The 100 most-cited articles in The Journal of Physical Chemistry C, 2018

The recent successes of the Materials Genome Initiative have opened up new opportunities for data-centric informatics approaches in several subfields of materials research, including in polymer science and engineering. Polymers, being inexpensive and possessing a broad range of tunable properties, are widespread in many technological applications. The vast chemical and morphological complexity of polymers though gives rise to challenges in the rational discovery of new materials for specific applications. The nascent field of polymer informatics seeks to provide tools and pathways for accelerated property prediction (and materials design) via surrogate machine learning models built on reliable past data. We have carefully accumulated a data set of organic polymers whose properties were obtained either computationally (bandgap, dielectric constant, refractive index, and atomization energy) or experimentally (glass transition temperature, solubility parameter, and density). A fingerprinting scheme that captures atomistic to morphological structural features was developed to…

Lithium intercalation into graphite is one of the electrochemically best studied solid-state reactions, and its application in lithium-ion batteries was a pioneering step in the development of advanced electrochemical storage devices. Therefore, one might expect that virtually any aspect of this important reaction has been examined both qualitatively and quantitatively. All the more, it is surprising that there are only a few experimental studies on the volume expansion of graphite, especially under cycling conditions. To the best of our knowledge, there exists no comprehensive set of structural data as a function of lithium content. Here, we present this missing information using combined results from electrochemical testing and operando X-ray diffraction. The changes in lattice parameters and unit cell volume are examined and related to the different intercalation stages and phase transition regimes. A total volume expansion (from space-group-independent evaluation) of 13.2% is observed when C 6 is fully lithiated to a composition of LiC 6, of which approximately 5.9% occur in the…

Construction of heterojunctions has aroused great interest recently in the photocatalysis field because of the special electronic band structure and unique physicochemical properties. In this work, a novel 0D/3D CuO/ZnO heterojunction was obtained via in situ deposition of CuO nanoparticles on the flowerlike ZnO surface using the wet chemistry method. After depositing CuO nanoparticles onto the ZnO, the CuO/ZnO heterojunction exhibits enhanced visible-light harvesting and effective separation of the photogenerated electron–hole pairs compared with those in the pure ZnO. The photocatalytic removal efficiency of phenol over the CuO/ZnO heterojunction is up to 78% under the irradiation of the light, which is ∼2 and ∼4 times higher than those of the pristine ZnO and CuO, respectively. This composite also presents good durability and stability for phenol degradation in the photocatalytic reactions. Additionally, in the photodegradation system of the CuO/ZnO heterojunction, the superoxide radicals ( • O 2 – ) and hydroxyl radicals ( • OH) are confirmed as the active species by the…

Recently, novel two-dimensional materials for solar water splitting have drawn enormous research attention for the interesting tunable electronic and optical properties. We investigate the geometry, electronic, and optical properties of the monolayer (ML) and multilayer Janus MoSSe with the first-principles calculations. We find that the ML Janus MoSSe is a semiconductor with a direct band gap of 2.14 eV, which is suitable for absorbing visual light efficiently. It also holds an appropriate band edge alignment with the water redox potentials. The biaxial strain could effectively modulate the electronic and optical properties of the ML MoSSe, from a direct semiconductor to the indirect one, even to be metal. As for the bilayer (BL) MoSSe, the stacking order could effectively affect the electronic, optical, and redox properties. The most stable stacking order is the A′B type, followed by the AA′, AA-SeSe, AA-SS, and AA-SSe stacking faults. For the BL MoSSe, they are all indirect semiconductors. The indirect band gap of the multilayer MoSSe decreases monotonously as the number of…

The Janus structures of transition metal dichalcogenides with an intrinsic dipole have been proposed as efficient photocatalysts for water splitting, and successfully synthesized recently. However, the mechanism for their superior photocatalytic activities are not understood. Here, we systematically investigate the photocatalytic activities of Janus molybdenum dichalcogenides (MoXY, X/Y = O, S, Se, and Te), by studying their band gaps, redox energy levels and electrons and holes separation, by first-principles calculations. The intrinsic dipoles in the Janus structures cause notable band bending to achieve favorable band edge positions relative to water redox potentials, which makes the Janus structures as efficient heterojunction photocatalysts. Electrons and holes are spatially separated on different surfaces of the Janus structure due to the internal electric field, which effectively inhibits the recombination of excitons and ensures photocatalytic activity with high efficiency.

The CO 2 electroreduction reaction (CO2RR) is a promising avenue to convert greenhouse gases into high-value fuels and chemicals, in addition to being an attractive method for storing intermittent renewable energy. Although polycrystalline Cu surfaces have long been known to be unique in their capabilities of catalyzing the conversion of CO 2 to higher-order C1 and C2 fuels, such as hydrocarbons (CH 4, C 2 H 4, etc.) and alcohols (CH 3 OH, C 2 H 5 OH), product selectivity remains a challenge. Rational design of more selective catalysts would greatly benefit from a mechanistic understanding of the complex, multiproton, and multielectron conversion of CO 2 . In this study, we select three metal catalysts (Pt, Au, Cu) and apply in situ surface enhanced infrared absorption spectroscopy (SEIRAS) and ambient-pressure X-ray photoelectron spectroscopy (APXPS), coupled to density-functional theory (DFT) calculations, to get insight into the reaction pathway for the CO2RR. We present a comprehensive reaction mechanism for the CO2RR and show that the preferential reaction pathway can be…

Graphitic carbon nitride (g-C 3 N 4 ) can be used as a photocatalyst to reduce CO 2 . Doping is an efficient strategy for improving the photocatalytic activity and tuning the electronic structure of g-C 3 N 4 . The sulfur-doped g-C 3 N 4 (S-doped g-C 3 N 4 ) as a promising photocatalyst for CO 2 reduction was investigated by density functional theory methods. The electronic and optical properties indicate that doping S enhances the catalytic performance of g-C 3 N 4 . From the reduction Gibbs free energies, the optimal path for CO 2 reduction reaction to CH 3 OH production catalyzed by S-doped g-C 3 N 4 is CO 2 → COOH* → CO → HCO* → HCHO → CH 3 O* → CH 3 OH. In comparison with g-C 3 N 4, doping S can alter the rate-determining step and reduce the Gibbs free energy from 1.43 to 1.15 eV. CO 2 reduction activity of S-doped g-C 3 N 4 is better than that of g-C 3 N 4, which is in well agreement with the experimental results. Our work provides useful insights into designing nonmetal-doped g-C 3 N 4 for photocatalytic CO 2 reduction reactions.

When it comes to energy density, all-solid-state batteries are seen as a promising technology for next-generation electrochemical storage devices. Nevertheless, the performance of all-solid-state cells is still very limited. The reasons are manifold, with insufficient ionic and electronic percolation within the composite cathode being a crucial one. In this work, we investigate percolation characteristics by three-dimensional microstructural modeling with the aim to define and understand boundary conditions for well-percolating networks. Utilizing spherical active material particles together with convex polyhedra as the solid electrolyte, ionic and electronic conduction clusters are determined and analyzed by means of percolation theory for varying macroscopic parameters, such as composition, porosity, particle size, and electrode thickness. Small active material particles turn out to enhance the effective electronic conductivity, offering high surface areas and thus more possibilities to connect particles, while porosity crucially affects ionic and electronic conduction…

This study aims to provide physical interpretations of electrochemical impedance spectroscopy (EIS) measurements for redox active electrodes in a three-electrode configuration. To do so, a physicochemical transport model was used accounting for (i) reversible redox reactions at the electrode/electrolyte interface, (ii) charge transport in the electrode, (iii) ion intercalation into the pseudocapacitive electrode, (iv) electric double layer formation, and (v) ion electrodiffusion in binary and symmetric electrolytes. Typical Nyquist plots generated by EIS of redox active electrodes were reproduced numerically for a wide range of electrode electrical conductivity, electrolyte thickness, redox reaction rate constant, and bias potential. The electrode, bulk electrolyte, charge transfer, and mass transfer resistances could be unequivocally identified from the Nyquist plots. The electrode and bulk electrolyte resistances were independent of the bias potential, while the sum of the charge and mass transfer resistances increased with increasing bias potential. Finally, these results and…

Ammonia (NH 3 ) is one of the most important industrial chemicals owing to its wide applications in various fields. However, the synthesis of NH 3 at ambient conditions remains a coveted goal for chemists. In this work, we study the potential of the newly synthesized single-atom catalysts, i.e., single metal atoms (Cu, Pd, Pt, and Mo) supported on N-doped carbon for N 2 reduction reaction (NRR) by employing first-principles calculations. It is found that Mo 1 -N 1 C 2 can catalyze NRR through the enzymatic mechanism with an ultralow overpotential of 0.24 V. Most importantly, the removal of the produced NH 3 is rapid with a free-energy uphill of only 0.47 eV for the Mo 1 -N 1 C 2 catalyst, which is much lower than that for ever-reported catalysts with low overpotentials and endows Mo 1 -N 1 C 2 with excellent durability. The coordination effect on activity is further evaluated, showing that the experimentally realized active site, single Mo atom coordinated by one N atom and two C atoms (Mo-N 1 C 2 ), possesses the highest catalytic performance. Our study offers new opportunities for…

Lead-based halide perovskites (APbX 3 ) are fascinating optoelectronic materials. Because of toxicity issues of Pb, Sn-based halide perovskites are studied, although less so, as an alternative. Adding SnF 2 often improves the properties of Sn halide perovskite-based devices. This effect is usually ascribed to suppression of Sn 2+ → Sn 4+ oxidation and/or decreased Sn vacancy concentration. These effects will change the doping, sometimes in opposite directions. Here we review the effect of addition of SnF 2 during the formation of ASnX 3 layers as observed by different groups, both to the properties of the layers themselves and to photovoltaic cells made from these layers. SnF 2 can affect many different properties of the ASnX 3 perovskites, including film morphology, doping, control over formation of unwanted crystal phases, material stability to various factors, and energy level positions. It also improves (in general) the performance of photovoltaic cells made with these layers. Besides focusing on all these issues, we also describe possible doping scenarios for the perovskites,…

Tafel-equation-based electrochemical analysis is widely employed in hydrogen evolution reaction (HER) to evaluate and characterize electrocatalysts. Tafel slope and exchange current density are the only two parameters that are invariably obtained and discussed with respect to Tafel equation in the literature. Herein, insights on Tafel constant in the analysis of HER are discussed, and its practical advantage is indicated. It is proposed that Tafel constant can be considered as the onset potential of HER. Tafel constant becomes the defining parameter between two electrocatalysts when other parameters such as Tafel slope or exchange current density become same. The significance of the same Tafel constants is illustrated using CoSe 2 and NiSe 2 . Variation of the Tafel constant within a series of Co (1– x ) Ni x Se 2 is examined. The occurrence of the same Tafel constants between two electrocatalysts leads to one electrocatalyst becoming the better catalyst in the lower current regime, whereas the other exhibits higher activity in the higher current regime. Furthermore, concepts…

