Showing papers in "The Journal of Physical Chemistry in 2020"

Synergistically Enhanced Electrochemical Performance of Ni-rich Cathode Materials for Lithium-ion Batteries by K and Ti Co-modification

Abstract: To improve the performance–price ratio of cathode materials, some modified Ni-rich (LiNi₀.₈Co₀.₁Mn₀.₁O₂) materials are identified by K and Ti single-doping or comodification using a simple coprecipitation method following by a proper post-treatment. The slab spacings for all the modified materials are broadened due to the doping of K and Ti. In particular, for K,Ti comodified LiNi₀.₈Co₀.₁Mn₀.₁O₂ (NCM-K-Ti), it possesses remarkably improved cycling and rate performances, i.e., 160.42 mAh g–¹ discharge capacity and 91.19% capacity retention at 1C after 200 cycles between 2.75 and 4.35 V. When being applied in graphite/NCM-K-Ti batteries, NCM-K-Ti keeps a high discharge capacity (173.74 mAh g–¹) and excellent capacity retention rate (91.62%) at 200th cycle at 0.2 C between 2.75 and 4.2 V. The enhanced mechanism is from the synergistic effect among codoping K and Ti and Li₂TiO₃ coating. Enlarging the interlayer spaces by K doping will open Li⁺ transporting channels to accelerate Li⁺ transport rate. Ti doping can strengthen Ni–O bonds to improve the structure stability. Furthermore, a certain Li₂TiO₃ coating as a protective layer suppresses side reactions on electrodes to improve the electrochemical performance.

Citric Acid Based Carbon Dots with Amine Type Stabilizers: pH-Specific Luminescence and Quantum Yield Characteristics

Abstract: We report the synthesis and spectroscopic characteristics of two different sets of carbon dots (CDs) formed by hydrothermal reaction between citric acid and polyethylenimine (PEI) or 2,3-diaminopyridine (DAP). Although the formation of amide-based species and the presence of citrazinic acid type derivates assumed to be responsible for a blue emission is confirmed for both CDs by elemental analysis, infrared spectroscopy, and mass spectrometry, a higher abundance of sp²-hybridized nitrogen is observed for DAP-based CDs, which causes a red-shift of the n-π* absorption band relative to the one of PEI-based CDs. These CD systems possess high photoluminescence quantum yields (QY) of ∼40% and ∼48% at neutral pH, demonstrating a possible tuning of the optical properties by the amine precursor. pH-Dependent spectroscopic studies revealed a drop in QY to < 9% (pH ∼ 1) and < 21% (pH ∼ 12) for both types of CDs under acidic and basic conditions. In contrast, significant differences in the pH-dependency of the n-π* transitions are found for both CD types which are ascribed to different (de)protonation sequences of the CD-specific fluorophores and functional groups using zeta potential analysis.

A Unified Push–Pull Model for Understanding the Ring-Opening Mechanism of Rhodamine Dyes

Abstract: Oxygen/carbon/silicon-rhodamine fluorophores and probes are ubiquitous in bioimaging and biosensing applications. Besides excellent brightness, photostability, and biocompatibility, these dyes possess a unique intramolecular spirocyclization equilibrium between nonemissive ring-closed and fluorescent ring-opened forms. Understanding the closed-ring/open-ring switching mechanism is critical for the rational designs of high-performance rhodamine dyes and probes. First-principles calculation-combined data search carried out herein quantitatively elucidates the importance of both substituent groups and environmental conditions in influencing the ring-opening process. Our analysis yields a unified push–pull model in elucidating the ring-opening mechanism of rhodamines. We demonstrate that this model produces excellent agreements with a broad range of experiments that involve different structural modifications in rhodamines and varied environmental conditions (i.e., solvent polarity, hydrogen bond donating strength, and acidity). We foresee that this push–pull model will provide important guidelines for understanding and designing rhodamine dyes and probes with required fluorescence-switching properties, such as autoblinking dyes and photoactivable dyes for super-resolution imaging and fluorogenic dyes for biochemical studies.

Construction of a Ternary Nanocomposite, Polypyrrole/Fe–Co Sulfide-Reduced Graphene Oxide/Nickel Foam, as a Novel Binder-Free Electrode for High-Performance Asymmetric Supercapacitors

Abstract: The development of asymmetric supercapacitors requires the design of electrode construction and the utilization of new electroactive materials. In this regard, an effective strategy is the loading of active materials on an integrated 3D porous graphene-based substrate such as graphene foam (GF). Herein, we successfully designed and fabricated a novel ternary binder-free nanocomposite consisting of polypyrrole, Fe–Co sulfide, and reduced graphene oxide on a nickel foam electrode (PPy/FeCoS-rGO/NF) via a facile, cost-effective, and powerful electrodeposition method for application in high-performance asymmetric supercapacitors. The monolithic 3D porous graphene foam (GF) obtained by the facile immersion method not only improves uniform growth of the FeCoS ultrathin porous nanosheet and conductive PPy film but also significantly boosts the mechanical stability, rate capability, and energy storage capacity. The results revealed that FeCoS interconnected nanosheets coated with a highly conductive PPy layer via the electrodeposition method are well decorated on the wrinkled surface of the graphene foam backbones. The PPy/FeCoS-rGO/NF exhibits excellent electrochemical performance with a high specific capacitance of 3178 F/g at 2 A/g and a good rate capability. The excellent electrochemical performance can be ascribed to the high surface area, superior electronic conductivity, low contact resistance between the PPy/FeCoS-rGO active layer and Ni foam current collector, short diffusion pathway for electrolyte ions, fast electron transfer, and effective utilization of active material during Faradaic charge-storage processes. Benefiting from their superior properties, a hybrid asymmetric supercapacitor is assembled by employing PPy/FeCoS-rGO/NF as the positive electrode and nickel foam coated with reduced graphene oxide (rGO/NF) as the negative electrode. Assembling the PPy/FeCoS-rGO//rGO device exhibits a high specific capacitance of 94 F/g at 1 A/g and an energy density of 28.3 Wh kg–¹ at a power density of 810 W kg–¹. Moreover, the asymmetric supercapacitor device shows an outstanding cycling performance with 97.5% capacitance retention after 2500 cycles. The obtained results demonstrate the PPy/FeCoS-rGO/NF electrode can be used as a promising electrode material for asymmetric supercapacitor applications.

