to the nanoscale. The results of previous studies on the effects of interfacial dislocations on the physical properties of ferroelectric thin films have been contradictory. Thus, this issue needs to be explored more deeply in the future. This study used aberration corrected scanning transmission electron microscopy (STEM) to study the atomic structure of a (010) PbTiO3/SrTiO3 heterointerface and found periodic misfit dislocations with a Burgers vector of a[001]. The extra planes at the dislocation cores could relieve the misfit strain between the two materials in the [001] direction and thus allowed the growth of such an atomically sharp heterointerface. Moreover, monochromated electron energy-loss spectroscopy with an atomic scale spatial resolution and high energy resolution was used to explore the charge distribution near the periodic misfit dislocation cores. The fine structure of the Ti L edge was quantitatively analyzed by linearly fitting the experimental spectra recorded at various locations near and at the misfit dislocation cores with the Ti3+ and Ti4+ reference spectra. Therefore, the accurate valence change of Ti could be determined, which corresponded to the charge distribution. The probable existence of an aggregation of electrons was found near the a[001] dislocation cores, and the density of the electrons calculated from the valence change was 0.26 electrons per unit cell. Based on an analysis of the fine structure of the oxygen K edge, it could be argued that the electrons aggregating at the dislocation cores came from the oxygen vacancies in the interior regions of the PbTiO3. This aggregation of electrons will probably increase the electron conductivity along the dislocation line. The physics of two-dimensional charge distributions at oxide interfaces have been intensively studied, however, little attention had been given to the one-dimensional charge distribution. Therefore, the results of this study can stimulate research interest in exploring the influence of the interfacial dislocations on the physics of ferroelectric heterointerfaces. ![]()
It is important to determine the effects of misfit dislocations and other defects on the domain structure, ferroelectricity, conductivity, and other physical properties of ferroelectric thin films to understand their ferroelectric and piezoelectric behaviors. Much attention has been given to ferroelectric PbTiO3/SrTiO3 or PbZr0.2Ti0.8O3/SrTiO3 heterointerfaces, at which improper ferroelectricity, a spin-polarized two-dimensional electron gas, and other physical phenomena have been found. However, those heterointerfaces were all (001) planes, and there has been no experimental studies on the growth of (010) PbTiO3/SrTiO3 heterointerface due to the 6.4% misfit between two materials. In this study, we selected an atomically flat (010) PbTiO3/SrTiO3 heterointerface grown using a two-step hydrothermal method as the research subject, and this is the first experimental report on that interface. Interfacial dislocations can play a significant role in causing dramatic changes in the Curie temperature and polarization distribution near the dislocation cores, especially when the size of a ferroelectric thin film is scaled down to the nanoscale. The results of previous studies on the effects of interfacial dislocations on the physical properties of ferroelectric thin films have been contradictory. Thus, this issue needs to be explored more deeply in the future. This study used aberration corrected scanning transmission electron microscopy (STEM) to study the atomic structure of a (010) PbTiO3/SrTiO3 heterointerface and found periodic misfit dislocations with a Burgers vector of a[001]. The extra planes at the dislocation cores could relieve the misfit strain between the two materials in the [001] direction and thus allowed the growth of such an atomically sharp heterointerface. Moreover, monochromated electron energy-loss spectroscopy with an atomic scale spatial resolution and high energy resolution was used to explore the charge distribution near the periodic misfit dislocation cores. The fine structure of the Ti L edge was quantitatively analyzed by linearly fitting the experimental spectra recorded at various locations near and at the misfit dislocation cores with the Ti3+ and Ti4+ reference spectra. Therefore, the accurate valence change of Ti could be determined, which corresponded to the charge distribution. The probable existence of an aggregation of electrons was found near the a[001] dislocation cores, and the density of the electrons calculated from the valence change was 0.26 electrons per unit cell. Based on an analysis of the fine structure of the oxygen K edge, it could be argued that the electrons aggregating at the dislocation cores came from the oxygen vacancies in the interior regions of the PbTiO3. This aggregation of electrons will probably increase the electron conductivity along the dislocation line. The physics of two-dimensional charge distributions at oxide interfaces have been intensively studied, however, little attention had been given to the one-dimensional charge distribution. Therefore, the results of this study can stimulate research interest in exploring the influence of the interfacial dislocations on the physics of ferroelectric heterointerfaces.
2020, 36(11): 190803
doi: 10.3866/PKU.WHXB201908035
Abstract:
The radioactivity and toxicity of actinides impede experimental investigation into their chemical properties in the condensed phase. The rapid development of computational methods and computational facilities allows for alternative experimental methods, including the use of a molecular force field, to gain insight into the coordination chemistry and dynamics of actinides. The key to this method is the force fieild parameters. In the present work, we report the development and validation of the AMBER (Assisted Model Building with Energy Refinement) force field parameters for Np4+, Am3+, and Cm3+ based on the experimentally determined ion-oxygen distance (IOD). The parameter set, together with that reported for Th4+, U4+, and Pu4+, was then applied to investigate the coordination chemistry and dynamics of these six actinide ions in the aqueous phase, in the absence and presence of counterions Cl-, NO3-, and CO32-. The simulations showed a shorter An-Ow coordination length for An4+ than for An3+, and for higher atomic numbers of ions in the same valence state. Th4+ preferentially existed in a 10-coordinated state, adopting a BCASP (bicapped square antiprism) conformation, while the other ions tended to be 9-coordinated with a CASP (capped square antiprism) conformation. The only exception was Cm3+, which adopted a TCTP (tricapped trigonal prism) conformation. The results also showed that the water molecules around An4+ were more ordered than those around An3+, as indicated by the smaller angles between the An-Ow vector and the dipole direction of the water ligand. This highly ordered structure of coordinated water affected their translation and rotation, i.e., the diffusion coefficient and rotational relaxation time of the water molecules around An4+ were smaller than those in the case of An3+ due to the stronger electrostatic interaction between An4+ and ligating water. The hydration free energies of the targeted actinide ions were also calculated by the FEP (free energy perturbation) method. An4+ underwent a greater degree of stabilization than did An3+ upon hydration; among the ions in the same oxidation states, those with a higher atomic number were better stabilized. In summary, the results of the simulations were consistent with the literature data in terms of the hydration structure, coordination of counterions, and hydration free energy of the actinide ions. The ability of the parameter set to describe the dynamics of water in the vicinity of actinides remains to be verified due to the lack of reference data. We tentatively propose that it may be used to investigate the coordination chemistry of actinides both in conformational analysis and binding strength, while special care should be taken when studying the kinetics of the solvated system. This work is expected to enrich our understanding of the solution behavior of An3+/An4+ at the force field level. ![]()
The radioactivity and toxicity of actinides impede experimental investigation into their chemical properties in the condensed phase. The rapid development of computational methods and computational facilities allows for alternative experimental methods, including the use of a molecular force field, to gain insight into the coordination chemistry and dynamics of actinides. The key to this method is the force fieild parameters. In the present work, we report the development and validation of the AMBER (Assisted Model Building with Energy Refinement) force field parameters for Np4+, Am3+, and Cm3+ based on the experimentally determined ion-oxygen distance (IOD). The parameter set, together with that reported for Th4+, U4+, and Pu4+, was then applied to investigate the coordination chemistry and dynamics of these six actinide ions in the aqueous phase, in the absence and presence of counterions Cl-, NO3-, and CO32-. The simulations showed a shorter An-Ow coordination length for An4+ than for An3+, and for higher atomic numbers of ions in the same valence state. Th4+ preferentially existed in a 10-coordinated state, adopting a BCASP (bicapped square antiprism) conformation, while the other ions tended to be 9-coordinated with a CASP (capped square antiprism) conformation. The only exception was Cm3+, which adopted a TCTP (tricapped trigonal prism) conformation. The results also showed that the water molecules around An4+ were more ordered than those around An3+, as indicated by the smaller angles between the An-Ow vector and the dipole direction of the water ligand. This highly ordered structure of coordinated water affected their translation and rotation, i.e., the diffusion coefficient and rotational relaxation time of the water molecules around An4+ were smaller than those in the case of An3+ due to the stronger electrostatic interaction between An4+ and ligating water. The hydration free energies of the targeted actinide ions were also calculated by the FEP (free energy perturbation) method. An4+ underwent a greater degree of stabilization than did An3+ upon hydration; among the ions in the same oxidation states, those with a higher atomic number were better stabilized. In summary, the results of the simulations were consistent with the literature data in terms of the hydration structure, coordination of counterions, and hydration free energy of the actinide ions. The ability of the parameter set to describe the dynamics of water in the vicinity of actinides remains to be verified due to the lack of reference data. We tentatively propose that it may be used to investigate the coordination chemistry of actinides both in conformational analysis and binding strength, while special care should be taken when studying the kinetics of the solvated system. This work is expected to enrich our understanding of the solution behavior of An3+/An4+ at the force field level.
