Showing papers in "The Journal of Physical Chemistry in 2019"

Theoretical Screening of Single-Atom-Embedded MoSSe Nanosheets for Electrocatalytic N₂ Fixation

Abstract: The NH₃ synthesis from N₂ electrochemical reduction under mild conditions is a great challenge. In this study, we investigated a series of transition-metal-embedded MoSSe nanosheets with S or Se vacancy based on the density functional theory calculations. Our results revealed that Mo-embedded MoSSe with S vacancy exhibits the best catalytic performance following the alternating mechanism. The potential-limiting step is the first hydrogenation of the adsorbed N₂ with a barrier of 0.49 eV. The relatively low ΔG of 0.58 eV for NH₃ desorption allows the rapid removal of the products from the catalyst. The high stability of the adsorbed N₂H species implies the great catalytic activity of the Mo-embedded MoSSe. By comparing the adsorption strength between single H atom and N₂ molecule on the active site, the Mo-doped MoSSe possesses a much higher selectivity on nitrogen reduction reaction. Our research may provide insightful ideas for the efficient electrochemical reduction of N₂ to NH₃ in ambient conditions.

Spectroscopic Fingerprints of Graphitic, Pyrrolic, Pyridinic, and Chemisorbed Nitrogen in N-Doped Graphene

Abstract: Doping and functionalization of graphene significantly modulate its properties and extend its application potential. Detailed and accurate chemical characterization of the final material is critical for understanding its properties and reliable design of new graphene derivatives. Spectroscopic methods commonly used for this purpose include Raman, Fourier transform infrared (IR), and X-ray photoelectron spectroscopy (XPS). However, the correct interpretation of observed bands is sometimes hampered by ambiguities when assigning measured binding energies or IR/Raman peaks to specific atomic structures. N-doped graphene has many potential applications but can contain several different chemical forms of nitrogen whose relative abundance strongly affects the doped material’s properties. We present clear spectroscopic fingerprints of the various chemical forms of nitrogen that can occur in N-doped/functionalized graphene to facilitate the identification and quantification of the different forms of N present in experimentally prepared samples. The calculated XPS binding energies of the N 1s state for graphitic, pyrrolic, pyridinic, and chemisorbed nitrogen in N-doped graphene are 401.5, 399.7, 397.9, and 396.6 eV, respectively, and hydrogenation of pyridinic N shifts its peak to 400.5 eV.

Optical Properties and Photocatalytic Applications of Two-Dimensional Janus Group-III Monochalcogenides

Abstract: Photocatalytic water splitting has received much attention for the production of renewable hydrogen from water, and two-dimensional (2D) materials show great potential for use as efficient photocatalysts. In this paper, the stabilities and electronic and optical properties of Janus group-III monochalcogenide M₂XY (M = Ga and In and X/Y = S, Se, and Te) monolayers were investigated using first-principles calculations. The band gaps of the Janus M₂XY monolayers are in the range of 1.54–2.98 eV, which satisfies the minimum band gap requirement of photocatalysts for overall water splitting. Indirect-to-direct band gap transitions occur in the M₂XTe (M = Ga and In and X = S and Se) monolayers. These transitions were induced by the valence band maximum at the Γ point, being composed of the pₓ and py orbitals of the M and Y atoms in M₂XTe instead of the pz orbitals of the M and X atoms in the MX and other M₂XY monolayers. The Janus M₂XY monolayers have a considerable optical absorption coefficient (∼3 × 10⁴/cm) in the visible light region and an even larger absorption coefficient (∼10⁵/cm) in the near ultraviolet region. This study not only highlights the efficient photocatalytic performance of the 2D MX and M₂XY monolayers but also provides an approach for tuning the band structures of 2D photocatalysts by forming Janus structures.

GeSe/BP van der Waals Heterostructures as Promising Anode Materials for Potassium-Ion Batteries

Abstract: Potassium-ion batteries have attracted attention because of their abundant resources and similar electrochemistry to Li-ion batteries (LIBs). In the present work, GeSe/black phosphorus (BP) heterostructures as promising anode materials for K-ion batteries (KIBs) have been systematically investigated by first-principles calculations. The results reveal that GeSe/BP exhibits a semiconductor-to-metal transition after incorporating K atoms, indicating enhanced conductivity compared to monolayer GeSe. The energy barrier for K atom diffusion on GeSe/BP surface is relatively lower than that on monolayer GeSe. In addition, the GeSe/BP heterostructure can accommodate up to five layers of K atoms with negative adsorption energy, which greatly improves the storage capacity. Hence, the GeSe/BP heterostructure has great potential for application in advanced electrode materials in KIBs.