Magnetic nanoparticle-mediated hyperthermia is a very promising therapy for cancer treatment. In this field, superparamagnetic iron oxide nanoparticles have been commonly employed because of their intrinsic biocompatibility, but they present some limitations that restrict their heating efficiency (specific absorption rate, SAR). Therefore, we have investigated how tuning the size and shape of these iron oxide nanoparticles can be useful to enhance their hyperthermia responses. Monodisperse and crystalline iron oxide nanoparticles have been synthesized by thermal decomposition in two different shapes (spheres and cubes) in a wide range of sizes, ∼10–100 nm. We have thoroughly characterized them both structurally (X-ray diffraction and transmission electron microscopy) and magnetically (physical property measurement system), and then we have analyzed their heating efficiency using a combination of calorimetric and AC magnetometry measurements (0–800 Oe, 300 kHz). We have been able to delimit a range of optimum sizes to maximize the heating efficiency of these nanoparticles depending…

This work was inspired by a previous report [ Janjua, M.R.S.A. Inorg. Chem. 2012, 51, 11306−11314] in which the nonlinear optical (NLO) response strikingly improved with double heteroaromatic rings. Herein, series of triphenylamine–dicyanovinylene based donor−π–acceptor dyes had been designed by structural tailoring of π-conjugated linkers and theoretical descriptions of their molecular NLO properties were reported. Density functional theory and time-dependent density functional theory calculations were performed on optimized geometries to elucidate the electronic structures, absorption spectra, and NLO properties and also to shed light on how structural modification influences the NLO properties. The simulated absorption spectra results indicate that all of the dyes showed the maximum absorbance wavelength in the visible region. The lowest unoccupied molecular orbital–highest occupied molecular orbital energy gaps of all of the dyes have been found smaller, which results in large NLO response. Calculation of natural bond orbital analysis reveals that electrons successfully migrated…

Organic–inorganic hybrid perovskite solar cells (PSCs) have received considerable attentions due to their low cost, easy fabrication, and high power conversion efficiency (PCE), which achieved a certified PCE of 22.7%. To date, most of high efficiency PSCs were fabricated based on organic hole transporting materials (HTMs) such as molecular spiro-MeOTAD or polymeric PTAA. However, poor stability of PSCs limits its large scale commercial application because of use of additives like tert -butylpyridine ( t -BP) and lithium salt. Moreover, relatively low-temperature degradation of organic HTMs is responsible for poor thermal stability of PSCs. Consequently, HTM play a crucial role in realization of efficient and stable PSCs. In order to improve the stability of PCSs, various inorganic HTMs have been developed and applied into PSCs. Recently, the devices based on CuSCN and Cu:NiOx HTMs have demonstrated PCEs over 20%, which is comparable to PCEs of devices based on organic HTMs. Most importantly, stability of PCSs are much improved by the inorganic HTM, which indicates clearly that…

Herein, the stabilities of aqueous Ti 3 C 2 T z (MXene) colloidal suspensions were studied as a function of pH and sodium chloride concentrations using ς-potential and dynamic light scattering measurements. Complete sedimentation was observed when the pH was changed to 5 or 10. In the low pH regime, protons saturate the surface functional groups, rendering the ς-potential less negative that, in turn, leads to aggregation. In the high pH regime, the ς-potential remained constant up to a pH of almost 12. As the molarity of NaCl increases from 0 to 0.04, the ς-potential goes from −35 to −22.5 mV. At a molarity of 0.02, sedimentation was observed. When the pH or NaCl concentration is high, sedimentation occurred, presumably, because of a reduction in the double-layer thickness. In all cases, the sediment comprised crumpled Ti 3 C 2 T z flakes. After adding charged nanoparticles to the colloidal suspension, at neutral pH, subsequent transmission electron microscope micrographs showed that the negative gold nanoparticles preferred the edges, whereas the positive ones preferred the…

Hexagonal boron nitride (h-BN) is widely used in two-dimensional electronics and serves as a host for single-photon emitters. We study the electronic structure of h-BN as a function of the number of layers and take into account different stacking configurations. Using first-principles calculations based on a hybrid functional, we find that the band gap of a single monolayer is direct, while for thicknesses above a monolayer, the band gap is indirect. By examining the positions of the band edges with respect to the vacuum level, we find this direct-to-indirect transition to be driven by a shift in the conduction-band minimum at the M point; this shift changes the band gap by 0.5 eV, going from a single monolayer to bulk. We analyze these results in terms of the orbital composition of the band edges at different high-symmetry points in the Brillouin zone.

The adsorption behavior and underlying mechanism of CO 2 and CH 4 binary mixture in shale kerogen significantly affect the CO 2 sequestration with enhanced gas recovery project (CS-EGR). In this study, we investigated the competitive adsorption behaviors of CO 2 and CH 4 in shale kerogen nanopores using grand canonical Monte Carlo (GCMC) method. Kerogen model takes into effect of matrix and slit nanopores and moisture content based on Ungerer’s molecular model and scanning electron microscope (SEM) analysis, and is successfully validated against experimental data. The effects of temperature, CO 2 and CH 4 distribution, moisture content, adsorption selectivity, and optimal formation for injection were discussed. The results show that adsorption amount of CH 4 on the kerogen increases with increasing pressure and decreases with increasing temperature. The adsorption selectivity of CO 2 over CH 4 is 2.53–7.25, which indicates that CO 2 is preferentially adsorbed over CH 4 under different temperatures. H 2 O prefers to adsorb inside the kerogen matrix and decrease the volumes of matrix…

The simulation of electrochemical reaction dynamics from first principles remains challenging, since over the course of an elementary step, an electron is either consumed or produced by the electrode. For example, the hydrogen evolution reaction begins with a simple proton discharge to a metal surface, but with conventional electronic structure methods, the simulated potential, which is manifested as the metal’s workfunction, varies over the course of the simulation as the electron is consumed in the new metal–hydrogen bond. Here, we present a simple approach to allow the direct control of the electrochemical potential via charging of the electrode surface. This is achieved by changing the total number of electrons in the self-consistent cycle, while enforcing charge neutrality through the introduction of a jellium counter charge dispersed in an implicit solvent region above the slab. We observe that the excess electrons localize selectively at the metal’s reactive surface and that the metal workfunction responds nearly linearly to the variation in electronic count. This linear…

A combination of time-resolved X-ray diffraction (TR-XRD), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and diffuse reflectance infrared Fourier transform spectroscopy was used to carry out in situ characterization of Cu/CeO 2 nanocatalysts during the hydrogenation of CO 2 . Morphological effects of the ceria supports on the catalytic performances were investigated by examining the behavior of copper/ceria nanorods (NR) and nanospheres. At atmospheric pressures, the hydrogenation of CO 2 on the copper/ceria catalysts produced mainly CO through the reverse water–gas shift (RWGS) reaction and a negligible amount of methanol. The Cu/CeO 2 -NR catalyst displayed the higher activity, which demonstrates that the RWGS is a structure-sensitive reaction. In situ TR-XRD and AP-XPS characterization showed significant changes in the chemical state of the catalysts under reaction conditions, with the copper being fully reduced and a partial Ce 4+ → Ce 3+ transformation occurring. A more effective CO 2 dissociative activation at high temperature and a preferential formation of…

The conducting salt in lithium-ion batteries, LiPF 6, can react with water contaminations in the battery electrolyte, releasing HF and further potentially harmful species, which decrease the battery performance and can become a health hazard in the case of a leakage. In order to quantify the hydrolysis products of LiPF 6 in a water-contaminated battery electrolyte (1 mol L –1 LiPF 6 in EC/DEC) and in aqueous solution, ion chromatography (IC), coulometric Karl Fischer titration (cKFT), and acid–base titration were used on a time scale of several weeks. The results show that the nature of the hydrolysis products and the kinetics of the LiPF 6 hydrolysis strongly depend on the solvent, with the main reaction products in the battery electrolyte being HF and HPO 2 F 2 . From the concentration development of reactants and products, we could gain valuable insight into the mechanism of hydrolysis and its kinetics. Since the observed kinetics do not follow simple rate laws, we develop a kinetic model based on a simplified hydrolysis process, which is able to explain the experimentally…

Functionalized MXenes hold promises in a variety of applications in which the dispensable functional groups are mixed. The functionalization is spontaneously realized through competitive adsorption of active species on the MX matrix during the acid etching process of MAX phases. Nevertheless, the knowledge of proportion and distribution of functional groups on MXenes, i.e., surface structures, is still limited. By high-throughput computation screening, ground-state stable structures of four kinds of typical MXenes—Ti 2 C T x, Ti 3 C 2 T x, Nb 2 C T x, and Nb 4 C 3 T x ( T = O, F, and OH)—with mixed functional group compositions are figured out for the first time. The multicomponent functional group patterns definitely demonstrate an obvious feature of spatial mixing at a given component. However, the heterogeneous structure has a near linear dependence on the functional group components in terms of free energy. Most functionalized MXenes are dynamically stable except for Nb 2 CF 2 and Nb 2 C(OH) 2 due to their competing displacive counterparts. Last but not least, Raman spectra of…

Solid electrolyte is vital for solid-state lithium batteries with higher energy and power densities. In this work, we prepared a poly(ethylene oxide) (PEO)-LiClO 4 –Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) composite solid electrolyte (CSE) membrane using a simple solution-casting method. It shows high Li ion conductivity (∼1 × 10 –3 S cm –1 ) and excellent mechanical and electrochemical performances. All solid-state lithium batteries were fabricated with the CSE membrane of PEO-50 wt % LATP, Li metal anode, and LiFePO 4 cathode. By softening the CSE membrane at 80 °C, its ionic conductivity is improved, and the interfacial resistance of the prepared solid-state batteries is greatly reduced. The solid-state battery shows remarkable rate capability and excellent cycling performance. Even after 500 cycles, a capacity of 109.3 mAh g –1 is still kept at 1C. Furthermore, the CSE membrane is used to assemble a flexible pouch cell. The pouch cell can work well at different bending states, and the discharge capacity remains higher than 100 mAh g –1 at a current density of 0.2C. These results…

The isotherm thickness and hydrogen-bonding interactions of water layers adsorbed on hydrophilic and hydrophobic surfaces were quantified and compared. The hydrophilic and hydrophobic surfaces were modeled with an OH-terminated native oxide layer on silicon and a HF-etched silicon terminated with hydrogen, respectively. The silicon substrate allows the use of attenuated total reflection infrared (ATR-IR) spectroscopy for quantitative measurement of adsorbed water without interferences from the gas phase water. On the hydrophilic Si–OH surface, the average thickness of the strongly hydrogen-bonded water layer increases up to ∼2 molecular layers as relative humidity (RH) increases, beyond which the weakly hydrogen-bonded structure is dominant. On the hydrophobic Si–H surface, the adsorbed water layer consists predominantly of the weakly hydrogen-bonded structure and its average thickness remains less than a monolayer even at RH = 90%. The differences in the thickness and structure of adsorbed water layers on hydrophilic versus hydrophobic surfaces found from ATR-IR measurements…

The carbon dioxide electrochemical reduction (CO 2 RR) to useful fuels and chemicals with renewable electricity offers a promising strategy for resolving energy security and environmental issues. Searching for low-cost catalysts with high efficiency and high selectivity is crucial to achieve this goal. Here, by means of comprehensive density functional theory computations, we systematically investigated the potential of several transition metal dimers supported on a porous C 2 N layer (Cu 2 @C 2 N) as the CO 2 RR electrocatalysts. Our results revealed that the Cu dimer can be stably embedded in the porous C 2 N monolayer because of the strong hybridization between Cu 3d orbitals and N 2p orbitals, thus ensuring its high stability. On the basis of the computed free energy changes, we found that Cu 2 @C 2 N exhibits superior performance for the CO 2 RR with a small limiting potential of −0.23 V and the CO 2 → HCOO* → HCOOH* → H 2 COOH* → H 2 CO* → H 2 COH* → CH 2 * → CH 3 * → CH 4 route is the most favorable, among which the hydrogenation of HCOO* to HCOOH* is the…

Abstract The surface oxygen vacancy formation energy (EOvac) is an important parameter in determining the catalytic activity of metal oxides. Estimating these energies can therefore lead to data-driven design of promising catalyst candidates. In the present study, we determine EOvac for various insulating and semiconducting oxides. Statistical investigations indicate that the band gap, bulk formation energy, and electron affinity are factors that strongly influence EOvac. Electrons enter defect states after O desorption, and these states can be in the valence band, mid-gap, or in the conduction band. Subsequent adsorption of O2, NO, CO, CO2, and H2 molecules on an O-deficient surface is also investigated. These molecules become preferentially adsorbed at the defect sites, and EOvac is identified as the dominant factor that determines the adsorption mode as well as a descriptor that shows good correlation with the adsorption energy.

