Showing papers in "Journal of Physical Chemistry C in 2023"

Application of Electrochemical Impedance Spectroscopy to Degradation and Aging Research of Lithium-Ion Batteries

Abstract: An in-depth understanding of battery degradation and aging in-Operando not only plays a vital role in the design of battery managing systems but also helps to ensure safe use and manufacturing optimization of lithium-ion batteries (LIBs) in large-scale applications. Electrochemical impedance spectroscopy (EIS) is a nondestructive method which unravels electrode kinetic processes inside the batteries in different time domains, including charge-transfer reactions, interfacial evolutions, and mass diffusions. It has become a powerful diagnosis and pre/prognosis tool in battery aging research, as it provides important insight into the changes of internal electrochemical processes by correlating the impedance evolution to degradation mechanisms. This review gives a critical overview on rapidly developing impedance techniques for degradation and aging investigation of Li-ion batteries. The EIS variations of LIBs at different aging conditions of calendar aging and accelerated aging are systematically summarized. In addition, the working principles, data validation, and modeling methods, including equivalent circuit model (ECM), distribution of relaxation times (DRT), and transmission line model (TLM), of classical EIS and dynamic EIS are elaborately concluded. Finally, the challenges and perspectives of further application of EIS in the aging research of LIBs are presented.

Site-Selective Atomic Layer Deposition on Rutile TiO2: Selective Hydration as a Route to Target Point Defects

Abstract: Routes to area- and especially site-selective atomic layer deposition (ALD) remain an enticing challenge in precision surface science, despite the potentially game-changing capability for many energy applications. An unparalleled level of surface reaction control is required to direct ALD to select sites on the same nominal material, for example, targeted growth on distinct phases, facets, step-edges, and/or defects. However, as a sequential surface synthesis method, ALD is uniquely suited to these challenges, including the possibility of selective deposition at defective surface atom arrangements. We computationally identify conditions for site-selective ALD through hydration of surface defects, including oxygen vacancies and titanium interstitials on low-index rutile TiO2 facets. First-principles computation is used to predict, as a function of temperature, the hydroxylation of defects that are targeted by proton-exchange-mediated ALD processes. In situ ellipsometric measurements of ALD Al2O3 nucleation on TiO2 (110) single crystals prepared with and without abundant oxygen vacancies demonstrate striking contrast, corroborating computational predictions and revealing a mechanistically clear path to site-selective ALD.

Additional Important Considerations in Surface-Enhanced Raman Scattering Enhancement Factor Measurements

Abstract: Despite considerable progress, the issues for precise evaluation of the surface-enhanced Raman scattering (SERS) substrate enhancement factor (EF) have not yet been completely solved until now. Herein, we verify that key aspects of probe molecules including initial concentration, footprint, and diffusion kinetics need to be taken into account for the experimental SERS EF measurement. The three factors collectively affect the surface coverage and thereby the determination of the number of probe molecules on the SERS substrate. We recommend that the measurement of the SERS EFs should be performed with complete surface coverage of probe molecules at steady-state equilibrium using relatively high initial concentrations. It has been demonstrated for the first time that the SERS EFs are independent of the initial concentration of probe molecules when exceeding the threshold initial concentration for complete surface coverage. This study highlights the importance of probe molecules in terms of diffusion kinetics and surface coverage for the accurate determination of the SERS EFs, which has yet to be addressed in the literature.

Chirality-Induced Spin Selectivity (CISS) Effect: Magnetocurrent–Voltage Characteristics with Coulomb Interactions I

Abstract: One of the manifestations of chirality-induced spin selectivity (CISS) is the appearance of a magnetocurrent. Magnetocurrent is the observation that the charge currents at finite bias in a two terminal device for opposite magnetizations of one of the leads differ. Magnetocurrents can only occur in the presence of interactions of the electrons either with vibrational modes or among themselves through the Coulomb interaction. In experiments on chiral molecules assembled in monolayers, the magnetocurrent seems to be dominantly cubic (odd) in bias voltage while theory finds a dominantly even bias voltage dependence. Thus far, theoretical work has predicted a magnetocurrent which is even bias. Here we analyze the bias voltage dependence of the magnetocurrent numerically and analytically involving the spin–orbit and Coulomb interactions (through the Hartree–Fock and Hubbard One approximations). For both approximations it is found that for strong Coulomb interactions the magnetocurrent is dominantly odd in bias voltage, confirming the symmetry observed in experiment.

