The 100 most-cited articles in The Journal of Physical Chemistry C, 2020

Abstract The inherently nanolaminated Ti3AlC2 is one of the most studied MAX-phase materials. MAX-phases consists of two-dimensional Mn+1Xn-layers (e.g., T3C2-layers) with strong internal covalent bonds separated by weakly interacting A-layers (e.g., Al-layers), where the repetitive stacking of the Mn+1Xn-layers and the A-layers suggests being the foundation for the unusual but attractive material properties of the MAX-phases. Although being an important parameter, the nature of the bonding between the Mn+1Xn-layers and the A-layers has not yet been established in detail. The X-ray photoelectron spectroscopy data presented in this paper suggest that the weak interaction between the Ti3C2-layers and the Al-layers in Ti3AlC2 is through electrostatic attraction facilitated by a charge redistribution of the delocalized electrons from the Ti3C2-layers to the Al-layers. This charge redistribution is of the same size and direction as between Ti atoms and Al atoms in TiAl alloy. This finding opens up a pathway to predict and improve MAX-phase materials properties through A-layer alloying,…

In this work, we develop a new microkinetic model for the oxygen reduction reaction (ORR) which incorporates field effects into the established computational hydrogen electrode model. We find that the field can significantly alter the binding energy of ORR adsorbates, particularly *OOH, *O 2, and *H 2 O 2 . By considering these effects, we find that the model gives polarization curves and rotating ring disk electrode currents consistent with those found experimentally on the Pt(111), Au(111), and Au(100) electrodes, allowing us to predict onset potentials for both overall ORR activity and activity toward hydrogen peroxide as well as reproduce experimental Tafel slopes. In particular, we resolve the peculiar behavior observed on the Au(100) surface, where increasing pH leads to greatly enhanced 4e-ORR activity, without strongly affecting 2e-ORR activity. We then use these predictions to better understand activity across all ORR catalysts, showing that weak-binding catalysts are more influenced by pH because of the strong field effects on the *OOH adsorbate. Finally, we argue that…

Electroreduction of CO 2 is one of the most potential ways to realize CO 2 recycle and energy regeneration. The key to promoting this technology is the development of high-performance electrocatalysts. Generally, high-throughput computational screening contributes a lot to materials innovation, but still consumes much time and resource. To achieve efficient exploration of electrocatalysts for CO 2 reduction, we created a machine learning model based on an extreme gradient boosting regression (XGBR) algorithm and simple features. Our screening model successfully and rapidly predicted the Gibbs free energy change of CO adsorption (Δ G CO ) of 1060 atomically dispersed metal–nonmetal codoped graphene systems, and greatly reduced the research cost. The competitive reaction, the hydrogen evolution reaction (HER), is also discussed with respect to such a screening model. This work demonstrates the potential of machine learning methods and provides a convenient approach for the effective theoretical design of electrocatalysts for CO 2 reduction.

The band gap is an important parameter that determines light-harvesting capability of perovskite materials. It governs the performance of various optoelectronic devices such as solar cells, light-emitting diodes, and photodetectors. For perovskites of a formula ABX 3 having a non-zero band gap, we study nonlinear mappings between the band gap and properties of constituent elements (e.g., electronegativities, electron affinities, etc) using alternating conditional expectations (ACE)—a machine learning technique suitable for small data sets. We also compare ACE with other machine learning methods: decision trees, kernel ridge regression, extremely randomized trees, AdaBoost, and gradient boosting. The best performance is achieved by kernel ridge regression and extremely randomized trees. However, ACE has an advantage that it presents its results in a graphic form, helping in interpretation. The models are trained with the data obtained from density functional theory calculations. Different statistical approaches for feature selection are applied and compared: Pearson correlation,…

Single-atom catalysts (SACs) are a new research frontier in electrocatalysis such as in the hydrogen evolution reaction (HER). Recent theoretical and experimental studies have demonstrated that certain M–N–C (metal–nitrogen–carbon) based SACs exhibit excellent performance for HER. Here we report a new approach to tune HER activity for SACs by changing the size and dimensionality of the carbon substrate while maintaining the same coordination environment. We screen the 3d, 4d, and 5d transition metal SACs in N-doped 2D graphene and nanographenes of several sizes for HER using first-principles density functional theory (DFT). Nanographenes containing V, Rh, and Ir are predicted to have significantly enhanced HER activity compared to their 2D graphene counterparts. We turn to machine learning to accurately predict the free energy of hydrogen adsorption (Δ G H ) based on various descriptors and compressed sensing to identify key descriptors for activity, which can be used to further screen for additional candidates.

Abstract The effect of spin polarization in conduction and in electric field-induced polarization was measured for double-stranded DNA oligonucleotides and oligopeptides of different lengths. These measurements were conducted using magnetic contact AFM, spin-dependent electrochemistry, spin-dependent polarization, and magnetoresistance studies. It was established that the spin-dependent conduction through chiral molecules depends on the voltage applied with a power of d, when d is larger than unity, and that there is a different voltage threshold for conducting each of the spin polarizations. In addition, there is no spin flipping during the conduction through the chiral system. The spin polarization depends linearly on the length, within the range of lengths studied, and it seems to scale like the optical activity. These results suggest the importance of the electric polarizability in the chiral-induced spin selectivity process. It was also shown that the preferred spin back scattering is suppressed, compared with the nonpreferred spin, probably as a result of the coupling between…

Low-dimensional metal halide perovskites possessing a large exciton binding energy have shown great promise in achieving efficient photonic emission required in the fields of lighting sources and display. Here, efficient dual self-trapped exciton (STE) emissions are directly observed in a low-dimensional inorganic copper iodine quasi-perovskite single crystal. The dual STEs have natural structure-oriented performance, showing a strong electron–phonon coupling. Temperature-dependent PL spectra and Raman spectra demonstrate a thermal-assisted radiative recombination of dark STEs, and based on such a mechanism, an ultrahigh photoluminescence quantum yield (PLQY∼100%) was obtained in bulk crystals with a zero-dimensional electronic structure, favoring photoexcited STEs to gain a large binding energy of up to 563 meV. All these results above show a great advancement in high-efficiency photonic emission through the low-dimensional electronic structure and STE radiative recombination.

Abstract Using first-principles calculations, we demonstrate a combination of two emergent fields, type II van der Waals heterostructures and Janus structures, for the purpose of optimizing the harvesting of solar and nano-electromechanical energy. The most stable stacking order in these nanoscale heterobilayers comprising Janus monolayers of transition metal dichalcogenides has been ascertained based on the interlayer binding energies. The binding energies in WSeTe/WSTe and MoSeTe/WSTe heterobilayers are found to be −27.93 and −25.67 meV/Å2 at an equilibrium interlayer layer distance of 3.25 and 3.32 Å, respectively, indicating the exothermicity in the process of heterobilayer formation, and hence, its experimental feasibility. The mechanical and dynamical stabilities have also been confirmed for these heterobilayers using the Born Huang stability criteria and phonon dispersion calculations. Our results unveil the mechanism underlying the electronic, piezoelectric, photocatalytic properties, and carrier mobility in these Janus heterobilayers. The power conversion efficiency in the…

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…

Bimetallic plasmonic nanoparticles enable tuning of the optical response and chemical stability by variation of the composition. The present numerical simulation study compares Ag–Au alloy, Ag@Au core–shell, and Au@Ag core–shell bimetallic plasmonic nanoparticles of both spherical and anisotropic (nanotriangle and nanorods) shapes. By studying both spherical and anisotropic (with LSPR in the near-infrared region) shapes, cases with and without interband transitions of Au can be decoupled. Explicit comparisons are facilitated by numerical models supported by careful validation and examination of optical constants of Au–Ag alloys reported in the literature. Although both Au–Ag core–shell and alloy nanoparticles exhibit an intermediary optical response between that of pure Ag and Au nanoparticles, there are noticeable differences in the spectral characteristics. Also, the effect of the bimetallic constitution in anisotropic nanoparticles is starkly different from that in spherical nanoparticles due to the absence of Au interband transitions in the former case. In general, the improved…

The efficient separation of photoexcited electrons and holes is crucial for improving the activity of photocatalytic hydrogen evolution. Herein, an efficient core–shell p–n heterojunction of ZnIn 2 S 4 @CuInS 2 microflowers has been devised and fabricated by two-step hydrothermal method. The results revealed that the marigold-like microspheres of ZnIn 2 S 4 @CuInS 2 heterojunction consisted of thin nanosheets, matched well in the lattice, and had a large interface contact area, which boosted charge separation and transfer for solar hydrogen production. Moreover, the intimate interfacial contact between n-type ZnIn 2 S 4 and p-type CuInS 2 resulted in the formation of unique p–n heterojunction, which further promoted charge separation due to the built-in electric field. As a consequence, the ZnIn 2 S 4 @CuInS 2 photocatalyst with 5 atom % CuInS 2 showed the highest production of H 2 evolution (about 1168 μmol·g –1 ) among all prepared photocatalysts, which was nearly 4-fold the amount of the hydrogen production for the pristine ZnIn 2 S 4 . Therefore, the core–shell p–n…

The reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with sodium borohydride (NaBH 4 ) in aqueous media is by far the most widely used reaction for testing the catalytic performance of a large variety of metal nanoparticles (MNPs) using UV/vis absorption spectroscopy. Huge differences in the kinetic rate constant of the catalytic 4-NP to 4-AP conversion as a function of material composition, size, and morphology have been reported by many different groups; so far, these have been exclusively attributed to the catalytic activity of the corresponding MNPs. In this study, we study the overlooked critical role of the pH value on the kinetics of this important reaction. We observe a strong pH dependence of both rate constant and reaction order, which we attribute to the pH-dependent hydrolysis of the reducing agent NaBH 4 in water. In each hydrolysis step, molecular hydrogen as a second reducing agent is produced. Overall, two competing pathways result in: slow hydride versus fast hydrogen reduction. Kinetic simulations based on a model including all relevant species are capable…

Despite the apparent simplicity of the hydrogen evolution reaction (HER) and the decades of research into it, controversy remains in the literature regarding the identity of the active site and the competition between the Heyrovsky and Tafel steps. In this work, we use charge-extrapolated ab initio simulations with explicit water in conjunction with mean-field microkinetic modeling to explore the mechanism for HER on both close-packed (111) and stepped (211) transition metals. First, we show that atop H*, beyond a monolayer of hollow H*, is unlikely to play a role in the HER mechanism, given its very positive adsorption energies. The energetics suggests the Volmer–Heyrovsky mechanism to predominate on fcc transition metals under typical operating conditions. We evaluate our theoretical results vs several experimental observations. We show that the Volmer–Heyrovsky mechanism predicts an activity volcano with its peak at a H* binding Δ G H* ≈ 0 eV, consistent with experiment. In contrast, the Volmer–Tafel volcano shows a broad rate plateau between Δ G H* ≈ 0 eV and Δ G H* ≈ – 0.4 eV.…

This work explores the use of chiral cobalt oxide thin film electrocatalysts for the oxygen evolution reaction and examines how their spin polarization properties might be used to control electrochemically generated intermediates at different pH values. These studies demonstrate that chiral cobalt oxide electrocatalysts reduce the reaction overpotential by 65 mV at 10 mA/cm 2, increase the oxygen yield by 1.4-fold at a fixed current density at pH 10, and decrease the production of hydrogen peroxide by 4.0-fold as compared to that of the corresponding meso-CoO x analogues. Additional studies in which the electrocatalyst is modified to make it paramagnetic exhibit a similar enhancement with an applied external magnetic field. The findings from these studies are described using a proposed mechanistic model, which unifies the favorable effects of chirality and magnetization. The results are attributed to the advantage of spin-polarized intermediates in facilitating the oxygen evolution reaction.