To investigate metal oxide surface catalysis, determining an appropriate Hubbard U -correction term is a challenge for the density functional theory (DFT) community and identifying realistic reaction intermediates and their corresponding X-ray photoelectron spectroscopy (XPS) shifts is a challenge for experimental researchers, when these methods are used independently. In this study, using CuO as a model transition metal oxide, we demonstrate that when DFT and XPS are applied synergistically, the determination of the U value and the identification of adsorbate/intermediate species on the surface (and their XPS shifts) can be done simultaneously. The experimental O 1s spectra of the as-synthesized CuO 2D-nanoleaves shows the presence of four different peaks with core level binding energies (CLBEs) of 529.7, 531.4, 533.2, and 534.6 eV. DFT is used to calculate the CLBE shifts for probable adsorbed moieties, in various adsorption configurations, on both, clean and vacancy defect containing surfaces. Comparison of experimental and theoretical CLBEs across the entire U value range of 0–9…

Presently, great attention is being directed toward the development of promising electrode materials for non-lithium rechargeable batteries which have the advantages of low cost, high energy storage density, and high rate capacity for substantial renewable energy applications. In this study, we have predicted that the C 3 N monolayer is a potential electrode material for Na- and K-ion batteries by first-principle calculations. The diffusion barriers are calculated to be as small as 0.03 eV for Na and 0.07 eV for K, which could lead to a very fast diffusion on the C 3 N monolayer surface, implying high mobility and cycle stability for batteries. The C 3 N monolayer is predicted to allow a high storage capacity of 1072 mAh/g by the inclusion of multilayer adsorption with an average voltage of 0.13 V for Na 2 C 3 N and 0.26 V for K 2 C 3 N systems, which is more promising than previously studied anode materials. All of these results ensure that the C 3 N monolayer could serve as an excellent anode material for Na- and K-ion batteries.

The (electro)chemical reactions between positive electrodes and electrolytes are not well understood. We examined the oxidation of a LiPF 6 -based electrolyte with ethylene carbonate (EC) with layered lithium nickel, manganese, and cobalt oxides (NMC). Density functional theory calculations showed that the driving force for EC dehydrogenation on oxides, yielding surface protic species, increased with greater Ni content in NMC. Ex situ infrared and Raman spectroscopy revealed experimental evidence for EC dehydrogenation on charged NMC surfaces. Protic species on charged NMC surfaces from EC dehydrogenation could further react with LiPF 6 to generate less-coordinated F species such as PF 3 O-like and lithium nickel oxyfluoride species on charged NMC particles and HF and PF 2 O 2 – in the electrolyte. Larger degree of salt decomposition was coupled with increasing EC dehydrogenation on charged NMC with increasing Ni or lithium deintercalation. An oxide-mediated chemical oxidation of electrolytes was proposed, providing new insights in stabilizing high-energy positive electrodes and…

MXenes, as an emerging family of conductive two-dimensional materials, hold promise for late-model electrode materials in Li-ion batteries. A primary challenge hindering the development of MXenes as electrode materials is that a complete understanding of the intrinsic storage mechanism underlying the charge/discharge behavior remains elusive. This article presents two key discoveries: first, the characteristics of the Ti 3 C 2 T x structure can be modified systematically by calcination in various atmospheres, and second, these structural changes greatly affect Li-ion storage behavior, which reveals the mechanism for lithium storage in Ti 3 C 2 T x MXene. Specifically, via ammonization, the interlayer spacing gets dilated and uniform, giving rise to only one redox couple. In stark contrast, there are two well-recognized redox couples corresponding to two interlayer spacings in pristine Ti 3 C 2 T x MXene, in which Li-ion (de)intercalation occurs between interlayers in a sequential manner as evidenced by ex situ X-ray diffraction (XRD). Notably, the XRD diffraction peaks shift hardly…

Upconversion (UC) luminescence materials doped with rare earth ions are extensively investigated as optical temperature probes by the fluorescence intensity ratio technique. However, most Er 3+ -doped materials are still suffering from low sensing sensitivity. In the present study, we attempt to develop high-sensing-sensitivity Er 3+ -doped materials based on the thermally coupled energy levels (TCLs) from Stark sublevels as well as the properties at subzero temperatures, for which there is continuous lack of research. Er 3+ /Yb 3+ codoped Ba 3 Y 4 O 9 (BYO) phosphors were produced via a solid-state reaction. Excited by 980 nm, various output colors, including bright green, yellow, and red, in BYO:Er 3+ /Yb 3+ phosphors as well as the relative emission intensities could be regulated through altering Yb 3+ concentrations. Subsequently, on the basis of all 12 pairs of TCLs especially from Stark sublevels of 2 H 11/2, 4 S 3/2, and 4 F 9/2 of Er 3+ ions, multiple temperature-sensing performances are evaluated over a wide range of 73–573 K. The results show that the maximum sensitivity…

Abstract Double perovskites, comprising two different cations, are potential nontoxic alternatives to lead halide perovskites. Here, we characterized thin films and crystals of Cs2AgBiBr6 by time-resolved microwave conductance (TRMC), which probes formation and decay of mobile charges upon pulsed irradiation. Optical excitation of films results in the formation of charges with a yield times mobility product, φΣμ > 1 cm2/Vs. On excitation of millimeter-sized crystals, the TRMC signals show, apart from a fast decay, a long-lived tail. Interestingly, this tail is dominant when exciting close to the bandgap, implying the presence of mobile charges with microsecond lifetimes. From the temperature and intensity dependence of the TRMC signals, we deduce a shallow trap state density of around 1016/cm3 in the bulk of the crystal. Despite this high concentration, trap-assisted recombination of charges in the bulk appears to be slow, which is promising for photovoltaic applications.

Shale gas has redefined the energy landscape in the world. The scientific community makes efforts to exploit shale gas resources with minimum environmental impact. Shale gas is mainly located in micropores and mesopores of shale rocks. Molecular simulations provide insights into the kerogen structure frameworks and hydrocarbon and nonhydrocarbon adsorption on the kerogen-accessible surface. In this work, a new method is introduced to create kerogen slit nanopores from the molecular scale with different surface roughness. The hybrid molecular dynamics-grand canonical Monte Carlo (MD-GCMC) simulations are used to investigate methane adsorption in a rigid and flexible kerogen matrix and slit nanopores at three different temperatures and various pressures. Molecular dynamics simulations are used to study CH 4 mobility in the flexible kerogen matrix and slit nanopores. Our simulation results show that the kerogen matrix is a dynamic system in which a coupling may exist between gas adsorption and kerogen matrix structure deformation. The chemical composition and flexibility of kerogen…

Improving the cyclic stability of lithium metal anodes is of particular importance for developing high-energy-density batteries. In this work, a remarkable finding shows that the control of lithium bis(fluorosulfonyl)imide (LiFSI) concentrations in electrolytes significantly alters the thickness and modulus of the related SEI layers, leading to varied cycling performances of Li metal anodes. In an electrolyte containing 2 M LiFSI, an SEI layer of ∼70 nm that is obviously thicker than those obtained in other concentrations is observed through in situ atomic force microscopy (AFM). In addition to the decomposition of FSI – anions that generates rigid lithium fluoride (LiF) as an SEI component, the modulus of this thick SEI layer with a high LiF content could be significantly strengthened to 10.7 GPa. Such a huge variation in SEI modulus, much higher than the threshold value of Li dendrite penetration, provides excellent performances of Li metal anodes with Coulombic efficiency higher than 99%. Our approach demonstrates that the FSI – anions with appropriate concentration can…

The process employed to discover new materials for specific applications typically utilizes screening of large compound libraries. In this approach, the performance of a compound is correlated to the properties of elements referred to as descriptors. In the effort described below, we developed a simple and efficient machine learning (ML) model for predicting adsorption energies of CH 4 related species, namely, CH 3, CH 2, CH, C, and H on the Cu-based alloys. The developed ML model predicted the DFT-calculated adsorption energies with 12 descriptors, which are readily available values for the selected elements. The predictive accuracy of four regression methods (ordinary linear regression by least-squares (OLR), random forest regression (RFR), gradient boosting regression (GBR), and extra tree regression (ETR)) with different numbers of descriptors and different test-set/training-set ratios was quantitatively evaluated using statistical cross validations. Among four types of regression methods, we have found that ETR gave the best performance in predicting the adsorption energies…

The power conversion efficiency of perovskite solar cells is drastically affected by photocarrier dynamics at the interfaces. Experimental measurements show quenching of the photoluminescence (PL) signal from the perovskite layer when it is capped with a hole transport medium (HTM). Furthermore, time-resolved PL (TRPL) data show a faster decay of the PL signal in the presence of the perovskite/HTM interface. The experimental decay is usually fitted using one or two exponential functions with an incomplete physical picture. In this work, an extensive model is used to extract the key physical parameters characterizing carrier dynamics in the bulk and at the interfaces. The decay of the TRPL signal is calculated in the presence of both defect-assisted recombination (Shockley Read Hall) and band-to-band radiative recombination where carrier extraction/recombination at the interfaces is described by interface recombination velocities. By proper curve fitting of the modeling results and the measured TRPL signal, meaningful optoelectronic parameters governing photophysical processes in…

Lead-free double perovskites have been proposed as promising nontoxic photovoltaic materials for the replacement of lead perovskites. While the latter ones reach remarkably high power conversion efficiencies (PCEs) above 23% in small lab devices, the lead-free double perovskites so far have severely underperformed, with PCEs below 3% for the prototypical system Cs 2 AgBiBr 6, in spite of considerable optimization efforts by several groups. Here, we present a detailed study of Cs 2 AgBiBr 6 thin films deposited on poly(methyl methacrylate) and mesoporous TiO 2 . Femtosecond UV–vis–NIR transient absorption experiments clearly identify the presence of excitons. In addition, strong electron–phonon coupling via Fröhlich interactions is observed in terms of pronounced coherent oscillation of a strong A 1g optical phonon mode of the double perovskite at 177 cm –1 . Similar behavior is also found for the related vacancy-ordered perovskite Cs 3 Bi 2 Br 9 and the parent compound BiBr 3 . Excitonic effects and electron–phonon coupling are known to induce unwanted electron–hole recombination…

