The 50 most-cited articles in The Journal of Physical Chemistry, 2018

The CuₓZrSe₂ intercalation single crystals have been synthesized and studied in a concentration range of x = 0–0.3, in which the semiconductor-metal transition was observed. The evolution of the electronic structure of CuₓZrSe₂ as a function of the copper content has been studied experimentally using the XPS and XAS methods. The obtained results indicate the gradual shift of the Fermi level with increasing copper content, which does not allow the metal–insulator transition to be associated with charge transfer to the conduction band. This transition is associated with the formation of the covalent bond between copper and the nearest selenium atoms and of the p–d hybridized states inside the Zr 4d/Se 4p band gap. First-principle calculations were performed in order to obtain a theoretical description of the density of states near the Fermi level and confirm this conclusion.

Binding affinities and stoichiometries of Na⁺ and Ca²⁺ ions to phospholipid bilayers are of paramount significance in the properties and functionality of cellular membranes. Current estimates of binding affinities and stoichiometries of cations are, however, inconsistent due to limitations in the available experimental and computational methods. In this work, we improve the description of the binding details of Na⁺ and Ca²⁺ ions to a 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayer by implicitly including electronic polarization as a mean field correction, known as the electronic continuum correction (ECC). This is applied by scaling the partial charges of a selected state-of-the-art POPC lipid model for molecular dynamics simulations. Our improved ECC-POPC model reproduces not only the experimentally measured structural parameters for the ion-free membrane, but also the response of lipid headgroup to a strongly bound cationic amphiphile, as well as the binding affinities of Na⁺ and Ca²⁺ ions. With our new model, we observe on the one side negligible binding of Na⁺ ions to…

In experimental studies, it has been observed that the presence of sodium dodecyl sulfate (SDS) significantly increases the kinetics of hydrate formation and the final water-to-hydrate conversion ratio. In this study, we intend to understand the molecular mechanism behind the effect of SDS on the formation of methane hydrate through molecular dynamics simulation. Hydrate formation conditions similar to that of laboratory experiments were chosen to study hydrate growth kinetics in 1 wt % SDS solution. We also investigate the effect of interactions with isolated SDS molecules on methane hydrate growth. It was observed that the hydrophobic tail part of the SDS molecule favorably interacts with the growing hydrate surface and may occupy the partial hydrate cages while the head groups remain exposed to water.

Electrolyte solutions of 1 M concentration are typically used in lithium ion batteries (LIB) for optimal performance. However, recently, superconcentrated electrolytes have been proposed to be a promising alternative to 1 M solutions. Despite their improved stability features, application of the concentrated electrolytes is hindered by their poor transport properties. We probe EC-LiPF₆ electrolyte system for a range of concentrations: 0.06 to 4 M using molecular dynamics simulations to study the effect of concentration on transport and structural properties. Molecular structure of the solution changes with concentration from a predominantly solvent separated ion pair (SSIP) configuration at the dilute limit to an aggregate rich configuration at high concentration. Depletion of SSIPs and formation of more aggregates at higher concentrations affect the transport properties. The present work provides insights into the relation between molecular structure and performance of the electrolyte solution and suggests ways to design novel concentrated electrolytes.

In the context of elucidating the mechanism by which siloxane-based sol–gel coatings adhere to the surface, the adsorption of a model silanol molecule, CH₃Si(OH)₃, and its oligomers (up to the trimer) on oxidized and fully hydroxylated aluminum substrates is described using density functional theory (DFT). To link our calculations with the synthesis of siloxane-based sol–gel coatings, the focus is given on the condensation mechanism. We find that the formation of a monodentate bonding mode with the hydroxylated surface via the condensation mechanism is exothermic by ≥0.5 eV in all considered cases. In contrast, the formation of a bidentate bonding mode is exothermic only for the trimer. However, taking entropic contributions into account, we find that the formation of the bidentate bonding mode is exergonic already for the dimer due to favorable entropic effects of a liberated water molecule during the reaction. In contrast, the reaction entropy is unfavorable for the monodentate formation because the effects of the immobilized silanol molecule counteract and surpass those of the…

The stability of enzymes is critical for their application in industrial processes, which generally require different conditions from the natural enzyme environment. Both rational and random protein engineering approaches have been used to increase stability, with the latter requiring extensive experimental effort for the screening of variants. Moreover, some general rules addressing the molecular origin of protein thermostability have been established. Herein, we demonstrate the use of molecular dynamics simulations to gain molecular level understanding of protein thermostability and to engineer stabilizing mutations. Carbonic anhydrase (CA) is an enzyme with a high potential for biotechnological carbon capture applications, provided it can be engineered to withstand the high temperature process environments, inevitable in most gas treatment units. In this study, we used molecular dynamics simulations at 343, 353, and 363 K to study the relationship between structure flexibility and thermostability in bacterial α-CAs and applied this knowledge to the design of mutants with…

This work investigates the role of the co-reactant for the atomic layer deposition of cobalt (Co) films using cobaltocene (CoCp₂) as the precursor. Three different processes were compared: an AB process using NH₃ plasma, an AB process using H₂/N₂ plasma, and an ABC process using subsequent N₂ and H₂ plasmas. A connection was made between the plasma composition and film properties, thereby gaining an understanding of the role of the various plasma species. For NH₃ plasma, H₂ and N₂ were identified as the main species apart from the expected NH₃, whereas for the H₂/N₂ plasma, NH₃ was detected. Moreover, HCp was observed as a reaction product in the precursor and co-reactant subcycles. Both AB processes showed self-limiting half-reactions and yielded similar material properties, that is, high purity and low resistivity. For the AB process with H₂/N₂, the resistivity and impurity content depended on the H₂/N₂ mixing ratio, which was linked to the production of NH₃ molecules and related radicals. The ABC process resulted in high-resistivity and low-purity films, attributed to the lack of…