Designing High Ionic Conducting NASICON-type Na₃Zr₂Si₂PO₁₂ Solid-Electrolytes for Na-Ion Batteries

Abstract: The present work investigates the synthesis and characteristics of a Na super-ionic conductor -type Sc³⁺- and Yb³⁺-doped Na₃Zr₂Si₂PO₁₂ solid electrolyte for application in solid-state Na-ion batteries. A significant improvement of Na-ion conductivity in Na₃Zr₂Si₂PO₁₂ has been achieved through crystal engineering and microstructure refinement. The presence of the monoclinic-ZrO₂ impurity phase adversely affecting the Na-ion conductivity is eliminated using the cubic-ZrO₂ precursor at the place of monoclinic-ZrO₂ in the conventional solid-state reaction method. Utilizing cubic-ZrO₂ also refined the microstructure with thin and microcrack-free grain boundaries. A replacement of 16.5 at. % of Zr⁴⁺ by Sc³⁺ in Na₃Zr₂Si₂PO₁₂ enhances the room-temperature total ionic conductivity from 0.61 to 0.96 mS·cm–¹. Replacing 11.11 at. % of Sc³⁺ by Yb³⁺ further improves the room-temperature ionic conductivity to 1.62 mS·cm–¹, which is >2.5 times higher than that of bare Na₃Zr₂Si₂PO₁₂. The strategic approach used to raise the ionic conductivity in the current work can be applied to other materials, paving way toward realizing high-performance solid-electrolytes for viable and economic Na-ion batteries. A room-temperature conductivity of 1.51 mS·cm–¹ for Sc³⁺/Yb³⁺-doped Na₃Zr₂Si₂PO₁₂ measured employing Na metal as electrodes confirms Na-ion conduction. Furthermore, a very low current density (∼10–⁷ A/cm²) in the cyclic-voltammetry profile of the Na|solid-electrolyte|Na cell demonstrates the suitability of Sc³⁺/Yb³⁺-doped Na₃Zr₂Si₂PO₁₂ as a solid-electrolyte for Na-ion batteries. A detailed analysis of these materials has been performed, and the possible reasons for the conductivity enhancement are discussed.

Rational Design of Phenothiazine-Based Organic Dyes for Dye-Sensitized Solar Cells: The Influence of π-Spacers and Intermolecular Aggregation on Their Photovoltaic Performances

Abstract: Power conversion efficiency (PCE) is one of the important factors in influencing the overall performance of dye-sensitized solar cells (DSSCs), and precise prediction of PCE is a feasible strategy for preparing highly efficient DSSCs devices. In this work, we designed a series of phenothiazine-based organic dyes by introducing different π-spacers including the 4-isopropyl-4H-dithieno[3,2-b:2′,3′-d]pyrrole (DTP) and 2,7-dihydronaphtho[1,2-d:5,6-d′]diimidazole (NDI) to tune their photovoltaic properties. The present studies reveal that the PCE value of the DTP-based dye is estimated to be 8.55%, in excellent agreement with the experimentally available value (8.19%) observed in the reported analogue. In comparison to DTP, the strong electron-deficiency NDI group induces a remarkable red-shifting of maximum absorption band, broadening the optical absorption into the near-infrared region. As a consequence, the NDI-based dye achieves an impressive PCE value of 15.51%, which is expected to be a potential organic dye applied in DSSCs.

Bloch Surface Waves and Internal Optical Modes-Driven Photonic Crystal-Coupled Emission Platform for Femtomolar Detection of Aluminum Ions