To improve the performance–price ratio of cathode materials, some modified Ni-rich (LiNi 0.8 Co 0.1 Mn 0.1 O 2 ) 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 LiNi 0.8 Co 0.1 Mn 0.1 O 2 (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 Li 2 TiO 3 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…

The simultaneous occurrence of gigantic piezoelectricity and Rashba effect in two-dimensional (2D) materials is unusually scarce. Inversion symmetry occurring in MX 3 (M = Ti, Zr, Hf; X = S, Se) monolayers is broken upon constructing their Janus monolayer structures MX 2 Y (X ≠ Y =S, Se), thereby inducing a large out-of-plane piezoelectric constant d 33 (∼68 pm/V) in them. d 33 can be further enhanced to a super high value of ∼1000 pm/V upon applying vertical compressive strain in the van der Waals bilayers constituted by interfacing these Janus monolayers. Therefore, d 33 in these Janus transition-metal trichalcogenide (TMTC) bilayers reach more than 4-fold times that of bulk ceramic PZT material (∼268 pm/V). The absence of a horizontal mirror symmetry and the presence of strong spin–orbit coupling cause Rashba spin-splitting in electronic bands in these Janus 2D monolayers, which shows up as an ultrahigh Rashba parameter, α R ∼ 1.1 eV Å. It can be raised to 1.41 eV Å via compressive strain. Most of the 2D materials reported to date mainly show in-plane electric polarization, which…

In this work, Sb 2 MoO 6 was applied to the field of photocatalysis and employed to modify g-C 3 N 4 for the first time so as to construct the Sb 2 MoO 6 /g-C 3 N 4 heterojunction. Results reveal that 7.5% Sb 2 MoO 6 /g-C 3 N 4 showed the best activity during photocatalytic RhB degradation and H 2 evolution in water. Rates of RhB degradation and H 2 evolution were 6.6 and 5.4 times higher than those of g-C 3 N 4, respectively. According to UV–visible absorption spectra, transient photocurrent curves, photoluminescence spectra, and electrochemical impedance spectra, outstanding performance of 7.5% Sb 2 MoO 6 /g-C 3 N 4 was benefited from increases in response of visible light and efficient separation of photogenerated carriers. Furthermore, a possible charge carriers’ transmission route was proposed based on the band gap position and characterization consequence.

Abstract The phase transition of Co3O4 into CoO is demonstrated by the impact of shock waves, and the mechanism is studied and reported. The required quantity of Co3O4 nanoparticle (NPs) is synthesized by coprecipitation method. 2.0 MPa transient pressure shock wave is utilized to understand the shock wave induced phase transition in Co3O4 NPs. XRD and Raman studies reveal that the phase transition occurs at 150 shocks in loaded condition. The obtained values of optical band gap energy show that there are changes in band gap energy with respect to the number of shock pulses. SEM images portray that the surface morphology and shape of the particles are considerably changed by the impact of shock waves due to the dynamic recrystallization. Vibrating sample magnetometer (VSM) shows the switchable magnetic phase transition from superpara–para–superpara magnetic behavior. The mechanism of the shock wave induced phase transition is described.

Abstract Fine-grained sediments and sedimentary rocks play important roles in a variety of modern energy technologies from petroleum geology to geological carbon sequestration and radioactive waste management. However, despite their utility and ubiquity, many of their properties remain poorly understood. In particular, the ability to predict the permeability and mechanics of these media remains a persistent fundamental challenge in the geosciences. In the present work, we show how large-scale classical molecular dynamics (MD) simulations can help interpret the properties of fine-grained sedimentary material. All-atom simulations containing 30 discrete clay particles are utilized to understand the evolution of a clay nanoparticle suspension during its progressive dehydration. Microstructural (pore size distribution, tortuosity, anisotropy), thermodynamic (enthalpy and free energy of hydration, anion exclusion), mechanical (total suction), and transport properties (diffusion coefficient tensors of water and sodium) are calculated and compared to the experiment. Overall, our results…

The designability of metal–organic frameworks (MOFs) offers a promising platform for development of multifunctional electrocatalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) which are long-desired in wide-range applications, such as overall water splitting, fuel cells, and metal–air batteries. On the basis of the recent experimental progresses, we proposed from first-principles a family of two-dimensional (2D) MOFs, consisting of transition metal (TM) atoms (TM = Fe–Zn) and 2,3,6,7,10,11-hexaiminotriphenylene (C 18 H 12 N 6 ) functional group (HITP), namely TM 3 (HITP) 2, with versatile multifunctional catalytic activity, which can be ascribed to synergistic effects of TM and organic ligands. Cu 3 (HITP) 2 can serve as a bifunctional catalyst for HER and OER, while Fe 3 (HITP) 2, Co 3 (HITP) 2, and Zn 3 (HITP) 2 are promising for both OER and ORR. The overpotentials of these TM 3 (HITP) 2 monolayers are comparable or even superior to those of the well-developed noble catalysts. The tunable catalytic activity in the…

The notion of “defect tolerance” has often been invoked to explain the excellent performance of halide perovskites in optoelectronic applications. However, this concept has not been rigorously defined or assessed. A common interpretation is that all of the energetically favorable intrinsic defects are shallow. On the basis of examples in the prototypical halide perovskite CsPbI 3, we show that this is not the case. The antisite defects Pb I and I Pb are energetically favorable, but also have levels deep in the band gap. Still, because of strong anharmonicity, they are not efficient nonradiative recombination centers. Our study demonstrates how to correctly evaluate the “defect tolerance” of halide perovskites.

Plasma-catalytic ammonia synthesis is receiving ever increasing attention, especially in packed bed dielectric barrier discharge (DBD) reactors. The latter typically operate in the filamentary regime when used for gas conversion applications. While DBDs are in principle well understood and already applied in the industry, the incorporation of packing materials and catalytic surfaces considerably adds to the complexity of the plasma physics and chemistry governing the ammonia formation. We employ a plasma kinetics model to gain insights into the ammonia formation mechanisms, paying special attention to the role of filamentary microdischarges and their afterglows. During the microdischarges, the synthesized ammonia is actually decomposed, but the radicals created upon electron impact dissociation of N 2 and H 2 and the subsequent catalytic reactions cause a net ammonia gain in the afterglows of the microdischarges. Under our plasma conditions, electron impact dissociation of N 2 in the gas phase followed by the adsorption of N atoms is identified as a rate-limiting step, instead of…

MXenes have attracted great attention as promising substrates for surface-enhanced Raman scattering (SERS) applications. However, the underlying SERS mechanism has not been a focus of any investigation. Herein, we report the first systematic experimental study on the SERS activity of titanium carbide (Ti 3 C 2 T X ) nanosheets with thicknesses ranging from 5 to 120 nm, using methylene blue (MB) as a probe molecule. The experimental and mathematical modeling results show that the Raman enhancement factor (EF) increases monotonically with the increasing thickness of Ti 3 C 2 T X nanosheets; however, it falls drastically around a sheet thickness of 0.8 and 1.0 μm under 532 and 633 nm laser excitations, respectively. The Raman EF reaches a maximum value around a thickness of 2.0 μm, suggesting that a maximum EF can be achieved with a 2.0 μm-thick Ti 3 C 2 T X film substrate. The thickness dependence of the Raman enhancement can be accounted for by the adsorption and intercalation of MB molecules into the interlayer spacing of Ti 3 C 2 T X . Furthermore, by combining experimental…

Nitrogen-doped carbon materials are promising metal-free catalysts for the electrochemical oxygen reduction reaction (ORR). A better theoretical understanding on the nature of the active site(s) would help further optimization of their ORR activity. Although quantum mechanical calculations have been widely employed to elucidate the active sites over various catalysts, these calculations are typically done assuming constant-charge conditions rather than the experimentally relevant constant-potential conditions. In this study, we employ the double-reference method to simulate the energetics of the ORR over pure and N-doped carbon materials under constant-potential conditions. We demonstrate that constant-potential calculations enable more accurate theoretical predictions, comparing well with existing experiments. Our key findings are (1) the zigzag edge of pure graphite is highly active for ORR, (2) the pyridinic N-doped armchair edge is highly active for ORR in alkaline media but not in acid, and (3) graphitic N can donate electrons to pyridinic N to enhance the ORR activity. These…

Abstract Hydrogen evolution reaction (HER) is vital for sustainable energy production and plays a key role in achieving a hydrogen economy. Herein, density functional theory calculations are used to screen for suitable HER catalysts among 24 two-dimensional double transition-metal (TM) carbide MXenes (chemical composition M2′M″C2Tx and M2′M2″C3Tx; M′ and M″ are two different metals, M′ = Cr, V, Ti, or Nb; M″ = Nb, Ta, Ti, or V; and T = O and/or OH) and determine their thermodynamic stability under HER-relevant conditions. The established surface Pourbaix diagrams, describing the chemistry on the basal planes of the MXenes, reveal the most stable terminations under the standard conditions (U = 0, pH = 0, p = 1 bar, and T = 298 K) for Mo2Mx″Cy, Ti2Mx″Cy, and Nb2Ta2C3 to be O-termination, whereas Cr2Mx″Cy and V2M2″C3 expose a mixed O- and OH-termination (M″ = Nb, Ta, Ti, or V; x = 1, y = 2, or x = 2, y = 3). Eighteen different carbides are predicted to be active HER electrocatalyst candidates, and Mo2NbC2O2 showed the lowest overpotential. The Pourbaix diagrams and free energy diagrams…

Over the past few decades, terahertz (THz) spectroscopy has become an established technique for studying the dielectric and transient photoconductive properties of materials. THz spectroscopy is a contact-free probe of electrical conductivity with subpicosecond time resolution, and it has proven especially useful for studying emerging materials. We highlight recent work used to study metal halide perovskites, metal oxides, metal–organic frameworks, and 2D materials in addition to providing an overview of methods in pump–probe THz spectroscopy. We focus on the ways in which THz spectroscopy can be used to study charge transport mechanisms and factors that might limit carrier mobility in emerging materials. This Perspective will provide a general understanding of pump–probe THz spectroscopy and how it can be applied to next-generation materials and will identify challenges and advantages in data processing to extract broadband complex conductivity spectra.