Pd or Pt alloyed with a secondary metal are the typical catalysts at the anode for the direct oxidation of methanol. The secondary metal is employed to diminish deactivation commonly ascribed to CO poisoning. Here we investigate the origin of the improved performance of Au–Pd core–shell and alloy nanocrystals as electrocatalysts for the methanol oxidation reaction (MOR), relative to Pd alone. Monodisperse Au–Pd core–shell nanocrystals were synthesized using H₂ as a mild reducing agent followed by annealing under a 5% H₂ atmosphere to produce the Au–Pd alloys. The nanocrystals were characterized using high-resolution electron microscopy to confirm their structures. The core–shell and alloy nanocrystals showed an improvement in specific activity with respect to pure Pd nanocrystals. Importantly, the stability was also improved by the inclusion of Au for both nanocrystals, being 2.7× higher for the alloy than for the core–shell after 30 min, while the activity is completely lost for the Pd nanocrystals within 10 min. We show that there is no evidence of CO formation for any of the…

Understanding the role of surface defects on the catalyst performance is of great significance for a good command of the catalytic mechanism on the real catalyst surface. This work reports the mechanistic study of CO oxidation on the defective Cu₂O(111) surface using density functional theory calculations. The effect of surface defects on surface catalytic activity was investigated by creating surface-adsorbed O atoms and Cu vacancies on the perfect Cu₂O(111) surface. Possible defective surface structures were found, and calculated results showed that Cu vacancy on the Cu₂O(111) surface could promote CO oxidation in two ways by (I) promoting the reaction between CO and lattice O with the energy barrier of 1.316 eV following the Mars–van–Krevelen mechanism; (II) promoting the reaction between CO and adsorbed O atoms below 575 K. For the perfect Cu₂O(111) surface, adding adsorbed O to the surface could either cause a strong surface reconstruction with the adsorbed O evolving into a lattice O or stay in the active state. The obtained results could explain the experimental observations…

A new type of two-dimensional layered semiconductor with weak electrostatic but not van der Waals interlayer interactions, Bi₂O₂Se, has been recently synthesized, which shows excellent air stability and ultrahigh carrier mobility. Herein, we combined theoretical and experimental approaches to study the Raman spectra of Bi₂O₂Se and related bismuth oxychalcogenides (Bi₂O₂Te and Bi₂O₂S). The experimental peaks lie at 160 cm–¹ in Bi₂O₂Se and at 147 and 340 cm–¹ in Bi₂O₂Te. They were fully consistent with the calculated results (159.89, 147.48, and 340.33 cm–¹) and assigned to the out-of-plane A₁g, A₁g, and B₁g modes, respectively. Bi₂O₂S was predicted to have more Raman-active modes due to its lower symmetry. The shift in the predicted frequencies of Raman-active modes was also found to get softened as the interlayer interaction decreases from bulk to monolayer Bi₂O₂Se and Bi₂O₂Te. To reveal the strain effects on the Raman shifts, a universal theoretical equation was established based on the symmetry of Bi₂O₂Se and Bi₂O₂Te. It was predicted that the doubly degenerate modes split under…

Halide perovskites, with prototype cubic phase ABX₃, undergo various phase transitions accompanied by rigid rotations of corner-sharing BX₆ octahedra. Using first-principles density functional theory calculations, we have performed a comprehensive investigation of all of the possible octahedral tilting in 18 halide perovskites ABX₃ (A = Cs, Rb, and K; B = Pb and Sn; and X = I, Br, and Cl) and found that the stabilization energies, i.e., energy differences between cubic and the most stable tilted phases, are linearly correlated with the tolerance factor t. Moreover, the tilt energies, i.e., energy differences between cubic and various tilted phases, are linearly correlated with the change of atomic packing fractions (Δη), indicating the importance of the atomic packing fraction on the stabilities of distorted perovskites. This work may provide guidance for searching proper perovskite ABX₃ to extend distorted perovskite for wide applications, such as thermochromics.

We investigate the impact of grain boundaries and interfaces on dynamics of photoexcited charge carriers in polycrystalline lead sulfide (PbS) films and at interfaces between polycrystalline PbS and ZnO by studying transient photoconductivity over sub-picoseconds to microseconds timescales using time-resolved terahertz spectroscopy and time-resolved microwave conductivity measurements. Narrow band gap bulk-like polycrystalline PbS with high absorption in the infrared paired with wide band gap metal oxide current collectors holds promise for infrared photodetectors and photovoltaics for converting infrared radiation to electricity. We find that grain boundaries in polycrystalline PbS suppress long-range conductivity and confine photoexcited carriers within individual crystallites. The mobility of photoexcited holes inside the ∼150 nm crystallites reaches 750 cm²/V s, and their lifetime exceeds hundreds of microseconds, while electrons get rapidly trapped at grain boundary states. The presence of PbS/ZnO interfaces dramatically reduces the lifetime of the photoexcited free holes in…

Perovskite-type oxyhydrides, BaTiO₃–ₓHₓ, have been recently shown to exhibit hydride-ion (H–) conductivity at elevated temperatures, but the underlying mechanism of hydride-ion conduction and how it depends on temperature and oxygen vacancy concentration remains unclear. Here, we investigate, through the use of quasielastic neutron scattering techniques, the nature of the hydride-ion dynamics in three metal hydride-reduced BaTiO₃ samples that are characterized by the simultaneous presence of hydride ions and oxygen vacancies. Measurements of elastic fixed window scans upon heating reveal the presence of quasielastic scattering due to hydride-ion dynamics for temperatures above ca. 200 K. Analyses of quasielastic spectra measured at low (225 and 250 K) and high (400–700 K) temperature show that the dynamics can be adequately described by established models of jump diffusion. At low temperature, ≤250 K, all of the models feature a characteristic jump distance of about 2.8 A, thus of the order of the distance between neighboring oxygen atoms or oxygen vacancies of the perovskite…