Interfacial Degradation of Calcium Silicate Hydrate and Epoxy under a Hygrothermal Environment: An Experimental and Molecular Model Study

Abstract: The adhesion at the concrete/epoxy interface is crucial to the reinforcement of carbon fiber-reinforced polymer (CFRP) sheets. The multiscale mechanism of adhesion, particularly under a hygrothermal environment, yet remains largely unclear. This paper combines molecular dynamics (MD) and synthesis experiments of a novel single-phase calcium silicate hydrate (C-S-H) and epoxy composite to investigate the combined effects of aggressive chemicals and temperature on the bonding properties. MD models were constructed under various conditions on C-S-H/epoxy composites, whose equilibrium configuration, dynamic property, bond connection, interfacial interaction energy, and mechanical properties were quantified. In addition, the experiments were carried out under different temperatures and humidity conditions to explore the macroscopic splitting tensile property of the composites and further validate the MD results. The MD results suggest that the Ca ions on the C-S-H surface are crucial to the adhesion by forming Ca–O and Ca–N bonds with epoxy. However, these bonds were reduced in the presence of water molecules, with a reduction also of the interfacial energy. The degradation effects are exacerbated by the presence of NaCl solution, as Na ions gather around the interfacial region of C-S-H/epoxy, further accumulating water molecules by forming hydrated ion clusters. This process is accelerated by elevated temperature. The experiment results suggest that without the influence of NaCl solution, a moist C-S-H surface turns to gain a stronger bonding with epoxy when the curing temperature is higher (40, 60 °C). Meanwhile, the bonding properties of a dry C-S-H surface and epoxy are less sensitive to temperature. This work provides new insight into understanding the bonding mechanism on the C-S-H/epoxy interface and may benefit the engineering application of CFRP-concrete reinforcement.

New Insights into Cu/Cu2O/CuO Nanocomposite Heterojunction Facilitating Photocatalytic Generation of Green Fuel and Detoxification of Organic Pollutants

Abstract: Cu/Cu2O/CuO nanocomposites were synthesized using the simple wet chemical approach for the production of dihydrogen as a potential fuel source and for the detoxification of dyes. The formation of Cu/Cu2O/CuO nanocomposites is confirmed by powder X-ray diffraction, whereas the W-H plot revealed the average particle size of nanocomposite approximately 17 nm, which is in good agreement with the Scherrer method and transmission electron microscopy analysis. The uniform distribution of Cu and O elements was supported by the elemental mapping of the nanocomposite. Band gaps of CuO and Cu2O were found to be 1.71 and 1.92 eV, respectively, using diffuse reflectance spectroscopy spectra and Kubelka–Munk functions. The oxygen vacancies in the nanocomposite are confirmed by various analytical spectroscopic techniques, such as electron paramagnetic resonance, Raman, photoluminescence, and X-ray photoelectron spectroscopy (XPS) spectra. The significant boost in the performance of the fabricated nanocomposite was observed and is attributed to the formation of a heterojunction and existence of oxygen vacancy. The nanocomposite demonstrated proficiency in the photocatalytic splitting of water for the production of hydrogen. The maximum hydrogen generation yield (68 μmol g–1) was observed for Cu/Cu2O/CuO nanocomposites along with NiO (co-catalyst) and methanol as a hole scavenger as well as an electron donor. Moreover, the degradation of congo red (CR) and malachite green (MG) dyes was also investigated and the efficiency of the nanocomposite was found to be 80 and 60%, respectively, after 120 min of light irradiation. The stability of nanocomposites after photocatalysis was investigated by the XPS spectrum of the nanocomposite. Explicitly, the area and broadening of the O 1s XPS spectrum demonstrated higher degradation of CR dye as compared to MG dye.

The Effect of Disorder on Endogenous MAS-DNP: Study of Silicate Glasses and Crystals

Abstract: In dynamic nuclear polarization nuclear magnetic resonance (DNP-NMR) experiments, the large Boltzmann polarization of unpaired electrons is transferred to surrounding nuclei, leading to a significant increase in the sensitivity of the NMR signal. In order to obtain large polarization gains in the bulk of inorganic samples, paramagnetic metal ions are introduced as minor dopants acting as polarizing agents. While this approach has been shown to be very efficient in crystalline inorganic oxides, significantly lower enhancements have been reported when applying this approach to oxide glasses. In order to rationalize the origin of the difference in the efficiency of DNP in amorphous and crystalline inorganic matrices, we performed a detailed comparison in terms of their magnetic resonance properties. To diminish differences in the DNP performance arising from distinct nuclear interactions, glass and crystal systems of similar compositions were chosen, Li2OCaO·2SiO2 and Li2CaSiO4, respectively. Using Gd(III) as polarizing agent, DNP provided signal enhancements in the range of 100 for the crystalline sample, while only up to around factor 5 in the glass, for both 6Li and 29Si nuclei. We find that the drop in enhancement in glasses can be attributed to three main factors: shorter nuclear and electron relaxation times as well as the dielectric properties of glass and crystal. The amorphous nature of the glass sample is responsible for a high dielectric loss, leading to efficient microwave absorption and consequently lower effective microwave power and an increase in sample temperature which leads to further reduction of the electron relaxation time. These results help rationalize the observed sensitivity enhancements and provide guidance in identifying materials that could benefit from the DNP approach.

