The 100 most-cited articles in The Journal of Physical Chemistry C, 2017

Electrochemical impedance spectroscopy (EIS) consists of plotting so-called Nyquist plots representing negative of the imaginary versus the real parts of the complex impedance of individual electrodes or electrochemical cells. To date, interpretations of Nyquist plots have been based on physical intuition and/or on the use of equivalent RC circuits. However, the resulting interpretations are not unique and have often been inconsistent in the literature. This study aims to provide unequivocal physical interpretations of electrochemical impedance spectroscopy (EIS) results for electric double layer capacitor (EDLC) electrodes and devices. To do so, a physicochemical transport model was used for numerically reproducing Nyquist plots accounting for (i) electric double layer (EDL) formation at the electrode/electrolyte interface, (ii) charge transport in the electrode, and (iii) ion electrodiffusion in binary and symmetric electrolytes. Typical Nyquist plots of EDLC electrodes were reproduced numerically for different electrode conductivity and thickness, electrolyte domain thickness, as…

In the near future, the targets for lithium-ion batteries concerning specific energy and cost can advantageously be met by introducing layered LiNi x Co y Mn z O 2 (NCM) cathode materials with a high Ni content ( x ≥ 0.6). Increasing the Ni content allows for the utilization of more lithium at a given cell voltage, thereby improving the specific capacity but at the expense of cycle life. Here, the capacity-fading mechanisms of both typical low-Ni NCM ( x = 0.33, NCM111) and high-Ni NCM ( x = 0.8, NCM811) cathodes are investigated and compared from crystallographic and microstructural viewpoints. In situ X-ray diffraction reveals that the unit cells undergo different volumetric changes of around 1.2 and 5.1% for NCM111 and NCM811, respectively, when cycled between 3.0 and 4.3 V vs Li/Li + . Volume changes for NCM811 are largest for x (Li) < 0.5 because of the severe decrease in interlayer lattice parameter c from 14.467(1) to 14.030(1) Å. In agreement, in situ light microscopy reveals that delithiation leads to different volume contractions of the secondary particles of (3.3 ± 2.4)…

Raman spectra of graphene oxides (GOs) with different chemical compositions and synthesized by oxidation of distinct starting materials were analyzed to relate the spectral features to structural properties. The chemical compositions of different graphene oxides were determined by X-ray photoelectron spectroscopy (XPS), and nanoplatelets were characterized by zeta potential (ζ) and dynamic light scattering (DLS) measurements. The results indicated that the chemical composition, size, and superficial charge of the nanoplatelets depend on the starting material. We found five reported bands (D, D′, G, D″, and D*) in the first-order Raman spectrum and three bands (2D, D + D′, and 2D′) in the second-order Raman spectrum that successfully interpret the Raman spectra between 1000 and 3500 cm –1 . Analysis of the bands allowed linear correlations to be found between the maximum positions of the 2D and D + D′ bands and between the relative intensities of the D and G bands ( I D / I G ) and the Csp 2 percentage. Moreover, our results demonstrate that the relative intensities of the D′ and D…

Cerium oxide is an important material for catalytic and fuel cell applications. We present an ab initio density functional theory (DFT) study of the vibrational properties of ceria focusing on the interpretation of Raman spectra of polycrystalline powder samples, with vibrational bands in the frequency region between 250 and 1200 cm –1 . The model systems include the oxidized CeO 2 as well as the reduced CeO 2– x and Ce 2 O 3 bulk materials together with the CeO 2 (111) and oxygen defective CeO 2– x (111) surfaces. The experimentally observed band at 250 cm –1 is assigned to a surface mode of the clean CeO 2 (111) surface, in agreement with our Raman spectra of ceria (CeO 2 ) powders with varying crystal size (Filtschew, A.; Hofmann, K.; Hess, C., J. Phys. Chem. C 2016, 120, 6694). The reduced model systems display signature vibrational bands in the 480–600 cm –1 region associated with the presence of oxygen defects and reduced Ce 3+ ions. In the high-frequency region between 800 and 900 cm –1, characteristic peroxide (O 2 2– ) stretching vibrations at the oxidized and defective…

Noble metal (gold or silver) nanoparticles or patterned films are typically used as substrates for surface-enhanced Raman spectroscopy (SERS). Two-dimensional (2D) carbides and nitrides (MXenes) exhibit unique electronic and optical properties, including metallic conductivity and plasmon resonance in the visible or near-infrared range, making them promising candidates for a wide variety of applications. Herein, we show that 2D titanium carbide, Ti 3 C 2 T x, enhances Raman signal from organic dyes on a substrate and in solution. As a proof of concept, MXene SERS substrates were manufactured by spray-coating and used to detect several common dyes, with calculated enhancement factors reaching ∼10 6 . Titanium carbide MXene demonstrates SERS effect in aqueous colloidal solutions, suggesting the potential for biomedical or environmental applications, where MXene can selectively enhance positively charged molecules.

Abstract Two major strategies are currently pursued to improve the energy density of lithium-ion batteries using LiNixCoyMnzO2 (NCM) cathode materials. One is to increase the fraction of redox active Ni (≥80%), which allows larger amounts of Li to be extracted at a given cutoff voltage (Umax). The other is to increase Umax, in particular for medium-Ni content NCM materials. However, the accompanying lattice changes ultimately lead to capacity fading in both cases. Here the structural changes occurring in Li1.02NixCoyMnzO2 (with x = 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) during cycling operation in the voltage range between 3.0 and 4.6 V vs Li are quantified by means of operando X-ray diffraction combined with detailed Rietveld analysis. All samples show a large decrease in unit cell volume upon charging, ranging from 2.4% for NCM111 (33% Ni) to 8.0% for NCM851005 (85% Ni). To make a fair comparison of the structural stability of the different NCM materials, energy densities as a function of Umax are estimated and correlated with X-ray diffraction results. It is shown that NCMs with a…

Abstract The anode solid electrolyte interface (SEI) on the anode of lithium ion batteries contains lithium carbonate (Li2CO3), lithium methyl carbonate (LMC), and lithium ethylene dicarbonate (LEDC). The development of a strong physical understanding of the properties of the SEI requires a strong understanding of the evolution of the SEI composition over extended timeframes. The thermal stability of Li2CO3, LMC, and LEDC in the presence of LiPF6 in dimethyl carbonate (DMC), a common salt and solvent, respectively, in lithium ion battery electrolytes, has been investigated to afford a better understanding of the evolution of the SEI. The residual solids from the reaction mixtures have been characterized by a combination of X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy with attenuated total reflectance (IR-ATR), while the solution and evolved gases have been investigated by nuclear magnetic resonance (NMR) spectroscopy and gas chromatography with mass selective detection (GC-MS). The thermal decomposition of Li2CO3 and LiPF6 in DMC yields CO2, LiF, and F2PO2Li. The…

Ni-rich LiNi x Co y Mn z O 2 (NCM) cathode materials have great potential for application in next-generation lithium-ion batteries owing to their high specific capacity. However, they are subjected to severe structural changes upon (de)lithiation, which adversely affects the cycling stability. Herein, we investigate changes in crystal and electronic structure of NCM811 (80% Ni) at high states of charge by a combination of operando X-ray diffraction (XRD), operando hard X-ray absorption spectroscopy (hXAS), ex situ soft X-ray absorption spectroscopy (sXAS), and density functional theory (DFT) calculations and correlate the results with data from galvanostatic cycling in coin cells. XRD reveals a large decrease in unit cell volume from 101.38(1) to 94.26(2) Å 3 due to collapse of the interlayer spacing when x (Li) < 0.5 (decrease in c -axis from 14.469(1) Å at x (Li) = 0.6 to 13.732(2) Å at x (Li) = 0.25). hXAS shows that the shrinkage of the transition metal–oxygen layer mainly originates from nickel oxidation. sXAS, together with DFT-based Bader charge analysis, indicates that the…

Copper electrodes, prepared by reduction of oxidized metallic copper, have been reported to exhibit higher activity for the electrochemical reduction of CO 2 and better selectivity toward C 2 and C 3 (C 2+ ) products than metallic copper that has not been preoxidized. We report here an investigation of the effects of four different preparations of oxide-derived electrocatalysts on their activity and selectivity for CO 2 reduction, with particular attention given to the selectivity to C 2+ products. All catalysts were tested for CO 2 reduction in 0.1 M KHCO 3 and 0.1 M CsHCO 3 at applied voltages in the range from −0.7 to −1.0 V vs RHE. The best performing oxide-derived catalysts show up to ∼70% selectivity to C 2+ products and only ∼3% selectivity to C 1 products at −1.0 V vs RHE when CsHCO 3 is used as the electrolyte. In contrast, the selectivity to C 2+ products decreases to ∼56% for the same catalysts tested in KHCO 3 . By studying all catalysts under identical conditions, the key factors affecting product selectivity could be discerned. These efforts reveal that the surface…

The ability of copper to catalyze the electrochemical reduction of CO 2 has been shown to greatly depend on its nanoscale surface morphology. While previous studies found evidence of irreversible changes of copper nanoparticle and thin film electrodes following electrolysis, we present here the first observation of the reversible reconstruction of electrocatalytic copper surfaces induced by the adsorbed CO intermediate. Using attenuated total internal reflection infrared and surface-enhanced Raman spectroscopies, the reversible formation of nanoscale metal clusters on the electrode is revealed by the appearance of a new C≡O absorption band characteristic of CO bound to undercoordinated copper atoms and by the strong enhancement of the surface-enhanced Raman effect. Our study shows that the morphology of the catalytic copper surface is not static but dynamically adapts with changing reaction conditions.

The stability of CH 3 NH 3 PbI 3 was investigated by observing the degradation in a coevaporated film irradiated by a blue laser in ultrahigh vacuum. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were employed to investigate the effects of irradiation on the surface. The core levels of CH 3 NH 3 PbI 3 were observed to shift toward a higher binding energy (BE) during the irradiation, suggesting that the surface became more n-type. A new metallic Pb component in the XPS spectrum appeared after 120 min of irradiation, indicating that the film had started to decompose. The decomposition saturated after about 480 min of irradiation when the ratio of metallic Pb to total Pb was about 33%. Furthermore, the film was no longer continuous after irradiation, as the elements gold and oxygen from the substrate were detected by XPS. SEM images also show a roughened surface after irradiation. The results strongly indicate that CH 3 NH 3 PbI 3 is sensitive to the laser irradiation and that the light induced decomposition is a self-limiting process.