Titanium dioxide has attracted considerable attention as a potential alternative anode material in lithium-ion rechargeable batteries. In recent years, the incorporation of oxygen vacancy into such anode materials has been demonstrated to improve electrical conductivity, cycling stability, and rate performance through experimental studies. In this work, lithium intercalation and diffusion behavior in pristine and oxygen-defective TiO₂ were studied by first principles based on density functional theory calculations. Total energies of possible intercalation sites were first calculated to find the most favorable site in the three commonly used polymorphs: anatase, rutile, and TiO₂(B). Furthermore, the energy barriers of possible paths for lithium diffusion from one stable site to another have been calculated by the climbing image nudged elastic band method. The electronic structures are also presented to compare conductivities of pristine and oxygen-defective TiO₂. Our results indicate that although all three phases show enhanced conductivity via oxygen-defect introduction, TiO₂(B) is…

Herein, we demonstrate a facile method for the preparation of microporous organic nanotubes (MONTs) based on a combination of hyper-cross-linking mediated self-assembling strategy and Scholl reaction by using polylactide-b-polystyrene diblock copolymers (PLA-b-PS) as precursors, in which the PS block forms the microporous organic frameworks of nanotubes through the C–C coupling reaction, while the PLA segment is completely degraded to produce the hollow mesoporous tubular structure. Owing to their high special surface area, robust organic framework and open-ended tubular structure, the obtained MONTs exhibit the outstanding adsorption capacity and efficiency for p-cresol and different organic vapors. Moreover, microporous carbon nanotubes (MCNTs) can be prepared by further carbonization. The resultant MCNTs as electrode materials of a supercapacitor display excellent electrochemical performance with specific capacitances of up to 192 F g–¹ at 0.5 A g–¹, with capacitance retention of 97% even after 5000 cycles at 10 A g–¹. The methodology provides a new route for fabrication of…

The excellent physical and chemical properties of aluminum oxynitride, Al(O)N, along with the tunable luminescence features of cerium ions (Ce) hold a promising future in white light sources based on chip technology. Manipulation of the emission color from blue to green, to yellow, and finally to white is demonstrated for the first time in Ce-doped Al(O)N thin films by tuning of the local structure, electronic structure, and film thickness. Analysis of the electronic structure by electron energy loss spectroscopy at the Ce-M₄,₅, Al-L₂,₃, O-K, and N-K edges and photoluminescence spectroscopies highlights the essential role of oxygen and postdeposition annealing in changing the valence state of Ce ions from Ce⁴⁺ (optically inactive) to Ce³⁺ (optically active) as well as the local field around Ce³⁺ and, finally, in activating excitation pathways through generation of specific defect complexes in the oxynitride matrix.

Paramagnetic lanthanide-based NPs are currently designed as magnetic resonance imaging (MRI) contrast agents to obtain optimal relaxivities at high magnetic fields of 7, 9.4, and 11.7 T where human imaging has been possible yielding high contrast to noise ratio in the MR images compared to the clinical field of 3 T. However, the underlying longitudinal (T₁) and transverse (T₂) relaxation mechanisms of the NP-based contrast agents based on the spatial motion and proximity of water protons with respect to the paramagnetic ions on the surface of NPs are still not well understood, specifically, in terms of contributions from inner, second, and outer spheres of coordination of water molecules to the NPs. Gd³⁺-based NPs, e.g., NaGdF₄, are promising T₁ contrast agents owing to the paramagnetic Gd³⁺ possessing a symmetric ⁸S₇/₂-state and slow electronic relaxation relevant to its efficiency to produce a positive (T₁) contrast. Here, water-dispersed NaGdF₄:Eu³⁺ (3 nm diameter, TEM) and NaYF₄–NaGdF₄:Eu³⁺ core–shell NPs (18.3 nm core diameter with 0.5 nm thick shell, TEM) were studied for…

While emission-based, single-particle microscopies and spectroscopies have been highly successful in revealing the properties of matter hidden by ensemble averages, their limits have now become apparent. To address recognized future needs and, in particular, the need to go beyond fluorescent specimens, single-particle extinction techniques have been developed. Motivating this has been the desire to acquire information about the electronic structure of nanoscale materials difficult to obtain otherwise using either ensemble or emission-based, single-particle measurements. These techniques are, however, nontrivial since single nanostructures attenuate only 0.0001–1% of the incident light. This Review Article describes the challenges associated with overcoming the low signal-to-noise ratios inherent to low-dimensional semiconductor nanostructure extinction measurements. It simultaneously describes the fundamental operating principles and achievements of photothermal heterodyne imaging (PHI) and spatial modulation spectroscopy (SMS), two of the most popular approaches to measuring…

This article deals with the synthesis of broad band absorption material which extends absorption response up to the infrared region by using plasmonic metal nanoparticles and upconverting host. The synergistic effect of surface plasmon resonance of the Au nanoparticle and upconversion property of (Yb,Er)NYF significantly enhances broad band absorption and photogenerated charge generation and transfer. Au as a plasmonic nanoparticle on (Yb,Er)NYF-TiO₂ plays multiple role of harvesting solar energy and exhibiting excellent catalytic activity. The synthesized heterostructure (Yb,Er)NYF-TiO₂/Au tested for donor-assisted photocatalytic hydrogen generation shows a steady and remarkably high hydrogen generation rate of 350 μmol h–¹ under solar AM 1.5 which is approximately 3.1 times higher than Au/TiO₂ (Degussa P-25). Moreover, a simple synthesis procedure has been rationally designed (Yb,Er)NYF-TiO₂/Au which shall provide a pathway for futuristic design of plasmon-enhanced broad band absorption material.