Abstract: The intrinsically lossy nature of plasmonic-based detection platforms necessitates the use of alternative nanophotonic platforms such as one-dimensional photonic crystals (1DPhCs) to exploit properties pertaining to photonic stop band (PSB), Bloch surface waves (BSWs), microcavity, and band-edge modes. We present a highly desirable confinement of internal optical modes (IOMs) and large surface electromagnetic (EM) field due to BSWs on a plasmon-free, metal template-free, photonic crystal-coupled emission (PCCE) platform ensuing 44-fold emission enhancements of the, otherwise, omnidirectionally emitting radiating dipoles. The effect of dielectric thickness in the PCCE platform has also been explored, and the optimized thicknesses for enhanced coupling of both BSWs and IOMs with the radiating dipoles have been obtained. Cavity engineering involving quantum emitters sandwiched in hot spots between 1DPhCs and Ag nanoparticles (AgNPs) has delivered ∼200-fold emission enhancements on account of the improved local density of states (LDOS) via exceptional EM field trapping by BSWs, IOMs, and localized surface plasmon resonance (LSPR) of plasmonic nanoparticles. Experimental results that are in strong agreement with the numerically calculated data validate this augmentation in enhancements due to the amplified coupling between the radiating dipoles and modes supported by 1DPhCs. Moreover, the tightly entrapped optical energy within the hot spots between AgNPs and 1DPhCs was adopted for sensing environmentally hazardous Al³⁺ ions at a 0.21 parts per quadrillion (ppq) limit of detection in drinking water samples with reliable and reproducible results, opening new avenues for investigating distinctive photonic crystal nanoarchitectures as a robust, practical, and user-friendly technology for multiplexed diagnostic fluorescence assays.

Plasma-Based CH₄ Conversion into Higher Hydrocarbons and H₂: Modeling to Reveal the Reaction Mechanisms of Different Plasma Sources

Abstract: Plasma is gaining interest for CH₄ conversion into higher hydrocarbons and H₂. However, the performance in terms of conversion and selectivity toward different hydrocarbons is different for different plasma types, and the underlying mechanisms are not yet fully understood. Therefore, we study here these mechanisms in different plasma sources, by means of a chemical kinetics model. The model is first validated by comparing the calculated conversions and hydrocarbon/H₂ selectivities with experimental results in these different plasma types and over a wide range of specific energy input (SEI) values. Our model predicts that vibrational–translational nonequilibrium is negligible in all CH₄ plasmas investigated, and instead, thermal conversion is important. Higher gas temperatures also lead to a more selective production of unsaturated hydrocarbons (mainly C₂H₂) due to neutral dissociation of CH₄ and subsequent dehydrogenation processes, while three-body recombination reactions into saturated hydrocarbons (mainly C₂H₆, but also higher hydrocarbons) are dominant in low temperature plasmas.

Photoinduced Charge Transfer in Donor-Bridge-Acceptor in One- and Two-photon Absorption: Sequential and Superexchange Mechanisms

Abstract: In this work, the one- (OPA) and two-photon absorption (TPA) and transition characteristics of the porphyrin and fullerene donor–acceptor systems linked by thiophene oligomers are investigated theoretically. In the study of the transition characteristics of OPA and TPA, a combination of 2D and 3D visualization methods is used. During the OPA and TPA transitions of this D–A system, there is both superexchange charge transfer without thiophene oligomers and sequential charge transfer through thiophene oligomers. On this basis, there exist also charge recombination TPA excitations of the previous two kinds of charge transfer into an intrinsic process. The degree of polymerization of the thiophene oligomer significantly affects the efficiency of superexchange charge transfer and participates in excitation in units of quarter-thiophene. Finally, a method to enhance the charge transfer efficiency during TPA excitation is proposed, which is local excitation enhanced charge transfer excitation.

Bulk and Monolayer ZrS₃ as Promising Anisotropic Thermoelectric Materials: A Comparative Study

Abstract: Different from two-dimensional (2D) transition-metal dichalcogenides, 2D transition-metal trichalcogenides with a quasi-one-dimensional chain offer additional advantages in electronics and optoelectronics. Based on the recent experimental synthesis of few layers of ZrS₃, we present a comparative study on the electron and phonon transport properties of bulk and monolayer ZrS₃ by using the first-principles calculations combined with the Boltzmann transport theory. The anisotropic electrical conductivity is obtained because of the different carrier effective masses along different directions, and consequently, a larger power factor along the y direction is achieved for both p-type and n-type. The multivalley degeneracy valence band contributes to the large Seebeck coefficient and the high power factor for holes. The carriers of electrons with higher mobility along the Γ–Y direction are responsible for the better transport properties of electrons than holes. In addition, the dimensionality reduction in the crystal structure enhances the phonon scattering and decreases the phonon group velocity and thus reduces the phonon thermal conductivity in monolayer ZrS₃. The calculated total Gruneisen parameter of 1.8 at 300 K for monolayer ZrS₃ is about 120% higher than that of bulk ZrS₃ (0.8). The optimized n-type thermoelectric figure of merit at 800 K for monolayer ZrS₃ reaches 2.44 along the y direction, while it is 1.75 for the n-type doping of bulk ZrS₃. These results indicate that ZrS₃ especially in the monolayer form is a promising anisotropic thermoelectric material.

Atomically Thin 1T-FeCl₂ Grown by Molecular-Beam Epitaxy

Abstract: Two-dimensional (2D) magnetic materials have attracted much attention due to their unique magnetic properties and promising applications in spintronics. Here, we report on the growth of ferrous chloride (FeCl₂) films on Au(111) and graphite with atomic thickness by molecular-beam epitaxy (MBE) and the layer-dependent magnetic properties by density functional theory (DFT) calculations. The growth follows a layer-by-layer mode with adjustable thickness from sub-monolayer to a few layers. Four types of moire superstructures of a single-layer FeCl₂ on graphite and two types of atomic vacancies on Au(111) have been identified based on high-resolution scanning tunneling microscopy (STM). It turned out that the single- and few-layer FeCl₂ films grown on Au(111) exhibit a 1T structure. The DFT calculations reveal that a single-layer 1T-FeCl₂ has a ferromagnetic ground state. The minimum-energy configuration of a bilayer FeCl₂ is satisfied for the 1T–1T structure with ferromagnetic layers coupled antiferromagnetically. These results make FeCl₂ a promising candidate as ideal electrodes for spintronic devices providing large magnetoresistance.