In 2D layered hybrid perovskite like (PEA) 2 PbX 4 (PEA = phenylethylammonium, X = Cl, Br, I), the high frequency dielectric constants for inorganic well layer (ε w ) and organic barrier (ε b ) are distinctly different. The dielectric contrast significantly influences their excitonic binding energy ( E ex 2D ). Here we vary ε w /ε b by varying both the halide anion and the organic cation and then correlate the influence of the dielectric contrast on E ex 2D . We estimate E ex 2D by employing temperature (5.4–300 K) dependent photoluminescence (PL) and find that the change in E ex 2D can be qualitatively monitored simply by measuring the PL lifetime at room temperature. E ex 2D increases, and therefore, PL lifetime decreases by varying halide ions from Cl to Br to I for (PEA) 2 PbX 4 . Notably, this trend is opposite the case of 3D Pb-halide perovskites, where the excitonic binding energy decreases for X = Cl to Br to I. The opposite trend for 2D perovskites is explained by dielectric confinement, where we find E ex 2D ∝ (ε w /ε b ) m, with m as an unknown positive number and ε w > ε…

Herein, we investigate the formation of a cathode electrolyte interphase (CEI) by electrolyte oxidation on a LiNi x M 1– x O 2 ( x > 0.5; M, transition metal) layered oxide (Ni-rich) cathode and compare this phenomenon with a Li-rich layered oxide (Li-rich) cathode. Our investigations focused on two electrochemical properties, the potential and kinetics of electrolyte oxidation, studied using hard X-ray photoelectron spectroscopy (HAXPES), soft X-ray absorption spectroscopy, and density functional theory calculations. HAXPES revealed that a thicker CEI formed on the Ni-rich cathode compared to that on the Li-rich cathode, despite the operation potential of the Ni-rich cathode being lower than that of the Li-rich cathode. Thus, the Ni-rich cathode induces the CEI formation through active oxidation of the electrolyte during charge–discharge cycles. The electronic state of the Ni-rich cathode indicates that the antibonding hybrid orbital of the transition metal 3d–O 2p corresponds to the lowest unoccupied molecular orbital energy level, that is, the hole, and lies near the highest…

In view of the limited ionic conductivity and low lithium transference number in classical poly(ethylene oxide) (PEO)-based salt-in-polymer electrolytes, employing alternative polymer architectures, e.g., polyester homopolymers or copolymers, is a promising approach. To shed light on the influence of the coordination properties of different polymer architectures and to identify their influence on Li ion transport, different polymeric structures are compared, i.e., poly(ε-caprolactone) (PCL), poly(trimethylene carbonate) (PTMC), and a PCL- co -PTMC random copolymer, combined with lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) at varying Li + /monomer ratios r . Electrophoretic NMR ( 1 H and 19 F eNMR) is applied to determine the electrophoretic mobilities of both ionic species, from which partial conductivities and Li transference numbers are calculated. In comparison to PEO-based electrolytes, the ester-based systems show a much higher lithium transference number (∼0.5 compared to ∼0.2), while the total ionic conductivity is lower. However, the partial lithium conductivities…

In this study, machine learning (ML) models combined with density functional theory (DFT) calculations and Gibbs free energy of hydrogen adsorption (Δ G H* ) were employed to facilitate the high-throughput screening of hydrogen evolution reaction (HER) catalysts in various MXene materials. The predicted Δ G H* values show a high-level accuracy via the random forest algorithm by using only simple elemental features. A total of 299 MXene materials were screened by DFT calculations and four ML models (Elman Artificial Neural Networks, kernel ridge regression, support vector regression, and random forest regression algorithms). Using the simple elemental information, the random forest algorithm shows a high-level predicted accuracy with a low testing root-mean-square error of 0.27 eV. Os 2 B- and S-terminated Sc n +1 N n ( n = 1, 2, 3) were discovered to be the active catalysts as Δ G H* approaches zero with wide hydrogen coverages (θ from 1/9 to 4/9). S functional groups play a crucial role in regulating the HER performance due to the antibonding states which are full of electrons.…

Electrochemical reduction of carbon dioxide (CO 2 ER) to high-energy-density multicarbon products is a quite promising technique for large-scale renewable energy storage, for which searching for stable, inexpensive, and efficient catalysts is a key scientific issue. In this work, the potential of an experimentally available single iron (Fe) atom supported on graphdiyne (Fe/GDY) as the CO 2 ER catalyst was explored by means of density functional theory (DFT) computations. Our results revealed that Fe/GDY exhibits high stability due to the strong hybridization between the Fe 3d orbitals and the C 2p orbitals of GDY. Interestingly, due to the small limiting potential of −0.43 V, the anchored Fe atom can effectively reduce CO 2 to CH 4 along the following pathway: CO 2 → HCOO* → HCOOH* → HCO* → H 2 CO* → H 3 CO* → O* + CH 4 → OH* → H 2 O, in which the hydrogenation of HCOOH* to HCO* is the potential-determining step. Furthermore, the unsaturated HCO* species on Fe/GDY can provide an active site for further coupling with CO to generate C 2 H 5 OH with a small activation energy for C–C…

Localized surface plasmon resonances have attracted much attention due to their ability to enhance light-matter interactions and manipulate light at the subwavelength level. Recently, alternatives to the rare and expensive noble metals Ag and Au have been sought for more sustainable and large-scale plasmonic utilization. Mg supports plasmon resonances, is one of the most abundant elements in earth's crust, and is fully biocompatible, making it an attractive framework for plasmonics. This feature article first reports the hexagonal, folded, and kite-like shapes expected theoretically from a modified Wulff construction for single crystal and twinned Mg structures and describes their excellent match with experimental results. Then, the optical response of Mg nanoparticles is overviewed, highlighting Mg's ability to sustain localized surface plasmon resonances across the ultraviolet, visible, and near-infrared electromagnetic ranges. The various resonant modes of hexagons, leading to the highly localized electric field characteristic of plasmonic behavior, are presented numerically and…

The low-yield and high-recombination rate of carriers are the main factors that limit the efficiency of traditional piezocatalysis and photocatalysis. In this paper, the Bi 0.5 Na 0.5 TiO 3 @TiO 2 composite catalyst with heterojunction was prepared and applied to obtain superior piezo-photocatalytic activity. On the one hand, the narrow band gap (2.77 eV) made it easy for the composite to excite photogenerated carriers under visible light. On the other hand, the piezoelectric field that originated from the bending deformation of Bi 0.5 Na 0.5 TiO 3 effectively promoted the separation and migration of carriers. Driven by ultrasonic vibration and visible light irradiation, the poled composite catalyst degraded over 97% of RhB dye solution through piezo-photocatalytic effect in 90 min. Compared with pure BNT and TiO 2, the enhanced catalytic performance of the BNT@TiO 2 composite indicated the superiority of the heterojunction.

In contrast to the energy-extensive Haber–Bosch process, green production of ammonia (NH 3 ) at ambient conditions remains as one of the main goals of the 21st century. In this work, we systematically studied single and double transition metal atoms (Fe, Co, Mo, Ru) anchored on graphyne (GY) for nitrogen (N 2 ) reduction by means of density functional theory (DFT). We demonstrated that Ru dimer anchored on two-dimensional (2D) GY exhibits superior catalytic activity for nitrogen reduction via an enzymatic pathway with the maximum free energy change (Δ G ) of 0.43 eV. Our findings propose highly reactive catalytic materials for efficient N 2 reduction while highlighting the importance of double-atom catalysts for nitrogen reduction reaction (NRR).

Abstract Chitosan was deposited on fumed silica without the addition of cross-linkers or activating agents. The chitosan surface layer has a high affinity toward organic molecules, e.g., Acid Orange 8 (AO8) dye, robust to a broad range of simulated conditions (variance with respect to temperature, time, and concentration of solute). Experimental equilibrium data were analyzed by the generalized Langmuir equation taking into consideration the energetic heterogeneity of the adsorption system. The effect of temperature on dye uptake and adsorption rate was studied. According to the calculated thermodynamic functions ΔG°, ΔH°, and ΔS° from the equilibrium data at different temperatures, the adsorption of AO8 onto chitosan–fumed silica composite is exothermic and spontaneous. The studies of temperature effect on adsorption equilibrium show that the maximum adsorption capacity (determined from the Langmuir–Freundlich equation) of synthesized composite toward AO8 is about one-third higher in the case of an isotherm measured at 5 °C than this value obtained for the isotherm measured at 45…

The present work investigates the synthesis and characteristics of a Na super-ionic conductor -type Sc 3+ - and Yb 3+ -doped Na 3 Zr 2 Si 2 PO 12 solid electrolyte for application in solid-state Na-ion batteries. A significant improvement of Na-ion conductivity in Na 3 Zr 2 Si 2 PO 12 has been achieved through crystal engineering and microstructure refinement. The presence of the monoclinic-ZrO 2 impurity phase adversely affecting the Na-ion conductivity is eliminated using the cubic-ZrO 2 precursor at the place of monoclinic-ZrO 2 in the conventional solid-state reaction method. Utilizing cubic-ZrO 2 also refined the microstructure with thin and microcrack-free grain boundaries. A replacement of 16.5 at. % of Zr 4+ by Sc 3+ in Na 3 Zr 2 Si 2 PO 12 enhances the room-temperature total ionic conductivity from 0.61 to 0.96 mS·cm –1 . Replacing 11.11 at. % of Sc 3+ by Yb 3+ further improves the room-temperature ionic conductivity to 1.62 mS·cm –1, which is >2.5 times higher than that of bare Na 3 Zr 2 Si 2 PO 12 . The strategic approach used to raise the ionic conductivity in the…