Persistent Dimer Emission in Thermally Activated Delayed Fluorescence Materials

Abstract: We expose significant changes in the emission color of carbazole-based thermally activated delayed fluorescence (TADF) emitters that arise from the presence of persistent dimer states in thin films and organic light-emitting diodes (OLEDs). Direct photoexcitation of this dimer state in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) reveals the significant influence of dimer species on the color purity of its photoluminescence and electroluminescence. The dimer species is sensitive to the sample preparation method, and its enduring presence contributes to the widely reported concentration-mediated red shift in the photoluminescence and electroluminescence of evaporated thin films. This discovery has implications on the usability of these, and similar, molecules for OLEDs and explains disparate electroluminescence spectra presented in the literature for these compounds. The dimerization-controlled changes observed in the TADF process and photoluminescence efficiency mean that careful consideration of dimer states is imperative in the design of future TADF emitters and the interpretation of previously reported studies of carbazole-based TADF materials.

Machine Learning Classical Interatomic Potentials for Molecular Dynamics from First-Principles Training Data

Abstract: The ever-increasing power of modern supercomputers, along with the availability of highly scalable atomistic simulation codes, has begun to revolutionize predictive modeling of materials. In particular, molecular dynamics (MD) has led to breakthrough advances in diverse fields, including tribology, catalysis, sensing, and nanoparticle self-assembly. Furthermore, recent integration of MD simulations with X-ray characterization has demonstrated promise in real-time 3-D characterization of materials on the atomic scale. The popularity of MD is driven by its applicability at disparate length/time scales, ranging from ab initio MD (hundreds of atoms and tens of picoseconds) to all-atom classical MD (millions of atoms over microseconds), and coarse-grained (CG) models (micrometers and tens of microseconds). Nevertheless, a substantial gap persists between AIMD, which is highly accurate but restricted to extremely small sizes, and those based on classical force fields (atomistic and CG) with limited accuracy but access to larger length/time scales. The accuracy and predictive power of classical MD simulations is dictated by the empirical force fields, and their capability to capture the relevant physics. Here, we discuss some of our recent work on the use of machine learning (ML) to combine the accuracy and flexibility of electronic structure calculations with the speed of classical potentials. Our ML framework attempts to bridge the significant gulf that exists between the handful of research groups that develop new interatomic potential models (often requiring several years of effort), and the increasingly large user community from academia and industry that applies these models. Our data-driven approach represents significant departure from the status quo and involves several steps including generation and manipulation of extensive training data sets through electronic structure calculations, defining novel potential functional forms, employing state-of-the-art ML algorithms to formulate highly optimized training procedures, and subsequently developing user-friendly workflow tools integrating these algorithms on high-performance computers (HPCs). Our ML approach shows marked success in developing force fields for a wide range of materials from metals, oxides, nitrides, and heterointerfaces to two-dimensional (2D) materials.

Impact of Preparation Method and Hydrothermal Aging on Particle Size Distribution of Pt/γ-Al₂O₃ and Its Performance in CO and NO Oxidation

Abstract: The influence of the preparation method and the corresponding particle size distribution on the hydrothermal deactivation behavior at 600–800 °C and performance during CO/NO oxidation was systematically investigated for a series of Pt/Al₂O₃ catalysts. Representative conventional (incipient wetness impregnation) and advanced preparation methods (flame spray pyrolysis, supercritical fluid reactive deposition, and laser ablation in liquid) were selected, which generated samples containing narrow and homogeneous but also heterogeneous particle size distributions. Basic characterization was conducted by inductively coupled plasma-optical emission spectrometry, N₂ physisorption, and X-ray diffraction. The particle size distribution and the corresponding oxidation state were analyzed using transmission electron microscopy and X-ray absorption spectroscopy. The systematic study shows that oxidized Pt nanoparticles smaller than 2 nm sinter very fast, already at 600 °C, but potential chlorine traces from the catalyst precursor seem to stabilize Pt nanoparticles against further sintering and consequently maintain the catalytic performance. Samples prepared by flame spray pyrolysis and laser ablation showed a superior hydrothermal resistance of the alumina support, although, due to small interparticle distance in case of laser synthesized particles, the particle size distribution increases considerably at high temperatures. Significant deceleration of the noble metal sintering process was obtained for the catalysts containing homogeneously distributed but slightly larger Pt nanoparticles (supercritical fluid reactive deposition) or for particles deposited on a thermally stable alumina support (flame spray pyrolysis). The correlations obtained between Pt particle size distribution, oxidation state, and catalytic performance indicate different trends for CO and NO oxidation reactions, in line with their structure sensitivity.

Molecular Dynamics Simulations of the Adsorption of Phthalate Esters on Smectite Clay Surfaces