Pt-supported TiO₂ catalysts have proved to be good catalysts for efficient hydrogen (H₂) production from photocatalytic water (H₂O) splitting and water–gas shift reactions. However, their origin for efficient H₂O conversion remains poorly understood. Here, we report a systematic study of low-temperature H₂ formation via H₂O conversion on Pt-deposited rutile-TiO₂(110) surfaces by means of spectroscopic and microscopic techniques in combination with first-principles calculations. We show that the low-temperature H₂ formation can occur facilely at ∼200 K via H₂O conversion on the Pt clusters/rutile-TiO₂(110) surfaces, which is initiated by H₂O dissociation at the surface defects, metal–oxide interfaces, and probably regular Ti⁴⁺ sites. More importantly, H₂O-assisted multistep H atom diffusion plays a crucial role for transferring H atoms toward Pt clusters, resulting in efficient H₂ evolution. These results enrich the understanding of H₂ formation via H₂O conversion on Pt/TiO₂ catalysts, which is helpful for understanding this reaction on other metal–oxide catalysts.

We present a combined differential scanning calorimetry (DSC) and infrared (IR), Raman, electron paramagnetic resonance (EPR), and NMR spectroscopy study of dimethylammonium zinc formate frameworks [(CH₃)₂NH₂][Zn(HCOO)₃] containing ¹⁴N and ¹⁵N nitrogen isotopes. The DSC reveals very small differences in the phase-transition temperatures for both compounds. The IR and Raman spectroscopies indicate different frequencies of the vibrational modes that involve a nitrogen atom. The temperature-dependent IR and Raman spectra also show a tiny isotope effect on the properties and temperature of the phase transition. The EPR measurements are performed for both compounds doped with small amount of paramagnetic Mn²⁺ ions. The obtained continuous-wave Mn²⁺ EPR spectra of the disordered phase are very similar for both frameworks, whereas the temperature dependence of the zero-field splitting parameter indicates a slightly different distortion of the MnO₆ octahedra in the low-temperature phase. The EPR line width analysis reveals that the activation energy of the (CH₃)₂NH₂⁺ cation hopping in the…

The electronic and vibrational properties of TiS₂, ZrS₂, and HfS₂ have been studied using dispersion-corrected hybrid density functional methods. The periodic trends in electronic band structures, electronic transport coefficients, IR and Raman spectra, and phonon dispersion relations were investigated. Comparison to the available experimental data shows that the applied DFT methodology is suitable for the investigation of the layered transition metal dichalcogenide materials with weak interlayer van der Waals interactions. The choice of damping function in the D3 dispersion correction proved to have a surprisingly large effect. Systematic investigation of the periodic trends within group 4 disulfides reveals that TiS₂ shows many differences to ZrS₂ and HfS₂ due to the more covalent M–S bonding in TiS₂. ZrS₂ and HfS₂ mainly show differences for properties where the atomic mass plays a role. All three compounds show similar Seebeck coefficients but clear differences in the relative electrical conductivity of cross- and in-plane directions. The transport and vibrational properties of…

We have grown epitaxial incommensurate α-Fe₂O₃ thin films on α-Al₂O₃(0001), SrTiO₃(111), and LaAlO₃(001) substrates, identifying hematite as single iron oxide phase stabilized. We demonstrate that a different lattice coupling behavior as a function of the selected oxide substrate mediates the epitaxial character. Single-oriented α-Fe₂O₃(0001) layers are obtained on α-Al₂O₃(0001) and SrTiO₃(111) substrates, whereas on the LaAlO₃(001) substrate, the hematite layer is found to grow along the r-plane to adapt its hexagonal lattice on the cubic lattice of the substrate, evidencing a single-oriented (1102) layer. In the film plane, crystallographic axes of α-Fe₂O₃(0001) are collinear with the α-Al₂O₃(0001) ones, while a rotation of 30° is found between those of α-Fe₂O₃(0001) and SrTiO₃(111). On LaAlO₃(001), α-Fe₂O₃(1102) adopts an in-plane orthorhombic structure rotated 45° respect to the substrate lattice. The crystallographic domain size and the crystalline order are dependent on the incommensurate lattice coupling mechanism. Larger values are obtained for layers grown on α-Al₂O₃(0001),…

ZnO is a member of a small class of semiconductors that includes In₂O₃, SnO₂, CdO, and InN, whose surfaces are highly unusual because their electronic bands bend downward to form a quantized potential well in which a two-dimensional electron gas is confined. At the O-polar ZnO(0001) surface, this effect arises from the adsorption of H adatoms which produces a hydroxyl-terminated surface. In this work, we investigate the effect of covalently anchored organic layers on the band bending at ZnO(0001) surfaces. We use aryldiazonium salt electrochemistry to deposit 4–5 nm thick layers of 4-nitrophenyl (NP) and 4-(trifluoromethyl)phenyl (TFMP) groups. Synchrotron X-ray photoelectron spectroscopy (XPS) showed that both NP and TFMP modifications permanently removed the downward band bending at the ZnO surface. This behavior can be explained by the direct covalent bonding of the aryl groups to the surface, also revealed by XPS analysis, and the electron-withdrawing character of both modifiers. Surprisingly, the in situ irradiation-induced reduction of the grafted NP groups to aminophenyl-like…

The understanding of ultrafast carrier relaxation process in doped semiconductor quantum dots (QDs) is very important for their potential applications in light-emitting diodes, optoelectronics. Here, we have studied the change in electronic properties of Cu-doped CdSe QDs upon light illumination. The light-induced effect leads to the enhancement of the band edge decay time and reduces the decay time of the dopant emission due to photocorrosion of Cu-doped CdSe QDs. The bleaching recovery kinetics and the hot electron cooling dynamics have been studied by using femtosecond transient absorption spectroscopy. It is observed that the electron cooling process of doped CdSe QDs is dependent on the dopant concentration and the cooling kinetics of doped CdSe QDs are found to be slower than undoped QDs. After light irradiation, the cooling processes of hot electron and recovery process in doped systems are modified.