The particle size-dependent surface optical (SO) phonon and magnon modes of NiO nanoparticles were extensively studied using room-temperature confocal Raman spectroscopy. The required nanoparticles were synthesized using a simple co-precipitation method, and the suitable particle sizes were obtained by varying the annealing temperature from 300 to 900 °C. The SO phonon modes of the prepared NiO nanoparticles were blue shifted and broadened as the nanoparticle size was reduced due to the lattice contraction and boundary relaxation, respectively. The presence of SO phonon modes was studied using the dielectric continuum (DC) model for a nearly spherical-like particle, and a violation of DC model was observed in the experimental SO mode frequency due to the presence of first order magnon background over the phonon modes. The magnon background and SO mode frequency differences were increased with the nanoparticle size. The absence of magnon and second-order phonon modes in smaller NiO nanoparticles indicates the reduced spin correlation between the next nearest neighbor Ni 2+ ions in…

Herein, we report a new strategy to effectively enhance the thermal conductivity of epoxy composites by constructing a three-dimensional thermally conductive network with 2D boron nitride nanosheets (BNNSs) and 0D boron nitride microspheres (BNMSs). In the composites, the BNMSs serve as the principal building blocks for constructing the thermally conductive network because of their large sizes, while the BNNSs are embedded within the interspaces of the BNMSs to improve the thermal pathway density. Experimental and finite element analysis reveals that the combined BNNSs and BNMSs (BNNSs/BNMSs) outperform individual BNNSs and BNMSs in terms of thermal conductivity enhancement. Notably, the BNNSs/BNMSs/epoxy composite with a filler loading of 30 wt % shows a thermal conductivity of 1.148 W m –1 K –1, which is approximately 28.0 and 51.5% higher than the corresponding data for BNNSs/epoxy and BNMSs/epoxy composites, respectively, and five times that of pure epoxy. In addition, the BNNSs/BNMSs/epoxy composites exhibit favorable dielectric properties, dynamic mechanical properties, and…

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…

The photocatalytic properties of cesium lead bromide (CsPbBr 3 ) perovskite nanocrystals make them attractive for designing light harvesting assemblies. Often ignored, the surface chemistry can dictate the excited state interactions of these semiconductor nanocrystals with charge-shuttling redox molecules. We have now explored the impact of CsPbBr 3 nanocrystal surface modification on the excited state interactions with methyl viologen (MV 2+ ) for three different ligand environments: prototypical oleic acid/oleylamine (OA/OAm) ligands, PbSO 4 -oleate capping, and didodecyldimethylammonium bromide (DDAB) ligands. Native OA/OAm ligands and PbSO 4 -oleate capping exhibit the strongest complexation with MV 2+, whereas the bulky DDAB ligand environment shows an order of magnitude weaker complexation. The electron transfer rate constants as measured from transient absorption spectroscopy vary in the range of 1.2–3.6 × 10 11 s –1 for different ligand environments. For DDAB-CsPbBr 3 NCs, the efficiency of electron transfer (Φ et ) is 73%. Despite a protective capping layer, PbSO 4 -oleate…

Electrolyte systems based on binary mixtures of organic carbonate ester cosolvents have limitations in ionic transport and thus limit extreme fast charge (XFC) and high-rate cycling of energy dense lithium-ion cells with thick electrodes (>80 μm per side) at ambient temperature and below. Here, we present LiPF 6 in methyl acetate (MA) as an ester-based liquid electrolyte that offers substantial improvements in ionic transport, doubling the conductivity of conventional electrolyte systems. Density functional theory-based molecular dynamics (DFT-MD) simulations give insights into the experimentally observed low solvation number for lithium ions in MA solutions and show a solution system with highly mobile, loosely bound ionic species. We show that MA-based electrolytes with suitable additive formulas enable high cycling rates and excellent low-temperature cycling performance in lithium-ion cell designs with thick electrodes but come with a trade-off in lifetime at elevated temperature. While there are inherent practical issues with MA as an electrolyte solvent, including a low flash…

Metal–organic frameworks (MOFs) have garnered interest as potential solid sorbent materials for postcombustion CO 2 capture. With a seemingly infinite design space, high-throughput computational screening of MOFs has developed into an effective tool for the development of new materials. In this work, machine learning (ML) has been used to develop accurate quantitative structure–property relationship (QSPR) models to rapidly predict the CO 2 working capacity and CO 2 /N 2 selectivity at the low-pressure conditions relevant to postcombustion carbon capture (0.15 bar CO 2, 0.85 bar N 2 ). A database of over 340 000 MOFs constructed from hundreds of types of building units arranged in over 1000 net topologies was used to train and test the models. Neural network ML models were optimized using six geometric descriptors along with three so-called chemical descriptors, namely, the atomic property-weighted radial distribution function (AP-RDF) and some variants thereof, the bag-of-atoms, and the chemical motif density descriptors. The ML models built using geometric descriptors alone…

Hematite (Fe 2 O 3 ) is a well-known oxide semiconductor suitable for photoelectrochemical (PEC) water splitting and industry gas sensing. It is widely known that Sn doping of Fe 2 O 3 can enhance the device performance, yet the underlying mechanism remains elusive. In this work, we determine the relationship between electronic structure, optical properties, and PEC activity of Sn-doped Fe 2 O 3 by studying highly crystalline, well-controlled thin films prepared by pulsed laser deposition (PLD). We show that Sn doping substantially increases the n-type conductivity of Fe 2 O 3, and the conduction mechanism is better described by a small-polaron hopping (SPH) model. Only 0.2% Sn doping significantly reduces the activation energy barrier for SPH conduction from at least 0.5 eV for undoped Fe 2 O 3 to 0.14 eV for doped ones. A combination of X-ray photoemission, X-ray absorption spectroscopy, and DFT calculations reveals that the Fermi level gradually shifts toward the conduction band minimum with Sn doping. A localized Fe 2+ -like gap state is observed at the top of the valence band,…

In the present study, we propose a new set of descriptors, appropriate for machine learning (ML) methods, aiming to predict accurately the gas adsorption capacities of nanoporous materials. The present work focuses on systems with nonnegligible electrostatic interactions between the materials’ framework and the guest gas. For that, the CO 2, H 2, and H 2 S gases are examined. The present approach is a generalization of our recent development for guest gases with no electrostatic interactions, such as CH 4 . For both types of systems, as ML descriptors we consider the adsorption probabilities by the materials’ framework of a small number of probe atoms with different van der Waals diameters. After examination and evaluation of various numerical schemes, probe atoms that carry in their centers an electric dipole are found to be the most appropriate for systems with electrostatic interactions. The accuracy of the present approach is assessed by comparing the ML predictions with a data set of reference results obtained after performing grand canonical Monte Carlo (GCMC) simulations.…

The standard model for diffuse charge phenomena in colloid science, electrokinetics, and biology is the Poisson–Boltzmann mean-field theory, which breaks down for multivalent ions and large surface charge densities because of electrostatic correlations. In this paper, we formulate a predictive continuum theory of correlated electrolytes based on two extensions of the Bazant–Storey–Kornyshev (BSK) framework: (i) a physical boundary condition enforcing continuity of the Maxwell stress at a charged interface, which upholds the contact theorem for dilute primitive-model electrolytes, and (ii) scaling relationships for the correlation length, for a one-component plasma at a charged plane and around a cylinder, as well as a dilute z:1 electrolyte screening a planar surface. In these cases, the theory accurately reproduces Monte Carlo simulation results from weak to strong coupling, and extensions are possible for more complex models of electrolytes and ionic liquids.

Ni-rich layered LiNi x Mn y Co z O 2 (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…

The practical and widespread application of photocatalysts is limited by the problem of visible-light absorption, the immediate recombination of photogenerated electron–hole pairs, and the low number of active sites on their surface. We synthesized a series of novel TiO 2 QDs/ZnBi 2 O 4 nanocomposites through the hydrothermal method to overcome these problems. The formation of nanocomposites and their features were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, photocurrent, field-emission scanning electron microscopy, transmission electron microscopy, electrochemical impedance spectroscopy, photoluminescence, Brunauer–Emmett–Teller, ultraviolet–visible diffuse reflection spectroscopy, and total organic carbon techniques. Among the different ratios of nanocomposites, the TiO 2 QDs/ZnBi 2 O 4 (10%) displayed the highest photocatalytic activity under visible-light irradiation, and the corresponding rhodamine B (RhB) degradation rate…

Gas hydrates have attracted substantial attention in energy and environmental research as a medium for energy storage and transport, gas separation, and carbon dioxide sequestration. However, the fundamental kinetics of gas hydrate formation remain poorly understood. Here, we report on a new approach for determining the reaction rate constant of hydrate formation. High-resolution X-ray computed tomography measurements were performed to monitor xenon hydrate formation on a water droplet, and a shell growth model was developed to quantify the kinetics of hydrate formation. The reaction rate constant of xenon hydrate formation was determined by combining the shell growth model and experimental data. The experimental reaction rate constant tended to increase with increasing temperature from 5.3 × 10 –7 to 1.65 × 10 –6 m/s in the range of 275.15–288.15 K corresponding to an activation energy of 56.6 kJ/mol. This approach is expected to be applicable to determining the reaction rate constant for other gas hydrates.