Band gap tunable hybrid organic–inorganic lead halide perovskites (APbX 3, A = CH 3 NH 3 + and NH 2 CH═NH 2 +, and X = Cl, Br, or I) have attracted significant attention in optoelectronic- and photovoltaic-related fields on account of their outstanding optoelectronic properties. Single crystals of hybrid perovskites, such as CH 3 NH 3 PbI 3 and CH 3 NH 3 PbBr 3, were certified to be advantageous over thin films as photodetectors. However, their resistance toward heat and moisture hinders their future development. Fully inorganic perovskite CsPbBr 3 stands a chance to fill the gap as a novel photodetector with perovskite structure. We revealed the growth of CsPbBr 3 single crystal was of a 2D nucleation mechanism. Similarities of d values and octahedra arrangements along [101] and [020] orientations restricted single-crystal growth. Under optimized conditions, orthorhombic CsPbBr 3 single crystals with (101) crystallographic facets were grown by using methyl alcohol as antisolvent from saturated DMSO solution. The optoelectronic properties of the single crystal were explored through…

Photoluminescence (PL) represents a sensitive tool for probing molecular adsorption and surface reactions in photocatalytic materials. Titanium dioxide (TiO 2 ) is one of the most widely used photocatalysis, and clarifying its basic PL mechanism can give important information. However, differently from other electronic and surface processes, the actual PL mechanisms of TiO 2 are not extensively studied. In this work, we address the topic by focusing our investigation on which are the different states that trigger the PL activity and on identifying the specific recombination pathways acting in the two stable TiO 2 polymorphs (rutile and anatase). On the basis of our experimental results on PL emission, PL excitation, and oxygen-induced and photoinduced PL modifications, we sketch an interpretative scheme for both the polymorphs. Excitation-resolved PL and recombination quenching caused by molecular oxygen evidence distinct contributions to anatase PL, originating from different kinds of hole-trapping and electron-trapping defects that we ascribe to surface and subsurface oxygen…

Ni-rich lithiated layered oxides composed of Ni, Co, and Mn (NCMs) have shown tremendous promise as cathode materials in lithium-ion batteries (LIB) for electro-mobility applications. The capacity of these materials increases with nickel content, but there is a concomitant decrease in stability and stable operating voltage during cycling. Hence, it is of great importance to probe ways to increase the nickel content without sacrificing other important aspects. In this study, we performed a detailed comparative theoretical study of Ni-rich NCMs to advance our understanding of the cycling and thermal stability. On the basis of extensive analysis of density of states, magnetic structure, bond covalency, molecular orbital diagrams, Bader atomic charges, and oxygen binding energies, we draw several crucial conclusions: as the NCM materials become increasingly rich in Ni, (1) the amount of high-valence Ni-ions increases (i.e., N 3+, Ni 4+ ), (2) Ni 4+ ions are readily reduced due to a low-lying LUMO, and hence can easily react with electrolyte species, (3) Ni 4+ -O bonds become…

Electrode potential–pH (Pourbaix) diagrams provide a phase map of the most stable compounds of a metal, its corrosion products, and associated ions in solution. The utility of these phase diagrams is that they enable the assessment of electrochemical stabilities, for example, of Ni metal and its derived oxides, hydroxides, and oxyhydroxides, against corrosion in aqueous environments. Remarkably, the Ni Pourbaix diagrams reported over the last 50 years are largely inconsistent with various electrochemical observations, which may be attributed to inaccurate experimental free energies of formation (Δ f G ) for the complex Ni-based compounds used in producing the available diagrams. Here we show that state-of-the-art density-functional theory (DFT) can be used to obtain accurate Δ f G values, which lead to Ni Pourbaix diagrams that are more consistent with direct electrochemical experiments: Electrochemical impedance spectroscopy and surface-enhanced Raman spectroscopy are used to characterize the electrochemical stabilities of NiO and Ni(OH) 2 formed on Ni, demonstrating the…

Polymer-based solid electrolytes containing ceramic nanoparticles are attractive alternatives to liquid electrolytes for high-energy density Li batteries. In this study, three different types of fillers have been dispersed in poly(ethylene) oxide (PEO) polymer matrices, and the effects on the resulting ionic conductivity of the nanocomposites have been examined. In this respect, the efficacy of one active, liquid-feed flame spray pyrolysis synthesized amorphous Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP), and two passive filler materials, TiO 2 and fumed silica nanoparticles, are compared. Nanocomposite electrolytes are prepared with up to 20 wt % particle loadings. PEO/LiClO 4 with 10 wt % LATP nanoparticles exhibits an ionic conductivity of 1.70 × 10 –4 S·cm –1 at 20 °C, the highest among the surveyed systems, despite exhibiting comparable or higher degrees of crystallinity and glass transition temperatures than the systems containing passive fillers. The ionic conductivity of the composites with LATP nanoparticles exceed that of the polymer matrix by 1 to 2 orders of magnitude. We…

Abstract Band structure calculations based on density functional theory (DFT) with local or gradient-corrected exchange-correlation potentials are known to severely underestimate the band gap of semiconducting and insulating materials. Alternative approaches have been proposed: from semiempirical setups, such as the so-called DFT+U, to hybrid density functionals using a fraction of nonlocal Fock exchange, to modifications of semilocal density functionals. However, the resulting methods appear to be material dependent and lack theoretical rigor. The rigorous many-body perturbation theory based on GW methods provides accurate results but at a very high computational cost. Hereby, we show that a linear correlation between the electronic band gaps obtained from standard DFT and GW approaches exists for most materials and argue that (1) this is a strong indication that the problem of predicting band gaps from standard DFT calculation arises from the assignment of a physical meaning to the Kohn–Sham energy levels rather than from intrinsic errors of the DFT methods and (2) it provides a…

Abstract We report here the plasma-enhanced chemical vapor deposition and electrocatalytic characterization of pure NiOx and NiOx(OH)y. Whereas NiOx is deposited if oxygen is used as a reactive gas, the use of air as a reactive gas leads to the deposition of the NiOx(OH)y, which is electrochemically more active than the NiOx. By recording X-ray photoelectron spectra from the as-deposited catalysts and after their electrochemical investigations, we determined that the electrochemical activity correlates with the amount of hydroxide sites on the surface. Such a behavior was already observed for CoOx and CoOx(OH)y. As a consequence, CoNiOx(OH)y was deposited using air as a reactive gas to study the influence of nickel on the electronic structure of CoOx(OH)y and its effect on the electrochemical activity.

MXenes represent a burgeoning family of two-dimensional (2D) functional materials with a variety of applications that highly rely on termination-mediated surface functionalization, but the understanding of termination is limited. Here, we take Ti 3 C 2 T 2 ( T = O, F, OH, and H) as an example of MXenes, to demonstrate how termination stabilizes the Ti 3 C 2 monolayer matrix by saturating the nonbonding valence electrons of the surface Ti atom by the low-energy orbitals of the termination. This is achieved by orbitally resolved density of states analysis by simply yet efficiently manipulating the internal coordination of the octahedral crystal field to match exactly the Cartesian coordination. Highly degenerate 3d orbitals of surface Ti split in such a way that it exhibits pseudogaps whose widths predict a stability order Ti 3 C 2 O 2 > Ti 3 C 2 F 2 > Ti 3 C 2 (OH) 2 > Ti 3 C 2 H 2 > Ti 3 C 2, consistent with Bader charge analysis, thermodynamic calculations, and experimental results. This new criterion could have implications in the general context of ubiquitous termination…

Silicon has long been regarded as a prospective anode material for lithium-ion batteries. However, its huge volumetric changes during cycling are a major obstacle to its commercialization, as these changes result in irreversible cracking and disconnection of the active mass from the current collector, as well as an excessive formation of a highly resistive solid electrolyte interphase. Multiple mechanical stress relief strategies that primarily use silicon nanostructurization have been previously developed. However, despite the significant improvements on the active material cycle life, using nanomaterials still results in complications, such as low conductivity, reduced volumetric energy density, and increased side reactions. This work provides a historical context for the development of silicon anodes and focuses on the surface chemistry and structural integrity of the electrode, thereby highlighting the most effective strategies reported recently for their optimization.

The nitrogenated porous two-dimensional (2D) material C 2 N has been successfully synthesized using a simple wet-chemical reaction, which provides a high-performance way to produce such 2D materials with novel electronic and optical properties. In this work, density functional theory (DFT) calculations were performed to investigate the structural, electronic, and optical properties of the layered C 2 N/MoS 2 van der Waals (vdW) heterojunction. The C 2 N/MoS 2 heterojunction was found to have a direct band gap of 1.30 eV and to present the typical type-II heterojunction feature, facilitating the effective separation of photogenerated electrons and holes. The calculated band alignment and enhanced optical absorption suggest that the C 2 N/MoS 2 heterojunction should exhibit good light-harvesting properties. The vertical strain can effectively tune the electronic properties and optical absorption of the C 2 N/MoS 2 heterojunction by changing the interaction between the p z orbital of C 2 N and the d z 2 orbital of MoS 2 . The moderate band gap, well-separated photogenerated electrons…

Efforts to tune the bulk physical properties of concrete are hindered by a lack of knowledge related to the atomic-level structure and growth of calcium silicate hydrate phases, which form about 50–60% by volume of cement paste. Here we describe the first synthesis of compositionally uniform calcium silicate hydrate phases with Ca:Si ratios tunable between 1.0 and 2.0. The calcium silicate hydrate synthesized here does not contain a secondary Ca(OH) 2 phase, even in samples with Ca:Si ratios above 1.6, which is unprecedented for synthetic calcium silicate hydrate systems. We then solve the atomic-level three-dimensional structure of these materials using dynamic nuclear polarization enhanced 1 H and 29 Si nuclear magnetic resonance experiments in combination with atomistic simulations and density functional theory chemical shift calculations. We discover that bridging interlayer calcium ions are the defining structural characteristic of single-phase cementitious calcium silicate hydrate, inducing the strong hydrogen bonding that is responsible for stabilizing the structure at high…

Abstract We have synthesized a ternary rGO/Fe3O4/PANI (rGFP) composite for binder-free, semiflexible, thin, all-solid-state supercapacitor device fabrication. A scalable soft-template technique has been adopted for the preparation of three-dimensional Fe3O4-decorated reduced graphene oxide (rGO)-doped polyaniline (PANI) nanorods that are unambiguously investigated under electron microscopes (FESEM and HRTEM). The presence of PANI in the nanocomposite is overwhelmingly supported by the absorption and vibration studies. Raman spectra convincingly show the presence of rGO in the nanocomposites, and the formation of Fe3O4 nanoparticles is confirmed by XRD and XPS results. The specific capacitance value that has been achieved for synthesized ternary rGFP nanocomposite is ∼283.4 F/g at 1.0 A/g current density and exhibited a maximum energy density of 47.7 W h/kg at a power density of 550 W/kg. Interestingly, after 5000 cycles the composite shows excellent life stability that is a 78% retention of the electrochemical property. To demonstrate the portable energy storage applicability, a…

We use density functional theory calculations to explore different polymorphs of a new class of 2D materials commonly known as MXenes, which are primarily carbides and nitrides of transition metals. The stability of the M 2 X, M 3 X 2, and M 4 X 3 polymorphs in their bare and functionalized forms is assessed via the calculated standard heat of formation. We find that most of the MXenes are metallic, and we investigate their performance as electrocatalysts for the hydrogen evolution reaction (HER) using the free energy of hydrogen adsorption at equilibrium coverage as an activity descriptor. For a given type of metal, we find that the hydrogen adsorption energy can vary by up to 0.5 eV depending on the number of metal layers in the structure, suggesting that the catalytic activity of MXenes can be tuned by controlling the layer thickness. On the basis of a combined stability and activity analysis of 72 different MXenes, we identify several new promising nonprecious HER electrocatalysts.