Charge carrier trapping by the surface defects of colloidal semiconductor nanocrystal (NC) is a ubiquitous process which limits the performance of NC-based photovoltaic and photocatalytic devices. Although several empirical approaches led to the enhancement of device efficiency via passivation of trap states, a systematic and unified description of trapping mechanism remains obscure. In this contribution, we present a detailed experimental investigation of the carrier dynamics of CdSe NCs with varying concentration of surface traps by means of time-resolved photoluminescence (PL) and absorption. Our study reveals that the rate of carrier cooling becomes faster as trap density increases because of the increased hot carrier trapping. A comparative dynamical study is also presented to demonstrate how trap states influence electron injection process. This enhanced understanding of the role of trap sates on charge carrier dynamics can provide valuable insight toward the rational development of more efficient NC-based optoelectronic devices.

Heterogeneous thermocatalysis is widely used for the removal of VOCs. However, it is energy-consuming because it must operate at high temperatures. Combining thermocatalysis with photocatalysis makes it possible for the efficient abatement of VOCs at low temperatures. In this work, Pd–Ag@CeO₂ photothermal catalyst was synthesized via synthesizing ceria-coated Ag nanoparticles followed by loading Pd nanoparticles by the liquid-phase reduction method. The catalyst has a core–shell structure which induces strong electron interactions between Pd/Ag and CeO₂ and significantly improves the light harvest, charge separation and dissociative adsorption of Pd–Ag@CeO₂ in the visible region. The Pd–Ag@CeO₂ catalyst has a good catalytic performance in toluene oxidation, showing the T₅₀ of Pd–Ag@CeO₂ at only 88 °C under visible light irradiation. Finally, the possible catalytic mechanism has also been proposed for the oxidation of toluene under visible light irradiation.

In this study, metal organic frameworks (MOFs) are prepared as a support to tether active selenium through melt-diffused method, in which MnO₂ nanoparticles are loaded on the surface of a Co-MOF (ZIF-67) through simple postmodification means. Selenium is uniformly dispersed on the surface of ZIF-67, and MnO₂ nanoparticles (∼5 nm in diameter) are coated on the surface of Se. As an analogue of sandwich, the MOF@Se@metal-oxide structure can inhibit the shuttle effect and dissolution of polyselenides in the electrolyte, which is confirmed by the cyclic voltammetry, galvanostatic discharge/charge, and electrochemical impedance tests. In addition, the cathode displays an excellent electrochemical performance. Especially, the discharge capacity of the ZIF-67@Se@MnO₂ cathode is 329 mA h g–¹ after 100 cycles at 1C (C = 675 mA g–¹) with the high capacity retention of 83.7%. Furthermore, the ZIF-67@Se@MnO₂ cathode displays stable capacities of 273 and 232 mA h g–¹ after 100 cycles at 2C and 5C, respectively.

Ligands are known to passivate the surface of semiconductor nanocrystals and consequently alter their Fermi energy and band positions. In this work, we have utilized scanning tunneling spectroscopy (STS) to record differential tunnel conductance spectra (dI/dV) of PbS nanocrystals; since dI/dV has a correspondence to the semiconductors’ density of states, their band edges with respect to the Fermi energy could be located directly from the dI/dV spectra. With a series of ligands in PbS quantum dots (QDs), ligand-dependent shift in Fermi energy has been observed. From the location of Fermi energy relative to the conduction and valence band edges, the STS measurements allowed determination of the semiconductor type, which could be correlated to the functional groups of the ligands. While thiol-based ligands introduced excess sulfur, resulting in lead-vacancies and thereby p-nature in PbS QDs, other ligands yielded n-type QDs due to their electron-donating nature. The p- and n-type nanocrystals, when cast sequentially to form pn- or np-homojunctions, resulted in current rectification…

The structural phase behavior of high-quality single crystals of methylammonium lead iodide (CH₃NH₃PbI₃ or MAPbI₃) was revisited by combining Raman scattering and photoluminescence (PL) measurements under high hydrostatic pressure up to ca. 10 GPa. The single crystals were specially grown with the final thickness needed for pressure experiments, retaining their high quality due to a less invasive preparation procedure, which avoids sample thinning. Both PL and Raman spectra show simultaneous changes in their profiles that indicate the occurrence of three phase transitions subsequently at around 0.4, 2.7, and 3.3 GPa. At the second phase transition, the Raman spectra exhibit a pronounced reduction in the line width of the phonon modes of the inorganic cage, similar to the changes observed at the tetragonal-to-orthorhombic phase transition occurring at around 160 K but ambient pressure. This behavior is interpreted as evidence for the locking of the organic cations in the cage voids above 2.7 GPa due to the reduced volume and symmetry of the unit cell. At the third phase transition,…

The studies on Pt- or Pt alloy-based catalysts for ethanol oxidation have been focused mostly on their activity, with much less emphasis on their stability. Dissolution of commercial Pt–SnO₂/C electrocatalyst for direct ethanol fuel cells was investigated on the atomic scale by employing two advanced electrochemical characterization techniques: (i) identical location scanning transmission electron microscopy (IL-STEM) and (ii) electrochemical flow cell connected to inductively coupled mass spectrometry (EFC–ICP–MS). IL-STEM provides electrochemically induced atomic scale insights into morphological and structural changes of Pt–SnO₂/C Janus type nanoparticles, and the second methodological configuration enables potential- and time-resolved dissolution monitoring of individual metal counterparts with extremely high sensitivity, even in the presence of ethanol. We observe that Sn is mostly dissolving in the anodic potential ramp, which is, as a rule, not affected by the presence of ethanol. Surprisingly, Pt dissolution gets dramatically enhanced in the presence of ethanol with the…