Induced hyperthermia has been demonstrated as an effective oncological treatment due to the reduced heat tolerance of most malignant tissues; however, most techniques for heat generation within a target volume are insufficiently selective, inducing heating and unintended damage to surrounding healthy tissues. Plasmonic photothermal therapy (PPTT) utilizes light in the near-infrared (NIR) region to induce highly localized heating in gold nanoparticles, acting as exogenous chromophores, while minimizing heat generation in nearby tissues. However, optimization of treatment parameters requires extensive in vitro and in vivo studies for each new type of pathology and tissue targeted for treatment, a process that can be substantially reduced by implementing computational modeling. Herein, we describe the development of an innovative model based on the finite element method (FEM) that unites photothermal heating physics at the nanoscale with the micron scale to predict the heat generation of both single and arrays of gold nanoparticles. Plasmonic heating from laser illumination is computed…

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 (FeCl 2 ) 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 moiré superstructures of a single-layer FeCl 2 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 FeCl 2 films grown on Au(111) exhibit a 1T structure. The DFT calculations reveal that a single-layer 1T-FeCl 2 has a ferromagnetic ground state. The minimum-energy configuration of a bilayer FeCl 2 is satisfied for the 1T–1T structure with ferromagnetic layers coupled antiferromagnetically. These results make FeCl 2 a promising candidate as ideal electrodes…

Synergistic interactions within metal clusters help to improve the catalytic activity of individual metal atoms and contribute to advance the understanding of mechanisms in multisite catalysis. Herein, the MoS 2 -supported Fe 2 cluster was chosen as the catalyst for nitrogen reduction reaction (NRR) to produce ammonia. First-principles calculations reveal that the Fe 2 /MoS 2 exhibits high catalytic activity for electrochemical NRR and sheds light on the enzymatic mechanism which bears a low overpotential of 0.21 V. Compared with a single Fe atom on MoS 2, the presence of a vicinal Fe atom (i.e., Fe 2 /MoS 2 ) enables side-on adsorption of N 2, which is propitious to effective N–N bond activation. Whereas the Fe 3 /MoS 2 catalyst, simply with one more Fe atom involved, reduces electron depletion among Fe atoms and hence represses N 2 adsorption. The origin of reasonable NRR catalysis of Fe 2 /MoS 2 is further unveiled by natural charge population and projected crystal orbital Hamilton population (pCOHP) analyses. This investigation provides an effective strategy for designing active…

Significant progress has been made in the field of a priori crystal structure prediction, with a number of recent remarkable success stories. Herein, we briefly outline the methods that have been developed for finding the global minimum structure and interesting local minima without the need for experimental information. Focus is placed on describing the XtalOpt evolutionary algorithm (EA) developed in our group toward this end. XtalOpt is published under well-known open-source licenses, and the EA searches can be analyzed via the Avogadro chemical editor and visualizer. We describe new algorithmic developments that have made it possible to predict the structures of ever-more complex crystalline lattices. Benchmark tests, which clearly illustrate how the new developments improve the success rate and accelerate the discovery of the global minimum structure, are performed. Finally, we describe how XtalOpt has been employed to predict novel ternary hydrides that have the propensity for high-temperature superconductivity under pressure.

O3-type layered transition metal oxides have shown great promise as high-capacity cathode materials for sodium-ion batteries in large-scale energy storage, due to their low cost and the abundance of sodium resources. However, the limited interlayer spacing and the unstable structure of the O3 phase result in inferior cycling stability and poor rate capability, which could even hinder their practical application process. Hereby, we present that doping with nonelectrochemically active Al in NaAl x (Ni 0.5 Mn 0.5 ) 1– x O 2 ( x = 0, 0.02, 0.06, 0.1) can effectively alleviate these issues. Among these materials, materials with x = 0.02 exhibited the best electrochemical behavior with improved capacity and better cycling property than other materials. Specifically, the discharge capacity of a 2 mol % Al-doped material possesses 63.2% capacity retention after 200 cycles at a current density of 240 mA g –1, which is 21.4% higher than that in NaNi 0.5 Mn 0.5 O 2 . In addition, a 2 mol % Al-doped electrode also shows outstanding rate capability and delivers 90 mAh g –1 at a high rate of 480…

Abstract In this study the optical spectroscopy, the excited state dynamics, and in particular the Tb3+ → Eu3+ energy transfer, have been investigated in detail both from the theoretical and experimental point of view in eulytite double phosphate hosts A3Tb­(PO4)3 (A = Sr, Ba) doped with Eu3+. It has been found that the energy transfer is strongly assisted by fast migration in the donor Tb3+ subset. Moreover, the transfer rates and efficiencies depend significantly on the nature of the divalent elements present in the structure and hence on the distances between Tb3+-Eu3+ nearest neighbors. It is shown that the competition between quadrupole–quadrupole and exchange interaction is crucial in accounting for the transfer rates.

Mineral and glass dissolution is a scientific topic that has been deeply investigated but is not understood completely; it continues to be of great interest in the geochemical and materials science communities. If the interfacial dissolution/reprecipitation mechanism seems to be applicable to most silicate minerals, the debate remains open concerning glass. Here, we studied two model glasses, a ternary borosilicate (CJ1) and the same glass doped with 4.1 mol % of Al 2 O 3 (CJ2). The two glasses were altered at 90 °C, pH 9, and in conditions far and close to saturation with respect to amorphous silica to determine the initial and residual rates. Moreover, a specific experiment was conducted for a short duration with a solution highly enriched with 18 O and 29 Si isotopes to understand how passivating gels form. SEM, TEM, and ToF-SIMS characterizations along with Monte Carlo simulations were used to understand the rate limiting reactions at play and infer the role of Al. We show that Al yields a slower matrix dissolution in dilute conditions. However, it slows down the formation and…

Abstract Excited-state symmetry-breaking charge separation (SB-CS) can offer an efficient pathway to solar energy capture and conversion. We synthesized a series of 1,6,7,12-tetrakis­(4-t-butylphenoxy)­perylene­(3,4:9,10)­bis­(dicarboximide) cyclophane dimers with m-xylylene, p-xylylene, and 4,4′-diyldimethane-1,1′-biphenyl spacers and studied them with steady-state and time-resolved optical spectroscopies. Photoinduced SB-CS occurs in all three cyclophanes in CH2Cl2, with the SB-CS rate decreasing as the interchromophore distance is increased. Time-resolved emission spectroscopy and kinetic modeling reveal that the charge-separated state exists in pseudoequilibrium with the excited state prior to decay. Notably, the meta-spaced cyclophane also undergoes SB-CS in toluene within ∼100 ps, despite the lack of charge stabilization by the low dielectric constant solvent. These results demonstrate that SB-CS can occur across long distances and in weakly polar environments, which offers the possibility of harnessing SB-CS for solar energy capture and conversion.

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-4 H -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.

Cr 3+ -activated persistent luminescent phosphors with a spinel structure are emerging materials in bio-imaging applications for their distinctive features of deep biotissue penetration and rechargeable near-infrared persistent emission. To realize the long-term and multicycle imaging purpose, reactivation using in situ external light with deep biotissue penetration is an alternative strategy, apart from the efforts on trap modulations for the host lattice. However, recharging with high-energy ultraviolet /visible photons will result in low activation efficiency because of the absorption by biological tissues. Here, we report a low-energy photon-rechargeable near-infrared persistent material, with rechargeable efficiency 400 times higher than that of the ZnGa 2 O 4:Cr 3+ reference material. The crystal field strength and band gap energies can be tailored by control of cation occupancy, contributing the red shift of both the persistent luminescence excitation and emission spectra of the optimized complex spinel samples. The persistent emission red shift is of interest for improved…

We report the two-photon excited nonresonant surface-enhanced hyper Raman scattering (SEHRS) spectra of six aromatic thiol molecules during their interaction with gold and silver nanostructures. SEHRS spectra were obtained from thiophenol, benzyl mercaptan, and phenylethyl mercaptan and from the three isomers 2-aminothiophenol (2-ATP), 3-aminothiophenol (3-ATP), and 4-aminothiophenol (4-ATP). All SEHRS spectra were excited off-resonance at a wavelength of 1064 nm and compared to surface-enhanced Raman scattering (SERS) spectra excited at 785 nm or at 633 nm. The SEHRS spectra show a different interaction of thiophenol, benzyl mercaptan, and phenylethyl mercaptan with silver and gold nanostructures. Density functional theory calculations were used to support band assignments, in particular, for the unknown SERS spectrum of 3-ATP, and identify a band of phenylethyl mercaptan as a vibrational mode unique to the SEHRS spectrum and very weak in the Raman and infrared spectra. 2-ATP, 3-ATP, and 4-ATP show a different interaction with gold nanostructures that was found to depend on pH.…

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…

Ni-rich NMC materials are a particularly promising class of Li-ion cathodes for various applications. LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) offers a unique balance of thermal stability and energy density, thus attracting attention for electric vehicle implementation. However, upon extended cycling, capacity fade is prevalent due to structural degradation, which is a major drawback for layered oxide cathodes. Therefore, exploring the underlying phenomena that drive detrimental structural response can lead to future improvements. For the first time, operando X-ray absorption spectroscopy (XAS) was performed on NMC622 pouch cells at three different stages. An extensive description of the first cycle (de)lithiation mechanisms was achieved through X-ray absorption near-edge structure analyses and extended X-ray absorption fine structure modeling. Transition metal specific electrochemical participation and structural variation revealed that much of the delivered capacity and distortion is a result of Ni redox behavior, while the local structure of Co and Mn are impacted due to their…

Abstract In this study, we present a new comprehensive methodology to quantify the catalytic activity of heterogeneous materials for the hydrogen evolution reaction (HER) using ab initio simulations. The model is composed of two parts. First, the equilibrium hydrogen coverage is obtained by an iterative evaluation of the hydrogen adsorption free energies (ΔGH) using density functional theory calculations. Afterward, the ΔGH are used in a microkinetic model to provide detailed characterizations of the entire HER considering all three elementary steps, i.e., the discharge, atom + ion, and combination reactions, without any prior assumptions of rate-determining steps. The microkinetic model takes the equilibrium and potential-dependent characteristics into account, and thus both exchange current densities and Tafel slopes are evaluated. The model is tested on several systems, from polycrystalline metals to heterogeneous molybdenum disulfide (MoS2), and by comparing to experimental data, we verify that our model accurately predicts their experimental exchange current densities and Tafel…

Atomically thin semiconductors such as monolayer MoS 2 and WS 2 exhibit nonlinear exciton–exciton annihilation at notably low excitation densities (below ∼10 excitons/μm 2 in exfoliated MoS 2 ). Here, we show that the density threshold at which annihilation occurs can be tuned by changing the underlying substrate. When the supporting substrate is changed from SiO 2 to Al 2 O 3 or SrTiO 3, the rate constant for second-order exciton–exciton annihilation, k XX [cm 2 /s], is reduced by 1 or 2 orders of magnitude, respectively. Using transient photoluminescence microscopy, we measure the effective room-temperature exciton diffusion coefficient in bis(trifluoromethane)sulfonimide-treated MoS 2 to be in the range D = 0.03–0.06 cm 2 /s, corresponding to a diffusion length of L D = 350 nm for an exciton lifetime of τ = 18 ns, which does not depend strongly on the substrate. We discuss possible mechanisms for the observed behavior, including substrate permittivity, long-range exciton–exciton or exciton–charge interactions, defect-mediated Auger recombination, and spatially inhomogeneous…