2020, 36(11): 191001
doi: 10.3866/PKU.WHXB201910010
Abstract:
Natural gas hydrates are considered as ideal alternative energy resources for the future, and the relevant basic and applied research has become more attractive in recent years. The influence of guest molecules on the hydrate crystal lattice parameters is of great significances to the understanding of hydrate structural characteristics, hydrate formation/decomposition mechanisms, and phase stability behaviors. In this study, we test a series of artificial hydrate samples containing different guest molecules (e.g. methane, ethane, propane, iso-butane, carbon dioxide, tetrahydrofuran, methane + 2, 2-dimethylbutane, and methane + methyl cyclohexane) by a low-temperature powder X-ray diffraction (PXRD). Results show that PXRD effectively elucidates structural characteristics of the natural gas hydrate samples, including crystal lattice parameters and structure types. The relationships between guest molecule sizes and crystal lattice parameters reveal that different guest molecules have different controlling behaviors on the hydrate types and crystal lattice constants. First, a positive correlation between the lattice constants and the van der Waals diameters of homologous hydrocarbon gases was observed in the single-guest-component hydrates. Small hydrocarbon homologous gases, such as methane and ethane, tended to form sI hydrates, whereas relatively larger molecules, such as propane and iso-butane, generated sⅡ hydrates. The hydrate crystal lattice constants increased with increasing guest molecule size. The types of hydrates composed of oxygen-containing guest molecules (such as CO2 and THF) were also controlled by the van der Waals diameters. However, no positive correlation between the lattice constants and the van der Waals diameters of guest molecules in hydrocarbon hydrates was observed for CO2 hydrate and THF hydrate, probably due to the special interactions between the guest oxygen atoms and hydrate "cages". Furthermore, the influences of the macromolecules and auxiliary small molecules on the lengths of the different crystal axes of the sH hydrates showed inverse trends. Compared to the methane + 2, 2-dimethylbutane hydrate sample, the length of the a-axis direction of the methane + methyl cyclohexane hydrate sample was slightly smaller, whereas the length of the c-axis direction was slightly longer. The crystal a-axis length of the sH hydrate sample formed with nitrogen molecules was slightly longer, whereas the c-axis was shorter than that of the methane + 2, 2-dimethylbutane hydrate sample at the same temperature. ![]()
Natural gas hydrates are considered as ideal alternative energy resources for the future, and the relevant basic and applied research has become more attractive in recent years. The influence of guest molecules on the hydrate crystal lattice parameters is of great significances to the understanding of hydrate structural characteristics, hydrate formation/decomposition mechanisms, and phase stability behaviors. In this study, we test a series of artificial hydrate samples containing different guest molecules (e.g. methane, ethane, propane, iso-butane, carbon dioxide, tetrahydrofuran, methane + 2, 2-dimethylbutane, and methane + methyl cyclohexane) by a low-temperature powder X-ray diffraction (PXRD). Results show that PXRD effectively elucidates structural characteristics of the natural gas hydrate samples, including crystal lattice parameters and structure types. The relationships between guest molecule sizes and crystal lattice parameters reveal that different guest molecules have different controlling behaviors on the hydrate types and crystal lattice constants. First, a positive correlation between the lattice constants and the van der Waals diameters of homologous hydrocarbon gases was observed in the single-guest-component hydrates. Small hydrocarbon homologous gases, such as methane and ethane, tended to form sI hydrates, whereas relatively larger molecules, such as propane and iso-butane, generated sⅡ hydrates. The hydrate crystal lattice constants increased with increasing guest molecule size. The types of hydrates composed of oxygen-containing guest molecules (such as CO2 and THF) were also controlled by the van der Waals diameters. However, no positive correlation between the lattice constants and the van der Waals diameters of guest molecules in hydrocarbon hydrates was observed for CO2 hydrate and THF hydrate, probably due to the special interactions between the guest oxygen atoms and hydrate "cages". Furthermore, the influences of the macromolecules and auxiliary small molecules on the lengths of the different crystal axes of the sH hydrates showed inverse trends. Compared to the methane + 2, 2-dimethylbutane hydrate sample, the length of the a-axis direction of the methane + methyl cyclohexane hydrate sample was slightly smaller, whereas the length of the c-axis direction was slightly longer. The crystal a-axis length of the sH hydrate sample formed with nitrogen molecules was slightly longer, whereas the c-axis was shorter than that of the methane + 2, 2-dimethylbutane hydrate sample at the same temperature.
2020, 36(11): 191104
doi: 10.3866/PKU.WHXB201911044
Abstract:
Two-dimensional graphene nanopores have proved to be a very effective molecular sieve with ultra-high molecular permeance due to the atomic thickness of graphene sheets. The mechanism of graphene nanopores for molecular sieving is generally the size-sieving effect of different molecules. However, high-selective molecular separation is difficult to realize based only on the size-sieving effect. Therefore, graphene nanopore-based membranes usually present high permeance but a moderate selectivity, such that the separation performance cannot far exceed those of traditional separation membranes. In this study, the effects of charges on graphene surfaces on the selective permeation of CO2/N2 mixtures through a graphene nanopore is studied using molecular dynamics simulations; its purpose to realize electrostatic effect-based selective molecular permeation through graphene nanopores and find a promising method to improve the selectivity of molecular separation. The simulation results show that graphene nanopores with negative charges have higher CO2 permeance and lower N2 permeance and, thus, present a high selectivity for the separation of the CO2/N2 mixtures. The graphene nanopore with positive charges, however, does not improve the selectivity. The electrostatic effect-based selectivity of graphene nanopores is related to the different molecular adsorption abilities on the graphene surface with charges. For negative charges, the adsorption ability of CO2 molecules increases and the number of permeated molecules via surface mechanism increases and the experience time during the permeation process also increases; ultimately the CO2 permeance increases with increasing the charge density. For the molecules permeated through the surface mechanism, they are firstly adsorbed onto the graphene surface and then diffuse to the pore region for the ultimate permeation; thus, their experience time is longer than that of the molecules permeated through a direct mechanism. Therefore, a longer experience time means a more significant contribution of the surface flux to the total flux. At high surface charge densities, the contribution of surface flux is dominated and thus the experience time is longer. For CO2 molecules, the permeation rates increase with increasing the surface charge density. Namely, a higher experience time corresponds to a higher permeation rate for CO2 molecules. A decrease of N2 permeance with increasing the charge density is correlated to the increasing CO2 permeance via the inhibition effects of non-permeating components on the permeation of permeating components. For positive charges, the adsorption abilities of CO2 and N2 molecules have no obvious variation with the charge density and their permeance is constant; therefore, the graphene nanopore still has no electrostatic effect-based selectivity. ![]()
Two-dimensional graphene nanopores have proved to be a very effective molecular sieve with ultra-high molecular permeance due to the atomic thickness of graphene sheets. The mechanism of graphene nanopores for molecular sieving is generally the size-sieving effect of different molecules. However, high-selective molecular separation is difficult to realize based only on the size-sieving effect. Therefore, graphene nanopore-based membranes usually present high permeance but a moderate selectivity, such that the separation performance cannot far exceed those of traditional separation membranes. In this study, the effects of charges on graphene surfaces on the selective permeation of CO2/N2 mixtures through a graphene nanopore is studied using molecular dynamics simulations; its purpose to realize electrostatic effect-based selective molecular permeation through graphene nanopores and find a promising method to improve the selectivity of molecular separation. The simulation results show that graphene nanopores with negative charges have higher CO2 permeance and lower N2 permeance and, thus, present a high selectivity for the separation of the CO2/N2 mixtures. The graphene nanopore with positive charges, however, does not improve the selectivity. The electrostatic effect-based selectivity of graphene nanopores is related to the different molecular adsorption abilities on the graphene surface with charges. For negative charges, the adsorption ability of CO2 molecules increases and the number of permeated molecules via surface mechanism increases and the experience time during the permeation process also increases; ultimately the CO2 permeance increases with increasing the charge density. For the molecules permeated through the surface mechanism, they are firstly adsorbed onto the graphene surface and then diffuse to the pore region for the ultimate permeation; thus, their experience time is longer than that of the molecules permeated through a direct mechanism. Therefore, a longer experience time means a more significant contribution of the surface flux to the total flux. At high surface charge densities, the contribution of surface flux is dominated and thus the experience time is longer. For CO2 molecules, the permeation rates increase with increasing the surface charge density. Namely, a higher experience time corresponds to a higher permeation rate for CO2 molecules. A decrease of N2 permeance with increasing the charge density is correlated to the increasing CO2 permeance via the inhibition effects of non-permeating components on the permeation of permeating components. For positive charges, the adsorption abilities of CO2 and N2 molecules have no obvious variation with the charge density and their permeance is constant; therefore, the graphene nanopore still has no electrostatic effect-based selectivity.