Abstract: The partitioning of organic contaminants between water and solid surfaces is a key process controlling their fate and transport in natural environments. A novel methodology was developed to predict the adsorption of organic contaminants by smectite clay minerals (high specific surface area adsorbents abundant in natural soils) using molecular dynamics (MD) simulations. The methodology models a stack of flexible Ca-montmorillonite lamellae in direct contact with a bulk aqueous reservoir and uses the metadynamics technique to facilitate the exploration of the free energy landscape. The methodology was tested and validated in the case of six phthalate esters, widely used chemical plasticizers with endocrine disrupting properties. Simulation predictions reveal strong phthalate adsorption, especially for the larger and more hydrophobic phthalates. Predicted partition coefficients (Kd values) are consistent with collected batch experimental data. Adsorption was observed on both the exterior basal surfaces and within the interlayer nanopore, with phthalate molecules predominately adopting a flat orientation on the clay surface. Intercalation was also detected in complementary X-ray diffraction (XRD) experiments. A strong inverse relationship between extent of adsorption and clay surface charge density was observed, as phthalate molecules preferentially occupied the more hydrophobic uncharged patches on each surface. Detailed analysis of the free energy of adsorption revealed that phthalate affinity for the clay surface results from a small favorable van der Waals contribution and a large favorable entropic contribution. Overall, this research demonstrates the substantial affinity of smectite clays for phthalate esters and establishes a computational methodology capable of predicting the water–clay partition coefficients of organic contaminants, a key parameter in environmental fate and transport models.

Theory of Impedance Spectroscopy for Lithium Batteries

Abstract: In this article, we derive and discuss a physics-based model for impedance spectroscopy of lithium batteries. Our model for electrochemical cells with planar electrodes takes into account the solid–electrolyte interphase (SEI) as a porous surface film. We present two improvements over standard impedance models. First, our model is based on a consistent description of lithium transport through electrolyte and the SEI. We use well-defined transport parameters, e.g., transference numbers, and consider convection of the center-of-mass. Second, we solve our model equations analytically and state the full transport parameter dependence of the impedance signals. Our consistent model results in an analytic expression for the cell impedance including bulk and surface processes. The impedance signals due to concentration polarizations highlight the importance of electrolyte convection in concentrated electrolytes. We simplify our expression for the complex impedance and compare it to common equivalent circuit models. Such simplified models are good approximations in concise parameter ranges. Finally, we compare our model with experiments of lithium metal electrodes and find large transference numbers for lithium ions. This analysis reveals that lithium-ion transport through the SEI has solid-electrolyte character.

Synthesis and Near-Infrared Emission of Yb-Doped Cs₂AgInCl₆ Double Perovskite Microcrystals and Nanocrystals

Abstract: Cs₂AgInCl₆ double perovskite (DP) is a direct band gap, stable, and environmentally benign material with a three-dimensional perovskite structure. However, the band gap (∼3.3 eV) is too wide for optoelectronic applications in the near-infrared (NIR) region. To achieve optical functionality in the NIR window, we have doped Yb³⁺ in both microcrystals and colloidal nanocrystals of Cs₂AgInCl₆ DPs. Our Yb-doped Cs₂AgInCl₆ DP microcrystals and nanocrystals show NIR emission at ∼994 nm (1.24 eV), whereas the band gap of the hosts is in the UV region. Therefore, the Yb-emission does not suffer from self-absorption. The microcrystals exhibit a single-exponential decay with 2.7 ms long lifetime corresponding to ²F₅/₂ → ²F₇/₂ transition of Yb³⁺ f electrons. The nanocrystals show biexponential decay (3 ms and 749 μs), probably because of the proximity of Yb to the surface of nanocrystals. Interestingly, NIR emission and perovskite structure of samples are stable for >3 months in ambient conditions for future optoelectronic applications.

Effect of a CdSe Layer on the Thermo- and Photochromic Properties of MoO3 Thin Films Deposited by Physical Vapor Deposition

Abstract: The influence of a cadmium selenide (CdSe) layer on the thermochromic and photochromic properties of molybdenum trioxide (MoO₃) thin films was studied. The films were deposited on glass substrates by thermal evaporation in vacuum using two different bilayer configurations, namely, substrate/MoO₃/CdSe (SMC) and substrate/CdSe/MoO₃ (SCM). The film thicknesses for the MoO₃ and CdSe layers were ca. 250 and 20 nm, respectively. The thermochromic effect was evaluated in the annealing temperature range from 25 to 225 °C, in the presence of air. The characteristic optical absorption band attributed to the color center formation, centered at 820 nm, indicated enhanced thermo- and photochromic effects for both bilayer systems relative to monolayer MoO₃ thin films. For the thermochromic effect, this improvement was more pronounced when CdSe was the upper layer, i.e., for the SMC system. Regarding the photochromic effect, the films were irradiated with UV light for several exposure times within the lapse of 30–180 min. While both bilayer systems presented better photochromic response than pure MoO₃ thin films, the SCM system exhibited better photochromic response. These results are explained in terms of the optical, structural, and surface chemistry properties of the films.

Near-Ultraviolet Dielectric Metasurfaces: from Surface-Enhanced Circular Dichroism Spectroscopy to Polarization-Preserving Mirrors

Abstract: Circular dichroism (CD) spectroscopy is an important technique to investigate the structural information about chiral molecules. The intrinsically weak chirality of molecules often requires long optical paths and high-concentrated molecules. Metasurfaces can generate localized strong chiral fields to enhance chiral light-molecule interactions near the surface. However, losses and structural chirality of metasurfaces can diminish the inherent CD of the chiral molecules and induce interfering spectral features that disturb the chiral analysis of the molecules. Herein, we realize achiral titanium dioxide metasurfaces to enhance the near-ultraviolet CD of chiral molecules. The device is completely lossless for wavelengths greater than 360 nm and produces intense chiral hotspots in the near-field. An 80-fold enhancement of optical chirality is numerically demonstrated, giving rise to a 50-fold enhancement of CD. In addition, we show that the metasurface can function as a polarization-preserving mirror and enhance optical chirality in the far-field. Our results provide a scheme to ultracompact CD spectrometers and chiral cavities.