MXenes have shown outstanding properties due to their highly active hydrophilic surfaces coupled with high metallic conductivity. Many applications rely on the intercalation between Ti 3 C 2 T x (T x describes the -OH, -F and -O- surface terminations) flakes by ions or molecules, which in turn might alter the Ti 3 C 2 T x surface chemistry and electrochemical properties. In this work, we show that the capacitance, rate capability, and charge carrier kinetics in Ti 3 C 2 T x MXene electrodes are remarkably enhanced after urea intercalation (u-Ti 3 C 2 T x ). In particular, the areal capacitance increased to 1100 mF/cm 2, which is 56% higher than that of pristine Ti 3 C 2 T x electrodes. We attribute this dramatic improvement to changes in the Ti 3 C 2 T x surface chemistry upon urea intercalation. The oxidation state and the oxygen bonding of individual Ti 3 C 2 T x flakes before and after urea intercalation are probed by soft X-ray absorption spectroscopy (XAS) at the Ti L- and O K-edges with 30 nm spatial resolution in vacuum. After urea intercalation, a higher Ti oxidation state…

The unique properties of chemical materials are usually related to the topology and size of their constituent unit. Inspired by a previous report on the synthesis of a separable catenane composed of two Möbius-conjugated nanohoops, we theoretically predicted the optical properties of the Möbius monomer, its Hückel analogue, and some structures derived from them. The underlying reasons for the effects of topology and size on molecular (hyper)polarizability and the absorption spectrum were explored from the electronic structure level by means of several analytical methods, including the two-level model, (hyper)polarizability density analysis, and the (hyper)polarizability contribution decomposition. This work is conducive to providing rational design ideas for those who are committed to discovering and synthesizing nanoscale cyclic molecules with special optical properties.

Abstract Plasma technology is gaining increasing interest for CO2 conversion. The gas temperature in (and after) the plasma reactor largely affects the performance. Therefore, we examine the effect of cooling/quenching, during and after the plasma, on the CO2 conversion and energy efficiency, for typical “warm” plasmas, by means of chemical kinetics modeling. For plasmas at low specific energy input (SEI ∼ 0.5 eV/molecule), it is best to quench at the plasma end, while for high-SEI plasmas (SEI ∼ 4 eV/molecule), quenching at maximum conversion is better. For low-SEI plasmas, quenching can even increase the conversion beyond the dissociation in the plasma, known as superideal quenching. To better understand the effects of quenching at different plasma conditions, we study the dissociation and recombination rates, as well as the vibrational distribution functions (VDFs) of CO2, CO, and O2. When a high vibrational–translational (VT) nonequilibrium exists at the moment of quenching, the dissociation and recombination reaction rates both increase. Depending on the conversion degree at…

This work investigates the ability of the deep-learning potential (DP) to describe structural, dynamic and energetic properties of crystalline and amorphous Li–Si alloys. Li–Si systems play an important role in the development of high-energy lithium ion batteries. One challenge in simulating Li–Si systems is to balance the proper description of complex Li–Si interactions and the system size. Molecular simulations implemented with DP provide a promising alternative to achieve this balance and enable us to investigate the fine details of Li–Si systems that the classical force fields cannot offer. We develop a DP for Li–Si systems with Li/Si ratio ranging from 0 to 4.2 based on a vast data set generated using the quantum mechanical calculations in an active learning procedure. Then we investigate the structural and dynamic properties of several crystalline and amorphous Li–Si systems using this developed DP. The DP can predict bulk densities, the radial distribution functions, and diffusivity of Li in amorphous Li–Si systems with an accuracy close to quantum mechanical calculations…

Since the 1970s, it has been shown that calcium phosphate crystals nucleate from one or several early amorphous calcium phosphate phases into several in vivo and in vitro systems. However, the precise chemical composition, structure, and transformation mechanism of these amorphous phases remain controversial. Here, we characterize the reaction mechanism and kinetics of formation of two phosphate crystals, brushite and hydroxyapatite, by using in situ Raman spectroscopy in batch reactors at 25 °C. We investigate three pH regimes to control the phosphate speciation in solution and used solutions with or without citric acid, a complexing agent that may stabilize the amorphous phases. As expected, brushite (CaHPO 4 ·2H 2 O) forms at pH < 9.8. Amorphous calcium phosphate (ACP: CaHPO 4 · n H 2 O) with a short lifetime (<2 min) and octocalcium phosphate (OCP: Ca 8 (HPO 4 ) 2 (PO 4 ) 4 ·5H 2 O) are the main transient phases prior to brushite nucleation that occurs after ∼8 min. At pH > 11, hydroxyapatite (HAp: Ca 10 (PO 4 ) 6 (OH) 2 ) nucleates after >35 min, depending on the experimental…

In this work, we have investigated the influence of alkali metal cations on the electrical double-layer (EDL) properties for various metal electrodes. Using electrochemical impedance spectroscopy, we demonstrate that those cations significantly affect the EDL capacitance in the case of single-crystalline Cu(111), Cu(100), Au(111), Pt(111), stepped Pt(775), and kinked Pt(12 10 5) electrodes in 0.05 M MeClO 4 (Me + = Li +, Na +, K +, Rb +, and Cs + ) electrolytes. For all the electrodes, the capacitance always linearly increases with decreasing hydration energy of Me + in the following order: Li + < Na + < K + < Rb + < Cs + . Moreover, we estimate the effective concentrations of the alkali metal cations near the electrode surfaces by correlating the capacitances with the relative permittivity. For all the electrodes, the concentrations near the electrode surface were calculated to be ∼60 to 80 times higher than in the bulk solutions.

The complete phase diagram of organic-cation solid solutions of lead iodide perovskites [FA x MA 1– x PbI 3, where MA stands for methylammonium, CH 3 NH 3, and FA for formamidinium, CH(NH 2 ) 2 ] with compositions x ranging from 0 to 1 in steps of 0.1 was constructed in the temperature range from 10 to 365 K by combining Raman scattering and photoluminescence (PL) measurements. The occurrence of phase transitions was inferred from both the temperature-induced changes in the optical emission energies and/or the phonon frequencies and line widths, complementing X-ray and neutron scattering literature data. For MA-rich perovskites ( x ≤ 0.2), the same structural behavior as for MAPbI 3 was observed with decreasing temperature: cubic Pm 3̅ m → tetragonal-I I 4/ mcm → orthorhombic Pnma . As the FA molecule is larger and more symmetric but less polar than MA, a tetragonal crystal structure is favored at low temperatures and FA compositions x > 0.4, to the detriment of the orthorhombic phase. As a consequence, with decreasing temperature, the phase transition sequence for FA-rich compounds…

We present a mean-field density functional theory (MFDFT) study of adsorption and desorption for nitrogen at 77 K in three-dimensional (3D) geometrical models of ordered and random mesoporous silicas obtained by electron tomography. Parameters of the lattice MFDFT model, such as reduced temperature, the ratio between the energies of fluid–solid and fluid–fluid interactions, and the lattice unit size, were investigated to achieve best qualitative agreement with experimental isotherms in the hysteresis region. Equilibrium and metastable equilibrium states were analyzed for 500 pressure values in the range of 0 < p / p 0 ≤ 1 for both adsorption and desorption, which allowed us to resolve subtle features of the isotherms. Calculated and experimental isotherms show good agreement in the hysteresis region, identifying type IV(a) isotherms with a H1 hysteresis loop for ordered silicas (SBA-15, KIT-6) and H2(a) hysteresis loop for random silica. Hysteresis loops are particularly narrow and hysteresis branches parallel and almost vertical for the ordered silicas. This indicates homogeneous…

Abstract Crystal structures and surface energies of transition metals are of fundamental importance to the activity, selectivity, and stability in heterogeneous catalysis, but the interplay between crystal structures and surface energies as well as its dependence on composition remains elusive. In the present work, we performed comprehensive density functional theory calculations of Co, Ni, Ru, Rh, Pd, Os, and Ir in both hexagonal close-packed (hcp) and face-centered cubic (fcc) phases and considered numerous surfaces to derive the equilibrium morphology and the surface energy. Irrespective of the transition metals considered, the fcc phase exposes mainly {111} and {100} facets, whereas the hcp phase exposes mainly {0001}, {101̅0}, and {101̅1} facets. For Co, Ru, and Os preferring the hcp bulk at ambient conditions, the corresponding surface energies are found higher to be than those in the fcc bulk, whereas for Ni, Rh, Pd, and Ir preferring the fcc bulk phase at ambient conditions, the opposite trend is found. A negative linear relationship of the surface energy difference between…

First-principles density functional theory (DFT) computations were adopted to assess the potential application of a boron carbide (BC 3 ) monolayer with point and topological defects as an anode material in alkali metal-based lithium (Li) ion rechargeable batteries. Results show that point defects (mono and bi vacancies) induce a large structural deformation upon Li intercalation which restricts their use for anode application. However, the Stone–Wales defect filled BC 3 monolayer shows high structural stability with a negative Li binding energy of −1.961 eV in comparison with −0.930 eV of its pristine form. It is also noticed that after adsorbing the Li atom, the semiconducting characteristics of both the pristine and Stone–Wales defect filled BC 3 monolayers are transformed into metallic, electrically conductive states. More importantly, the Li alkali metal atom shows fast diffusion on the surfaces of both the pristine and the Stone–Wales defect filled BC 3 monolayers with low energy barriers of 0.34 and 0.33 eV, respectively. Besides, both the pristine and Stone–Wales defect…

Determining the role of lattice oxygen in the oxygen evolution reaction (OER) is pivotal to understanding reaction mechanisms and predictive design of electrocatalysts based on transition metal oxides. Here, using well-defined, isotope ( 18 O)-enriched, epitaxial LaNiO 3 thin films as a model system, we show that dynamic lattice oxygen exchange occurs during OER. Time-of-flight secondary ion mass spectrometry studies reveal that lattice oxygen exchange can affect the top 2 nm of the LaNiO 3 films, but the surface largely remains crystalline and in the perovskite phase after OER. In addition, cyclic voltammetry and potentiostatic measurements show that OER kinetics are strongly pH-dependent, which is different from what is expected from the typical four concerted proton–electron transfer steps, most likely due to the involvement of lattice oxygen. Our findings suggest that the roles of lattice oxygen during OER and the mechanism of charge transfer in such systems need to be further studied in order to design more efficient and stable electrocatalysts.