2020, 36(11): 191206
doi: 10.3866/PKU.WHXB201912068
Abstract:
As the application of lithium-ion batteries in advanced consumer electronics, energy storage systems, plug-in hybrid electric vehicles, and electric vehicles increases, there has emerged an urgent need for increasing the energy density of such batteries. Lithium metal anode is considered as the "Holy Grail" for high-energy-density electrochemical energy storage systems because of its low reduction potential (-3.04 V vs standard hydrogen electrode) and high theoretical specific capacity (3860 mAh·g-1). However, the practical application of lithium metal anode in rechargeable batteries is severely limited by irregular lithium dendrite growth and high reactivity with the electrolytes, leading to poor safety performance and low coulombic efficiency. Recent research progress has been well documented to suppress dendrite growth for achieving long-term stability of lithium anode, such as building artificial protection layers, developing novel electrolyte additives, constructing solid electrolytes, using functional separator, designing composite electrode or three-dimensional lithium-hosted material. Among them, the use of electrolyte additives is regarded as one of the most effective and economical methods to improve the performance of lithium-ion batteries. As a natural polyphenol compound, tannic acid (TA) is significantly cheaper and more abundant compared with dopamine, which is widely used for the material preparation and modification in the field of lithium-ion batteries. Herein, TA is first reported as an efficient electrolyte film-forming additive for lithium metal anode. By adding 0.15% (mass fraction, wt.) TA into the base electrolyte of 1 mol·L-1 LiPF6-EC/DMC/EMC (1 : 1 : 1, by wt.), the symmetric Li|Li cell exhibited a more stable cyclability of 270 h than that of only 170 h observed for the Li|Li cell without TA under the same current density of 1 mA·cm-2 and capacity of 1 mAh·cm-2 (with a cutoff voltage of 0.1 V). Electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy, cyclic voltammetry (CV), and energy-dispersive X-ray spectroscopy (EDS) analyses demonstrated that TA participated in the formation of a dense solid electrolyte interface (SEI) layer on the surface of the lithium metal. A possible reaction mechanism is proposed here, wherein the small amount of added polyphenol compound could have facilitated the formation of LiF through the hydrolysis of LiPF6, following which the resulting phenoxide could react with dimethyl carbonate (DMC) through transesterification to form a cross-linked polymer, thereby forming a unique organic/inorganic composite SEI film that significantly improved the electrochemical performance of the lithium metal anode. These results demonstrate that TA can be used as a promising film-forming additive for the lithium metal anode. ![]()
As the application of lithium-ion batteries in advanced consumer electronics, energy storage systems, plug-in hybrid electric vehicles, and electric vehicles increases, there has emerged an urgent need for increasing the energy density of such batteries. Lithium metal anode is considered as the "Holy Grail" for high-energy-density electrochemical energy storage systems because of its low reduction potential (-3.04 V vs standard hydrogen electrode) and high theoretical specific capacity (3860 mAh·g-1). However, the practical application of lithium metal anode in rechargeable batteries is severely limited by irregular lithium dendrite growth and high reactivity with the electrolytes, leading to poor safety performance and low coulombic efficiency. Recent research progress has been well documented to suppress dendrite growth for achieving long-term stability of lithium anode, such as building artificial protection layers, developing novel electrolyte additives, constructing solid electrolytes, using functional separator, designing composite electrode or three-dimensional lithium-hosted material. Among them, the use of electrolyte additives is regarded as one of the most effective and economical methods to improve the performance of lithium-ion batteries. As a natural polyphenol compound, tannic acid (TA) is significantly cheaper and more abundant compared with dopamine, which is widely used for the material preparation and modification in the field of lithium-ion batteries. Herein, TA is first reported as an efficient electrolyte film-forming additive for lithium metal anode. By adding 0.15% (mass fraction, wt.) TA into the base electrolyte of 1 mol·L-1 LiPF6-EC/DMC/EMC (1 : 1 : 1, by wt.), the symmetric Li|Li cell exhibited a more stable cyclability of 270 h than that of only 170 h observed for the Li|Li cell without TA under the same current density of 1 mA·cm-2 and capacity of 1 mAh·cm-2 (with a cutoff voltage of 0.1 V). Electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy, cyclic voltammetry (CV), and energy-dispersive X-ray spectroscopy (EDS) analyses demonstrated that TA participated in the formation of a dense solid electrolyte interface (SEI) layer on the surface of the lithium metal. A possible reaction mechanism is proposed here, wherein the small amount of added polyphenol compound could have facilitated the formation of LiF through the hydrolysis of LiPF6, following which the resulting phenoxide could react with dimethyl carbonate (DMC) through transesterification to form a cross-linked polymer, thereby forming a unique organic/inorganic composite SEI film that significantly improved the electrochemical performance of the lithium metal anode. These results demonstrate that TA can be used as a promising film-forming additive for the lithium metal anode.