NiAl-LDH-Modified Core–Shell Rod-like ZnO@ZnS Heterostructures for Enhanced Photocatalytic Hydrogen Precipitation

Abstract: Photocatalytic hydrogen production technology is considered as an important approach to solve the problem of energy shortage. The ZnO@ZnS core–shell nanostructure can not only protect the nuclear material from chemical corrosion but also form heterojunctions to improve the catalytic performance; however, the photocatalysis of ZnO@ZnS only responds to the UV region, and its solar light utilization is low. In this paper, ZnO@ZnS rods were prepared and formed covalent bonding in the interface because of the in situ sulfidation process. Subsequently, the two-dimensional NiAl-LDH with positive charge was grown on the surface of ZnO@ZnS by electrostatic self-assembly due to the presence of negatively charged ZnS. So, the prepared ZnO@ZnS@NiAl-LDH composites have the stable interface structure. We compared the hydrogen production efficiency of ZnO, ZnO@ZnS, and ZnO@ZnS@NiAl-LDH under simulated sunlight. The maximum hydrogen production efficiency of the ZnO@ZnS@NiAl-LDH composite was 866.35 μmol g–1 h–1, which was 3.96 times higher than that of ZnO@ZnS (218.41 μmol g–1 h–1), and the sample also had a good stability and recyclability according to the results of the cycling test of photocatalytic H2 generation. Due to the presence of UV-responsive ZnO@ZnS and visible-light-responsive NiAl-LDH, ZnO@ZnS@NiAl-LDH can effectively utilize the entire solar spectrum. The interface structure affected the electron transfer rates so that the synergistic interaction between rod-shaped ZnO@ZnS and layered NiAl-LDH could improve the electron–hole separation rate and the transport rate of photogenerated charge pairs, thus optimizing the photocatalytic performance. This study provides new ideas for the research of ZnO-based materials as photocatalysts.

Fabrication of TiO2/CdS Heterostructure by Soluble Solid-State Titanium-oxo-Clusters for Fast Photocatalytic Degradation of Tetracycline

Abstract: Dependent on the solubility of titanium-oxo-clusters, the molar ratios of Ti/Cd in the composite can be adjusted precisely. Herein, a soluble solid-state titanium-oxo-cluster is employed as a Ti raw reactant to fabricate a TiO2/CdS photocatalyst with TiO2 nanoparticles isolated on the surface of CdS nanorods for highly efficient degradation of tetracycline (TC). The comparative experiment shows that the composite with the Ti/Cd molar ratio at 0.5 exhibits the best photocatalytic performance, which can degrade 95% TC with an initial concentration of 10 mg/L under 12 min visible-light irradiation. The excellent photocatalytic ability of the TiO2/CdS composites made of soluble titanium-oxo-clusters is superior to those of the pure CdS and TiO2, which is attributed to not only the synergic effect of TiO2/CdS heterostructure on improvement of photogenerated electron–hole pairs’ separation and lifetime but also the ideal dispersion of the isolated TiO2 nanoparticles on the surface of CdS nanorods. This work presents a facile way to achieve a highly active catalyst with controllable TiO2-involved heterostructure for photocatalytic degradation.

Hydration and Dehydration Behavior of Li4SnS4 for Applications as a Moisture-Resistant All-Solid-State Battery Electrolyte

Abstract: All-solid-state batteries are promising energy storage devices owing to their high safety and energy density. Sulfide-based solid electrolytes have high ionic conductivities and are ductile. Although sulfide electrolytes are candidates for all-solid-state batteries, they are moisture-sensitive. Li4SnS4 electrolytes can overcome some of these weaknesses and show the suppressed evolution of H2S gas in a humid environment. However, the suppression mechanism is unclear, making the design of moisture-resistant sulfide electrolytes challenging. Therefore, we investigated the structural changes of Li4SnS4 in humid N2. X-ray structural analysis revealed that Li4SnS4 was hydrated to Li4SnS4·4H2O without generating H2S gas. Furthermore, the hydrate could be reversibly dehydrated by heat treatment to the original sulfide, suggesting that the formation of stable hydrates suppresses the generation of H2S gas. These findings will aid in the design of moisture-resistant sulfide materials.

Phase Transitions in Inorganic Halide Perovskites from Machine-Learned Potentials

Abstract: The atomic scale dynamics of halide perovskites have a direct impact not only on their thermal stability but their optoelectronic properties. Progress in machine learned potentials has only recently enabled modeling the finite temperature behavior of these material using fully atomistic methods with near first-principles accuracy. Here, we systematically analyze the impact of heating and cooling rate, simulation size, model uncertainty, and the role of the underlying exchange-correlation functional on the phase behavior of CsPbX3 with X=Cl, Br, and I, including both the perovskite and the delta-phases. We show that rates below approximately 30 K/ns and system sizes of at least a few ten thousand atoms are indicated to achieve convergence with regard to these parameters. By controlling these factors and constructing models that are specific for different exchange-correlation functionals we then show that the semi-local functionals considered in this work (SCAN, vdW-DF-cx, PBEsol, and PBE) systematically underestimate the transition temperatures separating the perovskite phases while overestimating the lattice parameters. Among the considered functionals the vdW-DF-cx functional yields the closest agreement with experiment, followed by SCAN, PBEsol, and PBE. Our work provides guidelines for the systematic analysis of dynamics and phase transitions in inorganic halide perovskites and similar systems. It also serves as a benchmark for the further development of machine-learned potentials as well as exchange-correlation functionals.