In the past decade, noninvasive luminescent thermometry has become popular due to the limitations of traditional contact thermometers to operate at scales below 100 μm, as required by current demands in disparate areas. Generally, the calibration procedure requires an independent measurement of the temperature to convert the thermometric parameter (usually an intensity ratio) to temperature. A new calibration procedure is necessary whenever the thermometer operates in a different medium. However, recording multiple calibrations is a time-consuming task, and not always possible to perform, e.g., in living cells and in electronic devices. Typically, a unique calibration relation is assumed to be valid, independent of the medium, which is a bottleneck of the secondary luminescent thermometers developed up to now. Here we report a straightforward method to predict the temperature calibration curve of any upconverting thermometer based on two thermally coupled electronic levels independently of the medium, demonstrating that these systems are intrinsically primary thermometers. SrF…

The identification and quantification of defects are undoubtedly thorough challenges in the characterization of “defect-engineered” metal–organic frameworks (MOFs). UiO-66, known for its exceptional stability and defect tolerance, has been a popular target for defect-engineering studies. Herein, we show that synthesizing UiO-66 in the presence of an excess of benzoic acid is a reliable method for obtaining UiO-66 samples with a very high concentration of missing-cluster defects, allowing one to modulate specific properties (i.e., surface area and hydrophobicity). This was elucidated by a multitechnique marriage of experimental and computational methods: a combination of PXRD, dissolution/ 1 H NMR spectroscopy, and N 2 sorption measurements was used to quantify the defect loading, while vibrational spectroscopies (FTIR and Raman) allowed us to unequivocally identify the defect structure by comparison with DFT-simulated spectra and visual analysis of the computed vibrational modes.

The development of sodium and potassium batteries offers a promising way to meet the scaling and cost challenges of energy storage. However, compared to Li +, several intrinsic properties of Na + and K +, including their solvation and dynamics in typical organic electrolytes utilized in battery applications, are less well-understood. Here, we report a systematic investigation of Na + and K + in ethylene carbonate (EC) using first-principles molecular dynamics simulations. Our simulations reveal significant differences in the solvation structure and dynamical properties of Na + and K + compared to Li + . We find that, in contrast to Li + which exhibits a well-defined first solvation shell, the larger Na + and K + ions show more disordered and flexible solvation structures. These differences in solvation were found to significantly influence the ion dynamics, leading to larger diffusion coefficients of Na + and K + compared to Li + . Our simulations also reveal a clear and interesting analog in the behavior of the ions in EC and aqueous environments, particularly in the specific ion…

We study the electrochemical interface between rutile IrO 2 (110) and water to investigate how the inclusion of an explicit solvent influences the stabilities of adsorbed intermediates in the oxygen evolution reaction. Solvent is modeled by explicit nondissociated water molecules, and their structure is determined by a global optimization method. We find that the inclusion of an explicit solvent can significantly affect the geometry of adsorbed intermediates, changing from an interaction with the surface to an interaction with the water bilayer. These water structures consist of stacked octagonal sheets in an ordered network. Solvent stabilization is pronounced for adsorbed *OH and *OOH, which are capable of donating hydrogen bonds. We find little to no change in adsorbate binding energy as the number of layers of solvent is increased from 1 to 3, suggesting a single water bilayer is sufficient to describe the system. With either *O or *OH coadsorbates, the energetics of the reaction pathway are relatively unchanged with the inclusion of explicit solvent.

Electrospinning is a straightforward and versatile method to fabricate ultrafine fibers with unique physical and chemical properties. However, the chaotic nature of traditional electrospinning limits its applications in devices which usually need arranged or patterned micro/nanoscale fibrous structures. In order to improve the controllable deposition of electrospun fibers, near-field electrospinning (NFES) has been proposed and developed in recent years. With characteristics of position-controlled deposition, NFES significantly expands the range of fiber-fabrication uses including electronic components, energy harvesting, flexible sensors, and tissue engineering. In this paper, the basic principle and research advances of NFES have been briefly reviewed. In particular, we summarize the process parameters, polymer materials, as-spun fibrous structures, modified apparatus, and potential applications of NFES. Finally, future prospects on the development tendency and challenges of NFES are discussed.

We study the electronic structure and valence states in LiNi x Mn y Co z O 2 (NMC) materials and compare the resulting electronic, structural, mechanical, and thermal properties of a class of NMC compositions. The Jahn–Teller distortion in the transition metal (TM) octahedral complex allows us to determine the ionic states of the TM elements. The variation of Ni 2+ /Ni 3+ and Co 2+ /Co 3+ as the NMC composition changes alters the structural stability, electrical conductivity, lattice parameters, elastic modulus, and thermal stability. The theoretical predictions are in excellent agreement with the experimental results. Through intensive computational screening, we further show that long-range atomic ordering is absent in the NMC lattice due to the mixture of the ionic states and similar ionic radii of the TM elements. The first-principles modeling provides a theoretical foundation on a complete understanding of the physicochemical properties of NMC at the level of electronic structures.

Pd/zeolite passive NO x adsorber (PNA) materials were prepared with solution ion-exchange between NH 4 /zeolites (Beta, ZSM-5, and SSZ-13) and PdCl 2 solutions. The nature of Pd (dispersion, distribution, and oxidation states) in these materials was characterized with Na + ion exchange, TEM imaging, CO titration with FTIR, and in situ XPS. The NO x trapping and release properties were tested using feeds with different compositions. It is concluded that multiple Pd species coexist in these materials: atomically dispersed Pd in the cationic sites of zeolites and PdO 2 and PdO particles on the external surfaces. While Pd is largely atomically dispersed in ZSM-5, the small pore opening for SSZ-13 inhibits Pd diffusion such that the majority of Pd stays as external surface PdO 2 clusters. NO x trapping and release are not simple chemisorption and desorption events but involve rather complex chemical reactions. In the absence of CO in the feed, cationic Pd(II) sites with oxygen ligands and PdO 2 clusters are reduced by NO to Pd(I) and PdO clusters. These reduced sites are the primary NO…

Abstract The rapid development of nanomaterials with unique size-tunable properties forms the basis for a variety of new applications, including temperature sensing. Luminescent nanoparticles (NPs) have demonstrated potential as sensitive nanothermometers, especially in biological systems. Their small size offers the possibility of mapping temperature profiles with high spatial resolution. The temperature range is however limited, which prevents use in high-temperature applications such as, for example, nanoelectronics, thermal barrier coatings, and chemical reactors. In this work, we extend the temperature range for nanothermometry beyond 900 K using silica-coated NaYF4 nanoparticles doped with the lanthanide ions Yb3+ and Er3+. Monodisperse ∼20 nm NaYF4:Yb,Er nanocrystals were coated with a ∼10 nm silica shell. Upon excitation with infrared radiation, bright green upconversion (UC) emission is observed. From the intensity ratio between 2H11/2 and 4S3/2 UC emission lines at 520 and 550 nm, respectively, the temperature can be determined up to at least 900 K with an accuracy of 1–5…

Solid state luminescence enhancement (SLE) of conjugated organic materials has had a great impact in materials science, but a deep understanding has been rather limited to date. Here, we investigate a prototype example of SLE materials, cyano-substituted distyrylbenzene (DCS), by varying systematically and subtly the substitution pattern ( inter alia of the position of the cyano-substituent) to give largely different photoresponse in fluid and solid solution as well in the crystalline state. The combination of quantitative (ultra)fast optical spectroscopic techniques, appropriate quantum-chemical methods, and structural (X-ray) data allows us to elucidate and rationalize all details of the SLE process, including steric versus electronic factors, radiative versus nonradiative decay channels, and intra- versus intermolecular contributions, providing a first holistic picture of SLE.

Bimetallic electrocatalysts can improve the activity and selectivity over their monometallic counterparts by tuning the structure, morphology, and composition. However, there scarcely was a systematic model to understand the structural effect relationship on CO 2 electrochemical reduction reaction, especially for a product tuning process by introduction of a second metal to grow into outer layers. Herein, we report a structure-controlled model of the growth process of Ag@Cu bimetallic nanoparticles that are fabricated by a polyol method, that is, reducing mixtures of Ag + and Cu 2+ (excess amount) in ethylene glycol (reducing agent) in the presence of polyvinylpyrrolidone. Structural characterizations reveal that a series of Ag@Cu NPs are tuned from the Ag core, Cu modified Ag, to the Cu outer shell by controlling the heating time (0–25 min). Moreover, highly selective catalysts with the tuning reduction products from carbon monoxide to hydrocarbons can be realized. Different from the “dilution” effects between Ag and Cu, the volcanic curve for carbon monoxide production is detected…

The surprising ferroelectricity displayed by hafnia thin films has been attributed to a metastable polar orthorhombic ( Pca 2 1 ) phase. Nevertheless, the conditions under which this (or another competing) ferroelectric phase may be stabilized remain unresolved. It has been hypothesized that a variety of factors, including strain, grain size, electric field, impurities and dopants, may contribute to the observed ferroelectricity. Here, we use first-principles computations to examine the influence of mechanical and electrical boundary conditions (i.e., strain and electric field) on the relative stability of a variety of relevant nonpolar and polar phases of hafnia. We find that although strain or electric field, independently, do not lead to a ferroelectric phase, the combined influence of in-plane equibiaxial deformation and electric field results in the emergence of the polar Pca 2 1 structure as the equilibrium phase. The results provide insights for better controlling the ferroelectric characteristics of hafnia thin films by adjusting the growth conditions and electrical history.

To the league of rapidly expanding 2D materials, borophene is a recent addition. Herein, a combination of ab initio density functional theory (DFT) and nonequilibrium Green’s function (NEGF) based methods is used to estimate the prospects of this promising elemental 2D material for gas sensing applications. We note that the binding of target gas molecules such as CO, NO, NO 2, NH 3, and CO 2 is quite strong on the borophene surface. Interestingly, our computed binding energies are far stronger than several other reported 2D materials like graphene, MoS 2, and phosphorene. Further rationalization of stronger binding is made with the help of charge transfer analysis. The sensitivity of the borophene for these gases is also interpreted in terms of computing the vibrational spectra of the adsorbed gases on top of borophene, which show dramatic shift from their gas phase reference values. The metallic nature of borophene enables us to devise a setup considering the same substrate as electrodes. From the computation of the transmission function of system (gas + borophene), appreciable…

Recent success in Mn 2+ ion doping in cesium lead halide (CsPbX 3 ) nanocrystals opened the door to exploring new optical, magnetic and charge carrier transport properties mediated via exciton–dopant exchange coupling in this new family of semiconductor nanocrystals. Here, we studied the dynamics of energy transfer from exciton to Mn 2+ ions in Mn-doped CsPbCl 3 nanocrystals to gain an insight into the relative strength of exciton–Mn exchange coupling compared to more extensively studied Mn-doped II–VI quantum dots. The comparison of exciton–Mn energy transfer times in CsPbCl 3 nanocrystals and CdS/ZnS core/shell quantum dots suggests that exciton–Mn exchange coupling in CsPbX 3 is not far behind that of CdS/ZnS despite the lack of quantum confinement. With further progress in the synthesis of Mn-doped CsPbX 3 nanocrystals, such as imposing quantum confinement and expanding the range of host chemical composition, one could fully benefit from many properties of CsPbX 3 superior to those of other semiconductor nanocrystals for hosting magnetic dopants.