Because of the unique properties of plasma technology, its use in gas conversion applications is gaining significant interest around the globe. Plasma-based CO 2 and CH 4 conversion has become a major research area. Many investigations have already been performed regarding the single-component gases, that is, CO 2 splitting and CH 4 reforming, as well as for two-component mixtures, that is, dry reforming of methane (CO 2 /CH 4 ), partial oxidation of methane (CH 4 /O 2 ), artificial photosynthesis (CO 2 /H 2 O), CO 2 hydrogenation (CO 2 /H 2 ), and even first steps toward the influence of N 2 impurities have been taken, that is, CO 2 /N 2 and CH 4 /N 2 . In this Feature Article we briefly discuss the advances made in literature for these different steps from a plasma chemistry modeling point of view. Subsequently, we present a comprehensive plasma chemistry set, combining the knowledge gathered in this field so far and supported with extensive experimental data. This set can be used for chemical kinetics plasma modeling for all possible combinations of CO 2, CH 4, N 2, O 2, and H 2…

Nonradiative decay of localized surface plasmons results in the production of hot charge carriers and the generation of heat, both of which can affect the efficiency of plasmon-mediated photoelectrochemical processes. Unfortunately, decoupling the impact of each effect on measured photocurrents is extremely challenging because the relative contribution of the two plasmon decay pathways cannot be controlled or easily measured. Here, we present a methodology for exploring the roles of hot carriers and heat generation on plasmon-mediated photoelectrochemical processes using scanning electrochemical microscopy (SECM). Light is used to drive a redox reaction at a plasmonic substrate, while an ultra-microelectrode tip is positioned close to the substrate to read out both the reaction products and the mass transfer rate of the redox species. By controlling the potential at the tip and substrate electrodes, the roles of photoinduced reactions at the substrate and enhanced mass transport to the tip due to local heating can be isolated and investigated independently. We observe enhanced…

Adsorption and dissociation processes of gas molecules on bulk materials and nanomaterials are essential for catalytic conversion of carbon dioxide (CO 2 ). In this work, we systematically investigated the CO 2 adsorption and dissociation on low index surfaces of different transition metals by Density Functional Theory (DFT) calculations. A comparison study demonstrates that the open surfaces (Fe(100), Ni(100), Ni(110), Rh(100), and Ir(100)) have stronger interactions with CO 2 molecules than the close-packed surfaces. The order of energy barriers for CO 2 dissociation is Fe(110), Ir(100) < Ru(0001), Rh(100), Co(0001), Ni(100) < Os(0001), Ni(111) < Ir(111), Rh(111), Ni(110) < Fe(100), Pt(111) < Cu(100), Pd(111) < Cu(111). The interaction order between the dissociative CO*, O* species and the surfaces is Fe(100) > Fe(110) > Ru(0001) > Os(0001) > Ir(100), Rh(100) > Ni(110) > Co(0001) > Rh(111), Ir(111) > Ni(100), Ni(111) > Cu(100) > Pt(111) > Cu(111), Pd(111). In addition, we found that the change trend of adsorption energy is consistent with that of charge transfer amounts from the…

Generation of hydrogen by splitting water with the electrocatalytic approach could become a more sustainable way following the discovery of new materials, such as the 2D transition-metal carbides. Developing eco-friendly, low-cost, stable, and highly active nonprecious hydrogen evolution reaction (HER) catalysts is one of key factors for hydrogen energy economy. Two-dimensional metal carbide and nitride (MXenes) materials have shown characteristics of promising HER catalysts. Herein, we explored the conductive and thermal stability and electrocatalyst performance of four 2D ordered double MXenes M 2 ′ M″C 2, Cr 2 TiC 2, Cr 2 VC 2, Mo 2 TiC 2, and Mo 2 VC 2, and their corresponding oxygen (O*)- or hydroxyl (OH*)-terminated MXenes by using density functional calculations. Results indicated that all the above MXenes are conductive, which are favored to charge transfer during HER. Four MXenes are fully terminated by O* under standard conditions [pH = 0, p (H 2 ) = 1 bar, U = 0 V]. The Gibbs free energy for the adsorption of atomic hydrogen (Δ G H* ) on the O*-terminated M 2 ′ M″C 2…

Ion migration, which can be classified into cation migration and anion migration, is at the heart of redox-based resistive random access memory. However, the coexistence of these two types of ion migration and the resultant conductive filaments (CFs) have not been experimentally demonstrated in a single memory cell. Here we investigate the competition between metallic and vacancy defect CFs in a Ag/CH 3 NH 3 PbI 3 /Pt structure, where Ag and CH 3 NH 3 PbI 3 serve as the top electrode and memory medium, respectively. When the medium layer thickness is hundreds of nanometers, the formation/diffusion of iodine vacancy (V I ) CFs dominates the resistive switching behaviors. The V I -based CFs provide a unique opportunity for the electrical-write and optical-erase operation in a memory cell. The Ag CFs emerge and coexist with V I ones as the medium layer thickness is reduced to ∼90 nm. Our work not only enriches the mechanisms of the resistive switching but also would advance the multifunctionalization of resistive random access memory.

Abstract Despite considerable effort to improve upconversion (UC) in lanthanide-doped nanocrystals (NCs), the maximum reported efficiencies remain below 10%. Recently, we reported on low Er3+- and Yb3+-doped NaYF4 NCs giving insight into fundamental processes involved in quenching for isolated ions. In practice, high dopant concentrations are required and there is a trend toward bright UC in highly doped NCs. Here, additional quenching processes due to energy transfer and migration add to a reduction in UC efficiency. However, a fundamental understanding on how concentration quenching affects the quantum efficiency is lacking. Here, we report a systematic investigation on concentration-dependent decay dynamics for Er3+ or Yb3+ doped at various concentrations (1–100%) in core and core–shell NaYF4 NCs. The qualitative and quantitative analyses of luminescence decay curves and emission spectra show strong concentration quenching for the green-emitting Er3+ 4S3/2 and NIR-emitting 4I11/2 levels, whereas concentration quenching for the red-emitting 4F9/2 level and the IR-emitting 4I13/2…

In order to address an all-inorganic halide lead-free perovskite for potential photovoltaic applications, we carried out first-principles calculations of bandgaps of 260 all-inorganic halide perovskites belonging to the class ABX 3, with A = Li, Na, K, Rb, Cs, B = Pb, Sn, and Ge, and X = F, Cl, Br, I. Three most common crystal symmetries were chosen, including cubic, tetragonal, and two orthorhombic phases. The bandgap exhibited increase with the decreasing of the anions radius (I, Br, Cl, F) and lowering the symmetry of the structures. With consideration of multiple factors forming perovskites, we reported three all-inorganic lead-free halides perovskites including cubic-KSnCl 3, cubic-RbSnCl 3, and trigonal-NaGeBr 3 as candidates with desirable bandgap (1.24–1.44 eV) for photovoltaic applications.

Expanding the performance limit of current Li-ion batteries requires ion–ion and ion–solvent interaction, which governs the ion transport behavior of the electrolytes, to be fully understood as a matter of crucial importance. We herein examine the ionic speciation and conduction behavior of propylene carbonate (PC) electrolytes of 0.1–3.0 M LiPF 6 and LiBF 4 using Raman spectroscopy, dielectric relaxation spectroscopy (DRS), and pulsed-field gradient NMR (PFG-NMR) spectroscopy. In both LiPF 6 –PC and LiBF 4 –PC, free ions and a solvent-shared ion pair (SIP) are dominant species at dilute salt concentrations (<0.8 M), and SIP becomes dominant at intermediate concentrations (0.8–1.5 M). At higher concentrations (1.5–3.0 M), the solvent-shared dimer (SSD) and contact dimer (CD) are dominant in LiPF 6 –PC, whereas the contact ion pair (CIP), CD, and agglomerate (AGG) prevail in LiBF 4 –PC. Ionic conduction in 0.1–1.5 M LiPF 6 –PC and LiBF 4 –PC is governed by the migration of free ions and SIP. Notably, above 1.5 M of the two PC electrolytes, SSD participates in ionic conduction via the…

This work reports facile and well-controlled synthesis of a number of binary, ternary, and quaternary nanocomposites using combinations of metal oxides (CuO, NiO), multiwalled carbon nanotubes (MWCNTs), and conducting polymer polyaniline (PANI) for application as electrode in supercapacitor. X-ray diffraction and electron microscopic analyses confirmed the formation of different composites made of binary CuO@NiO, ternary CuO@PANI/MWCNT, NiO@PANI/MWCNT, and quaternary CuO@NiO/PANI/MWCNT nanocomposites. Such combination of materials has not been reported previously, and with specific capacitance of 1372 F g –1 and good cyclic stability (83% capacity retention after 1500 cycles), the quaternary nanohybrid electrode shows the best performance compared to all other binary and ternary electrodes tested and promises to be a very good electrode material for supercapacitor application. The improved performance of the quaternary nanocomposite is attributed to the well-designed structural advantages and the synergistic effects of the components that lead to significant reduction in the charge…

In the paper, the synthesis of ZnO/TiO 2 nanocomposites with different main parts (TiO 2 or ZnO) is studied. When TiO 2 is the main part of the ZnO/TiO 2 heterojunction photocatalysts (ZnO/TiO 2 ), the photocatalytic activity is decreased rapidly with the increase of the amount of ZnO. The reason may be attributed to the relative p–n junction (p-ZnO/n-TiO 2 ) produced between ZnO and TiO 2 . The migration directions of the electrons and holes in the relative p–n junction are opposite to the transfer directions of the photogenerated electrons and holes in the valence band (VB) and conduction band (CB). However, when ZnO is the primary part of the heterojunction photocatalysts (TiO 2 /ZnO), the photocatalytic activity of the samples increases with the increase of the TiO 2 amount up to 5% (95% ZnO/TiO 2 ). The reason may be that the migration directions of the electrons and holes in the relative p–n junction (p-TiO 2 /n-ZnO) are the same as the transfer directions of the photoexcited electrons and holes in VB and CB between the two semiconductors. It is proposed that the conductivity…

In the aqueous electrochemical reduction of CO2, the choice of electrolyte is responsible for the catalytic activity and selectivity, although there remains a need for more in-depth understanding of electrolyte effects and mechanisms. In this study, using both experimental and simulation approaches, we report how the buffer capacity of the electrolytes affects the kinetics and equilibrium of surface reactant species and the resulting reaction rate of CO2 with varying partial CO2 pressure. Electrolytes investigated include KCl (nonbuffered), KHCO3 (buffered by bicarbonate), and phosphate-buffered electrolytes. Assuming 100% methane production, the simulation successfully explains the experimental trends in maximum CO2 flux in KCl and KHCO3 and also highlights the difference between KHCO3 and phosphate in terms of pKaas well as the impact of the buffer capacity. To examine the electrolyte impact on selectivity, the model is run with a constant total current density. Using this model, several factors are elucidated, including the importance of local pH, which is not in acid/base…

Ever-proposed descriptors of catalytic activity for the oxygen evolution reaction (OER) were systematically investigated. A wide variety of stoichiometric perovskite oxides ABO 3 (A = Ca, Sr, Y, La; B = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) were examined as OER catalysts. The simplest descriptor, e g, electron number of transition-metal ions at the B-site, was not applicable for OER overpotentials (η) of the compounds tested in this study. Another descriptor, oxygen 2p band center relative to Fermi energy (ε 2p ), was not necessarily adequate for the most part of perovskite oxides. Eventually, a recently proposed descriptor, charge-transfer energy (Δ), displayed a linear relationship with η most reasonably. Because Δ values were obtained from theoretical calculations, not only by spectroscopic experiments, systematic exploration for a wide range of compounds including hypothetical ones could be allowed. This finding proposes the charge-transfer energy as the most helpful descriptor for the design of perovskite oxide catalysts for the OER.