Hydroxide Diffusion in Functionalized Cylindrical Nanopores as Idealized Models of Anion Exchange Membrane Environments: An Ab Initio Molecular Dynamics Study

Abstract: Anion exchange membranes (AEMs) have attracted significant interest for their applications in fuel cells and other electrochemical devices in recent years. Understanding water distributions and hydroxide transport mechanisms within AEMs is critical to improving their performance as concerns hydroxide conductivity. Recently, nanoconfined environments have been used to mimic AEM environments. Following this approach, we construct nanoconfined cylindrical pore structures using graphane nanotubes (GNs) functionalized with trimethylammonium cations as models of local AEM morphology. These structures were then used to investigate hydroxide transport using ab initio molecular dynamics (AIMD). The simulations showed that hydroxide transport is suppressed in these confined environments relative to the bulk solution although the mechanism is dominated by structural diffusion. One factor causing the suppressed hydroxide transport is the reduced proton transfer (PT) rates due to changes in hydroxide and water solvation patterns under confinement compared to bulk solution as well as strong interactions between hydroxide ions and the tethered cation groups.

Kinetic Relevance of Surface Reactions and Lattice Diffusion in the Dynamics of Ce–Zr Oxides Reduction–Oxidation Cycles

Abstract: Reduction–oxidation cycles in oxides are ubiquitous in oxygen storage and transport, chemical looping processes, and fuel cells. O-atom addition and removal are mediated by coupling reactions of oxidants and reductants at surfaces with diffusion of O-atoms within oxide crystals, with either or both processes as limiting steps. CeO2–ZrO2 solid solutions (CZO) are ubiquitous in practice. They are used here to illustrate general experimental strategies and reaction–diffusion formalisms for nonideal systems that enable assessments of the kinetic relevance of the steps that mediate O-atom addition and removal in these materials; these experiments are described within the context of models that describe the driving forces for reaction and diffusion rigorously in terms of oxygen chemical potentials (μO). These strategies assess the rate consequences of varying the fluid phase redox potential, through changes in the identity and pressures of the reactants and products used in redox cycles (O2; CO/CO2; H2/H2O; N2O/N2), of introducing dispersed metal nanoparticles that capture and react lattice O-atoms in CZO using CO or H2, and of imposing intervening dwells without reaction within redox cycles. O-removal rates depend on reductant pressures, even when CO/CO2 and H2/H2O ratios are chosen to maintain the same surface μO if surface reactions were quasi-equilibrated. These data, taken together with significant rate enhancements in O-removal when Pt nanoparticles are present at CZO crystal surfaces and with similar rates before and after inert dwells, demonstrate that reduction rates by both CO and H2 are limited by surface reactions without the presence of consequential spatial gradients in μO within CZO crystals. In contrast, O-addition rates to partially reduced CZO crystals are similar for N2O and O2 reactants and are not affected by the presence of Pt nanoparticles; O-addition rates are significantly higher after intervening inert dwells during CZO oxidation, indicative of spatial gradients in μO, which relax during nonreactive periods. These methods and models, illustrated here for CZO redox cycles at conditions relevant to oxygen storage practice, allow systematic assessments of the kinetic relevance of lattice diffusion and surface reactions for systems that use solids for the reversible storage and release of atoms, irrespective of the identity of the solids or the atoms (e.g., O, H, N, and S).

Lattice Dynamics and Electron–Phonon Coupling in Double Perovskite Cs2NaFeCl6

Abstract: Phonon–phonon and electron/exciton–phonon coupling play a vitally important role in thermal, electronic, as well as optical properties of metal halide perovskites. In this work, we evaluate phonon anharmonicity and coupling between electronic and vibrational excitations in novel double perovskite Cs2NaFeCl6 single crystals. By employing comprehensive Raman measurements combined with first-principles theoretical calculations, we identify four Raman-active vibrational modes. Polarization properties of these modes imply Fm3̅m symmetry of the lattice, indicative for on average an ordered distribution of Fe and Na atoms in the lattice. We further show that temperature dependence of the Raman modes, such as changes in the phonon line width and their energies, suggests high phonon anharmonicity, typical for double perovskite materials. Resonant multiphonon Raman scattering reveals the presence of high-lying band states that mediate strong electron–phonon coupling and give rise to intense nA1g overtones up to the fifth order. Strong electron–phonon coupling in Cs2NaFeCl6 is also concluded based on the Urbach tail analysis of the absorption coefficient and the calculated Fröhlich coupling constant. Our results, therefore, suggest significant impacts of phonon–phonon and electron–phonon interactions on electronic properties of Cs2NaFeCl6, important for potential applications of this novel material.

Electrolyte Optimization for Graphite Anodes toward Fast Charging

Abstract: As the most advanced energy storage devices, lithium ion batteries (LIBs) have captured a great deal of attention and have been developed swiftly during the past decades. However, the improved fast-charging performance is more urgent than ever for time-saving and convenience, which is generally limited by the graphite anode. Recent studies have revealed that the fast-charging performance of graphite anodes is highly dictated by the properties of the electrolyte. Therefore, the investigations on fast-charging graphite based on designs of electrolytes are summarized from two aspects: solid electrolyte interphase (SEI) structures and solvated lithium ion structures. Finally, challenges and prospects for further research toward fast-charging graphite anodes are proposed.