In this paper, the semisolid redox flow battery (SSRFB) in static mode is investigated through a kinetic Monte Carlo-based mechanistic model. Electrochemical reactions are said to occur in particle suspensions composed of carbon as conductive additive and silicon as active material, where silicon is known to undergo volume expansion on discharge. The coexistence of different physical phenomena in suspension is not trivial and leads to complex behavior. This work attempts to quantify physical complexity through parameter sensitivity analysis and some basic tools of graph theory. The systematic treatment employed herein not only expands the utility of mechanistic models, but also provides a more comprehensive theoretical understanding of these complex systems which can otherwise only be treated as black boxes. It is concluded that the primary source of complexity of the SSRFB is the competition between multiple phenomena and that quantifying the dynamics between parameters is as important as measuring a specific parameter. A systematic method of studying the dynamics is to…

Origin of mixed valence states from π-conjugated photoresponsive systems offers self-assembled nanostructures facilitating excellent optoelectronic properties. Despite various electron-rich moieties, tethered linear π-conjugated systems generate charged ions however limited to standard redox-active materials until now, which further extended to macrocyclic systems. Herein, we report three carbazole and triphenylamine-assisted phthalocyanine derivatives (1, 2, and 3) and discussed the nature of photobleaching/photoionization by illumination with an excitation wavelength of 350 nm. Detailed UV–visible absorption and cyclic voltammetry studies revealed that three derivatives are bleached during irradiation; however, 3 exhibits mixed valence species in CHCl₃ when compared to 1 and 2. Microscopic analysis suggested that ionized 3 forms nanospheres with an average diameter of 100–400 nm upon methanol vapor diffusion approach. Thus, photoresponsive mixed valence systems facilitated well-defined nanostructures, thereby advantageous in various electronic and biological applications.

Gel formation is described by a nonequilibrium self-assembly (SA) mechanism which considers the presence of precursors. Assuming that nonequilibrium structures appear and are maintained by entropy production, we developed a mesoscopic nonequilibrium thermodynamic model that describes the dynamic assembly of the structures. In the model, the evolution of the structures from the initially inactivated building blocks to the final agglomerates is governed by kinetic equations of the Fokker–Planck type. From these equations, we get the probability densities which enable one to know the measurable quantities such as the concentrations of the different components and the dynamic structure factor obtained in light-scattering experiments. Our results obtained are in very good agreement with the experiments. The model proposed can in general be used to analyze the kinetics of formation of nonequilibrium SA structures usually found in biomedicine and advanced materials.

Recent density functional theory (DFT) calculations have demonstrated the potential of mechanically bent graphene as a piezoelectric energy harvesting material. We develop a two-dimensional (2D) model of hexagonal materials and demonstrate quantitatively the potential of a single layer of graphene to function as an effective piezoelectric nanogenerator. The piezoelectricity of graphene stems from a dynamically generated surface charge density proportional to the local curvature of the graphene layer, and the proportionality constant is found from DFT calculations on a single layer of bent graphene. By virtue of different tailored mechanical and electrical loadings, explored in this work, it is demonstrated that graphene can be as effective an energy harvester as a single flat layer of 2D MoS₂, which is strongly piezoelectric due to its inversion-asymmetric unit cell. Demonstrations are carried out for graphene and MoS₂ using a 2D finite element model to determine the generated voltage, current, and power density.

The synthesis of bimetallic nanoparticles in microemulsions has been studied by simulation. The aim of this study was to understand the principles underlying kinetics in colloidal systems, which allowed us to further investigate the interaction between the intermicellar exchange rate and chemical reduction and ultimately to discover how these nanostructures may become. We present here results for three couples of metals with different reduction rates, the reduction rate of the faster metal being 10 times faster than the rate of the slower one in all cases. Under these conditions, the impact of slowing down chemical reductions on the final nanoarrangement can be studied. Our results show that a bimetallic nanostructure can be tuned by changing the microemulsion composition only when both metal reduction rates are quite fast in relation to the intermicellar exchange rate. If this is not the case, the compartmentalization of the reaction medium does not play any role because of chemically controlled kinetics, and the result is a fixed bimetallic arrangement that depends only on the two…

The structural and dynamical properties of water confined in highly ordered mesoporous silica (SBA-15) in the presence of ions having various kosmotropic properties (XCl₂ with X = Ba²⁺, Ca²⁺, and Mg²⁺) were characterized using thermogravimetric analysis, infrared spectroscopy, and quasi-elastic neutron scattering and were related to the surface density of ions obtained by sorption isotherms. The results highlight that the water network is perturbed by the confinement and that the dynamics of water molecules at a picosecond time scale is slowed by the presence of ions. These results are mainly explained by the presence of the interfacial layer where both the structure of the water network and the water dynamics are influenced by the surface ion excess and the kosmotropic properties of ions.