Reducing the driving force of exciton dissociation into charge-transfer states is one effective solution to minimize energy loss and thus to improve power conversion efficiencies for organic solar cells. Traditionally, the driving force should be larger than 0.3 eV to achieve efficient exciton dissociation. Recent experiments have shown that excitons can be effectively dissociated, whereas the energy offsets between donor and acceptor are extremely small, but the mechanisms are not understood yet. Here, we use system-optimized long-range corrected functional with solid-state electronic polarization to investigate exciton binding energies of 14 typical nonfullerene small molecule acceptors in organic solar cells. The results point to that the driving forces for dissociation of the acceptor excitons into charge-transfer states are linearly correlated to the exciton binding energies. The smaller the exciton binding energy, the lower driving force required. Moreover, primarily owing to the largest dielectric constants, IDT- or IDTT-based fused-ring acceptors have the smallest exciton…

Large-scale, clean, efficient, and sustainable hydrogen production from water is one of the major goals in solar-to-fuel conversion as the sun and water represent the two most abundant and geographically balanced free resources available on earth. Considering that most of the liquid water available on the earth’s surface is present in the form of seawater, H 2 generation from seawater splitting is highly desirable for large-scale practical and economical application. Herein, we report on the first demonstration of direct efficient overall solar-driven seawater splitting on p- GaN-based nanowire arrays without any external bias or sacrificial agents from various types of simulated seawater solutions. A stable solar-to-hydrogen (STH) conversion efficiency of 1.9% was obtained under concentrated irradiation, demonstrating its possible utilization for the large-scale, environmentally friendly, sustainable generation of clean solar fuel.

We present a simple, environment-friendly, and fast synthesis of nitrogen-doped graphene quantum dots (N-GQDs) on copper foil by chemical vapor deposition using exclusively chitosan, a cheap and nontoxic biopolymer, as a carbon and nitrogen precursor. We characterized the synthesized N-doped graphene quantum dots using Raman spectroscopy, XPS, AFM, HRTEM, and HRSEM and found them to be in the range 10–15 nm in diameter and 2–5 nm-thick with 4.2% of maximum nitrogen content. The proposed growth mechanism process includes three key steps: (1) decomposition of chitosan into nitrogen-containing compounds, (2) adsorption of reactive species (HCN) on the copper surface, and (3) nucleation to form N-doped graphene quantum dots. The synthesized N-GQDs exhibit photoluminescence (PL) emission in the visible band region, thus making them suitable for applications in nano-optoelectronics.

Electrolyte solutions of 1 M concentration are typically used in lithium ion batteries (LIB) for optimal performance. However, recently, superconcentrated electrolytes have been proposed to be a promising alternative to 1 M solutions. Despite their improved stability features, application of the concentrated electrolytes is hindered by their poor transport properties. We probe EC-LiPF 6 electrolyte system for a range of concentrations: 0.06 to 4 M using molecular dynamics simulations to study the effect of concentration on transport and structural properties. Molecular structure of the solution changes with concentration from a predominantly solvent separated ion pair (SSIP) configuration at the dilute limit to an aggregate rich configuration at high concentration. Depletion of SSIPs and formation of more aggregates at higher concentrations affect the transport properties. The present work provides insights into the relation between molecular structure and performance of the electrolyte solution and suggests ways to design novel concentrated electrolytes.

Abstract Dispersion-corrected periodic DFT calculations have been applied to elucidate the Langmuir–Hinshelwood (dissociative) and an Eley–Rideal/Mars–van Krevelen (associative) mechanism for ammonia synthesis over Co3Mo3N surfaces, in the presence of surface defects. Comparison of the two distinct mechanisms clearly suggests that apart from the conventional dissociative mechanism, there is another mechanism that proceeds via hydrazine and diazane intermediates that are formed by Eley–Rideal type chemistry, where hydrogen reacts directly with surface activated nitrogen, in order to form ammonia at considerably milder conditions. This result clearly suggests that via surface defects ammonia synthesis activity can be enhanced at milder conditions on one of the most active catalysts for ammonia synthesis.

The photoluminescence (PL) emission states of heteroatom-doped graphene quantum dots (GQDs) remain unknown, particularly the assignment of the low-energy excitation band (more than 330 nm). To address these issues, this work synthesized three different types of GQDs: undoped GQDs (UGQDs), nitrogen-doped GQDs (NGQDs), and boron-doped GQDs (BGQDs), with similar sizes, chemical compositions (types and compositions of surface functional groups), and defects using a constant potential electrolysis method. The PL emissive states in these GQDs and the effects of the dopant heteroatom on the PL were revealed based on the combination of spectroscopic methods and theoretical calculations. The results indicated that the GQDs exhibit multiemissive centers for the PL emission mechanism. An excitation-independent PL emission band (band I) results from a high-energy transition originating from the quantum confinement of the carbon core (carbon π–π* transitions in sp 2 domain), and an excitation-dependent PL emission band (band II) originates from a low-energy edge band transition, which is…

The slow kinetics of the oxygen evolution reaction (OER) is the main cause of energy loss in many low-temperature energy storage techniques, such as metal–air batteries and water splitting. A better understanding of both the OER mechanism and the degradation mechanism on different transition metal (TM) oxides is critical for the development of the next generation of oxides as OER catalysts. In this paper, we systematically investigated the catalytic mechanism and lifetime of ABO 3−δ perovskite catalysts for the OER, where A = Sr or Ca and B = Fe or Co. During the OER process, the Fe-based AFeO 3−δ oxides with δ ≈ 0.5 demonstrate no activation of lattice oxygen or pH dependence of the OER activity, which is different from the SrCoO 2.5 with similar oxygen 2p-band position relative to the Fermi level. The difference was attributed to the larger changes in the electronic structure during the transition from the oxygen-deficient brownmillerite structure to the fully oxidized perovskite structure and the poor conductivity in Fe-based oxides, which hinders the uptake of oxygen from the…

Solar cells and optoelectronics based on lead halide perovskites are generating considerable interest but face challenges with the use of toxic lead. In this study, we fabricate and characterize lead-free perovskites based on germanium and tin solid solutions, CH 3 NH 3 Sn (1– x ) Ge x I 3 (0 ≤ x ≤ 1). We show that these perovskite compounds possess band gaps from 1.3 to 2.0 eV, which are suitable for a range of optoelectronic applications, from single junction devices and top cells for tandems to light-emitting layers. Their thermodynamic stability and electronic properties are calculated for all compositions and agree well with our experimental measurements. Our findings demonstrate an attractive family of lead-free perovskite semiconductors with a favorable band-gap range for efficient single-junction solar cells.

A new structural model of hydrous alkali aluminosilicate gel (N-A-S-H) frameworks is proposed, in which charge-balancing extra-framework Al species are observed in N-A-S-H gels for the first time. This model describes the key nanostructural features of these gels, which are identified through the application of 17 O, 23 Na, and 27 Al triple quantum magic angle spinning solid-state nuclear magnetic resonance spectroscopy to synthetic 17 O-enriched gels of differing Si/Al ratios. The alkali aluminosilicate gel predominantly comprises Q 4 (4Al), Q 4 (3Al), Q 4 (2Al), and Q 4 (1Al) Si units charge-balanced by Na + ions that are coordinated by either 3 or 4 framework oxygen atoms. A significant proportion of Al 3+ in tetrahedral coordination exist in sites of lower symmetry, where some of the charge-balancing capacity is provided by extra-framework Al species which have not previously been observed in these materials. The mean Si IV –O–Al IV bond angles for each type of Al IV environments are highly consistent, with compositional changes dictating the relative proportions of individual…

Two-dimensional materials can be utilized to detect gas molecules in low concentration due to their high surface-to-volume ratios. In this respect, we investigate in the present work recently fabricated borophene, two-dimensional B, which has buckled and line-defective phases. Both are systematically studied for four gas molecules: NH 3, NO, NO 2, and CO. In each case, the adsorption energy is found to be high and borophene develops distinct wrinkles. Our results provide a thorough understanding of the interaction between borophene and the gas molecules. An excellent performance of borophene as gas sensor is demonstrated by simulating the material’s transport characteristics by means of the nonequilibrium Green’s function method.

Improving device lifetime and stability remains the stumbling block of the commercialization of hybrid perovskite-based devices (HPDs). Although extensive efforts have been paid, thermal property, one of the most crucial parameters in conventional solid-state electronic devices, has rarely been studied for HPDs. Here, we investigate the temperature-dependent ultralow thermal conductivity and ultrahigh thermal expansion of single-crystalline MAPbX 3 (MA = CH 3 NH 3 ), which are found distinct from traditional thin-film solar cells materials. Particularly, for MAPbI 3, thermal conductivity is observed being only 0.3 W·m –1 ·K –1 and linear thermal expansion coefficient along [100] direction is as high as 57.8 × 10 –6 K –1 (tetragonal) and much higher at the structural phase transition point. We attribute the ultralow thermal conductivity and ultrahigh thermal expansion to the weak chemical bonds associated with the soft perovskite materials. These unique properties can be very challenging for the multilayer device design, but their ultralow thermal conductivity may unveil a new…

The nanoporous and tunable nature of metal–organic frameworks (MOFs) has made them promising adsorbents for water adsorption applications such as water harvesting and adsorptive heat pumps. In these applications, water adsorption properties in MOFs play a crucial role. However, understanding their structural defects and how defects influence adsorption thermodynamics remains limited to date. In this work, by employing Monte Carlo techniques and first-principle density functional theory calculations, we investigate the effect of defects on the water adsorption properties in MOF-801 structures at an atomic level. Our calculations show that the adsorption isotherm in perfect MOF-801 (without defects) greatly deviates from that measured experimentally. With the introduction of defects with a high density, a reasonably good agreement can be achieved, suggesting that a high defect density in MOF-801 may be responsible for its hydrophilic adsorptive behaviors. Further, water adsorption properties in MOF-801 structures are found to depend on the spatial configuration of defects, and water…

Bulk Cs 3 Bi 2 I 9 exhibits zero-dimensional (0-D) perovskite crystal structure at the molecular level, providing scopes for novel optical properties compared to three-dimensional perovskite structures. Here, 0-D refers to the crystal structure irrespective of the size of the crystal. We have prepared colloidal Cs 3 Bi 2 I 9 nanocrystals and elucidated the unique optical properties arising from their 0-D crystal structure. Absorption spectrum at 10 K confirms that the electronic band gap of Cs 3 Bi 2 I 9 nanocrystals is at 2.86 eV, along with a sharp excitonic peak at 2.56 eV, resulting in a very high excitonic binding energy, E b X = 300 meV. Interestingly, we observe two peaks in the photoluminescence spectra at room temperature on both sides of the excitonic absorption energy. Because E b X (300 meV) ≫ effective phonon energy (36 meV), the phonon-mediated relaxation of carriers from conduction band minimum to the excitonic state is suppressed to an extent. Consequently, two photoluminescence peaks related to both the bulk band edge and the excitonic transitions are observed.…

Two-dimensional boron is promising as a tunable monolayer metal for nano-optoelectronics. We study the optoelectronic properties of two likely allotropes of two-dimensional boron using first-principles density functional theory and many-body perturbation theory. We find that both systems are anisotropic metals, with strong energy- and thickness-dependent optical transparency and a weak (<1%) absorbance in the visible range. Additionally, using state-of-the-art methods for the description of the electron-phonon and electron-electron interactions, we show that the electrical conductivity is limited by electron-phonon interactions. Our results indicate that both structures are suitable as a transparent electrode.