Tracking and Understanding Dynamics of Atoms and Clusters of Late Transition Metals with In-Situ DRIFT and XAS Spectroscopy Assisted by DFT

Abstract: Dynamic structural changes of single-atom catalysts (SACs) are key to many reactions that were reported to be catalyzed by supported single atoms. To understand these changes, systematic in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS) experiments were performed under reducing (1% CO) and oxidizing (1% CO + 1% O2) reaction atmospheres at room temperature over CeO2-supported late transition metals (Ru, Rh, Pd, Ir, and Pt) synthesized via two different methods (wet impregnation and precipitation) to account for the influence of surface properties. As a general trend, the CO vibrational frequencies downshifted under the CO atmosphere, which we assigned to the formation of clusters. Upon changing the gas mixture to more oxidizing (1% CO + 1% O2), single sites are retained as evidenced by the CO vibrational frequencies at higher wavenumbers. Among the investigated metals, Pt2+ and Pd2+ are more prone to cluster formation, and Rh3+ and Ru4+ are found to be stable as single sites following the order Rh > Ru > Ir > Pt > Pd. In combination with the density functional theory (DFT) calculations of CO vibrational frequencies, we were able to assign shifts to changes in the oxidation state of the metals. These findings thus serve as a benchmark for ceria-supported Pd, Pt, Ru, Ir, and Rh SACs.

A Polysulfide-Repulsive, In Situ Solidified Cathode–Electrolyte Interface for High-Performance Lithium–Sulfur Batteries

Abstract: Lithium–sulfur batteries are promising candidates for beyond-Li-ion electrochemical energy storage yet are hindered due to limited cycle lives. In case a liquid ether electrolyte is used, the S cathode suffers from an unstable electrode–electrolyte interface, at which soluble polysulfide intermediates form, dissolve, and shuttle between the two electrodes. When the cathode–electrolyte interface is solidified, the S cathode shows suppressed polysulfide dissolution. In this work, we show that a liquid polymer cathode additive, poly(hexamethylene diisocyanate), is able to react with soluble polysulfide species at the beginning of battery discharge to in situ form a solidified, polysulfide-anion-grafted organic cathode–electrolyte interface. In addition to serving as a physical barrier, the negatively charged interface helps to anchor polysulfides at the cathode surface via electrostatic repulsion. The solidified interface around the active S particles also forms an efficient, three-dimensional porous Li-ion conducting network to trigger improved electrode kinetics and avoid the formation of locally dead S. Benefiting from the improved interfacial electrochemistry, the Li–S battery shows admirable storage performance in terms of cycle life and rate capability to promise practical high-energy rechargeable batteries.

Copper Growth on a Stepped Nickel Surface: Electronic and Geometric Effects on CO Reactivity

Abstract: The deposition of Cu on a stepped Ni(119) surface was investigated by experimental and computational (DFT) methods. At ambient temperature, Cu grows thermodynamically stable in a layer-by-layer mode, occupying 4-fold hollow sites below the step edge. Although CO and Cu adsorption does not appear to be competitive in terms of stable surface sites, Cu was found to alter quantitatively the adsorption of CO. The Cu–Ni interaction is determined by both geometric (higher coordination number) and electronic (perturbation of surface electron density) effects. The latter is supported from the negative binding energy shifts observed for the Cu 2p3/2 photoelectron peak combined with the shift of the Cu 3d band center, which reflects a Ni-induced increase in the d-electron population of Cu metal atoms. CO preferably adsorbs on the step edges on the clean and submonolayer Cu-covered Ni(119) where the minimum electron density is observed. These results highlight the importance of geometric and electronic effects in a bimetallic system, which mimics real catalysts, where heteroatoms can electronically alter or selectively block certain surface sites and therefore change dramatically the overall reactivity of the surface.

The Dramatic Effect of Water Structure on Hydration Forces and the Electrical Double Layer