Quantitative Understanding of the Sluggish Kinetics of Hydrogen Reactions in Alkaline Media Based on a Microscopic Hamiltonian Model for the Volmer Step

Abstract: The sluggish kinetics of hydrogen evolution/oxidation reactions in alkaline media remains a technical barrier for alkaline membrane fuel cells and a scientific puzzle under heated discussion in fundamental electrocatalysis. Much attention has been centered around thermodynamic origins, whereas microscopic kinetics is less understood and a quantitative account of key factors is yet missing. To fill in this gap, a microscopic Hamiltonian model is developed for the alkaline Volmer step, an elementary step of hydrogen reactions, encompassing electronic interactions, bond breaking, solvent reorganization, and double-layer electrostatic effects. The model gives out a simple yet informative analytical formula for the activation barrier of the alkaline Volmer step, quantifying the contributions of various factors; roughly speaking, one quarter of the H–OH bond energy enters into the activation energy. This model elucidates that the larger activation energy seen at a more charged interface is not because it is more difficult to reorganize the solvents but rather because it consumes more work in bringing OH– to the double layer, namely, a larger work term. Previous strategies used to boost the activity of hydrogen reactions in alkaline media are rationalized in a coherent framework.

Computational Screening for ORR Activity of 3d Transition Metal Based M@Pt Core–Shell Clusters

Abstract: Core–shell nanoparticles are widely recognized as potential catalysts for oxygen reduction reaction (ORR) occurring at the cathode of proton exchange membrane (PEM) fuel cells. A comprehensive analysis of ORR activity of low-cost core–shell nanoparticles is still lacking from previous screening studies. To address this, a complete series of 3d metal based platinum core–shell nanoclusters are designed and scrutinized for ORR activity as well as stability. The adsorption behavior of ORR intermediates is observed to highly depend on the core–shell combination. The analysis of ORR energetics along the associative pathway shows a nonuniform trend in free energy changes and rate-determining steps. As compared to earlier reports, we show that a single intermediate binding energy is not enough for interpreting the ORR activity trends. Ti, Ni, and Cu based core–shell clusters are observed to have elevated activity as compared to bare platinum nanocluster and periodic platinum (111) surface. The origin of activity differences is explained via structural, charge transfer, and electronic structure analyses.

Boron-Decorated Graphitic Carbon Nitride (g-C₃N₄): An Efficient Sensor for H₂S, SO₂, and NH₃ Capture

Abstract: We investigated the interactions between nonboron (g-C₃N₄) and boron-decorated (B-g-C₃N₄) graphitic carbon nitride complexes with H₂S, SO₂, and NH₃ molecules by first-principles calculations. Our results show a highly superior selectivity toward the H₂S gas compared to the SO₂ molecule. In addition, by increasing the concentration of nitrogen defects at the edges of g-C₃N₄ from 1.785 to 3.572%, we noticed a complete H₂S dissociation, with the two hydrogen atoms chemisorbed on the g-C₃N₄ planes while the sulfur atom (S) remained in the gaseous phase. However, the efficiency of the D site is altered by the gas–gas interaction where a partial dissociation of H₂S occurs. Furthermore, our results show that doping g-C₃N₄ with the B atom was very efficient to fix the S atom on the g-C₃N₄ substrate. Moreover, decorating the g-C₃N₄ edges with the B atom enhanced selectivity toward the H₂S, SO₂, and NH₃ gases as they strongly chemisorbed on the B-g-C₃N₄ complex. Furthermore, our results in the gas–gas interaction show the same trend as the earlier results reported for a single gas adsorption. These results predict that B-g-C₃N₄ may be a better sensor for H₂S, SO₂, and NH₃ compared to pure and nitrogen monovacancy defect g-C₃N₄.

Accelerated Discovery of New 8-Electron Half-Heusler Compounds as Promising Energy and Topological Quantum Materials

Abstract: Rapid discovery of potential functional materials remains an open challenge. We often focus on exploring the properties of previously reported compounds, but avoid various unreported but chemically plausible compounds that might have promising properties. Here, we present a high throughput ab initio study of the I-III-IV class of half-Heusler alloys with 8 valence electrons, in the quest of finding potential (i) thermoelectric, (ii) topological insulating, and (iii) optoelectronic materials. Of various classes, 8-electron half-Heusler compounds are least studied, and hence our choice. By carefully choosing reliable and accurately simulated descriptors (such as formation energy, phonon dispersion, accurate band gaps), we have discovered 21 semiconducting compounds. Out of these 21 compounds, 6 were found to show excellent thermoelectric performance (figure of merit ZT > 0.8); other range from ZT = 0.2 to 0.8. Seventeen compounds were found to show robust topological insulating behavior confirmed by bulk band inversion, and surface conducting states. Two compounds show a relatively large band gap and can be promoted for possible optoelectronic applications with further band engineering. Our search model opens new avenues for the discovery of more novel materials in different and unexplored classes of systems. We strongly propose the experimental characterization of the above promising compounds to shed more light on the present findings.