The potential of a Ti 2 N monolayer and its Ti 2 NT 2 derivatives (T = O, F, and OH) as anode materials for lithium-ion and beyond-lithium-ion batteries has been investigated by the first-principles calculations. The bare and terminated monolayers are metallic compounds with high electronic conductivity. The diffusion barriers on bare Ti 2 N monolayer are predicted to be 21.5 meV for Li +, 14.0 meV for Na +, 7.0 meV for K +, 75.9 meV for Mg 2+, and 38.0 meV for Ca 2+, which are the lowest values reported for state-of-the-art two-dimensional energy storage materials. The functional groups on Ti 2 NT 2 increase the diffusion barriers by about 1 order of magnitude. The calculated capacities for the monovalent cations on Ti 2 N and Ti 2 NT 2 are close to that of the conventional graphite anode in lithium-ion batteries. In comparison, the capacities for Mg 2+ on Ti 2 N and Ti 2 NT 2 are more than 2000 mAh g –1 due to the two-electron reaction and multilayer adsorption of Mg 2+ . Comparison of the electrochemical performances of Ti 2 N and Ti 2 C suggests that Ti 2 N is a more promising…

Exciton many-body interaction is the fundamental light–matter interaction that determines the optical response of the new class of colloidal perovskite nanocrystals of the general formula CsPbX 3 [X = Cl, Br, or I]. However, the understanding of exciton many-body interactions manifested through the transient biexcitonic Stark effect at the early time scales and the Auger recombination process in this new class of materials still remains rather incomplete. In this Article, we studied the many-body exciton interactions under controlled conditions through ultrafast transient absorption spectroscopy. A large biexcitonic redshift ∼30 meV to the effect of hot excitations on the excitonic resonance is observed at the early time scales. From the fluence-dependent studies, it is evident that the samples have only single and biexciton lifetimes, suggesting that the band edges are 2-fold degenerate. This explicit experimental evidence for the exciton many-body interactions in CsPbBr 3 nanocrystals provides a powerful tool to explore the development of their prospective applications in…

Cesium lead iodide (CsPbI 3 ) has recently emerged as a promising solar photovoltaic absorber. However, the cubic perovskite (α-phase) remains stable only at high temperature and reverts to a photoinactive nonperovskite (δ-phase) CsPbI 3 at room temperature. In this work, the formation energies and transition energy levels of intrinsic point defects in γ- (more stable than α-phase) and δ-phases have been studied systematically by first-principles calculations. It is found that CsPbI 3 exhibits a unipolar self-doping behavior (p-type conductivity), which is in contrast to CH 3 NH 3 PbI 3 . Most of the intrinsic defects induce deeper transition energy levels in δ-phase than in γ-phase. This is due to the small Pb–I–Pb bond angles in δ-phase that results in the weak antibonding character of valence band maximum (VBM). However, the strong antibonding character of VBM plays a critical role in keeping defect tolerance in semiconductors. Therefore, these results indicate the importance of the large metal–halide–metal bond angle for the performance of perovskite solar cells.

Abstract Nitrogen-doped carbon (N/C) and graphene (N/G) were synthesized by the established conventional heat-treatment method, and the incorporation of nitrogen into the carbon matrix was confirmed by CHN analysis, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Electrochemical impedance spectroscopy (EIS) of the prepared catalysts in argon-saturated 0.1 M KOH was performed in a three-electrode rotating disk electrode (RDE) configuration. The capacitance derived from the low-frequency region of the EIS patterns was used to estimate the effective density of states [D(EF)] of carbon and its nitrogen-doped counterparts. Moreover, the carrier concentrations (ND) and flat band potentials of the samples were obtained by Mott–Schottky analysis. The metal-free catalyst samples were tested for possible oxygen reduction reaction (ORR) activity in oxygen-saturated 0.1 M KOH electrolyte, and the origin of the activity improvement with nitrogen doping of carbon/graphene can be explained on the basis of the effective density of states [D(EF)], carrier concentration (ND), and flat…

In this work, highly efficient Cu x –Mn composite catalysts (0 ≤ x ≤ 0.20) were synthesized by an improved hydrothermal–citrate complex method and tested in the catalytic total oxidation of CO and the removal of NO by CO. The influence of Cu on manganese oxide materials was characterized by several techniques, including FESEM, HRTEM, XRD, BET analysis, H 2 TPR, O 2 TPD, XPS, and DRIFTS. Possible reaction mechanisms for the NO + CO model reaction and CO oxidation were also tentatively proposed. The Cu-modified manganese oxide materials showed higher catalytic activity in CO oxidation and the selective catalytic reduction (SCR) of NO with CO than pure MnO x materials. The improved catalytic activity in CO oxidation observed for the copper–manganese oxide catalyst was associated with a greater amount of adsorbed oxygen species and high lattice oxygen mobility due to the formation of a Cu 1.5 Mn 1.5 O 4 spinel active phase (Cu x 2+ –Mn x 3+ –[O ( y – z ) ⊙ z ] species). Furthermore, in terms of the CO-SCR model reaction, the surface-dispersed Cu x + –O 2– –Mn y + active species could be…

Cesium lead halide pervoskite semiconductors are being extensively studied due to unprecedentedly high luminescence efficiency and concomitant narrow emission line width. Here, we report photophysical properties of CsPbBr 3 quantum dots having different sizes. Notably 5.5 nm sized CsPbBr 3 quantum dots reveal 95% photoluminescence (PL) quantum yield at room temperature. Moreover, signature of the stimulated emission is observed at low temperature for excitation fluence as low as ∼4.16 μW (Xe lamp excitation). Even though CsPbBr 3 quantum dots reveal the red shift in band gap at low temperature, similar to the single crystal, the exciton–phonon interaction is profoundly affected by the quantum size effects. Temperature-dependent optical studies reveal an anomalous decrease in exciton-LO phonon coupling in small-sized quantum dots aside from expected higher exciton binding energy. Observed stimulated emission in low-sized CsPbBr 3 quantum dots has implications in realizing a quantum dot based laser.

A simple and low-cost electrochemical deposition method is used to prepare reduced graphene oxide/polypyrrole/Cu 2 O–Cu(OH) 2 (RGO/PPy/Cu 2 O–Cu(OH) 2 ) ternary nanocomposites as the electrode material for supercapacitor application. First, graphene oxide–polypyrrole (GO/PPy) nanocomposite is electrochemically synthesized on Ni foam by electro-oxidation of pyrrole monomer in an aqueous solution containing GO and Tiron. Subsequently, the GO/PPy film is converted to the corresponding reduced form (RGO/PPy) by an effective and eco-friendly electrochemical reduction method. Then, a thin layer of Cu 2 O–Cu(OH) 2 is formed on RGO/PPy film by chronoamperometry. The RGO/PPy/Cu 2 O–Cu(OH) 2 nanocomposite is characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), atomic force microscopy (AFM), X-ray diffraction (XRD), and Fourier transform infrared spectra (FT-IR). SEM images show that Cu 2 O–Cu(OH) 2 nanoparticles are dispersed on the surface of RGO/PPy film with an average particle size of 50–70 nm. The electrochemical performance of the as-prepared…

Solution-processed organo-lead halide perovskite solar cells with deep pinholes in the perovskite layer lead to shunt-current leakage in devices. Herein, we report a facile method for improving the performance of perovskite solar cells by inserting a solution-processed polymer layer between the perovskite layer and the hole-transporting layer. The photovoltaic conversion efficiency of the perovskite solar cell increased to 18.1% and the stability decreased by only about 5% during 20 days of exposure in moisture ambient conditions through the incorporation of a poly(methyl methacrylate) (PMMA) polymer layer. The improved photovoltaic performance of devices with a PMMA layer is attributed to the reduction of carrier recombination loss from pinholes, boundaries, and surface states of perovskite layer. The significant gain generated by this simple procedure supports the use of this strategy in further applications of thin-film optoelectronic devices.

The mechanism responsible for the extremely long photoluminescence (PL) lifetimes observed in many lead halide perovskites is still under debate. While the presence of trap states is widely accepted, the process of electron detrapping back to the emissive state has been mostly ignored, especially from deep traps as these are typically associated with nonradiative recombination. Here, we study the photophysics of methylammonium lead bromide perovskite nanocrystals (PNCs) with a photoluminescence quantum yield close to unity. We show that the lifetime of the spontaneous radiative recombination in PNCs is as short as 2 ns, which is expected considering the direct bandgap character of perovskites. All longer (up to microseconds) PL decay components result from the rapid reversible processes of multiple trapping and detrapping of carriers with a slow release of the excitation energy through the spontaneous emission channel. As our modeling shows, the trap (dark) and excitonic states are coupled by the trapping–detrapping processes so that they follow the same population decay kinetics,…

Abstract Harvesting solar energy for artificial photosynthesis is an emerging area in alternative energy research. In the present article, we have investigated the photocatalytic properties of single-layer group IV–VI monochalcogenides, MXs (M = Ge, Si, Sn and X = S, Se) based on first-principles electronic structure calculations. Our dispersion corrected DFT calculations show that these materials have moderate cohesive energies (<120 meV/atom), which are indicative of favorable isolation of MX monolayers by mechanical, sonicated, or liquid-phase exfoliation. The calculated band gaps using hybrid density functional method (HSE06) reveal that all of the MXs show larger band gaps than the minimum energy required for the water splitting reaction (1.23 eV). Considering band edge alignments, all the MXs other than SiS have an acceptable alignment of conduction band minima but not the valence band maxima. We have evaluated the overpotentials for both oxygen and hydrogen evolution reactions. Interestingly, considering contribution from overpotentials, we have tuned the band alignments by…

Development of effective nonprecious metal and nitrogen codoped carbon catalysts for the oxygen reduction reaction (ORR) requires a fundamental understanding of the mechanisms underlying their catalytic activity. In this study, we employed the first-principles density functional theory calculations to predict some key parameters (such as activation energy for O–O bond breaking and free-energy evolution as a function of electrode potential) of ORR on three FeN 4 -type active sites with different local carbon structures. We find that the FeN 4 site surrounded by eight carbon atoms and at the edge of micropores has the lowest activation energy (about 0.20 eV) for O–O bond breaking among the three FeN 4 -type active sites for promoting a direct four-electron ORR. Consequently, our computational results suggest that introduction of micropores in the nonprecious metal catalysts could enhance their catalytic activity for ORR through facilitating the formation of FeN 4 –C 8 active sites with high specific activity.

Abstract Manganese oxides with rich redox chemistry have been widely used in (electro)catalysis in applications of energy and environmental consequence. While they are ubiquitous in catalyzing the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), redox processes occurring on the surface of manganese oxides are poorly understood. We report valence changes at OER- and ORR-relevant voltages of a layered manganese oxide film prepared by electrodeposition. X-ray absorption spectra were collected in situ in O2-saturated 0.1 M KOH using inverse partial fluorescence yield (IPFY) at the Mn L3,2-edges and partial fluorescence yield (PFY) at the O K-edge. Overall, we found reversible yet hysteretic Mn redox and qualitatively reproducible spectral changes by Mn L3,2 IPFY XAS. Oxidation to a mixed Mn3+/4+ valence preceded the oxygen evolution at 1.65 V vs RHE, while manganese reduced below Mn3+ and contained tetrahedral Mn2+ during oxygen reduction at 0.5 V vs RHE. Analysis of the pre-edge in O K-edge XAS provided the Mn–O hybridization, which was highest for Mn3+ (eg1). Our…

Metallic two-dimensional materials are emerging as promising catalysts for several reactions. This work reports a computational study of molybdenum nitride (MoN 2 ) nanosheet as a catalyst for ammonia synthesis at room temperature under the scheme of density functional theory. Pure MoN 2 shows excellent performance for dinitrogen adsorption and activation, but large energy input is needed to refresh the surface following the reaction. Iron (Fe) doping can effectively improve it to achieve full nitrogen reduction with a calculated overpotential of 0.47 V, indicating that Fe-doped MoN 2 is a promising catalyst for electrochemical synthesis of ammonia from water and air.