Abstract: : Forces between hydrophilic surfaces mediated by water are important in various systems from lipid membranes and solid surfaces to colloids and macromolecules, first discovered as a significant addition to DLVO forces at the nanoscale. These “hydration forces” have been studied in great detail experimentally using osmotic stress measurements, surface force apparatus, and AFM, and they have also been the subject of multiple theories and simulations. One spectacular feature observed in experimental and simulation studies was the nonmonotonic, oscillatory decay in the forces between atomically smooth surfaces. Forces between “rougher” surfaces exhibit only quasi-exponential, monotonic decay. Here we revisit this hydration force problem by exploring the consequences of an extended phenomenological Landau − Ginzburg approach that describes nonlocal correlations in water, linking them with the key features of the wave-number k -dependent nonlocal dielectric function of water. With corresponding boundary conditions, this theory predicts the observed oscillatory decay in hydration force between ideally flat surfaces, the oscillatory mode disappearing with just a tiny roughness of the surfaces (of mean height ca. of the size of a water molecule). This study also brings an important side message. Explanation of these observations appears only possible under an assumption of two modes of polarization in water, consistent with the behavior of the response function, i.e., Lorentzian at small k and resonance-like at higher k . This resolves the “force oscillation − non-oscillation” paradigm, which is a strong, although indirect indication of the existence of these two modes. We also consider other important subjects, such as how the distribution of ions near a charged surface reacts to the propensity for overscreening oscillations due to polarized water. This is important not only for the interactions between charged surfaces but also for the fundamental understanding of the structure of the electrical double layer at electrochemical interfaces. We show that even in dilute electrolytes, the distribution of ions in the vicinity of the polarized interface follows, although not literally, preferential positions corresponding to the potential wells caused by “resonance” water layering. For a sharp interface, the theory predicts that the decaying spatial oscillation profiles extend over a 1 to 2 nm distance from the interface. With the smearing of the interface and the corresponding suppression of the resonance water layering, oscillations in the spatial distribution of ions subside, resulting in a familiar Gouy − Chapman − Stern picture. At longer distances from the interface, whether smeared or not, the ion distribution profiles become Gouy − Chapman-like. The effect of the boundary conditions on water polarization at the interface goes beyond a trivial shift of the potential of zero charge. We show that they can dramatically affect the ion distribution near the charged surface. Last, but not least, we study how the interfacial water layering influences the double layer capacitance and show the effect of the boundary conditions on the slopes of Parsons − Zobel plots, resolving some recently discussed puzzles.

Determining the Geometric Surface Area of Mesoporous Materials

Abstract: Specific surface area is an important property of porous materials and a crucial index in multiple disciplines, which is mainly measured using nitrogen isotherms and the Brunauer–Emmett-Teller (BET) equation. The BET surface area is remarkably higher than the geometric surface area, which is defined as the surface area of the crystal geometry structure. However, many studies directly use the BET surface area as the geometric surface area. We proposed a simple, general, and accurate method that only needs the nitrogen adsorption isotherm as the input to calculate the geometric surface area of mesoporous materials. The proposed method considers the adsorptive potential influences and the physical state of adsorbed nitrogen in estimating the surface area. We utilized 18 isotherms generated by molecular simulations and actual experiments to cover a wide range of materials, including minerals, catalysts, and carbon-based tubes, and validate the effectiveness of the proposed method. Our results showed that the surface area determined by the proposed method is almost identical to the referenced geometric area. The surface area obtained by the proposed method can be typically considered a geometric surface area.

Effect of Electrolyte pH on Oxygen Bubble Behavior in Photoelectrochemical Water Splitting

Abstract: The issue of increased reaction resistance due to bubble growth has always been a major bottleneck limiting the efficiency improvement of photoelectrochemical water splitting. In this study, we developed a synchronized measurement system with a micro-high-speed camera and an electrochemical workstation to observe oxygen bubble evolution on the surface of a fixed TiO2 film electrode in situ. The intrinsic relationship between the nucleation and growth of oxygen bubbles and photocurrent at different pH values (1.0–13.0) was investigated. The results indicate that higher pH can promote bubble nucleation at lower potentials. Additionally, increasing pH from 1.0 to 13.0 at 0.1 V vs Ag/AgCl, the photocurrent in the bubble growth stage increases by about 35 times, and the average period of bubble growth decreases by about 15 times. Compared with pH = 9.0, the gas production rates of pH = 1.0 and pH = 13.0 are improved by 13 times and 22 times at 0.71 V vs RHE, respectively. Then, we developed a force balance model for oxygen bubbles at the anode surface, and the predicted bubble detachment diameters are in good agreement with the experimental results. The Marangoni force induced by the nonuniform distribution of dissolved oxygen was found to be increased with pH, which leads to the larger detachment diameter of bubbles. The results show that the strong alkali environment is an effective means to remove oxygen bubbles from the surface of the photoelectrode.

Origin of Magnetic Anisotropy in Nickelocene Molecular Magnet and Resilience of Its Magnetic Behavior

Abstract: The robustness of nickelocene’s (NiCp2, Cp = cyclopentadienyl) magnetic anisotropy and addressability of its spin states make this molecular magnet attractive as a spin sensor. However, microscopic understanding of its magnetic anisotropy is still lacking, especially when NiCp2 is deposited on a surface to make quantum sensing devices. Quantum chemical calculations of such molecule/solid-state systems are limited to density functional theory (DFT) or DFT+U (Hubbard correction to DFT). We investigate the magnetic behavior of NiCp2 using the spin-flip variant of the equation-of-motion coupled-cluster (EOM-SF-CC) method and use the EOM-SF-CC results to benchmark SF-TD-DFT. Our first-principle calculations agree well with experimentally derived magnetic anisotropy and susceptibility values. The calculations show that magnetic anisotropy in NiCp2 originates from a large spin–orbit coupling (SOC) between the triplet ground state and the third singlet state, whereas the coupling with lower singlet excited states is negligible. We also considered a set of six ring-substituted NiCp2 derivatives and a model system of the NiCp2/MgO(001) adsorption complex, for which we used SF-TD-DFT method. To gain insight into the electronic structure of these systems, we analyze spinless transition density matrices and their natural transition orbitals (NTOs). The NTO analysis of SOCs explains how spin states and magnetic properties are retained upon modification of the NiCp2 coordination environment and upon its adsorption on a surface. Such resilience of the NiCp2 magnetic behavior supports using NiCp2 as a spin-probe molecule by functionalization of the tip of a scanning tunneling microscope.