Effects of Fluorination on Exciton Binding Energy and Charge Transport of π-Conjugated Donor Polymers and the ITIC Molecular Acceptor: A Theoretical Study

Abstract: The intelligent addition of fluorine atoms in the chemical structure of conjugated polymers has been a popular approach to improve the efficiency of organic photovoltaic (OPV) devices. Recently, this strategy has been extended to nonfullerene acceptor (A) molecules in the best-performing bulk heterojunction (BHJ) devices. Yet, many details involved in the role of fluorination to enhance the photovoltaic response of organic semiconductors are still unclear. Here, we theoretically investigate the changes in the key properties of the representative fluorinated oligomers of polymers commonly used as donors (D) in BHJ-based OPVs. We then extend our analysis to consider the fluorination of ITIC, a very promising nonfullerene acceptor. We focus on the variation of the exciton binding energy (Eb) with the fluorination of an oligomer (molecule). Our calculations indicate that the fluorine substitution tends to lower the exciton binding energy which can enhance charge generation after light absorption. Considering the complexes of two oligomers (molecules), we also investigate the effects of fluorination on charge transport. We found that the intermolecular binding energy is considerably higher for the oligomers (molecules) with fluorine atoms. The increased electronic coupling tends to induce a better packing along the π–π direction which can explain the differences observed in the morphology of thin solid films. The calculation of the hole mobility for the oligomers (and electronic coupling for the acceptor molecules) showed higher values with fluorination. Our results are consistent with the space charge-limited current measurements performed in fluorinated conjugated materials and highlight the main reasons behind the better performance of fluorinated BHJ devices.

Anomalous Temperature-Dependent Exciton–Phonon Coupling in Cesium Lead Bromide Perovskite Nanosheets

Abstract: The role of exciton–phonon coupling in light emission in cesium lead bromide (CsPbBr₃) nanosheets is investigated with combined photoluminescence spectroscopy and the multimode Brownian oscillator model. A good agreement between theory and experiment in the low temperature range of 5–40 K enables us to determine several key parameters, including the dimensionless Huang–Rhys factor characterizing the exciton–longitudinal-optical phonon coupling strength and the damping constant accounting for the phonon bath (quasi-continuous acoustic phonons) dissipation. It is found that the Huang–Rhys factor of the free excitons peculiarly tends to diminish upon increasing the temperature in the interested low temperature range. However, the damping constant shows a linear increase with temperature in the interested temperature range. These new findings may deepen the understanding of the exciton–phonon coupling in CsPbBr₃ nanosheets and relevant solids.

Eco-Friendly and Safe Method of Fabricating Superhydrophobic Surfaces on Stainless Steel Substrates

Abstract: Industries are forced to explore more eco-friendly and safer processing methods to cope with increasing environmental issues. One of them is the fabrication of superhydrophobic stainless steel surfaces in the food industry. Current methods of fabricating superhydrophobic stainless steel surfaces have disadvantages, including using various toxic chemicals and fluorinated modifications, which cause hazards and environmental pollution, thereby limiting the application in the food industry. In this paper, an eco-friendly and safe method is proposed for fabricating superhydrophobic surfaces on stainless steel substrates using vacuum evaporation, chemical replacement reaction, and heating. The as-fabricated surface shows good superhydrophobicity with a water contact angle (WCA) of up to 155.7° as well as a water slide angle (WSA) as low as about 0°. Meanwhile, repellency to acidic and alkaline liquids, durability, self-cleaning, and antifouling properties are studied. Besides, the carbon content and its effect on wettability of the samples before and after heating at different temperatures are deeply investigated. The proposed method is an alternative approach for fabricating superhydrophobic stainless steel surfaces for application in the food industry without any environmental pollution.

Role of P-doping in Antipoisoning: Efficient MOF-Derived 3D Hierarchical Architectures for the Oxygen Reduction Reaction

Abstract: Developing nonprecious metal catalysts to efficiently catalyze the oxygen reduction reaction (ORR) is the nub for replacing precious metal catalysts and therefore of central importance for technological breakthrough. Herein, we reported that P-doping can improve the catalytic performance of the Fe–NC catalyst with antipoisoning ability. The P-doped Fe–NC catalyst showed a positive shift in the half-wave potential (>50 mV) and high resistance against poisoning molecules (SOₓ, NOₓ, and POₓ ions) for ORR in both alkaline and acidic media, as compared to the Fe–NC catalyst. Furthermore, an accelerated aging operation applied at the temperature ranging from 25 to 50 °C in 1 M H₃PO₄ demonstrated a good stability of the produced P-doped Fe–NC catalyst. In contrast, a drastic loss in the catalytic performance of Pt/C (20%) was observed in the same poisoning and aging conditions.