The use of surface organometallic chemistry on metal (SOMC/M) allows the controlled and stepwise variation of the platinum particle size in Pt/SiO₂ catalysts. This SOMC/M method is possible thanks to the better affinity of most organometallic compounds with the surface of zerovalent metal particles covered with hydrogen than their support. In this paper, Pt(acac)₂ was used as the organometallic precursor, silica as a support, and then hydrogen to reduce the adsorbed organometallic layer on top of the starting Pt nanoparticle. We partially succeeded in adding one Pt layer with a stepwise particle size increase of around 0.6 nm when going from the first (1G) to the second (2G) refilling run, as obtained from TEM and H₂ chemisorption analysis and then confirmed by DFT calculations. The metal loading could be kept at a very low level (<1–2 wt %), which is relevant for catalytic applications. The particle size distribution remained relatively narrow even after two refilling runs, allowing more precise relationships between particle size and catalytic properties to be established. The…

We examine the morphology of a donor molecule:fullerene organic photovoltaic (OPV) active layer, processed from the “green solvent” 2-methyl-THF (m-THF), as well as the conformation of the two OPV constituent components in the casting solution. We observe that the small molecule (X2) has a weak association with itself in chloroform solvent, whereas it does not self-associate in m-THF. Despite this difference, the morphologies of the final processed films are extraordinarily similar: there is negligible molecularly mixed phase in the layer, and the domain sizes of the pure X2 and pure fullerene are 15–20 nm. We attribute this similarity between the final films to the strong aggregation behavior of X2 upon drying; changes in solvent or solvent additive have therefore only a minor effect on the final bulk heterojunction morphology. This contrasts with the majority of other OPV molecular donor semiconductor systems, which need careful tuning of solvent and/or solvent additive to achieve the optimal morphology and photovoltaic performance. We argue that the absence of a mixed phase is a…

Orientational ordering within nanoscale (70–8 nm) thickness fluorinated ionomer films on Si substrates was investigated through the use of attenuated total reflection Fourier transform infrared (ATR–FTIR) spectroscopy in conjunction with electromagnetic field calculations. A spectral model was developed for Nafion thin films across the 1400–950 cm–¹ region from frequency-dependent, isotropic optical constants derived from Kramers–Kronig analysis of ionomer transmission infrared spectra. The model considered infrared light propagation within the parallel boundary regions between the Ge ATR crystal, the ionomer film, and the Si substrate supporting the film. The calculations reproduced overall polymer thickness-dependent changes in peak frequencies and band shapes observed in experimental spectra recorded with p- and s-polarized light. General trends were traceable to effects of anomalous dispersion and electric field enhancement within the nanoscale gap separating the Ge and Si phases. However, optical effects could not fully explain perturbations in spectra of the thinnest films,…

We discuss the structural, electronic, and vibrational properties of an adsorbed CO monolayer in the presence of liquid water at room temperature, as obtained by ab initio molecular dynamics simulations. The water molecules at closest distance from the substrate form a bilayer, where they are coadsorbed at empty surface sites and with relatively rigid orientation, at the difference with the bulk liquid above. Coadsorbed water strengthens the back-bonding to CO, which leads to the red shift of about 40 cm–¹. The synergy between the first bilayer and the bulk liquid induces further polarization of the CO bond, which changes the transition dipole and results in the CO intensity enhancement. It is also verified that no hydrogen bond is effectively present between CO and bulk water. These findings provide new insight into the physics of the surface/adsorbate/solvent interface, thus clarifying the experimental observation to be exploited for the design of improved catalysts.

We report on a new synthesis process for halide perovskite nanoplatelets and nanoplates that switches the production process of the cesium precursor from a fatty acid/cesium salt reaction to a cesium base/fatty acid ester reaction, thus enabling the reaction to occur in ambient conditions in minutes instead of hours. The saponification precipitation process reported here, as a result, does not require a vacuum oven or inert reaction environment in obtaining the cesium precursor, or any part of the reaction. Furthermore, the process creates a hygroscopic byproduct that results in a self-drying synthesis. The obtained perovskite nanocrystals exhibit a blue-green tunable emission that occurs via a quantum confinement effect, phase, and morphology change. The consequence of these physical processes is that the band gap is highly tunable with temperature and the resulting nanocrystals show remarkable optical properties, while greatly simplifying the production of halide perovskite nanoplatelets and nanoplates.

HfSe₂ is a very good candidate for a transition metal dichalcogenide-based field-effect transistor owing to its moderate band gap of about 1 eV and its high-κ dielectric native oxide. Unfortunately, the experimentally determined charge carrier mobility is about 3 orders of magnitude lower than the theoretically predicted value. This strong deviation calls for a detailed investigation of the physical and electronic properties of HfSe₂. Here, we have studied the structure, density, and density of states of several types of defects that are abundant on the HfSe₂ surface using scanning tunneling microscopy and spectroscopy. Compared to MoS₂ and WSe₂, HfSe₂ exhibits similar type of defects, albeit with a substantially higher density of 9 × 10¹¹ cm–². The most abundant defect is a subsurface defect, which shows up as a dim feature in scanning tunneling microscopy images. These dim dark defects have a substantially larger band gap (1.25 eV) than the pristine surface (1 eV), suggesting a substitution of the Hf atom by another atom. The high density of defects on the HfSe₂ surface leads to…

In situ X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and scanning tunneling microscopy (STM) were used to study the reduction of potassium-modified Cu₂O/Cu(111) by CO. By following the time evolution of the O 1s peak of Cu₂O, we determined that the apparent activation energy for Cu₂O reduction by 2 × 10–⁴ Torr CO is decreased by ∼30% in the presence of K. On the K-modified surface, both XPS and IRRAS data show the formation of a surface species identified by IRRAS as carbonate (CO₃²–), likely forming a K⁺-CO₃²– complex, which is stable up to 500 K. STM images show that K⁺-CO₃²– complexes form chains around reduced Cu islands, thereby hindering the mass transfer of Cu atoms and preventing the reconstruction of the surface. Theoretical calculations show that the formation of carbonate on the K-modified “44” Cu₂O structure is thermodynamically favorable compared to the formation of CO₂ on either the bare or K-modified surfaces.