Direct Experimental Evidence of Hot Carrier-Driven Chemical Processes in Tip-Enhanced Raman Spectroscopy (TERS)

Abstract: Nanoscale localization of electromagnetic fields using metallic nanostructures can catalyze chemical reactions in their immediate vicinity. Local optical field confinement and enhancement is also exploited to attain single-molecule detection sensitivity in surface- and tip-enhanced Raman (TER) spectroscopy. In this work, we observe and investigate the sporadic formation of 4-nitrobenzenethiolate upon TER imaging of a 4-nitrobenzenethiol (4NBT) monolayer on Au(111). Density functional theory (DFT), finite-difference time-domain (FDTD), and finite element method (FEM) calculations together confirm that this chemical reaction does not occur as a result of thermal desorption of the molecule, which requires temperatures in excess of 2100 K at the tip–sample junction. Our combined experimental and theoretical analyses strongly suggest that the chemical transformations observed throughout the course of TERS mapping is not driven by plasmonic photothermal heating, but rather by plasmon-induced hot carriers.

Tuning the Li/Ni Disorder of the NMC811 Cathode by Thermally Driven Competition between Lattice Ordering and Structure Decomposition

Abstract: Ni-rich layered LiNiₓMnyCozO₂ (NMC) cathodes for lithium-ion batteries are receiving a lot of attention owing to their promising large capacity, whereas the high content of Ni results in several issues including poor thermal stability and serious Li/Ni disorder. Although a little degree of the Li/Ni disorder may be beneficial for the structural stability of NMC cathodes and even migration of Li ions, a high degree of the Li/Ni disorder certainly deteriorates their electrochemical performances. Therefore, tuning the Li/Ni disorder is of great interest in the development of safer NMC cathodes with larger accessible capacity. Post-synthesis annealing is a facile and low-cost way to manipulate lattice defects, yet has not been utilized to optimize the Ni-rich NMC cathodes. In this work, we report that post-synthesis annealing can induce the competition between lattice ordering and structure decomposition. The thermal annealing promoted that lattice ordering would prevail until the decomposition of oxygen lattice. Once the annealing temperature reaches the critical temperature to form oxygen vacancies, Ni ions can easily migrate into the Li slab. The Li/Ni disorder can be facilely tuned through post-synthesis annealing to optimize the electrochemical performances of NMC cathodes.

Dipole–Dipole Interaction in Two-Photon Spectroscopy of Metallic Nanohybrids

Abstract: We have developed a theory for the two-photon fluorescence in nanohybrids made of an ensemble of metallic nanorod shells and an ensemble of quantum emitters. A metallic nanorod shell is made of a metallic rod and dielectric shell. We consider that the quantum emitters are four-level quantum systems. When a probe laser light falls on the metallic nanorod shells, the surface plasmon polariton electric field is produced at the interface between the metallic nanorod and dielectric shell. This electric field, along with the probe field, induces dipoles in the quantum emitters and nanorod shells. These dipoles interact with each other via the dipole–dipole interaction. The two-photon fluorescence has been calculated by using the quantum density matrix method in the presence of the dipole–dipole interaction (coupling). Analytical expressions of the two-photon fluorescence have been derived in the presence of the dipole–dipole interaction. We showed that that the two-photon process is made of two terms. The first term is the two-photon process due to the two probe field photons. On other the hand, the second term is made of one DDI field photon and one probe field photon. It is found that the surface plasmon polariton resonance energy is not resonant with the exciton energy, the two-photon fluorescence spectrum splits from one peak to three peaks. The splitting in the spectrum is due to the presence of the dressed states created in the system due to the strong dipole–dipole interaction. We also compared our theory with the experimental data of the metallic nanohybrid system made of metallic nanorod shells and quantum emitters (T790 molecules) and found a good agreement between the theory and the experiments.

Interaction of C₃–C₅ Alkenes with Zeolitic Brønsted Sites: π-Complexes, Alkoxides, and Carbenium Ions in H-FER

Abstract: We use a chemically accurate (4 kJ/mol) hybrid MP2:(PBE+D2) + ΔCCSD(T) method to determine relative stabilities of all possible π-complexes, alkoxides, and carbenium ions formed from propene, butene, and pentene with the Al(2)O(7) Bronsted acid site in H-FER. The energetic order is carbenium ions > tert-alkoxides > π-complexes as well as primary and secondary alkoxide species. Primary carbenium ions are not stationary points on the potential energy surface. The energetically most stable C3, C4, and C5 surface species are 2-propoxide, 2-butoxide, and the 2-methyl-2-butene π-complex with energies of −78, −81, and −85 kJ/mol, respectively, for formation from the corresponding alkenes. Compared to the present results, the widely applied PBE+D2 approach overbinds all species, and the energy differences are 18–24, 25–45, and 48–71 kJ/mol for π-complexes, alkoxides, and carbenium ions. Enthalpies and Gibbs free energies are calculated for 323 and 623 K within the harmonic approximation. The calculated adsorption enthalpy of trans-2-pentene, −93 kJ/mol, is in agreement with the experimental value, −92 kJ/mol [Schallmoser et al. J. Am. Chem. Soc. 2017, 139, 8646]. Entropy favors the more mobile species (carbenium ions, π-complexes), and the Gibbs free energy order becomes carbenium ions and tert-alkoxides > primary and secondary alkoxides > π-complexes.