Silicene has recently shown high electrochemical performance with discharging product Li 2 O(s) and high stability, avoiding discharging byproducts for nonaqueous lithium–oxygen batteries. At the fundamental level, little was known about the effect of defects existing in silicene surface and various solvents on the discharging and charging processes occurring in the batteries. Here, ab initio density functional theory is employed to explore the mechanisms of oxygen reduction to Li 2 O(s) (ORR) on discharge and the reverse reactions on pristine and defective silicenes including single vacancy (SV), double vacancies (DV), and Stone–Thrower–Wales (STW) defects. The influence of the permittivity of solvents on the adsorption energy of the ORR intermediates as well as the stability of the cathode materials in dimethyl sulfoxide (DMSO) and 1,2-dimethoxyethane (DME) is evaluated. The analysis of the calculated results suggests that the defects show higher overpotentials when compared with pristine silicene due to their stronger attraction with the ORR intermediates, especially for LiO 2…

We investigate the Raman spectrum of the low-temperature orthorhombic phase of the organic–inorganic halide perovskite CH 3 NH 3 PbI 3, by combining first-principles calculations with high-resolution low-temperature Raman measurements. We find good agreement between theory and experiment and successfully assign each of the Raman peaks to the underlying vibrational modes. In the low-frequency spectral range (below 60 cm –1 ), we assign the prominent Raman signals at 26, 32, 42, and 49 cm –1 to the Pb–I–Pb bending modes with either A g or B 2g symmetry and the signal at 58 cm –1 to the librational mode of the organic cation. Owing to their significant intensity, we propose that these peaks can serve as clear markers of the vibrations of the [PbI 3 ] − network and of the CH 3 NH 3 + cations in this perovskite, respectively. In particular, the ratios of the intensities of these peaks might be used to monitor possible deviations from the ideal stoichiometry of CH 3 NH 3 PbI 3 .

This paper outlines new design nanocomposites (Li 2 CoMn 3 O 8 /K10) for electrochemical hydrogen storage with an emphasis on the optimal conditions to achieve higher performance. Li 2 CoMn 3 O 8 /K10 nanocomposites were fabricated by loading different ratios of the Li 2 CoMn 3 O 8 (5%, 10%, and 20%) inside the montmorillonite K10. Electrochemical properties of the samples montmorillonite K10, Li 2 CoMn 3 O 8, and the respective nanocomposites were studied by chronopotentiometry charge–discharge techniques in alkaline medium. The Li 2 CoMn 3 O 8 nanostructures were synthesized by a facile combustion method in the presence of various carboxylic acids as fuel and capping agents. The influence of carboxylic acids on the size, morphology, and homogeneity of the samples was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray diffraction (XRD), energy-dispersive X-ray (EDX), and Fourier transform infrared (FT-IR) were applied to investigate the purity and chemical compositions of the samples. The electrochemical hydrogen storage performances of…

To construct efficient solar-driven devices, considerable efforts have been made to search for desirable photocatalysts for water splitting. Motivated by recent successful syntheses of single-layer blue phosphorus (BlueP) and Mg(OH) 2, we systematically investigate the stability, energy band structure, charge transfer, and potential photocatalytic properties of the BlueP/Mg(OH) 2 van der Waals heterostructure using the first-principles method. It is found that all the heterostructures considered possess similar electronic and optical properties and exhibit type-II band alignment and indirect band gap characteristics. In addition, the ground-state configuration (β-stacking) of the heterostructure is found to be a potential photocatalyst for water splitting. In particular, its band gap, band-edge positions, and optical absorption can be tuned by in-layer biaxial strain to improve the efficiency of the photocatalytic water splitting. Our findings provide a new possibility of designing efficient photocatalysts for water splitting.

Pb-free inorganic–organic perovskite materials - C 10 H 12 N 2 MnBr 4 (compound 1 ) and C 5 H 6 NMnBr 3 (compound 2 ) - with different structures were obtained by using pyridine and manganese bromide as raw materials. The single-crystal X-ray diffraction analyses revealed their different lattices in space groups. Compound 1 - Mn tetra-Br-coordinated with two pyridines - presented an isolated mononuclear structure. Its temperature-dependent photoluminescence spectra exhibited an emission band at 523 nm from the lowest d–d crystal field transition of Mn and a higher energy emission at 493 nm from its vibronic state at low temperature. Compound 2 - Mn octahedra-Br-coordinated with a minor pyridine - face-shares with linear chains, which were separated by pyridine molecules. Its emission peak occurred at around 650 nm, in which ferromagnetism was observed. The lifetimes of d–d transition for a single manganese ion and the Mn cluster were presented.

The catalytic properties of bulk-immiscible alloys are less explored than their bulk-miscible counterparts due to inherent difficulties in their synthesis and stabilization. The development of alternative synthetic methods can, however, provide routes toward bulk-immiscible nanoparticles with metastable randomly alloyed structures. In this study, we combine computational screening and a microwave-based synthesis method to target bulk-immiscible alloys that show enhanced reactivity over pure metals for the oxygen reduction reaction (ORR). A number of systems are identified theoretically as promising ORR catalysts. The theoretical predictions are experimentally verified for the RhAu system, for which a specific surface ensemble is identified as being highly active in the ORR.

There appears to be a controversy on whether remnant PbI 2 is beneficial to the performance of perovskite solar cells (PSCs). We have shown that PSCs with residual PbI 2 deposited by one-step antisolvent solution and two-step evaporation-solution method both have shown better performance than those without excess PbI 2 . X-ray diffraction with diverse X-ray incident angles combined with scanning electron microscopy and secondary-ion mass spectrometry is employed to identify the position of remnant PbI 2 . It reveals that residual PbI 2 is located at grain boundaries near the perovskite/hole-transporting layer interface area for the one-step antisolvent solution method, and the two-step evaporation-solution method situates the excess PbI 2 at grain boundaries and the electron transport layer/perovskite interface. The cell performance implies that grain boundary passivation is beneficial for promoting short-circuit current density, while interface passivation is more favorable to enhance open-circuit voltage and fill factor. The revealed passivation process indicates a deep…

Model Pd/SSZ-13 with high dispersion of Pd ions (0.1 and 1 wt % Pd) was synthesized. The material was characterized with Fourier transform infrared (FTIR) and cryo-scanning transmission electron microscopy. Adsorption of NO leads to the formation of Pd(II)–NO and Pd(I)–NO complexes as well as NO + species that replace residual H + (extra-framework) sites. These nitrosyl species have notable thermal stability, with resistance to decomposition under high vacuum at 200 °C. Addition of molecular oxygen to NO-containing stream improves NO x storage of the Pd/H-SSZ-13. In particular, addition of O 2 to NO slightly increases the amount of Pd(II)–NO complex with ν NO at ∼1865 cm –1, whereas the low frequency ν NO band at 1805 cm –1, assigned to Pd(I)–NO, decreases in intensity. Simultaneously, polydentate nitrate species appear in small amounts, contributing to the high temperature NO x release stage during a passive NO x adsorber (PNA) cycle. The concentration of NO + (characterized by the broad infrared band centered at 2170 cm –1 ), in the presence of O 2, increases in intensity…

Zeolite ZSM-5 is one of the most widely used zeolites, in particular in heterogeneous catalysis. This work investigates the incorporation of Al in the silica framework of monoclinic ZSM-5 and the Al speciation during steam treatment for four ZSM-5 samples with different Si/Al ratios, using ultrahigh field 27 Al NMR (22.3 T), 29 Si NMR, X-ray diffraction, and IR spectroscopy. 27 Al MQMAS NMR at 22.3 T allows identification and quantification of 10 distinct tetrahedral framework resonances which are assigned to the crystallographic sites by their average T–O–T angles, determined from powder XRD patterns for the same samples. A clear Al site preference is observed and found to be dependent on the Si/Al ratio of the parent zeolite. The framework sites facing the intersections in ZSM-5 are found to be most prone to dealumination, whereas Al in the straight and sinusoidal-shaped channels are more stable toward steam treatment at high temperatures. Extra-framework Al species with 5- and 6-fold coordination and two distorted tetrahedral AlO 4 sites have also been identified. The…

High-performance energy storage materials are of essential importance in advanced electronics and pulsed power systems, and the polymer dielectrics have been considered as a promising energy storage material, because of its higher dielectric strength and more excellent flexibility compared with that of inorganic ceramic dielectrics. However, the energy storage capability of pristine polymer has been limited by its low intrinsic dielectric permittivity and ordinary ferroelectric performance. Herein, this work demonstrates a favorable method to achieve a sandwich-structured poly(vinylidene fluoride) (PVDF)-based composite by the electrospinning, solution casting, thermal quenching, and hot-pressing process. This innovative method combines with the 0.5Ba(Zr 0.2 Ti 0.8 )O 3 -0.5(Ba 0.7 Ca 0.3 )TiO 3 nanofibers (BZT-BCT NFs), which possesses good ferroelectric hysteresis, and the hybrid particles of hexagonal boron nitride nanosheets (BNNSs) coated by ferroferric oxide (Fe 3 O 4 @BNNSs), which hold high breakdown strength ( E b ). It is worth mentioning that the Fe 3 O 4 particles…

Janus MoSSe monolayers have been recently synthesized by replacing S by Se on one side of MoS 2 (or vice versa for MoSe 2 ). Due to the different electronegativities of S and Se, these structures carry a finite out-of-plane dipole moment. As we show here by means of density functional theory calculations, this intrinsic dipole leads to the formation of built-in electric fields when the monolayers are stacked to form N -layer structures. For sufficiently thin structures ( N < 4), the dipoles add up and shift the vacuum level on the two sides of the film by ∼ N ·0.7 eV. For thicker films, the vacuum level shift saturates at around 2.2 eV due to compensating surface charges, which in turn leads to the formation of atomically thin n- and p-doped electron gasses at the surfaces. The doping concentration can be tuned between 5 × 10 12 and 2 × 10 13 e/cm 2 by varying the film thickness. On the basis of band structure calculations and the Mott–Wannier exciton model, we compute the energies of intra- and interlayer excitons as a function of film thickness, suggesting that the Janus…

Pyrene is an exemplary conjugated organic chromophore with a strong propensity for self-association through an excited-state process known as excimer formation. Pyrene “excimers” and molecular “excimers” more generally are strongly avoided in some applications, such as in light harvesting, yet have found widespread use in others, such as in sensing and structure determination. Despite this disparate view and despite their widespread use, a fundamental understanding of the structure and dynamics of these collective excitations remains outstanding. In this work, we shed key insights into the nature of excimer formation in crystalline pyrene. We developed a flash precipitation procedure incorporating a polymer additive that enabled us to prepare aqueous suspensions of crystalline pyrene nanoparticles. We provide evidence that the molecular-level packing in the nanoparticles is equivalent to the equilibrium packing of the single crystal and show that excimer formation is the primary excited-state decay pathway. We find that excimer formation in the crystalline pyrene nanoparticles…