Clarification of the Effects of Oxygen Containing Functional Groups on the Pore Filling Behavior of Discharge Deposits in Lithium–Air Battery Cathodes Using Surface-Modified Carbon Gels

Abstract: In lithium–air batteries (LABs), controlling the characteristics of the Li2O2 deposited during discharging can lead to the reduction of the large overpotential required for charging. The large overpotential is one of the most significant problems that needs to be solved to improve the cycle performance of LABs. Here, we focused on the effects of functional groups in the cathode carbon on the characteristics of the Li2O2 deposited during discharging and the cathode performance of LABs. In this study, 4 types of carbon gels (CGs) were prepared using different treatment methods to modify their surface properties. The types and amounts of oxygen-containing functional groups (OCFGs) existing within the CGs were clarified along with the number of edge H’s by a high-sensitivity temperature-programmed desorption (TPD) technique. The results of N2 adsorption analysis of discharged CGs suggested that, by increasing the number of OCFGs from 0.40 to 1.80 mmol g–1 through acid treatment, the ratio of Li2O2 deposited within the mesopores of the porous carbon particles can be increased from 1% to 60%. This significant change in the manner of Li2O2 deposition led to the reduction of the charging overpotential. Side reactions that are thought to deteriorate cycle performance tended to proceed in CGs having a large number of OCFGs. This negative effect could be reduced by removing carboxyl groups in the CGs through simple heat treatment at 300 °C in an inert atmosphere. Our study clarified the critical roles of OCFGs in the cathode during the discharging and charging of LABs. The obtained knowledge can be utilized for the development of a high-performance cathode for LABs.

High-Pressure Behavior of δ-Phase of Formamidinium Lead Iodide by Optical Spectroscopies

Abstract: The exceptional photovoltaic properties of hybrid organic–inorganic perovskites have attracted increasing interest in the past decades. Among these materials, FAPbI3 shows two structural phases: the high temperature perovskite α-phase, with direct bandgap close to the Shockley–Queisser limit, and the much less photoactive non-perovskite δ-phase, stable at ambient conditions. Although the presence of the δ-phase has been usually regarded as a limitation for FAPbI3 optoelectronic applications, recent studies have found that devices with increased stability and efficiency can be designed by mixing α- and δ-phases. This has brought out the need for a deeper understanding of the physical properties of δ-FAPbI3. In this paper, we present an original high-pressure Raman and photoluminescence study to address the effects of compression on the lattice and optoelectronic response of the sample. Also, based on the previous findings on different hybrid perovskites, our results for δ-FAPbI3 show that the cation configuration goes from a dynamically disordered regime at ambient conditions to a statically ordered phase at ∼1.5 GPa. On further increasing pressure, above 7 GPa, a statically disordered regime takes place, where the cations are locked at random orientations in the inorganic framework, giving rise to an amorphous-like state. Compared with α- FAPbI3, we found that the hexagonal δ-phase is less affected by external compression, as both the first detectable structural transition and the amorphous-like behavior occur at higher pressures.

Symmetry of the Hyperfine and Quadrupole Interactions of Boron Vacancies in a Hexagonal Boron Nitride

Abstract: The concept of optically addressable spin states of deep level defects in wide band gap materials is successfully applied for the development of quantum technologies. Recently discovered negatively charged boron vacancy defects (VB) in hexagonal boron nitride (hBN) potentially allow a transfer of this concept onto atomic thin layers due to the van der Waals nature of the defect host. Here, we experimentally explore all terms of the VB spin Hamiltonian reflecting interactions with the three nearest nitrogen atoms by means of conventional electron spin resonance and high frequency (94 GHz) electron-nuclear double resonance. We establish symmetry, anisotropy, and principal values of the corresponding hyperfine interaction (HFI) and nuclear quadrupole interaction (NQI). The HFI can be expressed in the axially symmetric form as Aperp = 45.5 MHz and Apar = 87 MHz, while the NQI is characterized by quadrupole coupling constant Cq = 1.96 MHz with slight rhombisity parameter n = (Pxx - Pyy)/Pzz = -0.070. Utilizing a conventional approach based on a linear combination of atomic orbitals and HFI values measured here, we reveal that almost all spin density (84 %) of the VB electron spin is localized on the three nearest nitrogen atoms. Our findings serve as valuable spectroscopic data and direct experimental demonstration of the VB spin localization in a single two dimensional BN layer.