Bismuth-based halide perovskite derivatives have now attracted huge attention for photovoltaic (PV) applications after the unparalleled success of lead-based halide perovskites. However, the performances of PV devices based on these compounds are poor, despite theoretical predictions. In this Article, we have investigated the electronic structure and defect formation energies of Cs 3 Bi 2 I 9 using density functional theory (DFT) calculations. The calculated electronic bandstructure indicates an indirect bandgap and high carrier effective masses. Our calculations reveal a large stability region for this compound; however, deep level defects are quite prominent. Even the varying chemical potentials from the stoichiometric region do not eliminate the presence of deep defects, ultimately limiting photovoltaic efficiencies.

Plasmonic copper sulfide-based colloidal nanocrystals (NCs) have attracted considerable attention due to their unique and versatile optical and electronic properties. In this Feature Article, we first introduce the optical properties of these p -type semiconductor nanostructures, particularly localized surface plasmon resonance (LSPR). We then discuss nanostructures of copper sulfides [CuS and Cu 2– x S, (0 ≤ x < 1)] with different crystal structures and optical properties. In addition to the synthesis and transformation between these copper sulfide phases, we review their doping or alloying with extrinsic cations, which can produce homogeneous alloy nanostructures, new crystal phases, and multidomain (e.g., core–shell or dimer) NCs. In general, divalent cations (e.g., Zn, Hg, Cd) do not form a homogeneous phase upon incorporation into copper sulfide NCs, but trivalent and tetravalent cations (e.g., In, Sn, Ga) can do so. Filling of the Cu vacancies responsible for p -type doping results in red-shifting and damping of the LSPR upon incorporation of extrinsic elements. Finally, we…

The removal of hydrogen sulfide (H 2 S) is essential in various industry applications such as purification of syngas for avoiding its corrosion and toxicity to catalysts. The design of adsorbents that can bear corrosion of H 2 S and overcome the competitive adsorption from carbon dioxide (CO 2 ) is a challenge. To obtain insight into the stability and adsorption mechanism of metal–organic frameworks (MOFs) during the H 2 S separation process, 11 MOF-based materials were employed for H 2 S capture from CO 2 . Density functional theory, molecular dynamic studies, and dynamic separation experiments were used to investigate selective H 2 S/CO 2 separation. Most of these MOFs showed one-off high capacity and selectivity to H 2 S. Complete reversible physical adsorption was proven on Mg-MOF-74, MIL-101(Cr), UiO-66, ZIF-8, and Ce-BTC. Incomplete reversible adsorption occurred on UiO-66(NH 2 ). Disposable chemical reaction happened on HKUST-1, Cu-BDC(ted) 0.5, Zn-MOF-74, MIL-100(Fe) gel, and MOF-5. Using breakthrough experiments, UiO-66, Mg-MOF-74, and MIL-101(Cr) were screened out to…

Size-selected Cu n catalysts ( n = 3, 4, 20) were synthesized on Al 2 O 3 thin films using mass-selected cluster deposition. A systematic study of size and support effects was carried out for CO 2 hydrogenation at atmospheric pressure using a combination of in situ grazing incidence X-ray absorption spectroscopy, catalytic activity measurement, and first-principles calculations. The catalytic activity for methanol synthesis is found to strongly vary as a function of the cluster size; the Cu 4 /Al 2 O 3 catalyst shows the highest turnover rate for CH 3 OH production. With only one atom less than Cu 4, Cu 3 showed less than 50% activity. Density functional theory calculations predict that the activities of the gas-phase Cu clusters increase as the cluster size decreases; however, the stronger charge transfer interaction with Al 2 O 3 support for Cu 3 than for Cu 4 leads to remarkably reduced binding strength between the adsorbed intermediates and supported Cu 3, which subsequently results in a less favorable energetic pathway to transform carbon dioxide to methanol.

Fast recombination of photoinduced charge carriers is a major problem in the case of semiconductor based photocatalysts, which must be solved for their potential application in photocatalysis. In this work, photostable CdS QDs/BiOI composites have been successfully fabricated by a two-step precipitation–deposition method. The prepared samples were characterized by X-ray diffraction (XRD), UV–vis diffuse reflection spectroscopy (UV–vis DRS), photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Mott–Schottky, and electrochemical impedance analysis. The potential applications of CdS QDs/BiOI composite materials have been tested toward decolorization of rhodamine B (RhB) solution and hydrogen generation under solar light and visible light irradiation, respectively. It has been observed that hydroxyl radicals, electrons, and holes played a major role in decolorization of RhB solution. Among all prepared photocatalysts, 4% CdS QDs/BiOI composite was able to decolorize 82% of RhB solution in 1 h and 203 μmol/h of H 2 under…

Anatase TiO 2 nanosheets (TiO 2 -NS) and nanospindles (TiO 2 -NSP) have been successfully prepared with F – and glacial acetic acid as structure-directing agents, respectively. The Fe 2 O 3 /TiO 2 -NS and Fe 2 O 3 /TiO 2 -NSP nanocatalysts were prepared by a wet incipient impregnation method with a monolayer amount of Fe 2 O 3 . All the catalysts were employed for the selective catalytic reduction of NO with NH 3 (NH 3 -SCR) in order to understand the morphology-dependent effects. It is interesting that the Fe 2 O 3 /TiO 2 -NS nanocatalyst exhibited better removal efficiency of NO x in the temperature range of 100–450 °C, which was attributed to more oxygen defects and active oxygen, acid sites, as well as adsorbed nitrate species based on Raman spectra, XPS, NH 3 -TPD, NO+O 2 -TPD, and in situ DRIFTS. The density functional theory (DFT) method was used to clarify the NO and NH 3 adsorption abilities over the catalyst models of Fe 2 O 3 /TiO 2 {001} and Fe 2 O 3 /TiO 2 {101}. The results showed that the NH 3 adsorption energy over the TiO 2 {001} (−2.00 eV) was lower than that over…

In this article, we report a study of the electrochemical performance and degradation mechanism of tin (Sn) nanoparticle anodes in potassium-ion batteries (KIBs). A high capacity of 197 mAh/g was found for the Sn nanoparticles in KIBs. In situ transmission electron microscopy characterization revealed a two-step potassiation mechanism: formation of a KSn phase after full potassiation and reversible nanopore formation during the cycling of Sn nanoparticles. However, significant capacity fading occurred after a few cycles, which was caused by the severe pulverization of the Sn nanoparticles. This work offers a fundamental understanding of the reaction and degradation mechanisms of alloying-type anodes for KIBs, shedding light on the development of high-performance KIBs.

We use high-throughput density functional theory calculations to screen lead-free perovskite-like materials with compositions A 2 BB′X 6, ABX 4, and A 3 B 2 X 9 for optoelectronic performance. We screen monovalent A and B′ cations from Na, K, Rb, Cs Cu, and Ag, trivalent B cations from Ga, In, and Sb, and monovalent anions from Cl, Br, and I. Our screening procedure is based on formation energy and hybrid HSE06 functional predicted bandgaps. We screened more than 480 compounds and found 10 compounds that have bandgaps in the 1.5–2.5 eV range. Of these 10 compounds, seven are new, not having been reported before. We further characterize effective masses, density of states, and absorption coefficients of these selected compounds for their suitability in optoelectronic applications. All 10 of these selected compounds are lead-free and are solution processable. These compounds pave a path forward for lead-free photovoltaics and light emission devices.

We investigated the temperature-dependent photoluminescence (PL) properties of colloidal CsPbX 3 (X = Br, I, and mixed Br/I) quantum dot (QD) samples in the 30–290 K temperature range. Temperature-dependent PL experiments reveal thermal quenching of PL, blue shifting of optical band gaps, and line width broadening for all CsPbX 3 QD samples with increasing temperature. Interestingly, side-peak emissions that are spectrally separated from the excitonic PL peaks were observed for both CsPbBr 3 and CsPb(Br/I) 3 QD samples at temperatures below ∼250 K. The side-peak emission for the CsPbBr 3 QD sample is located at a lower energy compared to the band-edge peak, whereas that of the Br-rich CsPb(Br/I) 3 alloy QD sample is located at a higher energy than that of the band-edge peak. We found that the CsPbBr 3 QDs have two emissive states, a band-edge state, and one involving shallow defects, which can be spectrally separated by narrowing the emission line widths at low temperature. In the case of the Br-rich CsPb(Br/I) 3 QD sample, the partial halide-segregation-induced heterogeneity of the…

The g-C 3 N 4 -based composite structure exhibits excellent photocatalytic performance. However, their photogenerated carrier transfer and photocatalytic reaction mechanism were unclear. In this study, a 2D/2D g-C 3 N 4 /SnS 2 heterojunction was systematically investigated by a hybrid density functional approach. Results indicated that the g-C 3 N 4 /SnS 2 heterojunction was a staggered band alignment structure, and band bending occurred at the interface. A built-in electric field from the g-C 3 N 4 surface to the SnS 2 surface was formed by interfacial interaction. During visible-light irradiation, excited electrons in the conduction band maximum (CBM) of SnS 2 easily recombined with the holes in the VBM of g-C 3 N 4 under the electric field force. As a result, photogenerated electrons and holes naturally accumulate at the CBM of g-C 3 N 4 and the valence band maximum (VBM) of SnS 2, respectively. The effective separation of holes and electrons in space was advantageous to them participating in catalytic reactions on a different surface. Consequently, a direct Z-scheme…

A size-selected series of water-soluble luminescent Ag–In–S (AIS) and core/shell AIS/ZnS quantum dots (QDs) were produced by a precipitation technique. Up to 10–11 fractions of size-selected AIS (AIS/ZnS) QDs emitting in a broad color range from deep-red to bluish-green were isolated with the photoluminescence (PL) quantum yield reaching 47% for intermediate fractions. The size of the isolated AIS (AIS/ZnS) QDs varied from ∼2 to ∼3.5 nm at a roughly constant chemical composition of the particles throughout the fractions as shown by the X-ray photoelectron spectroscopy. The decrease of the mean AIS QD size in consecutive fractions was accompanied by an increase of the structural QD imperfection/disorder as deduced from a notable Urbach absorption “tail” below the fundamental absorption edge. The Urbach energy increased from 90–100 meV for the largest QDs up to 350 meV for the smallest QDs, indicating a broadening of the distribution of sub-bandgap states. Both the Urbach energy and the PL bandwidth of the size-selected AIS QDs increased with QD size reduction from 3–4 to ∼2 nm, and a…

Toxic gaseous SO 2 adsorption on O-terminated M 2 CO 2 (M = Sc, Hf, Zr, and Ti) monolayers has been studied by means of first-principle calculations. It is demonstrated that monolayer Sc 2 CO 2 is most preferred for the SO 2 molecule with suitable adsorption strength and apparent charge transfer. Moreover, the electronic conductivity of Sc 2 CO 2 displays a sharp increase after the adsorption of SO 2 . In particular, the adsorption strength of SO 2 on Sc 2 CO 2 can be further enhanced or weakened by applying tensile strains or controlling external electronic fields, which is greatly desirable to realize the capture or reversible release of toxic SO 2 gas. These distinctive features endow the Sc 2 CO 2 monolayer with high selectivity and sensitivity as a potential candidate for SO 2 gas sensor, as well as efficient control for gaseous SO 2 capture.