Olivine-structured LiMPO₄ materials (M = Mn, Fe, Co, Ni, or mixtures) exhibit higher redox potentials than their layer oxide counterparts. This is due to the so-called inductive effect in the former, where the inner P–O bonds in the phosphate units make the M–O bond weaker than in the latter. A strategy to further increase the redox potentials in the olivines is to mix two metals. Along these lines, Kobayashi et al. have shown experimentally that Mn²⁺–Mn³⁺ and Fe²⁺–Fe³⁺ redox potentials approximately shift 0.1 V upon full substitution of Fe by Mn in LiₓMnyFe₁–yPO₄. Here, through density functional theory calculations, we found that the average metal–oxygen bond lengths (M = Mn, Fe) increase with increasing Mn content, resulting in a decrease in the covalency of the transition-metal–oxygen interaction. The decrease in the covalency can be linked with good qualitative agreement to the experimentally observed M²⁺–M³⁺ voltage-plateau positive shift. Finally, the impact of the Mn-content-dependent voltage plateaus and unit-cell volume on the energy densities of the active compound is…

We developed the ReaxFF force field parameters for Ge/O/H interactions, specifically targeted for the applications of Ge/GeO₂ interfaces and O-diffusion in bulk Ge. The original training set, taken from the Zheng et al. work, includes quantum mechanics (QM) data for equations of state and heats of formation of GeO and GeO₂ condensed phases as well as dissociation energies for single and double bonds of Ge and angle distortion of O–Ge–O. We expanded this training set with the additional crystal data containing the formation energies of different O-interstitial centers and the minimum energy migration pathway of O atoms in diamond Ge. After refitting the force field parameters based upon the extended training set, the ReaxFF results show that the equations of state and heats of formation of the GeO and GeO₂ condensed phases retain a good fit with the QM calculations. In addition, the ReaxFF correctly predicts the relative stability of the O-interstitial centers in the diamond Ge to be bond-center → split → tetrahedral → hexagonal from most stable to least stable with the energies…

The present study is a part of an experimental microkinetic approach (EMA) of the selective reduction of NOₓ to N₂ with NH₃ in excess O₂ on V₂O₅/WO₃/TiO₂ catalysts (NH₃-SCR process). In the temperature range of interest for NH₃-SCR (T ≥≈ 473 K) and for three TiO₂-based solids (sulfated and sulfate-free TiO₂ supports and a sulfated 0.7% V₂O₅/9% WO₃/TiO₂ catalyst), FTIR spectroscopy and volumetric measurements with a mass spectrometer are used to study the impacts of the NH₃–H₂O coadsorption on the coverages of (a) the molecularly adsorbed NH₃ species and (b) the molecularly and dissociated H₂O species on Lewis and Bronsted sites. Whatever the solid, it is shown that NH₃ dominates the molecular coadsorption on the Lewis sites. However, this does not prevent the dissociative H₂O chemisorption on a small amount of Lewis acidic sites, leading to an increase in the amount of OH groups. On the two sulfated solids, these OH groups increase the amount of adsorbed NH₄⁺ species as compared to the NH₃ adsorption equilibrium. For the sulfate-free TiO₂ solid having weak Bronsted sites, the switch…

A coverage-dependent growth of well-ordered copper halogenide structures as a result of fluorinated fullerene molecule adsorption on Cu(001) surface has been studied by means of scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) measurements. The experimental results clearly demonstrate that C₆₀Fₙ molecule on Cu(001) surface loses the fluorine atoms step by step. The gradual detachment of fluorine atoms leads originally to occurrence of two-dimensional gas phase on Cu(001) surface from which F-induced structures start to nucleate over time. The dynamics of further growth of F-induced structures directly depends on an initial coverage of Cu(001) surface by C₆₀Fₙ molecules. XPS and STM studies revealed that a stable cooper halogenide phase (2√2 × √2)-R45° is formed if fluorinated fullerene molecules are placed on Cu(001) surface for a time period of approximately a 100 h. It has been shown that a variation of C₆₀Fₙ initial coverage allows providing a controllable delivery of fluorine atoms to the surface.

Niobium pentoxide (Nb₂O₅), also known as niobia, has been applied in several areas among others in heterogeneous catalysis. This is due to both its high acidity (Bronsted acid and Lewis acid sites) and its Lewis acid sites tolerant to water. The structure and morphology of these sites present tunable quantity and strength; however, little attention has been given to its polymorphic forms and reactivity. In this work, the surface properties of stoichiometric B phase (B-Nb₂O₅), including the cleavage surfaces, structural, energetic, and electronic properties, and chemical reactivity toward water (H₂O) and hydrogen peroxide (H₂O₂), by means of periodic density functional theory (DFT), have been studied through DFT calculations. An initial investigation was carried out to determine cleavage surface of the B-Nb₂O₅. Our results show that the B-Nb₂O₅ (010)-2 surface is the most stable (surface energy 0.52 J m–²) of the surfaces studied. Projected density of state (PDOS) analysis showed that the niobium atom is a Lewis acid site. When H₂O was adsorbed on the (010)-2 surface, the molecular…

This paper deals with a study of the mechanism of hydrodeoxygenation (HDO) at 0.1 MPa and 300–350 °C of 2-methyltetrahydrofuran (2-MTHF) over a Ni₂P/SiO₂ catalyst. The study was conducted using in situ Fourier transform infrared (FTIR) spectroscopy to monitor the adsorbed species during reaction and in situ X-ray absorption fine structure (XAFS) to probe the oxidation state of the Ni component. The work is relevant to the upgrading of bio-oil derived from the pyrolysis of biomass. It was deduced that the initial interaction of the 2-MTHF was with OH groups on the catalyst surface, followed by adsorption on Ni sites. Ring-opening of the tetrahydrofuran led to the formation of the main products of the reaction, n-pentane by HDO and n-butane + CO by decarbonylation. The CO band appeared with a slight delay in the FTIR spectrum because the reaction to form n-pentane was favored and turned over about four times before one n-butane + CO was formed.