Abstract Magnetic nanoparticles of Fe3O4 doped by different amounts of Y3+ (0, 0.1, 1, and 10%) ions were designed to obtain maximum heating efficiency in magnetic hyperthermia for cancer treatment. Single-phase formation was evident by X-ray diffraction measurements. An improved magnetization value was obtained for the Fe3O4 sample with 1% Y3+ doping. The specific absorption rate (SAR) and intrinsic loss of power (ILP) values for prepared colloids were obtained in water. The best results were estimated for Fe3O4 with 0.1% Y3+ ions (SAR = 194 W/g and ILP = 1.85 nHm2/kg for a magnetic field of 16 kA/m with the frequency of 413 kHz). The excellent biocompatibility with low cell cytotoxicity of Fe3O4:Y nanoparticles was observed. Immediately after magnetic hyperthermia treatment with Fe3O4:0.1%Y, a decrease in 4T1 cells’ viability was observed (77% for 35 μg/mL and 68% for 100 μg/mL). These results suggest that nanoparticles of Fe3O4 doped by Y3+ ions are suitable for biomedical applications, especially for hyperthermia treatment.

Abstract Organic corrosion inhibitors are playing a crucial role to substitute traditional protective technologies, which have acute toxicity problems associated. However, why some organic compounds inhibit corrosion and others do not is still not well understood. Therefore, we tested different machine learning (ML) methods to distinguish efficient corrosion inhibitors for aluminum alloys commonly used in aeronautical applications. In this work, we have obtained information that can greatly contribute to automate the search for new and more efficient protective solutions in the future: (i) a ML algorithm was selected that is able to classify correctly efficient inhibitors (i.e., with more than 50% efficiency) and non-inhibitors (i.e., with lower or equal to 50% efficiency), even when information about different alloys at different pHs is included in the same data set, which can significantly increase the information available to train the model; (ii) new descriptors related to the self-association of the molecules were evaluated, but improvements to the predictive power of the…

Abstract The wettability of mineral surfaces is an important property influencing multiphase flow in soils and sedimentary rocks. In particular, for CO2 abatement technologies that rely on trapping supercritical CO2 in sedimentary formations, the wettability of relevant mineral surfaces by water is a poorly constrained fundamental property influencing stratigraphic and residual trapping. Theoretical studies have noted that adsorbed water films may hold a key to resolving many of the discrepancies in reported wettability data, but the transition from the droplet to the film is difficult to observe experimentally. The link between continuum and nanoscale observations can be elucidated using molecular dynamics (MD) and thermodynamic equations. We simulate water and CO2 at various pressures between quartz surfaces to probe the thickness of the adsorbed water film observed between the CO2 and quartz, and the radius of curvature of the fluid–fluid interface as a function of CO2 pressure. These results are discussed in the context of the relevant interfacial energies and Young’s equation…

Crystal impurities, such as atomic vacancies, are known to modulate the charge transport characteristics of two-dimensional (2D) materials. Here, we apply a first-principles-enriched tight-binding modelling approach to assess the influence of sulfur vacancies on the electronic structure and quantum transport characteristics of MoS 2 monolayers. To this end, an sp 3 d 5 orthogonal tight-binding (oTB) model of the pristine and defective MoS 2 monolayer is mapped with electronic structure calculations performed at the density functional theory level and subsequently used in the real-space Kubo–Greenwood (KG) scheme for charge transport simulations. The calculated charge carrier mobility is found to be sensitive to both the density and spatial arrangement of vacancies. Our oTB/KG simulations predict a drop of mobility by two orders of magnitude when the vacancy concentration is increased from 0.1 to 3%, in excellent agreement with experimental results. The simulation of realistic samples (including specific types of defects) pave a new route toward the accurate understanding and the…

The solid-electrolyte interphase (SEI) formed through the reductive decomposition of solvent molecules plays a crucial role in the stability and durability of lithium-ion batteries. Here, we investigate the initial process of SEI formation through reactive force field–molecular dynamics (ReaxFF-MD) simulations and density functional theory (DFT) calculations. ReaxFF-MD is used as a simulation protocol to predict the evolution of SEI components, and products are obtained in good agreement with the experimental results. DFT calculations are then used to model the reaction center. We find that one-electron reduction induces the similar breaking of the C—O bond in solvent ethylene carbonate (EC) and additive fluoroethylene carbonate (FEC). When another electron is added, EC decomposition produces gas CO + alkylcarbonate or ethylene (C 2 H 4 ) + carbonate (CO 3 2– ), whereas FEC decomposition generates lithium fluoride (LiF) and vinylene carbonate (VC) in addition to CO + alkylcarbonate. LiF and VC could also be regarded as important electrolyte additives to improve battery performance.…

The design of platinum group metal (PGM)-free catalysts is crucial to the application of energy conversion due to their excellent catalytic capability. Recently, M–N x (M = Mn, Fe, Co, etc.) single-atom catalysts embedded in graphene have been extensively studied with respect to catalyzing the oxygen reduction reaction (ORR). Although the ORR is operated in the liquid phase, few mechanistic studies have taken the solvation effect into consideration. In the present work, we have performed ab initio molecular dynamics (AIMD) simulations as well as density functional theory (DFT) calculations to investigate the influence of the solvation effect on the mechanisms of ORR on MnN 4 –graphene by using explicit water molecules. It is found that the solvent environment can effectively promote the charge transfer from the substrate to O 2, leading to the transformation from superoxide species to peroxide species. This also makes the ORR preferably proceed via a dissociative pathway, where O 2 can be easily adsorbed on the single Mn site in the form of a “side-on” type, leading to the probable…

Abstract The performance of carbon- and silicon-based solid lubricants is strongly affected by the tribochemical reactions, especially with hydrogen and oxygen. Even though understanding and unveiling these reaction dynamics are important to improve performance, these insights are difficult to be obtained by experiments. Molecular dynamics simulation using the reactive force field (ReaxFF) provides a way to study the detailed reaction dynamics at the atomic scale; however, there is no available ReaxFF parameter set which is able to handle all of the desired properties and tribochemical reactions regarding carbon- and silicon-based materials. Here, we succeed to develop a ReaxFF parameter set for the H/C/N/O/Si system which covers most of the current research targets on the carbon/silicon-based solid lubricants using the simulated annealing algorithm to fit with the first-principles calculations. This developed parameter set is able to well describe the chemical reactions of carbon- and silicon-based solids with conventional environmental reactants such as water, H2, and O2…

ZnS nanoparticles (NPs) were synthesized using a simple, green, and reproducible hydrothermal method.Transmission electron micrographs show polyhedral NPs having an average diameter of 21 nm; whereas the X-ray diffraction analysis is consistent with the exclusive presence of cubic ZnS; however no oxide could be detected.A comprehensive characterization of the NPs' surface was accomplished through X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), Raman, and thermogravimetric analysisdifferential scanning calorimetry, showing a fairly pure ZnS composition and a remarkable amount of adsorbed water molecules.The interaction capabilities of the surface were probed in situ by DRIFT using small molecules (CO, CO 2 , methanol, pyridine) as molecular probes.The same interactions were also theoretically studied with density functional calculations using a slab model based on the sphalerite ZnS ( 110) surface.By comparing theoretical and experimental vibrational shifts, insights on the nature of the interaction between molecular probes and…

Abstract CuBi2O4 exhibits significant potential for the photoelectrochemical (PEC) conversion of solar energy into chemical fuels, owing to its extended visible-light absorption and positive flat band potential vs the reversible hydrogen electrode. A detailed understanding of the fundamental electronic structure and its correlation with PEC activity is of significant importance to address limiting factors, such as poor charge carrier mobility and stability under PEC conditions. In this study, the electronic structure of CuBi2O4 has been studied by a combination of hard X-ray photoemission spectroscopy, resonant photoemission spectroscopy, and X-ray absorption spectroscopy (XAS) and compared with density functional theory (DFT) calculations. The photoemission study indicates that there is a strong Bi 6s–O 2p hybrid electronic state at 2.3 eV below the Fermi level, whereas the valence band maximum (VBM) has a predominant Cu 3d–O 2p hybrid character. XAS at the O K-edge supported by DFT calculations provides a good description of the conduction band, indicating that the conduction band…

In the past decades, shale gas has been recognized as the promising unconventional resource for global energy storage, and a clear understanding of the gas-transport characteristic within nonporous shale organic matter (i.e., kerogen) is fundamental for the effective development of shale reservoirs. In this regard, previous studies were generally conducted based on the ideally smooth nanochannels (e.g., graphite slit or tube) without considering the atomistic-scale roughness of the walls. Herein, using molecular dynamics (MD) simulations, we perform a systematical investigation on the gas-transport characteristic through amorphous organic nanopores constructed by realistic kerogen molecules. The results show that the gas-transport velocity in amorphous organic nanopores drops dramatically (40, 70, and 90%) only with tiny roughness factors (0.3, 0.6, and 1.2%) when compared with ideally smooth nanochannels. Further analysis of the potential energy surface and the particle trajectory justifies the entirely different gas-transport mechanisms in ideally smooth (surface diffusion) and…

Boron-dipyrromethene (BODIPY) molecules are widely used as laser dyes and have therefore become a popular research topic within recent decades. Numerous studies have been reported for the rational design of BODIPY derivatives based on their spectroscopic and photophysical properties, including absorption and fluorescence wavelengths (λabs and λfl), oscillator strength (f), nonradiative pathways, and quantum yield (ϕ). In the present work, we illustrate a theoretical, semi-empirical model that accurately predicts ϕ for various BODIPY compounds based on inexpensive electronic structure calculations, following the data-driven algorithm proposed and tested on the naphthalene family by us [Kohn, Lin, and Van Voorhis, J. Phys. Chem. C. 2019, 123, 15394]. The model allows us to identify the dominant nonradiative channel of any BODIPY molecule using its structure exclusively and to establish a correlation between the activation energy (Ea) and the fluorescence quantum yield (ϕfl). Based on our calculations, either the S1 → S0 or La → Lb internal conversion (IC) mechanism dominates in the…

ζ-Potential (ZP) is among key physical properties characterizing the behavior of nanoparticles in colloidal solutions. Despite many attempts to calculate and neatly interpret the ZP, a full understanding of various factors influencing its values has not been achieved yet, even for standard metal oxides, particularly when considering high ionic concentrations and the effect of temperature. This contribution extends our recent work [ Předota, M. Langmuir 2016, 32, 10189−10198], where we suggested a direct approach to calculate the ZP from nonequilibrium molecular dynamics (NEMD) simulations. Here, we investigate NaCl, RbCl, CaCl 2, SrCl 2, and Na 2 C 2 O 4 aqueous solutions interacting with TiO 2 and SiO 2 surfaces and show contrasting ZP behaviors of these metal oxides, elucidated by theoretical insights gained by molecular simulations. We show that both surface-specific and ion-specific properties play a key role in the observed electrokinetics. Additionally, we explore the concentration and temperature influence on the ZP of selected systems and discuss the measurement of the ZP of…