The method of X-ray photoelectron spectroscopy was used to study the interaction of the model Pt/TiO 2 catalysts with NO 2 and the following reduction of the oxidized Pt nanoparticles in vacuum, hydrogen, and methane. It was shown that, while interacting with NO 2 at room temperature, the metal Pt nanoparticles transform, first, into the phase which was tentatively assigned as particles containing subsurface/dissolved oxygen [Pt-O sub ], and then, into the PtO and PtO 2 oxides. If only the first state of platinum [Pt-O sub ] is formed, it demonstrates exclusively high reactivity toward hydrogen. For the samples containing simultaneously [Pt-O sub ], PtO, and PtO 2, the highest reaction ability was demonstrated by PtO 2; contrary to the other two oxidized states, it is reducing while kept in vacuum under X-ray irradiation. All three coexisting states of the oxidized platinum can be reduced when heated in vacuum as well as while interacting with hydrogen at room temperature. First, PtO 2 is reduced to PtO. PtO and [Pt-O sub ] begin being reduced after the complete consumption of PtO 2…

The optoelectronic properties of hybrid perovskites can be easily tailored by varying their components. Specifically, mixing the common short organic cation (methylammonium (MA)) with a larger one (e.g., butyl ammonium (BA)) results in 2-dimensional perovskites with varying thicknesses of inorganic layers separated by the large organic cation. In both of these applications, a detailed understanding of the dissociation and recombination of electron-hole pairs is of prime importance. In this work, we give a clear experimental demonstration of the interconversion between bound excitons and free charges as a function of temperature by combining microwave conductivity techniques with photoluminescence measurements. We demonstrate that the exciton binding energy varies strongly (between 80 and 370 meV) with the thickness of the inorganic layers. Additionally, we show that the mobility of charges increases with the layer thickness, in agreement with calculated effective masses from electronic structure calculations.

In the present study, we perform systematic density functional investigations on the single-atom Pt catalysts dispersed on CeO 2 (111), (110), and (100) surfaces. The structural and electronic properties of these SACs and their corresponding catalytic activity for CO oxidation are discussed. It is shown that the single-atom Pt substitution of a lattice Ce ion is thermodynamically stable on each ceria surface. On the (111) and (100) surfaces, the single Pt atom is found to exist at an oxidation state of +4 upon replacement of Ce 4+, while, on the (110) surface, it exhibits a planar structure with a reduced oxidation state of +2 due to the spontaneous formation of surface peroxide O 2 2– species. On all of the surfaces, CO oxidation is found to follow the Mars–van Krevelen mechanism. It is the single-atom Pt, rather than the lattice Ce, that acts as the electron acceptor/donor in the redox processes during the whole catalytic cycle. This work provides insights on the significant role of a single metal atom on reducible oxide of ceria and others.

Density functional theory (DFT) calculations are used in this study to screen for stable and active catalyst material among transition-metal nitrides (TMNs) for catalyzing the hydrogen evolution reaction (HER). Previously, we have predicted with DFT calculations that some TMNs form ammonia under electrochemical conditions, while others become poisoned or react to give ammonia and its parental metal. The TMNs of Sc, Ti, Y, Hf, Ta, and Mo, however, become fully covered with H adatoms without forming any ammonia. These TMNs are investigated further here, where we focus on the (100) facet of the rocksalt structure. We calculate the H coverage as a function of the applied potential. Activation energies of H 2 formation via the Tafel reaction are also calculated at varying applied potentials. The results nominate a few nitrides (TaN, HfN, MoN, and ScN) capable of catalyzing HER at low overpotentials of −0.09 to −0.34 V vs a reversible hydrogen electrode, with the activation energies of the Tafel reaction being found relatively similar to those reported on Pt(111).

The structure and composition of lithium ion solvation spheres of electrolyte solutions composed of common lithium salts (LiTFSI, LiPF 6, LiBF 4, and LiClO 4 ) dissolved in aprotic polar linear and cyclic carbonate solvents (propylene carbonate (PC) or dimethyl carbonate (DMC)) have been investigated via a combination of FTIR, 13 C NMR spectroscopy, and density functional theory (DFT). Results from the two different spectroscopic methods are in strong agreement with each other and with predictions from quantum chemistry calculations. The coordination of the carbonyl oxygen of the solvents to the lithium cation is observed by IR spectroscopy. The ratio of coordinated to uncoordinated PC and DMC has been used to determine solvent coordination numbers which range from 2 to 5 depending on salt, solvent, and concentration. The relative stability of the lithium–anion solvates were examined using DFT employing the cluster-continuum approach including changes to the intensity and frequency of the IR bands along with the populations of the cis–cis and cis–trans conformers of DMC in the…

This study is focused on the synthesis of a solar light driven novel poly(vinyl alcohol) (PVA)-assisted TiO 2 /Ti film via a facile one pot hydrothermal technique and then its photocatalytic applications by adding with peroxymonosulfate (PMS) for efficient degradation of ciprofloxacin (CIP). The key hydrothermal synthesis parameters like PVA-content, hydrothermal time ( H T ), and pH were optimized. The best photocatalytic performance of the synthesized material was observed when prepared with the conditions of PVA = 8 mg, H T = 3 h, and pH = 2.62. Results showed that 98.3% of CIP was degraded within 60 min by PVA-assisted TiO 2 /Ti film with 0.5 mM PMS added, compared to 89.38% and 88.86% degradation with 0.5 mM persulfate (PS) and 0.5 mM hydrogen peroxide (H 2 O 2 ), respectively. The photocatalytic degradation of CIP by PVA-assisted TiO 2 /Ti film with PMS added was dependent on the [PMS] 0 and increased with increase in [PMS] 0 . The k app values at [PMS] 0 of 0.2, 0.5, 0.8, and 1.0 mM were 0.029, 0.0582, 0.068, and 0.128 min –1, respectively. The photocatalytic degradation of…

The direct generation of efficient, tunable, and switchable circularly polarized laser emission (CPLE) would have far-reaching implications in photonics and material sciences. In this paper, we describe the first chiral simple organic molecules (SOMs) capable of simultaneously sustaining significant chemical robustness, high fluorescence quantum yields, and circularly polarized luminescence (CPL) ellipticity levels (| g lum |) comparable to those of similar CPL-SOMs. All these parameters altogether enable efficient laser emission and CPLE with ellipticity levels 2 orders of magnitude stronger than the intrinsic CPL ones.

We report the first in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) study of the binding of oxygenated species to the active sites of iron–nitrogen–carbon oxygen reduction reaction (ORR) electrocatalysts. To better interpret the results, DFT calculations were used to calculate absorption energies of reactants and intermediates on potential active sites and calculate the core level shifts for those. The observed oxygen binding to nitrogen coordinated to iron centers correlates with the enhanced measured ORR fuel cell activity of these materials with respect to metal-free analogs and sheds light on the ORR mechanism on PGM-free electrocatalysts.

Quantitative understanding of the photophysical processes is essential for developing novel thermally activated delayed fluorescence (TADF) materials. Taking as an example 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene, a typical TADF-active molecule, we calculated the interconversion and decay rates of the lowest excited singlet and triplet states at different temperatures as well as the prompt and delayed fluorescence efficiencies at 300 K at the first-principles level. Our results can reproduce well the experimentally available data. It is found that the reverse intersystem crossing rate ( k RISC ) is sharply increased by 3 orders of magnitude, while the other rates increase slightly or remain unchanged when the temperature rises from 77 to 300 K. Importantly, k RISC reaches up to 1.23 × 10 6 s –1 and can compete with the radiative and nonradiative decay rates of S 1 (1.11 × 10 7 and 2.37 × 10 5 s –1 ) at 300 K, leading to an occurrence of delayed fluorescence. In addition, our calculations indicate that it is the freely rotational motions of the carbazolyl between two cyano…

Lead-free pnictogen/noble metal halide double perovskites Cs 2 BiAgCl 6, Cs 2 BiAgBr 6, and Cs 2 SbAgCl 6 are some of the most promising environmentally friendly alternatives to lead–halide perovskites. However, due to their relatively large band gaps (1.9–2.2 eV), they are not yet competitive candidates for use in photovoltaic devices. In this work, we perform a systematic study of the thermodynamic stability of the entire family of Cs 2 BB′X 6 compounds (B = Bi and Sb, B′ = Cu, Ag, and Au, and X = Cl, Br, and I), and we explore the possibility of chemical mixing as a route to stabilize pnictogen/noble metal halide perovskites with low band gaps. Our calculations indicate that Cs 2 BiAg 1– x Cu x Cl 6 mixes should be amenable to synthesis and could reduce the band gap down to 1.6–1.9 eV.

We found for the first time a new origin of selection of perovskite crystallization pathways from DMF solutions containing MAI and PbI 2 to present here a comprehensive study of a full set of essential intermediate phases determining the perovskite’s morphology. For all three discovered structurally different intermediate phases forming at a given precursor ratio, we refined their crystal structures by synchrotron X-ray radiation and investigated dynamics and phase assemblage in the course of decomposition. As a result, we revealed a clear correlation between the composition of the intermediate phases, peculiarities of their crystal structure, and the morphology of the final perovskite films. Using the DFT method we calculated formation enthalpies of these intermediate phases and explained the preferential precipitation of DMSO-adduct rather than DMF-adduct in an antisolvent approach. This finding opens up a possibility of design-on-demand of perovskite materials using simple soft chemistry approaches.

The inability of conventional theoretical models to corroborate the well-known experimental activity of RuO 2 for the oxygen evolution reaction (OER) has recently been the subject of numerous research efforts. In this study, we use density functional theory calculations to investigate the possibility that surface defects formed during dissolution are responsible for the OER activity of RuO 2 . It has been well-established experimentally that RuO 2 undergoes dissolution during OER, yet little is known about the atomic structure or catalytic reactivity of the resulting defect sites. Through simulation of point defects, steps, and kinks derived from the RuO 2 (110) surface, we discover a 0.7 eV range in the primary descriptor for OER activity, with the most active sites outperforming those at the ideal (110) surface by nearly 0.5 eV. We postulate that these variations in reactivity are due to differences in the local electronic structure, and we investigate in more detail the electronic structure of two singly coordinated sites at the same Ru atom located at a kink. Finally, we…

Microwave plasmas are one of the most promising techniques for CO 2 conversion into value-added chemicals and fuels since they are very energy efficient. Nevertheless, experiments show that this high energy efficiency is only reached at low pressures and significantly drops toward atmospheric pressure, which is a clear limitation for industrial applications. In this paper, we use a zero-dimensional reaction kinetics model to simulate a CO 2 microwave plasma in a pressure range from 50 mbar to 1 bar, in order to evaluate the reasons for this decrease in energy efficiency at atmospheric pressure. The code includes a detailed description of the vibrational kinetics of CO 2, CO, and O 2 as well as the energy exchanges between them because the vibrational kinetics is known to be crucial for energy efficient CO 2 splitting. First, we use a self-consistent gas temperature calculation in order to assess the key performance indicators for CO 2 splitting, i.e., the CO 2 conversion and corresponding energy efficiency. Our results indicate that lower pressures and higher power densities lead to…