Plasmonic substrates play a crucial role in the confinement and manipulation of localized electromagnetic fields at the nanoscale. The large electromagnetic field enhancement at metal/dielectric interfaces is widely exploited in surface-enhanced fluorescence (SEF) and surface-enhanced Raman scattering (SERS) spectroscopies. Despite the advantage of near-field enhancement, unfortunately, in metals, the large absorption at optical frequencies induces local heating of the analyte fluid with possible damage of the biological material. In addition, in SEF plasmonic substrates, spacer layers are necessary to minimize undesired fluorescence quenching due to nonradiative decay, which strongly depends on the distance between molecules and metallic substrates. Therefore, the possibility of managing surface electromagnetic states mimicking surface-plasmon resonances in terms of spatial localization, high-field intensity, and dispersion characteristics, while avoiding metallic losses is of great interest. However, dielectric nanoantennas can currently provide limited possibilities in the…

Abstract The interface stability versus Li represents a major challenge in the development of next-generation all-solid-state batteries (ASSB), which take advantage of the inherently safe ceramic electrolytes. Cubic Li7La3Zr2O12 garnets represent the most promising electrolytes for this technology. The high interfacial impedance versus Li is, however, still a bottleneck toward future devices. Herein, we studied the electrochemical performance of Fe3+-stabilized Li7La3Zr2O12 (LLZO:Fe) versus Li metal and found a very high total conductivity of 1.1 mS cm–1 at room temperature but a very high area specific resistance of ∼1 kΩ cm2. After removing the Li metal electrode we observe a black surface coloration at the interface, which clearly indicates interfacial degradation. Raman- and nanosecond laser-induced breakdown spectroscopy reveals, thereafter, the formation of a 130 μm thick tetragonal LLZO interlayer and a significant Li deficiency of about 1–2 formula units toward the interface. This shows that cubic LLZO:Fe is not stable versus Li metal by forming a thick tetragonal LLZO…

Photogenerated charge separation is one of the key factors determining the solar energy conversion efficiency in photocatalysis and photoelectrocatalysis. Fabrication of phase junction has been demonstrated to be an effective strategy to construct the internal electric field for the charge separation. Phase junction is essentially a heterojunction, but more common in semiconductor-based photoelectric conversion systems, because most semiconductors exhibit the polymorphous structures. Because of the similar crystal structure between the two phases, phase junctions are more easily formed. The application of phase junction in photocatalysis and photoelectrocatalysis, especially the anatase–rutile TiO 2 and α–β Ga 2 O 3 phase junction, are summarized in this Feature Article. The internal electrical field across the phase junction provides enough driving force for the improved charge separation, evidenced by the time and spatial resolved characterizations. We conclude with a summary and perspectives on the design and application of phase junction in solar energy conversion systems.

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl 3 and FAPbBr 3, compared to those of MAPbCl 3 and previously reported MAPbBr 3, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX 3 system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results.

Three N-substituted 2-aminopyridine derivatives, namely, 6-(2,4-dihydroxyphenyl)-4-phenyl-2-(phenylamino)nicotinonitrile (DPPN), 6-(2,4-dihydroxyphenyl)-2-((4-hydroxyphenyl)amino)-4-phenylnicotinonitrile (DHPN), and 6-(2,4-dihydroxyphenyl)-2-((4-methoxyphenyl)amino)-4-phenylnicotinonitrile (DMPN) were investigated for their inhibitive effects on mild steel corrosion in 1 M HCl solution using electrochemical, surface, chemical, and theoretical studies. Results showed that the protection capabilities of inhibitors used in the study increase with increase in their concentrations and attained the maximum numerical values of 95.81, 96.24, and 96.63% for DPPN, DHPN, and DMPN, respectively, at the 20.20 × 10 –5 mol L –1 concentration. The results of the electrochemical impedance spectroscopy studies revealed that DPPN, DHPN, and DMPN molecules retard corrosion by adsorbing at the metal/electrolyte interface. Adsorption of the DPPN, DHPN, and DMPN molecules on the surface was found to obey the Langmuir adsorption isotherm model. Polarization measurement indicated that DPPN, DHPN, and DMPN…

Three donor–acceptor (D–A)-containing cruciform luminophores, named DHCS-BC, DMCS-BC, and DTCS-BC, with twisted molecular conformation have been synthesized. The D–A-type cross-conjugated compounds showed unique intramolecular charge transfer (ICT) properties. Interestingly, they showed substituent-dependent aggregation-induced emission (AIE) and mechanofluorochromic (MFC) behavior. The three cruciforms possessed evident AIE nature with the AIE factors of 9, 19, and >492 and high solid-state fluorescence quantum yields of 0.332, 0.425, and 0.492, respectively. Moreover, their AIE activities increased in the order DHCS-BC < DMCS-BC < DTCS-BC, which was consistent with the torsional degree of molecular spatial conformations induced by the steric hindrance between carbazole and substituted cyanostyryl and the ICT degrees of the three compounds. The solid emission studies illustrated that their fluorescence color could change reversibly between bright blue-green (485 nm), bright blue-green (479 nm), bright yellow (526 nm) and light yellow-green (524 nm), yellow-green (534 nm),…

Fourier transform infrared (FTIR) spectroscopy is broadly applied nowadays for probing the concentration and strength of acid sites in zeolite catalysts. Accuracy of direct determination of the quantity of different hydroxyl groups by FTIR method suffers from uncertainty of the integrated molar absorption coefficients, ε. The values of ε reported by different authors might differ by an order of magnitude. This paper provides an approach for reliable determination of the integrated molar absorption coefficients by combining 1 H magic-angle spinning (MAS) NMR and FTIR spectroscopy techniques. The concentration of Brønsted acid sites for the series of H-ZSM-5 and H-ZSM-23 zeolite samples with different Si/Al ratio has been reliably established with 1 H MAS NMR using methane and benzene as internal standards adsorbed on the studied samples. The data on the obtained concentration were further used to analyze same zeolite samples with FTIR spectroscopy and derive the information on the values of the integrated molar absorption coefficients. The coefficients ε have been reliably determined…

Laser-inducd fragmentation is a promising tool for controlling the particle size of ligand-free colloidal nanoparticles and to synthesize ligand-free gold nanoclusters. However, because the underlying mechanisms are not fully understood, increasing the yield of this process remains challenging. In this work, we examine the pulsed laser fragmentation of gold nanoparticles in liquid under statistical single-pulse conditions with high-fluence nanosecond pulses and correlate them with the educt particle size, number of pulses, and laser fluence. We conclusively prove that the fragmentation process of gold nanoparticles is a one-pulse and one-step event, which yields monomodal particles of ≪10 nm down to 2.8 +/- 0.1 nm when exceeding a pulse peak power of 1.6 × 10 12 W/m 2 and when all educt particles are larger than 13.4 nm. This size threshold for quantitative fragmentation fits well with the size limit of 13.1 nm calculated with respect to the evaporation–heat–energy balance. Furthermore, we found strong evidence that the number of irradiation cycles, varied within the regime of one…

A new type of two-dimensional layered semiconductor with weak electrostatic but not van der Waals interlayer interactions, Bi 2 O 2 Se, has been recently synthesized, which shows excellent air stability and ultrahigh carrier mobility. Herein, we combined theoretical and experimental approaches to study the Raman spectra of Bi 2 O 2 Se and related bismuth oxychalcogenides (Bi 2 O 2 Te and Bi 2 O 2 S). The experimental peaks lie at 160 cm –1 in Bi 2 O 2 Se and at 147 and 340 cm –1 in Bi 2 O 2 Te. They were fully consistent with the calculated results (159.89, 147.48, and 340.33 cm –1 ) and assigned to the out-of-plane A 1g, A 1g, and B 1g modes, respectively. Bi 2 O 2 S was predicted to have more Raman-active modes due to its lower symmetry. The shift in the predicted frequencies of Raman-active modes was also found to get softened as the interlayer interaction decreases from bulk to monolayer Bi 2 O 2 Se and Bi 2 O 2 Te. To reveal the strain effects on the Raman shifts, a universal theoretical equation was established based on the symmetry of Bi 2 O 2 Se and Bi 2 O 2 Te. It was…

We prepared Cu/a-ZrO 2 (a-ZrO 2: amorphous ZrO 2 ), Cu/m-ZrO 2 (m-ZrO 2: monoclinic ZrO 2 ), Cu/a-ZrO 2 /KIT-6, and Cu/t-ZrO 2 /KIT-6 (t-ZrO 2: tetragonal ZrO 2 ) by a simple impregnation method and examined the effect of the ZrO 2 phase on CO 2 -to-methanol hydrogenation. We discovered a-ZrO 2 -containing catalysts with high activity and selectivity in CO 2 -to-methanol hydrogenation. Next, we focused on Cu species formation on the above-described catalysts. While pure CuO was observed on Cu/m-ZrO 2 and Cu/t-ZrO 2 /KIT-6, copper-zirconium mixed oxide (Cu x Zr y O z ), not pure CuO, was formed on Cu/a-ZrO 2 and Cu/a-ZrO 2 /KIT-6, as evidenced by X-ray absorption spectroscopy (XAS) and the powder color. After reducing a-ZrO 2 -containing catalysts with H 2 at 300 °C, we observed highly dispersed Cu nanoparticles in close contact with a-ZrO 2 (or Cu x Zr y O z ). In addition, methanol vapor sorption revealed that methanol adsorbed more weakly on a-ZrO 2 than on m-ZrO 2 . Therefore, the high dispersion of Cu species and weak adsorption of methanol led to high activity and selectivity…

This work aims to explore inorganic boron as an electrochemical catalyst for nitrogen reduction reaction (NRR). The density functional theory approach has been employed to evaluate the free-energy profile for NRR on two stable boron monolayer types (α and β 12 ). Our results revealed that the rate-determining step for nitrogen reduction to ammonia over two-dimensional boron structures is the first reduction step. Hexagonal triangular boron atoms in the α-sheet exhibit the best catalytic performance in all possible positions on the boron monolayer sheet surface, with a maximum energy input of 0.77 eV vs stand hydrogen electrode.

Pt/CeO 2 single-atom catalysts have recently attracted increasing interest due to excellent thermal stability, high atom efficiency, and high activity in catalysis. In this study, by means of density functional theory (DFT) calculations, we systematically compare the stability and CO oxidation reactivity of Pt single atoms supported on CeO 2 (111) (Pt/CeO 2 ) and Ga-doped CeO 2 (111) (Pt/Ga–CeO 2 ). It was found that the formation of an oxygen vacancy (O V ) is very facile near a surface Ga-doping site (Pt/Ga–CeO 2 –O V ). Significantly, the stability of Pt single atoms anchored on the Ga site was enhanced compared with those on the bare ceria surface. In addition, our DFT results suggest a CO oxidation mechanism on Pt/Ga–CeO 2 –O V that differs from that on Pt/CeO 2 . In particular, the O V site plays an important role in activating the oxygen molecule, which then reacts with CO preadsorbed on Pt. The calculated energy barrier on Pt/Ga–CeO 2 –O V is about 0.43 eV lower than that on the undoped catalyst, suggesting an enhanced reactivity for CO oxidation. Experiments on CO oxidation…

Abstract It has become a significant challenge to select the best metal–organic frameworks (MOFs) for membrane-based gas separations because the number of synthesized MOFs is growing exceptionally fast. In this work, we used high-throughput computational screening to identify the top MOF membranes for flue gas separation. Grand canonical Monte Carlo and molecular dynamics simulations were performed to assess adsorption and diffusion properties of CO2 and N2 in 3806 different MOFs. Using these data, selectivities and permeabilities of MOF membranes were predicted and compared with those of conventional membranes, polymers, and zeolites. The best performing MOF membranes offering CO2/N2 selectivity > 350 and CO2 permeability > 106 Barrer were identified. Ternary CO2/N2/H2O mixture simulations were then performed for the top MOFs to unlock their potential under industrial operating conditions, and results showed that the presence of water decreases CO2/N2 selectivity and CO2 permeability of some MOF membranes. As a result of this stepwise screening procedure, the number of promising…

In a systematic investigation of scandium hydrides with high hydrogen content we predict seven phases of scandium hydrides (ScH 4, ScH 6, ScH 7, ScH 8, ScH 9, ScH 10, and ScH 12 ), which are stable above 150 GPa. Zero point energies are essential in determining the phases and pressure ranges within which they are stable. The interconversion of the various hydrides is intriguing; in one case there is a “return” to a lower hydrogen content hydride with increasing pressure. We argue that these hydrides may be synthesized by compressing mixtures of ScH 3 and H 2 above 150 GPa. New H bonding motifs are uncovered, including “H 5 ” pentagons or “H 8 ” octagons in ScH 9, ScH 10, and ScH 12 . High T c s are predicted for ScH 6, ScH 7, ScH 9, ScH 10, and ScH 12, with superconducting transition temperatures ( T c s) of 120–169 K above 250 GPa, as estimated by the Allen–Dynes modified McMillan equation.