2020, 36(11): 200401
doi: 10.3866/PKU.WHXB202004014
Abstract:
The structures of ionic liquids (ILs) based on 1-alkyl-3-methylimidazolium chloride [Cnmim]Cl (n = 2, 4, 6), (1-ethyl-3-methylimidazolium chloride [C2mim]Cl, 1-butyl-3-methylimidazolium chloride [C4mim]Cl, and 1-hexyl-3-methylimidazolium chloride [C6mim]Cl) were elucidated by1 H NMR and 13 C NMR experiments. The vaporization characteristics of these ILs were studied by thermogravimetric analysis. Dynamic and isothermal thermogravimetric experiments were conducted in this study. The purpose of the dynamic experiments was to determine the initial decomposition temperature of the experimental sample and the temperature range for the isothermal thermogravimetric experiments. The purpose of the isothermal experiments was to record the mass dependence of the sample on time in the experimental temperature range. The Langmuir equation and Clausius-Clapeyron equation were used to fit the experimental data and obtain the vaporization enthalpies of these ILs at the average temperature within the experimental temperature range. However, in order to expand the applicability of the estimated values and to compare them with the literature data, the vaporization enthalpy ΔHvap(Tav) measured at the average temperature was converted into vaporization enthalpy ΔHvap(298) at ambient temperature. The difference between the heat capacities of the ILs in the gaseous and liquid states at constant pressure, ΔlgCpmө proposed by Verevkin, was used in this conversion process. The experimental data for substance density and surface tension at other temperatures were obtained by referring to the literature. In addition, the data for density and surface tension at T = 298.15 K were obtained by applying the extrapolation method to the literature values for other temperatures. The vaporization enthalpy of the 1-octyl-3-methylimidazolium chloride IL [C8mim]Cl was estimated by using the new vaporization model we had proposed in our previous work and compared with the reference value. The estimated value for [C8mim]Cl was on the same order of magnitude as the reference value. We compared the vaporization enthalpies in the present study with those for the carboxylic acid imidazolium and amino acid imidazolium ILs ([Cnmim]Pro (n = 2-6) and [Cnmim]Thr (n = 2-6), respectively in our previous work. The results revealed that a change in the anion type affects the vaporization enthalpy of the ILs in the order amino acid imidazolium > carboxylic acid imidazolium > halogen imidazolium, when the cation is the same. Considering the structural differences between the three kinds of ILs, the abovementioned order may be related to the intermolecular hydrogen bonds. There were no intermolecular hydrogen bonds in the [Cnmim]Cl (n = 2, 4, 6) ILs studied here. Therefore, the vaporization enthalpy of [Cnmim]Cl (n = 2, 4, 6) was the lowest among the three kinds of ILs considered. ![]()
The structures of ionic liquids (ILs) based on 1-alkyl-3-methylimidazolium chloride [Cnmim]Cl (n = 2, 4, 6), (1-ethyl-3-methylimidazolium chloride [C2mim]Cl, 1-butyl-3-methylimidazolium chloride [C4mim]Cl, and 1-hexyl-3-methylimidazolium chloride [C6mim]Cl) were elucidated by
2020, 36(11): 190603
doi: 10.3866/PKU.WHXB201906034
Abstract:
The pair of molecular acidity and basicity is one of the most widely used chemical concepts in chemistry, biology, and other related fields. Nevertheless, quantitative determination of these intrinsic physical properties from the perspective of theory and computation is still an unresolved task at present. Earlier, we proposed to utilize the molecular electrostatic potential and natural atomic orbital from conceptual density functional theory for this purpose. Later, we also proposed utilizing quantities from the information-theoretic approach in the density functional reactivity theory such as Shannon entropy, Fisher information, and information gain to quantify electrophilicity, nucleophilicity, regioselectivity, and stereoselectivity. The latter was successfully applied later to five series of molecular systems for determining the molecular acidity, including singly and doubly substituted benzoic acids, benzenesulfonic acids, benzeneseleninic acids, phenols, and alkyl carboxylic acids, whose validity and effectiveness have been sufficiently corroborated. As a continuation of our recent efforts along this line, in this work, we generalize our previous approaches by combining these two approaches together as a new set of descriptors to quantify the molecular basicity. The applicability and usefulness of our new approach are demonstrated hereby by three types of amines, namely, primary, secondary, and tertiary amines, with a total of 179 systems. We show that this new set of descriptors, including the molecular electrostatic potential or its equivalence, the natural valence atomic orbital energy, and quantities from information-theoretic approach such as Shannon entropy, Fisher information, Ghosh-Berkowitz-Parr entropy, information gain, Onicescu information energy, and relative Rényi entropy, is able to accurately predict the experimental pKa values for the three types of amines. Our findings confirm that each of these quantities possesses strong linear correlation with the experimental pKa value, though less significantly than expected. Moreover, when combined, these quantities can yield accurate and quantitative models for determining the molecular basicity of all the three types of amines. The reason behind this is that all these descriptors are simple electron density functionals. According to the basic theorem of density functional theory, they should contain adequate information for the determination of all the physio-chemical properties in the ground state of molecular systems, including molecular acidity and basicity. Our present results predict that this new approach should be readily applicable to many other molecular species, thereby providing an effective and robust approach to appreciate chemical concepts such as acidity and basicity. ![]()
The pair of molecular acidity and basicity is one of the most widely used chemical concepts in chemistry, biology, and other related fields. Nevertheless, quantitative determination of these intrinsic physical properties from the perspective of theory and computation is still an unresolved task at present. Earlier, we proposed to utilize the molecular electrostatic potential and natural atomic orbital from conceptual density functional theory for this purpose. Later, we also proposed utilizing quantities from the information-theoretic approach in the density functional reactivity theory such as Shannon entropy, Fisher information, and information gain to quantify electrophilicity, nucleophilicity, regioselectivity, and stereoselectivity. The latter was successfully applied later to five series of molecular systems for determining the molecular acidity, including singly and doubly substituted benzoic acids, benzenesulfonic acids, benzeneseleninic acids, phenols, and alkyl carboxylic acids, whose validity and effectiveness have been sufficiently corroborated. As a continuation of our recent efforts along this line, in this work, we generalize our previous approaches by combining these two approaches together as a new set of descriptors to quantify the molecular basicity. The applicability and usefulness of our new approach are demonstrated hereby by three types of amines, namely, primary, secondary, and tertiary amines, with a total of 179 systems. We show that this new set of descriptors, including the molecular electrostatic potential or its equivalence, the natural valence atomic orbital energy, and quantities from information-theoretic approach such as Shannon entropy, Fisher information, Ghosh-Berkowitz-Parr entropy, information gain, Onicescu information energy, and relative Rényi entropy, is able to accurately predict the experimental pKa values for the three types of amines. Our findings confirm that each of these quantities possesses strong linear correlation with the experimental pKa value, though less significantly than expected. Moreover, when combined, these quantities can yield accurate and quantitative models for determining the molecular basicity of all the three types of amines. The reason behind this is that all these descriptors are simple electron density functionals. According to the basic theorem of density functional theory, they should contain adequate information for the determination of all the physio-chemical properties in the ground state of molecular systems, including molecular acidity and basicity. Our present results predict that this new approach should be readily applicable to many other molecular species, thereby providing an effective and robust approach to appreciate chemical concepts such as acidity and basicity.
2020, 36(11): 190803
doi: 10.3866/PKU.WHXB201908036
Abstract:
Due to their special polar structure, amphiphilic molecules are simple to process, low in cost and excellent in material properties. Thus, they can be widely applied in the preparation of functional film materials and bionics related to cell membranes. Therefore, amphiphilic organic semiconductor materials are receiving increasing attention in research and industrial fields. The structure of organic amphiphilic semiconductor molecules usually consists of three functional parts: a hydrophilic group, a hydrophobic group, and a linking group between them. The adjustment of their correlation to achieve the target performance is particularly important and needs experimental discussion regarding synthetic methodologies. In this work, we focused on the engineering of a substituent alkyl-chain, and an amphiphilic functional molecule (benzo[b]benzo[4, 5] thieno[2, 3-d]thiophene, named CnPA-BTBT, n = 3–11) was proposed and synthesized. This molecule links the hydrophobic semiconductor backbone and hydrophilic polar group through alkyl chains of different lengths. Fundamental properties were investigated by nuclear magnetic resonance (NMR) and ultraviolet-visible spectroscopy (UV-Vis) to conform the structure and the band gap properties of the designed organic semiconductor. Thermodynamic features were investigated by thermogravimetric analysis (TGA) and corresponding differential thermal gravity (DTG), which indicate that the functional molecule CnPA-BTBT (n = 3–11) has a great stability in ambient conditions. Moreover, the results show that the binding ability of the amphiphilic molecule to water molecules was regulated by the odd-even alternating effect of the alkyl chain and the intramolecular coupling with BTBT. Furthermore, differential scanning calorimetry (DSC) and polarized optical microscopy (POM) were used to study the material properties in detail. As the length of the alkyl chain increased, the functional molecule CnPA-BTBT (n = 3–11) gradually changed from "hard" species with no thermodynamic changes to a transition one with a pair of thermodynamic peaks, and eventually to a "soft" one as a typical liquid crystal with clear observation of Maltese-cross spherulites. The cooling and freezing points were further studied, and the values and trends of their enthalpy and corresponding temperature fluctuated and alternated due to the volume effect, odd-even alternating effect, flexibility, and other functions of the alkyl chain. Three molecular models were proposed according to the thermodynamic study results, namely the brick-like model, transition model, and liquid crystal model. This work presents in-depth discussion on material structure and corresponding thermodynamic properties, and it is an experimental basis for the design, synthesis, optimization, and screening of target performance materials. ![]()
Due to their special polar structure, amphiphilic molecules are simple to process, low in cost and excellent in material properties. Thus, they can be widely applied in the preparation of functional film materials and bionics related to cell membranes. Therefore, amphiphilic organic semiconductor materials are receiving increasing attention in research and industrial fields. The structure of organic amphiphilic semiconductor molecules usually consists of three functional parts: a hydrophilic group, a hydrophobic group, and a linking group between them. The adjustment of their correlation to achieve the target performance is particularly important and needs experimental discussion regarding synthetic methodologies. In this work, we focused on the engineering of a substituent alkyl-chain, and an amphiphilic functional molecule (benzo[b]benzo[4, 5] thieno[2, 3-d]thiophene, named CnPA-BTBT, n = 3–11) was proposed and synthesized. This molecule links the hydrophobic semiconductor backbone and hydrophilic polar group through alkyl chains of different lengths. Fundamental properties were investigated by nuclear magnetic resonance (NMR) and ultraviolet-visible spectroscopy (UV-Vis) to conform the structure and the band gap properties of the designed organic semiconductor. Thermodynamic features were investigated by thermogravimetric analysis (TGA) and corresponding differential thermal gravity (DTG), which indicate that the functional molecule CnPA-BTBT (n = 3–11) has a great stability in ambient conditions. Moreover, the results show that the binding ability of the amphiphilic molecule to water molecules was regulated by the odd-even alternating effect of the alkyl chain and the intramolecular coupling with BTBT. Furthermore, differential scanning calorimetry (DSC) and polarized optical microscopy (POM) were used to study the material properties in detail. As the length of the alkyl chain increased, the functional molecule CnPA-BTBT (n = 3–11) gradually changed from "hard" species with no thermodynamic changes to a transition one with a pair of thermodynamic peaks, and eventually to a "soft" one as a typical liquid crystal with clear observation of Maltese-cross spherulites. The cooling and freezing points were further studied, and the values and trends of their enthalpy and corresponding temperature fluctuated and alternated due to the volume effect, odd-even alternating effect, flexibility, and other functions of the alkyl chain. Three molecular models were proposed according to the thermodynamic study results, namely the brick-like model, transition model, and liquid crystal model. This work presents in-depth discussion on material structure and corresponding thermodynamic properties, and it is an experimental basis for the design, synthesis, optimization, and screening of target performance materials.