Assessing the Role of Pt Clusters on TiO₂ (P25) on the Photocatalytic Degradation of Acid Blue 9 and Rhodamine B

Abstract: The role of Pt on photocatalytic substrates such as TiO₂ (P25) for the decomposition of organic pollutants is still controversial in the scientific community. The well-observed behavior of an optimum catalytic activity as a function of the Pt loading is usually explained by the shift from charge separation to charge recombination behavior of Pt clusters. However, experiments supporting this explanation are still lacking to give a concise understanding of the effect of Pt on the photocatalytic activity. Here, we present an experimental study that tries to discriminate the different effects influencing the photocatalytic activity. Using atomic layer deposition in a fluidized bed reactor, we prepared TiO₂ (P25) samples with Pt loadings ranging from 0.04 wt % to around 3 wt %. In order to reveal the mechanism behind the photocatalytic behavior of Pt on P25, we investigated the different aspects (i.e., surface area, reactant adsorption, light absorption, charge transfer, and reaction pathway) of heterogeneous photocatalysis individually. In contrast to the often proposed prolonged lifetime of charge carriers in Pt-loaded TiO₂, we found that after collecting the excited electrons, Pt acts more as a recombination center independent of the amount of Pt deposited. Only when dissolved O₂ is present in the solution, charge recombination is suppressed by the subsequential consumption of electrons at the surface of the Pt clusters with the dissolved O₂ benefited by the improved O₂ adsorption on the Pt surface.

Signatures of Ion Pairing and Aggregation in the Vibrational Spectroscopy of Super-Concentrated Aqueous Lithium Bistriflimide Solutions

Abstract: Aqueous electrolytes in the super-concentrated regime, where water and ions are present in a similar concentration, have recently drawn considerable interest for their potential application in electrochemical systems, such as Li⁺ batteries. In this regime, the classical theories for the structure and properties of electrolyte solutions break down, and even such fundamental properties as the solution morphology on the molecular length scale can be unclear, with specific ion effects often driving the properties. Here, we study the role of ion–ion interactions in aqueous lithium bistriflimide (LiTFSI) solutions as a function of concentration, ranging from the dilute to the super-concentrated regimes. We use vibrational spectroscopy of the asymmetric −SO₂ stretch modes of the TFSI– anion, specifically the linear absorption, two-dimensional (2D) IR, and polarization-dependent 2D IR and IR transient absorption spectroscopies, as a method for probing for both the cation–anion interactions and anion–anion interactions in these solutions and demonstrate specific signatures of the formation of contact ion pairs and higher-order ion aggregates. These results are validated by comparison with molecular dynamics simulations of several concentrations of LiTFSI in water, and the spectral observables are interpreted with the analysis of the normal modes of small clusters with geometries consistent with solution-phase environments derived from these simulations. Using these techniques, we find evidence that contact ion pairs, both cation–anion and anion–anion pairs, begin to have substantial population at concentrations around 2 M (∼3 m) and that an extended, highly interconnected ion-rich network forms in the super-concentrated regime.

Insight into Selective Surface Reactions of Dimethylamino-trimethylsilane for Area-Selective Deposition of Metal, Nitride, and Oxide

Abstract: Area-selective deposition (ASD) is a promising bottom-up manufacturing solution for catalysts and nanoelectronic devices. However, industrial applications are limited as highly selective ASD processes exist only for few materials. “Passivation/deposition/defect removal” cycles have been proposed to increase selectivity, but cycling requires the passivation to be selective to the growth surface as well as the ASD-grown material. Dimethylamino-trimethylsilane (DMA-TMS) can passivate SiO₂ surfaces by covering them with −Si(CH₃)₃ groups. However, the interaction of DMA-TMS with materials other than SiO₂ and Si remains largely unknown and its compatibility with cycling is not yet understood. This work investigates the selectivity of metal, nitride, and oxide atomic layer deposition (ALD) to DMA-TMS-passivated SiO₂ as well as the surface chemistry and selectivity of the DMA-TMS reaction. The ALD coreagents O₂, NH₃, and H₂O show low reactivity with the −Si(CH₃)₃-terminated surface at temperatures up to 300 °C, but the selectivity of ALD strongly depends on the metal precursor and temperature. We demonstrate that DMA-TMS is a selective passivation agent for ASD of and on TiO₂, TiN, and Ru selective to SiO₂, by TiCl₄/H₂O, TiCl₄/NH₃, and EBECHRu/O₂ ALD, respectively. We investigate the DMA-TMS reaction on Ru and TiN/TiO₂ growth surfaces under conditions that passivate SiO₂. At least 77% of the area of the growth surface remains reactive for ALD, confirming the compatibility of DMA-TMS with cycling for ASD. We investigate the impact of changes in surface composition due to patterning before ASD and find that DMA-TMS removes F impurities on TiN and TiO₂ surfaces. DMA-TMS selectively passivates SiO₂ on three-dimensional (3D) nanopatterns, allowing preferential TiO₂ deposition on a nonpassivated growth surface. Thus, the selectivity of DMA-TMS shows great promise to expand the ASD material space as well as to increase selectivity during ASD cycles.