Investigating the Role of Silver Oxidation State on the Thermodynamic Interactions between Xenon and Silver-Functionalized Zeolites

Abstract: The molecular level understanding of the strong adsorption of Xe to silver-modified zeolites remains elusive. Here, we probe the effect of silver oxidation state on the thermodynamics of Xe sorption in silver-functionalized zeolites by measuring the enthalpies of adsorption after various treatments using inverse gas chromatography (IGC). The enthalpy of adsorption was measured for silver-functionalized chabazites (AgCHA) before and after hydrogen reduction and subsequent reoxidation. The sorption enthalpy (ΔH) for AgCHA was 35.2 kJ/mol, which decreased to 25.8 kJ/mol with hydrogen reduction. After reoxidation (O2-AgCHA), 95% of the binding strength was restored. Hydrogen reduction of the base chabazite (CHA) did not influence Xe adsorption. Henry’s law constant for Xe adsorption increased in the order AgCHA > O2-AgCHA > H2-AgCHA > CHA. A decrease in enthalpy and Henry’s constant with silver reduction and increase with reoxidation suggest that ionic silver is playing a role in Xe binding. The effect of reduction and reoxidation on the zeolite microstructure was analyzed using surface area analysis, powder X-ray diffraction (p-XRD), scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS), and X-ray photoelectron spectroscopy (XPS). These results lay the groundwork for better material design of strong noble gas adsorbents.

Ligand Energy Staff Gauge for Antenna Effect Study within Metal–Organic Frameworks

Abstract: Revealing the structure–optical property relationship of aggregation-induced emission (AIE) species (AIEgens) and achieving the synergy between AIE and antenna effect are challenging. Herein, we study the emission behaviors of three AIE ligands with tetraphenyl structures, 1,2,4,5-tetrakis(4-carboxyphenyl)benzene (L1), 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine (L2), and 1,1,2,2-tetrakis(4-carboxyphenyl)ethylene (L3), and find their ladder-shaped emission wavelength. In combination with terephthalic acid (L0), we construct a staff gauge from L0 to L3 to realize different sensitization for Ln3+ from the sensitization of both Eu3+ and Tb3+ to single Eu3+ and then to non-sensitization. We proposed the threshold value of 5000 cm–1 for complete sensitization from the different optical behaviors and energy levels of the lanthanide metal–organic frameworks (Ln-MOFs). Coplanar degree of L1–L3 is proposed to explain their optical behaviors and different sensitization. Thus, we revealed the connection among the energy levels, emission wavelength, and their effect on antenna effect for AIE ligands. L1-Eu MOF exhibits a dual emission with a porous property and is used for ratiometric fluorescent detection and removal of F–.

Competition Pathways of Energy Relaxation of Hot Electrons through Coupling with Optical, Surface, and Acoustic Phonons

Abstract: The hot electrons in carbon-based materials exhibit interesting ballistic transport behaviors for designing high-performance single-electron transistors. However, the cooling of such hot electrons back to the equilibrium state may be slowed down by the excessively populated optical phonons, thus limiting their ballistic transport. Therefore, a thorough understanding of the coupling between hot electrons and optical phonons is of critical importance. Here, by varying the thickness of a multilayer graphene film on a supporting substrate, we investigated the competition pathways of the energy relaxation of the photoinduced hot electrons through coupling with the optical, surface, and acoustic phonons. The difference in the τ2 values indicates that thickness plays an important role in the optical phonon population. For the multilayer graphene film thickness less than 3 nm, the super-collision model describes the hot electron cooling dynamics that is strongly affected by the surface phonons from the supporting substrate. As the multilayer graphene film thickness increases from 3 to 20 nm, the accumulation of the optical phonons induces a hot optical phonon effect, resulting in a bottleneck of cooling of the hot electrons. As the multilayer graphene film thickness further increases from 20 to 40 nm, a direct coupling between the hot electrons and acoustic phonons starts to dominate. The surface states of the interfaces at the inner layers of the multilayer graphene film contribute to this direct coupling as an additional cooling channel. The quantitative understanding of the energy relaxation pathways of the hot electrons offers insights into designing high-performance single-electron transistors.

Synaptic Plasticity of a Microfluidic Memristor with a Temporary Memory Function Based on an Ionic Liquid in a Capillary Tube

Abstract: Fluidic memristor devices have received tremendous attention for smooth resistance switching in artificial synapses due to the ion migration, concentration polarization, and redox reactions mechanism. Here we provide a novel method of preparing microfluidic memristor with superior stability, robustness, and ultralow cost. The structure of the two-terminal memristor device is Cu/[MMIm][Cl]: H2O/Cu, C5H9N2Cl. The ionic liquid of 1,3-dimethylimidazole chloride salt was used as representative IL to display resistive memory properties in a cylindrical microchannel of a capillary. The fabricated device shows hysteretic and bipolar I–V characteristics of memristor, which can respond to external stimuli, e.g., space length between two electrodes and applied voltage. Meanwhile, this artificial synapse can mimic synaptic plasticity under various pulse stimuli stably and repeatedly, which results in temporary memory behavior. Such device exhibits great potential value in the area of neuromorphic artificial synapses and memory states.