2020, 36(11): 191006
doi: 10.3866/PKU.WHXB201910067
Abstract:
The hydrophobic ionic liquid (IL) 1-propyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C3mim][NTf2]) was synthesized according to traditional methods. By adding different amounts of diethyl carbonate (DEC) solvent and lithium bis[(trifluoromethyl)sulfonyl]imide ([Li][NTf2]) salt to [C3mim][NTf2] IL, eight solution systems were prepared. First, the thermodynamic properties of the eight solution systems were characterized by differential scanning calorimetry (DSC). The semi-stable temperature of the system gradually disappeared with increasing lithium salt content, but the melting point temperature was not apparent in the experiment. These results indicate that DEC and lithium salts can dissolve in ILs within the tested temperature range. The basic properties of the eight systems, including thermodynamic and dynamic properties, were systematically studied at different temperatures. The variation in the self-diffusion coefficient of lithium ion ([Li]+) as a function of DEC concentration, density changes, viscosity, conductivity, and the viscosity/conductivity activation energy of the eight systems was calculated by the Vogel Fulcher Taman (VFT), Final Vogel Fulcher Taman (FVFT), and Arrhenius equations. The effect of temperature on the properties of the system was studied in detail. Within the temperature range measured herein, the deviation between the fitting equation and experimental value was small. Consequently, these equations were successfully used to calculate the properties of the system at various temperatures. All fitting parameters of the corresponding equations are provided herein. The viscosity for all systems decreased rapidly with increasing temperature, which increased the conductivity. Based on these experiments, the influence of DEC on the system microstructure was discussed in the context of the molecular dynamics simulation results. In particular, the interaction between [Li]+ and [NTf2]-/DEC was examined. In all solution systems, [NTf2]- coordinates to [Li]+ through only the O atom and not the N atom. Radial distribution function (RDF) analysis showed that the interaction between [Li]+ and [NTf2]- weakened with increasing DEC concentration. DEC molecules were observed in the first solvation layer of [Li]+ coordinating to [Li]+ through the carbonyl O atom. Although the interaction between [Li]+ and DEC was weakened, competition between [NTf2]- and DEC in the first solvation layer of [Li]+ was observed by the coordination number analysis of the O atom around [Li]+. Therefore, the introduction of DEC is beneficial for Li+ diffusion, which is consistent with the experimental results. ![]()
The hydrophobic ionic liquid (IL) 1-propyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C3mim][NTf2]) was synthesized according to traditional methods. By adding different amounts of diethyl carbonate (DEC) solvent and lithium bis[(trifluoromethyl)sulfonyl]imide ([Li][NTf2]) salt to [C3mim][NTf2] IL, eight solution systems were prepared. First, the thermodynamic properties of the eight solution systems were characterized by differential scanning calorimetry (DSC). The semi-stable temperature of the system gradually disappeared with increasing lithium salt content, but the melting point temperature was not apparent in the experiment. These results indicate that DEC and lithium salts can dissolve in ILs within the tested temperature range. The basic properties of the eight systems, including thermodynamic and dynamic properties, were systematically studied at different temperatures. The variation in the self-diffusion coefficient of lithium ion ([Li]+) as a function of DEC concentration, density changes, viscosity, conductivity, and the viscosity/conductivity activation energy of the eight systems was calculated by the Vogel Fulcher Taman (VFT), Final Vogel Fulcher Taman (FVFT), and Arrhenius equations. The effect of temperature on the properties of the system was studied in detail. Within the temperature range measured herein, the deviation between the fitting equation and experimental value was small. Consequently, these equations were successfully used to calculate the properties of the system at various temperatures. All fitting parameters of the corresponding equations are provided herein. The viscosity for all systems decreased rapidly with increasing temperature, which increased the conductivity. Based on these experiments, the influence of DEC on the system microstructure was discussed in the context of the molecular dynamics simulation results. In particular, the interaction between [Li]+ and [NTf2]-/DEC was examined. In all solution systems, [NTf2]- coordinates to [Li]+ through only the O atom and not the N atom. Radial distribution function (RDF) analysis showed that the interaction between [Li]+ and [NTf2]- weakened with increasing DEC concentration. DEC molecules were observed in the first solvation layer of [Li]+ coordinating to [Li]+ through the carbonyl O atom. Although the interaction between [Li]+ and DEC was weakened, competition between [NTf2]- and DEC in the first solvation layer of [Li]+ was observed by the coordination number analysis of the O atom around [Li]+. Therefore, the introduction of DEC is beneficial for Li+ diffusion, which is consistent with the experimental results.
2020, 36(11): 191100
doi: 10.3866/PKU.WHXB201911008
Abstract:
Cu-ZnO is broadly used as a catalyst in CO2 reduction to produce methanol, but fabricating small-sized Cu-ZnO catalysts with strong Cu-ZnO interactions remains a challenge. In this work, a simple, low-cost method is proposed to synthesize small-sized Cu-ZnO/SiO2 with high activity and controllable Cu-ZnO interactions derived from copper silicate nanotubes. A series of Cu-ZnO/SiO2 samples with different amounts of ZnO were prepared. The activities of the as-prepared catalysts for methanol synthesis were tested, and the results revealed a volcano relationship with the weight fraction of ZnO. At 523 K, the methanol selectivity increased from 20% to 67% when 14% ZnO was added to the Cu/SiO2 catalyst, while the conversion of CO2 increased first and then decreased with the addition of ZnO. The optimum space time yield (STY) of 244 g·kg-1·h-1 was obtained on C-SiO2-7%ZnO at 543 K under 4.5 MPa H2/CO2. Furthermore, the synergistic effect of Cu and ZnO was studied by high resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), and temperature-programmed reduction (TPR) analyses. The HRTEM images showed that the Cu particles come in contact with ZnO more frequently with increased addition of ZnO, indicating that the catalysts with higher ZnO contents have a greater probability of formation of the Cu-ZnO interface, which promotes the catalytical activity of Cu-ZnO/SiO2. Meanwhile, the HRTEM images, XRD patterns, and TPR results showed that the addition of excess ZnO leads to an increase in the size of the Cu particles, which in turn decreases the total number of active sites and further degrades the activity of the catalysts. The activation energy (Ea) for methanol synthesis and reverse water gas shift (RWGS) was calculated based on the results of the catalytical test. With the addition of ZnO, Ea for methanol synthesis decreased from 72.5 to 34.8 kJ·mol-1, while that for RWGS increased from 61.3 to 102.7 kJ·mol-1, illustrating that ZnO promotes the synergistic effect of Cu-ZnO. The results of XPS and in situ DRIFTS showed that the amount of Cu+ species decreases with the addition of ZnO, indicating that the Cu-ZnO interface serves as the active site. The Cu surface area and the turnover frequency (TOF) of methanol were calculated based on the H2-TPR curves. The TOF of methanol on the Cu-ZnO/SiO2 catalysts at 543 K increased from 1.5 × 10-3 to 3.9 × 10-3 s-1 with the addition of ZnO, which further confirmed the promotion effect of the Cu-ZnO interface on the methanol synthesis. This study provides a method to construct Cu-ZnO interfaces based on copper silicate and to investigate the influence of ZnO on Cu-ZnO/SiO2 catalysts. ![]()