Reaction Mechanism of the Aluminum Nanoparticle: Physicochemical Reaction and Heat/Mass Transfer

Abstract: A lack of clarity in the reaction mechanism of the aluminum nanoparticle (ANP) severely restricts its effective applications. By describing the physicochemical evolution of ANP burning in typical oxidizers (CO₂, H₂O, and O₂) at the nanoscale, three principal reaction modes including physical adsorption, chemical adsorption, and reactive diffusion were captured during the reaction. Initially, oxidizer molecules are physically and chemically adsorbed on the ANP surface until ignition in which reaction heat plays a more important role in contrast to heat transfer. Subsequently, partial oxidizer atoms adsorbed by surface diffuse across the shell to react with the Al core, presenting the dominant mode of reactive diffusion. It is assumed that the binding energy between Al and oxidizer atoms is in an inverse relation to atomic diffusivity but is positively correlated to reaction heat, resulting in various ANP structures and heat release rates. Our findings provide design guidelines to control various oxidizer supplies with respect to the reaction stages to balance the energy release and the residence time of ANP.

Influence of Substituent Groups on Chemical Reactivity Kinetics of Nonfullerene Acceptors

Abstract: Substitution (such as fluorine substitution, F) of terminal groups or atoms in nonfullerene acceptors (NFAs) is an effective strategy to tune the optoelectronic properties of nonfullerene electron acceptors and enhance their photovoltaic performance. However, how the substitution influences chemical reactivity or stability has not been reported. Here, we report that the terminal fluorine substitution would increase the chemical reactivity. This is supported by three different pieces of evidence: Frist, a device with fluorine-substituted NFA shows more severe “S” shape in the current density–voltage (J–V) characteristics under illumination when polyethylenimine is used as the interfacial layer and larger efficiency difference compared with that of the reference device with ZnO as an interfacial layer. Second, the fluorine-substituted NFA shows a higher reaction rate with monoethanolamine model compound, monitored by nuclear magnetic resonance (NMR) spectroscopy. Third, mass spectrometry of the products of the NFAs added with monoethanolamine indicates that the reaction is more complete compared with that of the NFAs without F substitution. In addition, we find that Y6 show good chemical stability with amines that suggests that proper design of the donor moiety of the NFA could also improve its chemical stability.

Computational Protocol for Precise Prediction of Dye-Sensitized Solar Cell Performance

Abstract: Numerous organic dyes have been developed for dye-sensitized solar cells (DSSCs). However, theoretical screening has not played a due role in designing new dyes. It is mainly attributed that there is rarely quantitative calculation and the inaccurate estimated values for short-circuit current density (JSC) and open-circuit photovoltage (VOC), especially for VOC. In this work, VOC is theoretically predicted by two different models for three D−π–A organic dyes (1, 2, and 3) with the same π bridge and acceptor as well as different donors. Although there is a slight deviation in their structures, their properties are successfully differentiated by accurate quantitative calculations. Dimethoxybenzene-substituted indoline is more suitable as donor than methoxy-substituted triphenylamine and methyl-substituted indoline when it combines with 8H-thieno [2′,3′:4,5]thieno[3,2-b]thieno[2,3-d]pyrrole (TTP) as π bridge and cyanoacrylic acid as acceptor. The properties of the donor are not only related to the core group but are also determined by the substituted group. A less than 10% deviation between theoretical and experimental results is an assurance to perform a reasonable prediction for photocurrent–photovoltage.

Combined Effects of Anion Substitution and Nanoconfinement on the Ionic Conductivity of Li-Based Complex Hydrides

Abstract: Solid-state electrolytes are crucial for the realization of safe and high capacity all-solid-state batteries. Lithium-containing complex hydrides represent a promising class of solid-state electrolytes, but they exhibit low ionic conductivities at room temperature. Ion substitution and nanoconfinement are the main strategies to overcome this challenge. Here, we report on the synthesis of nanoconfined anion-substituted complex hydrides in which the two strategies are effectively combined to achieve a profound increase in the ionic conductivities at ambient temperature. We show that the nanoconfinement of anion substituted LiBH₄ (LiBH₄–LiI and LiBH₄–LiNH₂) leads to an enhancement of the room temperature conductivity by a factor of 4 to 10 compared to nanoconfined LiBH₄ and nonconfined LiBH₄–LiI and LiBH₄-LiNH₂, concomitant with a lowered activation energy of 0.44 eV for Li-ion transport. Our work demonstrates that a combination of partial ion substitution and nanoconfinement is an effective strategy to boost the ionic conductivity of complex hydrides. The strategy could be applicable to other classes of solid-state electrolytes.

Understanding and Predicting the Cause of Defects in Graphene Oxide Nanostructures Using Machine Learning

Abstract: Machine learning is a powerful way of uncovering hidden structure/property relationships in nanoscale materials, and it is tempting to assign structural causes to properties based on feature rankings reported by interpretable models. In this study of defective graphene oxide nanoflakes, we use classification, regression, and causal inference to show that not all important structural features directly influence the concentration of broken bonds, as a representative property. We find that while the presence of oxygen is important for actual bond breakage the presence and distribution of hydrogen determines how often bond breakage occurs.