A series of Bodipy dimers with orthogonal conformation were prepared. The photophysical properties were studied with steady-state and time-resolved transient spectroscopies. We found the triplet-state quantum yield is highly dependent on the solvent polarity in the orthogonally linked symmetric Bodipy dimers, and the intersystem crossing (ISC) is efficient in solvents with moderate polarity. The photoinduced symmetry-breaking charge transfer (SBCT) in polar solvents was confirmed by femtosecond transient absorption spectroscopy, with the charge separation (CS) kinetics on the order of a few picoseconds and the charge recombination (CR) process occurring on the nanosecond time scale in dichloromethane. These observations are supported by the calculation of the charge separated state (CSS) energy levels, which are high in nonpolar solvents, and lower in polar solvents, thus the CR-induced ISC has the largest driven force in solvents with moderate polarity. These results clarify the mechanism of SOCT-ISC in the orthogonally symmetric Bodipy dimers. The acquired information, relating…

Defects play a pivotal role in the device performance of a photocatalytic, light-emitting, or photovoltaic system. Herein, graphitic carbon nitride (g-C 3 N 4 ) nanosheets are prepared at different calcination temperatures, and the evolution of defects in the system is studied by positron annihilation spectroscopy (PAS) and photoluminescence (PL) spectroscopy. Steady-state PL spectra show that free and defect-bound excitonic emission peaked at 2.78, 2.58, and 2.38 eV are dominant with above-band-gap excitation. Time-resolved PL studies reveal a significant enhancement of excitonic lifetime from 17.4 ns for free exciton to 27.4 ns in case of defect-bound exciton. We provide a direct correlation between the defects observed by PAS and those of the excitonic lifetime found from PL studies. Below-band-gap excitation activates defect emission, and it is characterized by a short carrier lifetime (∼0.14 ns). An excitation power-dependent PL study with 405 nm laser shows a progressive red shift and narrowing of the emission line. We have interpreted the different PL features with defect…

Both connate water and the injected water through hydraulic fracturing can coexist with methane inside shale nanopores where two-phase flow possibly occurs. Few studies have been pertaining to two-phase flow of water and methane in shale reservoirs at nanoscale. In this work, molecular dynamics simulations are employed to investigate two-phase flow of water and methane in slit-shaped silica nanopores with hydrophilic surfaces. A sandwich structure of water film–methane–water film or a structure of the methane gas bubble wrapped in water bridge exists in the nanopore because water wets the surfaces. Darcy’s law breaks down for methane single-phase flow in the nanopore because of the slippage near the surfaces. For two-phase flow of water and methane within the nanopore, water flow pattern varies in the form of water film, water bridge and water pillar at different applied accelerations (different pressure gradients), showing the breakdown of Darcy’s law for water flow. This is attributed to the inhomogeneous number density distribution near the surfaces, which arises from the…

Abstract Dark-colored shiny flakes of graphitic carbon nitride materials produced by reacting dicyandiamide C2N4H4 in a KBr/LiBr molten salt medium were determined to have a C/N ratio near 1.2:1. The compounds also contained 2.3–2.5 wt % H incorporated within N–H species identified by Fourier transform infrared spectroscopy. One recent study revealed analogous results for thin films produced by an similar synthesis method, while a previous investigation instead reported formation of crystalline gC3N4 flakes with a triazine-based graphitic carbon nitride (TGCN) structure. The structures of the materials produced here were studied using a combination of high resolution transmission electron microscopy, X-ray diffraction, IR and Raman and X-ray photoelectron spectroscopy, along with series of density functional theory (DFT) calculations carried out for a range of model layered structures. The results indicate the graphitic layered gCxNy materials contain a mixture of sp2-hybridized C–N and C–C bonded structures, with TGCN to graphene-like domains existing within the layers.…

In this study, electrodeposition of cobalt oxide (Co 3 O 4 ) from spent lithium-ion batteries is successfully enhanced by the magnetic field effect. In the presence of magnetic field, well-defined hierarchical Co 3 O 4 nanostructures with higher electroactive surface area are formed during the electrodeposition process. Electrochemical analysis shows that the enhanced Co 3 O 4 nanostructures exhibit excellent charge storage capabilities of 1273 F g –1 at 1 A g –1, approximately 4 times higher than the electrodeposited Co 3 O 4 that is formed without magnetic field effect. It also reveals the high cycling stability of enhanced Co 3 O 4 nanostructures, with 96% capacitance retention at 5000 charge discharge cycles. The results manifest the enhancement of Co 3 O 4 recovery from spent lithium-ion batteries, which can be the potential electrode material for supercapacitor application.

The 2–3 nm size-selected glutathione-capped Ag–In–S (AIS) and core/shell AIS/ZnS quantum dots (QDs) were produced by precipitation/redissolution from an aqueous colloidal ensemble. The QDs reveal broadband photoluminescence (PL) with a quantum yield of up to 60% for the most populated fraction of the core/shell AIS/ZnS QDs. The PL band shape can be described by a self-trapped exciton model implying the PL band being a sequence of phonon replica of a zero-phonon line resulting from strong electron–phonon interaction and a partial conversion of the electron excitation energy into lattice vibrations. It can be concluded that the position and shape of the PL bands of AIS QDs originate not from energy factors (depth and distribution of trap states) but rather from the dynamics of the electron–phonon interaction and the vibrational relaxation in the QDs. The rate of vibrational relaxation of the electron excitation energy in AIS QDs is found to be size-dependent, increasing almost twice from the largest to the smallest QDs.

All-inorganic cesium lead halide perovskite quantum dots (QDs) are attractive potential materials for high-performance optoelectronics because of their high photoluminescence quantum yield (PLQY), narrow emission widths, and tunable optical band gap. Hot injection is considered as a common method and is widely used for the preparation of the QDs. However, it suffers from the problems of time consumption and high cost for the mixed-halide CsPb(XY) 3 (XY is a combination of Cl and Br or Br and I) QDs because of its cumbersome preparation of precursors with different halide proportions. Here, the mixed-halide CsPb(XY) 3 QDs were synthesized by a simple and efficient way of mixing the single-halide CsPbX 3 (CsPbX 3; X = Cl, Br, and I) QDs stock solutions at room temperature. By modulating the ratio of stock solutions precisely, the cubic crystal structure of mixed-halide CsPb(XY) 3 QDs are obtained undergoing anion-exchange and lattice reconstruction processes. The room-temperature construction of QDs showed excellent properties of bright PL with the emission peaks tunable over the…

In the present work, phase transition mechanisms from semiconducting 2H phase to metallic 1T phase in MoS 2 nanosheets were studied using density functional theory (DFT) method. Various 2H → 1T phase transition mechanisms that consist of nucleation and propagation steps, which were simulated by collective rotational and rotational/translational movements, single atom translational movement, and the gliding movement of one row for sulfur (S) atoms, on both the basal plane and Mo- and S-edges with different S coverages were investigated. On the perfect basal plane, the 1T phase nucleation is unlikely due to the extremely high barrier of 2.25 eV/atom, whereas the presence of defective S vacancies on the basal plane dramatically facilitate the 1T phase nucleation and propagation around the defective sites by the collective rotational movement of three S atoms. On the 2H phase basal plane with two S vacancies, the kinetic barriers for the 1T phase nucleation are as low as of 0.66–0.77 eV/atom. Like the promoting effect of S vacancies on the phase transition over the basal plane, DFT…

Flexible dielectric materials with high electrical energy densities are of crucial importance in advanced electronics and electric power systems. The conventional methods for fabricating flexible dielectric materials with high electrical energy densities are introducing zero-, one-, and three-dimensional high-k inorganic nanofiller into a dielectric polymer matrix while less two-dimensional high-k nanofillers were included. Herein, two-dimensional (2D) high-k titanium dioxide nanosheets prepared by a one-step hydrothermal reaction were utilized to boost the energy storage performance of dielectric polymer nanocomposites. It was found that compared with the polymer matrix the nanocomposites not only exhibit an enhanced dielectric constant but also show suppressed dielectric loss, which is desirable for energy storage applications. The nanocomposite with 5 wt % 2D nanosheets exhibits a superhigh discharged energy density of 13.0 J/cm 3 at 570 MV/m, which is nearly four times greater than that of commercialized biaxially oriented polypropylene (BOPP) (3.6 J/cm 3 at 600 MV/m). In…

The pivotal role of CO 2 played in global temperature cycles has motivated the ongoing research on carbon capture and storage (CCS). Within this context, Li–CO 2 battery, alike the configuration of Li–O 2 battery, has been proposed as a novel energy storage device with the potential to reduce CO 2 . Nevertheless, the highly negative potential required for the electrochemical reduction of CO 2 adds difficulty to the achievement of energy efficient Li–CO 2 battery. Facilitating the electron transfer to this inert molecule, which largely dictates the discharge voltage and rate capability of a Li–CO 2 battery, is therefore necessary. Herein, three types of quinones have been surveyed, aiming to mediate the reduction of CO 2, which is expected to result in lower overpotential than with a direct electron transfer. We demonstrate by cyclic voltammetry that, in the presence of quinones, CO 2 reduction proceeds through an intermolecular interaction between CO 2 and quinone dianion. Importantly, such catalytic CO 2 reduction reaction is associated with the molecular structure of quinones, the…

The highest efficiency in perovskite solar cells is currently achieved with mixed-cation hybrid perovskites. The ratio in which the cations are present in the perovskite structure has an important effect on the optical properties and the stability of these materials. At present, the formamidinium cation is an integral part of many of the highest efficiency perovskite systems. In this work, we introduce a nuclear magnetic resonance (NMR) spectroscopy protocol for the identification and differentiation of mixed perovskite phases and of a secondary phase due to formamidinium degradation. The influence of the cooling rate used in a precipitation method on the FA/MA ratio in formamidinium–methylammonium lead iodide perovskites (FA x MA 1– x PbI 3 ) was investigated and compared to the FA/MA ratio in thin films. In order to obtain the FA/MA ratio from fast and accessible liquid-state 1 H NMR spectra, the influence of the acidity of the solution on the line width of the resonances was elucidated. The ratio of the organic cations incorporated into the perovskite structure could be reliably…