Cu-ZnO is broadly used as a catalyst in CO2 reduction to produce methanol, but fabricating small-sized Cu-ZnO catalysts with strong Cu-ZnO interactions remains a challenge. In this work, a simple, low-cost method is proposed to synthesize small-sized Cu-ZnO/SiO2 with high activity and controllable Cu-ZnO interactions derived from copper silicate nanotubes. A series of Cu-ZnO/SiO2 samples with different amounts of ZnO were prepared. The activities of the as-prepared catalysts for methanol synthesis were tested, and the results revealed a volcano relationship with the weight fraction of ZnO. At 523 K, the methanol selectivity increased from 20% to 67% when 14% ZnO was added to the Cu/SiO2 catalyst, while the conversion of CO2 increased first and then decreased with the addition of ZnO. The optimum space time yield (STY) of 244 g·kg-1·h-1 was obtained on C-SiO2-7%ZnO at 543 K under 4.5 MPa H2/CO2. Furthermore, the synergistic effect of Cu and ZnO was studied by high resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), and temperature-programmed reduction (TPR) analyses. The HRTEM images showed that the Cu particles come in contact with ZnO more frequently with increased addition of ZnO, indicating that the catalysts with higher ZnO contents have a greater probability of formation of the Cu-ZnO interface, which promotes the catalytical activity of Cu-ZnO/SiO2. Meanwhile, the HRTEM images, XRD patterns, and TPR results showed that the addition of excess ZnO leads to an increase in the size of the Cu particles, which in turn decreases the total number of active sites and further degrades the activity of the catalysts. The activation energy (Ea) for methanol synthesis and reverse water gas shift (RWGS) was calculated based on the results of the catalytical test. With the addition of ZnO, Ea for methanol synthesis decreased from 72.5 to 34.8 kJ·mol-1, while that for RWGS increased from 61.3 to 102.7 kJ·mol-1, illustrating that ZnO promotes the synergistic effect of Cu-ZnO. The results of XPS and in situ DRIFTS showed that the amount of Cu+ species decreases with the addition of ZnO, indicating that the Cu-ZnO interface serves as the active site. The Cu surface area and the turnover frequency (TOF) of methanol were calculated based on the H2-TPR curves. The TOF of methanol on the Cu-ZnO/SiO2 catalysts at 543 K increased from 1.5 × 10-3 to 3.9 × 10-3 s-1 with the addition of ZnO, which further confirmed the promotion effect of the Cu-ZnO interface on the methanol synthesis. This study provides a method to construct Cu-ZnO interfaces based on copper silicate and to investigate the influence of ZnO on Cu-ZnO/SiO2 catalysts.
2020, 36(11): 191203
doi: 10.3866/PKU.WHXB201912030
Abstract:
The development of human society and the continuously emerging environmental problems call for cleaner energy resources. Lithium-ion batteries, since their commercialization in the early 1990s, have been an important power source of mobile phones, laptops as well as other portable electronic devices. Their advantages include environment-friendliness, light weight, and no memory effect compared with lead-acid or nickel-cadmium batteries. Electrode materials play an important role in the performance of lithium-ion batteries. The traditional commercial anode material, graphite, has a theoretical specific capacity of 372 mAh·g-1 and working potential close to 0 V (vs Li+/Li), making it prone to the formation of lithium dendrite, which may cause short circuit especially when large current is applied. Another commercial anode material Li4Ti5O12, which also undergoes an intercalation reaction during lithiation process, has a theoretical specific capacity of 175 mAh·g-1 along with three lithium-ion intercalations per formula unit. This is relatively small, and it has a relatively high working potential of 1.55 V (vs Li+/Li), which reduces its output voltage and specific energy when assembled in full battery. To overcome the shortcomings mentioned above, it is essential to search for new anode materials that are low-cost, environment-friendly, and easy to synthesize. Silicate materials have gained widespread attention owing to their low cost and facile synthesis. Herein, we report for the first time a novel titanosilicate, NaTiSi2O6, synthesized by sol-gel and solid sintering. It is isostructural to pyroxene jadeite NaAlSi2O6, belonging to monoclinic crystal system with a space group of C2/c. By in situ pyrolysis and carbonization of glucose, nanosized NaTiSi2O6 mixed with carbon was successfully obtained with a specific surface area of 132 m2·g-1, calculated according to the Brunauer–Emmett–Teller formula. The specific charge/discharge capacity in the first cycle at current density of 0.1 A·g-1 is 266.6 mAh·g-1 and 542.9 mAh·g-1, respectively, with an initial coulombic efficiency of 49.1%. After 100 cycles, it retains a specific charge capacity of 224.1 mAh·g-1, corresponding to a capacity retention rate of 84.1%. The average working potential of NaTiSi2O6 is 1.2–1.3 V (vs Li+/Li), slightly lower than that of Li4Ti5O12. The reaction mechanism while charging and discharging was determined by in situ X-ray diffraction test as well as selected area electron diffraction. The results showed that NaTiSi2O6 undergoes an intercalation reaction during lithiation process, with two lithium-ion intercalations per formula unit. This makes NaTiSi2O6 a new member of the silicate anode material family, and may provide insights into the development of new silicate electrode materials in the future. ![]()
The development of human society and the continuously emerging environmental problems call for cleaner energy resources. Lithium-ion batteries, since their commercialization in the early 1990s, have been an important power source of mobile phones, laptops as well as other portable electronic devices. Their advantages include environment-friendliness, light weight, and no memory effect compared with lead-acid or nickel-cadmium batteries. Electrode materials play an important role in the performance of lithium-ion batteries. The traditional commercial anode material, graphite, has a theoretical specific capacity of 372 mAh·g-1 and working potential close to 0 V (vs Li+/Li), making it prone to the formation of lithium dendrite, which may cause short circuit especially when large current is applied. Another commercial anode material Li4Ti5O12, which also undergoes an intercalation reaction during lithiation process, has a theoretical specific capacity of 175 mAh·g-1 along with three lithium-ion intercalations per formula unit. This is relatively small, and it has a relatively high working potential of 1.55 V (vs Li+/Li), which reduces its output voltage and specific energy when assembled in full battery. To overcome the shortcomings mentioned above, it is essential to search for new anode materials that are low-cost, environment-friendly, and easy to synthesize. Silicate materials have gained widespread attention owing to their low cost and facile synthesis. Herein, we report for the first time a novel titanosilicate, NaTiSi2O6, synthesized by sol-gel and solid sintering. It is isostructural to pyroxene jadeite NaAlSi2O6, belonging to monoclinic crystal system with a space group of C2/c. By in situ pyrolysis and carbonization of glucose, nanosized NaTiSi2O6 mixed with carbon was successfully obtained with a specific surface area of 132 m2·g-1, calculated according to the Brunauer–Emmett–Teller formula. The specific charge/discharge capacity in the first cycle at current density of 0.1 A·g-1 is 266.6 mAh·g-1 and 542.9 mAh·g-1, respectively, with an initial coulombic efficiency of 49.1%. After 100 cycles, it retains a specific charge capacity of 224.1 mAh·g-1, corresponding to a capacity retention rate of 84.1%. The average working potential of NaTiSi2O6 is 1.2–1.3 V (vs Li+/Li), slightly lower than that of Li4Ti5O12. The reaction mechanism while charging and discharging was determined by in situ X-ray diffraction test as well as selected area electron diffraction. The results showed that NaTiSi2O6 undergoes an intercalation reaction during lithiation process, with two lithium-ion intercalations per formula unit. This makes NaTiSi2O6 a new member of the silicate anode material family, and may provide insights into the development of new silicate electrode materials in the future.