Bifunctional Nickel–Nitrogen-Doped-Carbon-Supported Copper Electrocatalyst for CO₂ Reduction

Abstract: Bifunctionality is a key feature of many industrial catalysts, supported metal clusters and particles in particular, and the development of such catalysts for the CO₂ reduction reaction (CO₂RR) to hydrocarbons and alcohols is gaining traction in light of recent advancements in the field. Carbon-supported Cu nanoparticles are suitable candidates for integration in the state-of-the-art reaction interfaces, and here, we propose, synthesize, and evaluate a bifunctional Ni–N-doped-C-supported Cu electrocatalyst, in which the support possesses active sites for selective CO₂ conversion to CO and Cu nanoparticles catalyze either the direct CO₂ or CO reduction to hydrocarbons. In this work, we introduce the scientific rationale behind the concept, its applicability, and the challenges with regard to the catalyst. From the practical aspect, the deposition of Cu nanoparticles onto carbon black and Ni–N–C supports via an ammonia-driven deposition precipitation method is reported and explored in more detail using X-ray diffraction, thermogravimetric analysis, and hydrogen temperature-programmed reduction. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDXS) give further evidence of the presence of Cu-containing nanoparticles on the Ni–N–C supports while revealing an additional relationship between the nanoparticle’s composition and the electrode’s electrocatalytic performance. Compared to the benchmark carbon black-supported Cu catalysts, Ni–N–C-supported Cu delivers up to a 2-fold increase in the partial C₂H₄ current density at −1.05 VRHE (C₁/C₂ = 0.67) and a concomitant 10-fold increase of the CO partial current density. The enhanced ethylene production metrics, obtained by virtue of the higher intrinsic activity of the Ni–N–C support, point out toward a synergistic action between the two catalytic functionalities.

Formation Thermodynamics, Stability, and Decomposition Pathways of CsPbX₃ (X = Cl, Br, I) Photovoltaic Materials

Abstract: Standard enthalpies of formation of CsPbX₃ (X = Cl, Br, I) perovskites from halides and from elements at 298 K were measured using solution calorimetry. Intrinsic and extrinsic stabilities of CsPbX₃ halides were analyzed and compared with those of CH₃NH₃PbX₃. The main difference between the stabilities of CsPbX₃ and CH₃NH₃PbX₃ halides was found to stem from the different chemical natures of cesium and methylammonium cations. Indeed, the enthalpies of formation of CsPbX₃ from binary constituent halides, ΔfH°ₕₐₗ, are only slightly more negative than those of CH₃NH₃PbX₃. Small values of ΔfH°ₕₐₗ imply that the entropic contribution to the Gibbs free energy of the formation of CsPbX₃ and CH₃NH₃PbX₃ is significant and, hence, of utmost importance for understanding the intrinsic stability of these compounds and their analogues. Regarding the extrinsic stability, the presence of gaseous O₂, H₂O, and CO₂ was shown to be crucial for the stability of the iodide, CsPbI₃, for which several decomposition reactions, exergonic at 298 K, were identified. At the same time, chloride, CsPbCl₃, and bromide, CsPbBr₃, are much less sensitive to these chemical agents. However, liquid water should degrade all of the CsPbX₃ halides.

Temperature-Dependent Na-Ion Conduction and Its Pathways in the Crystal Structure of the Layered Battery Material Na₂Ni₂TeO₆

Abstract: Na-ion conduction and correlations between Na-ion conduction pathways and the crystal structure have been investigated as a function of temperature in the layered battery material Na₂Ni₂TeO₆ by impedance spectroscopy and neutron diffraction, respectively. The impedance data reveal an ionic conductivity of σ ≈ 2 × 10–⁴ S/m at 323 K, which strongly enhances with increasing temperature and attains a high value of ∼0.03 S/m at 423 K. The temperature-dependent conductivity data show an Arrhenius-type behavior with an average activation energy (Eₐ) of ∼0.58(3) eV for T ≥ 383 K. By employing soft bond valence sum analysis of the neutron diffraction patterns, we experimentally demonstrate the site-specific Na-ion conductions through visualization of microscopic sodium-ion conduction pathways and verify the recent theoretical results of molecular dynamics simulation. Our results reveal two-dimensional Na-ion conduction pathways that are confined within the ab planes of Na layers. Crystal structural study indicates that the layered structure involving Na-ion layers is responsible for high ionic conductivity, and the local crystallographic environment of Na-ion sites is responsible for site-specific conductivity. Our study further reveals that, for up to 500 K, the ionic conduction is governed by the Na ions located at the Na1 and Na2 sites, whereas all the Na ions located at the three Na sites contribute to the conduction process above 500 K. Our neutron diffraction study also establishes that the crystal structure of Na₂Ni₂TeO₆ is stable for at least up to 725 K (the highest measured temperature), however, with an anisotropic thermal expansion (αc/αₐ ∼ 3).