Electronic Structure and Optical Properties of Designed Photo-Efficient Indoline-Based Dye-Sensitizers with D–A−π–A Framework

Abstract: Seven D–A−π–A-based indoline (IND) dyes that were designed via quantitative-structure–property relationship modeling have been comprehensively investigated using computational approaches to evaluate their prospect of application in future dye-sensitized solar cells (DSSCs). An array of optoelectronic properties of the isolated dye and dyes adsorbed on a TiO₂ cluster that simulates the semiconductor were explored by density functional theory (DFT) and time-dependent DFT methods. Light absorption spectra, vertical dipole moment, shift of the conduction band of semiconductor, excited state lifetime, driving force of electron injection, photostability of the excited state, and exciton binding energy were computed. Our study showed that the presence of an internal acceptor such as pyrido[3,4-b]pyrazine (pyrazine) would influence greater the open circuit voltage (VOC), compared to the benzothiadiazole moiety. Considering the balance between the VOC and JSC (short circuit current) along with the all calculated characteristics, the IND3, IND5, and IND10 are the most suited among the designed dyes to be used as potential candidates for the photo-efficient DSSCs. The present study provides the results of rational molecular design followed by exploration of photophysical properties to be used as a valuable reference for the synthesis of photo-efficient dyes for DSSCs.

Simulation Study of the Capacitance and Charging Mechanisms of Ionic Liquid Mixtures near Carbon Electrodes

Abstract: The performance of electric double-layer capacitors is strongly influenced by the choice of electrolyte, and electrolytes comprised of ionic liquid mixtures have shown promise for enabling high energy densities. Here we perform all-atom molecular dynamics simulations of ionic liquids containing 1-ethyl-3-methylimidazolium and different fractions of bis(trifluoromethylsulfonyl)imide and tetrafluoroborate, in conjunction with planar graphene sheets as electrodes. We demonstrate that relative ion–electrode van der Waals interactions play an important role in the population of ions adsorbed in the first interfacial layer near uncharged electrodes. Near charged electrodes, we find that the ionic liquid mixtures generally exhibit integral capacitances intermediate between the two pure ionic liquids. We characterize cumulative ion densities near electrodes carrying various surface charges, revealing different charging mechanisms for different ionic liquids, which we relate to the relative sizes of the ions. Finally, in the ionic liquid mixtures we identify an effective ion exchanging phenomenon wherein charging of the electrodes leads to different trends in the densities of the two types of anions in the first interfacial layer, which enhances counterion adsorption and improves capacitance at the negative electrode.

Stabilization of Super Electrophilic Pd⁺² Cations in Small-Pore SSZ-13 Zeolite

Abstract: We provide the first observation and characterization of super electrophilic metal cations on a solid support. For Pd/SSZ-13, the results of our combined experimental (Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, high-angle annular dark-field scanning transmission electron microscopy) and density functional theory study reveal that Pd ions in zeolites, previously identified as Pd⁺³ and Pd⁺⁴, are, in fact, present as super electrophilic Pd⁺² species (charge-transfer complex/ion pair with the negatively charged framework oxygens). In this contribution, we reassign the spectroscopic signatures of these species, discuss the unusual coordination environment of “naked” (ligand-free) super electrophilic Pd⁺² in SSZ-13, and their complexes with CO and NO. With CO, nonclassical, highly positive [Pd(CO)₂]²⁺ ions are formed with the zeolite framework acting as a weakly coordinating anion (ion pairs). Nonclassical carbonyl complexes also form with Pt⁺² and Ag⁺ in SSZ-13. The Pd⁺²(CO)₂ complex is remarkably stable in zeolite cages even in the presence of water. Dicarbonyl and nitrosyl Pd⁺² complexes, in turn, serve as precursors to the synthesis of previously inaccessible Pd⁺²–carbonyl–olefin [Pd(CO)(C₂H₄)] and Pd⁺²–nitrosyl–olefin [Pd(NO)(C₂H₄)] complexes. Overall, we show that the zeolite framework can stabilize super electrophilic metal (Pd) cations and show the new chemistry of the Pd/SSZ-13 system with implications for adsorption and catalysis.