Virtual screenings can accelerate and reduce the cost of discovering metal-organic frameworks (MOFs) for their applications in gas storage, separation, and sensing. In molecular simulations of gas adsorption/diffusion in MOFs, the adsorbate-MOF electrostatic interaction is typically modeled by placing partial point charges on the atoms of the MOF. For the virtual screening of large libraries of MOFs, it is critical to develop computationally inexpensive methods to assign atomic partial charges to MOFs that accurately reproduce the electrostatic potential in their pores. Herein, we design and train a message passing neural network (MPNN) to predict the atomic partial charges on MOFs under a charge neutral constraint. A set of ca. 2,250 MOFs labeled with high-fidelity partial charges, derived from periodic electronic structure calculations, serves as training examples. In an end-to-end manner, from charge-labeled crystal graphs representing MOFs, our MPNN machine-learns features of the local bonding environments of the atoms and learns to predict partial atomic charges from these…

Lithium (Li) dendrite growth poses serious challenges for the development of Li metal batteries. Replacing liquid electrolyte with solid composite electrolyte embedded with nanofiller additives can potentially suppress the Li dendrite growth. However, the underlying mechanism is still not fully understood, and most theoretical works focus on pure liquid electrolyte and ignore the mechanical strain effects. Here we developed a phase-field model to simulate the Li dendrite growth by incorporating the microstructure of the solid composite electrolyte and considering the mechanical effects of the electrolyte. Using aluminum oxide nanofiber embedded poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) as an example, we discovered two key factors, the elastic modulus and the electrolyte nanochannel width that govern the Li dendrite growth. The difference of the Young’s modulus between the Li metal and solid electrolyte acts as the additional mechanical driving force, which partially offsets the electrochemical driving force to either promote or inhibit the dendrite growth. We…

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 ZrS 3, we present a comparative study on the electron and phonon transport properties of bulk and monolayer ZrS 3 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…

Thermal stability of several representative self-assembled monolayers (SAMs) with phosphonic acid (PA) anchoring group on Al 2 O 3 substrates was studied by in situ high resolution X-ray photoelectron spectroscopy, complemented by thermogravimetric analysis of the relevant bulk materials and theoretical simulations. The range of thermal stability and the degradation pathways were found to be dependent on the chemical composition of the SAM forming molecules, with backbone-specific “weak links”. Whereas the anchoring groups were hardly affected by annealing up to 773 K (highest temperature applied for most of the samples), temperature-induced breakup of the bond between the backbone and anchoring group or a specific bond within the backbone was observed. The former is the case for nonsubstituted alkyl backbone, which is cleaved at temperatures above 673–773 K (depending on the backbone length), with the subsequent desorption of the released fragments. The latter is the case for the substituted (pentafluorophenoxy) or partly fluorinated alkyl backbone, with the bond cleavage occurring…

Abstract We study the quantum plasmonic features of gold and silver nanoparticles using TD-DFT+TB, a new density functional theory approach to the calculation of excited states, which combines a full DFT ground state with tight-binding approximations in the linear response calculation. In this framework, the optical properties of closed-shell Ag, Au and bimetallic Ag–Au nanoparticles with tetrahedral symmetry (with 20, 56, 120, and 165 atoms) and icosahedral structure (with 13, 55, and 147 atoms) were obtained and compared to full linear response time-dependent density functional theory (TD-DFT) as a reference and also to time-dependent density functional based tight binding (TD-DFTB) as a low-cost alternative approach. We find an excellent agreement of TD-DFT+TB calculated absorption spectra with the TD-DFT reference with errors less than 0.15 eV in peak positions, while TD-DFTB shows larger errors of about 1 eV. The computational cost for the ground state calculation is identical for TD-DFT and TD-DFT+TB, but the excited state calculation becomes about a hundred times faster when…

The photoactive MIL-125 and MIL-125-NH$_{2}$ Metal-Organic Frameworks (MOFs), despite a very similar crystalline structure, exhibit different optically behaviour. Luminescence in MIL-125 decays in about 1 ns while for its amino counterpart the lifetime of the charge-carriers is at least one order of magnitude larger. The origin of this difference is the key element for understanding the photocatalytic behaviour of MIL-125-NH2 when associated with active nanoparticles, behaviour that is completely absent in MIL-125. By performing advanced ab-initio electronic structure calculations, we find that charge-carriers interact differently in the two MOFs with subsequent effects on the luminescence lifetimes and their catalytic performances. To confirm the predictions of our model we synthesized a novel material in the MIL-125 family, MIL-125-NH2-[10%](OH)2, and confirm that our theory correctly predicts a faster decay compared to MIL-125-NH2.

High Resolution Image Download MS PowerPoint Slide In this work, the electronic and molecular structure, as well as the morphology, of innovative nanostructured materials whose optical properties respond to the presence of heavy metals in water samples was investigated. In particular, the here discussed silver nanoparticles (NPs) stabilized by a hydrophilic ligand (sodium 3-mercapto-1-propanesulfonate, 3MPS) are able to reveal the presence of mercury ions at ppm levels. The sensitivity of the proposed nanosensor to Hg(II) in 1–5 ppm range was ascertained by optical tests; then, the local chemistry and electronic structure of the nanostructured coordination compounds made of functionalized silver nanoparticles (NPs) and metal ions, and their correlation with the resulting nanoaggregates’ shape and morphology, were investigated by state-of-the-art synchrotron radiation (SR)-induced spectroscopies and transmission electron microscopy (TEM). In particular, SR-induced X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements allowed us to gather…

Over the past few years, gold nanostars have gained significant relevance in biology and medicine, in particular, in imaging and photothermal therapy, also owing to their biocompatibility. Furthermore, they have improved the sensitivity of diagnostic assays and have been integrated in lab-on-chip platforms for low-cost, field-deployable devices. However, despite their widespread use, they have not found applicability beyond the research laboratories. One of the main reasons for this limitation has been the almost exclusive focus, until recently, on applications alone and not on the understanding of the fundamental properties of these particles, which has hampered reproducibility, stability, and overall reliability. Recently, however, a resurgence in the fundamental research of gold nanostars has dramatically improved our understanding of their synthesis, functionalization, properties, and thus their applicability. In this Perspective, the literature exploring and improving our knowledge of the physicochemical properties of gold nanostars is critically presented and framed within the…

Expansion of clay minerals in expansive soil, such as montmorillonite (MMT), is critical to a broad class of problems in science and engineering. Cement is one of the most widely used materials in the treatment of expansive soil. However, at present, the expansion mechanism of the soil treated by cement has not been thoroughly investigated at the nanoscale level. In this paper, four types of cement-stabilized MMT (Na-MMT, K-MMT, Ca-MMT, and Mg-MMT) molecular models with different interlayer cations and surface charges are established and validated by experimental data. The maximum adsorption energies of the four different cement-stabilized MMT systems are then investigated through molecular simulations, and the optimum water ratios are proposed for the four cement-stabilized MMT systems to achieve proper volume stability. The numerical simulations also show that the volume change of cement-stabilized MMT depends not only on the water-absorbing capacity but also on the CSH–MMT interface energy as well as the competitive absorption of CSH and MMT. For different types of expansive…

In this study, new highly energetic coordination polymers (ECPs) were successfully prepared by a facile chemical modification method based on functionalized graphene oxide (GO) sheets. Simple cross-linked GO layers with triaminoguanidine ligand (T) may further be bonded with crystals containing T and transition metal ions (T–M) using cobalt, zinc, silver, or iron as coordination centers, generating 2D-layered energetic hybrid ECPs crystals. Successful functionalization of the prepared ECPs was confirmed by SEM, exhibiting very small crystals (<400 nm). The thermal decomposition of the different T–M complexes, with or without GO–T, was assessed by using simultaneous differential scanning calorimetry and thermogravimetric analysis. It has been revealed that the coordination reaction of GO–T with T–M leads to the production of highly thermostable ECPs. Furthermore, the incorporation of 10% of functionalized GO has improved the thermal stability of T–M crystals. Raman spectroscopy demonstrates that both D and G bands of these new ECPs have been shifted. The different chemical bonding is…

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 −SO 2 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…

Molten alkali chloride salts are a critical component in concentrated solar power and nuclear applications. Despite their ubiquity, the extreme chemical reactivity of molten alkali chlorides at high temperatures has presented a significant challenge in characterizing atomic structures and dynamic properties experimentally. Here, we investigate molten NaCl by performing high-temperature molecular dynamics simulations using a Gaussian approximation potential (GAP) trained on density functional theory (DFT) data sets. Our GAP model, trained with 1000 atomic configurations, arrives near DFT accuracy with a mean absolute error of 1.5 meV/atom, thus enabling fast analysis of high-temperature salt properties at large length (5000 ion pairs) and time (>1 ns) scales, currently inaccessible to ab initio simulations. Calculated structure factors and diffusion constants from our GAP model simulations show excellent agreement with experiments. Our results indicate that GAP models are able to capture the many-body interactions required to accurately model ionic systems.

A solid–state reaction method was used to prepare (Sr 2+, Ge 4+ ) co-doped CaCu 3 Ti 4 O 12 ceramics. A single-phase of CaCu 3 Ti 4 O 12 was detected in all the ceramics. An enormous evolution of grain growth in (Sr 2+, Ge 4+ ) co-doped CaCu 3 Ti 4 O 12 ceramics was observed, which was due to a liquid phase sintering mechanism. Theoretical calculations showed that Ge dopant ions are more likely substituted in Cu sites rather than Ti sites. High dielectric permittivity, ∼69,889, with a low dielectric loss tangent, ∼0.038, was achieved in a Ca 0.95 Sr 0.05 Cu 3 Ti 3.95 Ge 0.05 O 12 ceramic. Furthermore, dielectric permittivity at 1 kHz of this ceramic is more temperature-stable than that of the CaCu 3 Ti 4 O 12 and Ca 0.95 Sr 0.05 Cu 3 Ti 4 O 12 ceramics. The enhanced dielectric permittivity with reduced loss tangent in the co-doped ceramics originated from a metastable insulating phase created by a liquid phase sintering mechanism. The local insulating phase along the grain boundary layers can increase the grain boundary resistance as well as the conduction activation energy of the…