Ultrafast Carrier Dynamics in 2D CdSe Nanoplatelets–CsPbX₃ Composites: Influence of the Halide Composition

Abstract: Two-dimensional (2D) material-based composites are considered to be an important class of materials for light-harvesting applications because of their efficient charge separation. In this article, we have designed composites of 2D CdSe nanoplatelets (NPLs) and CsPbX₃ (X = mixture of Br and I or I) perovskite nanocrystals and investigated their ultrafast carrier dynamics using ultrafast spectroscopy. A time-resolved fluorescence upconversion study reveals that the electron transfer from CdSe NPLs to CsPbX₃ varies with changing the composition of perovskite from CsPbBr₁.₅I₁.₅ to CsPbI₃. From the transient absorption spectroscopic study, the shortening of the faster component of bleach recovery kinetics of CdSe NPLs along with the enhancement of growth time of CsPbX₃ NCs in composites indicates the ultrafast electron transfer from CdSe NPLs to CsPbX₃ NCs. The ultrafast electron transfer from 2D CdSe NPLs to CsPbX₃ NCs enhances in the following order: CsPbI₃ > CsPbBrI₂ > CsPbBr₁.₅I₁.₅. The dark current and photocurrent are 0.04 and 62.4 μA in the CdSe–CsPbI₃ composite. The dramatically improved photocurrent response in the CdSe–CsPbI₃ composite confirms the enhancement of their efficient charge separation because of the ultrafast electron transfer from CdSe NPLs to pervoskite NCs. Our finding reveals that the integration of 2D CdSe NPLs with perovskite NCs offers a great opportunity for the improvement of the efficiency of perovskite solar cells by engineering the interfacial charge-transfer dynamics.

Soft Nanocontainers Based on Hydroxyethylated Geminis: Role of Spacer in Self-Assembling, Solubilization, and Complexation with Oligonucleotide

Abstract: Self-organization of hydroxyethylated gemini surfactants with different spacer fragments 16-s-16(OH) (s = 4, 6, 10, and 12) was studied in single solutions and in binary surfactant-oligonucleotide systems. Despite the fact that aggregation activity and solubilization capacity of aggregates decrease with an increase in spacer length, gemini with the longer spacer demonstrate superior binding capacity toward oligonucleotide as compared to single head surfactant and gemini analogs with shorter spacers. The detailed study testified that gemini with longer spacers are characterized by a looser packing mode, a moist and more polar interior, and tend to show polymorphism. These features in combination with favorable geometry factor providing suitable orientation of components are probably responsible for the beneficial lipoplex formation in the case of longer spacers. The effectiveness of oligonucleotide-surfactant complexation changes in the same order as transfection efficacy mediated by these gemini reported earlier [Zakharova, L. et al. Colloids Surf. B, 2016, 140, 269–277], which indicates that physicochemical aspects probably play a key role upon the design of nonviral vectors and may be used for prediction of transfection efficacy mediated by amphiphilic agents.

MXene-Enhanced Deep Ultraviolet Photovoltaic Performances of Crossed Zn₂GeO₄ Nanowires

Abstract: The Zn₂GeO₄ crystal is an ideal semiconductor for deep ultraviolet (DUV) detection due to its wide band gap of ∼4.69 eV. To further improve its DUV performance, two-dimensional (2D) MXenes with high electrical conductivity, potentially tunable electronic structure, and nonlinear optical properties were applied on crossed Zn₂GeO₄ nanowire (NW) network materials. The results presented here show that the DUV detectors based on Zn₂GeO₄/MXene hybrid nanostructures exhibited excellent optoelectronic performances with a largest responsivity of 20.43 mA/W and external quantum efficiency (EQE) of 9.9% under 254 nm wavelength light illumination. The excellent optical performance is from the synergistic effect of MXene and Zn₂GeO₄ nanowires. The metallic property of MXene provides a fast electron transport for Zn₂GeO₄/MXene, which leads to a larger photocurrent and a fast photoresponse. The construction of unique semiconductive–conductive networks and large interfaces of Zn₂GeO₄ NWs, MXene layers, and the interfaces between them also promotes photoinduced electron–hole separation in the sample. Considering a large number of members in MXene, this study demonstrates a new strategy applicable for maximizing their applications in deep ultraviolet photodetectors.

Defect Engineering for Enhancement of Thermoelectric Performance of (Zr, Hf)NiSn-Based n-type Half-Heusler Alloys

Abstract: Defect engineering of thermoelectric (TE) materials enables the alteration of their crystal lattice by creating an atomic-scale disorder, which can facilitate a synergistic modulation of the electrical and phonon transport, leading to the enhancement of their TE properties. This work employs a compositional nonstoichiometry strategy for manipulation of Ni-vacancies and Ni-interstitials through Ni-deficient and Ni-excess compositions of (Zr, Hf)Ni₁±ₓSn-based half-Heusler (HH) alloys to realize a state-of-the-art TE figure-of-merit (ZT) of ∼1.4 at 873 K in 4 atomic % Ni-excess HH composition, which corresponds to a remarkable TE conversion efficiency of ∼12%, estimated using the cumulative temperature dependence model. These alloys are synthesized employing arc-melting followed by spark plasma sintering and are characterized for their phase, morphology, structure, and composition along with electrical and thermal transport properties to examine the implication of Ni-excess and Ni-deficiency on the TE properties of the synthesized Zr₀.₆Hf₀.₄NiSn HH alloy. A significant enhancement (∼30%) of ZT is observed in the low doping limit of Ni-excess HH compositions over their stoichiometric counterpart due to Ni-interstitials and in situ full-Heusler precipitation, which enable a strong phonon scattering for a drastic reduction in lattice thermal conductivity and lead to an enhancement of ZT. However, Ni-deficient HH compositions exhibit a deterioration in the TE properties owing to the vacancy-induced bipolarity. The defect-mediated optimization of electrical and thermal transport, thus, opens up promising avenues for boosting the TE performance of HH alloys.