Novel Mixed-Dimensional Photocatalysts Based on 3D Graphene Aerogel Embedded with TiO₂/MoS₂ Hybrid

Abstract: The ternary mixed-dimensional TiO₂/MoS₂/graphene-aerogel, zero-dimensional TiO₂ nanoparticles loaded on two-dimensional MoS₂ nanosheets, and TiO₂/MoS₂ hybrid embedded into the three-dimensional (3D) graphene aerogel, was successfully prepared by self-assembly hydrothermal reduction process. The macroscopic appearance of TiO₂/MoS₂/graphene-aerogel is recorded using a digital camera. The microstructure of the TiO₂/MoS₂/graphene-aerogel with three-dimensional interconnected porous network structure was characterized by scanning electron microscope and transmission electron microscope. UV–vis absorption spectra tests show that TiO₂/MoS₂/graphene-aerogel possesses enhanced light absorption properties. The photocatalytic performance of TiO₂/MoS₂/graphene-aerogel was tested by the photoelectrochemical system, and it is verified that the photocurrent density of the as-prepared TiO₂/MoS₂/graphene-aerogel is greatly improved to 105 μA/cm² at a voltage of 0.6 V, which is approximately 10 times than of sole TiO₂. The improved photocatalytic activity is due to the enormous specific surface area caused by the multiaperture structure of the unique 3D graphene aerogel, maximized reactive sites, good conductivity, and positive coupling effect with TiO₂/MoS₂ heterojunction. This study indicates that TiO₂/MoS₂/graphene-aerogel based photocatalysts have excellent potential for photocatalytic H₂ production and photocatalytic degradation.

Computational Screening of Roles of Defects and Metal Substitution on Reactivity of Different Single- vs Double-Node Metal–Organic Frameworks for Sarin Decomposition

Abstract: Understanding how different factors affect the electronic properties of metal–organic frameworks (MOFs) is critical to understanding their potential for catalysis and to serve as catalyst supports. In this work, periodic dispersion-corrected quantum mechanical calculations are performed to assess the catalytic activity of different Zr₆ vs Zr₁₂ MOFs for the heterogeneous catalytic hydrolysis of the chemical warfare agent sarin. Using a comprehensive series of extended periodic models capable of capturing long-range sarin/water/framework interactions in both Zr₆ and Zr₁₂ MOFs, the effects of numbers and morphologies of defective sites, as well as Zrᴵⱽ substitution with heavier Ceᴵⱽ, are thoroughly investigated. Our calculations show that hydrogen bonds involving both metal-oxide nodes and organic linkers can play important roles in the catalysis. Insights derived from this work should inform the design and realization of more effective and robust next-generation MOF-based heterogeneous catalysts.

Role of Dissolution Intermediates in Promoting Oxygen Evolution Reaction at RuO₂(110) Surface

Abstract: RuO₂ is one of the most active electrocatalysts toward oxygen evolution reaction (OER), but it suffers from rapid dissolution in electrochemical environments. It is also established experimentally that corrosion of metal oxides can, in fact, promote catalytic activity for OER owing to the formation of a surface hydrous amorphous layer. However, the mechanistic interplay between corrosion and OER across metal-oxide catalysts and to what degree these two processes are correlated are still debated. Herein, we employ ab initio molecular dynamics-based blue moon ensemble approach in combination with OER thermodynamic analysis to reveal a clear mechanistic coupling between Ru dissolution and OER at the RuO₂(110)/water interface. Specifically, we demonstrate that (i) dynamic transitions between metastable dissolution intermediates greatly affect catalytic activity toward OER, (ii) dissolution and OER processes share common intermediates with OER promoting Ru detachment from the surface, (iii) the lattice oxygen can be involved in the OER, and (iv) the coupling between different OER intermediates formed at the same Ru site of the metastable dissolution state can lower the theoretical overpotential of OER down to 0.2 eV. Collectively, our findings illustrate the critical role of highly reactive metastable dissolution intermediates in facilitating OER and underscore the need for the incorporation of interfacial reaction dynamics to resolve apparent conflicts between theoretically predicted and experimentally measured OER overpotentials.

Enhanced Photocatalytic Production of H₂O₂ by Nafion Coatings on S,N-Codoped Graphene-Quantum-Dots-Modified TiO₂

Abstract: Photocatalytic production of H₂O₂ requires simultaneous promotion of the formation and the suppression of the decomposition of H₂O₂. This work explored a promising strategy of Nafion (perfluorinated polymer with sulfonate groups) coatings to enhance photocatalytic H₂O₂ production. The presence of Nafion layer on the S,N-codoped graphene-quantum-dots-modified TiO₂ was characterized by transmission electron microscopy, Fourier transform infrared, and X-ray photoelectron spectroscopy measurements. The incorporation of Nafion coatings significantly improves the photocatalytic production of H₂O₂ (373 μM/h under simulated sunlight irradiation), which is about 70% higher than that without Nafion coatings. Both accelerated formation (34.8 μM/min) and dramatically inhibited decomposition (0.003 min–¹) of H₂O₂ contribute to the efficient production of H₂O₂. Moreover, the ratios of H₂O₂ formation rate in the presence and absence of Nafion layers are more significantly improved at the neutral pH, which are 1.2 and 1.7 at pH 3 and 6.5, respectively. Nafion coatings show the abilities to induce a strongly negative charged hydrophobic surface, which can enhanced the local proton activity and oxygen concentration on the catalyst surface. The enhanced production of H₂O₂ by Nafion coatings can be attributed to the unaltered two-electron oxygen reduction reaction (ORR) pathway, the promoted charge transfer, the enhanced proton activity and oxygen diffusion, and the blocked formation of surface peroxide complexes. This work provides an insight for the modulation of proton-coupled electron-transfer-dominated ORR in photocatalysis through surface modification of Nafion coatings.