Abstract Lithium dendrite growth dynamics on Cu surface is first visualized through a versatile and facile experimental cell by in operando transmission X-ray microscopy (TXM). Galvanostatic plating and stripping cycle(s) are applied on each cell. Upon plating/stripping at ∼1 mA cm–2, mossy lithium is clearly found growing and shrinking on the Cu surface as the application time increases. It is interesting to note that the aspect ratio (height/width) of deposited lithium has increased with charge passed during plating, indicating a faster growing from the base. In addition, the dendritic or mossy lithium has also been observed when various high current densities (25, 12.5, and 6.3 mA cm–2) are applied in different cycles, showing a severe dendritic lithium formation that could be induced by inhomogeneous current distribution. The clear structure of dead lithium is found after the cycling, which also shows a lower efficiency and higher hazard when a higher current density is applied. This work explores TXM as a useful tool for in operando dynamic visualization and quantitative…

Abstract The screening of surface charge by dissolved ions at solid–liquid interfaces─in the region of interfacial fluid known as the electrical double layer (EDL)─plays a recurrent role in surface science, from ion adsorption to colloidal mechanics to the transport properties of nanoporous media. A persistent unknown in theories of EDL-related phenomena is the structure of the Stern layer, the near-surface portion of the EDL where water molecules and adsorbed ions form specific short-range interactions with surface atoms. Here, we describe a set of synchrotron X-ray reflectivity (XRR) experiments and molecular dynamics (MD) simulations carried out under identical conditions for a range of 0.1 M alkali chloride (Li-, Na-, K-, Rb-, or CsCl) solutions on the basal surface of muscovite mica, a mineral isostructural to phyllosilicate clay minerals and one of the most widely studied reference surfaces in interfacial science. Our XRR and MD simulation results provide a remarkably consistent view of the structure and energetics of the Stern layer, with some discrepancy on the fraction of…

The loss of sulfur in the cathode of a lithium sulfur battery (LSB) severely hinders the practical application of LSBs, and so do the insulativity of S and its lithiation end products. The incorporation of MXene can significantly improve the performance of LSBs; however, the underlying mechanism at the atomic scale has not been deeply explored. In the present work, by using density functional theory calculations, we systemically studied the interactions of lithium (poly)sulfides (Li 2 S m ) on Ti-based bare MXenes (Ti n X n –1 ) and surface functionalized Ti 2 C with −F, −O, and −OH groups. Through analyzing the geometric and electronic structures, binding energies, and deformation charge densities of Li 2 S m adsorbed MXenes, we found that the strong Ti–S bonds dominate the interactions between Li 2 S m and MXenes. The strong Coulombic interactions help cathodes to confine S from dissolution. Besides, the conductivities of MXenes and Li 2 S m @MXenes are beneficial for the overall performance of the LSB. These will provide in-depth theoretical guidance support for the utilization…

We prepare a series of MAg 24 (SR) 18 (M = Ag/Pd/Pt/Au) nanoclusters (NCs) with similar core–inner shell–outer shell structures and investigate their crystal and solution photoluminescence. The core silver atom replacement by the Pd/Pt/Au atom obviously tunes the geometric and electronic structures of Ag 25 (SR) 18 NC. The crystal photoluminescence intensities sequence hints a core-atom-directing charge transfer from the ligands to the metal kernels. Both the calculated NPA charge and the measured Ag inner shell –S terminal bond length support the proposed mechanism. Further experiments show the solvent influence on the NCs photoluminescence supported by the blue-shift of emissions of MAg 24 (SR) 18 NCs and the solvent-dependent photoluminescence intensity sequences. Especially, for PtAg 24 (SR) 18, the quantum yield is almost 100-fold greater in CH 3 CN (18.6%) than in CH 2 Cl 2 (0.2%). However, the emission wavelengths of the series of NCs are barely influenced by the solvent type. This work indicates the importance of the core atom and the solvent to the photoluminescence of…

Nickel-rich layered lithiated Ni–Co–Mn oxides (NCMs) are emerging as the most promising candidates for next-generation Li-ion battery cathodes. Progress, however, is hindered by an incomplete understanding of processes that lead to performance-limiting impedance growth and reduced cycling stability. These processes typically involve surface reconstruction and O 2 release at the cathode surface, both of which are difficult to monitor in the working cell. We demonstrate that online electrochemical mass spectrometry can be used to measure the gas release from NCMs of varying Ni content at practically relevant potentials and under operando electrochemical conditions. We find that for cathode potentials up to 4.3 V (vs Li + /Li) there is virtually no trade-off between Ni-mediated specific-charge enhancement and parasitic surface reactions. However, at potentials greater than 4.3 V, surface-reconstruction processes giving rise to substantial CO 2 and O 2 release occur, implying that surface-reconstructed layers a few nanometers thick may form already after the first charge. Ni content and…

Carbon “quantum” dots (CDots), generally defined as small carbon nanoparticles with various surface passivation schemes, have emerged to represent a rapidly advancing and expanding research field. CDots are known for their bright and colorful fluorescence emissions, where the colorfulness is associated with the emissions being excitation wavelength dependent. In this work, CDots with 2,2′-(ethylenedioxy)bis(ethylamine) (EDA) for surface functionalization were studied systematically by using steady-state and time-resolved fluorescence methods. The observed fluorescence quantum yields are strongly excitation wavelength dependent, and the dependence apparently tracks closely the observed absorption profile of the EDA-CDots, whereas the excitation wavelength dependence of observed fluorescence lifetimes is much weaker, obviously decoupled from the quantum yields. Mechanistically, the presence of two sequential processes immediately following the photoexcitation of CDots leading to fluorescence is used to rationalize these effects, and the experimental results seem better explained by…

The mechanism and kinetics of interactions between dimethyl methylphosphonate (DMMP), a key chemical warfare agent (CWA) simulant, and Zr 6 -based metal organic frameworks (MOFs) have been investigated with in situ infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD), and DFT calculations. DMMP was found to adsorb molecularly to UiO-66 through the formation of hydrogen bonds between the phosphoryl oxygen and the free hydroxyl groups associated with Zr 6 nodes on the surface of crystallites and not within the bulk MOF structure. Unlike UiO-66, the infrared spectra for UiO-67 and MOF-808, recorded during DMMP exposure, suggest that uptake occurs through both physisorption and chemisorption. The XPS spectra of MOF-808 zirconium 3d electrons reveal a charge redistribution following exposure to DMMP. In addition, analysis of the phosphorus 2p electrons following exposure and thermal annealing to 600 K indicates that two types of stable phosphorus-containing species exist within the MOF. DFT calculations, used to guide the IR band assignments…

The influence of three different transition metals (Me = Fe, Co, Cu) on the oxygen reduction reaction (ORR) kinetics in acidic medium of Me–N–C catalysts synthesized using Me(II)-phthalocyanine as precursors is investigated in this work. Through a detailed electrochemical characterization using cyclic voltammetry and rotating ring-disk electrode, several kinetic parameters such as Tafel slope, reaction order for oxygen and proton, apparent activation energy, selectivity toward hydrogen peroxide production, and kinetics of reduction of adsorbed oxygen were determined. The behavior of these three catalysts is analyzed in detail. A comparison between each other of the catalysts, and with a Pt-based catalyst is done. The results obtained provide clear evidence of the important role played by each transition metal in the formation of more or less effective active sites. The ORR kinetics behavior can be well interpreted according to the occurrence of a redox-mediated coverage of the active sites at low overpotentials (close to the ORR onset), which has influence on the Tafel slope, as…

Sigmoidal kinetic curves have been reported for a number of cooperative phenomena in nature. These curves may be fit by purely mathematical functions that, however, do not correspond to any physical model. The 1997 Finke–Watzky (F–W) two-step model of slow, continuous nucleation (A → B, rate constant k 1 ) and fast, autocatalytic growth (A + B → 2B, rate constant k 2 ) provides both a physical model and a mathematical solution. As a result, the F–W two-step kinetic model has been successfully applied to a large number of cooperative phenomena throughout nature that display sigmoidal kinetic curves. Herein, we derive formulas for the first, second, and third derivatives of the concentration of product versus time, [B] t, expressed in terms of the F–W parameters k 1, k 2, and the initial concentration of monomer, [A] 0 . Mathematical expressions are then derived for key empirical parameters in sigmoidal curves, including the induction period and (maximum) slope, which are then examined under the limit k 1 ≪ k 2 [A] 0 where nucleation and growth are well-separated in time. The…

Considerable attention has been focused on layered double hydroxides (LDHs) for their applications in solar energy storage and conversion recently, but the in-depth investigation on the semiconducting properties of LDHs is limited. Herein, the electronic properties (band structure, density of states (DOS), surface energy, and band edge placement) of 14 kinds of M II n M III/IV –A–LDHs (M II = Mg, Co, Ni, Cu, Zn; M III = Cr, Fe; M IV = Ti; n = 2, 3, 4; A = Cl –, NO 3 –, CO 3 2– ) which contain transition-metal cations as well as their thermodynamic reaction mechanism toward the oxygen evolution reaction (OER) were studied using a density functional theory plus U (DFT + U) method. The calculation results indicate that the (003) plane is the most preferably exposed surface, and all these calculated LDHs are visible light responsive. The OER driving force and overpotential for these LDHs were obtained via their band edge placement and thermodynamic mechanism, and the results show that 10 of the calculated 12 LDHs (Ni 2 Ti–Cl–, Cu 2 Ti–Cl–, Zn 2 Ti–Cl–, Ni 2 Cr–Cl–, Zn 2 Cr–Cl–, Co 2…

We demonstrate using single molecule spectroscopy that inorganic CsPbI 3 perovskite quantum dots (PQDs) undergo an irreversible, photoaccelerated reaction with water that results in a blue-shift of the photoluminescence (PL) and ultimately to complete quenching of the emission. We find that decomposition does not take place in the presence of oxygen alone but that it requires light and water. We also analyze the blinking for some stable PQDs and find a continuous distribution of emission states with a linear correlation between intensity and lifetime. We postulate that, in addition to charging and discharging processes, blinking arises from the activation and deactivation of nonradiative recombination centers in the PQDs.