2020, 36(11): 200104
doi: 10.3866/PKU.WHXB202001048
Abstract:
Modified phosphate glasses can be used in all-solid-state batteries as solid electrolytes and cathodes due to their high ionic conductivity. The properties of fast ionic conducting glasses are strongly related to the structure of the glass networks. However, most previous works have focused on improving the ionic conductivity of such glasses by composition adjustments, while structural studies are scant. Structural investigations are essential to understand the composition dependence of the glass structure, which is valuable for improving the ionic conductivity and developing new ionic conducting glasses. In this work, phosphate ionic conducting glasses with compositions of 100LiO1/2-(100-x)PO5/2-xTeO2 (x = 0, 10, 20, 25, 30) were synthesized, and their structures were investigated using Raman and solid-state nuclear magnetic resonance (SSNMR) spectroscopy. When x = 0, Raman and 31P magic angle spinning (MAS) NMR spectra showed that most of the phosphorus species were Q0Te(2) species, while the concentration of Q0Te(1) species was negligible. QmTe(n) represents the phosphorus species with n bridging oxygen atoms (the oxygen atoms in P—O—P and P—O—Te linkages are both considered to be bridging oxygen atoms), and m Te atoms are connected to this [PO4] tetrahedron. When PO5/2 is substituted with TeO2, long P—O—P chains are broken into short chains, and Q0Te(2) species gradually transform into Q1Te(2) and Q0Te(1) species. Two-dimensional (2D) refocused incredible natural abundance double quantum transfer experiment (INADEQUATE) spectra proved that no isolated phosphorus species existed in the glasses; Q0Te(2), Q1Te(2), and Q0Te(1) species were connected with each other through P—O—P linkages. Three- and four-coordinated Te were observed in the static 125Te wideband uniform-rate smooth truncation quadrupolar Carr-Purcell-Meiboom-Gill (WURST-QCPMG) spectra. When the concentration of TeO2 was low, four-coordinated Te was dominant. However, with the increase in TeO2, the proportion of three-coordinated Te gradually increased, while that of four-coordinated Te decreased. The experimental contents of P—O—P, P—O—Te, and Te—O—Te linkages in these glasses were calculated from the deconvolutions of 31P and 125Te NMR spectra. Then, the experimental contents were compared with the theoretical contents calculated according to a random distribution model. It was found that the experimental contents of homonuclear P—O—P and Te—O—Te linkages were slightly higher than their corresponding theoretical values, while the experimental content of heteronuclear P—O—Te was slightly lower than the theoretical value. These results indicated a weak priority for homonuclear connectivities. In this glass system, Li+ ions preferred to stay around [PO4] tetrahedrons rather than tellurium oxygen polyhedrons. However, a small number of Li+ ions still interacted with tellurium oxygen polyhedrons to form [TeO3] units. During the substitution of PO5/2 by TeO2, the fractions of bridging oxygen atoms in these glasses were almost unchanged, resulting in a slight change in the glass transition temperature. This work provides a comprehensive description of glass networks, depending on their compositions, which could be valuable for improving the ionic conductivity and for designing new fast ionic conducting glasses through structural modifications. ![]()
Modified phosphate glasses can be used in all-solid-state batteries as solid electrolytes and cathodes due to their high ionic conductivity. The properties of fast ionic conducting glasses are strongly related to the structure of the glass networks. However, most previous works have focused on improving the ionic conductivity of such glasses by composition adjustments, while structural studies are scant. Structural investigations are essential to understand the composition dependence of the glass structure, which is valuable for improving the ionic conductivity and developing new ionic conducting glasses. In this work, phosphate ionic conducting glasses with compositions of 100LiO1/2-(100-x)PO5/2-xTeO2 (x = 0, 10, 20, 25, 30) were synthesized, and their structures were investigated using Raman and solid-state nuclear magnetic resonance (SSNMR) spectroscopy. When x = 0, Raman and 31P magic angle spinning (MAS) NMR spectra showed that most of the phosphorus species were Q0Te(2) species, while the concentration of Q0Te(1) species was negligible. QmTe(n) represents the phosphorus species with n bridging oxygen atoms (the oxygen atoms in P—O—P and P—O—Te linkages are both considered to be bridging oxygen atoms), and m Te atoms are connected to this [PO4] tetrahedron. When PO5/2 is substituted with TeO2, long P—O—P chains are broken into short chains, and Q0Te(2) species gradually transform into Q1Te(2) and Q0Te(1) species. Two-dimensional (2D) refocused incredible natural abundance double quantum transfer experiment (INADEQUATE) spectra proved that no isolated phosphorus species existed in the glasses; Q0Te(2), Q1Te(2), and Q0Te(1) species were connected with each other through P—O—P linkages. Three- and four-coordinated Te were observed in the static 125Te wideband uniform-rate smooth truncation quadrupolar Carr-Purcell-Meiboom-Gill (WURST-QCPMG) spectra. When the concentration of TeO2 was low, four-coordinated Te was dominant. However, with the increase in TeO2, the proportion of three-coordinated Te gradually increased, while that of four-coordinated Te decreased. The experimental contents of P—O—P, P—O—Te, and Te—O—Te linkages in these glasses were calculated from the deconvolutions of 31P and 125Te NMR spectra. Then, the experimental contents were compared with the theoretical contents calculated according to a random distribution model. It was found that the experimental contents of homonuclear P—O—P and Te—O—Te linkages were slightly higher than their corresponding theoretical values, while the experimental content of heteronuclear P—O—Te was slightly lower than the theoretical value. These results indicated a weak priority for homonuclear connectivities. In this glass system, Li+ ions preferred to stay around [PO4] tetrahedrons rather than tellurium oxygen polyhedrons. However, a small number of Li+ ions still interacted with tellurium oxygen polyhedrons to form [TeO3] units. During the substitution of PO5/2 by TeO2, the fractions of bridging oxygen atoms in these glasses were almost unchanged, resulting in a slight change in the glass transition temperature. This work provides a comprehensive description of glass networks, depending on their compositions, which could be valuable for improving the ionic conductivity and for designing new fast ionic conducting glasses through structural modifications.
2020, 36(11): 190904
doi: 10.3866/PKU.WHXB201909043
Abstract:
Molecular electronics is an important field for the application of nanotechnologies with an ultimate goal of building functional devices using single molecules or molecular arrays to realize the same functionality as macroscopic devices. To attain this goal, reliable techniques for measuring and manipulating electron transfer processes through single molecules are essential. There are various techniques and many environmental factors influencing single-molecule electronic conductance measurements. In this review, we first provide a detailed introduction and classification of the current well-accepted techniques in this field for measuring single-molecule conductance. All available techniques are summarized into two categories: the fixed junction technique and break junction technique. The break junction technique involves repeatedly forming and breaking molecular junctions by mechanically controlling a pair of electrodes moving into and out of contact in the presence of target molecules. Single-molecule conductance can be determined from the conductance plateaus that appear in typical conductance decay traces when molecules bind two electrodes during their separation process. In contrast, the fixed junction technique is to fix the distance between a pair of electrodes and measure the conductance fluctuations when a single molecule binds the two electrodes stochastically. Both techniques comprise different application methods and have been employed preferentially by different groups. Specific features of both techniques and their intrinsic advantages are compared and summarized in Section 4.Next, we systematically summarize the factors affecting the molecular conductance during the course of measurements, which are the focus of the current academic community in the field. As shown in the middle of the image above, the electrode, anchoring group, and target molecule are the three key elements in constructing a single-molecule junction. The contact geometry between the molecule and the electrode and the associated coupling strength can profoundly affect the conductance characteristics of single molecules. The properties of anchoring groups can determine whether a molecule is primarily transported by electrons or holes. A good selection of electrode materials can improve the yield of single-molecule junctions and facilitate strong electronic couplings with the target molecules. The conductance characteristics of saturated and conjugated molecules are quite different due to their diverse band gaps. Moreover, various substituents can be modified onto the backbone of the molecules, which can raise or lower the energy level of the frontier orbitals of molecules to different degrees, thereby affecting the conductance properties of target molecules. In addition to these factors for the key elements, the investigated molecules can be measured in a variety of environments, including organic solvents, high vacuum, aqueous solution, ionic liquids, or the atmosphere, and the work function of the metal contact may change in different environments. The change in work function can change the gap between the electrode Fermi energy and the frontier orbital of a target molecule, influencing the measured conductance. Additionally, both the electrical field orientation and the bias values applied have a significant effect on the molecular structures and thus their conductance characteristics. Temperature can also affect the charge transport when hopping dominates the transport mechanism. Meanwhile, pH affects the interactions between H+/OH- and the anchoring groups, which indirectly induces a change in the tunneling barrier.In this review, the influencing factors are comprehensively illustrated from two perspectives: internal factors (anchoring group, electrode, and target molecule) and external factors (voltage, temperature, solvent, pH value, and others). In addition, new approaches for modulating the molecular conductance (modulation of energy levels, light or heat stimulation, and others) that have been developed in recent years are reviewed, and investigations of chemical reactions at the single-molecule level using these methods are highlighted. Finally, the potential applications of these techniques and correlated modulating approaches are summarized and a prospective is provided for the field of single-molecule electronics. ![]()