Etching and Exfoliation Properties of Cr₂AlC into Cr₂CO₂ and the Electrocatalytic Performances of 2D Cr₂CO₂ MXene

Abstract: In the recent years, two-dimensional (2D) metal carbide and nitride (MXenes) materials have shown characteristics of promising electrocatalytic properties for hydrogen evolution reaction (HER). Here, by using density functional theory calculations, the etching and exfoliating properties of Cr₂AlC to Cr₂CO₂ and electrocatalytic (for HER) properties with and without carbon defect were studied. Results show that etching the pristine Cr₂AlC by HF solutions could intend to generate Cr₂C MXenes with O* termination, that is, Cr₂CO₂, and Cr₂CF₂ and Cr₂C(OH)₂ will translate into Cr₂CO₂ even if they were generated first during the etching reactions. The exfoliation energy of multilayer Cr₂CO₂ into monolayer Cr₂CO₂ MXene is large (2.78 eV/nm²), and the delamination process requires Li⁺ (LiF) cation. Carbon vacancy is easily generated during the etching and exfoliation reactions, and the formation energy of carbon vacancy (1.71 and 1.59 eV with and without considering charge correction) of Cr₂CO₂ is lower than that of common 2D materials, such as graphene (7.4 eV) and MoS₂ (2.12 eV for forming S vacancy and 6.2 eV for forming Mo vacancy). HER performances of Cr₂CO₂ were further studied considering the solvent effect. The studies indicated that the solvent can affect the performance of HER of Cr₂CO₂ at medium hydrogen coverage. Gibbs free energy of hydrogen adsorption (ΔGH*) increases at low hydrogen coverage, then reduces at medium hydrogen coverage, and finally increases at high hydrogen coverage. These results provide a guideline for experimentally synthesizing the 2D MXene materials and developing new promising HER catalysts for water splitting.

Coumarin as a Quantitative Probe for Hydroxyl Radical Formation in Heterogeneous Photocatalysis

Abstract: In this work, we have assessed coumarin as a quantitative probe for hydroxyl radical formation in heterogeneous photocatalysis. Upon reaction with the hydroxyl radical, coumarin produces several hydroxylated products, of which one, 7-OH-coumarin, is strongly fluorescent. The fluorescence emission is strongly affected by inner filtering due to the presence of coumarin. Therefore, we performed a series of calibration experiments to correct for the coumarin concentration. From the calibration experiments, we could verify that the inner-filtering effect can be attributed to the competing absorption of the fluorescence excitation light between coumarin and 7-OH-coumarin. Through judicious calibration for the inner-filtering effects, the corrected results for the photocatalytic system show that the rate of hydroxyl scavenging is constant with time for initial coumarin concentrations of ≥50 μM under the conditions of our experiments. The rate increases linearly with coumarin concentration, as expected from the Langmuir–Hinshelwood model. Within the coumarin concentration range used here, the photocatalyst surface does not become saturated. Given the fact that the highest coumarin concentration used (1 mM) in this work is quite close to the solubility limit, we conclude that coumarin cannot be used to assess the full photocatalytic capacity of the system, i.e., surface saturation is never reached. The rate of hydroxyl radical scavenging will, to a large extent, depend on the affinity to the surface, and it is therefore not advisable to use coumarin as a probe for photocatalytic efficiency when comparing different photocatalysts.

Wavelength-Dependent Optical Force Aggregation of Gold Nanorods for SERS in a Microfluidic Chip

Abstract: Optical printing of metal-nanoparticle–protein complexes in microfluidic chips is of particular interest in view of the potential applications in biomolecular sensing by surface-enhanced Raman spectroscopy (SERS). SERS-active aggregates are formed when the radiation pressure pushes the particle–protein complexes on an inert surface, enabling the ultrasensitive detection of proteins down to pM concentration in short times. However, the role of plasmonic resonances in the aggregation process is still not fully clear. Here, we study the aggregation velocity as a function of excitation wavelength and power. We use a model system consisting of complexes formed of gold nanorods featuring two distinct localized plasmon resonances bound with bovine serum albumin. We show that the aggregation speed is remarkably accelerated by 300 or 30% with respect to the off-resonant case if the nanorods are excited at the long-axis or minor-axis resonance, respectively. Power-dependent experiments evidence a threshold below which no aggregation occurs, followed by a regime with a linear increase in the aggregation speed. At powers exceeding 10 mW, we observe turbulence, bubbling, and a remarkable 1 order of magnitude increase in the aggregation speed. Results in the linear regime are interpreted in terms of a plasmon-enhanced optical force that scales as the extinction cross section and determines the sticking probability of the nanorods. Thermoplasmonic effects are invoked to describe the results at the highest power. Finally, we introduce a method for the fabrication of functional SERS substrates on demand in a microfluidic platform that can serve as the detection part in microfluidic bioassays or lab-on-a-chip devices.