Layered Li(Ni 1– x – y Mn x Co y )O 2 (NMC) oxides are promising cathode materials capable of addressing some of the challenges associated with next-generation energy storage devices. In particular, improved energy densities, longer cycle-life, and improved safety characteristics with respect to current technologies are needed. However, sufficient knowledge on the atomic-scale processes governing these metrics in working cells is still lacking. Herein, density functional theory (DFT) is employed to predict the stability of several low-index surfaces of Li(Ni 1/3 Mn 1/3 Co 1/3 )O 2 (NMC111) as a function of Li and O chemical potentials. Predicted particle shapes are compared with those of single crystal NMCs synthesized under different conditions. The most stable surfaces for stoichiometric NMC111 are predicted to be the nonpolar (104), the polar (012) and (001), and the reconstructed, polar (110) surfaces. Results indicate that intermediate spin Co 3+ ions lower the (104) surface energy. Furthermore, it was found that removing oxygen from the (012) surface was easier than from the…

We investigated, in depth, the interrelations among structure, magnetic properties, relaxation dynamics and magnetic hyperthermia performance of magnetic nanoflowers. The nanoflowers are about 39 nm in size, and consist of densely packed iron oxide cores. They display a remanent magnetization, which we explain by the exchange coupling between the cores, but we observe indications for internal spin disorder. By polarized small-angle neutron scattering, we unambiguously confirm that, on average, the nanoflowers are preferentially magnetized along one direction. The extracted discrete relaxation time distribution of the colloidally dispersed particles indicates the presence of three distinct relaxation contributions. We can explain the two slower processes by Brownian and classical Néel relaxation, respectively. The additionally observed very fast relaxation contributions are attributed by us to the relaxation of disordered spins within the nanoflowers. Finally, we show that the intrinsic loss power (ILP, magnetic hyperthermia performance) of the nanoflowers measured in colloidal…

Abstract Heteroatom doping is one of the most effective routes to adjust the physicochemical and optical properties of carbon dots (CDs). However, fluorine (F) doped CDs have been barely achieved. In this work, a F-doping strategy was proposed and adopted to modulate optical properties of CDs. A kind of F-doped CDs was synthesized by a solvothermal process using aromatic F bearing moiety as the F source and shows much longer maximum emissions (up to 600 nm, red fluorescence) than that of undoped CDs, indicating a large emission red-shift effect by F-doping. In addition, the F-doped CDs have remarkable water-solubility, high biocompatibility, as well as excellent stability even under broad pH range, ionic strengths, and light illumination and thus can be used as a novel probe for the highly efficient cell imaging of various normal cells and cancer cells. The F-doped CDs can selectively bind to Ag+. It therefore makes the F-doped CDs be a highly sensitive probe for the detection of Ag+ under both aqueous solution and various biological systems. The huge potential of this F-doping…

Using density functional theory combined with a semiempirical van der Waals dispersion correction, we have investigated the stability of lattice defects including boron vacancy, substitutional and interstitial X (X = H, C, B, N, O), and Σ5 tilt grain boundaries in borophene and their influence on the anisotropic mechanical properties of this two-dimensional system. The pristine borophene has significant in-plane Young’s moduli and Poisson’s ratio anisotropy due to its strong and highly coordinated B–B bonds. The concentration of B vacancy and Σ5 grain boundaries could be rather high given that their formation energies are as low as 0.10 and 0.06 eV/Å, respectively. In addition, our results also suggest that borophene can react easily with H 2, O 2, and N 2 when exposed to these molecules. We find that the mechanical strength of borophene is remarkably reduced by these defects. The anisotropy in Poisson’s ratio, however, can be tuned by some of them. Furthermore, the adsorbed H or substitutional C may induce negative Poisson’s ratio in borophene, and the substitutional C or N can…

In the present study, morphological and electrical properties of BaZrO 3 were investigated as a function of the ionic radii mismatch between Zr and the different B-site dopants (Al, Sc, In, Lu, Tm, Y, Gd, Sm, Nd, and La) for the same solute concentration and valence. Our study highlights the critical role of the ionic radius of the acceptor dopant on stability, sinterability, and proton conductivity of barium zirconate. From our study, Gd-doped barium zirconate emerges as a novel promising material for proton conductor electrolytes.

White or tunable photoluminescence of phosphors is of great significance for their practical applications in the light-emitting diodes. In this work, Ba 2 Y(BO 3 ) 2 Cl:Bi 3+,Eu 3+ phosphors were fabricated, and their photoluminescence properties were investigated. Three difference luminescence peaks located at 366, 410, and 490 nm were observed in the single Bi 3+ -doped Ba 2 Y(BO 3 ) 2 Cl phosphor under the excitation of ultraviolet light attributed to three various Bi 3+ sites occupied in the Ba 2 Y(BO 3 ) 2 Cl host. By tuning the excitation wavelength from three various Bi 3+ sites, the tunable luminescence properties were obtained. The energy transfer from Bi 3+ at various sites to Eu 3+ were observed in the Ba 2 Y(BO 3 ) 2 Cl:Bi 3+,Eu 3+ phosphors by an electric dipole–dipole interaction. The tunable and white luminescence was obtained by changing the Eu 3+ concentration or excitation wavelength. The light-emitting diodes with the white light emission were constructed by coupling Bi 3+ and Eu 3+ codoped Ba 2 Y(BO 3 ) 2 Cl phosphors with the UV chip, which indicated that the Bi…

Construction of a visible light driven Z -scheme-based photocatalytic system is a hot topic of research because of the potential to alleviate both energy and environmental issues. In the present study a series of surface vacancy mediated BiOI–CeO 2 nanocomposites were prepared and their crystallographic, morphological, optical, and electrochemical behavior were characterized through XRD, TEM, FESEM, UV–vis DRS, PL, ESI, and Mott–Schottky techniques. From the XPS analysis, it could be said that a high amount of oxygen defects and metallic bismuth were present on the composite than neat CeO 2 which was the major cause of enhancement in the photocatalytic activity. Further the photocatalytic efficiency of the as prepared samples was tested toward RhB decolorization/phenol oxidation as well as for O 2 gas evolution. It was observed that 40 wt % BiOI–CeO 2 nanocomposite exhibited highest photocatalytic activity among neat and other composites, i.e., 89% decolorization of 100 ppm RhB and produces 323 μmol/2h of O 2 under visible light illumination. To justify the enhanced photocatalytic…

ZnO/CuO nanofibers, with different CuO concentrations, were fabricated by one-step electrospinning of the polymer precursor and annealing in air. Scanning electron microscopy (SEM) showed smooth and beadless morphology for the synthesized nanofibers, while X-ray diffraction (XRD) analysis revealed formation of hexagonal and monoclinic crystalline structure phases for ZnO and CuO nanofibers, respectively. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of CuO on the surface of ZnO nanofibers. For further confirming the formation of chemical bonds, Fourier transform infrared (FT-IR) spectroscopy was employed. The effect of Cu contents in the overall electronic band structure of ZnO was explained by density functional theory (DFT) calculations. Diffuse reflectance spectroscopy (DRS) showed that the ZnO band gap energy reduced with increasing amount of CuO contents due to the presence of the Cu(3d) energy states above the valence band. Comparing the photocatalytic activity of ZnO/CuO nanofiber samples with different CuO concentrations under similar sunlight…

Abstract Changes of water state within the pore structure of cement paste due to temperature changes are followed by means of 1H nuclear magnetic resonance (NMR) relaxation analysis. The study shows that, with increasing temperature, the signal due to water contained in the smallest C–S–H interlayer spaces decreases while that from the larger gel pores, and to a lesser extent from the capillary pores, increases. On cooling, the opposite behavior is observed with complete reversibility. The observed changes in water populations appear to be instantaneous compared to the rate of temperature change in the samples. These changes are postulated to be responsible for macroscopically observed changes of relative humidity in pores during heating/cooling and are therefore key in understanding thermal deformations of cement-based materials. It is evident that the previous hypothesis of microstructural delayed water transport being responsible for macrostructural delayed thermal deformations can be rejected. Different microstructural mechanisms are discussed that could explain the…

Abstract The efficiency of Cu(In,Ga)Se2 (CIGS)-based solar cells can be markedly improved by controlled introduction of alkali metal (AM) atoms using post-deposition treatment (PDT) after CIGS growth. Previous studies have indicated that AM atoms may act as impurities or agglomerate into secondary phases. To enable further progress, understanding of atomic level processes responsible for these improvements is required. To this end, we have investigated theoretically the effects of the AM elements Li, Na, K, Rb, and Cs on the properties of the parent material CuInSe2. First, the effects of the AM impurities in CuInSe2 have been investigated in terms of formation energies, charge transition levels, and migration energy barriers. We found that AM atoms preferentially substitute for Cu atoms at the neutral charge state. Under In-poor conditions, AM atoms at the In site also show low formation energies and are acceptors. The migration energy barriers show that the interstitial diffusion mechanism may be relevant only for Li, Na, and K, whereas all the AM atoms can diffuse with the help…

We provide the first experimental evidence of oxygen vacancy defect induced ferromagnetism in undoped multiferroic CaMn 7 O 12 (CMO) manganite synthesized from the facile chemical combustion method. The obtained nanocrystalline is characterized by various techniques like TGA, FTIR, XRD, SEM-EDX, AFM, UV-visible, XPS, and SQUID, etc. to confirm the phase purity and crystallinity of CMO. Surface roughness increases with sintering time attributed to the increase of surface oxygen vacancy defects. X-ray photoelectron spectroscopy was carried out to confirm the oxidation state of constituent elements and also provides direct evidence of enhanced oxygen vacancies. UV–vis optical absorption used to infer band gap shift from 1.68 to 1.38 eV, respectively, is also attributed to increases in oxygen vacancy defects. Multiple magnetic phase transition temperatures of 90, 50, and 42 K, respectively, were obtained from the derivative of magnetization. A systematic decrease of full widths at half maxima (fwhm) of d M /d T vs T curves with sintering time indicates strengthening of ferromagnetism…

Potassium-ion batteries (KIBs), as alternatives to lithium-ion batteries (LIBs), have attracted increasing attention due to the abundance of K in the Earth’s crust. Here, using first-principles calculations, we have found that boron (B)-doped graphene is a promising anode material for KIBs. The studied B 4 C 28 structure has a large specific capacity of 546 mAh/g, a small migration barrier of 0.07 eV, and a moderate potassiation voltage of 0.82 V to suppress the formation of a SEI layer. Moreover, B-doped graphene with a doping concentration of 12.5 at. % is metallic with good electron conductivity that can improve rate performance. Also, B doping makes the substrate electron-deficient and results in significant charge transfer from K to the substrate, thus preventing K atoms from clustering, inhibiting dendrite growth, and leading to a good cycling stability.

A series of mixed-metal NH 2 –UiO-66(Zr/Ti) with different percentages of exchanged Ti have been prepared and studied by transient absorption spectroscopy (TAS). The photogenerated transients from mixed NH 2 –UiO-66(Zr/Ti) exhibit at short time scales two defined absorption bands, evolving to a continuous absorption band expanding from 300 to 700 nm at longer time scales. The observed spectral changes are compatible with an initial formation of Ti 3+ –O–Zr 4+ and its further transformation to Ti 4+ –O–Zr 3+ via metal–metal electron exchange, thus providing support to the role of substituted Ti as mediator to facilitate electron transfer from excited ligand to the (Zr/Ti) 6 O 4 (OH) 4 nodes in mixed NH 2 –UiO-66(Zr/Ti). The slow recombination of photogenerated electrons and holes in the mixed NH 2 –UiO-66(Zr/Ti) has been advantageously used for the construction of a photovoltaic cell fabricated with the mixed NH 2 –UiO-66(Zr/Ti), reaching a higher photon-to-current efficiency than NH 2 –UiO-66(Zr).

Abstract Nanodiamonds exhibit exceptional colloidal properties in aqueous media that lead to a wide range of applications in nanomedicine and other fields. Nevertheless, the role of surface chemistry on the hydration of nanodiamonds remains poorly understood. Here, we probed the water hydrogen bond network in aqueous dispersions of nanodiamonds by infrared, Raman, and X-ray absorption spectroscopies applied in situ in aqueous environment. Aqueous dispersions of nanodiamonds with hydrogenated, carboxylated, hydroxylated, and polyfunctional surface terminations were compared. A different hydrogen bond network was found in hydrogenated nanodiamonds dispersions compared to dispersions of nanodiamonds with other surface terminations. Although no hydrogen bonds are formed between water and hydrogenated surface groups, a long-range disruption of the water hydrogen bond network is evidenced in hydrogenated nanodiamonds dispersion. We propose that this unusual hydration structure results from electron accumulation at the diamond–water interface.