Molecular electronics is an important field for the application of nanotechnologies with an ultimate goal of building functional devices using single molecules or molecular arrays to realize the same functionality as macroscopic devices. To attain this goal, reliable techniques for measuring and manipulating electron transfer processes through single molecules are essential. There are various techniques and many environmental factors influencing single-molecule electronic conductance measurements. In this review, we first provide a detailed introduction and classification of the current well-accepted techniques in this field for measuring single-molecule conductance. All available techniques are summarized into two categories: the fixed junction technique and break junction technique. The break junction technique involves repeatedly forming and breaking molecular junctions by mechanically controlling a pair of electrodes moving into and out of contact in the presence of target molecules. Single-molecule conductance can be determined from the conductance plateaus that appear in typical conductance decay traces when molecules bind two electrodes during their separation process. In contrast, the fixed junction technique is to fix the distance between a pair of electrodes and measure the conductance fluctuations when a single molecule binds the two electrodes stochastically. Both techniques comprise different application methods and have been employed preferentially by different groups. Specific features of both techniques and their intrinsic advantages are compared and summarized in Section 4.Next, we systematically summarize the factors affecting the molecular conductance during the course of measurements, which are the focus of the current academic community in the field. As shown in the middle of the image above, the electrode, anchoring group, and target molecule are the three key elements in constructing a single-molecule junction. The contact geometry between the molecule and the electrode and the associated coupling strength can profoundly affect the conductance characteristics of single molecules. The properties of anchoring groups can determine whether a molecule is primarily transported by electrons or holes. A good selection of electrode materials can improve the yield of single-molecule junctions and facilitate strong electronic couplings with the target molecules. The conductance characteristics of saturated and conjugated molecules are quite different due to their diverse band gaps. Moreover, various substituents can be modified onto the backbone of the molecules, which can raise or lower the energy level of the frontier orbitals of molecules to different degrees, thereby affecting the conductance properties of target molecules. In addition to these factors for the key elements, the investigated molecules can be measured in a variety of environments, including organic solvents, high vacuum, aqueous solution, ionic liquids, or the atmosphere, and the work function of the metal contact may change in different environments. The change in work function can change the gap between the electrode Fermi energy and the frontier orbital of a target molecule, influencing the measured conductance. Additionally, both the electrical field orientation and the bias values applied have a significant effect on the molecular structures and thus their conductance characteristics. Temperature can also affect the charge transport when hopping dominates the transport mechanism. Meanwhile, pH affects the interactions between H+/OH- and the anchoring groups, which indirectly induces a change in the tunneling barrier.In this review, the influencing factors are comprehensively illustrated from two perspectives: internal factors (anchoring group, electrode, and target molecule) and external factors (voltage, temperature, solvent, pH value, and others). In addition, new approaches for modulating the molecular conductance (modulation of energy levels, light or heat stimulation, and others) that have been developed in recent years are reviewed, and investigations of chemical reactions at the single-molecule level using these methods are highlighted. Finally, the potential applications of these techniques and correlated modulating approaches are summarized and a prospective is provided for the field of single-molecule electronics.
2020, 36(11): 200307
doi: 10.3866/PKU.WHXB202003075
Abstract:
2020, 36(11): 200307
doi: 10.3866/PKU.WHXB202003076
Abstract:
2020, 36(11): 200400
doi: 10.3866/PKU.WHXB202004003
Abstract:
2020, 36(11): 200401
doi: 10.3866/PKU.WHXB202004011
Abstract:
2020, 36(11): 200402
doi: 10.3866/PKU.WHXB202004020
Abstract:
2020, 36(11): 200402
doi: 10.3866/PKU.WHXB202004022
Abstract:
2020, 36(11): 200404
doi: 10.3866/PKU.WHXB202004045
Abstract:
2020, 36(11): 200405
doi: 10.3866/PKU.WHXB202004056
Abstract:
2020, 36(11): 200405
doi: 10.3866/PKU.WHXB202004059
Abstract:
2020, 36(11): 200505
doi: 10.3866/PKU.WHXB202005052
Abstract:
2020, 36(11): 200604
doi: 10.3866/PKU.WHXB202006043
Abstract:
2020, 36(11): 200600
doi: 10.3866/PKU.WHXB202006005
Abstract:
2020, 36(11): 200606
doi: 10.3866/PKU.WHXB202006060
Abstract:
In 2005, the Science magazine, in commemorating the 125th anniversary of its founding, proposed 25 most challenging scientific issues in the future. One of the issues, "How far can we push chemical self-assembly?" has attracted the attention of scientists all over the world. During the 11th Five Year Plan period, NSFC convened scientists from various fields and proposed a major research project, "Controllable self-assembly system and its functionalization". Since the implementation of this project, Chinese scientists have developed and recognized a variety of noncovalent interactions, constructed numerous assembly building blocks with the "Chinese label", established a new assembly method similar to an organic "name reaction", realized the functions of multi-component and multi-level assemblies, and constructed a batch of controllable self-assembly systems having scientific importance and potential practical value. They have achieved great leap forward from following to original innovation, and brought the research of chemical self-assembly in China to move forward to the center of international stage. In this study, we examined the overall scientific goals, general layout of the plan, and ideas for implementation of the major research program "Controllable Self-Assembly System and its Functionalization", as well as an array of significant research accomplishments made possible through the funding received by the program.
In 2005, the Science magazine, in commemorating the 125th anniversary of its founding, proposed 25 most challenging scientific issues in the future. One of the issues, "How far can we push chemical self-assembly?" has attracted the attention of scientists all over the world. During the 11th Five Year Plan period, NSFC convened scientists from various fields and proposed a major research project, "Controllable self-assembly system and its functionalization". Since the implementation of this project, Chinese scientists have developed and recognized a variety of noncovalent interactions, constructed numerous assembly building blocks with the "Chinese label", established a new assembly method similar to an organic "name reaction", realized the functions of multi-component and multi-level assemblies, and constructed a batch of controllable self-assembly systems having scientific importance and potential practical value. They have achieved great leap forward from following to original innovation, and brought the research of chemical self-assembly in China to move forward to the center of international stage. In this study, we examined the overall scientific goals, general layout of the plan, and ideas for implementation of the major research program "Controllable Self-Assembly System and its Functionalization", as well as an array of significant research accomplishments made possible through the funding received by the program.
2020, 36(11): 200805
doi: 10.3866/PKU.WHXB202008058
Abstract:
Newly established in 2018, the UK Research and Innovation (UKRI) strengthens the strategic coordination of the UK research and innovation system by bringing together seven Research Councils, Research England, and Innovate UK. Through its nine organizations, UKRI funds multidisciplinary and interdisciplinary research in a number of priority areas. It also runs the Strategic Priorities Fund to support multidisciplinary and interdisciplinary research in strategic areas identified by government policies as well as the Global Challenges Research Fund to promote challenge-led interdisciplinary research needed by developing countries. The UKRI makes significant efforts to engage stakeholders in the development, design, and implementation of multidisciplinary and interdisciplinary programs. It has also developed a range of mechanisms to improve the evaluation of multidisciplinary and interdisciplinary projects. Chinese science and innovation funding agencies could draw upon the UKRI experience from four aspects to advance interdisciplinary research in China.
Newly established in 2018, the UK Research and Innovation (UKRI) strengthens the strategic coordination of the UK research and innovation system by bringing together seven Research Councils, Research England, and Innovate UK. Through its nine organizations, UKRI funds multidisciplinary and interdisciplinary research in a number of priority areas. It also runs the Strategic Priorities Fund to support multidisciplinary and interdisciplinary research in strategic areas identified by government policies as well as the Global Challenges Research Fund to promote challenge-led interdisciplinary research needed by developing countries. The UKRI makes significant efforts to engage stakeholders in the development, design, and implementation of multidisciplinary and interdisciplinary programs. It has also developed a range of mechanisms to improve the evaluation of multidisciplinary and interdisciplinary projects. Chinese science and innovation funding agencies could draw upon the UKRI experience from four aspects to advance interdisciplinary research in China.
